U.S. patent application number 13/059352 was filed with the patent office on 2012-03-15 for article for magnetic heat exchange and method of fabricating an article for magnetic heat exchange.
This patent application is currently assigned to Vacuumschmelze GmbH & Co., KG. Invention is credited to Matthias Katter, Volker Zellmann.
Application Number | 20120061066 13/059352 |
Document ID | / |
Family ID | 43050031 |
Filed Date | 2012-03-15 |
United States Patent
Application |
20120061066 |
Kind Code |
A9 |
Katter; Matthias ; et
al. |
March 15, 2012 |
ARTICLE FOR MAGNETIC HEAT EXCHANGE AND METHOD OF FABRICATING AN
ARTICLE FOR MAGNETIC HEAT EXCHANGE
Abstract
An article for magnetic heat exchange comprising a
magnetocalorically active phase with a NaZn.sub.13-type crystal
structure is provided by hydrogenating a bulk precursor article.
The bulk precursor article is heated from a temperature of less
than 50.degree. C. to at least 300.degree. C. in an inert
atmosphere and hydrogen gas only introduced when a temperature of
at least 300.degree. C. is reached. The bulk precursor article is
maintained in a hydrogen containing atmosphere at a temperature in
the range 300.degree. C. to 700.degree. C. for a selected duration
of time, and then cooled to a temperature of less than 50.degree.
C.
Inventors: |
Katter; Matthias; (Alzenau,
DE) ; Zellmann; Volker; (Linsengericht, DE) |
Assignee: |
Vacuumschmelze GmbH & Co.,
KG
Hanau
DE
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20110198069 A1 |
August 18, 2011 |
|
|
Family ID: |
43050031 |
Appl. No.: |
13/059352 |
Filed: |
May 6, 2009 |
PCT Filed: |
May 6, 2009 |
PCT NO: |
PCT/IB09/51854 PCKC 00 |
371 Date: |
April 25, 2011 |
Current U.S.
Class: |
165/185;
29/890.03 |
Current CPC
Class: |
C21D 1/74 20130101; C22C
2202/02 20130101; C22C 38/04 20130101; B22F 2998/10 20130101; H01F
1/015 20130101; B22F 2998/10 20130101; H01F 1/017 20130101; C22C
38/10 20130101; C22C 33/0278 20130101; B22F 3/10 20130101; B22F
2201/013 20130101; C22C 38/02 20130101; B22F 3/02 20130101; F28F
21/082 20130101; C22C 38/005 20130101; Y10T 29/4935 20150115 |
Class at
Publication: |
165/185;
29/890.03 |
International
Class: |
F28F 7/00 20060101
F28F007/00; B21D 53/02 20060101 B21D053/02 |
Claims
1. A method of fabricating an article for magnetic heat exchange,
comprising: providing a bulk precursor article comprising a
magnetocalorically active phase with a NaZn.sub.13-type crystal
structure, performing hydrogenation of the bulk precursor article
by: heating the bulk precursor article from a temperature of less
than 50.degree. C. to at least 300.degree. C. in an inert
atmosphere, introducing hydrogen gas only when a temperature of at
least 300.degree. C. is reached, maintaining the bulk precursor
article in a hydrogen containing atmosphere at a temperature in the
range 300.degree. C. to 700.degree. C. for a selected duration of
time, and cooling the bulk precursor article to a temperature of
less than 50.degree. C. to provide a hydrogenated article.
2. The method according to claim 1, wherein the cooling of bulk
precursor article to a temperature of less than 50.degree. C. is in
a hydrogen-containing atmosphere.
3. The method according to claim 2, wherein the selected duration
of time is 1 minute to 4 hours.
4. The method according to claim 2, wherein after hydrogenation,
the article comprises at least 0.21 wt % hydrogen.
5. The method according to claim 2, wherein after hydrogenation,
the article comprises a magnetic phase transition temperature of in
the range of -40.degree. C. to +150.degree. C.
6. The method according to claim 2, wherein the bulk precursor
article is cooled at a rate of 0.1K/min to 10K/min.
7. The method according to claim 1, further comprising, before
cooling the bulk precursor article to a temperature of less than
50.degree. C., replacing the hydrogen gas by inert gas.
8. The method according to claim 1, wherein the cooling of the bulk
precursor article to a temperature of less than 50.degree. C.
comprises cooling the bulk precursor article to a temperature in
the range 300.degree. C. to 150.degree. C. in a hydrogen containing
atmosphere, replacing the hydrogen by inert gas, and cooling the
bulk precursor article to a temperature of less than 50.degree.
C.
9. The method according to claim 7, wherein the selected duration
of time is 1 minute to 4 hours.
10. The method according to claim 7, wherein after hydrogenation,
the article comprises a hydrogen content in the range of 0.02 wt %
to 0.21 wt %.
11. The method according to claim 7, wherein the cooling of the
bulk precursor article is cooled at a rate of 1 K/min to 100
K/min.
12. The method according to claim 1, wherein the bulk precursor
article has initial outer dimensions before hydrogenation and the
article after hydrogenation has final outer dimensions, wherein a
difference between the initial outer dimensions and final outer
dimensions is less than 10%.
13. The method according to claim 1, wherein the introducing of
hydrogen gas is only when a temperature of 400.degree. C. to
600.degree. C. is reached.
14. A method of fabricating an article for magnetic heat exchange,
comprising: providing a bulk precursor article comprising a
magnetocalorically active phase with a NaZn.sub.13-type crystal
structure and at least 0.2 wt % hydrogen, performing a partial
dehydrogenation of the bulk precursor article by: heating the bulk
precursor article in an inert gas at a temperature of 150.degree.
