Title:
Heat sink with cooling channel
Kind Code:
A1


Abstract:
A heat sink apparatus is provided with a cooling channel which directs cooling air between groups of fins on either side of the cooling channel to a location adjacent that of an electronic device to be cooled. The increase in cooling efficiency achieved by providing cooling air to the critical location of the electronic device more than offsets any loss in cooling efficiency due to the elimination of fins required for creation of the cooling channel. Thus a heat sink apparatus can be designed providing increased cooling to selected locations without the use of heat pipes or the like.



Inventors:
Krassowski, Daniel W. (Columbia Township, OH, US)
Chen, Gary G. (Parma, OH, US)
Application Number:
10/098132
Publication Date:
09/18/2003
Filing Date:
03/13/2002
Assignee:
KRASSOWSKI DANIEL W.
CHEN GARY G.
Primary Class:
Other Classes:
257/E23.099, 165/121
International Classes:
F28F3/04; F28F13/06; H01L23/467; (IPC1-7): F28F7/00; F24H3/02; F28F1/00
View Patent Images:



Primary Examiner:
DUONG, THO V
Attorney, Agent or Firm:
Attn: James R. Cartiglia,Graftech Inc. (Brandywine West, Wilmington, DE, 19803, US)
Claims:

What is claimed is:



1. A heat sink apparatus, comprising: a base having first and second ends; a plurality of fins extending upward from the base; and a cooling channel defined between first and second groups of the fins, the cooling channel extending from the first end toward a location for an electronic device, the cooling channel having a channel width greater than a spacing between adjacent fins within each of the first and second groups, the cooling channel terminating upstream of the location for the electronic device.

2. The apparatus of claim 1, wherein: the plurality of fins includes intermediate fins located above the location for the electronic device in the path of the cooling channel, so that cooling air is directed by the cooling channel along the intermediate fins.

3. The apparatus of claim 1, wherein: the first and second ends of the base define a length of the base therebetween, the base having a width less than the length; and the fins extend parallel to the length of the base.

4. The apparatus of claim 1, wherein the channel width is at least 7 mm.

5. The apparatus of claim 1, wherein the channel width is at least 10 mm.

6. The apparatus of claim 1, wherein the channel width is at least three times the greatest spacing between adjacent fins within each of the groups of fins.

7. The apparatus of claim 6, wherein the cooling channel has a channel length at least one-half a distance from said first end to said location.

8. The apparatus of claim 1, wherein the cooling channel has a channel length at least one-half a distance from said first end to said location.

9. The apparatus of claim 8, wherein the channel length is at least 75% of the distance from said first end to said location.

10. The apparatus of claim 1, wherein the cooling channel has a channel length at least one-half the length of the base between the first and second ends.

11. The apparatus of claim 1, wherein the cooling channel is defined by an area on the base which area has no fins.

12. The apparatus of claim 1, wherein the cooling channel is defined by an area on the base having fins of shorter height than the fins of the groups of fins.

13. The apparatus of claim 1, wherein design parameters of the apparatus are such that a loss in cooling efficiency resulting from the presence of the cooling channel, as contrasted to having additional fins in the area of the cooling channel like the fins of the groups, is more than offset by an increase in cooling efficiency due to cooler air being channeled to the location of the electronic device.

14. The apparatus of claim 1, wherein the heat sink is at least partially constructed from a graphite material

15. The apparatus of claim 1, further comprising insulating walls defining sides of the cooling channel.

16. The apparatus of claim 1, in combination with: a cooling fan oriented to direct cooling air across the heat sink from the first end toward the second end of the base; and an electronic device in heat transfer communication with the location on the base, so that heat from the electronic device is transferred by the heat sink from the electronic device to the cooling air.

17. A heat sink apparatus, comprising: a base having a length and a width, the length being at least three times the width; and a plurality of parallel fins extending from the base parallel to the length of the base, the fins including first and second groups separated by a cooling channel extending from one end of the base at least one-half the length of the base and less than the entire length of the base.

