Cooking heater

Abstract

This cooking heater (50) comprises a heating chamber (51) for accommodating an object to be heated, wherein, in a metal member (100) constituting an outer surface of a heater (20) for heating the object to be heated disposed in the heating chamber (51), at least a surface facing the object to be heated in the heating chamber (51) is composed of a substrate (2) containing copper or iron as a main component and a coating layer (3) covering the surface of the substrate (2), the coating layer (3) has a ceramic layer (3S) containing, as a main component, an oxide of a metal element having an ionization tendency larger than that of copper or iron, a diffusion layer (4) in which the metal element is diffused into the copper or iron is provided at the interface between the substrate (2) and the coating layer (3), and the metal element is zinc or nickel.

Description

Cooking heater

[0001] The present disclosure relates to a cooking heater.

[0002] Many cooking heaters have a heating chamber for cooking grilled fish and the like. The heating chamber is generally also called a grill, a roaster, or an oven. Grilled fish (especially salt-grilled saury) is said to be delicious when the surface is well grilled by radiant heat from a high-temperature heat source and the inside of the fish is sufficiently heated by high-temperature air.

[0003] The heating chamber of a cooking heater generally has electric heaters such as sheathed heaters and radiant heaters above and below in the heating chamber (also called "resistance heaters" because they generate heat by Joule heat when an electric current flows through a resistor). The power supply terminals of the electric heaters are electrically connected to a power source provided outside the heating chamber, and power is supplied from the power source to the electric heaters to heat the electric heaters.

[0004] The electric heater has a structure in which a nichrome wire, which is a resistor, is surrounded by a metal pipe, and the nichrome wire is insulated from the metal pipe by an insulator (for example, powder of magnesium oxide) enclosed in the metal pipe. Generally, since metal has a low emissivity, there is a problem that sufficient radiant heat required for heat cooking cannot be obtained as it is. Therefore, a method of coating the surface of the metal pipe of the electric heater with a radiator is known.

[0005] For example, in Patent Document 1, in a heater in which a heating element is arranged along the tube axis inside a tubular enclosure, the enclosure is crimp-sealed at both ends so that the heating element is hermetically sealed inside the enclosure, and the heating element is characterized in that a resistance heating wire is arranged inside an infrared radiation tube. A heater is disclosed.

[0006] Japanese Patent Application Laid-Open No. 2001-291575

[0007] However, in the heater disclosed in Patent Document 1 above, there is a problem that variations in infrared radiation emitted from the heater are likely to occur, and when this is used as a heat source inside the heating chamber, uneven heating occurs depending on the location inside the chamber.

[0008] This disclosure provides a technology to solve the above-mentioned problems, and aims to provide a cooking heater having a heating chamber in which the coating layer on the surface of the metal component (such as a metal pipe) that constitutes the outer surface of the heater inside the heating chamber has excellent adhesion, and which can uniformly heat the inside of the chamber by far-infrared rays.

[0009] The cooking heater of this disclosure is a cooking heater having a heating chamber for housing an object to be heated, wherein of the metal members constituting the outer surface of the heater that heats the object to be heated and is placed in the heating chamber, at least the surface facing the object to be heated in the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material, the coating layer has a ceramic layer mainly composed of an oxide of a metal element that has a greater ionization tendency than copper or iron, and at the interface between the base material and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron, and the metal element is zinc or nickel. Furthermore, the cooking heater of the present disclosure is a cooking heater having a heating chamber for housing an object to be heated, wherein, of the metal members constituting the outer surface of the heater that heats the object to be heated and is placed in the heating chamber, at least the surface facing the object to be heated in the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material, the coating layer has a ceramic layer mainly composed of an oxide of a metal element having a greater ionization tendency than copper or iron, and at the interface between the base material and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron, and the coating layer has a metal layer made of the metal element between the base material and the ceramic layer.

[0010] The cooking heater according to this disclosure provides excellent adhesion of the coating layer on the surface of the metal member constituting the outer surface of the heater inside the heating chamber, and can uniformly heat the inside of the heating chamber by far-infrared rays.

[0011] This is a schematic cross-sectional view of a metal member according to Embodiment 1. This is a schematic cross-sectional view showing another example of a coating layer according to Embodiment 1. This is a schematic cross-sectional view showing an example of the crystal of the coating layer according to Embodiment 1. This is a schematic cross-sectional view showing another example of the crystal of the coating layer according to Embodiment 1. This is a schematic cross-sectional view showing an alloy layer formed in the diffusion layer of a metal member according to Embodiment 1. This is a schematic cross-sectional view showing another example of an alloy layer according to Embodiment 1. This is a schematic cross-sectional view of a metal member in which a metal layer remains on the coating layer according to Embodiment 1. This is a schematic diagram of an IH cooking heater as a cooking heater having a heating chamber according to Embodiment 1. This is a schematic top view of the electric heater of the cooking heater according to Embodiment 1. This is a schematic top view of a planar electric heater, which is another example of the electric heater of the cooking heater according to Embodiment 1. This is a diagram illustrating the location for measuring the in-plane distribution of the emissivity of the heater according to Embodiment 1. This is a flowchart showing the manufacturing process of a metal member according to Embodiment 1. This is a diagram showing the configurations of Examples 1 to 5 of the metal member according to Embodiment 1. This is a diagram showing the configurations of Examples 6 to 8 of the metal member according to Embodiment 1. This is a diagram showing the configurations of the metal members of Comparative Examples 1 to 3. This diagram shows the configurations of the metal members according to Embodiment 2, from Embodiment 9 to Embodiment 13.

[0012] Embodiment 1. First, before describing the cooking heater according to Embodiment 1 with reference to the figures, we will explain the metal members that constitute the outer surface of the heater used in the heating chamber of the cooking heater.

[0013] In the following description and drawings, various "layers" will be mentioned and described, but each "layer" does not have a strict boundary. Figure 1 is a schematic cross-sectional view of a metal member 100 that constitutes the outer surface of a heater used in the heating chamber of a cooking heater according to Embodiment 1. The metal member 100 consists of a base material 2 (hereinafter simply referred to as base material 2) mainly composed of copper or iron, a coating layer 3, and a diffusion layer 4.

[0014] The base material 2 constituting the metal component 100 is preferably made of a copper-based material because it has excellent thermal conductivity and is cost-effective. For example, in addition to pure copper, copper alloys such as brass and cupronickel can be used. From a cost perspective, iron-based materials are preferred. For example, in addition to pure iron, iron-based alloys such as stainless steel can be used.

[0015] In particular, for applications requiring high thermal conductivity, pure copper is preferred, with a thermal conductivity of 300 W / (m·K) or higher, and more preferably 350 W / (m·K) or higher. Furthermore, when use in a high vacuum is anticipated, oxygen-free copper is preferred among pure coppers. By using oxygen-free copper, it is possible to prevent a decrease in vacuum due to the release of oxygen dissolved in the copper.

