Materials & Coatings

Scattering of phonons by loosely bound atoms reduces thermal conductivity.

Crystalline phases of compounds of general composition (Cuw, CuxFey, or Tiz)Mo6Se8 [where w<4, x<2, y<1, and z<1] have been investigated for potential utility as thermoelectric materials for generation of electric power from waste-heat and other thermal sources. These phases are a subset of a set of ternary chalcogenide compounds named Chevrel compounds or phases after one of the researchers who first synthesized them in 1971. As explained below, relatively low thermal conductivity is an outstanding aspect of these compounds that makes them potentially attractive for thermoelectric applications.

Thermal Conductivities (as functions of temperature) of several candidate Chevrel phases were measured.Also shown for comparison are the corresponding quantities for Mo6Se8 and Mo2Re4Se8.

The figure of thermoelectric merit of a given material at a given temperature T is commonly denoted as ZT and is given by the equation ZT = α2T/ ρλ, where α is the Seebeck coefficient, ρ is the electrical resistivity, and λ is the thermal conductivity of the material at that temperature. Hence, as part of an effort to develop a material of large ZT, one should seek to minimize λ.

The present investigation is an extension of recent research on other thermoelectric materials that belong to a class of compounds called “rattling” semiconductors. The crystalline lattices of these materials contain cavities large enough to accommodate a variety of additional atoms, which are bound relatively loosely and are thus somewhat free to move around (“rattle”). It had been suggested that in such compounds, the rattling of the additional atoms would contribute more to scattering of phonons than to scattering of electrons and holes. Consequently, the rattling could be expected to contribute more to thermal than to electrical resistivity — an effect that would be favorable for obtaining a large value of ZT.

The (Cuw, CuxFey, or Tiz)Mo6Se8 Chevrel phases belong to the class of rattling materials. The electronic and thermal properties of these compounds could, potentially, be tailored through careful selection of the amounts and the manner of incorporation of the filling elements (Cu, Cu with Fe, or Ti). The compounds investigated thus far are Cu3.1Mo6Se8, Cu1.38Fe0.66Mo6Se8, and Ti0.9Mo6Se8, which exhibit p-type electrical conductivity and relatively low values of thermal conductivity (see figure). Of these compounds, the best was found to be Cu1.38Fe0.66Mo6Se8, for which ZT = 0.6 at a temperature of 1,150 K. This value of ZT is comparable to the ZT values of Si-Ge alloys in the same temperature range. One drawback of these three compounds has been low charge-carrier mobility. It will be necessary to increase charge-carrier mobilities in order to obtain ZT values greater than those of state-of-the-art thermoelectric materials. Research toward that end was underway at the time of reporting the information for this article.

This work was done by Thierry Caillat, Jean-Pierre Fleurial, G. Jeffrey Snyder, and Alexander Borshchevsky of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.

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Refer to NPO-21012, volume and number of this NASA Tech Briefs issue, and the page number.

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