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

Area of Science:

• Materials Science
• Thermoelectrics
• Solid-state Chemistry

Background:

• Sodium-lead telluride (Na$_{0.02}$Pb$_{0.98}$Te) is an efficient p-type thermoelectric material for medium-temperature power generation.
• Reliable, low-resistance electrical contacts are essential for fabricating functional thermoelectric devices.
• Direct metal contacts (Ni, Fe, Co) often lead to poor interfacial integrity or degradation of the thermoelectric material.

Purpose of the Study:

• To investigate the microstructural, electrical, mechanical, and thermochemical stability of Na$_{0.02}$Pb$_{0.98}$Te contacts with Ni, Fe, and Co.
• To evaluate the effectiveness of SnTe interlayers and composite buffer layers in improving contact properties.
• To identify an optimal contact structure for stable and efficient thermoelectric device fabrication.

Main Methods:

• Fabrication of Na$_{0.02}$Pb$_{0.98}$Te-metal contacts using a one-step vacuum hot pressing process.
• Incorporation of SnTe interlayers and composite buffer layers (Co + 75 vol % SnTe).
• Post-annealing stability tests at 723 K for 170 hours.
• Characterization of microstructural, electrical (specific contact resistance, $r_c$), and mechanical properties.

Main Results:

• Direct Ni, Fe, and Co contacts showed poor performance due to interfacial issues or material poisoning.
• SnTe interlayers improved $r_c$ and mechanical strength for Ni and Co contacts but did not prevent Ni diffusion.
• Fe/SnTe contacts lacked interfacial reaction, resulting in poor bonding.
• A composite buffer layer (Co + 75 vol % SnTe) with SnTe significantly enhanced mechanical stability for Co contacts.
• The optimized Co/Co + 75 vol % SnTe/SnTe/Na$_{0.02}$Pb$_{0.98}$Te contact achieved $r_c$ < 50 μΩ cm$^2$ and excellent post-annealing stability.

Conclusions:

• Direct metal contacts are unsuitable for Na$_{0.02}$Pb$_{0.98}$Te thermoelectric devices.
• SnTe interlayers and composite buffer layers are effective strategies for improving contact performance.
• The Co/Co + 75 vol % SnTe/SnTe/Na$_{0.02}$Pb$_{0.98}$Te contact structure offers a promising solution for stable, low-resistance electrical connections in thermoelectric applications.