JPH0660966A - Sheathed heater and manufacture thereof - Google Patents

Abstract

PURPOSE:To unseal metallic pipe end parts, and prevent a heat resistant insulating characteristic from deteriorating even if it is used at a comparatively high temperature using electrical insulating powder formed by capsuling magnesia powder by means of metallic alkoxide. CONSTITUTION:A sheathed heater is composed of a metallic pipe 10, a heating wire 12 composed of a coil shape nichrome wire having terminal bars 11 on both ends and electrical insulating powder 15 formed by capsuling magnesia powder 13 by means of metallic alkoxide 14. In such constitution, since the magnesia powder is capsuled by means of the metallic alkoxide, infiltration of moisture can be prevented even in a high moisture resistant atmosphere, and a moisture resistant characteristics is improved significantly, and since a both end part sealing process of the metallic pipe 10 can be dispensed with, significant cost reduction can be realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheathed heater used as a heating source for general cooking equipment and industrial electric furnaces, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, a sheathed heater has been widely used as a heat source for general cooking equipment such as an electric stove, a microwave oven, and a bread maker. This is because the sheathed heater is simple, highly safe, and excellent in heat resistance.

Generally, as shown in FIG. 3, a sheathed heater is prepared by filling a metal pipe 3 with a coiled heating wire 2 having terminal rods 1 at both ends thereof and electrically insulating powder 4 made of magnesia powder. Is sealed with a low melting point glass 5 and a heat resistant resin 6. In such a sheathed heater used at a relatively high temperature, the end portion of the metal pipe is sealed with a sealing material such as low melting point glass. This is because the magnesia powder, which is an electrical insulating material, has a high hygroscopic property, and both ends of the metal pipe must be completely sealed in order to secure the moisture resistance.

On the other hand, a low temperature sheathed heater does not need to be sealed because it adds silicon powder to magnesia powder to wet the interface of the magnesia powder to ensure water repellency, but the heat resistance is up to 400 ° C.

[0005]

As described in the prior art, the sheathed heater used at a relatively high temperature has both ends of a metal pipe sealed with a low melting point glass or the like. This is a highly hygroscopic property of magnesia powder, which is an electrical insulating material, and is an essential element for ensuring moisture resistance. Generally, the heat resistant temperature of low melting glass is about 250 ° C. Therefore, in the high temperature sheathed heater, the temperature of the metal pipe needs to be suppressed to 250 ° C. or less, so that the range of the heat generating portion with respect to the entire length is very narrow, which is uneconomical. In addition, in order to seal with low melting glass, a process of heating the glass pellets to a temperature above the softening point and melting and sealing is necessary, and in addition, an inspection process to check whether they are completely sealed is added, so the cost Get high Furthermore, low melting glass is vulnerable to mechanical shock,
There were issues such as the need for careful handling.

On the other hand, the low temperature sheathed heater in which silicon powder is added to magnesia powder has a heat resistance up to 400 ° C., and there is a problem that it cannot be used in a high temperature region of 400 ° C. or higher.

SUMMARY OF THE INVENTION The first object of the present invention is to solve the above problems and to provide a relatively high temperature sheathed heater which can be used at a temperature of 400 ° C. or higher with the end of a metal pipe being unsealed. I am trying.

A second object is to provide a method for producing the same.

[0009]

[Means for Solving the Problems] A first means for achieving the above-mentioned first object comprises a metal pipe, a coil-shaped heating wire, and an electrically insulating powder obtained by encapsulating magnesia powder with a metal alkoxide. Provide a sheath heater.

A second means for achieving the second object is to insert a coil-shaped heating wire in the center of the metal pipe, and the main component of the metal alkoxide for magnesia is 0.
A method for manufacturing a sheathed heater is provided, in which an electrically insulating powder mixed with a solution of 1 to 1.0 wt% is dried, filled, rolled, and fired in air at 400 to 600 ° C.

[0011]

The present invention encapsulates magnesia powder with a metal alkoxide, and improves the moisture resistance by adding the characteristics of metal alkoxide while taking advantage of the electrical insulation characteristics of magnesia. It is intended to be.

The metal alkoxide is produced by a so-called sol-gel method in which condensation (formation of siloxane bond) is promoted from a sol having terminal groups partially or wholly OH groups to form a gel (solid). And
A typical example is ethyl silicate [Si (OC ₂ H ₅ )].
However, since it is not possible to clear the electrical insulation characteristics, moisture resistance characteristics, and heat resistance characteristics only with ethyl silicate, the ethyl silicate is modified to produce vitrified SiO ₂ after heat treatment.
By significantly reducing the OH groups, a thin film with excellent water repellency can be formed. Therefore, by encapsulating the magnesia powder with a metal alkoxide, it is possible to prevent moisture from entering even in a high humidity resistant atmosphere, and to significantly improve the humidity resistant property.

Further, a coil-shaped heating wire is inserted in the center of the metal pipe, and a solution containing 0.1 to 1.0 wt% of the main component metal alkoxide is added to magnesia, mixed, and filled with a dried electric insulating powder. And after rolling, 400-600 ℃
By firing in the atmosphere, the step of sealing the end portion of the metal pipe can be eliminated, so that the cost can be significantly reduced.

