Ionic Transport in Gd[sub 3]Fe[sub 5]O[sub 12]- and Y[sub 3]Fe[sub 5]O[sub 12]-Based Garnets

Abstract

The oxygen permeability of dense garnet-type Gd 3Ϫx A x Fe 5 O 12Ϯ␦ (A ϭ Ca,Pr;x ϭ 0-0.8) and Y 3ϪxϪy Ca x Nd y Fe 5Ϫz Ni z O 12Ϫ␦ (x ϭ 0-0.5;y ϭ 0-0.25;z ϭ 0-1.0) membranes at 1173-1273 K is limited by the bulk ambipolar conductivity. The ion transference numbers, calculated from results on the oxygen permeation and total conductivity, vary from 1 ϫ 10 Ϫ5 to 5 ϫ 10 Ϫ3 , increasing with temperature. Ionic conduction in ferrite garnets, primarily determined by the oxygen vacancy concentration, increases with acceptor dopant additions. The activation energies for the oxygen ionic and electronic transport in air are in the ranges 176-224 and 20-81 kJ/mol, respectively. The ceramic microstructure of garnet-based materials has no essential effect on their ionic conductivity, which is low compared to that of perovskite-related ferrites. The low mobility of oxygen ions, probably limited by the ion transfer along the edges of Fe-O tetrahedra in the garnet lattice, is likely to result from a crooked diffusion pathway. Decreasing the A-site cation radius leads to a higher ionic conductivity of garnet phases. The thermal expansion coefficients of ferrite garnet ceramics at 300-1200 K in air are in the range (9.4-10.9) ϫ 10 Ϫ6 K Ϫ1 .

Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

“…The oxygen ionic conductivity under oxidising conditions was estimated from the data on total conductivity and oxygen permeability, as described elsewhere. 6,[17][18][19] All data on oxygen permeation presented in this work correspond to the membrane feed-side oxygen partial pressure (p 2 ) fixed at 0.21 atm (atmospheric air).…”

“…Note that negligible surface exchange limitations to oxygen transport are observed for numerous ferrite phases with moderate level of the ionic conductivity. 19 8. Fig.…”

Defect formation in Gd3Fe5O12-based garnets: a Mössbauer spectroscopy study

“…Due to the low concentration of charge carriers and the important role of lattice relaxation in the oxygen ion migration processes, this behavior results in similar activation energies for the ionic conductivity in all Gd 3 Fe 5 O 12 -based garnets. D Iron-containing oxide phases with A 3 B 5 O 12 garnet structure are of significant interest for numerous applications, including magnetic materials, lasers, phosphorescent sources, microwave and electrochemical devices [1][2][3][4][5]. The garnet materials possess unique optical, thermophysical and mechanical properties, in particular, excellent creep and radiation damage resistance, fracture toughness, moderate thermal expansion coefficients, high thermal conductivity and energy-transfer efficiency.…”

“…The garnet materials possess unique optical, thermophysical and mechanical properties, in particular, excellent creep and radiation damage resistance, fracture toughness, moderate thermal expansion coefficients, high thermal conductivity and energy-transfer efficiency. Most performance-determining parameters are strongly dependent on structural factors, such as the large size and complexity of the garnet unit cell, the absence of close-packed oxygen planes and the density of point defects [1,[4][5][6][7][8]. The garnet crystal lattice (Fig.…”

“…These corner-shared units are slightly distorted; their network forms dodecahedral cavities occupied by the A-site cations (rare-earth and alkaline-earth metals). Although the garnet structure may tolerate extensive doping in both A and B sublattices [1,4,5,8], information available in the literature on mechanisms of defect formation induced by incorporation of aliovalent cations is scarce. For example, while charge compensation of acceptor cations in Fe-containing garnets occurs via increasing concentration of oxygen vacancies and/or Fe 4+ [1,5], oxygen deficiency can hardly be associated with the very unlikely existence of threecoordinated iron cations.…”