A circulating fluidised bed for plasma-enhanced chemical vapor deposition on powders at low temperatures

The macroscopic properties of fine powders are strongly influenced by the properties of their surface. Low-temperature plasmas are often used for surface processes like cleaning, activating or coating. A circulating fluidised bed (CFB) for plasma-enhanced chemical vapour deposition (PECVD) on powders is introduced here. A low-temperature plasma is generated in a riser tube by coupling microwaves. As a model application, NaCl crystals were coated with a thin silicon oxide film and the deposition rate was measured. Since a similar treatment time for each particle is crucial for a homogeneous coating, the residence time distribution (RTD) in the reactor is estimated. It is shown that the process temperature can be easily adjusted to very low values. The data presented allows the design of industrial plasma processes on powders with similar electromagnetic properties. (C) 1999 Elsevier Science S.A. All rights reserved.