When a glassy material or its liquid state is subjected to sufficiently high pressure, significant changes can take place in the short- and medium-range structure, vibrational density of states, and physical properties. It is crucial to determine and understand the structure-property relations under high pressure from both scientific and technological perspectives, since the glass structures frozen-in under elevated pressure may give rise to properties unattainable under ambient pressure. However, the structural and topological origins of the pressure-induced changes in macroscopic properties are not yet well understood. Here, we address this problem by subjecting various isostatically compressed glasses to isothermal annealing at ambient pressure and monitor the relaxation of glass structure and properties as a function of time and temperature. For different glass systems, density is found to relax in a stretched exponential manner with an exponent close to the Phillips value of 3/5 for relaxation in three dimensions when both short- and long-range interactions are activated [1]. For a compressed soda lime borate glass, we find that upon annealing at 0.9T_g, the pressure-induced increase in boron coordination remains unchanged, while the pressurized values of macroscopic properties such as density, refractive index, and hardness are relaxing [2]. Hence, the pressure-induced changes in macroscopic properties are not necessarily attributed to changes in the short-range order in the glass, but rather to changes in overall atomic packing density and medium-range structures. Moreover, we show that the relaxation mechanism depends on the annealing temperature and different physical properties (e.g., density and hardness) are found to relax on different timescales. [1] M. M. Smedskjaer, S. J. Rzoska, M. Bockowski, J. C. Mauro, Journal of Chemical Physics140, 054511 (2014). [2] M. M. Smedskjaer, R. E. Yougnman, S. Striepe, M. Potuzak, U. Bauer, J. Deubener, H. Behrens, J. C. Mauro, Y. Z. Yue, Scientific Reports4, 3770 (2014).

This talk reports on two interesting new phenomena recently discovered in glasses.
The first part of the talk will discuss the elastic properties of vitreous silica submitted to high pressures in a diamond anvil cell. We show that the compressibility of a silica sample immersed in helium or neon fluid is much smaller than expected from its elastic properties measured by Brillouin light scattering. It results from gas atom penetration into the interstitial free volume of the glass network. This adsorption-induced expansion can be described by a generalized poromechanical model. The second part of the talk will address the long-standing question of the nature of the glassy state. It is commonly believed that far below the glass transition temperature, dynamics in glasses is frozen, i.e. relaxations occur on time scales too large to be observed. X-rays photon correlation spectroscopy is used to reveal the existence of surprisingly fast atomic rearrangements in a sodium silicate glass, even at room temperature. These results challenge our conception of the glassy state and call for a new microscopic theory.
References:
(1) C. Weigel, A. Polian, M. Kint, B. Rufflé, M. Foret, and R. Vacher, Vitreous Silica Distends in Helium Gas: Acoustic Versus Static Compressibilities, Phys. Rev. Lett. 109, 245504 (2012).
(2) B. Coasne, C. Weigel, A. Polian, M. Kint, J. Rouquette, J. Haines, M. Foret, R. Vacher, and B. Rufflé, submitted.
(3) B. Ruta, G. Baldi, Y. Chushkin, B. Rufflé, L. Cristofolini, A. Fontana, M. Zanatta, and F. Nazzani, Revealing the fast atomic motion of network glasses, Nat. Commun. 5, 3939 (2014).

The constituents of any network glass can be broadly classified as either network formers or network modifiers. Network formers, such as SiO₂, Al₂O₃, B₂O₃, P₂O₅, etc., provide the backbone of the glass network and are the primary source of its rigid constraints. Network modifiers play a supporting role, such as charge stabilization of the network formers or alteration of the network topology through rupture of bridging bonds and introduction of floppy modes. The specific role of the modifiers depends on which network formers are present in the glass and the relative free energies of modifier interactions with each type of network former site. This variation of free energy with modifier speciation is responsible for the so-called mixed network former effect, i.e., the nonlinear scaling of property values in glasses having fixed modifier concentration but a varying ratio of network formers. In this talk, a general theoretical framework is presented describing the statistical mechanics of modifier speciation in mixed network glasses. The model provides a natural explanation for the mixed network former effect and also accounts for the impact of thermal history and relaxation on glass network topology.

The development of effective new tools for structural characterization of disordered materials and systems is becoming increasingly important as such tools provide the key to understanding, and ultimately controlling, their properties. The relatively novel technique of correlograph analysis (i.e. the approach calculating angular autocorrelations within diffraction patterns) promises unique advantages for probing the local symmetries of disordered structures. Because correlograph analysis examines a component of the high-order four-body correlation function, it is more sensitive to medium-range orders than conventional diffraction methods. Generally, electron correlograph analysis would be implemented in a transmission electron microscope (TEM) operating in the STEM mode where a nanoscale probe can be controllably created and translated to regions of interest on the specimen. It can be primarily used for characterizing local diffraction symmetry of disordered solid materials, such as amorphous semiconductors, oxides, and metallic glasses. Like fluctuation electron microscopy, electron correlograph analysis is a statistical method, replying on a sufficiently large sampling to distinguish between random and truly persistent structural correlations. The Friedel correlation in the mean electron correlograph unveils the crystallinity of samples with medium-range orders, an important structural parameter that cannot be measured by other techniques. In the presentation, I will describe the practical experimental method and common systematic errors of electron correlograph analysis. First, I will discuss the effects of beamstop, diffraction center locating, and distortions on the electron correlograph analysis. Second, by using both experimental data and numerical simulations, I will elucidate why those seemingly real correlations showing up in one or a few correlographs do not actually reflect the local symmetries of disordered structures, and emphasize that reliable structural information about the sample can only be obtained from the mean correlograph, which is averaged over results from an adequate number of individual diffraction patterns (typically > 50).

I will review our previous work in B₂O₃ glass [1,2] as well as its recent extensions in the liquid phase and in other systems. Using ab-initio simulations, we predicted new low-density crystalline polymorphs [1] which provide a framework to understand the glass properties [2]. Thanks to NMR, the new crystals are shown to be structurally much more similar to the glass than is any of the known polymorphs. Some possible routes to synthetise these crystals will be mentionned. I will also discuss the implications of these findings for the liquid behavior. In particular, the possible existence of a liquid-liquid transition, much similar to that hypothetized in water will be raised.
[1] G. Ferlat et al., Nature Materials, 11, 925 (2012)
[2] G. Ferlat et al., Phys. Rev. Lett., 101, 065504 (2008)

Amorphous transparent conducting oxides (TCO) are of considerable technological interest due to their unusually high electron mobilities, excellent optical properties, wide thermal stability and ease of processing. Significant progress has been made regarding the synthesis and optimization of these thin films (notably Zn and Sn-doped In₂O₃) for a wide range of applications. However, the structural changes upon doping as well as the exact role played by the short and intermediate-range order in defining their valuable optical and electronic properties is still lacking. This knowledge is desirable as it can be leveraged to direct research efforts towards other materials more abundant than Indium, or even point towards compounds hitherto unexplored. Short-range order (typically < 5Å) can be obtained through analysis of extended x-ray absorption fine structure (EXAFS) data which yields a partial structure function centered on the absorber atom. A complementary technique yielding structural information on longer length-scales, i.e. medium-range order, is to analyze the complete pair distribution function (PDF) obtained from total X-ray scattering data. Essentially, total scattering data is the properly corrected and normalized elastic scattering i.e. I(Q) vs. Q, where Q is the scattering vector. The PDF is a real-space distribution of atoms obtained through an appropriately weighted Fourier transform of this function. Taken together, EXAFS and PDF data contain important structural information such as bond distances, bond angles, coordination numbers and structural coherence lengths which are otherwise challenging to access in poorly ordered materials. In this study, we analyze EXAFS and grazing-incidence PDF data on amorphous In-Zn-O films with varying In:Zn ratios. Samples of a given composition but possessing significantly different electronic conductivities displayed only subtle changes in both XAS and PDF data. Further, the PDF data show clear ordering up to almost 3 nm indicating that the films are not truly amorphous. These observations have implications for both theoretical and experimental studies and will be discussed along with other structural insights on the In-Zn-O thin films with the broader aim of arriving at a better understanding of the short and medium-range order in amorphous TCO materials.

Drawing inspiration from nature, where some organisms can control the short-range order of amorphous minerals, we successfully manipulated the short-range order of amorphous alumina by surface and size effects. By utilizing the Atomic Layer Deposition (ALD) method to grow amorphous nanometrically thin films, combined with state-of-the-art electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we show experimentally that the short-range order in such films is strongly influenced by size. This phenomenon is equivalent to the well-known size effect on lattice parameters and on the relative stability of different polymorphs in crystalline materials. Additionally, direct measurements of crystallization temperature have shown a dramatic increase in amorphous to crystalline phase transition temperature for thinner amorphous Al₂O₃ films. Finally, we show that the short-range order changes while still in the amorphous phase, before the amorphous to crystalline transformation takes place.
¹ Leonid Bloch, Yaron Kauffmann, and Boaz Pokroy. Crystal Growth & Design. In press 2014.

Originally, rigidity in glasses was formulated by J.C. Phillips in order to understand the relationship between cooling speed, which measures glass formation ability, and chemical composition. Although in the last years much of this picture has been confirmed by many experiments and simulations, still it is not clear how minimal cooling speed depends on the composition. Here a simple model is provided in which two key ingredients are considered in the glass, metastable states and their multiplicity. Metastable states are considered as in two level system models. However, their multiplicity and topology allows a phase transition in the thermodynamic limit, while a transition to the glass is obtained for fast cooling. By solving the corresponding master equation, the minimal speed of cooling required to produce the glass is obtained as a function of the distribution of metastable and stable states. This allows to understand cooling trends due to rigidity considerations in chalcogenide glasses as in metallic glasses doped with group IV impurities, like the Pd-Ni-P-Si compound.

We report that the properties of metallic glasses can be dramatically tailored by slow deposition at high temperatures without altering chemical composition. The glass transition and crystallization temperatures of the metallic glass can be increased as high as 83 K and 269 K, respectively, compared to the room-temperature deposited one and melt-spun ribbons. The high-temperature deposited glass also shows ultrahigh hardness, over 60 % higher than room-temperature deposited glass. Atomic structure characterization reveals that the exceptional properties of the high-temperature glasses are associated with abundance of medium range order contains mainly distorted icosahedral order with partially cubic symmetry. The finding of the high-temperature deposited glass gives insight on atomic mechanisms of metallic glass formation and has important impact on the technological applications of metallic glasses.

That shear causes dilatation in densely packed granular systems has been known and studied since the days of Osborne Reynolds (1885). A similar effect occurs in dense packings of hard spheres, which are a useful analogues of metallic glasses. It is now possible to follow this process directly in colloidal hard-sphere glasses, by tracking the invidual particles by confocal microscopy. In dense hard-sphere systems, the moduli are strong functions of the density (they diverge at close packing). These moduli can be measured from the thermal distribution of the local strain energy densities. These measurements show a decrease in the shear modulus (and hence density) upon plastic shear deformation, and a relaxation back to the initial state after the deformation stops. Microscopic mechanisms for these density changes as well as their implications for strain softening and shear instabilities in metallic glasses will be discussed.

Owing to the high cooling rates necessary for glass formation in metallic glasses, parts with a complex geometry have been difficult to cast directly. The metallic glasses' remarkable softening behaviour above the glass transition temperature T_g has led many to suggest, then develop, thermoplastic forming as an alternative manufacturing method for complex parts. Cerium-based BMGs, exhibiting very low T_g as well as a large supercooled liquid region (SCLR), provide an excellent platform on which to study such thermoplastic forming. In order to investigate and develop new thermoplastic forming processes, the SCLR of these alloys must first be appropriately characterised. Heating rate, processing temperature, and strain rate have been examined; their effects on viscosity, flow, and devitrification explored; and the results and conclusions are hereby reported.

Many materials have a high propensity to crystallize as this is the energetically most favorable state; it is thus difficult to make these materials amorphous. However, for certain applications, it would be highly beneficial to make these materials amorphous as many of their physicochemical properties differ from those of their crystalline couterparts. For example, amorphous materials have a much higher solubility than crystalline ones. I will present a microfluidic spray drier, we call it a microfluidic nebulator, that produces very small drops through the use of supersonic air. These small drops serve as containers to form small amorphous nanoparticles; the size of these nanoparticles is determined by the number of solute molecules contained in the drop. The nanoparticle structure is determined by the probability for a crystalline nucleus to form as the drop evaporates; this probability is typically very high in supersaturated solutions as crystalline solids readily form under these conditions. However, the formation of a crystalline nucleus itself entails some time delay. We demonstrate that the nebulator can kinetically suppress the formation of crystalline nuclei; thereby, it produces amorphous nanoparticles from many different materials, even from materials that have a very high propensity to crystallize.

One of the most intriguing issues in supercooled liquids and glassy materials is their dynamic relaxation behaviors, which underlies their physical and mechanical properties. Recently, Yu et al. found that the b-relaxations in metallic glass-forming liquids is sensitive to the chemical interactions among all the constituent atoms. In other words, b-relaxations in metallic glasses are linked to a large mixing enthalpy for the atoms.
We use mechanical spectroscopy to investigate the mechanical relaxation (a and b relaxations) in several typical bulk metallic glasses in isochronal route as well as isothermal mode: Pd-Ni-Cu-P, La-Ni-Cu-Al and Cu-Zr-Al-Dy systems. We investigated the dependence of the mechanical relaxation on the physical aging to study the evolution of the localized atomic mobility with the physical aging. The pronounced JG b relaxations in metallic glasses are associated to large negative values of the enthalpy of mixing among the constituting atoms, these results are in good agreement with the recent report by Yu et al.

Resistive switches offer the prospect of improved performance, efficiency and scalability over current data storage methods. Many device architectures have been proposed, reliant upon a wide variety of materials whose conductance switches in a non-volatile manner with the application of an applied field. Silicon-based switching materials are of particular interest in these devices as they offer the added potential for integration into existing CMOS infrastructures. It is of great importance that the underlying physics of switching is well-understood, such that device optimisation and integration into commercial hardware may be realised. Our device layers are sputter-deposited to create a 37nm thick, amorphous, non-stoichiometric silicon-rich layer of silica sandwiched between conductive electrodes. We report on the material changes leading to reversible resistive switching in silicon suboxide using secondary ion mass spectroscopy (SIMS), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and conductive atomic force microscopy (cAFM). Analysis with a range of techniques serves to highlight the broad dynamics of device behaviour, and supports the model of an ionic and defect-dependent switching mechanism which relies upon the presence of nanoscale grain boundaries within the amorphous silica switching layer.

Our recent research accomplishments led us believe that Ga substitution on Al sites need not permit it to remain in trivalent state only. The monovalent Ga atom, has atomic radius is greater than that of trivalent one. In contrast, pentavalent state of Ga has atomic radius smaller than that of trivalent one. The change of valency of Ga in metallic glass composition will lead to change in geometry of main cluster that promotes glass forming ability of the melt. Further, the mixed states of Ga may facilitate formation of clusters of different types leading to phase separation in the melt and thereby in metallic glasses. Keeping these in mind we have recently investigated two such compositions. They refer to Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ and Ce₇₅Al_25-xGa_x metallic glass compositions. The purpose of this presentation will be to bring out aforesaid features after Ga substitution on the nature of glass forming ability based on our studies on these alloy systems.
The first part of the presentation deals with the behavior of Zr_69.5Al_7.5minus;xGa_xCu₁₂Ni₁₁ (x = 0, 1.5, 7.5 at.%) alloys. A comparison between their nanohardness and reduced elastic modulus values of the as-synthesized glassy phase with their nanocomposites has been made. The indentation characteristics of a novel Ga substituted glass composition corresponding to x = 7.5 have shown significant improvement in regard to hardness and elastic modulus. The evidence of pile up has been observed in case of as-synthesized glassy ribbons. The load (P) versus depth (h) curves for as-synthesized melt-spun ribbons displayed the presence of displacement burst, which are known as pop-ins. The amount of energy per unit volume required for the shear band formation in glassy state has been estimated based on the pop-ins observed in P-h curve. This seems to decrease with Ga addition. Based on transmission electron microscopic observations of indented glassy specimen, the possibility of nanocrystallization has been ruled out.
The glass forming ability and indentation characteristics of melt-spun Ce₇₅Al_25-xGa_x (x = 0, 2, 4 and 6 at.%) metallic glasses will also be presented. The substitution of Ga decreases the glass transition temperature, while increases the supercooled liquid region. The small amount of Ga substitution has led to appearance of second diffuse halos in the X-ray diffraction (XRD) pattern. The observation of phase separation is thus asserted. The transmission electron microscopic images led us to conclude the formation of nano-amorphous domains after Ga substitution in a glassy matrix. The load dependent hardness behaviour of metallic glasses with and without Ga substitution has been studied. The substitution of Ga improves the microhardness property of Ce-Al alloy. The formation of shear bands around the indentation periphery has been observed. The value of yield strength and Meyer exponent of these alloys has been compared.

We investigate the crystallization of an amorphous eutectic Ni-P alloy, fabricated by electroless deposition as a 10 um thick continuous layer. The purpose of this work is to obtain a detailed understanding of the microscopic mechanisms of crystallization and phase formation upon heating. To this end, we combine a variety of complimentary characterization techniques. DSC (differential scanning calorimetry) during isothermal and isochronal heating reveals the crystallization kinetics. XRD (X-ray diffractometry) provides quantitative information on phase composition as a function of tempering time. Conventional-, high-resolution-, and analytical TEM (transmission electron microscopy) and TEM-based electron diffraction provide high-spatial-resolution information on phase nucleation and spatial distribution of atom species, particularly the crystallography of the nucleating crystalline phases (Ni3P and Ni) and the spatial distribution of phosphorus in the partially and completely crystallized alloy. Our results confirm earlier observations, according to which crystallization proceeds by formation of barrel-shaped grains. Internally, these exhibit a radial microstructure of of slightly misoriented subgrains of Ni3P containing Ni needles with unique ORs (orientation relationships) to Ni3P. However, the preferred Ni-Ni3P ORs we observe differ from those reported in the literature. Combining our observations on the structure and microstructure of partially and completely crystallized Ni-P with the observed crystallization kinetics leads to a new model for the microscopic mechanism of crystallization. This may be useful for understanding the effect of ternary alloying elements on the crystallization behavior and temperature.

