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The room temperature mechanical properties of a mixed conducting perovskite Sm1−xSrxCo0.2Fe0.8O3 (x ⳱ 0.2 to 0.8) were examined. Density, crystal phase, and microstructure were characterized. It was found that the grain size increased abruptly with increasing Sr content. Mechanical properties of elastic modulus, microhardness, indentation fracture toughness, and biaxial flexure strength were measured. Young’s modulus of 180–193 GPa and shear modulus of 70–75 GPa were determined. The biaxial flexure strength was found to decrease with increasing Sr content from ∼70 to ∼20 MPa. The drop in strength was due to the occurrence of extensive cracking. Indentation toughness showed a similar trend to the strength in that it decreased with increasing Sr content from ∼1.1 to ∼0.7 MPa m1/2. In addition, fractography was used to characterize the fracture behavior in these materials.

I. INTRODUCTION

Perovskite-type oxides [La1−xAxCo1−yFeyO3−␦ (A ⳱ Ca, Sr, and Ba)] have been extensively studied.1–10 The substitution of acceptors onto an A site will form oxygen vacancies and a change of valence state of the B-site cations to maintain electrical neutrality. The concentration of oxygen vacancies can also be tailored by substituting ions of similar sizes but lower valence on the B sites. As a result, these materials exhibit not only high ionic conductivity but also high electronic conductivity. Hence, they are so-called mixed conductors. The ionic conductivity of these materials exceeds that of yttrium-stabilized zironia by nearly an order of magnitude.1,2 At elevated temperatures, the high electronic and ionic conductivity makes these perovskite-type oxides attractive candidates for various applications, such as solid oxide fuel cells (SOFC), membrane reactors for syngas generation, oxygen separation, and the partial oxidation of hydrocarbons. To date, most of the research on La1−xSrxCo1−yFeyO3−␦ (LSCF) materials has focused on electrical properties,1– 4 oxygen permeability,5,6 oxygen transport and diffusion,7,8 nonstoichiometry and defect structure,9,10 membrane synthesis and characterization,11,12 and technological applications.13,14 In several studies, Sm2O3 has been noted for its high catalytic activity for partial oxidation of methane to higher hydrocarbons.15–18 To the knowledge of the authors, none of the mechanical properties have been reported for these mixed conductors. In this paper, room temperature mechanical properties including elastic properties, biaxial flexure strength, and indentation fracture toughness are presented. Structure characteriza-

tion as well as fractography were employed to identify the critical flaws and assess the microstructure relating to the mechanical properties.

II. EXPERIMENTAL A. Powder preparation, sintering, and microstructure characterization

Compositions within the system Sm1−xSrxCo0.2Fe0.8O3 (SSCF) (x ⳱ 0.2, 0.4, 0.6, and 0.8) were prepared using the glycine–nitrate combustion synthesis technique.19 After mixing Sm, Sr, Co, and Fe nitrate solutions in the appropriate proportions, glyc