18. The apparatus of claim 17, wherein: the plurality of fins includes intermediate fins located in the path of the cooling channel above a location for an electronic device, so that cooling air is directed by the cooling channel to the intermediate fins.

19. The apparatus of claim 17, wherein the cooling channel has a channel width of at least 7 mm.

20. The apparatus of claim 17, wherein adjacent fins of each group are spaced apart by less than 5 mm.

21. The apparatus of claim 17, wherein the cooling channel is defined by an area on the base which area has no fins.

22. The apparatus of claim 17, wherein the cooling channel is defined by an area on the base having fins of shorter height than the fins of the groups of fins.

23. A heat sink apparatus, comprising: a base having a length; and a plurality of parallel fins extending from the base, the plurality of parallel fins including first and second outer groups of fins extending the entire length of the base, and a third intermediate group of fins extending less than the entire length of the base, so that a cooling channel is defined between the first and second outer groups of fins in an area of the base not covered by the third intermediate group of fins.

24. The apparatus of claim 23, wherein the plurality of parallel fins has an equal spacing within each of the first, second and third groups.

25. The apparatus of claim 23, wherein the cooling channel has a channel length at least one half the length of the base.

26. The apparatus of claim 23, wherein design parameters of the apparatus are such that a loss in cooling efficiency resulting from the presence of the cooling channel, as contrasted to having additional fins in the area of the cooling channel like the fins of the groups, is more than offset by an increase in cooling efficiency due to cooler air being channeled to the location of the electronic device.

27. A method of cooling an electronic device, comprising: providing a heat sink having first and second groups of fins and having a cooling channel defined between the first and second groups; placing the electronic device in heat transfer communication with a location on the heat sink; channeling cooling air through the cooling channel to the location of the electronic device; and cooling the electronic device by transferring heat from the electronic device to the cooling air via the heat sink.

28. The method of claim 27, further comprising: providing more densely packed fins on the heat sink at the location of the electronic device than are provided upstream of the location.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a heat sink capable of managing the heat from a heat source such as an electronic device.

BACKGROUND OF THE INVENTION

[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 and systems as well as in other devices such as high power optical devices, relatively extreme temperatures can be generated. However, microprocessors, integrated circuits and other sophisticated electronic components typically operate efficiently only under a certain range of threshold temperatures. The excessive heat generated during operation of these components can not only harm their own performance, but can also degrade the performance and reliability of the overall system and can even cause system failure. The increasingly wide range of environmental conditions, including temperature extremes, in which electronic systems are expected to operate, exacerbates the negative effects of excessive heat.

[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] Several types of heat dissipating components are utilized to facilitate heat dissipation from electronic devices. The present invention is directly applicable to finned heat sinks.

[0005] These heat sinks facilitate heat dissipation from the surface of a heat source, such as a heat-generating electronic device, to a cooler environment, usually air. The heat sink seeks to increase the heat transfer efficiency between the electronic device and the ambient air primarily by increasing the surface area that is in direct contact with the air or other heat transfer media. This allows more heat to be dissipated and thus lowers the electronic device operating temperature. The primary purpose of a heat dissipating component is to help maintain the device temperature below the maximum allowable temperature specified by its designer/manufacturer.

[0006] Typically, the heat sinks are formed of a metal, especially copper or aluminum, due to the ability of metals like copper to readily absorb heat and transfer it about its entire structure. Copper heat sinks are often formed with fins or other structures to increase the surface area of the heat sink, with air being forced across or through the fins (such as by a fan) to effect heat dissipation from the electronic component, through the copper heat sink and then to the air.

[0007] The use of copper or aluminum heat dissipating elements can present a problem because of the weight of the metal, particularly when the heat transmitting area of the heat dissipating component is significantly larger than that of the electronic device. For instance, pure copper weighs 8.96 grams per cubic centimeter (g/cm3) and pure aluminum weighs 2.70 g/cm3.

