AU2001245505A1 - Flexible graphite capacitor element - Google Patents

Description

FLEXIBLE GRAPHITE CAPACITOR ELEMENT

Field of the Invention

This invention relates to an article of flexible graphite sheet, having a coating of

glassy carbon, which is fluid permeable in a transverse direction with enhanced isotropy

with respect to thermal and electrical conductivity and enhanced resistance to chemical

attack. This article can be used as an electrically conductive element in an electrical capacitor of the flow-through type. In a particular embodiment, natural cellulosic fibers are

included in the glassy carbon coating and are carbonized and activated.

Background of the Invention

Graphites are made up of layer planes of hexagonal arrays or networks of carbon

atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and

are oriented or ordered so as to be substantially parallel and equidistant to one another. The

substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as

basal planes, are linked or bonded together and groups thereof are arranged in crystallites.

Highly ordered graphites consist of crystallites of considerable size: the crystallites being

highly aligned or oriented with respect to each other and having well ordered carbon layers.

In other words, highly ordered graphites have a high degree of preferred crystallite

orientation. It should be noted that graphites possess anisotropic structures and thus exhibit

or possess many properties which are highly directional e.g. thermal and electrical

conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated

structures of carbon, that is, structures consisting of superposed layers or laminae of carbon

atoms joined together by weak van der Waals forces. In considering the graphite structure,

two axes or directions are usually noted, to wit, the "c" axis or direction and the "a" axes or directions. For simplicity, the "c" axis or direction may be considered as the direction perpendicular to the carbon layers. The "a" axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the "c" direction. The natural graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.

As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the "c" direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.

Natural graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or "c" direction dimension which is at least 80 or more times the original "c" direction dimension can be formed without the use of a binder into cohesive or integrated flexible graphite sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or "c" dimension which is at least 80 times the original "c" direction dimension into integrated flexible sheets by compression, without the use of any binding material is believed to be possible due to the excellent mechanical interlocking, or cohesion which is achieved between the voluminously expanded graphite particles.

In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent

flexibility, good strength and a very high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropic graphite sheet

material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or

compacting under a predetermined load and in the absence of a binder, expanded graphite

particles which have a "c" direction dimension which is at least 80 times that of the original

particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded

graphite particles which generally are worm-like or vermiform in appearance, once

compressed, will maintain the compression set and alignment with the opposed major

surfaces of the sheet. The density and thickness of the sheet material can be varied by

controlling the degree of compression. The density of the sheet material can be within the

range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The

flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the

alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet,

with the degree of anisotropy increasing upon roll pressing of the sheet material to increased

density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction

perpendicular to the opposed, parallel sheet surfaces comprises the "c" direction and the

directions ranging along the length and width, i.e. along or parallel to the opposed, major

surfaces comprises the "a" directions and the thermal, electrical and fluid diffusion

properties of the sheet are very different, by orders of magnitude, for the "c" and "a"

directions.

This very considerable difference in properties, i.e. anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket

applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid, e.g. gases or liquids, occurs more

readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in

most instances, provide for improved gasket performance, if the resistance to fluid flow

parallel to the major surfaces of the graphite sheet ("a" direction) were increased, even at the

expense of reduced resistance to fluid diffusion flow transverse to the major faces of the

graphite sheet ("c" direction). With respect to electrical properties, the resistivity of

anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces

("c" direction) of the flexible graphite sheet, and substantially less in the direction parallel to and between the major faces of the flexible graphite sheet ("a" direction). In applications

such as certain components for electrochemical cells, it would be of advantage if the

electrical resistance transverse to the major surfaces of the flexible graphite sheet ("c"

direction) were decreased, even at the expense of an increase in electrical resistivity in the

direction parallel to the major faces of the flexible graphite sheet ("a" direction).

With respect to thermal properties, the thermal conductivity of a 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 transverse to the upper and

lower surfaces.

Another carbon based material having unique properties is glassy carbon.

