JP3348798B2 - Method for producing silicon nitride - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing easily crushable silicon nitride for producing high quality silicon nitride powder at low cost.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of energy saving and high energy efficiency, silicon nitride sintered bodies have been studied as automobile engine parts such as turbo rotors, valves and swirl chambers and various industrial machine parts. The conditions required for the silicon nitride powder are also stricter as described below. (1) The α phase is mainly used. (2) Submicron fine particles. (3) High purity.
(4) Inexpensive.

[0003] Of these, regarding (1), in recent studies, a high-strength and high-toughness sintered body has been obtained from a silicon nitride powder mainly composed of a β-phase, which impairs other physical properties. It is desired to produce a silicon nitride powder mainly composed of an α phase or a β phase without any problem.

Various methods for producing silicon nitride have been proposed so far, and these are roughly classified into the following four methods. (A) A direct nitriding method in which metallic silicon is nitrided using a reaction gas such as nitrogen or ammonia. (B) A reduction nitriding method in which silica is nitrided using a reducing agent such as carbon and a reaction gas. (C) An imide thermal decomposition method for thermally decomposing silicon diimide generated from silicon tetrachloride. (D) A gas phase method in which a gas such as monosilane or silicon tetrachloride is reacted with a gas such as ammonia by heating such as laser or plasma.

[0005] Of these, the direct nitriding method and the reduction nitriding method are advantageous in terms of cost, and it has been said that the imide pyrolysis method and the gas phase method have excellent powder properties. That is, in the direct nitriding method, since silicon nitride powder is obtained by pulverizing an ingot, it is relatively difficult to achieve the above condition (2), and a purification step is required to obtain a high-purity product. The yield is not very good. In the reduction nitriding method, it is difficult to completely remove the internal oxygen contained in the raw material silica, and powder having poor sinterability is easily generated as compared with other production methods. In the imide thermal decomposition method or the gas phase method, expensive silicon tetrachloride or monosilane is usually used as a raw material, which is disadvantageous in cost as compared with the former two. Furthermore, in the imide thermal decomposition method, chlorine contained in silicon tetrachloride tends to remain, and in the gas phase method, it is difficult to obtain a laser or plasma device large enough to be used industrially.

At present, the most common method industrialized is the direct nitriding method. In the direct nitriding method, since the produced ingot is pulverized into silicon nitride powder, the powder properties are strongly affected by the ingot. An ingot is an aggregate of silicon nitride particles synthesized from metal silicon powder. Metal silicon powder as the main raw material is formed into a compact for improvement in handleability or filled in a reactor as a powder.Since the nitriding reaction of metal silicon is a large exothermic reaction, the generated silicon nitride particles are: A relatively strong aggregate, or ingot.

In order to produce a silicon nitride powder having a high specific surface area by the direct nitriding method, it is necessary to pulverize or further classify the ingot. In wet grinding, post-processes such as purification, filtration, drying, and disintegration of the pulverized material are required. In addition, hard materials to be pulverized, such as silicon nitride, are pulverized for a long period of time, resulting in severe wear of the pulverizing media and running. As the cost increases, a purification step for removing contaminated media and increased surface oxygen becomes indispensable. Furthermore, this purification step is expensive because of the acid treatment. On the other hand, dry pulverization does not have such a problem, but does not increase the specific surface area so much, and has problems such as mixing of abrasion powder of the medium and a large increase in surface oxygen.

To cope with such a problem, there have been many proposals for controlling a nitriding reaction to produce a silicon nitride powder having a high α ratio and a high specific surface area. For example, control of the reaction gas partial pressure and reaction temperature (for example, JP-A-48-102100, JP-A-49-94600, JP-A-52-117898, etc.), control of the rate of temperature rise (for example, JP-B4-54607, JP-A-54-24300, JP-A-63-170203, etc.), addition of a nitriding catalyst (for example, JP-A-2-248309), pores of a nitriding raw material. Rate control (for example, see JP-A-58-881
However, these methods are not sufficient because they increase the production process, increase the running cost and lower the yield.

[0009]

As described above, according to the conventional method, it is not possible to produce silicon nitride which has the above conditions (1) to (4) sufficiently and is industrially usable. It was difficult and the emergence of new technologies was awaited.
The present inventors have conducted various studies to meet such a demand, and as a result, have completed a method for producing easily crushable silicon nitride by a direct nitriding method.

[0010]

That is, the present invention comprises 30 to 100 parts by weight of silicon nitride powder having a specific surface area of 5 m ² / g or more based on 100 parts by weight of metal silicon powder, and has a bulk density of 1.0 g / g. cm ³ and in which raw nitride or less, the production method of the reaction rate 4% / hr or less of silicon nitride, which comprises nitriding by controlling the reaction rate or 0.5% / hr in 10% to 90% rate of nitride It is.