[0008] For example, in many applications, several heat sinks need to be arrayed on, e.g., a circuit board to dissipate heat from a variety of components on the board. If metallic heat sinks are employed, the sheer weight of the metal on the board can increase the chances of the board cracking or of other equally undesirable effects, and increases the weight of the component itself. For portable electronic devices, any method to reduce weight while maintaining heat dissipation characteristics is especially desirable.

[0009] Another group of materials suitable for use in heat sinks are those materials generally known as graphites, but in particular graphites such as those based on natural graphites and flexible graphite as described below. These materials are anisotropic and allow the heat sink to be designed to preferentially transfer heat in selected directions. Also, the graphite materials are much lighter in weight and thus provide many advantages over copper or aluminum.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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/cm3 to 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.

[0016] With heat sinks of any of these materials, special situations are sometimes encountered where additional cooling is necessary. Such problems are currently solved through the use of heat pipes. However, using a heat pipe adds cost, requires extensive machining, and because of reliability concerns a second heat pipe is often added to provide redundancy in the event of a failure.

[0017] What is needed is a way to increase cooling at selected locations on a heat sink without the cost and complication of adding heat pipes.

SUMMARY OF THE INVENTION

[0018] The present invention provides a heat sink design which in selected cases can provide improved cooling at selected locations on the heat sink, without the use of heat pipes. Take, for example, the case of an elongated heat sink having the electronic device which is to be cooled placed at a location a relatively long distance away from the end of the heat sink which receives cooling air from a fan. In this case the air which passes over the location of the electronic device is heated prior to reaching that location because it must pass over an extensive length of heated cooling fins. Thus, when the air reaches the location on the heat sink immediately above the device being cooled, the air will have already been heated by passing over the upstream portions of the heat sink, so that insufficient cooling occurs at the critical location.

[0019] The present invention provides a solution to this problem, by eliminating a portion of the fins which would normally be present on the heat sink thus defining a cooling channel which allows cooling air to flow directly from the fan to the location adjacent the electronic device being cooled. This solution is counterintuitive to typical heat sink design, in that it would normally be expected that a reduction in the surface area of the heat sink by removing fins would cause the heat sink to perform less well. It has been discovered, however, that in certain instances the reduction in cooling efficiency caused by elimination of fins is more than offset by the increase in cooling efficiency at the critical location on the heat sink, due to the cooler air.

[0020] Accordingly, a heat sink apparatus is provided which includes a base having first and second ends, a plurality of fins extending upward from the base, and a cooling channel defined between first and second groups of the fins. The cooling channel extends from the first end toward a location for an electronic device. The cooling channel has a channel width greater than the spacing between adjacent fins within each of the first and second groups. The apparatus is preferably used in combination with a cooling fan oriented to direct cooling air across the heat sink from the first end toward the second end of the base, and an electronic device in heat transfer communication with the location on the base, so that heat from the electronic device is transferred by the heat sink from the electronic device to the cooling air.

[0021] In yet another embodiment of the invention a heat sink apparatus includes a base having a length and a width, the length being at least three times the width. The apparatus includes a plurality of parallel fins extending from the base, the fins including first and second groups separated by a cooling channel terminating short of a location for an electronic device.

[0022] In another embodiment of the invention a method is provided for cooling an electronic device. The method includes providing a heat sink having first and second groups of fins and having a cooling channel defined between the first and second groups. An electronic device is placed in heat transfer communication with a location on the heat sink. Cooling air is channeled through the cooling channel to the location of the electronic device. The electronic device is cooled by transferring heat from the electronic device to the cooling air of the heat sink.

[0023] Thus it is an object of the present invention to provide an improved heat sink design for thermal management of electronic devices.

[0024] Still another object of the present invention is the provision of a heat sink having a cooling channel for directing cooling air to a location adjacent an electronic device to be cooled.

[0025] Still another object of the present invention is the provision of a heat sink which can provide additional cooling to selected locations without the use of heat pipes.

[0026] Still another object of the present invention is the provision of a heat sink design of economical construction which can be engineered for improved cooling at selected locations.

[0027] Other and further objects, features, and advantages of the present invention will be readily apparent to those skilled in the art, upon a reading of the following disclosure when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic plan view of a heat sink constructed in accordance with the present invention.

