A heating element and a method for manufacturing the same

Abstract: A heating element (1) for a cooking apparatus or a heating apparatus is manufactured by forming aluminium layers on a nickel or nickel-base alloy base or forming nickel layers and then aluminium layers on a metal base (2), and by oxidizing the aluminium layers into an aluminium oxide layers. At the oxidizing step, nickel-aluminium alloy layers with high corrosion resistance are also produced. Only aluminium layers, not nickel layers, may be formed on a metal base and oxidized into aluminium oxide layers to obtain a heating element with relatively low but practical corrosion resistance. The resultant heating element by any of the above processes which does not require a cover or other protection against corrosion is formed into a sheet and heated to provide radiation of about 800°C.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a heating element used for a cooking apparatus and a heating apparatus at temperatures of around 800°C, and a method for manufacturing the same. 2. Description of the Prior Art: As a heating element for a cooking apparatus and a heating apparatus, a silica tube heater has been conventionally used. A sheath heater and a "mica heater" where a heating wire is wound around a mica strip have also been used for a cooking apparatus. However, the above conventional heating elements have problems as follows. In the case of the silica tube heater where a heating wire wound in a toroidal shape is inserted into a silica tube, when the heater is used for cooking, for example, substances such as salt spattered from an object to be cooked may attach to the surface of the silica tube. As a result, the silica tube is likely to be devitrified, failing to provide a sufficient amount of extreme infrared radiation required for cooking, or the silica tube may even become broken. Thus, only a temperature of 700°C at highest is obtained from this heater. Further, it is difficult to form the silica tube heater into a sheet due to the structure thereof. In the case of the sheath heater where a heating wire is covered with a thermal resistant and corrosive resistant stainless steel plate, when used for cooking, the heater utilizes secondary radiation from the stainless steel sheath. This structure requires a longer time until a predetermined temperature is attained. Further, in order to obtain the radiation of 800°C, the temperature of the heating wire itself must be more than 800°C, which will shorten the life of the heating wire. Moreover, as in the case of the silica tube heater, it is difficult to form the sheath heater into a sheet due to the structure thereof. The mica heater can easily be formed into a sheet. However, since the heater is installed through an insulation on the outer surface of the wall of a cooking apparatus, heat conduction is not effective. Therefore, a longer time is required until a predetermined temperature is attained on the inner surface of wall of the cooking apparatus from which heat is radiated for cooking, and also it is difficult to obtain the radiation of a high temperature of about 800°C. To overcome the above-described problems, a bare metal heating element made of iron-chrome-aluminium, nickel-chrome or iron-nickel-chrome can be used to serve as a heating element which can be formed into a sheet and which can be rapidly and easily heated to provide radiation of 800°C suitable for a cooking apparatus and a heating apparatus. The bare metal heating element is highly thermal resistant and will not be damaged when exposed in an atmosphere of high temperature. However, when used for a cooking apparatus, corrosive substances such as salt spattered from an object to be cooked may attach to the metal heating element. If such a contaminated metal heating element is used under the condition of a temperature as high as 800 C, it will easily be corroded and damaged. The structures of the afore-mentioned heating elements had originally been developed to prevent such corrosion. To obtain a heating unit with stable organisation and bigger oxidation resistance in high temperature air JP-A-52-108 532 discloses a process for manufacturing an electric heating material in which after reducing an area of a compound material, the core material of which is a Fe-Cr-plate, both sides of it are covered by thin Al-plates by rolling process. The Al-plate of the area reduction process material is heated to diffuse into the core material. This electric heating material can also be obtained by applying a blanking process. Furthermore, it is already known a process for the production of a foil thermocouple having an elongate shape which is bent in the plane of the thermocouple. This process as known from GB-A-1 222 504 comprises forming by deposition on a metal foil a layer of a different metal, the junction between the two metals constituting the thermo-junction of the thermocouple, and the metal foil being provided with the elongate shape either before or after the deposition. This thermocouple may be made from any standard thermocouple materials which allow the deposition of one material on the other. Thus, for example, the thermocouple may be made by depositing nickel-chrome on nickel-aluminium foil. The objective of the present invention is to provide a heating element which is not only highly corrosive resistant at high temperature, but also can be formed into a sheet, and can be rapidly and easily heated to provide heat radiation of about 800°C. SUMMARY