Magnetocaloric Materials for Freezing, Cooling, and Heat-Pump Applications - Magnetocaloric Energy Conversion: From Theory to Applications - page 27

2.1.4 Near-Zero Hysteresis of the Magnetocaloric Effect

The MCMs should have as small a hysteresis as possible. The hysteresis occurs as
the magnetic hysteresis (during an alternating magnetic eld) and the thermal

hysteresis (during heating and cooling). It should be noted that the hysteresis is, in
general, related with the

rst-order phase transition and its structural changes, and
in general does not occur in a second-order phase transition materials (e.g. Gd),
which is a great advantage. However, both hystereses result in an energy loss and
therefore, an increase in the input work of the thermodynamic cycle as the result of
the entropy generation [ 33 ]. This can drastically reduce the MCE during the cycling
operation as well as the ef ciency of the magnetocaloric device. The impact of the

hysteresis on the performance of the magnetic refrigerator can be found in [ 34 , 35 ].

2.1.5 High Thermal Conductivity and Diffusivity

In general, the thermal conductivity and thermal diffusivity of the MCM should be
as high as possible, since it ensures a faster temperature response and a more intense
heat transfer between the material and the heat-transfer uid. However, the high

thermal conductivity of the MCM can also reduce the AMR

fl

' s performance due to

the heat fl ux along the direction of the temperature gradient in material, parallel to

the ow. This is especially pronounced in the case of a shorter AMR with an

ordered geometry (where the material in AMR is continuous along its length) and a
large temperature span. As shown in Nielsen and Engelbrecht [ 36 ], the optimal
thermal conductivity of the MCM applied in a parallel-plate AMR strongly depends
on the length of the AMR and the operating frequency. They showed that in the
case of a long AMR (200 mm) the thermal conductivity should be as high as
possible (up to 30 Wm − 1 K − 1 ), regardless of the operating frequency (up to 4 Hz)
and the temperature span, while in the case of a shorter AMR (50 mm) there is an
optimal thermal conductivity for each operating frequency (the higher the frequency
the higher the optimal thermal conductivity will be: around 10 Wm − 1 K − 1 at 1 Hz
and 30 Wm − 1 K − 1 at 4 Hz). For example, Gd and its alloys with Er and Tb have a
thermal

fl uid fl

conductivity around 10 Wm − 1 K − 1 ,La Fe Co Si alloys around

- - -

8Wm − 1 K − 1 and La MnO 3 ceramics around 1 Wm − 1 K − 1 . Ca Sr

- - -

2.1.6 Good Manufacturing Properties

It is desirable for the MCMs to have good manufacturing, casting, mechanical and
processing properties, which allow them to be fabricated into the desired shape,
suitable for use in an ef cient AMR. The impact of the geometrical properties of the