JPH0796444B2 - Method for producing silicon nitride powder - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a silicon nitride powder 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.

[Conventional technology]

Conventionally, as methods for producing silicon nitride powder, (1) direct metal nitriding method, (2) silica reduction nitriding method, and (3) silicon halide method are known. Probably because the powders produced by these methods have different production 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, and the powder of the method (2) is hardly sinterable but has high high-temperature bending strength, The powder of the method (3) is said to exhibit intermediate performance.

Regarding the amount of oxygen, the powder of the method (1) undergoes a crushing step, so that the total amount of oxygen is usually more than 2% by weight in most cases, but 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 with 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. As a matter of course, the sinterability of the powder and the characteristics of the sintered body are affected by the amount of powder oxygen, but in addition, various other factors such as specific surface area, crystallinity, particle shape, particle size (fine powder), etc. At present, the powder characteristics are entangled with each other, and it is hardly known how the powder characteristics of each of the above-mentioned manufacturing methods are related to the powder characteristics.

The present applicant has found that the formation of β-columnar crystals having a high aspect ratio, which is closely related to high temperature strength, is greatly influenced by the amount of oxygen in the powder, and filed Japanese Patent Application No. 63-277360. on the other hand,
It is expected that the amount of oxygen in the metal silicon powder obtained by the direct nitriding method is affected by the amount of fine powder. Therefore, it is necessary to consider not only the amount of oxygen but also the amount of fine powder to generate β columnar crystals having a high aspect ratio. There are four quantitative combinations of fine powder and oxygen in silicon nitride powder, one of which is low oxygen and small fine powder. ".

The remaining three combinations will be described below to deepen the significance of proposing the present invention.

First, most of the silicon nitride powder having a large amount of oxygen and a large amount of fine powder can be obtained by the direct nitriding method of metallic silicon powder. This is because this process results in fine powders rich in oxygen as the grinding step is unavoidable. Since this silicon nitride powder is easily sinterable, it has a feature that β-columnar crystals are very small, and the high temperature strength is not good, but the room temperature strength is excellent.

Secondly, a powder having a large amount of fine powder and a small amount of oxygen is currently available. However, it may be obtained when it is synthesized by the plasma laser method or the like and has strong oxidation resistance or is stabilized. The silicon nitride powder having such powder characteristics is probably composed of β columnar crystals having a very high aspect ratio.

Most of the silicon nitride powders having a small amount of fine powder and a certain amount of oxygen are obtained by the silicon halide method. The silicon nitride powder is easy to sinter with any sintering aid, and since the amount of fine powder is small, the β columnar crystal is also appropriately large. Therefore, it is widely used for research on high-temperature structural materials, but there is a difficulty in high cost resulting from the price of SiCl ₄ .

[Problems to be Solved by the Invention]

The present inventors have made various studies as to whether a silicon nitride powder having the above-mentioned powder characteristics such as the silicon halide method can be obtained by the direct nitriding method of metal silicon powder. The inventors have found that it is possible by nitriding it under a special atmosphere gas, and completed the present invention.

[Means for Solving the Problems]

 That is, the present invention is summarized below.

1. In a method for producing silicon nitride by nitriding metallic silicon powder in an atmosphere containing nitrogen and / or ammonia,
As a raw material, one containing 1 to 5 parts by weight of silicon oxide with respect to 100 parts by weight of metal silicon powder, and at least one kind of halide selected from the group consisting of alkali metal halides and alkaline earth metal halides,
A method for producing silicon nitride, characterized by continuously, intermittently or temporarily supplying and nitriding in a gas state.

2. The method for producing silicon nitride according to claim 1, wherein the halide is a fluoride of calcium or magnesium.

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

First, the silicon nitride powder produced by the present invention will be described. The oxygen content of the silicon nitride powder is 0.8 to 1.5% by weight. When the amount of oxygen is less than 0.8% by weight, when the amount of the usual sintering aid used is, for example, about 7 parts by weight of Y ₂ O ₃ —Al ₂ O ₃ system, insufficient sintering occurs and the strength development deteriorates. On the other hand, the amount of oxygen is 1.5
If the content exceeds 10% by weight, the α → β transition that occurs during sintering tends to occur from a low temperature, and since the amount of grain boundary phase formed by the sintering aid increases, the solubility in silicon nitride increases, As a result, the number of β nuclei increases, and it becomes difficult to obtain sufficiently grown β columnar crystals with a high aspect ratio.