V-Zr amorphous thin films potentially have a number of applications as coatings for components subject to aggressive chemical and physical conditions. In this work V-Zr thin films were produced by magnetron sputtering at room temperature and were subsequently characterized and modelled; using atomic scale techniques, to understand the alloys&’ stability and the relationship the amorphous material has to the crystalline Zr, V and V₂Zr phases.
Atomic scale modelling using density functional theory was used to investigate three compositions in the V-Zr system: equimolar VZr, the V rich V₂Zr and Zr rich VZr₂ compositions. Amorphous supercells were created and minimised under constant pressure with a variety of starting densities and supercell sizes. We find that the simulation conditions are pivotal in reproducing the experimentally observed structure. Bulk X-ray diffraction techniques were used as a first check to compare experiment and theory with encouraging results.
Ensembles of electron nanodifraction patterns were collected from the three formulations of amorphous V-Zr films. The electron beam was tuned to the diameter of the nearest-neighbour clusters. In this geometry, and collecting diffraction patterns from sub-4 nm thick regions, angular correlations in the diffraction patterns reflect the symmetries of nearest-neighbour polyhedra. [1] An angular autocorrelation technique was used to quantify the statistical incidence of symmetries in the diffraction patterns of each specimen as a function of the scattering angle. It was found that the symmetries in the diffracted intensities from all the films could not be explained by BCC local ordering alone, but required the presence of more complex intermetallic local ordering, such as the C15 Laves phase (V₂Zr). The experimental data matched simulations from the amorphous supercells.
[1] A. C. Y. Liu, M. J. Neish, G. Stokol, G. A. Buckley, L. A. Smillie, M. D. de Jonge, R. T. Ott, M. J. Kramer, and L. Bourgeois, Phys. Rev. Lett., 110, 205505 (2013)

The stability of undercooled liquids against crystallization is affected by initial viscosity of the stable liquids, viscosity change of the melt due to temperature drop, free energy difference between undercooled liquids and crystalline phases, liquid/solid interface energy, bulk density, non-uniform nucleation, cooling rate of particle and so many other factors. If the cooling rate is fast enough that diffusion among the atoms is limited, the amorphous phase is uniformly solidified. This behavior is called glass transition behavior, and several metallic alloys with such behavior have been discovered, which can be vitrified with only radiative cooling. For example, containerless high-temperature/high-vacuum electrostatic levitation (ESL) technique, which could minimize heterogeneous sites, is useful for investigation of the undercooled liquid behavior. In particular, several high glass-forming alloys, whose cooling rate is fast enough that diffusion among the atoms is limited, can be vitrified with only radiative cooling in these ESL. In this study, we measured the thermo-physical properties and Time-Temperature-Transformation (TTT) diagram for the crystallization of various amorphous metal system from liquid temperature to glass transition temperature, using ESL technique. After that, we analyzed atomic scale structural change of the samples with high energy X-ray scattering experiments to understand the influence of compositional change on glass-forming ability. Indeed, these results could provide a clear understanding of the exceptional stability against crystallization in amorphous metal based on kinetic, thermodynamic and structural principles. Undercooled liquids&’ thermophysical properties and their own structure variation have a pronounced influence on the glass forming ability and knowledge on it is essential in understanding more precisely the crystallization mechanism of amorphous metal.

Recently, by using the techniques of HREM, HAADF and EDX line mapping, it [1] was found that Pd-Ni-P bulk metallic glasses (BMGs) undergo amorphous phase separation (APS). Later, by using small angle X-ray scattering method, it [2] was confirmed that the observed APS is consistent with a spinodal mechanism. It is not clear, however, why Pd-Ni-P BMGs, an alloy system of negative heat of mixing, undergo amorphous spinodal decomposition (ASD). In this work, we attempt to measure the metastable liquid miscibility gap by employing the same technique as described in [1].
[1] S. Lan, Y.L. Yip, M.T. Lau, H.W. Kui, J. Non-Crystalline Solids 358 (2012) 1298-1302.
[2] Z.D. Wu, X.H. Lu, Z.H. Wu, H.W. Kui, J. Non-Crystalline Solids 385 (2014) 40-46.

A key challenge for understanding plasticity in amorphous materials is the link between microscopic (i.e., atomistic) and macroscale behaviour (i.e., the scale of the sample). To establish this link, it is useful to coarse-grain the microscopic details and construct a mesoscopic model which tries to capture important aspects from the atomistic scale without fully resolving the atomic motions, being thus computationally efficient. The common approach taken in mesoscale descriptions of amorphous plasticity is to consider plastic activity as a sequence of spatially and temporally localized events, known as shear transformations, which correspond to local rearrangements of atoms within the bulk material in response to the locally acting shear stress. This rearrangement in turn induces an internal (eigen-) stress field which - together with external stresses - determines the spatial and temporal pattern of shear transformation events which characterizes the plastic deformation dynamics of the amorphous structure. We present a mesoscale approach to amorphous plasticity based on a finite element framework with quasistatic, athermal dynamics analogous to lattice based models proposed in the literature, which use the (periodically continued) elastic Green's function of an infinite body to calculate internal stresses in systems with periodic boundary conditions.
The tensorial finite element formulation allows us to analyze a rich variety of collective phenomena such as strain localization and strain avalanches, comparing and benchmarking our results against the established lattice models. The main advantage of the finite element method over these models is that the FEM approach provides natural access to analysing the effects of surfaces and boundary constraints, and hence to physical mechanisms leading to finite size effects which cannot be captured by models which employ periodic boundary conditions. We analyse the results for the deformation behavior of 2D amorphous specimens under non-trivial loadings, such as shear or bending, and analyse the influence of strain gradients, surfaces, and finite size effects. We find that we can conceptually envisage systems with free surfaces as consisting of two distinct regions: an interior “bulk-like” domain where plastic activity evolves essentially as observed in a periodic system, and another “surface-dominated” domain where the interactions between shear transformations are suppressed and deformation behaviour is controlled by surface constraints and external tractions. We quantitatively analyse the deformation behaviour arising from the interplay of surface and bulk (statistical) size effects in 2D and present some preliminary results for 3D specimens.

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu / 100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with non-crystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

The alloy system, Pd-Ni-P, has a negative heat of mixing. It is therefore somewhat surprising that Pd-Ni-P bulk metallic glasses (BMGs) undergo amorphous phase separation (ASD) [1, 2]. An attempt has been made to measure the metastable liquid miscibility (MLMG) of undercooled molten Pd-Ni-P. In this work, X-ray diffraction method is employed to characterize undercooled specimens that have entered into the MLMG.
[1] S. Lan, Y.L. Yip, M.T. Lau, H.W. Kui, J. Non-Crystlline Solids 358 (2012) 1298-1302.
[2] Z.D. Wu, X.H. Lu, Z.H. Wu, H.W. Kui, J. Non-Crystalline Solids 385 (2014) 40-46.

A novel method is introduced to determine notch toughness of bulk metallic glasses (BMGs). Through thermoplastic forming of BMGs combined with Si photolithography, unprecedented control in fabricating BMG notch toughness samples can be achieved and many extrinsic influences (e.g., cooling rate, casting defects) can be drastically reduced. Our approach allows us to precisely vary the notch root radius with a high precision of ~ 1 mu;m. In order to systematically study the effect of notch root radius on the notch toughness of BMGs, notches with a broad range of radii from 3 mu;m to 380 mu;m are introduced into the test specimens. Through this method, we have fabricated ~ 100 samples with different notch radii (ρ = 3, 10, 25, 50, 100, 150, 230, 380 mu;m) in two different BMG systems, i.e., Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅ and Pd₄₃Cu₂₇Ni₁₀P₂₀ BMGs. We found that there exists a critical notch radius, ρ_c, above which the apparent notch toughness scales linearly with ρ^1/2. With decreasing notch radius below ρ_c, the notch toughness retains almost constant rather than degrading. However, this critical notch radius varies in the two different investigated BMGs that exhibit distinct toughness behavior. Such observed phenomenon can be well explained within the framework of characteristic critical distance theory and is similar to previously reported for crystalline metals, ceramics, and polymers, although the mechanistic origins are different. Our finding also suggests that BMGs can be highly tolerant to sharp flaws.

The presence of minute crystalline phases in metallic glasses is unfavorable for the reliable understanding of the intrinsic properties of the bulk metallic glasses(BMG) [1]. However, the detail of the sample characterization has often been omitted in the literature. In this study, the amorphous state of the Zr-Al-Cu-(Ni,Fe) BMG are thoroughly investigated using high intensity 50mu;m X-ray beam with a 2-dimensional detector in order to probe possible existence of minute crystalline phases that are not detectable by conventional X-ray equipments. So far, the results have shown that a small amount of crystalline phases exists in the cross section of most of the Zr-based BMG that were prepared in a conventional method. Experiments are currently being undertaken to fabricate Zr-based BMG without minute crystalline phases to perform further measurements such as mechanical properties. The detail of the results will be presented at the poster.
[1]Q.S Zhang et al. Acta Materialia 58 (2010)904-909

It is generally assumed that the glass transition temperature, T_g, of a bulk metallic glass, BMG, is only a shallow function of the composition. We give evidence for several metallic glasses derived from quarternary (Pt+TM)₆₆B₂₄(Si+Ge)₁₀ systems exhibiting strong changes of T_g (from 570 K to 650K) with varying the type of TM(=transition metal) used, resulting in virtually unchanged reduced glass transition temperature over a wide range of compositions.
The access to a wide range of T_g with minor levels of substitution opens the door to investigate whether the T_g can be shifted with regard to the critical fictive temperature, T_fc, or whether the latter stays put relative to the T_g. As proposed by Kumar et al.¹, the relative position of these two temperatures essentially defines whether a BMG will be ductile, brittle or cooling-rate sensitive in its mechanical behavior.
We further present mechanical properties derived from 3-point bending test, microhardness and elastic constants from elastic wave measurements.
1. Kumar, G., Neibecker, P., Liu, Y. H. & Schroers, J. Critical fictive temperature for plasticity in metallic glasses. Nat. Commun.4, 1536 (2013).

Mechanical properties in nano-sized metallic glasses have been widely investigated to clearly understand the fundamental deformation mechanism of metallic glasses. In particular, the change of yield strength and the occurrence of deformation mode transition (heterogeneous to homogeneous) with sample size reduction have been reported in previous experimental studies through the nano-pillar test results. Research on nano-mechanical responses of metallic glasses could lead to an in-depth understanding of the deformation mechanism related to the nucleation and propagation of shear bands. However, during the pillar fabrication using focused ion beam for nano-pillar test, Ga+ ion beam damage on pillar surface is difficult to avoid and the fabrication takes relatively long for multiple pillar preparations. To avoid these drawbacks of nano-pillar tests, in the present study we performed nano-compression tests of metallic glass nanoparticles prepared by FIB-less method. In our experiments, metallic glass nanoparticles have been fabricated by the selective dissolution technique from phase separated metallic glasses. From the two-amorphous alloys with droplet structure, the reactive matrix phase has been selectively dissolved by chemical process. The shape of the remaining particle phases was clearly spherical, and the sizes of fabricated particles have distribution in 40~500 nm diameter range. The effect of particle size and strain rate on mechanical properties of fabricated amorphous particles has been investigated from the compression test using in-situ compression tester with electron microscope imaging. Due to the distinct morphology of particles compared to that of nano-pillars, the stress-strain relations from particle compression test results are derived based on contact mechanics theory and contact area calculation. From the stress-strain curves and in-situ electron microscope video, we confirmed the metallic glass nanoparticles are not fractured at high strain over 50% and have favorable condition for homogenous-like deformation at strain rate near ~10^-3 s^-1 based on modified Griffith criterion. These results provide us with insights on evaluation of nano-mechanical properties and understanding of fundamental deformation mechanism of metallic glasses.

It is well known that applications of bulk metallic glasses (BMGs) for structural materials are limited by their brittleness in spite of desirable mechanical properties such as high yield strength, high elastic limit, and relatively low elastic modulus. To overcome this drawback, various composite microstructures with structural heterogeneity of micrometer to nanometer scale in the amorphous matrix have been developed. The heterogeneity can result in nucleation of multiple shear bands and interruption of their propagation, which can lead to enhanced plasticity. In the present study, in order to precisely control the amorphous heterogeneities or amorphous 2nd phases in the amorphous matrix, thermally activated growth behaviors of secondary amorphous phases in Cu-(Zr,Hf)-Al-(Y,Gd) phase separating metallic glasses have been investigated using in-situ heating SAXS experiment and additional intensive structural analysis. From the combination of two ternary systems, Cu-(Zr,Hf)-Al and Cu-(Y,Gd)-Al, phase separating metallic glasses with two distinct amorphous phases were fabricated in Cu-(Zr,Hf)-Al-(Y,Gd) quaternary system. Experimentally, the growth behavior of secondary amorphous phase was investigated by SAXS analysis at elevated temperature near first T_g and T_x. Also, the chemical composition of 2nd phase was evaluated using alloy contrast variation (ACV) method from comparison of SAXS/SANS patterns of various alloy compositions. From the results, the relationship between the composition/size of 2nd phases and the mechanical properties in the phase separating metallic glasses was analyzed. This provides us with a fruitful idea on how to control the properties of metallic glass composites from manipulating the 2nd phases in nanometer scale.

A structural amorphous metal (SAM) containing 0 to 30 vol% of W nanoparticles was tested by several techniques, including micro-bending tests. The density values reached up to 10.98 g/cc with the addition of W nano particles. Nanoindentation tests were performed in order to study the compositional effects on the hardness and modulus. Indentation mapping results show a significant increase in hardness values in the areas surrounding the W particles. Extensive analysis was performed around the nanoparticles in order to study if and how the nanoparticles could aid in crack deflection and improve the fracture toughness.

Rejuvenation is a structural excitation of glassy systems. Rejuvenation increases free volume and potential energy, and is one of the promising approaches to improve the deformability of amorphous metals, which generally exhibit brittle fracture. We here focus on the rejuvenation of ternary amorphous metal induced by thermal loading process. Using molecular dynamics techniques and embedded-atom method interatomic potentials, a ZrCuAl amorphous alloy model was constructed via melt-quenching process. Subsequent thermal loading simulations composed of reheating, isothermal annealing, and requenching were then conducted with various annealing temperatures and final quenching rates. Based on change in potential energy and volume during the thermal loading process, we evaluated level of aging and rejuvenation induced by the thermal loading; aging decreases and rejuvenation increases potential energy and volume. It is observed that the thermal loading induces rejuvenation when an annealing temperature is higher than a critical temperature T_c (T_c is higher than the glass transition temperature T_g) and a cooling rate after isothermal annealing is higher than that of initial melt-quenching process. This is because that erasing of the past aging history by heating at sufficiently high temperature is required to realize thermal rejuvenation. Moreover, a higher quenching suppressing aging during final quenching process is another necessary condition for thermal rejuvenation. We drew a rejuvenation map with respect to both annealing temperature and final cooling rates. The level of rejuvenation increases with increasing quenching rate and annealing temperature up to 1.3T_g. Changes in properties induced by rejuvenation were also investigated; aging increases and rejuvenation decreases topological short-range and medium-range order and elastic constants of amorphous metals. Our results could be useful information when we apply rejuvenation as a practical tool to control properties of amorphous metals.

Today, demand of electric power is expanded with population growth. We must effectively use electric power to build a sustainable society. When electric power comes to user from the generation plant, energy loss occurs in the transformer. Total energy loss is divided into iron loss and copper loss. Amorphous alloy gets much attention because of reduction in this iron loss. Planar Flow Casting (PFC) is well known as an industrial technique for rapid quenching to produce the amorphous ribbon used mainly as soft magnetic materials
However, this amorphous ribbon has defect called air pocket. The air pocket is considered to be formed by air layer induced by wheel revolution. The air pocket is concave defect and roles to make high the iron loss in electric energy loss. So, the purpose of our study is to investigate the influence of process parameters on the formation of the air pocket in order to fabricate more efficient amorphous ribbon in PFC process.
In PFC process, molten alloy is impinged strongly on rotating wheel at small gap between nozzle and wheel surface less than 1mm by high gas pressure to form wide amorphous ribbon. Applying the small gap results in the planar flow of molten alloy ejected from the nozzle, though the molten alloy is impinged strongly onto a wheel surface to get high cooling rate. Main process parameters are ejection gas pressure, gap between nozzle and wheel surface, and wheel surface velocity. We focus on wheel surface velocity, because it is a most convenient process parameter in operating PFC process. Clear differences in the size of the air pockets were apparent in casting under changing only wheel surface velocity. We would like to explain the influence of wheel surface velocity on the formation of air pockets.

In the past decades, there has been a large amount of scientific and industrial interest in metal matrix composites as a way to improve mechanical properties compared to the unreinforced alloys. Recently, new perspectives with 3D network reinforcement have been opened by the availability of ceramic scaffolds produced by freeze-casting of ceramic suspensions. Bulk metallic glasses (BMGs) have high yield strength and elastic strain limit combined with a good corrosion resistance. However, lack of macroscopic ductility could be an important drawback in many applications. The introduction of secondary phase into the glassy matrix encourages the formation of multiple shear bands and interferes with the propagating shear band, which delay failure and improve the toughness of the BMGs. In this study, Innovative BMG/ceramic composites produced by reactive infiltration of the melt into ceramic scaffolds prepared by freeze-casting technique. The ZrCuAgAlBe alloy system with the highest GFA in Zr-based BMGs was selected as a matrix because an interfacial reaction can deteriorate thermal stability of glassy matrix. The microstructure, including interfacial wetting behavior and interfacial adhesion between the scaffolds and the matrix, and mechanical properties of the BMG composites were systematically investigated.

Mechanical treatments, such as hot-and cold-working, are commonly used to optimize mechanical properties for polycrystalline metals, and there has long been interest in exploring similar possibilities for metallic glasses. For example, various mechanical treatments such as cold rolling or shot peening have been introduced to alter the glassy structure including structural anisotropy, dilatation and formation of free volume, which cause to improve plasticity of monolithic metallic glasses. High pressure torsion (HPT), in which a disk-shaped sample is deformed by torsional straining under pressure, has shown to be an efficient technique for grain refinement in the nanometer regime in various materials thereby significantly enhancing mechanical properties such as micro-hardness, tensile strength and ductility. Contrary to crystalline materials, it is recently proposed that HPT process can cause structural rejuvenation and mechanical softening of bulk metallic glasses, but there are still limited approaches to understand the effect of HPT on BMGs. In this study, spark plasma sintered BMG and their composites were subjected to a large amount of deformation by HPT process which enables us to apply severe deformation for brittle materials. As a result, HPT process increases the structural disordering and amount of excess free volume in the amorphous structure which suggests structural rejuvenation by heavy deformation. Interestingly, the tendency of rejuvenation changes by depending on the characteristics of secondary phases in BMG composites. Furthermore, the variations of thermal property and mechanical behavior of BMG and their composites after HPT process were systematically investigated. The understanding built-up from this study will be used to explain the successful control of following properties as well as microstructure in various BMG and their composites by severe plastic deformation technique.

