JPH11240778A - Ceramic lightweight composite material having closed pore and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high-strength alumina lightweight composite material having a low baking shrinkage percentage, excellent in dimensional stability, comprising closed pores, having a structure of a pore average diameter controlled to <=10 μm and having a low water absorbing power and a low thermal expansivity at a low cost and to provide a method for producing the alumina lightweight composite material. SOLUTION: This ceramic lightweight composite material is produced by uniformly mixing a solid volcanic glass powder having the average particle diameter controlled to <=10 μm and an alumina powder having the average particle diameter controlled to <=1 μm as a pore forming material and then baking the resultant mixture at >=1,250 deg.C and comprises closed pores. The method for reducing the weight comprises liquefying the volcanic glass particles surrounded by an unsintered alumina powder in advance in the course of baking and leaching the liquefied particles to the side of the alumina by a capillary phenomenon and thereby forming the closed pores.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a structure having a low shrinkage ratio, excellent dimensional stability, closed pores, a controlled average pore diameter of 10 μm or less, low water absorption,
The present invention relates to a low-thermal-expansion, low-cost, high-strength alumina lightweight composite and a method for producing the same.

[0002]

2. Description of the Related Art Japanese Patent Application No. 7-98839 proposes, as a prior art, a method of using a hollow body as a pore-forming material to reduce the weight of ceramics.

[0003]

A conventional ceramic lightweight composite using a hollow body or the like as a pore-forming material has a large sintering shrinkage ratio and is inferior in dimensional stability. There are problems that the number of open pores is larger than the number of closed pores, the water absorption rate is higher, the pore diameter is larger, the strength is lower, and the cost of the hollow body tends to be higher because the hollow body is expensive.
Further, when an organic hollow resin bead or the like is used,
Since toxic decomposition gas is emitted during firing, there is a major environmental problem. Further, since the hollow body has a low specific gravity, the specific gravity is easily separated particularly in a wet molding method, so that there is a problem that uniform mixing is difficult at the time of molding and the productivity is poor. Furthermore, in the case of dry molding using a hollow body, since the hollow body is easily damaged by the pressing pressure, there are problems such as the density of the molded body greatly changing by the pressing pressure, and the pores of the sintered body becoming non-uniform. was there.

[0004]

In order to solve these problems, a solid volcanic glass powder whose average particle diameter is controlled to 10 μm or less and an alumina powder whose average particle diameter is controlled to 1 μm or less are used as pore-forming materials. After uniform mixing with the body, baking at 1250 ° C or higher results in an average diameter of closed pores of 10 μm.
It became possible to produce an alumina lightweight composite controlled as follows. The method of weight reduction does not utilize the introduction of hollow bodies, the formation of pores due to burning out of combustibles, and the foaming phenomenon of glass containing volatile components, which are found in conventional methods. This is based on the fact that closed pores are formed by volatilization of the volcanic glass particles surrounded by the liquefaction and leaching to the alumina side by capillary action. For this reason, open pores (continuous vents) are unlikely to be generated, and a high-strength alumina lightweight composite composed of closed pores and having a low water absorption rate can be synthesized.

[0005] As a method for achieving both the lightness and strength of the alumina sintered body, there is a method of reducing the number of open pores which are liable to breakage, increasing the number of closed pores, and making the diameter of the closed pores as small as possible. . However, in the conventional method of simply introducing pores, it is difficult to achieve both lightness and strength because open pores are easily generated due to weight reduction. Therefore, as a result of intensive research on weight reduction, fine solid volcanic glass particles and finer alumina powder were mixed,
It has been found that by sintering at a high temperature of 1250 ° C. or more, it is possible to introduce closed pores without generating open pores.