Hereinafter, the present invention will be described in more detail. The most important feature of the present invention is that the control of the nitridation reaction, which has been proposed so far, is different from the use of a nitriding raw material and a nitriding material in view of the production of an easily crushable ingot. That is, the reaction conditions were strictly combined and optimized.

First, the nitriding raw material will be described. The nitriding raw material used in the present invention comprises 30 to 100 parts by weight of silicon nitride powder having a specific surface area of 5 m ² / g or more as an aggregate per 100 parts by weight of metal silicon powder. The mixture obtained by blending has a bulk density of 1.0 g / cm ³ or less.

[0013] The silicon nitride powder compounded as the aggregate,
Since it is difficult to separate it from the ingot after nitriding, as it so that no hindrance even remain as a product, its specific surface area must be at 5 m ² / g or more, preferably 6 m ² / g or more. The amount of the aggregate is adjusted so that the nitriding reaction to be described later can be controlled conveniently and the production efficiency can be increased.
30 to 100 parts by weight, preferably 40 to 100 parts by weight
It must be 90 parts by weight. That is, if the amount is less than 30 parts by weight, it is difficult to control the nitriding reaction, and
If the amount exceeds 0 parts by weight, the ratio of metallic silicon in the nitriding raw material decreases, and the production efficiency deteriorates.

As the metallic silicon powder, an ordinary powder used in the direct nitriding method is used. When producing high-purity silicon nitride, a high-purity product is used. Regarding the particle size of the metallic silicon powder, the specific surface area is preferably 0.2 to ⁵ m ² / g, particularly preferably 0.5 to ⁴ m ² / g.

To mix the metal silicon powder and the aggregate,
A method in which pulverization and mixing of these raw materials are performed at the same time may be adopted. In this case, it is necessary to give sufficient consideration to mixing of impurities, particularly impurities due to abrasion of the medium, and oxidation of metallic silicon during pulverization and mixing. Particularly, in the case of a high-purity product, pulverization and mixing are performed in a non-oxidizing atmosphere such as nitrogen or argon using a medium made of silicon nitride.

In the present invention, the bulk density of the nitriding raw material is a ratio of the weight to the apparent volume of the nitriding raw material, and it is necessary that the bulk density is 1.0 g / cm ³ or less in the present invention. If the raw material has a bulk density of more than 1.0 g / cm ³ , the nitriding reaction becomes uneven. The non-uniform nitriding reaction makes it difficult to control the nitriding reaction described later, and makes it difficult to synthesize an easily crushable ingot. From the viewpoint of synthesizing easily crushable ingots, it is preferable that the bulk density of the nitriding raw material is small.However, in this case, the handling of the nitriding raw material is hindered, and the operating efficiency of the nitriding furnace is reduced. 0.4
~1. 0g / cm ³ is preferably especially 0. 5~0.9g / cm ^3.

In order to obtain a nitriding raw material having a bulk density of 1.0 g / cm ³ or less, a method of filling the powder as it is in a container made of silicon nitride or silicon carbide, or a method of forming a molded body from the powder and foaming it A method of adding a substance which is burned off by heating to the powder compact to form a porous body can be adopted,
When large voids are introduced to reduce the average bulk density, a high bulk density portion is formed, which makes it difficult to control the reaction and varies the quality of the ingot. Therefore, the void diameter of the nitriding raw material is at most about 1 mm, especially 100 μm or less. It is desirable that

Examples of the substance which is burned off by heating used in the production of the porous body include organic substances such as styrene, acrylonitrile, poval, butyral, vinyl chloride and butadiene, and nitriding reaction temperatures such as ammonium chloride and ammonium fluoride. Inorganic substances which sublime below can be mentioned.

Another feature of the present invention is the nitriding reaction conditions.
This is the first effect of the present invention brought about by the combination with the above-mentioned nitriding raw material. That is,
Conventionally, there is a technique for controlling the nitridation reaction by regulating the reaction rate (for example, JP-A-56-50170), but the actual reaction rate is determined by the reaction rate within the reacting ingot or between the ingots. Since the effect is measured as an average value ignoring the variation, the effect is not sufficiently exhibited, and in many cases, an easily crushable ingot cannot be obtained. Such a phenomenon is the same in the technology in which the nitriding reaction is controlled only by the reaction temperature or the rate of temperature rise.

In the present invention, the reaction rate is controlled at 4% / hr at any stage during the nitriding reaction. Here, the reaction rate is a rate at which the metallic silicon component in the nitriding raw material is consumed per unit time by the nitriding reaction. That is, for example, when nitriding a nitriding raw material containing 1 kg of metallic silicon, a reaction rate of 1% / hr means that 10 g of metallic silicon is nitrided per hour. Such a reaction rate is a small value as compared with the conventional reaction rate.

