Publications

The pore size r{sub p} in a gel is determined by the extent of shrinkage of the gel network during drying. Shrinkage is driven by the collapse of the gel network in response to the capillary pressure P{sub c} exerted by the pore fluid. The extent of shrinkage depends on the balance between the capillary pressure P{sub c} in the pore fluid and the bulk modulus K{sub p} of the gel. The hydraulic pore radius, r{sub H} = 2V{sub p}/S{sub a}, where V{sub p} is the pore volume and S{sub a} is the apparent N{sub 2} BET surface area, is often used to characterize the pore size of a gel. A series of acid catalyzed silica gels dried in pore fluids with different {gamma}{sub lv}, showed that there is a limit to the minimum apparent r{sub H} obtainable in a gel, and when the volume fraction of porosity {phi} {le} 0.37, r{sub H} becomes constant and {approximately}0.8 nm. In contrast, experimental data show that the true pore size r{sub p} of gels continues to decrease when {phi} {le} 0.37. Analysis of their adsorption isotherms show that while r{sub H} apparently stays constant: (a) the BET C constant continues to increase, (b) the width and average of their pore size distributions continue to decrease, and (c) as shrinkage continues the gels eventually become non-porous to N{sub 2} at 77K but are still porous to CO{sub 2} at 273K. This paper reviews these results and addresses micropore formation in silica gels with the goal of determining how P{sub c} influences the final r{sub p}, and why r{sub p} and r{sub H} diverge when {phi} {le} 0.37.

We have used several sol-gel strategies to prepare supported inorganic membranes by a process that combines the features of slip-casting and dip-coating. To be viable the deposited membranes must exhibit both high flux and high selectivity. For porous membranes these requirements are met by extremely thin, defect-free porous films exhibiting a narrow size distribution of very small pores. This paper considers the use of polymeric silica and hybrid-organosilyl precursor sols in the context of the underlying physics and chemistry of the membrane deposition process. Since the average membrane pore size is ultimately established by the collapse of the gel network upon drying, it is necessary to promote polymer interpenetration and collapse during membrane deposition in order to achieve the very small pore sizes necessary for molecular sieving. For polymeric sols, this is accomplished using rather weakly branched polymers characterized by fractal dimension D < 1.5 under deposition conditions in which the silica condensation rate is minimized. By analogy to organic polymer sols and gels, we believe that the breadth of the pore size distribution can be influenced by the occurrence of micro-phase separation during membrane deposition. Minimization of the condensation rate not only fosters polymer collapse but should inhibit phase separation, leading to a narrower pore size distribution. The formation of microporosity through collapse of the gel network requires that small pores are achieved at the expense of membrane porosity. Incorporation of organic template ligands within a dense silica matrix followed by their removal allows us to independently control pore size and pore volume through the size and volume fraction of the organic template. Such strategies can be used to create microporous films with large volume fraction porosities.

The molecular-scale species distributions and intermediate-scale structure of silicate sols influence the microstructures of the corresponding thin films prepared by dip-coating. Using multi-step hydrolysis procedures, the authors find that, depending on the sequence and timing of the successive steps, the species distributions (determined by {sup 29}Si NMR) and intermediate scale structure (determined by SAXS) can change remarkably for sols prepared with the same nominal composition. During film formation, these kinetic effects cause differences in the efficiency of packing of the silicate species, leading to thin film structures with different porosities.

The porosity of sol-gel thin films may be tailored for specific applications through control of the size and structure of inorganic polymers within the coating sol, the extent of polymer reaction and interpenetration during film formation, and the magnitude of the capillary pressure exerted during the final stage of drying. By maximizing the capillary pressure and avoiding excessive condensation, dense insulating films may be prepared as passivation layers on silicon substrates. Such films can exhibit excellent dielectric integrity, viz., low interface trap densities and insulating properties approaching those of thermally grown SiO{sub 2}. Alternatively, through exploitation of the scaling relationship of mass and density of fractal objects, silica films can be prepared that show a variation in porosity (7--29 %) and refractive index (1.42--1.31) desired for applications in sensors, membranes, and photonics.

Inorganic thin films with molecular sieving properties have been formed by embedding microcrystals of zeolite Y and chabazite in a glassy silica matrix. The silica matrix was derived from sols prepared from tetraethylorthosilicate hydrolyzed under either acidic or basic conditions in alcoholic solution. Dip-coating, deposition of suspensions, or coating of zeolite dispersions with the glassy silica matrix were used to create the zeolite-silica films. The access of different probe molecules into the zeolitic part of the thin film was examined with in situ FTIR techniques and temperature-programmed-desorption studies. With all combinations of deposition techniques and silica matrices, the resulting films showed the molecular sieving properties of the parent zeolite. © 1989, Elsevier Science & Technology. All rights reserved.

This report summarizes research on the aging of Class I components in environments representative of nuclear power plants. It discusses Class IE equipment used in nuclear power plants, typical environments encountered by Class IE components, and aging techniques used to qualify this equipment. General discussions of radiation chemistry of polymers and accelerated aging techniques are also included. Based on the inadequacies of present aging techniques for Class IE equipment, a proposal for an experimental program on electrical cables is presented. One of the main purposes of the proposed work is to obtain relevant data in two areas of particular concern--the effect of radiation dose rate on polymer degradation, and the importance of synergism for combined thermal and radiation environments. A new model that allows combined environment accelerated aging to be carried out is introduced, and it is shown how the experimental data to be generated can be used to test this model.