Method for producing hexagonal boron nitride

Abstract

To provide a method for producing hexagonal boron nitride capable of easily and efficiently obtaining hexagonal boron nitride excellent in lubricity.SOLUTION: A method for producing hexagonal boron nitride comprises a mixing step of mixing a crude hexagonal boron nitride having an oxygen content of 0.5 mass% or more and 10.0 mass% or less with silicon dioxide and a firing step of firing the obtained mixture at 1800 to 2300°C under a non-oxidizing gas atmosphere.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing hexagonal boron nitride, and more particularly to a method for producing highly crystalline hexagonal boron nitride using silicon dioxide as a flux. Background Art

[0002] Hexagonal boron nitride (hereinafter also referred to as h-BN) has a layered structure similar to graphite, and is excellent in thermal conductivity, heat resistance, chemical stability, and solid lubricity. Therefore, it is used in high-temperature ceramics, heat-dissipating fillers filled in heat-dissipating sheet resins for electronic materials, and the like. Furthermore, due to its excellent solid lubricity, it has recently been used as an extender pigment for cosmetics for the purpose of improving the dispersibility of color pigments.

[0003] Similar to the graphite structure, h-BN grows crystals in the ab-axis direction to obtain a scaly particle shape. Therefore, it is considered that increasing the particle diameter in the ab-axis direction reduces the friction coefficient and improves lubricity.

[0004] Conventionally, high-purity h-BN powder is produced by mixing a boron compound such as boric acid or boron oxide with a nitrogen-containing substance such as melamine or urea, and performing heat treatment at a relatively low temperature in an ammonia or non-oxidizing atmosphere to synthesize crude h-BN powder with low crystallinity. The obtained crude h-BN powder is then heat-treated at a high temperature to volatilize and remove unreacted boron compounds. However, h-BN that has been highly purified by heat treatment at high temperature is stable, so crystallization is suppressed, and h-BN with sufficient crystal growth in the ab-axis direction could not be obtained. Accordingly, in Patent Documents 1 to 3, as a method for obtaining h-BN having a large particle diameter in the ab-axis direction, a method has been proposed in which additives such as alkali metal salts and alkaline earth metal salts are added to a mixed powder of a boron compound and a nitrogen-containing substance or crude h-BN powder, followed by heat treatment.

[0005] However, these methods leave residual additives or borates formed from the additives and boron compounds in the resulting h-BN. This can lead to reduced lubricity, and if incorporated into cosmetics, the residual substances may react with moisture such as sweat, potentially causing skin irritation. Therefore, these residual substances need to be removed by acid washing or other methods, which complicates the manufacturing process and makes complete removal difficult. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-263402 [Patent Document 2] Japanese Patent Application Publication No. 9-295801 [Patent Document 3] Patent No. 2922096 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] This invention has been made in view of the above circumstances, and aims to provide a method for producing hexagonal boron nitride that has excellent lubricity in a simple and efficient manner. [Means for solving the problem]

[0008] The inventors of the present invention conducted extensive research to solve the above problems and discovered that by mixing silicon dioxide, which is used in food additives and cosmetics, with crude h-BN powder having a predetermined oxygen content, and then heat-treating it, the particle size in the ab axis direction grows significantly, and at the same time, the added silicon component is volatilized and removed, resulting in an h-BN powder with excellent lubricity. This led to the present invention.

[0009] In other words, the present invention is 1. A mixing step of mixing crude hexagonal boron nitride with an oxygen content of 0.5% by mass or more and 10.0% by mass or less with silicon dioxide, The resulting mixture is then fired in a non-oxidizing gas atmosphere at 1800-2300°C. A method for producing hexagonal boron nitride having the following characteristics 2. A method for producing hexagonal boron nitride according to claim 1, using 0.1 moles to 2 moles of silicon dioxide per mole of crude hexagonal boron nitride. 3. The obtained hexagonal boron nitride is a method for producing hexagonal boron nitride according to 1 or 2, containing silicon. 4. A method for producing hexagonal boron nitride according to 3, wherein the silicon content is less than 100% by mass of the entire hexagonal boron nitride. To provide. [Effects of the Invention]

[0010] According to the present invention, by adding silicon dioxide as a flux to crude h-BN and heat-treating it, crystal growth occurs in the ab-axis direction while coarse particles derived from the flux components are volatilized and removed, thereby obtaining h-BN powder with excellent lubricity. Furthermore, if the flux component is retained, it is expected that the affinity of the h-BN powder to the organic binder will improve when it is incorporated into the organic binder. The h-BN powder of the present invention is useful as a heat dissipation sheet for electronic materials where thermal conductivity and heat resistance are required, as well as as a mold release agent or extender pigment for cosmetics where lubricity is required. [Brief explanation of the drawing]

