Control of grain size and crystal orientation for Bi-Sb-Te compounds

Controlling crystal orientation is one of important factors for improving the thermoelectric properties of Bi-Te compounds because the compounds have strong anisotropy in thermoelectric properties. Thermal conductivity by phonon should also be reduced for improving the thermoelectric figure of merit. Phonon of short wave length(hereafter: short wave phonon) is scattered at distorted atomic array sites which are often caused by substitute atoms. Phonon of long wavelength (long wave phonon) is scattered by grain boundaries. In this work, reducing grain size of Bi0.5Sb1.5Te3 was attempted, which was performed by mechanical alloying under ultra low oxygen atmosphere. The powder prepared by mechanical alloying was sintered by hot pressing at 650 K under an atmosphere of 4% H-2 with Ar balance (0.1 MPa). The crystal orientation in the sintered block was random. Then the block was deformed by ultra high pressure hot pressing at 650 K or 700 K or 750 K to obtain a desirable orientation for thermoelectric properties. Improvement of the crystal orientation in the deformed specimens was not confirmed by TEM. The results of electrical resistivity and Hall coefficient, however, revealed that the better orientation was performed in the meaning of electrical properties.