Polycarbosilane for silicon carbide fiber, method for producing the same, and method for producing silicon carbide fiber

Abstract

To provide a fiber material suited to a method for producing silicon carbide fibers endowed with excellent mechanical properties.　The polycarbosilane for silicon carbide fibers according to the present invention is a reaction product of a composition that contains a cyclic silane compound and has a weight average molecular weight (Mw) of from 10,000 to 16,000, a number average molecular weight (Mn) of from 1500 to less than 6000, and a ratio (Mw / Mn) of the weight average molecular weight (Mw) and number average molecular weight (Mn) of from 2.0 to less than 4.5. The method for producing a polycarbosilane for silicon carbide fibers according to the present invention synthesizes a polycarbosilane using a composition that contains a cyclic silane compound and subjects the reaction product to molecular weight adjustment treatment.

Description

Technical Field

[0001] The present invention relates to polycarbosilane for silicon carbide fibers, a method for producing the same, and a method for producing silicon carbide fibers.

Background Art

[0002] Since silicon carbide fibers are lightweight, heat-resistant, and high-strength, they have conventionally been applied to heat engines such as jet engines and gas turbines as a material to replace alloys.

[0003] As a method for producing silicon carbide fibers, for example, PCS infusibilized fibers are produced using a precursor polycarbosilane (hereinafter, "polycarbosilane" may also be referred to as "PCS"). After heating this in an inert gas atmosphere at a temperature not exceeding 1000°C, it is heated at a temperature of 1000 to 1500°C in an atmosphere of a mixed gas of hydrocarbon gas and inert gas to produce silicon carbide fibers (Patent Document 1). The above PCS infusibilized fibers are produced by subjecting PCS fibers to an infusibilization treatment of heating and oxidizing them in air. However, since such PCS infusibilized fibers contain a large amount of oxygen, the silicon carbide fibers obtained by firing them have a problem of inferior heat resistance. To solve this problem, Patent Document 2 proposes a method of irradiating with radiation in an oxygen-free atmosphere or in a vacuum for infusibilization. However, since expensive equipment is required for radiation irradiation, there has been a problem that the manufacturing cost of silicon carbide fibers becomes high.

[0004] In response to such problems, Patent Document 3 proposes a method of adjusting the molecular weight of PCS to produce a high molecular weight PCS that does not require an infusibilization treatment, and producing silicon carbide fibers by dry spinning using the PCS. It is described that silicon carbide fibers having a fiber diameter of 5 to 9 μm and a tensile strength of 2.5 GPa or more can be obtained by the method of Patent Document 3 (paragraphs

[0072] , paragraph

[0075] ).

Prior Art Documents

Patent Documents

[0005] Patent Document 1 Japanese Unexamined Patent Application Publication No. 1-314730 Patent Document 2 Japanese Unexamined Patent Application Publication No. 4-194028 Patent Document 3 Japanese Unexamined Patent Application Publication No. 2019-137935 Summary of the Invention Problems to be Solved by the Invention

[0006] However, the mechanical properties of the silicon carbide fibers produced by the method of Patent Document 3 are not practically sufficient. The tensile strength of silicon carbide fibers is a property that is greatly affected by the defects present in the fibers. Regarding the mechanism by which silicon carbide fibers break, when an external force is applied to the silicon carbide fibers, stress concentration occurs at the defects in the fibers, leading to fiber breakage. Therefore, as the fiber diameter decreases, the amount of defects per fiber decreases. Thus, when the structure of the silicon carbide fibers other than the fiber diameter is the same, the reduction in the fiber diameter reduces the amount of defects in the fibers and improves the tensile strength of the fibers. That is, the mechanical properties of silicon carbide fibers tend to improve as the fiber diameter decreases and to deteriorate as the fiber diameter increases.

[0007] On the other hand, regarding the manufacturing method, when manufacturing fibers with a small fiber diameter, it is necessary to use a thin spinning nozzle in the spinning process. As a result, the spinning amount of the fiber material per unit time decreases, and the production volume of the fibers decreases. Therefore, the reduction of the fiber diameter can be a factor that increases the manufacturing cost of the fibers. Accordingly, there is a demand for a manufacturing method capable of obtaining silicon carbide fibers having excellent mechanical properties such as tensile strength even for silicon carbide fibers with a large fiber diameter. As described above, the mechanical properties of silicon carbide fibers are greatly related to the fiber material and the manufacturing method. Therefore, an object of the present invention is to provide a fiber material suitable for a manufacturing method of silicon carbide fibers having excellent mechanical properties.

[0008] In view of the above problems, the present inventors have studied a method for preparing polycarbosilane (PCS), which is a precursor of silicon carbide. In the process of that research, PCS obtained by using a composition containing a cyclic silane compound and performing a treatment for adjusting the molecular weight (hereinafter referred to as "molecular weight adjustment treatment") was found to be a fiber material suitable for a method for producing silicon carbide fibers having high tensile strength and tensile modulus, and thus the present invention was achieved. Specifically, the present invention includes the following embodiments.

Means for Solving the Problems

[0009] (1) This embodiment is a polycarbosilane for silicon carbide fibers, which is a thermal decomposition condensation reaction product from a composition containing a cyclic silane compound, having a weight average molecular weight (Mw) of 10,000 or more and 16,000 or less, a number average molecular weight (Mn) of 1,500 or more and less than 6,000, and a ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 2.0 or more and less than 4.5.

[0010] (2) This embodiment is a method for producing the polycarbosilane for silicon carbide fibers according to (1) above, (a) a step of heating the composition in a liquid-phase reaction vessel at a first temperature of 300 to 600 °C to vaporize it; (b) a step of heating the gaseous composition obtained in the step (a) in a gas-phase heating region at a second temperature of 500 to 750 °C, which is 5 °C or more higher than the first temperature, to produce polycarbosilane; (c) a step in which the polycarbosilane produced in the step (b) is returned to the liquid-phase reaction vessel, and the gaseous components passing through the gas-phase heating region are cooled and returned to the liquid-phase reaction vessel, and then, (d) a step of heating the components returned to the liquid-phase reaction vessel at the first temperature to vaporize them; (e) a step of heating the gaseous compound obtained in the step (d) in the gas-phase heating region at the second temperature to produce polycarbosilane, (f) The step in which the polycarbosilane produced in the step (e) is returned to the liquid-phase reaction vessel, and the gaseous components that have passed through the gas-phase heating region are cooled and returned to the liquid-phase reaction vessel; (g) After repeating the steps (d), (e), and (f), a molecular weight adjustment step of performing a treatment for adjusting the molecular weight on the compound in the obtained liquid-phase reaction vessel; A method for producing polycarbosilane for silicon carbide fibers, comprising:

[0011] (3) This embodiment is the method for producing polycarbosilane for silicon carbide fibers according to the above (1) or (2), wherein the cyclic silane compound is dodecamethylcyclohexasilane.

[0012] (4) This embodiment includes a spinning step of spinning the polycarbosilane for silicon carbide fibers according to the above (1) to produce polycarbosilane fibers, and a firing step of firing the polycarbosilane fibers in a non-oxidizing atmosphere to produce silicon carbide fibers. The firing step includes (i) firing at 900 °C or higher and 1600 °C or lower, or (ii) primary firing at 900 °C or higher and less than 1200 °C, followed by secondary firing at 1200 °C or higher and 1600 °C or lower. A method for producing silicon carbide fibers.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide PCS for silicon carbide fibers, which is a fiber material suitable for the production of silicon carbide fibers having heat resistance and excellent mechanical properties. According to the present invention, since silicon carbide fibers with excellent mechanical properties can be obtained without applying an infusibilization treatment to the PCS fibers, it contributes to reducing the production cost of silicon carbide fibers.

Brief Description of the Drawings

[0014]

Figure 1

Mode for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be specifically described. The present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention. In this specification, the notation "X to Y" (X and Y are arbitrary numerical values) means "X or more and Y or less".