The particle size of the alumina powder is as follows:
It was found that when the thickness was less than μm, only closed pores were easily formed. Since the closed pore diameter depends on the volcanic glass particle diameter of the raw material, by controlling the average particle diameter of the raw material volcanic glass particles to 10 μm or less, the average diameter of the closed pores is very small, 10 μm or less. It was possible for the first time to achieve both lightness and strength. Regarding the firing temperature, if the firing temperature is low, sintering becomes insufficient, the number of open pores increases, and the water absorption rate increases.
By baking at 0 ° C. or higher, an alumina lightweight composite having almost closed pores and low water absorption was obtained.

With respect to firing shrinkage, when alumina powder and volcanic glass powder are used, firing at 1400 ° C. or lower shows the same firing shrinkage behavior as alumina powder alone.
When calcined at 0 ° C. or higher, alumina and silica in the volcanic glass reacted to generate mullite, so that the shrinkage was smaller than that of alumina powder alone, and excellent dimensional stability was exhibited. The generated mullite has a lower specific gravity and lower thermal expansion than alumina, and has a function of reducing the firing shrinkage and suppressing the thermal expansion coefficient of the sintered body. Further, regarding the strength, it has been confirmed that the alumina lightweight composite fired at 1300 ° C. to 1400 ° C. exhibits strength equal to or higher than the alumina sintered body sintered at the same temperature.

In order to exhibit the above-mentioned excellent characteristics, it is important to uniformly mix the raw material volcanic glass powder and alumina powder to obtain a dense compact. As a method therefor, a volcanic glass powder controlled to an average particle size of 10 μm or less, an alumina powder controlled to an average particle size of 1 μm or less, distilled water, a polymer electrolyte and a binder are mixed in an appropriate amount, and a gypsum mold is used. , And then dried after demolding to obtain a dense molded body. By sintering it at a high temperature of 1250 ° C. or more, an alumina lightweight composite controlled to an average pore diameter of 10 μm or less was obtained. Polymer electrolyte, alumina particles and volcanic glass particles in aqueous suspension,
The binder was used to uniformly disperse, and the binder was used to improve the strength of the molded article released from the gypsum mold and promote sintering. When the added amount was 2.9% by weight and 0.7% by weight, respectively, with respect to the powder weight, it was possible to synthesize a desired alumina lightweight composite.

[0009]

The pore formation in the alumina lightweight composite of the present invention is not caused by foaming of the volcanic glass, but during firing, the volcanic glass particles surrounded by unsintered alumina powder are first liquefied and the alumina side The pores are smaller than the original volcanic glass particle diameter because they are leached by capillary action. Since the pores are composed of closed pores, they have low water absorption. In addition, solid volcanic glass powder is inexpensive to manufacture and is far less expensive than hollow glass spheres, so that an alumina lightweight composite can be manufactured at low cost.

In the case of the wet molding method, since the specific gravity of the volcanic glass is larger than that of the hollow glass sphere, the difference in specific gravity is small and uniform mixing is easily performed when preparing a suspension mixed with alumina powder. Easy to mold and easy to mold. Further, in the case of the dry molding method, distilled water, a polymer electrolyte, and the like are not required, and there is an advantage that the production can be performed at lower cost than in the wet molding method. In addition, volcanic glass powder has been confirmed to release only moisture during firing, unlike organic beads, and is an environmentally superior material. That is,
The alumina lightweight composite according to the present invention has excellent features such as excellent dimensional stability, low thermal expansion, low water absorption, high strength, low cost, easy production, and environmental safety.

[0011]

[Example] A pulverized shirasu from Yoshida-cho, Kagoshima-gun (average particle size 8.2 μm, specific gravity 2.36) was used as the volcanic glass powder, and α-alumina powder manufactured by Sumitomo Aluminum Refining was used as the alumina powder. (Average particle size: 0.5 μm). The ratio of volcanic glass powder to alumina was 28.6% by weight, and the ratio of powder to distilled water was 42.9% by weight. An appropriate amount of an ammonium polyacrylate manufactured by Kyowa Sangyo as a polymer electrolyte and an acrylic emulsion manufactured by Mitsui Toatsu Chemicals are added as binders, and the mixture is rotated and mixed with alumina balls having a diameter of 19 mm in a plastic cylindrical container for 1 hour. Was prepared. After vacuum degassing, the mixture was cast into a gypsum mold and dried at 110 ° C. to obtain a molded product. The molded body was heated at a rate of 2.5 ° C. per minute in the atmosphere and kept at a temperature of 1300 to 1500 ° C. for 1 hour. As a comparison, the hollow glass sphere was 2
Similarly, a sintered body was obtained in the case of adding 8.6% by weight and the alumina powder alone.

