MXPA00002200A - Pitch-based carbon foam and composites - Google Patents

Abstract

A process for producing carbon foam or a composite is disclosed which obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphitic structure in the struts.The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications.

Description

COMPOUND MATERIALS AND FISH-BASED CARBON FOAM CROSS REFERENCE TO RELATED APPTIONS Not Appble DECLARATION REGARDING RESEARCH RESPONSIBILITY O FEDERAL DEVELOPMENT The Government of the United States has rights in this invention in accordance with contract no. DE-AC05- 960R22464 between the Department of Energy of the States United and the Energy Research Corporation Lockheed Martin.

BACKGROUND OF THE INVENTION The present invention relates to carbon foam and composite materials and more particularly to a process for the production of both. The extraordinarily mechanical properties of commercial carbon fibers are due to the unique graphitic morphology of the extruded filaments. See Edie, D.D., "Pitch and Mesofase Fibers", in Carbon Fibers, Filaments and Composites, Figueiredo (editor), Kluwer REF. 32785 Academic Publishers, Boston, pp. 43-72 (1990). Contemporaneously, advanced structural composite materials exploit these properties by creating a disconnected network of graphite filaments that are hung together by an appropriate matrix. The carbon foam that is derived from a fish precursor can be considered to be an interconnected network of graphite ligaments or columns as shown in Figure 1. As such interconnected networks, they represent a potential alternative as a reinforcement in structural composite materials. Recent developments of reinforced fiber composite materials have been driven by the requirements to improve strength, inflexibility, drag resistance and rigidity in structural engineering materials. Carbon fibers have led significant advances in these properties in composite materials of various polymer, metal and ceramic matrices. However, current apptions of carbon fibers have evolved from structural reinforcement to thermal management in the apption of the range from high density electronic modules to communication satellites. This has simulated research into novel reinforcement methods and composite material procedures. High thermal conductivity, low weight and low coefficient of thermal expansion are the primary interests in thermal management apptions. See Shih, ei, "Development of Carbon-Carbon Composites for Electronic Thermal Management Apptions", IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract Number F33615-93-C-2363 and Contract Number F29601 -93-C-0165 of AR Phillips Laboratory and Engle, GB, "High Thermal Conductivity C / C Composites for Thermal Management", IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory with Contract Number F33615-93 -C-2363 and AR Phillips Laboratory with Contract Number F29601-93-C-0165. Such apptions strive to achieve a type of sandwich in which a core material of low structural density (i.e. honeycomb or foam) is placed in the form of a sandwich between a front sheet of high thermal conductivity. Structural core cores are limited to low density materials to ensure that the weight limits do not exceed. Unfortunately, materials from carbon foams and honeycombs are the only materials available for use in high temperature apptions (> 1600 ° C). Carbon honeycomb materials of high thermal conductivity are extremely expensive to manufacture compared to honeycombs of low conductivity, therefore, a performance penalty is paid for low cost materials. High conductivity carbon foams are also more expensive to manufacture than low conductivity carbon foams, in part, due to the input materials. In order to produce carbon foams of high rigidity and high conductivity, invariably, a fish should be used as a precursor. This is because the fish is the only precursor which forms a highly aligned graphite structure which is a requirement for high conductivity. Typical processes use a blow technique to produce a fish precursor foam in which the fish melts and passes from a high pressure region to a low pressure region. Thermodynamically, this produces a "Flash", which causes low molecular weight compounds in the fish to evaporate (the boiling fish) resulting in a fish foam. See Hagar, Joseph W. and Max L. Lake, "Novel Hybrid Composites Based on Carbon Foams," Mat. Res. Soc. Symp. , Materials Research Society, 270: 29-34 (1992), Hagar, Joseph W. and Max L. Lake, "Formulation of a Mathematic Process Model for the Foaming of a Mesophase Precursor Coal", Ma t. Res. Soc. Symp. , Materials Research Society, 270: 35-40 (1992), Gibson, L.J. and M.F. Ashby, Cellular Solids: Structures &; Properties, Pergamon Press, New York (1988), Gibson, L.J., Mat. Sci. And Eng A110, 1 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36 (4), (1976), and Bonzom, A., P. Crepaux and E.J. Moutard, U.S. Patent 4,276,246, (1981). Then, the fish foam must be stabilized oxidatively by heating in air (or oxygen) for many hours, thereby cross-linking the structure and "placing" the fish so that it does not melt during carbonization. See Hagar, Joseph W. and Max L. Lake, "Formulation of a Mathematical Process Model for the Foaming of a Mesophase Carbon Forerunner, Mat. Res. Soc. Symp., Materials Research Society, 270: 35-40 (1992) and White, JL and PM Shaeffer, Carbon, 27: 697 (1989) This is a time consuming stage and can be an expensive stage that depends on the required particle size and equipment.The "settled" or oxidized fish is then carbonized In an inert atmosphere at temperatures as high as 1100 ° C. Then, the graphitization runs at temperatures as high as 3000 ° C to produce a graphite structure of high thermal conductivity, which results in a strong and very thermally conductive foam. they use a polymeric precursor, such as phenolic, urethane or mixtures thereof with fish, see Hagar, Joseph W. and Max L. Lake, "Idealized Strut Geometries for Open-Celled Foams", Mat. Res. Soc. Symp., Materials Research Society, 270: 41-46 (1992), Aubert, JW, (MRS Symposium Proceedings, 207: 117-127 (1990), Cowlard, F.C. and J.C. Lewis, J. of Mat. Sci. , 2: 507-512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1: 285-302, (1969). High pressure is applied and the sample is heated. At a specific temperature, the pressure is released, which in turn causes the liquid to form foam as volatile compounds that are released. The polymer precursors are vulcanized and then carbonized without a stabilization step. However, these precursors produce a "glazed" or vitrous carbon which does not exhibit graphite structure and thus has a low thermal conductivity and low rigidity. See Hagar, Joseph W. and Max L. Lake, "Idealized Strut Geometries for Open-Celled Foams," Ma t. Res. Soc. Symp. , Materials Research Society, 270: 41-46 (1992).

