Microstructure and thermomechanical properties of SrTi$_{1-x}$Fe$_{x}$O$_{3-δ}$ oxygen transport membranes and supports

Pure oxygen is considered to be an important commodity and its demand is expected to rise in the upcoming years. Therefore, interest in a new, scalable technology for air separation based on oxygen transport membranes has increased. Indeed, ceramic oxygen transport membranes can possess remarkable advantages compared to existing state-of-the-art processes for oxygen production in small and medium scale. Typically, such membranes have to be operated at 800 - 900 °C and under high pressure gradients, which challenges significantly the chemical and mechanical stability of the respective brittle ceramics components. Thus, the current work concentrates on the mechanical properties of a promising perovskite material for mixed conducting oxygen transport membranes based on SrTi1-xFexO3-δ, (STF-X) with x = 0.25, 0.35 and 0.5. The dense membrane materials, synthesized through solid state reaction route and processed via tape casting, were characterized with respect to their mechanical behavior via depth-sensitive indentation testing and ring-on-ring flexural tests. No tradeoff between functional and mechanical properties is observed for the studied dense materials. Ring-on-ring tests conducted at different loading rates gave access to subcritical crack growth sensitivity and aided the prediction of the materials’ lifetime through stress-time-probability diagrams, where SrTi0.65Fe0.35O3 revealed the best performance among the studied compositions. The mechanical properties of dense tape cast SrTi0.75Fe0.25O3-δ were derived at 900°C with help of ring-on-ring flexural tests carried out at two distinct loading rates: 1 and 100 N/min. The apparent elastic modulus appeared to be time dependent. The average fracture stress estimated on the basis of the data was rather loading rate invariant and higher than the same property obtained at room temperature. Thus, it appears that SrTi0.75Fe0.25O3-δ possesses an elastic behavior that might be associated with oxygen release and/or primary creep deformation. In addition, porous SrTi0.75Fe0.25O3-δ specimens, representative for use as porous substrate for supported asymmetric oxygen transport membranes, were produced through tape casting, freeze drying, and phase inversion casting. The first method yielded a random distributed porosity with spherical pores reaching a value of 32%. Freeze-drying and phase inversion, however, derived a channel-like porosity in the same level. Ring-on-ring bending tests were employed in order to derive elasticity and fracture stresses of the manufactured samples. In order to investigate the effect of anisotropy of the mechanical properties for freeze-dried specimens, compressive testing was used yielding apparent elastic modulus and fracture stresses in the transverse (out-of-plane) and longitudinal (in-plane) direction, verifying a strong dependency onto pore orientation. The lower mechanical stability in the in-plane direction appears to be associated with bending mode of the pore walls, being a result of a lower resistance to crack initiation. Acoustic emissions recorded during compressive tests indicated continuous damage of the pore walls before complete failure of the specimen, which could also be confirmed by complementary in-situ compressive tests in a scanning electronic microscope. Also, creep behavior of pressed, tape cast, and freeze-dried SrTi0.75Fe0.25O3-δ specimens was investigated and the experimental creep parameters were obtained. The results derived a stress exponent in the range of 2 with a large primary creep deformation (in the range of 0.03 to 0.6%). Therefore, it is suggested that SrTi0.75Fe0.25O3-δ undergoes a combination of diffusive and dislocation creep