Patent Publication Number: US-2023148780-A1

Title: Cooking device having a cooking vessel and a ceramic heater

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 16/782,318, filed Feb. 5, 2020, entitled “Cooking Device Having a Cooking Vessel and a Ceramic Heater,” which claims priority to U.S. Provisional Patent Application Ser. No. 62/802,955, filed Feb. 8, 2019, entitled “Heat Pipe Cooking Vessel,” the content of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present disclosure relates generally to cooking devices and more particularly to a cooking device having a cooking vessel and a ceramic heater. 
     2. Description of the Related Art 
     Manufacturers of cooking devices, such as rice cookers, are continuously challenged to improve heating time and heating effectiveness. Most low-end rice cookers, for example, utilize a wire coil heater, such as nichrome wire, potted with ceramic cement inside a stainless steel sheath embedded inside a cast aluminum body. These heaters generate heat by passing electrical current through the nichrome wire. These types of heaters often suffer from long warmup and cooldown times due to the high thermal mass provided by the electrical insulation materials and the relatively large metal components. Furthermore, cooking vessels used with wire coil heaters typically have relatively low thermal mass resulting in poor distribution of heat within the cooking vessel. 
     Some high-end rice cookers utilize induction heaters to directly warm the cooking vessel instead of relying on convection or thermal conduction. Induction rice cookers use induction heating where current is passed through a metal coil to create a magnetic field. The cooking vessel is positioned within the magnetic field to induce electrical current in the cooking vessel which, in turn, generates heat. With induction heating, the heating temperature may be controlled by adjusting the strength of the magnetic field allowing for shorter warmup and cooldown times to be achieved. However, induction heaters are generally expensive due to the cost of the electrical materials and components, and the control systems for induction heaters are relatively complex and generally expensive as a result. 
     Accordingly, a cost-effective cooking device having improved thermal efficiency is desired. 
     SUMMARY 
     A cooking device according to one example embodiment includes a base having a top surface positioned to contact a cooking vessel configured to hold food during cooking. The base includes a heater having a ceramic substrate and an electrically resistive trace on an exterior surface of the ceramic substrate. The heater is positioned to supply heat generated by applying an electric current to the electrically resistive trace to the top surface of the base for heating the cooking vessel to heat food in the cooking vessel. In some embodiments, the electrically resistive trace includes an electrical resistor material thick film printed on the exterior surface of the ceramic substrate. In some embodiments, the electrically resistive trace is positioned on a top surface of the ceramic substrate that faces upward toward the top surface of the base. 
     Embodiments include those wherein the heater includes a thermistor that is positioned on the ceramic substrate and in electrical communication with control circuitry of the heater for providing feedback regarding a temperature of the heater to the control circuitry of the heater. In some embodiments, the thermistor is positioned on a bottom surface of the ceramic substrate that faces away from the top surface of the base. 
     Embodiments include those wherein the base includes a heating plate that forms the top surface of the base. The heating plate is positioned in contact with the heater to transfer heat from the heater to the top surface of the base for heating the cooking vessel to heat food in the cooking vessel. In some embodiments, the heating plate includes a domed top surface for contacting a concave bottom surface of the cooking vessel. 
     Embodiments include those wherein the ceramic substrate has a polygonal shape. In some embodiments, the ceramic substrate has an octagonal shape. 
     Embodiments include those wherein the electrically resistive trace extends in a serpentine pattern across the exterior surface of the ceramic substrate. In some embodiments, the serpentine pattern of the electrically resistive trace has a generally circular outer perimeter. 
     A cooking device according to another example embodiment includes a housing having a receptacle and a base positioned along a bottom of the receptacle. A cooking vessel is removably positionable within the receptacle for containing food to be cooked. The cooking vessel contacts the base when the cooking vessel is positioned within the receptacle. The base includes a heater having a ceramic substrate and an electrical resistor material thick film printed on a surface of the ceramic substrate. The heater is positioned to supply heat generated by applying an electric current to the electrical resistor material to the cooking vessel when the cooking vessel is positioned within the receptacle. 
