CN1195672C - Turbostratic boron nitride powder and method for producing same - Google Patents

Abstract

To inexpensively produce high-purity boron nitride with a turbostratic structure that attains a high bulk density, when it is molded, shows excellent sinterability and gives sintered products of high strength in an industrially large scale by crystallizing a substantially amorphous boron nitride in the presence of a molten alkali borate in a non-oxidative atmosphere. A mixture, preferably containing a boron source (for example, boric anhydride and borax) and a nitrogen source (for example, urea) is heated under normal pressure condition or pressurized condition (in a tightly closed or almost closed reaction vessel), for example, the temperature is raised up from 250 deg.C to 900 deg.C in a multi-step manner to synthesize substantialy amorphous boron nitride. This amorphous boron nitride is crystallized by heating in the presence of a molten alkali borate, for example, borax as a flux agent in a non-oxidative atmosphere (including tightly closed or almost closed atmosphere in a vessel) at temperature, preferably lower than 1,500 deg.C, particularly at 1,200-1,400 deg.C for a prescribed time until the amorphous boron nitride crystallizes and attains substantially crystalline turbostratic structure.

Description

Turbostratic boron nitride powder, method for producing the same, and powder composition thereof

The present invention relates to a boron nitride powder having good sinterability, particularly high purity, and capable of mass production at low cost, particularly a boron nitride powder having a turbostratic structure, and a method for producing the same (capable of mass production).

Boron nitride is known to date as hexagonal boron nitride (hereinafter referred to as h-BN), rhombohedral boron nitride (hereinafter referred to as r-BN), amorphous boron nitride (hereinafter referred to as a-BN), turbostratic boron nitride (hereinafter referred to as t-BN), and high-pressure phase sphalerite-type c-BN and wurtzite-type w-BN. It is known that a-BN is formed when boron nitride is synthesized at a relatively low temperature of, for example, 900 ℃ or less, and that if boron nitride is synthesized at a high temperature or a-BN synthesized at a low temperature is subjected to heat treatment at a high temperature, the a-BN is crystallized and converted into h-BN of a stable hexagonal system. The primary particles of the h-BN powder are generally hexagonal plate-shaped.

In the powder X-ray diffraction pattern of the h-BN powder (as shown in FIG. 1), there are marked diffraction lines of [002], [100], [101], [102]and [004]. On the contrary, the X-ray diffraction pattern of a-BN boron nitride powder is generally as shown in FIG. 8, and the diffraction lines are widened (large half height width) at the positions corresponding to the [002]diffraction line, [100]diffraction line and [101]diffraction line of the powder X-ray diffraction pattern of h-BN powder when the [100]diffraction line and the [101]diffraction line are combined (for convenience of explanation, the diffraction line of the powder X-ray diffraction of boron nitride crystal having a crystal structure other than h-BN is referred to by the name of the diffraction line of the powder X-ray diffraction of h-BN powder). h-BN has a crystal structure in which hexagonal network layers of boron and nitrogen are laminated in an aa 'aa'. cndot.pattern, and if the lamination form of the hexagonal network layers composed of boron and nitrogen is changed, boron nitride crystals which are not hexagonal are formed, and boron nitride having no regularity in the lamination form (i.e., the relationship between the layers of the laminate) is generally called turbostratic boron nitride.

Broadly, a-BN is also a boron nitride having a turbostratic structure, and for example, as described in Vol.105(1989) No.2, p201 of the society of resources and materials, there is a classification method of classifying a-BN into a turbostratic boron nitride which should be referred to as amorphous boron nitride, and in the present invention, boron nitride having a relatively high crystallinity and a powder X-ray diffraction pattern thereof, and diffraction lines including [004]diffraction lines are referred to as t-BN. (in the present invention, boron nitride in which the full width at half maximum of 2 θ of the [004]diffraction line is not more than a predetermined value and the layered form (pattern) is not regular is referred to as crystalline t-BN, as described below.)

Boron nitride has similar properties to graphite, except that it is a good electrical insulator, since it has a layered structure similar to graphite. For example, the interlayer bond is weak, and the graphite is easily cleaved into a scale-like shape, has solid lubricity, is stable up to a high temperature in a non-oxidizing atmosphere, is difficult to sinter, is easy to machine asintered body, and has an oxidation temperature higher than that of graphite by about 500 ℃ (about 1000 ℃). Boron nitride has some useful properties as a material, and if boron nitride powder with good sinterability or high purity can be produced at low cost and the sintered body can be provided at low cost, many new applications that could not have been achieved for economic reasons have been realized.

The following methods are currently known for producing boron nitride:

(1) heating a mixture of borax and urea in an ammonia atmosphere (Japanese patent publication No. 38-1610);

(2) heating boric acid or ammonium borate together with a nitrogen-containing compound (urea, ammonia, melamine, dicyandiamide, etc.) (Japanese patent publication No. 48-14559, Japanese patent publication No. 5-47483, Japanese patent laid-open No. 7-172806, J.am.chem.Soc.vol.84 p4619-4622, 1963);

(3) heating boron powder in an atmosphere of nitrogen and ammonia (Japanese examined patent publication No. Hei 7-53610);

(4) make BCl₃And NH₃Synthesizing boron nitride by gas phase reaction under reduced pressure (Japanese patent laid-open No. 2-296706);

(5) thermally decomposing borazine or a borazine derivative (Japanese examined patent publication (Kokoku) No. 4-4966);

(6) J.Am.chem.Soc.vol.84 p4619-4622(1963) reports that boron nitride powder synthesized by heating urea and boric acid in ammonia at 500-950 ℃ has a turbostratic structure similar to that of turbostratic carbon.

In the development process, the invention encounters the following problems in the prior art:

the synthetic powder produced by the method (1) is not suitable for direct use in applications requiring electrical insulation because the product of the reaction at low temperatures is a-BN and the product of the reaction at high temperatures is h-BN, and the synthetic powder produced by the reaction contains a considerable amount of Na. When the synthesis is carried out at a low temperature of less than 1000 ℃ by the method (2), a-BN powder is obtained, and when the synthesis is carried out at a high temperature, boric acid promotes the crystallization of h-BN to form h-BN. According to the aspect of the present invention, although the synthesized a-BN powder does not contain impurities other than boric acid, h-BN is easily produced due to the presence of boric acid.

By adopting the methods (3) and (5), the boron powder and borazine, borazine derivatives as raw materials are expensive, resulting in an increase in the cost of the finally produced boron nitride. (4) The method (2) has problems of low productivity, deterioration of the working environment due to corrosive and irritating odor of hydrochloric acid gas and the like produced by the reaction, and increase of the equipment cost due to addition of a device for absorbing and removing the hydrochloric acid gas. In addition, none of these reports describes obtaining a t-BN powder having good crystallinity.

On the other hand, according to the method (6), from the viewpoint of the present invention, the boron nitride shows diffraction lines [002]and [10]having broad peaks (large half width) at the positions corresponding to the [002]diffraction line and the positions corresponding to the [100]diffraction line and the [101]diffraction line in the powder X-ray diffraction pattern, and the amorphous boron nitride which is not seen in the [004]diffraction line is merely described (fig. 1A). Further, it is reported that the transition from the turbostratic structure to the hexagonal system starts at about 1450 ℃ at a high temperature, the transition is completed at 1850 ℃ and partial three-dimensional regularization is generated as the crystallization proceeds (fig. 1B, C), and finally, complete three-dimensional regularization of the hexagonal system is achieved. In this case, the boron oxide remains inevitably.

As described above, none of the methods known so far is purposeful and capable of mass-producing turbostratic boron nitride (particularly, crystalline turbostratic boron nitride which is subjected to crystallization). Further, boron nitride having a turbostratic structure of such a nature that mass production is possible has not been known so far.

It is a basic object of the present invention to provide a novel turbostratic boron nitride and a novel method for producing the turbostratic boron nitride. More specifically, the present invention has an object to solve the above problems and to provide a boron nitride powder, particularly boron nitride having a turbostratic structure, which has good sinterability and is low in cost, and a method for producing the same which can be mass-produced.

On the other hand, the object of the present invention is to provide turbostratic boron nitride of higher purity, and to provide boron nitride having more uniform particle size (particularly boron nitride whose particle size can be controlled from a desired level of 1 μm or less to ultra-submicron).

