Symposium MD8 : Multiscale Behavior of Materials in Extreme Environments

The need for a better understanding of the behavior of materials in extreme conditions of temperature or irradiation is growing in many fields such as materials sciences, earth sciences, industrial process (refractories, glasses, ceramics), and nuclear sciences and techniques. This paper aims to review some recent experimental developments performed in CEMHTI lab. This acronym CEMHTI means “Extreme Conditions and Materials: High Temperature and Irradiation”, that shows that such conditions are the main motivation of this lab. The present paper will then review recent Raman scattering in situ developments in high temperature or/and irradiation conditions. For high temperatures, the main difficulty is thermal emission from the sample and its neighborhood, which can be filtered by time-resolved optical devices¹. Moreover this also allows separation of Raman from luminescence, which constitutes a frequent difficulty precluding Raman acquisition. The potentialities of the system will be illustrated by some examples on oxide materials: phase transitions of zirconia, structural relaxation of silicas², and reactivity of olivine ceramics.
For irradiation, a mobile Raman device, with a fiber-coupled probehead, was developed to monitor materials behavior under light ions beam (He²⁺, H⁺) supplied by a cyclotron. This system equiped with different sample chambers allows to investigate various conditions of irradiation: irradiation damages in solid actinide oxides (Th, U) ^3,4, radiolysis of solid/water interfaces (UO₂/H₂O)⁵, radiolytic corrosion of nuclear graphites (in a device combining high temperature, high pressure, and irradiation conditions). For uranium the understanding acquired in these experiments allows in a following step to consider more active materials⁴ such as PuO₂ and even spent nuclear fuels⁶.
This mobile device can be in fact used in much more situations than the only nuclear field. It only needs an optical access to the phenomenon to investigate, and a probehead adapted to the optical focus distance. As an example, Raman monitoring of carbon nanotubes growth in a PECVD reactor will be presented⁷.

(1) P. Simon et al., Journal of Raman Spectroscopy 2003; 34, 497.
(2) M. Dutreilh-Colas et al., J. Am. Ceram. Soc. 2011; 94, 2087.
(3) G. Guimbretiere et al., Applied Physics Letters 2013; 103, 041904.
(4) R. Mohun et al., Nuclear Instruments and Methods in Physics Research Section B 2015.
(5) A. Canizares et al., Journal of Raman Spectroscopy 2012; 43, 1492.
(6) C. Jegou et al., J. Nucl. Mater. 2015; 458, 343.
(7) T. Labbaye et al., Applied Physics Letters 2014; 105, 213109.

To fully utilize nuclear fuel its material properties must be maintained under extreme conditions. Thermal conductivity is particularly important owing to its role in mitigating centerline fuel melting. Firstly, the role of UO₂ magnetism in phonon scattering has been investigated experimentally. Furthermore, through the Callaway Model magnetism sufficiently accounts for the difference between molecular dynamics and experiment at low temperatures. By combining defect-phonon scattering (from molecular dynamics) with the magnetic contribution (from experiment), the Callaway Model can be parameterized to predict the degradation of UO₂ thermal conductivity due to different fission products. Zr, Xe, La and Pu are all predicted to influence thermal conductivity to different degrees depending on their valance state and atomic size relative to the host lattice. Furthermore, the model is extended to include magnetic scattering for predictions of thermal conductivity of mixed oxides, UPuO₂ and UThO₂.

Applications of glancing-incidence X-rays, to be specific the glancing-incidence diffraction for characterization of textured films and depth profiling of residual stresses are described. In a glancing-incidence experiment, the detector is in a horizontal plane parallel to the film surface to measure diffraction from lattice planes which make an angle to the surface. Especially for the analysis of thin films, x-ray diffraction techniques have been developed for which the primary beam enters the sample at very small angles of incidence. In its simplest variant, this configuration is called GIXRD: glancing incidence x-ray diffraction. The small entrance angle causes the path traveled by the impinging X-rays to increase significantly and the structural information contained in the diffraction pattern to come forth primarily from the thin film. The parafocusing geometry, as applied in conventional powder diffractometry, cannot be used in those experiments and devices for parallel x-ray optics need to be used (e.g. laterally graded multilayer mirrors).
In a stress/strain depth profiling experiment, values of strains and residual stresses averaged over different penetration depths beneath the film surface (or z-profiles) are obtained by asymmetric diffraction from crystal planes which are inclined to the film surface. The actual strain at different depths under the surface (z-profiles) is extracted from the average values using inverse Laplace and numerical inversion methods. The measurement of depth-dependent properties may be also generalized to the concentration of a chemical phase, the amorphous fraction, the crystallite size, microstrain or macrostrain or any other. On the basis of simple considerations one may define an attenuation length that allows expressing that depth-averaged property derived from a Bragg reflection and the the task of obtaining the local value of that interesting property function can be restated as a problem of inverse Laplace transformation.
Examples of such analyses of graded strain in sputtered W films are given. This work is partly supported by a grant of Investissements d’Avenir of LabEx PALM (ANR-10-LABX-0039-PALM).

A Ceramic-Ceramic composite fuel model has been proposed by several researchers as an alternative advanced nuclear fuel with the goal of increasing the thermal conductivity and reducing microstructure evolution due to defect accumulation. Understanding the structural modifications of ceramic composites with different grain sizes at elevated temperatures is therefore of fundamental importance. In this study, dynamical thermal effects were evaluated in model three phase composites of different grain sizes consisting of equal amounts of YSZ, MgAl₂O₄ and Al₂O₃ using in-situ high temperature x-ray diffraction (HTXRD) from room temperature to 1100°C. Crystallite size, strain and unit cell volume changes were determined as a function of temperature. Thermal expansion coefficients for each of these phases in a composite were also deduced and compared with those obtained for single phases.

Mechanism of cleavage crack propagation during testing of standard Charpy V-notch specimen at -196°C has been investigated in a low-carbon lath martensitic steel (modified 9Cr-1Mo steel) using electron back-scattered diffraction (EBSD) technique. The microstructural units of martensitic structure are characterized in terms of Kurdjumov-Sachs (K-S) crystallographic variants and subsequently the boundaries present in the martensitic structure (sub-block, block and packet boundaries) are identified to understand thoroughly the structure-property correlation of this steel. EBSD study of cleavage crack path indicated that block boundaries are more effective in cleavage crack retardation as compared to packet boundaries, whilst sub-block boundaries are ineffective in crack resistance. Moreover, characterizing boundaries in terms of misorientation (angle-axis) angle may be misleading to identify the role of boundary in cleavage crack resistance.

Tungsten carbide cermets are strong candidate materials for neutron shields in fusion reactors, due to their short mean-free-path for high-energy neutrons (Windsor et al, Nucl. Fusion 2015), excellent thermo-mechanical properties and manufacturability. However, they are susceptible to high temperature oxidation, which is problematic for reactor accident scenarios. For instance with a sudden air-ingress at high temperature, tungsten oxides can volatilise and release dangerous transmutation products. We have developed a successful process for depositing compositionally graded coatings with improved oxidation resistance. The coating formation mechanism could be of wide interest to the field of graded materials production, which is usually categorised into two main classes of processes (Suresh Int. Mat. Rev 1995): (i) constructive, i.e. coating or stacking of constituents in sequence; and (ii) transport-based, where diffusional phenomena create the compositional gradients. The coating process we present combines both constructive and transport processes simultaneously, resulting in the separation of key oxidation-resistant constituents to the surface. Crucially, this segregation process is fundamental to the superior oxidation performance, which outperform conventional coatings (e.g. boriding) by 2 or more orders of magnitude. We firstly compare the baseline oxidation studies of coated and pristine cermets by combining thermogravimetric analysis (between 600 and 1000 ^o C) with electron microscopy and X-ray diffraction, then reveal the mechanism of surface passivation, and finally demonstrate the coatings’ tomographic structure and formation mechanism.

