Process for producing silicon carbide fiber

Details Process for producing silicon carbide fiber Process for producing silicon carbide fiber

1998-12-21

2001-11-13

Meeks, Timothy (Department: 1762)

Coating processes

Particles, flakes, or granules coated or encapsulated

Applying superposed diverse coatings or coating a coated base

C427S215000, C427S249300, C427S249400, C427S249500, C427S249160, C427S255270, C427S255380, C427S255400, C427S372200, C427S397700

Reexamination Certificate

active

06316051

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing silicon carbide fibers. More particularly, the present invention relates to a process for producing silicon carbide fibers having excellent mechanical strength and heat resistance.
2. Description of the Related Art
As a simple process for producing a silicon carbide fiber, Japanese Unexamined Patent Publication No. 6-192,917 discloses a process for preparing a silicon carbide fiber by reacting a porous carbon fiber with a silicon monoxide (SiD) gas at a temperature of 800 to 2000° C.
Also, Japanese Unexamined Patent Publication No. 7-97,281 discloses a process for producing a silicon carbide material having a sheet form structure or three dimensional structure by reacting porous carbon fibers having a specific surface area of 100 to 3000 m
2
/g and formed into a sheet form structure or a three dimensional structure, for example, a honeycomb structure, with a silicon monoxide gas at a temperature of 800 to 2000° C.
Further, Japanese Unexamined Patent Publication No. 7-277,719 discloses a process in which porous carbon fibers having a specific surface area of 100 to 2500 m
2
/g and formed into a sheet form structure or a three dimensional structure are reacted with a silicon monoxide gas at a temperature of 800 to 2000° C., and then the resultant silicon carbide article in the form of fibers, a sheet or a three dimensional structure is heat-treated in a gas atmosphere comprising nitrogen and substantially no oxygen.
Still further, Japanese Unexamined Patent Publication No. 10-53924 discloses a process for producing a silicon carbide fiber in which process, an activated carbon fiber having a specific surface area of 100 to 2500 m
2
/g is reacted with a silicon monoxide gas at a temperature of 800 to 2000° C. under a reduced pressure, and the resultant silicon carbide fiber is coated with a coating containing as a component, a metal oxide. In this process, the metal oxide includes silicon oxide and boron oxide.
This Japanese publication further discloses a process for producing a silicon carbide fiber by reacting an activated carbon fiber with a silicon monoxide gas at a temperature of 800 to 2000° C. under a reduced pressure, heat treating the resultant silicon carbide fiber in an atmosphere containing, as a component, nitrogen at a temperature of 800 to 2000° C., and coating the heat treated silicon carbide fiber with a coating containing, as a component, a metal oxide. In this process, the nitrogen-containing atmosphere is defined to an atmosphere containing, as a component, an nitrogen element-containing gas, for example, a nitrogen gas or ammonia gas, and substantially not containing oxygen.
The above-mentioned prior arts are disadvantageous in that the resultant silicon carbide fiber is unsatisfactory in mechanical strength for composite materials in which the silicon carbide fiber is used as a reinforcing fiber.
Accordingly, there has been a strong demand for a process for producing a silicon carbide fiber having not only a high heat resistance but also a significantly enhanced mechanical strength.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a process for producing silicon carbide fibers having an excellent mechanical strength and a superior heat resistance.
The above-mentioned object can be attained by the process of the present invention, for producing silicon carbide fibers, which comprises the steps of:
reacting activated carbon fibers having a fiber thickness of 1 to 30 &mgr;m and a specific surface area of 700 to 1500 m
2
/g determined by a BET nitrogen absorption method, with a gas comprising at least one member selected from the group consisting of silicon and silicon oxides, at a temperature of 1200 to 1500° C. under a reduced pressure or in an inert gas atmosphere, to convert the activated carbon fibers to a silicon carbide fibers; and
heat-treating the silicon carbide fibers; in the presence of a boron-containing substance at a temperature of 1700 to 2300° C. in an inert gas atmosphere, to enhance the mechanical strength and heat resistance of the silicon carbide fibers.
In the process of the present invention, for producing silicon carbide fibers, preferably, before the heat-treating step, the boron-containing substance is carried on the silicon carbide fibers, and then the boron-containing substance-carrying silicon carbide fibers is subjected to the heat-treating step.
In this embodiment, the boron-containing substance carried on the silicon carbide fibers is preferably in an amount of 0.1 to 50 parts by weight, in terms of boron, per 100 parts by weight of the silicon carbide fibers.
In the process of the present invention for producing silicon fibers, in the heat-treating step, the boron-containing substance may be present in the state of a gas. In this case, the boron-containing substance in the state of a gas is preferably present in an amount of 0.01 to 1% by volume based on the volume of the inert gas.
In the process of the present invention, for producing silicon carbide fibers, the boron-containing substance preferably comprises at least one member selected from the group consisting of boron, boron carbide, borohydrides of alkali metals, borate esters, boron halides, boron hydrides, boric acid and alkali metal borates.
In the process of the present invention for producing silicon carbide fibers, preferably the inert gas for the reaction step and the heat-treating step respectively and independently from each other comprises at least one member selected from argon, helium and neon.
In the process of the present invention for producing silicon carbide fibers, the heat-treating step is preferably carried out in the presence of a carbon-containing substance in addition to the boron-containing substance.
In this case, the carbon-containing substance preferably comprises at least one member selected from carbon, carbon monoxide, carbon dioxide and other carbon compounds which generate at least one member selected from carbon, carbon monoxide and carbon dioxide by being heated in an inert gas atmosphere.
Also, in this case, preferably, before the heat-treating step, the carbon-containing substance is carried on the silicon carbide fibers.
In the process of the present invention for producing silicon carbide fibers, the carbon-containing substance preferably comprises at least one member selected from the group consisting of phenolic resins, carbon black, cellulose and cellulose derivatives.
In this case, the carbon-containing substance carried on the silicon carbide fibers is preferably in an amount of 0.1 to 10% by weight, in terms of carbon, based on the weight of the silicon carbide fibers.
In the process of the present invention for producing silicon carbide fibers, when the heat-treating step is carried out in the presence of a carbon-containing substance, the carbon-containing substance may be in the state of a gas.
In this case, the carbon-containing substance in the state of a gas is preferably present in an amount of 0.01 to 1% by volume, based on the volume of the inert gas.
In the process of the present invention for producing silicon carbide fibers, when the heat-treating step is carried out in the presence of a carbon-containing substance, the carbon-containing substance is preferably present in a weight ratio of carbon element contained in the carbon-containing substance to boron element contained in the boron-containing substance of 0.1:1 to 2.0:1.
In the process of the present invention for producing silicon carbide fiber, the activated carbon fibers are contained in a sheet-like structure or a three-dimensional structure.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the process of the present invention, it is essential that silicon carbide fibers converted from an activated carbon fiber having a fiber thickness of 1 to 30 &mgr;m and a specific surface area of 700 to 1500 m
2
/g determined by a BET nitrogen absorption method, by reacting the activated carbon fiber with a gas comprising at least one member

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