Search

Size matching between anvils and the pressure transmitting medium (PTM) is a key factor that affects pressure generation and sealing for a large volume cubic press. In this work, we studied the influence of PTM sizes from 30.5 mm to 34.5 mm at a fixed anvil geometry dimension (23.5 mm) on the pressure efficiency and sealing performance by measuring the pressure of the gasket and cell simultaneously at room temperature. Wires made of Bi, Tl, Ba, or Manganin were used for pressure calibration experiments within a pressure range of up to 6 GPa. It was found that a PTM with an edge length of 33.5 mm had the highest pressure-generation efficiency, but its sealing performance was the worst. Furthermore, it was confirmed that a PTM with an edge length of 32.5 mm had the best overall performance for a 23.5 mm anvil when both efficiency and sealing were considered. The results show that the pressure-generation efficiency and sealing performance are highly sensitive to PTM size. It is less rigorous to gauge the performance of the assembly only by the pressure-generation efficiency. This work provides practical guidelines and contributes to optimizing the design of the high-pressure assembly. [ABSTRACT FROM AUTHOR]

Plastic deformation of alumina (Al2O3) under high pressure was investigated by observing the shape changes of spherical particles, and the near fully dense transparent bulks were prepared at around 5.5 GPa and 900 °C. Through analyzing the deformation features, densities, and residual micro-strain of the Al2O3 compacts prepared under high pressures and temperatures (2.0-5.5 GPa and 600-1200 °C), the effects of plastic deformation on the sintering behavior of alumina have been demonstrated. Under compression, the microscopic deviatoric stress caused by grain-to-grain contact could initiate the plastic deformation of individual particles, eliminate pores of the polycrystalline samples, and enhance the local atomic diffusion at the grain boundaries, thus produced transparent alumina bulks. [ABSTRACT FROM AUTHOR]

Diamond and gold powders were compressed nonhydrostatically in a diamond anvil cell and examined by x-ray diffraction using a radial geometry to evaluate the evolution of stresses and strains in these materials to ultrahigh pressure. We found that near isostrain continuity developed across diamond and gold grains under uniaxial compression. The observed mean pressure of diamond powder reached to 360(40) GPa while it was only 31(1) GPa for the polycrystalline gold under the highest load. Polycrystalline diamond can support a microscopic deviatoric stress of 160(18) GPa at about 360 GPa. Due to the deformation of the diamond anvil culet, the macroscopic differential stress of the diamond sample was limited to about 43(8) GPa. There is no evidence that the diamond grains have yielded in our experimental pressure range. [ABSTRACT FROM AUTHOR]

Hexagonal boron nitride (hBN) was used as the additive in improving the hardness and thermal stability of polycrystalline diamond compact under high temperature and high pressure. Indentation testing on diamond/cubic boron nitride (cBN) composites indicated a Vickers hardness of 96 ± 7 GPa, rivaling the hardness of monocrystal diamond. Thermal gravimetric analysis (TGA) suggested that the composites of diamond/cBN are thermally stable in air up to ∼910 °C which is higher than that of pure polycrystalline diamond compact (∼750 °C). Raman, TGA, x-ray photoelectron spectroscopic and microstructure analyses all revealed the features of diamond/cBN composition beyond both diamond and cBN. [ABSTRACT FROM AUTHOR]

The grain size dependence of the mechanical properties of cBN-Si composites prepared using the high pressure infiltration method has been investigated. Indentation testing indicates that cBN-Si composites have hardness values of 38–43 GPa, which increase with increasing grain size and are harder than traditional polycrystalline cBN composites (PcBNs). Thermostability analyses display that cBN-Si composites with a grain size of > 9 μm also possess a higher temperature of oxidation, compared to traditional PcBNs, and the thermostability increases with increasing cBN grain size. Fracture toughness tests show that almost no cracks appear on the polished cBN-Si samples when the loading forces are increased to 294 N and the fracture toughness is better than for commercial samples. Scanning electron microscopy illustrates that deformations and close pores occurred easily between coarse BN grains, leading to denser cBN-Si compacts with better mechanical performances. [ABSTRACT FROM AUTHOR]

In this paper, two average tungsten carbide particle sizes of 2, 0.5 μm are placed respectively, in contact with a WC-16Co substrate, pressed at the pressure of 4.5–5.5 GPa, and heated to temperatures ranging from 1350°C to 1500°C in a large-volume cubic press. During the process Co was forced out of the WC-16Co substrate into the compressed powder. The resulting infiltrated samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), Vickers hardness and cutting performance tests. The results of XRD confirmed that the sintered bulks have WC and Co phases. The scanning electron microscopy (SEM) analysis reveals that the WC grains in well-sintered alloys are round in shape and cobalt with lower content is uniformly dispersed in the WC grain boundaries. The sintered sub-micron WC-Co alloy with a cobalt content of 3.8 wt% exhibits a prominent combination of high hardness value of 23.1 GPa and a large fracture toughness value of 8.6 MPa m½. The high-speed cutting tests indicating its cutting performance is significantly superior to the commercial YG6X (WC-6 wt%Co with WC grain size of 0.5 μm). [ABSTRACT FROM AUTHOR]