[0016] Generally, methods for forming a ceramic coating layer 3 on a substrate mainly composed of copper or iron include sputtering, which requires a complicated vacuum process, and thermal spraying, which requires an ultra-high temperature process.

[0017] However, these methods make it difficult to uniformly form the coating layer 3 on the surface of components with complex shapes, and the adhesion to the copper substrate may also be weak. Another method involves coating with a resin composite material in which ceramic fillers are mixed into a resin matrix, but this method has poor adhesion to substrates mainly composed of copper or iron, and also has low heat resistance, making it unsuitable for heater applications. Furthermore, even if the heat resistance problem is overcome, the resin decomposition gas is generated in a vacuum, which is a disadvantage as it cannot be applied to heaters used in a vacuum.

[0018] Therefore, in the metal member 100 of this embodiment 1, a plating film made of a metal element is formed on the surface of the base material 2, and a ceramic coating layer 3 is easily formed on the base material 2 by oxidizing the plating film in air. In this case, by using a plating film made of a metal element with a higher ionization tendency than copper or iron, when oxidized in air, only the plating film is oxidized, the base material 2 is not oxidized, and the thermal conductivity of the base material 2 does not decrease.

[0019] In this specification, the coating layer 3 refers to the plated film portion applied to the substrate 2, and the oxidized portion within the coating layer 3 is specifically referred to as the ceramic layer. In some cases, the entire coating layer 3 may be a ceramic layer, while in other cases, as described later, only a portion of the coating layer 3 may be a ceramic layer.

[0020] Furthermore, by forming a diffusion layer 4 between the coating layer 3 and the substrate 2 in which metal elements are mutually diffused, the adhesion between the substrate 2 and the coating layer 3 is strengthened. As a result, the metal member 100 of this embodiment 1 has excellent adhesion to the coating layer 3 and high emissivity without reducing the thermal conductivity of the substrate 2.

[0021] The coating layer 3 constituting the metal member 100 plays a role in improving the infrared radiation performance of the metal member 100. Therefore, it is preferable that it be made of a ceramic material with high emissivity. Furthermore, in the oxidation treatment step, which is one of the processes for forming the coating layer 3 on the substrate 2, it is preferable that it be an oxide of a metal element with a higher ionization tendency than copper or iron, from the viewpoint of preventing oxidation of the substrate 2. For example, ceramic materials such as zinc oxide, silica, and nickel oxide are used.

[0022] In particular, when applied to metal components 100 where radiation performance is important, zinc oxide with even higher radiation performance is preferable. By using zinc oxide, metal components with even higher radiation performance can be obtained. Furthermore, when considering the use of metal components in a high vacuum, or when using metal components after heat treatment processes under high vacuum, such as vacuum brazing, nickel oxide with a low vapor pressure is preferable from the viewpoint of preventing evaporation of metal elements in a vacuum.

[0023] By using nickel oxide, a highly reliable metal component 100 can be obtained that has high radiation performance and does not undergo evaporation of metal components in a vacuum. Figure 2 is a schematic cross-sectional view showing another example of the coating layer 3. As shown in Figure 2, the coating layer 3 may also be made of two different ceramic materials to form a laminated structure of coating layer 3A (first coating layer) and coating layer 3B (second coating layer). By making the coating layer 3 a two-layer laminated structure, a coating layer 3A with a coefficient of thermal expansion closer to that of the substrate 2 than the upper coating layer 3B can be formed on the lower layer on the substrate 2 side, and a coating layer 3B with a higher emissivity than coating layer 3A can be formed on the upper layer.

[0024] This makes it possible to form a coating layer 3 that has even better adhesion to the substrate 2 and even better radiation performance. In this embodiment, it is even more preferable that the ceramic layer, which is the coating layer 3 of the metal member 100, is crystalline.

[0025] By using a crystalline ceramic layer as the coating layer 3, the thermal conductivity within the ceramic layer is increased, and heat is efficiently transferred to the ceramic layer responsible for radiation, thus improving radiation performance. The crystalline state of the ceramic layer can be confirmed by X-ray diffraction. If diffraction peaks due to the crystalline structure are observed in the X-ray diffraction pattern, it can be confirmed as crystalline; if only a halo pattern is observed, it can be confirmed as amorphous.

[0026] Furthermore, the thickness of the coating layer 3 is preferably 0.5 μm or more and 20 μm or less, and more preferably 1 μm or more and 10 μm or less. If the thickness is less than 0.5 μm, sufficient radiation performance may not be obtained. On the other hand, if the film thickness exceeds 20 μm, the ceramic layer becomes brittle, and problems such as peeling may occur during long-term use.

[0027] Figure 3 is a schematic cross-sectional view showing an example of the crystalline grains 6 of the coating layer 3. Figure 4 is a schematic cross-sectional view showing another example of the crystalline grains 6 of the coating layer 3. The form of the ceramic layer, which is the coating layer 3, may be a dense film made of polycrystalline crystalline grains 6 as shown in Figure 3, or a porous film containing many voids 7 (micropores) between the crystalline grains 6 as shown in Figure 4, as long as it does not impede the radiation performance. From the viewpoint of adhesion to the substrate 2, it is good to use different materials for the ceramic layer. When using a ceramic layer with a large difference in thermal expansion coefficient from the substrate 2, a porous ceramic layer is preferable in order to alleviate thermal stress, which is a factor that impedes adhesion.

[0028] For example, when using zinc oxide, which has a low coefficient of thermal expansion, for the ceramic layer, which is the coating layer 3, making the ceramic layer porous lowers the apparent modulus of the ceramic layer, making it less likely for thermal stress to occur due to the difference in thermal expansion between the substrate 2 and the ceramic layer. Therefore, it becomes possible to further strengthen the adhesion with the substrate 2. On the other hand, when using nickel oxide, which has a small difference in coefficient of thermal expansion from the substrate 2, for the ceramic layer, even without making the ceramic layer porous, the thermal stress generated at the interface with the substrate 2 is small, and the possibility of delamination is low.

[0029] Therefore, selecting a dense ceramic layer with high strength in the ceramic layer itself is preferable from the viewpoint of improving the mechanical strength of the coating layer 3.

[0030] A diffusion layer 4 is formed at the interface between the substrate 2 and the coating layer 3, where the metallic components of the oxides of the metal elements constituting the coating layer 3 are diffused. A ternary metal oxide of Ni, Cu, and O, or Ni, Fe, and O, may be formed in the diffusion layer 4. By forming the diffusion layer 4, the adhesion between the substrate 2 and the coating layer 3 is improved through chemical effects.

