AU615755B2 - Improvements relating to microwave heating - Google Patents

Description

*1 ii COMMONWEALTH OF AUSTRI5 7 51 PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: Priority: 9 0 h'elated Art 00 9 00 lame of Applicant: ALCAN INTERNATIONAL LIMITED Acqdress of Applicant: 1188 Sherbrooke Street West, Montreal, Quebec H3A 3G2, Canada Actual Inventor: BRYAN COBLEY HEWITT and MELVILLE DOUGLAS BALL A AMM 4 atermark Patent Trademark Attorneys QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

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Arddress for Service Complete Specification for the Invention entitled: IMPROVEMvENTS RELATING TO MICROWAVE HEATING The following statement !s a full dzscription of this Invention, Including the best method of performing it known to US "1IMPROVEMENTS RELATING TO MICROWAVE HEATING" 0*000 000 0 0 0 000 0 0 0 00 000 a 00 0 00 00 0 00 40000O 0 0 a0 0 0 V 0 This invention relates to improvements in the heating of substances in a microwave oven. While the substances most commonly heated will be foodstuffs, and the examples below will therefore relate to foodstuffs, the present invention is not limited in this respect and can be used for heating other substances.

In normal microwave transparent containers, the microwave energy can enter t[hrough the top, bottom 6nd sides of the container. This is similar to the situation encountered during conventional oven cooking (or heating). With normal microwave foil containers the microwave energy can only enter through the top 15 (food tsurface).

Prepared foods are commonly reheated in a cooking utensil on a stove top, One characteristic of this type of reheating is that the heat enters the food through the bnttom of the container/utensil.

20 Heating the food from the bottom offers some advantages, as a result of the heat transfer mechanisms that take place. The food in contact with the base heats and becomes less dense. This provides a driving force for convective transport, the warm food rising 25 and being replaced by cooler food from nearer the surface. The extent of this convection depends oo the viscosity of the food. At a later stage of heating, bubbles of steara nucleate at or near the base and rise through the food. This transfers heat throughout the food as well as agitating the product.

With a view to simulating this type of "bottom heating" in a microwave oven, there has been proposed in k. Nakanaga U.S. pagent 4,661,672 Issued April 28, 1987, a rectangular container having a microwave energy shielding layer extending over the top and down A, w V -2at least the upper portions of the side walls, while the remainder, and particularly the bottom of the container, was made of a microwave transparent matefial, In this way, the microwave energy is caused to enter the container through its bottom, and possibly to some extent through the lower parts of the s id e w al Is This prior patent also discloses the feature of elevating the container so that its bottom is spaced above the floor of the microwave oven. A similar arrangement is disclosed. in K. Sugisawa et al European patent application 0,185,488 published June 25, 1986, although in this case the top of the container is only shielded at its edges, whereby to avoid excessive heating of the upper surface of the material located at the sides of the container.

For food loads that have low viscosity and hence al low substantial heat transfer by convection, WIcon vent ionalI conta iners w ill of ten perf orm s a tisf act onl Iy However, in many cases the characteristic 1Ionuniform heating that results from the dominant fundamental energy distribution will not be sufficient- 4 I ly equalised by convective heat transfer, and an unsatisfactory product will result. In particular, there will tend to be excessive heating at the edges S 25 and insufficient heating in the middle of the body of food. Viscous products such as meat stews or casseroles, lasagne, macaroni cheese, thick soups and chowders are particularly difficult in this respect..

The present invention seeks to minimise this difficulty, and in particular, to provide an arrangement in which the food product (or other substance) is not only heated at its undersurface (although not necessarily only at its undersurface), but is also heated more rapidly and/or more uniformly across its lateral dimensions.

-3- To this end, the present invention provides a stand for use with a container having at least one portion transparent to microwave energy including a bottom on which there is supported an undersurface of a substance to be heated. The stand comprises means for modifying the microwave field pattern to which the container is exposed, and means for supporting the container spaced above said field modifying means so that the undersurface of the substance is maintained at a predetermined distance from said field modifying m ea ns.

In the preferred form of the invention, the modifying means takes the form of means for generating a modified microwave field pattern having at least one 4 00 15 mode of microwave energy of higher order than the 00 fundamental modes of such energy.

