JP7122833B2 - Method for producing silicon carbide fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing silicon carbide fibers, and more particularly to a method for producing inexpensive silicon carbide fibers and silicon carbide fibers.

Silicon carbide fibers are lightweight, heat-resistant, and high-strength, so they are used in heat engines such as jet engines and gas turbines as a material to replace alloys.

Silicon carbide fibers can be produced by spinning polycarbosilane (PCS), which is a silicon carbide-based precursor polymer, and baking it at 1000 to 1600° C. in a non-oxidizing atmosphere such as nitrogen to form a ceramic.
In addition, the polycarbosilane fibers are subjected to an infusibilization treatment before firing so that the fibers do not melt and stick to each other even at a high temperature during firing so that the fiber shape can be maintained.

As the infusibilizing treatment, Patent Document 1 discloses that polycarbosilane fibers are heated to 50° C. to 400° C. in an oxidizing atmosphere to oxidatively crosslink the polycarbosilane to make it infusible.

However, in the infusibilization treatment by thermal oxidation, a large amount of oxygen is mixed into the silicon carbide fibers as an impurity. The mixed oxygen accelerates the coarse crystallization of silicon carbide by a carbon-thermal reduction reaction at high temperatures, thereby lowering the strength of the silicon carbide fiber.

In addition, Patent Document 2 discloses an infusibilization treatment by electron beam irradiation. According to the infusibilization treatment, oxygen is not introduced into the polycarbosilane fibers, so the oxygen content in the silicon carbide fibers is reduced to 1 mass. % or less, and a heat resistance of 1800° C. can be obtained.

U.S. Pat. No. 4,100,233 U.S. Pat. No. 5,322,822

However, the infusibilization treatment by electron beam irradiation requires expensive electron beam irradiation equipment, which makes it difficult to reduce the cost.

The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide a method for producing silicon carbide fibers that does not require electron beam irradiation equipment, is inexpensive, and does not lose strength at high temperatures. is to provide

As a result of intensive studies to achieve the above object, the present inventors adjusted the molecular weight of polycarbosilane to obtain a high-molecular-weight polycarbosilane that does not require infusibilization treatment, and the above-mentioned high-molecular-weight polycarbosilane that is difficult to melt-spun. The inventors have found that the above object can be achieved by dry spinning polycarbosilane, and have completed the present invention.

That is, the above problems are solved by the method for producing silicon carbide fibers of the present invention.
(1) a molecular weight adjusting step of adjusting the molecular weight of polycarbosilane;
a dry spinning process for fibrillating the polycarbosilane with a controlled molecular weight;
and a firing step of obtaining silicon carbide fibers from polycarbosilane fibers,
In the molecular weight adjustment step, the synthesized polycarbosilane is dissolved in a mixed solvent of a good solvent and a poor solvent, polycarbosilane having a molecular weight of less than 1000 is removed, and polycarbosilane having a molecular weight of 1000 to 2000 is less than 10% by mass. A method for producing silicon carbide fibers, characterized in that the remainder is a treatment to obtain a polycarbosilane having a number average molecular weight (Mn) of 6000 to 11000 .
(2) comprising a polycarbosilane synthesis step before the molecular weight adjustment step;
In the polycarbosilane synthesis step, the low boiling point component of polysilane in the liquid phase generated by thermal decomposition of polysilane, which is the raw material of polycarbosilane, is evaporated, and further heated in the gas phase to obtain polycarbosilane, and the polycarbosilane is obtained. The method for producing silicon carbide fibers according to item (1) above, which includes a process of returning the carbosilane to the liquid phase to increase the molecular weight.
(3) The silicon carbide fiber according to item (2) above, wherein the polycarbosilane synthesized in the polycarbosilane synthesis step is a polycarbosilane having a maximum molecular weight of 35,000 to 230,000. manufacturing method.
(4) Any one of items (1) to (3) above, wherein the mixed solvent is a hexane/acetone mixed solvent containing 70% to 80% by mass of acetone . of silicon carbide fibers.
(5) The carbonization according to any one of items (1) to (4) above, wherein the molecular weight adjustment step is a treatment for removing polycarbosilane having a molecular weight of less than 1000. A method for producing silicon fibers.
(6) The ratio (Mw/Mn) of the weight- average molecular weight (Mw) to the number-average molecular weight (Mn) of the polycarbosilane whose molecular weight has been adjusted in the molecular weight adjustment step is 1.2 to 3.7. The method for producing a silicon carbide fiber according to any one of items (1) to (5) above, characterized by:
(7) the dry spinning step comprises dissolving the molecular weight-controlled polycarbosilane in a solvent to prepare a dry spinning solution;
The method for producing a silicon carbide fiber according to any one of items (1) to (6) above, wherein the dry spinning solution has a viscosity of 10 to 40 Pa·s (25°C). .
(8) The method for producing silicon carbide fibers according to item (7), wherein the dry spinning solution contains 56 to 62% by mass of polycarbosilane.
(9) The firing step includes primary firing in a mixed gas of hydrogen gas and nitrogen gas or argon gas, and secondary firing in nitrogen gas or argon gas. A method for producing a silicon carbide fiber according to any one of items (1) to (8).

