U.S. patent application number 10/831385 was filed with the patent office on 2005-01-06 for resin-impregnated flexible graphite articles.
Invention is credited to Brady, John Joseph, Getz, George JR., Klug, Jeremy, Norley, Julian.
Application Number | 20050003200 10/831385 |
Document ID | / |
Family ID | 35320110 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050003200 |
Kind Code |
A1 |
Norley, Julian ; et
al. |
January 6, 2005 |
Resin-impregnated flexible graphite articles
Abstract
Composites are prepared from resin-impregnated flexible graphite
materials. Impregnated materials are compressed and cured at
elevated temperature and pressure to form structures suitable for
uses such as electronic thermal management (ETM) devices,
supercapacitors and secondary batteries.
Inventors: |
Norley, Julian; (Chagrin
Falls, OH) ; Brady, John Joseph; (Cleveland, OH)
; Getz, George JR.; (Parma Heights, OH) ; Klug,
Jeremy; (Brunswick, OH) |
Correspondence
Address: |
WADDEY & PATTERSON
414 UNION STREET, SUITE 2020
BANK OF AMERICA PLAZA
NASHVILLE
TN
37219
|
Family ID: |
35320110 |
Appl. No.: |
10/831385 |
Filed: |
April 23, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10831385 |
Apr 23, 2004 |
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09943131 |
Aug 31, 2001 |
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6777086 |
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Current U.S.
Class: |
428/413 ;
257/E23.11; 428/396; 428/408 |
Current CPC
Class: |
C04B 35/522 20130101;
C04B 2235/9607 20130101; B32B 37/156 20130101; B32B 2038/0076
20130101; H01M 4/665 20130101; B32B 2305/72 20130101; C04B 2235/96
20130101; B32B 2260/046 20130101; C04B 37/008 20130101; B32B
2315/02 20130101; C04B 35/63452 20130101; C04B 2237/38 20130101;
B32B 27/04 20130101; B32B 18/00 20130101; B32B 2309/12 20130101;
H01M 4/666 20130101; C04B 2235/77 20130101; C04B 2237/363 20130101;
C04B 2237/704 20130101; C04B 2237/385 20130101; B32B 2307/706
20130101; B32B 2309/02 20130101; B32B 2307/72 20130101; C04B
2237/525 20130101; C04B 2235/6581 20130101; C04B 2235/787 20130101;
Y02E 60/10 20130101; C04B 2235/48 20130101; B32B 2311/24 20130101;
B32B 9/007 20130101; C08J 5/24 20130101; C04B 2237/76 20130101;
H01L 23/373 20130101; B32B 2307/302 20130101; B32B 2309/022
20130101; C08J 5/042 20130101; H01L 2924/0002 20130101; C04B 35/645
20130101; B32B 2311/12 20130101; Y10T 428/30 20150115; F28F 21/02
20130101; Y10T 428/2971 20150115; B32B 9/00 20130101; C04B 35/521
20130101; C04B 35/536 20130101; B32B 38/08 20130101; Y10T 428/31511
20150401; C04B 2235/5208 20130101; C08J 2363/00 20130101; H01M
4/663 20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
428/413 ;
428/408; 428/396 |
International
Class: |
B32B 027/38; B32B
027/04 |
Claims
What is claimed is:
1. A resin/graphite composite comprising a resin impregnated
graphite article pressure cured at an elevated temperature.
2. The composite of claim 1 wherein the resin is an epoxy.
3. The composite of claim 1 wherein the graphite article is
pressure cured at a temperature of at least about 90.degree. C. and
at a pressure of at least about 7 Mpa.
4. The composite of claim 1 wherein the density of the cured
composite is greater than about 1.8 g/cm.sup.3.
5. The composite of claim 1 wherein the graphite article is
pressure cured at a temperature of below about 200.degree. C. and
at a pressure of below about 35 Mpa.
6. An electronic thermal management device comprising at least one
sheet of resin impregnated, compressed particles of exfoliated
graphite pressure cured at an elevated temperature.
7. The device of claim 6 wherein the graphite sheet is pressure
cured at a temperature of at least about 90.degree. C. and at a
pressure of at least about 7 Mpa.
8. The device of claim 6, wherein the device exhibits a thermal
conductivity which is anisotropic in nature and is greater than 300
W/mK in at least one plane.
9. The device of claim 8 wherein the anisotropic thermal
conductivity varies by a factor of at least 15 as between a plane
with a higher thermal conductivity and a plane with lower thermal
conductivity.
