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Thermopower on the Cheap

December 21, 2012

Things are precise in a laboratory environment. You weigh things out to five, or six, decimal places, you control temperature to a tenth of a degree, and you mark time to the second. Then, when the specification for your miracle material is sent to the factory, there's cigarette ash in the crucible, and things are over-annealed while the crew is on a coffee break. Six Sigma supposedly fixed most of these problems in the better companies, but scientists are never surprised when laboratory properties are not reproduced in commercial material.

As I wrote in a previous article (Coal, July 23, 2012), this isn't a problem for some materials, since they're just dug out of the ground and used in their pristine state. The most important of these is coal, and if we didn't have coal, we wouldn't have had the Industrial Revolution.

Another example is kieselguhr, also known as diatomaceous earth. This material, the fossil shells of diatoms, is a useful mixture of silica (SiO2, 80-90%), alumina (Al2O3, 2-4%) and iron oxide (Fe3O4, 0.5-2%). This nice mix of inorganic oxides makes diatomaceous earth a good material for thermally insulating high temperature furnaces.

Diatom FossilA diatom fossil from a sediment core of the Deep Sea Drilling Project.

Diatom fossils are just a few tens of micrometers in size.

Micrograph by Hannes Grobe, the Alfred Wegener Institute, via Wikimedia Commons.)

Diatomaceous earth is used as an abrasive, as a filtration medium, a support medium for catalysts, and a filler for plastics. The absorbency of diatomaceous earth makes it useful for cat litter, and also as a way to stabilize nitroglycerin to form dynamite.

Diatomaceous earth is about as close to a free lunch as a materials scientist can get, and it makes you wonder what other things might be made from other inexpensive minerals. A team of scientists from Michigan State University and UCLA has just developed a thermoelectric material based on an inexpensive mineral known as tetrahedrite.[1-2]

Tetrahedrite, one of the most abundant minerals, has the chemical formula, (Cu,Fe)12Sb4S13. As can be seen from the formula unit, it exits in both copper-rich and iron-rich forms, and bismuth (Bi) is known to substitute for the antimony (Sb). All this can be expected from the atomic radii of the atoms (Cu = 135 pm; Fe = 140 pm; Sb = 145, Bi = 160), although bismuth is a tight fit. The mineral takes its name from its tetrahedron-shaped crystals.

I've written about thermoelectric materials in several previous articles, most recently concerning the power source of the Curiosity rover (Curiosity Rover Power, November 5, 2012). Thermoelectric materials make use of the Seebeck effect to convert a temperature differential to electric power. Thermocouples, as used for temperature measurement, do this same trick, but not very efficiently. thermoelectric devices are constructed from junctions of n- and p-doped semiconductors (see figure), but the leading material for this purpose, bismuth telluride, is still relatively inefficient.

A thermoelectric cellA thermoelectric cell.

Bismuth and tellurium, common material components in such cells, are mildly toxic.

(Image by author, rendered using Inkscape.)

Says Donald Morelli, a professor of chemical engineering and materials science at Michigan State University and leader of the research team,
"Typically you'd mine minerals, purify them into individual elements, and then recombine those elements into new compounds that you anticipate will have good thermoelectric properties... But that process costs a lot of money and takes a lot of time. Our method bypasses much of that."[2]

Their material is quite unlike the typical thermoelectric material, which is composed of rare elements and demands careful doping to achieve high efficiency.[1] In fact, the research team was able to use natural tetrahedrite, itself, to make inexpensive thermoelectric devices; however, small chemical modifications produced highly efficient thermoelectric materials.[2]

It's reported that the dimensionless figure of merit of this material is near unity,[1] which puts it on par with the best thermoelectrics available (see figure). This research was supported by the Office of Science of the U.S. Department of Energy.[2] Thermoelectric modules are commercially available, so you can experiment with this technology yourself.[3]

Figure captionFigure of merit for a representative thermoelectric material (Copper-dispersed Bi0.5Sb1.5Te3.

(US Government Image, I.H. Kim, S.M. Choi, W.S. Seo and D.I. Cheong, Nanoscale Research Letters, 2012.)

References:

  1. Xu Lu, Donald T. Morelli, Yi Xia, Fei Zhou, Vidvuds Ozolins, Hang Chi, Xiaoyuan Zhou and Ctirad Uher, "High Performance Thermoelectricity in Earth-Abundant Compounds Based on Natural Mineral Tetrahedrites," Advanced Energy Materials, vol. 2, no. 11 (November, 2012), DOI: 10.1002/aenm.201200650.
  2. Energy savings - easy as dirt, heat, pressure, Michigan State University Press Release, November 27, 2012.
  3. Thermoelectric Power Generation Products, Tellurex Corporation.

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