As used herein and as described in U.S. Patent 5,476,679, the disclosure of which is

incorporated herein by reference, glassy carbon is a monolithic non-graphitizable carbon

with a high isotropy of the structure and physical properties and with a low permeability for

gases and liquids. Glassy carbon typically also has a pseudo-glassy appearance. Glassy

carbon can be formed from a non-graphitizing carbon-containing thermosetting resin such

as synthetic or natural resins. Thermosetting resins that become rigid on heating and do not significantly soften upon reheating and are particularly effective. The principal groups of

resins suitable for use in this invention are phenolics, polymers of furfural and furfuryl

alcohol, as well as urethanes, which are minimally useful due to low carbon yields. The

preferred phenolics are phenol-formaldehyde and resorcinol-formaldehyde. Furan based

polymers derived from furfural or furfuryl alcohol are also suitable for use in this invention.

The resin system should preferably give a carbon yield in excess of about 20% and have a

viscosity below about 200-300 cps. In addition to solutions of phenolics in furfural and

furfuryl alcohol, straight furfural or furfuryl alcohol can be used with a catalyst. For

example, a solution of furfural and an acid catalyst could be coated on a surface and then

cured and carbonized to form glassy carbon.

Glassy carbon can prevent diffusion of contaminants and since glassy carbon is

harder than graphite, glassy carbon will also provide protection from flaking, scratching and

other defects and glassy carbon, unlikely glass itself, is a relatively good conductor.

The aforedescribed materials, in combination, are advantageously employed in a

flow-through capacitor described in U.S. Patent 5,779,891, the disclosure of which is

incorporated herein by reference.

The flow-through capacitor, used in the separation and other treatment of fluids, and

more fully described hereinafter, comprises at least one anode and at least one cathode

adapted to be connected to a power supply, the capacitor arranged and constructed for use in

the separation, electrical purification, concentration, recovery or electrochemical treatment

or breakdown of solutes or fluids.

The capacitor includes one or more spaced apart pairs of anode and cathode electrodes incorporating a high surface area electrically conductive material and characterized by an open, short solute or fluid flow path, which flow paths are in direct communication with the outside of the capacitor.

Summary of the Invention hi accordance with the present invention, a graphite article is provided comprising a compressed mass of expanded graphite particles in the form of a sheet having parallel, opposed first and second surfaces, at least one of the parallel opposed surfaces having an adherent coating of glassy carbon. The coated sheet, in at least a portion thereof, has a plurality of transverse fluid channels passing through said sheet between the parallel, opposed first and second surfaces, the channels being formed by mechanically impacting a surface of the sheet to displace graphite within the sheet at a plurality of predetermined locations to provide the channels with openings at the first and second parallel opposed surfaces. In a preferred embodiment, the inner surface of the channels have an adherent coating of glassy carbon whereby chemical and erosive attack at the channel sidewalls is avoided. The article of the present invention is useful as an electrically conductive backing material and electrode for use in "flow through" type capacitors.

Brief Description of the Drawings

Figure 1 is a plan view of a transversely permeable sheet of flexible graphite having transverse channels without any coating;

Figure 1(A) shows a flat-ended protrusion element used in making the channels in the perforated sheet of Figure 1;

Figure 2 is a side elevation view in section of the sheet of Figure 1; Figures 2(A), (B), (C) and (D) show various suitable flat-ended configurations for

forming transverse channels in accordance with the present invention;

Figures 3, 3(A) shows a mechanism for making the article of Figure 1;

Figure 4 shows an enlarged sketch of an elevation view of a sheet of prior art

flexible graphite sheet material having a glassy carbon coating;

Figure 5 is a sketch of an enlarged elevation view of a glassy carbon coated article

formed of flexible graphite sheet in accordance with the present invention;

Figures 6, 7 and 8 are perspective views of prior art flow-through capacitor configurations;

Figures 9, 10 and 11 show articles in accordance with the present invention which

can be substituted for components of the flow-through capacitors of Figures 5, 6 and 7; and

Figure 12 is a scanning electron microscope photo (original magnification 54X) of

the upper surface of material of the type shown in Figure 5 which is in accordance with the present invention;

Figure 13 is an optical microscope view (original magnification 500X) of a polished cross-section of the material of Figure 12; and

Figure 14 is an optical microscope view (original magnification 200X) of a polished

cross-section of the material of Figure 12 which is at a different location from that of Figure 13.

