Silver nanoparticles provide a crucial advance in thermoelectricity for power generation

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

The work, described in a January 1 energy of nature, should accelerate the development of advanced modules for power generation and other applications. The use of silver nanoparticles has been tested for stability in modules composed of three different state-of-the-art thermoelectric materials and designed to operate over a wide temperature range.

Thermoelectric materials have attracted increasing interest due to their potential as a clean source of energy, created when the material converts heat — such as waste heat from power plants or other industrial processes — into electricity by harnessing the flow of heat from a warmer area to a cooler area. But in order to harness this ability, a material must be found that can connect the hot and cold sides of the material, both electrically and thermally, without degrading the material’s performance.

The bonding 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 device’s operating temperature to remain stable during device operation, said Zhifeng Ren, director of the Texas Center for Supraconductivity at UH and corresponding author of the paper. When the thermoelectric material operates at higher temperatures, the bonding layer will remelt.

However, it can also be a problem if the bonding material has too high a melting point, as high temperatures can affect the stability and performance of the thermoelectric materials during the bonding process. Thus, the ideal bonding material would have both a relatively low melting point for module assembly, so as not to destabilize the thermoelectric materials, and be able to withstand high operating temperatures without remelting.

For such a connecting material, silver has valuable properties with high thermal conductivity and high electrical conductivity. But it also has a relatively high melting point of 962 degrees Celsius, 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 returned to a bulk state after module assembly and regained the higher melting point for operation.

“If you process silver into nanoparticles, the melting point can be as high as 400 degrees or 500 degrees C, depending on the particle size. That means you can safely use the device at 600C or 700C as long as the operating temperature stays below the melting point of solid silver, or 962C,” said Ren, who is also the MD Anderson Professor of Physics at UH.

He worked on the project with five former students and postdocs from the Ren research group; They are now based at the Harbin Institute of Technology in Shenzhen, China and at the Beijing National Laboratory for Condensed Matter Physics at the Chinese Academy of Sciences in Beijing.

The researchers tested the silver nanoparticles with three known thermoelectric materials, each operating at a different temperature.

A lead-tellurium-based module operating at a low temperature of about 573 Kelvin up to about 823 K (300°C to 550°C) produced a heat-to-current conversion efficiency of about 11% and remained so after 50 heat cycles stable. according to the researchers.

They also used the silver nanoparticles as an interconnect material in modules that use low-temperature bismuth telluride and a high-temperature half-Heusler material, suggesting the concept would work for a variety of thermoelectric materials and purposes.

Different materials are used depending on the intended heat source, Ren said, to ensure the materials can withstand the heat applied. “But this paper proves that regardless of the material, we can use the same silver nanoparticles for the solder as long as the applied heat doesn’t go above 960 degrees C” to stay below the melting point of bulk silver, he said.