High Resolution and High Temperature Microscopy of Skutterudite Thermoelectrics

Thermoelectric materials are used widely by NASA as portable, robust, and stable sources of power generation, due to their simplicity of operation, and the utility of high temperature gradients in deep space. The efficiency of a thermoelectric material depends on being able to increase the electrical conductivity as far as possible while simultaneously reducing the thermal conductivity. In any single material this is challenging, since to a first order approximation electrical and thermal conductivity are proportional to one another. One class of compounds which has received particular scrutiny with respect to their promising thermoelectric behavior are the skutterudite compounds, with general formula MX3. Three approaches are commonly employed in order to reduce thermal conductivity in these compounds without prohibitively disrupting electronic conduction: (i) Doping on both the M and the X crystallographic sites; (ii) filling of the empty interstitial "cages" with "rattling" cations; and (iii) thermally treating the material so as to encourage the growth of nanostructured inclusions. Though all of these strategies involve locally altering the crystal and microstructure, high resolution microscopic characterization of these materials has not been extensive. Here we propose to perform a series of high resolution imaging and spectroscopy experiments on a class of skutterudite compounds with the formula CoxNi4-xSb12-ySny. Further, we will conduct in situ heating experiments in the transmission electron microscope to determine the mechanisms of void and secondary phase formation and to determine the structural phase transitions occurring during the synthetic process.