U.S. patent application number 10/054800 was filed with the patent office on 2003-06-26 for pitch-based carbon foam heat sink with phase change material.
Invention is credited to Burchell, Timothy D., Klett, James W..
Application Number | 20030115753 10/054800 |
Document ID | / |
Family ID | 27557456 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030115753 |
Kind Code |
A1 |
Klett, James W. ; et
al. |
June 26, 2003 |
Pitch-based carbon foam heat sink with phase change material
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.
Inventors: |
Klett, James W.; (Knoxville,
TN) ; Burchell, Timothy D.; (Oak Ridge, TN) |
Correspondence
Address: |
Gregory A. Nelson
AKERMAN, SENTERFITT & EIDSON, P.A.
P.O. Box 3188
West Palm Beach
FL
33402-3188
US
|
Family ID: |
27557456 |
Appl. No.: |
10/054800 |
Filed: |
January 23, 2002 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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10054800 |
Jan 23, 2002 |
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09489805 |
Jan 24, 2000 |
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09489805 |
Jan 24, 2000 |
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09093406 |
Jun 8, 1998 |
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6037032 |
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09093406 |
Jun 8, 1998 |
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08921875 |
Sep 2, 1997 |
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6033506 |
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09093406 |
Jun 8, 1998 |
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08923877 |
Sep 2, 1997 |
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09093406 |
Jun 8, 1998 |
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09337027 |
Jun 25, 1999 |
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6261485 |
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09337027 |
Jun 25, 1999 |
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08921875 |
Sep 2, 1997 |
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6033506 |
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09337027 |
Jun 25, 1999 |
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09136596 |
Aug 19, 1998 |
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6387343 |
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09136596 |
Aug 19, 1998 |
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08921875 |
Sep 2, 1997 |
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6033506 |
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Current U.S.
Class: |
29/890.03 |
Current CPC
Class: |
C09K 5/06 20130101; C04B
38/00 20130101; F28D 20/023 20130101; Y02E 60/14 20130101; Y02B
30/54 20130101; Y02P 20/10 20151101; B32B 5/18 20130101; Y10T
29/4935 20150115; F24F 5/0035 20130101 |
Class at
Publication: |
29/890.03 |
International
Class: |
F28D 017/00 |
Claims
What is claimed is:
1. A process of producing a carbon foam heat sink comprising:
selecting an appropriate mold shape; introducing pitch to an
appropriate level in said mold; purging air from said mold to form
a vacuum; heating said pitch to a temperature sufficient to
coalesce said pitch into a liquid; releasing said vacuum and
backfilling an inert fluid at a static pressure up to about 1000
psi; heating said pitch to a temperature sufficient to cause gases
to evolve and form a carbon foam; heating said carbon foam to a
temperature sufficient to coke the pitch; cooling said carbon foam
to room temperature and simultaneously releasing said inert fluid;
at least partially encasing said carbon foam; and at least
partially filling porous regions of said carbon foam with a phase
change material.
2. The process of claim 1 wherein said pitch is introduced as
granulated pitch.
3. The process of claim 1 wherein said pitch is introduced as
powdered pitch.
4. The process of claim 1 wherein said pitch is introduced as
pelletized pitch.
5. The process of claim 1 wherein said pitch is a synthetic
mesophase or isotropic pitch.
6. The process of claim 1 wherein said pitch is a petroleum derived
mesophase or isotropic pitch.
7. The process of claim 1 wherein said pitch is a coal-derived
mesophase or isotropic pitch.
8. The process of claim 1 wherein said pitch is a blend of pitches
selected from the group consisting of synthetic mesophase or
isotropic pitch, petroleum derived mesophase or isotropic pitch,
and coal derived mesophase or isotropic pitch.
9. The process of claim 1 wherein said pitch is a solvated
pitch.
10. The process of claim 1 wherein said purging is effected by a
vacuum step.
11. The process of claim 1 wherein said purging is effected by an
inert fluid.
12. The process of claim 1 wherein said vacuum is applied at less
than 1 torr.
13. The process of claim 1 wherein nitrogen is introduced as the
inert fluid.
14. The process of claim 1 wherein said pitch is heated to a
temperature in the range of about 500.degree. C. to about
1000.degree. C. to coke said pitch.
15. The process of claim 1 wherein said pitch is heated to a
temperature of about 800.degree. C. to coke said pitch.
16. The process of claim 1 wherein the temperature to coke said
pitch is raised at a rate of no greater than 5.degree. C. per
minute.
17. The process of claim 1 wherein said pitch is soaked at the
coking temperature for at least 1 5 minutes to effect said
coking.
18. The process of claim 1 wherein said pitch is heated to a
temperature of about 630.degree. C to coke said pitch.
