KR100515742B1 - Flexible graphite article and fuel cell electrode with enhanced electrical and thermal conductivity - Google Patents

Abstract

Disclosed is a graphite article 20 for use in the manufacture of a membrane electrode assembly 6 comprising a pair of electrodes and an ion exchange membrane 550 positioned between the electrodes. The at least one electrode is formed from a sheet of compacted mass of expanded graphite particles having a plurality of transverse fluid channels 20 passing through the sheet between the first and second opposing surfaces of the sheet, the one opposing surface being a membrane Adjacent to the ion exchange membrane 550 when using the electrode assembly 6. At least some fluid channels 20 are interconnected to enable flow of fluid therebetween.

Description

FLEXIBLE GRAPHITE ARTICLE AND FUEL CELL ELECTRODE WITH ENHANCED ELECTRICAL AND THERMAL CONDUCTIVITY}

The present invention relates to articles useful in electrode assemblies for electrochemical fuel cells. The electrode assembly of the present invention comprises a product having a flexible graphite sheet that is fluid permeable and isotropically enhanced with respect to thermal and electrical conductivity.

Graphites consist of a hexagonal array of carbon atoms or a layer plane of a network. The layer planes of carbon atoms arranged in this hexagonal system are substantially flat and are oriented or aligned such that they are substantially parallel and equidistant. Substantially flat, parallel equidistant, carbon atom sheets or layers, commonly referred to as base planes, are interconnected or bonded, and the groups are aligned in fine crystallites. Highly ordered graphites are composed of finely sized microcrystals, which have well aligned or oriented and well aligned carbon layers with respect to each other. In other words, highly ordered graphite has a highly preferred orientation of fine crystals. It should be noted that graphite has an anisotropic structure and thus exhibits or has many properties such as high directionality, in particular thermal and electrical conductivity and fluid diffusivity. In summary, graphite can be characterized as a laminated structure of carbon, i.e., a structure composed of carbon superimposed layers or laminas bonded together by weak van der Waals forces. Looking at the graphite structure, it can be seen that it usually has two axes or directions, that is, the "c" axis or direction and the "a" axis or direction. For simplicity, the "c" axis or direction may be considered in the direction perpendicular to the carbon layer. The "a" axis or direction can be considered a direction parallel to the carbon layer or a direction perpendicular to the "c" direction. Natural graphite suitable for the production of flexible graphite has a high degree of orientation.

As can be seen above, the bonding force in which parallel layers of carbon atoms are held together is only a weak van der Waals force. Graphite can be treated so that the spacing between the overlapped carbon layers or lamina can be significantly opened to provide a pronounced expansion in the direction perpendicular to the layers, i.e., the "c" direction, so that the lamination properties of the carbon layers are substantially It forms a retained expanded or swollen graphite structure.

Significantly expanded natural graphite flakes, particularly expanded to have a final thickness or “c” direction dimension of at least about 80 times or more of the original “c” direction dimension, are agglomerated or aggregated of the expanded graphite without the use of a binder. Flexible graphite sheet, for example, may be formed of a web, paper, strip, tape or the like. It is believed that forming graphite particles expanded by compression-integrated flexible sheets with a final thickness or “c” dimension, which is at least about 80 times the original “c” direction dimension, is possible without the use of any binding material. This is due to good mechanical mutual bonding or agglomeration between the bulky expanded graphite particles.

In addition to flexibility, it can be seen that the sheet material as can be seen above has a high anisotropy in terms of thermal and electrical conductivity and fluid diffusivity compared to the natural graphite starting material, This is because the orientation is directed substantially parallel to the opposing surfaces of the sheet by very high compression, for example roll pressing. As such, the resulting sheet material has good flexibility, good strength and very high orientation.

In summary, the process of producing flexible, binding-free anisotropic graphite sheet materials such as webs, papers, strips, tapes, foils, mats, and the like substantially reduces expanded graphite particles having a " c " direction dimension of at least about 80 times that of the original particles. Compressing or compressing under a predetermined load without a binder to form a flat, flexible, integrated graphite sheet. Expanded graphite particles, which are generally worms or vermiforms in appearance, once compressed, are set in compression and maintain alignment with the major opposing surface of the sheet. The density and thickness of the sheet material can vary depending on the degree of compression control. The density of the sheet material may be in the range of about 5 pounds to about 125 pounds per cubic foot. Flexible graphite sheet material exhibits a significant degree of anisotropy due to the alignment of the graphite particles parallel to the parallel major opposing surface of the sheet, and this degree of anisotropy also depends on the degree of roll pressing of the sheet material to increase its density. Will increase. In roll pressed anisotropic sheet material, the thickness, i.e., the direction perpendicular to the parallel and opposing sheet surface, comprises the "c" direction, and in the range of directions along the length and width, i.e. along or parallel to the main opposing surface. The range includes the "a" direction, and the thermal, electrical and fluid diffusion properties of the sheet are very different in order of magnitude depending on the "c" and "a" directions.

