HEXAGONAL BORON NITRIDE HAVING A REDUCED CONTENT OF WATER SOLUBLE BORON-CONTAINING IMPURITIES, AND PROCESS FOR ITS PRODUCTION

Abstract: A hexagonally crystalline boron nitride containing water-soluble boron containing impurity in such a magntidue that amount of boron to be contained in an extract after boiling at 100 °C of pure water is less than or equal to 100 µg/g-BN. In order to purify the boron nitride, a purification process is carried out according to the steps of preparing fine particles of boron nitride powder; dispersing the boron nitride powder in a disperser and stirred for a given period of time for removing water-soluble boron containing impurity; and drying the boron nitride in an atmosphere, in which vapor pressure is maintained lower than or equal to 10 mmHG and/or the process temperature is maintained lower than or equal to 100 °C.

Description: Field of the Invention The present invention relates generally to a novel and useful high purity boron nitride and production process thereof. More specifically, the invention relates to a hexagonally crystalline boron nitride which is essentially free of water-soluble impurities, particularly water-soluble boron containing impurities. More particularly, the invention relates to a hexagonally crystalline boron nitride which can be utilized in various fields, such as ceramic industries, cosmetics industries, electric industries, electronic industries, pharmaceutic industries and so forth. Description of the Background Art Boron nitride is appreciated as a useful material in various industrial fields because of its good heat conductivity, electric insulation ability, chemical stability, lubrication ability, lubricity, heat resistance and so forth. Hexagonally crystalline boron nitride generally has a lamellar, crystallographically hexagonal structure like that of graphite. In one application, hexagonally crystalline boron nitride powder is used as an additive in synthetic resins, lubricants and so forth or in solid form as jigs, electric insulators, molds and so forth. The demand for high purity hexagonally crystalline boron nitride has increased in recent years. For example, Japanese Patent (unexamined) Publication (Kokai) Showa 54-163398 discloses a synthetic rubber sheet having high heat conductivity and electric insulation ability and production thereof. Boron nitride powder is utilized as an additive for preparing the synthetic rubber. In the disclosure of the above-identified publication, it is suggested that the surface of the particles of the boron nitride powder utilized in preparation of the synthetic rubber should be smooth for obtaining better adherence on the synthetic rubber and to provide high heat conductivity and electric insulation ability. In order to provide the latter property, high purity crystalline boron nitride is required. On the other hand, Japanese Patent (examined) Publication 58-181708 discloses a process for preparation of a polygonally crystalline boron nitride by high pressure inversion utilizing boron nitride as starting material. A high yield of polygonally crystalline boron nitride cannot be obtained when the boron nitride starting material contains oxygen. Furthermore, when the oxygen is contained in the boron nitride, strength and optical transparency of the obtained polygonally crystalline boron nitride is degraded. In order to avoid these disadvantages, the disclosure of the above-mentioned publication suggests heating the boron nitride at about 2000 to 2200 °C to remove impurities including oxygen and thus to obtain high purity boron nitride. There are three mutually distinct processes for purifying boron nitride in the prior art. The first process has been proposed in Japanese Patent Publication (Kokai) Showa 58-60603. In the proposed process, boron nitride is subjected to high temperature to vaporize and/or burn away impurities. The second process, proposed in the Japanese Patent Publication (Kokai) Showa 59-107907 is to wash out impurities with an alkaline solution. In the third process, the high purity boron nitride is obtained by a chemical reaction between boron trichloride and ammonia according to the following equation : BCl₃ + NH₃ --------> BN + 3 HCl In the aforementioned first process, impurities such as nitric oxide and boron oxide can be vaporized and removed. When the amount of the boron nitride to be treated is small, the vaporized impurities can be removed from the atmosphere by thermal convection and/or diffusion. However, when a relatively large amount of