Mechanical behaviour of ceramic oxygen ion-conducting membranes

In devices such as solid oxide fuel cells (SOFCs), oxygen separators and membrane reactors, the mechanical properties of the ceramic components are particularly important for estimating reliability. Here we summarise some recent studies of the mechanical properties of oxygen ion-conducting ceramics for the applications mentioned. Young’s modulus and Poisson’s ratio of several materials of interest for SOFCs have now been characterised; including the effects of temperature and porosity on these parameters. The biaxial flexural strength and fracture toughness of tape-cast YSZ have been measured at room temperature and at a typical operating temperature of 900°C. Fracture toughness decreases with increasing temperature; from 1.61±0.12 MPa m0.5 at room temperature to 1.02±0.05 MPa m0.5 at 900°C. Flexural strength and fracture toughness were found to be quantitatively consistent with fracture being initiated at observed surface defects. YSZ has also been found to be susceptible to sub-critical crack growth. Many oxygen-ion conducting ceramics suffer from a chemically induced stress when subjected to a gradient in the thermodynamic activity of oxygen. Ceria and acceptor-doped lanthanum chromite are given as typical examples. One strategy for overcoming this problem in electrolytes is to laminate different materials. An example is given of CGO laminated with a thin layer of YSZ. The combined effects of improved microstructure and residual stress lead to the laminated membranes having higher apparent strength than single layer CGO membranes by a factor of between 1.75 and 4.06. Electrode/electrolyte laminates have been observed to have some unexpected behaviour.