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Permanent magnets are defined as solid materials that provide sufficiently high
magnetic flux and offer resistance to demagnetizing fields. High magnetic flux can be
controlled by changing the chemical composition of the permanent magnetic material but
the demagnetizing field resistance called coercivity depends upon the shape or crystal
anisotropies and the process in which microscopic regions of the material are further subdivided.
Permanent magnetic materials include a variety of ceramics, intermetallics and
alloys. Samarium-cobalt (SmCo) magnets are rare earth magnets and are known for their
high coercivity and Curie temperature. In this class of magnets, SmCo5 has a potential to
demonstrate highest coercivity due to its high magneto-crystalline anisotropy. However;
only 4% of the theoretical coercivity values are achieved so far. One method of improving
the coercivity is through alloying while the other is through process control. The latter
technique is used to improve the microstructure of the magnet.
In this research the improvement of coercivity of SmCo5 is focused through
process control. The microstructure of SmCo5 has been controlled through processing at
the stages of manufacturing i.e., casting and ball milling. In the first stage, the
microstructure was controlled using conformal cooling channels in the mold i.e., through
controlled solidification. The samples from this process were compared with the spin
casting technique. It was observed that the formation of Sm2Co7 and Sm5CO19 are
responsible for lowering the coercivity of the magnetic material. Therefore, the
solidification temperature was controlled to achieve better microstructure. The results
show that the casting produced at the lower temperatures had nano-sized peritectic
lamellar structure. These nano structures are belived to improve the coercivity of SmCo5
to 32.9 kOe, which is one of the highest reported value.
In the second part of this thesis, the focus was on the optimization of the ball
milling parameters i.e., the process by which fine powder is produced. Ball-milling affects
the shape, size distribution and mean particle size and consequently the final magnetic
properties. The Taguchi L9 experimentation was designed to determine the effect of
different parameters on the magnetic properties of the final product. It was noted that the
ball milling speed, ball to powder ratio and time are the most significant process
parameters which affects the coercivity of the final magnet. Best combination of these
parameters was time and ball to powder ratio.
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In this work novel mold design was introduced which can provide the casting with
fine microstructure. The coercivity values upto 32.9 kOe were achieved with the same
mold design. Further, the optimization of ball milling parameters through Taguchi was
done for SmCo5. Novel nano-structures were observed in SmCo5 before and after sintering
process.