# Patent

## Title
Heat spreader for emissive display device

## Abstract

A heat spreader for an emissive display device, such as a plasma display panel or a light emitting diode, comprising at least one sheet of compressed particles of exfoliated graphite having a surface area greater than the surface area of that part of a discharge cell facing the back surface of the device.

## Background
RELATED APPLICATION
This application is a continuation-in-part of copending and commonly assigned U.S. patent application having Ser. No. 10/685,103, filed in the names of Norley, Smalc, Capp and Clovesko on Oct. 14, 2003, entitled “Heat Spreader for Plasma Display Panel,” the disclosure of which is incorporated herein by reference.


TECHNICAL FIELD
The present invention relates to a heat spreader useful for an emissive display device, such as a plasma display panel (PDP) or a light emitting diode (LED), and the unique thermal issues occasioned by these devices.
BACKGROUND OF THE ART
A plasma display panel is a display apparatus which contains a plurality of discharge cells, and is constructed to display an image by applying a voltage across electrodes discharge cells thereby causing the desired discharge cell to emit light. A panel unit, which is the main part of the plasma display panel, is fabricated by bonding two glass base plates together in such a manner as to sandwich a plurality of discharge cells between them.
In a plasma display panel, each of the discharge cells which are caused to emit light for image formation generate heat and each thus constitutes a source of heat, which causes the temperature of the plasma display panel as a whole to rise. The heat generated in the discharge cells is transferred to the glass forming the base plates, but heat conduction in directions parallel to the panel face is difficult because of the properties of the glass base plate material.
In addition, the temperature of a discharge cell which has been activated for light emission rises markedly, while the temperature of a discharge cell which has not been activated does not rise as much. Because of this, the panel face temperature of the plasma display panel rises locally in the areas where an image is being generated. In addition, a discharge cell activated in the white or lighter color spectra generate more heat than those activated in the black or darker color spectra. Thus, the temperature of the panel face differs locally depending on the colors generated in creating the image. These localized temperature differentials can accelerate thermal deterioration of affected discharge cells, unless measures are taken to ameliorate the differences. In addition, when the nature of the image on the display changes, the location for localized heat generation changes with the image.
Further, since the temperature difference between activated and nonactivated discharge cells can be high, and the temperature difference between discharge cells generating white light and those generating darker colors also can be high, a stress is applied to the panel unit, causing the conventional plasma display panel to be prone to cracks and breakage.
When the voltage to be applied to the electrodes of discharge cells is increased, the brightness of the discharge cells increases but the amount of heat generation in such cells also increases. Thus, those cells having large voltages for activation become more susceptible to thermal deterioration and tend to exacerbate the breakage problem of the panel unit of the plasma display panel. LEDs present similar issues with respect to heat generation as do PDPs.
The use of so-called “high orientation graphite film” as thermal interface materials for plasma display panels to fill the space between the back of the panel and a heat sinking unit and to even out local temperature differences is suggested by Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374, however, no mention of the use or distinct advantages of flexible graphite sheets is made. In addition, U.S. Pat. No. 6,482,520 to Tzeng discloses the use of sheets of compressed particles of exfoliated graphite as heat spreaders (referred to in the patent as thermal interfaces) for a heat source such as an electronic component. Indeed, such materials are commercially available from Advanced Energy Technology Inc. of Lakewood, Ohio as its eGraf® 700 class of materials.