the boron nitride is to be treated creating a relatively large amount of vaporized decomposed impurities, boron oxide tends to be left in the atmosphere and is deposited on the surface of the crystalline boron nitride or accumulates between the boron nitride crystallites during the cooling step. Therefore, in processing large amounts of boron nitride, it is not practically possible to satisfactorily remove the impurities in the crystalline boron nitride. Furthermore, since the first process requires high temperatures for removing impurities, it is not practically applicable to the manufacture of high purity boron nitride. Furthermore, even when a small amount of boron nitride is treated to remove oxygen, carbon, boron oxide and other impurities, a water-soluble boron compound remains on the surface of the boron nitride crystallites. In the second process, ions in the alkaline solution are coupled with surface ions of the boron nitride and are retained in the resultant compound. These coupled ions cannot however be removed by washing with water. The resulting high alkalinity of the resultant boron nitride spoils its utility as a material for electronic products. Furthermore, water-soluble boron compounds can not be removed by washing with alkaline solutions. The presence of water-soluble boron or boron compounds has been proven by the inventor by experimentation. Thus, in the aforementioned Japanese Patent First Publication 59-107907, the boron nitride was boiled in water at 100 °C and 200 µg of boron compound was obtained per gram of boron nitride. The third process may well provide high purity boron nitride. However, this process does not provide good crystallization of hexagonally crystalline boron nitride. Furthermore, the boron nitride obtained from this third process has relatively poor electric insulation ability and poor heat conductivity and contains a relatively large amount of water-soluble boron compound. The water-soluble boron containing component in the boron nitride spoils the useful properties of the boron nitride and tends to reduce the number of fields in which it can be used. Therefore, in order to enable the boron nitride to be more widely used, it is preferable to minimize the water-soluble boron content to obtain a more highly purified boron nitride. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a high purity hexagonally crystalline boron nitride. Another object of the invention is to provide a boron nitride in which the content of water-soluble boron and/or boron-containing compounds is substantially reduced. A further object of the invention is to provide a process for obtaining the high purity hexagonal crystal boron nitride set forth above. In order to obtain such a high purity of boron nitride in which the content of water-soluble boron and/or boron-containing compounds is minimized, the hexagonal crystal boron nitride powder is washed with water having a surface active agent and/or water-soluble organic solvent dispersing agent. After washing, the resultant boron nitride is dried in an atmosphere of vapor pressure equal to or less than 1.33 x 10³ Pa (10 mmHg) and/or of temperature lower than or equal to 100 °C. Generally, the water-soluble boron is an unstable boron oxide. These impurities degrade adherence of the boron nitride to rubber when the boron nitride is used as an additive for producing silicon rubber. Furthermore, such impurities spoil heat resistance ability and electric insulation ability. Therefore, when boron nitride, which contains a substantially reduced amount of water-soluble, boron-containing impurities, is used as an additive, good adherence of the boron nitride to the rubber can be obtained. Furthermore, the silicon rubber, produced utilizing the boron nitride with reduced amount of water-soluble, boron containing impurities, exhibits satisfactorily high heat conductivity and electric insulation ability. Also according to the present invention a high yield can be obtained when it is used as starting material for preparation of cubically crystalline boron nitride. The water-soluble, boron-containing impurities in the boron nitride were measured by the following process. 2.5 g of boron nitride was dispersed in 10 cm³ of ethyl alcohol. 