JP7341038B2 - Method for manufacturing silicon nitride fiber - Google Patents

Description

The present invention relates to a method for manufacturing silicon nitride fibers.

Silicon nitride is known to be a material with high thermal conductivity. Therefore, compositions (for example, fibers and particles) made of silicon nitride are used in various industrial applications such as heat dissipation materials.

As a method for manufacturing such a silicon nitride composition, for example, Japanese Patent Application Laid-Open No. 9-30900 (Patent Document 1) discloses a method for manufacturing silicon nitride whiskers.

Japanese Patent Application Publication No. 9-30900

However, in the method for producing silicon nitride whiskers described in Patent Document 1, decomposition gas of the organosilicon polymer compound is generated by heating a molded product obtained by mixing an organosilicon polymer compound and solid carbon. Since whiskers are obtained by the reaction between this decomposed gas and nitrogen gas, the fiber diameter of the whiskers tends to vary depending on the environment in which the gases react with each other, and as a result, when the whiskers are combined with resin as a heat dissipation filler, the dispersibility is low. The thermal conductivity was poor, and the thermal conductivity varied widely, making it impossible to exhibit sufficient performance.

The present invention was made under these circumstances, and an object of the present invention is to provide a method for producing silicon nitride fibers having uniform fiber diameters.

The present invention comprises: "(i) a step of preparing a silicon alkoxide solution; (ii) a step of preparing a stringable sol solution by hydrolyzing and condensing the silicon alkoxide in the silicon alkoxide solution; (iii) A silicon nitride fiber comprising the steps of: preparing a silicon oxide fiber by electrospinning using the spinnable sol solution; (iv) firing the silicon oxide fiber in an ammonia gas or nitrogen gas atmosphere. ``Production method.''

The method for producing silicon nitride fibers according to the present invention uses a spinnable sol solution and prepares silicon oxide fibers by electrospinning, so the fiber diameter of the silicon oxide fibers tends to be uniform. Furthermore, the fiber diameter of silicon nitride fibers produced by firing the silicon oxide fibers in an ammonia gas or nitrogen gas atmosphere tends to be uniform.

Schematic cross-sectional view of electrostatic spinning device A plan view schematically showing the arrangement of silicon oxide fibers in a silicon oxide fiber sheet formed by electrostatic spinning. A plan view schematically showing the arrangement of silicon oxide fibers in a silicon oxide fiber sheet formed by a method other than electrostatic spinning.

The method for producing silicon nitride fibers of the present invention will be described in detail below.

First, the step (i) of preparing a silicon alkoxide solution will be explained.

The silicon alkoxide solution contains a solvent [for example, an organic solvent (for example, an alcohol such as ethanol, dimethylformamide)], water for hydrolyzing the silicon alkoxide contained in the silicon alkoxide solution, and a catalyst (for example, hydrochloric acid, etc.). ) may contain.

In addition, the silicon alkoxide solution contains nitrides such as hydrazine and diethanolamine that react with silicon oxide fibers, organic compounds (for example, polyvinylpyrrolidone) for imparting various functions such as flexibility adjustment, and inorganic components such as hydroxyapatite. , or may contain additives such as dyes. Note that these additives can be added before, during, or after the hydrolysis described below.

Furthermore, the silicon alkoxide solution may contain inorganic or organic fine particles. Examples of the inorganic fine particles include yttrium oxide, titanium oxide, manganese dioxide, copper oxide, activated carbon, and metals (e.g., platinum), and examples of the organic fine particles include dyes or pigments. . Further, the average particle diameter of the fine particles is not particularly limited, but is preferably 0.001 to 1 μm, more preferably 0.002 to 0.1 μm.

Next, the step (ii) of preparing a stringable sol solution by hydrolyzing and polymerizing the silicon alkoxide in the silicon alkoxide solution will be described.

The amount of water for hydrolyzing the silicon alkoxide contained in the silicon alkoxide solution varies depending on the molecular structure of the silicon alkoxide and is not particularly limited, but for example, in the case of tetraethoxysilane, it should be a stringable sol solution. The amount of water is preferably at most twice the mole of the alkoxide so that the amount of water can be reduced.

