GB2292873A

GB2292873A - Browning device for microwave oven

Info

Publication number: GB2292873A
Application number: GB9516700A
Authority: GB
Inventors: Manfred Kirschvink
Original assignee: EUPEN KABELWERK; Kabelwerk Eupen AG
Current assignee: EUPEN KABELWERK; Kabelwerk Eupen AG
Priority date: 1994-09-03
Filing date: 1995-08-15
Publication date: 1996-03-06
Also published as: FR2724101A1; FR2724101B3; GB9516700D0; DE4431475A1; DE4431475C2

Description

BROWNING DEVICE 2292873 The invention concerns a browning device used in

the cooking chamber of a microwave oven operated at conventional frequency and consisting of a dish or plate. This device, referred to generally as a "browning dish", is an accessory for a conventional microwave oven in which browning effects cannot be achieved without this device. The dish or plate has a preferably circular metal base part which has good thermal conductivity, the underside of which has a preferably circular layer of a material which absorbs high frequency waves. The browning device can be positioned in the cooking chamber of the microwave oven such that a parallel distance can be maintained between the underside of the layer on the metal base part and the top of the floor of the cooking chamber.

Such browning devices, the function of which depends on the absorption of microwave energy and its conversion into heat, are generally known.

DE 31 50 619 C2 describes a device for heating cooking products by means of microwave energy, where the device has a lower container part with metal wall sections. An upper container part attached by hinges to the lower container part also has metal wall sections. On the outside of the upper wall of the container upper part supporting itself on the product is a ferrite-containing body which releases heat under microwave radiation. In order to make optimum use of the heat released from the microwave-absorbing material in the area of the upper part of the product in order to heat the product, it is proposed that peripheral edges are fitted to the metal container parts and that unevennesses be created on the surface of the upper wall of the upper metal container part facing the product.

Browning devices are already available which consist of ceramic material and which depend for their function purely on dielectric losses. To this end, embedded in the heat resistant dielectric carrier material is a layer with very high losses. These browning devices are however very difficult to produce and are also effective only for a short time.

The browning device initially described has various advantages: it is easier to coat the underside of the metal base part with material which absorbs high frequency waves than to embed a high loss layer in ceramic material. A suitable material for the coating is a mixture of a ferrite powder and a binding agent. In the microwave oven, the use of ferrite causes an intrinsic temperature limitation due to the fading of the magnetic properties when the temperature is reached. The ferrite coating on underside of the metal base part prevents contact between the cooking product and the ferrite material and thus Curie the excludes any influence on the cooking product which could be associated with health risks.

In order however to achieve an adequate browning ef f ect on the cooking product, the microwave energy must be concentrated below the browning device. To achieve this concentration, essentially two solutions are normally applied in which either the microwave oven is specially designed such that at least one microwave inlet opening on the side of the cooking chamber is arranged on a plane below the browning device, where usually another microwave inlet is necessary for other uses of the microwave oven, or in which the browning device is fitted with additional aids in order to be placed at a certain level inside the cooking chamber such that a concentration of microwave energy below the browning device is guaranteed. Depending on the level required for the arrangement of the browning dish, the usable cooking space is considerably reduced.

DE 32 15 174 C2 describes a device for preparing pizza and similar meals in a microwave oven. This oven is fitted with a microwave-conducting plate supported on the floor with a microwave-absorbing layer of material attached to the underside of the plate which heats on contact with microwaves, and with a microwave-conducting lid supported on the base. To ensure an even temperature distribution over the cooking product and to protect against excessively high temperatures in the area of the product support, the base has support f eet Pointing downwards which hold the base of the plate at a distance from the floor of the microwave oven. The base also has a central opening in the vertical direction, and the plate has projecting side walls. These partly overlap with but are electrically isolated from the downward-pointing edge of the lid. Thus a certain quantity of microwave energy from the inside of the microwave oven is directed into the space enclosed by the plate and the lid.

The invention is based on the task of producing a browning device of the type initially described such that the brown ing device can be placed in the cooking chamber of a microwave oven of any design operated at conventional frequency and guarantee a good browning effect. In particular, there is no need for elther a complicated height adjustment of the browning device in the cooking chamber or an additional microwave inlet and/or one placed at a certain level in relation to the floor of the cooking chamber.