Alumina nano-powder was used as the cement in bonding coarse spherical particles to fabricate transparent alumina ceramics under high pressure and modest temperature. Observations on the microstructure, density and microhardness of Al 2 O 3 specimens prepared at 500–1200 °C under 5.0 GPa provided significant effects of additive nano-particles on the sintering of samples. The high pressure initiated plastic deformation of spherical particles, rearranged nano-particles filling the clearance among spherical particles and bonding each other to densify the products. The additive particles also contribute to grain boundary strengthening to obtain samples with enhanced transparency and microhardness. [ABSTRACT FROM AUTHOR]

Polycrystalline cubic boron nitride (PcBN) compacts with high hardness, high thermal stability and high cutting performance have been prepared through method of high pressure infiltration of silicon melt. The scanning electron microscopy (SEM) and metallographic observations indicates that cBN–cBN bonding appeared extensively in the sintered samples. The hardness tests give an average Vickers hardness of 43 GPa for the well-sintered cBN–Si compacts, which is much higher than the conventional PcBN composites (~ 30 GPa). The onset temperature of oxidation can reach to 1300 °C, about 300 °C than that of the commercial PcBN (~ 1000 °C). According to the results of turning test, the cutting performance of our sintered samples is also superior to the commercial PcBN. [ABSTRACT FROM AUTHOR]

ZrO 2 whiskers have been successfully synthesized at temperature of 1800 °C and high pressure of 15 GPa, using ZrO 2 powder as the starting material. After a high pressure and high temperature (HPHT) process, a bunch of long and idiomorphic ZrO 2 whiskers are obtained. The grown whiskers look like needles, and have silky luster. They are colorless and transparent, and exhibit hexagonal, triangular, quadrilateral, and irregular shaped cross sections. Through this HPHT method, whiskers with various diameters from 10 to 20 μm, and length from 200 to 1000 μm can be produced. The aspect ratio of length to diameter can be from 10 to 100. Growth mechanism of the ZrO 2 whiskers prepared under HPHT conditions are discussed, and the ZrO 2 whiskers growth may undergo a vapor–solid process. [ABSTRACT FROM AUTHOR]

Superhard materials are solids whose Vickers hardness is beyond 40 GPa. They have wide applications in industry such as cutting and polishing tools, wear-resistant coatings. Most preparations of superhard materials are conducted under extreme pressure and temperature conditions, not only for scientific investigations, but also for the practical applications. In this paper, we would introduce the recent progress on the design and preparations of novel superhard materials, mainly on nanopolycrystalline diamond, B-C-N superhard solid solutions, and cubic-SiN/diamond nanocomposites prepared under ultrahigh pressure and high temperature (HPHT), using multi-anvil apparatus based on the hinged-type cubic press. Bulk materials of all these superhard phases have been successfully synthesized and are systematically tested. We emphasize that ultra-HPHT method plays an important role in the scientific research and industrial production of superhard materials. It provides the driving forces for the light elements forming novel superhard phases as well as the way for sintering high-density nanosuperhard materials. [ABSTRACT FROM AUTHOR]

Diamond possesses a unique combination of excellent optical, thermal, and mechanical properties, and is therefore an ideal transparent ceramic material for harsh and extreme environments. Due to its important applications in technology, transparent diamond ceramic (TDC) has been explored and prepared by chemical vapor deposition (CVD) or direct conversions of non-diamond carbon precursors at high pressure and high temperature (HPHT), but the preparation of large-size TDC with high mechanical strength remains a challenge. Here, we report for the first time, a transparent polycrystalline diamond ceramic from diamond powder with a transmittance of ∼60 % at wavelengths of 400–1600 nm. The analyses of phase composition, residual stress and microstructure evolution of the sintered samples with different sintering conditions indicate that compression at high temperatures (>2000 ⁰C) facilitates the deformation of diamond grains, allowing for densification and diamond-diamond bonding formation. The sintering pressure of the diamond powders with an optimized particle size distribution was dramatically reduced from 16 GPa to 10 GPa. Our results, based on successfully preparing centimeter-sized TDC, set the standard and the precedent for the large-scale preparation of larger TDC in proximity to industrial conditions. [ABSTRACT FROM AUTHOR]

The pressure transmitting medium (PTM)-dependent structure stability of ∼12 nm nanoanatase TiO2under high pressure has been investigated usingin situsynchrotron X-ray powder diffraction techniques at room temperature. We found that the starting anatase phase (I41/amd) transforms into the monoclinic baddeleyite phase (P21/c) regardless of embedded in PTM or not. However, the beginning and ending pressure for the transformation under quasihydrostatic compression is found to be ∼7 GPa higher than that of under nonhydrostatic compression, and amorphization of sample occurs more easily during the former condition. This result indicates that the interface energy between the PTM and nanoparticles undoubtedly modifies the phase stability during high pressure experiments. [ABSTRACT FROM PUBLISHER]

