Improved Temperature Stability of Atomic Layer Deposition Coated Cellulose Nanocrystal Aerogels
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Improved Temperature Stability of Atomic Layer Deposition Coated Cellulose Nanocrystal Aerogels Sean W. Smith,1 Han Chan,2 Christian Buesch,3 John Simonsen,2 and John F. Conley Jr.1 1
School of Electrical Engineering and Computer Science, Oregon State University, 1148 Kelley Engineering Center, Corvallis, OR 97331, U.S.A.
2
Wood Science and Engineering, Oregon State University, 119 Richardson Hall, Corvallis, OR 97331, U.S.A.
3
School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, 204 Rogers Hall, Corvallis, OR 97331, U.S.A.
ABSTRACT Atomic layer deposition (ALD) was used to coat cellulose nanocrystal (CNC) aerogel scaffolds with a thin conformal layer of Al2O3. Electron probe microanalysis indicates that the penetration of Al2O3 into the aerogel was greater than 50 μm. Thermogravimetric analysis (TGA) shows that Al2O3 coated CNC aerogel composites have improved temperature and oxidation resistance. INTRODUCTION An aerogel is a sol gel in which the liquid solvent has been replaced by air without collapsing the suspension, creating a unique microstructure of loosely spaced fibres with very high porosity, low density, and high surface area [1]. CNCs are a renewable material with mechanical properties comparable with carbon fiber, making CNC aerogels attractive as a low cost renewable alternative for fiber reinforced polymers with potential applications such as plastic casings for cell phones and laptops [1]. However, the sensitivity of CNC aerogels to oxidation limits their ability to be incorporated into polymers which require high temperature processing (>200 °C) in oxygen containing environments [1]. ALD allows surface limited deposition of highly uniform and conformal thin films of inorganic oxides over high aspect ratio porous and large surface area structures. ALD Al2O3 layers as thin as 25 nm make excellent moisture permeation barriers [2]. ALD is a chemical vapor deposition like method in which the reactants are alternately dosed to the substrate, limiting the reaction to a chemisorbed surface layer. An ALD cycle consists of 4 basic steps: (1) A pulse of the first reactant is introduced; (2) Reaction products and excess reactants are purged away with an inert gas (N2); (3) A pulse of the second reactant is introduced which reacts with the chemisorbed layer of the first reactant; (4) Excess reactant and reaction products are purged away with inert gas (N2). These 4 steps are repeated to deposit a film of desired thickness. In this work, ALD is used to deposit conformal Al2O3 coatings on CNC aerogels. In order to coat a high aspect ratio structure such as an aerogel, the four steps described above are modified. After steps (1) and (3), additional exposure steps are added during which the substrate is allowed to soak in the reactant gasses to allow the reactants time to diffuse into the porous substrate. Similarly the purge step is extended to give the excess reactants and reaction products time to diffuse out again. Finally, a larger dose of reactant must often be used (
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