HK1128580B - Arc-resistant microwave susceptor assembly having overheating protection - Google Patents

Description

Arc resistant microwave susceptor assembly with overheat protection

This application claims priority to U.S. provisional applications 60/841107 and 60/840984, both filed on 29/2006, and U.S. provisional application 60/751544, filed on 19/12/2005, which are incorporated herein as part of this application for all purposes.

Technical Field

The present invention is directed to a susceptor assembly that prevents overheating when used in an unloaded microwave oven.

Cross reference to related applications

The subject matter disclosed herein is disclosed in the following co-pending applications filed concurrently with this application and assigned to the assignee of the present invention:

Field Director Assembly Having Arc-Resistant ConductiveVanes(CL-3630)。

background

Microwave ovens use electromagnetic energy at various frequencies to vibrate molecules in food products to generate heat. The heat thus generated heats or cooks the food. However, the item is not raised to a high enough temperature to char its surface to a crispy texture (and still keep the food edible).

To achieve these visual and tactile enjoyment, a susceptor formed from a substrate having a lossy susceptor material thereon may be placed adjacent to the surface of the food product. When exposed to microwave energy, the material of the susceptor is heated to a temperature sufficient to char and crisp the surface of the food item.

The walls of the microwave oven impose boundary conditions that cause a change in the energy distribution of the electromagnetic field within the oven volume. Electromagnetic fields, and in particular these variations in strength and directionality of the electric field component of the field, create relatively hot and cold regions in the furnace. These hot and cold areas cause the food product to heat or cook unevenly. The scorch and crisping effect is likewise not uniform if microwave susceptor material is present.

To counter this uneven heating effect, a turntable may be used to rotate the food product in a circular path within the oven. Each portion of the food product is exposed to a more uniform level of electromagnetic energy. However, this averaging effect occurs along a circumferential path, rather than a radial path. Thus, the use of a rotating disk still creates an uneven heating zone in the food.

This effect can be more fully understood from the illustrations of fig. 1A and 1B.

FIG. 1A is a plan view of the interior of a microwave oven showing five regions (H) of relatively high electric field strength ("hot zones")₁To H₅) And two regions C of relatively low electric field strength ("cold zones")₁And C₂. A food product F having an arbitrary random shape is placed on a susceptor S, which in turn is placed on a turntable T. The susceptor S is represented by a dashed circle and the turntable by a bold solid circle. Three representative locations on the surface of the food item F are indicated by points J, K and L. Points J, K and L are each located at radial position P of turntable T₁、P₂And P₃. As the turntable T rotates, each point moves along a circular path within the furnace, as indicated by the circular dashed lines.

As can be seen from FIG. 1A, during one full rotation, point J passes through a single region H of relatively high electric field strength₁. During the same revolution, the point K passes through a single small area H of relatively high electric field strength₅While the point L experiences three regions H of relatively high electric field strength₂、H₃And H₄. Thus, rotation of the turntable through one full rotation brings points J, K andeach of L is exposed to a different total amount of electromagnetic energy. The graph of fig. 1B shows the difference in energy exposure at each of these three points during a complete rotation.

Points J and L experience significantly more energy exposure than point K due to the number of hot areas encountered and cold areas avoided. If the area of the food product near the path of point J is deemed to be sufficiently cooked, then it is likely that the food product will be overcooked or overcooked (if susceptor is present) in the area near the path of point L. On the other hand, the area of the food product near the path of point K is likely uncooked.

Because uneven cooking caused by the presence of hot and cold regions is undesirable, it has been found advantageous to employ a susceptor assembly formed by a field director structure in combination with a susceptor. The field director structure includes one or more vanes, each having an electrically conductive portion on a cardboard support. The field director structure mitigates the effects of relatively high and low electric field strength regions within the microwave oven by redirecting and repositioning these regions so as to more uniformly heat, cook and char the food product. It has also been found advantageous to use the field director structure alone (i.e. without the susceptor).

When the susceptor assembly is placed in an "unloaded" microwave oven (i.e., a microwave oven in which no food or other items are present) and energy is applied to the oven, detrimental problems have been observed with susceptor overheating and/or overheating of the field director structure and/or arcing.

"susceptor superheat" (or similar term) means heating the lossy susceptor material to the point where the susceptor substrate burns.

"field director structure overheating" (or similar terms) means heating the cardboard support of the blade to the point where it burns. Such overheating may be caused by heat generated by the lossy susceptor material or the arc.

"arcing" (or similar term) is an electrical discharge that occurs when a high electric field exceeds the breakdown threshold of air. Arcing typically occurs near the conductive portions of the blade, particularly along the edges, especially at any sharp corner. Arcing may cause the paperboard support of the blade to discolor, burn, or, in the extreme, ignite and burn.

Most of the common countermeasures for preventing the generation of the arc are not feasible in the microwave oven application. These strategies are also not suitable for disposable packaging of convenience foods.

In view of the above, it is believed advantageous to provide a field director structure and susceptor assembly including a field director structure that prevents the generation of arcing, overheating of the field director, and overheating of the susceptor.

Disclosure of Invention

The present invention is directed to a susceptor assembly that does not overheat and prevents arcing when placed in an "unloaded" microwave oven, i.e., a microwave oven in which no food or other items are present. The microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength.

The susceptor assembly includes a generally planar susceptor having a base plane with an electrically lossy layer. A field director structure having one or more vanes is mechanically coupled to the susceptor. Each blade has a conductive portion that is substantially rectangular in shape having predetermined length and width dimensions and has a first end and a second end thereon. The conductive part of the blade may be formed by a metal foil having a thickness of less than 0.1 mm.

The conductive portion of each blade is positioned at least a predetermined close distance from an electrically lossy layer of a planar susceptor. The predetermined close distance is in a range from 0.025 times the wavelength to 0.1 times the wavelength. In a preferred embodiment, the predetermined close distance is defined by a boundary of a lower conductivity material disposed between the conductive portion of the blade and the lossy layer.

The first end of the conductive portion on each blade is disposed at a distance of at least a predetermined separation distance from the geometric center of the planar susceptor. The predetermined separation distance is at least 0.16 times the wavelength.

In addition to positioning the conductive portion of each blade at a predetermined close distance from the lossy layer, according to one embodiment of the invention, the corners of the conductive portions are rounded (rounded) at a radius that is at most and includes half the width dimension of the conductive portions. According to an alternative embodiment of the invention, the conductive part of the blade may be covered by a non-conductive material selected from the group consisting of polyimide tape, polyacrylic spray coating and polytetrafluoroethylene spray coating, instead of rounding it. According to another alternative embodiment of the invention, instead of the conductive part of the blade being rounded or covered, the conductive part of the blade may be formed by a metal foil having a thickness of less than 0.1 mm, wherein the metal foil is folded to at least twice the thickness along its periphery.

Drawings

The present invention will become more fully understood from the detailed description given herein below, taken in conjunction with the accompanying drawings which form a part of this application, wherein:

FIG. 1A is a graph showing regions of different electric field strengths in a microwave oven and showing the results of the measurements taken at corresponding radial positions P on the turntable₁、P₂And P₃A plan view of the path followed by the three discrete points J, K and L;

FIG. 1B is a graph illustrating the total energy exposure at each discrete point identified in FIG. 1A for one full rotation of the turntable;

FIG. 2 is a diagram of the susceptor assembly with portions of the planar susceptor exploded for clarity, showing various edge shapes of the blades of the field director structure, wherein the electrically conductive portions of the blades are directly adjacent to the planar susceptor;

FIG. 3 is a view similar to FIG. 2, showing a blade of a field director structure, wherein the electrically conductive portion of the blade is spaced from the planar susceptor;

figures 4A to 4C are plan views respectively showing a blade extending generally transversely across a generally straight edge, a curved edge (bent-edge) and an arcuate edge (curved-edge) of a planar susceptor in a direction offset from a generally radial line of the susceptor assembly;

figures 4D to 4F are plan views respectively showing the blade extending generally transversely across the generally straight, curved and arcuate edges of the planar susceptor in a direction transverse to the generally radial lines of the susceptor assembly;

FIGS. 5A and 5B are elevation views taken along view line 5-5 of FIG. 2, respectively showing a blade of a field director having a fixed connection and a flexible hinged connection to a planar susceptor, with the blade in the latter condition shown in a stowed and deployed position;

FIG. 6 is a graph showing the attenuation effect of a single transverse conductive blade on the component field vectors of the electric field component in the plane of a planar susceptor;

figure 7A is a plan view generally similar to figure 1A showing the effect of the field director structure of the susceptor assembly of the present invention on areas of high electric field strength, and again showing P at corresponding radial positions on the turntable₁、P₂And P₃Three discrete points J, K and L;

FIG. 7B is a graph similar to FIG. 1B showing the total energy exposure at each discrete point for one full rotation of the turntable, with the waveforms of FIG. 1B superimposed for ease of comparison;

figures 8A, 9A and 10A are pictorial views of various preferred embodiments of a susceptor assembly according to the present invention with portions of the planar susceptor exploded for clarity;

figures 8B, 9B and 10B are plan views of the susceptor assembly shown in figures 8A, 9A and 10A, respectively;

FIG. 11 is a diagram of a field director structure implemented using a single arcuate vane in accordance with the present invention;

FIG. 12 is a diagram of a field director structure implemented using planar blades with a single bend line therein, in accordance with the present invention;

FIGS. 13A and 13B are a front view and a pictorial view, respectively, of a field director structure implemented using a planar blade having two curved lines therein, in accordance with the present invention;

FIGS. 14 and 15 are pictorial views of two additional embodiments of a field director structure according to the present invention, each having a plurality of vanes flexibly connected to form a collapsible structure;

FIG. 16 is a pictorial view of a field director assembly in accordance with the present invention wherein at least one blade is supported on a non-conductive substrate;

FIGS. 17 and 18 are graphs of the results of examples 6 and 7, respectively;

FIG. 19 is a diagram showing various blade configurations of field director structures with different shapes and positions of the conductive portions;

figure 20 is a plan view of a susceptor assembly incorporating a six-vane field director structure used in examples 9 to 23;

FIG. 21 is an enlarged dimensional view showing a blade configuration having a rectangular conductive portion occupying the entire blade area;

FIG. 22 is an enlarged scale view showing a blade configuration having a generally rectangular conductive portion with rounded corners (rounded horns) and a surrounding non-conductive border portion;

FIG. 23 is an enlarged scale view showing a blade configuration having a generally rectangular conductive portion with rounded corners;

FIGS. 24, 25 and 26 are enlarged size views showing a blade blank (vane blank) having two spaced generally rectangular conductive portions with rounded corners and having a non-conductive border around each conductive portion;

FIG. 27 shows a typical superheat of the susceptor in examples 24-34;

FIG. 28 is an enlarged view showing typical overheating of the susceptor and melting of the protective polymeric coating on the susceptor;

FIG. 29 shows the results of examples 35-40; and is

FIG. 30 shows the results of examples 61-64.

Detailed Description

In the following detailed description, like reference numerals refer to like elements throughout the several views of the drawings.