Hot Isostatic Pressing (HIPping) was employed in an attempt to reduce the micro-porosity in bulk metallic glass (BMG) castings. A zirconium/copper-based alloy, shown to be very suitable for multi-scale tooling applications, was studied. Specimens in the form of rods were produced using arc melting and a drop casting technique, and their amorphous nature confirmed using X-ray diffraction. Isothermal and non-isothermal analyses were carried out using a differential scanning calorimeter to characterize the super cooled liquid region of the alloy. The resultant data were used to select the range of process parameters for use in the HIP treatment of the cast alloy. Porosity in the BMG samples was characterized by metallography, optical microscopy and quantitative image analysis. The area fraction of porosity in the as-cast BMG was found to be around 0.01%. Samples in the form of discs were cut from the cast BMG rods and subjected to HIPing at temperatures within their supercooled liquid range. The HIP experiments were dedicated to study the individual effects of pressure, temperature and time on porosity of BMG samples, in terms of the spatial and size distribution of pores. Post-HIP samples were found to be still amorphous, as confirmed by X-ray diffraction. Significant reduction in area fraction (by an order of magnitude) and number density of pores was achieved as the result of treating the BMG samples with HIP. Our findings confirm that HIP is an effective treatment to reduce significantly porosity levels in BMGs while also keeping the alloy amorphous.

Nanometer-scale structure in the solid state and dynamics in the supercooled liquid state are expected to have major effects on bulk metallic glass properties and process of the glass transition. We have measured aspects of both structure and dynamics using coherent electron nanodiffraction. We have developed the electron scattering equivalent of photon correlation spectroscopy, a technique we call electron correlation microscopy, which uses time-resolved nanodiffraction to measure the a relaxation time of glass-forming alloys heated into the supercooled liquid above T_g in the electron microscope. Experiments on Pd₄₀Ni₄₀P₂₀ show an average tau;_α varying from 25 s to 10 s between T_g and T_g + 25 K, consistent with previous measurements. Analysis of single coherent speckles suggests a broad distribution of relaxation times from different nanoscale regions of the glass.
We have used fluctuation electron microscopy, another coherent nanodiffraction technique, to study the static structure of Zr-Cu-Al BMGs. Experiments on Zr₅₀Cu₄₅Al₅ and Zr₅₀Cu₃₅Al₁₅ show a correlation between the thermal stability of nanoscale structure in the as-quenched state and glass-forming ability. Zr₅₀Cu₃₅Al₁₅ is the poorer glass former, and it shows weaker icosahedral-like nanoscale order and more stable crystal-like nanoscale order than Zr₅₀Cu₄₅Al₅, the better glass former.

The effects of thermo-mechanical treatments on the short- and medium-range order in Cu₆₄Zr₃₆ bulk metallic glasses were investigated by Molecular Dynamics simulations. In the first part of this talk, we will present investigations of structure-specific nearest-neighbor histograms and bond-angle distributions to highlight the unique role of Cu-centered icosahedra as compared to other types of Voronoi polyhedra. We find that in both, the glassy and supercooled liquid temperature regime, only icosahedra reveal a distinct tendency to form superclusters. The spatial distribution and interconnectivity of the icosahedra were also studied as a function of cooling rate and compared to a homogeneous icosahedral distribution. In the second part, we will present our studies on shear-band structure upon uniaxial deformation of the sample. During deformation the icosahedral fraction reduces and the superclusters become partially destructed. Via determination of an effective disorder temperature and investigations of the corresponding mean-square displacements, we are able to characterize the liquid-like structure of the shear band. This structural information is then used to discuss the enhancement of the self-diffusion coefficient in the shear band in comparison to that of the remaining matrix.

The dynamic-mechanical response of glasses provides important insights into atomic transport mechanisms. Relaxation processes involve a range of relaxation times, but their spectrum cannot be obtained explicitly from the data. Most previous studies assumed relaxation kinetics to be described by a stretched exponent, which amounts to an a priori assumption about the spectrum shape. We have recently been able to compute spectra from quasi-static anelastic relaxation data for a metallic glass, using nonlinear least-squares fits [1]. These spectra exhibited distinct peaks corresponding to an atomically-quantized hierarchy of shear transformation zones (STZs). We use these spectra to simulate the frequency-dependent loss modulus [2]. We show that the high-frequency (or low-temperature) tail observed in experiment corresponds to the same STZ mechanism as the rest of the curve: both a and b relaxations are due to STZs -- the tail is due to small, and therefore fast, STZs, which are reversible due to the small volume fraction they occupy. Many authors have shown that the time-temperature superposition principle was obeyed by loss-modulus curves measured at varying temperatures, yielding a single apparent activation energy. We show that this method yields Arrhenius behavior even for a broad range of STZ activation energies, and is thus not a reliable indicator. These results likely apply to any glass that does not exhibit intramolecular relaxations. Finally, we computed relaxation-time spectra from published frequency-dependent loss moduli for a metallic glass [3]. Using simulated data with noise, we validated the method by showing that the assumed input spectrum can be recovered. Arrhenius behavior is obtained for each STZ size, further confirming the STZ model. Above T_g, the typically high apparent activation energy is shown to be an artifact of the temperature dependence of the high-frequency shear modulus.
This work has been funded by NSF Grant DMR-1307884.
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1) J.D. Ju, D. Jang, A. Nwankpa, and M. Atzmon: J. Appl. Phys. 109, 053522 (2011).
2) J. D. Ju and M. Atzmon, MRS Comm., doi:10.1557/mrc.2014.12, Published online 12 May 2014.
3) J. D. Ju and M. Atzmon, Acta Mater.74, 183 (2014).

There is a clear interest, for bulk metallic glasses, in the atomic mobility is very sensitive to structural changes occurring during annealing or deformation. It is well known that amorphous materials are in out of equilibrium state below the glass transition temperature T_g. Modifications of enthalpy and entropy are induced by various treatments. For instance, annealing can induce physical aging, i.e. structural relaxation or even crystallization (when annealing time is long enough or annealing temperature high enough) and then an increase of short range order (SRL) or long range order (LRO). On the other hand, it is remarkable that plastic deformation (irreversible deformation) can increase the disorder in amorphous materials.
In the current work, the dynamic mechanical properties of Zr₅₆Co₂₈Al₁₆ bulk metallic glass have been investigated by dynamic mechanical analysis (DMA), high-resolution transmission electron microscopy (HRTEM) and nanoindentation technique. The influence of structural relaxation, plastic deformation and crystalline phases on atomic mobility in Zr₅₆Co₂₈Al₁₆ bulk metallic glass has been analyzed. In the amorphous state, dynamic mechanical properties of Zr₅₆Co₂₈Al₁₆ amorphous alloy are similar to that observed in other based bulk metallic glasses. At low temperature, the material is mainly elastic and driving frequency has no influence. In contrast, in the glass transition region, the visco-elastic component becomes very large. Then frequency has a major effect on the mechanical response. It has also been shown that the atomic mobility can be evaluated through the value of the loss factor. The atomic mobility increases by increasing the loss factor. Results are interpreted using a physical model, based on the existence of defects in the material, called quasi-point defects. Atomic mobility is reduced by structural relaxation or formation of quasi-crystalline or crystalline phases. In contrast, plastic deformation, introduced by cold rolling, increases the concentration of defects and therefore enhances the atomic mobility. The concentration of defects is evaluated by DMA and nanoindentation tests in the framework of the quasi-point defects theory.

Great progress has been made in understanding the atomic structure of metallic glasses, but there is still no clear connection between atomic structure and glass-forming ability. By adding three new concepts to the efficient cluster packing model, we show that glass-forming ability can be predicted from simple topology - the relative size and concentrations of atoms that make up a glass structure. This model distinguishes between topologies that cannot form glasses, topologies where marginal glasses can form, and specific topologies where bulk metallic glasses (BMGs) are most likely. The new concepts providing this predictive capability will be described and validated by comparing with previously known BMGs, and by predicting new BMGs. This model is not fully predictive, and still lacks a description of the chemical contribution to glass formation.

Using molecular dynamics (MD) simulations, we show that glass transition in a model phase separating amorphous metal alloy—Cu₅₀Nb₅₀—occurs by gelation. Below the glass transition temperature , a percolating network of icosahedral short-range order (ISRO) forms along transitional zones between regions of compositional enrichment in the material. This ISRO network is mechanically stiff, halts coarsening of compositional medium range order (CMRO), and constrains the dynamics of surrounding atoms, leading to anomalous diffusion. This ISRO network and its influence on the physical properties of the system bears striking resemblance to gelation in colloidal systems, in which a system-spanning, dynamically arrested network of locally preferred structures imparts stiffness. Gelation may be relevant to glass transition and microalloying in more conventional bulk metallic glasses.

In a three-dimensional model mimicking realistic Cu₆₄Zr₃₆ metallic glass, we demonstrate that quasilocalized low-frequency vibrational modes correlate strongly with fertile sites for shear transformations. We have also uncovered a direct link between these soft modes/spots and the local atomic packing structure: geometrically unfavored motifs (GUMs) constitute the most flexible local environments that encourage soft modes and high propensity for shear transformations, while local configurations preferred in this alloy, i.e., the full icosahedra (around Cu) and Z16 Kasper polyhedra (around Zr), contribute the least.

Metallic glasses (MGs) have attracted vast interest due to their extraordinary properties. Despite the bulk of theoretical and experimental efforts, their atomic structure and its relation to their properties are only vaguely known and a general model to this end is still missing. In order to describe the arrangement of atoms in MGs, various cluster-based structural models have been suggested (e.g. “efficient cluster packing” and “cluster-plus-glue-atom”). Many research groups have demonstrated the appositeness of these models through theoretical simulations in combination with experimental structure analysis. The decisive step i.e. the determination of the link between the structure and properties of MGs is, however, still missing. Recently a bottom-up approach to the fabrication of MGs by deposition of alloy metal clusters from the gas phase was proposed. Cluster-assembled metallic glasses (CAMGs) could play a key role in in determination of the structure-property relation in MGs since the structures and properties of their building blocks (alloy metal clusters) can be determined in state-of-the-art cluster science labs. Although the way is stony and steep, the first attempt to tackle this challenge (performed on CuZr binary alloy) has proven very promising.

An universal equation has been derived for the variable activation energy of viscous flow Q(T) of the generic Frenkel (or Eyring) equation of viscosity eta;(T)=A×exp(Q/RT) which has two constant asymptotes - high Q_H at low temperatures and low Q_L at high temperatures. Among different types of atomic mechanisms of viscous flow recent data on a viscous flow model based on network defects - broken bonds termed configurons - were analysed. The defect model used by many researchers states that higher the concentration of defects (e.g. configurons) the lower the viscosity. Additionally the defect model results in temperature relationship for viscosity eta;(T) which is a continuous function of temperature T both for glassy and liquid amorphous materials. The glass-liquid transition within configuron percolation theory (CPT) is treated as a percolation-type phase transition. It is accompanied by formation of percolation macroscopic clusters made up of configurons which are dynamic in nature. The characteristic linear size of dynamic clusters formed is given by correlation length which universally depends on formation Gibbs free energy of configurons G_d=H_dminus; TS_d ., where H_d and S_d are the enthalpy and the entropy of formation. It becomes macroscopic large at glass transition temperature T_g. Fractal-type medium range order is characteristic for correlation length sizes and homogeneous and isotropic disordered state is characteristic for macroscopic sizes. A reduction of topological signature (Hausdorff dimensionality) occurs at glass transition for the disordered bonding lattice: the dimensionality reduces from 3 for glass to fractal 2.4minus; 2.8 for melt. This is the main signature change which differentiates a glass from a liquid. Namely the signature change is responsible for drastic changes of material behaviour at glass transition (see e.g. J. Non-Cryst. Solids, 382, 79 (2013)). The CPT gives in an universal viscosity equation eta;(T)=A₁T[1+A₂exp(B/RT)][1+Cexp(D/RT)] valid at all temperatures. We show that a particular result from CPT is the universal temperature relationship for the activation energy of viscous flow:
Q(T)=Q_L+RT×ln[1+exp(-S_d/R) exp((Q_H-Q_L)/RT)]
Thus Q(T) depends on asymptotic energies Q_L at high temperatures (for the liquid phase), Q_H at low temperatures (for the glassy phase) and entropy of configurons S_d. This equation has two asymptotes, namely Q(TH, and Q(T>>T_g) = Q_L. The equation for Q(T) practically coincides in the transition range of temperatures with Sanditov&’s equation (JETP, 110, 675 (2010)). In addition we propose a simple analytical approach which provides numerical values of thermodynamic parameters of bonds (e.g. H_d and S_d) in glass forming materials from four shear viscosity coefficient data at four temperatures, two of which should be below and other two above glass transition temperature. This method aims to replace earlier used complex fitting procedures for the nonlinear viscosity equations to experimental data.

Amorphous polymers have structure on a range of length scales. At the smallest scales the packing of monomers affects the friction between them. At intermediate scales, the constraint that polymers cannot pass through each other leads to topological constraints called entanglements. Orientation of chains can be important on scales from bonds to the end-end separation. The talk will start by discussing the relation between monomer structure, packing and the yield stress. Coarse-grained potentials that remove atomic detail reproduce melt structure but dramatically suppress the yield stress, indicating the close connection between monomer friction and yield stress. The functional form of strain hardening is shown to be less sensitive to monomer friction and correlated instead with entanglements and the persistence length. Results for different strain histories can be collapsed when plotted against orientational order on large and short scales. Next the connection between entanglements and mechanical strength is revealed by studies of the evolution of interface strength with interdiffusion time. These are correlated with the real space distribution of entanglements as evaluated with the Primitive Path Algorithm. Both the maximum shear strength and tensile fracture energy scale linearly with the density of interfacial entanglements before saturating at bulk values. Entanglements are also critical to the formation and stability of bulk crazes, which increase the fracture energy by orders of magnitude.

The nature of the amorphous network in glass and highly crosslinked glassy polymers strongly controls thermo-mechanical properties. In silicate glasses, this network structure is modified by the inclusion of additional compounds, such as soda and lime, leading to changes in ductility. In highly crosslinked polymers, topology is controlled by the overall degree of reaction and processing conditions. Atomistic simulations of soda lime silicate and dynamically reacted epoxy network structures allow examination of defects and topology, i.e., sterically hindered polymerization vs. network scission via modifier species. This information is used to gain insight into the structure-property relationship of highly connected amorphous network materials. Networks in both systems are allowed to evolve over the course of the simulations, avoiding over-constrained structures. Analysis of our results in the framework of network theories for both polymers and inorganic glasses reveals commonalities between these systems.

Silicon-based polymer-derived ceramics (PDCs) are X-ray amorphous materials resistant to crystallization beyond 1400 ^oC. These ceramics exhibit many unusual properties, such as oxidation and corrosion resistance, as well as a resistance to creep at elevated temperatures. Amorphous PDCs are synthesized by careful pyrolysis of Si-containing preceramic polymers. The structure after pyrolysis has generally been described to contain nanodomains of amorphous carbon and amorphous Si-networks, where the Si-networks are comprised of SiO_xC_4-x and SiC_xN_4-x tetrahedral units in the SiOC and Si(B)CN PDC systems, respectively. The morphology and chemistry of the nanodomains can be fine-tuned so that the properties can be adjusted, therefore PDCs have been hypothesized for use in a variety of technological applications, such as semiconductors, barrier coatings, sensors, construction materials, and Li-ion battery anode materials.
SiOC and Si(B)CN PDCs were synthesized by pyrolysis of a series of preceramic polymers with systematically modified organic and/or functional groups to determine the influence on the final ceramic structure. Solid-state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy was carried out to identify distinct ²⁹Si molecular species and to correlate the molecular species to the intermediate-range structure. The data reveal for the first time that preceramic polymers with different functional groups result in dramatically different intermediate-range structures after pyrolysis. In the case of SiBCN PDCs synthesized using polysilylcarbodiimide as the preceramic polymer, it was found that the structure contains dense nanodomains of amorphous silicon nitride. This is in contrast to the structure found in the SiBCN PDCs synthesized using polysilazanes that contain an amorphous Si-C-N percolated network constructed of SiC_xN_4-x tetrahedral units, where silicon can be tetrahedrally bonded to either nitrogen, carbon, or a combination of the two. The detailed analyses of the short- and intermediate-range structures of PDCs synthesized with various preceramic polymers can help to explain the interesting properties that have been reported, such as thermal stability, electrical conductivity, and chemical durability.

We have used fluctuation electron microscopy (FEM) to measure medium range order / molecular packing in 40-85 nm thick indomethacin thin films. Vapor-deposited ultrastable indomethacin exhibits extraordinary thermodynamic and kinetic stability, with fictive temperature ~ 30 K below the glass transition temperature of a liquid-cooled indomethacin ordinary glass [1]. Periodic packing of molecules parallel to the film surface has been discovered in the ultrastable indomethacin glass, but not in an ordinary glass with x-ray diffraction [2]. We measured molecular packing perpendicular to film surface with FEM. Extremely low beam current of 1.5 pA at 200 kV was used to minimize beam damage, and the average electron diffraction from the sample is unchanged at total electron doses four times larger than required for an FEM experiment. A FEM variance peak centered near 0.1 Å^-1 was found in ultrastable indomethacin. It represents medium range order packing of molecules with interplanar distance ~1 nm, approximately the size of one indomethacin molecule. The variance from an ordinary indomethacin glass is significantly smaller, suggesting that ordinary glass is less heterogeneous. This is the first study of medium-range order in molecular glasses and may provide insight into the structural differences between the remarkable ultrastable thin films and ordinary glasses.
[1] Swallen, S. F., Kearns, K. L., Mapes, M. K., Kim, Y. S., McMahon, R. J., Ediger, M. D., Wu, T., Yu, L., Satija, S. Science 2007, 315, 353.
[2] Dawson, K. J., Zhu L., Yu, L., Ediger, M. D. J. Phys. Chem. B 2011, 115, 455.

The surface structure, mechanical properties, and alignment capabilities of oriented polymer substrates are of interest for nanomanufacturing and device applications. We have used atomic force microscopy (AFM) and the surface force apparatus (SFA) to study the effects of uniaxial stretching on chain orientation and nanoscale tribology of glassy polymer substrates. Examples will be shown of the different friction responses of semi-crystalline and amorphous polymers along and across the stretching direction, and how this friction response is altered as the strength of adhesion between the polymer and the countersurface is deliberately changed.

