On the thermal stability of CO2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

• Published in: Journal of magnetism and magnetic materials 2008-04, Vol.320 (7), p.1370-1376
• Main Authors: KRACUNOVSKA, S, TÖPFER, J
• Format: Article
• Language: English
• Subjects: Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Materials synthesis; materials processing; Physics
• ISSN: 0304-8853
• DOI: 10.1016/j.jmmm.2007.11.016

Co2Z hexaferrite Ba3Co2Fe24O41 was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T < 1300 deg C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 deg C and the Y-type ferrite at 1230 deg C. The Z-type material has its stability interval between 1300 and 1350 deg C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 deg C, intermediate grinding and sintering at 1330 deg C. The permeability of Co2Z-type ferrite of about mu=20 is stable up to several 100MHz, with maximum losses mu'' around 700MHz. Addition of 3wt% Bi2O3 as sintering aid shifts the temperature of maximum shrinkage down to 950 deg C and enables sintering of Z-type ferrite powders at 950 deg C. However, the permeability is reduced to mu=3. It is shown here for the first time that Co2Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co2Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 deg C.