U.S. patent application number 11/103348 was filed with the patent office on 2006-10-12 for sandwiched thermal article.
Invention is credited to David S. Flaherty, Gary D. Shives.
Application Number | 20060225874 11/103348 |
Document ID | / |
Family ID | 37082062 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060225874 |
Kind Code |
A1 |
Shives; Gary D. ; et
al. |
October 12, 2006 |
Sandwiched thermal article
Abstract
A thermal material for heat dissipation, which includes at least
one sheet of flexible graphite sandwiched about a non-graphite core
layer.
Inventors: |
Shives; Gary D.; (Brunswick,
OH) ; Flaherty; David S.; (Cleveland, OH) |
Correspondence
Address: |
WADDEY & PATTERSON
1600 DIVISION STREET, SUITE 500
NASHVILLE
TN
37203
US
|
Family ID: |
37082062 |
Appl. No.: |
11/103348 |
Filed: |
April 11, 2005 |
Current U.S.
Class: |
165/185 ;
257/E23.106; 257/E23.11 |
Current CPC
Class: |
H01L 2924/00 20130101;
H01L 23/373 20130101; H01L 2924/0002 20130101; F28D 2021/0029
20130101; F28F 21/02 20130101; H01L 2924/0002 20130101; H01L
23/3735 20130101 |
Class at
Publication: |
165/185 |
International
Class: |
F28F 7/00 20060101
F28F007/00 |
Claims
1. A thermal dissipation article, comprising at least two sheets of
graphite sandwiched about a core layer, capable of use for thermal
dissipation.
2. The article of claim 1, wherein the core layer comprises
materials selected from the group consisting of plastics, metals,
and composites or combinations thereof.
3. The article of claim 2, wherein the core layers comprises a
metallic material.
4. The article of claim 3, wherein the metallic material is tanged
or a mesh.
5. The article of claim 3, wherein the core layer comprises
aluminum.
6. The article of claim 1, wherein a portion of the core layer
extends beyond an edge of at least one of the sheets of
graphite.
7. The article of claim 1, further comprising a heat collection
article with which the article is in operative contact.
8. The article of claim 7, wherein the heat dissipation device
comprises a heat sink, a heat pipe, a heat plate or any combination
thereof.
9. The article of claim 1, which is in operative contact with a
heat source.
10. The article of claim 9, wherein the core layer is in direct
operative contact with the heat source.
11. The article of claim 1, which has an in-plane thermal
conductivity of at least about 140 W/m.degree. K.
12. The article of claim 11, which has a through-plane thermal
conductivity of no greater than about 12 W/m.degree. K.
13. A finstock for an electronic device thermal dissipation system,
comprising a core layer sandwiched between at least two sheets of
graphite.
14. The finstock of claim 13, wherein the core layer comprises
materials selected from the group consisting of plastics, metals,
and composites or combinations thereof.
15. The finstock of claim 14, wherein the core layer comprises a
metallic material.
16. The finstock of claim 15, wherein the metallic material is
tanged or a mesh.
17. The finstock of claim 13, wherein a portion of the core layer
extends beyond an edge of at least one of the sheets of
graphite.
18. The finstock of claim 15, wherein the core layer comprises
aluminum.
19. The finstock of claim 13, which has an in-plane thermal
conductivity of at least about 140 W/m.degree. K.
20. The finstock of claim 19, which has a through-plane thermal
conductivity of no greater than about 12 W/m.degree. K.
Description
TECHNICAL FIELD
[0001] The present invention relates to a sandwiched structure
having an isotropic core and being capable of use as a thermal
spreader or a finstock in the manufacture of heat sinks and other
thermal dissipation devices. By thermal spreader is meant a
material or article that functions to spread heat from a heat
source over an area greater than one or more of the surfaces of the
heat source; by finstock is meant a material or article that can be
utilized as, or to form, fins used to dissipate heat.
BACKGROUND OF THE INVENTION
[0002] 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, high capacity and
response memory components such as hard drives, electromagnetic
sources such as light bulbs in digital projectors, 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.
[0003] 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.
[0004] One group of relatively light weight materials suitable for
use in the dissipation of heat from heat sources such as electronic
components 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 thermal dissipation devices to be designed to
preferentially transfer heat in selected directions. Graphite
materials are much lighter in weight than metals like copper and
aluminum and graphite materials, even when used in combination with
metallic components, provide many advantages over copper or
aluminum when used to dissipate heat by themselves.