C. to 400.degree. C. for a selected duration of time, and rapidly
cooling the bulk precursor article to a temperature of less than
50.degree. C. in an inert atmosphere to produce an article for
magnetic heat exchange.
15. The method according to claim 14, wherein the rapid cooling of
the bulk precursor article is by quenching.
16. The method according to claim 14, wherein the selected duration
of time is extended and the hydrogen content of the bulk precursor
article is reduced as a function of dwell time.
17. The method according to claim 1, wherein the bulk precursor
article has at least one outer dimension greater than 5 mm.
18. The method according to claim 1, wherein the bulk precursor
article is polycrystalline.
19. The method according to claim 1, wherein the bulk precursor
article is sintered or reactive sintered.
20. The method according to claim 1, wherein the magnetocalorically
active phase is
La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z,
wherein M is at least one element from the group consisting of Si
and Al, T is at least one element from the group consisting of Co,
Ni, Mn and Cr, R is at least one element from the group consisting
of Ce, Nd and Pr, 0.ltoreq.a.ltoreq.0.5, 0.05.ltoreq.x.ltoreq.0.2,
0.ltoreq.y.ltoreq.0.2 and 0.ltoreq.z.ltoreq.3.
21. An article for use as a working medium in a magnetic heat
exchanger comprising a magnetocalorically active phase with a
NaZn.sub.13-type crystal structure and further comprising hydrogen,
wherein the article has at least one dimension greater than 5
mm.
22. The article according to claim 21, wherein the article is
polycrystalline.
23. The article according to claim 21, wherein the article is
sintered or reactive sintered.
24. The article according to claim 21, wherein the hydrogen is
accommodated interstitally in the NaZn.sub.13 crystal
structure.
25. The article according to claim 21, wherein the
magnetocalorically active phase is
La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z,
wherein M is at least one element from the group consisting of Si
and Al, T is at least one element from the group consisting of Co,
Ni, Mn and Cr, R is at least one element from the group consisting
of Ce, Nd and Pr, and 0.ltoreq.a.ltoreq.0.5,
0.05.ltoreq.x.ltoreq.0.2, 0.ltoreq.y.ltoreq.0.2 and
0.ltoreq.z.ltoreq.3.
26. The article according to claim 25, wherein
0.2.ltoreq.z.ltoreq.3.
27. A bulk precursor article comprising a magnetocalorically active
phase with a NaZn.sub.13-type crystal structure, wherein the
magnetocalorically active phase is
La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z,
wherein M is at least one element from the group consisting of Co,
Ni, Mn and Cr, R is at least one element selected from the group
consisting of Ce, Nd, and Pr, and 0.ltoreq.a.ltoreq.0.5,
0.05.ltoreq.x.ltoreq.0.2, 0.ltoreq.y.ltoreq.0.2 and z=0.
Description
BACKGROUND
[0001] 1. Field
[0002] The present application relates to an article for magnetic
heat exchange, in particular an article for use as a working medium
in a magnetic heat exchanger, and methods of fabricating an article
for magnetic heat exchange.
[0003] 2. Description of Related Art
[0004] Magnetic heat exchangers include a magnetocalorically active
material as the working medium to provide cooling and/or heating. A
magnetocalorically active material exhibits the magnetocaloric
effect. The magnetocaloric effect describes the adiabatic
conversion of a magnetically induced entropy change to the
evolution or absorption of heat. By applying a magnetic field to a
magnetocalorically active 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] The magnetic entropy of the material changes depending on
whether a magnetic field is applied or not owing to the difference
between the degrees in freedom of the electron spin system. With
this entropy change, entropy transfers between the electron spin
system and the lattice system.
[0006] A magnetocalorically active phase, therefore, has a magnetic
phase transition temperature T.sub.trans at which this phase change
occurs. In practice, this magnetic phase transition temperature
translates as the working temperature. Therefore, in order to
provide cooling over a wider temperature range, the magnetic heat
exchanger requires magnetocalorically active material having
several different magnetic phase transition temperatures.
[0007] A variety of magnetocalorically active phases are known
which have magnetic phase transition temperatures in a range
suitable for providing domestic and commercial air conditioning and
refrigeration. One such magnetocalorically active material,
disclosed for example in U.S. Pat. No. 7,063,754, has a
NaZn.sub.13-type crystal structure and may be represented by the
general formula La(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z, where
M is at least one element of the group consisting of Si and Al, and
T may be one of more of transition metal elements such as Co, Ni,
Mn and Cr. The magnetic phase transition temperature may be
adjusted by adjusting the composition.
[0008] In addition to a plurality of magnetic phase transition
temperatures, a practical working medium should also have a large
entropy change in order to provide efficient heating. However,
elemental substitutions which lead to a change in the magnetic
phase transition temperature can also lead to a reduction in the
entropy change observed.
SUMMARY
[0009] Therefore, it is desirable to provide a material for use as
the working medium in a magnetic heat exchanger which can be
fabricated to have a range of different magnetic phase transition
temperatures as well as a large entropy change. It is also
desirable that the material can be fabricated in a physical form
which can be incorporated reliably into a practical magnetic heat
exchanger.
[0010] The application provides an article for use as a working
medium in a magnetic heat exchanger which comprises a
magnetocalorically active phase with a NaZn.sub.13-type crystal
structure and hydrogen. The article has at least one dimension
which is greater than 5 mm (millimetres). In further embodiments,
the article has at least one dimension greater than 10 mm.
[0011] 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.
[0012] 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.