[0016] [1] Polycarbosilane for silicon carbide fiber The polycarbosilane for silicon carbide fiber according to this embodiment has a weight average molecular weight (Mw) of 10,000 or more and 16,000 or less, a number average molecular weight (Mn) of 1,500 or more and less than 6,000, and a ratio of the weight average molecular weight (Mw) to the number average molecular weight (Mn) (Mw / Mn) is 2.0 or more and less than 4.5, and is a thermal decomposition condensation reaction product from a cyclic silane compound.

[0017] The PCS for silicon carbide fiber according to this embodiment is an organic substance corresponding to a "precursor" applied to the production of silicon carbide fiber. In this specification, the PCS formed into a fibrous shape is described as "PCS fiber", and the fiber obtained by pre-firing the PCS fiber is described as "pre-fired PCS fiber". The "silicon carbide fiber" according to this embodiment is a silicon carbide fiber obtained by subjecting the PCS fiber to a firing treatment to be ceramized.

[0018] The PCS for silicon carbide fibers according to this embodiment is specified such that matters related to the molecular weight are within an optimal range and is specified to be a reaction product from a cyclic silane compound. The PCS is a reaction product generated by the thermal rearrangement and thermal decomposition condensation reaction of the cyclic silane compound as a raw material. By having these specific matters, it is possible to provide a fiber material suitable for manufacturing silicon carbide fibers having heat resistance and excellent mechanical properties. The PCS fibers made of the PCS can be fired to produce silicon carbide fibers without performing an infusibilization treatment. Furthermore, silicon carbide fibers having high tensile strength and tensile modulus can be obtained. The obtained silicon carbide fibers have high tensile strength and tensile modulus even at a large fiber diameter and have good mechanical properties.

[0019] In the process of manufacturing silicon carbide fibers, the PCS as a fiber material changes to silicon carbide. Along with this change, the bond energy between each atom increases, thereby improving the mechanical properties (such as tensile strength and tensile modulus) of silicon carbide. Therefore, theoretically, the closer the atomic ratio of carbon to silicon in silicon carbide is to the stoichiometric ratio, the more the mechanical properties of silicon carbide tend to improve.

[0020] On the other hand, in the process of the reaction in which the PCS changes to silicon carbide, when oxygen atoms coexist, it is known that defects are generated by the desorption of CO gas or SiO gas (see Patent Document 2). Also, in the manufacturing process of silicon carbide fibers, with the crystal growth due to the firing treatment, strain, voids, etc. are generated in the crystal and defects are formed. These defects cause the mechanical properties of the silicon carbide fibers to deteriorate.

[0021] The inventors of the present invention have studied the problems related to the improvement of the mechanical properties of silicon carbide. In the process of that research, it was found that silicon carbide fibers prepared from PCS fibers in which PCS having a relatively large molecular weight (hereinafter referred to as "large PCS") and PCS having a small molecular weight (hereinafter referred to as "small PCS") coexist have high mechanical properties. The detailed reason for this is not clear. The coexistence of large PCS and small PCS increases the degree of filling of PCS in the PCS fibers. Furthermore, in the process of changing from PCS to silicon carbide, while large PCS forms a strong skeletal structure by heating during the firing process, since there is a limit to its molecular movement, when gaps occur between molecules, it is difficult to eliminate such gaps. On the other hand, for small PCS, molecular movement due to heating during the firing process is relatively easy. When large PCS and small PCS coexist, the intermolecular gaps generated in the process of forming a strong skeletal structure by large PCS are filled by small PCS, and as a result, it is presumed that the crystallization of silicon carbide proceeds while forming an arrangement structure advantageous as the crystal structure of silicon carbide. It is considered that silicon carbide fibers having high mechanical properties were obtained by such a mechanism.

[0022] PCS with an excessively small content ratio of small PCS has difficulty in large molecular movement. As a result, in the process of changing from PCS to silicon carbide crystals, it is presumed that crystal defects such as voids are formed inside the silicon carbide fibers, and thus the mechanical properties of the silicon carbide fibers deteriorate. On the other hand, PCS with an excessively small content ratio of PCS having a large molecular weight may melt during the firing process, and thus it is presumed to be difficult to form a fiber structure.

[0023] (Weight average molecular weight (Mw)) The weight average molecular weight (Mw) of the PCS is preferably in the range of 10,000 or more and 16,000 or less. When the weight average molecular weight is less than 10,000, the tensile strength of the PCS fiber becomes low, which is not preferable. Therefore, the weight average molecular weight is preferably 10,000 or more, more preferably 11,000 or more, and still more preferably 12,000 or more. On the other hand, when the weight average molecular weight exceeds 16,000, a large amount of solvent for dissolving the PCS is required when performing dry spinning, which is not preferable. Therefore, the weight average molecular weight is preferably 16,000 or less, more preferably 15,000 or less, still more preferably 14,500 or less, and particularly preferably 14,000 or less.

[0024] (Number average molecular weight (Mn)) The number average molecular weight (Mn) of the PCS is preferably in the range of 1,500 or more and less than 6,000. When the number average molecular weight is less than 1,500, there is a risk that the PCS fiber melts and fuses when the PCS fiber is fired, which is not preferable. Therefore, the number average molecular weight is preferably 1,500 or more, more preferably 2,000 or more, and still more preferably 2,500 or more. On the other hand, when the number average molecular weight is 6,000 or more, a large amount of solvent for dissolving the PCS is required when performing dry spinning, which is not preferable. Therefore, the number average molecular weight is preferably less than 6,000, more preferably 4,000 or less, still more preferably 3,000 or less, and particularly preferably 2,000 or less.

[0025] (Ratio of weight average molecular weight to number average molecular weight (Mw / Mn)) In order to increase the packing density (packing ratio) of PCS molecules in PCS fibers, it is important to appropriately adjust the molecular weight and number of large PCS molecules, the molecular weight and number of small PCS molecules, and the respective ratios at these molecular weights and numbers. Note that the "packing density" and "packing ratio" described in this specification are descriptions meaning similar properties. The weight-average molecular weight (Mw) is greatly affected by the molecular weight distribution of large PCS molecules, while the number-average molecular weight (Mn) is greatly affected by the molecular weight distribution of small PCS molecules. Therefore, the ratio of the weight-average molecular weight to the number-average molecular weight (Mw / Mn) can be an index indicating the ratio of large molecules to small molecules in PCS. By appropriately adjusting the ratio (Mw / Mn) of PCS, the PCS fibers formed from the PCS can increase the packing density of PCS molecules inside them. As a result, silicon carbide fibers having excellent mechanical properties can be obtained by firing the PCS fibers.

[0026] The ratio (Mw / Mn) of the weight-average molecular weight to the number-average molecular weight of PCS is preferably in the range of 2.0 or more and less than 4.5. Hereinafter, this specification may also refer to this "ratio" as the "molecular weight ratio". When the molecular weight ratio (Mw / Mn) is less than 2.0, the silicon carbide fibers obtained using this PCS have a reduced tensile strength, which is not preferable. Therefore, the molecular weight ratio (Mw / Mn) is preferably 2.0 or more, more preferably 2.5 or more, and even more preferably 3.0 or more.

[0027] On the other hand, an increase in the molecular weight ratio (Mw / Mn) corresponds to a broadening of the molecular weight distribution. If the molecular weight distribution of PCS becomes excessively broad, the packing density of PCS molecules in the PCS fibers decreases, and the mechanical properties of the silicon carbide fibers prepared from the PCS fibers are reduced, which is not preferable. Therefore, the molecular weight ratio (Mw / Mn) is preferably less than 4.5, more preferably 4.0 or less, and particularly preferably 3.0 or less.

[0028] (Composition containing cyclic silane compound) PCS is generally manufactured using a polysilane compound as a raw material. The polysilane compound has a skeleton in which Sis are linked in a chain. In contrast, for the PCS for silicon carbide fibers according to the present embodiment, a composition containing a cyclic silane compound is used as the raw material for PCS. The cyclic silane compound has a skeleton in which Sis are linked cyclically.

