The present invention is generally related to heat transfer, and more particularly is related to a system and method for heating materials.
Technology utilized for providing heating surfaces has changed dramatically over time. One example of a material utilized for fabrication of heating surfaces is glass ceramics. Glass ceramics are useful for use as heating surfaces since glass ceramics provide a smooth and hard surface that may easily be cleaned, they are chemically stable at cooking and boiling temperatures, and due to glass ceramics typically having a low coefficient of thermal expansion, they are resistant to thermal shock.
Examples of heating systems that utilize glass ceramics include, but are not limited to, cook tops and laboratory hot plates. Typically, these heating systems utilize a large radiant heater as a heat source that is positioned below a surface of a material, such as a nickel chrome resistive element, used as a heating platform.
Unfortunately, use of present glass ceramic heating systems having a heating source positioned there below, is thermally inefficient. Specifically, the nickel-chrome resistive element has a thermal emissivity of approximately sixty percent (60%) or less, while the glass ceramic has a thermal transmittance of approximately eighty percent (80%) or less. In addition, a pot, or other device utilized for heating a substance therein, is required to be engineered in a shape and have surface properties that allow for high thermal absorption.
Attempts have been made to make heat transfer associated with the abovementioned glass ceramic heating system predominantly conductive. As an example, U.S. (U.S.) Pat. No. 4,039,777 (hereafter, “the '777 patent”), issued Aug. 2, 1977, to Fred E. Baker, discloses a heating system fabricated by cementing resistive heating wires to the glass ceramic, positioning wire elements contained within an insulating structure next to the glass ceramic, and configuring sheath type elements so that heat is conducted to the glass ceramic. Unfortunately, the heating system of the '777 patent is not thermally efficient. Specifically, the '777 system utilizes resistive heating wires located within a cement layer. The heating wires are intended to heat the entire cement layer. When the cement layer is heated, the heat is conducted to the glass ceramic. Unfortunately, heat is lost throughout the cement layer, thereby resulting in poor thermal efficiency. In addition, heat is poorly, and unevenly, displaced in the '777 patent system since the portions providing heat are limited to the heating wires, which are spaced apart a predetermined distance.
One example of a heating system is categorized as a thick film heater. U.S. Pat. No. 6,037,574, issued Mar. 14, 2000, to Lanham, et al., discloses an example of a thick film heater, which contains noble metals in a glass paste deposited on quartz, wherein, as is known by those having ordinary skill in the art, quartz is a low thermal expansion glass, or glass ceramic. In addition, Watlow Electric Manufacturing Company of St. Louis, Mo. manufactures thick film heaters. Unfortunately, thick film heaters suffer from performance problems due to a lack of molecular bonding of the paste to the glass ceramic. The lack of molecular bonding may result in poor thermal conductivity. Specifically, the coefficient of thermal expansion of the noble metal paste does not match the coefficient of thermal expansion of the glass ceramic, thereby resulting in cracking between the paste and the glass ceramic after repetitive heating and cooling of the thick film heater.
Another example of a heating system is categorized as a thin film heater. U.S. Pat. Nos. 5,616,266, issued Apr. 1, 1997, to Richard Cooper, and 6,376,816, issued Apr. 23, 2002, to Cooper, et al., disclose examples of thin film heaters. In addition, Thermo-Stone USA, LLC, of Marina, Calif. manufactures radiant thin film heaters. Unfortunately, thin film heaters do not deliver adequate power to provide for efficient use in cook top applications. Specifically, sputtering, evaporating, chemical vapor deposition (CVD), or other techniques of providing a thin film heater are inadequate because the resulting thin film heater does not provide adequate conductance properties to allow normal and high voltages, and associated currents, to be accommodated for by the resulting thin film heater.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
Embodiments of the present invention provide a system and method for heating materials. Briefly described, in architecture, one embodiment of the system, among others, can be implemented as follows. The system contains a first layer upon which a material may be placed for heating the material, wherein the first layer has sufficient conductivity to allow heat to travel through the first layer. The system also contains a heater layer provided on the first layer, which is capable of providing heat to the first layer for heating the material. In addition, the system has an insulator layer for protecting the heater layer from contaminants.
