A new material made of photonic crystals may be more insulating than a vacuum that completely lacks atoms.
Until now, vacuums were the go-to resource for reducing temperature transfer. That’s why a typical thermos has one — to keep beverages hot or cold. (Same goes for newer windows in your home.) Heat can be transferred from one material to another in three main ways: convection, conduction, and radiation. For two of the three, a vacuum reduces their effectiveness.
But in a recent study, scientists at Stanford University used a stack of photonic crystals layered within a vacuum to create a material with a thermal conductance of half that of a simple vacuum.
Led by Shanhui Fan, the researchers set out to create a material that addressed that pesky third source of heat transfer: infrared radiation. Photonic crystals, which consist of bands of nanostructures that affect how light travels through them, were the solution.
The crystals, found in both nature and the lab, can have band gaps that block of certain frequency ranges of light (infrared radiation is a form of invisible light).
The scientists determined that a 100-micron-thick structure made of a stack of 10 photonic crystal layers, each 1 micrometer thick and separated by 90-micrometer gaps of vacuum, could reduce the thermal conductance to about half that of a pure vacuum.
In a study, the researchers found that thermal conductance doesn’t depend on the thickness of the layers but the crystals’ index of refraction, or how fast light travels through them.
So what’s the importance of a better vacuum? As I mentioned above, smarter windows on cars, homes and offices is a start. With photonic crystals, smart windows could capture the sun’s heat for use as an energy source while allowing visible light to pass through uninterrupted.