[0029] The composition containing the cyclic silane compound according to the present embodiment preferably contains 50% by mass or more of the cyclic silane compound, and may contain 60% by mass or more, 80% by mass or more, or 90% by mass or more, or may contain 100% by mass of the cyclic silane compound. The above "cyclic silane compound" is a compound having a skeleton consisting only of Si-Si bonds as the main chain and the main chain forming a ring. The ring number of the cyclic silane compound used in the method for producing polycarbosilane is preferably 15 or less, more preferably 10 or less, and even more preferably 7 or less. The cyclic silane compound may be a single ring or may have a plurality of rings. The side chain of the cyclic silane compound may have any structure. Examples of the cyclic silane compound include octamethylcyclotetrasilane, decamethylcyclopentasilane, dodecamethylcyclohexasilane, and tetradecamethylcycloheptasilane. One or more compounds selected from this group of cyclic silane compounds can be used. From the viewpoint of raw material supply, the cyclic silane compound is preferably dodecamethylcyclohexasilane.

[0030] The above "composition" according to the present embodiment may contain a compound other than the cyclic silane compound. For example, dichlorodimethylsilane, which is a raw material for synthesizing the cyclic silane compound, its decomposition condensate, and a chain polysilane compound can be mentioned.

[0031] By the way, the lower the molecular weight of PCS, the more likely the softening point is to decrease. However, the present inventors have found that for the PCS according to the present embodiment, even PCS fibers prepared from PCS with a low molecular weight can obtain silicon carbide fibers without melting the PCS fibers in the firing process.

[0032] When producing silicon carbide fibers using PCS fibers for silicon carbide fibers according to this embodiment, the reason why the PCS fibers can be fired without melting without applying an infusibilization treatment to the PCS fibers is not clear. The PCS according to this embodiment is a PCS produced from a cyclic silane compound and having a specific molecular weight after being subjected to a molecular weight adjustment treatment. It is presumed that the PCS fibers obtained by spinning the PCS did not melt in the firing process because the molecular weight of the PCS increased in a short time. The mechanism of such an increase in molecular weight is not clear. In the thermal rearrangement and thermal decomposition condensation reactions in which the cyclic silane compound is ring-opened and changed to PCS, both ends of the ring-opened molecular chain become active reaction points, so it is presumed that the obtained PCS has high reaction activity.

[0033] The PCS for silicon carbide fibers according to this embodiment is a reaction product obtained from a composition containing a cyclic silane compound, and is characterized in that it is a PCS having a specific molecular weight range and molecular weight distribution. By spinning using a PCS in which the molecular weight distribution of the PCS is appropriately controlled, it is possible to increase the packing density of the PCS molecules in the PCS fibers. In order to increase the packing density, it is necessary to coexist a certain amount of low molecular weight PCS, while low molecular weight PCS tends to melt easily by heat treatment. The inventors of the present invention have obtained a PCS having a specific molecular weight range and molecular weight distribution from the cyclic silane compound contained in the raw material, and by using the PCS fibers spun from this PCS, it has been found that silicon carbide fibers having high mechanical properties can be obtained by firing without performing an infusibilization treatment on the PCS fibers.

[0034] [2] Method for producing polycarbosilane for silicon carbide fibers The method for producing polycarbosilane for silicon carbide fibers according to this embodiment is a method for producing polycarbosilane for silicon carbide fibers having the characteristics of the above [1], and has the following matters. (a) A step of heating the composition containing the cyclic silane compound at a first temperature of 300 to 600 ° C. in a liquid phase reaction vessel to vaporize it; (b) Heating the gaseous composition obtained in the step (a) in a gas-phase heating region at a second temperature of 500 to 750 °C, which is 5 °C or higher than the first temperature, to produce polycarbosilane; (c) A step in which the polycarbosilane produced in the step (b) is returned to the liquid-phase reaction vessel, and the gaseous components that have passed through the gas-phase heating region are cooled and returned to the liquid-phase reaction vessel; Next, (d) Heating and vaporizing the components returned to the liquid-phase reaction vessel at the first temperature; (e) Heating the gaseous compound obtained in the step (d) in the gas-phase heating region at the second temperature to produce polycarbosilane; (f) A step in which the polycarbosilane produced in the step (e) is returned to the liquid-phase reaction vessel, and the gaseous components that have passed through the gas-phase heating region are cooled and returned to the liquid-phase reaction vessel; (g) After repeating the steps (d), (e), and (f), a molecular weight adjustment step of performing a treatment for adjusting the molecular weight on the compound in the obtained liquid-phase reaction vessel; is included.

[0035] By the above production method, polycarbosilane having a weight average molecular weight (Mw) of 10,000 or more and 16,000 or less, a number average molecular weight (Mn) of 1500 or more and less than 6000, and a ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 2.0 or more and less than 4.5 can be produced.

[0036] (Liquid-phase Gas-phase Thermal Decomposition and Condensation Apparatus) The polycarbosilane for silicon carbide fibers according to this embodiment is preferably produced using a liquid-phase gas-phase pyrolysis condensation reaction (hereinafter, may be simply referred to as "pyrolysis condensation reaction"). The production of PCS by pyrolysis condensation reaction can be carried out, for example, using a liquid-phase gas-phase pyrolysis condensation apparatus 10 (hereinafter referred to as "pyrolysis condensation apparatus") schematically shown in FIG. 1. The pyrolysis condensation apparatus 10 has, as main components, a liquid-phase reaction vessel 1, a flow path 11 for gas-phase heating (hereinafter referred to as "heating flow path"), and a flow path 17 for gas-phase cooling (hereinafter referred to as "cooling flow path"). The liquid-phase reaction vessel 1 is, for example, a cylindrical vessel having a bottom surface, and is provided with a lid 5 for closing the opening at its upper part. The heating flow path 11 and the cooling flow path 17 are, for example, tubular structures.

[0037] (Liquid-phase reaction vessel) Inside the liquid-phase reaction vessel 1, there is a mixture 4 containing a composition containing a raw material cyclic silane compound and PCS of a reaction product. In order to heat the mixture 4 to a predetermined temperature (first temperature), a cylindrical liquid-phase heating means 2 is arranged so as to surround the liquid-phase reaction vessel 1, and a temperature measuring device 3 (for example, a thermocouple) for liquid-phase heating control is provided on the inner surface side of the liquid-phase heating means 2 facing the outer surface of the liquid-phase reaction vessel 1. As the heating means, any means for heating the liquid phase in the liquid-phase reaction vessel may be used. For example, a heater such as a mantle heater and a heating device in which the exterior is integrated can be used.

[0038] Stirring means having a stirring blade 8 and a driving motor 9, a liquid-phase temperature measuring device 6 (e.g., thermocouple) for measuring the temperature of the mixture 4, and an inert gas introduction pipe 7 for supplying an inert gas 20 into the internal space of the liquid-phase reaction vessel 1 are attached via the lid 5 of the liquid-phase reaction vessel 1. The tip of the liquid-phase temperature measuring device 6 and the stirring blade 8 are arranged so as to be immersed in the liquid phase of the liquid-phase reaction vessel 1. That the mixture 4 in the liquid-phase reaction vessel 1 is maintained at the first temperature can be confirmed by the liquid-phase temperature measuring device 6. A pressure gauge 22 is installed in the inert gas introduction pipe 7 to measure the pressure of the liquid-phase reaction vessel 1. By detecting the pressure in the liquid-phase reaction vessel 1, for example, an abnormal internal pressure caused by blockage of the heating flow path 11, the cooling flow path 17, the gas discharge pipe 19, etc. can be detected.