     A heater for use with a cooking device according to one example embodiment includes a ceramic substrate and an electrically resistive trace thick film printed on an exterior face of the ceramic substrate. The electrically resistive trace extends in a serpentine pattern across the exterior face of the ceramic substrate from a first end of the electrically resistive trace to a second end of the electrically resistive trace. The serpentine pattern of the electrically resistive trace has a generally circular outer perimeter. The heater also includes a first electrically conductive trace electrically connected to the first end of the electrically resistive trace and a second electrically conductive trace electrically connected to the second end of the electrically resistive trace. The first and second electrically conductive traces form respective first and second terminals providing respective first and second electrical connections for completing a circuit formed by the first and second electrically conductive traces and the electrically resistive trace. Some embodiments include one or more glass layers on the exterior face of the ceramic substrate that cover the electrically resistive trace electrically insulating the electrically resistive trace. Some embodiments include a thermistor positioned on a second exterior face of the ceramic substrate that is opposite the exterior face of the ceramic substrate on which the electrically resistive trace is positioned for providing feedback regarding a temperature of the heater to control circuitry of the heater. Embodiments include those wherein the ceramic substrate has a polygonal shape. In some embodiments, the ceramic substrate has an octagonal shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present disclosure and together with the description serve to explain the principles of the present disclosure. 
         FIG.  1    is a perspective view of a cooking device according to one example embodiment. 
         FIG.  2    is a schematic diagram of the cooking device according to one example embodiment. 
         FIG.  3    is an exploded perspective view of a heater assembly of the cooking device according to one example embodiment. 
         FIGS.  4  and  5    are plan views of a top surface and a bottom surface, respectively, of a heater of the heater assembly shown in  FIG.  3   . 
         FIG.  6    is a cross-sectional view of the heater shown in  FIGS.  4  and  5    taken along line  6 - 6  in  FIG.  4   . 
         FIG.  7    is a plan view of a top surface of a heater according to another example embodiment. 
         FIG.  8    is a cross-sectional view of a cooking vessel of the cooking device employing a heat pipe according to one example embodiment. 
         FIGS.  9 A- 9 C  are cross-sectional views of the cooking vessel shown in  FIG.  8    taken along line  9 - 9  in  FIG.  8    illustrating various example wick structures of the heat pipe. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings where like numerals represent like elements. The embodiments are described in sufficient detail to enable those skilled in the art to practice the present disclosure. It is to be understood that other embodiments may be utilized and that process, electrical, and mechanical changes, etc., may be made without departing from the scope of the present disclosure. Examples merely typify possible variations. Portions and features of some embodiments may be included in or substituted for those of others. The following description, therefore, is not to be taken in a limiting sense and the scope of the present disclosure is defined only by the appended claims and their equivalents. 
     Referring now to the drawings and particularly to  FIG.  1   , a cooking device  100  is shown according to one example embodiment. In the example embodiment illustrated, cooking device  100  includes a rice cooker. However, cooking device  100  may also include a pressure cooker, a steam cooker, etc. Cooking device  100  includes a housing  102 , a cooking vessel  120 , a lid  105 , a heater assembly  140 , and a user interface  109 . Housing  102  includes an upper portion having a receptacle  103  for receiving cooking vessel  120  and a lower portion within which heater assembly  140  is mounted. In the embodiment illustrated, heater assembly  140  forms a receiving base of receptacle  103  such that cooking vessel  120  contacts and rests on top of heater assembly  140  when cooking vessel  120  is positioned within receptacle  103  so that heat generated by heater assembly  140  heats cooking vessel  120 . 
     Cooking vessel  120  is generally a container (e.g., a bowl) having a food receptacle  121  in which food substances to be cooked, such as rice and water, are contained. That is, food receptacle  121  of cooking vessel  120  directly contacts and retains the food being cooked. Cooking vessel  120  may be composed of, for example, a metal having high thermal conductivity, such as stainless steel, aluminum or copper. Lid  105  covers the opening at a rim  122  of cooking vessel  120 . Lid  105  includes a handle  107  preferably composed of a material having low thermal conductivity to provide a safe surface for the user to hold when using lid  105 . User interface  109  is provided on a front portion of housing  102 . User interface  109  may include one or more buttons, dials, knobs, etc. for receiving user input and/or a display or indicator lights for providing information about the functioning and status of cooking device  100  to a user. Cooking device  100  also includes a power cord  112  for connecting cooking device  100  to an external power source  114 . 
