MXPA00012157A - Pitch-based carbon foam heat sink with phase change material - Google Patents

Abstract

A process for producing a carbon foam heat sink 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 graphic 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. The foam is encased and filled with a phase change material to provide a very efficient heat sink device.

Description

THERMAL DISSIFIER OF CARBON FOAM BASED IN TAR WITH A MATERIAL THAT CHANGES PHASE BACKGROUND OF THE INVENTION The present invention relates to porous carbon foam filled with materials that change phase and encapsulated to form a heat dissipating product, and more particularly with a process to produce them. Currently there are many applications that require the storage of large amounts of heat to cool or heat an object. Typically, these applications produce heat very quickly, so that dissipation through cooling blades, natural convection, or radiation can not dissipate heat fast enough, and thus, the object overheats. To alleviate this problem, a material with a large specific heat capacity, such as a heat sink, is placed in contact with the object when it heats up. During the heating process, the heat is transferred to the heat sink from the hot object, and when the temperature of the heat sink rises, it "stores" the heat more quickly than it can be dissipated into the environment through convection. Unfortunately, when the temperature of the heatsink rises, the heat flow from the hot object Ref: 125268 decreases, due to the lower temperature difference between the two objects. Therefore, although this method of energy storage can absorb large amounts of heat in some applications, it is not sufficient for all applications. Another method of absorbing heat is through a phase change of the material, rather than a change in temperature. Typically, the transformation of the phase of a material absorbs two orders of magnitude more of thermal energy than the heat capacity of the material. For example, the evaporation of 1 gram of water at 100 ° C absorbs 2,439 joules of energy, while changing the water temperature from 99 ° C to 100 ° C only absorbs 4.21 joules of energy. In other words, raising the temperature of 579 grams of water from 99 ° C to 100 ° C absorbs the same amount of heat as the evaporation of 1 gram of water at 100 ° C. The same tendency is found in the melting point of the material. This phenomenon has been used in some applications to absorb or release tremendous amounts of energy in situations where heat sinks will not work. Although a solid block of material that changes phase has a theoretically very large capacity to absorb heat, the process is not fast, due to the difficulties of heat transfer and this method can not be used in certain applications. However, the use of high thermal conductivity foam will overcome the disadvantages described above. If the high conductivity foam is filled with material that changes phase, the process can become very fast. Due to the extremely high conductivity in the foam column, when the heat comes into contact with the surface of the foam, it is quickly transmitted through the foam to a very large surface area of the material that changes phase. In this way, the heat is distributed very quickly through the material that changes phase, allowing it to absorb or emit extremely fast thermal energy without changing temperature, thus maintaining the driving force to transfer heat to its maximum. Heat sinks have been used in the aerospace community to absorb energy in applications such as missiles and aircraft where rapid heat generation is found. A material that has a high heat of fusion is encapsulated or wrapped in a graphite or metal envelope, typically aluminum, and placed in contact with the object that generates the heat. Since most materials that change phase have a low thermal conductivity, the rate of heat transfer through the material is limited, but this is displaced by the high energy absorption capacity of the phase change. When the heat is transmitted through a metallic or graphite envelope to the material that changes phase, the material that changes phase closer to the source of heat begins to melt. Since the temperature of the material that changes phase does not change until the material melts, the flow from the heat source to the material that changes phase remains relatively constant. However, as the heat continues to melt more material that changes phase, more liquid is formed. Unfortunately, the liquid has a much lower thermal conductivity, thus preventing an additional heat flow. In fact, the low total thermal conductivity of materials that change solid and liquid phases allows the speed of heat absorption and, thus, reduces the efficiency of the system. Recent developments of fiber reinforced compositions, including carbon foams, have been driven by the requirements of strength, stiffness, resistance to creep and toughness in structural design materials. Carbon fibers have led to significant advances in these properties in compositions of various polymeric, metallic and ceramic matrices. However, current applications of carbon fibers have evolved from structural reinforcement to thermal management in applications ranging from high density electronic modules to communication satellites.

