Method for producing polycarbosilane for silicon carbide fibers and method for producing silicon carbide fibers

Abstract

Provided is a method for producing polycarbosilane (PCS) for silicon carbide fibers that produces PCS in a few steps to improve the yield of PCS.　The present invention is a method that produces PCS for silicon carbide fibers without including a step that adjusts the molecular weight of the PCS, wherein the method includes (a) a step that heats and gasifies a composition containing a cyclic silane compound in a liquid-phase reactor 1, (b) a step that heats the gaseous composition obtained by step (a) to generate PCS, and (c) a step that returns the PCS generated by step (b) to the liquid-phase reactor 1 and cools and returns the gaseous components to the liquid-phase reactor 1, and each of these steps is repeated to produce PCS in which the number average molecular weight (Mn) is from 1250 to less than 6000 and the ratio (Mw / Mn) of the weight average molecular weight (Mw) and the number average molecular weight (Mn) is from 4.5 to 20.0.

Description

Technical Field

[0001] The present invention relates to a method for producing polycarbosilane for silicon carbide fibers 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, a PCS non-melting fiber is produced using a precursor polycarbosilane (hereinafter, "polycarbosilane" may also be referred to as "PCS"), and after heating it in an inert gas atmosphere at a temperature not exceeding 1,000°C, it is heated at a temperature of 1,000 to 1,500°C in a mixed gas atmosphere of a hydrocarbon gas and an inert gas to produce silicon carbide fibers (Patent Document 1). The above PCS non-melting fiber is produced by subjecting a PCS fiber to a non-melting treatment of heating and oxidizing it in air. However, since such a PCS non-melting fiber contains a large amount of oxygen, the silicon carbide fiber obtained by firing it has a problem of inferior heat resistance. To solve this problem, Patent Document 2 proposes a method of non-melting by performing radiation irradiation in an oxygen-free atmosphere or in a vacuum. However, since expensive equipment is required for radiation irradiation, there is a problem that the production cost of silicon carbide fibers becomes high.

[0004] In response to such problems, Patent Document 3 proposes a method for producing silicon carbide fibers by adjusting the molecular weight of PCS to produce a high molecular weight PCS that does not require a non-melting treatment and subjecting the PCS to dry spinning. However, the method of Patent Document 3 requires an additional step of adjusting the molecular weight using a solvent, so it is difficult to reduce the production cost of PCS, which is a precursor of silicon carbide fibers. In order to produce inexpensive carbon silicon fibers, it is desired to reduce the production cost of PCS, which is its precursor, and further, it is desired to reduce the production cost of silicon carbide fibers obtained from PCS. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 1-314730 [Patent Document 2] Japanese Patent Application Publication No. 4-194028 [Patent Document 3] Japanese Patent Publication No. 2019-137935 [Overview of the project] [Problems that the invention aims to solve]

[0006] As mentioned above, a challenge regarding the manufacturing cost of silicon carbide fibers is the high manufacturing cost of PCS (Precision Chemical Substances) for silicon carbide fibers, which is its precursor.

[0007] The present invention has been made in view of the above problems, and aims to provide a method for producing PCS for silicon carbide fibers at a low manufacturing cost by using raw materials such as silane compounds to produce PCS for silicon carbide fibers with a small number of steps and a high production yield. Furthermore, it aims to provide a method for producing silicon carbide fibers at a low cost using the PCS.

[0008] The inventors diligently conducted research to solve the above-mentioned problems. As a result, they found that PCS produced by liquid-phase gas-phase thermal decomposition condensation using a cyclic silane compound possesses a predetermined molecular weight without the need for molecular weight adjustment treatment. Furthermore, they found that silicon carbide fibers with good mechanical properties can be obtained by using the PCS, thus completing the present invention. Specifically, the present invention includes the following embodiments. [Means for solving the problem]

[0009] (1) This embodiment is a method for producing polycarbosilane for silicon carbide fibers without including a step to adjust the molecular weight of the polycarbosilane, (a) A step of heating a composition containing a cyclic silane compound 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 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) The polycarbosilane produced in step (b) above 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) A step of heating the component returned to the liquid-phase reaction vessel at the first temperature to vaporize it, (e) A step of heating the gaseous compound obtained in step (d) in a gas-phase heating region at the second temperature to produce polycarbosilane, (f) The polycarbosilane produced in step (e) above 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, Repeat steps (d), (e), and (f) above, This is a method for producing polycarbosilane for silicon carbide fibers, wherein the number average molecular weight (Mn) is 1250 or more and less than 6000, and the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn) is 4.5 or more and 20.0 or less.

[0010] (2) This embodiment is a method for producing polycarbosilane for silicon carbide fibers as described in (1) above, wherein the cyclic silane compound is dodecamethylcyclohexasilane.

[0011] (3) This embodiment includes a spinning step to produce polycarbosilane fibers by spinning the polycarbosilane for silicon carbide fibers described in (1) above, The process includes a firing step of firing the polycarbosilane fibers in a non-oxidizing atmosphere to produce silicon carbide fibers, The aforementioned firing process is a method for producing silicon carbide fibers, comprising (i) firing at a temperature of 900°C or higher and 1600°C or lower, or (ii) primary firing at a temperature of 900°C or higher and less than 1200°C, followed by secondary firing at a temperature of 1200°C or higher and 1600°C or lower. [Effects of the Invention]

[0012] According to the present invention, in the production of PCS for silicon carbide fibers, PCS with a specific molecular weight can be manufactured without molecular weight adjustment treatment, thereby improving work efficiency and yield (yield rate), and reducing manufacturing costs. Furthermore, since silicon carbide fibers with heat resistance and excellent mechanical properties can be manufactured in high yield without applying infusibility treatment to the PCS fibers, inexpensive silicon carbide fibers can be provided. [Brief explanation of the drawing]

[0013] [Figure 1] This diagram schematically shows a liquid-phase gas-phase pyrolysis apparatus used for polycarbosilane synthesis. [Modes for carrying out the invention]

[0014] The embodiments of the present invention will be described in detail below. The present invention is not limited to the embodiments described below, and can be implemented with appropriate modifications within the scope of the object of the present invention. In this specification, the notation "X~Y" (where X and Y are arbitrary numerical values) means "X or more and Y or less".

