Browsing by Author "Callender, Rhonda Lynn"

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    Advanced ceramic composites and coatings via alumoxane nanoparticles
    (1999) Callender, Rhonda Lynn; Barron, Andrew R.
    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.
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    Chemical control over ceramic porosity using carboxylate-alumoxanes
    (2005-08-30) Barron, Andrew R.; Bailey, Diane Amy; Wiesner, Mark Robert; Jones, Christopher Daniel; Callender, Rhonda Lynn; Rice University; United States Patent and Trademark Office
    This invention relates generally to a method for controlling the pore size, pore size distribution and porosity of aluminum-oxide based ceramics through the choice of substituents on carboxylate-alumoxanes and aluminum-oxide nanoparticles. The method allows for the formation of intra-granular pores in the nanometer range to be created in alumina and aluminum oxide ceramic bodies. The control over pore size and pore size distribution is accomplished through the use of different chemical substituents on the carboxylate-alumoxanes and aluminum-oxide nanoparticles. The size and distribution of pores within the alumina-oxide ceramic are dependent on the identity of the carboxylate substituents. In particular the formation of intra-versus inter-granular porosity is dependent on the identity of the carboxylate substituents. The invention also provides methods for the manufacture of ceramic coatings on ceramic and carbon fibers for composite applications and ceramic membranes with nanometer sized pores. The pore size, pore size distribution and porosity, and hence the strength, permeability and surface adhesion, of the ceramic coating is controlled by the choice of substituent on the carboxylate-alumoxane. Thermolysis of self supporting spun layers of the carboxylate-alumoxanes results in disks of alumina with controlled pore size, pore size distribution and porosity. In an alternative method a porous substrate is dipped or coated with a solution of the carboxylate-alumoxane, followed by thermolysis to produce a composite membrane.