JPH1129307A - Hexagonal boron nitride powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain coarse particles of hexagonal boron nitride powder having high crystallinity and a low specific surface area by specifying a graphitization index measured by a powder X-ray diffraction method and the BET specific surface area. SOLUTION: This hexagonal boron nitride powder has <=1.20 graphitization index measured by a powder X-ray diffraction method, and $2.5 m2<2> /g BET specific surface area. The hexagonal boron nitride powder is obtained by adding a calcium compound so that 15-50 wt.% liquid phase of calcium borate of the formula (CaO)x .B2 O3 [(x) is 1-3] based on the hexagonal boron nitrated formed in the crystallizing time expressed in the inner percentage may be formed, to a mixture containing boric acid and melamine regulated so that the B/N atomic ratio may be (1/1) to (1/6), maintaining the added mixture in the condition (e.g. 80'C, 80%, 10 hr) satisfying the formula T>=-20.log10 (t/4)+ (Ψ-100)<2> /20}+10 [T( deg.C) is a temperature; Ψ(%) is a relative humidity; t(hr) is a retention time] to provide melamine borate, and firing and crystallizing the melamine borate in a atmosphere of a non-oxidizing gas such as hydrogen gas, nitrogen gas and ammonia gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-crystal, low-specific-surface-area, coarse-grained hexagonal boron nitride powder.

[0002]

2. Description of the Related Art Hexagonal boron nitride powder has a layered structure similar to graphite and has excellent properties such as thermal conductivity, insulation, chemical stability, solid lubricity, and thermal shock resistance. It is applied to solid lubrication / release agents, resin and rubber fillers, heat-resistant and insulating sintered bodies.

A highly crystalline hexagonal boron nitride powder has a flaky morphology due to its layer structure similar to graphite, but generally has a specific surface area of more than 2.5 m ² / g and contains many fine particles.

[0006] Such hexagonal boron nitride powder having a high specific surface area and containing a large number of fine particles is bulky and has a low packing density, so that good handling cannot be obtained during hot press sintering. When performing various moldings such as CIP, extrusion or injection in the pre-process, there is a problem that a sufficient packing density cannot be obtained.

For this reason, the appearance of high-crystal, low-specific-surface-area, coarse-grained hexagonal boron nitride powder has been expected.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above, and has as its object to provide a coarse hexagonal boron nitride powder having a high crystallinity and a low specific surface area.

[0007]

That is, the present invention has a graphitization index (GI) of not more than 1.20 by a powder X-ray diffraction method and a specific surface area of not more than 2.5 m ² / g by a BET method. This is a hexagonal boron nitride powder.
Further, the present invention provides a mixture of boric acid and melamine having a B / N atomic ratio of 1/1 to 1/6, wherein a Ca compound is added to a mixture of hexagonal boron nitride produced during crystallization by 15 to 15%. 5
0% by weight of calcium borate _{(Ca O) x · B 2} O 3 in the liquid phase (where, X is 3 or less greater than 1) was added so as to produce a temperature T (° C.), relative humidity [psi ( %) And the holding time t (hr) are as follows: T ≧ −20 · lo
_{g 10 (t / 4) +} {(Ψ-100) 2/20} +60,
And melamine borate is formed under a condition satisfying the following conditions.
A hexagonal boron nitride powder obtained by firing and crystallizing at 0 ° C.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

Regarding the evaluation of the crystallinity of hexagonal boron nitride powder, the graphitization index (GI
= Graphitization Index). The GI can be determined by calculating the integrated intensity ratio, that is, the area ratio of the (100), (101), and (102) lines of the X-ray diffraction diagram by the following equation (J. Thomas, et.al, J. Am.Chem.S
oc. 84, 4619 (1962)), and the smaller the value, the higher the crystallinity. GI = [Area {(100) + (101)}] / [Area (102)]

As described above, GI is an index of the crystallinity of the hexagonal boron nitride powder. The higher the crystallinity, the smaller this value becomes.
It is said to be 1.60. However, in the case of hexagonal boron nitride powder having high crystallinity and sufficiently grown particles, the GI is further reduced because the powder is easily oriented.

