Coating particles with alumina nanolayers utilizing atomic layer deposition in a fluidized bed reactor

Jeffrey R. Wank
Abstract & Cover

There is a current need to provide simple methods to place conformal, pinhole-free, nanoscale-thickness films on fine particles. Such processing can be done using atomic layer deposition (ALD) in a fluidized bed reactor, as will be shown in this thesis. This work is the first application of ALD to coat bulk quantities of fine powders. A fundamental understanding of the fluidization of fine cohesive particles at reduced pressure along with the ALD processing of these particles is the major focus of this thesis. The applications for such nanocoated particles are broad and can be found in many different areas of materials science including microelectronics, defense, biomedical, consumer products, advanced materials, and others. The minimum fluidization velocity (tint) of fine cohesive particles at reduced pressure can be calculated using a balance of forces method. Two additional forces are added to a general force balance on a particle with an upward gas flow under vacuum conditions. The final equation is a quadratic in unif, and can be used to accurately describe umf for a variety of particle sizes, shapes, and densities. Additionally, as fine particles are coated with an alumina film, the cohesive forces between the particles will change. For the particles of interest in this thesis, the change in the cohesive force is small. 

Experiments for alumina deposition on 1.510-4 m (150 itm) diameter nickel particles, several different sized boron nitride (BN) particles (from 510-6 to 1.5.104 m (5 to 150 1.1m) average diameter), and fine (-5.10-6m (5 p.m)) iron particles were conducted using trimethylaluminum (TMA) and water as dosing reagents at 450 K. Successful deposition of alumina films, with thickness controllable at the angstrom level, was observed based upon TEM imaging, ICP-AES, XPS, particle size distributions, surface area analysis, and WDS imaging. Nickel particles are coated quite easily. For BN platelet particles, a small exposure (3.25.102 Pas• (2.5106 L)) of the reagent gases will coat the edge planes only. A larger dose of 1.3.104 Pa•s (1108 L) will coat the entire particle. After 10 ALD cycles, the exposure can be lowered back to 1.3.102 Pa•s (1.106 L) as the film is then growing on alumina and not BN. Improved interfacial adhesion between epoxy and the filler material is noted for alumina-coated BN particles. Nanocoated iron particles show improved oxidation resistance relative to uncoated particles, as long as the film is greater than 2.5.10-9 m (25 A) thick. 

Source of Information
University of Colorado Boulder
(Boulder, USA)
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