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

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.