αFe2O3, τ-Fe2O3, or α-FeOOll was autoclaved in a Ba(OH)2 solution at 250°C and 280°C respectively. The compositions of the reaction products depended on the Ba(OH)2/Fe2O3 ratio reaction temperature, and time. When α-Fe2O3 was used as the starting material. the constituents were barium hexaferrite, barium diferrite, unreacted α-Fe2O3, and an unknown compound which was identified as 3BaO·4Fe2O3 and its magnetic properties were 1H0= 400 Oc, σr2 6.48 cmu/g, and σr2= 9.60 cmu/g. When r-Fe2O3 or α-FeOOH was used as the starting material. and if the Ba(OH)2/Fe2O3. ratio was 0.33, the reaction product were mainly barium hexaferrite, while at other ratio α-Fe2O3 barium diferrite. and 3BaO·4Fe2O3 any also exist as by — products. When the slurry of Fe(OH)3 was autoclaved for the Sr and Feion concentrations at its SrO·6Fe2O3 stoichioaetric ratio(0.083), the reaction was not complete. The constituents of the product were strontium hexaferrite and α-Fe2O3. When the slurry of Fe(OH) was autoclaved containing lanthanum and calcium ions at 250°C. and the Ca/Fe ratio was 0.083, no lanthanum doped calcium hexaferrite was obtained, and the constituents of the reaction product were mainly α-Fe2O3 and an unknown compound if the solution contained only calcium ion, while one more unknown compound existed if the solution contained lanthanum ion. The observation of electromicrographs showed that the formation mechanism of barium hexaferrite was that α-Fe2O3, τ-Fe2O3, or α-FeOOH dissolved in the solution firstly and then barium hexaferrite nucleated and grew from the solution subsequently. © 1990 IEEE
