Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr-hexaferrite
Sas. Ebrahimi et al., Comparative effects of the hydrogen and nitrogen gas treatment and re-calcination (GTR) routes on the composition, microstructure, and magnetic properties of conventionally synthesized Sr-hexaferrite, J MATER SCI, 34(1), 1999, pp. 53-58
Optimized static hydrogen treated and recalcined (HTR) and static nitrogen
treated and recalcined (NTR) Sr-hexaferrite powders synthesized conventiona
lly in-house are compared with one another. The phase identification studie
s and lattice parameter measurements showed first that the Sr-hexaferrite d
ecomposed, forming iron oxide (Fe2O3), which was then reduced during the st
atic hydrogen or nitrogen treatment, and, second, that the hexaferrite phas
e was recovered albeit with a small change in the composition (as indicated
by the lattice spacings) after the re-calcination treatment in static air.
These effects were more pronounced in the hydrogen process than in the nit
rogen process. The main effect of this gas-treatment and re-calcination (GT
R) process on the microstructure of the Sr-hexaferrite was the transformati
on of the single-crystal particles into particles with a very fine sub-grai
n structure during the gas treatment, which resulted in the formation of po
lycrystalline hexaferrite particles with a much finer grain size during sub
sequent recalcination, compared to that of the initial hexaferrite powder.
This finer structure was responsible for the higher coercivities observed a
fter re-calcination. With regard to the hydrogen and nitrogen processes, th
e former resulted in a higher degree of oxide reduction and hence a higher
coercivity on re-calcination. The coercivity of the initial Sr-hexaferrite
increased from 310 kA/m (3.9 kOe) to similar to 400 kA/m (5 kOe) after HTR
and to 342 kA/m (4.3 kOe) after NTR. The initial magnetization behavior was
also different for the HTR- and NTR-processed powders, with the former exh
ibiting behavior characteristic of single domains. This was consistent with
the grain size being significantly less than the single-domain size (-1 mu
). (C) 1999 Kluwer Academic Publishers.