Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications - Magnetocaloric Energy Conversion: From Theory to Applications - page 26

temperature of the phase transition of the magnetic material between the ferromag-
netic and paramagnetic phases, which is related to the most pronounced magnetoc-
aloric effect. It should be noted that the magnetocaloric effects at temperatures that are
relatively far away from the Curie temperature are practically negligible (depending
on the width of the temperature range of the magnetocaloric effect).

2.1.2 The Intensity of the Magnetocaloric Effect

The most important criterion for the selection of a MCM is the intensity of its
magnetocaloric effect. The MCE manifests itself as the adiabatic temperature
change and/or isothermal entropy change, which are related through the speci

c
heat of the material (see Eqs. ( 1.14 )and( 1.19 )). It should be noted that for
applications of the MCM in the AMR its adiabatic temperature change is more
important than the isothermal entropy change. The material is, therefore, more
suitable for an application if it has a greater adiabatic temperature change on
account of the smaller isothermal entropy change. This is strongly related to the
heat-transfer characteristics between the material and the heat-transfer medium,
since the heat-transfer irreversibility losses can strongly reduce the device

'

s per-
formance in the case of a small adiabatic temperature change, as is also explained in
Sect. 4.5 . A detailed analysis of the impact of the adiabatic temperature change and
the isothermal entropy change on the AMR ' s performance is presented in [ 32 ].

2.1.3 The Wide Temperature Range of the Magnetocaloric
Effect

It is a great advantage for the MCM to have a (large) MCE over as wide a
temperature range as possible. This is especially important in an AMR where the
temperature span is established over the material. With a wide temperature range for
the MCE we ensure that the intense MCE occurs over the entire material, even in
the parts of the material that are temperature-wise away from its Curie temperature.
Since the great majority of currently known MCMs exhibit a MCE over a relatively
narrow temperature range, a layering of different MCMs with different Curie
temperatures along the length of the AMR (in a direction of the temperature gra-
dient) is required. As also explained and shown in Sect. 4.2 , the layering also
ensures an intense MCE over the entire length of the AMR (with the established
temperature pro le). It should also be noted that the MCE of second-order phase-

transition materials like Gd occurs over a relatively wide temperature range com-
pared to the rst-order phase-transition materials, e.g. Mn Fe P and La Fe Si

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alloys (see Sect. 4.2 ), where layering is therefore more important.