Symposium F.SF01—Materials for Extreme Conditions (MEC)

Wide bandgap (WBG) semiconductor-based electronics are becoming the center of interest due to their ability to operate at high temperatures and high voltages. Gallium Nitride (GaN), as one of the WBG semiconductors, is a strong candidate that can meet expectations in high-temperature electronic applications such as military systems, automotive and aerospace control units, gas and oil exploration drilling systems. The superior physical properties of GaN nanowires such as high direct bandgap, high breakdown voltage, and high thermal conductivity, as well as high surface area to volume ratio, make it even more significant material for harsh environments. In this work, we investigate the electrical transport properties of a back-gated single GaN nanowire field-effect transistor (GaNNW-FET) at elevated temperatures.
In order to analyze transport properties (I_DS-V_DS and I_DS-V_GS), electrical measurements were performed at temperatures ranging from room temperature to as high as 350°C. The device performs very well until 250°C, whereas it shows some reduction in current values beyond 300°C. In fact, the drain current increases by 2.1, 13.6 and 19.7 times at the temperatures of 100°C and 200°C, 250°C, respectively, with respect to room temperature current at the same bias voltage of 1 V. The enhancement of current is likely due to the reduction of contact resistance between the nanowire and electrodes as well as an increase in thermally excited carrier concentration. On the other hand, degradation of current is likely due to the increase in lattice scattering, lowering the carrier mobility, of the GaN nanowire. Moreover, the influence of high temperature on important transport properties such as transconductance, carrier concentration and carrier mobility will be presented in details. The device offers the following unique advantages: (i) stable operation at high temperatures (at 350°C), (ii) exhibiting an on/off current ratio of 5.5 x 10² and a high transconductance value of 3.09 µS at 350°C indicating a good gating effect even at high temperatures, and iii) offering solutions not only for high-power but also for low-power circuit and photonic applications at high temperature ambients (> 300^oC). In summary, GaNNW-FET proves to be an excellent device capable of operating at high temperatures enabling the development of high-performance nanoelectronic/photonic devices especially for harsh conditions.

Electrical short circuits caused by the growing tin whiskers can be mitigated by applying a polymeric conformal coating over the whisker-prone tin (Sn) surface which is now common in Pb-free electronics. These conformal coatings should be mechanically adaptable to resist the metallic whisker nucleation and growth while providing enough toughness to avoid cracking ahead of the growing tin whiskers. Importantly, the delamination of conformal coating would provide a space for tin whisker activities that will ultimately result in the vertical growth of tin whisker and its penetration through the coating; hence, the adhesion strength of the conformal coating to tin surface plays an important role. The selection of a conformal coating should consider all these mechanical characteristics because these coatings will be exposed to extreme environmental conditions over a long period. In this study, silicone rubbers were added to the polyurethane-based conformal coatings to form phase-separated nanosized domains for enhanced toughness, which were tested under high temperature and high humidity (HTHH) exposure (85^°C/85% relative humidity). Furthermore, in an effort to increase the adhesion of these coatings to the tin surface, silane coupling agents were also applied to the tin surface. The effect of long-term exposure with temperature and moisture (up to 5000 hours) on the chemical degradation of the polymeric coating and its adhesion to the tin surface was examined using Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA). Mechanical degradation of the nanocomposite coatings under HTHH conditions was evaluated at various length scales by universal testing machine and indentation techniques. In particular, interplays between the degradation of aged conformal coating and the growing tin whiskers under highly compressive stress in the tin layer are highlighted to understand the effectiveness of the conformal coating for tin whisker mitigation. For this, cross-sectional SEM images were obtained from the microtome-sliced coated tin coupons to monitor the growth of tin whiskers under various time stages under HTHH. This study, therefore, provides a potential source of developing an effective polymer coating strategy under harsh environments with optimal mechanical and adhesion properties that can well prevent or entrap the growth of tin whiskers and nodules beneath its coating.

(Honeywell Federal Manufacturing & Technologies, LLC operates the Kansas City National Security Campus for the United States Department of Energy / National Nuclear Security Administration under Contract Number DE-NA0002839)

Thermoelectric materials have been widely employed to convert heat into electricity and thermocouples. The state-of-art materials are usually working below 1000 celsius. Here, a boride with a thermally stable structure up to 3000°C is synthesized. The Seebeck coefficient (thermopower) of the boride is characterized over 2000 Celsius. The phonon/electron transports are investigated under extreme conditions. The investigation would explore applications of thermoelectric materials at ultra-high temperatures.

The goal of this work was to investigate the influence of grain boundary (GB) precipitates on the susceptibility of 7050 Aluminium alloy to Adiabatic shear band (ASB) during high strain rates using a Direct Impact Hopkinson Pressure Bar (DIHPB). It is demonstrated continuous distributed GB precipitates increase the propensity of ASB formation whereas discontinuously distributed GB precipitates coupled with large precipitation free zones decreases susceptibility to ASB formation of 7050 Aluminium. It is apparent that precipitates within the initial microstructure of 7050 Aluminium before deformation undergo fragmentation and/or dissolution during the occurrence of ASBs at high strain rate deformation.

At ambient and lower pressures elemental carbon sublimates to vapor near 4000K, even though quenched carbon droplets have been reported in laboratory laser ablation studies [1,2]. However, a subset of meteoritic carbon particles from red giant atmospheres show an “unlayered-graphene solid” core/graphite-rim structure [3,4], likely following condensation of super-cooled carbon droplets from the vapor phase, which nucleated ~40-Å graphene sheets (about 10^19/cc) on randomly-oriented 5-member rings before solidification and subsequent coating with a graphite rim. Similar core-rim particles (with ~15-Å sheets) can be formed by slow cooling of carbon vapor in the lab [5,6].
The difficulty of experimentation under extreme conditions in a lab remains as a challenge to study the properties of liquid carbon. Therefore, we investigated nucleation and growth of carbon rings and graphene sheets using density functional theory (DFT). Our computations target growth of carbon rings & graphene sheets at the experimental 1.8g/cc density estimate, by relaxing randomized positions of liquid-like 13/20/30/40/60/100-carbon atom clusters in a cubic supercell. Inter-atom distances characteristic of covalent vs. metallic interactions (with a gap in 1.7Å -2.0Å range) allow us to identify covalent “bonds” with small separation. Local energy minima at T = 0K show a mix of sp² and sp coordination numbers, plus few sp³ as in the literature which reports that liquid carbon favors sp coordination at lower densities, sp³ coordination at higher densities, and a mix of sp and sp² coordination at intermediate densities [7-11].
The formation of rings continues to be the same trend in all different sizes of atom clusters, where the ring sizes vary from triangle to heptagon but pentagons are more abundant than hexagons as possible nucleation seeds, also consistent with previous reports [10,11]. The properties of unlayered graphene sheets in a frozen liquid matrix, e.g. as a diffusion barrier, should be interesting to explore further both experimentally and computationally. Further work remains to see if such ab initio studies can confirm that abundant pentagonal rings at this density, during slow cooling of the liquid, can nucleate the growth of faceted pentacones as suggested by electron phase-contrast imaging of the presolar cores [12].

References
[1] D. Kasuya et al., (2002) J. Phys. Chem B 106, 4947.
[2] W.A. De Heer et al., (2005) Science 307:5711, pp. 907–910.
[3] P. Fraundorf and M. Wackenhut (2002) Ap. J. Lett. 578(2): L153-156
[4] T. Kevin Croat et al., (2014) Elements 10, 441-446.
[5] P. Fraundorf et al., (2019) HAL-02238804.
[6] P. Fraundorf et al., (2020). Microscopy and Microanalysis, 1-4. doi:10.1017/S1431927620022953
[7] Christopher B. Cannella and Nir Goldman (2015), J.Phys.Chem 119, 29-31.
[8] J. R. Morris et al., (1995) Phys. Rev. B 52.
[9] Christine J. Wu et al., (2002), Phys.Rev.Lett 89,135701
[10] N. A. Marks et al., (2002) Phys. Rev. B 65, 075411.
[11] Volker L. Deringer and Gábor Csányi (2017) Phys. Rev. B 95, 094203.
[12] Eric Mandell, "Electron Beam Characterization of Carbon Nanostructures" (2007) Ph.D. Dissertation at UM-St Louis/Rolla.

Pressure-induced phase transformations from body-centered cubic to hexagonal close-packed iron occur at pressuresabove 13 GPa. The contribution of allyoing elements may change the transition pressure. Several studies have documented the role of different alloying elements on diffusionless phase transitions. However, iron-carbon under high pressure conditions has not been analyzed in detail, although carbon forms the basis of modern steels. This study aims to determine the influence of interstitial carbon on the transformation mechanism. We performed molecular dynamics simulations using the Embedded Atom Method (EAM) to explore the potential relationship between carbon and the phase transition in iron. Our results show that octahedral sites are energetically favorable for interstitial carbon atoms. We findthat carbon atoms tend to be trapped in tetrahedral sites during the transformation, which results in an increasing threshold pressure.

Shock compression is widely used to investigate the mechanical responses of iron under dynamic loading. With shock waves, it is possible to investigate new, previously undiscovered regimes of material dynamics for the discovery of materials inaccessible at ambient conditions. It has been long known that -iron transforms to -iron under high pressure. Recently, molecular dynamics simulations have shown that plasticity occurs just before the parent phase transforms into -iron. To provide insights into the interplay of elastic and plastic activities during shocks in iron, we performed atomistic simulations of shock compression of nanocrystalline iron with a mean grain size of 20 nm comprising a total number of 267.5 million atoms. We observed elastic and plastic deformations before the phase transformation takes place. The plastic state is metastable and highly depends on the deformation rate. After a relaxation process of a few picoseconds, the structure transforms into a quasi-3D compressed state in which the new phase is stable. We found that with increasing ramp time of the piston, plastic relaxation is stronger.
Analysis of the X-ray diffraction patterns calculated from the atomistic structure using the Debye equation revealed pronounced anisotropy of the line broadening in hcp Fe that is due to stacking faults and dislocations.

Due to the size- and shape-dependent properties, nanoparticles have been successfully used as functional building blocks to fabricate 1-3D ordered assemblies for the development of artificial metamaterials. At ambient pressure, entropy driven self-assembly of monosized or binary nanoparticles generally results in polycrystalline 2- or 3D close-packed arrangements, and extensive efforts have been made to develop structural perfection of nanoparticle arrays or ‘single crystal-like’ domain structures with precise long range order for their definite advantages for electron transport. To date, fabrications of ordered nanoparticle assemblies have been relied on specific interparticle chemical or physical interactions such as van der Waals interactions, dipole-dipole interaction, chemical reactions, etc. Recently we have discovered a pressure-induced assembly method to engineer nanoparticle assembly at mesoscale and to fabricate new nanoparticle architectures without relying on specific nanoparticle interactions. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to reversibly shrink, allowing fine-tuning of interparticle separation distance. Under a uniaxial pressure field, nanoparticles are forced to contact and coalesce, forming hierarchical nanostructures. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures including nanorod, nanowire, and nanoporous network can be fabricated through the pressure-induced self-assembly method. Guided by computational simulations, we were able to rationalize the pressure-induced self-assembly of nanoparticle arrays for predictable nanostructures. Moreover, we discovered for the first time a transition from an ordered polycrystalline nanoparticle mesophase to quasi-single crystalline nanoparticle lattices induced by PDA process. Exerting pressure-dependent control over the structure of nanoparticle arrays provides a unique and robust system to understand collective chemical and physical characteristics and to develop novel electronic and photonic behavior for energy transduction related applications.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Developing elastic materials for technological applications in extreme conditions has been a challenging task. Normal metals have an elastic strain limit rarely exceeding 1%. A large pseudo-elastic strain up to 10% can be achieved in the so-called shape-memory alloys (SMAs). However, the intrinsic hysteresis and temperature sensitivity of the first-order phase transformation significantly hinder their usage in many critical areas. We recently discovered a new type elasticity: supercritical elasticity (SCE), in NiCoFeGa single crystals, which exhibit a large elasticity up to 15.2% strain, with non-hysteretic mechanical responses, a small temperature dependence and high-energy-storage capability and cyclic stability over a wide temperature and composition range [Chen et al., Nature Materials, (2020). https://doi.org/10.1038/s41563-020-0645-4]. Such a giant SCE is conceived to work even down to absolute zero Kelvin. In situ synchrotron X-ray diffraction measurements show that the SCE is correlated with a stress-induced continuous variation of lattice parameter accompanied by structural fluctuation. Neutron diffraction and electron microscopy observations reveal an unprecedented microstructure consisting of atomic-level entanglement of ordered and disordered crystal structures, which can be manipulated to tune the SCE. The discovery of the giant SCE and associated the entangled structure paves the way for exploiting elastic strain engineering and development of related functional materials for a wide variety of high-performance engineering applications, ranging from deep-space and deep-sea exploration to intelligent robotics.

Thermite reactions between a metallic fuel and a solid oxide can be thermally or mechanically activated. The ignition temperature in a thermally activated thermite reaction generally increases with heating rate but is ultimately limited by the onset of melting of the lower melting point constituent. In mechanically activated thermite reactions, there is often a structure-sensitive threshold pressure required to drive the reaction. Historically, thermally activated thermite reactions have been explained using diffusion-based reaction mechanisms while mechanically activated thermite reactions have been described using deformation-based mechanical mixing models. Here we probe mechanically activated thermite reactions at a single well-defined heterophase interface using a combination of hydrocode simulations and laser-induced particle impact testing, where a CuO powder particle strikes an aluminum target at supersonic velocities. No reactions are observed through in situ imaging; however, post-mortem characterization of embedded CuO particles reveals a thin nanocrystalline Al₂O₃ reaction layer, with a thickness roughly equal to that of a molten lubricating layer predicted to form during penetration. These results suggest that local melting at hot spots drives the ultrafast reactions observed in mechanically activated thermite reactions and that thermally and mechanically activated thermite reactions may in fact result from the same underlying mechanisms.

Surface stability of deposited sensing material is becoming a crucial issue in the field of opto-chemical sensor technology due to cracks/stresses. Porous silica-titania (ST) nanocomposite coating material is synthesized and doped with CdTe as transducers for sensing cyclic compounds. Ultra-thin coatings are obtained, ∼11 nm for ST nanocomposite, and 5 nm for encapsulated and heat-treated samples. BET analysis shows large surface area ∼441 m2/g ± 3 m2/g and pore diameter 58 Å of ST nanocomposite. It decreases down to ∼410 m2/g ± 2
m2/g, and pore diameter to 24 Å, after encapsulation. However, after heat treatment, it again increases up to 419 m2/g ± 2 m2/g and pore diameter to 38 Å. The sensitivity of 71.37 counts per unit change in the carbon chain with correlation determination coefficient R2 ∼0.89 at 468 nm is observed. The prepared sensor device shows excellent stability, signal-to-noise ratio, and good reversibility towards different hydrocarbons and non-hydrocarbons (inorganic materials).

Metal halide perovskites are promising for advanced photovoltaic and optoelectronic applications. Solar cells and light-emitting diodes based on metal halides have achieved an impressive power conversion efficiency of 25.2% and a high external quantum efficiency of over 20%, respectively. Importantly, exceptional structural turnability enables enhanced and/or emergent properties. By using appropriate organic and inorganic components, many perovskite variants with 2D, 1D, and 0D structures on the molecular level have been obtained. These electronically low-dimensional metal halides have exhibited rich emergent properties and significantly expanded the scope of halide perovskites, while critical challenges remain. Future design of high-performance metal halides calls for a deeper understanding of the structure-property relationships. In this talk, we will present our recent efforts in using external pressure to tune the structures and physical properties of low-dimensional metal halides. Using state-of-the-art high-pressure techniques coupled with in situ synchrotron-based and in-laboratory property measurements, we characterized the changes in their structural, electronic, and optical properties. Pressure-enhanced properties such as higher photoluminescence quantum yields, as well as pressure-induced novel phenomena including emergent photocurrent, were observed. Our findings demonstrate that high pressure is able to realize enhanced and/or emergent properties of various metal halides, and further our understanding of the underlying mechanisms.

Cyclotetramethylene-Tetranitramine (β-HMX) is an energetic molecular crystal widely used as a constituent for plastic bonded explosives (PBXs). At high pressures encountered under shock conditions, dislocation motion is inhibited and plastic deformation takes place primarily by shear localization. Atomistic simulations of shear localization are performed in β-HMX under isothermal-isobaric conditions. The post localization flow stress for different pressures (0 to 15 GPa) and strain rates (10⁷ to 10¹⁰ s^-1) and at different temperatures of the shear band is investigated. The flow stress decreases linearly with temperature and increases quadratically with the pressure and exponentially with the strain rates. Shear thinning is observed, i.e. the viscosity decreases with increasing the strain rate. It is concluded that the flow stress increases exponentially with the density of the amorphous shear band and hence it can be described as a function of this single variable. The pressure, temperature and strain rates control the flow stress through the density.
The critical stress for shear localization (CSSL) was investigated for different strain rates at different pressures. The critical stress depends significantly on pressure and weakly on strain rates.

The results of a comparative study of neat and doped silicon dioxide polymorphs (Fe, Ni or Cu) obtained by density functional theory (DFT) at 0 K and at ambient pressure, 10 GPa, and 20 GPa are presented. The metal atoms are introduced at substitutional and octahedral interstitial sites of the crystal, and the relaxed structures are analyzed and ranked according to the enthalpy. Changes in the enthalpy of the neat and doped crystals are estimated at selected temperatures using quantum mechanics molecular dynamics (DFT-MD). We report the modeled enthalpy of the neat and doped crystals, as well as the dependency of the unit-cell parameters, band gaps, and Raman-active modes on pressure, and compare some of these results to those obtained experimentally using a diamond anvil cell. In addition, we present our search results of the high-pressure phases of silicon dioxide using DFT - based evolutionary simulations and report on the stability of the silica phases, as evaluated by convex-hull construction.
Funding Acknowledgement: This work was supported in part by a grant of computer time from the DOD High-Performance Computing Modernization Program at the ARL, Navy, AFRL, and ERDC DoD Supercomputing Resource Centers.