[0011] [Figure 1] This chart shows the results of powder X-ray diffraction measurements of h-BN obtained in Examples 1-3 and Comparative Example 1. [Figure 2] This is a photograph showing a 4000x electron microscope image of the h-BN powder obtained in Example 1. [Modes for carrying out the invention]

[0012] [Method for producing hexagonal boron nitride] The present invention's method for producing h-BN is characterized by comprising the following steps. (1) A mixing step of mixing crude h-BN having an oxygen content of 0.5% by mass or more and 10.0% by mass or less with silicon dioxide (2) A firing step of firing the obtained mixture at 1800 to 2300°C under a non-oxidizing gas atmosphere

[0013] In the method for producing h-BN of the present invention, silicon dioxide mixed with crude h-BN forms a liquid phase with the crude h-BN in the heat treatment step, and crystal growth of h-BN in the ab-axis direction is promoted in this liquid phase.

[0014] (1) Mixing step The mixing step is a step of mixing crude h-BN having an oxygen content of 0.5% by mass or more and 10.0% by mass or less with silicon dioxide.

[0015] [Crude h-BN] The crude h-BN used as a raw material has low crystallinity and usually contains impurities other than boron nitride such as BNO and B₂O₃. As the crude h-BN used in the present invention, any product produced by a known method or a commercially available product can be used without particular limitation as long as the oxygen content thereof is 0.5% by mass or more and 10.0% by mass or less.

[0016] As a method for producing crude h-BN, for example, a method of synthesizing from a boron compound and a nitrogen-containing compound according to a conventionally known method can be mentioned. Specific examples of the boron compound include boric acid, boron oxide, borax, boron halide, borazine, borosiloxane, and the like, and one of these may be used alone, or two or more thereof may be used in combination. On the other hand, specific examples of the nitrogen-containing compound include organic nitrogen compounds such as melamine, urea, organic ammonium salts, and amide compounds; inorganic nitrogen compounds such as ammonia gas, ammonium salts of alkali metals or alkaline earth metals; elemental nitrogen such as nitrogen gas and liquid nitrogen; and mixtures thereof. These may be used alone, or two or more thereof may be used in combination. For example, a reaction method may involve mixing a predetermined amount of a boron compound with a predetermined amount of a nitrogen-containing compound, and then heating the resulting mixture at 1000°C or higher under an inert gas atmosphere such as nitrogen.

[0017] Alternatively, commercially available crude h-BN may be used, such as ABN (manufactured by Nisshin Rifratec Co., Ltd.).

[0018] The oxygen content in the raw material crude h-BN is preferably 0.5% by mass or more and 10.0% by mass or less, from the viewpoint of h-BN crystal growth, but preferably 1.0% by mass or more and 10.0% by mass or less, more preferably 1.5% by mass or more and 9.0% by mass or less, and even more preferably 1.5% by mass or more and 5% by mass or less. Crude h-BN with an oxygen content within the above range has low purity. When mixed with silicon dioxide used in this invention and heat-treated, the crude h-BN and silicon dioxide form a liquid phase, and h-BN crystals easily grow in the ab-axis direction within the liquid phase. When the oxygen content of the crude h-BN is below the above lower limit, the purity of the crude h-BN is high, and it does not form a liquid phase with silicon dioxide, making it difficult for h-BN crystals to grow in the liquid phase. Conversely, if it exceeds the above upper limit, the oxygen content of the h-BN after heat treatment increases, and the purity of the h-BN decreases. The above oxygen content values ​​were measured using an oxygen, nitrogen, and hydrogen analyzer (EMGA-900, manufactured by Horiba, Ltd.).

[0019] The shape of the crude h-BN is not particularly limited, but it is preferably in powder form. There are no particular restrictions on its particle size, however, if it is excessively large, unreacted crude h-BN may remain, reducing lubricity and impairing the smoothness during application. Therefore, the average particle size of the crude h-BN is preferably 5 μm or less, more preferably 4.5 μm or less, even more preferably less than 4 μm, and even more preferably less than 3.5 μm. There are no particular restrictions on the lower limit of the average particle size of the crude h-BN, but it is usually 1 μm or more, preferably 2 μm or more. The above average particle size is the value obtained by measuring the volume distribution with a laser diffraction particle size distribution analyzer (MASTERSIZER3000, manufactured by Malvern Panalytical), and the particle size of 50% of the measured volume distribution is taken as the average particle size of h-BN.

