Microstructure and thermoelectric properties of pristine and Al-doped ZnO ceramics fabricated by cost-effective and eco-friendly wet chemistry methods

We have shown that mainly due to the differences in microstructure and possibly the varying amount of crystalline defects in the samples, the thermoelectric performance of bulk ZnO samples obtained through wet chemistry methods is 4 times higher compared to the sample obtained from commercially available ZnO powder. By utilizing the developed chemical precipitation technique, we successfully synthesized a series of Al-doped samples with a nominal composition of Zn1–xAlxO (x = 0.02, 0.04, 0.06). The incorporation of Al into ZnO results in two significant effects: (1) partial substitution of Zn by Al, leading to an increase in the power factor, and (2) the formation of ZnAl2O4 spinel phase, which has negligible influence on electronic transport but acts as a scattering center for phonons in addition to the mass and strain fluctuations induced by the partial substitution of Zn with Al. Consequently, the thermoelectric performance was further improved by a factor of 3 through Al doping, achieving a zT value of approximately 0.12 at 1100 K for the sample with a nominal composition of Zn0.94Al0.06O.