JPH07102966B2 - Method for manufacturing silicon nitride - Google Patents

Description

The present invention relates to a method for producing silicon nitride capable of producing a sintered body having high strength at high temperature. Silicon nitride is used as a high temperature structural material for gas turbine members, nozzles, bearings and the like.

[Prior Art] Conventionally, as methods for producing silicon nitride powder, (1) metal silicon direct nitriding method, (2) silica reduction nitriding method, and (3) silicon halide method are known. Probably because the powders produced by these methods have different manufacturing histories, even if the amount of metal impurities, the amount of oxygen, the particle size, and the specific surface area are the same, the sinterability of the powder and the sintered body after sintering are There is a big difference in characteristics such as bending strength.

Generally, the powder produced by the method (1) is easily sinterable but has low high-temperature bending strength. The powder of the method (2) is hardly sinterable but has high high-temperature bending strength. The powder of method 3) is said to exhibit intermediate performance.

Regarding the amount of oxygen, since the powder of the method (1) undergoes a pulverization step, the total amount of oxygen is usually more than 2% by weight in most cases and is at least 1.5% by weight. If the method of (1) is passed through a step such as an acid treatment for removing impurities, the total oxygen amount is reduced, but it is still difficult to reduce it to less than 1.0% by weight. on the other hand,
Even in the case of the powder of the method (2), since silica powder is used as a raw material, there is residual silica, and the total amount of oxygen usually exceeds 2% by weight.

The above powders are currently available. Needless to say, the sinterability of the powder and the characteristics of the sintered body are affected by the amount of powder oxygen, but in addition, various powders such as specific surface area, crystallinity, particle shape, and particle size (fine powder) are also included. At present, the body characteristics are entangled, and it is hardly known how the powder characteristics of the above-mentioned production methods are related to the powder characteristics.

Japanese Examined Patent Publication No. 61-43311 discloses the relationship between the oxygen content of silicon nitride powder and high-temperature bending strength.

[Problems to be Solved by the Invention]

However, the above-mentioned invention proposes a sintered body which has a reduced amount of oxygen in silicon nitride powder and is excellent in high-temperature strength. However, because of adopting the HP sintering method, various characteristics other than oxygen, for example, a lump. In the method of crushing the ingot, the amount of fine powder unavoidable, the description of metal impurities, specific surface area, etc. are not clarified,
In the method of producing low oxygen powder, the cost is high due to the special treatment, the versatile HP is used for the sintering method as described above, and the reason why the high temperature strength is exhibited The reason for limiting the type of the sintering aid and the structure of the sintered body are not mentioned deeply.

An object of the present invention is to provide a method for manufacturing silicon nitride that solves the above problems.

[Means for Solving the Problems]

Briefly describing the present invention, the present invention relates to a method for producing silicon nitride, which comprises nitriding a metal silicon powder in an atmosphere containing nitrogen and / or ammonia to produce silicon nitride. And at least one kind of halide selected from the group consisting of alkaline earth metal halides is continuously, intermittently or temporarily supplied in a gaseous state.

As a result of various studies on the above points, the inventors of the present invention nitriding the metal silicon powder under a special atmosphere gas, the oxygen content of the silicon nitride powder, the specific surface area, the average particle diameter,
Along with producing a powder in which the amount of fine powder and metal impurities are controlled, a sintering aid that is well compatible with the silicon nitride powder is found, and a structure that greatly contributes to the high temperature strength development of the sintered body is also found, and the present invention is provided. completed.

The oxygen content of the silicon nitride powder in the present invention is 0.6% by weight or less, preferably 0.6% by weight to 0.2% by weight. Oxygen
If the content is more than 0.6% by weight, the α → β transition that occurs during sintering tends to occur from low temperature, and the amount of grain boundary phase formed by the sintering aid increases, so the nitriding is limited. This is because the solubility in silicon increases, and as a result, the number of β nuclei increases and it becomes difficult to obtain a sufficiently grown β columnar crystal having a high aspect ratio.

