JP2012148932A - Method for manufacturing hexagonal boron nitride sintered body, and hexagonal boron nitride sintered body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hexagonal boron nitride sintered body which can be sintered at a low temperature; and to provide a method for manufacturing the same.SOLUTION: A hexagonal boron nitride powder containing oxygen not only on the surface of the powder but also insde thereof is pressure formed to make a preformed body (preformation step), and the preformed body is sintered in a non-oxidizing gas or in vaccum at ≥600°C and <1,100°C (sintering step).

Description

  The present invention relates to a method for producing a hexagonal boron nitride sintered body capable of being sintered at a low temperature of 600 to 1100 ° C. and a hexagonal boron nitride sintered body.

  Hexagonal boron nitride (h-BN) is composed of boron (B) which is an element of group IIIb in the periodic table and nitrogen (N) which is an element of group Vb. These constituent elements are located immediately before and after the carbon (C) of group IVb and have a crystal structure similar to that of graphite. For example, the c-plane of hexagonal boron nitride is connected by a strong π bond, while the c-axis direction is connected by a van der Waals bond with a weak binding force, so that it has a cleavage plane with a plate-like crystal. ing. Further, like graphite, it has a high melting point, and at the same time has excellent properties such as lubricity, releasability, machinability and corrosion resistance. On the other hand, hexagonal boron nitride, unlike graphite, has high electrical insulation and oxidation resistance up to about 1000 ° C. Taking advantage of these features, it is used in metal melting crucibles, lubricants, high frequency electrical insulation materials, steelmaking nozzles, protective tubes, setters, and the like.

However, since hexagonal boron nitride has a high melting point and is a non-oxide, it has a problem that it is very difficult to burn and harden by baking (difficult to burn). In addition, even when trying to sinter using a sintering aid that plays the role of a binder, the melted sintering agent does not wet the hexagonal boron nitride surface due to poor wettability, and as a sintering aid There was also a problem that the role could not be fully demonstrated. Furthermore, there is also a problem that hexagonal boron nitride changes to boron oxide (B ₂ O ₃ ) in the presence of oxygen at a high temperature.

  In order to solve these problems, hexagonal boron nitride is sintered by the following method. That is, it is a method of sintering in a vacuum, an argon atmosphere, or a nitrogen atmosphere (generally in a nitrogen atmosphere) using a hot press apparatus or the like at a high temperature of 1500 to 2300 ° C. and a pressure exceeding 10 MPa.

However, this method has the following problems, and is difficult to manufacture and high in manufacturing cost.
(1) Since firing is performed in a pressurized state, it is necessary to provide a hot press mechanism in the furnace, and it is necessary to have a structure in which the sample is pressurized with a press ram, mold, punch, or the like by hydraulic driving while maintaining airtightness. .
(2) Since it is necessary for the hot press device to have a structure that can withstand high temperatures, not only materials with high temperature durability are used, but also a large amount of energy (generally electric power) is required for heating, and at the same time, a furnace A sufficient cooling mechanism and an enormous amount of cooling water are also required for protection.
(3) Due to the restriction of firing with a hot press, it is extremely difficult to produce large-sized products and articles with complicated shapes. In addition, since the production conditions are very severe, the environmental load is very large.

  Therefore, in order to manufacture a sintered body of hexagonal boron nitride, the manufacturing apparatus and running cost become very large, and the product itself has to be very expensive. For this reason, in spite of having a unique characteristic as described above, its use is inevitably limited. Furthermore, since it must be baked at a high temperature, the environmental load is very large, and it was necessary to take measures to reduce the environmental load.

  Various atmospheric pressure firing methods have been proposed in order to solve the problems of the pressure firing method by hot pressing, reduce the manufacturing cost, and facilitate the production of large-sized products and complex-shaped products. (See Patent Documents 1 to 6).

For example, in Patent Document 1, the addition of SiO ₂ as an auxiliary sintering of boron nitride, in order to SiO ₂ to improve the drawback of volatilization of SiO is reduced in a reducing atmosphere, these systems Further, the problem is solved by adding B ₂ O ₃ .

Further, in Patent Document 2, when boric anhydride (B ₂ O ₃ ) or aluminum nitride (AlN) is used as a binder for boron nitride, a mixed powder of aluminum and silicon or an alloy powder thereof is mixed with boron nitride. A method of firing in a nitrogen stream or in a slightly oxidizing atmosphere mainly containing nitrogen is described. According to this method, deterioration of high-temperature electrical characteristics and deterioration of corrosion resistance against the melt are prevented, and further, there are problems such as reduction in mechanical strength of the molded body and lack of corrosion resistance against the melt when AlN is used. It is stated that it can be solved.

