JP2019142757A - Method for producing boron carbide - Google Patents

Abstract

To provide a method for producing boron carbide capable of obtaining a firing body with desired shape.SOLUTION: A method for producing boron carbide 3 comprises: an adhesion step of precipitating boron source 2 consisting of boric acid and/or boron oxide from solution state to adhere it to a solid carbon source 1; and a firing step of firing the solid carbon source 1 through the adhesion step under inert gas atmosphere or reduction gas atmosphere.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing boron carbide.

Boron carbide (B ₄ C) is known as one of the hardest materials, and since mass production is possible, it has attracted attention as a lightweight and high-strength material. Conventionally, boron carbide has been used as a molded body by molding its powder, but it is known that the shape of the powder affects the strength of the molded body. As a method for synthesizing boron carbide, there is a method in which boron oxide or boric acid and carbon are reacted by a solid phase method at a high temperature of 2000 ° C. or higher.

Patent Document 1 discloses that a saccharide as a carbon source and boric acid or boron oxide as a boron source are dissolved in a solvent, and then the solvent is removed to form a mixed powder of the carbon source and the boron source. Is heated to 200 ° C. to 1100 ° C. to form an amorphous body composed of a B—O—C conjugate, and then this precursor is heated at 1300 ° C. or higher to obtain a B ₄ C crystal. A method for producing boron carbide is disclosed.

  On the other hand, there is an attempt to obtain a fired body having a predetermined shape when synthesizing boron carbide. For example, in Patent Document 2, a suspension in which a boron source is dispersed in viscose is spun to obtain an elementary fiber in which the boron source is present in a cellulose matrix, and then the elementary fiber is heated to 1800 ° C. to 2300 ° C. A method for producing a boron carbide ceramic fiber is disclosed in which boron carbide fiber is obtained by heat treatment at a high temperature of 0C.

JP 2003-277039 A Japanese Patent No. 5319767

  However, the production method of Patent Document 1 requires two steps of firing after dissolution and drying, and the process is complicated. Further, an additional process is required to obtain boron carbide having a predetermined shape. The method of Patent Document 2 includes a step of spinning a viscose suspension, so that the process is complicated and firing at a high temperature exceeding 2000 ° C. is required as in the conventional manufacturing method. Furthermore, since the boron carbide fiber finally obtained is a thread-like continuous body, there is room for examination as a raw material for producing a molded body.

  An object of this invention is to provide the manufacturing method of boron carbide which can obtain the boron carbide of a desired shape simply in view of said subject.

  As a result of intensive studies to solve the above problems, the present inventors have baked a solid carbon source having a desired shape in which a boron source uniformly adheres to the surface in an inert gas atmosphere or a reducing gas atmosphere. As a result, it was found that boron carbide having a desired shape was obtained, and the present invention was completed.

That is, the method for producing boron carbide of the present invention includes:
A deposition step of depositing a boron source comprising boric acid and / or boron oxide from a solution state and depositing it on a solid carbon source having a desired shape;
A firing step of firing the solid carbon source that has undergone the attachment step in an inert gas atmosphere or a reducing gas atmosphere;
It is characterized by having.

  According to the above manufacturing method, by firing the solid carbon source to which the boron source is attached, the shape of the solid carbon source can be maintained to some extent even after firing, so a solid carbon source having a predetermined shape is used. Thus, boron carbide having a desired shape can be obtained.

Although details of the reason why boron carbide having a desired shape can be obtained by such a simple manufacturing method are unknown, it is considered as follows. In general, it is known that the reaction between boron oxide and a carbon source follows the following reaction formula (1). Further, it is known that boric acid is dehydrated by heating to produce boron oxide.
2B ₂ O ₃ + 7C → B ₄ C + 6CO ↑ (1)

  Assuming the reaction according to the above reaction formula (1), the molar ratio of the boron atom in the boron source to the carbon atom in the solid carbon source is 4: 7. In the present invention, since the boron source is precipitated from the solution state (see FIG. 1 (a)) on the surface of the solid carbon source, this molar ratio is substantially maintained even in the solid carbon source (see FIG. 1 (b)) that has undergone the adhesion process. Has been. Then, the boron source existing in the vicinity of the surface of the solid carbon source reacts with the carbon source stoichiometrically without remaining unreacted, so that the carbonization of a desired shape is achieved while having a predetermined composition. It is considered that boron (see FIG. 1 (c)) is obtained.