According to the present invention, infusible high-molecular-weight polycarbosilane that can retain the fiber shape during firing is dry-spun, so infusibility treatment before firing is unnecessary, and inexpensive carbonization that does not require electron beam irradiation equipment. A method for producing silicon fibers and silicon carbide fibers can be provided.

4 is a graph showing an example of changes in polycarbosilane in the molecular weight adjustment process. 1 is an SEM photograph of silicon carbide fibers that are primarily sintered at a heating rate of 100° C./Hr. 4 is an SEM photograph of silicon carbide fibers that are primarily sintered at a heating rate of 50° C./Hr. 1 is a schematic diagram showing an example of a liquid phase-gas phase pyrolytic condensation apparatus; FIG. 4 is an FT-IR spectrum of polycarbosilane produced by the liquid-gas phase pyrolytic condensation method.

The method for producing silicon carbide fibers of the present invention will be described in detail.
The method for producing the silicon carbide fiber is
A molecular weight adjusting step of adjusting the molecular weight of polycarbosilane;
a dry spinning process for fibrillating the polycarbosilane with a controlled molecular weight;
and a sintering step of obtaining silicon carbide fibers from the polycarbosilane fibers.

<Molecular weight preparation step>
The above-mentioned molecular weight adjustment step removes low molecular weight components that cause fusion bonding from polycarbosilane, which has a broad molecular weight distribution from low molecular weight components to high molecular weight components. This is a step in which only high molecular weight components are used to prevent the fusion.
Specifically, low-molecular-weight components having a molecular weight of less than 1000 as determined by GPC (gel permeation chromatography) using polystyrene as a standard substance are removed by solvent fractionation to prevent fusion during firing.

The solubility of polycarbosilane in a mixed solvent of a good solvent and a poor solvent varies depending on the molecular weight.
For example, since the solubility in a hexane/acetone mixed solvent changes, by changing the acetone ratio of the hexane/acetone mixed solvent, it is possible to change the molecular weight range of the low molecular weight components to be dissolved in the hexane/acetone mixed solvent and removed. Hexane/methanol, hexane/ethanol, etc. can be mentioned as a combination of a mixed solvent of a good solvent and a poor solvent.

FIG. 1 shows the weight-average molecular weight of polycarbosilane having a weight-average molecular weight of 5,800 dissolved in hexane/acetone mixed solvents with different acetone ratios, and the change in the molecular weight of the precipitate measured by GPC.

