The use of solid-state refrigerators to cool appliances and electronic devices is a potential technological application for a theoretical research carried out at the University of Campinas (UNICAMP) in São Paulo State, Brazil. Even though this application is not anticipated by the study, which was based on computer simulations, such applications are a near possibility and could be an efficient and environmentally friendly substitute to vapor-compression refrigerators, which presently dominating the market and add to global warming and the depletion of ozone.
The research, led by Alexandre Fonseca with participation by his former student Tiago Cantuário, was part of the project “Carbon nanostructures: modeling and simulations”, aided by FAPESP. The findings are published in an article in the journal Annalen der Physik.
“Solid-state cooling is a young field of research with promising results. The method we investigated is based on the so-called elastocaloric effect (ECE), which makes use of temperature variations in a system in response to mechanical stress. We performed computer simulations of this effect in carbon nanotubes,” Fonseca told Agência FAPESP.
In the macroscopic realm, an analogous effect is witnessed when a rubber band warms up as it is quickly stretched and cools down again as it is freed. The effect happens if the deformation is applied to the material so that heat transfer does not take place into or out of the system, that is, when the process is adiabatic.
“We began our research on the basis of an article entitled ‘Elastocaloric effect in carbon nanotubes and graphene’, published in 2016 by Sergey Lisenkov and collaborators. It described a computer simulation study showing that when a small deformation was applied to carbon nanotubes, corresponding to up to 3% of their initial length, they responded with a temperature variation of up to 30 °C,” Fonseca said.
“In contrast with Lisenkov’s research, which simulated only simple strain and compressive force applied to the nanotubes, we reproduced the process computationally for a complete thermodynamic cycle. In our simulation, we considered two phases – nanotube strain and release – and two heat exchanges with two external reservoirs. We estimated the heat that would be extracted by the nanotube if it was in ideal contact with a certain medium. We obtained a good result for the performance coefficient compared with those of other experimentally tested materials.”
The performance coefficient is established as the heat extracted by a system from a particular region divided by the energy used to do so. With regards to a household refrigerator, for instance, it reveals the amount of heat extracted by the appliance from the internal environment in line with the electricity consumed. The leading household refrigerators have performance coefficients on the order of 8, which means they transfer eight times more thermal energy from inside to outside than the amount of electricity taken from the supply grid to carry out the exchange.
“Simulating the process for two different nanotubes, we obtained performance coefficients of 4.1 and 6.5. These are relatively good numbers compared with those for other heat exchange phenomena,” Fonseca explained.
Another benefit relates to molecular and atomic structure. “In the case of certain materials, the application of tensile strength makes the sample change phase by modifying its crystal structure. In the case of nanotubes, the thermal effect is due solely to expansion and relaxation of the structure, which is not modified. This is an advantage because phase changes make the material gradually lose its capacity to effect the function of interest. In the case of nanotubes, however, the process doesn’t produce any structural transformations capable of causing defects. The atoms are separated during expansion and return to their original positions with relaxation,” he said.
According to Fonseca, rupture tests have revealed carbon nanotubes can stretch as much as 20%. This deformation resistance integrated with high performance in elastocaloric effects makes carbon nanotubes appealing materials for the designing of nanoscale electronics.
The core problem in electronics is cooling. Our motivation was imagining a device that could use a simple cycle to extract heat from an appliance. Carbon nanotubes proved highly promising. They also have another virtue, which is that they’re small enough to be embedded in a polymer matrix, a desirable property at a time when manufacturers are investing in research and development to obtain flexible electronic devices such as foldable smartphones.
Alexandre Fonseca, Study Lead and Professor, UNICAMP.
All this is part of a bigger picture wherein vapor-compression refrigerators are substituted by solid-state refrigerators in the perspective of global climate change.