Article

SrxBa1−xNb2O6−δ Ferroelectric-Thermoelectrics: Crystal Anisotropy, Conduction Mechanism, and Power Factor

Applied Physics Letters (Impact Factor: 3.3). 01/2010; 96(3):031910. DOI: 10.1063/1.3291563

ABSTRACT

Nonstoichiometric tungsten bronze-structured ferroelectric SrxBa1-xNb2O6-d (SBN) single crystals were found to be a promising n-type thermoelectric oxide. Thermopower anomalies were observed at the phase transition temperatures, depending on the degree of reduction as well as crystal anisotropy. Above 500 K, heavily reduced SBN crystals show high thermoelectric power factors (~20 W/cm K2 at 516 K) with both thermopower and electrical conductivity higher parallel to the c-axis. It is noted that the power factor increases with temperature due to the semiconducting behavior with high carrier concentration. The carrier transport mechanism also varies with the degree of reduction and temperature.

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    • "Tungsten bronze (TTB) oxides are the largest dielectric family just after perovskites, and their interesting dielectric and ferroelectric properties have attracted systematic research activities towards gathering understanding on their structural and physical properties. The main promising applications for TTBs include nonlinear optics [1], thermoelectrics [2], electrocalorics for refrigeration applications [3], temperature stable multilayer capacitors (MLCC) [4], among others. The tetragonal tungsten bronze structure consists of layers of distorted BO 6 octahedra sharing corners in such a way that three different types of interstices (pentagonal A1, square A2 and trigonal C) are available for cation occupancy ( "
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    ABSTRACT: A promising n-type thermoelectric oxide, based on the tungsten bronze-structured ferroelectric SrxBa1–xNb2O6–δ (SBN, x~0.61), was investigated to enhance the thermoelectric power factor through templated grain growth (textured polycrystalline). In the reduced SBN textured, both the electrical conductivity (σ) and the magnitude of thermopower (S) are increased in the c axis: σ33 > σ11 and |S33| > |S11|, and consequently, the thermoelectric power factor (PF) increased significantly due to crystal anisotropy and grain boundary density reduction. It was found in randomly oriented polycrystalline ceramics that the thermoelectric properties are dominated by a-axis properties. A ferroelectric–thermoelectric anomaly is observed at 4mm–4/mmm phase transition temperature (TC) and depends on temperature and reduction degree, consistent with our earlier observations in single crystal SBN. Above TC, the carrier transport mechanism is controlled by polaron hopping conduction, and below TC the behavior depends on the degree of reduction. However, the magnitude of the Seebeck coefficient is dependent on the crystal anisotropy.
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