[0029] FIG. 2 is a side elevation view of the heat sink of FIG. 1 showing an electronic device mounted on the base of the heat sink.

[0030] FIG. 3 is a bottom view of the heat sink of FIG. 1 showing the electronic device mounted on the base.

[0031] FIG. 4 is an elevation section view taken along line 4-4 of FIG. 1.

[0032] FIG. 4A is a view similar to that of FIG. 4 showing an alternative embodiment of the invention wherein the cooling channel is defined by an area of relatively short fins.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] As noted, one material from which the heat sinks of the present invention may be constructed is graphite sheet material. Before describing the construction of the heat sinks, a brief description of graphite and its formation into flexible sheets is in order.

[0034] Preparation of Flexible Graphite Sheet

[0035] 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.

[0036] 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: 1g=3.45-d(002)0.095embedded image

[0037] 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.

[0038] 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%.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 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.

[0048] 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.

[0049] The flexible graphite sheet can also, at times, be advantageously treated with resin and the absorbed resin, after curing, enhances the moisture resistance and handling strength, i.e. stiffness, of the flexible graphite sheet as well as “fixing” the morphology of the sheet. Suitable resin content is preferably less than about 60% by weight, more preferably less than about 35% by weight, and most preferably from about 4% to about 15% by weight. Resins found especially useful in the practice of the present invention include acrylic-, epoxy- and phenolic-based resin systems, or mixtures thereof. Suitable epoxy resin systems include those based on diglycidyl ether or bisphenol A (DGEBA) and other multifunctional resin systems; phenolic resins that can be employed include resole and novolak phenolics.

[0050] When using graphite material for the heat sink, the transfer of heat from the heat source into the base of the heat sink may be enhanced by providing a high thermal conductivity insert in the base in a manner like that described in U.S. patent application Ser. No. ______, entitled HEAT DISSIPATING COMPONENT USING HIGH CONDUCTIVITY INSERTS, filed Dec. 13, 2001 by Krassowski, et al, the details of which are incorporated herein by reference. A cavity is formed through the thickness of the base and the high conductivity insert is received in the cavity. The insert may be an isotropic high thermal conductivity material such as copper or an anisotropic material such as graphite oriented to have high conductivity in the direction of the thickness of the base.

THE DETAILED EMBODIMENT OF FIGS. 1-4

[0051] Referring now to the drawings, and particularly to FIG. 1, a heat sink apparatus is shown and generally designated by the numeral 10. The apparatus 10 includes a base 12 having first and second ends 14 and 16 defining a length 18 therebetween. The base 12 has a width 20 less than the length 18. The base 12 has a location generally designated by dashed lines 22 in FIGS. 1 and 3, which location is defined for engagement with an electronic device 24 which is to be cooled by the heat sink 10.

[0052] The electronic device 24 may be any of the devices described above, and will be mounted in thermal transfer operative engagement with the base 12. The heat sink 24 may be attached to the base 12 in any conventional manner, which may include the use of a thin thermal interface 25 or a layer of phase change material or thermal grease therebetween. The thermal interface 25 may be, for example, a thin layer of flexible graphite material.

[0053] A plurality of parallel fins 26 extend upward from the base 12. The fins 26 in the embodiment shown in FIG. 1 are planar fins which extend parallel to the length 18 of the base 12. Fins 26 may be described as being in two groups 28 and 30. Each of the groups 28 and 30 may be described as an outer group of spaced fins having a spacing such as 32 defined between adjacent fins. The spacing 32 will typically be a uniform spacing between each of the fins, but it need not be a uniform spacing for purposes of the present invention.

[0054] The two groups of fins 28 and 30 are separated by a cooling channel 34 extending from the first end 14 toward the location 22. The cooling channel 34 has a channel width 36 which is substantially greater than the spacings 32 between adjacent fins within each of the two groups 28 and 30.

[0055] A fan 42 blows air in direction 43 across the length of heat sink 10.

