Thermoelectric properties of homologous p- and n-type boron-rich borides

Abstract

Thermoelectric properties of a series of layered homologous rare-earth boron carbonitrides: HoB 17CN, REB 22C 2N ( RE = Y , Er , Lu ) , and YB 28.5C 4, were investigated. Samples for measurements were prepared in the form of hot pressed or isostatically pressed and annealed single phase polycrystalline powder. This series of compounds has structures where B 6 octahedral and rare-earth atomic layers reside between an increasing number of B 12 icosahedral and C–B–C chain layers, and has structural analogy to boron carbide. Interestingly, a variation from p-type thermoelectric behavior for YB 28.5C 4 to n-type for REB 22C 2N and HoB 17CN was observed. This is the first non-doped compound among the boron-rich borides in which n-type thermoelectric behavior has been observed. Similar to other boron cluster compounds low values of the thermal conductivity κ ( κ ⩽ 0.02 W cm - 1 K ) were found. The origins of the low κ in such compounds has not been fully explained, but comparison among the homologous series shows that the thermal conductivity appears to increase as the number of boron cluster layers increases. This result indicates that the heavy rare-earth atoms residing in the boron matrix may play a role in depressing thermal conductivity in addition to other features common to boron cluster compounds. Although the absolute values of the determined figures of merit ZT are not large for hot pressed samples, the Seebeck coefficients and power factors for both n-type and p-type in this series show an increase at temperatures exceeding 1000 K.