Synthesis and thermoelectric characterization of polycrystalline Ni_1−x Ca _x Co₂O₄ (x=0–0.05) spinel materials

Abstract

Ca doped NiCo₂O₄ spinel materials were synthesized by conventional solid state reactions at 900 °C. Thermoelectric properties of polycrystalline products were characterized at high temperature range of ∼800 °C in air. d.c. conductivity of the prepared polycrystalline 5 mol % Ca doped NiCo₂O₄ was about 60 S m⁻¹ at 300 °C. The value of d.c. conductivity was increased with the temperature increasing. Thermoelectric voltage of polycrystalline Ni_1−x Ca _x Co₂O₄ (x=0–0.05) was positive at 300–800 °C, this showed p-type thermoelectric properties. The Seebeck coefficient of 5 mol % Ca doped NiCo₂O₄ was ca. 300 μV/K at 600 °C. The value of the Seebeck coefficient of Ni_1−x Ca _x Co₂O₄ polycrystalline products decreased with the increasing temperature. Thermal conductivity of 5 mol % Ca doped NiCo₂O₄ was ca. 2.2 W m⁻¹ K⁻¹ at 600 °C. The estimated thermoelectric figure-of-merit, Z, of 5 mol % Ca doped NiCo₂O₄ spinel polycrystalline product was about 3.5×10⁻⁵ K⁻¹ at 600 °C.