JPH052605B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing hexagonal boron nitride powder, which is known as a high-temperature material having excellent properties such as heat resistance, lubricity, electrical insulation, and thermal conductivity.
In particular, the present invention relates to a method for highly purifying hexagonal boron nitride powder using boric acid and/or boron oxide as starting materials.

As is well known, hexagonal boron nitride (hereinafter referred to as h-BN) has excellent heat resistance, lubricity, electrical insulation, thermal conductivity, etc. Therefore, in order to take advantage of these excellent properties, it is manufactured as a powder. is used for solid lubricants, mold release materials, etc., and the sintered body of h-BN powder is used for melting crucibles, electrical insulation materials,
It is used in many fields, including various electronic materials.

By the way, h using boric acid or boron oxide as a raw material
- As a conventional method for industrially producing BN powder, the first method is to heat these boron compounds to 700 to 1100°C in an ammonia gas stream to remove the boron in the raw materials. A method of nitriding is known. However, in the case of this method, when the boron oxide derived from the raw material melts during the heating process, it becomes an extremely viscous liquid, which reduces the contact interface with ammonia gas, and as a result, the reaction with ammonia gas is inhibited. It has the disadvantage of low reaction efficiency. In order to improve this, it is generally done to add and mix a filler with a high melting point such as calcium phosphate into the raw materials.In this case, although the reaction efficiency increases to some extent, it is still not at a fully satisfactory level. The reality is that it cannot be said. Further, when a filler is added in this manner, it is necessary to wash with hydrochloric acid or the like to remove the filler after the synthesis reaction is completed, but such a wet washing step using hydrochloric acid or the like has a problem of poor processing efficiency.

A second conventional method for industrially producing h-BN powder using boric acid or boron oxide as a raw material is to heat a mixture of these boron compounds and an inorganic or organic compound containing nitrogen at 800°C in a stream of ammonia or nitrogen gas. A method of reducing and nitriding by heating above is known. In this method, the obtained h-
BN powder is in a so-called crude state with a purity of about 80 to 90%, and therefore, after the synthesis reaction, crude h-
It is necessary to process the BN powder to make it highly purified to about 99% or more. As a method for this high purification, as disclosed in Japanese Patent Publication No. 47-43440, crude h-BN powder is heat-treated at 1600 to 2100°C in a stream of inert gas such as nitrogen or argon. Therefore, a method for removing impurities such as oxygen, hydrogen, and carbon by volatization, or
As described in the publication, crude h-BN powder is treated with an alkali metal compound such as sodium, potassium, or lithium, i.e., any of the oxides, hydroxides, carbonates, and borates of these alkali metals. The method used is to add a small amount and heat treat it at 1000℃ or higher. However, crude h-BN
The method of heating powder in an inert gas for high purity requires treatment at a high temperature of 1,600 to 2,100°C, which has the problem of significantly increasing operating and equipment costs. The method of adding an alkali metal compound to BN and heat treating it takes a long time and has a problem of low processing efficiency.

The present invention was made against the background of the above circumstances, and an object of the present invention is to provide a method for efficiently producing high-purity h-BN powder.

Specifically, the present invention provides a method for synthesizing crude h-BN powder using boric acid or boron oxide as the main raw material, and further purifying the crude h-BN powder. The object of the present invention is to provide a method that allows purification treatment to be carried out at a relatively low temperature and in a short period of time.

As a result of extensive experiments and studies by the present inventors in order to achieve the above-mentioned objectives, we have developed a conventional method (specially Kosho 47-
26600), an alkali metal compound is added and mixed to the starting materials at a stage before synthesizing crude h-BN, and the alkali metal compound is particularly anhydrous borax (Na ₂ B ₄ O ₇ ) or Using borax (Na ₂ B ₄ O ₇・10H ₂ O), crude h-
We have discovered that by adding and mixing carbonaceous powder to BN powder and performing heat treatment to obtain a high-purity compound, high purity can be achieved at a relatively low temperature and in a short heat treatment, and this invention has been made. It was reached.