[0013] A magnetic phase transition temperature is defined herein as
a transition from one magnetic state to another. Some
magnetocalorically active phases exhibit a transition from
antiferromagnetic to ferromagnetic which is associated with an
entropy change. Some magnetocalorically active phases such as
La(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z exhibit a transition
from ferromagnetic to paramagnetic which is associated with an
entropy change. For these materials, the magnetic phase transition
temperature can also be called the Curie temperature.
[0014] The magnetocalorically active phase may be described by the
formula
La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z. M is
at least one element from the group consisting of Si and Al, T is
at least one element from the group consisting of Co, Ni, Mn and
Cr, R is at least one rare earth metal such as Ce, Nd and Pr and
0.ltoreq.a.ltoreq.0.5, 0.05.ltoreq.x.ltoreq.0.2,
0.ltoreq.y.ltoreq.0.2 and 0.ltoreq.z.ltoreq.3.
[0015] An article with at least one dimension greater than 5 mm is
more practical to use in a heat exchanger than a magnetocalorically
active phase in the form of a powder. Although a powder has a
larger surface area which should, in principle, lead to a better
heat exchange with a heat exchange medium such as a fluid in which
it is in contact, in practice the use of powder has the
disadvantage that it must be contained within a further vessel and
not pumped around the heat exchanger system with the heat exchange
medium.
[0016] It is also found that the average particle size of the
powder tends to decrease over its working life since it is impacted
onto the side walls of the vessel due to the movement of the heat
exchange medium. Therefore, larger solid articles are desirable to
avoid these problems.
[0017] The article may be polycrystalline and may be a
polycrystalline sintered or reactive sintered article that is
fabricated by sintering or reactive sintering particles together to
produce a solid polycrystalline article.
[0018] The term "reactive sintered" describes an article in which
grains are joined to congruent grains by a reactive sintered bond.
A reactive sintered bond is produced by heat treating a mixture of
precursor powders of differing compositions. The particles of
different compositions chemically react with one another during the
reactive sintering process to form the desired end phase or
product. The composition of the particles, therefore, changes as a
result of the heat treatment. The phase formation process also
causes the particles to join together to form a sintered body
having mechanical integrity.
[0019] Reactive sintering differs from conventional sintering
since, in conventional sintering, the particles consist of the
desired end phase before the sintering process. The conventional
sintering process causes a diffusion of atoms between neighbouring
particles so as join the particles to one another. The composition
of the particles, therefore, remains unaltered as a result of a
conventional sintering process.
[0020] In further embodiments, the hydrogen is accommodated
interstitally in the NaZn.sub.13 crystal structure and comprises at
least one dimension greater than 10 mm. For example, the article
may comprise a reactive sintered polycrystalline plate having
dimensions of 11 mm.times.6 mm.times.0.6 mm. The article may
comprise a hydrogen content in the range 0.02 wt % to 0.3 wt % and
may have a magnetic phase transition temperature in the range
-40.degree. C. to +150.degree. C.
[0021] The present application, therefore, provides methods by
which an article having at least one dimension of greater than 10
mm which comprises a magnetocalorically active phase with a
NaZn.sub.13-type crystal structure and hydrogen can be
fabricated.
[0022] The magnetocalorically active phase may be described by
La.sub.1-aR.sub.a(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13H.sub.z,
wherein M is at least one element from the group consisting of Si
and Al, T is at least one element from the group consisting of Co,
Ni, Mn and Cr, R is at least one element from the group consisting
of Ce, Nd and Pr and 0.ltoreq.a.ltoreq.0.5,
0.05.ltoreq.x.ltoreq.0.2, 0.ltoreq.y.ltoreq.0.2 and
0.ltoreq.z.ltoreq.3, preferably 0.02.ltoreq.z.ltoreq.3.
[0023] In an embodiment, a method of fabricating an article for
magnetic heat exchange comprises hydrogenating a bulk precursor
article comprising a magnetocalorically active phase with a
NaZn.sub.13-type crystal structure is provided. The bulk precursor
article is initially hydrogen-free and is subsequently hydrogenated
by heating the bulk precursor article from a temperature of less
than 50.degree. C. to at least 300.degree. C. in an inert
atmosphere and introducing hydrogen gas only when a temperature of
at least 300.degree. C. is reached. The bulk precursor article is
maintained in a hydrogen containing atmosphere at a temperature in
the range 300.degree. C. to 700.degree. C. for a selected duration
of time, and afterwards cooled to a temperature of less than
50.degree. C. to produce a hydrogenated article.
[0024] The magnetocalorically active phase of the bulk precursor
article before hydrogenation comprises a hydrogen content, z, of
less than 0.02 wt %. In an embodiment, the temperature of less than
50.degree. C. is room temperature and may lie in the range of
18.degree. C. to 25.degree. C.
[0025] As used herein "bulk" is used to denote a precursor article
or a final product article other than a powder and specifically
excludes a powder. A powder includes a number of particles having a
diameter of 1 mm (millimetre) or less.
[0026] This method enables bulk precursor articles, which have been
previously fabricated, for example by melting and solidification
techniques as well as by sintering or reactive sintering powders to
form sintered or reactive sintered blocks, to be subsequently be
hydrogenated whilst retaining with mechanical properties of the
unhydrogenated block. In particular, it is found that if hydrogen
is introduced at temperatures lower than around 300.degree. C., the
bulk precursor article may disintegrate into pieces or at least
lose its previous mechanical strength. However, these problems may
be avoided by first introducing hydrogen when the bulk precursor
article is at a temperature of at least 300.degree. C.