The presence or absence of "stringiness" can be determined under the conditions shown below (determination method).
(Judgment method)
A spinning solution (solid content concentration: 10 to 50 wt%) is discharged from a metal nozzle (inner diameter: 0.4 mm) arranged horizontally to a grounded metal plate (discharge rate: 0.5 to 1.0 g/hr ), a voltage is applied to the nozzle (electric field strength: 1 to 3 kV/cm, polarity: positive application or negative application), and spinning is continued for 1 minute or more without causing solidification of the solution at the tip of the nozzle, Fibers are accumulated on a metal plate.
A scanning electron micrograph of the accumulated fibers was taken and observed. There were no droplets, the average fiber diameter (arithmetic mean of 50 points) was 5 μm or less, and the aspect ratio (average fiber length/average fiber diameter) was 100. If the conditions that allow the production of the above fibers exist, the spinning solution is determined to have "spinning properties." On the other hand, no matter how the conditions (i.e., concentration, throughput, electric field strength, and/or polarity) are varied and combined, if there are droplets, if the fiber morphology is not oily, and if the fiber morphology is not constant, the average fiber diameter exceeds 5 μm, or when the aspect ratio is less than 100 (for example, in the form of particles), and conditions for producing the fiber do not exist, the spinning solution is determined to be "not spinnable." In addition, the average fiber diameter of the present invention refers to the average value of the fiber diameters of 50 fiber cross sections, and if the fiber cross section is not a perfect circle, convert the cross section of the fiber to the diameter when the cross section is a perfect circle, Fiber diameter. Moreover, the average fiber length refers to the average value of the fiber lengths of 50 fibers.

Next, (iii) the step of preparing silicon oxide fibers by electrostatic spinning using the spinnable sol solution will be described.

This electrostatic spinning method is a method in which an electric field is applied to the spinning solution to stretch the spinning solution and form it into fibers.

The electrostatic spinning method will be briefly explained based on FIG. 1, which is a schematic cross-sectional view of an electrostatic spinning apparatus disclosed in JP-A No. 2005-194675.

The electrostatic spinning device of FIG. 1 includes a spinning dope supply device 1 that can supply a spinning dope to a nozzle 2, a nozzle 2 that discharges the spinning dope supplied from the spinning dope supply device 1, and a spinning dope that is discharged from the nozzle 2 and stretched by an electric field. A grounded collector 3 that collects silicon oxide fibers, a voltage application device 4 that can apply a voltage to the nozzle 2 to form an electric field between the nozzle 2 and the grounded collector 3, and the nozzle 2. The apparatus includes a spinning container 6 containing a collector 3, a gas supply device 7 capable of supplying gas at a predetermined relative humidity to the spinning container 6, and an exhaust device 8 capable of exhausting the gas inside the spinning container 6.

In the case of such an electrospinning device, the spinning dope is supplied to the nozzle 2 by the spinning dope supply device 1 . The supplied spinning stock solution is discharged from the nozzle 2, and is subjected to a stretching action by the electric field between the grounded collector 3 and the nozzle 2 applied by the voltage application device 4, and is transferred to the collector 3 while being turned into fibers. fly towards. Then, the flying silicon oxide fibers are directly accumulated on the collector 3.

The viscosity of the spinnable sol solution, which is a type of spinning stock solution, is preferably 0.01 to 10 Pa·s in order to perform electrostatic spinning efficiently. It is preferably 0.05 to 5 Pa·s, more preferably 0.1 to 3 Pa·s. This is because if the viscosity exceeds 10 Pa·s, it is difficult to spin thin fibers during electrostatic spinning, and if the viscosity is less than 0.01 Pa·s, the fiber shape itself tends to be difficult to obtain. In addition, when spinning a stringable sol solution using a nozzle, by setting the atmosphere at the tip of the nozzle to the same solvent gas atmosphere as the solvent of the silicon alkoxide solution, it is possible to spin a stringable sol solution with a viscosity exceeding 10 Pa. Even a filamentous sol solution may be able to be spun.

Note that, as the spinning dope supply device 1, for example, a syringe pump, a tube pump, a dispenser, etc. can be used. Further, instead of the nozzle 2, a sawtooth gear, a wire, a slit, etc. can also be used. Furthermore, although the collecting body 3 in FIG. 1 is in the form of a drum, it may be in the form of a conveyor. Furthermore, in FIG. 1, the collector 3 is grounded, but the nozzle 2 may be grounded and a voltage applied to the collector 3, or both the nozzle 2 and the collector 3 may be Although a voltage is applied, the voltage may be applied so as to have a potential difference.