A first aspect of the present invention provides a method for producing a turbostratic boron nitride powder, characterized by comprising a step (t-BN crystallization step or 2 nd step) of crystallizing substantially amorphous boron nitride into crystalline turbostratic boron nitride (t-BN) in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container) in the presence of an effective amount of a molten alkali metal borate as a flux.

Among them, the crystallization of BN is generally a transition to BN having a three-dimensional regular arrangement, and the t-BN crystallization referred to herein means a process in which a layer of BN gradually develops, turns within a layer plane are irregular, and at the same time, the parallelism between layers increases, and the interlayer spacing tends to be uniform, that is, crystalline t-BN is formed.

The t-BN crystallization step is preferably carried out at about 1500 ℃ or less for a predetermined time until crystalline t-BN is substantially formed, and more preferably carried out at a temperature of 1200 to 1400 ℃. In addition, in the first aspect of the present invention, it is preferable to use any amorphous boron nitride as the starting material in the step 2, as described in the second aspect of the present invention below, but not limited thereto. In addition, a product of an intermediate stage of crystallization having a partially disordered layer structure may be used, or a crystalline t-BN seed crystal may be used.

In a second aspect of the present invention, there is provided a method for producing a turbostratic boron nitride powder, characterized by comprising a step of synthesizing substantially amorphous boron nitride by reacting a mixture containing a boron source and a nitrogen source under atmospheric pressure or pressure (including in a closed or quasi-closed container) by heating (step 1 or BN synthesis step). The reaction can be accelerated by maintaining the pressure during heating at atmospheric pressure or higher, and the amount of the alkali metal borate can be reduced to a very small amount or none at all when high purity is required. The reaction product can be subjected to t-BN crystallization treatment without washing. The heating reaction may be carried out at 1200 ℃ or lower, preferably 950 ℃ or lower.

In a third aspect of the present invention, there is provided a method for producing a turbostratic boron nitride powder, characterized by comprising a step (step 1 or BN synthesis step) of synthesizing substantially amorphous boron nitride by maintaining a mixture containing an effective amount of an alkali metal borate as a flux in a nitrogen source and boron oxide constituting a boron source or a substance which generates boron oxide by heating in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container), and reacting the mixture by heating.

As the nitrogen source, urea is preferably used. The alkali metal borate may be used as a hydrate thereof, and sodium borate and/or a hydrate thereof may be used. The step 1 is preferably carried out by heating to a temperature of 1200 ℃ or lower, at least the melting temperature of the flux, or higher. The above reaction is more preferably carried out at a temperature within the range of 850-.

In the above second and third aspects, the non-oxidizing atmosphere in the 1 st step may be composed mainly of a thermally decomposed gas component of the starting mixture. The step 1 may be carried out in a reaction vessel without removing a generated gas by suction, and the non-oxidizing atmosphere may be atmospheric pressure or a slightly higher pressure than atmospheric pressure including a slight pressurization.

In addition, in the above-described second and third aspects, the ratio (N/B) of boron as a boron source to nitrogen as a nitrogen source in the mixture may be set to be nitrogen excess, and N/B may be set to be 1.01 or more, preferably 1.5 or more, more preferably 2 or more. For example, the urea/boric anhydride weight ratio is in the range of 1.5 or more, preferably 2 or more, and most preferably about 6/4 to 9/4(N/B is 1.75 to 2.6).

In the t-BN crystallization step (step 2), a nitrogen source may be further added (further heated). This facilitates the formation of a nitrogen atmosphere, and in particular facilitates the complete reaction of the unreacted boron source.

The amount of the alkali metal borate in the mixture is set to 0.01% by weight or more and 20% by weight or less (preferably 0.1 to 15% by weight, particularly 1 to 10% by weight) so as to promote the reaction, but it is preferable not to use the alkali metal borate in order to achieve high purity. In this case, the reaction conditions such as pressure and the N/B ratio of the charged mixture are adjusted to be optimized.

It is preferable to include a step (t-BN crystallization step or 2) of crystallizing the reaction product of the above-mentioned step 1 into crystalline turbostratic boron nitride (t-BN) in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container), at 1500 ℃ or less, preferably 1450 ℃ or less, more preferably 1200 ℃ or 1400 ℃. The present invention has an advantage that the t-BN crystallization step can be carried out without washing or purifying the reaction product in the step 1, and the washing or purification is somewhat more preferable.

It is preferable that the t-BN crystallization step (step 2) is preceded by a step of pulverizing the reaction product obtained in the step 1, but the step 2 may be immediately preceded by the step 1.

The crystalline turbostratic boron nitride obtained by the crystallization is washed with a solvent (aqueous cleaning solution, particularly an acidic aqueous solution) to remove residual impurities such as alkali metal borate. High purity can be achieved with water washing, which is a great advantage.

Preferably, the reaction vessel is set to a closed or quasi-closed vessel, and the temperature is gradually and/or stepwise increased to carry out the step 1. Thus, the reaction can be carried out during the temperature rise.

A step of heating a mixture containing a boron source, a nitrogen source and a flux to react (usually in the 1 st step) by adding a small amount of t-BN fine powder as a seed crystal may be included. Thus, the reaction efficiency or yield can be improved.

In a fourth aspect of the present invention, there is provided a crystalline turbostratic boron nitride powder, characterized in that the powder is produced by any one of the methods of the first, second, and third aspects.

In one example of the fourth aspect of the present invention, there is provided a crystalline turbostratic boron nitride powder produced by a method comprising the steps of: a step (1) of heating a mixture containing boron oxide as a boron source or a substance which generates boron oxide by heating, a nitrogen source containing urea, and an effective amount of an alkali metal borate in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container) to react and form a precursor of crystalline turbostratic boron nitride, and a step (2) of heating the reaction product in a non-oxidizing atmosphere to crystallize the reaction product into crystalline turbostratic boron nitride.

As another modification of the fourth aspect, there is provided a high-purity crystalline turbostratic boron nitride powder produced by a method comprising the steps of: the above-mentioned initial mixture does not contain alkali metal borate, and is heated and reacted under the condition of atmospheric pressure or pressurization (including closed or quasi-closed container), so as to produce the step 1 of precursor of crystalline turbostratic structure boron nitride and the above-mentioned step 2.

The crystalline turbostratic boron nitride powder obtained in the fourth aspect is characterized by having, in an X-ray diffraction pattern obtained using CuK α, a half-height width of 2 θ of a diffraction line corresponding to a [004]diffraction line of hexagonal boron nitride of 0.6 ° or less (particularly, 0.5 ° or less or 0.47 ° or less) in place of the [101]diffraction peak, the [002]diffraction peak, [004]diffraction peak and [100]diffraction peak corresponding to hexagonal boron nitride, in the X-ray diffraction pattern, the [10]diffraction peak of crystalline turbostratic boron nitride showing a substantially amorphous state and being connected to the lower part thereof on the large angle side of the [100]diffraction peak is present, and the [102]diffraction peak corresponding to hexagonal boron nitride is substantially not clearly present, and preferably, the diffraction peaks (S102 ) of the diffraction line corresponding to the [100]diffraction line, the [101]and the [102]diffraction line of hexagonal boron nitride in the X-ray diffraction pattern are present in a relationship of the respective intensities of S and S100:

S102/(S100+ S101). ltoreq.0.02 means that h-BN. is not substantially contained and, in addition, a crystalline turbostratic boron nitride powder having a diffraction line of 55 DEG. + -. 0.3 DEG in 2 theta equivalent to the [004]diffraction line of hexagonal boron nitride in the powder X-ray diffraction pattern obtained using CuK α line can be typically obtained, these are ones corresponding to high-purity crystalline t-BN, and the purity can usually be obtained at 99% or more, 99.5% or more, or even 99.8% or more.

In a fifth aspect of the present invention, there is provided boron nitride powder having a uniform particle size. Typically, the primary particles have a particle size of 3 μm or less, and the average primary particle size is 1 μm or less. In addition, a majority of the primary particles have a substantially spherical and/or substantially disc-like shape. The particles are smaller and have a substantially spherical shape, and thecrystals are developed and have a substantially disk shape.