[0027] The method may be used to fabricate articles having
differing hydrogen contents and, therefore, different magnetic
phase transition temperatures by adjusting the parameters used to
hydrogenate the article so that the hydrogen content of the article
differs.
[0028] In a first group of embodiments, a fully hydrogenated or
near fully hydrogenated article may be fabricated by cooling the
article to a temperature of less than 50.degree. C., for example to
room temperature, in a hydrogen-containing atmosphere. A fully or
near fully hydrogenated article is defined as one having a hydrogen
content, z, of 1.7 to 3.
[0029] The selected duration of heat treatment time at the
temperature in the range of 300.degree. C. to 700.degree. C. may
lie in the range 1 minute to 4 hours in the first group of
embodiments. After hydrogenation, the article may comprise at least
0.21 wt % hydrogen and a magnetic phase transition temperature,
T.sub.trans, in the range of -40.degree. C. to +150.degree. C.
[0030] The lower magnetic phase transition temperatures may be
obtained by substituting a portion of the element La by Ce, Pr
and/or Nd or by substituting a portion of the element Fe by Mn
and/or Cr. The higher magnetic phase transition temperatures may be
obtained by substituting a portion of the element Fe by Co, Ni, Al
and/or Si.
[0031] These magnetic phase transition temperatures and hydrogen
content are typical of a fully hydrogenated or near fully
hydrogenated material. The article may be cooled at a rate of 0.1
to 10K/min in a hydrogen containing atmosphere. Such a cooling rate
may be achieved by furnace cooling depending on the size and
construction of the furnace.
[0032] In a second group of embodiments, the parameters used to
carry out hydrogenation are adjusted in order to adjust the
hydrogen content of the article and adjust the magnetic phase
transition temperature of the article in the range of -40.degree.
C. to 150.degree. C. In the second group of embodiments, the bulk
precursor article is partially hydrogenated.
[0033] In an embodiment, the hydrogen gas is replaced by inert gas
before cooling the article to a temperature of less than 50.degree.
C. In other words, after the heat treatment in a hydrogen
containing atmosphere for the selected duration of time at a
temperature in the range of 300.degree. C. to 700.degree. C., the
hydrogen containing atmosphere is exchanged for inert gas at this
temperature before cooling begins.
[0034] This method produces a partially hydrogenated article, i.e.
an article with a hydrogen content which is less than that achieved
by the first group of embodiments described above which produce a
fully hydrogenated or near fully hydrogenated article. This
embodiment may be used to fabricate an article having a magnetic
phase transition temperature which is up to 60K higher than the
magnetic phase transition temperature of the hydrogen-free
precursor.
[0035] In a further embodiment, the article is cooled from the
dwell temperature in the range 300.degree. C. to 700.degree. C. to
a temperature in the range 300.degree. C. to 150.degree. C. in a
hydrogen containing atmosphere. The hydrogen is then replaced by
inert gas and the article cooled to a temperature of less than
50.degree. C.
[0036] This embodiment may be used to fabricate an article having a
magnetic phase transition temperature which is 60K to 140K higher
than the magnetic phase transition temperature of the hydrogen-free
precursor since the uptake of hydrogen may be larger than an
embodiment in which the hydrogen gas is exchanged for an inert gas
at the dwell temperature.
[0037] For this second group of embodiments, the selected duration
of time may be 1 minute to 4 hours. After hydrogenation, the
article may comprises a hydrogen content in the range of 0.02 wt %
to 0.21 wt %. The article may be cooled at a rate of 1 K/min to 100
K/min. This cooling rate is somewhat faster than that used to
produce a fully hydrogenated or near fully hydrogenated article.
Such a cooling rate may be provided by forced gas cooling of the
furnace and/or removing the heating jacket from the working chamber
of the furnace.
[0038] For both groups of embodiments described above, the method
may be further modified as follows.
[0039] The bulk precursor article has initial outer dimensions
before hydrogenation and the final article after hydrogenation has
final outer dimensions. In an embodiment, the difference between
the initial outer dimensions and final outer dimensions is less
than 10%. The article largely retains its initial dimensions since
it no longer disintegrates and loses its mechanical integrity
during the hydrogenation method. The final outer dimensions may
however differ slightly from the initial outer dimensions as a
result of the accommodation of hydrogen within the crystal lattice
of the magnetocalorically active phase of the article.
[0040] In further embodiments, hydrogen gas is introduced only when
a temperature of 400.degree. C. to 600.degree. C. is reached. These
embodiments can be used to provide an article after hydrogenation
with an improved mechanical strength.
[0041] In the above second group of embodiments, partially
hydrogenated articles are fabricated by adjusting the amount of
hydrogen introduced into the article during a single heat
treatment.
[0042] In a further method, pre-hydrogenated articles are provided
and then partially dehydrogenated to reduce the hydrogen content
and change the magnetic phase transition temperature of the
article.
[0043] This further method of fabricating an article for magnetic
heat exchange comprises providing a polycrystalline sintered or
reactive sintered article comprising a magnetocalorically active
phase with a NaZn.sub.13-type crystal structure and at least 0.2 wt
% hydrogen and performing at least a partial dehydrogenation of the
article. The at least partial dehydrogenation may be performed by
heating the article in inert gas at a temperature of 150.degree. C.
to 400.degree. C. for a selected duration of time, and rapidly
cooling the article to a temperature of less than 50.degree. C. in
an inert atmosphere. The article may be placed into a furnace
pre-heated to a temperature in the range of 150.degree. C. to
400.degree. C.
[0044] The initially fully hydrogenated or near fully hydrogenated
articles are partially dehydrogenated in order to fabricate
articles comprising a magnetic phase transition temperature between
that of hydrogen-free phase and the fully hydrogenated phase.