[0015] [1] 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 without including a step of adjusting the molecular weight of the polycarbosilane, and has the following features. (a) A step of heating a composition containing a cyclic silane compound 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 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 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 a 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, Repeating the steps (d), (e) and (f), [[ID=1 | 4]]To produce polycarbosilane having a number average molecular weight (Mn) of 1250 or more and less than 6000, and a ratio (Mw / Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 4.5 or more and 20.0 or less.

[0016] The PCS for silicon carbide fibers according to this embodiment is an organic substance corresponding to a "precursor" applied to the production of silicon carbide fibers. In this specification, the PCS formed into a fibrous shape is described as "PCS fiber", and the fiber obtained by pre-sintering the PCS fiber is described as "pre-sintered 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.

[0017] (Composition containing cyclic silane compound) Conventionally, PCS fibers have been treated to become infusible in order to suppress the melting of PCS fibers during the firing process. To omit this infusibility treatment, it is necessary to use PCS fibers that do not melt during the firing process. Among the PCS components contained in PCS fibers, low molecular weight PCS components have a low softening point and also play a role in lowering the overall softening point of PCS. Therefore, from the viewpoint of suppressing the melting of PCS fibers, it is conceivable to perform a treatment to remove low molecular weight PCS fibers. One such treatment is known as "molecular weight adjustment treatment" to remove low molecular weight components of PCS (see Patent Document 3). However, adding a molecular weight adjustment treatment increases manufacturing costs. Furthermore, as a result of removing low molecular weight PCS by molecular weight adjustment treatment, the yield of PCS decreases in terms of raw material yield.

[0018] The manufacturing method according to this embodiment is a method for producing PCS for silicon carbide fibers without performing the molecular weight adjustment treatment described above, and its main feature is that a composition containing a cyclic silane compound is used as a raw material for PCS. The inventors have found that silicon carbide fibers can be produced using PCS fibers obtained from a composition containing a cyclic silane compound, even if the PCS fibers have not undergone infusibility treatment, without melting during firing. Since the manufacturing method according to this embodiment produces PCS fibers without performing molecular weight adjustment treatment, PCS fibers can be obtained with fewer steps and a high yield, thereby reducing the manufacturing cost of PCS fibers.

[0019] The reason why the PCS fibers according to this embodiment can be fired without melting is unclear. PCS produced from a composition containing a cyclic silane compound using a liquid-phase gas-phase thermal decomposition apparatus is thought to have a high softening point. Furthermore, judging from the test results of the examples and comparative examples described later, it is presumed that the PCS fibers produced from a composition containing a cyclic silane compound did not melt during the firing process because the molecular weight of the PCS increased rapidly in a short time, and this increased softening point occurred as a result. The mechanism of this molecular weight increase is also unclear, but it is presumed to be due to the activation of both ends of the ring-opened molecular chain in the thermal rearrangement and thermal decomposition condensation reaction that causes the cyclic silane to open its ring and be converted into PCS.

[0020] PCS is generally manufactured using polysilane compounds as raw materials. Polysilane compounds have a skeleton in which silicon atoms are linked together in a chain. In contrast, the polycarbosilane for silicon carbide fibers according to this embodiment uses a composition containing a cyclic silane compound as a raw material for the PCS. Cyclic silane compounds have a skeleton in which silicon atoms are linked together in a ring.

[0021] The composition containing the cyclic silane compound according to this 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-mentioned "cyclic silane compound" is a compound in which the backbone consists only of Si-Si bonds as the main chain, and the main chain forms a ring. The number of cyclic silane compounds 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 monocyclic compound or may have multiple rings. The side chain of the cyclic silane compound may have any structure. Examples of cyclic silane compounds 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.

[0022] The "composition" according to this embodiment may include compounds other than cyclic silane compounds. Examples include dichlorodimethylsilane, which is a raw material for the synthesis of cyclic silane compounds, its decomposition condensates, and chain-like polysilane compounds.

[0023] (Liquid-phase gas-phase thermal decomposition condensation apparatus) The polycarbosilane for silicon carbide fibers according to this embodiment is preferably manufactured using a liquid-gas-phase thermal decomposition condensation reaction (hereinafter sometimes simply referred to as "thermal decomposition condensation reaction"). PCS production by thermal decomposition condensation reaction can be carried out, for example, using a liquid-gas-phase thermal decomposition condensation apparatus 10 (hereinafter referred to as "thermal decomposition condensation apparatus") schematically shown in Figure 1. The thermal decomposition condensation apparatus 10 has as its main components a liquid-phase reaction vessel 1, a flow path 11 for gas-phase heating (hereinafter referred to as the "heating flow path"), and a flow path 17 for gas-phase cooling (hereinafter referred to as the "cooling flow path"). The liquid-phase reaction vessel 1 is, for example, a cylindrical container with a bottom and a lid 5 at the top to close the opening. The heating flow path 11 and the cooling flow path 17 are, for example, tubular structures.

[0024] (Liquid-phase reaction vessel) A mixture 4 containing a composition including a cyclic silane compound as a raw material and PCS as a reaction product is placed inside a liquid-phase reaction vessel 1. To heat the mixture 4 to a predetermined temperature (first temperature), a cylindrical liquid-phase heating means 2 is arranged to surround the liquid-phase reaction vessel 1, and a temperature measuring instrument 3 (e.g., a thermocouple) for controlling the liquid-phase heating is provided on the inner surface of the liquid-phase heating means 2 facing the outer surface of the liquid-phase reaction vessel 1. The heating means can be any means that heats the liquid phase inside the liquid-phase reaction vessel, and for example, a heating device in which the heater and outer casing are integrated, such as a mantle heater, can be used.