The mechanical properties of polycrystalline materials are strongly governed by the presence of obstacles to dislocation motion. The grain boundaries (GBs) present an effective barrier to the dislocation motion, thereby strengthening the material. However, the presence of an aggressive environment such as hydrogen increases the susceptibility to intergranular fracture. There is a strong need to systematically investigate the role of corrosive environment on the dislocation-grain-boundary interactions from the perspective of failure mechanisms. In this work, we employ molecular dynamics to study the interactions between screw dislocations and several <111> tilt GBs in Fe. It was found that outcome of the dislocation-GB interaction depends strongly on the underlying GB structure. Further, there existed a strong correlation between the GB energy and the energy barrier for dislocation transmission. In other words, GBs with lower interfacial energy demonstrated a higher barrier for slip transmission, which are in agreement with previous experimental and modeling efforts. The introduction of hydrogen along the grain boundary causes the energy barrier for slip transmission to increase consistently for all of the grain boundaries examined. The energy balance for a crack initiation in the presence of hydrogen was examined with the help of our observations and previous findings. It was found that the presence of hydrogen increases the strain energy stored within the grain boundary which could lead to a transgranular-to-intergranular fracture mode transition.

A sequential multiscale model for finite element analysis of intergranular fracture process in metallic microstructures coupled with hydrogen embrittlement is presented. The two dimensional polycrystalline structure ahead of the notch in the specimen is represented implicitly and solved by a non-linear extended finite element method (XFEM) solver. In order to take into account the effect of concurrent stress-assisted hydrogen embrittlement of grain boundaries, a convection-diffusion equation is numerically solved on the network of grain boundaries to estimate the grain boundary degradation of cohesive strength. For that purpose, hydrostatic stress along the grain boundaries is introduced into the formulation of hydrogen transport. Grain boundary properties are quantified by atomistic simulations and employed in the XFEM framework using a modified cohesive zone model to bridge the different length scale calculations. Also a rate-dependent crystal plasticity model is incorporated into XFEM to account for orientation dependent elasto-plastic deformation. Effect of hydrogen coverage and microstructural properties, such as grain boundary character on the crack initiation and growth is studied.

The accurate prediction of phase transitions is one of the most important research areas in modern materials science as it often replace otherwise difficult or even impossible to get experimentally measured properties of selected phases. Density functional theory (DFT) was for a long time the main workhorse for such calculations. As a simple mean field method DFT employs different forms of approximate exchange-correlation functionals which often fails to describe accurately several phenomena as bonding, cohesion, conductivity, optical properties and other quantum effects. As a result one can encounter overstabilization of one phase compared to another, especially if two phases belong to different classes (e.g. metal and semiconductor). A recent example of such deficiency has been demonstrated in Sn: no bcc to hcp phase transition has been observed in Sn when dynamically compressed to 1.2 TPa while DFT predicts a transition to occur at 0.16-0.2 TPa [1].
To overcome the limitations of DFT, we have employed modern Quantum Monte Carlo (QMC) methods which treat the many body electron problem directly and allow us to systematically improve the results. Despite being more computationally demanding that a standard DFT, the accuracy that QMC provides can justify additional expenses.
We have applied one QMC flavor, diffusion quantum Monte Carlo (DMC), to study the pressure induced phase transitions in Sn. In order to get highly accurate results we systematically assess the effect of controllable approximations of DMC such as fixed node approximation, finite-size effects and the use of pseudopotentials. We then compare the reliable equations of state for several Sn phases with DFT all-electron and pseudopotential calculations to evaluate the accuracy of exchange-correlation functionals and the choice of effective core potential. Based on metrologically accurate equation of states we construct the pressure-temperature phase diagram and confirm dramatic effect of energetic phase stability on it. Based on our careful analysis we demonstrate that modern DMC calculations have a superior accuracy compared to DFT and allow accurate description of phase transitions.
[1] A. Lazicki et al., X-Ray Diffraction of Solid Tin to 1.2 TPa. Phys. Rev. Lett. 115, 075502 (2015)

We model the diffusional high temperature deformation of particulate composites using a discrete dislocation modelling framework. Interfacial diffusional deformation between the particles and the matrix is modelled by a stress-driven diffusion model while plastic deformation of the matrix involves both the glide of dislocations and their climb due to vacancy diffusion. The results demonstrate that while dislocation climb within the matrix reduces the strength of the composite, it results in the formation of dislocation cell structures within the matrix. These cell structures in turn bring additional length scales into the deformation process which results in a size dependence of the strength of composites. Moreover, the formation of cell structures is facilitated when the climb of dislocations coupled with the deformation of the particle/matrix interfaces due to diffusion. Parametric studies on the effect of the interfacial diffusion rates and the climb rates of the dislocation are presented to highlight the relative role of these parameters in the coupled deformation of the composites at high temperatures.

Shock produces pressures much more extreme than attainable under equilibrium conditions, albeit transiently, anisotropically, and in a combination of alternating compression and rarefaction waves. Shock can therefore induce transitions to high pressure and other phases, at pressures that may vary considerably from those under hydrostatic conditions. The formation of these phases is not necessarily fully reversible, leaving altered material as a signature. These provide the information from which the the atomic scale shock mechanism can be reconstructed. The standard methods of characterizing post-shock material include crystallography and transmission electron microscopy (TEM). Crystallography, however, is limited in that it only gives information on the average atomic positions of the periodic component of a material, being insensitive to aperiodic local lattice distortions and amorphous regions that exhibit short range order. TEM also depends to a lesser extent on long range order, and requires samples that are thin along the beam direction and so susceptible to elastic relaxation that may eliminate altered structures trapped by stress. Local structure methods, pair distribution function analysis and X-ray Absorption Fine Structure (XAFS) spectroscopy, that penetrate the material are therefore a useful and perhaps necessary complement to these more conventional techniques. This is demonstrated by XAFS studies of shocked Zr and Cu(Pb), in both of which the local structure gives different results than the diffraction. A common response of cubic materials to shock is the formation of hexagonal, omega phases in which the symmetry is lowered by changes in the lattice parameter along the unique direction, which is presumably the same as the shock propagation through the material. An issue, however, is whether this modified interplanar spacing is uniform throughout the material or varies between locations. Unless the domains are relatively large, diffraction will give the average spacing without necessarily indicating the amount of disorder, especially if some of the domains are below the diffraction limit in size. In the case of shocked Zr, we do find such a difference in the remnant omega phase, demonstrating that its domains are irregular with a substantial fraction being relatively small. Shocked Cu(Pb) was probed with a 2 micron beam that allowed the interiors of the grains to be mapped by XAFS. No difference was found as a function of position relative to the grain boundaries, but all of the probed spots showed the presence of an shell extraneous to the oriignal fcc structure that we interpet as an hcp distortion. That it is invisible in bulk diffraction measurements demonstrates that these domains are only nanometers in size and also that the distortion is disordered, possibly by being randomly oriented. These novel results therefore indicate that the forward and reverse transitions are more complex than previously believed.

