JP2612878B2 - Method for producing silicon carbide honeycomb structure - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a silicon carbide honeycomb structure.

[Prior Art] One end face of an infinite number of through holes connected in a honeycomb shape via a thin partition wall is filled with a plug material, for example, every other length and width, and sealed, and is adjacent to the sealed through hole. A ceramic honeycomb structure consisting of a porous partition wall filled with a plug material at the other end of the through hole and sealed, collects fine carbon contained in exhaust gas of various combustion equipment such as automobile diesel engines. It is known as an exhaust gas purifying device that removes excessively.

As a method for manufacturing such a honeycomb structure, the following method is conventionally known. For example, a method of making a ceramic thin plate having plasticity with inorganic particles that can be sintered, corrugating it and assembling it with another ceramic thin plate into a honeycomb shape and firing it, extruding pipes, binding and sintering. A method of forming a monolithic structure, a method of extruding a ceramic kneaded material having plasticity including an organic binder from a die designed to have a desired shape, forming a monolithic structure, and then drying and firing to form a honeycomb structure,
Alternatively, there is a method in which a honeycomb prototype is formed with an organic carrier, ceramic slurry is applied, impregnated, and fired.

[Problems to be Solved by the Invention] In any of the production methods described above, a medium liquid is usually added to a mixture of ceramic powder to impart plasticity to the mixture to form a plastic mixture. And a process of drying and removing the medium before firing.

However, when drying and removing such a medium solution, the surface of the honeycomb-shaped formed body, that is, both end faces and the outer side surface having the opening of the through hole are easier to dry than the inside of the formed body, so drying shrinkage occurs, Cracks are formed from the surface of the molded body to the inside, and even when used as an exhaust gas purifying apparatus as described above, the overfunction for collecting and removing fine carbon contained in the exhaust gas is extremely poor. There is.

As a means for solving such a problem, in a honeycomb structure mainly composed of cordierite, generally,
A method is used in which the inside of the honeycomb-shaped formed body is heated and dried uniformly with the surface to prevent cracks by heating with microwaves when performing the drying treatment.

On the other hand, a honeycomb structure containing silicon carbide as a main component shows superior properties such as heat resistance as compared with a honeycomb structure containing cordierite as a main component, but a honeycomb structure containing silicon carbide as a main component. In the body, even if the drying process is performed using microwaves as a heating source, the silicon carbide itself has semiconductor properties, so absorbs the microwaves and causes self-heating, so that the microwaves do not reach the inside of the generated form, As a result, there has been a problem that drying proceeds from the surface of the molded body to cause shrinkage and cracks in the molded body.

Therefore, as a method of drying and removing the medium liquid when manufacturing a honeycomb structure containing silicon carbide as a main component,
A method is used in which the green compact is dried under a constant temperature and humidity condition and gradually dried. However, this method requires a very long time for drying, and thus has a problem in productivity.

There is also a method of heating and drying the formed body under reduced pressure, but even in this case, there is a problem that heat does not conduct to the inside of the formed body and drying shrinkage still occurs from the surface of the formed body. .

Furthermore, in the above-mentioned drying method under reduced pressure, drying without heating was also performed. However, in this case as well, there was a problem that an extremely long drying time was required because the drying had to be performed gradually.

The present invention solves such a problem, and performs a drying process for removing the medium liquid from the formed form in a short time, without causing cracks in the formed form, and having a high-precision overfunction. An object of the present invention is to provide a manufacturing method capable of obtaining a silicon carbide honeycomb structure.

[Means for Solving the Problems] The method for producing a silicon carbide honeycomb structure of the present invention comprises:
A silicon carbide powder is used as a starting material and a crystal growth aid is added as necessary, and then a forming binder and a medium,
A plastic mixture obtained by adding a dispersant as needed is formed into a honeycomb-shaped formed body, and then a film having airtightness on an outer surface except an end face having an opening of a through hole of the formed body is formed. The method is characterized in that after being coated in close contact, the formed form is dried to remove the medium contained in the formed form by drying, and then the formed form is fired.

The crystal system of silicon carbide used as a starting material includes α
There are a type, a β type and an amorphous type, and any one of them, or a mixture of any two or more of them may be used as a starting material of the present invention.

In addition, examples of the auxiliary agent for crystal growth that is added as needed include aluminum, boron, iron, and carbon.

Further, as a molding binder, methyl cellulose,
Polyvinyl alcohol, water glass, etc.
Cyclohexane, benzene, methanol, ethanol,
Examples of the dispersant to which toluene, water, and the like are added as needed include ethylene glycol, dextrin, various surfactants, and the like, but are not particularly specified.

In addition, it is preferable that the ratio of the medium solution to the volume excluding the space in the plastic mixture is 5 to 30% by volume.
The reason is that if the content exceeds 30% by volume, molding cannot be performed because the mixture has fluidity. In addition, the dimensional change of the formed body due to the removal of the liquid medium increases, and the formed body may be cracked. On the other hand, if it is less than 5% by volume, the plasticity of the mixture is insufficient and the moldability is extremely poor. Above all, 10 ~
Preferably, it is 25% by volume.

As a method of forming the plastic mixture into a honeycomb shape,
Any method such as an extrusion molding method, a sheet molding method, and a press molding method may be employed.

The air-tight film is adhered to the outer surface except for the end surface having the opening of the through hole of the formed body formed in the honeycomb shape, and the film is dried because the film is vaporized and moved from the outer surface by this film. Therefore, vaporization and movement are performed from the inside of the formed body through the opening of the through-hole, and the surface of the molded body, that is, the outer surface is dried quickly even if it is dried in a short time. This is because it is possible to prevent that.

