Sintering Behavior, Microstructure, and Phase Composition of Sr(Fe,Co)O_3−δ Ceramics

Abstract

SrFe 1؊x Co x O 3؊␦ (x ‫؍‬ 0, 0.33) ceramics have been prepared in the temperature interval 900 -1350°C in air, nitrogen, and oxygen atmosphere. The effect of cation nonstoichiometry on densification behavior, microstructure, and phase composition has been investigated. Densification of Sr-deficient SrFeO 3؊␦ initiates at a lower temperature than near stoichiometric SrFeO 3؊␦ probably due to enhanced diffusion of Sr. However, for Sr-deficient samples Sr 4 Fe 6 O 13 is formed above ϳ775°C, causing a significant decrease in the sintering rate. It is therefore necessary to avoid Sr deficiency to obtain dense SrFeO 3؊␦ ceramics. The densification rate was significantly increased by Co substitution. Dense ceramics (>95% of theoretical density) of Sr-excess and Co-substituted SrFeO 3؊␦ were obtained by sintering in the temperature region 1000 -1200°C. The grain size increases by increasing temperature, decreasing partial pressure of oxygen and Co substitution. Exaggerated grain growth in the Co-substituted material occurred at high temperatures. Sintering above 1200°C caused all materials, apart from Co-substituted SrFeO 3؊␦ in oxygen, to swell and develop a porous microstructure. The swelling mechanism was related to heterogeneous phase equilibria, which is reductive in nature and leads to evolution of oxygen gas. The phase equilibria are governed by the cation nonstoichiometry of the materials. The present findings demonstrate the importance of controlling the cation stoichiometry of ternary transitionmetal oxides to achieve ceramics with the desired homogeneous microstructure. The mixed valence state of the transition metal may lead to both swelling during sintering and cracking during cooling due to reduction/oxidation during heating/ cooling.

Sintering behaviour of La1−xSrxFeO3−δ mixed conductors

“…Extreme expansion during sintering has also been reported for other related perovskites by the present group 9,11 and others groups. 10,12 It is worth noting that swelling is also observed in ceramics of high-T c superconductors [19][20] where the valence state of Cu is reduced during sintering.…”

“…This decrease is also in line with the sintering behaviour reported for LaFeO 3 9 and SrFeO 3−δ . 11 As for La09 Fig. 2.…”

“…This is also consistent with dilatometric studies of pure LaFeO 3 and SrFeO 3−δ powders. [9][10][11] The initial sintering temperature is increasing going from pure LaFeO 3 and SrFeO 3−δ , reflecting the difference in the melting temperature of the two pure compounds. So far it has not been possible to determine if the solid state sintering is governed by cation diffusion along grain boundaries or by bulk diffusion.…”

“…9 The densification behaviour of La 1−x Sr x FeO 3−δ has to our knowledge not been studied extensively, but reports on SrFeO 3−δ ceramics [10][11][12][13] show low densities and swelling at higher temperatures. Kleveland et al 11 have emphasized the importance of good control of the cation stoichiometry to avoid post sintering swelling. The swelling was caused by reductive heterogeneous phase equilibria involving secondary phases resulting in oxygen evolution and pore expansion.…”

Monoclinic Ferroelastic Domains in LaCoO3‐Based Perovskites

“…The monoclinic domains were shown to possess ferroelastic properties by in-situ X-ray diffraction and strain-stress measurements. were synthesized by the glycine-nitrate method or spray pyrolysis as described previously [21,35]. The nitrates were carefully characterized by thermogravimetry in order to determine the exact cation content to obtain stoichiometric materials.…”