Method for Producing a Microwave Absorbing Heater

Abstract: A microwave absorbing heater comprising a porous body containing silicon oxide having a porosity of 40 to 95%. A microwave absorbing heater comprising: a porous body having a porosity of 40 to 95% and composed of an inorganic electrical insulating material; and a silicon carbide film formed on the surface thereof. Since the microwave absorptivity is high, the dissipation of water from a heated object is easy and the heat dissipation is small, the heating efficiency is great. The thermal shock resistance is also high.

Description: The present invention relates to a method for producing a microwave absorbing heater which evolves heat by absorbing microwave. Particularly, it relates to a method for producing a heater which is excellent in the heat shock resistance and evolving characteristic. More particularly, it relates to a method for producing a heater which facilitates the diffusion of the vapor generated from the surface of a cooking material by heating and the scorching of the surface of the cooking material. By utilizing the phenomenon that a material is heated by absorbing a microwave, lumbers, cloths or plastics are dried and processed. Such materials are dried and processed mainly by utilizing the dielectric heating of the dipoles existent in the materials which are rocked by an alternating electric field produced by the microwave and are heated by the friction between molecules. Most of such microwave heating is based on the heat evolution of water molecules which are existent in the heated object and have a dipole moment. Therefore, microwave heating is generally used for heating or drying a material containing water. However, it is impossible to heat an object containing water molecules to a temperature of higher than 100°C due to the latent heat of vaporization, and when the water existing in the heated object is vaporized, since the water as a heating source is lost, the heating operation becomes gradual and the temperature of the heated object does not rise. In other words, it is impossible to heat an object to a temperature higher than 100°C merely by irradiating the object containing water with a microwave. Therefore, an object is conventionally heated to a high temperature by using a dielectric or a magnetic material which absorbs a microwave and evolves heat as a heater and bringing the object into contact with the heater or utilizing the radiant heat of the heater. As the heater, porcelains having heat resistance such as lead titanate porcelains, ferrite porcelains or soda-lime glass is conventionally used. Such conventional heaters, however, have problems in practical use. For example, they are poor in shock resistance, they are apt to produce a cracking by spattering of water during heating, and since the vapor produced from the surface of a cooking material remains on the surface thereof, it is difficult to scorch the surface. Although use of silicon carbide, which has an excellent shock resistance in spite of a poor dielectric heating as compared with e.g. ferrite has been investigated, since silicon carbide is difficult to mold or form, it suffers from various problems in producing a practical product. In addition, since the microwave absorptivity is not so excellent, the retention of water on the surface of a cooking material is a serious problem, and the improvement of a heating characteristic is demanded. EP-A-0432794 discloses a silicon carbide composite structure comprising a silicon carbide matrix having a specific resistivity higher than 10⁴ ohm/cm, and ceramic fibers or whiskers reinforcing said silicon carbide matrix. The silicon carbide is deposited in, on or around the set of the fibers or the whiskers by a chemical vapor deposition. This composite structure is used for cooking in a microwave oven, which is self-heated by dielectric absorbing the microwave energy radiated in the microwave-oven, and radiates an infrared radiation to the surface of the materials to be cooked. JP-A-3067489 discloses a microwave absorbing heating element comprising a high porous ceramic body and constituent particles, the ceramic body being coated with a microwave absorbing exothermic layer comprising silicon carbide by a bisquit-firing process. The element is heated into a red heat condition by absorbing microwaves. It is an object of the present invention to provide a method for producing a microwave absorbing heater which absorbs a microwave with good efficiency, easily diffuses the water content on the surface of a cooking material so as to easily scorch the surface of the cooking material. It is another object of the present invention to provide a method for producing a microwave absorbing heater which has an improved molding and forming processability, which has sufficient resistance to the thermal shock caused by spattering of water and which is unlikely to be broken in ordinary handling. The above objects have been solved by providing a method for producing a microwave absorbing heater consisting of porous silicon carbide having a porosity of 40 to 95%, said method comprises: a) carbonizing urethane foam to produce porous carbon having pores, b) producing silicon carbide on the inner surface of said pores and on the outer surface of said porous carbon by chemical vapor deposition, and c) removing said carbon by combustion. Since the microwave absorbing heater produced by the method of the present invention consists of silicon carbide and is a porous body having a large porosity, it prevents the vapor produced on the surface of a cooking material from remaining on the surface and it is excellent in thermal shock resistance. In addition, since the heat capacity is small, the microwave absorbing efficiency is high and the heat dissipation is small, the heating efficiency is prominently great. Since the heater itself is a porous body, the shock resistance is much superior to that of a heater made of a dense sintered body, and it is possible to provide a microwave absorbing heater having a thermal shock resistance (ΔT) of not less than 400°C. It is therefore possible to use a microwave absorbing heater safely for various uses without being broken. If the porosity of the porous body is less than 40%, the water produced on the surface of a cooking material remains on the surface, so that it takes a long time to scorch the surface and, in the worst case, the surface becomes soppy as the surface of boiled food. On the other hand, of the porosity exceeds 95%, the mechanical strength is insufficient for practical use. According to the present invention silicon carbide is deposited on the surface of the porous carbon by CVD and thereafter the carbon is removed by combustion. Preferably, the CVD is carried out as follows : Methyltrichlorosilane as Si and C source is caused to flow as a material gas and SiC is deposited on the surface (the outer and inner surfaces and the inner walls of the pores) of the porous carbon which is maintained at a temperature of about 1,000°C. Since the heater produced by the method of the invention consists of silicon carbide, it facilitates the diffusion of the vapor from a heated surface and removal of the water from the heated hood, thereby enabling the surface of the cooking material to be quickly scorched. In addition, since the microwave absorbing efficiency is high, the heat capacity is small and the heat dissipation is small, the microwave absorbing heater efficiently evolves heat by the irradiation of a microwave and it has a high thermal shock resistance. The present invention will be explained in more detail with reference to the following example. Example Porous bodies (Samples Nos. 1 to 3) of silicon carbide having the respective porosities shown in the Table were produced as follows : Urethane foam was carbonized to produce porous carbon. Si and C source such as methyl trichlorosilane was supplied as a material gas to the porous carbon which was maintained at 1,000°C to produce silicon carbide on the inner surface of the pores and the outer surface of the porous carbon. Thereafter, carbon was removed by combustion, thereby obtaining a porous body of silicon carbide having a predetermined porosity. Each of the porous bodies of silicon carbide was irradiated with a microwave at an output of 500 W for 2 minutes and the temperature of the surface was measured. The presence or absence of a crack during and after heating (including the case in which water is spattered), and the state of cooked food such as meat and vegetable which was irradiated with the microwave for 2 minutes on the porous body of silicon carbide was observed. The results are shown in the Table. As it is obvious from the table, in case of the porous body having a porosity of 97%, the mechanical strength was low and crack was produced, so that practical use thereof was impossible. In contrast, the porous bodies having a porosity in the range of from 40 to 95 % had a sufficient mechanical strength and the cooked food was scorched.

