Thermoelectric materials have great promise — if they can be made relatively simply and with inexpensive, safe materials. Just imagine the endless list of applications where waste heat energy could be converted to electrical energy. One problem, however, is that when the thermal conductivity of a material increases, its electrical conductivity tends to decrease. This lack of independence becomes a major problem when scientists and engineers are trying to develop more efficient thermoelectric devices.
Now, a group of Caltech researchers say they have devised a way to create a special silicon nanoscale mesh that can be used to slash thermal conductivity without affecting the flow of electrons. The Caltech team made the barrier from a 22-nm-thick sheet of silicon, which is then given a highly regular array of 11- or 16-nanometer-wide holes spaced 34 nm apart.
The nanomesh changes the way phonons (heat travels in material via phonons) behave, slowing them down.
According to a Caltech release, although the electrical conductivity of the nanomesh remained comparable to regular (bulk) silicon, its thermal conductivity was reduced to near the theoretical lower limit for silicon.
The researchers say they are setting their sights even higher. “Now that we’ve showed that we can slow the phonons down,” says James Heath, “who’s to say we can’t slow them down a lot more?” Heath is a professor of chemistry and leader of a research group at Caltech.
Slobodan Mitrovic, a postdoc in chemistry at Caltech, explains, “The nanomesh no longer behaves in ways typical of silicon.” Mitrovic and Heath are coauthors of a paper on the topic that appears in Nature Nanotechnology.
The key to their discovery exploits the fact that thermal conductivity and electrical conductivity behave independently at scales associated with phonons. They change the phonon band structure of a semiconductor thin film via the design of grid of holes in the silicon film that are comparable to, or shorter than, the phonon mean free path.
Mitrovic says the basic idea behind such structures is they provide a coherent mechanisms for reducing thermal conductivity that doesn’t depend on interface scattering, since they are made of a single-crystal, continuous material. He says that bulk-like electrical conductivity of the silicon is preserved by the nanomesh, and the development is a step towards a coherent mechanism for lowering thermal conductivity.
Heath’s group had previously demonstrated that methods create independence between thermal and electrical conductivity, however these methods use silicon nanowires that exploit interface scattering of phonons to separately decrease thermal conductivity.
According to papers authors, the benefit of the nanomesh structure is that it exhibits a substantially lower thermal conductivity than an equivalently prepared array of silicon nanowires, even though this array has a significantly higher surface-to-volume ratio.