10. The device of claim 6 wherein the pressure cured graphite sheet
has a density greater than about 1.85 g/cm.sup.3.
11. The device of claim 6 wherein the sheet of graphite has a resin
content of at least about 3% by weight.
12. The device of claim 11 wherein the sheet of graphite has a
resin content of from about 5% to about 35% by weight.
13. An anisotropic electronic thermal management device having a
thermal conductivity of greater than about 300 W/mK in an in plane
direction and a thermal conductivity of less than about 15 W/mK in
an out of plane direction and comprising at least one resin
impregnated sheet of compressed particles of exfoliated
graphite.
14. The device of claim 13 wherein the resin is epoxy.
15. The device of claim 13 wherein the resin impregnated sheet has
a density of at least about 1.85 g/cm.sup.3.
16. A method of forming a resin/graphite composite comprising
impregnated a graphite article with resin and curing the resin
under pressure and at an elevated temperature.
17. The method of claim 16 wherein the resin is an epoxy.
18. The method of claim 16 wherein the graphite article is pressure
cured at a temperature of at least about 90.degree. C. and at a
pressure of at least about 7 Mpa.
19. The method of claim 16 wherein the density of the cured
composite is greater than about 1.8 g/cm.sup.3.
20. The method of claim 16 wherein the graphite article is pressure
cured at a temperature of below about 200.degree. C. and at a
pressure of below about 35 Mpa.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of co-pending
application having Ser. No. 09/943,131 filed Aug. 31, 2001,
entitled "Laminates Prepared From Impregnated Flexible Graphite
Sheets", the disclosure of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] This invention relates to articles formed from
resin-impregnated, compressed particles of exfoliated graphite
(commonly referred to as flexible graphite), which are cured under
heat and pressure and are useful in applications such as heat
transporters used in electronic thermal management (ETM), or
current collectors for supercapacitors and secondary batteries.
BACKGROUND ART
[0003] Compressed exfoliated graphite articles are known in the
art, as are composite materials comprising resin-impregnated
graphite sheets. These structures find utility, for example, in
gasket manufacture.
[0004] In addition to their utility in gasket materials, graphite
composites also find utility as heat transfer or cooling apparatus.
The use of various solid structures as heat transporters is known
in the art. For example, Banks, U.S. Pat. Nos. 5,316,080 and
5,224,030 discloses the utility of diamonds and gas-derived
graphite fibers, joined with a suitable binder, as heat transfer
devices. Such devices are employed to passively conduct heat from a
source, such as a semiconductor, to a heat sink.
[0005] Graphite-based thermal management components offer several
advantages in electronic applications and can help eliminate the
potential negative impacts of heat generating components in
computers, communications equipment, and other electronic devices.
Graphite-based thermal management components include heat sinks,
heat pipes and heat spreaders. All offer thermal conductivity
comparable with or better than copper or aluminum, but are a
fraction of the weight of those materials, and provide
significantly greater design flexibility. Graphite-based thermal
management products take advantage of the highly directional
properties of graphite to move heat away from sensitive components.
Compared to typical aluminum alloys used for heat management, the
inventive graphite components can exhibit up to 300% higher thermal
conductivity, with values comparable to copper (.about.400 watts
per meter degree Kelvin, i.e., W/mK) or greater. Further, aluminum
and copper are isotropic, making it difficult to channel the heat
in a preferred direction.
[0006] The graphite material for use in this invention is graphite
material formed from compressed particles of exfoliated
graphite.
[0007] The following is a brief description of graphite and the
manner in which it is typically processed to form flexible
materials. Graphite, on a microscopic scale, is made up of layer
planes of hexagonal arrays or networks of carbon atoms. These layer
planes of hexagonally arranged carbon atoms are substantially flat
and are oriented or ordered so as to be substantially parallel and
equidistant to one another. The substantially-flat, parallel,
equidistant sheets or layers of carbon atoms, usually referred to
as graphene layers or basal planes, are linked or bonded together
and groups thereof are arranged in crystallites. Highly-ordered
graphite materials consist of crystallites of considerable size,
the crystallites being highly aligned or oriented with respect to
each other and having well ordered carbon layers. In other words,
highly ordered graphites have a high degree of preferred
crystallite orientation. It should be noted that graphites, by
definition, possess anisotropic structures and thus exhibit or
possess many characteristics that are highly directional, e.g.,
thermal and electrical conductivity and fluid diffusion.
