AVS 60th International Symposium and Exhibition
    Energy Frontiers Focus Topic Wednesday Sessions
       Session EN+SE+SS+TF-WeM

Paper EN+SE+SS+TF-WeM5
Correlation of Thermoelectric Properties with Local Chemical Environments in Reduced Polycrystalline and Single Crystalline SrxBa1-xNb2O6

Wednesday, October 30, 2013, 9:20 am, Room 101 A

Session: Thin Films for Energy
Presenter: C.S. Dandeneau, University of Washington
Authors: C.S. Dandeneau, University of Washington
Y. Yang, University of Washington
R.K. Bordia, University of Washington
M.A. Olmstead, University of Washington
F.S. Ohuchi, University of Washington
Correspondent: Click to Email
The excellent high-temperature stability of many oxide thermoelectrics (TEs) has led to the exploration of these materials for waste heat recovery applications. Of particular interest are oxides with a complex atomic structure involving random site occupation, which serves to decrease the thermal conductivity and improve the overall thermoelectric figure of merit. SrxBa1-xNb2O6 (SBN100x) has such a structure, and also exhibits a relaxor ferroelectric transition with an x-dependent Curie temperature near or above room temperature. In this work, the thermoelectric properties of reduced polycrystalline SBN fabricated via solution combustion synthesis (SCS) were compared with those of single crystal SBN. While reduction annealing increases the electrical conductivity (σ) and decreases the magnitude of the Seebeck coefficient (S), the conventional inverse relationship between S and σ with respect to temperature does not hold. In particular, the magnitude of S for SBN samples with various Sr:Ba ratios increases with temperature over the entire range of testing temperatures (d|S|/dT > 0), but dσ/dT is positive between room temperature and ~ 350°C, and then becomes negative at higher temperatures. To better understand the obtained TE data, extensive X-ray photoelectron spectroscopy (XPS) analysis was carried out both “in-situ” during reduction and “ex-situ” so as to elucidate the site occupancy of Sr and Ba ions and determine the variation of Nb valence in the reduced tungsten bronze crystal structure. The SBN carrier concentration was estimated via changes in the niobium valence state and the findings were correlated with Hall effect measurements on the n-type oxides. During reduction, Nb5+ and Nb4+ ions and a single oxygen vacancy (VO) may preserve the octahedral structure of SBN. However, once two oxygen atoms are missing from an octahedron and/or the central Nb cation is further reduced to Nb3+ or Nb2+ with a larger ionic radius, strong localized distortion counteracts the increased carrier density to decrease both mobility and S. A potential correlation between the ferroelectric and thermoelectric properties of SBN will also be discussed. This work is supported by a grant from the U.S. Department of Energy (DE-FE0007272).