This very significant difference in properties, depending on the orientation, known as anisotropy, can be a disadvantage in some applications. For example, in gasket applications where a flexible graphite sheet is used as the gasket material and is densely maintained between the metal surfaces in use, the diffusion of a fluid, such as a gas or liquid, may occur between the major surfaces of the flexible graphite sheet and It occurs more easily on the side parallel to the surfaces. This is in most cases a fluid flow parallel ("a" direction) to the major surfaces of the graphite sheet, even though it suffers from a decrease in resistance to fluid diffusion flow across the major surfaces of the graphite sheet ("c" direction). If the resistance is increased, it provides very good gasket performance. With respect to the electrical properties, the resistivity of the anisotropic flexible graphite sheet becomes high in the transverse direction (“c” direction) with respect to the main surface of the flexible graphite sheet, and parallel to the faces between the main faces of the flexible graphite sheet. Very small in one direction ("a" direction). In such a fuel cell and the use of a fluid flow field plate for sealing of the fuel cell, even if the electrical resistivity is increased in a direction parallel to the main surface of the flexible graphite sheet ("a" direction), the flexibility If the electrical resistance across the major faces of the graphite sheet (in the "c" direction) is reduced, it provides a great advantage.

With respect to the thermal properties, the thermal conductivity of the flexible graphite sheet in a direction parallel to the upper and lower surfaces of the flexible graphite sheet is relatively high, while it is relatively very low in the "c" direction across the upper and lower surfaces.

The above state is included in the present invention.

1 is a plan view of a transversely permeable flexible graphite sheet having interconnected transverse channels according to the present invention,

FIG. 1 (a) shows the flat terminated protrusion used to mark the channel pierced in the sheet of FIG. 1,

2 is a side cross-sectional view of the sheet of FIG. 1, FIG.

2 (a), (b), (c) are suitably varied flat termination configurations for transversely interconnected channels in accordance with the present invention;

 3, 3 (a) and 3 (b) show a mechanism for displaying the product of FIG. 1,

3 (c) and 3 (d) are enlarged perspective views of a part of the transversely flexible flexible graphite sheet according to the present invention;

FIG. 3 (e) is a photograph of a part of the flexible graphite sheet having a transverse transmissivity corresponding to FIG. 3 (c);

4 is an enlarged sketch of graphite particles expanded with the orientation of the flexible graphite sheet material,

5 is an enlarged sketch of a product composed of a flexible graphite sheet according to the present invention,

6, 7 and 7 (a) illustrate a fluid permeable electrode assembly comprising a transversely permeable article according to the present invention,

FIG. 8 is a magnified view of 100 times (original size) corresponding to part of the side sketch of FIG.

According to the present invention, there is provided a membrane electrode assembly for an electrochemical fuel cell, comprising a pair of electrodes and an ion exchange membrane positioned between the electrodes, the at least one electrode being formed from a sheet in which expanded graphite particles are compressed, There are a plurality of transverse fluid channels passing through the sheet between the first and second opposing surfaces of the sheet, one of the opposing surfaces adjoining the ion exchange membrane. Preferably, the transverse fluid channel is formed by mechanically pressing an opposing surface of the sheet to displace graphite within the sheet at predetermined points. The transverse fluid channel is closely spaced and separated by compressed walls of expanded graphite and allows for interconnection (such as having grooves) between adjacent channels to allow fluid to flow between adjacent channels. .

Graphite is a crystalline form of carbon with atoms covalently bonded in a flat layer plane with weak bonding force between crystalline faces. By treating particles of graphite, such as natural graphite flakes, for example with an intercalant of sulfuric acid and nitric acid solution, the crystal structure of the graphite reacts to form compounds of graphite and intercalant. The treated graphite particles are hereinafter referred to as "particles of intercalated graphite". When exposed to high temperatures, the intercalated graphite particles expand in an accordion-like fashion to a dimension of about 80 times or more of the original volume in the "c" direction, ie perpendicular to the crystal plane of the graphite. . Peeled graphite particles are generally referred to as worms because they are vermiform in appearance. The worms can be formed into various shapes and cut into compact sheets that are not the same as the original graphite flakes, by which a small transverse opening can be provided by applying a mechanical impact.