[0039] (Gas-phase heating region) One end of the heating flow path 11 is connected to the liquid-phase reaction vessel 1 via the lid 5 of the liquid-phase reaction vessel 1. The components vaporized from the mixture 4 in the liquid-phase reaction vessel 1 move to the heating flow path 11 and are heated to a predetermined temperature (second temperature) in the gas-phase heating region 14. As shown in FIG. 1, in order to heat the gas phase in the heating flow path 11, a gas-phase heating means 12 is arranged around the heating flow path 11. The gas-phase heating region 14 corresponds to the region heated by the gas-phase heating means 12 in the heating flow path 11. Since the gas-phase heating region is heated and an upward airflow is generated in the heating flow path, the vapor containing the components vaporized from the liquid-phase reaction vessel is drawn into the heating flow path by the chimney effect. As a result, a flow is formed from the liquid-phase reaction vessel into the heating flow path. The inert gas introduced into the liquid-phase reaction vessel also moves into the heating flow path along with the flow.

[0040] The structure of the gas-phase heating means 12 is not particularly limited. It is only necessary to be able to heat the gas components in the heating flow path 11 to a predetermined temperature. As shown in FIG. 1, a divided heater may be used. By controlling the divided heater, a wide isothermal zone is formed as the gas-phase heating region. It is preferable to cover and keep warm the portions other than the portion where the gas-phase heating means 12 is provided around the heating flow path 11 with a heat insulating material so that the temperature does not drop.

[0041] The temperature measuring device 13 for gas-phase heating control is installed on the inner surface side of the gas-phase heating means 12 facing the outer surface of the heating flow path 11. The temperature measuring device 15 (for example, a thermocouple) for measuring the temperature of the gas-phase heating region 14 is inserted into the heating flow path 11 from the end located on the opposite side of the end connected to the liquid-phase reaction vessel 1 in the heating flow path 11, and the tip of the temperature measuring device 15 is arranged to reach near the approximate center of the installation location of the gas-phase heating means 12.

[0042] A pressure gauge 16 for measuring the pressure of the gas phase in the heating flow path 11 is attached to the opposite end in the heating flow path 11. By means of the pressure gauge 16, for example, an abnormality in the internal pressure caused by blockage of the heating flow path 11, the cooling flow path 17, the gas discharge pipe 19, etc. can be detected. When it is necessary to control the amount of vapor flowing from the liquid-phase reaction vessel 1 into the heating flow path 11, an adjustment valve may be installed in the heating flow path 11 midway from the liquid-phase reaction vessel 1 to the gas-phase heating region 14.

[0043] (Cooling region) The cooling flow path 17 is connected to an opening located on the opposite side of the side connected to the liquid-phase reaction vessel 1 in the heating flow path 11. Among the components in the heating flow path 11 that have passed through the gas-phase heating region 14, the high-molecular-weight PCS having a boiling point of the second temperature or higher condenses in the heating flow path 11 to become a liquid and returns to the liquid-phase reaction vessel 1. The other gas components move from the heating flow path 11 into the cooling flow path 17, are cooled in the cooling region 18 in the cooling flow path 17 to change into a liquid, and then return to the liquid-phase reaction vessel 1. The cooling flow path 17 is such that substantially the whole of the cooling flow path 17 corresponds to the cooling region 18, and the vaporized components that have entered the cooling region 18 are gently liquefied by the heat dissipation of the cooling flow path 17.

[0044] If the temperature of the cooling region 18 is too low, the gas components in the cooling flow path 17 may change to a liquid and then become a highly viscous liquid or solidify, which may block the cooling flow path 17. It is preferable to cool the liquid flowing in the cooling flow path 17 to a temperature range suitable for easy flow. Therefore, the periphery of the cooling flow path 17 may be covered with a heat insulating material, or heated and kept warm as necessary.

[0045] Among the components entering the cooling region 18, there are also low-boiling components such as hydrogen, methane, and monosilane generated by the reaction in the gas-phase heating region 14. The low-boiling components are discharged to the outside together with the inert gas introduced from the inert gas introduction pipe 7 as the exhaust gas 21 through the gas discharge pipe 19 provided near the center of the cooling flow path 17. The exhaust gas 21 is appropriately treated outside.

[0046] (Liquid-phase gas-phase thermal decomposition condensation reaction) The polycarbosilane for silicon carbide fibers according to the present embodiment is produced based on a liquid-phase gas-phase thermal decomposition condensation reaction. Specifically, it is produced through the following steps (a) to (g). Hereinafter, each step will be described as "step (a)" to "step (g)".

[0047] Step (a): Step (a) is a step of heating a composition containing a cyclic silane compound at a first temperature of 300 to 600 °C in a liquid-phase reaction vessel to vaporize it. After the composition is placed in the liquid-phase reaction vessel 1, the substances in the liquid-phase reaction vessel 1 are heated at the first temperature by the liquid-phase heating means 2, and the composition is vaporized. Then, the gaseous composition moves into the heating flow path 11. When heating, in order to prevent oxidation of the raw materials, it is preferable to maintain a non-oxidizing gas atmosphere in the liquid-phase reaction vessel 1. For example, as shown in FIG. 1, the atmosphere gas may be replaced by supplying an inert gas 20.

[0048] Step (b): Step (b) is a step of heating the gaseous composition obtained in step (a) in a gas heating region 14 at a second temperature of 500 to 750 °C, which is 5 °C or more higher than the first temperature, to produce PCS. The inside of the heating channel 11 is heated by the gas heating means 12 to form the gas heating region 14 having the second temperature. The gaseous cyclic silane compound vaporized in step (a) and moved into the heating channel 11 synthesizes PCS by gas-phase reactions of thermal rearrangement and thermal decomposition condensation in the gas heating region 14. Note that the second temperature in the gas heating region 14 is specified based on the temperature measured near the center of the gas heating region 14.

[0049] Step (c): Step (c) is a step in which the gaseous components containing the PCS produced in step (b) are cooled and returned to the liquid-phase reaction vessel 1. The PCS produced by the gas-phase reaction in step (b) stays in the gas heating region 14, and a distribution occurs in its residence time. Therefore, a distribution also occurs in the molecular weight of the produced PCS, and PCS having various molecular weights is included. Further, among the reactants produced in the gas heating region 14, unreacted cyclic silane compounds, decomposition products of cyclic silane compounds, etc. are contained. Among these reactants, the high-molecular-weight PCS having a boiling point of the second temperature or higher condenses in the heating channel 11 to become a liquid and returns to the liquid-phase reaction vessel 1. The gaseous components containing other reactants move from the heating channel 11 to the cooling channel 17. Then, after being cooled in the cooling channel 17 and changed to a liquid, they return to the liquid-phase reaction vessel 1.

[0050] Step (d): Next, step (d) is a step of heating and vaporizing the components returned to the liquid-phase reaction vessel 1 at the first temperature. The components returned to the liquid-phase reaction vessel 1 include cyclic silane compounds, low-molecular-weight PCS, decomposition products of cyclic silane compounds, etc. The said components are heated at the first temperature as in step (a). Among the said components, the components having a boiling point lower than the first temperature are vaporized and move to the heating channel 11.

[0051] Step (e): Step (e) is a step of heating the gaseous compound obtained in step (d) in the gas-phase heating region 14 at the second temperature to produce PCS. The gaseous compound transferred to the heating channel 11 in step (d) is, as in step (b), in the gas-phase heating region 14 heated to the second temperature within the heating channel 11, and PCS is synthesized by thermal rearrangement and thermal decomposition condensation reactions.

[0052] Step (f): Step (f) is a step in which the polycarbosilane produced in step (e) is returned to the liquid-phase reaction vessel 1, and the gaseous components that have passed through the gas-phase heating region 14 are cooled and returned to the liquid-phase reaction vessel. Similar to step (c), among the PCS produced in step (e), the high-molecular-weight PCS having a boiling point of the second temperature or higher condenses and changes to a liquid, and returns to the liquid-phase reaction vessel 1. Other gaseous components move from the heating channel 11 to the cooling channel 17, change to a liquid within the cooling channel 17, and return to the liquid-phase reaction vessel 1.