     In one embodiment, during use, food receptacle  121  of cooking vessel  120  holds water and rice to cook, and heater  140  transfers heat to cooking vessel  120  to bring the water to boil. Once the water reaches a steady boil, the temperature of cooking vessel  120  remains generally stable. Once all of the water in cooking vessel  120  is absorbed by the rice and/or evaporated, the temperature of cooking vessel  120  tends to increase, triggering a mechanism inside cooking device  100  to either turn heater assembly  140  off or to switch to a reduced temperature warming cycle intended to keep the food in cooking vessel  120  warm. 
     With reference to  FIG.  2   , a schematic depiction of cooking device  100  is shown according to one example embodiment. Cooking device  100  includes heater assembly  140  including a heater  150  and a heating plate  145 . Heater  150  includes a substrate  152  to which at least one resistive trace  160  is secured. Heat is generated when electrical current provided by power source  114  is passed through resistive trace  160 . When cooking vessel  120  is disposed in receptacle  103 , cooking vessel  120  contacts and rests on top of heating plate  145 . Heating plate  145  is positioned in contact with, or in very close proximity to, heater  150  in order to transfer heat from heater  150  to cooking vessel  120 . In some embodiments, thermal grease is applied between heater  150  and heating plate  145  to facilitate physical contact and heat transfer between heater  150  and heating plate  145 . In some embodiments, a gap filler (e.g., silicon gap filler) or pad (e.g., graphite gap pad) is positioned between heater  150  and heating plate  145  to facilitate heat transfer between heater  150  and heating plate  145 . Heating plate  145  is composed of, for example, a metal having high thermal conductivity, such as forged aluminum. 
     Cooking device  100  includes control circuitry  115  configured to control the temperature of heater  150  by selectively opening or closing a circuit supplying electrical current to resistive trace  160 . Open loop or, preferably, closed loop control may be utilized as desired. In the embodiment illustrated, a temperature sensor  170 , such as a thermistor, is coupled to substrate  152  for sensing the temperature of heater  150  and permitting closed loop control of heater  150  by control circuitry  115 . Control circuitry  115  may include a microprocessor, a microcontroller, an application-specific integrated circuit, and/or other form integrated circuit. User interface  109  is communicatively coupled to control circuitry  115  via a communications link  110 . 
     In the embodiment illustrated in  FIG.  2   , control circuitry  115  includes a switch  117  connected between one end of resistive trace  160  and a first terminal  114   a  of power source  114 . Switch  117  may be, for example, a mechanical switch, an electronic switch, a relay, or other switching device. The other end of resistive trace  160  is connected to a second terminal  114   b  of power source  114 . The temperature of heater  150  is controlled by measuring the temperature of substrate  152  by temperature sensor  170  held in contact with substrate  152  and feeding temperature information from temperature sensor  170  to control circuitry  115  which, in turn, controls switch  117  to selectively supply power to resistive trace  160  based on the temperature information. When switch  117  is closed, current flows through resistive trace  160  to generate heat from heater  150 . When switch  117  is opened, no current flows through resistive trace  160  to pause or stop heat generation from heater  150 . In some embodiments, control circuitry  115  may include power control logic and/or other circuitries for controlling the amount of power delivered to resistive trace  160  to permit adjustment of the amount of heat generated by heater  150  within a desired range of temperatures. For example, in some embodiments, when the temperature of heater  150  is low (e.g., under 100 degrees Celsius), heater  150  is supplied with to 50% power and then gradually stepped up from 50% to 100% as the temperature of heater  150  increases. 
