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In order to investigate how to achieve volume plastic flow and what kind of deformation mechanism in the Vickers indentation of a magnesium oxide single crystal, the transmission electron microscopy (TEM) images were recorded. Significant crystal plane deviation was observed in the contact area between the sample and the indenter, while there was almost no deviation in other plastic zones. It was found that the deflecting lattice interface was realized by dislocation accumulation, and incoherent in the interface is coordinated by mismatch and multi-atom layer stacking. Some narrow holes in which both ends connected to boundaries were observed, and formation of the narrow holes is due to fracture and then re-bonding under high stress. There exists an amorphous trend in the area next to the indenter, which is generated by high stress extrusion. These mechanisms together enable plastic deformation of magnesium oxide single crystals in indentation experiments. [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]

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]

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]

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]

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]

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]

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]

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]

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]

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 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 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]

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]

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]

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]