JPS5926904A - Preparation of powdery raw material of silicon ceramic - Google Patents

Abstract

PURPOSE:To prepare fine powder of silicon ceramic raw material having high purity, easily, at a low cost, by reacting an alkali silicate with a mineral acid, mixing the obtained silica with carbon or a carbon compound, and calcining the mixture in a non-oxidative atmosphere. CONSTITUTION:An alkali silicate such as sodium silicate is mixed with a mineral acid such as sulfuric acid to obtain a mixture having an Si concentration of >=15wt%, and reacted under thorough agitation until the mixture becomes non- viscous liquid. The hydrogel thus obtained is filtered, washed with water, and dried to obtain highly pure silica. The silica is mixed with carbon such as carbon black and/or a carbon compound such as sucrose, and calcined at >=1,400 deg.C in reducing atmosphere or at <=1,600 deg.C in nitrogen gas atmosphere to obtain powdery raw material of ceramic composed of SiC or SiN. As an alternative method, the objective product can be obtained by reacting an alkali silicate with a mineral acid in the presence of a carbon source.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel method for producing silicon-based ceramic raw material powder, particularly silicon carbide powder and silicon nitride powder.

In recent years, ceramics as high-temperature mechanical materials, so-called engineering ceramics, have been attracting attention from the viewpoint of energy conservation and resource conservation (replacement of rare metals). Among them, silicon carbide and silicon nitride are considered to be one of the most promising materials, and many examples have already been put into practical use. Therefore, many methods have been researched to produce raw material powders for ceramics, and some have already been produced on an industrial scale.

The main manufacturing methods for silicon-based ceramic raw material powders are: (1) Method using metallic silicon as raw material) 1500℃ Si+O--m-→ 5iO 3f31 + 2N2 9Tom 513N4 (2) Gas phase reaction method ~ 2000℃ SiH4+ OH4 -----) SiU + 4H
2~1500℃ 3191014+ 4NH3* Si3N4+12
HO1(3) Pyrolysis method) 1300℃ 5i(OH3)4--→SiO+ 30H4~1500
°C 3Si(NH)2−−−4 si, N4+2MH1
(4) Silica reduction method ~2000C 8iO1+ 30-1→SiO+ 200 (1500℃ 5E+10.+, J+60−---→5ilN4+60
0 etc. are known.

Non-oxide silicon-based ceramics are generally non-sinterable, and therefore, these raw material powders must be made as fine as possible to make them active. Further, these ceramic sintered bodies are required to have high purity in relation to high-temperature strength, and a desirable purity is said to be 99.5'X or higher.

Comparing each manufacturing method from this point of view, the manufacturing method using metallic silicon as a raw material (high purity YS 1) is expensive and difficult to obtain fine product powder.

Production method (2) yields ultrafine powder with high purity, but the raw material gas used is expensive, the reaction yield is low, and, generally speaking, it is difficult to obtain crystalline powder. It is not easy to manufacture. Production method (3) makes it possible to obtain a highly pure product, but the raw material silicon compound is not a mass-produced product and is expensive. Manufacturing method (4) uses silica as a raw material; JP-A No. 56-75616 uses wet white carbon as the silica, and JP-A No. 51-Sho uses silica flour, which is a by-product during the production of ferrosilicon, etc. No. 70200, Special Publication Showa 48-2 using silica gravel
No. 2920, Japanese Unexamined Patent Application Publication No. 1983 using whitebait. -14071
No. 0 etc. are known as methods for producing silicon carbide powder,
Furthermore, as a method for producing silicon nitride powder, there is known a method such as JP-A-56-73604, which is characterized by wet white carbon. However, all of these silica raw materials contain about 10% impurities, and are therefore unsuitable for producing high-purity silicon-based ceramic raw material powder.

On the other hand, Si (, t1
Silica (trade name Aerosil) produced by hydrolysis in an oxyhydrogen flame is known, but the raw material qio
Since various bonding ends in the form of Al0I, TiJ, FeO2, etc. are likely to be mixed into 14, there remains a problem in the purity of the obtained silica, and the drawback is that it is expensive.

On the other hand, JP-A No. 57-42515, No. 42516, No. 82109, No. 88011, No. 88012, No. 8
No. 8019 discloses a method for producing silicon carbide and silicon nitride by treating carbon or a carbon precursor with a silicic acid liquid. In these methods, the silicic acid solution is prepared by acid decomposition of alkali silicate or cation exchange method, and in either case, the silica source and carbon source are sufficiently mixed, and some effects have been recognized. There are many problems in terms of the purity and economic efficiency of the products. That is, conventionally, in the acid decomposition of alkali silicates, Iqi
02 (This is carried out using an extremely dilute solution in which the degree of 1 degree is less than the amount of ta Since the hydrogel contains a large amount of by-product soluble salts, it is necessary to roughly crush the salt and wash it with water to completely wash it away.However, the gel obtained by this conventional method has an extremely high water content of 80% or more. This water cannot be removed by filtration.

