Interface Morphology And Performance Optimization Of Mg₂Si/Ni Compound Thermoelectric Joint

Thermoelectric material is a kind of functional materials which can convert heat into electricity directly.The thermoelectric module made of the thermoelectric materials includes generators and coolers with advantages as small size,lightweight,simple structure,lower noise and less pollution and so on.In the packaging process,the performance of thermoelectric joint of materials and diversion electrode directly affects the thermoelectric conversion efficiency.Therefore,it is great significance to find excellent electrode materials to optimize the contact resistance of thermoelectric joint.In this work,the Field Activated Pressure Assisted Sintering?FAPAS?method was used to realize the connection of Cu/FeNi₃,Mg₂SiNi₃ and Mg₂ Ni electrodes with Mg₂ Si,in which one-step synthesis and densification of Mg₂ Si were accomplished.Then The phase analysis?XRD?,CTE test,resistivity test of the synthesized Mg₂ Ni,Mg₂Si Ni₃ and FeNi₃ bulk materials were carried out.The results show that the prepared materials have dense structure,sharp of the peak shape.The CTE of Mg₂ Ni and Mg₂SiNi₃ have good matching with Mg₂ Si in the temperature range of 350?⁶00?.At about 400?,the Z-direction CTE of FeNi₃ is close to Mg₂ Si,and the CTE of R direction is relatively low.The resistivity of the three materials is much lower than that of Mg₂ Si,which can be kept below 9.0×10-7?·m.The microstructure,electrical contact property,mechanical property and thermal stability of the joints were analyzed by SEM?EDS?,four-probe method and shear test.The major results are shown as following:Mg₂Ni/Mg₂ Si interface appeared two diffusion layers,Mg₂SiNi₃ new phase layer and Mg-rich layer?including a small amount of Ni,Si?.As the dismatching of thermal expansion coefficient between the Mg-rich layer and Mg₂ Ni,the joint strength and contact resistance did not reach the desired value,so Mg₂ Ni is not the ideal electrode material.The interface of Mg₂ Si Ni₃/Mg₂ Si joints with 5min holding time appeared two diffusion layers,?-?Mg0.52Ni0.48?7Si4 and ?-Mg6Si7Ni16.With the prolonging of the holding time,the diffusion layer gradually thickened and the interface gradually became uniform.When the holding time reached 30 min,the maximum joint strength was 28 MPa?before aging?,the contact resistivity was 63 ??·cm²,and the interface after the aging was still well connected,no pores or cracks and other defects,and joint had excellent heat stability.Cu/FeNi₃/Mg₂ Si thermoelectric joint with sintering temperature of 650? has a thin diffusion layer,the strength does not meet the application requirements.Interface microstructure of 750?is same as the joint of 700?,both including three diffusion:?Mg?Fe?Ni?,? and ? ternary phase.The former diffusion layer is more uniform and neat,the strength is higher than the latter,the maximum strength before aging is 22 MPa.The joint interface of 800? is more complex,the three diffusion layers are:?Si,Fe,Ni?ternary phase,?Mg,Si,Ni,Fe?quaternary phase and ? phase.White spotted Ni2 Si is precipitated in the?Si,Fe,Ni?ternary phase diffusion layer,and a?Fe,Si,Ni?ternary phase is also precipitated in the quaternary phase.The minimum contact resistance measured was 0.72 m?·cm² before the aging.