TI builds on-chip thermal generator
The researches are working with TI on ways to build silicon ‘nanoblades’ that can be used for an on-chip thermoelectric generator. These could be built into a system-on-chip chip to power a processor and sensor from a temperature difference.
“Sensors go everywhere now. They can’t be constantly plugged in, so they must consume very little power,” said Dr Mark Lee, professor and head of the Department of Physics in the School of Natural Sciences and Mathematics at the University of Texas at Dallas. “Without a reliable light source for photovoltaic energy, you’re left needing some kind of battery — one that shouldn’t have to be replaced. If you have a steady temperature difference — even a small one — then you can harvest some heat into electricity to run your electronics.”
Sensors embedded beneath a traffic intersection provide an example of convenient thermoelectric power. “The heat from tires’ friction and from sunlight can be harvested because the material beneath the road is colder,” Lee said. “So no one has to dig that up to change a battery.”
However thermoelectric generation has been expensive, both in terms of cost per device and cost per watt of energy generated, he says. “The best materials are fairly exotic — they’re either rare or toxic — and they aren’t easily made compatible with basic semiconductor technology.”
Another reason is that the nanowires at the heart of a thermoelectric generator are too small to be compatible with today’s silicon process technology. To overcome this, Lee and his team developed the nanoblades, which are only 80nm long but more than eight times, 640nm, in width, which makes it comptabile with silicon process rules.
The nanoblade shape loses some thermoelectric ability relative to the nanowire. “However, using many at once can generate about as much power as the best exotic materials, with the same area and temperature difference,” said Lee.
Building the generator into a chip has other problems. Previous attempts failed because too much material was used. “When you use too much silicon, the temperature differential that feeds the generation drops,” said Lee. “Too much waste heat is used, and, as that hot-to-cold margin drops, you can’t generate as much thermoelectric power. There is a sweet spot that, with our nanoblades, we’re much closer to finding than anyone else. The change in the form of silicon studied changed the game.”
The team modelled the number of nanoblades per unit area that will produce the most energy without reducing the temperature difference. “We optimized the configuration of our devices to place them among the most efficient thermoelectric generators in the world,” said researcher Gangyi Hu. “Because it’s silicon, it remains low-cost, easy to install, maintenance-free, long-lasting and potentially biodegradable.”
Lee said the work was also novel because they used an automated industrial manufacturing line to fabricate the silicon integrated-circuit thermoelectric generators.
“We want to integrate this technology with a microprocessor, with a sensor on the same chip, with an amplifier or radio, and so on. Our work was done in the context of that full set of rules that govern everything that goes into mass-producing chips,” Lee said. “You can live with a 40% reduction in thermoelectric ability relative to exotic materials because your cost per watt generated plummets,” he said. “The marginal cost is a factor of 100 lower.”
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