Bulk materials Ba_(8)Ga_(16)In_(x)Ge_(30-x)(x=0.5,1.0,1.5)were prepared by High-Pressure and High-Temperature(HPHT)method and the crystal structure has been confirmed by X-ray diffraction and cell refinement.The actual In composition was much lower than the starting composition,and lattice constants increased with the increase of substitution.As the temperature increased,the Seebeck coefficient and electrical resistivity increased first and then decreased,while the thermal conductivity was the opposite,which leads to significant enhancement on thermoelectric properties of the clathrates.The substitution of indium elements decreased the seebeck coefficient and electrical resistivity,and also changed the microstructure of the compounds.A minimum thermal conductivity of 0.84Wm^(-1)1K^(-1)was obtained,and a good ZT value of 0.52 was achieved.The grain boundaries and lattice defects generated by high pressure can effectively scatter phonons of different frequencies,which reduce the lattice thermal conductivity.
We investigate the temperature field variation in the growth region of a diamond crystal in a sealed cell during the whole process of crystal growth by using the temperature gradient method （TGM） at high pressure and high temperature （HPHT）. We employ both the finite element method （FEM） and in situ experiments. Simulation results show that the temperature in the center area of the growth cell continues to decrease during the process of large diamond crystal growth. These results are in good agreement with our experimental data, which demonstrates that the finite element model can successfully predict the temperature field variations in the growth cell. The FEM simulation will be useful to grow larger high-quality diamond crystal by using the TGM. Furthermore, this method will be helpful in designing better cells and improving the growth process of gem-quality diamond crystal.
Thermal residual stress in Polycrystalline Diamond Compacts （PDCs） is mainly caused by the mismatch in the Coefficients of Thermal Expansion （CTE） between the polycrystalline diamond （PCD） layer and WC-Co substrate. In the PCD layer, the CTE of cobalt exhibit magnitudes four times larger than those of diamond. Cobalt content in the PCD layer has important effects on the thermal residual stress of PDCs. In this work, the effects of cobalt content on thermal residual stress in PCDs were investi- gated by the Finite Element Method （FEM）. The simulation results show that the thermal residual stress decreases firstly, and then increases with increasing cobalt content （1 vo1.%-20 vol.%）, which reaches a minimum value when the cobalt content is about 10 vol.%. The FEM analysis results are in agreement with our experimental results. It will provide an effective method for further designing and optimizing PDC properties.
LI ZhanChangJIA HongShengMA HongAnGUO WeiLIU XiaoBingHUANG GuoFengLI RuiJIA XiaoPeng
The temperature in the high-pressure high-temperature(HPHT) synthesis is optimized to enhance the thermoelectric properties of high-density Zn O ceramic, Zn_(0.98)Al_(0.02)O. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that HPHT can be utilized to control the crystal structure and relative density of the material.High pressure can be utilized to change the energy band structure of the samples via changing the lattice constant of samples, which decreases the thermal conductivity due to the formation of a multi-scale hierarchical structure and defects. The electrical conductivity of the material reaches 6×10^(4) S/m at 373 K, and all doped samples behave as n-type semiconductors. The highest power factor(6.42 μW·cm^(-1)·K^(-2)) and dimensionless figure of merit(z T = 0.09) are obtained when Zn_(0.98)Al_(0.02)O is produced at 973 K using HPHT, which is superior to previously reported power factors for similar materials at the same temperature. Hall measurements indicate a high carrier concentration, which is the reason for the enhanced electrical performance.
We present the work about the initiative fabrication of multi-scale hierarchical TiO2-x by our strategy,combining high pressure and high temperature(HPHT)reactive sintering with appropriate ratio of coarse Ti to nanosized TiO_(2).Ubiquitous lattice defects engineering has also been achieved in our samples by HPHT.The thermoelectric performance was significantly enhanced,and rather low thermal conductivity(1.60 W m^(-1)K^(-1))for titanium oxide was reported here for TiO1.76.Correspondingly,a high dimensionless figure of merit(zT)up to 0.33 at 700℃was realized in it.As far as we know,this value is an enhancement of 43%of the ever best result about nonstoichiometric TiO_(2)and the result is also exciting for oxide thermoelectric materials.The moderate power factor,the significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity are responsible for the excellent thermoelectric performance.We develop a facile strategy for preparing multi-scale hierarchical TiO_(2-x)and its superior ability to optimize thermoelectric performance has been demonstrated here.
Pyrite tailings are the main cause of acid mine wastewater.We propose an idea to more effectively use pyrite,and it is modified by exploiting the reducibility of metal represented by Al under high-pressure and high-temperature(HPHT)conditions.Upon increasing the Al addition,the conductivity of pyrite is effectively improved,which is nearly 734 times higher than that of unmodified pyrite at room temperature.First-principles calculations are used to determine the influence of a high pressure on the pyrite lattice.The high pressure increases the thermal stability of pyrite,reduces pyrite to highconductivity Fe7S8(pyrrhotite)by Al.Through hardness and density tests the influence of Al addition on the hardness and toughness of samples is explored.Finally we discuss the possibility of using other metal-reducing agents to improve the properties of pyrite.
Polycrystalline Cu_(2)Se bulk materials were synthesized by high-pressure and high-temperature(HPHT)technique.The effects of synthetic temperature and pressure on the thermoelectric properties of Cu_(2)Se materials were investigated.The results indicate that both synthetic temperature and pressure determine the microstructure and thermoelectric performance of Cu2Se compounds.The increase of synthetic temperature can effectively enhance the electrical conductivity and decrease the lattice thermal conductivity.A two-fold improvement in the power factor is obtained at synthetic temperature of 1000℃ compared to that obtained at room temperature.All b-Cu2Se samples exhibit low and temperatureindependent lattice thermal conductivity ranging from 0.3 to 0.5 Wm^(-1)K^(-1) due to the intrinsic superionic feature and the abundant lattice defects produced at high pressure.A maximum zT of 1.19 at 723 K was obtained for the sample synthesized at 3 GPa and 1000℃.These findings indicate that HPHT technology is an efficient approach to synthesize Cu_(2)Se-based bulk materials.
The high pressure and high temperature（HPHT） method is successfully used to synthesize jadeite in a temperature range of 1000℃–1400℃ under a pressure of 3.5 GPa. The initial raw materials are Na2SiO3·9H2O and Al2（SiO3）3.Through the HPHT method, the amorphous glass material is entirely converted into crystalline jadeite. We can obtain the good-quality jadeite by optimizing the reaction pressure and temperature. The measurements of x-ray diffraction（XRD）,scanning electron microscopy（SEM）, Fourier-transform infrared（FTIR） and Raman scattering indicate that the properties of synthesized jadeite at 1260℃ under 3.5 GPa are extremely similar to those of the natural jadeite. What is more, the results will be valuable for understanding the formation process of natural jadeite. This work also reveals the mechanism for metamorphism of magma in the earth.