Therefore, pe 1A/1 derived from alkali metal silicates
Most of these metal impurities are included in the hydrogel.9 Therefore, when producing ceramics using such gel as a calcium source, only products with a high metal impurity content can be obtained.

The presence of such metal impurities causes the generation of whiskers during powder production or abnormal grain growth during sintering, causing problems in terms of the physical properties of the sintered body. Furthermore, when using the darning method, as mentioned above, silica gel contains a large amount of water, as much as 80% or more, and the energy cost is extremely high to remove this large amount of water. Although it is inexpensive, it is an extremely economically disadvantageous manufacturing method when viewed as a whole process.

In addition, even when preparing a silicic acid solution using a cation exchange method, it is necessary to prevent gelation during ion exchange, and there are constraints on the concentration of silicic acid, so it is necessary to prepare a dilute solution. A very large amount of moisture must be removed before firing. Furthermore, it is extremely difficult to remove metal impurities such as Fe and A1 contained in the alkali silicate in the presence of a large amount of alkali, and this is not preferred as a method for producing high-purity ceramic raw material powder.

As a result of many years of research into manufacturing methods for raw material powders and powders for Noricon-based ceramics, the present inventors have established a manufacturing method that avoids all of the drawbacks of the various manufacturing methods described above.

That is, the first aspect of the present invention is to mix silica obtained by blending and reacting an alkali silicate and a mineral acid so that the sia2m degree is 15% by weight or more, and then mixing it with carbon and/or a carbon compound, followed by non-oxidizing This is a method for producing a silicon-based ceramic raw material powder, which is characterized by firing in a neutral atmosphere. A silicon-based ceramic which is characterized in that a mixture of carbon and/or a carbon compound and silica obtained by blending and reacting alkali silicate and mineral acid in a proportion by weight or more is desalted and then fired in a non-oxidizing atmosphere. This is a method for producing raw material powder.

The feature of the present invention is that regardless of the stage in which carbon and/or carbon compounds are added, S2O2 can be used as a silica raw material.
The concentration is s5! Silica obtained by mixing and reacting an alkali silicate and a mineral acid so that the amount is f%, preferably 20% by weight or more, is used. There is no particular upper limit for this SiQ2 concentration, but if the concentration is too high, it will be difficult to mix evenly.There are differences depending on the stirring device, but generally it is 50% by weight. The following are preferred. Various methods are possible for the preparation method in this case, and either mineral acid or alkali silicate can be diluted with water, but when using concentrated sulfuric acid as the mineral acid, It is preferable to dilute sulfuric acid because the heat of dilution can be removed in advance, making it easier to adjust the reaction temperature.

Examples of the alkali silicate used in the present invention include sodium silicate, potassium silicate, lithium silicate, etc., but inexpensive sodium silicate is generally used. Examples of mineral acids include sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid, with sulfuric acid being most commonly used.

Next, the progress of the silica production reaction in the present invention will be described in detail.

When the raw materials are prepared such that the alkali metal silicate and mineral acid have a concentration of 15% by weight or more and are in the acidic region, the whole becomes an ice block. When the mixture is stirred, the lumps loosen and become sloppy, which adheres to the stirring blade and becomes cake-like and sticky. If the stirring is continued further, water will ooze out and the mixture will become smooth. At this point, the reaction can be completed, followed by filtration, water washing, and drying. Under these conditions, the reaction is completed in about 1 hour, and the hydrogel obtained by filtration and water washing has a low water content of 60% or less. is obtained. As described above, according to the present invention, both temporary properties and washability are extremely good, and a small amount of washing water is sufficient. The reason for this is not necessarily clear, but it may be that the concentration of the raw materials used is high and that the crystallization of silica gel does not go through the sol.
This is thought to be due to the fact that it ends in an extremely short time even if it is routed through another route.

According to the present invention, the water content of the obtained silica can be lowered, so dehydration. The amount of water evaporated to be dried can be reduced to about 1/4 of the conventional method, making the process as a whole extremely economical. Further, according to the present invention, it is possible to provide a silica source that fully satisfies the purity required for silicon-based ceramics. That is, according to the present invention, metal impurities such as Fθ and A1 mainly derived from alkali silicate are hardly contained in the produced silica, making it possible to manufacture high-purity silicon-based ceramics. The reason for this is not necessarily clear, but S wholesale,
It is presumed that the silica gel produced by the reaction at a concentration of 15% by weight or more has a structure that makes it difficult to incorporate metal impurities. In addition, in the present invention, unlike silica gel obtained by conventional methods, the silica gel produced is easy to sieve and has good washability, so most of the metal impurities mentioned above can be transferred to the mother liquor side. be. For the purpose of the present invention, it is preferable to perform the reaction in an acidic region where metal ions are easily extracted from the ventral side.

Carbon sources used in the present invention include, for example, solid carbon such as carbon blank, petroleum coke, graphite, and activated carbon, as well as carbonaceous residues in a non-oxidizing atmosphere at a temperature of 200 to 1500°C. It also includes carbon compounds that are Car pump knock is the most common from the point of view of economic efficiency. Furthermore, when it is desired to obtain raw material powder with high purity, sucrose or the like is preferable.

