AgNi Alloy As a Suitable Barrier Layer Material for NbFeSb-Based Half-Heusler Thermoelectric Modules

As a type of moderate- and high-temperature thermoelectric materials, half-Heusler thermoelectric materials have a unique advantage in terms of their power factors and mechanical properties, and the figure of merit (ZT) of FeNb0.88Hf0.12Sb reaches 1.0 at 600°C. In this paper, after taking into account factors such as electrical conductivity, thermal conductivity, melting point, and metal activity, silver (Ag) was selected as the barrier layer material. The barrier layer and thermoelectric materials were welded together by atomic diffusion using spark plasma sintering. There was no obvious elemental diffusion on the connection surface after sintering, indicating that Ag did not impact the performance of FeNb0.88Hf0.12Sb. However, many cracks appeared on the joint surface during the aging process. In further experiments, we added a small amount of nickel (Ni) into the Ag to achieve a close connection between the barrier layer and FeNb0.88Hf0.12Sb. Moreover, we analyzed the interface of the intermetallic compounds after aging for 192 h, which showed that Ni3Nb and Ni6Nb7 compounds formed at the interface. The contact resistance was tested by a scanning probe method, and the resistivity of the barrier layer was determined to be 0.4 μΩ cm2, which is far less than that of other barrier layers. Moreover, the connection strength was greater than 40 MPa. When Ag0.9Ni0.1 was used as the barrier layer to compose the half-Heusler thermoelectric module, the thermoelectric conversion efficiency reached 7.33%, and there was no significant decrease in efficiency during the cyclic test.