JPH03101087A - Heat radiating ceramic - Google Patents

Abstract

PURPOSE:To suppress change of electric resistance of a heat radiating ceramic for a long duration by covering the whole surface of a conductive silicon carbide-based fiber bundle prepared from an organic siliceous polymer raw material with another ceramic so as to specify the whole surface area. CONSTITUTION:A center part (core part) is composed with conductive silicon carbide-based fiber bundle with a relatively low volume resistance and its surface is so covered with a ceramic having higher volume resistance than the core part as to suppress the surface area (equivalent to the surface area of the practical upper surface layer) of the whole heat radiating body of a heater to 0.03-0.1cm<2>/W per a prescribed heat generation. In this way, a ceramic heater which can relatively easily be given an optional shape, whose temperature can easily be increased and whose electric resistance hardly changes for a long period, is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ceramic heating element having heat resistance, oxidation resistance, etc. More specifically, it is a ceramic heating element that can withstand high temperatures and is made mainly of conductive silicon carbide fibers made from organosilicon polymers, and in particular, ceramic heating elements that last for a long time and whose electrical resistance is difficult to change. .

[Prior Art] It is a well-known fact that ceramics are used as electric heaters, especially electric heaters for high temperatures.

However, ceramics are hard and it is extremely difficult to make heaters of any type.

For example, there are various difficulties in creating a heater that snakes along the heated object. One possible solution to this problem is to use conductive ceramic fibers as a heating element.

On the other hand, it is also known that the materials used as conductive heating elements in electric heaters generally undergo oxidative deterioration and increase in resistance with use. This tendency is particularly noticeable in the case of ceramic heaters used at high temperatures, because the heaters are used at particularly high temperatures despite the fact that ceramics have high heat resistance. Therefore, the initial usage conditions of such heaters and the usage conditions at the end of their useful life are usually significantly different or different.

For example, silicon carbide is a well-known ceramic used as a heater material, and the life of this material is reached when the electrical resistance reaches two to three times the initial electrical resistance. For this reason, heaters using this type of material are largely limited to industrial applications where this adjustment is relatively easy to make. This problem becomes disadvantageous for fibers that are prone to deterioration, that is, fibers that have a large specific surface area.

In addition, the large specific surface area, which is a characteristic of fibers, means that they dissipate a large amount of heat, and there is also the problem that when used as a heater, it is difficult to raise the heat generation temperature.

[Problems to be Solved by the Invention] The problems to be solved by the present invention are to provide a novel heating element that eliminates the drawbacks of conventional ceramic heaters. That is, the object of the present invention is to provide a heating element that can be easily formed into any shape, is durable over a long period of time, has relatively little change in electrical resistance, and can easily raise its temperature. If such a heating element could be provided, it would be possible to easily use ceramic heaters in many fields such as household products.

[Means for Solving the Problems] The present inventors have made efforts to solve these problems, and have developed a ceramic heating element in which conductive silicon carbide fibers are used as the main conductor and this is covered with other ceramics. The inventors have discovered that the above problems can be solved and have arrived at the present invention.

The heating element of the present invention has a center portion (core portion) composed of a bundle of conductive silicon carbide fibers having a relatively low volume electrical resistivity, and a surface thereof preferably made of a ceramic material having a higher volume electrical resistivity. It is distinctive in that it is covered with It is preferable that the latter ceramic has a high volume resistivity, but it is not essential.

It is preferable that this phase having a high volume electrical resistivity is dense, especially glassy containing mainly 5iOz.

Generally, when ceramics are used for electrical resistance heating elements,
Ceramics are gradually eroded from the surface.

This erosion results in a substantial gradual reduction in the amount of conductor portion, leading to an increase in electrical resistance. Therefore, it is sufficient that the surface phase has extremely high corrosion resistance or that the overall resistance does not increase even if the surface phase deteriorates. In other words, the latter means that the surface phase only needs to have significantly greater volume resistivity than the internal phase. Moreover, in addition to the corrosion resistance of ceramics, the former is considered to be advantageous in that the actual contact area with the corrosive atmosphere is small. According to the results of research conducted by the present inventors, it has been found that glass is more preferable than sintered crystals, which are generally preferred as ceramics constituting the surface layer. The glassiness is thought to be due to the absence of gaps between crystal grains, like a sintered body of crystals. If it is a crystal, it is preferable that it has a high density, close to the theoretical value, and has no voids; for example, an aggregate of microcrystals that exhibits characteristics close to amorphous in X-ray analysis is preferable. Examples of such materials include glass and other amorphous ceramics, as well as alumina made by sintering fine powder.

However, in practice, these two phases, that is, the fiber phase in the center and the ceramic phase in the surface layer, will break at the interface unless their linear expansion coefficients are similar over a wide temperature range.

In order to meet these conditions, the core fibers are made from an organosilicon polymer, the surface ceramics are made from the same organosilicon polymer, and if possible, the volume resistivity of the former is lower than the volume resistivity of the latter. I can think of something to do with it.

