US3411949A - Method and apparatus for the manufacture of pyrolytic fibers - Google Patents

Description

Nov. 19, 1968 L HOUGH METHOD AND APPARATUS FOR THE MANUFACTURE OF PYROLYTIC FIBERS Filed Aug. 13, 1965 QWv vm n. N O n- ATTORNEY United States Patent O 3,411,949 METHOD AND APPARATUS FOR THE MANU- FACTURE F PYROLYTIC FIBERS Ralph L. Hough, Springfield, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Filed Aug. 13, 1965, Ser. No. 479,675 28 Claims. (Cl. 117-228) ABSTRACT OF THE DISCLOSURE A method for the manufacture of pyrolytic fibers comprising the steps of conducting a carbon containing, pyrolytic deposition gas from one of the methane, ethylene and acetylene series through a high voltage electric arc to nucleate a portion of the carbon therein, thereafter heating the gas with the nucleated carbon therein to a temperature within the range of from 500 to 1200 degrees centigrade, and thereafter passing the nucleated pyrolytic material through a second carbon deposition gas while heating the nucleated material to a temperature at which the carbon from said second deposition gas pyrolytically deposits upon the nucleated carbon.

The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me 0f any royalty thereon.

This invention relates to a method and apparatus for the manufacture of pyrolytic fibers.

Recent developments in the art of high temperature materials have involved a variety of new pyrolytic deposition techniques wherein a volatilized or ionized precursory material is caused to decompose or to undergo a gas phase plating reaction, usually at high temperatures, to .achieve a solid state, generally as a deposit or coating upon a substrate. While films, continuous laments and coatings of pyrolytic materials such as pyrolytic carbon or graphite, and various carbides, borides and nitrides have been available, very little has been done toward the development of improved techniques and devices for the manufacture of individual or discrete particulate' :filamentary fibers of such materials. On the other hand, because of the unique electrical, thermal and structural properties of the various pyrolytic materials, their availability in fibrous form has been desired for many uses including that as a reinforcing filler for plastic and pyrolytic composites.

While the prior art has touched upon the formation of carbon in finely divided particulate form, principally in the manufacture of carbon black, the carbon has generally been non-crystalline and is not truly a pyrolytic substance. Moreover, the particles generally formed by the prior art have not been truly filamentary or of a fibrous nature as a result of which they have not been available to fulfill the demands that have traditionally been made on the structural capabilities of natural or manmade textile fibers. On the other hand, in those few instances where filamentary graphite or other pyrolytic strands have been made available in fibrous form, the process and apparatus have been complicated, generally inefiicient and uneconomical and have not been 3,411,949 Patented Nov. 19, 1968 ICC subject to control of the type that has led to any uniformity of the product.

It is accordingly an object of the present invention to provide an improved method for the production of filamentary pyrolytic fibers.

Yet another object of the invention is to provide an apparatus for the manufacture of fibers composed solely of pyrolytic material.

Still another object of this invention is the provision of a method and apparatus for the efficient, economical and predictable production of a variety of pyrolytic fibers.

To achieve these and other objects and advantages which will appear from a reading of the following disclosure, the present invention teaches the nucleation of a pyrolytic-material-containing deposition gas by exposing it to high energy electric arcs the effect of which is to bring the molecules of the vaporized precursory material within the gas into such juxtaposition or conglomeration as to form reaction sites or condensation centers for the aggregation of further atoms or molecules thereupon. The invention then teaches the subjection of the nuclei thus formed to pyrolyzing conditions in a deposition atmosphere whereby a pyrolytic plating will occur upon the nucleated sites resulting in the development into solid discrete filamentary fibers. Modifications of the inven- 'tion involve particular forms and arrangements of the electrically energized nucleating means, intermediate handling and heat treating of the nucleated gas and the material selection for the final deposition upon the nuclei so that a great variety of fibrous structures from an equally great variety of precursory materials and pyrolytic substances may be obtained. In most preferred modifications of the invention, the process and apparatus are continuous to the extent that the sequence of the stage of fiber formation are influenced by the same continuous gas flow which may or may not be supplemented or altered by gaseous additions at one or more points along the continuous flow.

The invention thus generally described may be more clearly understood by reference to the following detailed description of certain preferred embodiments thereof in connection with which reference may be had to the appended drawings.

In the drawings:

FIGURE 1 is an elevational view in cross section of a preferred electrode-conduit unit rfor the nucleation of reaction sites from a gas flow.

FIGURE 2 is a schematic ow diagram illustrating a method and apparatus for manufacturing pyrolytic filaments.

