Abstract
Process of producing carbon/carbon composite Disclosed herein is an improved process for the fabrication of carbon/carbon composite consisting of carbon as the matrix material and carbon fiber dispersed in the matrix material as the reinforcing phase, wherein hybrid carbon fibers are chopped, split and then mixed with mesophase pitch using distilled water to form charge, said charge is molded, dried, hot pressed and carbonized. The pores created during carbonisation are then filled using resin impregnation, said carbon/carbon compact is thermoset and finally carbonized. This cycle of resin impregnation-thermosetting-carbonisation is repeated 3 times. In addition to devising a processing technology involving less processing time, invention addresses the problem of producing carbon/carbon composite with variable density and high conductivity. Fig 4
Information
| Application ID | 1713/CHE/2012 |
| Invention Field | CHEMICAL |
| Date of Application | 2012-05-02 |
| Publication Number | 03/2016 |
Applicants
| Name | Address | Country | Nationality |
|---|
Specification
PROCESS OF PRODUCING CARBON/CARBON COMPOSITE
FIELD OF INVENTION:
The invention is in the field of composite fabrication and relates to a method of fabricating inverse composite and more specifically to the fabrication of variable density and high conductivity carbon/carbon composite. It has wider application in aerospace and automobile industries, especially for the manufacturing of electrolyte/bipolar plate for Proton Exchange Membrane Fuel Cells.
BACKGROUND OF THE INVENTION:
Materials having low density and high strength are always in demand for aerospace applications. Carbon/carbon composite (hereinafter C/C composite) is an evolution in this class of engineering materials, which is ceramic in nature but exhibits brittle to pseudo plastic behavior based upon the structure. It is a unique material wherein carbon fibers (hereinafter CFs) are embedded in carbon matrix and is known as an inverse composite. Due to its excellent properties coupled with low density, high thermal and electrical conductivities, it is especially suitable for space applications such as nose-tips, elevons, wing leading edges of re-entry vehicles, rocket nozzles, aircraft brake discs, fasteners, pylons, vanes of hot gas motor, radiator plates, optical mirrors, etc. Apart from the above, C/C composite finds application in bio-medical and electronics industry.
Since C/C composite has high electrical and thermal conductivity and tailorable density, it is most suitable for making bipolar plate for fuel cells. At present, the practice employed to make such C/C composite bipolar plate is Liquid Phase Impregnation (LPI) wherein prepreg is impregnated with resin matrix followed by thermosetting and finally carbonization. At higher temperature, resin gets decomposed thermally and gasified resulting in pores in the matrix. These pores not only decrease the strength and density but also the electrical and thermal conductivities. In order to overcome this difficulty, the methods like Chemical Vapor Infiltration (CVI), Hot Isostatic Pressure Impregnation Carbonisation (HIPIC), etc are used. But, the high cost and longer process durations associated with these traditional methods make them commercially unviable.
An exhaustive prior art search revealed the existence of following patents in this domain, which are somewhat closer to our invention.
U.S. Pat. No. 5,871,838 by James et al. discloses a method for making C/C composite using chopped discontinuous CFs as reinforcement and carbonisable organic powder as matrix by vacuum molding, and hot pressing at 400°C and 2000 psi. This process has the limitation of producing C/C composite of density 1.1 g/cc at the maximum.
European Pat. No. 0 402 915 by Okura et al. discloses a method for manufacturing frictional C/C composite body using chopped discontinuous CFs as reinforcement and matrix with carbon fillers. But, the process yields C/C composite with less electrical conductivity, consuming more processing time.
US Pat. No.6,171,720 by Besmann et al. discloses a method for the fabrication of C/C composite bipolar plate using compression molding and phenolic resin as matrix and chopped discontinuous CFs as reinforcement material. This method also takes more processing time for producing C/C composite.
To sum up, the existing C/C composite realization processes have many limitations. They produce C/C composite with either less density or with less electrical conductivity and consume more processing time. They are not cost effective also. All the above drawbacks of the prior art evolved a need for development of a new process devoid of these problems.
OBJECTS OF THE INVENTION:
In order to address the drawbacks of the prior art, the present application discloses a process for producing C/C composite with the following objectives:
The main object of the invention is to devise a cost effective method of producing C/C composite of high density and high electrical conductivity.
Another object of the invention is to produce C/C composite with high electrical conductivity both in plane and through plane.
Yet another object of the invention is to devise a flexible process of producing C/C composite of variable densities.
Still another object of the invention is to devise a method of producing C/C composite that is devoid of chipping during machining.
Further object of the invention is to devise a processing technology using molding-sintering route.