[0008] Briefly, graphites may be characterized as laminated
structures of carbon, that is, structures consisting of superposed
layers or laminae of carbon atoms joined together by weak van der
Waals forces. In considering the graphite structure, two axes or
directions are usually noted, to wit, the "c" axis or direction and
the "a" axes or directions. For simplicity, the "c" axis or
direction may be considered as the direction perpendicular to the
carbon layers. The "a" axes or directions may be considered as the
directions parallel to the carbon layers or the directions
perpendicular to the "c" direction. The graphites suitable for
manufacturing flexible graphite articles possess a very high degree
of orientation.
[0009] As noted above, the bonding forces holding the parallel
layers of carbon atoms together are only weak van der Waals forces.
Natural graphites can be chemically treated so that the spacing
between the superposed carbon layers or laminae can be appreciably
opened up so as to provide a marked expansion in the direction
perpendicular to the layers, that is, in the "c" direction, and
thus form an expanded or intumesced graphite structure in which the
laminar character of the carbon layers is substantially
retained.
[0010] Graphite flake which has been chemically or thermally
expanded and more particularly expanded so as to have a final
thickness or "c" direction dimension which is as much as about 80
or more times the original "c" direction dimension, can be formed
without the use of a binder into cohesive or integrated sheets of
expanded graphite, e.g. webs, papers, strips, tapes, or the like
(typically referred to as "flexible graphite"). The formation of
graphite particles which have been expanded to have a final
thickness or "c" dimension which is as much as about 80 times or
more the original "c" direction dimension into integrated flexible
articles by compression, without the use of any binding material,
is believed to be possible due to the mechanical interlocking, or
cohesion, which is achieved between the voluminously expanded
graphite particles.
[0011] In addition to flexibility, the graphite material, as noted
above, has also been found to possess a high degree of anisotropy
to thermal and electrical conductivity and fluid diffusion,
somewhat less, but comparable to the natural graphite starting
material due to orientation of the expanded graphite particles
substantially parallel to the opposed faces of the material
resulting from very high compression, e.g. roll processing.
Material thus produced has excellent flexibility, good strength and
a very high degree or orientation. There is a need for processing
that more fully takes advantage of these properties.
[0012] Briefly, the process of producing binderless anisotropic
graphite material, e.g. sheets, articles, web, paper, strip, tape,
foil, mat, or the like, comprises compressing or compacting under a
predetermined load and in the absence of a binder, expanded
graphite particles which have a "c" direction dimension which is as
much as about 80 or more times that of the original particles so as
to form a substantially flat, integrated graphite article.
Typically, the article formed is a flexible, relatively thin (i.e.,
5 mm or less) sheet, although thicker articles are also capable of
being produced in this manner. The expanded graphite particles that
generally are worm-like or vermiform in appearance will, once
compressed, maintain the compression set and alignment with the
opposed major surfaces of the sheet. Properties of the article may
be altered by coatings and/or the addition of binders or additives
prior to the compression step. See U.S. Pat. No. 3,404,061 to
Shane, et al. The density and thickness of the material can be
varied by controlling the degree of compression.
[0013] Lower densities are advantageous where surface detail
requires embossing or molding, and lower densities aid in achieving
good detail. However, higher in-plane strength, thermal
conductivity and electrical conductivity are generally favored by
more dense sheets. Typically, the density of the material will be
within the range of from about 0.04 g/cm.sup.3 to about 1.4
g/cm.sup.3.
[0014] Graphite material made as described above typically exhibits
an appreciable degree of anisotropy due to the alignment of
graphite particles parallel to the major opposed, parallel surfaces
of the material, with the degree of anisotropy increasing upon roll
pressing to increased density. In roll-pressed anisotropic
material, the thickness, i.e. the direction perpendicular to the
opposed, parallel surfaces comprises the "c" direction and the
directions ranging along the length and width, i.e. along or
parallel to the opposed, major surfaces comprises the "a"
directions and the thermal, electrical and fluid diffusion
properties of the material are very different, by orders of
magnitude typically, for the "c" and "a" directions.
DISCLOSURE OF THE INVENTION
[0015] It is an object of the invention to provide a resin
impregnated graphite article suitable for use in electronic thermal
management (ETM), supercapacitors or secondary batteries.
[0016] It is a further object of this invention to provide graphite
structures having enhanced in-plane properties.
[0017] It is a further object of the invention to provide a
machinable graphite structure having relatively high thermal
conductivity in the "a" directions and relatively low conductivity
in the "c" direction.
