CA1038330A - Preparation of graphite fibers using plasma - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A continuous process for manufacturing graphite filaments, yarns and other graphite products from carbon-aceous precursors, which precursors are in the form of filaments, yarn, roving, cord or continuous filament town, by using plasma as a high temperature source.

Description

~Q38330 Background of the Invention ; The present lnvention relates to a contlnuous process and apparatus for manufacturing graphite fi-bers~ yarns and other products from carbonaceou~ pre-cursors whlch precursors are in the form of fibers, yarn, roving, cord or continuous filament tows.
Among the high performance industrlal mater-lals for relnforced composites, graphite f1bers have shown a considerable superiority over other materials in the same desirable properties category. Much in-terest has been focused on graphite fiber production because of certain excellent properties such as high corrosion and temperature resistance~ low density, high tensile strength, and the most important, high modulus. Graphite is one of the very few know ma-terialæ which possesses the characteristic of offer-ing higher strength at higher temperature.
Carbon ~ibers are the bas1c precursor mater-lals for graphlte fibers. The fundamental structural difference between these two high performance materials is that graphite fibers essentially consist of atomic carbon and possess a specific X-ray diffraction pattern characteristic of graphite, whereas carbon fibers which also consist of atomic carbon show an amorphous X-ray diffraction pattern. The carbon content in graphite fibers is much higher than in carbon fibers. In addi-tion, graphite fibers and yarns generally have higher tenacity, higher modulus and are electrically and thermally more conductive than carbon fibers.

_2_ Descriptlon of the Prior Art Preparation of graphite fibers from carbon-aceous materials has been disclosed in such repre-sentative United States Patents as 3,285,696, 3,305,315 and 3,313,596. Theæe patents di~close graphitization at temperatureæ up to 3,000C. in an inert atmoæphere.
United States Patent 3,453,362 discloses a method for preparing graphite fibers using an induc-tion furnace as a heating source with residence times up to 25 minuteæ.
United States Patent 3,313,597 diæcloses a method of pasæing fiberæ over a pair of electrical contacts or electrified rollers while applying elec-tric current through the fibers to raise their tempera-ture to the graphitization temperature. A controlled atmoæphere of carbon monoxide, hydrogen, ammonia or mixture thereof is generally maintained during pas~age of the yarn. Additionally, arrangementæ muæt be pro-vided to æcrape off or otherwiæe remove reæidue on the contact rollæ. Furthermore, a æerious problem of arc-ing occurs at the point of departure of the yarn from the rollæ reæulting in local overheating and evapora-tion of portionæ of the yarn. Consequently, a high incidence of inhomogeniety along the finished yarn predominates and, therefore, a wide distribution of individuaI filament strength values exist.
Another approach to the graphitization of car-bon fibers is by the use of a resistance heated tube furnace to provide the necessary heat for graphitiza-tion. The apparatus requires high power to insure ample reserves for the heat-up requirements and to guarantee the attainment of the required 5,000 to 5,500F. temperatures for satisfactory graphitization. The furnace utilizes an - expensive graphite tube which has a limited life, and its replacement is laborious. Tube erosion and a low rate of heating are other unfavorable factors. In general, the pyrolysis process i8 slow.
United States Patent 3,449,077 discloses a method for the direct production of graphite fihers by the use of a reducing flame, but is limited to preoxidized polybenzimidazole fibers.
The use of a laser beam for graphitization has been disclosed in German Patent 1,945,145.

Summary of the Invention The present invention therefore provides a method of preparing graphite fibers from carbonaceous precursors comprising the following steps:
continuously passing carbonaceous filaments into a closed ; chamber containing at least one porous anode and at least one cathode, continuously passing a non-oxidizing gas into said chamber through said porous anode while simultaneously applying suffi-cient electr~cal power between said porous anode and said cathode to form an arc through said gas emanating from said porous anode and thereby to initiate a plasma, maintaining said plasma at a temperature of between about 2800 and 3400C., advancing said carbonaceous filament under longitudinal stress through said plasma at a rate which allows transformation of said carbonaceous filament into graphite filament, ~, ~ - 3 -~, ~ ... .

~ ~ 1038330 and advancing said graphite filament to collection means outside said chamber.
The present invention represents a very practical method for the graphitization of carbonaceous materials which ' involves the use of plasma. The process is simple, rapid, ; efficient, inexpensive and does not require any bulky apparatus.
A plasma is essentially a highly, or fully, ionized , gas in which the conduction electrons have been stripped from i the atoms of the gas. In the plasma state, a gas is a mixture ~ 10 of electrons and positive ions, with some negative ions formed - when free electrons attach themselves to neutral atoms. The electrical conductivity of the gas is thus increased, but its resistance is still finite.
In accordance with certain aspects of the present invention, the graphitization assembly comprises a graphitization .~ .

