USRE28312E - Method for producing bokon-carbon fibers - Google Patents

Abstract

1. A METHOD FOR CONTINUOUSLY DEPOSITING NODE-FREE AMORPHOUR BORON TO UNIFORM THICKNESSES GREATER THAN 0.6 MIL ON A MOVING RESISTIVELY HEATED CARBON WIRE AS IT IS DRAWN THROUGH A SERIES OF REACTORS COMPRISING THE STEPS OF: MAINTAINING THE CARBON WIRE IN THE FIRST REACTOR AT A TEMPERATURE OF 1600*-2100*C.; EXPOSING THE CARBON WIRE WHILE AT SAID TEMPERATURE IN AT LEAST ONE REACTOR TO A GASEOUS STREAM CONSISTING ESSENTIALLY OF A CARBON-CONTAINING GAS ADMIXED WITH A DILUENT GAS TO EFFECT DEPOSITION OF PYROLYTIC GRAPHITE ON THE CARBON WIRE; MAINTAINING THE GRAPHITE-COATED CARBON WIRE IN A SUBSEQUENT REACTOR AT A TEMPERATURE OF 700*-1400*C., AND EXPOSING THE WIRE IN SAID SUBSEQUENT REACTOR TO A GASEOUS STREAM OF A BORON HALIDE ADMIXED WITH HYDROGEN TO EFFECT DEPOSITION OF ELEMENTAL BORON THEREON; SAID DEPOSIT OF PYROLYTIC GRAPHITE BEING DISPOSED TO PREVENT FRACTURE OF THE CARBON WIRE BY THE BORON.

Description

64-29 5 XQ REZ 8 v 3 l 2 5R 1975 M. BASCHE ETAL Re. 28,312

METHOD FOR PRODUCING HURON-CARBON FIBERS Original Filed March 27, 1969 United States Patent 28,312 METHOD FOR PRODUCING HURON-CARBON FIBERS Malcolm Basche, West Hartford, Conn., Roy Fanti, Springfield, Massn, Francis 5. Galasso, Manchester, and Urban E. Kuntz, East Hartford, Conn., and Richard D. Schile, Hanover, NJL, assignors to United Aircraft Corporation, East Hartford, Conn.

Original No. 3,679,475. dated July 25, 1972, Ser. No. 811,072, Mar. 27, 1969. Application for reissue Oct. 5, 1972, Ser. No. 295,393

Int. Cl. [344d 1/14, 1/18; HOlb 1/00 US. Cl. 117-216 4 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specillcation; matter prlnted in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to a method for producing boron fiber and more particularly relates to a method for continuously depositiong a relatively thick,

substantially constant diameter boron coating on a carbon filament.

It is known that filamentary boron may be produced by pyrolytic techniques in a process wherein the boron is chemically deposited on a resistively heated carbon monofilament which is exposed to a reactant gas consisting of boron trichloride admixed with hydrogen.

The use of carbon as a filamentary substrate for boron has been recognized as ofiering the potential of significant improvements in the field of composite materials. Carbon, which in the present disclosure also includes graphitic material, possesses desirable characteristics in the form of electrical conductivity, hot strength, apparent chemical compatibility with boron, low density and an attractive cost feasibility relative to presently used tungsten filamentary substrates. Although the potential of carbon as a substrate is thus recognized. realization of this potential has been limited by the degradation of the carbon fiber during the coating process. It has been observed that, although the deposition of boron on the carbon substrate can be initiated uniformly, the coating quickly takes on a hamboo-like appearance with periodic nodes of boron thickened circumfcrentially along the fiber. The areas of increased deposition are caused by the appearance of a plurality of hot spots along the fiber and subsequent tests have revealed that the hot spots are caused by fractures in the carbon core which produce an irreversible change in the electrical properties of the fiber. It was found that the fractures occur irrespective of whether the process be static or continuous and with the fiber at a uniform temperature. Further investigations have indicated that the substrate fracturing l5 attributable to an unexpected growth phenomenon. As the boron is deposited on the carbon it undergoes a period of expansion which. when unchecked. exceeds the strength of the carbon filament. and causes tracturing thereof. The exact cause and nature of this phenomenon is imperfectly understood at this time.

