University of Houston physicist Zhifeng Ren. Photo: University of Houston.
University of Houston physicist Zhifeng Ren. Photo: University of Houston.

Several high-performance thermoelectric materials have been discovered over the past two decades. But without efficient devices to convert the energy they produce into emission-free power, their promise has remained unfulfilled. Now, an international team of scientists led by a physicist at the University of Houston (UH) has reported a new approach to constructing thermoelectric modules, using silver nanoparticles to connect the modules’ electrode and metallization layers.

This work, reported in a paper in Nature Energy, should accelerate the development of advanced thermoelectric modules for power generation and other uses. The researchers tested the silver nanoparticles in modules fabricated from three different state-of-the-art thermoelectric materials designed to operate across a wide range of temperatures.

Thermoelectric materials have drawn increasing interest because of their potential as a source of clean energy. This is produced when the material converts heat – such as waste heat generated by power plants or other industrial processes – into electricity by exploiting the flow of heat current from a warmer area to a cooler area. But taking advantage of that ability requires finding a material that can connect the hot and cool sides of the material both electrically and thermally, without interfering with the material’s performance.

The connective material, or solder, is melted to create an interface between the two sides. That means the solder must have a higher melting point than the operating temperature of the device so that it remains stable while the device is working. If the thermoelectric material operates at hotter temperatures, the connective layer will re-melt.

But it can also be a problem if the melting point of the connective material is too high, because high temperatures can affect the stability and performance of the thermoelectric materials during the connection process. The ideal connective material would have a relatively low melting point during assembly of the module, so as not to destabilize the thermoelectric materials, but then be able to withstand high operating temperatures without re-melting.

Silver has properties that could make it a useful connective material, including high thermal conductivity and high electrical conductivity. But it also has a relatively high melting point of 962°C, which can affect the stability of many thermoelectric materials. For this work, the researchers took advantage of the fact that silver nanoparticles have a much lower melting point than bulk silver. The nanoparticles can then return to a bulk state after the module has been assembled, regaining the higher melting point for operations.

“If you make silver into nanoparticles, the melting point could be as low as 400°C or 500°C, depending on the particle size,” said Zhifeng Ren, professor of physics at UH, director of the Texas Center for Superconductivity and a corresponding author of the paper. “That means you can use the device at 600°C or 700°C with no problem, as long as the operating temperature remains below the melting point of bulk silver, or 962°C.”

The researchers tested silver nanoparticles with three well-known thermoelectric materials, each of which operates at a different temperature. They found that a lead tellurium-based module, which works at a low temperature of about 573K up to about 823K (300°C to 550°C), produced a heat-to-electricity conversion efficiency of about 11% and remained stable after 50 thermal cycles. They also used silver nanoparticles as the connective material in modules based on low-temperature bismuth telluride and a half-Heusler high-temperature material, indicating the concept would work for a variety of thermoelectric materials and purposes.

According to Ren, different thermoelectric materials could be used depending on the intended heat source to ensure the materials can withstand the applied heat. “But this paper proves that whatever the material, we can use the same silver nanoparticles for the solder as long as the applied heat does not go above 960°C,” to remain below the melting point of bulk silver, he said.

This story is adapted from material from the University of Houston, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.