Aluminosilicate glasses have wide technological applications as well as importance in geoscience. The atomic structures of the glasses are strongly influenced by the composition and chemistry such as modifier to alumina ratio and modifier cation field strength, which in term impact the properties such as chemical durability, mechanical strength, and ionic diffusion. The correlation of composition, structure and properties is otherwise complicated although significant progress has been made by characterizations such as solid state NMR. In this paper, we report the study of structure and structure-property correlations of sodium aluminosilicate glasses from peralumina to peralkali compositions by using molecular dynamics simulations with a set of highly effective partial charge pairwise potentials. Structure features such as aluminum coordination, oxygen tricluster, and silicon Q_n distributions as a function of composition were studied and correlated to the calculated mechanical properties. In addition, the field strength effect of modifier cations on aluminum coordination speciation was investigated. It was found that higher field strength cations facilitate higher coordinated Al species, in excellent agreement with experimental results. Implification of these structure changes on mechanical and thermodynamic behaviors will discussed

The mechanisms of density-driven structural collapse in prototypical network-forming glasses like B₂O₃, SiO₂, GeO₂ and GeSe₂ will be considered, where the debate is informed by the results obtained from in situ high-pressure neutron diffraction experiments made with a Paris-Edinburgh press [1-5]. The diffraction data are compared to new molecular dynamics simulations, made using schemes that suit the materials under investigation. In the case of oxide materials, we will show that the coordination number of network-forming structural motifs, which play a key role in defining the topological ordering, can be rationalized in terms of the oxygen packing-fraction over an extensive pressure and temperature range [6].
[1] A. Zeidler et al. J. Phys.: Condens. Matter 21 (2009) 474217.
[2] J. W. E. Drewitt et al. Phys. Rev. B 81 (2010) 014202.
[3] P. S. Salmon et al. J. Phys.: Condens. Matter 24 (2012) 415102.
[4] K. Wezka et al. J. Phys.: Condens. Matter 24 (2012) 502101.
[5] P. S. Salmon and A. Zeidler Phys. Chem. Chem. Phys. 15 (2013) 15286.
[6] A. Zeidler, P. S. Salmon, and L. B. Skinner, Proc. Natl. Acad. Sci. USA (in press).

Thermoset polymers have been extensively used as the matrix of fiber composites because of their high stiffness, high creep resistance and high thermal resistance. These favorable thermal and mechanical properties result from the three-dimensional amorphous network structure created by crosslinking chemical reactions during a carefully designed curing process. A fundamental understanding of the structural evolution and the relationship between material properties and molecular network structures is of great interests to the material community as well as aerospace industry.
Crosslinking degree and crosslink density are two important characteristics for amorphous thermoset polymers and they are related but not identical. Although great attentions have paid on their measurement by various methods, accurate determination of these characteristics is still a challenge for experimentalists. We have recently developed a procedure to mimic chemical reactions during crosslinking process of thermosets at atomistic scale employing molecular dynamics. The predicted density, coefficient of thermal expansion, glass transition temperature and elastic constants of the resulting polymer are in excellent agreement with experiments. However, the detailed topology of network structure has not been examined and structure-property relationship needs to be established.
In this talk, we will present our most recent progress in computational characterization of polymer network. The algorithm in our computational characterization will be described in detail. Our calculations indicate the network structures formed at different conditions could be very different even with the same crosslinking degree. There is a strong relationship between material properties and crosslink density. Some material properties are more sensitive to the structure details.

Random packing structure of colloidal system has been attracting attention for modeling amorphous materials because of its similarity to amorphous structure. Recently, vibration in the colloidal system is investigated, and the relationship between the structure of colloidal system and the density of state is discussed. In the previous studies, spontaneous motions of particles are generally observed. However, the motion is fast and the displacement is small, and direct monitoring of the each particle is not straightforward especially for three-dimensional colloidal system. In this study, we excite vibration using an external source, and observe a structural evolution caused by the vibration. By this method, contribution of a specific vibration to structural evolution is made measurable without monitoring the motions. We finally observe that there is a specific frequency that crystallizes a colloidal system.

Liquids and glasses are known to have “medium-range order (MRO)” beyond the nearest neighbors. However, the origin of the order and how to characterize it properly has not been well established. In oxides and polymers covalent bonds are considered to be the origin, but even metallic glasses, many of which do not have covalent bonds, still show strong MRO. In this case “caging” and “jamming” are considered to cause MRO, but the details are unclear. In this talk we discuss the role of the elastic field in establishing the MRO, particularly in the context of rapid increase in viscosity in the supercooled liquid with decreasing temperature. Viscosity can be expressed in terms of the shear stress correlation function in the Green-Kubo equation. If we express the macroscopic shear stress in terms of the atomic-level shear stress, we recognize that the increase in viscosity occurs through both spatial and temporal development of correlations among the local stresses [1]. We calculated the two-dimensional local stress-stress correlations for 2D liquids, and found that surprisingly there are strong anisotropic stress correlations up to high temperatures. The pattern of the stress correlations follows the Eshelby theory, which is a continuum theory of elastic inclusion, fairly well, as we predicted earlier [2]. However, it is modulated by the derivative of the pair-density function (PDF) and the calculated correlation is substantially weaker than the theory at short-range. The second point is the evidence that the elastic Eshelby field induces structural relaxation to increase stress correlation. The correlation at the nearest neighbor due to the Elshelby field is negative, but relaxation results in positive correlation, leading to frustrated but weakly biased local shear domain, similar to the polar nano-regions (PNR) in relaxor ferroelectrics. We discuss the similarities and differences among the spin-glass, relaxor ferroelectrics, strain glass and real glass, and the possibility of creating a unified theory of glasses as a class.
1. V. A. Levashov, J. R. Morris and T. Egami, Phys. Rev. Lett.106, 115703 (2011).
2. T. Egami and D. Srolovitz, J. Phys. F12, 2141 (1982).

Fragility is a fundamental concept that is often argued to be correlated with glass formability. It is defined from dynamical aspects of the liquid, generally based on the rate of increase of the viscosity as a function of the inverse temperature scaled to the glass transition temperature, T_g. Here a more fundamental scaling temperature, T_coop, is identified, which corresponds to the onset of cooperative motion in the liquid at high temperature. A universal high temperature limiting viscosity is also found, which is equal to the number density multiplied by Planck&’s constant. Scaling by T_Coop and the limiting viscosity gives a universal viscosity curve for all metallic liquids studied, independent of their fragility. The glass transition temperature can be accurately predicted from T_coop, indicating a deep connection between the onset of cooperative dynamics and the dynamical processes of the glass transition. Additionally, experimental studies of the temperature dependence of the liquid structure factor, S(q), show a correlation between the rate of structural ordering and the kinetic fragility. The magnitudes of the height of the first peaks in S(q) and the pair correlation function, g(r), show abrupt changes on approaching the glass transition temperature in fragile liquids. They change more continuously with decreasing temperature in strong liquids. The connection between the rate of structural ordering and fragility is confirmed by measurements of the high-temperature viscosities of these metallic liquids. This demonstrates a link between structure and dynamics in the liquids that is often assumed, but not previously demonstrated experimentally. Taken together, these results suggest that liquid metals are ideal for investigations of fundamental issues related to fragility and glass formation. (Partially Supported by the National Science Foundation (DMR-12-06707) and NASA (NNX10AU19G)).

Given its “off-equilibrium” nature, the liquid to glass transition is by essence a kinetic phenomenon which manifests by large thermodynamic variations during cooling/heatings cycles across the glass transition.
Here we will review the notion of reversibility in glasses, and present Molecular Dynamics simulations showing that certain glass-forming liquids are found to exhibit minuscule thermodynamic changes during such cycles, and therefore define glassy materials that can be viewed as “thermally reversing” given the obtained optimal volumetric or enthalpic recovery.
When the topology of the corresponding network structure is analyzed, it is found that such “optimal” liquids actually adapt under stress by experiencing larger bond-angle excursions indicative of a softening of underlying bond-bending interactions, and exhibit stress-free (isostatic) character. Additional anomalous behaviors are also found in dynamic and structural properties. Ultimately, these results show close connections with experiments on network glasses, and thermally reversing compositional windows, widely observed in chalcogenide, modified oxides and solid electrolyte glasses which are signatures of rigid but unstressed networks that form a so-called “intermediate phase”.These findings substantiate the notion of rigidity in disordered molecular systems, while also revealing new implications for the topological engineering of glasses.

Understanding the relaxational behavior of multi-component glass-forming metallic liquids is of critical importance to the development of novel alloy systems such as bulk metallic glasses (BMG). However, such relaxations are highly activated and complicated because of both quenching and chemical disorders, and thus remain to be investigated. Herein, we report observations of an unusual non-monotonic temperature dependence of the collective fast structural relaxation time beyond the melting temperature in a strong BMG (LM601 Zr_50.75Cu_36.23Ni_4.03Al_9.0) measured by quasi-elastic neutron scattering. Consequently, the slope of the Arrhenius plot of the relaxation time, the activation energy, shows a negative slope, which signals an apparent unrealistic negative activation energy. Hence, this could be a characteristic of some form of microscopic instability in the system. Heat capacity measurements at the same temperature range also show presence of an auxiliary peak. Classical Molecular Dynamics simulations of Cu-Zr-Al system, using Embedded Atom Potential (EAM), reveal a crossover from uncorrelated dynamics to correlated dynamics in α-relaxations above the melting temperature. These mechanisms in tandem are key dynamical features of this glass-forming metallic liquid system and can provide better insight into improving glass forming ability by understanding liquid state of these systems.

In this talk, we will discuss recent structural measurements of the effects of a change in the local coordination on doping with magnetic impurities based upon recent x-ray absorption spectroscopy measurements and well as density-functional simulations. Based upon the work of Skelton et al [1] which indicated a (different) magnetic ground states in the amorphous and crystalline states of GST, we have investigated the local structure about Fe and Mn atoms in as-deposited and crystalline GST. We report on the results of this modelling. We also discuss some recent results on the structural basis of resistance drift in the amorphous state of GST. PC-RAM device evaluation tests have shown that upon switching to the amorphous state, the measured sample resistivity drifts upwards with time following a power law. This phenomena is problematic for device performance especially for multilevel storage in a single bit as the resistance values are unstable with time. There has been much speculation in the literature as to the fundamental mechanisms with some speculating that mechanical stress plays a role while others attribute drift to structural relaxation although in the usual picture of structural relaxation, the annihilation of point defects would be expect to decrease, not increase resistivity. To address this question, we have measured both the resistivity of as-deposited amorphous GST samples under a drift-inducing annealing schedule as well as Ge L3-edge XANES. These measurements show a clear change in structure with time that correlates well with the observed resistivity change. These structural changes are interpreted as being due to the annihilation of tetrahedrally coordinated Ge-Ge bonds which lead to charge trapping a decrease in charge carriers for transport. Subsequent lattice relaxation towards an octahedral environment then leads to localisation of a lone-pair of electrons on the s-orbitals of Ge atoms. This process also suggests that pre-annealing the amorphous phase leads to faster crystallisation rates, namely that annihilation of tetrahedral sites occurs, ordering of pyramidal sites into the octahedral structure of the crystalline phase can proceed more quickly.
[1] J. M. Skelton and S. R. Elliott. In silico optimization of phase-change materials for digital memories: a survey of first-row transition-metal dopants for Ge2Sb2Te5. J Phys. C Solid State, 25(20):205801, 2013.

Amorphous or disordered materials are intrinsically defined by a lack of order, which typically refers to an idealized description of a corresponding crystalline phase that seldom exists in real materials. Starting from well-defined local structures - local order - at the scale of the coordination sphere of the constituent atoms, amorphous materials only appear as disordered because they lack of long-range order. The qualification of long-range ordering is itself strongly dependent upon the characterization techniques (e.g., cross section contrast in diffraction experiments). Many material properties (mechanical, optical, etc.) often depend on the controlled introduction of either disorder or order within the structure - e.g., solid solutions, defects, phase separation, nucleation & growth - that allow control of chemical and structural order in glasses, ceramics or glass ceramics.
We have recently introduced cutting-edge solid-state NMR methods involving the manipulation of multi-spin systems, which in conjunction with DFT-based computations, allow the qualification and quantification of local order in disordered system from the atomic to the nanometer scale in terms of geometrical and chemical order [1]. From these experiments and simulations, we show that amorphous solids often exhibit significant ordering that is a clue to understanding their macroscopic properties [2].
1. D. Massiot, R.J. Messinger, S. Cadars, M. Deschamps, V. Montouillout, N. Pellerin, E. Veron, M. Allix, P. Florian, F. Fayon, "Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR" Accounts Chem. Res. 46 1975-1984 2013
2. F. Fayon, C. Duée, T. Poumeyrol, M. Allix, D. Massiot, "Evidence of Nanometric-Sized Phosphate Clusters in Bioactive Glasses as Revealed by Solid-State 31P NMR" J. Phys. Chem. 117 2283-2288 2013

Observables of primary importance to most experiments derive from the electronic and vibrational properties, and charge carrier transport depends on these plus the electron-phonon coupling. In this talk, I review the electronic and vibrational consequences of topological structural disorder, describe universal features of the localized to extended (Anderson) transition for both electrons and phonons, and via ab initio computations of deformation potentials, describe electron-phonon couplings in amorphous Si and amorphous Se. The nature of generalized Wannier functions is reported for a-Si, and shown to be similar to the decay of analogously computed functions in diamond, a result that is also obtained for the single-particle density matrix using Kohn-Sham orbitals. These notions lead to a simplified model of photo-response that is helpful in explaining recent experiments in amorphous Si and amorphous Se.

Odd-even effects, the non-monotonic dependency of physical properties on odd/even number of structural units, are widely observed in homologous series of crystalline materials. However, such alternation is not expected for amorphous molecular materials because of absence of periodic packing. Herein, we report an unusual odd-even effect of glass transition temperature and relaxation time in a network-forming ionic glass homologue. To understand this phenomenon&’s molecular origin, we performed incoherent elastic neutron scattering measurements of the nanosecond atomic dynamics. The results indicated that the even-numbered cations showed much slower dynamics than neighboring odd-membered cations. The observed difference in mobility exists both around T_g and extends to the liquid temperature regime well above T_g. Further quasi-elastic neutron scattering measurements confirmed that the one-methylene unit structural difference causes up to 7 times difference in backbone dynamics. Pair distribution function (PDF) measured using neutron diffractometry indicates very small difference in structure of these odd/even ionic glasses. Our results suggest that odd-even effect, one of the most well-known structure- property relationships, exists in liquid and glassy state and has a dynamical origin.

Stress driven nucleation of nanocrystals in amorphous alloys has been a subject of intensive debate in the past decade. It has long been postulated that nanocrystals formed succeeding the occurrence of shear bands in deformed amorphous alloys. In this study, we show, via in situ nanoindentation of amorphous Cu₄₄Zr₄₄Al₁₂ alloy in a transmission electron microscope, that the formation of nanocrystals occurred at a ultra-low stress of 0.25 GPa in the elastic deformation regime, accompanied by load drops without evidence of shear bands [Y. Liu et al, Materials Research Letters, 2014. http://dx.doi.org/10.1080/21663831.2014.911778]. Furthermore, during successive loading, repetitive nanocrystal nucleation events were observed, and the stress required for nucleation kept on increasing to ~ 0.54 GPa, implying the occurrence of a ‘hardening&’ effect in amorphous alloy. This study provides direct evidence to advance our understanding on deformation induced nanocrystallization of amorphous alloys.

Size induced change in mechanical property is often reasoned based on defects and their reaction to external loading in small and confined samples, at least for crystalline materials. Different from crystalline materials, amorphous solids do not possess obvious extended structural defects, which in principle should make the disordered material insensitive to size effect. Nevertheless, there have been a slew of reports from experiments showing size effects on mechanical properties, namely, strengthening and enhanced plasticity in metallic glasses, while other show no or marginal effect. In this talk, I will go over an analysis to show how sample dimension could influence mechanical properties. Our emphasis is on surface induced state and its impact on stress-strain responses in nanoscale metallic glasses. We show that it is the complex interplay between the internal and external stresses that dominate the overall mechanical response in amorphous solids; and there exist a critical length scale at which the material should respond differently in two regimes: one dominated by a surface state and other by shear band.

It has long been conjectured that any metallic liquid can be vitrified into a glassy state provided that the cooling rate is sufficiently high. Experimentally, however, only metallic liquids within a finite compositional range can be successfully vitrified, due to the limited attainable cooling rates. Here, we report an experimental approach to achieve an ultrafast cooling rate, which enables formation of metallic glasses with compositions far beyond the glass-formation zone identified by conventional vitrification techniques, offering unique possibilities to study their structure-property relationships. Combining in-situ transmission electron microscopy observation and atoms-to-continuum modeling, we investigated the formation condition and the thermal stability of these novel metallic glasses. Our technique also exhibits great control over the reversible vitrification-crystallization processes, suggesting its potential in micro-electro-mechanical applications. The ultra-high cooling rate obtained in the present study makes it possible to explore the fast kinetics and structural behavior of supercooled metallic liquids.

Recent advances in metallic glass research demonstrated that the sample size is a tuning parameter to toughen metallic glasses, in addition to chemistry and processing. Shorter sample size not only delays fracture but also enhances the yield stress. The yielding of MG is likely to be controlled by the shear band propagation, with its size dependency not quantitatively understood. Due to the extreme localization of shear band in space and time, shear band propagation is difficult to study in experiments. Here, we employ molecular dynamics (MD) simulations to provide quantitative insights on how sample size will affect shear band propagation, without referring to any continuum level assumptions. We then discuss the asymptotic behavior of the stress/strain field during the propagation of the shear band and compare it to continuum level theory. Insights gained here will improve the understanding of the fundamental mechanics of shear band propagation.

Glass materials, including metallic glasses, typically fracture in tension at room temperature in a globally elastic manner. Although homogeneous tensile plasticity and necking of nanoscale metallic glasses have been reported, controversy exists regarding possible contributions from specimen preparation and testing techniques. Here, we show the separate effects of sample size reduction and extrinsic effects on the homogeneous tensile plasticity and necking of Pd₄₀Cu₃₀Ni₁₀P₂₀ glassy wires tested at room temperature. An intrinsic transition from catastrophic shear fracture to plasticity and necking was obtained in this glass when its diameter approaches the length scale of the shear-band nucleus size (i.e. 500 nm). Further reduction of the wire diameter to 267 nm produces complete ductile necking. Extrinsic effects introduced during sample preparation and/or testing produce entirely different results.