19. The process of claim 1 wherein said pitch is heated to a
temperature of about 50.degree. C. to about 100.degree. C. to
coalesce said pitch.
20. The process of claim 1 where said foam is cooled at a rate of
approximately 1.5.degree. C./min with the release of pressure at a
rate of approximately 2 psi/mn.
21. The process of claim 1 further including step of densifying
said foam.
22. The process of claim 1 wherein said phase change material is
acetic acid.
23. The process of claim 1 wherein said phase change material is a
paraffin wax.
24. The process of claim 1 wherein said phase change material is
germanium.
25. The process of claim 1 wherein said encasement material is
polyethylene.
26. The process of claim 1 wherein said encasement material is
aluminum.
27. The process of claim 1 wherein said encasement material is a
carbon/carbon composite.
28. A carbon foam heat sink product as produced by the process of
claim 1.
29. A process of producing a carbon foam heat sink comprising:
selecting an appropriate mold shape and a mold composed of a
material that the molten pitch does not wet; introducing said pitch
to an appropriate level in the mold; purging the air from said mold
to form a vacuum; heating said pitch to a temperature sufficient to
coalesce said pitch into a liquid; releasing said vacuum and
backfilling an inert fluid at a static pressure up to about 1000
psi; heating said pitch to a temperature sufficient to coke the
pitch; and cooling said foam to room temperature and simultaneously
releasing said inert fluid; at Least partially encasing said foam;
and at least partially filling porous regions of said foam with a
phase change material.
30. The process of claim 29 wherein said pitch is introduced as
granulated pitch.
31. The process of claim 29 wherein said pitch is introduced as
powdered pitch.
32. The process of claim 29 wherein said pitch is introduced as
pelletized pitch.
33. The process of claim 29 wherein said pitch is a synthetic
mesophase or isotropic pitch.
34. The process of claim 29 wherein said pitch is a
petroleum-derived mesophase pitch.
35. The process of claim 29 wherein said pitch is a coal-derived
mesophase pitch.
36. The process of claim 29 wherein said mold is pureed by a vacuum
applied at less than 1 torr.
37. The process of claim 29 wherein said mold is purged by an inert
fluid before heating.
38. The process of claim 29 wherein said phase change material is
acetic acid.
39. The process of claim 29 wherein said phase change material is a
paraffin wax.
40. The process of claim 29 wherein said phase change material is
germanium.
41. The process of clam 29 wherein said encasement material is
polyethylene.
42. The process of claim 29 wherein said encasement material is
aluminum.
43. The process of claim 29 wherein said encasement material is a
carbon/carbon composite.
44. A carbon foam heat sink product as produced by the process of
claim 29.
45. A process of producing a carbon foam heat sink comprising:
selecting an appropriate mold shape; introducing pitch to an
appropriate level in said mold; purging air from said mold to form
vacuum; heating said pitch to a temperature sufficient to coalesce
said pitch into a liquid; releasing said vacuum and backfilling an
inert fluid at a static pressure up to about 1000 psi; heating said
pitch to a temperature sufficient to cause gases to evolve and form
carbon foam; heating said carbon foam to a temperature sufficient
to coke the pitch; cooling said carbon foam to room temperature and
simultaneously releasing said inert fluid; placing facesheets on
the opposite sides of said carbon foam, adhering the facesheets to
said carbon foam; at least partially encasing said carbon foam and
facesheets; and at least partially filling porous regions of said
carbon foam with a phase change material.
46. The process of claim 45 wherein the adhering of the facesheets
to the carbon foam is effected by a molding step.
47. The process of claim 45 wherein the adhering of the facesheets
to the carbon foam is effected by a coating material.
48. The process of claim 45 wherein said phase change material is
acetic acid.
49. The process of claim 45 wherein said phase change material is a
paraffin wax.
50. The process of claim 45 wherein said phase change material is
germanium.
51. The process of claim 45 wherein said encasement material is
polyethylene.
52. The process of claim 45 wherein said encasement material is
aluminum.
53. The process of claim 45 wherein said encasement material is a
carbon-carbon composite.
54. The process of claim 45 wherein said facesheets material is a
carbon-carbon composite.
55. A composite carbon foam heat sink product produced by the
process of claim 45
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of an earlier
filed U.S. patent application Ser. No. 08/921,875, filed on Sep. 2,
1997, and U.S. patent application Ser. No. 08/1923,877, filed Sep.
2, 1997, both of which are herein incorporated in their entirety by
reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] The United States Government has rights in this invention
pursuant to contract No. DE-AC05-960R22464 between the United
States Department of Energy and Lockheed Martin Energy Research
Corporation.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to porous carbon foam filled
with phase change materials and encased to form a heat sink
product, and more particularly to a process for producing them.
