JPH0340966A - Conductive ceramic formed article and its production - Google Patents

Abstract

PURPOSE:To obtain a conductive ceramic formed article capable of being formed into an optional shape and which is not broken even when heated and cooled rapidly by filling a ceramic consisting essentially of silicon carbide into the pores of a thermal-shock-resistant porous ceramic having many pores communicating with the surface. CONSTITUTION:The porous ceramic is formed with a ceramic having >=400 deg.C thermal shock resistance such as silicon nitride, silicon carbide, Sialon, cordierite and zirconia and has many pores communicating with the surface. A conductive ceramic consisting essentially of silicon carbide is filled into the pores to form a phase communicating with the surface. Consequently, a conductive ceramic formed article is obtained and used as a ceramic heater, etc. The conductive ceramic is preferably obtained by calcining an organosilicon polymer (e.g. polysilane, polycarbosilane and polysilastyrene).

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel conductive ceramic molded article and a method for manufacturing the same. More specifically, the present invention relates to a novel conductive ceramic molded article that is a tough ceramic that can withstand high temperatures and is particularly useful as a heating element, and a method for producing the molded article industrially advantageously.

[Prior Art] It is well known that conductive ceramics are used as heating elements in electric heaters, particularly in high-temperature electric heaters. However, ceramic heaters are usually made of fired silicon carbide impregnated with silicon, and their workability is poor, making it difficult to form them into arbitrary shapes. In addition, this material is expensive and difficult to use for general purposes, but when it is protected by another insulating material as required in many applications, it is an inexpensive high-temperature material suitable as a protective support. There is a problem that the thermal tensile coefficient is different from that of alumina, which is an insulating material, making it difficult to integrate them. For this reason, the actual situation is that the above-mentioned materials are used only in limited industrial applications.

In order to solve this problem, the present inventors previously proposed a conductive ceramic composite molded product that is a combination of alumina and a conductive ceramic mainly composed of silicon carbide. Although this composite molded product certainly has various advantages, it is not suitable for applications such as auxiliary heaters in microwave ovens, where a predetermined voltage is applied to the air and the temperature rises in a short period of time, and in some cases, moisture etc. may fly away. When used as a heater, it is easy to break and has poor durability.

[Problems to be Solved by the Invention] The present invention seeks to provide a conductive ceramic molded product having a relatively arbitrary shape and a property of being resistant to breakage even when heated or cooled relatively quickly, and a method for manufacturing the same. It is. If such ceramics could be developed, they could be used in many fields, such as applications that require special shapes and physical properties, as well as applications where there is a possibility of rapid heating or the adhesion of flying debris, such as household products used to heat instant foods. It is thought that it will become possible to easily provide usable ceramic heaters.

[Means for Solving the Problems] As a result of intensive research into solving these problems, the present inventors have developed a porous ceramic pore that has a large number of pores that communicate with the surface and has a thermal shock resistance of 400°C or higher. It was discovered that the problem could be solved by filling the inside of the hole with ceramics mainly composed of silicon carbide, and the present invention was achieved based on this finding.

That is, the present invention comprises a first phase composed of a porous ceramic having a thermal shock resistance of 400° C. or higher and having a large number of pores communicating with the surface, and a first phase composed of a porous ceramic having a large number of pores communicating with the surface, and a first phase composed of a porous ceramic having a thermal shock resistance of 400° C. This is a conductive ceramic molded product characterized by comprising a second phase that is partially continuous to the surface of the molded product and is composed of a ceramic whose main component is silicon carbide.

The term "thermal shock resistance J" as used herein refers to the upper limit of the temperature difference at which ceramics are preheated and the high temperature ceramics do not break even if the ceramics are poured into water.

For example, if a ceramic heated to 200°C cracks when placed in water at a temperature of less than 10°C, but does not break when placed in water at 10°C, the thermal shock resistance is 190°C. Therefore, the thermal shock resistance of 400° C. or higher means that the above temperature difference is 400° C. or larger, and it means that the material can withstand rapid heating and cooling within this temperature range.

When the temperature of an auxiliary heater for a microwave oven increases rapidly in a short period of time, the operating temperature of the heating element ceramic of the heater, for example 800° C., corresponds to the required thermal shock resistance. According to the study results of the present inventors, the required thermal shock resistance of the porous ceramic in the present invention is lower than this, and it was found that it can withstand practical use at 400° C. or higher. The reason for this is
Conductive ceramics mainly composed of silicon carbide, which constitutes the second phase described below, have electrical resistance characteristics that are negative with respect to temperature at relatively low temperatures, and in the case of the composite ceramic according to the present invention, initially Excessive current does not flow,
Coupled with the problems of heat capacity and heat transfer rate, it is considered difficult to destroy. There are various types of ceramics with thermal shock resistance of 400°C or higher, including silicon nitride, silicon carbide, sialon, cordierite, and zirconia, which generally have low conductivity. It is something.

