Flexible graphite thermal management devices

FIELD OF THE INVENTION

[0001] The invention relates to thermal management devices, and more particularly, to such devices, which include flexible graphite and methods of making such devices.

BACKGROUND OF THE INVENTION

[0002] Thermal management devices, such as heat pipes, are devices which are known in the art of heat transfer. A heat pipe is essentially a closed system of heat transfer in which a small amount of liquid within a sealed and evacuated enclosure is cycled through an evaporation and condensation cycle. Heat entering the enclosure at one location on the casing evaporates liquid at that location, producing vapor, which moves to a cooler location on the casing where it is condensed. The movement of the vapor is motivated by a small vapor pressure differential between the evaporator and the condenser locations. The heat transfer is accomplished when the heat of vaporization, which produces the vapor, is essentially moved with the vapor to the condenser location where it is given up as the heat of condensation.

[0003] In order for the heat transfer to continue, the condensed liquid must be returned from the condenser to the evaporator where it will again be vaporized. Although this return can be accomplished by something as simple as gravity, capillary wicks have generally been used to permit heat pipes to be relatively independent of the effects of gravity. Such a wick extends from a location near the condenser, where the liquid originates, to a location at the evaporator where it is needed for evaporation.

[0004] With respect to material of construction, casings are traditionally made of copper or other metals, and are made with walls of sufficient thickness to assure that they are structurally sufficient to withstand the vapor pressures within the heat pipe, and that they are not porous to either the reduced vapor or non-condensable gases outside the heat pipe casing.

[0005] Considerable efforts have been expended to develop materials, which are both heat conductive and act as capillary structures for wicks. The most common such materials are metal screens used in multiple layers and metal powders sintered into a structure attached to the casing. The heat conductive property of such wicks has been considered important so that the heat entering the heat pipe will be conducted into and through the wick at the evaporator and vaporizes the liquid within the wick. It is also generally preferred that in heat pipe construction that the wick is attached to the casing wall at the evaporator, so that the input heat has direct access to the liquid in the wick.

[0006] However, there are applications for which the conventional heat pipe structure is not satisfactory. Metal casings and metal wicks add weight, rigidity, and electrical conductivity to heat pipes, but that makes them unusable in some situations. Portable computers, the so-called “laptops”, are one application in which traditional heat pipes are difficult to use. In such applications, weight and space are extremely critical factors. Furthermore, the costs of metal casings and sintered wicks are disadvantaged in the highly competitive market of portable computers. Furthermore, wick components from conventional materials are not resistant to corrosion. Therefore a need exists to find new materials of construction for the heat pipes.

SUMMARY OF THE INVENTION

[0007] One embodiment of the invention provides thermal management devices comprising a substantially fluid impermeable shell and a wick structure inside the shell. In certain preferred embodiments, the wick comprises a mass of expanded graphite.

[0008] Another embodiment of the invention provides methods of making thermal management devices having a wick structure formed from a mass of expanded graphite.

[0009] A further embodiment of the invention provides thermal management devices having a shell constructed from flexible graphite.

[0010] Additional embodiments of the invention, which will become apparent to those skilled in the art after reading this specification, include methods of making thermal management devices having a shell constructed from flexible graphite.

[0011] Thermal management devices of the present invention have many advantages when compared to their conventional counterparts, some of which include excellent weight, acceptable rigidity, and satisfactory thermal conductivity. Moreover, the wick structure of the inventive device also has improved corrosion resistance as compared to conventional wicking material.

[0012] Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0013] 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 into 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

[0014]
FIG. 1 is a plan view of the internal aspects of a cylindrical heat pipe.

[0015]
FIG. 2 is an exploded view of the elements of a vertical heat pipe.

[0016]
FIG. 3 is a plan view of one specific embodiment of an inventive heat pipe in a heat spreader assembly.

[0017]
FIG. 4 is a cross sectional view of one specific embodiment of a thermal management device with fins.

DETAILED DESCRIPTION

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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/cc to about 2.0 g/cc. 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 increased density. 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 and electrical properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.