[0031] Figure 5 is a schematic cross-sectional view showing an alloy layer 8 formed on the diffusion layer 4 of the metal member 100. Figure 6 is a schematic cross-sectional view showing another example of the alloy layer 8. Furthermore, from the viewpoint of improving the adhesion between the substrate 2 and the coating layer 3, it is even more preferable that an alloy layer 8 of copper or iron and the metallic component of the oxide of the metal element constituting the coating layer 3 is formed on the diffusion layer 4. The formation of the alloy layer 8 provides stronger chemical adhesion. In this case, the alloy layer 8 does not have to be formed as a continuous film of uniform thickness at the interface between the substrate 2 and the coating layer 3, but may be formed partially.

[0032] For example, when nickel oxide is used for the oxide layer, the alloy layer 8 is a Cu-Ni alloy or an Fe-Ni alloy. Since Cu and Ni or Fe and Ni are completely dissolved, the ratio of Cu to Ni in the Cu-Ni alloy and the ratio of Fe to Ni in the Fe-Ni alloy can be any ratio. Furthermore, the ratio of each metal element in the Cu-Ni alloy or Fe-Ni alloy may not be a single ratio, but may be a gradient, as shown in Figure 6, where the ratio of copper or iron gradually increases from the upper alloy layer 8a to the alloy layer 8b and alloy layer 8c on the substrate 2 side. When such a gradient is applied, the thermal expansion coefficient of the entire alloy layer 8 also becomes a gradient, so it acts as a buffer layer for thermal stress relaxation and further contributes to improving the adhesion between the substrate 2 and the coating layer 3. Also, when zinc oxide is used for the coating layer 3, the alloy layer is a Cu-Zn alloy or an Fe-Zn alloy.

[0033] Figure 7 is a schematic cross-sectional view of a metal member 100 in which a metal layer 5 remains on the coating layer 3. As shown in Figure 7, it is even more preferable that the metal member 100 is formed with a metal layer 5 made of the same metal component as the coating layer 3 remaining between the ceramic layer 3S, which is an oxide layer constituting the coating layer 3, and the substrate 2. The metal layer 5 is the lower layer portion of the metal that was plated to form the coating layer 3, which remained without oxidation during the oxidation treatment.

[0034] By leaving the unoxidized metal layer 5, a metal layer 5 with a thermal expansion coefficient intermediate between that of the substrate 2 and the ceramic layer 3S remains beneath the coating layer 3. This has the effect of mitigating thermal stress caused by the difference in thermal expansion coefficients between the substrate 2 and the coating layer 3, and further improving the adhesion between the substrate 2 and the metal layer 5. For example, when nickel oxide is used as the oxide layer for the coating layer 3, the metal layer is a nickel layer.

[0035] Furthermore, when zinc oxide is used for the ceramic layer 3S as the coating layer 3, the metal layer 5 is a zinc layer. When the metal member 100, in which the above-mentioned metal layer 5 is formed between the substrate 2 and the ceramic layer 3S, is used in a high vacuum, or when it is used after undergoing a heat treatment process under a high vacuum, such as vacuum brazing, a nickel layer with a lower vapor pressure than copper or iron is preferred in order to prevent evaporation of the metal layer 5. The thickness of the metal layer 5 is not particularly limited, but from the viewpoint of manufacturability, it is preferably 1 μm or more and 10 μm or less.

[0036] Next, a cooking heater 50 according to Embodiment 1 will be described. Figure 8 is a schematic diagram of an IH cooking heater 50 having a heating chamber 51. Figure 9 is a schematic top view of the electric heater 20A of the cooking heater 50. Figure 10 is a schematic top view of a planar electric heater 20B, which is another example of an electric heater in the cooking heater 50. In the following description, when simply referred to as heater 20, it refers to both the electric heater 20A and the planar electric heater 20B. Figure 11 is a diagram illustrating the location for measuring the in-plane distribution of the emissivity of the heater 20. The cooking heater 50 is, for example, an IH (Induction Heating) cooking heater (electromagnetic induction heating type cooker) with a built-in grill chamber as a heating chamber, an oven, a roaster, a toaster, etc.

[0037] As shown in Figure 8, the cooking heater 50 includes a heating chamber 51 in which the object to be heated is placed and contained on a receiving tray 52. ​​The heating chamber 51 is equipped with a heater 20 having a metal member 100 as a heat source. From the viewpoint of heating efficiency, it is preferable to place the heater 20 on the top surface or on the top and bottom surfaces of the heating chamber 51. Furthermore, when placed on the top surface of the heating chamber 51, the object to be heated can be heated uniformly by arranging the heater 20 so as uniformly as possible to cover the top surface of the heating chamber 51.

[0038] Alternatively, the heater 20 may be an exposed electric heater 20A as shown in Figure 9. In this case, the metal pipe 20P, which is formed by folding a wavy shape and contains a nichrome wire as the heating element of the electric heater 20A, is made of the metal member 100 described above. The cross-sectional shape of the metal pipe 20P perpendicular to the longitudinal direction may be round or rectangular.

[0039] Furthermore, as shown in Figure 10, when using a flat electric heater 20B in which a metal plate 21 is installed on the electric heater 20A, the metal plate 21 is made of the metal member 100 described above. By installing the plate 21 so that it faces the inside of the heating chamber 51 (the side facing the object to be heated), cleaning becomes easier. From the viewpoint of maintainability, the flat electric heater 20B is preferred. Note that the flat electric heater 20B may be constructed using only nichrome wire and the plate 21, without the metal pipe 20P.

[0040] In order to heat the inside of the heating chamber 51 uniformly, it is preferable that the variation in emissivity depending on the location of the electric heater equipped with the metal member 100 is small. If the variation in emissivity is large, the amount of infrared radiation emitted from the surface of the metal member 100 will vary, making it difficult to heat the object to be heated uniformly.

[0041] When the coating layer 3 according to the present embodiment is used for the heater 20, the variation in emissivity depending on the location on the surface of the metal member 100 of the heater 20 is within ±30%. If the variation in emissivity is within this range, it becomes possible to uniformly heat the object to be heated. Here, the variation in emissivity depending on the location on the surface of the metal member 100 of the heater can be calculated by determining the sampling locations of the data as follows.

[0042] First, as shown in FIG. 11, among the portions where at least the two diagonals of the surface (for example, the upper surface 50S) of the object to be heated in the heating chamber 51 where the heater 20 is installed and the heater 20 overlap when viewed as in FIG. 11, the emissivities of the five locations of the heater 20 closest to the four corners of the upper surface 5 OS and the intersection of the two diagonals of the upper surface 50S (the circled marks P) are measured respectively, and the average value is calculated. Next, the differences between the maximum value and the minimum value of the emissivities at the five locations and the average value are calculated, and can be calculated by dividing each difference by the average value. In the case of the planar electric heater 20B, the above five locations are determined in relation to the heat radiating body (nichrome wire). Positive variation in emissivity (%) = (maximum value - average value) ÷ average value × 100 Negative variation in emissivity (%) = (average value - minimum value) ÷ average value × 100 Note that the emissivity of the wall surface 50W of the heating chamber 51 is 20% or less.

