Thermoelectric properties of c-axis aligned Bi-Te materials

Single crystalline Bi-Te thermoelectric materials have a rhombohedral structure with van der Waals bonding along c-axis direction. This structure gives intrinsic anisotropy in thermoelectric properties like electrical resistivity, thermal conductivity, Seebeck coefficient, as well as electron/hall mobility. Recent research works report that nano structures in polycrystalline thermoelectric materials improve thermoelectric performance by reducing thermal conductivity or by enhancing Seebeck coefficient. We investigated the effect of adjusting crystal orientation in the microstructures containing sub micro-sized grains on the thermoelectric properties for polycrystalline Bi-Te materials. Bi-Te powder, prepared through the conventional pulverization process, was sufficiently dispersed in an appropriate solvent, and then was formed into c-axis aligned green body under a designated high magnetic field. The green bodies were sintered with spark-plasma-sintering machine. The degree of crystal alignment of sintered bodies was examined with the electron-back-scatter- diffraction SEM and the X-ray diffraction patterns. It was observed that for p-type thermoelectric Bi-Te materials, aligning crystal orientation properly made electrical resistivity in the preferred direction decreased with keeping Seebeck coefficient and thermal conductivity remained unchanged.