U.S. patent application number 12/595217 was filed with the patent office on 2010-05-13 for composite article with magnetocalorically active material and method for its production.
Invention is credited to Matthias Katter, Georg Werner Reppel.
Application Number | 20100116471 12/595217 |
Document ID | / |
Family ID | 40885088 |
Filed Date | 2010-05-13 |
United States Patent
Application |
20100116471 |
Kind Code |
A1 |
Reppel; Georg Werner ; et
al. |
May 13, 2010 |
COMPOSITE ARTICLE WITH MAGNETOCALORICALLY ACTIVE MATERIAL AND
METHOD FOR ITS PRODUCTION
Abstract
A composite article (1; 10; 40) comprises a plurality of
inclusions (5) of a magnetocalorically active material embedded in
a matrix (4) of a magnetocalorically passive material. The
inclusions (5) and the matrix (4) have a microstructure
characteristic of a compacted powder.
Inventors: |
Reppel; Georg Werner;
(Hammersbach, DE) ; Katter; Matthias; (Alzenau,
DE) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Family ID: |
40885088 |
Appl. No.: |
12/595217 |
Filed: |
December 27, 2007 |
PCT Filed: |
December 27, 2007 |
PCT NO: |
PCT/IB07/55316 |
371 Date: |
October 8, 2009 |
Current U.S.
Class: |
165/133 ;
264/122; 419/27; 419/30; 419/62; 419/65; 428/323; 428/328 |
Current CPC
Class: |
H01F 1/015 20130101;
B29C 67/24 20130101; B22F 3/02 20130101; F25B 2321/002 20130101;
F28F 13/16 20130101; Y10T 428/256 20150115; Y02B 30/66 20130101;
Y10T 428/25 20150115; H01F 1/012 20130101; B22F 3/12 20130101; F25B
21/00 20130101; Y02B 30/00 20130101; F28F 3/00 20130101 |
Class at
Publication: |
165/133 ;
264/122; 419/62; 419/65; 419/30; 419/27; 428/323; 428/328 |
International
Class: |
F28F 13/18 20060101
F28F013/18; B29C 70/00 20060101 B29C070/00; B29C 67/24 20060101
B29C067/24; B22F 1/00 20060101 B22F001/00; B22F 3/26 20060101
B22F003/26; B32B 5/16 20060101 B32B005/16 |
Claims
1. A composite article, comprising: (a) a solid matrix comprising a
magnetocalorically passive material and having a microstructure
comprising a plurality of particles of magnetocalorically passive
material that have lattice structure that exhibits strain, or
fractures, or both, from compression, and that, at least in part,
touch one or more immediately neighboring particles of
magnetocalorically passive material; and (b) a plurality of solid
inclusions comprising a magnetocalorically active material at least
partially embedded in the solid matrix of a magnetocalorically
passive material, wherein the inclusions either: (1) have a
microstructure comprising a plurality of particles of
magnetocalorically active material that have lattice structure that
exhibits strain, or fractures, or both from compression, and that,
at least in part, touch one or more immediately neighboring
particles of magnetocalorically passive material, or (2) form a
series of foils flakes or tapes embedded in the matrix.
2. Composite article according to claim 1, wherein the
microstructure of the matrix comprises particles having an average
particle size of less than 1000 .mu.m.
3. Composite article according claim 1, wherein the inclusions have
an average diameter of less than 1000 .mu.m.
4. Composite article according to claim 1, wherein the inclusions
further comprise a metallic corrosion protection coating.
5. Composite article according to claim 4, wherein the metallic
corrosion protection coating comprises one or more of Al, Cu, or
Sn.
6. Composite article according to claim 1, wherein the inclusions
further comprise an outer coating of an electrically isolating
material.
7. Composite article according to claim 1, wherein the matrix
further comprises an outer coating of an electrically isolating
material disposed on at least some of the particles of
magnetocalorically passive material.
8. Composite article according to claim 6, wherein the electrically
isolating material comprises a polymer or a ceramic or an inorganic
compound.
9. A composite article, comprising: (a) a solid matrix comprising a
magnetocalorically passive material having a sintered
microstructure comprising a plurality of grains of
magnetocalorically passive material that have lattice structure
that exhibits strain relief, and that, at least in part, touch one
or more immediately neighboring grains of magnetocalorically
passive material via an interface comprising interdiffused atoms
from the neighboring grains, resulting from sintering a compressed
powder of magnetocalorically passive material; (b) a plurality of
solid inclusions of comprising a magnetocalorically active material
at least partially embedded in the solid matrix.
10. (canceled)
11. (canceled)
12. Composite article according to claim 9, wherein the inclusions
further comprise a metallic corrosion protection coating.
13. Composite article according to claim 12, wherein the corrosion
protection coating comprises one or more of Al, Cu, or Sn.
14. Composite article according to claim 9, wherein the inclusions
further comprise an outer coating of an electrically isolating
material.
15. Composite article according to claim 9, wherein the grains of
the matrix further comprise an outer coating of an electrically
isolating material.
16. Composite article according to claim 14, wherein the
electrically isolating material comprises a polymer a ceramic, or
an inorganic compound.
17. Composite article according to claim 1, wherein the
magnetocalorically active material has a Curie temperature T.sub.c
in the range 220K to 345K.
18. Composite article according to claim 1, wherein the
magnetocalorically active material is one or more of Gd, a
La(Fe.sub.1-bSi.sub.b).sub.13-based phase, a Gd.sub.5(Si,
Ge).sub.4-based phase, a Mn(As, Sb)-based phase, a MnFe(P,
As)-based phase, a Tb--Gd-based phase, a (La, Ca, Pr, Nd,
Sr)MnO.sub.3-based phase, a Co--Mn--(Si, Ge)-based phase or a
Pr.sub.2(Fe, Co).sub.17-based phase.
19. Composite article according to claim 1, wherein the matrix
comprises one or more of the elements, Al, Cu, Ti, Mg, Zn, Sn, Bi
and Pb.
20. Composite article according to claim 1, wherein the matrix
comprises one or more of BeO, AlN, BN, or graphite.
21. Composite article according to claim 9, wherein the matrix
comprises grains having an average grain size of less than 1000
.mu.m.
22. Composite article according to claim 9, wherein the inclusions
have an average diameter of less than 1000 .mu.m.
23. Composite article according to claim 1, wherein the inclusions
are present in a volume fraction of inclusions between 25% and 98%,
of the volume of the composite article
24. Composite article according to claim 23, wherein the volume
fraction of inclusions is between 60% and 95%.
25. Composite article according to claim 1, wherein the matrix
comprises a soft magnetic material.
26. Composite article according to claim 25, wherein the soft
magnetic material comprises one or more of Fe, FeSi, Co or Ni.
27. Composite article according to claim 1, wherein the inclusions
are disposed in a plurality of layers, each layer comprising
inclusions of a magnetocalorically active material having a T.sub.c
which is different from the T.sub.c of the magnetocalorically
active material in an adjacent layer.
28. Composite article according to claim 1, further comprising a
surface having at least one channel therein.