The average particle size is very important, especially when normal pressure sintering is adopted, and generally it is preferable that it is small, 0.3 to 0.8 μm.
Is the range. If it exceeds 0.8 μm, the solubility of silicon nitride in the composite oxide caused by the reaction between the sintering aids such as yttrium oxide, magnesium oxide, aluminum oxide, etc. and oxygen contained in the silicon nitride powder is lowered, and the density is sufficiently dense. It does not change. On the other hand, 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, the number of β nuclei increases, and a sufficiently grown β columnar crystal with a high aspect ratio can be obtained. Will be difficult.

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

As described above, 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 and then supersaturated to precipitate as β-silicon nitride. 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%, β-columnar crystals having a high aspect ratio are hard to crystallize, and the high temperature strength cannot be 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. If it exceeds 500 ppm, the creep strength at high temperature will decrease if it is used for a long time at high temperature. Therefore, it is preferable that the metal impurities are as small as possible.

Regarding the specific surface area, when a shape is formed using various molding methods such as press molding, injection molding, slip casting, etc., it is generally 6 because of handling, viscosity and the like.
It is preferably ˜14 m ² / g. If it is less than 6 m ² / g,
The grain size becomes coarse, resulting in poor sinterability. on the other hand,
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. Furthermore, if it is less than 6 m ² / g, the particles become large, the sintered 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.

The fine powder amount is preferably 7% by volume or less when the fine powder amount is 0.2 μm or less as described in Japanese Patent Application No. 63-277360. Beyond 7% by volume, β
The number of nuclei of columnar crystals increases, and it becomes difficult to obtain sufficiently grown β columnar crystals with a high aspect ratio.

Next, a method for producing the silicon nitride powder of the present invention will be described.

The method for producing a silicon nitride powder of the present invention, while introducing an atmosphere gas containing nitrogen and or ammonia, nitriding the metal silicon powder, in the method of producing silicon nitride,
1 to 5 parts by weight of silicon oxide per 100 parts by weight of metallic silicon powder was used as a raw material, and was selected from alkali metal and alkali metal halides.
One kind or two or more kinds are continuously, intermittently or temporarily supplied in a gas state.

More specifically, the particle size of the metal silicon powder used in the present invention is preferably under 88 μm. Thus, the reason why metallic silicon having a relatively large particle size can be used is
As will be described later, not all solid silicon reaction with the nitrogen source is solid / gas reaction.
It is greatly involved in the progress of nitriding by the gas reaction. That is, according to the present invention, even if metallic silicon of about 88 μm is used, if part of the surface undergoes nitriding due to solid-gas reaction,
This is because the phenomenon of destruction of metallic silicon occurs due to the difference in density between metallic silicon and α-silicon nitride, resulting in fine metallic silicon.
However, if the thickness exceeds 88 μm, a similar phenomenon occurs, but the temperature at which the surface solid-gas reaction occurs becomes high, and free Si tends to remain under normal nitriding conditions, which is not preferable. There is no particular lower limit. That is, the use of a metal silicon powder having a smaller average particle size facilitates the production of SiO (G) described later and promotes gas-gas reaction, and thus makes it easier to produce the target granular α-silicon nitride ingot. Become.

In the present invention, it is necessary to appropriately increase the amount of oxygen in the powder as well as the granulation, so that oxygen in the metal silicon powder may be appropriately present. Specifically, since the amount of silicon oxide added to the raw material metal silicon powder is much larger, it may be contained up to about 0.5%.

The compounding amount of silicon oxide is 1 to 5 parts by weight with respect to 100 parts by weight of metallic silicon powder. If the amount is less than 1 part by weight, the target oxygen content in the silicon nitride powder does not reach 0.8% by weight, and if it is about 7 parts by weight of Y ₂ O ₃ —Al ₂ O ₃ system, the sintering will be insufficient. Strength development deteriorates. On the other hand, when the amount exceeds 5 parts by weight, the oxygen content in the silicon nitride powder is 1.5% by weight contrary to the above.
, The α → β transition that occurs during sintering is likely to occur from a low temperature, and since the amount of grain boundary phase formed by the sintering aid increases, the solubility in silicon nitride increases, As a result, the number of β nuclei increases, and it becomes impossible to obtain sufficiently grown β columnar crystals with a high aspect ratio. The type of silicon oxide is not particularly limited, but one having a specific surface area of 10 m ² / g or more is desirable. Specifically, it is white carbon, Aerosil, or the like.