[0056] The heat sink apparatus 10 is preferably of the type used in forced air convection, and typically the fins 26 have a thickness on the order of from 0.5 to 1.0 mm, and the spacing 32 between adjacent fins is typically on the order of from about 1 mm to about 4 mm. The spacing is typically three times the fin thickness. In general spacing 32 will usually be less than 5 mm. In a heat sink 10 having fins 26 and spacings 32 on the order just described, the channel width 36 is preferably at least 7 mm, it is more preferably at least 10 mm and even more preferably is about 15 mm. Another way of defining the preferred channel width is that the channel width is at least two times the spacing 32 between adjacent fins, and more preferably at least three to four times the spacing between adjacent fins of the groups on either side of the cooling channel.

[0057] The cooling channel 34 has a length 38. In general when using the design precepts of the present invention it is desired to extend the length 38 of cooling channel 34 to a point at or near the location 22 directly above the electronic device 24 which is to be cooled. This can be described as terminating the channel 34 at or upstream of the location 22. It is desired that a group of intermediate fins 40 located above the location 22 receive the cooling air from the cooling channel 34. Thus, one manner of describing the channel length 38 is that the channel length should extend a substantial portion of a distance from the first end 14, where the fan 42 is located, to the location 22. The length 38 should be at least one half the distance from end 14 to location 22, more preferably at least 75% of the distance from end 14 to location 22, and most preferably the length 38 should extend from the end 14 to the location 22.

[0058] Another manner of defining the length 38 of channel 34 is with reference to the length 18 of the base 12, and in such a description, the channel 34 can be described as having a channel length 38 at least one half the length 18 of the base 12.

[0059] It will also be understood that the definition of the location 22 includes the area immediately above the electronic device 24, and as indicated best in FIG. 3, the location 22 may extend somewhat beyond the perimeter of the electronic device 24.

[0060] As best seen in FIG. 4, the cooling channel 34 may be defined by an area on the base 12 which is completely free of fins. Alternatively, as shown in FIG. 4A, the cooling channel 34 may be defined by an area of relatively short fins 26A.

[0061] As seen in FIGS. 4 and 4A, the fins 26 have a fin height 27 above the base 12, which fin height 27 is typically on the order of 20 to 50 mm.

[0062] In one embodiment, the innermost fins 26B immediately on either side of cooling channel 34 may simply be cooling fins constructed in the same manner and of the same material as the other cooling fins 26. Alternatively, however, the innermost fins 26B may be constructed of an insulating material and serve as insulating walls 26B defining the sides of the cooling channel 34.

[0063] As previously mentioned, the heat sink 10 including its base 12 and fins 26 may be made of any conventional heat sink materials including copper, aluminum, graphite, and composites of the above.

[0064] The cooling channel 34 of the present invention has been found to be particularly effective in relatively elongated heat sinks. The electronic device 24 is located away from the end 14 which is adjacent the source of cooling air such as fan 42. Such an elongated heat sink may be characterized by a length 18 at least three times as great as the width 20.

[0065] With such an elongated heat sink, if the heat sink were of conventional design having fins across the entire width and the entire length, the air passing from fan 42 to the location 22 immediately above electronic device 24 could be so heated by heat from the fins that insufficient cooling is provided in the location 22. It has been determined that by essentially eliminating certain fins so as to define the cooling channel 34, cooling air from fan 42 will pass directly through the channel 34 with relatively little heating occurring to the air, and thus cool air passes through the fins 40 directly above the location 22. In certain situations, the loss of cooling efficiency by the elimination of fins from the area of cooling channel 34 is more than offset by the increased cooling efficiency due to cooler air passing over the location 22. Reduced operating temperatures of the electronic device 24 on the order of 1.5° to 3° C. may be achieved, which as will be appreciated by those skilled in the art may be very significant.

[0066] The following example shows one situation where the cooling channel of the present invention can be utilized effectively.

[0067] A heat sink design like that shown in FIGS. 1-4 was modeled using computational fluid dynamic modeling.