29. Composite article according to claim 28, wherein the at least
one channel is adapted to direct the flow of a heat exchange medium
in contact with the surface.
30. Composite article according to claim 1 further comprising an
outer protective coating.
31. Composite article according to claim 30, wherein the outer
protective coating comprises a polymer or a metal or an alloy.
32. Composite article according to claim 1, wherein the composite
article further comprises effective porosity.
33. Composite article according to claim 32, wherein the effective
porosity comprises at least one channel within the body of the
composite article providing flow communication from a first side of
the composite article to a second side of the composite
article.
34. Composite article according to claim 32, wherein the effective
porosity comprises 10 vol. % to 60 vol. % of the composite
article.
35. (canceled)
36. Heat exchanger comprising at least one composite article
according to claim 1.
37. A method of manufacturing a solid composite article,
comprising: providing a first powder comprising a
magnetocalorically active material, providing a second powder
comprising a magnetocalorically passive material, mixing the first
powder and the second powder together to form a powder mixture, and
compacting the powder mixture to form a solid composite article
comprising a solid matrix comprising the magnetocalorically passive
material and a plurality of solid inclusions comprising a
magnetocalorically active material at least partially embedded in
the solid matrix.
38. Method according to claim 37, further comprising: adding one or
more of a lubricant, an organic binder or a dispersant to the
powder mixture.
39. Method according to claim 37, wherein the first powder
comprises a plurality of particles which are have been coated with
a metallic corrosion protection coating before mixing the first
powder with the second powder.
40. Method according to claim 37, wherein the first powder
comprises a plurality of particles which have been coated with an
electrically isolating material before mixing the first powder with
the second powder (3).
41. Method according to claim 37, wherein the second powder
comprises a plurality of particles which have been coated with an
electrically isolating material before mixing the second powder
with the first powder.
42. Method according to claim 37, further comprising: coating a
plurality of particles of the first powder and a plurality of
particles of the second powder with an electrically isolating
material after mixing the first powder with the second powder.
43. Method according to claim 37, wherein the compacting is carried
out under a pressure of 10 MPa to 3000 MPa.
44. Method according to claim 37, wherein the compacting comprises
rolling or pressing.
45. Method according to claim 37, further comprising: enveloping
the powder mixture in a mantle before compaction.
46. Method according to claim 37, wherein the compacting is carried
out at room temperature or at a temperature between 30.degree. C.
to 250.degree. C.
47. Method according to claim 37, further comprising: heat treating
the composite article after the compacting.
48. Method according to claim 47, wherein the heat treating is
carried out at a temperature between 300.degree. C. and
1200.degree. C. for a time between 2 hours and 10 hours.
49. Method according to claim 47, wherein the heat treating is
carried out under vacuum or in a protective atmosphere.
50. Method according to claim 37, further comprising: compacting
the composite article a second time.
51. Method according to claim 37, further comprising: carrying out
a finishing procedure.
52. Method according to claim 37, further comprising applying an
outer protective coating to the composite article.
53. Method according to claim 52, wherein the applying the outer
protective coating comprises dipping, spraying, or
electro-deposition.
54. Method according to claim 52 to claim 53, wherein the applying
of the outer protective coating occurs after a sintering heat
treatment of the composite articles-carried-out.
55. Method according to claim 52, wherein the applying of the outer
protective coating occurs after compacting of composite
article.
56. Composite article according to claim 9, wherein the inclusions
either: (1) have a microstructure comprising a plurality of grains
of magnetocalorically active material that have lattice structure
that exhibits strain relief, and that, at least in part, touch one
or more immediately neighboring grains of magnetocalorically active
material via an interface comprising interdiffused atoms from the
neighboring grains, resulting from sintering a compressed powder of
magnetocalorically active material; or (2) form a series of foils,
flakes, or tapes embedded in the matrix.
57. Composite article according to claim 56, wherein the inclusions
have a microstructure comprising a plurality of grains of
magnetocalorically active material that have lattice structure that
exhibits strain relief, and that, at least in part, touch one or
more immediately neighboring grains of magnetocalorically active
material via an interface comprising interdiffused atoms from the
neighboring grains, resulting from sintering a compressed powder of
magnetocalorically active material.
58. A refrigeration system comprising the heat exchanger according
to claim 36.
59. An industrial, commercial, or domestic freezer comprising the
refrigeration system according to claim 58.
60. A climate control device comprising the heat exchanger
according to claim 36.
61. An air-conditioning unit comprising the climate control device
according to claim 60.
Description
BACKGROUND
[0001] 1. Field
[0002] Disclosed herein is a composite article with
magnetocalorically active material and to methods for producing a
composite article with magnetocalorically active material.
[0003] 2. Description of Related Art
[0004] The magnetocaloric effect describes the adiabatic conversion
of a magnetically induced entropy change to the evolution or
absorption of heat. Therefore, by applying a magnetic field to a
magnetocaloric material, an entropy change can be induced which
results in the evolution or absorption of heat. This effect can be
harnessed to provide refrigeration and/or heating.
[0005] In recent years, materials such as
La(Fe.sub.1-aSi.sub.a).sub.13, Gd.sub.5(Si, Ge).sub.4, Mn (As, Sb)
and MnFe(P, As) have been developed which have a Curie Temperature,
T.sub.c, at or near room temperature. The Curie Temperature
translates to the operating temperature of the material in a
magnetic heat exchange system. Consequently, these materials are
suitable for use in applications such as building climate control,
domestic and industrial refrigerators and freezers as well as
automotive climate control.
[0006] Magnetic heat exchange technology has the advantage that
magnetic heat exchangers are, in principle, more energy efficient
than gas compression/expansion cycle systems. Furthermore, magnetic
heat exchangers are environmentally friendly as ozone depleting
chemicals such as CFC's are not used.
[0007] Consequently, magnetic heat exchanger systems are being
developed in order to practically realise the advantages provided
by the newly developed magnetocaloric materials. Magnetic heat
exchangers, such as that disclosed in U.S. Pat. No. 6,676,772,
typically include a pumped recirculation system, a heat exchange
medium such as a fluid coolant, a chamber packed with particles of
a magnetic refrigerant working material which displays the
magnetocaloric effect and a means for applying a magnetic field to
the chamber.
[0008] Further developments of these systems have been directed
towards optimizing the composition of the magnetocaloric material
so as to increase the entropy change and to increase the
temperature range over which the entropy change occurs. This
enables smaller applied magnetic fields to be used to achieve
sufficient cooling and a stable refrigeration cycle to be achieved
over a larger temperature range.
[0009] These measures aim to simplify the design of the heat
exchange system since smaller magnetic fields can be produced by a
permanent magnet rather than an electromagnet or even a
superconducting magnet.
[0010] The magnetic refrigerant working material may also be
provided in the form of a composite. For example, U.S. Pat. No.
6,826,915 discloses a regenerative bed comprising a magnetic
refrigeration material including a binder comprising a metal or
alloy of high ductility and a magnetocaloric material of the
NiAs-type.
[0011] However, further improvements are desirable to enable a more
extensive application of magnetic heat exchange technology.
