Ni-Cu-Zn Ferrites for low temperature firing: II. Effects of powder morphology and Bi2O3 addition on microstructure and permeability

The morphology of Ni-Cu-Zn ferrite powders obtained by milling of a calcined raw materials mixture strongly effects the densification behavior during sintering at 900∘C. In order to obtain dense samples sub-micron powders with enhanced reactivity are required. The addition of Bi2O3 as sintering additive is beneficial: the density of samples sintered at 900∘C increases with the bismuth oxide concentration up to 0.75 wt.%. The process of liquid phase sintering was studied by dilatometry. The grain size of the sintered samples slightly increases for 0.25 wt.% Bi2O3 compared to bismuth-free samples, whereas for 0.3–0.5 wt.% Bi2O3 additions bimodal grain growth is observed with a significant fraction of very large grains. For > 0.5 wt.% Bi2O3 a homogeneous coarse-grained microstructure is obtained. The permeability increases for small bismuth oxide additions, but decreases for a Bi-oxide content of more than 0.5%. Maximum permeability of μi = 900 is observed for intermediate Bi2O3 concentrations.