Active Magnetic Regeneration - Magnetocaloric Energy Conversion: From Theory to Applications - page 151

4.7 Review of Processing and Manufacturing
Techniques for AMRs

It was shown earlier in this chapter that a ne AMR geometry (the micro-channel

range of wall uid voids that are well below 1 mm) with a

homogenous porosity is one of the crucial preconditions for ef

thickness and fl

cient operation. The
so far evaluated AMR geometries were limited to more or less simple solutions of
packed beds of powders, grains, or spheres and parallel-plate structures [ 5 ]. The

rst type of AMR suffers from high viscous losses, while the second has relatively
poor heat transfer properties [ 76 ]. The geometries that can have better thermohy-
draulic properties (e.g. corrugated plates, honeycomb structures, different foams)
are dif

cult to manufacture with currently available magnetocaloric materials and
conventional processing technologies. However, some recent achievements in
powder metallurgy, e.g. [ 128 ] might enable the fabrication of different micro-
channel AMRs, also with currently the most interesting La-based (i.e. La Fe

-

Co Si), Mn-based (i.e. Mn Fe P As; Mn Fe P Si) and perovskite-type man-

- - - - - - - -

ganese oxides based (i.e. La MnO 3 ) magnetocaloric materials.

In this section, some of the most promising applied fabrication techniques for
AMRs are reviewed. The presented methods are generally divided into two groups:
the fabrication of Gd-based AMRs, which are currently limited to packed-bed and
parallel-plate structures, and the fabrication of sintering-based AMRs, which in
general makes possible more advanced structures.

Ca Sr

- - -

4.7.1 Fabrication of Gd-based AMRs

Gadolinium is a metal with relatively good malleable and ductile properties. This
means that it can be formed into different geometries, like thin plates

(>50 m), wires, spheres, and cylinders using standard forming technologies

(Fig. 4.34 ). These are also the only four geometries (and powder) of Gd evaluated
in an AMR to date. The thin Gd plates or wires that are needed for an ef

100 ʼ

-

cient AMR

are too brittle to be further formed into more ef cient heat transfer geometries, like

corrugated plates, honeycomb structures or packed wire screens. However, it is to
be expected that further research in this

eld would lead to more advanced, Gd-
based AMRs. By Gd-based, we mean pure gadolinium or different gadolinium
alloys with other materials (mostly Er, Tb or Dy).
The fabrication of a packed-bed AMR is relatively straightforward. This holds
for all packed-bed AMRs and not only those which are Gd-based, while packed-bed
AMRs with any other magnetocaloric materials are limited to powder. However,
special attention should be given to the in ll of particles into the AMR housing. In

order to achieve as homogenous porosity as possible, the AMR should be lled

gradually and stacked in between, so the particles can nd their optimum positions.

Furthermore, the particles should be xed in their position so their movement