Oxygen Permeability and Thermal Expansion of Ferrite-Based Mixed Conducting Ceramics

Abstract

In order to evaluate promising directions in the development of mixed-conducting membrane materials for oxygen separation and partial oxidation of natural gas, a series of ferritebased ceramics were studied, including La 1-x Sr x Fe 1-y Ga y O 3-δ (x = 0.5 -0.8; y = 0 -0.4), La 1-x Sr x Fe 1-y Al y O 3-δ (x = 0.7 -1.0; y = 0 -0.5), La 0.3 Sr 0.7 Fe 0.7-x Al 0.3 Cr x O 3-δ (x = 0.1 -0.2), (Sr 2 Fe 3 ) 1-x (SrCo) x O z (x = 0 -0.8), CaFe 0.5 Al 0.5 O 2.5+δ and Ln 3-x Ca x Fe 5 O 12-δ (Ln = Gd, Y; x = 0 -0.5). The maximum oxygen permeation is observed for perovskite-type solid solutions with high oxygen deficiency, which exhibit, however, excessive thermal and chemically induced expansion. As for cobaltite-and gallate-based mixed conductors, the increase in ionic transport is accompanied with increasing limiting role of the surface exchange processes. The stability of perovskite-related ferrites in reducing atmospheres, which is comparable to that of LaFeO 3-δ and iron oxide, may be moderately increased or decreased by donor-or acceptor-type doping, respectively. In addition, the substitution of iron with cations having a more stable oxidation state, such as Ga 3+ , Al 3+ or Cr 3+/4+ , partly prevents the lattice expansion induced by oxygen nonstoichiometry variations, although the solubility of these dopants in the ferrite lattice is limited.

Oxygen Ion‐Conducting Materials

“…These primarily include perovskite-like (Ln,A)FeO 3Àd and their derivatives existing in all Ln-A-Fe-O systems, A 2 Fe 2 O 5AEd brownmillerites, (Ln,A) 3 Fe 5 O 12AEd garnets in the systems with relatively small Ln 3 þ cations, Ruddlesden-Popper series (Ln,A) n þ 1 Fe n O z , and a variety of other intergrowth compounds such as Sr 4 Fe 6 O 13AEd (see Refs [4,8,17,98,134,152,[157][158][159][160][161][162][163][164][165][166][167][168] and references cited therein). However, due to structural constrains and defect chemistry features limiting both ionic and electronic transport, in most cases an extensive iron substitution is necessary to achieve the total conductivity higher than 10-30 S cm À1 and partial ionic conductivity higher than 0.1 S cm À1 at temperatures above 700 K. For B-site-undoped ferrites, the maximum conductivity is characteristic of perovskite-related solid solutions, such as Ln 1Àx Sr x FeO 3Àd , where the highest level of electronic and ionic transport is known for Ln ¼ La and x % 0.5.…”

“…For most materials the oxygen fluxes are governed by both surface exchange and bulk ionic conduction, but can still be used to evaluate variations of the ionic transport properties in different systems due to the well-known correlations between the exchange kinetics and ion diffusivity [173][174][175]. It should also be noted that, in the Figure 9.12 Temperature dependence of the oxygen permeation fluxes through various mixed-conducting membranes, made of perovskite-related compounds with predominant electronic conductivity, under fixed oxygen chemical potential gradients [24,63,111,162,[169][170][171][172]. p 1 and p 2 are the oxygen partial pressures at the membrane permeate and feed sides, respectively.…”

“…If compared to manganite electrode materials, one important disadvantage of perovskite-related ferrites relates to a high chemical expansion, which provides a critical contribution to the apparent TECs due to oxygen losses at elevated temperatures, and may lead to a thermomechanical incompatibility with common solid electrolytes [117,162,176,177]. The layered ferrite phases -particularly the Ruddlesden-Popper series and brownmillerites -display modest stoichiometry changes and better thermomechanical properties compared to the perovskite compounds, although their compatibility with electrolyte ceramics is still limited [133,134,165,167,168,178].…”

Oxygen transport in ferrite-based ceramic membranes: Effects of alumina sintering aid

“…Kharton). and chemically-stable cations, respectively, such as Cr, Ti, Ga and Al on the B sublattice, and La for Sr. 2,4,[6][7][8][9][10][11][12] Particular compositions having attractive transport properties and stability are Sr 1−x La x Fe 1−y M y O 3−δ (M = Ga, Cr; x = 0.2-0.4; y = 0.2-0.4), SrFe 0.7 Al 0.3 O 3−δ and Sr 0.5 La 0.5 FeO 3−δ . The latter phase exhibits a maximum of electronic and oxygen ionic conductivity in the Sr 1−x La x FeO 3−δ system; a higher strontium content without B-site doping results in oxygen vacancy ordering and deterioration of transport properties.…”

Defect formation and transport in SrFe1-xAlxO3-δ

“…Unfortunately, Ga-containing materials, in addition to high cost of gallium precursors, possess a number of specific disadvantages, including gallium oxide volatilization at elevated temperatures and/or in reducing atmospheres [9] and possible interaction with catalysts containing Ni or Pt [9,10]. The use of aluminum instead of gallium was recently reported as a possible alternative that improves the stability of perovskite-type ferrites, keeping the high level of oxygen permeation, and eliminates unfavorable effects related to Ga [11][12][13].…”