Patent Publication Number: US-10782051-B2

Title: Magneto-caloric thermal diode assembly

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to heat pumps, such as magneto-caloric heat pumps. 
     BACKGROUND OF THE INVENTION 
     Conventional refrigeration technology typically utilizes a heat pump that relies on compression and expansion of a fluid refrigerant to receive and reject heat in a cyclic manner so as to effect a desired temperature change or transfer heat energy from one location to another. This cycle can be used to receive heat from a refrigeration compartment and reject such heat to the environment or a location that is external to the compartment. Other applications include air conditioning of residential or commercial structures. A variety of different fluid refrigerants have been developed that can be used with the heat pump in such systems. 
     While improvements have been made to such heat pump systems that rely on the compression of fluid refrigerant, at best such can still only operate at about forty-five percent or less of the maximum theoretical Carnot cycle efficiency. Also, some fluid refrigerants have been discontinued due to environmental concerns. The range of ambient temperatures over which certain refrigerant-based systems can operate may be impractical for certain locations. Other challenges with heat pumps that use a fluid refrigerant exist as well. 
     Magneto-caloric materials (MCMs), i.e. materials that exhibit the magneto-caloric effect, provide a potential alternative to fluid refrigerants for heat pump applications. In general, the magnetic moments of MCMs become more ordered under an increasing, externally applied magnetic field and cause the MCMs to generate heat. Conversely, decreasing the externally applied magnetic field allows the magnetic moments of the MCMs to become more disordered and allow the MCMs to absorb heat. Some MCMs exhibit the opposite behavior, i.e. generating heat when the magnetic field is removed (which are sometimes referred to as para-magneto-caloric material but both types are referred to collectively herein as magneto-caloric material or MCM). The theoretical Carnot cycle efficiency of a refrigeration cycle based on an MCMs can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. As such, a heat pump system that can effectively use an MCM would be useful. 
     Challenges exist to the practical and cost competitive use of an MCM, however. In addition to the development of suitable MCMs, equipment that can attractively utilize an MCM is still needed. Currently proposed equipment may require relatively large and expensive magnets, may be impractical for use in e.g., appliance refrigeration, and may not otherwise operate with enough efficiency to justify capital cost. 
     Accordingly, a heat pump system that can address certain challenges, such as those identified above, would be useful. Such a heat pump system that can also be used in a refrigerator appliance would also be useful. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
     In a first example embodiment, a magneto-caloric thermal diode assembly includes a magneto-caloric cylinder with a plurality of magneto-caloric stages. Each of the plurality of magneto-caloric stages has a respective Curie temperature. Each of the plurality of magneto-caloric stages also has a stack of magneto-caloric material blocks and metal foil layers distributed sequentially along an axial direction in the order of magneto-caloric material block then metal foil layer. A plurality of thermal stages is stacked along the axial direction between a cold side and a hot side. Each of the plurality of thermal stages includes a plurality of magnets and a non-magnetic ring. The plurality of magnets is distributed along a circumferential direction within the non-magnetic ring in each of the plurality of thermal stages. The plurality of thermal stages and the magneto-caloric cylinder are configured for relative rotation between the plurality of thermal stages and the magneto-caloric cylinder. 
     In a second example embodiment, a magneto-caloric thermal diode assembly includes a magneto-caloric regenerator with a plurality of magneto-caloric stages. Each of the plurality of magneto-caloric stages has a respective Curie temperature. Each of the plurality of magneto-caloric stages also has a stack of magneto-caloric material blocks and metal foil layers distributed sequentially along an axial direction in the order of magneto-caloric material block then metal foil layer. A plurality of thermal stages is stacked along the axial direction between a cold side and a hot side. The plurality of thermal stages and the magneto-caloric regenerator are configured for relative motion between the plurality of thermal stages and the magneto-caloric assembly. The magneto-caloric regenerator further includes a plurality of insulation blocks. The plurality of magneto-caloric stages and the plurality of insulation blocks are distributed sequentially along the axial direction in the order of magneto-caloric stage then insulation block within the magneto-caloric assembly. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures. 
         FIG. 1  is a refrigerator appliance in accordance with an example embodiment of the present disclosure. 
         FIG. 2  is a schematic illustration of certain components of a heat pump system positioned in the example refrigerator appliance of  FIG. 1 . 
         FIG. 3  is a perspective view of a magneto-caloric thermal diode according to an example embodiment of the present subject matter. 
         FIG. 4  is a section view of the example magneto-caloric thermal diode of  FIG. 3 . 
         FIG. 5  is a perspective view of the example magneto-caloric thermal diode of  FIG. 3  with certain thermal stages removed from the example magneto-caloric thermal diode. 
         FIG. 6  is a section view of the example magneto-caloric thermal diode of  FIG. 5 . 
         FIG. 7  is a perspective view of the example magneto-caloric thermal diode of  FIG. 5  with an insulation layer removed from the example magneto-caloric thermal diode. 
         FIG. 8  is a schematic view of the certain components of the example magneto-caloric thermal diode of  FIG. 3 . 
         FIG. 9  is a schematic view of the certain components of a magneto-caloric thermal diode according to another example embodiment of the present subject matter. 
         FIG. 10  is a schematic view of the certain components of a magneto-caloric thermal diode according to an additional example embodiment of the present subject matter. 
         FIG. 11  is an end, elevation view of a magneto-caloric cylinder according to an example embodiment of the present subject matter. 
         FIG. 12  is a side, elevation view of the example magneto-caloric cylinder of  FIG. 11 . 
         FIG. 13  is a side, elevation view of a magneto-caloric stage of the example magneto-caloric cylinder of  FIG. 11 . 
         FIGS. 14 through 16  are schematic views of a method for forming the magneto-caloric stage of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Referring now to  FIG. 1 , an exemplary embodiment of a refrigerator appliance  10  is depicted as an upright refrigerator having a cabinet or casing  12  that defines a number of internal storage compartments or chilled chambers. In particular, refrigerator appliance  10  includes upper fresh-food compartments  14  having doors  16  and lower freezer compartment  18  having upper drawer  20  and lower drawer  22 . Drawers  20 ,  22  are “pull-out” type drawers in that they can be manually moved into and out of freezer compartment  18  on suitable slide mechanisms. Refrigerator  10  is provided by way of example only. Other configurations for a refrigerator appliance may be used as well including appliances with only freezer compartments, only chilled compartments, or other combinations thereof different from that shown in  FIG. 1 . In addition, the magneto-caloric thermal diode and heat pump system of the present disclosure is not limited to refrigerator appliances and may be used in other applications as well such as e.g., air-conditioning, electronics cooling devices, and others. Thus, it should be understood that while the use of a magneto-caloric thermal diode and heat pump system to provide cooling within a refrigerator is provided by way of example herein, the present disclosure may also be used to provide for heating applications as well. 
       FIG. 2  is a schematic view of various components of refrigerator appliance  10 , including refrigeration compartments  30  (e.g., fresh-food compartments  14  and freezer compartment  18 ) and a machinery compartment  40 . Refrigeration compartment  30  and machinery compartment  40  include a heat pump system  52  having a first or cold side heat exchanger  32  positioned in refrigeration compartment  30  for the removal of heat therefrom. A heat transfer fluid such as e.g., an aqueous solution, flowing within cold side heat exchanger  32  receives heat from refrigeration compartment  30  thereby cooling contents of refrigeration compartment  30 . 
     The heat transfer fluid flows out of cold side heat exchanger  32  by line  44  to magneto-caloric thermal diode  100 . As will be further described herein, the heat transfer fluid rejects heat to magneto-caloric material (MCM) in magneto-caloric thermal diode  100 . The now colder heat transfer fluid flows by line  46  to cold side heat exchanger  32  to receive heat from refrigeration compartment  30 . 
     Another heat transfer fluid carries heat from the MCM in magneto-caloric thermal diode  100  by line  48  to second or hot side heat exchanger  34 . Heat is released to the environment, machinery compartment  40 , and/or other location external to refrigeration compartment  30  using second heat exchanger  34 . From second heat exchanger  34 , the heat transfer fluid returns by line  50  to magneto-caloric thermal diode  100 . The above described cycle may be repeated to suitable cool refrigeration compartment  30 . A fan  36  may be used to create a flow of air across second heat exchanger  34  and thereby improve the rate of heat transfer to the environment. 
     A pump or pumps (not shown) cause the heat transfer fluid to recirculate in heat pump system  52 . Motor  28  is in mechanical communication with magneto-caloric thermal diode  100  and is operable to provide relative motion between magnets and a magneto-caloric material of magneto-caloric thermal diode  100 , as discussed in greater detail below. 
     Heat pump system  52  is provided by way of example only. Other configurations of heat pump system  52  may be used as well. For example, lines  44 ,  46 ,  48 , and  50  provide fluid communication between the various components of heat pump system  52  but other heat transfer fluid recirculation loops with different lines and connections may also be employed. Still other configurations of heat pump system  52  may be used as well. 
     In certain exemplary embodiments, cold side heat exchanger  32  is the only heat exchanger within heat pump system  52  that is configured to cool refrigeration compartments  30 . Thus, cold side heat exchanger  32  may be the only heat exchanger within cabinet  12  for cooling fresh-food compartments  14  and freezer compartment  18 . Refrigerator appliance  10  also includes features for regulating air flow across cold side heat exchanger  32  and to fresh-food compartments  14  and freezer compartment  18 . 
     As may be seen in  FIG. 2 , cold side heat exchanger  32  is positioned within a heat exchanger compartment  60  that is defined within cabinet  12 , e.g., between fresh-food compartments  14  and freezer compartment  18 . Fresh-food compartment  14  is contiguous with heat exchanger compartment  60  through a fresh food duct  62 . Thus, air may flow between fresh-food compartment  14  and heat exchanger compartment  60  via fresh food duct  62 . Freezer compartment  18  is contiguous with heat exchanger compartment  60  through a freezer duct  64 . Thus, air may flow between freezer compartment  18  and heat exchanger compartment  60  via freezer duct  64 . 
     Refrigerator appliance  10  also includes a fresh food fan  66  and a freezer fan  68 . Fresh food fan  66  may be positioned at or within fresh food duct  62 . Fresh food fan  66  is operable to force air flow between fresh-food compartment  14  and heat exchanger compartment  60  through fresh food duct  62 . Fresh food fan  66  may thus be used to create a flow of air across cold side heat exchanger  32  and thereby improve the rate of heat transfer to air within fresh food duct  62 . Freezer fan  68  may be positioned at or within freezer duct  64 . Freezer fan  68  is operable to force air flow between freezer compartment  18  and heat exchanger compartment  60  through freezer duct  64 . Freezer fan  68  may thus be used to create a flow of air across cold side heat exchanger  32  and thereby improve the rate of heat transfer to air within freezer duct  64 . 
     Refrigerator appliance  10  may also include a fresh food damper  70  and a freezer damper  72 . Fresh food damper  70  is positioned at or within fresh food duct  62  and is operable to restrict air flow through fresh food duct  62 . For example, when fresh food damper  70  is closed, fresh food damper  70  blocks air flow through fresh food duct  62 , e.g., and thus between fresh-food compartment  14  and heat exchanger compartment  60 . Freezer damper  72  is positioned at or within freezer duct  64  and is operable to restrict air flow through freezer duct  64 . For example, when freezer damper  72  is closed, freezer damper  72  blocks air flow through freezer duct  64 , e.g., and thus between freezer compartment  18  and heat exchanger compartment  60 . It will be understood that the positions of fans  66 ,  68  and dampers  70 ,  72  may be switched in alternative exemplary embodiments. 
     Operation of heat pump system  52  and fresh food fan  66  while fresh food damper  70  is open, allows chilled air from cold side heat exchanger  32  to cool fresh-food compartment  14 , e.g., to about forty degrees Fahrenheit (40° F.). Similarly, operation of heat pump system  52  and freezer fan  68  while freezer damper  72  is open, allows chilled air from cold side heat exchanger  32  to cool freezer compartment  18 , e.g., to about negative ten degrees Fahrenheit (−10° F.). Thus, cold side heat exchanger  32  may chill either fresh-food compartment  14  or freezer compartment  18  during operation of heat pump system  52 . In such a manner, both fresh-food compartments  14  and freezer compartment  18  may be air cooled with cold side heat exchanger  32 . 
       FIGS. 3 through 8  are various views of magneto-caloric thermal diode  200  according to an example embodiment of the present subject matter. Magneto-caloric thermal diode  200  may be used in any suitable heat pump system. For example, magneto-caloric thermal diode  200  may be used in heat pump system  52  ( FIG. 2 ). As discussed in greater detail below, magneto-caloric thermal diode  200  includes features for transferring thermal energy from a cold side  202  of magneto-caloric thermal diode  200  to a hot side  204  of magneto-caloric thermal diode  200 . Magneto-caloric thermal diode  200  defines an axial direction A, a radial direction R and a circumferential direction C. 
     Magneto-caloric thermal diode  200  includes a plurality of thermal stages  210 . Thermal stages  210  are stacked along the axial direction A between cold side  202  and hot side  204  of magneto-caloric thermal diode  200 . A cold side thermal stage  212  of thermal stages  210  is positioned at cold side  202  of magneto-caloric thermal diode  200 , and a hot side thermal stage  214  of thermal stages  210  is positioned at hot side  204  of magneto-caloric thermal diode  200 . 
     Magneto-caloric thermal diode  200  also includes a magneto-caloric cylinder  220  ( FIG. 8 ). In certain example embodiments, thermal stages  210  define a cylindrical slot  211 , and magneto-caloric cylinder  220  is positioned within cylindrical slot  211 . Thus, e.g., each thermal stage  210  may include an inner section  206  and an outer section  208  that are spaced from each other along the radial direction R by cylindrical slot  211  such that magneto-caloric cylinder  220  is positioned between inner and outer sections  206 ,  208  of thermal stages  210  along the radial direction R. Thermal stages  210  and magneto-caloric cylinder  220  are configured for relative rotation between thermal stages  210  and magneto-caloric cylinder  220 . Thermal stages  210  and magneto-caloric cylinder  220  may be configured for relative rotation about an axis X that is parallel to the axial direction A. As an example, magneto-caloric cylinder  220  may be coupled to motor  26  such that magneto-caloric cylinder  220  is rotatable relative to thermal stages  210  about the axis X within cylindrical slot  211  with motor  26 . In alternative exemplary embodiments, thermal stages  210  may be coupled to motor  26  such that thermal stages  210  are rotatable relative to magneto-caloric cylinder  220  about the axis X with motor  26 . 
     During relative rotation between thermal stages  210  and magneto-caloric cylinder  220 , magneto-caloric thermal diode  200  transfers heat from cold side  202  to hot side  204  of magneto-caloric thermal diode  200 . In particular, during relative rotation between thermal stages  210  and magneto-caloric cylinder  220 , cold side thermal stage  212  may absorb heat from fresh-food compartments  14  and/or freezer compartment  18 , and hot side thermal stage  214  may reject heat to the ambient atmosphere about refrigerator appliance  10 . 
     Each of the thermal stages  210  includes a plurality of magnets  230  and a non-magnetic ring  240 . Magnets  230  are distributed along the circumferential direction C within non-magnetic ring  240  in each thermal stage  210 . In particular, magnets  230  may be spaced from non-magnetic ring  240  along the radial direction R and the circumferential direction C within each thermal stage  210 . For example, each of the thermal stages  210  may include insulation  232 , and insulation  232  may be positioned between magnets  230  and non-magnetic ring  240  along the radial direction R and the circumferential direction C within each thermal stage  210 . Insulation  232  may limit conductive heat transfer between magnets  230  and non-magnetic ring  240  within each thermal stage  210 . As another example, magnets  230  may be spaced from non-magnetic ring  240  along the radial direction R and the circumferential direction C by a gap within each thermal stage  210 . The gap between magnets  230  and non-magnetic ring  240  within each thermal stage  210  may limit or prevent conductive heat transfer between magnets  230  and non-magnetic ring  240  within each thermal stage  210 . 
     It will be understood that the arrangement magnets  230  and non-magnetic ring  240  may be flipped in alternative example embodiments. Thus, e.g., a steel and magnet ring may be thermally separate from non-magnetic blocks, e.g., aluminum blocks, within each thermal stage  210 . Operation magneto-caloric thermal diode  200  is the same in such configuration. 
     As may be seen from the above, thermal stages  210  may include features for limiting heat transfer along the radial direction R and the circumferential direction C within each thermal stage  210 . Conversely, thermal stages  210  may be arranged to provide a flow path for thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  200 . Such arrangement of thermal stages  210  is discussed in greater detail below. 
     As noted above, thermal stages  210  includes cold side thermal stage  212  at cold side  202  of magneto-caloric thermal diode  200  and hot side thermal stage  214  at hot side  204  of magneto-caloric thermal diode  200 . Thus, cold side thermal stage  212  and hot side thermal stage  214  may correspond to the terminal ends of the stack of thermal stages  210 . In particular, cold side thermal stage  212  and hot side thermal stage  214  may be positioned opposite each other along the axial direction A on the stack of thermal stages  210 . The other thermal stages  210  are positioned between cold side thermal stage  212  and hot side thermal stage  214  along the axial direction A. Thus, e.g., interior thermal stages  216  (i.e., the thermal stages  210  other than cold side thermal stage  212  and hot side thermal stage  214 ) are positioned between cold side thermal stage  212  and hot side thermal stage  214  along the axial direction A. 
     Each of the interior thermal stages  216  is positioned between a respective pair of thermal stages  210  along the axial direction A. One of the respective pair of thermal stages  210  is positioned closer to cold side  202  along the axial direction A, and the other of the respective pair of thermal stages  210  is positioned closer to hot side  204  along the axial direction A. For example, a first one  217  of interior thermal stages  216  is positioned between hot side thermal stage  214  and a second one  218  of interior thermal stages  216  along the axial direction A. Similarly, second one  218  of interior thermal stages  216  is positioned between first one  217  of interior thermal stages  216  and a third one  219  of interior thermal stages  216  along the axial direction A. 
     Each of the interior thermal stages  216  is arranged to provide a flow path for thermal energy along the axial direction A from cold side thermal stage  212  to hot side thermal stage  214 . In particular, magnets  230  of each of interior thermal stages  216  may be spaced from non-magnetic ring  240  of the one of the respective pair of thermal stages  210  along the axial direction A. Thus, e.g., magnets  230  of first one  217  of interior thermal stages  216  may be spaced from non-magnetic ring  240  of second one  218  of interior thermal stages  216  along the axial direction A. Similarly, magnets  230  of second one  218  of interior thermal stages  216  may be spaced from non-magnetic ring  240  of third one  219  of interior thermal stages  216  along the axial direction A. Hot side thermal stage  214  may also be arranged in such a manner. 
     By spacing magnets  230  of each of interior thermal stages  216  from non-magnetic ring  240  of the one of the respective pair of thermal stages  210  along the axial direction A, conductive heat transfer along the axial direction A from magnets  230  of each of interior thermal stages  216  to non-magnetic ring  240  of an adjacent one of thermal stages  210  towards cold side  202  along the axial direction A may be limited or prevented. In certain example embodiments, magneto-caloric thermal diode  200  may include insulation  250 . Magnets  230  of each of interior thermal stages  216  may be spaced from non-magnetic ring  240  of the one of the respective pair of thermal stages  210  along the axial direction A by insulation  250 . Insulation  250  may limit conductive heat transfer along the axial direction A from magnets  230  of each of interior thermal stages  216  to non-magnetic ring  240  of an adjacent one of thermal stages  210  towards cold side  202  along the axial direction A. 
     Magnets  230  of each of interior thermal stages  216  may also be in conductive thermal contact with non-magnetic ring  240  of the other of the respective pair of thermal stages  210 . Thus, e.g., magnets  230  of first one  217  of interior thermal stages  216  may be in conductive thermal contact with non-magnetic ring  240  of hot side thermal stage  214 . Similarly, magnets  230  of second one  218  of interior thermal stages  216  may be in conductive thermal contact with non-magnetic ring  240  of first one  217  of interior thermal stages  216 . Cold side thermal stage  212  may also be arranged in such a manner. 
     By placing magnets  230  of each of interior thermal stages  216  in conductive thermal contact with non-magnetic ring  240  of the other of the respective pair of thermal stages  210 , thermal energy flow along the axial direction A towards hot side  204  may be facilitated, e.g., relative to towards cold side  202 . In certain example embodiments, magnets  230  of each of interior thermal stages  216  may be positioned to directly contact non-magnetic ring  240  of the other of the respective pair of thermal stages  210 . For example, non-magnetic ring  240  of the other of the respective pair of thermal stages  210  may include projections  242  that extend along the axial direction A to magnets  230  of each of interior thermal stages  216 . 
     The above described arrangement of thermal stages  210  may provide a flow path for thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  200  during relative rotation between thermal stages  210  and magneto-caloric cylinder  220 . Operation of magneto-caloric thermal diode  200  to transfer thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  200  will now be described in greater detail below. 
     Magnets  230  of thermal stages  210  produce a magnetic field. Conversely, non-magnetic rings  240  do not produce a magnetic field or produce a negligible magnetic field relative to magnets  230 . Thus, each of the magnets  230  may correspond to a high magnetic field zone, and the portion of non-magnetic rings  240  between magnets  230  along the circumferential direction C within each thermal stage  210  may correspond to a low magnetic field zone. During relative rotation between thermal stages  210  and magneto-caloric cylinder  220 , magneto-caloric cylinder  220  may be sequentially exposed to the high magnetic field zone at magnets  230  and the low magnetic field zone at non-magnetic rings  240 . 
     Magneto-caloric cylinder  220  includes a magneto-caloric material that exhibits the magneto-caloric effect, e.g., when exposed to the magnetic field from magnets  230  of thermal stages  210 . The caloric material may be constructed from a single magneto-caloric material or may include multiple different magneto-caloric materials. By way of example, refrigerator appliance  10  may be used in an application where the ambient temperature changes over a substantial range. However, a specific magneto-caloric material may exhibit the magneto-caloric effect over only a much narrower temperature range. As such, it may be desirable to use a variety of magneto-caloric materials within magneto-caloric cylinder  220  to accommodate the wide range of ambient temperatures over which refrigerator appliance  10  and/or magneto-caloric thermal diode  200  may be used. 
     Accordingly, magneto-caloric cylinder  220  can be provided with zones of different magneto-caloric materials. Each such zone may include a magneto-caloric material that exhibits the magneto-caloric effect at a different temperature or a different temperature range than an adjacent zone along the axial direction A of magneto-caloric cylinder  220 . By configuring the appropriate number sequence of zones of magneto-caloric material, magneto-caloric thermal diode  200  can be operated over a substantial range of ambient temperatures. 
     As noted above, magneto-caloric cylinder  220  includes magneto-caloric material that exhibits the magneto-caloric effect. During relative rotation between thermal stages  210  and magneto-caloric cylinder  220 , the magneto-caloric material in magneto-caloric cylinder  220  is sequentially exposed to the high magnetic field zone at magnets  230  and the low magnetic field zone at non-magnetic rings  240 . When the magneto-caloric material in magneto-caloric cylinder  220  is exposed to the high magnetic field zone at magnets  230 , the magnetic field causes the magnetic moments of the magneto-caloric material in magneto-caloric cylinder  220  to orient and to increase (or alternatively decrease) in temperature such that the magneto-caloric material in magneto-caloric cylinder  220  rejects heat to magnets  230 . Conversely, when the magneto-caloric material in magneto-caloric cylinder  220  is exposed to the low magnetic field zone at non-magnetic rings  240 , the decreased magnetic field causes the magnetic moments of the magneto-caloric material in magneto-caloric cylinder  220  to disorient and to decrease (or alternatively increase) in temperature such that the magneto-caloric material in magneto-caloric cylinder  220  absorbs heat from non-magnetic rings  240 . By rotating through the high and low magnetic field zones, magneto-caloric cylinder  220  may transfer thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  200  by utilizing the magneto-caloric effect of the magneto-caloric material in magneto-caloric cylinder  220 . 
     As noted above, the high magnetic field zones at magnets  230  in each of thermal stages  210  (e.g., other than hot side thermal stage  214 ) is in conductive thermal contact with the low magnetic field zone at the non-magnetic ring  240  of an adjacent thermal stages  210  in the direction of hot side  204  along the axial direction A. Thus, the non-magnetic ring  240  of the adjacent thermal stages  210  in the direction of hot side  204  may absorb heat from the high magnetic field zones at magnets  230  in each of thermal stages  210 . Thus, thermal stages  210  are arranged to encourage thermal energy flow through thermal stages  210  from cold side  202  towards hot side  204  along the axial direction A during relative rotation between thermal stages  210  and magneto-caloric cylinder  220 . 
     Conversely, the high magnetic field zones at magnets  230  in each of thermal stages  210  (e.g., other than cold side thermal stage  212 ) is spaced from the low magnetic field zone at the non-magnetic ring  240  of an adjacent thermal stages  210  in the direction of cold side  202  along the axial direction A. Thus, the non-magnetic ring  240  of the adjacent thermal stages  210  in the direction of cold side  202  is thermally isolated from the high magnetic field zones at magnets  230  in each of thermal stages  210 . Thus, thermal stages  210  are arranged to discourage thermal energy flow through thermal stages  210  from hot side  204  towards cold side  202  along the axial direction A during relative rotation between thermal stages  210  and magneto-caloric cylinder  220 . 
     Magneto-caloric thermal diode  200  may include a suitable number of thermal stages  210 . For example, thermal stages  210  may include nine thermal stages as shown in  FIGS. 3 and 4 . In alternative example embodiments, thermal stages  210  may include no less than seven thermal stages. Such number of thermal stages  210  may advantageously permit magneto-caloric cylinder  220  to include a corresponding number of zones with different magneto-caloric materials and thereby allow magneto-caloric thermal diode  200  to operate over a wide range of ambient temperatures as discussed above. Magneto-caloric thermal diode  200  may have an odd number of thermal stages  210 . 
     Each of magnets  230  in thermal stages  210  may be formed as a magnet pair  236 . One of magnet pair  236  may be mounted to or positioned at inner section  206  of each thermal stage  210 , and the other of magnet pair  236  may be mounted to or positioned at outer section  208  of each thermal stage  210 . Thus, magneto-caloric cylinder  220  may be positioned between the magnets of magnet pair  236  along the radial direction Rat cylindrical slot  211 . A positive pole of one of magnet pair  236  and a negative pole of other of magnet pair  236  may face magneto-caloric cylinder  220  along the radial direction R at cylindrical slot  211 . 
     Cylindrical slot  211  may be suitably sized relative to magneto-caloric cylinder  220  to facilitate efficient heat transfer between thermal stages  210  and magneto-caloric cylinder  220 . For example, cylindrical slot  211  may have a width W along the radial direction R, and magneto-caloric cylinder  220  may having a thickness T along the radial direction R within cylindrical slot  211 . The width W of cylindrical slot  211  may no more than five hundredths of an inch (0.05″) greater than the thickness T of magneto-caloric cylinder  220  in certain example embodiments. For example, the width W of cylindrical slot  211  may about one hundredth of an inch (0.01″) greater than the thickness T of magneto-caloric cylinder  220  in certain example embodiments. As used herein, the term “about” means within five thousandths of an inch (0.005″) when used in the context of radial thicknesses and widths. Such sizing of cylindrical slot  211  relative to magneto-caloric cylinder  220  can facilitate efficient heat transfer between thermal stages  210  and magneto-caloric cylinder  220 . 
     Each thermal stage  210  may include a suitable number of magnets  230 . For example, each thermal stage  210  may include no less than ten (10) magnets  230  in certain example embodiments. With such a number of magnets  230 , may advantageously improve performance of magneto-caloric thermal diode  200 , e.g., by driving a larger temperature difference between cold side  202  and hot side  204  relative to a smaller number of magnets  230 . 
     Magnets  230  may also be uniformly spaced apart along the circumferential direction C within the non-magnetic ring  240  in each of thermal stages  210 . Further, each of thermal stages  210  may be positioned at a common orientation with every other one of thermal stages  210  within the stack of thermal stages  210 . Thus, e.g., first one  217  of interior thermal stages  216  may be positioned at a common orientation with third one  219  of interior thermal stages  216 , and hot side thermal stage  214  may be positioned at a common orientation with second one  218  of interior thermal stages  216 . As may be seen from the above, the common orientation may sequentially skip one thermal stage  214  with the stack of thermal stages  210 . Between adjacent thermal stages  210  within the stack of thermal stages  210 , each magnet  230  of thermal stages  210  may be positioned equidistance along the circumferential direction C from a respective pair of magnets  230  in adjacent thermal stages  210 . 
     The non-magnetic rings  240  of thermal stage  210  may be constructed of or with a suitable non-magnetic material. For example, the non-magnetic rings  240  of thermal stage  210  may be constructed of or with aluminum in certain example embodiments. In alternative example embodiments, the non-magnetic rings  240  of thermal stage  210  may be constructed of or with brass, bronze, etc. 
     Magneto-caloric thermal diode  200  may also include one or more heat exchangers  260 . In  FIG. 3 , heat exchanger  260  is shown positioned at the cold side  202  such that heat exchanger  260  absorbs heat from cold side thermal stage  212 . A heat transfer fluid may flow between heat exchanger  260  and cold side heat exchanger  32  via lines  44 ,  46  as discussed above. Another heat exchanger may be positioned hot side  204  such that a heat transfer fluid may flow between the heat exchanger and hot side heat exchanger  34  via lines  48 ,  50  as discussed above. The heat exchangers (including heat exchanger  260 ) may be solid-liquid heat exchangers with a port for heat transfer fluid. Alternatively, the heat exchangers could be direct to solid-gas heat exchangers. 
       FIG. 9  is a schematic view of the certain components of a magneto-caloric thermal diode  300  according to another example embodiment of the present subject matter. As shown in  FIG. 9 , magneto-caloric thermal diode  300  includes numerous common components with magneto-caloric thermal diode  200 . Thus, the description of magneto-caloric thermal diode  200  provided above is applicable to magneto-caloric thermal diode  300  except as otherwise noted. In addition, magneto-caloric thermal diode  200  may include one or more of the aspects of magneto-caloric thermal diode  300  discussed below. 
     In magneto-caloric thermal diode  300 , magneto-caloric cylinder  220  includes a plurality of magneto-caloric stages  222 . Magneto-caloric stages  222  are distributed along the axial direction A within magneto-caloric cylinder  220 . Each of magneto-caloric stages  222  may have a different magneto-caloric material. For example, the respective magneto-caloric material within each of magneto-caloric stages  222  may be selected such that the Currie temperature of the magneto-caloric materials decreases from hot side  204  to cold side  202  along the axial direction A. In such a manner, a cascade of magneto-caloric materials may be formed within magneto-caloric cylinder  220  along the axial direction A. 
     Each of magneto-caloric stages  222  may also have a respective length along the axial direction A. In particular, a length LM 1  of a first one  224  of magneto-caloric stages  222  may be different than the length LM 2  of a second one  226  of magneto-caloric stages  222 . It will be understood that each magneto-caloric stage  222  may have a different length in the manner described above for first one  224  and second one  226  of magneto-caloric stages  222  in certain example embodiments. However, in alternative example embodiments, one or more of magneto-caloric stages  222  may have a common length with first one  224  or second one  226  of magneto-caloric stages  222 . 
     Each of thermal stages  210  also having a respective length along the axial direction A. The length of each of thermal stages  210  corresponds to a respective one of magneto-caloric stages  222 . Thus, each of thermal stages  210  may be sized to match the respective one of magneto-caloric stages  222  along the axial direction A. The respective one of magneto-caloric stages  222  is disposed with each thermal stage  210 . 
     The length of each of magneto-caloric stages  222  along the axial direction A may be selected to assist with matching heat transfer power, e.g., such that each of magneto-caloric stages  222  accepts heat to one adjacent magneto-caloric stage  222  and rejects heat to the other adjacent magneto-caloric stage  222  along the axial direction A. Within each magneto-caloric stage  222 , the rejected heat may be slightly more than the accepted heat based on stage efficiency, and the length of each of magneto-caloric stages  222  along the axial direction A may be selected to complement the efficiency of each magneto-caloric stage  222 . 
     As an example, the length of each of magneto-caloric stages  222  may correspond to a respective Curie temperature spacing between adjacent magneto-caloric stages  222 . In particular, the Curie temperature spacing for the first one  224  of magneto-caloric stages  222  may be greater than the Curie temperature spacing for the second one  226  of magneto-caloric stages  222 . Thus, the length LM 1  of first one  224  of magneto-caloric stages  222  may be greater than the length of LM 2  of second one  226  of magneto-caloric stages  222 , e.g., in proportion to the difference between the Curie temperature spacing. As may be seen from the above, magneto-caloric stages  222  with larger Curie temperature spacing between adjacent magneto-caloric stages  222  may advantageously have an increased length along the axial direction A relative to magneto-caloric stages  222  with smaller Curie temperature spacing between adjacent magneto-caloric stages  222 . 
     As another example, the length of each of magneto-caloric stages  222  may correspond to an adiabatic temperature change (i.e., the strength) of the magneto-caloric stage  222 . In particular, the adiabatic temperature change of the first one  224  of magneto-caloric stages  222  may be less than the adiabatic temperature change of the second one  226  of magneto-caloric stages  222 . Thus, the length LM 1  of first one  224  of magneto-caloric stages  222  may be greater than the length of LM 2  of second one  226  of magneto-caloric stages  222 , e.g., in proportion to the difference between the adiabatic temperature changes. As may be seen from the above, weaker magneto-caloric stages  222  may advantageously have an increased length along the axial direction A relative to stronger magneto-caloric stages  222 . 
     As an additional example, the length of hot side thermal stage  214  along the axial direction A may be greater than the length of cold side thermal stage  212  along the axial direction A. Thus, magneto-caloric stages  222  at or adjacent hot side  204  may be longer along the axial direction A relative to magneto-caloric stages  222  at or adjacent cold side  202 . In such a manner, magneto-caloric thermal diode  300  may advantageously configured to account for losses in magneto-caloric stages  222 , e.g., where rejected heat is greater than accepted heat. 
     Magneto-caloric thermal diode  300  also includes multiple magneto-caloric cylinders  220  and multiple stacks of thermal stages  210  nested concentrically within each other. In particular, magneto-caloric thermal diode  300  includes a first magneto-caloric cylinder  310  and a second magneto-caloric cylinder  312 . Second magneto-caloric cylinder  312  is positioned within first magneto-caloric cylinder  310  along the radial direction R. Magneto-caloric thermal diode  300  also includes a first plurality of thermal stages  320  and a second plurality of thermal stages  322 . First thermal stages  320  are stacked along the axial direction A between cold side  202  and hot side  204 . Second thermal stages  322  are also stacked along the axial direction A between cold side  202  and hot side  204 . First thermal stages  320  are positioned within second thermal stages  322  along the radial direction R. 
     First and second thermal stages  320 ,  322  and first and second magneto-caloric cylinders  310 ,  312  are configured for relative rotation between first and second thermal stages  320 ,  322  and first and second magneto-caloric cylinders  310 ,  312 . First and second thermal stages  320 ,  322  and first and second magneto-caloric cylinders  310 ,  312  may be configured for relative rotation about the axis X that is parallel to the axial direction A. As an example, first and second magneto-caloric cylinders  310 ,  312  may be coupled to motor  26  such that first and second magneto-caloric cylinders  310 ,  312  are rotatable relative to first and second thermal stages  320 ,  322  about the axis X with motor  26 . In alternative exemplary embodiments, first and second thermal stages  320 ,  322  may be coupled to motor  26  such that first and second thermal stages  320 ,  322  are rotatable relative to first and second magneto-caloric cylinders  310 ,  312  about the axis X with motor  26 . 
     First thermal stages  320  define a first cylindrical slot  324 , and first magneto-caloric cylinder  310  is received within first cylindrical slot  324 . Second thermal stages  322  define a second cylindrical slot  326 , and second magneto-caloric cylinder  312  is received within second cylindrical slot  326 . Second cylindrical slot  326  is positioned inward of first cylindrical slot  324  along the radial direction R. 
     First magneto-caloric cylinder  310  and first thermal stages  320  operate in the manner described above for thermal stages  210  and magneto-caloric cylinder  220  to transfer thermal energy along the axial direction A from cold side  202  to hot side  204 . Similarly, second magneto-caloric cylinder  312  and second thermal stages  322  also operate in the manner described above for thermal stages  210  and magneto-caloric cylinder  220  to transfer thermal energy along the axial direction A from cold side  202  to hot side  204 . 
     Second magneto-caloric cylinder  312  and second thermal stages  322  are nested concentrically within first magneto-caloric cylinder  310  and first thermal stages  320 . In such a manner, magneto-caloric thermal diode  300  may include components for operating as multiple magneto-caloric thermal diodes  200  nested concentrically. First and second magneto-caloric cylinders  310 ,  312  may have identical cascades of magneto-caloric materials along the axial direction A. Thus, e.g., first and second magneto-caloric cylinders  310 ,  312  may have identical magneto-caloric materials along the radial direction R. By nesting second thermal stage  322  concentrically within first thermal stage  320 , a total cooling power of magneto-caloric thermal diode  300  may be increased relative to non-nested magneto-caloric thermal diodes. 
     First and second thermal stages  320 ,  322  may be arranged to provide a flow path for thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  300  in the manner described above for magneto-caloric thermal diode  200 . For example, each of first thermal stages  320  includes magnets  230  and non-magnetic ring  240 , and each of second thermal stages  322  includes magnets  230  and non-magnetic ring  240 . Magnets  230  and non-magnetic ring  240  may be arranged within first thermal stages  320  in the manner described above for magnets  230  and non-magnetic ring  240  of magneto-caloric thermal diode  200 . Magnets  230  and non-magnetic ring  240  may also be arranged within second thermal stages  322  in the manner described above for magnets  230  and non-magnetic ring  240  of magneto-caloric thermal diode  200 . 
     Each non-magnetic ring  240  within first thermal stages  320  may be in conductive thermal contact with a respective non-magnetic ring  240  within second thermal stages  322  along the radial direction R. For example, each non-magnetic ring  240  within first thermal stages  320  may be integral (e.g., at least partially formed from a single piece of material) with the respective non-magnetic ring  240  within second thermal stages  322  along the radial direction R. By placing each non-magnetic ring  240  within first thermal stages  320  in conductive thermal contact with the respective non-magnetic ring  240  within second thermal stages  322 , thermal energy flow along the radial direction R between first and second thermal stages  320 ,  322 . 
