Optimization of hydrogen dynamic heat treatment and re-calcination for preparation of strontium hexaferrite nanocrystalline powder

Abstract

Strontium hexaferrite is a hard magnetic material which under hydrogen treatment and re-calcination, its phase composition and also particles size and morphology change completely. Strontium hexaferrite was prepared by conventional route with calcination of strontium carbonate and hematite at 1100 °C for 1 h. Then strontium hexaferrite was heat treated in hydrogen dynamic atmosphere at various temperatures and gas flows for different times. Optimum conditions of hydrogen treatment were obtained at 850 °C with 60 cm 3/min flow for 1 h. Subsequent re-calcination was carried out at various temperatures for the optimum hydrogen treated powder and its optimum conditions were obtained at 1000 °C for 1 h. The effect of dynamic hydrogen treatment and re-calcination on the phase composition and particles size and morphology characterized by X-ray diffraction (XRD) and scanning and transmission electron microscopes (SEM and TEM). The results showed decomposition of strontium hexaferrite and reduction of the resultant hematite mainly to iron during hydrogen treatment. Nanocrystalline powder of strontium hexaferrite was also reformed after the re-calcination. The magnetic properties of the initial and final strontium hexaferrite powder were measured by a vibration sample magnetometer (VSM). The results showed about 30% increase in the coercivity by application of this process on the strontium hexaferrite powder.