Further, the boron nitride powder can be obtained such that the average particle diameter of the primary particles is 0.1 μm or less.

Typically, when the average particle size of the primary particles is X μm, a powder having a uniform particle size distribution in which 90% or more of the primary particles are in the range of 1/2X to 2X μm can be easily obtained.

The specific surface area of the boron nitride powder measured by adsorption method was 20m²(ii) a ratio of/g or more.

The crystalline turbostratic boron nitride powder of the present invention can easily obtain a powder containing substantially no boron oxide as a result of X-ray diffraction.

Further, the CuK α powder X-ray diffraction pattern of the obtained crystalline turbostratic boron nitride powder did not show a diffraction peak corresponding to the [102]diffraction line of hexagonal boron nitride.

Preferably, in the CuK α powder X-ray diffraction pattern of the obtained crystalline turbostratic boron nitride powder, a diffraction peak corresponding to a [101]diffraction line is substantially not present in a [10]diffraction line formed by a combination of diffraction lines corresponding to [100]and [101]of hexagonal boron nitride.

In a sixth aspect of the present invention, there is provided a powder composition comprising a substantially amorphous boron nitride powder having a blunt mountain peak shape in which a diffraction line corresponding to a [004]diffraction line of hexagonal boron nitride does not have a sharp peak in an X-ray diffraction pattern of a synthetic boron nitride powder using a CuK α line, the powder composition being obtained as a powder composition comprising an amorphous boron nitride powder, characterized by comprising a crystalline disordered layer structure boron nitride precursor having the following characteristics, wherein when the powder composition is heated at 1200-1400 ℃ in a non-oxidizing atmosphere to be subjected to t-BN crystallization treatment, the half height width of a diffraction line 2 θ corresponding to a [004]diffraction line of hexagonal boron nitride in an X-ray diffraction pattern of a synthetic boron nitride powder using a CuK α line in which the relationship between the intensities of S and S102 (representing the relationship between S100S and S102) of diffraction lines corresponding to the [100], [101]and [102]diffraction lines of hexagonal boron nitride exists.

S102/(S100+S101)≤0.02

In the seventh aspect of the present invention, coated boron nitride powder particles having a coating layer containing an alkali metal borate (preferably a coating layer mainly composed of an alkali metal borate) can be obtained. The powder particles are obtained in the form of an intermediate product, i.e. as the product of step 1 (or starting material of step 2). The coated particles in this state are most suitable for t-BN crystallization. The coated boron nitride may be substantially amorphous. However, t-BN containing substantial crystallinity is also useful (particularly as seed crystals). The seed crystal (or crystal nucleus) is not necessarily completely crystalline as long as it is substantially crystalline t-BN.

In the first and second aspects of the present invention, the method for producing a turbostratic boron nitride powder preferably comprises the steps of adding an alkali metal borate (flux) to boric acid (or boric anhydride) as a boron source and a nitrogen source (particularly urea), charging the resultant mixture into a reaction vessel with a lid, maintaining the reaction vessel in a non-oxidizing (particularly atmospheric pressure or a pressure higher than atmospheric pressure) atmosphere, and heating and reacting the mixture to synthesize boron nitride. The reaction is preferably carried out at a temperature of up to 950 c (up to the melting temperature of the flux, preferably in the range of 850 ℃ -&950 c, where the flux is included) to produce substantially amorphous boron nitride (particularly as a precursor to turbostratic boron nitride). As the nitrogen source, there may be mentioned compounds of nitrogen and 1 or more elements such as carbon, hydrogen and oxygen, and among them, compounds which become liquid at a heating temperature such as urea are preferable from the viewpoint of reaction efficiency. As the boron source, a compound of boron and oxygen is preferable from the viewpoints of reactivity, cost and safety. They may both be hydrates.

Boric acid can be produced by adding sulfuric acid to natural borates (mainly borax) and precipitating boric acid out of solution by utilizing its property of being less soluble in water. Boric anhydride can be obtained by dehydrating boric acid by heating, and is an inexpensive raw material, and the melting point of boric anhydride is 460 ℃. Further, urea can be directly synthesized from ammonia and carbon dioxide, so that a high-purity product can be easily obtained, and it can be used as a fertilizer, and therefore, it is very inexpensive and has a melting point of 135 ℃.

In the present invention, boric acid is B₂O₃The boric acid produced by hydration is generally called boric anhydride (boric oxide) by dehydration under heating. Therefore, boric acid may be used as a boron source instead of boric anhydride, and the same reaction may be carried out during heating. Examples of the alkali metal borate which is a flux usable as a raw material include lithium borate, sodium borate, and potassium borate. Among these alkali metal borates, sodium borate is preferred because itis inexpensive and can be easily removed by washing with water. Alkali metal borates, generally like borax, often contain crystal water, but when heated, the crystal water is removed and becomes anhydrous and melts at a relatively low temperature to form glassAnd (4) transforming. When an alkali metal borate is mixed in the mixture, the formed a-BN is easily converted into t-BN, so that the alkali metal borate has an action of promoting the conversion into crystalline t-BN.

Alkali metal borates such as borax (sodium tetraborate, Na)₂B₄O₇·10H₂O) is dehydrated at 200 ℃ to 350-400 ℃ to become anhydrous and melted at 741-878 ℃. Any of the nitrogen source and boron source can be used with a melting temperature lower than that of the compoundLow compounds, so that the raw material mixture first becomes a molten state upon heating and then becomes a scattered state as the temperature rises, and water vapor and carbon dioxide are released from the melt or the reaction intermediate (reaction system) during the reaction. In this case, the reaction product and the released product are nontoxic and nonflammable, and therefore, the structure of the production apparatus can be simplified. The reaction vessel is filled with the released vapor and carbon dioxide, etc. to generate gas, and if the apparatus is sealed (or quasi-sealed) to prevent the inflow of air (oxygen), the pressure will rise naturally by heating. In view of safety of the vessel, cost of the pressure-resistant vessel, and the like, it is preferable to provide a pressure relief valve so as to maintain an appropriate positive pressure or slight pressure (for example, 1.01 atm or more) in order to prevent the pressure from becoming excessively high. The pressure can be further adjusted to a pressure suitable for achieving a prescribed reaction according to the purpose of production. The upper limit of the pressure is not particularly limited, but may be 2.5kg/cm as an example in consideration of the pressure resistance of the reaction vessel, the manufacturing cost, and the like²The following. Particularly, when no flux is used, it is preferable to carry out the reaction under a high pressure in order to accelerate the reaction as much as possible.

The heating (heating) of the reaction in the step 1 should be carried out at a temperature of 450 ℃ or more, particularly 500 ℃ or more and 600 ℃ or less, and 1200 ℃ or less, preferably 1050 ℃ or less or 1000 ℃ or less (more preferably 800 ℃ to 950 ℃, more preferably 880 ℃ to 920 ℃, most preferably about 900 ℃) in order to carry out the BN synthesis reaction. However, since the reaction proceeds sufficiently at 950 ℃ or lower to synthesize BN, it is preferable that the step 1 is carried out by heating at a temperature not higher than the melting temperature of the flux. By heating above the melting temperature of the flux, the molten flux can coat (in the same amount as) the boron nitride particles produced (or, in the case where a sufficient amount of flux is present, the produced BN particles are dispersed in the molten flux matrix).

The reaction product of step 1, that is, the product of step 1 (turbostratic boron nitride precursor) is treated at the crystallization temperature of t-BN (preferably in a non-oxidizing atmosphere such as a nitrogen atmosphere) to convert it into crystalline t-BN. The treatment should be carried out under conditions (time, atmosphere and ambient conditions) under which the t-BN crystallization temperature is about 1500 ℃ or lower and substantially no h-BN is formed, usually under 1450 ℃ and in a non-oxidizing atmosphere (especially nitrogen atmosphere), especially under 1200-. The residual portion of the starting material (mixture) components incorporated therein at this time can prevent h-BN formation and efficiently perform t-BN formation. The specific mechanism is not yet fully understood, and it is believed that the presence of the alkali metal borate component plays a role. When the alkali metal borate is not used, it is preferable that BN synthesis and t-BN crystallization are sufficiently performed, and sufficient washing is advantageous in improving the purity. The washing may be carried out not only with water but also with an acidic aqueous solution, preferably with a volatile acid (e.g., HCl). To remove the remaining boron oxide, the cleaning solution was heated and finally washed with hot water. After the step of forming t-BN by crystallization is completed, the reaction product or the crystallized product is washed with water to remove residues such as alkali components, thereby obtaining purified t-BN powder. In this case, the residual component of the unreacted boron oxide can be easily removed, and the residual boron (or boron oxide) can be controlled to a low level.