A general method of making graphite sheets, such as foils, from flexible graphite is described in US Pat. No. 3,404,061 to Shane et al. In conventional particles of the method by Shane et al., Natural graphite flakes are intercalated by dispersing the flakes with a solution containing an oxidizing agent such as a mixture of nitric acid and sulfuric acid, for example. Intercalation solutions include oxidizing agents and other intercalating agents known in the art. Examples of these include nitric acid, potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium dichromate, perchloric acid, and the like. Oxidizing agents such as solutions and oxidizing mixtures; Or mixtures such as concentrated nitric acid and chlorate, chromic and phosphoric acid, sulfuric acid and nitric acid; Or a strong organic acid such as trifluoroacetic acid and a strong oxidizing agent soluble in the organic acid.

In a preferred embodiment, the intercalating agent is sulfuric acid or a solution of sulfuric acid and a mixture of phosphorous and oxidizing agents, in which case the oxidizing agent is nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodine or periodic acid, or the like. Things. Although relatively less preferred, intercalation solutions are metal halides such as ferric chloride and ferric chloride mixed in sulfuric acid, or halides such as bromine as a solution of bromic acid and sulfuric acid or bromine in organic solvents. It includes.

After the flakes are intercalated, excess solution is drained from the flakes and the flakes are washed with water. The amount of intercalation solution retained in the flakes after drainage is, by weight, from 20 to 150 portions (pph) of solution per 100 portions of graphite flakes, more typically about 50 to 120 pph. Alternatively, the amount of intercalation solution may be limited between 10 and 50 pph, which may be omitted in the washing step as described in US Pat. No. 4,895,713 and as indicated. Treated graphite particles are often referred to as "particles of intercalated graphite." For example, when exposed to high temperatures of about 700 ° C. to 1000 ° C. and higher, the intercalated graphite particles are in accordion-like fashion in the c direction, ie perpendicular to the crystal plane of the constituent graphite particles. Swell to about 80 to 1000 times or more of the original volume. Expanded (or exfoliated) graphite particles are generally referred to as worms because they are worms in appearance. The worm can be formed into various shapes and cut into a flexible sheet different from the original graphite flakes, which can be provided with a small transverse opening by applying a mechanical impact, which will be described in detail later.

Flexible graphite sheets and foils are agglomerates with good handling strength, such as roll pressing, suitably compressed to a thickness of 0.003 to 0.15 inches and a density of 0.1 to 1.5 grams per cubic centimeter. About 1.5-30% ceramic additives can be mixed with intercalated graphite flakes as described in US Pat. No. 5,902,762 to provide enhanced resin infusion to the final flexible graphite product. The additive includes ceramic fiber particles of 0.15 to 1.5 millimeters in length. The width of the particles is appropriate from 0.04 to 0.004 mm. Ceramic fiber particles are non-reactive and non-tacky to graphite and are stable up to temperatures up to 2000 ° F, preferably up to 2500 ° F. Suitable ceramic fiber particles are immersed in water, softened quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, calcium metasilicate fibers, calcium silicate aluminum aluminum silicate) is a form of naturally occurring mineral fiber such as fiber and aluminum oxide fiber.