Referring to fig. 2 and 3, there is shown a typical (stylized) view of a susceptor assembly, generally indicated by reference numeral 10, according to the present invention. The susceptor assembly 10 has a reference axis 10A extending through its geometric center 10C. In use, the susceptor assembly 10 is placed within a resonant cavity on the interior of a microwave oven M. The furnace M is shown in outline only in the figures. In operation, a source in the microwave oven generates electromagnetic waves having a predetermined wavelength. A typical microwave oven operates at a frequency of 2450MHz, producing waves having a wavelength on the order of 12 centimeters (12cm) (about 4.7 inches). The wall W of the microwave oven M imposes boundary conditions that cause the electromagnetic field energy distribution within the oven volume to change. This creates a standing wave energy pattern within the furnace volume.

The susceptor assembly 10 includes a conventional, generally planar susceptor 12, the susceptor 12 having a field director structure generally indicated at reference numeral 14 attached thereto. As will be described further herein, the field director structure 14 serves to redirect and relocate regions of high and low electric field strength of standing wave patterns within the furnace volume. When used in conjunction with a turntable, the position of the pass through redirection and repositioning regions changes continuously, further improving the uniformity of heating, cooking or browning of food products placed on the susceptor assembly 10 including the field director structure 16.

In the embodiment shown in fig. 2 and 3, the field director structure 14 is placed below the planar susceptor 12, however it should be understood that these relative positions may be reversed. Regardless of the relative positions of the field director structure 14 and the planar susceptor 12, respectively, typically a food product or other item to be heated, cooked or coked is placed in contact with the planar susceptor 12.

The profile of the planar susceptor 12 shown in the figures is substantially circular, however it may take on any predetermined desired form consistent with the food product to be heated, cooked or coked within the oven M. As shown in the circled detail in fig. 2, the planar susceptor 12 includes a substrate 12S having an electrically lossy layer 12C thereon. Typically, layer 12C is a thin coating of vacuum deposited aluminum.

The substrate 12S may be made of any of a variety of materials commonly used for this purpose, such as cardboard, paperboard, fiberglass, or polymeric materials such as polyethylene terephthalate, heat-stabilized polyethylene terephthalate, polyvinyl ester ketone, polyethylene naphthalate, cellophane, polyimide, polyetherimide, polyimide ester, polyarylate, polyamide, polyolefin, polyaramide, or polycyclohexylene dimethylene terephthalate. If electrically lossy layer 12C is self-supporting, substrate 12S may be omitted.

The field director structure 14 includes one or more vanes 16. In the embodiment shown in FIGS. 2 and 3, five blades 16-1 through 16-5 are shown. Figures 4A to 4F show a susceptor assembly 10 in which the field director structure 14 has a number N of blades 16, N ranging from 2 to 6. In general, any suitable number 1, 2, 3.. N of blades may be used, depending on the size of the planar susceptor, as well as the edge length, configuration, orientation, and arrangement of the blades.

For purposes of illustration, the blades shown in FIGS. 2 and 3 exhibit various edge profiles, as will be discussed.

The front and back faces of each blade define a surface area 16S. In fig. 2 and 3, the surface area 16S of each blade 16 is illustrated as being generally rectangular, however it should be understood that the surface area of the blade may be suitably arranged in any planar profile, such as triangular, parallelogram or trapezoidal. The surface area 16S of the blade may be curved in one or more directions, if desired.

At least a portion of the front and/or back surfaces of each blade 16 is electrically conductive. Any area with hatching in fig. 2 and 3 represents the conductive portion 16C of the blade 16. The non-conductive portion 16N of the blade 16 is represented by a stippled shading.

Each vane has a rim 16F extending between a first end 16D and a second end 16E. The edge 16F of the blade may present any type of profile. For example, the edges 16F of the blades may be straight, as shown for blades 16-1 through 16-3. Alternatively, the edge 16F of the vane may be curved or folded along one or more curved or fold lines 16L, as shown in vane 16-4. Further, the profile of the blade edge 16F may be arcuate, as shown for blade 16-5 (FIGS. 2 and 3) and blade 16-1' (FIG. 3).

The blade may have its first end 16D and its second end 16E placed at any predetermined respective starting and ending point on the planar susceptor 12. The distance along the blade's edge 16F between its first end 16D and its second end 16E defines the blade's edge length. The vanes in the field director structure 14 may have any desired edge length subject to the attendant limitations noted below with respect to the length of the conductive portion 16C.

The blade 16 may be integrally constructed of conductive foil or other material. In this case, the entire surface 16S of the blade is electrically conductive (e.g., blade 16-1 as shown in FIG. 2). The length and width of the conductive portion 16C thus correspond to the edge length and width of the blade.

Alternatively, the blade may be built up as a layered structure formed from a dielectric substrate having laminated or coated conductive material on some or all of the front and/or back of its surface area. One form of construction may use a paperboard substrate coated with an adhesive backed conductive foil tape.

If the conductive portion 16C is provided to be less than the full surface area of the blade, the conductive portion 16C itself may take any suitable shape, such as trapezoidal (as shown by blades 16-2 and 16-3) or rectangular (as shown by blades 16-4 and 16-5 and blade 16-1' in FIG. 3). The width dimension of the conductive portion 16C of the blade should be about 0.1 to about 0.5 times the wavelength generated in the microwave oven. The conductive portion 16C of the blade should have a length that is at least approximately close to a distance of approximately 0.25 times the wavelength of the electromagnetic energy generated in the microwave oven. An edge length of about 2 times the wavelength of the electromagnetic energy generated in the microwave oven defines a practical upper limit.

Regardless of the shape of the conductive portion, it may be desirable to round off the corners (radius) or "round off" to avoid arcing, as will be described in greater detail in connection with fig. 19.

The shape and length of the conductive portion of the blade, and the spacing of the conductive portion to the susceptor plane and other blades, allows the field attenuation effect of the blade to be more accurately tuned.

The blade may also be arranged to pass through the geometric center 10C, wherever its starting and ending points are. Fig. 2 shows the path of a straight-edged blade 16-1, which extends from a first end 16d, starting from the periphery of the adjacent susceptor, through the geometric center 10C. FIG. 3 illustrates a path for a curved-edge blade 16-1' extending through geometric center 10C from a first end 16D beginning near geometric center 10C. All other vanes in fig. 2 and 3 have paths that begin at a starting point near the geometric center 10C and extend outwardly therefrom.

The blades 16 extend in a generally radial direction relative to the geometric center 10C of the susceptor assembly 10. The blades 16 may be angularly spaced apart by equal or unequal separation angles about the center 10C. For example, the angle 18 between the blades 16-1 and 16-2 may be less than the angle 20 between the blades 16-2 and 16-3.

It should be understood that the term "generally radial" (or similar terms) does not require that each blade be precisely located on a radius from the center 10C. For example, the blades may be offset or inclined with respect to the radius. Fig. 4A to 4C show a straight-edged blade 16T, a curved-edged blade 16B and an arc-edged blade 16V, respectively, which are offset with respect to a radial line R emanating from the geometric center 10C. Similarly, fig. 4D to 4F show straight-edged blades 16T, curved-edged blades 16B and arc-edged blades 16V, respectively, which are inclined with respect to a radial line R emanating from the geometric center 10C. Other arrangements of vanes may be used to achieve the transverse orientation of the vanes 16 relative to the planar susceptor 12.

Each blade 16 is physically (i.e., mechanically) connected to the planar susceptor 12 at one or more connection points. The connection between the blade 16 and the planar susceptor 12 may be a fixed connection or a flexible hinged connection.

The fixed connection is shown in fig. 5A. In a fixed connection, the blade 16 is attached by a suitable adhesive 24 in a predetermined fixed orientation relative to the planar susceptor 12. The orientation of the vanes 16 is preferably at a pitch angle in a range between about 45 degrees (45 °) and about 90 degrees (90 °) relative to the planar susceptor, although smaller angular orientations may provide beneficial effects. In the most preferred example, the vanes 16 are substantially orthogonal to the planar susceptor 12.

The flexible hinge connection is shown in fig. 5B. In this arrangement, the blade 16 is attached to the planar susceptor 12 by a hinge 26. The hinge may be made of flexible tape. In the hinged connection, the blade 16 is movable from a stowed position (shown in phantom in fig. 5B) in which the plane of the blade is substantially parallel to the planar susceptor, to a deployed position (shown in solid outline in fig. 5B). The hinge may be provided with suitable stops to maintain the vanes in the deployed position at a desired tilt angle, preferably in a range between about 45 degrees (45 °) and about 90 degrees (90 °) relative to the planar susceptor, and most preferably substantially orthogonal to the planar susceptor 12.

Regardless of the form of construction, the configuration of the surface area of the blade, the shape of the conductive portion, the edge profile of the blade, the length of the edge of the blade, the length of the conductive portion on the blade, the path of the blade with respect to the center of the susceptor, and the orientation of the blade with respect to the plane of the susceptor, the conductive portion 16C of the blade 16 must be positioned no further than a predetermined close distance from the electrically lossy layer 12C of the planar susceptor 12. Typically, the predetermined close distance should not be greater than a distance of approximately 0.25 times the wavelength of the electromagnetic energy generated within the furnace. It will be appreciated that the predetermined close distance may be zero as long as food or other items are present, meaning that the conductive portion 16C of the blade is in electrical abutment with the lossy layer 12C of the planar susceptor.

In the exemplary embodiment shown in fig. 2, the lossy layer 12C is supported on a dielectric substrate 12S such that the edges of the conductive portion 16C of the blade are spaced from the lossy layer 12C only by the thickness of the substrate 12S. The vertical dimension of the non-conductive portion 16N may be used to control the height of the support planar susceptor 12 within the furnace M.

Alternatively, as can be seen from fig. 3, the non-conductive portion 12N of the blade may be disposed adjacent to the planar susceptor 12. This arrangement has the effect of separating the conductive portion 16C of the blade from the lossy layer 12C by a distance greater than the thickness of the substrate 12S. If desired, additional non-conductive portions 16N may be provided along opposite edges of the blade in order to obtain the height control benefits described above.

The planar susceptor 12 and the surface area 16S of the blade 16 intersect along an intersection line 12L, the intersection line 12L extending in a generally transverse direction with respect to the planar susceptor 12. When intersecting a planar susceptor 12, a straight-edged blade 16 will produce a straight intersection line 12L. A blade 16 having a curved or arcuate edge will produce a curved or arcuate line of intersection 12L, respectively, when intersecting the planar susceptor 12. The magnitude of the curve angle or the shape of the curve of the intersection line will depend on the angle of inclination of the blade to the planar susceptor, as the case may be. Whether the line of intersection is a straight line, a curved line or an arc, the conductive surface of the blade will extend along the line of intersection.

Having described various structural details of the susceptor assembly 10 according to the present invention, its effect on standing electromagnetic waves will now be discussed.