40 cm³ of pure water was added to form a slurry. The slurry was put into a flask with dry distillator. Elutriation of the water-soluble components in the boron nitride was then performed at 100 °C for 1 hour. The resultant solution was filtered. Ethyl alcohol in the solution is removed by evaporation. Pure water was then added to form 50 cc of solution. The total amount of boron in the solution was determined by the crucuminic acid test disclosed by A. Tolk and W. A. Tap in Talanta. Vol 16, No. 111, published in 1969. DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below in detail in terms of the preferred embodiment of the invention, which, however, should not be taken to limit the invention but should be understood to be for explanation and understanding only. FIRST EMBODIMENT According to the invention, the starting materials are an oxygen-containing boron compound and a nitrogen compound. Among the various oxygen-containing compounds, boric acid is preferred as the boron source in view of reducing the impurities left after reaction. Alternatively, dehydrated boric acid, ammonium borate and so forth can be the boron source. The nitrogen compound as the nitrogen source is selected from a group of compounds that are readily decomposable and removable at high temperatures, such as urea, melamine, dicyanodiamide, cyanuric acid, ammonium chloride and so forth. In general, the mixing ratio of these starting compounds in the powder mixture should be such that the molar ratio of nitrogen to boron is at least 1 to 1. As is well known, the crystalline structure of boron nitride is variable depending upon the conditions of synthesis. When boron nitride is synthesized by heating a mixture of boric acid and urea in an atmosphere of nitrogen gas, boron nitride is formed at a reaction temperature higher than or equal to 600 °C. When the reaction temperature is held lower than approximately 800 °C, the crystalline structure of the boron nitride formed is still not completely hexagonal but is referred to as "turbostratic" in crystallography, in which the adjacent layers are disposed at random positions relative to each other. When reaction temperatures exceed 800 °C, transition of the turbostratic to the hexagonal structure proceeds gradually. Simultaneously with transition of the structure, the boron nitride crystallites grow. During this transition, impurities, such as boron oxide, ammonium borate, oxygen, carbon and so forth, are removed to increase purity of the boron nitride. When the reaction temperature reaches about 1600 °C, the diameter of the primary particles of crystalline boron nitride reach about 1 µm or greater. When the reaction temperature reaches about 1800 °C, the purity of the boron nitride becomes 99 % or higher. At this time, the size of the crystalline boron nitride particles grows to a diameter of about 2 µm to 6 µm. The crystalline structure of boron nitride can be classified according to a method of measurement recommended by the 117th Committee of Japan Society for the Promotion of Science. The crystallite size can be expressed in terms of its average thickness (Lc) in the direction of the c-axis, and of its average diameter (La) along the a-axis. Higher accuracy may be obtained when crystallite size is determined in terms of Lc, because the peak for (002) has the highest intensity in the X-ray diffraction diagram. The crystalline structure of the boron nitride can be evaluated by the value of Lc as a measure. When the Lc is approximately 100 Angströms, the crystalline structure of the boron nitride is turbostratic. When the Lc is in a range of about 100 Angströms to 400 Angströms, the crystalline structure becomes partially three dimensional. When the Lc is greater than about 400 Angströms, the crystalline structure is completely three dimensional. The lubrication ability of the boron nitride is low if the Lc is less than 400 Angströms. Relative surface area is measured by nitrogen adsorption tests. The structurally completely graphitic boron nitride had a relative surface area of 4 m²/g to 10 m²/g and the boron nitride of quasi-graphitic structure had a relative surface area of 25 m²/g to 100 m²/g. Between the graphitic crystalline boron nitride and quasi-graphitic crystalline boron nitride, there is a substantial difference in hygroscopicity. For instance, the quasi-graphitic boron nitride powder was stored in a polyethylene container for a month. After one month, ammonia vapor was generated owing to hydrolysis of the quasi-graphitic boron nitride. Under the same conditions, no ammonia vapor was generated by the graphitic crystalline boron nitride. From this, it has been proven that the graphitic crystalline boron nitride has substantially higher hydrolysis resistance then that of the quasi-graphitic crystalline boron nitride. Such high