  According to the present invention, boron carbide having a desired shape can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS In the manufacturing method of the boron carbide of this invention, it is drawing which shows typically the state of each raw material and a sintered body, (a) shows the state of the raw material in a solution, (b) is the solid which adhered the boron source The state of a carbon source is shown, (c) has shown the state of the boron carbide obtained. It is a scanning electron microscope (SEM) photograph of the fibrous boron carbide (boron carbide fiber) obtained in Example 1, (a) shows the assembly state of a fiber, (b) shows the shape which expanded the fiber. (C) shows the shape of the surface of the fiber. It is a chart which shows the result of the X-ray diffraction (XRD) measurement of the sintered body obtained in Example 1, and is composed of a single phase of boron carbide (B ₄ C). It is a chart which shows the result of the X-ray diffraction (XRD) measurement of the sintered body obtained in Comparative Example 5, and is composed of boron carbide (B ₄ C), carbon (C), and an unknown crystal phase.

[Boron carbide]
Boron carbide obtained from the production method of the present invention has a crystal phase of boron carbide as a main phase, and all or almost all consists of a crystal phase of boron carbide.

  “The main phase” is a crystal phase of boron carbide means that all or almost all of the identified peaks are derived from boron carbide in the X-ray diffraction (XRD) measurement. Here, “almost all” means that a minute diffraction peak not identified as boron carbide is observed, or it cannot be denied that a small amount of an amorphous phase is mixed from a small halo background.

Boron carbide is ideally represented by the chemical formula: B ₄ C, but “boron carbide” obtained from the production method of the present invention is represented by the chemical formula: B _3.5 C to B ₁₀ C. It is a concept that indicates a solid solution over a wide composition range in which the amount of carbon atoms contained is from 10% by mass to 24% by mass. The chemical formula of each boron carbide contained in this solid solution is difficult to identify by X-ray diffraction (XRD) measurement, and can be identified from the quantitative result of the carbon content using a carbon analyzer or the like.

As described above, the boron carbide obtained from the production method of the present invention has a carbon atom content of 10% by mass (B ₁₀ C) to 24% by mass (B _3.5 C), and preferably contains carbon atoms. The amount is 18.2% by mass (B ₅ C) to 21.7% by mass (B ₄ C).

  Moreover, the boron carbide obtained from the production method of the present invention is a polycrystalline body in which primary particles are bonded, and the average particle size of the primary particles is preferably 50 nm to 1000 nm. The average particle size means an average value of measured values of the particle size (length of major axis) of several tens of particles in observation using a scanning electron microscope (SEM).

  Further, the boron carbide obtained from the production method of the present invention can have a shape derived from the shape of the solid carbon source used as a raw material, and depending on the shape of the solid carbon source, granular, spherical, fibrous, needle-like, Any shape such as a rod shape, a plate shape, and a thin plate shape can be taken.

[Production method]
The method for producing boron carbide of the present invention includes an adhesion step and a firing step, which will be described in detail below. 1A to 1C are the states of the raw materials and the fired body in the adhesion step (FIGS. 1A to 1B) and the firing step (FIG. 1C) of the production method of the present invention. Is schematically shown, but the ratio of each dimension is not accurate.

(Adhesion process)
The attachment step in the present invention is a step of depositing a boron source composed of boric acid and / or boron oxide from a solution state and attaching it to a solid carbon source. That is, as shown in FIG. 1A, the boron source is dissolved in the solution and the solid carbon source is not dissolved in the solution. As shown in FIG. The boron source is deposited and uniformly attached to the entire surface of the solid carbon source.

  In the present invention, when the solid carbon source has a hydroxyl group or the like, the hydroxyl group and boric acid can react, but “attachment” in the present invention includes a state in which both chemically react.

  As the boron source, one or more selected from the group consisting of orthoboric acid, metaboric acid, and boron oxide can be used. These can be dissolved in a solvent such as water while heating as necessary. At that time, in order to control the raw material ratio with high accuracy, it is preferable to dissolve completely. Moreover, in order to accelerate | stimulate melt | dissolution, it is preferable to heat a solution.

