Improvement in the thermoelectric properties of porous networked Al-doped ZnO nanostructured materials synthesized via an alternative interfacial reaction and low-pressure SPS processing

n-Type ZnO-based semiconducting materials are increasingly studied for thermoelectrics (TE) in the past due to their inexpensive and non-toxic nature coupled with their high Seebeck coefficient and stability at elevated temperatures. However, their high thermal conductivity limits their prospect for TE application. In this work, a novel, simpler and faster bottom-up approach to produce thermoelectric Al-doped ZnO ceramics from nanopowders produced by an interfacial reaction (double emulsion method) followed by consolidation by spark plasma sintering (SPS) is explored. This alternative interfacial reaction yielded porous aggregates composed of nanoparticles, which exhibited distinctly enhanced TE performance after densification by SPS at low pressure, due to the notable improvement in the power factor and pronounced suppression of the lattice thermal conductivity caused by the nano/micro-structure engineered by this bottom-up synthesis technique. This encompasses grain size reduction, inclusion of Al-rich nanoprecipitates, and nanoporosity, thus enabling scattering of phonons of different mean free paths. A maximum figure of merit ZT similar to 0.13 at 750 K was obtained for Zn0.97Al0.03O ceramics, a substantial enhancement to the previously reported values for the same composition synthesized by the traditional synthesis methods.