The present invention can also be viewed as providing methods for providing a heating system. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: thermally spraying a heater layer on a first layer, wherein the first layer is capable of supporting a material to be heated; and fabricating an insulator layer on the heater layer, wherein the insulator layer protects the heater layer from contaminants.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
The present invention provides a heating system and method that is capable of heating materials. For exemplary purposes, the present description is provided with reference to a cook top that is efficiently heated by thermal conduction, however, it should be noted that that the invention may be utilized on a wide variety of apparatus for the heating of materials with low thermal expansion. For exemplary purposes, the following describes the materials to be heated as low thermal expansion materials. It should be noted, however, that the present invention may be utilized to heat different thermal expansion materials, such as high thermal expansion materials. Examples of these apparatus include, but are not limited to, cook tops, cooking pots and containers, laboratory heaters, tubes, water heaters, semiconductor processing equipment, chemical processing systems, and medical devices.
In addition, the heating system avoids problems associated with thin film heaters and thick film heaters, due to deposition of at least one resistive layer via use of thermal spraying methods. An example of thermal spraying is described in U.S. patent application entitled, “Resistive Heaters and Uses Thereof,” by Abbott, et al., filed on Nov. 28, 2001, and having Ser. No. 09/996,183 (hereafter, “the '183 patent”), the disclosure of which is hereby incorporated by reference in its entirety. It should be noted that the '183 patent describes methods of depositing materials capable of generating high power, where the deposited materials bond by chemical action to a different material, such as, but not limited to, glass ceramic.
The thermal insulator 10, which is preferably located below the heating device 100, thermally insulates the heating device 100. Specifically, the thermal insulator 10 may reduce conduction, reduce radiation, or both reduce conduction and reduce radiation, as long as the thermal insulator 10 is capable of thermally insulating the heating device 100.
If the thermal insulator 10 reduces conduction, material utilized to provide the thermal insulator 10 may be selected from, for example, but not limited to, porous aluminum oxide or silica. In fact, a wide variety of commercial insulators having properties similar to porous aluminum oxide and silica may be selected. Specifically, the thermal insulator 10 may be provided by materials having low thermal conductivity.
Alternatively, if the thermal insulator 10 reduces radiation, material utilized to provide the thermal insulator 10 may be selected from different material having a very high reflectivity, such as, but not limited to, a metallic sheet and a metallic coating located on a glass substrate. Specifically, the thermal insulator 10 may be provided by materials having low thermal emissivity.
The power supply 30 may be a conventional power supply that is capable of providing a charge that is capable of powering the heating device 100. Specifically, a conventional power supply may provide a voltage of 110 volts (V) or 220 volts (V), such as a power supply located within a home that provides a charge to be channeled via a wall outlet connection. It should be noted, however, that a remote power supply can instead be utilized. In addition, a power supply that provides more than 220 volts, or less than 110 volts may be utilized.
Another variable that may be considered in determining thickness of the first layer 102 may be horizontal conductance of heat. Specifically, it is preferred that heat conduct to the top portion 102 of the first layer 102, and not more than a minimal horizontal distance. By minimizing horizontal conductance, heating of the top portion 102 is restricted to a specific pre-defined portion of the top portion 102.
For exemplary purposes, the first layer 102 may measure approximately 4 millimeters (mm) to 5 mm in thickness and have a coefficient of thermal expansion of less than 4×10E-6/° C. It should be noted however, that the first layer 102 may have a different thickness and have a different coefficient of thermal expansion. As is known by those having ordinary skill in the art, the coefficient of thermal expansion is the fractional increase in length per unit rise in temperature. Although other materials may be utilized, due to characteristics such as having a smooth and hard surface that may easily be cleaned, being chemically stable at cooking and boiling temperatures, and typically having a low coefficient of thermal expansion, the first layer 102 is preferably fabricated of a glass ceramic or other materials having similar characteristics. In addition, the first layer 102 may be fabricated of a material utilized for countertops such as, but not limited to, Corian®, by DuPont®, granite, or other materials if made thermally conductive.
The top portion 104 of the first layer 102 may be utilized as a heating surface. In addition, the first layer 102 also contains a bottom portion 106. In accordance with the first exemplary embodiment of the invention, the bottom portion 106 of the first layer 102 is a substrate, upon which a dielectric layer 110 is located. Specifically, a top portion 112 of the dielectric layer 110 is located on the bottom portion 106 of the first layer 102. A heater layer 120 is located on a bottom portion 114 of the dielectric layer 110. Specifically, a top portion 122 of the heater layer 120 is located on the bottom portion 114 of the dielectric layer 110. For exemplary purposes, the dielectric layer 110 may measure approximately 0.05 mm to 2.0 mm in thickness. It should be noted however, that the dielectric layer 110 may have a different thickness.