Displacement damage due to irradiation-matter interaction is the primordial reason for material degradation in radiation fields. The knowledge of the energy limits to generate damage has historically been generated using semi-empirical interaction potential in all but a few select materials, most notably in semi-conductors or insulators, where first principles molecular dynamics has been applied. Here, we will show recent developments in studying displacement damage, and the threshold energies, using first principles electronic structure theory in bcc metals, focusing on Fe and W for their importance in current and next generation nuclear energy applications. Even applying first principles methods, it is shown how important the choice of approximations are. The difference between the refractory W and softer Fe is highlighted and discussed. How to generate more reliable interatomic potentials for the critical short range interaction, between the screened Coulomb limit and the thermodynamic range of validity is treated explicitly. Optimization techniques to speed up these extremely costly calculations are also decribed. First results on the sensitivity to minor alloying elements will be discussed.

Compelling demands to reduce carbon dioxide emissions worldwide have created significant pressure to shift from extensive reliance on carbon-based energy sources to more carbon-neutral alternatives such as nuclear energy. Gen-IV is the next generation of nuclear reactor designs that requires structural materials to tolerate hostile environments that lead to undesirable microstructural evolution and deterioration in mechanical performance such as severe hardening and embrittlement. This necessitates the development of novel advanced structural materials that will be able to withstand higher operating temperatures, higher irradiation doses and extremely corrosive environments. Potential materials include ferritic-martensitic steels, austenitic stainless steels and oxide dispersion strengthened (ODS) steels.

Ion irradiation is often used in lieu of neutron irradiation for nuclear-reactor-based research. Studying the influence of neutron irradiation on nuclear reactor materials can often be costly and extremely difficult. Neutron-irradiated materials become radioactive in reactor-irradiation environments and require long irradiation exposure times to simulate real world use. Ion irradiation on the other hand requires significantly less time to achieve the same dose. However, the low penetration depth of ions restricts the amount of irradiated material for study, necessitating the development of small-scale mechanical testing techniques for ion-irradiated materials.

Our presentation will focus on small-scale mechanical testing of selected ion-irradiated candidate structural alloys for Gen-IV reactors, including: 800H, T91, nano-crystalline T91 (NCT91) and 14YWT. Fine-grained 14YWT is an exceptionally good representative of the ODS class of alloys due to its unique microstructure and nano-structured characteristics. 14YWT has not only high radiation resistance, owing partly to yttrium-rich nanoparticles that act as irradiation-induced defect sinks, but also desirable mechanical properties and stability at high temperatures.

The aforementioned candidate alloys were irradiated with 70 MeV Fe⁹⁺ bombarding ions, at 443°C to a dose of 20.680 dpa. The damage layer (ion penetration depth) is approximately 6.2 μm into the samples. Nanoindentation and in situ micro-compression testing were chosen to evaluate mechanical responses to ion-irradiation. Our initial results show that significant increases in hardness and yield stress observed in our ion-irradiated alloys are similar to neutron-irradiated alloys.

During the Charpy impact testing of controlled rolled high strength low alloy steel, a special type of crack is often found on the fracture surface, which propagates through the transverse plane with respect to the main fracture plane. This type of crack on the fracture surface is commonly known as splitting or fissure or delamination. The role of crystallographic texture on splitting is studied through combined scanning electron microscopy and electron backscattered diffraction analyses on the sample, finish rolled at different temperatures (820°C - 650°C). Through thickness texture band composed of cube (ND parallel to <001>) and gamma (ND parallel to <111>) fibre orientations has been found to be developed during the intercritical-rolling treatment. Splitting crack is found to propagate between these two texture bands on the main fracture plane owing to strain incompatibility. In order to understand the crack deflection at grain boundary, different combination of neighboring crystal orientations has been considered and possible tilt and twist angle has been evaluated. A new approach based on the angle between {001} planes of neighbouring crystals has been employed in order to estimate the ‘effective grain size’, which is used to determine the cleavage fracture stress on different planes of the sample. The severity of splitting is found to be directly related to the difference in cleavage fracture stresses between the ‘main fracture plane and ‘splitting plane’. Finally, a phenomenological model has been developed to represent the mechanism of splitting.

Owing to their lightweight, magnesium-based alloys such as AZ91D and AM60B are currently being incorporated into automotive designs. One of the major concerns in using these materials is their susceptibility to corrosion-induced damage. The corrosion behavior of cast Mg-Al alloys is dependent on the alloy composition and microstructure. Our research is aimed at understanding the compositional changes occurring on the surface of these alloys when exposed to aqueous environments, since these changes govern their long-term corrosion behavior.

We present experimental data for the corrosion behavior of a model system composed of elemental Mg with 5 at% aluminum in solid solution. Accelerated corrosion testing of this alloy shows the evolution of 100-micron size aluminum rich platelets that cover about 30% of the alloy surface. We investigate the mechanism for platelet evolution using Kinetic Monte Carlo simulations. These results show that this morphology evolves by a dealloying process involving the selective dissolution of Mg, which occurs via a layer-by-layer process involving step flow. The implications of our results for the corrosion behavior of the more complicated commercial alloys are discussed.

Light water reactors, which represent the vast majority of nuclear reactors in the world, use Zr-based alloys as fuel cladding material, due to their very low cross-section for thermal neutron absorption coupled with good mechanical properties and corrosion resistance at normal operating conditions. However, upon significant temperature increase, such as the one that occurred during the Fukushima Daiichi accident, these alloys experience significant decrease in strength and tend to burst at temperatures between 700 ^°C and 1100 °C, depending on the rod’s internal pressure. Most importantly, the exothermic Zr oxidation becomes self-catalytic at temperature above 1200 °C, resulting in significant hydrogen production and consequent explosion hazard. Because of this, efforts are ongoing worldwide to develop the so-called Accident Tolerant Fuels, i.e. fuel/cladding materials capable to survive loss of cooling for a longer period of time compared to Zr-based alloys.
One approach is to use advanced ferritic stainless steels as cladding materials, since they have the potential to overcome some of the limitation of Zr-based alloys. High temperature oxidation resistance and high temperature strength properties can be easily modified by controlling various alloying elements in ferritic steels in order to satisfy their application performance objectives. Considering the stress corrosion cracking (SCC), an important corrosion phenomenon in nuclear power plants, ferritic stainless steels have an excellent resistance compared to that of austenitic stainless steels, therefore the former is more suitable for the cladding materials in order to prevent the unpredictable failure in advance. Specifically, this study focuses on alumina-forming ferritic stainless steels since they were shown to have an excellent oxidation resistance at high temperature compared to Zr alloys. But, there is a limited amount of data on high temperature oxidation of these advanced ferritic stainless steels, it is necessary to establish data on high temperature oxidation behavior for these alloys under severe conditions.
With respect to high temperature oxidation of APM and APMT under dry air condition, weight gains at temperatures from 600 °C to 900 °C were similar or better to that for ZIRLO^® at 400 °C. The dense oxide layers, with no cracks, formed on the APM and APMT alloy surfaces at 1100 °C, which composed of ~66 at.% of O and ~33 at.% of Al and its thickness was ~3 µm. In particular, APMT showed lower rate constant over the whole temperature range between 600 °C and 1100 °C and had a lower activation energy than APM.
For high temperature oxidation behavior of APM and APMT under 100% steam condition, oxidation rate was higher compared to the equivalent tests done under dry air condition. Moreover, the grain size and the thickness of oxide layer of APM and APMT under 100% steam condition were larger and thicker than those under dry air condition.