It is said that the average particle size is very important especially when adopting pressureless sintering, and it is generally preferable that the average particle size is small. The average particle diameter of the present invention is in the range of 0.3 to 0.8 μm.
If it exceeds 0.8 μm, the solubility of silicon nitride in the composite oxide caused by the reaction of sintering aids such as yttrium oxide, magnesium oxide, aluminum oxide, etc. with oxygen contained in the silicon nitride powder will decrease, and the density will be sufficiently dense. Because it does not change. If it is less than 0.3 μm, the solubility in the grain boundary phase formed by the sintering aid becomes large, and as a result,
This is because the number of β nuclei becomes large, the densification does not become sufficiently dense, and it becomes difficult to obtain a sintered body that can withstand high temperature sufficiently by pressureless sintering.

The average particle diameter used in the present invention is the 50% diameter of the volume% measured by CAPA-700 manufactured by Horiba Ltd.

In general, when the oxygen content in the silicon nitride powder is low, the SH hump shear and the general sintering aids Y ₂ O ₃ and Al ₂ O ₃ are used.
KH Jack (SHHampshire, KHJack, Proceedings of British Ceramic Society, Proc.Brit.Ceram.Soc, Vol. 31, pp. 37-49 (1
981)] et al., It is difficult to sinter because a sufficient amount of liquid phase cannot be obtained. That is, the phase boundary reaction is rate-determining, and generally, a sintering aid, such as MgO, which has an effect of accelerating diffusion is added and sintered. In that case, the sinterability is improved, but the high aspect ratio β
The problem remains in columnar crystals. That is, the β columnar crystals having a high aspect ratio greatly affect the amount of fine powder as described in Japanese Patent Application No. 63-277360. That is, when the amount of fine powder increases, β columnar crystals having a high aspect ratio are less likely to occur, probably because the number of β nuclei generated in the liquid phase increases, and as a result, high temperature strength does not develop.

That is, the amount of fine powder is preferably 7% by volume or less when the amount of fine powder is 0.2 μm or less as described in Japanese Patent Application No. 63-277360. In other words, sintering is performed by adding a sintering aid that helps control the phase boundary reaction, and the β-columnar crystals with a sufficiently high sintering density and high aspect ratio are crystallized, that is, firing. The binding property is that the high temperature strength (σ _{1200 ° C} ) is 700 MPa or more. As such a sintering aid, MgO-based, MgO-Al ₂ O ₃ system, MgO
- rare earth oxide-based, MgO-Al ₂ O ₃ - rare earth oxide-based, MgO · Al ₂ O ₃ - include rare earth oxide.

The relationship between the β-columnar crystals with a high aspect ratio and the high temperature strength in the above-mentioned sintered body will be described in detail later. That is.

In addition, the α fraction is based on the fact that the sintering mechanism of silicon nitride is based on the fact that α-silicon nitride is once dissolved in the liquid phase, then becomes supersaturated and precipitates as β-silicon nitride, as described above. It is desirable that the α-phase content in the silicon powder is high. That is, the α phase content must be at least 80% or more. If it is less than 80%, the above-mentioned β columnar crystals having a high aspect ratio are hard to crystallize and the high temperature strength is not exhibited.

Content of metal impurities in silicon nitride powder, especially Fe, Al
The total content of Ca and Ca is preferably 500 ppm or less. This is because if it exceeds 500 ppm, the creep strength at a high temperature is prolonged, that is, the creep strength at a high temperature is lowered. Therefore, it is preferable that the metal impurities are as small as possible.

Regarding the specific surface area, when a shape is formed by using various molding methods such as press molding, injection molding, slip casting, etc., it is generally 6 to 6 for reasons such as handling and viscosity.
It is preferably 14 m ² / g. When it is less than 6 m ² / g, the grain size becomes coarse, and as a result, the sinterability is deteriorated, which is not preferable. If it exceeds 14 m ² / g, the powder itself becomes bulky, resulting in poor handling and high viscosity, which is an important factor for injection moldability, which is not preferable.

