Abstract:
The technology provides a shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices. The shimmed active magnetic regenerator comprises a combination of at least one magnetocaloric material and at least one shim comprising at least one passive material, such that when a magnetic field is applied the relative magnetization of the magnetocaloric material in combination with the shim is greater than the relative magnetization of the magnetocaloric material. The SAMR can achieve a relative magnetism of about 1 from a magnetic field less than approximately 3T.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 60/731,138, filed on 28 Oct. 2005 and entitled Shimmed Active Magnetic Regenerator for Use in Thermodynamic Devices, the contents of which are incorporated herein by reference. 
     
    
     FIELD  
       [0002]    The technology relates to magnetic regeneration to provide refrigeration. More specifically, it relates to active magnetic regeneration (AMR), using a combination of an AMR and shims of passive materials. 
       BACKGROUND  
       [0003]    There have been many developments in the field of refrigeration since the days when ice was cut from frozen lakes and rivers, stored under sawdust and delivered in the summer months to customers in order to increase the storage life of food. Most household freezers now rely on the use of CFCs/HCFCs/HFCs and gas compressors. While the refrigeration that is provided is very good, there is concern over the use of CFCs/HCFCs/HFCs, as they are considered to be environmentally unfriendly because of their ozone depletion potential and warming potential (GHG). Compressors on the other hand do not pose any environmental threat, however, they are prone to failure and are inefficient. 
         [0004]    Commercial regenerative refrigeration systems are used in a number of applications, including gas liquifiers and cryogenic refrigeration. Regenerators have been used in these thermodynamic devices for a substantial period of time. The regenerator materials in these devices can be magnetocaloric or passive materials with large heat capacity. Passive materials include iron and iron-nickel alloys, steels, lead, copper, bronze, and many other materials. In devices using passive magnetic or non-magnetic materials, a thermal wave-front propagates back and forth within the regenerator. The materials do not exhibit a significant reversible temperature change when subjected to a changing magnetic field. Hence, passive materials are unable to effectively provide a refrigeration cycle without the use of a fluid undergoing its own thermodynamic cycle. 
         [0005]    In contrast, when magnetocaloric materials are employed, the device, referred to as an Active Magnetic Regenerator (AMR), can provide refrigeration along the entire temperature span of the device. Each different material, if a plurality of materials is utilized, executes a small thermodynamic cycle near its Curie temperature. When all the materials are combined they may yield a cycle operating over an extended temperature range. The basic AMR concept is described in U.S. Pat. No. 4,332,135 by Barclay, et al. dated Jun. 1, 1982. 
         [0006]    Barclay&#39;s AMR technology can provide refrigeration along the entire temperature span of the device because each of the distributed segments of the regenerator executes its own refrigeration cycle. The “distributed refrigeration” feature is desirable in many applications such as liquefaction of cryogens. It is also advantageous when compared to gas refrigeration cycles where refrigeration is only provided when expansion of a fluid occurs at several distinct points in a cooling process. 
         [0007]    In AMR, the magnetic field is often generated with superconducting magnets. These magnets provide the necessary field strengths higher than 3 T. However, magnetic fields created using coiled conductors (superconductors) require a power supply and special ancillary equipment. Not only is this equipment bulky, but it can also be costly. 
         [0008]    Accordingly, there is a substantial amount of interest in creating magnetocaloric devices that use permanent magnets as the source of the magnetic field. Permanent magnets allow for the creation of more compact field generation without the need for an external current supply. The drawback of using permanent magnets is that the field strength is limited by the energy density of the magnet material (rare earth permanent magnets are the highest) and the volume of free-space over which the magnet field is at its highest is relatively small. Prototype permanent magnet AMR devices reported in the literature usually have usable field strengths of 2 T or lower, while most of the better magnetocaloric materials are magnetically saturated when subjected to applied fields greater than 2-3 T. 
         [0009]    Regardless of the magnet type, an AMR is subjected to demagnetization effects that can reduce the change in magnetization when the material experiences a field change. This reduces the magnetocaloric effect and, therefore, the effectiveness of an AMR refrigerator. In addition, if the AMR is composed of more than one material, interactions at the material interface can alter the expected magnetocaloric effect. Peksoy et. al numerically investigated this problem and its possible impacts (O. Peksoy and A. Rowe, Journal of Magnetism and Magnetic Materials, 288:424-432 (2005).) 
         [0010]    The flux density through any body is governed by Maxwell&#39;s equation for flux continuity ∇·B=0. The concept of a demagnetizing field, H d  , results from the mathematical description of magnetization in an arbitrarily shaped body. The net magnetization in a squat body subject to an applied field, H a , is less than in a long, thin specimen with the long axis parallel to the applied field. To help understand this behavior, the concept of a demagnetizing field is sometimes used (although not an actual field). In a material with uniform properties and shape, the demagnetizing field tends to be in a direction opposite to the magnetization. And, as a result, the flux density in a magnetic material can be less than one would expect if one just summed the field intensity and reported magnetization values (high-aspect ratio specimen). Demagnetization effects are most significant when a body is subjected to an applied field that does not cause magnetic saturation. Thus, permanent magnet AMR devices can suffer from reduced performance because of demagnetization effects. 
