5.0
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.
5.1
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.
5.2
Temperature
Temperature
effects fall into three categories:
|
• Reversible losses.
|
|
• Irreversible but recoverable losses.
|
|
• Irreversible
and unrecoverable losses.
|
5.2.1.
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 |
| Ferrite |
-0.2 |
0.3 |
|
5.2.2.
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.
5.2.3.
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
|
TCurie is the Curie temperature at which the elementary
magnetic moments are randomized and the material is demagnetized;
and
|
|
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.
|
| 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) |
| Ferrite |
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.
5.3
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.
5.4
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 ferrite magnets with coercive
forces of approximately 4KOe should be carefully evaluated in order
to assess the effect of external magnetic fields.
5.5
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.
5.6
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 ferrites
and Samarium Cobalt magnets are exposed to high temperature gradients
can cause fractures within the material and should be avoided.
TOP
6.0
Manufacturing Methods
Permanent
magnets are manufactured by one of the following methods:
|
• Sintering, (Rare Earths, ferrites, and Alnicos)
|
|
• Pressure Bonding or Injection Molding, (Rare Earths and Ferrites)
|
• Casting,
(Alnicos)
|
• Extruding,
(Bonded Neodymium and Ferrites)
|
• Calendering
(Neodymium and Ferrites)
|
The
sintering process involves compacting fine powders at high pressure
in an aligning magnetic field, then sintering to fuse into a solid
shape. After sintering, the magnet shape is rough, and will need
to be machined to achieve close tolerances. The intricacy of shapes
that can be thus pressed is limited.
Rare
Earth magnets may be die pressed (with pressure being applied in
one direction) or isostatically pressed (with equal pressure being
applied in all directions). Isostatically pressed magnets achieve
higher magnetic properties than die pressed magnets. The aligning
magnetic field for die pressed magnets can be either parallel or
perpendicular to the pressing direction. Magnets pressed with the
aligning field perpendicular to the pressing direction achieve higher
magnetic properties than the parallel pressed form.
Both
Rare Earth and Ferrite magnets can also be manufactured by pressure
bonding or injection molding the magnet powders in a carrier matrix.
The density of magnet material in this form is lower than the pure
sintered form, yielding lower magnetic properties. However, bonded
or injection molded magnets may be made with close tolerances "off-tool"
and in relatively intricate shapes.
Alnico
is manufactured in a cast or sintered form. Alnicos may be cast
in large or complex shapes (such as the common horseshoe), while
sintered Alnico magnets are made in relatively small sizes (normally
one ounce or less) and in simple shapes.
Flexible
Rare Earth or Ferrite magnets are made by calendering or extruding
magnet powders in a flexible carrier matrix such as vinyl. Magnet
powder densities and therefore magnetic properties in this form
of manufacture are even lower than the bonded or injection molded
form. Flexible magnets are easily cut or punched to shape.
TOP
7.0
Physical Characteristics and Machining of Permanent Magnets
Sintered
Samarium Cobalt and Ferrite magnets exhibit small cracks within
the material that occur during the sintering process. Provided that
cracks do not extend more than halfway through a section, they do
not normally affect the operation of the magnet. This is also true
for small chips that may occur during machining and handling of
these magnets, especially on sharp edges. Magnets may be tumbled
to break edges: this is done to avoid "feathering" of
sharp edges due to the brittle nature of the materials. Tumbling
can achieve edge breaks of 0.003" to 0.010". Although
Neodymium Iron Boron is relatively tough as compared to Samarium
Cobalt and Ferrite, it is still brittle and care must be taken in
handling. Because of these inherent material characteristics, it
is not advisable to use any permanent magnet material as a structural
component of an assembly.
Rare
Earth, Alnico, and Ferrite magnets are machined by grinding, which
may considerably affect the magnet cost. Maintaining simple geometries
and wide tolerances is therefore desirable from an economic point
of view. Rectangular or round sections are preferable to complex
shapes. Square holes (even with large radii), and very small holes
are difficult to machine and should be avoided. Magnets may be ground
to virtually any specified tolerance. However, to reduce costs,
tolerances of less than +0.001" should be avoided if
possible.
Cast
Alnico materials exhibit porosity as a natural consequence of the
casting process. This may become a problem with small shapes, which
are machined out of larger castings. The voids occupy a small portion
of the larger casting, but can account for a large portion of the
smaller fabricated magnets. This may cause a problem where uniformity
or low variation is critical, and it may be advisable either to
use a sintered Alnico, or another material. In spite of its slightly
lower magnetic properties, sintered Alnico may yield a higher or
more uniform net density, resulting in equal or higher net magnetic
output.
In
applications where the cosmetic qualities of the magnet are of a
concern, special attention should be placed on selecting the appropriate
material, since cracks, chips, pores, and voids are common in rigid
magnet materials.
