전자파 흡수체 응용을 위한 전기적 절연체 코팅 된 자성 철 나노입자

Abstract

The design of electromagnetic (EM) wave absorber materials is essential to enhance the absorption efficiency and overcome the degradation caused by eddy current in the high-frequency region. Although magneto-dielectric composites have proven to be suitable due to the high magnetic and dielectric loss, the demand for the development of the insulation and dispersion process is urgently required. We show two different methods for designing EM absorbers with the combination of magnetic nanoparticles and dielectric materials: (1) mixing magnetic particles with an insulating material, or (2) coating the particles with a nonmagnetic shell. We prepared Fe/MgO composites by an ultrasonic pyrolysis (USP) method, following a selective hydrogen reduction. Homogeneous oxide particles are first obtained and Fe/MgO composites are subsequently fabricated by a selective hydrogen reduction process. The magnetic properties of the composites are directly tuned by the magnetic contents controlled by changing the Fe³⁺/Mg²⁺ ratio in the precursor solution. The composites obtained at Fe³⁺ : Mg²⁺ ratio of 6 : 4 showed a high Ms value of 164.10 emu/g. Moreover, the maximum reflection loss reaches –65.6 dB at 7.7 GHz for the absorber with the thickness of 1.5 mm. Structural and magnetic study of the composites revealed that the higher Fe contents allowed the higher magnetic properties, and the Fe particles formed in the matrix or at the oxide surface with dispersion show better insulation properties in composite scale and the high absorption properties. In addition, we report on the synthesis and magnetic properties of Fe@SiO2 core-shell nanoparticles via a conventional sol-gel process. An amorphous SiO2 shells were uniformly formed on the commercial Fe nanoparticles with a surface oxide, and Fe@SiO2 particles were obtained by the post reduction in H2 atmosphere. The reaction times of the sol-gel process were adjusted to produce powders with different SiO2 thickness, and the tendency of the magnetic properties according to the SiO2 thickness was investigated. The thicker the SiO2 shell of Fe particles, the better the electromagnetic waves absorption was maintained at high frequencies. The absorption properties of the Fe particles without insulation reach the maximum absorption magnitude under -20 dB, but all of the particles coated with insulator exhibited a high absorption magnitude of more than -20 dB. The maximum absorption magnitude of -83.67 dB and the bandwidth of 2.2 GHz were obtained under the conditions of 15 nm SiO2 thickness.