In addition, if it is less than 6 m ² / g, the particles become large, the sintering density does not increase, and the high temperature strength cannot be improved. Further, in order to exceed 14 m ² / g, excessive pulverization is required, and as a result, the amount of fine powder increases, the number of β nuclei increases as described above, and β columnar crystals with a high aspect ratio are less likely to occur. is there.

Next, a method for manufacturing the above-described silicon nitride will be described.

Silicon nitride of the present invention, while introducing an atmosphere gas containing nitrogen and or ammonia, by nitriding the metal silicon powder,
In the method for producing silicon nitride, it is produced by continuously, intermittently or temporarily supplying one or more selected from alkali metal and alkali metal halides in a gaseous state.

Hereinafter, the present invention will be described in more detail.

The particle size of the metallic silicon powder used in the present invention is preferably under 88 μm. As described below, the reason why metallic silicon having a relatively large particle size can be used is that all the reactions between metallic silicon and the nitrogen source are not solid / gas reactions, but rather some solid / gas reactions on the surface. After that, it is greatly involved in the progress of nitriding by the gas-gas reaction. That is, in the present invention, even if
Even with metallic silicon of about μm, if part of the surface undergoes nitriding due to solid-gas reaction, the density difference between metallic silicon and α-silicon nitride causes a phenomenon of breaking metallic silicon, resulting in fine metallic silicon. Is. However, if it exceeds 88 μm, the same phenomenon occurs, but the temperature at which the solid-gas reaction occurs on the surface becomes high, and free Si tends to remain under normal nitriding conditions, which is not preferable. There is no particular lower limit. In other words, the use of metallic silicon powder having a smaller average particle size facilitates the production of SiO (G) described later and promotes the gas-gas reaction, which makes it easier to produce the target granular α-silicon nitride ingot. Become. Further, since the oxygen in the metal silicon powder is aimed at lowering the oxygen level as described in detail in the above-mentioned silicon nitride powder, it is desirable that the oxygen content in the metal silicon powder is as small as possible. In the present invention, it may be 0.2% or less.

As described above, the present invention lies in the application of the reaction mechanism for producing silicon nitride from metallic silicon powder in Japanese Patent Application No. 63-198987. Ie, the reaction of a metal silicon powder and nitrogen rather than solid-gas reaction, N ₂ -NH _3, or N ₂ through the O ₂
While knowing that the -H ₂ system is rate-controlled by the gas-gas reaction, various reaction equations for Si ₃ N ₄ formation involving SiO (G) were investigated. One example of this is shown below, but regarding the morphology in solid formation by such a gas-gas reaction, Kato [Akio Kato: Journal of Powder Engineering, Vol. 18, No. 1, pp. 36-45 (198
0)] has a great influence on the degree of supersaturation, that is, logkp. That is, the logk at 1300 ° C in the following equation (1)
While p is about 1.6, Equation (2) is about 46.

Therefore, it is presumed that the morphology of silicon nitride produced by the above two equations may be the whiskers of the equation (1) and the granularity of the equation (2).

3SiO (G) + 4NH ₃ (G) = Si ₃ N ₄ (S) + 3/2 O ₂ (G) + 6H ₂ (G) (1) 3Si (G) + 4NH ₃ (G) = Si ₃ N ₄ (S) + 6H ₂ (G) (2) In addition, taking the reaction equation up to the formation of Si (G) by taking CaF ₂ (G) as an example, the estimated reaction and the circulating reaction in the microscopic crystal field are expressed by equations (3) to ( 6).

Si (S) +1/2 O ₂ (G) → SiO (G) (3) SiO (G) + CaF ₂ (G) + H ₂ (G) → Si (G) + CaO (S) + 2HF (G) (4) 3Si (G) + 4NH ₃ (G) → Si ₃ N ₄ (S) + 6H ₂ (G)
(5) (CaO (S) + 2HF (G) → CaF ₂ (G) + H ₂ (G) +1/2 O ₂ (G)) (6) As is clear from this, halide gas entrained as a gas It is possible that is circulating in the microscopic crystal field. In fact, even if the amount of the halide gas is not more than the equivalent amount, the effect of the granulation is sufficiently recognized.

