JPH0829995B2 - Method for manufacturing foam ceramics - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing ceramics having a foamy appearance.

[Conventional technology]

Conventionally, ceramic fibers have been mainly used as a heat insulating material used at high temperatures. The reason is that the ceramic fiber is suitable for industrial mass production. That is, the ceramic fiber is manufactured by a method in which molten ceramic is dropped, and a high-speed air stream such as steam is blown onto this to make the melt into a filament shape, and the productivity is extremely good.

[Problems to be Solved by the Invention]

However, ceramic fibers have many problems as a heat insulating material.

The biggest problem among them is that ceramic fibers shrink when used at high temperatures. Due to this shrinkage, the heat insulating layer is cracked and the heat insulating effect is significantly reduced. this is,
This is due to the fact that the fibers are not fixed and that the fibers are glassy and mass transfer occurs when they crystallize.

Further, in the ceramic fiber heat insulating material, the space between the fibers is continuous, and the gas molecules can move freely, but the heat insulating effect is maintained by not causing a large air flow. However, even from the theory of heat conduction,
The free movement of gas molecules is fundamentally problematic as a heat insulating material.

Therefore, in the Japanese Patent Application No. 63-10921, the inventors of the present invention have a structure in which a large number of cell-shaped spaces in which a spherical dense polycrystalline ceramic film is isolated are surrounded and the ceramic film is continuous. Proposed foamy ceramics.
Such foam ceramics are produced, for example, by the following method. That is, various organic components such as ceramic particles and a binder are mixed in a solvent, foamed with a stirrer, and dried in a stable state to obtain a foamed molded body, which is then calcined and further fired. By doing so, foam ceramics can be produced. In this foamy ceramic, the cell-shaped space is isolated inside the material and is in an independent state, and the gas molecules only move within a limited range of one cell. Obtainable.

However, it has been found that it is difficult to control the fine structure when actually manufacturing such foam ceramics.
That is, when a raw material containing ceramic particles and an organic component is foamed, the film-forming substance tends to be spherical due to its surface tension. The film thus formed has a small thickness in the central portion of the partition between adjacent cell-shaped spaces. In some cases, a hole is formed in the center of the partition wall to connect adjacent cell-shaped spaces, and as a result, the above-described heat insulating effect cannot be obtained. Further, it is difficult to set the conditions under which the foamed ceramics in which both the size of the cell-shaped space and the thickness of the ceramics film are controlled can be manufactured. In particular,
Since the film thickness of the ceramic film is a factor that influences the characteristics such as the thermal conductivity of the foamed ceramics and the mechanical strength, it is extremely important to control it.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method capable of industrially and stably producing foamy ceramics having high performance as a heat-resistant heat insulating material.

[Means and Actions for Solving Problems]

The method for producing a foamy ceramic of the present invention encloses a large number of cell-shaped spaces in which a dense polycrystalline ceramic film is isolated by foaming a raw material containing ceramic particles and an organic component, and then firing the foamed raw material. In producing a foamed ceramic having a continuous structure of the ceramic film, the ceramic particles have a particle size similar to a desired film thickness of a dense polycrystalline ceramic film for isolating a cell-like space, It is characterized by using a mixture of coarse ceramic particles obtained by aggregating fine ceramic particles and fine ceramic particles.

The present invention was made based on the following findings. That is, in the middle of forming the ceramic film that separates the cell-shaped spaces, the film forming substance moves in the direction in which the film thickness in the central part of the partition wall becomes thin, and the thickness of the triple point of the film becomes large, and the film gradually moves. The movement of the film-forming substance stops at a certain point in time. In this case, if the mobility of the film-forming substance is sufficiently high, the film thickness of the thinnest portion is found to be almost the same as the particle size of the ceramic particles dispersed in the film-forming substance. Therefore, if ceramic coarse particles having a particle size similar to the target desired film thickness and agglomerated ceramic fine particles are used as the ceramic raw material, the film thickness of the ceramic film that becomes the partition of the cell-like space can be controlled. can do. However, if only the ceramic coarse particles are used, the particle gap becomes large. Therefore, the ceramic fine particles are added to fill the particle gap between the ceramic coarse particles. In order to homogenize the ceramic film, it is desirable to make the density of the ceramic coarse particles equal to the density of the ceramic fine particles filled in the particle gaps between the ceramic coarse particles.

