Rapidly Solidified Neodymium-Iron-Boron Magnets
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and cobalt are not the most desirable constituents for large-scale use, and in the late 1970s light rare-earth-iron materials began to be investigated as potential alternatives to Sm-Co alloys. The light rare-earth elements lanthanum, cerium, p r a s e o d y m i u m , and neodymium comprise over 95% of the rare-earth content of a typical ore body and, hence, are the most attractive economically. Development of a light rareearth-iron permanent magnet was hampered severely, however, by the absence of suitable Compounds, quintessential ingredients for conventional magnetic h a r d e n i n g . One approach for surmounting this obstacle was to employ rapid solidification tech-
niques to form nonequilibrium or metastable microstruetures. This line of research 1 ' 3 led to the discovery of neodymium-iron-boron alloys and to the identification4'5 of the novel ternary Compound, Nd2Fe14B, responsible for their intrinsic magnetic properties. The theoretical maximum energy produet for Nd2Fe14B is ~64 MGOe, versus -25 MGOe for SmCo5. Practical NdFe-B magnets having energy produets of at least 40 MGOe have been prepared from materials rapidly solidified by melt spinning,6 on which we focus here, as well as by traditional powder metallurgy techniques. 7 The prospect for achieving energy produets larger than ever before, coupled with the fact that neodymium is relatively abundant, has stimulated the considerable scientific and technological interest in Nd-Fe-B materials.
Crystal Structure and Intrinsic Properties of Nd2Fe,4B Neutron powder diffraction analysis provided the first determination of the exact stoichiometry, detailed crystal structure, and room temperature magn e t i c m o m e n t a r r a n g e m e n t of Nd2Fe14B.4 The crystallographic results were confirmed subsequently by two separate single-crystal x-ray investigations.8-9 Although isostructural R2Fe14B Compounds have been found to form with most of the other rare-earth ele-
Speedometer, Gauges and Digital
Sun Rool Motor Automatic Temperature Control
Tape Drive Motor Liquid Level Indicators Windshield Wiper Motor Windshield Washer Pump Headtlght Door Motor
Anti-Skid
wi 'Ilution Control
Fuel Pump Motor Window Lift Motor Seat Actuator Motor
Coolant Fan Motor Heat and Air Condltion Motor Starter Motor Throttle and Crankshatt Position Sensors
Figure 3. Automotive applications involving permanent magnets.
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Rapidly Solidified Neodymium-Iron-Boron Magnets
ments as well as yttrium, magnets having the largest observed energy products a r e b a s e d o n t h e n e o d y m i u m phase. Figure 2 displays the intricate unit cell of t h e Nd 2 Fe 1 4 B s t r u c t u r e , which has tetragonal lattice symmetry (space group P4 2 /mnm). Each cell contains 68 atoms, or four formula units. There are nine crystallographically distinct atomic positions: six iron, two rare earth, and one boron. Geometrie relationships among some of the atomic layers are illustrated in Figure 3. All the Nd and B atoms, but only four of the 56 Fe atoms, are located in the basal (z=0) and z
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