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John O | February 2017

Study shows that for one metal, electricity flows but heat does not


A study conducted by scientists from the U.S. Department of Energy Lawrence Berkeley National Laboratory and the University of California has demonstrated that the electrons in vanadium dioxide conduct electricity without conducting heat, which is another property of the material that does not conform to the standard rules of metals.

 

berkeley_600

Berkeley Lab scientists Junqiao Wu, Changhyun Ko, and Fan Yang are working at the nano-Auger electron
spectroscopy instrument at the Molecular Foundry, a DOE Office of Science User Facility.
(Marilyn Chung/Berkeley Lab)


According to an article on the Berkeley Lab website, this was an unexpected finding for the research team. Metallic vanadium oxide (already known for switching from an insulator to a meatal at 67°C) breaks the Wiedemann-Franz Law, which states that good conductors of electricity are also good conductors of heat.

 

The team used simulation and X-ray scattering experiments to determine the “proportion of thermal conductivity attributable to the vibration of the material’s crystal lattice, called phonons, and to the movement of electrons” and were stunned to see that the proportion was 10 times lower than would have been expected from the Wiedemann-Franz Law.

 

What the researchers found was that the electrons moved in unison as they would in a fluid rather than as individual particles, which is the standard for metals.

 

Physicist Junqiao Wu explained, “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 researchers also discovered that the metal’s thermal and electrical conductivity was tunable with the addition of other materials, such as metal tungsten, which lowered the phase change temperature that vanadium oxide becomes metallic and increased its thermal conductivity.

 

Researchers hope that this discovery could be used to stabilize temperatures in engines or, due to its transparency below 30°C and absorption of infrared light above 60°C, be used as coatings for windows.

 

The study was recently published in Science. The abstract read:

 

“In electrically conductive solids, the Wiedemann-Franz law requires the electronic contribution to thermal conductivity to be proportional to electrical conductivity. Violations of the Wiedemann-Franz law are typically an indication of unconventional quasiparticle dynamics, such as inelastic scattering, or hydrodynamic collective motion of charge carriers, typically pronounced only at cryogenic temperatures.

 

“We report an order-of-magnitude breakdown of the Wiedemann-Franz law at high temperatures ranging from 240 to 340 kelvin in metallic vanadium dioxide in the vicinity of its metal-insulator transition.

 

“Different from previously established mechanisms, the unusually low electronic thermal conductivity is a signature of the absence of quasiparticles in a strongly correlated electron fluid where heat and charge diffuse independently.”

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