       FIG.  3    shows heater assembly  140  including heating plate  145  and heater  150  according to one example embodiment.  FIG.  4    shows a top view of heater  150 , and  FIG.  5    shows a bottom view of heater  150 . In the example embodiment illustrated, heating plate  145  is formed as a circular disk having a domed upper surface  147  (also shown in  FIG.  2    with exaggerated scale for illustration purposes). In one embodiment, heating plate  145  has a diameter of about 162 mm, a central portion having a thickness of about 5 mm, and a circumferential edge having a thickness of about 1 mm. In other embodiments, heating plate  145  may have other shapes as long as heating plate  145  is positioned to spread heat from heater  150  across the bottom surface of cooking vessel  120 . The thermal conductivity and relative thinness of heating plate  145  result in a relatively low thermal mass, which reduces the amount of time required to heat and cool heating plate  145  and, in turn, cooking vessel  120 . 
     Heater  150  includes substrate  152  constructed from ceramic or the like, such as aluminum oxide (e.g., commercially available 96% aluminum oxide ceramic). Hereinafter, substrate  152  is referred to as ceramic substrate  152 . In some embodiments, heater  150  may include one or more layers of ceramic substrate  152 . Where heater  150  includes a single layer of ceramic substrate  152 , a thickness of ceramic substrate  152  may range from, for example, 0.5 mm to 1.5 mm, such as 1.0 mm. Where heater  150  includes multiple layers of ceramic substrate  152 , each layer may have a thickness ranging from, for example, 0.5 mm to 1.0 mm, such as 0.635 mm. In the embodiment illustrated, ceramic substrate  152  is octagonal in shape having an incircle diameter d of about 147 mm. However, ceramic substrate  152  may take other suitable shapes depending on the application, such as, for example, circular, hexagonal, square, etc. In general, the octagonal shape illustrated is easier to reliably manufacture on a commercial basis than, for example, a circular shape. 
     Ceramic substrate  152  includes a top surface  152   a  that faces heating plate  145  and a bottom surface  152   b  opposite top surface  152   a.  In the embodiment illustrated, resistive trace  160  is positioned on top surface  152   a  of ceramic substrate  152 . Resistive trace  160  includes a first end  160   a  and a second end  160   b.  In this embodiment, a pair of conductive traces  162   a,    162   b  are also positioned on top surface  152   a.  Conductive traces  162   a,    162   b  are connected to first and second ends  160   a,    160   b  of resistive trace  160 , respectively. Resistive trace  160  includes a suitable electrical resistor material such as, for example, silver palladium (e.g., blended 70/30 silver palladium). Conductive traces  162   a,    162   b  include a suitable electrical conductor material such as, for example, silver platinum. In the embodiment illustrated, resistive trace  160  and conductive traces  162   a,    162   b  are applied to ceramic substrate  152  by way of thick film printing. For example, resistive trace  160  may include a resistor paste having a thickness of 10-13 microns when applied to ceramic substrate  152 , and conductive traces  162   a,    162   h  may include a conductor paste having a thickness of 9-15 microns when applied to ceramic substrate  152 . Resistive trace  160  forms the heating element of heater  150 , and conductive traces  162   a,    162   b  provide electrical connections to resistive trace  160  in order to supply an electrical current to resistive trace  160  to generate heat. 
     In the example embodiment illustrated, resistive trace  160  follows a serpentine pattern extending from first end  160   a  to second end  160   b  along top surface  152   a  of ceramic substrate  152 . In this embodiment, the serpentine pattern formed by resistive trace  160  has a generally circular outer perimeter  161 . Conductive traces  162   a,    162   b  each form a respective terminal  163   a,    163   b  of heater  150 . Cables or wires  165   a,    165   b  are connected to respective terminals  163   a,    163   b  in order to electrically connect resistive trace  160  and conductive traces  162   a,    162   b  to, for example, control circuitry  115  and power source  114  in order to selectively close the circuit formed by resistive trace  160  and conductive traces  162   a,    162   b  to generate heat. Conductive trace  162   a  directly contacts first end  160   a  of resistive trace  160 , and conductive trace  162   b  directly contacts second end  160   b  of resistive trace  160 . Conductive traces  162   a,    162   b  both extend along an extension portion  155  of ceramic substrate  152  that extends from an edge  157  of ceramic substrate  152  in the example embodiment illustrated, but conductive traces  162   a,    162   b  may be positioned in other suitable locations on ceramic substrate  152  as desired, Portions of first and second ends  160   a,    160   b  of resistive trace  160  obscured beneath conductive traces  162   a,    162   b  in  FIG.  4    are shown in dotted line. In this embodiment, current input to heater  150  at, for example, terminal  163   a  by way of conductive trace  162   a  passes through, in order, resistive trace  160  and conductive trace  162   b  where it is output from heater  150  at terminal  163   b.  Current input to heater  150  at terminal  163   b  travels in reverse along the same path. 
