US20060225874A1

US20060225874A1 - Sandwiched thermal article

Info

Publication number: US20060225874A1
Application number: US11/103,348
Authority: US
Inventors: Gary Shives; David Flaherty
Original assignee: Advanced Energy Technology Inc
Current assignee: Graftech International Holdings Inc
Priority date: 2005-04-11
Filing date: 2005-04-11
Publication date: 2006-10-12
Also published as: WO2006132695A2; EP1875790A2; WO2006132695A3; EP1875790B1; EP1875790A4; ATE531246T1; ES2375375T3

Abstract

A thermal material for heat dissipation, which includes at least one sheet of flexible graphite sandwiched about a non-graphite core layer.

Description

TECHNICAL FIELD

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

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.
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.
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.
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).
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.
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.
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.
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.
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.
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/cm³to about 2.0 g/cm³. 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.
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.
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

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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° K, more preferably at least about 200 W/m° K and a through-plane thermal conductivity of no greater than about 12 W/m° K, more preferably no greater than about 10 W/m° K.
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.
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.
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

FIG. 1 is a partially broken away plan view of a first embodiment of the article of the present invention.
FIG. 2 is a cross-sectional view of the article of FIG. 1, taken along lines 2-2.
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.
FIG. 4 is a cross-sectional view of the article of FIG. 3, taken along lines 4-4.
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.
FIG. 6 is a cross-sectional view of the article of FIG. 5, taken along lines 5-5.
FIG. 7 is a plan view of another embodiment of the article of the present invention, wherein the core material is a mesh.
FIG. 8 is a cross-sectional view of the article of FIG. 7, taken along lines 8-8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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.
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: $g = \frac{3.45 - d (002)}{0.095}$
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.
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%.
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.
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.
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.
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° C. and 125° 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.
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.
Representative examples of saturated aliphatic carboxylic acids are acids such as those of the formula H(CH₂)_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.
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.
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° to 125° 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.
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° C. and especially about 700° C. to 1000° 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.
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/cm³). 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° C., preferably about 1400° 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.
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° C. and above and by the inclusion in the intercalant of a lubricious additive, as described in International Patent Application No. PCT/US02/39749.
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° C., because temperatures even 100° C. lower result in substantially reduced expansion.
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.
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° C., are at the high end of the range encountered in graphitization processes.
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.
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.
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.
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° C. and especially about 700° C. to 1200° 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.
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° C., preferably about 1400° 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.
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° 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).
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.
Also the processes of the present invention may use a blend of virgin materials and recycled materials.
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.
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.
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.
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.
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.
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.
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° 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° C. to about 800° C. for a time in the range of from at least about 10 and up to about 150 minutes or longer.
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.
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.
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.
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.
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.
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° 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° 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.
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.
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° K to about 450 W/m° 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.
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.
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.
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.
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.
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 20 a and 20 b having major surfaces and four edge surfaces; preferably, sheets 20 a and 20 b comprise sheets of compressed particles of exfoliated graphite. Sheets 20 a and 20 b 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.
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.
In another embodiment of the invention, illustrated in FIGS. 3 and 4, a portion (denoted 30 a) 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.
In yet another embodiment of the invention, illustrated in FIGS. 5 and 6, a portion of one of sheets 20 a and 20 b can be removed (generally before the formation of sheets 20 a and 20 b 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.
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 20 a and 20 b and mesh material 32.
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.
All cited patents, patent applications and publications referred to in this application are incorporated by reference.
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.

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° K.

12. The article of claim 11, which has a through-plane thermal conductivity of no greater than about 12 W/m° 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° K.

20. The finstock of claim 19, which has a through-plane thermal conductivity of no greater than about 12 W/m° K.