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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]

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]

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]

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]

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]

• 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]

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]

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]

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]

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]

A widely investigated allotrope of germanium, metastable germanium (ST12 Ge), has garnered sustained interest for quite a few years since it has been applied as a potential semiconductor component owing to its optical properties. In this study, a large pure bulk sample of ST12 Ge was synthesized through secondary pressure induced phase transition. The structure and purity of the sample were characterized via a combination of powder X-ray diffraction, Raman scattering, and X-ray photoelectron spectroscopy. Moreover, its thermal stability and optical properties were investigated. The bandgap structure, Vickers hardness, Raman spectrum, and other properties under pressure, were systematically investigated via first principles calculations. • First, in this study, phase-pure ST12 Ge bulks were prepared through secondary pressure induced phase transition. • Second, the structure and purity of the sample have been studied. The thermal stability and optical properties were investigated in details. • Third, first-principle calculations were used to calculate the its bandgap structure, Vickers hardness, Raman spectrum and other properties under pressure. [ABSTRACT FROM AUTHOR]

The strength of tungsten triboride (WB 3 ) was determined under nonhydrostatic compression up to 86 GPa using an angle-dispersive radial X-ray diffraction technique in a diamond-anvil cell (DAC). Analyze of diffraction data using lattice strain theory indicate that the ratio of differential stress to shear modulus ( t / G ) changes from 0.004 at ambient conditions to 0.078 at 86 GPa. Together with theoretical results on the high-pressure shear modulus, our results here show that WB 3 under uniaxial compression can support a differential stress of 26 GPa when it starts to yield to the plastic deformation at 40 GPa. The yield strength of WB 3 increases with increasing pressure, reaching a maximum value of 30 GPa at 77 GPa. By comparison, we find that the high-pressure strength of WB 3 is comparable to those of c -BC 2 N, B 6 O, and γ -Si 3 N 4 . [ABSTRACT FROM AUTHOR]

Nanophase materials have novel physical and chemical properties, differing from bulk materials. It is of exceptional interest to investigate the size effect on structural stability in nanocrystals. Here, we investigated pressure-induced phase transitions in nanosized Er2O3using angle-dispersive synchrotron X-ray diffraction up to 40.6 GPa. Nano-Er2O3has enhanced transition pressure and higher bulk modulus (K0) than its bulk counterparts. Amorphous Er2O3nanoclusters with traces of monoclinic phase are obtained upon compression. This is the first time that partial amorphous structure under compression was observed in nano-Er2O3, indicating a kinetic trapping of partial amorphous Er2O3on pressurizing. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we present a novel method to sinter tungsten bulk material under high pressure without any additives. The sintering temperatures were greatly reduced in our experiments. The sintered samples can achieve near-full densification (relative density of 99.5%), and have fracture toughness of about 50% higher than that from the traditional methods, without sacrificing hardness. Moreover, the grain sizes of the sintered samples maintained in the initio range. The sintering kinetics, microstructure evolutions, and relationships of mechanical properties versus pressure–temperature conditions were also discussed. [Copyright &y& Elsevier]

● GdB 6 is extremely stable under high pressure. ● Contrary to most other materials, the smooth X-ray diffraction rings of GdB 6 become spotty under high pressure. ● High-pressure spotty diffraction rings of GdB 6 are caused by the regional texture effect, which is reversible. ● Grain rotation behavior motivated by stress difference dominates the regional texturing in GdB 6. [Display omitted] As hard and refractory materials with high chemical resistance and mechanical strength, lanthanide hexaborides (LnB 6) have attracted much attention. Among the family of LnB 6 , gadolinium hexaboride (GdB 6) occupies a special position due to the half-filled 4 f shell of Gd. Here, using in situ synchrotron radiation angle-dispersive X-ray diffraction in a diamond anvil cell at room temperature, the structural stability and compression behavior of GdB 6 are investigated extensively, GdB 6 is observed to be structurally stable up to 73 GPa, and the bulk modulus of 177 GPa is obtained under hydrostatic compression. In this paper, an interesting observation of pressure-induced spotty diffraction rings of GdB 6 is reported, its formation mechanism can be well described by the reversible regional texture effect, which highlights the rearrangement of crystal grains under high pressure. The rearrangement mechanism has been well explained by investigating the pressure dependence of full width at half maximum, macro-differential stress and grain size. Collective grain rotation behavior motivated by stress difference is critical for the rearrangement process, the strong isotropy and strong stability of GdB 6 structure also provide necessary conditions for high-pressure grain rotation behavior. These results will help to promote the understanding of high-pressure structural properties of GdB 6 , and provide novel insights on the high-pressure grain behavior in hard materials with strong isotropy and three-dimensional skeleton constituted of strong covalent bonds. [ABSTRACT FROM AUTHOR]

Abstract: The directional growth experiments of graphite-like structured MoS2 crystallites have been conducted by utilizing a designed sample cell assembly under high pressure (2.0 and 5.0GPa) and high temperature (700°C). X-ray diffraction (XRD) and scanning electron microscope (SEM) are used to characterize the samples. The results show that the prepared nanocrystalline MoS2 (n-MoS2) crystals have a hexagonal layered structure. The crystal is uncovered to grow preferentially along the (002) plane, which indicates that the low-energy surface is the (002) plane of the crystal. The striking diffuse/broadening nature of Bragg reflection is also analyzed in details, and considered to be associated with the defect structures of the layers stacking and rotational disorder. Measurements of crystallite/grain size are performed by using XRD technique and SEM observation. The measurement results suggest that the traditional peak broadening analysis techniques, including Williamson–Hall formula and Scherrer equation, may not be suitable for the present poorly crystallized n-MoS2 situation. The results may be conducive to have an insight into the growth mechanism and defects analysis of the layer-structured materials. [Copyright &y& Elsevier]