OF THE INVENTION The heating element of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a metal base having nickel-aluminium alloy layers formed on the top and bottom surfaces thereof. A further invention is a method for covering the nickel or nickel base alloy with aluminium oxide layers. In another preferred embodiment, the metal base is nickel or a nickel-base alloy. In still another preferred embodiment, the nickel content in the nickel-base alloy is at least 50 %. Alternatively, the heating element of this invention comprises a metal base having nickel layers and aluminium oxide layers formed in this order on the top and bottom surfaces thereof. In a preferred embodiment, the aluminium layers or the aluminium alloy layers are formed by cladding. According to another aspect of the present invention, the method for manufacturing a heating element comprises the steps of forming aluminium layers on the top and bottom surfaces of a nickel or nickel-base alloy base, and oxidising the aluminium layers into aluminium oxide layers. In a preferred embodiment, the aluminium layers are formed by cladding the nickel or nickel-base alloy base with aluminium foils on both surfaces thereof. In another preferred embodiment, the method further comprises the step of cutting the nickel or nickel-base alloy base clad with the aluminium foils to a predetermined shape between the step of forming the aluminium layers and the step of oxidising the aluminium layers. In still another preferred embodiment, the aluminium foils are wider than the nickel or nickel-base alloy base. In still another preferred embodiment, the nickel content in the nickel-base alloy base is at least 50%. Alternatively, the method of this invention comprises the steps of forming nickel layers on the top and bottom surfaces of a metal base, forming aluminium layers on the nickel layers, and oxidizing the aluminium layers into aluminium oxide layers. In a preferred embodiment, the nickel layers and the aluminium layers are formed by cladding. In another preferred embodiment, the method further comprises the step of cutting the metal base clad with the nickel layers and the aluminium layers to a predetermined shape between the step of forming the aluminium layers and the step of oxidizing the aluminium layers. The heating element of the present invention with high corrosion resistance at high temperature can be used in a bare form without the necessity of providing a sheath, as required in the conventional heating elements, to protect against, for example, attachment of corrosive substances spattered from an object to be cooked. Further, the heating element without a sheath can easily be formed into a sheet heating element by stamping it into a desired pattern or the like. Also, since the specific heat of the heating element of the present invention can be small without an extra thickness for a sheath, the heating element can rapidly be heated to a high temperature of, for example, 800°C, and thereby radiation of high temperature is easily obtained. Thus, the present invention makes possible the objective of providing a heating element which can be formed into a sheet and which can be rapidly heated so as to provide radiation of high temperature. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a sectional view of a heating element of the present invention; Figure 2 is a sectional view showing a pretreated state of the heating element of Figure 1; Figure 3 is a chart showing the relationship between the nickel content and the corrosion resistance of the heating element of Figure 1; Figure 4 is a schematic sectional view of a cooking apparatus using the heating element of Figure 1; Figure 5 is a sheet heating element of the present invention stamped into a meander shape; Figure 6 is a sectional view of another heating element of the present invention; and Figure 7 is a sectional view showing a pretreated state of the heating element of Figure 6. DESCRIPTION OF THE PREFERRED EMBODIMENT Example 1 Referring to Figure 1, a heating element 1 comprises a metal base 2 made of nickel or a nickel-base alloy having aluminium oxide layers 3 and nickel-aluminium alloy layers 4 formed in this order on the top and bottom surfaces thereof. The heating element 1 of the above structure is formed by the following procedure. As shown in Figure 2, aluminium layers 5 are formed on both surfaces of the metal base 2 by spraying, plating, evaporation, spattering, cladding or any other appropriate method. When spraying or cladding is employed, a thicker layer is comparatively easily obtained. Especially, by cladding where an aluminium foil is rolled on and tightly attached to a heating element, an aluminium layer which is free from pinholes can be formed. Then, the aluminium layers 5 are oxidized into the aluminium oxide layers 3 with high corrosion resistance. The aluminium layers 5 can be oxidized by various methods such as anodic oxidation. Most practically, it is heated at high temperature in the atmosphere. When this method is employed, the nickel-aluminium alloy layers 4 which are also highly corrosive resistant can easily be formed between the metal base 2 and the aluminium oxide layers 3, thereby providing the resultant heating element with the property of high corrosion resistance. A heating element of the above structure was manufactured to assess the corrosion resistance as follows. A nickel or an alloy containing a varying content of nickel (chrome content: 21 ± 2%) was used as the metal