[0053] Repetition of steps (d), (e), and (f): By repeating each of the steps (d), (e), and (f), the low-boiling components are vaporized in the liquid-phase reaction vessel 1, compounds are produced by thermal rearrangement and thermal decomposition condensation reactions in the gas-phase heating region 14, and the reaction products in the gas-phase heating region 14 are refluxed to the liquid-phase reaction vessel 1, which are repeatedly carried out. As a result, since the production of high-molecular-weight PCS proceeds, the content ratio of high-molecular-weight PCS having a boiling point higher than the first temperature increases in the liquid-phase reaction vessel 1. On the other hand, the content ratios of other components decrease. That is, the content ratios of the composition containing the cyclic silane compound, the decomposition products of the composition, and PCS having a molecular weight less than a predetermined molecular weight that gives a boiling point higher than the first temperature decrease. Since the PCS that has reached the predetermined molecular weight is not vaporized at the first temperature in the liquid-phase reaction vessel 1, an excessive increase in the molecular weight is suppressed. By controlling the first temperature in the liquid-phase reaction vessel 1 and the second temperature in the gas-phase heating region 14, PCS having a predetermined molecular weight can be selectively produced.

[0054] Step (g): Step (g) is a molecular weight adjustment step of subjecting the compound in the obtained liquid-phase reaction vessel to a treatment for adjusting the molecular weight after repeating each of the steps (d), (e), and (f). The PCS prepared by the liquid-phase gas-phase thermal decomposition condensation reaction has a molecular weight distribution including from low molecular weight components to high molecular weight components. The molecular weight adjustment treatment is a screening treatment for suppressing the fusion of fibers without melting even at a high temperature for firing PCS fibers by removing low molecular weight components that cause fusion from PCS and leaving high molecular weight components.

[0055] Specifically, PCS is held in a solvent for a predetermined time. The type of the solvent is not particularly limited as long as it can dissolve PCS. For example, ethyl acetate, acetone, hexane, etc. can be mentioned. These solvents may be mixed. By changing the mixing ratio of the solvents, the molecular weight range of the low molecular weight components to be dissolved can be changed.

[0056] The method for producing PCS for silicon carbide fibers according to the present invention may include other steps other than the liquid-phase gas-phase thermal decomposition condensation method. The other steps are not particularly limited as long as the effects of the present invention are not impaired.

[0057] The method for producing PCS for silicon carbide fibers according to the present embodiment uses a "composition containing a cyclic silane compound" as a raw material for PCS.

[0058] (First temperature) The first temperature is the temperature at which the substances contained in the liquid-phase reaction vessel are heated, and can be set in the range of 300°C to 600°C. If the first temperature is less than 300°C, it is not preferable because it is difficult for low molecular weight PCS to vaporize. Also, the vaporization rate of the substances decreases, and the production rate of PCS decreases, so it is not preferable. Therefore, the first temperature is preferably 300°C or higher, more preferably 400°C or higher, and even more preferably 450°C or higher. On the other hand, if the first temperature exceeds 600°C, there is a risk of generating PCS with an excessive molecular weight, and furthermore, the production ratio of coagulated substances increases, so it is not preferable. Therefore, the first temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 500°C or lower.

[0059] (The second temperature) The second temperature is the temperature at which heating occurs in the gas-phase heating region in the heating flow path, and can be set in the range of 500°C to 750°C. If the second temperature is less than 500°C, the reaction rates of the thermal rearrangement and thermal decomposition condensation reactions of PCS decrease, leading to a decrease in productivity and difficulty in generating high molecular weight PCS, so it is not preferable. Therefore, the second temperature is preferably 500°C or higher, more preferably 525°C or higher, and even more preferably 550°C or higher. On the other hand, if the second temperature is higher than 750°C, it is not preferable because it is difficult to spin solids are generated and the viscosity of the produced polycarbosilane increases significantly, which may block the pipes of the manufacturing equipment. Therefore, the second temperature is preferably 750°C or lower, more preferably 725°C or lower, and even more preferably 700°C or lower.

[0060] The second temperature needs to be in a range higher than the first temperature. And the temperature difference between the second temperature and the first temperature can be set to 5°C or more. If the temperature difference is less than 5°C, the progress of the thermal rearrangement and thermal decomposition condensation reactions in the gas-phase heating region becomes slow, so it is not preferable. Therefore, the temperature difference is preferably 5°C or more, more preferably 30°C or more, even more preferably 60°C or more, and particularly preferably 90°C or more.

[0061] (Heating time) The process of heating the liquid-phase reaction vessel to the first temperature and the process of heating the gas-phase heating region of the gas-phase reaction tube to the second temperature may be continued until the PCS reaches a predetermined molecular weight. The time required to produce PCS having the target molecular weight varies depending on the type of cyclic silane compound used as the raw material, the first temperature, the second temperature, and the like. As the heating time extends, the molecular weight of the resulting PCS tends to increase. If the heating time is too short, the reaction time required for the production of high-molecular-weight PCS is insufficient, and high-molecular-weight PCS cannot be obtained sufficiently. Therefore, depending on the selected first temperature and second temperature, the heating time is preferably 4.0 hours or more, more preferably 5 hours or more, still more preferably 5.5 hours or more, and particularly preferably 6.0 hours or more. The heating treatment may be carried out continuously or in divided stages. When carried out in divided stages, the heating time is the cumulative sum of each heating time. On the other hand, after obtaining PCS having the required predetermined molecular weight, it is desirable to stop the heating to reduce the cost required for heating. This specification also describes this heating time as the "reaction time".

[0062] (Cooling temperature) The reactant that has passed through the gas-phase heating region is cooled in the cooling flow path. The cooling temperature only needs to be able to cool the gaseous components to the extent that they become liquid. If the cooling temperature is too low, the reactant solidifies or the viscosity of the liquid reactant increases, blocking the inside of the path, which is not preferable. The cooling temperature is preferably 50°C or higher and 300°C or lower. To maintain a predetermined cooling temperature, the cooling flow path may be heat-insulated.

[0063] (Non-oxidizing gas) The type of the atmosphere gas in the liquid-phase reaction vessel may be a non-oxidizing gas that does not react with the composition containing the cyclic silane compound and the reaction products such as PCS, and is not particularly limited. For example, an inert gas is preferable, and nitrogen gas or noble gas can be used alone or in combination.

[0064] The time for heating and reacting the liquid-phase reaction vessel and the gas-phase heating region can be appropriately adjusted according to the first temperature and the second temperature.

[0065] [3] Method for manufacturing silicon carbide fiber The method for manufacturing silicon carbide fiber according to this embodiment includes a spinning step of spinning the PCS for silicon carbide fiber obtained by the method for manufacturing PCS for silicon carbide fiber in [2] above to produce PCS fiber, and a firing step of firing the polycarbosilane fiber in a non-oxidizing atmosphere to produce silicon carbide fiber. The firing step includes (i) firing at 900°C or higher and 1600°C or lower, or (ii) primary firing at 900°C or higher and less than 1200°C, followed by secondary firing at 1200°C or higher and 1600°C or lower.

[0066] (Non-oxidizing gas) The type of non-oxidizing atmosphere gas in firing may be any non-oxidizing gas that does not react with PCS fiber and is not particularly limited. For example, an inert gas is preferable, and nitrogen gas or noble gas can be used alone or in combination.

[0067] (Spinning step) The spinning step is a step of forming PCS into fibers. General spinning methods include melt spinning method, dry spinning method, and wet spinning method, etc. The method for manufacturing silicon carbide fiber according to this embodiment preferably applies the dry spinning method. The dry spinning method is a method of adding a solvent to a precursor to prepare a precursor solution and spinning using the precursor solution. Dissolve PCS in a solvent to prepare a solution for dry spinning, and adjust the viscosity of the solution. Then, supply the solution for dry spinning to a spinning device to produce PCS fiber.