Figure 6 is a schematic representation in which an embodiment of the susceptor assembly 10 having a single straight-edged blade 16 is attached in a substantially orthogonal orientation relative to the lower surface of the planar susceptor 12. A set of cartesian axes is positioned to begin at the geometric center 10C of the assembly 10. The assembly 10 is arranged so that the planar susceptor 12 lies in an X-Y cartesian plane and the conductive portion 16C of the surface 16S of the blade 16 lies in an X-Z cartesian plane. An intersection line 12L defined along the connection between the blade 16 and the planar susceptor 12 extends transversely across the lossy layer 12C of the planar susceptor 12 and is oriented along the X-axis, as shown. The electrically conductive portion 16C of the surface 16S of the blade 16 is located a predetermined distance D in the Z-direction from the lossy layer on the planar susceptor 12. The conductive portion 16C of the surface 16S has a thickness (i.e., its Y dimension) greater than the depth of the skin effect (skinneffect) of the conductor at the microwave operating frequency.

The electromagnetic wave is composed of a magnetic field and an electric field that oscillate orthogonally to each other. At any given instant, the electromagnetic standing wave includes an electric field component E. At any instant, the electric field component E is oriented in a given direction within the cartesian space and may have any given value.

Electric fieldWhich itself may be decomposed into three component vectors, i.e.,andeach component vector is oriented along its respective corresponding coordinate axis. As the case may be, each component vector has a predetermined value in "x", "y", or "z" units depending on the value of the electric field E.

One corollary of the faraday's law of electromagnetism is the boundary condition that the tangential electric field at the interface surface between two media must be continuous across the surface. A particular example of such a dielectric interface is the interface between an ideal conductor and air. By definition, an ideal conductor must have zero electric field inside it. Thus, in particular, the tangential component of the electric field just inside the conductor surface must be zero. Therefore, the tangential electric field in air just outside the conductor must also be zero, according to the above-mentioned boundary continuity condition. So we have the general rule that the tangential component of the electric field at the ideal conductor surface is always zero. If the conductor is a good conductor, but not an ideal conductor, the tangential component of the electric field at the surface may be non-zero, but it remains very small. Thus, any electric field present just outside the surface of the good conductor must be substantially perpendicular to the surface.

The application of this laws of physics requires that, in the region of the surface of the blade 16 having the conductive portion 16C, only the component vectors of the electric field oriented perpendicular to the surface, i.e. the vectors, are allowedAre present.

The component vectors of the electric field that are not permitted to lie in any plane tangential to the surface of the blade (i.e. vectors)Sum vector). In fig. 6, the tangent plane is the plane of the conductive part of the blade surface.

If the blade 16 isConductive portion 16C is in electrical contact with lossy layer 12C, for the reasons discussed above, the component vector along intersection line 12LValue and component vector ofWill be zero. However, the conductive portion 16C is not in electrical contact with the lossy layer 12C, but instead is separated from the lossy layer 12C by a distance D. However, the electrically conductive part of the blade surface exerts a damping effect which has its most significant effect in the region of the electrically conductive part of the blade surface.

Thereby, the component vector of the electric field of the waveAndwith attenuated intensity "x" only_a"and" z_a". Intensity value "x_a"and" z_a"are each some intensity value less than" x "and" z ", respectively. Attenuation of the electric field component of the electromagnetic wave in a plane tangential to the blade surface results in an enhancement of the electric field component oriented perpendicular to the conductive portion of the blade surface. Thereby, the component vectorHaving an enhanced intensity value "y" greater than intensity value "y_e”。

Vector componentDepending on the magnitude of the distance D and the orientation of the conductive portion 16C relative to the lossy layer 12C. This attenuation effect is most pronounced when the distance D is less than one-quarter (0.25) of a wavelength, which for a typical microwave oven is a distance of about 3 centimeters (3 cm). At an angle of inclination of less than 90 degrees, the allowable field (i.e. with the conductive surface of the blade)Perpendicular field) will itself have a component acting in the plane of the susceptor.

This effect is used by the susceptor assembly 10 of the present invention to redirect and relocate areas of relatively high electric field strength within a microwave oven.

FIG. 7A is a typical plan view, generally similar to FIG. 1A, illustrating the effect of the blades 16 as they are carried by the disk T in the direction of rotation indicated by the arrow. The vanes are shown in outline and their thickness is exaggerated for clarity of explanation.

Considering the case at position 1, the blade first encounters the hot zone H near position 1₂. For the reasons explained previously, only electric field vectors having attenuated intensity are allowed to exist in the hot zone H covered by the blade 16₂Within the section (b). However, even if only the attenuated field is allowed to exist, the energy content of the electric field does not disappear. Instead, the damping effect in the region extending from the conductive part of the blade manifests itself by causing the electric field energy to relocate from its initial position a on the planar susceptor 12 to a displaced position a'. This energy repositioning is shown by displacement arrow D.

Similar results are obtained when the rotary sweep (sweep) brings the blade 16 to position 2. The damping effect of the blade again only allows the damped field to exist in the region extending from the conductive part of the blade. The energy in the electric field energy initially at location B on the planar susceptor 12 is displaced to location B 'as indicated by displacement arrow D'.

When the blade 16 sweeps the entire region H of relatively high electric field strength₁To H₅(FIG. 1A), similar energy relocation and redirection occurs.

The same effect will be obtained using the invention in a microwave oven with a mode stirrer apparatus (mode stirrer apparatus).

Fig. 7B is a graph illustrating the total energy exposure at each discrete point J, K and L for one full rotation of the turntable. On which the corresponding waveforms of the graph of fig. 1B are superimposed.

As is clear from fig. 7B, the presence of the susceptor assembly 10 with the field director 14 according to the present invention results in a substantially uniform total energy exposure. Thus, heating, cooking and browning of food products placed on the susceptor assembly 10 are improved over what exists in the prior art.

Figures 8A and 8B, 9A and 9B and 10A and 10B show a preferred configuration of a susceptor assembly according to the present invention.

FIGS. 8A and 8B illustrate a blade 16 having five straight edges²-1 to 16²-5 field director structure 14²Susceptor assembly 10². Five blades 16²-1 to 16²-5 are attached to the bottom surface (undercut) of the planar susceptor 12. The vanes are substantially orthogonal to the planar susceptor 12 and are equiangularly disposed about the center 10C. Blade 16²1 extending through the center 10C and the blades 16²-2 to 16²-5 starts in the vicinity of the centre 10C. Conductive part 16²C covers the entire surface of each blade. The field director 14 may be further modified if desired²The bottom edge of the blade is supported on a non-conductive planar support member 32.

The support member may be connected to all or some of the blades.

FIGS. 9A and 9B illustrate a blade 16 having two curved edges³-1 and 16³-2 field director structure 14³Susceptor assembly 10³. Two blades 16³-1 and 16³2 are attached to the bottom surface of the planar susceptor 12. The vanes are substantially orthogonal to the planar susceptor 12 and are equiangularly disposed about the center 10C. The blades intersect each other in the vicinity of the center 10C. Conductive part 16³C covers the entire surface of each blade. Again, if desired, the field director 14 may be further supported by a non-conductive planar support member 32³The bottom edge of the blade.

FIGS. 10A and 10B illustrate a blade 16 having six straight edges⁴-1 to 16⁴Field director structure 14 of-6⁴Susceptor assembly 10⁴. Six blades 16⁴-1 to 16⁴-6 is attached to the bottom surface of the planar susceptor 12. The vanes are substantially orthogonal to the planar susceptor 12 and are equiangularly disposed about the center 10C. All the blades start from the vicinity of the center 10C. Conductive part 16⁴C covers the entire surface of each blade. A non-conductive planar support member 32 may be used.

If desired, the blade 16⁴-1 and 16⁴4 itself may be formed by a section of non-conductive component 16⁴And N is connected. Component 16 is shown in dotted outline with dotted shading in FIG. 10A⁴N。

In a second aspect, the present invention is directed to various embodiments of a collapsible self-supporting field director structure embodying the teachings of the present invention.

Fig. 11, 12, 13A and 13B show field director structures formed from a single blade. In each embodiment, the blade has a folded back band, thereby making it possible to form the planar blade as a self-supporting structure oriented in a predetermined orientation with respect to a predetermined reference plane RP provided within the furnace M. The plane RP may conveniently be defined as the plane on which the surface of the turntable or the surface of the food or other item disposed within the oven lies.

In FIG. 11, a single curved blade 16 is used⁵Field director structure 14⁵. Blade 16⁵May be arcuate or may have first and second ends 16⁵D and 16⁵E at least one buckling or bending region 16 defined therebetween⁵And R is shown in the specification. Conductive part 16⁵C covers the entire surface of the blade. In use, the blade 16 may be made⁵Formed as a self-supporting structure arranged in a predetermined orientation relative to a predetermined reference plane RP.

The field director structure 14 shown in fig. 12⁶Middle blade 16⁶Having a single fold or bend line 16 therein⁶L-1. In use, may follow the bend line 16⁶L-1 folding or bending blades16⁶To define a self-supporting structure in a predetermined orientation relative to a predetermined reference plane RP within the furnace M. The same effect can be obtained by flexibly attaching two straight-edged blades along a flexible connecting line instead of a folding or bending line.

FIGS. 13A and 13B are illustrations using a wire having two bend lines 16, respectively⁷L-1 and 16⁷L-2 conductive planar blade 16⁷Implemented field director structure 14⁷Front view and illustration. Along the bending line 16⁷L-1 and 16⁷L-2 curved blades 16⁷Forming ears 16⁷E-1 and 16⁷E-2 for supporting the planar blade in a predetermined desired orientation relative to a predetermined reference plane RP within the furnace M.

Fig. 14 and 15 are diagrams of two additional embodiments of foldable self-supporting field director structures according to the present invention. Each field director structure has a blade array comprising a plurality of blades flexibly connected to form a structure that can be made self-supporting.

A field director structure 14 as shown in fig. 14 and 15⁸In, the blade array includes blades 16⁸-1 to 16⁸-5, each blade having an electrically conductive surface thereon. At the connection point 16⁸Each blade is flexibly connected to at least one other blade at F. The flexibly connected blades can fan towards and away from each other, as indicated by arrows 16⁸J is shown. In use, the field directors can be self-supporting as the blades in the array fan out from each other, with each blade in the array disposed at a predetermined orientation relative to a predetermined reference plane RP within the furnace. In a modified embodiment, the strut 16 may be used⁸S is connected to the free end of each of the at least three blades. The struts are made of any material that is transmissive to microwave energy.

The field director structure 14 shown in fig. 15⁹Comprising a pair of blades 16⁹-1 and 16⁹-2, each blade having an electrically conductive surface thereon. At the connection point 16⁹F flexibly connecting each blade to the other. Flexibly connected bladeCapable of fanning towards and away from each other, as indicated by arrows 16⁹J is shown. In use, the field directors can be self-supporting as the vanes in the array fan out of each other, with each vane in the array disposed at a predetermined orientation relative to a predetermined reference plane within the furnace.

Although the blade in each of the embodiments shown in figures 11 to 15 is shown in the form of a conductive portion extending over the entire surface of the blade, it will be appreciated that the conductive portion of any blade may take on any alternative shape.

It will also be appreciated that the field director structure of the present invention need not be collapsible but may be self-supporting by the use of suitable non-conductive support members. Fig. 16 is a diagram of a field director assembly, generally indicated by reference numeral 31. The field director assembly 31 shown in fig. 16 includes at least one blade 16 attached to a planar non-conductive support member 32 whereby the conductive surface of the blade is oriented in a predetermined orientation (shown as being generally orthogonal to the support member). If additional blades are provided, these additional blades are supported on the same support member. The blades may or may not be connected to each other as desired. The support member may be attached below or above the vanes.

