Atmospheric control of gel-oxide transformation in sol–gel derived Al2O3-Y2O3 fibers
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Via sol–gel processing metal–organic fibers were produced and dried up to 140 °C. For these gel fibers the influence of a treatment in different atmospheres was investigated for the temperature range of 200–850 °C. The atmospheres were nitrogen, water vapor, evaporated nitric and hydrochloric acid and evaporated hydrogen peroxide. In the presence of moisture and especially with acidic moisture fibers were transformed almost completely to their oxide composition (82 mol% Al2O3·18 mol% Y2O3). In these inorganic amorphous structures considerable differences were observed on several structural levels. On the atomic scale, the coordination of Al ions was investigated by 27Al MAS NMR and skeletal density by He-pycnometry. Porosity in the nm scale was characterized by N2-sorption. As a macroscopic effect of different treatment atmospheres, the longitudinal shrinkage was observed. For fibers treated at 500 °C the relative shrinkage varied by 100% (comparing water vapor and nitrogen atmosphere). No simple correlation between the release of organic constituents, the formation of porosity and the shrinkage could be found. These aspects were controlled by the rigidity of the inorganic network against atomic reconstitution. The kind of atmosphere was found to be an effective parameter to control various aspects of the xerogel structure.
KeywordsSol–gel fibers Pyrolysis Densification Al2O3-Y2O3
The authors are indebted to Dr. Rüdiger Bertermann (Institute of Inorganic Chemistry, University of Würzburg) for performing 27Al-MAS-NMR measurements and to Mr. Oliver Schüßler (Fraunhofer ISC) for synthesis of the sols.
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