[0025] A stirring mechanism having an agitator blade 8 and a drive motor 9, a liquid phase temperature measuring device 6 (e.g., a thermocouple) for measuring the temperature of the mixture 4, and an inert gas introduction pipe 7 for supplying inert gas 20 to 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 agitator blade 8 are positioned so as to be immersed in the liquid phase of the liquid phase reaction vessel 1. The liquid phase temperature measuring device 6 can confirm that the mixture 4 in the liquid phase reaction vessel 1 is maintained at a first temperature. A pressure gauge 22 for adjusting the flow rate of the inert gas 20 is installed in the inert gas introduction pipe 7. By detecting the pressure inside the liquid phase reaction vessel 1, it is possible to detect abnormalities in internal pressure caused by blockages in, for example, the heating channel 11, the cooling channel 17, and the gas discharge pipe 19.

[0026] (Vapor-phase heating region) One end of the heating channel 11 is connected to the liquid-phase reaction vessel 1 via the lid 5 of the liquid-phase reaction vessel 1. Components vaporized from the mixture 4 in the liquid-phase reaction vessel 1 move to the heating channel 11 and are heated to a predetermined temperature (second temperature) in the gas-phase heating region 14. As shown in Figure 1, gas-phase heating means 12 are arranged around the heating channel 11 to heat the gas phase within the heating channel 11. The gas-phase heating region 14 corresponds to the region within the heating channel 11 that is heated by the gas-phase heating means 12. As the gas-phase heating region is heated and an upward airflow is generated in the heating channel, the vapor containing the components vaporized from the liquid-phase reaction vessel is drawn into the heating channel by the chimney effect. As a result, a flow is formed from the liquid-phase reaction vessel to the heating channel. The inert gas introduced into the liquid-phase reaction vessel is also carried along with this flow and moves into the heating channel.

[0027] The structure of the gas-phase heating means 12 is not particularly limited. It is sufficient that it can heat the gaseous components in the heating channel 11 to a predetermined temperature. As shown in Figure 1, a divided heater may be used. By controlling the divided heater, a wide uniform zone is formed as the gas-phase heating region. It is preferable to cover the area around the heating channel 11 other than where the gas-phase heating means 12 is installed with insulating material to maintain the temperature and prevent a drop in temperature.

[0028] The temperature measuring instrument 13 for controlling the gas phase heating is installed on the inner side of the gas phase heating means 12, facing the outer surface of the heating channel 11. The temperature measuring instrument 15 (e.g., a thermocouple) for measuring the temperature of the gas phase heating region 14 is inserted into the heating channel 11 from the end opposite to the end of the heating channel 11 that connects to the liquid phase reaction vessel 1, and is positioned so that the tip of the temperature measuring instrument 15 reaches approximately the center of the installation location of the gas phase heating means 12.

[0029] A pressure gauge 16 is attached to the opposite end of the heating channel 11 for measuring the pressure of the gas phase within the heating channel 11. The pressure gauge 16 can detect abnormalities in internal pressure caused, for example, by blockages in the heating channel 11, cooling channel 17, and gas discharge pipe 19. When it is necessary to control the amount of steam flowing from the liquid-phase reaction vessel 1 into the heating channel 11, a control valve may be installed in the heating channel 11 between the liquid-phase reaction vessel 1 and the gas-phase heating region 14.

[0030] (cooling area) The cooling channel 17 is connected to an opening located on the opposite side of the heating channel 11 from the side connected to the liquid-phase reaction vessel 1. Of the components in the heating channel 11 that have passed through the gas-phase heating region 14, high molecular weight PCS having a boiling point of a temperature above the second temperature condenses into a liquid within the heating channel 11 and returns to the liquid-phase reaction vessel 1. The remaining gaseous components move from the heating channel 11 into the cooling channel 17, where they are cooled in the cooling region 18 and change into a liquid before returning to the liquid-phase reaction vessel 1. Almost the entire cooling channel 17 corresponds to the cooling region 18, and vaporized components that enter the cooling region 18 are slowly liquefied by the cooling of the cooling channel 17.

[0031] If the temperature of the cooling region 18 is too low, the gaseous components in the cooling channel 17 may change into a liquid, become a highly viscous liquid, or solidify, potentially blocking the cooling channel 17. It is preferable to cool the liquid flowing through the cooling channel 17 to a temperature range where it has a viscosity suitable for easy flow. For this reason, the area around the cooling channel 17 may be covered with insulating material, or it may be heated to maintain its temperature as needed.

[0032] The components that enter the cooling region 18 include low-boiling-point components such as hydrogen, methane, and monosilane, which are produced by the reaction in the gas-phase heating region 14. These low-boiling-point components, along with the inert gas introduced from the inert gas introduction pipe 7, are discharged to the outside as exhaust gas 21 through a gas discharge pipe 19 located near the center of the cooling flow path 17. The exhaust gas 21 is treated appropriately outside.

[0033] (Liquid-phase gas-phase thermal decomposition condensation reaction) The polycarbosilane for silicon carbide fibers according to this embodiment is manufactured based on a liquid-phase gas-phase thermal decomposition condensation reaction. Specifically, it is manufactured through the following steps (a) to (f). Hereinafter, each step will be referred to as "step (a)" to "step (f)".

[0034] Process (a): Step (a) is a step of heating a composition containing a cyclic silane compound in a liquid-phase reaction vessel at a first temperature of 300 to 600°C to vaporize it. After the composition is placed in the liquid-phase reaction vessel 1, the substance in the liquid-phase reaction vessel 1 is heated to the first temperature by the liquid-phase heating means 2, and the composition vaporizes. The gaseous composition then moves into the heating channel 11. When heating, it is preferable to maintain a non-oxidizing gas atmosphere inside the liquid-phase reaction vessel 1 in order to prevent oxidation of the raw materials. For example, as shown in Figure 1, the atmospheric gas may be replaced by supplying an inert gas 20.

