Improvement in thermoelectric performance of Cu-doped β-rhombohedral boron

The effects of Cu doping on the thermoelectric properties of β-rhombohedral boron have been systematically investigated using nominal compositions of Cu_xB₁₀₅ (x= 0-5). The electrical conductivity increases with increasing x up to 5 at.%, whereas the positive Seebeck coefficient decreases because of an increase in the carrier concentration. Consequently, the power factor is enhanced by a factor of four from 22 μW m^-1 K^-2 (x= 0) to 90 μW m^-1 K^-2 (x= 4) at 973 K. In addition, Cu doping of the interstitial D and E sites contributes a ~50% reduction in the thermal conductivity from 4.3 W m^-1 K^-1 (x= 0) to 2.1 W m^-1 K^-1 (x= 5) at 973 K because of increased numbers of phonon scattering events. The dimensionless figure of merit ZT is also enhanced by a factor of six from 0.006 (x= 0) to 0.038 (x= 4) at 973 K in the p-type material. The ZT value obtained is higher than that of the conventional thermoelectric boride B₄C.