It should also be understood that any embodiment of a field director structure falling within the scope of the present invention may be used with a separate planar susceptor (described earlier). It will also be appreciated that for some food products it may be desirable to place a second planar susceptor on the food product or wrap the food product with a flexible susceptor.

Examples 1 to 8

The operation of the field director structure and susceptor assembly according to the present invention will be more clearly understood from the following examples.

Introduction to

For all of the following examples, commercially available microwaveable pizzas were used in the cooking experiments (Microwave four cheese pizza, 280 grams).

The pizza in the package was supplied to a flat susceptor consisting of a thin layer of vapour-deposited aluminium sandwiched between a polyester film and a cardboard. The planar susceptor is used with various embodiments of the field director structure of the present invention, as will be discussed. The edges of the supplied paperboard were shaped to form an inverted U-shaped cooking pan with the planar susceptor spaced about 2.5cm above the turntable in the microwave oven. No crisping rings (for the edges of the coked pizza) are used that are provided with the pizza in the package.

In all cases, the planar susceptor was placed directly on the turntable of the microwave oven. Except for example 5, where the frozen pizza was cooked for 7.5 minutes at lower power, in all examples the frozen pizza was placed directly on the flat susceptor and cooked for 5 minutes at full power.

For comparison purposes, one group of 3 pizzas was cooked using only a planar susceptor without a field director structure, and the other group of 3 pizzas was cooked using a planar susceptor with a field director structure of the present invention.

The vanes of each field director were constructed using 0.002 inch (0.05mm) thick aluminum foil, cardboard, and tape.

For examples 1 to 7, the field director structure was placed in the space below the planar susceptor. For example 8, the field director structure was positioned over the pizza.

Coking and coking profile (profile) measurements

The percent and distribution of coking of the bottom crust of the pizza was measured according to the procedure described by Papadaikis, S.E. et al in "A Versatile and Inexpersive Technology for Measuring Color of Foods Technology, 54(12) pp.48-51 (2000). The lighting system was set up and an image of the cooked bottom shell was taken using a digital camera (Nikon, model D1). The color parameters were converted to the L-a-b color model, the preferred color model for food research, using commercially available image and graphics software programs. The percentage of char area is defined as the percentage of pixels with a luminance L value less than 153 (255 is brightest on a luminance scale of 0 to 255), according to recommendations from the referenced procedure. The coking profile (i.e., the percentage of coking area as a function of radial position) is calculated according to the method described in the referenced procedure.

The image of the bottom housing is divided into a plurality of concentric annular rings and an average L value is calculated for each annular ring.

It is believed that the following examples illustrate the coking and the improvement in coking uniformity obtained using the different field director configurations of the present invention.

Example 1

The cooking was carried out in the manner described in the introduction in a microwave oven of the General Electric (GE) brand number JES1036WF001 at 1100 WattsMicrowave four cheese pizza. When a field director is used, the field director structure according to fig. 14 is used (without the struts 16)⁸S). Blade 16⁸1 has a length dimension of 17.5 cm and a width dimension of 2 cm. Blade 16⁸-2 to 16⁸5 each having a length dimension of 8 cm and a width dimension of 2 cm.

As described, the bottom housing is imaged with a digital camera after cooking. The percentage of coked area was calculated from the image data using the described procedure. The average percentage of the coked area of the pizza that was not cooked using the field directors was determined to be 40.3%. The average percentage of the scorched area of the pizza cooked with the field directors was determined to be 60.5%.

Examples 2 to 5

The experiment described in example 1 was repeated in four microwave ovens of different manufacturers. The microwave oven manufacturer, model, full power wattage and cooking time for each example are summarized in table 1. The table reports the percent char area obtained with and without the use of field directors. It should be noted that the coking zone percentage is improved in all cases.

TABLE 1

Comparison of the percentage of area coked with and without the field director

Example 12345

Furnace brand GE Sharp Panasonic Whirlpool Goldstar

Wattage 1100110012501100700

Model # JES1036WF 001R-630 DW N5760WA MT4110SKQ MAL783W

Cooking time 5 minutes 6 minutes 7.5 minutes

Percentage of coking area

60.5 percent of field director, 70.7 percent of field director, 61.760.7 percent of field director and 51.4 percent of field director

No field director 40.3%, 55.2%, 50.3%, 15.3%, 31.5%

Example 6

280 g ofThe microwave four cheese pizza was cooked in a 1100 watt oven brand Sharp model R-630 DW. When a field director structure is used, a field director structure according to fig. 15 is used. Blade 16⁹-1 and 16⁹The length dimension of-2 is 22.9 cm and the width dimension is 2 cm. From the connection point 16⁹Each section of the F extended cambered vane has a radius of curvature of about 5.3cm and a camber angle of about 124 degrees.

As described, after cooking, an image of the bottom shell was taken with a digital camera and the percentage of the area that had been coked was calculated.

The average percentage of the scorched area of the pizza cooked without the use of field directors was 55.2%. The average percentage of the scorched area of the pizza cooked using the field directors was determined to be 73.8%. The coking profile was plotted and is shown in figure 17.

Example 7

The test described in example 6 was repeated using a 1300 watt oven, model NN5760WA, branded as Panasonic. The average percentage of the scorched area of the pizza cooked without the use of field directors was 50.3%. The average percentage of the scorched area of the pizza cooked using the field directors was determined to be 51.7%. It can be observed from the graph shown in fig. 18 that a substantially uniform coking profile is achieved as a result of the use of the present invention. It can be appreciated from a review of figure 18 that the coking profile along the radius is greatly improved by the use of the field director configuration.

Example 8

The test described in example 1 was repeated using a 700 watt microwave oven, brand Goldstar model MAL 783W. When a field director structure is used, the structure according to fig. 14 with the struts 16 is used⁸S field director structure. The legs are 5cm high and are placed on a turntable to support the field director just above the pizza. After the shell of the pizza rises, the field director structure only contacts the top of the pizza.

As described, after cooking (using the full power of the oven for 7.5 minutes) an image of the bottom shell was taken with a digital camera and the percentage of the area that had been coked was calculated.

The percentage of the scorched area of the pizza cooked without the field directors was 31.5%. The percentage of the scorched area of the pizza cooked using the field directors was determined to be 65.1%.

When a microwave susceptor assembly as described above is placed in an "empty" microwave oven (i.e., an oven without food or other items), several detrimental problems are observed. These problems are particularly evident in furnaces with high wattage (i.e., furnaces with power ratings typically greater than 900 watts). In some instances, microwave oven susceptor assemblies may overheat even when an article is present.

When the lossy layer 12C of the planar susceptor 12 overheats, melting or scorching of the substrate 12S may occur. The susceptor may overheat to the point that the susceptor substrate burns. The electrically conductive portions of the vanes of the field director structure may arc, particularly along the edges, and particularly at the corners. The creation of an arc discolors, burns or overheats the non-conductive (typically cardboard) support of the blade to the point of igniting a fire. Overheating of the field director structure may also be due to overheating of the susceptor material.

It is therefore considered advantageous to provide a field director structure and susceptor assembly including a field director structure that is "abuse tolerant", i.e. a structure that prevents arcing and/or overheating of the field director and/or overheating of the susceptor.

FIG. 19 shows a structure 14 with field directors¹⁰Susceptor assembly 10¹⁰A combination of (a) and (b). The blade depicted in FIG. 19 shows a blade for examples 9-64 below.

As described above with respect to fig. 2, the susceptor assembly 10¹⁰Comprising a generally planar susceptor 12 having a substrate 12B with a charged lossy layer 12C.

Field director structure 14¹⁰Having at least one, but preferably a plurality of blades 16¹⁰Each blade is mechanically connected to a planar susceptor 12. Each blade 16 shown in FIG. 19¹⁰-1 to 16¹⁰8 is a substrate 16 of electrically non-conductive material¹⁰N is formed. Each blade is generally rectangular in shape. Substrate 16¹⁰N is visible on some blades. Substrate 16¹⁰N may have a flame retardant component applied thereto.

It should be understood that field director structure 14¹⁰May optionally be used with a planar non-conductive support member 32 to define a field director assembly generally indicated by reference numeral 31.

Each blade 16¹⁰All have a surface 16¹⁰S, for clarity of illustration, only the blades 16 are labeled¹⁰6. Surface 16 of each blade¹⁰At least a part 16 of S¹⁰C is electrically conductive. As will be described below, each blade 16¹⁰Is electrically conductive part 16¹⁰C is positioned relative to the planar susceptor 12 and is configured in various ways to prevent overheating and arcing problems.

Each blade 16¹⁰Is electrically conductive part 16¹⁰C has a first end 15¹⁰D and a second end 15¹⁰E. Also for clarity, only at the blades 16¹⁰These ends are indicated on-6. First end 15¹⁰D and a second end 15¹⁰E defines a conductive portion 16¹⁰C predetermined length dimension. The conductive portion 16 of each blade¹⁰C also exhibits a predetermined width dimension. As already described above (e.g., in conjunction with fig. 2 and 3), the length dimension should be in the range of between about 0.25 to about 2 times the wavelength of the standing electromagnetic wave generated in the furnace. The width dimension should be in the range of about 0.1 to about 0.5 times the wavelength.

Blade 16¹⁰Conductive portion 16 of-1¹⁰C-1 occupies the entire rectangular surface. Conductive part 16¹⁰C-1 is adjacent to a planar susceptor 12. Blade 16¹⁰-1 is used forTypical blade configurations that overheat when the furnace is empty. While the susceptor 12 has blades 16¹⁰1, may also overheat when used together, leading to melting or scorching of the susceptor substrate 12S. Blade 16¹⁰The conductive part of-1 may form an arc along its edge or at its corners.

Blade 16¹⁰Conductive part 16 of-2¹⁰The shape of C-2 is also rectangular. The conductive part 16¹⁰C-2 occupies only a portion of the blade surface, leaving base 16¹⁰A portion of N is exposed to define a boundary 19L along the bottom edge. Conductive part 16¹⁰C-2 abuts the planar susceptor 12. Blade 16¹⁰The configuration of-2 is shown to limit, but not eliminate, overheating of the blades and susceptor when used in an empty furnace (examples 36, 39). When and with vanes 16¹⁰When the field director structures of-2 are used together, the susceptor 12 may also overheat, causing the substrate 12S to melt or char.

As will be explained, the blade 16¹⁰-3 to 16¹⁰-5，16¹⁰-7 and 16¹⁰-8 illustrates a conductive part 16 according to the invention¹⁰C, which may prevent problems with susceptor overheating and/or field director overheating and/or arcing.

