Article,
Controlling Crystallization and Sintering of α-Al2O3–YAG Fibers by Atmospheric Conditions
Journal of the American Ceramic Society, 94 (5): 1563--1567 (2011)
DOI: 10.1111/j.1551-2916.2010.04253.x
Abstract
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°C in nitrogen (“N2500”), water vapor (“H2O500”), or an atmosphere that changed from evaporated nitric acid (up to 300°C) to water vapor (“HNO3300·H2O500”). All other synthesis parameters were unchanged. Residues of carbon or nitrogen in the oxide composition were analyzed. Nitrogen sorption and 27Al 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°–1700°C. A high nucleation temperature and a vermicular morphology of large corundum grains were observed for the samples N2500 and HNO3300·H2O500. These samples could not be densified completely. Corundum crystallization at a lower temperature, and the formation of small compact grains was found for fibers H2O500, 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.