[0011] The hexagonal boron nitride powder of the present invention is characterized by having a GI value of 1.20 or less, is a powder having sufficiently grown particles, and having high crystallinity. Conventionally,
Hexagonal boron nitride powder having a GI value of 1.20 or less is not known.

The hexagonal boron nitride powder of the present invention has a BET
The powder has a low specific surface area and a coarse powder having a specific surface area of 2.5 m ² / g or less according to a method. This is more specific than the conventional high crystalline hexagonal boron nitride powder, which usually has a specific surface area of about 3 m ² / g or more.

The hexagonal boron nitride powder of the present invention has a B / N
A Ca compound is added to a mixture containing boric acid and melamine having an atomic ratio of 1/1 to 1/6, and 15 to 50% by weight of calcium borate (Ca 2 O 3) based on hexagonal boron nitride formed during crystallization. ) X · B ₂ O ₃ liquid phase (where X is greater than 1 and less than or equal to 3), and then the temperature T
(° C.), relative humidity Ψ (%) and holding time t (hr) are represented by the following relational expression T ≧ −20 · log ₁₀ (t / 4) + ｛(Ψ−100) ²
/ 20 ° + 60 to form melamine borate, which is further heated under a non-oxidizing gas atmosphere at a temperature of 1800 to 220
It can be produced by firing and crystallization at 0 ° C.

The boric acid used in the present invention includes orthoboric acid (H ₃ BO ₃ ), metaboric acid (HBO ₂ ), tetraboric acid (H ₂ B ₄ O ₇ ), boric anhydride (B ₂ O ₃ ), and the like.
General formula (B ₂ O ₃ ) · (H ₂ O) x [where x = 0 to 0
Among these compounds, one or more of the compounds represented by 3) are preferred, and among them, orthoboric acid is suitable for the present invention because it is easily available and has a good mixability with melamine.

The mixing of boric acid and melamine can be carried out using a general mixer such as a ball mill, ribbon blender, Henschel mixer and the like. The mixing ratio is such that the boron atom (B) of boric acid and the nitrogen atom (N) of melamine have a B / N atomic ratio of 1/1 to 1/6, preferably 1/2 to 1/4. It is. The B / N atomic ratio is 1
If the ratio is more than / 1, unreacted boric acid remains significantly after firing, and if it is less than 1/6, the unreacted melamine sublimates significantly during firing, and in any case, the hexagonal boron nitride powder of the present invention is produced. I can't.

According to the present invention, boric acid and
Ramin (C_ThreeN₆H₆The specific compounding ratio of boric acid is
Is orthoboric acid (H_ThreeBO_Three), Then H_ThreeBO_Three
/ C _ThreeN₆H₆Is a molar ratio of 6/1 to 1/1 and a weight ratio of
Is from 2.94 / 1 to 0.49 / 1.

In the present invention, calcium borate (CaO) x.B ₂ O, which is a crystallization catalyst for hexagonal boron nitride, is used.
Liquid phase ₃ (where X is greater than 1 and 3 or less) is 15 to 50% by weight based on the hexagonal boron nitride during crystallization.
After a Ca compound is added to a mixture of boric acid and melamine in advance, the mixture is mixed in an atmosphere where the temperature T (° C.), the relative humidity Ψ (%), and the holding time t (hr) satisfy the following relational expressions. To form melamine borate.

T ≧ −20 · log ₁₀ (t / 4) + ｛(Ψ
-100) ^2/20} + 60

Such an atmosphere can be easily formed using a thermo-hygrostat, a steam heating furnace, or the like.
Specific examples of temperature, relative humidity, and time are, for example, 80
° C, 80%, 10 hours, and the like. There are no particular restrictions on gases other than water vapor that form the atmosphere.
Examples include nitrogen gas and inert gas.

The Ca-based compound added in the present invention may be solid calcium borate, but a compound capable of forming calcium borate by reacting with boric acid, particularly calcium carbonate (Ca CO ₃ ), which is inexpensive and easily available, is used. preferable. Ca C
When O ₃ is used, boric acid must function not only as a raw material for hexagonal boron nitride, but also as a raw material for calcium borate liquid phase, but boric acid as a raw material for calcium borate liquid phase is used for hexagonal boron nitride. Even if CaCO ₃ is used, the mixing ratio of boric acid and melamine (C ₃ N ₆ H ₆ ) is much smaller than that of the raw material boric acid. When boric acid is orthoboric acid (H ₃ BO ₃ ), H ₃ BO ₃ / C ₃ N
₆ the H ₆ in a molar ratio of 6 / 1-1 / 1, 2.9 in weight ratio
It may be 4/1 to 0.49 / 1.

