Low temperature sintering of sub-stoichiometric Ni-Cu-Zn ferrites: Shrinkage, microstructure and permeability |
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Authors: | J MürbeJ Töpfer |
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Institution: | University of Applied Sciences Jena, Dept. SciTec, Carl-Zeiss-Promenade 2, 07745 Jena, Germany |
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Abstract: | 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 °C for z=0 towards lower temperatures down to T=900 °C for a sub-stoichiometric ferrite with z=0.02. Dense samples are obtained after firing at 900 °C for z>0 only. Sub-stoichiometric compositions (z>0) do not form single-phase spinel ferrites after sintering at 900 °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 μ=80 for stoichiometric ferrites (z=0) to μ=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 °C. Addition of 0.37 wt% Bi2O3 to a sub-stoichiometric ferrite (z=0.02) results in dense samples after firing at 900 °C; however, the microstructure formation is dominated by heterogeneous grain growth. |
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Keywords: | Soft ferrite Low temperature sintering Permeability Sub-stoichiometry |
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