Scientists at the Dept. of Energy’s Lawrence Berkeley National Lab and the University of California, Berkeley, say they’ve come up with a material that can conduct electricity without conducting heat. The findings, published in the Jan. 27 issue of the journal Science, could lead to applications such as thermoelectric systems that convert waste heat from engines and appliances into electricity.
Most metals, of course, are good conductors of electricity and heat as governed by the Wiedemann-Franz Law. That is not the case for metallic vanadium dioxide (VO2), a material already noted for its unusual ability to switch from an insulator to a metal when it reaches a balmy 67°C, or 152°F.
In the course of studying the properties of VO2, researchers were able to tease out the proportion of thermal conductivity attributable to the vibration of the material’s crystal lattice, called phonons, and to the movement of electrons. To their surprise, they found that the thermal conductivity attributed to the electrons is ten times smaller than what would be expected from the Wiedemann-Franz Law.
“The electrons were moving in unison with each other, much like a fluid, instead of as individual particles like in normal metals,” said study principal investigator Junqiao Wu, a physicist at Berkeley Lab’s Materials Sciences Div. and a UC Berkeley professor of materials science and engineering. “For electrons, heat is a random motion. 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 VO2is detrimental to heat transfer as there are fewer configurations available for the electrons to hop randomly between.”
Notably, the amount of electricity and heat that VO2 can conduct is tunable by mixing it with other materials. When the researchers doped single crystal VO2 samples with the metal tungsten, they lowered the phase transition temperature at which VO2 becomes metallic. At the same time, the electrons in the metallic phase became better heat conductors. This enabled the researchers to control the amount of heat that vanadium dioxide can dissipate by switching its phase from insulator to metal and vice versa, at tunable temperatures.
Such materials can be used to help scavenge or dissipate the heat in engines, or be developed into a window coating that improves the efficient use of energy in buildings, the researchers said.
“This material could be used to help stabilize temperature,” said study co-lead author Fan Yang, a postdoctoral researcher at Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility where some of the research was done. “By tuning its thermal conductivity, the material can efficiently and automatically dissipate heat in the hot summer because it will have high thermal conductivity, but prevent heat loss in the cold winter because of its low thermal conductivity at lower temperatures.”
Vanadium dioxide has the added benefit of being transparent below about 30°C (86°F), and absorptive of infrared light above 600°C (140°F).
Yang noted that there are more questions to be answered before vanadium dioxide can be commercialized, but said that this study highlights the potential of a material with “exotic electrical and thermal properties.”