[0023] Methods of manufacturing articles from graphite particles have been proposed. For example, U.S. Pat. No. 5,882,570 to Hayward discloses a method of grinding flexible unimpregnated graphite foil to a small particle size, thermally shocking the particles to expand them, mixing the expanded graphite with a thermoset phenolic resin, injection molding the mixture to form low density blocks or other shapes, then heat treating the blocks to thermoset the material. The resulting blocks may be used as insulating material in a furnace or the like.

[0024] WO 00/54953 and U.S. Pat. No. 6,217,800, both to Hayward, further describe processes related to those of U.S. Pat. No. 5,882,570. The Hayward processes are very limited in the scope of the source materials they use, and the type of end products they can produce. Hayward uses only unimpregnated graphite source materials, and his finished products are only formed by mixing the graphite particles with large proportions of resin and injection molding the mixture to form articles, which are then thermoset.

[0025] With respect to graphite, it is known that graphites are made up of layered planes of hexagonal arrays or networks of carbon atoms. These layered 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 are 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.

[0026] The invention may comprise providing source materials such as flexible sheets of graphite material. The source materials typically comprise graphite, a crystalline form of carbon comprising atoms covalently bonded in flat layered planes with weaker bonds between the planes. In obtaining source materials such as the above flexible sheets of graphite, particles of graphite, such as natural graphite flake, are typically treated 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 and provided with small transverse openings by deforming mechanical impact.

[0027] Graphite starting materials for the flexible sheets 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}

[0028] where d(002) is the spacing between the graphitic layers of the carbons in the crystal structure measured in Angstrom units. The spacing “d” between graphite layers is measured by standard X-ray diffraction techniques. The positions of diffraction peaks corresponding to the (002), (004) and (006) Miller Indices are measured, and standard least-squares techniques are employed to derive spacing which minimizes the total error for all of these peaks. Examples of highly graphitic carbonaceous materials include natural graphites from various sources, as well as other carbonaceous materials such as carbons prepared by chemical vapor deposition and the like. In many instances, natural graphite is preferred.

[0029] The graphite starting materials for the flexible sheets used in the present invention may contain non-carbon components so long as the crystal structure of the starting materials maintains the required degree of graphitization and they are capable of exfoliation. Generally, any carbon-containing material, the crystal structure of which possesses the required degree of graphitization and which can be exfoliated, is suitable for use with the present invention. Such graphite preferably has an ash content of less than twenty weight percent. In certain circumstances, the graphite employed will have a purity of at least about 94%. In other preferred circumstances, the graphite employed will have a purity of at least about 99%.

[0030] 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.

[0031] In one 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. The intercalation solution may even sometimes 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.

[0032] The quantity of intercalation solution may range from about 20 to about 150 pph, and typically from about 50 to about 120 pph. After the flakes are intercalated, any excess solution is typically drained from the flakes and the flakes are water-washed. Alternatively, the quantity of the intercalation solution may be limited to between about 10 and about 50 pph, which permits the washing step to be eliminated as taught and described in U.S. Pat. No. 4,895,713, the disclosure of which is also herein incorporated by reference.

[0033] 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 typically from about 0.1 to 5% by weight of the particles of graphite flake.

[0034] 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 often times 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.

[0035] 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 examples 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 examples 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 examples of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative examples 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 examples 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.

[0036] The intercalation solution will typically be aqueous and may contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In an 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 0.1% to about 10% by weight of the graphite flake.

[0037] 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.

[0038] 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 often times 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.

[0039] 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 0.05 mm to 4.00 mm and a typical density of about 0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5 to 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 0.1 to 1.5 millimeters. The width of the particles is suitably from about 0.05 to 0.001 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.

[0040] 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, or mixtures thereof. Suitable epoxy resin systems include those based on diglycidyl ether or bisphenol A (DGEBA) and other multifunctional resin systems; phenolic resins that can be employed include resole and novolac phenolics. In certain preferred embodiments, the glass transition temperature of the resin is compatible with the use temperature of the thermal heat management device. Nonetheless, the graphite sheet as prepared above may be cut and trimmed to form the desired articles. Flexible graphite treated with a resin is also referred to as “resin impregnated flexible graphite” or “impregnated flexible graphite”.

