GPa Pressure Research Articles

Pressure-dependent structural, electronic, optical, and mechanical properties of superconductor [formula omitted]: A first-principles study

The structural, electronic, mechanical, thermodynamic, and superconducting properties of a noncentrosymmetric heavy-fermion CeRh2As2 superconductor under pressure were studied using first-principles calculations based on density functional theory. The results indicate that the CeRh2As2 superconductor is mechanically and thermodynamically stable in the pressure ranges of 0–40 GPa. For the first time, this analysis provides invaluable, detailed insights into various physical features of this recently reported superconducting material CeRh2As2. It belongs to the well-known CaBe2Ge2-type family. The rising trend of elastic moduli with applied pressure indicates that CeRh2As2 stiffens up under applied pressure. Ductility of the studied superconductor rises significantly with pressure. The study of various anisotropy indices show that the compound is significantly anisotropic under ambient or applied pressure. Various mechanical features of CeRh2As2 are analyzed according to the results of elastic constants and adequately explained. The melting temperature rises with the applied pressure, making CeRh2As2 more suitable for high temperature applications. The electronic properties investigation greatly supports the calculated optical functions. As the applied pressure increases, the absorption and reflectivity spectra shift to higher energy regions. The computed TDOS at 0 GPa pressure at EF is ∼5.07 states/eV/f.u. and applied pressure has a negligible effect on the value of DOS. A pressure-induced increasing trend in the Debye temperature has also been observed.

Mechanical Properties of Nano SiC Reinforced Aluminum 7075-T6 Produced by Warm Powder Compaction using an Inert Gas

The micron-sized Al7075powder reinforced with nano-sized SiC particles is compacted at 4000C in an inert atmosphere using Nitrogen. It is shown that this technique can significantly improve the mechanical properties of the compacted powder characterized by measuring hardness, stress-strain curves and density. The maximum relative density was %98.8 and obtained for 2.3 GPa pressure and 0%SiC content. The minimum density was %93.95 and was obtained for 10%SiC content and 1.3 GPa pressure. The degree of porosity and the number and size of the void decreased as the compaction pressure increased and increased with the increase of SiC volume fraction. The maximum increase in ultimate strength was about 72% and occurred for the 5%SiC content and the 1.3 GPa compaction pressure. For 10%SiC volume fraction the strength improved by 53% for 2.6 GPa pressure. The effect of SiC content on ultimate strength was significantly higher than that of the compaction pressure. The maximum increase in Vickers hardness of the compacts was 100% and obtained for the samples with 10%SiC volume fraction and compacted at 2.6 GPa pressure. Again, the effect of SiC volume fraction on hardness improvement was found to be remarkably greater than that of compaction pressure.

Phase stability and thermoelectricity in topological Dirac semimetal Na3Bi

A vibrant research field in condensed matter is the study of topological materials, which range from topological insulators to Dirac semimetals and Weyl semimetals. Na3Bi is predicted to be Dirac semimetal with a Dirac node near the Fermi level. Hence, to validate the presence of Dirac cone and to check the stability of Na3Bi in cubic and hexagonal phases for its practical applications an ab-initio calculation has been performed. A structural phase transition has been observed from hexagonal to cubic phase at 0.89 GPa pressure for Na3Bi. The computed elastic constants satisfy Born-Huang stability criteria for both phases indicating the mechanical stability of the present compound. But it has been found that the P63/mmc phase of Na3Bi is dynamically unstable at ambient conditions therefore, the thermoelectric properties of c-Na3Bi have been investigated by employing Boltzmann transport theory. The low lattice thermal conductivity (1.89 W/mK) and high value of the figure of merit (0.32) at 1200K suggest a good thermoelectric response of the material at high temperatures. Hence, Na3Bi is a potential material that can be used in quantum electronic and thermoelectric devices.

