Effects of sintering temperature on thermoelectric figure of merit of trace Al2O3 doped n-type ZnO nanocomposites

Abstract

ZnO is a cost-effective and abundant oxide on the earth and has many emerging applications in the areas of photonics, electronics, optoelectronics and spintronics. However, its thermoelectric performance is not satisfactory although some strategies, for example, doping with group III elements to enhance its thermoelectric properties, have been explored. In this work, we reported facile preparation of Al₂O₃–ZnO (AOZO) nanocomposites by dispersing a trace quantity of Al₂O₃ nanoparticles into ZnO nanoparticles, followed by the spark plasma sintering (SPS) process at high temperature of 500–1000 °C. The SPS process formed more space ions and electrons embedded in low-energy grain boundaries, and induced multi-scattering centers in AOZO nanocomposites, leading to the enhacement of the Seebeck coefficient and reduction in lattice thermal conductivity while keeping electrical conductivity relatively constant. The effect of the sintering temperature of the AOZO nanocomposite on the thermoelectric properties was investigated, showing that the SPS temperature of ≥700 °C in general led to a high electrical conductivity and Seebeck coefficient at the same time. The computational simulation using Boltzmann transport theory with the relaxation time approximation revealed multi-scattering phonon mechanisms (polar optical phonon + ionized impurity + piezoelectric scattering) depending on the sintering temperature applied. Taking an optimal composition (Al₂O₃)_0.0016(ZnO)_0.9984 prepared by the SPS at 700 °C as an example, the enhanced thermoelectric figure of merit ZT value of up to 0.91 at 860 K was achieved, which is a record high for ZnO-based thermoelectric oxides. We envisage this simple method to prepare thermoelectric metal oxide-based nanocomposites would be useful to develop eco-friendly, low cost and efficient thermoelectric materials in the future.