Compositions – Defined liquid dielectric dispersed in defined web or sheet
Reexamination Certificate
1999-03-15
2001-07-17
Wu, Shean C. (Department: 1756)
Compositions
Defined liquid dielectric dispersed in defined web or sheet
C252S299010, C252S570000, C257S701000, C257S705000, C174S12000C
Reexamination Certificate
active
06261481
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an organic insulating composition, which is electrically insulative in nature and has an excellent thermal conductivity.
DESCRIPTION OF THE PRIOR ART
Almost all electrical appliances, from motors and generators to printed circuit boards and IC chips, are constituted of conductors, through which an electric current is passed, and insulating materials. In recent years, miniaturization of these electrical appliances has undergone rapid progress, for which the requirements of characteristic properties for insulating materials become very severe. Especially, the quantity of heat generated from conductors, which are packed with a high density owing to such miniaturization, increases, with the attendant problem as to how to dissipate the heat.
Conventionally, organic insulating compositions have been in wide use as an insulating material for various types of electrical appliances because of their high insulating performance and the ease in molding. However, organic insulating compositions are usually low in thermal conductivity, which is one of the factors impeding such dissipation of heat, as mentioned above. Accordingly, the need for organic insulating compositions having a high thermal conductivity is a very high.
For achieving high thermal conductivity, there is known method of utilizing conductive materials to an extent not impeding insulating properties. For instance, in Japanese Laid-open Patent Application No. Sho 61-2716, there are described thermoplastic plastics, wherein powders of light metals, such as aluminum, or powders of non-ferrous metals are filled in resins, ensuring good thermal conductivity and electrical insulation. Japanese Laid-open Patent Application No. Sho 63-175493 sets out a printed circuit board that makes use, as an insulating material, of electron conjugated aromatic polymers such as polythiophene. This approach utilizes the nature of electron conjugated aromatic polymers: such polymers serve as a conductor when used in combination with dopants; and, they serve as an insulator when no dopant is used. However, these methods make essential use of conductive substances, so that the dielectric breakdown voltage of the organic insulating compositions becomes very low.
Another method of achieving high thermal conductivity involves the use of inorganic ceramics of high thermal conductivity which are filled in organic insulating compositions. Examples of inorganic ceramics known in the art include silicon dioxide, aluminum oxide, magnesium oxide, beryllium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, aluminum fluoride, calcium fluoride and the like. When inorganic ceramics having both electrical insulating properties and high thermal conductivity are used, the dielectric breakdown voltage is improved, while achieving a high thermal conductivity. However, when inorganic ceramics are mixed with a monomer of the organic insulating composition, the resultant mixture has a much increased viscosity. Thus, the working properties become very poor, and it is difficult to make a fine structure. Moreover, it is known that organic insulating compositions filled with inorganic ceramics are lower in strength. In addition, composite organic insulating compositions, wherein two or more materials are mixed as set out above, are liable to cause separation at the interfaces between the two or more materials, with the possibility that, when these compositions are used over a long time, an abrupt lowering, such as of the dielectric breakdown voltage, may occur.
In Japanese Laid-open Patent Application No. Sho 61-296068, there is described a plastic compound of high thermal conductivity, in which polymer fibers oriented to a super-high degree are used. This makes use of the nature of polymer fibers, as set forth in POLYMER Vol. 19, P. 155 (1978), that super-highly oriented polymer fibers have a thermal conductivity which is improved along the fiber axis. However, the super-highly oriented polymer fibers are lower in thermal conductivity in directions orthogonal to the fiber axis. If the polymer fibers are randomly dispersed in an organic insulating composition, the thermal conductivity is not improved to a significant extent. When polymer fibers are allowed to align in one direction in an organic insulating composition, the resultant organic insulating composition exhibits excellent thermal conductivity along the direction of alignment of the fibers, but its thermal conductivity in the other directions is conversely lower. Further, the composition in this case consists of a composite insulating composition, which contains a mixture of two materials. As set out hereinabove, such a composition is liable to cause separation at the interface between materials.
For achieving high thermal conductivity of a single organic insulating composition, a method is known which utilizes the fact that the thermal conductivity along the direction of an oriented molecular chain becomes high. In Japanese Laid-open Patent Application Nos. Hei 1-149303, Hei 2-5307, Hei 2-28352, and Hei 2-127438, there are described methods of preparing organic insulating compositions comprising polyoxymethylene or polyimides in a state wherein an electrostatic voltage is applied thereto. In Japanese Laid-open Patent Application No. Sho 63-264828, sheets, of the type in which molecular chains of polypropylene, polyethylene or the like are aligned, are laminated so that the directions of the alignment are superposed and bonded together to obtain a laminate. Thereafter, the laminate is cut into thin pieces along a direction parallel to the direction of the alignment to obtain an organic insulating composition wherein the molecular chains are aligned in the longitudinal direction of the thin pieces. In these methods, the molecular chains are aligned in a given direction in order to make use of increase in conductivity along the molecular chains. However, such organic insulating compositions exhibit lower thermal conductivity in the other directions.
In ADVANCED MATERIALS, Vol. 5, p. 107 (1993) and German Laid-open Patent Application No. 4226994, there are described materials obtained by orienting a monomer, such as a diacrylate having a mesogen group, in one direction and subjecting it to a crosslinking reaction. In these materials, molecular chains are likewise aligned in one direction so as to utilize the thermal conductivity that becomes higher along the direction of the molecular chains. Accordingly, the resultant organic insulating compositions become lower in thermal conductivity in the other directions.
As a method which makes use of a single organic insulating composition and with which high thermal conductivity in all spatial directions can be achieved, it may occur to one to use an organic insulating composition which has be converted into a single crystal. However, it is actually very difficult to convert an organic insulating composition into a single crystal.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an organic insulating composition, which has good electric insulating properties and excellent thermal conductivity.
We have made it clear that low thermal conductivity ordinarily experienced in organic insulating compositions is ascribed to defects present in the organic insulating compositions. It has been found that when an organic insulating composition has a liquid crystal resin comprising a polymerization product of a resin composition containing a monomer which has a mesogen group, defects in the organic insulating composition are reduced in number, thereby providing an organic insulating composition having a thermal conductivity of 0.4 W/mK or over. In addition, the organic insulating composition has thermal conductivities in two or more directions including directions substantially vertical to each other, which are, respectively, 0.4 W/mK or over.
REFERENCES:
patent: 4837407 (1989-06-01), Nezu
patent: 5811504 (1998-09-01), Shiota et al.
patent: 206254 (1986-06-01), None
paten
Akatsuka Masaki
Ito Yuzo
Takezawa Yoshitaka
Antonelli Terry Stout & Kraus LLP
Hitachi Ltd
Wu Shean C.
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