Calcium borate (CaO) x.B
Specific composition of CaCO ₃ such that the liquid phase of ₂ O ₃ (where X is greater than 1 and 3 or less) is 15 to 50% by weight based on hexagonal boron nitride during crystallization. The ratio is determined by the amount of melamine and the amount of
Since the number of moles of boric acid reacting with the moles varies, it is necessary to appropriately change the number according to the firing method. However, if melamine does not volatilize at all, and 2 moles of boric acid always reacts with 1 mole of melamine to form hexagonal When it is assumed that crystalline boron nitride is formed, the specific mixing ratio of boric acid, melamine, and CaCO ₃ is 3.9 to 30.6 / 1.6 to 14.
3/1, 2.4 to 18.9 / 2.0 to 18.0 in weight ratio
/ 1.

In the present invention, a mixture of boric acid, melamine, and a Ca compound is held under conditions that the temperature T (° C.), the relative humidity Ψ (%), and the holding time t (hr) satisfy the above formula to form melamine borate. Let it. Melamine borate is not formed if any of the temperature, the relative humidity and the holding time are out of the range of the above formula.

By adding the Ca compound before the above holding, the Ca compound is uniformly mixed in the melamine borate. When boric acid, melamine, and a Ca compound are simply mechanically mixed, or when a Ca compound is mixed after forming melamine borate, water is further added to boric acid, melamine, and a Ca compound to form melamine borate and a Ca compound. When mixing is performed simultaneously, the mixing state of the Ca compound becomes non-uniform, and the hexagonal boron nitride powder after crystallization becomes non-uniform containing many fine particles, and in any case, the hexagonal crystal of the present invention is used. It is not possible to produce boron nitride powder.

Usually, when a hexagonal boron nitride raw material is fired, amorphous boron nitride is formed at a relatively low temperature (1000 ° C. or lower), and then the amorphous boron nitride is crystallized at a higher temperature to form a hexagonal boron nitride. Crystalline boron nitride forms. At this time, crystallization is promoted by coexisting a crystallization catalyst, but the size and shape of the generated hexagonal boron nitride particles vary depending on the type and amount of the catalyst used.

The crystallization catalyst used in the present invention, calcium borate _{(Ca O) x · B 2} O 3 ( where, X is 3 or less larger than 1), and is in liquid phase at the crystallization temperature . The amorphous boron nitride dissolves therein, and hexagonal boron nitride precipitates when the amount of dissolution reaches supersaturation. At this time, as the amount of the catalyst, that is, the amount of the liquid phase is larger, the distance between the adjacent hexagonal boron nitride particles is larger, so that a hexagonal boron nitride having a low specific surface area and coarse particles is easily generated. On the other hand, the molar ratio of the composition i.e. Ca O and B ₂ O ₃ catalyst is responsible for the shape of the particles. When X is 1 or less, that is, when the composition is rich in B ₂ O ₃ , the generated hexagonal boron nitride particles remarkably develop scale-like shapes. In such a case, since the strength in the thickness direction of the hexagonal boron nitride particles is reduced, the coarse particles are difficult to maintain the form and easily break, and the hexagonal boron nitride powder composed of the particles with the developed flake shape is There are few coarse particles and the specific surface area does not become 2.5 m ² / g or less. When X exceeds 3, that is, when a crystallization catalyst having a CaO-rich composition is used, the hexagonal boron nitride particles have an irregular shape with an unclear outline.

[0026] In the present invention, the crystallization catalyst as calcium borate _{(Ca O) x · B 2} O 3 in the liquid phase (X 3 or less larger than 1) in the presence of, and in the inner split the quantity 15-5
By crystallizing in the presence of a catalyst as large as 0% by weight, it becomes highly crystalline and thick (large in the C-axis direction), and a low specific surface area, coarse hexagonal boron nitride powder can be produced. . If the amount of the crystallization catalyst is less than 15% by weight,
Since the distance between adjacent hexagonal boron nitride particles is insufficient, it is not possible to obtain low specific surface area and coarse boron nitride powder. In addition, the amount of the crystallization catalyst is 50% by weight.
However, there is no advantage in increasing the amount of the catalyst any more.