From FIG. 1, as the acetone content in the hexane/acetone mixed solvent decreases, the molecular weight of the polycarbosilane dissolved in the hexane/acetone mixed solvent and eliminated increases, and the weight average molecular weight of the precipitated polycarbosilane increases. you know it will get bigger.

The hexane/acetone mixed solvent preferably contains 70% to 80% by mass, more preferably 70% to 78% by mass, of acetone.

When the acetone content is 80% by mass or less, most of the polycarbosilane having a molecular weight of 1000 or less, which causes fusion during firing, can be removed. On the other hand, if the content of acetone is less than 70% by mass, even high-molecular-weight polycarbosilane is removed, resulting in a reduced yield.

Fractionated using a hexane/acetone mixed solvent containing 80% by mass of acetone, dry-spun polycarbosilane raw silk is primarily calcined at 1000 ° C. in an H ₂ /Ar atmosphere (heating rate 100 ° C./Hr), and Ar FIG. 2 shows an SEM photograph of the silicon carbide fibers that were secondarily sintered at 1300° C. in the atmosphere. FIG. 3 shows an SEM photograph of silicon carbide fibers fired at a heating rate of 50° C./hr in the primary firing.

In the SEM photograph of FIG. 2, scars due to fusion bonding are observed in the silicon carbide fibers, but in the SEM photograph of FIG. 3, scars due to fusion bonding are not observed in the silicon carbide fibers.

In the silicon carbide fiber shown in FIG. 3, by lowering the rate of temperature rise in the primary firing, the increase in vapor concentration in the firing atmosphere of the low-molecular-weight polycarbosilane discharged from the polycarbosilane raw silk during firing is suppressed. It is considered that fusion between fibers could be avoided.

With a hexane/acetone mixed solvent containing 80% by mass of acetone, the low-molecular-weight polycarbosilane that causes fusion cannot be completely removed. Since the adhesion can be avoided, it can be seen that if the mixed solvent contains 80% by mass or less of acetone, the low-molecular-weight components that cause fusion during firing can be removed.

By the above molecular weight adjustment step, the average molecular weight shifts to the high molecular weight side and the molecular weight distribution becomes sharp.

The polycarbosilane after the above molecular weight adjustment step should be infusible, but preferably has a number average molecular weight (Mn) of 6,000 to 11,000.
The polycarbosilane with the above molecular weight distribution can be dry-spun, and can prevent fusion between fibers during firing, and can prevent fiber deformation due to firing because it has a small amount of low-molecular-weight gasification components and small volume shrinkage. .

In addition, the polycarbosilane after the molecular weight adjustment step has a weight average molecular weight (Mw) to number average molecular weight (Mn) ratio (Mw/Mn) of 1.2 to 1.2, depending on the average molecular weight of the polycarbosilane. 3.7 is preferred.

Further, the polycarbosilane after the molecular weight adjustment step may contain less than 10% by mass of polycarbosilane having a molecular weight of 1000 to 2000.
Polycarbosilane with a molecular weight of less than 1000 causes fusion during firing, but polycarbosilane with a molecular weight of 1000 to 2000 becomes a solvent component of the dry spinning solution described later and dissolves ultrahigh molecular weight polycarbosilane. It is considered to make it easier to

<Dry spinning process>
The dry spinning step is a step of making the polycarbosilane fibrous, and includes a process of dissolving the polycarbosilane with the adjusted molecular weight in a solvent to prepare a dry spinning solution.
Specifically, the dry spinning solution is discharged from a spinneret (nozzle), and the solvent is vaporized by natural drying or heat drying to obtain a polycarbosilane fiber.

In the dry spinning process, unlike melt spinning that spins at a high temperature of 300 ° C. or higher, spinning is possible at room temperature or without applying a high temperature, and a rapid temperature drop does not occur after spinning. It is possible to prevent variations in fiber diameter caused by changes.

The viscosity of the dry spinning solution is preferably 10 to 40 Pa·s (25° C.), although it depends on the diameter of the spinning nozzle.
If it exceeds 40 Pa·s, dry spinning is difficult, and if it is less than 10 Pa·s, the fiber diameter tends to vary.
The viscosity of the dry spinning solution is a value measured with a Brookfield viscometer.

The dry spinning solution having the above viscosity preferably contains 56% to 62% by weight of polycarbosilane having the above molecular weight. When the concentration of polycarbosilane is within the above range, deformation of the cross section of the fiber is prevented, and a silicon carbide fiber having a cross section close to a perfect circle can be obtained.

Examples of the solvent include aliphatic hydrocarbons, aromatic hydrocarbons, and halogenated hydrocarbons as long as they can dissolve polycarbosilane. It can be preferably used because of its excellent volatility.

Using the above dry spinning solution, raw silk can be obtained by, for example, spinning at a spinning nozzle diameter φ of 80 to 120 μm, a pressure of 0.5 MPa to 1.6 MPa, and a spinning speed of 60 to 110 m/min.

Polycarbosilane fibers contract about 30% in volume when fired, so silicon carbide fibers with a diameter of about 5 to 9 μm can be obtained from raw silk with a fiber diameter φ of 10 to 12 μm.

<Baking process>
The sintering step is a step of ceramicizing the polycarbosilane fibers to form silicon carbide fibers, and includes primary sintering and secondary sintering.

The primary firing is a process of firing the polycarbosilane fibers at approximately 1000° C. in a non-oxidizing atmosphere to eliminate excess carbon and form silicon carbide fibers.

The secondary firing is a process of firing silicon carbide fibers at 1300 to 1600° C. in a non-oxidizing atmosphere to promote crystallization and increase strength.
The secondary firing is preferably performed while applying tension as necessary. Firing while applying tension is effective in removing creases and the like from fibers, and is effective in obtaining high-strength fibers.

If the secondary firing temperature is less than 1300° C., the crystallization does not progress and the amorphous phase increases, resulting in a low elastic modulus and low strength. On the other hand, when the temperature exceeds 1600°C, crystallization progresses and the crystallite size becomes too large, the fiber modulus increases, the fiber becomes brittle, and the strength begins to decrease.

The primary firing can be performed in a mixed gas of hydrogen gas and nitrogen gas or argon gas in a non-oxidizing atmosphere, and the secondary firing can be performed in a mixed gas of nitrogen gas or argon gas in a non-oxidizing atmosphere. Although it can be carried out in gas, it is preferable to carry out the primary firing in a mixed gas of hydrogen gas and argon gas, and the secondary firing in argon gas.
By using argon gas to prevent the silicon carbide fibers from contacting oxides in the furnace, it is possible to obtain silicon carbide fibers with an oxygen content of 1% by mass or less and high heat resistance and high strength.

Silicon carbide fibers have the highest heat resistance when the molar ratio of carbon to silicon (C/Si) is 1, which is the stoichiometric ratio.
Silicon carbide fibers containing an excess amount of carbon lose their strength in an oxidizing environment such as the air because the excess carbon in the silicon carbide fibers is gradually gasified and released due to continuous heat load. However, when the primary firing conditions and the secondary firing temperature are within the above ranges, high-strength silicon carbide fibers having a carbon-to-silicon molar ratio (C/Si) of 0.9 to 1.1 can be obtained. .

<Polycarbosilane synthesis step>
The method for producing silicon carbide fibers of the present invention can include a polycarbosilane synthesizing step prior to the molecular weight adjusting step.
In the method for producing silicon carbide fibers of the present invention, any polycarbosilane containing a high molecular weight component that can prevent fusion between fibers without melting even at high temperatures during firing can be used. According to the silane synthesis process, the polycarbosilane having a high molecular weight can be easily synthesized.

The polycarbosilane synthesizing step is a step of synthesizing a high-molecular-weight polycarbosilane by a liquid-gas phase thermal decomposition condensation method.
FIG. 4 shows an example of a liquid-gas phase thermal decomposition/condensation apparatus.

Specifically, polysilane, which is a raw material of polycarbosilane, placed in the liquid phase reaction vessel 1 is heated at 300° C. to 500° C. to evaporate low-boiling components in the liquid phase generated by thermal decomposition.
The evaporated low-molecular-weight polysilane is further heated at 600 to 700° C. in the vapor-phase reaction quartz tube 2 to form low-molecular-weight polycarbosilane, which is then cooled and returned to the liquid-phase reaction vessel 1 .
Then, by condensing the low-molecular-weight polycarbosilanes together in the liquid-phase reaction vessel 1, polycarbosilane having a high molecular weight can be obtained.
By repeating a series of these reactions, the liquidus temperature rises as the low boiling point component decreases, and the polycarbosilane is gradually increased in molecular weight to obtain an ultra-high molecular weight polycarbosilane.

The above polysilane may be a polymer having a skeleton consisting only of Si—Si bonds, and conventionally known polysilanes having an alkyl group having 1 to 6 carbon atoms or a phenyl group in a side chain can be used.

The molecular weight of polycarbosilane can be adjusted by the synthesis time and the maximum heating temperature of the liquid phase reactor.
In the liquid phase-gas phase thermal decomposition condensation method, the polycarbosilane in the liquid phase reaction vessel increases in molecular weight as the synthesis time elapses. When the temperature of the liquid phase reactor exceeds 500° C., the polycarbosilane increases in molecular weight and viscosity, and finally gels and becomes insoluble.

In the polycarbosilane synthesis step, it is preferable to synthesize polycarbosilane having a maximum molecular weight of 35,000 to 230,000.

By setting the conditions for synthesizing polycarbosilane having a maximum molecular weight of 35,000 or more, the amount of low-molecular-weight components to be removed in the molecular-weight preparation step is reduced, and the yield is improved.
In addition, if polycarbosilane having a molecular weight of 230,000 or more is contained, the amount of polycarbosilane having a high molecular weight that is difficult to dissolve in a solvent increases. It is not preferable because the properties of the fiber may deteriorate.

<Silicon carbide fiber>
According to the above-described method for producing silicon carbide fibers of the present invention, inexpensive silicon carbide fibers that do not require infusibilization treatment before firing and do not require electron beam irradiation equipment can be obtained.
The silicon carbide fiber is composed of carbon, silicon, hydrogen, and oxygen, and has an oxygen content of 3% by mass or less.
The oxygen content is preferably 1% by mass or less. When the oxygen content is 1% by mass or less, the heat resistance is further improved.

The silicon carbide fiber preferably has a carbon to silicon molar ratio (C/Si) of 0.9 to 1.5, more preferably 0.9 to 1.1.
A C/Si molar ratio of 0.9 to 1.5 provides heat resistance, and a C/Si molar ratio of 0.9 to 1.1 provides further oxidation resistance.

Further, according to the method for producing silicon carbide fibers of the present invention, silicon carbide fibers having a fiber diameter of 6 to 15 μm can be spun, and the monofilament with the above fiber diameter has a tensile strength of 2.5 GPa or more, and 1500 A high-strength and high-heat-resistant silicon carbide fiber having a tensile strength of 2.0 GPa or more after a heat load of 10°C can be obtained.

EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the following examples.

<Polycarbosilane synthesis step>
[Example 1]
Polysilane (permethylpolysilane: manufactured by Nippon Soda Co., Ltd.) is placed in the liquid phase reaction vessel of the liquid-gas phase thermal decomposition condensation apparatus shown in FIG. The tube was heated to 600-700° C. and reacted for 5 hours to obtain polycarbosilane.

FIG. 5 shows the FT-IR spectrum of polycarbosilane.
From FIG. 5, it was confirmed that a polycarbosilane having a carbosilane skeleton was produced by the liquid-gas phase thermal decomposition condensation method.