Higher order mode generating means are known 4 00o per se. See, for example, R. Keefer Canadian patent 0 application Serial No. 485,142 filed June 25, 1985 patent application Serial No. 878171 filed June 1986 and European patent application No. 86304880 filed June 1986 and published December 30, 1986).

Such higher order mode generating means may take the form of one or more electrically conductive plates (or aperturez in an electrically conductive sheet) arranged ia symmotrical planar array. Examples of such structures are discussed below.

The term "mode"' is used in the specification and ciaims in its art-recognised sense, as meaning one4 of several states of electromagnetic Wave oscillation that may be sustained' in a given resonant system, each such state or type of vibration each mode) being ctraracterised by its own partictular electric and magnetic field configurations or patterns, The fundamental modes of the container and body are 11 0 4characterised by an electric field pattern (power distribution) confined or concentrated around the edge of the container (as viewed in a horizontal plane), these fundamental modes predominating in a system that does not include any higher order mode-generating means. The fundamental modes are defined by the geometry of the container and the contained body of material to be heated.

A mode of a higher order than that of the fundamental modes is a mode for which the electric field pattern (again, for convenience of description, considered as viewed in a horizontal plane) is concentrated around the periphery of an area smaller 0 "0 0° than that circumscribed by the electric field pattern ,0 15 of the fundamental modes. Each such electric field r* Q pattern may be visualised, with some simplification but nevertheless usefully, as corrE:, onding to a closed loop in the horizontal plane.

14 44 Alternatively, or additionally, the modifying means may take the form of means for enhancing the coupling of microwave energy into the undersurface of the substance to be heated. Such coupling enhancement 444o means are more fully described below.

In addition to its microwave transparent o 25 portion or portions, the container may have some portions that are reflective of microwave energy.

When bottom heating is to be the dominant mode of heating, the transparent portions will be principally 4 44 constituted by the bottom of the container. Thus, in one embodiment, the lid and side walls of the container can be reflective of the microwave energy, while the Lottom is transparent to such energy, so that all the energy enters the food by its undersurface. There may, however, be instances where it will be convenient to allow some of the microwave energy to enter the food I i; -A 1 'I through areas other than the undersurface, and such an arrangement is not excluded by the present invention.

For example, some foods such as baked goods, or those having a surface layer needing to be melted, e.g. a cheese layer on 1lasagne, or a potato layer on shepherd's pie, are ideally heated at the top and the bottom Simultaneously. In this case the container lid can be microwave transparent, or can simply be removed during the heating process. Nevertheless, in the preferred embodiments of the invention, the majority of the microwave energy will enter through the undersurface of the container to maximise the bottom heating effect, the advantages of which have been discussed above.

CI 10 015 In addition the invention does not exclude the 00 a boo apossibility tfhat may be desirable in some instances, namely that parts of the bottom of the container, for S 00 example the peripheral edge of such bottom, can be s0 h~ hi e1ded so a s to c oncentrate the mi1c rowa ve ene rgy i n the central portion of the undersurface of the substance being heated.

0 0 O Finally, it should be mention,.d that, in those examples where it is desired to avoid microwave energy entering the food at its lateral edges, the stand can a o0 include upwardly projecting metallic parts thac shield~ these lateral edges thus avoiding the need for the container itself to have reflective side walls.

In one preferred form of the invention, the "oo~acontainer will have a flat bottom, the field modifying means will be planar, and the supporting means will be so dimensioned as to support this flat bottom in a plane parallel to the field modifying means. As a result a predetermined spacing between. the container bottom and the field modifying mean,- is maintained uniform throughout the la*tral dimensions of the4 1, -6container.

The invention also consists of the assembly of a stand as described above and a container for holding the substance to be heated. This assembly can consist of two separate elements that are brought together for use, with the stand being available for re-use with the same or another container. Alternatively, these two elements can be joined together and sold as a single assembly, either for single or multiple use. For multiple use the assembly will constitute a permanent cooking vessel.