[0035] Process (b): Step (b) is a step of generating PCS by heating the gaseous composition obtained in step (a) in a gas-phase heating region 14 at a second temperature of 500 to 750°C, which is 5°C or more higher than the first temperature. The heating channel 11 is heated by the gas-phase heating means 12 to form a gas-phase heating region 14 having the second temperature. The gaseous cyclic silane compound that vaporized in step (a) and moved into the heating channel 11 is used to synthesize PCS in the gas-phase heating region 14 through gas-phase reactions of thermal rearrangement and thermal decomposition condensation. The second temperature in the gas-phase heating region 14 is determined based on the temperature measured near the center of the gas-phase heating region 14.

[0036] Process (c): Step (c) is a step in which the gaseous component containing the PCS generated in step (b) is cooled and returned to the liquid-phase reaction vessel 1. The PCS generated by the gas-phase reaction in step (b) remains in the gas-phase heating region 14, and a distribution occurs in its residence time. As a result, a distribution occurs in the molecular weight of the generated PCS, and PCS with various molecular weights are included. Furthermore, the reactants generated in the gas-phase heating region 14 contain unreacted cyclic silane compounds and decomposition products of cyclic silane compounds. Of these reactants, high molecular weight PCS with a boiling point above the second temperature condenses into a liquid in the heating channel 11 and returns to the liquid-phase reaction vessel 1. The gaseous component containing the other reactants moves from the heating channel 11 to the cooling channel 17. Then, it is cooled in the cooling channel 17 and changes into a liquid before returning to the liquid-phase reaction vessel 1.

[0037] Process (d): The next step (d) is to heat the components returned to the liquid-phase reaction vessel 1 at the first temperature to vaporize them. The components returned to the liquid-phase reaction vessel 1 include cyclic silane compounds, low molecular weight PCS, and decomposition products of cyclic silane compounds. These components are heated at the first temperature, similar to step (a). Components having a boiling point lower than the first temperature vaporize and move to the heating channel 11.

[0038] Process (e): Step (e) is a step in which the gaseous compound obtained in step (d) is heated in the gas-phase heating region 14 at the second temperature to produce PCS. The gaseous compound that moved to the heating channel 11 in step (d) is heated to the second temperature in the heating channel 11 in the gas-phase heating region 14, similar to step (b), and PCS is synthesized by thermal rearrangement and thermal decomposition condensation reactions.

[0039] Process (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), of the PCS produced in step (e), the high molecular weight PCS having a boiling point of the second temperature or higher condenses into a liquid and returns to the liquid-phase reaction vessel 1. The remaining gaseous components move from the heating channel 11 to the cooling channel 17, where they change into a liquid and return to the liquid-phase reaction vessel 1.

[0040] Repeating steps (d), (e), and (f): By repeating steps (d), (e), and (f) above, the low-boiling-point components vaporize in the liquid-phase reaction vessel 1, compounds are generated in the gas-phase heating region 14 by thermal rearrangement and thermal decomposition condensation reactions, and the reaction products from the gas-phase heating region 14 reflux to the liquid-phase reaction vessel 1. As a result, the generation of PCS with high molecular weights progresses, and the proportion 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 proportion of other components decreases. That is, the proportion of compositions containing cyclic silane compounds, decomposition products of the compositions, and PCS with a molecular weight below a predetermined level that gives a boiling point higher than the first temperature decreases. PCS that have reached a predetermined molecular weight do not vaporize at the first temperature in the liquid-phase reaction vessel 1, so an excessive increase in 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.

[0041] The method for producing PCS for silicon carbide fibers according to the present invention may include steps other than the liquid-phase gas-phase thermal decomposition condensation method. These other steps are not particularly limited as long as they do not impair the effects of the present invention.

[0042] (First temperature) The first temperature is the temperature at which the substance contained in the liquid-phase reaction vessel is heated, and can be set in the range of 300°C to 600°C. If the first temperature is below 300°C, it is undesirable because low molecular weight PCS is difficult to vaporize. Also, it is undesirable because the vaporization rate of the substance decreases and the PCS production rate decreases. 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 PCS with excessively large molecular weight being produced, and the proportion of solidified material produced increases, which is undesirable. Therefore, the first temperature is preferably 600°C or lower, more preferably 550°C or lower, and even more preferably 500°C or lower.

[0043] (Second temperature) The second temperature is the temperature at which the gas phase heating region in the heating channel is heated, and can be set in the range of 500°C to 750°C. If the second temperature is below 500°C, the reaction rates of the thermal rearrangement and thermal decomposition condensation reactions of PCS decrease, leading to a decrease in productivity and making it difficult to produce high molecular weight PCS, which is undesirable. 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 undesirable because it can lead to the formation of solids that are difficult to spin and a significant increase in the viscosity of the produced polycarbosilane, which may clog the piping of the manufacturing apparatus. Therefore, the second temperature is preferably 750°C or lower, more preferably 725°C or lower, and even more preferably 700°C or lower.

[0044] The second temperature must be in a range higher than the first temperature. 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 thermal rearrangement and thermal decomposition condensation reactions in the gas phase heating region will be slowed, which is undesirable. 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.

[0045] (Heating time) The process of heating the liquid-phase reaction vessel to a first temperature and the process of heating the gas-phase heating region of the gas-phase reaction tube to a second temperature should be continued until the PCS reaches a predetermined molecular weight. The time required to produce PCS with the target molecular weight varies depending on the type of cyclic silane compound used as a raw material, the first temperature, the second temperature, etc. As the heating time increases, the molecular weight of the resulting PCS tends to increase. If the heating time is insufficient, there will be insufficient reaction time to produce high molecular weight PCS, and a sufficient amount of high molecular weight PCS will not be obtained. Therefore, depending on the selected first and second temperatures, the heating time is preferably 4.0 hours or more, more preferably 5 hours or more, even more preferably 5.5 hours or more, and particularly preferably 6.0 hours or more. The heating treatment may be performed continuously or in stages. If performed in stages, the heating time will be the sum of the individual heating times. On the other hand, after obtaining PCS with the required predetermined molecular weight, it is desirable to stop heating to reduce the cost required for heating. In this specification, this heating time is also referred to as "reaction time".