[0043] The emissivity of the heater 20 provided with the metal member 100 is particularly greatly affected by the thickness of the coating layer 3 on the surface. When the thickness is large, the emissivity tends to be high, and when the thickness is small, the emissivity tends to be low. Therefore, in order to reduce the variation in emissivity at each location of the heater 20 provided with the metal member 100, it is effective to reduce the variation in the thickness of the coating layer 3.

[0044] The variation in the thickness of the coating layer 3 at each location on the surface of the metal member 100 of the heater 20 is preferably within ±50%. If the variation in the thickness of the coating layer 3 is within this range, the variation in emissivity tends to be small, and it becomes easier to uniformly heat the object to be heated.

[0045] Here, the variation in the thickness of the coating layer 3 at each location on the surface of the metal member 100 of the heater 20 can be calculated as follows. First, as shown in FIG. 11, among the portions where at least the two diagonal lines of the surface (for example, the upper surface 50S) of the object to be heated in the heating chamber 51 where the heater 20 is installed and the heater 20 overlap when viewed as in FIG. 11, the coating layer 3 thicknesses of the surfaces of the heater 20 at the four corners of the upper surface 50S and at the locations closest to the intersections of the two diagonal lines of the upper surface 50S (the circled points P), a total of five locations facing the object to be heated, are measured respectively, an average value is calculated, and the variation can be calculated by dividing each difference by the average value. Positive variation (%) in the thickness of the coating layer = (maximum value - average value) ÷ average value × 100. Negative variation (%) in the thickness of the coating layer = (average value - minimum value) ÷ average value × 100

[0046] Next, an example of the manufacturing method of the metal member 100 and the cooking heater 50 of the present embodiment will be described. FIG. 12 is a flowchart showing the manufacturing process of the metal member 100. First, copper or iron is processed into a desired shape and dimensions to produce the base material 2 (step S01). The shape of the base material 2 may be any shape that can function as the metal member 100 of the heater, such as a flat plate shape or a pipe shape.

[0047] The dimensions of the base material 2 are determined in consideration of the thickness of the coating layer 3 to be formed in later processes. Also, when manufacturing a metal member 100 with strict dimensional tolerances, after coating the base material 2 with another metal, surface polishing may be performed as a fine adjustment of the dimensions. By performing surface grinding treatment, it becomes possible to improve the surface smoothness and dimensional accuracy. The material of the base material 2 is preferably a copper-based material or an iron-based material. For example, in addition to pure copper, copper alloys such as brass and cupronickel can be used, and in addition to pure iron, stainless steel-based iron alloys can be used.

[0048] In particular, for applications that require high thermal conductivity, pure copper is preferred, and the thermal conductivity is preferably 300 W / (m·K) or more, and more preferably 350 W / (m·K) or more. Further, when use in a high vacuum is assumed, among pure coppers, oxygen-free copper is preferred.

[0049] Next, the surface of the substrate 2 is plated to form a plating layer (step S02). The plating film formed on the surface of the substrate 2 is preferably made of a metal element with a higher ionization tendency than copper or iron, from the viewpoint of preventing oxidation of the substrate 2. For example, zinc, silica, nickel, etc., can be used. By using a plating film made of a metal element with a higher ionization tendency than copper, when oxidized in air, only the plating film oxidizes, the substrate 2 is not oxidized, and the thermal conductivity of the substrate 2 does not decrease.

[0050] The plating process can be carried out using any known method, and electroless plating or electrolytic plating can be used. From the viewpoint of adhesion between the substrate 2 and the plating layer, the use of electroplating is even more preferable. Furthermore, the pretreatment for the plating process can be carried out using known methods such as degreasing, acid washing, and water washing, depending on the type of plating selected. For example, in electroplated nickel, a plating method using a Watt bath or a sulfamic acid bath can be used, but from the viewpoint of cost, plating using a Watt bath is preferred.

[0051] The plating layer may be formed in a single plating treatment or in multiple plating treatments. Furthermore, as shown in Figure 2, the coating layer 3 may be formed in a laminated structure of ceramic layers of different materials, or in a laminated structure of plating films of different materials. The thickness of the plating layer in the electrolytic plating treatment can be appropriately adjusted by conditions such as the applied electric field strength and immersion time in the treatment solution, and is preferably 1 μm to 30 μm, and more preferably 2 μm to 20 μm.

[0052] If the thickness is less than 1 μm, there is a risk that areas with extremely thin plating layers may occur due to variations in thickness. Also, if it exceeds 30 μm, the plating layer may become prone to peeling. Next, the plating layer is heat-treated in air and oxidized to form the coating layer 3 (ceramic layer 3S) (step S03).

[0053] The heat treatment method only needs to be able to oxidize the plating layer in the presence of oxygen, and batch-type electric furnaces, belt-type electric furnaces, etc., can be used. The oxidation temperature should be appropriately selected depending on the type of plating film chosen, and can also be determined by referring to the Ellingham diagram, which is an index of the oxidation-reduction temperatures of metals.

[0054] For example, if zinc plating is selected, oxidation treatment at 400°C to 600°C is preferred, and 450°C to 550°C is more preferred. Furthermore, if nickel plating is selected, 600°C to 850°C is preferred, and 650°C to 750°C is more preferred. In addition, the thickness of the ceramic layer 3S formed by oxidation of the plating layer is preferably 0.5 μm to 20 μm, and more preferably 1 μm to 10 μm. The thickness of the ceramic layer 3S can be adjusted as appropriate by changing the temperature and time of the oxidation treatment.

[0055] Furthermore, the metal layer 5 between the substrate 2 and the ceramic layer 3S can be formed by selecting heat treatment conditions that do not completely oxidize the plating layer. The diffusion layer 4 is formed during the oxidation treatment by the mutual diffusion of metal elements from the substrate 2 and the plating layer near the interface between the substrate 2 and the plating layer. Depending on the heat treatment conditions, the mutually diffused metal elements can also undergo a chemical reaction to form an alloy layer 8 within the diffusion layer 4.