This has stimulated reinforcement research and novel composition processing methods. The high thermal conductivity, low weight, and low coefficient of thermal expansion are the concerns in thermal management applications. See Shih, Wei, "Development of Carbon-Carbon Compositions for Electronic Thermal Management Applications", IDA Workshop, May 3-4, 1994, supported by AF Wright Laboratory under Contract Number F33615-93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165 and Engle, GB, "C / C Compositions of High Thermal Conductivity for Thermal Management", IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract F33615 -93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165. Such applications are striving toward a sandwich type method in which a low density structural core material (i.e., honeycomb or foam) is sandwiched between a high thermal conductivity coating sheet. Structural cores or centers are limited to low density materials to ensure that weight limits are not exceeded. Unfortunately, carbon foams and honeycomb carbon materials are the only materials available for use in high temperature applications (> 1600 ° C). The high thermal conductivity honeycomb materials are extremely expensive to manufacture compared to the low conductivity honeycombs, therefore, a performance penalty is the payment for low cost materials. High conductivity carbon foams are more expensive to manufacture than low conductivity carbon foams, in part, due to raw materials. Invariably, tar should be used as the precursor to produce carbon foams of high rigidity and high conductivity. This is because tar is the only precursor that forms a highly aligned graphite structure, which is a requirement for high conductivity. Typical processes use a blowing technique to produce a tar precursor foam in which the tar is melted and passed from a high pressure region to a low pressure region. Thermodynamically, this produces an "Instant Evaporation", thereby causing the low molecular weight compounds of the tar to evaporate (the tar boils), resulting in a tar foam. See, Joseph W. and Max L. Lake, "Novel Hybrid Compositions Based on Coal Foams", Ma t. Res. Soc. Symp. , Materials Research Society, 270: 29-23 (1992); Hagar, Joseph W. and Max L. Lake, "Formulation of a Process Model Using a Mathematical Process Model for Foaming a Mesophase Coal Precursor", Mat. Res. Soc. Symp., Material 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, US Patent 4,276,246 (1981). Next, the tar foam must be stabilized oxidatively by heating in air (or oxygen) for many hours, thereby reticulating the structure and "stabilizing the tar" so that it does not melt during carbonization. See Hagar, Joseph W. and Max L. Lake, "Formulation of a Process Model Using a Mathematical Process Model for Foaming a Mesophase Coal Precursor", Mat. Res. Soc. Symp., Material Research Society, 270: 35-40 (1992); and White, J.L. and P.M. Shaeffer, Carbon, 27: 697 (1989). This is a time-consuming step and can be an expensive step depending on the size of the part and the equipment required. The "stabilized" or oxidized tar is then carbonized in an inert atmosphere at temperatures as high as 1100 ° C. Next, grafitation is performed at temperatures as high as 3000 ° C to produce a graphite structure of high thermal conductivity, resulting in a rigid and thermally highly conductive foam.

Other techniques use a polymeric precursor, such as phenolic, urethane, or mixtures thereof with tar. See Hagar, Joseph W. and Max L. Lake, "Idealized Column Geometries for Open Cell Foams", Ma t. Res. Soc. Symp. , Material Research Society, 270: 41-46 (1992); Aubert, J.W., 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, thus causing the liquid to foam as the volatile compounds are released. The polymer precursors are cured and then carbonized without a stabilization step. However, these precursors produce a "glassy" or vitreous carbon, which does not exhibit graphite structure and, thus, has low thermal conductivity and low rigidity. See Hagar, Joseph W. and Max L. Lake, "Idealized Column Geometries for Open Cell Foams", Ma t. Res. Soc. Symp. , Material Research Society, 270: 41-46 (1992). In any case, once the foam is formed, it joins in a separate step to the coating sheet used in the composition. This can be an expensive step in the use of foam.

