On the sol–gel synthesis and characterization of strontium ferrite ceramic material

A simple oxalate based sol–gel process has been described to produce a highly stable anion deficient strontium ferrite for separation of oxygen from air. The method involves metal nitrates and oxalic acid precursors with ethanol and water as solvents, gel formation, digestion for 4 h, drying at 150 °C for 24 h, and finally decomposition at 800 °C in air. The resulting material (i) exhibits a single perovskite-type cubic (SrFeO3−ξ; ξ ∼ 0.13) phase with ao = 3.862 ± 0.002 Å, (ii) contains both the Fe4+ and Fe3+ species in 2.8:1 ratio, (iii) undergoes Fe4+ → Fe3+ reduction upon heating at 650 °C in rare gas ambient and transition to an orthorhombic phase with a ∼ ao√2, b ∼ 4ao, c ∼ ao√2, which reverts back to cubic phase with oxygen uptake at elevated temperatures, and (iv) acts as filter for air with excellent oxygen permeation, typical flux density value being 2.45 ml/cm2 min at 1000 °C.

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► Successful sol–gel preparation of cubic perovskite SrFeO3−ξ (ξ ∼ 0.13) with ao = 3.862 Å.
► Reduces to orthorhombic SrFeO2.5 via Fe4+ → Fe3+conversion with a ∼ ao√2, b ∼ 4ao, c ∼ ao√2.
► High stability and good oxygen permeation characteristics at 1000 °C.
► Phase reversibility upon heating in oxygen ambient at elevated temperatures.