JPH0782610A - Carbon fiber with high conductivity - Google Patents

Abstract

PURPOSE: To provide a method to increase conductivity, particularly thermal conductivity, of a graphite carbon fiber spun from an intermediate phase pitch. CONSTITUTION: The thermal conductivity and electric conductivity of a graphite carbon fiber spun from an intermediate phase pitch are improved by annealing the carbon for 15-120 minutes at 1,400-1,800 deg.C. The graphite carbon fiber has a thermal conductivity of at least 590 W/mK and a tensile strength of at least 465×10<3> lb/in<2> .

Description

Detailed Description of the Invention

The present invention is based on the mesophase pitc
It relates to a method for increasing the conductivity, especially the thermal conductivity, of graphitized carbon fibers spun from h). The present invention also provides
The product of this process, in particular carbon fiber, which has a combination of high thermal conductivity and high tensile strength.

There has been considerable research into producing carbon fibers with optimal physical properties. Two properties for which this study has often been focused are Young's modulus, which is a measure of tensile strength and stiffness. Improved tensile strength is achieved by eliminating large structural defects in the fiber. Fibers with radial microstructure are
It often tears in the machine direction, creating wedge-shaped voids in the fiber leading to low tensile strength. One approach to eliminating this longitudinal tear is to modify the microstructure of the fiber by using specially designed orifices to spin the pitch into the fiber. Elimination of radial microstructure eliminates the tendency of the fiber to tear in the machine direction. There are a number of spinning orifice geometries described in the art. One example of this approach is Jennings and Ross.
U.S. Patent Application No. 07 / 311,511.

The Young's modulus of carbon fibers has been increased by graphitizing the fibers by heat treating them at temperatures above 2000 ° C. Attempts to increase Young's modulus by graphitization at high temperatures have resulted in loss of tensile strength and at the same time increased stiffness. Co-pending US patent application Ser. No. 07 / 158,677 teaches a method of achieving increased Young's modulus while maintaining heating strength. However, it is not unusual for an improvement in one property to be accompanied by a loss in the other.

The strength and stiffness properties are valuable for structural applications. Graphitized carbon fibers also exhibit excellent electrical and thermal conductivity and are of value in applications where high conductivity is required. The thermal conductivity of most conductive carbon fibers exceeds that of metals such as copper and aluminum. This property allowed metal composites containing graphitized fibers with higher conductivity and lower density than metal. Such materials are particularly useful as heat sinks for electronic packages in avionics applications.

One approach to increasing the conductivity of carbon fibers has maximized the development of radial microstructure of the fibers.
Although Amoco reported very high conductivities for fibers discriminating from P130 and PX3, these fibers are believed to have radial microstructure, thus tearing in the machine direction, resulting in reduced strength. Carbon fibers having a combination of high conductivity and good tensile strength are not known in the art.

It would be desirable to be able to increase the conductivity of carbon fibers, especially the thermal conductivity, without sacrificing the strength of the carbon fibers.

Spun from mesophase pitch and 2400 ° C.
The thermal conductivity and electrical conductivity of graphitized carbon fibers at the above temperatures can be enhanced by annealing the fibers at a temperature of 1400 ° C to 1800 ° C for 15 to 120 minutes in an inert atmosphere. Aniling temperature of 1550 ° C to 1650 ° C and 15 to 7
A time of 5 minutes is preferred. The present invention also provides fibers made by the annealing method described above, as well as 590 W / m.
It has a thermal conductivity of more than K and is at least 465 × 10 ^3.
Includes fibers having a tensile strength of lb / in ² . More preferably the fibers are at least 500 × 10 ³ lb / i
having a tenacity of n ² and a thermal conductivity of 590 W / mK, even more preferably the fibers are at least 500 × 10 ³ lbs.
It has a high strength of / in ² and a thermal conductivity of at least 620 W / mK.

The method of preparing the mesophase pitch is described in detail in US patent application Ser. No. 07 / 158,677, the disclosure of which is incorporated herein by reference.

Spinning Spinning the prepared mesophase pitch, generally in the form of pellets, is placed in a screw extruder and passed through a spinneret to form fibers of round cross section, which are quenched in air.
It is then carried out by collecting the fibers as usual. It is preferred to use the spinneret and slotted shim arrangements shown in US patent application Ser. No. 07 / 311,511, the disclosure of which is incorporated herein by reference. The use of this device promotes the formation of fiber microstructures that are not radial and thus resist the formation of longitudinal cracks and voids that cause loss of tensile strength. Other spinnerets known in the art can be used as long as they produce fibers that do not have longitudinal cracks that often result in fibers having a radial microstructure.