     In some embodiments, heater  150  includes temperature sensor  170 , also referred. to as thermistor  170 , positioned in close proximity to a surface of heater  150  in order to provide feedback regarding the temperature of heater  150  to control circuitry  115 . In the embodiment shown, thermistor  170  is positioned on bottom surface  152   b  of ceramic substrate  152 . In the example embodiment illustrated, thermistor  170  is welded directly to bottom surface  152   b  of ceramic substrate  152 . In this embodiment, heater  150  also includes a pair of conductive traces  172   a,    172   b  that are each electrically connected to a respective terminal of thermistor  170 . Each conductive trace  172   a,    172   b  has a distal end that forms a respective terminal  173   a,    173   b  adjacent to an edge  158  of ceramic substrate  152 . Cables or wires  175   a,    175   b  are connected to terminals  173   a,    173   b  in order to electrically connect thermistor  170  to, for example, control circuitry  115  in order to provide closed loop control of heater  150 . In the embodiment illustrated, thermistor  170  is positioned at a central location of bottom surface  152   b  of ceramic substrate  152 . However, thermistor  170  and its corresponding conductive traces  172   a,    172   b  may be positioned in other suitable locations on bottom surface  152   b  of ceramic substrate  152 . 
     In some embodiments, heater  150  also includes a thermal cutoff (not shown), such as a bi-metal thermal cutoff, in contact with ceramic substrate  152  and connected in series with the heating circuit formed by resistive trace  160  and conductive traces  162   a,    162   b  permitting the thermal cutoff to open the heating circuit formed by resistive trace  160  and conductive traces  162   a,    162   b  upon detection by the thermal cutoff of a temperature that exceeds a predetermined amount. In this manner, the thermal cutoff provides additional safety by preventing overheating of heater  150 . 
       FIG.  6    is a cross-sectional view of heater  150  taken along line  6 - 6  in  FIG.  4   . As shown, heater  150  includes resistive trace  160  and conductive traces  162   a,    162   b  formed on ceramic substrate  152 .  FIG.  6    depicts a single layer of ceramic substrate  152 . However, ceramic substrate  152  may include multiple layers as depicted by dashed line  153 . In the embodiment illustrated, heater  150  includes one or more layers of printed glass  156  on top surface  152   a  of ceramic substrate  152 . In the embodiment illustrated, glass layer  156  covers resistive trace  160  and portions of conductive traces  162   a,    162   b  in order to electrically insulate such features to prevent electric shock or arcing. The borders of glass layer  156  are shown in dashed line in  FIG.  4   . In this embodiment, glass layer  156  covers resistive trace  160  and adjacent portions of ceramic substrate  152  such that glass layer  156  forms the majority of the top surface of heater  150  facing heating plate  145 . An overall thickness of glass layer  156  may range from, for example, 35-45 microns. 
     In the embodiment illustrated, heater  150  also includes one or more layers of printed glass  159  on bottom surface  152   b  of ceramic substrate  152  to minimize camber. The borders of glass layer  159  are shown in dashed line in  FIG.  5   . In this embodiment, glass layer  159  does not cover thermistor  170  and some portions of conductive traces  172   a,    172   b  because the relatively low voltage (in comparison with the voltages applied to resistive trace  160 ) applied to such features presents a lower risk of electric shock or arcing, An overall thickness of glass layer  159  may range from, for example, 35-45 microns. 