We have investigated the size-dependent high pressure behavior of nanocrystalline TiN using an angle-dispersive X-ray diffraction technique in a diamond-anvil cell at room temperature. No phase transition was observed during compression for each sample. A fit of the pressure versus volume data to a second Birch–Murnaghan equation yielded the following parameters for 16nm, 34nm and 80nm, zero-pressure volume, V 0=75.77Å3, 75.98Å3 and 76.06Å3, bulk modulus, B 0=320(7)GPa, 338(6)GPa and 287(3)GPa, respectively. This result along with a reanalysis of previous studies on TiN indicates that the bulk modulus first increases and then decreases with decreasing grain size. The compressibility of TiN has a minimum at ~34nm. [ABSTRACT FROM AUTHOR]

Phase stability of Ti3SiC2 was studied under high pressure and high temperature using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. From the results obtained, the decomposition temperature of Ti3SiC2 decreases quickly against pressure, and the low temperature limits of phase segregation of the sample Ti3SiC2 lie between 1100°C and 1000°C, 1000°C and 900°C, 900°C and 800°C, under high pressures of 3, 4 and 5GPa, respectively. Ti3SiC2 decomposes to generate TiC, SiC, and TiSi x . On the basis of the experimental results, we suggest two decomposition models to explain the phase decomposition of Ti3SiC2 at high pressure and high temperature. [ABSTRACT FROM AUTHOR]

High pressure and high temperature synthesized MoB2and WB2compacts were investigated using X-ray diffraction, energy dispersive spectroscope, scanning electron microscope, Vickers indentation test and ultrasonic measurements. Experiments showed that both MoB2and WB2compacts are phase pure and with a grain size of 100–200 nm. Vickers indentation test under a large loading force of 49 N showed that the Vickers hardness of MoB2and WB2are about 21 and 22 GPa, respectively. The bulk modulus and shear modulus are about 296 GPa, and 190 GPa for MoB2and 349 and 200 GPa for WB2through ultrasonic measurements. Our results indicate that MoB2and WB2are both hard materials with a hardness similar to that of tungsten carbide, which is widely used in industry. [ABSTRACT FROM PUBLISHER]

Abstract: Phase transitions of both α-BiNbO4 and β-BiNbO4 were investigated under high pressure and high temperature (HPHT) (0–5GPa and 300–1800°C, respectively) using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive spectroscope (EDS). The stable regimes for the BiNbO4 phases were also determined. Experiments showed that both α-BiNbO4 and β-BiNbO4 could convert to a new high pressure phase HP-BiNbO4 above 3GPa, and the transition temperatures decreased with increasing pressures. Below 3GPa, β-BiNbO4 keeps stable up to 1800°C, while α-BiNbO4 converts to β-BiNbO4 when temperature is higher than 1150°C. By heating the metastable HP-BiNbO4 phase at room pressure, HP-BiNbO4 converts to α-BiNbO4 at 600°C. Above 1150°C, HP-BiNbO4 would transform to β-BiNbO4. Phase relationships of Bi2O3, Nb2O5, α-BiNbO4, β-BiNbO4 and HP-BiNbO4 were also summarized. [Copyright &y& Elsevier]

Abstract: Well-crystallized graphite and high energy ball-milled graphite were treated under 16GPa and temperatures up to 2500°C. Our experiments indicated that the ball-milled graphite can completely transform to nano-polycrystalline diamond (NPD) at 2100°C, while the well-crystallized graphite required 2500°C. The samples were characterized by X-ray diffraction, optical microscope, scanning electron microscope, and Vickers indenter hardness test. We found that high pressure not only made the conversion of graphite to NPD thermodynamically preferred, but also reduced the needed activation energy barrier. By analyzing thermodynamics and kinetics of the transformation, we suggested a low boundary for NPD forming region based on the phase diagram of carbon, and the formation mechanisms of Nano-polycrystalline diamond. [Copyright &y& Elsevier]

A new method is introduced for investigating the compressibility of solids under high pressure by in situ electrical resistance measurement of a manganin wire, which is wrapped around the sample. This method does not rely on the lattice parameters measurement, and the continuous volume change of the sample versus pressure can be obtained. Therefore, it is convenient to look at the compressibility of solids, especially for the X-ray diffraction amorphous materials. The I–II and II–III phase transition of Bi accompanying with volume change of 4.5% and 3.5% has been detected using the method, respectively, while the volume change for the phase transition of Tl occurring at 3.67 GPa is determined as 0.5%. The fit of the third-order Birch–Murnaghan equation of state to our data yields a zero-pressure bulk modulus K 0=28.98±0.03 GPa for NaCl and 6.97±0.02 GPa for amorphous red phosphorus. [ABSTRACT FROM AUTHOR]

Abstract: High pressure in situ differential thermal analysis (DTA) was applied to study phase transitions of Na0.5Bi0.5TiO3 (NBT) up to 5GPa. DTA analysis at room temperature with pressure as the variable indicated NBT samples have two pressure induced phase transformations at about 0.7GPa and 2.5GPa respectively. Below 0.7GPa, two temperature induced phase transitions could be observed at around 220°C and 520°C as it was reported at ambient pressure. But under a pressure higher than 0.7GPa, these two temperature induced phase transitions would not happen. In situ electrical resistance measurements showed that the resistance of NBT changes smoothly, and the tendency is similar to that of electrical insulators like pyrophyllite. We suggest that the pressure induced phase transitions at 0.7GPa and 2.5GPa are both insulator to insulator phase transitions. [Copyright &y& Elsevier]

