JPS6325465B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a planar heating element that utilizes electrical energy, and provides a heat source for use in heaters, cookers, drying equipment, and the like. Conventionally, as heating elements that utilize this type of Joule heat, sheathed heaters, quartz tube heaters, infrared lamps, planar heating elements, and the like are known. A sheathed heater is created by inserting an electric heating coil into a metal tube, filling it with an insulator such as MgO, reducing the diameter by swaging, increasing the density of the insulator, and sealing the terminal. There is. This heater has a high normal operating temperature of around 900℃.
Although it has excellent corrosion resistance, it has drawbacks such as being rod-shaped, making it difficult to uniformly heat the object to be heated, and having a large heat capacity, resulting in poor start-up characteristics. A quartz tube heater has a simple structure consisting of an electric heating coil inserted into a quartz tube. Due to its low cost, it is often used in electric ovens, etc., but it is weak in mechanical shock and has no sealing end, so there are problems with moisture resistance, which limits the conditions of use and prevents uniform heating of the heated object. There's a problem. Infrared lamps emit a large amount of wavelengths in the near-infrared region, so they are poorly absorbed by objects to be heated, resulting in poor energy utilization efficiency. Furthermore, the cost is extremely high due to the use of tungsten wire and the inclusion of inert gas, which naturally limits its application to consumer devices. In recent years, planar heating elements have come into the spotlight as heating elements that meet the needs of thinning devices and uniform heating. However, most conventional planar heating elements have a structure in which the heater is wound around an insulating substrate such as mica, which has poor heat transfer to the heated object, and because the electrical heating material is not sealed, there are problems with moisture resistance. It was hot. In addition, in recent years, there are planar heating elements in which a conductive pattern is formed using conductive paste such as tungsten on raw sheets such as alumina, and the sheets are glued together and sintered, but the sintering temperature is high and the contact material melts. There were problems with removing the electrodes. In addition, there are drawbacks such as a large heat capacity of the heating element and a long rise time. Moreover, many of these had poor workability and productivity during manufacturing. In addition, there are heating elements constructed by forming conductive patterns between organic films such as silicone resin and polymide, and laminating them, but these heating elements have a heat resistance temperature of up to 250℃, and There are also problems with life characteristics. As described above, conventional planar heating elements have various problems, and there are a large number of technical problems to be solved. The following requirements are required for the sheet heating element. 1. It has a high far-infrared radiation function and has excellent energy usage efficiency. 2. Excellent dimensional accuracy of processing. 3 Heat capacity is small. 4. Terminals can be easily removed. 5. Objects to be heated can be heated uniformly. 6 Excellent heat and moisture resistance. 7. Excellent electrical properties (insulation resistance, dielectric strength), etc. 8. There is little variation in the resistance value of the electric heating material. Traditionally, when cooking food, etc., gas, oil,
A conduction heating method that uses the combustion flame of solid charcoal as its main component, a heating method that uses direct heat conduction to heat the thermal energy such as by baking on a hot plate, etc.
Alternatively, convection heating methods have been mainly used, such as in an oven, which heats the air inside the refrigerator and transfers thermal energy to the food in the high-temperature atmosphere.
However, in recent years, the effectiveness of infrared heating has been recognized, and it has come to be widely used for purposes such as heating and drying. Infrared rays are electromagnetic waves between visible light and microwaves, with a wavelength range of 0.72 to 1000 μm.
It is well absorbed by the object to be heated and has a great effect of activating the molecular movement of the object and generating heat. Therefore, since the heating method using infrared irradiation vibrates the constituent molecules of the heated object and generates self-heating, the thermal efficiency and energy usage efficiency during cooking are improved compared to the conventional heating method using conduction and convection. Energy saving effects can be expected. In particular, in order to effectively cook food, a heating source that has good infrared heating and emits infrared radiation corresponding to the absorption wavelength of the food is required, as can be understood from the absorption characteristics shown in Figure 1. be. Generally, the energy E radiated from an object is
It is expressed by the following Stephan Boltzmann's law: E=εσT ⁴ (ε: emissivity, σ: constant, T: temperature). In other words, radiant energy is determined by the temperature and emissivity of an object, and if the temperature is constant, the radiation source is a material with high emissivity, especially in the infrared wavelength region of 2 μm or more. Constructing from materials improves energy usage efficiency. The present invention takes into consideration the effects of infrared rays as described above, and aims to provide a planar heating element that has greatly improved properties such as far-infrared radiation efficiency, heat capacity, electrical characteristics, dimensional accuracy, and uniform heating. Main purpose. The planar heating element of the present invention mainly includes an electric heating element, a heat-resistant substrate whose surface on which the heating element is fixed is electrically insulating, and a glass frit that covers the heating element and is fixed to the substrate. and a heating element coating layer. FIG. 2 shows an example of the basic configuration of the planar heating element of the present invention. In the figure, 1 is a heat-resistant substrate, 2 is an electric heating element, and 3 is a heating element coating layer mainly composed of glass frit. The substrate 1 (a) is made of an electrically insulating material such as ceramic or glass. In b and c, a hollow layer 1b is formed on both sides or one side of the hollow substrate 1a. Fig. 3 shows another example of the structure, in which a first heating element coating layer 3a is formed on the substrate 1, and electric heat is generated by a second heating element coating layer 3b on the surface of the first heating element coating layer 3a. The body 2 is fixed. b is an example in which the substrate 1, the heating element coating layer 3a, the electric heating element 2, the heating element coating layer 3b, and the substrate 1 are laminated. In this case, it is necessary to use a substrate with excellent thermal conductivity for at least one surface serving as the heating surface. Further, c shows an example in which an electric heating element is brought into close contact with the substrate. In addition, in a, when a conductive substrate is used as the substrate, the first heating element coating layer 3a becomes an electrically insulating layer, and the layer 3a together functions as a substrate. Each component of the present invention will be explained in detail below. A Substrate The substrate applicable to the present invention may be a plate-like body made of an inorganic material and having at least an electrically insulating surface on which the heating element is fixed. For example, ceramics such as alumina, cordierite, and mullite,
Glass, mica, hollow substrates, etc. can be used, but hollow substrates are most suitable from the viewpoints of mechanical strength, electrical properties, heat capacity, radiation efficiency, etc. Therefore, the hollow substrate will be explained in detail below. In the configurations shown in FIGS. 2a to 2c, the order of superiority and inferiority is c>b>a in consideration of heater characteristics such as heat capacity and heater rise characteristics. Furthermore, although the structure a has excellent electrical insulation, it is inferior in terms of mechanical strength, heat shockability, and the like. However, if electrical insulation is particularly required, configuration a is suitable. Here, the configurations of b and c using hollow substrates which are excellent in various properties such as heater characteristics, mechanical strength, and heat shockability, and in terms of cost will be explained. (1) Metal base material for hollow board The metal base material for hollow board is aluminum, aluminum die-cast, cast iron, aluminized steel,
Low carbon steel, steel plate for hollow holes, nickel chrome steel,
Nickel chrome aluminum steel, stainless steel, etc. are used, and their selection is determined based on usage conditions, usage temperature, economic efficiency, shape of the base material, and workability. (2) Configuration of the hollow layer The configuration of the hollow substrate used as the insulating substrate is determined by taking into account the corrosion resistance and thermal oxidation properties of the metal substrate. For example, when using a steel plate or cast steel, both sides of the metal base material must be hollowed, as shown in FIG. 2b. When single-sided hollowing is performed, oxide scale is significantly generated on the non-hollowed side, resulting in poor corrosion resistance. However, stainless steel, aluminized steel, etc. may be hollowed on both sides as shown in b, but since these have excellent thermal oxidation resistance and corrosion resistance, the structure c is particularly applicable. (3) Enameling of materials Figure 4 shows a diagram of the process of enameling common materials. ○B. Material pre-treatment process It is necessary to remove oil and metal oxides applied to the metal during transportation or storage to prevent rust, and during the molding process before enameling.
The pretreatment process for this material has a large effect on the adhesion strength of the hollow layer. As shown in the figure, it is necessary to perform pretreatment appropriate for the material. ○ Hollow frit Once the type of material is determined, select a frit composition that has physical properties (coefficient of thermal expansion, softening temperature, and hollow firing temperature) suitable for the material, taking into account the material's coefficient of thermal expansion, melting point, transformation temperature, etc. There is a need. Table 1 shows the coefficient of thermal expansion of typical materials and their frits.