1 and 2, the compressed mass of expanded graphite particles is shown at 10 in the form of a rigid graphite sheet. The flexible graphite sheet 10 is provided with a channel 20, which preferably has a smoothed side, as shown at 67 in FIGS. 5 to 8, of the flexible graphite sheet 10. It passes between the parallel, opposing surfaces 30, 40 and is separated by a wall of compressed expandable graphite 3. The wall 3 is preferably provided with a groove 5 having a depth of 1/10 to 1/3 of the channel depth according to the invention. The channel 20 preferably has an opening 50 on one side of the opposing surface 30 that is larger than the opening 60 of the other opposing surface 40. Channel 20 may have a different configuration as shown by 20 ', 20 ", 20"' in Figures 2 (a), 2 (b), 2 (c), which are shown in Figures 1 (a) and Extending integrally with the compression roller of the pressing device of FIG. 3 as shown at 75, 175, 275, 375 in 2 (a), 2 (b), 2 (c) and suitably formed of a metal such as steel It is formed using flat end protrusion members of different shapes. Smooth flat ends of the channel-forming protrusion members 75, 175, 275, and 375 are shown at 77, 177, 277, and 377, and smooth flat ends of the groove-forming protrusion members 675, 775, 875, and 975. Are shown at 677, 777, 877, 977, the smooth bearing surface 73 of the roller 70 and the smooth bearing surface 78 of the roller 72 (or flat metal plate 79) in the flexible graphite sheet. Since the deformation and displacement of the graphite are ensured, there are preferably no rough or uneven edges or debris resulting from channel forming pressurization. Groove-forming protrusion members 675, 775, 875, and 975 cause deformation and displacement of graphite in the flexible graphite sheet. The preferred channel-forming protrusion member 77 reduces the cross section in the direction away from the compression roller 70 to provide a larger channel opening on the side of the initially pressed sheet. The development of the smooth, continuous surface 63 surrounding the channel opening 60 allows the fluid to flow freely into the channel 20 of the smooth side 67.

In a preferred embodiment, the opening at one opposing surface is larger than the opening at the other opposing surface, for example from 1 to 200 times larger in area, with converging sides such as 76, 276, 376. This is due to the use of a protrusion member. The transverse channel 20 is formed by mechanical pressing at predetermined points of the sheet 10 using the mechanism at predetermined points of the flexible graphite sheet 10, the mechanism being shown in FIG. 3. One roller comprises a pair of steel rollers 70, 72 having a prismatic protrusion 75 with a flat end, ie a flat end, the protrusion having a surface 30 of the flexible graphite sheet 10. Impact to displace the graphite and penetrate the sheet 10 to form the opening channel 20. In the present invention, the channel-forming protrusion 75 is formed between the channels 20 of the continuously aligned channels following the shape of the channel 20 shown in the sketch of FIG. 3 (c) and the picture of FIG. 3 (e). Is cross-linked by groove-forming protrusions 675 formed to interconnect the grooves 5. In addition, groove-forming protrusion members 675 ′ may be included as shown in FIGS. 3 (a) and 3 (b) such that parallel continuous transverse channels 20 as shown in FIG. 3 (d). Grooves 5 'are formed to interconnect. In practice, the two rollers 70, 72 may be provided with "out-of-register" projections, and a flat metal plate 79 may be used in place of the smooth surface roller 72. . 4 illustrates the orientation of a conventional prior art compression expanded graphite particle 80 substantially parallel to the opposing surfaces 130, 140 as an enlarged sketch of the flexible graphite sheet 110. The orientation of these expanded graphite particles 80 results in the anisotropic properties of the flexible graphite sheet, ie the electrical and thermal conductivity of the sheet is substantially parallel to the opposing surfaces 130, 140 ("a" direction). It is lower in the direction traversing the opposing surfaces 130, 140 (“c” direction). As shown in FIG. 3, in the pressing process of the flexible graphite sheet 10 forming the channel 20, the graphite is formed inside the flexible graphite sheet 10 by the flat end channel forming protrusion 75 at 77. Is displaced at and pushes the graphite laterally to move toward the smooth surface 73 of the roller 70 to disintegrate and deform the parallel orientation of the expanded graphite particles 80 as shown at 800 in FIG. Groove forming protrusions 675 together modify the parallel orientation of the expanded graphite particles. In the region of reference 800, the peripheral channel 20 and the groove 5 appear to be inclined in the collapse of the parallel orientation, and the non-parallel orientation is optically observed to be 100 times and larger. The effect of the displaced graphite is to be “die-molded” by the side 76 and the smooth surface 73 of the peripheral protrusion 75 of the roller 70 as shown in FIG. 5. This reduces the anisotropy of the flexible graphite sheet 10 and thus increases the electrical and thermal conductivity of the sheet 10 transversely with respect to the opposing surfaces 30, 40. Similar effects are achieved in truncated conical and parallel-sides peg-shaped flat end protrusions 275 and 175. The perforated gas permeable flexible graphite sheet 10 of FIG. 1 can be used for the electrodes of the electrochemical fuel cell 500 shown schematically in FIGS. 6, 7 and 7 (a).

6, 7 and 7 (a) schematically illustrate the basic elements of an electrochemical fuel cell, more complete descriptions of which are described in US Pat. Nos. 4,988,583 and 5,300,370 and International Patent Publication Nos. WO 95/16287 (1995, 6,15).

