| Solid Oxide Fuel Cell (SOFC) is an emerging technology that generates clean electrical power from fossil or hydrocarbon fuels for various kinds of applications, such as portable devices, transportations, distributed power plants and centralized power stations. Comparing with other kinds of designs for SOFC, the planar SOFC is more attractive due to its low manufacturing cost and high power density, which hasthe most promising future for commercial applications.The state-of-the-art anode material for planar anode-supported SOFC is Ni-Y2O3doped ZrO2(YSZ) cermet which is usually converted from the form of NiO-YSZ during the process of stack start-up in reducing atmosphere. However, the substrate of the cell is a ceramic composite of NiO-YSZ rather than the cermet of Ni-YSZ when the cells are assembled into a stack. Similar to other ceramics, NiO-YSZ composite is intrinsically brittle and susceptible to fracture. Thus, as an anode support it is facing a challenge of crack or fracture during stack assembling under a compressive load, leading to mixing of fuel and oxidant gases and in turn the failure of stack performance.In order to overcome the above problem of NiO-YSZ composite as an anode-supported material, this dissertation is aiming at the increase in fracture toughness for NiO-YSZ ceramic composite by adding LaNbO4, alleviating cell fracture during stack assembly at ambient temperature. Because LaNbO4is a ferroelastic material, and its monoclinic polymorph at room temperature is crystallographically oriented in two domains that are separated by a highly mobile boundary located at a lattice plane belong to the family of{204}Ⅰ/{402}Ⅱ. Under an applied load these two domains can switch, which is a possible toughening mechanism for ceramics. The research works in this dissertation include the following parts described as:synthesis and characterization of the LaNbO4power, the chemical compatibility between LaNbO4and NiO-YSZ, the mechanical properties of LaNbO4-NiO-YSZ composite, the domain switch toughening mechanism of LaNbO4in ceramic composite, evaluation and optimization of porous LaNbO4-NiO-YSZ as the anode support from mechanical properties, electrical conductivity and thermal expansion behavior. The major results obtained are described as follow:1) The calcined LaNbO4synthesized by solid-state reaction is subsequently ball-milled for48h to obtain a good distribution LaNbO4powder with about1.5μm average particle size. 2) LaNbO4, NiO and YSZ are uniformed distributed in the LaNbO4-NiO-YSZ composites, and the microstructure is refined with the increase of LaNbO4content. They are chemically stable and compatible without forming new phases by sintering at1500℃in air for6h.3) Sintering properties of LaNbO4-NiO-YSZ are not sensitive to the addition of LaNbO4. Despite that LaNbO4in LaNbO4-NiO-YSZ composites decreases their Vickers hardness and elastic modulus, the fracture strength of the composites remains unchanged with increasing LaNbO4content, depending on the randomly appeared flaw size in the specimens.4) With LaNbO4addition into the NiO-YSZ, fracture toughness KIC increases monotonously. For30wt%LaNbO4-NiO-YSZ, its Kic is3.1MPa m1/2, which is42%increase compared with that of the NiO-YSZ. The increase in fracture toughness is due to the overall grain refinement of the composite, crack deflection caused by fracture mode transition and the domain switch within monoclinic LaNbO4.5) The morphology change of LaNbO4domain structure under load in LaNbO4-NiO-YSZ composite is observed in-situ by using a TEM with a straining stage for the first time. The driving force for the domain switch under load is the difference of Gibbs free energy between two domains with different crystallographic orientations. This domain switch shields the propagating cracks by dissipating the stress build-up at the crack tip, resulting in toughness improvement of the composite.6) Comparing with conventional NiO-YSZ, porous LaNbO4-NiO-YSZ is regarded as an anode-supported material with better mechanical properties during stack assembling, compatible thermal expansion coefficient and suitably high electrical conductivity. Its coefficient of thermal expansion (CTE) before and after reduction decreases with addition of LaNbO4; which is closely matched with that of YSZ electrolyte. The electrical conductivity of LaNbO4-Ni-YSZ depends on its Ni content and shows metallic conducting behavior. For the purpose of using LaNbO4-NiO-YSZ cermet as the anode support, its electrical conductivity is higher than456.17S cm-1at temperatures below900℃after optimization of ratio of NiO, YSZ and LaNbO4content. |
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