Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Reexamination Certificate
2002-09-26
2004-09-21
Kiliman, Leszek B (Department: 1773)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
C428S403000, C428S407000, C427S212000, C427S228000, C264S029100
Reexamination Certificate
active
06794035
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to graphitized carbon powder and that has excellent thermal conductivity and a thermally conductive composition containing the powder.
With recent advancements, miniaturization, and lightening of electronic hardware, semiconductor packages have become more compact and more highly integrated and operated at higher speed. Therefore, the heat generated by the electronic hardware is a very important issue. Generally, to dissipate the heat from heat-generating components to outside, a molded article such as a sheet material made of a thermally conductive polymer composition and a liquid composition such as polymer grease and adhesive are placed between a radiator and one of the followings: a printed circuit board; a semiconductor package; a housing; a radiation plate and a heat sink.
Such thermally conductive compositions include a matrix, such as resin and rubber, and a filler that has high thermal conductivity in the matrix. Possible fillers include metal oxide, metal nitride, metal carbide, and metal hydroxide. Examples of such possible fillers include aluminum oxide, boron nitride, silicon nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide. However, such compositions do not necessarily have sufficient thermal conductivity.
In order to improve the thermal conductivity, various compositions have been proposed that include highly thermally conductive graphite powders or carbon fibers as filler in the matrix.
For example, Japanese Laid-Open Patent Publication No.62-131033 discloses a molded article made of thermally conductive resin in which the resin is filled with graphite powders. Japanese Laid-Open Patent Publication No.4-246456 discloses a composition of polyester resin containing carbon black or graphite. Japanese Laid-Open Patent Publication No.5-17593 discloses a thermally conductive molded article of great mechanical strength in which the carbon fibers are arranged in a certain direction and are impregnated with graphite powder and thermosetting resin. Japanese Laid-Open Patent Publication No.5-222620 discloses a thermally conductive material using pitch-based carbon fibers that have a specific cross section.
Japanese Laid-Open Patent Publication No.5-247268 discloses a rubber composition in which is mixed synthetic graphite having a particle size of 1 to 20 &mgr;m. Japanese Laid-Open Patent Publication No.9-283955 discloses a thermally conductive sheet in which the graphitized carbon fibers of specific aspect ratio are dispersed in polymer, such as silicone rubber. Japanese Laid-Open Patent Publication No.10-298433 discloses a composition and a radiation sheet in which silicone rubber has, mixed within it, spherical graphite powders having an interplanar spacing of crystals from 0.330 to 0.340 nm. Japanese Laid-Open Patent Publication No.2-242919 and Japanese Laid-Open Patent Publication No.7-48181 disclose certain pitch-based carbon fibers as highly thermally conductive carbon fibers.
However, for recent high performance electronic parts, due to increase in amount of heat generation, the need for greater thermal conductivity has increased. Therefore, the thermal conductivity is still insufficient for the above-mentioned compositions that include graphite powder or carbon fibers as thermally conductive filler.
The object of the present invention is to provide graphitized carbon powders that has excellent thermal conductivity and a thermally conductive composition including such graphitized carbon powders.
BRIEF SUMMARY OF THE INVENTION
A graphitized carbon powder is produced by carbonizing and expanding a pitch by heating the pitch to form carbonaceous foam and by graphitizing and pulverizing the carbonaceous foam.
A thermally conductive composition includes matrix and such graphitized carbon powders in the matrix.
A method of making a graphitized carbon powder includes carbonizing and expanding a pitch by heating the pitch to form a carbonaceous foam; and graphitizing and pulverizing the carbonaceous foam.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described in detail below.
1. Graphitized Carbon Powders
Graphitized carbon powders may be produced by carbonizing and expanding a pitch by heating to form a carbonaceous foam, graphitizing the carbonaceous foam, and then pulverizing the graphitized foam. Alternatively, graphitized carbon powders may be produced by carbonizing and expanding a pitch by heating to form a carbonaceous foam, pulverizing the carbonaceous foam, and then graphitizing the pulverized powders.
The present invention uses a special phenomenon of expansion or foaming. This phenomenon enables graphitized carbon powders to have a highly developed graphite structure and excellent thermal conductivity. Accordingly, a thermally conductive composition including such graphitized carbon powders as a filler also has excellent thermal conductivity.
When a pitch is carbonized and expanded by heated, voids are generated and the pitch is expanded along the perimeter of voids (i.e., the wall that defines each void in the expanded pitch). Thus, a carbonaceous foam is generated that has an expanded or foamed structure in which carbon layers are highly neatly arranged relative to the wall. When the carbonaceous foam is graphitized and pulverized, graphitized carbon powders can be obtained that have a highly developed graphite structure and excellent thermal conductivity.
Japanese Laid-Open Patent Publication No.3-164416 and Japanese Laid-Open Patent Publication No.2000-103610 disclose carbon powders that are formed by pulverizing a certain type of the carbon foam. These publications use the foaming phenomenon to generate the carbon powders. However, they are essentially conceptually different from the present invention in that the foaming is intended for improvement in anti-corrosive properties or chemical resistance or improvement in capacity or strength of an electrode. In addition, since these carbon powders are not graphitized, these powders cannot achieve the objective of the present invention, i.e. improved thermal conductivity.
A pitch as a raw material is obtained by heating tar which is generated by distilling organic materials such as stone coal, coal oil, or wood or by polymerizing polycyclic aromatic compounds such as naphthalene and anthracene. Typical types of such pitch include coal oil-pitch, stone coal-pitch, coal tar-pitch, coke-pitch, and synthetic pitch. Among them, coal oil-pitch and stone coal-pitch are preferred. Especially, isotropic pitch or mesophase pitch is preferred.
The graphitized carbon powders of the present invention are produced by the either method (a) or (b):
(a) a method including a process of carbonizing and expanding a pitch by heating to form a carbonaceous foam, a process of graphitizing the carbonaceous foam by heating to form a graphitized foam, and a process of pulverizing the graphitized foam to form graphitized carbon powders, or
(b) a method including a process of carbonizing and expanding a pitch by heating to form a carbonaceous foam, a process of pulverizing the carbonaceous foam to form carbonaceous powders, and a process of graphitizing the carbonaceous powders to form graphitized carbon powders.
Each process is explained below.
In the carbonizing and expanding process, a method for expanding a pitch is not particularly limited and a conventional expanding method can be applied. Exemplary expanding methods include (1) an expanding method by melting a pitch by heating the pitch in a pressurized state and reducing the pressure acting on the melted pitch from higher pressure to lower one and (2) an expanding method by adding a foaming agent or a volatile substance to a pitch.
In the former method, the higher pressure may be a pressure of any size if it is higher than lower pressu
Shimoyama Naoyuki
Tateda Shinya
Tobita Masayuki
Crompton Seager & Tufte LLC
Kiliman Leszek B
Polymatech Co. Ltd.
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