HK1091641B - Heat spreader for display device - Google Patents

Description

Radiator for display

Description of the invention

We are: national Timothy Clovesko, address: 5250 Columbia Road, Apt.407, North Omsted, Ohio 44070; american citizen Julian Norley, address: 17635 Plum Creek Trail, Chagrin Falls, Ohio 44023; martin Dvid Smalc, the national citizen: 5608 Ely Vista Drive, Parma, Ohio 44129; and american citizen Joseph paulcap, address: 10094 Juniper Court, Strongsville, Ohio 44136. We invented a new and useful "heat sink for displays".

Technical Field

The present invention relates to a heat sink suitable for use in displays, such as Plasma Display Panels (PDPs), Liquid Crystal Displays (LCDs), Light Emitting Diodes (LEDs), etc., and to the particular thermal problems created by these devices.

Technical Field

A plasma display panel is a display device having a plurality of discharge cells, and is configured to display an image by applying a voltage to electrode discharge cells to cause predetermined discharge cells to emit light. The panel unit, which is a main component of the plasma display panel, is manufactured by bonding two glass substrates sandwiching a plurality of discharge cells therebetween.

In the plasma display panel, each of the discharge cells, which is caused to emit light to form an image, generates heat, and thus each of them constitutes a heat source, so that the temperature of the entire plasma display panel is increased. Heat generated in the discharge cells is transferred to glass constituting the substrate, but is not easily conducted in a direction parallel to the panel due to the material property of the glass substrate.

In addition, the temperature of the discharge cells that have been activated to emit light increases significantly, while the temperature of the discharge cells that have not been activated does not increase much. Thus, the panel temperature of the plasma display panel is locally raised in the area where the image is formed. Moreover, discharge cells activated in the white or lighter color spectrum generate more heat than those activated in the black or darker color spectrum. Therefore, there is a local difference in the temperature of the screen depending on the color generated when forming an image. Unless measures are taken to improve these differences, these local temperature differences accelerate the thermal degradation process of the discharge cell affected by them. Further, when the image characteristics on the display change, the position of local heat generation also changes with the image.

Moreover, since the temperature difference between the activated and inactivated discharge cells is large and the temperature difference between the discharge cells emitting white light and the discharge cells emitting darker light is also large, the stress applied to the panel apparatus makes the conventional plasma display panel susceptible to cracks and breakage.

When the voltage applied to the electrodes of the discharge cells increases, the brightness of the discharge cells increases, but the amount of heat generated in the cells also increases. Therefore, those cells having a large activation voltage are more likely to be thermally deteriorated, and the problem of breakage of the panel cells of the plasma display panel tends to be exacerbated. LEDs have similar heating problems as PDPs. In addition to emissive displays, similar problems exist with other displays, such as LCDs, where hot spots can limit the effectiveness or lifetime of the device.

U.S. patent No.5831374 to Morita, Ichiyanagi, Ikeda, nishikhiki, Inoue, Komyoji and Kawashima suggests that a so-called "highly oriented graphite film" is used as a thermal interface material for a plasma display panel, filled between the rear surface of the panel and a heat dissipating unit, to reduce local temperature difference, however, there is no mention of the use of a flexible graphite sheet layer or its significant advantages. Furthermore, U.S. patent No.6482520 to Tzeng discloses the use of sheets of compressed particles of exfoliated graphite as heat spreaders (referred to in this patent as thermal interfaces) for heat sources such as electronic components. Indeed, such materials are available from Advanced Energy Technology Inc. of Lakewood, Ohio, for example, asSpreaderShield grade material.

Graphites are made up of layered planes of hexagonal arrays or networks of carbon atoms. These layered planes of hexagonally arranged carbon atoms are generally flat and are oriented or arranged in a substantially parallel and equidistant fashion from one another. Such substantially flat, parallel and equidistant sheets or layers of carbon atoms are commonly referred to as graphitic layers (graphienelayers) or basal planes (basal planes) which are linked or bonded together and are arranged in groups in the form of crystallites. Highly ordered graphites are made up of relatively large crystallites that are highly aligned or oriented with respect to each other and have layers of well-ordered carbon atoms. In other words, highly ordered graphites have a high degree of preferred crystal orientation. It should be noted that graphite has an anisotropic structure and thus exhibits or has many highly directional characteristics, such as in thermal and electrical conductivity.

Briefly, graphite is characterized by a layered structure of carbon atoms, i.e., a structure consisting of stacked or thin layers of carbon atoms joined together by weak van der waals forces. In view of this configuration of graphite, two axial directions or directions are generally mentioned, namely the "c" axis or direction and the "a" axis or direction. For simplicity, the "c" axis or direction may be considered to be the direction perpendicular to the carbon atom layers. The "a" axis or direction can be considered to be a direction parallel to the carbon atom layer or a direction perpendicular to the "c" direction. The graphites suitable for use in making flexible graphite sheets possess a very high degree of orientation.

As mentioned above, the bonding forces holding parallel layers of carbon atoms together are only weak van der waals forces. Natural graphites can be treated so that the spaces between the superposed or laminae of carbon atoms are appreciably opened up so as to have a marked expansion in the direction perpendicular to the plane of the layer, that is, in the "c" direction, and so as to form an enlarged or expanded graphite structure and to preserve substantially the laminar character of the layer of carbon atoms.

Graphite flake which is greatly expanded, particularly to a desired thickness or which has a dimension in the "c" direction which is about 80 or more times greater than the original dimension in the "c" direction, can be made without the use of a binder to form a coherent or integrated layer of expanded graphite, such as a network, paper, strip, tape, foil, shim, or the like (commonly referred to as "flexible graphite"). Without the use of any binding material, it is believed possible to produce integrated flexible sheets from compression of graphite particles that expand to a desired thickness or "c" direction dimension that is about 80 or more times greater than the original "c" direction dimension because of the sufficient mechanical connectivity or cohesion achieved between the voluminously expanded graphite particles.