Therefore, we searched for a gas compound that has a stronger affinity for oxygen than SiO (G). As a result, when nitriding the metal silicon powder while introducing an atmosphere gas containing nitrogen and / or ammonia, the gas of each halide of alkali metal and alkaline earth metal is intentionally supplied to generate silicon nitride. It was found that the morphology was granular, not whisker-like. Regarding the concentration of alkali metal and alkaline earth metal halides, for example, as can be seen from the above-mentioned estimated reaction formula, 1 mol of Si (S) is 1 mol of gas of alkaline earth metal halide, and ,
If it is an alkali metal halide, 2 mol or more is sufficient. In addition, since nitriding is not performed at one time, it may actually be less than that. Regarding the supply method, for example, a halide of an alkali metal and an alkaline earth metal is charged into another furnace, heated and sublimated, and nitrogen is supplied to a furnace charged with metallic silicon powder while controlling the concentration. Supplied as a companion. It is also possible to place a specified amount of the halide in the vicinity of the metal silicon powder in a furnace charged with the metal silicon powder, and supply the metal silicon powder by nitriding the metal silicon powder and sublimating the halide. The supply method is not limited to these. Further, the introduction form is not limited to this, and may be either separate introduction or the same mixed introduction.

Further, as the supply timing, the nitriding is performed at a temperature, that is, a temperature in the range of 1,150 ° C. to 1,450 ° C., continuously, intermittently or temporarily. This temporary supply can be achieved by, for example, performing a combined operation of opening and closing the introduction pipes to the above-mentioned furnace for sublimation and the furnace for nitriding.

It is understood that this temporary supply is supported by the fact that the introduced gas halide is circulated in the system in the microscopic crystal field, for example, as shown below. In fact, in terms of temperature, even if the halide was introduced up to 1,350 ° C., and then the supply of the gas was stopped, the form of silicon nitride formed was still the same as that continued to 1,450 ° C.

The above-mentioned alkali metal and alkali metal halide are, for example, fluorides of Li, Na, K, Mg, Ca, Sr, and Ba elements,
Chlorides and bromides. Particularly, fluorides of Ca, Mg and Li are preferable. The reason is that the affinity for oxygen is stronger than that of Si in the relationship between the standard formation energy of oxide formation and the temperature, and as a result, the above-mentioned granulation is further promoted.

The above halides may be used alone or in combination of two or more kinds. If it adds further, it is not limited to a nitriding furnace. For example, it can be applied to any of a batch furnace, a continuous push furnace, a continuous rotary furnace, a continuous screen furnace, and a fluidized bed.

Specific examples of the granular silicon nitride described above and the whisker-like silicon nitride described in Japanese Patent Application No. 63-198987 are shown in FIGS. 1 and 2. Further, FIG. 3 shows an example of a typical solid-gas reaction obtained by the direct nitriding method of Si, that is, a form exhibiting another shape. Each figure is a scanning electron microscope (SEM) photograph at a magnification of 3500 times.

As is clear from this, it can be seen that FIG. 1 is much smaller than the Si powder used and has a roundish granular shape, and also supports the occurrence of the qi-qi reaction.

It is clear that Qi-Qi reaction also occurred in Fig. 2, but Si
Maybe due to lack of supersaturation of O (G), it is in a form with many needle crystals. In FIG. 3, it can be clearly seen that the sintering of the Si powders first occurs, and then the nitrogen diffuses and the nitriding proceeds.

Such granulated silicon nitride makes it difficult to generate fine powder when pulverized, and as a result, it also leads to low oxygen, and easily manufactures the low-oxygen and low-fine silicon nitride powder which is the object of the present invention. can do.

