The use of strain and grain boundaries to tailor phonon transport properties: A first-principles study of 2H-phase CuAlO2. II

Transparent oxide materials, such as CuAlO2, a p-type transparent conducting oxide (TCO), have recently been studied for high temperature thermoelectric power generators and coolers for waste heat. TCO materials are generally low cost and non-toxic. The potential to engineer them through strain and nano-structuring are two promising avenues toward continuously tuning the electronic and thermal properties to achieve high zT values and low $cost/kWh devices. In this work, the strain-dependent lattice thermal conductivity of 2H CuAlO2 is computed by solving the phonon Boltzmann transport equation with interatomic force constants extracted from first-principles calculations. While the average bulk thermal conductivity is around 32W/(mK) at room temperature, it drops to between 5 and 15W/(mK) for typical experimental grain sizes from 3nm to 30nm. We find that strain can offer both an increase as well as a decrease in the thermal conductivity as expected; however, the overall inclusion of small grain sizes dictates the potential for low thermal conductivity in this material.