H.A. Ma

Simulation of pressure distribution in a pyrophyllite high-pressure cell by finite-element analysis

The pressure-transmitting behavior and pressure distribution of a pyrophyllite cell in a cubic-anvil large volume high-pressure apparatus is investigated by finite-element analysis (FEA). The mechanical equations describing pyrophyllite under high pressure are given, which are composed of the Drucker–Prager criterion and a linear equation of state. The related material parameters such as cohesive strength, angle of internal friction etc. are experimentally measured. The FEA simulation includes the non-uniformity deviatoric stress caused by plastic deformation and the isotropy hydrostatic pressure of the pyrophyllite cell caused by volume compression. The results demonstrate that, in such a pyrophyllite cell, almost 90{%} of the pressure is isotropy hydrostatic pressure, and the pressure gradient mainly arise from non-uniform deviatoric stresses. Simulated data are displayed using the contour and the path plots.

Crystal structures of C3N6H6 under high pressure

In situ high-pressure energy-dispersive X-ray diffraction (EDXRD) experiments on melamine (C3N6H6) have been carried out using diamond anvil cell (DAC) with synchrotron radiation source. In the pressure range from ambient pressure up to 14.7 GPa, two pressure-induced structure phase transitions, from monoclinic to triclinic structure at about 1.3 GPa and from triclinic to orthorhombic structure at about 8.2 GPa, are observed. The crystal structures of melamine at different pressures are built by using Materials Studio (MS) based on the principle of energy minimization.

Finite element analysis of high-pressure anvils according to the principle of lateral support

In order to extend the lifetime of an anvil made of tungsten carbide used in a large volume cubic high-pressure apparatus (CHPA), we analyzed the properties of an anvil with different magnitudes of interference, matching the steel supporting ring under high pressure using finite element method. It is found that the peak value and distribution of the von Mises stress does not change obviously when the magnitude of interference is smaller than 0.3mm. When the magnitude of interference reaches 0.3mm, the peak value of the von Mises stress and its distribution changes substantially, due to which the performance of the anvil can be significantly affected. Thus, the magnitude of interference ought to be between 0.1 and 0.25mm. The simulated results have been approved in many high-pressure experiments in CHPA.

cBN synthesis in the system of hBN–Mg and bonded water

Abstract The growing behavior of cubic boron nitride (cBN) crystal in the system of hBN–Mg with bonded water ((HO)2) was investigated at 5.0 GPa and 1100–1500 °C. The results show that, with the existence of bonded water, the grown cBN crystals can appear different color and the growth temperature decreases approximately 300 °C with the pressure fixed at 5.0 GPa by contrast with that in the hBN–Mg system. In addition, the cBN synthesis process is quite different from that when bonded water is absent or that when magnesium is absent.

Characterizations of single-phased cubic Mg0.5Zn0.5O prepared at high pressure and high temperature

We report the preparation of a single-phased cubic Mg0.5Zn0.5O semiconductor at high pressure and high temperature(HPHT) in this paper. Phase separation that was frequently observed in Mg0.5Zn0.5O alloys at a normal pressure was avoided in our case. The formation of the single-phased Mg0.5Zn0.5O resulted from the HPHT applied during the preparation process. The Mg0.5Zn0.5O has a maximum absorption at about 265 nm and a sharp absorption edge at about 300 nm. The single-phased Mg0.5Zn0.5O with an abrupt absorption edge obtained in this paper is of significance in realizing high-performance ultraviolet photodetectors based on MgZnO.

High-pressure synthesis and superconductivity of the Yb-substituted Ba 8−x Yb x Si 46 clathrates

Summary form only given. Clathrate materials are extended Si, Ge and Sn cage-like solids with sp³-hybridized networks. As a new generation of energy conversion materials, clathrates have received increasing attention over the past few years. Clathrates with an open network are tailorable materials containing polyhedral cages, which can capture alkali metal, alkaline-earth metal and europium atoms. These atoms on the center of cage arrange in such a nanoscale array. Superconductivity mediated by the Si framework was demonstrated in Ba₈Si₄₆ clathrate. In contrast to carbon in C₆₀, silicon atoms do not form sp²-like networks in clathrates. Therefore, superconductivity of Si clathrate with sp³ network should be unique. The strong hybridization of the Ba state with the Si framework conduction band state plays a key role in the superconductivity. The introduction of rare earth elements as guest atoms into the cages would create a strongly correlated electron system through Kondo interaction of the local 4f states with the conduction electrons. However, such influence on superconductivity still has remained elusive to date. Here we report the incorporation of Yb as a guest atom into the clathrate crystal structure, and relative magnetic measurements and Raman spectrum for these Yb-doped samples . The new clathrates Ba_8-xYb_xSi₄₆ were synthesized at a pressure of 3 GPa and temperature of 1113 K. X-ray powder diffraction data indicate that the cubic lattice parameter a decreases with Yb doping. EDS energy spectrum shows that the Yb peak is really evident above the background, which means a sizable amount of Yb incorporated into the clathrate phase. According to the result of EDS, the averaged chemical composition is Ba_7.92Yb_0.08Si₄₆, assuming that the number of Si atoms is fixed to 46. The temperature dependence of magnetization revealed that Ba_7.5Yb_0.5Si₄₆ is superconductive below T_C = 7 .3 K and shows Curie-paramagnetic behavior at high temperature. A Curie-Weiss fit yielded the negative Curie-Weiss temperature and the effective magnetic moment is 5.59 μ_B, which is approximately equal to the Yb³⁺ free ions value . As temperature decreases, electrical resistance decreases and drops to zero at 7.3 K . The high temperature resistance significantly deviates from the Bloch-Grüneisen law associated with simple metallic conductivity. In particular, the broad curvature around 175 K resembles a behavior frequently found in strongly correlated electron systems induced by rare earth incorporation. Room-temperature Raman-scattering spectrum is reported for Ba_7.5Yb_0.5Si₄₆ and compared to that of Ba₈Si₄₆. The peaks at 279 and 488 cm^-1 for Ba₈Si₄₆ show clear shift to 295 and 510 cm^-1 for the Yb doped Ba_7.5Yb_0.5Si₄₆. These shifts are caused by the incorporation of Yb into the cages. In contrast, the peaks at 125 and 358 cm^-1 do not show any shift .