In any case, once the foam is formed, then it is bonded in a separate step to the front sheet that is used in the composite material. This can be an expensive stage in the use of foam. The process of this invention overcomes these limitations, by not requiring a "blowing" or "pressure release" technique to produce the foam. In addition, an oxidation stabilization step is not required, as in other methods that are used to produce fish-based carbon foams with a highly aligned graffiti structure. This process is of less time of consumption, and consequently, it will be lower in costs and easy to manufacture. Finally, the foam can be produced with an integrated carbon sheet of high thermal conductivity on the surface of the foam, thereby producing a carbon foam with a smooth sheet on the surface to improve heat transfer. Brief Description of the Invention The general objective of the present invention is to provide carbon foam and composite materials from mesophase or isotropic fish such as synthetic, petroleum or pitch coal based fish.

Another objective is to provide a carbon foam and a composite material from fish which does not require an oxidative stabilization stage. These and other objects are completed by a method of producing carbon foam where an appropriate mold form is selected and preferably an appropriate mold release agent is applied to the mold walls. The fish is introduced at an appropriate level in the mold, and the mold is purged of air as by the application of a vacuum. Alternatively, an inert fluid could be used. The fish is heated to a temperature sufficient to coalesce the fish in a liquid which is preferably from about 50 ° C to almost 100 ° C above the softening point of the fish. The vacuum is released and an inert fluid is applied at a static pressure above almost 1000 psi. The fish is heated to a temperature sufficient to cause the gases to form and foam the fish. The fish is later heated to a temperature sufficient to coke the fish, which is cooled to room temperature with a simultaneous and gradual release of pressure. In another aspect, the previously described steps are used in a mold composed of a material in such a way that the molten fish does not wet it.

In yet another aspect, the objectives are accompanied by the carbon foam product that is produced by the methods mentioned herein that include a foam product with a smooth integral front sheet. In yet another aspect a product of carbon foam composite material is produced by the adhesion of the front sheets to a carbon foam that is produced by the process of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a micrograph illustrating typical charcoal foam with interconnected carbon ligaments and open porosity. Figs. 2-6 are micrographs of carbon foam derived from graphitized fish at 2500 ° C and at various magnitudes.

