JP2000048938A - Manufacture of carbon heating element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heating element for a household cooking appliances and a heating element for a household heater which can be operated for a long time. SOLUTION: This manufacturing method of a carbon heating element suitable for a household cooking appliances and a household heater which can be operated for a long time is accomplished by performing a precursor treatment to raise temperature and heat, in an inert gas atmosphere, a molding body provided by uniformly kneading graphite carbon, a resin material of which carbon residue yield after baking is almost 100%, and ceramics having a resistivity equal to or more than 1013 μΩ.cm, and thereafter carrying out baking in a vacuum condition at a temperature higher than that of the precursor treatment and higher than that when operating the heating element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a carbon heating element.

[0002]

2. Description of the Related Art Carbon materials are excellent in heat resistance, thermal shock resistance and corrosion resistance, and have the highest heat emissivity among all materials.
Further, since it has a very high melting point of 3800 ° C., it is very suitable for a heating element. Therefore, it is industrialized especially as a heating element for a high-temperature electric furnace of a semiconductor manufacturing apparatus.

[0003]

The heating element of the above-mentioned conventional construction has a problem that it is difficult to use it as a heating element for home cooking appliances or a heating element for a heater.

That is, the specific resistance is 1500 to 2000 μm.
Since it is as small as Ω · cm, in order to obtain a required heat value, the heat generating element is made thick and long, and the heat value is insufficient unless the current value is increased. That is, the power consumption of the heating element for a high-temperature electric furnace for semiconductor production using the heating element having this resistance value is about 50 kW, which greatly exceeds the allowable amount of a home power supply.

[0005]

SUMMARY OF THE INVENTION The present invention provides a graphite carbon,
A molded body obtained by uniformly kneading a resin material having a carbon residue yield of about 100% after firing and a ceramic having a specific resistance of 10 ¹³ μΩ · cm or more is heated and heated in an inert gas atmosphere. And then in a vacuum,
By performing calcination at a temperature at least higher than the temperature in the precursor treatment and higher than the temperature during operation of the heating element, carbon heat generation suitable for household cooking appliances and heaters capable of long-term operation. It is the method of body production.

[0006]

DETAILED DESCRIPTION OF THE INVENTION According to the first aspect of the present invention, graphite carbon, a resin material having a carbon residue yield of about 100% after firing, and a ceramic having a specific resistance of 10 ¹³ μΩ · cm or more are used. Precursor processing is performed in which the molded body obtained by uniformly kneading is heated and heated in an inert gas atmosphere, and then, in a vacuum, at least higher than the temperature in the precursor processing, and during the operation of the heating element. By performing the final firing at a temperature higher than the temperature, a method of manufacturing a carbon heating element suitable for household cooking appliances and heaters that can operate for a long period of time is provided.

The second aspect of the present invention is a method of manufacturing a carbon heating element which uses boron nitride as a ceramic, has good formability, can easily adjust a resistance value, and is easy to manufacture.

According to the third aspect of the present invention, the treatment of the precursor is carried out in an inert gas atmosphere at a temperature of at least 800 ° C., so that the resin material as a starting material is reduced to about 10%.
This is a method for producing a precursor of a carbon heating element which can be converted to 0% carbon and is suitable for household cooking appliances and heaters.

According to a fourth aspect of the present invention, the sintering is performed for 120 hours.
Operate under vacuum at 0 ° C or higher. The valve has a stable carbon structure for long-term use, so there is little change in specific resistance, electrical characteristics are stable, and there is little evaporation of impurities. And a method of manufacturing a carbon heating element capable of maintaining the mechanical strength of the carbon heating element.

[0010]

Embodiments of the present invention will be described below. In general, it is known that carbon has an intermediate structure in a wide range from a glassy amorphous structure to a hexagonal crystal structure. Naturally, it is also known that the specific resistance increases in the glassy state, decreases in the higher the crystallinity, and changes from the glassy state to the crystallinity as the carbonization firing temperature increases.

