JP5081488B2 - Hexagonal boron nitride powder - Google Patents

Description

  The present invention relates to a hexagonal boron nitride powder suitable for applications requiring thermal conductivity, such as an insulating heat dissipation material.

  Hexagonal boron nitride (h-BN) powder is used in a wide range of applications such as solid lubricating materials, insulating heat dissipation materials, and cosmetics. Conventionally, such a hexagonal boron nitride powder is obtained by reacting a boron compound such as boric acid or borate with a nitrogen compound such as urea or amine at a relatively low temperature to produce crude h-BN having low crystallinity. Generally, it is produced by a method of obtaining a powder and heating it at a high temperature to grow crystals (for example, Patent Document 1).

  On the other hand, h-BN powder has a layered structure similar to graphite, and (1) its thermal expansion coefficient is close to that of Si and is less likely to be distorted with respect to the Si substrate, which is a semiconductor material, (2) its dielectric constant is small, It has excellent characteristics such as low signal propagation delay, (3) high electrical insulation, excellent dielectric strength, and (4) high insulation of 2 to 3 times that of alumina. There is an increasing demand for fillers in the field. For example, a sheet or tape excellent in thermal conductivity (heat dissipation) and insulation obtained by adding h-BN to a resin material such as epoxy resin or silicon rubber has attracted attention.

  When h-BN powder is used for such applications, the substitution rate of h-BN powder with respect to the resin, that is, the filling property of h-BN powder affects the thermal conductivity, and the filling property of h-BN powder is increased. It is desired to obtain high thermal conductivity.

However, it cannot be said that the conventional h-BN powder has sufficient filling properties, and it is difficult to satisfy the required value for the thermal conductivity of a sheet or tape obtained by adding h-BN powder to a resin. is there. In addition, the conventional h-BN powder has a relatively high dielectric constant, and the signal propagation delay increases, which may reduce the reliability.
Japanese Patent Laid-Open No. 9-295801

  This invention is made | formed in view of this situation, Comprising: It aims at providing the hexagonal boron nitride powder with high filling property. It is another object of the present invention to provide hexagonal boron nitride powder having a low dielectric constant in addition to high filling properties.

In order to solve the above-mentioned problems, the present invention has a powder particle having a long side length / short side length ratio of 15 or less and a total of 50 powder particles having a long side length / short side length ratio of 10 or more. %, And after nitriding boron carbide, C contained in the powder is removed so that the C content in the powder is 0.5 mass% or less, and the long side length / short side length ratio of the powder particles Is an average value of 10 or less, the long side length of the powder particles is 10 μm or less, the average value of the long side length of the powder particles is 4 μm or less, and the secondary particles formed by aggregation of the powder particles The present invention provides a hexagonal boron nitride powder characterized in that the proportion of aggregated particles having a projected area of 100 μm ² or more is 5% or more by area ratio .

Furthermore, Te hexagonal boron nitride powder smell according to the present invention, the maximum length / minimum length ratio of the mean value of the projected area of 100 [mu] m ² or more aggregated particles of secondary particles powder powder particles are formed by aggregation Is preferably 5 or less.

  Furthermore, in the hexagonal boron nitride powder according to the present invention, boron carbide is brought to 1800 ° C. or higher in a nitrogen atmosphere, diboron trioxide and / or a precursor thereof are mixed, and baked to remove C. The C content is preferably 0.5 mass% or less.

According to the present invention, the long side length / short side length ratio of the powder particles is 1 5 or less, is the total of 50% or less long side length / short side length ratio is more than 10 powder particles Therefore, there are fewer flat and longer particles than in the past, and the filling property can be improved. For this reason, when it uses as a filler, thermal conductivity can be improved. In addition, after nitriding boron carbide, C contained in the powder is removed so that the C content in the powder is 0.5 mass% or less, so that the dielectric constant is small and stable, so that the signal propagation delay is small. And reliability can be improved .

Hereinafter, the present invention will be specifically described.
A crude h-BN powder having low crystallinity is obtained by reacting a boron compound such as boric acid or borate and a nitrogen compound such as urea or amine, which are disclosed in Patent Document 1 and the like, at a relatively low temperature. The h-BN powder obtained by heating this at a high temperature to grow a crystal has a long side length (the length of the particle in the C-axis direction) compared to the short side length (the thickness of the particle in the C-axis direction). Since the longest part) is long, the ratio of the long side length / short side length is large, and the long side length itself is large, the filling property is not sufficient.

