Fluidized-bed CVD

Fluidized-bed CVD is a special technique which is used primarily in coating particles such as nuclear fuel. A flowing gas imparts quasi-fluid properties to the particles. Fig. 7.3 shows a typical fluidized-bed CVD reactor.pl 

The fluidizing gas is usually methane, helium, or another non-reactive gas. Factorsto considerto obtain properfluidizationarethe density and size of the particles to be coated, and the velocity, density, and viscosity of the gases.P l If the velocity is too low, the particles will fall into the gas inlet if it is too high, they will be blown out of the bed. Heavy or large objects may require suspension in the bed. 

Pp = particle density pg = gas density G = acceleration of gravity Vo = superficial gas velocity /i = gas viscosity 

The meyor applications of pyrolytic carbon deposited by fluidized bed are found in the production of biomedical components such as heart valves and in the coating of uranium carbide and thorium carbide nuclear-fuel 

Hgure7.3. Schematicof afluidized-bed CVD reactorforthe deposition of pyrolytic graphite. ) 

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Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

Other compounds have been deposited by fluidized-bed CVD including zirconium carbide (from ZrCl4 and a hydrocarbon), hafnium carbide (from HfC and methane or propylene), and titanium carbide (from TiCl3 and propylene). 

This is a relatively special technique which combines the principles of fluidized - bed heating and CVD. It is primarily used to coat powders of very fine size with suitable films for special applications. The most prominent application of this technique is in the coating of nuclear fuel particles used in high-temperature gas-cooled reactors (HTGR). A typical fluidized-bed CVD reactor is shown schematically in figure 13.4. 

Deposition of a nickel layer hy fluidized bed CVD can improve sintering in powder metallurgy applications. The process can he used as a premixing step of the composite, for example on W particles. A similar technique can also he used to improve the wettabilty of ceramics like SiC hy the molten A1 during the fabrication of metal matrix composites. ... 

Fluidized-bed CVD is a version in which the substrate is suspended in a fluidized bed and yields fine-grained deposits because particle impact on the surface continuously creates new heterogeneous nucleation sites. Figure 6.27 shows a reactor schematic. 

Figure 4.55. SEM images of an electroluminescent phosphor particle, ZhS (used in backlight displays for cell phcHies, watches, etc.), before (a) and after (b) the deposition of an aluminum oxide thin film. This film is a transparent coating that prevents the phosphor particle from undergoing humidity-accelerated decay. A technique known as fluidized-bed CVD was used, where a carrier gas both delivered the precursors to a vertically aligned CVD chamber, and dispersed the powdery sample in order to expose all surface regions to the precursor vapors.

CoMoCAT (fluidized-bed CVD) and HiPCO (high-pressure CO CVD), have recently been developed for the commercial production of SWNTs (Figure 6.79). Though the experimental setup of these methods are significantly more complex than standard hot-walled CVD, these techniques are still considered an extension of... 

Reactor, fluidized bed (CVD) A means of floating, stirring, and mixing parts in a heated chamber using a flow of gas containing the chemical vapor precursor. Vibratory action can also be used to aid in moving the parts. Particles can be added to the parts to keep them separated during deposition. See also Pack cementation. 

Voudouris N, Christoglou C, Angelopoulos G N, Formation of aluminide coatings on nickel by a fluidized bed CVD process . Surf Coat TechnoL, 2001 141 275-282... 

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