JPH0337155A - Electrically conductive ceramic composite material and its production - Google Patents

Abstract

PURPOSE:To provide the subject composite material composed of a porous alumina phase having interconnected fine pores and a ceramic phase of silicon carbide forming a continuous phase filled in the fine pores, formable to an arbitrary shape, having a thermal expansion coefficient close to that of alumina and suitable as a heat-generating element for inexpensive electric heater, etc. CONSTITUTION:The objective electrically conductive ceramic composite material is produced by filling an electrically conductive ceramic material composed mainly of silicon carbide in the fine pores of a porous ceramic (having pore diameter of 1-100mum and porosity of 5-50vol.%, especially 10-40vol.%) containing continuous pores connected with each other and at least partly opened to the surface. The silicon carbide is preferably produced by using an organic silicon polymer [more preferably polycarbosilane or polycarbosilastyrene copolymer(PCSS), especially PCSS] as a precursor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a novel conductive ceramic composite and a method for manufacturing the same. More specifically, the present invention relates to a conductive ceramic composite containing alumina and silicon carbide that has excellent heat resistance and durability and is suitable as a heating element for an electric heater, and a method for industrially manufacturing the composite.

[Prior Art] It is well known that conductive ceramics are used as a heating element for electric heaters, especially as heating elements for high-temperature electric heaters (for example, Japanese Patent Laid-Open No. 57-152693,
Special Publication No. 60-16386, Special Publication No. 60-28781, etc.). Such conductive ceramics are usually fired products such as silicon carbide powder, but such silicon carbide powder has poor moldability and is difficult to form into arbitrary shapes, and is also expensive, making it difficult to use as a general-purpose heater. .

In addition, in many applications, it is required to protect the heating element with an insulator, but as a protective insulator, alumina, which is an insulator that can withstand high temperatures and is inexpensive, has a different coefficient of thermal expansion, so it cannot be integrated. The problem is that it is difficult.

For the reasons mentioned above, the reality is that conductive ceramics for heating elements, mainly made of silicon carbide, are used only in a limited field of industrial heaters.

[Problems to be Solved by the Invention] The present invention solves the problems of conductive ceramics made of silicon carbide powder fired bodies as described above, can be formed into a relatively arbitrary shape, and has thermal expansion similar to that of alumina. The purpose is to provide inexpensive conductive ceramics with a high coefficient of conductivity.

Furthermore, conductive ceramics that can be easily used as heating elements in fields that require special shapes or protective insulators, such as household heaters, etc. This is what we are trying to provide.

[Means for Solving the Problems] As a result of intensive research on the above-mentioned problems, the present inventors have developed a ceramic composite in which the pores of porous alumina having communicating pores are filled with ceramics mainly composed of silicon carbide. The inventors have discovered that the various problems of conventional conductive ceramics made of fired silicon carbide products can be solved by forming a ceramic body, and have thus arrived at the present invention.

That is, the present invention includes a first phase made of porous alumina having a large number of pores communicating with each other; and a second phase made of ceramics containing silicon carbide as a main component, which is connected to the surface.

The porous alumina that makes up the ceramic composite of the present invention has a large number of pores, so-called open cells, that are interconnected and at least partially connected to the surface, and the pore size is 1 to 100 μm. It is preferable that the porosity is distributed around 5 to 50% by volume,
Particularly preferred is 10 to 40% by volume. The components constituting porous alumina do not need to be pure alumina, and may contain small amounts of other components. Such porous alumina itself is insulating.

On the other hand, the ceramics filled in the pores of the porous alumina are conductive ceramics containing silicon carbide as a main component. Preferably, this ceramic has at least one organosilicon polymer selected from the group consisting of polysilane, polycarbosilane, polysilastyrene, and polycarbosilastyrene copolymer as a precursor; In addition, it contains a small amount of carbon, so it has suitable conductivity as a heating element, for example, has an electrical resistance of 10-2 to 101 Ω・cm.
It is a conductor of some degree. Among the above-mentioned organosilicon polymers, polycarbosilane and polycarbosilastyrene copolymer (PCSS) are particularly suitable.
s is optimal.

The conductive ceramic mainly composed of silicon carbide needs to be substantially filled in the pores of the porous alumina, and at least a part of it needs to reach the surface.

To obtain such a ceramic composite, first, for example, porous alumina, which has been molded and fired into a predetermined shape in advance, is impregnated with an organosilicon polymer dissolved in a solvent. Examples of the organosilicon polymer include polysilane, polycarbosilane, polysilastyrene, and polycarbosilastyrene copolymer. These polymers are dissolved in inexpensive solvents such as alkylbenzenes, specifically benzene, toluene, xylene, etc., and impregnated into porous alumina. Among these, polyolevosilane and polycarbosilastyrene copolymer (PCSS) are particularly preferred because they produce ceramics with high conductivity, and PCSS is particularly suitable.

