Environmental Engineering Reference
In-Depth Information
For example, permanent magnets exhibit strong flux retention in their Martensite
state of tetragonal crystals and strong ferromagnetic phase. In this phase, iron will
gradually lose its flux carrying properties as the temperature rises to the Curie tem-
perature of approximately 1,100 K, at which point its magnetization is zero. A
material that exhibits a strong phase transition, such as nickel-manganese-gallium,
will gradually lose magnetization as the Curie temperature (~320 K) is reached, then
drop precipitously at the Curie temperature and then reduce fairly quickly to zero
within a relatively narrow temperature band of only 100 K. Phase transitions involve
crystallographic or crystal symmetry transformation, which leads to sharp transitions
in magnetism from ferromagnetic to paramagnetic. For example, a Martensite
material heated above its Curie point will exhibit crystallographic change from tet-
ragonal to cubic crystal structure in its Austenite phase. Ferrites and samarium-
cobalt (SmCo) magnets have hexagonal crystal structure and the high energy rare
earth neodymium iron boron (NdFeB) magnets have tetragonal structure as noted.
Ferrites such as barium (~1955) and strontium (~1970) mixes are the cheapest, but
also the lowest energy, permanent magnets available. Alnico was commercialized
(~1935) and is about twice the price of ferrite and is the most stable of the permanent
magnet materials. Of the cobalt types, SmCo
2
is stable to 1,150
C. NdFeB grades
(~1980) have been developed to a stored energy level of 58 MGOe. Sumitomo
Special Metals developed NEOMAX, an NdFeB permanent magnet with
B
r
¼
1.53 T
(15,200 G),
H
ci
¼
784 kA/m (9.8 kO
e
) and
BH
mx
¼
460 kJ/m
3
(57.7 MGOe).
The ceramic ferrites have low magnetic properties (
B
r
,
H
c
), but are lowest in
cost too, have good temperature stability to
>
250
C and are corrosion resistant.
Sintered and hot formed fully dense anisotropic neodymium magnets are superior
to SmCo below 180
C. There needs to be solid technical justification such as
corrosion resistance and very high temperature (approaching 300
C) to justify the
expensive SmCo magnets. Alnico magnets were the first type used in electric
machines. This is because of their high flux that approximated the fields possible in
shunt wound dc motors of the day. However, because of very low coercive force of
alnico magnets, these early electric machines used novel pole magnetic structures
such as soft iron pole pieces adjacent to the armature or soft iron pole pieces
bonded to the alnico magnet and machined to the arc of the rotor (armature of dc
motor) [1]. With soft iron pole shoes of this design, the alnico magnet motor could
operate at up to six times the normal armature current of alnico only before
demagnetization. Even if the alnico did become demagnetized, it would be an easy
matter to simply re-magnetize it using coils designed just for this purpose as was
done in these early days.
5.1.1 Permanent magnets: A primer
With the availability of higher performance and low cost ceramic magnets, the
alnico magnet was displaced from use in electric machines but remained useful in
electronic metres as noted earlier. The industry compares various grades of per-
manent magnets based on their maximum energy product of remnant flux density,
