Plastic and nonmetallic article shaping or treating: processes – Laser ablative shaping or piercing
Reexamination Certificate
1998-12-23
2001-09-25
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Laser ablative shaping or piercing
C264S430000, C264S434000, C264S624000, C264S629000, C264S640000, C264S641000, C264S653000, C264S662000, C264S678000, C264S261000, C264S340000
Reexamination Certificate
active
06294125
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for changing the dielectric properties of a polymer impregnated and pyrolyzed ceramic matrix composite (CMC). More particularly, this invention relates to a CMC with improved dielectric properties and a method for its fabrication. The CMC can be fabricated with a dielectric constant and loss tangent that permits it to be used in aircraft and turbine engines.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,983,422 to Davis et al., Jan. 8, 1991, discloses a ceramic composite with a low dielectric constant and high mechanical strength. The composite is made from a liquid aluminum oxide precursor and a reinforcement fabric. The method for producing the ceramic composite comprises introducing the precursor into a reinforcement fabric, curing the precursor to set the desired geometry, and converting the cured precursor to an aluminum oxide ceramic by pyrolysis.
U.S. Pat. No. 5,198,302 to Chyung et al., Mar. 30, 1993, discloses a fiber reinforced ceramic matrix composite formed from a preform comprising silicon nitride fibers coated with boron nitride and alumina and a matrix material. When the preform is heated in an oxidizing atmosphere essentially free of water vapor, a composite with low dielectric loss and high strength is produced.
U.S. Pat. No. 5,318,930 to Leung et al., Jun. 7, 1994, discloses a fiber reinforced silicon carboxide composite with adjustable dielectric properties. The dielectric constants of the composites are adjusted by varying the reinforcing fibers and the carbon content of the black glass matrix.
U.S. Pat. No. 5,601,674 to Szweda et al., Feb. 11, 1997, discloses a method or making a fiber reinforced ceramic matrix composite that is oxidation stable. The method comprises heating a matrix mixture slurry interspersed about reinforcing fiber in an oxidizing atmosphere to yield a crystalline ceramic phase.
However, all of these composites contain oxide matrix materials, which suffer from the drawback of insufficient capability to retain mechanical strength at high process temperatures. One object of this invention is to provide a ceramic matrix composite that can withstand repeated exposure to high process temperatures (greater than 1,000° C.). A further object of this invention is to provide a ceramic matrix composite with low dielectric constant and loss factor, high mechanical strength, low observability, and low oxidation at high working temperatures in air.
SUMMARY OF THE INVENTION
This invention relates to a polymer impregnated and pyrolyzed ceramic matrix composite (CMC) having improved dielectric properties and a method for its fabrication. This invention further relates to a method of changing the dielectric properties of the CMC. The dielectric properties can be changed by incorporating at least 1 additive having dielectric properties different from the ceramic matrix and reinforcing fiber. The additive can be a filler incorporated in the ceramic matrix of the CMC. The additive can also be incorporated in the CMC by fabricating a uniform array of holes in the CMC, during or after the process of fabricating the CMC, and filling the holes with the additive.
DETAILED DESCRIPTION OF THE INVENTION
This invention relates to a polymer impregnated and pyrolyzed ceramic matrix composite (CMC) having improved dielectric properties, and a method for its fabrication. The CMC comprises
A) a reinforcing fiber that is capable of retaining its mechanical strength after processing at temperatures greater than 1,000° C.,
B) a ceramic matrix, and
C) an additive used to change the dielectric properties of the CMC.
U.S. Pat. No. 5,707,471 issued to Petrak et al. on Jan. 13, 1998, which is hereby incorporated by reference, discloses suitable reinforcing fibers and ceramic matrices, and methods for their preparation.
The reinforcing fibers that may be used in this invention are high-modulus fibers that are compatible with the interfacial coatings and ceramic matrices described herein. The fibers must be able to retain their mechanical strength after repeated process steps at temperatures of at least 1,000° C., preferably at least 1,300° C. These fibers are known in the art and are commercially available. Suitable fibers are disclosed in U.S. Pat. No. 5,707,471 and U.S. Pat. No. 5,279,780 to Lipowitz et al., Jan. 18, 1994, which is hereby incorporated by reference.