Nanocrystalline metals are comprised of a high density of grain boundaries, which act as sinks for radiation induced defects. Defect denuded zones should be more dominant in a nanocrystalline material, and this concept could lead to improved radiation tolerance. Advancement in the understanding of radiation damage has shown, however, that the concept of a constant or stable denuded zone does not seem to hold true. The efficiency of the grain boundaries as defect sinks play a significant role in the ability of a nanocrystalline metal to reduce radiation damage. To probe the relationship between denuded zone development, grain boundary efficiency, and dislocation behavior, precession electron diffraction was coupled with in situ irradiation to determine any connection between denuded zones and local dislocation densities. In-situ irradiation was performed in a transmission electron microscope (TEM) on freestanding nanocrystalline Fe samples using the i3TEM facility in the Department of Radiation Solid Interactions at Sandia National Laboratories. Automated crystallographic orientation mapping (ACOM) was also performed via NanoMEGAS ASTAR precession diffraction; dislocation densities were calculated from orientation data using the Nye tensor. Irradiations were performed with 10 keV Helium (He) ions at 300°C, and specific regions were monitored and analyzed throughout irradiation. Grains that appeared to have denuded zones at low dpa started to collapse at intermediate dpa ranges. At high dpa, many denuded zones were completely vanished. Grain boundary analysis using the Nye tensor method demonstrated high an increase in defect densities near the boundary with increasing dose, correlating with denuded zone collapse. The breakdown of denuded zones corresponded with an increase in geometrically necessary dislocations at the boundary, indicating that the boundary defect structure had been changed by the absorption of radiation defects.

We investigate semi-coherent interfaces between immiscible metals, AgCu and CuW, by means of Molecular Dynamics and Monte Carlo atomistic simulations relying on phenomenological n-body potentials taken from the literature or developed on purpose. The last are shown to comply with the experimental phase diagrams of the binary systems, namely by reproducing the large miscibility gap existing between their constituents. Furthermore, it is shown that at the thermodynamic equilibrium, unlike bulk systems, a smooth miscibility profile exists across hetero-phase interfaces, necessary for minimizing the excess free energy of the interface. These findings suggest that welds between not miscible metals are possible. Ongoing work focuses on the mechanical stability and resistance of such assembles.

We acknowledge partial support through grant ANR-10-LABX-0039-PALM of “Investissements d’Avenir of LabEx PALM”.

Energy transport via lattice vibrations (phonons) play a crucial role in several applications such as heat dissipation in semiconductors, waste heat energy conversion via thermoelectric materials, and phase transitions and cavitation phenomena in intensely heated nanofluids. Investigation of the temporal behavior of externally stimulated materials, under severe thermal non-equilibrium conditions, can lead to crucial insights for energy research.

Recently, experimental techniques have evolved to conduct time-dependent lattice dynamics measurements in nanomaterials. In particular, Bragg Coherent Diffraction Imaging (BCDI) in conjunction with optical pump-probe experiments via ultrafast x-ray lasers has been used to directly image the generation and subsequent evolution of coherent acoustic phonons within nanocrystals. In particular, experiments on bimetal (Au/Al) core-shell nanocrystals have revealed inhomogeneous effects in the lattice breathing upon heating with femtosecond x-ray lasers. Conventional theoretical models cannot be used to explain the physics of the phenomena in such non-equilibrium environments that involve extreme heat fluxes and temperature gradients.

Atomistic molecular dynamics (MD) simulation is an appropriate technique to investigate lattice dynamics in such environments, particularly in core-shell structures where interfacial effects can play an important role in phonon scattering. With the convergence of time and length scales accessible by both experiments and simulations, we are now able to integrate experimental observations with multi-million atom MD simulations to enhance the fundamental understanding of materials behavior under extreme environments.

In this talk, we focus on the MD simulations performed on core-shell bimetallic nanocrystals under the influence of extreme heat fluxes. We explore the vibrational modes that dominate lattice breathing and look into the effect the nanoparticle size and shape have on the same. In addition, we attempt to identify the origin of the inhomogeneous effects, observed experimentally, in the lattice breathing of core-shell structures. Finally we investigate the effects of lattice mismatch between the core and shell material, the shell-to-core size ratio and the interfacial structure to tune the lattice dynamics of core-shell nanocrystals.

At high strain rates (10³ – 10⁴ s^-1), metals experience a dramatic increase in flow stress as a result of phonon drag. This increase in flow stress accompanies a decrease in ductility which is undesirable for high energy impact materials such as shaped charge liners and ballistic penetrators. An effective means of limiting the effect of phonon drag is to keep the speed of dislocation propagation below a critical maximum. This can be achieved through the introduction of obstacles such as grain boundaries. Nanocrystalline materials show great potential for high energy impact materials due to the high number of grain boundaries which keep the mechanical response stable with respect to strain rate, but maintaining a nanocrystalline microstructure is a challenge under dynamic loading scenarios. Recently, a nanocrystalline Cu 10 at.% Ta alloy has been shown to maintain a refined microstructure under high temperature and dynamic loading conditions. Here, we further investigate the alloy through high strain rate testing on a split Hopkinson pressure bar to determine the stability of the mechanical properties with respect to strain rate. The microstructure is analyzed before and after deformation using high resolution transmission electron microscopy along with precession diffraction. Results show that the stable nanocrystalline microstructure correlates to stable flow stress behavior at strain rates up to 10⁴ s^-1. Further, twinning is observed after deformation indicating that the deformation mechanism of the nanocrystalline Cu-Ta alloy is largely affected by twin formation and its interactions at high strain rates leading to lower sensitivity of the flow stress to the strain rate.

Understanding the behavior of materials through systematic evaluation of microstructure with deformation at the macro and nano scale is critical for designing materials for engineering applications under extreme conditions. In this present study, microstructural evolution of a binary nanocrystalline copper - 10 at. % tantalum alloy prepared through mechanical alloying and consolidated through the equal channel angular extrusion (ECAE) process was investigated using high strain rate experiments (10³/s to 10⁴/s) at high temperatures (25 to 600 °C). The primary TEM observations revealed the presence of Ta nanodispersions in the as-received condition which has been observed to contribute to dramatic strengthening and exceptional thermal stability when the alloy is subjected to extreme conditions. Also, deformation induced coherent and incoherent twin boundaries are observed and the twin density reduces with increasing temperature relative to the 25 °C strained case. Furthermore, dislocation activity is observed inside a few large parent phase grains. Such microstructural changes contribute to the observed flow stress at high strain rates coupled with high temperatures where both twinning and dislocation based mechanisms contribute towards the plastic deformation of such alloys.