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 such as thermal and electrical conductivity.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal conductivity due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from high compression, making it especially useful in heat spreading applications. Sheet material thus produced has excellent flexibility, good strength and a high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/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 compression of the sheet material to increase orientation. In compressed 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.
Thus, what is desired is a light-weight and cost-effective thermal spreader for emissive display devices. The desired spreader should be capable of balancing the temperature differences over the area of the device contacted by the spreader to thereby reduce thermal stresses to which the panel would otherwise be exposed, and to compensate for changes in hot spot locations.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a heat spreader for an emissive display device such as a plasma display panel or a light emitting diode.
Another object of the present invention is to provide a heat spreader material which can be used in an emissive display device to ameliorate the temperature differences which occur in the panel during use.
Yet another object of the present invention is to provide a heat spreader material to a heat source such as a plasma display panel so that the temperature difference between any two locations on the panel is reduced as compared to a panel without the inventive heat spreader.
Another object of the present invention is to provide a heat spreader material which can be applied to a heat source or collection of heat sources such as a plasma display panel or light emitting diode and adhere with good thermal contact between the heat spreader and the device.
Still another object of the present invention is to provide a heat spreader material which can produced in sufficient volume and in a cost effective manner.
These objects and others which will be apparent to the skilled artisan upon reading the following description, can be achieved by providing a heat spreader for an emissive display device, comprising at least one sheet of compressed particles of exfoliated graphite having a surface area greater than the surface area of that part of a discharge cell facing the back surface of the device. The emissive display device can be a plasma display panel or a light emitting diode panel. More preferably, the at least one sheet of compressed particles of exfoliated graphite has a surface area greater than the surface area of that part of a plurality of discharge cells facing the back surface of the device. Advantageously, the heat spreader is a laminate comprising a plurality of sheets of compressed particles of exfoliated graphite.
In a preferred embodiment, the heat spreader has an adhesive thereon and a release material positioned such that the adhesive is sandwiched between the heat spreader and the release material. The release material and adhesive are selected to permit a predetermined rate of release of the release material without causing undesirable damage to the heat spreader. Indeed, the adhesive and release material provide an average release load of no greater than about 40 grams per centimeter at a release speed of one meter per second, more preferably no greater than about 10 grams per centimeter at a release speed of one meter per second.
Additionally, the adhesive preferably achieves a minimum lap shear adhesion strength of at least about 125 grams per square centimeter, more preferably an average lap shear adhesion strength of at least about 700 grams per square centimeter. The adhesive should result in an increase in through thickness thermal resistance of the adhesive/heat spreader material of not more than about 35% as compared to the heat spreader material itself. The thickness of the adhesive should be no greater than about 0.015 millimeters (mm), more preferably no greater than about 0.006 mm.
It is to be understood that both the foregoing general description and the following detailed description provide embodiments of the invention and are intended to provide an overview or framework of understanding and nature and character of the invention as it is claimed. The accompanying drawing is included to provide a further understanding of the invention and is incorporated in and constitute a part of the specification. The drawing illustrates various embodiments of the invention and together with the description serve to describe the principles and operations of the invention.