That is, the method for producing high-purity h-BN of the present invention uses boric acid or boron oxide as the main raw material to produce h-BN.
In producing BN, 0.5 to 5% carbonaceous powder is added to crude h-BN powder, which is produced by adding 5 to 50% by weight of anhydrous borax and/or borax powder to boric acid or boron oxide when mixing raw material powders.
Add 2.0% by weight and mix in an inert gas stream.
It is characterized by heat treatment at a temperature of 1300°C or higher.

The method of the present invention will be explained in more detail below.

In the method of this invention, as described above, in the step before synthesizing crude h-BN, boric acid or boron oxide as the main starting material and an inorganic or organic compound containing nitrogen as a nitriding agent are mixed. Anhydrous borax (Na ₂ B ₄ O ₇ ) or borax (Na ₂ B ₄
Add and mix powder of O ₇ .10H ₂ O). Then, the mixed powder is heated to 800 to 1000°C in a stream of inert gas such as ammonia gas or nitrogen to first synthesize crude h-BN.

Here, the Na content of anhydrous borax or borax is changed to the crude h-BN at a stage after the crude h-BN is synthesized.
It works effectively when purifying crude h-BN to a high degree, but by mixing it at a stage before synthesizing crude h-BN, as mentioned above, the crude h-BN can be purified after synthesis.
Compared to the conventional technology of adding and mixing an alkali metal compound to BN and subjecting it to heat treatment for high purity, the crude h
-Since the distribution of Na in the BN powder becomes uniform and finely distributed, the processing time for high-purification treatment of crude h-BN powder is shortened, and high purity of 99% or higher is relatively possible. The effect achieved in the low temperature range (about 1300 to 1500 degrees Celsius) is recognized. In addition, when anhydrous borax or borax is added, the melting point of these compounds (745°C) is higher than that of boron oxide (<450°C), so the crude h
- Even after boron oxide melts during the temperature raising process for synthesizing BN, it remains as a solid and acts as a filler, and if anhydrous borax or borax is melted,
The sodium oxide contained in these reacts with the boron oxide solution to lower the viscosity of the melt, increasing the gas-liquid interface with ammonia gas or inert gas. The reaction efficiency is improved in comparison. As mentioned above, anhydrous borax or borax contains Na in its raw materials.
Not only does it serve as a boron source, but it also acts as a boron source.

After synthesizing the crude h-BN powder as described above, the crude h-BN powder was pulverized, a small amount of carbonaceous powder was added and mixed, and the mixture was heated at 1300°C in an inert gas liquid such as nitrogen or argon. If the heat treatment is performed as described above, a high purity h-BN powder with a purity of 99% or more can be obtained.

In this way, a small amount of carbonaceous powder is added to the crude h-BN powder, and anhydrous borax or borax is added to the crude h-BN powder before synthesis, and then heated for high purity. By performing the treatment, the processing time to obtain high purity of 99% or more is shortened, and the processing temperature is also reduced to 1300%.
A low temperature of ~1500℃ is sufficient. That is, the first
The figure shows that boric acid and melamine are used as starting materials, (a) 10% by weight of anhydrous borax is added to the boric acid before synthesis of crude h-BN powder, and h-BN powder is added after synthesis.
In the case of the method of this invention, in which 1.5% by weight of carbonaceous powder is added to BN powder and heat treatment is performed for high purity, (b) anhydrous borax is mixed with boric acid at a stage before synthesis of crude h-BN powder. Added 10% by weight to the crude h after synthesis.
- When heat treatment is performed for high purity without adding carbonaceous powder to BN powder, (c) When anhydrous borax or borax is not added at the stage before synthesis of crude h-BN powder and synthesis Later crude h-
When no carbonaceous powder is added to the BN powder, the purity of the h-BN powder after heat treatment for high purity is determined by varying the heat treatment temperature in the heat treatment for high purity for the above three cases. This is what I investigated. As can be seen from FIG. 1, in the case of method (a) of the present invention in which both anhydrous borax is added before synthesis of the crude h-BN powder and carbonaceous powder is added to the crude h-BN powder, both anhydrous borax and carbon powder are added. At a much lower heat treatability temperature than in case (c) without additives.
It can be seen that a high purity of 99% or more was achieved, and that the high purity was achieved at a lower heat treatment temperature compared to the case (b) in which anhydrous borax was added alone. Therefore, it is clear that the addition of anhydrous borax before synthesis of the crude h-BN powder and the addition of carbonaceous powder to the crude h-BN powder are extremely effective in lowering the heat treatment temperature required for high purity. In this experiment, 102 parts by weight of melamine was mixed with 100 parts by weight of boric acid, and the heat treatment for synthesizing the crude h-BN powder was carried out by holding it at 900°C for 2 hours in an ammonia gas stream. The heat treatment time for high purity was kept constant at 30 minutes and was carried out in a nitrogen gas stream.

