U.S. patent application number 10/833928 was filed with the patent office on 2005-07-21 for composite heat sink with metal base and graphite fins.
Invention is credited to Frastaci, Michael, Getz, George JR..
Application Number | 20050155743 10/833928 |
Document ID | / |
Family ID | 36036670 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050155743 |
Kind Code |
A1 |
Getz, George JR. ; et
al. |
July 21, 2005 |
Composite heat sink with metal base and graphite fins
Abstract
A composite heat sink apparatus includes a metal base which has
a thermal conductivity of at least about 150 W/m.degree. K. The
metal base is preferably constructed either of copper of aluminum.
The heat sink apparatus further includes a plurality of fins
attached to the base, the fins being constructed of anisotropic
graphite material having a direction of relatively high thermal
conductivity perpendicular to the base.
Inventors: |
Getz, George JR.; (Parma
Heights, OH) ; Frastaci, Michael; (Parma,
OH) |
Correspondence
Address: |
Waddey & Patterson, P.C.
Bank of America Plaza
Suite 2020
414 Union Street
Nashville
TN
37219
US
|
Family ID: |
36036670 |
Appl. No.: |
10/833928 |
Filed: |
September 7, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10833928 |
Sep 7, 2004 |
|
|
|
10184841 |
Jun 28, 2002 |
|
|
|
6749010 |
|
|
|
|
Current U.S.
Class: |
165/80.3 ;
257/E23.103; 257/E23.11 |
Current CPC
Class: |
H01L 23/3672 20130101;
H01L 2924/00 20130101; F28F 21/02 20130101; F28F 3/02 20130101;
F28D 2021/0029 20130101; H01L 2924/0002 20130101; H01L 23/373
20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
165/080.3 |
International
Class: |
F28F 007/00 |
Claims
What is claimed is:
1. A heat sink apparatus, comprising: a metal base having a thermal
conductivity of at least about 150 W/m.degree. K; and a plurality
of fins attached to the base, the fins being constructed of a
resin-impregnated sheets of compressed particles of exfoliated
graphite pressure cured at an elevated temperature.
2. The apparatus of claim 1, wherein the fins are perpendicular to
the base.
3. The apparatus of claim 1, wherein the base is constructed of
copper.
4. The apparatus of claim 1, wherein the base is constructed of
aluminum.
5. The apparatus of claim 1, wherein: the base has a plurality of
parallel grooves formed therein; and the fins are planar fins, each
of the fine being closely received in one of the grooves.
6. The apparatus of claim 1, wherein the fins are constructed of
resin impregnated flexible graphite sheets pressure cured at a
temperature of at least about 90.degree. C. and at a pressure of at
least about 7 Mpa.
7. A heat sink apparatus, comprising: a copper base; and a
plurality of planar graphite fins attached to the base, the
graphite fins the graphite fins being formed of resin impregnated
sheets of compressed particles of exfoliated graphite material
pressure cured at an elevated temperature and having a relatively
high thermal conductivity within the plane of the fin and
relatively low thermal conductivity across a thickness of each fin,
so that the heat sink apparatus has a thermal performance
approximately equal to that of an all copper heat sink while having
a weight less than that of the all copper heat sink.
8. The apparatus of claim 7, wherein the graphite fins are
constructed of resin impregnated flexible graphite sheets pressure
cured at temperature of at least about 90.degree. C. and at a
pressure of at least about 7 Mpa.
9. The apparatus of claim 7, wherein: the base has a plurality of
parallel grooves formed therein; and the fins are planar fins, each
of the fins being closely received in one of the grooves.
10. A heat sink apparatus, comprising: an aluminum base; and a
plurality of graphite fins attached to the base, the graphite fins
being formed of resin impregnated sheets of compressed particles of
exfoliated graphite pressure cured at an elevated temperature and
extending from the base, the sheets having axes of relatively high
thermal conductivity greater than that of aluminum in the plane of
the sheet and having a relatively low thermal conductivity across a
thickness of the sheet material, the graphite sheet material having
a specific gravity no greater than that of aluminum, so that the
heat sink apparatus has a thermal performance greater than that of
a similar sized all aluminum heat sink while having a weight no
greater than that of the all aluminum heat sink.
11. The apparatus of claim 10, wherein the graphite fins are
constructed of resin impregnated flexible graphite sheets pressure
cured at temperature of at least about 90.degree. C. and at a
pressure of at least about 7 Mpa.
12. The apparatus of claim 10, wherein: the base has a plurality of
parallel grooves formed therein; and the fins are planar fins, each
of the fins being closely received in one of the grooves.
Description
RELATED APPLICATION
[0001] This application is a continuation-in-part of co-pending
application Ser. No. 10/184,841, filed Jun. 28, 2002 and entitled
"Composite Heat Sink With Metal Base And Graphite Fins," the
disclosure of which is incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a heat sink capable of
managing the heat from a heat source such as an electronic
device.