As an example of the fourth aspect of the present invention, a boron nitride powder having a specific crystalline turbostratic structure is provided by a method comprising the steps of heating a mixture containing boric acid (or boric anhydride) and urea and containing (or not containing) an alkali metal borate in a reaction vessel (particularly, a vessel in a closed or quasi-closed state) under a pressure atmosphere higher than atmospheric pressure to react and crystallizing the reaction product in a nitrogen atmosphere, and the step of heating the reaction product in a nitrogen atmosphere to crystallize the boron nitride having a turbostratic structure, the half height width of 2 theta of a diffraction line corresponding to the [004]diffraction line of hexagonal boron nitride in the X-ray diffraction pattern of a synthetic boron nitride powder using CuK α line is preferably 0.6 DEG or less (particularly, 0.5 DEG or less, typically, about 0.47 DEG or less), and the point is different from that (a-BN) close to amorphous boron nitrideor low crystalline boron nitride or a hexagonal boron nitride having a diffraction pattern showing a very small diffraction peak as a diffraction pattern, or a diffraction pattern showing a very small diffraction pattern, or a very small diffraction pattern showing a very small diffraction pattern, such as a diffraction pattern showing a diffraction pattern of 0.102-100 DEG or a diffraction pattern showing a very small diffraction peak as a diffraction pattern showing a small diffraction peak of S-100-h diffraction line.

In the sixth aspect of the present invention, crystalline t-BN having a uniform primary particle diameter and a narrow (concentrated) distribution is obtained. The primary particle size is mainly controlled by the purity and the heat treatment temperature (and also time) of the 2 nd step (t-BN crystallization step). In the submicron range, the average particle diameter can be controlled to 0.1 μm or less at 1300 ℃ or less, although the control cannot be carried out accurately without SEM measurement; the average grain diameter can be controlled to be 0.2-0.3 μm at 1300 ℃; the average particle size can be controlled to be below 1 μm at 1350 ℃; the minimum particle size is 0.01-0.02 μm, or less than 0.05 μm or 0.07-0.08 μm; the large particle size can be controlled at 3-4 μm (1400 deg.C). If the temperature is further increased, although it is possible to produce h-BN, the main body portion can be controlled to t-BN, which is a feature of the present invention. The primary particle diameter may be controlled to an arbitrary average particle diameter within the above range.

Step 2 is typically carried out in a non-oxidizing atmosphere such as nitrogen (which may be atmospheric pressure) at 1200-1400 ℃ for about 2 hours. For example, the temperature is raised from the room temperature to a predetermined holding temperature (for example, 1300 ℃) after the 1 st step for about 10 hours. The holding time is usually 10 minutes or more, preferably 30 minutes to 1 hour, and may be 2 hours or more, depending on the temperature, desired particle diameter, degree of crystallization, and the like. In this case, by adding a nitrogen source such as urea and heating, the yield and purity can be improved while BN-formation of residual boron oxide can be achieved together with a nitrogen atmosphere. Of course, the 1 st and 2 nd steps may be carried out successively, but it is preferable to knead or pulverize (slightly crush) the reaction product after the 1 st step, which is advantageous for homogenization.

In the seventh aspect of the present invention, as described above, a powder composition containing amorphous t-BN powder as a precursor of crystalline t-BN can be obtained. It is obtained as a product (intermediate product) in step 1 from a substance containing an alkali metal borate, and the amorphous t-BN particles are at least partially coated with or surrounded by an alkali metal borate flux in a dispersed form. In the present application, each numerical range represents any number included between the upper limit and the lower limit (or equal to or less than the upper limit) and includes at least any number within the range of 1 to 10 minutes in the entire range, and the description of intermediate numerical values is omitted simply for the sake of simplicity.

The alkali metal borate added to the mixture has a function of promoting the crystallization of a-BN to t-BN by making a part of the boron component a raw material of boron nitride, and is contained in the mixture in an amount of 0.01 wt% (preferably 0.1 wt% to 0.5 wt%) or more in order to sufficiently exhibit the function, calculated as the amount of the boron component added to the mixture excluding the water of crystallization. However, even if the amount of the compound is too large, the crystallization-promoting effect is not increased, and a large amount of time and a large amount of pure water are consumed in the washing. Therefore, the amount of the alkali metal borate to be blended is 20% by weight or less (preferably 15% by weight, 10% by weight, 5% by weight), and the amount to be blended can be arbitrarily adjusted within the above range.

However, when t-BN having a high purity is particularly required, the desired t-BN can be produced without adding an alkali metal borate. It is also possible to carry out the 2 nd step by washing only the alkali metal borate in the 1 st step, but in view of the removal loss of the remaining unreacted material (impurities) due to washing, it is advantageous to carry out the synthesis of BN without using the alkali metal borate from the 1 st step. In this case, as described previously, the reaction conditions are adjusted to promote the reaction. The promotion of the reaction can be achieved by increasing the upper temperature and time of step 1, in particular by increasing the pressure of the reaction atmosphere.

In addition, in step 2, as long as the temperature is kept, sufficiently uniform crystalline t-BN can be obtained even if the alkali metal borate is not present. If the washing is carried out by using the heated washing water, particularly the acidic washing water, at the same time, the crystalline t-BN with high purity can be finally obtained.

The reaction vessel may be one which can withstand the reaction temperature and reaction pressure without causing corrosion, and a reaction vessel which can be operated at a temperature of up to 950 ℃ may be one made of inexpensive steel or stainless steel. The non-oxidizing atmosphere may be a state in which the supply of oxygen is cut off. The pressure may be slightly increased during heating of the reaction vessel (in a state where the vaporized component of the steam is ejected). The above-mentioned atmosphere of a pressure higher than atmospheric pressure means a pressure at which oxygen in the air does not intrude into the reaction vessel at all, and from the viewpoint that the synthesis reaction can be promoted by forming a state in which oxygen is completely cut off, it is preferably 1.01 to 2.5 atmospheres, more preferably 1.05 to 2.0 atmospheres. Since the reaction can be sufficiently performed even if the pressure in the vessel is not higher than 2.5 atmospheres, the pressure is preferably 2.5 atmospheres or less, and a higher pressure may be used. The main reaction taking place in the reaction vessel is

The alkali metal borate remained mostly.

In order to efficiently convert the boron component in the mixture charged into the reaction vessel into boron nitride, the atomic ratio (N/B) of nitrogen to boron in the mixture in the reaction vessel should be such that nitrogen is excessive (preferably, excessive). The ratio of N/B is preferably set to 1.1 or more. The weight ratio urea/boric anhydride can be set between 6/4 and 9/4(N/B is 1.75 to 2.6). Generally, it is generally sufficient that about 10% by weight of urea remains with respect to urea (initial charge), but the ratio should be set higher when an alkali metal borate is not used and high purity is desired, and N/B is generally 2 or more, preferably 2.3 or more, and most preferably 2.6. + -. 0.2.

The reactant taken out of the reaction vessel after the synthesis reaction in the 1 st step (preferably less than 950 ℃) is a-BN coated with an alkali metal borate (also a reaction residue) or a reaction residue containing no alkali metal borate, and as an example, the a-BN coated with a sodium salt is formed into a sheet or a cake shape and is expanded in volume, so that it is preferable to perform crushing or crushing (kneading treatment) before entering the 2 nd crystallization step of crystallizing to form t-BN crystals so that a large amount of powdercan be filled. The target may be a degree of particle size of 1mm passing. If the pulverization is carried out at this stage, the purification of t-BN can be easily achieved by the washing carried out at the end.