Referring now to FIGURE 1, the device for nucleating a gaseous flow and thereby initiating development of pyrolytic fibers purely from a gaseous material is shown to ybe in the form of the gun 10 consisting of the electrode assembly designated generally by the number 11 and the barrel section 12. In the preferred embodiment illustrated, the spaced electrode surfaces 13 and 14 are provided by the core electrode 15 having the cylindrical portion 16 -concentrically positioned Within the cylindrical sleeve-electrode 17. The contours of the opposed electrode surfaces 13 and 14 as at 18 and 19 are such that the surfaces are of equal electrical potential throughout. Both of the electrode units 15 and 17 are composed of an electrically conductive material such as copper, silver or other metal;

and they in turn are held in such a position that the opposed surfaces 13 and 14 are radially spaced by the housing 20 which is of a dielectric material such as polytetrafluoroethylene. The housing 20, in addition to thus positioning the electrode units, also provides a cylindrical chamber 21 surrounding the electrode units and embracing the annular electrode 17 so that a gas introduced into the chamber 21, either by suitable opening through the housing 20 or lby the passage 22 through the core electrode surfaces 13 and 14.

The electrodes themselves are electrically energized by conventional high voltage means such as a high voltage generator 23 which results in the passage of a high voltage arc across the space between the electrode surfaces 13 and 14. Because the surfaces are contoured to be of equal potential throughout their length, there will be no preferred point at which the electricity will tend most to bridge the gap between the electrode surfaces as a result of which the arc will move lback and forth along and longitudinally of the electrode surfaces while passing transversely of the gas owing between them. In a preferred embodiment Iof the apparatus illustrated, the electrical energization of the electrodes is controlled by intermittently acting or pulsing switching means such as the electromagnetic make and break switch 24. In yet another lmodification of the invention, the core electrode is rotatable within the housing 29 and is rotatably driven, continuously or intermittently during the movement of the electric arc between the electrode surfaces. It has been found that the expedients of the intermittently acting current and the rotation of the one electrode surface relative to the other improve the continuity of fiber formation and uniformity of fibers formed according to this invention.

The balance -of the gun unit comprises the barrel section 12 extending longitudinally therefrom, the main body of which comprises the cylindrical tubular extension 25 which accommodates the flow of deposition gas emanating from the electrode assembly 11. This unit which may be conveniently composed of quartz, high temperature glass or other heat resistant material is preferably merely an extension of the chamber 21 within the electrode housing and is so proportioned relative to the dynamics of the flow of the deposition gas that a laminar, non-turbulent gas ow is achieved whereby the nucleated aggregations of the pyrolytic particles within the gas will be smoothly transported. As means for further influencing the flow of the deposition gas within the main stack 25 are the concentric tubular passages 26 and 27 through which other carrier or diluent gases such as hydrogen, argon, helium or the like may be introduced via the inlets 28 and 29 respectively. Since each of the concentric tubular sections 25, 26 and 27 are open at their downstream end from the electrode assembly, all of the gases in said chambers merge at the barrel opening 30 for the conclusion of the ber formation according to this invention as hereinafter described.

Referring now to FIGURE 2, the electrode unit 31 similar to that described in connection with FIGURE 1 and having the main deposition gas inlet 32 and auxiliary gas inlets 33 and 34 is shown to be connected by the conduit fitting 3S with a settling chamber 36 which also may be composed of quartz, high temperature glass or other heat resistant material and is primarily nothing more than an enlarged cylindrical unit of a relatively large diameter as compared to the diameter of the balance of the conduit assembly so that the rate of ow therethrough will be reduced and sooty agglomerations or other impurities resulting from the passage of the gas through the electric arc will tend, under the inuence of gravity, to settle out of the gaseous mixture which then leaves the settling chamber and enters into the preheating or heat-treatment zone 37 which may be nothing more than a tubular extension of the device thus far described. The gaseous mixture, including the nucleated reaction sites or ber seeds formed in the gun assembly 31, is heated as it passes through this zone to temperatures, preferably within the range of from 500 to 1200 degrees centigrade, as a result of which it is believed that such of the nucleated portions of the gas as tend to be mechanically adhered are fused into ilamentary proportions by the further carburization of a petroleumlike hydrocarbon ilm which is believed to be associated with the nucleated particles. A preferred pre-heating unit is shown to comprise the tubular'barrel 37 at least a part of which is composed of quartz, high temperature glass or other high temperature material which is optically transparent or will otherwise allow the passage of radiant heat energy therethrough and which is surrounded by one or more line heaters 3S of the radiant energy or infrared heating variety. The full effect of such heating energy may be concentrated upon the gas and particles within the conduit 37 by the strategic location of one or more reflectors 39.

