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ANALYSIS OF COERCIVITY AND OF THE MICROSTRUCTURE OF HIGH-TECH HARD MAGNETIC MATERIALS.

Objective


Permanent magnets based on the iron(14)neodymium(2)boron intermetallic compound have been produced by the 4 different techniques sintering, melt spinning, extrusion and mechanical alloying. Due to the deteriorating effects of the microstructure these different techniques have so far led to only 10-20% of the theoretically achievable coercive field. Investigations of magnetisation processes, hysteresis loops, magnetic after effects and domain patterns have been performed on these different materials as a function of temperature and different types of pretreatment. These magnetic measurements have been combined with studies of the microstructure by transmission electron microscopy. In particular, the role of nonmagnetic precipitation, misaligned grains, incompletely magnetically decoupled grains, structure of the intergranular phase have been studied in detail. From a comparison of the different types of permanent magnets a deeper insight into the role of the microstructure with respect to their hard magnetic properties has been obtained. Combining the results of the microstructural and the micromagnetic analysis of the different types of permanent magnets the procedures for optimising the coercive field have been determined.
The above mentioned types of magnets have been prepared with different compositions and additive elements. Melt spun materials have been prepared and analysed in a wide range of preparation parameters. By a well defined optimisation of the microstructure the coercive field of a melt spun iron(72)neodymium(17)boron(7.5)gallium(1.5)niobium magnet could be increased up to 940 kAm{-1} at a temperature of 150 C.

The microstructure of hard magnetic materials was investigated. It was found that:
large coercive fields require a perfect decoupling, an optimal alignment and perfect surfaces of grains;
these conditions can be approached by the combination of production techniques, suitable additives and annealing treatments;
the largest coercive fields are obtained by the melt spinning technique and a composition neodymium 17 iron 72 boron 7.5 gallium 1.5 niobium 2;
magnetic additives increase the coercive field at the expense of remanence, thus decreasing the energy product;
low melting metals such as gallium, aluminium and copper increase the viscosity of the intergranular neodymium rich phase, thus enhancing the magnetic decoupling between the grains;
gallium is more suitable than aluminium and copper because up to 1.4 atomic% gallium is nearly unsoluble in the hard magnetic phase neodymium 2 iron 14 boron while aluminium dissolves in neodymium 2 iron 14 boron where it reduces the remanence;
high melting refractory metals form high melting borides during the sintering process, thus increasing the magnetic decoupling and reducing the corrosivity;
in melt spun magnets the additives niobium, molybdenum and vanadium act as nuclei for a fine dispersed homogeneous grain structure;
the role of the microstructure is described by the microstructural parameters alpha and Neff;
largest alpha values (0.45 to 0.5) are obtained for sintered magnets;
large Neff values (1.2 to 2.0) are found in sintered and mechanically alloyed magnets;
by short time or flash annealing treatments the ideal conditions of a large alpha and a small Neff value can be approached;
the coercive field is nearly independent of the grain size.

The impossibility of approaching the ideal microstructure with alpha of 1 and Neff of 0 is attributed to 4 most effective deteriorating effects:
misaligned grains;
grains coupled by exchange and dipolar interactions;
strong demagnetization fields at sharp edges and corners;
imperfect grain surfaces.

The results detail the role of different microstructures which might help the magnets industry to produce optimally tailored alloys. In particular they clarify the role of magnetic additives, low melting additives and high melting additives in connection with different production techniques allowing the development of desired properties of the hysteresis loop.

Important emerging applications of the work include:
small size, high speed, high efficiency motors and dynamos;
general electrical motors and generators;
medical applications (imaging systems);
new electric cars or starter motors;
efficient, strong and high speed motors for disk drive units;
digital coding, multiflexing, pm digital arrays;
magnetic actuators and sensors;
micromotors, microdevices, magnetic lenses, magnetic confinement and new instrumentation.
Permanent magnets based on the Fe14Nd2B intermetallic compound will be produced by four different techniques : sintering, melt-spinning, extrusion and mechanical alloying. Due to the deteriorating effects of the microstructure these different techniques have so far to only 10-20 % of the theoretically achievable coercive field. Investigations of magnetization processes, hysteresis loops, magnetic after-effects and domain patterns will be performed on these different materials as a function of temperature and different types of pretreatment.
These magnetic measurements will be combined with studies of the microstructure by transmission electron microscopy. In particular, the role of non-magnetic precipitation, misaligned grains, incompletely magnetically decoupled grains, structure of the intergranular phase will be studied in detail. From a comparison of the different types of permanent magnets a deeper insight into the role of the microstructure with respect to their hard magnetic properties will be obtained. Combining the results of the microstructural and the micromagnetic analysis of the different types of permanent magnets the procedures for optimizing the coercive field be determined.

Coordinator

Max-Planck-Gesellschaft zur Förderung der Wissenschaften eV
Address
Seestraße 92
70174 Stuttgart
Germany

Participants (4)

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE
France
Address
Avenue Des Martyrs 25
38042 Grenoble
CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS
Spain
Address
Avenida Astrofisico Francisco Sanchez S/n
38206 La Laguna
Technische Universitat Wien
Austria
Address
Hauptstrasse 8
1040 Wien
Universita degli Studi di Parma
Italy
Address
Via Massimo D'azeglio 85
43100 Parma