[0018] These and other objects are accomplished by the present
invention, which provides structure comprising resin impregnated
graphite articles formed of compressed particles of exfoliated
graphite.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] This invention is based upon the finding that when articles
of epoxy impregnated graphite are compressed (such as by
calendering) and then cured at elevated temperatures and pressures,
the resultant material exhibits unexpectedly good mechanical and
thermal properties and also possesses good machinability.
[0020] Before describing the manner in which the invention improves
current materials, a brief description of graphite and its
formation into integrated articles, which will become the primary
substrate for forming the products of the invention, is in
order.
[0021] Preparation of Graphite Articles
[0022] Graphite is a crystalline form of carbon comprising atoms
covalently bonded in flat layered planes with weaker bonds between
the planes. By treating particles of graphite, such as natural
graphite flake, with an intercalant of, e.g. a solution of sulfuric
and nitric acid, the crystal structure of the graphite reacts to
form a compound of graphite and the intercalant. The treated
particles of graphite are hereafter referred to as "particles of
intercalated graphite." Upon exposure to high temperature, the
intercalant within the graphite decomposes and volatilizes, causing
the particles of intercalated graphite to expand in dimension as
much as about 80 or more times its original volume in an
accordion-like fashion in the "c" direction, i.e. in the direction
perpendicular to the crystalline planes of the graphite. The
exfoliated graphite particles are vermiform in appearance, and are
therefore commonly referred to as worms, and are sometimes referred
to herein as "particles of expanded graphite." The worms may be
compressed together into articles that, unlike the original
graphite flakes, can be formed and cut into various shapes and
provided with small transverse openings by deforming mechanical
impact.
[0023] Graphite starting materials for the inventive materials
include highly graphitic carbonaceous materials capable of
intercalating organic and inorganic acids as well as halogens and
then expanding when exposed to heat. These highly graphitic
carbonaceous materials most preferably have a degree of
graphitization of about 1.0. As used in this disclosure, the term
"degree of graphitization" refers to the value g according to the
formula: 1 g = 3.45 - d ( 002 ) 0.095
[0024] where d(002) is the spacing between the graphitic layers of
the carbons in the crystal structure measured in Angstrom units.
The spacing d between graphite layers is measured by standard X-ray
diffraction techniques. The positions of diffraction peaks
corresponding to the (002), (004) and (006) Miller Indices are
measured, and standard least-squares techniques are employed to
derive spacing which minimizes the total error for all of these
peaks. Examples of highly graphitic carbonaceous materials include
natural graphites from various sources, as well as other
carbonaceous materials such as carbons prepared by chemical vapor
deposition and the like. Natural graphite is most preferred.
[0025] The graphite starting materials for the materials used in
the present invention may contain non-carbon components so long as
the crystal structure of the starting materials maintains the
required degree of graphitization and they are capable of
exfoliation. Generally, any carbon-containing material, the crystal
structure of which possesses the required degree of graphitization
and which can be exfoliated, is suitable for use with the present
invention. Such graphite preferably has an ash content of less than
twenty weight percent. More preferably, the graphite employed for
the present invention will have a purity of at least about 94%. In
the most preferred embodiment, the graphite employed will have a
purity of at least about 98%.
[0026] A common method for manufacturing graphite sheets is
described by Shane et al. in U.S. Pat. No. 3,404,061, the
disclosure of which is incorporated herein by reference. In one
embodiment of the practice of the Shane et al. method, natural
graphite flakes are intercalated by dispersing the flakes in a
solution containing e.g., a mixture of nitric and sulfuric acid,
advantageously at a level of about 20 to about 300 parts by weight
of intercalant solution per 100 parts by weight of graphite flakes
(pph). The intercalation solution contains oxidizing and other
intercalating agents known in the art. Examples include those
containing oxidizing agents and oxidizing mixtures, such as
solutions containing nitric acid, potassium chlorate, chromic acid,
potassium permanganate, potassium chromate, potassium dichromate,
perchloric acid, and the like, or mixtures, such as for example,
concentrated nitric acid and chlorate, chromic acid and phosphoric
acid, sulfuric acid and nitric acid, or mixtures of a strong
organic acid, e.g. trifluoroacetic acid, and a strong oxidizing
agent soluble in the organic acid. Alternatively, an electric
potential can be used to bring about oxidation of the graphite.
Chemical species that can be introduced into the graphite crystal
using electrolytic oxidation include sulfuric acid as well as other
acids.