Xenon is quite a reactive noble gas, and xenon difluoride (XeF₂) being stable at ambient conditions is used in organic synthesis as fluorinating agent. Formation of XeF₂ is possible because xenon is bonded to highly electronegative fluorine atoms that take electrons from xenon filled outermost electron shell, forming cation Xe²⁺.
High-pressure behavior of XeF₂ (sp. gr. I4/mmm) is still under debate. Early powder X-ray diffraction and Raman experiments propose that XeF₂ should transform from a molecular compound to extended graphite-like layer structure at 50 GPa, and then above 70 GPa to a metallic fluorite structure. Contrary, the later experimental studies suggest just slight decrease in XeF₂ lattice symmetry, first to Immm and then to Pnma structure at 28 GPa and 59 GPa, respectively. Theoretical calculations predict that XeF₂ becomes unstable above 81 GPa, while high pressures should stabilize other xenon–fluorine compounds, such as XeF₄, XeF₆ and Xe₂F.
In order to resolve this issues, we investigated thermal stability of XeF₂ at pressures up to 54 GPa by means of single crystal X-ray diffraction in laser-heated diamond anvil cells. Single-crystal XRD opens the way to determine not only crystal structure of the material at truly extreme conditions, but also allows to refine its chemical composition. We report here the crystal structures of novel fluorides xenon, synthesized at high-pressure and high temperature conditions. In particular, the crystal structures of XeF₄ and Xe₃F₂, that both formed under heating-induced disproportionation of XeF₂ at 54 GPa. Similarly to ambient pressure structure, XeF₄ is a molecular compound composed of XeF₄ units with square planar geometry. Xe₃F₂ is not known at ambient conditions, and its structure is formed by isolated Xe atoms and linear XeF₂ molecules.

Recently, heterostructures are found to produce unprecedented combination of strength and ductility that is considered impossible from our textbook knowledge and materials history. Heterostructured (HS) materials consist of domains with dramatic strength differences. The superior mechanical properties have been attributed to back-stress strengthening and back-stress work hardening, which are related to strain gradient near the heterogeneous domain interfaces. Here I’ll discuss in-situ observations of strain gradient evolution near the domain interface and the dislocation mechanisms for the formation of strain gradient. It is found that contrary to our conventional understanding, negative strain gradient was developed near the interface. It is also observed under in-situ TEM that the Frank-Read dislocation sources are dynamically created and destroyed although postmortem examinations do not observe many Frank Read sources. These observations provide a new piece of scientific foundation for heterostructured materials, which is becoming a hot research field.

Cold spray, initially a coating technique, is being touted as a ‘near-net shape’ manufacturing technology that minimizes material waste by virtue of the high rate of deposition. During the cold spray process, metallic bulk components can be produced by spraying metal powders at high velocity, generating bonding through severe plastic deformation at temperatures well below the melting point of the powders. To fully understand the cold spray processing of metal powders, we systematically compare and study the microstructure evolution in Cu, Ni, Al, Ti, Ti-6Al-4V samples prepared by cold spray using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and nanoindentation. We show complex microstructure in these powder particles after cold spraying: nanocrystalline, nanotwins, annealing twins, gradient grains, deformation bands, dynamic/static recovery and recrystallization. The effects of gas temperature and powder velocity on the microstructure and mechanical properties in the cold sprayed samples are also discussed. Of particular interest are grain refinement, recrystallization and particle/particle bonding mechanisms of the cold sprayed powders.

Atomic features, such as surface roughness and dislocations, affect the mechanical strength and plastic deformation of materials under high shear rates. The precise role of how these atomic features influence the wear and tear of materials under nonequilibrium settings is not well understood. We use Objective Molecular Dynamics (OMD) to investigate the links between atomic features and high shear rates. This method has two advantages over traditional MD simulations: First, it simplifies solutions by exploiting the group invariance of the equations of molecular dynamics. For example, in OMD only a modest number of atoms are simulated (typically, 50-1000) and the motions of the other, typically infinite, set of atoms (the non-simulated atoms) are given by the group action on the simulated atoms. Second, it provides a framework for modeling continuum loads at an atomic scale. For example, a Lagrangian velocity gradient tensor is introduced to describe affine motion of simulated atoms. In today's talk, we demonstrate these advantages of OMD through two examples: 1) frictional behavior of sliding surfaces, and 2) dislocation movement under shear stress. Both of these computations were done under high shear rates, and demonstrate how atomic features, such as rough surfaces, edge dislocations, and vacancies, affect mechanical deformation. Broadly, we present a theoretical framework to explore the interplay between extreme mechanical loads at the continuum scale and mechanical deformation at the atomic scale.

Rhenium nitride is a promising candidate as superhard conductor. Different rhenium-to-nitrogen ratios and several phases result in challenging synthetic strategies for phase pure rhenium nitride. Novel heteroleptic rhenium(I) compounds, [fac-Re(I)(CO)₃(L)] (e.g., L= tfb-dmpda, (N,N-(4,4,4-trifluorobut-1-en-3-on)-dimethyl propylene diamine)), containing preformed Re−N bonds act as efficient precursors for selective growth of polycrystalline rhenium nitride (ReN) films by vapor phase deposition. This is the first known access to synthesize phase pure rhenium nitride by single source precursor approach without the need of additional gases. Interdependence of materials strength and thin film orientation is strongly influenced by external magnetic fields. Deposition of ReN films in presence of an external magnetic field showed an orientation effect with preferred growth of crystallites along〈100〉direction.

Inspired by recent progress in chemistry of compressed polyhydrides [1-2] and the discovery of high-temperature superconductivity above 250 K in the La–H system [3-4], we experimentally studied the high-pressure chemistry of Ba hydrides in four independent DACs. We successfully synthesized novel barium superhydride BaH₁₂ with a pseudocubic crystal structure, stabilized in the pressure range of 75–173 GPa. The compound was obtained by laser-heating of metallic barium with an excess of ammonia borane compressed to 173, 160, 146, and 90 GPa. The Ba sublattice structure of BaH₁₂ was resolved using the X-ray synchrotron diffraction, evolutionary structure prediction, and several postprocessing Python scripts, including an XRD matching algorithm. The ab initio calculations confirm a small distortion of the ideal fcc-barium sublattice to space group Cmc2₁ (or even P2₁), determined by the presence of additional weak reflections in the diffraction patterns. Discovered BaH₁₂ has unique combination of molecular H₂ and H₃ fragments formed due to Peierls-type distortion of one-dimensional H-chains, metallic conductivity, and even superconductivity with T_C = 20 K at 140 GPa, and one of the highest hydrogen content (>92 mol %) among all metal hydrides synthesized so far. Discovered barium dodecahydride is also the first known example of molecular metal amid polyhydrides with one-dimensional conductivity and superconductivity, realized in the H sublattice without its atomization. The results of these experiments confirm that the comparative stability of superhydrides increases with the growth of the period number of a hydride-forming element in the periodic table. [5]

References:
[1] D. Semenok et al. Mat. Today 2020, 33, 36.
[2] D. Zhou et al. J. Am. Chem. Soc. 2020, 142, 6, 2803–2811.
[3] M. Somayazulu et al. Phys. Rev. Lett. 2019, 122, 027001.
[4] A. Drozdov et al. Nature 2019, 569, 528.
[5] D. Semenok et al. Cur. Op. in Sol. State and Mat. Sci. 2020, 24, 2,100808.

Understanding melting processes in refractory materials under external stresses is important and can be of particular interest in harsh environment applications ranging from aerospace to nuclear and fusion energy where these materials have to simultaneously withstand the effect of high temperatures and complex stress states in such ways, that a melting process might be initiated in extreme conditions. However, most if not all of the prior research was focused on melting phenomena in the presence of hydrostatic compression.
In our work, we investigate the melting phenomenon in BCC tungsten under different shear stress types. We explore these relationships numerically by molecular dynamics simulations employing extended Finnis-Sinclair potential [1-2] and solid-liquid coexistence method and validate our results with theoretical and experimental findings previously reported in the literature for a hydrostatic compression case.
Regarding other stress states, no similar results have been reported yet for bcc tungsten. The melting behaviour was investigated for all studied stress states and compared both quantitatively and qualitatively on the basis of equivalent strain, least-square atomic-scale strain invariants, maximum shear stress, radial distribution function (RDF), thermal expansion and Lindemann melting criterion. For the majority of shear stress states, an abrupt decline of stress-induced melting point values was detected after certain critical stress values, which was explored with regard to the change of an RDF’s amplitude of the first peak. New high-temperature thermo-mechanical results are correlated with intricate structural changes taking place on the atomic scale during metal-melt phase transition.

Acknowledgements
The authors acknowledge the funding support of FDCR Grant No. 110119FD4501 by Nazarbayev University, Grant No. AP05130446 and State-Targeted Program No. BR05236454 by Kazakhstan Ministry of Education and Science.

References
[1] Akzhigitov D, Srymbetov T, Golman B, Spitas C and Utegulov Z N 2020 Melting of tungsten under uniaxial and shear stresses: molecular dynamics simulation Modelling Simul. Mater. Sci. Eng. [in press] https://doi.org/10.1088/1361- 651X/abaf39
[2] Dai X D, Kong Y, Li J H and Liu B X 2006 Extended Finnis–Sinclair potential for bcc and fcc metals and alloys Journal of Physics: Condensed Matter https://doi.org/10.1088/0953-8984/18/19/008

The deployment of Magnesium (Mg) alloys have increased significantly in the automotive, aerospace, and defense industries due to their lightweight structural integrity. In structural applications, these materials components are exposed to a wide range of strain rates such as explosive forming, high-speed impact, and shock loadings. These alloy components are placed in extreme loading environments where high strain rates are commonly achieved. Therefore, the evolution of failure mechanisms from strain localization and formation of adiabatic Shear Bands (ASBs) and the mechanical behavior at high strain rates is of desirable interest by the aerospace and automotive industries. The deformation behavior of Mg has extensively been studied under low strain rate loadings. However, the high strain rate deformation still requires further investigation. Deformation and damage accumulation in Mg alloys at high strain rate loading conditions such as impact is not fully understood. Thus, the correlation between the accuracy of the reported mechanical data and microstructural features that leads to the credence of failure mechanisms within Mg is not well understood. It is therefore critical to extending the understanding of the mechanism of failure of Mg at high strain rates of loadings by exploiting other techniques. In this work, the dynamic mechanical impact response and ASBs nucleation of heat-treated AZ31B Mg alloys at different strain rates and impact momentums are characterized in-situ during impact at room temperature using the Direct Impact Hopkinson Pressure Bar (DIHPB) coupled with an in-situ 3D Digital Image Correlation (DIC) and high speed and high-resolution thermal imaging techniques. DIC provides an independent measurement of surface strains directly on test specimens during deformation to monitor stain evolution and strain localization. It is found that the flow stress increased gradually to (~50 %) with increasing strain rates and impact momentums and the AZ31B Mg exhibits strain rate sensitivity at dynamic loadings. This is due to an increasingly selective grain refinement under dynamic compression. Evaluation of DIC results indicated that strain occurrence is linear at the onset of deformation but becomes heterogeneous at the later stage of deformation with multiple nucleation sites of strain localization leading to ASB at ~80% of the specimen length after ~340 us of deformation. The region of peak non-linearity of maximum strain concentration leads to ASB formation with characteristic fine grains, voids, and crack initiation. At most, ~22 °C temperature rise is observed. The temperature rise is, however, lower with respect to the homologous temperature of Mg, and a hence higher degree of strain hardening behavior is induced after deformation. These results suggest that ASB forms in AZ31B Mg alloy despite its brittle nature that leads to crack initiation and ultimate failure under high strain rates and large strains of deformation and can be used to extend the understanding of the mechanism of failure in Mg alloys.

The Dresden High Magnetic Field Laboratory (HLD) is a pulsed-field user facility, which provides external and in-house researchers with the possibility to perform a broad range of experiments in pulsed magnetic fields [1]. The HLD develops, manufactures, and operates the non-destructive pulsed magnets for the field range up to 90 T and beyond. A design of the magnets requires high-strength, high-conductivity wires with selected cross-sections and high-strength reinforcement materials. Currently some test coils are fabricated at the HLD for a prototype triple-coil magnet for magnetic fields of about 100 T. We will discuss the choice of materials for this pulsed magnet, material-based limitations, long standing experience with available materials, and material challenges for generating even higher magnetic fields by the non-destructive pulsed magnets.

[1] http://www.hzdr.de/hld

Ultra-High-Field (UHF) electromagnets require high current densities in high magnetic field with resulting intense Lorentz forces. A central part of developing higher field magnets is managing the Lorentz forces to keep stress and strain within the materials’ allowables.
The simplest UHF magnets are dc resistive ones. DC operations results in maximum performance being presently attained by high-strength copper and its alloys. Fatigue performance has started to emerge as a challenge in CuAg nano-composites. Pulsed resistive magnets are able to mix high-strength conductors and reinforcing materials with both high strength and high stiffness by appropriately adjusting the pulse length. High strength copper and a number of its alloys are productively employed, as are metallic and polymer/epoxy composite reinforcements. The highest field superconducting magnets employ the high-temperature superconducting (HTS) materials along with high strength reinforcement.
The present state of the art of material requriements for UHF magnets is briefly presented along with challenges for next-generation systems.

High magnetic fields have a long history of uncovering fundamentally important information about materials. They take their place among e.g. X-ray and neutron techniques, as a critically important tool of condensed matter physicists. The National High Magnetic Field Laboratory's Pulsed Field Facility at Los Alamos National Laboratory is one of few research centers in the world that can create and use ultra-high pulsed magnetic field for scientific research. The facility houses several types of non-destructive pulsed magnets which can provide the peak fields ranging from 60 tesla to 100 tesla for users. However, present frontiers in quantum matter, including topological matter and superconductivity, require magnetic fields beyond 100 tesla and several high field facilities around the world are currently working toward that ambitious goal. The power and stored energy required to generate a magnetic field increase exponentially with the field amplitude. Therefore the better conductors with higher mechanical strength and electrical conductivity are required to nondestructively generate magnetic fields above 100 tesla with acceptable risk and cost. Finite Element Modeling (FEM) simulations and our in-house mid-plane pulsed magnet design codes were employed to understand the requirements of advanced magnet materials to practically achieve magnetic fields beyond 100 tesla.

Magnetic field is a powerful tool to study new materials and new processes. The highest magnetic field generated with superconducting magnets made of NbTi and Nb₃Sn windings is 23.5 T. During the last years, technological breakthroughs have confirmed the feasibility of superconducting magnets generating more than 30 T thanks to the integration of high temperature superconductors. Still, higher magnetic fields are only available through copper-based magnets. Steady high magnetic field can be generated during several hours and implies a large electrical power (typically 30 MW for 40 T) whereas pulsed magnetic field can reach 100 T for some milliseconds with negligible power consumption. The French High Magnetic Field Facility (LNCMI) is developing both technologies, on its pulsed field site in Toulouse and its steady field site in Grenoble. Optimizing the high field magnets requires to search for the best material and associated shaping process for the realization of these highly constrained machines. The main high field magnet constraints are the mechanical constraints due to Lorentz forces and the thermal constraints coming from Joule losses.
For steady field magnets, copper alloys with high strength and high electrical conductivity are required for an operating temperature that can reached 150°C. To keep the windings of the magnet at a temperature acceptable for a given alloy a high velocity water flow is injected in it to manage heat fluxes as high as 10 MW.m^-2. Steady high field resistive magnets used two technologies: the Bitter and the polyhelix one. Bitter magnets are made of thin plates (from 0.2 to 1 mm) up to 1000 mm in diameter. In this shape Copper alloys with a high content of Silver (18 wt.%, yield strength up to 1000 MPa and electrical conductivity at room temperature of 50 MS.m^-1) have been industrialized by one company worldwide up to 250 mm in diameter. For larger diameters, copper plates with a possible addition of a low content of Silver are proposed by different companies with yield strength ranging from 400 to 450 MPa and a conductivity around 58 MS.m^-1. For the polyhelix technology, the LNCMI has set a R&D program to replace forged tubes (CuCrZr) with limited mechanical properties. Copper alloy tubes prepared by the Cold Spray technology are now in operation on the high field facility in Grenoble.
For pulsed field magnets, the highest priority is set on the mechanical properties of conductors at cryogenic temperature which have to cope with the increasingly severe mechanical constraints at fields beyond 60 T. To avoid excessive heating, the duration of the current that passes through a magnet is reduced to the order of 10-100 milliseconds. The LNCMI Toulouse disposes of several capacitor banks with a total energy of 22 MJ able to produce non-destructive fields over 60 T in 28 mm and close to 100 T in 8 mm bore. Beyond 100 T, it also features a Megagauss installation that makes use of dynamic effects in exploding copper single-turn coils to produce 200 T on a microsecond time scale. “In-house” copper/stainless steel macrocomposite conductors for wire wound pulsed field magnets generating magnetic fields up to 80 T are developed. High strength multiscale Cu/Nb nanocomposite wires produced by severe plastic deformation by ADB (Accumulative Drawing and Bundling) process are dedicated to the production of magnetic field over 90 T. New innovative composite copper based conductors, processed by powder metallurgy, reinforced with carbon nanotube or silver nanowires- are under development to reach non-destructive pulsed fields over 100 T.

The cold spray process was developed at the end of the 20th century and his principle is mainly based on the kinetic energy transmitted to micron size particles to allow their deposition. The particles remain solid and the deposits are generated by the particles impacting the substrate at high velocity. Working at low temperature, below the melting point of the deposited material, appears particularly interesting for deposition of oxygen sensitive materials like copper alloys. In comparison with conventional thermal spray processes, cold spray can retain the original properties of powder feedstock (grain size, nanostructured particles), prevent the adverse influence on the underlying substrate materials, and can produce very thick coatings due to compressive stresses.
To spray oxygen sensitive material like copper alloys, it is preferable to select an inert gas as propellant gas for cold spray deposition. Nitrogen is mainly used but, as the gas velocity is strongly linked to the gas mass, gases with low atomic weight like hydrogen or helium are particularly interesting to reach high velocities. Among those gases, helium is much better from the human safety point of view but its price is very high due to limited amounts on earth. A closed loop avoiding costly helium losses was implemented. Thus, deposition efficiency of cold spray copper alloys higher than 96 %, porosity below 0.1 % and oxygen content below 150 ppm were obtained. High deposition rates up to 7 kg/h were also reached.
Copper silver alloys present attractive mechanical properties combined with high thermal conductivity suitable for many applications like high field magnets. In the present work, CrCrZr and CuAg with various Ag percentage deposits were manufactured by cold spray using helium as propellant gas. The work hardening mechanisms are involved by comparison with classically forged materials. A large tailoring of the material’s properties can be obtained through specific heat treating. Residual stresses are very low after heat treating and allow machining helices without deformation. This process now allows to produce tubes for polyhelix magnets at LNCMI with a major performance gain.