Here, the granular silicon nitride obtained according to the present invention will be described in more detail. Rather than crushing whiskers or needle-shaped crystals having a small diameter as shown in FIG. 2 while physically bending, Since the pulverization proceeds in such a manner as to loosen the granular crystals as shown in FIG. 1, the generation of fine powder due to the pulverization of the ingot is small, and as a result, the oxidation of silicon nitride is suppressed, resulting in a low oxygen powder. That is, it becomes a silicon nitride powder close to equiaxed powder without fine powder as obtained by the silicon halide method.
Quantitatively, when crushed into coarse / medium crushed products using an ordinary coarse crusher and medium crusher, the coarse / medium crushed products, especially nitriding so that the specific surface area with a particle diameter of 0.2 mm is ² to 5 m ² / g. Silicon nitride that is silicon and has an oxygen content of 0.4% or less.

As the crusher for evaluating the pulverizability of silicon nitride, for example, the crusher described on pages 1279 to 1283 of Maruzen Co., Ltd., October 25, 1978, Chemical Engineering Handbook is used. That is, it is a coarse crusher such as a Dior crusher or a gyretory crusher, or a middle crusher such as a roll crusher, a roller mill or an edge runner.

The α-silicon nitride of the present invention having the small granular crystals obtained as described above is wet-processed by a conventional method, for example, after crushing / medium-crushing, using a ball mill, a vibration mill, a jet mill, an attritor mill, a pearl mill or the like.・ Dry pulverization to obtain α-silicon nitride powder. Regarding the fineness of powder, pay attention to at least the above-mentioned average particle diameter, and perform processing under appropriate conditions including a pulverizer.

Next, a silicon nitride powder composition suitable for obtaining a high-temperature high-strength silicon nitride sintered body using the above-described silicon nitride powder will be described.

The basic technical idea of the present invention is a silicon nitride powder obtained by the metal silicon direct nitriding method, and further, by using the atmospheric pressure sintering method of the silicon nitride raw material having low oxygen, the sintering density is sufficiently increased. At the same time, there is a regulation on the combination of the raw material silicon nitride powder and the sintering aid in increasing the aspect ratio of the β-columnar crystals constituting the sintered body.

Silicon nitride powder contains various impurities, most of which is oxygen. Further, to explain, this oxygen reacts with the oxide-based sintering aid to be added to form a grain boundary phase, and needless to say, its amount is preferably small. That is, low oxygen is very important for high temperature and high strength. Of course, the metal impurities affect the high temperature creep as described above, and therefore the smaller amount is preferable.

It is also noteworthy in the present invention that low oxygen plays an important role in nucleation in the α → β transition of silicon nitride. That is, because of the low oxygen, the solubility in the liquid phase of the sintering aid is lowered, heterogeneous nucleation occurs, and as a result, it is found that β columnar crystals with a high aspect ratio that may be considered abnormal are formed, It was found that this effect is more effective for high temperature and high strength development than the reduction of the amount of grain boundary phase described above.

Further, the present inventors have found that the fine powder in the raw material silicon nitride, for example 0.2% as described in Japanese Patent Application No. 63-277360.
It is considered that the smaller the amount of fine powder under μm is, the more oxygen is contained in this fine powder. However, it was also found that it contributes greatly to the heterogeneous nucleation of the α → β transition described above. It was

However, in general, when oxygen in the raw material silicon nitride powder is reduced, normal pressureless sintering, for example, Y ₂ O ₃ , Al ₂ O ₃ , SmO ₂ , CeO ₂ or the like, or a combination of two or more sintering aids is used. It is well known that even if the agent is added in an amount of 7 to 10%, a sufficient sintered body density cannot be obtained.

As described above, the silicon nitride powder of the present invention has optimum characteristics for the appearance of high temperature and high strength. However, it is very difficult to sinter under normal pressure because of low oxygen.

Therefore, this difficult sintering mechanism is considered to be the phase boundary reaction rate controlling described by Hampshire et al., And as a result of various studies on compounds that are said to break the rate controlling, magnesium oxide and / or its precursor is added. Then, it was found that the sintered density was sufficiently improved even at normal pressure. It was also found that the precursors were magnesium nitrate, magnesium carbonate, magnesium hydroxide and the like, and the addition amount of the silicon nitride powder composition was less than 5% by weight.