  In contrast, in the present invention, first, the long side length / short side length ratio of the powder particles is set to 20 or less. That is, there is substantially no particle having a long side length / short side length ratio exceeding 20. As described above, since there is no long side length / short side length ratio, there are few flat and long particles, and the powder particle filling property can be remarkably improved. Preferably, the long side length / short side length ratio of the powder particles is 15 or less.

  Further, the ratio of the long side length / short side length ratio of the powder particles is 20 or less, preferably 15 or less, and the ratio of the powder particles having the long side length / short side length ratio of 10 or more is 50% or less is preferable. Thus, the filling property can be further enhanced by reducing the number of particles having a long side length / short side length ratio of 10 or more. The ratio of powder particles having a long side length / short side length ratio of 10 or more is more preferably 40% or less, and even more preferably 30% or less.

  The powder filling property can also be improved by setting the average value of the long side length / short side length ratio of the powder particles to 10 or less. That is, by defining in this way, the ratio of particles having a long side length / short side length ratio is inevitably reduced, and the powder particle filling property can be improved. Preferably, the average value of the long side length / short side length ratio of the powder particles is 9 or less.

  The filling property of the powder is influenced not only by the ratio of the long side length / short side length but also by the value of the long side length itself, and the filling property is also improved by setting the long side length to 10 μm or less. Can do. That is, if there are those whose long side length exceeds 10 μm, such long particles are prevented from being densely packed, resulting in a low packing density, but such long particles are present. By not doing so, a high packing density can be obtained. A more preferable value of the long side length is 7 μm or less, more preferably 5 μm or less.

  Similarly, by setting the average value of the long side lengths of the powder particles to 4 μm or less, the filling property of the powder particles can be improved. A more preferable range of the average value of the long side length is 3 μm or less.

The above is the definition of the properties of the powder particles, but the powder filling property (packing density) can also be increased by the presence of aggregated particles having a predetermined size or more formed by aggregation of the powder particles. That is, the present invention is characterized in that there are aggregated particles having an area of a projected image by SEM of 100 μm ² or more. In this case, it is generally preferable that the field of view is in the range of 1 × 10 ⁵ μm ² or more.

Since the conventional h-BN powder has substantially no such agglomerated particles, the packing density cannot be increased by the agglomerated particles. However, in the present invention, the presence of agglomerated particles having a high density by itself, Inevitably, the packing density increases. The reason why the area of the agglomerated particles is 100 μm ² or more is that if the area is smaller than that, the effect of increasing the packing density is small.

The ratio of the aggregated particles having an area of 100 μm ² or more within the visual field is preferably 5% or more, more preferably 10% or more, in terms of area ratio. Further, such aggregated particles preferably have a maximum length / minimum length ratio of 5 or less on average. This is because the effect of increasing the packing density is greater when the maximum length / minimum length ratio is smaller than that of the flat shape.

The properties of the above powder particles, the properties and the abundance ratio of the agglomerated particles, etc. can be grasped by a scanning electron microscope (SEM) photograph. Specifically, regarding the properties of the powder particles, 50 or more powder particles that can clearly grasp the long side and the short side are arbitrarily extracted in a SEM photograph taken at a magnification at which the property of the powder particles can be grasped, for example, 5000 times. To measure. As for the aggregated particles, SEM photographs of a field of view of 1 × 10 ⁵ μm ² or more taken at a magnification at which the aggregated particles can be clearly grasped, for example, 200 ×, are image-processed by a computer, and using appropriate software, The number, area, and shape (maximum length, minimum length, etc.) of aggregated particles having an area of 100 μm ² or more are grasped. As such software, commercially available “Image Pro Plus” (trade name) can be suitably used.

  As described above, in the present invention, the requirements on the long side length / short side length ratio of the powder particles, the requirements on the long side length of the powder particles, and the requirements on the aggregated particles are separately defined. They may be filled at the same time, whereby a h-BN powder having a higher filling property can be obtained.

The method of producing h-BN powder of the present invention is not particularly limited, it may be suitably prepared by boron carbide (B 4 _C) as a starting material. Specifically, as a preferable production method, diboron trioxide (boric anhydride) and / or a precursor thereof are mixed with a product obtained by calcining boron carbide in a nitrogen atmosphere, and calcined to produce by-product carbon. The method of removing is illustrated.