The dielectric layer 110 is capable of reducing thermoelastic stresses in the heater layer 120. Preferably, the dielectric layer 110 has a coefficient of thermal expansion that is between a coefficient of thermal expansion of the first layer 102 and a coefficient of thermal expansion of the heater layer 120. Therefore, the coefficient of thermal expansion of the dielectric layer 110 is lower than the coefficient of thermal expansion of the heater layer 120. It should be noted, however, that the coefficient of thermal expansion of the dielectric layer 110 need not be between the coefficient of thermal expansion of the first layer 102 and the coefficient of thermal expansion of the heater layer 120.
In accordance with a second embodiment of the invention, the heating device 100 does not have a dielectric layer 110. Specifically, inclusion of the dielectric layer 110 may depend upon bulk resistivity of the first layer 102 and appropriate regulations pertaining to minimum dielectric strength between an electrically energized heating element and a conducting device, such as a metallic cooking pot, resting on the top portion 104 of the first layer 102. As an example, certain countries have regulations (e.g., Underwritters Laboratories, Inc. (UL) Standards) on a maximum leakage current allowed between the top portion 104 of the first layer 102 and a heating element. In these countries the dielectric layer 110 may be useful since dielectrics dissipate a minimum amount of energy, while supporting an electrostatic field.
FIG. 3A and
Returning to
Preferably, the coefficient of thermal expansion of the insulator layer 130 is lower, specifically, preferably slightly lower, than the coefficient of thermal expansion of the heater layer 120. If the coefficient of thermal expansion of the insulator layer 130 is lower than that of the heater layer 120, and the coefficient of thermal expansion of the dielectric layer 110 is lower than that of the heater layer 120, upon heating, the top portion 122 of the heater layer 120 is compressed, thereby preventing cracking of the heater layer 120 and ensuring proper thermal emission from the heater layer 120 to the first layer 102. It should be noted, however, that the coefficient of thermal expansion of the insulator layer 130 need not be lower than the coefficient of thermal expansion of the heater layer 120.
It should be noted that the insulator layer 130 preferably has low thermal conductivity, thereby providing a thermally insulating barrier. In addition, the insulator layer 130 preferably has low thermal emissivity to prevent the loss of heat. Still further, due to the above-mentioned properties, the insulator layer 130 may also serve as an electrical insulator to prevent exposure of the heater layer 120 to elements that may be flammable or provide a different hazardous situation.
For exemplary purposes,
As is shown by block 202, the dielectric layer 110 is deposited on the first layer 102. It should be noted that the dielectric layer 110 may be deposited as an uninterrupted area over the first layer 102 or in a pattern that is similar to a pattern utilized for the heater layer 120.
The dielectric layer 110 may be deposited via different methods, such as, but not limited to, thermal spraying, sputtering, evaporation, or thick film techniques such as screen printing or flowing of dielectric material as a paste, with subsequent heat treatment. Composition of the dielectric layer 110 preferably provides dielectric strength at operating temperatures of the heating device 100, a coefficient of thermal expansion that is similar to the coefficient of thermal expansion associated with the first layer 102, and allows for the dielectric layer 110 to chemically bond with the first layer 102.
As an example, the dielectric layer 110 may be fabricated from a combination of crushed and milled glass ceramic having a similar composition to the first layer 102, blended with approximately ninety-six percent (96%) purity fused silica in volumetric proportions that account for thermal expansion coefficients (resulting in a material having a coefficient of thermal expansion that is an average of a coefficient of thermal expansion of the glass ceramic and a coefficient of thermal expansion of the silica), bulk resistivities, and elastic moduli of the dielectric layer 110. Alternatively, the dielectric layer 110 may be a thermally sprayed blend of eucryptite and cordierite. Specifically, materials utilized to fabricate the dielectric layer 110 preferably are characterized by a low coefficient of thermal expansion and a high dielectric strength. It should be noted, however, that material utilized to fabricate the dielectric layer 110 may instead have a higher coefficient of thermal expansion and/or a lower dielectric strength.
As is shown by block 204, the heater layer 120 is deposited on the dielectric layer 110. An example of a method that may be used to fabricate the heater layer 120 is provided by the '183 patent, which has been incorporated by reference in its entirety. Specifically, as an example, a thermal spray gun and a gas source may be utilized where the gas source includes two or more gases that can be mixed in an arbitrary combination.
Electrical resistivity of the heater layer 120 is preferably 1×10E-4 to 0.1 ohm-cm. A desired electrical resistivity of the heater layer 120 may be formulated via use of an electrically conductive ceramic that is blended with several electrically insulating ceramics, such that an average threshold conduction level is reached by interconnecting conductive particles. The desired electrical resistivity of the heater layer 120 is then formulated by taking into account power and voltage requirements of the heating device 100, the nature of an electrical circuit providing power to the heating device 100 (i.e., parallel and series components), the geometric properties of current path length, and the cross-sectional area through which current passes within the heater layer 120, wherein the cross-sectional area comprises the thickness and width of the heater layer 120.
