Ni‐Cu‐Zn ferrites with high Curie temperature for multilayer inductors with increased operating temperatures

Abstract

Ni-Cu-Zn ferrites are used as soft magnetic materials for multilayer ferrite inductors (MLFI) because of their low sintering temperature and good magnetic performance up to MHz frequencies. 1-3 The cofiring temperature of multilayer inductors is typically restricted to ≤900°C because coil patterns are printed using Ag pastes. Correlations between ferrite compositions, their sintering behavior, and magnetic properties have been investigated, 4-7 and optimum ferrite compositions for low-temperature firing of multilayer inductors have been identified. A typical feature of low-firing Ni-Cu-Zn ferrite chemistries is a small deficiency of iron oxide, that is, less than 50 mol% Fe 2 O 3 is used in the starting oxide mixture. Such compositions are referred to as sub-stoichiometric here; their composition typically translates into a spinel formula with less than two Fe per formula unit, for example, Ni 0.20 Cu 0.20 Zn 0.62 Fe 1.98 O 3.99. It was shown that Fe-deficient sub-stoichiometric compositions with improved densification characteristics at 900°C constitute mixtures of a Cu-deficient stoichiometric spinel and a small concentration of CuO precipitated at grain boundaries. 8,9 In addition, bismuth oxide is frequently used as a sintering aid for Ni-Cu-Zn ferrites to enhance shrinkage and densification. 10,11 Additions of up to 2 wt% Bi 2 O 3 are effective for preparing low-temperature fired ferrites with different microstructures and permeabilities ranging from µ = 100 to 900 depending on the Bi 2 O 3 concentration 12,13. Multilayer inductors are passive devices which are used in many electronic components including filters, modules, DC-DC converters, transformers, or electromagnetic compatibility components. Requirements for MLFI include low profile, high power, low DC bias superposition, or higher operating temperature. Ni-Cu-Zn ferrites with low power loss, 14 or Fe-Si-Cr alloys are used for new generations of