Xiaopeng Jia

Enhanced thermoelectric performance of AgSbTe2 synthesized by high pressure and high temperature

Near single phase ternary bulk thermoelectric material AgSbTe2 was synthesized by high pressure and high temperature (HPHT) method. The temperature-dependent thermoelectric properties including Seebeck coefficient, electrical conductivity, and thermal conductivity were studied. The HPHT synthesized AgSbTe2 sample has higher thermoelectric performance in the measured temperature range than that of the same sample prepared at normal pressure. The enhanced thermoelectric properties should be attributed to the HPHT quenching which keeps partially the high electrical conductivity of AgSbTe2 under high pressure.

Effect of HPHT processing on the structure, and thermoelectric properties of Co4Sb12 co-doped with Te and Sn

Te–Sn co-doped Co4Sb12 bulk polycrystalline materials Co4Sb11.7−xTexSn0.3 have been prepared using a high pressure and high temperature method and then characterized using X-ray diffraction. The aim was to use the disorder of the lattice orientation generated by both high pressure and doping with Te and Sn to reduce the thermal conductivity. The thermoelectric properties were measured at room temperature. As expected, as the synthesis pressure increased, the Seebeck coefficient and electrical resistivity increased, but the thermal conductivity decreased greatly. A minimum thermal conductivity of 2.41 W m−1 K−1 was obtained at room temperature for Co4Sb11Te0.7Sn0.3 synthesized at 3 GPa.

Synthesis and characterization of hydrogen-doped diamond under high pressure and high temperature

To investigate the effect of the hydrogen element on diamond crystallization, hydrogen doped diamond crystals are synthesized at 5.0–6.0 GPa and 1250–1600 °C by adding ferrocene (C10H10Fe) to a system of carbon and Fe-based solvent–catalyst. The essential dependence of diamond morphology and nucleation on the composition of the crystallization medium is established in the P–T diagram. It is found that the growth region of the {111} face becomes wider, while the region of the {100} face becomes narrower and almost disappears with increasing concentration of C10H10Fe. The changing of the diamond growth habits can be explained by the effect of hydrogen from the decomposition of C10H10Fe on diamond crystallization. Fourier transform infrared absorption spectra reveal that the hydrogen-related peaks increase with an enhancement in the ratio of C10H10Fe additive. The observed Raman shift is due to the doping of hydrogen atoms into the diamond lattice. In addition, we find that the change of characteristics of the growth medium of the diamond crystal induced by hydrogen is an important factor for the changes of the synthesis conditions, the growth rate and the nitrogen concentration of the synthesized diamond.

Thermoelectric transport properties and crystal growth of BiSbTe3 bulk materials produced by a unique high-pressure synthesis

We report a significant enhancement in the figure of merit ZT of bulk BiSbTe3 fabricated by high pressure sintering technique under variable pressures. High pressure intervention during synthesis processes can effectively adjust the thermoelectric transport properties and crystal structures of the reaction products. The novel vibration modes produced by high pressure in Raman spectra can induce a positive role in the reduction of thermal conductivity. Meanwhile, the electrical resistivity can also obtain an efficient decrease due to the effect of high pressure on the texture and the preferred orientation. Because of these positive effects, the maximum ZT has reached 1.4 at 432 K from synthesized BiSbTe3 at 2.5 GPa in a short period. Therefore, we present that the appropriate high-pressure has a positive effect on the thermoelectric transport properties to produce the significant enhancement in thermoelectric properties of BiSbTe3.

Electrical properties of diamond single crystals co-doped with hydrogen and boron

In this paper, diamond single crystals co-doped with H and B were successfully synthesized under a fixed pressure of 6.0 GPa and temperature ranging from 1560 to 1600 K. In the synthesized diamond, hydrogen was found to be mainly incorporated as sp3 CH2, which had been detected by its characteristic stretching absorptions at 2920 cm−1 and 2850 cm−1 from the Fourier transform infrared spectra. Raman measurements indicated that the diamonds co-doped with B and H had a more compatible lattice structure than B-doped diamond. Hall effect measurements indicated that the co-doped diamonds showed p-type semiconductor behavior. Besides, the Hall mobility was nearly equivalent between B-doped and co-doped diamond crystals, while the concentrations of the carrier and conductivity of the co-doped diamonds were higher than that of the B-doped diamond crystals.

Finite element design of double bevel anvils of large volume cubic high pressure apparatus.