However, the article may be completely dehydrogenated as the
hydrogenation process is fully reversible if the hydrogenation and
dehydrogenation conditions are selected so as to prevent
decomposition of the magnetocalorically active phase with the
NaZn.sub.13-type crystal structure.
[0045] In an embodiment, the article is rapidly cooled by
quenching. This may be performed by rapidly moving the article from
the hot zone of a furnace to a peripheral end of the working
chamber outside of the hot zone. The article is then maintained in
the inert gas within the furnace chamber whilst being quenched.
Oxidation of the article can be avoided.
[0046] In an embodiment, the selected duration of time is extended
to reduce the hydrogen content of the fully or near fully article.
The hydrogen content of the article may be reduced generally
logarithmically with respect to increased time at the dwell
temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] Embodiments will now be described with reference to the
accompanying drawings.
[0048] FIG. 1 illustrates a graph of the change in entropy as a
function of temperature for partially hydrogenated articles,
[0049] FIG. 2 illustrates a graph of Curie temperature as a
function of gas exchange temperature for the articles of FIG.
1,
[0050] FIG. 3 illustrates a graph of the hydrogen content as a
function of Curie temperature for the articles of FIG. 1,
[0051] FIG. 4 illustrates a graph of entropy change as a function
of temperature for articles dehydrogenated at 200.degree. C. for
different times,
[0052] FIG. 5 illustrates a graph of Curie temperature as a
function of dehydrogenation time for the articles of FIG. 4,
[0053] FIG. 6 illustrates a graph of entropy change as a function
of temperature for articles dehydrogenated at 250.degree. C. for
different times,
[0054] FIG. 7 illustrates a graph of entropy change as a function
of temperature for articles dehydrogenated at 300.degree. C. for
different times,
[0055] FIG. 8 illustrates a comparison of Curie temperature as a
function of dehydrogenation time for the articles of FIGS. 4, 6 and
7, and
[0056] FIG. 9 illustrates a graph of entropy change as a function
of temperature for three articles with differing metallic element
compositions.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0057] An article for use as the working medium in a magnetic heat
exchanger may be fabricated by hydrogenating bulk precursor article
comprising a magnetocalorically active phase with a
NaZn.sub.13-type crystal structure.
[0058] In an embodiment, the bulk precursor article comprises one
or more La(Fe.sub.1-x-yT.sub.yM.sub.x).sub.13-based phases and
comprises 16.87 wt % La, 3.73 wt % Si, 4.61 wt % Co and remainder
iron. Each bulk precursor article has initial dimensions of around
11.5 mm.times.6 mm.times.0.6 mm and a magnetic phase transition
temperature of -18.5.degree. C., an entropy change of 9.4 J/(kgK)
for a magnetic field change of 1.6 T and 5.7% alpha-Fe
(.alpha.-Fe). The peak width (entropy change as a function of
temperature) is 13.7.degree. C.
[0059] The bulk precursor article is polycrystalline and may be
fabricated by sintering compacted powder comprising the
hydrogen-free magnetocalorcially active phase or by reactive
sintering precursor powders having an overall composition
corresponding to the desired hydrogen-free magnetocalorically
active phase to form the desired hydrogen-free magnetocalorically
active phase.
[0060] The .alpha.-Fe content was measured using a thermomagnetic
method in which the magnetic polarization of a sample heated above
its Curie Temperature is measured as the function of temperature of
the sample when it is placed in an external magnetic field. The
paramagnetic contribution, which follows the Curie-Weiss Law, is
subtracted and the content of alpha Fe is deduced from the
remaining ferromagnetic signal.
[0061] The bulk precursor articles were hydrogenated by wrapping 5
bulk precursor articles in iron foil, placing them in a furnace and
heating the bulk precursor articles from a temperature of less than
50.degree. C. to a selected temperature in the range 100.degree. C.
to 700.degree. C. in an inert atmosphere, in particular, in argon.
Hydrogen gas was introduced into the furnace only when the
temperature 100.degree. C. to 700.degree. C. was reached. Hydrogen
gas at a pressure of 1.9 bar was introduced into the furnace and
the article held in a hydrogen containing atmosphere at the
selected temperature for a selected duration of time or dwell time.
In this embodiment, the dwell time was 2 hours. Afterwards, the
articles were furnace cooled in the hydrogen containing atmosphere
at a mean cooling rate of about 1 K/min to a temperature of less
than 50.degree. C.
[0062] The articles heat treated at a temperature of 100.degree. C.
and 200.degree. C. were found to have disintegrated into powder and
the outermost portions of the article heat treated at 300.degree.
C. were observed to have broken away. The articles heat treated at
400.degree. C., 500.degree. C., 600.degree. C. and 700.degree. C.
were all found to be intact after the hydrogenation heat
treatment.
[0063] The magnetocaloric properties of entropy change, peak
magnetic phase transition temperature and peak width as well as the
measured alpha-iron content are summarized in Table 1.
TABLE-US-00001 TABLE 1 max, Entropy Peak Alpha change temp- Peak Fe
.DELTA.S'.sub.m.max, erature width content Sample Heat treatment
[J/(kg K)] [.degree. C.] [.degree. C.] [%] MPS-1030 none 9.36 -18.5
13.7 VZ0821-1A1 HST = 100.degree. C. 6.58 113.1 18.1 6.8 VZ0821-1B1
HST = 200.degree. C. 8.97 117.1 12.1 6.6 VZ0821-1C1 HST =
300.degree. C. 8.49 113.6 14.0 6.2 VZ0821-1D1 HST = 400.degree. C.
