JPH0393617A - Production of easily sinterable alumina - Google Patents

Abstract

PURPOSE:To improve the sinterability of alumina by hydrolyzing an alkali metal aluminate and calcining the produced Al(OH)3 under specific condition. CONSTITUTION:Al(OH)3 produced by the hydrolysis of an alkali metal aluminate and having an average particle diameter (d58) of <=8mum and a uniformity number (n) satisfying the formula I [K=log {log(100/0.01)}-log {log(100/50}=1.12345] is calcined by conventional calcination process in such a manner as to get <=10% (based on the whole alumina) of alumina which passes through Boehmite phase in the phase transition of Al(OH)3 to Boehmite. As an alternative method, Al(OH)3 having an average particle diameter (d58) of <=20mum and a uniformity number (n) satisfying the formula II is calcined in high vacuum (<=-600mmHg) in such a manner as to get <=10% (based on the whole alumina) of alumina which passes through Boehmite phase in the phase-transition of Al(OH)3 to Boehmite. The objective easily sinterable alumina having a sintered bulk density of >=90g/cm<3> at 1400 deg.C can be produced by the above methods.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing easily sinterable alumina. i.e. 14
The sintered bulk density is 3. 9 0
Alumina iIl that yields a sintered body of g/a1 or more
Regarding the manufacturing method. [Conventional technology] Alpha-alumina has traditionally had excellent physical properties such as heat resistance, corrosion resistance, corrosion resistance, electrical insulation, and thermal conductivity, so it has been used in semiconductor integrated circuit boards, cutting tools, aluminum alloy bearings, etc. It is used as a raw material for sintered bodies. In recent years, there has been significant technological progress in these applications, and the quality required for the raw material alumina has become stricter year by year. The particularly important physical properties required for raw materials for sintered bodies are that the primary particle size of α-alumina powder is small, there are no strongly agglomerated particles, the shape is uniform, and it can be sintered at low temperatures. Sintered bodies obtained when alumina with these physical properties is used as a raw material have excellent sintered density and mechanical strength, and can be sintered at low temperatures, reducing not only firing costs but also the construction costs of firing equipment. It has the advantage of being able to [Problem to be solved by the invention] However, alumina obtained by the Bayer method, which has been most industrially adopted so far, has an average primary particle size of several μm to
It is difficult to obtain average particles with an average particle size of 1 μm even if milling is carried out for a long time at several tens of μm, and the obtained alumina powder has reduced purity due to impurities mixed in from the crushing equipment and non-smoothness of the crushed surface of crystal bodies. Therefore, it was difficult to obtain the desired effect of improving physical properties. There is also a method of hydrolyzing aluminum compounds such as aluminum alkoxide to form alumina hydrate, which is then dried and calcined to obtain α-alumina. Although alumina hydrate with a small particle size can be obtained, agglomeration and solidification occur during dehydration and drying, and even if this is pulverized after firing, the desired α-alumina powder with a uniform particle shape and a small average particle size can be obtained. I can't do anything. Furthermore, α-
It is known that a homogeneous and dense sintered body can be obtained at a lower sintering temperature than before by mixing a colloidal solution of alumina and then forming and firing it (Japanese Patent Application Laid-Open No. 61-265!54).
However, this method has the disadvantage that since the mold is formed in a gel-like form, the molded body shrinks greatly during drying and sintering, making it difficult to obtain a precise molded body. Therefore, it is possible to obtain a powder by drying and firing the gel-like material and then pulverizing it and using this as the raw material powder for the precept form, but the powder obtained by this method has particles that aggregate and crystallize during the drying and firing process. Perhaps because a certain length is produced, fine powder cannot be obtained, and as a result, a homogeneous and dense compact cannot be obtained. In view of this situation, the present inventors conducted intensive studies to obtain homogeneous and easily sinterable alumina powder with a small average primary particle size that can produce a homogeneous and dense sintered body at a low sintering temperature. The reason why alumina obtained by the conventional Bayer method has poor sinterability is that aluminum hydroxide is We have come to the knowledge that the part of the aluminum that is transformed into crystalline boehmite and then transformed into α-alumina has an adverse effect, and we have completed the method of the present invention. [Means for Solving the Problems] That is, the present invention provides that when aluminum hydroxide obtained by hydrolyzing alkali aluminate is burnt to produce α-alumina, the alumina passing through the boehmite phase accounts for 10% of the total.
s#, which is characterized by burning so that it is less than %! The present invention provides a method for producing condensed alumina powder. Alumina passing through the boehmite phase accounts for 10% of the total
% or less, the average particle diameter dse is 8μ or less and the uniform number n is 1.
Aluminum hydroxide that satisfies the following formula n > K / {log{logl 5 -1o ( (d ss)}) can be fired as a raw material at the temperature at which aluminum hydroxide is normally fired. It is possible to produce sintered alumina.Also, by firing under reduced pressure, a similar easily sintered alumina can be produced using aluminum hydroxide, which has a relatively larger particle size than the above-mentioned aluminum ξ hydroxide, as a raw material. In other words, when firing under a high vacuum of -600 mmHg or more, water whose d5@ is 20μ or less and whose uniformity number n satisfies the following formula n>K/ (lox30-1og (dse)) is used. If aluminum oxide is used as a raw material for firing, less than 10% of the aluminum oxide will pass through the boehmite phase, making it possible to produce similar easily sintered alumina.

