Deformation behaviour and leakage tests of alternate sealing materials for SOFC stacks

Compression seals based on metal/mica composites were investigated under solid oxide fuel cell (SOFC) operating conditions. Since a compressive force has to be permanently applied in this sealing concept, the mechanical properties of the sealing materials and the SOFC components also became important for maintaining gas tightness. Leakage tests with a sandwich arrangement of mica paper and embossed metallic profiles show leak rates of <1 x 10(-4) mbar l/mm s under an applied normalized compressive load of 2.7 N/mm (0.7 MPa). Also under thermal cycles between ambient temperature and 800degreesC this leak rate does not change due to the elastic response of the seal. Compression tests with the seals in contact with SOFC membranes reveal no damage to the membranes. First results of leakage tests with SOFC stacks are presented, which may serve as a reference for future leakage tests of alternative sealing systems. (C) 2004 Elsevier B.V. All rights reserved

Deep and Shallow TiO2 Gap States on Cleaved Anatase Single Crystal 101 Surfaces, Nanocrystalline Anatase Films, and ALD Titania Ante and Post Annealing

Using photoelectron spectroscopy (PES), deep (around 1.2 eV below Fermi level) and shallow (around 0.2 eV below Fermi level) gap states are investigated in differently prepared TiO2 samples: In situ cleaved single anatase crystal TiO2 (101) surface, sintered slurry of nanocrystalline anatase, amorphous atomic layer deposited (ALD) titania, and nanocrystalline anatase transformed by annealing from ALD titania. Deep gap states are generally attributed to under-coordinated Ti atoms due to oxygen defects. The origin of shallow gap states is unclear. PES on in situ cleaved anatase TiO2 (101) surfaces show in part no or weak emission from deep, but always weak emission from shallow gap states. Amorphous ALD titania initially is free of gap states, but deep gap states are easily induced by exposure to synchrotron radiation, while shallow gap states do not form. Exposure to synchrotron radiation also induced deep gap states in in situ cleaved single crystal (101) surfaces and in the nanoporous anatase films, whereas emission from shallow gap states stays constant. Amorphous ALD films transform to anatase nanocrystals by annealing as shown by XRD and AFM and show deep and shallow gap state emissions similar to the nanoporous anatase films prepared by sintering spin coated anatase slurry. Atomic force microscopy (AFM) reveals the presence of terraces on the cleaved (101) surfaces. The comparison of the data collected on the differently prepared anatase and amorphous titania samples suggest assigning shallow gap states to under-coordinated Ti atoms located at edge sites. Conformal amorphous films that show no edges in AFM, accordingly have no shallow gap states