N. Martinez et al., A study of the evolution of the constituent phases and magnetic propertiesof hydrogen-treated Sr-hexaferrite during calcination, J MATER SCI, 34(10), 1999, pp. 2401-2406
A hydrogen treatment followed by calcination, has been developed in order t
o enhance the intrinsic coercivity of Sr-hexaferrite (SrFe12O19). Fully hyd
rogen-treated Sr-hexaferrite consists of a mixture of 73%, by weight, of al
pha Fe and 27% of Sr7Fe10O22 phases. Calcination of this material to reform
the SrFe12O19 phase occurs in two stages. Between room temperature and 600
degrees C, oxygen was absorbed resulting in a large increase in weight wit
h the formation of a mixture of SrFeO3-x, and Fe2O3 (alpha and gamma). Duri
ng the second stage, the intermediate phases reacted to form SrFe12O19 at a
temperature of between 700 and 800 degrees C. A partial desorption of oxyg
en occurred until calcination reached completion at 1000 degrees C. The mag
netization at 1100 kA m(-1) and the remanence were similar to those of the
untreated material, but, because of a much refined grain size, the intrinsi
c coercivity was considerably larger, with values around 400 kA m(-1). Grai
n growth occurs at temperatures >1000 degrees C, resulting in a decrease in
the intrinsic coercivity. (C) 1999 Kluwer Academic Publishers.