New materials carry current without heating up
Researchers at Duke and the University of California, Berkeley, have found that vanadium dioxide can conduct electricity without heating up, allowing significant advances in heat pump and power supply component design.
Other electrical engineers at Duke University have created the world’s first electromagnetic metamaterial made without any metal using boron-doped silicon. The device’s ability to absorb electromagnetic energy without heating up has direct applications in imaging, sensing and lighting.
For most metals, the Wiedemann-Franz Law states that good conductors of electricity are also good conductors of heat. The researchers at Duke and Berkeley used simulations and X-ray scattering experiments on VO2 nanobeams (shown above) to show the proportion of thermal conductivity attributable to the vibration of the material’s crystal lattice, called phonons, and to the movement of electrons was ten times smaller than what would be expected from the Wiedemann-Franz Law.
“This was a totally unexpected finding,” said Junqiao Wu, a physicist at Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of materials science and engineering, working with Olivier Delaire at DOE’s Oak Ridge National Laboratory and an associate professor at Duke University. “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 behaviour of novel conductors. “The electrons were moving in unison with each other, much like a fluid, instead of as individual particles like in normal metals,” he said.
The amount of electricity and heat that vanadium dioxide can conduct is tunable by mixing it with other materials. When the researchers doped single crystal vanadium dioxide samples with the metal tungsten, they lowered the phase transition temperature at which vanadium dioxide 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 around 67 ºC rather than several hundred degrees for other exotic materials. 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 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.”
For the metamaterial, the Duke researchers created a surface dimpled with cylinders (below) to absorb terahertz waves. While this specific frequency range sits between infrared waves and microwaves, the approach should be applicable for almost any frequency of the electromagnetic spectrum.
CREDIT Willie Padilla, Duke University
“People have created these types of devices before, but previous attempts with dielectrics have always been paired with at least some metal,” said Willie Padilla, professor of electrical and computer engineering at Duke University. “We still need to optimize the technology, but the path forward to several applications is much easier than with metal-based approaches.”
The researchers built a prototype consisting of hundreds of these optimized cylinders aligned in rows on a flat surface. Physical tests showed that the new “metasurface” absorbed 97.5 percent of the energy produced by waves at 1.011 terahertz.
The technology could be used for more efficient lighting. “We can produce a dielectric metasurface designed to emit light, without producing waste heat,” said Padilla. “Although we’ve already been able to do this with metal-based metamaterials, you need to operate at high temperature for the whole thing to work. Dielectric materials have melting points much higher than metals, and we’re now quickly trying to move this technology into the infrared to demonstrate a lighting system.”
Efficiently absorbing energy from electromagnetic waves is an important property for other applications such as thermal imaging devices operating in the terahertz range. “Heat propagates fast in metals, which is problematic for thermal imagers,” said Xinyu Liu, a doctoral student in Padilla’s laboratory. “There are tricks to isolate the metal during fabrication, but that becomes cumbersome and costly.”
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