base 2. Then, the metal base 2 was clad with aluminium foils on both surfaces thereof. The resultant aluminium-clad base was cut into a size required, and then heated at 900°C in the atmosphere for five hours so as to produce the aluminum oxide layers 3 on both surfaces thereof. In this way, the heating element 1 having a width of 6 mm and a thickness of 50 µ was obtained. Electricity was applied to the heating element 1 to raise the temperature of the surfaces thereof to 800°C. Under this condition, a 5% saline solution was dropped by 0.5 cc to the heating element 1 every two minutes, and the number of drops applied until the heating element 1 was corroded and cut apart was used to determine the level of the corrosion resistance of the heating element 1. This test was performed for ten samples, of which results are shown in Figure 3. In Figure 3, each two-direction arrow shows the range of the numbers of drops counted for the samples of each nickel content, and each mark · shows the average thereof. Each mark X shows a result obtained from a sample of the metal base without cladding of an aluminium foil. As is apparent from this figure, the corrosion resistance sharply increases when the nickel content exceeds 50%. This is because, with such a high nickel content, the corrosive resistant nickel-aluminium alloy layers 4 were further formed between the metal base 2 and the aluminium oxide layers 3. This formation was confirmed by analyzing the heating element 1 by an X-ray diffraction method, where NiAl and Ni₃Al were detected. In the above heating element 1, the aluminium layers 5 were formed on the top and the bottom surfaces of the metal base 2, but not on the side faces thereof (in the direction of the thickness thereof). However, after the heat treatment, the formation of the nickel-aluminium alloy layer 4 was also observed on the side faces. This is probably because burrs of the aluminium foils produced at the aforementioned cutting process after the cladding of the aluminium foils were spread onto the side faces of the metal base 2. Further, in the corrosion resistance test, most of the sample heating elements were finally cut apart by corrosion in the portions except the cut side ends thereof, which shows that the cut side ends are not especially inferior in corrosion resistance. When the base metal clad with the aluminium foils is not cut to a size of the heating element, but used as it is as a heating element, the aluminium foils can be slightly wider than the metal base, so that the nickel-aluminium alloy layer can also be formed on the side faces of the base metal as in the case described above. From the result of the aforementioned test, it is proved that the method of the present invention significantly improves the corrosion resistance of the resultant heating element. Next, an application of the above-described heating element 1 to a cooking apparatus will be described as follows. Figure 4 shows a microwave oven 6 which has a structure of a sheet heating element 7 comprising the heating element 1 of the invention being disposed on support members 9 placed on a microwave shielding plate 8. A control panel and other components of the microwave oven 6 not directly relating to this invention are not shown in the figure for simplicity. The microwave shielding plate 8 is made of a corrosive resistant stainless steel plate having a number of bores of a diameter of about 3 mm so that the total area of the bores should occupy about 60% of the total area of the plate. The microwave shielding plate 8 has two functions; shielding the sheet heating element 7 from receiving microwave radiated during microwave cooking and passing radiation from the heating element directly onto an object to be cooked during heat cooking. A nickel-chrome strip (NCHRW1) for heating was used as the metal base 2, which was clad with aluminium foils on both surfaces thereof. Then, the resultant strip was stamped into a meander shape and heated at 900°C in the atmosphere for five hours to produce the sheet heating element 7 having an output of 1.2 KW. When a rated voltage of 100 V was applied to the heating element 7, the temperature of the heating element reached 700°C in about one minute, and further rose to 800°C in three minutes, An object to be cooked, for example, a fish, was put in an oven chamber 10 for heat cooking. The fish was uniformly broiled under the radiation of 800°C for fifteen minutes. When a conventional sheet heating element, for example the mica heater, was employed to broil the fish, 25 minutes were required. Further, in any of the mica heater, the sheath heater and the silica tube heater, uniform browning as attained in the heating element of the invention was not possible. As described above, the sheet heating element 7 of the invention effects uniform browning over the object to be cooked, because the heating element 7 is formed as a sheet and the radiation from the sheet heating element 7 to the object is intensive. That is, since the radiation is sent in the direction vertical to the surface of the heating element 7, in the case of a strip heating element as shown in Figure 5, the radiation is theoretically sent only to an area right below the width A of the heating element 7. (The radiation in the horizontal direction along the heating element is not counted in this case since the thickness of the heating element is negligibly small compared with the width thereof.) Actually, however, the radiation is somewhat diffused since