[0041] The invention will now be described in terms of the aforementioned drawings in terms of a heat pipe, however the invention is not limited to a heat pipe and is applicable to other types of thermal management devices, e.g., a vapor chamber. Whenever possible like or similar reference characters will be used to describe like or similar elements of the drawings.

[0042]
FIG. 1 is a plan view of the internal aspects of a cylindrical heat pipe, generally designated 10. Depicted in FIG. 1 is a cylindrical heat pipe 12 arranged in a horizontal orientation with respect to the general path of flow of the working fluid. Heat pipe 12 includes a shell 14 and wick structure 16. Optionally, heat pipe 12 includes at least one fluid passage 18 external to wick structure 16, also represented by arrows E. Heat pipe 12 may also include at least one fluid passage 20 internal to wick structure 16, also represented by arrows I. Wick structure 16 may also comprise a plurality of approximately radial fluid passages, which allows the working fluid to travel at least from passage 18, through wick structure 16 to passage 20. Optionally, heat pipe 12 also includes an evaporator 22 and a condenser 26 at opposed ends of the heat pipe 12.

[0043] Heat pipe 12 may include a mating element 28 for a heat source 30. Optionally, mating element 28 is located on the same end of heat pipe 12 as evaporator 22. In certain preferred embodiments, mating element 28 has a surface in contact with a surface of heat source 30 and that the surface of mating element 28 is optionally the mirror image of the surface of heat source 30 in contact with mating element 28. In other preferred embodiments, mating element 28 is also in contact with an external portion of heat pipe 12. The contact between mating element 28 and heat pipe 12 may enhance heat transfer from heat source 30 to heat pipe 12. Suitable materials of construction of mating element 28 comprise at least flexible graphite, copper, aluminum, and combinations thereof. An example of a suitable material for mating element 28 comprises eGRAF™ HS 400 from Graftech Inc. One example of heat source 30 is a computer chip.

[0044] As illustrated in FIG. 1, heat pipe 12 may also include a plurality of fins 32 located on at least a portion of shell 14. Optionally, fins 32 are located on at least an end of pipe 12 which condenser 26 is located. With respect to an embodiment of heat pipe 12 that includes fins 32, preferably fins 32 are constructed from flexible graphite. Other suitable materials of fins 32 may comprise copper, aluminum, and any combination of the previously identified materials. Fins 32 are not limited to the embodiment depicted in FIG. 1. Any suitable configuration of fins 32 may be used as part of the invention. For example fins 32 may comprise a combination of elements of a base and a plurality of spaced apart fins extending vertically away from heat pipe 12. An embodiment of the invention may also include a fan to move air across fins 32 to assist with the dissipation of the heat absorbed by fins 32.

[0045] As shown in FIG. 1, heat is generated at heat source 30. The heat generated at heat source 30 is transmitted to the heat pipe 12 and evaporates the working fluid in at least evaporator 22. The vapor phase of the working fluid flows along passage 20 to condenser 26 as indicated by arrows I. The vapor working fluid is condensed into a liquid form at condenser 26 and the heat removed from the working fluid as a result of condensing is transmitted to fins 32 and dissipated into the ambient environment. The liquid form of the working fluid flows back to evaporator 22 along passage 18 in the direction indicated by arrows E. Typical flow mechanisms to transport liquid working fluid from condenser 26 to evaporator 22 include at least gravity, capillary action, or combinations thereof.

[0046] Optionally, heat pipe 12 may be operated at a pressure of less than atmospheric pressure. By adjusting the pressure inside heat pipe 12, the temperature at which the working fluid inside heat pipe 12 will vaporize may be adjusted to the specific change in temperature associated with the heat generated by heat source 30. For example, if the working fluid is water and heat pipe 12 is operating at a reduced pressure, the water will vaporize at a temperature of less than about 100° C.

[0047] A second embodiment of a heat pipe is illustrated in FIG. 2 and is generally designated 40. Optionally, heat pipe 40 may be arranged in a vertical orientation with respect to the path of travel of the working fluid. However, heat pipe 40 may be arranged in any configuration. Heat pipe 40 includes a shell, which comprises of the two sheets of material 44u and 44l. Sheets 44u and 44l are located on opposed ends of heat pipe 40. Sheet 44l may be referred to as an evaporator and sheet 44u may be referred to as a condenser.