Theoretical Predictions on Superconducting Phase above Room Temperature in Lutetium-Beryllium Hydrides at High Pressures

High-pressure structural search was performed on the hydrogen-rich compound LuBeH8 at pressures up to 200 GPa. We found an structure that exhibits stability and superconductivity above 100 GPa. Our phonon dispersion, electronic band structure, and superconductivity analyses in the 100–200 GPa pressure range reveal a strong electron–phonon coupling in LuBeH8, while the superconducting critical temperature T c shows a decreasing trend as the pressure increases, with T c = 255 K at 200 GPa and maximal T c = 355 K at 100 GPa. This study demonstrated the room-temperature superconductivity in -LuBeH8, thus enriching the family of ternary hydrides. These findings provide valuable guidance for identifying new high-temperature superconducting hydrides.

Dissolution-precipitation synthesis and cold sintering of mussel shells-derived hydroxyapatite and hydroxyapatite/chitosan composites for bone tissue engineering

In the present work, seafood by-products and derivates were exploited as raw materials to produce nanocrystalline calcium phosphates-based composites in light of the rising demand for waste recovery and valorisation. Mussel shells were transformed into hydroxyapatite by dissolution-precipitation synthesis at 45 °C, whereas chitosan from shrimp shells was introduced as a reinforcing biopolymer to produce hydroxyapatite/chitosan composites. The synthesised hydroxyapatite and hydroxyapatite/chitosan composite powders were cold sintered at room temperature under 1 GPa pressure for 10 min. The materials were consolidated up to ∼90% relative density and characterized mechanically. By increasing the polymer content up to 10 wt%, the flexural strength of the sintered pellets increases from ∼45 MPa to ∼57 MPa while the hardness decreases from ∼1.1 GPa to ∼0.8 GPa, thus better addressing the mechanical properties of cortical bone. Furthermore, hydroxyapatite/chitosan composites were proven to be bioactive, this demonstrating their potential use in bone tissue engineering applications.

Effect of Si on the evolution of plasticity mechanisms, grain refinement and hardness during high-pressure torsion of a non-equiatomic CoCrMnNi multi-principal element alloy

The present study unraveled the defining role of small silicon (Si) addition (5 atomic %) in dramatically altering the plasticity mechanisms, grain refinement and hardening response of a non-equiatomic CoCrMnNi multi-principal element alloy (MPEA) during high-pressure torsion (HPT) processing. Both the Si-free and the Si-added MPEAs had a face-centered cubic (FCC) structure and were subjected to a quasi-constrained HPT processing at 6 GPa pressure to different number of turns (0.5 and 5). Microstructure evolution was studied at the center and edge of the HPT-processed discs using X-ray diffraction line profile analysis (XLPA) and transmission electron microscopy (TEM). Si addition altered the predominant plasticity mechanism from micro-band formation to extensive occurrence of nano-twinning at the early stage of HPT processing. At later stages of HPT processing, both alloys exhibited deformation twinning but its propensity was considerably higher for the Si-added MPEA, as revealed by ∼50% higher twin fault probability. Additionally, the Si-added MPEA showed ∼30% higher dislocation density at any given stage of HPT processing compared to the Si-free MPEA. A significantly accelerated nano-structuring coupled with a finer saturation grain size was observed in the Si-added MPEA (34 nm for Si-free versus 23 nm for Si-added). These effects can be explained by the influence of Si addition on lowering the stacking fault energy (SFE) (from ∼40 mJ/m2 in Si-free to ∼20 mJ/m2 in Si-added MPEA) and increasing the solute pinning effect of Si on lattice defects. The plasticity mechanisms at nano-scale were also influenced by the presence of Si as confirmed by the formation of nano-twins and stacking faults inside the nano-grains for the Si-added and Si-free MPEAs, respectively. The differences in plasticity mechanisms and microstructure evolution resulted in enhanced hardness in the early stages of HPT processing for the Si-added MPEA, but the difference in hardness between the two alloys tended to be reduced at higher strains.