The firing and crystallization are performed under a non-oxidizing gas atmosphere.
It is performed at a temperature of 1800 to 2200 ° C. If the temperature is lower than 1800 ° C., the crystallization of hexagonal boron nitride does not sufficiently proceed, and a powder having high crystallinity, low specific surface area and coarse particles cannot be obtained. If the temperature exceeds 2200 ° C., hexagonal boron nitride is decomposed.

As the gas forming the non-oxidizing gas atmosphere, nitrogen gas, ammonia gas, hydrogen gas, hydrocarbon gas such as methane and propane, and rare gas such as helium and argon are used. Among them, nitrogen gas which is easily available, inexpensive, and has a large effect of suppressing the decomposition of hexagonal boron nitride in a high temperature range of 2000 to 2200 ° C. is optimal.

As the firing furnace, a batch furnace such as a muffle furnace, a tubular furnace, and an atmosphere furnace, and a continuous furnace such as a rotary kiln, a screw conveyor furnace, a tunnel furnace, a belt furnace, a pusher furnace, and a vertical continuous furnace are used. . These can be used properly according to the purpose. For example, a batch furnace is used when small quantities of hexagonal boron nitride of many types are manufactured, and a continuous furnace is used when large quantities of a certain type are manufactured.

The hexagonal boron nitride powder produced as described above undergoes post-treatment steps such as pulverization, classification, removal (purification) of catalyst solidified by acid treatment, washing and drying, if necessary. Provided for practical use.

The hexagonal boron nitride powder of the present invention is a homogeneous material having high crystallinity and containing almost no fine particles, and therefore has a low bulk and a high packing density, and is suitable as a raw material for producing a hot pressed sintered body. . In other words, in general, to produce a boron nitride sintered body by hot pressing, a high-crystal boron nitride powder is mixed with a low-crystal powder, and the crystallization of the low-crystal powder is accelerated under high temperature and pressure to obtain a strong and dense powder. The production of a simple sintered body has been performed. For this reason, it is advantageous to increase the initial packing density as much as possible, so raw material powders having the lowest possible bulk are selected and used.However, the conventional hexagonal boron nitride powder has a high bulk, so the limit is naturally limited. In many cases, troubles often occurred during the filling operation before hot press firing and during the hot press. On the other hand, if the hexagonal boron nitride powder of the present invention is used, such troubles can be reduced.

Furthermore, the hexagonal boron nitride powder of the present invention has a low bulk and a high packing density, and is therefore suitable as a raw material for producing a normal pressure sintered body. When producing a normal pressure sintered body containing boron nitride by normal pressure sintering, generally, a low crystal powder is mixed with a high crystal boron nitride powder and then CIP-molded, or a binder is added and then molded and debindered. It is fired later. In the case of CIP molding, sufficient molding density cannot be obtained during normal pressure sintering with powder having a high bulk. In addition, since a large amount of binder is required to form a bulky powder by an extrusion molding method or an injection molding method, not only is it difficult to remove the binder, but also the voids in the molded body after the removal of the binder become large, which is disadvantageous. . On the other hand, if the hexagonal boron nitride powder of the present invention is used, such a problem can be solved.

[0033]

The present invention will be described more specifically with reference to examples and comparative examples.

Example 1 50 kg of orthoboric acid, 40 kg of melamine, and 10 kg of calcium carbonate as a Ca compound were mixed with a Henschel mixer, and the mixture was kept in a thermo-hygrostat at a temperature of 80 ° C. and a relative humidity of 80% for 10 hours. Melamine was formed. Further, thereafter, firing and crystallization were performed at 1900 ° C. in a batch atmosphere furnace under a nitrogen atmosphere.

The obtained fired product is pulverized and subjected to X-ray diffraction.
Investigation of the composition of
Ca O · B which is a coagulated product of the catalyst_TwoO_ThreeAnd 2Ca OB
_TwoO _ThreeWas observed. This is washed with nitric acid and crystallized.
The removal of coagulated solidified catalyst resulted in a weight loss of 35% by weight.
There was little.

