CA1333547C - Metal coating of inorganic fibers and solid particulates - Google Patents

Abstract

Glass fibers newly formed from a molten glass mass or other hot, non-metallic, heat resistant substrates such as solid particulates are initially coated with a metal by thermal decomposition using latent heat of the fibers and then the metal coating is increased in thickness by further thermal decomposition using heat generated by inducing electrical energy into the initially coated fibers at megahertz frequencies.

Description

METAL COATING OF INORGANIC FIBERS AND SOLID PARTICULATES

The present invention is concerned with metallization of heat resistant inorganic materials such as fibers and particulates and, more particularly, with metallization of glass or glass-like fibers.

BACKGROUND AND PROBLEM

Recent regulations applied on a national and international scale limiting the EMI (Electromagnetic Interference) levels emitted by various electronic devices has stimulated development of various composite materials and coatings to reduce the EMI emissions to the required levels. The efficiency of EMI absorbing constituents of such composites improves with the conductivity and with the aspect ratio of the particles. A number of products have been introduced to the market for this purpose. They include: nickel plated graphite fibers, stainless steel fibers, nickel fibers, nickel plated mica, nickel plated mineral fibers, etc. Generally speaking, the currently available materials are either expensive or poorly effective. Nickel coated glass fiber produced in a single operation could be marketed at a much lower cost and could be therefore used in a wide range of 13335~7 consumer product applications. EMI shielding conductive fibers are usually sold in chopped bundles joined by a suitable binder or sizing to el~ ~nAte the environmental hazards of handling loose individual fibers.
The use of sized glass fibers in structural composites is well established. Glass rovings from nickel coated fibers can be used in similar applications, where electrical conductivity (or other metallic characteristic) of the structural element is required for some reason, e.g. EMI shielding or rapid curing by induced current.
Glass fibers can also be used for reinforcing light metal composites and special grade glass has been developed for this purpose. Wetting of the glass is, however, difficult and is greatly facilitated by using nickel plated fibers.
Methods of plating separately formed glass fibers as have been suggested in the past, e.g. electroless plating from aqueous solution, are complicated by the necessity to apply a protective sizing on freshly formed glass fibers to protect them from rapid deterioration. Such sizing has to be removed as completely as possible immediately prior to electroless plating as any residual sizing will interfere with the plating process and the quality of the product. Isolated disclosures of gas plating glass fibers and assisting the process of gas plating by induction heating such as appear in U.S. Patent No. 2,867,552 of 1959 to H.J. Homer have been found to be inoperative.
There is also a need to provide products such as high quality nickel-coated graphit~ fibers and nickel-coated inorganic and heat resistant organic particulates at relatively reasonable cost compared to the present cost of producing such materials.

OBJECT OF THE lNV~NllON

It is an object of the invention to provide an operative, novel process for gas plating glass fibers with a metal such as nickel.
It is a further object of the invention to provide an operative, novel process for gas plating non-metallic fibers and particulates stable at temperatures at least up to about 300C.

_ 3 _ 1333547 PC-3122 DRAWINGS

Figure 1 of the drawing depicts schematically the process of the present invention as applied specifically to glass.
Figure 2 of the drawing depicts schematically the process of the present invention as applied specifically to heat resistant particulates.