Thin film metallic glasses (TFMGs) have recently attracted attention with a potential to mitigate the brittle-like mechanical behavior of the bulk counterparts, leading to superior mechanical properties among which a rupture strength close to the theoretical limit and large elastic deformation [1]. Moreover, TFMGs have shown to sustain outstanding plastic deformation above 10% involving homogeneous deformation, instead of shear bands-induced severe localization [2]. Most studies deal with FIB-milled micro-scale specimens involving the potential risk of defect generation. The on-going debate on length-scale effects on shear bands formation is not fully settled.
Here, we investigate the mechanical properties of submicron Zr₆₅Ni₃₅ TFMGs by using and innovative method based on an on-chip internal stress actuated microtensile testing set-up [3]. The on-chip test structures are produced using microfabrication techniques involving cleaning, deposition, lithography, and etching. After release from the substrate, the test specimens are subjected to uniaxial tension. Tests are simultaneously performed on thousands of specimens, deformed up to different strain levels to obtain the stress-strain curve up to fracture, while avoiding FIB-milling.
Zr₆₅Ni₃₅ (% at.) TFMGs have been deposited by DC-magnetron sputtering with different thicknesses smaller than 400 nm. The amorphous structure was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The elastic properties were investigated by Brillouin spectroscopy, while nano-indentation has been used to extract the hardness and activation volume.
Then tensile tests show very large ductility up to 12% involving elastic deformation up to 4% and up to 8% of homogenous plastic deformation before failure. The measured elastic modulus is in agreement with the Brillouin spectroscopy data. Besides the TFMGs thickness, the effect of different actuator-specimen sizes on film mechanical properties has also been tested. It is worth noting that large ductility is obtained for specimens up to 6 micrometers wide and up to 100 micrometers long which is significantly more than earlier results reported in the literature [2]. On this basis, we present an effective model to describe the mechanical properties of TFMGs and their changes with respect to bulk counterparts.
References
[1] L. Tian et al., Nat. Commun. 3:609 (2012) pp. 1-6.
[2] Q. Deng et al., Acta Mater. 59 (2011) 6511.
[3] M. Coulombier et al., Rev. Sci. Instrum. 83 (2012) 105004.

Introducing a soft crystalline phase into an amorphous alloy can promote the compound's ductility. Here we synthesized multilayered nanolaminates consisting of alternating amorphous Cu₅₄Zr₄₆ and nanocrystalline Cu layers. The Cu layer thickness was systematically varied in different samples. Mechanical loading was imposed by nanoindentation and micropillar compression. Increasing the Cu layer thickness from 10 nm to 100 nm led to a transition from sharp, cross-phase shear banding to gradual bending and co-deformation of the two layer types (amorphous/nanocrystalline). Specimens with a sequence of 100 nm amorphous Cu₅₄Zr₄₆ and 50 nm Cu layers show a compressive flow stress of ~2.57 GPa, matching the strength of pure CuZr metallic glass, hence exceeding the linear rule of mixtures. In pillar compression, 40% strain without fracture was achieved by the suppression of preliminary shear band propagation. The results show that inserting a ductile nanocrystalline phase into a metallic glass prevents catastrophic shear banding. The mechanical response of such nanolaminates can be tuned by adjusting the layer thickness.

Metallic glasses typically fail via shear band formation and exhibit minimal macroscopic plasticity. However, shear band formation can be suppressed when nm-scale layers of metallic glass are embedded within alternating layers of crystalline metals. The resulting nanolaminate composites exhibit both high strength and significantly larger strains to failure compared to larger scale metallic glass samples. In principle, the high density of interfaces in the nanolaminate can serve as nucleation sites for both dislocations and shear bands, but they also can suppress macroscopic failure by mitigating the propagation of these defects across layers. Presently, little is known about these effects on the strength of metallic glasses; rather, only the bulk strength or hardness of such nanolaminates is reported.
This work makes use of a new method to determine the individual layer flow strengths in nanolayered composites—not just bulk composite strength—by coupling finite element simulations of nanoindentation with experimental nanohardness and micropillar compression data. In particular, nanohardness measurements are found to be sensitive to the mismatch in strength between the constituents whereas micropillar compressive strength is not. For Cu/PdSi nanolaminates, the method reveals that 20 nm thick PdSi layers have ~4-5 times the flow strength of neighboring 20 nm Cu layers. Further, the 20 nm thick PdSi layers are ~1.5x the strength of monolithic PdSi pillars. This contrasts with prior work on monolithic PdSi pillars showing little sample size dependence on flow strength. The findings suggest that size-dependent flow strength in metallic glass may be dependent on whether the material is bounded by free surface (e.g., micropillars) or crystalline material (e.g., nanolaminates).

Metallic glass is generally considered as isotropic in terms of structure and mechanical properties. But this perception is only valid in samples free of stress and without initial structural inhomogeneity introduced in processing. We have shown that under external loading, structural symmetry is generally broken in the initially isotropic metallic glass samples and subsequently the mechanical properties become anisotropic. This finding connects the early experimental observation of structural anisotropy to the underlying mechanical anisotropy. But this phenomenon is explored only in the continuum regime. In this work, we present new results from extensive atomistic simulation that show the mechanical anisotropy varies in metallic glass samples with the smallest length scale down to nanometers. We characterize the variations of the mechanical anisotropy and its effects on overall mechanical properties.

Chalcogenide phase change alloys (Ge₂Sb₂Te₅, GeTe and related materials) are the subject of extensive experimental and theoretical research because of their use in optical (DVD) and electronic (phase change memories, PCM) storage devices [1]. Both applications rest on the fast (10-100 ns) and reversible transformation between the crystalline and amorphous phases induced by heating either via laser irradiation (in DVD) or Joule effect (in PCM). The two states of the memory can be discriminated thanks to the large difference in optical reflectivity and electronic conductivity of the two phases. What makes the crystallization kinetics so fast in materials of this class is, however, still a matter of debate. Since both nucleation and crystal growth are extremely fast, these materials provide an ideal playground for atomistic simulations of the crystallization kinetics. To this end, we employed an interatomic potential generated by fitting a huge database of ab-initio energies with a Neural Network method [2,3]. This potential allows simulating several thousand atoms for several tens of ns by still keeping an accuracy close to that of the underlying framework based on Density Functional Theory.
In this talk, we will present the results of molecular dynamics (MD) simulations of the homogeneous and heterogeneous crystallization of the supercooled liquid and the overheated amorphous phase of the GeTe compound [4]. The simulations reveal that the nucleation is not rate limiting in the temperature range 500-700 K of interest for PCM operation. The growth velocity of supercritical crystalline nuclei is also very high in this temperature range because of a large self-diffusion coefficient down to temperatures close to the glass transition T_g. This feature is a consequence of the high fragility of the supercooled liquid which also leads to the breakdown of Stokes-Einstein relation between viscosity and diffusivity [5,6]. Isoconfigurational analysis of MD trajectories demonstrates that this breakdown is due to the presence of dynamical heterogeneities in the atomic motion. The most mobile particles tend to cluster in domains that contain a significant number of chains of homopolar Ge-Ge bonds which are also responsible for the structural relaxation below T_g leading to the drift in the electrical resistance with time.
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[2] G. C. Sosso, G. Miceli, S. Caravati, J. Behler, and M. Bernasconi, Phys. Rev. B 85, 174103 (2012).
[3] J. Behler and M. Parrinello, Phys. Rev. Lett. 14, 146401 (2007).
[4] G. C. Sosso, G. Miceli, S. Caravati, F. Giberti, J. Behler, and M. Bernasconi, J. Phys. Chem. Lett. 4, 4241 (2013).
[5] J. Orava et al., Nat. Mater. 11, 279 (2012).
[6] G. C. Sosso, J. Behler, and M. Bernasconi, Physica Status Solidi B 249, 1880 (2012)

Amorphous Ge_xTe_100-x alloys were obtained over a broad composition range (12 le; x le; 44.6) by thermal co-evaporation. Their structure was investigated by X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements. Experimental datasets were fitted simultaneously by the reverse Monte Carlo simulation technique. The models obtained by simulation indicate a complete random covalent network for compositions x > 24 while some chemical ordering can be observed for compositions x le; 24, the only ones which can be obtained in the glassy form when quenched from the melt by twin roller quenching. In any case, Te is mainly twofold coordinated and the majority of Ge atoms have four neighbours. The number of Ge-Ge and Te-Te bonds evolves monotonically with composition: Ge-Ge bonds are already present for x=24, much below the threshold expected in the case of chemically ordered system; as for the Te-Te bonds, they exist all the way through the amorphous domain up to to Ge_44.6.Te_55.4.
The behavior of reciprocal and real space properties showed thresholds and anomalies which could be explained in connection with the topological model. These findings also pointed to a relationship with the composition dependences of some physico-chemical properties: the thermal stability ΔT and the optical band gap E_g exhibited maxima around x = 20-25.
Acknowledgement
The work was carried out in the framework of the Project ANR-2011-BS08-012-01 (TEAM).

Phase-Change Materials (PCMs) are employed in optical and electronic data storage due to their ability to form a stable glass under ambient conditions, but also crystallize with growth rates of 1 m/s above 550 K [1]. This behavior is typical for a fragile glass forming material and together with the inverse melt-quenching process enables rapid and reversible switching at elevated temperatures. The resulting contrast in optical reflectivity and electric conductivity enables the later data retrieval.
Crystallization is composed of nucleation and growth processes, which have distinct temperature dependencies. To design and optimize a memory device based on these processes it is essential to decouple them - already because the volume transformed per unit time by nucleation is volume-independent, whereas it scales approximately linear with volume in a growth process. Upon down-scaling the active area in a memory device, the scaling law of both effects have to be taken into account.
To decouple both mechanisms under the conditions of rapid crystallization, we employed fs-duration x-ray pulses emitted by a free electron laser (LCLS) to take snapshots of the scattering patterns sub-ps to few µs after the initialization of the crystallization event. A fs-laser was employed to excite the PCM in the sample, generating quasi-isothermal conditions after few ns and finally crystallizing or amorphizing the material depending on the incident fluence. The information from the Scherrer patterns cover a wide range of reciprocal space up to 5 Å^-1 and allow counting the number of grains and the transformed volume as a function of delay time and fluence for the PCMs under investigation, among them Ge₁₅Sb₈₅. This enables decoupling the nucleation from the total volume transformation process of crystallization. The temporal resolution covers the full range of crystallization times.
[1] M. Salinga et al., (2013). Nat. Comm., 4, p. 2371

One of the most difficult issues for the structural characterizations for non-crystalline multi-component materials is to determine partial structure factors, S_ij(Q), because n(n+1)/2 scattering experiments with different scattering cross-sections are necessary for n-component materials. For example, only two conventional experiments of x-ray and neutron total scattering are not satisfied to obtain S_ij(Q) for binary alloys, which need three independent scattering experiments.
Three decades have already passed since anomalous x-ray scattering (AXS) using synchrotron radiation sources was expected as a promising tool for investigating partial atomic structures in non-crystalline multi-component materials [1]. Compared with a related method for structural analysis, x-ray absorption fine structure (XAFS), however, AXS is still rarely used owing to difficulties of experiments and data analyses. We have developed a new detecting system [2], which can fully utilize intense x-ray fluxes from third-generation synchrotron radiation facilities. Using it, we carried out AXS experiments at the beamline BM02 of the ESRF on several chalcogenide glasses [3], amorphous DVD material [4] room-temperature superionic glasses [5], and bulk metallic glasses (BMG) [6], and differential structure factors, Δ_kS(Q), were obtained with good statistical quality.
Experimental Δ_kS(Q) data were analyzed using reverse Monte Carlo (RMC) modeling [7] to obtain partial structure factors S_ij(Q), and corresponding partial pair distribution functions g_ij(r), from which three-dimensional atomic configurations were evaluated. The feasibility of the combination of AXS and RMC was already shown by Waseda et al. [8].
In this presentation, I will show results of chalcogenide glasses and room-temperature superionic glasses, and discuss the relation between atomic structures and ion conducting properties.
These AXS works were performed with many coworkers, in particular, Dr. J.-F. Berar and Dr. N. Boudet of CNRS-Grenoble. I would like to thank Dr. D. Raoux for several suggestions on AXS technique at the beginning phase of these AXS studies.
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[2] S. Hosokawa and J.-F. Berar, AIP Conf. Proc. 879, 1743 (2007); S. Hosokawa et al., Eur. Phys. J. Special Topics 208, 291 (2012).
[3] S. Hosokawa et al., Z. Phys. Chem. 216, 1219 (2002); J. Non-Cryst. Solids 352, 1517 (2006); Phys. Rev. B 84, 014201 (2011).
[4] S. Hosokawa et al., J. Appl. Phys. 111, 083517 (2012).
[5] T. Usuki et al., Nucl. Instrum. Met. Phys. Res. B 238, 124 (2005); J. Non-Cryst. Solids 352, 1514 (2006).
[6] S. Hosokawa et al., Phys. Rev B 80, 174204 (2009); Eur. Phys. J. Special Topics 208, 291 (2012); Int. J. Mater. Res. 103, 1108 (2012).
[7] R. L. McGreevy and L. Pusztai, Molec. Simul. 1, 359 (1988).
[8] Y. Waseda et al., J. Phys.: Condens. Matter 12, A195 (2000).

Compared to oxide based glasses, vitreous materials composed of chalcogen elements (S, Se, Te) show large transparency windows in the infrared. Indeed, chalcogenide glasses can be transparent from the visible up to 12-15 µm, depending on their compositions. This is due to the lower phonon energies of chalcogenides, which are also responsible for enhanced luminescence of rare-earth ions embedded in such matrices. Thus, sulfide glasses, for instance, allow light emission at wavelengths not accessible with silica. In addition, chalcogenide glasses contain large polarisable atoms and external lone electron pairs which induce exceptional non-linear properties. Consequently, the non-linear properties can be 100 or 1000 times as high as the non-linearity of silica.
The presentation deals with the last results in terms of optical properties of chalcogenide glasses and fibers with respect to glass composition. Structural aspects are also discussed.

Chalcogenide semiconductors are promising candidates for data storage applications since they possess a pronounced optical contrast between the amorphous and the crystalline state and a high recrystallization speed [1]. Up to now, many materials were identified to be suitable for rewritable data storage applications and the most studied are Te-based alloys such as GeSbTe. Ge₁₅Te₈₅ glasses were among the first materials employed by S. Ovshinsky to investigate various switching effects relevant to phase-change memory devices [2]. Even if at present they are not anymore considered for applications, they are very suitable for fundamental studies. They can be synthesized using various methods: melt quenching, co evaporation or sputter deposition. For this reason, the Ge₁₅Te₈₅ system is well suited for the investigation of the influence of thermal history and preparation method on the atomic structure. The fact that it is a binary system simplifies this task.
In this contribution, we discuss the thermal stability of various Ge₁₅Te₈₅ glasses elaborated using different preparation recipes. We have followed their structural and vibrational changes starting from room temperature up to their crystallization. The crystallization process is also studied since it can give information on the tendency to phase separation.
Our experimental strategy consists in using differential scanning calorimetry (DSC) simultaneously coupled with x-ray absorption spectroscopy (XAS) [3]. Indeed, the best way to monitor the thermodynamic state of the sample during an X-ray experiment is the use of a DSC as sample environment. Since we probe the structure at a given thermodynamic state, we are able to follow precisely the structural changes around the glass transition. Our special attention will be given to the changes of interatomic distances and the local disorder as extracted from the displacement parameter.
[1] S. R. Ovshinsky, Phys. Rev. Let., 21(20),1450 (1968).
[2] Phase Change Materials and Applications, ed. by S. Raoux, and M. Wuttig, Springer, Berlin (2008).
[3] P. Zalden, et al. J. of Synchrotron Rad. 19(5), 806 (2012).
Acknowledgments :
Support from Agence Nationale de la Recherche (Grant No ANR-11-BS08-0012) is acknowledged.

Phase-change materials can be switched reversibly between a crystalline and amorphous phase by application of electrical pulses. During this process, amorphous phase-change materials undergo a field-driven “threshold switching” process in which a threshold applied electric field causes a sharp increase in conductivity. The atomic mechanisms behind this threshold process have not been resolved by conventional electrical bias measurements, which are limited to pulse durations on the order of a nanosecond. Laser-generated single-cycle THz frequency electric field pulses can be used to bias materials on a sub-picosecond time scale in an all-optical setting. Here we study the THz-driven response of amorphous Ge₂Sb₂Te₅, a common chalcogenide phase-change material. Metallic nanoslit structures are used to enhance the THz field strength, enabling the application of fields above the DC switching threshold.
Measuring the time-resolved transmittance of a near-bandgap infrared probe beam, we observe an instantaneous THz-induced electroabsorption response, anisotropic in the probe beam polarization, which can be related to the localized carrier states in the amorphous semiconductor. Field-driven heating of the sample is also observed. We show that this heating effect is enhanced by increasing the free carrier concentration through optical pumping of the material in conjunction with the THz excitation. This carrier multiplication might be comparable to the mechanism observed during the electrically driven threshold switching. The non-Ohmic conduction typically seen in amorphous phase-change materials is not observed during this ultrafast field-driven heating, although the fields applied here are on the order of the DC threshold switching field.

Elastic constants including the Poisson&’s ratio of glass are simple to define and easy to measure, and are directly related to the interatomic forces and potentials, embodying its local structure and bonding information. Unfortunately, structural models of glasses generated by molecular dynamics (MD) simulations using most force fields, including both pair-interactions and many-body force fields for oxide glasses and metallic glasses, overestimate the Poisson&’s ratio. This overestimation may come from the super-fast cooling rates used for sample preparations in MD or due to the deficiencies of the force fields. Our systematic study will show how high-fidelity atomic structural models of glasses can be obtained by reaching the correct Poisson&’s ratio.

Despite significant recent advances like the Gorilla Glasscopy; from Corningcopy;, glasses still break, which limits their field of application. Similar challenges are also faced by the industry of cement, an ubiquitous material in our society. To design tougher materials, it is critical to understand the relationship between their structure and their ability to resist cracking. Here, we report on molecular dynamics simulations of permanently densified sodium-silicate glasses (NS2) and calcium-silicate-hydrates (CSH), the binding phase of cement. Their complex structures are described as simple mechanical trusses following the framework of the topological constraints theory. For both of these materials, we report the existence of a rigidity transition, driven by composition for CSH, and by pressure for NS2. The fracture toughness of NS2 and CSH is computed and shows maxima. We show that this toughening is achieved for materials characterized by an isostatic network, rigid but stress-free, as observed experimentally for germanium-selenium glasses. In addition, we identify a ductile to brittle transition correlated to the isostatic to stressed-rigid transition. This optimal fracture toughness arises from a reversible molecular network, allowing optimal stress relaxation and crack blunting behaviors. This opens the way to the discovery of high-performance materials, designed at the molecular scale.

