Sol-Gel Film Formation by Dip Coating
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SOL-GEL FILM FORMATION BY DIP COATING ALAN J. HURD and C. JEFFREY BRINKER Sandia National Laboratories, Albuquerque,
NM
87185-5800
ABSTRACT The physical aspects of sol-gel film formation are discussed, including the steady state film profile during dip coating, evaporation, and capillary phenomena. It is argued that, since the evaporation rate increases singularly near a sharp boundary (analogous to an electric field singularity near a sharp conductor), the film profile near the drying line falls off precipitously, following the inverse form of the evaporation singularity. Finally, the large tensile pressures in the solvent during the final stage of drying of a porous film are discussed from the point of view of controlling the degree of capillary collapse.
INTRODUCTION Manufactured coatings are fundamental to our Information Age Society. From Wall Street to the local library, much of the world's production of new wealth and wisdom is necessarily archived on magnetic, optical, or photographic coatings. Expanding markets can be expected for coatings on optics, integrated circuits, microsensors (including bioactive layers), and separation membranes. To track the demand of these and other future applications, the materials and processing of coatings deserve exploration. Sol-gel films, belonging to the broad class of coatings applied via a liquid carrier, have enjoyed recent attention. Although they have been studied since WWII [1], their use has not been widespread in spite of potential advantages, not the least of which is simplicity. Whereas deposition from the gas phase requires expensive vacuum equipment, sol-gel coatings can usually be applied with a minimum of investment while often surpassing the conventional coating in quality. Deposition by dip coating proceeds through overlapping stages: When a substrate is withdrawn slowly from a sol, containing polymeric or colloidal species in suspension, a film of liquid becomes entrained on the surface. The film thins through gravitational draining, capillary-driven flows, and, most importantly, evaporation, culminating in a well defined drying line beyond which lies a nearly dry film. When the recession speed of the drying line (relative to the substrate) matches the withdrawal rate, steady state conditions (relative to the reservoir) prevail. In the meantime, the particles in the entrained liquid, which are often reactive and tend to gel, experience a rapidly concentrating environment. Whereas the dilute sol in the reservoir might gel in months, the entrained sol has only seconds to react, albeit in less dilute states. The sol may pass through a transient gel-like state before the drying line passes; during the final stage of evaporation, large capillary stresses can develop in the network, causing it to collapse partially. The remnant porosity is evidence for the fleeting "gel" state, and varying its structure through chemical and physical means serves to tune the film characteristics. Thus sol-gel thin films can differ greatly in structure from bulk xerogels or
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