Development of Co-Fe Based Nanostructured Soft Magnetic Materials for High Temperature Applications

Abstract

Nanocrystalline FINEMET type alloys derived from amorphous precursors exhibit superior soft magnetic properties suitable for various types of applications like transformer cores, choke coils, magnetic sensors etc. However these materials lose their ferromagnetic stability at elevated temperature due to their low Curie temperature (below 700K) and hence are unsuitable for high temperature applications. To overcome this limitation, additional effort has been made to develop materials with high Curie temperature coupled with superior soft magnetic properties. It is observed that crystalline CoFe based alloy (HIPERCO) exhibits high Curie temperature. Hence, Willard et al. introduced a series of CoFe based alloys by the incorporation of cobalt in amorphous FeZrB alloy systems, which resulted in optimum material composition of Co44Fe44Zr7B4Cu1 termed as HITPERM alloy. Even though this type of alloy had fairly high Curie temperature but its soft magnetic properties tended to deteriorate due to the presence of oxidation prone Zr in the system and put a limitation to use. In the present work, an attempt is made to develop CoFe- based nanocrystalline alloys with good soft magnetic properties and high Curie temperature for high temperature applications. In the FeSiNbB system, the element cobalt was incorporated with an aim to increase the Curie temperature and also enhance the soft magnetic properties. To achieve the optimum alloy composition, a series of CoxFe72xSi4Nb4B20 (x = 10, 20, 36, 50 at%) alloys were prepared in the form of amorphous ribbons by the melt spinning technique. Partial substitution of Co by Fe in CoxFe72-xSi4Nb4B20 (x = 10, 20, 36, 50 at%) series has an effect on the onsets of primary and secondary crystallization temperature. The size of nanoparticles tends to be smaller with increasing cobalt content. It was found that the alloy with stoichiometry x = 36 at% i.e., Co:Fe :: 50:50 exhibits better soft magnetic properties at elevated temperature compared to other measured alloys (x = 10, 20, 50 at%). The Co36Fe36Si4Nb4B20 alloy exhibits excellent soft magnetic properties with coercivity less than ~106 Am-1, susceptibility ~0.5X103, high Curie temperature ~1000K and low power loss in its nano-crystalline state (annealed at 925K). This is due to the lowering of magneto crystalline anisotropy attributed to the formation of bcc-FeCo nanoparticles. The combination of high Curie iv temperature and superior soft magnetic properties makes this developed material a potential candidate for high temperature soft magnetic applications. For further improvement of soft magnetic properties, Al was incorporated in the optimum Co36Fe36Si4Nb4B20 alloy. The effect of Al substitution for Si in the above mentioned alloy system i.e. Co36Fe36Si4-yAlyNb4B20 (y = 0, 0.5, 1.0, 1.5, 2.0 at%) has been studied. The onset of crystallization temperature increases with Al content up to (y ≤ 1), beyond which (1 < y < 2) it follows a decreasing trend. The glass transition temperature (Tg) lowers with increasing Al content. The alloy with Al = 1 at% exhibits low and stable coercivity values in its nanocrystalline state. This may be attributed to the reduction of magneto crystalline anisotropy and at the same time smaller nanoparticles of bcc-(CoFe)SiAl reduce effective anisotropy. The nanocrystalline alloy with Al = 1 at% (annealed at 925K) exhibits Curie temperature of 1100K in comparison to that of 1000K in the alloy without Al (y = 0 at%). It indicates higher ferromagnetic ordering at 925K in the alloy with y = 1 at% than the alloy without Al (y = 0 at%). Similarly the alloy with Al = 1 at% exhibited very low coercivity i.e. ~66 A/m whereas that without Al (y = 0) showed coercivity around ~106 A/m annealed at 925K. The optimum Co36Fe36Si3Al1Nb4B20 nanocrystalline alloy exhibited better corrosion resistance in its nanocrystalline state due to the highest charge transfer resistance on the formation of bcc-(FeCo)SiAl nanoparticles. The spin wave stiffness constant and the mean square range of the exchange interaction of nanocrystalline Co36Fe36Si3Al1Nb4B20 alloy were found to decrease by magnetic softening on annealing at 850K. The excellent soft magnetic properties of Co36Fe36Si3Al1Nb4B20 alloy with the formation of nano-phases may be softening the spin waves. In addition, the smaller grain size of nanoparticles may also have an effect on magnetic softening.