Yinjuan Liu

Stress control of heterogeneous nanocrystalline diamond sphere through pressure-temperature tuning

The hollow nanocrystalline diamond (NCD) sphere, a promising ablator material for inertial confinement fusion capsule, is generally fabricated by the chemical vapor deposition method. Herein, we report on a method to transform hydrogenated tetrahedral amorphous carbon coatings on spherical molybdenum (Mo) substrates into nanocrystalline diamond films via a designed high pressure high temperature (HPHT) treatment that balances the mismatch in the thermal expansion coefficient between a diamond coating and the Mo substrate through the difference in the bulk modulus. The results show that the density and strength of the diamond shell increase significantly and the residual stress is eliminated as well. The methodology of the designed HPHT treatment can not only provide an alternative way to fabricate NCD spheres but also can apply to other heterogeneous material stress control applications.

Experimental study on the pressure-generation efficiency and pressure-seal mechanism for large volume cubic press

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

Thermal insulation performance of monoclinic ZrO2 and cubic ZrO2–CaO solid solution under high pressure and high temperature

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

Synthesis of single-crystal perovskite PbCrO3 through a new reaction route at high pressure

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

High pressure infiltration sintering behavior of WC-Co alloys

ABSTRACT 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).