KR0132765B1 - Method of processing mullite whisker - Google Patents

Abstract

Mixed powder which is produced through mixing a boehmite sol and a silica sol, gelating and drying them is reacted with ammonium hexafluorosilicate to prepare mullite whisker, which is useful as reinforcing materials for matrix materials such ad ceramics and metal materials, as it shows no aggregation. To the mixed powder produced by mixing, gelating and drying the boehmite sol and the silica sol as a alumina-silica source whose fill densities are no more than 0.5g/mm3, is added 30 to 50 wt% of ammonium hexafluorosilicate or 1 to 30 wt% of aluminum fluoride based on the mullite as a fluorine source, and which are heat-treated for 0.5 to 10 hours at a temperature of 900 to 1500 deg. C for producing the mullite whisker.

Description

Manufacturing method of mullite needle crystal

1 is an electron micrograph of a mullite needle crystal prepared by adding aluminum fluoride.

2 is an electron micrograph of a mullite needle crystal prepared by adding ammonium hexafluorosilicate.

3 is a schematic diagram of an apparatus for preparing mullite needle crystals designed to trap fluorine-containing toxic gases generated when ammonium hexafluorosilicate is added.

* Explanation of symbols for main parts of the drawings

1, 2: tube furnace 3: ammonium hexafluorosilicate raw powder

4: alumina-silica raw material powder 5: fluorine-containing gas collecting device

The present invention relates to a method for producing mullite needle crystals. More specifically, the present invention relates to a method for producing agglomerated mullite crystals.

Mullite is an aluminum silicate compound, a typical stoichiometric mullite is expressed as 3Al ₂ O ₃ SiO _{2, according} to Pask et al. (Ceram. Inter., 9 [4] 107-113 (1983)). The 3Al ₂ O ₃ / SiO ₂ ratio forms a solid solution in the range 1.5 to 2.87.

Mullite keeps its stability at room temperature up to high temperature, and has been spotlighted as a high-temperature structural material because of its excellent creep resistance and thermal shock resistance, and especially, the ratio of the length to the width of the particles, that is, the aspect ratio is 10 or more. The mullite whisker, which is a single crystal, can be applied as a reinforcing material for matrix materials such as ceramic materials, metal materials, plastics, and the like, which require high mechanical strength and toughness at high temperatures. Since the mullite needle crystal is in the form of an oxide, there is no fear of deterioration of the physical properties due to oxidation in the case of SiC whiskers. The replacement effect can be expected.

Up to now, a method for producing mullite acicular crystals has been known (JP 03212299) by adding aluminum fluoride, a compound containing fluorine, to a mixture of an alumina source and a silica source and heat-treating in a sealed container (JP 03212299).

However, the above method has a problem that the prepared mullite needle crystals are entangled with the unreacted particles or the agglomeration of the mullite needle crystals is difficult to uniformly mix on the substrate, and aluminum fluoride is added to obtain the needle crystals. When the amount is small, a high temperature reaction temperature is required. On the contrary, when the reaction temperature is low, a large amount of aluminum fluoride needs to be added.

In order to solve the above-mentioned problems with the manufacturing method of mullite acicular crystal, a method of adding a mixture of an alumina source and a silica source, aluminum fluoride, and a separate carbon (C) as a spacer (JP 01261700) Developed.

However, the above method has the following problems.

First, since the fluorine-containing toxic gas is generated during the production of mullite needle crystals, the amount of aluminum fluoride added should be further minimized.

Second, aluminum fluoride is an expensive material.

Third, it is difficult to develop a process for collecting and treating a fluorine-containing toxic gas generated in manufacturing mullite needle crystals.

Fourth, a post-treatment step of washing with hydrofluoric acid solution is required to increase the purity after mullite needle crystal production.

Accordingly, it is an object of the present invention to provide a manufacturing method for minimizing the addition amount of aluminum fluoride in a simple process without the addition of a spacer, with respect to the production of mullite needles without aggregation.

It is still another object of the present invention to provide a manufacturing method for replacing aluminum fluoride with a material which is capable of capturing a fluorine-containing toxic gas with low cost in connection with the production of agglomerated mullite needles.

In the present invention, alumina-silica source is agglomeration-free mullite by adding 1 to 30% by weight of aluminum fluoride at a reaction temperature of 900 to 1500 ° C. using a fine mixed powder having a fill density of 0.5 g / cm 3 or less. It relates to a process for producing acicular crystals.

The present invention also relates to a method for producing mullite needle crystals using ammonium hexafluorosilicate ((NH 4) 2 SiF 6), which is a low cost fluorine source.

The present invention will be described in detail below.

In the present invention, aluminum fluoride or ammonium hexafluorosilicate is used as a fluorine source for heat treatment with an alumina-silica source in the preparation of mullite needle crystals. Although no detailed role of aluminum fluoride is known in the method for preparing mullite needle crystals, it is generally known that the fluorine component of aluminum fluoride plays an important role. However, since aluminum fluoride is a very expensive material as mentioned above, it is advantageous to use ammonium hexafluorosilicate as a low-cost fluorine source in addition to aluminum fluoride.