We subject an aged Cu-24 wt%Ag ingot to heavily cold drawing to develop a nanocomposite. The as-drawn wire is composed of Cu-rich proeuctectic components and Ag-rich eutectic components. A fine lamellar structure are developed in the proeuctectic component embedded a high density of Ag fibers. The average lamellar spacing is 20±6 nm. Ag fiber width is below 5nm. The eutectic component has a relative coarse structure with Ag grain size around 100 nm. This bimodal size distributed Ag fibers yield ultra-high bending plasticity—up to 59% bending strain at the outermost edge-- 15 times the tensile elongation of the wire (3.6%). During our bending tests, dynamic recovery and partial recrystallization occurr primarily in the eutectic component, and are more frequently at the inner edge than the outer edge. The high bending strain causes some of the thick Ag fibers to become discontinuous and lose their original alignment. This structural evolution increases local plasticity, resulting in unexpectedly high achievable bending strain. This is unusual in nano-sized, Ag-fiber-reinforced high-strength composite.

There is a demand for stronger yet lighter conducting copper wires in high-technology domains of global interest where weight is at a premium, such as power and aerospace engineering as well as in niche applications, such as materials for high-field magnets.
Many proposed solutions involve the preparation of copper-matrix composite wires or copper-rich alloyed wires with ultrafine but complex microstructures. Wires are prepared by wire-drawing billets (cylinders). This is a severe plastic deformation method and may be followed by appropriate thermal treatments. There are many variants but the importance of the characteristics of the cylinder is not to be neglected. The usual routes for cylinder preparation involve melting-solidification at some point, and therefore a relatively high temperature, either for alloying or for the preparation of the pure copper sample to be used in a composite.
We have proposed an alternative method based on powder metallurgy routes, involving the consolidation of a suitable powder into a cylinder by sintering at a relatively low temperature. This is done by spark plasma sintering (SPS), which has several advantages over pressureless sintering and hot-pressing, including rapid heating and cooling rates. Therefore, lower sintering temperatures and shorter holding times make it possible to avoid grain growth and prepare copper cylinders with micrometer-sized grains, typically ten times lower than conventional copper cylinders.
SPS typically differs from hot-pressing by the application of a dc pulsed current to the pressing die and powder compact, while a uniaxial pressure is simultaneously applied. Interestingly, cylinders with only about 90% of their full density were found suitable for the rest of the study, because a too high density hampers the deformability of the cylinder during wire-drawing, resulting in sample breaking.
The basics of SPS will be presented and we will show several examples of the preparation of wires of pure copper, carbon nanotube - copper composites and silver nanowire - copper composites. The microstructure of the powders, cylinders (typically 8 mm in diameter and 33 mm long) and wires will be presented as well as the mechanical properties and electrical resistivity of the wires measured at 293 K and 77 K.

Advanced electrical and electronic devices sometimes require that conductor’s material properties should be counteracting, e.g. high electrical conductivity and high mechanical strength. Traditional high strength Cu-matrix composites consists of Cu-based matrix, which contains a uniformly distributed nanoscaled particles of intermetallic compounds or oxides. The required microstructure is usually formed by the doping of the initial Cu matrix alloy by the combination of elements which would form the intermetallic compounds during the aging heat treatment at specific temperature range of 300-500 ^o C (Cu-Zr-Nb; Cu-Ni-Si; Cu-Cr-Zr alloys) or to form fine oxides through the process of internal oxidation (Cu-Al alloys). These submicron particles of oxides and intermetallic compounds altogether with increase of ultimate strength lead to substantial decrease of fatigue properties. The paper describes the alternative approach to development of the advanced high strength Cu-matrix composites with the application of the plastic nanostructured Cu-Nb in situ microcomposites as a strengthening element. As an example the principally new design of composite Contact Wire for high speed railways has been proposed to attain at least 20% better combination of ultimate tensile strength (610-660 MPa) and high conductivity (more than 80% IACS). Composite conductor contains the bundles of nanostructured anomalously high strength Cu-Nb filamentary cores and outer layer made of low doped high conductivity Cu-Zr-Nb alloy with improved tribological properties. The microcomposite Contact Wire has very high fatigue properties due to filamentary architecture of microstructure of Cu-Nb strengthening component. This Contact wire was designed for the use in the scope of Overhead Contact Line CS-400 for railways with the speeds of 400 km per hour. The results of laboratory complex tests of contact wire are presented.

2-dimensional (2-D) sharp interfaces with distinct boundaries demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for enhanced behaviors such as strength, radiation damage tolerance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. 3-D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. Here, we focus on the thermal stability and mechanical behavior of nanolayered Cu/Nb containing interfaces with 3-D character. By co-sputtering the bimaterial interfaces between the constituent pure phases, the resulting compositional gradient gives rise to new interphase boundary structures, which have been analyzed and quantified via S/TEM and Atom Probe Tomography. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3-D interfaces is significantly greater than those containing 2-D interfaces. Mechanical anisotropy, as well as shear banding is observed during pillar compression with retention of continuous layers across the shear band. We will present our recent results on deformation of such 3-D interfaces and structures, and describe their structural evolution mechanistically through the use of atomistic simulations.

LNCMI-Toulouse produces some of the most intense non-destructive pulsed magnetic fields in the world with a European record of 98.8 Tesla and aims at reaching more than 100 Tesla. The generation of pulsed magnetic fields higher than 60 T requires the use of coils wound of wires with exceptional properties. The wires need a high conductivity to limit the heating and they must have a high strength to be able to resist the Lorentz forces. The Lorentz forces are tangential to the coil and proportional to the square of the magnetic field. The resulting von Mises stress is 1 GPa at 60 T and higher than 2.2 GPa at 100 T.

Classical methods for strengthening copper wires involve the preparation of composite wires, such as copper-stainless steel (Cu-SS) and copper-niobium (Cu-Nb) or alloyed wires such as copper/silver (Cu/Ag). Cu-Nb and Cu/Ag wires present a nanostructured microstructure with fine and elongated grains. Cu-Nb microstructure is made up of pure Cu and pure Nb grains, while the Cu/Ag microstructure presents Cu-rich and Ag-rich solid solution. These wires exhibit a UTS value higher than 1 GPa but they are distinguished by their high resistivity. The aim of this work is the preparation of composite wires based on pure Cu and pure Ag.

LNCMI and CIRIMAT explore the design and preparation of novel Cu-based nanocomposite wires with the use of powder metallurgy, spark plasma sintering (SPS) and wire-drawing at room temperature. This preparation method is adapted to the preparation of Ag-Cu composite wires. Ag nanowires (200 nm in diameter and 30 µm in length) were synthetized and mixed with a commercial micrometric (about 1µm) Cu powder. Samples containing 1, 5 and 10 vol. % Ag were prepared. Cu and Ag-Cu cylinders (diameter 8 mm, length 33 mm) are prepared by SPS. Cylinders are sintered at 400, 500 or 600 °C. The so-obtained cylinders serve as precursors to wire-drawing. Their diameter is reduced by wire-drawing at room temperature, in several passes, thus producing progressively finer wires (diameter in the range 1 - 0.2 mm).

The wires show a lamellar microstructure with ultrafine Cu grains (200 - 700 nm for a 0.5 mm diameter wire) elongated over several micrometers. The Ag nanowires are dispersed along the grain boundaries of Cu. Cu and Ag grains exhibit major <111> and minor <100> orientations like most of wire-drawn face centered cubic metals.

The electrical resistivity and tensile strength were measured at 293 K and 77 K. The tensile strength for the composite wires is more than twice the value measured for the corresponding pure Cu wires. Interestingly, the wires containing only 1 vol.% Ag offer the best combination of high strength (1100 MPa at 77 K) and low electrical resistivity (0.50 µΩ.cm at 77 K).

We have obtained wires with excellent mechanical properties while greatly reducing the electrical resistivity compared to Cu/Ag alloy. The present 1 vol. % Ag-Cu composite wires compare favorably with Cu/Ag alloy wires containing about 20 times more Ag.

Nanostructured reinforcement materials, which have been successfully used in high field magnets and at cryogenic temperatures, reach their most desirable strength level when their scale of microstructure is below 100 nanometers. This refined microstructure can be achieved by plastic deformation and heat treatment, which create obstacles, such as dislocations and precipitates, for dislocation motions. Both deformation and heat treatment also introduce microstructure and property anisotropy. Investigating both anisotropy and microstructure changes helps us to understand the plasticity of these materials, particularly at cryogenic temperatures. The goal of our research is to relate microstructural scale and anisotropy to the mechanical and physical properties of these materials. This presentation describes microstructure in different orientations, with emphasis on the impact of high anisotropy and fine microstructure on properties.
This work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

Since the prediction of metallization and superconductivity of hydrogen and hydrogen-rich materials at high pressures /1,2/, there has been a sustained effort to make advances in high pressure sciences (theory, experiments and techniques) to realize this dream. In comparison to the much larger research landscape spawned by the discovery of high T_c cuprate superconductors, the high pressure (and high temperature) superconductivity field has been slow to encapsulate the attention of many. Partly, this has to do with the paucity of experimental probes that can be broguth to bear and to a major extent, it is the question of transitioning discovery to application. Nevertheless, the discovery of superconductivity in H₃S, YH₆ and LaH₁₀ /3,4,5/ and the phenomenal rise in transition temperatures has begged the question of revisiting these two questions. These discoveries have been aided by theoretical predictions and those can be used to make technological feasibility a likelihood /6-9/. However, there are experimental limitations that need to be surmounted to understand the physics and chemistry of these materials that constitute and important feedback to these simulations /10,11/. This talk will aim at connecting these dots and discus the rapidly evolving picture we have of these fascinating materials.

References:
1. Ashcroft, N. W., Metallic hydrogen: A high-temperature superconductor? Phys. Rev. Lett. (1968) 21, 1748.
2. Ashcroft, N. W., Hydrogen Dominant Metallic Alloys: High Temperature Superconductors? Phys. Rev. Lett. (2004) 92, 187002.
3. Drozdov, A. P., et. al, Conventional Superconductivity at 203 Kelvin at High Pressures in the Sulfur Hydride System. Nature, (2015) 525, 7567
4. Kong, P., et. al, Superconductivity up to 243 K in yttrium hydrides under high pressure. (2019) arXiv:1909.10482
5. Somayazulu. M. S., et. al, Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures. Phys. Rev. Lett, (2019) 122, 027001.
6. Y. Li, et. al, The Metallization and Superconductivity of Dense Hydrogen Sulfide, J. Chem. Phys. (2014) 140, 174712.
7. Hanyu Liu et. al, Potential high-T_c superconducting lanthanum and yttrium hydrides at high pressure, PNAS (2017), 114, 6990.
8. Ying Sun et. al, Route to a Superconducting Phase above Room Temperature in Electron-Doped Hydride Compounds under High Pressure, Phys. Rev. Lett., (2019) 123, 097001.
9. Jose A Flores-Livas et. al, A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials, (2019) arXiv:1905.06693
10. Shirin Mozaffari et. al, Superconducting phase diagram of H3S under high magnetic fields, Nat. Comm. (2019), 10. 2522. 10.1038/s41467-019-10552-y.
11. A. D. Grokowiak et. al, Hot Hydride Superconductivity above 550 K, (2020) arXiv:2006.03004

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature [1].
Fermi-surface, transport and thermodynamic signatures of the pseudogap phase are reminiscent of a transition into a phase with antiferromagnetic order, but evidence for an associated long-range magnetic order is still lacking. Here we report measurements of the thermal Hall conductivity κ_xy in the normal state of four different cuprates and show that a large negative κ_xy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p*[2]. It is also a property of the Mott insulator at p ≈ 0, where κ_xy has the largest reported magnitude of any insulator. Since this negative κ_xy signal grows as the system becomes increasingly insulating electrically, it cannot be attributed to conventional mobile charge carriers. Nor is it due to magnons, since it exists in the absence of magnetic order.
In order to identify the heat carriers responsible for this negative κ_xy, we turn to transverse heat transport along the c-axis: κ_zy. Here we show that the thermal Hall conductivity at p = 0 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO₂ planes, i.e. κ_zy(T) ≈ κ_xy(T). This shows that the negative thermal Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes. At p > p*, we observe no c-axis Hall signal, i.e. κ_zy(T) = 0, showing that phonons have zero Hall response outside the pseudogap phase. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators [4].

[1] Proust & Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409 (2019).
[2] Grissonnanche et al., Nature 571, 376 (2019).
[3] Grissonnanche et al., Nature Physics (to appear); arXiv:2003.00111 (2020).
[4] Kasahara et al., Nature 559, 227 (2018).

The high-temperature superconductors (HTS), e.g., RE-Ba₂Cu₃O_x (RE = rare earth) can sustain significant transport current at high background magnetic field and elevated temperatures, enabling various applications including fusion energy generation, next generation high-energy physics particle accelerators, and medical accelerators for carbon therapy. However, reliable protection against catastrophic superconducting-to-normal transitions (quenches) remains to be addressed for successful applications of HTS magnet technology. To address this issue, we investigated the potential of vanadium oxide (VO_x) coatings as a temperature-dependent switching medium to self-regulate current sharing. VO_x coatings (1.70 ≤ x ≤ to 2.07) were deposited by reactive cathodic arc deposition on insulating glass and commercial REBCO tapes to study their electrical properties. The process gas was monitored in situ by a residual gas analyzer. Effects of varying process parameters such as argon-to-oxygen ratio and substrate manipulator speed on coating properties were studied. The coatings were X-ray amorphous but Raman spectrometry showed short-range crystalline ordering. Hall effect measurements found a decrease in resistivity of VO_x by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process did not damage the REBCO tapes as evidenced by a negligible change in critical current after versus before coating. The results from current-sharing experiments and circuit analysis suggest that the VO_x coating can effectively self-regulate current sharing in REBCO magnets and enable self-protection during quenches. This work was supported by the Office of Science, Office of Fusion Energy Sciences, U.S. Department of Energy under Contract No. DE-AC02- 05CH11231 and the Laboratory Directed Research and Development (LDRD) funding from the Lawrence Berkeley National Laboratory provided by the Director. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Bubble – a precursor of undersense nanostructure or fuzz formation is crystal lattice defect and is approached by Frenkel pair formation mechanism. One needs to have crystal lattice that inhibits Frenkel Pair generation by ejection of W self-interstitial atom by He cluster and its bonding with vacancy. However, this mechanism is very commonly observed in nuclear materials. It is applicable in conventional nuclear materials where α and β particles, γ rays and neutron flux strike the material. Plasma does not constitute of aforementioned and thus it is not very applicable mechanism. In the present study, author addresses this problem of development of plasma facing materials by primitive method of self-interstitial solid solution strengthening. Actual mechanism of bubble formation in studied and a suitable lattice is selected and proposed which inhibits possible Frenkel pair generation thus inherently promotes formation of plasma facing material.

*Corresponding author: [email protected]

When a beam of swift heavy ions (SHI) impinges onto a material, the formation of defects called ion tracks is possible as a result, either in the bulk or on the surface of the material. As a consequence, effects produced by means of SHI irradiation that lead to structural changes of the material can be used to modify the properties of the material via defects engineering.
SHI damage introduction to materials can result in complex behavior, especially in radiation harsh environments. To assess the final damage of the material, the effect of the energy deposition from the ions to the atoms within the material must be taken under consideration. Deposition of energy occurs through two main channels: either to the electronic subsystem of the material (electronic stopping power) or to the nuclei (nuclear stopping power). Here we investigate the influence of ion beams with different stopping powers, used in a sequence, on defects formation. It is known that sequential irradiation of ion beams can lead to the SHIBIEC effect (Swift Heavy Ion Beam Induced Epitaxial Crystallization) i.e. the annealing of defects that occurs during the second irradiation step. Alternatively, the anti-SHIBIEC effect is also possible, i.e. the enhanced defect formation during the second step of irradiation.
Sequential ion irradiation of GaN has not been investigated so far. This material is an important semiconductor for high-power electronics. The results presented here are of importance for basic research in this field as well. In our previous work [1], we found no evidence of ion track formation in the bulk after SHI irradiation using 23 MeV I and 90 MeV Xe beams. However, recently we were able to introduce additional disorder into moderately damaged GaN crystals using the same 23 MeV I and 90 MeV Xe beams, in cases where GaN samples have been pre-irradiated with 2 MeV Au or with 900 MeV Xe ion beams. Two different ion beams are expected to introduce damage into GaN either by nuclear or electronic stopping, respectively [2,3]. We observe the difference in total damage in the material depending on the fluence of the beams applied. In this contribution, we report new results of sequential ion irradiation of GaN based on RBS/c (Rutherford Backscattering Spectroscopy in Channeling) and TEM (Transmission Electron Microscopy) measurements.

[1] M. Karlušić et al., Response of GaN to energetic ion irradiation - conditions for ion track formation, J. Phys. D: Appl. Phys. 48 (2015) 325304
[2] S.O. Kucheyev et al., Ion implantation into GaN, Mat. Sci. Eng. 33 (2001) 51
[3] M. Sall et al., Track formation in III-N semiconductors irradiated by swift heavy ions and fullerene and re-evaluation of the inelastic thermal spike model, J. Mater. Sci. (2015) 50:5214–5227

Materials possessing a defect-fluorite pyrochlore structure can have a range of useful properties that are sought after, which include radiation tolerance, nuclear waste immobilization, and phase stability at elevated temperatures. In this study, we demonstrate for the first time the synthesis and a detailed analysis of defect-fluorite pyrochlore sodium niobate (NaNbO₃) nanoparticles. This analysis included an investigation into the stability of these nanoparticles at elevated temperatures, as well as to neutron irradiation. A surfactant-assisted solvothermal method was used to prepare nanoparticles of NaNbO₃. This solution-phase approach results in the formation of crystalline nanoparticles of a defect-fluorite pyrochlore NaNbO₃ at relatively low temperatures. The products had an average diameter of ~74 ± 11 nm. The nanoparticles adopted a defect-fluorite pyrochlore phase and matched the cubic space group. This pyrochlore form of NaNbO₃ was stable up to at least 500 ^oC. The nanoparticles transformed into the cubic and rhombohedral perovskite phases of NaNbO₃, along with the introduction of a pseudo-hexagonal Nb₂O₅ at higher temperatures. These defect-fluorite pyrochlore nanoparticles of NaNbO₃ also exhibited a resistance to radiation induced amorphization. The dimensions, phase, and crystallinity of the defect-fluorite pyrochlore nanoparticles after exposure to a flux of neutrons were comparable to the as-synthesized product. The thermal stability and radiation tolerance of these pyrochlore nanoparticles could be useful in the design of thermally resilient materials, high temperature catalysts, and durable materials for the handling and storage of radioactive waste.

