JP4273227B2 - Manufacturing method of microwave heating element - Google Patents

Description

  The present invention relates to ceramics having a function of absorbing microwaves and converting them into heat.

  In general, oxide ceramics are electrical insulators, but recently, ceramic products for electromagnetic cookers have been developed because they can use Joule heat induced by electromagnetic induction by combining them with metals.

  Even ceramics with a composition used in ordinary general ceramic products for daily use are known to generate some heat when irradiated with microwaves. The product used for is not developed.

As a method of heating ceramics using microwaves and application to products,
Japanese Patent Laid-Open No. 6-13174 discloses a microwave oven equipped with a ceramic base material in which carbon black is mixed and dispersed in a substrate. In JP-A-5-121165, a heating element in which a metal conductive material is plated on one surface of a porous ceramic such as silicon carbide. In JP-A-8-309276, a heating element in which a heating material such as titanium oxide or ferrite is applied to ceramics using a resin as a binder. In JP-A-10-50473, a heating element in which a container is mixed and filled with a semiconductor such as granular silicon carbide or a dielectric.

  However, any of them is a method of simply mixing, dispersing, plating or coating a conductive material or a semiconductor, and the structure of the heating element is also different from the present invention.

As a method similar to the present invention for obtaining exothermic ceramics by firing,
In Japanese Patent Publication No. 8-10620, as a method of manufacturing a planar heating element using a ceramic as a base material, a ceramic that integrally obtains a semiconductive layer or a conductive layer on a ceramic base material by high-temperature firing at 1200 ° C. or higher and a manufacturing method thereof . In Japanese Patent Laid-Open No. 2-82484, an organic polymer is applied and a silicon carbide film is formed on the surface by baking.

  However, Patent Document 5 obtains a heat generation effect by energizing as a means, requires a high temperature of 1200 ° C. or higher, and completely mentions a microwave absorption effect and a detailed manufacturing method. However, the composition of the heating element is fundamentally different between Patent Document 6 and the present invention.

  When resistance heat is generated by direct energization of the conventional method, there is a risk of electric leakage if the human body is electrocuted or wet. Further, in induction heating, it is necessary to form a conductive layer such as a metal on the surface after firing the ceramic, and the ceramic is easily broken by thermal shock.

  Regarding carbon black dispersed exothermic ceramics applied to the components of the microwave oven disclosed in Patent Document 1, the use time, temperature, atmosphere, and the like are limited due to the risk of carbon combustion, and there is a concern about the destruction of the substrate due to thermal shock.

  The heating element obtained by plating the metal of Patent Document 2 generates heat, but cannot be used because it sparks in a microwave oven.

  In the method of obtaining the heat generating layer using the resin of Patent Document 3, the use is limited because the resin is melted or decomposed when the heat generation temperature is high. In Patent Document 4, a granular microwave absorption heating element is mixed. Thus, a dense exothermic ceramic having an arbitrary shape is not obtained by integral firing.

  In the method of manufacturing a planar heating element based on ceramics according to Patent Document 5, a high temperature of 1200 ° C. is necessary, and when applied to microwave heating, the ceramic surface is reoxidized during the cooling process by a normal firing method. It is easy and αFe2O3 is formed thick, and the microwave absorption heat generation effect is low.

  There is a method of forming a silicon carbide film on the surface by applying an organic polymer of Patent Document 6 and firing, but this must be a ceramic substrate such as alumina to prevent reaction with the substrate, There is a problem that the degree of adhesion with the heat generating layer is low.

  As described above, in the conventional technology, a heat-generating ceramic that has a high effect of absorbing and generating microwaves and that has good adhesion between the base material and the heat-generating portion and hardly undergoes oxidation or breakage even at high temperatures has not been obtained.

  In order to solve these problems, the present invention applies a glaze or a glaze-like material to a molded base material and fires it, as in a normal ceramic manufacturing method, A large amount of microwave absorbing material is produced in the vitrified soot layer or on the surface.

  In addition, this makes the base material and the heat generating layer integrated, and makes it difficult for defects such as peeling and cracking of the heat generating layer due to thermal shock during use and external force during handling to occur.

  It is possible to use various metals or metal oxides as the raw material for the glaze material applied on the base material. However, as a metal that is an inexpensive raw material and easily precipitates in the glassy material or the surface thereof, metallic iron Or an iron oxide or a metal iron compound is used.

  In order to deposit metallic iron and magnetite from glazes mainly composed of iron oxides, it is said that a firing temperature of 1200 ° C and a strong reducing atmosphere are required. In order to produce and precipitate metallic iron crystals from the melt, a silicate mineral such as a clay mineral that produces amorphous silicic acid in the firing process is blended in the base material or glaze material.