A double bevel anvil of the cubic high pressure apparatus (CHPA) was developed, adopting tungsten carbide as the anvil material. We have performed finite element analyses of conventional single bevel anvil and double bevel anvil. The results indicate that the double bevel anvil has two advantages. Firstly, to gain the same chamber pressure, the oil pressure of CHPA using double bevel anvil decreases about 10.8% than that using single bevel anvil. Secondly, double beveling can maintain the pressurized seal stability of the sample chamber, which is often sacrificed with improve the pressure of sample chamber. The results of finite element analyses are well consistent with the experimental results at CHPA (SPD-6x1200 type).

Diamond crystallization and growth in N–H enriched environment under HPHT conditions

Advances in understanding the state and transformation of impurity-related defects in natural diamond have been achieved by means of high-pressure and high-temperature annealing experiments. However, the existing literature featuring complex behaviors of aggregated nitrogen and hydrogen-related defect (3107 cm−1 center) has remained controversial. Their formation mechanism during diamond crystallization is still unclear, thus further investigation is required. In this work, we have successfully synthesized a variety of high-quality diamonds containing nitrogen impurities with IaA (nitrogen pairs), IaB (group of four nitrogen atoms around a vacancy) and Ib (single-substitutional nitrogen) characteristics ranging from <1 ppm to 3380 ppm at pressures ranging from 5.0 GPa to 6.3 GPa and temperatures of 1300–1650 °C. Our results provide new experimental evidence for the aggregation of nitrogen impurities (A- and B-centers) during diamond growth. We notice that the hydrogen is easily trapped by nitrogen atoms to form nitrogen–hydrogen complexes (–NH, –NH2, –NH3) when the nitrogen tends to form C-centers in hydrogen-enriched environments. Additionally, we observed that the high reaction temperature and formation of A-centers during diamond growth play important roles in the formation of the 3107 cm−1 center. We believe the current results could be helpful for further understanding and constructing a clear model of impurity-related defects, and thus provide us deeper insights into the genesis of natural diamond.

Crystallization of HPHT diamond crystals in a floatage system under the influence of nitrogen and hydrogen simultaneously

Employing floatage as a driving force for diamond growth, the crystallization of diamond crystals in a Fe–Cr–C system co-doped with nitrogen and hydrogen elements is established at a static pressure of ~6.5 GPa and a temperature range of 1335–1485 °C. Under the influence of nitrogen and hydrogen incorporated into the diamond structure simultaneously, a rich morphological diversity of diamond specimens is produced, such as hexagonal slice-shape, trapezoidal slice-shape, strip shape and triangular slice-shape crystals. Observation of the infrared spectra of the as-grown crystals indicates that a dramatic enhancement in the simultaneous incorporation of hydrogen and nitrogen atoms into the diamond structures is present in the strip-shape specimens, confirmed by the fact that relatively high absorption coefficients of the peaks at 1130 cm−1, and 1344 cm−1 are accompanied with high absorption coefficients of the bands at 2850 cm−1 and 2920 cm−1. Hydrogen-related absorption in the three-phonon region further indicates that hydrogen atoms exist in the diamond structures as sp3 bonded –CH2– and –CH3 group forms. At atmospheric pressure, these hydrogen-containing structures are rather stable and can sustain high temperatures of up to 1800 °C. Nitrogen donors are universally observed as an isolated substitutional form in the crystals, while minor paired-form nitrogen atoms are readily formed in the strip shape crystals or other crystals crystallized at higher temperature.

Fast Preparation and Low-Temperature Thermoelectric Properties of CoSb3

Polycrystalline p-type CoSb3 was synthesized by the high-pressure method. The microstructure and temperature-dependent thermoelectric properties of CoSb3 were investigated. X-ray diffraction and scanning electron microscopy showed that single-phase CoSb3 with fine grain size could be quickly synthesized under high pressure. The carrier concentrations of CoSb3 could be tuned by more than a factor of 10 by changing the pressure during synthesis. With the increase of the synthetic pressure, the Seebeck coefficient and resistivity of CoSb3 increase while the thermal conductivity decreases.

Ultrahard stitching of nanotwinned diamond and cubic boron nitride in C2-BN composite

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at moderate pressures and high temperatures provides a pathway to such materials. Here we report the fabrication of Cx-BN nanocomposites, measuring up to 10 mm in longest dimension, by reacting nanodiamond with pre-synthesized cBN in a large-volume press. The nanocomposites consist of randomly-oriented diamond and cBN domains stitched together by sp3-hybridized C-B and C-N bonds, leading to p-type semiconductivity. Dislocations near the sutures accommodate lattice mismatch between diamond and cBN. Nanotwinning within both diamond and cBN domains further contributes to a bulk hardness ~50% higher than sintered cBN. The nanocomposite of C2-BN exhibits p-type semiconductivity with low activation energy and high thermal stability, making it a functional, ultrahard substance.