Environmental Engineering Reference
In-Depth Information
2.2.1 Gd and Its Alloys
Gadolinium (Gd) is de
nitely the most common MCM for magnetic refrigeration
near room temperature. It is the only pure element that exhibits a MCE near room
temperature (
*
293 K). Furthermore, its magnetocaloric properties are fairly good
T
ad
= 3.3 K, c
H
= 300 Jkg
−
1
K
−
1
,
s
M
= 3.1 Jkg
−
1
K
−
1
at magnetic
(
eld change of
1T[
38
]), making it a strong candidate for use in magnetic refrigeration. Actually,
gadolinium has already been thoroughly investigated and characterized for use as a
constituent of various AMRs in different magnetic refrigeration devices as it is
reviewed later in Chap.
7
on magnetic prototypes. As a result, Gd is known as a
kind of reference material when considering different MCM candidates for an AMR
design. However, one of the most important factors when choosing Gd is its purity.
As was shown by Dan
D
D
'
kov et al. [
39
], different impurities in Gd may signi
cantly
alter its magnetocaloric properties.
However, introducing different amounts of other elements to make alloys with
Gd can also have positive effects. Especially in terms of designing layered AMRs.
For instance, different ratios of Gd and Mn in Gd
Mn alloys can lower the T
C
to
278 K without any drastic changes in the MCE, as was shown by Jayaraman et al.
[
40
,
41
]. Furthermore, for example, in Gd
-
R alloys, where R is some other rare-
earth element (Tb, Dy, Ho, Er) the T
C
may also be shifted to lower temperatures
[
42
]; however, without any drastic changes in their MCEs. For instance, Ka
-
til et al.
š
[
43
] presented the MCE in Gd
Tb alloys. By changing the Tb content they could
shift the Curie temperature of Gd
-
Tb alloys in the temperature range from 269 to
294 K, with an average adiabatic temperature change of approximately 2.5 K for a
1 T magnetic
-
eld change.
On the other hand, the Curie temperature T
C
may also be shifted above that of pure
Gd, which can be suitable in magnetic heat pumping or magnetic power generation.
For instance, Couillaud et al. [
44
] presented the magnetocaloric properties of two
MCMs, Gd
Si. The former has a T
C
of 348 K and a magnetic
entropy change
D
s
M
of 2.5 Jkg
−
1
K
−
1
from 0 to 1.5 T, while the latter has a T
C
of
252 K and
Sc
Ge and Gd
Sc
-
-
-
-
D
s
M
of 2 Jkg
−
1
K
−
1
when changing the magnetic
eld from 0 to 1.5 T.
Furthermore, Law et al. [
45
] showed that different ratios of elements in Fe
B
alloys can lead to an increase in T
C
above 400 K. However, the magnetic entropy
change for the materials with a Curie temperature around such high temperatures can
decrease to approximately 1 Jkg
−
1
K
−
1
for magnetic
Gd
Cr
-
-
-
eld change from 0 T to 1.5 T.
However, there is a group of Gd-based alloys that exhibit a remarkable MCE,
noticeably higher than that of pure Gd and the above-mentioned alloys. These are
the
Ge alloys. These alloys exhibit the so-called giant magnet-
ocaloric effect. The giant MCE was discovered in 1997 in Gd
5
Si
2
Ge
2
by Pecharsky
and Gschneidner [
22
]. The same researchers later showed that by varying the Si-to-
Ge ratio and by introducing small amounts of Ga into the Gd
rst-order Gd
Si
-
-
Ge the giant MCE
may be tuned in the temperature range between approximately 20 and 305 K [
42
].
The MCE (in terms of magnetic entropy change) of Gd
Si
-
-
Si
Ge is near room
-
-
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