the surface of the heating element is uneven, which as a result, enables the uniform browning of the object to be cooked. Thus, in the case of a strip heating element, radiation is more intensive in the direction vertical to the heating element, and therefore effective heating can be accomplished by placing the object to be cooked in this direction. In the case of the sheath heater or the silica heater where a heating element with a sheath is round in section, heat is radiated to every direction in the space. Therefore, the amount of radiation reaching the object to be cooked is small, that is, the heating efficiency is low when the sheath heater or the silica heater is used compared with when the strip heating element is used, under the condition that both heating elements have the same output and the same temperature of radiation. In the above-described structure of the microwave oven 6, salt or other substances spattered from an object to be cooked may attach to the surface of the heating element 7. However, this will not cause any practical problem for the heating element 7 of the invention because it is highly corrosive resistant. The corrosion resistance of the heating element 7 was tested by repeating a cycle of alternating the microwave cooking of a 5% saline solution and the heat cooking of a fish. The result was that the heating element 7 of the invention had been little damaged after 500 cycles, while a sheet heating element made of the nickel-chrome strip stamped without the treatment according to the invention had been corroded and damaged after 60 cycles. From the above results, it was found that the heating element of this example had high corrosion resistance and was little deteriorated after a long period of usage. Example 2 Another example of the present invention will be described with reference to Figures 6 and 7 as follows. Referring to Figure 6, a heating element 11 comprises a metal base 12 having nickel-base layers 13 and aluminium oxide layers 14 formed in this order on the top and bottom surfaces thereof. The metal base 12 is made of iron-chrome, iron-chrome-aluminium or stainless steel such as SUS 430, SUS 430A or SUS 444. The heating element 11 of the above structure is formed in the following procedure. First nickel layers 15 and then aluminium layers 16 are formed on both surfaces of the metal base 12 by spraying, plating, evaporation, spattering, cladding or any other suitable method, as described in Example 1. Then, the aluminium layers 16 are oxidized into the aluminium oxide layers 14 most practically by heating at high temperature in the atmosphere. At the same time, layers of the nickel-aluminium alloy can be formed, as described in Example 1. The nickel-aluminium alloy layers can be sufficiently formed by heating at 500 to 700°C in an environment of an inactive gas, not in the atmosphere, and then heating at a temperature of 800°C or higher in the atmosphere. The nickel-base layers 13 of this example are the nickel layers 15 wholly or partly changed to the nickel-aluminium alloy. The aluminium layers 16 should be completely oxidized because, during the use of the heating element, any unoxidized aluminium which is conductive remaining in the heating element is exposed to high temperature and oxidized into aluminium oxide which is insulating, causing a gradual increase of resistance of the heating element. The heating element of the above structure was manufactured to assess the corrosion resistance as follows. First, the metal base 12 made of iron-chrome (FCHRW1) was clad with nickel foils and then aluminium foils on both surfaces thereof. The total thickness of the clad base was 50 µ with 42 µ for the metal base 12, 4 µ for the two nickel layers and 4 µ for the two aluminium layers, and the width thereof was 6 mm. The clad base was then heated at 900°C in the atmosphere for five hours so as to produce the aluminum oxide layers 14 on both surfaces. The formation of the nickel-aluminium alloy layers between the metal base 12 and the aluminium oxide layers 14 was confirmed by an X-ray diffraction method. Electricity was applied to the thus obtained heating element 11 to raise the surface temperature thereof to 800°C. The same corrosion resistance test as that in Example 1 was performed. As a result, the number of drops applied to the above heating element 11 until it was corroded and cut apart was 80. The same test was also performed on the following three comparative samples; a heating element made of iron-chrome and heated at 900°C in the atmosphere for five hours, a heating element made of SUS 430 and SUS 444 on which the same steps of cladding of nickel and aluminium and of thermally oxidizing the aluminium as in Example 2 were performed, and a heating element made of the same material as the above on which the step of cladding of nickel and aluminium was not performed but the step of heating was performed. The results were 10, 80, and 10, respectively. These results indicate that the corrosion resistance at high temperature shown in the heating element of this invention is irrespective of the type of the base metal. Next, an application of the above-described heating element 11 to the microwave oven 6 shown in Figure 4 will be described as follows. An iron-chrome strip (FCHRW1) for heating was used as the metal base 12, which was clad with nickel foils and aluminium foils on both surfaces thereof. Then, the resultant strip was stamped into a meander shape and heated at 900°C in the atmosphere for