DESCRIPTION OF THE lNV~NllON

A process for coating a non-metallic fiber or particulate with an endothermically vapor-deposited metal comprising initially introducing hot fiber or particulate (i.e. the substrate) into an atmosphere containing thermally decomposable metal compound to thereby initiate metal deposition on said substrate. Thereafter the amount of metal on the substrate is increased by maintaining the substrate in an atmosphere containing decomposable metal compound while electrically inducing in the initially coated substrate at a frequency at least in the megahertz (MHz) range sufficient energy to maintain the metal surface thereof at a temperature sufficiently high to support decomposition of the metal compound on the surface.
The process of the present invention advantageously employs as a starting material heat resistant, e.g. an inorganic, non-metallic fiber. For practical purposes such a fiber can be made of graphite or can be made of glass. "Glass" is defined for the purposes of this specification and claims as "an inorganic material usually comprising oxides which can be cooled from the molten state to a relatively stable non-crystalline, apparently solid state". As commonly encountered, "glass" comprises silica, an alkali metal oxide, an alkaline earth metal oxide and optional amounts of alumina, boron oxide and other metal oxldes. However, glass can comprise essentially silica or can be based entirely on oxides such as boron oxide or phosphorus oxide. Por purposes of this specification and claims, "fiber" is considered to be an elongated mass of material having a diameter or major cross sectional dimension of the order of 10-50 micrometers. The fiber is considered to be hot if it is at a temperature significantly in excess of the temperature of 4- 1333S~7 PC-3l22 decomposition of the decomposable metal compound and up to that temperature at which the fiber decomposes, sinters or melts.
Specifically with respect to glass, a glass is deemed to be molten when it has a vlscosity of less than about 100,000 centipoises.
The thermally decomposable metal compound used in accordance with the present invention is advantageously nickel carbonyl, Ni(C0)4, which rapidly decomposes into metal and carbon monoxide under about atmospheric pressure at about 150C. However, other metal compounds thermally decomposable to metal and a combining moiety can be used. Materials such as, for example, carbonyls and nitrosyls, both pure and mixed of nickel, iron, chromium, molybdenum, tungsten, etc., copper acetyl acetonate, hydrides such as stibine and arsine, carbonyl halides such as osmium carbonyl bromide, ruthenium carbonyl chloride, metal alkyls such as trimethyl aluminum, triethyl tin, etc. can be employed in the present invention. Those skilled in the art will appreciate that by controlling the thermally decomposable materials in an atmosphere in contact with hot substrate, e.g. fiber or particulate, various products such as complex composite structures, alloys and the like can be produced.
For example, if hot glass fiber is introduced into an atmosphere containing both nickel and iron carbonyl, an alloy can be deposited on the glass. An example of the utility of such a product would be the deposition of a low expansion nickel-iron alloy containing say 36% nickel so as to match reasonably well the thermal expansion of the metal and the glass. Another example of a useful, complex composite structure would be introdlf-ing hot glass into an atmosphere cont~ining nickel carbonyl and, after producing an initial layer of nickel, introducing the still hot or inductively heated fiber into an atmosphere containing chromium carbonyl Cr(C0)6 to deposit a layer of chromium atop the layer of nickel.
It is essential in the process of the present invention that inductive heating of initially plated fiber be done at MHz frequencies. It has been discovered that disclosures mentioned hereinbefore such as those by Homer simply were not operable due to the fact inductive heating frequencies available to him were too low.
The process of the present invention has been found to be 1333S~7 operative using an induction heating coil operating at 13.6 MHz, said coil and associated electronics being the product of Leco Corporation and sold for the general purpose of analyzing solids for carbon and sulfur content. Generally speaking frequencies in excess of about 5 MHz are operative for purposes of the invention.
Considering nickel carbonyl, Ni(C0)4, as a model material thermally decomposable to metal for use in the present invention, metal is deposited on glass fibers from the gas phase by virtue of the endothermic reaction:
Ni(C0)4 Ni + 4C0 It is known that in the presence of a catalyst, e.g. nickel, carbon monoxide can disproportionate according to the formula:

2C0 C + CO2 The result of these two reactions proceeding simultaneously is deposited metallic nickel along with some small amount of elemental carbon. As taught in U.S. Patents Nos. 3,694,186 and 3,820,977 the amount of carbon codeposited along with nickel can be ;n; ;~ed by including in the atmosphere cont~ining nickel carbonyl an oxide of nitrogen such as N20 (nitrous oxide), N0 (nitric oxide), N203 (nitrogen trioxide) or N02 (nitrogen peroxide). Assuming that nickel carbonyl is present in a minor amount in a carrier gas such as carbon monoxide, an oxide of nitrogen can be present in the carrier gas/carbonyl mixture in an amount of about 1 to about 1500 ppm. The carrier gas is advantageously carbon monoxide but can be any gas inert with respect to the metal compound decomposition reaction, e.g. nitrogen or argon. It is important that the gas/carbonyl mixture be substantially free of oxygen, halides, hydrogen halides, dust or aerosol particles or other substances which can nucleate decomposition of nickel carbonyl to form a powder product. Likewise, the wall of the apparatus in which coating of fiber with metal is carried out should be cool or washed by gas relatively free of nickel carbonyl in order to prevent unwanted plating of such walls with nickel.