The shape of the container may be that of a conventional tray in which frozen food is commonly o~o sold, i.e. a relatively shallow, rectangular or round tray with a flat bottom, side walls and a flat removable lid. However, one of the advantages of bottom heating, is that the normal limitations on °o product depth are much less important. Since other I oo heat transfer mechanisms (convection, steam bubbles) are being encouraged, deeper loads (similar to those which would be used in a stove top saucepan) can be o satisfactorily dealt with. This represents a real advantage. It is also worth noting that microwave heating from the bottom will be better than normal 25 stove top heating, because the penetration of the microwave heating obviates the need to stir the product. In normal stove top cooking, the heat energy o*o is transferred to the food through the base by 4 o conduction. Rapid heating of the food normally requires the temperature of the base of the utensil to be raised to a high tewperature. To avoid burning the food in contact with the base, low power settings can be used (which extend the heating time) or, alternatively, the food must be stirred frequently (for viscous foods).

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7 According to a still further aspect, there is provided a method of heating a substance by microwave energy, said method comprising the steps of confining such substance in a partially shielded container that is of such a nature that at least some and preferably the majority of the energy enters the substance through its undersurface, while modifying the microwave field pattern by means located a predetermined distance below such undersurface to enhance the coupling of microwave energy into such undersurface and/or to improve the uniformity of heating in the lateral dimensions.

In order that the invention may be better understood, several embodiments thereof will now be o0o described by way of example only, and with reference to o 15 the accompanying drawings in which:- Figure 1 shows a vertical central section of an 4ao: assembly of a container and a stand therefor, 4 4 according to an embodiment of the invention; Figure 2 is a section on II-II in Figure 1; Figure 2A is a modification of Figure 2; Figure 3 is a vertical central section of a 444 l modified stand according to a further embodiment of the 4o O invention; Figure 4 is a view on IV-IV in Figure 3; ae 25 Figure 4A is a modification of Figure 4; Figure 5 shows a view similar to Figure 1 of an 4 alternative embodiment; r Figure 6 is a similar view of yet another 0. embodiment; Figure 7 is a side view of an alternative form of stand; i s Figure 8 is a side view of a still further alternative form of stand;

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Figures 9A and 9B are diagrams illustrating the positions of temperature sensors used in the tests i t A -8illustrated in Figures 10 and 11; Figures 10(a) and and 11(a) and are comparative graphs comparing the performance of different arrangements; Figure 12 is a vertical central section illustrating an application of the invention to a multi-compartment container; Figures 13 to 15 each show alternative stand constructions; Figure 15A is a graph related to Figure 15; and Figure 16 to 18 each show still further alternative stand constructions.

Referring to the drawings, Figures 1 and 2 show a container 10 having a bottom 12 of a suitable 4444 oo 15 microwave transparent material, e.g. fibreboard or a plastic material, side walls 14 of metal foil or of a laminate containing metal foil 15, and a lid 16 also of 4:o, metal foil or of a fibreboard laminate including metal E foil 17, the lid being held in place by a fold down rim 18. The design of the lid and rim is such that there is no possibility of arcing. A food load 20 is supported in the container with its undersurface 21 on 4 44 the bottom 12. This container 10 can be circular, rectangular, or any other convenient shape in plan o 25 view. In Figures 1 and 2, it has been assumed that the container 10 is circular. Figure 2A shows a rectangular stand for a rectangular container.

The cooking assembly includes a stand 22 on *o which the container 10 is designed to be seated, su.h stand 22 consisting of a base 24, side walls 26 and a rim 28 with an inwardly sloping portion 30, all made of a microwave transparent material. The base 24 is formed with either a continuous peripheral depression or a series of such depressions forming feet 25 that serve to elevate the base 24. Centrally of the base I k 4 9 4 o4 4 «c 4 C4 e 44 4 4~ 4 4 44 f oooe 4 44 4 44 a 4B 6 44 O «L a i 24, there is a plate 32 of conducting material, e.g.

aluminium, that will serve to modify the microwave field pattern and generate the higher order modes. In Figure 2, the plate 32 is circular; ii: Figure 2A it is rectangular. The dimensions of the stand 22 are such that the spacing S between the undersurface 21 of the food load 20 and the upper surface of the plate 32 is set at an optimum value for the conditions. The choice of the value for this spacing S is discussed below. Since the undersurface of th food into which the microwave energy is being propagated lies in continuous contact with the bottom 12 of the container, this spacing S is uniform across the lateral dimensions X and Y of the container.

