In the conventional solid-state synthesis of barium hexaferrite (BaFe12O19) before complete crystallization of BaFe12O19, an intermediate phase of barium-monoferrite (BaFe2O4) was formed. In some process (e.g. mechanical activation) this phenomenon was not occurred and BaFe12O19 formed without formation of BaFe2O4 phase. Indeed, formation of barium-monoferrite bypassed
BaFe12O19 and BaFe2O4 are neighboring phases with a comparable low (1336°C) eutectic in between, see the phase diagram attached here which is taken from "Phase Diagrams for Ceramists", entry 12599. During normal sintering processes mixtures show often a tendency to run in the direction of such eutectics; and then you have the intermediate formation of BaFe2O4. If thermal activation replaces (partially) high temperature, such segregation phenomena are reduced, and you can get the desired phase directly.
BaFe12O19 and BaFe2O4 are neighboring phases with a comparable low (1336°C) eutectic in between, see the phase diagram attached here which is taken from "Phase Diagrams for Ceramists", entry 12599. During normal sintering processes mixtures show often a tendency to run in the direction of such eutectics; and then you have the intermediate formation of BaFe2O4. If thermal activation replaces (partially) high temperature, such segregation phenomena are reduced, and you can get the desired phase directly.
Dear Doctor Klimm,
thank you for your comprehensive answer. I will be so grateful if you would share the complete data of the reference.
Sincerely yours,
Amin Hodaei
The Japanese paper is in the attachment. Bibliographic data are here: https://www.jstage.jst.go.jp/article/jinstmet1952/60/6/60_6_547/_article
According to our experience, the presence of Ba mono ferrite together with alpha-Fe2 O3 as secondary phases is an indication that the sintering temperature was not high enough, and sintering at higher temperatures (normally > 1100 C)
Dear Doctor Klimm,
I would like to thank you so much for sharing such valuable information.
Sincerely yours,
Amin Hodaei
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