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This dissertation research focuses on two related technical subjects that are important to the successful development of fiber optic sensors (FOSs) for applications in fossil fuel and biomass based power plants. The first is the development of multilayer FOS, which consists of a chemically inert and thermally stable nanoporous silicalite overcoat as a protective layer and a gas sensitive doped-ceramic thin film coated on a fiber optic device. The effects of the individual layers on the multilayer FOS performance are investigated and the FOSs have been tested for high temperature gas detection in syngas from coal gasification. The second is the development of ceramic thin films as sapphire fiber claddings to enhance the optical waveguide efficiency for extreme temperature (>1000 oC) FOS development. The multilayer FOSs fabricated in this work consisted of a silicalite protective overcoat and a proton-conducting perovskite-type oxide (SrCe0.8Zr0.1Y0.1O2.95, SCZY) hydrogen sensing film coated on a long-period fiber grating (SCZY-LPFG). The H2 sensitivity of the SCZY-LPFG was found to increase with the thickness of the SCZY film with and without the silicalite protective layer. The sensitivity of the multilayer sensor was found to decrease with increasing the thickness of the protective silicalite overcoat. The experimental observations have been discussed and explained based on the LPFG cladding models. The silicalite layer is expected to effectively protect the SCZY sensing film from the airborne fine ash (mineral) particles and large contaminating molecules. The zeolite film coated SCZY-LPFG (Z-SCZY-LPFG) H2-sensor was successfully tested for online monitoring of H2 concentration in real syngas streams from coal gasification with accuracy comparable to gas chromatographer (GC).In this work, spinel MgAl2O4 film has been investigated as sapphire optical fiber cladding. The spinel MgAl2O4 is a well-known refractory ceramic material with a high melting point of ~2135 oC, inertness in basic and acidic environments, thermal expansion coefficient and refractive index close to sapphire. These basic properties of the spinel are desirable for constructing cladding layers for sapphire optical waveguides. Moreover, the MgO-Al2O3 binary phase diagram contains a unique phase equilibrium regime between spinel MgAl2O4 and sapphire (alpha-Al2O3) where a spinel MgAl2O4 phase with RMgO/Al2O3 of 1.0 can be chemically stable with the sapphire phase at temperatures up to 1250oC. Spinel MgAl2O3 thin films have been successfully coated on the 75-um-diameter single crystalline sapphire fiber and sapphire wafer at relatively low temperature |