15 The stand 22 may be a re-usable kitchen appliance that is constructed of a sturdy plastic or glass, or t may be a more cheaply made disposable element that is sold with the container 10 either as a separate item to be assermled in the oven or as a fixture secured to the bottom of the container The size and arrangement of the plate 32 centrally of the base 24 in Figures 1 and 2, is similar to arrangements of conducting plates shown in the Keefer patent application referred to above. If it is preferred to generate still higher order modes of microwave energy at the bottom 12 of the container, an array of a larger number of smaller plates 34 can be provided on the base 24' of a modified stand 22' shown in Figures 3 and 4 and designed for use with a rectangular container, this array of plates 34 being generally similar to that shown mounted on a container lid in Figure 10B of said Keefer patent. This latter arrangement is well suited to the heating of relatively shallow food loads, since the higher order modes may not penetrate as fa, into the food load as the 10 fundamental modes. On the other hand, they achieve enhanced uniformity of heating across the lateral dimensions of the container.

As explained in the Keefer patent application, an array of plates, such as the plates 34, can be replaced by an array of apertures in a metallic sheet that otherwise covers the surface. Figure 4A shows a suitable array of apertures 36 in a conductive plate 38 on the base of a stand 22", or the whole stand may be conductive, e.g. made of aluminium.

Figure 5 shows a further modification in which a stand 22 made of aluminium has upwardly extended sloping end and side walls 27, and a base 39 containiing apertures 36. A container 11 with a food load 20 has a°a 15 end walls 13 that nest snugly within the walls 27 to ZOo support the container with its bottom 12 and hence the undersurface 21 of the food load a predetermined

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t distance S above the base 39. The container 11 has a lid 16, In this arrangement the metallic walls 27 of the stand provide lateral shielding for the food load, so that the container 11 can be made entirely of a microwave transparent material. The lid 16 may be °o metallic, if top shielding is required, or microwave transparent, if such shielding is not required, or some 0 25 combination thereof, if partial shielding is required.

Figure 6 shows an application of a somewhat similar construction, as applied to a re-usable cooking vessel 41 made of glass with a metallised outer surface 0o o layer 43 having apertures 45 in the portion 47 thereof that extends across the bottom surface of the bottom portion 49. This bottom portion 49 of the utensil 41 is relatively thick compared to its sides whereby to provide the necessary spacing S' between Its upper bottom surface that supports the undersurface of the food load (not shown) and its bottom surface 47.

Figure 7 shows an alternative arrangement in which a stand 40 consists of a flat base 42 supporting four posts 44 on which the container 10 is placed.

Conductive plates 46 are located on the upper surface of the base 42 for generating the higher order modes.

Figure 8 shows another construction of stand made of a solid plastic or glass slab 52 on the upper surface of which the container 10 will be placed.

Legs 54 hold the slab 52 above the oven floor, and conductive plates 56 are secured to the underside of the slab 52.

Figures 9A and 9B show how the tests reproduced in Figures 10 and 11 were conducted. As shown in Figure 9A, four temperature probes A, B, C, D were 040 15 inserted into the food load 20, approximately centrally 00 0of both lateral dimensions of the container, and at :4 varying depths, probe A being nearest the undersurface of the food and probe 0 nearest the top surface.

'4 0 0Figure 9B shows the locations of four temperature probes C, E, F, G inserted into the food load, all. at the same depth, i.e. at approximately one quarter depth, and respectively located at approximately the 0o 4)f CIO centre,. the left end, the right end and the side 4, 0 0(located at the back when placed in the microwave oven) of th~a Container.

Figure 10(a) shows the temperatures measured by probes A-D when heating a load of about 680 grams of canned beef and vegetable stew for 15 minutes in a 700 watt microwave oven in a convention~al circular foil Container, I.e. one having the following dimensions: outside top diameter 181 mm; inside top diameter I71 mat; bottom diameter 140 mm; slant depth 38 mm, and capacity 796 ml.

Figure 10(b) shows the s4m experiment whenI Conducted In a similar Container m~odtfied to make the energy witnin a container having at least one portion including a bottom portion transparent to microwave energy, on which bottom portion there is supported an i A. :/2 12 lid and sides microwave reflective and the bottom microwave transparent, and mounted on a stand as shown in Figure 2 having a single circular aluminium plate 32 with a diameter of 55 mm.