[Table] In order to prevent separation of the hollow surface due to the difference in thermal expansion coefficient between the material and the hollow layer, it is necessary to select a frit with a thermal expansion coefficient suitable for the material. Preferred hollow frits used in the present invention will be described later. ○C Holo-slip adjustment process Once the type of frit is determined, mill additives, matte forming materials, insulating substances, surfactants, water, etc. are mixed in appropriate amounts in addition to the frit, and processed using a ball mill, etc. to make a slurry (slip). D. Coating, drying, and baking process The method for applying the prepared slip to the material is usually a spray method or a dip method, but a brush coating method or a bar code method may also be used. In the drying process, the coated surface is dried using air drying or a drying oven at 50 to 150°C. Next, baking is performed. Firing is performed in a batch furnace or continuous furnace set at a predetermined temperature. (4) Fritz The frit used in the present invention has not only the working temperature and expansion rate, but also the desirable requirements of the sheet heating element, such as the small heat capacity of the sheet heating element, excellent dimensional accuracy, and thermal oxidation. Typical compositions of preferred low softening point frits are shown in Table 2, and typical composition examples are shown in Table 3, based on the viewpoints of resistance to damage and easy removal of heating element terminals. As the hollow substrate constituting the planar heating element of the present invention, one using a low softening point frit is suitable. The reason for this will be explained using an example in which a steel plate is used as the base material. Conventional hollow machining is performed at high temperatures of 800-900℃, which is the _A1 transformation point of steel (723℃).
That's all. When steel reaches the _A1 transformation point or higher, its structure transforms from alpha iron to gamma iron, accompanied by a volume reduction. If hollowing is performed at a temperature above this temperature, the firing distortion of the steel sheet will be large, causing distortion and deformation.
For this reason, in conventional hollow machining, the selection of the thickness of the material has become an important factor, and only base materials with a thickness of 0.6 mm or more can be used. On the other hand, as can be seen from the preferable frit composition examples of the present invention shown in Table 3, the firing temperature is below the _A1 transformation point. In other words, by using a low softening point frit,
Less firing distortion due to steel transformation, 0.6~0.3mm
It is possible to use a base steel plate with a thin plate thickness, and the heat capacity of the planar heating element can be reduced. Therefore, the start-up characteristics of the heating element are significantly improved. Furthermore, since firing can be performed below the _A1 transformation point described above, processing yield is improved, dimensional accuracy is excellent, and post-processing is easy.

