Optimized nanocrystalline strontium hexaferrite prepared by applying a methane GTR process on a conventionally synthesized powder

Abstract Optimization of the effective re-calcination parameters in a gaseous heat treatment and re-calcination (GTR) process for producing nanocrystalline Sr-hexaferrite powder using CH 4 has been investigated for the first time in this research. The initial Sr-hexaferrite powder was prepared by a conventional route with calcination of the mixture of SrCO 3 and α-Fe 2 O 3 at 1100 °C for 1 h. Then the resultant powder was isothermally heat treated in CH 4 dynamic atmosphere at 950 °C with a gas flow of 15 cc/min for 30 min. Finally the resultant powder was re-calcined at various temperatures for different times. The rate of heating and cooling was 10 °C/min. Due to the gas heat treatment, the hard magnetic nature of the material changed from hard to soft with changes in the phase composition, particle size and morphology. In the second step of the process, the soft magnetic nature of the intermediate material returned from soft to hard again by re-calcination. However, the resultant nanocrystalline Sr-hexaferrite powder had a higher coercivity compared to that of the initial powder. The results showed significant changes in morphology and crystallite size of the initial powder during re-calcination process which made a great increase of about 17% in its coercivity. The crystallite size of the resultant Sr-hexaferrite was measured lower than 50 nm.