Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications - Magnetocaloric Energy Conversion: From Theory to Applications - page 30

temperature at least two times higher than that of pure Gd. However, due to its rst-

order nature, Gd Ge alloys display a high magnetic hysteresis, which can

drastically contribute to the parasitic losses in the magnetocaloric device.
However, the main drawback of Gd (and its second-order transition alloys) is its
price, which limits its practical application. However, its magnetocaloric, thermal
and manufacturing properties and the absence of hysteresis make it currently the
best MCM for room-temperature magnetic refrigeration.

Si

- -

2.2.2 La Fe Si-Based MCMs

- -

La Si-based MCMs are well represented in magnetic refrigeration and are

considered to be one of the possible alternatives to the expensive Gd-based MCMs.
The basis of La

Fe

- -

Si materials is a hypothetical compound LaFe 13 , which does
not exist. However, by substituting a certain proportion of the Fe for Si or Al one
can make a stable compound. For instance, in 2001 Hu et al. [ 46 ] discovered a

- Fe -

rst-
order transition at 208 K in the compound LaFe 11.4 Si 1.6 . Later researchers dis-
covered that the Curie temperature can be tuned by adding H to the structure of
La

Si, as, for example, was presented by Fujita et al. in 2003 [ 47 , 48 ]. Fur-
thermore, researchers also found that the T C may also be tuned by partially
substituting Fe with Al, Co or Mn. This was presented by several authors, such as
Katter et al. [ 49 ], Hansen et al. [ 50 ] and Bj

- Fe -

ø rk et al. [ 38 ]. These kinds of substi-

tutions may also alter a material rst to second order.

Nowadays, there is a substantial number of different La

' s transition from

Si-based MCMs,
which were thoroughly reviewed by Shen et al. [ 24 ]. One of the major issues
regarding such materials is their long-term stability. However, this can be avoided
by properly processing the material [ 51 ]. La

Fe

- -

Si-based MCMs have a great
potential to be used in layered AMRs, since their T C may be tuned in a temperature
range from approximately 200 to 340 K. Regarding their magnetocaloric properties,
La

Fe

- -

Si-based materials exhibit a larger magnetic entropy change than that of Gd.
It may vary from approximately 5 to 12 Jkg − 1 K − 1 (regarding magnetic

Fe

- -

eld change
of 1.6 T) [ 49 , 51 ], depending on the material. The adiabatic temperature change is,
for a magnetic eld change from 0 to 1.4 T, in the range of 2.8 K [ 38 ]. La Fe Si-

- -

based materials have a substantially higher speci c heat than Gd (from approx.

1,200 Jkg − 1 K − 1 at 0 T to 700 Jkg − 1 K − 1 at 1.4 T [ 38 , 51 ]).
The reasons why La

Si-based MCMs are so appealing for use in magnetoca-
loric devices lie mostly in their low cost (in comparison to Gd). Some of the materials
also exhibit no or low magnetic hysteresis, which is positive from the device-per-
formance point of view. Moreover, the technology for producing such materials and
then processing them is available for large-scale industrial production [ 49 ].

- Fe -