  Even if a metal known as a microwave absorbing substance or a compound thereof is blended as a glaze raw material and fired in the normal manner in the atmosphere, it remains as a metal due to oxidation in the firing process or reaction with the substrate. Therefore, the reducing process is maintained during the cooling process in firing.

  1150 When the atmosphere in the kiln is reduced by applying a glaze containing silicic acid mainly composed of metallic iron or iron compound, or applying a glaze containing only metallic iron or iron compound and not containing silicic acid to the ceramic substrate, Iron oxide and active silicic acid react at temperatures around ℃ to form a melt with a Fayalite (2FeO · SiO2) composition.

  In the case of a composition containing a large amount of iron oxide in the glaze layer, magnetite crystals are precipitated from the melt during the cooling process. However, in an oxidizing atmosphere at high temperatures, ferric oxide crystals with a silvery white surface develop. Part of the iron element is also consumed.

  However, if the cooling process is also in a reducing atmosphere, the iron element remains abundant as magnetite or metallic iron without being reoxidized, and forms a layer having a high microwave absorption capacity.

  When this is irradiated with microwaves, the energy of the metallic iron and magnetite generated in or on the vitrified layer of the ceramic surface is converted into heat by electron motion.

If the irradiation time is prolonged, the heat is stored and the soot layer continues to rise in temperature, but since the crystals are supported in the glass and not oxidized, the heat generation function lasts for a long time, and the ceramic substrate and soot layer Since the intermediate layer that is inevitably formed during the firing process relieves thermal stress, it exerts an effect of hardly causing peeling and cracking.

  As a glaze raw material, a petrol (mainly ferric oxide / αFe2O3), which is mainly composed of iron oxide, is the cheapest and easy to handle. It is desirable to use clay minerals that are effective for lowering the temperature of the steel.

  As the ceramic base to which the glaze raw material is applied, since a material containing a highly reactive silicate mineral is required for melting at low temperature, a normal ceramic base using a clay mineral is desirable.

  The firing condition is that it is necessary to be incorporated into the glaze layer that has been vitrified so that metallic iron and magnetite are not re-oxidized.

  A mixture of ferric oxide (20 to 100% by weight), alumina fine powder (0 to 20% by weight) and silica stone fine powder (0 to 80%), bentonite added to each by 5% by weight, and then a glaze. As an example, it was applied to a clay-based pottery base and fired at a maximum temperature of 1150 ° C to 1200 ° C.

  Firing is performed using a gas kiln, and the firing atmosphere in the furnace is controlled by changing the air ratio of propane gas. The reduction starting temperature is in the range of 1150 ° C to 1200 ° C. A comparative study was conducted.

  Table 1 shows the results of measuring the surface temperature of the glazed surface quickly by irradiating the obtained specimen with microwaves using a microwave oven with a frequency of 2.45 GHz for 30 seconds.

  As a result, the temperature of the substrate did not change so much, and the mixing ratio of alumina and silicon oxide contained in the glaze was not significantly different from the exothermic temperature, but glaze containing 80% or more of ferric oxide was applied. It was found that the temperature of the glazed surface fired at 1150 ° C. or higher markedly increased.

  Further, when the reducing atmosphere was maintained even during cooling, the maximum temperature reached 400 ° C. or higher, and the exothermic temperature was higher than that of silicon carbide that has been known to have a high exothermic effect.

  As a result of thermal analysis and X-ray diffraction analysis of the surface of the ceramic which had good heat generation, the presence of magnetite and metallic iron was confirmed as crystals.

  In addition, it was confirmed that those with a large amount of crystal formation and those in which metallic iron was precipitated in the soot layer by the method of maintaining the reducing atmosphere during cooling generated heat to a higher temperature.

  It can be used in a wide range of fields, such as dishes for cooking in a microwave oven that can get a burnt surface, efficient snow melting tiles that can quickly heat only the necessary parts that are in contact with snow, and electromagnetic wave absorbing tiles for construction that have excellent durability.

It is a schematic diagram of the cross-sectional structure of exothermic ceramics.

Explanation of symbols

1 Glazed layer
2 Metallic iron 3 Magnetite 4 Ceramic substrate (base)
5 middle class

Claims (1)

A step of applying a glaze containing 80% by weight or more of ferric oxide on the surface of a ceramic substrate containing a silicate mineral, and a maximum temperature of 1150 for a ceramic substrate subjected to the glaze in a reducing atmosphere using propane gas A process in which metallic iron and magnetite crystals are formed so as to be supported in a vitrified layer on the surface of the fired ceramics by maintaining a reducing atmosphere in the cooling process after firing at a temperature in the range of 1 ° C. to 1200 ° C. the equipped, microwave heating element, characterized in that to produce a microwave heating element crystals of metallic iron and magnetite generate heat the glaze absorb microwaves than 400 ° C. by microwave irradiation Production method.

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