The metal is called vanadium dioxide, and with the discovery of another one of its unusual properties, a fundamental law has just been broken. We’re referring to the Wiedemann-Franz Law — the one that states that good conductors of electricity are necessarily good conductors of heat too. The same one that explains why motors and electrical appliances heat up when used frequently.
It’s supposed to be a law that governs all metals. But as researchers from Berkeley Lab’s Materials Sciences Division and University of California, Berkeley found out, vanadium dioxide seems to be an exception.
Apart from its unique abilities to switch from an insulator to a conductive metal when it reaches 67 degrees Celsius (152 degrees Fahrenheit), absorb infrared light when it exceeds 60 degrees Celsius (140 degrees Fahrenheit), and turn transparent when it goes below 30 degrees Celsius (86 degrees Fahrenheit), vanadium dioxide appears to have another special ability. Despite being a good conductor of electricity, it’s not a good conductor of heat. And that’s a good thing.
By performing simulations and X-ray scattering experiments, the research team discovered that the electrons in vanadium dioxide can conduct electricity, but the rate at which it can conduct heat (in relation to the rate it can conduct electricity) is “ten times smaller than what would be expected from the Wiedemann-Franz Law.”
The explanation lies in the way the electrons in vanadium dioxide move. Compared with typical metals that are able to transfer heat effectively because electrons can move around freely and randomly, jumping between one configuration and another, the electrons in vanadium dioxide move in a coordinated manner, like a ‘marching band’. This type of motion is unfavorable for heat transfer — the electrons can’t jump randomly because they’re forced to follow a united motion, like a fluid instead of independent particles.
Aside from low thermal conductivity (heat transfer rate), the researchers also found out that by mixing vanadium dioxide with other metals, its conductive ability can be altered. To be more specific, when they tried to combine vanadium dioxide with tungsten, they were able to lower the temperature normally required for vanadium dioxide to turn metallic. Plus, they were also able to make the metal form of vanadium dioxide conduct heat better. This form of control — changing conductivity rate through changes in temperature — can pave the way for the creation of materials that can help prevent engines and motors from overheating. It can also be used to develop more energy-efficient building materials, like window coatings that can deflect heat during summer, and keep heat in during the winter.
The research team admits that more tests are in order before vanadium dioxide can be used for commercial purposes. But it’s a promising material nonetheless, one that can potentially help with our energy problems.
The study entitled “Anomalously low electronic thermal conductivity in metallic vanadium dioxide” has recently been published in the journal Science.
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