[0068] The solvent for dissolving PCS in the solution for dry spinning is not particularly limited as long as it can dissolve PCS. For example, aromatic hydrocarbons such as benzene, toluene, ethylbenzene, xylene, and mesitylene, aliphatic hydrocarbons such as hexane, heptane, octane, and nonane, and halogenated hydrocarbons such as chloroform and dichloromethane can be mentioned. From the viewpoint of excellent solubility and volatility of PCS, toluene or xylene is preferable.

[0069] The concentration of the solution for dry spinning can be adjusted as appropriate. For example, the concentration can be selected in the range of 50 to 70 wt%.

[0070] The solution viscosity of the solution for dry spinning can be adjusted as appropriate according to the nozzle diameter of the spinning device. For example, when the nozzle diameter is 65 μm, the solution viscosity at 25°C is preferably 10 to 30 Pa·s. The solution viscosity is determined by a known measurement method. The solution viscosity can be measured, for example, using an E-type viscometer.

[0071] The spinning device used in the spinning process can apply the spinning device and spinning conditions commonly used in this technical field. The solution for dry spinning is supplied to the spinning device, and for example, spinning is performed under the conditions of a spinning nozzle diameter of 65 μm, a discharge pressure of 2 to 3.5 MPa, and an extrusion rate of the PCS solution of 10 to 30 mg / min to obtain a predetermined PCS fiber.

[0072] (Firing process) The method for producing silicon carbide fibers according to this embodiment includes a firing process of firing the PCS fibers in a non-oxidizing atmosphere to produce silicon carbide fibers. The firing process is a process of obtaining silicon carbide fibers by firing the PCS fibers generated in the spinning process in a non-oxidizing atmosphere to convert them into ceramics.

[0073] The firing process preferably includes (i) firing at 900°C or higher and 1600°C or lower, or (ii) primary firing at 900°C or higher and less than 1200°C, followed by secondary firing at 1200°C or higher and 1600°C or lower. It is preferable to generate silicon carbide fibers by these firing treatments.

[0074] The firing process in (ii) above is carried out in two stages: primary firing and secondary firing. The primary firing in (ii) above is a process mainly aimed at firing PCS fibers at a temperature of 900 °C or higher and less than 1200 °C in a non-oxidizing atmosphere, removing hydrogen atoms and excessive carbon atoms from PCS, and changing PCS fibers into silicon carbide fibers. By primary firing of PCS fibers, the PCS fibers change into silicon carbide (SiC) fibers, and the tensile strength of the fibers increases along with this chemical reaction. If the temperature is less than 900 °C, the degree of change into silicon carbide is insufficient, and the tensile strength of the silicon carbide fibers becomes low. Therefore, the temperature of the primary firing is preferably 900 °C or higher. Also, from the viewpoint of achieving the purpose of the primary firing, the temperature of the primary firing may be set to less than 1200 °C.

[0075] The secondary firing in (ii) above is a process mainly aimed at firing silicon carbide fibers at a temperature of 1200 °C or higher and 1600 °C or lower in a non-oxidizing atmosphere, promoting the crystallization of silicon carbide, and aiming for high strength. If the temperature exceeds 1600 °C, crystallization proceeds excessively and the crystallite size becomes too large. As a result, the tensile modulus of elasticity of the fibers increases, the fibers become brittle, and the mechanical strength begins to decrease. Therefore, the temperature of the secondary firing is preferably 1600 °C or lower. Also, from the viewpoint of promoting the secondary firing efficiently, the secondary firing temperature may be set to 1200 °C or higher.

[0076] The non-oxidizing atmosphere for the primary firing is not particularly limited as long as it is an atmosphere of a non-oxidizing gas in which PCS does not oxidize. As the non-oxidizing gas, nitrogen, noble gas, and their mixtures can be used. Note that within the heating temperature range of the primary firing, silicon and nitrogen hardly react. The non-oxidizing atmosphere for the secondary firing is not particularly limited as long as it is a non-oxidizing gas atmosphere in which silicon does not react. Since there is a risk that silicon reacts with nitrogen and is nitrided at high temperatures, the firing process is preferably carried out in a noble gas such as argon.

[0077] The firing process in (i) above is a one-step firing process, which is carried out in a temperature range of 900 °C or higher and 1600 °C or lower. Depending on the heated firing temperature, it includes the case where it stops at the primary firing in (ii) above, or the case where it further reaches the secondary firing in (ii) above. At the heating temperature corresponding to the primary firing, a high tensile strength is mainly obtained by the change to silicon carbide fibers. At the heating temperature corresponding to the secondary firing, in addition to the change to silicon carbide fibers, a higher tensile modulus of elasticity is obtained by the progress of the crystallization of silicon carbide.

[0078] For the firing process in (i) above, a non-oxidizing gas atmosphere corresponding to the heating temperature may be adopted. At the heating temperature corresponding to the primary firing in (ii) above, nitrogen, noble gas, and their mixtures can be used so that PCS does not oxidize. At the heating temperature corresponding to the secondary firing in (ii) above, noble gases such as argon and their mixtures can be used so that silicon does not oxidize.

[0079] Before each firing of (i) or (ii) in the firing process, a preliminary firing may be carried out in a non-oxidizing atmosphere at 500 °C to less than 900 °C. The preliminary firing is a process for removing excess carbon atoms. The gas forming the non-oxidizing atmosphere is not particularly limited as long as it is a non-oxidizing gas. As the non-oxidizing gas, nitrogen, noble gas, hydrogen, and their mixtures can be used. If excess carbon exists in the manufacturing process of silicon carbide fibers, carbon atoms are desorbed by firing, which causes a decrease in the tensile strength of the silicon carbide fibers. Therefore, it is desirable to mix hydrogen with nitrogen or noble gas as the atmosphere gas during the preliminary firing to reduce the excess carbon content in the PCS fibers before the primary firing. The hydrogen content in the gas atmosphere during the preliminary firing is preferably 30% by volume to 70% by volume, and more preferably 50% by volume to 70% by volume.

[0080] (Mechanical Properties of Silicon Carbide Fibers) The silicon carbide fibers produced using the PCS for silicon carbide fibers according to this embodiment have high tensile strength and tensile modulus. Regarding the tensile strength, silicon carbide fibers having a tensile strength of 2.2 GPa or more, 2.5 GPa or more, 2.7 GPa or more, 2.8 GPa or more, or 2.9 GPa or more can be obtained. Regarding the tensile modulus, silicon carbide fibers having a high tensile modulus of 210 GPa or more, 230 GPa or more, 260 GPa or more, 300 GPa or more, or 330 GPa or more can be obtained.

[0081] Furthermore, the silicon carbide fibers according to this embodiment have the above-mentioned high tensile strength at a fiber diameter (diameter) of 10.0 μm or more. As described above regarding the problems of the prior art, as the fiber diameter of silicon carbide fibers increases, the tensile strength of the fibers tends to decrease. The silicon carbide fibers according to this embodiment can provide a useful material in that they have excellent mechanical properties even with a large fiber diameter.

Examples

[0082] Hereinafter, examples according to the present invention will be described. The scope of the present invention is not limited to the following description.

[0083] The methods for measuring the physical property values of molecular weight, solution viscosity, tensile strength, tensile modulus, and fiber diameter will be described below. The physical property values according to the present invention and the examples are based on the numerical values obtained by these measurement methods.

[0084] <Molecular weight> In accordance with the method specified in JIS K7252-1:2016 (ISO16014-1:2012), the weight-average molecular weight (Mw) and number-average molecular weight (Mn) were measured. Specifically, the molecular weight of PCS was measured using a liquid chromatogram (HPLC manufactured by Shimadzu Corporation) and columns manufactured by Showa Denko K.K. (one each of KF-604, KF-602, and KF-601 were connected in sequence from the pump side and used). Toluene was used as the measurement solvent, the sample solution concentration was 0.5 wt%, the flow rate to the analysis unit and reference was 0.40 mL / min, the oven temperature was 40 °C, and the molecular weight was measured using a differential refractometer as the detector.