  Further, in Patent Document 3, a hexagonal boron nitride sintering method in which a molded body having a density as high as possible is formed in a molding stage before firing, and firing is performed in an inert atmosphere in a low expansion coefficient mold. Is described. According to this method, it is described that the disadvantage that the sample does not become densified because the sample expands during firing when firing without pressure is described.

  Further, in Patent Document 4, the calcinability of hexagonal boron nitride is improved by adding a large amount of a counterpart material (sintering aid or binder) to improve mechanical strength. Furthermore, by controlling the raw material particle size, the type and amount of the sintering aid, and the firing temperature, it is intended to improve the high temperature stability of boron nitride and to have practically usable characteristics.

  Furthermore, Patent Document 5 attempts to produce large-sized products and complex-shaped products at low cost by containing an appropriate amount of an alkaline earth metal borate and firing it at normal pressure in a non-oxidizing atmosphere.

  Further, in Patent Document 6, it is said that carbon and boron carbide are dispersed and contained in a fired body mainly composed of boron nitride, so that the fired body by the normal pressure firing method has a lower strength at a high temperature, particularly a thermal shock resistance. We are trying to solve the problem.

Japanese Patent Publication No. 47-38047 Japanese Patent Publication No. 48-43648 Japanese Patent Application Laid-Open No. 61-132563 JP-A 63-303862 Japanese Patent No. 2614874 JP 2001-146477 A

  However, in the method for firing hexagonal boron nitride described in Patent Documents 1 to 6, the firing temperature was as extremely high as 1,200 to 2,100 ° C. For this reason, the amount of energy consumption is increased, the running cost is expensive, the production efficiency is not good, the price of the product is expensive, and the environmental load cannot be reduced.

  The present invention has been made in view of the above-described conventional circumstances, and provides a hexagonal boron nitride sintered body that can be fired under normal pressure and can be sintered at a low temperature, and a method for producing the same. It is intended to provide.

  In order to solve the above-described conventional problems, the present inventors considered adding boron oxide as a sintering aid for hexagonal boron nitride. This is because boron oxide has a lower melting point than other sintering aids such as feldspar and is expected to lower the sintering temperature. However, contrary to expectations, it was difficult to sinter even if hexagonal boron nitride and boron oxide were mixed and pre-molded and sintered in a non-oxidizing atmosphere. As a result of repeated studies on this sintering failure, the inventors thought that sufficient results could not be obtained for the following reason.

  That is, even when hexagonal boron nitride and boron oxide are mixed, boron oxide is not homogeneously dispersed, or when hexagonal boron nitride particles are crushed during mixing and a fracture surface appears. It was estimated that the wettability was poor because there was no boron oxide on the fracture surface, and this was the reason why sintering was difficult.

  As a result of further research, oxygen is contained not only in the surface of each particle of the hexagonal boron nitride powder used for sintering (that is, one grain of hexagonal nitride). In the case where boron oxide is dispersed in the inside of boron crystal particles), it was found that sintering can be performed at a low temperature without using a sintering aid, and the present invention was completed. It came to do.

  That is, the method for producing a hexagonal boron nitride sintered body of the present invention is a method for producing a boron nitride sintered body in which hexagonal boron nitride is sintered in a non-oxidizing gas atmosphere or in vacuum, Each particle of the hexagonal boron nitride powder to be used contains oxygen not only on the surface but also inside, and the sintering temperature is 600 ° C. or higher and lower than 1100 ° C.

In the method for producing a hexagonal boron nitride sintered body according to the present invention, since sintering is performed in a non-oxidizing gas atmosphere or in a vacuum, oxidation of the hexagonal boron nitride can be prevented. Here, the non-oxidizing gas refers to a gas that does not react with boron nitride at the time of sintering, and includes nitrogen gas in addition to rare gas such as argon gas and helium gas.
Further, since each particle of the hexagonal boron nitride powder as a raw material contains oxygen not only on the surface but also inside (that is, in each particle constituting the hexagonal boron nitride powder, boron oxide Even if no sintering aid is used, each particle of hexagonal boron nitride powder has boron oxide present not only on the surface but also inside the sintered material. Sintering effect is also obtained inside one grain of hexagonal boron nitride. As a result, it is possible to sinter just by pre-molding before sintering without using a hot press. It becomes possible. Further, since the melting point of boron oxide is as low as 480 ° C., hexagonal boron nitride can be sintered at a low temperature of 600 ° C. or higher and lower than 1100 ° C.

