JPH0784324B2 - Mullite precursor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a mullite precursor and a method for producing the same.

In the specification of the present invention, the "mullite precursor" means a material that becomes a mullite powder by firing.
“Mullite powder” means a mullite sintered body by molding and firing, that is, 3Al ₂ O ₃ · 2SiO ₂
The Al ₂ O ₃ / SiO ₂ weight ratio of 71.8 / 28.2, as well as the weight ratio of 65/35 to 80/20.

[Prior Art] It is well known that when mullite powder with few impurities and impurity phases (cristobalite, corundum phase, etc.) is used as a sintered body, it has high strength, thermal stability, and creep resistance. Has been.

Examples of precursors for obtaining mullite powder containing less such impurities and impurity phases (cristobalite, corundum phase, etc.) include those obtained by mixing alumina powder with kaolin, those obtained by the alkoxide method, and those obtained by coprecipitation.

[Problems to be Solved by the Invention] However, since these mullite precursors have different precipitation pHs and precipitation formation times of Al and Si, it is difficult to simultaneously generate a precipitate from a mixed solution. Local variations in the ratio are likely to occur. This is remarkable especially when a high-concentration raw material is used. When the mullite precursor is fired, it is difficult to obtain a mullite phase single phase due to local variation in Al / Si ratio, and cristobalite (SiO ₂ ) phase and corundum (Al ₂ O ₃ ) phase are simultaneously precipitated as impurity phases. It's easy to do.

Such a mullite precursor is fired to obtain a mullite powder, and the mullite sintered body obtained by molding and firing the powder has a strength higher than that of the mullite single phase because of the presence of an impurity phase and a glass phase, There is a problem that thermal stability and creep resistance are low.

The object of the present invention is that the mullite powder obtained by firing has a low content of cristobalite phase or corundum phase which is an impurity phase. It is to provide a mullite precursor.

[Means and Actions for Solving the Problems] As a result of intensive studies for achieving the above-mentioned object, the present inventors have found that in the oxide and / or hydroxide mainly containing aluminum and silicon, aluminum and silicon The composition ratio is 65/35 to 80/20 in terms of Al ₂ O ₃ / SiO ₂ conversion weight ratio, and further, the substance containing 6.7 to 20 wt% of sulfur as an acid radical is a cristobalite phase and corundum which are impurity phases after firing. The present invention has been completed by discovering that it is a mullite precursor capable of obtaining a mullite powder having few phases.

The details will be described below.

The mullite precursor in the present invention is an oxide and / or hydroxide mainly containing aluminum and silicon.

The composition ratio of aluminum and silicon in the oxide and / or hydroxide is 65/35 to 80 / in terms of Al ₂ O ₃ / SiO ₂ weight ratio.
It shall be 20. The converted weight ratio is preferably 70/30 to 75/25.

Further, the oxide and / or hydroxide contains 6.7 to 20 wt% of sulfur as an acid radical. Preferably, the sulfur content is 10 to 16.7 wt%. this is,
This is because if it is lower than this range, a cristobalite phase other than the mullite phase is likely to be generated after firing, and if it is higher than this range, a corundum phase is easily generated, and in any case, it is difficult to obtain a mullite single phase.

The mullite precursor in the present invention is, for example, Al _a (OH) _b (SO ₄ ) _c (SiO ₂ ) _d · nH ₂ O a = 6.0 b = 18.0−2c c = {0.067 (306 + 60d) to 0.2 (306 + 60d) } / 32 d = 2.74 to 1.27 n ≧ 0.

In producing the mullite precursor in the present invention, as the aluminum source, for example, aluminum hydroxide, a sulfuric acid acidic slurry such as alumina or an aqueous solution such as aluminum sulfate can be preferably used, and as the silica source, for example, silica. Sodium acid, silicic acid, colloidal silica slurry, silica fine powder and the like can be preferably used.

The above two raw materials are mixed so as to have an Al ₂ O ₃ / SiO ₂ equivalent weight ratio of 65/35 to 80/20, preferably 70/30 to 75/25.

