JP3344663B2 - Method for producing high α-type 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 an inexpensive method for producing high α-type silicon nitride.

[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.

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.

[0004] Of these, the most popular method at present is the direct nitriding method. In the direct nitriding method, the produced ingot is pulverized into silicon nitride powder, so that the powder characteristics, particularly the α-rate, are almost determined by the ingot. An ingot is an aggregate of silicon nitride particles synthesized from metal silicon powder. The metal silicon powder as a main raw material is usually formed into a compact to improve handling properties or charged into a reactor as a powder. However, since the nitriding reaction of metal silicon is a large exothermic reaction, the silicon nitride particles formed are Is a relatively strong aggregate, or ingot.

[0005] There are wet methods and dry methods for grinding ingots, but they have advantages and disadvantages. In wet grinding, post-processes such as purification, filtration, drying, and disintegration of the pulverized material are required. In addition, grinding of the hard material to be pulverized, such as silicon nitride, is performed for a long time. As a result, a purification step for removing mixed media and increased surface oxygen becomes indispensable.
Furthermore, this purification step is expensive because it is an acid treatment.
In contrast, dry grinding does not have such a problem,
The specific surface area does not increase so much, and there are problems such as mixing of abrasion powder of the media and a large increase in the surface oxygen. Coarse particles of several tens μm or more remain unless classified to reduce the yield. . Even if the coarse particles slightly remain, they become large defects of the sintered body and reduce strength and toughness.

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, a low oxygen content and a high specific surface area. For example, control of reaction gas partial pressure and reaction temperature (for example,
-102100, JP-A-49-94600, JP-A-52-117
898), control of heating rate (for example,
No. 607, JP-A-54-24300, JP-A-63-170203
And the addition of a nitriding catalyst (for example, see JP-A-2-248309).
And the control of the porosity of the raw material molded article (for example, Japanese Patent Application Laid-Open No. 58-88109), but it was not sufficient.

[0007]

As described above, according to the conventional method, it is necessary to produce silicon nitride which is capable of satisfying the above conditions (1) to (4) and which can be used industrially at relatively low cost. Was difficult and the emergence of new technologies was awaited. The present inventors have conducted various studies to respond to such a demand, and as a result, have found that a predetermined amount of a nitriding raw material may be nitrided while strictly controlling the reaction rate, and have completed the present invention. is there.

[0008]

That is, the present invention provides a method for manufacturing a silicon nitride by heating a nitride material containing metallic silicon in an atmosphere containing a reaction gas comprising nitrogen and / or ammonia. A method for producing high α-type silicon nitride, characterized in that a reactor is filled with a nitriding raw material containing a metal silicon content of 10 kg or more per 1 m ^{3 of} gas volume in the furnace and nitrided under the following conditions. (1) Reaction rate 4% / hr or less. (2) Reaction rate 0.5% /
hr or more. (3) Increase in reaction rate at a nitridation rate of less than 50%.
6% / hr ² or less.

The present invention will be described in more detail below. A major feature of the present invention is that a sealable reaction furnace is used, and the amount of metallic silicon contained in a specific amount of a nitrided raw material is strictly controlled with respect to the gas volume. While reacting. Since the nitridation reaction of metallic silicon is an exothermic reaction as much as 170 kcal per mole of silicon nitride generated, if this is not controlled, this reaction heat leads to a runaway reaction in which nitridation proceeds at an accelerated rate. The molten metallic silicon remains unnitrided.
Conventionally, various measures have been proposed to suppress the runaway reaction and allow the nitridation reaction to proceed gently. However, in the present invention, the runaway amount is controlled more strictly than in the conventional method by controlling the amount of reaction gas supplied into the furnace. It is intended to suppress the reaction.

More specifically, in the present invention, when a nitriding raw material is filled in a sealable reaction furnace, the volume of 1 m ³ in which a reaction gas can exist in the furnace is reduced by the amount of metal in the nitriding raw material. The silicon content is at least 10 kg. Normally, the volume of the reaction furnace refers to the volume of a portion where the temperature, atmosphere, and the like can be sufficiently controlled, that is, the effective volume, but this must be distinguished from the volume in which the reaction gas in the present invention can exist, that is, the gas volume. . For example, voids in the furnace material, spaces near the furnace wall where the temperature is low, and the like are not included in the effective volume but are included in the gas volume.