[0046] (cooling temperature) The reactants that have passed through the gas-phase heating region are cooled in a cooling channel. The cooling temperature should be sufficient to cool the gaseous components to a liquid state; if the cooling temperature is too low, the reactants will solidify or the viscosity of the liquid reactants will increase, which is undesirable as it can block the channel. The cooling temperature is preferably between 50°C and 300°C. The cooling channel may be kept warm to maintain the predetermined cooling temperature.

[0047] (Non-oxidizing gas) The type of atmospheric gas in the liquid-phase reaction vessel is not particularly limited, as long as it is a non-oxidizing gas that does not react with the composition containing the cyclic silane compound and the reaction products such as PCS. For example, an inert gas is preferred, and nitrogen gas or noble gases can be used alone or in combination.

[0048] The time for heating the liquid-phase reaction vessel and the gas-phase heating region to allow the reaction to occur can be appropriately adjusted according to the first and second temperatures.

[0049] (Number average molecular weight (Mn)) The number-average molecular weight (Mn) of PCS is preferably in the range of 1250 or more and less than 6000. If the number-average molecular weight is less than 1250, there is a risk that the PCS fibers will melt and fuse together when the PCS fibers are fired, which is undesirable. Therefore, the number-average molecular weight is preferably 1250 or more, more preferably 1300 or more, and even more preferably 1500 or more. On the other hand, if the number-average molecular weight is 6000 or more, a large amount of solvent is required to dissolve the PCS when dry spinning is performed, which is undesirable. Therefore, the number-average molecular weight is preferably less than 6000, more preferably 4000 or less, even more preferably 3000 or less, and particularly preferably 2000 or less.

[0050] (Ratio of weight-average molecular weight to number-average molecular weight (Mw / Mn)) The ratio of the weight-average molecular weight to the number-average molecular weight (Mw / Mn) of PCS is preferably in the range of 4.5 to 20.0. Hereinafter, this specification may refer to this "ratio" as the "molecular weight ratio." When the number-average molecular weight (Mn) is small and the molecular weight ratio (Mw / Mn) is less than 4.5, the degree of dispersion of the molecular weight distribution is small, so the molecular weight of the PCS as a whole is in a small range. Therefore, PCS fibers obtained from this PCS are undesirable because they may melt during firing. Accordingly, the molecular weight ratio (Mw / Mn) is preferably 4.5 or higher, more preferably 5.0 or higher, even more preferably 5.5 or higher, and particularly preferably 6.0 or higher.

[0051] On the other hand, when the molecular weight ratio (Mw / Mn) exceeds 20.0, the degree of dispersion of the molecular weight distribution is large, so the proportion of molecular weight components with a large molecular weight relative to the average molecular weight increases in the PCS, and the proportion of components with low solubility in the solvent for dry spinning increases. Therefore, this is undesirable as it leads to a decrease in spinnability in the spinning process of the PCS. Furthermore, the proportion of solids that do not dissolve in the solvent increases, which increases the proportion of components that are difficult to spin and may lead to a decrease in yield, so this is undesirable. For this reason, the molecular weight ratio (Mw / Mn) is preferably 20.0 or less, more preferably 18.0 or less, even more preferably 14.0 or less, and particularly preferably 10.0 or less.

[0052] The method for producing PCS for silicon carbide fibers according to this embodiment can produce PCS for silicon carbide fibers without including a step to adjust the molecular weight of the PCS (hereinafter referred to as the "molecular weight adjustment step"). The above molecular weight adjustment step refers, for example, to the "step of removing low molecular weight components that cause fusion from PCS with a broad molecular weight distribution including low molecular weight components to high molecular weight components, and leaving only high molecular weight components that do not melt even at high temperatures during firing and prevent fusion between fibers" as described in paragraphs

[0016] and

[0017] of Patent Document 3.

[0053] [2] Method for producing silicon carbide fibers The method for producing silicon carbide fibers according to this embodiment includes a spinning step of spinning the silicon carbide fiber PCS obtained by the method for producing silicon carbide fiber PCS in [1] above to produce PCS 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 more and 1600°C or less, or (ii) primary firing at 900°C or more and less than 1200°C, followed by secondary firing at 1200°C or more and 1600°C or less.

[0054] (Non-oxidizing gas) The type of non-oxidizing atmosphere gas used in firing is not particularly limited, as long as it is a non-oxidizing gas that does not react with the PCS fibers. For example, an inert gas is preferred, and nitrogen gas or noble gases can be used alone or in combination.

[0055] (Spinning process) The spinning process is the process of turning PCS into fibers. Common spinning methods include melt spinning, dry spinning, and wet spinning. In the silicon carbide fiber production method according to this embodiment, dry spinning is preferably applied. Dry spinning is a method of preparing a precursor solution by adding a solvent to a precursor, and then spinning using this precursor solution. PCS is dissolved in a solvent to prepare a dry spinning solution, and the viscosity of the solution is adjusted. Next, the dry spinning solution is put into a spinning apparatus to produce PCS fibers.

[0056] The solvent used to dissolve PCS in the dry spinning solution is not particularly limited, as long as it can dissolve PCS. Examples include 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. From the viewpoint of excellent solubility and volatility of PCS, toluene or xylene is preferred.

[0057] The concentration of the dry spinning solution can be adjusted as appropriate. For example, the concentration can be selected within the range of 50 to 70 wt%.

[0058] The viscosity of the dry spinning solution can be appropriately adjusted to match the nozzle diameter of the spinning apparatus. 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 can be determined by known measurement methods. This solution viscosity can be measured, for example, using an E-type viscometer.

[0059] The spinning apparatus used in the spinning process can be the same as that commonly used in this art, and spinning conditions can be applied. By supplying a dry spinning solution to the spinning apparatus and spinning under conditions such as a spinning nozzle diameter of 65 μm, a discharge pressure of 2 to 3.5 MPa, and an extrusion rate of PCS solution of 10 to 30 mg / min, a predetermined PCS fiber can be obtained.

