Design Guide
Permanent
Magnet Stability
The
ability of a permanent magnet to support an external
magnetic field results from small magnetic domains "locked"
in position by crystal anisotropy within the magnet
material. Once established by initial magnetization,
these positions are held until acted upon by forces
exceeding those that lock the domains. The energy required
to disturb the magnetic field produced by a magnet varies
for each type of material. Permanent magnets can be
produced with extremely high coercive forces (Hc)
that will maintain domain alignment in the presence
of high external magnetic fields. Stability can be described
as the repeated magnetic performance of a material under
specific conditions over the life of the magnet.
Factors
affecting magnet stability include time, temperature,
reluctance changes, adverse fields, radiation, shock,
stress, and vibration.
Time
The
effect of time on modern permanent magnets is minimal.
Studies have shown that permanent magnets will see changes
immediately after magnetization. These changes, known
as "magnetic creep", occur as less stable domains
are affected by fluctuations in thermal or magnetic energy,
even in a thermally stable environment. This variation
is reduced as the number of unstable domains decreases.
Rare Earth magnets are not as likely to experience this
effect because of their extremely high coercivities. Long-term
time versus flux studies have shown that a newly magnetized
magnet will lose a minor percent of its flux as a function
of age. Over 100,000 hours, these losses are in the range
of essentially zero for Samarium Cobalt materials to less
than 3% for Alnico 5 materials at low permeance coefficients.
Temperature
Temperature
effects fall into three categories:
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Reversible losses.
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Irreversible but recoverable
losses.
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Irreversible
and unrecoverable losses.
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Reversible
losses.
These
are losses that are recovered when the magnet returns
to its original temperature. Reversible losses cannot
be eliminated by magnet stabilization. Reversible losses
are described by the Reversible Temperature Coefficients
(Tc), shown in table 5.1. Tc is
expressed as % per degree Centigrade. These figures
vary for specific grades of each material but are representative
of the class of material as a whole. It is because the
temperature coefficients of Br and HC are
significantly different that the demagnetization curve
develops a "knee" at elevated temperatures.
| Table
5.1 Reversible Temperature Coefficients of Br
and HC |
| Material |
Tc
of Br |
Tc
of HC |
| NdFeB |
-0.12 |
-0.6 |
| SmCo |
-0.04 |
-0.3 |
| Alnico |
-0.02 |
0.01 |
| Ceramic |
-0.2 |
0.3 |
Irreversible
but recoverable losses.
These
losses are defined as partial demagnetization of the magnet
from exposure to high or low temperatures. These losses
are only recoverable by remagnetization, and are not recovered
when the temperature returns to its original value. These
losses occur when the operating point of the magnet falls
below the knee of the demagnetization curve. An efficient
permanent magnet design should have a magnetic circuit
in which the magnet operates at a permeance coefficient
above the knee of the demagnetization curve at expected
elevated temperatures. This will prevent performance variations
at elevated temperatures.
Irreversible
and unrecoverable losses.
Metallurgical
changes occur in magnets exposed to very high temperatures
and are not recoverable by remagnetization. Table 5.2
shows critical temperatures for the various materials,
where
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TCurie
is the Curie temperature at which the elementary
magnetic moments are randomized and the material
is demagnetized; and
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Tmax
is the maximum practical operating temperatures
for general classes of major materials. Different
grades of each material exhibit values differing
slightly from the values shown here.
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| Table
5.2 Critical Temperatures for Various Materials |
| Material |
TCurie
|
Tmax*
|
| Neodymium
Iron Boron |
310
(590) |
150
(302) |
| Samarium
Cobalt |
750
(1382) |
300
(572) |
| Alnico |
860
(1580) |
540
(1004) |
| Ceramic |
460
(860) |
300
(572) |
| (Temperatures
are shown in degrees Centigrade with the Fahrenheit
equivalent in parentheses.) |
*Note
that the maximum practical operating temperature is dependent
on the operating point of the magnet in the circuit. The
higher the operating point on the Demagnetization Curve,
the higher the temperature at which the magnet may operate.
Flexible
materials are not included in this table since the binders
that are used to render the magnet flexible break down
before metallurgical changes occur in the magnetic ferrite
powder that provides flexible magnets with their magnetic
properties.
Partially
demagnetizing a magnet by exposure to elevated temperatures
in a controlled manner stabilizes the magnet with respect
to temperature. The slight reduction in flux density
improves a magnet's stability because domains with low
commitment to orientation are the first to lose their
orientation. A magnet thus stabilized will exhibit constant
flux when exposed to equivalent or lesser temperatures.
Moreover, a batch of stabilized magnets will exhibit
lower variation of flux when compared to each other
since the high end of the bell curve which characterizes
normal variation will be brought in closer to the rest
of the batch.
Reluctance
Changes
These
changes occur when a magnet is subjected to permeance
changes such as changes in air gap dimensions during
operation. These changes will change the reluctance
of the circuit, and may cause the magnet's operating
point to fall below the knee of the curve, causing partial
and/or irreversible losses. The extents of these losses
depend upon the material properties and the extent of
the permeance change. Stabilization may be achieved
by pre-exposure of the magnet to the expected reluctance
changes.
Adverse
Fields
External
magnetic fields in repulsion modes will produce a demagnetizing
effect on permanent magnets. Rare Earth magnets with
coercive forces exceeding 15 KOe are difficult to affect
in this manner. However, Alnico 5, with a coercive force
of 640 Oe will encounter magnetic losses in the presence
of any magnetic repelling force, including similar magnets.
Applications involving Ceramic magnets with coercive
forces of approximately 4KOe should be carefully evaluated
in order to assess the effect of external magnetic fields.
Radiation
Rare
Earth materials are commonly used in charged particle
beam deflection applications, and it is necessary to
account for possible radiation effects on magnetic properties.
Studies (A.F. Zeller and J.A. Nolen, National Superconducting
Cyclotron Laboratory, 09/87, and E.W. Blackmore, TRIUMF,
1985) have shown that SmCo and especially Sm2Co17
withstand radiation 2 to 40 times better than NdFeB
materials. SmCo exhibits significant demagnetization
when irradiated with a proton beam of 109
to 1010 rads. NdFeB test samples were shown
to lose all of their magnetization at a dose of 7 x
107 rads, and 50% at a dose of 4 x 106
rads. In general, it is recommended that magnet materials
with high Hci values be used in radiation
environments, that they be operated at high permeance
coefficients, Pc, and that they be shielded
from direct heavy particle irradiation. Stabilization
can be achieved by pre-exposure to expected radiation
levels.
Shock,
Stress, and Vibration
Below
destructive limits, these effects are very minor on
modern magnet materials. However, rigid magnet materials
are brittle in nature, and can easily be damaged or
chipped by improper handling. Samarium Cobalt in particular
is a fragile material and special handling precautions
must be taken to avoid damage. Thermal shock when Ceramics
and Samarium Cobalt magnets are exposed to high temperature
gradients can cause fractures within the material and
should be avoided.
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