Abstract
Equilibrium partial oxygen pressure of SrFe 18O 27 ferrite phase was determined between 1250° and 1460°C by flowing CO2-O2 mixed gas or in air. In air this phase is stable in the temperature between 1400° and 1460°C. At 1350°C the partial oxygen pressure ranges from 6×10-2 to 3×10-2 atm, and at 1300°C from 4 × 10-2 to 2 × 10-2 atm. The polycrystalline ferrites prepared by firing at different temperatures and oxygen pressures contained various amount of ferrous ions. Chemical analysis revealed that the nonstoichiometric parameter x in molar expression SrFe2+2xFe3+18-2xO28-x varied from 1.0 to 0.4. Partial molar free energy of solution of oxygen into the ferrite lattice was calculated as ΔG=6.2×10 3 +6.7T-12xT cal/mole O 2 and standard free energy change of the oxidation of ferrite phase was ΔG°=-8.3×104+46.2T cal/mole O 2. From D. C. resistivity measurement the activation energy of the conduction was calculated to be 0.05 eV. Ferromagnetic Curie temperature was 520°C and the values of coercive force were 6-17 oersteds.
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