BRIEF DESCRIPTION OF THE DRAWING
The FIGURE shows a system for the continuous production of resin-impregnated flexible graphite sheets.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Graphite is 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, where 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 expanded (otherwise referred to as 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.
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
  =
  
    
      3.45
      -
      
        d
        ⁡
        
          (
          002
          )
        
      
    
    0.095
  




where d(002) is the spacing between the graphitic layers of the carbons in the crystal structure measured in Angstrom units. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of diffraction peaks corresponding to the (002), (004) and (006) Miller Indices are measured, and standard least-squares techniques are employed to derive spacing which minimizes the total error for all of these peaks. Examples of highly graphitic carbonaceous materials include natural graphites from various sources, as well as other carbonaceous materials such as graphite prepared by chemical vapor deposition, high temperature pyrolysis of polymers, or crystallization from molten metal solutions, and the like. Natural graphite is most preferred.

The graphite starting materials for the flexible sheets 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 an ash content of less than twenty weight percent. More preferably, the graphite employed for the present invention will have a purity of at least about 94%. In the most preferred embodiment, the graphite employed will have a purity of at least about 98%.
A common method for manufacturing graphite sheet is described by Shane et al. in U.S. Pat. No. 3,404,061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al. method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing e.g., a mixture of nitric and sulfuric acid, advantageously at a level of about 20 to about 300 parts by weight of intercalant solution per 100 parts by weight of graphite flakes (pph). The intercalation solution contains oxidizing and other intercalating agents known in the art. Examples include those containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent soluble in the organic acid. Alternatively, an electric potential can be used to bring about oxidation of the graphite. Chemical species that can be introduced into the graphite crystal using electrolytic oxidation include sulfuric acid as well as other acids.
In a preferred embodiment, the intercalating agent is a solution of a mixture of sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or periodic acids, or the like. Although less preferred, the intercalation solution may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent.
The quantity of intercalation solution may range from about 20 to about 350 pph and more typically about 40 to about 160 pph. After the flakes are intercalated, any excess solution is drained from the flakes and the flakes are water-washed.
Alternatively, the quantity of the intercalation solution may be limited to between about 10 and about 40 pph, which permits the washing step to be eliminated as taught and described in U.S. Pat. No. 4,895,713, the disclosure of which is also herein incorporated by reference.
The particles of graphite flake treated with intercalation solution can optionally be contacted, e.g. by blending, with a reducing organic agent selected from alcohols, sugars, aldehydes and esters which are reactive with the surface film of oxidizing intercalating solution at temperatures in the range of 25° C. and 125° C. Suitable specific organic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol, decylalcohol, 1,10 decanediol, decylaldehyde, 1-propanol, 1,3 propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfate. The amount of organic reducing agent is suitably from about 0.5 to 4% by weight of the particles of graphite flake.
The use of an expansion aid applied prior to, during or immediately after intercalation can also provide improvements. Among these improvements can be reduced exfoliation temperature and increased expanded volume (also referred to as “worm volume”). An expansion aid in this context will advantageously be an organic material sufficiently soluble in the intercalation solution to achieve an improvement in expansion. More narrowly, organic materials of this type that contain carbon, hydrogen and oxygen, preferably exclusively, may be employed. Carboxylic acids have been found especially effective. A suitable carboxylic acid useful as the expansion aid can be selected from aromatic, aliphatic or cycloaliphatic, straight chain or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids which have at least 1 carbon atom, and preferably up to about 15 carbon atoms, which is soluble in the intercalation solution in amounts effective to provide a measurable improvement of one or more aspects of exfoliation. Suitable organic solvents can be employed to improve solubility of an organic expansion aid in the intercalation solution.