  The solid carbon source is preferably insoluble in the solvent used. The solid carbon source may be a carbon material or a material that is carbonized by heating in an inert gas atmosphere or a reducing gas atmosphere. The solid carbon source is a solid material composed of carbon, a solid material composed of hydrocarbon, or a solid composed of carbohydrate. Materials and the like. Specifically, celluloses such as cellulose, alkylcellulose, and acetylcellulose; carbons such as carbon fiber, carbon black, and graphite; phenolic resins such as novolac and resole; or pitch-based materials such as petroleum pitch and coal pitch Is preferred.

  Among these, from the viewpoint of performing a stoichiometric reaction at a low calcination temperature, a solid material composed of carbohydrates such as celluloses is preferable.

  The shape of the solid carbon source can be any shape such as granular, spherical, fibrous, needle-like, rod-like, plate-like, and thin-plate-like.

  The molar ratio (B: C) of boron atoms in the boron source and carbon atoms in the solid carbon source used in the attachment step is preferably 14:25 to 10:14, and 4: 7 to 10 : 17 is more preferable. Further, the molar ratio (B: C) of the boron atom in the boron source after deposition and the carbon atom in the solid carbon source is preferably in the same range.

  If the molar ratio of boron atom to carbon atom (B: C) is given, all of the boron atoms become boron carbide, and the carbon that has not become boron carbide is a boron source as in reaction formula (1) above. Therefore, only boron carbide can be obtained in the next firing step.

  As a method of attaching a boron source to a solid carbon source, a method of evaporating and drying the solvent by heating, a method of evaporating and drying the solvent by heating and decompression in a container, and after applying or impregnating a solution to the solid carbon source, The method of evaporating a solvent, the method of repeating these, etc. are mentioned.

  The concentration in the solution at the time of dissolution of the boron source (not including the solid carbon source) is preferably 3% by mass to 25% by mass from the viewpoint of the efficiency of evaporation to dryness in the adhesion step.

(Baking process)
The firing step in the present invention is a step of firing the solid carbon source that has undergone the adhesion step in an inert gas atmosphere or a reducing gas atmosphere. Through this firing step, the boron source and the solid carbon source cause a chemical reaction represented by the above reaction formula (1), and boron carbide can be obtained.

  In that case, it is preferable that the shape of the solid carbon source is held as the shape of the obtained fired body. As a result, the shape of the obtained boron carbide can be controlled to some extent by the shape of the solid carbon source.

  The firing temperature is preferably 1300 ° C to 1900 ° C, more preferably 1300 ° C to 1800 ° C, and still more preferably 1300 ° C to 1500 ° C from the viewpoint of firing at a low temperature while sufficiently increasing the reaction rate. The firing time is preferably 30 minutes to 20 hours, more preferably 2 hours to 10 hours, although it depends on the firing temperature.

  Firing is performed in an inert gas atmosphere or a reducing gas atmosphere, and argon gas, neon gas, nitrogen gas, carbon monoxide, or a mixed gas thereof can be used. In addition, from the viewpoint of accelerating the progress of the reaction by substituting the gas, it is preferable to perform the firing while flowing or substituting the inert gas or the reducing gas.

  In order to maintain the shape of the solid carbon source before firing as boron carbide, a solid carbon source having a boron source deposited on the surface obtained in the attaching step may be placed in a mold and fired while being pressed. The firing temperature at this time is the same 1300 ° C. to 1900 ° C. as described above, and the pressure is preferably 1 MPa to 30 MPa, more preferably 5 MPa to 20 MPa. The mold used in the pressure firing is preferably made of carbon, silicon nitride, silicon carbide, or boron carbide from the viewpoint of mixing of impurities and thermal expansion.

  In the present invention, it is possible to perform a calcination step after the adhesion step and before the firing step. In that case, for example, heating may be performed at 300 ° C. to 800 ° C. for 10 minutes to 3 hours. The calcination step is also preferably performed in an inert gas atmosphere or a reducing gas atmosphere.

  The boron carbide obtained in the firing step may be washed with water. Even if the boron source remains in the boron carbide, the water cleaning can remove it. The amount of water used for the water washing is 100 parts by mass to 1000 parts by mass when the temperature of the washing water is 25 degrees and the temperature of the washing water is 80 ° C. with respect to 100 parts by mass of the fired product obtained in the firing step. Is 50 to 300 parts by mass.

  The washed boron carbide is dried if necessary. Examples of the drying means include constant temperature drying, hot air drying, vacuum drying, freeze drying and the like.