In the present invention, the nitriding raw material is placed in the effective volume portion, and the filling amount is expressed as an amount relative to the gas volume. For example, at a reaction temperature of 1250 ° C., the amount of nitrogen gas per 10 kg (357 mol) of metal silicon is only 8.6 mol (0.240 kg) at the maximum. In this state, even if the reaction proceeds at an accelerated rate, the runaway reaction does not occur because only 361 g (3.61%) of metallic silicon can be reacted if the furnace is sealed. If the filling amount of the metal silicon in the nitriding raw material is further increased to more than 10 kg / m ^{3 and} the nitrogen gas partial pressure of the reaction gas is made lower than 1 atm, the proportion of the metal silicon that can react is further reduced.

The gas volume in the present invention can be determined by measuring a change in the furnace pressure when a fixed amount of gas is added to the sealed furnace. For example, if the pressure in the furnace is increased to 2 atm 1 atm when adding 1.5 m ³ of gas, the gas volume becomes 1.5 m ^3. The filling amount is the weight of metal silicon in the nitriding raw material charged per 1 m ³ of the gas volume. In the present invention, the filling amount is required to be at least 10 kg, preferably 12 kg or more.

[0013] The sealable reactor according to the present invention is a furnace having a structure capable of shutting off the gas inlet and outlet, and having a structure in which the atmosphere outside the furnace does not affect the atmosphere inside the furnace when shut off. There is no particular restriction on the structure, material and the like as long as the filling amount of metallic silicon in the nitriding raw material can secure 10 kg or more per 1 m ³ of gas volume. However, as will be described later, when measuring the reaction rate by changing the furnace pressure, a pressure resistance to pressurization or decompression of about several hundred to 1000 mmaq is required, but this requirement is a normal batch type nitriding furnace. And a continuous furnace can be handled by providing a pressure control chamber or the like.

Next, in the present invention, the following three reaction conditions must be satisfied. (1) Reaction rate 4% / hr or less. (2) Reaction rate 0.5% /
hr or more. (3) Increase in reaction rate at a nitridation rate of less than 50%.
6% / hr ² or less.

[0015] The reaction rate in the present invention is a rate at which metallic silicon in a nitriding raw material charged in a reaction furnace is consumed per unit time by a nitriding reaction. For example,
When a nitriding reaction is performed on 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. Strictly speaking, the reaction rate should be determined based on the amount of metallic silicon reacting in a unit of minute time. However, since its measurement is technically difficult, in the present invention, the reaction rate and its increase Is preferably as short as possible, and the time for measuring
It is 0 minute or less, preferably 10 minutes or less, particularly 5 minutes or less.

In the present invention, the reaction rate is determined by measuring the amount of a reaction gas consumed within a certain period of time.
It is assumed that 1.5 times the amount of metallic silicon is consumed according to ₂ → Si ₃ N ₄ , and the method of measuring the amount by converting it to weight is simple and accurate. In this case, the amount of reaction gas consumed can be measured by determining the difference between the amount of reaction gas supplied to the reaction furnace and the amount of reaction gas discharged outside the system, and the amount of gas discharged outside the system can be measured. When there is no such measurement, 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 predetermined time. For example, assuming that the consumed gas amount of 22.4 liters was measured in 5 minutes in terms of the standard state, the metal silicon reacted during that time was:
1.5 moles (42 g), 50 hours per hour
4 g. If the filling amount of the metallic silicon in the nitriding raw material at this time is 20 kg, the reaction rate is 2.5% / hr.
Becomes

Since the nitriding of metal silicon is a large exothermic reaction, the calorific value increases as the reaction rate increases,
There is a variation in the reaction between the portion where heat is easily stored and the portion where heat is easily released. A large amount of heat causes sintering of the generated silicon nitride primary particles to be difficult to pulverize. In the present invention, the reaction rate is controlled to 4% / hr or less, preferably 3% / hr or less, with respect to the filling amount of the nitriding raw material.

On the other hand, when the reaction rate is reduced, the reaction time is inevitably increased, and the furnace efficiency is reduced. For example, 1%
The reaction time takes 100 hours if nitriding at an average rate of / hr, and 1000 hours at 0.1% / hr. In the present invention, in order to achieve practical furnace efficiency, the second reaction condition is to make the reaction rate at a nitriding rate of 10 to 90% 0.5% / hr or more. Nitriding rate 10-90
% In the reaction region other than%, that is, in the early stage and the end period of the reaction, the reaction rate may be less than 0.5% / hr, but that is not a problem. Since the nitridation rate in the present invention is an integral value of the reaction rate, it can be obtained from the integrated amount of consumed metallic silicon.

