All of the electronics devices you use, all the wires, the cars, basically anything using energy heats up and loses part of its useful energy as heat – wasted heat, through well-cooled radiators. Thermoelectric devices have recently suffered technological advances and are now more able to harness the lost heat energy and transform it into electricity. A new material made out of silicon can serve as the ultimate heat insulator and still conduct electricity.
The material, invented by researchers from the Caltech (California Institute of Technology) represents the latest discovery in the field, and features a thin film with a grid-like arrangement of nanometer-sized holes. This design makes it harder for heat to pass through, and makes the resulted silicon almost reach its theoretical limit as a thermal insulator, while also letting electricity pass like it does through normal silicon.
“In terms of controlling thermal conductivity, these are pretty sophisticated devices,” says James Heath, the Elizabeth W. Gilloon Professor and professor of chemistry at Caltech, who led the work. A paper about the research will be published in the October issue of the journal Nature Nanotechnology.
The phenomenon is explained through quantum physics means. Phonons, the quantized packets of vibration (resembling photons) deliver heat from one point to another in the material. Because of their small size, nanowires have a lot of surface area compared to their volume. Phonons have the property of scattering off surfaces and interfaces, and the large area keeps them from making it through a nanowire without losing energy and bouncing between the surfaces. This property makes the nanowire able to resist heat but still conduct electricity.
Heath had studied silicon nanowires in the past, and they are actually the starting points for his research on silicon nano-holes. He discovered, for instance, that narrowing nanowires too much can have a detrimental effect on its electric properties.
The nanomesh, on the other hand, made from a 22 nm-thick silicon sheet, changes the way phonons behave, slowing them down. The mesh structure actually lowers the speed limit, otherwise set by the properties of silicon. “The nanomesh no longer behaves in ways typical of silicon,” says Slobodan Mitrovic, a postdoctoral scholar in chemistry at Caltech.
Compared to nanowires, the nanomesh is half as conductive thermally, despite having a much higher surface-area-to-volume ratio. Experiments further made on a thin film and on a grid-like silicon sheet, but with holes 100 times larger than the ones from the original nanomesh, pointed out that they had thermal conductivities about 10 times higher than that of the nanomesh.
“Now that we’ve showed that we can slow the phonons down,” Heath says, “who’s to say we can’t slow them down a lot more?” This discovery seems to hold the key to ultra-efficient thermoelectric devices, because there will be enough heat to drive them even where there’s little. The efficiency will then not be given only by the efficiency of the thermoelectric devices, but also by the heat insulation the silicon nanomesh provides. And this could help construction engineers making buildings greener, too.