SUMMARY
[0012] Disclosed herein are embodiments of a composite article
comprising magnetocalorically active material which can be reliably
and cost effectively manufactured. Also disclosed herein are
embodiments of methods by which the composite article may be
produced.
[0013] A particular embodiment relates to a composite article
comprising a plurality of inclusions of a magnetocalorically active
material embedded in a matrix of a magnetocalorically passive
material. At least the matrix has a microstructure characteristic
of a compacted powder. Desirably, the inclusions and the matrix
have a microstructure characteristic of a compacted powder.
[0014] In another particular embodiment is disclosed a composite
article having a solid matrix comprising a magnetocalorically
passive material, and having a microstructure characteristic of a
sintered powder, and having a plurality of inclusions comprising a
magnetocalorically active material at least partially embedded in
the matrix. Desirably, the inclusions may also have a
microstructure characteristic of a sintered powder.
[0015] In another particular embodiment is disclosed a method of
manufacturing a composite article comprising: [0016] providing a
first powder comprising a magnetocalorically active material,
[0017] providing a second powder comprising a magnetocalorically
passive material, [0018] mixing the first powder and the second
powder together to form a powder mixture, and [0019] compacting the
powder mixture to form a composite article.
[0020] In another embodiment is disclosed a heat exchanger
comprising one or more of the composite articles disclosed herein.
This heat exchanger may form part of a refrigeration system, such
as a individual, commercial, or domestic freezer, or may form part
of a climate control device, such as an air conditioning unit.
BRIEF DESCRIPTION OF DRAWINGS
[0021] Embodiments described herein will now be described in more
detail with reference to the drawings.
[0022] FIG. 1 is a schematic diagram showing a composite article
according to a first embodiment disclosed herein,
[0023] FIG. 2 is a schematic diagram showing a composite article
according to a second embodiment disclosed herein,
[0024] FIG. 3 is a schematic diagram showing a cross-sectional view
along the line A-A of the composite article of FIG. 2,
[0025] FIG. 4 is a schematic diagram showing a composite article
according to a third embodiment disclosed herein,
[0026] FIG. 5 is a schematic diagram showing a composite article
according to a fourth embodiment disclosed herein.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0027] Herein, the term "matrix" is defined as the
magnetocalorically passive material, irrespective of its volume
fraction and distribution.
[0028] A "magnetocalorically active material" is defined herein as
a material which undergoes a change in entropy when it is subjected
to a magnetic field. The entropy change may be the result of a
change from ferromagnetic to paramagnetic behaviour, for example.
The magnetocalorically active material may exhibit in only a part
of a temperature region an inflection point at which the sign of
the second derivative of magnetization with respect to an applied
magnetic field changes from positive to negative.
[0029] A "magnetocalorically passive material" is defined herein as
a material which exhibits no significant change in entropy when it
is subjected to a magnetic field.
[0030] "Inclusion" is defined herein as a particle or grain having
a first composition which is at least partially embedded in the
matrix comprising a different second composition. Inclusion also
includes an agglomerate of particles or grains of a first
composition which is at least partially embedded in the matrix
comprising a second different composition material. A particle or
agglomerate which has a portion exposed from the matrix, for
example this exposed portion may provide an outer surface of the
composite article, is included in this definition of
"inclusion."
[0031] A "microstructure characteristic of a compacted powder" can
be identified by examining a polished cross-section of the
composite article using light microscopy or Scanning Electron
Microscopy. A compacted powder has a microstructure which comprises
a plurality of particles which at least in part touch their
immediate neighbours, i.e. the contiguous particles, without
interdiffusion of atoms between the contiguous particles.
[0032] The particles of a compacted powder also have lattice
structure which exhibits strain and/or fractures which have
occurred as a result of the pressure applied during compaction.
These structural defects can be observed using Transmission
Electron Microscopy.
[0033] The composite article described herein has the advantage
that it can be simply produced by mixing together a first powder of
a magnetocalorically active material and a second powder of a
magnetocalorically passive material and then compacting the powder
mixture to form a composite article. The composite article may be
used in a magnetic heat exchange system as the magnetic working
material without undergoing a subsequent heat treatment. Therefore,
the inclusions and the matrix of the composite article after
compacting have a "microstructure characteristic of a compacted
powder."
[0034] The matrix of the composite article may be thought of as a
binder which holds the inclusions of magnetically active material
together to form a composite article. Thus magnetocalorically
active material, which, particularly in the case of materials such
as Gd, La(Fe.sub.i-bSi.sub.b).sub.13-based phases, Gd.sub.s(Si,
Ge).sub.4-based phases, Mn(As, Sb)-based phases, MnFe(P, As)-based
phases, Tb--Gd-based phases, (La, Ca, Pr, Nd, Sr)MnO.sub.3-based
phases, Co--Mn--(Si, Ge)-based phases and Pr.sub.2(Fe,
Co).sub.17-based phases is brittle, can be formed into a composite
article having sufficient mechanical stability to be used in a heat
exchanger system due to the presence and properties of the matrix
of magnetocalorically passive material.
[0035] The composite article has the advantage that the
magnetocalorically active material and magnetocalorically passive
material may be fabricated using known techniques. Moreover, these
materials may be fabricated separately from one another before
being mixed together to form the starting mixture for the composite
article. Consequently, the fabrication of the two powders can be
optimised separately from the optimisation of the composite
article.
[0036] By controlling the compaction process appropriately, the
composite article can be produced having the dimensions desired for
a particular magnetic heat exchange system. The composite article
may be produced having a near-net shape simply by compacting the
powder mixture. The composite article is, therefore, easily
produced using a simple, cost-effective manufacturing process.
[0037] The magnetocalorically active material may be used as
supplied or as fabricated or may be given an additional heat
treatment in order to improve the magnetocaloric properties of the
powder before it is mixed with the magnetocalorically passive
powder which later forms the matrix.
[0038] In a first embodiment, the matrix has an average particle
size of less than 1000 .mu.m. In a further embodiment, the
inclusions have an average diameter of less than 1000 .mu.m.
[0039] The average particle size of the matrix is defined herein as
the mean of the longest dimension of each of 20 randomly selected
particles in a polished cross-section of the composite article. In
embodiments where the matrix has a microstructure characteristic of
a sintered powder, this is essentially synonymous with the grain
size of the matrix.
[0040] The average diameter of the inclusions is defined herein as
the mean of the longest dimension of each of 20 randomly selected
inclusions in a polished cross-section of the composite
article.
[0041] In a further embodiment disclosed herein, a composite
article is provided which comprises a plurality of inclusions of a
magnetocalorically active material embedded in a matrix of a
magnetocalorically passive material, wherein the matrix has a
microstructure characteristic of a sintered powder.
[0042] The "microstructure characteristic of a sintered powder" can
be identified by examining a polished cross-section of the
composite article using light microscopy or Scanning Electron
Microscopy. A sintered powder has a microstructure which comprises
a plurality of grains which at least in part touch their immediate
neighbours, i.e. the contiguous grains, wherein the interface
between contiguous grains comprises material that results from
interdiffusion of atoms of the materials of the contiguous
grains.
