Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-03-18
2001-10-16
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S448000
Reexamination Certificate
active
06303096
ABSTRACT:
PITCH BASED CARBON FIBER
The present invention relates to a pitch based carbon fiber. In particular, it relates to a pitch based graphite fiber excellent in either a mechanical strength such as compression strength, tensile modulus and so on or thermal conductivity.
The pitch based graphite fiber of the present invention is suitably used as structural materials for space for which high dimensional stability and thermal shock resistance are required, and heat-dissipating materials for electronic devices.
High performance carbon fibers are generally classified into PAN-based carbon fibers prepared from polyacrylonitrile (PAN) as starting material and pitch based carbon fibers prepared from pitches as starting material, and they are widely used as e.g., materials for aircrafts, materials for sporting goods and materials for buildings, by taking advantages of their mechanical properties of high specific strength and high specific modulus of elasticity.
In addition to the above-mentioned properties, high thermal conductivity is required for the application to e.g., materials for space for which dimensional stability and thermal shock resistance under a large temperature difference are required, or heat-dissipating materials for electronic devices, and for such usage, carbon fibers to which a graphitization treatment is conducted are used. Many studies have been made to improve the thermal conductivity of the carbon fibers.
However, the thermal conductivity of commercially available PAN-based carbon fibers is less than 200 W/mK and insufficient. On the other hand, it has been generally considered that with pitch based graphite fibers, high thermal conductivity can readily be accomplished as compared with PAN-based carbon fibers.
However, the thermal conductivity of commercially available pitch based carbon fibers is usually less than 700 W/mK.
As a method for improvement, a method has been proposed in which graphite fibers having a high thermal conductivity which exceeds 1000 W/mK are produced by regulating the softening point of the pitch, the spinning temperature and the baking temperature (U.S. Pat. No. 5,266,295, European patent 481762 and JP-A-9-119024).
However, in such carbon fibers having a thermal conductivity of more than 1000 W/mK, the mechanical strength is generally insufficient. In considering the cause, the facts that the spread La of graphite crystallites constituting a graphite fiber is large and there are many fibers having cracks in their cross-sectional surface, may be affected. Further, the mechanical strength handling properties deteriorate, with a result that productivity decreases and physical properties decrease because of to partial breakage of fibers. Further, U.S. Pat. No. 5,721,308 proposes carbon fibers in which the thermal conductivity in a range of 500-1500 W/mK is specified. However, the thermal conductivities of the carbon fibers disclosed concretely in Examples are all less than 700 W/mK although the mechanical strengths are good.
As described above, it is difficult to say that carbon fibers having high thermal conductivity and mechanical properties with good balance has been reported realistically. Although carbon fibers having a thermal conductivity exceeding 1000 W/mK are partially commercialized, productivity is low and manufacturing cost is high. Further, since they do not have a sufficient mechanical strength independently, they are used at present for a heat radiation plate in combination with PAN-based carbon fibers having a thermal conductivity of less than 200 W/mK.
On the other hand, there has been an increased expectation of a heat-dissipating plate having higher emission properties(thermal conductivity property) with high densification of integration circuits. Under such circumstances, even though the thermal conductivity is in a range of about 700-1000 W/mK, they have a thermal conductivity of about twice that of copper which is a typical metallic material having high conductivity. If carbon fibers can be prepared which have such thermal conductivity and a sufficient mechanical strength, they can solely be processed in the form of heat-dissipating plates, and there is a remarkable development in this technical field. Accordingly, an object of the present invention is to provide a carbon fiber excellent in both mechanical strength and thermal conductivity.
The inventors of this application have made extensive studies about the structure of carbon fibers and the relationship of thermal conductivity and mechanical properties and so on, and have noticed in particular, the relation of the spread La of graphite crystallites constituting a fiber in the layer plane direction of the film to the orientation angle &PSgr; in the direction of fiber axis. As a result of the study on the conditions for producing a carbon fiber having characteristics in response to this, they have found a completely new carbon fiber which can sufficiently be distinguished in terms of physical properties from the conventional carbon fiber, and the present invention has been achieved.
Namely, the graphite fiber according to the present invention is a pitch based carbon fiber characterized in that the spread La of graphite crystallites constituting a fiber in the direction of layer plane is 1000 angstroms or less; the orientation angle &PSgr; in the direction of fiber axis is 10° or less, and the following relationship formulas (1) and (2) are satisfied:
0.70La−46&PSgr;>50 (1)
0.55La−76&PSgr;<500 (2).
In the following, the present invention will be described in further detail. In addition that the spread La of graphite crystallites constituting a fiber in the direction of layer plane and the orientation angle &PSgr; is the direction of fiber axis satisfy simultaneously the above-mentioned relationship formulas (1) and (2), they preferably satisfy simultaneously the below-mentioned formulas (3) and (4):
0.70La−46&PSgr;>120 (3)
0.55La−76&PSgr;<350 (4).
A carbon fiber deviating from the relationship formula (2) is poor in mechanical strength and is difficult to use although a higher value can be expected to a certain extent for the thermal conductivity. Further, a carbon fiber deviating from the relationship formula (1) can not have a high thermal conductivity of not less than 700 W/mK although the mechanical strength is good.
For the carbon fiber of the present invention, the spread La of graphite crystallites in the direction of layer plane is 1000 angstroms or less, preferably, 900 angstroms or less, and the orientation angle &PSgr; in the direction of fiber axis is 10° or less, preferably, 6° or less. The lower limit of the orientation angle &PSgr; is not particularly limited. However, it is realistically about 2°.
When La becomes large, the mechanical strength of the fiber is generally decreased. In one of main causes, a radial crack type fiber wherein when fibers are embedded in resin, and a cross-sectional surface of a fiber is polished and observed with a microscope, the cross-sectional surface of the fiber, which is originally circular is partly cut to show a sectorial shape, is apt to be produced. The carbon fiber is generally used in a shape of a tow of fibers wherein several hundreds to several thousands of fibers are gathered into one piece. The proportion of the number of radial crack type fibers in a tow of carbon fibers comprised of pitch based graphite fibers of the present invention is generally 20% or less, preferably, 10% of less. Further, the tow of graphite fibers is usually subjected to weaving to form a cloth or a process of impregnating resin to form a pre-preg. When the strength is low, a thread may be broken so that handling properties become poor. Accordingly, the proportion of the radial crack type fibers should be low for the purpose of increasing the strength of the tow of graphite fibers.
On the other hand, when La becomes smaller, the mechanical strength is generally increased, however, there is a problem that the thermal conductivity is decreased. In a g
Fukagawa Toshihiro
Muroi Mika
Yamamoto Iwao
Yoshiya Akihiko
Hendrickson Stuart L.
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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