Recently, several techniques have been developed to improve the etlecuvencss of the basic continuous process through the close control of process conditions. in one of these methods, a continuous coating of node-fr e :uuor phous boron is achieved by carefully limiting resident exposure of the carbon substrate in the reactor to a time period shorter than that at which fracturing occurs At present however. the thickness of node-free boron which can be deposited on a one mil carbon filament by this technique is limited to a maximum of .6 mil to give a composite fiber of 2.2 mils.

SUMMARY OF THE INVENTION The present invention relates to the production of relatively large, constant diameter composite fibers. approximating 4 mils, in an improved process wherein filamentary carbon is modified through pretreatment. The lllVCllIlOll contemplates a process wherein a non-reactive structural barrier is provided between the carbon and the boron in the nature of an electrically conductive precoating ol pyrolytic graphite. In one particular embodiment of the invention, the pyrolytic graphite is deposited in thin layers to provide for relative slippage, without fracture, between the inner carbon bonded layer and the outer boron bonded layer to prevent hot spotting and provide a technique wherein relatively thick node-free boron coatings are achieved in reproducible fashion.

BRIEF DESCRIPTION OF THE DRAWING In the detailed description which follows. it will be convenient to make reference to the drawing which shows. in cross sectional view, an elevation of a reactor usable in the practice of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawing, a reactor 10 is shown and described below. It is to be understood, however. that although a single reactor is shown which is suitable for both carbon and boron deposition, a plurality of such reactors are preferably disposed. in sequence in the practice of the present invention. The reactor 10 comprises a tubular containment vessel 12 having dual gas inlets I4. 16 at the upper end and dual gas outlets or exhaust ports 18, 20 at the lower end thereof. During the deposition of pyrolytic graphite, the inlets l4 and 16 are utilized as a feed for a reactant gas mixture comprising a diluent gas, as for example the inert gas argon, and a C3l'bOll-COl1lZlll'ling gas such as methane. During the boron deposition, the inlets l4 and 16 are utilized as the feed for a reactant gas mixture comprising a boron halide and hydrogen. The containment vessel is typically formed of quartz or Pyrex. although a wide variety of other dielectrics and glasses are suitable. The gas inlet 16 and outlet 20 penetrate and are electrically connected to the metallic end plugs 24 and 26 which provide the end closures for the containment vessel and also. provide a convenient means through which the power may be supplied to the wire for resistance heating purposes.

The end plugs 24 and 26 are respectively formed to provide a well 30 and 32 for containing a conductive sealant 34. such as mercury. The mercury serves the dual purpose of p oviding a gas seal around the wire where it penetrates the end plugs and further providing elec tricnl contact between the wire and the end plugs. tllrtiupt the gas tubes 20 and 26. the leads 23 and Z5. and the DC power source 36. The end plugs are further prmulcu with an annular surface groove 38. which communicates with the mercury well 34 in the plugs through passage-- ways 40 and 42. to provide sealing between the plug and the abutting wall of the containment vessel where-u; gas

3 is prevented trom escaping from the reactor around the periphery or the plutrs.

The respective plugs are further each formed with centrally oriented oritict-s. 44 and 46. which are large crtuttuit to permit free passage of the wire therethrough but which. in combination with the wire. are small enough to retain the mercury. through stll'fitCt? tension forces. in the re pective ttells. 'l he end plug can be modified to include an orilieed ruby. tungsten or other suitable insort through which the wire passes and which provide the sealant retainment function previously mentioned.