The results illustrate dramatically how the I: more uniform heating of the invention enables all levels in the food to assume an acceptable temperature, 0o i.e. at least 80 C, within 6 minutes, in contrast to 1 the 15 minutes of Figure i10 Figure 11(a) and respectively show the Sreadings obtained from probes C, E, F and G in a rectangular container having the following dimensions: outside top 146 x 121 mm; inside top 130 x 105 mm; bottom 115 x 89 mm; slant depth 38 mm; and capacity 455 15 ml. The first test was conducted with a microwave j transparent base, but no higher order mode generating a- stand (Figure and then with such stand (Figure 411(b)). The load was about 400 grams of a frozen S* Chili-con-Carne product. Figure 11(a) shows that the 20 outer regions of the product had thawed and heated to on acceptable temperature (60 C) in nine ten minutes, while the central region was still frozen until after about eleven minutes had elapsed. Acceptable temperatures were not achieved in the central region until after about 15 minutes. It should be noted that, at this time, some regions around the edge of the container had been boiling for about five minutes, which is undesirable. The erratic temperature variations during the rapid heating part of the curves ;j 4 .o r 30 are indicative of turbulence caused by bubbles of steam rising through the product.

In the equivalent container used in conjunction with a higher order mode generating stand (Figure the heating behaviour obtained 1$i very different. The mode generating device in this case r /ti 13 onG4 4 404 0 0 4 eo os I 00 404 4 40 44 00 Sa -e e o a b0 044 44 4 0 00 0 0 44 0 t0 6 4 44 i «r i was a single foil block as shown in Figure 2A, the block being rectangular, 55 x 30 mm. In this case it is noticeable that the centre region thawed and heated in a much shorter tiime than before. Furthermore, the overall heating behaviour is noticeably more uniform.

Thus the fastest region to heat was only boiling for about one minute befo all the measured temperatures had reached an acceptable temperature (60 C).

In another test (not illustrated) when using a stanrdard container, the initial weight of a load of Chinese style chicken fried rice that had been precooked and frozen was 330.8 grams and its final weight was 239.5 grams, for a weight loss of 91.3 grams, i.e.

27.6%, over a ten minutes heating time. In a 15 corresponding test when the container was placed on a stand as shown in Figure 4, the initial weight was 329.5 grams and the final weight 318.8 grams, for a weight loss of 10.7 grams, or over a seven minutes heating time which was all that was necessary.

20 This reduced weight loss is a further advantage of the present invention.

Figure 12 illustrates how a multi-compartment container 60 having two different food loads 62,64 can be mounted on a common stand 66. Depending on the different natures of the two food loads and the amount of microwave energy that it is desired they should each absorb, the conditions can be adjusted appropriately.

For example, the portion 68 of the stand 66 situated below the food load 62 may employ a single higher order 30 mode generating conductive plate 70, while the portion 72 situated below the food load 64 may employ multiple plates 74. Alternatively, in an example not illustrated, one of the portions of the stand 66 may not include any means for generating higher order modes and the food load associated with such portion may be r P:l_ The following statement is a fuii description of this invention, including the best method of performing it known to US 14 entirely shielded from the microwave energy. This latter arrangement would be especially appropriate if the fully shielded food load is required to remain cold.

As far as spacing is concerned, there will be a requirement for a certain minimum spacing between the conducting plates (or foil surround, in the case of apertures) and the metal of the oven floor, in order to avoid arcing. It is for this reason that the embodiment of Figure 1, and many of the other embodiments, are provided with feet 25. However, if the oven has a sufficiently thick glass tray on its floor, or a separate microwave transparent rack is used, such feet can be dispensed with, e.g. the vessel 15 of Figure 6. Such arcing-avoidance spacing will typically be required to be at least 3mm. It should o, t also be mentioned that, in a case where the stand is not provided with feet and is placed directly on a glass tray on the oven floor, i.e. with mainly glass 1 20 and little air between the conducting material and the oven floor, the array of plates or apertures may require dimensional modification to take into account so* the dielectric constant of the glass.

The followindr considerations should be taken into account when selecting the preferred value for the \oo spacing between the undersurface of the food and the field modifying means, i.e. the spacing S when in air (Figures 1 or 5) or S' when in a plastic or glass material (Figures 6 or 8).