Fig. 7 is a micrographic MBE of the foam produced by the process of this invention. Fig. 8 is a graph illustrating the volume of cumulative intrusion against the pore diameter. Fig. 9 is a graph illustrating the differential log of the volume of the intrusion against the pore diameter.

Fig. 10 is a graph illustrating the temperatures at which the volatiles are emitted from the natural fish. Fig. 11 is an X-ray analysis of the graphitized foam that is produced by the process of this invention. Figures 12 A-C are photographs illustrating the foam that is produced with aluminum crucibles and the smooth structure or front sheet that it develops. Fig. 13A is a schematic view illustrating the production of a carbon foam composite material made in accordance with this invention. Fig. 13B is a perspective view of the composite material of the carbon foam of this invention.

Detailed Description of the Invention In order to illustrate the product and composite material of the carbon foam of this invention, the following Examples are set forth. They do not intend to limit the invention in any way.

EXAMPLE I Fish powder, pellets or pellets are placed in a mold with the final desired shape of the foam. These fish materials can be solvated if desired. The mesophase fish of Mitsubishi ARA-24 was used in this Example. A proprietary mold of release agent or film is applied to the sides of the mold to allow the removal of the particles. In this case, the Boron Nitride atomizer and the Dry Graphite Lubricant were used separately as a mold release agent. If the mold is made of pure aluminum, the mold release agent is not necessary since the molten fish does not wet the aluminum and therefore will not stick to the mold. Similar mold materials that do not wet the fish can be found and therefore, will not need mold release. The sample is evacuated to less than 1 torr and then heated to a temperature of approximately 50 to 100 ° C above the softening point. In this case where the mesophase fish of Mitsubishi ARA24 was used, 300 ° C was sufficient. At this point, the vacuum is released to a compound to protect the nitrogen surface and then a pressure above 1000 psi is applied. The temperature of the system is then raised to 800 ° C, or at a temperature sufficient to coke the fish which is from 500 ° C to 1000 ° C. This is performed at a speed of no more than 5 ° C / min and preferably at almost 2 ° C / min. The temperature is maintained for at least 15 minutes to get a safe soak and then the powerful oven is turned off and cooled to room temperature. Preferably the foam was cooled at a rate of about 1.5 ° C / min with pressure release at a rate of about 2 psi / min. The temperatures of the final foam for three runs of product were 500 ° C, 630 ° C and 800 ° C. During the cooling cycle, the pressure is released gradually to atmospheric conditions. The foam was then heated treated at 1050 ° C (carbonized) under a compound to protect the nitrogen surface and then heated treated in separate runs at 2500 ° C and 2800 ° C (graphitized) in Argon. The carbon foam produced with this technique was examined with photomicrography, electronic scanning microscope (EBM), X-ray analysis and mercury porphysimetry. As can be seen in Figures 2-7, the interference patterns under cross-polarized light indicate that the foam columns are completely graphite. That is, the whole fish was converted to graphite and aligned along the axis of the columns. These columns are also similar in size and are interconnected through the foam. This could indicate that the foam could have high rigidity and good strength. As seen in Fig. 7 by the MBE micrograph of the foam, the foam is cellularly open which means that the porosity is not closed. Figures 8 and 9 are results of the mercury porphysimetry tests. These tests indicate that the pore sizes are in the range of 90-200 microns. A thermogravimetric study of the natural fish was carried out to determine the temperature at which the volatiles are emitted. As you can see in Figure 11, the fish loses about 20% of this mass quite quickly in the temperature range between almost 420 ° C and almost 480 ° C. Although this was performed at atmospheric pressure, the addition of 1000 psi of pressure will not significantly deviate this effect. Therefore, while the pressure is at 1000 psi, the gases are rapidly emitted during heating through the temperature range of 420 ° C to 480 ° C. The gases produce a foam effect (as boiling) in the molten fish. While the temperature increases more at temperatures in the range from 500 ° C to 1000 ° C (which depends on the specific fish), the sparkling fish becomes coked (or rigid), thus producing a solid foam that is derived of the fish. Consequently, foam generation has occurred prior to the release of pressure and therefore, this process is very different from the prior art. The samples of the foam were maquilaled in specimens to measure the thermal conductivity. The volume of thermal conductivity was in the range from 58 W / m ° K to 106 W / m ° K. The average density of the samples was 0.53 g / cm3. When the weight is taken into account, the specific thermal conductivity of the fish derived from the foam is up 4 times greater than that of copper. More derivations can be used to estimate the thermal conductivity of the columns by themselves to be close to 700 W / m ° K. This is comparable to carbon fibers of high thermal conductivity that are produced from this same mesophase fish ARA24. X-ray analyzes of the foam were performed to determine the crystalline structure of the material. The X-ray results are shown in Figure 11. From these data, the space of the graphene layer (doo2) was determined to be 0.336 nm. The coherence length (La, 1010) was determined to be 203.3 nm and the stacked height was determined to be 442.3 nm.