SUMMARY OF THE INVENTION:
The invention presents a cost effective and rapid process for the fabrication of C/C composite with density up to 1.72 g/cc and electrical conductivity up to 1200 S/cm. The method involves steps including the following: The hybrid (Polyacrylonitrile (hereinafter PAN) and pitch) CFs are chopped to desired aspect ratio using indigenously designed Fiber Milling unit. Chopped CFs are then split using air pressure. The split CFs and mesophase pitch powder are mixed with distilled water to form charge. Then, the charge is molded in to a green cake and dried to get the C/C preform. The C/C preform is then hot pressed and carbonized. The pores created during carbonisation are filled using resin impregnation. The C/C compact is then thermoset. Finally, carbonisation of the thermoset C/C compact is done. This cycle of resin impregnation-thermosetting-carbonisation is repeated 3 times to obtain C/C composite of predetermined specifications.
The method according to the invention as summarized above is a highly cost effective method for producing C/C composite of very high density up to 1.72 g/cc in mere three processing cycles. Also, the method produces C/C composite of high electrical conductivity up to 1200 S/cm in plane and 600 S/cm through plane. Further, the method is flexible enough to produce C/C composite of variable densities.
The method according to the invention solves the problem of chipping generally observed during machining of C/C composite using chopped CFs randomly dispersed in carbon matrix as reinforcement. Also, the method employs mesophase pitch during sintering process that delivers high electrical conductivity C/C composite in a cost effective manner. The method also facilitates achievement of uniform properties throughout the material, by employing chopped CFs as reinforcement with optimum aspect ratio.
BRIEF DESCRIPTION OF THE DRAWING:
The invention will be described in greater detail with reference to the embodiments of the invention, as illustrated in the accompanying drawing and table, expressed as following:
Figure 1 shows the 200 X SEM micrograph of a section of molded-sintered C/C compact,
Figure 2 shows the 200 X SEM micrograph of a section of molded-sintered-carbonized C/C compact,
Figure 3 shows the C/C composite realized from the rapid process,
Figure 4 shows the Process flow chart,
Figure 5 shows the temperature vs time graph of the sintering process,
Figure 6 shows the temperature vs time graph of carbonization process,
DETAILED DESCRIPTION OF THE INVENTION:
The invention describes a method of manufacturing C/C composite, which is not limited to, but has a major application in fabricating electrolyte/bipolar plate in Proton Exchange Membrane Fuel Cells. It is developed based on the molding of the Hybrid charge, which comprises PAN and pitch based CFs, and mesophase pitch, into various shapes and sizes according to the mold. The process flow chart for the fabrication of C/C composite in accordance with the invention is shown in figure 4. To enhance the strength and conductivity of C/C composite, the hybrid form of CFs is used.
The process involved in the fabrication of tailorable density and high conductivity C/C composite is described below:
Petroleum pitch is used to develop the mesophase pitch using, preferably but not restricted to thermotropic route. A pitch is a thick, viscoelastic polymer and a petroleum pitch is a pitch that is derived from petroleum.
The petroleum pitch is inspected and tested for various parameters like carbon-hydrogen-nitrogen (CHN), aromaticity, softening point, quinoline insolubility, domain size and density. The pitch and PAN based CFs are tested for various control parameters such as carbon contents, filament diameter, density, tensile strength, etc.
The Mesophase synthesis is done by converting the petroleum pitch to a mesophase pitch using, preferably, a thermotropic route. Mesophase pitch preparation is a precursor for preparing CFs and matrix. There are numerous methods for such a synthesis of mesophase pitch from petroleum pitch. The synthesis could involve heating the petroleum pitch under a rapid nitrogen flow to remove effectively low molecular weight non-mesogen species which formed an isotropic phase. It may also involve the crushing and heat treatment of raw petroleum pitch.
The hybrid PAN and pitch CFs are chopped to desired size, preferably having an aspect ratio of 300 to 2500 through a Fiber Milling unit.
Next, the charge is prepared by selecting the matrix to reinforcement ratio followed by selection of PAN to pitch fiber ratio. The matrix is formed by the mesophase pitch and the reinforcement is formed by a suitable weight ratio between the PAN based and pitch based CFs. The chopped CFs are split mechanically using air pressure ranging between 0.5 and 3 bar, preferably in a dessicator unit. The split CFs in which the quantity of pitch being 2 to 3 times that of the PAN and mesophase pitch powder are mixed with distilled water in the mixing chamber of a molding unit. Uniformly mixed charge is transferred to the molding chamber associated with die for molding the green cake. The green cake is dried at 60°C for 10 to 16 hrs to remove the water content using custom built air oven. C/C preform is taken out from oven after drying.
The C/C preform is loaded in a hot press. Hot-pressing is a pressure sintering operation. It comprises of hot pressing preform at 0.1 to 0.5°C/min at a pressure ranging between 10 to 16 MPa and temperature between 700 to 800°C to form a semi pyrolyzed and compact part. This sintering process eliminates the necessity of further impregnation of mesophase pitch to enhance density and conductivity of C/C compact, which in turn reduces the processing cost. The temperature vs time graph of sintering of C/C composite is as shown in figure 5.
