Continuous method of producing silicon carbide fibers

Abstract: This invention provides a continuous method for production of polycrystalline ceramic fibers from silicon oxycarbide (SiCO) ceramic fibers. Said method comprises heating an amorphous ceramic fiber containing silicon and carbon in an inert environment comprising boron oxide and carbon monoxide at a temperature sufficient to convert said amorphous ceramic fiber to said polycrystalline fiber. The presence of carbon monoxide during the heating process of the ceramic fiber, unexpectedly achieves higher production rates on a continuous process.

Description: Advanced ceramic fibers are needed for use as reinforcement for ceramic and metal matrix composites. Fiber reinforcement enhances toughness, strength, stiffness and creep resistance for these composites. Silicon carbide fiber has long been sought as a potential reinforcement because it possesses high elastic modulus, excellent thermal stability, attractive thermal expansion characteristics, and relatively low density. Often the desired application temperature exceeds the fiber capability, i.e., the fiber strength degrades significantly in just a few hours. For example, Nicalon™ (SiCO) and Tyranno™ (SiCOTi) silicon oxycarbide fibers degrade at temperatures above 1200°C. Several methods have been developed to make silicon carbide (SiC) based continuous fibers. One approach consists of depositing SiC by chemical vapor deposition (CVD) on a core of carbon monofilament. However, because of the large diameter of these fibers, they are poorly suited for ceramic matrix composite (CMC) reinforcement. Also, they are too stiff to be woven into fabric as required by CMC applications. Another approach involves the melt-spinning of organosilicon polymers to form precursor fibers. The precursor fibers are then pyrolytically transformed into ceramic fibers. The final products have not been pure SiC fiber. This process is used to produce the Nicalon™ and Tyranno™ fibers mentioned above. A third method, disclosed in U.S. Patent 5,279,780 produces near-stoichiometric polycrystalline silicon carbide (SiC) fibers that withstand temperatures up to 1400°C., while still maintaining reasonable strength. That process comprises heating an amorphous or microcrystalline ceramic fiber containing silicon, carbon and oxygen in an environment comprising a volatile sintering aid. The sintering aid is used by merely introducing it into the pyrolysis environment in its volatile state or it is diluted in an inert carrier gas, such as argon or helium. The process is run as a batch or continuous system. When this process is run continuous, it is still limited to slow line rates because more time is needed for the boron to diffuse into the fibers. We have now found that when carbon monoxide is present in the reactor with the carrier gas, high rates of production are achieved while still obtaining fibers with dense microstructure and good physical properties, such as tensile strength and modulus. It is therefore an object of this invention to provide a continuous method for the production of polycrystalline ceramic fibers from silicon oxycarbide (SiCO) ceramic fibers. This invention is a method for the continuous production of polycrystalline ceramic fibers from silicon oxycarbide (SiC and SiCOTi) ceramic fibers comprising heating an amorphous ceramic fiber containing silicon and carbon in an environment comprising boron oxide and carbon monoxide at a temperature sufficient to convert said ceramic fiber to said polycrystalline ceramic fiber. By having carbon monoxide present during heating of the ceramic fiber, it is surprisingly possible to achieve higher production rates on a continuous process. This invention pertains to a method for the continuous production of polycrystalline silicon carbide (SiC) fibers wherein said method comprises heating an amorphous ceramic fiber comprising silicon and carbon in an inert environment comprising boron oxide and carbon monoxide. Fig. 1 represents a schematic diagram of the process of the instant invention. As seen in Fig. 1, our process comprises introducing an amorphous fiber(1) into a first furnace (2). The environment in said furnace is comprised of carbon monoxide, boron oxide and optionally an inert gas (e.g. argon or helium). The fibers (1) are continuously fed through furnace (2), with a residence time in said furnace sufficient to diffuse at least 0.1 wt% of boron into said fibers. The presence of carbon monoxide allows for shorter residence times in the furnace (2) and ultimately higher production speeds. The fibers (1) exit furnace (2) and optionally enter a second furnace (3) where they are further densified. The fibers (1) then exit the second furnace (3) and are collected for use by conventional procedures, such as winding onto spools (4). The operating temperature in the first furnace (2) is typically in the range of 1500 to 1600°C. while the operating temperature in the second furnace (3) is in the range of 1900 to 2000 °C. The conversion and densification steps may be combined and completed in the first furnace (2). This is normally done by heating fiber (1) to higher temperatures during the treating process in furnace (2). The ceramic fibers useful in this invention are not critical and nearly any amorphous ceramic fiber that contains silicon and carbon is suitable. By "amorphous", we mean fibers that are primarily amorphous (not crystalline), including those fibers which contain very small crystallites (nano/microcrystals) in an amorphous continuous phase. The amorphous ceramic fibers herein contain silicon and carbon. Examples are amorphous