High permeability Ni–Cu–Zn ferrites through additive-free low-temperature sintering of nanocrystalline powders

Nanocrystalline Ni–Cu–Zn ferrite powders Ni0.20Cu0.20Zn0.62Fe1.98O3.99 were prepared by thermal decomposition of an oxalate precursor. The particle size is 6 nm and 350 nm, respectively, for powders obtained through calcinations at 350 °C or 750 °C. The shrinkage behavior significantly changes with particle size; the temperature of maximum shrinkage rate is TMSR = 700 °C for particles of 6 nm size and increases to TMSR = 880 °C for particles 350 nm in size. Dense samples with a permeability of μ = 780 are obtained by sintering at 900 °C for 2 h. Mixtures of nanocrystalline and sub-micron powders allow tailoring of the shrinkage behavior. A maximum permeability of μ = 840 is obtained after sintering of a 1:1-mixture at 900 °C. This demonstrates the potential of nanocrystalline ferrites for co-firing without additives at 900 °C and integration of ferrite inductors into LTCC modules.