Researchers have discovered a metal that can conduct electricity without conducting heat, paving the way for new materials that convert waste heat into electricity and window coatings that can help promote energy efficiency and temperature stability, among other applications.
Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and at the University of California, Berkeley have discovered that electrons in vanadium dioxide (VO2) can conduct electricity without conducting heat, bucking traditional rules in materials science.
Typically, a principle called Wiedemann-Franz Law governs the relationship between electrical and thermal conductivity in the majority of metals. The law states that good conductors of electricity are also good conductors of heat.
But metallic VO2—which already is known for its nontypical ability to switch from an insulator to a metal when it reaches 152 degrees Fahrenheit—also is different in this respect, which was “a totally unexpected finding,” said study principal investigator Junqiao Wu, a physicist at Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of materials science and engineering.
“We experimentally find a textbook law, the Wiedemann-Franz law, is dramatically violated in metallic VO2 due to independent transport or diffusion of heat and charge by electrons moving in the form of a fluid, instead of the regular behavior as individual particles,” Fan Yang, an engineering graduate student at UC Berkeley who also worked on the project, told Design News.
Wu, Yang and others co-authored a paper on the work published in the journal Science. Olivier Delaire at DOE’s Oak Ridge National Laboratory and an associate professor at Duke University, among others, also collaborated on the project and the paper.
|Vanadium dioxide (VO2) nanobeams synthesized by Berkeley researchers show unique electrical and thermal properties shown by the metal, which was discovered to conduct electricity without conducting heat. In this false-color scanning electron microscopy image, thermal conductivity was measured by transporting heat from the suspended heat source pad (red) to the sensing pad (blue). The pads are bridged by a VO2 nanobeam. (Credit: Junqiao Wu/ Department of Energy’s Lawrence Berkeley National Laboratory)|
The team used results from simulations and X-ray scattering experiments to discover the proportion of thermal conductivity attributable to the vibration of VO2’s crystal lattice, called phonons, and to the movement of electrons in the material, finding that the thermal conductivity attributed to the electrons is 10 times smaller than what would be expected from the Wiedemann-Franz Law.
“For electrons, heat is a random motion,” Wu said. “Normal metals transport heat efficiently because there are so many different possible microscopic configurations that the individual electrons can jump between. In contrast, the coordinated, marching-band-like motion of electrons in vanadium dioxide is detrimental to heat transfer as there are fewer configurations available for the electrons to hop randomly between.”
The work has a number of ramifications for both science and material applications, Yang said. “This study will stimulate scientific interests on studying exotic electrical and thermal properties of related materials,” he said.
It also can inspire the development of new materials that can be used to stabilize temperature in existing devices and machines by adding the mineral Tungsten (W) to the VO2, Yang said.
“For instance, for car radiators, when the car is running, it gets very hot, so we need a high-thermal-conductivity material to quickly dissipate the heat; while the car is not running on a very cold day, we need low thermal conductivity material to prevent the engine from freezing,” he explained. “W-doped VO2 is exact this type of material with higher thermal conductivity at high temperatures and lower thermal conductivity at slightly low temperatures. Thus, such material can be used to stabilize temperature of a system.”
The discovery also could lead to new material coatings such as those that can be painted on windows to stabilize temperature in houses or offices, creating more energy-efficient buildings, Yang said.
“By tuning its thermal conductivity, it can efficiently dissipate heat out in the hot summer, but prevent heat loss in the cold winter inside the house,” he said. “Since the material can tune its properties by itself, it will be a very cost-effective way to reduce home energy consumption.”
Scientists will continue to experiment with their findings to investigate more thoroughly the charge- and heat-transport mechanisms of VO2, as well as to improve its temperature-stabilizing capability, Yang said.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York. She currently resides in a village on the southwest coast of Portugal.