[0004] There are currently many applications that require the
storage of large quantities of heat for either cooling or heating
an object. Typically these applications produce heat so rapidly
that normal dissipation through cooling fins, natural convection,
or radiation cannot dissipate the heat quickly enough and, thus,
the object over heats. To alleviate this problem, a material with a
large specific heat capacity, such as a heat sink, is placed in
contact with the object as it heats. During the heating process,
heat is transferred to the heat sink from the hot object, and as
the heat sink's temperature rises, it "stores" the heat more
rapidly than can be dissipated to the environment through
convection. Unfortunately, as the temperature of the heat sink
rises the heat flux from the hot object decreases, due to a smaller
temperature difference between the two objects. Therefore, although
this method of energy storage can absorb large quantities of heat
in some applications, it is not sufficient for all applications
Another method of absorbing heat is through a change of phase of
the material, rather than a change in temperature. Typically, the
phase transformation of a material absorbs two orders of magnitude
greater thermal energy than the heat capacity of the material. For
example, the vaporization of 1 gram of water at 100.degree. C.
absorbs 2,439 joules of energy, whereas changing the temperature of
water from 99.degree. C. to 100.degree. C. only absorbs 4.21 Joules
of energy. In other words, raising the temperature of 579 grams of
water from 99.degree. C. to 100.degree.C. absorbs the same amount
of heat as evaporating 1 gram of water at 100.degree. C. The same
trend is found at the melting point of the material. This
phenomenon has been utilized in some applications to either absorb
or evolve tremendous amounts of energy in situations where heat
sinks will not work.
[0005] Although a solid block of phase change material has a very
large theoretical capacity to absorb heat, the process is not a
rapid one because of the difficulties of heat transfer and thus it
cannot be utilized in certain applications. However, the
utilization of the high thermal conductivity foam will overcome the
shortcomings described above. If the high conductivity foam is
filled with the phase change material, the process can become very
rapid. Because of the extremely high conductivity in the struts of
the foam, as heat contacts the surface of the foam, it is rapidly
transmitted throughout the foam to a very large surface area of the
phase change material. Thus, heat is very quickly distributed
throughout the phase change material, allowing it to absorb or emit
thermal energy extremely quickly without changing temperature, thus
keeping the driving force for heat transfer at its maximum.
[0006] Heat sinks have been utilized 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
melting is encased in a graphite or metallic case, typically
aluminum, and placed in contact with the object creating the heat.
Since most phase change materials have a low at thermal
conductivity, the rate of heat transfer through the material is
limited, but this is offset by the high energy absorbing capability
of the phase change. As heat is transmitted through the metallic or
graphite case to the phase change material, the phase change
material closest to the heat source begins to melt. Since the
temperature of the phase change material does not change until all
the material melts, the flux from the heat source to the phase
change material remains relatively constant. However, as the heat
continues to melt more phase change material, more liquid is
formed. Unfortunately, the liquid has a much lower thermal
conductivity, thus hampering heat flow further. In fact, the
overall low thermal conductivity of the solid and liquid phase
change materials limits the rate of heat absorption and, thus,
reduces the efficiency of the system.
[0007] Recent developments of fiber-reinforced composites,
including carbon foams, have been driven by requirements for
improved strength, stiffness, creep resistance, and toughness in
structural engineering materials. Carbon fibers have led to
significant advancements in these properties in composites of
various polymeric, metal, and ceramic matrices.
[0008] However, current applications of carbon fibers have evolved
from structural reinforcement to thermal management in application
ranging from high-density electronic modules to communication
satellites. This has stimulated research into novel reinforcements
and composite processing methods. High thermal conductivity, low
weight, and low coefficient of thermal expansion are the primary
concerns in thermal management applications. See Shih, Wei,
"Development of Carbon-Carbon Composites for Electronic Thermal
Management Applications," IDA Workshop, May 3-5, 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,
G.B., "High Thermal Conductivity C/C Composites for Thermal
Management," DA 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 towards a sandwich type approach in which a low-density
structural core material (i.e. honeycomb or foam) is sandwiched
between a high thermal conductivity facesheet. Structural cores are
limited to low density materials to ensure that the weight limits
are not exceeded. Unfortunately, carbon foams and carbon honeycomb
materials are the only available materials for use in high
temperature applications (>1600.degree. C.). High thermal
conductivity carbon honeycomb materials are extremely expensive to
manufacture compared to low conductivity honeycombs, 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 starting
materials.
[0009] In order to produce high stiffness and high conductivity
carbon foams, invariably, a pitch must be used as the precursor.
This is because pitch is the only precursor which forms a highly
aligned graphitic structure which is a requirement for high
conductivity. Typical processes utilize a blowing technique to
produce a foam of the pitch precursor in which the pitch is melted
and passed from a high pressure region to a low pressure region.
