CA1095680A

CA1095680A - Process for making high strength flexible graphite foil

Info

Publication number: CA1095680A
Application number: CA300,924A
Authority: CA
Inventors: John W. Borkowski
Original assignee: Stackpole Carbon Co
Current assignee: Stackpole Carbon Co
Priority date: 1978-04-11
Filing date: 1978-04-11
Publication date: 1981-02-17

Abstract

Abstract of the Disclosure Finely-divided graphite flake is heated to a high temperature in the presence of boron or a boron-containing com-pound to cause the boron to permeate the crystal structure of the flake, which is then subjected to an intercalating agent and rapidly heated, resulting in an expanded or vermicular graphite with a bulk density substantially less than what might be expected if the flake were not pretreated with boron. The expanded graphite then is compressed to form a preform that is reduced in thickness by rolling or molding to form a higher-strength flexible foil than heretofore.

Description

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The way in which expanded or vermicular graphite is made is well known, Briefly, an intercalating agent, such as mixtures of nitric and sulfuric acids, or nitric acid and potassium chlorate, is introduced between the 13minae of graph-ite flakes, which are then rapidly heated to a high temperature, resulting in an expanded graphite with a bulk density, for example, of approximately 0.30 pounds per cubic foot~ The expanded or venmicular graphite then can be compressed, and rolled or molded, to form a flexible graphite foil. A foil so produced, having a density of one gram per cubic centimeter, for example, has a tensile strength of approximately 1000 pounds per square inch, which for some applications may not be great enough.
It is an object of this invention to provide a process for making a flexible graphite foil with a much higher tensile strength than heretofore realized.
In accordance with this invention, finely-divided graphite flake, typically -10 ~200 mesh but preferably fine enough to pass through a 28 mesh screen and be stopped by a 100 me~h screen, is 810wly heated in the presence of boron to a temperature between 1700C and 3000C, preferably about 2750C.
The boron atmosphere in the furnace, in the presence of which the flake is heated, can be produced either by heating boron, or a boron-containing compound, surh as boric acid; or by heat-ing graphite flake previously wet-treated with a saturated solution of boric acid or other boron-~ontaining cpmpound; or ~k ,., S~

by delivering to a preheated furnace containing the flake an ine!rt carrier gas such as argon saturated with a gaseous boron-containing compound such as boron trichloride, The heating is continued for approximately one-half to two hours or until the boron has substantially completely permeated the crystal struc-ture of the graphite flake and which usually occurs in about one hour.
The boron-treated flake then is treated in a convention-al manner with an intercalating agent such as mentioned above, and rapidly heated to a temperature between about 350CC and 1100C~ preferably about 950C, causing the graphite flake to expand~ The resulting expansion is much greater than what might normally be expected if the ~lake had not been pretreated with boron, For example, bulk densities of material that had been intimately premixed with dry boric acid and wet-treated with a saturated solution of boric acid were O.lQ and 0O03 pounds per cubic foot, respectively. The bulk density of the expanded graphite prepared from wet-treated flake was, in fact, so low that sufficient material was difficult to accumu-late for further processing~
~ Although the reason for the greater expansion of the boron-trëated flake is not fully understood~ it is believed that the boron introduced between the crystal planes of the flake reduces the bondlng ~trength between the planes and there-by makes the graphite more susceptible to intercalation attack.
After expansion, the vermicular graphite is compressed to make a preform that is then rolled or molded to ~orm a higher 1~956i8~

strength flexible graphite foil compared with foil made by con-ventional processes. The strength appears to be xelated to the amou.nt of residual boron in the foil. For example, foil samples with a density of one gram per cubic centimeter and boron levels of 0.013 and 0.16 weight percent had tensile strengths of 1800 pounds per square inch and 2200 pounds per square inch, respect-ively, Although the reasons for the higher strengths are not fully understood, it is believed that they are the result of improved mechanical interlocking of the expanded flakes achieved by using expanded flake with a greater degree of expansion than heretoforeO

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for making a high strength flexible graphite foil, comprising heating finely divided graphite flake in the presence of boron to a temperature between about 1700°C
and 3000°C until the boron has substantially completely per-meated the crystal structure of the graphite flake, then sub-jecting the graphite flake to an intercalating agent and rapidly heating to a temperature between 300°C and 1100°C to expand the flake until it has a bulk density substantially lower than expanded flake not pretreated with boron, compressing the expanded graphite to make a preform, and further compressing the preform to form a flexible graphite foil.

2. A process according to claim 1, in which said graphite flake is fine enough to pass through a 28 mesh screen prior to treatment with boron.

3. A process according to claim 1, in which said first-mentioned heating is in the presence of a boron-containing compound.

4. A process according to claim 3, in which said compound is boric acid.

5. A process according to claim 3, in which said heating of the graphite flake after subjecting it to an inter-calating agent is to a temperature of about 950°C, and the bulk density of the expanded graphite is substantially 0.10 pounds per cubic foot.

6. A process according to claim 5, in which approx-imately 0.013 weight percent of said foil is boron and the tensile strength of the foil is substantially 1800 pounds per square inch.

7. A process according to claim 5, in which approx-imately 0.16 weight per percent of said foil is boron and the tensile strength of the foil is substantially 2200 pounds per square inch.

8. A process according to claim 1, in which before said first-mentioned heating the graphite flake is wet-treated with a boron-containing solution.

9. A process according to claim 8, in which said heating of the graphite flake after subjecting it to an inter-calating agent is to a temperature of about 950°C, and the bulk density of the expanded graphite is substantially 0.03 pounds per cubic foot.

10. A process according to claim 1, in which the boron is derived by saturating an inert carrier gas with a gaseous boron-containing compound and then delivering the saturated gas to a preheated furnace containing the graphite flake.

11. A process according to claim 10, in which said gaseous boron-containing compound is boron trichloride.