Preparation, thermal stability and permeability behavior of substituted Z-type hexagonal ferrites for multilayer inductors
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Co2Z-type hexagonal ferrites with iron excess Ba3Co2 − x Fe24 + x O41 (0 ≤ x ≤ 0.8) and deficiency Ba3Co2 + y Fe24 − y O41 (0 ≤ y ≤ 0.6) were prepared by an oxalate coprecipitation technique. This synthesis route leads to almost single phase Z-type ferrites for x = 0 after calcination and sintering at 1330 °C. The Z-type formation is enhanced for x > 0 and single phase ferrites are obtained for 0.4 ≤ x ≤ 0.8. The permeability of Z-type ferrites varies with composition x: Maximum permeability of μ′ = 28 is observed for 0.4 ≤ x ≤ 0.6 for samples sintered at 1330 °C. The frequency dispersion shows broad peaks of μ″ stretching from 200 MHz to >1 GHz. For iron deficient samples 0 ≤ y ≤ 0.6 multi-phase mixtures were obtained. For Ag-based multilayer inductor applications sintering at 950 °C is required. Co2Z ferrites with Fe excess are not stable at this temperature as demonstrated by XRD. The permeability of samples sintered at 950 °C is drastically reduced to μ′ = 3. This demonstrates that these materials are not able to provide sufficient permeability for multilayer inductors for high-frequency operations since they are not compatible with the low temperature ceramic cofiring technology.
KeywordsSoft ferrites Co2-Z hexagonal ferrites Permeability
PACS75.50 Gg 81.20 Ev 81.40 Rs
The authors thank the Bundesministerium für Bildung und Forschung (Germany) for financial support (grant 03WKF22E). We acknowledge the contributions of Mrs. M. Friedrich (FHJ) for SEM, Mr. J. Bierlich for TG/DTA measurements and Dr. R. Müller (IPHT Jena) for high temperature VSM measurements.
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