U.S. patent application number 10/608733 was filed with the patent office on 2004-12-30 for enhanced magnetocaloric effect material.
Invention is credited to Lewis, Laura J. Henderson.
Application Number | 20040261420 10/608733 |
Document ID | / |
Family ID | 33540664 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040261420 |
Kind Code |
A1 |
Lewis, Laura J. Henderson |
December 30, 2004 |
Enhanced magnetocaloric effect material
Abstract
A magnetocaloric effect heterostructure having a core layer of a
giant magnetocaloric material and an elastically stiff material
layer coated on at least one surface of the magnetocaloric material
layer. The elastically stiff material layer restricts volume
changes of the core layer during application of a magnetic field to
the heterostructure. A magnetocaloric effect composite powder
including a plurality of core particles of a giant magnetocaloric
material. Each of the core particles is encapsulated within a
coating of elastically stiff material that restricts volume changes
of the core particles during application of a magnetic field
thereto. A method for enhancing the magnetocaloric effect within a
giant magnetocaloric material including the step of coating a
surface of the magnetocaloric material with an elastically stiff
material. The elastically stiff material restricts volume changes
of the magnetocaloric material during application of a magnetic
field thereto.
Inventors: |
Lewis, Laura J. Henderson;
(Calverton, NY) |
Correspondence
Address: |
BROOKHAVEN SCIENCE ASSOCIATES/
BROOKHAVEN NATIONAL LABORATORY
BLDG. 475D - P.O. BOX 5000
UPTON
NY
11973
US
|
Family ID: |
33540664 |
Appl. No.: |
10/608733 |
Filed: |
June 30, 2003 |
Current U.S.
Class: |
62/3.1 ; 141/121;
148/101; 148/301 |
Current CPC
Class: |
F25B 2321/001 20130101;
Y02B 30/00 20130101; H01F 10/10 20130101; H01F 1/017 20130101; F25B
21/00 20130101; Y02B 30/66 20130101 |
Class at
Publication: |
062/003.1 ;
148/301; 148/101; 141/121 |
International
Class: |
F25B 021/00; H01F
001/03; B65B 001/04; B67C 003/02; B65B 003/04; H01F 001/04; H01F
001/14; H01F 001/16; H01F 001/18 |
Goverment Interests
[0001] This invention was made with Government support under
contract number DE-AC02-98CH10886, awarded by the U.S. Department
of Energy. The Government has certain rights in the invention.
Claims
1. A magnetocaloric effect heterostructure comprising: a core layer
of a giant magnetocaloric material; and an elastically stiff
material layer coated on at least one surface of said core layer,
said elastically stiff material layer restricting volume changes of
said core layer during application of a magnetic field to said
heterostructure.
2. A magnetocaloric effect heterostructure as defined in claim 1,
wherein said core layer of giant magnetocaloric material is a
compound of Gd.sub.5(Si.sub.1-x Ge.sub.x).sub.4.
3. A magnetocaloric effect heterostructure as defined in claim 1,
wherein said elastically stiff coating layer is a low-coercivity,
high-magnetization material.
4. A magnetocaloric effect heterostructure as defined in claim 3,
wherein said coating layer of low-coercivity, high-magnetization
material comprises an element selected from the group consisting of
iron, cobalt, nickel and magnetic oxides.
5. A magnetocaloric effect heterostructure as defined in claim 1,
wherein said elastically stiff material layer is coated on opposite
surfaces of said core layer.
6. A magnetocaloric effect heterostructure as defined in claim 1,
wherein said elastically still material layer substantially
encapsulates said core layer.
7. A magnetocaloric effect heterostructure as defined in claim 1,
wherein said coating layer is applied to said core layer using a
chemical vapor deposition process.
8. A magnetocaloric effect composite powder comprising a plurality
of core particles of a giant magnetocaloric material, each of said
core particles being encapsulated within a coating of elastically
stiff material, said elastically stiff coating restricting volume
changes of said core particles during application of a magnetic
field thereto.
9. A magnetocaloric effect composite powder as defined in claim 8,
wherein said giant magnetocaloric material of said core particles
is a compound of Gd.sub.5(Si.sub.1-x Ge.sub.x).sub.4.
10. A magnetocaloric effect composite powder as defined in claim 8,
wherein said elastically stiff coating is a low-coercivity,
high-magnetization material.
11. A magnetocaloric effect composite powder as defined in claim
10, wherein said coating of low-coercivity, high-magnetization
material comprises an element selected from the group consisting of
iron, cobalt, nickel and magnetic oxides.
12. A magnetocaloric effect composite powder as defined in claim 8,
wherein said core particles are substantially spherical.
13. A magnetocaloric effect composite powder as defined in claim
12, wherein said core particles have a diameter of about 30 .mu.m
and said coating has a thickness between 50 nm and 200 nm.