13335~7 Attention is also directed to the use of substances such as H2, N0, PF3, PH3, NH3 or halogens to catalyze the decomposition of iron pentacarbonyl as disclosed in U.S. Patent No. 4,056,386 and to the use of N0, N203 and N02 for the identical purpose as disclosed in U.S. Patent No. 3,694,187. The present invention contemplates the use of these substances in small amounts when plating glass fiber or any other heat resistant substrate with iron and the use of any substance capable of catalyzing the decomposition of a compound thermally decomposable to produce metal when plating glass fiber or substrate with any metal or combination of metals.
Speaking again particularly with respect to nickel, the latent heat content of a newly formed glass fiber 10 micrometers in diameter from a molten glass mass is sufficient to cause the deposition of up to about 0.2 micrometer thick layer of nickel on the fiber. In accordance with the present invention, a fiber is provided with a nickel (or other metal) coating at least about one micrometer thick, advantageously about 1-3 micrometers thick and even coatings much thicker. Accordingly for purposes of the present invention to provide a ini adequate coating of nickel it is necessary to supply to the fiber by means of induction heating an amount of heat at least about 5 times the latent heat in a newly formed glass fiber.
Thus the input of heat to the fiber by induction heating is a significant feature of the present invention.
In order to give those of normal skill in the art a greater appreciation of the advantages of the invention, reference is made to Figure 1 of the drawing which de?icts schematically the process of the present invention as applied to glass fiber. Referring now thereto, glass 11 is melted in furnace 13 and forced by head or head plus applied pressure through holes 15 in spinneret 17. Glass melting furnace 13 is supported on gas impervious, non-metallic tubular column 19 made, for example, of silica or other heat resistant material. Tubular column 19 has ports 21 and 23 for ingress of gas containing nickel carbonyl and egress of gas depleted in nickel carbonyl. Tubular column 19 can also be provided with port 25 adjacent spinneret 17 through which a wash gas such as nitrogen or carbon monoxide can pass to wash the hot, outer surface of spinneret 17 with a gas free of nickel carbonyl and thus to rin; l~e deposition 13335~7 of nickel on the outer surface of spinneret 17. Internally cooled induction coil 27 surrounds tubular column 19 and is connected to MHz frequency generator 28. Near the bottom of tubular column 19 is located baffle 29 separating the active plating chamber 31 above baffle 29 from purge chamber 33 below baffle 29 and above baffle 35.
Purge chamber 33 has inlet 37 and outlet 39 for purge gas and inlet 41 for sizing. Below baffle 35 tubular column 19 ends being supported in position by means not shown. Pinch rolls 43 are mounted below the open but baffled end of tubular column 19.
In operation, molten glass 11 from furnace 13 is forced through holes 15 in spinneret 17 to form tow 45 of several hundred fibers. Initially these fibers are fed through tubular column 19 through ports 47 and 49 in baffles 29 and 35 respectively and between pinch rolls 43. Pinch rolls 43 pull on tow 45 so as to produce fibers of the desired diameter. When the apparatus is operating satisfactorily to produce the proper glass fiber, plating is initiated by introduction of gas containing nickel carbonyl into plating chamber 31. Megahertz frequency generator 28 operating at, for example, 13.6 MHz is activated energizing coil 27. The quantity of nickel carbonyl passing into plating chamber 31 in unit time is controlled vis-a-vis the amount of glass fiber emerging from spinneret 17 such that a coating of nickel at least about 1 micrometer thick is formed on each of the glass fiber of tow 45. It is within contemplation of the present invention to provide means, such as a vibrating means, within plating chamber 31 to enhance the separation of fibers of tow 45 and thus facilitate metal deposition.
Generally speaking, coil 27 maintains the fibers of tow 45 at a temperature of at least about 150C and advantageously in the range of about 180C to about 240C. After the fibers of tow 45 have been properly coated with nickel, they are sized in purge chamber 33 or at another down stream location from plating chamber 31. In this context, "sizing" means coating the fibers with a binder which is either readily removable or compatible with the ultimate usage of the fiber. The purpose of "sizing" in this sense is to bind individual fibers into more or less loosely aggregated flocks so as to facilitate the handling of chopped fibers and ;n~ ;~e dusting and atmospheric levitation of individual fibers. For general purposes a water-removable size could be polyvinyl alcohol applied as an aqueous aerosol. For ultimate use in organic binder fiystems useful sizes could be polystyrene, polymethyl methacrylate, stage 2 phenol- or urea-formaldehyde resin and the like all applied by means of an aerosol.
Once coated fiber tow 45 is purged of residual nickel carbonyl and is sized, it exits through port 49 into the open atmosphere, passes between pinch rolls 43 and is then either spooled or chopped. As indicated hereinbefore, products of the process of the present invention have numerous uses including EMI shielding materials, reinforcing means in resinous and metallic systems, electrical conductors in fabrics, felts, concretes and the like, magnetically responsive means in plastic systems, in-situ susceptors for resin curing and the like.
Graphite fiber can be treated essentially in the same manner as glass fiber except that graphite as fiber is heated by any appropriate means in an inert, e.g. argon atmosphere instead of being formed from a melt as depicted in Figure 1. Graphite fiber exiting from a heating chamber, like the emerging glass fiber tow 45 in Figure 1, is initially contacted with gas containing a volatilized, decomposable, metal compound, e.g. Ni(CO)4, and then subjected to a megahertz frequency induction field in the presence of an atmosphere containing a decomposable metal compound.
An apparatus for coating solid particulates, e.g. sand, alumina, mica, zirconia, tetrafluoroethylene powder, etc. with metal such as nickel is depicted schematically i~ Figure 2. Referring now thereto, particulate 53 is fluidized in non-metallic fluid bed 55 by a gas consisting of carbon monoxide and nickel carbonyl. The gas is forced through line 57 by pump 59 through porous grate 61. The gas is not heated except to the extent that it is heated by compression.
Particulate 53 enters fluid bed 55 from hopper 63 by means of sealed screw auger 65 operated by motor 67. Particulate 53 passes into standpipe 69 and is heated up to about 300~C or higher (depending on the nature of particulate 53) by means of electrical resistance heating jacket 71. Particulate 53 is retained in the vicinity of jacket 71 for a residence time necessary to achieve the desired temperature by constriction 73 in standpipe 69. Constriction 73 can `- 13335~7 be of any construction and advantageously is vibratable so as to meter hot particulate 53 into fluid bed 55. On entering fluid bed 55 hot particulate 53 reacts with nickel carbonyl to form an initial thin layer of nickel on the surface of the particles. Thereafter while in residence in fluid bed 55, particulate 53 is heated by inductive coupling with internally cooled coil 27 energized by megahertz frequency generator 28 to increase the amount of nickel on the particle surface. Gases exit from fluid bed 55 through line 75 which in turn leads into cyclone 77 with off-gas line 79. Entrained solids in exiting gases are returned to standpipe 69 through return line 81. Product comprising particulate 53 coated with metal, e.g.
nickel, is removed via line 83 into container 85 which can be isolated by valve 87. In Figure 2, line 83 is depicted as coming off near the bottom of fluid bed 55 on the premise that particulate 53 is substantially uniform in particle size and is relatively low density material such as silica or alumina. A relatively thick deposit of heavy nickel on such a particle will, on the average, cause such a particle to be levitated at a higher gas velocity, i.e. near the bottom of a diverging fluid bed.
While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention, those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