[0056] The heater 20 and cooking heater 50 to which the metal member 100 of this embodiment 1 is applied can be manufactured by known methods. For example, a sheathed heater, which is a type of heater 20 of this embodiment, can be manufactured by known methods including an insertion step of inserting an electric heating wire into a metal pipe made of the metal member 100 via an electrically insulating powder such as magnesium oxide, a step of reducing the diameter of the metal pipe using a diameter reduction machine such as a swager or roll mill, a step of annealing the metal pipe to make it easier to bend, and a step of bending the metal pipe into a desired heater shape.

[0057] Furthermore, the induction cooking heater 50 of this embodiment 1 shown in Figure 8 can be manufactured by a known method that includes the steps of assembling an induction heating unit 54, which consists of a wound coil (not shown) for generating magnetic force and a glass top plate 53, into a housing 55, attaching a sheathed heater (heater 20) to the heating chamber 51, and connecting the terminals of the sheathed heater to a power supply circuit.

[0058] Next, the details of the metal member 100 constituting the outer surface of the heater 20 will be described by examples and comparative examples, but this disclosure is not limited thereto. Figure 13 shows the configuration of the metal member 100 in Examples 1 to 5. Figure 14 shows the configuration of the metal member 100 in Examples 6 to 8. Figure 15 shows the configuration of the metal member in Comparative Examples 1 to 3. Each figure shows the configuration of the metal member, including the presence or absence of a base material, coating layer, diffusion layer, alloy layer, and metal layer, and the relative evaluation results compared to Example 1 are shown.

[0059] [Example 1] A plate material (50 mm x 50 mm x 5 mm) of oxygen-free copper (C1020) was prepared as a base material 2, and a nickel plating layer with a thickness of 0.3 μm was formed on the surface of the base material 2 by electroplating. Next, the plating layer was oxidized by heat treatment at 700°C for 40 minutes in air using a batch-type electric furnace, and an evaluation metal member 100 having a coating layer 3 (all ceramic layers) with a thickness of 0.3 μm and an alloy layer 8 was obtained.

[0060] [Example 2] The procedure was the same as in Example 1, except that the nickel plating layer was formed to a thickness of 3 μm, the process was heat-treated in air at 700°C for 9 hours, and the coating layer 3 (all ceramic layers) was 3 μm thick.

[0061] [Example 3] The procedure was the same as in Example 1, except that the nickel plating layer was formed to a thickness of 18 μm, the process was heat-treated in air at 800°C for 13 hours, and the coating layer 3 (all ceramic layers) had a thickness of 18 μm.

[0062] [Example 4] The procedure was the same as in Example 1, except that the nickel plating layer was formed to a thickness of 25 μm, the process was heat-treated in air at 800°C for 18 hours, and the coating layer 3 (all ceramic layers) was 25 μm thick.

[0063] [Example 5] The same procedure as in Example 1 was followed, except that the nickel plating layer was formed to a thickness of 3 μm, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 1.5 μm and a metal layer 5 with a thickness of 1.5 μm.

[0064] [Example 6] The same procedure as in Example 1 was followed, except that the nickel plating layer was formed to a thickness of 8 μm, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 2.5 μm and a metal layer 5 with a thickness of 5.5 μm.

[0065] [Example 7] A plate material (50 mm x 50 mm x 5 mm) of oxygen-free copper (C1020) was prepared as a base material 2, and a zinc plating layer with a thickness of 3 μm was formed on the surface of the base material 2 by electroplating. Next, the plating layer was oxidized by heat treatment at 500°C in air for 12 hours using a batch-type electric furnace, and an evaluation metal member 100 having a coating layer 3 (all ceramic layers) with a thickness of 3 μm, a diffusion layer 4 and an alloy layer 8 was obtained.

[0066] [Example 8] The procedure was the same as in Example 7, except that the zinc plating layer was formed to a thickness of 6 μm, the coating layer 3 was heat-treated at 500°C in air for 7 hours, and the coating layer 3 consisted of a ceramic layer 3S with a thickness of 2 μm and a metal layer 5 with a thickness of 4 μm.

[0067] [Comparative Example 1] A plate material (50 mm x 50 mm x 5 mm) of oxygen-free copper (C1020) was prepared as a base material, and a metal member for evaluation was obtained without forming a coating layer on the surface.

[0068] [Comparative Example 2] A plate material (50 mm x 50 mm x 5 mm) of oxygen-free copper (C1020) was prepared as a base material, and without plating the surface, a metal component for evaluation was obtained by heat treatment at 500°C in air for 12 hours using a batch-type electric furnace.

[0069] [Comparative Example 3] A plate material (50 mm x 50 mm x 5 mm) of oxygen-free copper (C1020) was prepared as a base material, and a nickel oxide target was sputtered onto its surface using the sputtering method to obtain an evaluation metal member having a coating layer with a thickness of 2 μm.

[0070] Next, the metal members obtained in the above examples and comparative examples were evaluated as follows: (1) Emissivity For the metal members obtained in Examples 1 to 8 and Comparative Examples 1 to 3 above, the surface emissivity was measured using an emissivity measuring device as an indicator of radiant performance. The results of this emissivity are shown in Figures 13 to 15 as relative values ​​of the emissivity obtained for the metal members of each example or comparative example ([emissivity obtained for the metal members of each example or comparative example] / [emissivity obtained for the metal member of Example 1]) with respect to the emissivity obtained for the metal member 100 of Example 1.

[0071] (2) Cycas Strength The cycas strength of the coating layer 3 was measured as an indicator of adhesion for the metal members obtained in Examples 1 to 8 and Comparative Examples 1 to 3 described above. The results of this cycas strength are shown in Figures 12 to 15 as relative values ​​of the cycas strength obtained for the metal members of each example or comparative example (value of [cycas strength obtained for the metal members of each example or comparative example] / [cycas strength obtained for the metal member of Example 1]) with the cycas strength obtained for the metal member of Example 1 as the standard.

[0072] (3) Thermal conductivity of base material 2 The thermal conductivity of base material 2 was evaluated using the laser flash method for the metal members obtained in Examples 1 to 8 and Comparative Examples 1 to 3 described above. If the obtained thermal conductivity value was equivalent to the thermal conductivity of untreated oxygen-free copper (350 W / (m·K)), it was marked as ○, and if it decreased by 10% or more, it was marked as ×.

[0073] (4) Evaporation of metal components in a vacuum The metal components obtained in Examples 1 to 8 and Comparative Examples 1 to 3 described above were subjected to heat treatment (800°C) in a high vacuum (10⁻³ Pa) simulating brazing conditions using a vacuum heating furnace, and the presence or absence of evaporation of metal components was investigated. The presence or absence of evaporation was evaluated by EPMA (Electron Probe Micro Analyzer) analysis of the cross-section and XRD (X-ray Diffraction) analysis of the surface before and after heat treatment. If there was no change in the elemental distribution and XRD pattern of the metal layer, it was evaluated as no evaporation; if there was a change, it was evaluated as evaporation.

