ALBERTO BOLLERO REAL

5.1 Nanocrystalline powder processing
The HDDR process utilizes a reversible hydrogen induced phase transformation
which yields highly coercive NdFeB- and SmFe-type powders, very suitable for the
production of both bonded and fully dense hot pressed permanent magnets [103-105]. In
the case of Nd 2 Fe 14 B, according to equation (5.1) the conventional-HDDR process consists
of two stages: firstly, the disproportionation into a finely divided mixture of neodymium
hydride, iron and ferroboron typically at 800°C and 10 5 Pa hydrogen and secondly, the
desorption at a similar temperature leading to the recombination of the original Nd 2 Fe 14 B
phase, but now with a much refined grain size.
Nd 2 Fe 14 B + (2 ± δ)H 2 ⇔ 2NdH 2 ± δ + 12Fe + Fe 2 B ± ∆H (5.1)
The value of δ depends on temperature and hydrogen pressure and ∆H is the reaction
enthalpy. The reactive milling technique applied in this work is shown schematically in
Fig. 5.1. It is performed in the same way as the intensive milling technique but now argon
is substituted by hydrogen at high pressure and the milling is performed at enhanced
550 - 850°C
II
temperature
I
~ 300°C
vacuum
~ 5 × 10 5 Pa
hydrogen
time
Fig. 5.1: Schematic representation of the reactive milling procedure (I) and the subsequent
recombination step in a vacuum furnace (II).
temperature. The milling creates new surfaces to absorb the hydrogen. The energy input
via the ball collisions might lead to local temperature peaks providing the kinetics for
metal atom diffusion. The result is a very effective hydrogenation and a subsequent
disproportionation of thermodynamically very stable compounds due to the high pressure
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