[0020] [Silicon dioxide] As mentioned above, silicon dioxide (silica) forms a liquid phase with crude h-BN in the calcination process described later, and crystal growth of h-BN in the ab-axis direction is promoted in this liquid phase. The silicon dioxide used to generate this liquid phase is not particularly limited and may be amorphous silica, crystalline silica, synthetic silica, or natural silica. Examples of amorphous silica include fumed silica, precipitated silica, and other amorphous synthetic silica; diatomaceous earth, opal, etc. Examples of crystalline silica include quartz-type, tridymite-type, cristobalite-type crystalline synthetic silica; natural quartz, tridymite, cristobalite, silica, and silica sand, etc. These may be used individually or in combination of two or more. Among these, amorphous synthetic silica is preferred, and precipitated amorphous synthetic silica is more preferred. Furthermore, the shape of silicon dioxide is not particularly limited, but powder form is preferred. The particle shape is also not particularly limited and may be spherical, pulverized, or amorphous. The average particle size is also not particularly limited, but 0.1 to 50 μm is preferred, and 0.5 to 30 μm is more preferred. The above average particle size is the mass-average value D obtained by particle size distribution measurement by laser diffraction. 50 This is the value obtained as (or median diameter).

[0021] The amount of silicon dioxide used is preferably 0.1 moles to 2 moles per mole of crude h-BN, more preferably 0.3 moles to 1.7 moles, and even more preferably 0.5 moles to 1.5 moles. If the amount of silicon dioxide used is less than the lower limit, some of the crude h-BN may remain unreacted, and the flake-like particles may not be obtained uniformly. Also, if the amount of silicon dioxide used exceeds the upper limit, a large amount of silicon dioxide that did not form a liquid phase remains, which may increase friction and reduce lubricity.

[0022] [Other additives] In the method for producing h-BN of the present invention, in addition to the crude h-BN and silicon dioxide described above, other additives may be added as long as they do not impair the effects of the present invention. Examples of other additives include one or more inorganic substances such as carbon black, boron carbide, silicon carbide, and other carbides. By using these compounds in combination, the oxygen concentration of the resulting h-BN can be reduced without impairing its lubricity, thereby achieving high purity.

[0023] [Dispersion medium] In the mixing step, when mixing crude h-BN, silicon dioxide, and other additives as needed, a dispersion medium may be added as necessary, provided that it does not hinder the effects of the present invention. Specific examples of dispersion media include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, and cyclohexanone; ethers such as tetrahydrofuran, diethyl ether, diisopropyl ether, and dibutyl ether; aliphatic hydrocarbons such as n-heptane, n-hexane, and cyclohexane; and aromatic hydrocarbons such as benzene, toluene, and xylene. These may be used individually or in combination of two or more. Among these, ketones are preferred, and acetone is more preferred.

[0024] [Mixing method] The mixing method is not particularly limited, and examples include adding silicon dioxide, other additives used as needed, and a dispersion medium to the crude h-BN raw material and mixing them. The mixing means are also not particularly limited, and examples include ball mills, agitators, mortars, etc. As a mixing method, for example, one method involves adding a predetermined amount of silicon dioxide and other additives as needed to crude h-BN, and then uniformly mixing it in an agate mortar with a dispersion medium such as acetone in an amount 1 to 5 times the volume of the crude h-BN. The resulting mixture is preferably in powder or paste form and is preferably dried at 50-100°C.

[0025] (2) Firing process The calcination process involves heating a mixture of crude h-BN obtained in the mixing process, silicon dioxide, and other additives used as needed, at 1800-2300°C under a non-oxidizing gas atmosphere.

[0026] [Non-oxidizing gases] Here, non-oxidizing gases include nitrogen gas, argon gas, ammonia gas, hydrogen gas, carbon monoxide gas, etc., and one of these can be used alone or in combination of two or more. The nitrogen content of the resulting h-BN will differ depending on the type of atmospheric gas used. In order to synthesize high-purity h-BN, nitrogen gas or a mixed gas of nitrogen gas and other gases is particularly suitable, with nitrogen gas being more preferable.

[0027] [Heating conditions] The heat treatment temperature is 1800 to 2300°C, preferably 1800 to 2200°C, and more preferably 1900 to 2200°C. If the heat treatment temperature is below the lower limit, crude h-BN and silicon dioxide cannot form a liquid phase, resulting in insufficient crystal growth of h-BN in the liquid phase. If the heat treatment temperature exceeds the upper limit, volatilization of the liquid phase or decomposition of h-BN occurs before sufficient crystal growth of h-BN can occur. The heat treatment time is not particularly limited, but is preferably 1 hour to 15 hours, and more preferably 1 hour to 10 hours. If the heat treatment time is less than the lower limit, crystal growth may be insufficient, and if it exceeds the upper limit, there is a risk of h-BN decomposition.