five hours to produce a sheet heating element 7′ having an output of 1.2 KW. When a rated voltage of 100 V was applied to the heating element 7′, the temperature of the heating element reached 700°C in about one minute, and further rose to 800°C in three minutes. An object to be cooked, for example, a fish, was put in the oven chamber 10 for heat cooking. The fish was broiled with uniform browning under the radiation of 800°C for fifteen minutes. The same corrosion resistance test as in Example 1 was performed on the microwave oven comprising the sheet heating element 7′ of this example, and the same favorable results as in Example 1 were obtained. From the above results, it was found that the heating element of this example had high corrosion resistance and was little deteriorated after a long period of usage. In the aforementioned examples, the strip-shaped metal base was used. However, the metal base applicable to the present invention is not limited to the strip shape, but a linear metal base can also be used to obtain the same high corrosion resistance when the structure and the method of the present invention is applied thereto. Example 3 In this example, a metal base made of iron-chrome, iron-chrome-aluminium or stainless steel such as SUS 430, SUS 430A or SUS 444 was used. On the top and bottom surfaces of the metal base were formed aluminium layers in the same manner as shown in Example 1. The aluminium layers were then oxidized into aluminium oxide layers in the same manner as shown in Example 1. The same test as in Example 1, that is, dropping a 5% saline solution was performed on the resultant heating element, and the result was 20 drops until the heating element was corroded and cut apart, indicating that the corrosion resistance of the heating element of this example was about twice as high as that of a heating element lacking an aluminium oxide layer. It was also found that when the aluminium layers were formed by cladding of aluminium foils on the metal base as in Examples 1 and 2, the resultant heating element showed stable results in the corrosion resistance test. The heating element obtained in this example was formed into a sheet heating element in the same manner as in Example 1 for the application to a microwave oven, and the same test as in Example 1 was performed. After 500 cycles, some small corrosion was observed on the surface of the heating element, but such corrosion did not cause inferiority in the cooking ability nor any other practical problems. In the above examples, the aluminium layer was formed and oxidized in order to obtain the aluminium oxide layer. It should be appreciated that an aluminium-base alloy can also be used instead of aluminium to attain the above objective.

Claims: A heating element comprising a metal base (2, 12) having nickel-aluminium alloy layers (4) formed on the top and bottom surface thereof. A heating element according to claim 1, wherein the nickel-aluminium alloy layers (4) are covered with aluminium oxide layers (3, 14). A heating element according to claim 1 or 2, wherein the metal base (2, 12) is nickel or a nickel-base alloy. A heating element according to claim 3, wherein the nickel content in the nickel-base alloy is at least 50%. A method for manufacturing a heating element (1, 7, 7′, 11) comprising the steps of: forming aluminium layers (5, 16) on the top and bottom surfaces of a nickel or nickel-base alloy base (2, 12), and oxidizing the aluminium layers (5, 16) into aluminium oxide layers (3, 14). A method for manufacturing a heating element (1, 7, 7′, 11) according to claim 5, wherein the aluminium layers (5, 16) are formed by cladding the nickel or nickel-base alloy base with aluminium foils. A method for manufacturing a heating element (1, 7, 7′, 11) according to claim 6, further comprising the step of cutting the nickel or nickel-base alloy base clad with the aluminium foils to a predetermined shape between the step of forming the aluminium layers (5, 16) and the step of oxidizing the aluminium layers (5, 16). A method for manufacturing a heating element (1, 7, 7′, 11) according to claim 6, wherein the aluminium foils are wider than the nickel or nickel-base alloy base. A method for manufacturing a heating element (1, 7, 7′, 11) according to any of claims 5 to 8, wherein the nikkel content in the nickel-base alloy base is at least 50%. A heating element comprising a metal base (2, 12) having nickel layers (15) and aluminium oxide layers (3, 14) formed in this order on the top and bottom surfaces thereof. A method for manufacturing a heating element (1, 7, 7′, 11) comprising the steps of: forming nickel layers (15) on the top and bottom surfaces of a metal base (2, 12), forming aluminium layers (5, 16) on the nickel layers (15), and oxidizing the aluminium layers (5, 16) into aluminium oxide layers (3, 14). A method for manufacturing a heating element (1, 7, 7′, 11) according to claim 11, wherein the nickel layers (15) and the aluminium layers (5, 16) are formed by cladding. A method for manufacturing a heating element (1, 7, 7′, 11) according to claim 12, further comprising the step of cutting the metal base (3, 12) clad with the nickel layers (15) and the aluminium layers (5, 16) to a predetermined shape between the step of forming the aluminium layers (5, 16) and the step of oxidizing the aluminium layers (5, 16).

Inventors: MAKI MASAO, YAMASHITA KAZUO, MAKI, MASAO, YAMASHITA, KAZUO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H05B3
IPC codes: H05B3