Search for: All records

Abstract Nanostructuring to reduce thermal conductivity is among the most promising strategies for designing next‐generation, high‐performance thermoelectric materials. In practice, electrical grain boundary resistance can overwhelm the thermal conductivity reduction induced by nanostructuring, which results in worse overall performance. Since a large body of work has characterized the transport of both polycrystalline ceramics and single crystals of SrTiO₃, it is an ideal material system for conducting a case study of electrical grain boundary resistance. An effective mass model is used to characterize the transport signatures of electrical grain boundary resistance and evaluate thermodynamic design principles for controlling that resistance. Treating the grain boundary as a secondary phase to the bulk crystallites explains the transport phenomena. Considering that the interface can be engineered by controlling oxygen partial pressure, temperature, and the addition of extrinsic elements into the grain boundary phase, the outlook for SrTiO₃as a nanostructured thermoelectric is promising, and thezTcould be greater than 0.5 at room temperature. 

Abstract The discovery of a semiconducting ground stateX_yYZ(y= 0.8 or 0.75) in nominally 19‐electron half‐Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. By systematically investigating the temperature dependence of off‐stoichiometry (x) in V_0.8+xCoSb, Nb_0.8+xCoSb, and Ti_0.75+xNiSb it is found thatxinvariably increases with increasing temperature, leading to an n‐type self‐doping behavior. In addition, there is also a large phase width (range ofx) associated with each phase that is temperature dependent. Thus, unlike in typical 18‐electron half‐Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19‐electron half‐Heuslers links its transport properties to synthesis conditions. The temperature dependence ofxandnare understood using density functional theory based defect energies (E_d) and phase diagrams.E_dare calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimizednto engineer TE properties in phase‐pure V_0.8+xCoSb, and the previously unexplored Ta_0.8+xCoSb.