Investigation of structure development in disordered materials: Aggregation, gelation and sintering of colloidal iron and titanium oxide.

摘要

The microstructure of colloidal-based disordered materials has been investigated. The primary objective was to study the processes involved in the formation and evolution of structure in these systems, and to draw some inferences regarding the relationship between structure and material properties. Studies focus on colloidal hydrous oxides of iron and titanium, and the critical processes of aggregation, xerogel formation and interparticle sintering of gels. The results of three distinct studies are summarized below. The effects of short-range van der Waals and electrostatic forces on the growth and structure of {dollar}alpha{dollar}-FeOOH (goethite) aggregates were studied using dynamic and static light scattering. Electrolyte-induced aggregation could be defined in terms of reaction or diffusion-limited kinetics. These regimes exist below and above the critical coagulation concentration, respectively, as determined by stability measurements. Scattering exponents reflect long-range density correlations consistent with a fractal structure for the aggregates. Anisotropic association is postured as a model to explain variance of the measured fractal dimension from predicted values.; Novel unsupported ceramic membranes derived from goethite and hydrolytic iron oxide polymer have been prepared by the sol-gel technique. These materials are a preliminary step in the fabrication of supported membranes for chemical separations and gas-phase catalytic reactors. Dehydration of the initial solid phase during firing results in a transformation to {dollar}alpha{dollar}-{dollar}rm Fesb2Osb3{dollar} (hematite). Both mesoporous and microporous membranes have been formed. Mean pore diameters less than 2nm have been achieved. These materials were characterized by nitrogen adsorption, fourier transform infrared spectroscopy and electron microscopy.; Small-angle x-ray scattering (SAXS) has been used to investigate the microstructure evolution during firing of sol-gel derived nanophase titania xerogels. These materials are microporous with an initial primary grain size of about 5nm. The interpretation of SAXS data for dense porous systems is detailed. Direct analysis methods and microstructural scattering models are applied. SAXS curves were found to exhibit a pronounced maximum at firing temperatures below 550{dollar}spcirc{dollar}C, indicating the presence of strong spatial correlations. Scattering results are compared to electron microscopy and nitrogen adsorption methods.