[0085] <Solution viscosity> The viscosity of the solution for dry spinning was measured using an E-type viscometer (ARES-G2 manufactured by TA Instruments) with a test jig of a φ25 mm stainless steel parallel plate, a solution thickness of 0.5 mm, a temperature of 25 °C, and a shear rate of 200 sec -1 under the following conditions.

[0086] <Tensile strength and tensile modulus> In accordance with the measurement method of JIS R7606:2000, the tensile strength and tensile modulus of the silicon carbide fiber were measured. Ten silicon carbide fibers were randomly selected, and the tensile strength and tensile modulus of each silicon carbide fiber were measured, and the numerical value obtained by averaging the 10 measured values was adopted.

[0087] <Fiber diameter> The fiber diameter was measured using 10 fibers used for the measurement of tensile strength and tensile modulus with an optical microscope (VHX-5000 manufactured by Keyence Corporation) at a magnification of 2000 times, and the numerical value obtained by averaging the measured values was adopted.

[0088] (Example 1) (1) Production of PCS for silicon carbide fiber Polysilcarbosilane (PCS) was produced using a pyrolysis condensation apparatus 10 as shown in Fig. 1. First, dodecamethylcyclohexasilane (DMCHS) was placed into the liquid-phase reaction vessel 1 as the raw material cyclic silane compound. The temperature inside the liquid-phase reaction vessel 1 is hereinafter referred to as the "first temperature". A first temperature of 485 °C was selected. The inside of the liquid-phase reaction vessel 1 was heated to the first temperature to vaporize the cyclic silane compound. The vaporized cyclic silane compound moved into the heating channel 11 and passed through the gas-phase heating region 14. The temperature of the gas-phase heating region 14 is hereinafter referred to as the "second temperature". A second temperature of 600 °C was selected. The gas-phase heating region 14 was heated to the second temperature, and in the gas-phase heating region 14, PCS having various molecular weights was generated by the thermal rearrangement and pyrolysis condensation reaction of the cyclic silane compound.

[0089] After the substance in the heating channel 11 passed through the gas-phase heating region 14, it moved into the cooling channel 17. Among the PCS generated in the gas-phase heating region 14, the high molecular weight PCS having a boiling point of the second temperature or higher condensed in the heating channel 11 to become a liquid and returned to the liquid-phase reaction vessel 1. The gaseous components other than the high molecular weight PCS were cooled in the cooling channel 17 and returned to the liquid-phase reaction vessel 1.

[0090] The components and unreacted cyclic silane compound that returned to the liquid-phase reaction vessel 1 were vaporized again in the liquid-phase reaction vessel 1 heated to the first temperature. The vaporized cyclic silane compound moved to the heating channel 11, PCS was generated in the gas-phase heating region 14 heated to the second temperature, and furthermore, the high molecular weight of PCS proceeded. Among the high molecular weight PCS generated in the heating channel 11, the high molecular weight PCS having a boiling point of the second temperature or higher was condensed and returned to the liquid-phase reaction vessel 1, and the gaseous components that passed through the gas-phase heating region 14 were cooled in the cooling channel 17 and returned to the liquid-phase reaction vessel 1. In this way, a cyclic reaction of heating, vaporizing the liquid-phase reaction vessel, generating PCS in the gas-phase heating region, and proceeding with the high molecular weight of PCS, and returning to the liquid-phase reaction vessel was performed.

[0091] The process of heating the liquid-phase reaction vessel to the first temperature and the process of heating the gas-phase heating region to the second temperature were carried out for a heating time of 6.3 hours (hereinafter referred to as the "reaction time"), and the above-mentioned circulation reaction was continued. Thereafter, the heating was stopped, and the liquid-phase reaction vessel and the cooling flow path were allowed to cool to room temperature by natural cooling, and polycarbosilane (PCS) having a predetermined molecular weight was obtained in the liquid-phase reaction vessel.

[0092] For the obtained PCS, the following molecular weight adjustment treatment was further performed. Ethyl acetate having a mass five times that of PCS was added to PCS to prepare a mixture. Next, the mixture was heated and stirred at 50 ° C, and then the liquid was removed. By repeating this operation four times, low-molecular-weight PCS dissolved in ethyl acetate was removed. Then, ethyl acetate was removed from the remaining mixture to obtain PCS.

[0093] (2) Preparation of PCS fibers The obtained PCS was dissolved in xylene as a solvent to prepare a solution for dry spinning. After filtering and removing coagulants and the like in the solution, the PCS fibers were obtained by winding up the fibers extruded from the spinneret (nozzle) with a nozzle diameter of 65 μm using the solution.

[0094] (3) Preparation of silicon carbide fibers The PCS fibers were fired by the following procedure to produce silicon carbide fibers. The PCS fibers were heated to 500 ° C at a rate of 150 ° C / h in a nitrogen atmosphere. Thereafter, pre-firing was performed by heating from 500 ° C to 800 ° C at a rate of 100 ° C / h in a mixed gas atmosphere containing 40% by volume of argon gas and 60% by volume of hydrogen gas. Next, after heating from 800 ° C to 1000 ° C at a rate of 150 ° C / h in an argon gas atmosphere, firing was performed by holding at 1000 ° C for 1 hour. After firing, the heating was stopped and the temperature was allowed to cool to room temperature to obtain silicon carbide fibers.

[0095] The production conditions and physical property values of PCS are shown in Table 1. The physical property values of the solution for dry spinning of PCS and the physical property values of the obtained silicon carbide fibers are shown in Table 2.

[0096] (Example 2) Silicon carbide fibers were obtained in the same procedure as in Example 1, except that the PCS fibers produced in Example 1 were heated from 800°C to 1400°C at a rate of 150°C / h and then held at 1400°C for 1 hour.

[0097] (Example 3) After manufacturing PCS with the same procedure as in Example 1, with the first temperature for PCS production being 475°C, the second temperature being 650°C, and the reaction time being 4.4 hours, the obtained PCS was subjected to molecular weight adjustment treatment with ethyl acetate four times. Then, eight times the mass of hexane was added to the treated PCS to prepare a mixture. After stirring the mixture at room temperature (25°C), the liquid was removed to remove the low molecular weight PCS dissolved in hexane. Then, hexane was removed from the remaining mixture to obtain PCS with adjusted molecular weight. Using the obtained PCS, silicon carbide fibers were produced in the same procedure as in Example 1.

[0098] (Example 4) PCS with adjusted molecular weight and silicon carbide fibers were obtained in the same procedure as in Example 1, except that the reaction time for PCS production was 6.5 hours.

[0099] (Example 5) Using the PCS fibers prepared in Example 4, silicon carbide fibers were obtained in the same procedure as in Example 2.

[0100] (Example 6) PCS with adjusted molecular weight was obtained in the same procedure as in Example 1, except that the first temperature for PCS production was 480°C, the reaction time was 6.0 hours, a mixed solvent consisting of 70% by mass of acetone and 30% by mass of hexane was used as the solvent for the molecular weight adjustment treatment, and the number of molecular weight adjustment times was 1.

[0101] (Example 7) A PCS with adjusted molecular weight was obtained by the same procedure as in Example 6, except that a mixed solvent consisting of 80% by mass of acetone and 20% by mass of hexane was used as the solvent for the molecular weight adjustment treatment.

[0102] (Example 8) The silicon carbide fiber produced in Example 1 was further heat-treated. Since the silicon carbide fiber of Example 1 was obtained by firing at 1000°C, it corresponds to the one subjected to primary firing. Using the silicon carbide fiber, the temperature was raised to 1000°C at a rate of 150°C / h in a nitrogen atmosphere, and then the temperature was raised from 1000°C to 1400°C at a rate of 150°C / h in an argon gas atmosphere, and then held at 1400°C for 1 hour to perform secondary firing. After the secondary firing, heating was stopped and the sample was allowed to cool to room temperature to obtain silicon carbide fibers.

