Thermoelectric Properties And Microstructure Characterization Of Strontium Titanate Matrix Composites

With the rapid development of industry,the energy depletion and environmental pollution are becoming more and more serious.Therefore,to explore new energy sources and improve the efficiency of energy conversions are the key to solving these problems.Thermoelectric materials,as a kind of functional materials,can realize the conversion of electric power and heat energy directly,which is different from traditional thermal electric conversion systems.Thermoelectric materials possess many advantages,such as the small volume,no noise,no pollution and portability.And their properties can be evaluated by a dimensionless thermoelectric figure of merit ZT=S²?T/K,where S,?,K and T refer Seebeck coefficient,electrical conductivity,thermal conductivity and absolute temperature,respectively.The SrTiO₃ oxide,as a thermoelectric material in high temperature region,has the advantages of high chemical stability,low cost and simple preparation.But the thermoelectric performance of SrTiO₃ is not desirable due to its high electrical conductivity.With similarity,SiGe thermoelectric material has shown an excellent thermoelectric property so as to using in the fields of aerospace.Unfortunately,the high cost and high thermal conductivity of SiGe limit its further wide application.Therefore,if combining the two materials?SrTiO₃ and SiGe?,their advantages and disadvantages could form a complementation to obtain good performance.In this work,we combine P-doped SiGe?n-type?and La-Nb doped SrTiO₃?n-type?.The depressing of thermal conductivity and optimization of power factor?PF=S²??are achieved by enhancing the phonon scattering and controlling doping concentrations,respectively,so as to obtain a good composite material with a high ZT value.The fabrication process of La-Nb doped SrTiO₃ powder,SiGe powder and SrTiO₃-SiGe composite powder are firstly studied.The result shows that both hydrothermal synthesized La-Nb doped SrTiO₃ powder and ball milling synthesized SiGe powder are pure phases.Ball milling synthesized Si_0.8Ge_0.2P_0.02.02 is added into the precursor solution of SrTiO₃ firstly,and then the hydrothermal method is used to synthesize the SrTiO₃-SiGe composite powder,this procedure is favorable to obtain the composite powder with a complex interface structure.The effect of doping concentrations,sintering pressure,TiO₂ impurity and C powder on the properties of La-Nb doped SrTiO₃ bulk is investigated.The result shows that the addition of the C powder and the micro-pressure sintering are favorable for the enhancement of thermoelectric properties.The mixture powder of 7 mol%La and 7 mol%Nb doped SrTiO₃ and C are sintered at micro pressure into a bulk sample,which exhibits the best thermoelectric performance of ZT=0.43 at 1000K.The STO-SiGe bulk sample is obtained by the micro-pressure sintering of hydrothermal synthesized STO-SiGe composite powders.The effect of SiGe content,sintering pressure,Zr dopant and C powder on the electrical transport performance and microstructure of the composite bulk sample is studied.The result shows that the bulk sample obtained by micro pressure sintering of mixture of C powder,3mol%SiGe and97mol%SrTiO₃ with 10mol%La-10mol%Nb-1mol%Zr doping shows the best thermoelectric performance.At 1000K,the power factor is 1.18 mwm^-1k^-2,the thermal conductivity is 2.68 wm^-1k^-1,and the ZT value is 0.45.To compare with 10 mol%La-10mol%Nb SrTiO₃ bulk sample,the thermal conductivity of composite material is reduced by 19%with doping,and ZT increased by about 22%.