Structural phase stability, elastic properties, and electronic structure of pressure-induced CaC2: A first-principles study

This work investigates the structural phase transition of CaC2 material under 0–150 GPa pressure by using first-principles calculation. The enthalpy difference results suggest that the C2/c structure transforms into the Cmcm structure at 0.08 GPa, while the C2/m structure transforms into the Cmcm structure at 0.11 GPa. The Cmcm structure transforms into the Immm structure at 13.70 GPa and the P6/mmm structure at 110.65 GPa. Moreover, the enthalpy values of C2/c, C2/m, I4/mmm, and Cmcm are very close between 0 and 0.11 GPa, indicating that these four structures may coexist in this pressure range. The change of volume with pressure shows that the volume decreases with the increased pressure, especially it decreases sharply at the pressure transition point. In addition, the results show that the elastic modulus of different structures increases linearly with the increase of pressure in their respective pressure stable ranges, but it will increase sharply at the pressure transition point, indicating that CaC2 exhibits better stiffness under high pressure than under low pressure. Finally, the change of the electronic structure of CaC2 with pressure is also analyzed, the results show that C2/c and C2/m are indirect band gap semiconductor structures between 0–0.11 GPa, but the band gap becomes 0 ⅇV at the first pressure transition point 0.11 GPa, and it is always 0 ⅇV in the pressure range of 0.11–150 GPa, which means that it becomes a metal phase. This work will provide a valuable reference for the experiment and application of CaC2 under high pressure.

Deformation of two-phase aggregates with in situ X-ray tomography in rotating Paris-Edinburgh cell at GPa pressures and high temperature.

High-pressure (>1 GPa) torsion apparatus can be coupled with in situ X-ray tomography (XRT) to study microstructures in materials associated with large shear strains. Here, deformation experiments were carried out on multi-phase aggregates at ∼3-5 GPa and ∼300-500°C, using a rotational tomography Paris-Edinburgh press (RoToPEc) with in situ absorption contrast XRT on the PSICHE beamline at Synchrotron SOLEIL. The actual shear strain reached in the samples was quantified with respect to the anvil twisting angles, which is γ ≤ 1 at 90° anvil twist and reaches γ ≃ 5 at 225° anvil twist. 2D and 3D quantifications based on XRT that can be used to study in situ the deformation microfabrics of two-phase aggregates at high shear strain are explored. The current limitations for investigation in real time of deformation microstructures using coupled synchrotron XRT with the RoToPEc are outlined.

An X-ray and Neutron Scattering Study of Aqueous MgCl2 Solution in the Gigapascal Pressure Range

The structure of electrolyte solutions under pressure at a molecular level is a crucial issue in the fundamental science of understanding the nature of ion solvation and association and application fields, such as geological processes on the Earth, pressure-induced protein denaturation, and supercritical water technology. We report the structure of an aqueous 2 m (=mol kg−1) MgCl2 solution at pressures from 0.1 MPa to 4 GPa and temperatures from 300 to 500 K revealed by X-ray- and neutron-scattering measurements. The scattering data are analyzed by empirical potential structure refinement (EPSR) modeling to derive the pair distribution functions, coordination number distributions, angle distributions, and spatial density functions (3D structure) as a function of pressure and temperature. Mg2+ forms rigid solvation shells extended to the third shell; the first solvation shell of six-fold octahedral coordination with about six water molecules at 0 GPa transforms into about five water molecules and one Cl− due to the formation of the contact ion pairs in the GPa pressure range. The Cl− solvation shows a substantial pressure dependence; the coordination number of a water oxygen atom around Cl− increases from 8 at 0.1 MPa/300 K to 10 at 4 GPa/500 K. The solvent water transforms the tetrahedral network structure at 0.1 MPa/300 K to a densely packed structure in the GPa pressure range; the number of water oxygen atoms around a central water molecule gradually increases from 4.6 at 0.1 MPa/298 K to 8.4 at 4 GPa/500 K.