Currently, reactor performance is largely constrained by the limitations of the

materials involved in these reactors. The fuel is particularly limiting due to fission gas

generation, changes in thermal conductivity, microstructure changes within the fuel, fuel

swelling, and fuelcladding chemical interaction (FCCI). The fuel cladding is also under extreme temperature and dose, in excess of 150 DPA for some samples. Highly irradiated fuel is radially inhomogeneous in composition, microstructure, and temperature. In this work, high burnup

mixed oxide fuel with local burnups of 3.4-23.7% FIMA were destructively examined as part of a

research project to understand the performance of oxide fuel at extreme burnups. Optical

metallography, transmission electron microscopy and electron back-scatter diffraction were

performed to further study the microstructure and chemical composition of the irradiated fuel and cladding

.

The optical micrographs were used to generate finite-element meshes in order to model the

effective thermal conductivity of the irradiated fuel as a function of burnup, radial position, and

temperature.

The fueltocladding gap closed significantly in samples with burnups below 7-9% FIMA.

Samples with burnups in excess of 7-9% FIMA had a reopening of the fueltocladding gap and

evidence of joint oxidegain formation. Additionally, high burnup structure was observed in the

two highest burnup samples (23.7 and 21.3% FIMA). The microstructural modeling of the

effective thermal conductivity found close (10-20%) agreement between the calculated effective

thermal conductivities and the semiempirical based analytical models, validating the finite-

element mesoscale approach to microstructural modeling of effective thermal conductivities in

irradiated fuel.

We present an elaborate phase field model for investigating the grain growth process in porous UO₂. The model takes into account the interactions between pores and grain boundaries, which strongly affect the grain growth kinetics. Moreover, the model takes into consideration the effect of anisotropy in surface and grain boundary energies and mobilities and the effect of temperature gradients on the kinetics of grain growth. Furthermore, using a formal asymptotic analysis, the phase field model was matched to its sharp-interface counterpart and all the model parameters were uniquely determined. Therefore, the model is able to obtain accurate growth rates that can be compared with experiments. The simulations of the model are carried out using MARMOT, the mesoscale simulator developed at Idaho National Laboratory. The model is used to quantify the effect of pore drag on the kinetics of grain growth in UO₂. The effects of pore fraction and configurations, anisotropy of thermodynamic and kinetic parameters, and temperature gradient on the grain growth process were investigated.

This research uses an atomistic approach to study microstructural morphology and evolution, to investigate how temperature and alloy concentration affect ordering. A semi-empirical Modified Embedded Atom Method (MEAM) is used to describe Uranium and Zirconium interactions. Interactions are simulated using molecular dynamics (MD) and Monte Carlo (MC) methods to investigate the properties and equilibrium configurations of the high temperature body-centered-cubic (bcc) uranium-zirconium (U-Zr) alloys, the fundamental metallic alloy fuel for the next generation of fast nuclear reactors. This setup is able to successfully replicate some of the base thermo-physical and microstructural characteristics seen during fabrication and irradiation of the U-Zr metallic fuels. These simulations aid the fundamental understanding of the microstructure evolution of U-Zr alloys, which has proven to be complex and has not been well understood. The MEAM potential shows the thermodynamic driving force to the lamellar structure for the melt-casted uranium-rich alloys and the finely acicular microstructure of the water quenched uranium-rich alloys seen experimentally. In addition, when the uranium-rich U-Zr alloy is equilibrated at a lower temperature, the lamellar/acicular microstructures begin to spheroidize as seen in experiments. In the intermediate region, the ordering seen is able to facilitate the structure to the partially ordered δ-UZr₂ phase seen experimentally. Lastly, the zirconium-rich region is able to successfully show the thermodynamic driving force to the acicular, Widmanstätten, and martensitic needles type microstructures seen experimentally.

During in-reactor irradiation, actinide fission produces large quantities of defects and fission products, which have a significant influence on the structural, thermal and mechanical properties of nuclear fuels and claddings. A better understanding of atomic transport (self-diffusion and diffusion of fission gases) in these materials is central to getting further insight into the irradiation driven micro-structural changes.

An efficient approach to unravel basic mechanisms is to couple separate effect experiments and modelling at the atomic scale. We will present two examples of investigations where electronic structure calculations coupled to experimental observations have yielded a better understanding of defect and fission gas behaviour in nuclear ceramics.

We will first show the application of the two component density functional theory (TCDFT) to the interpretation of positron annihilation spectroscopy (PAS) results. This has enabled to identify equilibrium and irradiation induced defects in silicon carbide (SiC) [1,2,3] and uranium dioxide (UO₂) [3,4].

We will then present the use of the results of electronic structure calculations in the DFT+U framework as input for X-ray absorption spectroscopy (XAS) spectra simulations to identify the most favourable incorporation site for fission gases (xenon and krypton) in ion-irradiated UO₂ [5,6].


[1] J. Wiktor, G. Jomard, M. Torrent, M. Bertolus, Phys. Rev. B 87, 235207 (2013)
[2] J. Wiktor, X. Kerbiriou, G. Jomard, S. Esnouf, M.-F. Barthe, M. Bertolus, Phys. Rev. B 89, 155203 (2014)
[3] J. Wiktor, Ph.D., Université Aix-Marseille (2015)
[4] J. Wiktor, G. Jomard, M. Torrent, M.-F. Barthe, M. Freyss, M. Bertolus, Phys. Rev. B 90, 184101 (2014)
[5] R. Bès, P.M. Martin, E. Vathonne, R. Delorme, C. Sabathier, M. Freyss, M. Bertolus, P. Glatzel, Appl. Phys. Lett. 106, 114102 (2015)
[6] P.M. Martin, E. Vathonne, G. Carlot, R. Delorme, C. Sabathier, M. Freyss, P. Garcia, M. Bertolus, P. Glatzel, O. Proux, J. Nucl. Mater. 466, 379 (2015)

Uranium-rare earths complex compounds are important materials in nuclear energy, they are considered as possible waste forms for actinide disposal. Many of those compounds have structures related to the ideal fluorite. Depending on composition and annealing conditions, long- or intermediate-range ordering may occur in these compounds. In this respect, some of these structures may be good candidates for enhancing chemical stability and for accommodating large quantities of defects (substitutional, vacancies, interstitials, …). Interestingly, high solubility of rare earth elements is desirable in nuclear fuels because they constitute a significant fraction of the isotopes produced by fission. In this work, we examine the structural flexibility of the (La,U)O_2±x system, with a particular focus on a compound with stoichiometry, La₆UO₁₂. Numerous oxides with this 6:1:12 stoichiometry exhibit highly-ordered structures that are derivatives of the fluorite (CaF₂) structure. They are typically referred to in the literature as delta (δ) phase compounds. Anion and cation ordering in La₆UO₁₂ might occur in a number of different ways. We have explored several possible ordered structures, which we derived using a symmetry group-subgroup theory approach. Different ordering patterns for the anions and cations lead to different electronic structures. To discuss these differences and their specific features in a quantitative way, we have assessed and applied AIM-type tools to the electron charge density obtained by DFT for each of our postulated structures, and obtained robust estimates for Bader’s charges and the characteristics of the critical points of the Laplacian of the charge density.

Predicting accurately fission gas release (FGR) from high burn up fuels during off-normal conditions, such as a loss of coolant accident (LOCA), remains a significant and important challenge. A clear progress would be to identify and evaluate the basic mechanisms promoting this FGR.