30 The optimum spacing will depend in part on the 4* properties of the foodstuff (for example, -he dielectric properties will change the phase shift which occurs on reflection). A possible range for the spacing S in air is from about 3 to 30 mm. A spacing 3C S of 15 mm (with air separating the foil structure from the container base) has been successfully used in practice. As indicated, this spacing will depend on the dielectric constant of the material between the foil array and the bottom of the food load. The following table gives examples of modifications to the mm spcing that would be appropriate if materials of different dielectric constant were present between the bottom of the food and the foil array structure.

Material Air Silica Glass 15 Polyethylene Plexiglass Dielectric Constant (Relative Permittivity) 1.0 3.78 2.25 2.6 Spacing S' 15 mm 7.72 mm 10 mm 9.3 mm 000 ar 4 41 41 Tests have also been carried out to measure the effect of the invention on total power absorption. A S 20 rectangurlar container (with a microwave transparent base) and a stand with the 9-block foil array structure as in Figure 4 was used. Power measurements were made using water as the load.

Test 1 Container placed directly on the oven glass plate 4i 4 44 4 4 44 measured power 271.5 watts Test 2 Container raised 30 mm above the glas plate (no foil array) measured power 268.2 watts Test 3 Container raised 30 mm above the |lass plate (with the 9-block array as in Figure 4 located midway, i.e. 15 mm from the food undersurface) measured power 307.2 watts This corresponds to an improvement in power

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16 absorption of approximately 13%. Increased power absorption is useful (reduced cooking time), in addition -to the improvement in heating uniformity th at many of the embodiments of the present invention provide.

In the examples described so far it has been assumed that the stand will have a flat bottom. it is, however, within the scope of the invention to employ a stand embodying higher order mode generating means incorporating a stepped structure, e.g. a stepped structure of one of the types disclosed in R. Keefer Canadian patent applications Serial Nos. 508,812 filed May 9, 1986; 536,589 filed May 7, 1987; and 544,007 filed August 7, 1987 patent applications Serial Nos. 943,563 filed December 18, 1986 and 044,588 filed April 30, 1987 and European patent applications Nos.

87304120.6 filed May 8, 1987 and published November 19, 1987 and 87309398.3 filed October 23, 1981 and At 4 published June 22, 15H). Some of the patent 44 20 applications j~vst referred to also disclose a container having a wall a bottom wall) having a modified portion that has a different electrical thickness from that of adj acent port ions of the wall1, the elIectr icalI thickness being defined as a function of the actual 2 5 spatial thickness of the wall and the dielectric constant of the wall material. Such a wall structure comprising appropriately arranged contiguous wall portions of respectively different electrical 4 thicknesses can serve to generate at least one mode of 444 30 a higher order than the fundamental modes. In the pireseiit invention, higher order mode generating means located in the stand can Utilise such an arrangement of various portions of differing electrical thickness instead of the foil plates or apertures described above.

D D I uIIucIIllIIi Ldi uuUt U 1,1t! UUIILdlner ana Dooy are 4 17 Figure 13 shows a stand with such a structure based on portions 75, 76 of different physical thickness, while Figure 14 shows a structure in which portions 77, 78 have the same physical thickness, but a different electrical .thickness by virtue of having different dielectric constants, respectively designated L and H for low and high.

Figure 15 shows a structure in which apertures are formed in a conducting base 67 supported by non- 4 10 conducting supports 69, a central aperture 65a being formed in a raised portion 67a of the base, whereby its distance S2 from the undersurface of a food load (not shown) in a container 10 is less than the distance S$ of the remainder of the base 67. Figure 15A shows the S15 effect on the power P conveyed to the load as a o i functior of S. Curve 61 is for larger apertures boo while curve 63 is for smaller apertures.

'o:4 A plan view of Figures 13, 14 or 15 would show ,,the portions 75, 76 or 77, 78, or the apertures 20 forming a nine block array similar to Figure 4, although this array can be modified as required.