[Example]
Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. However, the present invention is not limited to the contents of the examples and comparative examples. In addition, measurement of various physical properties and the like in each example and comparative example was performed as follows.

<SEM (scanning electron microscope) photography>
SEM observation was performed on the fired bodies obtained in the examples and comparative examples, and photographs were taken of typical images at each magnification (scale is shown in the photograph).

  The average primary particle size was determined by measuring the length of the major axis of 20 primary particles randomly selected in the SEM photograph and calculating the average value.

<Identification by X-ray diffraction (XRD)>
For the fired bodies obtained in Examples and Comparative Examples, a powder X-ray diffraction measurement sample pressed with a pressure of 70 kgw was prepared with a powder sample molding machine (TK-750, manufactured by Tokyo Kagaku). Next, each of the obtained samples for measurement was measured with an X-ray diffractometer (D8-Advanced, manufactured by BRUKER). The measurement conditions at that time were a target CuKα, a tube voltage of 35 kV, a tube current of 350 mA, a scanning range of 10 to 80 ° (2θ), a step width of 0.02 °, and a scanning speed of 0.13 seconds / step. From the obtained chart, the crystal phase was identified by associating the crystal phase having the face spacing corresponding to each peak (2θ).

<Quantitative analysis of carbon atoms>
For the fired bodies obtained in the Examples and Comparative Examples, the mass of carbon atoms was quantified by a carbon / sulfur analyzer (EMIA-220V2, manufactured by Horiba, Ltd.), and the mass of carbon atoms based on the total mass measured in advance. % Was calculated.

[Example 1] (boron carbide fiber, target B: C molar ratio 4: 1, firing temperature 1400 ° C.)
2.472 g of orthoboric acid (manufactured by Kanto Kagaku, 99.5%) and 1.89 g of cellulose fiber (fiber length 100 μm to 1000 μm, fiber width 1 μm to 15 μm) (raw material B: C molar ratio is 4: 7), container It was put in 30 g of water, and this was warmed to 50 ° C. or higher to dissolve all orthoboric acid. After the orthoboric acid was dissolved, the water was further evaporated by heating to evaporate to dryness, and a mixture of boric acid and cellulose in which boric acid was precipitated and coated on the surface of the cellulose fiber was obtained. At this time, almost all boric acid was attached to the cellulose fibers, so the B: C molar ratio in the mixture was almost the same as the molar ratio of the raw materials.

  Thereafter, using a tubular electric furnace, the dried mixture of boric acid and cellulose is held for 6 hours while being heated to 1400 ° C. while flowing an inert gas (argon), thereby obtaining a fiber-like fired body. It was. An SEM photograph of this fired body is shown in FIG. Further, FIG. 3 shows a chart obtained by X-ray diffraction (XRD) measurement.

  From these results, it was found that a boron carbide fiber having a fiber length of 100 μm to 1000 μm and a fiber width of 1 μm to 15 μm in which primary particles of about 100 nm exist was obtained.

  Furthermore, in Table 1, the identification result by X-ray diffraction (XRD) measurement and the measurement result of carbon content were shown with the manufacturing conditions of the fiber-like sintered body. Table 1 shows the theoretical carbon content (mass%) corresponding to the target B: C molar ratio (for example, when the B: C molar ratio is 4: 1, the carbon content is 21.7 mass). %).

[Example 2] (Example of target B: C molar ratio 4.5: 1)
In Example 1, all the same conditions as in Example 1 except that the target B: C molar ratio 4: 1 is changed to the target B: C molar ratio 4.5: 1 by changing the feed ratio of the raw materials. A fiber-like fired body was prepared, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Example 3] (Example of target B: C molar ratio 5: 1)
In Example 1, all the same conditions as in Example 1 except that the target B: C molar ratio 4: 1 is changed to the target B: C molar ratio 5: 1 by changing the feed ratio of raw materials. A fired body was prepared, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

