JPH0610083B2 - Method for producing easily sintered alumina - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a method for producing easily sinterable alumina.

That is, the sintered bulk density is 3.90 at a firing temperature of 1400 ° C. or lower.
The present invention relates to a method for producing alumina, which yields a sintered body of g / cm ³ or more.

[Conventional technology]

Since α-alumina has been excellent in physical properties such as heat resistance, corrosion resistance, wear resistance, electrical insulation, and thermal conductivity, it has been used as a sintered material for semiconductor integrated circuit boards, cutting tools, wear resistant bearings, etc. Has been done. In recent years, these applications have made remarkable technological advances, and the quality required for the raw material alumina has become stricter year by year.

The most important physical property required for the raw material for the sintered body is α
-The primary particle size of the alumina powder is small, there are no strong agglomerated particles, the shape is uniform, and it can be sintered at low temperature. The sintered body obtained when alumina having such physical properties is used as a raw material has excellent sintering density and mechanical strength and can be sintered at a low temperature, which reduces the firing cost as well as the construction cost of the firing facility. It has the advantage that it can be done.

[Problems to be Solved by the Invention]

However, in the alumina obtained by the Bayer method which has been most industrially adopted so far, the average primary particle is several μm to
It is difficult to obtain particles with an average particle size of 1 μm even after pulverizing for several tens of μm for a long time, and the obtained alumina powder has a reduced purity due to impurities mixed in from the pulverizer and the non-smoothness of the crushed surface of the crystal form. It was difficult to obtain the desired effect of improving the physical properties. There is also a method of hydrolyzing an aluminum compound such as an aluminum alkoxide to form an alumina hydrate, and drying and firing this to obtain α-alumina. In this method, the average primary particle diameter by hydrolysis is Although a hydrated alumina hydrate having a small particle size can be obtained, the desired α-alumina powder having a uniform particle shape and a small average particle size cannot be obtained even if it is aggregated and solidified during dehydration drying and pulverized after firing. Absent.

Further, it is known that a homogenous and dense sintered body can be obtained at a lower firing temperature than before by mixing a colloidal solution of α-alumina as seeds with a colloidal solution of boehmite and firing the mixture (JP-A-61). -26554). However, this method has a drawback in that a molded body is largely shrunk at the time of drying and sintering because it is molded in a gel state, and it is difficult to obtain a precise molded body. Therefore, it is conceivable to dry and calcine the gel-like material to obtain a powder, and use this powder as a raw material powder for a molded body. However, the powder obtained by this method is agglomerated and crystallized during the drying and baking process. Fine particles may not be obtained, possibly because of the growth, and as a result, a homogeneous and dense compact cannot be obtained.

In view of such a situation, the present inventors have earnestly studied to obtain a uniform easily sinterable alumina powder having a small average primary particle diameter that can obtain a homogeneous and dense sintered body at a low sintering temperature, Alumina obtained by the conventional Bayer method is inferior in sinterability because a part of aluminum hydroxide crystallizes in the process of obtaining alumina by firing aluminum hydroxide obtained by hydrolysis of alkali aluminate. The inventors have come to the knowledge that the portion that transforms to the characteristic boehmite and then transforms to α-alumina has an adverse effect, and has completed the method of the present invention.

[Means for Solving the Problem] That is, according to the present invention, when aluminum hydroxide obtained by hydrolyzing an alkali aluminate is fired to form α-alumina, the total amount of alumina passing through the boehmite phase is 10%.
Another object of the present invention is to provide a method for producing a readily sinterable alumina powder, which is characterized in that it is fired so as to have a content of at most%.

Then, the total amount of alumina passing through the boehmite phase is 10
%, The average particle size d ₅₀ is 8 μm or less, and the uniform number n is
The following equation n> K / {log15-log (d 50)} aluminum hydroxide that satisfies, may be fired at a temperature of firing normally aluminum hydroxide as a raw material, sinterability alumina can be produced by this method.

Further, by firing under reduced pressure, similar easily sinterable alumina can be produced by using aluminum hydroxide having a particle size relatively larger than that of the above aluminum hydroxide as a raw material.

That is, in the case of firing in a high vacuum of -600 mmHg or more, aluminum hydroxide having d ₅₀ of 20 μ or less and an even number n satisfying the following expression n> K / {log30-log (d ₅₀ )} If this is fired as a raw material, the proportion of the material passing through the boehmite phase is 10% or less, and a similar easily sinterable alumina can be manufactured.

The uniform number n is a numerical value representing the uniformity of the particle size distribution, and is obtained as the slope of the graph of the particle size distribution plotted on the Rosin-Rammler diagram. The Rosin-Rammler diagram is a logarithmic probability paper for powders, which can be obtained from the Japan Powder Industry Association, but if the particle size is D and the sieve top is R (%), log on the x-axis and y-axis.
It is a scale with D and log {log (100 / R)}.

