Thermoelectric properties of some metal borides

Polycrystalline AlMgB₁₄ and some hexaborides (CaB₆, SrB₆, YbB₆, SmB₆, and CeB₆) were synthesized to examine their thermoelectric properties. Single phase of orthorhombic AlMgB₁₄, which contains B₁₂ icosahedral clusters as building blocks, was obtained at sintering temperatures between 1573 and 1823 K. Seebeck coefficient (α) and electrical conductivity (σ) of the phase were about 500 μV/K and 10⁻¹ 1/Ω m at room temperature, respectively. These values are comparable to those of metal-doped β-rhombohedral boron. On the other hand, metal hexaborides with divalent cation possessed large negative α ranging from −100 to −270 μV/K at 1073 K. Calculated power factors of CaB₆ and SrB₆ exceeded 10⁻³ W/K² m within the entire range of temperature measured. As a result, they can be thought as promising candidates for n-type thermoelectric material.