Dense and half-dense NiZnCo ferrite ceramics: Their respective relevance for antenna downsizing, according to their dielectric and magnetic properties at microwave frequencies
Résumé
Spinel ferrite Ni0.5Zn0.3Co0.2Fe1.98O4x nanoparticles were synthesized by co-precipitation
method, and samples were realized by moulding and annealing at key temperatures (TM¼800 C,
900 C, 1050 C, determined beforehand through shrinkage measurements) going with calcining
and sintering processes. Annealing at 800 C and 900 C led to half-dense ceramics (porosity 50
vol. %), whereas bulky ferrite was obtained after annealing at 1050 C. Elemental analysis, X-ray
diffraction and ion chromatography analysis were performed. Complex dielectric permittivity (e*)
and magnetic permeability (l*) were investigated up to 6 GHz. With increasing TM, a decreasing
amount of Fe2þ was observed, going with increasing sample density. Coupled effects of the Fe2þ
concentration and of the porosity, both on dielectric and magnetic properties, were chiefly investigated
and discussed. The materials show almost constant permittivities (e0 ¼5.0, 6.0, and 14.8 for
TM¼800 C, 900 C and 1050 C, respectively). The bulk value at f¼1 GHz (e0 ¼14.8) can be
interpreted well according to Shannon’s theory. The permittivities of the half-dense ceramics are
discussed on the basis of Bruggeman’s Effective Medium Theory. The materials annealed at
800 degC and 900 degC show almost constant magnetic permeabilities in the frequency range from 0.2
to 1GHz (l0 ¼3.4 and 6.0 for TM¼800 degC and 900 degC). The observed permeability behavior is
typical of monodomain particles, except for the sample annealed at 1050 degC, for which domain
wall contribution to l* is suspected because of non-negligible losses at low frequency
(l00 ¼1.3–1.8 at f<0.3 GHz). This finding is supported by estimations of the upper and lower values
for the critical grain size, on the basis of Brown–Van der Zaag’s theory. Facing bulk ceramics,
and in view of using Ni0.5Zn0.3Co0.2Fe1.98O4x ferrite as substrate for antenna miniaturization, the
electromagnetic properties of half-dense ceramics materials seem to be very competitive at frequencies
beyond 0.2 GHz, and up to 0.7–0.8 GHz.