Halide removal from BaM (BaFe12O9) 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 degreesC, 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 degreesC/3 It, and the subsequent effects upon microstructure and magnetic properties. The fibers were prefired to 400 degreesC 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 degreesC, becoming single-phase SrM or virtually pure phase BaM at 700 degreesC, 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 degreesC had M-s and H-c values comparable to random M ferrite fibers fired to 1000 degreesC in air, while steaming over a temperature range from 400 to 800 degreesC/3 h gave products with improved magnetic properties, with SrM fibers having an M-s of 81.4 emu g(-1) and H-c of 457 kA m(-1)