Meihua Hu

Synthesis and characterization of boron and nitrogen co-doped diamond crystals under high pressure and high temperature conditions

In this study, diamond crystals co-doped with boron and nitrogen were synthesized via a temperature gradient method at 5.3–5.8 GPa and 1300–1550 °C by adding B and N dopants to a system of carbon and an Fe-based solvent catalyst. The obtained co-doped diamond crystals were characterized via optical microscopy (OM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Our results indicated that the obtained B/N-co-doped diamond crystals were green in color and nearly transparent. The surface characteristics were different for the (100) and (111) crystal faces. In the B/N-co-doped diamonds, B, C, and N combined with each other and formed stable chemical bonds. The nitrogen concentration of the co-doped diamond crystals was much higher than that of the single doped crystals, and the form of nitrogen atoms was also different. The Raman characteristic peak of the co-doped diamond crystals almost did not shift as compared to that of the undoped or single doped crystals.

High temperature thermoelectric properties of PbTe prepared by high pressure method

Abstract Highly dispersed olive-like NiS particles were synthesized in a liquid-liquid biphasic system at room temperature, where nickel xanthate in organic solvents (toluene and benzene) and sodium sulfide in water solution were used as nickel and sulfide sources, respectively. NiS particles were formed at the stabilized phase interface. The structures, chemical composition and optical characteristics of the products were investigated by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy. The organic solvents obviously influenced the morphology of the NiS particles. The olive-like NiS with smooth surface and sharp ends was obtained at benzene/water interface, while spindle-like NiS particles with rough surface and circle ends were formed when using toluene as a solvent. Analogously, chainlike Bi2S3 nanowires were produced at chloroform/water interface. The effect of the experiment parameters including reaction time, solvent and concentration of reactants on the size and morphology of the products was discussed in detail and a possible formation mechanism was suggested.

Hydrothermal synthesis and thermoelectric properties of PbS

Abstract In this paper, hydrothermal approach combined with high pressure sintering method was employed to synthesize PbS. The X-ray diffraction results show that single phase PbS can be obtained by a simple hydrothermal method. The scanning electron microscope results show that the PbS sample has nearly cubic shape and preserves well crystallized and coarse grains after high pressure sintering. The thermoelectric performance of PbS obtained in this study is comparable to that of a PbS sample prepared by conventional method. The carrier type and concentration of PbS can be tuned effectively by doping with Bi. The maximum figure of merit for PbS doped with 1 mol% Bi reaches 0.44 at 550 K, which is about 30 % higher than that of undoped PbS. These results indicate that hydrothermal method provides a viable and controllable way of tuning the electrical transport and thermoelectric properties for PbS.

Large single crystal diamond grown in FeNiMnCo–S–C system under high pressure and high temperature conditions*

Large diamonds have successfully been synthesized from FeNiMnCo–S–C system at temperatures of 1255–1393 °C and pressures of 5.3–5.5 GPa. Because of the presence of sulfur additive, the morphology and color of the large diamond crystals change obviously. The content and shape of inclusions change with increasing sulfur additive. It is found that the pressure and temperature conditions required for the synthesis decrease to some extent with the increase of S additive, which results in left down of the V-shape region. The Raman spectra show that the introduction of additive sulfur reduces the quality of the large diamond crystals. The x-ray photoelectron spectroscopy (XPS) spectra show the presence of S in the diamonds. Furthermore, the electrical properties of the large diamond crystals are tested by a four-point probe and the Hall effect method. When sulfur in the cell of diamond is up to 4.0 wt.%, the resistance of the diamond is 9.628×105 Ωcm. It is shown that the large single crystal samples are n type semiconductors. This work is helpful for the further research and application of sulfur-doped semiconductor large diamond.

Fast preparation and thermoelectric properties of Zn4Sb3 by HPHT

In this paper, crack-free bulk thermoelectric material Zn4Sb3 was prepared rapidly by high pressure and high temperature (HPHT) method. Near a single-phase Zn4Sb3 specimen was obtained using nominal stoichiometric powder mixtures, which were indexed by powder X-ray diffraction. The temperature-dependent thermoelectric properties including the Seebeck coefficient and electrical resistivity were studied. The maximum power factor of Zn4Sb3 specimen prepared by HPHT reaches 10.8 μW/(cmK2) at 637 K, which is comparable to the published data. The results show that the HPHT offers potential processing route to produce the thermoelectric material Zn4Sb3 quickly and effectively.