High Entropy Materials as Brazing Filler Metals for Thermoelectric Devices

Brazing is a 5000 year-old joining process. Components are joined by heating them above the liquidus of a filler metal placed between them which forms a metallurgical bond as it solidifies. Brazed joints are relatively high strength and are often electrically conductive. As materials applications evolve, ever more is demanded of filler metals and hence applications arise in which current filler metals cannot perform sufficiently. In these cases, development of new custom-designed filler metals becomes the means of progress. The aim of this work is to design filler metals to replace ISO17672:Ag-155 when joining skutterudite thermoelectrics to copper within automotive heat-recovery devices. To do this, the class of High Entropy Alloys is explored as a source of new filler metal compositions.
Initial work here reported concludes that Ra=0.09μm is the optimum surface roughness for wetting of Ag-155 on copper. EDX data demonstrates that excessive silver diffusion is the principal inadequacy of filler metal Ag-155; leading to the conclusion that the replacement filler metal should not contain silver.
Replacement filler metals are designed and explored. They can form low void percentage (<15%) metallurgical bonds between copper and nickel, contain 4 phases and melt between 528-681⁰C, with a characteristic constituent diffusion distance through nickel <70% that of filler metal Ag-155. Brazed joints formed with these fillers have lower bond strength than joints formed with Ag-155, but showed no clear decline in strength after thermal cycling; although the joint failure rate reached 40% within 20-30 cycles. Contact resistance of joints between skutterudite thermoelectrics brazed with these alloys was measured as 2.02×10−5Ωcm-2, approximately half that of joints formed with Ag-155 and below the target maximum contact resistance of 5.0×10−5Ωcm-2
Overall this work demonstrates the potential of some HEAs as filler metals and provides insights into designing HEA systems to fill niche filler metal applications.