Thermodynarnically, this produces a "Flash," thereby causing the
low molecular weight compounds in the pitch to vaporize (the pitch
boils), resulting in a pitch 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 Mathematical Process
Model Process Model for the Foaming of a Mesophase Carbon
Precursor," Mat. Res . Soc. Symp., Materials Research Society,
270:3540 (1992); Gibson, L. U. and M. F. Ashby, Cellular Solids:
Structures & Properties, Pergamon Press, N.Y. (1988); Gibson,
L. J., Mat. Sci. and Eng A110, 1 (1989); Krnippenberg and B.
Lersmacher, Phillips Tech. Rev., 36(4), (1976); and Bonzom, A., P.
Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Then,
the pitch-foam must be oxidatively stabilized by heating in air (or
oxygen) for many hours, thereby, cross linking the structure and
"stabilizing," the pitch so it does not melt during, carbonization.
See Hagar, Joseph W. and Max L. Lake, "Formulation of a
Mathematical Process Model Process Model for the Foaming of a
Mesophase Carbon Precursor, Mat. Res. Soc. Symp., Materials
Research to Society, 270:3540 (1992); and White, J. L., and P. M.
Shaeffer. Carbon, 27:697 (1989). This is a time concurring step and
can be an expensive step depending on the part size and equipment
required. The "stabilized" or oxidized pitch is then carbonized in
an inert atmosphere to temperatures as high as 1100.degree. C.
Then, graphitization is performed at temperatures as high as
3000.degree. C. to produce a high thermal conductivity graphitic
structure, resulting in a stiff and very thermally conductive
foam.
[0010] Other techniques utilize a polymeric precursor, such as
phenolic, urethane, or blends of these with pitch. See Hagar,
Joseph W. and Max L. Lake, "Idealized Strut Geometries for
Open-Celled Foams," Mat. Res. Soc. Symp., Materials Research
Society, 270:4146 (1992); Aubert, J. W., MRS Symposium Proceedings,
207:117-127 (1990); Cowlard, F. C. and J. C. Lewis, 3. of Mat.
Sci., 2:507-512 (1967); and Noda, T., Liagaai and S. Yamada, J. of
Non-Crystalline Solids, 1:285-302, (1969). High pressure is applied
and the sample is heated. At a specified temperature, the pressure
is released, thus causing the liquid to foam as volatile compounds
are released. The polymeric precursors are cured and then
carbonized without a stabilization step. However, these precursors
produce a "glassy" or vitreous carbon which does not exhibit
graphitic structure and, thus, has low thermal conductivity and low
stiffness. See Hagar, Joseph W. and Max L. Lake, "Idealized Strut
Geometries for Open-Celled Foams," Mat. Res. Soc. Symp., Materials
Research Society, 270:4146 (1992).
[0011] In either case, once the foam is formed, it is then bonded
in a separate step to the facesheet used in the composite. This can
be an expensive step in the utilization of the foam.
[0012] The extraordinary mechanical properties of commercial carbon
fibers are due to the unique graphitic morphology of the extruded
filaments. See Edie, D. D., "Pitch and Mesophase Fibers," in Carbon
Fibers, Filaments and Composites, Figueiredo (editor), Kluwer
Academic Publishers, Boston, pp. 43-72 (1990). Contemporary
advanced structural composites exploit these properties by creating
a disconnected network of graphitic filaments held together by an
appropriate matrix. Carbon foam derived from a pitch precursor can
be considered to be an interconnected network of graphitic
ligaments or struts, as shown in FIG. 1. As such interconnected
networks, they represent a potential alternative as reinforcement
in structural composite materials.
[0013] The process of this invention overcomes current
manufacturing limitations by avoiding a "blowing" or "pressure
release" technique to produce the foam. Furthermore, an oxidation
stabilization step is not required, as in other methods used to
produce pitch based carbon foams with a highly aligned graphitic
structure. This process is less time consuming, and therefore, will
be lower in cost and easier to fabricate. The foam can be produced
with an integrated sheet of high thermal conductivity carbon on the
surface of the foam, thereby producing a carbon foam with a smooth
sheet on the surface to improve heat transfer.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is production of encased
high thermal conductivity porous carbon foam filled with a phase
change material wherein tremendous amounts of thermal energy are
stored and emitted very rapidly. The porous foam is filled with a
phase change material (PCM) at a temperature close to the operating
temperature of the device. As heat is added to the surface, from a
heat source such as a computer chip, friction due to re-entry
through the atmosphere, or radiation such as sunlight, it is
transmitted rapidly and uniformly throughout the foam and then to
the phase change material. As the material changes phase, it
absorbs orders of magnitude more energy than non-PCM material due
to transfer of the latent heat of fusion or vaporization.
