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The phase transition of PbNiO3 from LiNbO3 structure to the perovskite structure has been observed between 4.4 and 9.5 GPa, which was investigated by high-pressure synchrotron x-ray powder diffraction at room temperature. Compression behavior of the two phases has been analyzed, and the c axis is the soft direction for LiNbO 3-type PbNiO3. The bulk modulus of the LiNbO3-type PbNiO3 is K0=184(3) GPa, K0′=4.5(9), and the perovskite type is K0=166(8) GPa, K0′=4.9(8). [ABSTRACT FROM AUTHOR]

Carbon has been proposed to be a potential light element in the Earth's solid inner core. However, its presence and concentration in the core remain limited constrained. In this study, we measured the equation of state of an iron‐carbon alloy with ∼1.5 wt.% (∼6.64 at.%) carbon up to ∼185 GPa and ∼3,750 K via shock compression. The Hugoniot equation of state of the Fe‐1.5wt.%C alloy was determined to be Us (km/s) = 3.94 (0.15) + 1.62 (0.08) Up with an initial density of 7.77 (0.03) g/cm3, where Us and Up are the shock wave velocity and particle velocity, respectively. Hugoniot data combined with the Mie‐Grüneisen equation of state allow us to build the thermal equations of state of the Fe‐C alloy at high pressure‐temperature (P‐T) conditions. Investigations on the pressure‐density relation of Fe‐C alloys by the shock experiments and synchrotron X‐ray diffraction measurements by Pamato et al. (2020), https://doi.org/10.1029/2020jb020159 have indicated that carbon should occupy interstitial sites of iron lattice at high P‐T conditions. The thermal equation of state of the Fe‐1.5wt.%C alloy provided here shows that Fe‐1.5wt.%C is ∼1.65% denser than the inner core under conditions relevant to the inner‐core boundary. This study indicates that iron alloyed with an upper bound of ∼2.2 (0.3) wt.% (∼10.7 [1.6] at.%) carbon may match with the Preliminary Reference Earth Model (PREM) in the Earth's inner core. Thus, interstitial carbon in iron helps to explain the observed density deficit of the inner core, although other light elements are also expected. Plain Language Summary: Carbon (C), as an essential life element, is likely to be present in the Earth's core. Previous investigations revealed that carbon would be alloyed interstitially in the hexagonal close‐packed (hcp) iron lattice at high pressures which occurs in the Earth's core and thus affect the thermodynamic and seismic velocities of the core in a different way. In this study, we determined the density of Fe‐1.5wt.%C alloy up to ∼185 GPa and ∼3,750 K via hypervelocity impact experiments. Experimental results were used to develop reliable thermal equations of state for Fe‐1.5wt.%C alloy under the Earth's inner‐core conditions. Our results support that a small amount of carbon may occupy the interstitial sites of hcp‐iron at extreme pressure and temperature, and the proposed model suggests an upper bound of ∼2.2 wt.% carbon in the Earth's inner core assuming it as the major light element. Key Points: Hugoniot equation of state of Fe‐1.5wt.%C alloy has been investigated to ∼185 GPa and ∼3,750 K via shock compressionA certain amount of carbon (e.g., 1.5 wt.%) in solid iron takes the interstitial sites under conditions relevant to the Earth's inner coreAn upper bound of ∼2.2 wt.% carbon is suggested in the Earth's inner core based on the density deficit of the Earth's core [ABSTRACT FROM AUTHOR]

We observed an isostructural phase transition in the solid nitrogen λ -N2 at approximately 50 GPa accompanied by anomalies in lattice parameters, atomic volume and Raman vibron modes. The anomalies are ascribed to a slight reorientation of the nitrogen molecules, which does not seem to affect the monoclinic symmetry (space group P 21/ c). Our ab initio calculations further confirm the phenomena, and suggest an optimized structure for the λ -N2 phase. In addition, a new high-pressure amorphous phase of η ′-N2 was also discovered by a detailed investigation of the pressure-temperature phase diagram of nitrogen with the aim of probing the phase stability of λ -N2. Our result may provide helpful information about the crystallographic nature of dissociation transitions in diatomic molecular crystals (H2, O2, N2, etc). [ABSTRACT FROM AUTHOR]

Abstract: We report phase transition and stability of MoS2 with and without the presence of sulfur melt under high-pressure and high-temperature conditions. Rhombohedral (3R) phase is found to be a high-temperature phase of MoS2 at high pressures. Excess sulfur melt catalyzes the hexagonal (2H) to rhombohedral (3R) phase transformation and lowers the conversion temperature by more than 280K. Boundary between 2H and 3R phases has been delineated with a negative slope. Based on experimental observations, sulfur-catalyzed 2H→3R transformation mechanisms are proposed involving atomic exchange between MoS2 and sulfur, which is different from the case of without excess sulfur that proceeds through rotation and translation of the S–Mo–S sandwich layers. [Copyright &y& Elsevier]

Polycrystalline diamond composites (PDC) have been used as a cutting material in a multitude of fields, from mechanical processing to oil and gas drilling. In this study, PDC samples were prepared under high pressure and high temperature (HPHT) conditions within a pressure interval of 5.5 to 10.5 GPa by sintering diamond powder (average grain size ~3 μm) with WC-10 wt% Co substrate. Samples obtained with varying sintering pressures and temperatures were characterized using Vickers hardness tester, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results revealed that: (1) within a specified material system, each distinct sintering pressure condition corresponds to a unique optimal sintering temperature that yields the pinnacle of consolidation efficiency in samples. This optimal temperature exhibits a linearly increasing trend with rising sintering pressure; (2) at the intersection where the sintering pressure attains its optimal match with temperature, an increase in pressure fosters more intimate contact and stronger inter-grain bonding among diamond particles, thus amplifying the D D bond formation effect, which consequently leads to a linear enhancement in the Vickers hardness values of PDC with increasing sintering pressure; (3) simultaneously, under conditions of optimal temperature matching, elevating the sintering pressure notably decreases the cobalt content within the polycrystalline diamond layer of PDC samples. These findings furnish crucial experimental evidence for optimizing PDC material properties, significantly contributing to the advancement of both theoretical exploration and technological innovation within the field, carrying profound academic value and practical guidance implications. [Display omitted] • At a given pressure, there is an optimal temperature for the peak sintering performance of PDC. • The optimal sintering temperature exhibits a linear increase with rising pressure. • Under optimal temperature conditions, increased pressure improves sample performance. [ABSTRACT FROM AUTHOR]