hydrolysis resistance is believed to occur for the following reasons : 1) Since the graphitic or hexagonally crystalline boron nitride has a substantially smaller relative surface area than the quasi-graphitic crystalline boron nitride there is less reacting surface area ; 2) boron on the surface of the graphitic crystalline boron nitride is firmly coupled with the internal structure and has a high coupling energy so that erosion by vapor may not easily occur ; 3) since a very small amount of hydrophilic substances, such as CO, H are contained in the high purity hexagonally crystalline boron nitride, it has low hydroscopicity. In view of the above-mentioned properties, the hexagonally crystalline boron nitride is preferred as a starting material for preparing the boron nitride of the present invention. The hexagonally crystalline boron nitride selected as the starting material, will be hereafter referred to as "pre-process boron nitride" and the boron nitride of the present invention will be hereafter referred to as "treated boron nitride". Before the washing treatment stage the pre-process boron nitride has to be crushed and classified. The pre-process boron nitride particles greater than or equal to 50 µm in diameter are removed. Preferably, pre-process boron nitride particles that are as small as primary particles are used. In order to effectively wash out water-soluble boron-containing impurities, the boron nitride has to be sufficiently wettable and dispersable. On the other hand, the wash liquid has to be selected to have satisfactory washing characteristics. In general, crystalline boron nitride has poor wettability and dispersion characteristics that are similar to those of graphite. Therefore, in order to provide sufficient wettability and dispersion ability, treatment with a surface active agent and/or dispersion agent is necessary. Experiments have been performed using various surface agents and reagents. As a result of the experiments, it has been found that anionic, nonionic, cationic surface active agents or water-soluble organic solvents having a Hydrophile-Lipophile Balance (HLB) value of 10 to 16 are useful. The surface active agent used in the present invention can be selected from for example : polyoxyethylene(5)sorbitan monostearate, polyoxyethylene(4)sorbitan trioleate, polyoxyethylene glycol 400 monooleate, triethanolamine oleate, polyethylene glycol 400 monooleate, polyoxyethylene(9)nonylphenol, polyethylene glycol 400 monolaurate, polyoxyethylene(4) sorbitan monolaurate. The preferred concentration of the surface active agent is in the range of 0.001 wt% to 1 wt%. When the concentration of the surface active agent exceeds 1 wt%, higher dispersion efficiency can not be obtained. Therefore, from the economical viewpoint, the maximum concentration should be limited at 1 wt%. On the other hand, when the concentration of the surface active agent is less than 0.001 wt%, the desired dispersion effects cannot be obtained. On the other hand, the water-soluble organic solvent may be selected from acetone, acetylacetone, ethyl amine, acetaldehyde, phenol, alcohols, such as methyl alcohol, ethyl alcohol, glycol, glycerin, and so forth. Preferably, methyl alcohol or ethyl alcohol is selected in view of the cost and availability. For dispersing the boron nitride in the solvent, first the boron nitride is dispersed in the surface active agent and/or the water-soluble organic solvent to form highly concentrated slurry. Water is added to the slurry. This process requires less surface active agent and/or water-soluble organic solvent and can produce a satisfactorily high degree of dispersion. Good dispersion can also be obtained by utilizing a mixture of the surface active agent and the water-soluble organic solvent. The treatement temperature is in the range of 10 °C to 100 °C. When the disperser, i.e. the surface active agent and/or the water-soluble organic solvent, has a boiling point lower than 100 °C, the treatment temperature is preferably lower than the boiling point of the disperser. On the other hand, in view of the efficiency with which the water-soluble boron and boron-containing compounds can be washed out, the treatment temperature has to be higher than or equal to 10 °C. As will be appreciated, slurry containing a lower concentration of dispersed boron nitride will exhibit higher washing performance. In order to prepare lower concentration boron nitride slurry, a larger amount of surface active agent and/or water-soluble organic agent is required. This