Patent Publication Number: US-2020281398-A1

Title: Double-walled crockery provided with a pouch filled with phase-change material

Description:
FIELD OF THE INVENTION 
     The present invention relates to double-walled crockery with a wall to be heated or cooled, and in particular to double-walled crockery provided with a pouch filled with phase-change material. 
     BACKGROUND 
     The use of phase-change material as heat storage material in crockery, wherein the phase-change material is able to absorb heat in a relatively short time and to relinquish this heat again over a longer period of time, is known. An example of such an application is the use of phase-change material in a plate on which hot food is served. By making use of phase-change material the plate will cool less quickly and the food will remain warm for longer. A cavity filled with phase-change material is typically provided in the plate. 
     Typical phase-change materials used for this and similar applications in crockery are solid at room temperature and transpose to a liquid phase at higher temperatures at which they absorb heat. When the phase-change material is in the liquid phase, there is the danger that particles of the phase-change material can escape through the micropores of the crockery. In the liquid phase the phase-change material can moreover move freely in the cavity under the influence of movements of the crockery, whereby the release of heat to a wall of the crockery to be heated will decrease and whereby an irregular and/or varying distribution of weight of phase-change material will occur in the crockery. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention have the object of providing the phase-change material in the crockery in a manner such that a better exchange of heat is obtained between a wall of the crockery to be heated or cooled and the phase-change material when compared to known applications of phase-change material as heat storage material in crockery. 
     A first aspect of the invention relates to double-walled crockery comprising a first wall to be heated or cooled, and a second wall, wherein between the first wall and the second wall is located a space in which a pouch filled with a phase-change material is accommodated. The pouch is arranged against the first wall of the crockery and a thermally insulating layer is present between the pouch and the second wall of the crockery. 
     By providing the phase-change material in a filled pouch the freedom of movement of the phase-change material in the crockery is limited by the pouch. Because the pouch is arranged against the first wall of the crockery, a better heat exchange between the pouch and the first wall to be heated or cooled is obtained when compared to known applications of phase-change material in crockery wherein a thermally insulating layer, for instance an air layer, is present between the phase-change material and the first wall to be heated or cooled. The pouch is arranged against the first wall of the crockery, while a thermally insulating layer is present between the pouch and the second wall of the crockery. This thermally insulating layer ensures that the amount of heat leaving the crockery via a path other than via the first wall is limited. This has the advantage that an efficient and targeted heat exchange can take place between the phase-change material and the first wall of the crockery and that the second wall of the crockery does not heat up, or only does so to limited extent. 
     In a preferred embodiment the pouch is elastic and flexible to an extent such that the pouch can also stretch when the phase-change material expands during transition from the one phase to the other, and such that the pouch becomes taut again when the phase-change material contracts during a reverse phase transition. The flexibility of the pouch further ensures that it can be arranged fittingly against walls of different shapes to be heated or cooled without air spaces forming between the pouch and the wall. 
     In a possible embodiment a first wall part of the pouch arranged against the first wall is optionally elastic, while a second wall part of the pouch facing toward the second wall is elastically flexible. In another possible embodiment this second wall part of the pouch facing toward the second wall of the crockery is not elastic, but has a surface area such that an expansion of the phase-change material is possible. 
     In an embodiment the double-walled crockery is for instance a plate and the above stated measures ensure that the first wall of the plate on which food is located is kept warm or heated by means of the pouch and that the second wall for instance the bottom wall of the plate, remains cool so that it is possible to take hold of the plate by the bottom wall without the danger of burn injuries. Although the example of a plate is given here, the described measures can be applied to diverse types of crockery with at least one wall for heating or cooling. Examples thereof are: oven dishes, cups, dessert plates, soup bowls, dessert coupes, pizza plates, spoons and so on. 
     In a preferred embodiment of the double-walled crockery the pouch is filled with phase-change material not accommodated in capsules. 