• This article systematically investigated the pressure and temperature effects on recrystallization behavior of cBN grains. • The results show that the recrystallization temperature of cBN under 8 GPa is approximately 1650 °C and increases by approximately 100 °C with every 2 GPa increase in pressure. • Once grain recrystallization starts, the strengthening effects of grain refinement and defect structure are greatly weakened. • This work is helpful to understand the sintering mechanism and control the microstructure and mechanical properties of sintered polycrystalline cBN compacts. • In addition, the melting curve for cBN under high pressure is inferred according to the empirical relationship between recrystallization temperature and melting temperature, and the phase diagram for boron nitride is revised based on this new melting curve. The recrystallization behaviour of micron-sized cubic boron nitride (cBN) was studied by analysing the grain size and morphology of samples treated at 8−16 GPa/1500–2200 °C. The results show that the recrystallization temperature of cBN under a pressure of 8 GPa is approximately 1650 °C and increases by approximately 100 °C with every 2 GPa increase in pressure. Once grain recrystallization starts, the grains grow abnormally quickly as the temperature rises, and the strengthening effects of grain refinement and defect structure are greatly weakened. The recrystallization behaviour of cBN at high pressure is helpful to understand the sintering mechanism and control the microstructure and mechanical properties of sintered polycrystalline cBN compacts. In addition, the melting curve for cBN under high pressure is inferred according to the empirical relationship between recrystallization temperature and melting temperature, and the phase diagram for boron nitride is revised based on this new melting curve. [ABSTRACT FROM AUTHOR]

Herein, we report transparent LaGdZr 2 O 7 nano-grained ceramics with an average grain size of 10 nm prepared using the high-pressure sintering method at 5.0 GPa/450 °C. These ceramics showed improved plastic deformation as compared to transparent micro-grained LaGdZr 2 O 7 ceramics. Moreover, the fracture toughness of the nano-grained ceramics reached up to 3.4 MPam1/2, which is 79 % higher than that of the micro-grained samples. This improvement in the fracture toughness of the nano-grained ceramics was associated with the presence of a large number of weak grain boundaries in them. [ABSTRACT FROM AUTHOR]

High-hardness TaN compacts with a diameter and height of 6 mm were synthesized through the phase transformation of CoSn-type into WC-type TaN under experimental conditions of high temperature and high pressure. The Vickers hardness of WC-type TaN compacts obtained at the pressure of 5 GPa and the temperature of 1873 K is found to be ~21.5 ± 1.5 GPa with a load of 3 Kg, which is comparable to that of the pure tungsten carbide compact (~20 GPa) and higher than those of ultrahigh temperature ceramics ZrB 2 and HfB 2 under the same load. The high hardness of WC-type TaN compacts presented in this work is attributed to the deviatoric strain and well-bonded nanograins. The synthesized WC-type TaN compacts also possess a high oxidation temperature (1072 K). These results suggest that the WC-type TaN with high hardness and high thermal stability holds great promise for industrial applications. [ABSTRACT FROM AUTHOR]

Abstract: Internal residual metal phases (mainly cobalt) were effectively removed from polycrystalline diamond compacts (PDCs) by electrolysis to improve their high temperature wear resistance. Through turning granite (dry cutting), we have checked the wear resistance of PDCs with different residual metal removal depth (RMRD). The relationship between the measured wear rate and RMRD was obtained, and the results showed that PDCs treated by electrolysis have a significant improvement in wear resistance. X-ray diffraction (XRD) and scanning electron microscopy (SEM) observation of PDCs'' wear surface indicated that diamond–graphite phase transformation occurs for the samples with residual metal phase. The wear mechanism is discussed, and our study suggests that the graphitization is the main wear process of PDCs with internal residual metal phases. [Copyright &y& Elsevier]

Porous diamonds have unique application prospects due to their flexible surface functionalities, high surface-area-to-volume ratio and high self-sharpening ability. Porous diamonds are usually prepared through microwave plasma chemical vapor deposition (MPCVD) techniques or through etching by transition metals, which are expensive and inefficient. Here, we report a simple and efficient method to grow large millimetre-scale porous single-crystal diamonds using high-pressure and high-temperature (HP-HT) techniques. The porous diamond crystal growth is driven by minimizing the surface energy through the attachment of small crystals to a large seed crystal surface in a metal solvent. Industrial-scale production of single-crystal porous diamonds is possible with this discovery. The porous single-crystal diamond is prepared by using high pressure and high temperature (HP-HT) technology. XRD, SEM, Raman spectroscopy and HRTEM observations illustrated the high quality of the porous diamond surface layer and confirmed the obtained diamonds as porous single crystals. [Display omitted] [ABSTRACT FROM AUTHOR]

