Synthesis of Boron-Rich Metal Borides and Their Thermoelectric Properties

摘要

Polycrystalline metal borides AlMgB_(14), CaB_6, and MB_2 (M: transition metals) were synthesized and their thermoelectric properties were examined. Single phase of orthorhombic AlMgB_(14), which contains B_(12) icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 K and 1823 K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500 (μV/K) and 10~(-1) (1/Ωm) at room temperature, respectively. These values are comparable to those of metal-doped - rhombohedral boron. Synthesized AlB_2-type diborides (MB_2; M=Ti, V, Cr, and Mn) exhibited metallic conduction. The MB_2's α varied with number of valence electrons, and showed a maximum α in VB_2. The variation was similar to that predicted for pseudogap system. The CaB_6 also possessed metallic conduction, but the α of the phase was as large as 200 (μV/K) with negative sign.