_ 3(a) -.i .

chamber containing one or more (typically three) water-cooled anodes and one water-cooled cathode. The three a~odes, capable of operating at 400 amps are oriented at a 120 angle to each other and are located on the ~ame plane. The termination point of each anode in the chamber iB shielded with a porous graphite tip through which the gas pas~es. The cathode assembly equlpped with a tunesten tlp iæ positioned at the bottom of the chamber such that the tip (made of tungsten) is perpen-dicular to a plane passing through the anodes. One or more of the anodes can be operated in con~unction with one cathode to achieve the required temperature.
To start the operation, power is applied to the electrodes and the ~low of gas is regulated to maintain an arc between them after a high frequency starter has been activated. Spacing of the anodes with respect to the cathode is not crltical as long a~ an arc is ini-tiated to ~orm the plasma.
The arc ionizes the gas quite rapidly result-ing in a plasma which conducts current through the chamber gas providing a uni~orm heating flame. Tem-perature control to some extent is done by varying the gas ~low rate, although limits do exist. Too high a gas flow without a sufficient current provides a cooling effect which makes it very difficult to main-tain the plasma state. The principal means of tempera-ture control, however, is by control of the power level.
Temperatures can be measured by use of an optical py-rometer used to view through a window fitted in the graphitization chamber.

Once the plasma state is formed and the re-quired temperature of about 2800_3000C. is attained, carbon yarns under longitudlnal stress are passed over the electrodes at a speed suitable for graphitl-zation usually 10 to 26 feet per minute, preferably 15 to 18 feet per minute. Preheating the yarn before it enters the plas~a helps to remove moisture from the yarn and also protect it ~rom thermal shock.
The use of a plasma as the heat source offers the advantage of an instantaneous high heat source and no tlme is lost in cooling the system for repair purposes s1nce the system can be shut off ln a matter of seconds for ~oinlng the broken ends of carbon fi-bers if necessary. Additionally, extremely high tem-peratures can be obtained ~ust by changing the flow of gas or current or both through the electrodes, dependlng on requirements. An inert atmosphere is automatically maintained in the graphitizatlon chamber, as the gas used is inert. Additonal inert gas may be introduced, however. As an illustration, a temperature of 3175-3200C. was obtained in a matter of a few seconds with an argon flow of 80 SCFH (Standard Cubic Feet per Hour) and a current of 175 amperes through each of three electrodes.
The same temperature can be obtained with the focused beam of a 100 watt C02 laser, as described in Germany Patent 1,945,145 noted above, but with the lasbr beam the intense heat is so localized that yarns under tension usually break due to vaporization of surface filaments. Secondly, the low thermal conduc-_6_ 1038~30 tivity of the surrounding gas inhibits the proper trans-fer of heat necessary for the uniform heating of the carbon yarn when passing throueh the focused beam of the laser. Consequently, partially graphitized fila-ments in the yarn are obtained, with many flaws. Such problems do not exist when graphitizing with plasma which imparts a very efficient and uniform heat to the material under controlled conditions.
Almost all gases can be ionized to form a plasma, but the specific selection of the type of non-oxidizing gas depends upon the type of application as well as the type of system used for generating the plasma. Inert gases such as He, Ne, Ar, Kr, Xe or Rn, can be used as well as H2, CO, C02, N2 and mixtures of two or more of these gases. In this particular process of graphitiza-tion, argon gas is preferred as it is an inert gas and can be ionlzed with low power to obtain the required temperatures. Reactive gases such as 2~ H2 or Cl can also be efficiently and reliably heated to over 3200C., but the life of the electrodes, under those conditions is known to be ~ust a few hours.

Description of the Drawing Other advantages of the invention will become apparent upon reading the following detailed descrip-tion and upon reference to the drawings, in which Fig.
1 is a schematic of the process of the present inven-tion.
As shown in Fig. 1, spool (1) dispenses carbon yarn (2) which is passed over let-off pulley (3) from the supply package and then passes oven tension rolls --6_ ~7- 1 0 ~ 3 ~
(4) and (5). Tension control system (5) offers the feasibility of applying tension up to 15 pounds per end of yarn, running at a speed of 60 feet per minute.
Longitudinal stress on the yarn and graphitization time can be ad~usted easily by varying the tension and the speed. At this time, the precursor carbon yarn (2) may be passed through a separately heated oven (not shown) to preheat it before it enters slit (6) in graphitization chamber (7). The preheating of the yearn removes the moisture therein and protects the yarn from thermal shock. Fig. l further shows two anodes (8) and (9) each having three lines, two of which supply water and electricity (lO) (13), gas (ll) (14) and the third which acts as return water and electricity lines (12) (15). At the bottom of the chamber is cathode assembly (16) having a tungsten tip cathode (17). Lines (18) (l9) (20) perform the same functions as the three lines used in anodes (8) (9). Partial preheating of carbon yarn is also ob-tained due to the temperature gradient in chamber (7).
In center of the chamber (7), the yarn attains the graphitization temperature of at least 2900C. while longitudinal stress on the ya-rn is continuously main-tained.