[0048] Heat pipe 40 also includes a wick structure 46. As illustrated, wick structure 46 includes four (4) different plate structures 46a-46d. It should be noted that heat pipe 40 is not limited to any particular number of plates. As for the construction of plate structures 46a-46d, it is preferred that the path of flow of the working fluid is altered from one plate to the adjacent plate. With respect to plates 46a and 46b, it is shown that the flow pathway of plate 46a is about the exact opposite of the flow pathway of the working fluid through plate 46b. For example, plate 46a has a central aperture and spoke like flow pathways that extend radially outward from the central aperture. In contrast, plate 46b includes a central hub and spoke like support members that extend radially outward from the central hub. Furthermore, preferably plate 46c provides a flow path that is complimentary to that of plate 46b. The same is true between the relationship plates 46cand 46d. Wick structure 46 is constructed in a manner such that the working fluid is transported within heat pipe 40 by at least capillary action.

[0049] With respect to one embodiment of the invention, preferably heat pipe 12 comprises a substantially fluid impermeable shell 14, preferably shell 14 is a vacuum tight enclosure, and a wick structure 16 is inside shell 14. Optionally, shell 14 of heat pipe 12 may be constructed from flexible graphite. Optionally, shell 14 is constructed from flexible graphite having a density of at least about 1.6 g/cc, typically at least about 1.7 g/cc, more typically at least about 1.9 g/cc, and even more typically at least about 2.0 g/cc.

[0050] The flexible graphite used to form shell 14 may or may not be resin impregnated flexible graphite. Furthermore, shell 14 may comprise more than one sheet of flexible graphite. In a further embodiment of shell 14, an interior surface of shell 14 may include at least one channel, preferably a plurality of channels. Vacuum tight is used herein to mean that vacuum was used to remove at least some, preferably substantially all, of the non-working fluid from shell 14. The non-working fluid is defined herein to mean at least a fluid that is present in shell 14 that is not the working fluid, e.g., air.

[0051] In the case that shell 14 is constructed from flexible graphite, various techniques may be used to form the desired shaped shell. For example, compression may be used to form sheets of flexible graphite into the desired shape for shell 14. A typical pressure used to compress flexible graphite comprises about 4 bars or less, typically about 2 bars or less. In a second technique, an adhesive may be used to form the flexible graphite into the desired shape for shell 14. In certain preferred circumstances, the adhesive is not soluble in the working fluid or vice versa. In the third technique, the flexible graphite is rolled into a cylindrical shape and a lengthwise seam of heat pipe 12 is formed through the use of compression or an adhesive and a plug may be used at each one of the axial openings of the cylinder to form shell 14.

[0052] In another technique, one or more flexible graphite sheets may be rolled into a tube shape and the ends of the tube may be plugged to form shell 14. Optionally, the sheets may be embossed to form channels on an interior surface of shell 14. In another alternative, the flexible graphite sheets may be corrugated. A further technique, sheets of flexible graphite may be wrapped around a mandrel to form shell 14. In wrapping the sheets, the sheets may be wrapped in any configuration, for example but limited to spirally wrapping the sheets. In the above techniques, the flexible graphite sheets may be resin impregnated or nonresin impregnated. With respect to the resin impregnated sheets, the resin may be cured prior to or after forming shell 14.

[0053] In another embodiment, shell 14 is constructed from a three-dimensional piece of flexible graphite. A passage is machined into the piece. Preferably the passage does not extend completely through the piece. The open end of the passage may be sealed by any one of the aforementioned techniques of an adhesive, pressing, or a plug.

[0054] In certain preferred embodiments, wick structure 16 comprises a porous material, more preferably a mass of expanded graphite, even more preferably flexible graphite having a density of no more than about 1.5 g/cc. Flexible graphite is used herein to describe a mass of expanded graphite that has been formed into a sheet. More preferably, the flexible graphite of wick structure 16 has a density of no more than about 1.1 g/cc, even more preferably less than about 1.0 g/cc, and most preferably no more than about 0.5 g/cc. It is even further preferred that the flexible graphite has a density of at least about 0.25 g/cc. One example of such flexible graphite is GRAFOIL® available from Graftech Inc. of Lakewood, Ohio.

