1999 Volume 63 Issue 11 Pages 1400-1406
β-rhombohedral boron (β-boron) is expected to be one of the candidates for high-temperature thermoelectric materials because of its hopping conduction mechanism. To reveal the controllability of the thermoelectric properties of boron-rich semiconductors which consisted mainly of B12 icosahedral clusters, β-boron was doped with V, Cr, Fe, Co or Zr, and the composition and temperature dependence of the electrical conductivity and the Seebeck coefficient were investigated. V or Cr dopant which preferentially occupies the A1 sites in β-boron structure causes an increase of the electrical conductivity and a negative Seebeck coefficient. On the other hand, Zr dopant which does not occupy the A1 sites causes a large positive Seebeck coefficient. The electrical conductivity and Seebeck coefficient of Co- and Zr-doubly-doped p-type β-boron are nearly determined by the Co content. These results have been discussed with the concept of metallic-covalent bonding conversion for the B12 cluster. The temperature dependence of the electrical conductivity of metal-doped β-boron is well explained as the variable range hopping conduction. The Seebeck coefficient as well as the electrical conductivity increases with increasing temperatures up to room temperature, contrary to the cases of ordinary metals and semiconductors. Consequently, the power factors of both Co-doped p-type β-boron and V- or Cr-doped n-type β-boron increase with increasing temperature, and materials whose power factor is 3-4 orders of magnitude larger than that of pure β-boron at room temperature were obtained by doping with about 1 at% metal atoms.