The rapid evolution in technology and its integration with humans requires the development of novel processes that enable the incorporation of the crucial electronic materials, such as the wide bandgap semiconductors (Ga₂O₃ and GaN) into the wearable electroinc systems. Wide bandgap semiconductors form the building blocks of the current high-power high-frequency electronics and the upcoming mmWave fifth-generation (5G) device technologies. To make such technologies integrable with wearable/flexible systems, it is critical that high quality and in particular as-grown crystalline thin films of these materials can be produced on highly temperature-sensitive substrates at very low temperatures.
In this work, we report on the low-temperature, as-grown crystalline Ga₂O₃ films on various substrates (Si, sapphire, glass, and Kapton) via a hollow-cathode plasma-ALD system. Triethylgallium (TEG) and Ar/O₂ plasma were employed as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed at 160 – 240 °C substrate temperature and 30 – 200 W rf-power range. Additionally, each unit AB-type ALD-cycle was followed by an in situ Ar-plasma annealing treatment, which consisted of Ar-plasma exposure for 20 seconds scanned over 50 – 300 W rf-power. Both in situ and ex situ ellipsometry were employed to measure the thickness and optical properties of the films. The thickness of the films without Ar-annealing stage, ranged between 20.74 – 39.30 nm and as-grown refractive indices were between 1.75 - 1.67 within the scanned plasma power range. X-ray diffraction (XRD) showed that Ga₂O₃ films grown without in situ plasma annealing exhibited amorphous structure irrespective of both substrate temperature and rf-power. However, with the in situ Ar-annealing phase incorporated, the thickness of the films ranged between 22.9 – 31.4 nm with refractive indices of 1.75 – 1.79. The increased refractive index (1.79) and reduced thickness gain (31.4 nm) at 250 W Ar-annealing power indicates possible densification and crystallization of the films. Indeed, XRD and XRR confirmed that in situ Ar-plasma treated films grow in a monoclinic β-Ga₂O₃ crystal phase with further improving crystallinity and film density (from 5.07 to 5.80 g/cm³) with increasing Ar-annealing plasma power. Additionally, we will discuss the impact of varying in situ Ar-annealing plasma chemistries (Ar-only, O₂-only, and Ar/O₂) coupled with x-ray photoelectron spectroscopy (XPS) measurements, which will provide additional insight into the elemental composition of the films that might help to understand the changes in the structural, optical, and electrical properties. A future outlook will be provided to overcome the challenges to achieve device quality layers on temperature-sensitive flexible substrates.

Organosilicon polymer derived ceramic (PDC) materials and its derivatives have gained interest as an emerging class of materials capable of providing mechanical stability and heat dissipation upon operation in extreme environments. PDC's found various applications such as electrode material for Li-ion batteries, ultra-high temperature composites and others [1]. Being X-ray amorphous, PDC’s consist of nanodomains of different structures and compositions, demonstrating nanoscale heterogeneity that influence their unique mechanical, chemical and functional properties. Conventionally, PDC coatings are prepared by wet-chemical dipping, spinning thermal-spraying or by plasma-assisted chemical vapor deposition (CVD), followed by high temperature pyrolysis [2].
In this work, molecular layer deposited (MLD) hybrid organic-inorganic films serve as precursors for the synthesis of SiAlCO PDC composite coatings. Alumina-polysiloxane films were grown using sequential surface reactions between 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V₄D₄) and trimethylaluminum (TMA). The deposition was conducted at temperatures between 120 and 220 ^oC. In-situ quartz crystal microbalance (QCM) monitoring showed linear mass increase with the number of MLD cycles with the highest mass gain rate of 24 ng/cm²/cycle at 180 ^oC. The QCM study revealed a self-limiting surface chemistry. An infrared analysis (FTIR) of the deposited films showed vibrational features characteristic of organosilicon polymers: Si-CH₃, Si-O-Si, Si-CH=CH₂ and Si-O-Al peaks. High-resolution transmission electron microscopy (HRTEM) imaging confirmed the conformal nature of the deposited MLD film.
In order to increase mass yield during the polymer-to-ceramic transformation, cross-linking of the growing polymer chains is needed. Cross-linking was achieved at 200^oC by introducing a third precursor, di-tert-butyl peroxide (TBPO), into the MLD cycle, following a V₄D₄/TBP/TMA sequence. A positive mass gain after TBPO dosing, observed by QCM, indicates the formation of radicals and cross-linking. The cross-linking occurs through the activation of the vinyl groups. Characteristic signals, related to the generated polyethylene chains, were identified by ATR-FTIR and XPS. The formed Si-C-C-Si units are expected to improve the mechanical and thermal properties of the cross-linked SiAlCO PDCs, since the Si–C and C–C bonds are not affected by thermal depolymerization reactions.
X-ray and optical characterization techniques were employed to analyze the structural changes and composition of the films after pyrolysis. After pyrolysis at 900^oC in an Ar atmosphere, the composite thin film remained amorphous. The resulting film contained uniformly distributed Si, Al, O, and C. Raman spectroscopy showed presence of graphitic carbon within the film. This MLD-derived SiAlCO thin film is expected to have exceptional uniformity, high temperature stability, creep, and corrosion resistance.
[1] P. Colombo, et al., J. Am. Ceram. Soc., 93 (7), 1805–1837 (2010)
[2] G. Borroso, et al, J. Mater. Chem. A., 7 (5), 1936–1963 (2019)

Magnetic nanoparticles (MNPs) have attracted significant attention in the oil industry in the past decade due to their potential applications including reservoir sensing and mapping, enhanced oil recovery, drilling and completion improvement, oil-water separation in produced water, oil spill clean-up, and directional transport and local heating. For these specific applications, the dispersity and stability of the MNPs in suspension are of great importance. Surface coating and specific functionalization of the MNPs are typically used to improve their stability and to reduce aggregation for the successful use of the nanoparticles. However, many of the well-established methods and procedures of surface coating chemistry for lab applications are not suitable for harsh environmental conditions such as at extreme high salinity for industrial oil applications. In this research, we synthesized magnetite MNPs with tuned particle sizes to optimize their superparamagnetic properties, and developed a new procedure for surface functionalization to enhance their colloidal stability at high salinity. The functionalized magnetite MNPs have been characterized by XRD, HR-TEM, cryo-TEM, dynamic light scattering and zeta potentials. Colloidal stabilities have been studied in a wide pH range and salinity with 5000-230000 ppm of TDS (Total Dissolved Solids). The results have demonstrated that the covalently bonded zwitterionic functional groups and cross-linked saccharide highly enhance the colloidal stability in high salinity brines.

The polarization response of ionic and electronic defects in complex oxides under high electric field is critical for applications such as memristors and ferroelectrics, but historically challenging to model or measure. Here we developed a localized Hubbard U approach that resolves the dilemma of maintaining in-gap localized defect states for an oxygen vacancy while simultaneously retaining non-linear dielectric behavior of perovskite oxides. With this approach, we obtained accurate dielectric constant and electronic configuration for SrTiO₃ under electric field, using density functional theory and Berry phase calculations. We identified a direct relationship between oxygen vacancy polarizability and lattice strain, which can be generalized to other materials with similar crystal structure. For example, a neutral oxygen vacancy in BaTiO₃ and in SrTiO₃ strained to exhibit the same lattice constants showed identical polarization behavior under the same electric field. To trace the site polarizability difference between defected crystals and their defect-free counterparts, we utilized maximally localized Wannier functions to obtain the site-decomposed polarization. These simulations illustrated that not only the vacant site, but also the neighboring ions around the vacancy have significant influence on the system polarization with field applied either parallel or perpendicular to the Ti – V_O^x – Ti chain. Together, this model and identification of driving factors in defect displacement under electric field contribute generalizable approaches to study and optimize materials in memristors, ferroelectrics, electrocaloric refrigeration and flash sintering.

Selective solar absorbers with high performance are the key to concentrated solar power systems. Optical metamaterials are emerging as a promising strategy to enhance selective photon absorption, however, the high-temperature resistance (500 C) remains as one of the main challenges for practical applications. Here, a multilayered metamaterial system (Al₂O₃/W/SiO₂/W) based on metal-insulator-metal resonance effect has been demonstrated with high solar absorptance over 92%, low thermal emittance loss below 6%, and significant high-temperature resistance: it has been proved that the optical performance remains 94% after 1-hour thermal annealing under ambient environment up to 500 C, and 94% after 96-hour thermal cycle test at 400 C. Outdoor tests demonstrate that a peak temperature rise (193.5 C) can be achieved with unconcentrated solar irradiance and surface abrasion resistance test yields that SSAs have a robust resistance to abrasion attack for engineering applications.

Alumina-forming austenitic stainless steels (AFAs) are a class of alloys that are showing promise for use in high temperature applications, particularly for use in power generation. AFAs have been shown to demonstrate good high temperature strength, great oxidation resistance due to a protective alumina scale, and are cheaper than nickel-base alloys. While showing great promise, these alloys need to continue to be studied and improved to be practical for use. In particular, the deformation behavior of these alloys is not well understood. The austenitic matrix can contain various precipitates, including: L1₂ structured precipitates, B2 precipitates, Laves precipitates, carbides, and others. The grain boundary (GB) also can become covered by Laves and/or B2 precipitates. In a complex alloy with different types and sizes of precipitates, it can be difficult to determine what effect each of these precipitates are having on the high temperature strength.
In this work, we studied the deformation mechanisms in the model AFA alloy Fe-20Cr-30Ni-2Nb-5Al (at%). This alloy displays many characteristics of AFA alloys, and thus is a good alloy for scientific studies. This alloy contains L1₂, Laves, and B2 precipitates in the matrix as well as Laves and B2 precipitates on the GBs. Creep tests, high temperature tensile tests, and high temperature strain-rate jump tests were performed on this alloy after various heat treatments to determine the deformation mechanisms – including strain-rate jump tests with and without L1₂ precipitates (but otherwise similar microstructures) to determine the effect of these precipitates on the high temperature strength. In addition, tests were performed on alloys with different grain size, but otherwise similar microstructure, to determine the effect of GB precipitation. Analysis of the stress – strain rate relationships was used to predict deformation mechanisms and to fit the experimental data to models. In addition, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed on samples after mechanical testing to determine deformation behavior. In-situ straining experiments in the TEM and SEM were also performed. This study found that nano-sized L1₂ precipitates provided the majority of the strengthening in this alloy. At low stresses dislocations by-passed these particles by climb, and at high stresses dislocations started to form Orowan loops around the particles. While dislocations piled up at larger Laves and B2 precipitates, these precipitates were not as effective at strengthening the alloy. In addition, work was performed to determine if there was a GB strengthening effect due to coverage of the GB by precipitates.

Acknowledgements
This research was supported by National Science Foundation (NSF) Grant DMR 1708091.

Zinc ferrite nanoparticles are currently of much interest for their potential applications in extreme conditions like high temperature: e.g. in metal casting, in water splitting reaction for H₂ production (solar fuels) or as high - temperature desulfurizing sorbents [1-3]. Bulk ZnFe₂O₄ (ZF) should have paramagnetic nature around room temperature, when it is the normal spinel with Zn²⁺ incorporated mostly in the tetrahedral lattice sites. In the case of nanoparticles, the situation is more complex due to high probability of inversion in the cation distribution between octahedral and tetrahedral sites, which may lead to ferrimagnetism or superparamagnetism [4-6]. It was suggested that during heating Zn²⁺ ions can move from tetrahedral site to octahedral site whereas Fe³⁺ ions redistribute within the octahedral and tetrahedral sites in order to reduce the strain [7]. It was also speculated the possibility of reduction of Fe³⁺ to Fe²⁺ during high temperature treatment. In order to verify these hypotheses we have performed a comprehensive study of structural and magnetic properties of stoichiometric and non-stoichiometric zinc ferrite Zn_xFe_3-xO₄ (NZF) nanoparticles synthesized using co-precipitation and thermal decomposition method [2, 8] as well as their evolution after thermal treatment. Nanoparticles with different zinc to iron ratio were obtained and systematically probed with volume (XRD, VSM, TG/DSC), microscopic (TEM) and element sensitive probes (ICP-OES, Mossbauer spectroscopy, XPS, XAFS). Magnetic studies proved paramagnetic response of stoichiometric ZF nanoparticles, while a superparamagnetic behaviour is observed in strongly non-stoichiometric (as synthesized) nanoparticles. After thermal treatment at 1400 ^oC in inert atmosphere, significant changes in the saturation magnetization were observed in the case of NZF. This is related with an increase of the amount of formally Fe²⁺ ions. The estimated saturation magnetization of as synthesized NZF nanoparticles, of approx. 50 emu/g, is increased upon thermal treatment (at 1400 ^oC in inert atmosphere) up to approx. 130 emu/g. As such we have confirmed that zinc concentration and method of synthesis plays important role in magnetic properties of non-stoichiometric zinc ferrite nanoparticles. Moreover, the reduction of Fe³⁺ to Fe²⁺ during high temperature treatment has been observed in NZF nanoparticles annealed in inert atmosphere.
Acknowledgements: This work was supported by the National Science Centre, Poland, Grant No: 2016/23/D/ST8/00013.
References
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Actinide and lanthanide fluorite oxides, UO₂, CeO₂, ThO₂, form an important class of ceramic energy materials with applications ranging from nuclear fuels to solid oxide fuel cells. Of these systems, ThO₂ (thoria), has been the least explored and offers unique features including an extremely high melting temperature and a fixed tetravalent oxidation state. Many potential application environments for thoria include high radiation fields or temperatures which either promote or directly generate lattice defects. Such defects drastically influence thermal transport, a controlling safety and performance property in many applications. To date, little experimentally-validated understanding has been generated on the role of defect formation and clustering on thermal transport in thoria due to the lack of high-quality starting material in which these effects may be explored. In this work, high-quality oriented single crystals of thoria are grown using hydrothermal synthesis and then exposed to proton irradiation at both room temperature and 600C. The thermal conductivity of the defected surface layer in these crystals is measured using an all-optical spatial domain thermoreflectance (SDTR) technique and sub-electron-microscopy-resolution defects are characterized using Raman spectroscopy. Together, these methods provide important initial insight into the types of defects formed under ionizing radiation, defect clustering, and how these defects impact thermal transport in high-quality thoria.

Helium is the most inert element in the periodic table and generally considered to be unreactive under ambient conditions. On the other hand, helium is one of the main components of giant icy planets’ atmosphere. It is unclear whether the helium can diffuse into the depths and react with the mantle materials (water, ammonia and methane). In this talk, I will introduce some of our recent theoretical predictions on helium compounds, including helium-water, helium-ammonia, helium-methane and helium-alkali metal oxides/sulfides, etc [1-4], using our own machine learning accelerated crystal structure search method [5]. In addition, we also found that some of these compounds exhibit exotic dynamical properties at high pressure and high temperature, including the superionic, plastic states and their co-existence.

REFERENCE
Hao Gao, Cong Liu, Andreas Hermann, Richard J. Needs, Chris J. Pickard, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, "Coexistence of plastic and partially diffusive phases in a helium-methane compound", Natl. Sci. Rev. (2020). DOI: 10.1093/nsr/nwaa064
Cong Liu, Hao Gao, Andreas Hermann, Yong Wang, Maosheng Miao, Chris J. Pickard, Richard J. Needs, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, “Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature”, Phys. Rev. X 10, 021007 (2020).
Cong Liu, Hao Gao, Yong Wang, Richard J. Needs, Chris J. Pickard, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, “Multiple superionic states in helium-water compounds”, Nature Physics 15, 1065 (2019).
Hao. Gao, Jian Sun*, Chris. J. Pickard, Richard. J. Needs, “Prediction of pressure-induced stabilization of noble-gas-atom compounds with alkali oxides and alkali sulfides”, Phys. Rev. Mater. 3, 015002 (2019).
Kang Xia, Hao Gao, Cong Liu, Jianan Yuan, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, “A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search”, Sci. Bull. 63, 817 (2018).

The high-pressure chemistry of nitrogen and nitrogen-rich compounds have been in a focus of many studies in the recent years due to both fundamental and practical interest and due to the improvement of high-pressure synthetic and characterization techniques. Poly-nitrogen compounds are usually considered as potential high energy density materials (HEDM) due to the remarkable difference in the average bond energy between the single N–N bond, the double N=N bond , and the triple N≡N bond. Numerous polynitrogen compounds with various nitrogen polymeric networks were theoretically predicted, but there are only a very few experimental attempts to obtain such compounds.

In the present work we have systematically studied reactions between transition metals and nitrogen at pressures up to 130 GPa in laser-heated diamond anvil cells. Despite very high synthesis pressure Hf, W, Re and Os react with nitrogen forming porous metal-inorganic frameworks Hf₄N₂₂, WN₁₀, ReN₁₀ and Os₅N₃₄ of various topologies. The frameworks are built of metals interconnected via polymeric nitrogen linkers, while the pores are filled with dinitrogen molecules N₂. Lighter transition metals (Fe) form non-porous polynitrides featuring polymeric nitrogen chains, while alkali metals react with nitrogen at extreme conditions with the formation of pentazolate salts. In this contribution we will discuss novel synthetic routes to energetic polynitrogen compounds and crystal-chemical rules governing their formation.

Na1/2Y1/2Cu3Ti4O12 (NYCTO) ceramics with giant dielectric constant (ε′) were synthesized by simple reactive sintering. NYCTOnanopowder was first synthesized using high energy ball-mill. Then the pelletized powder was sintered in air at temperatures in the range 975 °C to 1050 °C for 10–20 h. The obtained ceramics showed pure CaCu3Ti4O12 (CCTO)-like cubic phase as revealed by x-ray diffraction measurements. Field effect-SEM observations showed that the grain size increases from 2 μmto5 μmwith increasing sintering temperature. NYCTO samples sintered at temperatures higher than 975 °C showed giant dielectric constant (103–104) over most of the frequency range. The minimum dielectric loss (tanδ) of∼0.055 at 300 K has been approved for the ceramic sample sintered at 1050 °C. Impedance and modulus spectra of the current samples showed two relaxations related to semiconductor (grain) and high resistance (grain-boundaries) elements. The activation energy for conduction located in the range 0.1–0.5 eV highlighted the role of single ionized oxygen vacancies in the dielectric properties of the investigated NYCTO ceramics.