The pH of the mixed aqueous solution is 3 or less from the start to the end of the mixing, and is 3 or less, preferably 1 to 3 after the completion of the mixing. If it is too high, a coarse precipitate of aluminum hydroxide is formed and a mullite precursor having a uniform composition cannot be obtained, and if it is too low, a large amount of a neutralizing agent needs to be used in the subsequent neutralization step. This is because the amount of alkali that accompanies the mullite precursor increases and the burden on the cleaning process increases.

When the pH after mixing does not become 3 or less, an acid may be added to either or both of the aluminum source and the silicon source to adjust the pH of the raw materials before mixing.

Alkali is added to the mixed slurry.

The amount of the alkali added is 2.10 per 1 gram atomic weight of contained Al.
~ 2.55 gram equivalents, preferably 2.25 to 2.40 gram equivalents. By setting this range, 6.7 to 20 wt% of sulfur as an acid radical can be present in the mullite precursor.

After completion of the neutralization, the silicon compound takes a long time to precipitate, so that a uniform precipitation-containing aqueous solution of an aluminum compound and a silicon compound can be obtained by leaving it for a certain period of time, preferably 1 hour or more, more preferably 6 hours or more with stirring. .

After completion of stirring, the uniform precipitate-containing aqueous solution is filtered and washed,
Dry to obtain a mullite precursor. As a filtration method, a usual method such as vacuum filtration can be adopted. The washing is preferably performed with water, more preferably warm water washing. The washing is preferably carried out until the impurities in the mullite precursor become 0.1 wt% or less of the total of Al ₂ O ₃ and SiO _2, and 0.01 wt%
It is more preferable to wash until the following. If the drying is performed so that the water content is about 30 wt% or less, there will be no problem in the production of mullite powder. 50-200 ℃, especially 150-200 by oven drying, spray dryer, etc.
It is better to dry in the atmosphere of ℃.

By firing the dried mullite precursor or a pulverized product thereof at 1000 ° C. to 1600 ° C. in the atmosphere, mullite powder having less cristobalite phase and corundum phase as impurity phases can be obtained.

By molding the mullite powder, preferably after crushing and molding and firing at 1500 ° C. to 1800 ° C., it is possible to obtain a mullite sintered body having a small amount of impurity phases and therefore excellent strength.

[Operation] It is not always clear why the mullite precursor of the present invention is a precursor of mullite powder having less cristobalite phase and corundum phase, which are impurity phases, but it is presumed to be as follows. That is, the sulfur component present in the mullite precursor as an acid radical is, for example,
It is considered that they are present in the mullite precursor in the same state as the complex compound of [Al ₁₃ O ₄ (OH) ₂₄ (H ₂ O) ₁₂ (SO ₄ ) ₄ ] ⁺ ions.

The mullite precursor is thermally decomposed by firing, and the residual sulfur component has a high thermal decomposition temperature. Therefore, the mullite precursor is thermally decomposed at about 1000 ° C. at which the mullite precursor is transformed into mullite powder.
It is considered that a state in which a complex oxide of Si and Si is easily generated appears, a cristobalite phase or a corundum phase that is an impurity phase is hard to be generated, and a mullite single phase is likely to occur.

[Effects of the Invention] As is clear from the above description, the mullite precursor of the present invention is a precursor of mullite powder having less cristobalite phase or corundum phase as an impurity phase.

The mullite precursor is fired to form mullite powder,
By further firing, a mullite sintered body having excellent strength and the like can be manufactured. This mullite sintered body is suitably used as a high temperature engineering ceramic, a ceramic substrate, or the like.

