Preparation of Nanostructured Membranes by Reactive Depositions from Supercritical Fluids

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Watkins - 9811088 The development of functional, nanostructured metal composite membranes will have a substantial economic impact on the separation, purification and recovery of gases as well as novel membrane reactor applications for oxygen-enriched combustion, controlled partial oxidation and equilibrium limited dehydrogenation reactions. Despite progress that has been achieved to date, precise control of deposit microstructure, composition and distribution within the support has not been realized. The challenge for porous-inorganic support/metal composites remains the development of methodologies for the controlled infiltration and deposition of nanostructured metal and metal alloys within interior pores at a prescribed depth and thickness. These architectures are inaccessible by current techniques including chemical vapor deposition, metal sputtering and electroless plating due to constraints of these processes that do not allow for optimization of deposit microstructure or composition. Polymer/metal composites membranes are plagued by poor and to rectify this deficiency, processes leading to gradient interfaces are required. The PI's plan to integrate materials synthesis, characterization and multi-scale simulations for the development of a processing strategy based on the concept of chemical fluid deposition (CFD). CFD is a form of metal deposition that involves the chemical reduction of soluble organometallic compounds in supercritical carbon dioxide solution. The key to the process is physicochemical properties of the solvent which lie intermediate to those of liquids and gases. Transport and reduction in solution offers low process temperatures, high reagent concentrations and eliminates precursor volatility requirements associated with vapor phase techniques. The absence of surface tension and low viscosity of supercritical fluid (SCF) solutions are well suited to the delivery of relatively high concentrations of organometallic precursors within porous environments. Deposi tion is triggered by the introduction of a reducing agent to produce metal deposits. This reaction scheme can be used to dictate deposit composition and microstructure via direct control of nucleation and growth kinetics by precise adjustments in stoichiometry of the soluble precursors. Mass-transport limitations are largely eliminated due to high precursor concentration and favorable transport properties of the SCF solution which is similar to those of a gas. The permeability of polymer substrates to SCF/organometallic solutions can be exploited to produce composite membranes with adherent, gradient polymer/metal interfaces and metal interlayers.