[0005] For instance, Tzeng, in U.S. Pat. No. 6,482,520 teaches a
graphite based thermal management system which includes a heat sink
formed of a graphite article formed so as to have a heat collection
surface and at least one heat dissipation surface. Krassowski and
Chen take the Tzeng concept a step further in International Patent
Application No. PCT/US02/38061, where they teach the use of high
conducting inserts, formed, for instance, of a metal like copper or
aluminum, in a graphite base. Indeed, the use of sheets of
compressed particles of exfoliated graphite (i.e., flexible
graphite) has been suggested as thermal spreaders, thermal
interfaces and as component parts of heat sinks for dissipating the
heat generated by a heat source (for example, see, U.S. Pat. Nos.
6,245,400; 6,503,626; and 6,538,892).
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] However, the flexible nature of graphite materials makes it
difficult to form complex structures or shapes unless the materials
are first impregnated with a resin or the like. Such complex shapes
are desirable when the materials are to be used, for example, as
complex fin shapes or configurations to maximize heat transfer and
dissipation. In addition, the attachment of graphite fins to
metallic bases is also problematic, since graphite cannot be
soldered into place in the same way metallic fins can. Moreover,
while the anisotropic nature of graphite articles can be employed
advantageously to move or spread heat, their relatively low
through-plane conductivity (as compared to in-plane conductivity)
can retard heat spreading through the graphite structure.
[0013] Accordingly, there is a continuing need for improved designs
for graphitic materials for heat dissipation solutions for
electronic devices which provide the weight and thermal advantages
of graphite elements, with the formability, isotropy and other
advantages of metallic elements.
SUMMARY OF THE INVENTION
[0014] The present invention provides a thermal spreader material
and finstock for thermal solutions for dissipating the heat from an
electronic component. The inventive article comprises anisotropic
sheets of compressed particle of exfoliated graphite (sometimes
referred to with the term of art "flexible graphite") sandwiched
around non-graphitic materials, especially metallic materials like
aluminum or copper, advantageously in the form of a mesh. As used
herein, the term "flexible graphite" also refers to sheets of
pyrolytic graphite, either singly or as a laminate. The flexible
graphite sheets employed in the inventive article have an in-plane
thermal conductivity substantially higher than its through-plane
thermal conductivity. In other words, the article of the present
invention has a relatively high (on the order of 10 or greater)
thermal anisotropic ratio. The thermal anisotropic ratio is the
ratio of in-plane thermal conductivity to through-plane thermal
conductivity.
[0015] By sandwiching the non-graphite material between layers of
graphite sheets, the thermal properties of graphite are maintained,
while providing additional benefits, such as isotropic thermal
spreading and moldability or formability, as well as improved
attachment to a heat sink base. Most preferably, the non-graphite
core layers comprise a metallic material, especially aluminum.
Although aluminum is not as thermally conductive as copper,
aluminum is preferred due to its lighter weight as compared to
copper. Advantageously, when the non-graphite layer comprises a
metal material, and the inventive article is employed as finstock,
the metal core material can extend beyond the graphite layers and
provide a substrate for soldering of the finstock to a metallic
base or the like. In addition, the use of a metallic core layer
permits the resulting structure to be molded and/or formed into
complex shapes that meet specific space demands. The use of a
metallic core also makes use of the isotropic nature of the metal
to more efficiently spread heat along the graphite outer layers;
indeed, it may be advantageous to expose a portion of the metallic
core layer through one or both of the graphite outer layers to
permit direct operative contact between the core layer and the heat
source, in order to facilitate thermal spreading thorugh the core
and the graphite layers, when the inventive article is employed as
a thermal spreader.
[0016] The use of a metal mesh or tanged metal core material is
considered most advantageous, especially when the inventive article
is employed as finstock. The spaces in the mesh or tanged metal
provide a passageway for resin to flow between the graphite outer
layers to more securely adhere the finstock sandwich together.
Moreover, there can also be some graphite "flow" through the metal
core passageways, again to provide more secure sandwich formation
and adherence. Additionally, the tangs in a tanged metal core can
also assist in securely maintaining the structure of the finstock
sandwich of the present invention. When employed as a thermal
spreader, a metal mesh or tanged metal core can be employed for the
above-noted reasons; the use of a solid metal sheet can also be
advantageously used.
