Device  level assessment of Ni and Ni45Cu55 as potential electrodes in Mg2(Si,Sn)-based TEGs for waste heat recovery

Kurzfassung

Mg2(Si,Sn)-based solid solutions are a promising class of materials for mid-to-high temperature waste heat recovery owing to their non-toxicity, low mass density, and low price. Previous work has shown that n-Mg2(Si,Sn) degrades due to Mg-loss and a corresponding reduction of charge carrier concentration, with the best performing Sn-rich compositions showing limited stability at temperatures ≥ 400 °C posing significant challenges in developing a fully Mg2(Si,Sn)-based TEG. In our study, we propose that n-Mg2Si could serve as a suitable n-type leg for constructing a stable Mg2X-TEG that operates at temperatures up to 450 °C. Testing different electrode materials we found that usage of Ni45Cu55 and Ni results in low electrical contact resistance (< 5 µΩ.cm2) and does not alter the thermoelectric properties of n-Mg2Si, in contrast to other electrodes (eg. Al, Cu, Ag). In a differential experiment we fabricated two 2x2 TE modules that used the same electrode-TE combination for the p-type legs but a different electrode (Ni or Ni45Cu55) for their n-type legs . Both TEGs were subjected to a temperature difference of up to 375K allowing for a direct comparison between Ni and Ni45Cu55 electrodes for n-Mg2Si at the device level concerning the changes under thermal load and cyclic thermal stability. Our results prove that Ni45Cu55 is a better working electrode for n-Mg2Si and provides a stable and robust design for silicide-based thermoelectric generators. We find the strongest difference between the two employed electrode at the device level, implying that testing at the device level provides more valuable insights than tests conducted on individual legs. We report a maximum power density of 0.79 W/cm2 at ∆T ~ 375K which is among the highest performance of Mg2X-TEG reported in literature. A constant property model was used to simulate the performance of the TEG device, and a good match was found between the measured data and theoretical expectations when using Ni45Cu55 as the electrode for n-Mg2Si. We find that the measured efficiency of the prototype is comparable to that of previously reported fully Mg2(Si,Sn)-based TEGs due to reduced internal losses.