[0055] One way the density of the flexible graphite may be measured is by an immersion density test. In this test the sample of flexible graphite is weighed and the weight is recorded. Next the sample is immersed in a predetermined volume of water. The volume of water dispersed by the immersion of the sample is recorded. The density is determined by dividing the weight of the sample by the volume of water dispersed. How to measure density is not limited to above immersion density test.

[0056] The flexible graphite of wick structure 16 may be resin impregnated or non-impregnated flexible graphite. In the case that structure 16 is resin impregnated. In certain preferred embodiments, structure 16 is impregnated in a manner to introduce porosity into structure 16. Under such circumstances, it is more preferred that the resin impregnate enhances capillary flow and/or diffusion of the working fluid.

[0057] Optionally, flexible graphite wick structure 16 may further include a metal wire incorporated into at least a segment of wick structure 16. Examples of suitable metal wire include copper, aluminum, stainless steel, titanium, and combinations thereof. The metal wire may be incorporated into wick structure 16 by various methods.

[0058] One method is that the metal wire may be wrapped around at least a portion of an exterior of wick structure 16. In a second method, the metal wire is adhesively bonded to at least a portion of the an interior or an exterior of wick structure 16. In a third method, a laminate of the flexible graphite and the metal wire is formed. At least a portion of the laminate includes the metal wire.

[0059] Wick structure 16 may also include one or more sheets of flexible graphite. Furthermore, each sheet of flexible graphite may comprise about two or more layers of flexible graphite.

[0060] In one specific embodiment, wick structure 16 includes a sheet of crinkled flexible graphite. Preferably, the crinkles comprise microcrinkles. More preferably, the microcrinkles have an amplitude of about 1 mm or less. Crinkling gears may be used to from the crinkles. This embodiment of wick structure 16 may include a second sheet of flexible graphite. In some circumstances, the second sheet of flexible graphite is not crinkled.

[0061] In another embodiment, wick structure 16 includes at least one channel. The channel(s) may comprise a first portion sized to facilitate vapor flow, and a second portion sized to facilitate liquid flow. Wick structure 16 may be embossed to form the channels in structure 16. The size of the channels may be uniform or vary. If the size of the channels varies, some channels can be sized to facilitate vapor flow and others to facilitate liquid flow. As for structure 16, in one embodiment, wick structure 16 is attached to evaporator 22 of heat pipe 12.

[0062] Alternatively, wick structure may be constructed from other materials than expanded graphite. Suitable alternative materials include metals such as aluminum, copper, iron, nickel, titanium, and combinations thereof. The alternate materials may be used instead of or in combination with the expanded graphite.

[0063] Heat pipe 12 may also include a working fluid circulating inside shell 14. Preferred examples of the working fluid include at least one of methanol, ethanol, other alcohols, water, and, fluorocarbons (e.g., Freon®). In one embodiment, the construction of the heat pipe comprises evacuating the shell-wick structure assembly of the heat pipe and then back filling the heat pipe with at least enough fluid to fill the voids in wick structure 16.

[0064] In certain preferred embodiments, the amount of the working fluid in heat pipe 12 comprises enough to saturate wick structure 16. More preferably, the amount of working fluid charged into shell 14 comprises about ten percent (10%) more than what is needed to saturate wick structure 16.

[0065] The amount of working fluid may optionally comprise up to about twenty percent (20%) more than what is needed to saturate wick structure 16. In another embodiment, the amount of working fluid in heat pipe 12 comprises the volume of condenser 26, more preferably about 10% more than the volume of condenser 26. One technique to evacuate shell 14 is to pull vacuum on an interior of shell 14. A function of evacuating shell 14 is to remove as much residual air or other non-working fluid from shell 14.