[0027] In a preferred embodiment, the intercalating agent is a
solution of a mixture of sulfuric acid, or sulfuric acid and
phosphoric acid, and an oxidizing agent, i.e. nitric acid,
perchloric acid, chromic acid, potassium permanganate, hydrogen
peroxide, iodic or periodic acids, or the like. Although less
preferred, the intercalation solution may contain metal halides
such as ferric chloride, and ferric chloride mixed with sulfuric
acid, or a halide, such as bromine as a solution of bromine and
sulfuric acid or bromine in an organic solvent.
[0028] The quantity of intercalation solution may range from about
20 to about 150 pph and more typically about 50 to about 120 pph.
After the flakes are intercalated, any excess solution is drained
from the flakes and the flakes are water-washed. Alternatively, the
quantity of the intercalation solution may be limited to between
about 10 and about 50 pph, which permits the washing step to be
eliminated as taught and described in U.S. Pat. No. 4,895,713, the
disclosure of which is also herein incorporated by reference.
[0029] The particles of graphite flake treated with intercalation
solution can optionally be contacted, e.g. by blending, with a
reducing organic agent selected from alcohols, sugars, aldehydes
and esters which are reactive with the surface film of oxidizing
intercalating solution at temperatures in the range of 25.degree.
C. and 125.degree. C. Suitable specific organic agents include
hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol, 1,10
decanediol, decylaldehyde, 1-propanol, 1,3 propanediol,
ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose,
sucrose, potato starch, ethylene glycol monostearate, diethylene
glycol dibenzoate, propylene glycol monostearate, glycerol
monostearate, dimethyl oxylate, diethyl oxylate, methyl formate,
ethyl formate, ascorbic acid and lignin-derived compounds, such as
sodium lignosulfate. The amount of organic reducing agent is
suitably from about 0.5 to 4% by weight of the particles of
graphite flake.
[0030] The use of an expansion aid applied prior to, during or
immediately after intercalation can also provide improvements.
Among these improvements can be reduced exfoliation temperature and
increased expanded volume (also referred to as "worm volume"). An
expansion aid in this context will advantageously be an organic
material sufficiently soluble in the intercalation solution to
achieve an improvement in expansion. More narrowly, organic
materials of this type that contain carbon, hydrogen and oxygen,
preferably exclusively, may be employed. Carboxylic acids have been
found especially effective. A suitable carboxylic acid useful as
the expansion aid can be selected from aromatic, aliphatic or
cycloaliphatic, straight chain or branched chain, saturated and
unsaturated monocarboxylic acids, dicarboxylic acids and
polycarboxylic acids which have at least 1 carbon atom, and
preferably up to about 15 carbon atoms, which is soluble in the
intercalation solution in amounts effective to provide a measurable
improvement of one or more aspects of exfoliation. Suitable organic
solvents can be employed to improve solubility of an organic
expansion aid in the intercalation solution.
[0031] Representative examples of saturated aliphatic carboxylic
acids are acids such as those of the formula H(CH.sub.2).sub.nCOOH
wherein n is a number of from 0 to about 5, including formic,
acetic, propionic, butyric, pentanoic, hexanoic, and the like. In
place of the carboxylic acids, the anhydrides or reactive
carboxylic acid derivatives such as alkyl esters can also be
employed. Representative of alkyl esters are methyl formate and
ethyl formate. Sulfuric acid, nitric acid and other known aqueous
intercalants have the ability to decompose formic acid, ultimately
to water and carbon dioxide. Because of this, formic acid and other
sensitive expansion aids are advantageously contacted with the
graphite flake prior to immersion of the flake in aqueous
intercalant. Representative of dicarboxylic acids are aliphatic
dicarboxylic acids having 2-12 carbon atoms, in particular oxalic
acid, fumaric acid, malonic acid, maleic acid, succinic acid,
glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid,
1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid,
cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids
such as phthalic acid or terephthalic acid. Representative of alkyl
esters are dimethyl oxylate and diethyl oxylate. Representative of
cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic
carboxylic acids are benzoic acid, naphthoic acid, anthranilic
acid, p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl
acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoic acids
and, acetamidobenzoic acids, phenylacetic acid and naphthoic acids.
Representative of hydroxy aromatic acids are hydroxybenzoic acid,
3-hydroxy- 1-naphthoic acid, 3-hydroxy-2-naphthoic acid,
4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,
5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and
7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic
acids is citric acid.