     The use of phase-change material accommodated in micro- or macro-capsules is known, A higher heat capacity can however be obtained by making use of phase-change material not accommodated in capsules when compared to the same amount of material accommodated in capsules. This is because air is often present between the micro- or macro-capsules, and this results in a thermally insulating effect. Phase-change material not accommodated in capsules can moreover be placed in simple and efficient manner in a pouch of appropriate form, whereby a more efficient contact surface can be obtained for heat exchange between the phase-change material in the pouch and the wall of the crockery to be heated, when compared to crockery provided with phase-change material accommodated in capsules. 
     In an exemplary embodiment of the double-walled crockery the thermally insulating layer is an air layer. Air (s a good thermal insulator and the insulating air layer will ensure that the amount of heat leasing the crockery via a path other than via the first wall to be heated is limited. This has the advantage that an efficient and targeted heat exchange can take place between the phase-change material and the first wall of the crockery and that the second wall of the crockery does not heat up in undesirable manner. 
     The pouch can be held in different ways in the position arranged against the first wall. The advantage of holding the pouch in position is that movements of the crockery will have substantially no effect on the pouch and the position thereof against the first wall. An efficient and targeted heat exchange between the pouch and the first wall of the crockery is thus possible in all conditions. In an exemplary embodiment of the double-walled crockery the pouch is glued against the first wall of the crockery. Glueing the pouch against the first wall of the crockery results in a glue layer between the pouch and the first wall. The resulting glue layer typically has a thermally insulating effect, though it typically insulates considerably less than air, and this glue layer can be relatively thin. The glue layer will therefore typically only have a delaying effect on the heat exchange between the pouch and the first wall of the crockery, while an insulating air layer between the pouch and the first wall of the crockery would seriously interfere with this heat exchange. 
     In an exemplary embodiment of the double-walled crockery the thermally insulating layer comprises an elastic pressing material configured to press the pouch against the first wall of the crockery. As described above, the thermally insulating layer between the pouch and the second wall of the crockery has in the first place an insulating function which contributes toward a targeted and efficient heat exchange between the pouch and the first wall of the crockery. The elastic pressing material in the thermally insulating layer moreover ensures that the pouch is held in position against the first wall of the crockery. The elastic pressing material can thus be an alternative to glueing the pouch against the first wall. Both measures can however also be combined with each other. In addition, other attaching and clamping techniques are also possible which ensure that the pouch is attached or pressed against the first wall of the crockery. 
     The elastic pressing material preferably has a heat conduction coefficient of less than 0.9 W/(m·K), and preferably of less than 0.5 W/(m·K). Examples of elastic pressing materials which can be used are wood, cork, polyethylene etc. It will however be apparent to the skilled person that other common elastic materials can also be used for the above described measure. The elastic pressing material preferably has a heat conduction coefficient which is lower than the heat conduction coefficient of the material of which the crockery consists, such as for instance chinaware, earthenware, metal or glass. The elastic pressing material is for instance a foam. The foam preferably has a heat conduction coefficient of less than 0.5 W(m·K), more preferably of less than 0.3 W/(m·K) and most preferably of less than 1.5 W/(m·K). Foam is light and the use of foam as elastic pressing material will have a negligible effect on the overall weight of the crockery. 
     In a preferred embodiment of the double-walled crockery a high vacuum prevails in the filled pouch, wherein a pressure of between 1×10 −1  and 1×10 4  Pa prevails in the pouch. 
     Because a high vacuum prevails in the pouch, insulating air is prevented from being able to lodge between the phase-change material and the walls of the pouch, or between different parts of the phase-change material. The phase-change material in the vacuum pouch is moreover protected from possible contaminants, possible degradation of the phase-change material, and igniting of the phase-change material is prevented by the high vacuum in the pouch. 
     In an exemplary embodiment of the double-walled crockery the pouch is arranged against the first wall of the crockery such that the layer of phase-change material has a thickness, as seen in a direction transversely of the first wall of the crockery, of between 2 mm and 10 mm. The space between the first wall and the second wall of the crockery is often limited. It is therefore advantageous, with the smallest possible quantity of phase-change material, to obtain the most efficient possible heat exchange which meets the needs of a specific type of crockery. A layer of a suitable thickness can be selected depending on the type of crockery and on the desired heating or cooling of the first wall. 
     In a preferred embodiment of the double-walled crockery the pouch filled with phase-change material releases per square centimetre of surface area a heat energy of between 50 joule and 200 joule, preferably of between 100 joule and 180 joule. 
     In an exemplary embodiment of the double-walled crockery the first wall of the crockery is substantially flat and the pouch is arranged against a central part of the first wall such that a border part of the first wall is left clear. When the crockery is for instance a plate with a substantially flat, circular first wall, it is advantageous to arrange the pouch against the central part of the first wall since more food will come to lie in the central part of the plate than at the border part of the plate. By leaving a border part of the first wall clear the pouch has the opportunity to stretch along the first wall from the central part to the border part when the phase-change material in the pouch melts. This avoids pressure being built up in the space between the first wall and the second wall of the plate. It is moreover more advantageous for the heat exchange with the first wall that the pouch stretches along the first wall instead of away from the first wall. 
     In another exemplary embodiment of the double-walled crockery the first wall of the crockery is bowl-shaped and the pouch comprises a pan formed in bowl shape which is configured to be arranged against the first bowl-shaped wall. Because the pouch has a part formed in bowl shape, the pouch can be arranged in efficient manner against the first wall of the crockery. In the case of a coffee or tea cup the pouch with the part formed in bowl shape can for instance be arranged in advantageous manner simultaneously against a bottom part of the first wall and against a standing side part of the first wall. It will be apparent to the skilled person that the same principle can be applied for any random shape of the first wall, wherein the pouch can have a wall part with a corresponding shape in order to guarantee good contact between the pouch and the first wall. 
     In a preferred embodiment the contact surface between the first wall and the pouch is larger than a third of the total outer surface of the pouch. This guarantees that the most efficient possible heat exchange can take place between the pouch and the first wall, and the amount of heat released by the pouch in directions other than in the direction of the first wall is limited. 
     In an exemplary embodiment of the double-walled crockery the first wall is a wall to be heated and the phase-change material has a inching temperature of between 40° C. and 65° C., preferably between 50° C. and 60° C. Phase-change material with such a melting temperature is highly suitable for use in crockery intended for hot foods. The foods will thus be kept at temperature without being cooked or fried any further. Typical phase-change materials used are paraffin, salt hydrates and/or eutectic materials. 
     In an alternative exemplary embodiment of the double-walled crockery the first wall is a wall to be heated and the phase-change material has a melting temperature of between 65° C. and 95° C., preferably of between 70° C. and 80° C. Phase-change material with such a melting temperature is highly suitable for use in crockery intended for hot beverages such as coffee, tea, drinking chocolate and so on. 
     In another exemplary embodiment of the double-walled crockery the first wall is a wall to be cooled and the phase-change material has a melting temperature of between 0° C. and 14° C., preferably between 4° C. and 8° C. Phase-change material with such a melting temperature is highly suitable for use in crockery intended for cold dishes, such as ice creams, or for cold beverages. In a preferred embodiment of the double-wailed crockery the pouch comprises a first and a second film, wherein the phase-change material is situated between the first and second films. The films of the pouch can comprise different materials just as long as the pouch has the necessary flexibility and/or dimensions and the films can keep the phase-change material enclosed. The freedom of movement of the phase-change material in the crockery is thus limited by the pouch. At least one of the first and second film is preferably elastically flexible to an extent such that the pouch can also stretch when the phase-change material expands at the transition from the solid phase to the liquid phase, and such that the pouch becomes taut again when the phase-change material contracts during a reverse phase change. In another possible embodiment the second film of the pouch facing toward the second wall of the crockery is not elastic, but has a surface area such that an expansion of the phase-change material is possible. It will be apparent to the skilled person that different materials meet the above stated requirements and that the films can for instance be made of different types of plastic and/or metal, such m aluminium. Films moreover have the advantage that they can be laminated in advantageous manner, whereby different material properties can be combined in one film. A pouch can for instance thus be formed in advantageous manner which consists of a welding layer, for instance of polypropylene, laminated with an aluminium foil. The aluminium foil ensures that the pouch is substantially impermeable to molecules. 
