Study On The Fabrication Process And Properties Of Bi₂Te₃-based Thermoelectric Leg

Recycling the waste heat by the thermoelectric(TE)materials have attracted extensive attention due to the increase of the vehicle exhausts and the industrial excess heat.At present,the thermoelectric legs have disadvantages of low efficiency and bad mechanical properties,which is bad for the long term usage of the thermoelectric device.To meet the demands of high-efficiency and stability,we should improving the thermoelectric properties and the mechanical properties of the thermoelectric legs.And improving the fabrication process of the thermoelectric legs is the key to improve the thermoelectric and mechanical properties of them.Thus,theories and experiments are carried out to study the fabrication process of the thermoelectric legs based on the Bi2Te3 materials in this dissertation.The research work is supported by the National Natural Science Foundation of China(Grant No.51175460)and Program for New Century Excellent Talents in University(Grant No.NCET-13-0518).In Chapter 1,the research background and significance of the dissertation were introduced,the research status and development trend of the fabrication process of bulk thermoelectric materials,and the structure optimization process of the thermoelectric legs were investigated in detail.Then,the main research contents and architecture between the chapters of this dissertation were put forward.In Chapter 2,aiming at the disadvantages of the low thermoelectric conversion efficiency of the single thermoelectric leg,a segmented thermoelectric leg composed of Bi2Te3 and PbTe was proposed.A numerical model was established to analyse the thermoelectric performance of the BbTe3/PbTe segmented leg.Based on the model,an optimization analysis was carried out to decide the length ratio of Bi2Te3 to PbTe towards the maximum thermoelectric conversion efficiency and the maximum thermoelectric output power,respectively.The corresponding distributions of the temperature,voltage,and heat flux inside the thermoelectric leg were also studied.In Chapter 3,the semi-solid temperature range of the Bi2Te3-xSex materials was measured and a semi-solid powder forming method was developed to fabricate the Bi2Te3-based bulk materials.The obtained bulk materials have pores with mesoscale size and reduce the lattice thermal conductivity significantly.The effects of the semi-solid powder forming on the microstructure was studied.And the thermoelectric properties of the bulk materials made by semi-solid powder forming and other methods were also compared.In Chapter 4,Bi2Te3-based bulk materials with refined grain size and high density were fabricated by ultrasonic assisted hot-pressing method.The mechanism of the formation of the dislocations were studied.Based on the observation results of the microstructure,the relationship between the ultrasonic treating time and the final grain size of the bulk material was revealed.The ultimate grain size was refined after the ultrasonic assisted hot-pressing.The thermoelectric properties of the bulk materials were improved due to the grain size refining.In Chapter 5,an ultrasonic assisted semi-solid processing method was proposed based on the research of the semi-solid processing and the ultraosnic processing method.As a combined effect of the semi-solid and ultrasonic processing,the grain size was refined and the morphology of the grains was modified to be oblate-like.The lattice thermal conductivity was further decreased due to the strong scattering of the phonons.Meanwhile,the mechanical properties like the bending strength and the Vickers hardness were also improved for the bulk materials.In Chapter 6,the segmented Bi2Te3/PbTe thermoelectric leg(STL)was fabricated with the connection layer of Sn5Pb92.5Ag2.5.The effect of the thickness of connection layer on the contact resistance was studied.The results indicated that the SnsPb92.5Ag2.5 layer worked well in connecting the thermoelectric bulk materials.A performance-testing platform was built to measure the thermoelectric and mechanical properties of the segmented Bi2Te3-PbTe thermoelectric leg with the Sn5Pb92.5Ag2.5 connection layer.Results showed that connecting the STL by Sn5Pb92.5 Ag2.5 with a proper thickness can reduce the contact electrical resistance and improve the mechanical connection strength.In Chapter 7,the chief work of this dissertation was summarized,and the further research works were prospected.