Metal And Metal Alloy Electrode Contact Effects In Mg₃Sb₂ Baesd Medium Temperature Thermoelectric Devices

Thermoelectric?TE?materials are a kind of functional materials that can convert heat into electricity.Devices fabricated using TE materials can realize TE power generation and TE cooling.Mg₃Sb₂-based TE materials have attracted much attention due to their low price,environmentally friendly and excellent TE properties in the medial temperature region.However,because of the active Mg and Sb elements,the high coefficient of thermal expansion?CTE?,and the volatility of Mg₃Sb₂ at high temperature,there are few reports on Mg₃Sb₂-based device.Therefore,it is of great significance to find the suitable electrode connecting materials and connection method for Mg₃Sb₂-based materials.In this paper,we realized the connection of the electrode and the Mg₃Sb₂-based materials by using one-step spark plasma sintering?SPS?technology.The morphology of the interface and the growth law of the mesophase for different electrode materials joints were studied.The differences in electrical contact properties,mechanical properties and thermal stability of the joints were compared.The effects of alloying on joint properties were investigated.The main findings were as follows:We prepared different connections with different metal electrodes,like Ag,Cu,Ni,Ti,and Fe on Mg₃Sb₂-baesd materials by one-step SPS and investigated their properties,including microstructure,shear strength,and contact resistance.It was found that a?'-AgMg phase formed at Ag/Mg₃Sb₂ joint,and a Cu₂Mg phase formed at Cu/Mg₃Sb₂joint due to the intensive diffusion of Mg,resulting in the connection failure.In the case of Ni/Mg₃Sb₂,the seriously mismatched CTE between Ni and Mg₃Sb₂ led to many obvious cracks and the formation of the Ni/Ni₂Mg/NiMgSb/Mg₃Sb₂ structureat the interface,causing the joint failure.So does the Ti/Mg₃Sb₂ joint.Furthermore,the thermal stability of Ti/Mg₃Sb₂ joint was not good.With the increase of aging time,the contact resistance increased and the shear strength decreased.The joint failed after aging for 200 hours.We also found the cracks in the Fe/Mg₃Sb₂ joint.During the aging process,the Sb-rich Mg₃Sb₂ layer formed at the interface,resulting in the increase of contact resistance and the decrease of shear strength.And the joint failed after 200 hours of aging.The work function and CTE of the electrode materials were adjusted to match with Mg₃Sb₂ through alloying,Ti-Al alloy,Fe-Mn alloy,and 304 stainless steel alloy were studied,respectively.It was found that alloying Al and Ti reduced the work function of Ti electrode and successfully inhibited the formation of Ti-Sb mesosphere.However,the CTE of Ti-Al alloy is still mismatched with that of Mg₃Sb₂,which caused a large number of cracks in the joint,resulting in a decreased strength and increased contact resistance.The work function of the Fe electrode was reduced by alloying with Mn,and the CTE of Fe-Mn alloy is close to that of Mg₃Sb_2.There was no crack at the interface of Fe-Mn/Mg₃Sb₂ joint after sintering.The FeMn20 and FeMn40 joints have low contact resistivity and excellent mechanical properties,but the thermal stability is not good.After aging for 200 hours,the joint failed due to the enrichment of Sb at the interface.Using 304 stainless steel as the electrode material,the electrode work function was reduced while the CTE compatibility was greatly improved.The304S/Mg₃Sb₂ joint prepared by one-step SPS connection method has excellent electrical contact properties and mechanical properties,and the joint has good thermal stability.After sintering,the interface only has a diffusion layer of about 1?m,and the contact resistivity is only⁵.62??·cm².After aging for 100 hours,the interfacial diffusion layer keeps at 1?m,and the contact resistivity is⁶.14??·cm².After aging for 200 hours,Fe and Cr diffused and formed an intermediate layer of Mg-Fe-Cr.The contact resistivity increased to²5.19??·cm² while the strength of the joints kept well?>30 MPa?.