The extraordinary mechanical properties of commercial carbon fibers are due to the unique graphite morphology of the extruded filaments. See Edie, D.D., "Tar Fibers and in Mesophase" in Carbon Fibers, Filaments and Composites, Figuereido (editor), Kluwer Academic Publishers, Boston, pp. 43-72 (1990). Contemporary advanced structural compositions exploit these properties by creating a disconnected network of graphite filaments held together by an appropriate matrix. The carbon foam derived from a tar precursor can be considered an interconnected network of ligaments or columns, as shown in Figure 1. Such interconnected networks represent a potential alternative as reinforcement in structural composite materials. The process of this invention overcomes current manufacturing limitations by avoiding a "blowing" or "pressure release" technique to produce the foam. In addition, a stabilization step by oxidation is not required, as in other methods used to produce tar-based carbon foams with a highly aligned graphitic structure. This process consumes less time, and therefore, it will be cheaper and easier to manufacture. 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 An object of the present invention is the production * porous carbon foam, high encapsulated or wrapped thermal conductivity filled with a phase-changing material where tremendous amounts of thermal energy are stored and emitted very quickly. The porous foam is filled with a material that changes phase (PCM) at a temperature close to the operating temperature of the device. When heat is added to the surface, from a heat source, such as an integrated computer chip, the friction due to re-entry through the atmosphere, or radiation such as sunlight, is transmitted quickly and uniformly to through the foam and continuation to the material that changes phase. As the material changes phase, it absorbs orders of magnitude of higher energy than the non-PCM material due to the transfer of latent heat from melting or evaporation. On the contrary, the filled foam can be used to emit energy quickly when placed in contact with a cold object. The non-limiting modalities described here are a device for rapidly thawing frozen foods or freezing thawed foods, a design to prevent satellite overheating or store thermal energy when they experience cyclical heating during orbit, and a design to cool the front parts during a hypersonic flight or space re-entry. Another object of the present invention is to provide carbon foam and a composition of a mesophase or isotropic tar such as synthetic tar or tar based on petroleum or coal tar. Another object is to provide a carbon foam and a tar composition that does not require an oxidative stabilization step. These and other objects are achieved by a method for producing a carbon foam heatsink where an appropriate mold form is selected, and preferably, a suitable mold release agent is applied to the mold walls. The tar is introduced at an appropriate level in the mold, and the mold is purged of air by applying a vacuum, for example. Alternatively, an inert fluid could be employed. The tar is heated to a temperature sufficient to coalesce the tar in a liquid, which is preferably from about 50 ° C to about 100 ° C above the point of tar softening. The vacuum is released and an inert fluid is applied at a static pressure of approximately 1000 psi (6894.7 kPa). The tar is heating to a temperature sufficient to cause gases to escape from the foam and tar. The tar is further heated to a temperature sufficient to coke the tar and the tar is cooled to room temperature with a simultaneous and gradual release of pressure. The foam is then filled with a material that changes phase and encapsulated or wrapped to produce a product that stores heat, efficiently. In another aspect, the steps described above are employed in a mold composed of a material such that the molten tar does not adhere to the surface of the mold. In yet another aspect, the objectives are achieved by means of a carbon foam product produced by the methods described herein, including a foamed product with a smooth integral coating sheet. In another aspect a carbon foam composite product is still produced by adhering facing sheets to the carbon foam produced by the process of this invention. Figure 1 is a sectional section of a heat dissipating device for defrosting food using acetic acid as the material that changes phase.

Figure 2 is a sectional section of a heat sink to prevent overheating of satellites during cyclic orbits. Figure 3 is a sectional section of a heat sink used in the front of a space shuttle. Figure 4 is a micrograph illustrating typical carbon foam with interconnected carbon ligaments and open porosity. Figures 5-9 are micrographs of carbon foam derived from graphite tar at 2500 ° C and various amplifications. Figure 10 is a SEM micrograph of the carbon foam produced by a process of this invention. Figure 11 is a diagram illustrating the volume of cumulative intrusion against the pore diameter. Figure 12 is a diagram illustrating the logarithm of the differential intrusion volume versus the pore diameter. Figure 13 is a graph illustrating the temperatures at which volatile compounds are released from untreated tar. Figure 14 is an X-ray analysis of the graphitized foam produced by the process of this invention.

Figures 15A-C are photographs illustrating the foam produced with aluminum crucibles and the smooth structure of the coating sheet it produces. Figure 16A is a schematic view illustrating the production of a carbon foam composition made in accordance with this invention. Figure 16B is a perspective view of the carbon foam composition of this invention.

DETAILED DESCRIPTION OF THE INVENTION To illustrate the carbon foam heat sink product of this invention, the following examples are set forth. They do not intend to limit the invention in any way.