Electrical resistivity of FeSi alloy under high pressure

The electrical resistivity of FeSi alloy under the influence of pressure is studied based on the Matthiessen's rule and saturation electrical resistivity model. Numerical calculations were conducted for FeSi up to a pressure of 100 GPa and compared with experimental data when possible. The results show that the electrical resistivity of the material diminishes gradually with pressure and saturates at high pressure. Furthermore, in this paper, we also investigated the effects of impurity concentration on the electrical resistivity of FeSi alloy. Our numerical calculations conducted for 40 and 80 Gpa pressures show that when the Si concentration is less than 20%, the electrical resistivity of the alloy increases rapidly and linearly with the Si concentration. Our electrical resistivity values are consistent with those of the recent experimental measurements.

Shock response of unidirectional carbon polymer composite up to pressures of 200 GPa

Carbon fiber reinforced composite materials are often used to protect spacecraft from hypervelocity impacts with micrometeoroids or space debris. Therefore, shock-wave properties of these materials at pressures corresponding to orbital impact velocities are of interest. Experimental studies of shock compressibility of unidirectional carbon fiber polymer composite with longitudinal and transverse fiber orientation relative to the direction of shock-wave propagation have been carried out in the pressure range of up to 200 GPa. Particle velocity profiles on the composite surface-water window interface were recorded with a multichannel laser interferometer. The formation of a two-wave structure with a precursor amplitude from 1.5 to 3 GPa was observed with longitudinally oriented fibers. We show that Hugoniot of the composite material almost does not depend on the orientation of the carbon fibers, except for low pressures, when the particle velocity does not exceed 1 km/s. The graphite/diamond phase transition and the destruction of epoxy resin result in a characteristic kink on the Hugoniot curve with a distinct two-phase state region in the 23–35 GPa pressure range.

The origin of shifted fermi level and improved thermoelectric performance of monolayer BiCuSeO under pressures

The layered BiCuSeO has been widely investigated because of high thermoelectric (TE) performance. However, there is few reports about the TE property of two-dimensional (2D) BiCuSeO so far. In this work, it is found that the Fermi level passes through the conduction band of [Bi2O2]2+ layer and the valence band of [Cu2Se2]2- layer simultaneously. Therefore, there is no band gap appeared for monolayer BiCuSeO, which would lead to very poor TE performance. First-principles calculation shows that negative hydrostatic pressure could weaken the intra-layer bonding and enhance the inter-layer coupling. Thus, the Fermi level would gradually get out of conduction and valence bands along with the increased pressure. To improve the TE performance of monolayer BiCuSeO, negative hydrostatic pressures are applied and successfully increased its band gap values from 0 eV to 0.85 eV. At the same time, an obvious band convergence near conduction band minimum (CBM) is found under the −2.22 GPa pressure. Besides, the band convergence near the Fermi level under negative pressure will greatly improve the TE performance of n- and p-type monolayer BiCuSeO.

Efficiency Optimization of Ge‐V Quantum Emitters in Single‐Crystal Diamond upon Ion Implantation and HPHT Annealing

AbstractThe authors report on the characterization at the single‐defect level of germanium‐vacancy (GeV) centers in diamond produced upon Ge− ion implantation and different subsequent annealing processes, with a specific focus on the effect of high‐pressure‐high‐temperature (HPHT) processing on their quantum‐optical properties. Different post‐implantation annealing conditions are explored for the optimal activation of GeV centers, namely, 900 °C 2 h, 1000 °C 10 h, 1500 °C 1 h under high vacuum, and 2000 °C 15 min at 6 GPa pressure. A systematic analysis of the relevant emission properties, including the emission intensity in saturation regime and the excited state radiative lifetime, is performed on the basis of a set of ion‐implanted samples, with the scope of identifying the most suitable conditions for the creation of GeV centers with optimal quantum‐optical emission properties. The main performance parameter adopted here to describe the excitation efficiency of GeV centers as single‐photon emitters is the ratio between the saturation optical excitation power and the emission intensity at saturation. The results show an up to eightfold emission efficiency increase in HPHT‐treated samples with respect to conventional annealing in vacuum conditions, suggesting a suitable thermodynamic pathway toward the repeatable fabrication of ultra‐bright GeV centers for single‐photon generation purposes.