Example 1 With the temperature kept at 30 ° C., 234 g (3.0 mol) of aluminum hydroxide having an average particle size of 1 μm was added to 1.5 mol of a sulfuric acid aqueous solution having a concentration of 1 mol / well and stirred for 6 hours. While stirring the aqueous solution,
7 mol of 1 mol / sodium silicate in terms of Al ₂ O ₃ / SiO ₂ equivalent weight ratio
It was added so as to have a composition of 3/27 and stirred for 1 hour. The pH of the mixed aqueous solution was 2.5. Concentration 4mol / in the mixed aqueous solution
2.5 g of sodium hydroxide aqueous solution per 1 gram atomic weight of Al
Added until 5 grams equivalent. After adding the alkali, the mixture was left for 6 hours with stirring. Then, the mixture was filtered and washed with distilled water until the impurities in the mullite precursor became 0.1 wt% or less of the total of Al ₂ O ₃ and SiO ₂ . The obtained precipitate was left standing and dried at 180 ° C. in the atmosphere for 24 hours to obtain a mullite precursor. The obtained mullite precursor has an average Al ₂ O ₃ / SiO ₂ weight ratio of 73.8.
/26.2, the content of sulfur component was 7.5 wt%.

The obtained mullite precursor was fired at 1450 ° C. to obtain mullite powder. This mullite powder is shown in FIG. 1 which shows the intensity ratio of each peak to the peak of maximum intensity from the X-ray diffraction pattern under the conditions of X-ray: Cu, Kα X-ray generation condition: 35 Kv, 25 mA.

Example 2 While maintaining the temperature at 30 ° C. and stirring 1.5 mol of an aqueous solution of aluminum sulfate having a concentration of 1 mol / mol, sodium silicate having a concentration of 1 mol / mol had a composition of 73/27 in terms of Al ₂ O ₃ / SiO ₂ conversion weight ratio. Was added to. The pH of the mixed aqueous solution was 3. An aqueous solution of sodium hydroxide having a concentration of 4 mol / was added to the mixed aqueous solution until 2.40 gram equivalents per 1 gram atomic weight of Al. After the addition, the mixture was left for 6 hours with stirring. After that, the impurities in the mullite precursor were filtered to 0.1 wt% of the total of Al ₂ O ₃ and SiO _2.
It was washed with distilled water until it became less than or equal to%. The obtained precipitate is
The mullite precursor was obtained by standing and drying at 180 ° C. in the atmosphere for 24 hours. The obtained mullite precursor had an average weight ratio of Al ₂ O ₃ / SiO _{2 of} 72.2 / 27.8 and a sulfur component content of 14.5 wt%.

The obtained mullite precursor is fired under the same conditions as in Example 1 to obtain mullite powder, and the powder X-ray diffraction pattern obtained by measuring the powder under the same conditions as in Example 1 is shown in FIG.

Comparative Example It was carried out under the same conditions as in Example 2 except that the amount of sodium hydroxide aqueous solution added was 2.85 gram equivalents per 1 gram atomic weight of Al.

The obtained mullite precursor had an average Al ₂ O ₃ / SiO ₂ weight ratio of 71.1 / 28.9 and a sulfur content of 0.83 wt%.

The obtained mullite precursor is fired under the same conditions as in Example 1 to obtain mullite powder, and the powder X-ray diffraction pattern obtained by measuring the powder under the same conditions as in Example 1 is shown in FIG.

[Brief description of drawings]

FIG. 1 shows the intensity ratio of each peak to the peak of maximum intensity from the powder X-ray diffraction patterns of the mullite powders obtained in Examples 1, 2 and Comparative Example. The meanings of the symbols are as follows: M: Mullite A: Corundum

Claims (2)

[Claims] 1. In an oxide and / or hydroxide mainly containing aluminum and silicon, the composition ratio of aluminum and silicon is 65/35 to 80 / in terms of Al ₂ O ₃ / SiO ₂ conversion weight ratio.
A mullite precursor having a content of 20 and containing 6.7 to 20 wt% of sulfur as an acid radical. 2. An aluminum compound and a silicon compound,
The method according to claim 1, wherein the solution is dissolved in a mixed solution having a pH of 3 or less and mixed, an alkali is added to the solution to neutralize, and the obtained coprecipitated precipitate is washed and dried. Item 6. A method for producing a mullite precursor according to the item.