WO2007048243A1

WO2007048243A1 - Shimmed active magnetic regenerator for use in thermodynamic devices

Info

Publication number: WO2007048243A1
Application number: PCT/CA2006/001759
Authority: WO
Inventors: Andrew Rowe; Armando Tura; Ozan Peksoy
Original assignee: University Of Victoria Innovation And Development Corporation
Priority date: 2005-10-28
Filing date: 2006-10-26
Publication date: 2007-05-03
Also published as: US20100107654A1; CA2627675A1

Abstract

The invention 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

Shimmed Active Magnetic Regenerator for Use in Thermodynamic Devices

Cross-Reference to Related Application

This application claims the benefit of U.S. Provisional Patent Application No. 60/731 ,138, titled "SHIMMED ACTIVE MAGNETIC REGENERATOR FOR USE IN THERMODYNAMIC DEVICES" and filed October 28, 2005, which is incorporated herein by reference.

Field

The invention 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

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.

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 nonmagnetic 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.

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 United States Patent 4,332,135 by Barclay, et al. dated June 1, 1982.

Barclay's AMR invention 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.

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.

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.

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).)

The flux density through any body is governed by Maxwell's equation for flux continuity V • 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₍, , is less than in a long, thin specimen with the long axis parallel to the applied field [I]. 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.

It is an object of the present invention to overcome the deficiencies in the prior art. Summary

The present invention 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.

In one embodiment of the invention, 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.

In another embodiment of the invention, 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.

In another embodiment of the invention, 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.

In another embodiment of the invention, 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 and the shim is located proximate to the AMR.

In one aspect of the invention, the passive material is non-magnetocaloric.

In another aspect of the invention, the passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

In another aspect of the invention, the passive material is iron.

In another aspect of the invention, 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.

In another aspect of the invention, 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.

In another aspect of the invention, the magnetocaloric material is shaped to have a body, the body having a side wall, a warm end and a cold end.

In another aspect of the invention, the at least one shim is located proximate to the warm end, the cold end or both the warm and the cold end.

In another aspect of the invention, the at least one shim is located within the body.

In another aspect of the invention, the at least one shim is proximate to the side wall.

In another aspect of the invention, the shim envelopes the side wall. In another aspect of the invention, the shim envelopes the magnetocaloric material.

In another aspect of the invention, the relative magnetization exceeds about 1.

5

In another aspect of the invention, the magnetic field is less than approximately 3T.

In another aspect of the invention, the magnetic field is less than approximately 3T and the relative magnetism exceeds about 1.

I O

In another embodiment of the invention, 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

15 shim from the at least one passive material, and placing the at least one shim proximate to the AMR.

In one aspect of the method of the invention, the AMR is shaped to have a body, the body having a side wall, a warm end and a cold end. 0

In another aspect of the method of the invention, the at least one shim is located proximate to the warm end, the cold end or both the warm and the cold end.

In another aspect of the method of the invention, the shim is located within the body. 5

In another aspect of the method of the invention, the shim is proximate to the side wall.

In another aspect of the method of the invention, the shim envelopes the side wall.

30

In another aspect of the method of the invention, the shim envelopes the AMR. In another aspect of the method of the invention, the passive material is non- magnetocaloric.

In another aspect of the method of the invention, the passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

In another aspect of the method of the invention, the passive material is iron.

In another aspect of the method of the invention, 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.

In another aspect of the method of the invention, 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.

In another embodiment of the invention, an active magnetic thermodynamic device is provided that comprises a permanent magnet and a shimmed active magnetic regenerator.

Figures

Figure 1 (a) shows a schematic of a typical active magnetic regenerator structure of the prior art.

Figure 1 (b) shows a shimmed active magnetic regenerator of in accordance with an embodiment of the invention.

Figure 2 shows the relative magnetization through the regenerator of Figure 1 (b) when the magnetic field varies between 0.5 T and 5 T. Figure 3 shows the relative magnetization with various magnetic field changes for a shimmed active magnetic regenerator consisting of Gd and Gd _J4Tb₂₆ operating between 270 and 306 K, in accordance with an embodiment of the invention.

Figure 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 invention.

Figure 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 invention (b), an active magnetic regenerator with passive material between layers, in accordance with an embodiment of the invention (c), and an active magnetic regenerator with passive material around the circumference, in accordance with an embodiment of the invention (d).

Detailed Description

Definitions: Magnetocaloric effect (MCEV. 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.

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.

T_H is the temperature on the warm extremity of the AMR and Tc 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 15 following,

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. 0

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 5 20 K are common, but not generally desirable.

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.

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 0.2K/T and even more specifically more than about 0.5K/T.

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).

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:

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.

Figure 1 (a) shows a schematic of a typical active magnetic regenerator structure of the prior art and Figure 1 (b) shows the same regenerator with the addition of a shim composed of a passive material on either end.

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 Figure 1 are shown below in Figure 2 and Figure 3.

An alternative way to look at the performance increase is in terms of relative magnetization as shown in Figures 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. Figure 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 Figure 3.

Figure 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.

Figure 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 Figure l(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.

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)

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 AMRs can consist of one or more magnetocaloric materials. Examples of these arrangements are shown in Figure 5. In Figure 5 (a), A and B refer to different magnetocaloric materials. In Figure 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.

Figure 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.

Active magnetic regenerators are usually constructed using a shell of nonconducting material to provide a structural container for the magnetocaloric material. These shells are often composite materials using glass or phenolic in epoxy matrices.

The foregoing is a description of a number of embodiments of the invention. As would be known to one skilled in the art, variations that do not alter the scope of the invention would be contemplated. For example, a variation on the shims show in Figure 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 Gd₅(Si ι_-xGe_x)₄ and La(Fe i_-xSi_x)i₃H_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.