Examples of suitable fibers include silicon carbide, silicon nitride, silicon carbide deposited on a carbon core, aluminum borate, aluminum oxide, silicon oxide, silicon carbide containing titanium, silicon oxycarbides, silicon oxycarbonitrides, carbon, and the like. Silicon carbide fibers are preferred because they can withstand processing temperatures of 1,300° C. or greater. Generally, the fibers have a modulus greater than 100 GPa, preferably greater than 150 GPa. The fibers may contain any number of filaments per tow and typically have a diameter in the range of 5 to 500 micrometers.
Examples of specific fibers include NICALON® and HI-NICALON® fibers, which are silicon oxycarbide fibers with diameter of 10 to 20 micrometers manufactured by Nippon Carbon; SCS-6® fibers comprising silicon carbide deposited on a carbon core with diameter of about 143 micrometers, which are manufactured by Textron; NEXTEL® 312, NEXTEL® 440, NEXTEL® 480, NEXTEL® 610, and NEXTEL® 720 fibers, which are alumina-boria-silica fibers with diameters of 10 to 12 micrometers manufactured by 3M; SiO
2
fibers with diameter of 8 to 10 micrometers manufactured by J. P. Stevens; Al
2
O
3
—SiO
2
fibers with diameter of 9 to 17 micrometers manufactured by Sumitomo, TYRRANO® fibers, which are silicon carbide fibers containing titanium with diameter of 8 to 10 micrometers manufactured by Ube; and silicon carbide fibers with diameter of 6-10 micrometers manufactured by Textron.
The fibers can have interface coatings. Suitable interface coatings are disclosed in U.S. Pat. No. 5,707,471. Examples of interface coatings include carbon, boron nitride, a two-layer coating comprising silicon nitride on top of boron nitride, silicon carbide, silicon nitride, aluminum nitride, and combinations thereof. The interface coatings are typically 0.05 to 1.0 micrometers thick.
The fibers may be used in nearly any length and may be arranged in the ceramic matrix in nearly any manner desired to reinforce the CMC. The fiber reinforcement can have various forms. The fibers can be continuous or discontinuous. The continuous fibers may be woven into a 2-dimensional fabric or cloth using different weaves such as plain, satin, leno, and crowfoot. The fibers may also be shaped as a 3-dimensional pre-form. Other forms of continuous fiber reinforcement are exemplified by braids, stitched fabrics, and unidirectional tapes and fabrics.
Discontinuous fibers suitable for this invention include milled fibers, whiskers, chopped fibers, and chopped fiber mats. A combination of continuous and discontinuous fibers may be used in the CMC.
The ceramic matrix of the CMC is derived from a curable preceramic polymer. “Curable” means that the polymer can be deep section infusibilized (cured) in the composite under moderate conditions by means such as mild heat, radiation, curing catalysts, or curing agents. Curability inhibits the composite from delaminating during pyrolysis.
Curable preceramic polymers are known in the art and can be manufactured by known techniques, such as those disclosed in U.S. Pat. No. 5,707,471. Examples of suitable preceramic polymers include polysilazanes, polycarbosilanes, polysiloxanes, polysilanes, polymetallosiloxanes, and the like. Polysilazanes are preferred. Suitable polysilazanes include hydridopolysilazanes, vinyl-modified polysilazanes, silacyclobutasilazane, vinyl-modified poly(disilyl)silazanes, and borosilazanes. Preferred polysilazanes include boro-modified polysilazanes and vinyl-modified polysilazanes.
Other curable silicon-containing preceramic polymers can also be used in this invention. One skilled in the art would know how to select suitable curable preceramic polymers and methods for their
Bridgewater Todd Jeffery
Petrak Daniel Ralph
Szweda Andrew
Brown Catherine U.
Dow Corning Corporation
Fiorilla Christopher A.
Severance Sharon K.
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