14. A magnetocaloric effect composite powder as defined in claim 8,
wherein said coating is applied to said core particles using a
chemical vapor deposition process.
15. A method for enhancing the magnetocaloric effect within a giant
magnetocaloric material comprising the step of coating a surface of
said giant magnetocaloric material with an elastically stiff
material, said elastically stiff material restricting volume
changes of said giant magnetocaloric material during application of
a magnetic field thereto.
16. A method as defined in claim 15, wherein said giant
magnetocaloric material is a compound of Gd.sub.5(Si.sub.1-x
Ge.sub.x).sub.4.
17. A method as defined in claim 15, wherein said elastically stiff
coating is a low-coercivity, high-magnetization material.
18. A method as defined in claim 17, wherein said coating of
low-coercivity, high-magnetization material comprises an element
selected from the group consisting of iron, cobalt, nickel, and
magnetic oxides.
19. A method as defined in claim 15, wherein said giant
magnetocaloric material is coated on opposite surfaces.
20. A method as defined in claim 15, wherein said giant
magnetocaloric material is substantially encapsulated by said
coating.
21. A method of enhancing the magnetocaloric effect within a giant
magnetocaloric material comprising the step of restricting volume
changes of said giant magnetocaloric material during application of
a magnetic field thereto.
22. A method as defined in claim 21, wherein said volume changes of
said giant magnetocaloric material is restricted by a coating of an
elastically stiff material.
23. A method as defined in claim 22, wherein said elastically stiff
material is a low-coercivity, high-magnetization material.
24. A method as defined in claim 23, wherein said low-coercivity,
high-magnetization material comprises an element selected from the
group consisting of iron, cobalt, nickel, and magnetic oxides.
25. A method as defined in claim 21, wherein said giant
magnetocaloric material is a compound of Gd.sub.5(Si.sub.1-x
Ge.sub.x).sub.4.
Description
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to materials that
exhibit thermodynamically first-order magnetic transitions (i.e.,
magnetic state change accompaned by changes in the volume of the
material) and, more particularly, to a method of enhancing the
magnetocaloric effect (MCE) in such giant magnetocaloric
materials.
[0003] The magnetocaloric effect (MCE) describes the adiabatic
conversion of a magnetically-induced entropy change to the
evolution or absorption of heat, with a corresponding rise or
decrease in temperature. FIG. 1 provides a schematic illustration
of the magnetocaloric effect in a material 100. It is usual to
quantify the MCE by the entropy change achieved by a change of
magnetic field 102. The entropy change is determined from dc
magnetic measurement by using the Maxwell relation: 1 ( S ( T , H )
H ) T = ( M ( T , H ) T ) H [ 1 ]
[0004] which can then yield a magnetic entropy change
.DELTA.S.sub.M: 2 S M ( T , H ) = H 1 H 2 ( M ( T , H ) T ) H H . [
2 ]
[0005] In the previous expressions [1] and [2], T is the absolute
temperature and H is the applied magnetic field. Typical
(adiabatic) magnetocaloric temperature changes under an applied
field change of 7 T range from .DELTA.T.sub.ad.apprxeq.2.5 K at
T=10 K for Nd to .DELTA.T.sub.ad.apprxeq.12 K at T.about.180 K for
Dy.
[0006] Systems employing the magnetocaloric effect are important
for energy-efficient, low-CO.sub.2 emission refrigeration, air
conditioning for vehicles and buildings, as well as for sensitive
temperature/heat sensor applications. Materials with a large
magnetocaloric effect may also be utilized as heat pumps and,
compared with the conventional vapor-cycle refrigerator, the
magnetic refrigerator is environmentally benign and has a number of
advantages which include high efficiency, low mechanical vibration
and compact size.
[0007] Briefly, an active magnetic regenerator (AMR) refrigerator
employs a porous bed of a magnetic refrigerant working material
that acts as both the coolant that produces refrigeration and the
regenerator for the heat transfer fluid. As the magnetic working
material is subjected to the application of a magnetic field, the
particles of the material warm in an adiabatic manner from the MCE
and absorb heat from the environment. As fluid flows through the
particle bed from the cold end to the hot end, the working material
warms the fluid via heat transfer. The heat from the fluid is
removed at the hot heat sink in the heat exchanger. After the fluid
flow is stopped, the magnetic field is removed which then causes
the magnetic working material to cool. The hot fluid is forced back
to the now-cool porous bed of material where it is cooled by the
bed. Remnant heat is removed from the fluid by the cold sink in the
cold heat exchanger.
[0008] The potential applications of magnetic refrigeration are
wide-ranging; with properly optimized performance, it is expected
that they will be employed in building climate control, frozen food
processing plants and supermarket chillers. Utilization of these
materials can be envisioned in automotive and aircraft climate
control, with an especially promising application of automotive
climate control for zero-emission electric vehicles.
