Sintering Behavior, Microstructure, and Phase Composition of Sr(Fe,Co)O3−δ Ceramics

Abstract

SrFe1−xCoxO3−δ (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 SrFeO3−δ initiates at a lower temperature than near stoichiometric SrFeO3−δ probably due to enhanced diffusion of Sr. However, for Sr‐deficient samples Sr4Fe6O13 is formed above ∼775°C, causing a significant decrease in the sintering rate. It is therefore necessary to avoid Sr deficiency to obtain dense SrFeO3−δ ceramics. The densification rate was significantly increased by Co substitution. Dense ceramics (>95% of theoretical density) of Sr‐excess and Co‐substituted SrFeO3−δ 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 SrFeO3−δ 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 transition‐metal 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.