Effect of NaNO3 foaming agent on barium ferrite hollow microspheres prepared by self-reactive quenching technology

Al + Fe2O3 + BaO2 and NaNO3 as the reactive system and the foaming agent, respectively, are used to prepare barium ferrite hollow microspheres (BFHMs) by self-reactive quenching technology based on flame spraying technology, self-propagating high-temperature synthesis (SHS) technology and quick chilling technology. Effects of NaNO3 on the particle size, morphology, phase structure and microwave absorption properties of BFHMs are investigated through SEM, XRD, particle size analyzer, high-speed camera and vector network analyzer. The results show that, after adding 10 wt.% NaNO3, the average particle size of BFHMs decreases initially from 28 μm to 6 μm, as well as the particle distribution gets narrower. Micro-nano lamellar crystals appear on the surface of BFHMs, with the dimension ranging from 500 nm to 2 μm. BaFe2O4 and Fe3O4, which have spinel structures, can be seen in XRD, and they are beneficial for microwave absorption properties. The real part (ɛ′), imaginary part (ɛ″) of permittivity, and the imaginary part of permeability (μ″) increases in 0.5-18 GHz. The real part (μ′) of permeability increases in 0.5-10.3 GHz, while decreases in 10.3-18 GHz. The microwave absorption properties are improved greatly, and the minimum reflectivity decreases from −3.1 dB to −9.8 dB. The reasons for improvement of microwave absorption properties after adding NaNO3 foaming agent may be the decreasing of the particle size of BFHMs, the appearance of ferrites (BaFe2O4 and Fe3O4) with spinel structure and special micro-nano tabular crystals. Magnetoplumbite-type barium ferrites (BaFe12O19) hollow microspheres are obtained after heat-treatment and the microwave absorption properties are further improved.