Claims

To solve this task, the invention is based on a browning device of the

type described in the preamble of Claim 1 and it is proposed that:

the parallel distance 11P11 between the underside of the layer (6) of the metal base part (4) and the top of the cooking chamber floor (8) is up to 6 mm, - the diameter I'D" of the surf ace of the metal base part (4) conforms to the floor area of the cooking chamber (8) and the diameter Ildl of the surface of the layer (6) is at most 20 mm smaller than diameter I'D", and the thickness "H" of the layer (6) is 0. 5 - 3 mm and the material of the layer (6) consists of ferrite powder and a binding agent, where the ferrite powder proportion is at least 40 vol %, preferably at least 60 vol %.

The optimum function of the browning device is determined according to the invention exclusively by its parallel distance P, diameters D and d and thickness H and the material properties of the layer. Only these parameters can affect the E- field distribution such that good browning effects can be achieved within short cooking times. In particular, a high permittivity and permeability of the coating material can have a positive influence on the distribution of microwave energy in the cooking area below the browning device.

The drawing shows a design example of a browning device according to the invention, described in more detail below:

As Fig. 1 shows, the browning device consists of a circular dish 1 which according to Fig. la is placed in a cuboid microwave oven 2 shown in diagram form which has a side microwave inlet 3.

Dish 1 is formed from a single part with a circular metal base part 4 and a metal casing part 5 extending conically upwards. on its underside, the metal base part 4 has a coating 6 consisting of ferrite powder and a binding agent. Dish 1 also has three feet 7, on which the browning device is positioned securely on the floor 8 of the cooking chamber 9 defined by the microwave oven
2.

The cooking chamber 9 acts as a resonant chamber which is supplied with microwave energy through the microwave inlet 3 designed as a wave guide. A cavity effect is created in the form of an E-110 wave, where the E-field vector lies vertical to floor 8 of the cooking chamber 9.

In order to describe this arrangement with a circular dish 1 in precise arithmetic terms, the cuboid resonant chamber is transformed into a cylindrical resonant chamber as shown in Fig. 2. This also gives a resonant chamber with an E-field vector vertical to the base of the resonant chamber, since the cavity, as in Fig. 1, is also supplied with microwave energy through a wave guide arranged in the side in the form of microwave inlet
3.

In one position of dish 1 at the height of the microwave inlet 3, it is conceivable that a wave will propagate itself along dish 1 in the plane of axis llrll. In this model position, the dish 1 forms an element of a so-called resonant line which is short-circuited by the walls of the resonant chamber at the start and end.

To determine the dimensions relevant to dish 1, we proceed as follows:

First the dimensions of the cylindrical resonant chamber in Fig. 2 are determined according to the dimensions of the base of the specified cuboid resonant chamber in Fig. 1. Radius llr.,," of the cylindrical cavity in Fig. 2 is selected such that the base area is the same f or both base shapes of the resonant chamber.

The propagation of a wave along dish 1 can be treated in the form of a socalled resonant line. This consists of an annulus shown in Fig. 3 as a section through plane llrll in Fig. 2, which is delimited by the edge of the dish 1, the wall of the cylindrical resonant chamber, and by the dish 1 itself.

The corresponding equivalent circuit diagram in Fig. 4 shows that the line ends of impedance llz," on both sides also enclose the impedance lIz,,,11 of dish 1. The impedance of the annulus is enclosed by a short circuit represented by the wall of the cylindrical resonant chamber.

8 - The impedances can be calculated in the known manner by determining the inductances and capacitances of the arrangement, as the relevant dimensions 'Ir H 3R 11 and thickness IIHII of the layer and its material properties at the working frequency of the microwave oven are known.

The equivalent circuit diagram shown in Fig. 4 can also be used to calculate the relative E-field distribution along the dish 1 and hence the field distribution along the resonant line can be optimised by changing diameter I'D" of dish 1 and/or thickness IIHII and/or the material properties of the layer.

The next step is to determine the absolute E-field distribution in the cooking chamber of the microwave oven.

Dish 1 with layer 6 on metal base part 4 represents a plate capacitor which establishes the voltage or E-field distribution along the vertical axis of the cavity.

From the theoretical E-field distribution shown diagrammatically in Fig 5, the energy distribution can also be determined. It must be taken into account that thickness "H" of layer 6 essentially influences the distribution of microwave energy in the area below dish 1 because of the high permittivity and permeability of the coating material. This distribution however is also influenced by the distance maintained between layer 6 of the metal base part 4 and the floor 8 of the cooking chamber 9, which is determined by the height of the feet 7.

