Cheaper thermoelectric from silicon nanowires

Rough silicon nanowires synthesized by Berkeley National Lab researchers demonstrated high performance thermoelectric properties even at room temperature when connected between two suspended heating pads.

Nearly all of the world’s electrical power, approximately 10 trillion Watts, is generated by heat engines, giant gas or steam-powered turbines that convert heat to mechanical energy, which is then converted to electricity. Much of this heat, however, is not converted but is instead released into the environment, approximately 15 trillion Watts.

Bulk silicon is a poor thermoelectric material at room temperature, but by substantially reducing the thermal conductivity of our silicon nanowires without significantly reducing electrical conductivity, we have obtained ZT values of 0.60 at room temperatures in wires that were approximately 50 nanometers in diameter,” said Yang. “By reducing the diameter of the wires in combination with optimized doping and roughness control, we should be able to obtain ZT values of 1.0 or higher at room temperature.”

Here is chart that shows how the more expensive bismuth telluride and its alloys get a lot better when 1D wires are 3 nanometer or less in diameter and a lot lot better when less than 1 nanometer in diameter.

The ability to dip a wafer into solution and grow on its surface a forest of vertically aligned nanowires that are consistent in size opens the door to the creation of thermoelectric modules which could be used in a wide variety of situations. For example, such modules could convert the heat from automotive exhaust into supplemental power for a Freedom CAR-type vehicle, or provide the electricity a conventional vehicle needs to run its radio, air conditioner, power windows, etc.

When scaled up, thermoelectric modules could eventually be used in co-generating power with gas or steam turbines.

ZT around 5 or 10 would kick ass and would let 20-35% of the waste heat from say a nuclear or coal power plant to be captured. 20-35% of 15TW of waste heat would be 3-5TW.

A ZT of 1-2 would be 10-18% of the waste heat.

The goal of the freedomcar project is a ZT of 10 in 2014.

Figure (a) is a cross-sectional scanning electron microscope image of an array of rough silicon nanowires with an inset showing a typical wafer chip of these wires. Figure (b) is a transmission electron microscope image of a segment of one of these wires in which the surface roughness can be clearly seen. The inset shows that the wire is single crystalline all along its length.

If the silicon nanowire pans out for inexpensive bulk production at ZT of 1 to allow for 10% of waste heat to be converted to electricity, then if applied to the 104 nuclear reactors in the USA then about 15GW could be captured. Retrofitting the old plants with thermoelectric around pipes and around the core would not effect operations and would be like building ten new large reactors. 100 GW is generated by the existing US reactors now but about 150GW of waste heat is generaly lost. This would be 120 billion kwh on existing reactors which would be more than double all of the non-hydro renewable power in the USA now.

A ZT of 10 would be able to capture 35% of the waste heat and would be like the electricity from 35 new reactors generating 53GW.

Nature article: Silicon nanowires as efficient thermoelectric materials

Here we report efficient thermoelectric performance from the single-component system of silicon nanowires for cross-sectional areas of 10 nm 20 nm and 20 nm 20 nm. By varying the nanowire size and impurity doping levels, ZT values representing an approximately 100-fold improvement over bulk Si are achieved over a broad temperature range, including ZT 1 at 200 K

IEEE spectrum has an article on this work.

An array of nanowires [green] convert heat from the temperature difference between two slivers of a microchip. Current in flowing through a heater [red] causes the temperature difference.

Both research teams [Caltech and the University of California, Berkeley] found that they could decrease silicon’s thermal conductivity—and therefore increase the conversion efficiency—by fashioning the material into nanowires with diameters of 10 to 100 nanometers and introducing defects in the silicon that slowed the flow of phonons—the acoustic vibrations in the crystal lattice of a material that carry heat.

“Defects are important here,” says Peidong Yang, a materials scientist at Berkeley. “They can block the phonon transport from one end to the other end, so the thermal conductivity can be drastically reduced.”

Yang says his group engineered defects into the nanowires at three different length scales. First, by fashioning the bulk silicon into nanowires, they made the material very small compared with the phonons so that the size of the wires themselves affected how the phonons could move. They also made the surface of the wires rough, introducing a set of defects at a smaller scale. Finally, they doped the silicon with boron to introduce defects at an atomic level.

James Heath induced a greater drop in thermal conductivity by making his nanowires even smaller than Yang’s—only 10 to 20 nm in diameter. Normally, a wire would carry two types of phonons, he explains: one that causes the wire’s diameter to expand or contract, and one that causes it to lengthen or shorten. Like a rubber band that gets thinner when stretched, the two work in opposition. But when the nanowires get small enough, the two types merge into a single type of phonon, and that slows down the heat transport even more.

Unfortunately, when Heath made the wires 10 nm wide, which gave him the best results for thermal conductivity, the electrical conductivity crucial to thermoelectric conversion also dropped.

Thinking on a larger scale, Yang would like to see systems that convert the waste heat from car engines or power plants.

Both teams are pressing ahead to see what they can achieve next. The researchers believe a material with a ZT of 3 or 4 would be very appealing commercially. Heath hopes to apply his findings to other materials that might start out with better properties than silicon and be improved further. He’s doing work with silicon germanium, for instance, which has much lower thermal conductivity than pure silicon.

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