Conversely, the filled foam can be utilized to emit energy rapidly
when placed in contact with a cold object.
[0015] Non-limiting embodiments disclosed herein are a device to
rapidly thaw frozen foods or freeze thawed foods, a design to
prevent overheating of satellites or store thermal energy as they
experience cyclic heating during orbit, and a design to cool
leading edges during hypersonic flight or re-entry from space.
[0016] Another object of the present invention is to provide carbon
foam and a composite from a mesophase or isotropic pitch such as
synthetic, petroleum or coal-tar based pitch. Another object is to
provide a carbon foam and a composite from pitch which does not
require an oxidative stabilization step.
[0017] These and other objectives are accomplished by a method of
producing carbon foam heat sink wherein an appropriate mold shape
is selected and preferably an appropriate mold release agent is
applied to walls of the mold. Pitch is introduced to 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 pitch is heated to a temperature sufficient to
coalesce the pitch into a liquid which preferably is of about
50.degree. C. to about 100.degree. C. above the softening point of
the pitch. The vacuum is released and an inert fluid applied at a
static pressure up to about 1000 psi. The pitch is heated to a
temperature sufficient to cause gases to evolve and foam the pitch.
The pitch is further heated to a temperature sufficient to coke the
pitch and the pitch is cooled to room temperature with a
simultaneous and gradual release of pressure. The foam is then
filled with a phase change material and encased to produce an
efficient heat storage product.
[0018] In another aspect, the previously described steps are
employed in a mold composed of a material such that the molten
pitch does not adhere to the surface of the mold.
[0019] In yet another aspect, the objectives are accomplished by
the carbon foam product produced by the methods disclosed herein
including a foam product with a smooth integral facesheet.
[0020] In still another aspect a carbon foam composite product is
produced by adhering facesheets to the carbon foam produced by the
process of this invention.
[0021] FIG. 1 is section cut of a heat sink device for thawing food
using acetic acid as the phase change material.
[0022] FIG. 2 is a section cut of a heat sink to prevent
overheating of satellites during cyclic orbits.
[0023] FIG. 3 is a section cut of a heat sink used on the leading
edge of a shuttle orbits.
[0024] FIG. 4 is a micrograph illustrating typical carbon foam with
interconnected carbon ligaments and open porosity.
[0025] FIG. 5-9 are micrographs of pitch-derived carbon foam
graphitized at 2500.degree. C. and at various magnifications.
[0026] FIG. 10 is a SEM in micrograph of carbon foam produced by
the process of this invention
[0027] FIG. 11 is a chart illustrating cumulative intrusion volume
versus pore diameter.
[0028] FIG. 12 is a chart illustrating log differential intrusion
volume versus pore diameter.
[0029] FIG. 13 is a graph illustrating the temperatures at which
volatiles are given off from raw pitch.
[0030] FIG. 14 is an X-ray analysis of the graphitized foam
produced by the process of this invention.
[0031] FIGS. 15 A-C are photographs illustrating foam produced with
aluminum crucibles and the smooth structure or face sheet that
develops.
[0032] FIG. 16A is a schematic view illustrating the production of
a carbon foam composite made in accordance with this invention.
[0033] FIG. 16B is a perspective view of the carbon foam composite
of this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] In order to illustrate the carbon foam heat sink product of
this invention, the following examples are set forth. They are not
intended to limit the invention in any way.
EXAMPLE 1
Device for Thawing Food
[0035] Acetic acid has a heat of melting of 45 J/g at a melting
point of 11.degree. C. The heat of melting of food, primarily ice,
is roughly 79 J/g at 0.degree. C. Therefore, take a block of foam
and fill it with liquid acetic acid at room temp. The foam will be
encased in a box made from an insulating polymer such as
polyethylene on all sides except the top. The too of the
foam/acetic acid block will be capped with a high thermal
conductivity aluminum plate that snaps into place thus sealing the
foam/acetic acid inside the polymer case (illustrated in FIG. 1).
If the foam block is 10-in..times.15-in..times.0.5-in. thick, the
mass of foam is 614 grams. The mass of acetic acid that fills the
foam is roughly 921 grams. Therefore, when a piece of frozen meat
is placed in contact with the top of the aluminum block, the foam
will cool to the freezing point of the acetic acid (11.degree. C.).
At this point, the heat given off from the acetic acid as it
freezes (it also remains at 11.degree. C.) will be equivalent to 49
KJ. This heat is rapidly transferred to the frozen meat as it thaws
(it also remains at 0.degree. C.). This amount of heat is
sufficient to melt roughly 500 grams (1lb.) of meat.
EXAMPLE 2
Heat Sink To Prevent Overheating Of Satellites During Cvclic
Orbits.
[0036] Produce a carbon-carbon composite with the foam in which the
foam is a core material with carbon-carbon face sheets (FIG. 2).
