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Professor Mercouri Kanatzidis holds up his device that can harvest 14 percent of waste heat as usable electricity.  (Source: Northwestern University)
New lead-based compound could see a variety of scenarios -- including helping power the machines in the absence of sunlight

A new material from researchers at Northwestern University could offer a way to capture and recycle waste heat better than ever before [press release].  The material can convert a record 14 percent of the waste heat passing through it to usable electric energy.

When manmade devices perform work, be it a computer or a car, they produce heat.  That heat is ultimately lost, reducing the energy efficiency of our devices.  Some have cleverly exploited this fact, using waste heat to offer desirable comfort heating.  But ultimately, the only good solution is to try to somehow recapture that heat in a usable form.  To do that, the right material was necessary.

Semiconductors have long been considered a promising candidate, as they can produce electricity when heated.  Lead telluride (composed of lead and tellurium ions on a lattice) was considered one of the most promising candidates, as it was relatively efficient in accomplishing the heat to electricity transformation.  

But attempts to improve that efficiency via various techniques, such as nano-inclusions resulted in an undesirable side effect -- increased scattering of electrons, reducing overall conductivity.  Obviously, if you're converting heat to electricity, you have to funnel it out of the device, so this was unacceptable.

The NU team, lead by Chemistry professor Mercouri Kanatzidis discovered that by using a special type of nano-inclusion, the scattering could actually be reduced.  The trick was to use special crystals of rock salt (SrTe).  Professor Kanatzidis sums up, "It has been known for 100 years that semiconductors have this property that can harness electricity. To make this an efficient process, all you need is the right material, and we have found a recipe or system to make this material."

Materials Science professor Vinayak Dravid also assisted in the study.  He describes the results, stating, "We can put this material inside of an inexpensive device with a few electrical wires and attach it to something like a light bulb. The device can make the light bulb more efficient by taking the heat it generates and converting part of the heat, 10 to 15 percent, into a more useful energy like electricity."

The study on the promising material earned a place [abstract] in the prestigious peer-reviewed journal Nature Chemistry.

So the material seems great, but what about its commercialization prospects?

Well, lead telluride is relatively rare , but occurs naturally in mountain deposits as the mineral Altaite.  Significant deposits have been found in the Altai mountains of northeast Asia;  Zyrianovsk, Kazakhstan; the Ritchie Creek Deposit in Price County, Wisconsin; the Koch-Bulak gold deposit in Kazakhstan; Moctezuma, Mexico; and Coquimbo, Chile.

Given that air or liquid bearing waste heat can be channeled through a relatively small area, a little telluride (say in a heatpipe on a computer component) could go a long ways, recycling almost a sixth of the wasted energy.

Strontium is very abundant, so coming up with sufficient quantities of the nano-inclusion material shouldn't be as big an issue.

Aside from making existing devices more efficient, the material could be used to make new low voltage electronic devices, powered by waste heat from the human body.



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Some Notes
By nstott on 1/20/2011 12:21:56 PM , Rating: 2
* The lead and tellurium are 'in' the crystalline lattice, not 'on' the crystalline lattice. Even so, the wording is awkward. Semiconductor-grade lead telluride is crystalline. I'm not sure how much more you need to say unless you're writing a textbook.

* SrTe is NOT a special form of rock salt. Rock salt is rock salt, also known as halite, which is a crystalline form of sodium chloride (NaCl). The strontium telluride nanocrystals that they are growing have a "rock salt" crystal structure, which is to say that the periodic placement of Sr and Te relative to each other is the same as the Na and Cl ions in rock salt (the interionic/interatomic distances are different, though). Crystal structure plays an important role on material properties. For example, the most abundant crystalline form of nickel is face-centered cubic (FCC), which is magnetic, but the hexagonal close-packed (HCP) form of nickel is nonmagnetic with respect to its bulk properties.

* The abundance of natural lead telluride deposits are not so important to commercialization. The amounts of lead and tellurium in any form are what are important. The semiconductor-grade PbTe crystalline films are grown by MBE from separate Pb and Te precursors. That isn't to say it's impossible to take natural lead telluride crystals and cleave them properly.




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