JPH0948670A - Silicon nitride powder and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a silicon nitride powder stably, without fouling the inside of a heating furnace for a reaction, without existing oxides or oxynitrides on its surface and high in purity and α-forming ratio. SOLUTION: This silicon nitride powder having >=96% α-forming rate and <=0.4wt.% metallic impurity, is obtained by heating a metallic silicon powder containing oxygen with an aluminum oxide powder presenting without a contact thereto under a nitriding atmosphere.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-purity α-type silicon nitride powder and a method for producing the same.

[0002]

2. Description of the Related Art α-type silicon nitride powder is synthesized by reacting metallic silicon powder with a substance serving as a nitrogen source, for example, nitrogen gas and directly nitriding it. This method is suitable for mass production on an industrial scale because the synthesis process is relatively simple.

[0003] This metallic silicon powder generally contains a trace amount of oxygen as an unavoidable impurity due to restrictions in refining silica, which is a silicon source, and the surface thereof is extremely easily oxidized and is usually oxidized on the surface layer portion. Since silicon is formed, most of the silicon powders actually used contain about several wt% of oxygen.

In order to obtain high-purity silicon nitride powder by directly nitriding metallic silicon, it is necessary to remove this oxygen. However, it is difficult to selectively remove only oxygen, and it is usually removed by high-temperature vaporization in the form of silicon monoxide or an oxide with a trace metal contained as impurities other than silicon.

The reaction of synthesizing α-type silicon nitride by directly nitriding metallic silicon from which surface oxygen has been removed in this way is an exothermic reaction, but since the melting point of metallic silicon is around 1410 ° C., an excessive reaction occurs. When this occurs, the metallic silicon is melted and its specific surface area is extremely reduced, so that the synthesis reaction is difficult to proceed.

Therefore, various measures have been studied and implemented in order to stably synthesize α-type silicon nitride. That is, the reaction is performed for a long time in a low temperature region, or the reaction temperature is suppressed from rising by lowering the nitrogen partial pressure of the atmosphere so that the reaction temperature of nitriding does not cause a sudden increase in the raw material temperature, and the nitriding reaction heat prevents the raw material temperature from rapidly increasing. In addition to the above measures, by mixing gases such as H ₂ , NH ₃ and the like, which also have the effect of reducing surface oxygen of Ar, He and metallic silicon in the atmosphere, the thermal conductivity of the atmosphere gas is increased. It is also done to prevent the heat from being stored. Further, when high-purity silicon nitride is not required, the addition of calcium or iron oxides or nitrides to metallic silicon promotes the nitriding reaction and stabilizes α-type silicon nitride with high productivity. It is also done by synthesizing.

[0007]

When silicon oxide is formed on the surface of metallic silicon, when the direct nitriding is carried out, the nitriding gas and metallic silicon are directly contacted by the oxide layer on the surface.
In some cases, the reaction was hindered, the nitriding reaction decreased, unnitrided silicon was present, and a long reaction time was required. Furthermore,
At high temperature, the silicon oxide on the surface of metallic silicon decomposes and vaporizes as SiO gas, and the generated SiO gas reacts with the heater material in the heating device to corrode the heater, and also the cooling part of the heating device or the low temperature part. Since it tends to be deposited on the member, the cooling effect of the device may be reduced or the life of the device may be shortened. Further, the SiO gas may react with the produced silicon nitride to produce a silicon oxynitride compound on the surface of the silicon nitride. In this way, the silicon nitride powder obtained by directly nitriding metallic silicon containing a large amount of oxygen on the surface thereof produces oxynitride or remains unreacted metallic silicon, resulting in high-purity and high-alpha silicon nitride. I couldn't get it.

[0008]

In order to solve the above problems, the present inventors have found that when metallic silicon is heated in the presence of aluminum oxide powder in a nitriding atmosphere, the silicon oxide formed on the surface of the metallic silicon. The vaporized SiO 2 is selectively captured rapidly by the aluminum oxide powder with little effect on the heating furnace material including the heater, and at least silicon having no oxygen on its surface is used for the nitriding reaction,
The inventors have found that high-purity silicon nitride can be obtained, and that this nitriding starts from a relatively low temperature of about 1200 ° C., so that α-type silicon nitride is likely to be formed, and the present invention has been completed.

That is, according to the present invention, a metal silicon powder containing oxygen is heated in an nitriding atmosphere together with an aluminum oxide powder coexisting without being brought into contact with the powder, and the α-conversion rate is 9.
The silicon nitride powder is characterized by containing 6% or more and 0.4% by weight or less of metal impurities.

