Layered Ga2O3(ZnO)m based homologous compounds for thermoelectric applications

Abstract

The electrical and thermal transport properties of ZnO based layered compounds were investigated for mid-range thermoelectric applications. Ga2O3(ZnO)m based materials were prepared via solid state reaction with sintering temperatures in the range 1623 to 1673 K. The processing conditions were investigated to understand the factors controlling thermoelectric response. The thermal properties of all the compositions were analysed by means of the laser flash and differential scanning calorimetry. The electrical conductivity and Seebeck coefficients were determined using the fourprobe technique and the differential method respectively. The grain size and morphology was analysed by means of SEM observations. XRD and TEM techniques were employed to characterise the crystal structures of the homologous compounds. The thermal and electrical transport properties were correlated with the microstructures and crystal structures of the material. It is difficult to fully characterise the structures of the Ga2O3(ZnO)m homologous compounds due to experimental limitations arising from the closeness of the atomic number of Zn and Ga. Using aberration corrected TEM, it was found that gallium forms half occupied twin boundaries and is also agglomerated halfway between these twin boundaries, forming Ga rich inversion boundaries. The width of the nanoscale structural features was engineered to improve the thermoelectric properties of the compounds. The thermal conductivity decreased from 3.5 W/m·K to 1.9 W/m·K at room temperature when the value of 𝑚 decreased from 15 to 9; by increasing the value of 𝑚, the width of the nano-twinned region increased. The inherent plate-like grain shape of these layered compounds hinders the densification process. The addition of B2O3 was found to significantly improve the sinterability; both, improving densification and thermoelectric response. Subsequently, it was found that the co-addition of B2O3 and Nd2O3 helped to fully densify the Ga2O3(ZnO)m homologous compounds. All the samples reported had a relative density higher than 86% of the theoretical density. The highest density of 97% was achieved by the co-addition of B2O3 and Nd2O3; this is significantly higher than values reported in the literature for equivalent compositions. The crystal structures and thermoelectric properties of the Ga1-xInxO3(ZnO)9 solid solution were found to be sensitive to small differences in indium content. An improved electrical conductivity of 40 S/cm at 900 K was achieved in (Ga0.8In0.2)2O3(ZnO)9, this is 4 times higher than that found in the In-free samples. This improvement led to a 𝑍𝑇 of 0.07 at 900 K. This is the highest 𝑍𝑇 reported for the Ga1- xInxO3(ZnO)m compounds. Fast cooling after sintering the In2O3(ZnO)m homologous compounds led to improved thermoelectric response, increasing the electrical conductivity by 50% of their value when they are slow cooled. The addition of small amounts of Fe2O3 affected the electrical properties of the Ga2O3(ZnO)9 homologue; XPS revealed the oxidation state of Fe was 2+ and 3+ . It was found that Fe ions acted as deep donors, increasing the Seebeck Coefficient at room temperature, but decreasing the electrical conductivity.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Freer (Supervisor) & Robert Cernik (Supervisor)