Paper: Rough Silicon Nanowires as High Performance Thermoelectric Materials

[OKIsItJustMe]

http://www.cchem.berkeley.edu/pdygrp/pubpdf/155.pdf

Enhanced thermoelectric performance of rough silicon nanowires

Allon I. Hochbaum^1*, Renkun Chen^2*, Raul Diaz Delgado1, Wenjie Liang¹, Erik C. Garnett¹, Mark Najarian³, Arun Majumdar^2,3,4 & Peidong Yang^1,3,4

Approximately 90 per cent of the world’s power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT . 1, since the parameters of ZT are generally interdependent¹. While nanostructured thermoelectric materials can increase ZT . 1 (refs 2–4), the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here we report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT 5 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.

...

In conclusion, we have shown that it is possible to achieve ZT = 0.6 at room temperature in rough Si nanowires of ~50 nm diameter that were processed by a wafer-scale manufacturing technique. With optimized doping, diameter reduction, and roughness control, the ZT is likely to rise even higher. This ZT enhancement can be attributed to efficient scattering throughout the phonon spectrum by the introduction of nanostructures at different length scales (diameter, roughness and point defects). The significant reduction in thermal conductivity observed in this study may be a result of changes in the fundamental physics of heat transport in these quasi-one-dimensional materials. By achieving broadband impedance of phonon transport, we have demonstrated that the EE Si nanowire system is capable of approaching the limits of minimum lattice thermal conductivity in Si. Modules with the performance reported here, and manufactured from such a ubiquitous material as Si, may find wide-ranging applications in waste heat salvaging, power generation, and solid-state refrigeration. Moreover, the phonon scattering techniques developed in this study could significantly augment ZT even further in other materials to produce highly efficient solid-state thermoelectric devices.

...

---

¹Department of Chemistry, ²Department of Mechanical Engineering, ³Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA. ⁴Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.

^*These authors contributed equally to this work.

Entire paper available at the link

[Politicub]

1. Enhanced thermoelectric performance of rough silicon nanowires for Clinton!

Woo woo!

:rofl:

[greenman3610]

2. what's it mean?

[skids]

4. TECs and TEGs are pretty cool.

TECs are what are in those electric beverage coolers. A flat disc that will get very cold on one side (and hot on the other) when you run electricity through it. It sucks the heat out of the beverage. You can literally take a can of water, put it on top of a TEC and freeze it solid.

TEGs are the same thing but used the other way around (optimized for use as a generator). You can literally take an ice cube, put it on top of a TEG in your hand, and electricity comes out of it.

The problem is that so far TECs/TEGs have been to expensive and not efficient enough so the only place they are used a lot is cooling CPUs in hotrodded computers.

They were looking into efficient TEGs using Bizmuth Telluride, but then they discovered "quantum wells" and this is sort of the same concept as a quantum well. They should be a lot cheaper and will provide a market for Silicon manufacturers after thin film takes over the solar industry.

Here's some more stuff from a company that has done some initial research into quantum well thermoelectrics -- the ones they sell I don't think are quantum well, since they have low efficiency.

http://hi-z.com <--- be sure to click on the red button, neat stuff there.

Once there are highly efficient TECs, we'll be able to do away with heat pumps in refrigerators -- you'll have refrigerators with no moving parts (maybe a small fan is all). In fact you can get those now, but they suck more power than a regular fridge.

Once there are highly efficient (and cheap enough) TEGs, hybrid vehicles will be able to turn some of the waste heat from the gas engine into electricity for the electric motors, and get even higher mileage. And there are hundreds of other uses.

The best price I have seen on current generation TEGs is a company called Tellurex.

[skids]

3. Thanks, I've been meaning to post this but always too tired...

...after getting home from work.

Nanotech thermoelectrics are definitely a technology worth keeping track of. I just hope they make it to market given the economy crashing is going to stifle all sorts of worthy techs.