The in situ axial X-ray diffraction patterns of four ceramic powder samples (MgO, Al 2 O 3 , AlN, and cBN) that were compressed in a diamond anvil cell under uniaxial non-hydrostatic conditions were recorded. The microscopic deviatoric stress as a function of the pressure was determined from the X-ray diffraction peak broadening analysis: the curves increased approximately linearly with the pressure at the initial compression stage and then levelled off under further compression. Pressure-induced transparency was observed in all of the samples under compression, and the pressure at the turning point on the curves of the microscopic deviatoric stress versus pressure corresponded to the pressure at which the samples became transparent. Analysis of the microstructural features of the pressure-induced transparent samples indicated that the compression caused the grains to fracture, and the broken grains bonded with each other. We demonstrated that the ceramics' pressure-induced transparency was a process during which the grains were squeezed and broken, the pores were close between the grains, and the broken grains were re-bonded under compression. [ABSTRACT FROM AUTHOR]

We report a method for strengthening microcrystalline alumina compaction by work hardening under conditions of high pressure and temperature, in which the hardness significantly improved. Micro-sized spherical alumina powders were treated without additives under 5.5 GPa–14 GPa and 600 °C–1200 °C. A polycrystalline sample sintered at 12 GPa and 900 °C yielded a Vickers hardness of 30.0 GPa, which was 1.5 times that of single-crystal alumina and the highest value reported to date for alumina. By analysing the density, hardness, and microstructure of the sintered samples, the results demonstrated that under ultra-high pressure and below the recrystallisation temperature of alumina, plastic deformation of the grains produced many substructures such as stacking faults and lattice distortions showing obvious work hardening effects. Combined with well-bonded crystalline and amorphous grain boundaries, the hardness of the samples was unexpectedly enhanced. [ABSTRACT FROM AUTHOR]

Pressure is the core of high-pressure science and technology, and the accuracy of pressure calibration is of much importance for high-pressure experiments and production. Although the pressure limit of the large volume press (LVP) continues to increase, there are no well solutions for in situ pressure calibration. In this study, using in situ high-pressure electrical performance measurement technology, two ideal calibration standard materials in powder conductors, cadmium phosphide (Cd3P2) and zinc telluride (ZnTe) with stable physical and chemical properties and obvious resistance change, are applied to pressure calibration in the LVP. In situ high-pressure synchrotron radiation x-ray diffraction was used to verify the phase transition pressure point of Cd3P2. The introduction of powder conductors for pressure calibration commits to establish a pressure system, which is safer, more stable to operate, and more accurate in experimental measurements for the LVP. [ABSTRACT FROM AUTHOR]

Polycrystalline cubic boron nitride (PcBN) has been synthesized at 14 GPa and high temperatures of 1300–2000 °C in a two-stage multi-anvil cell. Sintered PcBN synthesized at 1700–1800 °C and 14 GPa with a grain size of ∼200 nm is optically transparent with a transmittance of ∼70% at wavelengths of 400–1500 nm and has the Vickers hardness of ∼63–69 GPa. Analyses with scanning and transmission electron microscopy reveal that PcBN can be strengthened by introducing nanometer-scale grains and microscopic defects at high pressure and temperature. The optical transparency of the bulk PcBN synthesized at high pressure and temperature can be explained by the very thin intergranular films between grains. The present sintered PcBN is the second-hardest transparent material after diamond and can be used for windows in extreme conditions. [ABSTRACT FROM AUTHOR]

The solubility of NaCl in water is one of the most important thermo-physical properties. However, the solubility behavior of NaCl in water is poorly understood at high pressure and low temperature. Herein, we performed high pressure neutron diffraction and Raman scattering to investigate the solubility of NaCl in water and the corresponding structural change, respectively. With the pressure increasing, the solubility almost increases linearly below 0.2 GPa, beyond which it starts to level off and reaches its maximum at about 0.61 GPa. The Raman spectra suggested that the deformation of the hydrogen-bonded network in the NaCl aqueous solution promotes the formation of ions pairing and is responsible for the solubility increase of NaCl in water. Finally, we used a two-dimensional Mercedes-Benz model to descript the picture of solubility behavior of NaCl in water at high pressure. [ABSTRACT FROM AUTHOR]

Pressure is an important thermodynamic parameter in addition to temperature and chemical composition during material sintering process. However, few experimental images intuitively describing the densification process of grains during compression, especially for the high melting point ceramics. Here we observed deformation evolution and mechanical response of spherical alumina particles after high pressure treatment. By analyzing the plastic deformation and grain boundary characters of spherical alumina particles, and the boundary stress and yield stress variation of grains under various pressures and temperatures, a clear picture of the high-pressure powder densification and sintering process is depicted. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Diamond is the hardest naturally occurring material found on earth but single crystal diamond is brittle due to the nature of catastrophic cleavage fracture. Polycrystalline diamond compact (PDC) materials are made by high pressure and high temperature (HPHT) technology. PDC materials have been widely used in several industries. Wear resistance is a key material property that has long been pursued for its valuable industrial applications. However, the inevitable use of catalysts introduced by the conventional manufacturing process significantly reduces their end-use performance and limits many of their potential applications. In this work, an ultra-strong catalyst-free polycrystalline diamond compact material has been successfully synthesized through innovative ultra-high pressure and ultra-high temperature (UHPHT) technology. These results set up new industry records for wear resistance and thermal stability for PDC cutters utilized for drilling in the oil and gas industry. The new material also broke all single-crystal diamond indenters, suggesting that the new material is too hard to be measured by the current standard single-crystal diamond indentation method. This represents a major breakthrough in hard materials that can expand many potential scientific research and industrial applications. [ABSTRACT FROM AUTHOR]

Creep behavior at room temperature was firstly observed in nanocrystalline Gd 2 Zr 2 O 7 ceramic (NC-GZO) having an average grain size about 7.8 nm. The creep strain rate sensitivity of NC-GZO was calculated, and many dislocations can be observed in the strained region. The mechanism causing this room temperature creep behavior was associated with the dislocations slipping which most likely occurs though grain boundaries sliding. [ABSTRACT FROM AUTHOR]

