Improvement in Magnetic Properties of Samarium-Cobalt (1:5) Alloy through Controlled Material Processing

Abstract

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 sub-divided. 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. 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.