     In addition to providing electrical insulation, laminating the ceramic heater of the present disclosure with glass layers  156 ,  159  provides increased resistance to thermal shock. In some embodiments, heater  150  is fabricated by fiber laser scribing the perimeter of heater  150  to further increase thermal shock resistance. Fiber laser scribing tends to provide a more uniform singulation surface having fewer microcracks along the separated edge in comparison with conventional carbon dioxide laser scribing. 
     Heater  150  may be constructed by way of thick film printing. For example, in one embodiment, resistive trace  160  is printed on fired (not green state) ceramic substrate  152 , which includes selectively applying a paste containing resistor material to top surface  152   a  of ceramic substrate  152  through a patterned mesh screen with a squeegee or the like. The printed resistor is then allowed to settle on ceramic substrate  152  at room temperature. The ceramic substrate  152  having the printed resistor is then heated at, for example, approximately 140-160 degrees Celsius for a total of approximately 30 minutes, including approximately 10-15 mins at peak temperature and the remaining time ramping up to and down from the peak temperature, in order to dry the resistor paste and to temporarily fix resistive trace  160  in position. The ceramic substrate  152  having temporary resistive trace  160  is then heated at, for example, approximately 850 degrees Celsius for a total of approximately one hour, including approximately 10 minutes at peak temperature and the remaining time ramping up to and down from the peak temperature, in order to permanently fix resistive trace  160  in position. Conductive traces  162   a,    162   b  are then printed on top surface  152   a  of ceramic substrate  152 , which includes selectively applying a paste containing conductor material in the same manner as the resistor material. The ceramic substrate  152  having the printed resistor and conductor is then allowed to settle, dried and fired in the same manner as discussed above with respective to resistive trace  160  in order to permanently fix conductive traces  162   a,    162   b  in position. Glass layer(s)  156  on top surface  152   a  are then printed in substantially the same manner as the resistors and conductors, including allowing the glass layer(s)  156  to settle as well as drying and firing the glass layer(s)  156 . In one embodiment, glass layer(s)  156  are fired at a peak temperature of approximately 810 degrees Celsius, slightly lower than the resistors and conductors. Conductive traces  172   a,    172   b  for thermistor  170  are printed on bottom surface  152   b  of ceramic substrate  152  in substantially the same manner as conductive traces  162   a,    162   b,  and glass layer(s)  159  are printed on bottom surface  152   b  of ceramic substrate  152  in substantially the same manner as glass layer(s)  156 . Thermistor  170  is then mounted to ceramic substrate  152  in a finishing operation with the terminals of thermistor  170  directly welded to conductive traces  172   a,    172   b.    
     Thick film printing resistive trace  160  and conductive traces  162   a,    162   b  on fired ceramic substrate  152  provides more uniform resistive and conductive traces in comparison with ceramic heaters having resistive and conductive traces printed on green state ceramic. The improved uniformity of resistive trace  160  and conductive traces  162   a,    162   b  provides more uniform heating across heating plate  145  as well as more predictable heating of heater  150 . 
     While the example embodiment illustrated in  FIGS.  3 - 5    includes heater  150  having an octagonal shape, in other embodiments, heater  150  may have other forms and shapes as desired. For example, with reference to  FIG.  7   , a heater  1150  may have a circular shape according to one example embodiment. Thermistor  170  is disposed on a surface of ceramic substrate  152  opposite the surface along which resistive trace  160  is disposed in the embodiment shown in  FIG.  5   , but thermistor  170  and/or its corresponding conductive traces may be disposed on the same side of ceramic substrate  152  as resistive trace  160  so long as they do not interfere with the positioning of resistive trace  160  and conductive traces  162   a,    162   h.  For example, in  FIG.  7   , a thermistor  1170  is positioned on the same surface as resistive trace  160  (e.g., top surface  1152   a  of ceramic substrate  1152 ). In some embodiments, corresponding conductive traces of thermistor  170  may be disposed on the bottom surface (opposite top surface  1152   a ) of ceramic substrate  1152  while thermistor  1170  is positioned on top surface  1152   a  thereof. In this embodiment, heater  150  may include vias that are formed as through-holes substantially filled with conductive material extending through ceramic substrate  1152  from top surface  1152   a  to the bottom surface of ceramic substrate  1152  in order to electrically connect the terminals of thermistor  1170  on top surface  1152   a  to their corresponding conductive traces on the bottom surface. 