Representative examples of saturated aliphatic carboxylic acids are acids such as those of the formula H(CH2)nCOOH wherein n is a number of from 0 to about 5, including formic, acetic, propionic, butyric, pentanoic, hexanoic, and the like. In place of the carboxylic acids, the anhydrides or reactive carboxylic acid derivatives such as alkyl esters can also be employed. Representative of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid and other known aqueous intercalants have the ability to decompose formic acid, ultimately to water and carbon dioxide. Because of this, formic acid and other sensitive expansion aids are advantageously contacted with the graphite flake prior to immersion of the flake in aqueous intercalant. Representative of dicarboxylic acids are aliphatic dicarboxylic acids having 2–12 carbon atoms, in particular oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid, 1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid or terephthalic acid. Representative of alkyl esters are dimethyl oxylate and diethyl oxylate. Representative of cycloaliphatic acids is cyclohexane carboxylic acid and of aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- and p-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoic acids and, acetamidobenzoic acids, phenylacetic acid and naphthoic acids. Representative of hydroxy aromatic acids are hydroxybenzoic acid, 3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid, 4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid, 5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and 7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids is citric acid.
The intercalation solution will be aqueous and will preferably contain an amount of expansion aid of from about 1 to 10%, the amount being effective to enhance exfoliation. In the embodiment wherein the expansion aid is contacted with the graphite flake prior to or after immersing in the aqueous intercalation solution, the expansion aid can be admixed with the graphite by suitable means, such as a V-blender, typically in an amount of from about 0.2% to about 10% by weight of the graphite flake.
After intercalating the graphite flake, and following the blending of the intercalated graphite flake with the organic reducing agent, the blend can be exposed to temperatures in the range of 25° to 125° C. to promote reaction of the reducing agent and intercalated graphite flake. The heating period is up to about 20 hours, with shorter heating periods, e.g., at least about 10 minutes, for higher temperatures in the above-noted range. Times of one-half hour or less, e.g., on the order of 10 to 25 minutes, can be employed at the higher temperatures.
The above described methods for intercalating and exfoliating graphite flake may beneficially be augmented by a pretreatment of the graphite flake at graphitization temperatures, i.e. temperatures in the range of about 3000° C. and above and by the inclusion in the intercalant of a lubricious additive.
The pretreatment, or annealing, of the graphite flake results in significantly increased expansion (i.e., increase in expansion volume of up to 300% or greater) when the flake is subsequently subjected to intercalation and exfoliation. Indeed, desirably, the increase in expansion is at least about 50%, as compared to similar processing without the annealing step. The temperatures employed for the annealing step should not be significantly below 3000° C., because temperatures even 100° C. lower result in substantially reduced expansion.
The annealing of the present invention is performed for a period of time sufficient to result in a flake having an enhanced degree of expansion upon intercalation and subsequent exfoliation. Typically the time required will be 1 hour or more, preferably 1 to 3 hours and will most advantageously proceed in an inert environment. For maximum beneficial results, the annealed graphite flake will also be subjected to other processes known in the art to enhance the degree expansion—namely intercalation in the presence of an organic reducing agent, an intercalation aid such as an organic acid, and a surfactant wash following intercalation. Moreover, for maximum beneficial results, the intercalation step may be repeated.
The annealing step of the instant invention may be performed in an induction furnace or other such apparatus as is known and appreciated in the art of graphitization; for the temperatures here employed, which are in the range of 3000° C., are at the high end of the range encountered in graphitization processes.
Because it has been observed that the worms produced using graphite subjected to pre-intercalation annealing can sometimes “clump” together, which can negatively impact area weight uniformity, an additive that assists in the formation of “free flowing” worms is highly desirable. The addition of a lubricious additive to the intercalation solution facilitates the more uniform distribution of the worms across the bed of a compression apparatus (such as the bed of a calender station conventionally used for compressing (or “calendering”) graphite worms into flexible graphite sheet. The resulting sheet therefore has higher area weight uniformity and greater tensile strength, even when the starting graphite particles are smaller than conventionally used. The lubricious additive is preferably a long chain hydrocarbon. Other organic compounds having long chain hydrocarbon groups, even if other functional groups are present, can also be employed.
More preferably, the lubricious additive is an oil, with a mineral oil being most preferred, especially considering the fact that mineral oils are less prone to rancidity and odors, which can be an important consideration for long term storage. It will be noted that certain of the expansion aids detailed above also meet the definition of a lubricious additive. When these materials are used as the expansion aid, it may not be necessary to include a separate lubricious additive in the intercalant.