High-pressure induced the morphological evolution from lath to integrated martensite with {112} nano-twins of 30MnSi steel

In this paper, the evolution of martensitic morphology and hardening mechanism of 30MnSi steel quenched at atmospheric and 3, 4 GPa pressure were investigated using EBSD, Mössbauer, HR-TEM. The experimental results show that an integrated martensite of lath and grain-preferred plate is induced after high-pressure quenching, instead of the almost entirely lath martensite that arises from normal quenching. The intricate substructures of martensite, which include tangled dislocations, 60.0°<111> twin boundaries formed by twin-related K–S variants, and {112}<111>-type twins with nanoscale ω-Fe (hexagonal), are characterized in 3, 4 GPa-samples. As the quenching pressure rises, the dislocation density, the percentage of twin boundaries and the amount of {112}<111>-type twins all exhibit an increasing trend. In order to clarify the nature of transformation twins, the ω-lattice mechanism for high-pressure quenching martensite was discussed. Furthermore, the hardness of 30MnSi steel is considerably increased from 480 HV (normal quenching) to 766 HV (4 GPa-sample), and the relationship between microstructure and hardening was established. Based on our findings, high-pressure quenching has proved to have promising applications in the extensive control of martensitic transformation.

Pressure-induced physical properties of alkali metal chlorides Rb2NbCl6: A density functional theory study

Using density functional theory-based first-principles simulations, detailed physical properties of the tetragonal phase alkali metal halide Rb2NbCl6 under pressure were explored for the first time. The structural, mechanical, and thermodynamic stability were confirmed by the Born stability requirements and the negative values for the formation energy. The analysis of Pugh’s and Poisson’s ratios and Cauchy’s pressure reveals that Rb2NbCl6 is ductile under the pressures in consideration. As the applied pressure rises, the elastic moduli show a rising trend, which indicates that Rb2NbCl6 stiffens up. According to several anisotropy indices, the compound is noticeably anisotropic both in ambient and under pressure. The machinability index suggests that the material under study is highly machinable. Several mechanical features of Rb2NbCl6 are analyzed according to the results of elastic constants and adequately explained. Since the melting temperature rises with applied pressure, Rb2NbCl6 is more suitable for high-temperature applications. The computed total density of states (TDOS) at 0 GPa pressure at EF is ∼5.07 states/eV/f.u., and applied pressure has a negligible effect on the value of DOS. The study of electronic properties provides significant support for interpreting the optical function. As the applied pressure rises, the reflectivity and absorption spectra shift to higher energy regions. High-reflectivity spectra suggest that the material would be an excellent choice for coatings that lower solar heating. The authors of this study expect that the fascinating findings of this investigation will give researchers and engineers a helpful foundation.

Experimental research on impact adiabatic relationship of U71Mn steel

Little is known about the changes in physical and chemical properties of U71Mn steel during supersonic operation. In this paper, the impact adiabatic relationship of U71Mn steel in the range of 0.5~1km/s impact velocity was studied by flyer impact experiment. The data shows that with the increase of the impact velocity, the Hugoniot pressure of U71Mn steel increases significantly, but the increment of the Hugoniot temperature is relatively small. U71Mn steel does not undergo phase transformation under the impact of 0~23.5GPa pressure range.

Evaluating the pressure dependence of PZT structures using a virtual reality environment