In the upper temperature range, 1100°C to 1200°C, previous experimental work has already been performed with specific dedicated annealing tests, measuring the FGR kinetics, combined with post-tests fuel microstructural examinations. For fuels up to 70GWd/t under simulated LOCA conditions, the main outcome is that gas from the release burst mainly originates from gas accumulated at grain boundaries during base irradiation.

In the lower temperature range, below 1000°C, no particular FGR mechanism has been clearly established. Recent results on irradiated fuel suggest that grain boundary rupture does not occur significantly. To further progress, this work aims at evaluating two complementary mechanisms which could be involved during the accident: defects annealing and mechanical stress release. The main goal of this paper is to highlight how these FGR mechanisms, in the 600-800°C temperature range, can be studied thanks to thin film samples. The methodology consists in creating different states of defects in simulated samples and to study their influence on gas migration.

We believe that an interesting way to gain further understanding of those basic mechanisms is to separate these effects and to perform irradiations on materials simulating the nuclear fuel having thin film geometry. Mesoporous or dense CeO₂ and UO₂ model materials were selected for these studies, optimizing the film thickness to separate the effects of damage and implantation of heavy ions and specifically separating the mechanical stress related to the creation of defects from those related to ion implantation. Differences between UO₂ thin films and CeO₂ ones will be discussed to assess the accuracy of CeO₂ films as a surrogate of nuclear fuel, focusing on the specific manufacturing characteristics. Sample characterization will be performed by glancing angle X-ray diffraction to analyze the microstructure and measure the strain existing in those films. Results will be discussed and compared to gas release models in irradiated fuel.

High resolution electron microscopy investigations of selected coated particles from the first advanced gas reactor experiment (AGR-1) at Idaho National Laboratory has provided important information on the fission product distribution and chemical composition within the SiC and inner pyrolytic carbon layers and the interface between them. Particles from AGR-1 were irradiated to an average burn-up of 15.3% fissions per initial metal atom, time-averaged, volume-averaged temperature of 1092°C, a time-averaged, peak temperature of 1166°C, and an average fast fluence of 3.22x10²⁵ n/cm². Of particular concern is the release of silver from the particles. Recent observations have suggested that grain-boundary transport is a primary method for Ag transport through the SiC. However, the complex phases that the fission products comprise and their combined role in governing mass transport through the TRISO layers require further understanding. This report describes detailed, high-resolution analytical electron microscopy studies of fission product movement through the TRISO layers with a particular emphasis on the microstructural aspects related to Ag transport.

Electronic effects are of significant importance in a wide variety of fields where high energy irradiation processes take place, including nuclear applications, the semiconductor industry and material characterization. We use MD simulations in combination with the inelastic thermal spike model to study the combined effects of the nuclear and the electronic energy loss on the damage production due to ion irradiation in ceramics. We investigate the role of pre-existing disorder in nanoscale ion track formation in SrTiO₃ and we find a significant synergy between the inelastic and elastic energy loss. These findings are in agreement with experimental results and our results reveal the ion track formation mechanism in this material. Additionally we investigate the role of the electronic effects during Kr and Xe ion irradiation of ZrSiO₄, finding that the electronic energy loss has additive impacts in the damage production, which can explain the amorphization efficiency of these ions in previously reported experimental results.
Our work highlights the importance of the electronic effects and the need to take them into account in order to approach irradiation effects in a more realistic way and predict the materials’ performance under irradiation.
This work was supported by the U.S. DOE, BES, MSED.

Rare Earth Oxide (REO) based materials are used in, or are contenders for numerous settings in materials science. For instance, they are studied for nuclear energy applications such as nuclear waste forms, strengthening elements in ODS steels and solid-state neutron detectors. As thin epitaxial layers, they represent, intrinsically or after ion doping, possible high-k gate dielectrics for metal-oxide-semiconductor devices; they can also be buffers for growth of alternative semiconductors, the choice of which depends on the phase of the REO underneath. For all these applications, an understanding of the behavior of these materials under ion irradiation is of upmost interest. A few studies have been conducted on some irradiated polycrystalline REOs (namely Gd₂O₃, Er₂O₃, Dy₂O₃, Y₂O₃), and the main result is that the initial cubic structure is not stable and transforms into a monoclinic structure. However, no study has been carried out so far on model, single-crystal-like materials and no complete phase-change kinetics has been reported.

In the present work [1], we studied high crystalline-quality Gd₂O₃ epitaxial layers deposited on silicon. These layers were irradiated with 4 MeV Au²⁺ ions in a broad fluence range (from 2.5x10¹³ to 5x10¹⁵ cm^-2). We used ion channeling, X-ray diffraction and Raman spectroscopy to monitor the irradiation-induced effects. Results show that a cubic-to-monoclinic phase change also takes place in these layers. More strikingly, the phase change is found to be depth dependent, whereas the energy deposition profile is constant. A defect migration towards the surface is invoked to explain this finding. Besides, the phase-change kinetics follows a two-step process. An XRD analysis shows that the monoclinic layer is nanocrystalline. Therefore, it is suggested that defect accumulation in the near-surface region occurs in the first step, generating highly disordered, energetically unstable nanometer-scale regions. The release of the corresponding stored energy takes place through a phase transition in the second step.

N. Mejai, A. Debelle, L. Thomé, G. Sattonnay, A. Boulle, D. Gosset, R. Dargis, A. Clark, Appl. Phys. Lett. 107 (2015) 131903.

Periodic mesoporous materials possess high surface to volume ratio and nano-scale sized pores, making them potential candidates for heterogeneous catalysis, ion exchange, gas sensing and other applications. In this study, we use in situ small angle x-ray scattering (SAXS) and molecular dynamics (MD) simulations to investigate the mechanical and hydrothermal stability properties of periodic mesoporous SBA-15 silica and SBA-15 type aluminosilica (Al-SBA-15) to extreme conditions. The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica possess amorphous frameworks and have similar pore size distribution (pore size ~9-10 nm). The in situ SAXS measurements were made at the B1 beamline, at the Cornell High Energy Synchrotron Source (CHESS). The mesoporous SBA-15 silica and Al-SBA-15 aluminosilica specimens were loaded in a diamond anvil cell (DAC) for pressure measurements, and, separately, with water in the DAC for hydrothermal measurements to high P-T conditions (to 280 °C and ~ 200 MPa). Analyses of the pressure-dependent SAXS data show that the mesoporous Al-SBA-15 aluminosilica is substantially more mechanically stable than the SBA-15 silica. Hydrothermal measurements show a small net expansion of the pores at elevated P-T conditions, due to dissolution of water into the pore walls. Under elevated P-T conditions, the Al-SBA-15 aluminosilica shows significantly greater hydrothermal stability than the SBA-15 silica. Our MD simulations show that the bulk modulus value of periodic mesoporous SBA-15 silica varies exponentially with percentage porosity. Molecular dynamics simulations are being made in order to better understand how the pore architecture and the chemical composition of the host structure govern the stability properties of the mesoporous materials.

