JP7627484B2 - Method for producing boron nitride - Google Patents

Description

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

Boron nitride has high thermal conductivity and is chemically and thermally stable, which is why it is increasingly being used as a thermally conductive filler.
Boron nitride can be produced by reacting a compound containing boron with a compound containing nitrogen. For example, Patent Document 1 describes a method of producing hexagonal boron nitride primary particle aggregates by two-stage calcination of melamine borate under specific conditions.

JP 2018-104253 A

Boron nitride is known to have a large anisotropy in thermal conductivity due to its crystal structure and scale shape. Therefore, when it is used as a thermally conductive filler in resin, there is a problem that the thermal conductivity is easily reduced due to the orientation of boron nitride in the resin.
Furthermore, when filling a resin with boron nitride as a filler, it is necessary to control the particle size of the melamine borate in order to further improve the filling property.

The present invention was made in consideration of the above circumstances, and aims 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 step of calcining the melamine borate in the presence of a flux at a temperature of 1000°C or higher and 1700°C or lower to obtain boron nitride.
[2] The method for producing boron nitride according to [1], wherein the step of obtaining melamine borate comprises a step of ultrasonically treating a saturated aqueous solution of boric acid and melamine.
[3] The method for producing boron nitride according to [1] or [2], wherein in the firing step, the flux is mixed and fired in a molar ratio (melamine borate/flux) of 1 or more and 4 or less to the melamine borate.
[4] The method for producing boron nitride according to any one of [1] to [3], wherein the flux is lithium carbonate or lithium hydroxide.

The present invention provides a method for producing boron nitride that allows control of the shape and size of boron nitride particles.

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

<Method for producing boron nitride>
The method for producing boron nitride of 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 inventors have found through their research that calcining melamine borate in the presence of a flux makes it possible to produce boron nitride that maintains the particle size and shape of the melamine borate.The present invention relates to the technical idea of controlling the particle shape and size of the produced boron nitride within a desired range by controlling the particle shape and size of the melamine borate in the process of obtaining the melamine borate.

[Step for 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" refers to 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 size and shape of the melamine borate particles are maintained. Therefore, the size and shape of the melamine borate are controlled to obtain boron nitride of the desired size and shape.

For example, when it is desired to produce boron nitride having a particle size of about 5 μm to 10 μm, it is preferable to produce melamine borate having a particle size of about 5 μm to 10 μm.
The above particle size range can be appropriately changed depending on the desired size of boron nitride.

When producing boron nitride secondary particles having an elongated columnar 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 of the minor axis diameter to the major axis diameter, of about 1 to 1.5.
The above aspect ratio range can be appropriately changed depending on the desired size of the boron nitride.

The size and shape of melamine borate particles can be controlled by controlling the recrystallization conditions and post-treatment. Recrystallization conditions include heating and cooling conditions of a saturated aqueous solution, or ultrasonic treatment conditions of a saturated aqueous solution. Post-treatment can include dry grinding or mechanical grinding using a ball mill, etc.

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

(Heating and cooling process)
First, a predetermined amount of boric acid is completely dissolved in pure water heated to about 95° C. Next, a predetermined amount of melamine is added and stirred until the melamine is completely dissolved. This operation produces a saturated aqueous solution.

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

The saturated aqueous solution is allowed to cool, whereby crystals of melamine borate are precipitated.
As an example of the cooling conditions, it is preferable to immerse the beaker containing the obtained saturated aqueous solution in a 1 L beaker containing hot water at 50° C. or more and 100° C. or less, and leave it at room temperature for about 12 hours or more and 24 hours or less to slowly cool the saturated aqueous solution.

The precipitate obtained is dried to obtain melamine borate. The precipitate may be dried naturally or in a thermostatic chamber at room temperature or below 100°C.

By the heating and cooling process, melamine borate particles with a long and thin columnar shape are obtained. The minor axis diameter of these melamine borate particles is about 5 μm to 10 μm, and the major axis diameter is about 10 μm to 2 cm.

(Ultrasonic treatment and suction filtration process)
In this embodiment, it is preferable to ultrasonically treat the saturated aqueous solution obtained by the same method as above. In this case, for example, an ultrasonic cleaner may be used to perform the ultrasonic treatment. Crystals of melamine borate are precipitated by the ultrasonic treatment. For example, an ultrasonic cleaner can be used for the ultrasonic treatment.

By changing the conditions of ultrasonic treatment, the shape and size of the melamine borate particles produced can be controlled. By ultrasonic treatment, columnar particles are crushed, and melamine borate particles with minor and major axis diameters of several tens of micrometers are obtained.

