چكيده به لاتين
Abstract:
It is necessary to explore methods to minimize the radar signal reflected from a
vehicle. Microwave absorbing materials including of dielectric and magnetic materials and conducting polymers and composites have attracted a great attention as electromagnetic interference shielding in electronic devices. Among the magnetic materials, the magnetic spinel and hexagonal ferrites are widely used for their strong absorption, wide working frequency range, thin layer and light weight. Spinel and hexagonal ferrites can absorb a narrow band of electromagnetic wave in mega and giga hertz range, respectively, making it difficult to selection a single material as a perfect absorber material. Therefore, the magnetic hard/soft ferrite composites with complementary properties can be a promising solution. Among all types of hexagonal ferrites, W-type hexaferrite possesses the highest saturation magnetization and anisotropy field and high ferromagnetic resonance frequencies of which make it as a strong candidate for the microwave absorber applications. Solution combustion synthesis method has benefits of the facile operation, low cost, energy-efficient and a short reaction time. In this work ZnCo-substituted strontium W-type hexaferrite (SrZnCoFe16O27) has been prepared by solution combustion synthesis method. The effects of fuel content (φ = 0.75, 1 and 1.5) on phase evolution, microstructure and magnetic properties were characterized by X-ray diffractometry, infrared spectroscopy, electron microscopy and vibrating sample magnetometry techniques. Fourier transform infrared analysis and theoretical calculations were conducted to determine and control the concentration of metal citrates in solution precursors. Then in situ nanocomposites of SrZnCoFe16O27/M wt.%CoFe2O4 (M=0, 25, 50) were prepared by one-pot solution combustion synthesis method. Single phase SrZnCoFe16O27 powders with the large platelet-like particles were achieved at φ= 1.5 following calcination at 1200 °C, while the M-type hexaferrite and spinel ferrite phases were formed at φ < 1.5. With the increase of fuel content, the saturation magnetization increased from 64.8 to 76.8 emu/g and coercivity firstly increased from 355 to 585 Oe and then decreased up to 34.1 Oe. After synthesis of single phase, the effect of CoFe2O4 formation on phase evolution, microstructure, magnetic and microwave absorption properties were studied by X-ray diffractometer, electron microscope and vibrating sample magnetometer techniques. Microstructural studies revealed that the SrZnCoFe16O27/M wt.%CoFe2O4 composites consist of platelet-like SrZnCoFe16O27 particles being mixed with cubic-like spinel particles. The saturation magnetization and coercivity of SrZnCoFe16O27/M wt.%CoFe2O4 composite increases from 74 to 81 emu/g and from 105 to 129 Oe with increasing of M from 25 to 50 respectively. The absorption results show reflection loss of -33.6 dB in 10.4 GHz and 2.5 GHz bandwidth of -20 dB for a thickness of 2.6 mm for a 0wt.% compound of Cobalt ferrite. Reflection loss of -8.2 dB in 10.2
GHz and a 1.4 GHz bandwidth of 4.5 dB for a thickness of 4 mm for a 25wt.% compound of Cobalt ferrite and reflection loss of -7 dB in 10.1 GHz and a 1.1 GHz bandwidth of -4.9 dB for a thickness of 3.5 mm for 50wt.% compound of cobalt ferrite.
Keywords: Microwave absorber, Solution combustion, W-type Strontium hexaferrite, Cobalt ferrite