Further, according to the present invention, the α-conversion rate obtained by heating metallic silicon powder containing oxygen together with aluminum oxide powder coexisting without contacting the powder in a nitriding atmosphere is 9
The method for producing a silicon nitride powder is characterized in that 6% or more and 0.4% by weight or less of metal impurities.

BEST MODE FOR CARRYING OUT THE INVENTION

The commercially available metallic silicon powder is extremely susceptible to oxidation, and generally, silicon oxide having an amorphous structure is mainly formed mainly on the surface portion, and this silicon oxide layer is once formed on the surface. And it becomes stable with almost no oxidation. The metallic silicon that is the object of the present invention is in such a state, and contains approximately 3% by weight or less of oxygen mainly in its surface portion.

Further, the silicon nitride powder of the present invention comprises 96% or more of α-type crystals, and its purity is, for example, Fe, Al, Ca which is inevitably contained in the metallic silicon refining stage.
Impurities made of metal such as metal and silicon are 0.4 wt% or less.

Such a silicon nitride powder is manufactured by the following method. That is, as the raw material, metallic silicon powder containing oxygen in an amount of approximately 0.5 to 3% by weight on the surface as described above is used. The particle size is preferably smaller, for example, 105 μm or less. Metallic silicon having a large particle size is not preferable because the nitriding reaction takes a long time and the inside may not be completely nitrided. This metallic silicon powder containing oxygen is placed in a heating furnace having a heating chamber capable of being isolated from the atmosphere. The heating furnace may be any one as long as it has a heating chamber that can be isolated from the atmosphere and can supply an atmosphere gas such as nitrogen to the heating chamber. For example, a batch furnace, a pusher furnace, a tunnel furnace that can control the atmosphere. A belt furnace or the like can be used.

As a method of installing the aluminum oxide powder to be coexistent with the metallic silicon powder in the heating furnace, the metallic silicon powder is placed in a container made of a material such as reaction-sintered silicon nitride and placed in the heating furnace. The aluminum oxide powder may be laid directly on the hearth, or may be placed in an open container made of a material that does not react with any of the hearth material and aluminum oxide. The aluminum oxide powder and the metal silicon powder in the heating furnace are preferably installed at positions as close to each other as possible without contacting each other.For example, in the case of a batch furnace, the aluminum oxide powder is loaded on the bottom shelf plate, Metallic silicon powder is loaded on the upper shelf. Alternatively, in the case of a continuous furnace, another storage container having a vent hole containing aluminum oxide powder may be installed on the same shelf plate. It is preferable that the coexisting aluminum oxide powder has a larger area occupied by the aluminum oxide powder in the furnace than the absolute amount thereof, because the silicon oxide trapping capacity that can be trapped at one time increases. It is usually sufficient to use an amount of about 10% of the weight of metallic silicon to be treated.

The aluminum oxide powder used in the present invention is
Both α-type and γ-type crystals may be used, but the highly active γ-type is more preferable. Further, the impurities contained in aluminum oxide may be contained as long as they do not affect the nitriding of metal silicon, the removal and absorption of silicon oxide from metal silicon, and the purity of generated silicon nitride. . Such acceptable impurities may be those which are usually contained in so-called low-priced commercial products, and their content is about 5%. The smaller the particle size of such aluminum oxide is, the larger the surface area is because the aluminum oxide has a large surface area, and the vaporized silicon oxide is easily trapped, which is preferable. The aluminum oxide powder used in the present invention is preferably in the form of single particles, but aggregated particles or powder compacts can also be used.

The temperature required for the nitride formation reaction by direct nitriding of metal silicon powder is 1200 ° C. or lower, and the nitriding reaction does not occur or is extremely slow. Therefore, a temperature exceeding 1200 ° C. is excellent. Although it is possible to generate a nitride with a high nitriding efficiency, 1410 ° C. or higher is preferable in order to stably obtain a higher nitriding rate. Again 1500
If the temperature exceeds ℃, β-type silicon nitride is easily synthesized, which is not preferable. Further, the nitriding atmosphere in the present invention may be an atmosphere in which silicon metal can be directly nitrided at a predetermined temperature to generate a nitride, and nitrogen is the main component, and argon, helium, hydrogen, ammonia, etc. A mixed gas mixed in a small amount may be used, but nitrogen gas alone is the best from the viewpoint of suppressing the corrosion of the furnace material and heater material, and also from the viewpoint of being relatively inexpensive. The silicon nitride powder of the present invention can be produced by directly nitriding metallic silicon under the above-mentioned installation and heating conditions, and then naturally cooling the reaction product to near room temperature in the furnace.