The lubricious additive is present in the intercalant in an amount of at least about 1.4 pph, more preferably at least about 1.8 pph. Although the upper limit of the inclusion of lubricous additive is not as critical as the lower limit, there does not appear to be any significant additional advantage to including the lubricious additive at a level of greater than about 4 pph.
The thus treated particles of graphite are sometimes referred to as “particles of intercalated graphite.” Upon exposure to high temperature, e.g. temperatures of at least about 160° C. and especially about 700° C. to 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 compression molded together into flexible sheets having small transverse openings that, unlike the original graphite flakes, can be formed and cut into various shapes, as hereinafter described.
Alternatively, the flexible graphite sheets of the present invention may utilize particles of reground flexible graphite sheets rather than freshly expanded worms. The sheets may be newly formed sheet material, recycled sheet material, scrap sheet material, or any other suitable source.
Also the processes of the present invention may use a blend of virgin materials and recycled materials.
The source material for recycled materials may be sheets or trimmed portions of sheets that have been compression molded as described above, or sheets that have been compressed with, for example, pre-calendering rolls. Furthermore, the source material may be sheets or trimmed portions of sheets that have been impregnated with resin, but not yet cured, or sheets or trimmed portions of sheets that have been impregnated with resin and cured. The source material may also be recycled flexible graphite PEM fuel cell components such as flow field plates or electrodes. Each of the various sources of graphite may be used as is or blended with natural graphite flakes.
Once the source material of flexible graphite sheets is available, it can then be comminuted by known processes or devices, such as a jet mill, air mill, blender, etc. to produce particles. Preferably, a majority of the particles have a diameter such that they will pass through 20 U.S. mesh; more preferably a major portion (greater than about 20%, most preferably greater than about 50%) will not pass through 80 U.S. mesh. Most preferably the particles have a particle size of no greater than about 20 mesh.
The size of the comminuted particles may be chosen so as to balance machinability and formability of the graphite article with the thermal characteristics desired. Thus, smaller particles will result in a graphite article which is easier to machine and/or form, whereas larger particles will result in a graphite article having higher anisotropy, and, therefore, greater in-plane electrical and thermal conductivity.
If the source material has been resin impregnated, then preferably the resin is removed from the particles. Details of the resin removal are further described below.
Once the source material is comminuted, and any resin is removed, it is then re-expanded. The re-expansion may occur by using the intercalation and exfoliation process described above and those described in U.S. Pat. No. 3,404,061 to Shane et al. and U.S. Pat. No. 4,895,713 to Greinke et al.
Typically, after intercalation the particles are exfoliated by heating the intercalated particles in a furnace. During this exfoliation step, intercalated natural graphite flakes may be added to the recycled intercalated particles. Preferably, during the re-expansion step the particles are expanded to have a specific volume in the range of at least about 100 cc/g and up to about 350 cc/g or greater. Finally, after the re-expansion step, the re-expanded particles may be compressed into flexible sheets, as hereinbefore described.
Flexible graphite sheet and foil are coherent, with good handling strength, and are suitably compressed by, e.g. compression molding, to a thickness of about 0.025 mm to 3.75 mm and a typical density of about 0.1 to 1.5 grams per cubic centimeter (g/cc). Although not always preferred, 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. When used, a 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 novolak phenolics.
With reference to the FIGURE, a system is disclosed for the continuous production of resin-impregnated flexible graphite sheet, where graphite flakes and a liquid intercalating agent are charged into reactor 104. More particularly, a vessel 101 is provided for containing a liquid intercalating agent. Vessel 101, suitably made of stainless steel, can be continually replenished with liquid intercalant by way of conduit 106. Vessel 102 contains graphite flakes that, together with intercalating agents from vessel 101, are introduced into reactor 104. The respective rates of input into reactor 104 of intercalating agent and graphite flake are controlled, such as by valves 108, 107. Graphite flake in vessel 102 can be continually replenished by way of conduit 109. Additives, such as intercalation enhancers, e.g., trace acids, and organic chemicals may be added by way of dispenser 110 that is metered at its output by valve 111.