Studying the mechanical and thermal properties as well as the relationship between microstructure evolution and strengthening mechanisms is crucial for obtaining superior high-temperature refractory molybdenum disilicide-silicon carbide composites. In this study, molybdenum disilicide and silicon carbide powders with different volume ratios were used to fabricate molybdenum disilicide-silicon carbide composites via high pressure and high temperature at 5.5 GPa/1773 K. The introduction of the second phase improved the materials' mechanical properties. Transmission electron microscopy demonstrated that severe plastic deformation generated numerous micro-defects and grain refinement during the high pressure sintering process. The second phase enhancement also improved the samples' performances. Under high pressure and high temperature sintering process, the thermal and mechanical properties of the materials increased dramatically, including a well Vickers hardness (~20.6 GPa), fracture toughness (~10.3 MPa·m1/2) and thermal stability (>1673 K). [ABSTRACT FROM AUTHOR]

We report experimental determination of compressibility and thermoelasticity for CrN based on in situ high pressure/high temperature synchrotron x-ray diffraction measurements. The pressure-induced cubic-to-orthorhombic phase transition is observed at ∼ 5 GPa in high-quality CrN samples synthesized using a high pressure reaction route. Equation of state, bulk modulus and axial compressibility of this material are obtained by analysis of high pressure x-ray diffraction data collected at room temperature. Both cubic and orthorhombic phases have a similar bulk modulus value of 257 (5) and 262 (6) GPa, respectively. For orthorhombic CrN, the b-axis has the most axial incompressibility, which should be associated with magnetic properties. In addition, the isothermal equation of states is also determined at different temperatures of 300, 400, 600, 800, 1000, and 1200 K, respectively. Some of the important thermoelastic properties of cubic CrN are thus derived, including temperature derivative of bulk modulus ∂ K T / ∂ T = − 2.95 (5) × 10 − 2 G P a ⋅ K − 1 and volumetric thermal expansivity a T ≈ a 0 + a 1 ⋅ T where a0 = 1.95 (16)×10−5 K−1 and a1 = 4.39 (3)×10−9 K−1. [ABSTRACT FROM AUTHOR]

Pure-phase polycrystalline cadmium tetraphosphide (CdP4) has been successfully synthesized under high pressures in a large volume cubic press, and the pressure-temperature (P-T) formation boundary of CdP4 from the Cd-P binary system has been determined. The equation of state of monoclinic CdP4 has also been evaluated by synchrotron-radiation angle-dispersive X-ray diffraction (ADXRD) under quasihydrostatic compression up to 26.4 GPa in a diamond-anvil cell. The ADXRD data yield a bulk modulus B0 = 94 ± 9 GPa with a pressure derivative B0′ = 8.0. The experimental data are discussed and compared to the results of first-principles calculations. [ABSTRACT FROM AUTHOR]

Cubic boron nitride (cBN), a typical superhard material, has garnered significant interest for both fundamental scientific research and technical applications. Here, nanostructured cBN with high-density nanotwins and stacking faults was synthesized by rapid quenching using hexagonal boron nitride (hBN) as the starting material at a moderate temperature (2200 K) and low pressure (13 GPa). This synthetic nanostructured cBN exhibits high Vickers hardness (78 GPa), high fracture toughness (11.8 MPa·m1/2), and extremely high oxidization temperature (1565 K). We propose that the refined grain structure and excellent mechanical properties of the cBN synthesized in this study is attributed mainly to dislocation slips and (111) mechanical nanotwins that dominate this phase transformation. [ABSTRACT FROM AUTHOR]

We report a method combining the top-down and bottom-up approaches for the fabrication of nanocrystalline ceramic, which can break down the Y 2 O 3 nanorods and consolidate the broken nanoparticles to 99.1% relative density. The lab-mad nanorods with a length of about 500 nm and a diameter of about 20 nm, exhibit better compressibility and sinterability than the spherical nanopowders when the compaction pressure exceeds 0.4 GPa and the sintering condition is 5 GPa/500 °C, respectively. The obtained nanocrystalline ceramic shows a maximum transmittance of 81.3% in the infrared region, hardness as high as 10.5 GPa and fracture toughness of 2.21 MPa m1/2. Image, graphical abstract [ABSTRACT FROM AUTHOR]

High-performance polycrystalline cubic boron nitride (PcBN) was sintered without binders at 1500 °C in a pressure range from 11 to 15 GPa using commercial micrometre cubic boron nitride (cBN) with a diameter of approximately 2–4 μm. The results demonstrated that the sample sintered at 12 GPa and 1500 °C had the best mechanical properties and thermal stability. Its average Vickers hardness, fracture toughness, and thermal stability was 63 GPa, 15 MPa m1/2, and 1315 °C, respectively. The considerable improvement in the mechanical properties was mostly attributed to the high compactness, close bonding between grains, and the sample's internal defect structures. The relatively small specific surface area of the micron grains provides an advantage due to its high thermal stability. The amorphous regions observed in the sample's local areas may provide a new strengthening mechanism under high pressure and temperature. [ABSTRACT FROM AUTHOR]

Translucent nanocrystalline Gadolinium Zirconate (Gd 2 Zr 2 O 7) ceramic with grain size below 10 nm was successfully fabricated using high pressure sintering technology at 5 GPa and low temperature of 550 °C. The bilinear compressive stress-strain relationship of this sample was firstly extracted from nanoindentation procedure based on the partial unloading method. It indicated that the plastic deformation did not originate from dislocation sliding and was controlled by the grain boundary sliding. Moreover, the compressive yield strength, σ y = 7.8 GPa of the translucent nanocrystalline Gd 2 Zr 2 O 7 ceramic was also determined by the intersection of the theoretical elastic line and the indentation compressive stress-strain line. [ABSTRACT FROM AUTHOR]

