Improving the Figure of Merit in N-Type Thermoelectric Oxides Using Low Temperature Synthesis and Cold Sintering

Thermoelectric materials convert heat directly into electricity according to the Seebeck effect. Thermoelectric generators (TEGs) use two types of TE materials: n-type (electron dominated conductor) and p-type (hole dominated conductor) to increase the efficiency of electricity generation from a given temperature gradient. High electrical and low thermal conductivities are required to enhance the figure of merit, ZT. These characteristics are often mutually exclusive in naturally occurring materials. Some p-type materials have been found with greater than unity ZT however, current n-type materials are seriously underperforming their p-type counterparts. This work shows a study of two methods designed to enhance ZT in an n-type oxide by reducing thermal conductivity. These are: controlled impurities to enhance phonon scattering and a novel cold sintering method designed for complex stoichiometry oxides with incongruent dissolution rates which allows nano-scale grains to be retained in a sintered ceramic, increasing the density of grain boundaries which scatter phonons. Nanoscale particles are produced using by ionic liquid synthesis method, what’s more a new compositional series was made using this method: La0.15-xEuxSrTiO3. The results demonstrate the cooperative fine tuning of thermoelectric properties to improve ZT. The initial densities of ceramics made using the novel cold sintering method are relatively low, ~55%, but this technique has the potential to be greatly improved. These results show novel compositions and methods which make incremental steps towards improving the efficiency of TEGs, in order to begin recuperating some of the 72% of primary energy lost globally in the form of waste heat.