Mixed conductivity of garnet phases based on gadolinium ferrite


Incorporation of Pr and Ca into the garnet lattice of Gd3Fe5O12 (+/- delta) was found to increase the electronic conductivity, and to decrease the thermal expansion coefficients and oxygen ion transference numbers. The ionic conductivity of Gd2.2Pr0.8Fe5O12+/-delta ceramics is similar to that of undoped gadolinium ferrite, whilst Gd2.5Ca0.5Fe5O12-delta exhibits slightly higher ionic transport, suggesting an enhanced vacancy contribution. The activation energies for ionic conduction in Gd3Fe5O12-based materials vary in the narrow range 211-224 kJ/mol. Due to a significantly lower activation energy for the total conductivity (24-81 kJ/mol), the oxygen ion transference numbers in air increase with temperature, from 4 x 10(-5) - 5 x 10(-3). Steady-state oxygen permeation fluxes through dense garnet membranes are limited by bulk ambipolar conductivity. The thermal expansion coefficients of the ferrite ceramics (100-1000 degreesC were found linear and in the range 10.36-10.86 x 10(-6) K-1. (C) 2003 Elsevier Ltd. All rights reserved.



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Materials Science


Shaula, AL; Kharton, VV; Marques, FMB

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