JP2022111748A - Method for producing boron nitride - Google Patents

Abstract

To provide a method for producing boron nitride capable of controlling the shape and size of boron nitride particles.SOLUTION: A method for producing boron nitride comprises a step of obtaining melamine borate by slowly cooling or ultrasonic treatment of a saturated aqueous solution of boric acid and melamine, and a firing step of firing the melamine borate in the presence of a lithium compound as a flux at a temperature of 1000 to 1700°C to obtain boron nitride. The method can give large-diameter columnar particles of boron nitride, which therefore has a filling property to a resin that is better than that of conventional scaly particles and has excellent thermal conductivity.SELECTED DRAWING: None

Description

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

Boron nitride has high thermal conductivity and is chemically or thermally stable. For this reason, it is being used as a heat-conducting filler.
Boron nitride can be produced by reacting a compound containing boron with a compound containing nitrogen. For example, Patent Literature 1 describes a method of producing hexagonal boron nitride primary particle aggregates by two-stage firing of melamine borate under specific conditions.

JP 2018-104253 A

Boron nitride is known to have a large anisotropy of thermal conductivity due to its crystal structure and scale shape. Therefore, when filled into a resin as a thermally conductive filler, there is a problem that the thermal conductivity tends to decrease due to the orientation of the boron nitride in the resin.
In addition, when filling a resin with boron nitride as a filler, it is required to control the particle size of the melamine borate in order to further improve the filling property.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing boron nitride that can control the shape and size of boron nitride particles.

The present invention includes the following [1] to [4].
[1] A method for producing boron nitride, comprising a step of obtaining melamine borate, and a baking step of obtaining boron nitride by baking the melamine borate at a temperature of 1000° C. or more and 1700° C. or less in the presence of a flux. .
[2] The method for producing boron nitride according to [1], wherein the step of obtaining the melamine borate includes a step of ultrasonicating a saturated aqueous solution of boric acid and melamine.
[3] In the firing step, the flux is mixed with the melamine borate at a molar ratio (melamine borate/flux) of 1 or more and 4 or less, and then fired. [1] or [2] ] The manufacturing method of the boron nitride as described in ].
[4] The method for producing boron nitride according to any one of [1] to [3], wherein the flux is lithium carbonate or lithium hydroxide.

ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of boron nitride which can control the shape and size of the particle|grains of boron nitride can be provided.

FIG. 2 shows an X-ray diffraction pattern of melamine borate; 1 is a scanning electron micrograph of melamine borate. 1 is a diagram showing an X-ray diffraction pattern of boron nitride produced in Example 1. FIG. 1 is a scanning electron micrograph of boron nitride produced in Example 1. FIG. 2 is a scanning electron micrograph of melamine borate produced in Example 2. FIG. 4 is a scanning electron micrograph of boron nitride produced in Example 2. FIG.

<Method for producing boron nitride>
The method for producing boron nitride according to the present embodiment includes a step of obtaining melamine borate and a firing step of firing the melamine borate in the presence of a flux.
The present inventors have found that sintering melamine borate in the presence of a flux can produce boron nitride in which the particle size and shape of melamine borate are maintained. The present invention is based on the technical idea of controlling the shape and size of the produced boron nitride particles within a desired range by controlling the shape and size of the melamine borate particles in the process of obtaining melamine borate. It depends.

[Step of obtaining melamine borate]
Melamine borate can be obtained, for example, by recrystallizing melamine borate from a saturated aqueous solution containing boric acid and melamine. Hereinafter, the term "saturated aqueous solution" means a saturated aqueous solution containing boric acid and melamine.

When the melamine borate is fired in the presence of a flux in the subsequent firing step, the particle size and shape of the melamine borate are maintained. Therefore, the size and shape of melamine borate are controlled in order to obtain boron nitride of desired size and shape.

For example, when producing boron nitride with a particle size of about 5 μm or more and 10 μm or less, it is preferable to produce melamine borate with a particle size of about 5 μm or more and 10 μm or less.
The above particle size range can be changed as appropriate depending on the desired size of the boron nitride.

Secondary particles having a columnar elongated shape and an aspect ratio, which is the ratio of the minor axis diameter to the major axis diameter, of about 1 to 1.5. It is preferable to produce melamine borate having an aspect ratio, which is the ratio to the shaft diameter, of about 1 to 1.5.
The range of the aspect ratio can be appropriately changed depending on the desired size of the boron nitride.

The size and shape of the melamine borate particles can be controlled by controlling recrystallization conditions and post-treatment. Conditions for recrystallization include heating and cooling conditions for a saturated aqueous solution or ultrasonic treatment conditions for a saturated aqueous solution. As a post-treatment, a dry pulverization method or mechanical pulverization such as a ball mill can be used.