As a method for examining whether or not oxygen is contained in the hexagonal boron nitride, it can be confirmed by measuring in the depth direction using an ion milling method in Auger electron spectroscopy. By this method, the inventors manufactured a sintered body using hexagonal boron nitride in which the presence of oxygen was confirmed at a depth of 300 nm in terms of SiO ₂ , thereby increasing the relative density of the sintered body. It has been confirmed that the mechanical strength is excellent. Further preferred is hexagonal boron nitride in which the presence of oxygen is confirmed at a depth of 600 nm, and most preferred is hexagonal boron nitride in which the presence of oxygen is confirmed at a depth of 900 nm.

  The oxygen content of the hexagonal boron nitride is preferably 10% by weight or more.

  In the method for producing a hexagonal boron nitride sintered body of the present invention, as a pre-molding step, hexagonal boron nitride powder containing oxygen not only on the surface but also inside the particles is pressure-molded and pre-molded. As a sintering process, sintering is performed at 600 ° C. or higher and lower than 1100 ° C. in a non-oxidizing gas atmosphere or vacuum.

  The average particle size of the hexagonal boron nitride as the raw material is preferably 10 μm or less. This is because if the average particle size is reduced to 10 μm or less, the sintered body becomes dense and a sintered body having high mechanical strength can be obtained at a lower temperature.

  In the present invention, in the sintering step, the sintering can be performed without particularly applying pressure. For this reason, it is not necessary to prepare a complicated apparatus such as a hot press apparatus, the equipment cost of the manufacturing apparatus can be reduced, and the manufacturing cost can be reduced.

  The inventors have confirmed that a sintered body having a relative density of 60% or more can be obtained by the method for producing a hexagonal boron nitride sintered body of the present invention. Further, from observation of the obtained sintered body with a scanning electron microscope, it has been confirmed that the hexagonal boron nitride crystals in the sintered body have a non-plate-like form.

As described above, according to the present invention, a hexagonal boron nitride sintered body having a high relative density and excellent mechanical strength can be produced at a low temperature of 600 ° C. or more and 1000 ° C. or less. Manufacturing cost and environmental load can be kept low.
In addition, since pressure firing like hot pressing is not required, large-sized products and complex-shaped products can be manufactured easily and efficiently with low cost.

It is the elemental analysis result of the depth direction by the Auger electron spectroscopy measurement of the boron nitride powder particle used in Example 1 and Example 2. 3 is an external appearance photograph of fired bodies of Example 1 and Example 2. 2 is a scanning electron micrograph of a fracture surface of fired bodies of Example 1 and Example 2. FIG. It is a chart of the XRD measurement of the sintered body and raw material of Example 1 and Example 2. It is the elemental analysis result of the depth direction by the Auger electron spectroscopy measurement of the boron nitride powder particle used in the comparative example 1 and the comparative example 2. 3 is an external appearance photograph of fired bodies of Comparative Example 1 and Comparative Example 2. 3 is a scanning electron micrograph of fractured surfaces of fired bodies of Comparative Examples 1 and 2. FIG. 3 is a chart of XRD measurement of fired bodies and raw materials of Comparative Example 1 and Comparative Example 2.

The hexagonal boron nitride used in the present invention is distributed not only on the surface of the hexagonal boron nitride particles but also inside (that is, boron oxide is dispersed inside the hexagonal boron nitride crystal particles). The hexagonal boron nitride is not particularly limited. Specifically, for example, a commercially available product includes hexagonal boron nitride (manufactured by Octom Co., Ltd., trade name: SFM, purity of 40% by weight as BN, and 57% by weight of B ₂ O ₃ ). The presence or absence of oxygen on the surface of the hexagonal boron nitride powder particles can be confirmed by Auger electron spectroscopy. As for the presence of oxygen inside, the inside of the powder particles can be measured by etching the observation surface of the boron nitride particles with an ion etching apparatus.

