Thermo-mechanical investigations of reoxidation stable material concepts for solid oxide fuel cells

Vasechko, Viacheslav; Singheiser, Lorenz (Thesis advisor)

Aachen (2014)
Dissertation / PhD Thesis

Abstract

The solid oxide fuel cell (SOFC) design concept has a strong impact on the chemical and mechanical stability of the actual ceramic cell. The mechanical loads induced by static and transient operation of the device are mainly sustained by the cell substrate. Two means of implementation of the mechanical support design have received considerable attention in recent years. The first route is to increase the thickness of one of the core layers (anode or electrolyte or cathode), whereas the second one is based on the deposition of the functional layers on an inert, for example, metallic substrate. Hence, one of the aims of this current work is to compare metal-supported and anode-supported SOFC concepts focusing on planar designs. A novel materials’ configuration of the anode-supported fuel cell is mechanically analyzed, where anode and anode substrate are produced using pure ceramic materials instead of the typically-used Ni-8YSZ composite, i.e. is a full-ceramic. The metal-supported fuel cell is produced using the graded anode structure (anode active layer and anode intermediate layer) deposited on the porous ITM alloy substrate. The concepts are compared via FEM simulations performed with the ANSYS 14.0 Workbench. In an initial step the mechanical properties necessary for the simulations are assessed from a concise literature review. An additional outcome of the review is the development of models to predict the mechanical data of, for example, Ni-8YSZ composite as a function of porosity and Ni content. The missing data for the novel materials (Y-doped strontium titanate as anode active layer and anode substrate material for the full-ceramic concept and porous ITM alloy for the metal-supported concept) are determined via thermomechanical testing, where characterizations are performed from room temperature up to SOFC operation-relevant (annähernd 800ºC) temperatures. The thermomechanical testing of the SYT material includes the determination of Young’s modulus, characteristic strength, Poisson’s ratio, thermal expansion coefficient and creep rate measurements. Additionally, the chemical stability of the Y-doped strontium titanate is investigated using differential thermal analysis and X-ray diffraction indicating that the mechanical behavior is affected by a phase transition at elevated temperatures. Interestingly, dislocation slip bands are observed during the microindentation tests in grains exceeding a discrete size. Finally, material defects are investigated with light, stereo, confocal and scanning electron microscopes to derive some hints for the materials’ production optimization. In addition an attempt to enhance the strength of the SYT material is performed. Equal volumes of pure SYT and 3YSZ ceramic are mixed. The subsequent mechanical characterization of the sintered composite is performed at room temperature with the associated fractographic analysis. Porous and dense ITM material grades were investigated in terms of Young’s modulus, ultimate tensile strength, Poisson’s ratio (also for the dense ITM), thermal expansion coefficient and creep rate. A ferromagnetic-paramagnetic transition influences the mechanical properties (Young’s modulus, TEC, Poisson’s ratio). The Poisson’s ratio is affected by the rolling direction similar as already reported previously for the ultimate tensile strength and yield stress of dense ITM. The creep measurements focusing on the porous ITM verify a high stress-sensitivity that has also been reported for the dense ITM before. An explanation of the creep mechanism still needs further investigations.

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