Then, the reaction product of the step 1, which is mainly composed of a-BN and contains (or does not contain) an alkali metal salt, is heated at a t-BN crystallization promoting temperature under a temperature condition at which h-BN does not occur, and the step 2 is carried out to convert the a-BN into t-BN crystals. The heating temperature should be below about 1500 ℃ so as to generate substantially no h-BN, and is preferably set at 1200-1400 ℃, more preferably 1250-1350 ℃. The temperature may be changed depending on the allowable amount of the residual boron oxide (or oxygen), the target purity, the particle diameter, and the like (generally, the temperature may be controlled in units of 10 minutes to 1 hour at a predetermined temperature). In general, the temperature is raised and the heating time is at least 1 hour or more, preferably 10 hours or more, and the temperature is raised and the heating is carried out for 12 to 13 hours, more preferably about 16 hours until the t-BN crystallization is sufficiently completed. In addition, in the 1450-1500 ℃ temperature region where h-BN is likely to form, special attention is paid to the treatment time (control is performed in units of minutes, as the case may be).

In this case, as the vessel for containing the reaction product, a vessel made of a material which can withstand the heat treatment temperature, such as heat-resistant steel and a refractory ceramic such as alumina, mullite or cordierite, and generally referred to as a shell, can be used. The atmosphere may be a non-oxidizing atmosphere, i.e., a state in which oxygen entry is blocked. Therefore, it is preferable that the boron nitride does not absorb oxygen when the container is heated, and the container should be in a closed state or a quasi-closed state in order to prevent oxygen in the air from entering. For this purpose, the container should be provided with a cover made of the same heat-resistant material as the container. The temperature for the crystallization of a-BN to t-BN is preferably 1280-1350 ℃. In order to complete the crystallization of the specific t-BN crystal within a practical period of time, the heating temperature is 1200 ℃ or higher, but if the heating temperature is too high, h-BN is simultaneously produced, so that the heating temperature is 1450 ℃ or lower, preferably 1400 ℃ or lower, more preferably 1300. + -. 10 ℃ in order to prevent the h-BN from being mixed.

In order to reduce the oxygen content (B) in the t-BN powder₂O₃The remainder), the transformation to t-BN crystallization should be carried out in an atmosphere of nitrogen or the like in which a-BN is not brought into contact with oxygen in the air, and in the case where a-BN is coated with an alkali metal salt such as sodium borate, the closed or quasi-closed state is formed using a container with a lid, whereby oxygen in the air can be prevented from entering at least to some extent, and in this state, heating is carried out to convert it to oxygen (B)₂O₃Residual fraction) of t-BN crystals. The reactant charged into the container with the lid is put into a heating crystallization furnace such as an electric furnace together with the container, heated to a predetermined temperature, and heated for a predetermined time. If the heating time is too short, the conversion to t-BN crystallization cannot be completed. The heating time is generally sufficient to be 10 hours, and can be appropriately adjusted depending on the relationship with the crystallization temperature. The temperature is raised by heating for usually 10 hours or more, preferably 12 to 13 hours, more preferably 16 hours, whereby sufficient t-BN crystallization can be achieved. In this case, the temperature is maintained at the maximum set temperature for a predetermined time (generally 10 minutes or longer, preferably 30 minutes or longer, more preferably 1 to 2 hours). In the heating in the 2 nd step, it is preferable to raise the temperature to a predetermined t-BN crystallization temperature in stages or slowly (for example, in 10 hours) in order to further react the unreacted boron source.

The reaction product having completed the t-BN crystallization contains unreacted residues as impurities and may contain alkali metal salts such as sodium salts, and therefore should be washed and purified with an aqueous washing solution. Conventionally, in order to remove these impurities, cleaning with an alcohol such as ethanol has been carried out so as not to incorporate oxygen into boron nitride by reacting boron nitride with water, but the t-BN powder of the present invention in which t-BN crystallization has sufficiently proceeded is purified by water cleaning, and hence the t-BN purification can be carried out by inexpensive water cleaning. Since the purity of the washing water used affects the purity of the boron nitride powder after purification, it is preferable to use distilled water, ion-exchanged pure water, or deoxidized water. The extent to which washing has proceeded can be determined by measuring the pH of the wash water. Since the washing water can be speeded up by increasing the temperature of the washing water and increasing the solubility of boron oxide, sodium salt, etc. in water, it is preferable to use warm water (usually 80 to 85 ℃ C.) within a range not adversely affecting the synthesized t-BN. As the cleaning solution, an acidic cleaning solution can be used, and it is preferable to use a volatile or thermally decomposable acid (HCl or an organic acid) having a small residual trace, whereby high-purity t-BN can be obtained.

The boron nitride powder purifiedby washing with water as described above is a high-purity t-BN powder, and the primary particles are very fine and substantially all particles having a particle size of 3 to 4 μm or less. In order to achieve both good moldability and sinterability, the average primary particle diameter of the t-BN powder is preferably 1 μm or less, more preferably 0.5 μm or less, 0.3 μm or less, 0.2 μm or less, and the like, and most preferably 0.1 μm or less. Since the primary particles of the obtained t-BN powder are very fine and secondary particles are generally formed, it is difficult to measure them by a precipitation type particle size distribution measuring apparatus. For this purpose, t-BN powder dispersed in a liquid is photographed using an electron microscope and measured from the image of the photograph after printing. As an example, fig. 4 shows the measurement values obtained by using a LA-700 particle size analyzer manufactured by HORIBA, in which the median diameter is 0.3 μm, the cumulative particle diameter is 95.2% or less, and the 90% particle diameter is 0.75 μm. Considering that a considerable amount of agglomerate is contained therein, the actual primary particle size is still smaller than this.

the average particle diameter of the t-BN powder can be determined as a cell size La in the a-axis direction and a cell size Lc in the c-axis direction by the Scherrer's formula (see J.Am.chem.Soc.vol.84 p4620, 1963) from the full width at half maximum of the powder X-ray diffraction pattern. The unit cell size is substantially identical to the value found from the electron micrograph. As observed from the particle size distribution curve shown in FIG. 4 and the SEM micrographs shown in FIGS. 3, 5 and 6, the primary particle size distribution of t-BN obtained by the production process of the present invention is very narrow, and synthesis can be carried out in a particle size range where the particle size distribution is very narrow. The t-BN powdercan be obtained in various levels of narrow particle size distribution, for example, a powder having 95% or more of primary particles of t-BN in the range of 0.3 to 1 μm, a powder having 0.1 μm or less, a powder having 0.02 to 0.07 μm, a powder having 0.2 to 0.6 μm, and other particle size ranges can be obtained.

In a preferred embodiment of the method for producing t-BN powder of the invention, the temperature is raised in stages in the reaction vessel to perform the synthesis reaction of boron nitride (step 1 is performed in three or more stages). For example, a batch-type reaction vessel (particularly, a tank-type vessel) is charged with a raw material mixture, and the temperature of the reaction vessel is raised successively while the reaction vessel is moved from one heating apparatus to another heating apparatus. The a-BN powder synthesized in this way can be easily converted into the t-BN powder intended by the present invention.

In the process for producing t-BN powder of the present invention, it is preferred to add a small amount of t-BN powder as a seed crystal to the mixture of the starting materials in the 1 st step (or 2 nd step). The addition of a small amount of seed crystals can promote the transition to t-BN crystallization, rapidly synthesize high-purity t-BN powder and improve the yield. When the amount of the seed crystal added is too small, the effect is not so large, and therefore, it is preferable to add 0.1% by weight or more to the mixture for removing water, and when it is too large, the effect is not more, so that the amount of the seed crystal added is 3% by weight or less, preferably 0.2 to 1% by weight. The seed crystals may not be completely crystalline, and may be intermediates. The reaction product of the step 2 (in some cases, the reaction product of the step 1) may be used in the step 1 and/or the step 2 either directly or after pulverization or, in some cases, after washing with water, and this is advantageous in improving the yield.

When the t-BN powder of the present invention is analyzed by X-ray diffraction analysis using CuK α powder, it is found that the intensity of the diffraction line corresponding to the [004]diffraction line of hexagonal boron nitride is extremely high, and crystallization has been carried out, and the full width at half maximum of 2 theta is not only small but 0.6 DEG or less (typically 0.47 DEG or less), and that either no [102]diffraction line is found or even if it is found to be extremely small, it is a t-BN powder having a high purity.