[0043] Furthermore, as a result of the sintering process, the
grains will generally have undergone strain relief. This strain
relief can be observed using Transmission Electron Microscopy
techniques since the dislocation density and lattice strain within
the grains that have undergone strain relief are lower than those
of compacted unsintered powder.
[0044] The composite article comprising a matrix having a
microstructure characteristic of a sintered powder has the
advantage that the mechanical strength of the composite article is
increased due to the sintering process. This simplifies the
handling of the composite article during any later production
steps, for example surface finishing of the composite article,
during storage and delivery of the composite article, as well as
during the assembly of the composite article into a magnetic heat
exchange system. Furthermore, in the case that the magnetic heat
exchange system is of the type in which the magnets remain
stationary and the magnetic working material is physically moved
relative to the magnets, there is reduced likelihood of cracking
and damage to the magnetic working material as a result of this
movement when the composite article described herein is used.
[0045] The composite article comprising a matrix having a
microstructure characteristic of a sintered powder is also easily
produced using known powder processing techniques. For example, a
powder of a magnetocalorically passive material may be mixed with
magnetocalorically active material, and the powder mixture may be
compacted and then sintered by applying heat.
[0046] This embodiment has the advantage that the composite article
may be manufactured by embedding magnetocalorically active material
in powder of a magnetocalorically passive material. This
intermediate may then be sintered to produce the composite
article.
[0047] In an embodiment, the inclusions comprising a
magnetocalorically active material are provided in the form of a
powder. In this embodiment, the inclusions as well as the matrix
have a microstructure characteristic of a sintered powder.
[0048] Alternatively, the inclusions may be provided in forms other
than that of powder. For example, the inclusions may be provided in
the form of a skeleton structure which is embedded in a powder of a
magnetocalorically passive material or in the form of a series of
foils, flakes, filaments or tapes which are embedded in a powder of
a magnetocalorically passive material which forms the matrix of the
composite article.
[0049] The matrix and possibly the inclusions may have a grain
structure characteristic of a sintered powder. This characteristic
grain structure can be identified using light microscopy and or
Scanning Electron Microscopy techniques. The most appropriate
identification technique may depend on the average grain size. The
grain structure of a sintered powder is characterised by the grain
boundaries between adjacent grains, in particular between
contiguous grains comprising the same material. These grains are
atomically joined to one another due to interdiffusion between the
contiguous grains at the interface and generally have a
misorientation with respect to one another. The microstructure of
the grains and of the grain boundaries can be observed using
Transmission Electron Microscopy techniques.
[0050] The average grain size of the sintered composite article, in
particular of the matrix, is also typically greater than the
average particle size from which the article was produced. For
example, the average grain size of the matrix is greater than the
average particle size of the magnetocalorically passive powder.
[0051] In an embodiment, the magnetocalorically active material has
a Curie temperature T.sub.c in the range 220K to 345K. The
operating temperature of the magnetocalorically active material,
when used in a magnetic heat exchange system, is approximately that
of its Curie temperature. A magnetocalorically active material with
a Curie temperature in the range 220K to 345K is suitable for
applications such as domestic and commercial freezer systems,
refrigeration, air conditioning or climate control systems
depending on the desired operating temperature and operating
temperature range.
[0052] The magnetocalorically active material is one of Gd, a
La(Fe.sub.1-bSi.sub.b).sub.13-based phase, a Gd.sub.5(Si,
Ge).sub.4-based phase, a Mn(As, Sb)-based phase, a MnFe(P,
As)-based phase, a Tb--Gd-based phase, a (La, Ca, Pr, Nd,
Sr)MnO.sub.2-based phase, a Co--Mn--(Si, Ge)-based phase and a
Pr.sub.2(Fe, Co).sub.17-based phase. These basic compositions may
further comprise other chemical elements which may substitute
partially or in full for the listed elements. These phases may also
comprise elements which are accommodated at least in part
interstitially within the crystal structure, for example, hydrogen.
These phases may also include impurity elements and small amounts
of elements such as oxygen.
[0053] The matrix may comprise a variety of materials. In an
embodiment, the matrix comprises a material which has a higher
thermal conductivity than the thermal conductivity of the
inclusions of the magnetocalorically active material. This has the
advantage that the efficiency of the heat exchange between the
magnetocalorically active material and the heat exchange medium is
increased.
[0054] In an embodiment, the matrix comprises a metal or an alloy
which may comprise one or more of the elements Al, Cu, Ti, Mg, Zn,
Sn, Bi and Pb.
[0055] The matrix may, alternatively or in addition, comprise a
ceramic such as one or more of BeO, AlN, BN and graphite. In a
further embodiment, the matrix comprises a metal or alloy and a
ceramic. In a more particular embodiment, the ceramic may be
provided in the form of inclusions within a metal or alloy
matrix.
[0056] In a further embodiment, some or all of the inclusions may
comprise a metallic corrosion protection coating. This corrosion
protection coating may comprise one or more of Al, Cu and Sn. The
corrosion protection coating may be applied to increase the working
life of the magnetocalorically active phase since the corrosion and
degradation of the magnetocalorically active material into
non-magnetocalorically active phases is at least slowed, or even
prevented entirely, over the working lifetime of the
magnetocalorically active material, due to the corrosion protection
coating.
[0057] In a further embodiment, the inclusions comprise an outer
coating of an electrically isolating material. In the case of a
composite article comprising inclusions and matrix having a
microstructure characteristic of a compacted powder, the particles
of the matrix may also comprise an outer coating of an electrically
isolating material.
[0058] In the case of a composite article having a matrix with a
microstructure characteristic of a sintered powder, the grain
boundaries between the grains of the matrix as well as between the
inclusions and the matrix may comprise an electrically isolating
layer. The electrically isolating material may comprise a polymer
or a ceramic such as BeO, AlN, BN or inorganic compounds such as
silicates, oxides and phosphates.
[0059] A mixture of a ceramic and a metal or alloy included in the
matrix or in the form of an outer coating of the inclusions and/or
particle or grains of the matrix has the advantage that the average
electrical resistivity of the matrix is increased with respect to a
matrix that does not include the ceramic, e.g., a matrix primarily
compound of a metal or alloy. This reduces the effect of eddy
currents during the application and removal of the magnetic field
which further increases the efficiency of the magnetic heat
exchange system.
[0060] The electrically isolating coating may also provide
corrosion protection for the magnetocalorically active material
since it provides a physical barrier between the magnetocalorically
active particle and the surrounding environment.
[0061] An electrically isolating coating has the advantage that the
layer thickness may be small. Therefore, a thin ceramic coating may
be provided on the particles to electrically isolate the particles
from the contiguous particles without increasing the brittleness of
the matrix.
[0062] The material of the matrix may be selected in order to
optimise a variety of additional properties. For example, the
material of the matrix may be selected so as to be easily worked by
mechanical deformation techniques such as extrusion, rolling,
drawing and swaging, for example. The material may, therefore, be
ductile and easily workable at room temperature with or without
intermediate annealing at temperatures of a few hundred .degree. C.