BACKGROUND OF THE INVENTION
[0003] With the development of more and more sophisticated
electronic devices, including those capable of increasing
processing speeds and higher frequencies, having smaller size and
more complicated power requirements, and exhibiting other
technological advances, such as microprocessors and integrated
circuits in electronic and electrical components and systems as
well as in other devices such as high power optical devices,
relatively extreme temperatures can be generated. However,
microprocessors, integrated circuits and other sophisticated
electronic components typically operate efficiently only under a
certain range of threshold temperatures. The excessive heat
generated during operation of these components can not only harm
their own performance, but can also degrade the performance and
reliability of the overall system and can even cause system
failure. The increasingly wide range of environmental conditions,
including temperature extremes, in which electronic systems are
expected to operate, exacerbates the negative effects of excessive
heat.
[0004] With the increased need for heat dissipation from
microelectronic devices, thermal management becomes an increasingly
important element of the design of electronic products. Both
performance reliability and life expectancy of electronic equipment
are inversely related to the component temperature of the
equipment. For instance, a reduction in the operating temperature
of a device such as a typical silicon semiconductor can correspond
to an increase in the processing speed, reliability and life
expectancy of the device. Therefore, to maximize the life-span and
reliability of a component, controlling the device operating
temperature within the limits set by the designers is of paramount
importance.
[0005] Several types of heat dissipating components are utilized to
facilitate heat dissipation from electronic devices. The present
invention is directly applicable to finned heat sinks.
[0006] These heat sinks facilitate heat dissipation from the
surface of a heat source, such as a heat-generating electronic
device, to a cooler environment, usually air. The heat sink seeks
to increase the heat transfer efficiency between the electronic
device and the ambient air primarily by increasing the surface area
that is in direct contact with the air or other heat transfer
media. This allows more heat to be dissipated and thus lowers the
electronic device operating temperature. The primary purpose of a
heat dissipating component is to help maintain the device
temperature below the maximum allowable temperature specified by
its designer/manufacturer.
[0007] Typically, the heat sinks are formed of a metal, especially
copper or aluminum, due to the ability of metals like copper to
readily absorb heat and transfer it about its entire structure.
Copper heat sinks are often formed with fins or other structures to
increase the surface area of the heat sink, with air being forced
across or through the fins (such as by a fan) to effect heat
dissipation from the electronic component, through the copper heat
sink and then to the air.
[0008] The use of copper or aluminum heat dissipating elements can
present a problem because of the weight of the metal, particularly
when the heat transmitting area of the heat dissipating component
is significantly larger than that of the electronic device. For
instance, pure copper weighs 8.96 grams per cubic centimeter
(g/cm.sup.3) and pure aluminum weighs 2.70 g/cm.sup.3.
[0009] For example, in many applications, several heat sinks need
to be arrayed on, e.g., a circuit board to dissipate heat from a
variety of components on the board. If metallic heat sinks are
employed, the sheer weight of the metal on the board can increase
the chances of the board cracking or of other equally undesirable
effects, and increases the weight of the component itself. For
portable electronic devices, any method to reduce weight while
maintaining heat dissipation characteristics is especially
desirable.
[0010] Another group of materials suitable for use in heat sinks
are those materials generally known as graphites, but in particular
graphites such as those based on natural graphites and flexible
graphite as described below. These materials are anisotropic and
allow the heat sink to be designed to preferentially transfer heat
in selected directions. Also, the graphite materials are much
lighter in weight and thus provide many advantages over copper or
aluminum.
[0011] Graphites are 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 graphites 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 possess anisotropic structures and thus
exhibit or possess many properties that are highly directional e.g.
thermal and electrical conductivity and fluid diffusion.
[0012] 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 sheets possess a very high degree
of orientation.
[0013] 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 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.
[0014] Graphite flake which has been greatly 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, foils, mats 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
sheets 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.
[0015] In addition to flexibility, the sheet material, as noted
above, has also been found to possess a high degree of anisotropy
with respect to thermal and electrical conductivity and fluid
diffusion, comparable to the natural graphite starting material due
to orientation of the expanded graphite particles and graphite
layers substantially parallel to the opposed faces of the sheet
resulting from very high compression, e.g. roll pressing. Sheet
material thus produced has excellent flexibility, good strength and
a very high degree of orientation.
[0016] Briefly, the process of producing flexible, binderless
anisotropic graphite sheet material, e.g. 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, flexible, integrated graphite sheet.