Example 1: Device for Defrosting Foods Acetic acid has a heat of fusion of 45 J / g at a melting point of 11 ° C. The heat of fusion of the food, mainly the ice, is of approximately 79 J / g to 0 ° C. Therefore, a block of foam is taken and filled with liquid acetic acid at room temperature. The foam will be encapsulated or wrapped in a box made of an insulating polymer such as polyethylene on all sides except the top. The upper part of the foam / acetic acid block will be covered with an aluminum plate of high thermal conductivity which is held in place thereby sealing the foam / acetic acid into the polymeric case (illustrated in Figure 1). If the foam block is 10 inches x 15 inches x 0.5 inches (25.4 centimeters by 38.1 centimeters x 1.25 centimeters) thick, the mass of the foam is 614 grams. The mass of acetic acid that fills the foam is approximately 921 grams. Therefore, when a piece of frozen meat is placed in contact with the upper part of the aluminum block, the foam will be cooled to the freezing point of acetic acid (11 ° C). At this point, the heat released from the acetic acid as it freezes (also remains at 11 ° C) will be equivalent to 49 KJ. This heat is transferred quickly to the frozen meat as it thaws (it also remains at 0 ° C). This amount of heat is sufficient to melt approximately 500 grams (1 pound) of meat.

Example 2: Thermal Dissipation to Prevent Overheating of Satellites During Cyclic Orbits To produce a carbon-carbon composition with foam in which the foam is a central material with carbon-carbon coating sheets (Figure 2). Fill the foam core with a suitable phase-changing material, such as a paraffin wax, that melts around the maximum operating temperature of the satellite components. One method to do this would be to drill a hole in a surface of the carbon-carbon coating sheets and fill the liquid-phase changing material in the porous foam in vacuum. Once refilled, the sample can be cooled (the material that changes phase solidifies) and the hole can be covered with an epoxy cap or screw type. The epoxy and any other sealing material must be able to withstand the operating temperature of the application. The composition of the foam core will then be mounted on the side of the satellite that is exposed to the sun during orbit. As the satellite revolves around the earth and is exposed to the sun, the radiant energy in the sun will begin to heat the composite panel or board to the melting point of the material that changes phase. At this point, the panel or board will not increase its temperature when the material that changes phase melts. The amount of radiant energy that the panel or board can absorb will depend on the thickness and external dimensions of the panel or board. This can be easily calculated and designed through knowledge of satellite orbit times, so that the material never fully melts and, thus, never exceeds the melting temperature. Then, when the satellite stops seeing the sun, it will begin to radiate heat to space and the material that changes phase will begin to freeze. The cycle will repeat itself once the material sees the sun again.

Example 3: Heat dissipator paralas. Front Parts At present, space shuttles experience extreme heat during re-entry. Specifically, the forward parts of the ship can reach 1800 ° C and the lower part of the ship can reach temperatures as high as 1200 ° C. If a foam core composition is placed on the surface of the front and on the surface of the bottom of the fuselage (Figure 3), it could absorb enough energy to dramatically reduce the maximum temperature of the hot areas. This will also allow a faster re-entry or (gradual slip slope) and maintain the current maximum temperatures. In this case, the material that changes phase could very likely be an alloy, for example of anion-silicon ger, which melts around 800-900 ° C and does not evaporate to a temperature much higher than the maximum temperature of the ship. For example, Germanium has a heat of formation (heat of fusion) of 488 J / g. In this way 1.0 Kg of Germanium will be required to reduce the temperature of 1 Kg of a coal-carbon thermal shield existing at 688 ° C. In other words, if the existing carbon-coal were placed pound-for-pound with foam filled with germanium, the maximum temperature of the heat shield would be only about 1131 ° C instead of about 1800 ° C during re-entry, depending on the duration of the thermal load.