As an example, in the conventional method, there are not many references referring to the reaction rate, but by analogy with the rate of temperature rise and reaction time, if the reaction temperature is set to 1000 ° C. or higher, the reaction rate is as follows. In the example of 63802, 17-40% /
hr, 20-25% in the example of Japanese Patent Publication No. 4-54607.
/ Hr, and in Japanese Patent Publication No. 44-3210, 12.
5-20% / hr, 6.7% in JP-A-54-24300
/ Hr.

When the reaction rate increases, the generated reaction heat increases, and the reaction in the ingot fluctuates. As a result, sintering of the primary particles of silicon nitride proceeds. Speed is 4% /
However, if the reaction rate is too low, the reaction time becomes longer, and the efficiency of the reaction furnace deteriorates. Therefore, in the present invention, the reaction rate at a nitriding rate of 10 to 90% is controlled to 0.5% / hr or more.
At the beginning and end of the reaction, the reaction rate may be lower than 0.5% / hr.

In the present invention, the reaction rate is measured, for example, by measuring the amount of reaction gas consumed in nitriding per unit time, that is, the difference between the amount of gas supplied to the reaction furnace and the amount of gas discharged outside the system. Can be done by In this case, when there is no reaction gas discharged from the reaction furnace to the outside of the system, the measurement can be performed by measuring a change in the gas composition in the furnace, a change in the furnace pressure, and the like within a certain period of time.
Strictly speaking, the reaction rate should be determined based on the amount of consumed reactant gas per unit time, but the measurement is technically difficult. It is determined by measuring the amount of gas consumed per unit short time. The shortest possible unit is
The time is 30 minutes or less, preferably 10 minutes or less, and particularly 5 minutes or less.

For example, when a consumed gas of 22.4 liters is measured in 5 minutes in terms of a standard condition, the amount of metal silicon reacted during that time is 1.5 mol (42 g). Become. At this time, if the charged amount of the metal silicon in the nitriding raw material is 20 kg, the reaction rate is 2.5% / hr. Since the nitriding rate in the present invention is an integrated value of the reaction rate, it can be calculated from the integrated value of the consumed reaction gas amount, that is, the integrated amount of the consumed metal silicon.

The reaction rate in the present invention is most effective when the reaction temperature and the atmosphere are adjusted. The following shows an example. In the nitridation reaction in the present invention, metal silicon reacts with the nitrogen component of the reaction gas. If the temperature is raised to the temperature at which the reaction proceeds, the reaction rate can be controlled to a predetermined value. If the reaction rate increases and the consumption increases more than the supplied reaction gas, the furnace pressure decreases.If the furnace pressure is adjusted by adding an inert gas, the reaction gas partial pressure in the furnace decreases and the reaction rate increases. Become smaller. Conversely, if the reaction rate decreases and the supply exceeds the consumption, the furnace pressure will increase.If the supply of the reaction gas is continued while discharging gas outside the furnace, the reaction gas partial pressure in the furnace will increase. The reaction rate increases. If such an operation is performed while increasing the temperature, the recovery of the reaction rate is quickened, and the control is facilitated.

The pulverization of the silicon nitride produced according to the present invention can be performed by either a wet method or a dry method, but the dry method is well suited to the present invention. In the case of the dry method, it is preferable to use a grinding aid, for example, triethylamine, n-butylamine, methylethylketoton, acetonitrile and the like, and the amount of use is about 0.2 to 3% by weight. It is.

[0027]

EXAMPLES The present invention will be specifically described below with reference to examples and comparative examples. Examples 1 to 5 Comparative Examples 1 to 5 Commercially available high-purity metallic silicon powder was pulverized to an average diameter of 9.1 μm by a ball mill using silicon nitride balls.
Silicon nitride powder having various specific surface areas as an aggregate was mixed with 100 parts by weight of the metal silicon powder at a ratio shown in Table 1, and mixed with a ball mill to obtain a nitride raw material powder.

1 kg of this nitriding raw material powder was formed by the following method so as to have a bulk density shown in Table 1. That is, in Example 1, a 60% by weight aqueous solution of a nitriding raw material powder was granulated by a spray drier, dried and degreased, and then dried to give a particle size of 100%.
Those having a size of not more than μm were collected and filled in a 16 × 16 cm silicon nitride container to a thickness of 6 cm. In Examples 2 and 3 and Comparative Examples 1 and 2, the nitriding raw material powder was directly placed in a 16 × 16 cm silicon nitride container. In this case, the thickness was 5 cm. In Examples 4 and 5 and Comparative Examples 3 and 4, the nitriding raw material powder was placed in a wooden frame of 16 × 16 cm, pressed by hand to a thickness of 4 cm, and then placed on a silicon nitride plate. In Comparative Example 5,
The nitriding raw material powder was die-pressed to 16 × 16 × 2.5 cm and placed on a silicon nitride plate.