The compression strength of the samples was measured to be 3. 4 MPa and the compression coefficient was measured to be 73.4 MPa. The foam sample was easily maked and could easily be handled without fear of damage, indicating good strength. It is important to note that when this fish is heated in a similar manner, but only under atmospheric pressure, the fish foam dramatically more than when under pressure.

In fact, the resulting foam is so fragile that it could not yet be handled for testing.

EXAMPLE II An alternative to the method of Example I is to use a mold made of aluminum. In this case two molds were used, an aluminum weighing plate and a sectioned soda can. The same process set in motion in Example I is employed except that the final coking temperature was only 630 ° C, as to prevent the aluminum from melting. Figures 12A-C illustrate the ability to use complex shape molds for the production of foam of complex shape. In one case, shown in Fig. 12 A top of a soda can was removed and the remaining can could be used as a mold. No release agent was used. It was observed that the shape of the resulting part conformed to the shape of the soda can, even after graphitization at 2800 ° C. This demonstrates the dimensional stability of the foam and the ability to produce parts formed near the net. In the second case, as shown in Figs. 12 B and C using an aluminum weighing pan, a very smooth surface formed on the surface that contacts the aluminum. This is directly attributable to the fact that the molten fish does not wet the aluminum surface. This could allow one to produce complex shaped parts with smooth surfaces such as to improve the contact area for the bond or for the improvement of heat transfer. This smooth surface will act as a front sheet and, consequently, a foam core composite material can be fabricated on site with the manufacture of the front sheet. Since it is manufactured together and an integral material is not bound from the separation surface, the thermal stress will be lower, resulting in a stronger material.

The following examples illustrate the production of a composite material employing the foam of this invention.

EXAMPLE III The fish-derived carbon foam was produced by the method described in Example I. Referring to FIG. 13, the carbon foam 10 was then machined in a 2"x2" xl / 2 block. Two pieces 12 and 14 of a pre-plug comprising Hercules AS4 carbon fibers and IZ Fibirite Polyetheretherketone thermoplastic resin also of size 2"x2" xl / 2 were placed in the upper and lower part of the foam sample, and placed in a graphite mold 16 matched for compression by the graphite plunger 18. The sample of the composite material was heated under an applied pressure of 100 psi at a temperature of 380 ° C at a rate of 5 ° C / min. The composite material was then heated under a pressure of 100 psi at a temperature of 650 ° C. The core-shaped sandwich panel of foam core generally was then removed from the mold and carbonized under nitrogen at 1050 ° C and then graphitized at 2800 ° C, resulting in a foam with carbon-carbon front sheets attached to the surface. The composite material generally 30 is shown in Figure 13 B.

EXAMPLE IV The carbon foam derived from fish was produced with the method described in Example I. It was then machined in a block of 2"x2" xl / 2. Two pieces of carbon-carbon material, 2"x2" xl / 2, were lightly covered with a mixture of 50% ethanol, 50% Durez® phenolic resin available from Occidental Chemical Co. The foam block and the carbon-carbon material were positioned together and placed in a mold as indicated in Example III. The sample was heated to a temperature of 150 ° C at a rate of 5 ° C / min and soaked at room temperature for 14 hours. The sample was then carbonized under nitrogen at 1050 ° C and then graphitized at 2800 ° C, which results in a foam with carbon-carbon front sheets attached to the surface. This is also generally shown at 30 in Figure 13B.

