Mechanical properties of mixed conducting La_0.5Sr _0.5Fe_1-x Co _x O_3-δ (0≤x≤1) materials

Young's modulus, strain-stress behavior, fracture strength, and fracture toughness of La_0.5Sr_0.5Fe_1-xCo _xO_3-δ. (0≤×;≤1) materials have been investigated in the temperature range 20-1,000°C. Young's moduli of La _0.5Sr_0.5Fe_0.5Co_0.5O _3-δ and La_0.5Sr_0.5CoO _3-δ, measured by resonant ultrasound spectroscopy, were 130±1 and 133±3 GPa, respectively. The nonlinear stress-strain relationship observed by four-point bending at room temperature was inferred as a signature of ferroelastic behavior of the materials. Above the ferroelastic to paraelastic transition temperature, the materials showed elastic behavior, but due to high-temperature creep, a nonelastic respond reappeared above ∼800°C. The room temperature fracture strength measured by four-point bending was in the range 107-128 MPa. The corresponding fracture toughness of La_0.5Sr_0.5Fe_0.5Co_0.5O _3-δ, measured by single edge V-notch beam method, was 1.16±0.12 MPa•m^1/2. The measured fracture strength and fracture toughness were observed to increase with increasing temperature. The fracture mode changed from intragranular at low temperature to intergranular at high temperature. Tensile stress gradient at the surface of the materials caused by a frozen-in gradient in the oxygen content during cooling was proposed to explain the low ambient temperature fracture strength and toughness.