[0103] (Example 9) PCS with adjusted molecular weight and silicon carbide fibers were obtained by the same procedure as in Example 1, except that a cyclic silane mixture (containing 89.7 wt% of dodecamethylcyclohexasilane (DMCHS), 6.9 wt% of decamethylcyclopentasilane (DMCPS), and 1.9 wt% of tetradecamethylcycloheptasilane (TDMCHS)) was used instead of dodecamethylcyclohexasilane (DMCHS), and the reaction time for PCS production was 5.0 hours.

[0104] (Comparative Example 1) PCS with adjusted molecular weight and silicon carbide fibers were obtained by the same procedure as in Example 1, except that polydimethylsilane (PDMS) was used instead of dodecamethylcyclohexasilane (DMCHS), and the reaction time for PCS production was 5.0 hours.

[0105] (Comparative Example 2) PCS with adjusted molecular weight and silicon carbide fibers were obtained by the same procedure as in Comparative Example 1, except that the reaction time for PCS production was 8.0 hours.

[0106] (Comparative Example 3) Except that the first temperature for PCS production was 480 °C, the reaction time was 6.0 hours, and no molecular weight adjustment treatment was performed, PCS and silicon carbide fibers were obtained by the same procedure as in Example 1.

[0107] (Comparative Example 4) Except that the first temperature for PCS production was 475 °C and the reaction time was 5.0 hours, PCS with adjusted molecular weight and silicon carbide fibers were obtained by the same procedure as in Example 1.

[0108] For the obtained silicon carbide fibers in the above Examples and Comparative Examples, the weight average molecular weight, number average molecular weight, solution viscosity (Pa·s), fiber diameter (μm), tensile strength (GPa), and tensile modulus (GPa) were measured by a predetermined measurement method. The solution viscosity (Pa·s) of the solution for dry spinning, the diameter (μm) of the PCS fiber, and the diameter (μm) of the silicon carbide fiber were measured by a predetermined measurement method. The solution concentration (wt%) of the solution for dry spinning was calculated from the blending ratio. These results are shown in Table 1 and Table 2.

[0109] In the column of "Molecular Weight Adjusting Solvent" in Table 1, "EtOAc" is ethyl acetate, "n-H" is n-hexane, "ACE" is acetone, "80%ACE / 20%n-H" is a mixed solvent of 80 mass% acetone and 20 mass% n-hexane, and "70%ACE / 30%n-H" is a mixed solvent of 70 mass% acetone and 30 mass% n-hexane.

[0110] The firing treatments of Examples 1 to 5 and Comparative Examples 1 to 4 correspond to the one-step treatment of (i) in this embodiment, and the firing temperature column in Table 2 shows the firing temperature. The firing treatment of Example 8 corresponds to the two-step treatment of (ii) in this embodiment, and the firing temperature column in Table 2 shows the temperatures of the primary firing and the secondary firing.

[0111]

Table 1

[0112]

Table 2

[0113] (Evaluation) For Examples 1 to 9 included in the scope of the present invention, molecular weight adjustment treatment was performed to produce PCS having a molecular weight within the scope of the present invention. Among these, the silicon carbide fibers obtained by firing the PCS fibers of Examples 1 to 5, Example 8, and Example 9 showed high values in terms of tensile strength and tensile modulus, and had good mechanical properties. Note that the silicon carbide fibers of Example 2, Example 5, and Example 8 were obtained by firing at a higher firing temperature than the other examples. Therefore, since crystallization progressed, the tensile modulus increased.

[0114] In Comparative Examples 1 and 2, PCS was produced using polydimethylsilane (PDMS), a chain silane compound, instead of a cyclic silane compound as a raw material for PCS production. The number average molecular weight (Mn) of the PCS in Comparative Example 1 was in a range higher than that of the present invention, while the weight average molecular weight (Mw) and molecular weight ratio (Mw / Mn) were in ranges lower than those of the present invention. The weight average molecular weight, number average molecular weight, and molecular weight ratio of the PCS in Comparative Example 2 were within the scope of the present invention. The silicon carbide fibers obtained using the PCS fibers composed of the PCS in Comparative Examples 1 and 2 were both in a range where the tensile strength was less than 2.2 GPa, and showed lower mechanical properties compared to the silicon carbide fibers of Examples 1 to 5, 8, and 9.

[0115] In Comparative Examples 3 and 4, PCS was produced using a raw material of a cyclic silane compound. The molecular weight ratio (Mw / Mn) of the PCS in Comparative Example 3 was in a range higher than that of the present invention. The weight molecular weight (Mw) and molecular weight ratio of the PCS in Comparative Example 4 were in ranges lower than those of the present invention. The silicon carbide fibers obtained by firing the PCS fibers composed of the PCS in Comparative Examples 3 and 4 were both in a range where the tensile strength was less than 2.2 GPa, and showed lower mechanical properties compared to the silicon carbide fibers of Examples 1 to 5, 8, and 9.

[0116] According to the above, the present invention has shown a useful effect in that it can provide PCS for silicon carbide fibers, which is a fiber material suitable for the production of silicon carbide fibers having heat resistance and excellent mechanical properties. Furthermore, the present invention has shown a useful effect in that it can produce silicon carbide fibers having heat resistance and excellent mechanical properties without applying an infusibilization treatment to the PCS fibers.

Explanation of Signs

[0117] 1 Liquid-phase reaction vessel 2 Liquid-phase heating means 3 Temperature measuring device for liquid-phase heating control 4 Mixture 5 Lid 6 Liquid-phase temperature measuring device 7 Inert gas introduction pipe 8 Stirring blade 9 Driving motor 10 Liquid-phase gas-phase pyrolysis condensation device 11 Flow path for gas-phase heating 12 Gas-phase heating means 13 Temperature measuring device for heating control 14 Gas-phase heating region 15 Temperature measuring device 16 Pressure gauge 17 Flow path for gas-phase cooling 18 Cooling region 19 Gas discharge pipe 20 Inert gas 21 Exhaust gas 22 Pressure gauge 30 Heat insulating material

Claims

1. A polycarbosilane for silicon carbide fibers, which has a weight average molecular weight (Mw) of 10,000 or more and 16,000 or less, a number average molecular weight (Mn) of 2,500 or more and less than 6,000, and a ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) of 2.0 or more and less than 4.5, and is a reaction product from a composition containing dodecamethylcyclohexasilane.

2. A method for producing the polycarbosilane for silicon carbide fibers according to Claim 1, comprising: (a) heating the composition in a liquid phase reaction vessel at a first temperature of 300 to 600 °C to vaporize it; (b) heating the gaseous composition obtained in step (a) in a gas phase heating region at a second temperature of 500 to 750 °C, which is 5 °C or more higher than the first temperature, to produce polycarbosilane; (c) returning the polycarbosilane produced in step (b) to the liquid phase reaction vessel, and cooling and returning the gaseous components that have passed through the gas phase heating region to the liquid phase reaction vessel; Then, (d) heating and vaporizing the components returned to the liquid phase reaction vessel at the first temperature; (e) heating the gaseous compound obtained in step (d) in the gas phase heating region at the second temperature to produce polycarbosilane; (f) returning the polycarbosilane produced in step (e) to the liquid phase reaction vessel, and cooling and returning the gaseous components that have passed through the gas phase heating region to the liquid phase reaction vessel; (g) after repeating steps (d), (e) and (f), performing a molecular weight adjustment process of subjecting the compound in the obtained liquid phase reaction vessel to a treatment for adjusting the molecular weight; A method for producing a polycarbosilane for silicon carbide fibers, comprising the above steps.

3. A spinning step of spinning the polycarbosilane for silicon carbide fibers according to claim 1 to produce polycarbosilane fibers, and a firing step of firing the polycarbosilane fibers in a non-oxidizing atmosphere to produce silicon carbide fibers, wherein the firing step includes (i) firing at 900°C or higher and 1600°C or lower, or (ii) primary firing at 900°C or higher and less than 1200°C and then secondary firing at 1200°C or higher and 1600°C or lower, a method for producing silicon carbide fibers.

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