The expanded graphite particles that generally are worm-like or
vermiform in appearance, once compressed, will maintain the
compression set and alignment with the opposed major surfaces of
the sheet. The density and thickness of the sheet material can be
varied by controlling the degree of compression. The density of the
sheet material can be within the range of from about 0.04
g/cm.sup.3 to about 2.0 g/cm.sup.3. The flexible graphite sheet
material exhibits an appreciable degree of anisotropy due to the
alignment of graphite particles parallel to the major opposed,
parallel surfaces of the sheet, with the degree of anisotropy
increasing upon roll pressing of the sheet material to increase
orientation. In roll pressed anisotropic sheet material, the
thickness, i.e. the direction perpendicular to the opposed,
parallel sheet 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 sheet are very different, by orders of magnitude,
for the "c" and "a" directions.
[0017] There is a continuing need for improved heat sink designs
which provide relatively high thermal conductivity and relatively
low weight as compared to prior designs.
SUMMARY OF THE INVENTION
[0018] The present invention provides a heat sink apparatus which
comprises a metallic base having a thermal conductivity of at least
about 150 W/m.degree. K, and a plurality of fins attached to the
base, the fins being constructed of anisotropic graphite material
having a direction of relatively high thermal conductivity
perpendicular to the base.
[0019] In specific embodiments of the invention the base may be
constructed either of copper or aluminum.
[0020] Accordingly, it is an object of the present invention to
provide an improved heat sink design for thermal management of
electronic devices.
[0021] Still another object of the present invention is the
provision of a composite heat sink design having a metal base and
having fins constructed of anisotropic graphite material.
[0022] And another object of the present invention is the provision
of a composite heat sink having a copper base with graphite fins,
which provides a thermal performance approximately equal to that of
an all copper heat sink while having a weight less than that of the
all copper heat sink.
[0023] And another object of the present invention is the provision
of a heat sink apparatus having an aluminum base and a plurality of
graphite fins, so that the heat sink apparatus has a thermal
performance greater than that of a similar sized all aluminum heat
sink while having a weight no greater than that of the all aluminum
heat sink.
[0024] Other and further objects, features, and advantages of the
present invention will be readily apparent to those skilled in the
art, upon a reading of the following disclosure when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic plan view of a heat sink constructed
in accordance with the present invention.
[0026] FIG. 2 is an elevation section view taken along line 2-2 of
FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] As noted, one material from which the heat sinks of the
present invention may be constructed is graphite sheet material.
Before describing the construction of the heat sinks, a brief
description of graphite and its formation into flexible sheets is
in order.
[0028] Preparation of Flexible Graphite Sheet
[0029] 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. The worms may be
compressed together into flexible sheets that, unlike the original
graphite flakes, can be formed and cut into various shapes.
[0030] Graphite starting materials suitable for use in the present
invention 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
[0031] 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 graphite prepared by chemical vapor
deposition, high temperature pyrolysis of polymers, or
crystallization from molten metal solutions and the like. Natural
graphite is most preferred.
[0032] The graphite starting materials used in the present
invention may contain non-graphite 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 a purity of at least about eighty 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%.
[0033] A common method for manufacturing graphite sheet is
described by Shane et al. in U.S. Pat. No. 3,404,061, the
disclosure of which is incorporated herein by reference. In the
typical 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.
[0034] 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.
[0035] The quantity of intercalation solution may range from about
20 to about 350 pph and more typically about 40 to about 160 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 40 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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 20 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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 1000.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 flexible sheets
that, unlike the original graphite flakes, can be formed and cut
into various shapes.
[0049] Flexible graphite sheet and foil are coherent, with good
handling strength, and are suitably compressed, e.g. by roll
pressing, to a thickness of about 0.075 mm to 3.75 mm and a typical
density of about 0.1 to 1.5 grams per cubic centimeter
(g/cm.sup.3). 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 flexible
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.
[0050] The flexible graphite sheet can also, at times, be
advantageously treated with resin and the absorbed resin, after
curing, enhances the moisture resistance and handling strength,
i.e. stiffness, of the flexible graphite sheet as well as "fixing"
the morphology of the sheet. Suitable resin content is preferably
less than about 60% by weight, more preferably less than about 35%
by weight, and most preferably from about 4% to about 15% by
weight. Resins found especially useful in the practice of the
present invention include acrylic-, epoxy- and phenolic-based resin
systems, or mixtures thereof. Suitable epoxy resin systems include
those based on diglycidyl ether or bisphenol A (DGEBA) and other
multifunctional resin systems; phenolic resins that can be employed
include resole and novolac phenolics.
[0051] 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.
[0052] Also the processes of the present invention may use a blend
of virgin materials and recycled materials.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] 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.
Preparation of Preferred Graphite Materials
[0059] The graphite fins of the heat sinks described below are
preferably constructed from a resin impregnated graphite material
in the manner set forth in the U.S. patent application filed Apr.
23, 2004 of Norley et al. entitled "RESIN-IMPREGNATED FLEXIBLE
GRAPHITE SHEETS", assigned to the assignee of the present
invention, having docket number P1048-1/N1169 the details of which
are incorporated herein by reference.
