- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
Production of Polycrystalline Materials by Sintering of Nanodispersed Diamond Nanopowders at High Pressure. Review
摘要： The investigations dealing with the problems of forming polycrystalline materials with a high level of physical-and-mechanical properties using the method of diamond powder sintering at high pressure have been considered. The compacting mechanism of diamond nanopowders of different genesis under exposure to high pressure and high temperature is described, special features of pore structure formations are considered, and the critical role of the specified factor in sintering is shown. The need of degassing and modification of the diamond particle surface during the preparation for sintering is noted. The effectiveness of tungsten and its compounds is shown as an activating addition for producing wear-resistant thermostable diamond nanocomposites.
关键词： high pressure,nanopowders of tungsten and its compounds,diamond nanopowders,diamond nanocomposites,sintering
更新于2025-09-10 09:29:36
-
Fragmentation and stress diversification in diamond powder under high pressure
摘要： Studying the fragmentation and re?nement of diamond powder as well as the diversi?cation in the intergranular stress is crucial to produce a high-quality polycrystalline diamond. In this paper, using different micron-size diamond powders as the initial materials, the samples were compressed under different pressures at ambient temperature. The fragmentation behavior of the diamond powder was investigated by scanning electron microscopy and with a laser particle size analyzer. The results show that the fragmentation of diamond comprises three stages with increasing pressure: (i) fracturing of edges and corners, (ii) cracking of the crystal plane, and (iii) re?nement of particle disorder; the particle deformation tends to become relatively stable after a certain pressure. In situ high-pressure synchrotron X-ray diffraction was used to study the intergranular stress distribution under non-hydrostatic compression to 35.1 GPa. A heterogeneous stress distribution was found in compressed diamond bulk, in which under the highest load, the maximum stress reached 69.5 GPa, whereas the minimum stress was only 18.8 GPa.
关键词： diamond powder,fragmentation,polycrystalline diamond,high pressure,intergranular stress
更新于2025-09-10 09:29:36
-
Elastic and thermophysical properties of BAs under high pressure and temperature
摘要： The pseudopotential plane-wave approach in the framework of the density functional theory, and the density functional perturbation theory with the generalized gradient approach for the exchange-correlation functional has been used to calculate the structural phase stability, elastic constants and thermodynamic properties of boron-arsenide (BAs) compound. The BAs compound transforms from the zincblende phase to rock-salt structure; the phase transition pressure was found to be 141.2 GPa with a volume contraction of around 8.2 %. The thermodynamic properties under high pressure and temperature up to 125 GPa and 1200 K respectively were also determined, analyzed and discussed in comparison with other data of the literature. The systematic errors in the static energy were corrected using the bpscal EEC method. Our results agree well with those reported in the literature, where for example, our calculated melting temperature (2116 K) deviates from the theoretical one (2132.83 K) with only 0.8 %, and the deviation between our result (1.86) of the Grüneisen parameter and the theoretical one (1.921) is only around 3.2 %.
关键词： Phase transition,Thermodynamic properties,High-pressure,High-temperature,Boron arsenide
更新于2025-09-09 09:28:46
-
by means of high-pressure–high-temperature Raman experiments
摘要： We report here high-pressure–high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by TF ?Sch(K) = ?166(8)P (GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at TZ?Sch(K) = ?107(8)P (GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. The introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.
关键词： zircon,scheelite,BiVO4,fergusonite,high-temperature,phase transition,high-pressure,ab initio calculations,Raman experiments,vibrational patterns
更新于2025-09-09 09:28:46
-
High pressure synthesis of A2NiO2Ag2Se2 (A = Sr, Ba) with a high spin Ni2+ in square planar coordination
摘要： Square-planar coordinate Ni2+ in oxides are exclusively limited to a low spin state (S = 0) owing to extensive crystal field splitting. Here we report layered oxychalcogenides A2NiIIO2Ag2Se2 (A = Sr, Ba) with the S = 1 NiO2 square lattice. The structural analysis revealed that the Ni2+ ion is under-bonded by a significant tensile strain from neighboring Ag2Se2 layers, leading to the reduction in crystal field splitting. Ba2NiO2Ag2Se2 exhibits a G-type spin order at 130 K, indicating fairly strong in-plane interactions. The high-pressure synthesis employed here possibly assists the expansion of NiO2 square lattice by taking the advantage of the difference in compressibility in oxide and selenide layers.
