Silicon carbide fiber and method of manufacturing the same

Abstract: A silicon carbide fiber having a low specific electric resistance value and excellent mechanical properties including high tensile strength and tensile modulus comprises an amorphous carbon film which is formed on the surface thereof. In a method of manufacturing the same an antimelt-treated polycarbosilane fiber is heated at a temperature not exceeding 1000 o C in an atmosphere of inert gas and thereafter heated at a temperature of above 1000 o C and up to 1500 o C in an atmosphere of mixture of hydrocarbon gas and inert gas.

Description: BACKGROUND OF THE INVENTION The present invention relates to a silicon carbide fiber having a low specific electric resistance value and excellent mechanical properties including high tensile strength and tensile modulus, and to a method of manufacturing the same. There has been known heretofore a method of manufacturing a silicon carbide fiber comprising, for example, steps of spinning a polycarbosilane material into threads or yarns, heating the spun yarns at a temperature ranging between 50°C and and 400°C in an oxidizing gas atmosphere to produce antimelt-treated fibers and heating said antimelt-treated fibers at a temperature of above 1000°C and up to 1500°C in an atmosphere of inert gas such as a nitrogen gas. The silicon carbide fiber obtained as above can be made to have various specific electric resistance values by changing the heating condition in which the antimelt-treated fibers are further heated in an inert gas atmosphere. This silicon carbide fiber manufactured as above has excellent mechanical properties including high tensile strength of 200-350 kg/mm² and high tensile modulus of 18-25 t/mm² in its specific electric resistance range of 10²-10⁸ (Ω· cm). However, a silicon carbide fiber so manufactured according to the foregoing method as to have a low specific electric resistance value such as 10⁻¹ - 10² (Ω·cm) has such a problem that its tensile strength is lowered to 50 - 100 kg/mm² and its tensile modulus also lowered to 5 - 10 t/mm². Therefore, it is the object of the present invention to provide a silicon carbide fiber having a low specific electric resistance and excellent mechanical properties, thereby solving the above-mentioned problem. SUMMARY OF THE INVENTION A silicon carbide fiber manufactured according to the present invention is characterized in that an amorphous film is formed on the surface thereof. As for the manufacturing method thereof according to the present invention, it is characterized in that an antimelt-treated polycarbosilane fiber is heated at a temperature not exceeding 1000°C in an atmosphere of inert gas and thereafter heated at a temperature of above 1000°C and up to 1500°C in an atmosphere of the mixture of hydrocarbon and inert gas. A silicon carbide fiber according to the present invention comprises silicon for 55-60% by weight and carbon for 28-33% by weight, having the same internal composition as another silicon carbide fiber manufactured by Nippon Carbon K.K. and known under the registered trademark name of "NICALON", said NICALON fiber being chiefly composed of fine crystals of β-SiC. In addition, the silicon carbide fiber of the present invention has an amorphous carbon film formed on the surface thereof and measures generally 8-20 µm in diameter. Thickness of the amorphous film formed on the surface of the fiber ranges approximately from 1 nm (nanometer) to 1 µm (micrometer). The antimelt-treated polycarbosilane fibers are heated for 1 to 10 hours in an atmosphere of inert gas such as nitrogen gas, for example. The hydrocarbon gas in the mixture gas used in the subsequent high-temperature heating treatment of said antimelt-treated fiber having been heated as above can be, for example, a propane gas, butane gas, ethane gas, etc. while the inert gas may be nitrogen gas and argon gas, for example. Then, the hydrocarbon content of the mixture gas may be varied according to desired thickness to be obtained of the amorphous carbon film to be formed on the surface of the fiber, said hydrocarbon content being generally in the range of 0.01 to 50 vol.%. The heating time length during which the antimelt-treated fiber is heated in the mixture gas atmosphere can be also varied according to desired thickness of the film to be formed on the fiber and is generally 5 seconds to 6 hours. Since the silicon carbide fiber of the present invention has an amorphous carbon film formed on the surface thereof, it has a lower specific electric resistance and yet maintains good mechanical properties including high tensile strength and high tensile modulus substantially equal to those of the conventional silicon carbide fiber mentioned in the foregoing. According to the silicon carbide fiber manufacturing method of the present invention, an amorphous carbon film is formed on the surface of the fiber by heating an antimelt-treated polycarbosilane fiber at a temperature not exceeding 1000°C in an atmosphere of inert gas and then at a temperature of above 1000°C and up to 1500°C in an atmosphere of the mixture gas of hydrocarbon gas and inert gas. BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 is a characteristic diagram showing relationships between specific electric resistance and tensile strength, of silicon carbide fibers. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS: The present invention will now be explained more in detail by comparing embodiment examples of the present invention with pertinent comparative examples in the following. EMBODIMENT EXAMPLE 1 Polycarbosilane (having an average molecular weight of 2000 and the melting point of 220°C) was melted and spun, and then heated at 180°C for one hour in atmospheric air for an antimelt treatment to obtain an antimelt-treated polycarbosilane fiber measuring 20 µm in diameter. This antimelt-treated polycarbosilane fiber was further heated at 1000°C for 8 hours in a nitrogen gas atmosphere. The antimelt-treated polycarbosilane fiber so heated is then further heated at 1200°C for 2 hours in the atmosphere of mixture gas containing propane gas for 0.5 vol.% and nitrogen gas for 99.5 vol.%, thereby obtaining a silicon carbide fiber of the present invention. The silicon carbide fiber manufactured according to the foregoing method was measured of its specific electric resistance, tensile strength, and tensile modulus. The results obtained are as shown in Table 1. EMBODIMENT EXAMPLE 2 A silicon carbide fiber was manufactured in the same manner as in the EMBODIMENT EXAMPLE 1 above except for the final high-temperature heating treatment in which the mixture gas contained propane gas for 10 vol.% and nitrogen gas for 90 vol.% and the heating time length was 3 hours. Its specific electric resistance, tensile strength, and tensile modulus were measured in the same manner as in the EMBODIMENT EXAMPLE 1. The results obtained are as shown in Table 1. COMPARATIVE EXAMPLE 1 Polycarbosilane (having an average molecular weight of 2000 and the melting point of 220°C) was melted and spun, and then heated at 180°C in atmospheric air for an antimelt treatment to obtain an antimelt-treated polycarbosilane fiber measuring 20 µm in diameter. This antimelt-treated polycarbosilane fiber was further heated at 1200°C in nitrogen atmosphere for 8 hours to obtain a silicon carbide fiber. Its specific electric resistance, tensile strength, and tensile modulus were measured in the same manner as in the above EMBODIMENT EXAMPLE 1. The results obtained are as shown in Table 1. EMBODIMENT EXAMPLE 1 EMBODIMENT EXAMPLE 2 COMPARATIVE EXAMPLE 1 Specific Electric Resistance (Ω·cm) 8 x 10⁻¹ 3 x 10⁻¹ 7.8 x 10³ Tensile Strength (kg/mm²) 310 290 300 Tensile Modulus (t/mm²) 19.2 19.0 20 As is clear from Table 1 above, it is confirmed that the silicon carbide fiber manufactured according to either of the EMBODIMENT EXAMPLES 1 and 2 of the present invention has the tensile strength and tensile modulus substantially equivalent to those of the silicon carbide fiber manufactured according to the conventional COMPARATIVE EXAMPLE 1 and yet has a lower specific electric resistance as compared with that of the latter. EMBODIMENT EXAMPLE 3 Various silicon carbide fibers were manufactured in the same manner as in the EMBODIMENT EXAMPLE 1 except that the heating temperature and time length for the heat treatment in the mixture gas atmosphere were varied, each of said various silicon carbide fibers thus having a different specific electric resistance. Each of these silicon carbide fibers so obtained was measured of the specific resistance and tensile strength in the same manner as in the EMBODIMENT EXAMPLE 1. Relationship between the specific electric resistance and tensile strength of each of said silicon carbide fibers on the basis of the results obtained is shown as a curve A in Figure 1. COMPARATIVE EXAMPLE 2 Various silicon carbide fibers were manufactured in the same manner as in the COMPARATIVE EXAMLE 1 except that the heating temperature and time length for the heat treatment applied to the antimelt-treated polycarbosilane fiber were varied, each of said various silicon carbide fibers thus having a different specific electric resistance. Each of these silicon carbide fibers so obtained was measured of the specific electric resistance and tensile strength in the same manner as in the EMBODIMENT EXAMPLE 1 above. Based on the results so obtained, relationship between the specific electric resistance and tensile strength is as shown by a curve B in Figure 1. As is clear from Figure 1, it is confirmed that each of the silicon carbide fibers manufactured according to the EMBODIMENT EXAMPLE 3 has a lower specific electric resistance and yet possesses a high tensile strength, whereas the tensile strength of a conventional silicon carbide fiber sharply decreased with a decrease in its specific electric resistance. EMBODIMENT EXAMPLE 4 The silicon carbide fiber manufactured according to the EMBODIMENT EXAMPLE 1 above was combined with an epoxy resin to manufacure a fiber-reinforced plastic having the 55% fiber content by volume and reinforced in two directions, 0°/90°. This fiber-reinforced plastic was then measured of this relative dielectric constant, dielectric loss and tensile strength to produce the results as shown in Table 2. Further, a two-layer electric wave absorber was prepared, which comprised an absorption layer made of this fiber-reinforced plastic and a surface layer made of a glass fiber-reinforced plastic composed of glass fiber and an epoxy resin. This two-layer wave absorber was measured of its wave absorption characteristic. The results thereof are as shown in Table 2. For the wave absorption characteristic, return loss (dB) was measured in the entire microwave frequency range of 8 - 12 GHz. COMPARATIVE EXAMPLE 3 The silicon carbide fiber manufactured according to the COMPARATIVE