Fluidized bed coating process with liquid reagent

Details Fluidized bed coating process with liquid reagent Fluidized bed coating process with liquid reagent

1997-04-09

2001-02-13

Beck, Shrive (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

C427S591000, C427S595000, C427S900000, C427S901000

Reexamination Certificate

active

06187379

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a method for coating various substrates and, more particularly, to a method for coating particles or components by using a fluidized bed having a liquid reagent.
BACKGROUND OF THE INVENTION
Fluidized bed coating processes are used to deposit carbon (C), silicon (Si), siliconcarbide (SiC), and other ceramics and metals onto particles and components used in low observable and medical applications, for example. The fluidized bed typically is a bed of particles that are lifted and agitated by a rising stream of a process gas. At low velocities, the amount of lifting is slight and the bed behaves as a boiling liquid, hence the term fluidized bed. The bed is typically contained within a furnace, or reactor heated inductively, resistively, or otherwise.
In chemical vapor deposition (CVD) at least a part of the gas used to fluidize the bed is a reactive gas that break downs when it contacts the heated particles into coating materials such as carbon, silicon or silicon carbide. Often times, for example, methane is used, perhaps along with argon as a carrier. The term chemical vapor infiltration, or CVI, is used when the particles or articles being coated are not merely surface coated, but the reagent enters into the porous substrate and is deposited therein.
In the above described conventional fluidized bed, the entire bed and vessel are hot. Thus, the interior surface of the vessel, which is commonly made out of graphite, as well as the particles, will become coated by the reagent. As such, the vessel and the particles must be frequently replaced.
Some work has been done in the past using liquid reagents with CVD or CVI processes to achieve coating of filaments and infiltration of porous preforms. For example, U.S. Pat. No. 3,850,689 to Basche et al. teaches a process to deposit silicon carbide on filaments by passing the hot filament through a liquid organo silicon halide, wherein the temperature of the hot surface is high enough to cause the organo silicon halide immediately surrounding the hot surface to film boil, decompose, and deposit a silicon carbide coating on the filament. Two other U.S. Pat. No. 5,389,152 to Thurston et al. and U.S. Pat. No. 4,472,454 to Houdayer et al., teach processes for the densification of a porous preform that involve immersing a conductive preform in a liquid hydrocarbon or other precursor liquid. An induction coil is used to heat the preform so that the precursor liquid immediately surrounding the preform decomposes and carbon, or another element, is infiltrated into the preform. These processes are taught using a stationary preform.
Thus, there exists a need in the art for a coating method using chemical vapor deposition or chemical vapor infiltration from a fluidized bed in order to quickly, economically, and completely coat a substrate.
SUMMARY OF THE INVENTION
An object of the present inventive method is to permit coating of a substrate by means of chemical vapor infiltration or chemical vapor deposition using a fluidized bed with a liquid reagent.
Another object of the present invention is to provide a coating method that permits the substrate, or preform, to freely move about in a liquid reagent in order to more fully and evenly coat the substrate.
An additional object of the present invention is for a coating method which only coats the conductive particles/objects and avoids coating nonconductive reactor walls and other nonconductive particles in a liquid reagent.
Briefly described, a preferred embodiment of a method for coating a substrate with a chemical compound using a fluidized bed comprises, initially, a precursor liquid. This precursor liquid serves as a reagent in the chemical vapor deposition or chemical vapor infiltration process and is housed in a furnace, or reactor.
Once the precursor liquid is placed in the reactor, the substrate, whether particles or articles to be coated, should be preferably immersed in the precursor liquid. In the preferred embodiment, the substrate is permitted to freely circulate within this liquid. However, it would be possible to suspend the substrate in the liquid from a securing device such that the substrate will not freely circulate within the liquid.
While the substrate is immersed in the precursor liquid, a non-reactive, preferably inert, gas is passed through a gas injector in the bottom of the reactor. This non-reactive gas bubbles up through the liquid and aids in fluidization of the bed. Fluidization may also be augmented by placing non-reactive, non-conducting particles within the liquid. Particles such as glass or ceramic beads would be appropriate and would aid in fludizing the bed.
In the preferred embodiment, inductive heating of the substrate is performed by an induction coil placed directly adjacent to the exterior of the reactor walls. Alternatively, the induction coil can be placed inside the reactor, within the precursor liquid. In either aspect, the induction coil will be driven by a generator to emit a high frequency alternating current such that only the substrate to be coated will be heated. Of course, other devices for heating the substrate may be used with equal effectiveness. For example, the substrate may be heated by resistive heating, microwave heating, or even a laser. When the substrate is heated to high enough level, chemical vapor deposition or chemical vapor infiltration occurs and the desired chemical compound is deposited on the substrate.
In another aspect of the present invention, the liquid in the reactor may be inert and the gas may be the reactive compound in the process. Alternatively, the liquid in the reactor and the gas bubbled through the liquid may react in some manner to cause a compound, or element, to be deposited onto the substrate by either chemical vapor deposition or chemical vapor infiltration.
In an alternative embodiment of the present invention, the reactor unit is adapted to recirculate the precursor liquid in order to aid in fluidization of the bed. In fact, recirculation of the precursor liquid can comprise the primary means of fluidization. In this way, injecting a gas into the reactor is no longer necessary to this alternative embodiment of the present invention.
As an improvement to the preferred embodiments, the vapor escaping from the fluidized bed may be recaptured by a reflux condensor or other appropriate vapor recapturing device. Alternatively, or additionally, a stirring device, such as an auger or propeller, may be placed within the precursor liquid in order to agitate the liquid and more evenly coat the substrate.
An advantage of the present invention is that the method described herein permits use of an auger or other stirring device to agitate the liquid reagent and the substrate. This gives a more thorough coating of the substrate in a lesser time.
Another advantage of the present invention is that sticking of substrates together or to the fluidizing media is reduced or eliminated since only the conducting components are being coated. This means that more substrates may be coated at one time.
An additional advantage of the present invention is that the liquid reagent yields faster deposition rates as compared to gas fluidized beds.
Another advantage of the invention is that coating of both larger and smaller substrates may be possible as compared to a gas fluidized bed. For example, small particles would be less likely to be expelled from a liquid fluidized bed since they are “wetted” and since the gas flow (if any) required for fluidization is reduced.
Yet another advantage of the present invention is that a continuous coating process is possible since the reactor walls and the nonconductive fluidizing media are not coated, or coated to a much lesser extent than involved in prior art fluidized bed depositions.
Another advantage of the present invention is that a variety of gas injectors can be used. Design of a gas injector for the novel method described herein will be less complex since the liquid reagent will assist in suspending the substrate particles or ar

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