Further, since hexagonal boron nitride used in the present invention has oxygen not only on the surface but also inside each particle, oxygen is also distributed on the new surface produced by crushing and grinding. For this reason, the wettability is good even if the oxidation treatment is not particularly performed. There is no limitation on the method for performing the crushing / pulverizing treatment, but wet method is preferable. However, when water is used as a wet medium, boron oxide (B ₂ O ₃ ) contained in boron nitride elutes into water, and therefore a medium containing water should be avoided. Preferably, it is carried out in an organic solvent such as ethanol or isopropyl alcohol. The ball mill used for the crushing / pulverizing treatment is preferably a planetary ball mill using an alumina ball of 5 mm or less, but is not limited thereto.

Hereinafter, examples of the present invention will be described in detail.
Example 1
In Example 1, hexagonal boron nitride powder (manufactured by Octom Co., Ltd., trade name: SFM, purity of 40% by weight as BN, 57% by weight of B ₂ O ₃ ) was used as a raw material. This boron nitride powder had an oxygen content of 25% by weight in a quantitative analysis by fluorescent X-ray. Further, elemental analysis of the hexagonal boron nitride in the depth direction was performed by Auger electron spectroscopy. The result is shown in FIG. In FIG. 1, peaks near 200 eV, 410 eV, and 540 eV correspond to peaks of boron (B), nitrogen (N), and oxygen (O), respectively. In addition, the measurement result before ion etching (that is, the most front profile in each of the B, N, and O profiles) is the analysis result on the particle surface. Further, the powder particles are etched and removed from the surface for 10 minutes every 10 minutes under the conditions that the ion etching apparatus is 30 nm / 1 minute in terms of SiO ₂ (the acceleration voltage of the ion gun is 3 kV and the emission current for generating ions is 20 mA). The powder was also analyzed. As a result, it was found that a considerable amount of oxygen atoms existed at least up to 900 nm in terms of SiO ₂ .

<Pre-molding process>
Next, the hexagonal boron nitride powder was filled in a cylindrical mold having a diameter of about 16 mm and a thickness of about 7 mm, and a pre-molded body was produced at 200 MPa using CIP (Cold Isostatic Press).

<Baking process>
This pre-molded body was fired in a nitrogen atmosphere at a heating rate of 5 ° C./min and held at 1000 ° C. for 1 hour. Thereafter, the furnace was cooled to obtain a hexagonal boron nitride sintered body.

(Example 2)
In Example 2, the firing temperature in the firing step was 800 ° C. Others are the same as those in the first embodiment, and a description thereof will be omitted.

-Evaluation-
(Appearance observation and observation by scanning electron microscope)
The appearance photograph of the hexagonal boron nitride fired bodies obtained in Example 1 and Example 2 is shown in FIG. Moreover, the scanning electron micrograph of those fracture surfaces is shown in FIG. 2 and 3, it can be seen that the hexagonal boron nitride sintered bodies of Examples 1 and 2 are homogeneous boron nitride sintered bodies. In the fracture surface shown in FIG. 3, plate-like crystals observed in normal hexagonal boron nitride were not observed. This is presumed that the hexagonal boron nitride used in Example 1 and Example 2 is caused by the presence of boron oxide even inside the particles. And it is thought that the boron oxide which exists even inside a particle | grain shows the role of a sintering auxiliary agent, and enables sintering at low temperature, such as 800 degreeC and 1000 degreeC.

(Relative density and bending strength)
The density of the boron nitride sintered bodies of Example 1 and Example 2 was calculated from the size and weight of the sintered body. The theoretical density was 2.27 g / cm ³ . Furthermore, the three-point bending strength was also measured. The results are shown in Table 1.

  As shown in Table 1, the relative density of both Example 1 and Example 2 exceeded 60%, and the bending strength was as high as 13.5 MPa in Example 1 and 19.0 MPa in Example 2. It was. The reason why such a high-strength boron nitride sintered body was obtained is considered that boron oxide contained in boron nitride used as a raw material functioned as a sintering aid. However, even when a mixture of boron oxide powder and high-purity boron nitride powder was sintered, such a high-strength sintered body could not be obtained. From this, it was found that hexagonal boron nitride powder containing oxygen not only on the surface but also inside each particle must be used for sintering.

(XRD measurement)
XRD of the hexagonal boron nitride sintered bodies of Example 1 and Example 2 was measured. As a result, as shown in FIG. 4, the diffraction peak of hexagonal boron nitride was clearly recognized in both sintered bodies.

(Comparative Example 1)
In Comparative Example 1, “99% purity hexagonal boron nitride powder (UHP manufactured by Showa Denko)” was used. Other conditions were the same as in Example 1 and sintering was performed at 1000 ° C.