[0009] Magnetic refrigeration technology can accomplish those
objectives in an environmentally-friendly manner, without the use
of ozone-depleting chemicals such as CFCs (halogenated
chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons), HFCs
(hydrofluorocarbons), PFCs (fluorocarbons) and SF.sub.6 (sulfur
hexafluoride), other hazardous chemicals (NH.sub.3) and the
production of additional greenhouse gases. The energy efficiency
resulting from use of technologies employing magnetic refrigeration
is anticipated to reduce the amount of energy consumed as well as
reduce CO.sub.2 emissions. Thus, two significant benefits of
magnetic refrigeration technology are the replacement of CFC's
(which will reduce the potential for global warming) and designing
climate control in electric vehicles. The latter technology will
allow a greater fraction of the available automobile power to be
used for transportation rather than be exhausted for climate
control.
[0010] With those advantages in mind, significant challenges to the
technological development of these systems exist. Probably the most
daunting technological hurdle remaining is the development of a
cost-effective MCE material requiring practical magnetic fields and
size considerations.
[0011] All magnetic materials, to a greater or lesser degree,
exhibit a magnetocaloric effect. However, some materials, by virtue
of a unique electronic structure or physical nanostructure, may
display a significantly enhanced MCE, which may potentially be
harnessed for technological application. In contrast to the MCE
found in paramagnetic (PM) materials, the large MCE exhibited by
ferromagnetic (FM) materials near their magnetic phase transition
temperature (the Curie temperature T.sub.C) renders them suitable
as working materials for magnetic cooling at temperatures
T>.about.20 K, and up to the target temperatures appropriate for
commercial, industrial and home refrigeration application and heat
pump devices, 200 K-400 K (approximately -70.degree.
C..about.130.degree. C.).
[0012] Further enhancements of the ferromagnetic MCE are possible
if the magnetic transition at T.sub.C is accompanied by a
crystallographic lattice distortion, as is often found in
strongly-correlated electron systems. This lattice distortion may
be either a contraction or an expansion of the atomic lattice, and
may or may not include a change of symmetry. Such coupled
magnetic-crystallographic changes are often referred to as
first-order magnetic phase changes. The enhancement of the MCE
arises because of the overall greater entropy change that occurs
with a combined crystallographic and magnetic phase change. While
materials such as amorphous and nanocrystalline alloys,
intermetallic compounds, perovskite-type oxides and, recently,
pnictides and carbides, have all been topics of vigorous study over
the past 5 years by virtue of their interesting MCE properties, by
far the most promising MCE material is the intermetallic compound
series based on the composition Gd.sub.5(Si.sub.xGe.sub.1-x).sub.4,
where 0.ltoreq.x.ltoreq.0.5.
[0013] The magnetocaloric effect produced by this family of
compounds, heretofore referred to as GdSiGe, has been christened as
"giant" because of its extremely large magnitude. In particular,
the MCE of the GdSiGe alloys is reversible and exceeds that of any
other known magnetic material by a factor of two. Another
noteworthy characteristic of the GdSiGe family is that the Curie
temperature, and hence the MCE, may be tuned with compositional
variation. This feature allows the working temperature of the
magnetic refrigerator to vary from 30 K to 276 K by adjusting the
Si:Ge ratio. Moreover, the GdSiGe series of compositions is
"metamagnetic", which means that the magnetic transition from
ferromagnetic to paramagnetic behavior at the Curie point can be
induced by applied field and pressure as well as increased
temperature.
[0014] However, magnetocaloric materials made from gadolinium (Gd)
and its alloys are generally very expensive and require a very
large and, therefore, impractical magnetic fields on the order of
2-10 T to yield a large magnetocaloric effect. For example, the
typical AMR refrigerator described above, utilizing approx. 3 kg of
Gd spheres, operates near room temperature in applied magnetic
fields between 1.5 T and 5 T. This design provides a temperature
span of 38 degrees for a field change of 5 T, and it generates up
to 600 W of cooling power in a 5 T field with an efficiency that
approaches 60% at 5 T. While these may be impressive figures, a
magnetic field of 5 T can only be generated with a superconducting
magnet that needs liquid helium to operate. Thus, the main drawback
impeding the successful exploitation of the GdSiGe alloys is that
the magnetic field magnitude required to obtain the spectacular
magnetocaloric effect is simply too high for wide-spread
commercial, home and transportation sector use.
[0015] U.S. Pat. No. 5,743,095 to Gschneider, Jr. et al. discloses
an improved Gd.sub.5(Si.sub.1-xGe.sub.x).sub.4 magnetic refrigerant
that provides a high magnetocaloric effect and a high regenerator
efficiency parameter. It is stated that the inclusion of a
magnetically-soft alloying element, such as Mn, Fe, Co or Ni, to
the Gd.sub.5(Si.sub.1-xGe.sub.x) compound optimizes the
magnetocaloric effect properties of the refrigerant. However, the
magnetic fields necessary to influence the magnetocaloric effect of
this improved compound is still obtainable only with
liquid-He-cooled superconducting magnets, which are not practical
additions in typical applications.
