Mechanical reliability and oxygen permeation of Ce0.8Gd0.2O2-δ- FeCo2O4 dual phase membranes
Due to the COVID-19 crisis measures the PhD defence of Fanlin Zeng will take place online (until further notice).
The PhD defence can be followed by a live stream.
Fanlin Zeng is a PhD student in the research group Inorganic Membranes (IM). His supervisor is prof.dr.ir. W.A. Meulenberg from the Faculty of Science and Technology (TNW).
Dual phase oxygen transport membranes, consisting of ionic and electronic conducting phases, exhibit great potential in high-purity oxygen generation due to their high stability under harsh application atmospheres. Oxygen-ion conductive fluorite oxides (e.g. Ce0.8Gd0.2O2-δ) and electron conductive spinel phases (e.g. FeCo2O4) are promising material candidates for such a dual phase oxygen transport membrane. Mechanical properties (e.g. elastic modulus, hardness, strength and subcritical crack growth behaviour) and oxygen permeation of the membrane are important parameters regarding reliability for future applications. These parameters have close relationships with composition and microstructural characteristics, like grain size, phase distribution and defects (e.g. microcracks). However, these relationships are currently not fully understood. Therefore, in this thesis, the influence of composition, grain size and microstructural defects on mechanical properties are investigated for Ce0.8Gd0.2O2-δ-FeCo2O4 membranes. Milling procedures during powder fabrication and ceramic sintering profiles are optimized to overcome the formation of unfavorable microstructural defects. Furthermore, the effects of grain size and phase distribution on oxygen permeation are discussed for a 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 membrane.
Chapter 1 of this thesis presents currently known potential applications and basic concepts of oxygen transport membranes (e.g. mechanism of oxygen transport and material candidates). Promising prospects for dual phase oxygen transport membranes are reflected, in particular those for the Ce0.8Gd0.2O2-δ-FeCo2O4 composites.
Chapter 2 reports on investigations and quantifications of phase compositions and microstructural features including volume fractions, grain sizes, and contiguity for the different phases in zCe0.8Gd0.2O2-δ-(1-z)FeCo2O4 (z = 50, 60, 70, 85 or 90 wt%) composites. The characterizations reveal a multi-phase system containing Ce1-xGdxO2-δ’ (x » 0.1), and FeyCo3-yO4 (0.2 < y < 1.2), CoO and Gd0.85Ce0.15Fe0.75Co0.25O3 phases in the membranes. A novel model is derived to calculate the ambipolar conductivity using the quantified phase constituents and microstructural features. These calculations results indicate that, if the grain sizes of all phases in the composites are identical, a 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 composite exhibits the highest ambipolar conductivity. Besides, both experimental data and calculated results indicate a rather poor ambipolar conductivity of the membrane containing a significant amount of large grains.
The mechanical limits regarding application in particular strength and lifetime are presented in Chapter 3 for zCe0.8Gd0.2O2-δ-(1-z)FeCo2O4 (z = 50 or 85 wt%) composites containing a significant amount of large grains. In general, the fracture strengths of as-sintered membranes are reduced by the presence of tensile residual stresses and microcracks. In particular, for the 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 composite, after an operation time of 10 years, the failure stress, inducing a failure probability of 1 %, significantly decreases from ~ 48 MPa to ~ 2 MPa, which appears to be a result of tensile residual stresses and microcracks.
To improve the mechanical properties and ambipolar conductivity, the grain size of the membranes is reduced by starting with smaller initial particle sizes of the powder mixtures. These powder mixtures with reduced particle sizes are subsequently used to sinter Ce0.8Gd0.2O2-δ-FeCo2O4 composites that are further investigated as outlined in Chapters 4-7.
Studies of the effects of sintering profiles on microstructural and mechanical characteristics of 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 composite are presented in Chapter 4. The results indicate that the optimal sintering temperature appears to be 1200 °C with a holding time of 10 h, since the microstructure sintered at this temperature possesses a density exceeding 99 %, relatively small grains and small surface defects, which contributes to a high average flexural strength of approximately 266 MPa. This optimal sintering profile is further applied to sinter the Ce0.8Gd0.2O2-δ-FeCo2O4 composites as described in Chapters 5-7.