6, 7 and 7 (a), the fuel cell 500 has a plastic electrolyte, such as surfaces 601 and 603, for example platinum 600, as shown in FIG. 7 (a). A solid polymer ion exchange membrane (550) catalyst coated with; Perforated flexible graphite sheet electrode 10 according to the present invention; And flow field plates 1000 and 1100 adjacent to each electrode 10. Pressurized fuel is circulated through the groove 1400 of the fuel flow field plate 1100 and pressurized oxidant is circulated through the groove 1200. In operation, the fuel flow field plate 1100 becomes an anode and the oxidant flow field plate 1000 becomes a cathode so that an electrical potential, ie, a voltage, develops between the fuel flow field plate 1100 and the oxidant flow field plate 1000. The electrochemical fuel cell described above is combined with other fuel cell stacks to provide a predetermined level of electrical force, such as in US Pat. No. 5,300,370 described above.

Operation of the fuel cell 500 is such that the electrode 1 10 is porous to fuel and oxidant fluids such as hydrogen and oxygen such that these components contact the catalyst 600 as shown in Figure 7 (a). It is necessary to be able to easily pass the electrode 10 from 1200, and to be able to move protons derived from hydrogen through the ion exchange membrane 550. In the electrode 10 of the present invention, the channel 20 is positioned to cover the grooves 1400 and 1200 of the flow field plate adjacently so that the compressed gas from the groove passes through the small opening 60 of the channel 20 It is discharged to the large opening 50 of 20. As shown by "7" in Figures 6 and 7, when the channel 20 is closed, fluid from the adjacent channel can flow through the groove to maintain gas catalyst contact adjacent to the blocked channel. The initial velocity of the gas in the small opening 60 becomes greater than the gas flow in the large opening 50, resulting in a decrease in the velocity of the gas when it comes into contact with the catalyst 600 and an increase in the residence time of the gas catalyst contact and the exchange membrane. The gas exposure area at 550 is maximized. In addition to these features, the increased electrical conductivity of the flexible graphite electrode of the present invention enables more efficient fuel cell operation.

FIG. 8 is a photograph (100 times original size) of a body of flexible graphite corresponding to the sketched portion of FIG. 5.

The material shown in the article of FIGS. 1-5 and the photograph 100 times of FIG. 8 is a parallel flat opposite to the surface 130, 140 of the material of FIG. 4 as compared to the thermal and electrical conductivity in the transverse direction. Surfaces 30 and 40 may exhibit increased thermal and electrical conductivity laterally, in which expanded natural graphite particles that are not aligned with opposing flat surfaces are not optically detected.

A sample of 0.01 inch thick flexible graphite sheet having a density of 0.3 grams / cc, shown in FIG. 4, is mechanically pressed by a similar device as in FIG. 3 to provide channels of different sizes in the flexible graphite sheet. The transverse ("c" direction) electrical resistance of the sheet material sample is measured and the results are presented in the table below.

Also in accordance with the present invention, the transverse gas permeability of the channeled graphite sheet sample is measured using Gurley Model 4118 for gas permeability measurement apparatus.

A sample of the channeled flexible graphite sheet according to the invention is placed in the bottom opening (3/8 inch diameter) of the vertical cylinder (3 inch diameter cross section). The cylinder was filled with 300 cc of air, and the weight piston (5 ounces) was set at a predetermined position at the top of the cylinder. The gas flow rate through the channeled sample is measured as a function of time for the piston's drop and the results are shown in the following table.

Flexible graphite sheet

(0.01 inch thick; density = 0.3 gms / cc)

No channels 1600 channels per square inch-0.02 inches wide at the top; 0.005 inch wide from the bottom 250 channels per square inch-0.02 inches wide at the top; 0.007 inch wide from the bottom Transverse Electrical Resistance (micro ohms) 80 8 0.3 Diffusion Rate-Second - 8 sec 30 seconds

In the present invention, for a flexible graphite sheet having a thickness of 0.003 inches to 0.015 inches adjacent to the channel and a density of 0.5 to 1.5 grams per square centimeter, the preferred channel density is from 1000 to 3000 channels per square inch and the preferred channel size is It is a channel having an area ratio of large channel opening to small channel opening from 50: 1 to 150: 1.