In one aspect of the present invention, the step of obtaining melamine borate is a step of heating and cooling a saturated aqueous solution.
In one aspect of the present invention, the step of obtaining melamine borate is a step of subjecting a saturated aqueous solution to ultrasonication followed by suction filtration.

(Process of heating and cooling)
First, a predetermined amount of boric acid is completely dissolved in pure water heated to about 95°C. Next, add a predetermined amount of melamine and stir to completely dissolve the melamine. This operation yields a saturated aqueous solution.

The amount of boric acid is, for example, 1 g or more and 5 g or less, preferably 2 g or more and 4 g or less, per 100 mL of distilled water.
The amount of melamine is, for example, 0.5 g or more and 4 g or less, preferably 1 g or more and 3 g or less, per 100 mL of distilled water.

Crystals of melamine borate are deposited by allowing the saturated aqueous solution to cool.
An example of the cooling conditions is that the beaker containing the resulting saturated aqueous solution is immersed in a 1 L beaker filled with hot water of 50° C. or more and 100° C. or less, left at room temperature for about 12 hours or more and 24 hours or less, and slowly saturated. It is preferred to cool the aqueous solution.

The resulting precipitate is dried to obtain melamine borate. As a method for drying the precipitate, natural drying may be performed, or drying may be performed using a constant temperature bath at room temperature or 100° C. or less.

By the heating and cooling step, particles of melamine borate having an elongated columnar shape are obtained. The melamine borate particles have a minor axis diameter of about 5 μm to 10 μm. Also, the major axis diameter is about 10 μm to 2 cm.

(Ultrasonic treatment and suction filtration step)
In the present embodiment, it is preferable to sonicate the saturated aqueous solution obtained by the same method as above. In this case, for example, an ultrasonic cleaning machine may be used for ultrasonic treatment. Crystals of melamine borate are precipitated by the ultrasonic treatment. An ultrasonic cleaner, for example, can be used for the ultrasonic treatment.

By changing the ultrasonic treatment conditions, the shape and size of the produced melamine borate particles can be controlled. The ultrasonic treatment crushes the columnar particles to obtain melamine borate particles each having a short axis diameter and a long axis diameter of several tens of μm.

Conditions for ultrasonic treatment include frequency and treatment time. The frequency is preferably adjusted in the range of 20 kHz or more and 50 kHz or less. The treatment time is preferably adjusted in the range of 1 minute or more and 10 minutes or less.

The combination of frequency and processing time is, for example, 20 kHz or more and 50 kHz or less and 1 minute or more and 10 minutes or less.

The longer the treatment time, the smaller the size of the resulting melamine borate and the higher the aspect ratio.
Also, the higher the frequency, the smaller the size of the obtained melamine borate.

For example, when 1 g of melamine borate is added to 100 mL of saturated solution, when ultrasonic treatment is performed for 5 minutes at power consumption of 430 W, high frequency output of 300 W, and oscillation frequency of 38 kHz, the particle size of boric acid is several tens of μm. Particles of melamine are obtained.

After ultrasonication, the crystals obtained are preferably suction filtered to obtain particles of melamine borate.

The shape and size of the produced melamine borate particles can be confirmed by observation with a scanning electron microscope.

The ultrasonic treatment makes it easier to obtain particles of melamine borate having an aspect ratio of about 1 to 1.5, which is the ratio of the minor axis diameter to the major axis diameter.

[Baking process]
The firing step is a step of firing melamine borate in the presence of a flux.
By firing melamine borate in the presence of a flux, boron nitride can be produced while maintaining the shape and size of the melamine borate particles.

The firing temperature is 1000° C. or higher and 1700° C. or lower, preferably 1100° C. or higher and 1600° C. or lower, and more preferably 1200° C. or higher and 1500° C. or lower.
When the firing temperature is within the above range, it is easy to obtain boron nitride that maintains the size and shape of melamine borate.
When the firing temperature is equal to or higher than the above lower limit, crystal growth of boron nitride tends to proceed.
If the firing temperature exceeds the above upper limit, the crystallization of boron nitride proceeds and it becomes easy to grow into scales. For this reason, in order to obtain particulate boron nitride, it is preferable to bake at a temperature of 1700° C. or lower.

In the present embodiment, the term "firing temperature" means the highest temperature of the atmosphere in the firing furnace during the firing process.

The time for holding at the above firing temperature is preferably 30 minutes or more and 2 hours or less, and preferably 50 minutes or more and 90 minutes or less.

In this embodiment, the firing process is preferably carried out under an inert gas atmosphere. Examples of inert gas include nitrogen gas and argon gas.