The GI value and the BET specific surface area of the hexagonal boron nitride powder after removing the crystallization catalyst were measured. G
The I value was measured using an X-ray diffractometer “GF-2013” manufactured by Rigaku Corporation, and the BET specific surface area was measured by QUANTACHR.
OME manufactured "QUANTASORB-Jr OS Jr-
1 ". As a result, the GI value was 0.99,
The BET specific surface area was 1.8 m ² / g.

Further, the loose bulk density of the hexagonal boron nitride obtained above was measured using a powder tester manufactured by Hosokawa Micron Corporation (a vibrating sieve having a sieve of 710 μm was used for charging the powder). However, 0.24g / cm
^{Was 3} . The CIP density of this powder pressed at 2.7 ton / cm ² was 2.1 g / cm ³ .

Examples 2 to 4 Boric acid, melamine and a Ca compound were mixed, humidified and heated under various conditions shown in Table 1 to form melamine borate, and then calcined and crystallized to form a crystallization catalyst. The powder was removed to produce a hexagonal boron nitride powder, and the performance was evaluated in the same manner as in Example 1. Table 2 shows the results.

COMPARATIVE EXAMPLE 1 Boric acid, melamine and a Ca compound were mixed, humidified and heated under the conditions shown in Table 1 to form melamine borate.
00 was subjected to calcination-crystallization at ° C., the solidification of the crystallization catalyst in the calcined product Ca O · B ₂ O ₃ and 2Ca O & B
A peak of ₂ O ₃ was observed. The weight reduction rate of the fired product by washing with nitric acid was 45% by weight. GI value, BET specific surface area, loose bulk density and CIP of this powder
Table 2 shows the measurement results of the density.

Comparative Example 2 A raw material having a smaller proportion of the Ca compound than in Example 1 was fired and crystallized at 1950 ° C. As a result, a peak of a coagulated product of the crystallization catalyst of only B ₂ O ₃ was observed in the calcined product. The proportion was 4% by weight based on the weight reduction rate of the fired product by washing with nitric acid. GI value of this powder, BE
Table 2 shows the measurement results of the T specific surface area, the loose bulk density and the CIP density.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

According to the present invention, high crystal, low specific surface area,
It is possible to obtain a hexagonal boron nitride powder which is a coarse particle.

The hexagonal boron nitride powder of the present invention may be used alone, or may be other hexagonal boron nitride powder having different crystallinity, or a nitride, oxide or boron other than the hexagonal boron nitride powder. When mixed with ceramic powders such as carbides and carbides, the operability is improved when producing a hot-press sintered body, and excess binder remains when producing a normal-pressure sintered body. It is possible to manufacture a high-quality ceramic sintered body having few voids and few voids.

The hexagonal boron nitride powder of the present invention comprises a raw material for producing a boron nitride sintered body, a lubricant such as grease, a resin and / or
Alternatively, it can be used as a rubber filler or the like.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yukio Kuroda 1 Shinkaicho, Omuta-shi, Fukuoka Prefecture Inside the Omuta Plant of Denki Kagaku Kogyo Co., Ltd.

Claims (2)

[Claims] 1. A graphitization index (G) determined by a powder X-ray diffraction method.
I) is 1.20 or less, and the specific surface area by BET method is 2.5 m ² / g or less. 2. A mixture containing boric acid and melamine having a B / N atomic ratio of 1/1 to 1/6 is mixed with a Ca compound in an amount of 15 to 15 with respect to hexagonal boron nitride produced during crystallization. 5
0% by weight of calcium borate _{(Ca O) x · B 2} O 3 in the liquid phase (where, X is 3 or less greater than 1) was added so as to produce a temperature T (° C.), relative humidity [psi ( %) And the holding time t (hr) are as follows: T ≧ −20 · log ₁₀ (t / 4) + ｛(Ψ−100) ²
/ 20 ° + 60 to form melamine borate, which is further heated under a non-oxidizing gas atmosphere at a temperature of 1800 to 220
A hexagonal boron nitride powder obtained by firing and crystallizing at 0 ° C.