Fill the foam core with a suitable phase change material, such as a
paraffin wax, that melts around the maximum operating temperature
of the satellite components. One method to perform this would be to
drill a hole in one surface of the carbon-carbon face sheets and
vacuum fill the phase change material in the liquid state into the
porous foam. Once filled, the sample can be cooled (the phase
chance material solidifies) and the hole can be plugged with an
epoxy or screw-type cap. The epoxy and any other sealant must be
able to withstand the operating temperature of the application. The
foam-core composite will then be mounted on the side of the
satellite that is exposed to the sun during orbit. As the satellite
orbits the earth and is exposed to the sun, the radiant energy from
the sun will begin to heat the composite panel to the melting point
of the phase change material. At this point, the panel will not
increase in temperature as the phase change material melts. The
amount of radiant energy the panel can absorb will be dependent on
the thickness and outer dimensions of the panel. This can be easily
calculated and designed through knowledge of the orbit times of the
satellite such that the material never completely melts and, thus,
never exceeds the melt temperature. Then, when the satellite leaves
the view of the sun, it will begin to radiate heat to space and the
phase change material will begin to freeze. The cycle will repeat
itself once the satellite comes into view of the sun once
again.
EXAMPLE 3
Heat Sink for Leading Edges
[0037] Currently, the shuttle orbiter experiences extreme heats
during reentry. Specifically, the leading edges of the craft can
reach 1800.degree. C. and the belly of the craft can reach
temperatures as high as 1200.degree. C. If a foam core composite
panel is placed at the surface of the leading edges and at the
surface of the belly (FIG. 3), it would be able to absorb enough
energy to dramatically reduce the maximum temperature of the hot
areas. This also would permit a faster re-entry or (steeper glide
slope) and maintain the current maximum temperatures. In this case
the phase change material would most likely be an alloy, e.g.
geranium-silicon, which melts around 800-900C. and does not
vaporize until much higher than the maximum temperature of the
craft.
[0038] For example, Cermanium has a heat of formation (heat of
melting) of 488 J/g. This would require 1.0 Ka of Germanium to
reduce the temperature of 1 Kg of existing carbon/carbon
heat-shield by 668.degree. C. In other words, if the existing
carbon-carbon were replaced pound-for-pound with geranium Filled
foam, the maximum temperature of the heat shield would only be
about 1131.degree. C. instead of about 1800.degree. C. during
re-entry, depending on the duration of thermal loading.
EXAMPLE 4
[0039] Pitch powder, granules, or pellets are placed in a mold with
the desired final shape of the foam. These pitch materials can be
solvated if desired. In this example, Mitsubishi ARA-24 mesophase
pitch was utilized. A proper mold release agent or film is applied
to the sides of the mold to allow removal of the part. In this
case, Boron Nitride spray and Dry Graphite Lubricant were
separately used as a mold release agent. If the mold is made from
pure aluminum, no mold release agent is necessary since the molten
pitch does not adhere to the aluminum and, thus, will not stick to
the mold. Similar mold materials may be found that the pitch does
not adhere and, thus, they will not need mold release. The sample
is evacuated to less than 1 torr and then heated to a temperature
approximately 50 to 100.degree. C. above the softening point. In
this case where Mitsubishi ARA24 mesophase pitch was used,
300.degree. C. was sufficient. At this point, the vacuum is
released to a nitrogen blanket and then a pressure of up to 1000
psi is applied. The temperature of the system is then raised to
800.degree. C., or a temperature sufficient to coke the pitch which
is 500.degree. C. to 1000.degree. C. This is performed at a rate of
no greater than 5.degree. C./min. and preferably at about 2.degree.
C./min. The temperature is held for at least 15 minutes to achieve
an assured soak and then the furnace power is turned off and cooled
to room temperature. Preferably the foam was cooled at a rate of
approximately 1.5.degree. C./min. with release of pressure at a
rate of approximately 2 psi/min. Final foam temperatures for three
product runs were 500.degree. C., 630.degree. C. and 800.degree. C.
During the cooling cycle, pressure is released gradually to
atmospheric conditions. The foam was then heat treated to
1050.degree. C. (carbonized) under a nitrogen blanket and then Heat
treated in separate runs to 2500.degree. C. and 2800.degree. C.
(graphitized) in Argon.
[0040] Carbon foam produced with this technique was examined with
photomicrography, scanning electron microscopy (SEM, X-ray
analysis, and mercury porisimetry. As can be seen in the FIGS.
5-10, the isochromatic regions under cross-polarized light indicate
that the Struts of the foam are completely graphitic. That is, all
of the pitch was converted to graphite and aligned along the axis
of the struts. These struts are also similar in size and are
interconnected throughout the foam. This would indicate that the
foam would have high stiffness and good strength. As seen in FIG.