The past 20 years developments on various applications, such as diots, optical power out, transparent electrods, izolation materials on glass, LCD secreen and smart windows, in oxide semiconductor technology have been developed [King, 2011]. Finding new physical properties cause an increase recent studies on oxide semiconductors. Because of having wide band gap, oxide semiconductors are excellent transparent (> 90%) in the range of UV region and IR regions. In the past the wide band gap of oxide semiconductors was considering as a disadvantage for conductivity, nowadays it is realized that the low formation entalpy of point defects in oxide semiconductors cause a conductivity at the oxide semiconductors. The point defects generated in the crystal lattice of the oxide semiconductor allow formation of shallow and deep energy levels. Formed new energy levels causes increased number of free electrons in the lattice and allows the new electronic transitions. Those newly acquired electronic configuration will help the development on the optoelectronic devices.
We focus on zinc gallate (Zn,Co)Ga₂O₄ oxide semiconductors, which have limited study on. (Zn,Co)Ga₂O₄ are direct band gap semiconductors with value 4.4 – 4.7 eV (at room temperature) [López, 2014; Oshima, 2014], transparent to the visible light and high refractive index materials [Oshima, 2014]. The physical properties make (Zn,Co)Ga₂O₄ semiconductors be suitable for applications in solar cells, gas sensors, chemical sensors, surface acoustic wave, pressure sensors, transparent conductive, anti-reflective coating and luminescent materials. The aim of this study is to show the point defects dependent gas (H₂ and O₂) sensing ability above the room temperature and opto electronic behavior of ZnGa₂O₄ thin films, fabricated via RF magnetron sputtering system. The sensing ability were perfomed with temperature dependent I-V and C-V curves. Furthermore, the post deposition annealing atmosphere were changed to manage the point defect amounts and types in thin films. The study include investigation of structural characterization (XRD, SEM, XPS, FTIR ..etc) of (Zn,Co)Ga₂O₄ semiconductors and point defects dependent physical property variation (optical, thermal transport and DC conductivity). (Zn,Co)Ga₂O₄ crystal structure usually has n-type structure. Oxygen vacancies (V_o), interstitial Zn (Zn_i) and Ga (Ga_i) atoms or oxygen sub-lattice in the Zn or Ga atoms (Zn_o or Ga_o) are the origin of n-type behavior [King, 2011; Li, 2014; Oshima, 2014].

[1] King, P. D. C., et.al., (2011), J. Phys.: Condens. Matter., 23, 334214.
[2] Oshima, et.al., (2014), Journal of Crystal Growth, 386, 190–193.
[3] López, et. al. (2014) , Materials Research Express 1, 025017

Fast and repeatable transitions between the crystalline and amorphous solid phases of so-called GST materials are the basis for storing large amount of information in rewritable devices in the electronics industry. In GST systems, the Ge matrix enables fast transitions when stimulated via heat discharges and thus glassy-to-rystalline phase transformations in pure Ge have been the subject of many experimental works. In particular, observations made with ultrafast time-resolved electron microscopy reveal a complex micorstructure characterized by several distinct crystallization regimes. In this work, we present a mesoscale model based on a phase field microstructural simulator fitted entirely to atomistic calculations, including bulk and interfacial free energies, as well as interface mobilities, to simulate Ge recrystallization under laser-spot heating. In addition, we perform direct molecular dynamics simultions of the growth of nanoscale crystalline Ge grains embedded in a glassy Ge matrix. These simulations provide valuable physical insights that we then use to inform the mesoscale simulator to enhnce its fidelity. This atomistically-informed mesoscale approach is then employed to simulate Ge crystallization under laser-spot heating conditions. We discuss the impact on the results of the nucleation rate and compare the simulated timescales to experimental measurements.

Silicon carbide is a prototypical nuclear ceramic material. It is also attractive for high-temperature electronics applications. Understanding radiation damage processes in SiC is crucial for exploiting unique properties of this material in both nuclear and electronics applications. Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by the mobility and interaction of ballistically-generated point defects. The mechanism of defect interaction processes, however, remains poorly understood.

Here, we use a novel pulsed ion beam method to study radiation defect dynamics in SiC (3C and 4H) bombarded with 500 keV Ar ions in the temperature range of 25 - 250 °C. Results reveal that the defect recombination efficiency monotonically increases with temperature. In contrast, the defect lifetime (and, hence, the defect relaxation rate) exhibits a non-monotonic temperature dependence with a maximum of ~5 ms at ~100 °C, indicating a change in the dominant defect interaction mechanism at this temperature. For the 150-250 °C range, the dominant defect relaxation process has an activation energy of 0.25 eV. In contrast to the strong temperature dependence of the defect lifetime, the defect diffusion length exhibits a weak temperature dependence, reflecting a weak temperature dependence of the concentration of defect traps. These results have important implications for understanding and predicting radiation damage in SiC and for the development of radiation-resistant materials via interface-mediated defect reactions.

This work was performed under the auspices of the US DOE by LLNL under contract DE-AC52-07NA27344.

Interaction of energetic ions with crystalline solids leads to lattice disorder which degrades material quality. Formation of stable ion induced structural defects often proceeds via so called dynamic annealing, involving the diffusion and interaction of mobile point defects. While critically important for understanding ion induced defect generation, this dynamic regime of defect evolution after the thermalization of ion collision cascades remains poorly understood for most materials.

Gallium arsenide is a direct bandgap semiconductor with high resistance to radiation damage, making it ideal for applications in extreme environments. However, little is known about the mechanisms of dynamic annealing in gallium arsenide. Here, a pulsed ion beam method is used to study radiation defect dynamics in gallium arsenide. Specifically, we use a pulsed 500 keV Ar ion beam to determine the characteristic time constant (τ) and diffusion length (L_d) of dynamic annealing in gallium arsenide over a temperature range from -20 to 80 °C, enabling the determination of the activation energy of the dominant dynamic annealing process. Our results elucidate the dynamic annealing characteristics of gallium arsenide and demonstrate the utility of the pulsed ion beam method in measuring dynamic annealing characteristics in a range of technologically important materials.

This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Spontaneous formation of complex geometric patterns is an interesting phenomenon that provides fundamental insights into underlying roles of symmetry breaking, anisotropy and non-linear interactions. Here we present dynamic, highly ordered topographic patterns on the surface of diamond that span multiple length scales and have a symmetry controlled by the chemical species of a precursor gas used in electron beam induced etching (EBIE).

We provide an anisotropic etch rate kinetics model that fully explains the observed patterns, and reveals an electron energy transfer pathway that has been over-looked by existing EBIE theory. We therefore propose a fundamental modification, whereby the critical role of energetic electrons is to transfer energy to surface atoms of the solid rather than to surface-adsorbed precursor molecules.

EBIE is a high resolution, direct-write nanofabrication technique in which a precursor gas and an electron beam are used to realize etching. A key advantage of EBIE is the ability to etch materials such as diamond that are resistant to conventional chemical etch processes, without introducing damage to the substrate as observed in ion sputtering techniques. As a result, EBIE has recently been used to fabricate components for photonic and electronic applications. Our findings can be harnessed to engineer specific surface patterns under various electron beam irradiation environments for controlled wetting, optical structuring and other emerging applications that require nano and micro-scale surface texturing.

A portion of this work was funded by FEI Company and the Australian Research Council (Project Number DP140102721). A portion of this work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. I.A. is the recipient of an Australian Research Council Discovery Early Career Research Award (Project Number DE130100592).