Abstract: Powder mixture of graphite and silicon nitride with hexagonal structure (α/β-Si3N4) was ball-milled in nitrogen and compressed under ~16GPa/1800°C and ~18GPa/2000°C, to investigate the possibility for replacing Si atoms by C atoms in cubic spinel silicon nitride (c-Si3N4). The sintered samples were characterized by X-ray diffraction (XRD), hardness measurement, optical microscope, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results show that the well-sintered compacts were composed of nanocrystalline c-Si3N4 and nanocrystalline diamond, and neither C3N4 and CSi2N4 nor other reaction products of Si3N4 and C were observed. Vickers hardness test shows that the average hardness of the samples is 41–42GPa. As a comparison, phase-pure c-Si3N4 bulks also have been synthesized and their hardness was confirmed to be about 31GPa. The pure c-Si3N4 sintered bulks exhibit a high frangibility, but their fracture toughness could be largely improved by adding carbon (nanocrystalline diamond) in the sintered compacts. [Copyright &y& Elsevier]

A hybrid pressure cell of pyrophyllite and magnesium oxide (HPCPM) used in large volume cubic presses is presented. In the HPCPM, a cubic frame which is made of pyrophyllite with face-centered square holes works as gaskets, and a heteromorphosis magnesium oxide works as the pressure-transmitting medium. Our experimental results indicated that the pressure-generation efficiency using the HPCPM was improved by about 40% than that using the traditional pyrophyllite pressure cell without decreasing the anvil truncation size (without sacrificing the sample volume). The HPCPM could pressurize samples of 1000 mm3 volume up to about 8 GPa, which is significantly higher than that available using the traditional pressure cell, which reports a highest pressure of about 6 GPa. [ABSTRACT FROM AUTHOR]

Single-phase BC4N can be formed at high pressure and temperature, starting with either a mixture of diamond and cubic boron nitride or graphite and hexagonal boron nitride. The samples exhibit extreme Vickers hardness (85±4GPa), reaching the hardness of single crystal diamond. Within a random solid solution model, the X-ray diffraction patterns of cubic BC x N solid solutions were calculated. The results indicate that the prepared B–C–N materials are continuous solid solutions with a zinc-blende structure. [Copyright &y& Elsevier]

We propose a sintering mechanism of transparent Y3Al5O12 nanoceramics under high pressure. Through analysis of the porosity, grain size and residual stress for the nanostructured YAG compacts prepared at high pressure and modest temperature (HPMT, 2.0–5.0GPa and 300–500°C), the nanosintering behavior was systematically investigated. High pressure can initiate the plastic deformation of nanograins to eliminate pores and enhance the local atomic diffusion among grain boundaries, thus producing fine nanostructured and near fully dense transparent YAG bulks. [Copyright &y& Elsevier]

Quantitative measurements of pressure gradients have been made in the pyrophyllite and magnesium oxide (MgO) pressure-transmitting mediums using large-volume cubic apparatus. The relationships of pressure gradients along the axis of symmetry of the pyrophyllite and MgO cubes versus corresponding cell pressures have been established at room temperature. Our experimental results indicated that the pressure gradients increase as the cell pressure increases and that the pressure gradient in the MgO pressure-transmitting medium is about twice as high as that in the pyrophyllite pressure-transmitting medium at the same cell pressure. When the cell pressure reaches 5.5 GPa, the pressure gradient along the axis of symmetry of the pyrophyllite cube is about 50 MPa/mm. When the cell pressure reaches 7.7 GPa, the pressure gradient along the axis of symmetry of the MgO cube is about 140 MPa/mm. The experimental data obtained in the present work not only adapt to the cubic apparatus but also provide reference for the tetrahedral press and the double-stage multi-anvil apparatus (the octahedral compression). [ABSTRACT FROM PUBLISHER]

Abstract: Nanosintering behaviour of MgAl2O4 ultra-fine powder was investigated under high pressure. We found that the nanosintering mechanism under high pressure was quite different from that under ambient pressure. To shed light on the mechanism, the residual stress, grain size, and boundary situation were investigated using HRTEM and XRD, respectively. Our results show high pressure can restrain grain growth and initiate plastic deformation to eliminate pores and/or additional phases existing in triple junctions of the grains. However, the conventional sintering process is controlled by grain growth to avoid imperfections between grain boundaries when preparing transparent ceramics. [Copyright &y& Elsevier]

Cell pressures in the cubic press have been determined by means of differential thermal analysis (DTA) based on the known fusion curve of aluminum. The results compared favorably with those of the well-known fixed-point method, based on pressure-induced phase transitions in bismuth (I-II, 2.55 GPa) and thallium (II-III, 3.68 GPa). The high-pressure fusion curve of lead was measured using our calibrated results, which agreed well with those of the previous work. All of this was made possible by the development of a successful cell assembly for the DTA analyses in the cubic press. Details of the design are described in the present work. [ABSTRACT FROM AUTHOR]

