A new series of SrCo 0.4 Fe 0.6-x Zr x O 3-δ (0 e x e 0.2) and SrCo 0.95-x Fe x Zr 0.05 O 3-δ (0.1 e x e 0.8) oxides were synthesized by the solid-state reaction method. The incorporation of Zr and its effect on the crystal chemistry, oxygen permeation, and structural stability of SrCo 0.4 Fe 0.6-x Zr x O 3-δ have been investigated. Formation of the solid solutions with the cubic perovskite-type structure was found in the SrCo 0.4 Fe 0.6-x Zr x O 3-δ oxide system in the range of 0 e x < 0.1. The incorporation of the proper content of Zr into SrCo 0.4 Fe 0.6 O 3-δ suppressed the oxygen loss at high temperature and could stabilize the cubic structure of perovskite in a low oxygen partial pressure atmosphere, while the oxygen permeability of SrCo 0.4 Fe 0.6-x Zr x O 3-δ membranes decreased with increasing Zr content. A total of 5 mol % of Zr cation dissolution was considered to be enough for the structural stability without deleteriously affecting the oxygen flux. Meanwhile, an investigation of the effect of the Co content on the oxygen permeation and structural stability of SrCo 0.95-x Fe x Zr 0.05 O 3-δ series oxides showed that, with increasing Co content, the oxygen permeability increased, but no phase transition occurred in low oxygen partial pressure for all of the samples investigated. Our results indicated that the content of Zr played a critical role in the stability of Sr-Co-Fe-Zr-O series oxides. When good structural stability and oxygen permeation properties are combined, these Sr(Co,Fe,Zr)O 3-δ oxides are considered to be promising candidates as oxygen separation membranes.
The oxygen nonstoichiometry, transport properties, and structure stability of ZrO 2 -promoted SrCo 0.4 Fe 0.6 O 3-δ (SCFZ) were investigated. The nonstoichiometry of SCFZ increased with increasing temperatures and decreasing oxygen partial pressures. The results of X-ray diffraction and oxygen desorption experiments showed that the addition of ZrO 2 in SrCo 0.4 Fe 0.6 O 3-δ (SCF) stabilized the phase structure under low oxygen partial pressure. The chemical diffusion coefficient of oxygen in SCFZ in a temperature range of 928-1178 K was obtained using the weight relaxation technique. A simple transport equation correlating oxygen flux to the oxygen diffusion coefficient was deduced. Model calculations, based on the transport equation in conjunction with data of oxygen permeation and oxygen nonstoichiometry, were performed. The rates of oxygen permeation fluxes measured at various temperatures and time showed that the long-term operation stability of the SCF membrane with the addition of ZrO 2 was greatly improved. The oxygen permeation flux, however, was slightly reduced.
The effects of ZrO addition with ®arious particle sizes 1, 3, and 56 m and 2 ( ) ®arious amounts 0, 1, 3, 5, 7, 9 wt. % on structure, oxygen permeation, and stability ( ) of SrCo Fe O SCF were in®estigated. XRD and EDX analysis re®ealed that the 0 .4 0 .6 3 -␦ dissolution of Zr cation into the B site of SCF phase occurred with the addition of ZrO , resulting in a lattice expansion of SCF at ele®ated temperatures. The dissolution 2 amount of Zr in the SCF phase increased with a decrease of ZrO particle size and an 2 increase in the amount of ZrO added, and, therefore, resulted in a decrease of oxygen 2permeation flux of the membranes and an increase in their structural stability in a helium atmosphere. This study indicated that adding a certain amount of Zr cation in SCF oxide was an effecti®e route to sustain the structural stability of SCF in low oxygen partial pressure, and the optimum ZrO addition was 3 wt.% with a particle size of 1 2 m, which greatly impro®ed the structural stability without significantly changing the oxygen permeability of SCF.
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