In this paper, a pressurization-insulation and pre-sealing (PIPS) system is designed to increase the cell pressure of the widely used large volume cubic press without sacrificing cell volume. The sample chamber was sandwiched between a pair of tungsten carbide anvils used as the pressurization system. Ultra-high pressure in the cavity was up to about 12 GPa, and the pressure limit had increased by 100% in contrast with that of an anvil-gasket (AG) system. Furthermore, the confining pressure around the sample chamber was supported by grade 304 stainless steel and a zirconia–calcium oxide solid solution before a press load of 2.8 MN was applied as well as by four surrounding anvils. The relationship between the sample chamber pressure and the press load for this system was calibrated at room temperature using transitions in zinc telluride. With samples of similar volumes, the proposed system retained not only stability but also uniform pressure and temperature fields, in contrast with the AG system and the anvil-preformed gasket cubic press pressurization system. The results of more than 20 experiments show that the proposed PIPS system can operate stably under a press load of 4.2 MN, corresponding cell pressure of 10 GPa, and temperature in the cell exceeding 1800 °C by using graphite as a heater. [ABSTRACT FROM AUTHOR]

High pressure obviously decreased the sintering temperature of high melting point ceramics and produced bulks with excellent mechanical and thermal properties. In this study, the effect of pressure alone on the densification process of alumina ceramics was demonstrated. The translucent alumina bulks with unexpected hardness were obtained by compressing α-Al 2 O 3 powder under ultra-high pressure at room temperature. Through analyzing microstructure and hardness of cold compressed samples, the results reveal that ultra-high pressure could crush alumina grains under appropriate pressure, and initiate plastic deformation of grains when the applied pressure exceeds the yield strength of alumina. Thus to enhance the hardness of the samples by grain boundary strengthening and plastic deformation at higher pressure. [ABSTRACT FROM AUTHOR]

Cold bonding of metals necessitates high normal/frictional loads, or ultrahigh-vacuum treated atomically flat ductile surface. In case of ceramics composed of ionic and/or covalent bonds, the high diffusion barrier of ions or atoms gives them a constraint to bond together at room temperature. However, from this investigation, it has been found that alumina crystals could be well bonded at room temperature under high pressure. Also, it has been noticed that if the pressure is capable of yielding and breaking down alumina crystals, it could then re-bond the fractured crystals by reducing the distance of their surface atoms to meet the condition for bonding. Overall, we demonstrate a process for the cold bonding of alumina under high pressure with grain fracturing and re-bonding. [ABSTRACT FROM AUTHOR]

High‐pressure structural evolution of CeOCl, a lanthanide oxychloride, was investigated by in situ high‐pressure Raman scattering and first‐principles calculations. We report the novel observation of discontinuities in Raman wavenumber at the pressure of approximately 7 GPa, which are related to an isostructural phase transformation caused by an increase of Bader charge transfer from Ce to Cl and O. In addition, the mode‐Grüneisen parameters (of CeOCl) were given and discussed. This is helpful for us to better understand applications of the Ce‐based compounds. [ABSTRACT FROM AUTHOR]

Hafnium diboride (HfB2) is one of the most promising hard-brittle ceramic materials with unique physical properties. Here, we have synthesized the well-crystallized HfB2 by a high-pressure solid-state reaction and employ in situ high-pressure synchrotron radiation angle-dispersive X-ray diffraction to investigate the size-effect of HfB2. An abnormal physical behavior of HfB2 under high pressure is observed. The microsized HfB2 shows slight anisotropy along a and c axes; however, the nanosized HfB2 reveals a different compression behavior with pressure-induced shell structural transition from a disordered symmetric amorphous shell state to an asymmetric amorphous shell state. In addition, the results indicate that the pressure calibrations are sensitive to the grain size. The present findings offer insights into the physical behaviors of different sized HfB2, which may also provide valuable information for other transition metal borides under high pressure. [ABSTRACT FROM AUTHOR]

We analyzed the compression process of the centrally caved Bridgman-anvil cell by a simplified mechanics model and proposed that the pressure limitation for a given configuration is governed by the compressibility and compression ratios (V0/V) of the gasket and sample. Then we replaced the middle part of the sample that is prone to undergo plastic deformation with a diamond plate, which will obviously change the stress state of the sample and gasket. The results of ex situ and in situ pressure testing experiments show that this way doubles the efficiency of the pressure generation and largely expands the pressure limitation. The pressure of sample Ni can reach 29.4 GPa in the double toroidal sintered-diamond anvil cell with a sample volume of 17 mm3 even at the loading force of 1500 kN. [ABSTRACT FROM AUTHOR]

The compression behavior and stress state of nanocrystalline tungsten boride (WB) were investigated using radial x-ray diffraction (RXRD) in a diamond-anvil cell under non-hydrostatic compression up to 60.4 GPa. The compression properties and stress state are analyzed using lattice strain theory. Experiments were conducted at beamline X17C of the National Synchrotron Light Source. The radial x-ray diffraction data yield a bulk modulus that is qualitatively consistent with density functional theory calculations and demonstrate that WB is a highly incompressible material. A maximum differential stress, t, of about 14 GPa can be supported by nanocrystalline WB at the highest pressure. This corresponds to about 5% of the shear modulus, G, which is smaller than the values of t/G (∼8%-10%) observed for BC2N, B6O, TiB2, and γ-Si3N4 at high pressures. Thus, while WB is highly incompressible, its strength is relatively low at high pressures compared to other hard ceramics. [ABSTRACT FROM AUTHOR]