       FIG. 10  is a schematic view of the certain components of a magneto-caloric thermal diode  400  according to an additional example embodiment of the present subject matter. As shown in  FIG. 10 , magneto-caloric thermal diode  400  includes numerous common components with magneto-caloric thermal diodes  200 ,  300 . Thus, the description of magneto-caloric thermal diodes  200 ,  300  provided above is applicable to magneto-caloric thermal diode  400  except as otherwise noted. In addition, magneto-caloric thermal diodes  200 ,  300  may include one or more of the aspects of magneto-caloric thermal diode  400  discussed below. 
     Like magneto-caloric thermal diode  300 , magneto-caloric thermal diode  400  includes multiple magneto-caloric cylinders  220  and multiple stacks of thermal stages  210  nested concentrically within each other. In particular, magneto-caloric thermal diode  400  includes a first magneto-caloric cylinder  410  and a second magneto-caloric cylinder  412 . Second magneto-caloric cylinder  412  is positioned within first magneto-caloric cylinder  410  along the radial direction R. Magneto-caloric thermal diode  400  also includes a first plurality of thermal stages  420  and a second plurality of thermal stages  422 . First thermal stages  420  are stacked along the axial direction A between cold side  202  and hot side  204 . Second thermal stages  422  are also stacked along the axial direction A between cold side  202  and hot side  204 . First thermal stages  420  are positioned within second thermal stages  422  along the radial direction R. First and second thermal stages  420 ,  422  and first and second magneto-caloric cylinders  410 ,  412  are configured for relative rotation between first and second thermal stages  420 ,  422  and first and second magneto-caloric cylinders  410 ,  412 . 
     Second magneto-caloric cylinder  412  and second thermal stages  422  are nested concentrically within first magneto-caloric cylinder  410  and first thermal stages  420 . In such a manner, magneto-caloric thermal diode  400  may include components for operating as multiple magneto-caloric thermal diodes  200  nested concentrically. First and second magneto-caloric cylinders  410 ,  412  may have different cascades of magneto-caloric materials along the axial direction A. Thus, e.g., first and second magneto-caloric cylinders  410 ,  412  may have different magneto-caloric materials along the radial direction R. By nesting second thermal stage  422  concentrically within first thermal stage  420 , a total temperature span of magneto-caloric thermal diode  400  relative to non-nested magneto-caloric thermal diodes. 
     First and second thermal stages  420 ,  422  may be arranged to provide a flow path for thermal energy along the axial direction A from cold side  202  to hot side  204  of magneto-caloric thermal diode  400  in the manner described above for magneto-caloric thermal diode  200 . For example, each of first thermal stages  420  includes magnets  230  and non-magnetic ring  240 , and each of second thermal stages  422  includes magnets  230  and non-magnetic ring  240 . Magnets  230  and non-magnetic ring  240  may be arranged within first thermal stages  420  in the manner described above for magnets  230  and non-magnetic ring  240  of magneto-caloric thermal diode  200 . Magnets  230  and non-magnetic ring  240  may also be arranged within second thermal stages  422  in a similar manner to that described above for magnets  230  and non-magnetic ring  240  of magneto-caloric thermal diode  200  except that the arrangement of second thermal stage  422  may be reversed along the axial direction A. 
     In addition, the non-magnetic ring  240  in the one of first thermal stages  420  at cold side  202  may be in conductive thermal contact with the non-magnetic ring  240  in the one of second thermal stages  422  at cold side  202  along the radial direction R. For example, the non-magnetic ring  240  in the one of first thermal stages  420  at cold side  202  may be integral (e.g., at least partially formed from a single piece of material) with the one of second thermal stages  422  at cold side  202  along the radial direction R. By placing the non-magnetic ring  240  in the one of first thermal stages  420  at cold side  202  in conductive thermal contact with the one of second thermal stages  422  at cold side  202 , thermal energy flow along the radial direction R between first and second thermal stages  420 ,  422  at cold side  202 . 
     Other than at cold side  202 , each non-magnetic ring  240  in first thermal stages  420  may be spaced from a respective non-magnetic ring  240  in second thermal stages  422  along the radial direction R. For example, other than at cold side  202 , each non-magnetic ring  240  in first thermal stages  420  may be spaced from the respective non-magnetic ring  240  in second thermal stages  422  along the radial direction R by insulation  430 . By spacing each non-magnetic ring  240  in first thermal stages  420  from the respective non-magnetic ring  240  in second thermal stages  422  other than at cold side  202 , thermal energy flow along the radial direction R between first and second thermal stages  420 ,  422  may be limited. 
       FIG. 11  is an end, elevation view of a magneto-caloric cylinder  500  according to an example embodiment of the present subject matter.  FIG. 12  is a side, elevation view of magneto-caloric cylinder  500 . Magneto-caloric cylinder  500  may be used in any suitable magneto-caloric heat pump. For example, magneto-caloric cylinder  500  may be used in magneto-caloric thermal diode  200  as magneto-caloric cylinder  220 . As discussed in greater detail below, magneto-caloric cylinder  500  includes features for anisotropic thermal conductance. 
     As shown in  FIG. 12 , magneto-caloric cylinder  500  includes a plurality of magneto-caloric stages  510 . Magneto-caloric stages  510  may be annular in certain example embodiments. Each of magneto-caloric stages  510  has a respective Curie temperature. Thus, e.g., each of magneto-caloric stages  510  may have a different magneto-caloric material. In particular, the respective magneto-caloric material within each of magneto-caloric stages  510  may be selected such that the Currie temperature of the magneto-caloric materials changes along the axial direction A. In such a manner, a cascade of magneto-caloric materials may be formed within magneto-caloric cylinder  500  along the axial direction A. 
     Accordingly, magneto-caloric cylinder  500  can be provided with magneto-caloric stages  510  of different magneto-caloric materials. Each magneto-caloric stage  510  may include a magneto-caloric material that exhibits the magneto-caloric effect at a different temperature or a different temperature range than an adjacent magneto-caloric stage  510  along the axial direction A. By configuring the appropriate number and/or sequence of magneto-caloric stages  510 , an associated magneto-caloric thermal diode can be operated over a substantial range of ambient temperatures. 