Detailed Description of the Invention

Graphite is a crystalline form of carbon comprising atoms covalently bonded in flat

layered planes with weaker bonds between the planes. By treating particles of graphite, such as natural graphite flake, with an intercalant of, e.g. a solution of sulfuric and nitric acid, the crystal structure of the graphite reacts to form a compound of graphite and the

intercalant. The treated particles of graphite are hereafter referred to as "particles of

intercalated graphite." Upon exposure to high temperature, the particles of intercalated

graphite expand in dimension as much as 80 or more times its original volume in an

accordion-like fashion in the "c" direction, i.e. in the direction perpendicular to the

crystalline planes of the graphite. The exfoliated graphite particles are vermiform in

appearance, and are therefore commonly referred to as worms. The worms may be

compressed together into flexible sheets which, unlike the original graphite flakes, can be

formed and cut into various shapes and provided with small transverse openings by deforming mechanical impact.

A common method for manufacturing graphite sheet, e.g. foil from flexible graphite

is described by Shane et al in U.S. Pat. No. 3,404,061, the disclosure of which is

incorporated herein by reference. In the typical practice of the Shane et al method, natural

graphite flakes are intercalated by dispersing the flakes in a solution containing an oxidizing

agent of, e.g. a mixture of nitric and sulfuric acid. The intercalation solution contains

oxidizing and other intercalating agents known in the art. Examples include those

containing oxidizing agents and oxidizing mixtures, such as solutions containing nitric acid,

potassium chlorate, chromic acid, potassium permanganate, potassium chromate, potassium

dichromate, perchloric acid, and the like, or mixtures, such as for example, concentrated

nitric acid and chlorate, chromic acid and phosphoric acid, sulfuric acid and nitric acid, or

mixtures of a strong organic acid, e.g. trifluoroacetic acid, and a strong oxidizing agent

soluble in the organic acid.

In a preferred embodiment, the intercalating agent is a solution of a mixture of

sulfuric acid, or sulfuric acid and phosphoric acid, and an oxidizing agent, i.e. nitric acid, perchloric acid, chromic acid, potassium permanganate, hydrogen peroxide, iodic or

periodic acids, or the like. Although less preferred, the intercalation solutions may contain

metal halides such as ferric chloride, and ferric chloride mixed with sulfuric acid, or a

halide, such as bromine as a solution of bromine and sulfuric acid or bromine in an organic solvent.

After the flakes are intercalated, any excess solution is drained from the flakes and

the flakes are water- washed. The quantity of intercalation solution retained on the flakes

after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. Alternatively, the quantity

of the intercalation solution maybe limited to between 10 to 50 parts of solution per

hundred parts of graphite by weight (pph) which permits the washing step to be eliminated

as taught and described in U.S. Pat. No. 4,895,713, the disclosure of which is also herein

incorporated by reference. The thus treated particles of graphite are sometimes referred to

as "particles of intercalated graphite." Upon exposure to high temperature, e.g. 300°C and

up to 700°C to 1000°C and higher, the particles of intercalated graphite expand as much as

80 to 1000 or more times its original volume in an accordion-like fashion in the c-direction, i.e. in the direction perpendicular to the crystalline planes of the constituent graphite

particles. The expanded, i.e. exfoliated graphite particles are vermiform in appearance, and

are therefore commonly referred to as worms. The worms may be compressed together into

flexible sheets which, unlike the original graphite flakes, can be formed and cut into various

shapes and provided with small transverse openings by deforming mechanical impact as hereinafter described.

Flexible graphite sheet and foil are coherent, with good handling strength, and are

suitably compressed, e.g. by roll-pressing, to a thickness of 0.003 to 0.15 inch and a density of 0.1 to 1.5 grams per cubic centimeter. From about 1.5-30% by weight of ceramic

additives, can be blended with the intercalated graphite flakes as described in U.S. Patent

5,902,762 (which is incorporated herein by reference) to provide enhanced resin

impregnation in the final flexible graphite product. The additives include ceramic fiber

particles having a length of 0.15 to 1.5 millimeters. The width of the particles is suitably

from 0.04 to 0.004 mm. The ceramic fiber particles are non-reactive and non-adhering to

graphite and are stable at temperatures up to 2000°F, preferably 2500°F. Suitable ceramic

fiber particles are formed of macerated quartz glass fibers, carbon and graphite fibers,

zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral

fibers such as calcium metasihcate fibers, calcium aluminum silicate fibers, aluminum oxide

fibers and the like.