In addition to flexibility, the above-described sheet materials have also been found to have a high degree of anisotropy with respect to thermal conduction due to the orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet as a result of high compression, making them particularly useful in heat dissipation applications. The resulting sheet material has excellent flexibility, high strength and high degree of orientation.

Briefly, the process of making flexible, binderless and anisotropic graphite sheet material, e.g. a 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 having a "c" direction dimension which is about 80 or more times greater than the original "c" direction dimension to form a substantially flat, flexible, integrated graphite sheet. Generally, the expanded graphite particles, which are worm-like or vermiform in appearance, once compressed, will remain compressively deformed and aligned 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 is in the range of about 0.04g/cc to 2.0 g/cc.

The flexible graphite sheet material exhibits a significant degree of anisotropy due to the alignment properties of the graphite particles parallel to the major opposed parallel surfaces of the sheet, and the degree of anisotropy increases upon compression of the sheet material to increase its orientation. In the compressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed parallel sheet surfaces, constitutes the "c" direction, while the directions along the length and width, i.e. the directions along or parallel to said opposed major surfaces, constitute the "a" direction, for which the thermal and electrical properties of the sheet differ by a very large order of magnitude.

However, there is a major concern in the electronics industry that the use of graphite-based materials can lead to exfoliation of the graphite particles, which as a result can mechanically (i.e., in the same manner as dust) affect the operation and performance of the device, and more importantly, can cause electrical interference with the operation of the emissive display due to the electrical conductivity of the graphite. While these concerns are believed and suggested to be unnecessary, they still exist.

Also, the use of adhesives to attach graphite heat sinks to emissive displays can sometimes be disadvantageous. More specifically, in situations where rework (i.e., removal and replacement of the heat spreader) is required, the adhesive bond is stronger than the structural integrity of the graphite sheets, so that the graphite sheets cannot always be cleanly removed from the panel without the use of a doctor blade or other similar tool, which is time consuming and may damage the graphite sheets, the panels, or both.

Thus, there is a need for a light weight and cost effective heat sink for an emissive display, and in particular a heat sink that is compartmentalized to prevent graphite particles from flaking off and that can be effectively removed from the device when needed. The heat sink should be able to balance the temperature differential in the area of the device that the heat sink is exposed to, thereby reducing thermal stresses to which the screen is otherwise subjected, and also to act to reduce hot spots, even in those locations where hot spots are not fixed.

Brief description of the invention

It is therefore an object of the present invention to provide a heat sink for a display, such as a plasma display panel, a light emitting diode, or a liquid crystal display.

It is another object of the present invention to provide a heat sink material that can be used in a display to improve the temperature differences that occur during its use.

It is yet another object of the present invention to provide a heat sink material for one or more units of a heat source, such as a plasma display panel. So that the temperature difference between any two locations on the panel is smaller than if the inventive heat sink were not present on the panel.

It is another object of the present invention to provide a heat spreader material that can be applied to a heat source or a collection of heat sources, such as a plasma display panel or a light emitting diode, and that has good thermal contact adhesion with the device.

It is yet another object of the present invention to provide a heat spreader material that can be isolated to prevent or reduce the likelihood of graphite particles flaking off.

It is yet another object of the present invention to provide a heat sink material that can be adhered to and removed from a heat source with minimal damage to the heat sink or heat source.

Another object of the invention is to provide a heat sink which can be produced in a sufficiently large scale and cost-effective manner.

These and other objects, which will become apparent to those skilled in the art upon reading the following description, are achieved by providing a heat spreader for a display, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite (exfoliated graphite) having a surface area greater than the surface area of the components of the device, such as the discharge cells, facing the back of the device. The display may be an emissive display, such as a plasma display panel or a light emitting diode panel, or other types of displays, such as a liquid crystal display. More preferably, the surface area of the at least one sheet of compressed particles of exfoliated graphite is greater than the surface area of the portion of the plurality of discharge cells facing the back of the device. Advantageously, the heat spreader is a thin layer structure comprising a plurality of sheets of compressed particles of exfoliated graphite with a protective layer thereon to prevent the graphite particles from flaking off therefrom. In a preferred embodiment, the surface of the heat spreader has a facing layer (facing), such as aluminum or copper, on top to further seal the heat spreader and facilitate the rework process.

In a preferred embodiment, the heat spreader has an adhesive thereon and is provided with a release material such that the adhesive is sandwiched between the heat spreader and the release material. The removal material and adhesive are selected such that the removal material does not cause undesirable damage to the heat spreader when removed at a predetermined rate. In practice, the adhesive and release material will have an average release load (release load) of no greater than about 40 grams per centimeter at a removal rate of one meter per second, and more preferably no greater than about 10 grams per centimeter at a removal rate of 1 meter per second.

In addition, the adhesive preferably achieves a minimum overlap bond shear strength (lap shear adhesion strength) of at least about 125 grams per square centimeter, and more preferably an average overlap bond shear strength of at least about 700 grams per square centimeter. The adhesive should raise the through-thickness thermal resistance of the adhesive/heat spreader material by no more than about 35% compared to the heat spreader material itself. The thickness of the adhesive should be no greater than about 0.015 millimeters (mm), and more preferably no greater than about 0.005 mm.

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 and nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to describe the principles and operations of the invention.

Brief Description of Drawings

FIG. 1 is a partially broken away top perspective view of an embodiment of the inventive heat sink.

Fig. 2 is a partially broken away top perspective view of another embodiment of the inventive heat sink.

Fig. 3 is a side view of another embodiment of the inventive heat sink.

Figure 4 illustrates a system for continuously manufacturing resin-impregnated flexible graphite sheet layers.

Detailed description of the preferred embodiments

Graphite is a crystalline form of carbon atoms, consisting of covalently bonded atoms in flat layered planes with weaker bonding forces between the planes. In obtaining a raw material, such as the flexible graphite sheet described above, the graphite particles, such as natural graphite flakes, are typically treated with an intercalant of, for example, a solution of sulfuric and nitric acids, wherein the crystal structure of the graphite reacts to form a compound of graphite and the intercalant. The treated graphite particles are hereinafter referred to as "intercalated graphite particles". Upon exposure to high temperatures, the intercalant within the graphite decomposes and volatilizes away, causing the particles of intercalated graphite to expand in volume up to about 80 times or more in accordion-like fashion in the "c" direction, i.e., the direction perpendicular to the crystal planes of the graphite, than their original volume. The expanded (also known 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 or cut into various shapes and have very small transverse openings by deforming mechanical impact forces.

Graphite starting materials suitable for use in the flexible sheets of the present invention include highly graphitic carbon materials capable of intercalating organic and inorganic acids as well as halogens and then expanding with heat. These highly graphitic carbon materials most preferably have a degree of graphitization of about 1.0. The term "degree of graphitization" as used in the description of the present disclosure relates to the g value obtained according to the following formula:

where d (002) is the spacing between carbon atom graphite layers in the crystal structure, measured in angstroms. The spacing d between graphite layers can be measured by standard X-ray diffraction techniques. The diffraction peak positions corresponding to the Miller indices of (002), (004), and (006) were measured and the spacing was obtained using standard least squares, which minimizes the total error for all of these peaks. Examples of the high graphitic carbon material include natural graphite derived from various raw materials, and other carbon materials such as graphite prepared by chemical vapor deposition, pyrolysis of polymers, or melt crystallization of molten metals, and the like. Natural graphite is most preferred.

The graphite starting materials for the flexible sheets of the present invention may contain non-graphite components so long as the crystal structure of the starting materials maintains the desired degree of graphitization and they are capable of exfoliation. Generally, any carbon-containing material whose crystal structure has the desired degree of graphitization and which is capable of delamination is suitable for use in the present invention. Such graphite preferably has an ash content of less than twenty weight percent. More preferably, the graphite employed in the present invention has a purity of at least about 94%. In the most preferred embodiment, the graphite employed has a purity of at least about 98%.

One conventional method of making graphite sheets is described by Shane et al in U.S. patent No.3404061, the disclosure of which is incorporated herein by reference. In the typical practice of the Shane et al process, flakes of natural graphite are intercalated by dispersing the flakes in a solution containing a mixture of, for example, nitric and sulfuric acids, preferably at an intercalant solution level of about 20 to about 300 parts by weight per 100 parts by weight of graphite flake (pph). The intercalant solution contains an oxidizing agent and other intercalants as is well known in the art. Examples include solutions containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, potassium perchlorate, and the like, or mixtures, such as concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or mixtures of a strong organic acid, such as trifluoroacetic acid, with a strong oxidizing agent dissolved in the organic acid. Alternatively, the oxidation process of graphite can be carried out using an electrical potential. Chemical species that can be introduced into the graphite crystal using an electrolytic oxidation process include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is 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 not preferred, the intercalation solution may contain metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or halides such as bromine and bromine in sulfuric acid solution and bromine in an organic solvent.

The amount of intercalation solution may range from about 20 to about 350pph, more typically from about 40 to about 160 pph. After intercalation of the flakes, any excess solution is drained from the flakes and the flakes are washed with water.

Alternatively, the amount of intercalation solution may be limited to between about 10 and about 40pph, which may allow the elimination of a washing step, as taught and disclosed in U.S. Pat. No.4895713, the disclosure of which is also incorporated herein by reference.

The particles of graphite flake treated with the intercalation solution can be selectively contacted, e.g., by mixing, with a reducing organic agent selected from the group consisting of alcohols, sugars, aldehydes, and esters, which react with the surface film of the oxidizing intercalation solution at temperatures in the range of 25 c to 125 c. Suitable specific organic agents include cetyl alcohol, stearyl alcohol, 1-octanol, 2-octanol, decyl alcohol, 1, 10-decanediol, decanal, 1-propanol, 1, 3-propanediol, ethylene glycol, polypropylene glycol, glucose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethylglyoxylate, diethylglyoxylate, methyl formate, ethyl formate, ascorbic acid, and lignin-derived compounds, such as lignin sulfate. Suitable amounts of organic reducing agent are about 0.5 to 4% by weight of the graphite flake particles.

The use of an expansion aid before, during or immediately after intercalation can also provide improvements. In these improvements, the delamination temperature can be reduced and the expansion volume (also known as the "worm volume") increased. The expansion aid herein is preferably an organic material that is sufficiently soluble in the intercalation solution to achieve improved expansion properties. Further, such organic materials containing carbon, hydrogen and oxygen, preferably only oxyhydrogen, may be used. We have found that carboxylic acids are particularly effective. Suitable carboxylic acids for use as expansion aids may be selected from aromatic, aliphatic or cycloaliphatic, straight or branched chain, saturated and unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids having at least 1 carbon atom, preferably up to about 15 carbon atoms, which are soluble in the intercalation solvent in an amount effective to provide a measurable improvement in one or more aspects of the delamination process. Suitable organic solvents may be employed to improve the solubility of the organic expansion aid in the intercalation solution.

Representative examples of saturated aliphatic carboxylic acids are, for example, those of the formula H (CH)₂)_nThose acids of COOH wherein n is a number from 0 to about 5 include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, and the like. In place of the carboxylic acids, it is also possible to use anhydrides or reactive carboxylic acid derivatives, such as alkyl esters. Representative of alkyl esters are methyl formate and ethyl formate. Sulfuric acid, nitric acid, and other known aqueous intercalants are capable of decomposing formic acid, ultimately forming water and carbon dioxide. Thus, formic acid and other sensitive expansion aids are preferably contacted with the graphite flake prior to immersion of the flake in the aqueous intercalant. Representative dicarboxylic acids are aliphatic dicarboxylic acids having 2 to 12 carbon atoms, especially oxalic acid, fumaric acid, malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid, 1, 5-glutaric acid, 1, 6-adipic acid, 1, 10-sebacic acid, 1, 4-cyclohexanedicarboxylic acid and aromatic dicarboxylic acidsDicarboxylic acids, such as phthalic acid or terephthalic acid. Representative alkyl esters are dimethylglyoxylate (dimethylglyoxylate) and diethylglyoxylate (diethyloxylate). Representative of the cycloaliphatic acids are cyclohexane carboxylic acids, the aromatic carboxylic acids of which are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m-and p-tolyl acids (tolylacids), methoxy and ethoxy benzoic acids, acetoacetanilic acids and, acetamiaminobenzoic acids, phenylacetic acids and naphthoic acids. Representative hydroxyaromatic 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. Among these polycarboxylic acids, citric acid is most preferred.

The intercalation solution is aqueous and preferably contains an amount of expansion aid of from about 1 to about 10%, the amount being effective to enhance delamination. In embodiments where the expansion aid is contacted with the graphite flake prior to or after immersion 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.

Following intercalation of the graphite flake, the intercalated graphite flake is then mixed with an organic reducing agent and the mixture is exposed to a temperature change in the range of 25 ℃ to 125 ℃ to promote reaction of the reducing agent with the intercalated graphite flake. The heating time may be up to about 20 hours, with shorter heating times, e.g., at least about 10 minutes, and higher temperatures within the above range. Times of one and a half hour or less, for example, about 10 to 25 minutes, may be employed at higher temperatures.

The above-described methods of intercalating and delaminating graphite flakes may be advantageously enhanced by pretreating the graphite flakes at graphitization temperatures, i.e., temperatures of about 3000 c and above, and adding a lubricious additive to the intercalant.

Pretreatment or annealing of graphite flake prior to intercalation and delamination processes can significantly improve expansion (i.e., increase expansion volume by up to 300% and above). In fact, it is desirable that the expansion be increased by at least about 50% as compared to a similar treatment without the annealing step. The temperature used in the annealing step should not be significantly below 3000 c, since a reduction in temperature, even 100c, will result in a considerable reduction in expansion.

The annealing process of the present invention is for a time sufficient to allow the flakes to have a high degree of expansion during intercalation and subsequent delamination. Typically, the time required is 1 hour or more, preferably 1 to 3 hours, and is best carried out in an inert environment. For optimum effect, the annealed graphite flakes may also be treated to enhance the degree of expansion by other methods known in the art-i.e., the intercalation reaction is carried out in the presence of an organic reducing agent, an intercalation aid such as an organic acid, and a surfactant to wash the subsequent intercalation. Moreover, the intercalation step may be repeated for optimum effect.

The annealing step of the present invention may be carried out in an induction furnace or other such equipment known and commonly used in the graphite art; the temperature used here is in the range of 3000 c, which is the upper limit encountered during the graphitization treatment.

Since it has been found that worms (worm) produced by pre-intercalation annealing sometimes "cluster" together, which can adversely affect areal weight uniformity, there is a great need for an additive that can help form "free-flowing" worms. The addition of a lubricious additive to the intercalation solution helps to distribute the worms more evenly on the lathe of a compression device, such as a rolling platform bed that is commonly used to compress (or "roll") graphite worms into flexible graphite sheets. Even when the original graphite particles are smaller than conventionally used, the resulting sheet has a higher area weight uniformity and a greater tensile strength. 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, most preferably a mineral oil, especially considering that mineral oils are less susceptible to spoilage and odor development, which is an important consideration for long term storage. It is noted that some of the expansion aids detailed above also meet the requirements of the lubricant additives. When these materials are used as expansion aids, it may not be necessary to include a separate lubricious additive in the intercalant.

The amount of lubricious additive present in the intercalant is at least about 1.4pph, more preferably at least about 1.8 pph. Although the upper limit of the lubricant additive is not as important as the lower limit, the inclusion of the lubricant additive does not appear to have any more significant advantage at levels above about 4 pph.

The graphite particles so treated are sometimes referred to as "intercalated graphite particles". Upon exposure to elevated temperatures, for example, at least about 160 c, and particularly from about 700 c to 1000 c or higher, the intercalated graphite particles expand in accordion-like form up to about 80 to 1000 or more times their original volume in the c-direction, i.e., the direction perpendicular to the crystal planes of the constituent graphite particles. The expanded, i.e., exfoliated, graphite particles appear as worms and are therefore commonly referred to as worms. The worms may be stamped together into flexible sheets having small transverse openings that, unlike the original graphite flakes, can be formed and cut into various shapes, as will be described in more detail below.

Alternatively, the flexible graphite sheets of the present invention may utilize reground flexible graphite sheets rather than just expanded worms. The sheet may be newly formed sheet material, recycled sheet material, shredded sheet material, or any other suitable raw material.

The process of the present invention may also employ a mixture of virgin and recycled materials, or all recycled materials.

The feedstock for the recycled material may be a sheet or scrap portion of a sheet that has been subjected to the compression molding process described above, or a sheet that has been compressed, for example, by a roller. Furthermore, the starting material may be a resin impregnated but not yet cured sheet or scrap of sheets, or a resin impregnated and cured sheet or scrap of sheets. The feedstock 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 mixed with natural graphite flakes.

Once the raw material of flexible graphite sheet is available, it can be comminuted by known methods or devices, such as jet mills, air mills, blenders, and the like, to form particles. Preferably, the majority of the particles have a diameter such that they pass through a20 u.s. mesh; more preferably, a substantial portion (greater than about 20%, most preferably greater than about 50%) cannot pass through an 80u.s. mesh. Most preferably, the particles have a particle size of no greater than about 20 mesh.

The comminuted particle size can be selected such that the processability and formability of the graphite article can be balanced with the desired thermal properties. Thus, smaller particles result in graphite articles that are easier to process and/or shape, while larger particles result in graphite articles that have higher anisotropy, and therefore higher in-plane electrical and thermal conductivity.

Once the raw materials are comminuted, any resin is removed if necessary, and then allowed to re-expand. The re-expansion process may be accomplished using the intercalation and delamination methods described above, which are described in U.S. patent No.3404061 to Shane et al and U.S. patent No.4895713 to grelnke et al.

Typically, after intercalation, the particles are delaminated by heating the intercalated particles in a furnace. In this delamination step, intercalated natural graphite flakes may be added to the recovered intercalated particles. Preferably, during the re-expansion step, the particles expand to a specific volume in the range of at least about 100cc/g up to 350cc/g or even greater. Finally, after the re-expansion step, the re-expanded particles may be compressed into flexible sheets, as described later.

The flexible graphite sheet and foil are bonded together, have good processing strength, and are suitable for compression to a thickness of about 0.025mm to 3.75mm by methods such as compression molding, and have a typical density of about 0.1 to 1.5 grams per cubic centimeter (g/cc). Although not always preferred, the flexible graphite sheet may also sometimes be advantageously treated with a resin, the absorbed resin, after curing, increasing 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, suitable resin content is preferably at least about 5% by weight, more preferably from about 10 to 35% by weight, and desirably up to about 60% by weight. Resins found particularly 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 may be used include resole and novolak phenolic resins.

Referring to fig. 4, a system for the continuous production of resin-impregnated flexible graphite sheet is disclosed in which graphite flakes and a liquid intercalant are loaded into reactor 104. More specifically, vessel 101 is used to contain a liquid intercalant. Vessel 101, suitably made of stainless steel, may be constantly replenished with liquid intercalant via line 106. Vessel 102 contains graphite flake which is introduced into reactor 104 with the intercalant from vessel 101. The respective rates at which the intercalant and graphite flake are fed into the reactor 104 may be controlled, for example, by valves 108, 107. The graphite flakes in the container 102 may be continuously replenished through the conduit 109. Additives, such as intercalation enhancers, e.g., trace acids, and organic chemicals may be added through dispenser 110 and metered at its output through valve 111.

The resulting intercalated graphite particles, which are moist and acid coated, are introduced (e.g., via conduit 112) into a rinse tank 114 to rinse the particles, preferably with water entering and exiting rinse tank 114 at 116,118. The washed intercalated graphite flakes are then passed through a drying chamber 122, such as through line 120. Additives such as buffers, antioxidants, contaminant reducing chemicals may be added to the flow of intercalated graphite flakes through vessel 119 to modify the surface chemistry of the exfoliation during expansion and use and to improve the release of gases that cause expansion.

The intercalated graphite flakes are dried in the dryer 122, preferably at a temperature of about 75 c to about 150 c, essentially to avoid any expansion or distension of the intercalated graphite flakes. After drying, the intercalated graphite flake is injected into flame 200 as a stream of fluid, for example, continuously into collection vessel 124 through conduit 126, and then injected as a stream of fluid into flame 200 in expansion vessel 128, as shown at 2. Additives such as ceramic fiber particles comprising macerated quartz glass fibers, carbon and graphite fibers, zirconium, boron nitride, silicon carbide and magnesium oxide fibers, naturally occurring mineral fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide fibers, and the like, may be added from vessel 129 to the stream of intercalated graphite particles advancing entrained in the non-reactive gas introduced at 127.

After passing through the flame 200 in the expansion chamber 201, the intercalated graphite particles 2 expand more than 80 times in the "c" direction, forming an expanded form 5 which is "vermicular"; the additive introduced from 129 and mixed with the particles of intercalated graphite is not substantially affected when passing through the flame 200. The expanded graphite particles 5 can be passed through a gravity separator 130 in which the heavy ash natural mineral particles are separated from the expanded graphite particles and then fed into a wide-top hopper 132. The separator 130 may be bypassed when not needed.

The expanded, i.e., exfoliated, graphite particles 5, along with any additives, fall freely into hopper 132, randomly dispersed and enter compression station 136, such as through 134. The compression platform 136 includes opposed converging, spaced apart moving porous belts 157, 158 for receiving the exfoliated expanded graphite particles 5. Due to the decreasing spacing between the opposed moving belts 157, 158, the exfoliated expanded graphite particles are compressed into a flexible graphite mat, as shown at 148, having a thickness of, for example, about 25.4 to 0.075mm, especially about 25.4 to 2.5mm, and a density of about 0.08 to 2.0g/cm³. Gas (es)The scrubber 149 may be used to remove and clean the gases emanating from the expansion chamber 201 and the hopper 132.

Pad 148 is passed through a vessel 150 and is impregnated with liquid resin sprayed from spray nozzle 138, the resin conveniently being "pulled through" the pad by vacuum chamber 139, after which the resin is preferably dried in a dryer 160 to reduce the viscosity of the resin, and the resin impregnated pad 143 is then consolidated in a calender mill 170 to produce rolled flexible graphite sheet 147. Gases and fumes from the vessel 150 and dryer 160 are preferably collected and purged in a scrubber 165.

After compaction, the resin in the flexible graphite sheet 147 is at least partially cured in a curing oven 180. Alternatively, partial curing may be performed prior to compaction, although post-compaction curing is preferred.

However, in one embodiment of the invention, the flexible graphite sheet is not resin impregnated, in which case the box vessel 150, the dryer 160 and the curing oven 180 can all be eliminated.

Plasma display panels produced today are on the order of 1 meter in size (diagonal measurement). Therefore, heat sinks for cooling and improving the hot spot effect of such panels also need to be relatively large, on the order of about 270 mm by about 500 mm, or as large as about 800 mm by 500 mm, or even larger. In a plasma display panel, as described above, there are thousands of cells each containing plasma gas. When a voltage is applied to each cell, the plasma gas then reacts with the phosphor in each cell to produce colored light. The plasma display can become very hot due to the higher power required to ionize the gas to produce the plasma. Furthermore, depending on the color in a specific area of the panel, hot spots may be formed on the screen, which may cause premature breakdown of the phosphor to shorten the lifetime of the display and cause thermal stress to the panel itself. Therefore, a heat sink is needed to reduce these hot spot effects.