Hereinafter, the present invention will be described in detail by dividing the case of using aluminum fluoride as the fluorine source and the case of using ammonium hexafluorosilicate.

The manufacturing method of the mullite needle crystal which uses aluminum fluoride as a fluorine source, and is not agglomeration without a separate spacer is demonstrated.

In connection with the present invention, the alumina source includes aluminum salts such as Al (NO ₃ ) ₃₉ H ₂ O and AlCl ₃ obtained from heat treatment, aluminum alkoxides such as Al (OC ₃ H ₇ ) ₃ , boehmite, boehmite sol, Alumina sol, γ-alumina, and the like may be used, and likewise, silicon alkoxides such as Si (OC ₂ H ₅ ) ₄ obtained as silica during heat treatment, silica gel, silica sol, and fumed silica may be used as the silica source. In addition, as a material containing both an alumina source and a silica source, aluminum silicates, for example, an alumina silica clay clay such as kaolinite may be used. In particular, in the present invention, a boehmite sol and a silica sol are used as starting materials to uniformly mix the alumina source and the silica source to increase the reactivity with aluminum fluoride, and use a mixed-gelled-dry mixed powder.

In the present invention, a mixed powder of fine alumina-silica source having a packing density of 0.5 g / cm 3 or less is used to prepare a mullite needle-free crystal. In the case of using a mixed powder having a packing density of 0.5 g / cm 3 or more, agglomeration occurs and mullite needle crystals are hardly separated. Since the fine powder has a specific surface area and high reactivity with aluminum fluoride, it is possible to reduce the amount of aluminum fluoride added and lower the reaction temperature, and in particular, due to its low packing density, sufficient mullite needle crystals can be grown between the sample particles and the particles. It provides space and is efficient in producing high quality mullite needles without agglomerating spacers.

In addition, in the present invention, in order to obtain a uniform mullite needle crystal and simplify the manufacturing process, the aluminum fluoride of the mixed powder of alumina-silica source is not mixed and heat-treated in a reaction vessel. The mixed powder of alumina-silica source and aluminum fluoride can be heat treated with or without mixing, but the latter method is a mixed powder of alumina-silica source with the fluorine component generated from aluminum fluoride as the mullite needle crystal grows. It is advantageous in producing needle-shaped crystals having a uniform shape since it is in contact with.

The amount of aluminum fluoride added is 1 to 30% by weight, preferably 1 to 10% by weight based on mullite. If the amount of aluminum fluoride is less than 1% by weight (when the length of the mullite needle crystal and the equipment is too small and more than 30% by weight), the amount of toxic generated gas is increased, which is a problem in the working environment. The heat treatment temperature and the heat treatment time are 900 to 1500 ° C, 0.5 to 10 hours, preferably 1100 to 1300 ° C and 1 to 4 hours, respectively, depending on the amount of aluminum fluoride added.

A method of producing agglomerated mullite needles using ammonium hexafluorosilicate as a fluorine source will be described.

When preparing mullite needle crystals using ammonium hexafluorosilicate, a mixed powder of alumina-silica source is prepared in the same manner as in the above method.

The heat treatment method for synthesizing mullite acicular crystals from the mixed powder of alumina-silica source and ammonium hexafluorosilicate can be carried out in the same manner as in the case of adding aluminum fluoride or using the apparatus of FIG.

In Fig. 3, 1 and 2 are tube furnaces, 3 is ammonium hexafluorosilicate raw powder, 4 is a mixed powder of alumina-silica source, and 5 is a device for collecting by dissolving fluorine-containing toxic gas in basic aqueous solution.

Ammonium hexafluorosilicate, used as a fluorine source, is a naturally occurring mineral called cryptohalite, a low-cost material, and a material containing silicon, which is a member of mullite, dissolved in water. In particular, aluminum fluoride decomposes at about 1040 ° C., whereas ammonium hexafluorosilicate decomposes at about 300 ° C., which is much lower than this, so that fluorine is easily gasified. Therefore, the pyrolysis of ammonium hexafluorosilicate in the apparatus of the third degree, i.e., around 300 ° C, is injected into the reaction vessel together with the carrier gas to react with the mixed powder of alumina-silica source and to capture fluorine-containing toxic gas generated It is possible to use an apparatus for fairing.

When mullite needle crystals are used with ammonium hexafluorosilicate, the amount of ammonium hexafluorosilicate added is 3 to 50% by weight, preferably 5 to 20% by weight relative to mullite. When the addition amount of ammonium hexafluorosilicate is 3% by weight or less (when the shape of the mullite needle crystal is insufficient and 50% by weight or more), the amount of toxic generated gas is increased, which is a problem in the working environment. The heat treatment temperature and the heat treatment time are 900 to 1500 ° C, 0.5 to 10 hours, preferably 1100 to 1300 ° C and 1 to 4 hours, respectively, depending on the amount of the ammonium hexafluorosilicate added.