Tungsten will be the plasma-facing material in the ITER divertor. Hydrogen blistering has been observed in plasma-facing materials, but to date, all electronic structure and molecular mechanics simulations have indicated that hydrogen atoms repel one another in perfect crystals. The influence of grain boundaries on hydrogen retention is also not well-understood. We conducted simulations intended to elucidate the mechanisms of blister formation. These studies were accelerated using direct implantation, as we have used in the past for helium, so that computational time ordinarily required to simulate hydrogen atoms that reflect is instead used to simulate diffusion. Consistent with prior observations, we observe that hydrogen is generally repelled by other hydrogen atoms in single-crystal tungsten and we observe neither self-clustering nor cluster growth of any kind. Near grain boundaries, however, the excess volume present allows hydrogen to cluster. Eventually, sufficient hydrogen concentration can build up and weaken the grain boundary, possibly resulting in delamination of the grains and forming a blister.

The present work focuses on understanding the microstructural development during sintering of nanopowders under pressure and electric fields/currents. For this purpose, a method has been developed which combines flash sintering (FS) and spark plasma sintering (SPS) [1]. This FS-SPS combination enables us to combine the advantages of both techniques: to force the electric current through the sample (FS) and to sinter pellet-shaped samples under mechanical loads (SPS). We explored the novel device on the model material Al doped ZnO and discovered a different microstructure, compared to conventional SPS. Grain growth and second phase formation were emphasized by FS-SPS. It remains controversial if the observed microstructure is a result of an intrinsic electric current/field effect or of a temperature-distribution. However, this method helps to understand mechanisms involved in electric current or field assisted sintering techniques and possibly produces new microstructures and, hence, materials.

Acknowledgement:
We acknowledge the support from the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft)—Priority Programme (SPP, Schwerpunktprojekt) 1959/1 “Manipulation of matter controlled by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials” with Project Nos. WI 981/16-1 and WO 577/13-1.

References:
[1] C. Gorynski, U. Anselmi-Tamburini and M. Winterer, Rev. Sci. Instrum. 91, 015112 (2020); doi: 10.1063/1.5119059

Designing ever stronger nanostructured materials that can even withstand harsh environments has become a major goal in the last decades. However, the increase in strength commonly comes hand in hand with a severe reduction in ductility, which at the same time also limits the toughness of these materials. Due to the high fraction of interfaces in such nanostructured materials, it is safe to assume that the processes limiting ductility and toughness as well as their response to harsh environments are controlled by the respective interface structures. However, a detailed understanding is at this point still lacking.
Bulk nanostructured materials were synthesized by severe plastic deformation and subject to tailored interface modifications. To investigate the structural properties of these nanomaterials at high stress and strain, we utilize quantitative miniaturized deformation and fracture experiments performed in situ in scanning or transmission electron microscopes. These highly resoled techniques not only allow to measure material performance, but also to directly identify the underlying deformation and failure mechanisms at the same time.
Using this experimental approach, we will first address the nanoscale processes taking place during deformation and fracture of bcc nanocrystalline metals with modified interfaces. Building on this, the discussion will be extended to the behavior of nanocomposite structures and the influence of an irradiative environment.

Swift heavy ion irradiation and high-energy ball milling are far-from-equilibrium processing techniques which provide access to a unique phase space not achievable by means of traditional synthesis approaches. High-resolution neutron total scattering experiments were utilized to gain fundamental insight into the multi-scale structural details of A₂B₂O₇ pyrochlore oxides prepared by these extreme conditions and their subsequent evolution under high-temperature treatment. Pair distribution function analysis revealed unconventional atomic arrangements characterized by an unexpectedly high degree of local order. For instance, the atomic arrangement of ball-milled titanate pyrochlore oxides is nearly identical to the same pyrochlore composition synthesized by solid-state reaction and subsequently amorphized by intense ion irradiation (1). Interestingly, the same atomic arrangement is found in pyrochlore oxides that exhibit distinct long-range radiation responses (order-disorder versus crystalline-amorphous) (2). These phenomena are proposed to be the result of fundamental chemical rules which stringently dictate the atomic configuration in disordered materials (3, 4). This description provides insight into far-from-equilibrium processing of oxides and helps to better elucidate the structural properties and heterogeneity of disordered complex oxides from the local atomic arrangement to the macroscale.

1. E. C. O’Quinn et al., Advanced characterization technique for mechanochemically synthesized materials: neutron total scattering analysis. J. Mater. Sci. 53, 13400–13410 (2018).
2. J. Shamblin et al., Similar local order in disordered fluorite and aperiodic pyrochlore structures. Acta Mater. 144, 60–67 (2018).
3. D. Drey, Disorder in Ho₂Ti_2-xZr_xO₇: Pyrochlore to Defect Fluorite Solid Solution Series. RSC Adv. In Press (2020).
4. E. C. O’Quinn et al., Predicting short-range order and correlated phenomena in disordered crystalline materials. Sci. Adv. 6, 1–8 (2020).

Due to the size- and shape-dependent properties, nanoparticles have been successfully used as functional building blocks to fabricate 1-3D ordered assemblies for the development of artificial metamaterials. At ambient pressure, entropy driven self-assembly of monosized or binary nanoparticles generally results in polycrystalline 2- or 3D close-packed arrangements, and extensive efforts have been made to develop structural perfection of nanoparticle arrays or ‘single crystal-like’ domain structures with precise long range order for their definite advantages for electron transport. To date, fabrications of ordered nanoparticle assemblies have been relied on specific interparticle chemical or physical interactions such as van der Waals interactions, dipole-dipole interaction, chemical reactions, etc. Recently we have discovered a pressure-induced assembly method to engineer nanoparticle assembly at mesoscale and to fabricate new nanoparticle architectures without relying on specific nanoparticle interactions. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle arrays can be manipulated to reversibly shrink, allowing fine-tuning of interparticle separation distance. Under a uniaxial pressure field, nanoparticles are forced to contact and coalesce, forming hierarchical nanostructures. Depending on the orientation of the initial nanoparticle arrays, 1-3D ordered nanostructures including nanorod, nanowire, and nanoporous network can be fabricated through the pressure-induced self-assembly method. Guided by computational simulations, we were able to rationalize the pressure-induced self-assembly of nanoparticle arrays for predictable nanostructures. Moreover, we discovered for the first time a transition from an ordered polycrystalline nanoparticle mesophase to quasi-single crystalline nanoparticle lattices induced by PDA process. Exerting pressure-dependent control over the structure of nanoparticle arrays provides a unique and robust system to understand collective chemical and physical characteristics and to develop novel electronic and photonic behavior for energy transduction related applications.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Developing elastic materials for technological applications in extreme conditions has been a challenging task. Normal metals have an elastic strain limit rarely exceeding 1%. A large pseudo-elastic strain up to 10% can be achieved in the so-called shape-memory alloys (SMAs). However, the intrinsic hysteresis and temperature sensitivity of the first-order phase transformation significantly hinder their usage in many critical areas. We recently discovered a new type elasticity: supercritical elasticity (SCE), in NiCoFeGa single crystals, which exhibit a large elasticity up to 15.2% strain, with non-hysteretic mechanical responses, a small temperature dependence and high-energy-storage capability and cyclic stability over a wide temperature and composition range [Chen et al., Nature Materials, (2020). https://doi.org/10.1038/s41563-020-0645-4]. Such a giant SCE is conceived to work even down to absolute zero Kelvin. In situ synchrotron X-ray diffraction measurements show that the SCE is correlated with a stress-induced continuous variation of lattice parameter accompanied by structural fluctuation. Neutron diffraction and electron microscopy observations reveal an unprecedented microstructure consisting of atomic-level entanglement of ordered and disordered crystal structures, which can be manipulated to tune the SCE. The discovery of the giant SCE and associated the entangled structure paves the way for exploiting elastic strain engineering and development of related functional materials for a wide variety of high-performance engineering applications, ranging from deep-space and deep-sea exploration to intelligent robotics.

Metal halide perovskites are promising for advanced photovoltaic and optoelectronic applications. Solar cells and light-emitting diodes based on metal halides have achieved an impressive power conversion efficiency of 25.2% and a high external quantum efficiency of over 20%, respectively. Importantly, exceptional structural turnability enables enhanced and/or emergent properties. By using appropriate organic and inorganic components, many perovskite variants with 2D, 1D, and 0D structures on the molecular level have been obtained. These electronically low-dimensional metal halides have exhibited rich emergent properties and significantly expanded the scope of halide perovskites, while critical challenges remain. Future design of high-performance metal halides calls for a deeper understanding of the structure-property relationships. In this talk, we will present our recent efforts in using external pressure to tune the structures and physical properties of low-dimensional metal halides. Using state-of-the-art high-pressure techniques coupled with in situ synchrotron-based and in-laboratory property measurements, we characterized the changes in their structural, electronic, and optical properties. Pressure-enhanced properties such as higher photoluminescence quantum yields, as well as pressure-induced novel phenomena including emergent photocurrent, were observed. Our findings demonstrate that high pressure is able to realize enhanced and/or emergent properties of various metal halides, and further our understanding of the underlying mechanisms.

Recently, heterostructures are found to produce unprecedented combination of strength and ductility that is considered impossible from our textbook knowledge and materials history. Heterostructured (HS) materials consist of domains with dramatic strength differences. The superior mechanical properties have been attributed to back-stress strengthening and back-stress work hardening, which are related to strain gradient near the heterogeneous domain interfaces. Here I’ll discuss in-situ observations of strain gradient evolution near the domain interface and the dislocation mechanisms for the formation of strain gradient. It is found that contrary to our conventional understanding, negative strain gradient was developed near the interface. It is also observed under in-situ TEM that the Frank-Read dislocation sources are dynamically created and destroyed although postmortem examinations do not observe many Frank Read sources. These observations provide a new piece of scientific foundation for heterostructured materials, which is becoming a hot research field.

Thermite reactions between a metallic fuel and a solid oxide can be thermally or mechanically activated. The ignition temperature in a thermally activated thermite reaction generally increases with heating rate but is ultimately limited by the onset of melting of the lower melting point constituent. In mechanically activated thermite reactions, there is often a structure-sensitive threshold pressure required to drive the reaction. Historically, thermally activated thermite reactions have been explained using diffusion-based reaction mechanisms while mechanically activated thermite reactions have been described using deformation-based mechanical mixing models. Here we probe mechanically activated thermite reactions at a single well-defined heterophase interface using a combination of hydrocode simulations and laser-induced particle impact testing, where a CuO powder particle strikes an aluminum target at supersonic velocities. No reactions are observed through in situ imaging; however, post-mortem characterization of embedded CuO particles reveals a thin nanocrystalline Al₂O₃ reaction layer, with a thickness roughly equal to that of a molten lubricating layer predicted to form during penetration. These results suggest that local melting at hot spots drives the ultrafast reactions observed in mechanically activated thermite reactions and that thermally and mechanically activated thermite reactions may in fact result from the same underlying mechanisms.

Surface stability of deposited sensing material is becoming a crucial issue in the field of opto-chemical sensor technology due to cracks/stresses. Porous silica-titania (ST) nanocomposite coating material is synthesized and doped with CdTe as transducers for sensing cyclic compounds. Ultra-thin coatings are obtained, ∼11 nm for ST nanocomposite, and 5 nm for encapsulated and heat-treated samples. BET analysis shows large surface area ∼441 m2/g ± 3 m2/g and pore diameter 58 Å of ST nanocomposite. It decreases down to ∼410 m2/g ± 2
m2/g, and pore diameter to 24 Å, after encapsulation. However, after heat treatment, it again increases up to 419 m2/g ± 2 m2/g and pore diameter to 38 Å. The sensitivity of 71.37 counts per unit change in the carbon chain with correlation determination coefficient R2 ∼0.89 at 468 nm is observed. The prepared sensor device shows excellent stability, signal-to-noise ratio, and good reversibility towards different hydrocarbons and non-hydrocarbons (inorganic materials).

The results of a comparative study of neat and doped silicon dioxide polymorphs (Fe, Ni or Cu) obtained by density functional theory (DFT) at 0 K and at ambient pressure, 10 GPa, and 20 GPa are presented. The metal atoms are introduced at substitutional and octahedral interstitial sites of the crystal, and the relaxed structures are analyzed and ranked according to the enthalpy. Changes in the enthalpy of the neat and doped crystals are estimated at selected temperatures using quantum mechanics molecular dynamics (DFT-MD). We report the modeled enthalpy of the neat and doped crystals, as well as the dependency of the unit-cell parameters, band gaps, and Raman-active modes on pressure, and compare some of these results to those obtained experimentally using a diamond anvil cell. In addition, we present our search results of the high-pressure phases of silicon dioxide using DFT - based evolutionary simulations and report on the stability of the silica phases, as evaluated by convex-hull construction.
Funding Acknowledgement: This work was supported in part by a grant of computer time from the DOD High-Performance Computing Modernization Program at the ARL, Navy, AFRL, and ERDC DoD Supercomputing Resource Centers.

Cyclotetramethylene-Tetranitramine (β-HMX) is an energetic molecular crystal widely used as a constituent for plastic bonded explosives (PBXs). At high pressures encountered under shock conditions, dislocation motion is inhibited and plastic deformation takes place primarily by shear localization. Atomistic simulations of shear localization are performed in β-HMX under isothermal-isobaric conditions. The post localization flow stress for different pressures (0 to 15 GPa) and strain rates (10⁷ to 10¹⁰ s^-1) and at different temperatures of the shear band is investigated. The flow stress decreases linearly with temperature and increases quadratically with the pressure and exponentially with the strain rates. Shear thinning is observed, i.e. the viscosity decreases with increasing the strain rate. It is concluded that the flow stress increases exponentially with the density of the amorphous shear band and hence it can be described as a function of this single variable. The pressure, temperature and strain rates control the flow stress through the density.
The critical stress for shear localization (CSSL) was investigated for different strain rates at different pressures. The critical stress depends significantly on pressure and weakly on strain rates.

Atomic features, such as surface roughness and dislocations, affect the mechanical strength and plastic deformation of materials under high shear rates. The precise role of how these atomic features influence the wear and tear of materials under nonequilibrium settings is not well understood. We use Objective Molecular Dynamics (OMD) to investigate the links between atomic features and high shear rates. This method has two advantages over traditional MD simulations: First, it simplifies solutions by exploiting the group invariance of the equations of molecular dynamics. For example, in OMD only a modest number of atoms are simulated (typically, 50-1000) and the motions of the other, typically infinite, set of atoms (the non-simulated atoms) are given by the group action on the simulated atoms. Second, it provides a framework for modeling continuum loads at an atomic scale. For example, a Lagrangian velocity gradient tensor is introduced to describe affine motion of simulated atoms. In today's talk, we demonstrate these advantages of OMD through two examples: 1) frictional behavior of sliding surfaces, and 2) dislocation movement under shear stress. Both of these computations were done under high shear rates, and demonstrate how atomic features, such as rough surfaces, edge dislocations, and vacancies, affect mechanical deformation. Broadly, we present a theoretical framework to explore the interplay between extreme mechanical loads at the continuum scale and mechanical deformation at the atomic scale.

Rhenium nitride is a promising candidate as superhard conductor. Different rhenium-to-nitrogen ratios and several phases result in challenging synthetic strategies for phase pure rhenium nitride. Novel heteroleptic rhenium(I) compounds, [fac-Re(I)(CO)₃(L)] (e.g., L= tfb-dmpda, (N,N-(4,4,4-trifluorobut-1-en-3-on)-dimethyl propylene diamine)), containing preformed Re−N bonds act as efficient precursors for selective growth of polycrystalline rhenium nitride (ReN) films by vapor phase deposition. This is the first known access to synthesize phase pure rhenium nitride by single source precursor approach without the need of additional gases. Interdependence of materials strength and thin film orientation is strongly influenced by external magnetic fields. Deposition of ReN films in presence of an external magnetic field showed an orientation effect with preferred growth of crystallites along〈100〉direction.

Inspired by recent progress in chemistry of compressed polyhydrides [1-2] and the discovery of high-temperature superconductivity above 250 K in the La–H system [3-4], we experimentally studied the high-pressure chemistry of Ba hydrides in four independent DACs. We successfully synthesized novel barium superhydride BaH₁₂ with a pseudocubic crystal structure, stabilized in the pressure range of 75–173 GPa. The compound was obtained by laser-heating of metallic barium with an excess of ammonia borane compressed to 173, 160, 146, and 90 GPa. The Ba sublattice structure of BaH₁₂ was resolved using the X-ray synchrotron diffraction, evolutionary structure prediction, and several postprocessing Python scripts, including an XRD matching algorithm. The ab initio calculations confirm a small distortion of the ideal fcc-barium sublattice to space group Cmc2₁ (or even P2₁), determined by the presence of additional weak reflections in the diffraction patterns. Discovered BaH₁₂ has unique combination of molecular H₂ and H₃ fragments formed due to Peierls-type distortion of one-dimensional H-chains, metallic conductivity, and even superconductivity with T_C = 20 K at 140 GPa, and one of the highest hydrogen content (>92 mol %) among all metal hydrides synthesized so far. Discovered barium dodecahydride is also the first known example of molecular metal amid polyhydrides with one-dimensional conductivity and superconductivity, realized in the H sublattice without its atomization. The results of these experiments confirm that the comparative stability of superhydrides increases with the growth of the period number of a hydride-forming element in the periodic table. [5]

References:
[1] D. Semenok et al. Mat. Today 2020, 33, 36.
[2] D. Zhou et al. J. Am. Chem. Soc. 2020, 142, 6, 2803–2811.
[3] M. Somayazulu et al. Phys. Rev. Lett. 2019, 122, 027001.
[4] A. Drozdov et al. Nature 2019, 569, 528.
[5] D. Semenok et al. Cur. Op. in Sol. State and Mat. Sci. 2020, 24, 2,100808.

Xenon is quite a reactive noble gas, and xenon difluoride (XeF₂) being stable at ambient conditions is used in organic synthesis as fluorinating agent. Formation of XeF₂ is possible because xenon is bonded to highly electronegative fluorine atoms that take electrons from xenon filled outermost electron shell, forming cation Xe²⁺.
High-pressure behavior of XeF₂ (sp. gr. I4/mmm) is still under debate. Early powder X-ray diffraction and Raman experiments propose that XeF₂ should transform from a molecular compound to extended graphite-like layer structure at 50 GPa, and then above 70 GPa to a metallic fluorite structure. Contrary, the later experimental studies suggest just slight decrease in XeF₂ lattice symmetry, first to Immm and then to Pnma structure at 28 GPa and 59 GPa, respectively. Theoretical calculations predict that XeF₂ becomes unstable above 81 GPa, while high pressures should stabilize other xenon–fluorine compounds, such as XeF₄, XeF₆ and Xe₂F.
In order to resolve this issues, we investigated thermal stability of XeF₂ at pressures up to 54 GPa by means of single crystal X-ray diffraction in laser-heated diamond anvil cells. Single-crystal XRD opens the way to determine not only crystal structure of the material at truly extreme conditions, but also allows to refine its chemical composition. We report here the crystal structures of novel fluorides xenon, synthesized at high-pressure and high temperature conditions. In particular, the crystal structures of XeF₄ and Xe₃F₂, that both formed under heating-induced disproportionation of XeF₂ at 54 GPa. Similarly to ambient pressure structure, XeF₄ is a molecular compound composed of XeF₄ units with square planar geometry. Xe₃F₂ is not known at ambient conditions, and its structure is formed by isolated Xe atoms and linear XeF₂ molecules.