The development of the carbon-based nanostructures such as graphene and carbon nanotubes (CNTs) for various advanced technological applications including nanoelectronics, and field-emission display devices are based on the methods to modify these carbon nanosystems with definitiveness. Various methods such as grafting, doping, functionalization, and mechanical strain have been used to modify these unique structures for electron field applications. Apart from these methods, the modification by ions is one such method that can be employed for precise alteration and tailoring of CNTs and graphene, resulting in some beneficial effects.
The structure modification under stress behaviour and ion irradiation conditions have been depicted. Ion irradiation has been explored as a tool to tailor the structure and electron mission properties on CNTs and multilayer graphene (MLGs) as a function of wall thickness.
In the present work, possibility of achieving enhanced electron field-emission properties of stress-induced carbon nanotubes (CNTs) and multilayer graphene (MLGs) by ion modification is investigated. CNTs and MLGs are synthesized by microwave plasma enhanced chemical vapour deposition. Micro-Raman spectroscopy is used as a potent technique for in-depth investigations of stress induced CNTs and MLGs. It is found that iron used as a catalyst, compresses at particular fluence and induces stresses in CNTs and MLGs to modify these structures, supported by high-resolution transmission electron microscopy (HRTEM) studies. The stresses are explained by the buckling wavelength.
The structural modification in carbon nanostructures is well related to the electron field emission properties Furthermore, the stresses induced in exotic nanostructures are studied for investigating wetting properties, which are well-corroborated with electron emission characteristics. It is found out that less-wetted CNTs and MLGs display enhanced emission properties with turn-on voltages of 1.4 and 2.6 V/μm, respectively, in comparison to hydrophilic CNTs and MLGs with turn-on voltages of 2.8 and 3.4 V/μm, respectively.

References:
(1) Wei, J.; Jia, Y.; Shu, Q.; Gu, Z.; Wang, K.; Zhuang, D.; Zhang, G.;Wang, Z.; Luo, J.; Cao, A.; Wu, D. Double-Walled Carbon Nanotube Solar Cells. Nano Lett. 2007, 7, 2317−2321.
(2) Sharma, H.; Agarwal, D. C.; Sharma, M.; Shukla, A. K.; Avasthi, D. K.; Vankar, V. D. Tailoring of Structural and Electron Emission Properties of CNT Walls and Graphene Layers Using High-Energy Irradiation. J. Phys. D: Appl. Phys. 2013, 46, 315301−315308.
(3) Misra, A.; Tyagi, P. K.; Rai, P.; Mahopatra, D. R.; Ghatak, J.;Satyam, P. V.; Avasthi, D. K.; Misra, D. S. Axial Buckling and Compressive Behavior of Nickel-Encapsulated Multiwalled Carbon Nanotubes. Phys. Rev. B 2007, 76, 014108−014113.

Fiber optic temperature sensors are used in a variety of harsh environment applications. We have explored use of such temperature sensors in commercial gas turbines to measure the temperature various regions of interest within the turbine system. More specifically, fiber optic temperature rakes were designed and installed on a commercial gas turbine under full load conditions. This work will focus on failure mechanisms observed at multiple length scales that impact the performance of high temperature optical fiber sensors. It was found that Au-coated silica fibers, which are a standard in the industry, undergo various failure modes when subjected to combinations of high temperature and high vibration. More specifically, the Au coating becomes soft/ductile as the temperature is increased. We also observed that the Au coating is not well bonded to the silica fiber, as expected since there are no adhesion layers present. These effects lead to significant damage of the fiber optic under high vibrations. We also found that vibrations from the gas turbine couple into fundamental modes of the fiber optic probe assembly, which was analyzed by detailed dynamic mechanical analysis. This led to the fiber impacting the internal wall of the probe assembly, which caused further damage and failure of the fiber and the Au coating. The silica fibers returned from the field also exhibited significant twisting throughout most of their length. This suggests the fibers reached temperatures above their strain point (about 1000 C for pure silica glass) which is explained by either a) the strain point has been significantly reduced by the presence of the Ge dopant, or b) the temperature was higher than expected in the gas turbine exhaust region. It was also hypothesized that complex anelastic effects may play a role under the high temperature, high vibration environment experienced by the probes. Detailed structural analysis of the fiber optic temperature sensors by scanning electron microscopy, TOF-SIMS, and X-ray microscopy will be presented to corroborate the above simulations and proposed damage mechanisms. Finally, we note that the fiber Bragg gratings (FBG) present within the temperature probes provided promising temperature data, and were in fact not damaged/erased by the high temperature environment.

Understanding the kinetics of shock-compressed SiO₂ is of great importance for mitigating optical damage for high-intensity lasers and for understanding meteoroid impacts. Experimental work has placed some thermodynamic bounds on the formation of high-pressure phases of this material, but the formation kinetics and underlying microscopic mechanisms are yet to be elucidated. Here, by employing multiscale molecular dynamics studies of shock-compressed fused silica and quartz, we find that silica transforms into a poor glass former that subsequently exhibits ultrafast crystallization within a few nanoseconds. We also find that, as a result of the formation of such an intermediate disordered phase, the transition between silica polymorphs obeys a homogeneous reconstructive nucleation and grain growth model. Moreover, we construct a quantitative model of nucleation and grain growth, and compare its predictions with stishovite grain sizes observed in laser-induced damage and meteoroid impact events.

There is a critical need to strategically deploy sensors on industrial equipment such as turbines, compressors, and boilers operating in harsh environments above 1000°C in order to improve their safety, efficiency, and reliability. However, the development of small, robust, wireless sensor devices to operate in such environments is hampered by the lack of morphologically and chemically stable thin films that can be used as electrodes and other electronic components in such environments. At micron length scales relevant for electronic devices, commonly stable thin film materials such as Pt are thermodynamically driven to agglomerate and dewet the substrate at temperatures above 700°C, thereby eliminating electrically conductive pathways. To improve film stability, nanolaminates comprised of Pt and Zr layers were grown on sapphire substrates, and these films have been tested at temperatures between 800–1300°C in air. Upon high temperature oxidation, ZrO₂ nanograins are formed and they serve to hinder the agglomeration of Pt grains on a sapphire substrate. As an alternative material, zirconium diboride (ZrB₂) is an ultra-high temperature ceramic with metallic-like electrical conductivity and is well known to be a stable bulk material in reducing environments. ZrB₂ thin films less than 300 nm thick have been grown on sapphire using e-beam evaporation and they were found to be morphologically and electrically stable at least to 1000°C in vacuum, and they exhibit very little grain growth over time. EXAFS analysis of the films reveals nanoscale crystallographic structure very similar to that of bulk ZrB₂, despite limited long-range order. When annealed in air, however, ZrB₂ quickly oxidizes to ZrO₂ and the film becomes insulating. Thus, a capping layer strategy has been adopted, wherein the ZrB₂ films are protected from oxidation by either an extremely thin (<50 nm) amorphous Al₂O₃ layer grown by ALD or a crystalline h-BN layer grown by sputter deposition. The low rate of oxygen diffusion in Al₂O₃ retards film oxidation, and theoretical and experimental work reported in the literature has demonstrated that h-BN is extremely oxidation resistant at high temperatures. Pt-Zr and ZrB₂ thin films on sapphire substrates employing these Al₂O₃ and h-BN capping layers have been tested at temperatures up to 1300°C under varying levels of oxygen exposure in order to characterize the early stages of oxidation. The results indicate that the capping layer strategy is crucial in order to utilize ZrB₂ films in oxidizing environments.