This review summarizes the crystal structures, microstructures, electronic structures, physical/chemical properties, and effective methods to enhance the thermoelectric performance of the BiCuSeO system.

Impedance spectroscopy measurements evidence superionic Li + mobility (>10 À3 S cm À1 ) at room temperature and fast ionic mobility for Na + (5 Â 10 À6 S cm À1 ) in high entropy oxides, a new family of oxide-based materials with the general formula (MgCoNiCuZn) 1ÀxÀy Ga y A x O (with A ¼ Li, Na, K). Structural investigations indicate that the conduction path probably involves oxygen vacancies.

p -type BiCuSeO, a layered oxyselenide composed of conductive (Cu2Se2)2− layers alternately stacked with insulating (Bi2O2)2+ layers, shows an enhancement of the electrical conductivity after substituting Bi3+ by Sr2+, from 470 S m−1 (BiCuSeO) to 4.8×104 S m−1 (Bi0.85Sr0.15CuSeO) at 293 K. Coupled to high Seebeck coefficients, this leads to promising values of the thermoelectric power factor that exceeds 500 μW m−1 K−2 at 873 K. Moreover, the thermal conductivity of these layered compounds is lower than 1 W m−1 K−1 at 873 K. Maximum ZT values reach 0.76 at 873 K, making this family promising for thermoelectric applications in the medium temperature range.

BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200-400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power generation applications.The thermoelectric (TE) energy conversion technology, which can be used to convert wasted heat into electricity, has received much attention in the past decade. The efficiency of TE devices is characterized by the dimensionless figure of merit, ZT ¼ (S 2 s/k)T, where S, s, k, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively. Until now, several classes of bulk materials with high ZT values have been discovered, 1,2 including nanostructured BiSbTe alloys, 3 filled skutterudites, 4 zinc antimonide, 5 AgPb 18+x SbTe 20 , 6 Tl doped PbTe 7 or (AgSbTe 2 ) 1Àx (GeTe) x alloys, 8 but they lack thermal and chemical stabilities in air. Therefore, TE oxides are expected to play an important role in extensive applications for waste heat recovery, on the basis of their potential advantage over heavy metallic alloys of chemical and thermal robustness. To date, several families of oxides have been developed as promising TE materials. Typical TE oxides include Ca 2.8 Ag 0.15 Lu 0.05 Co 4 O 9+d

We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu 2 Se 2 ) 2 À and insulating (Bi 2 O 2 ) 2 þ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi 3 þ with Ca 2 þ . The resulting materials exhibit a large positive Seebeck coefficient of B þ 330 lV K À1 at 300 K, which may be due to the 'natural superlattice' layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of B4.74 lW cm À1 K À2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (B0.9 W m À1 K À1 ) to 923 K (B0.45 W m À1 K À1 ). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young's modulus, EB76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, cB1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of B0.9 at 923 K for Bi 0.925 Ca 0.075 CuSeO.

1 Introduction Configurational entropy as driving force for stabilizing, at a given temperature, different structures has been used in several studies to tailor new materials. When this entropy is large enough, e.g. when mixing four or five metals like in high entropy alloys [4,5] or five cations in binary oxides [11], a high symmetry structure is formed at high temperature (>850 °C for the case of a mixture of five binary oxides). At high temperature this mixture forms a solid solution with rock salt structure which can be quenched and is (meta)stable at room temperature due to the small diffusion coefficients. This solid solution of binary oxides was somehow unexpected since several of the oxides used (Mg, Ni, Co, Cu and Zn oxides) do not exhibit solid solutions in their binary phase diagrams. For the

No abstract

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi 3+ by Sr 2+ induces a strong decrease of the electrical resistivity up to the solubility limit reached for x = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu 1−x OS. We show that the thermoelectric properties of Bi 1−x Sr x CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials.