EDI-T® :
Technological Surprises to Sharpen Cutting
Edges of
Understanding magnetic materials and their applications
is the foundation of EDI-T® battery-free & consumption-free LED
flashlights. The duality between electricity and magnetism provides a
degree of freedom for magnetic circuit design. Experience, intuition and
the latest software modelling tools enable our engineers to optimise designs and to determine performance characteristics prior to cutting
magnets or assembling systems. The ever more sophisticated requirements of
science and industry have fuelled continuous material science and advances.
These advanced materials and enabling technologies open the door to new exciting magnet
developments, applications and requirements.
Advances in materials science seem never ending. There
are various factors to be considered when choosing a permanent magnet
material for your applications. Factors such as operating temperature, demagnetising
effects, energy requirements, cost limitations,
environmental characteristics and available space all need to be taken
into account; and to the magnetics novice, the decisions can seem
overwhelming. One important consideration is that while material is key,
the utilization of an optimised design is of primary importance. There are
4 major families of permanent magnetic materials commercially available.
They range from hard ferrite (ceramic), which is cheap and low energy,
alnico, samarium-cobalt (SmCo), to neodymium-iron-boron (NdFeB), which is
expensive and high energy. Each family of materials has several grades
with a range of magnetic properties. Today, application fields of these
permanent magnets are extended to new industrial fields, such as
electronic clocks, speakers, relays, motors, printers, communications
equipment, MRI, MagLev trains, etc.
FERRITE
magnets, sometimes referred to as ceramic magnets because of their product
process, are the least expensive class of permanent magnetic materials. It
became commercially available in the mid 1950s, and has since found its
way into countless applications including are shaped magnets for motors,
magnetic chucks, and magnetic tools. The raw material, iron oxide, for
these magnets is mixed with either strontium or barium and milled with a
ceramic binder and magnets are produced through a compression or extrusion
moulding technique that is followed by a sintering process. The nature of
the manufacturing process results in a product that frequently contains
imperfections such as cracks, porosity, chips, etc. Fortunately, these
imperfections rarely interfere with magnet performance. To enhance a
ceramic magnet's performance, the ferrite compound may be biased by a
magnetic field using the pressing process. This biasing induces a
preferred direction of magnetization within the magnet, significantly
reducing its performance in any other orientation. Consequently, ceramic
magnets are available in both orientated (anisotropic) and non-orientated
(isotropic) grades. Ceramic magnets are inherently brittle, and it is
highly recommended that they not be utilized as structural elements in any
application. The dimensional repeatability of as pressed components is
difficult to control, consequently, components requiring tight tolerances
necessitate secondary grinding operations to assure conformity.
ALNICO
was developed in the early 1930s. During World War II it was used in
military electronic applications. After the war it quickly spread into
civilian versions of these applications and replaced magnet steel in many
applications. High induction levels, with good resistance to demagnetisation
and stability due to its low temperature coefficient, at a
reasonable cost made Alnico the material of choice. A high working
temperature limit (550 degrees Celsius) makes Alnico especially well
suited for sensitive automotive and aircraft sensor applications. Other
popular Alnico applications include: instruments, security sensors,
magnetos, electronic distributors, separators, electron tubes, travelling
wave tubes, radar, holding magnets, coin acceptors, generators and motors,
clutches and brakes, relays, controls, receivers, telephones, microphones,
bell ringers, guitar pickups, loudspeakers, security systems, and cow
magnets. Alnico is made by alloying aluminium, nickel and cobalt with iron.
Some grades also contain copper and/or titanium. The alloying process is
casting or sintering. These constituents, the process and the heat
treatment needed to optimise magnetic properties produce hard and brittle
parts that are best shaped or finished by abrasive grinding. Case parts
are generally under 70 pounds and may be used as is, but polar surfaces
are usually ground flat and parallel. Sintering is confined to high volume
parts in sizes under one cubic inch and an effective press length to
diameter ratio under four.
Samarium
Cobalt (Sm-Co) is the first commercially viable rare earth
permanent and is considered to still be the premium material for many high
performance applications. Formulated in the 1960s, it came as a
revolutionary product, initially tripling the general product of other
materials available at the time. Sm-Co materials come in energy products
from 16 MGOe up to 33 MGOe. Their high resistance to demagnetising influences and excellent thermal stability has ensured Sm-Co as the
premium choice for the most demanding motor applications. In addition, the
corrosion resistance is significantly higher than, for example, NdFeB. Its
corrosion resistance has also offered a high degree of comfort to those
looking to use magnets in medical applications. On a "per pound"
basis, Sm-Co is the most expensive permanent magnetic material. However,
because of its high energy product, it has achieved considerable
commercial success by decreasing the required volume of magnet material to
fulfil a certain task. Sm-Co can typically be used up to 300 degrees
Celsius, though, of course, its actual performance at that temperature is
governed strongly by the design of the magnetic circuit. The approximately
linear demagnetisation curve of Sm-CO materials allows repeatable
performance over a wide range of operating conditions. As with all
permanent magnets, extreme caution must be exercised when handling
magnetized samples. Sm-Co can be prone to chipping and should not be used
as structural components in an assembly.
Sintered NEODYMIUM-IRON-BORON
(NdFeB) magnets are the most powerful commercialised permanent magnets
available today, with maximum energy product ranging from 26 MGOe to 52
MGOe. NdFeB is the third generation of permanent magnet developed in the
1980s. It has a combination of very high remanence and coercivity, and
comes with a wide range of grades, sizes and shapes. With its excellent
magnetic characteristics, abundant raw material and relatively low prices,
NdFeB offers more flexibility in designing of new applications or
replacing the traditional magnetic materials to achieve high efficiency,
low cost and more compact devices. A powder metallurgy process is used in
producing sintered NdFeB magnets. Although sintered NdFeB is mechanically
stronger than Sm-Co magnets and less brittle than other magnets, it should
not be used as structural components. Selection of NdFeB is limited by
temperature due to its irreversible loss and moderately high reversible
temperature coefficient. The maximum application temperature is 200
degrees Celsius for high coercivity grades. NdFeB magnets are more prone
to oxidation than any other magnet alloys. If NdFeB magnet is to be
exposed to humidity, chemically aggressive media such as acids, alkaline
solutions, salts and harmful gases, coating is recommended. It is not
recommended in a hydrogen atmosphere.