In order to minimize residual tensile stress in the heater layer 120, which may arise from the thermal spraying process, and in order to minimize compressive stress in the heater layer 120, which may arise during use of the heating device 100, thermal expansion of the heater layer 120 is preferably tailored to match thermal expansion of the material to which the heater layer 120 is bonded. Specifically, thermal expansion of the heater layer 120 is tailored to match thermal expansion of the dielectric layer 110. Tailoring of thermal expansion of the heater layer 120 may be accomplished by blending the electrically conductive ceramic component, which typically has a higher thermal expansion than the first layer 102, with insulative components, which have a thermal expansion coefficient and elastic modulus that will lower the overall thermal expansion of the heater layer 120.
Thermoelastic stress, attributed to thermal expansion, can be further reduced by ensuring that the heater layer 120 operates at the lowest possible temperature for a given power level, thereby minimizing excessive thermal expansion. For example, in looking at a heat equation for conduction, shown below as equation 1 (Eq. 1), wherein Q is heat flux, A is cross sectional area of the heater layer 120 located on the first layer 102, K is thermal conductance,
is the thermal gradient with x as the unit vector normal to the surface across which the heat flux is being measured, wherein ΔT is change in temperature as represented by the coefficient of thermal expansion, it is desirable to have minimal thermal expansion of the heater layer 120, as represented by a low coefficient of thermal expansion. It should be noted, however, that the heater layer 120 needs to be provided with enough power so that the heater layer 120 emits the required Q. Therefore, the larger A is, the smaller ΔT is, which is desirable for the heater layer 120 since, as is mentioned above, minimal thermal expansion of the heater layer 120 is desirable.
To minimize thermoelastic stress, and to allow chemical bonding to either the dielectric layer 110 or the first layer 102, the heater layer 120 may contain a zirconium boride electro-conductive ceramic blended with pure silica, for lowering the coefficient of thermal expansion, and pure silicon, for both lowering the coefficient of thermal expansion and providing a reactant with either the dielectric layer 110 or the first layer 102. Other material combinations, such as, but not limited to, molybdenum silicide blended with eucriptite and silicon, or silicon carbide blended with fused silica and mullite, would suffice as a replacement for zirconium boride.
As is shown by block 206, the insulator layer 130 is deposited on the heater layer 120. The insulator layer 130 may be deposited on the heater layer 120 via thermal spraying. As an example, the insulator layer 130 may be thermally sprayed mullite. Alternatively, the insulator layer 130 may be a borosilicate glass deposited by thick film techniques such as, but not limited to, etching or cutting. In addition, the insulator layer 130 may be a ceramic cement, a refractory paint, or an aerogel material. It should be noted that to reduce radiative thermal losses, the material utilized to fabricate the insulator layer 130 may be blended with a metallic component, or the insulator layer 130 may be covered with a reflective metallic component.
As is shown by
As is shown by
It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.
This application claims priority to U.S. Provisional Application entitled, “Resistive Heater For Low Thermal Expansion Materials,” having Ser. No. 60/433,539, filed Dec. 14, 2002, now abandoned, which is entirely incorporated herein by reference.
Number | Name | Date | Kind |
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3610888 | Button | Oct 1971 | A |
3883719 | Hurko | May 1975 | A |
4039777 | Baker | Aug 1977 | A |
4300990 | Maurer | Nov 1981 | A |
5007818 | Barancik et al. | Apr 1991 | A |
5155340 | Morita et al. | Oct 1992 | A |
5504307 | Hayashi et al. | Apr 1996 | A |
5616266 | Cooper | Apr 1997 | A |
5869808 | Hyllberg | Feb 1999 | A |
5938957 | Tanahashi et al. | Aug 1999 | A |
5973298 | Kallgren | Oct 1999 | A |
5981913 | Kadomura et al. | Nov 1999 | A |
6037574 | Lanham et al. | Mar 2000 | A |
6376816 | Cooper et al. | Apr 2002 | B2 |
6444957 | Kitagawa et al. | Sep 2002 | B1 |
Number | Date | Country |
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354094346 | Jul 1979 | JP |
408069868 | Mar 1996 | JP |
Number | Date | Country | |
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20040112892 A1 | Jun 2004 | US |
Number | Date | Country | |
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60433539 | Dec 2002 | US |