[0017]

It is considered that the aluminum oxide in the present invention captures the silicon oxide formed on the surface of metallic silicon by the following actions. That is, the silicon oxide formed on the surface of the metallic silicon is vaporized as SiO at about 1000 ° C. or higher, and the silicon at the separation point immediately after the separation of SiO is in an extremely high energy state, and the reaction activity is increased. Since the nitrogen that is present in the form of gas exists in a gaseous state, the partial pressure of nitrogen in the system may relatively decrease with a rapid increase in the amount of SiO gas, and the reaction may slow down.
Furthermore, this SiO gas is more active than nitrogen gas, and it is often seen that it reacts with the heater and / or the furnace wall material in the heating furnace during heating, and the stable SiO gas is stable during the cooling process after heating.
_It easily becomes ₂ and precipitates on the heater and / or furnace wall material.

On the other hand, in this high temperature process, if the aluminum oxide powder which is in a higher active state than the heater and the furnace wall material coexists, the SiO gas reacts with the aluminum oxide powder more quickly than the heater and the furnace wall material. done, also it can supply a source of oxygen SiO ₂ reduction, and SiO ₂
Since Al ₂ O ₃ and Al ₂ O ₃ easily form a stable compound or solid solution, there is no re-desorption of SiO gas after the reaction and it can remain stable until the cooling process. Further, since the aluminum oxide powder has a surface area which is sufficiently larger than that of the heater and the wall material of the furnace, it becomes a reaction source sufficient to react with the total amount of generated SiO gas. In this way, the aluminum oxide powder reacts with the SiO gas more quickly and preferentially than the nitrogen gas, and the amount of unreacted residual SiO gas in the furnace can be kept to a minimum level.

From the above, the coexistence of the aluminum oxide powder facilitates the nitriding reaction of metallic silicon by direct nitriding, and the furnace material in the heating furnace is hardly contaminated, and the nitriding atmosphere is maintained. In addition, a high-purity α-type silicon nitride powder having almost no oxide or oxynitride on the surface can be obtained in a stable and high yield.

[0020]

EXAMPLES The present invention will be described in detail below with reference to Examples and Comparative Examples. Example 1 Metallic silicon powder 800 having a particle size of 44 μm or less, containing 2.0% by weight of oxygen (all% are% by weight unless otherwise specified) and 0.2% of metal impurities centered on the surface portion.
g to a shelf plate made of reaction-sintered silicon nitride having a thickness of 1.2 cm, each having a length of about 15 cm, a width of about 25 cm, and a thickness of about 1 c.
200 g each was loaded so as to be m. Further, 80 g of aluminum oxide powder containing 0.05% of metal impurities and having a particle size of 50 μm or less was placed on a shelf plate made of the same reaction-sintered silicon nitride as above, about 15 cm in length, about 25 cm in width, and maximum thickness. It was loaded so that it would be about 0.5 cm. Next, columns of reaction-sintered silicon nitride capable of installing the above-mentioned respective shelf plates one by one over 5 stages at intervals of approximately 5 cm above the hearth of a batch type resistance heating furnace having a heating chamber volume of 0.05 m ³ were assembled. A shelf board loaded with aluminum oxide was installed in the lowest stage, and a shelf board loaded with metallic silicon was installed in each of the second to fifth steps from the bottom. The outline of this installation state is shown in FIG.

[0021]

[Figure 1]

After the installation, the atmosphere in the furnace was replaced with nitrogen gas having a purity of 99.99%, the nitrogen atmosphere was maintained at about 1 atm, and the temperature rising rate was from room temperature to 1000 ° C at 500 ° C / hour, 1000 ° C.
Heat up to 1410 ° C to 100 ° C / hour,
After holding at 1410 ° C. for 1 hour, it was naturally cooled in the furnace to near room temperature. After cooling, the nitriding reaction product of the metallic silicon powder was subjected to qualitative and quantitative analysis by X-ray diffraction,
The reaction product is silicon nitride containing 0.2% of unreacted residual silicon and 0.1% of metal impurities other than silicon.
The conversion rate was 97%. Further, at least by visual observation, the heater of the heating furnace and the furnace wall were not deteriorated, and no new product was observed on the heater or the furnace material. still,
From the cooled aluminum oxide powder, the presence of SiO ₂ other than Al ₂ O ₃ initially contained as impurities was detected by powder X-ray diffraction, and a small amount of Si-A
I-O type compounds and Si-Al-O-N type compounds were also detected.