The resulting intercalated graphite particles are soggy and acid coated and are conducted (such as via conduit 112) to a wash tank 114 where the particles are washed, advantageously with water which enters and exits wash tank 114 at 116, 118. The washed intercalated graphite flakes are then passed to drying chamber 122 such as through conduit 120. Additives such as buffers, antioxidants, pollution reducing chemicals can be added from vessel 119 to the flow of intercalated graphite flake for the purpose of modifying the surface chemistry of the exfoliate during expansion and use and modifying the gaseous emissions which cause the expansion.
The intercalated graphite flake is dried in dryer 122, preferably at temperatures of about 75° C. to about 150° C., generally avoiding any intumescence or expansion of the intercalated graphite flakes. After drying, the intercalated graphite flakes are fed as a stream into flame 200, by, for instance, being continually fed to collecting vessel 124 by way of conduit 126 and then fed as a stream into flame 200 in expansion vessel 128 as indicated at 2. Additives such as ceramic fiber particles 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 can be added from vessel 129 to the stream of intercalated graphite particles propelled by entrainment in a non-reactive gas introduced at 127.
The intercalated graphite particles 2, upon passage through flame 200 in expansion chamber 201, expand more than 80 times in the “c” direction and assume a “worm-like” expanded form 5; the additives introduced from 129 and blended with the stream of intercalated graphite particles are essentially unaffected by passage through the flame 200. The expanded graphite particles 5 may pass through a gravity separator 130, in which heavy ash natural mineral particles are separated from the expanded graphite particles, and then into a wide topped hopper 132. Separator 130 can be by-passed when not needed.
The expanded, i.e., exfoliated graphite particles 5 fall freely in hopper 132 together with any additives, and are randomly dispersed and passed into compression station 136, such as through trough 134. Compression station 136 comprises opposed, converging, moving porous belts 157, 158 spaced apart to receive the exfoliated, expanded graphite particles 5. Due to the decreasing space between opposed moving belts 157, 158, the exfoliated expanded graphite particles are compressed into a mat of flexible graphite, indicated at 148 having thickness of, e.g., from about 25.4 to 0.075 mm, especially from about 25.4 to 2.5 mm, and a density of from about 0.08 to 2.0 g/cm3. Gas scrubber 149 may be used to remove and clean gases emanating from the expansion chamber 201 and hopper 132.
The mat 148 is passed through vessel 150 and is impregnated with liquid resin from spray nozzles 138, the resin advantageously being “pulled through the mat” by means of vacuum chamber 139 and the resin is thereafter preferably dried in dryer 160 reducing the tack of the resin and the resin impregnated mat 143 is thereafter densified into roll pressed flexible graphite sheet 147 in calender mill 170. Gases and fumes from vessel 150 and dryer 160 are preferably collected and cleaned in scrubber 165.
After densification, the resin in flexible graphite sheet 147 is at least partially cured in curing oven 180. Alternatively, partial cure can be effected prior to densification, although post-densification cure is preferred.
In the preferred embodiment, however, the flexible graphite sheet is not resin-impregnated, in which case vessel, 150, dryer 160 and curing oven 180 can be eliminated.
Although this application is written in terms of the application of heat spreaders to plasma display panels, it will be recognized that the inventive method and heat spreader are equally applicable to other emissive display device heat sources, or heat source collections (equivalent in relevant function to the collection of individual discharge cells making up the plasma display panel) such as light emitting diodes.
Plasma display panels are now being produced at sizes of 1 meter and above (measured from corner to corner). Thus, heat spreaders used to cool and ameliorate the effects of hot spots on such panels are also required to be relatively large, on the order of about 270 millimeters×about 500 millimeters, or as large as about 800 millimeters×500 millimeters, or even larger. In a plasma display panel, as discussed above, hundreds of thousands of cells, each containing a plasma gas, are present. When a voltage is applied to each cell, the plasma gas then reacts with phosphors in each cell to produce colored light. Since significant power is required to ionize the gas to produce the plasma, the plasma display can become very hot. Moreover, depending on the color in a particular region of the panel, hot spots can be created on the screen which can result in premature breakdown of the phosphors which can shorten display life as well as cause thermal stresses on the panel itself. Therefore, a heat spreader is needed to reduce the effect of these hot spots.