is more expensive owing to the amount of solvent. Therefore, in view of the cost-vs-performance, the maximum allowable boron nitride concentration in the slurry is 50 % by weight. When the boron nitride concentration exceeds 50 wt% in the slurry, the viscosity of the slurry becomes too high resulting in difficulty in stirring. In addition, such slurry has excessive friction between the stirring impeller and the wall of the container causing material of the impeller and the wall to melt into the slurry as impurities. On the other hand, in view of the cost and drying efficiency, the minimum concentration of the dispersed boron nitride in the slurry should be 2.5 % by weight. The slurry may be stirred at a normal rate, high speed stir, dispersion utilizing shearing force and so forth. The manner of stirring can be selected depending on the type of washing device in use. Furthermore, washing of the boron nitride can be performed by continuous processing or batch processing. The timing of termination of the washing operation may be determined so that the content of the water-soluble boron and boron compounds in the resultant boron nitride is at the desired value. After completing the washing operation, the washed boron nitride is obtained by dewatering the slurry e.g. by centrifugal dewatering, vacuum dewatering, pressure dewatering or by settling out. The wet boron nitride obtained is then dried. It is essential not to induce hydrolysis of the boron nitride during the drying process. As will be appreciated, when hydrolysis occurs in the presence of H₂O, the boron nitride is converted into B₂O₃ and NH₃ according to the following equation : 2 BN + 3 H₂O --------> B₂O₃ + 2 NH₃ Hydrolysis of boron nitride may occur when the following conditions are satisfied : a) when the vapor pressure exceeds 1.33 x 10³ Pa (10 mmHg) ; and b) when the temperature of the drying atmosphere is higher than 100 °C. If the vapor pressure is lower that or equal to 1.33 x 10³ Pa (10mmHg) hydrolysis and oxidation of boron nitride, according to the following equation : 2 BN + (3/2)O₂ --------> B₂O₃ + N₂ will occur at an atmospheric temperature of 500 °C. Therefore, by maintaining a vapor pressure lower than or equal to 1.33 x 10³ Pa (10 mmHg), the drying of the boron nitride can be done at a temperature lower than or equal to 500 °C. On the other hand, as long as the atmospheric temperature is maintained at lower than 100 °C, hydrolysis will not occur at any vapor pressure. The atmospheric temperature inducing hydrolysis is much lower than that inducing oxidation. In order to determine whether liquid water or H₂O vapor caused a greater amount of hydrolysis, two experiments were performed. In one experiment, boron nitride powder was immersed in 100 °C water for 24 hours. In the other experiment, the wet boron nitride obtained from the aforementioned washing process was dried at 100 °C. The boron content of the water in the former experiment was about 10 µg/g-BN, which was less than one-tenth of that in the extract derived from the dried boron nitride. Therefore it will be understood that H₂O vapor may have a stronger affect on hydrolysis than liquid H₂O. In order to reduce the effects of the H₂O vapor, it is preferable to dry the wet boron nitride at a temperature lower than or equal to 100 °C. Such relatively low temperature drying can be performed by a freeze-drying process and/or air-drying process. It is preferable when drying the wet boron nitride to maintain a vapor pressure lower than or equal to 1.33 x 10³ Pa (10 mmHg) and to maintain an atmospheric temperature lower than or equal to 100 °C. EXAMPLES 1 TO 6 The hexagonally crystalline boron nitride to be purified was crushed by means of a jet-mill. The crushed particles of boron nitride were classified by means of an airflow classifier to obtain a sample of boron nitride particles of less than 25 µm in diameter, having 82 nm (820 Angströms) of Lc, 99 % purity, and 4.5 m²/g relative surface area. The obtained sample was dispersed in 10 l anionic surface active agent in ethyl alcohol as disperser which was diluted by an ion-exchange liquid to a concentration of 0.5 wt%. The boron nitride was boiled with the disperser and washed and stirred therein for about 1 hour. After washing, the slurry was subjected to vacuum filtering to obtain a solid residue. Washing and filtering were repeated 5 times . The last three cycles of the washing operation were performed with pure ion-exchange liquid in order to remove the surface active agent. After five cycles of