Sintering behavior, microstructure, and phase composition of Sr(Fe,Co)O3-delta ceramics

SrFe1-xCoxO3-delta (x = 0, 0.33) ceramics have been prepared in the tempera ture interval 900-1350 degreesC in air, nitrogen, and oxygen atmosphere, Th e effect of cation nonstoichiometry on densification behavior, microstructu re, and phase composition has been investigated. Densification of Sr-defici ent SrFeO3-delta initiates at a lower temperature than near stoichiometric SrFeO3-delta probably due to enhanced diffusion of Sr, However, for Sr-defi cient samples Sr4Fe6O13 is formed above similar to 775 degreesC, causing a significant decrease in the sintering rate, It is therefore necessary to av oid Sr deficiency to obtain dense SrFeO3-delta ceramics. The densification rate was significantly increased by Co substitution. Dense ceramics (>95% o f theoretical density) of Sr-excess and Co-substituted SrFeO3-delta mere ob tained by sintering in the temperature region 1000-1200 degreesC. The grain size increases by increasing temperature, decreasing partial pressure of o xygen and Co substitution. Exaggerated grain growth in the Co-substituted m aterial occurred at high temperatures. Sintering above 1200 degreesC caused all materials, apart from Co-substituted SrFeO3-delta 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 e volution of oxygen gas. The phase equilibria are governed by the cation non stoichiometry of the materials. The present findings demonstrate the import ance of controlling the cation stoichiometry of ternary transition-metal ox ides to achieve ceramics with the desired homogeneous microstructure. The m ixed valence state of the transition metal mag lead to both swelling during sintering and cracking during cooling due to reduction/oxidation during he ating/cooling.