Taichao Su

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.

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.

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.

Effects of high pressure sintering on the microstructure and thermoelectric properties of BiCuSeO

ABSTRACT Polycrystalline BiCuSeO oxides are prepared by solid-state reaction followed by pressureless sintering (PLS) and high pressure sintering (HPS) methods. Both the experimental results and the density functional theory calculations indicate that the crystal defect concentrations of BiCuSeO can be reduced under the effect of high pressure. By comparing with the PLS sample, a larger power factor and a lower thermal conductivity can be obtained for the HPS sample. The maximum figure of merit ZT∼0.4 @ 800 K was obtained for the HPS sample, which is about 3 times higher than the PLS sample. These results indicate that the effect of high pressure is beneficial to modifying the microstructure and improving the thermoelectric performance of BiCuSeO oxyselenide.

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.

High-temperature thermoelectric properties of AgxYyCa2.8Co4O9 + δ ceramics

Thermoelectric ceramics, AgxYyCa2.8Co4O9 + δ (0 ≤ x, y ≤ 0.15), have been prepared using a sol-gel method, followed by pressureless sintering. The phase structures and microstructures of oxides were investigated at room temperature. Thermoelectric properties including Seebeck coefficient, electrical conductivity and thermal conductivity were measured from 303 to 973 K. It was found that the samples contain nanostructured Ag phase with grain size of 100 nm. The Ag0.05Y0.15Ca2.8Co4O9 + δ sample has the maximum figure of merit (ZT ~ 0.14 @ 973 K), which is about 27% higher than that of Ca3Co4O9 + δ.

Thermodynamic, Structural and Thermoelectric Properties of AgSbTe2 Thick Films Developed by Melt Spinning

Cubic AgSbTe2 compound is a metastable phase within Ag2Te-Sb2Te3 pseudo-binary phase diagram and theoretically rapid cooling molten elements to room temperature may be an effective way to obtain it. In this work, thick films composed of 5–10 nm fine grains were developed by a melt spinning technique. The formation mechanism of the nanostructure and its influences on the thermoelectric properties have been studied and correlated. Differential scanning calorimetry (DSC) analysis shows that the as-prepared films exhibit distinct thermodynamic properties when prepared under different cooling rates and doping element. A small amount of Se doping is effectively capable of inhibiting the emergence of the Ag2Te impurity and optimizing the electrical transport properties. All films have positive large Seebeck coefficient, but rather small positive or negative Hall coefficient, indicating a multicarrier nature of transport consisting of both holes and electrons. A power factor of ~1.3 was achieved at 500 K for Se-doped film for its excellent electrical conductivities. This result confirms that a combination of Se doping and melting spinning technique is an effective way to obtain high phase-pure AgSbTe2 compound and reveal its intrinsic transport properties routinely masked by impurities in sintering or slow-cooling bulk samples.

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.