       SUMMARY  
       [0011]    The present technology provides a shimmed active magnetic regenerator. This regenerator reduces the effects of demagnetization by using passive ferromagnetic materials in addition to magnetocaloric materials in a magnetic regenerator. Passive materials can act as thermal masses that dampen the magnetocaloric effect. However, if suitably fabricated and located in the regenerator structure, passive materials can increase the performance of the AMR. 
         [0012]    In one embodiment of the technology, a shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices is provided. The shimmed active magnetic regenerator comprises a combination of at least one magnetocaloric material and at least one shim. The shim comprises at least one passive material, such that when a magnetic field is applied the relative magnetization of the magnetocaloric material in combination with the shim is greater than the relative magnetization of the magnetocaloric material. 
         [0013]    In another embodiment of the technology, a shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices is provided. The shimmed active magnetic regenerator comprises a combination of an at least one magnetocaloric material and an at least one shim. The shim comprises at least one passive material, and is located proximate to the magnetocaloric material. 
         [0014]    In another embodiment of the technology, a shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices is provided. The shimmed active magnetic regenerator comprises a combination of an active magnetic regenerator (AMR) and an at least one shim that comprises at least one passive material, such that when a magnetic field is applied the relative magnetization of the shimmed AMR is greater than the AMR without shims. 
         [0015]    In another embodiment of the technology, a shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices is provided. The shimmed active magnetic regenerator comprises a combination of an active magnetic regenerator (AMR) and at least one shim that comprises at least one passive material and the shim is located proximate to the AMR. 
         [0016]    In one aspect of the technology, the passive material is non-magnetocaloric. 
         [0017]    In another aspect of the technology, the passive material is selected from the group consisting of iron, steel and nickel-iron alloys. 
         [0018]    In another aspect of the technology, the passive material is iron. 
         [0019]    In another aspect of the technology, the magnetocaloric materials are selected from the group consisting of Gd, Tb, Dy, alloys of Gd, Tb and Dy, rare-earth elements, alloys of rare-earth and transition metals. 
         [0020]    In another aspect of the technology, the magnetocaloric material comprises Gd, an at least one alloy of Gd or a combination of Gd and an at least one alloy of Gd. 
         [0021]    In another aspect of the technology, the magnetocaloric material is shaped to have a body, the body having a side wall, a warm end and a cold end. 
         [0022]    In another aspect of the technology, the at least one shim is located proximate to the warm end, the cold end or both the warm and the cold end. 
         [0023]    In another aspect of the technology, the at least one shim is located within the body. 
         [0024]    In another aspect of the technology, the at least one shim is proximate to the side wall. 
         [0025]    In another aspect of the technology, the shim envelopes the side wall. 
         [0026]    In another aspect of the technology, the shim envelopes the magnetocaloric material. 
         [0027]    In another aspect of the technology, the relative magnetization exceeds about 1. 
         [0028]    In another aspect of the technology, the applied magnetic field is less than approximately 3T. 
         [0029]    In another aspect of the technology, the applied magnetic field is less than approximately 3T and the relative magnetism exceeds about 1. 
         [0030]    In another embodiment of the technology, a method of manufacturing a shimmed active magnetic regenerator (AMR) is provided. The method comprises selecting a magnetocaloric material, constructing an active magnetic regenerator (AMR) from the magnetocaloric material, selecting a passive material, constructing an at least one shim from the at least one passive material, and placing the at least one shim proximate to the AMR. 
         [0031]    In one aspect of the method of the technology, the AMR is shaped to have a body, the body having a side wall, a warm end and a cold end. 
         [0032]    In another aspect of the method of the technology, the at least one shim is located proximate to the warm end, the cold end or both the warm and the cold end. 
         [0033]    In another aspect of the method of the technology, the shim is located within the body. 
         [0034]    In another aspect of the method of the technology, the shim is proximate to the side wall. 
         [0035]    In another aspect of the method of the technology, the shim envelopes the side wall. 
         [0036]    In another aspect of the method of the technology, the shim envelopes the AMR. 
         [0037]    In another aspect of the method of the technology, the passive material is non-magnetocaloric. 
         [0038]    In another aspect of the method of the technology, the passive material is selected from the group consisting of iron, steel and nickel-iron alloys. 
         [0039]    In another aspect of the method of the technology, the passive material is iron. 
         [0040]    In another aspect of the method of the technology, the magnetocaloric materials are selected from the group consisting of Gd, Tb, Dy, alloys of Gd, Tb and Dy, rare-earth elements, alloys of rare-earth and transition metals. 
         [0041]    In another aspect of the method of the technology, the magnetocaloric material comprises Gd, an at least one alloy of Gd or a combination of Gd and an at least one alloy of Gd. 
         [0042]    In another embodiment of the technology, an active magnetic thermodynamic device is provided that comprises a permanent magnet and a shimmed active magnetic regenerator. 
     