Further, as described above, the property of the grain boundary phase is important for high temperature strength development, and it is natural that it is preferable that the decomposition temperature is higher as well as the amount. It is also an important point to crystallize a high-melting-point grain boundary phase composed of a binder, for example, Al ₂ O ₃ and / or silicon nitride. As rare earth oxides, Y, La, Ce,
Oxides such as Pr, Nb, Sm and Eu can be mentioned.

As described above, the silicon nitride powder composition of the present invention, which comprises the silicon nitride powder, magnesium oxide or the precursor thereof, and the sintering aid containing the rare earth oxide, is indispensable for achieving high temperature and high strength. Incidentally, the sintering aid in the silicon nitride powder composition is preferably within 10 wt%, further, Mg in the sintering aid
It is appropriate that the content of the compound is 4% by weight or less in terms of MgO.

In addition, to be more specific, since the silicon nitride powder of the present invention has low oxygen content, a system that independently forms a liquid phase other than the above-mentioned sintering aid, or a system that forms a liquid phase in a slightly low temperature range is used. A sintering aid may be used.

For example, the former is to use zinc oxide, nickel oxide, etc., and the latter is, for example, in the case of Y ₂ O ₃ -Al ₂ O ₃ , a sintering aid having a composition rich in Al ₂ O ₃ up to a YAG composition. Is to use. Regarding the amount of addition, for example, even if it is low oxygen,
If the amount of sintering aid is large, the glass phase will increase, so 12% by weight or less is appropriate.

Next, a silicon nitride-based sintered body obtained by firing the above-described silicon nitride powder composition will be described.

A feature of the present invention is that a large number of β columnar crystals having a large aspect ratio generated by α → β transition can be seen. Ie β
The composition of the silicon nitride sintered body is inhomogeneous with respect to the size of the columnar crystals, but the inhomogeneity is homogeneous in the region of, for example, several tens of μm, and its structure plays an important role in developing high temperature strength. It has been found to be doing.

In general, high temperature strength development is mainly researched on strengthening of the grain boundary phase, for example, selection of a sintering aid capable of synthesizing a high melting point grain boundary phase, crystallization of vitreous material, and most of the research. The approach was from the sintering side. Therefore, from the approach of the powder side, for example, powders with different oxygen, powders with different specific surface areas, powders with different crystallinity, etc. It was found that there is a strong correlation between the high temperature strength per hit and the β-columnar crystals with a high aspect ratio that make up the sintered structure. That is, (30 μm) ×
(25 μm) In the field of view of the sintered body, the average aspect ratio of the five β-columnar crystals that are considered to have a large aspect ratio is 10
It was found that the high-temperature strength exhibited 250 MPa or more per unit density if it was above. That is, it was felt that if the sintering could be performed to a value close to the theoretical density, the expression of 700 MPa or more could be sufficiently considered at the strength of 1200 ° C. The density up inhibition is oxygen in the silicon nitride powder described above and is a fine powder. Further, the promotion of the density up inhibition is magnesium oxide in the above-mentioned silicon nitride powder composition.

The above-described silicon nitride sintered body of the present invention has a structure in which the sintered body density is sufficiently improved, and is composed of inhomogeneous β columnar crystals, and a part of the inhomogeneous β columnar crystals has an aspect ratio. The sintered body has a large ratio, and its microscopic heterogeneous structure is macroscopically homogeneous.

Regarding the method for producing the silicon nitride sintered body of the present invention, the silicon nitride powder composition of the present invention is pulverized and mixed by a dry method or a wet method, and then molded into a required shape, and then under an inert gas such as nitrogen or argon. 5 at a temperature of 1,600-1,800 ℃ under gas
It can be obtained by holding for about an hour. The type of molding method, such as press molding, injection molding, slip casting, pressure, filling powder, etc., is not particularly limited.

As described in detail above, the present invention examines the characteristics of silicon nitride powder, specifically, the amount of oxygen, the specific surface area, the average particle size, the amount of fine powder, and the relationship between the metal impurities and the high temperature strength, and particularly the amount of oxygen and the fine powder. The amount shows a correlation of high temperature strength development.
In addition to finding a sintering aid that is familiar with the above characteristics, β-columnar crystals with a very high aspect ratio were observed in the sintered body structure, and it was found that they play an important role in high temperature strength development. Further, such a powder is economically obtained by nitriding a metal silicon powder under a special atmosphere gas, and then pulverizing by a conventional method, not by a silicon halide method.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

The measured values shown in each example were obtained by the following method.