     In a preferred embodiment of the double-walled crockery the first film and second film of the pouch are at least 40 μm thick, preferably between 50 μm and 150 μm thick. Films of such thickness ensure that the pouch provides a high degree of protection and strength, while the pouch remains flexible. 
     In a further preferred embodiment of the double-walled crockery the first film and second film of the pouch comprise a welding layer and a barrier layer, wherein the welding layer of the first film is welded to the welding layer of the second film. The barrier layer is selected such that it does not allow passage of molecules of the phase-change material, particularly at higher temperatures at which the phase-change material becomes liquid. The welding layer is selected such that it is weldable in simple manner, whereby the first film and second film can be welded to each other at a peripheral part of the first and second film. The welding layer preferably comprises polypropylene and the barrier layer comprises polyamide. The first film and/or the second film preferably comprises a layer manufactured from aluminium. A film can for instance thus be obtained in advantageous manner which consists of a welding layer, for instance of polypropylene, laminated with an aluminium foil as barrier layer. The aluminium foil ensures that the pouch is substantially impermeable to molecules. 
     The crockery is preferably manufactured from ceramic material, for instance chinaware, earthenware, glass and so on. In an embodiment the double-walled crockery comprises a first part of ceramic material and a second part of ceramic material, wherein the first part comprises the first wall and the second part comprises the second wall. The first part is attached against the second part in order to form the double-walled crockery. The first and second parts can for instance be any of the following: an upper part of a plate and a lower part of a plate; an inner part of a cup and an outer part of a cup; an upper part of a spoon and a lower part of a spoon, and so on. In other embodiments the first and second walls form part of one integrally formed double-walled article. 
     In an advantageous embodiment the crockery comprises at least two parts of ceramic material which are attached to each other in sealed manner, for instance by an attaching means, in order to bound said space in which the pouch is accommodated. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above stated and other advantageous properties and objectives of the invention will become more apparent, and the invention better understood, on the basis of the following detailed description when read in combination with the accompanying drawings, in which: 
         FIG. 1A  shows a cross-section of an exemplary embodiment of an article of crockery, in particular a plate, provided with a pouch filled with phase-change material according to the invention; 
         FIG. 1B  shows a simplified cross-section of an exemplary embodiment of an article of crockery, in particular a plate, provided with a pouch filled with phase-change material according to the invention; 
         FIG. 2  shows a simplified cross-section of an alternative exemplary embodiment of an article of crockery provided with a pouch filled with phase-change material according to the invention; 
         FIG. 3  shows a simplified cross-section of an alternative exemplary embodiment of an article of crockery provided with a pouch filled with phase-change material according to the invention; 
         FIG. 4  shows a simplified cross-section of an alternative exemplary embodiment of an article of crockery provided with a pouch filled with phase-change material according to the invention, wherein the first wall of the crockery is substantially flat and the pouch is arranged against a central part of the first wall; 
         FIG. 5  shows a cross-section of an alternative exemplary embodiment of an article of crockery provided with a pouch filled with phase-change material according to the invention, wherein the first wall of the crockery is bowl-shaped and the pouch comprises a part formed in bowl shape which is configured to be arranged against the first bowl-shaped wall; and 
         FIGS. 6A and 6B  show simplified cross-sections of embodiments of double-walled crockery, wherein the crockery comprises a first pan and a second part of ceramic material. 
