The thermoelectric method of directly converting thermal into electrical energy is attracting increased attention for such applications and thus enhancing its efficiency is of great importance.
The performance of thermoelectric devices is assessed by the dimensionless figure of merit ZT of the material, defined as ZT =α2σT/k, where α, σ, k and T are the Seebeck coefficient, the electrical and thermal conductivities, and the absolute temperature, respectively. The thermal conductivity is a combination of thermal conductivity via electrons, κe, and via phonons, κl. The main difficulty in improvement of the efficiency of a thermoelectric device is due to the complex relation between σ, α and k. Improving the performance of thermoelectric materials is usually done either by improving the power factor, α2σ, or by applying phonon scattering methods in order to lower the thermal conductivity.
The most efficient n-type thermoelectric materials for temperatures up to 300°C are currently Bi2TexSe3-x based. By optimizing the preparation process while considering the inherent crystallographic anisotropy, the efficiency of such materials can be further improved. In this study the Bi2Te2.4Se0.6 composition was optimized by CHI3 doping, preferred alignment of the crystallographic orientation, and lattice thermal conductivity minimization. The synthesis route included rocking furnace melting, ball-milling or melt spinning, and hot pressing with optimal parameters for enhancement of the thermoelectric figure of merit, ZT, at temperatures higher than 200oC, commonly applied in low temperature power generation applications
The transport properties in the directions parallel and perpendicular to the pressing direction were examined. In the direction perpendicular to the pressing axis, a maximal ZT of ~0.9 was obtained at ~175oC and at ~210 oC, which is as far as we know among the highest ever reported for n-type Bi2TexSe3-x based alloys.