Chemical vapor deposition and powder formation using thermal spray

Abstract

A method for chemical vapor deposition using a very fine atomization or vaporization of a reagent containing liquid or liquid-like fluid near its supercritical temperature, where the resulting atomized or vaporized solution is entered into a flame or a plasma torch, and a powder is formed or a coating is deposited onto a substrate. The combustion flame can be stable from 10 torr to multiple atmospheres, and provides the energetic environment in which the reagent contained within the fluid can be reacted to form the desired powder or coating material on a substrate. The plasma torch likewise produces the required energy environment, but, unlike the flame, no oxidizer is needed so materials stable in only very low oxygen partial pressures can be formed. Using either the plasma torch or the combustion plasma, coatings can be deposited and powders formed in the open atmosphere without the necessity of a reaction chamber, but a chamber may be used for various reasons including process separation from the environment and pressure regulation.

Description

RELATED CASES [0001] This application is a continuation of U.S. patent application Ser. No. 09 / 921,437 filed Mar. 8, 2001, which is a divisional of U.S. patent application Ser. No. 09 / 293,867 filed Apr. 16, 1999, (now abandoned), which is a divisional of U.S. patent application Ser. No. 08 / 691,853, filed Aug. 2, 1996, now U.S. Pat. No. 5,997,956, which claims the benefit of U.S. Provisional Application Ser. No. 60 / 002,084, filed Aug. 4, 1995, the contents of all of which are hereby incorporated in their entirety by this reference.II. FIELD OF THE INVENTION [0002] This invention relates to methods of powder formation and thin film deposition from reagents contained in liquid or liquid-like fluid solutions, whereby the fluid solution, near its supercritical point temperature, is released into a region of lower pressure causing a superior, very fine atomization or vaporization of the solution. Gasses are entrained or fed into the dispersed solution and rapidly flow into a flame or plas...

AI Technical Summary

Benefits of technology

[0019] A reduction in the supercritical temperature of the reagent containing fluid demonstrated superior coatings. Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature. To ease the formation of a liquid or fluid solution which can only exist at pressures greater than ambient, the chemical precursor(s) are optionally first dissolved in primary solvent that is stable at ambient pressure. This solution is placed in a pressure capable container, and then the secondary (or main) liquid or fluid (into which the primary solution is miscible) is added. The main liquid or fluid has a lower supercritical temperature, and results in a lowering of the maximum temperature needed for the desired degree of nebulization. By forming a high concentration primary solution, much of the resultant lower concentration solution is composed of secondary and possible additional solution compounds. Generally, the higher the ratio of a given compound in a given solution, the more the solution properties behave like that compound. These additional liquids and fluids are chosen to aid in the very fine atomization, vaporization or gasification of the chemical precursor(s) containing solution. Choosing a final solution mixture with low supercritical temperature additionally minimizes the occurrence of chemical precursors reacting inside the atomization apparatus, as well as lowering or eliminating the need to heat the solution at the release area. In some instances the solution may be cooled prior to the release area so that solubility and fluid stability are maintained. One skilled in the art of supercritical fluid solutions could determine various possible solution mixtures without undue experimentation. Optionally, a pressure vessel with a glass window, or with optical fibers and a monitor, allows visual determination of miscibility and solute-solvent compatibility. Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.

[0020] The resulting powder size produced by the methods and apparatuses of the present invention can be decreased, and therefore, improved by: 1) decreasing the concentration of the initial solution; 2) decreasing the time in the hot gasses; 3) decreasing the size of the droplets formed; and / or 4) increasing the vapor pressure of the reagent used. Each of the variables has other considerations. For instance, economically, the concentration of the initial solution should be maximized to increase the formation rate, and lower vapor pressure reagents should be used to avoid the higher costs of many higher vapor pressure reagents. Decreasing the time in the hot gasses is countered by the required minimum time of formation of the desired phase. Decreasing the size of the droplets formed can entail increased fluid temperature which is countered by possible fluid reaction and dissolution effects. Similarly, coating formation has parallel effects and relationships.

[0021] Another advantage is that release of fluids near or above their supercritical point results in a rapid expansion forming a high speed gas-vapor stream. High velocity gas streams effectively reduce the gas diffusion boundary layer in front of the deposition surface which, in turn, improves film quality and deposition efficiency. When the stream velocities are above the flame velocity, a pilot light or other ignition means must be used to form a steady state flame. In some instances two or more pilots may be needed to ensure complete combustion. With the plasma torch, no pilot lights are needed, and high velocities can be easily achieved by following operational conditions known by one of ordinary skill in the art.

[0022] The solute containing fluid need not be the fuel for the combustion. Noncombustible fluids like water or CO2, or difficult to combust fluids like ammonia, can be used to dissolve the precursors or can serve as the secondary solution compound. These are then expanded into a flame or plasma torch which provides the environment for the precursors to react. The depositions can be performed above, below or at ambient pressure. Plasma torches work well at reduced pressures. Flames can be stable down to 10 torr, and operate well at high pressures. Cool flames of even less than 500° C. can be formed at lower pressures. While both can operate in the open atmosphere, it can be advantageous to practice the methods of the invention in a reaction chamber under a controlled atmosphere to keep airborne impurities from being entrained into the resulting coating. Many electrical and optical coating applications require that no such impurities be present in the coating. These applications normally require thin films, but thicker films for thermal barrier, corrosion and wear applications can also be deposited.

[0023] Further bulk material can be grown, including single crystals, by extending the deposition time even further. The faster epitaxial deposition rates provided by higher deposition temperatures, due to higher diffusion rates, can be necessary for the deposition of single crystal thick films or bulk material.

Problems solved by technology

Many of these fluids are not stable as liquids at STP, and must be combined in a pressure cylinder or at a low temperature.

Conversely, if in-line filters become clogged or precipitant is found remaining in the main container, an incompatibility under those conditions may have occurred.

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