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Controlling crystallization and sintering of alpha-Al2O3-YAG fibers by atmospheric conditions

: Krüger, R.; Löbmann, P.


Journal of the American Ceramic Society 94 (2011), Nr.5, S.1563-1567
ISSN: 0002-7820
Fraunhofer ISC ()
Fasern; Sintern; Werkstoff

Amorphous sol-gel-derived fibers of 82 mol% Al2O3-18 mol% Y2O3 were synthesized for structural and sintering investigations. Three sets of samples were fabricated by treatment up to 500 degrees C in nitrogen ("N(2)500''), water vapor ("H(2)O500''), or an atmosphere that changed from evaporated nitric acid up to 300 degrees C) to water vapor ("HNO(3)300 center dot H(2)O500''). All other synthesis parameters were unchanged. Residues of carbon or nitrogen in the oxide composition were analyzed. Nitrogen sorption and Al-27 magic angle spinning nuclear magnetic resonance experiments were performed to investigate porosity and Al ion coordination of the xerogel fibers. Crystallization and densification of these pretreated samples were characterized using differential thermal analysis/TG and SEM at 1300 degrees-1700 degrees C. A high nucleation temperature and a vermicular morphology of large corundum grains were observed for the samples N(2)500 and HNO(3)300 center dot H(2)O500. These samples could not be densified completely. Corundum crystallization at a lower temperature, and the formation of small compact grains was found for fibers H(2)O500, which could be sintered to full density. The individual crystallization and sintering behaviors are ascribed to the respective structures in the amorphous intermediate products. The amount of mesopores, internal surface area, and the Al coordination are regarded as the main controlling factors. A low content of residual carbon alone is not sufficient for good sintering results.