The uniformity 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, available from the Japan Powder Industry Association, but if the particle diameter is D and the sieve surface is R (%), then
It has a scale of zD and log {log{log (100/R)). Also, in the above formula, K=log {log{log (100/0.01)
)−log {log{log (100/50))=
1. It is 12345. The present invention will be explained in more detail below. Aluminum hydroxide usually obtained by the Bayer process is alumina trihydrate, whose mineral name is gibbsite or hydralgilite, and is a powder with a visible particle size of about 30 to 100 μm. When these aluminum hydroxides are incinerated in the atmosphere using a conventional method, a portion of the aluminum hydroxide undergoes a phase transition to α-alumina through crystallizing boehmite (Electrochemistry Vol. 28, pp. 358-364). The proportion of this transition can be easily measured by analysis using a differential thermal balance. That is, as shown in Fig. 1, differential thermal analysis (D
Calculate the amount of boehmite from the weight loss in the temperature range in which the boehmite produced dehydrates and loses crystal water during the reaction corresponding to the endothermic peak in A). Calculate the amount of boehmite and calculate the proportion of the original sample. can be calculated. When the amount of boehmite transition is measured using this method for aluminum hydroxide obtained by the Bayer method, it is usually 2.
The value is between 5 and 55%. The inventors of the present invention have investigated in detail the phenomenon in which the sintering properties of the final α-alumina change when the amount of boehmite transition changes, and have found that, unexpectedly, when the amount of boehmite transition is below a certain level. We discovered that the sintering characteristics could be significantly improved and completed the present invention. The experimental facts that formed the basis of the present invention are described below. Caustic soda concentration 160 g/l. A l 203
Aluminum hydroxide (fine particle aluminum hydroxide H-43 manufactured by Showa Denko K.K.) with equal number n = 3.8, average particle size dss = Q, 52p was added to a sodium aluminate solution with a concentration of 120 g / l as seeds. An appropriate amount was added, kept under stirring at a predetermined temperature, and then filtered and washed to obtain aluminum hydroxide with a uniform number n of about 4 and an average particle size of 1.2 to 40μ as shown in Table 1. ．． Using the same sodium aluminate solution as above, precipitation was continued using the above H-43 on the initial seeds using a seed circulation type precipitation facility that combined a three-stage precipitation tank and a two-stage cyclone and shaker. As a result, aluminum hydroxide having a uniform number n of about 2 and an average particle size of 3 to 40 μm was obtained. Using a differential thermal analyzer manufactured by Rigaku Denki Co., Ltd., these aluminum hydroxides were used to measure the amount of boehmite produced when heated in an air atmosphere and the amount of boehmite produced when heated in a -600 mmHg atmosphere. Yukuri is also listed in Table I. In addition, these aluminum hydroxides were
Using 0719 as a reference, we processed alumina using the following procedure and compared its sintering properties. Aremi ゜ (1) Under atmospheric pressure and -600 m using an atmosphere firing furnace
Heat each aluminum hydroxide at 4OO@C under aHg vacuum for 2 hours. (2) 200g of the heat-treated product is added to lwt% citric acid solution.
Suspend at a concentration of /l, filter, wash with warm water, and dry. (3) Maintained at a specified temperature for 2 hours under atmospheric pressure in an electric furnace using a Kanthal heating element, and the BET specific surface area was approximately 10 m2.
/g of α-alumina (4). Each fired product was placed in a cylindrical rotating bowl with a diameter of 300 ml and a length of 300 am, and an anoremina bowl of 24 k. and sample anoremina 2.4k. Filled with powder and subjected to pulverization for 16 hours. Add 5% paraffin to each of the pulverized aluminas prepared above, mix thoroughly, weigh 7 g, put in a mold, and shape at a pressure of 500 kg/cm2.
After degreasing and holding at 1400°C for 2 hours, the sintered bulk density was measured. The measurement results are shown in Table 1. (Left below) As can be seen from Table 1, when the amount of boehmite transition is 10% or less, even if the sintering temperature is as low as 1400"C, the bulk density of the sintered body is over 3.90 g/cm3, which is extremely high. It can be seen that the sintering activity is high.It is clear that the amount of boehmite transfer cannot be determined only by the average particle size of aluminum hydroxide, but is influenced by the number of equal particles and the atmosphere during firing. In this case, the amount of boehmite transfer is small even if the average particle size is relatively large, but if the number of equal particles is small, the amount of boehmite transfer is large even if the average particle size is small.In addition, under reduced pressure, the amount of boehmite transfer decreases overall and the relative The sintered bulk density at 1400°C is high even with relatively large particles and a low uniformity number.The size of the uniformity number n is basically influenced by the proportion of coarse particles in the aluminum hydroxide. Specifically, from the average particle size and uniform number of raw materials and the above results, when fired in an atmospheric atmosphere, the average particle diameter is 15 μ, -600 m+
*For Hg, it is necessary that there are almost no particles with a particle size of 30μ or more (for example, 0.01% or less). From the above results, as stated in the claims, in the process of producing alumina by burning out aluminum hydroxide obtained by hydrolyzing alkali aluminate, aluminum hydroxide becomes alumina. By sintering so that less than 10% of the alumina passes through the boehmite phase during phase transition, it has become possible to produce alumina with unprecedented sintering properties. The above conditions can be achieved by selecting the particle size distribution of aluminum hydroxide used as a raw material depending on the firing atmosphere. As the precursor of alumina, aluminum hydroxide manufactured by the conventional Bayer method or its improved method, which has already been established as a commercial production process, can be used, and the manufacturing cost can be reduced because the conventional process can be used for the alumina preparation process. It's also cheap. The present invention is industrially extremely useful because the alumina produced by this method can be sufficiently sintered at low temperatures. The content of the present invention will be explained in detail below using Examples, but the technical scope of the present invention is not limited thereto. [Example] Example 1 After dissolving aluminum hydroxide obtained by the Bayer method in industrial caustic soda, dilution and clarifying filtration were performed to obtain Na 2
0: A I 20 3 molar ratio 1.5. Na2012
A 0 g/l sodium alkyl solution was prepared.