Further, as the voltage application device 4, for example, a DC high voltage generator or a Van de Graaff generator can be used, so that the spinning dope can be spun into fibers without causing dielectric breakdown of the air. It is preferable to apply the electric field so that the electric field strength is 0.2 to 5 kV/cm. Further, the polarity of the applied voltage may be either positive or negative, but the polarity is appropriately selected according to the characteristics of the spinning stock solution so as to suppress the spreading of the silicon oxide fibers in the collector.

In the electrostatic spinning apparatus shown in FIG. 1, a gas supply device 7 (for example, a propeller fan, a sirocco fan, an air compressor, a blower with a temperature and humidity adjustment function, etc.) and an exhaust device 8 (for example, a fan) are installed in the spinning container 6. Because they are connected, the atmosphere inside the spinning container 6 can be kept constant, so a silicon oxide fiber sheet with uniform fiber diameter can be manufactured.

The silicon oxide fiber sheet formed by the electrostatic spinning method in this manner is in the state of silicon oxide gel-like fibers in which the silicon oxide sol solution is gelled. In order to increase the rigidity and strength of the silicon oxide fiber sheet, to improve the handling of the silicon oxide fiber sheet, and to make it easier to manufacture silicon oxide fibers with uniform fiber length, heat treatment is performed to increase the oxidation Silicon dry gel fibers or silicon oxide sintered fibers are preferred. This heat treatment can be performed using, for example, an oven, a sintering furnace, or the like.

In addition, when preparing silicon oxide fibers, it is possible to obtain silicon oxide fibers with short fiber lengths and uniform fiber lengths, and as a result, it is possible to obtain silicon nitride fibers with short fiber lengths and uniform fiber lengths. Since this is possible, it is preferable to form a silicon oxide fiber sheet by an electrostatic spinning method, and then press the silicon oxide fiber sheet with a press to prepare the silicon oxide fiber.

The reason why silicon oxide fibers with short fiber length and uniform fiber length can be obtained will be explained below. When a silicon oxide fiber sheet is formed by electrospinning, the average pore diameter of the silicon oxide fiber sheet is small and the pore diameters are uniform. In other words, the fact that the average pore diameter is small and the pore diameters are uniform means that the distances between the intersections of silicon oxide fibers are short and the distances between the intersections are uniform.

Regarding this point, FIG. 2 is a plan view schematically showing the arrangement of silicon oxide fibers in a silicon oxide fiber sheet formed by an electrostatic spinning method, and FIG. Referring to FIG. 3, which is a plan view schematically showing the arrangement of silicon oxide fibers, the electrospinning method produces oxidized fibers with small average pore diameters and uniform pore diameters, as shown in FIG. Since a silicon fiber sheet can be formed, the distance between the intersections of silicon oxide fibers is short and the distances between the intersections are uniform. For example, when looking at c5, which is the intersection point between fibers, the distances to b5, c4, c6, and d4, which are the intersection points between silicon oxide fibers adjacent to c5, are relatively short, and the distances are almost the same. . On the other hand, silicon oxide fiber sheets formed by methods other than electrospinning have large variations in pore diameter, as shown in FIG. For example, when looking at C5, which is an intersection between fibers, as a reference, the distances from B5, C4, C6, and D4, which are intersections between silicon oxide fibers adjacent to C5, vary widely.

Next, this silicon oxide fiber sheet is pressed and pulverized using a press. As mentioned above, the silicon oxide fiber sheet formed by electrospinning has a small average pore diameter, uniform pore diameters, short distances between the intersections of silicon oxide fibers, and uniform distances between the intersections. Therefore, when the silicon oxide fiber sheet in this state is pressurized with a press to prevent the orientation of the silicon oxide fibers from changing, the intersections of the silicon oxide fibers are strongly pressed, and the silicon oxide fibers become stiff. Coupled with the fact that it is high and difficult to deform, it is easy to break at intersections between silicon oxide fibers, so it is possible to produce silicon oxide fibers with short fiber lengths and uniform fiber lengths. In other words, the intersection of silicon oxide fibers overlaps each other, and microscopically corresponds to a point where the thickness of the silicon oxide fiber sheet becomes thicker, so the pressure from the press is applied to the intersection between silicon oxide fibers. It acts preferentially on intersections. Therefore, silicon oxide fibers with short fiber length and uniform fiber length can be produced.

