Icrostructure And Properties Of The Interface Between Mg₂Si Thermoelectric Materials And Cu/Ni Composite Electrode

Thermoelectric material is a kind of functional material, which can convert heat into electricity directly. Thermoelectric device made of the thermoelectric materials can be used as generators or coolers with advantages of less pollution, lower noise, more compact and safe and so on. Thermoelectric joint that is mainly composed of thermoelectric materials and electrode is one of the most important part of thermoelectric devices which determines their performance and life.In this work, Cu/Ni/Mg₂Si joint were prepared by Field Activated Pressure Assisted Sintering（FAPAS） method and Spark Plasma Sintering（SPS） method, respectively, in which synthesis and densification of Mg₂Si were accomplished during the bonding between Mg₂Si and Cu/Ni simultaneously. In order to explore the relationship between microstructure and performance of the joint, the effect of bonding temperature and dwelling time on microstructure, process of growing of new phase were studied under different process conditions, and then the contact electrical resistance, mechanical properties and thermal stability of the joints were measured and evaluated as well.The results show that Cu, Ni and Mg₂Si have similar thermal expansion coefficient, which is beneficial to reduce the residual stress in the joint produced during heating and cooling processes, and improve its life and stability of service.Under the function of external coupling fields, Cu9 Si and Cu5Mg₂Si intermetallic compounds are observed in the interface of Cu/Mg₂Si joint due to strong interactive diffusion between Cu foil and Mg₂Si compounds. By introducing Ni as barrier layer, the diffusion and interface reaction can be supressed effectively due to its higher diffusion activation energy and lower solubility of Si, Mg, which makes the Ni/Mg₂Si interface more stable.Cu/Ni/Mg₂Si interfaces obtained under the FAPAS process are connected well with each adjacent layer and have good thermal shock resistance. The microstructure and growing mechanism of new phase are affected mainly by the bonding temperature. When it is lower than 750℃, the interface is composed of Mg₂Si Ni3 and Mg2 Ni, in which the former is prior to the latter. When the temperature reaches 750 ℃, Mg₂Si Ni3 is the only phase due to the thermodynamic instability of Mg2 Ni. The contact resistances of the joints prepared in different temperature are different, and the minimum magnitude is 112μΩ?cm2 for that bonded at 700℃ for 15 min. It increases with the thickness of interface when the aging time is elongated with linear dependence on square root of time.Cu/Ni/Mg₂Si are bonded well by the formation of new phase including Ni2 Si, η and ω compounds in interface under the SPS process, in which Ni2 Si is scattered in η phase matrix as particle and then grows into web shape when the temperature is increased. With the increase of aging time, the contact resistance of the joint is increased while the shear strength is decreased. The minimum magnitude of 1.28mΩ?cm2 has been reached by the specimens which were bonded at 800℃ for 15 min. While the best shear strength of 19.81 MPa has been reached by the specimens that was bonded at 750℃ for 30 min and then experienced a 600-hour-aging.