GB2024787A

GB2024787A - Flexible graphite foil

Info

Publication number: GB2024787A
Application number: GB7829207A
Authority: GB
Original assignee: Stackpole Carbon Co
Current assignee: Stackpole Carbon Co
Priority date: 1978-07-07
Filing date: 1978-07-07
Publication date: 1980-01-16
Also published as: GB2024787B

Abstract

A flexible foil is made by heating finely divided graphite flake in the presence of boron, subjecting the results to an intercalation treatment and then heating and compressing. Boron permeates the crystal structure of the flake and this leads to a reduction in the density of the flake. A much greater expansion of the flake can be achieved by this method.

Description

SPECIFICATION Flexible graphite foil The present invention relates to a process for making a high strength flexible graphite foil.

A process for producing expanded or vermicular graphite is well known. An intercalating agent (such as a mixture of nitric and sulphuric acids or a mixture of nitric acid and potassium chlorate) is introduced between the laminae of graphite flakes, and the flake is then rapidly heated to a high temperature. This results in an expanded graphite material having a bulk density of, for example, about 4.9 x 10-3 gm/cm3 (about 0.30 pounds per cubic foot). The expanded or vermicular graphite can then be compressed, and rolled or moulded to form a flexible graphite foil.

Atypical foil having a density of about 1 gm/cm3, has a tensile strength of about 70.39 Kg/cm2 (about 1,000 pounds per square inch), and this may not be great enough.

We have now discovered a process for making a high strength flexible graphite foil having a much higher tensile strength than could be achieved by the old methods.

Thus, the present invention consists in a process for making a high strength flexible graphite foil, which comprises: (a) heating finely divided graphite flake (preferably pass 10 mesh stop 200 mesh and more preferably pass 28 mesh stop 100 mesh) in the presence of boron such that boron permeates the crystal lattice of the graphite flake, (b) subjecting the graphite flake to intercalation, (c) heating, preferably rapidly, to expand the flake, and (d) compressing the result to form a foil. The compression preferably takes place in two stages, firstly to produce a preform which is further compressed to form a flexible graphite foil.

We prefer that the temperature to which the finely divided graphite flake is heated in stage (a) is from 1700 to 3,0000C, more preferably about 2,750"C. The temperature to which the expanded flake is heated in stage (c) is preferably from 300 to 11 00 C, more preferably about 950"C.

Heating in the presence of boron in stage (a) can be done in a variety of ways. We prefer that the flake is heated in an atmosphere containing boron, and this atmosphere can be produced by heating boron or a boron-containing compound such as boric acid.

Another way is to wet-treat the graphite flake with a solution of a boron-containing compound, and boric acid is also suitable here. It is preferable to use a saturated solution of the boron-containing compound. Afurther method is to contact heated flake with an inert gas carrying (and preferably saturated with) a gaseous compound of boron, such as boron trichloride. Although we prefer to use just one of these methods a suitable combination of more than one can be used. Heating is generally carried out until the crystal lattice of the graphite flake is substantially completely permeated with boron, and this will usually occur after from 0.5 to 2.0 hours, more usually about 1 hour.

In practice the boron-treated flake is usually intercalated in a conventional manner, for instance by using one of the intercalating agents mentioned above. The material is then rapidly heated to a temperature between 350 and 1100 C, preferably about 950cm, causing the graphite flake to expand.

The expansion is much greater than what might have been expected had the flake not been pretreated with boron. This effect is illustrated by the following data cone: the bulk densities of material that was imtimately premixed with dry boric acid on the one hand and wet-treated with a saturated solution of boric acid on the other hand were 1.64 x 10-3 and 4.9 x 10-4 gm/cm3 (0.10 and 0.03 pounds per cubic foot) respectively. The bulk density of the expanded graphite prepared from wet-treated flake was so low that sufficient material was difficult to accumulate for further processing.

Although we do not wish to be limited by any theory, we believe that the great expansion of the boron-treated flake is due to introduction of boron between the crystal planes of the flake. This reduces the strength of the bonds between adjacent plains and thereby makes the graphite more susceptable to intercalation attack.

After expansion, the vermicular graphite may be compressed to make a preform that is then rolled or moulded to produce a graphite foil having a higher strength than conventional graphite foils. The strength of the foil appears to be dependent on the amount of residual boron in the foil. For example, foil samples with a density of 1 gm/cm3 and boron concentrations of 0.013 and 0.16 percent by weight had tensile strengths of 126.5 and 154.9 Kg/cm2 (1800 and 2200 pounds per square inch) respectively. The reasons for the higher strengths are not fully understood, but we believe that they are due to improved mechanical interlocking of the expanded flakes which is achieved by using flake having a higher degree of expansion.

1. A process for making a high strength flexible graphite foil, which comprises (a) heating finely divided graphite flake in the presence of boron such that boron permeates the crystal lattice of the graphite flake, (b) subjecting the graphite flake to intercalation, (c) heating to expand the flake, and (d) compressing the result to form a foil.

2. A process according to claim 1, in which the graphite flake is heated in stage (a) to a temperature of from 1700 to 3,0000C.

3. A process according to claim 2, in which the graphite flake is heated in stage (a) to about 2,750"C.

4. A process according to any one of the preceding claims, in which the graphite flake is heated in stage (a) in an atmosphere containing boron.

5. A process according to claim 4, in which the atmosphere is produced by heating boron.

6. A process according to claim 4 or claim 5, in which the atmosphere is produced by heating a boron-containing compound.