After hot pressing, the mold with the C/C compact is inspected for pitch oozing. The mold is unloaded from the hot press and loaded onto an extraction fixture to remove the C/C compact from the mold under pressure. Figure 1 shows the 200 X SEM micrograph of a section of the molded-sintered C/C compact.
The C/C compact is then loaded in the carbonisation furnace. The carbonisation is carried out under inert atmosphere at 0.5 to 2°C/min for a duration of 1 to 3 hours to complete pyrolysis of the hydrocarbons to form carbon matrix. Figure 2 shows the 200 X SEM micrograph of a section of the molded-sintered-carbonized C/C compact. The carbonisation is done in the presence of nitrogen at a temperature from about 1000 to 1100°C. Figure 6 shows the temperature vs time graph of carbonization of C/C composite.
The pyrolyzed C/C composite is allowed to undergo impregnation, which is a densification process, that assists in tailoring the properties of the C/C composite. Impregnation is used to fill the pores created during carbonisation. In impregnation, phenolic resin is used to fill the pores under a pressure of 5 to 8 bar for 2 to 5 hrs. After impregnation, C/C composite is thermoset at 0.3 to 2°C/min at 150 to 250°C for a duration of 10 to 18 hours followed by carbonisation. This cycle of resin impregnation-thermosetting-carbonisation is repeated at least 3 times. The C/C composite hence realized is shown in figure 3.
Validation
The invention is further validated by experiments. A total of 35 experiments were carried out by varying the length of CFs, ratio b/w CFs, ratio b/w reinforcement to matrix, mixing time, mixing medium, molding time, drying time and temperature, sintering time and temperature and pressure and heating rate and cooling rate, pyrolysis time and temperature and heating rate and cooling rate, impregnation time and temperature and pressure and heating rate. These variations couldn't result significant change in the achievable properties.
The best combination of properties was achieved in the experiment where the following settings were employed. CFs were chopped using Fiber Milling equipment and mesophase pitch was synthesized in the custom built synthesis system through a thermotropic route. A charge containing matrix (mesophase pitch) and reinforcement (3mm length for pitch based and 9mm for PAN based, chopped CFs in the ratio of 70: 30 wt % respectively) in the ratio of (50: 50 wt %) was prepared. Charge was prepared by mixing the chopped CFs and powder mesophase pitch in distilled water, followed by molding the same in a die of size (70mm x 50mm x 20mm). After molding, the charge was kept along with die in an air oven for 20 hours at a temperature of 60°C to remove the water content. The dried preform was then loaded in the hot press. Sintering was carried out at 0.2°C/min at 700°C for 3 hours. Slow heating rate was employed for the smooth liberation of hetero atoms. Sintered and semi-carbonised C/C compact was loaded in a carbonisation furnace. Pyrolysis was carried out up to 700°C and at 1°C/min from 750 to 1050°C with a soaking of 1 hour at 1050°C. Carbonized C/C compact was impregnated in carbonisable organic (phenolic resin) under a pressure of 8 bar for 5 hours to fill the pores created during pyrolysis. Impregnated compact was thermoset for 18 hours at 220°C. Thermoset compact was carbonized. This cycle of resin impregnation-thermosetting-carbonisation was repeated 3 times.
Properties of the resulting C/C composite realized through this process are shown in Table-1 below. A reduction in the length of CFs results in more uniformity in green cake and C/C compaction. Compressive strength was found significantly improved compared to other experiments. It was observed that short length CFs act as filler, making it tightly packed.
TABLE 1
The foregoing description is a specific embodiment of the present invention. It should be appreciated that this embodiment is described for purpose of illustration only, and that numerous alterations and modifications may be practiced by those skilled in the art without departing from the spirit and scope of the invention. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.
WE CLAIM:
1. A method of producing C/C composite, comprising the steps of:
synthesising mesophase pitch from petroleum pitch;
chopping hybrid CFs in desired aspect ratio, where said hybrid CFs are PAN and pitch CFs;
splitting the chopped CFs using air pressure;
preparing a charge by mixing the split CFs and mesophase pitch with distilled water;
molding the charge into a green cake;
drying the green cake to obtain the C/C preform;
hot pressing the preform to a semi pyrolyzed C/C compact part;
carbonising the compact to form a carbon matrix;
impregnating pores of the C/C compact with phenolic resin; and
thermosetting and carbonizing the C/C compact.
2. The method as claimed in claim 1, wherein the petroleum pitch is converted into mesophase pitch by crushing and heat treatment of the petroleum pitch.