Abstract: The coordination and structure changes in LiBO2 have been studied at high pressure and temperature up to 5GPa and 1500°C using in-situ high-pressure differential thermal analysis, infrared absorption spectra and X-ray diffraction. The layer framework structure of α-LiBO2 is found to be compressed easily along the direction of c-axis, resulting in the formation of tetra-coordinated BO4 units. The phase transition boundaries between α- and γ-LiBO2 as well as between amorphous LiBO2 hydrate and γ-LiBO2 have negative pressure–temperature slopes. The conditions for transformation from α- to γ-LiBO2 are lower than that necessary to transform amorphous LiBO2 hydrate to γ-LiBO2. Moreover, the melting curve of LiBO2 has also been determined and has a positive pressure–temperature slope. Upon quenching from high pressure, LiBO2 may not contain [3]B–O–[3]B rings but contain more fraction of [4]B units with increasing pressure. [Copyright &y& Elsevier]

Abstract: This work presents a comprehensive study on phase transitions in LiAlO2 system at high pressures and temperatures (0.5–5.0GPa and 300–1873K, respectively), as well as the phase stability for polymeric phases of LiAlO2 in the studied P–T space by X-ray diffraction (XRD). Besides the previously described polymorphic hexagonal α-phase, orthorhombic β-phase and tetragonal δ-phase, a possible new phase of LiAlO2 was observed after the tetragonal γ-LiAlO2 sample was treated at 5.0GPa and 389K. The stable regimes of these high-pressure phases were defined through the observation of coexistence points of the polymeric phases. Our results revealed that LiAlO2 could experience structural phase transitions from γ-LiAlO2 to its polymorphs at lower pressures and temperatures compared to the reported results. Hexagonal α-LiAlO2 with highly (003) preferential orientation was prepared at 5.0GPa and 1873K. [Copyright &y& Elsevier]

Pyrophyllite is widely used both as a gasket material and as a pressure-transmitting medium, especially for a large-volume press. Its pressure-transmitting efficiency mainly depends on the following factors: geometry of anvil and pyrophyllite block, chemical composition, density and strength. For a long time, most studies have been devoted to the first two factors, whereas the third one has been studied less. In this work, we report that the pressure-transmitting efficiency of pyrophyllite is improved by precompressing. The results show that pyrophyllite was fully densified when the pressure reached about 1 GPa. The press load for Ba I-II transition in the cubic large-volume pressure cell is reduced more than 8×105 N by using the densified pyrophyllite, compared with the unprecompressed pyrophyllite. Namely, the press-load utilization efficiency was improved above 15% when the precompressed pyrophyllite is used as a pressure-transmitting medium. [ABSTRACT FROM AUTHOR]

Amorphous metallic alloys lack long-range atomic order and consequently exhibit excellent homogeneity, no microstructure discontinuities, and no sharp x-ray diffraction peaks. Moreover they have higher tensile fracture strength and hardness than those of traditional crystalline metals. These excellent physical properties make bulk metallic glasses good candidates for high pressure gaskets for in situ x-ray/neutron diffraction experiments. We tested the Pd[sub 40]Ni[sub 40]P[sub 20] amorphous alloy as a gasket material in three experiments. The behavior of the Pd[sub 40]Ni[sub 40]P[sub 20] amorphous alloy under a hydrostatic/nonhydrostatic pressure of over 100 GPa was investigated by in situ synchrotron x-ray diffraction. The pressure gradient of the amorphous metallic gasket in the diamond anvil cell was measured on the ruby fluorescence scale during compression and decompression. The results show that bulk metallic glasses are good high-pressure gasket materials for in situ x-ray diffraction experiments. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The concurrent enhancement of toughness, electrical conductivity, and thermal stability in polycrystalline diamond composites represents a formidable challenge. Here, a novel diamond-TiC composites (referred to as DTC) with high fracture toughness and conductivity were synthesized through a high-temperature and high-pressure reaction sintering process utilizing two precursors: diamond mixtures containing TiH 2 and Ti-coated diamonds. On the one hand, the inherent bonding and encasing of TiC onto the diamond matrix form a heterogeneous structure, providing high strength and excellent fracture toughness up to 11.6 MPa·m1/2. On the other hand, the established oxygen barrier and current pathway offer a thermal stability of 765 °C and high conductivity up to 837 S·m−1. Remarkably, this rare combination of mechanical, electrical, and thermal properties has been realized within a single material under relatively moderate sintering conditions, thus positioning diamond-TiC composites as a promising ceramic for electrical applications in high-stress environments. [ABSTRACT FROM AUTHOR]

