| Perovskite-type Sr Ti O3 is reported to be one of the best performance of n-type oxide thermoelectric materials.For Sr Ti O3-based thermoelectric materials,the study of La element doping is relatively mature,but its conductivity decreases with increasing temperature and thermal conductivity is very high,resulting in the thermoelectric properties still low.So in this paper,we designed La and Bi co-doped and Sr vacancy to improve the thermoelectric properties of Sr Ti O3-based ceramics.In this paper,Sr Ti O3 powder was prepared by an improved solid phase method.Then,the corresponding ceramic materials were sintered at 1500? and5%H2/95%N2 reduction atmosphere.Besides,through the analysis of phase composition,microstructure,electrical conductivity,Seebeck coefficient and thermal conductivity of the materials,the effect of La and Bi co-doped and Sr vacancies on the structure and thermoelectric properties of ceramics was investigated.Firstly,the La0.1Bix Sr1-x Ti O3(x=0,0.05,0.075,0.1)ceramic's sintering process,composition,structure and thermoelectric properties were studied.The result indicated that the longer holding time can make the samples with low doping concentration(x=0~0.05)obtain better sintering effect,while shorter holding time can get smaller grain size so as to reduce the thermal conductivity and improve material performance.Moreover,the best thermoelectric properties were obtained in the sample with 4 hours holding time and 0.075 Bi doping content.The maximum power factor is 692?W·m-1·K-2 at 500?,and highest ZT value is 0.172 at 500 ?,which is 130% higher than La0.1Sr0.9Ti O3 ceramics without Bi.Secondly,four types and two series of ceramics with Sr vacancies and La,Bi co-doped which nominal composition was La0.1Bix Sr0.9-1.25 x Ti O3,La0.1Bix Sr0.9-1.5x Ti O3,La0.1Bix Sr1-1.25(0.1+x)Ti O3 and La0.1Bix Sr1-1.5(0.1+x)Ti O3(x=0,0.05,0.075,0.1 @4h;x=0,0.025,0.05,0.065 @6h)were prepared.The results showed that the impurity phase Bi2O3 was detected when Bi doping content was exceeded 0.05 and the presence of Sr vacancies can increase the maximum amount of Bi addition.At the same time,the Sr vacancies also enhance the sintering effect and refine the grain.Finally,the thermoelectric properties of the four types and two series of samples were tested.The results showed that the conductivity and Seebeckcoefficient of the samples prepared under two different conditions were slightly different,but they all get the best performance at y=1-1.25(0.1+x)and x=0.05,and the best power factor is 1213?W·m-1·K-2(300?)when holding time is 4h and1034?W·m-1·K-2(500?)when holding is 6h,which is 420% and 340% higher than La0.1Sr0.9Ti O3 and 75%and 49% higher than La0.1Bix Sr1-x Ti O3.The ZT values of the two are 0.235 and 0.227 at 500?,which are 260% and 250% higher than La0.1Sr0.9Ti O3 and 37% and 32% higher than La0.1Bix Sr1-x Ti O3 respectively.Thus indicated that the synergistic effect of Bi-La co-doped and Sr vacancies can effectively improve the thermoelectric properties of Sr Ti O3 based ceramics.Since the values La0.1Bi0.05Sr0.8125 Ti O3 samples prepared with the same composition and different holding time are very close,and within the test error range,the stability and repeatability of the material properties studied in this thesis are demonstrated. |
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