JPS6177608A - Production of fine powdery alpha silicon nitride - Google Patents

Abstract

PURPOSE:To produce fine powdery silicon nitride having superior particle characteristics with good yield inexpensively by using Mg and an Mg compd. as nitridation accelerator in the reduction nitridation process of silicon oxide, and using fine silicon nitride powder having a specified specific surface area. CONSTITUTION:Silicon nitride is produced by heat-treating a mixture of silicon oxide powder and carbon powder at high temp. in an atmosphere contg. N2. In this case, at least one selected from Mg and Mg compds. and fine silicon nitride powder having 15-100m<2>/g specific surface area (determined by BET process) are added to the above-described mixture. By this method, inexpensive SiO2 having coarse particles having ca. 1-100mu median particle size is also useful. Moreover, uniform and fine alpha-Si3N4 particles having <=1mu particle size and almost spherical shape are obtd.

Description

Detailed Description of the Invention The present invention relates to a method for producing α-type silicon nitride (α-”13N4) fine powder. More specifically, a method for obtaining high-grade, fine α-type silicon nitride fine powder with good yield and at low cost. It provides:

Silicon nitride sintered bodies have excellent heat resistance and high temperature strength, and are used as high-strength heat-resistant materials and high-precision wear-resistant materials that can achieve higher temperatures, lighter weight, and higher efficiency in heat engines such as diesels and gas turbines. It is expected that The thermal and mechanical properties of these sintered bodies largely depend on the properties of the raw material powder for the sintered bodies. Supply is desired.

Among the silicon nitride synthesis methods, the reduction nitridation method of silicon oxide has the following advantages: the reaction operation is relatively easy, it does not corrode the equipment, it does not use raw materials that are hazardous to explosions, etc., and it has a high proportion of α-type silicon nitride. This method is attracting attention as an industrially advantageous method because silicon nitride can be easily obtained.

However, even if this method uses carefully selected silicon oxide powder and carbon powder as raw materials, it usually only yields silicon nitride powder with a size of several μm, and in some cases, needle-shaped crystals and rod-shaped particles are mixed. The problem is that uniform α-type silicon nitride fine powder with a shape close to a spherical shape of 1 μm or less cannot be obtained, and when the carbon/silicon oxide ratio in the raw material is small, the nitriding reaction rate is low and unreacted silicon oxide remains. There is a problem. In addition, these problems are caused by the central particle size of 1 μm
The more coarse-grained silicon oxide powder is used, the more pronounced this becomes, and it becomes a major barrier to obtaining α-type silicon nitride powder at a lower cost.

In order to increase the nitriding reaction rate, a method of adding oxides such as iron, manganese, magnesium, etc. as a catalyst (Ceramic Industry Association Journal vol. 1.85 [11j 1977 P, 587-542)
) has been proposed. However, the nitriding rate can be improved even if iron oxide, magnesium 62ide, calcium oxide, manganese dioxide, cobalt oxide, chromium oxide, and vanadium pentoxide, which are mentioned here as substances that promote the nitriding reaction, are added as catalysts. However, the particle diameter of the silicon nitride produced is usually several μm, and needle-like crystals and rod-like particles are mixed therein. This tendency is more pronounced as silicic acid oxide with a larger particle size is used as a raw material. Further, when iron oxide, manganese dioxide, cobalt oxide, and chromium oxide are added, silicon carbide is likely to be produced together, and when vanadium pentoxide is added, β-type silicon nitride is likely to be produced. In other words, although the substances described here are effective as catalysts for promoting the nitriding reaction, there is a problem in controlling the particle size and shape of the silicon nitride particles that are produced. It has little effect on the purpose of producing fine powder.

A method of adding silicon nitride fine powder of 2 μm or less as a method of accelerating the nitriding reaction and controlling the particle shape (Japanese Patent Publication No. 54-28917, Japanese Patent Application Laid-Open No. 58-91005)
No. Publication, Proceedings of the 1st Next Generation Industrial Infrastructure Technology Symposium, November 11, 1988, p. 27-46) has been proposed.

However, as described in the above-mentioned publications and literature, this method requires that the particle size of the raw material silicon oxide powder be 20-40 m.
The effect is remarkable if it is a fine powder of μ, but the particle size is 1 μm.
When using the above coarse-grained silicon oxide powder, the nitriding reaction rate is slow, the α-type silicon nitride content is low, and the particle shape of the silicon nitride produced cannot be controlled, resulting in needle-like crystals and rod-like particles. In other words, this method is not effective when the particle size of silicon oxide used as a raw material is as large as 1 μm or more.