The mullite needle crystal prepared by the present invention is not agglomerated and can be usefully used as a reinforcing material for matrix materials such as ceramic materials, metal materials, plastics, etc., where mechanical strength and toughness at high temperatures are greatly required. .

Example 1

A mixed sol having a stoichiometric mullite composition was prepared by adding silica sol (Ludox AS40, Dupont) to a sol peptized with Bohemite (Catapal B, Vista Chemical Co., Ltd). The mixed sol of the alumina-silica source prepared by gel-drying the mixed sol was placed in alumina crucible with aluminum fluoride and heat-treated in an electric furnace for 2 hours to synthesize mullite needles. The mullite needle crystals synthesized under the experimental conditions shown in Table 1 were observed by scanning electron microscopy to measure the average whisker length and thin equipment, and the scanning electron micrographs of representative mullite needle crystals are shown in FIG. In addition, the phase was confirmed by X-ray diffraction analysis and presented in Table 1.

The mullite needle crystals invented according to the present invention had lengths of 5 to 100 µm and 7 to 30, respectively, depending on the production conditions. As mentioned above, the amount of aluminum fluoride added is not only an expensive material, but is preferably minimized since a gas containing fluorine is generated after the heat treatment reaction. Therefore, in the synthesis of mullite acicular crystals using aluminum fluoride, the length and fineness of the aluminum fluoride are greatly dependent on the amount of aluminum fluoride, the heat treatment temperature, and the heat treatment time. Do.

According to the present invention, when the heat treatment time was 2 hours, the amount of aluminum fluoride added to prepare the mullite needle crystals of fine equipment 7 or more at a heat treatment temperature of 1200 ° C. or more was minimized to 2 wt%.

In addition, the mullite needle crystal prepared by the present invention was suitable as a reinforcing material for producing a composite because there was no aggregation between each other.

Example 2

By using ammonium hexafluorosilicate as the fluorine source, the mullite needle crystals were synthesized by heat treatment for 2 hours under the experimental conditions shown in Table 2 using the same method as in Example 1 and using the apparatus of FIG.

The mullite acicular crystals were identified by X-ray diffraction analysis. The representative shapes of the mullite acicular crystals were observed by scanning electron microscopy, and the representative shapes of the mullite acicular crystals were 5-50 μm and 7 depending on the manufacturing conditions. It was -20.

In addition, the mullite needle crystal prepared by the present invention was suitable as a reinforcing material for producing a composite because there was no aggregation between each other.

Claims (12)

Mullite acicular crystal, characterized in that 1 to 30% by weight of aluminum fluoride is added to a mullite as a fluorine source in a mixed powder having a packing density of 0.5 g / cm 3 or less with heat treatment at 900 to 1500 ° C. for 0.5 to 10 hours. How to prepare. The method according to claim 1, wherein aluminum source, aluminum alkoxide, boehmite, boehmite sol, alumina sol, γ-alumina are used as the alumina source. The process of claim 2 wherein the aluminum salt is Al (NO ₃ ) ₃₉ H ₂ O and AlCl ₃ . The method of claim 2, wherein the aluminum alkoxide is Al (OC ₃ H ₇ ) ₃ . The method according to claim 1, wherein silicon alkoxide, silica gel, silica sol, and fumed silica are used as the silica source. The method of claim 5, wherein the silicon alkoxide is Si (OC ₂ H ₅ ) ₄ . The method of claim 1, wherein kaolinite is used as a material containing both an alumina source and a silica source. The method according to claim 1, wherein a mixed powder of boehmite sol and silica sol is mixed-gelled-dry as the alumina-silica source. A method for producing a mullite needle crystal characterized in that ammonium hexafluorosilicate is used as a fluorine source in a mixed powder having a packing density of 0.5 g / cm 3 or less with an alumina-silica source. 10. The process according to claim 9, wherein ammonium hexafluorosilicate is added in an amount of 3 to 50% by weight relative to mullite. The method of claim 9, wherein the heat treatment temperature is 900 to 1500 ° C. and the heat treatment time is 0.5 to 10 hours. Pyrolysis of ammonium hexafluorosilicate as the fluorine source and the tube electric furnaces (1, 2) are connected through a tube, each of which is a mixture of ammonium hexafluorosilicate powder (3) and alumina-silica source (4). Through the process of injecting together with air into the reaction vessel composed of a device (5) inherent and dissolving the fluorine-containing toxic gas in a basic aqueous solution and collecting the fluorine-containing toxic gas generated by reacting with the mixed powder of alumina-silica source Method for producing a mullite needle crystal characterized in that the manufacturing.