Example 4 Tar powder, pellets or pellets are placed in a mold with the desired final shape of the foam. These tar materials can be solvated if desired. In this example, Mitsubishi ARA-24 mesophase tar was used. An appropriate mold release agent or film was applied to the sides of the mold to allow removal of the part. In this case, they were used separately sprayed with Boron Nitride and Dry Graphite Lubricant as mold release agent. If the model is made of pure aluminum, the mold release agent is not necessary, since the molten tar does not adhere to the aluminum and, thus, will not stick to the mold. Mold materials similar to those that tar does not adhere to can be found and, thus, will not need to be released from the mold. The sample is evaluated at less than 1 torr and then heated to a temperature of approximately 50 to 100 ° C above the softening point. In this case where Mitsubishi ARA24 mesophase tar was used, 300 ° C were sufficient. At this point, the vacuum is released to a stream of nitrogen and then a pressure of up to 1000 psi (6894.7 kPa) was applied. The temperature of the system was then raised to 800 ° C, or a temperature sufficient to coke the tar which is from 500 ° C to 1000 ° C. This is carried out at a speed not higher than 5 ° C / min and preferably at approximately 2 ° C / min. The temperature was maintained for at least 15 minutes to achieve accelerated saturation and then the power supply of the oven was interrupted 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 (13.8 kPa / min). The final foaming temperatures for the three products tested were 50 ° C, 630 ° C and 800 ° C. During the cooling cycle, the pressure was released gradually to atmospheric conditions. The foam was then thermally treated at 1050 ° C (carbonized) under a stream of nitrogen and then heated in separate tests at 2500 ° C and 2800 ° C (graphite) in Argon. The carbon foam produced with this technique was examined with photomicrography, scanning electron microscopy (SEM), X-ray analysis and mercury porphysimetry. As can be seen in Figures 5-10, the isochromatic regions under the cross-polarized light indicate that the columns of the foam are completely graphitic. That is to say, that all the tar was converted to graphite and aligned along the axis of the columns. Those columns are also similar in size and are interconnected through the foam. This indicates that the foam would have high rigidity and good strength. As seen in Figure 10 by the SEM micrograph of the foam, the foam is open cellular, which means that the porosity is not closed. Figures 11 and 12 are the results of mercury porosimetry tests. These tests indicate that the pore sizes are in the range of 90-200 microns. A thermogravimetric study of the untreated tar was carried out to determine the temperature at which the volatiles were evaporated. As can be seen in Figure 14, the tar loses almost 20% of its mass very rapidly in the temperature range of between about 420 ° C and about 480 ° C. Although this was done at atmospheric pressure, the addition of 1000 psi (6894.7 kPa) of pressure will not displace this effect significantly. Therefore, even if the pressure is 1000 psi (6894.7 kPa), gases are rapidly released during heating through the temperature range of 420 ° C to 480 ° C. The gases produce a foaming effect (like boiling) on the molten tar. As the temperature increases further at temperatures ranging from 500 ° C to 1000 ° C (depending on the specific tar), the foamed tar coats (or becomes rigid), thereby producing a solid foam derived from tar . Consequently, the foaming has to occur before the release of pressure and, therefore, this process is very different from that of the prior art. Specimens of foam were machined into specimens to measure thermal conductivity. The apparent thermal conductivity ranged from 58 W / m »° K to 106 W / m * ° K. The average density of the samples was 0.53 g / cm3. When the weight was taken into account, the specific thermal conductivity of the tar derived from the foam is more than 4 times that of copper. Additional derivations can be used to estimate the thermal conductivity of the columns themselves, which are almost close to 700 W / m ° ° K. This is comparable with the high thermal conductivity of the carbon fibers produced by this same tar in mesophase ARA24. X-ray analysis of the foam was carried out to determine the crystalline structure of the material. The results of the x-rays are shown in Figure 14. From these data, it was determined that the separation of the graphene layer (d002) was 0.336 nm. It was determined that the coherence length (La'ioo) was 203.3 nm and it was determined that the height of the stack was 442.3 nm. The compression strength of the samples measured 3.4 MPa and the measured compression module was 73.4 MPa. The foam mass was easily machined and could be easily handled without damage, indicating good strength. It is important to note that when this tar is heated in a similar way, but only under atmospheric pressure, tar foam dramatically more than when under pressure. Indeed, the resulting foam is so fragile that it could still not be handled to carry out the tests. Molding under pressure serves to limit the growth of the cells and produce a useful material.

