DE69109940T2 - THROUGH MICROWAVE ABSORPTION HEAT GENERATING MATERIAL. - Google Patents

Abstract

This invention relates to a material which heats and cooks foods from outside by absorbing microwaves of an electronic range and by generating heat. A conductive substance or a nonconductive compound containing transition metals of atomic number 21-29 is mixed in a base material having an Fe group oxide as a principal component. Since the material has a rapid heat generating speed, the temperature in heat-generation is high, and further the material is cheap; it is effective as a material generating heat by absorbing microwaves.

Description

The present invention relates to a microwave absorbing heat generating material which generates heat as a result of absorbing microwave energy and offers excellent heat generating characteristics when used in an electric furnace.

An electron oven is a device in which water molecules and the like contained in a food to be cooked are subjected to frictional vibrations by microwave rays, so that food cooking is carried out by the temperature rise generated by the frictional heat. An advantage of the oven is that food cooking can be carried out in a short time since the food is heated by an internal heat source.

However, since the food is not cooked by conduction or radiation from the surface, as is the case with a gas cooker or an electric resistance heater, the surface of the food cannot be braised (browned) and does not become crispy (roasted). Therefore, these ovens are not suitable for cooking foods whose surface must be browned or crispy, e.g. grilled fish, roasted meat and pizza.

To eliminate this disadvantage, a method for heating food from the surface is known which uses a heat-generating material in an existing electron oven which absorbs microwaves and generates heat, giving the food a browned or crispy appearance.

However, the rate of heat generation of this heat generating material is slow and is generally insufficient for the material to be useful as a heat generating material in an electron oven where rapid food preparation is required. US-A-4 190 757 describes in this connection a heating food package for browning or roasting food wrapped therein when the package is heated in a microwave oven. The package comprises a package body comprising a microwave transparent, non-attenuating dielectric sheet material into which particles of a microwave absorbing material of varying particle size and a binder binding the particles together have been introduced. The particles may be Fe₃O₄ and the binder sodium silicate. Other classes of materials that may be used include zinc oxide, chromium oxide, cobalt oxide, manganese oxide and nickel oxide. The oxides can all be used individually.

It is an object of the invention to provide a new microwave-absorbing, heat-generating material which can heat up in a relatively short time and reach a higher temperature.

According to the present invention, there is provided a microwave-absorbing heat-generating material in the form of a one-piece body comprising an oxide of the Fe group as a base material, characterized in that an electrically non-conductive oxide selected from oxides of transition metals having the atomic number of 21 to 29, excluding iron, is mixed with the base material in an amount of not more than 0.7 mole per mole of Fe, and with which a surface temperature higher than that achieved without the transition metal oxide can be achieved by heating with microwaves.

The invention also provides a method for improving local heat generation at the surface of a A method of heating a foodstuff in a microwave oven comprising operating the oven with a foodstuff and a microwave absorbing, heat generating material according to the invention disposed therein.

The term "Fe group oxide" as used herein means an oxide of Fe as such, e.g. FeO, Fe2O3, Fe3O4, or a complex oxide in which part of Fe is replaced by another metal element.

Thus, the microwave absorbing, heat generating materials of the present invention comprise an electrically non-conductive compound of a transition metal having an atomic number of 21 to 29, excluding Fe, mixed with a base material consisting basically of an oxide of the Fe group, thereby forming a solid product. The amount of the electrically non-conductive oxide present preferably does not exceed 0.5 moles per mole of iron (Fe) in the base material.

In other words, the present invention has found that oxides of transition metals having the atomic number of 21 to 29 generally effectively improve heat generation in Fe group oxide materials.

The production of an oxide of the Fe group of the base material as a mixture with electrically non-conductive oxides, thus obtaining a solid or integral body, can be carried out when the resulting mixture is sintered after molding into a desired configuration. Alternatively, a cold-curing adhesive can be used. In the case of solidification by sintering, the sintering is carried out at about 800 to 1200 ºC to a sufficient degree.

The following three mechanisms are believed to occur in the heat generation by microwave absorption according to the invention, namely heat generation by magnetic loss, dielectric loss and ohmic loss.

Ferrite, which is a prior art microwave absorbing heat generating material, generates heat as a result of microwave absorption through, among other things, magnetic loss. However, the process cannot effectively convert microwave energy into heat energy and thus neither a sufficient level of generation nor a sufficiently high temperature can be obtained.

The reason why the rate of heat generation of the microwave absorbing heat generating material used in the invention is extremely high is presumably because, in addition to the heat generation by magnetic loss, the heat generation by ohmic loss or the heat generation by dielectric loss is lower as a result of the addition of the above-described electrically non-conductive substance.