The following method can be adopted as a specific method for producing the foam ceramics of the present invention. As a general method, coarse ceramic particles, fine ceramic particles,
Dispersing materials, binders, foam stabilizers were mixed well in a solvent to prepare slips, which were then whipped with a stirrer and dried in a stable state to obtain a foamed molded product, which was then calcined. Examples of such a method include burning organic components such as, and firing at a higher temperature (higher than the operating temperature of the foam ceramics).

In the present invention, the material forming the ceramic film is
In order to maintain the independence of each cell and to maintain the strength as a material, it is required to be dense, have high strength, and have high heat insulating properties. Examples of such materials include oxide-based ceramics such as alumina, zirconia, magnesia, mullite, and spinel, and non-oxide-based ceramics such as silicon nitride and silicon carbide. Of these ceramics, alumina-based ceramics are particularly desirable. Alumina-based ceramics are the most commonly used ceramics and have excellent strength and heat resistance. Alumina has a higher thermal conductivity than silica and the like, but since the cross-sectional area of the ceramic film is smaller than the cross-sectional area of the space, the amount of heat transmitted through the film is small,
The heat insulation effect is satisfied. When the alumina-based ceramics is used at a high temperature, crystal grains may grow and mechanical strength may decrease. In order to prevent this, it is effective to add a small amount of magnesia of 100 ppm to 0.2 wt% to alumina. Further, when a small amount of magnesia is contained in alumina, it is also effective in suppressing abnormal particle growth during firing during the production of the foam ceramics of the present invention.

In the present invention, various methods can be adopted as a method for preparing coarse particles in which the ceramic fine particles are aggregated. In general, the ceramic fine particles and the binder may be made into coarse particles with a granulator such as a spray dryer and then calcined. Alternatively, the coarse particles obtained by granulation as described above may be pressurized, then roughly crushed and sized with a sieve. Further, it is also possible to harden the ceramic fine particles and the binder by a slip casting method, coarsely pulverize this, sieving with a sieve, and calcination to prepare coarse particles. It is necessary that the coarse ceramic particles do not loosen in the solvent, and therefore calcination is effective as described above, but if the binder used when agglomerating the fine particles does not dissolve in the solvent, No baking is necessary. Further, the temperature condition in the case of calcination is preferably a temperature range in which firing shrinkage does not occur so that the density of coarse particles is likely to be equal to the packing density of fine particles.

When preparing the slip by mixing the raw materials, it is desirable to prepare the slip having a low viscosity with a small amount of solvent. In this case, regarding the ceramic fine particles, it is desirable that the primary particles are sufficiently loosened.

The binder is added in order to impart a strong strength to the foamed molded body. That is, although cracks are likely to occur in the foam-like molded article during drying shrinkage, the binder can prevent the occurrence of cracks. The binder may be an organic binder generally used for press molding of ceramics, but a binder for slip casting that does not extremely increase the viscosity of slip is desirable.

As the dispersant, polyacrylate, polyaluminum chloride, etc. are effective. However, the sodium salt generally used as a dispersant is inconvenient because it brings in different metal ions into the ceramics, and an ammonium salt that does not bring in different metal ions is desirable.

The foam stabilizer is used to stabilize the foam so that it does not disappear. The foam stabilizer may be a so-called soap, but it is preferable that it does not contain metal ions, and for example, ammonium stearate is suitable.

The method for producing the foamed ceramics of the present invention is not limited to the above-mentioned method of foaming a slip, and a method similar to a usual method for producing a foamed material can be adopted. In this case, a kneaded material is prepared by mixing coarse ceramic particles, fine ceramic particles, and a foaming agent in a plastic such as polyurethane or polystyrene, or rubber, which is foamed, solidified, and then fired.

By using the method of the present invention, the thickness of the ceramic film can be controlled, and the cell-shaped space can be isolated by the ceramic film having a sufficiently large thickness. For this reason, there is no hole in the center of the partition wall so that adjacent cell-shaped spaces are not connected, and the cell-shaped spaces are reliably separated inside the material to be in an independent state, and the movement of gas molecules is one cell. Since it is limited to the inside, an excellent heat insulating effect can be obtained. Moreover, since the film thickness of the ceramic film is controlled, the thermal conductivity and other characteristics of the foamed ceramic can be controlled to some extent.