[0016] Focusing solely on magnetoresistive materials, U.S. Pat. No.
5,767,673 to Batlogg et al. describes an improved magnetoresistance
obtained in a thin single crystal perovskite
La.sub.2/3Ca.sub.1/3MnO.sub.- 3 at extremely low fields when two
magnetically-soft ferromagnetic (Mn,Zn)Fe.sub.2O.sub.4 bars were
used in close proximity to the perovskite manganite. It is stated
that the magnetically soft material can be thinly layered on the
magnetoresistive core or it can be mixed with a magnetoresistive
material to produce the improved magnetoresistive element. In each
case, it is disclosed that the magnetically soft material placed in
close proximity to the magnetoresistive core serves to increase the
magnetic field experienced by the magnetoresistive core resulting
in an increased magnetoresistive effect at low applied magnetic
fields.
[0017] While such advances have been made in the field of
magnetoresistance, it is clear that innovative material design and
engineering is needed to lower the applied magnetic field necessary
to realize the optimum MCE in magnetocaloric materials. In
particular, there is a great motivation to bridge the gap between
giant magnetocaloric materials and the state-of-the-art AMR
permanent magnet refrigerator design.
SUMMARY OF THE INVENTION
[0018] The present invention involves the application of a
conformal thin layer of an elastically stiff material to a giant
magnetocaloric material to effectively amplify the magnetic field
applied to the magnetocaloric material and thereby lower the
critical field necessary to realize a large MCE without adversely
affecting the functionality of the material.
[0019] Thus, the present invention is a magnetocaloric effect
heterostructure having a core layer of a giant magnetocaloric
material and an elastically stiff material layer coated on at least
one surface of the magnetocaloric material layer. The elastically
stiff material layer restricts volume changes of the core layer
during application of a magnetic field to the heterostructure.
[0020] Preferably, the core layer of magnetocaloric material is a
compound of Gd.sub.5(Si.sub.1-x Ge.sub.x).sub.4 and the elastically
stiff coating layer is a low-coercivity, high-magnetization
material selected from the group consisting of iron, cobalt, nickel
and magnetic oxides. Additionally, the elastically stiff material
layer is preferably coated on opposite surfaces of the core layer
and, more preferably, substantially encapsulates said core
layer.
[0021] In a preferred embodiment, the present invention is in the
form of a magnetocaloric effect composite powder including a
plurality of core particles of a giant magnetocaloric material.
Each of the core particles is encapsulated within a coating of
elastically stiff material that restricts volume changes of the
core particles during application of a magnetic field thereto.
[0022] Again, the material of the core particles is a compound of
Gd.sub.5(Si.sub.1-x Ge.sub.x).sub.4 and the elastically stiff
coating is a low-coercivity, high-magnetization material selected
from the group consisting of iron, cobalt, nickel and magnetic
oxides. Also, the core particles are preferably substantially
spherical and have a diameter of about 30 .mu.m, whereas the
coating has a thickness between 50 nm and 200 nm.
[0023] The present invention further involves a method for
enhancing the magnetocaloric effect within a giant magnetocaloric
material including the processing step of coating a surface of the
magnetocaloric material with an elastically stiff material. The
elastically stiff material restricts volume changes of the
magnetocaloric material during application of a magnetic field
thereto.
[0024] In another method of enhancing the magnetocaloric effect
within a giant magnetocaloric material according to the present
invention, volume changes of a giant magnetocaloric material are
restricted during application of a magnetic field thereto.
Preferably, the volume changes of the magnetocaloric material is
restricted by a coating of an elastically stiff material.
[0025] As a result of the present invention, it is possible to
significantly lower the applied magnetic fields necessary to obtain
a large heating or cooling effect in giant magnetocaloric materials
such as the Gd.sub.5(Si.sub.1-xGe.sub.x).sub.4 family of compounds.
It is envisioned that this composite architecture can be easily
deployed in many materials geometries and forms. The present
invention can produce an effect that resembles amplification of an
applied magnetic field and an increased or enhanced magnetocaloric
effect.
[0026] Other objects, advantages and features of the present
invention will become apparent from the following detailed
description considered in conjunction with the accompanying
drawings. It is to be understood, however, that the drawings are
designed as an illustration only and not as a definition of the
limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a schematic diagram illustrating the
magnetocaloric effect (MCE) in materials.
[0028] FIG. 2 is a cross-sectional view of an enhanced
magnetocaloric effect material formed in accordance with the
present invention.
[0029] FIG. 3 is a cross-sectional view of the preferred embodiment
of the enhanced magnetocaloric effect material of the present
invention.
[0030] FIG. 4 is a graph showing selected magnetization curves at
three representative temperatures for prepared samples of the
enhanced magnetocaloric effect material of the present
invention.