    
     
       FIGURES  
         [0043]      FIG. 1(   a ) shows a schematic of a typical active magnetic regenerator structure of the prior art. 
           [0044]      FIG. 1(   b ) shows a shimmed active magnetic regenerator of in accordance with an embodiment of the technology. 
           [0045]      FIG. 2  shows the relative magnetization through the regenerator of  FIG. 1(   b ) when the magnetic field varies between 0.5 T and 5 T. 
           [0046]      FIG. 3  shows the relative magnetization with various magnetic field changes for a shimmed active magnetic regenerator consisting of Gd and Gd 0.74 Tb 0.26  operating between 270 and 306 K, in accordance with an embodiment of the technology. 
           [0047]      FIG. 4  shows experimental results of a gadolinium shimmed magnetic regenerator with and without the presence of passive material on the ends of the regenerator, in accordance with an embodiment of the technology. 
           [0048]      FIG. 5  shows a conventional active magnetic regenerator of the prior art using two layered magnetocaloric materials (a), an active magnetic regenerator with additional passive material on the top and bottom, in accordance with an embodiment of the technology (b), an active magnetic regenerator with passive material between layers, in accordance with an embodiment of the technology (c), and an active magnetic regenerator with passive material around the circumference, in accordance with an embodiment of the technology (d). 
       
    
    
     DETAILED DESCRIPTION 
     Definitions: 
       [0049]    Magnetocaloric effect (MCE): the reversible adiabatic temperature change displayed by a material when subjected to a change in applied magnetic field. Gd is a good conventional magnetocaloric material with an MCE that is on the order of 2-3 K/Tesla (temperature change per unit applied field). A magnetocaloric material with 2-3 K/T is good, 3-4 K/T is excellent, and less than 2 K/T is moderate. Materials with MCEs exceeding 2 K/T are preferable. 
         [0050]    Active Magnetic Regenerator (AMR): a porous structure made up of one or more magnetocaloric materials. When subjected to a time-varying, reversing flow of fluid, and a periodic change in applied magnetic field, an AMR performs a net amount of magnetic work, develops a temperature gradient through the porous structure, and pumps heat from one side of the structure to another side. An AMR must be able to generate a temperature span that exceeds the magnetocaloric effect of the material used to make the AMR. This is an absolute minimum. Preferably, the AMR should generate a temperature span that is many times the peak magnetocaloric effect of any of the constituent materials. (For example, Gd with an applied field of 2 T will have a peak MCE of approximately 5 K; thus, an AMR using Gd should be able to generate a temperature span exceeding 5 K.) The best metric for performance combines both the temperature span achieved and the cooling power, Q c . An AMR that performs well will make the value of the following relationship greater than zero, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Q 
                       C 
                     
                      
                     
                       ( 
                       
                         
                           
                             T 
                             H 
                           
                           
                             T 
                             C 
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                   
                   . 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    T H  is the temperature on the warm extremity of the AMR and T C  is the temperature on the cold extremity of the AMR. The difference between these two temperatures is the temperature span. In addition, an AMR that performs well will maximize the following, 
         [0000]    
       