     Magneto-caloric cylinder  500  also includes a plurality of insulation blocks  520 . Magneto-caloric stages  510  and insulation blocks  520  may be stacked and interspersed with one another along the axial direction A within magneto-caloric cylinder  500 . In particular, magneto-caloric stages  510  and insulation blocks  520  may be distributed sequentially along the axial direction A in the order of magneto-caloric stage  510  then insulation block  520  within magneto-caloric cylinder  500 . Thus, e.g., each magneto-caloric stage  510  may be positioned between a respective pair of insulation blocks  520  along the axial direction A within magneto-caloric cylinder  500 . 
     Insulation blocks  520  may limit conductive heat transfer along the axial direction A between magneto-caloric stages  510 . In particular, insulation blocks  520  may limit conductive heat transfer along the axial direction A between magneto-caloric stages  510  with different Currie temperatures. Insulation blocks  520  may be constructed of a suitable insulator, such as a plastic. Insulation blocks  520  may be annular in certain example embodiments. Thus, e.g., each insulation block  520  may be a plastic ring. 
       FIG. 13  is a side, elevation view of one of magneto-caloric stages  510 . Although only one of magneto-caloric stages  510  is shown in  FIG. 13 , the other magneto-caloric stages  510  in magneto-caloric cylinder  500  may be constructed in the same or similar manner to that shown in  FIG. 13 . As discussed in greater detail below, magneto-caloric stage  510  may be constructed such that conductive heat transfer along the radial direction R is greater than conductive heat transfer along the axial direction A. Thus, magneto-caloric stage  510  may be constructed such that the thermal conductance of magneto-caloric stage  510  is greater along the radial direction R relative to the thermal conductance of magneto-caloric stage  510  along the axial direction A. 
     In  FIG. 13 , magneto-caloric stage  510  includes a plurality of magneto-caloric material blocks  530  and a plurality of metal foil layers  540 . Magneto-caloric material blocks  530  and metal foil layers  540  are stacked and interspersed with one another along the axial direction A in magneto-caloric stage  510 . In particular, magneto-caloric material blocks  530  and metal foil layers  540  may be distributed sequentially along the axial direction A in the order of magneto-caloric material block  530  then metal foil layer  540 . Thus, e.g., each metal foil layer  540  may be positioned between a respective pair of magneto-caloric material blocks  530  along the axial direction A within magneto-caloric stage  510 . 
     In each magneto-caloric stage  510 , the magneto-caloric material blocks  530  may be constructed of a respective magneto-caloric material that exhibits the magneto-caloric effect. Thus, e.g., the magneto-caloric material blocks  530  within each magneto-caloric stage  510  may have a common magneto-caloric material composition. Conversely, as noted above, each of magneto-caloric stages  510  may have a different magneto-caloric material composition. 
     Metal foil layers  540  may be provide a heat flow path within magneto-caloric stage  510 . In particular, metal foil layers  540  may have a greater thermal conductance than magneto-caloric material blocks  530 . Thus, heat may conduct more easily along the radial direction R, e.g., through metal foil layers  540 , compared to along the axial direction A, e.g., through magneto-caloric material blocks  530 . 
     As shown in  FIG. 13 , metal foil layers  540  may be spaced apart from one another along the axial direction A within magneto-caloric stage  510 , e.g., by magneto-caloric material blocks  530 . Conversely, metal foil layers  540  may extend, e.g., continuously, along the radial direction R from an inner surface  512  of magneto-caloric stage  510  to an outer surface  514  of magneto-caloric stage  510 . Inner surface  512  of magneto-caloric stage  510  may be positioned opposite outer surface  514  of magneto-caloric stage  510  along the radial direction R on magneto-caloric stage  510 . In particular, inner and outer surfaces  512 ,  514  of magneto-caloric stage  510  may be cylindrical and may be positioned concentric with each other. With metal foil layers  540  arranged in such a manner, heat may conduct more easily along the radial direction R comparted to along the axial direction A within magneto-caloric stage  510 . 
       FIGS. 14 through 16  are schematic views of a method for forming magneto-caloric stage  510 . As shown in  FIG. 14 , a first plurality of magneto-caloric powder  532  may be loaded into a press  550  on top of a first metal foil layer  542 , and a second metal foil layer  544  may then be loaded into press  550  on top of first plurality of magneto-caloric powder  532 . A second plurality of magneto-caloric powder  534  may also be loaded into press  550  on top of second metal foil layer  544 , and a third metal foil layer  546  may then be loaded into press  550  on top of second plurality of magneto-caloric powder  534 . Additional layers of magneto-caloric powder and metal foil may be loaded into press  550  in a similar manner. 
     Turning to  FIGS. 15 and 156 , after loading press  550  with magneto-caloric powder and metal foil layers, a piston  552  of press  550  may then be operated to compress first plurality of magneto-caloric powder  532 , second plurality of magneto-caloric powder  534 , first metal foil layer  542 , second metal foil layer  544 , third metal foil layer  546 , etc. together. Pressing first plurality of magneto-caloric powder  532 , second plurality of magneto-caloric powder  534 , etc. generates magneto-caloric material blocks  530 . 
     Metal foil layers  540  may act as a binder between adjacent magneto-caloric material blocks  530 . Thus, magneto-caloric stage  510  may have greater mechanical strength than magneto-caloric stages without metal foil layers  540 . Metal foil layers  540  may be constructed of a suitable metal. For example, metal foil layers  540  may be aluminum foil layers. The percentage of metal foil layers  540  may also be selected to provide desirable thermal conductance and mechanical binding. For example, a total volume of metal within magneto-caloric stage  510  may be about ten percent (10%), and, e.g., the remainder of the volume of magneto-caloric stage  510  may be magneto caloric material, binder, etc. within magneto-caloric material blocks  530 . As used herein the term “about” means within nine percent of the stated percentage when used in the context of volume percentages. 
     As noted above, the thermal conductance along the radial direction R within magneto-caloric stage  510  may be greater than the thermal conductance along the radial direction A. Thus, an associated thermal diode with magneto-caloric cylinder  500 , such as magneto-caloric thermal diode  200 , may harvest caloric effect (heat) more quickly compared to thermal diodes with magneto-caloric cylinders lacking metal foil layers. In such a manner, a power density of the associated thermal diode may be increased relative to the thermal diodes with magneto-caloric cylinders lacking metal foil layers. 
     It will be understood that while described above in the context of magneto-caloric cylinder  500 , the present subject matter may also be used to form magneto-caloric regenerators with any other suitable shape in alternative example embodiments. For example, the present subject matter may be used with planar and/or rod-shaped regenerators having anisotropic thermal conductance. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.