[0031] FIG. 5 is a graph showing the temperature dependence of the
initial .chi..sub.dc of the samples shown in FIG. 4.
[0032] FIG. 6 is a graph showing the onset field differences
.DELTA.H.sub.onset between the samples shown in FIG. 4.
[0033] FIG. 7 is a graph showing the entropy changes determined for
internal field changes of 1 T and 5 T for the samples shown in FIG.
4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] In general, the present invention is a magnetocaloric effect
(MCE) heterostructure having two different phase components. FIG. 2
shows a cross-section of an enhanced MCE heterostructure material
10 of the present invention in its simplest form. The enhanced MCE
heterostructure 10 includes a first component consisting of a giant
magnetocaloric effect compound core 12. As mentioned above, a
"giant magnetocaloric material" is a magnetic material that
undergoes a coupled magnetic and structural change under the
application of a magnetic field. The MCE core 12 of the present
invention is preferably a compound of Gd.sub.5(Si.sub.1-xGe.sub.x-
).sub.4. The Gd-based core 12 is coated on at least one surface
with a second component consisting of an elastically stiff material
14. An "elastically stiff" material, as defined herein, is a
medium- to high-strength material that can restrict volume changes
of the core 12 when a magnetic field is applied thereto. It has
been found that any material having a Modulus of Elasticity of more
than about 0.1.times.10.sup.6 psi will meet this criteria. However,
preferable elastically stiff materials will typically have a
Modulus of Elasticity in the range of 10.times.10.sup.6
psi-40.times.10.sup.6 psi and may include iron (Fe), cobalt (Co),
copper (Cu), nickel (Ni), titanium (Ti), silicon (Si), aluminum
(Al) and alloys thereof. Moreover, as will be discussed further
below, it has been found that low-coercivity (magnetically soft),
high-magnetization materials within this group, such as iron,
cobalt, nickel and ferro- or ferrimagnetic magnetic oxides, are
particularly well suited as a coating material 14 for the Gd-based
compound core 12.
[0035] The magnetic field amplification coating 14 may be applied
to slices of an arc-melted Gd.sub.5(Si.sub.1-5Ge.sub.2.5) core 12.
In the embodiment shown in FIG. 2, the coating 14 is present as two
conformal thin layers, having an approximate thickness of 100 nm
(0.1 .mu.m), coated on opposite sides of the Gd-based core 12,
whereas the Gd-based core has an approximate thickness of 0.6 mm.
Thus, in this manner, a magnetic nanocomposite is created.
[0036] In the case where the coating 14 is a magnetically soft
material, both components have distinct magnetocaloric properties
at their respective ordering temperatures. As mentioned above, the
Gd-based compound core alone exhibits a "giant" magnetocaloric
effect around its magnetic ordering transition due to a first-order
coupled magnetic-crystallographic transformation driven by the
reversible destruction and reformation of specific covalent
Si(Ge)--Si(Ge) bonds. The crystallographic transformation occurs
via a martensitic-like collective shear movement of sub-nanometer
thick building slabs. The family of Gd.sub.5(Si.sub.1-xGe.sub.x)
compounds is metamagnetic (i.e., the crystal structure
transformation from the ferromagnetic orthorhombic
Gd.sub.5Si.sub.4-type (Pnma) structure to paramagnetic monoclinic
Gd.sub.5Si.sub.2Ge.sub.2-type (P1121/a) structure can be easily
induced by changes of temperature, magnetic field and pressure in
the vicinity of the Curie temperature). While the metamagnetic
character is reversible, it does have a temperature and field
hysteresis consistent with the first-order thermodynamic nature of
the phase change. Gd.sub.5Si.sub.1-5Ge.sub.2.5 is a magnetically
heterogeneous system in the region of its first-order phase
transition temperature characterized by coexistence of a
magnetically-ordered (ferromagnetic) and a disordered
(paramagnetic) phase. This two-phase coexistence gives rise to a
unique two-stage initial magnetization behavior, wherein the first
stage corresponds to magnetization of the ferromagnetic phase via
domain-wall movement and magnetization rotation processes, and the
second stage signals a field-induced paramagnetic-to-ferromagnetic
(metamagnetic) transition that occurs at a critical onset field
H.sub.onset.
[0037] It has been found that the application of a thin layer of a
strain- or deformation-resistant coating material 14 to both
surfaces of a slice of Gd.sub.5(Si.sub.1-5Ge.sub.2-5) core 12
effectively lowers the magnitude of the applied magnetic field
required to induce the magnetocaloric effect within the core
without significantly altering the magnitude of the induced MCE
effect. The two phases of the resulting heterostructure complement
each other to provide a "kick" to the MCE effect.