[0032] The intercalation solution will be aqueous and will
preferably contain an amount of expansion aid of from about 1 to
10%, the amount being effective to enhance exfoliation. In the
embodiment wherein the expansion aid is contacted with the graphite
flake prior to or after immersing in the aqueous intercalation
solution, the expansion aid can be admixed with the graphite by
suitable means, such as a V-blender, typically in an amount of from
about 0.2% to about 10% by weight of the graphite flake.
[0033] After intercalating the graphite flake, and following the
blending of the intercalant coated intercalated graphite flake with
the organic reducing agent, the blend is exposed to temperatures in
the range of 25.degree. to 125.degree. C. to promote reaction of
the reducing agent and intercalant coating. The heating period is
up to about 2 hours, with shorter heating periods, e.g., at least
about 10 minutes, for higher temperatures in the above-noted range.
Times of one-half hour or less, e.g., on the order of 10 to 25
minutes, can be employed at the higher temperatures.
[0034] The above described methods for intercalating and
exfoliating graphite flake may beneficially be augmented by a
pretreatment of the graphite flake at graphitization temperatures,
i.e. temperatures in the range of about 3000.degree. C. and above
and by the inclusion in the intercalant of a lubricious
additive.
[0035] The pretreatment, or annealing, of the graphite flake
results in significantly increased expansion (i.e., increase in
expansion volume of up to 300% or greater) when the flake is
subsequently subjected to intercalation and exfoliation. Indeed,
desirably, the increase in expansion is at least about 50%, as
compared to similar processing without the annealing step. The
temperatures employed for the annealing step should not be
significantly below 3000.degree. C., because temperatures even
100.degree. C. lower result in substantially reduced expansion.
[0036] The annealing of the present invention is performed for a
period of time sufficient to result in a flake having an enhanced
degree of expansion upon intercalation and subsequent exfoliation.
Typically the time required will be 1 hour or more, preferably 1 to
3 hours and will most advantageously proceed in an inert
environment. For maximum beneficial results, the annealed graphite
flake will also be subjected to other processes known in the art to
enhance the degree expansion--namely intercalation in the presence
of an organic reducing agent, an intercalation aid such as an
organic acid, and a surfactant wash following intercalation.
Moreover, for maximum beneficial results, the intercalation step
may be repeated.
[0037] The annealing step of the instant invention may be performed
in an induction furnace or other such apparatus as is known and
appreciated in the art of graphitization; for the temperatures here
employed, which are in the range of 3000.degree. C., are at the
high end of the range encountered in graphitization processes.
[0038] Because it has been observed that the worms produced using
graphite subjected to pre-intercalation annealing can sometimes
"clump" together, which can negatively impact area weight
uniformity, an additive that assists in the formation of "free
flowing" worms is highly desirable. The addition of a lubricious
additive to the intercalation solution facilitates the more uniform
distribution of the worms across the bed of a compression apparatus
(such as the bed of a calender station conventionally used for
compressing, or "calendering," graphite worms into an integrated
graphite article). The resulting article therefore has higher area
weight uniformity and greater tensile strength. The lubricious
additive is preferably a long chain hydrocarbon, more preferably a
hydrocarbon having at least about 10 carbons. Other organic
compounds having long chain hydrocarbon groups, even if other
functional groups are present, can also be employed.
[0039] More preferably, the lubricious additive is an oil, with a
mineral oil being most preferred, especially considering the fact
that mineral oils are less prone to rancidity and odors, which can
be an important consideration for long term storage. It will be
noted that certain of the expansion aids detailed above also meet
the definition of a lubricious additive. When these materials are
used as the expansion aid, it may not be necessary to include a
separate lubricious additive in the intercalant.
[0040] The lubricious additive is present in the intercalant in an
amount of at least about 1.4 pph, more preferably at least about
1.8 pph. Although the upper limit of the inclusion of lubricous
additive is not as critical as the lower limit, there does not
appear to be any significant additional advantage to including the
lubricious additive at a level of greater than about 4 pph.
[0041] The thus treated particles of graphite are sometimes
referred to as "particles of intercalated graphite." Upon exposure
to high temperature, e.g. temperatures of at least about
160.degree. C. and especially about 700.degree. C. to 1200.degree.
C. and higher, the particles of intercalated graphite expand as
much as about 80 to 1000 or more times their original volume in an
accordion-like fashion in the c-direction, i.e. in the direction
perpendicular to the crystalline planes of the constituent graphite
particles. The expanded, i.e. exfoliated, graphite particles are
vermiform in appearance, and are therefore commonly referred to as
worms. The worms may be compressed together into articles that,
unlike the original graphite flakes, can be formed and cut into
various shapes and provided with small transverse openings by
deforming mechanical impact as hereinafter described.