In the practice of the present invention, the flexible graphite sheet can often be usefully treated as a resin and the absorbed resin enhances the moisture resistance and handling strength after curing, such as the rigidity of the flexible graphite sheet. The suitable resin content is preferably 20 to 30% by weight, and up to 60% by weight is appropriate.

The products of the present invention can be used as electrically thermal connection elements for integrated circuits in computer applications, as conformal electrical contact pads, and as electrically activated grids of ice making devices.

The foregoing description has been described to enable those skilled in the art to practice the invention. It is not intended to limit all possible variations and changes in the matter apparent to those skilled in the art, which appear in the description. However, it is intended that all modifications and changes fall within the scope of the present invention, which is limited by the following claims. It is to be understood that the claims herein include components and steps in any apparatus or method effective for the purposes intended in the present invention, unless the context clearly indicates otherwise.

Claims (20)

A membrane electrode assembly comprising a pair of electrodes and an ion exchange membrane positioned between the electrodes, At least one of the electrodes is formed of a compressed mass sheet of expanded graphite particles and has a plurality of transverse fluid channels-the plurality of transverse fluid channels being the first and the first sheet of the compact sheet. Penetrating the sheet between a second opposing surface, separated by walls of the compressed expanded graphite particles, at least a portion of the walls allowing adjacent channels to be interconnected, one of the opposing surfaces being the ion Membrane electrode assembly, adjacent to the exchange membrane. The membrane electrode assembly of claim 1, wherein the transverse fluid channels are formed by mechanically pressing an opposing surface of the sheet to displace graphite within the sheet at a plurality of predetermined points. The membrane electrode assembly of claim 1, wherein at least a portion of the adjacent channels can be interconnected by grooves formed in at least a portion of the walls. 4. The membrane electrode assembly of claim 3, wherein the interconnecting grooves are formed by mechanically pressing an opposing surface of the sheet at walls separating the adjacent channels to enable fluid flow between the adjacent channels. 2. The membrane electrode assembly of claim 1, wherein the compact of expanded graphite particles is characterized by expanded graphite particles adjacent the channels extending obliquely relative to opposing surfaces of the sheet. The membrane electrode assembly of claim 1, wherein the channel openings on the second opposing surface of the sheet are surrounded by a smooth graphite surface. 2. The membrane electrode assembly of claim 1, wherein the channel openings on the first opposing surface are larger than the channel openings on the second opposing surface. 8. The membrane electrode assembly of claim 7, wherein the channel openings in the first opposing surface are 50 to 150 times larger in area than the channel openings in the second opposing surface. The membrane electrode assembly of claim 1, wherein 1000 to 3000 channels per square inch are present in the sheet. The membrane electrode assembly of claim 1, wherein the graphite sheet has a thickness of 0.003 inches to 0.015 inches around the channels and has a density of 0.5 to 1.5 grams per cubic centimeter. A compressed mass sheet of expanded graphite particles and having a plurality of transverse fluid channels, the plurality of transverse fluid channels being interposed between the first and second opposing surfaces of the compact sheet. Penetrating a sheet, separated by walls of the compressed expanded graphite particles, at least a portion of the walls allowing adjacent channels to be interconnected. 12. The graphite article of claim 11 wherein the transverse fluid channels are formed by mechanically pressing an opposing surface of the sheet to displace the graphite within the sheet at a plurality of predetermined points. 12. The graphite article of claim 11, wherein at least a portion of the adjacent channels can be interconnected by grooves formed in at least a portion of the walls. 14. The graphite article of claim 13 wherein the interconnecting grooves are formed by mechanically pressing an opposing surface of the sheet at walls separating the adjacent channels to enable fluid flow between the adjacent channels. 12. The graphite article of claim 11 wherein the compact of expanded graphite particles is characterized by expanded graphite particles adjacent the channels extending obliquely with respect to opposite surfaces of the sheet. 12. The graphite article of claim 11 wherein the channel openings at the second opposing surface of the sheet are surrounded by a smooth graphite surface. 12. The graphite article of claim 11 wherein the channel openings at the first opposing surface are larger than the channel openings at the second opposing surface. 18. The graphite article of claim 17 wherein the channel openings on the first opposing surface are 50 to 150 times larger in area than the channel openings on the second opposing surface. 12. The graphite product of claim 11, wherein 1000 to 3000 channels per square inch are present in the sheet. 12. The graphite product of claim 11, wherein the graphite sheet has a thickness of 0.003 inches to 0.015 inches around the channels and a density of 0.5 to 1.5 grams per cubic centimeter.