[0038] FIG. 3 shows the preferred embodiment of a MCE nanocomposite
20 formed in accordance with the present invention. In order to
more fully explore and optimize the results of the enhanced
magnetocaloric effect investigations in magnetic nanocomposites it
is preferred to fully encapsulate individual spherical particles 22
of the giant magnetocaloric material Gd.sub.5(Si,Ge).sub.4 with a
coating 24 of an elastically stiff material, as discussed above.
The coating 24 is preferably conformal, homogeneous, uniform and of
precise thickness. As also discussed above, soft magnetic materials
are particularly well-suited as coating materials. The core
particles 22 can be commercial-grade Gd.sub.5(Si,Ge).sub.4 with
nominal diameter of about 30 microns and a variety of coating
compositions 24 and thicknesses can be applied. For example,
coatings of 50 nm, 100 nm and 200 nm with compositions of Fe,
Ni/Fe, Fe/Co, Al and Si can be applied to produce different
samples. Thus, a MCE powder consisting of a plurality of
nanocomposite particles 20 can be produced that can be utilized in
a variety of applications.
[0039] The magnetic composite powder can be made by coating the
GdSiGe particles 22 with an elastically stiff coating 24 using
chemical vapor deposition. A Fast-Fluidized Bed Chemical Vapor
Deposition (FFBCVD) process is ideal to achieve these processing
parameters and objectives. Generally, chemical vapor deposition
(CVD) is a plating method that relies on the chemical reaction of a
vapor at a surface to form solid structural deposits. Since this is
done on an atom-by-atom basis, impurity levels are typically less
than 0.1% and densities are 99.9%. The CVD process utilizes a
gaseous compound of the element to be deposited, which is then
flowed over a heated substrate, resulting in thermal decomposition
or reduction of the gaseous compound and subsequent deposition of
the material onto the substrate. The first layer forms at
nucleation sites and, after the substrate is fully coated, growth
continues on the deposit's crystal faces.
[0040] Successful CVD--producing dense, adherent coatings--depends
upon experimentally determining optimal deposition parameters.
These parameters include: the gaseous compound of the material to
be deposited, substrate temperature, gas concentration, flow,
pressure and geometry within the reaction chamber, coating
thickness and substrate material. For the coating to have high
integrity and adhesion to the substrate, the substrate either must
have a similar coefficient of expansion to that of the deposited
material or must form a strong chemical or metallurgical bond with
it. The thinner the coating, the less similar the coefficients of
expansion need be. Where coating and substrate form no
intermetallic bond and have widely differing coefficients of
expansion, a good bond often can be achieved by using a thin
interlayer of a third material.
[0041] Fluidized beds, and more preferably fast-fluidized coating
apparatus, are ideal tools for producing the fine powder materials
of the present invention. Operating a fluidized bed above the
transport limit (fast fluidization) or in the turbulent
fluidization region may enable fine powders and whiskers, chopped
fibers and such to be fluidized with high product yields. This
technology eliminates problems with heat and mass transfer and
handling fine and cohesive powders by using high gas shear
velocities and turbulent gas mixing technologies. In a
fast-fluidized bed, high gas velocities are used to entrain the
small particles, which are then filtered out of the gas entrainment
stream and re-fed to the bottom of the reactor.
[0042] While not being bound by theory, it is believed that, in
both the embodiments described above, both mechanical and magnetic
mechanisms underlie the enhancement of the entropy change in the
giant magnetocaloric compound Gd.sub.5Si.sub.1-5Ge.sub.2.5. In
particular, it has been found that the application of a
ferromagnetic elastically stiff coating to the magnetocaloric
compound has the effect of mechanically restricting the significant
volume change that accompanies the magnetic transition occurring
upon application of a magnetic field, thereby displacing the effect
to different temperatures and fields. In magnetic terms, it is also
believed that the stray field of a soft magnetic (e.g., Fe) layer
serves to amplify the externally applied field. Measurements of the
magnetostatic effect of the Fe coatings were carried out by
comparing the room-temperature paramagnetic response obtained from
the single-phase Gd.sub.5Si.sub.1-5Ge.sub.2.5 and the
heterostructured Gd.sub.5Si.sub.1-5Ge.sub.2.5 specimens. No
discernable difference in the response of the two specimens was
detected, indicating that the magnetostatic contribution is
extremely small and unlikely to produce the large changes noted in
the ferromagnetic response. The enhanced field sensitivity found in
the heterostructured samples is not due to a lowered energy barrier
to reverse magnetic domain nucleation, because measured full field
cycles indicate that the heterostructured samples display a
stabilization of magnetization upon magnetic field reversal, not a
reduction of the magnetization relative to the uncoated sample.
Ferromagnetic exchange coupling between the Fe layer and the
Gd.sub.5Si.sub.1-5Ge.sub.2.5 is conceivable because of the very
large value, approaching 50 nm, of the domain wall width in Fe
despite the presence of a layer of Gd oxide that is undoubtedly
intervening between the Fe and the Gd.sub.5Si.sub.1-5Ge.sub.2.5
interfaces.