In the synthesis of silicon nitride by the reduction and nitriding reaction of silicon oxide, the raw material cost is a very important proportion of the production cost.

In particular, the price of silicon oxide used as a raw material depends on its particle size, etc. Fine silicon oxide powder with a particle size of 20-40 mμ is expensive, and in order to significantly reduce production costs, coarse powder with a particle size of 1 μm or more is used. There is a strong desire to develop a method that can use granular silicon oxide. In view of these circumstances, the present inventors have developed Mg as a reduction/nitriding reaction catalyst in a reduction/nitriding method using silicon oxide and carbon as raw materials. and Mg compounds, and a BFT specific surface area of 15 to 100.
When adding fine silicon nitride powder of
Even when silicon oxide coarse particles with a particle size of 1 μm or more are used, the synergistic effect of these additives makes it possible to obtain uniform α-type silicon nitride fine powder with a high nitriding rate and a shape close to a spherical shape with a center particle diameter of 1 μm or less. It has been found that this can be obtained efficiently, and the present invention has been achieved.

That is, the present invention provides a method for producing silicon nitride by heat-treating a mixture of silicon oxide powder and carbon powder at high temperature in a nitrogen-containing atmosphere, in which at least one selected from Mg and Mg compounds and BET are added to the mixture. The present invention provides a method for producing α-type silicon nitride fine powder, which comprises adding silicon nitride fine powder having a specific surface area of 15 to 100 d/1/.

According to the present invention, silicon nitride fine powder with good particle properties can be obtained at low cost, and its industrial value is extremely large.

The present invention will be explained in detail below.

The silicon oxide powder used in the present invention preferably has a center particle size of 100 μm or less and is as pure as possible.

Even if silicon oxide fine powder with a center particle size of 1 μm or less is used, according to the present invention, a nearly spherical uniform α with a center particle size of 1 μm or less can be obtained.
type silicon nitride fine powder can be obtained, but its price is 10 μm compared to silicon oxide powder with a center particle size of 1 to 100 μm.
Since the price is almost twice as high and it is not possible to obtain A-type chambered silicon fine powder at a cheaper price, from an industrial perspective, the central particle size is 1 to 1.
Coarse particles of 00 μm are preferred. Also, the center particle size is 100
When using silicon oxide powder of μm or larger, in order to ensure uniform mixing with carbon powder, etc., either increase the mixing time in a ball mill, etc. to obtain a pulverizing effect, or oxidize it in advance with a ball mill, vibration mill, etc. It is necessary to use the silicon powder after pulverizing it to 100 μm or less. When silicon oxide powder contains impurities such as B, Al, and Zn compounds, these act to suppress reduction and nitrogen reactions, while V, Nb, Ta, Cr, Mo, W,
Mn, Fe, Co, Ni.

It is desirable that impurities such as Cu compounds are contained in the raw silicon oxide powder as little as possible because they cause SiC to form and also facilitate the formation of needle-shaped crystals.

Therefore, it is desirable to use silicon oxide that does not contain impurities containing these metals in a total amount of 0.8% by weight or more of each metal element. Examples of such silicon oxide powder include anhydrous silicic acid, quartz, cristobalite, quartz glass, and silica gel, but it is most preferable to use natural quartz powder because it can be obtained at low cost.

Similarly, it is desirable to use a carbon powder that does not contain impurities containing the above-mentioned metals in an amount of 0.8% by weight or more based on the total amount of each metal element. A typical example thereof is powder such as acetylene black furnace black. Moreover, the particle size can be used from several millimicrons. In terms of handling, if it can be powdered during mixing, granulated 0,3~
It is advantageous to use granules of about 1.5III or press-molded granules.

If the carbon content is less than 0.4 parts by weight per 1 part by weight of silicon oxide powder, the reduction/nitriding reaction formula 3SiO2+6G+2N
, →S 13N4 + 6 CO is less than the reaction equivalent (and unreacted S i O.

remains. On the other hand, if the amount is more than 4 parts by weight, the reaction yield of α-type silicon nitride decreases, and a large amount of unreacted carbon remains, making it difficult to remove and increasing the cost, which is not preferable. Therefore, the amount of carbon powder added should be 0.4~
The amount is preferably 4 parts by weight, more preferably 0.5 to 1.2 parts by weight.