Claims

1. A shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices, the shimmed active magnetic regenerator comprising a combination of an at least one magnetocaloric material and an 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.

2. The SAMR of claim 1 wherein the passive material is non-magnetocaloric.

3. The SAMR of claim 2 wherein the passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

4. The SAMR of claim 3 wherein the passive material is iron.

5. The SAMR of claim 1 wherein 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.

6. The SAMR of claim 3 wherein 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.

7. The SAMR of claim 4 wherein 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.

8. The SAMR of claim 6 wherein the magnetocaloric material is shaped to have a body, the body having a side wall, a warm end and a cold end.

9. The SAMR of claim 8 wherein said at least one shim is located proximate to said warm end, said cold end or both said warm and said cold end.

10. The SAMR of claim 8 wherein said shim is located within said body.

1 1. The SAMR of claim 8 wherein said shim is proximate to said side wall. 5

12. The SAMR of claim 11 wherein said shim envelopes said side wall.

13. The SAMR of claim 8 wherein said shim envelopes said magnetocaloric material.

I O

14. The SAMR of claim 1, wherein said relative magnetization exceeds about 1.

15. A shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices, said shimmed active magnetic regenerator comprising a

15 combination of an at least one magnetocaloric material and an at least one shim comprising at least one passive material, said shim located proximate to said magnetocaloric material.

16. The SAMR of claim 15 wherein said passive material is non-magnetocaloric. 0

17. The SAMR of claim 15 wherein said passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

18. The SAMR of claim 17 wherein said passive material is iron.

25

19. The SAMR of claim 15 wherein said 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 metallic elements.

30 20. The SAMR of claim 17 wherein said 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.

21. The SAMR of claim 20 wherein said magnetocaloric material comprises Gd, an at least one alloy of Gd or a combination of Gd and an at least one alloy of Gd.

22. The SAMR of claim 20 wherein said magnetocaloric material is shaped to have a body, said body having a side wall, a warm end and a cold end.

23. The SAMR of claim 22 wherein said at least one shim is located proximate to said warm end, said cold end or both said warm and said cold end.

24. The SAMR of claim 22 wherein said shim is located within said body.

25. The SAMR of claim22 wherein said shim is proximate to said side wall.

26. The SAMR of claim 25 wherein said shim envelopes said side wall.

27. The SAMR of claim 22 wherein said shim envelopes said magnetocaloric material.

28. The SAMR of claim 15, wherein said relative magnetization exceeds about

29. A shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices, said shimmed active magnetic regenerator comprising a combination of an active magnetic regenerator (AMR) and an at least one shim comprising 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.

30. The SAMR of claim 29 wherein said passive material is non-magnetocaloric.

31. The SAMR of claim 29 wherein said passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

32. The SAMR of claim 31 wherein said passive material is iron.

33. The SAMR of claim 29 wherein said AMR is comprised of materials 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.

34. The SAMR of claim 31 wherein said AMR is comprised of materials 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.

35. The SAMR of claim 34 wherein said AMR is comprised of materials selected from the group consisting of Gd, an at least one alloy of Gd or a combination of Gd and an at least one alloy of Gd.

36. The SAMR of claim 34 wherein said AMR is shaped to have a body, said body having a side wall, a warm end and a cold end.

37. The SAMR of claim 36 wherein said at least one shim is located proximate to said warm end, said cold end or both said warm and said cold end.

38. The SAMR of claim 36 wherein said shim is located within said body.

39. The SAMR of claim 36 wherein said shim is proximate to said side wall.

40. The SAMR of claim 39 wherein said shim envelopes said side wall.

41. The SAMR of claim 36 wherein said shim envelopes said AMR.

42. The SAMR of claim 29, wherein said relative magnetization exceeds about 1.

43. The SAMR of claim 42 wherein said magnetic field is less than approximately 3T.

44. The SAMR of claim 14 wherein said magnetic field is less than approximately 3T.

45. The SAMR of claim 28 wherein said magnetic field is less than approximately 3T.

46. A shimmed active magnetic regenerator (SAMR) for use in active magnetic thermodynamic devices, said shimmed active magnetic regenerator comprising a combination of an active magnetic regenerator (AMR) and an at least one shim comprising at least one passive material, said shim located proximate to said AMR.

47. The SAMR of claim 46 wherein said magnetic field is less than approximately 3T and said relative magnetism exceeds about 1.

48. A method of manufacturing a shimmed active magnetic regenerator (AMR), said method comprising selecting a magnetocaloric material, constructing an active magnetic regenerator (AMR) from said magnetocaloric material, selecting a passive material, constructing an at least one shim from said at least one passive material, and placing said at least one shim proximate to said AMR.

49. The method of claim 48 wherein said AMR is shaped to have a body, said body having a side wall, a warm end and a cold end.

50. The method of claim 49 wherein said at least one shim is located proximate to said warm end, said cold end or both said warm and said cold end.

51. The method of claim 49 wherein said shim is located within said body.

52. The method of claim 49 wherein said shim is proximate to said side wall.

53. The method of claim 52 wherein said shim envelopes said side wall.

54. The method of claim 49 wherein said shim envelopes said AMR.

55. The method of claim 48 wherein said passive material is non- magnetocaloric.

56. The method of claim 48 wherein said passive material is selected from the group consisting of iron, steel and nickel-iron alloys.

57. The method of claim 56 wherein said passive material is iron.

58. The method of claim 56 wherein said 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.

59. The method of claim 58 wherein said magnetocaloric material comprises Gd, an at least one alloy of Gd or a combination of Gd and an at least one alloy of Gd.

60. An active magnetic thermodynamic device, comprising a permanent magnet and a shimmed active magnetic regenerator.