[0043] It is envisioned that this composite architecture can be
easily deployed in many materials geometries and forms. Established
technology exists to provide a thin ferromagnetic metallic coating
onto layers and particles of many compositions. Very important
advantages can be envisioned with the implementation of this
development. In the case of the giant magnetocaloric materials such
as the Gd.sub.5(Si.sub.1-xGe.sub.- x).sub.4 family of compounds
these results strongly suggest that it is possible to significantly
lower the applied magnetic fields necessary to obtain a large
heating or cooling effect.
[0044] The following is an illustrative example demonstrating the
foregoing:
EXAMPLE
[0045] A Gd.sub.5Si.sub.1-5Ge.sub.2.5 sample core slice was
prepared by conventional arc melting procedures using 99.9 percent
Lunex rare earths and 99.999 percent silicon and germanium. The
weight loss after melting was less than 1 percent and no subsequent
heat treatment was carried out. Laboratory Cu--K.sub..alpha. x-ray
diffraction studies revealed that the sample was well-crystallized
and single-phase within the estimated limits of detection (5 vol %)
and was isotropic, exhibiting no texture. Slices of
Gd.sub.5S.sub.1-5Ge.sub.2.5 of approximate thickness 0.6 mm were
cut with a water-cooled slow-speed wire saw and the surfaces were
carefully polished with fine emery paper. Iron (Fe) layers of
nominal thicknesses 0.1 .mu.m or 0.2 .mu.m were applied to both
surfaces of the core slices by a chemical vapor deposition
technique, with very small volume ratios of
Fe:Gd.sub.5Si.sub.1-5Ge.sub.2.5 of 0.03 vol % for the 0.1 .mu.m
layers and was 0.06 vol % for the 0.2 .mu.m layers. During the Fe
evaporation process the substrates were water-cooled and the
chamber pressure was about 5.times.10.sup.-6 Torr. The deposition
rate was .about.3-4 .ANG./sec as monitored by a quartz crystal
oscillator. FIG. 2 provides a schematic illustration of the
heterostructured sample architecture.
[0046] Magnetization measurements were made on both an uncoated
Gd.sub.5Si.sub.1-5Ge.sub.2.5 sample core and the coated
heterostructure of Gd.sub.5Si.sub.1-5Ge.sub.2.5+Fe in the
temperature range 165 K to 235 K using a SQUID magnetometer. The
uncoated Gd.sub.5Si.sub.1-5Ge.sub.2.5 specimen was cut into a
single prism with dimensions 0.5 mm.times.0.5 mm.times.6.0 mm,
while the coated slices were cut into uniform coupons of dimensions
1.26 mm.times.1.26 mm.times.0.6 mm and stacked to mimic the prism
shape. Care was taken to determine the appropriate demagnetization
factors of the specimens as accurately as possible. To this end,
the specimens were experimentally corrected for demagnetization
effects by using the demagnetization factor measured from Fe slices
of the same dimensions and geometry as the specimens. An inherent
assumption in the application of this procedure is that the stacked
heterostructure slices act as a monolithic magnetic solid.
[0047] In the case of the heterostructure with the 0.1 .mu.m-thick
Fe layer, ten slices were stacked together for measurement with the
applied field perpendicular to the iron layer with a
demagnetization factor N.sub.d=0.083. With the field applied
parallel to the iron layers two slices were stacked together that
produced a demagnetization factor of N.sub.d=0.2475. In the case of
the heterostructure with 0.2 .mu.m Fe layer, two slices were
stacked together for measurement with the applied field both
perpendicular and parallel to the iron layer, and the
demagnetization factors are calibrated to be 0.319 and 0.2475,
respectively. The demagnetization factor of the single
Gd.sub.5Si.sub.1-5Ge.sub.2.5 prism was determined to be
N.sub.d=0.022. The estimated error of the calculated entropy change
.DELTA.S, based on consideration of the uncertainties in the
measured parameters, is on the order of 2% in the vicinity of the
zero-field Curie temperature.
[0048] Magnetization curves and .DELTA.S determinations were made
for a total of five sample configurations: the single-phase
Gd.sub.5Si.sub.1-5Ge.sub.2.5 prism and the heterostructured samples
of 0.1-.mu.m and 0.2-.mu.m Fe thicknesses measured with the applied
field oriented both parallel and perpendicular to the Fe film
plane. FIG. 4 shows selected magnetization curves at three
representative temperatures: at T=188 K, below the zero-field Curie
point; at the zero-field Curie point of 191 K and in the
paramagnetic region at 215 K. For the sake of clarity only data for
the parallel configuration is included in FIG. 4 (the data obtained
from samples in the perpendicular configuration are similar). The
expected two-stage magnetization development of the samples in the
magnetically-ordered state is evident.