EXAMPLE V The carbon foam derived from the fish was produced by the method described in Example I. The foam sample was then densified with carbon by the chemical vapor infiltration method for 100 hours. The density increased to 1.4 g / cm3, the flexural strength was 19.5 MPa and the flexural coefficient was 2300 MPa. The thermal conductivity of the natural foam was 50 W / m ° K and the thermal conductivity of the densified foam was 94 W / m ° K.

EXAMPLE VI The fish-derived carbon foam was produced by the method described in Example I. The foam sample was then densified with epoxy by the vacuum impregnation method. The epoxy was vulcanized at 150 ° C for 5 hours. The density was increased to 1.37 g / cm3 and the flexural strength was measured to be 19.3 MPa. It is obvious that other materials, such as metals, ceramics, plastics or reinforced fiber plastics could be attached to the surface of the foam of this invention to produce a foam core composite material with acceptable properties. It is also obvious that ceramics or glass, or other materials could be impregnated in the densification foam. Based on the data taken to date of the carbon foam material, several observations can be made and the important features of the invention are: The fish-based carbon foam can be produced without an oxidative stabilization step, thereby saving time and costs. The high graphitic alignment in the foam columns is achieved by graphitization at 2500 ° C, and consequently the high thermal conductivity and stiffness will be exhibited by the foam, which makes it suitable as a core material for thermal applications . High compressive forces should be achieved with mesophase fish-based coal foams, which makes them suitable as a core material for structural applications. Foam core composite materials can be manufactured at the same time the foam is generated, saving time and costs. Rigid monolithic preforms can be made with significant open porosity suitable for densification by the Chemical Vapor Infiltration method of ceramic and carbon infiltrants. Rigid monolithic preforms can be made with significant open porosity suitable for activation, which produces a monolithic activated carbon. 7. It is obvious that by the variation of the applied pressure, the size of the bubbles that form during foam formation will change and, consequently, the density, strength and other properties can be affected.

The following alternative methods and products can also be carried out by the process of this invention: 1. Manufacture of preforms with complex shapes for densification by CVI or Foundry Impregnation. 2. Monoliths of activated carbon. 3. Optical absorber 4. Low density heating elements. 5. Firewall material. 6. Low secondary electron emission surfaces for high energy physical applications.