Compressed particle sheets of exfoliated graphite, particularly laminated sheets of compressed particles of exfoliated graphite, have been found to be particularly suitable for use as heat sinks for displays such as plasma display panels. More specifically, one or more layers of compressed particles of exfoliated graphite, referred to herein as flexible graphite sheets, are placed in thermal contact with the back surface of the plasma display panel so that the flexible graphite sheets cover the multiple heat sources (i.e., discharge cells) within the panel. In other words, the surface area of the flexible graphite sheet is larger than that of one discharge cell on the back of the plasma display panel; in practice, the surface area of the flexible graphite sheet is larger than the surface area of the discharge cells on the back side of the plasma display panel. Thus, since the inventive heat sink comprises a flexible graphite material having the described features, heat can be emitted from hot spots that may appear in different parts of the plasma display as the image displayed by the panel changes.

Due to the characteristics of the flexible graphite sheet material, it is more suitable than other materials, even other forms of graphite, thus reducing the contact resistance between the heat spreader and the plasma display panel and achieving better thermal contact than prior art heat spreaders applying equal pressure.

The flexible graphite sheet heat spreader of the present invention can reduce the thermal temperature difference (i.e., Δ T) between different locations on the display. In other words, with the inventive flexible graphite heat spreader, the temperature difference between hot spots on the plasma display panel, such as the temperature difference between a white image-forming site and a nearby dark image-forming site, is reduced relative to the Δ T that would be present without the flexible graphite sheet. Therefore, thermal stress to which the plasma display panel is otherwise subjected is reduced, and the life and effectiveness of the panel are extended. Moreover, the entire device can be operated at a higher temperature and with higher image quality due to the reduction of hot spots (i.e., thermal peaks).

In practice, the graphite heat spreader may preferably have a layer of adhesive fabricated thereon to adhere the heat spreader to the plasma display panel, particularly during assembly of the plasma display panel. The adhesive must then be covered with a release liner such that the adhesive is sandwiched between the release liner and the graphite sheet layer so that the graphite heat spreader can be stored and transported before it is adhered to the plasma display panel.

The use of graphite sheets (or laminates) with a release liner and coated with an adhesive requires certain conditions to be met if applied in a process for manufacturing a large number of displays. More specifically, the release liner must be capable of being quickly removed from the sheet without causing the layers of graphite to separate. Layer separation occurs when the removal liner actually pulls the adhesive and some of the graphite off the sheet upon removal, resulting in loss of graphite, damage to the graphite sheet itself, and a reduction in the adhesive required to adhere the graphite sheet to the display, as well as making its appearance unsightly and unsightly.

However, in turn, although the adhesive and the release liner are selected so that removal of the release liner from the adhesive/graphite sheet layer does not cause the graphite layers to separate, the adhesive must still be strong enough so that the graphite sheet layer remains in place on the device and ensures good thermal contact between the heat sink(s) and the device when the panel is in various orientations.

In addition, the adhesive does not significantly degrade the thermal performance of the heat spreader. In other words, the application of a relatively thick layer of adhesive can affect the thermal performance of the heat sink, as the adhesive can affect the conduction of heat from the plasma display panel or other display to the heat sink.

Thus, the combination of adhesive and release liner must achieve a balance of providing a removal force of no greater than about 40g/cm, more preferably about 20g/cm, and most preferably about 10g/cm when removed at a speed of about 1m/s, as measured, for example, by a Cheminstruments HSR-1000 high speed removal force tester. For example, if it is desired to remove the removal liner at a rate of about 1m/s to meet the requirements of mass production displays such as plasma display panels, the removal liner should have an average removal force of no greater than about 40g/cm, more preferably about 20g/cm, and most preferably about 10g/cm, so that removal of the removal liner at this removal rate does not cause separation of the graphite layers. For this reason, the thickness of the adhesive should preferably be no greater than about 0.015mm, most preferably no greater than about 0.005 mm.

Another factor to balance is the adhesive strength of the adhesive, which as mentioned above should be sufficient to hold the heat sink in place on the display during manufacture and to ensure good thermal contact between the heat sink and the device. To achieve the desired tack, the minimum lap bond shear strength of the adhesive should be at least about 125g/cm²More preferably, the average lap bond shear strength is at least about 700g/cm²For example, by means of a ChemInstructions TT-1000 tensile tester.

Nevertheless, as noted above, the adhesive should not substantially affect the thermal performance of the heat spreader. This means that the presence of the adhesive should not result in an increase in through-thickness thermal resistance of the heat spreader of more than about 100% compared to when the heat spreader material itself is free of the adhesive. Indeed, in a more preferred embodiment, the adhesive does not cause a thermal resistance increase of greater than about 35% as compared to a heat spreader material without the adhesive. Thus, the adhesive must meet the requirements of unloading and average overlap bond shear strength while being thin enough to avoid an undesirable increase in thermal resistance. To achieve this, the adhesive should be no thicker than about 0.015mm, and more preferably no thicker than about 0.005 mm.

To achieve the balance described above to meet the needs of heat sink manufacture for applications in the manufacture of large volumes of displays such as plasma display panels, wherein the heat sink is a sheet or laminate of compressed particles of exfoliated graphite having a thickness of no more than about 2.0mm and a density of between about 1.6 and about 1.9 grams per cubic centimeter, the desired results are achieved using commercially available pressure sensitive acrylic (acrylic) adhesives of the desired thickness in combination with a release liner made from a silicon coated kraft paper such as L2 or L4, available from Sil Tech, a division of Technicote inc. Accordingly, a heat spreader composition is provided comprising a heat spreader material, such as a stack or sheet of compressed particles of exfoliated graphite, having an adhesive thereon, at a thickness such that there is no substantial loss in the thermal properties of the heat spreader material, and having a release layer such that the adhesive is sandwiched between the heat spreader material and the release material. In operation, the removal material may then be removed from the heat spreader/adhesive composition, which is then applied to a display, such as 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 are produced, at least one heat sink/adhesive composition is applied to each of the plurality of plasma display panels.

