Thermoelectric Oxides: Effect of Doping in Delafossites and Zinc Oxide

The electrical resistivity (ρ) and Seebeck coefficient (S) of the three delafossites CuFe_0.9Cr_0.1O₂, CuCr_0.98Mg_0.02O₂, and CuRh_0.9Mg_0.1O₂ have been measured and their power factor (PF) calculated. These p-type oxides show PF values at 800 K from 1.4 × 10⁻⁴ W K⁻² m⁻¹ to 6.9 × 10⁻⁴ W K⁻² m⁻¹. In contrast to delafossites containing Fe or Cr, for which ρ exhibits a regime, the Rh-based delafossite shows a metallic regime from 5 K to 1000 K. This points toward the role of the transition-metal electronic configuration in the transport properties. Otherwise, similar PF values are obtained in the case of the n-type Al-doped ZnO. For these oxides, the ρ and PF values are minimum and maximum, respectively, for x = 0.01. However, the presence of spinel impurities even for x = 0.01 in Zn_1−x Al _x O or for x = 0.02 in CuCr_1−x Mg _x O₂ calls into question the role of the doping element in the physical properties. This should motivate a deeper insight into the physics of thermoelectric oxides.