Mg or its compounds used in the present invention include metal magnesium and water-soluble compounds such as magnesium nitrate, magnesium chloride and magnesium sulfate;
Magnesium hydroxide, magnesium oxide, magnesium carbonate, basic magnesium carbonate, magnesium isopropoxide, magnesium nitride, etc. can be used, but raw material powders such as coarse silicon oxide powder and carbon powder can be used to make the mixing more uniform. Among the above, it is preferable to use a water-soluble Mg compound, since water is added thereto and mixed using a wet ball mill or the like. If it is insoluble in water, it can be added after being dissolved in an acidic aqueous solution in advance.

In addition, the above-listed substances may be added alone or two or more types, but the amount added is 0.0005 parts by weight of silicon oxide powder in terms of the weight of Mg element.
It is desirable that the amount is in the range of 0.1 parts by weight. 0.00
If the addition amount is less than 0.05 parts by weight, there is almost no effect on promoting the reduction/nitriding reaction and controlling the shape and atomization of the generated α-3i3N4 (on the other hand, if the amount is more than 0.12 parts by weight, the α-
5i3N contains a large amount of Mg and is not preferred as a raw material for a sintered body. A more preferable addition amount is o, oot
-0.03 parts by weight. Moreover, at this time, Be, Sr, Ca, and Zr1Ti are added together with Mg.

Although metals such as Hf, Sn, and Ge or their compounds may coexist, it is not preferable that the total amount of each metal element I exceeds 0.1 part by weight.

The silicon nitride fine powder used in the present invention is an α-type silicon nitride fine powder having a BET specific surface area of 15 to 10077//V, and preferably has an α phase content of 90% or more.

Even if the particle has a center particle size of 1 μm or less, if its BET specific surface area is less than 1571 f/f, the effect of the present invention will not be exhibited, and the center particle size of the α-type nitrosilicon produced will be large to 1 μm or more. Moreover, needle-like crystals and rod-like particles become mixed. This phenomenon is particularly noticeable when silicon oxide coarse particles of 1 μm or more are used as the raw material.

Moreover, even if the specific surface area exceeds 10077//y, no further improvement in the effect is observed. On the other hand, it is difficult to manufacture, resulting in cost and industrial disadvantages, so it is preferably 100rrr'79 or less. More preferably, it is in the range of 15 to 50 fi/y.

In addition, if a fine silicon nitride powder with an α phase content of less than 90% and a large amount of β phase or amorphous phase is used, the α phase content of the silicon nitride produced will be low, and acicular crystals,
Since rod-shaped particles are mixed, more than 90% α-
It is preferred to use fine phase content silicon nitride powder.

The particle size of the α-5i3N4 fine powder added in the present invention is usually a center particle size of 1 μm or less, preferably 0.8 to 0.8 μm.
It is m.

The α-type silicon nitride fine powder obtained by the present invention has a center particle size of 0.
．． Even in the case of fine particles of 3 to 0.5 μm, the specific surface area is L
Since it is considerably smaller than 5yd/I, it is usually applied to a crusher such as a vibrating mill that has a strong impact destructive force (WX impact value 3G to 15G) until the specific surface area becomes 15rr179 or more.
It is preferable to obtain

Since the particle size of silicon nitride powder having a diameter of 1 μm or less hardly changes even when subjected to these crushers, it is thought that the specific surface area increases by roughening the particle surface.

In addition, since the destructive force of commonly used ball mills is small, the specific surface area hardly increases after about 200 hours.

During pulverization, due to the material of the pulverizer such as a vibrating mill, A
Metal impurities such as I, Fe, Ni, and W are mixed. When such a silicon nitride fine powder is used, its effect is not noticeable and needle-like crystals and rod-like particles are mixed in the silicon nitride produced.

In such cases, α-3i3N, which has been processed with a crusher such as a vibrating mill and adjusted to have a BET specific surface area of 15 g/f to 100 g/f, is used after cleaning the fine powder with mineral acid containing hydrofluoric acid. , it is desirable to use.

Furthermore, when the particles are subjected to a pulverizer, the surface layer of the particles may be covered with oxides, so the above-mentioned washing is preferable in order to remove these.