     It will be appreciated that the example embodiments illustrated and discussed above are not exhaustive and that the heater of the present disclosure may include resistive and conductive traces in many different patterns and locations on ceramic substrate  152 , including resistive traces on one or more of the exterior surfaces (top surface and/or bottom surface) of ceramic substrate  152  and/or an intermediate surface of ceramic substrate  152 , as desired. Other components (e.g., a thermistor) may be positioned on either the top surface or the bottom surface of the heater as desired, including on the same surface as the resistive traces or an opposite surface. 
       FIG.  8    shows a cooking vessel  120  suitable for use with heater assembly  140  according to one example embodiment. In the embodiment illustrated, cooking vessel  120  includes an inner shell  125  and an outer shell  130 . An outside surface  125   b  of inner shell  125  forms food receptacle  121  of cooking vessel  120 . Inner shell  125  and outer shell  130  have corresponding side walls  126 ,  131  and corresponding bottom walls  127 ,  132  separated by a gap  129  to form a dual-wall vessel. In this embodiment, bottom wall  132  of outer shell  130  has a slightly concave outside surface  130   b  that substantially matches domed upper surface  147  of heating plate  145 . The use of a heating plate  145  having a domed upper surface  147  in contact with a concave outside surface  130   b  of the bottom wall  132  of cooking vessel  120  helps reduce bowing of bottom wall  132  of cooking vessel  120  during heating in comparison with a cooking vessel having a flat bottom surface in contact with a fiat top surface of a heating plate or heater. This, in turn, helps upper surface  147  of heating plate  145  maintain consistent contact with outside surface  130   b  of the bottom wall  132  of cooking vessel  120  for heat transfer. Inner shell  125  and outer shell  130  are integrally joined or welded, e.g., at rim  122 , forming a sealed volume between inner and outer shells  125 ,  130  that includes gap  129 . In some embodiments, the sealed volume is formed under reduced pressure relative to atmospheric pressure, such as a partial vacuum. 
     In the example embodiment illustrated, a heat pipe  134  is provided between inner and outer shells  125 ,  130 , including between side walls  126 ,  131  and between bottom walls  127 ,  132 . In the embodiment shown, corresponding inside surfaces  125   a,    130   a  of inner and outer shells  125 ,  130  are lined with wick structures  135  containing a relatively small amount of working fluid, such as water. The wick structures  135  may be constructed from materials that allow capillary action of the working fluid within the sealed volume as discussed below. In  FIGS.  9 A- 9 C , various example wick structures for use with cooking vessel  120  are illustrated. Each of  FIGS.  9 A- 9 C  is a cross-sectional view of cooking vessel  120  taken along line  9 - 9  in  FIG.  8   . In the embodiment shown in  FIG.  9 A , the wick structure includes sintered or arc sprayed metal  135   a,  such as copper or aluminum, provided on inside surfaces  125   a,    130   a  of inner and outer shells  125 ,  130 . In the embodiment shown in  FIG.  9 B , a screen or wire mesh  135   b  is provided on each of the inside surfaces  125   a,    130   a  of inner and outer shells  125 ,  130  to form the wick structure. In the embodiment shown in  FIG.  9 C , grooves  135   c  are formed on each of the inside surfaces  125   a,    130   a  of inner and outer shells  125 ,  130  to provide the wick structure. Each groove  135   c  extends substantially vertically along a respective side wall  126 ,  131  and may continue substantially horizontally along a respective bottom wall  127 ,  132 . While the example embodiments illustrated include a heat pipe  134  that includes one or more wick structures  135  and a working fluid, in other embodiments, heat pipe  134  includes a working fluid (e.g., water) contained between inner and outer shells  125 ,  130 , but no wick structure. 