In this embodiment, graphite powder is used as graphite carbon as a main component, and as a binder for binding graphite carbon, the carbon residue yield after firing is almost 100%. A thermosetting resin with a small shrinkage is used. Once carbon has changed into a crystalline state like graphite, it is structurally stable up to about 3800 ° C., and is therefore stable at the temperature during firing and the temperature during use. Further, as the binder, as a result of various studies by the inventors,
From thermosetting resin compounds such as furfuryl alcohol resin, furfuryl alcohol / furfural copolymer resin, furfural resin such as furfural / phenol copolymer resin, phenolic resin such as resol and novolak, xylene resin, and toluene resin. The selected monomer or prepolymer has been found to be suitable. In other words, the resin group has very good compatibility with graphite, and the graphite is sufficiently dispersed uniformly in the molded body during molding. With the above configuration, the graphite powder and the binder are firmly bonded to each other, and exhibit high mechanical strength after carbonization and firing.

Hereinafter, a method of manufacturing the heating element of the present embodiment will be described. The graphite powder and the binder are kneaded and uniformly dispersed using a mixer or the like, and a shearing force is applied using a three-roll mill or the like to sufficiently dissolve and bind them, thereby obtaining graphite. An organic substance is physicochemically bonded to the surface of the primary particles of the powder. Using this as a starting material, heat treatment is performed after molding to form a heating element.

However, in this state, the specific resistance of the obtained resistor is small, and the releasability is extremely poor at the time of extrusion molding with a plunger or the like. Therefore, a carbon rod having a predetermined resistance value and a predetermined shape cannot be obtained.

Therefore, as a result of various studies, the present inventors have found that the addition of ceramics to the starting material makes it possible to easily adjust a specific resistance value and to form a resistor having good moldability. The ceramics are uniformly mixed with graphite and a binder, have good moldability, have low frictional resistance during extrusion molding, have excellent mold releasability, and have a temperature of 10 ¹⁴ to 2200 ° C. in an inert atmosphere. Boron nitride, which has a high specific resistance of 10 ¹⁵ μΩ · cm, is the most suitable.

Since the thermal conductivity of boron nitride is approximately 20 to 30 W / (m · K), which is almost the same as that of stainless steel, heat is well transmitted to the entire molded body. Boron nitride is a substance whose structure is very similar to graphite. Graphite belongs to a hexagonal system, and the lattice constant of the crystal is a = 2.4704.
{, C = 6.7244}. On the other hand, boron nitride belongs to the same hexagonal system, and a = 2.5044 ° and c = 6.656.
2Å. The lattice constant mismatch Δa with respect to graphite is 0.034 °, which is only 1.4%.
Similarly, the mismatch Δc is −0.0682 °, and −
It is only 1.0%. That is, it can be kneaded and dispersed in the same manner as graphite during molding. Molybdenum disulfide, which is a typical substance generally used as a mold release agent, is easily oxidized and decomposed at 600 ° C. or higher, and cannot be used in this embodiment. That is, the coefficient of friction rapidly increases. On the other hand, boron nitride has a constant frictional resistance around 0.2 up to around 1000 ° C., and is rich in shape-forming properties and releasability.

The specific resistance value which is extremely important as a resistor is 1500 to 2000 μΩ · cm for graphite alone.
It has a property closer to a conductor than a heating element. On the other hand, boron nitride has a large specific resistance of 10 ¹⁴ to 10 ¹⁵ μΩ · cm up to around 2200 ° C. in an inert atmosphere, and belongs to the class of insulator. As a result of various studies by the inventors, the thickness of the heating element was set to about 1 mm in diameter, the total length of the heating element was set to 290 mm, and the specific resistance value was 10,000 to 1
It has been found that, in order to produce a material of 8000 μΩ · cm, if graphite is 100 parts by weight, boron nitride should be 50 to 100 parts by weight. As the heating element, a material having a high rise rate and a high heat conductivity that uniformly generates heat is desired. However, as described above, boron nitride is exceptionally large as a ceramic, having a large 20 to 30 W / (m · K). Since it has thermal conductivity and is almost equivalent to stainless steel as a metal material, it can be said that it is an ideal compound material with respect to formability and electric resistance adjustment and thermal conductivity.

In summary, by adding boron nitride to the raw material of the present heating element, a desired high specific resistance value can be freely designed only by changing the compounding ratio, and the same behavior as graphite at the time of material kneading can be obtained. The dispersibility and the releasability are extremely improved.