Example 5 An alternative 3l. The method of Example 4 is to use a mold made of aluminum. In this case two molds, an aluminum weight and a sectioned soda can were used. The same process as set forth in Example 4 was used except that the final coking temperature was only 630 ° C, to prevent melting of the aluminum. Figures 15A-C illustrate the ability to use molds of complex shape to produce foam of complex shape. In one case, shown in Figure 15A, the top of the soda can was removed and the remaining can used as a mold. No release agent was used. Note that the shape of the resulting part conforms to the shape of the soda can, even after grafitation at 2800 ° C. This demonstrates the dimensional stability of the foam and the stability to produce almost pure formed parts. In the second case, as shown in Figures 15 B and C using an aluminum weight, a very smooth surface was formed on the surface in contact with the aluminum. This is directly attributable to the fact that the molten tar does not adhere to the aluminum surface. This will allow us to produce complex shaped parts with smooth surfaces to improve the contact area for bonding or improve heat transfer. This smooth surface will act as a coating sheet and, thus, a foam core composition can be manufactured in itself with the manufacture of the coating sheet. Since this is manufactured as a whole and is an integral material without interferences, thermal stresses 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 6 Tar-derived carbon foam was produced by the method described in Example 4. Referring to Figure 16a, the carbon foam 10 was then machined into a 2"x2" xl / 2"block (5.08cm x 5.08). cm x 1.27cm) Two pieces 12 and 14 of preparation comprised of carbon fibers Hercules AS4 and thermoplastic resin of polyether ether ketone ICI Fibirite also of 2"x2" xl / 2"(5.08cm x 5.08cm x 1.27cm) in size placed on the top and bottom of the foam sample, and all were placed in a paired graphite mold 16 for compression by a graphite piston 18. The composite sample was heated under an applied pressure of 100 psi (689.5 kPa) at a temperature of 380 ° C at a speed of 5 ° C / min. The composition was then heated under pressure at 100 psi (689.5 kPa) at a temperature of 650 ° C. The panel or board sandwiching the foam core generally 20 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 facing sheets bonded to the surface . The composition generally 30 is shown in Figure 16B.

Example 7 Tar-derived carbon foam was produced by the method described in Example 4. This was then machined into a 2"x2" xl / 2"block (5.08cm x 5.08cm x 1.27cm) Two pieces of material of carbon-carbon, 2"x2" xl / 2"(5.08cm x 5.08cm x 1.27cm), were lightly coated with a mixture of 50% ethanol, 50% phenolic resin Durez8 available from Occidental Chemical Co. The block The foam and the carbon-carbon material were placed together and placed in a mold according to what was indicated in Example 6. The mixture was heated to a temperature of 150 ° C and at a rate of 5 ° C / min and kept at temperature for 14 hours. The mixture was then carbonized under nitrogen at 1050 ° C and then graphite at 2800 ° C, resulting in a foam with carbon-carbon facing sheets bonded to the surface. This is also shown generally at 30 in Figure 16B.

Example 8 Tar-derived carbon foam was produced with the method described in Example 4. The foam sample was then densified with carbon by the chemical vapor permeation method for 100 hours. The density was increased to -1.4 g / cm3, the flexural strength was 19.5 MPa and the flexural modulus was 2300 MPa. The thermal conductivity of the untreated foam was 58 W / m * ° K and the thermal conductivity of the densified foam was 94 W / m ° ° K.

Example 9 Tar-derived carbon foam was produced by the method described in Example 4. The foam sample was then densified with epoxy by the vacuum impregnation method. The epoxy was cured at 250 ° C for 5 hours. The density was increased to 1.37 g / cm3 and the measured bending strength was 19.3 MPa. Other possible embodiments may include materials, such as metals, ceramics, plastics, or fiber reinforced plastics bonded to the surface of the foam of this invention to produce a foam core composite with acceptable properties. Additional possible embodiments include ceramics, glass, or other materials impregnated in the foam for densification. Based on the data taken to date of the carbon foam material, several observations can be made which outline the important features of the invention, which include: 1. Tar-based carbon foam can be produced without a stabilization step oxidative, thus saving time and costs. 2. The highly graffitic alignment of the columns in the foam is achieved after gravitation at 2500 ° C, and in this way the foam will exhibit high thermal conductivity and rigidity, making it suitable as a core material for thermal applications. 3. High compression strengths should be achieved with coal tar foams in mesophase, making them suitable as a core material for structural applications. 4. The foam core compositions can be manufactured at the same time the foam is generated, thereby saving time and costs. 5. Rigid monolithic preforms with a significantly open porosity suitable for densification can be made by the Chemical Vapor Infiltration method of ceramic and carbon infiltrants. 6. Rigid monolithic preforms with a significantly open porosity suitable for activation can be made, producing a monolithic activated carbon. 7. It is obvious that by varying the applied pressure, the size of the bubbles formed during foaming will change and, thus, density, strength and other properties can be affected. The following alternative methods and products may also be affected by the process of this invention: 1. The manufacture of preforms with complex shapes by densification by CVl or Fusion Impregnation. 2. Activated carbon monoliths with high thermal conductivity. 3. Optical absorber 4. Low density heating elements. 5. Wall material against fire. 6. Low secondary electronic emission targets for high energy physical applications.

