Preparation And Thermoelectric Properties Of Aluminum-tungsten Doubledoped Zinc Oxide Ceramics

Thermoelectric Oxides are promising for high-temperature thermoelectric application due to their excellent stability at high-temperature,low toxicity,no pollution and abundant reserve.The ZnO thermoelectric material is one of the reported n-type oxide thermoelectric materials with high thermoelectric performance.However,the intrinsic ZnO has low eletrical conductivity and large thermal conductivity.In this paper,the thermoelectric properties of ZnO were improved by co-doping Al and W.Al_xZn_1-x-xO（x=0,0.01,0.02,0.04）powders were synthesized by a sol-gel method,and then were sintered at 1200℃,1300℃ and 1400℃ in three sintering atmosphere（Air,nitrogen and 10%H₂/Ar）,respectively.The effects of sintering atmosphere and Al doping on the composition,morphology and thermoelectric properties of ZnO were investigated.The experiment results showed that the Al_xZn_1-x-xO ceramics could not be sintered directly in 10%H₂/Ar atmosphere.The high density ceramics could be obtained by being sintered at 1400℃ in Air,nitrogen or pre-calcined the powder first in 10%H₂/Ar and then sintered in nitrogen（marked as H₂+N₂ sintering atmosphere）.For the samples sintered in the three sintering atmospheres,the doping of Al improved the thermoelectric properties of ZnO,and at 773 K,the power factors of the samples with x=0.02 reached maximum.The power factor of Al_xZn_1-x-xO ceramics sintered in N₂ or H₂+N₂ atmosphere was much higher than that of samples sintered in Air atmosphere.Among them,the samples sintered in H₂+N₂ atmosphere had optimal thermoelectric performance,and the power factor of Al_0.02Zn_0.98O reached a maximum of 5.86×10^-4W·m^-1·K^-2,which was 2.3 times higher than that of ZnO.Zn_0.98-y.98-y Al_0.02W_y O（y=0,0.005,0.01,0.02）ceramics were sintered in N₂atmosphere at 1200℃,1300℃ and 1400℃,respectively.The effects of Al and W co-doping on the composition,microstructure and thermoelectric properties of ZnO were investigated by XRD,SEM and thermoelectric parameters test.For the samples sintered at 1200℃,the doping of W could promote the growth of grain and acted as a sintering aid.But compared to sintered samples at 1300℃ and1400℃,the thermoelectric properties of the samples sintered at 1200℃ were low.For the samples sintered at 1300℃and 1400℃,the electrical conductivity and power factor were significantly increased,and thermal conductivity was decreased,as the doping amount of W was y=0.005,which resulted in a significant increase in ZT value.However,the porosity increased and density reduced with increasing the doping of W,which decreased the thermoelectric properties.TheZn_0.975Al_0.02W_0.005O sample sintered at 1400℃ had a maxium power factor of5.69×10^-4W·m^-1·K^-2at 773 K,which was 50%higher than that of Al_0.02Zn_0.98O prepared at the same condition.Finally,the ZT value of Al_0.02W_0.005Zn_0.975O sample sintered at 1400℃ reached a maximum value of 0.047 at 773 K,which was 66%higher than that of Al_0.02Zn_0.98O.