Preferably, the alumina prepared in advance is porous, with appropriate pore size and appropriate porosity. It is preferable that the pores have a center size of 1 to 100 μm; if the pores are too small, impregnation may be difficult or re-firing as described below may be difficult. On the other hand, if the pores are 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%; if it is too small, it will be difficult to improve the conductivity of the conductive ceramic, and if it is too large, it will often be easily broken.

Porous alumina may have a planar shape, a rod shape, a block shape, or the like. Regarding heat theory, special shapes, for example, in order to conduct electricity uniformly, the present inventors previously proposed a patent application filed in 1983-29.
No. 209 may be used.

The porous alumina uniformly impregnated with the organosilicon polymer is then heated and refired in an inert atmosphere to a temperature at which the organosilicon polymer turns into a ceramic, forming a conductive material mainly composed of silicon carbide inside the porous alumina. It is converted into ceramics and becomes the desired electrically conductive ceramic composite.

[Effects of the Invention] According to the present invention as described above, a conductive ceramic composite of a relatively arbitrary shape and having physical properties similar to alumina that can be covered with a protective insulator such as alumina can be obtained relatively easily. This conductive ceramic composite has various desirable properties such as conductivity and mechanical properties, and has excellent durability.
It can withstand use at 1000°C. Moreover, it is easy to obtain any shape, is inexpensive, and has a thermal expansion coefficient similar to that of ceramic protective insulators, especially alumina, so it can be easily integrated, so it can be used in a wide range of applications such as household items. .

[Example 1 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 A commercially available porous alumina plate was obtained. When immersed in water, the weight increased by 30%, and when the state of the pores was observed using an R microscope, it was mainly composed of a large number of communicating pores with a pore diameter of 10 to 50 μm.

On the other hand, a polycarbosilastyrene copolymer was obtained in the following manner. 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 with a softening point of 190 to 200°C.

100 parts of this copolymer was dissolved in t and ooo parts of toluene, and the above alumina was impregnated with this copolymer solution. −
After being soaked day and night and dried, it was impregnated again and dried.

This sample was heated at a maximum temperature of 1,300° C. for a total of 32 hours (slowly increasing the temperature from room temperature to 600° C., then increasing the heating rate to 1,300° C., and returning to room temperature. > in a nitrogen stream. The obtained ceramic composite had a weight of 115 parts and a volume resistivity of 15 Ωcm. When electricity was passed through this ceramic composite, the surface temperature decreased to 1.0
It was possible to raise the temperature to 00°C or higher.

Alumina (estimated to be made by the alkoxide method, manufactured by Mitsubishi Mining Cement) with a thickness of 50 μm was obtained, and glass was bonded to the above ceramic composite sample using an adhesive.
It was made of insulator-coated conductive ceramics. For adhesion, a thin glass sheet was sandwiched between the sample and the alumina thin sheet, and the temperature was raised to 1° to 200°C in an argon stream to melt and bond.The resulting multilayer body had good adhesion between the eyebrows. It became strong.

Example 2 A commercially available ceramic plate for use as an electrolytic partition was obtained. When immersed in water, the weight increased by 28%, indicating that it was dense porous alumina.

Polycarbosilane (rearranged product of barmethylpolysilane) having a pour point of 241° C. was dissolved in xylene to make a 5% solution, and 100 parts by weight of the above ceramic was impregnated with the solution.

- After day and night impregnation, drying, impregnation again and drying.

This sample was fired in the same manner as in Example 1 at a maximum temperature of 1j00°C. The obtained composite ceramic had a weight of 109 parts,
The volume resistivity was 182 Ωcm. This product was useful as a heating element for a high-temperature heater.

Example 3 Porous alumina (called “machinable ceramics”)
I got it from Nippon Tsusho.

This ceramic was impregnated with the polycarbosilastyrene solution used in Example 1. Similar to Example 1, impregnation,
Repeated drying.

The weight of the obtained sample was 117 parts based on 100 parts of the original porous alumina (machinable ceramics).This sample was fired at a maximum temperature of 1,300°C in a nitrogen stream.The obtained ceramic The weight of the composite is 109
The volume resistivity was 42 Ωcm. When electricity was passed through this ceramic composite, it became red hot, confirming its usefulness as a heating element for a high-temperature heater.

Claims (4)

[Claims] (1) A composite comprising a first phase made of porous alumina having a large number of pores communicating with each other, and the pores of the porous alumina being filled and at least a part of which forms a continuous phase. 1. A conductive ceramic composite comprising: a second phase made of ceramics containing silicon carbide as a main component, which is connected to the surface; (2) The conductive ceramic composite according to claim 1, wherein the second phase is a fired product of an organosilicon polymer. (3) A method for producing a conductive ceramic composite, which comprises impregnating pre-fired porous alumina with a solution of an organosilicon polymer, drying and firing. (4) The method for producing a conductive ceramic composite according to claim (3), wherein the organosilicon polymer is at least one of polysilane, polycarbosilane, polysilastyrene, or polycarbosilastyrene copolymer.