The spinning speed is generally 100 to 1000 m.
/ Min, and the fibers usually have a diameter of about 8-20 microns.

Stabilization The next step in treating freshly spun or raw fiber is stabilization. The method and apparatus of U.S. Pat. No. 4,576,810 is used. As is known in the art, freshly spun fibers are collected in conventional manner on a spinning spool or bobbin. U.S. Pat. No. 4,527,754
The issue illustrates bobbins useful in this operation.
A finish, such as a silicone oil finish, can be applied to the as-spun fiber and then wound onto a bobbin.

The fibers on the bobbin are stabilized by heating in air or a mixture of oxygen and an inert gas. As the stabilization method is an exothermic oxidation reaction, care must be taken to prevent the reaction from going too fast and going too far. Generally, the temperature of the reaction gas is gradually increased to a temperature of 200 to 340 ° C. The rate of temperature increase will depend on the concentration of oxygen in the reaction gas and the rate at which heat generated by the reaction can be transferred from the yarn to the bobbin.

Carbonization and graphitization Next, the stabilized carbon fiber is first heated at 800 to 1000 ° C. for 0.1 to 1 minute, and then 1000 to 2000 ° C.
Preferably about 0.3 to 3 at 1500 to 1950 ° C.
Carbonize for minutes. Both carbonization and graphitization are carried out in an inert atmosphere. Carbonization can be carried out on fibers which are slowly placed on trays placed in a closed furnace in a batchwise operation or, as a continuous operation, by drawing a tow of fibers through a long furnace. Stabilized and graphitized fiber, identified as Fiber G, type E-35, is suitable for graphitization and annealing according to the present invention and is available from E. I. DuPont.

The graphitization is carried out on the yarn under tension in a batchwise operation. Carbonized fiber is about 2400-3
Heat to 300 ° C, preferably 2600-3000 ° C. Graphitization is generally at least 1 minute, but longer times appear to have a deleterious effect on either carbonization or graphitization.

Various electric furnaces can be used to carry out the graphitization process. Examples are Tamann electric furnaces or Centorr Associa
tes) furnace. After the carbonization and graphitization steps, the yarn is generally cooled to room temperature. Graphitized carbon fibers suitable for annealing according to the present invention are available from EI DuPont. Such fibers are fiber G, type E-105, E-120 or E-130.
Have been identified.

[0016] After Aniringu graphitization, typical carbon fiber is 200 to 550
Has a thermal conductivity of W / mK, and the fibers have structural defects,
For example, if there are no longitudinal cracks, it generally depends on the conditions of graphitization, but is generally 480 × 10 ³ ~.
It has a tensile strength in the range of 570 × 10 ³ lb / in ² (3.3 to 3.9 GPa). Such conductivity would be approximately 3700 siemens / cm. It is surprising that an additional heat treatment below the graphitization temperature modifies these properties.

Graphitized carbon fiber is 1400 ° C. to 1
It has now been discovered that annealing at temperatures of 800 ° C., preferably 1550 ° C. to 1650 ° C., increases the conductivity and conductivity of the fibers.

The thermal conductivity of the fibers rises rapidly upon annealing, but usually the thermal conductivity of the fibers decreases after the treatment lasts for more than 1 hour, and after 2 hours of annealing, most of the treatment benefits are To lose. In some cases,
While longer annealing times were observed to be beneficial, generally the greatest increase in thermal conductivity was seen in the early stages of annealing, and longer heating times
It is very expensive but still does not give rise to high thermal conductivity.
After all, this method takes 15 to 120 minutes, preferably 15 to 7 minutes.
Includes an annealing time of 5 minutes.

The annealing process described above produces carbon fibers with increased conductivity. For example, the thermal conductivity of the annealed carbon fibers reached at least 590 W / mK, preferably 620 W / mK, and high thermal conductivity as high as 740 W / mK was observed. Aniling sometimes causes some loss of tensile strength. Annealed carbon fiber exceeds 465 × 10 ³ lb / in ² , preferably 500.
It has a strength exceeding × 10 ³ lb / in ² .