Next, the third reaction condition will be described. The increase in the reaction rate in the present invention means an increase in the above reaction rate per hour. For example, if the reaction rate for a certain 10 minutes is 1.5% / hr and the reaction rate for the next 10 minutes is 1.6% / hr, the reaction rate increases by 0.1% / hr in 10 minutes. Therefore, the increase per hour is 0.6% / hr ² . A large increase in the reaction rate indicates that the nitridation is proceeding at an accelerated rate. Even if the reaction rate is 4% / hr or less, the reaction is not sufficiently controlled, and Can not achieve. Since such a phenomenon is more remarkable in the first half than in the second half of nitriding, in the present invention, the increase in the reaction rate when the nitriding rate is less than 50% is strictly 0.6% / hr ^2.
It is preferably controlled to 0.5% / hr ² or less.

When the temperature of the nitriding raw material is raised to a reaction temperature in a reaction gas atmosphere, nitriding is started. The temperature varies depending on the particle size and surface state of the metallic silicon, the partial pressure of the reaction gas, and the like, but is about 1050 to 1150 ° C. In order to control the rate of increase of the reaction rate to 0.6% / hr ² or less, it is necessary to lower the partial pressure of the reaction gas in the atmosphere at the reaction start temperature and sufficiently reduce the rate of temperature rise. Specifically, the reaction gas has a partial pressure of 0.5 atm or less, that is, 50 vol% or less, preferably 0.4 atm or less for a mixed gas under 1 atm, and the temperature rising rate is 20 ° C./hr or less, preferably 10 ° C. or less. / Hr or less.

Next, an example of the method for adjusting the reaction rate of the present invention will be described. According to the present invention, the nitriding raw material is filled into the sealable reaction furnace in an amount of 10 kg or more in terms of metal silicon per 1 m ^{3 of} gas volume in the furnace. Therefore, if the reaction gas is not continuously or intermittently supplied, the nitriding reaction is not performed. Does not progress. After the reaction starts,
The reaction proceeds when a reaction gas necessary for obtaining a predetermined reaction rate is supplied, but the reaction proceeds. However, when the reaction rate is increased and the consumption is larger than the supplied reaction gas, the furnace pressure is reduced. At this time, if the pressure of the furnace is adjusted by supplying an inert gas, the partial pressure of the nitrogen gas in the furnace is reduced and the reaction rate is reduced. Conversely, if the reaction rate decreases and the supply exceeds the consumption, the furnace pressure increases.If the supply of the reaction gas is continued while discharging gas outside the furnace, the reaction gas partial pressure in the furnace increases. The reaction rate increases. Further, if this operation is performed while raising the temperature, the recovery of the reaction rate is quick and the control is easy.

The nitriding raw material used in the present invention is a mixed powder or formed body of 100 to 70% by weight of metal silicon powder and 0 to 30% by weight of silicon nitride powder as an aggregate. The metal silicon may be ordinary industrial metal silicon, but a high-purity product is used to produce a high-purity silicon nitride powder.
If the particle size is too fine, the grinding process will be burdened,
If the surface is too coarse, the nitridation reaction becomes difficult. Therefore, the specific surface area is preferably 0.2 to ⁵ m ² / g, particularly preferably 0.5 to ⁴ m ² / g.

In order to produce low-oxygen, high-specific-surface-area, and high-purity silicon nitride powder, a high-grade silicon nitride powder is also used. In mixing metal silicon powder and silicon nitride powder, sufficient consideration is necessary for mixing of impurities, especially impurities due to abrasion of the media, and oxidation of metal silicon. Pulverize and mix in a non-oxidizing atmosphere using media made from

On the other hand, the reaction gas used in the present invention is nitrogen and / or ammonia. These are usually used as a mixture with an inert gas such as argon or a hydrogen gas for controlling the reaction.

The pulverization of the ingot produced according to the present invention can be carried out by any of a wet method and a dry method, but the dry method is well suited for achieving the object of the present invention. In this case, from the viewpoint of reducing oxygen and increasing the specific surface area, under a non-oxidizing atmosphere such as nitrogen, ammonia, and argon, a non-oxidizing pulverization auxiliary agent capable of preventing aggregation of the silicon nitride powder,
For example, it is preferable to add and grind triethylamine, n-butylamine, acetonitrile, methyl ethyl ketone, and the like, and the amount of use is about 0.2 to 3% by weight. As the pulverizing medium, those mainly composed of silicon nitride are preferable.

[0026]

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 20 parts by weight of silicon nitride powder (trade name “SN-9FW” manufactured by Denki Kagaku Kogyo Co., Ltd.) as an aggregate was mixed with 100 parts by weight of a commercially available high-purity metallic silicon powder. To obtain a nitriding raw material. This was filled in a sealed reactor having a gas volume of 1997 liters in the amount shown in Table 1, and after evacuating and replacing with nitrogen gas, 30% by volume of nitrogen and 5% of argon were added.
A mixed gas of 0% and 20% of hydrogen was supplied to start the temperature rise.
The heating rate was 5 ° C./hr from a temperature of 1150 ° C.

The reaction rate was measured at the inlet and outlet of the reactor by using an integrating gas flow meter at every 5 minutes, and the difference was determined from the weight of metal silicon calculated as the gas consumption. . The reaction initiation temperature was 1
After the start of the reaction, the flow rate of the nitrogen gas and the argon gas was adjusted to control the reaction rate and the increase in the reaction rate when the nitriding ratio was less than 50%. Maximum reaction rate, minimum reaction rate at a nitriding rate of 10 to 90%,
Table 1 shows the maximum value of the increase in the reaction rate and the reaction time when the nitriding ratio is less than 50%.

[0028]

[Table 1]

After the completion of nitriding, the mixture was allowed to cool to room temperature while flowing nitrogen gas, and the synthesized ingot was taken out. The obtained ingot was coarsely and medium crushed to 0.2 mm or less, and then crushed in a vibration mill using silicon nitride balls for 1 hour in a nitrogen atmosphere to produce silicon nitride powder. Α of the obtained powder
The conversion, specific surface area and residual coarse particles were measured as follows. Table 2 shows the results.

(1) α conversion: powder X using CuKα ray
Line diffraction, the diffraction line intensity Ia ₁₀₂ of the (102) plane of the α phase
And a (210) plane of Ia _210, beta phase (101) and the diffraction intensity Ib ₁₀₁ of plane (210) plane diffraction intensity Ib ₂₁₀ of was calculated by the following equation. α conversion rate (%) = [(Ia ₁₀₂ + Ia ₂₁₀ ) / (Ia ₁₀₂ + Ia ₂₁₀ +
_{Ib 101 + Ib 210)] ×} 100 (2) specific surface area: at Yuasa Ionics Co. Kantasobu, helium - was measured by flow-through 1-point method of a mixed gas of nitrogen as a standard gas. (3) Residual coarse particles: The ratio of the dry weight of the residue on the sieve when 200 g of the powder is sieved with water at 25 μm to the original powder.

Further, the sinterability of the obtained silicon nitride powder was evaluated as follows. 6% by weight in silicon nitride powder
Of Y ₂ O ₃ powder and 3.2% by weight of Al ₂ O ₃ powder, and an aqueous slurry of 50% by weight of the mixed powder is prepared by using an organic binder. Die press molding and C of 3.5t / cm ²
IP molding was performed. It was heated under nitrogen pressure of 12 kgf / cm ² and fired at 1750 ° C. × 4 hours. For the obtained sintered body, the four-point bending strength at room temperature was measured in accordance with JIS R1601. Table 2 shows the results.
Shown in

[0032]

[Table 2]

The following can be seen from Tables 1 and 2. The silicon nitride (Examples 1 to 5) produced according to the present invention can produce a silicon nitride powder having a high α-conversion rate by ordinary dry pulverization, has little residual coarse particles, and has a specific surface area of 10 m ² /. g or more. Also, the strength of the sintered body obtained using this silicon nitride powder is sufficiently high. On the other hand, in Comparative Examples 1 and 5 in which the filling amount of metallic silicon was small, Comparative Example 2 in which the maximum reaction rate was too high, and Comparative Example 3 in which the increase in the reaction rate was large, the reaction control was not sufficiently performed. Rate is low and the specific surface area of the obtained silicon nitride powder is not very high. Furthermore, since the coarse particles also remain, the strength of the sintered body is lower than that of the example, and in order to use as a fine ceramics raw material, it is necessary to classify and further pulverize for a long time or to perform additional pulverization in a wet method. Become. In Comparative Example 4 in which 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 takes almost a week and productivity is poor.

[0034]

According to the present invention, the use of metallic silicon, which is a relatively inexpensive raw material, does not require a complicated pulverizing step or a complicated post-treatment such as a wet refining treatment. Silicon nitride powder for use as a raw material for fine ceramics can be produced relatively inexpensively.

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

Claims (1)

(57) [Claims] When a silicon nitride is produced by heating a nitriding raw material containing metallic silicon in an atmosphere containing a reaction gas composed of nitrogen and / or ammonia, a gas volume of 1 m in a furnace capable of being sealed is used. ^{3. A} method for producing high α-type silicon nitride, comprising filling a nitriding raw material containing at least 10 kg of metallic silicon with respect to ³ and nitriding the same under the following conditions. (1) Reaction rate 4% / hr or less. (2) Reaction rate 0.5% /
hr or more. (3) Increase in reaction rate at a nitridation rate of less than 50%.
6% / hr ² or less.