Microstructure and thermoelectric properties of α-and γ-Al2O3 doped ZnO under high pressure

Using high pressure and high temperature (HPHT) technology to improve the thermoelectric properties of oxides is a feasible solution. To further understand the micro-physical mechanism improving the thermoelectric performance of ZnO in a higher pressure environment, the effects of α and γ lattice structure Al₂O₃ (α-Al₂O₃, γ-Al₂O₃) on doped ZnO were systematically studied. ZnO samples with different Al doping ratios (α-x, γ-x; x = 0.02, 0.04, 0.06, 0.08) were prepared by HPHT. Test results show that the high pressure and high temperature synthesis method effectively improves the solid solubility of α-Al₂O₃ and γ-Al₂O₃ in ZnO, among which γ-Al₂O₃ is more easily dissolved into the zinc oxide crystal lattice. Under the same doping ratio, the electronic conductivity of the sample synthesized with γ-Al₂O₃ is higher than that of the sample synthesized with α-Al₂O₃. In addition, the ZnAl₂O₄ phase precipitated out of the sample with increased Al doping. Although the ZnAl₂O₄ phase decreases the electrical properties of the sample, a small amount of ZnAl₂O₄ is uniformly dispersed in the ZnO sample, which can inhibit the growth of crystal grains, and assist grain refinement. The optimized high pressure sintering temperature was 1123 K, and the zT value of γ-0.04 was 0.17 at 973 K.