Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Patent
1995-11-22
1998-07-14
McCamish, Marion E.
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
428364, 264345, 264346, 501 951, B32B 900
Patent
active
057801541
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a boron nitride fiber and a process for production thereof.
More particularly, the present invention relates to a boron nitride fiber having a tensile strength larger than that of any boron nitride fiber known heretofore, as well as to a process for production of the fiber.
BACKGROUND ART
Boron nitride fibers are known. None of known boron nitride fibers, however, has a sufficiently large tensile strength, and no boron nitride fiber having a sufficiently large tensile strength is known yet.
A boron nitride fiber having a sufficiently large strength can be used, for example, as a reinforcing fiber for ceramic material.
Ceramic materials, having a high strength and moreover being stable up to high temperatures, are expected to be applied as a high-temperature structural material which no plastic or metal material can replace. While the ceramic materials have excellent thermal and mechanical properties, they have inherent brittleness which causes cracking easily. Owing to this inherent brittleness of ceramic, fracture of ceramic takes place catastrophically. Therefore, the ceramic materials are not reliable for use as a structural material which must retain a given structure, and are not in wide use.
In order to overcome the brittleness of ceramic, it is effective to blend a ceramic with a reinforcing material to convert the ceramic into a composite material having an improved toughness. As the reinforcing material, there have been studied spherical particles, platy particles, whiskers, continuous fibers, etc. It is particularly effective to blend a ceramic with a continuous fiber for improved toughness, and it is known that the method can increase the fracture toughness of a ceramic to about the same level as that of aluminum alloy. Prospective continuous fibers used as a reinforcing material for converting a ceramic into a composite material are ceramic fibers (e.g. a silicon carbide fiber and an alumina fiber) and a carbon fiber.
Both the ceramic fibers and the carbon fiber, however, have respective drawbacks and are not fully satisfactory as a fiber used as a reinforcing material for converting a ceramic into a composite material. For example, the ceramic fibers, which have a polycrystalline structure consisting of fine crystals, come to possess a significantly reduced tensile strength caused by the growth of the crystals when the ceramic fibers are exposed to high temperatures. In general, when a reinforcing fiber is blended with a ceramic to obtain a composite material, it is necessary to heat them at a high temperature of one thousand and several hundred degrees (centigrade) or above. As a result, a ceramic fiber, when used as a reinforcing material for ceramic to obtain a composite material, causes reduction in tensile strength during the process for obtaining the composite material and it is difficult to obtain a composite material of improved toughness.
Meanwhile, the carbon fiber exhibits little structural change at high temperatures and retains its tensile strength even when heated to about 2,000.degree. C. Consequently, after the heat treatment to obtain a carbon fiber reinforced ceramic matrix composite, the carbon fiber can retain its strength, which makes it possible to use a carbon fiber as a reinforcing material for ceramic matrix composite material of improved toughness. However, the carbon fiber is oxidized and loses its weight in air at temperatures of about 400.degree. C. or above; therefore, the resulting carbon fiber-reinforced ceramic cannot be used at high temperatures in air or in an oxidizing atmosphere.
Thus, there is not yet developed any reinforcing fiber capable of reinforcing a brittle material (e.g. ceramic) and endowing the material with high toughness without impairing the useful properties of the material.
In contrast, a boron nitride fiber, when containing no impurity (e.g. boron oxide) which promotes crystal growth, hardly exhibits structural change (e.g. gram growth of crystals) even at high temperatures and is presum
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Fazen et al. "Thermally Induced Borazine Dehydropolymerization Reactions, Synthesis And Ceramic Conversion Reactions Of A New High-Yield Polymeric Precursor To Boron Nitride", Chemistry of Materials, vol. 2, 96-97 (1990).
Lynch et al., "Transition-Metal-Promoted Reactions Of Boron Hydrides. 10..sup.1 Rhodium-Catalyzed Syntheses Of B-Alkenylborazines", Journal of American Chemical Society, vol. 109, 5867-5868 (1987).
Rees, Jr., et al., "High-Yield Synthesis Of B.sub.4 C/BN Ceramic Materials By Pyrolysis Of Polymeric Lewis Base Adducts Of Decaborane(14)", Journal of America Ceramic Society, vol. 71, C194 (1988).
Okano Yoshio
Yamashita Hiroya
Gray J. M.
McCamish Marion E.
Tokuyama Corporation
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