Thus, it will be seen that the present invention provides for the manufacture of fish-based carbon foam for structural and thermal composite materials. The process involves the manufacture of a graffiti foam of a mesophase or isotropic fish which can be synthetic base, petroleum or coal tar. A mixture of these fish can also be used. The simplified process uses a high-pressure high-temperature furnace and does not require an oxidative stabilization stage. The foam has a relatively uniform pore size distribution ("100 microns), very small closed porosity and density of approximately 0.53 g / cm3. The mesophase fish narrows along the columns of the foam structure and thereby produces a graffiti structure highly aligned in the columns. These columns will exhibit thermal conductivities and rigidity similar to the very expensive high performance carbon fibers (such as P-120 and K1100). Thus, the foam will exhibit high rigidity and thermal conductivity at a very low density («0.5 g / cc). This foam can be formed in place as a core material for high temperature sandwich panels for both thermal and structural applications, thereby reducing manufacturing time. By the use of an isotropic fish, the resulting foam can easily be activated to produce a large surface area of activated carbon. The activated carbon foam will not experience the problems associated with granules such as attrition, channeling and long pressure surges.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (24)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A carbon foam production process, characterized in that it comprises: the selection of an appropriate form of mold; introducing fish to an appropriate level in a mold; the air purge of the mold; heating the fish at a temperature sufficient to coalesce the fish in a liquid; vacuum release and the application of an inert fluid at a static pressure above about 1000 psi; the heating of the fish at a temperature sufficient to cause the gases to be emitted and form foam of the fish; heating the fish to a temperature sufficient to coke the fish; and cooling the foam to room temperature with a simultaneous pressure release. 2. The process of claim 1, characterized in that the fish is introduced as a granulated fish. 3. The process of claim 1, characterized in that the fish is introduced as a fish powder. 4. The process of claim 1, characterized in that the fish is introduced as pelleted fish. 5. The process of claim 1, characterized in that the fish is a synthetic or isotropic mesophase fish. 6. The process of claim 1, characterized in that the fish is a mesophase fish derived from petroleum or isotropic. The process of claim 1, characterized in that the fish is a mesophase fish derived from carbon or isotropic. The process of claim 1, characterized in that the fish is a mixture of fish selected from a group consisting of synthetic or isotropic mesophase fish, mesophase fish derived from petroleum or isotropic and mesophase fish derived from carbon or isotropic. 9. The process of claim 1, characterized in that the fish is a solvated fish. 10. The process of claim 1, characterized in that the purging is effected by a vacuum stage. 11. The process of claim 1, characterized in that the purging is effected by an inert fluid. 12. The process of claim 1, characterized in that the vacuum is applied to less than 1 torr. The process of claim 1, characterized in that the nitrogen is introduced as the inert fluid. 14. The process of claim 1, characterized in that the fish is heated to a temperature in the range of almost 500 ° C to almost 1000 ° C to coke it. 15. The process of claim 1, characterized in that the fish is heated to a temperature of almost 800 ° C to coke it. 16. The process of claim 1, characterized in that the temperature for coking the fish rises at a speed no greater than 5 ° C per minute. 17. The process of claim 1, characterized in that the fish is soaked at a coking temperature of at least 15 minutes to effect coking. 18. The process of claim 1, characterized in that the fish is heated to a temperature of almost 630 ° C to coke it. 19. The process of claim 1, characterized in that the fish is heated to a temperature of almost 50 ° C to almost 100 ° C to coalesce it. The process of claim 1, characterized in that the foam is cooled at a rate of approximately

1. 5 ° C / min with the release of pressure at a rate of approximately 2 psi / min. 21. The process of claim 1, characterized in that it also includes the step of densification of the foam. 2

2. A carbon foam produced as produced by the process of claim 1. 2

3. A carbon foam as produced by the process of claim 1 with a smooth front sheet. 2

4. A process for the production of carbon foam, characterized in that it comprises: the selection of an appropriate mold form and a mold composed of a material that the molten fish does not wet; the introduction of fish at an appropriate level in the mold; the air purge of the mold; heating the fish at a temperature sufficient to coalesce it in a liquid; the release of vacuum and the application of an inert fluid at a static pressure above almost 1000 psi; heating the fish at a temperature sufficient to coke it; and cooling the foam to room temperature with a simultaneous release of pressure. The process of claim 24, characterized in that the fish is introduced as a pelletized fish. The process of claim 24, characterized in that the fish is introduced as a powder fish. The process of claim 24, characterized in that the fish is introduced as pelleted fish. The process of claim 24, characterized in that the fish is a synthetic or isotropic mesophase fish. The process of claim 24, characterized in that the fish is a mesophase fish derived from petroleum. The process of claim 24, characterized in that the fish is a mesophase fish derived from coal. The process of claim 24, characterized in that the mold is purged by a vacuum application of less than 1 torr. The process of claim 24, characterized in that the mold is purged by an inert fluid before heating. A production process of a carbon foam core composite material, characterized in that it comprises: the selection of an appropriate form of mold; introducing fish to an appropriate level in a mold; the air purge of the mold; heating the fish at a temperature sufficient to coalesce it in a liquid; the release of vacuum and the application of an inert fluid at a static pressure above 1000 psi; heating the fish to a temperature sufficient to cause the gases to emit and form fish foam; heating the fish at a temperature sufficient to coke it; the cooling of the foam at room temperature with a simultaneous release of pressure; place the front sheets on the opposite sides of the carbon foam; and the adhesion of front sheets to the carbon foam. The process of claim 33, characterized in that the adhesion of the front sheets to the carbon foam is effected by the molding step. The process of claim 33, characterized in that the adhesion of the front sheets of the carbon foam is effected by a coating material. In a carbon foam composite material product, characterized in that it is produced by the process of claim 33.