Further, the space confined in the ceramics sintered at a high temperature has the same atmospheric pressure as the atmosphere at that temperature, and thus becomes a depressurized state when cooled to room temperature. Therefore, if the foamed ceramic is fired at a temperature higher than the temperature at which it is used, the pressure inside the cell is reduced at the temperature used, the number of collisions of gas in the cell is reduced, and the heat insulating effect is improved.

Furthermore, if the atmosphere at the time of firing is made vacuum, the cell-shaped space of the foam ceramics can be made vacuum. Hereinafter, this method will be described. That is, the packing density of the ceramic particles in the film portion at the stage of the foamed molded body is about 35 to 60% of the theoretical density, and the molded body has air permeability. In this molded product, a part of the film is filled with organic components such as a binder, a dispersant, and a foam stabilizer. If this is calcined in air, the organic components will not be burned out,
The packing density of the ceramic particles remains at about 35 to 60% as described above and remains unchanged. By putting this in a vacuum furnace and exhausting the gas around the calcined body, the gas molecules trapped in the bubbles pass through the gaps of the ceramic particles forming the film and gradually escape from the calcined body. Finally, it becomes a vacuum. When the temperature is further raised in this state and, for example, about 1400 ° C. for alumina ceramics, the air permeability is lost and the cell-like space becomes a closed space. Then, at higher temperatures, for example
Bake at 1800 ° C and cool to room temperature. In the foam ceramics produced in this way and having a cell-shaped space in a vacuum, the heat insulating effect can be further improved.

〔Example〕

 Examples of the present invention will be described below.

Example 1 100 parts of 99.9% alumina powder having a particle size of 0.2 μm, 0.5 part of stoichiometric spinel, 100 parts of deionized water, and 2 parts of PVA were mixed with a ball mill for one day. This was granulated with a spray dryer. This granulated powder is pressed at a pressure of 1 ton / cm ^{2 to} obtain a green compact, which is roughly crushed in a mortar and then the particle size is 10 to 20.
One having a particle size of 5 μm and one having a particle size of 5 to 10 μm were classified. These were calcined in air at 800 ° C. for 2 hours to prepare coarse particles.

Alumina coarse particles (particle size 10 ~
70 parts, alumina fine particles 30 parts, ammonium stearate 1 part, ammonium polyacrylate 1 part, acrylic binder 10 parts, ion-exchanged water 20
Whisk with a stirrer while mixing the parts. After the bubbles stabilize
It was dried with a drier to obtain two types of molded bodies (each having different alumina coarse particles).

These molded bodies were fired in air at 800 ° C. for 2 hours to burn off organic substances such as binders. Furthermore, in vacuum, 16
Two types of foam alumina ceramics were obtained by firing at 50 ° C. for 2 hours.

Regarding these foam-like ceramics, the thickness of the ceramic film was 15 μm on one side and 7 μm on the other side, and the average diameter of the cell-like spaces was 50 μm in both cases. Further, the thickness of the ceramics film was almost equal to the size of the classified coarse alumina particles having the largest diameter that had been shrunk by firing. Further, the ceramic film had little variation in film thickness.

Example 2 In the same manner as in Example 1, coarse alumina particles having maximum diameters of 20 μm and 10 μm (particle diameters of 10 to 20 μm and 5 to 10 μm) were obtained.

Alumina coarse particles (particle size 10 to 20 μm or 5 to 10 μm) 70
Parts, 30 parts of alumina fine particles, 20 parts of unvulcanized rubber, and 1 part of dinitropentamethylenetetramine as a foaming agent.
These were heated to 140 ° C. and foamed. These foams were fired in air for 2 hours to obtain two types of foam ceramics.

Regarding these foam ceramics, the thickness of the ceramic film was 15 μm on one side and 7 μm on the other side, and the average diameter of the cell-shaped spaces was 100 μm in both cases.

〔The invention's effect〕

As described above in detail, by using the method of the present invention, it is possible to stably produce foamed ceramics having a controlled film thickness, and to industrially supply a heat insulating material having excellent properties.

Claims (1)

[Claims] 1. A raw material containing ceramic particles and an organic component is foamed and then fired to enclose a large number of cell-shaped spaces in which the dense polycrystalline ceramic film is isolated, and the ceramic film is continuously formed. In producing the foamed ceramics having the structure described above, as the ceramics particles, ceramics fine particles having a particle size similar to the desired film thickness of the dense polycrystalline ceramics film isolating the cell-like space were aggregated. A method for producing foam ceramics, which comprises using a mixture of coarse ceramic particles and fine ceramic particles.