Halide removal from BaM (BaFe12O19) and SrM (SrFe12O19) ferrite fibers via a steaming process

Aligned fibers of barium and strontium M hexaferrite (BaFe12O19 and SrFe12O19) were manufactured from an aqueous inorganic sol-gel-based spinning process, but halides have been found to be retained up to 1000 °C, inhibiting the formation of the hexagonal ferrite phases. Therefore an investigation was carried out into the removal of the halides at lower temperatures through steaming between 500 and 900 °C/3 h, and the subsequent effects upon microstructure and magnetic properties. The fibers were prefired to 400 °C to remove all organic components, and in all cases the steaming process resulted in loss of alignment of the fibers. It was found that the M phase began to form at only 600 °C, becoming single-phase SrM or virtually pure phase BaM at 700 °C, confirming that halides had indeed delayed M phase formation. Both materials had a grain size below 100 nm, but other unusual surface features not seen before on ferrite fibers were observed. The fibers steamed at 700 °C had Ms and Hc values comparable to random M ferrite fibers fired to 1000 °C in air, while steaming over a temperature range from 400 to 800 °C/3 h gave products with improved magnetic properties, with SrM fibers having an Ms of 81.4 emu g-1 and Hc of 457 kA m-1.