The Hugoniot elastic limit (HEL) is widely adopted as an important criterion for assessing the dynamic strength of materials, representing the transition stress from elastic to plastic response prior to failure under shock compression. Nano-polycrystalline diamond currently holds the highest HEL of 208 (±14) GPa. Here, we report a diamond-TiC composite (∼11.5 wt. % TiC) showing an ultrahigh HEL of at least 195 (±3.5) GPa, which is comparable to that of nano-polycrystalline diamond. All measured velocity profiles on the diamond-TiC free surface exhibited a single-wave structure at shock pressures of 48–195 GPa. Moreover, the measured Us–Up (shock wave velocity–particle velocity) relation can be linearly fitted, indicating no elastic–plastic transition or solid–solid phase transition up to a shock pressure of 195 GPa. The diamond-TiC composite's compression ratio was similar to that of TiC but significantly higher than that of diamond. These extraordinary dynamic responses are intrinsically attributed to the unique microstructure in which diamond polycrystals are encased in a TiC matrix, providing protection against yielding. Our findings not only developed a mechanically reliable, lightweight, and high-performance armor material at low synthesis costs, but also provided new insights into the shock compression behavior of diamond composites. [ABSTRACT FROM AUTHOR]

Hardness is an important indicator of the wear resistance of tool materials. The study of the effect of hardness on the wear resistance of tool materials has significant practical applications. In this experimental work, a series of polycrystalline cubic boron nitride (PcBN) and diamond (PCD) compacts with different hardness are sintered under high temperature and pressure, and these samples were fashioned into turning tools, used for edge wear testing in the granite turning process. Based on the measurements above, the relationship between hardness and wear resistance in two materials with identical grain sizes (1–2 µm) was examined, and the results reveal a significant positive correlation between the hardness and wear resistance of the two materials. Remarkably, the relationship between hardness and wear resistance in two materials complies with the same type of fitting function. But incredibly, the wear resistance of the two materials exhibits inconsistency as the hardness increases. Specifically, below 40 GPa Vickers hardness, both materials exhibit comparable wear resistance. However, above 40 GPa, the difference in wear resistance between the two materials becomes more pronounced with increasing hardness. At the Vickers hardness of 70 GPa, the abrasion ratio of the PCD is approximately 3.5 times that of the PcBN, and it is anticipated that the disparity between them will continue to expand within the broader range of hardness. Further analysis reveals that the intrinsic hardness of the crystal grains significantly impacts the wear resistance of polycrystalline materials. [ABSTRACT FROM AUTHOR]

In-situ time-of-flight neutron powder diffraction and electrical resistance measurements are performed to determine the behaviors of PbS under pressure and temperature up to 5 GPa and 773 K. The phase diagram in our experimental range is shown based on electrical resistance measurements. Both the two approaches indicate that only the starting cubic PbS phase remains stable when the temperature reaches above 573 K. The Rietveld refinements of the neutron diffraction patterns verify that the orthorhombic (ortho) PbS phase has a Cmcm space group. The fitting of experimental data with the Birch-Murnaghan equation of state at 300 K, yields the bulk modulus of the cubic phase as K0T = 60.0 ± 0.5 GPa with its pressure derivative K0′ fixed to 4.2. [ABSTRACT FROM AUTHOR]

We report the preparation of nanostructured diamond-TiC composites with high fracture toughness and high hardness starting from a ball-milled mixture of nano-sized Ti3SiC2 and submicron-sized diamond by simultaneously tuning the pressure-temperature conditions. The phase segregation of Ti3SiC2 at pressure of 5.5 GPa were investigated by X-ray diffraction and high resolution transmission electron microscopy, we found that the Ti3SiC2 could decompose into nanosized TiC and amorphous Ti-Si at 600-700 °C. The subsequent reaction between diamond and Ti-Si led to an amorphous Ti-Si-C matrix in which diamond and TiC crystals are embedded. With a loading force of 98 N, the measured fracture toughness KIC and Vicker's hardness HV of the synthesized composites reach up to 14 MPa m1/2 and 45.5 GPa, respectively. Our results demonstrate that the nanocrystalline/amorphous bonding matrix could largely enhance the toughness of the brittle composites. [ABSTRACT FROM AUTHOR]

Abstract: The Vickers hardness of pressure synthesized WB compacts was measured using a diamond indentation method. Under the applied load of 4.9N, our test gave a maximum Vickers hardness of 30.1GPa for WB compacts. The hardness of WC-8wt.% Co composites was also measured under the same load for comparison, which was found to be much lower than that of WB. The Young''s modulus, bulk modulus, and shear modulus of WB and WC-8wt.% Co were further investigated by ultrasonic measurements. The results suggest that WB might be a promising hard alloy material for industrial applications. [Copyright &y& Elsevier]

Abstract: In general, the high-density state is a preferred state of materials under high pressures. In this study, however, when the LiBO2–Al2O3 system quenched from melts at the cooling rate of 50–60 °C/s under high pressures, low density - Al2O3 was synthesized from high density - Al2O3, with a 7.1% volume expansion of Al2O3. While those samples which are quenched at the same cooling rate under a lower pressure appeared to be glassy. The minimal pressure synthesized the low-density - Al2O3 or the maximal glass forming pressure was decreased with increasing content of Al2O3 below its saturation point in LiBO2 melt. [ABSTRACT FROM AUTHOR]

An anvil-preformed gasket system has been developed to extend the pressure range for the widely used large volume cubic press without sacrificing the sample volume. The relationship of the sample chamber pressure versus press load for this system was calibrated at room temperature using transitions in Bi, Tl, and Ba. With similar sample volumes (8-11 mm in diameter and 8 mm in length), the anvil-preformed gasket system can generate pressures up to about 8.5 GPa, significantly higher than 6 GPa, which was generally the maximum pressure for the conventional anvil-gasket system. The details on the optimized design for the anvil-preformed gasket system are given in this note. [ABSTRACT FROM AUTHOR]