[0060] (Firing process) The method for producing silicon carbide fibers according to this embodiment includes a firing step in which the PCS fibers are fired in a non-oxidizing atmosphere to produce silicon carbide fibers. The firing step is a step in which silicon carbide fibers are obtained by firing the PCS fibers produced in the spinning step in a non-oxidizing atmosphere to convert them into ceramics.

[0061] The firing process is preferably (i) fired at 900°C to 1600°C, or (ii) first fired at 900°C to less than 1200°C, followed by second firing at 1200°C to 1600°C. It is preferable to produce silicon carbide fibers through these firing processes.

[0062] The firing process described in (ii) above is carried out in two stages: primary firing and secondary firing. The primary firing in (ii) above is a process in which PCS fibers are fired in a non-oxidizing atmosphere at a temperature of 900°C or higher but less than 1200°C, with the main purpose being to remove hydrogen atoms and excess carbon atoms from the PCS and change the PCS fibers into silicon carbide fibers. Primary firing of PCS fibers changes the PCS fibers into silicon carbide (SiC) fibers, and this chemical reaction increases the tensile strength of the fibers. Below 900°C, the degree of change to silicon carbide is insufficient, and the tensile strength of the silicon carbide fibers is low, so it is preferable that the primary firing temperature be 900°C or higher. Furthermore, from the viewpoint of achieving the purpose of primary firing, the primary firing temperature may be set to less than 1200°C.

[0063] The secondary firing described in (ii) above is a process primarily aimed at increasing the strength of silicon carbide fibers by firing them in a non-oxidizing atmosphere at a temperature between 1200°C and 1600°C to promote the crystallization of silicon carbide. If the temperature exceeds 1600°C, crystallization proceeds excessively, causing the crystallite size to become too large, which increases the tensile modulus of the fibers, making them brittle and causing a decrease in mechanical strength. Therefore, it is preferable that the secondary firing temperature be 1600°C or lower. Furthermore, from the viewpoint of efficiently carrying out the secondary firing, the secondary firing temperature may be set to 1200°C or higher.

[0064] The non-oxidizing atmosphere for the primary calcination is not particularly limited, as long as it is a non-oxidizing gas atmosphere that prevents oxidation of PCS. Examples of non-oxidizing gases include nitrogen, noble gases, and mixtures thereof. Within the heating temperature range of the primary calcination, silicon and nitrogen hardly react. The non-oxidizing atmosphere for the secondary calcination is not particularly limited, as long as it is a non-oxidizing gas atmosphere that prevents silicon from reacting. However, since silicon may react with nitrogen and undergo nitridation at high temperatures, the calcination process is preferably carried out in a noble gas atmosphere such as argon.

[0065] The firing process described in (i) above is a single-stage firing process carried out in a temperature range of 900°C to 1600°C. Depending on the heated firing temperature, the process may end with the primary firing described in (ii) above, or it may proceed to the secondary firing described in (ii) above. At the heating temperature corresponding to the primary firing, high tensile strength is obtained mainly through the transformation into silicon carbide fibers. At the heating temperature corresponding to the secondary firing, in addition to the transformation into silicon carbide fibers, an even higher tensile modulus is obtained due to the progression of silicon carbide crystallization.

[0066] The firing process described in (i) above may be carried out using a non-oxidizing gas atmosphere appropriate to the heating temperature. At the heating temperature corresponding to the primary firing described in (ii) above, nitrogen, noble gases, and mixtures thereof can be used to prevent oxidation of the PCS. At the heating temperature corresponding to the secondary firing described in (ii) above, noble gases such as argon and mixtures thereof can be used to prevent oxidation of silicon.

[0067] Prior to performing the firing described in (i) or (ii) above in the firing process, pre-firing may be carried out in a non-oxidizing atmosphere at a temperature of 500°C to less than 900°C. This pre-firing is a process to remove excess carbon atoms. The gas that forms the non-oxidizing atmosphere is not particularly limited as long as it is a non-oxidizing gas. As non-oxidizing gases, nitrogen, noble gases, hydrogen, and mixtures thereof can be used. If excess carbon is present in silicon carbide fibers during the manufacturing process, carbon atoms will be removed during firing, causing a decrease in the tensile strength of the silicon carbide fibers. Therefore, it is desirable to mix hydrogen with nitrogen or noble gases as the atmosphere gas during pre-firing to reduce the excess carbon content in the pre-fired PCS fibers before primary firing. The hydrogen content in the gas atmosphere during pre-firing is preferably 30% to 70% by volume, and more preferably 50% to 70% by volume. [Examples]

[0068] The following describes examples of the present invention. The scope of the present invention is not limited to the following description.

[0069] The methods for measuring molecular weight, solution viscosity, tensile strength, tensile modulus, and fiber diameter are described below. The physical properties related to the present invention and the examples are based on values ​​obtained by these measurement methods.

[0070] <Molecular weight> The weight-average molecular weight (Mw) and number-average molecular weight (Mn) were measured in accordance with the method specified in JIS K7252-1:2016 (ISO 16014-1:2012). Specifically, the molecular weight of PCS was measured using a liquid chromatogram (Shimadzu HPLC) and Showa Denko columns (one of each column connected in the order of KF-604, KF-602, and KF-601 from the pump side). Toluene was used as the measurement solvent, the sample solution concentration was 0.5% by weight, the flow rate to the analyzer and reference was 0.40 mL / min, and the oven temperature was 40°C. A differential refractometer was used as the detector to measure the molecular weight.

[0071] <Solution viscosity> The viscosity of the dry spinning solution was measured using an E-type viscometer (ARES-G2, manufactured by T.A. Instruments Corporation) with a test fixture consisting of a φ25mm stainless steel parallel plate, a solution thickness of 0.5mm, a temperature of 25℃, and a shear rate of 200 sec. -1 The measurements were taken under the following conditions.

[0072] <Tensile strength and tensile modulus> The tensile strength and tensile modulus of silicon carbide fibers were measured in accordance with the measurement method of JIS R7606:2000. Ten silicon carbide fibers were randomly selected, and the tensile strength and tensile modulus of each silicon carbide fiber were measured. The average of the obtained measurements from the ten fibers was adopted.

