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

Abstract

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 T1 (temperature in the first sintering stage), t1 (holding time at T1), T2 (temperature in the second sintering stage), and t2 (holding time at T2). The effects of each setting parameter on the density, grain growth, degree of orientation and magnetic properties were systematically investigated. The results revealed that T1 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 Br = 440.4 mT, Hcj = 328.2 kA/m and (BH)max = 37.6 kJ/m3, were obtained under the optimum processing condition: T1 = 1200 °C, T2 = 1000 °C, t1 = 5 min, and t2 = 2 h.