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. That is, the reaction between the metal silicon powder and nitrogen is
Together knows that it is limited by the gas-gas reaction of N ₂ -NH _3, or N ₂ -H ₂ system rather via the O ₂ from the gas reaction, SiO
The reaction formula of Si ₃ N ₄ formation involving (G) was further investigated. One example is shown below. Regarding the morphology in solid production by such 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, formula (2) is about 46.

Therefore, it is presumed that the morphology of silicon nitride produced by the equations (1) and (2) is whiskers and granular.

3SiO (G) + 4NH ₃ (G) = Si ₃ N ₄ (S) + 3 / 2O ₂ (G) + 6H ₂ (G) (1) 3Si (G) + 4NH ₃ (G) = Si ₃ N ₄ (S) + 6H ₂ (G)
(2) In addition, taking CaF ₂ (G) as an example of the reaction formula until Si (G) is generated, the estimated reaction and its circulating reaction in the microscopic crystal field are shown in Formulas (3) to (6).

Si (S) + 1 / 2O 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 / 2O ₂ (G)) (6) As is clear from this, the halide gas entrained as a gas is It is also possible that they are circulating in a 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. Although it is not clear about controlling the oxygen in the silicon nitride ingot or the silicon nitride powder by adding silicon oxide to the metal silicon powder, generation of SiO (G) from SiO ₂ (S) ( It is presumed that higher temperature is required than the equation (3), and in that case, the reaction of the equation (1) occurs preferentially rather than the equation (4), so that oxygen may be easily taken up in the reaction field. .

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 the atmosphere gas containing nitrogen and / or ammonia, the gas of each halide of the alkali metal and the alkaline earth metal is intentionally supplied to generate the silicon nitride. It was found that the morphology was granular, not whisker-like. As for the concentration of the 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 alkaline earth metal halide gas, or ,
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 added to the furnace in which the metal silicon powder is charged while controlling the concentration. Supplied as a companion. Alternatively, it is also possible to place a specified amount of the halide in the vicinity of the metal silicon powder in a furnace containing the metal silicon powder, and supply the metal silicon powder by nitriding 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 halide of the gas introduced as described above is circulated in the system in the microscopic crystal field. 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 produced was still the same as that during continuous introduction up to 1,450 ° C.

Examples of the alkali metal and the alkali metal halide include fluorides, chlorides and bromides of Li, Na, K, Mg, Ca, Sr and Ba elements. 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. Furthermore, it is not limited to the nitriding furnace, and any of a batch furnace, a continuous pusher furnace, a continuous rotary furnace, a continuous screw furnace, and a fluidized bed can be applied.

Specific examples of the grain structure of the granular silicon nitride produced by the present invention 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 shows a scanning electron microscope (SE
M) It is a photograph.

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

It is clear that Qi-Qi reaction also occurred in Fig. 2, but Si
Maybe due to lack of supersaturation degree of O (G), it has many needle-like 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.

When such a granulated silicon nitride is pulverized, fine powder is less likely to be produced, and an appropriate amount of oxygen is uniformly distributed in each particle.

Here, the granular silicon nitride obtained according to the present invention will be described in more detail. As shown in FIG. 2, whiskers or needle-like crystals having a small diameter are not crushed while physically bending, Since the crushing progresses in the form of loosening the granular crystals as shown in the figure, the generation of fine powder due to the crushing of the ingot is reduced. That is, it becomes a silicon nitride powder close to equiaxed powder without fine powder as obtained by the silicon halide method.
Quantitatively, nitriding to obtain a specific surface area of ² to 5 m ² / g when the coarse / medium crushed product is crushed to a coarse / medium crushed product using an ordinary coarse crusher and medium crusher, especially a particle size of 0.2 mm. Silicon,
Further, it is silicon nitride having an oxygen content of 0.4 to 1.2%.

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 jaw crusher or a gyratory crusher, or a medium crusher such as a roll crusher, a roller mill or an edge runner.