[0074] As shown in Figures 13 and 14, the metal members 100 of Examples 1 to 8 use nickel and zinc, which have a higher ionization tendency than the base material 2, resulting in good thermal conductivity of the base material 2. Furthermore, the large ratio of emissivity and the large ratio of Cycas strength indicate good radiation performance and adhesion strength.

[0075] On the other hand, in Comparative Example 1, which lacks a coating layer, the radiation performance is extremely low. Furthermore, in Comparative Example 2, where the surface of the substrate 2 was oxidized as is, the copper oxide film formed on the surface of the substrate 2 peeled off, indicating that it did not form a coating.

[0076] Therefore, the radiation performance did not improve. Furthermore, in Comparative Example 3, where nickel oxide was coated onto the surface of substrate 2 by sputtering, it can be seen that the adhesion was very low because there was no diffusion layer on substrate 2 and therefore no chemical adhesion.

[0077] Furthermore, comparing Examples 1-4 with Example 5, it can be seen that the luminescence performance is even better when the nickel oxide in the coating layer 3 is crystalline. Also, comparing Examples 1-4 with Example 6, it can be seen that Example 6, which has a thick metal layer 5 between the ceramic layer 3S and the substrate 2 to mitigate the difference in thermal expansion, has even better adhesion between the coating layer 3 and the substrate 2.

[0078] Focusing on the difference in the material of the coating layer 3, zinc oxide exhibits even better radiation performance than nickel oxide. However, focusing on the presence or absence of evaporation of metal components in a vacuum, a comparison between Example 6 and Example 8, both of which have a metal layer 5, shows that the nickel oxide in Example 6 does not undergo evaporation of metal components in a vacuum and is suitable for use in high vacuum. On the other hand, the zinc oxide in Example 8 undergoes evaporation of metal components in a vacuum and is not suitable for use in high vacuum.

[0079] According to the cooking heater of Embodiment 1, in a cooking heater having a heating chamber for containing an object to be heated, at least the surface of the metal member constituting the outer surface of the heater that heats the object to be heated, which is placed in the heating chamber, that faces the object to be heated in the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material, the coating layer has a ceramic layer mainly composed of an oxide of a metal element that has a greater ionization tendency than copper or iron, and at the interface between the base material and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron, and the metal element is zinc or nickel, so that the coating layer on the surface of the metal member constituting the outer surface of the heater in the heating chamber has excellent adhesion and can uniformly heat the inside of the heating chamber by far-infrared rays, and a cooking heater 50 can be provided. Furthermore, a metal member can be obtained that has excellent adhesion between the base material 2 and the coating layer 3 and has high emissivity without reducing the thermal conductivity of the base material 2. In addition, since oxidation of the base material 2 can be prevented, a cooking heater 50 with high heater durability can be provided.

[0080] Furthermore, in a cooking heater having a heating chamber for housing an object to be heated, at least the surface of the metal member constituting the outer surface of the heater that heats the object to be heated, which is placed inside the heating chamber, that faces the object to be heated inside the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material, the coating layer has a ceramic layer mainly composed of an oxide of a metal element that has a greater ionization tendency than copper or iron, and at the interface between the base material and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron, and the coating layer consists of the metal element between the base material and the ceramic layer, so that the adhesion of the coating layer to the surface of the metal member constituting the outer surface of the heater inside the heating chamber is excellent and the inside of the heating chamber can be uniformly heated by far-infrared rays, thus providing a cooking heater 50. In addition, by leaving an unoxidized metal layer 5 on the base material 2 side within the coating layer 3, there is a metal layer 5 having a thermal expansion coefficient intermediate between the thermal expansion coefficients of the base material 2 and the coating layer 3, so that the effect of mitigating thermal stress due to the difference in thermal expansion coefficients between the base material 2 and the coating layer 3 can be obtained. This makes it possible to provide a cooking heater 50 with high durability for the heater 20.

[0081] Furthermore, since the in-plane thickness variation of the surface of the coating layer facing the object to be heated is less than ±50% at each of the five locations closest to the heater, namely the four corners of the upper surface of the heating chamber and the intersection of the two diagonals of the upper surface, the variation in the emissivity of the heater 20 can be reduced.

[0082] Furthermore, since the variation in the in-plane emissivity of the surface of the coating layer facing the object to be heated is ±30% or less at each of the five locations closest to the heater, namely the four corners of the upper surface of the heating chamber and the intersection of the two diagonals of the upper surface, the variation in the amount of infrared radiation emitted from the surface of the metal member 100 is reduced, and a cooking heater 50 that can uniformly heat the object to be heated can be provided.

[0083] Furthermore, since the emissivity of the walls inside the heating chamber is 20% or less, it is possible to suppress the unnecessary absorption of infrared rays emitted from the heater by the walls, and as a result, uneven heating can be suppressed and the object to be heated can be heated uniformly.

[0084] Furthermore, since the diffusion layer contains an alloy layer made of copper or iron and the aforementioned metal element, an even stronger chemical adhesion between the coating layer 3 and the substrate 2 can be obtained.

[0085] Furthermore, since the aforementioned metal element has a lower vapor pressure than copper, evaporation of the metal element in a vacuum can be prevented.

[0086] Furthermore, since the thickness of the ceramic layer is between 0.5 μm and 20 μm, both the radiation performance and durability of the metal component 100 can be achieved. This makes it possible to provide a high-quality and highly durable cooking heater 50.

[0087] Furthermore, since the oxide of the metal element is crystalline, a ceramic layer 3S with high radiative performance can be formed. This makes it possible to provide a cooking heater 50 equipped with a heater 20 with high emissivity.

[0088] Furthermore, the coating layer consists of two layers, and the thermal expansion coefficient of the first coating layer on the substrate side is closer to the thermal expansion coefficient of the substrate than the thermal expansion coefficient of the upper second coating layer. Therefore, a coating layer 3 with even better adhesion to the substrate 2 can be formed. This makes it possible to provide a cooking heater 50 equipped with a highly durable heater 20.

[0089] Furthermore, since the emissivity of the second coating layer is higher than that of the first coating layer, a coating layer 3 with even higher radiation performance can be formed. This makes it possible to provide a cooking heater 50 equipped with a heater 20 with even higher radiation performance.

[0090] Furthermore, since the heater is an electric heater in which the heating element is built into a metal pipe as the metal component, a low-cost cooking heater 50 can be provided.

[0091] Since the heater is a flat heater in which the metal plate is installed on the heating element side of the heating element, it is possible to provide a cooking heater 50 that is easy to clean.

