Broadening of EM Energy-Absorption Frequency Band by Micrometer-to-Nanometer Grain Size Reduction in NiZn Ferrite

Prior to its use for microwave absorption, NiZn ferrite with certain compositions and with ultrafine microstructures (submicron/nanoscale) have been fabricated to investigate the best chemical composition and microstructure for such absorption. A mixture of iron oxide $({\rm Fe}_{2}{\rm O}_{3})$, nickel oxide (NiO) and zinc oxide (ZnO) was weighed according to the targeted proportion, milled using the mechanical alloying technique and sintered at a temperature of 900$\ ^{\circ}{\rm C}$ for 10 h to form nickel zinc ferrite $({\rm Ni}_{\rm x}{\rm Zn}_{1-{\rm x}}{\rm Fe}_{2}{\rm O}_{4})$. X-ray diffractometry (XRD), scanning transmission electron microscopy (STEM) and field emission electron microscopy (FeSEM) were used to investigate the crystalline phase formation, particle size and surface morphology, respectively. The toroidal samples were further measured using an Agilent 4291B impedance analyzer with the frequency range from 1 MHz to 1 GHz to investigate the materials complex permeability component of $\mu^{\prime}$ and $\mu^{\prime\prime}$. The XRD results show that at 900$\ ^{\circ}{\rm C}$ the full phase of nickel zinc ferrite was formed. The average particle size was 89.1 nm. The resulting morphology was a homogeneous microstructure with small grain size and a uniform grain size distribution via the mechanical alloying technique. A significantly important result was established: that it was possible to extend the energy absorption frequency range by reducing the grain size from micrometer to nanometer, using samples of the same chemical composition.