The α-silicon nitride produced by the present invention having the small granular crystals obtained as described above is prepared by a conventional method, for example, after crushing / medium crushing, a ball mill, a vibration mill, a jet mill, an attritor mill, Wet and dry pulverize with a pearl mill 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.

〔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.

In addition, the measured value shown in each example was based on the following method.

(1) Oxygen (% by weight): According to TC-136 type O / N simultaneous analyzer manufactured by LECO.

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

(3) particle size ([mu] m): manufactured by HORIBA, Ltd. C _A P _A -700
by.

(4) α fraction (%): by X-ray diffraction of Geiger flux RAD-II B type manufactured by Rigaku Denki KK

(5) Metal impurities (PPm) (Fe, Al, Ca): JIS-G-132
Compliant with 2.

(6) Relative density (%): By Archimedes method.

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

Examples 1 to 7, Comparative Examples 1 to 4 100 parts by weight of metallic silicon powder having a metal Si purity of 99.98% by weight is α
Mixed powder of 10 parts by weight of silicon nitride powder with a fraction of 90% and a specific surface area of 20 m ² / g and SiO ₂ (specific surface area of 50 m ² / g) in the ratio shown in Table 1.
2.5 kg was molded into a nitriding specimen having a bulk specific gravity of about 150 × 150 × 20t shown in Table 1 and filled in an electric furnace.

Nitriding was carried out as shown in Table 1, but at that time, FaF ₂ (G) (18-36) / (18-36) / was fed from another electric furnace filled with 2720 g of solid CaF ₂ and kept at 1,200 ° C. Hr
It was heated and nitrided while being accompanied by argon gas so that the temperature became (25 ° 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 thereof, 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 to 0.2 mm with coarse crushing / medium crushing (jaw crusher and top grinder).
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 therein, wet-milled for 20 hours, acid-treated with hydrochloric acid and hydrofluoric acid, filtered, dried and crushed to produce a silicon nitride powder for 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, this silicon nitride powder was mixed with Y having an average particle size of 1.3 μm.
₂ O ₃ , average particle diameter 1.4 μm Al ₂ O ₃ , average particle diameter 1.2 μm
MgO was added at the ratio shown in Table 2, 1,1,1-trichloroethane was added, and the mixture was wet mixed in a ball mill for 4 hours, dried, and then molded into a 6 × 10 × 60 mm shape at a molding pressure of 100 kg / cm ^2. After molding by die, CIP molding was performed at a molding pressure of 2700 kg / cm ² . These molded bodies were set in a carbon crucible, N ₂ gas atmosphere,
Sintered bodies were obtained by firing under the conditions shown in Table 2. 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. For Example 12, the halogen gas introduction temperature was set to 1,15
The temperature was 0 to 1,350 ° C. In addition, in Example 17, the method of introducing the halogen gas was not from another furnace, but solid CaF ₂ was molded and placed beside the nitriding specimen in the same 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 (using a sintering aid of Y ₂ O ₃ = 5% and Al ₂ O ₃ = 2% at 1750 ° C.
X6 Hr normal pressure sintering) was performed to manufacture and evaluate a sintered body. The results are shown in Table 5.

[Effects of the Invention] The silicon nitride powder produced according to the present invention has a small amount of fine powder and appropriately contains oxygen. Therefore, even if any sintering aid is used, the sinterability is very excellent and β-columnar crystals are also suitable. Very high, high temperature strength is developed.

[Brief description of drawings]

1, 2 and 3 show the grain structure of the silicon nitride obtained in Example 1, Comparative Example 1 and Comparative Example 3 at a magnification of 3500 times.
It is a SEM photograph.

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Claims (2)

[Claims] 1. A method for producing silicon nitride by nitriding metal silicon powder in an atmosphere containing nitrogen and / or ammonia, wherein 1 to 5 parts by weight of silicon oxide is contained as a raw material with respect to 100 parts by weight of metal silicon. And at least one of the halides selected from the group consisting of alkali metal halides and alkaline earth metal halides.
A method for producing a silicon nitride powder, which comprises nitriding a seed in a gas state continuously, intermittently or temporarily. 2. The method for producing a silicon nitride powder according to claim 1, wherein the halide is a fluoride of calcium or magnesium.