Environmental Engineering Reference
In-Depth Information
The characteristic of a second-order phase transition is the continuous change of
the magnetization around the Curie temperature (Fig. 2.1 a), while in the
rst-order
phase transition the magnetization changes discontinuously at some temperature
(Fig. 2.1 b) with associated structural-deformation.
Regarding the magnetic entropy change, in second-order materials the magnetic
entropy change increases with a larger magnetic
rst-order magnetic
materials the entropy change only increases drastically to a certain value of mag-
netic
eld. In the
eld the magnetic entropy change becomes
considerable over a wider temperature range.
Adiabatic temperature changes for both phase transitions increase their values
with increasing
eld. However, with a larger
eld. However, in second-order materials the peak is broader than
in
rst-order materials. Another important distinction between second- and
rst-
order is in the speci
c heat (Fig. 2.1 c, d). In second-order materials the speci
c heat
is sharply peaked with a lower
eld and then decreases and broadens the peak
without any signi
cant shift in the peak temperature, while in
rst-order materials
the speci
c heat signi
cantly changes its peak-temperature position with larger
magnetic
elds, whilst not changing the peak values drastically.
Since this topic is focussed more on an engineering approach to research and the
design of magnetocaloric devices, the next pages of this chapter will present dif-
ferent MCMs from the engineering point of view rather than that of a material
scientist. Some of the MCMs that are, at least at the moment, the most promising,
will be presented. In this way, an engineer reading this topic could get some initial
impression about which direction she or he could focus her or his research and
design of magnetic devices. Furthermore, some important issues and aspects
regarding other characteristics (e.g. thermal, mechanical, chemical properties) will
also be discussed.
2.1 General Criteria for the Selection of the Magnetocaloric
Material
The MCMs as the coolants and the regeneration materials represent the most crucial
elements of the magnetic refrigerator. Therefore, it is very important to apply the
best material possible for a particular application. In general, they should have the
following properties (see also e.g. [ 31 ]):
2.1.1 Suitable Curie Temperature of the Material
A precondition for the application of a MCM for a particular application is the
suitability of its Curie temperature. With this we ensure that the MCE occurs at
the required temperature or temperature range. The Curie temperature represents the
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