Cold spray, initially a coating technique, is being touted as a ‘near-net shape’ manufacturing technology that minimizes material waste by virtue of the high rate of deposition. During the cold spray process, metallic bulk components can be produced by spraying metal powders at high velocity, generating bonding through severe plastic deformation at temperatures well below the melting point of the powders. To fully understand the cold spray processing of metal powders, we systematically compare and study the microstructure evolution in Cu, Ni, Al, Ti, Ti-6Al-4V samples prepared by cold spray using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and nanoindentation. We show complex microstructure in these powder particles after cold spraying: nanocrystalline, nanotwins, annealing twins, gradient grains, deformation bands, dynamic/static recovery and recrystallization. The effects of gas temperature and powder velocity on the microstructure and mechanical properties in the cold sprayed samples are also discussed. Of particular interest are grain refinement, recrystallization and particle/particle bonding mechanisms of the cold sprayed powders.

Understanding melting processes in refractory materials under external stresses is important and can be of particular interest in harsh environment applications ranging from aerospace to nuclear and fusion energy where these materials have to simultaneously withstand the effect of high temperatures and complex stress states in such ways, that a melting process might be initiated in extreme conditions. However, most if not all of the prior research was focused on melting phenomena in the presence of hydrostatic compression.
In our work, we investigate the melting phenomenon in BCC tungsten under different shear stress types. We explore these relationships numerically by molecular dynamics simulations employing extended Finnis-Sinclair potential [1-2] and solid-liquid coexistence method and validate our results with theoretical and experimental findings previously reported in the literature for a hydrostatic compression case.
Regarding other stress states, no similar results have been reported yet for bcc tungsten. The melting behaviour was investigated for all studied stress states and compared both quantitatively and qualitatively on the basis of equivalent strain, least-square atomic-scale strain invariants, maximum shear stress, radial distribution function (RDF), thermal expansion and Lindemann melting criterion. For the majority of shear stress states, an abrupt decline of stress-induced melting point values was detected after certain critical stress values, which was explored with regard to the change of an RDF’s amplitude of the first peak. New high-temperature thermo-mechanical results are correlated with intricate structural changes taking place on the atomic scale during metal-melt phase transition.

Acknowledgements
The authors acknowledge the funding support of FDCR Grant No. 110119FD4501 by Nazarbayev University, Grant No. AP05130446 and State-Targeted Program No. BR05236454 by Kazakhstan Ministry of Education and Science.

References
[1] Akzhigitov D, Srymbetov T, Golman B, Spitas C and Utegulov Z N 2020 Melting of tungsten under uniaxial and shear stresses: molecular dynamics simulation Modelling Simul. Mater. Sci. Eng. [in press] https://doi.org/10.1088/1361- 651X/abaf39
[2] Dai X D, Kong Y, Li J H and Liu B X 2006 Extended Finnis–Sinclair potential for bcc and fcc metals and alloys Journal of Physics: Condensed Matter https://doi.org/10.1088/0953-8984/18/19/008

The deployment of Magnesium (Mg) alloys have increased significantly in the automotive, aerospace, and defense industries due to their lightweight structural integrity. In structural applications, these materials components are exposed to a wide range of strain rates such as explosive forming, high-speed impact, and shock loadings. These alloy components are placed in extreme loading environments where high strain rates are commonly achieved. Therefore, the evolution of failure mechanisms from strain localization and formation of adiabatic Shear Bands (ASBs) and the mechanical behavior at high strain rates is of desirable interest by the aerospace and automotive industries. The deformation behavior of Mg has extensively been studied under low strain rate loadings. However, the high strain rate deformation still requires further investigation. Deformation and damage accumulation in Mg alloys at high strain rate loading conditions such as impact is not fully understood. Thus, the correlation between the accuracy of the reported mechanical data and microstructural features that leads to the credence of failure mechanisms within Mg is not well understood. It is therefore critical to extending the understanding of the mechanism of failure of Mg at high strain rates of loadings by exploiting other techniques. In this work, the dynamic mechanical impact response and ASBs nucleation of heat-treated AZ31B Mg alloys at different strain rates and impact momentums are characterized in-situ during impact at room temperature using the Direct Impact Hopkinson Pressure Bar (DIHPB) coupled with an in-situ 3D Digital Image Correlation (DIC) and high speed and high-resolution thermal imaging techniques. DIC provides an independent measurement of surface strains directly on test specimens during deformation to monitor stain evolution and strain localization. It is found that the flow stress increased gradually to (~50 %) with increasing strain rates and impact momentums and the AZ31B Mg exhibits strain rate sensitivity at dynamic loadings. This is due to an increasingly selective grain refinement under dynamic compression. Evaluation of DIC results indicated that strain occurrence is linear at the onset of deformation but becomes heterogeneous at the later stage of deformation with multiple nucleation sites of strain localization leading to ASB at ~80% of the specimen length after ~340 us of deformation. The region of peak non-linearity of maximum strain concentration leads to ASB formation with characteristic fine grains, voids, and crack initiation. At most, ~22 °C temperature rise is observed. The temperature rise is, however, lower with respect to the homologous temperature of Mg, and a hence higher degree of strain hardening behavior is induced after deformation. These results suggest that ASB forms in AZ31B Mg alloy despite its brittle nature that leads to crack initiation and ultimate failure under high strain rates and large strains of deformation and can be used to extend the understanding of the mechanism of failure in Mg alloys.

Magnetic field is a powerful tool to study new materials and new processes. The highest magnetic field generated with superconducting magnets made of NbTi and Nb₃Sn windings is 23.5 T. During the last years, technological breakthroughs have confirmed the feasibility of superconducting magnets generating more than 30 T thanks to the integration of high temperature superconductors. Still, higher magnetic fields are only available through copper-based magnets. Steady high magnetic field can be generated during several hours and implies a large electrical power (typically 30 MW for 40 T) whereas pulsed magnetic field can reach 100 T for some milliseconds with negligible power consumption. The French High Magnetic Field Facility (LNCMI) is developing both technologies, on its pulsed field site in Toulouse and its steady field site in Grenoble. Optimizing the high field magnets requires to search for the best material and associated shaping process for the realization of these highly constrained machines. The main high field magnet constraints are the mechanical constraints due to Lorentz forces and the thermal constraints coming from Joule losses.
For steady field magnets, copper alloys with high strength and high electrical conductivity are required for an operating temperature that can reached 150°C. To keep the windings of the magnet at a temperature acceptable for a given alloy a high velocity water flow is injected in it to manage heat fluxes as high as 10 MW.m^-2. Steady high field resistive magnets used two technologies: the Bitter and the polyhelix one. Bitter magnets are made of thin plates (from 0.2 to 1 mm) up to 1000 mm in diameter. In this shape Copper alloys with a high content of Silver (18 wt.%, yield strength up to 1000 MPa and electrical conductivity at room temperature of 50 MS.m^-1) have been industrialized by one company worldwide up to 250 mm in diameter. For larger diameters, copper plates with a possible addition of a low content of Silver are proposed by different companies with yield strength ranging from 400 to 450 MPa and a conductivity around 58 MS.m^-1. For the polyhelix technology, the LNCMI has set a R&D program to replace forged tubes (CuCrZr) with limited mechanical properties. Copper alloy tubes prepared by the Cold Spray technology are now in operation on the high field facility in Grenoble.
For pulsed field magnets, the highest priority is set on the mechanical properties of conductors at cryogenic temperature which have to cope with the increasingly severe mechanical constraints at fields beyond 60 T. To avoid excessive heating, the duration of the current that passes through a magnet is reduced to the order of 10-100 milliseconds. The LNCMI Toulouse disposes of several capacitor banks with a total energy of 22 MJ able to produce non-destructive fields over 60 T in 28 mm and close to 100 T in 8 mm bore. Beyond 100 T, it also features a Megagauss installation that makes use of dynamic effects in exploding copper single-turn coils to produce 200 T on a microsecond time scale. “In-house” copper/stainless steel macrocomposite conductors for wire wound pulsed field magnets generating magnetic fields up to 80 T are developed. High strength multiscale Cu/Nb nanocomposite wires produced by severe plastic deformation by ADB (Accumulative Drawing and Bundling) process are dedicated to the production of magnetic field over 90 T. New innovative composite copper based conductors, processed by powder metallurgy, reinforced with carbon nanotube or silver nanowires- are under development to reach non-destructive pulsed fields over 100 T.

Ultra-High-Field (UHF) electromagnets require high current densities in high magnetic field with resulting intense Lorentz forces. A central part of developing higher field magnets is managing the Lorentz forces to keep stress and strain within the materials’ allowables.
The simplest UHF magnets are dc resistive ones. DC operations results in maximum performance being presently attained by high-strength copper and its alloys. Fatigue performance has started to emerge as a challenge in CuAg nano-composites. Pulsed resistive magnets are able to mix high-strength conductors and reinforcing materials with both high strength and high stiffness by appropriately adjusting the pulse length. High strength copper and a number of its alloys are productively employed, as are metallic and polymer/epoxy composite reinforcements. The highest field superconducting magnets employ the high-temperature superconducting (HTS) materials along with high strength reinforcement.
The present state of the art of material requriements for UHF magnets is briefly presented along with challenges for next-generation systems.

High magnetic fields have a long history of uncovering fundamentally important information about materials. They take their place among e.g. X-ray and neutron techniques, as a critically important tool of condensed matter physicists. The National High Magnetic Field Laboratory's Pulsed Field Facility at Los Alamos National Laboratory is one of few research centers in the world that can create and use ultra-high pulsed magnetic field for scientific research. The facility houses several types of non-destructive pulsed magnets which can provide the peak fields ranging from 60 tesla to 100 tesla for users. However, present frontiers in quantum matter, including topological matter and superconductivity, require magnetic fields beyond 100 tesla and several high field facilities around the world are currently working toward that ambitious goal. The power and stored energy required to generate a magnetic field increase exponentially with the field amplitude. Therefore the better conductors with higher mechanical strength and electrical conductivity are required to nondestructively generate magnetic fields above 100 tesla with acceptable risk and cost. Finite Element Modeling (FEM) simulations and our in-house mid-plane pulsed magnet design codes were employed to understand the requirements of advanced magnet materials to practically achieve magnetic fields beyond 100 tesla.

The Dresden High Magnetic Field Laboratory (HLD) is a pulsed-field user facility, which provides external and in-house researchers with the possibility to perform a broad range of experiments in pulsed magnetic fields [1]. The HLD develops, manufactures, and operates the non-destructive pulsed magnets for the field range up to 90 T and beyond. A design of the magnets requires high-strength, high-conductivity wires with selected cross-sections and high-strength reinforcement materials. Currently some test coils are fabricated at the HLD for a prototype triple-coil magnet for magnetic fields of about 100 T. We will discuss the choice of materials for this pulsed magnet, material-based limitations, long standing experience with available materials, and material challenges for generating even higher magnetic fields by the non-destructive pulsed magnets.

[1] http://www.hzdr.de/hld

We subject an aged Cu-24 wt%Ag ingot to heavily cold drawing to develop a nanocomposite. The as-drawn wire is composed of Cu-rich proeuctectic components and Ag-rich eutectic components. A fine lamellar structure are developed in the proeuctectic component embedded a high density of Ag fibers. The average lamellar spacing is 20±6 nm. Ag fiber width is below 5nm. The eutectic component has a relative coarse structure with Ag grain size around 100 nm. This bimodal size distributed Ag fibers yield ultra-high bending plasticity—up to 59% bending strain at the outermost edge-- 15 times the tensile elongation of the wire (3.6%). During our bending tests, dynamic recovery and partial recrystallization occurr primarily in the eutectic component, and are more frequently at the inner edge than the outer edge. The high bending strain causes some of the thick Ag fibers to become discontinuous and lose their original alignment. This structural evolution increases local plasticity, resulting in unexpectedly high achievable bending strain. This is unusual in nano-sized, Ag-fiber-reinforced high-strength composite.

The cold spray process was developed at the end of the 20th century and his principle is mainly based on the kinetic energy transmitted to micron size particles to allow their deposition. The particles remain solid and the deposits are generated by the particles impacting the substrate at high velocity. Working at low temperature, below the melting point of the deposited material, appears particularly interesting for deposition of oxygen sensitive materials like copper alloys. In comparison with conventional thermal spray processes, cold spray can retain the original properties of powder feedstock (grain size, nanostructured particles), prevent the adverse influence on the underlying substrate materials, and can produce very thick coatings due to compressive stresses.
To spray oxygen sensitive material like copper alloys, it is preferable to select an inert gas as propellant gas for cold spray deposition. Nitrogen is mainly used but, as the gas velocity is strongly linked to the gas mass, gases with low atomic weight like hydrogen or helium are particularly interesting to reach high velocities. Among those gases, helium is much better from the human safety point of view but its price is very high due to limited amounts on earth. A closed loop avoiding costly helium losses was implemented. Thus, deposition efficiency of cold spray copper alloys higher than 96 %, porosity below 0.1 % and oxygen content below 150 ppm were obtained. High deposition rates up to 7 kg/h were also reached.
Copper silver alloys present attractive mechanical properties combined with high thermal conductivity suitable for many applications like high field magnets. In the present work, CrCrZr and CuAg with various Ag percentage deposits were manufactured by cold spray using helium as propellant gas. The work hardening mechanisms are involved by comparison with classically forged materials. A large tailoring of the material’s properties can be obtained through specific heat treating. Residual stresses are very low after heat treating and allow machining helices without deformation. This process now allows to produce tubes for polyhelix magnets at LNCMI with a major performance gain.

There is a demand for stronger yet lighter conducting copper wires in high-technology domains of global interest where weight is at a premium, such as power and aerospace engineering as well as in niche applications, such as materials for high-field magnets.
Many proposed solutions involve the preparation of copper-matrix composite wires or copper-rich alloyed wires with ultrafine but complex microstructures. Wires are prepared by wire-drawing billets (cylinders). This is a severe plastic deformation method and may be followed by appropriate thermal treatments. There are many variants but the importance of the characteristics of the cylinder is not to be neglected. The usual routes for cylinder preparation involve melting-solidification at some point, and therefore a relatively high temperature, either for alloying or for the preparation of the pure copper sample to be used in a composite.
We have proposed an alternative method based on powder metallurgy routes, involving the consolidation of a suitable powder into a cylinder by sintering at a relatively low temperature. This is done by spark plasma sintering (SPS), which has several advantages over pressureless sintering and hot-pressing, including rapid heating and cooling rates. Therefore, lower sintering temperatures and shorter holding times make it possible to avoid grain growth and prepare copper cylinders with micrometer-sized grains, typically ten times lower than conventional copper cylinders.
SPS typically differs from hot-pressing by the application of a dc pulsed current to the pressing die and powder compact, while a uniaxial pressure is simultaneously applied. Interestingly, cylinders with only about 90% of their full density were found suitable for the rest of the study, because a too high density hampers the deformability of the cylinder during wire-drawing, resulting in sample breaking.
The basics of SPS will be presented and we will show several examples of the preparation of wires of pure copper, carbon nanotube - copper composites and silver nanowire - copper composites. The microstructure of the powders, cylinders (typically 8 mm in diameter and 33 mm long) and wires will be presented as well as the mechanical properties and electrical resistivity of the wires measured at 293 K and 77 K.

Advanced electrical and electronic devices sometimes require that conductor’s material properties should be counteracting, e.g. high electrical conductivity and high mechanical strength. Traditional high strength Cu-matrix composites consists of Cu-based matrix, which contains a uniformly distributed nanoscaled particles of intermetallic compounds or oxides. The required microstructure is usually formed by the doping of the initial Cu matrix alloy by the combination of elements which would form the intermetallic compounds during the aging heat treatment at specific temperature range of 300-500 ^o C (Cu-Zr-Nb; Cu-Ni-Si; Cu-Cr-Zr alloys) or to form fine oxides through the process of internal oxidation (Cu-Al alloys). These submicron particles of oxides and intermetallic compounds altogether with increase of ultimate strength lead to substantial decrease of fatigue properties. The paper describes the alternative approach to development of the advanced high strength Cu-matrix composites with the application of the plastic nanostructured Cu-Nb in situ microcomposites as a strengthening element. As an example the principally new design of composite Contact Wire for high speed railways has been proposed to attain at least 20% better combination of ultimate tensile strength (610-660 MPa) and high conductivity (more than 80% IACS). Composite conductor contains the bundles of nanostructured anomalously high strength Cu-Nb filamentary cores and outer layer made of low doped high conductivity Cu-Zr-Nb alloy with improved tribological properties. The microcomposite Contact Wire has very high fatigue properties due to filamentary architecture of microstructure of Cu-Nb strengthening component. This Contact wire was designed for the use in the scope of Overhead Contact Line CS-400 for railways with the speeds of 400 km per hour. The results of laboratory complex tests of contact wire are presented.

LNCMI-Toulouse produces some of the most intense non-destructive pulsed magnetic fields in the world with a European record of 98.8 Tesla and aims at reaching more than 100 Tesla. The generation of pulsed magnetic fields higher than 60 T requires the use of coils wound of wires with exceptional properties. The wires need a high conductivity to limit the heating and they must have a high strength to be able to resist the Lorentz forces. The Lorentz forces are tangential to the coil and proportional to the square of the magnetic field. The resulting von Mises stress is 1 GPa at 60 T and higher than 2.2 GPa at 100 T.

Classical methods for strengthening copper wires involve the preparation of composite wires, such as copper-stainless steel (Cu-SS) and copper-niobium (Cu-Nb) or alloyed wires such as copper/silver (Cu/Ag). Cu-Nb and Cu/Ag wires present a nanostructured microstructure with fine and elongated grains. Cu-Nb microstructure is made up of pure Cu and pure Nb grains, while the Cu/Ag microstructure presents Cu-rich and Ag-rich solid solution. These wires exhibit a UTS value higher than 1 GPa but they are distinguished by their high resistivity. The aim of this work is the preparation of composite wires based on pure Cu and pure Ag.