The timing of adding carbon and/or carbon compounds in the present invention is not particularly limited, and they may be removed into an aqueous alkali silicate solution or mineral acid before the neutralization reaction, or they may be added after the neutralization is completed. be. Furthermore, after obtaining the silica gel, it may be dry mixed with the obtained silica gel. However, in order to improve the uniformity of mixing with silica and obtain a good silicon-based ceramic raw material powder, it is of course preferable to add it in a liquid system.

Generally, silica gel obtained by neutralization reaction of an aqueous alkali silicate solution contains a large amount of by-product soluble salts, so it is necessary to remove the sol or gel by water washing, salting out, or electrochemical method. Since the filterability is extremely easy, when the obtained slurry is filtered and washed, metal ions such as Fθ and A1, which are cine particles, are transferred to the mother liquor side without being incorporated into the silica or the silica-carbon source mixture. Therefore, a raw material for producing high-purity silicon-based ceramics can be obtained.

The silica or silica-carbon source mixture obtained in this way is dehydrated and dried, and in some cases after mixing with a carbon source, it is fired in a reducing atmosphere or a nitriding gas atmosphere to form silicon carbide or silicon nitride powder. The obtained powder is of high purity with very little metal impurities such as Fθ and A1, and is obtained as fine particles with a single particle diameter of 1 μm or less.

As described above, according to the present invention, high-purity single-fine 7-recon thread ceramic powder can be obtained easily and inexpensively.

The present invention will be explained in more detail with reference to Examples below.

Example 1-3 98% sulfuric acid or 3002 prepared by diluting it in advance was charged into a 2-t kneader (type 2 blades) so that the SiO*# content was 15 to 25%, and No. 5 was added while stirring. A solution of sodium silicate (S i02 /Na2O molar ratio 3.1.5 iot concentration 29% by weight) was added at a rate of ioo y/min, and the pH of the reactant was brought to around the set value at a reaction temperature of 35°C. After finishing painting with IJum and mixing well for 10 minutes, the pH was measured again and all the reactants were evaporated, separated with a centrifuge, washed, and dried at 1'05℃. The raw material silica was obtained. A commercially available carbon blank was added to this material so that the a/Si atomic ratio would be a predetermined value, and then mixed overnight in a Teflon ball mill. This mixture was fired to obtain silicon-based ceramic powder.

The neutralization conditions for sodium silicate, the water content of the produced silica gel, the firing conditions, and the physical properties of the produced silicon ceramic powder are shown in Table 1 along with the following Examples and Comparative Examples. Table 2 shows the amount of impurities in No. 3 sodium silicate and carbon black used in Examples and Comparative Examples.

Example 4 Silicon carbide was obtained in the same manner as in Examples 1 to 3, except that carbon black was completely added to 98% sulfuric acid in advance so that the S102 concentration was 25%.

Example 5 Silicon nitride was obtained in the same manner as in Example 4 except for the amount of carbon blank added and the firing conditions.

Comparative Examples 1 to 3 Examples except that 30'X sulfuric acid and No. 5 sodium silicate diluted with water to adjust the S102 concentration to 14% were used as raw materials, and the S102 concentration during the reaction was set to 5 to 15. 1
A silicon-based ceramic powder was obtained by the same operation as in Steps 5 to 5.

As is clear from Table 1 showing the results of the above Examples and Comparative Examples, the present invention is economical because the amount of water in the produced silica gel is extremely small. Furthermore, the cleaning and insertion properties are good, and high-purity, fine-grained silicon-based ceramic raw material powder can be easily produced.

Table 2 Impurity content in raw materials used 1) By atomic absorption method 2) By fluorescence X-ray analysis method Patent applicant: Central Glass Co., Ltd.

Claims (1)

[Scope of Claims] (S10. Silica obtained by blending and reacting alkali silicate and mineral acid so that the concentration is 15% by weight or more is mixed with carbon and/or carbon compounds, and then non-oxidizing A method for producing silicon-based ceramic raw material powder characterized by firing in an atmosphere. (2) In the presence of carbon and/or carbon compounds, the S102 concentration in the total amount excluding the carbon source is 15% by weight or more. A method for producing a silicon-based ceramic raw material powder, which comprises desalting a mixture of carbon and/or a carbon compound and silica obtained by blending and reacting an alkali silicate and a mineral acid, and then firing the mixture in a non-oxidizing atmosphere. (3) The method for producing a silicon-based ceramic raw material powder according to claim 1 or 2, wherein the silicon-based ceramic raw material powder is silicon carbide and is fired at a temperature of 1400° C. or higher in an inert gas atmosphere. (4) The method for producing silicon-based ceramic raw material powder according to claim 1 or 2, wherein the silicon-based ceramic raw material powder is silicon nitride, and the silicon-based ceramic raw material powder is fired at a temperature of 1600° C. or lower in a nitrogen gas atmosphere.