Material scientists at the Department of Energy’s Lawrence Berkeley National Laboratory test the properties of metallic vanadium dioxide. Photo by Marilyn Chung/Berkeley Lab
Jan. 26 (UPI) -- Researchers have discovered a metal that fails to comply with the Wiedemann-Franz Law, the rule that suggests good conductors of electricity will also be good conductors of thermal energy.
Metallic vanadium dioxide easily carries an electric current, but fails to conduct heat as expected.
"This was a totally unexpected finding," Junqiao Wu, a professor of materials science and engineering at the University of California, Berkeley, said in a news release. "It shows a drastic breakdown of a textbook law that has been known to be robust for conventional conductors. This discovery is of fundamental importance for understanding the basic electronic behavior of novel conductors."
The revelation -- detailed in the journal Science -- furthers the oddball reputation of metallic vanadium dioxide.
Previous studies showed the substance is an insulator at room temperature, but becomes a conductor once it reachers a temperature of 152 degrees Fahrenheit.
Researchers used electron scanning imagery to observe the movement of heat energy across the material. Modeling and lab experiments helped scientists determine how much heat was being carried by electrons and how much was being propagated by the vibration of the material's unique crystal lattices, or phonons.
"The electrons were moving in unison with each other, much like a fluid, instead of as individual particles like in normal metals," explained Wu. "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 vanadium dioxide is detrimental to heat transfer as there are fewer configurations available for the electrons to hop randomly between."
Scientists were able to lower the threshold for vanadium dioxide's electric conductivity by mixing it with other materials, like metal tungsten. Mixing also encouraged the vanadium dioxide's electrons to carry heat more effectively.
"This material could be used to help stabilize temperature," said Fan Yang, a postdoctoral researcher at Berkeley Lab's Molecular Foundry. "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."