[0073] <Fiber diameter> The fiber diameter was determined by measuring the diameter of silicon carbide fibers at a magnification of 2000x using an optical microscope (VHX-5000) manufactured by Keyence Corporation, using 10 fibers that were used for measuring tensile strength and tensile modulus. The average of the obtained measurements was then adopted.

[0074] (Example 1) (1) Production of PCS for silicon carbide fibers Polycarbosilane (PCS) was produced using a pyrolysis condensation apparatus 10 as shown in Figure 1. First, dodecamethylcyclohexasilane (DMCHS) was placed in 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." 485°C was selected as the first temperature. The liquid-phase reaction vessel 1 was heated to the first temperature, and the cyclic silane compound was vaporized. The vaporized cyclic silane compound moved into the heating channel 11 and passed through the gas-phase heating region 14. The temperature inside the gas-phase heating region 14 is hereinafter referred to as the "second temperature." 600°C was selected as the second temperature. The gas-phase heating region 14 was heated to the second temperature, and PCS with various molecular weights were produced in the gas-phase heating region 14 by thermal rearrangement and pyrolysis condensation reactions of the cyclic silane compound.

[0075] The substance in the heating channel 11 moved to the cooling channel 17 after passing through the gas-phase heating region 14. Of the PCS generated in the gas-phase heating region 14, the high molecular weight PCS having a boiling point above the second temperature condensed into a liquid in the heating channel 11 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.

[0076] The components and unreacted cyclic silane compounds that returned to the liquid-phase reaction vessel 1 were vaporized again in the liquid-phase reaction vessel 1, which was heated to a first temperature. The vaporized cyclic silane compounds moved to the heating channel 11, where PCS was generated in the gas-phase heating region 14, which was heated to a second temperature, and furthermore, the molecular weight of the PCS increased. Of the high molecular weight PCS generated in the heating channel 11, the high molecular weight PCS with a boiling point above the second temperature condensed and returned to the liquid-phase reaction vessel 1, while 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 occurred involving heating of the liquid-phase reaction vessel, vaporization, generation of PCS in the gas-phase heating region, progression of high molecular weight PCS, and return to the liquid-phase reaction vessel.

[0077] The process of heating the liquid-phase reaction vessel to a first temperature and the process of heating the gas-phase heating region to a second temperature were carried out for a heating time of 6.5 hours (hereinafter referred to as "reaction time"), and the above-mentioned circulating reaction continued. After that, heating was stopped, and the liquid-phase reaction vessel and cooling channel were allowed to cool to room temperature, and polycarbosilane (PCS) having a predetermined molecular weight was obtained in the liquid-phase reaction vessel.

[0078] (2) Preparation of PCS fibers The obtained PCS was dissolved in xylene solvent to prepare a solution for dry spinning. After filtering out solidified material and other contaminants from the solution, PCS fibers were obtained by winding up the fibers extruded from the 65 μm diameter spinneret (nozzle) using the solution.

[0079] (3) Preparation of silicon carbide fibers Silicon carbide fibers were produced by calcining the PCS fibers according to the following procedure. The PCS fibers were heated to 500°C at a rate of 150°C / h in a nitrogen atmosphere. Then, pre-calcination 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, calcination was performed by heating from 800°C to 1000°C at a rate of 150°C / h in an argon gas atmosphere, and then holding at 1000°C for 1 hour. After calcination, heating was stopped and the fibers were allowed to cool to room temperature to obtain silicon carbide fibers.

[0080] Table 1 shows the manufacturing conditions and physical properties of PCS. Table 2 shows the physical properties of the PCS dry spinning solution and the obtained silicon carbide fibers.

[0081] (Example 2) Silicon carbide fibers were obtained using the same procedure as in Example 1, except that the firing temperature was set to 1400°C.

[0082] (Example 3) Silicon carbide fibers were obtained by the same procedure as in Example 1, except that the first temperature for PCS production was 480°C and the reaction time was 6.0 hours. (Example 4) Silicon carbide fibers were obtained using the same procedure as in Example 1, except that the reaction time for PCS production was 7.3 hours.

[0083] (Comparative Example 1) PCS was prepared using 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 8.0 hours. PCS fibers were then prepared using the obtained PCS, and these PCS fibers were subjected to a calcination treatment. However, the PCS fibers melted during calcination, and silicon carbide fibers could not be obtained.

[0084] (Comparative Example 2) PCS was produced using the same procedure as in Example 1, except that the first temperature for PCS production was 475°C and the reaction time was 5.0 hours. Then, PCS fibers were prepared using the obtained PCS and the PCS fibers were fired. However, the PCS fibers melted during firing, and silicon carbide fibers were not obtained.

[0085] (Comparative Example 3) The PCS prepared in Comparative Example 1 was subjected to the following molecular weight adjustment procedure (hereinafter referred to as the "molecular weight adjustment procedure"). Ethyl acetate in an amount five times the mass of the PCS was added to the PCS to prepare a mixture. Next, the mixture was heated and stirred at 50°C, and then the liquid was removed. This operation was repeated four times to remove the low molecular weight PCS dissolved in the ethyl acetate. Then, the ethyl acetate was removed from the remaining mixture to obtain PCS with adjusted molecular weight. Next, using the obtained PCS, PCS fibers were prepared in the same procedure as in Example 1, and the PCS fibers were calcined to obtain silicon carbide fibers.

[0086] (Comparative Example 4) Using the PCS prepared in Comparative Example 2, molecular weight adjustment treatment was performed in the same manner as in Comparative Example 3. Then, using the obtained PCS, PCS fibers were prepared in the same manner as in Example 1, and the PCS fibers were calcined to obtain silicon carbide fibers.

[0087] For the silicon carbide fibers obtained 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 using a predetermined measurement method. The solution viscosity (Pa·s) of the dry spinning solution, the diameter of the PCS fibers (μm), and the diameter of the silicon carbide fibers (μm) were measured using a predetermined measurement method. The solution concentration (wt%) of the dry spinning solution was calculated from the blending ratio. These results are shown in Tables 1 and 2.