As a further alternative, the higher order mode C generating means employed in a stand according to the •6 opresert invention can take the form shown used on a container in R. Keefer U.S. patent application Serial No. 051078 filed May 15, 1987 (Canadian application filed May 12, 1988). This alternative is illustrated by the plan view of a circular stand in Figure 15 where the portion 79 is a shaped piece of foil on a microwave J 30 transparent base Higher order modes of microwave energy can also be generated by a stepwise discontinuity of lossiness between a pair of regions of a susceptor. Such a susceptor, which may constitute a separate element or may form a wall component of a container, is disclosed 7 C 18 in R. Keefer Canadian patent application Serial No.

552,110 filed November 18, 1987. in accordance with the present invention such a susceptor structure can be used in the stand to provide higher order mode generating means, as well as to generate heat that can be conveyed to the container and the food or other material therein. Such a structure is shown in Figure 17, where the portions 81 and 82 have different l'~ssiness. A plan view of Figure 17 could show the portions 81, 82 as a single block array, similar to Figures 2 or 2A, or the portions 81, 82 could be strips extending fully across a rectangular container.

An arrangement for retaining and concentrating microwave energy in a container, i.e. enhancing the coupling of such energy into the cortainer, is described in R. Keefer Canadian patent No. 1,228,126 500 040 0 issued October 13, 1987 patent 4,6561,325 issued 0 ,3aApril 7, 1987). A similar arrangement can be embodied o~oo in a stand in accordance with the present invention, as 0 20 illustrated, for example, in Figure 18 which shows a a, V stand with a substrate 83 of a dielectric material having a relatively low dielectric loss factor, e.g.

polyethylene polyester film. On this substrate 83 00 0 there is an array of conductive plates or islands 84, e.g. aluminium foil. The total surface area of the 9405,metallic islands should preferably be between 50 and of the surface area of the substrate. Figure 18 shows the substrate 83 on a stand having a foot portion and a rim 86 for supporting a container. The plates should cooperatively provide a dielectric constant greater than 10, and the spacing between such array and the undersurface of the substance to be heated in the container (not shown) should be between[ one-fifteenth and one-sixth of the wavelength of the 1 '1 19 microwave energy, which is approximately between 8 and mm in air. This arrangement may also serve at the same time to generate some higher order modes of microwave energy. However, in view of the relatively large number of plates 84 used in the 20-block array shown in Figure 18, the height of the higher order modes will be greater than that of the modes generated by the single and nine-block arrays illustrated in other views. These very high order modes will penetrate a shorter distance into the food, and hence the advantage of the Figure 18 embodiment flows more from the increased coupling of energy into the food that from higher order mode generation, although the latter phenomenon will contribute to some extent to the 15 overall improvement in performance.

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Claims (18)

