Two-step sintering of M-type strontium ferrite with high coercivity

Two-step sintering (TSS) was applied on the M-type Sr-hexaferrite to control grain growth and form a desirable microstructure during the final stage of sintering. The process involved T-1 (temperature in the first sintering stage), t(1) (holding time at T-1), T-2 (temperature in the second sintering stage), and t(2) (holding time at T-2). The effects of each setting parameter on the density, grain growth, degree of orientation and magnetic properties were systematically investigated. The results revealed that T-1 was the main factor that primarily controlled microstructure and magnetic properties. Compared with conventional sintering (CS) process, the magnets with smaller average grain size were achieved by TSS, which suggested that two-step sintering could improve the coercivity of Sr-hexaferrites. It was shown that the high integrated magnetic properties for Sr-hexaferrites, including B-r = 440.4 mT, H-cj = 328.2 kA/m and (BH)(max) = 37.6 kJ/m(3), were obtained under the optimum processing condition: T-1 = 1200 degrees C, T-2 = 1000 degrees C, t(1) = 5 min, and t(2) = 2 h.