The aforementioned annealing also increases the conductivity of the carbon fiber. Prior to annealing, the graphitized carbon fiber having a conductivity of about 3700 siemens / cm is about 4 times after annealing for 20 minutes at 1600 ° C.
It exhibits a conductivity of 400 siemens / cm. The conductivity of such fibers will reach a conductivity of about 5600 siemens / cm after 1 hour of annealing.

The graphitized fiber should be under an inert atmosphere during annealing. It is preferred that the fibers are essentially under tension or under tension.
The fibers on the bobbin can also be annealed if the expansion properties of the bobbin are carefully matched to the fiber.

Thermal conductivity can be measured by the techniques described in the following document: "Apparatus for Conducance for thin fibers.
tivity Measurements On Thin Fibers) ",
L. Piraux, P. Coopmans
mans) and J. P. By Issi, Measurement (Meas
urement), Vol. 5, No. 1 (1987) pp. Two
-5. The electrical resistance was measured by the 4-probe technique,
The sample was mounted for measurement of thermal conductivity. The conductivity was estimated from the resistance.

[0023]

EXAMPLES Example 1 Carbon fibers commercially available from E. I. DuPont, identified as Fiber G, Type E-35, can be annealed as described below. The fibers are first unwound and graphitized by pulling them onto a suitable tray for placement in a Cetorr Associates furnace. The fibers are slowly heated to a temperature of approximately 2850 ° C. under a nitrogen atmosphere. It takes 5-6 hours to reach the desired temperature. Hold the fiber at this temperature for 10 minutes, then 2
Cool slowly to room temperature over ~ 3 hours. Remove the fibers from the oven and place in a bag. Prior to annealing, the expected fiber properties are: thermal conductivity, about 530 W / mK; conductivity, about 3700 siemens / cm; strength / elongation / modulus (T / E / M), about. Four
80 × 10 ³ lb / in ² (3.3 GPa) /0.37%/
130 × 10 ⁶ lb / in ² (894 GPa). Fibers make them Center Associates (Cetorr Associates)
s) It can be annealed by placing it on a tray compatible with the furnace. Place the tray and fiber in a furnace under a nitrogen atmosphere and keep the fiber temperature at 1600 ° C.
To It takes about 2 hours to reach the desired temperature. The fiber is held at 1600 ° C for 20 minutes and then cooled. The cooling time is slightly less than 2 hours. After annealing, the expected fiber properties are: thermal conductivity, about 660 W / mK; conductivity, about 4400 siemens / cm; T / E / M (1 inch gauge length), about 4
70 × 10 ³ lb / in ² (3.2 GPa) /0.35%/
130 × 10 ⁶ lb / in ² (894 GPa).

Example 2 If the annealed fibers of Example 1 are further annealed at 1600 ° C. for a further 40 minutes, for a total of 60 minutes, these fibers will have the following expected properties: thermal conductivity. , About 740 W / mK; conductive, about 5
600 siemens / cm; T / E / M, about 550 × 10
³ lb / in ² (3.8 GPa) /0.38%/140×1
0 ⁶ lb / in ² (960 GPa).

The main features and aspects of the present invention are as follows.

1. A method for improving the conductivity of graphitized fibers, which comprises annealing graphitized carbon fibers spun from an intermediate phase pitch at a temperature of 1400 ° C. to 1800 ° C. for 15 to 120 minutes in an inert atmosphere.

2. Annealing at 1550 ° C to 1650
A method according to claim 1 which is carried out at a temperature of ° C for 15 to 75 minutes.

3. Graphitized, annealed carbon fibers made by the method of paragraph 1 above.

4. Graphitized annealed carbon fiber made by the method described in paragraph 2 above.

5, a graphitized carbon fiber having a thermal conductivity of at least 590 W / mK and a tensile strength of at least 465 × 10 ³ lb / in ² .

6, at least 500 × 10 ³ lb / in ²
6. The carbon fiber according to the above item 5, which has a tensile strength of.

7. The carbon fiber according to the above item 6, which has a thermal conductivity of at least 620 W / mK.

Claims (2)

[Claims] 1. A graphitized carbon fiber spun from a mesophase pitch at a temperature of 1400 ° C. to 1800 ° C.
A method for improving the conductivity of graphitized fibers, characterized by annealing for 5 to 120 minutes in an inert atmosphere. ^2. A graphitized carbon fiber having a thermal conductivity of at least 590 W / mK and a tensile strength of at least 465 × 10 ³ lb / in ² .