With reference to Figure 1 and Figure 2, a compressed mass of expanded graphite

particles, in the form of a flexible graphite sheet is shown at 10. The flexible graphite sheet

10 is provided with channels 20, which are preferably smooth-sided as indicated at 67 in

Figure 5, and which pass between the parallel, opposed surfaces 30, 40 of flexible graphite

sheet 10. The channels 20, in a particular embodiment, have openings 50 on one of the

opposed surfaces 30 which are larger than the openings 60 in the other opposed surface 40.

The channels 20 can have different configurations as shown at 20' - 20"" in Figures 2(A),

2(B), 2(C), 2(D), which are formed using flat-ended protrusion elements of different shapes

as shown at 75, 175, 275, 375, 475 in Figures 1(A) and 2(A), 2(B), 2(C), 2(D), suitably

formed of metal, e.g. steel and integral with and extending from the pressing roller 70 of the

impacting device shown in Figure 3. The smooth flat-ends of the protrusion elements,

shown at 77, 177, 277, 377, 477, and the smooth bearing surface 73, of roller 70, and the

smooth bearing surface 78 of roller 72 (or alternatively flat metal plate 79), ensure deformation and displacement of graphite within the flexible graphite sheet, i.e. there are

little or no rough or ragged edges or debris resulting from the channel-forming impact. For

some applications, preferred protrusion elements have decreasing cross-section in the

direction away from the pressing roller 70 to provide larger channel openings on the side of

the sheet which is initially impacted. The channels 20 in the sheet of compressed expanded

graphite particles are dimensioned to increase the surface area of the sheet, i.e. the side wall

area of a channel exceeds the surface area removed by formation of the channel; when the

ratio of the average width of the channel to the thickness of the sheet is equal to or less than

"one", the surface area is increased by a factor of 2 (or more as the ratio decreases). The

development of smooth, unobstructed surfaces 63 surrounding channel openings 60, enables

the formation of a smooth, conformal, glassy carbon coating 68 and free flow of fluid into

and through smooth-sided (at 67) channels 20. In a particular embodiment, openings one of

the opposed surfaces are larger than the channel openings in the other opposed surface, e.g.

from 1 to 200 times greater in area, and result from the use of protrusion elements having

converging sides such as shown at 76, 276, 376. The channels 20 are formed in the flexible

graphite sheet 10 at a plurality of pre-determined locations by mechanical impact at the

predetermined locations in sheet 10 using a mechanism such as shown in Figure 3

comprising a pair of steel rollers 70, 72 with one of the rollers having truncated, i.e. flat-

ended, prism-shaped protrusions 75 which impact surface 30 of flexible graphite sheet 10 to

displace graphite and penetrate sheet 10 to form open channels 20. In practice, both rollers

70, 72 can be provided with "out-of-register" protrusions, and a flat metal plate indicated at

79, can be used in place of smooth-surfaced roller 72. Figure 4 is an enlarged sketch of a

sheet of flexible graphite 110, having a glassy carbon coating 68; graphite sheet 110 shows a

typical prior art orientation of compressed expanded graphite particles 80 substantially parallel to the opposed surfaces 130, 140. This orientation of the expanded graphite

particles 80 results in anisotropic properties in flexible graphite sheets; i.e. the electrical

conductivity and thermal conductivity of the sheet being substantially lower in the direction

transverse to opposed surfaces 130, 140 ("c " direction) than in the direction ("a" direction)

parallel to opposed surfaces 130, 140. hi the course of impacting flexible graphite sheet 10

to form channels 20, as illustrated in Figure 3, graphite is displaced within flexible graphite

sheet 10 by flat-ended (at 77) protrusions 75 to push aside graphite as it travels to and bears

against smooth surface 73 of roller 70 to disrupt and deform the parallel orientation of

expanded graphite particles 80 as shown at 800 in Figure 5. This region of 800, adjacent

channels 20, shows disruption of the parallel orientation into an oblique, non-parallel

orientation is optically observable at magnifications of 100X and higher, hi effect the

displaced graphite is being "die-molded" by the sides 76 of adjacent protrusions 75 and the

smooth surface 73 of roller 70 as illustrated in Figure 5. This reduces the anisotropy in

flexible graphite sheet 10 and thus increases the electrical and thermal conductivity of sheet