7.46 112.3 16.3 7.0 VZ0821-1E1 HST = 500.degree. C. 8.18 120.0 13.8
7.5 VZ0821-1F1 HST = 600.degree. C. 8.67 118.5 12.9 6.6 VZ0821-1G1
HST = 700.degree. C. 2.14 44.9 18.0 45.6
[0064] Articles heated at a hydrogenation temperature between
100.degree. C. and 600.degree. C. have an increased magnetic phase
transition temperature of between 112.degree. C. and 120.degree. C.
compared to a value of -18.5.degree. C. for the unhydrogenated bulk
precursor article. For a hydrogenation temperature of 700.degree.
C., an increased alpha-iron proportion as well as a lower magnetic
phase transition temperature of around 45.degree. C. and increased
peak width of 18.degree. C. was observed indicating that the
magnetocalorically active phase has partially decomposed.
[0065] The hydrogen content was determined using chemical methods
for the samples and the measured values are summarised in Table 2.
The hydrogen content of all the articles lies within 0.2325 wt %
and 0.2155 wt %.
TABLE-US-00002 TABLE 2 Sample Hydrogen content [%] ratio La:H
comparison, 0.0090 unhydrogenated state HST = 100.degree. C. 0.2310
1:1.91 HST = 200.degree. C. 0.2325 1:1.92 HST = 300.degree. C.
0.2325 1:1.92 HST = 400.degree. C. 0.2210 1:1.82 HST = 500.degree.
C. 0.2195 1:1.81 HST = 600.degree. C. 0.2185 1:1.80 HST =
700.degree. C. 0.2155 1:1.78
[0066] The magnetic phase transition temperature of an article for
use as the working medium in a magnetic heat exchanger translates
into its operating temperature. Therefore, in order to be able to
provide cooling and/or heating over a large temperature range, a
working medium comprising a range of different magnetic phase
transition temperatures is desirable.
[0067] In principle, by hydrogenating bulk samples so that the
hydrogen content of the article varies, i.e. by partially
hydrogenating the article, different magnetic phase transition
temperatures may be provided. Therefore, a plurality of articles of
different magnetic phase transition temperature may be used
together as the working medium in the magnetic heat exchanger so as
to increase the operating range of the heat exchanger.
[0068] In a first group of experiments, the hydrogenation
conditions were adjusted in order to control the amount of hydrogen
taken up by the article so that articles of differing hydrogen
content and differing magnetic phase transition temperatures can be
produced.
[0069] Five bulk precursor articles having a size and composition
as listed above were wrapped in iron foil and heated in inert gas
to a hydrogenation temperature in the range 300.degree. C. to
500.degree. C. At the hydrogenation temperature, the inert gas was
exchanged for 1.9 bar of hydrogen and the articles held at the
hydrogenation temperature for 10 minutes. After 10 minutes, the
hydrogen was exchanged for inert gas, the heating element was
removed from the furnace and the working chamber of the furnace
cooled with forced air as fast as possible to a temperature below
50.degree. C.
[0070] For two samples, hydrogenation was carried out at
350.degree. C. and 450.degree. C., respectively, and the samples
cooled to 200.degree. C. and 250.degree. C., respectively, before
the hydrogen was exchanged for argon.
[0071] For hydrogenation temperatures of 350.degree. C. and above,
the articles were found to be intact. Also the two samples at which
the gas exchange took place at 200.degree. C. and 250.degree. C.,
but which were initially heated in a hydrogen-containing atmosphere
at a temperature above 350.degree. C. were also found to be intact
after the heat treatment.
[0072] The measured magnetocaloric properties of the samples are
summarised in Table 3. The entropy change of the samples was
measured for a magnetic field change of 1.6 T and the results are
illustrated in FIG. 1.
TABLE-US-00003 TABLE 3 Entropy Peak Peak change temperature width
.DELTA.S'.sub.m.max, T.sub.C .DELTA.T.sub.WHH Sample Heat treatment
[J/(kg K)] [.degree. C.] [.degree. C.] MPS-1030 None starting
material 9.4 -18.5.degree. C. 13.7 VZ0821-1L1 TH = 500.degree. C.,
10 min, gas exchange 6.6 -3.2.degree. C. 20.3 VZ0821-1M1 TH =
450.degree. C., 10 min, gas exchange 6.6 -2.9.degree. C. 19.8
VZ0821-1N1 TH = 400.degree. C., 10 min, gas exchange 6.9
14.2.degree. C. 19.4 VZ0821-1O1 TH = 350.degree. C., 10 min, gas
exchange 7.4 17.8.degree. C. 17.5 VZ0821-1P1 TH = 300.degree. C.,
10 min, gas exchange 6.8 37.6.degree. C. 18.9 VZ0821-1R1 TH =
450.degree. C., OK auf 250.degree. C. 7.3 88.7.degree. C. 16.8 gas
exchange VZ0821-1S1 TH = 350.degree. C., OK auf 200.degree. C. 7.7
97.0.degree. C. 15.9 gas exchange
[0073] The relationship between the magnetic phase transition
temperature and the gas exchange temperature is also illustrated in
FIG. 2. FIG. 2 shows a general trend that with increasing gas
exchange temperature, the magnetic phase transition temperature
decreases. In the temperature region from 250.degree. C. to
300.degree. C. a strong dependence of the magnetic phase transition
temperature with the gas exchange temperature is observed.
[0074] The hydrogen content of the samples was determined using
chemical techniques and the results are summarised in Table 4 and
FIG. 3. FIG. 3 illustrates a generally linear relationship between
the magnetic phase transition temperature and the measured hydrogen
content of the samples.
