Highly Stable Metal─Na_0.02Pb_0.98Te Contacts for Medium Temperature Thermoelectric Devices

In the medium temperature (600-850 K) range, Na_0.02Pb_0.98Te is a highly efficient p-type thermoelectric compound. Device fabrication utilizing this compound for power generation demands highly stable low-contact resistance contacts with metal electrodes. This work investigates the microstructural, electrical, mechanical, and thermochemical stability of Na_0.02Pb_0.98Te-metal (Ni, Fe, and Co) contacts made by a one-step vacuum hot pressing process. Direct contact mostly resulted in either an interface with poor mechanical integrity, as in Co and Fe, or poisoning of the TE compound, as in the case of Ni, which results in high specific contact resistance (r_c). In Ni and Co, adding a SnTe interlayer lowers the r_c and strengthens the contact. It does not, however, effectively stop Ni from diffusing into Na_0.02Pb_0.98Te. The bonding is poor in the Fe/SnTe/Na_0.02Pb_0.98Te contacts due to the absence of any reaction at the Fe/SnTe interface. A composite buffer layer Co + 75 vol % SnTe with SnTe improves the mechanical stability of the Co contact with moderately lesser r_c than pure SnTe alone. However, a similar approach with Fe does not yield stable contact. The Co/Co + 75 vol % SnTe/SnTe/Na_0.02Pb_0.98Te contact exhibits r_c less than 50 μΩ cm² and has good microstructural and mechanical stability after annealing at 723 K for 170 h.