PNNL

Abstract

The synthetically tailored morphologies of mesoporous silica thin films are important for applications to sensing, separations, and catalysis. Sol-gel methods using surfactant templates can produce a variety of mesoporous phases, including one composed of long cylindrical micelles aligned parallel to the substrate. This hexagonal phase is particularly useful for analysis of the thin film stresses that produce the circular-elliptical-rectangular micelle shape evolution as the film dries. A simple linear elastic model employing an effective medium concept is proposed to describe this shape evolution using a minimal set of parameters: mesopore aspect ratios are predicted from the film’s known Poisson ratio and its measured thickness strain. The model may be of general utility for guiding the structural design of thin film mesoporous materials. Additional analysis reveals that the material fails when the internal stress system departs significantly from the plane stress condition. This occurs for severe gel shrinkage when the micelle shape exceeds its maximum aspect ratio, defined by conservation of volume and the geometric constraints of adjacent micelles. Localized stresses associated with micelle pressurization cause failure of the small silica ligaments between micelle ends, thus limiting the range of mesopore aspect ratios attainable for the structural tailoring of these materials.