TABLE-US-00004 TABLE 4 sample Hydrogen content [%] ratio La:H
comparison 0.0090 GAT = 500.degree. C. 0.0324 1:0.27 GAT =
450.degree. C. 0.0337 1:0.28 GAT = 400.degree. C. 0.0576 1:0.48 GAT
= 350.degree. C. 0.0621 1:0.51 GAT = 300.degree. C. 0.0818 1:0.68
GAT = 250.degree. C. 0.1615 1:1.33 GAT = 200.degree. C. 0.1750
1:1.44
[0075] Curie temperatures in the range of -3.2.degree. C. and
97.degree. C. and hydrogen contents in the range of 0.0324 wt % and
0.1750 wt % were obtained.
[0076] This method therefore, enables polycrystalline sintered or
reactive sintered articles for use as the working medium in the
heat exchanger to be fabricated with differing magnetic phase
transition temperatures and differing hydrogen content.
[0077] A set of articles having differing Curie temperatures may be
used together as the working medium of a magnetic heat exchanger in
order to extend the operating range of the magnetic heat exchanger.
The magnetic heat exchanger is able to heat and/or cool over a
temperature range generally corresponding to the range of the
magnetic phase transition temperatures of the working medium.
[0078] In a second set of embodiments, articles with differing
magnetic phase transition temperatures were fabricated by
dehydrogenating fully hydrogenated or near fully hydrogenated bulk
precursor articles comprising the magnetocalorically active phase
described above.
[0079] The hydrogenated bulk precursor articles were fabricated by
heating the samples in an inert gas to 450.degree. C. and, at
450.degree. C., exchanging the inert gas for 1.9 bar of hydrogen.
After a dwell time of two hours at 450.degree. C. in the hydrogen
atmosphere, the samples were furnace cooled in a hydrogen
atmosphere to a temperature of less than 50.degree. C.
[0080] To partially dehydrogenate the now fully hydrogenated or
near fully hydrogenated articles, the articles were heated at one
of three different temperatures 200.degree. C., 250.degree. C. and
300.degree. C. for different times in air. In particular, 10
samples were placed in a preheated oven and then the samples
removed individually after a different dwell time in a range of 10
minutes to 1290 minutes. The magnetocaloric properties of the
samples were measured.
[0081] The results for samples heated at a temperature of
200.degree. C. are summarised in Table 5. The entropy change at 1.6
T measured for these articles is illustrated in FIG. 4 and the
dependence of the magnetic phase transition temperature as a
function of dwell time at 200.degree. C. is illustrated in FIG.
5.
TABLE-US-00005 TABLE 5 Entropy Peak Alpha Dwell change Peak width
Fe time at .DELTA.S'.sub.m.max, temperature .DELTA.T.sub.WHH
content sample 200.degree. C. [J/(kg K)] T.sub.C [.degree. C.]
[.degree. C.] [%] VZ0826-1A1 none 8.30 113.6 14.3 6.7 VZ0826-1B1 10
min 7.91 111.3 15.0 7.6 VZ0826-1C1 30 min 7.62 101.5 15.7 8.6
VZ0826-1D1 60 min 7.37 93.1 16.1 8.2 VZ0826-1E1 120 min 7.00 95.6
17.3 8.4 VZ0826-1F1 240 min 6.87 82.3 18.5 8.3 VZ0826-1G1 390 min
6.45 64.0 19.2 8.9 VZ0826-1H1 810 min 6.30 55.9 20.0 8.6 VZ0826-1I1
1290 min 6.32 46.9 19.9 8.6
[0082] The entropy change measured at 1.6 T for samples heated for
different times at 250.degree. C. and 300.degree. C. are
illustrated in FIGS. 6 and 7 and summarized in Tables 6 and 7.
TABLE-US-00006 TABLE 6 Entropy Peak Alpha- Dwell change Peak width
Fe time at .DELTA.S'.sub.m.max, temperature .DELTA.T.sub.WHH
content Sample 250.degree. C. [J/(kg K)] T.sub.C [.degree. C.]
[.degree. C.] [%] VZ0826-1A1 none 8.30 113.6 14.3 6.7 VZ0827-1B1 10
min 6.24 98.2 21.0 8.3 VZ0827-1C1 30 min 4.42 85.4 33.0 8.8
VZ0827-1D1 60 min 6.01 55.9 20.4 10.1 VZ0827-1E1 120 min 5.68 46.5
22.5 9.5 VZ0827-1F1 240 min 5.37 29.6 23.7 10.4 VZ0827-1G1 480 min
4.95 17.4 26.2 11.1 VZ0827-1H1 960 min 4.20 15.5 33.3 11.9
TABLE-US-00007 TABLE 7 Entropy Peak Alpha- Dwell change Peak temp-
width Fe time at .DELTA.S'.sub.m.max, erature T.sub.C
.DELTA.T.sub.WHH content Sample 300.degree. C. [J/(kg K)] [.degree.
C.] [.degree. C.] [%] VZ0828-1A1 none 8.16 117.0 14.5 6.9
VZ0828-1B1 10 min 5.38 81.8 23.4 10.1 VZ0828-1C1 30 min 4.76 56.2
29.0 11.2 VZ0828-1D1 60 min 4.99 45.0 26.1 10.4 VZ0828-1E1 120 min
4.63 26.8 29.5 10.5 VZ0828-1F1 240 min 4.44 4.0 30.3 12.5
[0083] The Curie temperature as a function of dwell time for
articles heated at the three different temperatures are illustrated
in the comparison of FIG. 8.
