Effects of sintering on the microstructure and electrical properties of ZnO-based thermoelectric materials

Both the spark plasma sintering (SPS) method and reducing atmosphere sintering (N-2 + CO) method could greatly improve the electrical performance of ZnO-based thermoelectric materials. However, there is no comparison of these two methods which is essential for further improvements to the thermoelectric performance. In this article, a systematic comparison of the microstructures and electrical properties of the ZnO-based thermoelectric materials prepared by the SPS method, the reducing atmosphere sintering (N-2 + CO) method, and the conventional sintering method was presented. The ZnO-based thermoelectric materials prepared by the reducing atmosphere sintering (N-2 + CO) method showed the highest power factor with a moderate carrier concentration and a high Hall mobility, while the sample prepared by the SPS method exhibited the highest electrical conductivity of 2.3 x 105 S.m(-1) with a lower power factor. The ultra-high electrical conductivity of the SPS processed sample was mainly due to the increased solubility of Ti, which led to a higher carrier concentration. Moreover, the grain-boundary structure, which is important for the electrical performance, was also systematically analyzed by EBSD and CL. It can be concluded that the reducing atmosphere sintering (N-2 + CO) method is more suitable for thermoelectric applications, while the SPS method is an effective way to produce highly conductive ZnO ceramics. (C) 2017 Elsevier Ltd. All rights reserved.