Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability

We have studied sub-stoichiometric Ni-Cu-Zn ferrites with iron deficiency (i.e., < 50mol% Fe2O3) of composition Ni0.20Cu0.20Zn0.60+zFe2-zO4-(z/2) with 0 <= z <= 0.06. The temperature of maximum shrinkage rate is shifted from T=1000 degrees C for z=0 towards lower temperatures down to T=900 degrees C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 degrees C for z > 0 only. Sub-stoichiometric compositions (z > 0) do not form single-phase spinel ferrites after sintering at 900 degrees C, but rather represent mixtures of CuO and a stoichiometric ferrite with slightly modified composition. The formation of small amounts of CuO at grain boundaries is demonstrated by XRD and SEM. The permeability is increased from mu=80 for stoichiometric ferrites (z=0) to mu=660 for z=0.02. The formation of CuO during sintering of sub-stoichiometric ferrites supports densification and is a prerequisite for low temperature firing of multilayer inductors. Addition of 1 wt% Bi2O3 as liquid phase sintering aid is required to provide sufficient densification of the stoichiometric ferrite (z=0) at 900 degrees C. Addition of 0.37 wt% Bi2O3 to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 degrees C; however, the microstructure formation is dominated by heterogeneous grain growth. (C) 2011 Elsevier B.V. All rights reserved.