Article

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

(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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Available from: Soonil Lee
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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: Temperature stable dielectrics of tungsten bronze Sr4Nd2Ti4Nb6O30 (SNTN) with maximized dielectric performance are achieved with thick films prepared by electrophoretic deposition. 30 μm thick SNTN films sintered at 1300 °C, exhibit permittivity ɛ > 375, loss tangent tanδ < 0.01 and stable to ±7.5% of the room temperature value in the temperature range of −95 °C to 280 °C. This permittivity is ∼34% higher than that for bulk ceramics (∼280) processed under the same conditions. Contrary to the microstructure of ceramics, SNTN thick films exhibit anisotropy of the grain growth with increasing sintering temperature. It is proposed that the observed anisotropy is responsible for the maximization of the dielectric properties and is due to the anisotropic crystal structure of SNTN and to the sintering under constraint. The main contribution of the c axis vibration to the dielectric constant in tungsten bronze SNTN is confirmed. These results are relevant because via tailoring the substrate constraint and sintering conditions the grain anisotropy of SNTN thick films can be controlled and thus the dielectric properties.
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ABSTRACT: We propose an explanation for the high electrical conductivity of the ferroelectric strontium-barium niobate. As the temperature T approaches the ferroelectric transition T c, the static dielectric constant $\varepsilon(0)$ diverges when a soft mode occurs. This divergence of $\varepsilon(0)$ reduces the donor binding energy, and increases the effective Bohr radius of the donor. The electrons bound to the donors become unbound, and the material becomes conductive.
No preview · Article · Jul 2012 · Journal of Electronic Materials
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##### Article: Thermoelectric Power Factor Enhancement of Textured Ferroelectric SrxBa1−x Nb2O6−δ Ceramics
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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.
Full-text · Article · Jan 2011