Engineering the Evaporation Driven Assembly of Nanostructured Ceramics
This report details the results of computational and experimental investigations into the formation of well-defined nanoporous metal oxide materials by evaporation-driven assembly (EDA) of nonionic surfactants and polymerizing metal alkoxides. Coarse-grained lattice Monte Carlo simulations show the effects of confining the 2D hexagonal columnar phase (HCP) of nonionic surfactants and polar solvents in cavities of varying shape and surface chemistry. Generally, hydrophilic surfaces align micelles parallel to themselves and hydrophobic surfaces adsorb a monolayer of surfactant that then aligns the micelles parallel to the surface. Chemically neutral surfaces (with equal, weak interactions with the head and tail of the surfactant) align micelles normal to themselves. A wide variety of pore sizes and shapes are simulated, and the energetics of the system are consistent with a simple sum of a bulk energy term plus an interfacial energy contribution due to partitioning of components to the surface. Based on the simulations, conditions are predicted for forming thin films with orthogonally tilted HCP (o-HCP) pores, wires with pores running across them, wires with helical nanopores, spheres with radially oriented nanopores, and multilamellar spheres. Surface roughness is also explored, and found to possibly play a role in creating defects and orienting the micelles in unusual orientations (such as (110) parallel to the surface). A fine lattice method is developed and tested for Lennard-Jones chain fluids. The prediction from the lattice Monte Carlo method of conditions for the formation of o-HCP films is tested experimentally. Crosslinking a layer of the triblock copolymer P123 onto porous or nonporous supports leads to the formation of silica and titania films with orthogonally tilted nanopore arrays. The orientation of the pores is confirmed with a combination of XRD, TEM, SEM and GISAXS experiments. o-HCP silica supported on macroporous alumina is confirmed to be accessible by showing that the silica acts as a nanofiltration membrane to filter out 20 nm gold particles, while allowing a fraction of 5 nm gold particles to pass through the membrane. The kinetics of curing of the films are studied by FTIR, and a >20 minute induction time is found before condensation begins in P123-templated films cured under humid air. This induction period allows us to form thick, well-aligned o-HCP films by sandwiching the as-deposited films between two chemically neutral surfaces. Finally, a multiscale coupled dynamics Monte Carlo (DMC) / finite element method is described that can model the simultaneous curing and drying of sol-gel derived silica films. The model tracks particles of fluid in a film with a 1D drying / diffusion model. The concentration in the tracked particle is used as input for the DMC model, which allows the structure distribution and gel point of the sol to be calculated. Drying regime maps are developed for ideal polycondensation and polycondensation with first-shell substitution effects (which are known to exist in silica polymerization). Cyclization (another important nonideal feature of silica polymerization) is incorporated, and it is shown that gradients in structure can occur in silica films when cyclization is allowed. This multiscale model can be used to predict and avoid conditions where “skinning” (formation of a gel layer at the top surface of a coating) occurs, which will help to avoid the occurrence of defects in sol-gel silica coatings.
- Research Organization:
- University of Kentucky Research Foundation
- Sponsoring Organization:
- USDOE - Office of Energy Research (ER)
- DOE Contract Number:
- FG02-03ER46033
- OSTI ID:
- 908144
- Report Number(s):
- DOE/ER/46033-1
- Country of Publication:
- United States
- Language:
- English
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