[0084] Generally, the magnetic phase transition temperature is
reduced for increasing dwell time. Furthermore, for increased
temperature, the reduction in the magnetic phase transition
temperature occurs more quickly. The relationship between magnetic
phase transition temperature and dwell time is approximately
logarithmic for all three temperatures.
[0085] For a temperature of 250.degree. C. and 300.degree. C., the
change in entropy is slightly reduced and the peak width is
increased for the partially dehydrogenated samples in comparison to
the fully hydrogenated precursor sample. This indicates that the
dehydrogenation may be more inhomogeneous than that achieved at
200.degree. C. although the dehydrogenation occurs more quickly.
Additionally, the alpha iron content was found to increase at
250.degree. C. and 300.degree. C. which may indicate that some of
the magnetocalorically active phase has decomposed due to
oxidation.
[0086] FIG. 9 illustrates a graph of entropy change as a function
of temperature for three articles with differing metallic element
compositions. The magnetocaloric properties are summarized in Table
8.
TABLE-US-00008 TABLE 8 Entropy change Peak Peak Alpha-Fe
.DELTA.S.sub.m.max, temperature width content [J/(kg K)] T.sub.PEAK
[.degree. C.] .DELTA.T.sub.WHH [.degree. C.] [%] Nr. 1 11.10 29.81
9.76 3.53 Nr 2 20.24 70.64 6.24 4.35 Nr. 3 8.97 117.06 12.09
6.58
[0087] Sample Nr. 1 has a composition of 17.88 wt % La, 4.34 wt %
Si, 0.03 wt % Co and 1.97 wt % Mn, rest Fe. The Co and Mn is
substituted for Fe. Sample 1 was sintered at 1120.degree. C. and
then annealed at 1050.degree. C. Sample Nr 1 was subsequently
hydrogenated by heating it from room temperature to 500.degree. C.
in an argon atmosphere and exchanging the gas for 1.9 bar of
hydrogen at 500.degree. C. After a dwell time of 15 min in the
hydrogen atmosphere at 500.degree. C., the sample was furnace
cooled at an average cooling rate of 1K/minute in the hydrogen
atmosphere to a temperature of less than 50.degree. C.
[0088] Sample Nr. 2 has a composition of 17.79 wt % La, 3.74 wt %
Si, 0.06 wt % Co and 0 wt % Mn, rest Fe. The Co is substituted for
Fe. Sample 2 was sintered at 1100.degree. C. and then annealed at
1040.degree. C. Sample Nr 2 was subsequently hydrogenated by
heating it up from room temperature to 500.degree. C. in an argon
atmosphere and exchanging the gas for 1.9 bar of hydrogen at
500.degree. C. After a dwell time of 15 min in the hydrogen
atmosphere at 500.degree. C., the sample was furnace cooled at an
average cooling rate of 1K/minute in the hydrogen atmosphere to a
temperature of less than 50.degree. C.
[0089] Sample Nr. 3 has a composition of 18.35 wt % La, 3.65 wt %
Si, 4.51 wt % Co and 0 wt % Mn, rest Fe. The Co is substituted for
Fe. Sample 1 was sintered at 1080.degree. C. and then annealed at
1030.degree. C. Sample Nr 3 was subsequently hydrogenated by
heating it from room temperature to 500.degree. C. in an argon
atmosphere and exchanging the gas for 1.9 bar of hydrogen at
500.degree. C. After a dwell time of 15 min in the hydrogen
atmosphere at 500.degree. C., the sample was furnace cooled at an
average cooling rate of 1K/minute in the hydrogen atmosphere to a
temperature of less than 50.degree. C.
[0090] Table 8 illustrates that as the Co content is increased, the
magnetic transition temperature increases. Sample 1 which includes
Mn substitutions has a lower magnetic transition temperature.
[0091] A working medium for a magnetic heat exchanger is provided
which comprises at least one article which comprises a
NaZn.sub.13-type crystal structure and hydrogen. The article may
have at least one outer dimension which is at least 5 mm. For a
working medium which includes two or more of these articles, the
articles may have differing hydrogen contents and differing Curie
or magnetic phase transition temperatures. The articles may be
fully- or near fully hydrogenated as well as partially
hydrogenated.
[0092] The partially hydrogenated articles may be produced by
adjusting the temperature at which hydrogenation is carried out as
well as by exchanging the hydrogen atmosphere for an inert
atmosphere at the hydrogenation temperature or at temperatures
above about 150.degree. C. during the cooling of the article from
the hydrogenation temperature.
[0093] For both fully-hydrogenated as well as partially
hydrogenated articles, hydrogen is introduced into the furnace
containing the articles only once the furnace has been heated up to
a temperature above 300.degree. C. This prevents the physical
disintegration of the bulk precursor article so that a solid bulk
article comprising hydrogen can be provided. Furthermore, the
entropy change is largely unaffected by the hydrogenation treatment
so that the hydrogenated article can provided an efficient working
medium for a magnetic heat exchanger.
[0094] In a further method, fully or near fully hydrogenated
articles are dehydrogenated to remove some or all of the hydrogen.
Since the magnetic transition temperature depends on the hydrogen
content, articles of different magnetic phase transition
temperature may be provided by controlling the degree of
dehydrogenation. Increased dwell times at temperatures in the range
150.degree. C. and 400.degree. C. lead to decreasing hydrogen
content and decreasing magnetic transition temperature.
[0095] The invention having been described herein with respect to
certain of its specific embodiments and examples, it will be
understood that these do not limit the scope of the appended
claims.
* * * * *