The appropriate amount of α-5i3N4 fine powder to be added is 0.01 to 1 part by weight per 1 part by weight of silicon oxide powder. α-5i
3N, if the amount of fine powder added is less than 0.01 part by weight,
This effect is hardly seen, and only α-5i3N4 powder with a diameter of 1/1 μm or more can be obtained in which needle-like crystals and rod-like particles are mixed. In addition, when there is a large amount of tXX, the newly generated α-3i
Since the amount of additive α-5i3N4 is higher than that of 3N4, the production efficiency is higher, and therefore it is more preferably 0.01 to 0.
It is in the range of 1 part by weight.

In the present invention, any known method can be used to uniformly mix the above raw materials and additives, and is not particularly limited, but preferably silicon oxide powder, carbon powder, M
Wet mix g or Mg compound and silicon nitride powder with water.

As a mixing method, mixing means such as a ball mill or a ceramic mixer can be used, but Fe.

It is necessary to select the material so that impurities such as At which may harm the reaction are not mixed in. Usually, in the case of a ball mill, it is preferable to use balls coated with quartz glass, silicon nitride or plastic, and mix in a plastic pot.

In addition, carbon powder has a bifurcated shape of several hundred micrometers or less and has a small specific gravity, making it difficult to handle.
(Using ζ granulated or press-molded particles,
A method of mixing this with other raw materials by the above-mentioned means is preferred.

When mixing is carried out wet, the mixture is dried, but it is preferable to use a method such as spray drying or a rotary evaporator to prevent silicon oxide and carbon powder from separating due to differences in specific gravity during drying.

The mixture is heat treated in an atmosphere containing nitrogen and subjected to a reduction/nitriding reaction, but the atmosphere is N2. NH8
, N2-H, , N, -Ar, etc., can be used. Heat treatment temperature is 1,400℃
~1,600°C, preferably 1.450-1,550°C
The range can be selected. If the temperature is lower than 1,400°C, it will take a long time for the nitriding reaction to proceed sufficiently, and if the temperature exceeds 1,600°C, a large amount of SiC will be produced. Including economic points, 1.
, 450 DEG C. to 1,550 DEG C. for 2 to 6 hours. Furthermore, after the reduction/nitridation reaction, heat treatment is performed in an oxidizing atmosphere to remove remaining excess carbon, but this treatment is generally performed at 600-800°C.
1 to 4 hours is appropriate.

The method of the present invention not only has a catalytic effect on the reduction/nitriding reaction of Mg or its compounds, but also has a BET effect on these substances.
Since the atomization effect is produced by the synergistic effect of α-3i3N4 fine powder with a specific surface area of 15 to 100y#/f, even if inexpensive coarse silicon oxide with a particle size of 1 to 100μm is used, the particle size is 1μ.
A uniform α-5i3N4 fine powder having a shape close to a spherical shape of less than m can be easily obtained. Further, the α-5i3N4 fine powder obtained according to the present invention has the property of being well dispersed in water and alcoholic solvents such as isopropyl alcohol.

According to the present invention, raw material powder for silicon nitride sintered bodies having excellent heat resistance and high-temperature strength can be produced industrially more advantageously.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited thereto.

Example 1 Commercially available quartz sand powder (center particle size 6 pm) was used as silicon oxide powder.
, BET specific surface area 1.2 d/da), and a commercially available acetylene black pressed product was used as the carbon powder.

As a compound of Mg, Mg (NOx)2.6H
was used. Silicon nitride powder has a commercially available center particle size of 9.
．． 5μm, BET specific surface area 17vf/fa phase content 96
% of α-8i3N4 fine powder (LC-12 manufactured by Starck)
) was used. These powders were made into the composition shown in Table 1,
Water was added and wet ball mill mixing was performed for 2 hours using a plastic covered ball and a plastic pot.

The obtained slurry-like mixture was dried using a rotary evaporator under heating and reduced pressure while rotating.

The dried mixture was placed in a graphite container and heated at temperatures of 500°C and 1,550°C for 4 to 6 hours without flowing N2 gas.
5in2 was reduced and nitrided by heat treatment for a period of time. The obtained powder was further heat-treated in air at 700° C. for 4 hours, and unreacted C was burned off to obtain Si3N4 fine powder.

For each Si3N4 fine powder synthesized in this way, the average particle size, N content, α-5i3N4 content (X
) was measured, and the values are shown in Table 1.

Example 2 It was synthesized in Example 1 as silicon nitride powder.

Center grain! Powders with a diameter of 0.5 μm, a BET specific surface area of 7 d/9, and an α phase content of 98% were subjected to wet vibration mill treatment for 75 hours and 100 hours using isopropyl alcohol as a dispersion medium using silicon nitride balls and pots. Using. The silicon nitride powder subjected to the 75-hour vibration mill treatment had a center particle size of 0.5 μm, a BET specific surface area of 17 rtUI, and an α phase content of 96%, and was designated as silicon nitride powder A.