In the process of the present invention. a plurality of reactors [U are serially disposed and a filamentary substrare 50 is drawn therethrough from a feed reel 52 to a take up reel 54 which maintain the wire under a slight tension as it passes through the orifice openings. Power from DC source 30 to the filament may be conveniently controlled by a resistor 56 although other means are suitable in .arryinc at the process wherein graphite is deposited on the carbon substrate in the reactor, conditions conventionally used for etlecting pyrolytic deposition of graphite may be used. For example, the carbon filament substrate may be resistively heated to a temperature in the range of 1600 to 2100' C. preferably 1900 to 2000 C. Temperatures above 1600 C. are needed to insure the formation t graphite rather than pure carbon. The reaction may be carried out at a pressure of one atmosphere A reactant gas which is introduced into the re actant chamber can be any carbon-containing gas suitable tor depositing pure carbon in graphitic form. In particular. methane. in an amount to 50 mol percent has given satisfactory results. The restricted concentration 01 methane in the reactant gas mixture is designed to prevent nodules troin forming from too high a concentratinn and in prevent the formation of soot. The reactant gas mixture also includes a diluent gas such as nitrogen. hydrogen or one of the inert gases. Argon, in an amount of St) to 9t) mol percent has been particularly metal A prererrett ratio in the reactant gas mixture is mol percent methane and 80 mol percent argon. Similarly, those conditions suitable for effecting pyrolyti'c boron deposition may also be used. For example. the graph|te-coated carbon substrate may be resistively heated to a temperature in the range of 700 to 1400' C., preterahly 1100" to 1300" C. The reaction may be carried our .11 a pressure of one atmosphere and the reactant gases may contain a boron containing gas leg. boron trichloride) in an amount of 15 to 75 mol percent and a reducing gas. preferably hydrogen. in an v amount 85 to mol percent. A preferred ratio of gases is 4o"? boron trichloride and b0 mol percent hydrogen.

During one investigation, a one mil carbon monofilamerit. trom Great Lakes Carbon Corporation. having a clean surface substantially free of imperfection. a circular cross section and an electrical resistance between 500 to 25th) ohms per men. was coated with two layers oi. p iolytic graphite prior to boron deposition. The mono tilament was passed through two reactors such as do scribed above. each having an etfective length of 3% inches In each reactor. the argon was provided at 800 cwmiri and the (H at IOU cc./min. with a wire speed ot l U tt..='hr. A substantially constant current or' 145 ma gave a substrate tcmpeiatuie of 1900" C. The original diamete r t the lilamerii was 1.05 mil and. utter the first pylfi l' tpiitk' coating. the diameter Wt'LS 1.16 mils. Alter ir-t ritifld oyrogrtiphite coating. the diameter was L3 nuts The pyrolytic graphite coated fiber as then passed hrough a boron reactor. The ichicwd with boron trichloiidc tccd of JUL) cc. min. and a hydrogen teed ot htltl (L/il'ltfl with .t tibcr speed ot tlt Hwhl through the a inch reactor The tibcr temperature was approximately 1200 C and the diameter measured $.11 mils The second boron layer as at:

tirsr boron layer as eomplihcd with a boron trichloride feed 0? 400 CC./Iflll'l. and a hydrogen feed of 600 ce/min with a fiber speed of liO ft./hr. The fiber temperature was 1170 during deposition and the diamctcr measured 2.9 mils. The third boron layer was achieved with a BC1 feed of 4nd cc./rnin and a hydrogen feed of 600 cc./m|ri with a fiber speed 0t 1S0 ftn'hr. The fiber temperature during this pass was maintained a about i200 C. There were no hot spots and no breakage of the carbon monolilamerit and the diameter of the final composite filament was smooth and uniform and measured 3.73 mils constant within 100i]! inch By way or comparison, a boron-carbon fiber was produced without pyrographite precoatings on the carbon A one mil carbon monofilament was run through the reactor at a speed of 230 fL/hr. and at a temperature of. H70 C with a BC]; feed or 400 cc./miri and an H; feed of 600 cc /min The resulting composite (X hibitcd the undesirable bamboo structure as previously discussed. The fiber had an average diameter ol 3 mils with nodes as large as 4 mils in diameter which were spaced 7 to it) mils apart What has been set forth above is intended primarily as exemplary to enable those skilled in the art in the practice of the invention and it should therefore be understood that, within the scope of the appended claims, the invention may be practiced in other ways than as specifically described.