10 in the direction transverse to the opposed surfaces 30, 40. A similar effect is achieved

with frusto-conical and parallel-sided peg-shaped flat-ended protrusions 275 and 175 and

protrusions 375, 475. The glassy carbon coating 68 on the surfaces of flexible graphite

sheet 10 is achieved by deforming a glassy carbon coated flexible graphite sheet, such as

shown in Figure 4, or by treating a channeled sheet such as shown in Figure 1, with a resin

solution and subsequently converting the resin to glassy carbon. The glassy carbon coated

perforated fluid permeable flexible graphite sheet 10 of Figure 5 can be used in an electrode

and electrically conducting backing material in a flow-through capacitor of the type shown

schematically in Figures 6, 7 and 8 and disclosed in U.S. Patent 5,779,891. Figure 6 shows a prior art stacked washer flow-through capacitor as shown in the

above-noted U.S. Patent 5,779,891, whose high surface area electrodes contain a backing

layer. The electrodes consist in combination of electrically conductive high surface area

material 1 and conductive backing 2. The end electrodes may be either single or double

sided, whereas the intermediate electrodes are preferably double sided. The electrical

contact between the high surface area layer 1 and the conductive backing layer 2 is

preferably a compression contact, which is afforded by the screw on end cap 7 tightened

around central rod or tube 5 around threads 6. The electrodes are present in even numbers

to form at least one anode/cathode pair. The anode and cathodes 10 formed are separated by

spacers 5. Integral leads 4 extend from conductive backing (2).

These leads may be joined together to connect separately, in parallel alignment to

themselves, the alternate anode and cathode layers, or they may be gathered together to

accomplish the same purpose and to form an electrical lead.

Fluid flow is between the spaced apart electrodes and through the holes 9 and then

out through the central tube 8. Instead of a tube with holes, a ribbed rod may be substituted

with fluid flow alongside the longitudinal ribs.

Figure 7 shows a prior art washer style flow-through capacitor with high surface area

electrodes that are sufficiently conductive that no conductive backing is required. Integral

lead 4 are attached to high surface area conductive material 1, which forms alternating

anode cathode pairs separated by spacers as shown in U.S. Patent 5,779,891.

Figure 8 depicts a spiral wound capacitor (disclosed in U.S. Patent 5,779,891)

utilizing conductive high surface area material 1, optional conductive backing 2, and

spacing material 3 in a setting or open mesh form. Electric leads 4 extend from the

electrodes formed from material 1 or the optional conductive backing 2. The capacitor may optionally be wound around a structural central rod 5. This capacitor is preferably made

short and fat, with the width wider than the length of the capacitor as measured down the

central axis.

hi the practice of the present invention, an article 112 such as shown in Figure 9 is

provided for use as an electrode in the flow-through prior art capacitor of Figure 6, in

substitution for the electrode 1 shown in Figure 6. Also, article 112 of Figure 9 is provided

for use as an electrically conductive backing material in substitution of electrically

conducting backing material 2 of Figure 6. The article 112 of Figure 9 comprises a

perforated sheet of compressed expanded graphite 100, corresponding to sheet 10 of Figures

1 and 2, having transverse, open channels 20 formed as described hereinabove and an

adherent coating of glassy carbon 68. The article 212 of Figure 10 is provided for use as an

electrode, or electrically conductive backing material in the flow-through capacitor of Figure

