Development of Strontium Hexaferrite Magnets from Celestite Ore and Blue Dust

Abstract

Among the class of permanent magnets, M-type ferrites are by far the most important permanent magnetic materials. The M-type ferrites or hexaferrites are characterized by compound having general formula MFe12O19 (M = Sr, Ba, Pb) with hexagonal magnetoplumbite or M- structure. Their hard magnetic nature is due to large magnetocrystalline anisotropy with hexagonal c axis as the preferred direction. The most characteristic feature that underlines the great economic success of M- type ferrite magnets is the low price per unit of available magnetic energy. This is particularly due to the relatively low cost and wide availability of the raw materials. Compared with the alnico magnets, M-type ferrites have high coercivity (Hc) and a moderate remanence (Br) resulting in moderate energy product (BH)max. Strontium ferrite has higher magnetocrystalline anisotropy compared to barium or lead ferrite. Commercially, strontium ferrite magnets are available in the sintered (Isotropic/ Anisotropic) and plastic bonded form. Apart from chemical composition, the magnetic properties of sintered strontium ferrite depend upon on the processing technique and microstructure. The most important microstructural features are high degree of grain alignment, high density and small grain size. These characteristics are dependent on powder processing, sintering temperature, sintering additives and powder alignment in the magnetic field. The present study focuses on preparation of sintered strontium hexaferrite (SrFe12O19) magnets from natural ores celestite and blue dust and from pure strontium carbonate and iron oxide. The magnets were made by conventional calcination method. Synthesis of strontium hexaferrite by mechanoalloying was also carried out.