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

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

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 resulting melamine borate.

For example, if 1 g of melamine borate is added to 100 mL of saturated solution, ultrasonic treatment for 5 minutes with a power consumption of 430 W, a high-frequency output of 300 W, and an oscillation frequency of 38 kHz will produce melamine borate particles with a particle size of several tens of μm.

After ultrasonic treatment, it is preferable to suction filter the resulting crystals to obtain melamine borate particles.

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

By using ultrasonic treatment, it becomes easier to obtain melamine borate particles with an aspect ratio, which is the ratio of the short axis diameter to the long axis diameter, of approximately 1 to 1.5.

[Firing process]
The calcination step is a step in which the melamine borate is calcined in the presence of a flux.
Calcination of melamine borate in the presence of a flux can produce boron nitride which retains the particle shape and size of the melamine borate.

The firing temperature is from 1000° C. to 1700° C., preferably from 1100° C. to 1600° C., and more preferably from 1200° C. to 1500° C.
When the firing temperature is within the above range, boron nitride that maintains the size and shape of the melamine borate is easily obtained.
When the firing temperature is equal to or higher than the lower limit, the crystal growth of boron nitride is likely to proceed.
If the firing temperature exceeds the upper limit, the crystallization of boron nitride proceeds and it becomes easier for the boron nitride to grow in a scale-like shape. Therefore, in order to obtain particulate boron nitride, it is preferable to fire the boron nitride at a temperature of 1700° C. or less.

In this embodiment, "firing temperature" refers to the maximum temperature at which the atmosphere in the firing furnace is maintained during the firing process.

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

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

In the firing process, the rate of temperature rise until the firing temperature is reached is 200°C/hour or more and 400°C/hour or less, and preferably 250°C/hour or more and 350°C/hour or less.

In this embodiment, the "heating rate" is calculated from the time from when heating starts in the firing device until the maximum holding temperature is reached, and the temperature difference from the temperature at the start of heating in the firing furnace of the heating device to the pore holding temperature.

After firing, the resulting fired product 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 a commonly used flux, and specific examples thereof include lithium borate, sodium borate, potassium borate, boron oxide, sodium nitrate, sodium chloride, potassium chloride, lithium chloride, sodium carbonate, potassium carbonate, 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, as well as ammonium salts corresponding thereto.

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

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

By firing in the presence of a flux, boron nitride crystals can be grown at low temperatures below 1700°C. This makes it difficult for the boron nitride to grow in a scaly form, and makes it possible to produce boron nitride that maintains the shape and size of the melamine borate particles.

<Boron nitride>
The boron nitride produced by the production method of this embodiment maintains the particle shape and size of the raw material melamine borate.
The boron nitride of this embodiment is in the form of secondary particles, which are aggregates of primary particles. The secondary particles are porous and have minute pores with a pore size of about 10 nm.

The boron nitride of this embodiment is, for example, a secondary particle having a columnar elongated shape, with a minor axis diameter of about 5 μm to 10 μm and a major axis diameter of about 10 μm to 2 cm.
The boron nitride of this embodiment is, for example, a secondary particle having a minor axis diameter and a major axis diameter each of several tens of μm, and is a particle having an aspect ratio of about 1 to 1.5.

The boron nitride of this embodiment has smaller anisotropy of thermal conductivity than conventional flake-shaped boron nitride, so it is expected that the thermal conductivity of the thermal conductor as a whole will be improved. In addition, because the size and shape of the boron nitride particles can be controlled, it can be used in a wide range of applications, such as thin insulating films with excellent thermal conductivity and interlayer insulating films for devices.

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

Example 1
[Step for obtaining melamine borate]
3 g of boric acid was added to 100 mL of distilled water heated to 80° C., and the mixture was stirred until completely dissolved. Then, 2 g of melamine was added and stirred until completely dissolved. A stirrer was used for stirring. As a result, a saturated aqueous solution of boric acid and melamine was obtained.

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

An X-ray diffraction pattern was obtained for the precipitate 1. The result is shown in Figure 1(a).

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

Furthermore, as shown in Figure 1(c), it was confirmed that it was melamine borate, ICDD No. 076-6949. From the results in Figure 1(c), it was confirmed that Precipitate 1 was melamine borate 1.

Figure 2 shows an SEM photograph (magnification 3000 times) of the obtained melamine borate 1. As shown in Figure 2, melamine borate 1 had a columnar, elongated shape with a minor axis diameter of approximately 5 μm to 10 μm.