Sheets of compressed particles of exfoliated graphite, especially laminates of sheets of compressed particles of exfoliated graphite, have been found particularly useful as heat spreaders for plasma display panels. More particularly, one or more sheets of compressed particles of exfoliated graphite, referred to herein as sheets of flexible graphite, are placed in thermal contact with the back of a plasma display panel, such that the flexible graphite sheet overlays a plurality of heat sources (i.e., discharge cells) in the panel. In other words, the surface area of the flexible graphite sheet is larger than the surface area of a discharge cell at the back of the plasma display panel; indeed, the surface area of the flexible graphite sheet is larger than the surface area of a plurality of discharge cells at the back of the plasma display panel. Moreover, because of the nature of the flexible graphite material from which the inventive heat spreader is formed, it will spread the heat from hot spots which may arise in different locations on the plasma display panel, as the image displayed by the panel changes.
Because of the nature of flexible graphite sheet materials, in that they are more conformable than other materials, even other forms of graphite, the contact resistance between the heat spreader and the plasma display panel is reduced and better thermal contact can be achieved than when using prior art heat spreaders applied with equivalent application pressures.
The flexible graphite sheet heat spreader of the present invention acts to reduce the heat difference (i.e., ΔT) between locations on the plasma display panel. In other words, the temperature difference between a hot spot on the panel, such as a location where a white image is created, and an adjacent location where a darker image is created, is reduced by the use of the inventive flexible graphite heat spreaders, as compared to the ΔT had the flexible graphite sheet not been present. Therefore, thermal stresses to which the plasma display panel would otherwise have been exposed are reduced, extending panel life and effectiveness. Moreover, since hot spots (i.e. thermal spikes) are reduced, the entire unit can be run at a higher temperature, with resulting image improvement.
In practice, it may be advantageous for the graphite heat spreaders to be produced with a layer of adhesive thereon to adhere the heat spreader to the plasma display panel, especially during the plasma display panel assembly process. A release liner must then be used to overlay the adhesive, with the adhesive sandwiched between the release liner and the graphite sheet, to permit storage and shipping of the graphite heat spreader prior to adhesion to the plasma display panel.
The use of an adhesive coated graphite sheet (or laminate of sheets) with a release liner has certain requirements which should be met if it is to be practical in a high volume plasma display panel manufacturing process. More particularly, the release liner must be capable of being removed from the sheet at high speed without causing delamination of the graphite. Delamination occurs when the release liner in effect pulls the adhesive and some of the graphite off the sheet as it is being removed, resulting in a loss of graphite, impairment of the graphite sheet itself, and diminution of adhesive needed to adhere the graphite sheet to the plasma display panel, as well as an unsightly and unfortunate appearance.
With that however, though the adhesive and release liner should be selected to permit release of the release liner from the adhesive/graphite sheet without delamination of the graphite, the adhesive must still be strong enough to maintain the graphite sheet in position on the plasma display panel while the panel assumes any of a variety of orientations and to ensure good thermal contact between the heat spreader(s) and the panel.
In addition, significant diminution of the thermal performance of the heat spreader must not be caused by the adhesive. In other words, an adhesive applied in a layer that is of substantial thickness can interfere with the thermal performance of the heat spreader, since the adhesive would interfere with the conduction of heat from the plasma display panel to the heat spreader.
Thus, the adhesive and release liner combination must achieve a balance such that they provide a release load no greater than about 40 g/cm, more preferably about 20 g/cm and most preferably about 10 g/cm, at a release speed of about 1 m/s, as measured, for instance, on a ChemInstruments HSR-1000 high speed release tester. For example, if it is desired to remove the release liner at a speed of about 1 m/s in order to match the high volume manufacturing requirements of the plasma display panel, the average release load of the release liner should be no greater than about 40 g/cm, more advantageously, about 20 g/cm, and most advantageously about 10 g/cm, in order to permit removal of the release liner without causing graphite delamination at that release speed. To achieve this, the adhesive should most preferably be no greater than about 0.006 mm in thickness.
Another factor to be balanced is the adhesion strength of the adhesive which as noted above, should be sufficient to maintain the heat spreader in position on the plasma display panel during the plasma display panel manufacturing process and to ensure good thermal contact between the heat spreader and the plasma display panel. In order to achieve the required adhesion, the adhesive should have a minimum lap shear adhesion strength of at least about 125 g/cm2, more preferably an average lap shear adhesion strength of at least about 700 g/cm2, as measured, for instance, on a ChemInstruments TT-1000 tensile tester.