As described above, in the carbon heating element of this embodiment, as a starting material, the binder is 99 to 20 parts by weight, preferably 5 to 50 parts by weight, preferably 1 to 80 parts by weight of the graphite powder.
The binder is 45 to 50 parts by weight based on parts by weight. Further, boron nitride is used in an amount of 2.5 to 5 parts by weight with respect to 5 parts by weight of graphite powder, and 25 to 50 parts by weight with respect to 50 parts by weight of graphite powder. At this time, the specific resistance can be increased by increasing the amount of boron nitride.

Examples of the binder include furfuryl alcohol resins, furfuryl alcohol / furfural copolymer resins, furan resins such as furfural / phenol copolymer resins, phenol resins such as resole and novolak, xylene resins, and toluene. A compound containing at least one or more monomers or prepolymers selected from a thermosetting resin compound group such as a resin is used. These are sufficiently kneaded by a mixer, and the obtained kneaded material is subjected to physicochemical bonding between graphite, boron nitride and an organic substance while applying a shearing force using a roll mill. Next, after being formed into a sheet shape, it is extruded with a plunger. In the present embodiment, it is extruded into a round bar or pipe having a diameter of 0.8 to 2 mm. On this occasion,
The added graphite and boron nitride act as a release agent and can accurately form a carbon rod having a predetermined shape.

The molded body obtained in this manner is cut into a suitable length, and while being slowly dried in an air oven at a temperature rising rate of 5 to 10 ° C./hour, it is kept at 180 ° C. for 1 hour to obtain a liquid component. Allow to dry completely. Then, the temperature is raised to 300 ° C. at a rate of 5 ° C./hour in an inert gas atmosphere such as nitrogen or argon by a horizontal tubular furnace. Thereafter, the temperature is raised to 800 ° C. or more, preferably to 1000 ° C.
The temperature was raised at a rate of 0 ° C./hour, and after maintaining this state for 3 hours, it was allowed to cool in a furnace to complete the precursor treatment.

The completed precursor is then fired. The purpose of firing is roughly divided into two. The first is to remove impurities. Second, by performing a heat treatment at a temperature higher than the actual use temperature of the heating element, a structural change does not occur even during long-term actual use, and a constant resistance value, that is, a constant power consumption can be maintained. Is to do so.

According to the analysis by the inventors, most of graphite, especially natural graphite, contains mineral components as impurities. These impurities include Si, Fe, Ca,
Al, K and the like. When these are contained, the oxidation start temperature of carbon is lowered by about 100 ° C. as compared with purified graphite without impurities, and the carbon becomes active, and it is also confirmed that it is extremely unsuitable as a carbon heating element. I have.

For the reasons described above, the impurities need to be thoroughly removed. For this reason, in the present embodiment, the final baking is performed at a temperature higher than the processing temperature of the precursor, at a temperature higher than the operating temperature of the heating element, and under reduced pressure vacuum. Specifically, 120
It is performed at a temperature of 0 ° C. or more under reduced pressure vacuum.
Therefore, impurities can be removed.

By setting the firing temperature, the characteristics when used as a heating element are very stable. That is, it is possible to prevent the vitreous carbon generated by the precursor treatment from gradually changing its structure into crystals while the heating element is operating. FIG. 1 is a characteristic diagram showing the relationship between the carbonization firing temperature and the volume resistivity. As can be seen from this figure, the final firing temperature is higher than the processing temperature of the precursor,
When the temperature is higher than the actual operating temperature, the change in the volume resistivity is very small.

In this embodiment, the temperature is set between 1500 and 1600.
When the final firing was performed under reduced pressure and vacuum (1/1000 Torr), a heating element having an appropriate specific resistance of 10,000 to 18000 μΩ · cm was obtained as the heating element.

Table 1 shows that the heat treatment was performed at 1600 ° C. in an inert gas atmosphere and that the heat treatment was performed at 1600 ° C. under vacuum (1 / 1000T
5 shows the results of composition analysis of the carbon heating element when heat treatment was performed at orr). In each case, the temperature was maintained for 3 hours. As can be seen from the analysis data, it can be proved that the excess impurities are almost completely removed under reduced pressure vacuum.