This matrix material enables a composite article to be fabricated
which can be easily worked after the initial fabrication of the
composite article. This has the advantage that the composite
article may be formed by mechanical deformation techniques into the
desired final form.
[0063] Alternatively, or in addition, the material of the matrix
may have a high thermal conductivity. This has the advantage that
the efficiency of the heat transfer from the magnetocalorically
active material to the heat exchange medium which is in contact
with outer surfaces of the composite article during the operation
of the composite article in a magnetic heat exchange system is
increased.
[0064] The mechanical properties of the matrix may also be adjusted
by selection of a single material or by selection of a mixture of
materials, for example one or more ceramics and one or more metals
or alloys. Thus the mechanical strength of the composite article
can be increased in order to further limit possible damage to the
composite article during its use in a magnetic heat exchange
system.
[0065] In a particular embodiment the composite article comprising
at least a matrix having a microstructure characteristic of
sintered powder may have a matrix comprising an average grain size
of less than 1000 .mu.m and/or inclusions having an average
diameter of less than 1000 .mu.m.
[0066] The relative proportions of the magnetocalorically active
material and of the magnetocalorically passive material may also be
selected to provide the desired heat exchange characteristics for a
desired total volume of the composite article. For example, the
lower limit of the volume fraction of magnetocalorically active
material may be limited by the desired heat exchange capacity,
whether that be cooling capacity or heating capacity.
[0067] The upper limit of the volume fraction of magnetocalorically
active material may be limited by the production of the composite
article and/or its workability and/or corrosion stability. For
example, if the volume fraction of the magnetocalorically passive
material is too low, then the advantages produced by using a
ductile material in order to improve the workability of the
compacted article and/or the sintered composite article may not be
achieved to the desired degree.
[0068] If the magnetocalorically active material corrodes in the
heat exchange medium, the matrix may not provide a sufficiently
good coverage of the inclusions to prevent the magnetocalorically
active material coming into contact with the heat exchange medium
if the fraction of the magnetocalorically passive material
providing the matrix is too low and the volume fraction of the
magnetocalorically active material is too high. The volume fraction
of inclusions may be between 25% and 98%, preferably between 60%
and 95%.
[0069] In a further embodiment, the matrix comprises a soft
magnetic material. This has the advantage that the effective
magnetic air gap between the magnets and the composite article in
the magnetic heat exchange system is reduced. Therefore, the
cooling and/or heating effect provided by the magnetocalorically
active material of the composite article can be increased. In a
particular embodiment, the soft magnetic material may comprise one
or more of Fe, FeSi, Co or Ni.
[0070] In a further embodiment, the composite article comprises a
plurality of magnetocalorically active materials, each having a
different Curie temperature. This may be provided by adjusting the
composition of a single magnetocalorically active phase or by
providing different magnetocalorically active phases.
[0071] In a first arrangement of such an embodiment, the plurality
of magnetocalorically active materials are distributed essentially
homogeneously throughout the volume of the composite article.
[0072] In another arrangement of such an embodiment, the composite
article comprises a plurality of layers, each layer comprising
inclusions of a magnetocalorically active material having a T.sub.c
which is different to the T.sub.c of the magnetocalorically active
material in an adjacent layer. In a more particular arrangement,
the magnetocalorically active material of each of the layers is
selected, along with the order in which the layers are arranged, in
order that the T.sub.c progressively increases from one end of the
composite article to the other.
[0073] The use of composite articles comprising a plurality of
magnetocalorically active materials having different T.sub.c's, has
the advantage that the operating range of the heat exchanger in
which the composite article is used is increased. The Curie
temperature T.sub.c translates to the operating temperature and,
since a range of T.sub.c's are provided, the operating range of the
heat exchanger is increased. This enables the heat exchanger to
provide cooling and/or heating over a wider operating temperature
range and to provide cooling and/or heating from a starting
temperature to a smaller/larger lowermost/uppermost temperature,
respectively, than that possible using magnetocalorically active
material having a single T.sub.c.
[0074] In a particular embodiment, the composite article may
further comprise an effective porosity. "Effective porosity" is
used herein to describe a porosity of the composite article which
has a measurable effect on the efficiency of the magnetic heat
exchange.
[0075] The effective porosity comprises at least one channel within
the body of the composite article in flow communication from a
first side of the composite article to a second side of the
composite article. The effective porosity may be in the range of 10
vol. % to 60 vol. %.
[0076] In a more particular embodiment, the effective porosity may
be provided in the form of a series of interconnected channels in
flow communication with each other forming a hollow network or
skeleton type structure within the body of the composite article.
The heat exchange fluid can then flow through the hollow network
from one side of the composite article to the other.
[0077] In a particular embodiment, the effective porosity may be
provided by loosely compacting the powder or by loosely compacting
the powder followed by sintering to form in each case a body with a
density of less than 100% such that at least a portion of the
unoccupied volume provides an interconnected hollow network though
which the heat exchange medium can flow.
[0078] These embodiments of a composite article having effective
porosity have the advantage that the surface area of the composite
article in contact with the heat exchange medium is increased. The
heat exchange fluid is in contact with inner surfaces, that is the
surfaces of the channels providing the porosity which are
positioned within the body of the composite article, as well as
with the overall outer surface of the composite article. Thus, the
contact area between the composite article and the heat exchange
fluid is increased. Consequently, the efficiency of the magnetic
heat exchange may be further increased.
[0079] In a particular embodiment, the composite article may
further comprise at least one channel. The channel may be provided
in the form of a through-hole which is surrounded by the composite
article or may be provided in the form of a channel in an outer
surface of the composite article. One or more channels have the
advantage of increasing the surface area of the composite article
which can further improve the heat exchange efficiency between the
composite article in contact with the heat exchanger medium and the
heat exchange medium. The channel may be formed by extrusion or
profile rolling, for example.
[0080] In a further embodiment, the channel is adapted to direct
the flow of the heat exchange medium. The position of the channel
is determined by the design of the heat exchange system in which
the composite article is to operate. The channel may be adapted to
direct the flow of the heat exchange medium with reduced or,
optimally, minimum turbulence, in order to increase the efficiency
of the heat exchange.
[0081] In a particular embodiment, the composite article may be a
component of a heat exchanger, a cooling system, an air
conditioning unit for a building or a vehicle, in particular an
automobile, or a climate control device for a building or an
automobile. The climate control device may be used as a heater in
winter and as a cooler in summer by reversing the direction of the
heat-exchange fluid (e.g., fluid coolant or heat exchanger medium).
This is particularly advantageous for automobiles and other
vehicles as the space available within the chassis for
accommodating the climate control system can be limited by the
design of the vehicle.
[0082] In a particular embodiment, composite article may also
comprise an outer protective coating. The outer protective coating
may comprise a metal, an alloy or a polymer. The material of the
outer protective coating may be chosen so as to be chemically, as
well as mechanically, stable during the lifetime operation of the
composite article in the heat exchange medium. If the coating is
applied to the finished article, in a particular embodiment it is
not subjected to higher temperatures, for example during sintering,
or working of the composite article. In this case, a polymer with a
relatively low decomposition temperature or melting temperature may
be used.
[0083] In certain embodiments, heat exchange medium may comprise
ethanol, or glycol, mixtures of water, ethanol or glycol, or an
alternative material with a high thermal conductivity, in order to
increase the efficiency of the heat exchange between the heat
exchange medium and the composite article. The heat exchange medium
may be corrosive to the magnetocalorically active material and/or
the magnetocalorically passive material of the matrix. Therefore,
an additional outer protective coating may be used to provide
additional protection.
[0084] Also disclosed herein are methods of manufacturing a
composite article. A particular embodiment of a method comprises:
[0085] providing a first powder comprising a magnetocalorically
active material, [0086] providing a second powder comprising a
magnetocalorically passive material, [0087] mixing the first powder
and the second powder together to form a powder mixture, and [0088]
compacting the powder mixture to form a composite article.
[0089] The first powder and the second powder may be mixed using
known powder mixing techniques, such as ball mixing or attrition
milling. The average particle size of the first powder and the
second powder may also be reduced as a result of the mixing
process.
[0090] The powder mixture may be compacted to form the composite
article using known methods. For example, the powder may be placed
into a die and compacted by applying pressure. The powder may also
be compacted by rolling the powder mixture itself, by rolling a
partially compacted or pre-compacted article or by rolling an
encapsulated preform containing the powder mixture or an at least
partially compacted powder mixture. To produce a preform, the
powder mixture may be enveloped in a mantle before compaction using
a method such as a powder-intube type method.
[0091] The composite article comprising a mixture of compacted
powders of differing compositions may be referred to as a green
body since the composite article has not been subjected to a heat
treatment process. In a first embodiment, the compacted powder
mixture is not subjected to further heat treatment after its
production and is used in a magnetic heat exchanger in this
unsintered condition.
[0092] This embodiment has the advantage that the composite article
is formed directly from a powder mixture without additional heat
treatment. The manufacturing process is simple and cost-effective,
since the use of one or more additional heat treatments on the
composite article is avoided.
[0093] In a more particular embodiment, the first powder comprises
a plurality of particles which are coated with a metallic corrosion
protection coating before the first powder is mixed with the second
powder. The metallic corrosion protection coating may be applied to
the first powder by electro-plating or electroless plating for
example.
[0094] In a further embodiment, the particles of the first powder
are coated with an electrically isolating material before the first
powder is mixed with the second powder.
[0095] If, in a particular embodiment, it is desired to
electrically isolate the particles or grains of the matrix, this
can be performed by coating the particles of the second powder with
an electrically isolating material before the second powder is
mixed with the first powder.
[0096] In order to reduce the effect of eddy currents further, in
particular embodiments the particles of both the first and second
powder may comprise an electrically isolating layer. This can be
applied to the powders before they are mixed together. The
composition of the electrically isolating layer of the first powder
may differ from the composition of the electrically isolating layer
of the second powder. This enables the coating to be selected
specifically for the differing compositions of the two powders.
This may be advantageous to improve the adhesion of the coating to
the particle by the appropriate selection of the composition of the
electrically isolating material.
[0097] Alternatively, the first powder may be mixed with the second
powder and then afterwards the particles of the powder mixture are
coated with the electrically isolating material. In this case both
the particles of the first and second powder comprise an
electrically isolating layer of the same composition.
[0098] In a particular embodiment, the coating of the electrically
isolating material also functions as a corrosion protection
coating. In such an embodiment, an additional corrosion protection
coating may or may not be included.
[0099] In an embodiment, additionally one or more of a lubricant,
an organic binder and a dispersant are added to the powder mixture.
These additive components may be used to improve mixing of the
first powder and the second powder, in the case of a binder and/or
dispersant, and/or to improve the density of compaction which is
achieved during the compaction step, in the case of a lubricant.
The lubricant can, for example, enable the particles to slide over
one another more effectively, thus increasing the density of the
compacted powder mixture and the density of the composite article
both in the compacted state as well as in the sintered state in
embodiments where sintering occurs.
[0100] In a particular embodiment, powder mixture may be compacted
under a pressure in the range of 10 MPa to 3000 MPa. The density of
the composite article generally increases with increasing applied
pressure. This relationship is, however, not linear so that a
practical upper limit to the pressure may arise above which the
increase in density is not outweighed by the increased complexity
involved in applying the higher pressure. If the pressure applied
is too low, then the compacted composite article may not have the
desired mechanical integrity for reliable use in a magnetic heat
exchange system. The pressure may be applied using a method such as
rolling or pressing.
[0101] In a further embodiment, the powder mixture is enveloped in
a mantle before the powder mixture is compacted. The powder mixture
may also be partially compacted to form a self supporting body
before being enveloped in the mantle and further compacted within
the mantle.
[0102] The powder mixture may be compacted at room temperature.
Alternatively, the powder may be compacted as a temperature of
between 30.degree. C. and 250.degree. C.
[0103] An elevated temperature during compacting has the advantage
that the density of the green body can be increased. Dependent on
the temperature, a degree of sintering may also occur during the
compaction process so that the mechanical integrity of the
composite article is further increased. If additives are provided
in the powder mixture, they may also be removed during the
compaction process at an elevated temperature due to their
evaporation and/or decomposition into gaseous components.
[0104] Particular embodiments relate to methods to provide a
composite article in which at least the second powder providing the
matrix is sintered and has a microstructure characteristic of a
sintered powder.
[0105] In a further embodiment, after the powder mixture is
compacted to form a green composite article, a heat treatment is
carried out on the compacted composite article. The heat treatment
may be carried out such that the powder particles of the matrix
sinter together, thus improving the mechanical strength of the
composite article.
[0106] In a particular embodiment, the heat treatment may be
carried out at temperatures between 300.degree. C. and 1200.degree.
C. for a time of between two hours and 10 hours. The temperature
and time may be selected so as to optimise the sintering process
and densification of the composite article and, typically, depends
on the material used for the magnetocalorically active material and
of the magnetocalorically passive material.
[0107] The heat treatment may also be performed in order to improve
the magnetocaloric properties of the magnetocalorically active
material. For example, the T.sub.c may be increased or the
sharp-ness of the transition providing the entropy change can be
increased.
[0108] In particular embodiments, this heat treatment may be
carried out under vacuum or in a protective atmosphere. A
protective atmosphere may be provided by a noble gas, such as
argon, or by a gas which is inert with respect to the composite
article. The gas may be selected so as to avoid degradation of the
magnetocalorically active material and/or magnetocalorically
passive material due to be elevated temperature used for the heat
treatment. The gas may also be selected so that residual undesired
elements such as oxygen and carbon are removed during the heat
treatment.
[0109] During the heat treatment, the composition of the
magnetocalorically active material may be modified by the
interstitial incorporation of elements, such as hydrogen and/or
nitrogen, delivered by the atmosphere.
