SECTION 12.6. MANUFACTURING TECHNOLOGIES OF RARE-EARTH-BASED MAGNETS 121offers the possibility to reach almost perfect particle alignment in the alignment step (see
also Fig. 12.6.1). Numerous investigations have shown that the ultimate sintered magnets
have a sufficiently high coercivity only if the particles are present in sufficiently
small grains. The coercivity in permanent magnets is nucleation controlled
(see Section 12.4). One of the reasons why small particle sizes favor high coercivities is that
the smaller the particle the lower the probability that it will contain an imperfection acting
as nucleation center. It can be never completely avoided that some of the fine particles
present contain nucleation centers and hence a fraction of the particles will be prone to
magnetization reversal in a demagnetizing field. The magnetization reversal will affect,
however, only the latter particles and not spread into the whole magnet body. The overall
magnetization reversal will therefore be very modest for small particle sizes and may even
remain unnoticed.
In order to obtain an anisotropic magnet with the highest possible magnetization in
a given direction, the powder particles have to be aligned after milling by means of an
external magnetic field. After the magnetic alignment, the powder is pressed isostatically to
yield a compact powder that, after sintering, has a sufficiently high density. It is commonly
assumed that the degree of particle alignment does not change during isostatic pressing.
Generally speaking, it is desirable to apply a high compacting pressure, but this pressure
should not be chosen too high because it may then cause severe particle misorientation.
Particle alignment and pressing can also be performed simultaneously. A non-magnetic die
is used in this case, the desired magnetization direction being determined by the direction
of the magnetic field set up in the cavity of the die.
The sintering step is essential for attaining high values of the ultimate magnetization
and coercivity. Isostatic pressing or die-pressing alone is known to lead to densities of
only 80% of the theoretical density. Liquid-phase sintering leads to much higher densities,
up to 99% of the theoretical density. In that case, the overall composition of the alloy is
chosen in such a way that after casting, small amounts of a low-melting alloy component
are present. Sintering is then performed at a temperature low enough for the main phase
to remain solid. Only the second phase melts and makes mass transport pos
sible during sintering with the ultimate result that all voids disappear and all
grains are surrounded by a thin layer of the low-melting intergranular material. At room
temperature and above, the intergranular material is non-magnetic. It magnetically isolates
the grains and prevents magnetization reversal to spread into the whole mag
net body if in one (or more) of the grains a domain wall is nucleated in a demagnetizing
field. We mentioned already that the presence of very small, magnetically well-isolated,
particles is important for achieving high coercivity. The liquid-phase sintering has a sec
ond equally important advantage. The disappearance of voids and the concomitant high
density implies a high magnetization per unit volume or per unit mass. This is of prime
importance for the manufacture of magnets with high energy products because, as we
showed already in Section 12.3, the energy product of (ideal) permanent magnets is pro
portional to the magnetization squared. A third advantage of the liquid-phase sintering is
the absence of porosity in the ultimate magnet body, making it more resistant to corrosion
and giving it a substantially higher mechanical strength than would have been obtained by
pressing alone. More sophisticated manufacturing routes have been reviewed by Buschow
(1998).