[0042] The graphite materials prepared as described are coherent,
with good handling strength, and are suitably compressed, e.g. by
molding or roll-pressing, to a thickness of about 0.075 mm to 30 mm
and a typical density of about 0.1 to 1.5 grams per cubic
centimeter (g/cc). From about 1.5-30% by weight of ceramic
additives can be blended with the intercalated graphite flakes as
described in U.S. Pat. No. 5,902,762 (which is incorporated herein
by reference) to provide enhanced resin impregnation in the final
graphite product. The additives include ceramic fiber particles
having a length of about 0.15 to 1.5 millimeters. The width of the
particles is suitably from about 0.04 to 0.004 mm. The ceramic
fiber particles are non-reactive and non-adhering to graphite and
are stable at temperatures up to about 1100.degree. C., preferably
about 1400.degree. C. or higher. Suitable ceramic fiber particles
are formed of macerated quartz glass fibers, carbon and graphite
fibers, zirconia, boron nitride, silicon carbide and magnesia
fibers, naturally occurring mineral fibers such as calcium
metasilicate fibers, calcium aluminum silicate fibers, aluminum
oxide fibers and the like.
[0043] As noted above, the graphite materials are also treated with
resin and the absorbed resin, after curing, enhances the moisture
resistance and handling strength, i.e. stiffness, of the material
as well as "fixing" the morphology of the sheet. The amount of
resin within the epoxy impregnated graphite articles should be an
amount sufficient to ensure that the final cured structure is dense
and cohesive, yet the anisotropic thermal conductivity associated
with a densified graphite structure is preserved or improved.
Suitable resin content is preferably at least about 3% by weight,
more preferably about 5 to 35% by weight, and suitably up to about
60% by weight. Resins found especially useful in the practice of
the present invention include acrylic-, epoxy- and phenolic-based
resin systems, fluoro-based polymers, or mixtures thereof. Suitable
epoxy resin systems include those based on diglycidyl ether of
bisphenol A (DGEBA) and other multifunctional resin systems;
phenolic resins that can be employed include resole and novolac
phenolics. Optionally, the flexible graphite may be impregnated
with fibers and/or salts in addition to the resin or in place of
the resin. Additionally, reactive or non-reactive additives may be
employed with the resin system to modify properties (such as tack,
material flow, hydrophobicity, etc.).
[0044] In a typical resin impregnation step, the flexible graphite
material is passed through a vessel and impregnated with the resin
system from, e.g. spray nozzles, the resin system advantageously
being "pulled through the mat" by means of a vacuum chamber.
Typically, but not necessarily, the resin system is solvated to
facilitate application into the flexible graphite. The resin is
thereafter preferably dried, reducing the tack of the resin and the
resin-impregnated article.
[0045] Alternatively, the flexible graphite of the present
invention may utilize particles of reground flexible graphite
materials rather than freshly expanded worms. The reground
materials may be newly formed material, recycled material, scrap
material, or any other suitable source.
[0046] Also the processes of the present invention may use a blend
of virgin materials and recycled materials.
[0047] The source material for recycled materials may be articles
or trimmed portions of articles that have been compression molded
as described above, or sheets that have been compressed with, for
example, pre-calendering rolls, but have not yet been impregnated
with resin. Furthermore, the source material may be impregnated
with resin, but not yet cured, or impregnated with resin and cured.
The source material may also be recycled flexible graphite fuel
cell components such as flow field plates or electrodes. Each of
the various sources of graphite may be used as is or blended with
natural graphite flakes.
[0048] Once the source material of flexible graphite is available,
it can then be comminuted by known processes or devices, such as a
jet mill, air mill, blender, etc. to produce particles. Preferably,
a majority of the particles have a diameter such that they will
pass through 20 U.S. mesh; more preferably a major portion (greater
than about 20%, most preferably greater than about 50%) will not
pass through 80 U.S. mesh. Most preferably the particles have a
particle size of no greater than about 20 mesh. It may be desirable
to cool the flexible graphite when it is resin-impregnated as it is
being comminuted to avoid heat damage to the resin system during
the comminution process.
[0049] The size of the comminuted particles may be chosen so as to
balance machinability and formability of the graphite article with
the thermal characteristics desired. Thus, smaller particles will
result in a graphite article which is easier to machine and/or
form, whereas larger particles will result in a graphite article
having higher anisotropy, and, therefore, greater in-plane
electrical and thermal conductivity.
