Advanced ceramic composites and coatings via alumoxane nanoparticles
Callender, Rhonda Lynn
Barron, Andrew R.
Doctor of Philosophy thesis
The objective of this research is the development of an environmentally-benign process for the synthesis and fabrication of alumina-based ceramic precursors and advanced ceramic materials. Carboxylate-alumoxanes, [Al(O)x(OH) y(O2CR)z]n, were synthesized by the reaction of boehmite, [Al(O)(OH)]n, with acetic acid (HO2 CCH3), methoxyacetic acid (HO2CCH2OCH 3), methoxyethoxyacetic acid (HO2CCH2OCH 2CH2OCH3) and methoxyethoxyethoxyacetic acid [HO 2CCH2(OCH2CH2)2OCH 3]. Carboxylate-alumoxanes can be considered inorganic-organic hybrid materials consisting of an aluminum-oxygen backbone with carboxylate substituents. These are infinitely stable at ambient conditions in solid and solution. In addition, they show no propensity to segregation or polymerization and are readily processed in aqueous or hydrocarbon medium. Upon thermolysis the carboxylate-alumoxanes are converted to alumina. The physical and spectroscopic properties of the carboxylate-alumoxanes have been determined. The potential environmental impact of the new alumoxane methodology will be discussed. Carboxylate-alumoxanes are reacted with metal acetylacetonate complexes, M(acac)n, to form metal-doped nanoparticles and aluminum acetylacetonate via a transmetalation reaction. This allows the facile formation of highly crystalline materials such as calcium hexaluminate (CaAl12O 19, hibonite) and lanthanum hexaluminate. The formation of highly phase pure materials is proposed to be due to the presence of atomic scale mixing within the metal doped alumina nanoparticle structure of the carboxylate-alumoxane. The potential of the carboxylate alumoxanes as interlayer coatings in ceramic matrix composites (CMCs) was investigated. Sapphire, SiC, carbon, and KevlarRTM fibers and carbon/KevlarRTM fabric have been dip-coated by aqueous and CHCl3 solutions of carboxylate-alumoxane nanoparticles and fired to 1400°C to form uniform alumina and aluminate coatings. Optimum solvent, dip/dry, and firing sequences were determined for the formation of crack-free coatings. Coatings produced were stable to thermal cycling under air at temperatures of 1400°C. The ability of the carboxylate-alumoxanes to provide crack infiltration and repair of damaged coatings is demonstrated.