[0014]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to FIG. SUS-321 as the metal pipe 10
A heating wire 12 made of a coil-shaped nichrome wire having terminal bars 11 at both ends is prepared and inserted into the center of the metal pipe 10, and magnesia powder 13 is added to the metal alkoxide 14 [ethyl ether] as shown in FIG. The electrical insulating powder 15 encapsulated with trade name L5007 (Nissan Chemical Co., Ltd.) whose main component is lucicate was filled and rolled and aged. After that, when heat treatment is performed for 30 minutes in an atmosphere of 500 ° C., the metal alkoxide on the surface of the magnesia powder changes into a stable glass phase.
The sheathed heater thus obtained has a moisture resistance test (40
As a result of being left for 7 days at ℃ -95% RH, a value of 1000 MΩ or more was obtained as the moisture resistance insulation resistance. As a comparative example, when the electrical insulating powder 15 was only magnesia, the moisture resistance insulation resistance was 0.01 MΩ or less.

(Embodiment 2) Next, regarding a method for manufacturing a sheathed heater, a heating wire 12 made of a coil-shaped nichrome wire having a terminal rod 11 is inserted in the center of a metal pipe 10 and is inserted into a magnesia powder 13. A method of mixing the metal alkoxide solution of 0.1 to 1.0 wt% (solid content), filling the dried electrically insulating powder 15, rolling and reducing, and then firing at a temperature of 400 to 600 ° C. will be described.

The electrically insulating powder 15 used in this example is a powder prepared by adjusting the particle size of electro-melted magnesia to a predetermined particle size and uniformly mixing 0.5 wt% (solid content) of a SiO ₂ metal alkoxide.

Here, the encapsulation method will be described. There are a solution type and a powder type of the metal alkoxide 14, and the synthesis method differs depending on the metal. The powder type can be uniformly dispersed in magnesia by a dry mixer, while the solution type is a mixture of magnesia and metal alkoxide that is sprayed onto the magnesia, then uniformly mixed and dried. The obtained product is used as an electrically insulating powder. After processing this electric insulating powder into a sheathed heater, heat treatment (in this embodiment, 500 ° C. × 3
(0 minutes), the metal alkoxide undergoes a semi-melting process, and the magnesia particles are encapsulated with the metal alkoxide.

The metal alkoxide used has an average coefficient of thermal expansion of 135 × 10 ⁻⁷ / ° C., which is close to the coefficient of thermal expansion of magnesia and stable to heat cycles. Moreover, the volume resistivity at the time of heating is 1 × 10 ¹¹ Ω-cm at 600 ° C.
It is also high and has excellent thermal insulation properties.

Further, the firing step is an important step of coating the surface of the magnesia powder with a metal alkoxide, and in this example, the firing was carried out at 500 ° C. for 30 minutes. The reason why the firing temperature is set to 400 to 600 ° C. is that at 400 ° C. or less, vitrification of the metal alkoxide does not proceed and it is not stable. Further, when the temperature is 600 ° C. or higher, the water repellency tends to decrease, which is not preferable.

On the other hand, the amount of the metal alkoxide added is set to 0.1 to 2.0 wt% as the solid content because the effect of improving the moisture resistance effect is small at 0.1% or less, and the addition amount of the metal alkoxide film coated with magnesia particles is less than 1.0%. The reason for this is that the film thickness becomes too thick and cracks occur in the film during the heat cycle, resulting in a decrease in the moisture resistance insulation resistance.

As a result of the moisture resistance test, the sheathed heater manufactured according to the present embodiment as described above has a moisture resistance insulation resistance of 1000 MΩ or more.

The metal alkoxide in the examples of the present invention is generally represented by RO-M (R is an alkyl group, M is a metal element), and representative ones are ethyl silicate, orthomethyl silicate, ortho n-propyl. There are silicates, ortho-n-butyl silicate, etc., and the metal elements include Ti, Ta, Fe, Al, Cu, Ba,
W, Ga, Sn, Zr, etc. may be mentioned.

[0023]

INDUSTRIAL APPLICABILITY As described above, the present invention improves the hygroscopic property of magnesia by using the electrically insulating powder in which the magnesia powder is encapsulated with the metal alkoxide, and makes it possible to eliminate the sealing.

By using a mixture of magnesia powder and metal alkoxide solution as an electrically insulating powder,
The hygroscopicity of magnesia is improved, a relatively high temperature sheathed heater can be obtained without the glass sealing step, the effective heat generation length can be lengthened, and the economic effect is great.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a sheathed heater according to the present invention.

FIG. 2 is a sectional view of an electrically insulating material in which magnesia powder is encapsulated with a metal alkoxide.

FIG. 3 is a sectional view of a conventional sheathed heater.

[Explanation of symbols]

 10 Metal Pipe 12 Heating Wire 13 Magnesia Powder 14 Metal Alkoxide 15 Electrical Insulating Powder

Claims (2)

[Claims] 1. A sheathed heater comprising a metal pipe, a coil-shaped heating wire, and an electrically insulating powder obtained by encapsulating magnesia powder with a metal alkoxide. 2. A coil-shaped heating wire is inserted in the center of a metal pipe, and an electrically insulating powder in which a solution containing 0.1 to 1.0 wt% of a metal alkoxide as a main component with respect to magnesia is mixed is dried and filled. A method for manufacturing a sheathed heater obtained by firing in air at 400 to 600 ° C. after rolling.