In the firing step, the rate of temperature increase until reaching the firing temperature is preferably 200° C./hour or more and 400° C./hour or less, preferably 250° C./hour or more and 350° C./hour or less.

In the present embodiment, the “heating rate” means the time from the time when the temperature rise in the firing device starts until the maximum holding temperature is reached, and the temperature at the time when the temperature rise in the firing furnace of the heating device starts. and the temperature difference up to the holding temperature.

After firing, the fired product obtained may be washed with water.

In the firing step, the melamine borate is fired in the presence of a flux.
The flux is not particularly limited as long as it is commonly used, but specific examples include lithium borate, sodium borate, potassium borate, boron oxide, sodium nitrate, sodium chloride, potassium chloride, lithium chloride, sodium carbonate, carbonate potassium, lithium carbonate, sodium sulfate, potassium sulfate, lithium sulfate, sodium acetate, potassium acetate, lithium acetate, sodium oxalate, potassium oxalate, lithium oxalate, sodium citrate, potassium citrate, lithium citrate, sodium oxide, Potassium oxide, lithium oxide, sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium fluoride, potassium fluoride, lithium fluoride, and mixtures thereof and their corresponding ammonium salts.

In this embodiment, it is preferable to use lithium carbonate or lithium hydroxide as the flux.

In the firing step, it is preferable to mix and fire the flux at a molar ratio (melamine borate/flux) of 1 to 3 with respect to melamine borate.

By firing in the presence of a flux, boron nitride crystals can be grown under low temperature conditions of 1700° C. or less. As a result, boron nitride is less likely to grow into scales, and boron nitride in which the shape and size of melamine borate particles are maintained can be produced.

<Boron nitride>
Boron nitride produced by the production method of the present embodiment maintains the shape and size of the particles of melamine borate as a raw material.
The boron nitride of the present embodiment is secondary particles that are aggregates of primary particles. The secondary particles are porous and have fine pores with a pore size of about 10 nm.

The boron nitride of the present embodiment is, for example, secondary particles having an elongated columnar shape and having a short axis diameter of about 5 μm or more and 10 μm or less and a long axis diameter of about 10 μm or more and 2 cm or less.
The boron nitride of the present embodiment is, for example, secondary particles each having a short axis diameter and a long axis diameter of several tens of μm, and having an aspect ratio of about 1 to 1.5.

Since the boron nitride of the present embodiment has less anisotropy in thermal conductivity than conventional scaly boron nitride, it can be inferred that the thermal conductivity of the entire thermal conductor is improved. In addition, since the particle size and shape of boron nitride can be controlled, it can be applied to a wide range of applications, such as thin insulating films with excellent thermal conductivity and interlayer insulating films for devices.

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

<Example 1>
[Step of obtaining melamine borate]
3 g of boric acid was added to 100 mL of distilled water heated to 80° C. and completely dissolved by stirring. 2 g of melamine was then added and completely dissolved by stirring. A stirrer was used for stirring. This gave a saturated aqueous solution of boric acid and melamine.

The resulting beaker containing the saturated aqueous solution was immersed in a 1 L beaker containing hot water at 80° C. and allowed to stand at room temperature for about 12 hours to slowly cool the saturated aqueous solution. Upon cooling, crystals of melamine borate precipitated.
The precipitated melamine borate crystals were taken out in a petri dish and allowed to stand at room temperature for 12 hours to air dry. Thus, a precipitate 1 was obtained.

An X-ray diffraction pattern of the obtained deposit 1 was obtained. The results are shown in FIG. 1(a).

As shown in FIG. 1(b), when referring to the powder X-ray diffraction database (ICDD), it was confirmed to be C ₄ H ₃ NO ₂ of ICDD No. 050-2384.

Furthermore, as shown in FIG. 1(c), it was confirmed to be melamine borate of ICDD No. 076-6949. From the results of FIG. 1(c), it was confirmed that the deposit 1 was melamine borate 1.

A SEM photograph (magnification: 3000) of the obtained melamine borate 1 is shown in FIG. As shown in FIG. 2, melamine borate 1 had an elongated columnar shape with a minor axis diameter of about 5 μm to 10 μm.

[Baking process]
Mixture 1 was obtained by mixing 2 mmol of melamine borate 1 and 1 mmol of lithium carbonate for 10 minutes. Mixture 1 was placed in a square carbon crucible and fired using a Keramax box electric furnace. The molar ratio of flux to melamine borate (melamine borate/flux) was two.

First, the temperature was raised to 1400° C. over 4 hours and 20 minutes at a temperature elevation rate of 300° C./hour, and held at 1400° C. for 1 hour. After that, the temperature was lowered to 0° C. over 4 hours and 20 minutes at a temperature lowering rate of 300° C./hour. Nitrogen gas was passed through at a flow rate of 0.2 L/min during all of the heating, holding at 1400° C., and cooling steps.
As a result, a baked product 1 was obtained. The baked product 1 was washed with water to obtain the baked product 1.