As described above, the heating element of the present invention consists of a core made of a fiber bundle of conductive silicon carbide fibers and a surface layer made of another ceramic layer covering the core, and the fiber bundle of the core constitutes the surface layer. Fixed by ceramics.

The fibers that make up the core are made by heating precursor fibers made by spinning organosilicon polymers such as polysilane, polycarbosilane, polysilastyrene, and polycarbosilastyrene copolymers to make them infusible, and then spinning them under an inert atmosphere. By firing at a high temperature, it is made into a ceramic fiber mainly made of silicon carbide. Among these, it is particularly preferable to use a polycarbosilastyrene copolymer as a raw material, which is made infusible using iodine in substantially the absence of oxygen, and then fired.

The conductive silicon carbide fiber used in the present invention is preferably in a range of electrical resistance. If fibers with high electrical resistance are used, a product with a small specific surface area relative to the cross section must be made for practical use, and such a product would be extremely difficult to make in practice. On the other hand, if the electrical resistance is too low, it is easy to cause excessive current to flow and damage the heater unless special equipment and techniques are used when energizing and heating begins. Therefore, this fiber has a volume resistivity of 10-
About 3 to 102 Ωcm, particularly preferably 10-2 to 2O
It is preferable to have a resistance of 3 Ωcm.

On the other hand, the material of the surface layer is not limited as long as it is made of a different ceramic, but as mentioned above, it is preferably dense and has a relatively high electrical resistance.

As a specific example of the heating element of the present invention made of such a composite, for example, silicon carbide fibers starting from organosilicon polymers such as polysilane, polycarbosilane, polysilastyrene, and polycarbosilastyrene,
Examples include those covered with O2-based glass or amorphous silicon carbide-based ceramics made of polysiloxane, polysilazane, or the like as a starting material.

Al1 similar to these with alkoxide added as a raw material
For example, it may be covered with alumina mainly composed of 03 or boron ceramics mainly composed of B2O3.

These are generally made by bundling conductive silicon carbide fibers obtained by spinning, infusible, and firing an organosilicon polymer as a raw material, evenly coating the surface of the fiber with the ceramic precursor described above, and firing it again. It can be manufactured by a method.

Fibers have an extremely large specific surface area, and the surface area per filament is about 18rrf'/&, but silicon carbide fibers are commercially available fibers, and SiC fibers under published research.
Even when connected to a household power source (100 volts), including the fibers, it does not easily become red hot. In the heating element of the present invention, the fibers are consolidated into a bundle and the surface of the fiber bundle is coated with other ceramics, thereby increasing the surface area of the entire heater heating element (corresponding to the surface area of the surface layer) to generate the required heat. 0.03 for the amount
It is necessary to keep it to ~0.1' cd/watt.

With this configuration, it is possible to stably achieve 700 to 1
The temperature can now be maintained at 200'C.

In view of the gist of the present invention, as long as the above-mentioned conditions are satisfied, the type of raw material for the ceramics forming the surface layer is not critical. However, it is preferable that the volume resistivity of the silicon carbide fiber is lower than the volume resistivity of the ceramic covering it. Otherwise, the conductive performance of the ceramic heating element will drop significantly as the ceramic heating element deteriorates, which is undesirable.

The shape of the heating element of the present invention is usually linear, but rod-shaped,
It may be in the form of a tape or a sheet. Tape-like and sheet-like products are manufactured by arranging fiber bundles in parallel, covering them with ceramics, and hardening them.

[Effects of the Invention] The ceramic heating element of the present invention has advantages such as being able to be formed into any desired shape relatively easily, being able to easily raise the temperature, being durable over a long period of time, and having relatively little change in electrical resistance. This is a ceramic heater using silicon carbide fibers.

[Examples] 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 Polysilastyrene was obtained by polymerizing equimolar amounts of dichlorodimethylsilane and dichlorophenylmethylsilane in toluene with the addition of metallic sodium. This polysilastyrene was treated at 400°C in a nitrogen atmosphere to have a softening point of 240.
A polycarbosilastyrene copolymer at ℃ was obtained. This polysilastyrene copolymer (PCSS>) was melt-spun in a nitrogen stream at a spinning temperature of 280° C. and a spinning speed of 600 m/min.

Next, iodine was allowed to adsorb/act on the spun yarn. Thereafter, ammonia gas was applied to the material to form a raw material for firing.

The Kono treatment system was fired at 1,200° C. under a load of 10 g/10,000 de. The obtained ceramic fiber had a solid resistivity of 1 Ωam.

This fiber (strand) was wound around a frame of fluororesin "Teflon", immersed in a 10% solution of hydrofluoric acid for 10 minutes, and washed with water. The volume resistivity was also 1 Ωcm.

