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Intelligent Machines

Heat-Harvesting Nanomaterials

A new design could help thermoelectric materials find broader use.

Engineers have long dreamed of harvesting waste heat from a microchip or car engine and turning it into usable electricity. But thermoelectric materials, which convert heat into electricity, have never been efficient enough to move beyond a few niche applications.

Nanomesh: A thin film of silicon riddled with regular nanoscale holes could be an efficient thermoelectric material.

Now researchers at Caltech have demonstrated new materials that could boost the efficiency of thermoelectric devices. They’ve demonstrated the design using silicon, but say it should improve the performance of other thermoelectric materials, too.

A good thermoelectric material conducts electricity very well but conducts heat poorly. But most naturally occurring materials that conduct electricity well also conduct heat well. So researchers in the Caltech group, led by chemistry professor James Heath, have tried to make nanoscale designs that sever the relationship between thermal and electrical conductivity in abundant materials such as silicon.

In 2008, Heath’s group showed that arrays of silicon nanowires are as efficient at converting heat into electricity as the more exotic materials used in commercial thermoelectric devices. The nanowires are highly conductive to electricity, but they divert heat by causing packets of heat energy called phonons to bounce off. But the efficiency boost wasn’t sufficient enough to make these materials viable for widespread use.

Now the Caltech researchers have come up with a new nanoscale design for thermoelectric materials. They believe their design works by a different mechanism–instead of diverting the phonons, it slows them down considerably. The researchers have demonstrated the nanomesh design in thin films of silicon riddled with a regular array of nanoscale pores. Compared to an unpatterned silicon film, the nanomesh conducts 10 times less heat. These results are described in the journal Nature Nanotechnology.

Further experiments are needed to verify the total thermoelectric efficiency of the nanomesh and to establish why it is such a good thermal insulator. The researchers say that the patterned silicon is a metamaterial–that is, it exhibits properties not found in naturally occurring materials. “We’ve managed to tap into the wave nature of phonons to slow them down,” says Caltech researcher Slobodan Mitrovic.

The researchers need to do more experiments to prove that the material is indeed slowing down phonons, or to establish how it actually works. In addition, the group is developing the chemical-etching technique they used to make the nanomeshes in silicon to apply it to other thermoelectric materials that are more efficient to start with.

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