Magnet
Sales & Manufacturing has extensive experience in the machining
and handling of all permanent magnet materials. In house machining
facilities allow the ability to deliver prototype to production
quantities with short lead times.
TOP
8.0
Coatings
Samarium
Cobalt, Alnico, and Ferrite materials are corrosion resistant, and
do not require to be coated against corrosion. Alnico is easily
plated for cosmetic qualities, and Ferrites may be coated to seal
the surface, which will otherwise be covered by a thin film of ferrite
powder (although not a problem for most applications).
Neodymium
Iron Boron magnets are susceptible to corrosion and consideration
should be given to the operating environment to determine if coating
is necessary. Nickel or tin plating may be used for Neodymium Iron
Boron magnets, however, the material must be properly prepared and
the plating process properly controlled for successful plating.
Plating houses experienced in the plating of NdFeB materials are
difficult to locate, and must be furnished with the necessary information
for proper preparation and control of the process. Aluminum chromate
or cadmium chromate vacuum deposition has been successfully tested,
with coating thickness as low as 0.0005". Teflon and other
organic coatings are relatively inexpensive and have also been successfully
tested. A further option for critical applications is to apply two
types of protective coatings or to encase the magnet in a stainless
steel or other housing to reduce the chances of corrosion.
TOP
9.0
Assembly Considerations
Magnet
Sales & Manufacturing Inc. has manufacturing capabilities to
manufacture complex magnet pole pieces and housings to provide a
complete magnet assembly. The following points should be considered
when designing magnet assemblies.
9.1
Affixing Magnets to Housings
Magnets
can be successfully affixed to housings using adhesives. Cyanoacrylate
adhesives that are rated to temperatures up to 350
F
with fast cure times are most commonly used. Fast cure times avoid
the need for fixtures to hold the magnets in place while the bond
cures. Adhesives with higher temperature ratings are also available,
but these require oven curing, and fixturing of the magnets to hold
them in place. If magnet assemblies are to be used in a vacuum,
potential out-gassing of the adhesives should be considered.
9.2
Housing Design
Magnet
Sales & Manufacturing is equipped with state of the art CNC
and EDM equipment allowing the manufacture of complex housings.
Effective magnet locating sections should be included in housing
designs to support and locate magnets precisely.
9.3
Mechanical Fastening
When
arrays of magnets must be assembled, especially when the magnets
must be placed in repelling positions, it is very important to consider
safety issues. Modern magnet materials such as the Rare Earths are
extremely powerful, and when in repulsion they can behave as projectiles
if adhesives were to break down. We strongly recommend that in these
situations mechanical fastening be included in the design in addition
to adhesives. Potential methods of mechanical retention include
encasement, pinning, or strapping the magnets in place with non-magnetic
metal components. The Design Engineering team at Magnet Sales &
Manufacturing is experienced in magnet housing and fastening designs,
and we will be pleased to assist in arriving at an appropriate design.
9.4
Potting
Magnet
assemblies may be potted to fill gaps or to cover entire arrays
of magnets. Potting compounds cure to hard and durable finishes,
and are available to resist a variety of environments, such as elevated
temperatures, water flow, etc. When cured, the potting compounds
may be machined to provide accurate finished parts.
9.5
Welding
Assemblies
that are required to be hermetically sealed can be welded using
either laser welding (which is not affected by the presence of magnetic
fields) or TIG welding (using appropriate shunting elements to reduce
the effect of magnetic fields on the weld arc). Special care should
be taken when welding magnetic assemblies so that heat dissipation
of the weld does not affect the magnets.
TOP
10.0
Magnetization
Permanent
magnet materials are believed to be composed of small regions or
"domains" each of which exhibit a net magnetic moment.
An unmagnetized magnet will possess domains that are randomly oriented
with respect to each other, providing a net magnetic moment of zero.
Thus a magnet when demagnetized is only demagnetized from the observer's
point of view. Magnetizing fields serve to align randomly oriented
domains to give a net, externally observable field.
10.1
Objective of Magnetization
The
objective of magnetization is initially to magnetize a magnet to
saturation, even if it will later be slightly demagnetized for stabilization
purposes. Saturating the magnet and then demagnetizing it in a controlled
manner ensures that the domains with the least commitment to orientation
will be the first to lose their orientation, thereby leading to
a more stable magnet. Not achieving saturation, on the other hand,
leads to orientation of only the most weakly committed domains,
hence leading to a less stable magnet.
Anisotropic
magnets must be magnetized parallel to the direction of orientation
to achieve optimum magnetic properties. Isotropic magnets can be
magnetized through any direction with little or no loss of magnetic
properties. Slightly higher magnetic properties are obtained in
the pressing direction.
