| is one of the thermoelectric materials with the most applicable prospect in moderate and high temperature. In order to improve further its thermal property, the effect of different elements doping on the thermoelectric property of Ca3Co4O9has been extensively investigated. In this thesis, the effect of Na or/and Ca on the thermoelectric property of Ca3Co4O9 has been studied by doping Na at Ca-site or/and Zn at Co-Site using sol-gel technique.The influence of calcination temperature on the formation of Ca3Co4O9 phase of the precursor powders has been analyzed. It is found that powders with pure Ca3Co4O9 phase can only be obtained when calcination temperature is 750～850℃. Hence, the calcination temperature is 750℃in this thesis. SEM observation shows that the Ca3Co4O9 sintered at 900℃consists of layer grains with grain size of 2-3μm; furthermore, the distribution of layer structure is homogeneous. The relationship between electric resistance and temperatures indicates that the conductive behavior of obtained Ca3Co4O9 is like metal from room temperature to 100℃, like semiconductor from 100 to 480℃, and like metal again above 480℃. The Seebeck coefficient S increases continuously with increasing temperature during the measuring temperature range. The positive S indicates that the Ca3Co4O9 is p-type semiconductor. The heat conductivity of Ca3Co4O9 increases slightly with temperature. The ZT value of Ca3Co4O9 is near 0.1 at 500℃.The influence of Na doping on microstructure and thermoelectric property of (NaxCa1-x)3Co4O9 (x=0,0.02,0.05,0.1,0.15 and 0.2) has been studied. It is found that Na doping can improve the microstructure. The minimum level of electric resistance (9.4～10mΩ·cm) of (NaxCa1-x)3Co4O9 can be obtained at x=0.05, which is apparently smaller than that (13.1-14mQ-cm) of Ca3Co4O9. It suggests that the some Ca substituted by Na can improve the conductivity. The maximum S is found to be 258.8μV·K-1 at 550℃in (Na0.1Ca0.9)3Co4O9, which is much higher than that of 9. The heat conductivity of Ca3Co4O9 increases with Na doping. In all, (NA0.1Ca0.9)3Co4O9 has optimal thermoelectric property with ZT value of 0.14 at 500℃, which is 40% higher than that of Ca3Co4O9.The influence of Zn doping on microstructure and thermoelectric property of Ca3(Co1-xZnx)4O9 (x=0,0.05,0.1 and 0.15) has also been studied. It is found that the ZnO peaks appear in XRD patterns once the Zn amount higher than 0.5; furthermore, the peak position of Ca3Co4O9 shifts slightly to higher diffraction angle. The SEM observation shows that the grain size of Ca3(Co1-xZnx)4O9 decreases to 1～2μm with Zn doping. In addition, the layer structure and orientation of grain sizes are not apparent. The Co substituted by Zn can increase of electric resistance and S of Ca3(Co1-xZnx)4O9 . The results show that the Ca3(Co0.9Zn0.1)409 has the maximum power factor 357.5μW·m-1·K-2 (520℃), which is apparently higher than that (246.3μW·m-1·K-2) of Ca3Co4O9. It suggests that the Zn doping in Ca3Co4O9 can also improve the thermoelectric property, but with a smaller effects than that of Na doping.The results of (NaxCa1-x)3(Co1-yZny)4O9 (x=0.1,0.15,y=0.05,0.1) with Na and Zn co-doping show that the grain size increases further to 4-6μm. The layer structure is apparent but with a thinner thickness. Unfortunately, both the conductivity and Seebeck coefficient can not be apparently improved. The power factor of all (NaxCa1-x)3(Co1-yZny)4O9 is smaller than that of Ca3Co4O9 due to the apparently higher thermoelectric resistance.In all, the Ca-site substituted by Na in Ca3Co4O9 can improve significantly the thermoelectric property. Co-site substituted by Zn in Ca3Co4O9 can also improve the thermoelectric property, but with a smaller effect. However, the co-doping of Na and Zn in Ca3Co4O9 results in the decrease of thermoelectric property. |
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