In this method, boron carbide is fired in a nitrogen atmosphere, and the generation of boron nitride powder proceeds based on the following formula (1). For this purpose, it is necessary to provide sufficient temperature, time, and nitrogen partial pressure. .
(1/2) B ₄ C + N ₂ → 2BN + (1/2) C (1)

  Thermodynamically, the reaction of formula (1) is an exothermic reaction, and boron nitride should be generated under a condition where the nitrogen partial pressure is 0.1 MPa, but in reality, the reaction hardly proceeds at 1800 ° C. or lower. Therefore, a temperature of 1800 ° C. or higher is required.

  The particle size of the boron carbide powder is preferably 44 μm or less from the viewpoint of reactivity. When the particle size exceeds 44 μm, the completion of the reaction of the above formula (1) may be delayed, which lowers the yield. From the viewpoint of avoiding such a decrease in yield, it is preferable to set the powder having a particle size of 44 μm or less in a range of 90% by mass or more.

  Diboron trioxide and / or a precursor thereof (hereinafter also referred to as diboron trioxide or the like) is mixed into the fired product of boron carbide. At that time, a solvent can be added to the ball mill to carry out a wet process, but it is preferable to use a dry mixer such as a V-blender. Mixing is performed until uniform. Specifically, it is sufficient that the mixture becomes a homogeneous gray color by visual observation.

  The precursor of diboron trioxide is a boron compound that can be converted to diboron trioxide by heating, and specifically includes ammonium salt of boric acid, orthoboric acid, metaboric acid, tetraboric acid, and the like. Of the diboron trioxide and its precursors, preferred is diboron trioxide.

The mixing amount of diboron trioxide and the like is determined by the amount of boron carbide. That is, it suffices if there is a sufficient amount that can be consumed and removed by the reaction of the later-described formula (2) of the total amount of carbon generated by the reaction of the formula (1).
C + B ₂ O ₃ → B ₂ O ₂ (gas) + CO (gas) (2)

  In the calcination for removing the by-product carbon, the by-product carbon is oxidized to carbon monoxide and removed by the reaction of the above formula (2).

The reaction of the above formula (2) proceeds at 1500 ° C. or higher, and the evaporation of diboron trioxide proceeds at 1600 ° C. or higher. Since both generate gas, the reaction and evaporation can be effectively advanced by the effective removal of the generated gas. In order to further proceed the reaction, the flow rate of nitrogen gas is increased, and the generated carbon monoxide, anhydrous boric acid (B ₂ O ₂ ) gas, and diboron trioxide (B ₂ O ₃ ) gas are actively used. It is more effective when discharged outside.

  In the present invention, when producing hexagonal boron nitride powder using boron carbide as a starting material, carbon (C) is mixed as an inevitable impurity in the term-filling hexagonal boron nitride powder as a product. Since carbon is a conductive substance, it affects the dielectric constant, and if there is too much carbon, the dielectric constant rises, resulting in signal propagation delay. For this reason, the content of carbon (C) is preferably 0.5 mass% or less, and more preferably 0.2 mass% or less.

Examples of the present invention will be described below.
Commercially available boron carbide powder with a purity of 98% by mass is passed through a 44 μm sieve, 101.8 g of the raw material powder passed through the sieve is put into a carbon crucible having an inner diameter of 90 mm and a height of 100 mm, and baked at 2000 ° C. for 10 hours in a nitrogen atmosphere. did. The amount of the fired product was 176.6 g.

  From the calcined product, 69.3 g was taken and mixed with 35.2 g of commercially available diboron trioxide. The mixing at this time was performed by rotating for 30 minutes under the condition of 1 Hz using a V-blender having an internal volume of 1 L.

  The obtained powdery mixture was placed in a carbon crucible having an inner diameter of 90 mm and a height of 100 mm and baked at 2000 ° C. for 10 hours in a nitrogen stream to obtain a second baked product. This second baked product was a white aggregate, and as a result of being subjected to X-ray diffraction after pulverization, it was confirmed that it was almost completely h-BN as shown in FIG. Moreover, the carbon content of the powder was measured by an infrared absorption method using a tubular electric resistance furnace (manufactured by Shimadzu Corporation), and the oxygen content was measured using an ON simultaneous analyzer (manufactured by Horiba, Ltd .; model EMGA-550). Was measured by an inert gas-impulse heating melting thermal conductivity detection method. As a result, it was confirmed that the oxygen content was as small as 0.2% by mass and the carbon content was as small as 0.01% by mass, and high-purity h-BN was obtained.