[0110] In a further embodiment, a second compaction process is
carried out on the composite article. The second compaction process
may be carried out on the compacted composite article resulting
from the first compaction, that is the green body, or on the
composite article after it has been subjected to a heat treatment
and sintered. A second compaction process may be used to further
increase the density of the composite article. A second compaction
process may be advantageous if additives, such as a lubricant,
binder and/or dispersant, are used in the powder mixture which is
removed or burnt out during the first heat treatment. This can
result in porosity within the composite article which can, if
desired, be reduced by means of a second compaction process.
[0111] The degree of compaction achieved during a second compaction
or sizing process may range from 5% to 50%.
[0112] Therefore, in a particular embodiment of the method, the
compacted composite article is first given a heat treatment at a
relatively low temperature to remove additives, and is then
subjected to a second compaction process, and a second heat
treatment at a higher temperature during which sintering takes
place. The result is a composite article in which at least the
matrix has a microstructure characteristic of a sintered
powder.
[0113] After the production of a composite article, whether it
comprises compacted powder or sintered powder, one or more further
finishing procedures may be carried out. A finishing procedure may
be one in which the final profile of the composite article is
produced. The final profile may be produced by grinding and/or
polishing the outer surface of the composite article, for
example.
[0114] An additional possible finishing procedure may comprise
further working of the composite article in order to form the final
desired dimensions. For example, the composite article may be
subjected to the further mechanical deformation process such as
extruding, rolling, drawing and/or swaging, for example. One or
more channels for directing the flow of the heat exchange medium
may be formed in the composite article during one of these
finishing procedures.
[0115] In a particular embodiment, the composite article may be
also be coated with an outer protective coating. This outer
protective coating may be applied by dipping, spraying or
electro-plating.
[0116] In a more particular embodiment, the outer protective
coating may be applied to the composite article after the sintering
heat treatment is carried out. This has the advantage that
thermally sensitive materials may be used for the protective
coating, since the coating is not subjected to the sintering heat
treatment.
[0117] In a further embodiment, the outer protective coating is
applied to the compacted composite article. This method may be used
e.g., when the composite article is to remain unsintered. However,
if e.g., the composite article is to be sintered, the protective
coating may be applied before the sintering heat treatment in order
to protect the intermediate product.
[0118] FIG. 1 illustrates a composite article 1 according to a
first embodiment described herein. The composite article 1
comprises a first powder 2 comprising a magnetocalorically active
material and a second powder 3 comprising a magnetocalorically
passive powder. In this particular example of this embodiment, the
magnetocalorically active powder 2 consists essentially of
La(Fe.sub.1-a-bCo.sub.aSi.sub.b).sub.13 with a T.sub.c of
20.degree. C. and the magnetocalorically passive powder 3 consists
essentially of copper. Copper has a high thermal conductivity and
is ductile so that the composite article 1 can be worked by
mechanical deformation processes if desired.
[0119] The two powders 2, 3 are mixed together and compacted in a
press to produce composite article 1 which is self-supporting and
has a sufficient degree of mechanical strength to be used as the
working component in a non-illustrated magnetic heat exchange
system. The magnetic heat exchange system may be of a known
type.
[0120] The copper particles 3 providing the magnetocalorically
passive powder provide a matrix 4 in which inclusions 5 of
magnetocalorically active powder 2 are embedded. The inclusions 5
are distributed and embedded within the matrix 4. Some of the
inclusions 5 of magnetocalorically active material 2 have portions
6 which are exposed from the matrix 4 and provide regions of the
outer surface 7 of the composite article 1.
[0121] The inclusions 5 and the matrix 4 each have a microstructure
which is characteristic of a compacted powder.
[0122] The copper powder 3 providing the matrix 4 has a higher
thermal conductivity than the magnetocalorically active material 2.
The matrix 4, in addition to functioning as a binder, also improves
the heat exchange efficiency between the magnetocalorically active
inclusions 5 and the heat exchange medium within which the
composite article 1 is positioned when it is in use in a magnetic
heat exchange system.
[0123] FIG. 2 illustrates a composite article 10 according to a
second embodiment disclosed herein.
[0124] The composite article 10 comprises three layers, first layer
11, second layer 12, and third layer 13. Each layer 11, 12, 13
comprises a plurality of inclusions 14, each consisting essentially
of a magnetocalorically active material, which is embedded in a
matrix 15 comprising magnetocalorically passive material 16. In
this particular example of this embodiment, the magnetocalorically
passive material 16 providing the matrix 15 of each of the layers
11, 12, 13 consists essentially of copper. The boundary between
adjacent layers is indicated in the figure by a dashed line 17.
[0125] In a particular example of this embodiment, the composite
article 10 is built up from three layers 11, 12, 13. The lower
layer 13 comprises inclusions 14 of a first magnetocalorically
active material 18 with a T.sub.c of 3.degree. C. The second layer
12 is positioned adjacent to the first layer 13 and comprises
inclusions 14 of second magnetocalorically active material 19 with
a T.sub.c of 15.degree. C., which is greater than the T.sub.c of
3.degree. C. of the first magnetocalorically active material 18 of
the first layer 13. The third layer 13, which is positioned
adjacent to the second layer 12, comprises inclusions 14 of a third
magnetocalorically active phase 20 having a T.sub.c of 29.degree.
C. which is greater than the T.sub.c of 15.degree. C. of the second
magnetocalorically active material of the second layer. The T.sub.c
of the inclusions 14 of the composite 10 increases from one side of
the composite article 10 to the other, i.e., from bottom to top in
the orientation of FIG. 2.
[0126] The illustrated example of this embodiment of composite
article 10 also comprises an outer coating 21 which comprises a
polymer. The polymer can be applied to the composite article 10
after the sintering process by spraying the composite article 10
with the polymer. The outer coating 21 provides a protective
coating which prevents the magnetocalorically active materials 18,
19 and 20 from coming into contact with the atmosphere and heat
exchange medium of the heat exchange system, thus preventing
corrosion of the magnetocalorically active materials 18, 19 and 20
and of the composite article 10.
[0127] The use of a plurality of layers 11, 12, 13 each having a
progressively higher T.sub.c has the advantage that the operating
temperature range of the composite article 10 is increased compared
to that of a composite article comprising a magnetocalorically
active material having a single T.sub.c.
[0128] In a further embodiment not illustrated in the figures, the
composite article comprises a plurality of materials each having a
different T.sub.c. These plurality of materials are mixed
throughout the volume of the composite article rather than being
arranged in layers as illustrated in FIG. 2.
[0129] In contrast to the embodiment illustrated by FIG. 1, the
composite article 10 is subjected to a heat treatment which results
in sintering of the powder 15 and the matrix 16. Therefore, in this
example of this embodiment, the matrix 16 has a microstructure
characteristic of a sintered powder.
[0130] The composite article 10 is fabricated by first fabricating
three different powder mixtures. The first powder mixture comprises
first magnetocalorically active material 18 with a T.sub.c of
3.degree. C. and copper powder, which will form matrix 15, the
second powder comprises second magnetocalorically active material
19 with a T.sub.c of 15.degree. C. and copper powder and the third
powder mixture comprises third magnetocalorically active material
20 with a T.sub.c of 29.degree. C. and copper powder.