[0049] It is noted that the Fe layers have basically no effect at
T=215 K when the Gd.sub.5Si.sub.1.5Ge.sub.2.5 alloy is in the
paramagnetic state. However, in the magnetically-ordered state, in
both parallel and the perpendicular sample geometries, the
heterostructured sample data exhibit two main features that are
attributed to the Fe layers. The first feature is that the
magnetization development, as quantified by the magnitude of the
initial dc magnetic susceptibility .chi..sub.dc, initiates at
significantly lower fields in the heterostructured samples. Thus
domain wall movement and rotation of magnetization in the
ferromagnetic component of the Gd.sub.5Si.sub.1-5Ge.sub.2.5 sample
is promoted by the iron layer.
[0050] FIG. 5 displays the temperature dependence of the initial
.chi..sub.dc of the samples, where it can be seen that the Fe
layers cause an increase of the initial susceptibility in the
ferromagnetic component of the Gd.sub.5Si.sub.1.5Ge.sub.2.5 alloy,
effectively allowing it to be magnetized more readily. This
susceptibility enhancement is especially evident at the lower
temperatures and decreases monotonically with increasing
temperature.
[0051] The second feature originating from the presence of the Fe
layers is that the field-induced paramagnetic to ferromagnetic
phase transition shifts to lower onset fields H.sub.onset. The
paramagnetic-ferromagnetic onset field is determined as the minimum
of the field derivative of the magnetization curve, dM/dH.sub.i.
The minimum in the curve signals the critical onset field at which
the paramagnetic phase begins to change into the ferromagnetic
phase, increasing the magnetic susceptibility in that field range.
The onset field difference .DELTA.H.sub.onset between the
single-phase and the heterostructured Gd.sub.5Si.sub.1-5Ge.sub.2.5
data reaches a maximum at approximately 4500 G at 187 K, then
decreases with increasing temperature as shown in FIG. 6. The
thicker Fe layer shifts the onset field to lower field values.
[0052] The increased field sensitivity of the magnetization
processes and the metamagnetic transitions in the
Gd.sub.5Si.sub.1-5Ge.sub.2.5 compound conferred by the thin
ferromagnetic coating results in an enhancement of the
magnetocaloric effect. FIG. 7 displays the entropy changes
determined for internal field changes of 1 T and 5 T. It can be
seen in FIG. 7 that the selected heterostructured samples display a
larger magnetic entropy change than the uncoated
Gd.sub.5Si.sub.1-5Ge.sub.2.5 in the vicinity of the metamagnetic
transition temperature. For the sake of clarity of presentation,
only the data from samples measured with the field parallel to the
0.1 .mu.m-thick Fe layers are shown (the entropy changes for the
same samples measured with the field perpendicular to the Fe layers
fall between the uncoated Gd.sub.5Si.sub.1.5Ge.sub.2.5 data and the
Fe-coated parallel data).
[0053] The maximum effect is the 11% enhancement achieved at
H.sub.int=5 T with the field applied perpendicular to the 0.1 .mu.m
Fe layers. Although thicker iron layers shift the metamagnetic
transition to lower field values, they do not contribute to an
enhanced magnetocaloric effect. This result indicates that
magnetocaloric effect enhancement is sensitive to the thickness of
the Fe layer, and presents the possibility of obtaining an even
more amplified MCE from optimized Fe layer thicknesses.
[0054] Thus, it has been found that in the metamagnetic transition
temperature range the Fe layer situated in a parallel geometry
lowers the onset field of the metamagnetic transition by 4240 Oe
for a 0.1 .mu.m layer thickness and by 4940 Oe for a 0.2 .mu.m
layer thickness from the base value of 8700 Oe at 187 K in the
absence of Fe layers. Furthermore, the 0.1 .mu.m layer of Fe
oriented perpendicular to the applied field is found to enhance the
entropy change, and thus the magnetocaloric effect, by
approximately 11% above its base value at 191.5 K.
[0055] These results conclusively demonstrate that very thin layers
of Fe can produce an effect that resembles amplification of the
applied magnetic field. The effects of Fe layers with thicknesses
of 0.1 .mu.m and 0.2 .mu.m are demonstrated by an increased initial
dc susceptibility .chi..sub.dc and a shift of the onset field
H.sub.onset to lower fields to induce the
paramagnetic-ferromagnetic metamagnetic transition in the
Gd.sub.5Si.sub.1-5Ge.sub.2.5 compound. Furthermore, the 0.1 .mu.m
coating produces an increased entropy change, indicative of an
increased magnetocaloric effect, in the vicinity of the
Gd.sub.5Si.sub.1-5Ge.sub.2.- 5 Curie transition. All of these
effects are observed when the applied field is oriented in
geometries both parallel and perpendicular to the iron layers.
[0056] Thus, while there have been described what are presently
believed to be the preferred embodiments of the invention, those
skilled in the art will realize that changes and modifications may
be made thereto without departing from the spirit of the invention,
and is intended to claim all such changes and modifications as fall
within the true scope of the invention.
* * * * *