An X-ray diffraction pattern of the obtained baked product 1 was obtained. FIG. 3(a) shows the X-ray diffraction pattern of the baked product 1 before washing with water. FIG. 3(b) shows the X-ray diffraction pattern of the fired product 1 after washing with water. No peak attributed to lithium carbonate was observed in both FIGS. 3(a) and 3(b).

As shown in FIG. 3(c), when referring to ICDD, it was confirmed that the material was boron nitride, ICDD No. 034-0421. From the results of FIG. 3(c), it was confirmed that the fired product 1 was boron nitride 1.

FIG. 4(a) shows an SEM photograph (magnification: 2000 times) of boron nitride 1, and FIG. 4(b) shows an SEM photograph (magnification: 20000 times). As can be seen from FIG. 4(a), the boron nitride 1 had an elongated columnar shape with a minor axis diameter of about 5 μm to 10 μm. That is, it was confirmed that the shape and size of melamine borate 1 were maintained.

From FIG. 4(b), it was confirmed that the melamine borate 1 has a porous structure having a plurality of pores with a pore size of about 10 nm.

<Example 2>
[Step of obtaining melamine borate]
3 g of boric acid was added to 100 mL of distilled water heated to 80° C. and completely dissolved by stirring. 2 g of melamine was then added and completely dissolved by stirring. A stirrer was used for stirring. This gave a saturated aqueous solution of boric acid and melamine.

The resulting saturated aqueous solution was ultrasonically treated for 5 minutes to precipitate crystals of melamine borate. The obtained crystals were suction-filtered to obtain a deposit 2.

An X-ray diffraction pattern of the obtained deposit 2 was obtained. From the results, it was confirmed that deposit 2 was melamine borate.

A SEM photograph (500x) of the obtained melamine borate 2 is shown in FIG. 5(a). A SEM photograph (1000 times) of the obtained melamine borate 2 is shown in FIG. 5(b). As shown in FIG. 5(a), melamine borate 2 had a particle shape, and the short axis diameter and long axis diameter of the particles were each about 20 μm. The aspect ratio (minor axis diameter/major axis diameter) of melamine borate 2 was about 1.

[Baking process]
Mixture 2 was obtained by mixing 4 mmol of melamine borate 2 and 2 mmol of lithium carbonate for 10 minutes. Mixture 2 was placed in a square carbon crucible and fired using a Keramax box electric furnace. The molar ratio of flux to melamine borate (melamine borate/flux) was two.

First, the temperature was raised to 1400° C. over 4 hours and 40 minutes at a temperature elevation rate of 300° C./hour, and held at 1400° C. for 1 hour. After that, the temperature was lowered to 0° C. over 4 hours and 20 minutes at a temperature lowering rate of 300° C./hour. Nitrogen gas was passed through at a flow rate of 0.2 L/min during all of the heating, holding at 1400° C., and cooling steps.
As a result, a baked product 2 was obtained. The baked product 2 was washed with water to obtain the baked product 2.

An X-ray diffraction pattern of the obtained fired product 2 was obtained. As a result, it was confirmed that the fired product 2 was boron nitride 2 .

FIG. 6(a) shows an SEM photograph (magnification: 1000 times) of boron nitride 2, and FIG. 6(b) shows an SEM photograph (magnification: 2000 times).
From FIG. 6(a), the boron nitride 2 had a particle shape, and the minor axis diameter and the major axis diameter of the particles were each about 20 μm. Moreover, the aspect ratio (minor axis diameter/major axis diameter) of the boron nitride 2 was about 1. In other words, it was confirmed that the shape and size of melamine borate 2 were maintained.

FIG. 6(c) shows an SEM photograph of boron nitride 2 at a magnification of 40,000. From FIG. 6(c), it was confirmed that the melamine borate 2 has a porous structure having a plurality of pores with a pore size of about 10 nm.

Claims (4)

obtaining melamine borate;
a firing step of firing the melamine borate at a temperature of 1000° C. or higher and 1700° C. or lower in the presence of a flux to obtain boron nitride. 2. The method for producing boron nitride according to claim 1, wherein the step of obtaining the melamine borate comprises sonicating a saturated aqueous solution of boric acid and melamine. The nitriding according to claim 1 or 2, wherein in the firing step, the flux is mixed with the melamine borate at a molar ratio (melamine borate/flux) of 1 or more and 4 or less and fired. A method for producing boron. The method for producing boron nitride according to any one of claims 1 to 3, wherein the flux is lithium carbonate or lithium hydroxide.

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