Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally
Sas. Ebrahimi et al., Characterization and optimization of the coercivity-modifying nitrogenation and re-calcination process for strontium hexaferrite powder synthesized conventionally, J MATER SCI, 34(1), 1999, pp. 45-52
Strontium hexaferrite powder synthesized conventionally in-house from stron
tium carbonate and hematite (Fe2O3) without using additives has been treate
d in a static nitrogen atmosphere and subsequently calcined in static air.
The phase identification studies by means of X-ray diffraction (XRD) and th
ermal magnetic analysis (TMA) indicated the decomposition of the strontium
hexaferrite and the reduction of the resultant iron oxide (Fe2O3) during th
e reaction with nitrogen. High-resolution scanning electron microscopy (HRS
EM) studies show that the reduction occurring during nitrogenation results
in the conversion of some of the large grains into much finer sub-grains. S
trontium hexaferrite, Fe3O4, and Sr7Fe10O22 were the main phases obtained a
fter reduction. However, weak traces of other phases, such as Fe2O3, were a
lso detected. The hexaferrite phase re-formed on subsequent calcination. Th
e magnetic measurements indicated a significant decrease in the intrinsic c
oercivity during nitrogenation due to the formation of Fe3O4. However, afte
r a re-calcination process, the remanence and maximum magnetization (i.e.,
magnetization at 1100 kA/m) exhibited values close to the initial values be
fore treatment, but the value of the intrinsic coercivity was higher than t
hat prior to nitrogenation. Examination of the re-calcined microstructure s
howed that this could be attributed to the fine grains that originated from
the fine sub-grain structures formed in the powder particles during nitrog
enation. The optimum time, initial gas pressure, and temperature of nitroge
nation and the optimum temperature of re-calcination were investigated usin
g a vibrating sample magnetometer (VSM), XRD, and HRSEM.
The optimum temperature for nitrogenation was 950 and 1000 degrees C for re
-calcination. The optimum time and initial nitrogen pressure were 5 h and 1
bar, respectively. The highest intrinsic coercivity obtained after re-calc
ination was similar to 340 kA/m. (C) 1999 Kluwer Academic Publishers.