Magnetic Field Sources - Magnetocaloric Energy Conversion: From Theory to Applications - page 55

Fig. 3.15 The temperature
dependence of the magnetic
characteristics (magnetization
— dashed lines and magnetic
flux density) of a permanent
magnet

In the second case (Fig. 3.14 b), the coercivity is smaller than the remanence of
the permanent-magnet material (e.g. AL CO) and it does not differ much from

the intrinsic coercivity. Such a magnet will resist a relatively small

NI

- -

eld of

demagnetization. If magnetic elds that are higher than those at double the value of

the (BH) max are applied, this will lead to a full demagnetization of the magnet.
Therefore, it is very important in a magnet

' s design that the demagnetization of a

magnet does not occur.
A high intrinsic coercivity usually means magnets with rare-earth materials and
magnets with an intrinsic magnetocrystalline anisotropy. In materials that are based
on shape anisotropy (such as AL - NI - CO materials), these will not possess a high
coercivity.
As was noted at the beginning of this section, the maximum energy product
(BH) max is not the only criterion on which the selection of an appropriate magnet
material should be made. Additionally, one should also consider the magnitude of
the reverse magnetic elds that do not harm the properties of the magnet. Therefore,

the recoil line is one of the important criteria, especially in structures where very
high changes in the demagnetization magnetic can occur (for

more information see Buschow and de Boer [ 15 ] and Campbell [ 14 ]).
In certain cases magnets can be exposed to high temperatures in the environment
(note also that eddy currents may cause the heating of a magnet assembly). In this
particular case one should consider the temperature dependence of the magnet
material in terms of the remanence and the coercivity (see Fig. 3.15 ) as an example.

eld in the “ air gap ”

3.2.1 Permanent Magnet Materials

Permanent-magnet materials can generally be divided into groups of ceramic
materials, Al

Co materials, rare-earth materials and polymer-bonded materials.
The last group at present does not provide appropriate characteristics to be suc-
cessfully applied in magnetocaloric energy conversion. We will brie

- Ni -

fl y present the

rst three groups of permanent-magnet materials.