Pinwen Zhu

High-pressure pyrolysis study of C3N6H6: a route to preparing bulk C3N4

In order to prepare bulk C3N4, high-pressure pyrolysis of melamine (C3N6H6) at different temperatures was carried out. The products were characterized by C, N, H element analysis, Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffractometry. The results of the analysis reveal that graphitic phase C3N4 has been synthesized. It provides a novel route to synthesis of the theoretical superhard cubic C3N4 and other C3N4 phases from organic compounds by a high-pressure and high-temperature method.

A new method of synthesis for thermoelectric materials: HPHT

There is a widespread interest in searching for materials that are used to synthesize more efficient thermoelectric devices for cooling and power generation applications. We report on lead telluride (PbTe) compounds which have been synthesized by the high pressure and high temperature method. The thermoelectric properties in PbTe synthesized by high pressure and high temperature were strongly increased. A ZT value of 0.87 for PbTe measured at ambient temperature and pressure has been achieved in our experiment. This value is nearly 20 times larger than the reported values for PbTe synthesized at ambient pressure, and approximately equal to that of Bi2Te3. These results indicate that the technique of high pressure and high temperature is a useful tool to obtain materials of improved properties, and highly efficient thermoelectric materials may be expected.

Enhanced thermoelectric properties of PbTe alloyed with Sb2Te3

We report the enhanced thermoelectric properties of PbTe alloyed with antimony telluride (Sb2Te3). The resistivities and absolute values of the Seebeck coefficient for PbTe alloyed with Sb2Te3 increase while the thermal conductivities decrease with rising temperature. A maximum figure of merit, ZT, of 1.17 has been achieved for PbTe alloyed with 0.8xa0mol% Sb2Te3 at 560xa0K. This value is about 30% higher than the ones reported previously for bulk PbTe. The enhanced thermoelectric properties may be due to the low thermal conductivities which are induced by alloying with Sb2Te3 and much smaller than that reported for bulk PbTe.

Giant improved thermoelectric properties in PbTe by HPHT at room temperature

Abstract We report a large increase in the thermoelectric properties of n-type lead telluride compounds synthesized by applying pressure tuning and temperature tuning simultaneously. The value of the thermoelectric figure of merit, ZT for PbTe measured at ambient temperature and pressure, 0.87, has been achieved. This value is nearly 20 times larger than that of PbTe synthesized at the ambient pressure, and approximately equal to that of Bi2Te3. It is suggested that this method has potential application in obtaining good quality thermoelectric materials.

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.

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 (ue605(HO)2ue605) 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.

Thermoelectric properties of PbTe prepared at high pressure and high temperature

Lead telluride (PbTe) with rock-salt structure was successfully obtained by a high-pressure and high-temperature (HPHT) method. The orientation of the PbTe samples varies with pressure increase. The results—a decrease in the Seebeck coefficient, resistivity and thermal conductivity of PbTe with pressure but an increase in the thermoelectric power figure σS2—indicate that the figure of merit Z of PbTe samples can be improved several times over by using HPHT.

Superhard MgB2 bulk material prepared by high-pressure sintering

Superhard MgB2 bulk material with a golden metallic shine was synthesized by high-pressure sintering for 8 h at 5.5 GPa and different temperatures. Appropriate pressure and temperature conditions for synthesizing polycrystalline MgB2 with high hardness were investigated. The samples were characterized by means of atomic force microscopy and x-ray diffraction. The Vickers hardness, bulk density, and electrical resistivity were measured at room temperature.

High thermal stability of diamond–cBN–B4C–Si composites*

Improving the thermal stability of diamond and other superhard materials has great significance in various applications. Here, we report the synthesis and characterization of bulk diamond–cBN–B4C–Si composites sintered at high pressure and high temperature (HPHT, 5.2 GPa, 1620–1680 K for 3–5 min). The results show that the diamond, cBN, B4C, B x SiC, SiO2 and amorphous carbon or a little surplus Si are present in the sintered samples. The onset oxidation temperature of 1673 K in the as-synthesized sample is much higher than that of diamond, cBN, and B4C. The high thermal stability is ascribed to the covalent bonds of B–C, C–N, and the solid-solution of B x SiC formed during the sintering process. The results obtained in this work may be useful in preparing superhard materials with high thermal stability.

Synthesis of bulk nanocrystalline Pb–Sn–Te alloy under high pressure

Pb–Sn–Te bulk nanocrystalline (NC) materials are prepared successfully by quenching melts under high pressure. The mean particle size is about 100 nm and the crystal structure is NaCl type. The mechanism of formation of the bulk NC alloy is explained: there is an increasing of the nucleation rate and a decrease in the growth rate of nuclei with increase of pressure during the solidification processes. The thermoelectric properties of Pb–Sn–Te bulk NC alloy are enhanced. This method is promising for producing thermoelectric materials with improved high-energy conversion efficiency.