[Example 2] Oxygen of 2.0 with the surface portion as the center
A rack made of reaction-sintered silicon nitride having a thickness of 1.2 cm, containing 800 g of metal silicon powder having a particle size of 44 μm or less and containing 0.2% by weight of metal impurities and 0.2% of metal impurities. Approximately 15 cm long and 25 c wide on each of the four plates
m and a thickness of about 1 cm, 200 g each was loaded.
Further, 80 g of aluminum oxide powder containing 0.05% of metal impurities and having a particle size of 50 μm or less was placed on a shelf plate of the same reaction sintered silicon nitride as above, about 15 cm in length and about 25 c in width.
m, and the maximum thickness was about 0.5 cm. Next, columns of reaction-sintered silicon nitride capable of installing the above-mentioned respective shelf plates one by one over 5 stages at intervals of approximately 5 cm above the hearth of a batch type resistance heating furnace having a heating chamber volume of 0.05 m ³ were assembled. A shelf board loaded with aluminum oxide was installed in the lowest stage, and a shelf board loaded with metallic silicon was installed in each of the second to fifth steps from the bottom. After installation, set the furnace atmosphere to a purity of 9
It is replaced with 9.99% nitrogen gas, the nitrogen atmosphere is kept at about 1 atm, and the temperature rising rate is 500 ° C / from room temperature to 1000 ° C.
Hour, 1000-1200 ℃ up to 100 ℃ / hour, 1200
Up to -1400 ° C. was heated to 50 ° C./hour, and from 1400 to 1500 ° C. was heated to 100 ° C./hour, and immediately after reaching 1500 ° C., it was naturally cooled in the furnace to near room temperature. After cooling, the nitriding reaction product of the metallic silicon powder was analyzed in the same manner as in Example 1. As a result, the reaction product was free of unreacted residual silicon and contained 0.1% of metal impurities other than silicon. Containing silicon nitride, and the α conversion rate was 97%. Further, at least by visual observation, neither the heater of the heating furnace nor the deterioration of the furnace material and new products were observed. Further, from the cooled aluminum oxide powder, powder X-ray diffraction was performed to obtain SiO.
₂ , Si-Al-O-based compounds, and Si-Al-O-N
A system compound was detected.

[Comparative Example 1] The same analysis as in Example 2 was performed on the nitriding reaction product of the metal silicon powder produced in the same manner as in Example 2 except that the aluminum oxide powder was not provided. In this reaction product, some of the silicon was melted, and unreacted residual silicon was 30%, metal impurities other than silicon were 0.1%, and silicon oxynitride (Si ₂ O
N ₂ ) was silicon nitride containing 0.05% of each, and the α conversion rate was 80%. Further, at least by visual observation, a white fiber-like substance was deposited on the heater of the heating furnace and almost the entire surface of the furnace wall. Precipitation of this material was also found on the surface of the shelf plate and a part of the generated silicon nitride powder.
When this precipitate was examined by powder X-ray diffraction, it was found that SiO
₂ has been detected.

COMPARATIVE EXAMPLE 2 Instead of installing aluminum oxide powder, 0.05% of metal impurities were contained and the particle size was 50.
80 g of aluminum nitride powder having a size of less than or equal to μm is placed on a shelf plate made of reaction-sintered silicon nitride to measure about 15 cm in length and about 25 cm in width
Install the loaded one so that the thickness is about 0.5 cm,
Other conditions and methods were the same as in Example 1 with respect to the nitriding reaction product of the metallic silicon powder produced in the same manner as in Example 1. As a result, the reaction product showed that unreacted residual silicon was present. It was silicon nitride containing 42% and 0.1% of metal impurities other than silicon, and its α conversion rate was 97%. Further, at least by visual observation, a white fiber-like substance was deposited on the heater of the heating furnace and almost the entire surface of the furnace wall. Precipitation of this material was also found on the surface of the shelf plate and a part of the generated silicon nitride powder. When this precipitate was examined by powder X-ray diffraction, SiO ₂ was detected.

[Brief description of drawings]

FIG. 1 is an explanatory diagram in which metallic silicon powder and aluminum oxide powder are installed in a heating furnace.

[Explanation of symbols]

 1 Nitriding Gas Inlet 2 Heater 3 Gas Outlet Valve 4 Hearth 5 Aluminum Oxide Powder 6 Metal Silicon Powder 7 Powder Loading Shelf Board

Claims (2)

[Claims] 1. A metal silicon powder containing oxygen is heated in a nitriding atmosphere together with an aluminum oxide powder made to coexist without being brought into contact with the powder, and the α-conversion rate is 96% or more, and the metal impurities are 0.1. Silicon nitride powder characterized by being 4% by weight or less. 2. A metal silicon powder containing oxygen is heated together with an aluminum oxide powder coexisting without being in contact with the powder in a nitriding atmosphere to have an α-conversion rate of 96% or more and a metal impurity of 0.1. 4% by weight or less, a method for producing a silicon nitride powder.