         
           
             
               
                 
                   η 
                   ≡ 
                   
                     
                       
                         Q 
                         C 
                       
                        
                       
                         ( 
                         
                           
                             
                               T 
                               H 
                             
                             
                               T 
                               C 
                             
                           
                           - 
                           1 
                         
                         ) 
                       
                     
                     W 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Where W is the work input to the device. A good AMR will have values for η greater than 0.5. η should always be greater than 0 and will never exceed 1. 
         [0051]    Temperature span: the maximum absolute temperature difference between the extremities of an active magnetic regenerator. For applied magnetic fields of 2 T or less, an AMR generating a temperature span greater than 50 K is very good. AMRs producing temperature spans of 20-40 K are good. Temperature spans of less than 20 K are common, but not generally desirable. 
         [0052]    Relative Magnetization: a measure of regenerator effectiveness. Defined as the ratio of actual material magnetization to the magnetization determined from material susceptibility curves. A value of 1 is expected and good for an AMR material. With applied fields of less than 2 T, relative magnetizations can fall in the range of 0.8, and are undesirable. Relative magnetizations higher than 1.0 are uncommon, but can be produced. A relative magnetization of 1-1.2 would be good; greater than 1.2 would be excellent. 
         [0053]    Magnetocaloric Material: a material displaying a reversible, magnetic field induced, temperature change or, magnetocaloric effect. A magnetocaloric material has a magnetocaloric effect of more than about 0.1 K/T, more specifically more than about 0.2 K/T and even more specifically more than about 0.5 K/T. 
         [0054]    Passive material: a material that experiences a force when subjected to an applied magnetic field, but does not display a significant magnetocaloric effect (less than about 0.1 K/T, preferably less than about 0.05 K/T and even more preferably less than about 0.01 K/T). 
         [0055]    Non-magnetocaloric: a material in which the reversible temperature change due to a changing magnetic field is small (less than about 0.1 K/T, preferably less than about 0.05 K/T and even more preferably less than about 0.01 K/T). Non-magnetocaloric materials included passive materials in addition to other non-magnetic materials. 
       Examples  
       [0056]    The addition of passive materials can increase performance by creating a larger temperature span or cooling power than an AMR made up of magnetocaloric material only and contained in a non-magnetic structure. 
         [0057]      FIG. 1(   a ) shows a schematic of a typical active magnetic regenerator structure of the prior art and  FIG. 1(   b ) shows the same regenerator with the addition of a shim composed of a passive material on either end. 
         [0058]    For an active magnetic regenerator to perform well, when the magnetic field strength is changed, a large change in magnetization should occur. When a material is operating near the Curie point, the variation in magnetization as a function of field and temperature creates the magnetocaloric effect and allows for a magnetic cycle to be created. A simulation of the impacts of applying various magnetic fields to the regenerators in  FIG. 1  are shown below in  FIG. 2  and  FIG. 3 . 
         [0059]    An alternative way to look at the performance increase is in terms of relative magnetization as shown in  FIGS. 2 and 3 . A passive shim can increase performance if the relative magnetization is increased as compared an AMR without shims. This in practice appears as in increase in the parameter defined by Equation 1. Regardless, a performance improvement occurs when the relative magnetization is increased.  FIG. 2  shows the relative magnetization through the regenerator when the magnetic field varies between 0.5 T and 5 T. For the fields less than 3 T, the relative magnetization can deviate quite significantly from a value of one. Values less than one indicate that the material is not magnetized as high as would be expected. Some parts of the regenerator have a relative magnetization greater than one, near the middle for example, which can be good depending on the operating conditions and properties of the adjacent material. In general, increasing the relative magnetization so it is close to 1 or greater is desirable. The impacts of adding a layer of passive ferromagnetic material on either end of the regenerator are shown in  FIG. 3 . 
         [0060]      FIG. 3  suggests that the use of passive material has increased the relative magnetization in the AMR. The improvements are most significant for lower field strengths where the relative magnetization near the ends of the regenerator is greater than 1. 
         [0061]      FIG. 4  shows experimental results of a gadolinium AMR with and without the presence of shims composed of passive material on the ends of the regenerator (as per  FIG. 1(   b )). The line connecting the triangular markers shows the temperature span achieved as a function of different operating points (warm temperature). Larger temperature spans are desirable. The line connecting the square data points shows the temperature span resulting from the use of passive material (“shim”) on the warm end of the regenerator while the diamond markers show the temperature span when shims are on both ends of the AMR. In both cases, the temperature span has increased significantly for operating points above the Curie temperature of Gd. 
         [0062]    The concept of flux shimming a magnetic regenerator generally means using magnetic materials (not displaying a significant magnetocaloric effect) to augment the magnetic field seen by the magnetocaloric material. The advantages of this are:
       1. better performance because of larger magnetocaloric effect,   2. the use of less magnetocaloric material (reduced costs),   3. resulting in a smaller volume over which a magnetic field must be generated
           a. smaller magnet (lower cost and weight),   b. possibility of higher magnetic field (increased performance)   
               