[Example 2]
A polycarbosilane was obtained in the same manner as in Example 1, except that the synthesis time was changed to 20 hours.
The FT-IR spectrum of the polycarbosilane of Example 2 was similar to that of Example 1.

[Example 3]
Polycarbosilane was obtained in the same manner as in Example 1, except that the temperature of the liquid phase reaction vessel was raised by 10°C.
The FT-IR spectrum of the polycarbosilane of Example 3 was similar to that of Example 1.

[Example 4]
As the polycarbosilane of Example 4, PCS-UH manufactured by NGS Advanced Fiber Co., Ltd. was prepared.

(Measurement of molecular weight)
The molecular weights of the polycarbosilanes of Examples 1 to 4 were determined by GPC (Gel Permeation Chromatography) measurement under the following conditions.

・ Apparatus: GPC-8020 (manufactured by Tosoh Corporation)
・Column: TSK G2000HXL and G4000HXL (manufactured by Tosoh Corporation)
・Temperature: 40℃
・ Solvent: THF (tetrahydrofuran)
・Flow rate: 1.0 mL/minute

1 mL of polycarbosilane with a concentration of 0.5% by mass was injected, and from the molecular weight distribution of polycarbosilane measured under the above conditions, the number of polycarbosilane bodies was calculated using a molecular weight calibration curve created from a monodisperse polystyrene standard sample. An average molecular weight Mn and a weight average molecular weight Mw were calculated. Table 1 shows the measurement results.

<Molecular weight preparation step>
The polycarbosilane of Example 1 was subjected to solvent fractionation using a hexane/acetone mixed solvent containing 78% by mass of acetone.
Further, the polycarbosilanes of Examples 2 and 3 were subjected to solvent fractionation using a hexane/acetone mixed solvent containing 80% by mass of acetone.
Furthermore, the polycarbosilane of Example 4 was subjected to solvent fractionation using a hexane/acetone mixed solvent with an acetone ratio of 70% by mass.

The molecular weights of the polycarbosilanes prepared in Examples 1 to 4 were measured.
Table 2 shows the measurement results.

<Dry spinning process>
The polycarbosilane of Examples 1 and 2 whose molecular weight was adjusted was completely dissolved in a large amount of xylene to obtain a low-concentration dry spinning solution. Evaporation and adjustment to the desired concentration gave a dry spinning solution.
The above dry spinning solution was dry-spun under the conditions shown in Table 3 to obtain polycarbosilane raw silk having a fiber diameter φ of 10 to 12 μm.

<Baking process>
The polycarbosilane raw silk of Examples 1-2, 2, 4-1 and 4-2 was sintered under the conditions shown in Table 4 while applying tension to obtain silicon carbide fibers.

・Primary firing: temperature 1000°C, heating rate 50-100°C/hr
Atmosphere 40-60 Vol% H ₂ /Ar 250 ml/min
・Secondary firing: temperature 1300-1500°C, heating rate 100°C/hr
Atmosphere Ar250ml/min

<Evaluation>
Silicon carbide fibers were evaluated by the following methods. Table 4 shows the evaluation results together with the firing conditions.

(Measurement of oxygen content and (C/Si))
The C, H, O content of the silicon carbide fiber was measured using SC-144DR manufactured by Leco and EMGA-930 manufactured by HORIBA, and Si was calculated as the remaining amount excluding C, H, O. A molar ratio (C/Si) was determined.

(Measurement of tensile strength)
The tensile strength of the monofilament was measured by combining a digital force gauge manufactured by Imada Co., Ltd. and a load/displacement measuring unit.

From Table 4, it was confirmed that the silicon carbide fiber manufacturing method of the present invention has excellent heat resistance with a tensile strength of 2.5 GPa or more and a tensile strength of 2.0 GPa or more after a heat load at 1500°C.