关键词： oxychalcogenide,spin state,high pressure synthesis,strain,square lattice
更新于2025-09-09 09:28:46
-
Reference Module in Materials Science and Materials Engineering || Liquid Crystals Under High Pressure
摘要： From a historical perspective, the first high pressure experiments were carried out soon after the discovery of liquid crystals, although the field became more active much later. Despite being considered as equivalent thermodynamic variables, temperature (T) and pressure (P) act differently on liquid crystals (LC), with the former affecting the rotational and vibration energy dependent parameters, and the latter altering the intermolecular distances. This feature, in combination with the fact that relatively low pressures bring about significant changes in the properties of LCs, has provided impetus for the studies described here. These investigations have not only augmented the knowledge on LCs obtained at atmospheric pressure, but have provided valuable insights into the understanding of complex fluids (Chandrasekhar and Shashidhar, 1979; Pollmann, 1999; Urban, 2014); the studies have been carried out using apparatus such as optical transmission, DTA, calorimetry, dielectric, NMR, XRD, volumetry, and Raman, detailed in these articles. For the description of the structures of the LC mesophases, numbering more than 40, the reader may refer to the article.
关键词： ferroelectric properties,mesomorphism,liquid crystals,nematic-isotropic transition,high pressure
更新于2025-09-09 09:28:46
-
Synthesis of Transparent Nano-Ceramics under Ultra-High Pressure; 透明ナノセラミックスの超高圧合成;
摘要： Various transparent nano-ceramics have been synthesized using direct conversion from bulk glass starting materials under pressures higher than ～10 GPa and at high temperatures in Kawai-type Multi-anvil Apparatus (KMA). Rapid nucleation followed by slow crystal/grain growth is the key to synthesize such nano-ceramics at the ultra-high pressure regime. Thus synthesized novel ceramics have interesting features, such as higher hardness and lower thermal conductivity relative to those of the corresponding single crystals, in addition to their high optical transparencies, which may be useful for some scientific and industrial applications.
关键词： transparent nano-ceramics,KMA,ultra-high pressure,polycrystalline material,hardness
更新于2025-09-09 09:28:46
-
High-temperature high pressure synthesis of monoclinic Y2O3
摘要： In this work, synthesis of cubic Y2O3 by calcining yttrium based coordination polymer (Y-CP) which were obtained by microwave-assisted method was reported. Monoclinic Y2O3 was obtained by treating the cubic Y2O3 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 Y2O3: Eu3+ sample was investigated. This method can be used to prepare other rare earth oxide with various phase.
关键词： Coordination polymer,luminescence,Y2O3,high temperature high pressure,Phase transformation
更新于2025-09-04 15:30:14
-
High Pressure effect on fluorescence lifetime <i>τ</i> for magnetic dipole <sup>5</sup> D <sub/>0</sub> → <sup>5</sup> F <sub/>1</sub> transitions in YAG:Eu <sup>3+</sup>
摘要： The fluorescence lifetime for magnetic dipole 5D0→7F1 transition in yttrium aluminum garnet doped with Eu3+ (YAG:Eu3+) crystal was studied under the pressure of up to 10.4 GPa at room temperature. The fluorescence lifetime τ (5D0→7F1 transition) slowly decreased with pressure. The pressure effect on τ (5D0→7F1 transition) was explained with a model which considered pressure effect on line position: inter-ionic distance, ion volume, molecular volume, ion polarizability, molecular polarizability, sample refractive index, and surrounding hydrostatic medium refractive index. The fluorescence lifetime τ calculated by the presented model was in close correspondence with the experimental values.
关键词： rare earth,fluorescence lifetime,YAG:Eu3+,optical properties,glass ceramics,High pressure
更新于2025-09-04 15:30:14
-
Pressure-Induced Phase Transitions in TiO <sub/>2</sub> Rutile Nanorods
摘要： TiO2 nanorods (diameter of 6-8 nm) with the rutile-type structure have been investigated by Raman spectroscopy under high pressure. Additional bands in the Raman spectrum at ambient pressure are interpreted as resulting from defects that break the Raman selection rules as the simulated vibrational density of states (vDOS) is in good agreement with the experimental spectrum. Under pressure, the rutile structure transforms to the baddeleyite one at P~16 GPa. After pressure cycle, the α-PbO2 phase is recovered. The sequence of phase is discussed in terms of thermodynamics and kinetics based on crystallographic relationships. Contrary to other 1D TiO2 nanomaterials that adopt a metastable structure, the nanorods studied in this work show little impact of the morphology on the phase transitions sequence compared to the bulk TiO2.
关键词： phase transitions,vibrational density of states,TiO2 nanorods,high pressure,Raman spectroscopy
更新于2025-09-04 15:30:14