Regarding this point, FIG. 2 is a plan view schematically showing the arrangement of silicon oxide fibers in a silicon oxide fiber sheet formed by an electrostatic spinning method, and FIG. Referring to FIG. 3, which is a plan view schematically showing the arrangement of silicon oxide fibers, for example, the intersections a1 to a3, b1 to b5, c1 to c6, d1 to d6, and the intersections between fibers in FIG. At e1 to e5, the two silicon oxide fibers intersect, so the thickness is approximately twice as thick as the area where they do not intersect. Therefore, when the silicon oxide fiber sheet in FIG. 2 is pressurized by a press, the pressure is preferentially applied to the intersections a1 to a3, b1 to b5, c1 to c6, d1 to d6, and e1 to e5 between the fibers. In addition, due to the stiffness of the silicon oxide fiber, the silicon oxide fiber breaks at the intersections a1 to a3, b1 to b5, c1 to c6, d1 to d6, and e1 to e5 between the fibers. Therefore, silicon oxide fibers with short fiber length and uniform fiber length can be manufactured.

On the other hand, in the case of the silicon oxide fiber sheet formed by a method other than the electrospinning method in FIG. Since the two silicon oxide fibers intersect with each other, the thickness is approximately twice as thick as that of the part where they do not intersect. Therefore, when the silicon oxide fiber sheet in FIG. 3 is pressurized by a press, the pressure is preferentially applied to the intersections A1 to A3, B1 to B5, C1 to C7, D1 to D6, and E1 to E5 between the fibers. In addition, due to the stiffness of the silicon oxide fiber, the silicon oxide fiber breaks at the intersections A1 to A3, B1 to B5, C1 to C7, D1 to D6, and E1 to E5 between the fibers. Therefore, silicon oxide fibers with uniform fiber length cannot be manufactured.

Note that the pressing force when pressing with a press machine is not particularly limited, and an appropriate pressing force is selected by confirming the pressing force, fiber length, and CV value of the fiber length through experiments.

Finally, (iv) the step of firing the silicon oxide fiber in an ammonia gas or nitrogen gas atmosphere will be described.

This baking can be carried out using, for example, an oven or a baking furnace. If the firing temperature is low, the growth of silicon nitride crystals can be suppressed and silicon nitride fibers that are flexible and easy to handle can be realized. preferable. The lower limit of the firing temperature is not particularly limited as long as silicon nitride can be produced, but it is preferably 1000° C. or higher so that the organic components contained in the silicon oxide fibers are decomposed. The firing time is adjusted appropriately to generate sufficient silicon nitride and exhibit excellent thermal conductivity, and to prevent silicon nitride crystals from growing too much and resulting in silicon nitride with low flexibility. Specifically, it is preferably 1 to 5 hours. During this firing step, a catalyst such as a nitride that reacts with silicon oxide or carbon for the purpose of promoting the reaction from silicon oxide to silicon nitride may be included.

EXAMPLES The present invention will be explained below with reference to Examples, but the present invention is not limited to these Examples.

(Example 1)
Tetraethyl orthosilicate, water, and hydrochloric acid were mixed at a molar ratio of 1:2:0.0025, and the mixture was heated and stirred at a temperature of 80° C. for 15 hours. Then, the mixture was concentrated using an evaporator until the silicon oxide concentration reached 44 wt%, and the viscosity was increased to 200 to 300 mPa·s to obtain a silicon oxide sol solution.
Thereafter, the silicon oxide sol solution was used for spinning under the following spinning conditions, and then degreased under the following degreasing conditions to obtain a silicon oxide fiber sheet (fabric weight: 26.0 g/m ² ) with an average fiber diameter of 1 μm. .
(Spinning conditions)
・Discharge amount from nozzle: 1g/hour ・Distance between nozzle tip and drum collector: 10cm
・Temperature and humidity inside the spinning container: 25℃/30%RH
・Voltage applied to nozzle: +10kV
(Degreasing conditions in a degreasing furnace)
・500℃/2 hours Next, about 1 g of this silicon oxide fiber sheet was taken, the silicon oxide fiber sheets were stacked to make a thickness of 1.5 cm, and then pulverized by pressing at a pressure of 2 MPa for 30 seconds using a press machine. Thus, silicon oxide staple fibers having an average fiber diameter of 1.0 μm, a CV value of fiber diameter of 0.189, an average fiber length of 99.2 μm, a CV value of fiber length of 0.186, and an aspect ratio of 99.0 were prepared.
Furthermore, this silicon oxide short fiber and carbon as a catalyst were fired in a firing furnace at 1,400°C under a nitrogen atmosphere for 5 hours, and further fired at 650°C for 5 hours under an air atmosphere to remove the carbon. Silicon nitride fibers with a fiber diameter of 0.8 μm, a CV value of fiber diameter of 0.189, an average fiber length of 92.3 μm, a CV value of fiber length of 0.223, and an aspect ratio of 116 were produced.
Note that the CV value of the fiber length and fiber diameter is the value obtained by dividing the standard deviation of the fiber length or fiber diameter by the average fiber length or average fiber diameter. Note that the "standard deviation" is a value obtained from the fiber length and fiber diameter of 50 fibers.