Abstract High-pressure strengthening is reported to be an effective approach to strengthen materials in addition to traditional nanocrystalline strengthening. Numerous studies have proven that high pressure is an important factor for improving the mechanical properties of materials. Here, we sintered high-purity sub-micron tungsten carbide (sm-WC) ceramics without any additives under a high pressure of 10 GPa and temperatures ranging from 1000 °C to 1500 °C. Numerous microdefects (stacking faults and twins) were introduced inside the grains, and the Vickers hardness of the well-sintered WC bulks reached a high value of 33 GPa, which could be compared with that of single crystal and nanocrystalline counterparts. Thus, the high-pressure strengthening can be applied to the WC system and improves the mechanical properties. The strengthening mechanism is that the high pressure can make the grains take place bulk yield, promote a severe plastic deformation, and introduce numerous microdefects. These defects act as obstacles against dislocations glide during deformation, and thus improve the mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract A series of pure HfB 2 ceramics have been prepared by sintering nano-grained powder using high-energy ball milling at 700–1600 °C and 5.5 GPa. The HfB 2 ceramics are characterized via various techniques for their residual stress, grain size, density, microstructures and defects, hardness, fracture toughness, thermal stability, and oxidation resistance. All properties strongly depend on the treatment temperature, but the exact manner of dependence for each property varies. The results identified that the HfB 2 ceramic sintered at a relatively low temperature of 1000 °C and 5.5 GPa – a bulk pure nano-grained composite for the first time – has the best overall performance. It has a relative density of 99.6%, a Vickers hardness of 26 GPa, a fracture toughness of 5.2 MPa m1/2, and excellent thermal stability and oxidation resistance at high temperatures. Additional strengthening and stabilizing effects are provided by microstructures and defects such as large-angle grain boundaries, stacking faults and twinning. Simultaneous high temperature and high pressure is an effective sintering route for HfB 2 ceramics with grain-size ranging from nanometer to micron. [ABSTRACT FROM AUTHOR]

Polycrystalline diamond in the presence of silicon with different sintering times was investigated under high pressure and high temperature of 5.0 GPa and 1400 °C, respectively, using a multi-anvil apparatus. In this research, phase analysis demonstrated that the content of SiC increased and the amount of Si decreased clearly with the sintering time extending by X-ray diffraction (XRD). The characterization of the sintered body demonstrated that the electrical resistance decreased as the reaction time rose. Specimens with good conductivity of 17 Ω·cm−1 sintered for 20 min at 1400 °C under 5.0 GPa were successfully obtained. The measured Vickers hardness and thermal stability of the synthesized diamond-SiC composites in the present study were >40 GPa and 1450 K, respectively. • Diamond–SiC composites were investigated by sintering different times in the range of 1–20 min at 5.0 GPa and 1400 °C. • SiC was densely packed between the diamond's grain boundaries, preventing the contact between the oxygen and diamond. • Extending the sintering time, and the graphitization of diamond occurred in low pressure zone are gradually in process. [ABSTRACT FROM AUTHOR]

Highlights • Monoclinic Y 2 O 3 was obtained by treating the cubic Y 2 O 3 under high temperature high pressure. • The equipment used is a large volume cubic multi-anvil high pressure apparatus. • The cubic Y 2 O 3 used in this work was obtained by calcining Y-based coordination polymer precursor. Abstract In this work, synthesis of cubic Y 2 O 3 by calcining yttrium based coordination polymer (Y-CP) which were obtained by microwave-assisted method was reported. Monoclinic Y 2 O 3 was obtained by treating the cubic Y 2 O 3 under high temperature high pressure (HTHP) on a large volume cubic multi-anvil high pressure apparatus. The effect of temperature on the preparation was studied. The morphology and crystal phase of all samples have been characterized by the scanning electron microscope (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) spectra. What more, the photoluminescence (PL) property of the Y 2 O 3 :Eu3+ sample was investigated. This method can be used to prepare other rare earth oxides with various phases. [ABSTRACT FROM AUTHOR]

The thermal insulation performance of cubic ZrO2-CaO solid solution (CSZ) (16 mol % CaO) and monoclinic ZrO2 (MZ) was investigated in a two-stage multi-anvil press under a pressure of 10 GPa and temperatures up to 1800°C through in situ temperature measurements with the same cell assembly. The results show that the cell temperatures with CSZ as the thermal insulator were always higher than that when thermal insulator was MZ at the same heating power, and the maximum deviation value was 600°C (50% higher) within the measurable range. These two materials were analyzed using X-ray diffraction (XRD), thermal conductivity measurement and scanning electron microscopy. It revealed that the point defects existed in CSZ dominated the thermal insulation performance at high pressure, the same as what at normal pressure. It was also found that the heating process was more controllable when using CSZ as thermal insulator under high pressure. [ABSTRACT FROM AUTHOR]

Magnesium oxide (MgO) doped with cobalt (Co) semi-sintered pressure-transmitting medium (MCOPM) was prepared at 1200°C in air, starting with a pre-compressed mixture of MgO and cobalt oxide (CoO). The samples were characterized using powder X-ray diffraction, scanning electron microscopy, and thermal conductivity measurements. The results demonstrate that during the sintering process, the exchange of metal ions between the mixture powders introduced lattice defects into the MgO lattice structure, and an MgO-CoO solid solution was formed. Compared to commercial semi-sintered MgO pressure-transmitting medium (SSMPM), the temperature-generation efficiency of MCOPM-17 (MgO + 17 mol% CoO) improved by about 220% at the high pressure of 12 GPa with a cell assembly in a two-stage multi-anvil large volume press. Moreover, the thermal conductivity of CoO-doped MgO decreased with increasing CoO doping. [ABSTRACT FROM AUTHOR]