【table】

[Table] (5) Electrical properties of the hollow layer of the hollow substrate As a factor that determines the electrical properties of the planar heating element of the present invention, as shown in Fig. One of the important points is the electrical properties (insulation resistance, dielectric strength voltage, etc.) of the hollow layer. As a result of various studies, the present inventors have found that, in addition to the thickness of the hollow layer, the surface gloss of the hollow layer is an important factor that determines the electrical properties. First, the thickness of the hollow layer will be explained. The thickness of the hollow layer is related not only to the electrical properties described above but also to the adhesion of the hollow layer. As the hollow layer, the above-mentioned opalescent frit was used, and the film thickness was varied, and the adhesion and
Table 4 shows the results of comparing the electrical properties. Adhesion was determined using a PEI adhesion strength tester at 90
% or more is indicated by ○, 80 to 90% is indicated by △, and 80% or less is indicated by ×. Further, the electrical properties were measured according to the insulation resistance test method and the dielectric strength test method stipulated in the Electrical Appliance and Material Control Law. 500V for insulation resistance test
Using an insulation resistance meter, insulation resistance of 1MΩ or more is marked with ○, and those with insulation resistance of less than 1MΩ are marked with ×. In the dielectric strength test, those with dielectric strength of 1.5 KV or more were marked as ○, and those with a dielectric strength of 1.5 KV or higher were marked as ×.