(Example 2)
Approximately 1 g of the silicon oxide fiber sheet (fabric weight: 26.0 g/m ² ) produced in the same manner as in Example 1 was taken, the silicon oxide fiber sheets were stacked to a thickness of 1.5 cm, and then a pressure of 10 MPa was applied using a press. It was crushed by applying pressure for 30 seconds to obtain an average fiber diameter of 1.0 μm, a CV value of fiber diameter of 0.192, an average fiber length of 12.0 μm, a CV value of fiber length of 0.247, and an aspect ratio of 12.0. Silicon oxide staple fibers were prepared.
Furthermore, this silicon oxide short fiber and carbon as a catalyst were fired in a firing furnace at 1,400°C under a nitrogen atmosphere for 5 hours, and further fired at 650°C for 5 hours under an air atmosphere to remove the carbon. Silicon nitride fibers were produced with a fiber diameter of 0.8 μm, a CV value of fiber diameter of 0.208, an average fiber length of 10.0 μm, a CV value of fiber length of 0.286, and an aspect ratio of 12.5.

(Example 3)
Approximately 1 g of the silicon oxide fiber sheet (fabric weight: 26.0 g/m ² ) produced in the same manner as in Example 1 was taken, and after stacking the silicon oxide fiber sheets to a thickness of 1.5 cm, a press machine was used to press the silicon oxide fiber sheet at a pressure of 5 MPa. It was crushed by applying pressure for 30 seconds to obtain an average fiber diameter of 1.0 μm, a CV value of fiber diameter of 0.178, an average fiber length of 42.3 μm, a CV value of fiber length of 0.297, and an aspect ratio of 49.0. Silicon oxide staple fibers were prepared.
Furthermore, this silicon oxide short fiber was fired in a firing furnace at 1400°C in an ammonia atmosphere for 5 hours to obtain an average fiber diameter of 0.8 μm, a CV value of fiber diameter of 0.252, an average fiber length of 39.1 μm, and a fiber length of 39.1 μm. A silicon nitride fiber with a CV value of 0.293 and an aspect ratio of 49.0 was produced.

From the results of the Examples, it was found that the silicon nitride fibers produced by the production method satisfying the configuration of the present invention had a low CV value of the fiber diameter, and silicon nitride fibers with uniform fiber diameters could be realized. In addition, if a silicon oxide fiber sheet is formed by electrospinning, and then the silicon oxide fiber sheet is pressurized with a press to prepare silicon oxide fibers, silicon oxide fibers with a low CV value and uniform fiber length can be obtained. It has been found that fibers and silicon nitride fibers can be realized.

The silicon nitride fiber of the present invention is a silicon nitride fiber with a uniform fiber diameter, and since silicon nitride is a substance with excellent thermal conductivity, for example, silicon nitride can be used as a filler in a resin to improve thermal conductivity. A resin composite with excellent properties can be realized, and the resin composite can be used, for example, as a thermally conductive sheet that radiates heat.

1 Spinning dope supply device 2 Nozzle 3 Collector 4 Voltage application device 5 Spinning space 6 Spinning container 7 Gas supply device 8 Exhaust devices a1 to a3, b1 to b5, c1 to c6, d1 to d6, e1 to e5 Intersections between fibers A1-A3, B1-B5, C1-C7, D1-D6, E1-E5 Intersections between fibers

Claims (1)

(i) preparing a silicon alkoxide solution;
(ii) preparing a stringable sol solution by hydrolyzing and polymerizing the silicon alkoxide in the silicon alkoxide solution;
(iii) using the spinnable sol solution to form fibers by electrospinning and heat-treating to prepare silicon oxide fibers;
(iv) firing the silicon oxide fiber in an ammonia gas or nitrogen gas atmosphere;
A method for producing silicon nitride fiber, including:

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