EXAMPLE 1 above was combined with an epoxy resin to manufacture a fiber-reinforced plastic having the 55% fiber content by volume and reinforced in two directions, 0°/90°. This fiber-reinforced plastic was measured of its relative dielectric constant, dielectric loss and tensile strength in the same manner as in the EMBODIMENT EXAMPLE 4 above to obtain the results as shown in Table 2. A two-layer electric wave absorber was then prepared, which comprised an absorption layer made of this fiber-reinforced plastic and a surface layer made of a glass fiber-reinforced plastic composed of glass fibers and an epoxy resin. The two-layer wave absorber was then measured of its wave absorption characteristic. The results obtained are as shown in Table 2. EMBODIMENT EXAMPLE 4 COMPARATIVE EXAMPLE 3 Relative Dielectric Constant 35 5.2 Dielectric Loss 1.2 1.2 Tensile Strength (kg/mm²) 75 70 Wave Absorption Characteristic (dB) 20 4 As is apparent from Table 2 above, it is confirmed that the fiber-reinforced plastic using a silicon carbide fiber of the present invention can provide a superb tensile strength even when its relative dielectric constant and dielectric loss are 35 and 1.2 respectively. It is also confirmed that the two-layer wave absorber having an absorption layer composed of the silicon carbide fiber of the present invention has an excellent wave absorption characteristic. Compared with the above-mentioned wave absorber, another two-layer wave absorber using an absorption layer composed of the conventional silicon carbide fiber showed a very poor wave absorption characteristic. As seen from the foregoing, the silicon carbide fiber according to the present invention can find a wider field of industrial application including use as a fiber material for a fiber-reinforced plastic and for a wave absorption layer in a two-layer wave absorber. EMBODIMENT EXAMPLE 5 The silicon carbide fiber manufactured according to the EMBODIMENT EXAMPLE 1 above was combined with borosilicate glass to manufacture a fiber-reinforced glass ceramic having the 45% fiber content by volume and reinforced in one direction. The fiber-reinforced glass ceramic so obtained was measured of its tensile strength and fracture toughness. The results obtained are as shown in Table 3. COMPARATIVE EXAMPLE 4: The silicon carbide fiber manufactured according to the COMPARATIVE EXAMPLE 1 was combined with borosilicate glass to manufacture a fiber-reinforced glass ceramic having the 45% fiber content by volume and reinforced in one direction. The tensile strength and fracture toughness of this fiber-reinforced glass ceramic were measured in the same manner as in the EMBODIMENT EXAMPLE 5 above to obtain the results also as shown in Table 3. EMBODIMENT EXAMPLE 5 COMPARATIVE EXAMPLE 4 Tensile Strength (kg/mm²) 135 50 Fracture Toughness (MNm -3/2 ) 30 10 As is clear from Table 3 above, it is confirmed that, as compared with a fiber-reinforced glass ceramic using a conventional silicon carbide fiber, one using the silicon carbide fiber of the present invention has an improved mechanical strength. As described in the foregoing, the silicon carbide fiber according to the present invention has an amorphous carbon film formed on the surface thereof, so that it has not only the lower specific electric resistance but also the higher tensile strength and tensile modulus. Further, according to the method of manufacturing the same, an antimelt-treated polycarbosilane fiber is heated at a temperature not exceeding 1000°C in an atmosphere of inert gas and thereafter heated at a temperature of above 1000°C and up to 1500°C in an atmosphere of mixture of hydrocarbon gas and inert gas so that a silicon carbide fiber having an amorphous carbon film formed on the surface thereof and possessing lower specific electric resistance and superb mechanical strength can be manufactured easily. Accordingly, a fiber-reinforced plastic using the silicon carbide fiber of the present invention can make an electric wave absorber featuring a higher wave absorption characteristic and excellent mechanical strength. When combined with a ceramic, it never chemically reacts with the ceramic, so that there can be obtained a fiber-reinforced ceramic having greater strength and fracture toughness.

Claims: A silicon carbide fiber having an amorphous carbon film formed on the surface thereof, said silicon carbide fiber obtained by heating an antimelt-treated polycarbosilane thread at a temperature not exceeding 1000° C in an atmosphere of inert gas, and thereafter heating said thread at a temperature of above 1000°C and up to 1500° C in an atmosphere of hydrocarbon gas and inert gas, wherein said hydrocarbon gas content is in the range of 0.01 to 50 vol.%, said silicon carbide fiber having a specific electric resistance of 10⁻¹ - 10²Ω·cm, a tensile strength of 200 - 350kg/mm² and a tensile modulus of 18 - 25 t/mm². The silicon carbide fiber of claim 1, wherein the thickness of the amorphous carbon film is 1 nm to 1 µm.

Inventors: ICHIKAWA HIROSHI NIPPON CARBON, TOKUTOMI KATSUYA NIPPON CARBON, USHIKOSHI KENJI NIPPON CARBON, YAMAUCHI HIROSHI NIPPON CARBON, ICHIKAWA, HIROSHI NIPPON CARBON CO.,LTD., TOKUTOMI, KATSUYA NIPPON CARBON CO.,LTD., USHIKOSHI, KENJI NIPPON CARBON CO.,LTD., YAMAUCHI, HIROSHI NIPPON CARBON CO.,LTD.
Applicants: NIPPON CARBON CO LTD, NIPPON CARBON CO., LTD.
Application year: 1989
Main classification: C04B41
IPC codes: C04B41, C04B35, C01B31, D01F9