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 (VO 2) 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 VO 2—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 VO 2 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 VO 2’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