Auger electron spectroscopy measurement was performed on the hexagonal boron nitride powder having a purity of 99% by weight used in Comparative Example 1, and elemental analysis in the depth direction was performed. The result is shown in FIG. In FIG. 8, peaks around 184 eV and 405 eV correspond to peaks of boron (B) and nitrogen (N), respectively. In addition, oxygen should appear in the vicinity of 540 eV if present, but was not recognized. The measurement result before ion etching (that is, the frontmost profile in each of the B, N, and O profiles) is the analysis result on the particle surface. Further, the powder particles are etched and removed from the surface for 10 minutes every 10 minutes under the conditions that the ion etching apparatus is 30 nm / 1 minute in terms of SiO ₂ (the acceleration voltage of the ion gun is 3 kV and the emission current for generating ions is 20 mA). The powder was also analyzed. As a result, oxygen atoms were hardly detected from the surface to at least 900 nm in terms of SiO ₂ .

(Comparative Example 2)
In Comparative Example 2, “99% purity hexagonal boron nitride powder (UHP manufactured by Showa Denko)” was used. Other conditions were the same as in Example 2 and sintering was performed at 800 ° C.

-Evaluation-
(Appearance observation and observation by scanning electron microscope)
An appearance photograph of the sintered bodies of Comparative Examples 1 and 2 thus obtained is shown in FIG. Moreover, the scanning electron micrograph of those fracture surfaces is shown in FIG. From these figures, it can be seen that the hexagonal boron nitride sintered bodies of Comparative Example 3 and Comparative Example 4 have a uniform boron nitride sintered body. Further, from the fracture surface of FIG. 10, plate-like crystals observed in normal hexagonal boron nitride were observed.

(Relative density and bending strength)
The density of the boron nitride sintered bodies of Comparative Examples 1 and 2 was calculated from the dimensions and weight of the sintered bodies, and a test by three-point bending was performed. The theoretical density was 2.27 g / cm ³ . The results are shown in Table 2.

  From Table 2, although it was possible to prepare a sintered body having a relative density of 60% or higher at a temperature lower than the melting point of boron nitride, it was found that the bending strength was lower than that of Examples 1 to 4. This is presumably because the high-purity hexagonal boron nitride as the raw material had almost no boron oxide functioning as a sintering aid.

(XRD measurement)
Further, XRD of the hexagonal boron nitride sintered bodies of Comparative Example 1 and Comparative Example 2 was measured. As a result, as shown in FIG. 11, the diffraction peak of hexagonal boron nitride was clearly recognized in both sintered bodies.

  The present invention is not limited to the description of the embodiments of the invention. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

  According to the method of the present invention, a hexagonal boron nitride-based fired body can be produced at a lower temperature than conventional methods, and can be used for members such as a lubricant, an electrical insulating material, and a heat-resistant material.

Claims (7)

  A method for producing a boron nitride sintered body in which hexagonal boron nitride is sintered in a non-oxidizing gas atmosphere or in a vacuum, and each particle of hexagonal boron nitride powder as a raw material is not only a surface. A method for producing a hexagonal boron nitride sintered body characterized in that oxygen is contained therein and the sintering temperature is 600 ° C. or higher and lower than 1100 ° C. The presence of oxygen is confirmed at the depth of at least 300 nm in terms of SiO _{2 in} the measurement of the depth direction using the ion milling method in Auger electron spectroscopic analysis in the particles of the hexagonal boron nitride powder. The method for producing a hexagonal boron nitride sintered body according to claim 1.   The method for producing a hexagonal boron nitride sintered body according to claim 1 or 2, wherein the hexagonal boron nitride has an oxygen content of 10 wt% or more. A pre-molding step for pressure-molding hexagonal boron nitride powder containing oxygen not only in the surface of the particles but also in the interior,
A sintering step of sintering the pre-molded body in a non-oxidizing gas atmosphere or in a vacuum at 600 ° C. or higher and lower than 1100 ° C .;
The method for producing a hexagonal boron nitride sintered body according to any one of claims 1 to 3, wherein:   The method for producing a hexagonal boron nitride sintered body according to claim 3 or 4, wherein the sintering step performs sintering without applying pressure.   A hexagonal boron nitride sintered body manufactured by the manufacturing method according to claim 1 and having a relative density of 60% or more.   A hexagonal boron nitride produced by the production method according to any one of claims 1 to 6, wherein the hexagonal boron nitride crystals in the sintered body have a non-plate-like form. Sintered body.