[0092] Embodiment 2. The cooking heater 50 according to Embodiment 2 will be described below, focusing on the differences from Embodiment 1. Note that for parts other than those described below, the same configuration as in Embodiment 1 is valid. The base material 2 constituting the metal member 100 of Embodiment 2 is preferably made of a copper-based material because it has excellent thermal and electrical conductivity and is cost-effective. For example, in addition to pure copper, copper alloys such as brass and cupronickel can be used.

[0093] In particular, for applications requiring high thermal and electrical conductivity, pure copper is preferred, with a volume resistivity of 2.5 μΩ·cm or less, and more preferably 1.8 μΩ·cm or less. Furthermore, when use in a high vacuum is anticipated, oxygen-free copper is preferred among pure coppers. By using oxygen-free copper, it is possible to prevent a decrease in vacuum due to the release of oxygen dissolved in the copper.

[0094] In the metal member 100 of this second embodiment, a plating film made of nickel, a metal element with a greater ionization tendency than copper, is formed on the surface of the substrate 2, and a ceramic coating layer is easily formed on the substrate 2 by oxidizing the plating film in air. Generally, the plating solution used for nickel plating contains sulfur components, and therefore the nickel plating film contains sulfur components. The sulfur components contained in the nickel film affect the oxidation-reduction potential of the nickel film, and if it contains a large amount of sulfur components, the nickel film becomes very easily oxidized.

[0095] Therefore, when the nickel plating film is oxidized in air, a rapid oxidation reaction may occur, which can reduce the adhesion between the substrate 2 and the ceramic layer 3S that constitutes the coating layer 3. For this reason, in the metal member 100 of this second embodiment, it is preferable to use a nickel plating film with a relatively low sulfur content to suppress the rapid oxidation reaction and to reduce the sulfur content of the coating layer 3 after oxidation.

[0096] Specifically, the sulfur content of the coating layer 3 in this second embodiment is preferably 1000 ppm or less. More preferably 600 ppm or less, even more preferably 400 ppm or less, and most preferably 200 ppm or less.

[0097] In this second embodiment, the ceramic layer 3S constituting the coating layer 3 of the metal member 100 is preferably composed of nickel oxide particles with an average particle size of 1 μm or less, and more preferably composed of nickel oxide particles with an average particle size of 0.5 μm or less. By setting the particle size of the nickel oxide particles as described above, the strength of the ceramic layer 3S is increased, and the effect of a ceramic layer 3S that is less likely to peel off from the substrate 2 is obtained.

[0098] Furthermore, from the viewpoint of further improving adhesion with the substrate 2, it is preferable that the ceramic layer 3S has a large number of micropores (voids 7 as described above). In this case, the average size of the micropores is preferably equal to or less than the average particle size of the nickel oxide constituting the ceramic layer 3S, with an average size of 1 μm or less, and more preferably 0.5 μm or less. By setting the average size of the micropores in this way, the effect of mitigating thermal stress due to the difference in thermal expansion with the substrate 2 is created without reducing the strength of the ceramic layer 3S, thereby improving the adhesion between the substrate 2 and the ceramic layer 3S.

[0099] In this embodiment 2, it is preferable that a copper-nickel alloy layer 8 is formed in the diffusion layer 4 of the metal member 100, and in this case, it is preferable that the content ratio of copper to nickel constituting the alloy is within a certain range. Specifically, the ratio of copper to nickel constituting the alloy is preferably 40:60 to 60:40 by mass, and more preferably 45:55 to 55:45. By forming an alloy layer 8 with a copper-nickel content ratio within a certain range in this way, the alloy layer 8 becomes chemically stable, and even when the ceramic layer 3S becomes thicker, the adhesion to the substrate 2 can be further improved.

[0100] Next, an example of a method for manufacturing the metal member 100 of this second embodiment will be described. The method for manufacturing the metal member 100 of this second embodiment is based on the flowchart in Figure 12, which is the method for manufacturing the metal member 100 of the first embodiment, so only the differences will be described here.

[0101] Step S02 involves applying a plating treatment to the surface of the substrate 2 to form a plating layer. From the viewpoint of preventing oxidation of the substrate 2, nickel, a metallic element with a higher ionization tendency than copper, is used for the plating film formed on the surface of the substrate 2. From the viewpoint of adhesion between the substrate 2 and the plating layer, it is preferable to use electroplating for the nickel plating treatment. Furthermore, the pretreatment for the plating treatment process can be carried out by known methods such as degreasing, acid washing, and water washing. In addition, in electroplated nickel, a plating treatment method using a Watt bath or a sulfamic acid bath can be used, but from the viewpoint of cost, plating treatment using a Watt bath is preferred.

[0102] In electroplating nickel, a brightener is generally added to the plating solution. However, some brighteners contain large amounts of sulfur, which can lead to a large amount of sulfur being incorporated into the nickel plating layer. Sulfur can cause corrosion of the plating layer, which forms the coating layer 3. Therefore, from the viewpoint of suppressing the sulfur content of the nickel plating layer after plating, it is preferable to use a brightener that does not contain sulfur. Furthermore, if it is necessary to use a brightener containing sulfur, such as saccharin, it is preferable to use as little as possible.

[0103] Furthermore, in electroplated nickel, there are additives that incorporate carbon components into the nickel plating film after the plating process. In the metal member 100 of this second embodiment, it is preferable to add such an additive to the plating solution and then perform the plating process. For example, it is preferable to use an additive such as butynediol.

[0104] By incorporating carbon components into the plating film, during the oxidation treatment process (step S03), when the carbon components burn and gasify and escape, the grain growth of nickel oxide particles constituting the ceramic layer 3S is suppressed, making it easier for fine particles to form. Furthermore, as the combustion gas of the carbon components escapes, micropores are more easily formed in the ceramic layer 3S, thus enabling the formation of a coating layer 3 with even better adhesion to the substrate 2.

[0105] Specific examples are described below. [Example 9] The procedure was the same as in Example 1, except that a nickel plating layer was formed on the substrate 2 to a thickness of 8 μm using a plating solution to which 1 mL / L of saccharin, a brightening agent, was added, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 3 μm and a metal layer 5 with a thickness of 5 μm.

[0106] [Example 10] The procedure was the same as in Example 1, except that a nickel plating layer was formed to a thickness of 8 μm using a plating solution to which 3 mL / L of saccharin, a brightening agent, was added, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 3 μm and a metal layer 5 with a thickness of 5 μm.

[0107] [Example 11] The procedure was the same as in Example 1, except that a nickel plating layer was formed to a thickness of 8 μm using a plating solution to which 5 mL / L of saccharin, a brightening agent, was added, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 3 μm and a metal layer 5 with a thickness of 5 μm.

[0108] [Example 12] The procedure was the same as in Example 1, except that a nickel plating layer was formed to a thickness of 8 μm using a plating solution to which 15 mL / L of saccharin, a brightening agent, was added, the coating layer 3 consisted of a ceramic layer 3S with a thickness of 3 μm and a metal layer 5 with a thickness of 5 μm.

