Preparation, thermal stability and permeability behaviour of Z-type hexagonal ferrites for multilayer inductors

Co 2 Z hexaferrite Ba 3 Co 2 Fe 24 O 41 with iron excess Ba 3 Co 2-x Fe 24+x O 41 (0 ≤ x ≤ 0.8) and deficiency Ba 3 Co 2+y Fe 24-y O 41 (0 ≤ y ≤ 0.6) were prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies showed that both synthesis routes lead to an almost single phase Z-type ferrites for x = 0 after calcination and sintering at 1330°C. For iron deficient samples, 0 ≤ y ≤ 0.6, the multi-phase mixtures were observed. The permeability of Z-type ferrites varied with composition x. The maximum permeability of µ’ = 37 was observed for the sample with x = 0.6. Addition of Bi 2 O 3 as sintering aid shifts the temperature of maximum shrinkage to lower temperatures and dense samples were obtained after sintering at 950°C. However, the permeability is only µ’ = 3. It is shown here that Co 2 Z ferrites are not stable under 1300°C and that partial thermal decomposition causes a reduction of the permeability.