[0017] The inventive sandwich can be formed by a variety of
methods. For instance, the graphite sheet or laminate of sheets can
be disposed about the core material layers and the edges of the
graphite layers adhered together using an adhesive or impregnated
resin. In the alternative, the edges of the outer layers can be
folded together to form the sandwich, or, an adhesive material can
be applied to the surfaces of the graphite layers and/or the core
layer, to adhere the graphite to the core material. In addition,
where the core layer is formed of a mesh or tanged material, the
passageways can be advantageously employed for adhesion of the
sandwich layers, as discussed hereinabove.
[0018] The inventive sandwich thermal solution comprises two major
surfaces and four edge surfaces between the major surfaces, at
least one of the major surfaces of which can be arrayed in
operative contact with a heat source (in the case of a thermal
spreader); or at least one of the edge surfaces of which can be
arrayed in operative contact with a heat collection article or
material, such as the base of a heat sink. For example, an edge of
finstock can be fit into a slot or the like formed in the heat
collection article or material as described hereinbelow. If the
core of the sandwich is a metal, the inventive sandwich can be
formed such that a portion of the metal extends from the edge fit
into the heat collection article or material, allowing it to be
soldered into the heat collection article or material (if formed of
the appropriate material), for use as a finstock for optimum heat
transfer and reduced contact resistance between the finstock and
the heat collection article or material. If soldering is not
appropriate or desired, the finstock can be pressure fit or
attached to the heat collection article or material using a
suitable adhesive or resin.
[0019] Once fit into the heat collection article or material, the
remaining surface area of the finstock functions to dissipate heat
transferred to the finstock from the heat collection article or
material. For instance, heat is transferred to the inventive
finstock article from the heat collection article or material, and
the heat is then conducted along the finstock due to the in-plane
thermal conductivity of the inventive finstock. Air or another
fluid can be passed along or across the surface area of the
inventive finstock material to carry heat away from the heat
source.
[0020] As noted, the inventive finstock can be attached to a heat
collection article or material, such as a heat sink base, via
welding or soldering (in the case of a metallic core layer that
extends beyond an edge of the finstock) or melting thereto (in the
case of a plastic core layer that extends beyond an edge of the
finstock). In the alternative, the inventive material can be formed
into a series of discrete fins, which can be mounted to a heat sink
base individual by, for instance, forming channels in the heat sink
base and pressure fitting or soldering the individual fins into the
channels to maximize thermal contact between the base and the
fins.
[0021] The formable nature of the inventive sandwich permits the
formation of complex fin shapes and structures. For instance,
folded or loop structures which optimize contact with the heat sink
base while still providing substantial heat dissipation surface
area are possible using the sandwich structure of the present
invention.
[0022] When used as a thermal spreader, one of the major surfaces
is fit against the heat source, such as a chipset, hard drive,
etc., by adhesives or the like. As noted, direct operative contact
between the core layer and the heat source, such as by removing a
portion of one or both of the graphite outer layers or forming the
graphite outer layers with the requisite "void", can be
advantageous, especially if the core layer is isotropic, such as a
metal.
[0023] The combination anisotropic graphite layers with the
isotropic core layer can be used to effectively spread the heat
form a heat source over a wider area to reduce hot spots and other
undesirable effects. The inventive thermal spreader can also
effectively dissipate heat from a heat source by spreading the heat
from one location, effectively moving it to another location. In
addition, the thermal spreader of the present invention can also
function to shield one component in an electronic device from the
heat generated by a heat source in an electronic device, because of
the relatively low through-plane thermal conductivity of the
graphite outer layers.
[0024] Another benefit of the use of flexible graphite/metal
sandwich in the inventive thermal solution lies in the potential of
the inventive article to block electromagnetic and radio frequency
(EMI/RF) interference. It is believed that the thermal solutions of
this invention will function to shield components of the device in
which it is positioned from EMI/RF interference, in addition to
performing the thermal dissipation or spreading function that is
its primary purpose.
[0025] Accordingly, it is an object of the present invention to
provide an improved thermal solution for facilitating the
dissipation or spreading of heat from a component of an electronic
device.
[0026] Still another object of the present invention is the
provision of a thermal solution having a sufficiently high thermal
anisotropic ratio to function effectively for heat dissipation from
a heat collection article or material.
[0027] Yet another object of the present invention is the provision
of a formable thermal solution which provides heat dissipation in
an environment where available space is otherwise impractical.