[0131] A layer 11 of the first powder mixture is placed into a die,
followed by a layer 12 of the second powder mixture and finally
followed by a layer 13 of the third powder mixture. The layered
arrangement of the powder mixtures is compacted under a pressure of
600 MPa and is then given a sintering heat treatment at
1000.degree. C. in order to produce a sintered composite article
10.
[0132] A plurality of generally parallel channels 22 are then
formed in the outer surface 23 of the composite article by profile
rolling. The channels 22 are illustrated in FIG. 3.
[0133] The channels 22 each have dimensions and are arranged in the
outer surface 23 so as to be adapted to direct the flow of a heat
exchange medium in the direction of the desired heat transfer when
the composite article 10 is positioned in a non-illustrated heat
exchange system. The channels 22 are arranged to extend from the
low T.sub.c first layer 11 to the high T.sub.c third layer 13.
[0134] In the particular embodiment illustrated, composite article
10 is then sprayed with a polymer to provide the outer protective
coating 21.
[0135] In a further non-illustrated embodiment, additionally, one
or more of a lubricant, an organic binder and a dispersant are
added to the powder mixture before its compaction, preferably
before the mixing procedure. A lubricant can help to increase the
density of the compacted composite article 1, 10, as described
above. A binder and/or a dispersant can help to provide a uniform
distribution of inclusions 5, 14 within the matrix 4, 15.
[0136] Although the layered structure has been illustrated in FIG.
2 as comprising three layers, embodiments including any number of
layers, including only a single layer, may be produced by similar
processes for the production of a sintered composite article.
Similarly, a compacted unsintered green composite article with
layers comprising magnetocalorically active material of different
T.sub.c may also be produced using the above process for layering
of the different powders. The process differs from that described
in connection with the embodiment of FIG. 2 in that no heat
treatment is performed on the layered composite article after the
compaction process.
[0137] FIG. 4 illustrates a composite article 30 according to a
third embodiment. The composite article 30 is similar to the
composite article 1 of the first embodiment illustrated in FIG. 1,
but differs from the composite article 1 of the first embodiment in
that the inclusions 5 of first powder of magnetocalorically active
material 2, as well as the matrix 4 (containing in this example,
copper particles as the second powder 3) are coated with an
electrically isolating material 31.
[0138] For clarity of illustration, in FIG. 4 the inclusions 5 of
the magnetocalorically active powder 2 are shaded black while the
magnetocalorically passive copper particles 3 of the matrix 4 are
unshaded. It should also be noted that the thickness of the
electrically isolating coating as well as the size of the particles
has been exaggerated for illustrative purposes.
[0139] The inclusions 5 and the particles of the second powder 3 of
the matrix 4 are electrically isolated from one another by the
electrically isolating coating 31. The provision of the
electrically isolating coating 31 has the advantage that the
production of eddy currents during the application and removal of a
magnetic field are reduced, which leads to an increase in the
efficiency of the magnetic heat exchange system.
[0140] The electrically isolating coating 31 may contain a polymer
or a ceramic, for example. The particles of the two powders 2, 3
may be coated with an electrically isolating material before they
are mixed together or the two powders 2, 3 may first mixed together
and the electrically isolating coating 31 applied to the outer
surfaces of the particles of the powder mixture.
[0141] In a further example of this embodiment, the composite
article 30 is subjected to a heat treatment which results in
sintering of the powder 3 providing the matrix 4, so that the
matrix 4 has a microstructure characteristic of a sintered powder.
In this embodiment, the electrically isolating coating 31 is
present at the grain boundaries between the grains of the sintered
matrix 4 as well as at the grain boundaries between the inclusions
5 and the sintered matrix 4.
[0142] The electrically isolating coating 31 may also provide
protection against corrosion. This is particularly advantageous for
composites in which the inclusions 5 of the magnetocalorically
active material are coated with an electrically isolating coating
or layer, since some magnetocalorically active materials tend to
corrode upon exposure to the atmosphere instead of, or in addition
to, upon exposure to the heat transfer medium.
[0143] FIG. 5 illustrates a composite article 40 according to a
fourth embodiment. The composite article 40 differs from the
previous embodiments in that the composite article further
comprises an effective porosity 41.
[0144] For clarity of illustration, in FIG. 5 the inclusions 5 of
the magnetocalorically active powder 2 are shaded black while the
magnetocalorically passive copper particles 3 are unshaded.
[0145] In the fourth embodiment, the composite article 40 comprises
an effective porosity which enables the heat exchange fluid to pass
through the composite article 40. The porosity 41 comprises a
plurality of interconnected channels 42 providing a skeleton type
structure of channels 42 throughout the composite article 40, a
sufficient number of which are open at one or two ends at the outer
surface 7 of the composite article 40. The heat exchange medium is
thus able to flow from one side 43 of the composite article 41 to
the opposite side 44 as indicated by the arrows through the network
of channels 42. In FIG. 5 only one through channel 42 and a branch
channel 47 from the through channel 42 of the network providing the
porosity 41 are illustrated. However, it will be understood that
other arrangements, numbers of through channels, and degrees of
interconnection, are possible
[0146] The heat exchange fluid is able to contact the inner
surfaces 45 provided by the porous structure 42 as well as the
outer surface 7 of the composite article 40.
[0147] The surface area of the composite article 40 is increased
due to the presence of the effective porosity 41 and, furthermore,
the contact area between the composite article 40 and the heat
exchange medium is increased. Consequently, the efficiency of the
magnetic heat exchange may be further increased.
[0148] In the particular example of the fourth embodiment
illustrated, the inclusions 5 comprising magnetocalorically active
material further comprise a metallic corrosion protection coating
46. The corrosion protection coating 46 is copper and is arranged
between the outer surface of the magnetocaloritally active material
of the individual inclusions 5 and the electrically isolating
coating 31. The inclusions 5, therefore, comprise two coatings.
This has the advantage that the properties of each coating layer
can be optimized separately.
[0149] An additional corrosion protection layer 46 is useful in a
composite article 40 having an effective porosity 41 since some of
the inclusions 5 border on the channels 42 and form a portion of
the surface 45 of the channel 42. The corrosion protection coating
46 provides additional protection for these portions of the
inclusions 5 which are not completely embedded in the matrix 3 and
which can come into contact with the environment and the heat
exchange fluid.
[0150] It should also be noted that the thickness of the corrosion
protection coating 46 and the electrically isolating coating 31, as
well as the size of the particles 3, 5 and the channels 42, 47, has
been exaggerated for illustrative purposes in FIG. 5.
[0151] In a further non-illustrated embodiment, the inclusions 5
include only a metallic corrosion protection coating 46, that is
the inclusions 5 are not provided with an additional outer
electrically isolating layer 31.
[0152] The composite article 1; 10; 30; 40 may be used as the
working component of a heat exchanger, a refrigeration system, a
climate control device, an air-conditioning unit, or an industrial,
commercial or domestic freezer.
[0153] The invention having been thus described with reference to
certain specific embodiments and examples thereof, it will be
understood that this is illustrative, and not limiting, of the
appended claims.
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