In addition, Figure 2 shows crude h-BN as in (a) above.
10% by weight of anhydrous borax was added at the stage before powder synthesis, and 1.5% by weight of carbonaceous powder was added to the synthesized h-BN powder.
When the crude h
-The purity of h-BN powder after the treatment was investigated by variously changing the treatment time while keeping the treatment temperature constant at 1500° C. for purifying the -BN powder. From FIG. 2, it is clear that the heat treatment time for high purity is also significantly shortened in case (a) of the present invention. The other conditions in the experiment shown in FIG. 2 were the same as those in the experiment shown in FIG.

Each process condition and material compounding condition in the method of the present invention as described above will be further explained.

First, the amount of anhydrous borax or borax added to the starting material boric acid or boron oxide must be within the range of 5 to 50% by weight based on the weight of boric acid or boron oxide. The reason is as follows.
In other words, if the amount is less than 5% by weight, the effects of these additions, that is, the dispersion of Na in the entire reaction system, cannot be said to be good, and high purity cannot be achieved at a relatively low temperature range of 1300 to 1500°C. The reaction rate also does not improve much compared to the case without addition. In addition, if more than 50% by weight is added, the dispersion state of Na will be good, but the Na content will be volatilized during the heat treatment for high purity in a relatively low temperature range, which is one of the important features of the method of this invention. The time required for heat treatment becomes longer, and in some cases, there is a risk that Na content may remain in the powder after heat treatment. Here, anhydrous borax or borax is
Of course, one of them may be used alone or both may be used in combination.

The amount of the nitrogen-containing inorganic or organic compound added to boric acid or boron oxide as a starting material is about 70 to 200 parts by weight per 100 parts by weight of boric acid or boron oxide, as in conventionally known methods. Inorganic or organic compounds containing nitrogen include inorganic compounds such as urea, thiourea, biuret, guanidine, etc.
Alternatively, organic compounds such as melamine, dicyandiamide, semicarbazide cyanuric acid, etc. can be used.

As mentioned above, the heat treatment for the mixed powder obtained by adding anhydrous borax or borax to an inorganic or organic compound containing boric acid or boron oxide and nitrogen as starting materials, that is, heat treatment for synthesizing crude h-BN, is a conventional method. Similarly, it may be carried out in a stream of ammonia or inert gas within the range of 800 to 1000°C. If the treatment temperature is less than 800°C, the reaction will not proceed completely and the yield will drop significantly. Meanwhile 1000℃
Although the reaction rate can be increased if the reaction rate is exceeded, it is not economically advantageous. Note that the processing time may be about 30 minutes to 5 hours.

The amount of carbonaceous powder added to the obtained crude h-BN powder needs to be within the range of 0.5 to 2.0% by weight based on the weight of the crude h-BN powder.
If the amount of carbonaceous powder added is less than 0.5% by weight, it will be difficult to obtain the desired high-purity h-BN powder in a short time and at a relatively low temperature. On the other hand, if the amount of carbonaceous powder added exceeds 2.0% by weight, depending on the heat treatment conditions, it may remain as a carbon source or impurity in the powder after treatment, which is not appropriate. Here, as the carbonaceous powder added to the crude h-BN powder, any carbonaceous powder can be used, from amorphous to highly graphitized powder, but preferably amorphous is more effective. It is true.

The heat treatment for high purification of the crude h-BN powder mixed with carbonaceous powder in this way can be carried out in a stream of inert gas such as nitrogen or argon, as in the conventionally known method. . Regarding the processing temperature, as mentioned above, in the method of this invention, high purity of 99% or more can be achieved at a temperature lower than conventional methods. The temperature must be at least 1300° C., since this becomes extremely long and goes against the other objective of the present invention, which is to shorten the heat treatment time. On the other hand, the upper limit of the treatment temperature is not particularly limited, but in order to obtain powder with high purity of 99% or more at low cost, it is desirable to set it to less than 1600°C, preferably less than 1500°C. Furthermore, the heat treatment time for high purity is difficult to determine due to the relationship with the treatment temperature, but usually 30 minutes to 2.5 hours is sufficient.
Of course, if the treatment temperature is the same, high purity can be achieved in a much shorter time than in the conventional method.