[0060] According to the Norley et al. process, flexible graphite
sheets prepared as described above and having a thickness of about
4 mm to 7 mm, or higher, are impregnated with a thermosetting resin
such as an epoxy, acrylic or phenolic resin system. Suitable epoxy
resins include diglycidyl ether of bisphenol A (DGEBA) resin
systems; other multifunctional epoxy resins systems are also
suitable for use in the present invention. Suitable phenolic resin
systems include those containing resole and novolac resins. The
sheets are then calendered to a thickness of up to about 3 mm, more
preferably about 0.35 mm to 0.5 mm, at which time the calendered,
epoxy impregnated flexible sheets have a density of about 1.4
g/cm.sup.3 to about 1.9 g/cm.sup.3.
[0061] The amount of resin within the epoxy impregnated graphite
sheets should be an amount sufficient to ensure that the final
assembled and cured layered structure is dense and cohesive, yet
the anisotropic thermal conductivity associated with a densified
graphite structure has not been adversely impacted. Suitable resin
content is preferably at least about 3% by weight, more preferably
from about 5% to about 45% by weight depending on the
characteristics desired in the final product.
[0062] In a typical resin impregnation step, the flexible graphite
sheet 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 sheet. The resin
is thereafter preferably dried, reducing the tack of the resin and
the resin-impregnated sheet.
[0063] One type of apparatus for continuously forming
resin-impregnated and calendered flexible graphite sheet is shown
in U.S. Pat. No. 6,432,336, the disclosure of which is incorporated
herein by reference.
[0064] 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 density 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.
[0065] 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.
The Detailed Embodiment of FIGS. 1-2
[0066] Referring now to the drawings, and particularly to FIGS. 1
and 2, a heat sink apparatus is shown and generally designated by
the numeral 10. The heat sink apparatus 10 includes a metal base 12
having a thermal conductivity of at least 150 W/m.RTM. K.
Preferably the metal base 12 is constructed of either copper or
aluminum. A copper base 12 will have a thermal conductivity of
approximately 350 W/m.RTM. K or higher. An aluminum metal base 12
will have a thermal conductivity of approximately 150 W/m.RTM. K or
higher.
[0067] The heat sink apparatus 10 further includes a plurality of
fins such as 14A-H.
[0068] The fins 14 are constructed of flexible graphite sheet
material, and preferably are constructed from a resin-impregnated
flexible graphite sheets.
[0069] As previously noted, the graphite sheet material is
anisotropic and has a relatively high thermal conductivity of
approximately 400 W/m.RTM. K. in the plane of the sheet, and has a
very much lower thermal conductivity across the thickness of the
sheet. Thus, the fins when constructed of the sheet material have a
relatively high thermal conductivity within the plane of the fin
which is generally perpendicular to the orientation of the base
12.
[0070] The graphite material from which the fins are constructed is
considerably lighter than a comparable size copper fin, and is also
lighter than a comparable size aluminum fin. Pure copper weighs
8.96 gm/cm.sup.3 and pure aluminum weighs 2.70 gm/cm.sup.3. The
density of the graphite sheet material, on the other hand, can be
within the range of from about 0.04 gm/cm.sup.3 to about 2.0
gm/cm.sup.3. The preferred resin-impregnated graphite material
described above has a density of approximately 1.94
gm/cm.sup.3.
[0071] Thus when using a copper base 12, with the graphite fins 14,
the heat sink apparatus 10 will have a thermal performance
approximately equal to that of an all copper heat sink while having
a weight less than that of the all copper heat sink.
[0072] Similarly, when utilizing an aluminum base 12 with the
graphite fins 14, the heat sink apparatus 10 will have a thermal
performance greater than that of a similar size all aluminum heat
sink while having a weight of less than and certainly no greater
than that of an all aluminum heat sink.
[0073] Preferably, the fins 14 are attached to the base 12 by
machining a plurality of grooves such as 16A-H in the base 12, with
the fins 14 each having their lower edges closely received within
the respective groove 16.
[0074] The fins 14 may be held in place within the groove 16 by a
friction fit, a thermal shrink fit, or by the use of adhesive.
[0075] An electronic device 18 which is to be cooled by the heat
sink apparatus 10 is schematically illustrated in FIG. 2 and
engages the lower surface of the base 12. The electronic device 18
may be thermally connected to the base 12 by a layer of thermal
grease or adhesive or by a thermal interface layer constructed of a
thin sheet of graphite material.
[0076] Thus it is seen that the apparatus of the present invention
readily achieves the ends and advantages mentioned as well as those
inherent therein. While certain preferred embodiments of the
invention have been illustrated and described for purposes of the
present disclosure, numerous changes in the arrangement and
construction may be made by those skilled in the art, which changes
are encompassed within the scope and spirit of the present
invention as defined by the appended claims.
* * * * *