[Table] As is clear from this table, the hollow film thickness is preferably 50 to 500 μm, and more preferably 100 to 400 μm in consideration of workability and other factors. Another method for improving electrical properties is to fill the hollow layer with an electrically insulating material such as Al ₂ O ₃ , TiO ₂ , ZrO ₂ , MgO, etc. However, if too much of the electrically insulating material is added, minute pinholes are generated in the hollow layer, which tends to deteriorate the electrical characteristics. The present inventors investigated this point and found that this phenomenon is largely related to the surface glossiness of the hollow layer. Using the above-mentioned opalescent frit, samples with different surface glosses were prepared by adding Al ₂ O ₃ in various proportions into the hollow layer, and the insulation resistance and dielectric strength voltage were investigated. The results are shown in Table 5. Note that the thickness of the hollow layer is approximately 150 μm. Further, the surface gloss of the hollow layer was measured using a gloss meter (VG-107 manufactured by Nippon Denshoku).

[Table] As is clear from the results, it was found that the surface gloss of the hollow layer constituting the planar heating element must be 0.5 or more. B. Heating element coating layer The heating element coating layer is mainly composed of glass frit, and serves to bond the ribbon-shaped heating element to an insulating substrate such as a ceramic substrate, glass substrate, or hollow substrate, and at the same time provides electrical insulation and infrared radiation. It is a layer. (1) Glass frit used for heating element coating layer It is preferable to use a low softening point frit for the heating element coating layer. As mentioned above, this improves the dimensional accuracy and processing yield of the product, and at the same time, the firing temperature is 150~150℃ compared to conventional hollow machining technology.
Since the temperature can be lowered by 300℃, there is less thermal oxidation on the terminals of the heating element, which improves workability and
It is also advantageous in terms of life and cost. In particular, conventional enamels are fired at a temperature of 800 to 850°C, which causes electrical heating materials such as Ni-Cr and stainless steel to undergo thermal oxidation, resulting in large variations in resistance and shortened service life due to local oxidation. There were problems such as significant deterioration. Also,
There was oxidation of the electrode lead-out terminal and oxidation of the connecting part between the electric heating material and the terminal, and it was not possible to construct it as a planar heating element. These problems can be solved by using a low softening point frit. A glass frit with a low softening point is selected taking into account the coefficient of thermal expansion, softening temperature, etc. of the substrate. When using a hollow substrate, it is desirable to use the frit used for the hollow substrate in terms of processability and cost. (2) Composition of glaze forming heating element coating layer Typical composition examples (weight ratio) of glaze forming heating element coating layer are shown below.

