Fabrication of wavy type porous triple-layer SC-SOFC via in-situ observation of curvature evolution during co-sintering

Wavy type Single Chamber Solid Oxide Fuel Cells (SC-SOFCs) have been shown to be conducive to improving the effective electrochemical reaction area contributing to higher performance, compared with planar type SC-SOFCs of the same diameter. This study presents a fabrication process for wavy type SC-SOFCs with a single fabrication step via co-sintering of a triple-layer structure. The monitoring and observation of the curvature evolution of bi- and triple-layer structures during co-sintering has resulted in an improved process with reduced manufacturing time and effort, as regards the co-sintering process for multi-layer structures. Investigation using in-situ monitoring helps different shrinkage behaviours of each porous layer to minimise mismatched stresses along with avoidance of severe warping and cracking. In the co-sintering of the multi-layer structures, the induced in-plane stresses contribute to curvature evolution in the structure, which can be utilised in the design of a curved multi-layer structure via the co-sintering process. For intermediate temperature SOFCs, the materials used are NiO/CGO for anode; CGO for electrolyte; and LSCF for cathode. These materials are tape-casted with 20µm thickness and assembled for bi- and triple-layer structures by hot pressing. Sintering mismatch stresses have been analysed in bi-layer structures, consisting of NiO/CGO-CGO and CGO-LSCF. The maximum sintering mismatch stress was calculated at interface of bi-layer structure in the top layer. In order to achieve the desired wavy type triple-layer structure, flexible green layers of each component were stacked up and laid on alumina rods to support the curvature during the process. In-situ observation, to monitor the shrinkage of each material and the curvature evolution of the structures, was performed using a long focus microscope (Infinity K-2). With these values, the main factors such as viscosity, shrinkage rate of each material, and curvature rate are investigated to determine the sintering mismatch stresses. This enables the prediction of curvature for triple-layer structure and the prediction is validated by in-situ monitoring of the triple-layer structure co-sintering process. Zero-deflection condition is confirmed to maintain initial shape during co-sintering and helps to minimise the development of undesired curvature in the triple-layer structure. Performance testing of the wavy cell was carried out in a methane-air mixture (CH4:O2 =1:1). In comparison with a planar SC-SOFC, it showed higher OCV which might be attributed to not only macro deformation, but also microstructural distribution affecting the effective gas diffusion paths and electrochemical active sites

Ferroelectric domains in barium titanate by Bragg coherent X-ray diffraction imaging

My PhD work focused on studying the domain structures and the strain fields inside barium titanate (BaTiO3) nanocrystals. The results on the domain structure study have already been published. The results on the stripe-like strain fields inside nanocrystals are finalized and there is a plan for publication. The first question my PhD work wants to address is what the domain structures inside BTO nanoparticles exist and how they evolve with temperature and when crossing the phase transition. Bragg coherent X-ray diffraction imaging (BCDI) experiments on nominal 200 nm size BTO nanoparticles were carried out at the Diamond I13-1 beamline and the Advanced Photon Source 34-ID-C beamline. The 90° domain walls were tracked in detail when crossing the tetragonal-cubic phase transition. This is presented in Chapter 3. Upon studying the domain structure inside BTO nanocrystals, some unexpected stripe-like strain fields were found. Crystals with clear facets were chosen to restore resolve the crystallographic direction, after which the strain field direction and periodicity were studied in detail. This is shown in Chapter 4. To understand the temperature dependence of the strain stripes, in-situ BCDI experiments were done at ESRF ID-01 beamline. Faceted BTO nanocrystals were chosen for temperature study. The strain stripes were found to be stable and preserved at both tetragonal and cubic phase with at elevated temperatures. This is illustrated in Chapter 5. The Finite element analysis (FEA) approach was utilized to understand the origins of the strain stripes. Different piezoelectric blocks were defined to simulate the domain structures inside a BTO crystal. 180° domain walls were found to give more strain stripes features than 90° domain walls in the simulation. This is covered in Chapter 6. The same patch of BTO nanocrystals were also studied using an X-ray Free-electron Laser as a function of time delay after laser excitation. Rather than seeing any significant thermal expansion effects, the diffraction peaks were found to move perpendicular to the momentum transfer direction. This suggests a laser driven rotation of the crystal lattice, which is delayed by the aggregated state of the crystals. Internal deformations associated with crystal contacts were also observed. These are shown in Chapter 7