The Fabrication And Properties Of Several Thermoelectric Legs Based On Combustion Synthesis

Thermoelectric（TE）materials can realize the direct conversion between heat and electricity,which shows great prospects to be applied in the power generation field,such as the recovery of industrial waste heat,exhaust waste heat of cars and hybrid solar photovoltaics-TE power generation system as well as the spot-cooling application.However,TE materials hasn’t been adopted to large-scale commercial applications due to their high cost.On the one hand,current preparation process of TE devices are quite complicated,which will affect the reliability of TE devices and greatly increase the preparation cost.On the other hand,the cutting process is inevitable during the current preparation process of TE devices,which lead to serious waste of the raw material especially for the micro TE devices.The low yield ratio also increases the preparation cost significantly.Therefore it is of great significance for developing the low-cost controllable preparation of TE devices with high properties to reduce its fabrication cost by simplifying the preparation process and increasing the utilization rate of raw material.In order to reduce the preparation cost of TE devices,we explore two approaches based on the self-propagation high-temperature synthesis（SHS）technique.The first approach is that we start from the raw element material powder of TE material and electrode material,the SHS reaction of TE material is initiated by in-situ heating during the sintering process,then the densification of both the TE material and the electrodes and the connection between them are realized simultaneously in the sintering apparatus,thus the TE leg is fabricated in just one step.The second approach is to realize the preparation of TE material and electrode material layer by layer using the selective laser melting（SLM）technique,so as to prepare TE devices directly without cutting process.The TE powder used for SLM process is synthesized by SHS technique.In this approach,the key is to investigate the effect of SLM process on the phase composition,microstructure and thermoelectric transport properties of the TE functional semiconductor materials.This research focuses on the Cu₂Se,CoSb₃ and Bi₂Te₃ TE materials which can be synthesized by the SHS technique.The two approaches are investigated respectively,and the main research content and results are shown as follow.Firstly,the Cu and Se powder were used as the raw materials for TE material,while the Ni and Al powder were used as the raw material for electrodes.We explored the feasibility to fabricate the Cu₂Se thermoelectric leg by sintering apparatus in just one step.The reaction mechanism of both Cu₂Se and the electrode material,the effect of both sintering temperature and the Se content on the phase composition,microstructure,TE properties and the interface properties were investigated systematically.Results show that the Cu₂Se is synthesized by SHS process,and the electrode material is obtained by solid state reaction.Higher sintering temperature leads to more copper ion migration,resulting in more notable copper deficiency in the Cu₂Se material.The deposited copper at the interface is likely to form Al-Cu alloy,resulting in larger specific contact resistivity.Appropriate Se deficiency can reduce the electrical conductivity as well as the thermal conductivity,while increase the Seebeck coefficient simultaneously,resulting in higher figure of merit ZT.Finally,the Cu₂Se_1.99.99 sample sintered at 500℃by PAS shows the maximum ZT value of 1.29 at 773 K,which is higher than the samples prepared by other methods at the same temperature.The specific contact resistivity is just 32μΩ·cm²,showing that the TE leg possesses excellent contact properties.Secondly,the Co,Sb and Te powder were used as the raw materials for TE material,and the Ti and Al powder were used as the raw materials for electrodes.We explored the feasibility to fabricate the CoSb₃ thermoelectric leg by PAS apparatus in just one step.The reaction mechanism of CoSb₃ material,the effect of sintering temperature and the Te content on the phase composition,microstructure,TE properties and the interface properties were investigated systematically.Results show that a small amount of Co-Sb compounds are formed by solid state reaction before 450℃from Co,Sb elemental material,then the CoSb₃ compound is obtained by SHS reaction.The CoSb₃ single phase can be obtained when the sintering temperature is higher than 550℃,higher sintering temperature shows no obvious effect on the TE properties and the interfacial contact resistivity.The single phase CoSb_3-xTe_x material can be obtained when the Te doping content x is lower than 0.15,and higher Te content leads to larger electrical conductivity and lower thermal conductivity due to the strong phonon scattering,which is caused by the Te substitution of Sb atom.Finally,the sample exhibits the maximum ZT value of 0.82 at 773 K,the interfacial specific contact resistivity is lower than 1μΩ·cm².Based on this result,n-CoSb₃/p-Cu₂Se thermoelectric couple is fabricated by the PAS one-step method starting from elemental raw material powder,which greatly simplified the preparation process of TE devices.Finally,the feasibility of preparing n-type Bi₂Te_2.7Se_0.3.3 material by SHS-SLM technique has been preliminarily explored.The effect of laser parameters on the phase composition of TE material,the forming morphology of single track and single layer were investigated.Furthermore,a multilayer Bi₂Te_2.7Se_0.3.3 sample was prepared,and the microstructure and TE properties of the SLM samples were characterized.Results show that higher laser energy leads to the serious volatilization of Se and Te element,resulting in the appearance of Bi elementary phase.With the increase of laser energy,the width of single track shows an obvious increase,and some pores are found on the surface due to the vaporization.During the single layer printing process,cross remelting using low laser energy density is found to be a good strategy to reduce the formation of microcrack while maintaining good layer surface quality.The multilayer sample prepared by the SLM method shows a relative density over 95%,a lot of fine grains are found and the crystal grain shows no obvious orientation.The characterization of the composition and properties of SLM sample shows that the Seebeck coefficient distributes inhomogeneously due to the different extent of vaporization at different locations.However,the Seebeck coefficient of SLM sample distribute homogeneously after annealing at 500℃for 1 h.This result indicates that the SLM technique is feasible for the preparation of TE materials,which offers a very good foundation for TE devices fabrication using the additive manufacturing technique.