3. The method as claimed in claim 1, wherein the petroleum pitch and the hybrid CFs are inspected and tested prior to the process
4. The method as claimed in claim 1, wherein the aspect ratio of CFs is in the range of 300 to 2500.
5. The method as claimed in claim 1, wherein the CFs are split under air pressure ranging between 0.5 and 3 bar.
6. The method as claimed in claim 1, wherein the quantity of pitch CFs is at least 2 to 3 times the quantity of the PAN CFs for the preparation of the charge.
7. The method as claimed in claim 1, wherein preparation of the charge involves mixing mesophase pitch with reinforcement comprising pitch and PAN fibres in equal quantity.
8. The method as claimed in claim 1, wherein the green cake is dried at 60°C for 10to16hrs.
9. The method as claimed in claim 1, wherein hot pressing is performed at 0.1 to 0.5°C/min at a pressure ranging between 10 to 16 MPa and temperature between 700 to 800°C.
10. The method as claimed in claim 1, wherein carbonisation is done at 0.5 to 2°C/min at 1000 to 1100°C for a duration of 1 to 3 hours.
11. The method as claimed in claim 1, wherein impregnation is carried out under 5 to 8 bar for 2 to 5 hrs.
12. The method as claimed in claim 1, wherein thermosetting is done at 0.3 to 2°C/min at 150 to 250°C for a duration of 10 to 18 hours.
13. The method as claimed in claim 1, wherein the steps of impregnation, thermosetting and carbonizing is repeated thrice.
14. The method as claimed in claim 1, wherein the C/C composite has a density ranging up to 1.72 g/cc
15. The method as claimed in claim 1, wherein chopped CFs are randomly dispersed in carbon matrix for chipping free machining of the composite.
16. The method as claimed in claim 1, wherein mesophase pitch is employed during hot pressing process for high electrical conductivity.
17. C/C composite obtained according to the method as claimed in any one or more of the preceding claims.
Documents
| Name | Date |
| 1713-CHE-2012 POWER OF ATTORNEY 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 FORM-3 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 FORM-18 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 CLAIMS 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 DRAWINGS 02-05-2012.pdf | 2012-05-02 |
| abstract1713-CHE-2012..jpg | 2013-05-23 |
| 1713-CHE-2012-FER.pdf | 2017-01-10 |
| 1713-CHE-2012 DESCRIPTION (COMPLETE) 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 FORM-8 02-05-2012.pdf | 2012-05-02 |
| 1713-CHE-2012 FORM-2 02-05-2012.pdf | 2012-05-02 |
| Examination Report Reply Recieved [07-07-2017(online)].pdf | 2017-07-07 |
| 1713-CHE-2012 FORM-1 02-05-2012.pdf | 2012-05-02 |
| Drawing [07-07-2017(online)].pdf | 2017-07-07 |
| 1713-CHE-2012 CORRESPONDENCE OTHERS 02-05-2012.pdf | 2012-05-02 |
| Other Document [07-07-2017(online)].pdf | 2017-07-07 |
| 1713-CHE-2012 ABSTRACT 02-05-2012.pdf | 2012-05-02 |
| Description(Complete) [07-07-2017(online)].pdf_207.pdf | 2017-07-07 |
| Form 26 [07-07-2017(online)].pdf | 2017-07-07 |
| Description(Complete) [07-07-2017(online)].pdf | 2017-07-07 |
| 1713-CHE-2012-HearingNoticeLetter-(DateOfHearing-02-12-2019).pdf | 2019-11-18 |
| 1713-CHE-2012-HearingNoticeLetter-(DateOfHearing-30-12-2019).pdf | 2019-12-09 |
| 1713-CHE-2012-REQUEST FOR ADJOURNMENT OF HEARING UNDER RULE 129A [28-11-2019(online)].pdf | 2019-11-28 |
| 1713-CHE-2012-Written submissions and relevant documents (MANDATORY) [11-01-2020(online)].pdf | 2020-01-11 |
| Claims [07-07-2017(online)].pdf | 2017-07-07 |
| 1713-CHE-2012-IntimationOfGrant11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-Marked up Claims_Granted 331700_11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-Drawings_Granted 331700_11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-Claims_Granted 331700_11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-Correspondence to notify the Controller (Mandatory) [27-12-2019(online)].pdf | 2019-12-27 |
| 1713-CHE-2012-Description_Granted 331700_11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-PatentCertificate11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-RELEVANT DOCUMENTS [21-09-2021(online)].pdf | 2021-09-21 |
| 1713-CHE-2012-Abstract_Granted 331700_11-02-2020.pdf | 2020-02-11 |
| 1713-CHE-2012-RELEVANT DOCUMENTS [22-09-2022(online)].pdf | 2022-09-22 |
Orders
| Applicant | Section | Controller | Decision Date | URL |