[0068] Case No. 1 was for an elongated copper heat sink with no cooling channel. The heat sink was modeled having a length 18 of 280 mm, a width 20 of 70 mm, a base thickness of 8 mm, a total of 27 fins having fin thickness of 0.635 mm, a spacing between fins of 2.0 mm, and fin height of 37 mm. The heat source was modeled as having a size of 40 mm by 40 mm and having a power output of 150 W. The copper material for the heat sink had a thermal conductivity of 391 W/m ° C.

[0069] In Case No. 2, the design is similar to No. 1 except the nine centralmost fins have had a portion thereof removed along the length 38 as seen in FIG. 1, which length is 168 mm. The channel width 36 is equal to 26 mm.

[0070] Case No. 3 is similar to Case No. 2 except the model utilized graphite material having a thermal conductivity of 400 W/m ° C. in the plane of the base and utilizing a copper insert in the base directly above the heat source, like that described above with reference to U.S. patent application Ser. No. ______, entitled HEAT DISSIPATING COMPONENT USING HIGH CONDUCTIVITY INSERTS.

[0071] Case No. 4 utilized the same materials as Case No. 3, but added a cooling channel having dimensions like that of Case No. 2.

[0072] Case No. 5 is similar to Case No. 1 except that the material of the heat sink modeled was aluminum having a thermal conductivity of 209 W/m ° C.

[0073] Case 6 utilized the same aluminum material as Case No. 5, but added a cooling channel having dimensions like that described above for Case No. 2.

[0074] Utilizing the same ambient conditions for each of the six cases, the maximum temperature of the base 12 in the location 22 (Tmax) and the thermal resistance (Rsa) were calculated and are shown in the following Table I. 1

TABLE I
CaseHeat SinkModelTmaxRsa
No.MaterialOption(° C.)(° C./W)
1CopperOriginal60.440.24
Design
2Cooling58.430.23
Channel
3GraphiteOriginal62.270.25
Design
4Cooling60.480.24
Channel
5AluminumOriginal68.040.29
Design
6Cooling65.250.27
Channel

[0075] As seen in Table I, when comparing Cases 2 and 1, the addition of the cooling channel to the copper heat sink lowered Tmax from 60.44° C. to 58.43° C., thus providing a temperature decrease of 2.01° C.

[0076] Looking now at the graphite material example in Cases 3 and 4, the addition of the cooling channel in Case 4, as compared to the design without the cooling channel in Case 3, resulted in a decrease in Tmax from 62.27° C. to 60.48° C. or an improvement of 1.79° C.

[0077] Finally, comparing Cases 6 and 5, the addition of the cooling channel to the aluminum heat sink resulted in the most significant decrease, from 68.04° C. to 65.25° C. for an improvement of 2.79° C.

[0078] Thus it is seen that in a heat sink having dimensions and a geometry like that described in the example above, the addition of a cooling channel upstream of the location of the heat source provided significant decreases in the maximum temperature of the heat sink at the heat source, which are all within a range of from about 1.5° C. to about 3.0° C. for the example model.

[0079] Also, the elimination of fins in the area of the cooling channel reduces the weight of the heat sink by as much as about 15%. This is an advantage in many products.

[0080] Methods of utilizing the heat sink of the present invention include:

[0081] (a) providing the heat sink 10 having first and second groups 28 and 30 of fins 26 and having the cooling channel 34 defined between the first and second groups of fins 28 and 30;

[0082] (b) placing the electronic device 24 in heat transfer communication with the location 22 on the heat sink 10;

[0083] (c) channeling cooling air from the fan 42 through the cooling channel 34 to the location 22 of the electronic device 24; and

[0084] (d) cooling the electronic device 24 by transferring heat from the electronic device 24 to the cooling air via the heat sink 10.

[0085] Thus it is seen that the apparatus and methods of the present invention readily achieve the ends and advantages mentioned as well as those inherent therein. While certain preferred embodiments of the invention have been illustrated and described for purposes of the present disclosure, numerous changes in the arrangement and construction of parts and steps may be made by those skilled in the art, which changes are encompassed within the scope and spirit of the present invention as defined by the appended claims.