     In one embodiment, during use, the working fluid cycles between an evaporation zone  180  near or around the lower region of cooking vessel  120  that is directly heated by heating plate  145  and a condensation zone  190  around the upper region of cooking vessel  120 . In particular, as cooking vessel  120  is heated by heater assembly  140  (e.g., by outside surface  130   b  of bottom wall  132  of outer shell  130  receiving heat from heater assembly  140 ) the working fluid within the evaporation zone  180  (e.g., working fluid within the wick structures  135  between bottom walls  127 ,  132  of inner and outer shells  125 ,  130  and between side walls  126 ,  131  of inner and outer shells  125 ,  130  in the lower region of cooking vessel  120 ) absorbs heat  183  and changes state from liquid to vapor  138 . Driven by pressure and temperature differences between the lower (hotter) region and upper (cooler) region, vapor  138  travels from the evaporation zone  180  to the condensation zone  190  along the gap  129  between wick structures  135 . When vapor  138  arrives at the condensation zone  190 , it condenses back into liquid form releasing latent heat  185  through inner and outer shells  125 ,  130  at the upper region of cooking vessel  120 . Condensed liquid  139  at the condensation zone  190  travels back to the evaporation zone  180  via wick structures  135  due to capillary action. As the vaporization and condensation cycle repeats, heat is transferred from locations near the heat source to the rest of the sealed volume of cooking vessel  120  (i.e., from between bottom walls  127 ,  132  of inner and outer shells  125 ,  130  to between side walls  126 ,  131  of inner and outer shells  125 ,  130 ) resulting in an improved temperature uniformity within cooking vessel  120 . 
     The present disclosure provides a ceramic heater having a low thermal mass in comparison with the heaters of conventional cooking devices. In particular, a thick film printed resistive trace on a ceramic substrate provides reduced thermal mass in comparison with conventional wire coil heaters. The use of a thin heating plate, such as forged aluminum, also provides reduced thermal mass in comparison with the cast aluminum bodies of conventional wire coil heaters. The low thermal mass of the ceramic heater of the present disclosure allows the heater, in some embodiments, to heat to an effective temperature for use in a matter of seconds (e.g., less than 5 seconds), significantly faster than conventional wire coil heater cooking devices. The low thermal mass of the ceramic heater of the present disclosure also allows the heater, in some embodiments, to cool to a safe temperature after use in a matter of seconds (e.g., less than 5 seconds), again, significantly faster than conventional wire coil heater cooking devices. 
     Further, embodiments of the heater of the cooking device of the present disclosure operate at a more precise and more uniform temperature than conventional cooking devices because of the closed loop temperature control provided by the thermistor in combination with the relatively uniform thick film printed resistive and conductive traces. The low thermal mass of the ceramic heater permits greater energy efficiency in comparison with conventional wire coil heaters. The improved temperature control and temperature uniformity also improve the performance of the cooking device of the present disclosure. In this manner, embodiments of the cooking device of the present disclosure achieve high thermal and energy efficiency and high-end end performance comparable to induction heating cooking devices, but at a greatly reduced cost in comparison with conventional induction heating cooking devices. 
     The present disclosure further provides a heat pipe cooking vessel for use with the ceramic heater. The heat pipe structure within the cooking vessel provides improved thermal conductivity in comparison with conventional aluminum or copper cooking vessels allowing for a more uniform temperature distribution and effective heat transfer. Coupled with the low thermal mass of the ceramic heater, the heat pipe cooking vessel provides improved temperature uniformity relative to conventional cooking devices. 
     While the example embodiment discussed above includes a ceramic heater used in conjunction with a heat pipe cooking vessel, it will be appreciated that the ceramic heater and the cooking vessel of the present disclosure may be used separately from each other in different heating and/or cooking applications. That is, the ceramic heater of the present disclosure may be used with a conventional cooking vessel, and the heat pipe cooking vessel of the present disclosure may be used with conventional heaters. 
     The foregoing description illustrates various aspects of the present disclosure. It is not intended to be exhaustive. Rather, it is chosen to illustrate the principles of the present disclosure and its practical application to enable one of ordinary skill in the art to utilize the present disclosure, including its various modifications that naturally follow. All modifications and variations are contemplated within the scope of the present disclosure as determined by the appended claims. Relatively apparent modifications include combining one or more features of various embodiments with features of other embodiments.