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Phase transition generates rapid changes of transport parameters and poor mechanical property, and thus restricts the application of thermoelectric materials. AgBiSe2 exhibits cubic phase at above 580 K with high-symmetry structure and low lattice thermal conductivity, indicating the potentiality of high thermoelectric performances. In this work, the cubic structure of AgBiSe2 was achieved at ambient conditions by alloying with PbS, enhancing the configurational entropy at both cationic and anionic sites. The cubic structure was rather stable after several measurement cycles. Nb substitution at cationic sites effectively reduced band gap, and increased both carrier concentration and effective mass. All samples exhibited relatively low lattice thermal conductivity (0.68–0.34 W/(m·K)) in the temperature range of 300–773 K, due to the nanoscale inhomogeneity and the random distribution of multiple species at some atomic sites. A maximum zT of 0.65 at 773 K was obtained for (Ag0.99Nb0.01BiSe2)0.8(PbS)0.2 sample. The entropy-driven structural stabilization is a promising strategy to achieve stable structure for practical thermoelectric applications.

Polycrystalline Cu2Se bulk materials were synthesized by high-pressure and high-temperature (HPHT) technique. The effects of synthetic temperature and pressure on the thermoelectric properties of Cu2Se materials were investigated. The results indicate that both synthetic temperature and pressure determine the microstructure and thermoelectric performance of Cu2Se compounds. The increase of synthetic temperature can effectively enhance the electrical conductivity and decrease the lattice thermal conductivity. A two-fold improvement in the power factor is obtained at synthetic temperature of 1000 °C compared to that obtained at room temperature. All β-Cu2Se samples exhibit low and temperature-independent lattice thermal conductivity ranging from 0.3 to 0.5 Wm−1K−1 due to the intrinsic superionic feature and the abundant lattice defects produced at high pressure. A maximum zT of 1.19 at 723 K was obtained for the sample synthesized at 3 GPa and 1000 °C. These findings indicate that HPHT technology is an efficient approach to synthesize Cu2Se-based bulk materials. Keywords: Thermoelectric performance, High pressure, Cu2Se

Bulk materials Ba8Ga16InxGe30-x (x = 0.5, 1.0, 1.5) were prepared by High-Pressure and High-Temperature (HPHT) method and the crystal structure has been confirmed by X-ray diffraction and cell refinement. The actual In composition was much lower than the starting composition, and lattice constants increased with the increase of substitution. As the temperature increased, the Seebeck coefficient and electrical resistivity increased first and then decreased, while the thermal conductivity was the opposite, which leads to significant enhancement on thermoelectric properties of the clathrates. The substitution of indium elements decreased the seebeck coefficient and electrical resistivity, and also changed the microstructure of the compounds. A minimum thermal conductivity of 0.84 Wm−1K−1 was obtained, and a good ZT value of 0.52 was achieved. The grain boundaries and lattice defects generated by high pressure can effectively scatter phonons of different frequencies, which reduce the lattice thermal conductivity.

We present the work about the initiative fabrication of multi-scale hierarchical TiO2-x by our strategy, combining high pressure and high temperature (HPHT) reactive sintering with appropriate ratio of coarse Ti to nanosized TiO2. Ubiquitous lattice defects engineering has also been achieved in our samples by HPHT. The thermoelectric performance was significantly enhanced, and rather low thermal conductivity (1.60 W m−1 K−1) for titanium oxide was reported here for TiO1.76. Correspondingly, a high dimensionless figure of merit (zT) up to 0.33 at 700 °C was realized in it. As far as we know, this value is an enhancement of 43% of the ever best result about nonstoichiometric TiO2 and the result is also exciting for oxide thermoelectric materials. The moderate power factor, the significantly reduced thermal conductivity and the remarkable synergy between electrical properties and thermal conductivity are responsible for the excellent thermoelectric performance. We develop a facile strategy for preparing multi-scale hierarchical TiO2-x and its superior ability to optimize thermoelectric performance has been demonstrated here.