[Firing process]
2 mmol of melamine borate 1 and 1 mmol of lithium carbonate were mixed for 10 minutes to obtain mixture 1. Mixture 1 was placed in a square carbon crucible and fired using a Keramax box electric furnace. The molar ratio of the flux to the melamine borate (melamine borate/flux) was 2.

First, the temperature was raised to 1400° C. over 4 hours and 20 minutes at a temperature rise rate of 300° C./hour, and then held at 1400° C. for 1 hour. Thereafter, the temperature was lowered to 0° C. over 4 hours and 20 minutes at a temperature drop rate of 300° C./hour. Nitrogen gas was passed through at a flow rate of 0.2 L/min during all of the temperature rise, holding at 1400° C., and temperature drop steps.
This resulted in the production of fired product 1. Fired product 1 was washed with water to obtain fired product 1.

An X-ray diffraction pattern was obtained for the resulting fired product 1. Figure 3(a) shows the X-ray diffraction pattern of fired product 1 before washing with water. Figure 3(b) shows the X-ray diffraction pattern of fired product 1 after washing with water. No peaks attributed to lithium carbonate were confirmed in either Figure 3(a) or (b).

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

Figure 4(a) shows an SEM photograph (magnification 2000x) of boron nitride 1, and Figure 4(b) shows an SEM photograph (magnification 20000x). From Figure 4(a), it can be seen that boron nitride 1 has a columnar, elongated shape with a minor axis diameter of approximately 5 μm to 10 μm. In other words, it was confirmed that the shape and size of melamine borate 1 were maintained.

Figure 4 (b) confirms that melamine borate 1 has a porous structure with multiple pores with a pore size of approximately 10 nm.

Example 2
[Step for obtaining melamine borate]
3 g of boric acid was added to 100 mL of distilled water heated to 80° C., and the mixture was stirred until completely dissolved. Then, 2 g of melamine was added and stirred until completely dissolved. A stirrer was used for stirring. As a result, a saturated aqueous solution of boric acid and melamine was obtained.

The resulting saturated aqueous solution was sonicated for 5 minutes to precipitate melamine borate crystals. The resulting crystals were filtered under suction to obtain precipitate 2.

An X-ray diffraction pattern was obtained for the precipitate 2. The results confirmed that precipitate 2 was melamine borate.

Figure 5(a) shows an SEM photograph (500x) of the obtained melamine borate 2. Figure 5(b) shows an SEM photograph (1000x) of the obtained melamine borate 2. As shown in Figure 5(a), melamine borate 2 had a particulate shape, with the minor axis diameter and major axis diameter of the particles each being approximately 20 μm. The aspect ratio (minor axis diameter/major axis diameter) of melamine borate 2 was approximately 1.

[Firing process]
4 mmol of melamine borate 2 and 2 mmol of lithium carbonate were mixed for 10 minutes to obtain mixture 2. Mixture 2 was placed in a square carbon crucible and fired using a Keramax box electric furnace. The molar ratio of the flux to the melamine borate (melamine borate/flux) was 2.

First, the temperature was raised to 1400° C. over 4 hours and 40 minutes at a temperature rise rate of 300° C./hour, and then held at 1400° C. for 1 hour. Thereafter, the temperature was lowered to 0° C. over 4 hours and 20 minutes at a temperature drop rate of 300° C./hour. Nitrogen gas was passed through at a flow rate of 0.2 L/min during all of the temperature rise, holding at 1400° C., and temperature drop steps.
This resulted in the production of fired product 2. Fired product 2 was washed with water to obtain fired product 2.

An X-ray diffraction pattern was obtained for the resulting fired product 2. This confirmed that 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).
6(a), the boron nitride 2 had a particle shape, and the minor axis diameter and major axis diameter of the particle were each about 20 μm. 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 the melamine borate 2 were maintained.

Figure 6(c) shows an SEM photograph of boron nitride 2 at 40,000 times magnification. From Figure 6(c), it was confirmed that melamine borate 2 has a porous structure with multiple pores with a pore diameter of about 10 nm.

Claims (2)

Obtaining melamine borate;
A calcination step of calcining the melamine borate in the presence of a flux at a temperature of 1000° C. or more and 1700° C. or less to obtain boron nitride,
In the firing step, the melamine borate and the flux are mixed and fired in a molar ratio (melamine borate/flux) of 1 to 3,
The method for producing boron nitride, wherein the flux is lithium carbonate or lithium hydroxide . The method for producing boron nitride according to claim 1, wherein the step of obtaining melamine borate comprises a step of ultrasonically treating a saturated aqueous solution of boric acid and melamine.

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