Claims: A method for producing a microwave absorbing heater consisting of porous silicon carbide having a porosity of 40 to 95%, said method comprises: a) carbonizing urethane foam to produce porous carbon having pores, b) producing silicon carbide on the inner surface of said pores and on the outer surface of said porous carbon by chemical vapor deposition, and c) removing said carbon by combustion. The method of claim 1, wherein said chemical vapor deposition is carried out by using methyl trichlorosilane.

Inventors: FUJIKAWA TOSHITAKA C O MITSUBI, IIDA KEIICHI C O MITSUBISHI MA, INUKAI TAKAO C O MITSUBISHI MA, KOGURE NOBORU C O MITSUBISHI M, TACHIBANA KANICHI C O MITSUBIS, TSUKADA KAZUO C O MITSUBISHI M, FUJIKAWA, TOSHITAKA, C/O MITSUBISHI MATERIALS CORP, IIDA, KEIICHI, C/O MITSUBISHI MATERIALS CORP., INUKAI, TAKAO, C/O MITSUBISHI MATERIALS CORP., KOGURE, NOBORU, C/O MITSUBISHI MATERIALS CORP., TACHIBANA, KANICHI, C/O MITSUBISHI MATERIALS CORP., TSUKADA, KAZUO, C/O MITSUBISHI MATERIALS CORP.
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1991
Main classification: H05B6
IPC codes: B65D81, C09K5, C01B31, H05B6