Optimization of the coercivity-modifying hydrogenation and re-calcination processes for strontium hexaferrite powder synthesized conventionally

Strontium hexaferrite powder, synthesised conventionally in-house from strontium carbonate (SrCO3) and hematite (Fe2O3) without additives, has been treated in a static hydrogen atmosphere and subsequently calcined in static air under different conditions. The optimum time, temperature, and initial pressure of hydrogenation and the optimum temperature of re-calcination for a fixed time of 1 h were determined using a combination of X-ray diffraction, vibrating sample magnetometer, and high-resolution scanning electron microscope techniques.Increasing the temperature, initial pressure, and time of hydrogenation up to the determined optimum values resulted in the decomposition of the strontium hexaferrite into Fe2O3 and Sr7Fe10O22, together with a more marked reduction of the resultant Fe2O3 to Fe. This was accompanied by the conversion of the initial single-crystal particles into very fine sub-grains, which is the reason for the higher coercivities obtained after re-calcination. Increasing the hydrogenation and re-calcination parameters beyond the optimum values, however, generally resulted in grain growth, which decreased the final magnetic properties. Increasing the re-calcination temperature to 1000 °C resulted in completion of the hexaferrite reformation. Beyond this temperature, however, the coercivity decreased due to grain growth.The optimum conditions were as follows: hydrogenation at 700 °C for 1 h under an initial pressure of 1.3 bar and then re-calcination in air at 1000 °C for 1 h. The highest coercivity obtained after re-calcination was around 400 kA/m. The remanence and saturation magnetization values were very similar to their initial values before the hydrogen treatment.