10 by the SEM micrograph of the foam, the foam is open cellular
meaning that the porosity is not closed. FIGS. 11 and 12 are
results of the mercury porisimetry tests. These tests indicate that
the pore sizes are in the range of 90-200 microns.
[0041] A thermogravimetric study of the raw pitch was performed to
determine the temperature at which the volatiles are evolved As can
be seen in FIG. 14, the pitch loses nearly 20% of its mass fairly
rapidly in the temperature range between about 420.degree. C. and
about 480.degree. C. Although this was performed at atmospheric
pressure, the addition of 1000 psi pressure will not shift this
effect significantly. Therefore, while the pressure is at 1000 psi,
gases rapidly evolved during heating through the temperature range
of 420.degree. C. to 430.degree. C. The gases produce a foamina
effect (like boiling) on the molten pitch. As the temperature is
increased further to temperatures ranging from 500.degree. C. to
1000.degree. C. (depending on the specific pitch), the foamed pitch
becomes coked (or rigid), thus producing a solid foam derived from
pitch. Hence, the foaming has occurred before the release of
pressure and, therefore, this process is very different from
previous art.
[0042] Samples from the foam were machined into specimens for
measuring the thermal conductivity. The bulk thermal conductivity
ranged from 58 Wm.cndot.K to 106 W/m.cndot.K. The average density
of the samples was 0.53 g/cm.sup.3. When weight is taken into
account, the specific thermal conductivity of the pitch derived
from foam is over 4 times. greater than that of copper. Further
derivations can be utilized to estimate the thermal conductivity of
the struts themselves to be nearly 700 W/m.cndot.K This is
comparable to high thermal conductivity carbon fibers produced from
this same ARA24 mesophase pitch.
[0043] X-ray analysis of the foam was performed to determine the
crystalline structure of the material. The x-ray results are shown
in FIG. 14. From this data, the graphene layer spacing (d.sub.002)
was determined to be 0.336 nm. The coherence length (L.sub.3,100)
was determined to be 203.3 nm and the stacking height was
determined to be 442.3 nm.
[0044] The compression strength of the samples were measured to be
3.4 MPa and the compression modulus was measured to be 73.4 MPa.
The foam sample was easily machined and could be handled readily
without fear of damage, indicating good strength.
[0045] It is important to note that when this pitch is heated in a
similar manner, but only under atmospheric pressure, the pitch
foams dramatically more than when under pressure. In fact, the
resulting foam is so fragile that it could not even be handled to
perform tests. Molding under pressure serves to limit the growth of
the cells and produces a usable material.
EXAMPLE 5
[0046] An alternative to the method of Example 4 is to utilize a
mold made from aluminum. In this case two molds were used, an
aluminum weighing dish and a sectioned soda can. The same process
as set forth in Example 4 is employed except that the final coking
temperature was only 630.degree. C., so as to prevent the aluminum
from melting.
[0047] FIGS. 15 A-C illustrate the ability to utilized complex
shaped molds for producing complex shaped foam. In one case, shown
in FIG. 15 A, the top of a soda can was removed and the remaining
can used as a mold. No release agent was utilized. Note that the
shape of the resulting part conforms to the shape of the soda can,
even after graphitization to 2800.degree. C. This demonstrates the
dimensional stability of the foam and the ability to produce near
net shaped parts.
[0048] In the second case, as shown in FIGS. 15 B and C employing
an aluminum weight dish, a very smooth surface was formed on the
surface contacting the aluminum. This is directly attributable to
the fact that the molten pitch does not adhere to the surface of
the aluminum. This would allow one to produce complex shaped parts
with smooth surfaces so as to improve contact area for bonding or
improving heat transfer. This smooth surface will act as a face
sheet and, thus, a foam core composite can be fabricated in-situ
with the fabrication of the face sheet. Since it is fabricated
together and an integral material no interface joints result,
thermal stresses will be less, resulting in a stronger
material.
[0049] The following examples illustrate the production of a
composite material employing the foam of this invention.
EXAMPLE 6
[0050] Pitch derived carbon foam was produced with the method
described in Example 4. Referring to FIG. 16A the carbon foam 10
was then machined into a block 2".times.2".times.1/2". Two pieces
12 and 14 of a prepeg comprised of Hercules AS4 carbon fibers and
ICI Fibirite Polyetheretherkeytone thermoplastic resin also of
2".times.2".times.1/2" size were placed on the top and bottom of
the foam sample, and all was placed in a matched graphite mold 16
for compression by graphite plunger 18. The composite sample was
heated under an applied pressure of 100 psi to a temperature of
380.degree. C. at a rate of 5.degree. C./min. The composite was
then heated under a pressure of 100 psi to a temperature of
650.degree. C. The foam core sandwich panel generally 20 was then
removed from the mold and carbonized under nitrogen to 1050.degree.