[0028] These objects and others which will be apparent to the
skilled artisan upon reading the following description, can be
achieved by providing an article for use in a thermal dissipation
system for, e.g., an electronic device (like a laptop computer),
where the inventive article comprises sheets of flexible graphite
sandwiched about a non-graphite core layer, preferably a metal such
as copper or aluminum, and most preferably a mesh or tanged metal
core layer. The inventive article preferably has an in-plane
thermal conductivity of at least about 140 W/m.degree. K, more
preferably at least about 200 W/m.degree. K and a through-plane
thermal conductivity of no greater than about 12 W/m.degree. K,
more preferably no greater than about 10 W/m.degree. K.
[0029] Advantageously, the inventive system further includes a heat
collection device, such as a heat sink, heat pipe, heat plate or
combinations thereof, positioned in a location not directly
adjacent to the first component and further wherein the present
invention (when used as finstock) is in operative contact with the
heat collection device to dissipate heat conducted to the finstock
from the heat collection device.
[0030] Alternatively, the inventive sandwich can provide for direct
operative contact between the core layer and a heat source to
facilitate its thermal spreading capabilities, when employed as a
thermal spreader.
[0031] It is to be understood that both the foregoing general
description and the following detailed description present
embodiments of the invention, and are intended to provide an
overview or framework for understanding the nature and character of
the invention as it is claimed. The accompanying drawings are
included to provide a further understanding of the invention, and
are incorporated in and constitute a part of this specification.
The drawings illustrate various embodiments of the invention, and
together with the description serve to explain the principles and
operations of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is a partially broken away plan view of a first
embodiment of the article of the present invention.
[0033] FIG. 2 is a cross-sectional view of the article of FIG. 1,
taken along lines 2-2.
[0034] FIG. 3 is a plan view of another embodiment of the article
of the present invention, having the core material extending form
one of the edges thereof.
[0035] FIG. 4 is a cross-sectional view of the article of FIG. 3,
taken along lines 4-4.
[0036] FIG. 5 is a plan view of another embodiment of the article
of the present invention, having the core material partially
exposed through one of the major surfaces thereof.
[0037] FIG. 6 is a cross-sectional view of the article of FIG. 5,
taken along lines 5-5.
[0038] FIG. 7 is a plan view of another embodiment of the article
of the present invention, wherein the core material is a mesh.
[0039] FIG. 8 is a cross-sectional view of the article of FIG. 7,
taken along lines 8-8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] As noted, the inventive article is a sandwich whose outer
layers are formed from sheets of compressed particles of exfoliated
graphite, commonly known as flexible graphite. 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.
[0041] 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: g = 3.45 - d .function. ( 002 ) 0.095 ##EQU1## 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.
[0042] 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%.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] The thusly 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.
[0052] 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.
[0053] 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,
as described in International Patent Application No.
PCT/US02/39749.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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 flexible
graphite sheet. The resulting sheet 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.
[0058] 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.
[0059] 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.
[0060] 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 flexible sheets
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.
[0061] 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/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 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.
[0062] 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
at least about 5% by weight, more preferably about 10 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.). In order to maximize the thermal conductivity of the
resin-impregnated materials, the resin can be cured at elevated
temperatures and pressure. More particularly, cure at temperatures
of at least about 90.degree. C. and pressures of at least about 7
megapascals (MPa) will produce graphite materials having superior
thermal conductivities (indeed, in-plane thermal conductivities in
excess of those observed with copper can be achieved).
[0063] Alternatively, the flexible graphite sheets of the present
invention may utilize particles of reground flexible graphite
sheets rather than freshly expanded worms, as discussed in
International Patent Application No. PCT/US02/16730. The sheets may
be newly formed sheet material, recycled sheet material, scrap
sheet material, or any other suitable source.
[0064] Also the processes of the present invention may use a blend
of virgin materials and recycled materials.
[0065] The source material for recycled materials may be sheets or
trimmed portions of sheets 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 sheets or
trimmed portions of sheets that have been impregnated with resin,
but not yet cured, or sheets or trimmed portions of sheets that
have been impregnated with resin and cured. The source material may
also be recycled flexible graphite proton exchange membrane (PEM)
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.
[0066] Once the source material of flexible graphite sheets 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 sheet when it is
resin-impregnated as it is being comminuted to avoid heat damage to
the resin system during the comminution process.
[0067] 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.