This research aims to investigate the origin of the universal low temperature elastic properties of amorphous solids. A full description of the physical processes that cause the observed thermal and elastic behavior, as well as the cause for universality in these properties, has eluded investigators for over forty years. In order to investigate the role that localized vibrational modes have in these anomalous properties, we take advantage of recent simulations that have shown the incorporation of lattice planes of more massive atoms at random locations in a host lattice leads to a high number of such modes. These structures have been realized experimentally using sputter deposition to create Si thin films in which 20% of the host lattice&’s planes were randomly substituted with Sn planes, a structure known as a random multilayer (RML) film. With a mass ratio of four, this material theoretically has ~90% of its vibrational modes localized. We have confirmed the desired structure and thickness (113.4 nm) of the RML using TEM. In order to achieve short-range order, as in the simulations, rapid thermal annealing (RTA) at 450°C was used, assisted by the metal induced crystallization (MIC) effect from Sn layers, to crystallize the film. This was verified using XRD, where strong Si peaks were observed, with the absence of Sn peaks indicating that Sn agglomeration in the sample was minimal. Using a high quality (Q) factor silicon double-paddle oscillator (DPO) of thickness 300mu;m, measurements of internal friction (Q^-1) were made on the RML to characterize the effect of vibrational mode localization on elastic properties. When driven in its antisymmetric mode, the DPO has Q^-1 asymp; 2.2 x 10^-8, which allows for the extraction of Q^-1 for a thin film deposited on the oscillator at thicknesses down to 5 nm. Q^-1 for the RML film was measured in the temperature range 340mK to 150K. A temperature independent plateau in Q^-1 was observed between 1.5K and 5.5K, having a magnitude of Q^-1 asymp; 3.2x10^-4. The temperature dependence and magnitude of the internal friction are identical to that found in all amorphous solids. In addition, measurements of the relative variation in the speed of sound displayed a linear temperature dependence above 7.7K. Known as the Bellessa effect, this behavior is also observed in all amorphous solids. To our knowledge, this is the first time that the universal behavior found in the elastic properties of amorphous solids has been observed in an artificially created crystalline solid where a significant fraction of the vibrational normal modes are believed to be localized.

Often solid glass at room temperature is treated as a perfect model system for describing brittle fracture in materials. Based on Zachriasen&’s random network model of atomic structure of prototypic alkali silicate glass, the weak alkali-NBO links and the strong Si-O-Si bonds are distributed randomly, thereby producing on average a homogeneous solid. Then the fracture is presumed to follow the path of maximum local stress as set by external conditions. However, in recent years there is increasing evidence that the alkali ions may be forming clusters or channels, as aptly described by Greaves&’ modified random network model. Clearly, this segregated distribution of alkali-NBO bonds, if valid, will have fundamental repercussions on how we treat fracture of such multicomponent glasses. So, we have performed experiments to verify its validity by directly observing the surface composition of glass samples that are fractured in situ in ultra-high vacuum, which enables analysis by low energy ion scattering (LEIS). This technique detects only the top monolayer of atoms.
LEIS measurements of binary trisilicate glasses modified with Na₂O, Cs₂O and BaO have shown that there is a strong accumulation of the modifying alkali cations on the freshly fractured glass surface, apparently due to their preferential migration to the surface during fracture, aided by the strain energy released upon breaking of the weak alkali metal-NBO bonds. The Si/O ratio shows usually little deviation from the nominal glass batch value, falling within the estimated errors.
LEIS measurements of ternary silicate glasses (modified with Na₂O, Cs₂O, CaO and BaO) have also shown similar trends, with the fracture surfaces being enriched with Na and Cs compared to Ca and Ba. However, the as-fractured surface of a glass with both Na and Cs was depleted of Na compared to Cs. While the Na or Cs enrichments may be attributed to their preferential migration towards the fracture surface, in view of the much higher ionic mobility of the alkali compared to the alkaline-earth cations, the depletion of Na relatively to Cs appeared somewhat surprising, with Na⁺ appearing less mobile than Cs⁺, despite the size factor. It was also found that after prolonged sputtering cycles, the surface composition often fails to reach the nominal (batch) stoichiometry with respect to the modifier cations.

The microcracking sequence (radial, median, lateral, and ring-like) arising at the glass surface under sharp contact loading and the extent to which these cracks develop is intimately related to the way the material attempts to relax the corresponding stress field. Two processes which are known to occur upon indentation are densification and isochoric shear flow. The contribution of both mechanisms were quantitatively assessed for glasses belonging to different chemical systems and indentation-cracking maps are provided which offer guidelines to the design of glasses with better surface damage resistance based on their elastic properties and hardness.

Imaging the atomic structure of disordered materials in real space has long remained elusive. While techniques now exist to probe the local atomic ordering in glass [1,2], most experimental methods rely on a challenging extraction of real-space information from reciprocal-space techniques. Here, we investigate atomically-thin SiO₂, a strong network glass former with a newly-discovered two-dimensional (2D) phase [3-6]. We show that, for 2D amorphous materials, it is possible to image their structures and structural-rearrangements at atomic resolution using aberration-corrected transmission electron microscopy.
Our images strikingly resemble Zachariasen&’s 1932 cartoon models, realizing an 80-year-old vision for easily understandable amorphous systems. Importantly, we can image large regions simultaneously, giving us microscopic access into the structural variations that characterize glassy materials. From our images, we can determine atomic coordinates for thousands of atoms at a time. We extract ring statistics and pair distribution functions, two standard measures of medium-range-ordering that can be directly compared with theoretical models [7]. Using quantitative analytical methods in the TEM, we are able to solve the full structure and bonding of this 2D glass.
Using real-space imaging, we can also image the atomic-scale structural rearrangements excited by the high-energy electron beam. We directly observe elastic and plastic deformations and track the motions of each atom over time. With these unique data sets, we can determine local strain field around individual ring rearrangements, which we find can be modeled surprisingly well with a continuum mechanics approach. We also observe larger-scale shear that is enabled by localized regions of plastic deformation. Together our work demonstrates that 2D glasses are promising model systems for gaining atom-by-atom insight into amorphous materials.
[1] A. Hirata et al., Nat Mater, 10, 28-33 (2010).
[2] P. Voyles and J. Gibson, J Electron Microsc, (2000).
[3] P.Y. Huang et al., Nano Lett, 12, 1081-1086 (2012).
[4] D. Löffler et al., Phys. Rev. Lett, 105, 146194 (2010).
[5] L. Lichtenstein et al., J Phys Chem C, 116, 20426-20432 (2012).
[6] P.Y. Huang et al., Science, 342, 224-227 (2013).
[7] M. Wilson et al., Phys. Rev. B, 87, 214108 (2013).
[8] This work was conducted in collaboration with Simon Kurasch, Anchal Srivastava, Jonathan S. Alden, Viera Skakalova, Jani Kotakoski, Arkady V. Krasheninnikov, Ashivni Shekhawat, Alex A. Alemi, Robert Hovden, Qingyun Mao, Paul L. McEuen, James P. Sethna, Jannik C. Meyer, Jurgen Smet, Ute Kaiser, and David A. Muller

Melt-quenching is a deceptively simple method to realize bulk glasses. Less obvious is how long and what temperature should a dry melt of a given batch size be reacted at to achieve compositional homogeneity. Furthermore, what does ‘homogeneous&’ imply in this context? Important breakthroughs in the field have emerged from Raman spectroscopy profiling experiments in the chalcogenides. Experiments reveal that the isostatically rigid network compositions formed in the intermediate phase possess unusually low fragility indices (m ~ 15 in Ge-Se). The super-strong character of these select melts presents a bottleneck to melt homogenization with full homogeneity achieved after several days of high temperature reaction instead of tens of hours; a discovery that is supported by molar volume, fragility and enthalpy of relaxation measurements at T_g and by theory. Once homogenized, the three topological (elastic) phases, flexible, intermediate and stressed-rigid are readily observed in the Ge-Se and As-Se systems.

Materials science seeks to correlate microstructure with (mechanical) properties.
This has been successfully shown in some limited cases, however, for most technological relevant materials the microstructures and their fabrication are too complex for a systematic study. As a novel approach, in between real microstructures and virtual experiments, we propose to study microstructure-property relationships with artificial microstructures. This approach allows us to individually and independently vary parameters and thereby determine their individual effects on mechanical properties. The artificial microstructures are fabricated through a two-step process; silicon lithography is used to fabricate the mold and thermoplastic forming to replicate the mold into a bulk metallic glass structure. A vast range of shapes comprising of length scales ranging from a few microns to millimeters can be fabricated. Examples where this approach is useful include toughening mechanism in metallic glasses, transition from plastic deformation to elastic buckling in metallic glass heterostructures, and flaw tolerances of microstructures.
This technique is also useful to investigate the fracture toughness of metallic glasses. Our results suggest that previously reported fracture behaviors of metallic glasses are often overshadowed by extrinsic, sample preparation effects. Introduced method mitigates such effects and allows for precise and repeatable determination of fracture toughness, its scatter, effect of notch radius, and effects of processing conditions.

Many new drug candidates being developed today, although highly potent, have very low solubility in biorelevant media that may limit their bioavailability or efficacy. One strategy currently used to overcome this issue is to take advantage of the increased solubility of the drug compound&’s amorphous form as compared to the crystalline form. However, pure amorphous drug compounds tend to have poor physical stability in the solid-state and/or rapidly crystallize. To mitigate these issues, the drug compounds are often mixed with water soluble amorphous polymers to yield a single phase mixture, referred to in the pharmaceutical industry as an amorphous solid dispersion. These dispersions can be prepared on commercial scales using hot-melt extrusion or spray drying technologies.
In this presentation, we study the effect of drug loading and moisture content on the mechanical properties of these dispersions using quasi-static nanoindentation and nano-dynamic mechanical analysis (nanoDMA). In some cases, we have found that although the drug molecule, which has a lower glass transition temperature than the polymer, reduces the glass transition temperature (Tg) of the mixture upon addition, the hardness, reduced modulus and storage modulus increase as the drug is added to the polymer. Thus the drug molecule acts as an antiplasticizer by reducing the Tg but increasing the hardness and stiffness. This behavior persists up to a critical drug concentration after which these mechanical properties decrease on further addition and the drug acts as a plasticizer. In addition to thermal analysis and mechanical testing, we have utilized infrared spectroscopy and dynamic vapor sorption to provide insight into potential structural aspects of the drug-polymer mixture. From a practical standpoint, we have shown the significance of this behavior by preparing the same drug-polymer dispersions using hot-melt extrusion. When the extrudate is milled, the resulting particle size distribution is related to the brittleness of the material. As predicted by the nanoindentation measurements, increasing the drug loading leads to harder but more brittle material and thus a decrease in the median particle size.

Recently, group IV elements based second-order nonlinear optical materials have attracted significant attention toward the realization of CMOS-compatible optical modulators[M1] and frequency converters. There have been numerous reports on exploration of CMOS-compatible materials with large second order nonlinearity. One of the successful approaches is breakdown of the centrosymmetry of Si crystal by strain engineering via silicon nitride (SiN) deposition[1]. Another promising canditate is amorphous thin films prepared by conventional deposition systems. So far, strong SHG has been observed from several kinds of CMOS-compatible amorphous thin films such as SiN thin films grown by plasma enhanced chemical vapor deposition (PECVD) and silicon monoxide (SiO) thin films deposited by electron beam deposition[2,3]. These amorphous thin films can be simply fabricated by conventional CMOS technologies used for Si based microelectronics and it must be a great advantage for real applications. However, the origin of the large second order nonlinearity has not been elucidated and the parameters, i.e., film composition and structure and deposition processes, to achieve large second nonlinearity have not been clarified.
In this work, we propose Ge doped SiO₂ (Ge_x(SiO₂)_1-x) thin films as a new CMOS-compatible amorphous thin films exhibiting large second-order nonlinearity[4]. We show that the second-order nonlinearity of SiO₂, which vanishes in the electric-dipole approximation due to the centrosymmetric structure, can be significantly enhanced by Ge doping. The effective second-order nonlinear coefficient (d_eff) estimated by Maker-fringe measurements reaches 5.48 pm/V. This value is more than twice as large as that of β-BaB₂O₄ (BBO). Electron spin resonance (ESR) measurements reveal that the thin films possess two kinds of oxygen vacancy defects, i.e., SiE&’ and GeP_b centers. The comparison between the d_eff values and the ESR signal intensities suggests that GeP_b centers are the most probable origin of the large second-order nonlinearity of Ge_x(SiO₂)_1-x thin films. [1]M.Cazzanelli,et.al., NatureMater.11,148(2012) [2]T. Ning, et.al., Appl. Phys. Lett. 100, 161902 (2012) [3]S.V.Andersen, et.al. Opt.Express20, 13857 (2012). [4]Ibuki Kawamura, et al., Appl. Phys. Lett. 103, 201117 (2013)

CMOS-compatible materials with large second-order nonlinearity are of great interest in relation to the realization of on-chip photonics devices such as multipolar wavelength converters, optical switches and electro-optical modulators. Especially, recent discovery of large second harmonic generation (SHG) of amorphous thin films, which had been believed not to exhibit second order nonlinearity due to their centrosymmetric structures, has accelerated exploration of new thin film materials with high second-order nonlinearity. Among several CMOS-compatible amorphous thin films, such as SiN_x, SiO_x, and Ge doped SiO₂, SiNx thin films have most intensively been investigated due to the large nonlinearity. The susceptibility tensor is reported to be 68.8 pm/V, which is comparable to that of traditional second-order crystal like β-BaB₂O₄ (BBO). However, the origin of the large nonlinearity has not been elucidated. In this work, we investigate second-order nonlinearity of SiN_x thin films with a wide range of excess Si concentration from 46.8% to 65.6% by the observation of the SHG and prepared by a sputtering method. We discuss the origin via detailed characterization by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy.
SiN_x thin films were deposited on a silica substrate by reactive magnetron sputtering of a Si target in a N₂/Ar atmosphere. The composition was controlled by the N₂:Ar ratio. SHG was measured by using mode-locked Ti: sapphire femtosecond laser (wavelength: 800nm, pulse width: 10nm, frequency: 82MHz) as an excitation source. SHG was observed for all the samples. The intensity depended strongly on Si concentration and was the largest at the Si concentration of 48.2%. The effective second-order nonlinear coefficient (d_eff) increases with increasing excess Si concentration and reached 5.9 pm/V at the Si concentration of 65.6%. This value is twice as large as that of BBO crystal. Second-order nonlinear tensors were obtained by polarization resolved Maker fringe measurements, in which transmitted SHG signal intensity was monitored as a function of the angle of incident light. We found that the tensor component d₃₃ was the largest component in all the samples, implying that the samples possess in-plane isotropy. We studied the correlation between the d_eff value and the intensities of XPS signals of Si 2p core levels from five different valence states, i.e. Si⁰, Si¹⁺, Si²⁺, Si³⁺, Si⁴⁺, and those of ESR signals from different kinds of defects. The best correlation was observed between d_eff values and Si⁰ XPS intensities, suggesting that Si clusters formed in non-stoichiometric silicon nitride are a strong candidate for the origin of strong second order nonlinearity of amorphous SiN_x.

There is extensive research to develop next-generation Li-ion batteries containing silicon electrodes because theoretically silicon absorbs high concentrations of lithium compared with graphite which is the common anode material. Upon high lithiation, amorphous Si (a-Si) anode can expand up to 400% that leeds to fracture of the electrode and prevent having many charging cycles for the battery. Understanding the nature of lithiation/delithiation of a-Si anode using realistic simulations is crucial and requires large numbers of atoms and long time scales which is generally inaccessible with first-principle approaches. These simulations can be carried out using model interatomic potentials that can capture the dependence of structure on chemical composition. Compared with the empirical interatomic potentials, a promising approach to extend the time scales of simulations without sacrificing the accuracy is using artificial neural networks (ANN) to construct the potential energy surface. Using environment-dependent atomic energy contributions and ab-initio density functional theory data for training, we developed a high-dimensional ANN to represent the potential energy surface of Li-Si alloys. Our calculations show that the developed ANN potential can accurately predict structure and total energies of Li, Si and their alloys with various concentrations of Li.

Amorphous polymers are used in many heat transfer-critical applications, yet heat conduction in amorphous polymer has not been fully understood. In this work, we use perturbation theory to analyze the nature of heat carriers and calculate the thermal conductivity of amorphous polymers. Within this framework, thermal conductivity can be efficiently calculated given the equilibrium atomic positions and force constants calculated from lattice dynamics. We were able to incorporate both harmonic and anharmonic contributions in the calculation. Using polyethylene bundle as an example, we show that the results predicted by this model agree well with those obtained from non-equilibrium molecular dynamics simulations, yet a much lower computational cost was needed.

The details of amorphous structures of extended CO solids obtained by isotropic compression of solid CO phases in the range of 0.01 - 25 GPa were studied theoretically. DFT simulations were performed for 128, 432, and 1024 atom models. Structures of random networks found at zero temperature were used for equilibration at finite temperatures up to 50 ps by employing first principles MD. We found that the polymerization begins at 6-8 GPa and a random network of 4-7 atom rings obtained above 15 GPa could exist down to 0.01 GPa. Pressure induced changes in the topological characteristics of the random network based on ring statistics, radial distribution function, and the average number of the nearest neighbors were studied. Vibration analysis of the systems at various pressures and calculated IR intensities and Raman activities, identifying contributions to the intensity from several main motifs of the amorphous structures were performed. N2 molecules are not incorporated in significant numbers in the amorphous poly-CO network up to 20 GPa. We identified that the highest IR spectrum intensities of amorphous p-CO at 15 GPa are originated with intact CO molecules, carbonyl groups of chains and lactones of polymeric structure. N2 molecules were not incorporated in significant numbers in the amorphous poly-CO network. He atoms appeared to facilitate complete formation of the random structure at a lower pressure than that for pure poly-CO, which is almost completely polymerized at a pressure of 18 GPa. He atoms also helped stabilize the structure while lowering the pressure down to 100 Bar with only few CO molecules detaching in the process.