10.2
Magnetizing Equipment
Magnetization
is accomplished by exposing the magnet to an externally applied
magnetic field. This magnetic field may be created by other permanent
magnets, or by currents flowing in coils. Using permanent magnets
for magnetization is only practical for low coercivity or thin sections
of materials. Removal of the magnetized specimen from the permanent
magnet magnetizer can be problematic since the field cannot be turned
off, and fringing fields may adversely affect the magnetization
of the specimen.
The
two most common types of magnetizing equipment are the DC and capacitor
discharge magnetizers.
10.2.1
DC Magnetizers
DC
magnetizers employ large coils through which a current is applied
for a short duration by closing a switch. The current flowing through
the coil produces a magnetic field, which is usually directed by
the use of iron cores and pole pieces, and magnets are placed in
the gap between the pole pieces. DC magnetizers are only practical
for magnetizing Alnico materials, which have a low magnetizing force
requirement, or small sections of Ferrite materials.
10.2.2
Capacitor Discharge Magnetizers
Capacitor
discharge magnetizers employ capacitor banks that are charged, and
then discharged through a coil. Provided the coil has a resistance,
R, which is greater than , where L is the inductance and C the capacitance,
the current flowing though the coil will be unidirectional. Extremely
high magnetizing fields (in the range of 100 KOe) can be achieved
using special coils and power supplies.
10.3
Saturation Fields Required
Some
Rare Earth magnets require very high magnetizing fields in the 20
to 50 KOe range. These fields are difficult to produce requiring
large power supplies in conjunction with carefully designed magnetizing
fixtures. Isotropic bonded Neodymium materials require fields in
the high 60 KOe range to be fully saturated. However, fields in
the 30 KOe range may achieve 98% of saturation. Ferrites require
fields in the order of 10 KOe, while Alnicos require fields in the
range of 3 KOe for saturation. Because of the ease by which Alnico
5 can become inadvertently demagnetized, it is preferable for this
material to be magnetized just prior to or even after final assembly
of the magnet into the device.
10.4
Multiple Pole Magnetization
In
certain cases, it may be desirable to magnetize a magnet with more
than one pole on a single pole surface. This may be accomplished
by constructing special magnetizing fixtures. Multiple pole magnetizing
fixtures are relatively simple to build for Alnico and Ferrite,
but require great care in design and construction for Rare Earth
materials.
Magnetizing
with multiple poles will sometimes eliminate the need for several
discrete magnets, reducing assembly costs, although a cost will
be incurred for building an appropriate magnetizing fixture. Multiple
pole fixtures for Rare Earth magnets may cost several thousand dollars
to build, depending on the size of the magnet, the number of poles
required, and the fields necessary to achieve saturation.
10.5
The Orientation Direction
Some
applications require magnets oriented in a particular direction
with a high degree of accuracy. This direction may or may not coincide
with a geometrical plane of the magnet. For anisotropic materials
the orientation direction can normally be held within 3° of the
nominal with no special precautions. However, more precise requirements
may need special measurement and testing. This is achieved by the
use of Helmholtz coils, which measure the total flux in various
axes, and thence calculating the resultant magnetic moment vector.
Materials must be cut and machined taking into account the actual
angle of orientation to achieve the required accuracy. Isotropic
materials may be magnetized in any direction, and therefore pose
no problem in this regard.
TOP
11.0
Measurement and Testing
It
is important that incoming inspection of magnetic characteristics
be clearly and properly specified. End point characteristics (such
as Br or HC) cannot be directly observed;
therefore inspection personnel should not expect to measure 8,500
Gauss on a SmCo 18 magnet even though the Br is specified
at 8,500 Gauss.
A test
method or combination of test methods should be based upon the criticality
of the requirement, and the cost and ease of performing tests. Ideally,
the test results should be able to be directly translated into functional
performance of the magnet. A sampling plan should be specified which
inspects the parameters which are critical to the application. A
brief description of some common test methods follows below.
11.1
B-H Curves
B-H
curves may be plotted with the use of a permeameter. These curves
completely characterize the magnetic properties of the material
at a specific temperature. In order to plot a B-H curve, a sample
of specific size must be used, then cycled through a magnetization/demagnetization
cycle. This test is expensive to perform due to the length of time
required to complete. The test is destructive to the sample piece
in many cases, and is not practical to perform on a large sample
of finished magnets. However, when magnets are machined from a larger
block, the supplier may be requested to provide B-H curves for the
starting raw stock of magnet material.
11.2
Total Flux
Using
a test set up consisting of a Helmholtz coil pair connected to a
fluxmeter, total flux measurements can be made to obtain total dipole
moments, and interpolated to obtain close estimates of Br,
HC, and BHmax. The angle of orientation of
the magnet can also be determined using this method. This is a quick
and reliable test, and one that is not overly sensitive to magnet
placement within the coil.