  A 5000-times SEM photograph of the h-BN powder thus obtained was taken. FIG. 2 shows SEM images of four different fields of view at this time. From such SEM photographs, 68 powder particles that can clearly grasp the long side (longest part) and the short side (thickness direction) are arbitrarily extracted, and the short side length, the long side length, The long side length / short side length ratio was measured. The results are shown in Table 1.

  As shown in Table 1, the largest long side length / short side length ratio was 14.6, and the ratio of those having a value of 10 or more was 25%. Moreover, the average value of long side length / short side length ratio was 8.2. As the value of the long side length itself, the longest one was 4.96 μm, and the average was 2.78 μm. That is, it was confirmed that h-BN powder satisfying the scope of the present invention was obtained.

  For comparison, a conventional boron compound such as boric acid or borate and a nitrogen compound such as urea or amine are reacted at a relatively low temperature to obtain a crude h-BN powder having low crystallinity. Similarly, a 5000 times SEM photograph was taken with respect to the powder obtained by heating at a high temperature to grow crystals. FIG. 3 shows SEM images of four different fields of view at this time. From such SEM photographs, 72 powder particles that can clearly grasp the long side (longest part) and the short side (thickness direction) are arbitrarily extracted, and the short side length, the long side length, The long side length / short side length ratio was measured. The results are shown in Table 2.

  As shown in Table 2, the largest long side length / short side length ratio was 25.3, and the ratio of those having a value of 10 or more was 71%. The average value of the long side length / short side length ratio was 12.5. As the value of the long side length itself, the longest one was 15.54 μm, and the average was 5.88 μm.

  Comparing these figures and tables, in the example of the present invention, there is no long side length / short side length ratio of 15 or more, the ratio is 10 or more, the average value is small, It can be seen that while the side length itself is small, the comparative example has a long side length / short side length ratio of 20 or more, the average value is large, and the long side length itself is large. For this reason, a clear difference between the two can be recognized in the SEM photograph.

Next, the agglomerated particles of the above-described h-BN powder of the present invention were investigated.
The analysis part is cut out from the SEM photograph (original drawing) taken at 200 times, and the aggregated particles are made whitish due to the charge-up, and the aggregated particles are grasped by the color tone. in area white the other part the part to extract 100 [mu] m ² or more of the blackened, using a commercially available software "image Pro plus" (trade name), area of 100 [mu] m ² or more aggregated particles The number, area, and shape (maximum length and minimum length) of the Note that the size (field of view) of the analysis unit was 355 μm × 580 μm. Such an operation was performed in 10 fields of view from # 1 to # 10. The results are shown in Table 3. Also, for # 1, # 5, and # 10 of these, (a) 200 times original SEM photograph (original drawing), (b) SEM photograph of analysis section therein, (c) analysis section image What was processed is shown in FIGS.

As shown in Table 3 and FIGS. 4 to 6, the h-BN powder of the example of the present invention has aggregated particles having an area of 100 μm ² or more, and the area ratio of such aggregated particles in the field of view is 8. 6 to 19.7%, all showing values exceeding 5%, and most showed values exceeding 10%. The maximum length / minimum length ratio values were all 5 or less on average.

In addition, about the h-BN powder of the comparative example, the aggregate particle | grains of 100 micrometers ² or more were not seen from the SEM photograph image | photographed by 200 time.

  Next, a composite sheet with a resin was prepared using the h-BN powder of the present invention example and the h-BN powder of the comparative example. Epoxy resin “Epicoat 807” (manufactured by Japan Epoxy Resin Co., Ltd.) was used as the resin, and modified alicyclic amine grade “Epicure 113” (manufactured by Japan Epoxy Resin Co., Ltd.) was used as the curing agent. As a result of obtaining the h-BN powder substitution rate (filling rate) of the sheet from the viewpoint of ensuring the fluidity that can be degassed, it was about 33% by mass in the comparative example, whereas it was about 50% by mass in the present invention example. Thus, it was confirmed that the examples of the present invention have remarkably good filling properties as compared with the comparative examples. When the thermal conductivity of these sheets was compared, the sheet using the h-BN powder of the comparative example was 1.2 W / m · K, whereas the sheet using the h-BN powder of the present invention example was It was 1.75 W / m · K, and it was confirmed that the thermal conductivity increased by 46%.

