Shiho Wang

Drying of alkoxide gels?Observation of an alternate phenomenology: Code: H6

The process of drying of a porous material as per the current phenomenological theory can be divided into two stages. At first the body shrinks by an amount equal to the volume of liquid that evaporates, and the liquid-vapor interface remains at the exterior surface of the body. The second stage begins when the body becomes too stiff to shrink and the liquid recedes into the interior, leaving air filled pores near the surface. We shall refer to this phenomenology as the drying front model. In our investigation of drying of alkoxide silica gels of less than 50 Angstroms pore radius, we have observed a different drying pattern, in which even after the gel body stops shrinking, drying continues to occur by evaporation on the exterior surface of the gel body, causing spontaneous nucleation of partially or fully dried opaque clusters, randomly distributed in the interior parts of the gel. These clusters than increase in number and size till they coalesce to form an opaque body. Upon further drying, the gel returns to its transparent form. We postulate that this is possible only if the rate of fluid flow in the pores by diffusion is faster than that by Darcys flow, as well as the evaporation rate at the surface of the gel body. We shall refer to this as the cluster drying model. We shall present results of pin-hole drying experiments on cylindrical alkoxide gels showing that for identical gels the evaporation rate can be increased to change the phenomenology from cluster drying to one that exhibits both phenomenology simultaneously and finally to that of the drying front phenomenology. We shall also show the effect of gel pore size distribution on the phenomenology of drying under identical drying conditions. Finally, we will present evidence that for successful drying of large cylindrical alkoxide gels, drying conditions favoring cluster drying phenomenology is desirable.

Accelerated subcritical drying of large alkoxide silica gels

Fracture during drying has been the key hurdle in fabrication of large monolithic silica glass from alkoxide gels. Although existing literature suggests pore enlargement, aging, chemical additives, supercritical drying and freeze drying as helpful in avoiding fracture during drying, successful accelerated sub-critical drying of large silica monoliths from alkoxide gels has not yet been reported. In the present approach, acid catalyzed sols of TEOS, ethanol and water (pH equals 2) were cast as cylindrical rods in plastic molds of 8.0 and 10.0 cm diameter with volumes of 2000 cc and 3000 cc respectively. The resultant gels were aged for about 7 days and dried in a specially designed chamber under sub-critical conditions of the pore field. We have obtained monolithic dry gels in drying times of 3 - 7 days for sizes of 2000 - 3000 cc. The dry gels have narrow unimodal pore size distributions, with average pore radius of about 20 angstroms as measured by BET. Although capillary stress during drying increases with reduction of pore size, it was found that in this approach it is easier to dry gels of smaller pore size.

Silica Glass Monoliths from Alkoxide Gels; An Old Game With New Results,

Cylindrical monolithic silica dry gel bodies, up to 250 grams in weight, are routinely produced from alkoxide sols free of colloidal particles, using tetraethylorthosilicate precursor. These dry gel bodies, after sintering in controlled chlorinated atmosphere, yielded clear, bubble free, dense glass rods 3.0-4.0 cm in diameter and 1 5-20 cm in length. Uv-vis--ir spectra, of properly dehydrated and sintered glass samples tested from 1 80 nm to 3200 urn, showed no detectable absorption peak at 2700 nm and a UV band edge at around 185 nm; implying that resultant glasses are reasonably free of impurities and hydroxyl ions. Effect of sol composition on gel ultrastructure was carefully investigated. It was found that by careful choice of sol composition, type and amount of catalyst and aging conditions, it is possible to tailor the gel ultrastructure for ease of drying. For example, we have been able to produce gels with unimodal pore distributions, and average pore sizes in the range from 8 to 300 A and surface area in the range from 150 - 1100 m2/gram. As a result of this ability to tailor gel ultrastructure, including pore size, bulk density and skeletal density of the gel bodies, we have been able to optimize gel ultrastructure to maximize its strength, so that it can withstand capillary forces generated during the drying processes. The result of this preliminary investigation has led us to believe that high quality fused silica glass of much larger sizes can be produced by the alkoxide route; and experiments to scale up the process is under way. Up to date results of this investigation will be presented at the conference.