Correlation of the Formation and the Decomposition Process of the BSCF Perovskite at Intermediate Temperatures

Abstract

The mixed ionic-electronic conductor (MIEC) (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3-δ (BSCF) is a renowned material with applications in membrane reactors and as cathodes in solid-oxide fuel cells. Despite BSCF's large oxygen permeabilities, long-time phase instability at intermediate temperatures has been reported. However, the mechanism of this decomposition is still unclear. Here, we present a study of the synthesis of BSCF and compare our results with those obtained from long-time decomposition. Rietveld and Le Bail analysis as well as transmission electron microscopy studies were applied to investigate the reaction sequence in BSCF formation. We are now able to draw the following conclusion about the reaction mechanism: the formation as well as decomposition is due to a reversible reordering of the hexagonal AO 3 -layer stacking sequence in the cubic perovskite, which can occur if the cubic BSCF is kept at temperatures below T ) 1173 K for long time periods, thereby leading to the decomposition of BSCF into a three-phase mixture. The driving force for this reaction was identified to occur at the cobalt site because cobalt prefers a low-spin configuration in the 3+ oxidation state. This reaction occurs only at temperatures below T ) 1173 K because of the oxidation of cobalt at low temperatures.

High temperature X-ray diffraction and thermo-gravimetrical analysis of the cubic perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ under different atmospheres

“…Švarcová et al reported that the cubic BSCF become unstable with respect to a hexagonal polymorph below 850-900 ºC under oxidizing conditions 12 . The transition from the cubic to the hexagonal polymorph is quite complex, involving formation of several phases [13][14][15][16][17][18][19] . The structural phase transformation has also been observed by compressive creep measurements 20,21 , showing that cation diffusion is slowed down by formation of the hexagonal polymorph, reflecting the slow kinetics of the phase reaction below 900 ºC.…”

Design and experimental investigation of oxide ceramic dual-phase membranes

“…However, to realize the process, the membrane is required possessing both high permeation flux and stability under syngas atmosphere. Among the developed materials, cobalt-doped Ln x (Ba,Sr,Ca) 1−x Co y Fe 1−y O 3−ı (Ln: rare earth metal, 0 ≤ x ≤ 1, 0 ≤ y ≤ 1) usually have high oxygen permeation fluxes but are unstable under reducing environments and long-term operation at high temperatures [11][12][13][14][15][16][17][18][19][20]. Ln x (Ba,Sr,Ca) 1−x Fe y (Zn,Al,Ga,Ti,Zr,Ce) 1−y O 3−ı (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) usually possesses high stability under reducing environments but poor permeation fluxes [21][22][23][24][25][26][27][28][29][30][31].…”

Dense ceramic oxygen permeable membranes and catalytic membrane reactors

“…However, as a result of some decomposition of the bulk phase and a tiny surface segregation at 850°C, there is a very slow decrease on the oxygen permeation flux in the 1000 h operation [17]. However, in a medium temperature window between 500 and 800°C, stability problems are reported for BSCF [17,[20][21][22][23][24][25][26][27]. At these temperatures, the cubic BSCF phase decomposes partially in a cobalt-rich hexagonal Ba 0.5±0.1 Sr 0.5±0.1 CoO 3Àd and in a cobaltrich trigonal phase [23,25,26].…”