A 10% solution of polycarbosilastyrene (PC8S) copolymer, the same as that used for spinning, dissolved in toluene was repeatedly applied to the fiber bundle wrapped around this fluororesin frame, and then dried. Apply 93 parts of PCSS to 100 parts of fiber. After drying, it was separated from the fluororesin frame and fired as it was in a nitrogen stream. For firing, the temperature was raised at a rate of 25°C/hr up to 600°C.
The temperature was increased from 300°C to 100°C/hr. After cooling, the sample taken out was a composite consisting of silicon carbide-based ceramics with a core made of silicon carbide fiber bundles and a dense surface. This is calculated with a 2 volume fiber ratio of 62%, a volume resistivity of 5.6 Ωcm at room temperature, and a surface area of 0.06 cm so that the temperature will reach 800°C when connected to a 100 volt power source. / Watt (cross-sectional area: 6.5 Xl0-'alv, length: 5 cm),
By directly connecting it to a household power source, it was able to raise the temperature to 800 degrees Celsius and generate continuous heat. The calorific value of this heating element is 16.7 watts/-
It was 2.

Example 2 The silicon carbide ceramic fibers (strands) used in Example 1 were wrapped around a Teflon frame in exactly the same manner, treated with hydrofluoric acid, and washed with water.The resulting fiber bundle was coated with the same polycarbosilane. Coated.Finally 10 fibers
Apply 183 parts of polycarbosilane to 0 parts. Separate this from "Teflon" and heat it at 180℃ in the atmosphere for 30 minutes.
time, then held at 210°C for 3 hours to make it air infusible (
Introduction of oxygen was carried out.

This sample was fired in the same manner as in Example 1. The obtained sample had a volume fiber ratio of 52% and a volume resistivity of 16.3Ω at room temperature.
cm, specific surface area 9&/g, calorific value 17.8 w)/al
l < 0.056 (equivalent to all/watt), it could generate red heat when connected directly to a household power source.

Example 3 The ceramic fibers (strand 3) used in Example 1 were wrapped around a Teflon frame in exactly the same manner as in Example 1, treated with hydrofluoric acid, and washed with water.The resulting fiber bundle was given a melting point of 248°C. of polycarbosilane (a thermally rearranged product of permethylpolysilane) was applied so that the final amount was 90 parts of polycarbosilane per 100 parts of fiber.

This sample was fired in the same manner as in Example 1. The obtained sample had a volume fiber ratio of 62% and a volume resistivity of 9.2Ωc at room temperature.
m, specific surface area 20 cJ/g, calorific value 15.8 W)/
& (equivalent to 0,063 ail/watt) and could generate red heat when connected directly to a household power source.

Example 4 The ceramic fiber (strand) used in Example 1 was heated, wrapped around a Teflon frame in exactly the same manner, treated with hydrofluoric acid, washed with water, and thoroughly dried.

The fiber bundle obtained by cutting was placed on paraffin paper, and aluminum alkoxide (AI (OC4H[l) a
) was applied and impregnated.

This sample was fired in the same manner as in Example 1. The sample that could not be obtained had an estimated volume fiber ratio of about 64%, a volume resistivity of 10.2 Ωcm at room temperature, and a specific surface area of 20 cnY/g.
It has a calorific value of 14.7 watts/CIl+ (equivalent to 0,068 cJ/watt I) and cannot generate red heat when directly connected to a household power source.

Example 5 Commercially available silicon carbide fibers were obtained. The volume resistivity of this fiber at room temperature was 102 Ωcm.

This fiber was successively coated and solidified using the polycarbosilane solution used in Example 3, and finished into a rod shape with a diameter of 12 mm and a length of 50 mm.

This sample was fired in the same manner as in Example 1. The obtained sample had "cracks", but the estimated volumetric fiber content was 40%.
The volume resistivity at room temperature is 120 Ωcm, the apparent specific surface area is 1.7 c//g, and the calorific value is 10.6 W)/c.
nY (equivalent to 0,094 cJ/watt I), it was possible to generate heat by connecting directly to a household power source.

Claims (6)

[Claims] (1) A heating element comprising a fiber bundle of conductive silicon carbide fibers made from an organosilicon polymer as a conductive heating element, and the entire surface of the fiber bundle is coated with other ceramics, and the total surface area is 0.03 to 0.1cm^
A ceramic heating element characterized by having a power of 2/watt. (2) The ceramic heating element according to claim (1), wherein the organosilicon polymer is polycarbosilastyrene. (3) Claim (1) wherein the ceramic covering the fiber bundle of silicon carbide fibers is a ceramic whose main component is silicon carbide made from an organic silicon polymer.
) or the ceramic heating element described in (2). (4) Claim (1) wherein the ceramic covering the fiber bundle of silicon carbide fibers is vitreous containing SiO_2.
) or the ceramic heating element described in (2). (5) The ceramic heating element according to claim 1 or 2, wherein the ceramic covering the fiber bundle of silicon carbide fibers is an alumina ceramic containing Al_2O_3. (6) The ceramic heating element according to claim (1) or (2), wherein the ceramic covering the fiber bundle of silicon carbide fibers is a boron-based ceramic containing B_2O_3.