washing and filtration, a residue was developed on a stainless steel tray. The tray containing the washed boron nitride was put into a vacuum drier and dried under various conditions as shown in the appended table 1. In order to check hydrolysis characteristics, the dried boron nitride powder was put into boiling water. After boiling the resultant boron nitride, the boron content of the water was measured. The amounts of boron in the water per 1 g of boron nitride are shown in the table 1. EXAMPLES 7 TO 11 Boron nitride particles of less than 25 µm in diameter, having 82 nm (820 Angströms] of Lc, 99 % purity, and 4.5 m²/g relative surface area, i.e. identical to those used in the foregoing examples 1 through 6 were used. The particles were washed utilizing ion-exchange water and surface active agents A to E. The surface active agents A to E were prepared by polyoxyethylene 2 heptyl-undecylethyl system materials with mutually different degrees of polymerization. The samples were dispersed in solutions of the surface active agents and the ion-exchange water. Dispersion of the boron nitride powder was performed in the same manner as that in the examples 1 through 6. When ethyl alcohol was used, 1 kg of sample boron nitride powder was at first dispersed in 2 l of ethyl alcohol. After forming the dispersion, 8 l of ion-exchange water was added. When the surface active agent was used with ethyl alcohol, the sample was at first dispersed in ethyl alcohol. The solution of surface active agent and ion-exchange water was added thereafter. The washing and filtering of the boron nitride sample were conducted in identical manner to that of the examples 1 to 6. The washed sample was placed on a stainless steel tray and put in a vacuum drier. The vacuum pressure was set at 1.33 Pa (10⁻² Torr.). The drying operation continued for 12 hours. The amounts of boron obtained in the water per 1 g of boron nitride are shown in table 2. SECOND EMBODIMENT Highly crystalline hexagonal boron nitride can be further purified by the second embodiment of the purification process according to the invention. In this embodiment, the boron nitride material is firstly dispersed in a disperser. A water-soluble organic solvent is used as the disperser. In the dispersing process, the particles of the pre-process boron nitride are crushed into fine particles of equivalent size to the primary particles, i.e. particles of about 1 µm diameter. The resultant wet boron nitride powder is dried. The dried powder is heat treated for further removal of oxygen and carbon. Like the foregoing first embodiment, the preferred water-soluble organic solvents are acetone , acetylacetone, ethyl amine, acetaldhyde, phenol, alcohols, such as methyl alcohol, ethyl alcohol, glycol, glycerin, and so forth. Among the above-listed organic solvants methyl alcohol or ethyl alcohol are preferred in view of the cost and availability. Dispersion of the boron nitride powder in the disperser may be assisted by adding the surface active agents. As the surface active agent, anionic, nonionic, cationic surface active agents or water-soluble organic solvents having a Hydrophile-Lipophile Balance (HLB) value of 10 to 16 are preferred as the disperser for use in the second embodiment of the boron nitride purification process. More particularly, the surface active agent used in the present invention can be selected from for example : polyoxyethylene(5)sorbitan monostearate, polyoxyethylene(4)sorbitan trioleate, polyoxyethylene glycol 400 monooleate, triethanolamine oleate, polyethylene glycol 400 monooleate, polyoxyethylene(9)nonylphenol, polyethylene glycol 400 monolaurate. polyoxyethylene(4)sorbitan monolaurate. The preferred concentration of the surface active agent is in a range of 0.001 wt% to 1 wt%. When the concentration of the surface active agent exceeds 1 wt%, higher dispersion efficiency can not be obtained. Therefore, from the economical viewpoint, the maximum concentration is limited to 1 wt%. On the other hand, when the concentration of the surface active agent is less than 0.001 wt%, the desired dispersion effect connot be obtained. The washing of the boron nitride for removal of the boron containing impurities is performed in identical manner and conditions as set forth with respect to the foregoing first embodiment. The dried boron nitride from which a substantial amount of boron-containing impurity is removed during the washing process, is further treated by the heat treatment process as set forth above. This heat treatment process