Study On The Join,interfacial Microstructure And Thermal Stability Of Electrode/CoSb₃ In The Skutterudite Thermoelectric Elements

Thermoelectric power generation devices can transform heat energy into electric energy directly.However,at present,there are some problems in most thermoelectric power generation devices,such as low conversion efficiency and poor reliability.For the devices used in medium and high temperature power generation,the interface between electrodes and thermoelectric materials at the hot side is the key issue to ensure the stable and reliable operation of devices.CoSb₃-based skutterudite（SKD）is one of the most promising thermoelectric materials in the medium temperature region,which has a high ZT value.However,the rapid interdiffusion between Sb and common electrode materials at high temperature often results in the damage of device.Therefore,it is necessary to study the microstructure and stability of the CoSb₃/electrode interface.In this study,Fe-Ni alloy/CoSb₃ joint,TC4 titanium alloy/CoSb₃joint and Mo-Cu/CoCrFeNi/CoSb₃ joint were prepared by vacuum hot pressing sintering.The microstructure evolution and thermal stability of the CoSb₃/electrode interface before and after aging were studied.The main contents were as follows:（1）Pure Feand pure Ni as electrode materials were selected and the Fe/（Co,Fe）Sb/（Co,Fe）Sb₂/CoSb₃ interface structure formed at the interface of the sintered Fe/CoSb₃ joint.However,due to the great difference of thermal expansion coefficient（CTE）between Feand CoSb₃,the fracture occurred in CoSb₃ matrix.The element diffusion at the Ni/CoSb₃ interface was very fast.The thickness of the diffusion layer after sintering was greater than 70μm,and the Ni/Ni₅Sb₂/（Co,Ni）Sb/CoSb₃ interface structure formed at the interface.Due to the rapid diffusion of elements and the mismatched CTE,a large crack occurred between Ni and Ni₅Sb₂.Therefore,in order to alleviate the thermal stress caused by the mismatch of CTE,Fe-Ni alloy with adjustable CTE was selected as the electrode material.The CTE of 4J50 Fe-Ni alloy foil was similar to that of CoSb₃.It was found that（Co,Fe,Ni）Sb phase formed at the interface of Fe-Ni/CoSb₃ joint with good bonding and no crack occurred,and there was a Fe-rich layer near the Fe-Ni side.After aging at 500℃,the thickness of diffusion layer increased obviously,and the Fe-rich layer gradually became loose and cracks occurred.After the accelerated aging at550℃,serious cracking occurred in the joint due to changes of CTE.（2）The widely studied titanium alloy was selected as the electrode material,and it was found that the interface morphology of Ti/CoSb₃ joint was related with the sintering densification rate of Ti and CoSb₃.The interface of TC4/CoSb₃ joint was well bonded,and diffusion layer can hardly be observed at the interface.After aging at 500℃,the island diffusion layer began to appear at the interface.With the increase of aging time,the island diffusion layer became the layered structure.Finally,the TC4/Ti Sb/Ti Sb₂/Ti CoSb/CoSb₃ interface structure formed,and the growth of diffusion layer was very slow.After aging at 550℃for 8 days,a layer of Ti₃Al phase formed on the side near TC4.Ti₃Al phase was beneficial to decreasing the interface diffusion rate,but increased the brittleness of the interface thus reduced the bonding strength of the joint.（3）Mo-Cu alloy as electrode and CoCrFeNi high-entropy alloy as diffusion barrier layer were selected.It was found that the three materials had a good bonding,and the CoCrFeNi barrier layer successfully blocked the diffusion between Mo-Cu and CoSb₃.The Mo-Cu/CoCrFeNi interface was firmly bonded and stable.Only one intermetallic compound（（Co,Cr,Fe,Ni）Sb）formed at the CoCrFeNi/CoSb₃ interface,and the interface structure was simple.After a long time of aging,the interfacial composition and structure changed little,and the diffusion layer grew slowly.The diffusion layer growth rate D was 5.18×10^-16 m²/s at 550℃.According to the fitting experimental data,the diffusion layer thickness was predicted to be less than 100μm after 200 days of aging at 550℃.（4）The thermal shock cycle test showed that the number of thermal shock of Fe-Ni/CoSb₃joint decreased greatly after thermal aging,and the TC4/CoSb₃ joint had better thermal stability,while the Mo-Cu/CoCrFeNi/CoSb₃ joint had the highest number of thermal shock and excellent thermal stability.