On the other hand, silicon nitride powder treated with a wet vibration mill for 100 hours has a center particle size of 0.5 μm, a BET specific surface area of 21 vf/f, and α
The phase content was 96%, and this was designated as silicon nitride powder B.

Si3N4 powder was synthesized according to the procedure of Example 1 using silicon nitride powders A and B as silicon nitride powders.

Average particle size, N content, α-3i for each powder
Table 1 shows the 3N4 content.

Comparative Example 1 The powders synthesized in Examples 1 and 2 were used as silicon nitride powder. The powder synthesized in Example 1 had a center particle size of 0.5μ.
Silicon nitride powder C was prepared with m, BET specific surface area of 7 d/f and α phase content of 98%.

Furthermore, the powder synthesized in Example 2 has a center particle size of 0.4 μm BE.
It was made into silicon nitride powder with a T specific surface area of 9 d/9 and an α phase content of 99%.

Using silicon nitride powders C and D as silicon nitride powders, Si3N4 powder was synthesized according to the procedure of Example 1. Unlike No. 2, the obtained silicon nitride powder had a large particle size and contained needle-like crystals in each powder.

Example 8 Silicon nitride powder synthesized in Example 1 with a center particle size of 0.
A powder with a diameter of 5 μm, a BET specific surface area of 7 g/f, and an α phase content of 98% was subjected to wet vibration milling for 100 hours using isopropyl alcohol as a solvent and a silicon nitride ball and a high alumina pot to obtain a powder. t-0 This silicon nitride powder was added to a mixed solution of 50% hydrofluoric acid aqueous solution and 70% nitric acid aqueous solution at a volume ratio of 1:5 to a concentration of 509/l, stirred for 1 hour, and then washed and dried. was used. This powder has a center particle size of 0.5μm1BET
The specific surface area was 22g/f1α phase content was 96% and the A1 content was 0.02%, and this was designated as silicon nitride powder E.

Si3N4 powder was synthesized according to the procedure of Example 1 using silicon nitride powder E as the silicon nitride powder. Table 1 shows the average particle diameter N content, α-3i3N, and content of each powder.

Example 4 As silicon oxide powder, commercially available silicic anhydride (center particle size 17
μm, BET specific surface area o, 5nt7y), and acetylene black granules were used as the carbon powder. Mg(NOx)z·6H20 was used as the Mg compound, and silicon nitride powder B was used as the silicon nitride powder.

Using these powders, Si3N was prepared according to the procedure of Example 1.
4 powders were synthesized, and the average particle size, N
The content rate and α-5i3N4 content rate were measured and shown in Table 1.

Example 5 Si3N powder was synthesized according to the procedure of Example 1 using the same powder as used in Example 1 except that Mg (OH)2 was used as the Mg compound.

Average particle size, BET specific surface area, N
Table 1 shows the content rate and α-3i3N4 content rate.

Comparative Example 2 Using the same powder as used in Example 1 and without adding silicon nitride powder, Mg (NO3)! - Si and N4 powders were synthesized according to the procedure of Example 1 in the case where 6H20 was not added or when neither of them was added. Table 1 shows the average particle size, particle shape, N content, and α-5i3N4 content of each powder.

[Brief explanation of the drawing]

FIG. 1 is an electron micrograph of the α-type silicon nitride fine powder obtained in Example 1-1, and FIG. 2 is an electron micrograph of the silicon nitride powder obtained in Comparative Example 1-9.

Claims (1)

[Claims] 1) A method for producing silicon nitride by heat-treating a mixture of silicon oxide powder and carbon powder at high temperature in a nitrogen-containing atmosphere, in which at least one compound selected from Mg and Mg compounds is added to the mixture. species, and BET specific surface area 15-100
A method for producing α-type silicon nitride fine powder, which comprises adding silicon nitride fine powder having m^2/g. 2) The mixture contains 0.0005 to 0.1 part by weight of at least one selected from Mg and Mg compounds, calculated as Mg element weight, and 0.000.1 part by weight of silicon nitride fine powder per 1 part by weight of silicon oxide powder. α according to claim 1, characterized in that it contains 0.01 to 1 part by weight and 0.4 to 4 parts by weight of carbon powder.
Method for producing silicon nitride fine powder.