ceramic fiber comprised of a material selected from the group consisting of SiC, SiCO, SiCN, SiCON, SiCOTi. It is preferred that the silicon and carbon be present in near stoichiometric or moderately carbon rich amounts. Other elements, however, can also be present. These elements include oxygen, nitrogen, titanium, zirconium, chlorine, or any other element which is either volatilized out of the fiber during pyrolysis or remains in the fiber without affecting the integrity of the resulting fiber. Amorphous silicon carbide fibers useful in our claimed invention are further described in U.S. Patent 5,279,780. These fibers are exemplified by SiC, SiCO, SiCN, SiCON, SiCOTi and similar fibers. Preferred are SiCO and SiCOTi fibers. These fibers are commercially available as Nicalon™ fibers from Nippon Carbon (SiCO fibers with a diameter of 10-20 micrometers) and Tyranno™ fibers from UBE Industries (SiCOTi fibers with a diameter of 8-12 micrometers). Commercially available fibers often contain a sizing to prevent their premature bonding together. Although not necessary, it is desirable to remove this sizing before our treatment. This is accomplished by methods known in the art such as chemically dissolving the sizing or heating the fiber to a temperature sufficient to remove the sizing. Methods for manufacturing the above amorphous ceramic fibers are well known in the art. The most common approach generally comprises spinning organosilicon polymers (with or without ceramic powder additives) followed by infusibilization and pyrolysis. Alternative approaches, however, such as various sol-gel processing techniques, CVD techniques, and the like are also suitable. These methods are described in numerous texts, literature references and patents. The amorphous ceramic fibers used in our process are continuous and are of any available length. However, since our method involves the continuous treatment of ceramic fibers to convert them to a silicon carbide (SiC) fiber, it is preferable that said fibers be of sufficient length to achieve the benefits of this invention. The process of this invention comprises heating said amorphous ceramic fiber in an environment comprising a boron oxide, preferably B 2 O 3 , and carbon monoxide. The boron oxide is introduced into the environment in its volatile state. This is accomplished, for example, by placing boron oxide in the furnace and allowing it to vaporize when heated. Alternatively, boron oxide is volatilized outside the furnace and then introduced into the furnace as a vapor. When boron oxide is introduced as a vapor, it can be used neat, it can be diluted in a carrier gas (e.g. inert gases such as argon, helium, etc.) or it can be added under a vacuum. Generally, the time that the fiber is exposed to boron oxide is not critical so long as exposure continues at least until the desired conversion and densification is achieved or until sufficient boron oxide is incorporated into the fiber to produce conversion and densification during the subsequent thermal treatment without any additional boron oxide. The amount of boron oxide present in the environment generally is not critical so long as sufficient boron oxide is present to convert and densify the fiber. Normally, the resultant polycrystalline fibers have 0.1 to 3.5 wt% of boron incorporated therein after conversion and densification. The environment in which the fibers are heated also comprises carbon monoxide. The carbon monoxide is introduced with an inert carrier gas (e.g. argon or helium) or it is introduced neat. The amount of carbon monoxide present in the furnace is not critical. Usually, the carbon monoxide is present in the furnace in an amount of 20 to 100 mole %, preferably 20 to 70 mole%. At lower concentrations of carbon monoxide, lower production speeds are the result. At higher concentrations of carbon monoxide, higher production speeds are achieved; however, a carbon coating may also form on the fiber. The ceramic fibers are thereafter heated at a temperature sufficient to convert the ceramic fiber to polycrystalline fiber. By "polycrystalline", we mean a fiber having at least 75% crystallinity with a small average crystallite size (e.g. less than 1 micrometer). This temperature is in the range of 1500 to 2200°C., and most preferably in the range of 1500 to 1600°C. If the fibers are simultaneously converted and densified, then higher temperatures can be used, preferably in the range of 1900 to 2000°C. The residence time of the fibers in the furnace (time that the fibers are heated) is not critical so long as they allow for diffusion of the boron oxide into the fiber and volatile elements, such as oxygen, are substantially removed from the fibers. This time will be dependent on the length of the furnace, concentration of boron oxide, concentration of carbon monoxide and operating temperature. One skilled in the art will be able to determine the optimum time without undue experimentation. Following heating in the environment of boron oxide and carbon monoxide, the fiber may undergo a second heating step to further convert and densify. This densification is typically conducted in an environment that does not contain any boron oxide and carbon monoxide (e.g. pure argon or helium). Densification is usually performed in a separate furnace by heating the fiber with the boron incorporated therein to a temperature of between 1800 and 2200°C., preferably from 1900 to 2000°C. However, densification can be carried out in the same furnace as used to incorporate the boron into the fiber. Any furnace suitable for the continuous production of fibers may be used herein. However, due