Phase Formation, Microstructure and Permeability of Fe-Deficient Ni-Cu-Zn Ferrites, (I): Effect of Sintering Temperature

We have studied the densification, 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 temperatures from 900 degrees C to 1150 degrees C. The shrinkage is shifted from 1000 degrees C for z = 0 towards lower temperatures and reaches its maximum rate at 900 degrees C for z = 0.02. Stoichiometric ferrites show regular growth of single-phase ferrite grains if sintered at T-s <= 1100 degrees C. Sintering at 1150 degrees C leads to the formation of a small amount of Cu2O, triggering exaggerated grain growth. Fe-deficient compositions (z > 0) form Cu-poor stoichiometric ferrites coexisting with a minority CuO phase after sintering at 900-1000 degrees C. At T-s >= 1050 degrees C, CuO transforms into Cu2O, and exaggerated grain growth is observed. The formation of Cu oxide second phases is investigated using XRD, SEM, and EDX. The permeability of the ferrites increases with sintering temperature up to a maximum permeability of mu = 230 for z = 0 or mu = 580 for z = 0.02, respectively, at T-s = 1000 degrees C. At higher sintering temperatures, the permeability decreases, which is due to the formation of a microstructure with intra-crystalline porosity in large grains, and a non-magnetic Cu oxide grain boundary phase.

Automated summary

The study investigates the effect of stoichiometry and Fe-deficiency on the properties of Ni-Cu-Zn ferrites, and observes phase formation, microstructure changes, and permeability variations under different sintering temperatures and compositions.