Suitable materials for layer 6 on the underside of the metal base part 4 are preferably mixtures of ferrite powder and plastics, where the granulometry of the ferrite powder has a maximum grain size of 200 - 250 pm. In order to ensure a high fill ratio of the mixture, a grain distribution with an adequate number of fine grains should be sought. Mn-Zn- ferrite can be used, also mixed with Ni-Zn-ferrite, in order to ensure optimum permeability and permittivity of the mixture at the operating frequency of the microwave oven. Depending on the composition of the mixture, the Curie temperature is also influenced, i.e. the temperature limit beyond which, as is known, the permeability of magnetic materials diminishes strongly and the magnetic losses in the mixture become insignificantly small.

To bind the ferrite powder together, binding agents having 2o an organic or inorganic base are suitable where these can resist a permanent usage temperature above the Curie temperature of the electromagnetic powder used.

Suitable organic-based binding agents are plastics which, in addition to having a corresponding temperature resistance, can also bind a high proportion of the HF-absorbing electromagnetic substance. These are for example: PAN, PPO, PSU, PI, PC, PS, PTFE, PVW, PA, SIR and stabilized EP and PUR resins.

suitable inorganic binding agents are in particular ceramic materials and cements.

The influence of the fill ratio of the ferrite powder on the electromagnetic properties, i.e. the permittivity and permeability, is shown in Figs. 6 and 7.

Fig 6. shows the curve of the complex permittivity llell and the complex permeability 11t" of an Mn-Zn ferrite powder mixture with a ferrite powder volume fill ratio of 50%.

At 2500 MHz, the following values apply:

ReF,: 28; ImF-: 12 Rep: 1.8; Impt: 1.4 This gives a product of Re VE -xp = 90.6.

Fig. 7 shows the values of an Mn-Zn ferrite powder mixture with a volume fill ratio of 73%.

At 2500 MHz, the following values apply:

ReF,: 48; Imrc: 22 Rept: 2.2; Imt: 2.2 11 - This gives a product of Re If-x[t = 217.8.

In the above calculation process, material properties c and p and the parameter 11Re F-- -xptlf given are also used in determining the impedance of the conductive element represented by the dish 1. The function of the dish 1 is therefore particularly dependent on the material properties of the layer 6; a relatively thin layer 6 with a good heat conduction to the metal base part 4 of dish 1 can only be achieved with a high product Re [c x p].

Further coating materials are the new so-called "synthetic" dielectrics, which have high dielectric losses and a high temperature stability. These are in particular ceramic materials which cause high permittivities and losses at high frequencies due to the well dispersed TiC powder with a grain size in the nm range.

Experiments with a browning device according to the invention with the following features:

diameter I'D" of metal base part 4 of dish 1: 250 mm. diameter I'd" of layer 6: 230 mm thickness IIHII of layer 6: 1.8 mm parallel distance IIPII between layer 6 and floor 8 of the cooking chamber 9: 6 -mm 12 - material of layer 6 fill ratio of 60% : Mn-Zn ferrite powder with a volume used in a conventional microwave oven have shown that f or example a pizza of normal thickness placed on dish 1 can be prepared within 7 minutes and optimum browning obtained.

CLAIM Browning device suitable foruse in the cooking chamber of a microwave oven operated at conventional frequency with a side microwave inlet, consisting of a dish or plate with a preferably circular metal base part which has good thermal conductivity and the underside of which has a preferably circular layer of a material which absorbs high frequency waves, where the browning device can be placed in the cooking chamber such that a parallel distance can be maintained between the underside of the layer on the metal base part and the top of the f loor of the cooking chamber, characterised in that:

the parallel distance 11P11 between the underside of the layer (6) of the metal base part (4) and the top of the cooking chamber floor (8) is up to 6 mm, diameter I'D" of the surface of the metal base part (4) conforms to the floor area of the cooking chamber (8) and the diameter I'd" of the surface of the layer (6) is at most 20 mm smaller than diameter I'D", and thickness "H" of layer (6) is 0.5 to 3 mm and the material of layer (6) consists of ferrite powder and a binding agent, where the ferrite powder proportion is at least 40 vol %, preferably at least 60 vol %.