Date of Award

5-1-1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics & Astrophysics

Abstract

The aggregation of alumina particles, created by hydrolysis of aluminum nitrate solutions with sodium carbonate, was studied by small-angle x-ray scattering (SAXS) techniques. The OH/Al ion ratios for the solutions ranged from 1.5 to 2.15. The hydrolysis process leads to formation of primary alumina subunits which rapidly aggregate to form mass fractal structures. The SAXS measurements, made at various stages of the aging process, showed that in addition to fractal aggregation, there was also a second process active in the solutions, which led to a break-up of the fractal structure. The scattering results were analyzed in terms of a modified Fisher-Burford Equation to obtain information regarding the aggregation process and the break-up of the aggregation.The fractal dimension of the aggregates varied from 1.7 to 2.2 depending on the OH/Al ion ratio of the solution. The maximum radius of gyration for the aggregates of about 50 A was found in solutions having the highest OH/Al ion ratio. A rate equation to describe the kinetics of aggregation was discussed.The second process in the solutions was the formation of non-fractal particles which were probably Al$\sb{13}$ ions, (AlO$\sb4$ Al$\sb{12}$(OH)$\sb{24}$(OH$\sb2$)$\sb{12}$) $\sp{7+}.$ This process which was strongly temperature dependent, required six months to reach the final equilibrium state at 23$\sp\circ$C and less than two hours at 70$\sp\circ$C. Rate constants for the process were determined for three different aging temperatures. The activation energy governing the decay was approximately 50 KCalories/mole. The observed scattering cross section for the Al$\sb{13}$ ion had a broad maximum at a non-zero scattering angle. This feature is consistent with the expected scattering from a particle having an adsorbed surface layer with electron density less than that of the solvent. Evidence was presented indicating that the outer layer may be a monolayer of water molecules.

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