[Table] a is an example of the composition of a regular glossy hollow (80 or higher by light selection). In b, Al ₂ O ₃ , which is an insulating substance, is added to the heating element coating layer for the purpose of further improving the electrical characteristics. Other insulating materials include TiO ₂ , ZrO ₂ , MgC, BeO,
MgAl ₂ O ₄ , SiO ₂ and the like are also effective. The amount of the insulating material added is preferably 50 parts by weight or less per 100 parts by weight of the frit. The reason for this is that if the amount exceeds 50 parts by weight, the adhesion will be extremely reduced and the heating element coating layer will peel off during actual use. In c, an infrared radiation material NiO is added to the heating element coating layer for the purpose of further improving the infrared radiation characteristics. In addition, MnO ₂ , Co ₃ O ₄ , carbon, Cu ₂ O,
Cr ₂ O ₃ , Fe ₂ O ₃ and the like are also effective. The amount of the infrared emitting material added is preferably 50 parts by weight or less per 100 parts by weight of the frit. Furthermore, when used together with an insulating substance, the total amount thereof is 50 parts by weight or less. The reason for this is that, as mentioned above, the heating element coating layer peels off. Note that the thermal expansion coefficient of the heating element coating layer is 1
When, the range of 0.8 to 1.5 is preferable. (3) Electrical properties of the heating element coating layer The electrical properties of the heating element coating layer are important factors that determine the electrical properties of the planar heating element of the present invention. The electrical properties of the heating element coating layer were similar to those described above in the section of the electrical properties of the hollow substrate. That is, the thickness of the heating element coating layer is 50~
The thickness is preferably 500 μm, and the surface gloss of the heating element coating layer needs to be 0.5 or more. The reason is as mentioned above. What must be noted here is that when using a hollow substrate as an insulating substrate, the total thickness of the hollow layer and heating element coating layer must be
It must be 500μm or less. This is regulated in terms of adhesion strength. C Electric Heating Material The electric heating material applicable to the present invention is a metal ribbon. When encapsulating the heating element in the heating element coating layer, if it is not thin, the heat capacity of the planar heating element will be large, which is contrary to the purpose of the present invention. (1) Material of electric heating material Any metal or alloy that can be made into a thin strip can be used as the material of electric heating material, but especially Fe, Al, Fe-
Cr, Fe-Cr-Al, Ni-Cr, and stainless steel are suitable. Among them, Fe, Fe-Cr system, Ni-Cr
The Fe-Cr-Ni system is superior. Metallic aluminum and iron-chromium-aluminum systems tend to evaporate during firing of the heating element coating layer and cause pinholes in the heating element coating layer. For iron-based materials, low carbon steel with a low carbon content is suitable. When selecting the material, it is necessary to consider heat resistance and compatibility with the heating element coating layer. (2) Making metal into thin strips In addition to the usual methods of cold rolling and hot rolling, metals can also be made into thin strips by ultra-quenching. The thickness of the ribbon can be 10 to 150 μm, but the preferred range is 20 to 100 μm. 10μm
The following thin strips are difficult to process and have poor workability. Furthermore, if the thickness is 150 μm or more, there are problems such as cracks appearing in the heating element coating layer and warpage occurring in the planar heating element when heat cycles are applied. (3) Processing method of electric heating material Etching method and press processing method are suitable as a method for forming a thin metal strip into a desired pattern. The etching method can be used when production quantities are small, and the press processing method can be used for mass production. FIG. 5 shows an example of a patterned electric heating material. The film thickness and pattern shape of the electric heat generating material can be arbitrarily set in consideration of the rated power, heat generating area, temperature distribution, etc. D Manufacturing method of planar heating element Figure 6 shows a typical manufacturing process of a planar heating element. A pre-prepared hollow strip is applied to one side of the insulating substrate (b), and an electric heating material manufactured based on the design is placed on the coated surface (c). When placing the electric heating material, it is best that the applied surface is not dry. This is because the electric heating material is in primary contact with the coating surface, making it easier to position the electric heating material. Next, to form the outer heating element coating layer,
Apply Holoslip (d), then air dry or
Dry at 50-150℃ to remove moisture (e). After drying,
Bake at 550-850℃ for about 10 minutes (f). The firing temperature varies depending on the composition of the slip. Through the above steps, the planar heating element of the present invention is completed. Examples of the present invention will be described below. Example 1 A glaze having the composition shown in Table 7 was milled for 2 hours to adjust the slip, and this was applied to a surface area of 1000 cm ² on one side.
It was applied to the surface of an aluminum die-cast hot plate base material to a thickness of about 150 μm, dried for 10 minutes in a dryer at about 120°C, and then baked at 550°C for about 10 minutes.