     
    
    
     DETAILED EMBODIMENTS 
     A first embodiment of an article of double-walled crockery provided with a pouch filled with phase-change material is illustrated in  FIG. 1A . This is a double-walled article of crockery  100 , more specifically a plate  100 , comprising a first wall  110  to be heated or cooled and a second wall  120 , wherein between first wall  110  and second wall  120  a space  130  is present in which a pouch  140  filled with a phase-change material  145  is accommodated. Pouch  140  is arranged against first wall  110  of the crockery and a thermally insulating layer  150  is present between pouch  140  and second wall  120  of the crockery.  FIG. 1B  shows a simplified view of the article of crockery  100  of  FIG. 1A . and  FIG. 1B  shows in particular the part of crockery  100  indicated with the broken line border K in  FIG. 1A .  FIG. 1B  shows the same elements as  FIG. 1A , but now without these being linked to a specific type of crockery. It will be apparent to the skilled person that the elements and measures shown in  FIG. 1B  can be applied in different types and shapes of crockery, such as oven dishes, cups, dessert plates, soup bowls, dessert coupes, pizza plates, spoons and so on. In order to keep the figures as clear as possible, measures which are universally applicable in diverse types of crockery will be illustrated on the basis of figures similar to  FIG. 1B . When determined measures apply only to specific types of crockery, this will be stated in the text. 
       FIGS. 1A and 1B  show a first wall  110  and second wall  120  of the article of crockery  100 , wherein walls  110 ,  120  are substantially parallel to each other. It will however be apparent to the skilled person that the principles of the invention can likewise be applied when walls  110 ,  120  are oriented in other manner relative to each other. Walls  110 ,  120  are shown for the sake of simplicity as flat walls. The skilled person will however appreciate that the principles of the invention can likewise be applied when walls  110 ,  120  have a different shape. 
     The phase-change material  145  used in pouch  140  can be any known phase-change material and, depending on the type of crockery, a specific phase-change material may be preferred to another phase-change material. A phase-change material may thus be selected for instance on the basis of the melting point thereof, since a higher or lower melting point can be advantageous for optimal heating or cooling of liquids or foods. Organic phase-change materials based on paraffins are particularly suitable for use in a pouch  140  filled with phase-change material  145  for use in double-walled crockery. 
     By providing phase-change material  145  in a pouch  140  the freedom of movement of phase-change material  145  in crockery  100  is limited by the pouch. Pouch  140  is embodied for instance in an elastically flexible material such that pouch  140  can also stretch when phase-change material  145  expands during transition from the one phase to the other, and such that pouch  140  becomes taut again when phase-change material  145  contracts during a reverse phase transition. Because pouch  140  is arranged against first wall  110 , a better heat exchange is obtained between pouch  140  and the wall HO to be heated or cooled when compared to known applications of phase-change material in crockery wherein an insulating layer, such as for instance an air layer, is present between the phase-change material and the wall to be heated or cooled. Pouch  140  can moreover be positioned in advantageous manner against the first wall of the crockery, whereby a very targeted heal exchange can take place. In addition, a thermally insulating layer  150  is situated between pouch  140  and second wall  120  of the crockery. This thermally insulating layer  150  ensures that the amount of heat leaving the crockery via a path other than via first wall  110  is limited. This has the advantage that an efficient and targeted heat exchange can take place between the phase-change material and the first wall of the crockery and that second wall  120  of the crockery does not heat up unnecessarily, or only does so to limited extent. A first wall part of the pouch arranged against first wall  110  can be optionally elastic, while a second wait part of pouch  140  facing toward second wall  140  is preferably elastic and flexible. In another possible embodiment this second wall part of pouch  140  facing toward second wall  120  of the crockery is not elastic, but has a surface area such that an expansion of phase-change material  145  is possible. Thermally insulating layer  150  is shown in  FIGS. 1A and 1B  as a layer situated against second wall  120 . Thermally insulating layer  150  can however also be situated closer to or against pouch  140 . In an embodiment the whole space  130  between pouch  140  and second wall  120  is filled with a thermally insulating material, for instance air. This material, for instance air, then forms the thermally insulating layer. In the shown embodiment of  FIG. 1A  the double-walled crockery is for instance a plate  100 , and the above stated measures ensure that first wall  110  of the plate, wherein the food is on the upper side of first wall  110 , is kept warm or heated by means of pouch  140  located against the underside of first wall  110 , while the second wall, for instance live bottom wall of the plate, remains cool so that it is possible to take hold of the plate by the bottom wall without the danger of burn injuries. 
