Predicting\nthe Realizable Maximum Power Factor Using\nthe <i>Jonker</i> and <i>Ioffe</i> Formulation:\nAl-Doped ZnO Triangular Microcrystals with Graphite Inclusion Case\nStudy

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