 
         [0068]    We have used solid magnetic disks as shims fabricated using 1018 carbon steel and with holes drilled near the outer radius to allow for gas flow. The central portion of the disk was solid and had a thickness of ˜1 cm. to prove the concept experimentally and numerically. Other configurations that reduce or minimize eddy-currents are contemplated. These alternative embodiments include using shims composed of magnetic material in particle form or laminations of high aspect ratio material as is used in electric motor core construction. Also, the shim material can be arranged on the ends of the AMR, around the circumference (perimeter), or within the AMR between material layers or mixed with magnetocaloric material. There can be many layers of shims and magnetocaloric materials, for example, as shown, two layers, and for example, up to about four layers, or for example up to ten layers, or for example more than ten layers. Mixing with magnetocaloric material may be beneficial particularly when it is used between layers of different magnetocaloric materials. This technique may create a region with a more gradual variation in magnetic susceptibility and therefore lower demagnetization. The 
         [0069]    AMRs can consist of one or more magnetocaloric materials. Examples of these arrangements are shown in  FIG. 5 . In  FIG. 5(   a ), A and B refer to different magnetocaloric materials. In  FIG. 5(   c ) the regenerator is shown with a shim between two magnetocaloric materials and on one end. It could be on the warm end or on the cold end, but for typical operating conditions it is more effective to have a shim on the warm end of the AMR instead of the cold end. 
         [0070]    FIG.  5 —A conventional active magnetic regenerator using two layered magnetocaloric materials (a), a magnetic regenerator with additional passive material on the top and bottom (b), an AMR with passive material between layers (c), an AMR with passive material around the circumference (d), shims around all of the magnetocaloric material (e), and shim between layers of magnetocaloric material. 
         [0071]    Active magnetic regenerators are usually constructed using a shell of non-conducting material to provide a structural container for the magnetocaloric material. These shells are often composite materials using glass or phenolic in epoxy matrices. 
         [0072]    The foregoing is a description of a number of embodiments of the technology. As would be known to one skilled in the art, variations that do not alter the scope of the technology would be contemplated. For example, a variation on the shims show in  FIG. 5  would be to use passive material in place of the fibres making up a composite shell. Thus, the shell material could act simultaneously as a structural container and as a shim providing magnetic field augmentation on the magnetocaloric material. The passive materials can be, for example, but not to be limited to iron, a variety of steels (excluding the 300 series stainless steels which have low magnetic susceptibility), nickel-iron alloys such as mumetal™, and supermalloy™. In general, the material should be magnetically soft so there is little hysteresis. The addition of the layer of passive material is important in that it creates a smoother transition in magnetic permeability where the magnetocaloric material ends. The thickness of the layer depends on the type of passive material (magnetic properties), strength of the applied field, the magnetocaloric material, and the temperature the material is operating at. There is no maximum thickness, but there will be some optimum amount and having more will reduce performance. The magnetocaloric materials can be, for example, but not limited to, Gd, Tb, Dy, Er, alloys consisting of these and other rare-earth elements, and alloys of rare-earths and transition metals. In addition, new magnetocaloric materials consisting of Si, Ge, Fe, Mn, La and other metallic elements are known to display a magnetocaloric effect, examples being Gd 5 (Si 1-x Ge x ) 4  and La(Fe 1-x Si x ) 13 H y . In general, the passive materials are located adjacent to any discontinuities in magnetic permeability. Hence the perimeters of any magnetocaloric material regions are areas where passive materials can be placed. For a regenerator consisting of layers of magnetocaloric material, this means that passive materials between the layers may also increase the efficacy of the regenerator. In terms of fabrication, the passive materials must have small cross-sectional area perpendicular to the direction of the changing magnetic field vector. Preferably, the relative magnetization will exceed one.