Newswise — HOUSTON – Numerous high-functioning thermoelectric substances were found during the previous twenty years, but their potential has remained unrealized due to the absence of efficient mechanisms to convert the energy they generate into eco-friendly electricity. Recently, a group of global researchers, led by a physicist from the University of Houston and some of his ex-pupils, has disclosed a novel strategy for building thermoelectric modules. They employed silver nanoparticles to link the modules' electrodes and metallization layers.
The study, detailed in a publication released on May 1 in the journal Nature Energy, is expected to hasten the progress of cutting-edge modules for power generation and other applications. The stability of silver nanoparticles was evaluated in modules created from three distinct, state-of-the-art thermoelectric components, intended to function over an extensive temperature range.
The possibility of utilizing thermoelectric materials as a clean energy source has garnered greater attention due to their ability to convert heat, including waste heat generated by power plants or other industrial activities, into electricity by capitalizing on the movement of thermal current from a hotter region to a cooler one. However, exploiting this capability necessitates identifying a substance that can electrically and thermally connect the hot and cold sides of the material, without disrupting its performance.
To establish a junction between the two sides, the interconnecting material or solder is liquefied. For this reason, the melting point of the solder must be higher than the device's operating temperature to remain steady during device operation, as stated by Zhifeng Ren, the director of the Texas Center for Superconductivity at UH and one of the corresponding authors of the paper. If the thermoelectric material operates at higher temperatures, the interconnecting layer will liquefy again.
However, it can be problematic if the interconnecting material has an excessively high melting point, as elevated temperatures during the connection process can impact the stability and performance of the thermoelectric materials. Consequently, the optimal interconnecting material should possess a relatively low melting point for module assembly to avoid destabilizing the thermoelectric materials, yet it should also endure high operating temperatures without liquefying again.
Silver possesses advantageous characteristics as an interconnecting material due to its high thermal and electrical conductivity. However, its high melting point, which stands at 962 degrees Celsius, can destabilize numerous thermoelectric materials. The researchers capitalized on the fact that silver nanoparticles have a substantially lower melting point than bulk silver for this research. After module assembly, the nanoparticles transitioned back to a bulk state, regaining the higher melting point needed for device operation.
According to Ren, who is also a professor of physics at UH's M.D. Anderson, "If silver is converted into nanoparticles, the melting point can decrease to roughly 400 or 500 degrees Celsius, depending on the size of the particle. This implies that the device can be operated at temperatures of up to 600 or 700 degrees Celsius without issue, as long as the operating temperature remains beneath the melting point of bulk silver, which is 962 degrees Celsius." He collaborated on this research with five former students and post-doctoral researchers from the Ren research group, who are now based at the Harbin Institute of Technology in Shenzhen, China, and the Beijing National Laboratory for Condensed Matter Physics at the Chinese Academy of Sciences in Beijing.
The team of researchers tested the silver nanoparticles with three widely recognized thermoelectric materials, each of which operates at distinct temperatures.
The researchers found that a lead tellurium-based module, which operates at a relatively low temperature of approximately 573 Kelvin up to roughly 823 Kelvin (equivalent to 300 degrees Celsius to 550 degrees Celsius), achieved a heat-to-electricity conversion efficiency of roughly 11%. Furthermore, the module remained stable after undergoing 50 thermal cycles.
In addition, the team employed the silver nanoparticles as a connective material in modules utilizing low-temperature bismuth telluride and high-temperature half-Heusler materials, indicating that this approach could be applied to a wide range of thermoelectric materials and applications.
Ren highlighted that the selection of materials for the thermoelectric modules depends on the desired heat source and that different materials are utilized to guarantee their capability to withstand the applied heat. He further explained that the research demonstrated the potential of using silver nanoparticles as the solder material, irrespective of the specific thermoelectric material employed, as long as the applied heat remains below the melting point of bulk silver, which is 960 degrees Celsius.
The paper has several co-authors, including Li Yin, Fan Yang, Xin Bao, Zhipeng Du, Xinyu Wang, Jinxuan Cheng, Hongjun Ji, Jiehe Sui, Xingjun Liu, Feng Cao, Jun Mao, Mingyu Li, and Qian Zhang, who are all affiliated with the Harbin Institute of Technology. Additionally, Wenhua Xue, who is affiliated with both the Harbin Institute of Technology and the Beijing National Laboratory for Condensed Matter Physics, and Yumei Wang, who is affiliated with the Beijing National Laboratory for Condensed Matter Physics, also contributed to the paper.
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