HK1078731B - Heat spreader for display device - Google Patents

Description

Heat sink for display device

Description of the invention

We are known as Timothy Clovesko, the United states of America, resident in North Omsted, 5250 Columbia Avenue Apt.407 (44070); julian Norley of the United states citing Chagrin Falls, 17635 Plum Creek Trail (44023), Ohio; a novel and useful "heat sink" for display devices is invented by Martin David Smlc, a citizen living in Parma, 5608 Ely Vista Drive (44129), and Joseph Payl Capp, a citizen living in Strongsville, 10094 Juniper Coirt (44136).

RELATED APPLICATIONS

This application is a co-pending and commonly assigned U.S. patent application Ser. No.10/685,103 entitled "Heat sink for Plasma Display Panel" (Heat Spreader for Plasma Display Panel), filed 10, 14, 2003, and a partial continuation of U.S. patent application Ser. No.10/844,537 filed 5, 12, 2004, filed for "Heat sink for Emissive Display Device" (Heat Spreader for Emissive Display Device), the contents of which are hereby incorporated by reference.

Technical Field

The present invention relates to a heat sink for display devices such as Plasma Display Panels (PDPs), Liquid Crystal Displays (LCDs), Light Emitting Diodes (LEDs), etc., and to the unique thermal problems caused by these devices.

Background

A plasma display panel is a display device including a plurality of discharge cells, and constitutes a display image by applying voltages across electrode discharge cells to cause desired discharge cells to emit light. A panel device, which is a main part of a plasma display panel, is manufactured by bonding two glass substrates together so as to sandwich a plurality of discharge cells therebetween.

In the plasma display panel, the discharge cells that emit light to be used for image formation are caused to generate heat, so that each of the discharge cells constitutes a heat source, causing the temperature of the plasma display panel to rise as a whole. The heat generated in the discharge cells is transferred to the glass constituting the substrate, but it is difficult to conduct heat in a direction parallel to the panel surface due to the nature of the glass substrate material.

In addition, the temperature of the discharge cells excited to emit light is significantly increased, while the temperature of the discharge cells not excited is not increased so much. Thereby, the plate surface temperature of the panel of the plasma display panel locally rises in the area where the image is generated. In addition, discharge cells activated in the white or lighter color spectrum generate more heat than discharge cells activated in the black or darker color spectrum. Thus, the temperature of the panel surface locally differs depending on the color produced when the image is generated. These local temperature differences accelerate the thermal degradation of the affected discharge cells unless some heat dissipation measures are taken to improve the differences. Further, when the properties of an image on a display are changed, the local heat generation position varies with the image.

In addition, since the temperature difference between the excited and the non-excited discharge cells is high, in fact, the temperature difference between the discharge cells generating white light and the discharge cells generating dark color light is also high, and the panel parts are subject to stress, resulting in the conventional plasma display panel being easily broken and damaged.

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 excitation voltage are more prone to thermal degradation and exacerbate the cracking problem of the panel device of the plasma display panel. LEDs have the same problems as PDPs with respect to heat generation. The same problem exists in display devices other than emissive display devices, such as LCDs, where bright spots can limit the efficiency or lifetime of the device.

U.S. patent No.5,831,374 to Morita, Ichiyanagi, Ikeda, nishikhiki, Inoue, Komyoji and Kawashima proposes the use of a so-called "highly oriented graphite film" as a thermal contact material for a plasma display panel to fill the space between the back surface of the panel and the heat sink and to reduce the local temperature difference, however, the use of flexible graphite sheets or the unique advantages are not mentioned. Further, U.S. Pat. No.6,482,520 to Tzeng discloses the use of compressed particle sheets of exfoliated graphite as heat sinks for heat sources such as electronic components (referred to in this patent as thermal interfaces). Indeed, eGraf is commercially available from Advanced energy technology, Lakewood, OhioSpreaderShield type material is used as the material.

Graphites are made up of hexagonal arrays or network layered planes of carbon atoms. These hexagonal layered planes in which the carbon atoms are arranged are generally flat and are oriented or ordered generally parallel and equidistant to each other. Generally flat, parallel equidistant sheets or layers of carbon atoms, often referred to as graphene layers or basal planes, are bonded or bonded together, with the groups arranged into grains. Highly ordered graphites consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to one another and having well-ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred grain orientation. It should be noted that graphite has an anisotropic structure and thus exhibits or has many properties that are highly directional, such as thermal and electrical conductivity.

Briefly, graphites are characterized as carbon-layered structures, that is, structures comprised of superposed or thin layers of carbon atoms bonded together by weak van der waals forces. When considering graphite structures, two axes or directions are often noted, namely the "c" axis or direction and the "a" axis or direction. For simplicity, the "c" axis or direction may be assumed to be the direction perpendicular to the carbon layers. The "a" axis or direction is assumed to be the direction parallel to the carbon layers or the direction perpendicular to the "c" direction. The graphites suitable for making flexible graphite sheets possess a very high degree of orientation.

As mentioned 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 increased 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 graphite structure in which the laminar character of the carbon layers is substantially retained.

Greatly expanded or particularly significantly expanded graphite sheets, such as webs, papers, strips, tapes, foils, mats or the like (commonly referred to as "flexible graphite") having a final thickness or "c" direction dimension that is about 80 times the original "c" direction dimension can be formed without the use of a binder in the bonded or integrated sheets of expanded graphite. It is believed that the graphite particles expanded to a final thickness or "c" direction dimension that is about 80 times the original "c" direction dimension can be formed into integrated flexible sheets by compression without the use of any binding material due to the mechanical interlocking or agglomeration achieved between the plurality of expanded graphite particles.

In addition to flexibility, as noted above, it has also been found that due to the orientation of the expanded graphite particles and the graphite layers being substantially parallel to the opposed surfaces of the sheet as a result of high pressure, the heat conduction is highly anisotropic, making the sheet particularly useful in heat dissipation applications. The sheet produced has excellent flexibility, good strength and high orientation.

Briefly, the process of making a flexible, binderless anisotropic graphite sheet material, e.g. a web, paper, strip, tape, foil, mat, or the like, comprises compressing or extruding 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 times that of the original particles, thereby to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles, which are typically worm-like or vermiform in shape, once compressed, will remain compressively deformed and aligned with the opposite major surfaces of the sheet. The density and thickness of the sheet material are varied by controlling the degree of compression. The density of the sheet may range from about 0.04g/cc to about 2.0 g/c.

The flexible graphite sheet exhibits significant anisotropy due to the orientation of graphite particles parallel to the opposed, parallel major surfaces of the sheet, with the degree of anisotropy increasing as the sheet is compressed to increase orientation. In compressed anisotropic sheets, the thickness, i.e., the direction perpendicular to the opposed parallel sheet surfaces, comprises the "c" direction, and the length and width directions, i.e., the direction along or parallel to the opposed major surfaces, comprises the "a" direction, the thermal or electrical properties of the sheets are very different, by orders of magnitude, for the "c" and "a" directions.

However, a concern in the electronics industry is that the use of graphite-based materials in general can cause graphite particles to flake off, with the result that the flakes can mechanically (i.e., in the same manner as dust particles) interfere with device operation and function, and the graphite flakes can electrically interfere with the emissive display noise due to the conductive nature of graphite. Although these concerns are believed to have been shown to be inadequate, they still exist.

Furthermore, it is sometimes disadvantageous to use an adhesive to secure the graphite heat spreader to the emissive display device. Specifically, when rework is required (i.e., removal and replacement of the heat spreader), the adhesive should be stronger than the structural integrity of the graphite sheet; in such a case, it is not always possible to cleanly peel the graphite sheet without the use of a doctor blade or other tool, which can be time consuming and can damage the graphite sheet, the display panel, or both.

Accordingly, there is a need for a lightweight and cost effective heat sink for an emissive display device, particularly one that prevents graphite particles from flaking off when used alone, and that can be effectively removed from the device when desired. The heat sink required should be capable of balancing the temperature differences over the device areas that the heat sink contacts, thereby reducing the thermal stress otherwise experienced by the display panel, and can be used to reduce the hot spots even when the hot spots are not fixed in position.

Disclosure of Invention

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

Another object of the present invention is to provide a heat sink material for a display device to improve the temperature difference generated during the use process.

It is a further object of the present invention to provide a heat spreader material for one or more cells of a heat source, such as a plasma display panel, whereby the temperature difference between any two locations on the display panel is reduced compared to a display panel without the heat spreader of the present invention.

It is a further object of the present invention to provide a heat sink material that can be applied to a heat source or a group of heat sources such as a plasma display panel or a light emitting diode and that provides good thermal contact bonding between the heat sink and the plasma display panel.

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

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

It is a further object of the present invention to provide a heat sink that can be manufactured in large quantities and in a cost-effective manner.

These and other objects, which will become apparent to those skilled in the art upon a reading of the following description, are achieved by providing a display device comprising a heat spreader comprising at least one sheet of compressed particles of graphite having a surface area greater than the surface area of the portion of the rear surface facing the device, e.g., the discharge cells. The display device may be an emissive display device such as a plasma display panel or a light emitting diode display panel, or another display device such as a liquid crystal display device. 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 rear surface of the device. Advantageously, the heat spreader is a laminate comprising a plurality of sheets of compressed particles of exfoliated graphite, and has a protective coating thereon to prevent graphite particles from flaking off therefrom. In a preferred embodiment, the surface of the heat spreader is provided with a facing sheet, such as aluminum or copper sheet, to further seal the heat spreader and facilitate rework.

In a preferred embodiment, the heat spreader has an adhesive and a release material thereon, with the adhesive disposed between the heat spreader and the release material. The release material and the adhesive are selected such that the release material does not cause undesirable damage to the heat dissipating material when the release material is released at a predetermined rate. In practice, the adhesive and release material should produce an average release force of no greater than about 40 grams per centimeter at a separation speed of about 1 meter/second, and preferably no greater than about 10 grams per centimeter at a separation speed of about one meter per second.

In addition, the adhesive preferably achieves at least about 125 grams per square centimeter (g/cm)²) Preferably an average lap shear bond strength of at least about 700 grams per square centimeter. The adhesive should increase the thermal resistance of the adhesive/heat spreading material combination in the thickness direction by no more than about 35% compared to the heat spreading material itself. The adhesive should be no greater than about 0.015 millimeters (mm) thick, and preferably no greater than about 0.005mm thick.

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, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.

In view of the above, the present invention provides a display device comprising a heat spreader having two major surfaces, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein all of the at least one major surface of the heat spreader is in direct thermal contact with the display device, and wherein the heat spreader further comprises a facing layer adhered to one of the major surfaces of the at least one sheet of compressed particles of exfoliated graphite and disposed between the sheet of compressed particles of exfoliated graphite and the display.

In other aspects, the facing layer comprises a metal. The heat spreader includes a laminate including a plurality of compressed particle sheets of the exfoliated graphite. The heat spreader includes at least one layer of non-graphite material other than the facing layer. The non-graphite material layer includes a metal, a polymer, or an insulating material. The at least one sheet of compressed particles of exfoliated graphite has edge faces, and at least one of the edge faces is coated with a protective coating sufficient to prevent exfoliation of the graphite particles. The display device includes a light emitting diode. The heat spreader also includes an adhesive layer.

The present invention also provides a display device comprising a heat spreader having two major surfaces, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein all of at least one of the major surfaces of the heat spreader is in direct thermal contact with the display device, wherein at least a portion of the at least one sheet of compressed particles of exfoliated graphite is coated with a protective coating sufficient to prevent exfoliation of the graphite particles.

In other aspects, the protective coating comprises a metal or a thermoplastic material. The protective coating has a thickness of no greater than about 0.025 mm. The protective coating is effective to electrically isolate the coated portion of the at least one compressed particle sheet of exfoliated graphite. The heat spreader also includes an adhesive layer. The display device includes an emissive display device, and a surface area of the at least one compressed particle sheet of exfoliated graphite is greater than a surface area of a portion of the discharge cell facing a back side of the emissive display device. The emissive display device is a plasma display panel. At least one of the major surfaces is coated with a protective coating sufficient to prevent exfoliation of the graphite particles. The protective coating comprises a metal or a thermoplastic material. The protective coating has a thickness of no greater than about 0.025 mm. The protective coating is effective to electrically isolate the coated major surface of the at least one compressed particle sheet of exfoliated graphite. The display device further includes an adhesive layer disposed between the protective coating and the at least one compressed particle sheet of exfoliated graphite. The display device includes an emissive display device, and a surface area of the at least one compressed particle sheet of exfoliated graphite is greater than a surface area of a portion of the discharge cell facing a back side of the emissive display device. The emissive display device is a plasma display panel. The display device further includes an adhesive layer disposed between the protective coating and the display device. The display device includes a light emitting diode.

Drawings

Fig. 1 is a top perspective view, partially broken away, of one embodiment of a heat sink of the present invention.

Fig. 2 is a top perspective view, partially broken away, of another embodiment of the heat spreader of the present invention.

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

Fig. 4 shows a system for continuously producing resin-impregnated flexible graphite sheets.

Detailed Description

Graphite is a crystalline form of carbon that includes atoms covalently bonded between planes with weaker bonds to flat layered planes. In obtaining the raw materials for the above-described flexible graphite sheets, graphite particles, such as natural graphite flakes, are typically treated with a solution of an intercalant, such as sulfuric and nitric acids, in which 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, volatilizes, and expands the size of the intercalated graphite particles in a zigzag fashion to about 80 or more times the original volume in the "c" direction, i.e., in the direction perpendicular to the crystalline planes of the graphite. The expanded (otherwise known as exfoliated) graphite particles are vermiform in appearance and are therefore often referred to as worms. The worms may be compressed together to form flexible sheets that, unlike the original graphite flakes, can be formed and cut into various shapes and formed into small transverse openings by deforming mechanical impact.

The graphite starting material for the flexible sheets suitable for use in the present invention comprises a highly graphitic carbonaceous material capable of intercalating organic and inorganic acids and halogens and expands upon exposure to heat. These highly graphitic carbonaceous materials preferably have a degree of graphitization of about 1.0. As used in this specification, the term "degree of graphitization" means a value g according to the following formula:

where d (002) is the spacing, measured in angstroms, between graphitic layers of carbon in the crystal structure. The spacing d between graphite layers is measured by standard X-ray diffraction techniques. The positions of the diffraction peaks corresponding to the (002), (004) and (006) Miller Indices (Miller Indices) are measured and the spacing that minimizes the total error of all these peaks is derived using standard least squares techniques. Highly graphitic carbonaceous materials include, for example, natural graphites from a variety of sources, as well as other carbonaceous materials such as graphites prepared by chemical vapor deposition, pyrolysis of polymers, or crystallization of molten metal solutions, among others. More preferably natural graphite.

The graphite starting material of the flexible sheet used in the present invention may contain non-graphite components as long as the crystal structure of the starting material maintains the required degree of graphitization and can be exfoliated. Generally, any carbon-containing material, the crystal structure of which has 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. Preferably, the graphite employed in the present invention has a purity of at least about 94%. In the most preferred embodiment, the graphite employed is at least about 98% pure.

One conventional method for manufacturing graphite sheets is described by Shane et al in U.S. patent No.3,404,061, the contents of which are incorporated herein by reference. In a representative embodiment of the Shane et al method, natural graphite flakes are intercalated by dispersing the flakes in a solution containing, for example, a mixture of nitric and sulfuric acid, preferably at a level of from 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. For example, substances containing oxidizing agents and oxidizing mixtures, such as substances containing nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like; or mixtures, such as concentrated nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid; or a mixture of a strong organic acid, such as acetic trifluoride, and a strong oxidizing agent dissolved in the organic acid. Alternatively, an electrical potential may be used to oxidize 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 sulfuric acid, or a mixture of sulfuric acid and phosphoric acid, and an oxidizing agent, such as 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 halides such as bromine, a solution of bromine and sulfuric acid, or bromine in an organic solvent.

The amount of intercalation solution may range from about 20 to about 350pph, preferably from about 40 to about 160 pph. After the sheet was embedded, excess solution was drained from the sheet and the sheet was rinsed with water.

Alternatively, the amount of intercalation solution may be limited to between about 10 and about 40pph, which may eliminate the need for a rinsing step as taught and described in U.S. Pat. No.4,895,713, the contents of which are also incorporated herein by reference.

The particles of graphite flake treated with intercalation solution may be selected from alcohols, sugars, aldehydes and esters which react with the surface film of oxidizing intercalation solution at temperatures in the range of 25 c to 125 c, optionally by contacting with an organic reducing agent, for example by blending. Suitable specific organic agents include cetyl alcohol, stearyl alcohol, 1-octanol, 2-octanol, decyl alcohol, 1, 10 decanediol, decanal, 1-propanol, 1,3 propanediol, 1, 2 ethylene glycol, polypropylene glycol, glucose, fructose, lactose, sucrose, potato starch, ethylene glycol monostearate, diethylene glycol dibenzoate, propylene glycol monostearate, glycerol monostearate, dimethyl oxalate, diethyl oxalate, methyl formate, ethyl formate, ascorbic acid and lignin-derived compounds, such as sodium lignosulfonate. 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 expanding acid before, during or immediately after intercalation may also bring about an improved effect. These improvements include lowering the delamination temperature and increasing the expansion volume (also referred to as "volume of the convolution"). The swelling acid herein is preferably an organic material that is sufficiently soluble in the intercalation solution to improve swelling. Further limiting, such organic materials containing carbon, hydrogen, and oxygen may be preferably, but not exclusively, employed. Carboxylic acids have been found to be particularly effective. Suitable carboxylic acids for the swelling acid may be selected from aromatic, aliphatic or cycloaliphatic, branched or branched, saturated or unsaturated monocarboxylic acids, dicarboxylic acids and polycarboxylic acids having at least one carbon atom, preferably up to about 15 carbon atoms, dissolved in the intercalation solution in an amount effective to produce a measurable improvement in one or more aspects of exfoliation. Suitable organic solvents may be employed to enhance the solubility of the organic swelling acid in the intercalation solution.

Representative examples of saturated aliphatic carboxylic acids are those having the formula H (CH)₂)_nAcids of COOH, wherein n is a number from 0 to about 5, include formic, acetic, propionic, butyric, valeric, caproic, and the like. Instead of 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 to water and carbon dioxide. For this reason, it is advantageous to contact formic acid and other sensitive swelling acids before the flakes are immersed in the aqueous intercalant. Representative dicarboxylic acids are aliphatic dicarboxylic acids having 2 to 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 alkyl esters are dimethyl oxalate and diethyl oxalate. Representative of cycloaliphatic acids are cyclohexane carboxylic acids and aromatic carboxylic acids are benzoic acid, naphthoic acid, anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m-and p-methylbenzoic acid, methoxy and ethoxybenzoic acid, acetoacetamidobenzoic acid, acetamidobenzoic acid, phenylacetic acid and naphthoic acid. 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 the polycarboxylic acids, citric acid is predominant.

The intercalation solution contains water and preferably contains an amount of swelling acid of from about 1% to about 10%, this amount being effective to enhance exfoliation. In this embodiment, the expanding acid is contacted with the graphite flake prior to or after immersing the flake in the aqueous intercalation solution. The expansion acid may be mixed 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.

After intercalating the graphite flake, the intercalated graphite flake is mixed with an organic reducing agent and the mixture is then placed at a temperature in the range of 25 c to 125 c to promote reaction of the reducing agent with the 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. At higher temperatures, times of one-half hour or less, e.g., on the order of 10 to 25 minutes, may be employed.

The above described process of intercalating and exfoliating graphite flake is facilitated by pre-treating the graphite flake at graphitization temperatures, i.e., temperatures in the range of about 3000 c and above, and by the inclusion of a lubricious additive in the intercalant.

The pretreatment or annealing of the graphite sheet results in a significant increase in the amount of expansion (i.e., increase in expansion volume to 300% or greater) when the sheet is subsequently subjected to intercalation and exfoliation. Indeed, it is desirable that the expansion be increased by at least 50% as compared to a similar treatment without the annealing step. The temperature used for the annealing step should not be significantly below 3000 c, since even a 100c reduction in temperature results in a significant reduction in expansion.

The annealing of the present invention is performed for a time sufficient to result in an increase in the degree of expansion of the sheet upon intercalation and subsequent delamination. Generally, the time required is 1 hour or more, preferably 1 to 3 hours, and more preferably, the treatment is carried out in an inert atmosphere. To achieve the greatest beneficial effect, the annealed graphite flake will also be subjected to other treatments known in the art for increasing the degree of expansion-namely intercalation in the presence of an organic reducing agent, an intercalating acid such as an organic acid, and a surfactant rinse after intercalation. Furthermore, the embedding step may be repeated for maximum benefit.

The annealing step of the invention is carried out in an induction electric furnace or other such apparatus well known in the art of graphitization; temperatures in the range of 3000 c are used here, at the high end of the range encountered during graphitization.

Because it has been observed that worms made using graphite that has been pre-intercalated with annealing can sometimes "clump" together, negatively impacting regional weight uniformity, additives that aid in the formation of "no flow" worms are highly desirable. The addition of a lubricious additive to the intercalation solution facilitates more uniform distribution of the worms on the platens of a compression apparatus, such as the platens of a calendering station conventionally used to compress (or "calender") graphite worms into flexible graphite sheets. The resulting sheet thus has higher area weight uniformity and greater tensile strength even when the raw graphite particles are smaller than those conventionally used. The lubricious additive is preferably a long chain hydrocarbon. Organic compounds having long chain hydrocarbon groups, even in the presence of other functional groups, may also be employed.

More preferably, the lubricious additive is an oil, most preferably a mineral oil, especially considering that mineral oils are less prone to rancidity and odor development, which is an important consideration for long term storage. Note that some of the swelling acids described above also satisfy the definition of lubricious additive. When these materials are used as the expansion acid, the lubricant additive is not necessarily contained in the intercalator alone.

The lubricious additive is present in the intercalant in an amount of at least about 1.4pph, preferably at least about 1.8 pph. Although the upper limit of the lubricant content is not as critical as the lower limit, it appears that the inclusion of the lubricant additive in excess of about 4pph does not provide significant additional advantages.

Such treated graphite particles are sometimes referred to as "intercalated graphite particles". Upon exposure to elevated temperatures, e.g., at least about 160 c, and particularly from about 700 c to 1000 c and higher, the intercalated graphite particles expand in a Z-shape to about 80 to 1000 or more times their original volume in the c-direction, i.e., in a direction perpendicular to the crystal planes that make up the graphite particles. Expanded, i.e., exfoliated, graphite particles are vermiform in appearance and are therefore often referred to as worms. The worms may be molded together into flexible sheets having small transverse openings, as opposed to the original graphite sheets, and the separator material may be formed and cut into various shapes, as described below.

Alternatively, the flexible graphite sheets of the present invention may utilize reground flexible graphite sheets rather than freshly expanded worms. The sheets may be newly formed sheets, recycled sheets, scrap sheets, or any other suitable source.

And the process of the present invention may use a mixture of fresh material and recycled material or all recycled material.

The starting material for the recycled material may be a sheet that has been subjected to the above-described molding or a trimmed portion of the sheet, or a sheet that has been compressed by a pre-calendering roller. Further, the raw material may be a sheet or a trimmed portion of a sheet that has been injected with resin but has not yet been cured, or a sheet or a trimmed portion of a sheet that has been injected with resin and cured. The starting material may also be a recycled flexible graphite PEM fuel cell component such as a flow field plate or electrode. Each of the various graphite starting materials may be used, or mixed with natural graphite flakes.

Once the raw material of flexible graphite sheets is obtained, it may be comminuted by known processes or devices, such as jet milling, air milling, blenders, and the like, to produce particles. Preferably, the majority of the particles have a diameter in excess of 20u.s. mesh; more preferably, the major portion (greater than about 20%, more preferably greater than about 50%) does not exceed 80u.s. mesh. Most preferably, the particles have a particle size of no greater than about 20 mesh.

The size of the milled particles can be selected in such a way as to balance machinability and formability of the graphite particles with the desired thermal properties. Thus, smaller particles produce graphite articles that are easier to process and/or shape, while larger particles will produce graphite articles with higher anisotropy, and thus higher in-plane electrical and thermal conductivity.

Once the raw materials are ground, any resin is removed if necessary and then re-expanded. The re-expansion may be performed using the embedding and stripping processes described above and in U.S. Pat. Nos. 3,404,061 to Shane et al and 4,895,713 to Greinke et al.

Typically, the particles are delaminated after intercalation by heating the intercalated particles in an oven. During this delamination step, intercalated natural graphite flakes may be added to the regenerated intercalated particles. Preferably, during the re-expansion step, the particles expand to have a specific volume in the range of at least about 100cc/g to about 350cc/g or more. Finally, after the re-expansion step, the re-expanded particles are compressed into flexible sheets, as described above.

Flexible graphite and foil are agglomerated to have good processing strength and are compressed, such as by compression molding, to a thickness of about 0.025mm to 3.75mm, with 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 sometimes be advantageously treated with a resin, and after curing, the adsorbed resin enhances the moisture resistance and processing 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, suitably up to 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 diglycidyl ether or bisphenol a (dgeba) based resins and other multifunctional resin systems; phenolic resins which may be used include resole and novolak phenols.

Referring to fig. 4, a system for the continuous manufacture of resin-embedded flexible graphite sheets is disclosed wherein graphite flakes and a liquid intercalant are filled into reactor 104. More specifically, the container 101 is adapted to contain a liquid intercalant. The container 101 is suitably made of stainless steel and is continuously replenished with liquid intercalant by means of a conduit 106. Vessel 102 contains graphite flakes and is introduced into reactor 104 along with the intercalant from vessel 101. Such as by controlling the respective rates of introduction of the intercalator and graphite flakes into the reactor 4 via 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, where metering is performed at the output of dispenser 110 through valve 111.

The resulting embedded graphite particles are wetted and coated with acid and directed (such as through conduit 112) to a rinse tank 114 where the particles are rinsed, preferably with water entering and exiting the rinse tank 114 at reference numerals 116, 118. The rinsed embedded graphite flakes are then passed, such as through conduit 120, to a drying chamber 122. Additives such as buffers, antioxidants, and decontamination chemicals may be added from container 119 to the flow of the embedded graphite sheets to modify the surface chemistry of the delamination during expansion and to utilize and modify the gas discharge that causes expansion.

The intercalated graphite sheets are dried in a dryer 122, preferably at a temperature of about 75℃ to about 150℃, to generally avoid any swelling or expansion of the intercalated graphite sheets. After drying, the embedded graphite sheets are continuously fed into the flame 200 as a fluid stream by, for example, using conduit 126, and then fed into the flame 200 in expansion vessel 128 as a fluid stream, as shown at reference numeral 2. Additives such as macerated quartz glass fibers, carbon and graphite fibers, ceramic fiber particles composed of 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 may be added from vessel 129 to the stream of embedded graphite particles propelled by entrainment without the introduction of reactive gas at reference numeral 127.

The embedded graphite particles 2 expand more than 80 times in the "c" direction after passing through the flame 200 in the expansion chamber 201 and assume a "worm-like" expanded form 5; the additives introduced from reference numeral 129 and mixed with the stream of embedded graphite particles are substantially unaffected while passing through the flame 200. The expanded graphite particles 5 may be passed through a gravity separator 130 wherein the heavier ash-like natural mineral particles are separated from the expanded graphite particles and then passed into a wider, truncated funnel 132. The separator 130 may be bypassed when not needed.

The expanded, i.e. exfoliated, graphite particles 5, together with any additives, fall freely in the hopper 132 and, for example, through a chute134 are optionally dispersed and enter a compression station 136. The compression station 136 includes separate opposed, converging moving porous belts 157, 158 to receive the exfoliated expanded graphite particles 5. As the space between the oppositely moving belts 157, 158 is reduced, the exfoliated expanded graphite particles are compressed into a flexible graphite mat, as indicated at 148, having a thickness of from, for example, about 25.4 to 0.075mm, especially from about 25.4 to 2.5mm, and a density of from about 0.08 to about 2.0g/cm³. A gas scrubber 149 may be used to remove and clean the gas emanating from the expansion chamber 201 and funnel 132.

The mat 148 passes through a vessel 150 and liquid resin is injected from spray nozzle 138, the resin preferably being "pumped through the mat" using vacuum chamber 139, and the resin is then preferably dried in dryer 160, reducing the viscosity of the resin, and the injected resin mat 143 is then thickened into rolled flexible graphite sheet 147 in calender mill 170. The gases and fumes from the vessel 150 and the dryer 160 are preferably collected and cleaned in a scrubber 165.

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

However, in one embodiment of the invention, the flexible graphite sheet is not embedded with resin, in which case the container 150, dryer 160, and curing oven 180 may be omitted.

Plasma display panels are now manufactured with dimensions of more than 1 meter (angle to angle measurement). Thus, the heat spreader plates used to cool and improve the hot spot effect on such display panels also need to be quite large, on the order of about 270 mm by about 500 microns, or on the order of about 800 microns by 500 microns or more. In the 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 reacts with the phosphor in each cell, producing colored light. Plasma displays can be very hot due to the considerable energy required to ionize a gas to produce a plasma. In addition, depending on the color in a particular area of the display panel, hot spots are generated on the screen, which can lead to premature phosphor failure, shortened display life, and thermal stress on the display panel itself. Thus, a heat sink is required to reduce the effects of these hot spots.

Compressed particles of exfoliated graphite sheets, and in particular laminates of compressed particles of exfoliated graphite, have been found to be particularly useful as heat sinks for displays such as plasma displays. More specifically, one or more sheets of compressed particles of exfoliated graphite, referred to herein as flexible graphite sheets, are placed in thermal contact with the back side of the plasma display panel, with the flexible graphite sheets covering the multiple heat sources (i.e., discharge cells) in the display panel. In other words, the surface area of the flexible graphite sheet is larger than the surface area of the discharge cells at the rear side of the plasma display panel; in practice, the flexible graphite sheet has a surface area larger than that of the plurality of discharge cells at the rear side of the plasma display panel. Therefore, due to the properties of the superior graphite material forming the heat spreader of the present invention, it can dissipate heat of hot spots generated in different positions of the plasma display panel when the image displayed on the panel is changed.

Due to the properties of the flexible graphite sheet material, i.e. more compliant than other materials and even graphite in other forms, the contact resistance between the heat spreader and the plasma display panel is reduced and a better thermal contact is obtained than when using prior art heat spreaders and applying an equivalent pressure.

The flexible graphite sheet heat spreader of the present invention is used to reduce the heat difference (i.e., Δ T) between multiple locations on a display. In other words, by using the flexible graphite heat spreader of the present invention, the temperature difference between hot spots on the plasma display panel, e.g., between a location where a white image is generated and an adjacent location where a darker image is generated, can be reduced compared to Δ T when the flexible graphite sheet is not present. Thus, thermal stress to which the plasma display panel is otherwise subjected is reduced, and the lifetime and utility of the panel are extended. Furthermore, due to the reduced hot spots (i.e., thermal peaks), the entire part can be operated at higher temperatures, resulting in improved images.

In fact, it is advantageous for the graphite heat spreader to be manufactured, particularly during plasma display panel assembly, to bond the heat spreader to the plasma display panel with a layer of adhesive thereon. It is thus necessary to cover the adhesive with a release liner, sandwich the adhesive between the release liner and the graphite sheet, and store and transport the graphite heat spreader before it is adhered to the plasma display panel.

The use of an adhesive coated graphite sheet (or laminate of sheets) with a release liner is desirable for meeting certain requirements when used in a large scale plasma display panel manufacturing process. In particular, the release liner must be capable of being removed from the sheet at high speed without delaminating the graphite. Delamination occurs when the release liner actually detaches the adhesive and some of the graphite flakes from the sheet upon removal, resulting in graphite waste, damage to the graphite sheet itself, and a reduction in the adhesive required to adhere the graphite sheet to the plasma display panel, as well as an unsightly and undesirable appearance.

Nevertheless, since the adhesive and release liner must be selected so that the adhesive/graphite sheet can be separated from the release liner without delamination of the graphite, the strength of the adhesive must be sufficient to hold the graphite sheet in place on the plasma display panel while the panel assumes any of a variety of orientations to ensure good thermal contact between the heat spreader and the panel.

Furthermore, the adhesive must not cause a significant reduction in the thermal properties of the heat sink. In other words, the adhesive applied in a layer of greater thickness may affect the thermal properties of the heat spreader, as the adhesive may affect the conduction of heat from the plasma display panel or other display device to the heat spreader.

Thus, the adhesive and release liner combination must be in a balance such that they produce a release load of no greater than about 40g/cm, preferably about 20g/cm, and most preferably about 10g/cm, at a separation speed of about 1m/s, as measured on, for example, a Cheminstruments HSR-1000 high speed separation tester. For example, if removal of the release liner at a rate of about 1m/s is desired to comply with large scale manufacturing requirements for display devices such as plasma display panels, the release liner should have an average release load of no greater than about 40g/cm, preferably about 20g/cm, and more preferably about 10g/cm, so as not to cause graphite delamination when the release liner is removed at that release rate. To achieve this, the thickness of the adhesive is preferably no more than about 0.015mm, more preferably no more than about 0.005 mm.

Another factor to be balanced is the adhesive strength of the adhesive, which must be sufficient to hold the heat spreader in place on the plasma display panel during the plasma display panel manufacturing process to ensure good thermal contact between the heat spreader and the plasma display panel. To achieve the desired bond, the adhesive must have at least about 125g/cm as measured on, for example, a ChemInstructions TT-1000 tensile tester²More preferably an average lap shear bond strength of at least about 700g/cm²。

Throughout the above, the adhesive should not substantially affect the thermal properties of the heat spreader. This means that the presence of the adhesive does not cause the thermal resistance in the thickness direction of the heat spreader to increase by more than 100% compared to the heat dissipating material itself without the adhesive. Indeed, in a more preferred embodiment, the adhesive does not increase the thermal resistance by more than about 35% as compared to a heat dissipating material without the adhesive. Thus, the adhesive must meet the release load requirements and the average overlap shear bond strength requirements while being sufficiently thin to avoid an undesirable extreme increase in thermal resistance. To meet these requirements, the thickness of the adhesive should not exceed about 0.015mm, preferably not exceed 0.005 mm.

In order to achieve the desired balance when the heat spreader is applied to a display device such as a plasma display panel in a high volume manufacturing process, the heat spreader is a compressed particle sheet or stack of sheets 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, a pressure sensitive acrylic adhesive of the desired thickness is commercially available in combination with a release liner made of silicone-coated Kraft paper, such as the L2 or L4 release liners commercially available from techicoteinc. Accordingly, a heat spreader composite is provided that includes a heat spreader material, such as a compressed particle sheet or a laminate of sheets of exfoliated graphite, having a binder thereon in a thickness such that the thermal properties of the heat spreader material are not substantially compromised, and a release layer such that the binder is disposed between the heat spreader material and the release material. Then, in use, the release material may be removed from the heat spreader/adhesive combination and the heat spreader material/adhesive combination is then applied to a display device, such as a plasma display panel, such that the adhesive bonds the heat spreader material to the plasma display panel. In addition, at least one heat spreader/adhesive combination is applied to each of the plurality of plasma display panels when the plurality of plasma display panels are manufactured.

When flexible graphite laminates are employed as the heat spreader of the present invention, other laminates may also be included to improve the mechanical or thermal properties of the laminate. For example, a stack of thermally conductive metallic aluminum or copper may be interposed between the flexible graphite layers in order to increase the thermal dissipation of the stack without sacrificing the low contact resistance exhibited by the graphite; other materials, such as polymers, may also be used to reinforce or increase the strength of the laminate. Furthermore, whether monolithic or laminated, the graphite material may be provided with a thin plastic sheet or, in an alternative, a backing layer formed of a thin dried resin coating to improve handling of the material and/or reduce damage to the sheet during transport or application to a display device without compromising the heat dissipation capabilities of the heat sink of the present invention. Insulating material layers may also be employed.

Furthermore, the surface of the heat sink intended to abut the display device may have a material finish to improve the thermal and/or rework properties of the heat sink of the present invention. Preferably, aluminum or copper is used in the form of a metal, and most preferably, aluminum. Although there may be some heat loss for greater contact resistance (because the compliant graphite surface is not in contact with the device surface when the facing is used), this can be compensated for by the thermal isotropy of the metal lining. However, not only is it the veneer bonded to the device surface, which facilitates removal of the heat sink of the present invention for rework or other purposes, since the metal veneer is structurally stronger than the bond, allowing for quick and damage free removal of the heat sink from the display device surface.

As shown in fig. 1, a flexible graphite sheet or laminate, generally designated 10, for use in the heat spreader of the present invention, once formed, can be cut to a desired shape, in most cases rectangular. The heat sink 10 has two major surfaces 12 and 14 and at least one edge (or side) face, and typically four edge faces 16a, 16b, 16c, 16d if the heat sink 10 is rectangular (obviously, when the heat sink 10 is cut into a shape other than a square, such as a circle or more complex shape, there will be a different number of edge faces).

Referring now to fig. 1-3, heat spreader 10 advantageously includes a protective coating 20 to prevent graphite particles from flaking off of the flexible graphite sheet or laminate from which heat spreader 10 is constructed, as opposed to separating the graphite particles. The protective coating 20 also facilitates effective isolation of the heat sink 10 from electrical interference caused when conductive materials (graphite) are included in the electronic device. The protective coating 20 may comprise any suitable material sufficient to prevent exfoliation of the graphite material and/or to electrically isolate the graphite, such as a thermoplastic material, e.g., polyethylene, polyester, or polyimide, a paraffin wax, and/or a varnish material. Indeed, when grounding is desired, the protective coating 20 may comprise a metal such as aluminum, as opposed to electrical isolation.

Advantageously, to achieve the desired sheet resistance and/or electrical isolation, the protective coating 20 should preferably be at least about 0.001mm thick. Although there is no true maximum thickness of the protective coating 20, the protective coating 20 should not exceed about 0.025mm thick, with a thickness of no more than about 0.005mm being preferred for effective functioning.

When the heat spreader 10 is applied to a display device such as a plasma display panel, the major surface 12 of the heat spreader 10 is the surface that is in operative contact with the display panel. Also, in many applications, contact between major surface 12 and the display panel serves to "seal" major surface 12 from graphite flaking, thereby eliminating the need to coat major surface 12 with protective coating 20. Likewise, it is not necessary to electrically isolate the major surface 12 if the major surface 14 is electrically isolated from the rest of the electronic device in which the heat sink 10 is provided. However, for processing or other considerations, in some embodiments, a protective coating 20 may be applied to both major surfaces 12 and 14 of heat spreader 10, with heat spreader 10 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 protective coating 20 can be provided to the heat spreader 10 by a number of different methods. For example, once the flexible graphite sheet or laminate is cut to a prescribed size and shape to form heat spreader 10, the material used to form protective coating 20 can be coated on individual heat spreaders 10 to flow completely around major surface 14 and edge faces, etc., and extend beyond the side faces, etc., to form a protective laminar boundary around heat spreader 10, as shown in fig. 1. To this end, the protective coating 20 may be applied by a variety of coating methods well known to those skilled in the art, such as spraying, roll coating, and thermal lamination.

In another embodiment, as shown in fig. 2, a protective coating 20 may be applied to the heat sink 10 to cover one or more of the edge surfaces 16a, 16b, 16c, 16d (e.g., depending on which edge surface is exposed, potentially flaking off and/or creating electrical interference). The protective coating 20 may be applied by mechanical mapping and laminating.

In yet another embodiment of the present invention, and as shown in FIG. 3, a protective coating 20 is applied to the heat spreader 10, coating only the major surface 14. A particularly preferred method of manufacturing the heat spreader 10 of the present embodiment is to coat a flexible graphite sheet or laminate with the protective coating 20, such as by roll coating, lamination with an adhesive, or thermal lamination, and then cut the flexible graphite sheet or laminate into the desired heat spreader 10 shape. By this method, manufacturing efficiency is maximized and waste of the protective coating 20 is minimized during the manufacturing process.

Generally, for most applications, the coating process will protect the coating 20 sufficiently stronglyIs bonded to the heat sink 10. However, if desired, or in the case of a relatively non-stick protective coating 20, such as MylarA polyester material and a Kapton polyimide material (both available from e.i. du Pont de Nemours and Company, Wilmington Delaware), an adhesive layer 30 may be applied between the heat spreader 10 and the protective coating 20, as shown in fig. 3. A suitable adhesive is an adhesive that facilitates bonding the protective coating 20 to the heat sink 10, such as an acrylic or latex adhesive. The adhesive layer 30 may be applied to either or both of the heat spreader 10 and the protective coating 20. It is advantageous that the adhesive layer 30 be as thin as possible while still maintaining adhesion between the protective coating 20 and the heat spreader 10. Preferably, the adhesive layer 30 has a thickness of no greater than about 0.015 mm.

Furthermore, in another embodiment, the heat spreader 10 may include a facing layer interposed between the surface 12 of the heat spreader 10 and the surface of the display device. As noted above, the facing layer is preferably a metal, such as aluminum, and may be adhered to the surface 12 of the heat sink 10 using an adhesive layer applied between the surface 12 and the facing layer, as shown in FIG. 1. Suitable adhesives are acrylic or latex adhesives and may be applied to either or both of the heat sink surface 12 and the facing layer. Of course, the adhesive is applied as thinly as possible while still maintaining adhesion between the facing layer and the surface 12, preferably to a thickness of no greater than about 0.015 mm.

Further, as shown in fig. 1, a facing layer may be provided along with the protective coating 20 to seal the graphite heat sink 10 between the facing layer and the protective coating 20. Specifically, if the finish layer extends beyond the edges of the heat sink 10, etc., a protective coating may be applied around the heat sink 10 and the finish layer. Alternatively, a material such as aluminum tape, sealing the edges between the facing layer and the protective coating 20, or the like may be used.

Although the present application has been described with respect to a heat sink applied to a plasma display panel, it will be appreciated that the method and heat sink of the present invention are equally applicable to other emissive display device heat sources, or heat source concentrators (functionally equivalent in relation to the collection of individual discharge cells that make up a plasma display panel), such as light emitting diodes, and other display devices that produce localized high temperature regions or hot spots, such as liquid crystal displays.

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

Example 1

The thermal properties of a panasonic plasma television (model: TH42PA20) using an acrylic heat sink attached to the back side of the plasma display panel were analyzed under different screen conditions as follows. White and black patterns are generated on the display and the screen surface temperature is measured using an infrared camera. The background was black in all cases. The pattern includes: 1) three white lines evenly spaced horizontally on the screen (23.9% screen illumination) and 2) a 4 x 3 array of evenly spaced white dots (4% screen illumination). After testing the device with the conventional acrylic heat sink, the acrylic heat sink was removed and replaced with a flexible graphite heat sink having a thickness of 1.4mm and an in-plane thermal conductivity of about 260W/m ° K. The plasma display was then tested again under the same conditions as described above, and the results are listed in table 1.

TABLE 1

Pattern(s)	Heat radiator	Tmax	White colourRange of pattern T	Environment(s)
					White line pattern	Acrylic acid	49.3	30	24.1
White line pattern	Flexible graphite sheet	48.6	34.4	23.5
					White dot array pattern	Acrylic acid	51.8	30.4	24.3
White dot array pattern	Flexible graphite sheet	39.3	28.3	23.4

Example 2:

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

TABLE 2

Pattern(s)	Heat radiator	Tmax	White pattern T range	Environment(s)
					White line pattern	Aluminum/silicone resin	61.4	32.9	25.2

White line pattern

Flexible graphite sheet

55.1

33.9

24.9

These examples illustrate the benefits of using flexible graphite heatsinks over conventional heatsink techniques in terms of the maximum temperature observed (Tmax) and the temperature range (T-range).

All cited patents and publications mentioned in this application are incorporated herein by reference.

The invention described can obviously be varied in a number of 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 (24)

1. A display device comprising a heat spreader having two major surfaces, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein all of at least one of the major surfaces of the heat spreader is in direct thermal contact with the display device, and wherein at least one of the major surfaces of the heat spreader further comprises a facing layer attached to the major surface of the heat spreader, the facing layer being intended to abut the display device and be disposed between the sheet of compressed particles of exfoliated graphite and the display device to improve the thermal and/or rework properties of the heat spreader.

2. The display apparatus of claim 1, wherein the facing layer comprises a metal.

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

4. The display apparatus of claim 1, wherein the heat spreader comprises at least one layer of non-graphite material disposed between the exfoliated graphite, except for the facing layer.

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

6. The display device of claim 1, comprising an emissive display device, and wherein the surface area of the at least one compressed particle sheet of exfoliated graphite is greater than the surface area of the portion of the discharge cell facing the back side of the emissive display device.

7. The display device of claim 6, wherein the emissive display device is a plasma display panel.

8. The display apparatus of claim 1, wherein at least one of the major surfaces is coated with a protective coating sufficient to prevent exfoliation of the graphite particles.

9. The display device of claim 8, wherein the protective coating comprises a metal or a thermoplastic material.

10. The display device of claim 9, wherein the protective coating has a thickness of no greater than 0.025 mm.

11. The display apparatus of claim 8, wherein the protective coating is effective to electrically isolate the coated major surface of the at least one compressed particle sheet of exfoliated graphite from an environment.

12. The display apparatus of claim 1, wherein the at least one sheet of compressed particles of exfoliated graphite has edge faces, and at least one of the edge faces is coated with a protective coating sufficient to prevent exfoliation of the graphite particles.

13. The display device of claim 8, further comprising an adhesive layer disposed between the protective coating and the at least one compressed particle sheet of exfoliated graphite.

14. The display device of claim 8, further comprising an adhesive layer disposed between the protective coating and the display device.

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

16. The display device of claim 1, wherein the heat spreader further comprises an adhesive layer thereon.

17. A display device comprising a heat spreader having two major surfaces, the heat spreader comprising at least one sheet of compressed particles of exfoliated graphite, wherein all of at least one of the major surfaces of the heat spreader is in direct thermal contact with the display device, wherein at least a portion of the at least one sheet of compressed particles of exfoliated graphite is coated with a protective coating sufficient to prevent exfoliation of the graphite particles.

18. The display device of claim 17, wherein the protective coating comprises a metal or a thermoplastic material.

19. The display device of claim 17, wherein the protective coating has a thickness of no greater than 0.025 mm.

20. The display apparatus of claim 17, wherein the protective coating is effective to electrically isolate the coated portion of the at least one compressed particle sheet of exfoliated graphite from an environment.

21. The display device of claim 17, wherein the heat spreader further comprises an adhesive layer thereon.

22. The display device of claim 17, comprising an emissive display device, and wherein the surface area of the at least one compressed particle sheet of exfoliated graphite is greater than the surface area of the portion of the discharge cell facing the back side of the emissive display device.

23. The display device of claim 22, wherein the emissive display device is a plasma display panel.

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