Chapter 5 reports the elastic modulus and hardness of zCe0.8Gd0.2O2-δ-(1-z)FeCo2O4 (z = 50, 60, 70, 85 or 90 wt%) composites as well as of the individual phases in these membranes. The mechanical properties were determined by nanoindentation tests at room temperature. It unveils that the magnitude of the elastic moduli of the different phases is in the order Gd0.9Ce0.1Fe0.8Co0.2O3 > Ce1-xGdxO2-δ » FexCo3-xO4 > CoO, and difference in hardness values are also in the same order. The average elastic modulus and hardness of the composites are in the ranges of ~ 214-223 GPa, and ~ 10.5-11.5 GPa, respectively. The elastic modulus of the composites marginally decreases with increasing iron cobalt spinel content, while the hardness values of the composites are affected to a slightly stronger extent by porosity rather than by the compositional variation. Any compositional effect appears to diminish above a porosity of around 1%.
Chapter 6 focuses on fracture strength and its relationship to residual stresses of zCe0.8Gd0.2O2-δ-(1-z)FeCo2O4 (z = 50, 70 or 85 wt%) composites. The strength was determined by ring-on-ring bending tests, and the residual stress and residual stress gradient were derived from X-ray diffraction (based on the method) and the indentation method. The results reveal that the strength of composites increase with decreasing spinel content. The low fracture strength of the composites with high spinel content is attributed to tensile residual stress gradients and microcracks at the surface. It is found that for 50 wt% Ce0.8Gd0.2O2-δ-50 wt% FeCo2O4 composite, the residual tensile stress decreases when a dwelling step at 850 °C for 100 h is applied after sintering. However, it is proposed to limit the spinel content to a nominal value of 15 wt% in order to eliminate the residual tensile stress and thereby yielding a material of high mechanical strength.
In Chapter 7 oxygen permeances are discussed for 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 membranes with three different microstructures. The first membrane has a small average grain size (~ 0.4 μm) and a homogeneous distribution of the two phases, while the second membrane has bigger grains and a less homogeneous phase distribution. The results indicate that the membrane with a fine and homogeneous microstructure exhibits higher oxygen permeance (~ 1.7 × 10-8 mol·cm2·s-1 at ~ 900 °C, measured for a bare membrane with a thickness of ~ 0.96 mm) than a membrane with larger grains due to good connections between ionic/electronic conducting phase and large triple phase boundaries (TPBs) at the surface. Finally, a dry powder mixing method is introduced to prepare the third membrane. This membrane has a unique microstructure with relative straight paths for ionic and electronic conduction, resulting in significantly improved oxygen permeance (~ 2.7 × 10-8 mol·cm2·s-1 at ~ 900 °C, measured for a surface-activated membrane with a thickness of ~ 0.96 mm), if compared with the other two membranes after the limiting effect from surface exchange is overcome by a ~ 10 μm porous La0.58Sr0.4Co0.2Fe0.8O3-δ layers.
Finally, Chapter 8 reflects the overall findings, and give perspectives for future research. It is reflected that the membranes with a high iron cobalt spinel content suffer from low mechanical strength due to the existence of microcracks and residual tensile stresses. A crack-free microstructure with only small surface defects and small grain size is obtained for 85 wt% Ce0.8Gd0.2O2-δ-15 wt% FeCo2O4 membrane using the optimized powder preparation procedure and sintering profile. This microstructure exhibits significantly improved strength and oxygen permeation. Future improvements in oxygen permeation without compromising mechanical strength can be realized by further reducing the grain size without inducing microstructural defects like microcracks and pore agglomerates. It is recommended to use nano-sized powder mixtures and/or apply a two-step sintering profile. Besides, it is necessary to investigate the mechanical stability of the membrane after a long-term operation.