Formation of secondary phases and diffusion of cations in diffusion couples of yttria-stabilized zirconia and lanthanum manganite substituted with 0 to 60 mol% strontium have been studied by scanning electron microscopy and energy dispersive X-ray spectroscopy. Only the primary phases were observed after 120 h at 1200°C, while formation of secondary phases was identified already after 1 h heat treatment at 1350°C. The phase composition of the reaction layer altered from La 2 Zr 2 O 7 to SrZrO 3 at increasing Sr content in La x Sr 1−x MnO 3 . The thickness of the reaction layer was increasing with heat treatment time. In diffusion couples of La 0.4 Sr 0.6 MnO 3 formation of manganese oxide was observed in the perovskite layer after 1 h heat treatment at 1350°C, while isolated grains of SrZrO 3 relatively deep inside the zirconia were observed after longer heat treatment time. Diffusion of Mn into zirconia was observed preferenced along grain boundaries in the early stage of the interface reaction.

LaCoO 3 and La 0.8 Ca 0.2 CoO 3 ceramics show a nonelastic stress-strain behavior during four-point bending experiments where hysteresis loops are observed during loading-unloading cycles. Permanent strain is stored in the material after unloading, and a mechanism related to ferroelastic domain switching in the rhombohedral perovskite is proposed. Domain switching in the materials has been confirmed using X-ray diffractometry. Fracture toughnesses of La 0.8 Ca 0.2 CoO 3 measured using single-edge notched beam and single-edge V-notched beam methods coincide and are equal to 2.2 MPa⅐m 1/2 at room temperature and decrease to ϳ1 MPa⅐m 1/2 at temperatures >300°C. A decrease in fracture toughness is consistent with ferroelastic behavior, because the rhombohedral distortion decreases with increasing temperature.

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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.

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