In addition, even after the release of the reaction heat, further microwave absorption probably occurs based on secondary and tertiary products generated by these reactions and contributes to heat generation.

The following examples illustrate the invention.

example 1

Powders of oxides of Ti, Cr, Mn, Co, Ni, Cu, Er and Ba were added to the Fe₃O₄ powder in an element ratio of 0 to 0.7 mole per mole of Fe. The mixture was subjected to press molding and then sintered at 800 to 1200°C to produce samples with a diameter of 30 mm and a thickness of 2 mm. The samples placed in an electron furnace operating at a frequency of 2450 MHz and an output power of 1500 W were then heated with microwaves for 30 s. The surface temperature of the samples was measured with a radiation thermometer. The measurement results are shown in Fig. 1 of the accompanying drawings.

As can be seen from Fig. 1, except for the oxides of Sr and Ba, a tendency for a temperature increase as a result of the addition of the metal oxide is evident, although on the other hand the temperature becomes lower when the element added in the form of an oxide exceeds 0.7 mole per mole of Fe. It can thus be stated that the use of oxides of a transition metal in the fourth period (atomic number 21 to 29), e.g. Ti, Cr, Mn, Co, Ni or Cu, results in a high temperature characteristic being obtained.

Example 2

Oxide powders of Ti, Cr, Mn, Co, Cu, Sr and Ba were added to the Fe3O4 powder in a metal element ratio of 0.3 mol per mol of Fe. The mixtures were subjected to press molding and then sintered at 800 to 1200°C, and the thus prepared samples with a diameter of 30 mm and a thickness of 1 to 8 mm were heated by microwaves in the electron furnace of Example 1 for 30 s. The surface temperatures were measured by a radiation thermometer. The measurement results are shown in Fig. 2 of the accompanying drawings.

However, when an oxide of Sr or Ba was used, the surface area was actually reduced when the thickness of the heat-generating body was small. However, when a transition metal oxide was used, high performance was obtained even with a thin sample.

Example 3

Metal powders of Ti, Mn and Sr were added to the Fe₃O₄ powder in an element ratio of 0.3 mol to 1 mol Fe. The metal powders obtained by press-forming the mixtures and subsequent Samples prepared by sintering the molded bodies were ground. The ground powder was then coated with an inorganic binder on one side surface of a crystal glass plate (Li₂O-Al₂O₃-SiO₂ group) having high resistance to heat influence and having a diameter of 150 mm and a thickness of 0.5 mm to obtain a heat generating film, and thus a heat generating body was prepared. The coated plates were heated by microwaves in the electron furnace used in Example 1 for 30 seconds, and the surface temperature was measured with a radiation thermometer. The measurement results are shown in Table 1. Table 1 Added elements Average temperature (ºC)

In addition, a commercially available frozen pizza with a diameter of 150 mm was placed on the above-described heat generating body and baked in an electron oven. The baked state of the topping ingredients on the surface of the pizza as well as a pizza crust on the back side were observed. The results of the observations are shown in Table 2. Table 2 Preparation time Added elements inside outside : well baked Δ: insufficiently baked X: over baked

The heat-generating body, which was made by adding Ti or Mn, had a high temperature. When baking a pizza, the pizza crust could be crispy and the whole food could be cooked evenly and had a good taste.

As described in detail above, the heat generating material of the present invention is effective in thawing frozen food and shortening the cooking time because it has a remarkably high heat generating speed as compared with the conventional heat generating material. In addition, a high temperature suitable for cooking can be obtained with a smaller amount of material than the conventional heat generating material, and thus the use is economical.

In summary, the material of the present invention can thus be effectively used in an electron oven as a material for externally heating and cooking food by absorbing microwaves of an electric oven and generating surface heat at the time of cooking in an electron oven.

Claims (5)

1. Microwave-absorbing, heat-generating material in the form of a one-piece body, which comprises an oxide of the Fe group as the base material, characterized in that an electrically non-conductive oxide, selected from oxides of transition metals with an atomic number of 21 to 29, excluding iron, is mixed with the base material in an amount of not more than 0.7 mole per mole of Fe, and with which a surface temperature can be achieved when heated with microwaves which is higher than that achieved without the transition metal oxide. 2. A microwave absorbing heat generating material according to claim 1, which contains not more than 0.5 mole of the transition metal oxide per mole of Fe. 3. Microwave absorbing heat generating material according to claim 1 or 2, which is in the form of a sintered product. 4. A microwave absorbing heat generating material according to claim 1 or 2, which is in the form of a solid body which is given integrity by introducing an adhesive material. 5. A method for improving local heat generation on the surface of a food heated in a microwave oven, comprising operating the oven with the food and a microwave-absorbing, heat-generating material according to any preceding claim disposed therein.