The present invention is provided for the manufacture of a tar-based carbon foam heat sink for structural and thermal compositions. The process involves the manufacture of a graphite foam from mesophase or isotropic tar that can be synthetic, petroleum or coal tar based. A mixture of these pitches can also be used. The simplified process uses a high pressure and high temperature furnace and therefore does not require an oxidative stabilization step. The foam has a relatively uniform distribution of pore sizes (~ 100 microns), very little closed porosity, a density of approximately 0.53 g / cm3. The separation of the mesophase increases along the columns of the foam structure and therefore produces a graffiti structure highly aligned in the columns. These columns will exhibit thermal conductivities and rigidities similar to expensive high-performance carbon fibers (such as P-120 and K1100). In this way, the foam will exhibit high rigidity and high thermal conductivity at a very low density (~ 0.5 g / cc). This foam can be formed in place as a core or core material for high temperature sandwich panels for both thermal and structural applications, thereby reducing manufacturing time. Using an isotropic graphite, the resulting foam can be easily activated to produce an activated carbon of high surface area. The activated carbon foam will not experience the problems associated with granules such as attrition, channeling and large pressure drops. 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 (55)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process for producing a carbon foam heat sink, characterized in that it comprises: selecting an appropriate mold form; introduce tar at an appropriate level in the mold; purge air from the mold to form a vacuum; heating the tar to a temperature sufficient to coalesce the tar in a liquid; releasing the vacuum and re-filling an inert fluid at a static pressure to approximately 1000 psi (6894.7 kPa); heat the tar to a temperature sufficient to cause gases to escape and form a carbon foam; heating the carbon foam to a temperature sufficient to coke the tar; cooling the carbon foam to room temperature and simultaneously releasing the inert fluid; encapsulate or at least partially envelop the carbon foam; and at least partially filling the porous region of the carbon foam with a material that changes phase.
2. 2. The process according to claim 1, characterized in that the tar is introduced as granulated tar.
3. 3. The process according to claim 1, characterized in that the tar is introduced as pulverized tar.
4. 4. The process according to claim 1, characterized in that the tar is introduced as tar in the form of pellets.
5. 5. The process according to claim 1, characterized in that the tar is mesophase or synthetic isotropic tar.
6. 6. The process according to claim 1, characterized in that the tar is mesophase or isotropic tar derived from petroleum.
7. 7. The process according to claim 1, characterized in that the tar is a mesophase or isotropic tar derived from coal.
8. 8. The process according to claim 1, characterized in that the tar is a mixture of tars selected from the group consisting of mesophase or synthetic isotropic tar, mesophase or isotropic tar derived from petroleum, and mesophase or isotropic tar derived from coal.
9. 9. The process according to claim 1, characterized in that the tar is a solvated tar.
10. 10. The process according to claim 1, characterized in that the purging is effected by a vacuum step.
11. 11. The process according to claim 1, characterized in that the purging is effected by means of an inert fluid.
12. 12. The process according to claim 1, characterized in that the vacuum is applied to less than 1 torr.
13. 13. The process according to claim 1, characterized in that the nitrogen is introduced as the inert fluid. The process according to claim 1, characterized in that the tar is heated to a temperature in the range of about 500 ° C to about 1000 ° C to coke the tar. 15. The process according to claim 1, characterized in that the tar is heated to a temperature of about 800 ° C to coke the tar. 16. The process according to claim 1, characterized in that the temperature for coking the tar is raised to a temperature not higher than 5 ° C per minute. 17. The process according to claim 1, characterized in that the tar is held at the coking temperature for at least 15 minutes to effect the coking. 18. The process according to claim 1, characterized in that the tar is heated to a temperature of about 630 ° C to coke the tar. 19. The process according to claim 1, characterized in that the tar is heated to a temperature of about 50 ° C to about 100 ° C to coalesce the tar. The process according to claim 1, characterized in that the foam is cooled at a falling speed of 1.5 ° C / min with pressure release at a rate of approximately 2 psi / min (13.8 kPa / min). 21. The process according to claim 1, characterized in that it also includes the step of densifying the foam. 22. The process according to claim 1, characterized in that the material that changes phase is acetic acid. 23. The process according to claim 1, characterized in that the material that changes phase is a paraffin wax. 24. The process according to claim 1, characterized in that the material that changes phase is germanium. 