Low-Pressure Plasma Process for Nanoparticle Coating

Summary form only given. Nanoparticles of various materials are building blocks and important constituents of ceramics and metal composites, pharmaceutical and food products, energy related products such as solid fuels and batteries, and electronics related products. The ability to manipulate the surface properties of these particles through deposition of one or more materials can greatly enhance their applicability. In this work, we discuss a low-pressure, nonequilibrium plasma process for deposition on the surface of nanoparticles. Low-pressure plasmas offer several advantages as compared to more traditional approaches. The particles are negatively charged, which prevents their agglomeration. Further, operating at room temperatures opens the possibility of coating a larger variety of materials. An overview of the experimental study is presented; however, the main thrust of the work is on theoretical modeling and numerical simulation. The modeling approach takes into account various phenomena including plasma particle distribution, nanoparticle dynamics, species transport and chemical reactions leading to the surface deposition. The plasma modeling is conducted via both particle-in-cell (PIC) method and ´fluid´ transport equations for ions and electrons. Whereas the PIC approach is mainly limited to a single nanoparticle, due to excessive computational cost, it is able to provide a detailed fundamental understanding of the charging and shielding process. For practical purposes, the method of choice is via the solution of the ´fluid´ transport equations augmented by an ionization model. The nanoparticle dynamics is modeled by considering a variety of forces acting on the particle. These include gravitational, electric, ion drag, and neutral drag forces. The effect of an external magnetic field on the trapping of nanoparticles in the plasma sheath is also discussed. It is shown that, for the same reactor pressure, larger particles can be trapped in the sheath by- application of an external magnetic field. This information could play a critical role in the design of low-pressure reactors which operate based on particle trapping. Finally, the chemical reaction modeling is discussed by considering a CH₄/H₂ plasma. The reaction model considers neutral species (CH₄ and H₂) introduced in the reactor, along with positive ions, radicals (which are the species that contribute to the growth of the nanoparticle surface) and other neutral molecules produced by reaction of the above. The 20 species in the reaction pool, plus electrons, constitute a reaction network, consisting of 31 reactions that accounts for ionization and dissociation reactions.