[0068] Once the source material is comminuted, 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.
[0069] 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 flexible sheets, as
hereinafter described.
[0070] If the starting material has been impregnated with a resin,
the resin should preferably be at least partially removed from the
particles. This removal step should occur between the comminuting
step and the re-expanding step.
[0071] In one embodiment, the removing step includes heating the
resin containing regrind particles, such as over an open flame.
More specifically, the impregnated resin may be heated to a
temperature of at least about 250.degree. C. to effect resin
removal. During this heating step care should be taken to avoid
flashing of the resin decomposition products; this can be done by
careful heating in air or by heating in an inert atmosphere.
Preferably, the heating should be in the range of from about
400.degree. C. to about 800.degree. C. for a time in the range of
from at least about 10 and up to about 150 minutes or longer.
[0072] Additionally, the resin removal step may result in increased
tensile strength of the resulting article produced from the molding
process as compared to a similar method in which the resin is not
removed. The resin removal step may also be advantageous because
during the expansion step (i.e., intercalation and exfoliation),
when the resin is mixed with the intercalation chemicals, it may in
certain instances create undesirable byproducts.
[0073] Thus, by removing the resin before the expansion step a
superior product is obtained such as the increased strength
characteristics discussed above. The increased strength
characteristics are a result of in part because of increased
expansion. With the resin present in the particles, expansion may
be restricted.
[0074] In addition to strength characteristics and environmental
concerns, resin may be removed prior to intercalation in view of
concerns about the resin possibly creating a run away exothermic
reaction with the acid.
[0075] Once the flexible graphite sheet is comminuted, it is formed
into the desired shape and then cured (when resin impregnated) in
the preferred embodiment. Alternatively, the sheet can be cured
prior to being comminuted, although post-comminution cure is
preferred.
[0076] Optionally, the flexible graphite sheet used to form the
inventive finstock can be used as a laminate, with or without an
adhesive between laminate layers. Non-graphite layers may be
included in the laminate stack, although this may necessitate the
use of adhesives, which can be disadvantageous, since it can slow
thermal dissipation across the plane of the laminate stack. Such
non-graphite layers may include metals, plastics or other
non-metallics such as fiberglass or ceramics.
[0077] As noted above, the thusly-formed sheets of compressed
particles of exfoliated graphite are anisotropic in nature; that
is, the thermal conductivity of the sheets is greater in the
in-plane, or "a" directions, as opposed to the through-sheet, or
"c" direction. In this way, the anisotropic nature of the graphite
sheet directs the heat along the planar direction of the thermal
solution (i.e., in the "a" direction along the graphite sheet).
Such a sheet generally has a thermal conductivity in the in-plane
direction of at least about 140, more preferably at least about
200, and most preferably at least about 250 W/m.degree. K and in
the through-plane direction of no greater than about 12, more
preferably no greater than about 10, and most preferably no greater
than about 6 W/m.degree. K. Thus, the thermal solution has a
thermal anistropic ratio (that is, the ratio of in-plane thermal
conductivity to through-plane thermal conductivity) of no less than
about 10.
[0078] The values of thermal conductivity in the in-plane and
through-plane directions of the laminate can be manipulated by
altering the directional alignment of the graphene layers of the
flexible graphite sheets used to form the thermal solution,
including if being used to form a laminate, or by altering the
directional alignment of the graphene layers of the laminate itself
after it has been formed. In this way, the in-plane thermal
conductivity of the thermal solution is increased, while the
through-plane thermal conductivity of the thermal solution is
decreased, this resulting in an increase of the thermal anisotropic
ratio.
[0079] One of the ways this directional alignment of the graphene
layers can be achieved is by the application of pressure to the
component flexible graphite sheets, either by calendering the
sheets (i.e., through the application of shear force) or by die
pressing or reciprocal platen pressing (i.e., through the
application of compaction), with calendering more effective at
producing directional alignment. For instance, by calendering the
sheets to a density of 1.7 g/cc, as opposed to 1.1 g/cc, the
in-plane thermal conductivity is increased from about 240
W/m.degree. K to about 450 W/m.degree. K or higher, and the
through-plane thermal conductivity is decreased proportionally,
thus increasing the thermal anisotropic ratio of the individual
sheets and, by extension, any laminate formed therefrom.
[0080] Alternatively, if a laminate is formed, the directional
alignment of the graphene layers which make up the laminate in
gross is increased, such as by the application of pressure,
resulting in a density greater than the starting density of the
component flexible graphite sheets that make up the laminate.