[0027]

[Table 1]

Next, the results of checking the characteristics as a heating element of a completed carbon heating element manufactured according to this embodiment will be described. In the sample used in the experiment, the heating element was placed in a quartz tube having an outer diameter of about 10 mm and a thickness of about 1 mm, evacuated to vacuum, and then instantaneously cooled to 14 mm.
The mixture was heated at 00 ° C. to remove impurities and moisture adhering to the carbon heating element during storage.
rr encapsulated. The characteristic of this heating element is that the specific resistance is 18000 μΩ · cm, the diameter of the heating element is 1.2 mm,
The exothermic length was 280 mm, and the exothermic temperature was 1200 ° C. at 100 V-380 W. About this sample 10
When a cycle test was conducted at 0 V for 2 minutes and a 2-minute pause, the current was applied for 8760 hours (equivalent to 1 year) without disconnection, and the power consumption changed within 10% of the initial level. However, a heating element having a high temperature could be obtained.

Further, since the carbon heating element has a large emissivity of 0.8, the inside of the refrigerator is large like a microwave oven, and is suitable for a heating element of a cooker heated by radiant heat. For example, when the present heating element is incorporated in a microwave oven, and when toasting is baked with the same power consumption, it can be cooked in three minutes, which used to take six minutes in the past. For this reason, when the carbon heating element of this embodiment is used, significant energy savings can be achieved. Also,
When used as a heating element of an electric heater, 10
In order to raise the temperature by 35 ° C. compared to room temperature, the power consumption of the heating element is 167 W,
A conventional heater, for example, a quartz tube heater using Ni-Cr as a heating element required 186 W. That is, when the heating element of this embodiment is used, power consumption can be reduced by about 10%.

[0030]

According to the first aspect of the present invention, graphite carbon, a resin material having a carbon residue yield of about 100% after firing, and a ceramic having a specific resistance of 10 ¹³ μΩ · cm or more can be uniformly prepared. Precursor processing of heating and heating the molded body obtained by kneading in an inert gas atmosphere is performed, and then, in vacuum, at least higher than the temperature in the precursor processing,
In addition, a method of manufacturing a carbon heating element suitable for household cooking appliances and heaters that can operate for a long period of time can be realized by performing firing at a temperature higher than the temperature during operation of the heating element. .

According to the second aspect of the present invention, a method of manufacturing a carbon heating element which uses boron nitride as a ceramic, has good formability, can easily adjust a resistance value, and is easy to manufacture, can be realized. .

According to the third aspect of the present invention, the treatment of the precursor is carried out in an inert gas atmosphere at a temperature of at least 800 ° C., so that the resin material as a starting material can be carbonized by 95% or more. Thus, a method for producing a precursor of a carbon heating element suitable for a cooking appliance or a heater can be realized.

According to the invention described in claim 4, the firing is performed for 120 hours.
At a temperature of 0 ° C. or higher, it is performed under vacuum, and even for long-term use, the carbon structure is stable, the change in specific resistance is small, the electric characteristics are stable, and the evaporation of impurities is small, and A method of manufacturing a carbon heating element capable of maintaining mechanical strength can be realized.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between temperature and resistance of a carbon heating element according to an embodiment of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Shuzo Tokuma 1006 Kadoma Kadoma, Kadoma, Osaka Prefecture Inside Matsushita Electric Industrial Co., Ltd. 72) Inventor Mori Yasuhisa 1006 Kadoma, Kazuma, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] (1) A graphite carbon, a resin material having a carbon residue yield of about 100% after firing, and a specific resistance of 10 ¹³ μΩ ·
cm or more of ceramics is uniformly kneaded, and a molded body obtained is heated and heated in an inert gas atmosphere to perform a precursor treatment, and then in a vacuum, at least higher than the temperature in the precursor treatment, and A method for producing a carbon heating element, wherein firing is performed at a temperature higher than the temperature at which the heating element operates. 2. The method according to claim 1, wherein the ceramic is boron nitride. 3. The method for producing a carbon heating element according to claim 1, wherein the treatment of the precursor is performed at a temperature of at least 800 ° C. in an inert gas atmosphere. 4. The method for producing a carbon heating element according to claim 1, wherein the calcination is performed at a temperature of 1200 ° C. or higher under a vacuum.