How The Photonic Thermos Works

There is science behind your thermos, but that is the old science. The new science says you can keep your coffee hot for a lot longer using photonic crystals, but that is only a part what this new material could do for us in the future. Keeping coffee hot is step one, using the sun’s heat as energy source is the next step.

So far, the vacuum was basically the only and the best insulator that a thermos could use to keep the liquid inside warm. It seems that is about to change as scientists have found another structure, called a photonic crystal, which can keep the heat from dissipating in a better and more effective way than the vacuum can.

The team that made this discovery published a fascinating article in the October issue of the “Physical Review B”, where they have explained thoroughly the phenomenon. It seems that these photonic crystals have a lot of potential in the computing and communications technologies and that their thermal properties could be used in the future for capturing the sun’s heat and transforming it into usable energy for everyone.

Your normal thermos keeps the liquid’s heat from dissipating by creating a vacuum between the two walls (one inner and one outer). The process is not perfect because the light that is radiated through the walls (all warm bodies radiate a range of infrared frequencies) will take some energy away with it. And by energy, this time we mean heat.

This was the starting point for the team led by Shanhui Fan from Stanford University in California. As photonic crystals are already known light frequency blockers, last year they have wondered if they could make these crystals block the infrared frequencies that a worm body radiates. And by studying different silicon and vacuum layer variations, they have found out that a 100 micron thick stack that alternated vacuum layers and 10 layers of silicon (those silicon layers are 1 micron thick) reduced the thermal conductance to half. This basically means your coffee will stay warm for longer that it does in the thermos you have today.

In the paper published this month, the same team took a different approach: rather that studying specific cases, they managed to complete a complete analysis of the matter. And while most scientist that study this problem usually focus on calculating the frequency ranges that are blocked by the crystals (also called band gaps), Fan and his colleagues used a method called statistical theory to calculate the percent of all the frequency that these photonic crystals allow to pass through. The result was highly unusual.

It seems that this percentage does not change depending on the thickness of the layers (which is unusual because the structure is very important when dealing with photonic crystals, as Fan states). Instead, it only depends on the speed of light in the solid layers (also called an index of refraction). This is a big discovery that also puts researchers in a dilemma as they can adjust the conductance only by varying the materials, and not the way the layers are placed.

Further on the team plans on studying photonic crystal that have irregular structures. If their assumptions are correct, these crystals can be even more effective insulators. Fan is highly convinced that studying the control of heat flow by using photonic crystal can lead to major breakthrough in the way we can use the sun’s heat as an energy source.