[0066] In one preferred embodiment, the atmosphere inside the heat pipe reaches equilibrium of liquid and vapor. As heat enters at the evaporator, this equilibrium shifts to the vapor side and increases the pressure inside the heat pipe. Under the increased pressure, the vapor may diffuse to the condenser, where slightly lower temperatures cause the vapor to condense and give-up its latent heat of vaporization. The condensed fluid is then transferred back to the evaporator by preferably capillary forces developed in wick structure 16, diffusion, or gravity forces.

[0067] The continuous cycling of the working fluid transfers large quantities of heat with low thermal gradients. Preferably, the heat pipe's operation is passive, driven only by the heat that is transferred. Benefits of passive operation include excellent reliability and superior useful life.

[0068] The inventive heat pipe may be included into a heat spreader assembly as shown in FIG. 3, generally designated 50. The assembly 50 comprises a heat pipe 52. In certain preferred embodiments, heat pipe 52 includes at least one of a shell or a wick structure comprised of flexible graphite as described above.

[0069] As shown in FIG. 3, assembly 50 may also optionally include a base unit 54. Preferably base unit 54 is located at the end of assembly 50, which includes the condenser, not shown. Suitable materials of construction of base unit 54 comprise flexible graphite, copper, aluminum, and combinations thereof. Base unit 54 includes a surface 56 which may be used to attach a plurality of fins, not shown, to assembly 50. Furthermore, assembly 50 may include a mating element 58. Preferably, mating element 58 is located at an end of heat pipe 52 with the evaporator and is in contact with the heat source. Suitable materials of construction of mating element 58 comprise the same as mating element 28 noted above.

[0070] In one embodiment of assembly 50, all three of the shell of heat pipe 52, base 54, and mating element 58 are constructed from flexible graphite, e.g., eGraf™ from Graftech Inc. In certain preferred embodiments, the flexible graphite for at least one of the pipe 52, base 54, and element 58 comprises a laminate. The laminate may be constructed from high density (in certain embodiments, preferably at least about 1.6 g/cc, more preferably at least about 1.7 g/cc, and even more preferably at least about 1.9 g/cc) sheets of flexible graphite cemented together. Alternatively, the flexible graphite laminate may be constructed from a plurality of resin impregnated flexible graphite sheets that have been hot pressed and cured to form a substantially monolithic structure.

[0071] Illustrated in FIG. 4 is a cross sectional view of an embodiment of a thermal management device 60 and a heat source 70. The device 60 includes a shell formed from lower base 62 in contact with heat source 70. Preferably, base 62 is constructed of a conductive material, e.g., copper, aluminum, or alloys thereof. The shell further includes an upper element 66 comprises of flexible graphite. The upper element 66 and lower base 62 may be joined at interface 64 by any suitable technique such as the use of an adhesive like an epoxy. In certain preferred embodiments, device 60 includes a plurality of fins 32 extending from a top surface of element 66.

[0072] Device 60 may, optionally, also include one or more internal support elements 68. The internal support elements are not limited to any particular shape or any particular material of construction. Support elements 68 could be constructed from a conductive material such as cooper, aluminum, expanded graphite, or combinations thereof. In an alternate embodiment, support elements 68 may comprise one or more fins that extend down from upper element 66.

[0073] The invention also applies to other types of thermal management devices such as a vapor chamber. A vapor chamber is similar to a heat pipe many ways. A vapor chamber like a heat pipe uses the latent heat of vaporization of a working fluid to transfer heat from a heat source to a location that is colder than the heat source. In operation, the working fluid in the vapor chamber is vaporized at some location inside the vapor chamber and travels to a cooler location with in the vapor chamber and condenses at such cooler location.

[0074] In certain preferred embodiments, the vapor chamber comprises at least a shell similar to the shell a heat pipe. Under these circumstances, the shell of the vapor chamber comprises flexible graphite. Typically, the vapor chamber will also comprise a working fluid. The working fluid of the vapor chamber may be the same as the working fluid of the heat pipe described above. Optionally, the vapor chamber may include one of more internal supports. Preferably, the internal supports are made from some type of thermally conductive material such as flexible graphite, copper, aluminum, or combinations thereof. Optionally, at least a portion of the outer surface of the vapor chamber may include a plurality of fins, as described with respect to the heat pipe.

Flexible graphite thermal management devices

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