Blade 16¹⁰-3 is a substrate 16¹⁰N adjacent to the example of a blade of the planar susceptor 12. In this example, the conductive portion 16¹⁰C-3 is positioned on the blade such that a top boundary 19T of the non-conductive substrate material is exposed along the edge of the blade adjacent the susceptor 12. The boundary 19T is used to make the blade 16¹⁰Conductive part 16 of-3¹⁰C-3 is spaced from susceptor 12 by a predetermined close distance 21D. Dimension 21D, measured in a direction normal to the plane of susceptor 12, is in the range of 0.025 to 0.1 times the wavelength of a standing electromagnetic wave generated in a microwave oven in which susceptor assembly 10 is used¹⁰. That is, dimension 21D should be at least 0.025 times the wavelength. Also, dimension 21D should not be greater than 0.1 times this wavelength (i.e., dimension 21D ≦ 0.1 times the wavelength). It should be noted that the maximum distance mentioned before17D and the maximum distance indicated by reference character D in fig. 6 (i.e., 0.25 times the wavelength) are dimensions determined with a clear understanding of the contents in the microwave oven using the blade.

Blade 16¹⁰Conductive portion 16 of-4¹⁰C-4 is sized such that its base 16¹⁰Portions of N are exposed to define radially inner and outer boundaries 19D and 19E, respectively. Further, the upper and lower boundaries 19T and 19L of the base material 16N are exposed.

Blade 16¹⁰-5 is a conductive part 16¹⁰C-5 is generally rectangular (similar to conductive portion 16)¹⁰C-4) but with rounded corners. Rounding the corner with a radius dimension 15R, the radius dimension 15R being at its maximum and including a conductive portion 16¹⁰Half the width dimension of C-5 (i.e., 15R ≦ 0.5 times the width). When the corners are rounded, the length of the conductive portion is defined by the radial extent of the conductive portion. Blade 16¹⁰5 also has boundaries 19T, 19L, 19D, 19E (similar to blade 16)¹⁰Those shown around C-4). The size of the lower boundary 19L is indicated by reference numeral 21L.

Blade 16¹⁰-6 also presents a conductive portion 16 with rounded corners¹⁰And C-6. However, the conductive portion 16¹⁰C-6 extends the full width of the blade and abuts the planar susceptor 12. Conductive part 16¹⁰C-6 is not spaced a predetermined close distance from the planar susceptor 12.

Blade 16¹⁰7 is the conductive part 16 of the blade¹⁰One example of C-7 being made of a metal foil, 16¹⁰The locations indicated by C-7F are folded to define a thickness along their perimeter of at least twice. Boundaries 19T, 19L, 19D, 19E (similar to blade 16)¹⁰Those boundaries shown around 4) are along the conductive portion 16¹⁰The periphery of C-7.

Blade 16¹⁰Conductive portion 16 of-8¹⁰C-8 occupies its entire rectangular surface. For the blade, the conductive part 16¹⁰The requisite spacing 21D of C-8 from susceptor 12 is by use of mountingAn arrangement is obtained in which the blade is physically arranged separate from the susceptor.

Of course, it should also be appreciated that the requisite spacing 21D may also be provided by a separation distance from the susceptor and by appropriately sized bounded vanes (i.e., vanes 16)¹⁰-3，16¹⁰-4，16¹⁰-5 or 16¹⁰-7) of the boundary width.

When a plurality of blades are used, as shown in fig. 19 and 20, the first end 15 of the conductive portion of each blade is as appropriate¹⁰D is disposed at a predetermined separation distance 21S from the geometric center 12C of the planar susceptor 12 or the geometric center 32C of the planar support member 32. The separation distance 21S measured in a direction parallel to the plane of the susceptor 12 or the support part 31 should be such that the susceptor assembly 10 is used¹⁰At least 0.16 times the wavelength of the standing electromagnetic wave generated in the microwave oven.

It has been found that the conductive portion 16 of each blade is formed from a single piece of material¹⁰First end 15 of C¹⁰D is positioned a predetermined separation distance 21S from the geometric center 12C of the planar susceptor 12 to mitigate against the occurrence of susceptor overheating near the center of the susceptor (examples 18, 19, 20-22). It has been found that locating the conductive portion of the blade at a predetermined close distance 21D from the electrically lossy layer of the planar susceptor (whatever the way this spacing is achieved) mitigates the occurrence of overheating of the susceptor (examples 35, 37). The occurrence of susceptor overheating may be further mitigated by providing a lower boundary 19L (examples 36, 39).

According to the invention, the combination of the arrangement of the conductive portions of the vanes at a predetermined separation distance 21S, together with the arrangement of the conductive portions of the vanes at a predetermined close distance 21D from the planar susceptor, prevents the occurrence of overheating of the susceptor when used in an unloaded microwave oven.

Also, according to the present invention, positioning the conductive portion of the blade at a predetermined close distance 21D from the electrically lossy layer of the planar susceptor and rounding the corners of the conductive portion with a radius 15R prevents arcing when used in an unloaded microwave oven.

Also in accordance with the present invention, by positioning the electrically conductive portion of the blade at a predetermined short distance 21D from the electrically lossy layer of the planar susceptor and covering the blade 16 with a non-conductive material¹⁰-3 to 16¹⁰-5，16¹⁰-7，16¹⁰The conductive portion of any of-8 prevents arcing in an unloaded microwave oven, and the non-conductive material is a polyacrylic spray coating or a polytetrafluoroethylene spray coating or a polyimide tape.

Still according to the invention, the conductive part of the blade is arranged at a predetermined short distance 21D from the electrically lossy layer of the planar susceptor and the peripheral thickness of the conductive part of the foil is increased (to provide for a blade 16 with an increased thickness¹⁰The manner shown on-7) will prevent the occurrence of electric arcs when used in an unloaded microwave oven.

Examples 9 to 23

The following examples describe tests conducted to determine the mitigation or elimination of overheating and/or arcing problems. Examples 9-23 used a General Electric (General Electric) model JES1456BJ01, 1100 watt microwave oven. The test is performed with the oven empty, i.e. without food or other items in the oven. These examples are summarized in table 2 herein.

Example 9 is a comparative example where the conductive portion of a single blade has no boundaries and no rounding of the corners.

Examples 10-13 and 14-17 tested the effect of a non-conductive coating on the conductive portion of an individual blade. In examples 10-13, the conductive portion was 3/4 "(0.75"; 19mm) wide, with rounded corners; the conductive portions of examples 14-17 were 1 "wide (25.4mm) with rounded corners.

Examples 18-20 test the effect of varying the center gap between diametrically opposed conductive portions on arcing and overheating.

Examples 21-22 test alternative materials for the conductive portion. Example 23 the effect of flame retardant treatment of paperboard on arcing and burning was tested.

Example 9

In this example, the blade 16 according to FIG. 19¹⁰1 configuring and positioning a single blade with respect to a susceptor. An enlarged view of the size of this vane is shown in figure 21. Conductive portions of aluminum foil with square corners, 3-1/2 '(3.5') long and 1 '(88.9 mm. times.25.4 mm) wide backed with adhesive, 0.002' (0.05mm) thick, from Merco Co., Hackenscack, NJ, were applied to cellulose paper board of the same size. The board is from International Paper corporation (International Paper) (grade code 1355, 0.017/180# Fortress Uncoated Cup Stock). The blade was then taped with a polyimide tape (from E.I. DuPont de Nemours and Company) having a thickness of 0.001' (0.025mm)Polyimide tape) is provided withThe bottom surface of a commercial susceptor assembly of a microwave cheesecake pizza (280 grams). This configuration results in arcing within 28 seconds when exposed to an unloaded microwave oven.

Examples 10 to 13

In these examples, the blade 16 according to FIG. 19¹⁰-5 configuring and positioning the single blade with respect to the susceptor. An enlarged view of the size of this vane is shown in fig. 22.

Examples 10-12 provide a protective covering of non-conductive material over an aluminum conductive portion to prevent arcing. The form without the cover-example 13 was also tested as a control.

The conductive portion of each vane, 3-1/2 "(3.5"; 88.9mm) long by 3/4 "(0.75"; 19.2mm) wide, was cut from the same adhesive backed 0.002 "(0.05 mm) aluminum foil used in example 9, and was applied to the same 4" x 1 "(101.6 mm x 25.4mm) rectangular cellulose paperboard as in example 9. The conductive portion is 3/4 "(0.75"; 19.2mm) wide to ensure that the non-conductive covering covers all edges of the aluminum conductive portion. The top border of the 1/8 "(0.125"; 3.2mm) of the cardboard was exposed on the conductive portion. 1/8 "(0.125"; 3.2mm) has a boundary dimension of about 0.025 times the wavelength. All corners of the conductive portion are rounded with a radius 3/8 "(0.375"; 9.6 mm).

The lower border of the cardboard 1/8 "(0.125"; 3.2mm) is also exposed under the conductive portion. A border of 1/4 "(0.25"; 6.4mm) of paperboard was exposed on each end.

Different non-conductive materials were used as the cover as follows:

example 10-0.001 "(0.025 mm) polyimide tape 1" (x (25.4mm) thick (under the trademark DuPont de Nemours and Company, E.I. DuPont)Sale)

Example 11 polyacrylic acid spray coating from Minwax

Example 12 Polytetrafluoroethylene spray coating (under the trademark "DuPont de Nemours and companySale)

Example 13 uncoated

None of the blades shown arc when exposed to no load in the microwave for 2 minutes.

Examples 14 to 17

In these examples, the blade 16 according to FIG. 19¹⁰-6 configuring and positioning the single blade with respect to the susceptor. An enlarged view of the size of this vane is shown in figure 23.

Examples 14-16 evaluated the same nonconductive protective covering disposed over the aluminum conductive portion as in examples 10-12, respectively, but the aluminum conductive portion was 1 "(25.4 mm) wide as the paperboard. The uncovered form, example 17, was also tested as a control. In each case, the conductive portion was an aluminum foil 3-1/2 "(3.5"; 88.9mm) long, 1 "(25.4 mm) wide, backed with adhesive, and 0.002" (0.05mm) thick, applied to a rectangular cellulose paperboard 4 "× 1" (101.6mm × 25.4mm) as used in examples 10-13. The conductive portion rounds all corners at a radius of 1/2 "(0.5"; 12.7mm) and the cardboard exposes a 1/4 "(0.25"; 6.4mm) border at both ends.

Different non-conductive materials were used as the cover as follows:

example 14-polyimide tape 0.001 "(0.025 mm) thick, 1" (25.4mm) wide (under the trademark "DuPont de Nemours and Company from E.ISale)

Example 15 polyacrylic acid spray coating from Minwax

Example 16 Polytetrafluoroethylene spray coating (trade Mark from E.I. DuPont de Nemours and company)Sale)

Example 17-no coating.

In example 14, the surface of the conductive portion was covered with a polyimide tape. The top and bottom edges were not covered with polyimide tape.

In examples 15-16, the surfaces of the conductive portions were covered with polyacrylic acid or polytetrafluoroethylene spray coatings, respectively. The top and bottom edges of the aluminum conductive portion are covered with only an incidental over-coating (incidenal over-spray) of a polyacrylic or polytetrafluoroethylene coating.

In examples 14, 16 and 17, the bottom edge of the conductive part generated an arc at the center. When exposed to a microwave oven empty, arcing occurs in a very short time. In example 15, no arcing occurred.