[Table] A thin strip heating element (1.2KW) made of Ni-Cr alloy with a film thickness of 50 μm as shown in Fig. 5 was placed on the surface of the substrate on which the hollow layer was formed, and the same slip as above was applied with a spray gun. It was applied to a thickness of approximately 250 μm, dried and fired to form a heating element coating layer. Furthermore, a fluororesin dispersion was applied to this surface to a thickness of approximately 30 μm, dried at 120°C, and then heated to 380°C.
The plate was fired for 20 minutes to obtain a hot plate using the heating element of the present invention. As a comparative example, a commercially available hot plate in which a sheathed heater was embedded in aluminum die casting was used. For these hot plates, the surface temperature is
Table 8 shows the results of comparing the time required for the temperature to rise to 250°C and the difference between the maximum and minimum temperature on the hot plate surface 10 minutes after energization.

[Table] It can be seen that the hot plate according to the present invention has better stand-up characteristics than the comparative example, and also has uniform heat dissipation. Furthermore, when we actually cooked hot cakes using this hot plate, we were able to cook the hot cakes evenly, with no uneven baking or scorching, no matter where on the hot plate we cooked. As described above, the heating element of the present invention is effective in the construction of cooking appliances, electric appliances such as electric pots, rice cookers, etc. that utilize conductive heat transfer. Example 2 A slip was prepared by milling the glaze having the composition shown in Table 6b for 2 hours, and this was made into a slip with a size of 150×100.
It was applied to both sides of a 0.4 mm thick SPP steel plate to a thickness of about 150 μm using a spray gun, dried in a dryer at about 120°C for about 10 minutes, and then baked at 690°C for about 10 minutes. Next, on one side, a thickness of Ni-Cr alloy is applied.
A 50μm thin strip heating element (600W) was placed, and a slip made by milling the glaze with the composition shown in c in Table 6 was applied over it to a thickness of approximately 250μm using a spray gun, and after drying. Fired. This planar heating element was installed in an electric oven and compared with an electric oven using a quartz tube heater. Figure 7 compares the temperature change over time at the center of the meat when 300 g of chicken thigh meat is cooked. In addition, we compared the time required to cook two thick slices of bread approximately 2 cm thick, 300 g of thigh meat, and the degree of heat penetration after 10 minutes when cooking 1 cm thick beef. It is shown in Table 9.

[Table] As is clear from the table, when the planar heating element of the present invention is used, compared to conventional products, the cooking time is faster and the heat penetrates into the food very well.
Furthermore, when looking at the degree of grilling of the actual cooked food, the food cooked using the planar heating element of the present invention was grilled without any unevenness or the like. In other words, it can be seen that the planar heating element of the present invention can cook food uniformly, has a high far-infrared radiation function, and has excellent energy utilization efficiency. In this way, it is also effective for cooking appliances that utilize radiant heat transfer, such as toasters, and heaters such as panel heaters and electric stoves.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the infrared absorption characteristics of the components constituting the cooked food, Figs. 2 and 3 are cross-sectional views showing an example of the structure of the sheet heating element of the present invention, and Fig. 4 is the manufacture of the hollow substrate. Process diagram, Figure 5 is a plan view of an embodiment of the electric heating material, Figure 6 is a manufacturing process diagram of a typical planar heating element of the present invention, and Figure 7 is a diagram of actual cooking using an electric oven. It is a figure showing a temperature change of an object. DESCRIPTION OF SYMBOLS 1... Substrate, 2... Electric heating element, 3... Heating element coating layer.

Claims (1)

[Scope of Claims] 1. An electric heating element made of a thin metal strip formed in a desired pattern including lead terminals, a hollow substrate whose surface on which the heating element is fixed is electrically insulating at least, and the heating element and a heating element coating layer mainly composed of glass frit, which is coated with glass frit and fixed to the substrate. 2. The planar heating element according to claim 1, wherein the heating element coating layer is a hollow layer. 3. The planar heating element according to claim 1, wherein the thermal expansion coefficient of the heating element coating layer is in the range of 0.8 to 1.5, where the thermal expansion coefficient of the heating element is 1. 4 The thickness of the hollow layer of the hollow substrate is 100 to 400 μm
A planar heating element according to claim 1.