The porous ceramic constituting the first phase is made of the ceramic described above and has a large number of pores (so-called open cells) that communicate with each other, and at least some of the pores are connected to the surface. It is preferable to have pores in the interior, and it is not preferable to have only independent pores (closed cells) inside.

On the other hand, the conductive ceramic mainly composed of silicon carbide constituting the second phase is preferably obtained by firing an organosilicon polymer, and such a ceramic contains a small amount of carbon in addition to silicon carbide. Because it contains a moderate electrical conductivity, for example, volume resistivity is 1
It has a conductivity of about 02 to 10-1 ΩcI+.

The ceramic molded article of the present invention is one in which the pores of a porous ceramic serving as a base are filled with a conductive ceramic whose main component is silicon carbide,
It has electrical conductivity as a whole.

When the pores of porous ceramics are filled with conductive ceramics whose main component is silicon carbide, the conductive ceramic phase takes on a complicated curved shape rather than a straight line. Even if there is a slight difference in fat expansion or contraction, it will be absorbed. In addition, since it exhibits flexibility as a whole, it can absorb some differences in fat expansion coefficient, and no problem occurs even when laminated with other ceramics as a protective insulating material, such as heat-resistant alumina.

To obtain the composite ceramic molded article of the present invention, first, a porous ceramic molded article is manufactured. For example, a porous ceramic molded product can be obtained by pre-forming into an arbitrary shape and firing, or a porous molded product can be obtained by molding a porous ceramic into an arbitrary shape. Next, this porous ceramic molded article is impregnated with an organosilicon polymer dissolved in a solvent and dried.

Preferred organosilicon polymers include, for example, polysilane, polycarbosilane, polysilastyrene, polycarbosilastyrene copolymer, and the like.

These polymers are dissolved and impregnated in inexpensive solvents such as alkylbenzenes, specifically benzene, toluene, xylene, and the like. Among these, polycarbosilane and polycarbosilastyrene copolymer (PC3S) are preferable because they produce ceramics with particularly high conductivity.
ss is optimal.

Preferably, the ceramic prepared in advance is porous and has appropriate pore size and appropriate porosity. The pores preferably have a center size of 1 to 100 μm; if the pores are too small, impregnation or re-firing may be difficult. If it is too large, the conductive phase tends to peel off, making it difficult to obtain uniformity as a conductive ceramic. The porosity is preferably 5 to 50%, particularly 10 to 40%; if it is too small, it is difficult to improve the conductivity of the conductive ceramic, and if it is too large, it is often prone to breakage.

Porous forms include planar, rod-like, block-like, and the like. In order to conduct heat uniformly, a special shape may be used, such as the one proposed by the present inventors in Japanese Patent Application No. 63-29209.

[Effects of the Invention] According to the present invention, a device having a relatively arbitrary shape that can be rapidly energized;
Provided is a conductive ceramic molded product that has substantially excellent thermal shock resistance and is suitable as a heating element for a heater. This conductive ceramic has various desirable properties, can be easily obtained in any shape, is inexpensive, and can be integrated with other ceramic-based protective insulators, so it can be used in a wide range of applications such as household products.

[Example] Next, Examples and Comparative Examples of the present invention will be given, but the present invention is not limited thereto. In addition, unless otherwise specified, "parts" in each example are parts by weight.

Example 1 Commercially available silicon carbide (manufactured by Mitsui Toatsu; MSC-2'i11
00 parts, carbon powder 2 parts, carbomethoxycellulose 13
sample with a small amount of water added to it at a pressure of 800 kg.
/cd. The obtained sample was placed in a nitrogen stream.
1. Calcined at 300°C. The fired product was porous ceramic, brittle, and had a volume resistivity of 10 3 Ωcm.

Next, polycarbosilastyrene was produced as follows. That is, using equimolar amounts of dichlorodimethylsilane and dichloromethylphenylsilane, metal sodium was added and polymerized in toluene to obtain polysilastyrene. This polysilastyrene was treated at 400°C in a nitrogen atmosphere to obtain a polycarbosilastyrene copolymer having a softening point of 190 to 200°C.

