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Extended phase homogeneity and electrical properties of barium calcium titanate prepared by the wet chemical methods

Jayanthi, S and Kutty, TRN (2004) Extended phase homogeneity and electrical properties of barium calcium titanate prepared by the wet chemical methods. In: Materials Science and Engineering: B, 110 (2). pp. 202-212.

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Ca-substituted $BaTiO_3$ with extended homogeneity range up to $\sim50$ mol% $CaTiO_3$ have been prepared by three different chemical routes namely carbonate-oxalate (COBCT), gel-carbonate (GCBCT), and gel-to-crystallite conversion (GHBCT) followed by heat treatment above $1150^oC.$ X-ray powder diffraction (XRD) data show continuous decrease in the tetragonal unit cell parameters as well as $c_0/a_0$ ratio with $CaTiO_3$ content, which are in accordance with the substitution of smaller sized $Ca^{2+}$ ions at the barium sites. The microstructure as well as the dielectric properties are greatly influenced by the cationic ratio, α = (Ba + Ca)/Ti. The grain size decreases with $CaTiO-3$ content for the stoichiometric samples (α = 1), whereas ultrafine microstructure is observed in the case of off-stoichiometric samples (α > 1) for the whole compositional range of $CaTiO_3$ concentrations. Sharper $\varepsilon r - T$ characteristics at lower calcium content and broader $\varepsilon r - T$ with decreased $\varepsilon max,$ in the higher calcium range are observed in the case of α = 1. Whereas nanometer grained ceramics exhibiting diffuse $\varepsilon r - T$ characteristics are obtained in the case of α > 1. The positive temperature coefficient of resistivity (PTCR) is realized for barium calcium titanate ceramics having 0.3 at.% Sb as the donor dopant for higher $CaTiO_3$ (typically 30 mol%) containing samples (α = 1), indicating that $Ca^{2+}$ ions do not behave as acceptors if they were to substitute at the $Ti^{4+}$ sites. Whereas the off-stoichiometric (α > 1) ceramics retained high resistivity, indicative of the Ti-site occupancy for $Ca^{2+}$ in fine grain ceramics.

Item Type: Journal Article
Publication: Materials Science and Engineering: B
Publisher: Elsevier
Additional Information: The copyright belongs to Elsevier.
Keywords: Ceramics;Permittivity;Stoichiometry and homogeneity;Metastable phases
Department/Centre: Division of Chemical Sciences > Materials Research Centre
Date Deposited: 05 May 2006
Last Modified: 19 Sep 2010 04:26
URI: http://eprints.iisc.ac.in/id/eprint/6577

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