Pulsed-bed atomic layer deposition setup for powder coating

Atomic layer deposition (ALD) provides a method for coating conformal, pinhole-free, chemically bonded, and ultra-thin films on particle surfaces. ALD is based on one or more cycles, each cycle comprising two half-reactions. As such, ALD is a process inherently discrete in time, where the coating thickness can be controlled as a function of number of cycles. A popular scheme for achieving uniform coats on powders is to combine ALD reactors with fluidization conditions. However, fluidization is not easy to attain because it is strongly dependent on particle size, density, morphology, and surface roughness. This article proposes that a pulsed-bed, instead of a continuous fluidization, is easier to achieve in most ALD reactors. Taking advantage of the discrete nature of the ALD process, with simple changes in the configurations of purge and carrier gases, the pulsed-bed mode can be completed. An adaptation made to a regular ALD reactor to work in this mode is presented. The inclusion of a capsule for powder, valve relocations, and control of times were all necessary modifications. It was found that the pulsed-bed is a very convenient alternative for research purposes, since it can coat powders of different morphological characteristics, such as carbon nanotubes, flower-like ZnO micro-arrays, and YCrO3 particles.