Examples of the present invention and comparative examples using conventional methods will be described below.

Example 1 100 parts by weight of boric acid and melamine as starting materials
Using 102 parts by weight, anhydrous borax powder corresponding to 10% by weight of boric acid was added and dry mixed. This mixed powder was filled into an iron reaction vessel and heat-treated in an ammonia gas stream at 900°C for 2 hours to synthesize h-BN. The purity of the obtained crude h-BN powder was 90.8%. 2.3g of amorphous graphite powder was added to 150g of this crude h-BN powder, and after dry mixing, it was filled into a graphite crucible and heat treated in a Tammann furnace at 1500℃ for 2 hours in a nitrogen stream. As a result, high purity h- with a purity of 99.9% was obtained.
129 g of BN powder was obtained.

Example 2 Same as Example 1, 100 parts by weight of boric acid and 102 parts by weight of melamine
Borax powder in an amount corresponding to 20% by weight of boric acid was added to the weight part and dry mixed. This mixed powder was packed into an iron reaction vessel and heated to 900°C in an ammonia gas stream.
h-BN was synthesized by heat treatment under conditions of holding at °C for 3 hours. The purity of the obtained crude h-BN powder is 88.6
It was %. 2.6% amorphous graphite powder was added to 150g of this crude h-BN powder, and after dry mixing, it was filled into a graphite crucible and heat treated in a Tammann furnace at 1500°C for 2 hours in a nitrogen stream. As a result, 120 g of high purity h-BN powder with a purity of 99.8% was obtained.

Comparative Example After dry mixing 100 parts by weight of boric acid and 102 parts by weight of melamine as starting materials, the mixed powder was filled into an iron reaction vessel and mixed in a stream of ammonia gas.
Crude h-BN was produced under conditions of holding at 900°C for 2 hours. The purity of the crude h-BN powder was 84.4%. 13g for 150g of this crude h-BN powder
After adding and mixing sodium carbonate powder, it was filled into a graphite crucible and heated in a nitrogen stream in a Tammann furnace.
As a result of heat treatment at 1500° C. for 3 hours, 118 g of h-BN powder with a purity of 99.6% was obtained.

In the above comparative example, no carbonaceous powder was added, and sodium carbonate was added and mixed as an alkali metal compound to the synthesized crude h-BN powder. According to Examples 1 and 2, h-BN with higher purity than the comparative example
It is clear that powder can be obtained in a shorter time and with better yield.

As is clear from the above explanation, according to the method for producing high-purity h-BN powder of the present invention, anhydrous borax or borax is used as an alkali metal compound, and the anhydrous borax or borax is used at a stage before the synthesis of crude h-BN. By adding and mixing a small amount of carbonaceous powder to the crude h-BN powder after synthesis, and performing heat treatment for high purity, it is possible to Crude h-BN by heating
Not only can the powder be highly purified, but the yield of high-purity h-BN powder can also be increased. Therefore, according to the method of the present invention, the manufacturing cost of high-purity h-BN powder can be significantly reduced compared to the conventional method. can be reduced.

[Brief explanation of the drawing]

Figure 1 shows the heat treatment temperature in heat treatment for high purity of crude h-BN powder and h-BN after treatment.
Figure 2 is a correlation diagram showing the relationship between the purity of the powder and the purity of the h-BN powder after the treatment and the heat treatment time in heat treatment for purifying crude h-BN powder. It is.

Claims (1)

[Claims] 1. A crude product produced by adding 5 to 50% by weight of anhydrous borax and/or borax powder to the boric acid or boron oxide when mixing the raw material powder when producing hexagonal boron nitride using boric acid or boron oxide as the main raw material. High-purity hexagonal boron nitride powder is produced by adding 0.5 to 2.0% by weight of carbonaceous powder to hexagonal boron nitride powder and heat-treating the mixture at a temperature of 1300°C or higher in an inert gas flow. Production method.