Predicting the Realizable Maximum Power Factor Using the <i>Jonker</i> and <i>Ioffe</i> Formulation: Al-Doped ZnO Triangular Microcrystals with Graphite Inclusion Case Study

Among the popular TE materials, selenides and tellurides are the benchmarks of high-efficiency systems. However, for high-temperature applications (>700 K), it is required to rely on the silicides and the oxides due to their exceptional thermal stability. ZnO is among the first few oxides in the field of thermoelectricity. Al-doped ZnO is a proven material for its high-temperature thermoelectric applications. However, the high grain boundary resistance limits further improvement of the efficiency of this oxide. Band engineering and band modification are successful approaches in lowering the grain boundary resistance. The addition of graphite and graphite-based materials at the grain boundaries is shown to serve this purpose. In this work, graphite powder is added in varying proportions to Al-doped ZnO triangular microcrystals. Thus, prepared materials are characterized to confirm the formation and investigate the nature of interface, morphology, etc. TE parameters such as electrical conductivity, Seebeck coefficient, and thermal conductivity of those materials also have been measured. The theoretical calculation of TE efficiency zT often differs from the actual experimental results due to the wide range of preparation methods, leading to changes in porosity, nature and density defects, and several other factors. In this paper, an effort has been made to estimate the maximum achievable power factor (PFmax) from the measured TE parameters of this set of samples by Jonker and Ioffe analysis. Based on the predicted PFmax, an appropriate material composition has been identified to achieve the same. Subsequently, including the measured parameters, the TE efficiency (zT) is calculated. Further, a sudden dip observed in the thermal conductivity in the high-temperature range (625–1000 K) of the prepared undoped ZnO graphite composite is investigated in this paper.