In the above equation, K = log {log (100 / 0.01)}-log {log (100/50)} = 1.12345.

Hereinafter, the present invention will be described in more detail.

Usually, aluminum hydroxide obtained by the Bayer method is an alumina trihydrate whose mineral name is gibbsite or hydrargillite, and is a powder having an apparent particle diameter of about 30 to 100 μm. When these aluminum hydroxides are fired in the air by a usual method, a part of them undergoes a phase transition to α-alumina through crystalline boehmite (Electrochemistry 28: 358-364).

The rate of this transition can be easily measured by analysis with a differential thermal balance. That is, as shown in FIG. 1, differential thermal analysis (D
In the reaction corresponding to the endothermic peak of (TA), the amount of boehmite is calculated from the weight reduction in the temperature range where the formed boehmite loses water of crystallization due to dehydration, and the ratio of this to the original sample is calculated. Can be calculated.

When the boehmite transfer amount of aluminum hydroxide obtained by the above-mentioned Bayer method is measured by such a method, it is usually 2
It is a value of 5 to 55%.

The present inventors, when the amount of boehmite transfer changes,
As a result of detailed examination of the phenomenon in which the sintering characteristics of the finally obtained α-alumina change, it was discovered that when the boehmite transition amount falls below a certain level, the sintering characteristics are significantly improved. The invention was completed.

The experimental facts on which the present invention is based will be described below. Sodium aluminate solution having a caustic soda concentration of 160 g / l and an Al ₂ O ₃ concentration of 120 g / l has an equal number n = 3.8 and an average particle diameter d.
₅₀ = 0.52μ aluminum hydroxide (Showa Denko KK)
Finely divided aluminum hydroxide H-43) was added as a seed in an appropriate amount, and the mixture was kept stirring at a predetermined temperature and then washed by filtration. As shown in Table 1, the uniform number n was about 4 and the average particle size was about 4. 1.2 to 40 μ of aluminum hydroxide was obtained.

Using a sodium aluminate solution having the same composition as described above, precipitation was continued using H-43 as the initial seeds by a seed circulation type precipitation equipment combining a three-stage precipitation tank and a two-stage cyclone & thickener. As a result, aluminum hydroxide having an even number n of about 2 and an average particle diameter of 3 to 40 μm was obtained.

The amount of boehmite produced when heated in an air atmosphere and the amount of boehmite produced when heated in an atmosphere of -600 mmHg were measured by a differential thermal analyzer manufactured by Rigaku Denki Co., Ltd. The results are also shown in Table 1.

Further, these aluminum hydroxides are used as disclosed in JP-A-55-14.
With reference to No. 0719, alumina was treated by the following procedure, and the sintering characteristics were compared.

Alumina preparation method (1) Atmospheric pressure and -600mmHg
Each aluminum hydroxide is heated under reduced pressure at 400 ° C. for 2 hours.

(2) 200 g of the heat-treated product in a 1 wt% citric acid solution /
It is suspended at a concentration of 1 and separated by filtration, washed with warm water and dried.

(3) A BET specific surface area of about 1 is maintained at a predetermined temperature for 2 hours under atmospheric pressure by an electric furnace using a Kanthal heating element.
It is 0 m ² / g of α-alumina.

(4) Using a cylindrical rotary ball mill having a diameter of 300 mm and a length of 300 mm, 24 Kg of 20 mmφ alumina balls and 2.4 Kg of sample alumina are filled in each fired product and pulverized for 16 hours.

To each pulverized alumina prepared as described above, 5% of paraffin is added by external splitting and thoroughly kneaded, and then 7 g is weighed and placed in a mold and molded at a molding pressure of 500 Kg / cm ² . After degreasing this 1400
After holding at 0 ° C for 2 hours, the sintered bulk density was measured. The measurement results are shown in Table 1.

As can be seen from Table 1, when the boehmite transition amount is 10% or less, the bulk density of the sintered body is 3.90 g / cm ³ or more and the sintering activity is extremely high even if the sintering temperature is as low as 1400 ° C. I understand.

It is also apparent that the amount of boehmite transition cannot be regulated only by the average particle diameter of aluminum hydroxide, and that it depends on the uniform number and the atmosphere during firing. When the uniform number is large, the amount of boehmite transfer is small even if the average particle size is relatively large, but when the uniform number is small, the amount of boehmite transfer is large even if the average particle size is small. Further, under reduced pressure, the amount of boehmite transition is reduced as a whole, relatively large particles, and even if the number is low, the baked bulk density at 1400 ° C. increases. The size of the uniform number n is basically influenced by the proportion of coarse particles of aluminum hydroxide. Specifically, based on the average particle diameter and the uniform number of the raw materials and the above results, the firing in the air atmosphere was 15 μ, −
At 600 mmHg, it is necessary that almost no particles having a particle size of 30 μ or more exist (for example, 0.01% or less).

