IEEE Electrification Magazine - March 2017 - 24
NdFeB magnets by increasing the magnetocrystalline
anisotropy, while at the same time reducing the remanence due to reduction of the magnetic moment of the
material, it can be expected that the remanence and
coercivity in the center of the magnet will be similar to
these for the base magnet prior to diffusion. On the
other hand, in the corner of the magnet, it can be
expected that the coercivity will increase due to the high
amount of Dy at the corner grain boundaries, while the
remanence will be somewhat reduced due to diffusion
into the grains.
The prior examples illustrate how EPMA analysis can
be very helpful in understanding the distribution of the
diffused HRE element into a specific magnet and possibly
set expectations for the BH curves corresponding to different regions of magnets with GBD. The exact measurements of the BH curves in the various regions of magnets
with HRE element diffusion has its own challenges.
B (T) B-H (T)
very center and from the corners of the same magnet, as
shown in Figure 5(a), and measure the BH curves for these
cubic samples using a vibrating sample magnetometer
(VSM), thus sampling the two regions with the biggest difference in amounts of diffused HRE. In the preparation of
the cubic samples, care should be taken so that the sides of
the cubes are as close to 2 mm as possible; attention should
also be paid to the perpendicularity of the samples. When
preparing the corner samples, the sides with diffusion
should not be ground to preserve the HRE-rich area located
close to the surface of the magnets; ideally, the samples
should be prepared from magnets with unrounded corners.
Figure 5(b) shows the demagnetization BH curves measured on corner and center 2-mm cubes machined from
magnets with GBD of Tb. The cubes were premagnetized
prior to each VSM measurement by an 85 kOe Magnetic Instrumentation pulsed magnetizing coil, and demagnetization curves were measured along the alignment
direction. The magnetizations 4rM as functions of
magnetic Properties Using
applied field H were measured on the cubes using a LakeVibrating Sample magnetometer
Shore Cryotronics Model 7410 VSM and the magnetic flux
As seen by EPMA, the concentration of the HRE element (Dy
density values were then calculated, correcting for the selfor Tb) often is highest in the corners of the magnet, a fortudemagnetizing effect.
nate coincidence since in EV and HEV rotor operation, the
The difference in the coercivities between the samples is
corners are most often subjected to the highest demagneapparent, with the corner samples having on the order of
tizing fields and thus are most likely to demagnetize first.
100 kA/m higher coercivities at 160 °C. This is consistent with
One approach to understanding the BH curve distribution
the higher amount of Tb in the grain boundaries of the corin diffused magnets is to machine 2-mm cubes from the
ner samples. These corner and center BH curves should be
considered together with our findings from EPMA. The cubic sample
Center
size of 2 mm per side was selected
a
for ease of sample preparation and to
Corner
ensure a large enough volume-tosurface ratio to allow for the measurement of meaningful BH curves.
However, on the scale of the Tb dif(a)
fusion observed in some commer1.2
cially available magnets, 2 mm may
160 °C
1.1
be too large to represent the region
1
richest in HRE element. In the case of
0.9
the corner sample, the BH curve
0.8
includes many regions, some of
0.7
which contain more grain-boundary
0.6
Tb than others. One corner of the
0.5
cube has a very large amount of Tb
0.4
while the opposite corner, depend0.3
ing on the diffusion parameters,
might have no diffused Tb at all. To
Corner Samples 0.2
Center Samples 0.1
measure a set of BH curves on a
0
sample from only the region richest
-1,000 -900 -800 -700 -600 -500 -400 -300 -200 -100
0
in Tb, however, a much smaller sam-0.1
H (kA/m)
-0.2
ple will be needed, one that would be
(b)
extremely hard to produce or to
measure reliably.
Figure 5. (a) A schematic of magnet locations of samples for VSM measurements. (b) The second
Another very apparent characquadrant BH curves for corner and center 2-mm cubic samples machined from magnets with
Tb GBD measured by VSM.
teristic of the data presented in
24
I E E E E l e c t r i f i c ati o n M agaz ine / march 2017
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