Example 4 (Example of target B: C molar ratio 3.5: 1)
In Example 1, all the same conditions as in Example 1 except that the target B: C molar ratio 4: 1 is changed to the target B: C molar ratio 3.5: 1 by changing the feed ratio of raw materials. A fiber-like fired body was prepared, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Example 5] (Example of firing temperature of 1800 ° C)
In Example 1, except that the firing temperature was changed from 1400 ° C. to 1800 ° C., a fiber-like fired body was produced under the same conditions as in Example 1, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Comparative Example 1] (Example with a firing temperature of 1000 ° C.)
In Example 1, except that the firing temperature was changed from 1400 ° C. to 1000 ° C., fiber-like fired bodies were produced under the same conditions as in Example 1, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Comparative Example 2] (Example of firing temperature of 1200 ° C)
In Example 1, except that the firing temperature 1400 ° C. was changed to 1200 ° C., a fiber-like fired body was produced under the same conditions as in Example 1, and the physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Comparative Example 3] (Example of target B: C molar ratio 2: 1)
In Example 1, all the same conditions as in Example 1 except that the target B: C molar ratio 4: 1 is changed to the target B: C molar ratio 2: 1 by changing the feed ratio of the raw materials. A fired body was prepared, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Comparative Example 4] (Example of target B: C molar ratio 6: 1)
In Example 1, all the same conditions as in Example 1 except that the target B: C molar ratio 4: 1 is changed to the target B: C molar ratio 6: 1 by changing the feed ratio of the raw materials. A fired body was prepared, and physical properties and the like were measured under the same conditions. The results are shown in Table 1.

[Comparative Example 5] (Baking temperature 2200 ° C., example of rayon fiber)
As described in Patent Document 1 (Japanese Patent No. 5319767), rayon fibers with boron oxide attached were fired at a firing temperature of 2200 ° C. to obtain a fiber-like fired body. At that time, 50 g of boron oxide was added to 50 g of water to obtain a boron oxide slurry. Rayon fibers (Daiwabow rayon nonwoven fabric was defibrated) were immersed in the slurry for 10 minutes, and the rayon adhered with boron oxide. Fiber was obtained.

  That is, in Example 1, a fiber-like fired body was produced under the same conditions as in Example 1 except that the rayon fiber to which boron oxide prepared as described above was attached was fired at a firing temperature of 2200 ° C. The physical properties were measured under the same conditions. The results are shown in Table 1. Further, FIG. 4 shows a chart obtained by X-ray diffraction (XRD) measurement.

As the results of Table 1 show, Examples 1 to 5 are the measurement results of carbon content (mass%) that is almost the same as the theoretical carbon content (mass%), and the target B: C molar ratio is It turned out that it was almost achieved. And from the identification result by XRD, only the crystal phase of boron carbide (B ₄ C) was obtained.

  On the other hand, in Comparative Example 1 where the firing temperature was 1000 ° C. and Comparative Example 2 where the firing temperature was 1200 ° C., a crystalline phase of boron oxide and a crystalline phase of carbon were generated. Further, in Comparative Example 3 having a target B: C molar ratio of 2: 1, a carbon crystal phase and an unknown crystal phase were generated. In Comparative Example 4 where the target B: C molar ratio was 6: 1, an unknown crystal phase was generated. Further, in Comparative Example 5 where the firing temperature was 2200 ° C., a deviation from the target B: C molar ratio occurred, and a carbon crystal phase and an unknown crystal phase occurred.

Thus, as described in Japanese Patent No. 5319767, in the method of baking at a high temperature, a different phase other than B ₄ C was observed. In the present invention, a single phase of B ₄ C can be obtained at a low firing temperature by dissolving boron oxide at the time of charging the raw material and uniformly mixing with the fiber.

1: Solid carbon source (cellulose fiber)
2: Carbon source (boric acid)
3: Boron carbide (boron carbide fiber)

Claims (5)

A deposition step of depositing a boron source composed of boric acid and / or boron oxide from a solution state and adhering to a solid carbon source;
A firing step of firing the solid carbon source that has undergone the attachment step in an inert gas atmosphere or a reducing gas atmosphere;
A method for producing boron carbide, comprising:   The said baking process is performed at 1300 degreeC-1900 degreeC, The manufacturing method of the boron carbide of Claim 1 characterized by the above-mentioned.   The method for producing boron carbide according to claim 1, wherein the shape of the solid carbon source is maintained as the shape of the obtained fired body.   The molar ratio (B: C) of boron atoms in the boron source and carbon atoms in the solid carbon source used in the attachment step is 14:25 to 10:14. The manufacturing method of the boron carbide of any one of -3.   The said solid carbon source is cellulose, carbon, phenol resins, or a pitch-type material, The manufacturing method of the boron carbide of any one of Claims 1-4 characterized by the above-mentioned.

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