is performed in order to further remove water-soluble boron containing impurity which adheres on the outer surface of the boron nitride particles in the drying process. Additionally, in the heat treatment process, sodium, alkaline substances on the surface of the boron nitride crystallite can be removed. Furthermore, in the heat treatment process, oxygen and carbon are contained in the boron nitride crystallite in a form of cyano group compound. Therefore, it is essential to perform the heat treatment at a temperature sufficiently high to remove the oxygen and carbon. On the other hand, during the heat treatment, the boron nitride should not be oxidized. For this, the heat treatment has to be performed in non-oxidizing atmosphere. In order to prevent the boron nitride from being oxidized or from causing decomposition, and in order to effectively remove oxygen by reduction, N₂, NH₃ and H₂ atmospheres are preferred. In addition, oxidation and decomposition of the boron nitride may also be prevented by performing the heat treatment in a vacuum. In order to determine the optimum temperature for heat treatment, heat treatment experiments were performed at different temperatures. The composition was measured by way of infrared absorption. At the treatment temperature of 1600 °C, the amounts of the B-N-O and the CN groups are reduced. The upper limit of the treatment temperature was the pyrolysis temperature, i.e. 3000°C, in theory. However, since the heat treatment is normally performed in a graphite crucible, carbon is usually generated at treatment temperatures higher than or equal to 2000 °C. Therefore, where a graphite crucible is used for the heat treatment, the upper limit of treatment temperature is lower than 2200 °C. When a high purity boron nitride crucible is used in combination with a tungsten heater, the upper limit of the treatment temperature can be raised to 2500 °C. The process time is variable depending upon the treatment temperature. As will be appreciated, the process time may be shortened by using higher treatment temperatures. Furthermore, the process time may also be variable depending upon the amount of boron nitride in the crucible. According to this embodiment, the purified hexagonally crystalline boron nitride will contain less than or equal to 1000 ppm of carbon, less than or equal to 1000 ppm oxygen and less than or equal to 100 ppm water-soluble boron compound. Such a high purity hexagonally crystalline boron nitride is useful as a material for cosmetics and pharmaceutical additives. Also, such a boron nitride can be used as filler material for sintering Si₃N₄, AlN and SiC. When the boron nitride produced by the second embodiment of the purification process according to the present invention is used as the filler material, much less oxygen and carbon are generated during the sintering operation. Therefore, a high quality sintered body can be obtained. EXAMPLES 12 TO 16 AND COMPARATIVE EXAMPLES 6 AND 7 In a manner similar to the foregoing examples of the first embodiment, the high purity boron nitride powder was crushed by means of a jet-mill. The crushed particles of boron nitride were classified by means of an airflow classifier to obtain a sample of boron nitride particles of 25 µm in diameter, having 820 Angströms of Lc, 99 % purity, and 4.5 m²/g relative surface area. The classified boron nitride powder was mixed with ethanol in a ball mill for 30 hours at a mixing ratio of 1 to 5. After this, the ethanol was removed by drying in a drier. Then, the residual boron nitride was crushed into a fine powder. The fine boron nitride powder 5 cm in depth was placed in a high purity nitride crucible at a bulk density of 0.18 g/cm³. Heat treatment was performed at different temperatures and in various atmospheres. The carbon, oxygen and water-soluble boron content of the resultants are shown in the appended table 3. EXAMPLES 17 TO 20 The same sample is washed in a solvent in a ball mill for 30 hours at a 1 to 5 mixing ratio. After washing, the sample was vacuum dried. Then, the boron nitride was crushed into a fine powder. The fine boron nitride powder was filled to a depth of 5 cm into a high purity nitride crucible at a bulk density of 0.18 g/cm³. The boron nitride powder in the crucible was subjected to heat treatment at 2000 °C for 2 hours. The result of this experiment is shown in the appended table 4. As can be seen from the above, high purity and useful hexagonally crystalline boron nitride can be obtained by the process according to the invention.