The new melting curve of cobalt (Co) was determined to be 12 GPa using the in situ high-pressure temperature measurement method (HPTM) in a high-volume cubic press using two melting criteria: first, plateaus in the temperature vs power functions in situ experiments, and second, the texture changes in the samples before and after high-pressure high-temperature (HP-HT) treatments with a scanning electron microscope (SEM). The slope of the melting point with increasing pressure was approximately 33 K/GPa in our experimental pressure range and progressively decreased under further compression. Our new melting curve of Co under high pressure is reasonably consistent with the simulation results using one-phase and two-phase calculations and has a similar trend with the reported melting curves of Fe and Ni. • We show the in situ high-pressure temperature measurement method in the large volume cubic press that was an effective and simple method to measure the melting point. • Two melting criteria were used: firstly, plateaux in temperature vs. power functions in in situ experiments and secondly, the texture changes for the samples before and after high-pressure high-temperature treatments with a scanning electron microscope. • The slope of melting point with increasing pressure was found to be about 33 K/GPa in our experimental pressure range and progressively decreases under a further compression. • Our new melting curve of Co under high pressure is reasonably consistent with the simulation results using one-phase and two-phase calculations and has a similar trend with the reported melting curves of Fe and Ni. [ABSTRACT FROM AUTHOR]

The Vickers hardness of boron suboxide single crystals was measured using a diamond indentation method. Under a loading force of 0.98 N, our test gave an average Vickers hardness of 45 GPa. The average fracture toughness was measured as 4.5 MPam[sup 1/2]. We also measured the hardness of the cubic boron nitride and sapphire single crystals for comparison. The average measured hardness for boron suboxide was found to be very close to that of cubic boron nitride under the same loading force. Our results suggest that the boron suboxide could be a new superhard material for industrial applications, surpassed in hardness only by diamond and cubic boron nitride. [ABSTRACT FROM AUTHOR]

The cold compression behavior of alumina samples, with average grain sizes of 350 nm, 7.9 μm, and 18.2 μm, has been investigated under non-hydrostatic conditions by using a diamond anvil cell. After treatment with various pressures, the in situ axial X-ray diffraction patterns, transparency variations, and microstructures after decompression of the samples were also analyzed. The results showed that the submicron grains became easily translucent. The larger the grain size, the higher the pressure needed to compress the sample to transparency. On the other hand, for a similar grain size, the various geometrical shapes of the grains did not affect the pressure dependency on transparency. The elastoplastic characteristics of the alumina grains of different sizes under cold compression were found to be consistent with the transparency variation. As a conclusion, alumina particles under cold compression were found to be applicable in the pressure-assisted preparation of alumina transparent ceramics. [ABSTRACT FROM AUTHOR]

Micro-grained diamond-SiC composites have been prepared by sintering a mixture of micro-grained diamond and Si powder using high-temperature and high-pressure method at 1100–1500 ° C , and 5.5 GPa. Plate impact experiments in reverse—and forward—impact geometry were used to investigate the shock response of the micro-grained diamond-SiC composites within a shock pressure range from 22 to 195 GPa. The obtained plot of shock velocity (D) vs particle velocity (u) indicated that the double elastic wave responses are observed in the pressure range of 22–170 GPa, which is consistent with the previously reported diamond-SiC composites containing a small amount of tungsten carbide (WC) under shock compression up to ∼ 110 GPa [Li et al., J. Appl. Phys. 128, 245901 (2020)]. The results of shock experiments demonstrate that the Hugoniot elastic limit of micro grained diamond in SiC matrix is as high as 170 GPa, which is nearly twice as high as that of single crystal diamond. [ABSTRACT FROM AUTHOR]

The enhanced optical properties of chemical vapor deposited (CVD) diamond have been pursued in academia and industry for many applications. However, the barrier of CVD technology limits the application field of diamond. Herein, the performance of CVD polycrystalline diamond thick films was improved by high-pressure and high-temperature (HPHT) treatment. The microstructures of CVD polycrystalline diamond films before and after HPHT treatments were thoroughly examined using optical microscope, UV–visible and infrared absorption, Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). It is found that the transparency of the CVD samples at 10 GPa increases dramatically with processing temperatures, from the original opacity to almost full optical transparency. Through spectroscopic and microstructural analyses, the modification mechanism of CVD polycrystalline diamond under HPHT conditions is proposed. The results show that the HPHT treatment can significantly enhance the optical properties of the starting CVD polycrystalline diamond films. [ABSTRACT FROM AUTHOR]