LNCMI and CIRIMAT explore the design and preparation of novel Cu-based nanocomposite wires with the use of powder metallurgy, spark plasma sintering (SPS) and wire-drawing at room temperature. This preparation method is adapted to the preparation of Ag-Cu composite wires. Ag nanowires (200 nm in diameter and 30 µm in length) were synthetized and mixed with a commercial micrometric (about 1µm) Cu powder. Samples containing 1, 5 and 10 vol. % Ag were prepared. Cu and Ag-Cu cylinders (diameter 8 mm, length 33 mm) are prepared by SPS. Cylinders are sintered at 400, 500 or 600 °C. The so-obtained cylinders serve as precursors to wire-drawing. Their diameter is reduced by wire-drawing at room temperature, in several passes, thus producing progressively finer wires (diameter in the range 1 - 0.2 mm).

The wires show a lamellar microstructure with ultrafine Cu grains (200 - 700 nm for a 0.5 mm diameter wire) elongated over several micrometers. The Ag nanowires are dispersed along the grain boundaries of Cu. Cu and Ag grains exhibit major <111> and minor <100> orientations like most of wire-drawn face centered cubic metals.

The electrical resistivity and tensile strength were measured at 293 K and 77 K. The tensile strength for the composite wires is more than twice the value measured for the corresponding pure Cu wires. Interestingly, the wires containing only 1 vol.% Ag offer the best combination of high strength (1100 MPa at 77 K) and low electrical resistivity (0.50 µΩ.cm at 77 K).

We have obtained wires with excellent mechanical properties while greatly reducing the electrical resistivity compared to Cu/Ag alloy. The present 1 vol. % Ag-Cu composite wires compare favorably with Cu/Ag alloy wires containing about 20 times more Ag.

Nanostructured reinforcement materials, which have been successfully used in high field magnets and at cryogenic temperatures, reach their most desirable strength level when their scale of microstructure is below 100 nanometers. This refined microstructure can be achieved by plastic deformation and heat treatment, which create obstacles, such as dislocations and precipitates, for dislocation motions. Both deformation and heat treatment also introduce microstructure and property anisotropy. Investigating both anisotropy and microstructure changes helps us to understand the plasticity of these materials, particularly at cryogenic temperatures. The goal of our research is to relate microstructural scale and anisotropy to the mechanical and physical properties of these materials. This presentation describes microstructure in different orientations, with emphasis on the impact of high anisotropy and fine microstructure on properties.
This work was performed at the National High Magnetic Field Laboratory, which is supported by the National Science Foundation Cooperative Agreement No. DMR-1644779 and the State of Florida.

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature [1].
Fermi-surface, transport and thermodynamic signatures of the pseudogap phase are reminiscent of a transition into a phase with antiferromagnetic order, but evidence for an associated long-range magnetic order is still lacking. Here we report measurements of the thermal Hall conductivity κ_xy in the normal state of four different cuprates and show that a large negative κ_xy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p*[2]. It is also a property of the Mott insulator at p ≈ 0, where κ_xy has the largest reported magnitude of any insulator. Since this negative κ_xy signal grows as the system becomes increasingly insulating electrically, it cannot be attributed to conventional mobile charge carriers. Nor is it due to magnons, since it exists in the absence of magnetic order.
In order to identify the heat carriers responsible for this negative κ_xy, we turn to transverse heat transport along the c-axis: κ_zy. Here we show that the thermal Hall conductivity at p = 0 is roughly isotropic, being nearly the same for heat transport parallel and normal to the CuO₂ planes, i.e. κ_zy(T) ≈ κ_xy(T). This shows that the negative thermal Hall response must come from phonons, these being the only heat carriers able to move as easily normal and parallel to the planes. At p > p*, we observe no c-axis Hall signal, i.e. κ_zy(T) = 0, showing that phonons have zero Hall response outside the pseudogap phase. The microscopic mechanism by which phonons become chiral in cuprates remains to be identified. This intrinsic phonon Hall effect provides a new window on quantum materials and it may explain the thermal Hall signal observed in other topologically nontrivial insulators [4].

[1] Proust & Taillefer, Annu. Rev. Condens. Matter Phys. 10, 409 (2019).
[2] Grissonnanche et al., Nature 571, 376 (2019).
[3] Grissonnanche et al., Nature Physics (to appear); arXiv:2003.00111 (2020).
[4] Kasahara et al., Nature 559, 227 (2018).

Since the prediction of metallization and superconductivity of hydrogen and hydrogen-rich materials at high pressures /1,2/, there has been a sustained effort to make advances in high pressure sciences (theory, experiments and techniques) to realize this dream. In comparison to the much larger research landscape spawned by the discovery of high T_c cuprate superconductors, the high pressure (and high temperature) superconductivity field has been slow to encapsulate the attention of many. Partly, this has to do with the paucity of experimental probes that can be broguth to bear and to a major extent, it is the question of transitioning discovery to application. Nevertheless, the discovery of superconductivity in H₃S, YH₆ and LaH₁₀ /3,4,5/ and the phenomenal rise in transition temperatures has begged the question of revisiting these two questions. These discoveries have been aided by theoretical predictions and those can be used to make technological feasibility a likelihood /6-9/. However, there are experimental limitations that need to be surmounted to understand the physics and chemistry of these materials that constitute and important feedback to these simulations /10,11/. This talk will aim at connecting these dots and discus the rapidly evolving picture we have of these fascinating materials.

References:
1. Ashcroft, N. W., Metallic hydrogen: A high-temperature superconductor? Phys. Rev. Lett. (1968) 21, 1748.
2. Ashcroft, N. W., Hydrogen Dominant Metallic Alloys: High Temperature Superconductors? Phys. Rev. Lett. (2004) 92, 187002.
3. Drozdov, A. P., et. al, Conventional Superconductivity at 203 Kelvin at High Pressures in the Sulfur Hydride System. Nature, (2015) 525, 7567
4. Kong, P., et. al, Superconductivity up to 243 K in yttrium hydrides under high pressure. (2019) arXiv:1909.10482
5. Somayazulu. M. S., et. al, Evidence for Superconductivity above 260 K in Lanthanum Superhydride at Megabar Pressures. Phys. Rev. Lett, (2019) 122, 027001.
6. Y. Li, et. al, The Metallization and Superconductivity of Dense Hydrogen Sulfide, J. Chem. Phys. (2014) 140, 174712.
7. Hanyu Liu et. al, Potential high-T_c superconducting lanthanum and yttrium hydrides at high pressure, PNAS (2017), 114, 6990.
8. Ying Sun et. al, Route to a Superconducting Phase above Room Temperature in Electron-Doped Hydride Compounds under High Pressure, Phys. Rev. Lett., (2019) 123, 097001.
9. Jose A Flores-Livas et. al, A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials, (2019) arXiv:1905.06693
10. Shirin Mozaffari et. al, Superconducting phase diagram of H3S under high magnetic fields, Nat. Comm. (2019), 10. 2522. 10.1038/s41467-019-10552-y.
11. A. D. Grokowiak et. al, Hot Hydride Superconductivity above 550 K, (2020) arXiv:2006.03004

Bubble – a precursor of undersense nanostructure or fuzz formation is crystal lattice defect and is approached by Frenkel pair formation mechanism. One needs to have crystal lattice that inhibits Frenkel Pair generation by ejection of W self-interstitial atom by He cluster and its bonding with vacancy. However, this mechanism is very commonly observed in nuclear materials. It is applicable in conventional nuclear materials where α and β particles, γ rays and neutron flux strike the material. Plasma does not constitute of aforementioned and thus it is not very applicable mechanism. In the present study, author addresses this problem of development of plasma facing materials by primitive method of self-interstitial solid solution strengthening. Actual mechanism of bubble formation in studied and a suitable lattice is selected and proposed which inhibits possible Frenkel pair generation thus inherently promotes formation of plasma facing material.

Helium is the most inert element in the periodic table and generally considered to be unreactive under ambient conditions. On the other hand, helium is one of the main components of giant icy planets’ atmosphere. It is unclear whether the helium can diffuse into the depths and react with the mantle materials (water, ammonia and methane). In this talk, I will introduce some of our recent theoretical predictions on helium compounds, including helium-water, helium-ammonia, helium-methane and helium-alkali metal oxides/sulfides, etc [1-4], using our own machine learning accelerated crystal structure search method [5]. In addition, we also found that some of these compounds exhibit exotic dynamical properties at high pressure and high temperature, including the superionic, plastic states and their co-existence.

REFERENCE
Hao Gao, Cong Liu, Andreas Hermann, Richard J. Needs, Chris J. Pickard, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, "Coexistence of plastic and partially diffusive phases in a helium-methane compound", Natl. Sci. Rev. (2020). DOI: 10.1093/nsr/nwaa064
Cong Liu, Hao Gao, Andreas Hermann, Yong Wang, Maosheng Miao, Chris J. Pickard, Richard J. Needs, Hui-Tian Wang, Dingyu Xing, and Jian Sun*, “Plastic and Superionic Helium Ammonia Compounds under High Pressure and High Temperature”, Phys. Rev. X 10, 021007 (2020).
Cong Liu, Hao Gao, Yong Wang, Richard J. Needs, Chris J. Pickard, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, “Multiple superionic states in helium-water compounds”, Nature Physics 15, 1065 (2019).
Hao. Gao, Jian Sun*, Chris. J. Pickard, Richard. J. Needs, “Prediction of pressure-induced stabilization of noble-gas-atom compounds with alkali oxides and alkali sulfides”, Phys. Rev. Mater. 3, 015002 (2019).
Kang Xia, Hao Gao, Cong Liu, Jianan Yuan, Jian Sun*, Hui-Tian Wang*, Dingyu Xing, “A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure search”, Sci. Bull. 63, 817 (2018).

2-dimensional (2-D) sharp interfaces with distinct boundaries demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for enhanced behaviors such as strength, radiation damage tolerance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. 3-D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. Here, we focus on the thermal stability and mechanical behavior of nanolayered Cu/Nb containing interfaces with 3-D character. By co-sputtering the bimaterial interfaces between the constituent pure phases, the resulting compositional gradient gives rise to new interphase boundary structures, which have been analyzed and quantified via S/TEM and Atom Probe Tomography. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3-D interfaces is significantly greater than those containing 2-D interfaces. Mechanical anisotropy, as well as shear banding is observed during pillar compression with retention of continuous layers across the shear band. We will present our recent results on deformation of such 3-D interfaces and structures, and describe their structural evolution mechanistically through the use of atomistic simulations.

The high-temperature superconductors (HTS), e.g., RE-Ba₂Cu₃O_x (RE = rare earth) can sustain significant transport current at high background magnetic field and elevated temperatures, enabling various applications including fusion energy generation, next generation high-energy physics particle accelerators, and medical accelerators for carbon therapy. However, reliable protection against catastrophic superconducting-to-normal transitions (quenches) remains to be addressed for successful applications of HTS magnet technology. To address this issue, we investigated the potential of vanadium oxide (VO_x) coatings as a temperature-dependent switching medium to self-regulate current sharing. VO_x coatings (1.70 ≤ x ≤ to 2.07) were deposited by reactive cathodic arc deposition on insulating glass and commercial REBCO tapes to study their electrical properties. The process gas was monitored in situ by a residual gas analyzer. Effects of varying process parameters such as argon-to-oxygen ratio and substrate manipulator speed on coating properties were studied. The coatings were X-ray amorphous but Raman spectrometry showed short-range crystalline ordering. Hall effect measurements found a decrease in resistivity of VO_x by at least three orders of magnitude when the temperature increased from 80 to 300 K. The coating process did not damage the REBCO tapes as evidenced by a negligible change in critical current after versus before coating. The results from current-sharing experiments and circuit analysis suggest that the VO_x coating can effectively self-regulate current sharing in REBCO magnets and enable self-protection during quenches. This work was supported by the Office of Science, Office of Fusion Energy Sciences, U.S. Department of Energy under Contract No. DE-AC02- 05CH11231 and the Laboratory Directed Research and Development (LDRD) funding from the Lawrence Berkeley National Laboratory provided by the Director. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

*Corresponding author: [email protected]

When a beam of swift heavy ions (SHI) impinges onto a material, the formation of defects called ion tracks is possible as a result, either in the bulk or on the surface of the material. As a consequence, effects produced by means of SHI irradiation that lead to structural changes of the material can be used to modify the properties of the material via defects engineering.
SHI damage introduction to materials can result in complex behavior, especially in radiation harsh environments. To assess the final damage of the material, the effect of the energy deposition from the ions to the atoms within the material must be taken under consideration. Deposition of energy occurs through two main channels: either to the electronic subsystem of the material (electronic stopping power) or to the nuclei (nuclear stopping power). Here we investigate the influence of ion beams with different stopping powers, used in a sequence, on defects formation. It is known that sequential irradiation of ion beams can lead to the SHIBIEC effect (Swift Heavy Ion Beam Induced Epitaxial Crystallization) i.e. the annealing of defects that occurs during the second irradiation step. Alternatively, the anti-SHIBIEC effect is also possible, i.e. the enhanced defect formation during the second step of irradiation.
Sequential ion irradiation of GaN has not been investigated so far. This material is an important semiconductor for high-power electronics. The results presented here are of importance for basic research in this field as well. In our previous work [1], we found no evidence of ion track formation in the bulk after SHI irradiation using 23 MeV I and 90 MeV Xe beams. However, recently we were able to introduce additional disorder into moderately damaged GaN crystals using the same 23 MeV I and 90 MeV Xe beams, in cases where GaN samples have been pre-irradiated with 2 MeV Au or with 900 MeV Xe ion beams. Two different ion beams are expected to introduce damage into GaN either by nuclear or electronic stopping, respectively [2,3]. We observe the difference in total damage in the material depending on the fluence of the beams applied. In this contribution, we report new results of sequential ion irradiation of GaN based on RBS/c (Rutherford Backscattering Spectroscopy in Channeling) and TEM (Transmission Electron Microscopy) measurements.

[1] M. Karlušić et al., Response of GaN to energetic ion irradiation - conditions for ion track formation, J. Phys. D: Appl. Phys. 48 (2015) 325304
[2] S.O. Kucheyev et al., Ion implantation into GaN, Mat. Sci. Eng. 33 (2001) 51
[3] M. Sall et al., Track formation in III-N semiconductors irradiated by swift heavy ions and fullerene and re-evaluation of the inelastic thermal spike model, J. Mater. Sci. (2015) 50:5214–5227

The rapid evolution in technology and its integration with humans requires the development of novel processes that enable the incorporation of the crucial electronic materials, such as the wide bandgap semiconductors (Ga₂O₃ and GaN) into the wearable electroinc systems. Wide bandgap semiconductors form the building blocks of the current high-power high-frequency electronics and the upcoming mmWave fifth-generation (5G) device technologies. To make such technologies integrable with wearable/flexible systems, it is critical that high quality and in particular as-grown crystalline thin films of these materials can be produced on highly temperature-sensitive substrates at very low temperatures.
In this work, we report on the low-temperature, as-grown crystalline Ga₂O₃ films on various substrates (Si, sapphire, glass, and Kapton) via a hollow-cathode plasma-ALD system. Triethylgallium (TEG) and Ar/O₂ plasma were employed as metal precursor and oxygen co-reactant, respectively. Growth experiments have been performed at 160 – 240 °C substrate temperature and 30 – 200 W rf-power range. Additionally, each unit AB-type ALD-cycle was followed by an in situ Ar-plasma annealing treatment, which consisted of Ar-plasma exposure for 20 seconds scanned over 50 – 300 W rf-power. Both in situ and ex situ ellipsometry were employed to measure the thickness and optical properties of the films. The thickness of the films without Ar-annealing stage, ranged between 20.74 – 39.30 nm and as-grown refractive indices were between 1.75 - 1.67 within the scanned plasma power range. X-ray diffraction (XRD) showed that Ga₂O₃ films grown without in situ plasma annealing exhibited amorphous structure irrespective of both substrate temperature and rf-power. However, with the in situ Ar-annealing phase incorporated, the thickness of the films ranged between 22.9 – 31.4 nm with refractive indices of 1.75 – 1.79. The increased refractive index (1.79) and reduced thickness gain (31.4 nm) at 250 W Ar-annealing power indicates possible densification and crystallization of the films. Indeed, XRD and XRR confirmed that in situ Ar-plasma treated films grow in a monoclinic β-Ga₂O₃ crystal phase with further improving crystallinity and film density (from 5.07 to 5.80 g/cm³) with increasing Ar-annealing plasma power. Additionally, we will discuss the impact of varying in situ Ar-annealing plasma chemistries (Ar-only, O₂-only, and Ar/O₂) coupled with x-ray photoelectron spectroscopy (XPS) measurements, which will provide additional insight into the elemental composition of the films that might help to understand the changes in the structural, optical, and electrical properties. A future outlook will be provided to overcome the challenges to achieve device quality layers on temperature-sensitive flexible substrates.

Materials possessing a defect-fluorite pyrochlore structure can have a range of useful properties that are sought after, which include radiation tolerance, nuclear waste immobilization, and phase stability at elevated temperatures. In this study, we demonstrate for the first time the synthesis and a detailed analysis of defect-fluorite pyrochlore sodium niobate (NaNbO₃) nanoparticles. This analysis included an investigation into the stability of these nanoparticles at elevated temperatures, as well as to neutron irradiation. A surfactant-assisted solvothermal method was used to prepare nanoparticles of NaNbO₃. This solution-phase approach results in the formation of crystalline nanoparticles of a defect-fluorite pyrochlore NaNbO₃ at relatively low temperatures. The products had an average diameter of ~74 ± 11 nm. The nanoparticles adopted a defect-fluorite pyrochlore phase and matched the cubic space group. This pyrochlore form of NaNbO₃ was stable up to at least 500 ^oC. The nanoparticles transformed into the cubic and rhombohedral perovskite phases of NaNbO₃, along with the introduction of a pseudo-hexagonal Nb₂O₅ at higher temperatures. These defect-fluorite pyrochlore nanoparticles of NaNbO₃ also exhibited a resistance to radiation induced amorphization. The dimensions, phase, and crystallinity of the defect-fluorite pyrochlore nanoparticles after exposure to a flux of neutrons were comparable to the as-synthesized product. The thermal stability and radiation tolerance of these pyrochlore nanoparticles could be useful in the design of thermally resilient materials, high temperature catalysts, and durable materials for the handling and storage of radioactive waste.