     In a preferred embodiment of the double-walled crockery the pouch  140  is filled with phase-change material  145  not accommodated in capsules, whereby a more efficient contact surface can be obtained for heat exchange between phase-change material  145  in pouch  140  and the wall  110  of the crockery to be heated, when compared to crockery provided with phase-change material accommodated in capsules. 
     In a preferred embodiment of the double-walled crockery a high vacuum prevails in pouch  140  filled with phase-change material  145 . A pressure of between 1×10 −1  and 1×10 −7  Pa then prevails in pouch  140 . Because a high vacuum prevails in pouch  140 , insulating air is prevented from being able to lodge between phase-change material  145  and the walls of pouch  140 , or between different parts of phase-change material  145 . Phase-change material  145  in vacuum pouch  140  is moreover protected from possible contaminants, possible degradation of the phase-change material, and igniting of phase-change material  145  is prevented by the high vacuum in the pouch. 
     In an exemplary embodiment of the double-walled crockery the pouch  140  is arranged against first wall  110  of the crockery such that the layer of phase-change material  145  has a thickness, as seen in a direction transversely of first wall  110  of crockery  100 , of between 2 mm and 10 mm. The space  130  between first wall  110  and second wall  120  of the crockery is often limited. It is therefore advantageous, with the smallest possible quantity of phase-change material, to obtain the most efficient possible heat exchange which meets the requirements of a specific type of crockery. A layer of a suitable thickness can be selected depending on the type of crockery and on the desired heating or cooling of first wall  110 . 
     In a preferred embodiment of the double-walled crockery the pouch  140  filled with phase-change material  145  releases per square centimetre of surface area a heat energy of between 50 joule and 200 joule, preferably of between 100 joule and 180 joule. 
     In a preferred embodiment the contact surface between first wall  110  and pouch  140  is larger than a third of the total outer surface of pouch  140 . The contact surface between the first wall and the pouch is typically larger than 0.4 times the total outer surface of pouch  140 , and for instance about half the total outer surface. This guarantees that the most efficient possible heat exchange can take place between pouch  140  and first wall  110 , and the amount of heat released by pouch  140  in directions other than in the direction of first wall  110  is limited. The contact surface between first wall  110  and pouch  140  can be favourably influenced by providing a pouch  140  filled with phase-change material  145  of suitable thickness and/or by advantageous shaping of a part of pouch  140 . 
       FIG. 2  shows an exemplary embodiment of the double-walled crockery wherein pouch  240  is glued against first wall  210  of the crockery. Pouch  240  can be held in different alternative ways in the position arranged against first wall  210 . The advantage of holding pouch  240  in position is that movements of the crockery will have substantially no influence on pouch  240  and the position thereof against first wall  210 . An efficient and targeted heat exchange between pouch  240  and first wall  210  of the crockery is thus possible in all conditions. Glueing pouch  240  against the first wall of the crockery results in a glue layer  241  between pouch  240  and first wall  210 . The resulting glue layer  241  typically has a thermally insulating effect, though it typically insulates considerably less than air, and this glue layer  241  can be relatively thin. 
     In the embodiment of  FIG. 2  thermally insulating layer  250  is an air layer present in space  250  between pouch  240  and first wall  210  of the crockery. As indicated above, air is a good thermal insulator and thermally insulating air layer  150  will ensure that the amount of heat leaving the crockery via a path other than via the first wall  210  to be heated is limited. This has the advantage that an efficient and targeted heal exchange can take place between phase-change material  245  and first wall  210  of the crockery and that second wall  220  of the crockery does not heat up in undesirable manner. 
