William C. Ackerman

Preparation of low-density xerogels at ambient pressure

Abstract For successful large-scale commercialization of aerogel technology to occur, aerogel costs must be dramatically reduced. Using a previously developed model for predicting gel shrinkage during drying, a number of approaches for producing aerogels under ambient pressure conditions are presented. The extent of drying shrinkage is governed by two parameters, the dimensionless group, P = Asγ cos(θ)mρ0/K0, which represents the relative magnitudes of capillary pressure and gel stiffness, and the parameter, m, which describes the stiffness variation with density. For P less than 1, the density increase upon drying is less than 10%. Necessary steps to produce aerogels which are commercially viable include: (1) the use of low-cost precursors such as sodium silicate; (2) the production of aerogel granules to greatly reduce diffusion and heat transfer-related process timescales; (3) development of a continuous aerogel process technology which employs a minimum of solvents (number and volume), does not require expensive high-pressure supercritical or surface modification techniques, which has short process times at each step and which is built upon existing unit operations. If possible, structural relaxation after the drying critical point should be enabled to yield lower density materials for a given P value.

Adsorption Studies Of Pure And Modified Imogolite As A Potential Pore Size Standard.

Abstract Imogolite is a microporous tubular aluminosilicate having the unique property of forming tubes of a single length and fixed inner diameter. With proper processing, the tubes can self assemble into aligned, densely-packed arrays exhibiting a high degree of microporosity orientated in a single dimension. These tubes have a nominal inner diameter of 0.8 nm but the diameter may be increased by replacing a portion of the silicon with germanium, or chemically modified via silylation reactions with the active silanol groups on the internal tube surface. These materials are attractive as pore size standards since they form continuous tubes (not throats and cavities typical of a zeolite), the pores are uniform in size, and the pore size may be varied or modified in a controlled fashion. We discuss the characterization of imogolite tube bundles by a range of adsorption experiments including nitrogen and carbon dioxide and by 129xe NMR. Gas phase silylation experiments are also explored in relation to changes in surface area, pore size, size distribution, total pore volume, and adsorption kinetics.

The Mechanism of Haze and Defectivity Reduction in a New Generation of High Performance Silicon Final Polishing Slurries

The mechanism of haze reduction during silicon polishing using a new generation of additives has been explored. These additives are thought to decrease haze by adsorbing to the wafer surface and increasing the activation energy of the reaction between the silanolates on the silica particle surface with the surface silicon. This leads to greater selectivity between the peaks and valleys resulting in a net decrease in surface roughness.