Haiqiang Liu

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.

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.

Effects of Pressure on the Microstructure and Simultaneous Optimization of the Electrical and Thermal Transport Properties of Yb0.5Ba7.5Ga16Ge30

The thermoelectric (TE) properties of n-type polycrystalline Yb0.5Ba7.5Ga16Ge30 bulks can be optimized by high-pressure and high-temperature (HPHT) sintering. After HPHT sintering, abundant nanograins are randomly distributed in the sample. Grains are refined by HPHT, with the grains being smaller with higher pressure. In comparison with the arc-melted sample, the samples obtained by quenching under high pressure possess a great number of nanograins and lattice structural disorders. Lower thermal conductivity is benefited by our deliberately engineered microstructures via HPHT, and the minimum thermal conductivity is 0.86 W m-1 K-1 at 773 K. The thermal conductivity and electrical properties are optimized simultaneously by raising the reactive sintering pressure. In comparison with the arc-melted sample (0.56), a maximum zT value of 1.13 at 773 K is obtained for the Yb0.5Ba7.5Ga16Ge30 sample fabricated at 5 GPa. This demonstrates that HPHT provides an effective strategy to improve TE performance through simultaneously enhancing electrical and thermal transport properties and should be applicable to other thermoelectric 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.