Thermoelectric devices convert heat to energy and vice versa, enabling you to harness the otherwise wasted energy from computers, light bulbs, and even people. While pushed for use in refrigerators and other heating/cooling machines, the current designs aren’t efficient enough for widespread commercial use or are made from rare, expensive, environmentally harmful materials.
However, this may change as researchers at the California Institute of Technology (Caltech) have developed a new material – made out of silicon – that could lead to more efficient thermoelectric devices. This new type of nanomesh is composed of a thin film with a grid-like arrangement of tiny holes – making it difficult for heat to travel through the material and lower its thermal conductivity to near silicon’s theoretical limit. The design also allows electricity to flow as well as it does in unmodified 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.
To make thermoelectric materials efficient, the thermal conductivity needs to be lowered while not affecting the electrical conductivity. Heath and his team previously accomplished this using silicon nanowires, which impede heat while allowing electrons to flow freely.
Heat travels via phonons – quantized packets of vibration that are like photons, which are quantized packets of light waves. As phonons move along the material, they deliver heat from one point to another. Because of the tiny size of nanowires, they have a lot of surface area relative to their volume – making it harder for phonons to make it through a nanowire without bouncing astray. As a result, a nanowire resists heat flow but remains electrically conductive.
The researchers are experimenting with different materials and arrangements of holes so that they can optimize their design. “One day, we might be able to engineer a material where you not only can slow the phonons down, but also exclude the phonons that carry heat all together,” says Slobodan Mitrovic, a postdoctoral scholar in chemistry at Caltech. “That would be the ultimate goal.”