When a flexible graphite laminate is used as the inventive heat sink, additional laminates may be included to improve the mechanical or thermal properties of the laminate. For example, a laminate of a thermally conductive metal such as aluminum or copper may be interposed between flexible graphite layers to increase the thermal ductility of the laminate without sacrificing the low contact resistance exhibited by graphite; other materials, such as polymers, may also be used to reinforce or otherwise improve the strength of the laminate. Furthermore, the graphite material, whether single-layered or laminated, may have a backing layer, such as a thin layer of plastic or, in the alternative, a thin coating of dry resin, in order to improve the processability of the material and/or to reduce damage during transport or when applied to a display, without compromising the thermal ductility of the inventive heat spreader. Insulating material layers may also be employed.

In addition, the surface of the heat sink intended to abut the display may have a material facing to enhance the thermal performance and/or rework capability of the inventive heat sink. Most preferred is a metal such as aluminum or copper, most preferably aluminum. Although some heat may be lost due to greater contact resistance (since the appropriate graphite surface does not contact the device surface when such a facing is used), this can be offset by the thermal isotropy of the metal facing. More importantly, however, because the overlay can stick to the surface of the device, this facilitates the removal of the inventive heat sink for rework or other purposes, as the structure of the metal overlay is stronger than the adhesive connection, allowing the heat sink to be quickly and non-destructively removed from the display surface.

After formation, as shown in fig. 1, the flexible graphite sheet or laminate used in the inventive heat spreader may be cut into the desired shape, most often rectangular, as shown at 10. The heat sink 10 has two major surfaces 12 and 14 and at least one edge surface, typically four edge surfaces 16a, 16b, 16c, 16d if the heat sink 10 is rectangular (obviously, when the heat sink 10 is cut to a shape other than square, for example, circular or more complex shapes, it will have a different number of edge surfaces).

Referring now to fig. 1-3, heat spreader 10 preferably further includes a protective layer 20 to prevent graphite particles from flaking off or otherwise separating from the flexible graphite sheets or laminates that comprise heat spreader 10. The protective layer 20 also preferably effectively isolates the heat sink 10 from electrical interference caused by the inclusion of conductive materials (graphite) in the electronic device. The protective layer 20 may comprise any suitable material sufficient to prevent exfoliation of the graphite material and/or to electrically isolate the graphite, for example a thermoplastic material such as polyethylene, polyester or polyimide, a wax-like and/or lacquer material. Indeed, when grounding is desired, the protective layer 20 may comprise a metal, such as aluminum, as opposed to electrical isolation.

Preferably, to achieve the desired peel resistance and/or electrical isolation, the protective layer 20 should preferably be at least about 0.001mm thick. Although protective layer 20 does not have a true maximum thickness, in order to function effectively, protective layer 20 should be no greater than about 0.025mm thick, preferably no greater than about 0.005mm thick.

When the heat sink 10 is applied to a display such as a plasma display panel, the major surface 12 of the heat sink 10 is the surface that is in operative connection with the panel. Thus, in many applications, contact between the major surface 12 and the screen will serve to "seal" the major surface 12 from graphite flaking, thus eliminating the need to cover the major surface 12 with the protective layer 20. Likewise, given that major surface 14 is electrically isolated from the rest of the electrical devices in which heat spreader 10 is located, electrical isolation of major surface 12 is not required. However, for processing or other considerations, in some embodiments, both major surfaces 12 and 14 of heat spreader 10 may be coated with a protective layer 20 that is interposed between the graphite sheet and any adhesive used on major surface 12 to adhere heat spreader 10 to a plasma display panel (not shown).

The heat spreader 10 may provide the protective layer 20 by several different methods. For example, once the flexible graphite sheet or laminate is cut to size and shape to form the heat spreader 10, the material comprising the protective layer 20 may be coated onto the individual heat spreader 10 so as to flow over and extend beyond the entire major surface 14 and edge surfaces or the like, thereby forming a protective spall boundary around the heat spreader 10, as shown in fig. 1. To this end, the protective layer 20 may be applied by various coating methods familiar to those skilled in the art, such as spray coating, roll coating, and thermal lamination.

In an alternative embodiment, as shown in fig. 2, the protective layer 20 applied to the heat spreader 10 may be made to cover one or more of the edge surfaces 16a, 16b, 16c, 16d (depending, for example, on which surface is exposed and thus may flake off and/or cause electrical interference). The protective coating 20 may be applied by mechanical patterning and lamination methods to accomplish this process.

Yet another embodiment of the present invention, as shown in fig. 3, is to have a protective layer 20 applied to the heat spreader 10 covering only the major surface 14. One particularly advantageous method of manufacturing the heat spreader 10 of this embodiment is to coat the protective layer 20 with a flexible graphite sheet or laminate by roll coating, adhesive lamination, or thermal lamination, and then cut the flexible graphite sheet or laminate into the desired heat spreader 10 shape. In this way, production efficiency is maximized and waste of the protective layer 20 during the manufacturing process is reduced.

Generally, for most applications, the coating process is strong enough to adhere the protective layer 20 to the heat sink 10. However, if desired, or for a relatively non-tacky protective layer 20, for exampleSuch asPolyester materials and Kapton polyimide materials (both available from E.I du Pont de Nemours and company of Wilmington, Delaware) may be used to apply the adhesive layer 30 between the heat spreader 10 and the protective layer 20, as shown in FIG. 3. A suitable adhesive, such as an acrylic or latex adhesive, helps to adhere protective layer 20 to heat sink 10. The adhesive layer 30 may be coated on one or both of the heat spreader 10 and the protective layer 20. Advantageously, the thinner the adhesive layer 30, the better, while maintaining adhesion between the protective layer 20 and the heat spreader 10, it is preferred that the thickness of the adhesive layer 30 be no greater than about 0.015 mm.

