Effect of calcination temperature on structure and thermoelectric properties of CuAlO2 powders

Abstract

Copper aluminum oxide (CuAlO2) with delafossite phase was synthesized by the Pechini method using different calcination temperatures to evaluate its influence on the structure and thermoelectric material properties. X-ray diffraction and Raman spectroscopy confirm that delafossite phase was formed at 1100 °C with the presence of 2H-CuAlO2 and Al2O3 impurities, while at lower calcination temperatures (900 and 1000 °C), a mixture of CuO + CuAl2O4 (spinel phase) was observed. Energy-dispersive X-ray elemental maps display an even distribution of copper, aluminum and oxygen in the sample calcined at 1100 °C. Direct optical band gap, E g = 3.6 eV, was calculated from reflectance diffuse spectra by Kubelka–Munk and Tauc methods. An absorption band at 1.7 eV accounts for defect levels, masking the characteristic indirect transition. The thermoelectric properties, such as Seebeck coefficient, and thermal and electrical conductivities of the sample calcined at 1100 °C were measured at different temperatures. Hall voltage and positive values of the Seebeck coefficient (425.8–434.4 µV K−1) confirm the material’s p-type character. The independence of the Seebeck coefficient on the operation temperature indicates a small polaron electrical conduction mechanism. Thermal conductivity decreases exponentially with the temperature from 43.45 to 23.9 W m−1 K−1, where the principal contribution is due to phonons. Figure of merit ZT of sample calcined at 1100 °C between 100 and 800 °C increases from 1.42 × 10−8 to 4.94 × 10−4 in the order of the literature reports. From the Arrhenius plot ln(σT) versus 1000/T, an activation energy E a = 0.32 eV for the electrical conductivity was calculated.