C. and then graphitized to 2800.degree. C., resulting in a foam
with carbon-carbon facesheets bonded to the surface. The composite
generally 30 is shown in FIG. 16B.
EXAMPLE 7
[0051] Pitch derived carbon foam was produced with the method
described in Example 4. It was then machined into a block
2".times.2".times.1/2". Two pieces of carbon-carbon material,
2".times.2".times.1/2", were coated lightly with a mixture of 50%
ethanol, 50% phenolic Durez.COPYRGT.Resin available from Occidental
Chemical Co. The foam block and carbon-carbon material were
positioned together and placed in a mold as indicated in Example 6.
The sample was heated to a temperature of 150.degree. C. at a rate
of 5.degree. C./min and soaked at temperature for 14 hours. The
sample was then carbonized under nitrogen to 1050.degree. C. and
then graphitized to 2800.degree. C., resulting in a foam with
carbon-carbon facesheets bonded to the surface. This is also shown
generally at 30 in FIG. 16B.
EXAMPLE 8
[0052] Pitch derived carbon foam was produced with the method
described in Example 4. The foam sample was then densified with
carbon by the method of chemical vapor infiltration for 100 hours.
The density increased to 1.4 g/cm.sup.3, the flexural strength was
19.5 MPa and the flexural modulus was 2300 MPa. The thermal
conductivity of the raw foam was 58 W/m.cndot.K and the thermal
conductivity of the densified foam was 94 W/m.cndot.K.
EXAMPLE 9
[0053] Pitch derived carbon foam was produced with the method
described in Example 4. The foam sample was then densified with
epoxy by the method of vacuum impregnation. The epoxy was cured at
150.degree. C. for 5 hours. The density increased to 1.37
g/cm.sup.3 and the flexural strength was measured to be 19.3
MPa
[0054] 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 material with acceptable properties. Additional possible
embodiments include ceramics, glass, or other materials impregnated
into the foam for densification.
[0055] Based on the data taken to date from the carbon foam
material, several observations can be made outlining important
features of the invention that include:
[0056] 1. Pitch-based carbon foam can be produced without an
oxidative stabilization step, thus saving time and costs.
[0057] 2. High graphitic alignment in the struts of the foam is
achieved upon graphitization to 2500.degree. C., and thus high
thermal conductivity and stiffness will be exhibited by the foam,
making them suitable as a core material for
thermal-applications.
[0058] 3. High compressive strengths should be achieved with
mesophase pitch-based carbon foams, making them suitable as a core
material for structural applications.
[0059] 4. Foam core composites can be fabricated at the same time
as the foam is generated, thus saving time and costs.
[0060] 5. Rigid monolithic preforms can be made with significant
open porosity suitable for densification by the Chemical Vapor
Infiltration method of ceramic and carbon infiltrants.
[0061] 6. Rigid monolithic preforms can be made with significant
open porosity suitable for activation, producing a monolithic
activated carbon.
[0062] 7. It is obvious that by varying the pressure applied, the
size of the bubbles formed during is the foaming will change and,
thus, the density, strength, and other properties can be
affected.
[0063] The following alternative procedures and products can also
be effected by the process of this invention:
[0064] 1. Fabrication of preforms with complex shapes for
densification by CVI or Melt Impregnation.
[0065] 2. Activated carbon monoliths with high thermal
conductivity.
[0066] 3. Optical absorbent.
[0067] 4. Low density heating elements.
[0068] 5. Firewall Material
[0069] 6. Low secondary electron emission targets for high-energy
physics applications.
[0070] The present invention provides for the manufacture of
pitch-based carbon foam heat sink for structural and thermal
composites. The process involves the fabrication of a graphitic
foam from a mesophase or isotropic pitch which can be synthetic,
petroleum, or coal-tar based. A blend of these pitches can also be
employed. The simplified process utilizes a high pressure high
temperature furnace and thereby, does not require and oxidative
stabilization step. The foam has a relatively uniform distribution
of pore sizes (-100 microns), very little closed porosity, and
density of approximately 0.53 g/cm.sup.3. The mesophase pitch is
stretched along the struts of the foam structure and thereby
produces a highly aligned graphitic structure in the struts These
struts will exhibit thermal conductivities and stiffness similar to
the very expensive high performance carbon fibers (such as P-120
and K1100). Thus, the foam will exhibit high stiffness 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, thus reducing
fabrication time. By utilizing an isotropic pitch, the resulting
foam can be easily activated to produce a high surface area
activated carbon. The activated carbon foam will not experience the
problems associated with granules such as attrition, channeling,
and large pressure drops.
* * * * *