From the above results, in the process for producing alumina by firing aluminum hydroxide obtained by hydrolyzing alkali aluminate as described in the claims, when aluminum hydroxide undergoes a phase transition to alumina, By firing so that the alumina passing through the boehmite phase is 10% or less of the whole, it has become possible to produce alumina having excellent sintering characteristics as never before. The above conditions can be achieved by selecting the particle size distribution of aluminum hydroxide used as a raw material according to the firing atmosphere. As the precursor of alumina, aluminum hydroxide manufactured by the conventional Bayer process or a process in which a commercial production process has already been established as an improved method can be used, and since the conventional process can be used for the preparation process of alumina, the manufacturing cost is also high. cheap. Since the alumina produced by this method can obtain a sufficient sintered body at a low temperature, the present invention is extremely useful industrially. Hereinafter, the content of the present invention will be described in detail with reference to Examples, but the technical scope of the present invention is not limited thereto.

〔Example〕

Example 1 Aluminum hydroxide obtained by the Bayer method was dissolved in industrial caustic soda, and diluted and clarified by filtration to obtain Na ₂ O:
Al ₂ O ₃ molar ratio of 1.5, was prepared sodium aluminate solution Na ₂ O120g / l. This liquid has an average particle size of 1.0
3 g of aluminum hydroxide (microparticle aluminum hydroxide H-42 manufactured by Showa Denko Co., Ltd.) having a uniform number of 3.8
/ L was added and the mixture was kept at 50 ° C. for 48 hours with stirring to precipitate agglomerated aluminum hydroxide having a mean particle size of 24 μm and a uniform number of 3.9. Next, this was filtered and washed, and then kneaded with a paddle dryer and dried, whereby the agglomeration was released and aluminum hydroxide having an average particle diameter of 6.8 μ and an equal number of 3.7 was obtained. This product had a boehmite transition rate under atmospheric pressure of 8.3%. 50 in a rotary kiln
After calcination at 0 ° C., suspension in warm water, filtration and washing again, and calcination with a rotary kiln at a residence time of 1150 ° C. for 1 hour, alumina having a BET specific surface area of 9.8 m ² / g and an α conversion of 98% was obtained. . When this was subjected to pulverization and a sintering test by the method described above, the sintered bulk density was 3.95 at 1400 ° C.
A sintered body of g / cm ³ was obtained.

Comparative Example 1 The aluminum hydroxide precipitated in Example 1 was filtered, washed, suspended in industrial water, and spray-dried to obtain aggregated aluminum hydroxide having an average particle size of 22 μ and a uniform number of 3.4. This product has a boehmite transition rate of 2 under atmospheric pressure.
It was 1%. This was baked in a rotary kiln at 500 ° C, suspended in warm water, filtered and washed again, and then baked in a rotary kiln at a residence time of 1180 ° C for 1 hour.
Alumina having an ET specific surface area of 10.2 m ² / g and an α conversion of 98% was obtained. When this was subjected to pulverization and a sintering test by the method described above, it had a sintered bulk density of 3.66 g / cm at 1400 ° C.
A sintered body of ³ was obtained.

〔The invention's effect〕

As described above, the alumina produced by the method of the present invention has a sintered bulk density at 1400 ° C. of 3.90 g / cm 3.
^It is as high as ³ or more and has excellent sinterability.

[Brief description of drawings]

FIG. 1 is a graph showing the state of heat absorption and weight loss during the baking process of aluminum hydroxide.

Claims (3)

[Claims] 1. A process for producing alumina by calcining aluminum hydroxide obtained by hydrolyzing alkali aluminate, in which aluminum hydroxide passes through the boehmite phase when the aluminum hydroxide undergoes a phase transition to alumina. Of 10
A method for producing easily sinterable alumina, which comprises firing so that the content becomes less than or equal to%. 2. The average particle diameter d ₅₀ is 8 μm or less, and the uniform number n is the following expression n> K / {log15−log (d ₅₀ )} where K = log {log (100 / 0.01)} − log { 2. The method for producing easily sinterable alumina according to claim 1, wherein aluminum hydroxide satisfying log (100/50) = 1.12345 is fired by a usual firing method. 3. An aluminum hydroxide having an average particle diameter d ₅₀ of 20 μm or less and an even number n satisfying the following expression n> 1.123 45 / {log30−log (d ₅₀ )} under high vacuum of −600 mmHg or more. The method for producing easily sinterable alumina according to claim 1, wherein the method is as follows.