In situ regulation of microstructure and microwave-absorbing properties of FeSiAl through HNO3 oxidation

Wrapping insulation of coatings is effective for enhancing the microwave-absorbing properties (MAPs) of ferromagnetic absorbents (FMAs). However, the process is still limited by the low bonding strength with the matrix. Herein, an in situ regulation strategy based on the preparation of thin thickness and strong adhesion insulating layers through HNO3 oxidation was developed to address the limitations. The oxidation process of FeSiAl (FSA) powders was carried out by HNO3 following three main steps. First, the original oxide layer first reacted with HNO3 to form Fe3+ and Al3+. Second, the oxide layer composed of Al2O3 and Fe3O4 was preferentially formed due to the negative change in Gibbs free energy. Finally, the oxide and pigment-deposition layers were subjected to competitive growth and dissolution accompanied by the dissolution of Fe and Al atoms. Oxidation time up to 10 min resulted in the formation of a bilayer structure with a thickness of ∼50 nm on the FSA surface, as well as an outer layer crammed of Al(OH)3 and Fe(OH)3, and an inner layer containing mixed Fe2O3, Fe3O4, Al2O3, and SiO2. The MAPs of as-treated FSA achieved minimum reflection loss (RL) of −25.90 dB at 13.36 GHz, as well as absorption bandwidth of 5.61 GHz (RL < −10 dB) at 10.13–15.74 GHz and thickness of 2.5 mm. In sum, the developed route looks promising for the preparation of high-performance FMAs.

Structural, Magnetic, and X-Band Microwave Absorbing Properties of Ni-Ferrites Prepared Using Oxidized Mill Scales

This study aims to evaluate the structural, magnetic, and microwave absorbing properties at the X-band region of oxidized mill scales as by-product derived from a steel making process by means of a facile solid-state reaction. The oxidized mill scales were heated at 600 °C for 4 h followed by mixing with NiO. A calcination process took place at 900 °C and sintering process were conducted at 1260 °C with a milling process conducted in between the heating process. X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS) were employed to evaluate the structural properties of the Ni-ferrites samples. Remacomp measurement were conducted to evaluate the magnetic properties and vector network analyzer (VNA) to measure its microwave properties. A single phase of NiFe2O4 was confirmed by XRD data. The site occupancies derived from the Rietveld refinement shows that the Ni:Fe:O ratio deviates from the 1:2:4 ratio as that suggests vacancies formed in the Ni2+ and Fe3+ that lowers the unit cell density to 5.08 g/cm3 that further confirmed by EDS measurement. The coercivity of 11 kOe is also higher than the bulk NiFe2O4¬ prepared by the chemical grade raw materials. The reflection data of the microwave properties at X-band of 8-12 GHz do not shows significant absorptions. This study suggests that the selected preparation method yields a single phase, however with the significant crystallographic defects and has less ‘soft’ magnetic properties compared to NiFe2O4 prepared using chemical grade by previous study.