Binary ε -Fe 3 N 1.098 and ternary ε -Fe 2.322 Co 0.678 N 0.888 were synthesized as spherical bulk materials using novel high-pressure solid-state metathesis reactions (HPSSM). The structural and magnetic properties of the two nitrides were investigated using neutron powder diffraction (NPD), a vibrating sample magnetometer (VSM), and first-principle calculations. We found that at high pressure and high temperature (HPHT), nitrogen atoms enter the interstitial 2d sites of the nitrogen-rich ε -Fe 3 N 1.098 structure, space group P 6 3 22, which were previously reported to be vacant. The stoichiometry of the nitride (N/Me ratio) and the level of disorder have a significant influence on the site occupancies and local magnetic moments in these iron-based nitrides. The substitution of Fe by Co in ε -Fe 2.322 Co 0.678 N 0.888 did not reduce the mean magnetic moment ( μ m ) per metal atom at the Wyckoff 6 g site. NPD refinements showed that the magnetic moment per Fe atom in ε -Fe 3 N 1.098 (at 1.8 (4) μ B ) was slightly higher than that of ε -Fe 2.322 Co 0.678 N 0.888 , where the moment per Fe/Co atom is 1.7(2) μ B . [ABSTRACT FROM AUTHOR]

Measuring the pressure of a gasket (Pg) and cell (Pc) in situ is the key point to understanding the mechanism of pressure-generation and pressure-seal for the widely used large volume cubic press. However, it is a challenge to measure Pg due to the large deformation in the gasket zone and the complex rheological behavior of the pressure transmitting medium. Herein, a method of in situ electric resistance measurement has been developed to measure Pg. The open circuit failure in electric resistance measurement was avoided by using powder electrodes which could match the mould-pressed pyrophyllite cube in rheological behavior during compression. The relationships between press-load vs. Pc and press-load vs. Pg were obtained through in situ electric resistance measurements of bismuth, thallium, barium, and manganin. It was found that Pg exceeded Pc at around 5 GPa and Pc generated in the large volume cubic press was limited to the rapid rise of Pg above 5 GPa. Furthermore, the maximum ΔP (ΔP = Pc − Pg) above 0.9 GPa has been observed when Pc was released to a pressure range of 3–4 GPa, and this also leads to a large probability of high pressure cavity seal failure. [ABSTRACT FROM AUTHOR]

Abstract: Ferroelectrics with perovskite‐like layered (PL) structure are well‐known for their high Tc and the application prospect of high‐temperature‐piezoelectric sensing. In this study, the PL‐structure Eu2Ti2O7 was prepared by 1‐step high‐pressure sintering, which show the pyrochlore structure of Eu2Ti2O7 would change into PL structure at 11 GPa, 1300°C. The PL‐structure Eu2Ti2O7 is metastable, which will change back to pyrochlore structure at about 900°C in the air. The PL‐structure Eu2Ti2O7 was confirmed as a high‐temperature ferroelectric material for the first time. The ferroelectric domain switching was directly observed using piezoelectric force microscope. The piezoelectric constant of the PL Eu2Ti2O7 ceramic was measured as 0.7‐0.9 pC/N and its thermal depoling temperature (Td) was determined as 800°C, which is associated with the PL‐pyrochlore transition. [ABSTRACT FROM AUTHOR]

Crystal solubility is one of the most important thermo-physical properties and plays a key role in industrial applications, fundamental science, and geoscientific research. However, high-pressure in situ measurements of crystal solubility remain very challenging. Here, we present a method involving high-pressure neutron diffraction for making high-precision in situ measurements of crystal solubility as a function of pressure over a wide range of pressures. For these experiments, we designed a piston-cylinder cell with a large chamber volume for high-pressure neutron diffraction. The solution pressures are continuously monitored in situ based on the equation of state of the sample crystal. The solubility at a high pressure can be obtained by applying a Rietveld quantitative multiphase analysis. To evaluate the proposed method, we measured the high-pressure solubility of NaCl in water up to 610 MPa. At a low pressure, the results are consistent with the previous results measured ex situ. At a higher pressure, more reliable data could be provided by using an in situ high-pressure neutron diffraction method. [ABSTRACT FROM AUTHOR]

As a new member in the family of Mott system, perovskite PbCrO3 has recently been uncovered to exhibit fantastic structural transition under pressure, coupled with magnetic, electronic, and ferromagnetic transitions, which provide many opportunities for understanding of correlated system. However, it is still challenging to synthesize high-quality single-crystal PbCrO3, leading to the limited exploration of this Mott compound. In this work, we formulate a new high-pressure reaction route for preparation of high-quality PbCrO3 crystals between PbCl2 and Na2CrO4 at high pressure of 5-10 GPa and at high temperature of 750-1500°C. Because of the formation of reaction byproduct NaCl, the final product can readily be separated by washing with water. The obtained sample is in the form of single crystal with crystallite size up to 200 μm. In addition, combined with X-ray diffraction measurement, a tentative pressure-temperature synthesis diagram of PbCrO3 is mapped out from the reaction between PbCl2 and Na2CrO4 and the reaction mechanism is also explored in detail. [ABSTRACT FROM AUTHOR]

Single-crystal platinum diselenide (PtSe) is successfully synthesized through direct reaction of metal platinum foil and selenium powder under high pressure and high temperature. Characterizations are performed by scanning electron microscopy observation, X-ray diffraction technique and atomic force microscopy, which show that the largest crystallite size is more than 15 microns. Our studies not only lead to a new method for the growth of single-crystal PtSe under high pressure, but also provide a new technique and method in expanding the family of transition-metal dichalcogenides. [ABSTRACT FROM AUTHOR]

Pure diamond compacts with submicron grain size were prepared by sintering diamond powder at 14.0 GPa and 1600–2000 °C using a multi-anvil apparatus. The synthesis conditions were much milder than those required when using graphite as the precursor. The sintered polycrystalline diamond (PCD) displayed excellent mechanical properties comparable to those of nano-polycrystalline diamond. The highest Vickers hardness measured for PCD was 125.0 GPa. During sintering, the small diamond grains between larger grains transformed into graphite first at moderate sintering temperature, and then the graphite transformed into diamond at 14.0 GPa and higher temperature. The large amounts of nano-twins and stacking faults inside the diamond grains contributed to the high Vickers hardness of PCD. [ABSTRACT FROM AUTHOR]

