In 1984 the revolutionizing magnetic properties of NdFeB were discovered simultaneously and independently by Dr. Masato Sagawa in Japan, and by Dr. John Croat in the USA. Both produced materials based on the same magnetic phase, but employed different processing routes.
The results of the different processing routes resulted in materials of roughly the same composition, with differing microstructures. These permanent magnets utilize cheaper and more abundant raw materials than the Samarium Cobalt magnets (SmCo), which was the dominant at the time.
State of art in 1984
Production of Sm-Co permanent magnets had been increasing steadily. They had become widely applied to the fields of the electronics industry. To obtain larger scale adaptation of strong permanent magnets, it was necessary to develop a material containing little or no Sm and Co due to the low availability of these elements. As alternative materials, there had been a growing interest in alloys consisting of Iron (Fe) and Rare Earths (RE), especially those of which large amounts were available: the light rare earths, LRE.
Due to that LRE and Fe binary compounds have high magnetic properties, they were strong candidates for high performance permanent magnets competing with the SmCo magnets.
However, the RE-Fe permanent magnets had not been realized yet due to three reasons: Fe and RE form few stable phases, they are difficult to magnetise, and they easily lose their magnetisation at high temperatures.
Adhering to the equilibrium, RE-Fe binary phases did not appear promising; so two alternative approaches where thought of to possibly achieve a breakthrough.
- Extend the search to metastable or non-equilibrium phases instead of limiting it within the equilibrium phases. This is the route followed by Croat.
- Extend the search to ternary or quaternary systems (adding one or two new elements to the mix) instead of limiting it within the binary system. Even though the LRE and Fe form few stable compounds, a variety of stable phases might exist in ternary or quaternary systems. This is the route followed by Sagawa.
Dr. JohnÂ Croat of General Motors in the USA developed RE-Fe and RE-Fe-B alloys by using a route that tends to form metastable phases. Meanwhile, Dr. Masato Sagawa Sumitomo Special MetalsÂ in Japan synthesized numerous compounds based on RE-Fe and small amounts of other elements. Finally, he found a new ternary compound consisting of Nd, Fe and B with remarkable magnetic properties for a permanent magnet material.
Sumitomo Special Metalsâ Route
InÂ Japan, Sumitomo Special Metals developed a powder metallurgy processing route, which initially gave the highest ever observed energy product (the maximum amount of magnetic energy stored in a magnet).
The processing route for sintered NdFeB based magnets is shown in Figure 1. The as-cast ingot must first be broken into powder. This is achieved by exposing the ingot to hydrogen, which is absorbed at the surface. The hydrogen enters the material in the spaces between the atoms and causes an expansion which generates stress in the ingot and the alloy breaks down into a fine powder. The powder is then broken up further by a jet milling stage.
Each powder particle is a single crystal, which can be aligned in a magnetic field. This alignment is held in place by pressing the powder into a green compact, which is not fully dense. The compact is then heated in vacuum to at 1060 Â°C for 1 hour; sintering occurs and the compact densifies, with the assistance of a liquid formed by the melting of the Nd-rich phase. After sintering, the magnets are cooled down and then heat-treated in order to achieve the optimum magnetic properties.
The magnet must then be machined to get the right dimensions for the intended application. After this, the next stage in the processing is to provide a protective barrier on the surface of the magnets. Finally, the magnets are magnetised and tested prior to shipping to the customer.
Figure 1:Â The processing route for sintered NdFeB permanent magnets
General Motorsâ Route
Meantime, on the other side of the world General Motors developed a rapid solidification process to produce powder that afterwards would have used in resin bonded magnets.
The melt-spinningÂ process which was used to produce a ribbon like powdered material appears in the Figure 2.Â In this process, molten alloy is ejected onto the surface of a rotating water cooled wheel, which cool down the material at a rate of one million Â°C/s. The microstructure and magnetic properties of the NdFeB ribbons formed are very sensitive to this cooling rate.
Figure 2:Â Schematic representation of the melt-spinning process and MQ magnet production
This powder cannot be sintered to produce fully dense magnets without destroying the magnetic properties, but can beÂ employed in one of the following three ways:
The melt spun ribbon is blended with a resin to produce an isotropic bonded permanent magnet, what means that can be magnetised along any direction.
The powder is pressed at a temperature about 700 Â°C achieving a fully dense magnet with higher magnetic properties than MQ-I.
The material is heated in a die cavity and then it is slowly deformed. Such magnets are 100% dense and because of the alignment they have higher magnetic properties than MQ-II.
What did happen next?
Bonded and fully dense magnets have their own advantages and drawbacks. Bonded magnets will be selected if isotropic properties and low cost are required. Sintered magnet route should be selected if a high remanence (magnetization remaining after an exciting magnetic field has been removed) and anisotropy is required in large volume.
NdFeB immediately attracted considerable technological interest because of its excellent magnetic properties as well as economic advantages over Sm-Co materials.Â From EREAN we focus on both routes: sintered and resin bonded magnets.
J. J. Croat, J. F. Herbst, R. W. Lee and F. E. Pinkerton, F. E., âHigh-energy product Nd-Fe-B permanent magnetsâ. Applied Physics Letters, 44, 148-149 (1984). http://dx.doi.org/10.1063/1.94584
D. Brown, B. Ma and Z. Chen., âDevelopments in the processing and properties of NdFeB-type permanent magnetsâ. Journal of Magnetism and Magnetic Materials, 248, 432-440 (2002). http://dx.doi.org/10.1016/S0304-8853(02)00334-7
J. J. Croat, J. F. Herbst, R. W. Lee and F. E. Pinkerton, “Pr-Fe and Nd-Fe-based materials: A new class of high-performance permanent magnets”, J. Appl. Phys. 55, 2078 (1984). http://dx.doi.org/10.1063/1.333571
M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, “New Material for Permanent Magnets on a Base of Nd and Fe”, J. Appl. Phys.55, 2083 (1984). http://dx.doi.org/10.1063/1.333572
J. J. Croat, âObservation of large roomâtemperature coercivity in meltâspun Nd0.4Fe0.6â. Applied Physics Letters, 39, 357 (1981). http://dx.doi.org/10.1063/1.92728
GITAM, http://www.gitam.edu/eresource/Engg_Phys/semester_2/magnetic/hard.htm, consulted on 16/05/2014.
A. J. Williams, âHydrogen absorption and desorption studies on NdFeB type alloys used for the production of permanent magnetsâ. Materials Science and Engineering PhD, University Of Birmingham (1994).
S. McCain, âCharacterisation of the Aqueous Corrosion Process in NdFeB Melt Spun Ribbon and MQI Bonded Magnetsâ. Materials Science and Engineering PhD, University of Birmingham (2011). http://etheses.bham.ac.uk/3680/
E. P. Furlani, âPermanent Magnet and Electromechanical Devices. Materials, Analysis and Applicationsâ. Academic Press, 52-53 (2001). http://dx.doi.org/10.1016/B978-012269951-1/50002-4