Yuewen Zhang

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.

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.

Suppressing adverse intrinsic conduction of Bi2Te3 thermoelectric bulks by Sb and Cu co-substitutions via HPHT synthesis

Chemical substitution combined with high-pressure tuning (high pressure and high temperature, HPHT) was applied to synthesize Bi2Te3 polycrystalline bulks. The synthesis time was sharply shortened to about half an hour, which is a distinct advantage over conventional synthesis methods for mass production. Acceptor-like antisite and substitutional defects owing to Sb and Cu substituting Bi sites were responsible for the increase in carrier concentration. Pressure-induced multiple textures and microstructures contributed to phonon scattering, including grain boundaries, lattice distortions, and dislocations. A maximum ZT value of 1.20 was achieved for Cu0.005Bi0.5Sb1.495Te3 at 473 K. Moreover, an available ZT value of 1.17 was obtained in a wide temperature range of 423–523 K. The increases in carrier concentration and band gap effectively suppressed the adverse intrinsic conduction and delayed the onset of the bipolar effect. Cu-substituted samples exhibit new vibrational modes in Raman spectroscopy, which implies that the substitutions induced significant changes in lattice vibrations.

Improved thermoelectric performance of Bi2Te3-xSex bulk materials produced by the preparation of high-pressure

The purpose of this research is to study the effect of Se doping content on the improvement in thermoelectric properties of Bi2Te3-xSex bulk materials produced by high-pressure role. The Bi2Te3-xSex bulk materials can be successfully synthesized within 30 min due to high-pressure role, which significantly shortened the synthesis time. The typical textures of the reaction products exhibit abundant cracked crystal planes and special layer structures with increasing Se content, which can coordinate electrical and thermal transport in the Bi2Te3-xSex samples to achieve an optimal thermoelectric performance. As a result, a Bi2Te2.73Se0.27 bulk material obtained a maximum ZT value of 1.03 at 344 K. These results suggest that the low Se doping content with high-pressure can improve the figure of merit of Bi2Te3-xSex materials.

One-step synthesis of type-I silicon clathrate Ba8Si46 under high pressure and high temperature

In this paper, the silicon clathrate Ba8Si46 was successfully synthesized using low-cost antioxidative azide Ba(N3)2 and Si as precursors by means of high pressure and high temperature (HPHT). The clathrate phase was one-step synthesized by high pressure chemical method within a short time. The reaction temperature and pressure were optimized to achieve good-quality crystalline products with a composition of Ba7.7Si46 and its transition temperature (Tc) is about 8.3 K. The new preparation route presented in this paper provides an alternative to the multistep HPHT synthesis applied so far. One-step synthesis of type-I silicon clathrate Ba8Si46 by high pressure chemical method can shorten the synthesis period of time greatly.

Rapid synthesis and thermoelectric properties of clathrate Ba8Cu6Ge15Si25 by high temperature and high pressure

Ba8Cu6Ge15Si25 were successfully synthesized by a simple high pressure and high temperature (HPHT) method to investigate the microstructures and thermoelectric (TE) properties. After high pressure synthesis, a highly dense bulk material with lots of fine-layered structure, lattice defects and disorders has been obtained. As expected, the thermal conductivity decreased greatly and the ZT value has been improved significantly, which reaches up to 0.43 at around 773 K. Comparing with other methods, HPHT could shorten the synthesis time from several days to half an hour. It reveals that the HPHT method will become an effective approach for optimizing the TE performance of these materials.

High-Pressure and High-Temperature Synthesis and Pressure-Induced Simultaneous Optimization of the Electrical and Thermal Transport Properties of Nonstoichiometric TiO1.80

We developed suitable high-pressure and high-temperature (HPHT) conditions for improvement of the thermoelectric properties of nonstoichiometric TiO1.80. X-ray diffraction, scanning transmission microscopy, transmission electron microscopy, and ultraviolet spectral measurements demonstrate that the crystal structures and microstructures are strongly modulated by our HPHT. The electrical properties and thermal conductivity are improved simultaneously by raising the reactive sintering pressure. The band gap was narrowed, contributing to the increase of the electrical properties with the pressure. In addition, relatively low thermal conductivities were obtained here as a result of a full spectrum of phonon scattering, benefiting from our deliberately engineered microstructures via HPHT. As a consequence, a high dimensionless figure of merit (zT) of 0.36 was obtained at 700 °C in the sample fabricated at 5 GPa. As far as we know, this is higher than all of the results of nonstoichiometric titanium oxide by other means and an enhancement of 57% of the best ever result. HPHT offers us a promising alternative for optimization of the thermoelectric properties, and it is worthwhile to popularize it.

Thermoelectric properties of Ni0.15Co3.85Sb12 and Fe0.2Ni0.15Co3.65Sb12 skutterudites prepared by HPHT method

Abstract N-type polycrystalline skutterudite compounds Ni0.15Co3.85Sb12 and Fe0.2Ni0.15Co3.65Sb12 with the bcc crystal structure were synthesized by high pressure and high temperature (HPHT) method. The synthesis time was sharply reduced to approximately half an hour. Typical microstructures connected with lattice deformations and dislocations were incorporated in the samples of Ni0.15Co3.85Sb12 and Fe0.2Ni0.15Co3.65Sb12 after HPHT. Electrical and thermal transport properties were meticulously researched in the temperature range of 300 K to 700 K. The Fe0.2Ni0.15Co3.65Sb12 sample shows a lower thermal conductivity than that of Ni0.15Co3.85Sb12. The dimensionless thermoelectric figure-of-merit (zT) reaches the maximal values of 0.52 and 0.35 at 600 K and 700 K respectively, for Ni0.15Co3.85Sb12 and Fe0.2Ni0.15Co3.65Sb12 samples synthesized at 1 GPa.

HPHT synthesis, structure and electrical properties of type-I clathrates Ba{sub 8}Al{sub x}Si{sub 46−x}

Abstract Clathrate compounds Ba8AlxSi46−x were successfully synthesized using the method of high-pressure and high-temperature (HPHT). In this process, we used BaSi2 as one of the starting materials in place of Ba metals, which reduces the complexity of the program caused by the extremely high chemical reactivity. By using this method, the processing time was reduced from few days to an hour. X-ray diffraction and structural refinement indicated this composition crystallized in type-I clathrate phase. Bond length analysis showed the Ba atoms in small dodecahedron had spherical thermal ellipsoids while those in large tetrakaidecahedron displayed anisotropic thermal ellipsoids. The negative Seebeck coefficient indicated transport processes were dominated by electrons as carriers, and increased with the increasing temperature. The electrical properties, including Seebeck coefficient and Power factor, were greatly enhanced by Al substitution.

Investigation of electrical transport properties of Bi0.5Sb1.5Te2.7Se0.3 alloys prepared by high-pressure method

The quaternary Bi0.5Sb1.5Te2.7Se0.3 alloys have been successfully synthesized within 25 min by high-pressure method. The pressure-dependent electrical transport properties of the as-prepared Bi0.5Sb1.5Te2.7Se0.3 alloys are carefully investigated at room temperature. The measurement results indicate that the electrical resistivity and the Seebeck coefficient reveal a strong correlation with the increase of synthesis pressure. The carrier concentration and mobility are modulated effectively due to the effects of synthesis pressure and composition, leading to an improvement in the power factor of Bi0.5Sb1.5Te2.7Se0.3. These results suggest that the utilization of pressure during the synthesis process provides an effective and controllable strategy to optimize the electrical transport properties of the (Bi,Sb)2(Te,Se)3 alloys.