Rc. Pullar et al., Halide removal from BaM (BaFe12O9) and SrM (SrFe12O19) ferrite fibers via a steaming process, J MATER RES, 16(11), 2001, pp. 3162-3169
Aligned fibers of barium and strontium M hexaferrite (BaFe12O19 and SrFe12O
19) were manufactured from an aqueous inorganic sol-gel-based spinning proc
ess, but halides have been found to be retained up to 1000 degreesC, inhibi
ting the formation of the hexagonal ferrite phases. Therefore an investigat
ion was carried out into the removal of the halides at lower temperatures t
hrough steaming between 500 and 900 degreesC/3 It, and the subsequent effec
ts upon microstructure and magnetic properties. The fibers were prefired to
400 degreesC to remove all organic components, and in all cases the steami
ng process resulted in loss of alignment of the fibers. It was found that t
he 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 inde
ed 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 compara
ble to random M ferrite fibers fired to 1000 degreesC in air, while steamin
g 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).