Indeed, a final density for the laminated article of at least about
1.4 g/cc, more preferably at least about 1.6 g/cc, and up to about
2.0 g/cc can be obtained in this manner. The pressure can be
applied by conventional means, such as by die pressing or
calendering. Pressures of at least about 60 MPa are preferred, with
pressures of at least about 550 MPa, and more preferably at least
about 700 MPa, needed to achieve densities as high as 2.0 g/cc.
[0081] Surprisingly, increasing the directional alignment of the
graphene layers can increase the in-plane thermal conductivity of
the graphite laminate to conductivities which are equal to or even
greater than that of pure copper, while the density remains a
fraction of that of pure copper. Additionally, the resulting
aligned laminate also exhibits increased strength, as compared to a
non-"aligned" laminate.
[0082] Once the flexible graphite material is formed, whether as a
single sheet or a laminate, it is then sandwiched about the core
layer. As noted above, the core layer can comprise a plastic
material, especially a conductive plastic material, but is more
preferably metallic, and most preferably aluminum. In the preferred
embodiment, the core layer is a mesh or a tanged metal material.
This core layer should each be no more than about 10 mm in
thickness, most preferably no more than about 7.5 mm in thickness,
to keep the inventive sandwich as thin as practically possible.
[0083] As discussed above, the sandwich can be formed by using
adhesives, or by folding or crimping the outer layers about
themselves, thus encapsulating the core layer between the graphite
material, preferably with a portion of the core material extending
from an edge of the graphite outer layers when soldering attachment
to a base or the like is desired. In the most preferred embodiment,
the outer graphite layers are adhered to each other, with the
adhesive applied only where the two outer layers meet each other,
to avoid any diminution of heat transfer between the outer layer(s)
and the core.
[0084] Referring now to the drawings, and particularly to FIGS. 1
and 2, an embodiment of the present invention is shown and
generally designated by the numeral 10. Thermal article 10
comprises a sandwich having formed of sheets 20a and 20b having
major surfaces and four edge surfaces; preferably, sheets 20a and
20b comprise sheets of compressed particles of exfoliated graphite.
Sheets 20a and 20b are sandwiched about core layer 30. At least a
portion of article 10 is positioned in operative contact with a
heat source (not shown) such as a chipset and/or a heat collection
article or material (not shown), such as a heat sink, such that
heat generated by the heat source is spread by article 10; and/or
heat collected by a heat collection article is conducted into
article 10 as finstock and is thereby dissipated.
[0085] Moreover, because of the formable nature of the metallic
core layer 30 of sandwich 10, sandwich 10 can be formed into
complex shapes, so as to maximize or optimize contact with a heat
source or heat sink and thus improve thermal dissipation.
[0086] In another embodiment of the invention, illustrated in FIGS.
3 and 4, a portion (denoted 30a) of core layer 30 can extent beyond
the edges of article 10. In this manner, if core layer 30 is a
metal, it can be soldered into a heat sink or the like to improve
contact and attachment of article 10 to the heat sink, when article
10 is used as finstock.
[0087] In yet another embodiment of the invention, illustrated in
FIGS. 5 and 6, a portion of one of sheets 20a and 20b can be
removed (generally before the formation of sheets 20a and 20b into
article 10), to thus expose a portion of core layer 30. In this
way, more intimate contact (i.e., direct operative contact) between
a heat source and core layer 30 can be achieved, thereby increasing
heat conduction into core layer 30 and, thus, through article
10.
[0088] In still another embodiment of the invention, illustrated in
FIGS. 7 and 8, core layer is formed of a mesh material 32, such as
a copper mesh, to improve thermal and physical contact between
sheets 20a and 20b and mesh material 32.
[0089] Thus, by use of the present invention, effective heat
dissipation or spreading can be accomplished using the weight and
anisotropic advantages of graphite and the formability of metals
like aluminum. These functions cannot be accomplished by more
traditional heat dissipation materials like copper or aluminum
which, because of their high density, are often undesirable for
weight-sensitive applications.
[0090] All cited patents, patent applications and publications
referred to in this application are incorporated by reference.
[0091] The invention thus being described, it will be obvious that
it may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the present
invention and all such modifications as would be obvious to one
skilled in the art are intended to be included within the scope of
the following claims.
* * * * *