7. A process according to any one of the preceding claims, in which the graphite flake is wet-treated with a saturated solution of a boron-containing compound.

8. A process according to claim 6 or claim 7, in which the boron-containing compound is boric acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Flexible graphite foil The present invention relates to a process for making a high strength flexible graphite foil. A process for producing expanded or vermicular graphite is well known. An intercalating agent (such as a mixture of nitric and sulphuric acids or a mixture of nitric acid and potassium chlorate) is introduced between the laminae of graphite flakes, and the flake is then rapidly heated to a high temperature. This results in an expanded graphite material having a bulk density of, for example, about 4.9 x 10-3 gm/cm3 (about 0.30 pounds per cubic foot). The expanded or vermicular graphite can then be compressed, and rolled or moulded to form a flexible graphite foil. Atypical foil having a density of about 1 gm/cm3, has a tensile strength of about 70.39 Kg/cm2 (about 1,000 pounds per square inch), and this may not be great enough. We have now discovered a process for making a high strength flexible graphite foil having a much higher tensile strength than could be achieved by the old methods. Thus, the present invention consists in a process for making a high strength flexible graphite foil, which comprises: (a) heating finely divided graphite flake (preferably pass 10 mesh stop 200 mesh and more preferably pass 28 mesh stop 100 mesh) in the presence of boron such that boron permeates the crystal lattice of the graphite flake, (b) subjecting the graphite flake to intercalation, (c) heating, preferably rapidly, to expand the flake, and (d) compressing the result to form a foil. The compression preferably takes place in two stages, firstly to produce a preform which is further compressed to form a flexible graphite foil. We prefer that the temperature to which the finely divided graphite flake is heated in stage (a) is from 1700 to 3,0000C, more preferably about 2,750"C. The temperature to which the expanded flake is heated in stage (c) is preferably from 300 to 11 00 C, more preferably about 950"C. Heating in the presence of boron in stage (a) can be done in a variety of ways. We prefer that the flake is heated in an atmosphere containing boron, and this atmosphere can be produced by heating boron or a boron-containing compound such as boric acid. Another way is to wet-treat the graphite flake with a solution of a boron-containing compound, and boric acid is also suitable here. It is preferable to use a saturated solution of the boron-containing compound. Afurther method is to contact heated flake with an inert gas carrying (and preferably saturated with) a gaseous compound of boron, such as boron trichloride. Although we prefer to use just one of these methods a suitable combination of more than one can be used. Heating is generally carried out until the crystal lattice of the graphite flake is substantially completely permeated with boron, and this will usually occur after from 0.5 to 2.0 hours, more usually about 1 hour. In practice the boron-treated flake is usually intercalated in a conventional manner, for instance by using one of the intercalating agents mentioned above. The material is then rapidly heated to a temperature between 350 and 1100 C, preferably about 950cm, causing the graphite flake to expand. The expansion is much greater than what might have been expected had the flake not been pretreated with boron. This effect is illustrated by the following data cone: the bulk densities of material that was imtimately premixed with dry boric acid on the one hand and wet-treated with a saturated solution of boric acid on the other hand were 1.64 x 10-3 and 4.9 x 10-4 gm/cm3 (0.10 and 0.03 pounds per cubic foot) respectively. The bulk density of the expanded graphite prepared from wet-treated flake was so low that sufficient material was difficult to accumulate for further processing. Although we do not wish to be limited by any theory, we believe that the great expansion of the boron-treated flake is due to introduction of boron between the crystal planes of the flake. This reduces the strength of the bonds between adjacent plains and thereby makes the graphite more susceptable to intercalation attack. After expansion, the vermicular graphite may be compressed to make a preform that is then rolled or moulded to produce a graphite foil having a higher strength than conventional graphite foils. The strength of the foil appears to be dependent on the amount of residual boron in the foil. For example, foil samples with a density of 1 gm/cm3 and boron concentrations of 0.013 and 0.16 percent by weight had tensile strengths of 126.5 and 154.9 Kg/cm2 (1800 and 2200 pounds per square inch) respectively. The reasons for the higher strengths are not fully understood, but we believe that they are due to improved mechanical interlocking of the expanded flakes which is achieved by using flake having a higher degree of expansion. CLAIMS

9. A process according to any one of the preceding claims, in which the graphite flake is heated in stage (a) in the presence of boron by contacting heated graphite flake with an inert gas carrying a compound of boron.

10. A process according to claim 9, in which said compound of boron is boron trichloride.

11. A process according to any one of the preceding claims, in which heating in stage (a) is continued for from 0.5 to 2 hours.

12. A process according to any one of the preceding claims, in which substantially all of the graphite flake is able to pass through a 28 mesh screen.

13. A process according to any one of the preceding claims, in which heating (c) is rapid heating to a temperature between 300 and 1100 C.

14. A process according to claim 13, in which heating (c) is to a temperature of about 950"C.

15. A process according to claim 6, in which the atmosphere is produced by heating dry boric oxide and heating (c) is to a temperature of about 950"C and causes the flake to expand to a density of about 1.6 x 10-3 gm/cm3.

16. A process according to claim 7, in which the boron-containing compound is boric oxide and heating (c) is to a temperature of about 950"C and causes the flake to expand to a density of about 4.9 x 10-4 gm/cm3.

17. A process according to claim 1 substantially as hereindescribed.

18. A graphite foil when produced buy a process according to any one of the preceding claims.