Organosilicon polymer derived ceramic (PDC) materials and its derivatives have gained interest as an emerging class of materials capable of providing mechanical stability and heat dissipation upon operation in extreme environments. PDC's found various applications such as electrode material for Li-ion batteries, ultra-high temperature composites and others [1]. Being X-ray amorphous, PDC’s consist of nanodomains of different structures and compositions, demonstrating nanoscale heterogeneity that influence their unique mechanical, chemical and functional properties. Conventionally, PDC coatings are prepared by wet-chemical dipping, spinning thermal-spraying or by plasma-assisted chemical vapor deposition (CVD), followed by high temperature pyrolysis [2].
In this work, molecular layer deposited (MLD) hybrid organic-inorganic films serve as precursors for the synthesis of SiAlCO PDC composite coatings. Alumina-polysiloxane films were grown using sequential surface reactions between 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V₄D₄) and trimethylaluminum (TMA). The deposition was conducted at temperatures between 120 and 220 ^oC. In-situ quartz crystal microbalance (QCM) monitoring showed linear mass increase with the number of MLD cycles with the highest mass gain rate of 24 ng/cm²/cycle at 180 ^oC. The QCM study revealed a self-limiting surface chemistry. An infrared analysis (FTIR) of the deposited films showed vibrational features characteristic of organosilicon polymers: Si-CH₃, Si-O-Si, Si-CH=CH₂ and Si-O-Al peaks. High-resolution transmission electron microscopy (HRTEM) imaging confirmed the conformal nature of the deposited MLD film.
In order to increase mass yield during the polymer-to-ceramic transformation, cross-linking of the growing polymer chains is needed. Cross-linking was achieved at 200^oC by introducing a third precursor, di-tert-butyl peroxide (TBPO), into the MLD cycle, following a V₄D₄/TBP/TMA sequence. A positive mass gain after TBPO dosing, observed by QCM, indicates the formation of radicals and cross-linking. The cross-linking occurs through the activation of the vinyl groups. Characteristic signals, related to the generated polyethylene chains, were identified by ATR-FTIR and XPS. The formed Si-C-C-Si units are expected to improve the mechanical and thermal properties of the cross-linked SiAlCO PDCs, since the Si–C and C–C bonds are not affected by thermal depolymerization reactions.
X-ray and optical characterization techniques were employed to analyze the structural changes and composition of the films after pyrolysis. After pyrolysis at 900^oC in an Ar atmosphere, the composite thin film remained amorphous. The resulting film contained uniformly distributed Si, Al, O, and C. Raman spectroscopy showed presence of graphitic carbon within the film. This MLD-derived SiAlCO thin film is expected to have exceptional uniformity, high temperature stability, creep, and corrosion resistance.
[1] P. Colombo, et al., J. Am. Ceram. Soc., 93 (7), 1805–1837 (2010)
[2] G. Borroso, et al, J. Mater. Chem. A., 7 (5), 1936–1963 (2019)

Magnetic nanoparticles (MNPs) have attracted significant attention in the oil industry in the past decade due to their potential applications including reservoir sensing and mapping, enhanced oil recovery, drilling and completion improvement, oil-water separation in produced water, oil spill clean-up, and directional transport and local heating. For these specific applications, the dispersity and stability of the MNPs in suspension are of great importance. Surface coating and specific functionalization of the MNPs are typically used to improve their stability and to reduce aggregation for the successful use of the nanoparticles. However, many of the well-established methods and procedures of surface coating chemistry for lab applications are not suitable for harsh environmental conditions such as at extreme high salinity for industrial oil applications. In this research, we synthesized magnetite MNPs with tuned particle sizes to optimize their superparamagnetic properties, and developed a new procedure for surface functionalization to enhance their colloidal stability at high salinity. The functionalized magnetite MNPs have been characterized by XRD, HR-TEM, cryo-TEM, dynamic light scattering and zeta potentials. Colloidal stabilities have been studied in a wide pH range and salinity with 5000-230000 ppm of TDS (Total Dissolved Solids). The results have demonstrated that the covalently bonded zwitterionic functional groups and cross-linked saccharide highly enhance the colloidal stability in high salinity brines.

The polarization response of ionic and electronic defects in complex oxides under high electric field is critical for applications such as memristors and ferroelectrics, but historically challenging to model or measure. Here we developed a localized Hubbard U approach that resolves the dilemma of maintaining in-gap localized defect states for an oxygen vacancy while simultaneously retaining non-linear dielectric behavior of perovskite oxides. With this approach, we obtained accurate dielectric constant and electronic configuration for SrTiO₃ under electric field, using density functional theory and Berry phase calculations. We identified a direct relationship between oxygen vacancy polarizability and lattice strain, which can be generalized to other materials with similar crystal structure. For example, a neutral oxygen vacancy in BaTiO₃ and in SrTiO₃ strained to exhibit the same lattice constants showed identical polarization behavior under the same electric field. To trace the site polarizability difference between defected crystals and their defect-free counterparts, we utilized maximally localized Wannier functions to obtain the site-decomposed polarization. These simulations illustrated that not only the vacant site, but also the neighboring ions around the vacancy have significant influence on the system polarization with field applied either parallel or perpendicular to the Ti – V_O^x – Ti chain. Together, this model and identification of driving factors in defect displacement under electric field contribute generalizable approaches to study and optimize materials in memristors, ferroelectrics, electrocaloric refrigeration and flash sintering.

Selective solar absorbers with high performance are the key to concentrated solar power systems. Optical metamaterials are emerging as a promising strategy to enhance selective photon absorption, however, the high-temperature resistance (500 C) remains as one of the main challenges for practical applications. Here, a multilayered metamaterial system (Al₂O₃/W/SiO₂/W) based on metal-insulator-metal resonance effect has been demonstrated with high solar absorptance over 92%, low thermal emittance loss below 6%, and significant high-temperature resistance: it has been proved that the optical performance remains 94% after 1-hour thermal annealing under ambient environment up to 500 C, and 94% after 96-hour thermal cycle test at 400 C. Outdoor tests demonstrate that a peak temperature rise (193.5 C) can be achieved with unconcentrated solar irradiance and surface abrasion resistance test yields that SSAs have a robust resistance to abrasion attack for engineering applications.

Alumina-forming austenitic stainless steels (AFAs) are a class of alloys that are showing promise for use in high temperature applications, particularly for use in power generation. AFAs have been shown to demonstrate good high temperature strength, great oxidation resistance due to a protective alumina scale, and are cheaper than nickel-base alloys. While showing great promise, these alloys need to continue to be studied and improved to be practical for use. In particular, the deformation behavior of these alloys is not well understood. The austenitic matrix can contain various precipitates, including: L1₂ structured precipitates, B2 precipitates, Laves precipitates, carbides, and others. The grain boundary (GB) also can become covered by Laves and/or B2 precipitates. In a complex alloy with different types and sizes of precipitates, it can be difficult to determine what effect each of these precipitates are having on the high temperature strength.
In this work, we studied the deformation mechanisms in the model AFA alloy Fe-20Cr-30Ni-2Nb-5Al (at%). This alloy displays many characteristics of AFA alloys, and thus is a good alloy for scientific studies. This alloy contains L1₂, Laves, and B2 precipitates in the matrix as well as Laves and B2 precipitates on the GBs. Creep tests, high temperature tensile tests, and high temperature strain-rate jump tests were performed on this alloy after various heat treatments to determine the deformation mechanisms – including strain-rate jump tests with and without L1₂ precipitates (but otherwise similar microstructures) to determine the effect of these precipitates on the high temperature strength. In addition, tests were performed on alloys with different grain size, but otherwise similar microstructure, to determine the effect of GB precipitation. Analysis of the stress – strain rate relationships was used to predict deformation mechanisms and to fit the experimental data to models. In addition, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed on samples after mechanical testing to determine deformation behavior. In-situ straining experiments in the TEM and SEM were also performed. This study found that nano-sized L1₂ precipitates provided the majority of the strengthening in this alloy. At low stresses dislocations by-passed these particles by climb, and at high stresses dislocations started to form Orowan loops around the particles. While dislocations piled up at larger Laves and B2 precipitates, these precipitates were not as effective at strengthening the alloy. In addition, work was performed to determine if there was a GB strengthening effect due to coverage of the GB by precipitates.

Acknowledgements
This research was supported by National Science Foundation (NSF) Grant DMR 1708091.

Zinc ferrite nanoparticles are currently of much interest for their potential applications in extreme conditions like high temperature: e.g. in metal casting, in water splitting reaction for H₂ production (solar fuels) or as high - temperature desulfurizing sorbents [1-3]. Bulk ZnFe₂O₄ (ZF) should have paramagnetic nature around room temperature, when it is the normal spinel with Zn²⁺ incorporated mostly in the tetrahedral lattice sites. In the case of nanoparticles, the situation is more complex due to high probability of inversion in the cation distribution between octahedral and tetrahedral sites, which may lead to ferrimagnetism or superparamagnetism [4-6]. It was suggested that during heating Zn²⁺ ions can move from tetrahedral site to octahedral site whereas Fe³⁺ ions redistribute within the octahedral and tetrahedral sites in order to reduce the strain [7]. It was also speculated the possibility of reduction of Fe³⁺ to Fe²⁺ during high temperature treatment. In order to verify these hypotheses we have performed a comprehensive study of structural and magnetic properties of stoichiometric and non-stoichiometric zinc ferrite Zn_xFe_3-xO₄ (NZF) nanoparticles synthesized using co-precipitation and thermal decomposition method [2, 8] as well as their evolution after thermal treatment. Nanoparticles with different zinc to iron ratio were obtained and systematically probed with volume (XRD, VSM, TG/DSC), microscopic (TEM) and element sensitive probes (ICP-OES, Mossbauer spectroscopy, XPS, XAFS). Magnetic studies proved paramagnetic response of stoichiometric ZF nanoparticles, while a superparamagnetic behaviour is observed in strongly non-stoichiometric (as synthesized) nanoparticles. After thermal treatment at 1400 ^oC in inert atmosphere, significant changes in the saturation magnetization were observed in the case of NZF. This is related with an increase of the amount of formally Fe²⁺ ions. The estimated saturation magnetization of as synthesized NZF nanoparticles, of approx. 50 emu/g, is increased upon thermal treatment (at 1400 ^oC in inert atmosphere) up to approx. 130 emu/g. As such we have confirmed that zinc concentration and method of synthesis plays important role in magnetic properties of non-stoichiometric zinc ferrite nanoparticles. Moreover, the reduction of Fe³⁺ to Fe²⁺ during high temperature treatment has been observed in NZF nanoparticles annealed in inert atmosphere.
Acknowledgements: This work was supported by the National Science Centre, Poland, Grant No: 2016/23/D/ST8/00013.
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Actinide and lanthanide fluorite oxides, UO₂, CeO₂, ThO₂, form an important class of ceramic energy materials with applications ranging from nuclear fuels to solid oxide fuel cells. Of these systems, ThO₂ (thoria), has been the least explored and offers unique features including an extremely high melting temperature and a fixed tetravalent oxidation state. Many potential application environments for thoria include high radiation fields or temperatures which either promote or directly generate lattice defects. Such defects drastically influence thermal transport, a controlling safety and performance property in many applications. To date, little experimentally-validated understanding has been generated on the role of defect formation and clustering on thermal transport in thoria due to the lack of high-quality starting material in which these effects may be explored. In this work, high-quality oriented single crystals of thoria are grown using hydrothermal synthesis and then exposed to proton irradiation at both room temperature and 600C. The thermal conductivity of the defected surface layer in these crystals is measured using an all-optical spatial domain thermoreflectance (SDTR) technique and sub-electron-microscopy-resolution defects are characterized using Raman spectroscopy. Together, these methods provide important initial insight into the types of defects formed under ionizing radiation, defect clustering, and how these defects impact thermal transport in high-quality thoria.

Designing ever stronger nanostructured materials that can even withstand harsh environments has become a major goal in the last decades. However, the increase in strength commonly comes hand in hand with a severe reduction in ductility, which at the same time also limits the toughness of these materials. Due to the high fraction of interfaces in such nanostructured materials, it is safe to assume that the processes limiting ductility and toughness as well as their response to harsh environments are controlled by the respective interface structures. However, a detailed understanding is at this point still lacking.
Bulk nanostructured materials were synthesized by severe plastic deformation and subject to tailored interface modifications. To investigate the structural properties of these nanomaterials at high stress and strain, we utilize quantitative miniaturized deformation and fracture experiments performed in situ in scanning or transmission electron microscopes. These highly resoled techniques not only allow to measure material performance, but also to directly identify the underlying deformation and failure mechanisms at the same time.
Using this experimental approach, we will first address the nanoscale processes taking place during deformation and fracture of bcc nanocrystalline metals with modified interfaces. Building on this, the discussion will be extended to the behavior of nanocomposite structures and the influence of an irradiative environment.

The high-pressure chemistry of nitrogen and nitrogen-rich compounds have been in a focus of many studies in the recent years due to both fundamental and practical interest and due to the improvement of high-pressure synthetic and characterization techniques. Poly-nitrogen compounds are usually considered as potential high energy density materials (HEDM) due to the remarkable difference in the average bond energy between the single N–N bond, the double N=N bond , and the triple N≡N bond. Numerous polynitrogen compounds with various nitrogen polymeric networks were theoretically predicted, but there are only a very few experimental attempts to obtain such compounds.

In the present work we have systematically studied reactions between transition metals and nitrogen at pressures up to 130 GPa in laser-heated diamond anvil cells. Despite very high synthesis pressure Hf, W, Re and Os react with nitrogen forming porous metal-inorganic frameworks Hf₄N₂₂, WN₁₀, ReN₁₀ and Os₅N₃₄ of various topologies. The frameworks are built of metals interconnected via polymeric nitrogen linkers, while the pores are filled with dinitrogen molecules N₂. Lighter transition metals (Fe) form non-porous polynitrides featuring polymeric nitrogen chains, while alkali metals react with nitrogen at extreme conditions with the formation of pentazolate salts. In this contribution we will discuss novel synthetic routes to energetic polynitrogen compounds and crystal-chemical rules governing their formation.

Swift heavy ion irradiation and high-energy ball milling are far-from-equilibrium processing techniques which provide access to a unique phase space not achievable by means of traditional synthesis approaches. High-resolution neutron total scattering experiments were utilized to gain fundamental insight into the multi-scale structural details of A₂B₂O₇ pyrochlore oxides prepared by these extreme conditions and their subsequent evolution under high-temperature treatment. Pair distribution function analysis revealed unconventional atomic arrangements characterized by an unexpectedly high degree of local order. For instance, the atomic arrangement of ball-milled titanate pyrochlore oxides is nearly identical to the same pyrochlore composition synthesized by solid-state reaction and subsequently amorphized by intense ion irradiation (1). Interestingly, the same atomic arrangement is found in pyrochlore oxides that exhibit distinct long-range radiation responses (order-disorder versus crystalline-amorphous) (2). These phenomena are proposed to be the result of fundamental chemical rules which stringently dictate the atomic configuration in disordered materials (3, 4). This description provides insight into far-from-equilibrium processing of oxides and helps to better elucidate the structural properties and heterogeneity of disordered complex oxides from the local atomic arrangement to the macroscale.

1. E. C. O’Quinn et al., Advanced characterization technique for mechanochemically synthesized materials: neutron total scattering analysis. J. Mater. Sci. 53, 13400–13410 (2018).
2. J. Shamblin et al., Similar local order in disordered fluorite and aperiodic pyrochlore structures. Acta Mater. 144, 60–67 (2018).
3. D. Drey, Disorder in Ho₂Ti_2-xZr_xO₇: Pyrochlore to Defect Fluorite Solid Solution Series. RSC Adv. In Press (2020).
4. E. C. O’Quinn et al., Predicting short-range order and correlated phenomena in disordered crystalline materials. Sci. Adv. 6, 1–8 (2020).

Na1/2Y1/2Cu3Ti4O12 (NYCTO) ceramics with giant dielectric constant (ε′) were synthesized by simple reactive sintering. NYCTOnanopowder was first synthesized using high energy ball-mill. Then the pelletized powder was sintered in air at temperatures in the range 975 °C to 1050 °C for 10–20 h. The obtained ceramics showed pure CaCu3Ti4O12 (CCTO)-like cubic phase as revealed by x-ray diffraction measurements. Field effect-SEM observations showed that the grain size increases from 2 μmto5 μmwith increasing sintering temperature. NYCTO samples sintered at temperatures higher than 975 °C showed giant dielectric constant (103–104) over most of the frequency range. The minimum dielectric loss (tanδ) of∼0.055 at 300 K has been approved for the ceramic sample sintered at 1050 °C. Impedance and modulus spectra of the current samples showed two relaxations related to semiconductor (grain) and high resistance (grain-boundaries) elements. The activation energy for conduction located in the range 0.1–0.5 eV highlighted the role of single ionized oxygen vacancies in the dielectric properties of the investigated NYCTO ceramics.

Tungsten will be the plasma-facing material in the ITER divertor. Hydrogen blistering has been observed in plasma-facing materials, but to date, all electronic structure and molecular mechanics simulations have indicated that hydrogen atoms repel one another in perfect crystals. The influence of grain boundaries on hydrogen retention is also not well-understood. We conducted simulations intended to elucidate the mechanisms of blister formation. These studies were accelerated using direct implantation, as we have used in the past for helium, so that computational time ordinarily required to simulate hydrogen atoms that reflect is instead used to simulate diffusion. Consistent with prior observations, we observe that hydrogen is generally repelled by other hydrogen atoms in single-crystal tungsten and we observe neither self-clustering nor cluster growth of any kind. Near grain boundaries, however, the excess volume present allows hydrogen to cluster. Eventually, sufficient hydrogen concentration can build up and weaken the grain boundary, possibly resulting in delamination of the grains and forming a blister.

The present work focuses on understanding the microstructural development during sintering of nanopowders under pressure and electric fields/currents. For this purpose, a method has been developed which combines flash sintering (FS) and spark plasma sintering (SPS) [1]. This FS-SPS combination enables us to combine the advantages of both techniques: to force the electric current through the sample (FS) and to sinter pellet-shaped samples under mechanical loads (SPS). We explored the novel device on the model material Al doped ZnO and discovered a different microstructure, compared to conventional SPS. Grain growth and second phase formation were emphasized by FS-SPS. It remains controversial if the observed microstructure is a result of an intrinsic electric current/field effect or of a temperature-distribution. However, this method helps to understand mechanisms involved in electric current or field assisted sintering techniques and possibly produces new microstructures and, hence, materials.

Acknowledgement:
We acknowledge the support from the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft)—Priority Programme (SPP, Schwerpunktprojekt) 1959/1 “Manipulation of matter controlled by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials” with Project Nos. WI 981/16-1 and WO 577/13-1.

References:
[1] C. Gorynski, U. Anselmi-Tamburini and M. Winterer, Rev. Sci. Instrum. 91, 015112 (2020); doi: 10.1063/1.5119059