With all that, as noted above, the adhesive should not substantially interfere with the thermal performance of the heat spreader. By this is meant, the presence of the adhesive should not result in an increase in the through-thickness thermal resistance of the heat spreader of more than about 100% as compared to the heat spreader material itself, without adhesive. Indeed, in the more preferred embodiment, the adhesive will not lead to an increase in the thermal resistance of more than about 35% as compared to the heat spreader material without adhesive. Thus, the adhesive must meet the release load requirements and average lap shear adhesion strength requirement while being thin enough to avoid an undesirably high increase in thermal resistance. In order to do so, the adhesive should be no thicker than about 0.015 mm, more preferably no thicker than about 0.006 mm.
In order to achieve the balancing described above needed for the production of a heat spreader useful for being applied to a plasma display panel in a high volume manufacturing process, where the heat spreader is a sheet or laminate of sheets of compressed particles of exfoliated graphite having a thickness no greater than about 2.0 mm and a density between about 1.6 and about 1.9 grams per cubic centimeter, an Aroset 3300 pressure sensitive acrylic adhesive commercially available from Ashland Chemical in the desired thickness combined with a release liner made of silicone-coated Kraft paper such as an L2 release liner commercially available from Sil Tech, a division of Technicote Inc., can achieve the desired results. Thus, a heat spreader composite is provided which comprises a heat spreader material such as a sheet or laminate of sheets of compressed particles of exfoliated graphite, having an adhesive thereon in a thickness such that the thermal performance of the heat spreader material is not substantially compromised, with a release layer positioned such that the adhesive is sandwiched between the heat spreader material and the release material. In operation then, the release material can be removed from the heat spreader/adhesive combination and the heat spreader material/adhesive combination then applied to a plasma display panel such that the adhesive adheres the heat spreader material to the plasma display panel. Furthermore, when a plurality of plasma display panels is being produced, the at least one of the heat spreader/adhesive combinations is applied to each of the plurality of plasma display panels.
When a flexible graphite laminate is employed as the inventive heat spreader, other laminate layers can also be included, to improve the mechanical or thermal properties of the laminate. For instance, a laminate layer of a thermally conductive metal like aluminum or copper can be interposed between layers of flexible graphite in order to increase the thermal spreading characteristics of the laminate without sacrificing the low contact resistance exhibited by graphite; other materials, such as polymers, can also be employed to reinforce or otherwise improve the strength of the laminate. In addition, the graphite material, whether a single sheet or a laminate, can be provided with a backing layer of, for instance, a thin plastic sheet or, in the alternative, a thin coating of dried resin, to improve handling of the material and/or reduce damage to the sheet during shipment or application to the panel, without compromising the thermal spreading capabilities of the inventive heat spreader. A layer of an insulating material can also be employed.
All cited patents and publications referred to in this application are incorporated by reference.
The invention thus being described, it will obvious that it may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the present invention and all such modifications as would be obvious to one skilled in the art are intended to be included in the scope of the following claims.

## Claims

1. An emissive display device having a plurality of discharge cells, the emissive display device comprising a heat spreader having two major surfaces, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite and an adhesive, wherein substantially all of one of the major surfaces of the heat spreader is in direct thermal contact with the emissive display device such that it overlays a plurality of the discharge cells and the adhesive adheres the heat spreader to the emissive display device sufficiently to maintain the heat spreader in position regardless of the orientation of the emissive display device.

2. The emissive display device of claim 1 which comprises a plasma display panel.

3. The emissive display device of claim 1 wherein the heat spreader comprises a laminate comprising a plurality of sheets of compressed particles of exfoliated graphite.

4. The emissive display device of claim 1 wherein the heat spreader comprises at least one layer of a non-graphitic material.

5. The emissive display device of claim 4 wherein the non-graphitic layer comprises a metal, a polymer or an insulating material.

6. The emissive display device of claim 1 wherein the adhesive achieves a minimum lap shear adhesion strength of at least about 125 grams per square centimeter.

7. The emissive display device of claim 1 wherein the adhesive results in an increase in through thickness thermal resistance of the heat spreader of not more than about 35% as compared to the heat spreader without adhesive.

8. The emissive display device of claim 1 which comprises light emitting diodes.