The test results are more specifically shown below:

example 14-conductive part of the vane is 0.001' (0.025mm) thickTape covering, arcing after 16 seconds of exposure

Example 15-conductive part of blade was spray coated with polyacrylic acid and no arcing occurred within 2 minutes

EXAMPLE 16 conductive portion of blade PolytetrafluoroethyleneSpray coating, arc generation after 12 seconds exposure

Example 17-conductive part of blade uncovered, arcing after 17 seconds of exposure

Figure 20 is a plan view of a susceptor assembly incorporating the 6-vane field directors used in examples 18-23. It will be appreciated from fig. 20 that the end-to-end gap ("gap") between the conductive portions of diametrically opposed blades is twice the separation distance 21S.

Example 18

In this example, each of the 6 blades of the field director of fig. 20 is a blade 16 according to fig. 19¹⁰-5 is provided with a conductive part.

As shown in FIG. 24, the conductive portion of each of the 3 blade blanks has a length by width of 3-1/2 "(3.5") by 3/4 "(0.75"), i.e., 88.9mm by 19.2mm, and all corners are rounded with a radius of 3/8 "(0.375"; 9.6 mm). The conductive portions were cut from the same adhesive backed aluminum foil 0.002 "(0.05 mm) thick as used in the previous examples 9-17. Two of these conductive portions were placed on 8 '. times.1' (203.2 mm. times.25.4 mm) rectangular cellulose paper sheets used in examples 9-17 such that the conductive portions were exposed above, below, and at the outer ends to 1/8 '(0.125'; 3.2mm) boundaries of the paper sheets. An end-to-end gap of 3/4 "(0.75"; 19.2mm) was left between the inner ends of each conductive portion.

Each of the 3 blade blanks was then bent in the middle to form a V-shape and positioned under the susceptor with the apex of each V-shape at the center of the susceptor, thereby defining a separation distance 21S of 3/8 '(0.375'; 9.6mm) (FIG. 19). The V-shaped blade blank is bonded to the bottom surface of the susceptor using a water soluble Adhesive such as model BR-3885 from Basic Adhesive, inc. These blanks are positioned so that the blades are equally spaced in a radial spoke pattern. The fully assembled susceptor assembly was arranged so that the pairs of conductive portions were directly opposed at an end-to-end gap of 3/4 "(0.75"; 19.2 mm).

There was no discernable arcing when the susceptor assembly was exposed to an empty microwave oven, but the susceptor assembly did ignite when the center paperboard substrate overheated within 47 seconds.

Example 19

In this example, each of the 6 blades of the field director of fig. 20 is provided with a blade 16 according to fig. 19¹⁰-5 conductive parts.

The blade in this example was constructed from the blade blank illustrated in fig. 25 in the same manner as in example 18. The blade stock was an identical rectangular cellulose paperboard of 8 '. times.1-1/4' (203.2 mm. times.31.7 mm). The conductive portion was 3-3/8 "(3.375"; 85.7mm) long and 1 "(25.4 mm) wide, with all corners rounded at a radius of 1/2" (0.5 "; 12.7 mm). The conductive portion is attached to the blank so that the 1/8 "(0.125"; 3.2mm) border of the paperboard is exposed above, below and at the outer ends of the conductive portion. A 1 "(25.4 mm) end-to-end gap is left between the inner ends of each conductive portion.

As in example 18, 3 of these V-folded blade blanks were bonded to the bottom surface of the susceptor to define a separation distance 21S (FIG. 19) of 1/2 '(0.5'; 12.7 mm).

There was also no discernable arcing when the susceptor assembly was exposed to an empty microwave oven, but the assembly did ignite when the cardboard blade at the center was overheated within 1 minute 18 seconds.

Example 20

In this example, each of the 6 blades of the field director of FIG. 20 is in accordance with blade 16 of FIG. 19¹⁰-5 is provided with a conductive part.

The blade in this example was also constructed from the blade blank illustrated in fig. 26 in the same manner as in examples 18 and 19. The blade stock was the same 8 '. times.1-1/4' (203.2 mm. times.31.7 mm) rectangular cellulose paper board. The conductive portion was 3-1/8 '(79.4 mm) long and 1' (25.4mm) wide, with all corners rounded at a radius of 1/2 '(0.5'; 12.7 mm). The conductive portion is attached to the blank of cardboard such that the 1/8 "(0.125"; 3.2mm) border of the cardboard is exposed above, below and at the outer ends of the conductive portion. Between the inner ends of each conductive portion there is left an end-to-end gap of 1-1/2 "(1.5"; 38.1 mm).

As with examples 18 and 19, 3 of these V-folded blade blanks were bonded to the bottom surface of the susceptor to define a separation distance 21S (FIG. 19) of 3/4 '(0.75'; 19.2 mm).

When the susceptor assembly was exposed to a microwave oven for 5 minutes, no arcing occurred, and there was no combustion.

Example 21

The test of example 20 was repeated using the conductive portion shown in fig. 26. The conductive portion of this example is an Avery-Dennison with Avery-Dennison Specialty Tape Division, Painesville, OH0817 is made of aluminum foil with a thickness of 0.002' (0.05mm) backed with adhesive.

When the susceptor assembly was exposed to an unloaded microwave oven for 5 minutes, no arcing occurred, and there was no combustion.

Example 22

The test of example 20 was repeated using the conductive portion shown in fig. 26. The conductive portion of this example was made from 0.002 "(0.05 mm) thick aluminum foil backed with adhesive from Shurtape, Hickory, NC, Shurtape AF 973.

When the susceptor assembly was exposed to an unloaded microwave oven for 5 minutes, no arcing occurred, and there was no combustion. The aluminum foil performance of this tape was acceptable except that the adhesive was spread.

Example 23

Example 23 tests were conducted to avoid spontaneous ignition of the vanes using a flame retardant composition. The Flame retardant used is from FlameProducts of Houston, TX called Paper Seal^TMThe water-based resin of (1). The susceptor assembly was constructed as in example 18, with the gap at the center between each pair of conductive portions being 3/4 "(0.75"; 19.2mm) as shown in figure 24, thereby defining a separation distance 21S (figure 19) of 3/8 "(0.375"; 9.6 mm).

The cardboard blank is immersed in a bath of flame retardant liquid and dried for one day before adhering the conductive parts, assembling the susceptor assembly.

When the unloaded susceptor assembly was exposed to the microwave oven for 5 minutes, no arcing occurred. Unlike example 18, the assembly did not catch fire, indicating that the fire retardant treatment of the cardboard was sufficient to prevent burning.

The tests of examples 9-23 are summarized in table 2.

TABLE 2 evaluation of arcing and overheating (N/A means "unusable")

From examples 9-23, it can be observed that:

1. the combination of rounded corners on the conductive portion and the border of at least 1/8 "(0.125"; 3.2mm) (about 0.025 times the wavelength of standing waves present in the microwave oven) of cardboard (i.e., low conductivity material) that completely surrounds the uncovered conductive portion of the blade prevents arcing. It should be noted that the boundary serves to separate the conductive portion of the blade from the susceptor by a predetermined close distance (examples 18-23);

2. the combination of at least the boundary (predetermined close distance) of 1/8 "(0.125"; 3.2mm) and 3/4 "(0.75"; 19.2mm) (about 0.16 times the wavelength of the standing wave occurring in the microwave oven) of the separation distance of the inner end of the conductive portion from the geometric center of the susceptor (i.e., a center gap of 1-1/2 "(1.5"; 38.1mm) between the opposing conductive portions) prevents overheating and spontaneous combustion of the paperboard of the susceptor assembly when exposed to an unloaded microwave oven (examples 20-22);

3. the combination of a boundary (predetermined close distance) of at least 1/8 "(0.125"; 3.2mm) and a non-conductive covering of the conductive portion prevents arcing (examples 10-12). However, as can be seen from examples 14-16, when the conductive portion is covered with a non-conductive covering and without a boundary, an arc is generated; and

4. the application of the fire retardant to the paperboard prevented spontaneous ignition due to overheating was 3/8 "(0.375"; 9.6mm) spaced from the geometric center of the susceptor (about 0.08 wavelength), i.e., the center gap between the opposing conductive portions was 3/4 "(0.75"; 19.2 mm).

Examples 24 to 64

General comments

In examples 24-64 below, a susceptor assembly similar to that shown in fig. 20 was used in a microwave oven to cookMicrowave four-cheese pizza(280 g). The results of these tests are set forth in tables 3, 4A, 4B and 5 below.

Examples 24-50 and examples 61-64 were conducted to evaluate the effect of various blade designs on susceptor overheating removal during cooking of pizzas in various microwave ovens. The remaining tests (i.e., examples 51-60) were conducted to evaluate the effect of various paddle designs on the scorching of various pizza cooked in a microwave oven.

As shown in FIG. 20, each susceptor assembly comprised 6 identical susceptor mounted blades equally spaced sixty (60) degrees apart, each conductive portion of the blade being spaced from the geometric center of the susceptor by a distance 21S of 3/8 "(0.375"; 9.6 mm).

The substrate of the susceptor assembly under test is made of various materials. 4 different susceptor substrate materials were tested in combination with 2 different metallization thicknesses forming the lossy conducting layer.

The conductive portion of each blade was made using an adhesive backed aluminum foil of 0.002 "(0.05 mm) thickness applied to cellulose board blades from the international paper described previously in connection with examples 9-20. Each conductive portion is 3-1/2 "(3.5"; 88.9mm) in length, but is of a different width. Table 3, Table 4A, Table 4B and Table 5 each contain a column of alphabetical designators representing "blade type" under test. Each indicator represents the blade type as depicted in fig. 19, the "width" dimension and "boundary" of the conductive portion, as follows:

indicator symbolBlade typeWidth of Boundary of

FIG. 19

A blades 16¹⁰-11.0 "none

(25.4mm)

B blade16¹⁰-3 0.75″19T

(19.2mm) 0.25″(6.4mm)

C blade 16¹⁰-2 0.75″ 19L

(19.2mm) 0.25″(6.4mm)

D blade 16¹⁰-11.25 "none

(31.7mm)

E blade 16¹⁰-3 1.0″ 19T

(25.4mm) 0.25″(6.4mm)

F blade 16¹⁰-2 1.0″ 19L

(25.4mm) 0.25″(6.4mm)

G blade 16¹⁰-3 0.875″19T

(22.2mm) 0.125″(3.2mm)

H blade 16¹⁰-3 0.9375″19T

(23.8mm) 0.0625″(1.6mm)

Tables 3, 4A, 4B and 5 also contain a list of alphanumeric indicators that indicate the "furnace" used for the test. Each indicator corresponds to a particular microwave oven manufacturer and model as follows:

indicator furnace manufacturer, model

F-950 Frigidaire, FMV 156DBA, 950 Watt

GE-1100 General Electric, JES1456B J01, 1100 Watts

GS-700 Goldstar MAL783W, 700 Watts

S-1000 Sharp R-150F, 1000 Watts

S-1000 Sharp R-630DW, 1100 Watts

Table 3, table 4A, table 4B, and table 5 contain columns indicating the "susceptors" (i.e., substrate 12S and layer 12C) used.

Susceptors included in some examples in table 3, table 4A and table 4B below were labeled "control". The "control" receptor is the aforementioned sensor equipped withSusceptor for microwave cheesecake (280 g). The "control" susceptor comprised a paperboard substrate.