to the corrosive nature of boron oxide, it is preferred to use a furnace with a carbon lining. It should be noted that the above process describes the conversion of preformed ceramic fibers (i.e. from a commercial source) to polycrystalline silicon carbide fibers. However, it is contemplated herein that our claimed invention can be incorporated into a standard fiber manufacturing process wherein, for example, fibers are spun, cured, and then pyrolyzed in the presence of both boron oxide and carbon monoxide. So that those skilled in the art can better understand and appreciate the invention taught herein, the following examples are presented to further demonstrate our invention defined in the claims. The experimental apparatus consisted of two furnaces, fiber feed spool and a take up winder. The first furnace was a Lindberg™ Model 54474 fitted with a 7.6 cm (3 inch) ID and 152.4 cm (60 inch) long alumina tube. The alumina tube was lined with a 118.1 cm (46.5 inch) long and 4.4 cm (1.75 inch) diameter Grafoil™ carbon tube with BN spacers situated at both ends of said carbon tube to prevent the tube from touching the hot alumina furnace tube. The second furnace was a carbon lined Astro furnace. To start an experiment, fiber was first strung through the first furnace and the protective tubes at both the inlet and outlet of the furnace. A boat made out of boron nitride containing 10 g of boron oxide powder was situated at 30.5 cm (12 inches) from the entrance of the furnace tube. Carrier gases (argon or argon/CO) at 0.46 m/s (1.5 ft/sec) velocity were turned on to blanket the furnace from air exposure. The winder was then set at a desired uptake speed for a desired throughput rate. To minimize fiber line breakage during start-up, the following procedure was followed for the furnace heat-up period. The three zones were first ramped up at 7°/minutes to 1300°C. The middle and the exit zones were thereafter ramped up at the same rate to the treatment temperature (i.e. 1500°C. or higher) while maintaining the entrance zone at 1300°C until the middle and exit zones reached the desired temperature. The entrance zone was next ramped up to the desired temperature. The fiber was continuously fed through the furnace. Following treatment with boron oxide in the first furnace, the fibers were continuously densified at a temperature of 1920°C. by feeding said fibers through the second furnace in an environment of argon. Tensile Strength and Modulus were determined by pulling the fibers at a constant rate of extension to the breaking point pursuant to ASTM D 3379. Example 1 Tyranno® Lox M fibers were continuously treated according to the above procedure using a mixture of argon and CO as the carrier gas. The amount of CO used in the run is shown in Table 1. The results of the runs and furnace temperature are also reported in Table 1. Comparison Example 1 Tyranno® Lox M fibers were continuously treated as above using argon as the gas (no CO). Useful fibers were obtained at a line speed of 1.4 inches/minute (3.6 cm/min). When the speed was increased to 2.6 inches/minute (6.6 cm/minute), the tensile strength and the modulus were reduced. When the furnace temperature was increased to 1600°C. and the line speed was increased to 4.8 inches/minute (12.2 cm/minute), the fiber was very weak and could not be tested for physical properties. Results are reported in Table 1 (Samples C1-C3). Comparison Example 2 Nicalon® fibers were continuously treated as above. The fibers were treated at a rate of 0.67 ft/minute (20.4 cm/minute) at a furnace temperature of 1580°C. using 100% CO as the carrier gas. The resulting SiC fibers had a tensile strength of 2498 MPa and a modulus of 359 GPa. In the absence of any CO, the fibers were very weak, brownish rather than the typical black color and could not be tested for physical properties. Continuous treatment of Tyranno® fibers. Sample Speed (cm/min) Strength (MPa) Modulus (GPa) Furnace Temp (°C) CO (mol%) 1 20.4 3802 420 1550 45 2 20.4 3202 365 1550 40 3 20.4 2477 324 1550 8 4 61.0 2891 261 1525 25 5 61.0 2919 365 1600 70 to 100 C1 3.6 2877 366 1500 0 C2 6.6 1380 214 1500 0 C3 12.2 nd nd 1600 0 nd=not determined

Claims: A method for the production of polycrystalline ceramic fibers comprising heating an amorphous ceramic fiber comprised of silicon and carbon in an environment comprising a boron oxide at a temperature of 1500°C to 2200°C to convert said amorphous ceramic fiber to said polycrystalline ceramic fiber, characterised in that the environment also comprises carbon monoxide. The method of claim 1 wherein the amorphous ceramic fiber additionally comprises elements selected from oxygen, nitrogen, titanium, zirconium, chlorine and boron. The method of claim 1 wherein the amorphous ceramic fiber comprises a material consisting of SiC, SiCO, SiCN, SiCON, SiCOTi. The method of claim 2 wherein the amorphous ceramic fiber comprises silicon, carbon and oxygen. The method of claim 1 wherein the boron oxide is B 2 O 3 . The method of claim 1 wherein the environment comprises boron oxide and a gas that contains carbon monoxide in an amount of 20 to 100 moles. The method of any of claims 1 to 6, wherein the temperature is in the range of 1500°C to 1600°C. The method of any of claims 1 to 6, wherein the amorphous fibers are simultaneously converted and densified at a temperature in the range of 1900°C to 2000°C.

Inventors: BARNARD THOMAS DUNCAN, NGUYEN KIMMAI THI, RABE JAMES ALAN, BARNARD, THOMAS DUNCAN, NGUYEN, KIMMAI THI, RABE, JAMES ALAN
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1998
Main classification: C04B35
IPC codes: D06M11, C04B35, D01F9, D06M101