1. 2. A stand according to claim 1, wherein said modifying means is planar and said supporting means is dimensioned to support a flat bottom of the container lying in a plane parallel to said modifying means.
2. 3. A stand according to claim 2, wherein said modifying means comprises at least one plate of electrically conductive material lying in a plane parallel to said bottom.
3. 4. A stand according to claim 3, including means for spacing said electrically conductive material from an oven floor. A stand according to claim 2, wherein said modifying means comprises an array of a plurality of plates of electrically conducting material lying In a plane parallel to said bottom. 0 L!yi I 3U%" UP i U±Uii1Z I Uriliiiy TeeL 43 LnaL serve to elevate the base 24. Centrally of the base rw ww-- 21
4. 6. A stand according to claim 2, wherein said modifying means comprises at least one aperture in a sheet of electrically conductive material lying in a plane parallel to said bottom.
5. 7. A stand according to claim 2, wherein said modifying means comprises an array of a plurality of apertures in a sheet of electrically conductive material lying generally in a plane parallel to said bottom.
6. 8. A stand according to claim 7, wherein at least one of said apertures is in a portion of said sheet '1 located at a distance from said undersurface different from the distance of the remainder of said sheet from a said undersurface. 15 9. A stand according to claim 1, wherein said modifying means comprises portions having different electrical thicknesses from one another. A stand according to claim 9, wherein said portions have different physical thicknesses from one another.
7. 11. A stand eicording to claim 9, wherein said portions have different dielectric constants from one another.
8. 12. A stand according to claim 1, wherein said modifying means comprises portions having different lossinesses from one another. I .13. A stand according to claim 1, wherein the stand includes upwardly extending, conductive outer portions for shielding side walls of the container.
9. 14. A stand arcording to any one of the preceding claims wherein said distance between said undersurface and said field modifying means is at least 3 mm. A stand according to claim 14, wherein said distance is in the range of 3 to 30 mm.
10. 16. A stand according to claim 14, wherein the space between the modifying means and the container L /l 9 w i .;f 44 j -Y: I- 22 4 .44t 94 4 4< rf II I 4 I <4 4 4rp bottom is air and said distance is approximately mm.
11. 17. A stand according to claim 14, wherein the space between the modifying means and the container bottom is polyethylene and said distance is approximately 10 mm.
12. 18. A stand according to claim 14, wherein the space between the modifying means and the container bottom is glass and said distance is in the range from 7 to 10 mm.
13. 19. A stand according to any one of the preceding claims, further comprising means for enhancing the coupling of microwave energy into the undersurface of said substance, and means for supporting the container spaced above said coupling enhancement means, the arrangement being such that said undersurface is a distance of between one-fifteenth and one-sixth of the wavelength of the microwave energy in the medium between the container and the coupling enhancement means above said coupling enhancement means, and wherein said coupling enhancement means comprises a substrate of dielectric material with an array of conductive plates covering at least the majority of the surface area of said substrate, the substrate and the plates co-operatively providing a dielectric constant greater than
14. 20. A stand according to claim 19, wherein said distance is in the range 8 to 20 mm. '21, In combination a container for use in a microwave oven having a microwave field pattern, said container having at least one portion including a bottom portiJn transparent to microwave energy, on which bottom portion there is supported an undersurface of a substance to be heated, and a stand as claimed in any one of the .4 ili-- a 23 rac 04 00 0 ,a aB 4 4*4 B 00 0 *4 4 it W 0 a an al a, at a ai a preceding claims.
15. 22. A vessel comprising the combination according to claim 21 wherein the container and the stand are formed as an integral unit.
16. 23. A method of.heating a substance by microwave energy within a container having at least one portion including a bottom portion transparent to microwave energy, on which bottom portion there is supported an undersurface of the substance to be heated, said method comprising the steps of confining said substance in the container such that at least some of the energy enters the substance through said undersurface thereof, and at a location beneath said undersurface 15 modifying the microwave field pattern to cause at least one mode of microwave energy of higher order than the fundamental modes of such energy to propagate in said substance to provide by means of such higher order mode a significant portion of the heating of such substance, to improve the uniformity of heating of said substance, said location being spaced a distance beneath sai( rdersurface measured in millimeters of at lea3,t divided by the square root of the dielectric constant of the medium in such space.
17. 24. The method of claim 23, wherein the majority of all the microwave energy enters the subtance through said undersurface. The method of either one of claims 23 or 24, whereirt said undersurface is flat and said predetermined distance is uniform, 26, The method of any one of claims 23 to Including the further step of: at a location between one-fifteenth and one-sixth of the wavelength of the microwave energy In the medium between said location and said undersurface r;i Fr, Eat i- V lh ,I I; i: i 24 beneath said undersurface modifying the microwave field pattern to enhance the coupling of microwave energy into said undersurface, wherein said coupling enhancement is achieved by means of a substrate of dielectric material with an array of conductive plates covering at least the majority of the surface areas of said substrate, the substrate and tne plates cooperatively providing a dielectric constant greater than
18. 27. A method of heating a substance by microwave energy within a container having at least one portion including a bottom portion transparent to microwave energy, on which bottom portion there is supported an undersurface of the substance to be heated, said t 15 method comprising the steps of: confining said substance in the container such that at least some of the energy enters the substance through said undersuiface thereof, and at a location between one-fifteenth and one-sixth of the wavelength of the microqwave energy in the medium between said location and said undersurface S;beneath said undersurface modifying the microwave field pattern to enhance the coupling of microwave energy into said undersurface, wherein said coupling enhancement is achieved by means of a substrate of dielectric matortal with an array of conductive plates covering at least the majority of the surface areas of said substrate, the substrate and the plates cooperatively providing a dielectric constant greater than DATED this 22nd day of May 1991. ALCAN INTERNATIONAL LIMITED WATERMARK PATENT A4EMARK ATTORNEYS 2nd Floor The Atrium 290 Burwood Road Hawthorn. Vie. 3 122