Further, regarding the silicon carbide fibers fired under the conditions shown in Table 5 while applying tension to the polycarbosilane raw silk of Examples 1-1 to 1-3, Example 2, and Examples 4-1 and 4-2 , confirmed the dispersion of the fiber diameter.

From Tables 4 and 5, it can be seen that the method for producing silicon carbide fibers of the present invention can obtain silicon carbide fibers having a reduced diameter of 5 to 9 μm.

Further, the polycarbosilane of Example 1 was fractionated using a hexane/acetone mixed solvent containing 80% by mass of acetone, and the raw silk of dry-spun polycarbosilane was calcined under the conditions shown in Table 6 while applying tension. The composition of silicon carbide fibers was investigated.

From Table 6, if the primary firing is performed up to 1000 ° C. in an H ₂ /Ar atmosphere, the molar ratio of carbon and silicon (C/Si) is 0.9 to 1.1, and the oxygen content is 0.83% by mass or less. of silicon carbide fibers can be obtained.

REFERENCE SIGNS LIST 1 liquid phase reaction vessel 2 gas phase reaction quartz tube 3 electric furnace 4 cooler 5 cooler 6 cooler 7 cock 8 _N2 gas cylinder 9 flow meter 10 thermocouple 11 thermocouple 12 mantle heater 13 gas outlet

Claims (9)

A molecular weight adjusting step of adjusting the molecular weight of polycarbosilane;
a dry spinning process for fibrillating the polycarbosilane with a controlled molecular weight;
and a firing step of obtaining silicon carbide fibers from polycarbosilane fibers,
In the molecular weight adjustment step, the synthesized polycarbosilane is dissolved in a mixed solvent of a good solvent and a poor solvent, polycarbosilane having a molecular weight of less than 1000 is removed, and polycarbosilane having a molecular weight of 1000 to 2000 is less than 10% by mass. A method for producing silicon carbide fibers, characterized in that the remainder is a treatment to obtain a polycarbosilane having a number average molecular weight (Mn) of 6000 to 11000. A polycarbosilane synthesis step is provided before the molecular weight adjustment step,
In the polycarbosilane synthesis step, the low boiling point component of polysilane in the liquid phase generated by the thermal decomposition of polysilane, which is the raw material of polycarbosilane, is evaporated, and further heated in the gas phase to obtain polycarbosilane, and the polycarbosilane is obtained. 2. The method for producing silicon carbide fibers according to claim 1, comprising a process of returning the carbosilane to the liquid phase to increase the molecular weight. 3. The method for producing silicon carbide fibers according to claim 2, wherein the polycarbosilane synthesized in the polycarbosilane synthesis step has a maximum molecular weight of 35,000 to 230,000. The method for producing silicon carbide fibers according to any one of claims 1 to 3, wherein the mixed solvent is a hexane/acetone mixed solvent containing 70% by mass to 80% by mass of acetone. The method for producing silicon carbide fibers according to any one of claims 1 to 4, wherein the molecular weight adjusting step is a treatment for removing polycarbosilane having a molecular weight of less than 1,000. The ratio (Mw/Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) of the polycarbosilane whose molecular weight has been adjusted in the molecular weight adjustment step is 1.2 to 3.7. A method for producing silicon carbide fibers according to any one of claims 1 to 5. The dry spinning step comprises dissolving polycarbosilane with a controlled molecular weight in a solvent to prepare a dry spinning solution,
The method for producing silicon carbide fibers according to any one of claims 1 to 6, wherein the dry spinning solution has a viscosity of 10 to 40 Pa·s (25°C). 8. The method for producing silicon carbide fibers according to claim 7, wherein the dry spinning solution contains 56 to 62% by weight of polycarbosilane. 1-, wherein the firing step includes primary firing in a mixed gas of hydrogen gas and nitrogen gas or argon gas, and secondary firing in nitrogen gas or argon gas. 9. A method for producing a silicon carbide fiber according to any one of 8.

Images