Drying can be performed under any pressure conditions. That is, when under reduced pressure, the drying speed is high, and the difference in drying speed between the outer surface of the molded body and the inside thereof is small.When under pressure, the drying speed is reduced under reduced pressure. Although it is inferior, the thermal conductivity is increased, so that even when heated, the difference in temperature between the parts of the formed body is small, and the entire molded body is uniformly dried.

Examples of the airtight membrane include polyethylene,
A synthetic resin film such as Saran, polypropylene, Teflon, or nylon, or a metal thin film such as aluminum or copper can be used.

Here, the film having airtightness means a film having airtightness to such an extent that the medium solution can prevent the vaporized molecules from passing therethrough. Further, this film is a thin film which is in good contact with the honeycomb-shaped formed body, and preferably has a thickness of 0.1 mm or less. Furthermore, it is preferable that the contact area is such that at least 90% of the area of the outer surface of the honeycomb-shaped formed body is covered.

According to the production method of the present invention, the desired silicon carbide-based honeycomb structure, the honeycomb-shaped formed body thus obtained by drying and removing the medium, is usually filled with plugs at desired positions at both ends of the through hole. After firing.
As the firing conditions, it is preferable to put the above-mentioned formed body in a heat-resistant container and fire in a non-oxidizing atmosphere.

The reason is that it is possible to promote the movement of silicon carbide atoms between silicon carbide particles by evaporation-recondensation and / or surface diffusion, and the partition walls of the through-holes of the honeycomb-like structure are complicated by plate-like crystals of silicon carbide. This is because the structure can be entangled in a proper state and the overfunction can be improved.

Examples of heat-resistant containers for firing the formed body include graphite, silicon carbide, zirconium oxide, tungsten carbide, titanium carbide, magnesium oxide, molybdenum carbide,
A container made of any one selected from molybdenum, tantalum carbide, tantalum, zirconium carbide, and a graphite-silicon carbide composite can be used. This is because these containers do not melt within the firing temperature range described below.

The firing temperature is preferably from 1800 to 2500 ° C. The reason is that, when the temperature is lower than 1800 ° C., the growth of silicon carbide particles is insufficient, the firing is incomplete, and it becomes difficult to obtain a high-strength sintered body. Also, this is because the surface area increases and the oxidation amount increases. On the other hand, when the temperature exceeds 2500 ° C., the sublimation decomposition of silicon carbide crystal becomes active, and the developed crystal becomes thinner on the contrary, and it is difficult to obtain a high-strength sintered body.

[Example] 100 parts by weight of silicon carbide powder having an average particle diameter of 0.27 μm,
0.1 parts by weight of boron and 0.1 parts by weight of aluminum were added, and 5 parts by weight of methylcellulose powder, 0.1 part by weight of condensed ammonium naphthenate, and 25 parts by weight of water were added.
After mixing in a pressure kneader for 5 hours, the mixture was stored in a sealed container for 1 day. Next, this mixture was subjected to vacuum degassing to obtain a plastic mixture in which silicon carbide was uniformly decomposed.

Then, by extrusion, diameter 200mm, length 150mm,
A honeycomb-shaped formed body having 200 through-holes per square inch was obtained. The outer surface of the resulting green body is immediately
It was coated with a thin film of polypropylene having a thickness of μm so as to be in close contact with it, and drying from the outer surface was suppressed. At this time, the coating area of the thin film was 98% of the outer surface of the formed body.

When the formed product in this state was inserted into a 35 torr vacuum dryer and dried, the drying was completed in about 5 hours, and no crack was generated in appearance. Then, the formed body was inserted into a Tamman furnace and fired at 2000 ° C.

Both end faces of the through-holes of the obtained silicon carbide honeycomb structure were sealed with raw rubber every other length and width, immersed in water, and air pressure was applied from one end face.

As a result, it was confirmed that bubbles were generated from the entire structure at an air pressure of 0.55 kg / cm ² or more, and uniform pores were formed throughout the entire structure.

[Comparative Example] The same as the above example, but drying was performed without covering the outer surface of the honeycomb-shaped formed body with a thin film made of polypropylene. A crack of about 0.1 mm x 5 mm was confirmed on the outer surface of the.

The crack is filled with a paraffin was aerated test under the same conditions as described above in Example, bubbles are generated from the interior of the structure pneumatically 0.01 kg / cm ^2, cracks occur to the inside of the structure It was confirmed that.

[Effects of the Invention] As is clear from the above description, according to the method for manufacturing a silicon carbide-based honeycomb structure of the present invention, cracks do not occur in the honeycomb-shaped formed body due to the drying and removal treatment of the medium. Has a uniform pore size throughout the structure,
A silicon carbide-based honeycomb structure having excellent overfunction can be manufactured.

Further, the processing time required for drying and removing the medium solution can be shortened, so that the productivity of the honeycomb structure can be improved.

Claims (1)

(57) [Claims] Claims: 1. Mainly using silicon carbide powder as a starting material,
Next, a plastic mixture obtained by adding a molding binder and a medium solution is formed into a honeycomb-shaped formed body, and then the formed body has airtightness on an outer surface except an end face having an opening of a through hole. A method for producing a silicon carbide-based honeycomb structure, comprising drying a formed liquid after removing the medium contained in the formed form by drying the formed form after the film is adhered, and firing the formed form.