(1) Oxygen (% by weight): By LECO TC-136 O / N simultaneous analyzer.

(2) Specific surface area (m ² / g): According to the Cantersorb Jr BET 1-point method manufactured by Yuasa Ionics.

(3) particle size ([mu] m): According to Horiba Ltd. C _A P _A -700.

(4) α fraction (%): X of Kaiger Frax RAD-II B type manufactured by Rigaku Denki Co., Ltd.
By line diffraction.

(5) Metal impurities (ppm): Based on JIS-G-1322.

(Fe, Al, Ca) (6) Relative density (%): By Archimedes method.

(7) Three-point bending strength (MPa): According to Shimadzu Corporation Autograph AG-2000 A type.

Examples 1 to 7, Comparative Examples 1 to 4 100 parts by weight of metallic silicon powder having a metallic Si purity of 99.98% by weight is α.
10 parts by weight of silicon nitride powder having a specific surface area of 20 m ² / g with a fraction of 90% 2.5 kg of mixed powder was molded into a nitriding specimen having a bulk specific gravity of about 150 × 150 × 20 t shown in Table 1 and charged into an electric furnace. .

Nitriding was performed as shown in Table 1, but at that time, from another electric furnace filled with 2720 g of solid CaF ₂ and kept at 1,200 ° C, CaF ₂ (G) (18 to 36) / Hr (2
It was heated and nitrided while being accompanied by argon gas so that the temperature became 5 ° C.

In Comparative Examples 1 to 4, heating and nitriding were performed under the conditions shown in Table 1 without introducing CaF ₂ (G).

SEM observation of the obtained silicon nitride revealed that
No. 9 was all granulated, but Comparative Examples 1 to 4
Had a whisker and other shapes. As one example, SEM photographs of Example 1, Comparative Example 1 and Comparative Example 3 are shown in FIGS. 1, 2 and 3, respectively.

The obtained silicon nitride is crushed and crushed (Jiyo crusher and top grinder) to crush 0.2 mm below.
50 mm of crushed products under 0.2 mm in a ball mill with an internal volume of 1 and 4φ Fe
A ball 0.5 and 100 g of water were put in, wet pulverized for 20 hours, then acid-treated with hydrochloric acid and hydrofluoric acid, over-dried and crushed to produce silicon nitride powder as a sintering raw material. The obtained silicon nitride powder was measured for oxygen, average particle size, specific surface area, 0.2 μm or less, and Fe + Al + Ca content, and the results are shown in Table 2.

Next, the average particle size of this silicon nitride powder is 1.3 μm
Y ₂ O ₃ and average particle size 1.4 μm Al ₂ O ₃ , average particle size 1.2 μm
5% by weight, 2% by weight and 3% by weight of MgO are added, respectively, 1,1,1-trichloroethane is further added, and the mixture is wet mixed in a ball mill for 4 hours, dried, and then molded at a molding pressure of 100 kg / cm ^2. After molding into a 6 × 10 × 60 mm shape, CIP molding was performed at a molding pressure of 2700 kg / cm ² . These compacts were set in a carbon crucible and fired under N ₂ gas atmosphere under the conditions shown in Table 2 to obtain sintered compacts. After the obtained sintered body was ground, the relative density and the three-point bending strength at normal temperature and high temperature were measured. The results are shown in Table 2.

Examples 8-9 In order to know the effect of Fe, Al and Ca as metal impurities,
When the silicon nitride powder of Example 1 was sintered, Fe-Al-Ca alloy powder prepared in advance was added as shown in Table 3, and a sintered body was manufactured under the same conditions as in Example 1. did. The properties of the obtained sintered body are shown in Table 3.