25. The process according to claim 1, characterized in that the encapsulation material is polyethylene. 26. The process according to claim 1, characterized in that the encapsulation material is aluminum 27. The process according to claim 1, characterized in that the encapsulation material is a carbon-carbon composition. carbon foam heat sink, characterized in that it is produced by the process according to claim 1. 29. A process for producing a carbon foam heat sink, characterized in that it comprises: selecting an appropriate mold form and a mold composed of a material that the molten tar does not wet, introduce the tar to an appropriate level in the mold, purge the air from the mold to form a vacuum, heat the tar to a temperature sufficient to coalesce the tar in a liquid, release the vacuum and replenish an inert fluid at a static pressure to approximately 1000 psi (6894.7 kPa); heat the car foam at a temperature sufficient to coke the tar; and cooling the foam to room temperature and simultaneously releasing the inert fluid; encapsulate or at least partially envelop the carbon foam; and at least partially filling the porous regions of the foam with a material that changes phase. 30. The process according to claim 29, characterized in that the tar is introduced as granulated tar. 31. The process according to claim 29, characterized in that the tar is introduced as pulverized tar. 32. The process according to claim 29, characterized in that the tar is introduced as tar in the form of pellets. 33. The process according to claim 29, characterized in that the tar is mesophase or synthetic isotropic tar. 34. The process according to claim 29, characterized in that the tar is mesophase or isotropic tar derived from petroleum. 35. The process according to claim 29, characterized in that the tar is a mesophase tar derived from coal. 36. The process according to claim 29, characterized in that the mold is purged by a vacuum applied to less than 1 torr. 37. The process according to claim 29, characterized in that the mold is purged by an inert fluid before heating. 38. The process according to claim 29, characterized in that the material that changes phase is acetic acid. 39. The process according to claim 29, characterized in that the material that changes phase is a paraffin wax. 40. The process according to claim 29, characterized in that the material that changes phase is germanium. 41. The process according to claim 29, characterized in that the encapsulation material is polyethylene. 42. The process according to claim 29, characterized in that the encapsulation material is aluminum. 43. The process according to claim 29, characterized in that the encapsulation material is a carbon-carbon composition. 44. A carbon foam heat dissipative product, characterized in that it is produced by the process according to claim 29. 45. A process for producing a carbon foam heat sink, characterized in that it comprises: selecting an appropriate mold form; introduce tar at an appropriate level in the mold; purge air from the mold to form a vacuum; heating the tar to a temperature sufficient to coalesce the tar in a liquid; releasing the vacuum and re-filling an inert fluid at a static pressure to approximately 1000 psi (6894.7 kPa); heating the tar to a temperature sufficient to cause gases to escape and forms a carbon foam; heating the carbon foam to a temperature sufficient to coke the tar; cooling the carbon foam to room temperature and simultaneously releasing the inert fluid; Place facing sheets on opposite sides of the carbon foam; Adhere the lining sheets to the carbon foam; encapsulate or at least partially envelop the carbon foam and the coating sheets; and at least partially filling the porous regions of the carbon foam with a material that changes phase. 46. The process according to claim 45, characterized in that the adhesion of the coating sheets to the carbon foam is effected by a molding step. 47. The process according to claim 45, characterized in that the adhesion of the coating sheets to the carbon foam is effected by means of a coating material. 48. The process according to claim 45, characterized in that the material that changes phase is acetic acid. 49. The process according to claim 45, characterized in that the material that changes phase is a paraffin wax. 50. The process according to claim 45, characterized in that the material that changes phase is germanium. 51. The process according to claim 45, characterized in that the encapsulation material is polyethylene. 52. The process according to claim 45, characterized in that the encapsulation material is aluminum. 53. The process according to claim 45, characterized in that the encapsulation material is a carbon-carbon composition. 54. The process according to claim 45, characterized in that the surface sheet material is a carbon-carbon composition. 55. A composition of the carbon foam heat sink product, characterized in that it is produced by the process according to claim 45.