100 parts of this copolymer was dissolved in 1.000 parts of toluene and impregnated into the above ceramic. - Repeated soaking, drying, soaking again, and drying day and night. The final weight after drying was 127 parts based on 100 parts of the ceramic immediately after firing.

This sample was heated to a maximum temperature of 1.300°C for 32 hours.
The product was fired in a nitrogen stream for several hours (the temperature was gradually raised from room temperature to 600°C, then the temperature increase rate was increased to 1,300°C, and then returned to room temperature). The resulting ceramic weighs 12
3 parts, and the volume resistivity was 2.3 Ωcm. When electricity is passed through this composite ceramic, the surface temperature becomes 1.00.
It was possible to generate heat above 0°C.

Next, alumina (manufactured by Mitsubishi Mining Cement; said to be made by the alkoxide method) with a thickness of 50 μm was obtained, and glass was bonded to the above sample as an adhesive to form an insulator-coated conductive ceramic. For adhesion, a thin sheet of glass was sandwiched between the prototype sample and a thin sheet of alumina, and the process was carried out for one hour in an argon stream.
, 200'C to melt and bond. This laminate maintained good adhesion over a long period of time and had excellent durability.

Example 2 Polycarbosilane (rearranged permethylpolysilane) with a pour point of 241'C was dissolved in xylene, and 50 parts of silicon nitride powder (manufactured by Showa Denko, DENSI-N) and
25 parts of potassium titanate whiskers (manufactured by Otsuka Chemical; "Tismo") were dispersed, cast, and dried.

This was further hot-pressed at 270°C and 1 ton/- to form a green sheet.

This was fired at 1.200°C in a nitrogen stream. The obtained porous ceramic sheet had a strength of 15 kg/-. The calculated void is 1-2.3/3.2 = 28%.

The above polycarbosilane (rearranged product of barmethylpolysilane) was dissolved in xylene to form a 5% solution, and 100 parts of the above ceramic was immersed in this solution to impregnate it with the polycarbosilane. - Soaked day and night, dried, impregnated again and dried.

This sample was heated to a maximum temperature of 1,300 in the same manner as in Example 1.
Calcined at ℃. The obtained composite ceramic has a weight of 10
Example 3 25 parts of the polycarbosilastyrene copolymer used in Example 1 was dissolved in toluene, and silicon nitride powder (manufactured by Showa Denko,
DENSI-N) and 25 parts of silicon nitride whiskers (manufactured by Ube Industries, Ltd., 5N-WB) were dispersed, cast, and dried. Furthermore, this was heated at 270°C for 1 t.
on/co? The green sheet was heated and pressed at 1.500°C in a nitrogen stream.

The obtained porous ceramic sheet has a strength of 20 kg/d.
, and the volume resistivity was 10'Ωcm. The calculated gap is 1
-2.3/3.2 = 28%.

This porous ceramic was impregnated with the polycarbosilastyrene copolymer solution used in Example 1, and impregnation and drying were repeated in the same manner as in Example 1.

The weight of the resulting sample was 117 parts based on 100 parts of the original sample. This sample was washed in a nitrogen stream.
Baked at a maximum temperature of 1.300°C. The weight of the obtained composite ceramic was 109 parts, and the volume resistivity was 0.5 Ωcm. This composite ceramic sheet became red hot when electricity was passed through it, and did not break even after repeated rapid heating.

Claims (4)

[Claims] (1) A first phase composed of a porous ceramic composed of a ceramic having a thermal shock resistance of 400°C or more and having a large number of pores communicating to the surface, and at least a part of the pores filled with the first phase. 1. A conductive ceramic molded product characterized in that the second phase is continuous to the surface of the molded product and is composed of a conductive ceramic whose main component is silicon carbide. (2) The ceramic that constitutes the first phase and has a thermal shock resistance of 400°C or higher is silicon nitride, silicon carbide,
At least one type of non-conductive ceramic selected from the group consisting of sialon, cordierite, and zirconia,
The conductive ceramic molded article according to claim 1, wherein the second phase is a conductive ceramic whose main component is silicon carbide obtained by firing an organosilicon polymer. (3) Conductive ceramic molding characterized by impregnating a porous ceramic with a thermal shock resistance of 400°C or higher obtained by firing in advance with a solution of an organosilicon polymer, and firing the molded product again after drying. method of manufacturing the product. (4) The organosilicon polymer is at least one solvent-soluble organosilicon polymer selected from the group consisting of polysilane, polycarbosilane, polysilastyrene, and polycarbosilastyrene copolymer. manufacturing method.