The nitriding raw material obtained above is put into a reactor,
After replacing the inside of the furnace with nitrogen gas, nitrogen gas 1 liter / m
The temperature was raised while flowing at a rate of 0.2 l / min of hydrogen gas at 0.2 in / min. The temperature was raised to a maximum temperature of 1420 ° C. while controlling to maintain the reaction rate shown in Table 1, thereby completing nitriding. The nitridation reaction started at a temperature of around 1140 ° C. in each case. The reaction rate is controlled by measuring the reaction rate by the following method.
The temperature was raised at the rate of r, and conversely, when it was desired to decrease the temperature, the temperature was stopped, and argon gas was flowed at a rate of 1 liter / min instead of nitrogen gas.

The reaction rate is determined by measuring the amount of gas supplied to the reactor and the amount of exhaust gas from the reactor every 5 minutes with an integrated gas flow meter, and taking the difference as the amount of consumed reaction gas, 3Si + 2N ₂ → S
The consumption of metal silicon was calculated assuming that the reaction was performed with i ₃ N ₄ , and the consumption was calculated as a ratio per hour to the metal silicon content in the charged nitriding raw material.

After completion of nitriding, the mixture was allowed to cool to room temperature while flowing nitrogen gas, and the resulting ingot was coarsely and medium-crushed in a silicon nitride mortar, passed through a 0.5 mm sieve, and the specific surface area of the ingot was measured using the sieve. did. Then, using a silicon nitride ball and a vibration mill lined with silicon nitride, the coarsely and medium-crushed ingot was further pulverized for 1 hour under a nitrogen atmosphere to produce silicon nitride powder. About the obtained silicon nitride powder, the specific surface area and 100 g of
The residue of coarse particles was measured by measuring the dry weight of the residue on the sieve when sieved with a sieve of μm.

Further, 5 parts were added to the silicon nitride powder obtained above.
By weight, Y ₂ O ₃ powder of 3.5 wt% and Al ₂ O ₃ powder of 3.5 wt% were mixed, and an aqueous slurry of 55 wt% of the mixed powder was prepared using an organic binder. After granulating and drying it with a spray drier, press-molding the mold, further performing CIP molding of 3 t / cm ² , raising the temperature under nitrogen pressure of 10 kgf / cm ² , and firing at 1750 ° C. × 8 hours. Was. The four-point bending strength of the obtained sintered body at room temperature was measured according to JIS R1.
601 was measured. Table 2 shows the results.

[0033]

[Table 1]

[0034]

[Table 2]

According to Tables 1 and 2, according to Examples 1 to 5 of the present invention, an ingot having a high specific surface area can be obtained. A silicon nitride powder having the same was produced. In addition, almost no coarse particles remained in the silicon nitride powder, and the sintered body strength was 100 kgf / mm ² or more. On the other hand, Comparative Example 1 using an aggregate having a small specific surface area, Comparative Example 2 in which the amount of the aggregate was not appropriate, Comparative Example 3 in which the bulk density of the nitriding raw material was high, In Comparative Example 4, which was large, the specific surface area of the ingot was small and the pulverizability was poor, so that the specific surface area of the obtained silicon nitride powder was also small, and residual coarse particles were also observed. Further, the strength of the sintered body was small, and the raw material powder of fine ceramics was low. In Comparative Example 5 where the minimum reaction rate was not appropriate, the reaction time was long, and if the time for gas replacement and furnace cooling was included, 2 batches of nitriding were performed.
It took weeks and the productivity was poor.

[0036]

According to the present invention, the pulverizability is remarkably improved as compared with the ingot obtained by the conventional direct nitriding method, and a long or special refining treatment such as pulverization / pulverization or wet pulverization is required. Therefore, a high-quality silicon nitride powder can be produced at low cost.

──────────────────────────────────────────────────の Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C01B 21/068 C04B 35/626 CA (STN)

Claims (1)

(57) [Claims] 1. A nitriding raw material containing 30 to 100 parts by weight of a silicon nitride powder having a specific surface area of 5 m ² / g or more and a bulk density of 1.0 g / cm ³ or less per 100 parts by weight of a metal silicon powder is reacted. Nitriding rate of 10-9 at a rate of 4% / hr or less
A method for producing silicon nitride, comprising nitriding while controlling the reaction rate at 0% to 0.5% / hr or more.