Controlling Fine Metal Particle Surface Reactivity with Atomic Layer Deposition (ALD)

摘要

Very fine (～5 μm) iron (Fe) particles are coated with an ultrathin, conformal, pinholefree alumina nanolayer on the order of 1-10 nm using atomic layer deposition (ALD) in a fluidized bed reactor. While fluidizing, the individual primary particles form aggregates that are approximately 96 μm based upon laser imaging. Minimum fluidization velocity is calculated with additional forces of cohesion, vibration, and vacuum added. The primary particles are found to have a very thin (～3-5 nm) layer of native oxide, most likely α-Fe2O3 or hematite. The hematite and alumina coating require different considerations for minimum fluidization calculations due to their surface characteristics. After the coating process, the particle size distributions and surface areas do not change, indicating that the coatings do not bind the aggregates together. All individual primary particles are coated, regardless of aggregation during fluidization, due to the nature of the ALD process. Uncoated particles begin to oxidize in air at 473 K. Once the coating is thicker than 25 ?, the particles will not oxidize in air at temperatures up to 723 K. Oxidation temperature is even higher for thicker coatings.