[0028] [Heating furnace] The heating furnace used for heating is not particularly limited as long as it can heat to the above temperature, and examples include gas furnaces, electric furnaces, muffle furnaces, laser heating furnaces, etc. Since the heat treatment is carried out in a non-oxidizing gas atmosphere, it is generally preferable to first evacuate the furnace with a vacuum pump, then introduce a non-oxidizing gas to the desired pressure, and then heat the material to the desired temperature. Therefore, it is preferable that the heating furnace be capable of creating a vacuum inside the furnace and has a device for supplying a non-oxidizing gas into the furnace. As such a heating furnace, for example, a vacuum heating and firing furnace such as the VESTA manufactured by Shimadzu Corporation can be used.

[0029] [Heating method] More specifically, regarding the heating method, for example, the vacuum level inside the furnace is set to 10 -1 After evacuating to approximately Pa, a non-oxidizing gas is introduced to 0.9 MPa and continued until the end of the heat treatment. The flow rate of the non-oxidizing gas is not particularly limited and depends on the size of the furnace, but 0.5 L / min or more is preferable. Furthermore, the pressure of the non-oxidizing gas inside the furnace is preferably between atmospheric pressure and 10 MPa, as the evaporation rate of the liquid phase differs depending on the gas pressure inside the furnace. After introducing a non-oxidizing gas, the temperature is preferably raised to a predetermined heat treatment temperature at a rate of 1 to 50°C / min, more preferably 5 to 50°C / min, and even more preferably 5 to 20°C / min. After heating for the above heat treatment time, the temperature is preferably lowered to room temperature at a rate of 1 to 50°C / min, more preferably 5 to 50°C / min, and even more preferably 5 to 20°C / min.

[0030] [Hexagonal boron nitride] The hexagonal boron nitride obtained by the above manufacturing method may contain silicon. The silicon may be included as silicon dioxide or the like, and the silicon (silicon atom) content is preferably less than 100% by mass of the total hexagonal boron nitride obtained, more preferably 50% by mass or less, and even more preferably 10% by mass or less.

[0031] Furthermore, the shape of the hexagonal boron nitride obtained by the above manufacturing method is not particularly limited, but powder form is preferred. The shape of the particles is also not particularly limited and may be flaky, teardrop-shaped, plate-shaped, spherical, needle-shaped, amorphous, etc., but flaky form is preferred. While the particle size is not particularly limited, in the case of flake-shaped particles, the average major axis is preferably 1 μm or more, more preferably 3 μm or more, even more preferably 3.5 μm or more, even more preferably 4 μm or more, and also preferably 10 μm or less, more preferably 8 μm or less, and even more preferably 6 μm or less. The aspect ratio is also not particularly limited, but 100 to 2000 is preferred. The above major axis and aspect ratio of hexagonal boron nitride are values ​​measured by scanning electron microscope (SEM). [Examples]

[0032] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to the following examples. In the example below, the oxygen content of the crude h-BN powder was measured using an oxygen, nitrogen, and hydrogen analyzer (EMGA-900, manufactured by Horiba, Ltd.). Furthermore, the average particle size was measured using a laser diffraction particle size distribution analyzer (MASTERSIZER3000, manufactured by Malvern Panalytical).

[0033] [Example 1] 0.25 g (0.01 mol) of crude h-BN powder (ABN, manufactured by Nisshin Rifratec Co., Ltd.) with an oxygen content of 4% by mass and an average particle size of 3 μm, 0.6052 g (0.01 mol) of silicon dioxide powder (sedimentary amorphous, special grade, manufactured by Kanto Chemical Co., Ltd.), and acetone (3.0 g) were added and stirred in an agate mortar, and this mixed powder was dried in a drying oven at 80°C. The dried powder was placed in a BN crucible and heat-treated using a vacuum pressurized firing furnace (VESTA, manufactured by Shimadzu Corporation) to obtain flaky h-BN. The heat treatment conditions were as follows: the vacuum level inside the furnace was set to 10. -1 After evacuating to approximately Pa, nitrogen gas at 0.9 MPa was introduced into the furnace. The temperature was raised from room temperature to 2000°C at a rate of 5°C / min, held at 2000°C for 5 hours, and then cooled down at a rate of 5°C / min. When the composition of the obtained calcined material was examined by powder X-ray diffraction, no crystalline phases other than h-BN were detected. The obtained diffraction lines are shown in Figure 1. When the particle shape of this h-BN powder was observed using a scanning electron microscope (SEM), flaky particles with an average major diameter of 5 μm were identified. The SEM image is shown in Figure 2. To evaluate the lubricity of the obtained h-BN, the h-BN powder was placed on a glass plate, and its fluidity was observed by tilting the glass plate at a 45° angle. The h-BN powder slid off the glass plate. These results are shown in Table 1.

