Microstructural Optimization of Low-Temperature-Fired Ni–Zn–Cu Ferrites Using Calcination

The purpose of this work was to examine the effect of low-temperature sintering on the microstructural optimization of Ni–Zn–Cu ferrites. This behavior was evaluated with respect to calcination temperature. The amount of unreacted hematite produced during calcination was quantitatively analyzed using a new calibration curve obtained using X-ray powder diffractometry. The formation of a spinel phase was initiated above 550° C, and completed above 850° C. The microstructural optimization of Ni–Zn–Cu ferrite samples was achieved using powder calcined at 750° C, and these specimens showed a maximum permeability of 700. A variation in microstructure with the change of the calcination temperature is ascribed as the cause of this optimization. Two mechanisms for the optimization are also suggested. One is the formation of hard agglomerates at high calcination temperatures and the second is the inhibition of grain growth due to the residual hematite in powders calcined at low temperatures. Microstructural optimization was achieved through a trade-off between these two mechanisms.