       FIG. 3  shows in simplified manner a preferred embodiment of the double-walled crockery wherein thermally insulating layer  350  comprises an elastic pressing material  350  configured to press pouch  340  against first wall  310  of the crockery. As described above, thermally insulating layer  350  between pouch  340  and second wall  320  of the crockery has in the first place an insulating function which contributes toward a targeted and efficient heat exchange between pouch  340  and first wall  310  of the crockery. The elastic pressing material  350  in thermally insulating layer  350  moreover ensures that pouch  340  is held in position against first wall  310  of the crockery. Elastic pressing material  350  can thus be an alternative to glueing pouch  340  against the first wall as shown in  FIG. 2 . Both measures can however also be combined with each other in advantageous manner. Elastic pressing material  350  preferably has a heat conduction coefficient of less than 0.9 W/(m·K), and more preferably of less than 0.5 W/(m·K). Examples of elastic pressing materials  350  which can be used are wood, cork, polyethylene etc. It will however be apparent to the skilled person that other common elastic materials can also be used for the above described measure. The elastic pressing material preferably has a heat conduction coefficient which is lower than the heat conduction coefficient of the material of which the crockery consists, such as for instance chinaware, earthenware, metal or glass. Elastic pressing material  350  can also be a foam. The foam used preferably has a heat conduction coefficient of less than 0.5 W/(m·K), more preferably of less than 0.3 W/(m·K) and most preferably of less titan 1.5 W/(m·K). Foam is light and the use of foam as elastic pressing material  350  will have a negligible effect on the overall weight of the crockery. It is hereby possible to avoid the crockery becoming too heavy, whereby it would no longer be easy to handle. Typical foams which can be used as elastic pressing material  350  are for instance PU foam, EVA foam or polystyrene foam. 
       FIG. 4  shows an embodiment of the double-walled crockery wherein first wall  410  of the crockery is substantially flat and pouch  440  is arranged against a central part  411  of first wall  410  such that a border part  412  of first wall  410  is left clear. For the sake of clarity the reference numerals  411 ,  412  in  FIG. 4  are placed at the upper side of first wall  410 , although it will be apparent that pouch  440 , central part  411  against which pouch  440  is arranged and the free border part  412  are situated against the underside of first wall  410 . When the article of crockery is for instance a plate with a substantially flat, circular first wait  410 , it is advantageous to arrange pouch  440  against central part  411  of first wall  410  since more food will come to lie hi the central part of the plate than at the border part of the plate, and it is therefore more useful to have the heat exchange take place at the centre of first wall  410 . By leaving a border part  412  of first wall  410  clear the pouch  440  has the opportunity to stretch along first wall  410  from central part  411  to border part  412  when phase-change material  440  in the pouch melts. This avoids pressure being built up in space  430  between the first wall and the second wall of the plate. It is moreover more advantageous for the heat exchange with first wall  410  that pouch  440  stretches along first wall  410  instead of away from first wall  410 . 
     In an exemplary embodiment of the double-walled crockery the first wall is a wall to be heated and the phase-change material has a melting temperature of between 40° C. and 65° C., preferably between 50° C. and 60° C. Phase-change material with such a melting temperature is highly suitable for use in crockery intended for hot foods. The foods will thus be kept at temperature without being cooked or fried any further. For other types of crockery wherein higher temperatures are desirable, such as for instance oven dishes, phase-change materials can be used with a melting point lying between 120° C. and 250° C. It will be apparent to the skilled person that the principles and measures according to embodiments of the invention can be applied for different types of phase-change material, wherein the melting temperature of the respective phase-change materials can vary from 40° C. to 300° C. 