[0050] Once the source material is comminuted (if the source
material has been resin impregnated, then preferably the resin is
removed from the particles), it is then re-expanded. The
re-expansion may occur by using the intercalation and exfoliation
process described above and those described in U.S. Pat. No.
3,404,061 to Shane et al. and U.S. Pat. No. 4,895,713 to Greinke et
al.
[0051] Typically, after intercalation the particles are exfoliated
by heating the intercalated particles in a furnace. During this
exfoliation step, intercalated natural graphite flakes may be added
to the recycled intercalated particles. Preferably, during the
re-expansion step the particles are expanded to have a specific
volume in the range of at least about 100 cc/g and up to about 350
cc/g or greater. Finally, after the re-expansion step, the
re-expanded particles may be compressed into coherent materials and
impregnated with resin, as described.
[0052] Graphite materials prepared according to the foregoing
description can also be generally referred to as compressed
particles of exfoliated graphite. Since the materials are
resin-impregnated, the resin in the sheets needs to be cured before
the sheets are used in their intended applications, such as for
electronic thermal management.
[0053] According to the invention, resin-impregnated graphite
materials prepared as described above are compressed to the desired
thickness and shape, commonly a thickness of about 0.35 mm to 0.5
mm, at which time the impregnated mats have a density of about 1.4
g/cm.sup.3 to about 1.9 g/cm.sup.3.
[0054] One type of apparatus for continuously forming
resin-impregnated and compressed flexible graphite materials is
shown in International Publication No. WO 00/64808, the disclosure
of which is incorporated herein by reference.
[0055] Following the compression step (such as by calendering), the
impregnated materials are cut to suitable-sized pieces and placed
in a press, where the resin is cured at an elevated temperature.
The temperature should be sufficient to ensure that the lamellar
structure is densified at the curing pressure, while the thermal
properties of the structure are not adversely impacted. Generally,
this will require a temperature of at least about 90.degree. C.,
and generally up to about 200.degree. C. Most preferably, cure is
at a temperature of from about 150.degree. C. to 200.degree. C. The
pressure employed for curing will be somewhat a function of the
temperature utilized, but will be sufficient to ensure that the
lamellar structure is densified without adversely impacting the
thermal properties of the structure. Generally, for convenience of
manufacture, the minimum required pressure to densify the structure
to the required degree will be utilized. Such a pressure will
generally be at least about 7 megapascals (Mpa, equivalent to about
1000 pounds per square inch), and need not be more than about 35
Mpa (equivalent to about 5000 psi), and more commonly from about 7
to about 21 Mpa (1000 to 3000 psi). The curing time may vary
depending on the resin system and the temperature and pressure
employed, but generally will range from about 0.5 hours to 2 hours.
After curing is complete, the composites are seen to have a density
of at least about 1.8 g/cm.sup.3 and commonly from about 1.8
g/cm.sup.3 to 2.0 g/cm.sup.3.
[0056] Although the formation of sheets through calendering or
molding is the most common method of formation of the graphite
materials useful in the practice of the present invention, other
forming methods can also be employed. For instance, the exfoliated
graphite particles can be compression molded into a net shape or
near net shape. Thus, if the end application requires an article,
such as a heat sink or heat spreader, assuming a certain shape or
profile, that shape or profile can be molded into the graphite
article, before or after resin impregnation. Cure would then take
place in a mold assuming the same shape; indeed, in the preferred
embodiment, compression and curing will take place in the same
mold. Machining to the final shape can then be effected.
[0057] The temperature- and pressure-cured graphite/resin
composites of the present invention provide for the first time a
graphite-based composite material having in-plane thermal
conductivity rivaling or exceeding that of copper, at a fraction of
the weight of copper. More specifically, the inventive composites
exhibit in-plane thermal conductivities of at least about 300 W/mK,
with through-plane thermal conductivities of less than about 15
W/mK, more preferably less than about 10 W/mK. Such materials will
be extremely useful in heat dissipation applications, such as in
heat sinks, heat spreaders, heat pipes and the like, especially
where the weight of copper would be disadvantageous.
[0058] The above description is intended to enable the person
skilled in the art to practice the invention. It is not intended to
detail all of the possible variations and modifications that will
become apparent to the skilled worker upon reading the description.
It is intended, however, that all such modifications and variations
be included within the scope of the invention that is defined by
the following claims. The claims are intended to cover the
indicated elements and steps in any arrangement or sequence that is
effective to meet the objectives intended for the invention, unless
the context specifically indicates the contrary.
* * * * *