11.3
Flux Density
Flux
density measurements are made using a gaussmeter and an appropriate
probe. The probe contains a Hall Effect device whose voltage output
is proportional to the flux density encountered. Two types of probe
construction (axial, where the lines of flux traveling parallel
to the probe holder, and transverse where the lines of flux
traveling perpendicular to the probe holder, are measured) allow
the measurement of flux density of magnets in various configurations.
The placement of the probe with respect to the magnet is critical
in order to obtain comparable measurements from magnet to magnet.
This is accomplished by building a holding fixture for the magnet
and probe, so that their positions are fixed relative to each other.
11.4
Flux Maps
Using
special scanners equipped with 3-axis Hall probes, magnetic arrays
can be mapped, to capture flux densities in x, y, and z directions
with a specified number of data points across the entire array.
The resulting data can then be output as a flux contour map, as
flux vectors, or as a data table for further analysis.
11.5
Pull Tests
This
is a commonly used test for magnets. The pull of the magnet is proportional
to B2, and is therefore very sensitive to the value of
B. Variations in B occur due to variations in the inherent properties
of the magnet itself, as well as environmental effects such as temperature,
composition and condition of the material that the magnet is being
tested on, measurement equipment, and operator. Since B decays exponentially
from a zero air gap, small inadvertently introduced air gaps between
the magnet and the test material can have a large effect on the
measured pull. It is therefore recommended that pull be tested at
a positive air gap. Performing pull tests at a number of air gaps,
and plotting results as air gap vs. (pull)1/2
, provides a more accurate description of the pull characteristics
of the magnet. Extrapolating from this pull at zero air-gap may
be calculated.
11.6
Other Functional Tests
These
should be determined according to the application and after discussion
with the supplier. They may involve complex tests such as a profile
of flux density along a specified axis, flux uniformity requirements
within a defined volume, or relatively simple tests such as a torque
test.
TOP
12.0
Handling and Storage
Handle
magnets with care!
Personnel
wearing pacemakers should not handle magnets.
Magnets
should be kept away from sensitive electronic equipment.
Modern
magnet materials are extremely strong magnetically and somewhat
weak mechanically. Any person required to handle magnets should
be appropriately trained about the potential dangers of handling
magnets. Injury is possible to personnel, and magnets themselves
can easily get damaged if allowed to snap towards each other, or
if nearby metal objects are allowed to be attracted to the magnets.
Materials
with low coercive forces such as Alnico 5 must be carefully handled
and stored when received in a magnetized condition. When stored,
these magnets should be maintained on a "keeper" which
provides a closed loop protecting the magnet from adverse fields.
Bringing together like poles in repulsion would lead to irreversible,
though re-magnetizable, losses.
Samarium
Cobalt should be carefully handled and stored due to the extremely
brittle nature of the material.
Uncoated
Neodymium magnets should be stored so as to minimize the risk of
corrosion.
In
general, it is preferable to store magnetized materials under vacuum-sealed
film so that the magnets do not collect ferromagnetic dust particles
over time, since cleaning this accumulated dust is time consuming.
TOP
13.0
Quick Reference Specification Checklist
When
requesting design assistance, information should establish adverse
conditions to which the magnet may be subjected - for example unusual
temperatures, humidity, radiation, demagnetizing fields produced
by other parts of the magnetic circuit, etc. The various magnet
materials react differently under different environmental conditions,
and it is most likely that a material can be selected which will
maximize the chances of success, provided that all relevant information
is conveyed.
The
following checklist may be helpful in constructing and communicating
specifications for permanent magnets:
|
Material type
|
|
Nominal, minimum and/or maximum magnetic properties
(Br, HC, Hci, BHmax)
|
Geometry
and tolerances of magnet
|
Orientation
direction (and tolerance of orientation direction if critical)
|
Whether
to be supplied magnetized or not
|
Marking
requirements
|
Coating
requirements
|
Acceptance
tests or performance requirements
|
Inspection
sampling plan
|
Packaging
and identification
|
NdFeB |
39H |
12,800 |
12,300 |
21,000 |
40 |
150 |
SmCo |
26 |
10,500 |
9,200 |
10,000 |
26 |
300 |
NdFeB |
B10N |
6,800 |
5,780 |
10,300 |
10 |
150 |
Alnico |
5 |
12,500 |
640 |
640 |
5.5 |
540 |
Ferrite |
8 |
3,900 |
3,200 |
3,250 |
3.5 |
300 |
Flexible |
1 |
1,600 |
1,370 |
1,380 |
0.6 |
100 |
| * Tmax (maximum
practical operating temperature) is for
reference only. The maximum practical operating
temperature of any magnet is dependent on the circuit
the magnet is operating
in. |
TOP
