A film made of tiny carbon nanotubes (CNT) may be a key material in developing clothing that can heat or cool the wearer on demand. Researchers at North Carolina State University (NC State) have found that the novel CNT film possesses a combination of thermal, electrical and physical properties that make it an appealing candidate for next-generation smart fabrics.
The researchers were also able to optimize the thermal and electrical properties of the material, allowing it to retain these desirable properties even when exposed to air for many weeks. Moreover, these properties were achieved using processes that were relatively simple and did not need excessively high temperatures.
"Many researchers are trying to develop a material that is non-toxic and inexpensive, but at the same time is efficient at heating and cooling," said Tushar Ghosh, professor of textiles in NC State's Wilson College of Textiles and co-corresponding author of a paper on this work in ACS Applied Energy Materials. "Carbon nanotubes, if used appropriately, are safe, and we are using a form that happens to be inexpensive, relatively speaking. So it's potentially a more affordable thermoelectric material that could be used next to the skin."
"We want to integrate this material into the fabric itself," said Kony Chatterjee, a PhD student at NC State and first author of the paper. "Right now, the research into clothing that can regulate temperature focuses heavily on integrating rigid materials into fabrics, and commercial wearable thermoelectric devices on the market aren't flexible either."
To cool the wearer, Chatterjee explained, CNTs have properties that would allow heat to be drawn away from the body when an external source of current is applied. "Think of it like a film with cooling properties on one side of it and heating on the other," Ghosh said.
The researchers measured the material's ability to conduct electricity, as well as its thermal conductivity, or how easily heat passes through it. One of the biggest findings was that the material has a relatively low thermal conductivity – meaning heat would not travel back to the wearer easily after leaving the body to cool it. That also means that if the material were used to warm the wearer, the heat would travel with a current toward the body and not pass back out to the atmosphere.
The researchers were able to accurately measure the material's thermal conductivity through a collaboration with the lab of Jun Liu, an assistant professor of mechanical and aerospace engineering at NC State. Using a special experimental design, they could more accurately measure the material's thermal conductivity in the direction that the electric current is moving within the material.
"You have to measure each property in the same direction to give you a reasonable estimate of the material's capabilities," said Liu, a co-corresponding author of the paper. "This was not an easy task; it was very challenging, but we developed a method to measure this, especially for thin flexible films."
The researchers also measured the ability of the material to generate electricity from a difference in temperature, or thermal gradient, between two environments, which could be used for heating, cooling or to power small electronics. Liu said that while these thermoelectric properties were important, it was also crucial that they found a material that was flexible, stable in air and relatively simple to make.