The "susceptor" included in some examples in tables 3 and 5 below is labeled by a reference designator comprising a first value bearing a hyphen and a second value. The first value represents the polymeric base material of the susceptor and the second value represents the thickness of the metallization of the susceptor loss layer (vacuum deposited aluminum) based on its measured optical density.

The first number represents the polymer base material, as follows:

first digital thin film substrate type

10 polyethylene terephthalate 300 Specification (gauge)

(without heat treatment) (to be from e.i. dupont de

Nemours and Company under the trade markGo out

Sale)

300 gauge thermally stable thin 12 polyethylene terephthalate

Membranes (from e.i. dupont de Nemours and

company trade mark isST-507 sale)

A 13 polyethylene naphthalate (PEN) film of 2 mil,

under the trademark DuPont Teijin FilmsSell it

The second numerical value represents an optical density thickness measurement of a vacuum deposited metallized coating of aluminum as follows:

second digital metal coating thickness

30.3 optical Density

40.4 optical Density

Thus, for example 29 in Table 3, the susceptor labeled "12-3" represents a 300 gauge polyethylene terephthalate heat stable film with a susceptor substrate: (ST-507 film) (indicated by the first number "12") and the optical density of the aluminum vacuum deposited metallization is 0.3 (indicated by the second number "3").

Examples 24 to 34

As described above, a susceptor assembly having type A blades is used to cook in an S-1000 oven or an F-950 ovenMicrowave four cheese pizza (280 grams). As can be seen from table 3, 4 types of susceptor base materials were used. Cooking times varied between 5-6 minutes. All bladed susceptor assemblies were superheated in unison at the center. For each susceptor base material used, the severity of overheating increases with increasing cooking time. Examples of overheating include burning and melting spots on the susceptor surface, which in some cases can result in melted susceptor material flowing to the bottom of the pizza, as can be seen in fig. 27 and 28.

Examples 35 to 40

In examples 35-40, cardboard borders with 1/4 "(0.25"; 6.4mm) added to the top or bottom of the conductive portion of the blade were tested to evaluate their potential for eliminating overheating at the center of the susceptor. As summarized in table 3 below, in this series of tests,the microwave cheesecake was cooked in an S-1000 microwave oven for 6 minutes using a susceptor having a 12-3 base. Field director assemblies exhibiting different blade types a, B, C, D, E and F were tested. Example 35 employs type B blades; example 36 employs type C blades; example 37Adopting type D blades; example 38 employs type E blades; example 39 employs type F blades; example 40 employs type a blades.

The results are summarized in table 3.

TABLE 3 susceptor superheat evaluation

Example number	Blade type	Sensor	Furnace with a heat exchanger	Cooking time minute second	Results (for receptors)
						24	Is free of	Control	S-1000	6:00	Without overheating
25	A	Control	S-1000	6:00	Superheating
						26	A	Control	S-1000	5:00	Superheating
27	A	10-4	S-1000	6:00	Superheating
						28	A	10-4	S-1000	5:00	Superheating
29	A	12-3	S-1000	5:30	Superheating
						30	A	13-4	S-1000	5:30	Superheating
31	Is free of	Control	F-950	6:00	Without overheating

32	A	Control	F-950	5:30	Superheating
						33	A	12-3	F-950	5:30	Superheating
34	A	13-4	F-950	5:30	Superheating
						35	B	12-3	S-1000	6:00	Without overheating
36	C	12-3	S-1000	6:00	Limited superheat
						37	D	12-3	S-1000	6:00	Superheating
38	E	12-3	S-1000	6:00	Without overheating
						39	F	12-3	S-1000	6:00	Limited superheat
40	A	12-3	S-1000	6:00	Superheating

Table 3 illustrates that for a bladed susceptor having a separation distance defined between the interior of the conductive portion and the geometric center of the susceptor, the incorporation of a top boundary between the susceptor and the top edge of the conductive portion of the blade structure (blade types B and E) consistently prevents overheating of the susceptor. Bladed (blade types a and C) susceptors without any boundary consistently resulted in overheating at the center of the susceptor. Bladed susceptors having a lower boundary (but no tip boundary) of non-conductive material along the conductive portions of the blades (blade types C and F) somewhat reduce the severity of susceptor overheating, but do not completely eliminate the problem. These results for examples 35-40 are shown in FIG. 29.

Examples 41 to 60

A series of cooking tests were performed with the 5 microwave ovens indicated above. These tests used susceptors with type A and type B blades to evaluate the effect of adding 1/4 "(0.25"; 6.4mm) wide top boundaries of cardboard along the conductive portion of the blade. Examples 41-50 (summarized in Table 4A) and 51-60 (summarized in Table 4B) each used the same test conditions. Examples 41-50 evaluated the superheat.

Examples 51-60 evaluate overall microwave cooking performance, particularly the ability of this configuration of susceptor assembly to consistently char the bottom of pizza. The percent of pizza coking (% char) was measured in the same manner as described in connection with examples 1-8. The measured% scorch was averaged over 3 pizza samples.

TABLE 4A superheat evaluation

Example number	Blade type	Sensor	Furnace with a heat exchanger	Cooking time minute second	Superheating
						41	A	Control	S-1100	5:00	Is that
42	B	Control	S-1100	5:00	Whether or not
						43	A	Control	S-1000	5:00	Is that
44	B	Control	S-1000	5:00	Whether or not
						45	A	Control	F-950	6:00	Is that
46	B	Control	F-950	6:00	Whether or not
						47	A	Control	G-1100	5:00	Is that
48	B	Control	GE-1100	5:00	Whether or not
						49	A	Control	GS-700	7:00	Is that
50	B	Control	GS-700	7:00	Whether or not

Table 4B cooking performance evaluation

Example number

Blade type

Sensor

Furnace with a heat exchanger

Cooking time minute second

Average% coking

Superheating

51

A

Control

S-1100

5:00

53％

Is that

52

B

Control

S-1100

5:00

46％

Whether or not

53

A

Control

S-1000

5:00

42％

Is that

54

B

Control

S-1000

5:00

37％

Whether or not

55

A

Control

F-950

6:00

69％

Is that

56

B

Control

F-950

6:00

63％

Whether or not

57

A

Control

G-1100

5:00

42％

Is that

58

B

Control

GE-1100

5:00

26％

Whether or not

59

A

Control

GS-700

7:00

19％

Is that

60

B

Control

GS-700

7:00

22％

Whether or not

The results shown in tables 4A and 4B demonstrate that for a bladed susceptor having a separation distance defined between the interior of the conductive portion and the geometric center of the susceptor, the top margin of the paperboard incorporating 1/4 "(0.25"; 6.4mm) along the conductive portion of the blade (type B) consistently prevented overheating at the center of the susceptor. However, as can be seen from table 4B, the overall cooking performance of the susceptor with type B lobes is reduced (as evidenced by the lower average percent scorch).

Examples 61 to 64

Examples 61-64 evaluate the effect of the width of the top boundary of the sheet between the susceptor and the top edge of the conductive portion of the blade on the overheating of the susceptor. This series of tests was also performed in an S-1000 microwave ovenMicrowave four cheese pizza cooking was performed for 6 minutes. The susceptor assembly had a base material of 12-3 and blade types a, B, G and H.

These results for examples 61-64 are illustrated in FIG. 30 and summarized in Table 5.

TABLE 5 estimation of the effect of top boundary on superheat

Example number	Blade type	Sensor	Furnace with a heat exchanger	Cooking time minute second	Susceptor overheating
						61	A	12-3	S-1000	6:00	Is that
62	B	12-3	S-1000	6:00	Whether or not
						63	G	12-3	S-1000	6:00	Whether or not
64	H	12-3	S-1000	6:00	Is that

These tests indicate that for a bladed susceptor having a separation distance defined between the interior of the conductive portion and the geometric center of the susceptor, a top boundary of the paperboard between the susceptor and the top edge of the conductive portion of the blade structure of at least 1/8 "(0.125"; 3.2mm) (i.e., blade types B and G) is required to prevent overheating of the susceptor.

In summary, for a bladed susceptor having a separation distance defined between the interior of the conductive portion and the geometric center of the susceptor, the conclusions drawn from examples 24-64 are as follows:

1. the boundary between the susceptor and the top edge of the electrically conductive portion of the blade having a width of at least 1/8 "(0.125"; 3.2mm) prevents overheating of the susceptor. It should be noted that the boundary serves to space the electrically conductive part of the blade from the susceptor by a predetermined close distance;

2. regardless of the substrate used, for susceptor assemblies using vanes with tip boundaries less than 1/8 "(0.125"; 3.2mm), overheating occurred at the center of the susceptor. This result was observed for all teammates using microwave ovens.

3. The severity of overheating (burning and melting) increases with increased cooking time, higher levels of metallization of the susceptor substrate or higher microwave oven power.

Preventing arcing

When a field director structure having one or more electrically conductive portions is present in an energized microwave oven (with or without a susceptor), the conductive portion(s) cause interference with the standing wave electric field in the oven. The conductive portion(s) concentrate the electric field along its edges, producing a local electric field strength that is much higher than the basic electric field within the furnace (i.e., the field strength prior to the introduction of the conductive portion (s)). These higher field strengths are generally insufficient to cause air breakdown as long as the furnace is loaded.

However, when the oven is empty (i.e., no food or other items are present), the basic electric field may increase to a level that exceeds that present with food or other items. In the unloaded case, the local field strength along the edge of the conducting part may be high enough to exceed the breakdown threshold of air, so that an electric discharge in the form of an arc is generated.

When a field director structure without a susceptor is used, it is believed that the conductive portion should be separated from the planar support member by a boundary of a lower conductivity material (e.g., a dielectric) by at least a predetermined close distance. Preferably the border surrounds the conductive portion. The presence of the boundary reduces the local electric field strength at the edge. The magnitude of this reduction is approximated by the following equation:

wherein E is_lIs the local electric field before the boundary is added;

E_l' is a local electric field with a boundary;

ε_r' is the relative dielectric constant of the boundary material; and is

ε_r"is the relative dielectric loss of the boundary material.

In essence, due to the presence of the surrounding boundary, the local field decays so that the breakdown threshold of air is not exceeded, thereby preventing the generation of arcs.

When using a field director with a susceptor, the lossy layer of the susceptor also serves to prevent arcing. The lossy layer absorbs a portion of the microwave energy in the furnace and converts it to heat. This absorption reduces the electric field strength in the furnace. The heat flows into the food or other item present.

However, when the oven is empty, there is no food or other item in the oven to dissipate the heat generated by the lossy layer. This results in rapid overheating, which damages the lossy layer, significantly reducing its conductivity. This reduces the ability of the lossy layer to absorb microwave energy.

Without absorption by the lossy layer, the electric field strength in the furnace increases, and high field conditions along the edges of the conductive portions may then exceed the breakdown threshold of air, causing an arc-type discharge to occur.

When the conductive portion(s) of the field director structure are separated from the lossy layer by a boundary of dielectric material, the boundary is said to reduce the local electric field strength at the edge.