The structure and properties of (Na₂O)_0.23(Si₂O)_0.77 sodium silicate glass are studied by combined ab-initio and classical molecular dynamics simulations to identify the sources of electronic traps in the band gap. Structures from classical molecular dynamics melt-quench simulations are taken as initial configurations for first-principles density functional theory relaxation, from which electronic properties are determined. An ensemble of fifty glass structures, each containing 648 atoms, is prepared in order to perform statistical analyses. The inverse polarization ratio is used as a metric to characterize localized states in the band gap. Structures with varying amounts of local disorder (defects) are compared. The effects of annealing and details of the melt-quench procedure on the local disorder will lend insight into efforts to reduce highly localized trap states in amorphous materials. Control of the electronic properties of amorphous insulators and semiconductors is essential for the advancement of many technologies, such as photovoltaics and scintillators.
Prepared by LLNL under Contract DE-AC52-07NA27344.

We report on the dependence of the dark conductivity and room temperature Raman spectra on boron and hydrogen incorporation on thin films of a-Si prepared by plasma enhanced chemical vapor deposition. Samples were grown at various boron concentrations and hydrogen dilution of the silane precursor, and at differing substrate temperatures, to examine the effect of each on the conductivity and structural properties of the resulting thin films. The temperature dependent conductivity generally follows the hopping conduction model σ = σ₀ exp[-(T₀/T)^p]. Above a critical temperature, the dominant conduction mechanism is Mott variable range hopping conductivity (M-VRH) where p = 1/4 and carrier hopping depends on energy. At lower temperatures, a coulomb gap appears and Efros-Shklosvkii hopping conduction ( p = ½) is observed and hopping depends on both spatial proximity and energy. In the Mott temperature regime the conductivity prefactor shows a correlation with the characteristic temperature T₀ according to the Meyer-Neldel compensation rule. For structural characterization, transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short and mid-range order to the effects of boron and hydrogen incorporation. With an increase of hydrogen and/or substrate temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of short range order. The electrical and structural parameters obtained from modeling will be presented and correlations between the conductivity and structural order will be discussed.
This work was done under the ARO grant W911NF-10-1-0410, William W. Clark Program Manager

Polymers typically have low thermal conductivity that makes them unfavorable for thermal management in microelectronic circuits. However, precise control of the molecular weight, molecular structures and crystallinity can improve thermal transport. The goal of this project is to investigate the dependence of molecular structures, molecular weights, and crystallinity on the thermal conductivity and thermal boundary conductance of several polymer systems. Well-characterized polymer standards of polystyrene (PS), polyvinyl acetate (PVAc), and polyvinyledene fluoride (PVDF) are deposited with the Langmuir-Blodgett (LB) technique. For this study we vary the substrate, polymer thickness and molecular weight of the polymer standards. Frequency domain thermoreflectance (FDTR) is used to measure the thermal conductivity of the nanometer thick polymer films and the thermal boundary conductance of metal/polymer/dielectric and metal/polymer/metal contacts. Experimental results obtained provide insights on factors determining heat conduction in polymeric materials and directions for developing high performance thermal interface materials.

The analysis and optimization of the mechanical performance of amorphous polymers are critical for numerous industrial and biomedical applications. In the absence of external deformation, the molecular motion slows down and a polymer glass gradually evolves towards an equilibrium state in a process called physical aging, which affects mechanical properties of the material. In turn, the effects of physical aging can be removed by application of mechanical stresses or by heating above the glass transition temperature and then cooling back down. The well-known Eyring model, based on effective energy barriers for molecular motion, however, does not include the effects of dynamical heterogeneity, strain localization, and strain hardening.
In this presentation, the dynamics of structural relaxation in a model polymer glass subject to spatially-homogeneous, time-periodic shear deformation is investigated using molecular dynamics simulations. We consider a coarse-grained bead-spring model of short polymer chains below the glass transition temperature. It is found that at small strain amplitudes, the segmental dynamics is nearly reversible over about 10000 cycles, while at strain amplitudes above a few percent, polymer chains become fully relaxed after a hundred cycles. At the critical strain amplitude, the transition from slow to fast relaxation dynamics is associated with the largest number of dynamically correlated monomers as indicated by the peak value of the dynamical susceptibility. The analysis of individual monomer trajectories showed that mobile monomers tend to assist their neighbors to become mobile and aggregate into relatively compact transient clusters.

With growing demand on amorphous metal-oxide semiconductor for application in transparent electronics, amorphous zin-tin-oxide (a-ZTO) has been considered as a promising material due to its elemental abundance and high electron mobility. Previously, we have demonstrated 1) high field-effect mobilities in a-ZTO n-channel thin-film transistor deposited by low-temperature atomic layer deposition (ALD)¹ and, 2) significant enhancement in the open-circuit voltage of Cu₂O/ZnO heterojunction solar cells using ALD a-ZTO as electron-blocking layers.² In both studies, we observed a degradation in device performance with increasing [Sn] content in a-ZTO. In particular, the electron mobility as measured by field and Hall-effect degrades with increasing [Sn] content. The significant impact of the [Zn]/[Sn] ratio in a-ZTO thin-films on device performance suggests a strong correlation between the atomic structure and electronic transport properties of the films. Since amorphous materials can exhibit a continuum of structures, a probe of local order is needed.
In this work, we investigate the effect of a-ZTO thin-film atomic structure on electron transport properties by using synchrotron-based extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES). We perform EXAFS and XANES at the Zn and Sn K-edges to probe the local chemical neighborhoods of Zn and Sn atoms in a-ZTO films of different [Zn]/[Sn] ratios, respectively. We observe that the increase in structural disorder in a-ZTO as measured by EXAFS and XANES coincides with a decrease in measured electron mobility, suggesting that the degradation in electron mobility may be correlated with structural changes. Our observations are consistent with theoretical predictions which indicate that the decrease in mobility can be due to reduced hybridization strength of Zn-4s and O-2p orbitals in the presence of structural disorder which can limit electron conduction pathways.^3
1 J. Heo, S. Bok Kim, and R.G. Gordon, Appl. Phys. Lett. 101, 113507 (2012).
² Y.S. Lee, J. Heo, S.C. Siah, J.P. Mailoa, R.E. Brandt, S.B. Kim, R.G. Gordon, and T. Buonassisi, Energy Environ. Sci.6, 2112 (2013).
³ D.-Y. Cho, J.H. Kim, and C.S. Hwang, Appl. Phys. Lett.98, 222108 (2011).

The large deformation behavior of individual amorphous polystyrene (PS) nanofibers was investigated as a function of the molecular and structural length scales. Glassy PS fibers with diameters of 150-5,000 nm were electrospun from seven monodisperse PS powders with molecular weights between 13,000 and 9,000,000 g/mol. Individual nanofibers were tested using a MEMS based microscale testing platform under an optical microscope over six decades of strain rate between 0.0001 and 150 /s. The uniaxial stress-strain response demonstrated unusual but also repeatable post-yielding behavior including strain-softening, necking and strain-hardening, unlike the brittle behavior of bulk PS. The aforementioned deformation mechanisms were exclusive to specific combinations of specimen size and molecular weight (molecular dimensions). Extremely low and high molecular weights resulted in glassy or craze-assisted brittle failure while intermediate molecular weight sustained stable necking leading to ductilities exceeding 100%, and strain hardening that increased the fiber strength by 200% compared to bulk PS. A size dependent brittle-to-ductile transition occurred within the intermediate molecular weight regime wherein thin fibers showed pronounced strain-hardening behavior and 100-300% increase in failure strength which was reduced with increasing fiber diameter until brittle failure ensued. Additionally, experiments over six decades of strain rate showed that nanofibers of specific combinations of molecular weight and diameter can sustain stable necking at localized strain rates reaching 20,000 /s, thereby resulting in rate-independent elongation and drastically improved capacity for energy dissipation.

The photocarrier lifetime is often viewed as one of the most critical parameters describing the photophysics and the performance of photovoltaic and photodetecting systems. The importance of photocarrier lifetime in photosensing systems originates from the need to extract the charges prior to their recombination. A multitude of techniques, including the wide-spread method of transient photovoltage (TPV), is used to quantify the lifetime of photogenerated charge carriers in various amorphous and disordered systems such as organic molecules, polymers, nanoparticle-based films. However, regardless of the methodology used, it is demonstrated that the photocarrier lifetime, as a parameter, is inadequate for describing the nature of recombination in disordered films with Langevin-type or close to Langevin-type recombination.
Furthermore, the differences between non-Langevin-type (crystalline) and Langevin-type (disordered, amorphous) photocarrier recombination and lifetime are demonstrated and discussed. Presently, numerous high impact academic publications use the photocarrier lifetime to compare the two independent disordered photosensing systems and conclude about the differences in photocarrier recombination. It is shown that the photocarrier lifetime does not describe the nature of recombination in close to Langevin-type systems because it is determined by the photocarrier mobility and the physical separation between the positive and negative charges (photocarrier density). It is concluded that the mobility-lifetime product, often used to characterize the photocarrier drift distance in photosensing systems, cannot reliably determine the charge transport length scales in the low mobility disordered systems. It is shown that in order to describe the charge transport lengths adequately, it is critical to use the product of mobility-recombination coefficient. The implications of this conclusion for device performance are discussed.

Molecular dynamics simulations have been applied for many years for the prediction of the structure and properties of soft materials such as polymers, surfactants, and organic liquids, and force field parameters have been developed, allowing one to predict thermophysical properties such as density quite accurately. There is a strong desire for the ability to predict the diffusion behavior and solubility of molecules in polymers for applications such as food packaging, and in drug delivery applications. Recent developments allow one to study the diffusion rate of molecules in complex materials such as amorphous polymers through the application of an externally applied constant force. The diffusion coefficient is calculated by the induced drift velocity. Recent work has also been done in predicting the solvation free energy of molecules in solvent or polymer environments. Calculations have been perform with the application of a thermodynamic integration method in which a coupling parameter is applied which controls the degree of interaction between the solute molecules and the solvent environment. This method is applied by first removing the atomic charges and scaling the van der Waals interactions, and then gradually scaling up the electrostatic interactions. A series of dynamics simulations are performed at a number of coupling parameter values to predict the derivative of free energy at each value. The free energy difference between adjacent coupling parameter points is estimated and the total solvation free energy is then obtained. Examples of the use of these methods for the prediction of diffusion and solubility of small molecules in amorphous polymers will be described.

The deformation behavior of amorphous silica spheres of diameter 1 micron has been studies under 5 kV to 30 kV electron beam using an in situ nanomechanical instrument. A dramatic change in the plastic strain and tendency to fracture has been observed with variation in electron beam intensity and applied load. The particles showed negligible plastic strain with beam off and as high as 60% plastic strain under 20 kV beam with similar loading condition. The mechanical properties of such amorphous particles have also been investigated with passing current through the particles and heating the particles at temperatures up to 400 ⁰C. Understanding mechanical properties as a function of temperature and current flow will help to determine the stability and strength of amorphous materials and devices where silica is an integral component.

The glass transition temperature (T_g) of polymers confined at the nanometer length scale can deviate significantly from the bulk. Recent studies of aqueous suspended polystyrene (PS) nanoparticles and freely-standing PS and PMMA #64257;lms showed that the decrease of T_g with confinement occurs due to the free surface effects.We investigated the dynamics of poly (methyl methacrylate) - PMMA - nanospheres near the glass transition by means of frequency modulation electrostatic force microscopy techniques. The dielectric response of single PMMA nanospheres with diameters between 20 nm to 60 nm was obtained through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field and by measurement of the polarization noise spectrum. We carried out measurements in a frequency range from 10^-2 Hz to 10² Hz and at temperatures from 283K to 401K. From the frequency spectrum we determined the position of α and β peaks as a function of temperature and calculated the corresponding relaxation times. We compared these local measurements with the bulk measurements acquired on a film made of the same polymer. We observed that for the PMMA nanospheres, the α process is faster and much more Arrhenius than in bulk. We found that T_g is nearly independent of the size of the nanoparticles and is ~ 25K lower than Tg for bulk. Our results are consistent with the model of the spheres with a bulk-like core behavior and a surface shell which exhibits an enhanced mobility.

We studied the mechanisms of thermal transport in amorphous silicon using classical molecular dynamics simulation and wavelet transform techniques. First, we developed a new simulation method for predicting the frequency-dependent thermal diffusivity based on a transient heating event and decay. Then, to understand the relationship between structure and thermal properties in amorphous silicon, we tested different interatomic potentials and amorphous structure preparation methods. We found that the potential and preparation method affected the predicted frequency-dependent diffusivities. The details of the frequency-dependent diffusivity agree with the predictions of the Allen-Feldman theory, whereas the Cahill-Pohl model captures the overall trend.

Ion beam irradiation of iron phosphate glasses was investigated as model system for understanding radiation damage in nuclear waste glasses induced by actinide decay. Iron phosphate glasses are being considered as an immobilisation matrix for high level nuclear waste (HLW), including minor actinides and plutonium residues, due to their high chemical durability and ability to incorporate diverse chemical compositions. Iron borophosphate glasses are of particular interest as the addition of boron, which has a high thermal neutron absorption cross-section, increases glass thermal stability and provides criticality control. α-decay of the incorporated actinides results in the formation of α-particles (He nuclei) and energetic (~100KeV) daughter recoil nuclei. Interactions between recoil nuclei and glass atoms may result in atomic displacements, forming collision cascades, potentially altering glass network polymerisation and cation valance state. Such modifications may affect glass durability and, therefore, its long term performance as an immobilisation matrix. In this study, heavy ion implantation (e.g. 2MeV Kr or 2MeV Au irradiation) was used to provide an analogue for α-recoil damage. Iron borophosphate glasses, with nominal molar composition 60P₂O₅ - (40 - x) Fe₂O₃ - xB₂O₃, for which x = 0, 10, 20, were irradiated at room temperature, producing a damaged region extending from the surface of the samples to a depth of approximately 1µm. In order to probe only the damaged region, surface sensitive techniques were employed. The effects of simulated α-recoil damage were investigated by probing the speciation and valence of Fe, and by examining the glass structure. In this contribution, we report on structural and chemical modifications as a consequence of heavy ion irradiation, elucidated using Reflectance Fourier-Transform Infrared (FT-IR), Mossbauer, and X-ray absorption spectroscopies.

Compositionally and Structurally graded amorphous carbon nitride (CSG-CN_x) film was prepared on Si(100) and SiO₂ glass substrates by reactive radio frequency magnetron sputtering technique using a graphite target into N₂ plasma. The films with various compositions were prepared by changing the deposition temperature from RT to 873 K. The graded structure was fabricated to increase in photoconductivity of amorphous carbon nitride films. The composition ratio and chemical bonding structure were estimated using X-ray photoelectron spectroscopy. The optical band-gap and absorption coefficient were obtained by UV transmittance spectroscopy. The photoconductivity of single layered a-CN_x films increased with elevating the deposition temperature due to a decreasing nitrogen concentration and an increasing sp² bonding fraction. Simultaneously, the optical band-gap of the a-CN_x films decreased with increasing the temperature. We changed the temperature gradually from 723 K to 873 K for the fabrication of CSG-CN_x with higher photoconductivity. The photoconductivity of CSG-CN_x film was about 100 times higher than that of the single layered film prepared at 723 K. It was found that the compositionally and structurally graded structure in a-CN_x films is effective to improve the photoconductivity.

Flash lamp annealing (FLA) is a promising thin film crystallization of a-Si thin films with pulse duration in the order of milliseconds. Micrometer-order-thick a-Si films can be fully heated without damaging the substrates like plastic and low-cost glass.
In this work, we have reported a detailed study of flash lamp crystallized a-Si thin films deposited by PECVD using spectroscopic ellipsometry and Raman spectroscopy to understand the crystallization process at different FLA conditions.

In this study, amorphous lithium lanthanum titanate (LLTO) powders were prepared by a sol-gel method with an all alkoxide based route. Results showed that unlike its crystalline counterpart which turns into an electronic conductor in direct contact with lithium metal, amorphous LLTO remains to be an electronic insulator and hence is compatible with lithium metal although it also undergoes lithium insertion and the consequent reduction of Ti⁴⁺ to Ti³⁺. This striking difference between crystalline and amorphous LLTO could be ascribed to the atomic configuration difference, ordered versus disordered. The electronic states of a disordered system could be localized. It was found that the transfer ratio of the lithium ion conduction of amorphous LLTO is over 82%. Our results on electrochemical measurements indicated that amorphous LLTO is stable up to 12V, which can potentially be used with high voltage cathode materials.

Large scale Molecular Dynamics (MD) simulations of shock response of a-SiO₂ are carried out to understand the wave propagation behavior in a-SiO₂ as a function of shock pressure using various interatomic potentials. Planar shock waves are generated under conditions of impact of a piston with velocities of 500 m/s, 1000 m/s, 1500 m/s, and 2000 m/s. The response of the material varies with the peak pressures generated at the various impact velocities. For example, an impact velocity of 2 km/s is observed to cause melting of the material behind the shock front that is later ejected from the front end of the system due to the interaction of the reflected tensile wave. No failure due to spallation is observed for the impact velocities considered here under planar shock. The pressure wave propagation, the atomic scale deformation response and the effect of interatomic potentials will be presented in this poster. A comparison of the deformation response defined by long-range coulomb interactions with that defined by the short range bond-order interactions will be discussed.

Recent studies have showed that the Possion&’s ratio (nu;) correlates to the intrinsic ductility in glassy solids. We aim to investigate whether this correlation can be found for polymeric materials using molecular dynamics simulations. Here, we employed a new reactive polymer model, in which a glassy polymer is synthesized from monomers in silico. The glassy polymer exhibits a final length and weight distribution matching well with the Flory-Schulz distribution. This is in contrast to the conventional Krener-Grest bead-spring model, which does not consider poly-dispersity. The uniaxial tensile tests were conducted on the linear, cross-linked and network polymers synthesized by this model. Our simulations show that as the density of cross-linker increases, both the ductility and the Poisson&’s ratio reduce.

