| As a new type of thermoelectric material,ZnO has many remarkable advantages,such as good thermal and chemical stability,abundant resource of the material and no pollution,it has prospective applications in the field of medium and high-temperature thermoelectrics.However,compared with the traditional thermoelectric materials,its thermoelectric properties need to be further improved.As it has been reported that doping can change the microstructures of ZnO which regulate the resistivity and Seebeck coefficient to increase its power factor,that finally improve its thermoelectric properties.In this research,solid state reaction technique was employed to prepare Ga-doped ZnO,Ti-doped ZnO and Ga-Ti co-doped ZnO.The microstructures,electrical properties and thermoelectric properties were investigated.The relationship between the microstructures,electrical properties and thermoelectric properties was discussed.Simultaneously,the effects of different dopants on the microstructures,electrical properties and thermoelectric properties of ZnO which were prepared under the same preparation conditions were compared.The conclusions obtained by analysis of experiments are as follows:(1)Ga doping can significantly improve the thermoelectric properties of ZnO.When the Ga addition content was in the range of 0.2-10 at.%,the relative density of Ga-doped ZnO increased first and then decreased with the amount of Ga addition increasing,the internal point defects decreased first and then increased,the carrier concentration increased first and then decreased and the resistivity decreased first and then increased.As the addition amount of Ga was 0.5 at.%,the relative density reached the maximum.The samples of Zn0.998Ga0.002O prepared at 1200℃ and 1300℃ showed better conductivity,the room temperature resistivity of it was 3.31×10-3 Ω·cm and 2.51×10-3 Ω·cm,respectively.While the improvement of conductivity between Zn0.998Ga0.002O and undoped ZnO which were sintered at 1200℃ was higer than that of 1300℃.The carrier concentration was 59.1,42.9 times higher than that of undoped ZnO.Obviously,Ga doping can markedly increased the carrier concentration of ZnO,When the test temperature range was 350-800 K,the resistivity and the absolute value of Seebeck coefficient of Zn0.998Ga0.002O and Zn0.98Ga0.02O were lower than that of undoped ZnO,the power factor was obviously improved.The power factor of Zn0.998Ga0.002O at 800 K was 2.29×10-4 W·m-1·K-2,which was 6.3 times higher than that of undoped ZnO(3.62×10-5 W·m-1·K-2).(2)Appropriate amount of Ti doping can improve the thermoelectric properties of ZnO,but the magnitude of improvement was lower than Ga doping.The addition amount of Ti was in the range of 0.2 at.%to 1 at.%.As the Ti addition amount increased,the relative density of Ti-doped ZnO increased,the oxygen vacancy defects and carrier concentration decreased,then the resistivity gradually increased.The room temperature resistivity of Zn0.998Ti0.002O prepared at 1200℃ was 8.86×10-2 Ω-cm,which showed better conductivity,and its resistivity was 60%lower than that of undoped ZnO.Meanwhile,its Hall mobility was 6.3 times higher than that of undoped ZnO,clearly,Ti doping can obviously increased the Hall mobility of ZnO.The power factor of Zn0.998Ti0.002O prepared at 1200℃ was higher than that of undoped ZnO,while the power factor of Zn0.99Ti0.01O decreased.The power factor for Zn0.998Ti0.002O at 800 K was 3.91 × 10-5 W·m-1·K-2,which was 8%higher than that of the undoped ZnO.(3)A proper amount of Ga-Ti co-doping can further improve the thermoelectric propertiesof ZnO.When the addition amount of Ga was 0.2 at.%and the addition amount of Ti was in the range of 0.1-0.3 at.%,the relative density of Ga-Ti co-doped ZnO increased as Ti content increased,the oxygen vacancy content decreased,the resistivity increased and the power factor gradually decreased.When the test temperature range was 350-800 K,the power factor of Ga-Ti co-doped ZnO was obviously higher than that of undoped ZnO and Ti-doped ZnO.Meanwhile,the power factor of Zn0.997Ga0.002Ti0.001O was always higher than that of Zn0.998Ga0.002O at 350-700 K.The power factor for Zn0.997Ga0.002Ti0.001O at 800 K was 2.07×10-4 W·m-i·K-2,which was slightly lower than that of Zn0.998Ga0.002O.While the addition content of Ti was 0.2 at.%and 0.3 at.%the power factor of Ga-Ti co-doped ZnO was lower than that of Zn0.998Ga0.002O. |
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