Pb–Zr–Ti–O (PZT) perovskites span a large solid-solution range and have found widespread use due to their piezoelectric and ferroelectric properties that also span a large range. Crystal structure analysis via Rietveld refinement facilitates materials analysis via the extraction of the structural parameters. These parameters, often obtained as a function of an additional dimension (e.g., pressure), can help to diagnose materials response within a use environment. Often referred to as “in-situ” studies, these experiments provide an abundance of data. Viewing structural changes due to applied pressure conditions can give much-needed insight into materials performance. However, challenges exist for viewing/presenting results when the details are inherently three-dimensional (3D) in nature. For PZT perovskites, the use of polyhedra (e.g., Zr/Ti–O6 octahedra) to view bonding/connectivity is beneficial; however, the visualization of the octahedra behavior with pressure dependence is less easily demonstrated due to the complexity of the added pressure dimension. We present a more intuitive visualization by projecting structural data into virtual reality (VR). We employ previously published structural data for Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 as an exemplar for VR visualization of the PZT R3c crystal structure between ambient and 0.62 GPa pressure. This is accomplished via our in-house CAD2VR™ software platform and the new CrystalVR plugin. The use of the VR environment enables a more intuitive viewing experience, while enabling on-the-fly evaluation of crystal data, to form a detailed and comprehensive understanding of in-situ datasets. Discussion of methodology and tools for viewing are given, along with how recording results in video form can enable the viewing experience.

Melting Curve of Cobalt using Molecular Dynamics Simulation

Molecular dynamics simulation is used to estimate the melting point of cobalt using the embedded atomic model (EAM) potential by heat until melting, void, hysteresis and interface methods. For instance, the estimated melting temperature are 2102 K, 1944.15 K, 1731 K and 1725±25 K using these methods, respectively. Then, the melting points at different pressures are calculated. A graph depicting the variation of melting point with pressure is drawn and compared with the available simulation and experimental results. The melting point at a low-pressure range is similar to the previous diamond anvil cell experiments. Besides, using the Simon equation, we calculated the melting slope at 0 GPa pressure of 36 K/GPa for one phase and 40 K/GPa for two-phase methods.

Synthesis and characterization of novel NiTi–Ni3Ti/SiC nanocomposites prepared by mechanical alloying and microwave-assisted sintering process

Equiatomic Ni–Ti alloy reinforced with 0, 2.5, 5, and 10 vol % SiC nanoparticles was synthesized through mechanical alloying and microwave sintering process. In this method, a mixture of Ti and Ni powders and SiC nanoparticles (0–10 vol%) were mechanically milled for 30 h, compressed under 1 GPa pressure, and then sintered at 900–1100 °C by microwave heating. Microstructural analysis of the synthesized samples was performed using X-ray diffraction and scanning/transmission electron microscopy, and their hardness was determined using a Vickers microhardness tester. Structural studies revealed the formation of a complex phase structure consisting of the austenite (B2–NiTi), the martensite (B19′-NiTi), and the nickel-rich (Ni3Ti) and titanium-rich (NiTi2) phases. HR-TEM investigations confirmed an increased Ni3Ti phase in the austenitic NiTi matrix nearby the martensitic NiTi phase. It was found that adding SiC nanoparticles to Ni–Ti increased the amount of hard Ni3Ti phase, promoted the stability of the martensite phase at room temperature, and retarded the matrix grain growth during the sintering process. Microhardness tests showed that when the content of SiC nanoparticles increased up to 5 vol %, the microhardness increased considerably to 9 GPa and then declined. The high microhardness of synthesized nanocomposites is due to the formation of Ni3Ti and NiTi martensitic phases and their nanocrystalline structure.

Pressure‐Induced Semiconductor‐to‐Metallic Transition of Monoclinic KCa2Nb3O10 Layered Perovskite: A Theoretical DFT Insight

AbstractThe structural, electronic, optical, and semiconducting properties of monoclinic KCa2Nb3O10 layered perovskite at pressures ranging from 0 to 400 GPa are investigated using a first‐principles method based on density functional theory (DFT). The calculated lattice parameters and other properties are compared with available experimental values, and good agreement with them is found. The investigation of the electronic properties demonstrates that the compound's direct bandgap nature at the Z‐point appears for all pressure and the compound becomes metallic at 400 GPa. The most influential contribution to the total and partial density of states comes from Nb‐4d and O‐2p states at the Fermi level under applied pressure. The study of optical functions displays that, at 400 GPa pressure, the material is particularly reflective, with the maximum absorption in the infrared band. The outcomes of this study suggest that pressure has a significant effect on the structural, electronic, and optical properties of KCa2Nb3O10 perovskite that may be promising for optoelectronic applications.