Examples 10 to 11 This example was conducted to find out the effect of the fineness of silicon nitride. When wet-milling the silicon nitride obtained in Example 1, the wet-milling time was set to 2 Hr (Example 10), 4
A silicon nitride powder was obtained in the same manner as in Example 1 except that 0Hr (Example 11) was used to produce a sintered body. The results are shown in Table 4.

Examples 12 to 17 and Comparative Example 5 In this example, the kind of halogen gas was changed.

The same raw material, nitriding sample, and filling amount as in Example 1 were used and the heat nitriding was performed under the nitriding conditions shown in Table 5. The method of introducing the halogen gas was the same as in Example 1. In addition,
For Example 12, the introduction temperature of the halogen gas was 1,150 ° C.
It was set to ~ 1,350 ° C. In addition, in Example 17, solid CaF ₂ was molded and placed beside the nitriding specimen in the same furnace as the method of introducing the halogen gas, not from another furnace, and nitriding was performed while sublimating. Regarding Comparative Example 5, introduction of halogen gas was not performed.

The obtained silicon nitride was wet-ground in the same manner as in Example 1 to obtain a silicon nitride powder. The powder characteristics obtained are shown in Table 5.

The obtained silicon nitride powder was sintered in the same manner as in Example 1 to manufacture and evaluate a sintered body. The results are shown in Table 5.

The grain structure diagrams of the β columnar crystals having a high aspect ratio (Example 1) as in the present invention and the β columnar crystals having a low aspect ratio of the comparative example (Comparative Example 5) are shown in FIGS. 4 and 5, respectively. .

4 and 5 are SEM photographs with a magnification of 7500 times.

Fig. 4 consists of an inhomogeneous homogeneous structure with many β-columnar crystals with an aspect ratio of 10 or more, while Fig. 5 consists of a homogeneous composition with a relatively low aspect ratio (approximately 5). I understand.

Examples 18-26, Comparative Examples 6 and 7 In this example, the type and amount of the sintering aid was changed. In Examples 18 to 26, sintered bodies were produced in the same manner as in Example 1 except that the sintering aid was changed to that shown in Table 6.

In Comparative Examples 6 and 7, the silicon nitride powder used in Examples 25 and 26 was used instead of that in Comparative Example 5. The results are shown in Table 6.

Examples 27-33, Comparative Examples 8 and 9 In this example, the sintering conditions were changed. Example 1
Using the silicon nitride powder composition of, under the conditions shown in Table 7,
A sintered body was manufactured in the same manner as in Example 1 except that sintering was performed. In Comparative Examples 8 and 9, the powder of Comparative Example 5 was used instead of the silicon nitride powder of Example 1. The results are shown in Table 7.

[Effect of the invention] The silicon nitride powder produced by the present invention is low oxygen, Mg
When pressure-sintered with an O—Al ₂ O ₃ —Y ₂ O ₃ -based sintering aid, the powder has a high-temperature bending strength at 1200 ° C. of 700 MPa or more.

This is a result of controlling the powder properties related to the generation and growth of β-columnar crystals in the sintered body.

[Brief description of drawings]

FIGS. 1, 2, and 3 are SEM photographs at a magnification of 3,500 showing the morphology of the silicon nitride particle structures obtained in Example 1 and Comparative Examples 1 and 3, and FIGS. Magnification showing a structural diagram of the grain structure of the silicon nitride sintered bodies obtained in Example 1 and Comparative Example 5.
It is a SEM photograph of 7,500 times.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C04B 35/626 (72) Inventor Hideki Hirotsuru 1 Shinkaimachi, Omuta City, Fukuoka Prefecture Electrochemical Industry Co., Ltd. Omuta Plant (56) References JP 54-22000 (JP, A) JP 64-37469 (JP, A)

Claims (1)

[Claims] 1. A method for producing silicon nitride by nitriding a metal silicon powder in an atmosphere containing nitrogen and / or ammonia, comprising a halide selected from the group consisting of alkali metal halides and alkaline earth metal halides. A method for producing silicon nitride, which comprises continuously, intermittently, or temporarily supplying at least one kind in a gas state.