Thermomigration and Soret effect in Na xCoO2 as thermoelectric material: Preparation and characterization of sodium cobaltate thin films.

Sodium cobaltate (Na xCoO2, NCO) is a model type thermoelectric material for high temperature applications, which is representative for the class of non-stoichiometric and mixed ion-electron conducting thermoelectrics (e.g., Cu2 - xSe). The present study deals with the kinetic instability of the originally homogeneous chemical composition when a temperature gradient is applied - a common situation in thermoelectric materials with a mobile component (element), but rarely considered in thermoelectric materials research so far. In order to investigate a well reproducible system, highly epitaxial thin films of Na xCoO2 with an atmosphere protective capping layer of alumina are prepared via pulsed laser deposition on sapphire (001) and lanthanum aluminate (111). A self-designed non-isothermal set-up allows the precise determination of the heat of transport for mobile sodium as 8.3 kJ mol−1 and, therefore, the quantification of thermodiffusion (i.e., the Ludwig-Soret effect in the stationary state). The experiments also allow to estimate the chemical diffusion coefficient of sodium at 422 K as about [ABSTRACT FROM AUTHOR]