Positron annihilation techniques have been applied to study the pore diameters and volumes of 11 crystalline and amorphous zeolites. The data, consisting of the number of counts per channel recorded on a multichannel analyzer, were analyzed by means of the POSITRONFIT program. The data were presented as four lifetimes, T, to T~, with their associated intensities, I , to I,. The pore volumes reported here were based on a quantum mechanical model. The derived volumes are in reasonable agreement with pore volumes obtained with X-ray and other data. The intensity of our fourth positron lifetime correlates with Brunauer-EmmettTeller measurements of the surface area of the powders. These results are consistent with two studies by other authors.

Dynamic oscillatory experiments are used to monitor the gelation of the borosilicate systems prepared through the sol-gel process from metal alkoxides. The rheological experiments show that tan δ = G”/G’ is independent of frequency at the gel point in agreement with the results of others on organic gelling systems. The dynamic moduli at the gel point followed power-law behavior with respect to frequency. The power-law exponent is found to be ∼0.70. The apparent fractal dimension, dp, of the network cluster at the gel point is determined. The dF values for the samples ranged from 2.5 to 3.8 depending on the final structure of the evolved products at the gel point. The large values (dF > 3) exclude a simple geometric interpretation of the results. The effect of processing parameters, such as composition of reactants and temperature, on the resulting microstructures near the gel point is discussed.

Microwave processing was carried out on SiO2 -B2O3 solutions and gels prepared by sol-gel methods. Monolithic gels were prepared from alcoholic solutions of trimethylborate and tetraethylorthosilicate using a two-step hydrolysis process. A novel technique of Liquid State Processing (LSP) was employed for the first time, and it was found to be faster and more effective than the conventional processing techniques. The structural evolution of the dried products was followed using FTIR. The effect of processing was examined via surface area analysis (BET), electron microscopy, and FTIR. The microwave drying has been compared with conventional oven drying and vacuum drying techniques. Shorter processing times, improved microstructures, and unique properties have been obtained.