What is claimed is: 1 A method for continuously depositing node-free amorphour boron to uniform thicknesses greater than 0.6 mil on a moving resistively heated carbon wire as it is drawn through a series of reactors comprising the steps of.

maintaining the carbon wire in the first reactor at a temperature of 1600-2l00 C.;

exposing the carbon wire while at said temperature in at least one reactor to a gaseous stream consistin essentially of a carbon-containing gas admixed with a diluent gas to effect deposition of pyrolytic graph ite on the carbon wire;

maintaining the graphite-coated carbon wire in a subsequent reactor at a temperature of 700-1400 C, and

exposing the wire in said subsequent reactor to a gaseous stream of a 1307"! halide admixed with by drogen to effect deposition of elemental boron thereon;

said deposit of pyrolyttc graphite being disposed to prevent fracture of the carbon wire by the boron. Z The method of claim 1 wherein said carbon-com taining gas is methane and said diluent gas [5 argon. said argon being present in an amount of 90 mol percent 3 The nvention of claim 1 wherein the pyrolvtic graphite coating is deposited in at least two layers 4 A "Iflh ll! lo conrmuouslv dcportmig none-free u' IOTD IUuS boron to uniform thicknesses greater rtum 0.0 "it! on a mount; resistive! mited carbon titre as H U drown Trough a sertes 0f reactors comp ising the ilepa mmatrrrurning "re carbon wire in the first reactor at a temperature rulficient to eflecr deposition 0/ pyr0- I\ :10 grriphtre thereon, said temperature being abate 1601) (l erpnsi'i the carbon wire while at .nri'd temperrirurr' in at least one reactor 10 i1 gaseous Stream POILHXHIU; essentially of a carbon-containing gtir ridmtxcd' ii H ri titlltt'll! gas to eflerr deposition of pyrolyrir. griipliirr on r te cu /101i wire;

mmrimiutu the .i,'mpltirt'-roiiri-tf carbon wire tn .1 subdream of a boron halide (tr/mun] wit/i livilro -t'ri 1c iii-u! tlt'PlMl/[Oll of t'it'mi'rtlul boron thereon. and

5 mid deposlr w pyw y xrapime being dis osed :0 3,567,826 prevent fracture a! {he carbon WHL' by the boron. 3,369,920 3,464,843 References Cited 3 79 2 5 following rcferenccs, cited bv the Examiner, are 5 of record m the patented file of lhws patcnt or [he onginal pu enl UNITED STATES PATENTS 2,767,289 lO/I956 RObII'lSOH IN-D1610 3,226,248 12/1965 Talley [IT-DIG. 10

2/1968 2/1968 9/1969 l l/l969 9/1970 2/l97l 6 Heestand at al. 11 -46 (7 G Bourdeau 117-46 CG Baschc -7 ll7--4h (l G Morelock ll7--4h L1G Turkat ll7 6 CG Morclock 117-1316v 10 MICHAEL SOFOCLEOUS, Primary Exammcr US. Cl. XR.

117 -46 (1.6.. 69, 93, 106 R, DIG, 10; 423- 4

Claims (1)

1. A METHOD FOR CONTINUOUSLY DEPOSITING NODE-FREE AMORPHOUR BORON TO UNIFORM THICKNESSES GREATER THAN 0.6 MIL ON A MOVING RESISTIVELY HEATED CARBON WIRE AS IT IS DRAWN THROUGH A SERIES OF REACTORS COMPRISING THE STEPS OF: MAINTAINING THE CARBON WIRE IN THE FIRST REACTOR AT A TEMPERATURE OF 1600*-2100*C.; EXPOSING THE CARBON WIRE WHILE AT SAID TEMPERATURE IN AT LEAST ONE REACTOR TO A GASEOUS STREAM CONSISTING ESSENTIALLY OF A CARBON-CONTAINING GAS ADMIXED WITH A DILUENT GAS TO EFFECT DEPOSITION OF PYROLYTIC GRAPHITE ON THE CARBON WIRE; MAINTAINING THE GRAPHITE-COATED CARBON WIRE IN A SUBSEQUENT REACTOR AT A TEMPERATURE OF 700*-1400*C., AND EXPOSING THE WIRE IN SAID SUBSEQUENT REACTOR TO A GASEOUS STREAM OF A BORON HALIDE ADMIXED WITH HYDROGEN TO EFFECT DEPOSITION OF ELEMENTAL BORON THEREON; SAID DEPOSIT OF PYROLYTIC GRAPHITE BEING DISPOSED TO PREVENT FRACTURE OF THE CARBON WIRE BY THE BORON.

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