Extending beyond the heat treatment zone 37 is a continuation of the genenal conduit through which the deposition gases are passing which includes the pyrolyzing zone 40 in which the heat-treated, nucleated components of the gas are heated or otherwise energized to the extent that a gas phase plating reaction between the pyrolytic components of the gas and the nucleated reaction sites therein will occur. As a result of this plating reaction the fiber seeds, having -been nucleated, heat treated, and thus integrated into iilamentary form, will receive one or more coatings of pyrolytic material thereby growing in thickness to produce fibers of lengths and diameters comparable tothe staples lof naturally occurring bers such as cotton, wool, etc. While a variety of heating means, including for example, in-line radiant heaters acting through a quartz tube in the manner of the radiant heat treating source 38 and reflector 39, may be used to bring the nucleated substrate to a temperature at which the deposition reaction will occur, a preferred means for heating the gas and the fibers to such a temperature is shown in FIGURE 2 to comprise the induction-coupled plasma torch 41 which consists of the energized induction coil 42 surrounding the quartz or other transparent tube 40 and connected to a high frequency generator operating at from four to thirty megacycles per second. Where the quartz tube is on the order of twenty-six millimeters in outside diameter and the induction coil is ytive or so turns of three-sixteenths-inch diameter copper tubing, a ten kilowatt alternating or direct current generator 43 has been found to provide suicient power. In lieu of the tubing, a plate-type coil using tive turns in pancake fashion may also be inductively coupled to the gas at or near thermal equilibrium to provide the beneficial fiber growth.

Where, according to the method and in the use of the apparatus of this invention, it is desired to form pyrolytic graphite fibers for example, acetylene gas may be introduced into the electrode gun 31 at the inlet passage 32 in such quantities that it will represent an adequate carbon source for the nucleation and the subsequent deposition of graphite upon the nucleated brous substrates. Other gases to be so used in the formation of pyrolytic graphite include the methane series gases, ethylene series gases, the acetylene series gases and the volatile aromatics. Such gas may -represent the entire gas iiow or it may be part of a mixture with a suitable carrier gas such as hydrogen or an inert gas such as argon, neon, krypton or helium in proportions of as low as forty parts by volume of the carbon-containing gas to sixty parts by volume of the carrier gas. In certain instances it may be desirable to supply additional gas, either of the carrier variety or additional carbon-containing gas, by injecting the same into the system via the inlet openings 33 andV 34 so that they will mix with the nucleated gas emitting from the discharge end of the barrel 25 of the electrode gun or by introduction of additional gas subsequent to the heat treating operation via the inlet passage 44. In certain situations where it might be desired to form the fibers of a nucleated carbon or graphite substrate having another pyrolytic coating such as a boride, nitride or carbide thereon, different pyrolytic-material-containing gases may be inserted. Thus for example, where a silicon carbide coating is desired upon the carbon substrate fibers, a vaporized silicon halide may be introduced into the gas stream at the inlet 44 so that it will react with the nucleated carbon and the carbon supplied by the acetylene gas or other carbon-containing gas in the pyrolyzing chamber to form the silicon carbide coating. Where graphite fibers are ultimately desired and the source gas is acetylene, the electrode gun and the entire heat treating and pyrolyzing system may be operated at pressures as low as four-tenths of an atmosphere. On the other hand, the precise reaction temperatures and most eiiicient pressures and flow rates will vary depending upon the materials being used and the properties and characteristics sought in the finished fibers, all according to known or readily determinable preferred procedures.

While the foregoing invention has been described in considerable detail in connection with certain preferred embodiments thereof, it is to be understood that the foregoing particularization has been for the purpose of illustration only and does not limit the scope of the invention as it is defined in the subjoined claims.

I claim:

1. A method for the manufacture of pyrolytic iibers comprising the steps of conducting a carbon containing, pyrolytic deposition gas from one of the methane, ethylene and acetylene series through a high voltage electric arc to nucleate a portion of the carbon therein, thereafter heating the gas with the nucleated carbon therein to a temperature within the range of from 500 to 1200 degrees centigrade, and thereafter passing the nucleated pyrolytic material through a second carbon deposition gas while heating the nucleated material to a temperature at which the carbon from said second deposition gas pyrolytically deposits upon the nucleated carbon.

2. A method according to claim 1 wherein said second deposition gas is a mixture of the first gas and an additional gas containing the same pyrolytic material.

3. A method according to claim 1 wherein said second deposition gas is a mixture of the first deposition gas and additional gas containing a pyrolytic material which is different from that in said iirst gas.

4. A method according to claim 1 wherein the gas containing the nucleated pyrolytic carbon is heated to said temperature by passing the said rst deposition gas through an optically transparent tube in heating proximity to a radiant energy heater.

5. A method according to claim 1 wherein the nucleated pyrolytic carbon is finally heated to the temperature at which the material from the second deposition gas will deposit thereon by an electrically energized induction coil which is inductively coupled to said second deposition gas.

6. A method according to claim 5 wherein said electrically energized induction coil is operated at a frequency within the range of from four to thirty megacycles per second.