In this study, the structure of concentrated D-sorbitol-water mixtures is studied by wide- and small-angle neutron scattering (WANS and SANS) as a function of temperature. The mixtures are prepared using both deuterated and regular sorbitol and water at a molar fraction of sorbitol of 0.19 (equivalent to 70% by weight of regular sorbitol in water). Retention of an amorphous structure (i.e., absence of crystallinity) is confirmed for this system over the entire temperature range, 100-298 K. The glass transition temperature, Tg, is found from differential scanning calorimetry to be approximately 200 K. WANS data are analyzed using empirical potential structure refinement, to obtain the site-site radial distribution functions (RDFs) and coordination numbers. This analysis reveals the presence of nanoscaled water clusters surrounded by (and interacting with) sorbitol molecules. The water clusters appear more structured compared to bulk water and, especially at the lowest temperatures, resemble the structure of low-density amorphous ice (LDA). Upon cooling to 100 K the peaks in the water RDFs become markedly sharper, with increased coordination number, indicating enhanced local (nanometer-scale) ordering, with changes taking place both above and well below the Tg. On the mesoscopic (submicrometer) scale, although there are no changes between 298 and 213 K, cooling the sample to 100 K results in a significant increase in the SANS signal, which is indicative of pronounced inhomogeneities. This increase in the scattering is partly reversed during heating, although some hysteresis is observed. Furthermore, a power law analysis of the SANS data indicates the existence of domains with well-defined interfaces on the submicrometer length scale, probably as a result of the appearance and growth of microscopic voids in the glassy matrix. Because of the unusual combination of small and wide scattering data used here, the present results provide new physical insight into the structure of aqueous glasses over a broad temperature and length scale, leading to an improved understanding of the mechanisms of temperature- and water-induced (de)stabilization in various amorphous pharmaceutical systems involving proteins, pharmaceuticals, and biological objects. (This presentation reflects the views of the author and should not be construed to represent FDA&’s views or policies.)

Thermoplastic polymer composites based on mixtures of amorphous and semicrystalline polymers are known to develop synergistic mechanical properties via the development of a thermally induced network structure referred to in the literature as “mechanical grafting.” In this work we studied the amorphous olefinic polymer (polystyrene) as a binary composite member combined with polyethylene at compositions near the phase inversion point. In this region, a fine co-continuous structure is usually observed and we sought to assess the thermal stability of these composites with regard to coalescence towards coarser, less functional structures.
Blends were prepared by melt processing pellets of polystyrene and polyethylene in a single screw Brabender extruder with L/D 30:1 at 230 C and 100 rpm at several compositions containing 30 - 45 percent of the polystyrene. A 3 mm die insert was used to produce strands of specimen that we cut to 30 cm lengths. Annealing was conducted at temperatures ranging from ambient to well above the polystyrene glass transition temperature for periods of time of up to three months. Specimens were removed periodically for scanning electron microscope examination and image analysis to characterize the evolving morphology.
Results show that all blend compositions are stable well below the glass transition temperature of the polystyrene, as expected since the kinetically constrained nature of amorphous solids below the glass transition do not allow for additional relaxation within the time periods studied. However, when the blends were aged above the glass transition (Tg = 90 C) slow changes in the morphology began to occur. Current work assesses the degree to which mechanical properties such as elastic modulus and strength are affected by these relatively minor changes. When the blends were aged at or above the polyethylene melting point (Tm = 125 C) dramatic effects of coalescence could be observed, even at the “mechanically grafted” compositions near the phase inversion. Our presentation will show the evolution of the morphology as a function of annealing time and temperature and will present the most recent data regarding the effects of these changes on mechanical properties. At the highest temperatures studied, the evolution of the morphology to coarse, dispersed phase systems occurred. Clearly under these conditions the blends are of little practical engineering use. However, even for the low temperature materials, we seek to define the structure/property relationships as they evolve during the annealing treatments. Such data are highly useful in engineering design since elevated temperatures in the range of 50 - 125 C are often found in the design envelopes of engineered materials. As such, this study will contribute to our knowledge of the maximum continuous operating temperatures for commercial polymer blends.

Although a single polymer chain can have a thermal conductivity (κ) as high as several hundreds of W/mK,¹ polymers in an amorphous bulk form are poor thermal conductors (0.1 to 0.5 W/mK) due to weak van der Waals pathways for heat transfer between polymer chains. Adding thermally conductive fillers (e.g., silver particles, CNTs) to a polymer matrix at a concentration above the percolation threshold can yield high κ in polymers, yet this method can significantly increase material cost (nylon6,6: $2/kg vs. CNT: $1000/kg) and alter other important (e.g., electrical, optical) properties. Crystallization of polymers can yield high κ in small volumes such as nanofibers², but these high values are limited to the direction of chain alignment and require specialized fabrication techniques that are not compatible with bulk-scale manufacturing processes such as injection molding. While increased bond strength has been shown to yield a significant increase in interface thermal conductance at the monolayer scale³, low values of κ in cross-linked⁴ or hydrogen bonding capable polymers (e.g., nylon6,6: 0.25 W/mK) suggest that bonding strength alone does not dictate κ in amorphous polymers.
Rather, a homogenous distribution of strong bonds (e.g., hydrogen bonds) is crucial to the formation of the continuous thermal pathways that yield high thermal conductivity in a bulk amorphous polymer. By mixing hydrogen bond donating and accepting polymers that are miscible at the molecular level, we show that κ can be engineered to surpass 1.5 W/mK, which is the largest isotropic κ yet measured among amorphous polymers. Furthermore, the molecular structure of the hydrogen bonding linker is found to strongly affect the formation of thermal connections, allowing us to tune κ in the opposite manner (for lower thermal conductivity). For example, a heavy molecular unit between the hydrogen bonding moiety and the polymer backbone resulted in decrease in κ, suggesting a means to reduce κ in polymers for thermoelectric applications⁵.
References
¹ A. Henry and G. Chen, Phys. Rev. Lett.101, 235502 (2008).
² S. Shen, A. Henry, J. Tong, R. T. Zheng, and G. Chen, Nature Nanotech.5, 251 (2010).
³ P. J. O'Brien, S. Shenogin, J. X. Liu, P. K. Chow, D. Laurencin, P. H. Mutin, M. Yamaguchi, P. Keblinski, and G. Ramanath, Nature Mater.12, 118 (2013).
⁴ O. Yamamoto and H. Kambe, Polym. J.2, 623 (1971).
⁵ G.-H. Kim, L. Shao, K. Zhang, and K. P. Pipe, Nature Mater.12, 719 (2013).

Amorphous metal oxides (AMOs) are a potential key channel-layer material in the fabrication of thin film transistors (TFTs) for future electronic applications due to their unique combination of optical transparency, relatively high carrier mobility, low deposition temperature, and low-cost. Considerable attention has been paid to the tuning and optimizing electrical properties while little research has been done on the thermal properties of AMOs, especially the crystallization process as a function of growth conditions.
The deposition (substrate) temperature for AMOs films grown by pulsed laser deposition (PLD) is a factor known to control the crystallinity with films becoming more amorphous at lower deposition temperature. Below a certain deposition temperature, the metal oxide (MO) becomes AMO. From our study, for In₂O₃ films grown by PLD, not only do physical and electrical properties change as the deposition temperature is lowered within the amorphous growth region, but so does the kinetics of the amorphous-to-crystalline annealing processes.
The crystallization process of amorphous In₂O₃ (a-In₂O₃) films deposited as a function of deposition temperature under isothermal-anneal conditions has been studied using in situ grazing incidence wide-angle X-ray scattering (GIWAXS). It is found that the higher deposition temperature within the amorphous growth region, the lower the temperature needed for crystallization. Degree of crystallinity has been extracted as a function of time from time-sequenced 2D diffraction patterns and comparing with level-set simulation based on the Avrami equation, we are able to gain an understanding of the amorphous-crystalline transition processes, including the velocity of the amorphous-crystal interface, whether the nucleation is heterogeneous or homogenous nucleation, rate of nucleation, nucleation density, and more importantly, the initial crystalline volume fraction of the as-deposited films. Thus we are not only able to use the crystallization studies to gain insight into the crystallization process but to also gain insight into the structure of the amorphous film. Several additional techniques, such as X-ray reflectivity (XRR) measurements combined with X-ray photoelectron spectroscopy (XPS) to probe the density of the films, high-resolution transmission electron microscope (HRTEM) images to directly measure nanocrystalline inclusions sizes and molecular dynamics (MD) simulation, are used to compliment the crystallization study.

Metallic glasses (MG) in engineering services are usually under various stress states. Correct understanding of their stability is thus important from both scientific and technological point of view. Here we combine both synchrotron X-ray experiment and computer simulation to investigate the thermal and mechanical stability of geometrically confined metallic glass under anisotropic stresses. We demonstrate that the energy landscape of well-equilibrated MG is greatly perturbed under applied stress. As a result, the kinetic relaxation barriers of MG are greatly reduced, leading to various structural instabilities (e.g., mechanical and thermal instability). We reveal that atomic diffusion kinetics as well as certain atomic vibrational modes is greatly fastened in comparison to stress-free MGs, causing a much reduced glass transition temperature. The possibility of room temperature devitrification behavior will also be discussed based on metadynamics simulation of the nucleation process of stressed MG.

Glass exhibits a significant change in microstructure and properties when subjected to high pressure, since the short- and intermediate-range structures of a glass are tunable through compression. Understanding the link between the microscopic structure and macroscopic properties of glasses under high pressure is important, since the glass structures frozen-in under elevated pressure may give rise to properties unattainable under ambient pressure. Chemical strengthening of glass through K⁺-for-Na⁺ ion exchange is currently receiving significant interest due to the increasing demand for stronger and more damage resistant glasses. However, the interplay among isostatic compression, pressure-induced changes in alkali diffusivity, compressive stress generated through ion exchange, and the resulting mechanical properties are poorly understood. In this work, we employ a specially designed gas pressure chamber to compress bulk glass samples isostatically up to 1 GPa at elevated temperature before or after the ion exchange treatment of an industrial sodium-magnesium aluminosilicate glass. Compression of the samples prior to ion exchange leads to a decreased Na⁺-K⁺ inter-diffusivity, increased compressive stress, and slightly increased hardness. Compression after the ion exchange treatment changes the shape of the potassium-sodium diffusion profiles and significantly increases glass hardness. We discuss these results in terms of the underlying structural changes in network-modifier environments and overall network densification.

Titanium-based bulk metallic glasses (BMGs) exhibit strength values superior to conventional crystalline alloys, often combined with a large elastic limit and rather low Young&’s modulus, as well as excellent corrosion resistance and good biocompatibility. However, the major drawback for structural and functional applications is their limited room temperature ductility and toughness due to the localized deformation processes linked to shear banding. Recent reports have demonstrated that the mechanical properties of a BMG depend on its internal structure, i.e., the intrinsic brittleness or plasticity is determined by the way that different atoms are mixed, packed and bonded in a monolithic BMG. In that sense, minor changes in atomic species (e.g. through micro-alloying) may result in significant structural changes of a BMG and accordingly influence its mechanical properties. In the present study we propose a novel strategy for the design of ductile Ti-based BMGs through minor substitution using exceptional “soft” atoms. Since the elastic properties of constituent elements are generally taken into account for the design of ductile metallic glasses, we selected indium (In) as substituting atom in order to increase the heterogeneity of BMGs because of its low Young&’s modulus of 10.6 GPa, large Poisson&’s ratio of 0.45 and its electronic structure, due to which mainly s and p orbitals contribute to the metallic bonding. In addition, its relatively large atomic radius of 167 pm might also influence the atomic-level structure of the BMG. In this study, a Ti40Zr10Cu36Pd14 alloy (at.%) is selected as a model glass to further improve its plasticity due to the change of internal structure upon partially substituting copper atoms by different contents of indium atoms. The beneficial effect of In addition on the glass-formation, amorphous phase stability and mechanical properties was assessed by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, compression tests and ultrasonic measurements. The results reveal that the plastic strain of Ti-Zr-Cu-Pd BMGs can be significantly enhanced from 2.7% to 8.3% via 4 at.% indium substitution. The addition of In “soft” atoms with “soft” elastic properties may form regions of lower stiffness due to weak bonding with the other elements, thus triggering enhanced atomic-scale heterogeneity, which improves the overall plasticity. Support by the EC (BioTiNet, 264635), DFG (SFB-Transregio 79) and the National Science Foundation of China is gratefully acknowledged.

The modeling of metallic glass mechanical response, including the prediction of failure, requires the establishment of numerically tractable continuum descriptions of viscoplasticity that incorporate relevant atomistic mechanisms and can be parameterized to metallic glass microstructure. Here we deploy the shear transformation zone (STZ) theory [1] to make quantitative predictions of deformation and failure processes in amorphous solids. This is done in the context of a thermodynamic theory wherein the local disorder is quantified in terms of an effective temperature. [2] Molecular dynamics (MD) simulation is used to parameterize the model. A highly optimized fully Eulerian implementation of the STZ theory is implemented [3] to investigate monolithic metallic glasses and metallic glass crystal composite materials subjected to very large strains, such as those that arise in failure processes such as strain localization. We then perform cross-comparisons between continuum theory and MD predictions using structural parameters that can be independently measured in MD. The onset of failure depends sensitively on the coarse-grained stochastic field that represents the structural inhomogeneity, the details of which determine the mechanical response as well as the onset of instabilities. The properties of this stochastic field are further studied to provide insights into the structure of amorphous solids.
[1] M.L. Falk, J.S. Langer, “Deformation and Failure of Amorphous Solidlike Materials,” Annual Review of Condensed Matter Physics,2, 353 (2011).
[2] E. Bouchbinder, J.S. Langer, “Nonequilibrium thermodynamics of driven amorphous materials. III. Shear-transformation-zone plasticity,” Phys. Rev. E,80, 031133 (2009).
[3] C.H. Rycroft, Y. Sui and E. Bouchbinder, “An Eulerian projection method for quasi-static elasto-plasticity,” (2013) in press.

The studies on dynamics and deformation in glassy materials are particularly challenging because of their strongly disordered atomic structure. Here by probing the changes in the atomic displacements and stresses at saddle points of the potential energy landscape (PEL), we show that the thermally activated deformation is triggered by subnano-scale rearrangements of a small number of atoms, typically less than 10 atoms. Surprisingly, the individual triggers are invariant of the cooling history or elastic structure of the system. However, the organizations between different trigger centers can be varied and are related to the overall stability of the system. It is observed that such different organization patterns might be related to the spatial distribution of “soft” atoms within the lowest 5% atomic shear modulus. This finding allows a semi-quantitative construction of PEL and brings a new perspective to the age-old study of the mechanical properties of glasses.

Shear band is considered as the deformation unit in bulk metallic glasses (BMGs), which is similar to the dislocation in crystalline. However, shear induced softening and catastrophic failure significantly undercuts their application as structural materials. So far, the usual ways, i.e., promotion of multiple shear bands or preventing the rapid propagation of one dominant shear band, are significant limitations in improving the ductility, especially for bulk samples in tension. Here, we show an improved plasticity when shearing is suppressed in the notched samples. Furthermore, it is found that the deformation of Zr- and La-based BMGs strongly depends on the possion&’s ratio under triaxial stress state, and the reason was further investigated by free volume model. The present findings greatly enrich our understanding of the deformation behavior of metallic glasses, and provide a total new way of designing a better structure for their engineering applications.

Bulk metallic glasses (BMGs) deform inhomogeneously at low homologous temperatures with confinementinto localized shear-bands. Whilst the dynamics of shear-bands has been addressed in previous studies (shear-band initiation [e.g. Phys. Rev. Lett. 107, 185502 (2011)], shear-band propagation [e.g. Acta.Mater. 59, 3205, (2009)], and shear-band arrest [e.g. Appl. Phys. Lett. 100, 071904 (2012)]), little is yet known about structural changes due to shear-band activation. Here we investigate the nanomechanical properties in two dimensions along single shear-band lines. We find that the hardness locally decreases up to 30%, but more importantly, our 2D mapping reveals the extent of softened regions on the scale of hundreds of micrometers. Tracing the spatial variation of both hardness and elastic modulus evidences strong fluctuations along and normal to the shear-bands. Further analysis indicates that the softening is a result of long range stress fields that originate from the shear-bands, as well as from mesoscopic cavities. These stress fields are found to be relieved as a function of time, leading to the spontaneous formation of diffuse shear-bands, indicating the existence of internal stresses around shear-bands that are of the order of the yield strength of the material itself.

The effects of changes in material composition and test techniques on the fracture toughness of a variety of metallic glasses will be reviewed. In addition, more recent work on bulk metallic glass composites will be presented. The effects of changes in test temperature, loading rate and type, were examined in tension and toughness for some compositions and these will also be summarized.

The nucleation and propagation of shear bands in metallic glasses can be affected by the introduction of crystalline phases into the amorphous matrix. Using molecular dynamics simulations on Cu-Zr based glass with crystalline precipitates, we investigate conceivable mechanisms of influencing shear bands in crystal-glass composites. First, tensile test simulations of the bulk glass with fcc copper nanoprecipitates show that the crystal-glass interfaces promote the formation of shear transformation zones and therefore serve as a source for shear band nucleation. This increased shear band nucleation rate enhances the homogeneity of deformation. Second, we investigate if a precipitate could act as a barrier to shear band propagation. We control the location of shear band nucleation using a stress concentrator on a free surface and put a precipitate in the way of the shear band. We use fcc copper and B2 CuZr precipitates of varying size. Depending on the size of the precipitates and the stress needed to nucleate a dislocation in the precipitate, we identify different propagation mechanisms under tensile strain: The shear band is stopped by the precipitate, the shear band wraps around the precipitate, or the precipitate takes part in the plastic deformation.

Bulk metallic glasses (BMGs) have attracted much attention with superior properties such as high strength, large elastic strain limit (~2%), corrosion resistance, etc. However, monolithic BMGs typically fail in a brittle manner due to absence of slip systems in crystalline metals, such as dislocations, grain boundaries, twin, etc. This catastrophic failure is a serious flaw that limits structural applications of BMGs. A large number of BMG matrix composites with improved ductility have been developed with a variety of ductile or hard secondary phases. Because these BMG composites showed macroscopic strain softening phenomenon, it is necessary to develop special composite structure with significant work-hardening capability. Recently, it has been reported that some BMG composites with transformation mediated CuZr secondary phases showed advantageous work hardening behavior even in tensile deformation. In the present study, we fabricated Ex-situ BMG composites with various types of secondary phases such as soft crystalline Ta, hard ceramic SiC, and transformation mediated Ni-Ti-Cu secondary phases by spark plasma sintering. BMG composites with Ni-Ti-Cu 2nd phases showed the highest work hardening ability supporting prior knowledge that martensitic transformation of 2nd phase is a key factor of work hardening. The effect of secondary phases on deformation behavior during compression test will be systematically discussed with in-situ neutron diffraction compression test. By tailoring types and transformability of secondary phases in BMG composites, we could fabricate more work-hardenable BMG composites and offer a promising guideline for design of BMG composites with high damage tolerant reliability in service.