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 Magnetic Materials
ALNICO ALLOYS
The Alnico alloys were developed in the 1930s, when Mishima discovered an alluminium-nickel-iron alloy with good magnetic properties. The behavior of these alloys derives mainly from the shape anisotropy of elongated rod-shape grains of Fe-Co, originated by precipitation-hardening in a non-magnetic phase of Ni-Al, that hinders the domain-wall motion. The main disadvantage of these alloys is the hardness and brittleness, so they can be only shaped by casting or sintering. Although Alnico’s magnetic characteristics have been overcomed by modern magnets, they are widely used, expecially for their little temperature coefficients.
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Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
| ALNICO |
0.5 ¸ 1.3
|
50 ¸ 120
|
15 ¸ 40
|
300
|
-0.02
|
0.02 +0.02
|
900
|
|
BONDED FERRITE
These materials, also known as oxide or ceramic magnets, are the most widely used magnets, especially for their low cost. They were developed in 1950s, owing to theorical consideration that the coercivity of a single-domain material is proportional to the anisotropy. Ferrites are produced by powder metallurgy, in form of aggregation of single-domain particles (about 1 m m) of BaO× 6Fe2O3 or SrO× 6Fe2O3. The final product could be isotropic or anisotropic, depending from the process.
|
Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
|
BONDED FERRITE
|
0.2
|
50÷120
|
7
|
800
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-0.2
|
+0.4
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-
|
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BONDED NdFeB
In the last years, bonded materials have seen a strong improvement because of their lower cost and weight and acceptable performances. In bonded magnets ferrite or NdFeB powder are mixed with epoxy resin using techniques such as cold rolling, injection molding or extrusion. Bonded magnets have lower magnetic properties, but they can be formed in complex shapes and with best mechanical tolerances.
|
Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
|
BONDED NdFeB
|
0,5÷0,7
|
1000
|
70
|
2500
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-0.08
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-0.4
|
-
|
|
FERRITE
These materials, also known as oxide or ceramic magnets, are the most widely used magnets, especially for their low cost. They were developed in 1950s, owing to theorical consideration that the coercivity of a single-domain material is proportional to the anisotropy. Ferrites are produced by powder metallurgy, in form of aggregation of single-domain particles (about 1 m m) of BaO× 6Fe2O3 or SrO× 6Fe2O3. The final product could be isotropic or anisotropic, depending from the process.
|
Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
|
FERRITE
|
0.4
|
150÷300
|
25÷30
|
800
|
-0.2
|
+0.4
|
450
|
|
NdFeB
This material was discovered in 1980s, because of economical aspects that prevented the massive use of Sm-Co magnets. The new magnet alloy, constituted mainly of Nd2Fe14B phase, has greater coercivity and BHmax than Sm-Co alloy. Two principal production methods have been developed:
- sintering method, by Sagawa of Sumitomo Special Metals (Japan);
- rapidly quenching method from melt, by General Motors.
The principle advantage respect Sm-Co is the lower cost, although NdFeB has a rather low Curie temperature (about 300 °C) and therefore a higher temperature coefficient.
|
Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
|
NdFeB
|
1.1÷1.2
|
1000÷2000
|
250
|
2500
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-0.1
|
-0,5
|
310
|
|
SAMARIUM COBALT
Samarium-Cobalt magnets were developed in 1960s as a result of a research of a new magnet material based an alloys of transition metals Fe, Co, Ni, and rare-earth elements.
Two class of alloys were developed: SmCo5 and Sm2Co17.
Despite their good magnetic properties, the cost of these alloys limits their applications.
|
Material
|
Br
( T )
|
HcJ
( kA/m )
|
BHmax
( kJ/m3 )
|
Hsat
( kA/m )
|
aBr
( % / °C )
|
aHcJ
( % / °C )
|
TCurie
( °C )
|
|
SmCo5
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0,8÷1
|
1500÷2500
|
180
|
2000
|
-0.04
|
-0.04
|
700
|
|
Sm2Co17
|
1÷1.1
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2000 |
200 |
3500 |
-0.03
|
-0.03
|
800 |
|
About Magnetic Materials...
Magnetism is still somewhat mysterious.
We neither can see or feel it and none of our senses are able to perceive it. Actual permanent magnets are divided in five main groups: AlNiCo alloys, Ferrite, Samarium-Cobalt magnets, Neodymimum-Iron-Boron, Bonded magnets.
Permanent magnets are used as passive generators of magnetic field. To produce and mantain the magnetic field, a material should have both high residual induction and high coercivity.
The quality of a magnet is judged in relation to remanence Br, coercive force HcJ and HcB, maximum energy product BHmax and to physical characteristics such as temperature dependence, corrosion resistence, resistivity, brittleness.
The magnets are also divided in two types: isotropic and anisotropic.
This is also closely related to another important factor that has to be considered: the directions of magnetization.
The permanent magnets have a very extensive application area: automotive, electronics, energy engineering, household appliances, light engineering, measurement and control engineering, environmental systems.
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