Claims: A hexagonally crystalline boron nitride having an average particle size of approximately 0.5 to 20 µm, containing impurity in such a magnitude that the amount of boron to be contained in an extract after boiling at 100 °C for one hour of a mixture of water and 20 wt% ethyl alcohol is less than or equal to 100 µg/g-BN. A hexagonally crystalline boron nitride as set forth in claim 1 which contains less than or equal to 1000 ppm of oxygen. A hexagonally crystalline boron nitride as set forth in claim 1 which contains less than or equal to 1000 ppm of carbon. A hexagonally crystalline boron nitride as set forth in claim 1, which contains less than or equal to 1000 ppm of oxygen and less than or equal to 1000 ppm of carbon. A hexagonally crystalline boron nitride as set forth in claim 1, which is in the form of a fine particle powder. A hexagonally crystalline boron nitride as set forth in claim 1, has a particle size equivalent to primary particles of boron nitride. A method for purifying a hexagonally crystalline boron nitride, comprising the steps of : preparing fine particles of boron nitride powder, dispersing said boron nitride powder in a disperser which is an aqueous solution of a surface active agent having a Hydrophile-Lipophile Balance (HLB) value of 10 to 16, in the range of 0.001 to 1 wt% and/or a water soluble organic solvent and stirring for a given period of time at a temperature in the range of 10 °C to 100 °C for removing water-soluble boron-containing impurity, and drying the boron nitride in an atmosphere, in which vapor pressure is maintained lower than or equal to 1.33 x 10³ Pa (10 mmHg) and/or the process temperature is maintained lower than or equal to 100°C A method for purifying the hexagonally crystalline boron nitride as set forth in claim 7, wherein the purified boron nitride contains water-soluble boron containing impurity in such a magnitude that the amount of boron contained in an extract after boiling at 100 °C with pure water is less than or equal to 100 µg/g-BN. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 8, in which said disperser is a solution of a surface active agent, organic solvent or their mixtures. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 9, wherein said organic solvent is selected from acetone, acetylacetone, ethylamine, acetaldehyde, phenol and alcohols. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 9, wherein said surface active agent is selected among anionic surface agents, nonionic surface agents and cationic surface agents. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 11, wherein said surface active agent is selected from polyoxyethylene(5)sorbitan monostearate, polyoxyethylene(4)sorbitan trioleate, polyoxyethylene glycol 400 monooleate, triethanolamine oleate, polyethylene glycol 400 monooleate, polyoxyethylene(9)nonylphenol, polyethylene glycol 400 monolaurate, and polyoxyethylene(4) sorbitan monolaurate. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 11, wherein the preferred concentration of the surface active agent is in a range of 0.001 wt% to 1 wt%. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 8, in which said disperser is an organic solvent selected from acetone, acetylacetone, ethylamine, acetaldehyde, phenol and alcohols. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 9, wherein said disperser is a surface active agent selected from polyoxyethylene(5)sorbitan monostearate, polyoxyethylene(4)sorbitan trioleate, polyoxyethylene glycol 400 monooleate, triethanolamine oleate, polyethylene glycol 400 monooleate, polyoxyethylene(9)nonylphenol, polyethylene gylcol 400 monolaurate, and polyoxyethylene(4) sorbitan monolaurate. A method for purifying the boron nitride as set forth in claim 14, which further comprises a step of performing heat treatment on the boron nitride, after said drying, for removing water-soluble boron containing impurity on the surface of boron nitride crystallite. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 16, wherein said heat treatment is performed at a temperature higher than or equal to 1600 °C. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 17, in which said hexagonally crystalline boron nitride obtained by said heat treatment contains less than or equal to 1000 ppm of oxygen. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 17, in which said hexagonally crystalline boron nitride obtained by said heat treatment contains less than or equal to 1000 ppm of carbon. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 17, in which said hexagonally crystalline boron nitride is in a form of fine particle powder. A method for purifying the hexagonally crystalline boron nitride as set forth in claim 17, in which said hexagonally crystalline boron nitride has an average particle size of approximately 0.5 to 20 µm.

Inventors: KOSHIDA, TAKAHISA C/O TECHNICAL RESEARCH DIVISION, OGASAWARA, TAKESHI C/O TOKYO HEAD OFFICE, UCHIMURA, RYOJI C/O TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1987
Main classification: C01B21
IPC codes: C01B21