Preventing overheating

When a field director structure having two electrically conductive portions is present in an energized microwave oven, a concentrated field is generated in the space between the two electrically conductive portions. When a material with a medium dielectric loss factor, such as a cardboard planar support member or susceptor, is placed in or adjacent to the area between the conductive portions, the concentrated field causes the material to heat up quickly. The concentration (concentration) of the field is a function of the spacing separating the conductive portions. This is the case for cardboard if the conductive parts are close enough, which concentrated field may cause the material to overheat enough to ignite a fire. Increasing the spacing between the conductive portions reduces the concentration of the field, thereby preventing overheating.

Modifications of the invention will occur to those skilled in the art having the benefit of the teachings herein. Such modifications are to be construed as being within the scope of the invention as defined by the appended claims.

Claims (50)

1. A susceptor assembly for heating an article in a microwave oven, the susceptor assembly comprising:

a generally planar susceptor having a geometric center, the planar susceptor including an electrically lossy layer; and

a field director structure having a plurality of vanes, each of the plurality of vanes being mechanically coupled to the susceptor, at least a portion of each vane being electrically conductive, the electrically conductive portion of the vane being disposed at least a predetermined distance from an electrically lossy layer of the planar susceptor,

the conductive portion of each blade having a first end and a second end, the first end of the conductive portion on each blade being disposed at a distance of at least a predetermined separation distance from the geometric center of the planar susceptor,

thereby preventing overheating of the susceptor and overheating of the field director structure when the susceptor assembly is used in an unloaded microwave oven.

2. The susceptor assembly of claim 1 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the predetermined separation distance is at least 0.16 times the wavelength.

3. The susceptor assembly of claim 1, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the predetermined distance is at least 0.025 times the wavelength.

4. The susceptor assembly of claim 3, wherein the predetermined separation distance is at least 0.16 times the wavelength.

5. The susceptor assembly of claim 1, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the predetermined distance is no greater than 0.1 times the wavelength.

6. The susceptor assembly of claim 5, wherein the predetermined separation distance is at least 0.16 times the wavelength.

7. The susceptor assembly of claim 1, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the predetermined distance is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

8. The susceptor assembly of claim 7, wherein the predetermined separation distance is at least 0.16 times the wavelength.

9. The susceptor assembly of claim 1 wherein a boundary of material having a conductivity that is less than the conductivity of the conductive portion of each blade surrounds the conductive portion of each blade.

10. The susceptor assembly of claim 9 wherein the conductive portion of the vane has a predetermined width dimension and corners thereon, the corners of the conductive portion being rounded with a radius up to and including one-half of the width dimension.

11. The susceptor assembly of claim 9 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, wherein the boundary has a predetermined width dimension, and

wherein the width of the boundary is at least 0.025 times the wavelength.

12. The susceptor assembly of claim 9 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, wherein the boundary has a predetermined width dimension, and

wherein the boundary has a predetermined width dimension, wherein the width of the boundary of the lower conductivity material is no greater than 0.1 times the wavelength.

13. The susceptor assembly of claim 9 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, wherein the boundary has a predetermined width dimension, and

wherein the boundary has a predetermined width dimension, wherein the width of the boundary of the lower conductivity material is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

14. The susceptor assembly of claim 1 wherein the conductive portion of each blade is covered with a non-conductive material.

15. The susceptor assembly of claim 14 wherein the electrically non-conductive covering is selected from the group consisting of polyimide tape, polyacrylic spray coating, and polytetrafluoroethylene spray coating.

16. The susceptor assembly of claim 1 wherein the conductive portion of each vane comprises a metal foil having a thickness of less than 0.1 millimeters, and wherein the metal foil is folded to at least twice the thickness along its perimeter length.

17. The susceptor assembly of claim 1 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the conductive portion of each blade has a width dimension of about 0.1 to about 0.5 times the wavelength.

18. The susceptor assembly of claim 1 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the conductive portion of each blade has a length dimension of from about 0.25 to about 2 times the wavelength.

19. The susceptor assembly of claim 1 wherein the conductive portion of the blade has a predetermined width dimension and corners thereon, the corners of the conductive portion being rounded with a radius up to and including one-half of the width dimension.

20. A susceptor assembly for use in a microwave oven operable to generate a standing electromagnetic wave having a predetermined wavelength, the susceptor assembly comprising:

a substantially planar susceptor having a geometric center, the planar susceptor comprising an electrically lossy layer;

a field director structure having at least six vanes, each of the at least six vanes being mechanically coupled to the susceptor, each vane being substantially orthogonal with respect to the planar susceptor,

at least a portion of each blade being electrically conductive, the electrically conductive portion of each blade having a first end and a second end,

the first end of the conductive portion on each blade is disposed at a distance of at least a predetermined separation distance from the geometric center of the planar susceptor,

the separation distance from the geometric center of the planar susceptor is at least 0.16 times the wavelength,

disposing the electrically conductive portion of the vane at least a predetermined distance from the electrically lossy layer of the planar susceptor, wherein the predetermined distance is at least 0.025 times the wavelength,

thereby preventing overheating of the susceptor and overheating of the field director structure when the susceptor assembly is used in an unloaded microwave oven.

21. A susceptor assembly for heating an article in a microwave oven, the susceptor assembly comprising:

a substantially planar susceptor comprising an electrically lossy layer;

at least one blade mechanically connected to the susceptor, at least a portion of the blade being electrically conductive, the electrically conductive portion of the blade having a predetermined width dimension and corners thereon, the corners of the electrically conductive portion being rounded with a radius that is at most and includes half of the width dimension,

the electrically conductive part of the blade is arranged at least a predetermined distance from the electrically lossy layer of the planar susceptor,

thereby preventing arcing in the vicinity of the conductive portions when the susceptor assembly is used in an unloaded microwave oven.

22. The susceptor assembly of claim 21 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is at least 0.025 times the wavelength.

23. The susceptor assembly of claim 21 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is not greater than 0.1 times the wavelength.

24. The susceptor assembly of claim 21 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

25. The susceptor assembly of claim 21 wherein a boundary of material having a conductivity that is less than the conductivity of the conductive portion of each blade surrounds the conductive portion of the blade.

26. The susceptor assembly of claim 25, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and wherein the boundary has a predetermined width dimension, wherein the width of the boundary of lower conductivity material is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

27. The susceptor assembly of claim 21 wherein the conductive portion of the blade is covered with a non-conductive material.

28. The susceptor assembly of claim 27, wherein the electrically non-conductive covering is selected from the group consisting of a polyimide tape, a polyacrylic spray coating, and a polytetrafluoroethylene spray coating.

29. The susceptor assembly of claim 21 wherein the conductive portion of the blade comprises a metal foil having a thickness of less than 0.1 millimeters, and wherein the metal foil is folded along its periphery to at least twice its thickness.

30. The susceptor assembly of claim 21 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength and wherein the conductive portion of the blade has a width dimension of about 0.1 times to about 0.5 times the wavelength.

31. The susceptor assembly of claim 21 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the conductive portion of each blade has a length dimension, and wherein the length dimension is in a range from about 0.25 to about 2 times the wavelength.

32. A susceptor assembly for heating an article in a microwave oven, the susceptor assembly comprising:

a substantially planar susceptor comprising an electrically lossy layer;

at least one blade mechanically connected to the susceptor, at least a portion of the blade being electrically conductive, the electrically conductive portion being covered with an electrically non-conductive material,

the electrically conductive part of the blade is arranged at least a predetermined distance from the electrically lossy layer of the planar susceptor,

thereby preventing arcing in the vicinity of the conductive portions when the susceptor assembly is used in an unloaded microwave oven.

33. The susceptor assembly of claim 32, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is at least 0.025 times the wavelength.

34. The susceptor assembly of claim 32, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is not greater than 0.1 times the wavelength.

35. The susceptor assembly of claim 32, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

36. The susceptor assembly of claim 32 wherein a boundary of material having a conductivity that is less than the conductivity of the conductive portion of each blade surrounds the conductive portion of the blade.

37. The susceptor assembly of claim 36 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, wherein the boundary has a predetermined width dimension, and

wherein the width of the boundary of the lower conductivity material is in the range from 0.025 times the wavelength to 0.1 times the wavelength.

38. The susceptor assembly of claim 32, wherein the electrically non-conductive covering is selected from the group consisting of a polyimide tape, a polyacrylic spray coating, and a polytetrafluoroethylene spray coating.

39. The susceptor assembly of claim 32 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength and wherein the conductive portion of the blade has a width dimension of about 0.1 times to about 0.5 times the wavelength.

40. The susceptor assembly of claim 32, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the conductive portion of each blade has a length dimension, and wherein the length dimension is in a range from about 0.25 to about 2 times the wavelength.

41. A susceptor assembly for heating an article in a microwave oven, the susceptor assembly comprising:

a substantially planar susceptor comprising an electrically lossy layer;

at least one blade mechanically connected to the susceptor, at least a portion of the blade being electrically conductive, wherein the electrically conductive portion of the blade comprises a metal foil having a thickness of less than 0.1 mm, and wherein the metal foil is folded along its periphery to at least twice its thickness,

the electrically conductive part of the blade is arranged at least a predetermined distance from the electrically lossy layer of the planar susceptor,

thereby preventing arcing in the vicinity of the conductive portions when the susceptor assembly is used in an unloaded microwave oven.

42. The susceptor assembly of claim 41, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is at least 0.025 times the wavelength.

43. The susceptor assembly of claim 41, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is not greater than 0.1 times the wavelength.

44. The susceptor assembly of claim 41, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the predetermined distance is in a range from 0.025 times the wavelength to 0.1 times the wavelength.

45. The susceptor assembly of claim 41, wherein a boundary of material having a conductivity that is less than a conductivity of the conductive portion of each blade surrounds the conductive portion of the blade.

46. The susceptor assembly of claim 45, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, wherein the boundary has a predetermined width dimension, and

wherein the width of the boundary of the lower conductivity material is in the range from 0.025 times the wavelength to 0.1 times the wavelength.

47. The susceptor assembly of claim 41, wherein the conductive portion is covered with a non-conductive covering.

48. The susceptor assembly of claim 41 wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength and wherein the conductive portion of the blade has a width dimension of about 0.1 times to about 0.5 times the wavelength.

49. The susceptor assembly of claim 41, wherein the microwave oven is operable to generate a standing electromagnetic wave having a predetermined wavelength, and

wherein the conductive portion of each blade has a length dimension, and wherein the length dimension is in a range from about 0.25 to about 2 times the wavelength.

50. A susceptor assembly for use in a microwave oven operable to generate a standing electromagnetic wave having a predetermined wavelength, the susceptor assembly comprising:

a generally planar susceptor having a geometric center, the planar susceptor including an electrically lossy layer;

at least six blades, each of the blades being mechanically connected to the susceptor, each blade being substantially orthogonal with respect to the planar susceptor,

at least a portion of each blade is electrically conductive,

the conductive portion of the blade has a predetermined width dimension and corners thereon, the corners of the conductive portion are rounded with a radius that is at most and includes half of the width dimension,

disposing the electrically conductive portion of the vane at least a predetermined distance from the electrically lossy layer of the planar susceptor, wherein the predetermined distance is at least 0.025 times the wavelength,

thereby preventing arcing in the vicinity of the conductive portion when the susceptor assembly is used in an unloaded microwave oven.