Study On Preparation And Properties Of ZnO Based Thermoelectric Materials

Thermoelectric materials, which can realize thermoelectric conversion directly without any mechanical equipments and noise, are attractive as a kind of save-energy materials. Zinc Oxide (ZnO), as a fine n-type semiconductor thermoelectric material, has promising applications in the high temperature region because of its advantages in following aspects: abundant resource of the material, low cost, environmentally friendly, good thermal stabilities and chemical stabilities.ZnO based thermoelectric materials were prepared with the method of solid state reaction (SSR). Phase composition, crystalline structure and power factor were studied for ZnO samples doped with different elements (S, Fe, Ni) under the aid of XRD and a home made thermoelectric test device, the optimized doping concentration for each element has been obtained. The effect of sintering temperature on thermoelectric properties was investigated with the measurements of surface morphology and power factor for S doped samples sintered at various temperatures. The co-doping of S with fixed concentration and nano-TiO2 particle with various concentrations in ZnO was taken out to research the influence of nano-TiO2 particle on the electrical properties for ZnO based thermoelectric materials.Results of the study for ZnO based thermoelectric materials sintered at 1273K show that undoped ZnO is an n-type semiconductor. The conductivity, Seebeck coefficient and power factor for undoped ZnO at 1073K are 9.5?-1cm-1, -159.3μV/K, 2.4×10-5 Wm-1K^-2 respectively. Iron is a good n-type dopant for ZnO. The optimized doping concentration in ZnO is 2at.%, and the power factor of ZnO:Fe(x=0.02) at 1073K is 1.7×10-4Wm-1K^-2, which is 7 times as much as that of undoped ZnO. ZnO doped with Ni is still n-type semiconductor, but the doping efficiency of which is worse than that of Fe. The optimized doping concentration for Ni in ZnO is 1at.%, and the power factor of ZnO:Ni(x=0.01) at 1073K is 0.7×10-4Wm-1K^-2. ZnO:S is n-type semiconductor, S is more effective than Fe, Ni in improving the power factor of ZnO. The optimized concentration of S in ZnO is 3at.%, and the sample ZnO:S (x=0.03) has the highest power factor at 1073K (2.5×10-4 Wm-1K^-2).Among the samples of ZnO:S (x=0.03) sintered with various temperatures (1173K,1273K,1373K), the one sintered at 1273K has the highest power factor at 1073K. The co-doping of nano-TiO2 particle with S does not change the n-type semi-conducting characterization of ZnO. The original conductivity of ZnO:S (x=0.03) has been maintained due to the tiny solution of TiO2 in ZnO and the optimized doping concentration for TiO2 is 0.3at.%. The power factor for the co-doped sample ZTOS0.3% (S: 3at.%, TiO2: 0.3at.%) at 1073K is one half of that for ZnO:S (x=0.03). The decrease of power factor here is as moderate as those for reported thermoelectric materials incorporated with effective nano-particles, which suppressed the thermal conductivity greatly and improved ZT to some extent. Hence, an increase of ZT for ZnO based thermoelectric materials doped with nano-TiO2 is promising.