Phase Formation, Microstructure, and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites (II): Effect of Oxygen Partial Pressure

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We have investigated the phase formation, microstructure, and permeability of stoichiometric and Fe-deficient Ni-Cu-Zn ferrites of composition Ni0.30Cu0.20Zn0.50+zFe2−zO4−(z/2) with 0 ≤ z ≤ 0.06 sintered at 1000 °C in various oxygen partial pressures pO2, which range from 0.21 atm down to 10−5 atm. The density of the sintered samples is almost independent of the pO2, whereas the grain size of the Fe-deficient ferrites decreases in more reducing atmospheres. Stoichiometric ferrites show a regular growth of single-phase ferrite grains if sintered in air. Sintering at pO2 ≤ 10−2 atm leads to the formation of a small amount of Cu2O at grain boundaries and triple points. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites, which coexist with a minority CuO phase homogeneously distributed between the grains after sintering in air. At pO2 ≤ 10−2 atm, the CuO grain boundary phase starts to transform into Cu2O, which concentrates at some triple points at pO2 = 10−2 atm, and it is more homogeneously distributed between the ferrite grains at the lower pO2. Formation of the Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability at 1 MHz of the stoichiometric ferrites (z = 0) is between µ′ = 200 and µ′ = 300 within the studied range of the pO2. The permeability at 1 MHz of the Fe-deficient samples decreases with the pO2, e.g., from µ′ = 750 at pO2 = 0.21 atm to µ′ = 320 at pO2 = 10−5 atm for z = 0.02, respectively.