IEEE Electrification Magazine - March 2017 - 26

Such discrepancies
are strongly
dependent on the
diffusion process
and its execution and
illustrate the need
to fully understand
the diffusion element
distribution as
related to specific
magnets.

1.02
1.01
1
0.99
0.98
0.97
0.96
0.95
0.94
0.93

magnets are shown as the red line.
Regions AB1, AB3, and AB9 are prepared from the corners of two diffused magnets and are expected to
have similar magnetic properties. As
the Tb concentration in the grain
boundaries of the corner regions is
generally the highest, coercivities in
these regions are also expected to
exceed the coercivity measured for
the whole magnet. However, there is a
significant difference between these
regions, and the coercivities measured for these corner regions are
lower than the coercivity measured
on the whole magnet. Such discrepancies are strongly dependent on the
diffusion process and its execution
and illustrate the need to fully understand the diffusion element distribution as related to specific magnets. It is also important to
understand the damage introduced during the preparation of the regions and its contribution to the discrepancies in the data.
Such differences may in part be also due to measurement error. There are many opportunities to introduce
air-gaps, surface damage, and reduced alignment to the
magnetic direction when preparing the individual
regions. Corner radii, if present in the original samples,
will affect the measurements, and interior regions with
no rounded edges are prone to breaking, making repeat
measurements and measurements at different temperatures difficult.
The method of slicing small pieces of the diffused
magnet and stacking them together to measure the BH
curves of the representative regions inherently contains
multiple sources of possible error. Therefore, these curves
should be used for simulations only after good understanding of the distribution of the diffusion element and
the possible sources of measurement error.

Br 160 °C
Average
Whole Magnet

AB1

AB3

AB9

Hcj (Normalized)

Br (Normalized)

Another broad assumption is often
made about the uniformity of the diffusion layers in the magnet. Let's consider the case of a magnet with Tb
diffusion performed on six sides of a
magnet. Suppose that the diffusion
layer on each side of the magnet is
uniform and similar in Tb concentration and penetration from the surface
for all six sides of the magnet. It was
already evident in our EPMA discussion that the corner of the magnet
gets more HRE element than the
sides. Thus, one can imagine that the
concentration of Tb, and therefore the
coercivity, is highest in the corners of
the magnet in regions 1, 3, 7, and 9,
shown in yellow in Figure 6. In
regions 2 and 8 (green), the concentration of Tb is expected to be slightly
lower than in the corners; it is expected to be lower still
in regions 4 and 6 (red). Thus, region 5 (gray), which represents the very center of the magnet, may contain very
little diffused Tb in the grain boundaries, or none at all
depending on the diffusion process. This region is
expected to have the lowest Hcj, and if no Tb has reached
its grain boundaries, it will be representative of the base
magnet material before Tb diffusion. This detailed division of the diffused magnet into regions assumes that
the different regions from the same group (for example,
all the yellow regions) have similar magnetic properties.
However, the diffusion process varies widely between
suppliers, and the logic that the regions shown in the
same color on Figure 6 should have similar properties
sometimes proves untrue.
Figure 7 shows the Br and Hcj values measured on
three yellow regions obtained from two different magnets
(A and B) that have undergone the same diffusion process. The corresponding values for the whole diffused
magnet as averaged from the measurement of five

1.1
1.05
1
0.95
0.9
0.85
0.8
0.75
0.7

Hcj 160 °C
Average
Whole Magnet

AB1

(a)
Figure 7. (a) The Br and (b) Hcj of regions AB1, AB3, and AB9 measured using a hysteresisgraph.

26

I E E E E l e c t r i f i c ati o n M agaz ine / march 2017

AB3
(b)

AB9



Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017

IEEE Electrification Magazine - March 2017 - Cover1
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IEEE Electrification Magazine - March 2017 - 1
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https://www.nxtbook.com/nxtbooks/pes/electrification_december2022
https://www.nxtbook.com/nxtbooks/pes/electrification_september2022
https://www.nxtbook.com/nxtbooks/pes/electrification_june2022
https://www.nxtbook.com/nxtbooks/pes/electrification_march2022
https://www.nxtbook.com/nxtbooks/pes/electrification_december2021
https://www.nxtbook.com/nxtbooks/pes/electrification_september2021
https://www.nxtbook.com/nxtbooks/pes/electrification_june2021
https://www.nxtbook.com/nxtbooks/pes/electrification_march2021
https://www.nxtbook.com/nxtbooks/pes/electrification_december2020
https://www.nxtbook.com/nxtbooks/pes/electrification_september2020
https://www.nxtbook.com/nxtbooks/pes/electrification_june2020
https://www.nxtbook.com/nxtbooks/pes/electrification_march2020
https://www.nxtbook.com/nxtbooks/pes/electrification_december2019
https://www.nxtbook.com/nxtbooks/pes/electrification_september2019
https://www.nxtbook.com/nxtbooks/pes/electrification_june2019
https://www.nxtbook.com/nxtbooks/pes/electrification_march2019
https://www.nxtbook.com/nxtbooks/pes/electrification_december2018
https://www.nxtbook.com/nxtbooks/pes/electrification_september2018
https://www.nxtbook.com/nxtbooks/pes/electrification_june2018
https://www.nxtbook.com/nxtbooks/pes/electrification_december2017
https://www.nxtbook.com/nxtbooks/pes/electrification_september2017
https://www.nxtbook.com/nxtbooks/pes/electrification_march2018
https://www.nxtbook.com/nxtbooks/pes/electrification_june2017
https://www.nxtbook.com/nxtbooks/pes/electrification_march2017
https://www.nxtbook.com/nxtbooks/pes/electrification_june2016
https://www.nxtbook.com/nxtbooks/pes/electrification_december2016
https://www.nxtbook.com/nxtbooks/pes/electrification_september2016
https://www.nxtbook.com/nxtbooks/pes/electrification_december2015
https://www.nxtbook.com/nxtbooks/pes/electrification_march2016
https://www.nxtbook.com/nxtbooks/pes/electrification_march2015
https://www.nxtbook.com/nxtbooks/pes/electrification_june2015
https://www.nxtbook.com/nxtbooks/pes/electrification_september2015
https://www.nxtbook.com/nxtbooks/pes/electrification_march2014
https://www.nxtbook.com/nxtbooks/pes/electrification_june2014
https://www.nxtbook.com/nxtbooks/pes/electrification_september2014
https://www.nxtbook.com/nxtbooks/pes/electrification_december2014
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