Claims (11)

1. A process for coating a non-metallic substrate directly with at least one endothermically vapor-deposited metal comprising initially introducing substrate into an atmosphere containing at least one thermally decomposable metal compound, said substrate at introduction into said atmosphere having a temperature significantly in excess of the temperature of decomposition of said thermally decomposable metal compound, to thereby initiate metal deposition on said substrate and thereafter increasing the amount of metal on said substrate by maintaining said substrate in an atmosphere containing decomposable metal compound while electrically inducing, at a frequency at least in the megahertz range, in said initially coated substrate sufficient energy to maintain the metal surface thereof at a temperature sufficiently high to support decomposition of said metal compound on said surface.

2. A process as in claim 1 wherein said non-metallic substrate is fibrous.

3. A process as in claim 2 wherein said fibrous substrate is inorganic.

4. A process as in claim 3 wherein said inorganic, nonmetallic fiber is a glass fiber.

5. A process as in claim 1 wherein said non-metallic substrate is a particulate

6. A process as in claim 1 wherein said at least one thermally decomposable metal compound is selected from the group consisting of metal carbonyls, metal alkyls, metal nitrosyls, metal acetylacetonates, metal hydrides and metal carbonyl halides.

7. A process as in claim 6 wherein said at least one thermally decomposable metal compound is contained in an inert carrier gas.

decomposable metal compound is contained in an inert carrier gas.

8. A process as in claim 6 wherein said at least one thermally decomposable metal compound is a metal carbonyl.

9. A process as in claim 8 wherein said metal carbonyl includes nickel carbonyl.

10. A process as in claim 9 wherein nickel carbonyl is thermally decomposed to deposit a layer of nickel at least about one micrometer thick on glass fiber.

11. A process as in claim 5 wherein heat resistant particulate is inorganic and is coated while in a fluid bed levitated by said thermally decomposable metal compound in said inert carrier gas.