2~ It has been found that the very low denier carbon yarns, even under proper longitudinal stress frequently fail to withstand the direct impingement of plasma resulting in either yarn breakage or flaws in the graphitized yarn. To overcome this minor prob-lem, the system can easily be modified by using a small ~g;
.~ i trough or tube (71) of extre~ely high temperature re-sistant material such as graphite, boron nitride or tungsten in the graphitization zone without causlng a significant temperature drop. However, a neutral atmosphere must be maintained to prevent any rapid deterioratlon due to oxidation.
Graphite yarns emerge through slit (21) and are wound up on spool (22). Temperatures in the chamber (7) can be measured with an optical pyrometer by view-ing through the window (23) fitted in the graphitiza-tion chamber (7). (Optical pyrometer not shown in the drawing).
Once the plasma is stabilized and the required temperature of about 2900-3000C. is obtalned, the ,-yarn is positioned in the plasma and the yarn speed through the chamber is set easily to produce graphite yarn~ continuously. To avoid breakage of yarn at the start, the yarn can be run at a higher speed than normal until the plasma is stabilized.
The system described herein offers extreme flexibility of processing cohditions with regard to temperature, yarn speed, number of yarns and amount of tension. This system offers a wide range of processing conditions which can be used to produce products with a wide range of properties.
It should be understood that various aspects of the process as mentioned above, including argon gas as the exemplary material for the plasma, are equally applicable to any other system using plasma as heating source for graphitization.

B

10;~4330 The invention is illustrated but not limlted thereto by the ~ollowing examples:

Example I
Two ply carbon yarn having 2.0 Z x 2.0 S twist was graphitized using two anodes and one cathode. Cur-rents were 130 and 175 amperes. Argon gas ~low was contlnuously maintalned at 80 SCFH through each elec-trode. The yarn was graphitized under tenslon at the rate of 15 ft./minute. Results were:
Avg. Breaking Avg. Tensile Elongation Modulus TensionStrength (psi)(~) (psi) ; 500 gms.98.5 x 103 0.59 16.8 x 106 Example II
The yarn of Example I was graphitized with all three anodes in operation. Gas flow was maintained at 80 SCFH while current flow was 190 amperes through each electrode. Yarn was graphitized at 17 ft./minute.
The temperature recorded was 3175C. The yarn was pre-heated to 1300F. before entering the graphitization zone. Results were:

Tensile Avg. Breaking Strength Elongation Modulus Tension(psi) (~) (psi) ; 25 425 gms.154.2 x 103 0.90 17.3 x 10 Example III
The yarn of Example I was graphitized under the same conditions as in Example II except for an increase in tension. Results were:

-10- 10;~8330 .
Avg. Breaking Tensile Strength Elongation Modulus Tension (psl) (%) (psi) 750 gms.152.3 x 103 0.47 32.7 x 106 Example IV
The yarn o~ Example I was graphitized by passing through a plasma-heated tungsten tube at 3350C. All three anodes were ln operation. Gas flow was 85 SCFH
and current was 2000 amperes through each electrode.
Results were:
Avg. Breaking Tensile Strength Elongation Modulus Tension (psi) (~) (psi) 680 gms.119 x 103 o.66 18.2 x 106 ~10--

Claims (8)

We claim:

1. A method of preparing graphite filaments from carbonaceous precursors comprising the following steps:
continuously passing carbonaceous filaments into a closed chamber containing at least one porous anode and at least one cathode, continuously passing a non-oxidizing gas into said chamber through said porous anode while simul-taneously applying sufficient electrical power between said porous anode and said cathode to form an arc through said gas emanating from said porous anode and thereby to initiate a plasma, maintaining said plasma at a temperature of be-tween about 2800° and 3400°C., advancing said carbonaceous filament under longi-tudinal stress through said plasma at a rate which allows transformation of said carbonaceous filament into graphite filament, and advancing said graphite filament to collec-tion means outside said chamber.

2. The method of preparing graphite filaments as defined in claim 1 wherein said chamber contains three anodes.

3. The method of preparing graphite filaments as defined in claim 2 wherein said anodes are oriented at an 120° angle to each other and are located on the same plane.

4. The method of preparing graphite filaments as defined in claim 3 wherein said gas is selected from the group consisting of He, Ne, Ar, Kr, Xe, Rn, H2, CO, CO2, N2 and mixtures of two or more of these gases.

5. The method of preparing graphite filaments as defined in claim 4 wherein the termination point of each said anode in the chamber is shielded with a por-ous graphite tip through which said gas passes.

6. The method of preparing graphite filaments as defined in claim 5 wherein said cathode is equipped with a tungsten tip.

7. The method of preparing graphite filaments as defined in claim 6 wherein said carbonaceous fila-ment is advanced at the rate of 10 to 26 feet per minute.

8. The method of preparing graphite filaments as defined in claim 6 wherein said carbonaceous fila-ment is advanced at the rate of 15 to 18 feet per minute.