Mechanical-performance of polycrystalline cubic boron nitride (PcBN) sintered using micron-sized powder particles might exceed that of samples produced from submicron crystals. However, no effective method has been found yet. In this study, commercial cubic boron nitride (cBN) powder (∼5 μm) was used to sinter high-performance PcBN without a binder. Two processes, isothermal compression (IC) and isothermal isobaric sintering (IIS), were used and compared. PcBN sintered with the former method possesses much more excellent mechanical performance, with the average Vickers hardness and indentation crack resistance of 73 GPa and 8.4 MPa·m1/2. The new temperature and pressure path adopted in the IC process can largely improve the hardness of PcBN sintered even with micro-sized powder. The quantitative analysis results of the defect structure in samples can be used as a reference for controlling the defect structure density to improve the material performance. • High hardness polycrystalline cubic boron nitride was manufactured. • New temperature and pressure path for preparing high performance superhard materials. • Positive effect of defect density on sample hardness is quantified. [ABSTRACT FROM AUTHOR]

Exploring the factors affecting the wear resistance of superhard materials is interesting and has practical industrial applications. Characterizing the hardness and grain size of superhard materials is more convenient and simple than measuring their wear resistance. Therefore, we expect to obtain a quantitative relationship between the hardness and grain size of superhard materials and their wear resistance. In this work, a series of polycrystalline cubic boron nitride (PcBN) compacts with different grain sizes and hardnesses were sintered under high temperature and pressure. Furthermore, turning tools were manufactured using these PcBN samples. The edge wear of the turning tool during the turning of granite was examined. Based on aforementioned measurements, the relationship between hardness and wear resistance is examined with same grain size (1–2 μm) and the influence of PcBN grain size on wear resistance is investigated under the same hardness (44.0 ± 1.2 GPa). The results show that the wear resistance of PcBN exhibits a regression parabolic and linearly increasing trends with the increase in hardness and grain size, respectively. Further investigation reveals that the compactness of PcBN materials, the bonding strength between the crystal grains, and the depth of the embedded crystal grains are critical factors affecting the wear resistance. These works can serve as a practical guide for the preparation as well as research and development (R&D) of PcBN and are expected to be extended to the R&D and application of other superhard materials and hard ceramics. [ABSTRACT FROM AUTHOR]

We report the combined experimental and theoretical investigations on high pressure structural stability of niobium carbide (NbC). The compressibility of NbC has been measured using angle-dispersive synchrotron X-ray diffraction in a diamond anvil cell (DAC) at room temperature and high pressure (up to 38.0 GPa), complemented with first-principles density-function theory calculations. The results imply that NbC shows bulk modulus of = 281 (6) GPa with a pressure derivative = 6.2 (1.5) for its strong covalent bonding. In addition, indentation testing on the well-sintered bulk NbC obtained at 5.0 GPa/1400°C yielded a Vickers hardness of 19.2 GPa and fracture toughness of 7.7 MPa m1/2at applied load of 10 kgf, demonstrating that binderless NbC prepared under high pressure should be a prospective hard material. [ABSTRACT FROM PUBLISHER]

Multicomponent alloys prepared using different experimental methods under atmospheric pressure have been widely studied. However, research on multicomponent alloys involving methods using high pressures and high temperatures is still in its infancy. In the high-pressure CoCrFeNiMo x alloy, the combination of each element atom and the change in the sample crystal structure indicate a relationship between the evolution mechanism of the multi-element alloy microstructure and high pressure. We observe that molybdenum (Mo) atoms enter the medium-entropy alloy (MEA) lattice near its theoretical recrystallization temperature to form a high-entropy alloy (HEA). The diffusion of Mo atoms requires the energy provided by the temperature to overcome the low-entropy effect under high pressure, Mo atom reacts with MEA near its recrystallization temperature to form HEA means the formation of HEA under high pressure follows the specific steps. High pressure inhibits grain growth and generates numerous dislocations in the crystal, thereby preparing the high-pressure CoCrFeNiMo x alloy with a high hardness of 3.7 ± 0.12 GPa, which is harder than the CoCrFeNiMo x alloy prepared under atmospheric pressure. Additionally, the generalized high-pressure HEA constant is introduced, which is 1 ≥ P ≥ 0.17, proving that high-pressure promotes the synthesis of HEAs. This constant provides a new method for studying the thermodynamic behavior of HEAs under extreme conditions. • High-entropy allys are synthesized under extreme condition. • The formation of high-entropy allys follow the fixed step. • A generalized high-pressure high-entropy alloys constant is proposed. [ABSTRACT FROM AUTHOR]

Highlights • The nucleation and growth process of bismuth were studied by our experiments. • We observed an abnormal grain coarsening behavior of bismuth under high pressure. • Low nucleation rates and recrystallization mainly lead the abnormal grain coarsening. Abstract It is well known the atomic diffusion caused by high temperature leads to conspicuous grain coarsening, which is of crucial importance to the properties of materials. High pressure usually slows down the diffusion rate of atoms and refines the grain size. In this study, using synchrotron angle dispersive x-ray diffraction (ADXRD) techniques, we observed an abnormal grain coarsening behavior of bismuth (Bi) stemming from the high pressure phase transition at room temperature, demonstrating that the grain coarsening can also be triggered by pressure. Two factors mainly give raise to this unusual coarsening behavior of Bi: (1) the nucleation rate of new phase is very slow during transformation according to the kinetics study of the phase transition in Bi, which inhibits the grain refinement; (2) the liquid interphase in the phase transition process enables the recrystallization of Bi under high pressure. Our experiment results are consistent with the previous theoretical calculations, which is crucial for describing the nucleation and growth process and clarifying the phase transition mechanisms in Bi. [ABSTRACT FROM AUTHOR]