The firing shrinkage rate of the alumina lightweight composite up to 1500 ° C. is 9.9%, which is smaller than that using a hollow glass spherical body (firing shrinkage rate of 16.4%). (Sintering shrinkage rate of 13.1%) and showed excellent dimensional stability. 1300 ° C, 1400
C. and 1500.degree. C., the alumina lightweight composite had a bulk specific gravity of 2.80, 2.97 and 2.93, respectively, and the pores formed inside the sintered body had a water absorption of almost 0%. From this, it was found that it was composed of closed pores. As a result of observing the fracture surface, it was observed that each of the lightweight composites was composed of closed pores, the pore shape was dependent on the contour of the volcanic glass particles, and the pore average diameter was 10 μm or less.

The thermal expansion coefficient of the alumina lightweight composite sintered at 1500 ° C. is 5.7 × 0.000001 / K (25
８００800 ° C.), which is smaller than the value of only alumina,
Excellent low thermal expansion. 1300 ° C, 140
The bending strength (average value of three tests) of the alumina lightweight composite sintered at 0 ° C. was 190.7 MPa and 249.3 MPa, respectively, and the sintered body of alumina alone was 158.9 MPa and 24
It was 9.7 MPa. In other words, the strength of the alumina lightweight composite was similar to that of alumina alone at 1400 ° C., and the strength of the alumina lightweight composite was higher than that of alumina alone at 1300 ° C.

[0014]

The lightweight alumina composite obtained by the present invention is at least 25% lighter than a pure alumina sintered body, has a water absorption of almost 0%, and has an average pore diameter of closed pores of 10 μm. It is controlled as follows. In addition, the firing shrinkage is small and excellent in dimensional stability, and the fired body has low thermal expansion,
It has high strength and can be manufactured safely at low cost by a simple manufacturing method. Due to these excellent physical properties, heat insulating materials for high temperature that require thermal shock resistance, electronic materials such as alumina substrates for ICs that require hermeticity (low water absorption), low dielectric constant and high strength, and low water absorption It can be applied to alumina tableware, alumina crafts, exterior products, etc., which require high efficiency and low cost, and is expected to lead to the effective use of shirasu, which is said to have 90 billion tons in southern Kyushu.

Continued on the front page (72) Inventor Kazuto Hamaishi 1445-1 Oda, Hayato-cho, Aira-gun, Kagoshima Prefecture Inside the Kagoshima Prefectural Industrial Technology Center

Claims (2)

[Claims] 1. A method in which a solid volcanic glass powder controlled to an average particle size of 10 μm or less and an alumina powder controlled to an average particle size of 1 μm or less are uniformly mixed, molded, and fired at 1250 ° C. or more. This is a lightweight sintered body composed of closed pores, and volcanic glass particles surrounded by unsintered alumina powder are liquefied first during sintering and leached out toward the alumina side by capillary action, forming closed pores and lightweight. A high-strength alumina lightweight composite, wherein the average diameter of the closed pores is controlled to 10 μm or less. 2. A solid volcanic glass powder controlled to an average particle size of 10 μm or less and an alumina powder controlled to an average particle size of 1 μm or less are uniformly mixed, molded, and then fired at 1250 ° C. or more. On the way, volcanic glass particles surrounded by unsintered alumina powder are first liquefied and leached toward the alumina side by capillary action to form closed pores, and the average diameter of the closed pores is controlled to 10 μm or less. A method for producing a high-strength alumina lightweight composite, comprising: