High thermoelectric power factor in Cu-Ni alloy originate from potential barrier scattering of twin boundaries

Constantan alloy (Cu-Ni) has been known for a long time in thermocouples due to its thermal power property. In this study, we show an enhancement in thermoelectric performance of Cu₅₆Ni₄₂Mn₂ alloy by introducing nanoscale twins into its microstructure. Comparing to arc-melted ingot (without nanoscale twins), the ball milled and hot pressed (BM-HP) samples with twinning showed a higher Seebeck coefficient of ~-72.5μVK^-1 (an increase of ~12% at 873K), a larger power factor of ~102μWcm^-1K^-2 (an increase of ~21% at 873K), and hence a higher ZT of ~0.19 (an increase of ~34% at 873K). A high output power density of ~53.4Wcm^-2 is calculated from the high power factor even though the conversion efficiency is lower than 3% due to the low ZT. TEM characterization shows there is a large quantity of nanoscale twins with spacing of 50-200nm. It is very likely that low-energy carriers are selectively scattered by the twin boundaries (i.e., potential barrier scattering) thus lead to enhanced Seebeck coefficient. The improved thermoelectric performance of nano-twinned Cu-Ni alloy suggests constantan could be promising in thermoelectric power generation where the power output density is more important than the conversion efficiency.