Next, in the case of producing BN using B ₄ C as a starting material, in order to investigate the influence of carbon (C) present as an inevitable impurity in the highly filled BN powder as a product, BN having different carbon contents The influence on electrical properties was investigated using powder.

  When producing the h-BN powder of the present invention, test materials shown in Table 4 were prepared with different opportunities. Using these test materials, the epoxy resin was blended at a mass ratio of 60:40, and the volume resistance value and the dielectric constant were measured. The results are shown in Table 5.

Originally, the dielectric constant of the powder sample itself should be measured. However, since the electrode cannot be attached unless it is molded, a sample in which BN powder is dispersed in a resin was prepared and measured. Whether or not the resin is uniformly dispersed was confirmed by measuring the volume resistivity from the direct current characteristics shown in Table 5. As a result, even if 1% or more of carbon was contained as an impurity in the BN powder, it showed a value of 1 × 10 ¹⁴ Ω · cm or more, indicating that it was an insulator. I found out.

  On the other hand, in the alternating current measurement, when the carbon content was 0.5 mass% or less, the dielectric constant ε was 4.0 or less, indicating good properties. The dielectric constant is expected to increase as the carbon content increases due to the influence of the inherent conductive material. In order to stably produce h-BN powder having a low dielectric constant, the carbon content is reduced to 0. It turned out that it is preferable to set it as 5 mass% or less.

  In preparing the sample, a curing agent was mixed with the epoxy resin, a predetermined amount of hexagonal boron nitride powder was mixed for 5 minutes, and defoamed for 1 minute to prepare a sample. As an epoxy resin, Japan Epoxy Resin: Epicoat 807 was used, and as a cured resin, Japan Epoxy Resin: EpiCure 113 was used.

  The volume resistivity was evaluated by a direct current three terminal method (JIS C2141) at room temperature (26 ° C., humidity 40%) and in the atmosphere. The dielectric constant was evaluated by a two-terminal method at room temperature (26 ° C., humidity 42%), in the atmosphere, and at a frequency of 1 MHz.

  Since the hexagonal boron nitride powder of the present invention has high filling properties, it is suitable for use as a filling material that is added to a resin material to form a sheet or tape excellent in thermal conductivity (heat dissipation) and insulation. It can also be used for various other applications such as solid lubricating materials, cosmetics, and refractories.

The X-ray-diffraction chart of the h-BN powder of the example of this invention. The SEM photograph of 5000 times the h-BN powder of the example of the present invention. The SEM photograph of 5000 times the h-BN powder of a comparative example. The figure which shows what processed the SEM photograph of the 200 times SEM photograph of # 1, the SEM photograph of the analysis part, and the analysis part in the example of the present invention. The figure which shows the SEM photograph of # 200 of # 5 in the example of this invention, the SEM photograph of the analysis part in it, and the thing which image-processed the analysis part. The figure which shows what carried out the image process of the SEM photograph of 200 times as many as # 10 among the examples of this invention, the SEM photograph of the analysis part in it, and the analysis part.

Claims (3)

After the long side length / short side length ratio of the powder particles is 15 or less, the powder particles having a long side length / short side length ratio of 10 or more are 50% or less of the total, and after nitriding boron carbide The C content in the powder is reduced to 0.5 mass% or less by removing C contained in the powder, and the average value of the long side length / short side length ratio of the powder particles is 10 or less. The long side length of the agglomerated particles is 10 μm or less, the average value of the long side lengths of the powder particles is 4 μm or less, and the projected area of the secondary particles formed by agglomeration of the powder particles is 100 μm ² or more. A hexagonal boron nitride powder having an area ratio of 5% or more in area ratio . The projected area of the secondary particles of the powder particles are formed by aggregation of claim 1, wherein the average value of the maximum length / minimum length ratio of 100 [mu] m ² or more aggregated particles is 5 or less Hexagonal boron nitride powder. After boron carbide is heated to 1800 ° C. or higher in a nitrogen atmosphere, diboron trioxide and / or a precursor thereof are mixed and baked to remove C to make the C content 0.5 mass% or less. The hexagonal boron nitride powder according to claim 1 or 2 , characterized by the above.

Images