       FIG. 5  illustrates an embodiment of the double-wailed crockery wherein first wall  510  of the crockery is bow l-shaped  531  and wherein pouch  540  comprises a part formed in bowl shape which is configured to be arranged against first bowl-shaped wall  510 ,  513 , Because pouch  540  has a part formed in bowl shape, pouch  540  can be arranged in efficient manner against first wall  510 ,  513  of the crockery. In the case of a coffee or tea cup the pouch  540  with the part formed in bowl shape can for instance be arranged in advantageous manner simultaneously against a bottom part  513  of first wall  510  and against a standing side part  514  of the first wall. It will be apparent to the skilled person that the same principle can be applied for any random shape of first wall  510 , wherein pouch  540  can have a wall part with a corresponding shape in order to guarantee good contact between pouch  540  and first wall  510 . 
     In a preferred embodiment the contact surface between first wall  510  and pouch  540  is larger than a third of the total outer surface of pouch  540 . The contact surface between first wall  510  and pouch  540  can be favourably influenced by providing a pouch  540  filled with phase-change material  545  of suitable thickness and/or advantageously shaping a part of pouch  540 . 
     In an exemplary embodiment of the double-walled crockery the first wall  510  is a wall to be heated and phase-change material  545  has a melting temperature of between 65° C. and 93° C. preferably of between 70° C. and 80° C. Phase-change material  545  with such a melting temperature is highly suitable for use in crockery intended for hot beverages such as coffee, tea, drinking chocolate and soon. 
     In an alternative exemplary embodiment of the double-walled crockery the first wall  510  is a wall to be cooled and phase-change material  545  has a melting temperature of between 0° C. and 14° C., preferably between 4° C. and 8° C. Phase-change material  545  with such a melting temperature is highly suitable for use in crockery intended for cold dishes, such as ice creams, or for cold beverages. 
     In a preferred embodiment of the double-walled crockery the pouch  140  comprises a first and a second film, wherein phase-change material  145  is situated between the first and second films. The films of the pouch can comprise different materials just as long as the pouch has the necessary flexibility, elasticity and/or dimensions and the films can keep the phase-change material enclosed. The freedom of movement of the phase-change material in the crockery is thus limited by the pouch. It will be apparent to the skilled person that different materials meet the above stated requirements and that the films can for instance be made of different types of plastic or aluminium. Films moreover have the advantage that they can be laminated in advantageous manner, whereby different material properties can be combined in one film. The films can have one or more of the above described properties. 
       FIG. 6A  illustrates a plate with an upper first part  601  substantially with the shape of a “normal” plate, against which a substantially flat round second lower part  602  is attached. First part  601  comprises first wall  610  and second part  602  comprises second wall  620 . Pouch  640  is arranged, preferably substantially centrally, against an underside of first wall  610  of first part  601 . The upper first part  601  is provided on the underside thereof with a round support edge  611  which makes contact with the surface on which the plate is placed. Provided on the inner side of support edge  611  is a support flange  612  against which a peripheral edge  622  of second part  602  is attached, for instance by glueing. When second part  602  is attached against first part  601 , a space  630  in which a thermally insulating layer, for instance an air layer, is present lies between pouch  640  and second wall  620  of second part  602 ,  FIG. 6B  illustrates a similar article of crockery as that of  FIG. 6A , with the difference that first upper part  601  in  FIG. 6B  has a substantially flat bottom side and is attached to an upper edge  622  of a second lower part  602 . Second part  602  comprises second wall  620  and has a bottom edge  621  on which the plate is supported. 
     In other embodiments the first and the second wall can form part of one integrally formed double-walled piece which is finished with further parts so as to form the article of crockery. Note that the skilled person will appreciate that the variants shown in  FIGS. 1A and 5  are typically also manufactured from two or more parts which are attached to each other so as to form a closed article of crockery in which the pouch is accommodated. 
     The skilled person will appreciate that the invention is not limited to the above described embodiments, and that many modifications and variants are possible within the scope of the invention, which is defined solely by the following claims.