Influence of grain size and spatial volume fraction variation in ZnO based functionally graded thermoelectric devices

In the present study, a novel coupled analytical - finite element model is developed to examine the effect of spatially varying volume fractions of differently sized ZnO grains on the performance of functionally graded thermoelectric devices (FGTEDs). The volume fraction distribution of grains along the device length is defined using the power's law function. The volume fraction variation parameter (n) is varied from 0 to ∞, and the corresponding power output of the device is systematically analysed and compared. The grain sizes ranging from 0.18 µm to 5.75 µm are considered in the temperature range 120 K to 675 K. The results indicate that the FGTEDs with a higher volume fraction of larger grains yields a greater power output. A maximum power output of 2.6341 x 10-4 W and a peak figure of merit of 0.006 at 345 K are achieved. Furthermore, a sensitivity analysis performed reveals that the electrical resistivity has the most significant influence on the power output, followed by the Seebeck coefficient and the thermal conductivity. This study underscores the crucial role of grain size distribution in optimizing the performance of ZnO based FGTEDs.