[0088] The "PCS yield (wt%)" listed in Table 1 is a value expressed in wt% calculated from the product of the mass of the obtained polycarbosilane (PCS) divided by the mass of the raw material composition containing the cyclic silane compound (hereinafter referred to as the "raw material composition") and the molecular weight adjustment yield. The "molecular weight adjustment yield" is a value calculated from the mass of the PCS obtained by performing molecular weight adjustment treatment a predetermined number of times, divided by the mass of the PCS before the molecular weight adjustment treatment. PCS yield (wt%) = (mass of PCS ÷ mass of raw material composition) × molecular weight adjustment rate × 100 Molecular weight adjustment rate = PCS mass after molecular weight adjustment treatment ÷ PCS mass before molecular weight adjustment treatment

[0089] The "Total Yield (wt%)" listed in Table 2 is a value calculated from the product of the PCS yield and the SiC fiber yield. The "SiC Fiber Yield" is a value calculated from the mass of silicon carbide fibers obtained by firing at 1000°C divided by the mass of PCS fibers. Total yield (wt%) = PCS yield (wt%) × SiC fiber yield SiC fiber yield = Mass of SiC fibers after firing ÷ Mass of PCS fibers

[0090] [Table 1]

[0091] [Table 2]

[0092] (evaluation) Examples 1 to 4, which fall within the scope of the present invention, demonstrate the production of PCS without molecular weight adjustment treatment, and the production of silicon carbide fibers using the PCS, with good PCS yield and overall yield. Furthermore, the silicon carbide fibers obtained by calcination in Examples 1 to 4 exhibited high values ​​in tensile strength or tensile modulus, and possessed good mechanical properties. Note that the silicon carbide fibers of Example 2 were obtained by calcination at a higher calcination temperature than in the other examples. As a result, crystallization progressed, and the tensile modulus increased.

[0093] Comparative Example 1 used polydimethylsilane (PDMS), a chain-like silane compound, as the raw material for PCS production, instead of a cyclic silane compound. As a result, the resulting PCS fibers melted during firing. Now, let's compare Example 4 with Comparative Example 1. As shown in Table 1, Example 4, which used a raw material containing a cyclic silane compound, had a reaction time of 7.3 hours, which was shorter than the reaction time of Comparative Example 1 (8.0 hours). Nevertheless, the molecular weight (Mw and Mn) of the PCS obtained in Example 4 was larger than that of the PCS obtained in Comparative Example 1. Judging from this difference in molecular weight, it can be said that PCS produced from a raw material containing a cyclic silane compound, such as in Example 4, has the property of increasing molecular weight in a short time. It is presumed that the PCS fibers made from the PCS of Example 4 increased in molecular weight during the firing process and transformed into PCS fibers with a high softening point. As a result, it is considered that Example 4 was able to produce silicon carbide fibers without melting of the PCS fibers.

[0094] Comparative Example 2 involved producing PCS using a cyclic silane compound as a raw material. However, the PCS fibers of Comparative Example 2 melted during the firing process, making it impossible to produce silicon carbide fibers. The number-average molecular weight and molecular weight ratio of the PCS in Comparative Example 2 were both outside the range of the present invention, resulting in the formation of PCS with a high softening point. Consequently, it is believed that the PCS fibers melted due to the heating during the firing process.

[0095] Comparative Examples 3 and 4 underwent molecular weight adjustment treatment during PCS production. Because low molecular weight PCS were removed by the molecular weight adjustment treatment, the proportion of raw materials used in PCS production decreased, resulting in a lower PCS yield. Consequently, the overall yield of silicon carbide fibers also decreased.

[0096] Based on the above, the present invention has demonstrated a useful effect in improving the yield in a method for producing PCS for silicon carbide fibers. Furthermore, the present invention has demonstrated a useful effect in that silicon carbide fibers with excellent mechanical properties can be produced in high yield without applying an infusibility treatment to the PCS fibers. [Explanation of Symbols]

[0097] 1. Liquid-phase reaction vessel 2 Liquid phase heating means 3. Temperature measuring instrument for liquid-phase heating control 4 mixture 5 Lid 6 Liquid temperature measuring device 7. Inert gas introduction pipe 8. Agitator blades 9. Drive motor 10. Liquid-phase gas-phase thermal decomposition condensation apparatus 11 Flow path for gas-phase heating 12. Gas-phase heating means 13. Temperature measuring instrument for heating control 14. Gas-phase heating region 15 Temperature measuring device 16. Pressure gauge 17 Flow channels for gas-phase cooling 18 Cooling area 19 Gas discharge pipe 20 Inert gas 21 Exhaust gas 22 Pressure gauge 30 Insulation

Claims

1. A method for producing silicon carbide fibers without including a step to adjust the molecular weight of polycarbosilane, (a) A step of heating a composition containing a cyclic silane compound 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 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) The polycarbosilane produced in step (b) above 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) A step of heating the component returned to the liquid-phase reaction vessel at the first temperature to vaporize it, (e) A step of heating the gaseous compound obtained in step (d) in the gas phase heating region at the second temperature to produce polycarbosilane, (f) The polycarbosilane produced in step (e) above 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, Repeat steps (d), (e), and (f) above, A method for producing silicon carbide fibers, comprising: a step of producing a polycarbosilane having a number average molecular weight (Mn) of 1250 or more and less than 6000, and a ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn) of 4.5 or more and 20.0 or less; a spinning step of spinning the produced polycarbosilane for silicon carbide fibers to produce polycarbosilane fibers; and a firing step of firing the polycarbosilane fibers in a non-oxidizing atmosphere to produce silicon carbide fibers.

2. The method for producing silicon carbide fibers according to claim 1, wherein the cyclic silane compound is dodecamethylcyclohexasilane.

3. The method for producing silicon carbide fibers according to claim 1 or 2, wherein the firing step includes (i) firing at 900°C or more and 1600°C or less, or (ii) primary firing at 900°C or more and less than 1200°C, followed by secondary firing at 1200°C or more and 1600°C or less.

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