Coating processes – Direct application of electrical – magnetic – wave – or... – Pretreatment of substrate or post-treatment of coated substrate
Reexamination Certificate
2001-11-30
2002-10-15
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Pretreatment of substrate or post-treatment of coated substrate
C427S201000, C427S255250, C427S255280, C427S255290, C427S255380, C427S255390, C427S255394, C427S561000, C427S562000, C427S564000, C427S570000, C427S576000, C427S580000
Reexamination Certificate
active
06465052
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing a nano-porous coating on a substrate. In particular, the invention provides a method that is capable of mass-producing coatings for sensor, membrane, and electrode applications.
BACKGROUND OF THE INVENTION
Porous solids have been utilized in a wide range of applications, including membranes, catalysts, sensor, energy storage (electrodes), photonic crystals, microelectronic device substrate, absorbents, light-weight structural materials, and thermal, acoustical and electrical insulators. These solid materials are usually classified according to their predominant pore sizes: (i) micro-porous solids, with pore sizes <1.0 nm; (ii) macro-porous solids, with pore sizes exceeding 50 nm (normally up to 500 &mgr;m); and (iii) meso-porous solids, with pore sizes intermediate between 1.0 and 50 nm. The term “nano-porous solid” means a solid that contains essentially nanometer-scaled pores (1-1,000 &mgr;m) and, therefore, covers “meso-porous solids” and the lower-end of “macro-porous solids”.
A number of methods have previously been used to fabricate macro- or meso-porous inorganic films. Meso-porous solids can be obtained by using surfactant arrays or emulsion droplets as templates. Latex spheres or block copolymers can be used to create silica structures with pore sizes ranging from 5 nm to 1 &mgr;m. Nano-porous silica films also can be prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate. When forming such nano-porous films by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and additional water to cause polymerization or gelation and to yield sufficient strength so that the film does not shrink significantly during drying.
Another method for providing nano-porous silica films was based on the concept that film thickness and density (porosity, or dielectric constant) can be independently controlled by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, e.g., partially hydrolyzed and condensed oligomers of tetraethoxysilane (TEOS), is applied to a suitable substrate and polymerized by chemical and/or thermal methods until it forms a gel. The second solvent, called the Pore Control Solvent (PCS) is usually then removed by increasing the temperature until the film is dry. The density or porosity of the final film is governed by the volume ratio of low volatility solvent to silica. It has been found difficult to provide a nano-porous silica film having sufficiently optimized mechanical properties, together with a relatively even distribution of material density throughout the thickness of the film.
Still another method for producing nano-porous inorganic materials is by following the sol-gel techniques, whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. Continued reactions within the sol will lead to a critical chemical state in which one or more molecules within the sol eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this chemical state, called the gel point, the material begins to become a gel. Hence, a gel may be defined as a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term “gel” as used herein means an open-pored solid structure enclosing a pore fluid. Removal of the pore fluid leaves behind empty pores.
The following publications represent the state-of-the-art of the methods for the preparation of nano-porous films or coatings:
1. O. D. Velev, et al.“Porous silica via colloidal crystallization,” Nature, 389 (Oct. 1997) 447-448.
2. K. M. Kulinowsky, et al. “Porous metals from colloidal templates,” Advanced Materials, 12 (2000) 833.
3. P. R. Coronado, et al., “Method for rapidly producing micro-porous and meso-porous materials,” U.S. Pat. No. 5,686,031 (Nov. 11, 1997).
4. S. C. Jha, et al., “Composite porous media,” U.S. Pat. No. 6,080,219 (Jun. 27, 2000).
5. M. Moskovits, et al. “Nanoelectric devices,” U.S. Pat. No. 5,581,091 (Dec. 3, 1996).
6. R. L. Bedard, et al., “Semiconductor device containing a semiconducting crystalline nanoporous material,” U.S. Pat. No. 5,594,263 (Jan. 14, 1997).
7. D. L. Gin, et al., “Highly ordered nanocomposites via a monomer self-assembly in situ condensation approach,” U.S. Pat. No. 5,849,215 (Dec. 15, 1998).
8. T. J. Pinnavaia, et al. “Porous inorganic oxide materials prepared by non-ionic surfactant templating route,” U.S. Pat. No. 5,622,684 (Apr. 22, 1997).
9. C. J. Brinker, et al., “Method for making surfactant-templated, high-porosity thin films,” U.S. Pat. No. 6,270,846 (Aug. 7, 2001).
10. P. J. Bruinsma, et al., “Mesoporous-silica films, fibers, and powders by evaporation,” U.S. Pat. No. 5,922,299 (Jul. 13, 1999).
11. R. Leung, et al., “Nanoporous material fabricated using a dissolvable reagent,” U.S. Pat. No. 6,214,746 (Apr. 10, 2001).
12. R. Leung, et al., “Low dielectric constant porous films,” U.S. Pat. No. 6,204,202 (Mar. 20, 2001).
13. K. Lau, et al., “Nanoporous material fabricated using polymeric template strands,” U.S. Pat. No. 6,156,812 (Dec. 5, 2000).
14. S. K. Gordeev, et al., “Method of producing a composite, more precisely nanoporous body and a nanoporous body produced thereby,” U.S. Pat. No. 6,083,614 (Jul. 4, 2000).
Despite the availability of previous methods for preparing nano-porous silica films, an urgent need exists for a more general method capable of producing a greater variety of metal compounds and ceramic materials in a thin film or coating form. Furthermore, most of the prior art techniques for the preparation of porous coatings are slow and tedious and, hence, not amenable to mass production.
The present invention has been made in consideration of these problems in the related prior arts, and its object is to provide a cost-effective method for directly forming a nano-porous coating onto a solid substrate. In order to produce a uniform, thin, and nano-porous metal compound or ceramic coating on a substrate, it is essential to produce depositable clusters that are on the nanometer scale prior to striking the substrate. These clusters must be capable of partially adhering to each other through parting sintering, liquid bonding, and/or vapor bonding between clusters.
In one embodiment of the present invention, a method entails producing ultra-fine clusters of metal compound or ceramic species and directing these clusters to impinge upon a substrate, permitting these clusters to become solidified thereon to form a thin coating layer. These nano clusters are produced by operating a twin-wire arc nozzle in a chamber to produce metal vapor clusters and by introducing a reactive gas (e.g., oxygen) into the chamber to react with the metal clusters, thereby converting these metal clusters into nanometer-sized ceramic (e.g., oxide) clusters. The heat generated by the exothermic oxidation reaction can in turn accelerate the oxidation process and, therefore, make the process self-sustaining or self-propagating. The great amount of heat released can also help to maintain the resulting oxide clusters in the vapor, liquid, and/or high-temperature solid state. Rather than cooling and collecting these clusters to form individual powder particles, these nanometer-sized vapor clusters can be directed to form an ultra-thin oxide coating onto a solid substrate.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention is a method for producing an optically transparent and electrically conductive coating onto a substrate. The method includes three primary steps: (a) operating a twin-wire arc nozzle to provide a stream of nano-sized metal vapor clusters into a coating chamber in which the substrate is disposed; (b) introducing a stream of oxygen-containing gas into this chamber to impinge
Nanotek Instruments, Inc.
Pianalto Bernard
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