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February 27, 2007 19:30


Russian scientists have come out with a semiconducting material with thermal conductivity two times lower than any known analogue has. The material is a successful intermediate on the way to thermoelectric refrigeration, in other words, to safe noiseless refrigerators, which require no cooling agents.

These unique substances consist of geometrically adjusted cells, which contain single atoms or ions, having no strong covalent binding with neighbouring atoms. Not long ago clathrates (the word clathrate origins from Latin clatratus meaning protected with lattice) were exotic even for expert chemists, but now scientists dont use quotation marks while describing said compounds and consider clathrates to be highly promising thermoelectric materials. Scientists from Moscow State University have successfully developed semiconducting clathrates with anomalously low thermal conductivity (0.45 W/mK), which is 2.5 times lower than any known commercial material has.

If you imagine a simplest device, consisting of two connected semiconductors of n- and p-type, says the head of the project professor Shevelkov, you know that when electric current passes from n-semiconductor to p-semiconductor, charge carriers electrons (n) or holes (p) move away from operating contact and carry heat with them. Therefore, operating contact, which is also known as active element, cools down. A pile of such simple devices can provide cooling for about 150 degrees Centigrade.

To produce such devices, scientists need appropriate materials with seemingly contradicting properties they should have high electrical conductivity and low thermal conductivity. Existing materials are not sufficiently effective their characteristics allow making cooling devices for laser and electronic equipment and cool boxes. In order to compete with ordinary refrigerator compressors thermoelectric materials should be at least twice as effective as they are now. Semiconducting clathrates are one of possible solutions.

Simple description of suggested technique is that heat and electricity carriers in such units are separated in space: clathrate frame has electrical conductivity for moving charge carrriers and atoms, located in vesicles, dissipate heat with their fluctuations, thus providing low thermal conductivity for a material.

This type of materials clathrates with tin and phosphorus atoms as a frame and iodine, chlorine and tellurium atoms sitting in vesicles scientists from Moscow are currently studying. The work is a hard one synthesizing clathrates with given composition and proving that all atoms are located strictly on their places is a difficult task. Gaining required properties, high thermoelectric figure of merit, for instance, is even more difficult. However, scientists announced first successful experiments on developing semiconducting materials with anomalously low thermal conductivity.

Current research of the think-tank is focused on several directions development of materials, able to maintain their properties under extremely low temperatures (less than 160 K) for future development of thermoelectric cooling units for superconducting materials among them. And we should not forget about developing materials, which could be used in industrial cold-storage plants and household refrigerators of new generation. Mentioned refrigerators have one great advantage they do not contain moving particles and hazardous cooling chemical agents freons, thus being immensely safe and environmentally friendly.

Source:
    Russian Science News

Kizilova Anna


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