The half height width of 2 theta of a diffraction line corresponding to the [002]diffraction line of hexagonal boron nitride in the X-ray diffraction pattern of CuK α powder is 0.6 DEG or less in t-BN powder, meaning that crystals are well developed, further, in h-BN powder, primary particles of the powder are hexagonal plate-like and have a disordered layer structure in t-BN powder, and therefore, no orientation is exhibited in the a-axis direction of the crystal, and as can be seen from FIG. 3, the primary particles of t-BN are disk-like (large particles) or substantially spherical as can be seen from FIGS. 3, 5 and 6 in the case where the particle size is small, the half height width of 2 theta of [002]diffraction line of CuK α is preferably 0.5 DEG or less in the t-BN powder of the present invention, but the values can be controlled to be large or small by the combination of the 1 st step and the 2 nd step and the setting of conditions of the present invention, which is one advantage of the production method of the present invention.

In the t-BN powder of the present invention, it is found that S102/(S100+ S101) is not more than 0.02 among areas S100, S101 and S102 occupied by respective diffraction lines corresponding to the [100], [101]and [102]diffraction lines of hexagonal boron nitride in a powder X-ray diffraction pattern. Since the [102]diffraction line is a diffraction line occurring when the hexagonal lattice is regularly repeated, the above-mentioned relationship means t-BN of a turbostratic structure in which the arrangement of the hexagonal lattice layers is completely irregular or hardly regular (there is no regularity in alignment of angles and positions between layers).

The measurement method of the areas S100, S101 and S102 may be performed by an area meter, or by cutting off the portions of the diffraction lines [100], [101]and [102](upper side of the base line) from the X-ray diffraction pattern recorded on the recording paper with scissors, and weighing each piece of paper cut off with a precision balance, and considering the weight of each piece of paper as S100, S101 and S102. The value of S102/(S100+ S101) is preferably 0.01 or less. If S102 is very small, no diffraction line corresponding to [102]of h-BN is seen in the powder X-ray diffraction pattern. (see FIGS. 2 and 7)

The t-BN powder of the invention is characterized in that the oxygen content (i.e., the amount of impurities) in the t-BN powder is very small and B is not visible at least on an X-ray diffraction pattern₂O₃The diffraction peak of (1). The purity of the boron nitride powder obtained can be up to 99% by weight or more, preferably 99.5% by weight or more and 99.8% by weight or more, most preferably 99.9% by weightThe above. t-BN powder having a small boron oxide (oxygen) content is advantageous in that it has high sintering activity, and since the volume density of a molded body is high when it is compacted by press molding or the like, it is easy to sinter, and the shrinkage at sintering is small, whereby a sintered body having high dimensional accuracy can be easily produced. Further, high-purity single-phase t-BN is very attractive in consideration of various applications which fully exhibit its original characteristics.

The specific surface area of the t-BN powder measured by the gas adsorption method is preferably 20m²More than g, preferably 23 to 25m²More than g.

Additionally, in a seventh aspect of the present invention, a specific boron nitride powder (or composition) is provided that is substantially amorphous. The amorphous boron nitride powder is characterized in that it can be converted into a predetermined crystalline t-BN at a high yield and a high efficiency when subjected to a t-BN crystallization treatment at 1200-1400 ℃.

The composition is characterized in that it is generally obtained as the product of step 1 and contains an effective amount of the specified amorphous boron nitride powder. Although the reaction residue and, in some cases, the residue of the alkali metal boride are included in the product as it is, it is needless to say that a purified intermediate can be obtained by washing. From a particular perspective, this also represents the usefulness of step 1 as independent itself. The repetition of step 1 by recycling (addition of seed crystal) and the like is also advantageous in improving the yield and adjusting the particle size.

Incidentally, although the reaction product itself in the step 1 can be used as a precursor of the turbostratic boron nitride, it is needless to say that the reaction product itself can be used for synthesizing other (crystalline) boron nitride such as h-BN, other composite compounds (or sintered bodies), composite ceramics, or starting materials thereof, when it is used alone or when t-BN is mainly used.

Examples

The following specifically describes embodiments of the present invention, but the following embodiments are only examples of the present invention and the present invention is not limited to these embodiments.

Example 1

With 3.5kg of boric anhydride (B)₂O₃) 5.3kg of urea ((NH)₂)₂CO), 0.63kg borax (Na)₂B₄O₇·10H₂O) as a starting material, loading the mixture into a stainless steel reaction vessel with a cover and a diameter of 530mm, and placing the reaction vessel into a furnace at 500 ℃, 500-The reaction was carried out at 900. + -. 10 ℃ for 10 minutes (total 1 hour) at 10 minutes per stage of temperature rise. Steam starts to be sprayed at about 100 ℃, part of components starts to melt at 200 ℃, the reaction proceeds, and the product bubbles and continuously releases gas. The water vapor was mainly released before 350 ℃ and 400 ℃, and the release of the generated gas was reduced when the temperature was maintained at 900 ℃ for 10 minutes. After naturally cooling in this state, the reaction vessel was opened to remove the reactant from the reaction vessel. At this point, the reactant in the reaction vessel has become B₂O₃The dried cake was roughly in the form of a dried cake. The reaction product was crushed in a reaction vessel, taken out by vacuum suction, and crushed by a crusher to obtain a powder having a size of 1mm (step 1 above). The resultant powder was used as a starting material in the following step 2.

Transferring it to a belt made of ceramic (alumina) refractory materialThe lid container (lightly covered with lid) was loaded into the electric furnace together with the lidded container. Introduction of N into an electric furnace₂Or CO₂Forming non-oxidizing atmosphere, heating from normal temperature to 1300 deg.C for 10 hr, holding at 1300 deg.C for 2 hr, and naturally cooling.

Taking out the powder from the container with cover, neutralizing with 80-85 deg.C ion-neutralized exchange water (hot water), stirring thoroughly, pulverizing, washing to remove alkali component, neutralizing with acid (HCl), washing with water, and drying. About 0.6 to 0.65kg of t-BN is obtained per 10kg of the starting material from step 2 after washing. This corresponds to the formation of t-BN in an amount of about 28.5% or more, based on the weight of the starting boron in step 1, in yields of up to 70% or more and in high purity. In addition, there is a weight loss of 10-20% from the product of step 1 to the heat treatment of step 2.

Example 2

Although example 2 is a sample different from example 1, it is a crystalline t-BN powder obtained by substantially the same operation as in example 1, and the powder is analyzed by CuK α powder X-ray diffraction.the X-ray diffraction pattern of the resulting synthetic powder is shown in FIG. 2. when the X-ray diffraction pattern of the known h-BN shown in FIG. 1 is compared with the powder X-ray diffraction pattern of FIG. 2, boron nitride in the powder X-ray diffraction pattern of FIG. 2 is subjected to t-BN crystallization to a considerable extent, and steep diffraction lines of about 26.6 DEG and about 55 DEG are respectively observed at positions corresponding to [002]diffraction line and [004]diffraction line of h-BN in FIG. 1. however, no steep diffraction line is found at a position corresponding to [102]diffraction line of h-BN. in the case where the diffraction line of h-BN is not found at a position (41.55 DEG), and the case where the diffraction line of h-BN is superposed with a steep diffraction angle of 101.101 DEG, the high degree of the peak of the crystallization of the powder is shown as a high purity in the invention.

The position and full width at half maximum of 2 θ of each diffraction line in the powder X-ray diffraction pattern of FIG. 2 were measured, and it was found that the [002]diffraction line was at 26.58 °, the [004]diffraction line was at 55.0 ° and the full width at half maximum was 0.47 °.

Example 3

Fig. 3 shows SEM enlarged photographs (× 20000 times and × 10000 times) of t-BN fine powder obtained in the same manner as in example 1. As can be seen from the SEM photograph of FIG. 3, the average particle diameter of the primary particles of the t-BN synthetic powder was about 0.45 μm, and the particle diameter of the primary particles was substantially in the range of 0.3 to 0.75. mu.m. Further, it was found that the primary particles did not exhibit a hexagonal plate-like crystal particle shape peculiar to a hexagonal system which was observed in the primary particles of h-BN, but exhibited a disk-like shape (large) or a substantially spherical shape (small) peculiar to crystalline t-BN crystals.

Example 4

A t-BN powder was synthesized in the same manner as in example 1 except that a t-BN powder containing a large amount of dispersed primary particles synthesized in the same manner as in example 1 was used as a seed crystal and 1 wt% of the seed crystal was added to the raw material. In this example, the first reaction step proceeded rapidly, and the yield of t-BN finally produced was further improved. The yield of BN produced is up to 80% or more relative to the charged boric anhydride.

Example 5

A dispersion was prepared from the t-BN powder produced under the same conditions as in example 1, and the particle size distribution was measured, and the results are shown in FIG. 4. The measurement was carried out using a HORIBALA-700 particle size analyzer. As a result of the measurement, 95.2% of particles having a median diameter of 0.30 μm and a particle diameter of 1 μm or less were integrated, and 90% of particles had a particle diameter of 0.75. mu.m. The measurement cannot be said to be completeSince it was confirmed that the average particle diameter of the primary particles (measured in the case where a considerable portion was agglomerated) was 0.3 μm or less. The specific surface area thereof was 23.4m²SEM photographs of other samples similarly obtained are shown in fig. 5. The particles are substantially disc-shaped or spherical, and have an average primary particle diameter of about 0.3 μm, and the primary particle diameter is substantially in a narrow range of 0.2 to 0.45. mu.m.

Example 6

BN was synthesized in the same manner as in example 1 except that the mixing ratio of boric anhydride and urea was changed to 4: 9 (weight ratio) and heating was carried out for 1.5 hours in the 1 st step without using borax, the resultant was held at a final temperature of 920 ℃ and 950 ℃ for 15 minutes, and the inside of the closed vessel was sufficiently narrowed to form a pressurized state. The step 2 was carried out under substantially the same conditions as in example 1, and washing was carried out in the same manner to obtain very high-purity t-BN. An SEM photograph thereof is shown in fig. 6. The shape was found to be roughly spherical, with an average primary particle size of about 0.25 μm, with the primary particle size mostly ranging from 0.2 to 0.3 μm, and essentially ranging from 0.15 to 0.38 μm (i.e., about 0.1 to 0.4 μm). Further, the mixing ratio (weight ratio) of boric anhydride to urea may be 4: 6 to 4: 9, and the best results are obtained when the mixing ratio is 4: 9.

Example 7

FIG. 7 shows an X-ray diffraction pattern of t-BN sample prepared under the same conditions as in example 6.

As can be seen by comparing FIG. 7 with the powder X-ray diffraction pattern of FIG. 1, the boron nitride in the powder X-ray diffraction pattern of FIG. 7 has undergone considerable t-BN crystallization to [002]of h-BN as in FIG. 1]Diffraction line sum of [100]The positions corresponding to the diffraction lines show steep diffraction lines of 26.7 degrees and 41.8 degrees respectively. However, it was found that [002]was present in comparison with the corresponding diffraction line position of h-BN]The position of the diffraction line is shifted a little to the high angle side, at [102]with h-BN]The position (50 °) corresponding to the diffraction line is completely free of diffraction lines. In addition, [100]in the presence of h-BN]At the position (41.8 °) corresponding to the diffraction line, there is a steep diffraction line, although not too high. The diffraction line is [101]of h-BN]The high angle side of the diffraction line is slightly elongated through the shoulder peak angle (hereinafter referred to as [10]]Diffraction line) but [101]]No definite prominence of the diffraction lines was present. This means that it is possible to use,the synthetic boron nitride powder is a high-purity unidirectional t-BN powder which has undergone t-BN crystallization. The powder of FIG. 7 is an example of the high-purity crystalline t-BN powder of the invention (especially, ultra-submicron fine particles of the order of 0.2 to 0.3 μm). It can be seen from the low background that the t-BN single phase is high in purity. That is, the diffraction lines in FIGS. 2 and 7 do not show B at all₂O₃The diffraction peak of (1).

ADVANTAGEOUS EFFECTS OF INVENTION

By adopting the inventionIt is possible to provide a high-purity single-phase t-BN fine powder which is not amorphous and has been crystallized by t-BN in a large scale at a low cost in an industrial production manner. The crystalline t-BN micropowder obtained by the method of the present invention is the only product which can be industrially mass-produced and can be used as a starting materialfor the t-BN itself or a sintered body thereof or even other ceramic materials. Further, since the volume density is high and the sinterability is good when a compact is formed, a boron nitride sintered body having a high relative density and therefore a high strength can be obtained by sintering the t-BN powder as a raw material. In addition, the residual impurities (B) can be arbitrarily controlled (reduced)₂O₃Or oxygen content), so that the use according to the purpose can be developed. Since the t-BN powder can be produced at low cost, applications which have not been achieved for economic reasons in the past can be realized, and a boron nitride sintered body having good properties can be used, and the t-BN powder produced by the present invention is industrially very valuable. The present invention is very useful as an industrial production method of such boron nitride.

In addition, a composition containing substantially amorphous boron nitride as a precursor of boron nitride (particularly t-BN) can be efficiently produced in the step 1 of the present invention. The 1 st step can achieve high efficiency particularly when the gas atmosphere is at atmospheric pressure or higher. Compared with the conventional reduced pressure synthesis method, the method has made a great technical progress. Further, the composition has a characteristic that it can be converted into a turbostratic boron nitride (particularly, a specific t-BN) with high efficiency through the subsequent 2 nd step (heating t-BN for crystallization). The product obtained in the step 1 can be directly used, can also be used as a starting material of BN (h-BN and the like) except crystalline t-BN, can also be used as a starting material of other composite ceramic materials, and has wide application prospect.

In addition, the alkali metal borate used as a flux is originally intended to prevent oxidation mainly, and is thought to act asa catalyst or a promoter for the transition to t-BN (fine crystallization) because it inhibits the growth of crystal grains. In other words, the transition to h-BN can be prevented. In addition, the method is easy to wash and is beneficial to improving the purity. However, it is completely surprising that the present invention can achieve high purity and miniaturization while ensuring a uniform primary particle diameter without using a flux.

Brief description of the drawings

FIG. 1 shows an example of a known X-ray diffraction pattern of conventional h-BN.

FIG. 2 is a powder X-ray diffraction chart of an example of the t-BN powder of the invention. []The internal index represents the peak corresponding to h-BN, and the internal index represents the peak corresponding to t-BN.

FIG. 3 is a Scanning Electron Microscope (SEM) photograph (X20000, X10000, expression multiple X2/3) showing the crystal structure of t-BN powder according to one embodiment of the invention.

FIG. 4 is a graph showing a particle size distribution of t-BN powder according to an embodiment of the invention.

FIG. 5 is a Scanning Electron Microscope (SEM) photograph (X20000, X10000, expression magnification X2/3) showing the crystal structure of t-BN powder according to one embodiment of the invention.

FIG. 6 is a Scanning Electron Microscope (SEM) photograph (X20000, X10000, expression multiple X2/3) showing the crystal structure of t-BN powder according to another example of the present invention (without flux).

FIG. 7 is an X-ray diffraction pattern of another embodiment of the present invention. []The internal index represents the peak corresponding to h-BN, and the internal indexrepresents the peak corresponding to t-BN.

Fig. 8 is an example of an X-ray diffraction pattern of amorphous BN (a-BN) (prior art method).

Claims (42)

1. A method for producing a turbostratic boron nitride powder, characterized by comprising a step (t-BN crystallization step or 2 nd step) of crystallizing substantially amorphous boron nitride into crystalline turbostratic boron nitride (t-BN) in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container) in the presence of an effective amount of a molten alkali metal borate as a flux.

2. The method for producing a turbostratic boron nitride powder according to claim 1, wherein the t-BN crystallization step is carried out at about 1500 ℃ or lower for a predetermined time until substantially crystalline t-BN is obtained.

3. The method for producing a turbostratic boron nitride powder according to claim 1 or 2, characterized in that the crystallization is carried out at a temperature of 1200-1400 ℃.

4. A method for producing a turbostratic boron nitride powder, characterized by comprising a step of synthesizing substantially amorphous boron nitride by reacting a mixture containing a boron source and a nitrogen source under atmospheric pressure or pressure (including a closed or quasi-closed container) by heating (step 1 or BN synthesis step).

5. A method for producing a turbostratic boron nitride powder according to claim 4, characterized in that the boron source is an oxide of boron or a substance which generates an oxide of boron upon heating.

6. The method for producing a turbostratic boron nitride powder according to claim 4, characterized by comprising a step (t-BN crystallization step or 2) of crystallizing the reaction product of the above-mentioned step 1 until it is substantially converted into crystalline t-BN in a predetermined time at a temperature of about 1500 ℃ or lower and in a non-oxidizing atmosphere (including a nitrogen atmosphere) to form crystalline turbostratic boron nitride (t-BN).

7. A method for producing a turbostratic boron nitride powder according to claim 5, characterized in that the mixture is kept in a non-oxidizing atmosphere in the presence of an effective amount of an alkali metal borate as a flux and heated to react.

8. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7, characterized in that urea is used as the nitrogen source.

9. A method of producing a turbostratic boron nitride powder according to any one of claims 4 to 7, characterized in that the step 1 is carried out by heating to a temperature of less than 1200 ℃.

10. A method for producing a turbostratic boron nitride powder according to claim 7, characterized in that the heating of the step 1 is carried out at least to a melting temperature of the flux or more.

11. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7 and 10, characterized in that the reaction is carried out by heating to a temperature in the range of 850-.

12. A method for producing a turbostratic boron nitride powder according to any one of claims 1, 2, 7 and 10, characterized in that the alkali metal borate is sodium borate and/or a hydrate thereof.

13. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7, 10, characterized in that the atmosphere in the step 1 is mainly composed of a thermal decomposition gas component of the starting mixture.

14. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7 and 10, characterized in that the step 1 is carried out without suction-exhausting a gas generated in the reaction vessel.

15. The method for producing a turbostratic boron nitride powder according to any one of claims 7 and 10, characterized in that the atmosphere in the step 1 is atmospheric pressure or a pressure higher than atmospheric pressure including a minute pressurization.

16. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7 and 10, characterized in that nitrogen as a nitrogen source is made excessive for boron as a boron source in the mixture.

17. The method for producing turbostratic boron nitride powder according to any one of claims 1, 2 and 6, characterized in that a nitrogen source is further added in the t-BN crystallization step.

18. A method for producing a turbostratic boron nitride powder according to any one of claims 1, 2, 7 and 10, characterized in that the alkali metal borate is added to the mixture in an amount of 0.01 to 20% by weight.

19. The method for producing a turbostratic boron nitride powder according to claim 4, characterized by comprising a step (t-BN crystallization step or 2) of crystallizing the reaction product obtained in the step 1 into crystalline turbostratic boron nitride (t-BN) by maintaining the reaction product at 1200-1400 ℃ in a non-oxidizing atmosphere (including an atmosphere in a closed or quasi-closed container).

20. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7, 10 and 19, characterized by further comprising a step of pulverizing the reaction product of the step 1 before the step of t-BN crystallization (step 2).

21. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7, 10 and 19, characterized in that the 2 nd step is continuously carried out after the 1 st step.

22. The method for producing a turbostratic boron nitride powder according to any one of claims 1, 2, 4 to 7, 10 and 19, characterized in that the turbostratic boron nitride is washed with an aqueous washing liquid after the t-BN crystallization step to remove impurities.

23. The method for producing a turbostratic boron nitride powder according to any one of claims 4 to 7, 10 and 19, characterized in that the step 1 is carried out while raising the temperature slowly and/or in stages.

24. The method for producing a turbostratic boron nitride powder according to any one of claims 1, 2, 4 to 7, 10 and 19, characterized by comprising a step of adding t-BN fine powder as a seed crystal, and heating and reacting a mixture containing a boron source, a nitrogen source and a flux.

25. Crystalline turbostratic boron nitride powder, characterized in that it is produced by the process according to any one of claims 1, 2, 4 to 7, 10, 19.

26. A crystalline turbostratic boron nitride powder according to claim 25, characterized in that it is produced by a process comprising the steps of: a step (step 1) of forming a precursor of crystalline turbostratic boron nitride by the method according to claim 7, and a step (step 2) of heating the reaction product in a non-oxidizing atmosphere to crystallize the reaction product into crystalline turbostratic boron nitride.

27. A crystalline turbostratic boron nitride powder according to claim 25, characterized in that it is produced by a process comprising the steps of: a step (step 1) of forming a precursor of crystalline turbostratic boron nitride by the method according to claim 4, and a step (step 2) of heating the reaction product in a non-oxidizing atmosphere to crystallize the reaction product into crystalline turbostratic boron nitride.

28. A crystalline turbostratic boron nitride powder characterized in that the half-height width of 2 [ theta]of a diffraction line corresponding to a [004]diffraction line of hexagonal boron nitride in an X-ray diffraction pattern using CuK α is 0.6 DEG or less.

29. A crystalline turbostratic boron nitride powder according to claim 28, having [002]diffraction peak, [004]diffraction peak and [100]diffraction peak corresponding to hexagonal boron nitride in the X-ray diffraction pattern, having [10]diffraction peak indicating substantially amorphous turbostratic boron nitride connected to the large angle side of the [100]diffraction peak in place of the [101]diffraction peak, and substantially no diffraction peak corresponding to [102]of hexagonal boron nitride.

30. A crystalline turbostratic boron nitride powder according to any one of claims 28 to 29, characterized in that there is a relationship of S102/(S100+ S101) ≦ 0.02 between the areas S100, S101 and S102 (representing the intensities of diffraction lines) occupied by respective diffraction lines corresponding to the [100], [101]and [102]diffraction lines of hexagonal boron nitride in the X-ray diffraction pattern.

31. A crystalline turbostratic boron nitride powder according to any one of claims 28 to 29, characterized in that a diffraction line in a powder X-ray diffraction pattern using CuK α line, which corresponds to 2 θ of the [004]diffraction line of hexagonal boron nitride, is 55 ° ± 0.3 °.

32. A crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that the boron nitride powder has a primary particle diameter of 3 μm or less, an average primary particle diameter of 1 μm or less, and a majority of the primary particles have a substantially spherical and/or substantially discotic particle shape.

33. A crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that the average particle diameter of the primary particles of the boron nitride powder is 0.1 μm or less.

34. A crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that 90% or more of the primary particles are in the range of 1/2X to 2X μm, assuming that the average primary particle size is X μm.

35. A crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that it is measured by adsorptionThe specific surface area of the boron nitride powder was determined to be 20m²/g。

36. A crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized by being substantially free of boron oxide and having a high purity as measured by X-ray diffraction.

37. The crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that no diffraction peak corresponding to the [102]diffraction line of hexagonal boron nitride appears in the CuK α powder X-ray diffraction pattern of the turbostratic boron nitride.

38. The crystalline turbostratic boron nitride powder according to any one of claims 25 to 29, characterized in that a diffraction peak corresponding to a [101]diffraction line is substantially absent at a [10]diffraction line synthesized from diffraction lines corresponding to [100]and [101]of hexagonal boron nitride in a CuK α powder X-ray diffraction pattern of the turbostratic boron nitride powder.

39. A powder composition comprising an amorphous boron nitride powder, which composition comprises a substantially amorphous boron nitride powder having ablunt peak shape, but not having a sharp peak in a diffraction line corresponding to a [004]diffraction line of hexagonal boron nitride in an X-ray diffraction pattern of a synthetic boron nitride powder obtained by using a CuK α line, wherein when the powder is subjected to a t-BN crystallization treatment by heating at 1200 ℃ in a non-oxidizing atmosphere, the synthetic boron nitride powder obtained by using a CuK α line has a full width at half maximum of 2 theta of a diffraction line corresponding to a [004]diffraction line of hexagonal boron nitride in an X-ray diffraction pattern in which areas (intensity of the diffraction lines) S100, S101 and S102 of the respective diffraction lines corresponding to [100], [101]and [102]diffraction lines of hexagonal boron nitride are in a relation of S102S 100+ S101/(S100 + S02) (. ltoreq.0.02).

40. Coated boron nitride powder particles, characterized in that the particles have a coating layer comprising an alkali metal borate.

41. The coated boron nitride powder particle of claim 40, wherein said boron nitride comprises substantially amorphous boron nitride.

42. The coated boron nitride powder particle according to claim 40, wherein said boron nitride comprises substantially crystalline turbostratic boron nitride.

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