8, in substitution for electrodes 1, and electrically conductive backing material 2. The

article 212 of Figure 10 comprises a perforated sheet of compressed expanded graphite 100

having open transverse channels 20, formed as described hereinabove, and an adherent

glassy carbon coating 68 and is substitutable for the electrode 1, and backing material 2 of

the capacitor of Figure 8. Figure 11 shows an electrode 112 for a flow-through capacitor in

accordance with the present invention having an outer coating 95 of activated carbon

particles bonded to glassy carbon coating 68 of sheet 100, formed of compressed expanded

graphite particles and having transverse channels 20. The activated carbon particles are

formed in situ by contacting a resin coated surface of flexible graphite sheet 100 with

particles of natural cellulosic precursors, e.g. shredded paper, wood pulp, straw, cotton, and

activating and carbonizing the cellulosic precursor in the course of heating and curing the

resin coating to form glassy carbon. Additionally, natural cellulosic precursors, as described above can be admixed with the resin prior to coating of the flexible graphite sheet so that

after heating, curing and activating the activated cellulosic precursors are incorporated and

embedded within the glassy, carbon coating which enhances the development of a relatively

thick activated glassy carbon coating. Procedures for activating and carbonizing cellulosic

precursors is disclosed in U.S. Patent 5,102,855, the disclosure of which is incorporated herein by reference.

In producing an article in accordance with the present invention, a sheet of

compressed expanded graphite particles having transverse channels, as illustrated in Figure

1 and Figure 2, is treated, e.g. by dipping, with a solution of non-graphitizing, organic thermo-setting resin, e.g. liquid resol phenolic resin in furfural which may advantageously

include the cellulosic precursors noted above. The solution covers and penetrates the

surface of the sheet and is subsequently dried and heated to cure and thermoset the resin and

thereafter heated to temperatures of 500°C and higher, e.g. up to about 1600°C, to convert

the thermoset resin to glassy carbon.

In preparing a high surface area electrode such as shown in Figure 11, particles of natural cellulosic materials, e.g. in the form of shredded newspaper, cotton linters, wood pulp, and

the like are treated with an activating agent and applied to or incorporated within a resin

coated sheet of compressed expanded graphite particles before the resin has fully dried.

Thereafter the resin-coated sheet, with applied or incorporated natural cellulosic particles

bonded thereto or embedded therein, is heated to cure and thermoset the resin and convert

the resin to glassy carbon; in the course of this heat treatment, the applied and incorporated

natural cellulosic particles are converted to high surface area activated carbon.

Preparation of glassy carbon surface to protect against corrosion, erosion and distortion (change in flatness): a) A sheet of compressed expanded graphite particles is coated with a

thermosettable organic resin by means of roll, spray, gauge, or dip methods depending upon

the coating thickness desired;

b) The coated sheet is heated to dry and set the resin at a temperature of 130 to

235°C.

c) The dried sheet is mechanically impacted to form transverse channels as described hereinabove.

d) The channeled sheet is heat treated in an inert or halogen atmosphere to

500°C-1600°C to form the glassy carbon coating.

A high surface area strongly adhering coating is obtained by including 2 to 20 weight

percent cellulosic material (e.g. milled newspaper) in the thermosettable resin. The

cellulosic material suitably includes an activating material, e.g. phosphoric acid, and the

cellulosic char, formed in and on the glassy carbon coating, and the surface of the glassy

carbon coating is activated by heating in an oxidizing atmosphere at 700°C for a few

minutes.

Figure 12 is a photograph (original magnification 54X) of a body of flexible graphite having a glassy carbon coating corresponding to a portion of the sketch of Figure 5.

The article of Figure 5, representative of the material of Figure 12, can be shown to

have increased thermal and electrical conductivity in the direction transverse to opposed

parallel, planar surfaces 30, 40 as compared to the thermal and electrical conductivity in the

direction transverse to surfaces 130, 140 of prior art material of Figure 4 in which particles

of expanded natural graphite unaligned with the opposed planar surfaces are not optically

detectable. A sample of a sheet of glassy carbon coated flexible graphite 0.01 inch thick having

a density of 0.3 grams/cc, representative of Figure 4, was mechanically impacted by a device

similar to that of Figure 3 to provide channels in the flexible graphite sheet.