[0109] [Example 13] The procedure was the same as in Example 1, except that a nickel plating layer was formed to a thickness of 8 μm using a plating solution to which 20 mL / L of saccharin, a brightening agent, was added, the plating was heat-treated in air at 700°C for 7.5 hours, and the coating layer 3 consisted of a ceramic layer 3S with a thickness of 3 μm and a metal layer 5 with a thickness of 5 μm.

[0110] Next, the metal members 100 obtained in Examples 9 to 13 were evaluated in the same manner as in Examples 1 to 8 and Comparative Examples 1 to 3. Furthermore, the sulfur content in the coating layer 3 was analyzed by GD-OES (Glow Discharge Optical Emission Spectrometry) analysis. The results are shown in Figure 16. The metal members 100 of Examples 9 to 13 have excellent heat dissipation and adhesion to the substrate 2 because they possess a nickel oxide ceramic layer 3S and a diffusion layer 4, alloy layer 8, and metal layer 5, which are effective in improving adhesion. Moreover, it can be seen that Examples 9 to 12, in which the sulfur content of the coating layer 3 is 1000 ppm or less, exhibit even better adhesion of the coating layer 3 to the substrate 2.

[0111] According to the cooking heater of Embodiment 2, the metal element is nickel, and the coating layer has a sulfur content of 1000 ppm or less, so a corrosion-resistant coating layer can be formed. This makes it possible to provide a cooking heater 50 equipped with a highly durable heater 20.

[0112] Furthermore, the metal element is nickel, and the ceramic layer is composed of nickel oxide particles with an average particle size of 1 μm or less and has micropores with an average size of 1 μm or less. This creates an effect that reduces thermal stress due to the difference in thermal expansion with the substrate without reducing the strength of the ceramic layer. As a result, the adhesion between the substrate and the ceramic layer is improved, and a cooking heater 50 equipped with a highly durable heater 20 can be provided.

[0113] Furthermore, the metal element is nickel, and the diffusion layer consists of a copper-nickel alloy layer with a copper-nickel content ratio of 40:60 to 60:40. This ensures that the alloy layer is chemically stable and improves adhesion to the substrate even when the ceramic layer becomes thicker. This makes it possible to provide a cooking heater 50 equipped with a highly durable heater 20.

[0114] While this disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to the application of a particular embodiment, but are applicable individually or in various combinations to the embodiments. Accordingly, countless variations not illustrated are envisioned within the scope of the art disclosed in this specification. For example, these include modifying, adding or omitting at least one component, or extracting at least one component and combining it with a component from another embodiment.

[0115] 100 Metal component, 2 Base material, 3, 3A, 3B Coating layer, 3S Ceramic layer, 4 Diffusion layer, 5 Metal layer, 6 Crystal grain, 7 Void, 8, 8a, 8b, 8c Alloy layer, 50 Cooking heater, 50W Wall surface, 50S Top surface, 51 Heating chamber, 52 Drip tray, 53 Glass top plate, 54 Induction heating section, 55 Housing, 20 Heater, 20A Electric heater, 20B Flat electric heater, 20P Metal pipe, 21 Flat plate.

Claims

In a cooking heater having a heating chamber for containing an object to be heated, Of the metal members constituting the outer surface of the heater that heats the object to be heated and is placed inside the heating chamber, at least the surface facing the object to be heated inside the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material. The coating layer has a ceramic layer mainly composed of an oxide of a metal element with a greater ionization tendency than copper or iron, and at the interface between the substrate and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron. The aforementioned metal element is zinc or nickel in the cooking heater. In a cooking heater having a heating chamber for containing an object to be heated, Of the metal members constituting the outer surface of the heater that heats the object to be heated and is placed inside the heating chamber, at least the surface facing the object to be heated inside the heating chamber consists of a base material mainly composed of copper or iron and a coating layer covering the surface of the base material. The coating layer has a ceramic layer mainly composed of an oxide of a metal element with a greater ionization tendency than copper or iron, and at the interface between the substrate and the coating layer, there is a diffusion layer in which the metal element is diffused into the copper or iron. The aforementioned coating layer is a cooking heater having a metal layer made of the metal element between the substrate and the ceramic layer. The cooking heater according to claim 1 or claim 2, wherein the in-plane thickness variation of the surface of the coating layer facing the object to be heated is ±50% or less at each of the five locations on the heater closest to the four corners of the upper surface of the heating chamber and the intersection of the two diagonals of the upper surface, for a total of five locations. A cooking heater according to any one of claims 1 to 3, wherein the variation in emissivity of the surface of the coating layer facing the object to be heated is ±30% or less at each of the five locations on the heater closest to the four corners of the upper surface of the heating chamber and the intersection of the two diagonals of the upper surface, for a total of five locations. The cooking heater according to any one of claims 1 to 4, wherein the emissivity of the wall surface inside the heating chamber is 20% or less. A cooking heater according to any one of claims 1 to 5, further comprising an alloy layer consisting of copper or iron and the aforementioned metal element within the diffusion layer. The cooking heater according to claim 2, wherein the aforementioned metal element is a metal element with a lower vapor pressure than copper or iron. The cooking heater according to any one of claims 1 to 7, wherein the thickness of the ceramic layer is 0.5 μm or more and 20 μm or less. The cooking heater according to any one of claims 1 to 8, wherein the oxide of the metal element is crystalline. The cooking heater according to any one of claims 1 to 9, wherein the coating layer consists of two layers, and the thermal expansion coefficient of the first coating layer on the substrate side is closer to the thermal expansion coefficient of the substrate than the thermal expansion coefficient of the upper second coating layer. The cooking heater according to claim 10, wherein the emissivity of the second coating layer is higher than the emissivity of the first coating layer. The aforementioned metallic element is nickel, The cooking heater according to any one of claims 1 to 11, wherein the coating layer has a sulfur content of 1,000 ppm or less. The aforementioned metallic element is nickel, The cooking heater according to any one of claims 1 to 12, wherein the ceramic layer is composed of nickel oxide particles with an average particle size of 1 μm or less and has micropores with an average size of 1 μm or less. The aforementioned metallic element is nickel. The cooking heater according to any one of claims 1 to 13, wherein the diffusion layer comprises a copper or iron-nickel alloy layer having a copper or iron-nickel content ratio of 40:60 to 60:

40. The cooking heater according to any one of claims 1 to 14, wherein the heater is an electric heater in which a heating element is built into a metal pipe as the metal member. The cooking heater according to any one of claims 1 to 14, wherein the heater is a flat heater in which the flat plate as the metal member is installed on the side of the heating element to the object to be heated.

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