Effects Of Coatings On Interface Microstructure And Output Power Of Thermoelectric Modules

Based on the Seebeck effect,thermoelectric materials are able to convert heat energy into electricity energy via recovering the waste heat or under the temperature difference,thus they are beneficial to our society today due to increasing serious problems of environmental pollution and resource shortage.Since the Bi₂Te₃-based thermoelectric materials show the best thermoelectric properties at room temperature,they have attracted much concern nowadays.In fact,the thermoelectric properties of Bi₂Te₃-based module depend on not only inherent properties of thermoelectric materials,but also the reliability of solder joints which are utilized for connecting the electrodes and Bi₂Te₃-based substrates,thus it is significant for modules to improve the reliability of solder joint.In this study,the atom diffusion barriers are established via introducing different types of coatings into interfaces of solder joints,and hence enhance the interface bonding performance and the electrical conductivity of thermoelectric legs.In the first part of this study,we systematically investigate morphologies of three joint systems(SAC305/Bi₂(Te,Se)₃,SAC305/Ni/Bi₂(Te,Se)₃ and SAC305/Cu/Bi₂(Te,Se)₃),and compare the effects of Ni and Cu electrodeposits on the interfacial reaction under aging treatment at 160 ^oC for various time.Results reveal that with increasing aging time,the intermetallic compounds(IMCs)at the SAC305/Bi₂(Te,Se)₃ interface excessively grow,and eventually show the embrittlement after aging treatment for 240 hours.This severe growth behavior of IMC layer can be effectively inhibited by Ni and Cu coatings.Due to the diffusion of Cu atoms and poor binding force between Cu coating and Bi₂(Te,Se)₃ substrate,Cu coating is detected separated from the substrate after aging for 240 hours,and this phenomenon results in failure of solder joints.Inversely,the Ni coating can not only efficiently inhibit the severe atom diffusion,but also show the excellent binding performance with Bi₂(Te,Se)₃ substrate throughout the whole aging stage.Thus,the Ni coating is confirmed to be the better barrier which could enhance the reliability of solder joints and protect the Bi₂(Te,Se)₃ substrate.Given the excellent binding force between the Ni coating and Bi₂Te₃-based alloys,a systematic investigation is carried out to evaluate the effect of Ni electrodeposit on the microstructure evolution and electrical resistance of the p-type Bi₂Te₃-based solder joint.Results indicate that Sn Te phase is the major and thermodynamically stable product in the SAC305/(Bi,Sb)₂Te₃ system,and its porous structure determined the presence of micro-tunnels,which facilitated the diffusion process of Sn and Te atoms,therefore the Sn Te layer continuously grew in the whole reflowing duration.In this system,the growth of Sn Te layer is demonstrated to follow the linear law,and be reaction-controlled.In the SAC305/Ni/(Bi,Sb)₂Te₃system,it is remarkable that the Ni barrier inhibits the rapid growth of the IMC layer,and the dominated product is detected to be(Ni,Cu)₃Sn₄ whose dense distribution can effectively suppress the diffusion of Sn atoms.Meantime,the growth of the Ni-Cu-Sn ternary IMC layer follows the parabolic laws and is mainly volume diffusion-controlled.After liquid-solid aging treatment,the electrical resistance remains at 2.203 m?for Ni TL(the TE leg with the Ni electrodeposit)system while that of TL(the TE leg without the Ni electrodeposit)system rapidly increases to a higher value about 4.879 m?,so it can reasonably conclude that the Ni electrodeposit is of great importance in depressing the electrical resistance.The third part focuses on exploring the effects of coatings on the performance of the Bi₂Te₃-based thermoelectric modules.In this work,Bi₂Te₃-based TE modules without barrier(TM),with Ni(TM_Ni)and with Ni/Au(TM_Ni/Au)are fabricated via reflow-soldering.Experimental results reveals that at the interface between SAC305solder and p-or n-type Bi₂Te₃ sample,the Ni or Ni/Au barrier can efficiently inhibit severe growth of the intermetallic compound(IMC)phase,and Ni/Au composite coating shows the more excellent performance of inhibiting effect.The results of power generation ability test reveal that TM_Ni exhibits higher value of output P under the temperature difference(?T)of 70 ^oC-90 ^oC due to its higher values of open circuit voltage(V)and output current(I).On the other hand,since the lowest electrical resistance(R)and relatively low output current of TM_Ni/Au were measured during the test,the TM_Ni/Au is suggested to generate less heat,meaning it consumed less energy,and hence be theoretically more efficient in improving the conversion efficiency.