With reference to Figure 12, the electron microscope view (500X) is a top plan view

of the surface of a glassy carbon coated sheet of flexible graphite, i.e. compressed expanded

particles of natural graphite, indicated at 10', having a glassy carbon coating indicated at

68'. Channels 20' extend transversely through the flexible graphite sheet 10'. Figures 13

and 14 are optical microscope views (500X) and (200X) respective of the cross-section of the material of Figure 12 along planes corresponding to 1300, 1400 of Figure 12.

The samples of Figures 13, 14 were prepared by epoxy potting pieces of the material

of Figure 12 and polishing the potted material with increasingly finer grit paper and

finishing with diamond powder paste. In both Figures 13 and 14, epoxy potting material

(dark gray) is indicated at 200 and pits and grooves, i.e. voids, due to graphite removal

during polishing are indicated (dark black) at 210. The non-parallel orientation of the

graphite is indicated at 800'. The glassy carbon coating is indicated at 68'. Figure 14 shows

a channel 20' with channel openings 50', 60'.

The transverse (across the thickness) electrical resistance of a sample of a sheet of

flexible graphite having a coating of glassy carbon was measured prior to channel

formation. The result is shown in the following table.

Also, the transverse gas permeability of channeled flexible graphite sheet samples, in

accordance with the present invention, was measured, using a Gurley Model 4118 for Gas Permeability Measurement.

Samples of channeled flexible graphite sheet in accordance with the present

invention were placed at the bottom opening (3/8 in. diam.) of a vertical cylinder (3 inch diameter cross-section). The cylinder was filled with 300 cc of air and a weighted piston (5

oz.) was set in place at the top of the cylinder. The rate of gas flow through the channeled

samples was measured as a function of the time of descent of the piston and the results are

shown in the table below.

Flexible Graphite Sheet With Carbon Coating

(Average Thickness = 5 Micron)

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

In the present invention, for a flexible graphite sheet having a thickness of .003 inch

to .015 inch adjacent the channels and a density of 0.3 to 1.5 grams per cubic centimeter, the

preferred channel density is from 100 to 3000 channels per square inch.

In the practice of the present invention, the flexible graphite sheet can, at times, be

advantageously treated with resin and the absorbed resin, after curing, enhances the

moisture resistance and handling strength, i.e. stiffness of the flexible graphite sheet. Resin

content is preferably 10 to 30% by weight, suitably up 60%) by weight.

The article of the present invention can further be used as electrical and thermal

coupling elements for integrated circuits in computer applications, as conformal electrical

contact pads and as electrically energized grids in de-icing equipment. The above description is intended to enable the person skilled in the art to practice

the invention. It is not intended to detail all of the possible variations and modifications

which will become apparent to the skilled worker upon reading the description. It is

intended, however, that all such modifications and variations be included within the scope

of the invention which is defined by the following claims. The claims are intended to cover

the indicated elements and steps in any arrangement or sequence which is effective to meet

the objectives intended for the invention, unless the context specifically indicates the

contrary.

Claims (1)

1. WHAT IS CLAIMED IS:

   1) A fluid permeable graphite article comprising a compressed mass of expanded

   graphite particles in the form of a sheet having parallel, opposed first and second surfaces, at

   least a portion of said sheet having a plurality of transverse fluid channels passing through

   said sheet between said first and second parallel, opposed surfaces, said channels being

   formed by mechanically impacting the first surface of said sheet at a plurality of locations to provide said channels with openings at both of said first and second parallel, opposed

   surfaces, at least one of said parallel opposed surfaces having an adherent coating of glassy

   carbon.

   2) Article in accordance with claim 1 wherein the channels are bounded by an inner

   surface having an adherent coating of glassy carbon.

   3) Article in accordance with claim 1 wherein 100 to 3000 channels per square inch are

   present in said sheet.

   4) Article in accordance with claim 1 wherein said graphite sheet has a thickness of

   0.003 inch to 0.015 inch adjacent said channels and a density of 0.3 to 1.5 grams per cubic centimeter.

   5) Article in accordance with claim 1 wherein carbonized and activated natural

   cellulosic particles are bonded to said glassy carbon coating. 6) Article in accordance with claim 1 wherein at least a portion of the glassy carbon

   coating is activated.