Furthermore, in another embodiment, the surface 12 of the heat sink 10 comprises a veneer layer interposed between the surface 12 of the heat sink 10 and the surface of the display. As mentioned above, the veil is preferably a metal, such as aluminum, which is adhered to the surface 12 by a layer of adhesive applied between the heat spreader 10 and the veil, as shown in FIG. 1. Suitable adhesives are acrylic or latex adhesives that may be applied to either or both of the heat spreader surface 12 and the veil. Of course, the thinner the adhesive is applied the better, while still maintaining adhesion between the veil and the surface 12, preferably no more than about 0.015mm thick.

Further, as shown in fig. 1, the veil may cooperate with the protective layer 20 to seal the graphite heat spreader 10 disposed between the veil and the protective layer 20. More specifically, if the veil exceeds the edges of the heat spreader 10, etc., a protective layer may be applied around the heat spreader 10 and extend to the veil. Alternatively, a material such as aluminum tape may be used to seal the edges or the like between the veil and protective layer 20.

Although the present application is described in terms of a heat sink for use on a plasma display panel, it should be understood that the inventive method and heat sink are equally applicable to other emissive display heat sources, or sets of heat sources (whose corresponding function is equivalent to the set of individual discharge cells that make up the plasma display panel), such as light emitting diodes, and other displays that produce localized high temperature regions or hot spots, such as liquid crystal displays.

The following examples illustrate the operation and effect of one embodiment of the present invention, but are illustrative only and not intended to limit the scope or breadth of the claimed invention.

Example 1

The thermal characteristics of a Song plasma television, model No. TH42PA20, were analysed under different screen conditions, using an acrylic heat sink attached to the back of the plasma display screen. A black and white pattern was formed on the display and its screen surface temperature was measured with an infrared camera. The background was black in all cases. The pattern includes: 1) three evenly spaced white lines, horizontally across the screen (23.9% screen illumination) and; 2)4 x 3 white dots (4% screen illumination) evenly spaced. After testing the device with a conventional acrylic heat sink, the acrylic heat sink was removed and replaced with a 1.4mm thick flexible graphite heat sink having an in-plane thermal conductivity of about 260W/m ° K. The plasma display panel was then tested again under the same conditions as described above, and the results are shown in Table 1.

TABLE 1

Mode(s)	Heat radiator	Tmax	T range of white mode	Ambient environment
					White line pattern	Acrylic acid	49.3	30	24.1
White line pattern	Flexible graphite	48.6	34.4	23.5
					White dot matrix pattern	Acrylic acid	51.8	30.4	24.3
White dot matrix pattern	Flexible graphite	39.3	28.3	23.4

Example 2

The thermal properties of NEC plasma displays, model Plasmsync 42 "42 XM2 HD, were analyzed under different screen conditions using an aluminum/silicone heat sink attached to the back of the plasma display. A black and white pattern was formed on the display and its screen surface temperature was measured with an infrared camera. The background was black in all cases. The pattern includes: 1) three evenly spaced white lines, horizontally across the screen (23.9% screen illumination) and; 2)4 x 3 white dots (4% screen illumination) evenly spaced. After testing the device with a conventional aluminum/silicone heat sink, the aluminum/silicone heat sink was removed and replaced with a 1.4mm thick flexible graphite heat sink having an in-plane thermal conductivity of about 260W/m ° K. The display was then tested again under the same conditions as previously described and the results are given in table 2.

TABLE 2

Mode(s)	Heat radiator	Tmax	T range of white mode	Ambient environment
					White line pattern	Aluminum/silicone	61.4	32.9	25.2
White line pattern	Flexible graphite	55.1	33.9	24.9

These examples show that the use of flexible graphite heat sinks is superior to conventional heat sink technology in terms of the maximum temperature observed (Tmax) and the temperature range (T range).

All cited patents and publications referred to in this application are incorporated by reference.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims (17)

1. A display, comprising:

a. a display comprising a plurality of heat sources; and

b. a heat spreader comprising at least one layer of compressed particles of exfoliated graphite and a facing layer which enables the heat spreader to be cleanly removed from the display, the heat spreader having two major surfaces, the surface area of the heat spreader being greater than the portion of the surface area of the back surface of the display which forms the local area elevated temperature, wherein one major surface of the heat spreader is substantially entirely in thermal contact with the display such that the facing layer is located between the at least one layer of compressed particles of exfoliated graphite and the display, and wherein the heat spreader itself reduces the temperature differential between the locations on the display.

2. The display of claim 1 wherein said heat spreader comprises a stack of compressed particles comprising a plurality of layers of exfoliated graphite.

3. The display of claim 2, wherein the stack comprises a layer of non-graphite material.

4. The display of claim 1 wherein the heat spreader is adhered to the display by an adhesive that achieves a minimum overlap bond shear strength of at least about 125 grams per square centimeter.

5. The display of claim 4 wherein said adhesive achieves a minimum overlap bond shear strength of at least about 700 grams per square centimeter.

6. The display of claim 4, wherein the adhesive has a thickness of no greater than about 0.015 mm.

7. The display of claim 1 wherein at least one major surface of the heat spreader is covered with a protective layer sufficient to prevent exfoliation of the graphite particles.

8. The display of claim 7, wherein the protective layer comprises a metal or a thermoplastic material.

9. The display of claim 8, wherein the protective layer has a thickness of no greater than about 0.025 mm.

10. The display of claim 7 wherein said protective layer effectively electrically isolates the wrapped major surfaces of said at least one layer of compressed particles of exfoliated graphite.

11. The display of claim 7 wherein the compressed particles of the at least one layer of exfoliated graphite have edge faces and at least one edge face is covered with a protective layer sufficient to prevent exfoliation of the graphite particles.

12. The display of claim 7 further comprising a layer of binder interposed between said protective layer and said compressed particles of at least one layer of exfoliated graphite.

13. The display of claim 1 wherein the veil comprises a metal, a polymer, or a thermal interface material.

14. The display of claim 13, wherein the thermal interface material comprises an adhesive.

15. The display of claim 1, wherein the display is a liquid crystal display.

16. The display of claim 1, wherein the display is a plasma display panel.

17. The display of claim 1, wherein the display comprises a light emitting diode.