We have synthesized the well-performance bulk diamond–SiC composite by the high-temperature and high-pressure method and employed the light gas gun launched flyer plates to investigate shock compression response of the diamond–SiC composite. The plots of shock velocity vs particle velocity demonstrate the presence of double elastic waves in the diamond–SiC composite under shock compression. The first elastic wave travels at 13.1 ∼ 13.6 km / s and leads to yielding at ∼ 12.75 GPa. The second elastic wave propagates at 12.8 ∼ 13.0 km / s and does not display the yielding up to ∼ 110 GPa. Such a Hugoniot elastic limit is apparently higher than that of the single crystal diamond. The strengthening mechanism underlying the diamond–SiC composite has been discussed. By simulations of the lattice-spring model, the results revealed that under shock compression, the silicon carbide matrix yields first in the composite, resulting in damage to the substructure surrounding the diamond particles. This damage releases the intense shear stress and protects the diamond from severe crushing. Due to diamonds being hydrostatically confined by the silicon carbide matrix, both dislocation migration and slip band sliding are suppressed significantly, which enhances the strength of diamonds. [ABSTRACT FROM AUTHOR]

In the present work, the orthorhombic (α) BiNbO 4 have been investigated with the in-situ angle-dispersive x-ray diffraction (ADXRD) measurements under high pressure. Pressure-induced phase transition was observed in α-BiNbO 4 , and compressibility of the unit cell axes (a, b and c axes) of α-BiNbO 4 shows an anisotropic with pressure increasing. The phase transition routine and the effect of pressure in the phase evolution of BiNbO 4 were also explored. The results indicate that pressure-induced phase transition is reversible, in contrast, pressure and temperature-induced phase transition is irreversible in system of BiNbO 4 , but the phase remains stable without pressure. High pressure plays a critical role and provides a driving force for phase transition of BiNbO 4 . [ABSTRACT FROM AUTHOR]

Although it has been nearly 70 years since iron-based catalysts were used to synthesize diamond from graphite at high pressure and high temperature (HPHT), the graphite to diamond transformation mechanism in this process is still a mystery. It is found that the formation of a eutectic melt between carbon and an iron-based catalyst plays a critical role in the graphite to diamond transformation at HPHT. In this work, the eutectic temperature of Fe–Ni–C (diamond and graphite) was determined using the in situ high-pressure temperature measurement (HPTM) and the high-pressure differential thermal analysis (HPDTA) method up to 8 GPa in a large-volume cubic press. The experimental results of XRD, Raman, SEM and XPS indicate that the transformation of graphite to diamond occurs in the Fe–Ni–C eutectic melt: the C–C bonds in graphite are broken in the eutectic melt, and the free carbon atoms in the melt form sp3 bond states in the thermodynamically stable region of diamond. In this process, the role of iron-based catalysts is to reduce the environmental conditions, in which the C–C bonds in graphite are destroyed, and to make the carbon atoms that are separated from the sp2 bond state form the sp3 bond state in the diamond P–T phase stable region. [ABSTRACT FROM AUTHOR]

Phase transitions of the title compound are investigated by high pressure in situ DTA up to 5 GPa. [ABSTRACT FROM AUTHOR]

Nano-polycrystalline diamond (NPD) can be obtained by direct conversion of various carbons at high temperature and high pressure (HPHT). However, the ultra-high synthesis condition is an obstacle for large-scale production of NPD. This work aimed to reduce the synthesis condition. Annealed precursors fabricated by annealing detonation nano-diamond in vacuum were used for synthesizing NPD at HPHT conditions of 1600–1800 °C, 10–15 GPa and holding time of 1–10 min. High-purity NPD with high Vickers hardness of 147 ± 17 GPa was synthesized at 1800 °C and 10 GPa for 10 min. The synthesis conditions were much lower than that using graphite precursors; in particular, the pressure was dramatically reduced to 10 GPa from 15 GPa. X-ray diffraction and transmission electron microscopy analyses suggested that most carbon particles in the annealed precursors contain a diamond core and the residual diamond core is the key for the reduction in synthesis pressure. Meanwhile, the onion-like carbon became metastable at HPHT because of the compression from the surrounding carbon particles and then transformed to diamond or graphite. Our findings proved that pure NPD with high hardness can be synthesized in proximity to industrial conditions. [ABSTRACT FROM AUTHOR]

Abstract: GaN crystals are successfully obtained through solid-state metathesis (SSM) reaction between sodium gallium oxide (NaGaO2) and boron nitride (BN) under high pressure and high temperature. X-ray diffraction (XRD) pattern indicates that the attained GaN crystals possess a hexagonal wurtzite-type structure. Scanning electron microscopy (SEM) is used to estimate the size and morphology of GaN crystals, and results show that GaN grains with the size over 100μm can be prepared at 5GPa and 1600°C. Moreover, pressure–temperature (P–T) formation region of GaN has been discussed. Our results suggest a promising novel route for synthesizing GaN crystals from SSM reactions under high pressure. [ABSTRACT FROM AUTHOR]

Abstract: The method of differential thermal analysis is applied to the study of phase segregation of Ti2AlC at high hydrostatic pressure up to 5GPa. It is very useful for detecting the phase decomposition temperature and the thermal stability of materials under high pressure. The phase decomposition temperature of Ti2AlC detected was 890±10°C at 5GPa and 1030±10°C at 4GPa. The presented in situ high-pressure differential thermal analysis (HPDTA) result is in good agreement with our previous XRD results. [Copyright &y& Elsevier]