   7) Article in accordance with claim 1 wherein carbonized and activated natural

   cellulosic fibers are incorporated within said glassy carbon coating.

   8) A fluid permeable graphite article comprising a compressed mass of expanded

   graphite particles in the form of a sheet having parallel, opposed first and second surfaces,

   said sheet having a plurality of transverse fluid channels passing through said sheet between

   said first and second parallel, opposed surfaces, said channels being formed by mechanically

   impacting the first surface of said sheet at a plurality of locations to displace graphite within

   said sheet at said locations and provide said channels with openings at both of said first and

   second parallel, opposed surfaces, at least one of said parallel opposed surfaces having an

   adherent coating of glassy carbon, and said channels are bounded by an inner surface having

   an adherent coating of glassy carbon.

   9) A fluid permeable graphite electrically conductive backing material for use in a

   flow-through, electrical capacitor, comprising a compressed mass of expanded graphite

   particles in the form of a sheet having parallel, opposed first and second surfaces, said sheet

   having a plurality of transverse fluid channels passing through said sheet between said first

   and second parallel, opposed surfaces, said channels being formed by mechanically

   impacting the first surface of said sheet at a plurality of locations to provide said channels

   with openings at both of said first and second parallel, opposed surfaces, at least one of said

   parallel opposed surfaces having an adherent coating of glassy carbon. 10) Backing material in accordance with claim 9 wherein both of said parallel opposed

   surfaces have an adherent coating of glassy carbon.

   11) Backing material in accordance with claim 9 wherein 100 to 3000 channels per

   square inch are present in said sheet.

   12) Backing material in accordance with claim 9 wherein said graphite sheet has a

   thickness of 0.003 inch to 0.015 inch adjacent said channels and a density of 0.3 to 1.5

   grams per cubic centimeter.

   13) Backing material in accordance with claim 9 wherein at least a portion of the glassy

   carbon coating is activated.

   14) Backing material in accordance with claim 9 wherein carbonized and activated

   natural cellulosic particles are bonded to said glassy carbon coating.

   15) Backing material in accordance with claim 9 wherein carbonized and activated

   natural cellulosic particles are incorporated within said glassy carbon coating.

   16) A fluid permeable graphite electrically conductive backing material for use in a

   flow-through electrical capacitor comprising a compressed mass of expanded graphite

   particles in the form of a sheet having parallel, opposed first and second surfaces, said sheet

   having a plurality of transverse fluid channels passing through said sheet between said first and second parallel, opposed surfaces, said channels being formed by mechanically

   impacting the first surface of said sheet at a plurality of locations to displace graphite within

   said sheet at said locations and provide said channels with openings at both of said first and

   second parallel, opposed surfaces, at least one of said parallel opposed surfaces having an

   adherent coating of glassy carbon.

   17) A fluid permeable graphite electrode for use in a flow-through electrical capacitor

   comprising a compressed mass of expanded graphite particles in the form of a sheet having

   parallel, opposed first and second surfaces, said sheet having a plurality of transverse fluid channels passing through said sheet between said first and second parallel, opposed

   surfaces, said channels being formed by mechanically impacting the first surface of said

   sheet at a plurality of locations to provide said channels with openings at both of said first

   and second parallel opposed surfaces, at least one of said parallel opposed surfaces having

   an adherent coating of glassy carbon.

   18) Electrode in accordance with claim 17 wherein both of said parallel opposed

   surfaces have an adherent coating of glassy carbon with respect to said parallel opposed surfaces.

   19) Electrode in accordance with claim 17 wherein carbonized and activated natural

   cellulosic fibers are bonded to said glassy carbon coating.

   20) Electrode in accordance with claim 17 wherein carbonized and activated natural

   cellulosic particles are incorporated within said glassy carbon coating. 21) Electrode in accordance with claim 17 wherein 100 to 3000 channels per square inch are present in said sheet.

   22) Electrode in accordance with claim 17 wherein said graphite sheet has a thickness of 0.003 inch to 0.015 inch adjacent said channels and a density of 0.3 to 1.5 grams per cubic centimeter.

   23) Electrode in accordance with claim 17 wherein at least a portion of the glassy carbon coating is activated.