7. A method for the manufacture of pyrolytic materials comprising passing a carbon containing deposition gas through an elongated electric are zone wherein the arc strikes transversely of the gas tiow but moves from one place to another longitudinally of said elongated arc zone whereby at least a portion of the pyrolytic material in said gas is nucleated and Vthereafter and while the nucleated material is suspended in and being carried by the deposition gas flow, heating the gas and the nucleated particles to a temperature at which a vapor phase deposition plating reaction occurs between the nucleated particles and the pyrolytic material in said deposition gas.

8. A method according to claim 7 wherein said arc is a high voltage arc energized with from 10,000 to 100,000 volts.

9. A method according to claim 7 wherein the electrical energy causing said arc is intermittently applied in pusles with a frequency of from 0.1 to 10.0 pulses per second.

10. A method according to claim 7 wherein additional pyrolytic-material-containing gases are admixed with the gas carrying the nucleated pyrolytic material prior to the time that the nucleated particles are heated to the temperature at which the plating reaction occurs.

11. A method according to claim 7 wherein said carbon-containing deposition gas is one of that class which consists of a methane series gas, an ethylene series gas, an acetylene series gas and the volatile aromatics.

12. A method according to claim 11 wherein said carbon-containing deposition gas is admixed with an inert carrier gas in the proportion of up to forty parts by volume of the carbon-containing gas to sixty parts by volume of the carrier gas. i

13. A method according to claim 12 wherein said carrier gas is one of that class which consists of nitrogen, argon, helium, krypton, neon and hydrogen.

14. An apparatus for the manufacture of pyrolytic fibers comprising :at least a pair of spaced longitudinally extending equipotential electrode surfaces, means for passing a pyrolytic-material-containing deposition gas between said surfaces, and simultaneously acting means for causing an electric arc to move between said surfaces.

15. An apparatus according to claim 14 wherein said pair of electrode surfaces is radially spaced and concentrically aligned.

16. An apparatus according to claim 14 which includes conduit means for carrying a laminar flow of the pyrolytic-material-containing deposition gas after the same has passed through said electrode surfaces.

17. An apparatus for the -manufacture of pyrolytic fibers comprising a conduit assembly, means for introducing and causing the iiow of a pyrolytic-material-containing deposition gas through said assembly, at least a pair of transversely spaced, longitudinally extending electrode surfaces, tiow-control lmeans for causing substantially all of said deposition gas to ow between such surfaces, means for electrically energizing said electrodes and causing a high voltage arc to jump from one electrode surface to the other, and pyrolyzing means downstream from said electrode surfaces for heating said gas to the temperature at which it will pyrolytically deposit the pyrolytic material contained therein.

18. An apparatus according to claim 17 wherein preheating means for heating said g-as to a temperature of from 500 to 1200 degrees centigrade are positioned between said electrode surfaces and said pyrolyzing means.

19. An apparauts according to claim'18 wherein said preheating means comprise -an optically transparent portion ofthe wall of said conduit and a radiant heater acting through said wall.

20. An apparatus according to claim 19 wherein said radiant heater is an infrared line heater.

21. An apparatus according to claim 17 wherein said pyrolyzing means comprise an optically transparent section of the wall of said conduit and an electrically energized induction coil adjacent said wall and inductively coupled to the gas therein.

22. An apparatus according to claim 17 comprising means between said pre-heating means and said pyrolyzing means for introducing additional deposition gas to the gas flowing within said conduit.

23. An apparatus according to claim 17 comprising a settling chamber forming a part of said conduit between said electrode surfaces and said pyrolyzing means.

24. An apparatus according to claim 23 wherein said settling chamber comprises an enlarged portion of said conduit whereby the linear rate of ow of the gases therethrough is diminished and heavy particles are allowed to settle therefrom.

25. An apparatus according to cl-aim 17 comprising means for intermittently energizing said electrode vsurfaces and causing the arc to jump across the space between them.

26. An apparatus according to claim 17 wherein said electrodes are concentrically aligned cylindrical members having radially spaced opposed surfaces through which said deposition gas is made to pass.

27. An apparatus according to claim 26 wherein said opposed surfaces are electrically equipotential surfaces.

28. An apparatus according to claim 26 wherein means are provided for rotating one of said electrodes relative to the other While the deposition gas is passing between them. Y

References Cited UNITED STATES PATENTS 2,796,331 6/1957 Kauffman et al. .Z3-209.4 2,957,756 10/1960 Bacon 23-209.2 3,009,783 11/1961 Sheer et al. 204-173 X 3,344,051 9/1967 Latham 204-173 10 ALFRED L. LEAV1TT,Primary Exam-mer.

I. H. NEWSOME, Assistant Examiner.

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