[0034] [Example 2] h-BN was obtained in the same manner as in Example 1, except that the amount of silicon dioxide powder added was 0.3026 g (0.5 moles per mole of crude h-BN). The composition of the obtained calcined product was examined by powder X-ray diffraction in the same manner as in Example 1, and no crystalline phases other than h-BN were detected. The obtained diffraction lines are shown in Figure 1. Furthermore, evaluations were performed in the same manner as in Example 1, and the results are shown in Table 1.

[0035] [Example 3] h-BN was obtained in the same manner as in Example 1, except that the amount of silicon dioxide powder added was 0.9079 g (1.5 moles per mole of crude h-BN). The composition of the obtained calcined product was examined by powder X-ray diffraction in the same manner as in Example 1, and no crystalline phases other than h-BN were detected. The obtained diffraction lines are shown in Figure 1. Furthermore, evaluations were performed in the same manner as in Example 1, and the results are shown in Table 1.

[0036] [Comparative Example 1] In Example 1, the procedure was carried out in the same manner as in Example 1, except that the heat treatment temperature was set to 1700°C, to obtain h-BN. When the composition of the obtained calcined material was examined by powder X-ray diffraction, an amorphous peak was observed around 2θ = 15~30° in addition to the h-BN diffraction line. The obtained diffraction lines are shown in Figure 1. SEM observation of the h-BN particle shape revealed an average major axis of 0.4 μm, and no crystal growth in the ab-axis direction was observed. Furthermore, when the lubricity was evaluated in the same manner as in Example 1, the h-BN powder remained on the glass plate without sliding. The evaluation results are shown in Table 1.

[0037] [Comparative Example 2] In Example 1, the procedure was carried out in the same manner as in Example 1, except that silicon dioxide was not used, to obtain h-BN. When the obtained h-BN particle shape was observed using SEM, the average major axis was 0.6 μm, and no crystal growth in the ab-axis direction was confirmed. Furthermore, when the lubricity was evaluated in the same manner as in Example 1, the h-BN powder remained on the glass plate without sliding. The evaluation results are shown in Table 1.

[0038] [Comparative Example 3] h-BN was obtained in the same manner as in Example 1, except that crude h-BN powder (product name: RBN, manufactured by Nisshin Rifratec Co., Ltd.) with an oxygen content of 0.4% by mass was used. When the obtained h-BN particle shape was observed using SEM, the average major axis was 1 μm, and no crystal growth in the ab axis direction was confirmed. Furthermore, when the lubricity was evaluated in the same manner as in Example 1, the h-BN powder remained on the glass plate without sliding. The evaluation results are shown in Table 1.

[0039] [Comparative Example 4] h-BN was obtained in the same manner as in Example 1, except that a crude h-BN powder with an oxygen content of 11.2% was used, which was obtained by mixing 100 parts by mass of boric acid, 150 parts by mass of melamine, and 10 parts by mass of calcium carbonate, then heat-treating it at 1200°C under a nitrogen atmosphere, washing it in 3 mol / L hydrochloric acid, and drying it. When the obtained h-BN particle shape was observed using SEM, the average major axis was 2 μm, and no crystal growth in the ab axis direction was confirmed. Furthermore, when the lubricity was evaluated in the same manner as in Example 1, the h-BN powder remained on the glass plate without sliding. The evaluation results are shown in Table 1.

[0040] [Table 1] *: Percentage of silicon dioxide powder relative to the total amount of crude h-BN powder and silicon dioxide powder. Lubricity: ○: Slid down the glass plate, ×: Stayed on the glass plate without sliding.

Claims

1. A mixing step of mixing crude hexagonal boron nitride with an oxygen content of 0.5% by mass or more and 10.0% by mass or less with silicon dioxide, The resulting mixture is then fired in a non-oxidizing gas atmosphere at 1800-2300°C. A method for producing flaky hexagonal boron nitride having the following characteristics.

2. A method for producing hexagonal boron nitride according to claim 1, wherein 0.1 moles to 2 moles of silicon dioxide are used per mole of crude hexagonal boron nitride.

3. A method for producing hexagonal boron nitride according to claim 1 or 2, wherein the obtained hexagonal boron nitride contains silicon.

4. A method for producing hexagonal boron nitride according to claim 3, wherein the silicon content is less than 100% by mass of the total hexagonal boron nitride obtained.

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