This liquid has an average particle size of 1. Add 3g/1 of aluminum hydroxide (Showa Denko am grain aluminum hydroxide H-42) with a uniform number of 3.8 and
Hold with stirring for a period of time, average particle size 24μ, uniform number 3.
9 coagulated aluminum hydroxide was precipitated. Next, this was filtered and washed, and then dried while being kneaded with a paddle dryer, and the agglomeration was dissolved and the average particle size was 6.8μ, and the number of even particles was 3.
．． 7 aluminum hydroxide was obtained. The boehmite conversion rate of this material under atmospheric pressure was 8.3%. After firing this in a rotary kiln at 500'C, it was suspended in warm water, filtered and washed again, and then heated in a rotary kiln at 1150'C.
When burnt out with a residence time of 1 hour, the BET specific surface area was 9.
Alumina of 8 m2/g and a gelatinization rate of 98% was obtained. This is powdered using the method described above. When subjected to crushing and sintering tests, a sintered body with a sintered bulk density of 3.95 g/am3 was obtained at 1400°C. Comparative Example 1 When the aluminum hydroxide precipitated in Example 1 was filtered and washed, suspended in industrial water and spray-dried, agglomerated aluminum hydroxide with an average particle size of 22 μm and an equivalent number of 3.4 was obtained. The boehmite transition rate of this material under atmospheric pressure is 2
It was 1%. This was fired in a rotary kiln at 500''C, suspended in hot water, filtered and washed again, and fired in a rotary kiln at 1180''C for 1 hour, resulting in a BET specific surface area of 10.2m? /g, alumina with a gelatinization rate of 98% was obtained. When this was subjected to a crushing and sintering test using the method described above, the sintered bulk density was 3.
A sintered body of 66 g/am3 was obtained. [Effects of the Invention] As explained above, the alumina produced by the method of the present invention has a sintered bulk density of 3.90z/c1 at 1400°C.
It has excellent sinterability. 4.

[Brief explanation of drawings]

Figure 1 shows During the firing process of aluminum hydroxide

Claims (3)

[Claims] (1) In the process of producing alumina by calcining aluminum hydroxide obtained by hydrolyzing alkali aluminate, when aluminum hydroxide undergoes a phase transition to alumina, alumina that passes through the boehmite phase accounts for 10% of the total % or less. (2) The average particle diameter d_5_0 is 8 μ or less, and the uniform number n is calculated by the following formula n>K/{log15-log(d_5_0)} where K
=log{log{log(100/0.01)}-l
The method for producing easily sinterable alumina according to claim 1, characterized in that aluminum hydroxide satisfying the following formula is fired by a normal firing method. (3) The average particle diameter d_5_0 is 20μ or less and the uniform number n is calculated by the following formula n>1.12354/{log30-log(d_5_
0)} aluminum hydroxide that satisfies -600mm
The method for producing easily sinterable alumina according to claim 1, characterized in that the firing is performed under a high vacuum of Hg or higher.