Preparation Of Ultrafine Feni Alloy Powders By Thermal Decomposition Process And Their Microwave Absorbing Properties

Abstract:With the rapid development of wireless communication technology in modern society, the environmental and safety issues caused by electromagnetic radiation and electromagnetic interference are more and more serious. Meanwhile, stealth ability is an urgent requirement for the modern military equipment; stealth weaponry has become one of the important directions of current world military high-tech development. Microwave absorbing materials as a means of absorbing and attenuating electromagnetic wave were studied extensively. FeNi alloy is expected to be an excellent absorbing material due to its excellent electromagnetic properties. Fiber morphology and porous structure is a hot spot for magnetic metal absorbing materials. However, the present methods for preparing FeNi alloy fibers have some shortcomings such as great difficulty, small-scale and difficult to control the process. In addition, the preparation of porous ultrafine FeNi alloy powder has not related reports. Therefore, it is significant to develop a simple and large-scale method to obtain a uniform morphology porous and fibrous FeNi alloy powder. In this paper, the porous and fibrous FeNi alloy powder with different compositions were prepared for the first time by an oxalate coprecipitation-thermal decomposition reduction process, and their absorbing properties were studied systematically. The main research results are as follows:According to the mass balance and simultaneous balance principle, the precipitation-coordination equilibrium thermodynamic models of Fe2+-Ni2+-NH3-C2O42--H2O system and Fe2+-Ni2+-C2N2H8-C2O42--H2O system were established. The models were calculated by Matlab programs, and the change laws of the concentration of each substance under different conditions were illuminated in detail. The granular FeNi oxalate precursors with different morphologies and components are prepared in the Fe2+-Ni2+-NH3-C2O42--H2O system. At the conditions of25%ethanol solvent, dumping feed, pH=5, metal ions concentration of 0.4mol/L and temperature of60℃, the FeNi precursor particles with good dispersibility and uniform particle size can be obtained. With the increasing content of Fe in the precursor, the precursor particle morphology has undergone an evolution of cuboid, dodecahedron, octahedron, cube and cuboid. The fibrous FeNi oxalate precursors with different components and large aspect ratios are prepared in the Fe2+-Ni2+-C2N2H8-C2O42--H2O system. At the conditions of pH=6.3, concentration of0.05mol/L, temperature of60℃, the FeNi precursor fibers with good dispersibility and uniform aspect ratio can be obtained. The growth mechanism of precursor fiber was studied by IR and XRD, and the results show that in the precipitation process, ethylene diamine entered into the precursor and a new oxalate crystal structure of [NixFe1-x(C2H8N2)(C2O4)] was formed, which is the key to obtain a fibrous morphology.The thermal decomposition thermodynamic of FeNi oxalate was investigated and a H2reducing atmosphere was determined. When the hydrogen partial pressure is greater than0.29%, NiO can be reduced to Ni, and Fe3O4can be reduced to Fe when the hydrogen partial pressure is greater than90.52%. The thermal decomposition process of the precursors were studied by using TG-DSC technology. The results show that, the thermal decomposition process of the precrsor prepared at ammonia system is:FexNi1-xC2O4·2H2Oâ†'FexNi1-xC2O4â†'Ni+Fe3O4; and the thermal decomposition process of the precrsor prepared at ethylene amine system is [FexNi1-x(C2N2H8)y]C2O4·zH2Oâ†'[FexNi1-x(C2N2H8)y]C2O4â†'FexNi1-xC2O4â†'Ni+Fe3O4.The thermal decomposition kinetics of the precursors was studied by a non-isothermal kinetics method. The apparent activation energy of thermal decomposition reaction was calculated by Starink method, and the kinetic mechanism function was determined by combining Tang-Liu-Zhang-Wang-Wang temperature integral approximation and master curve method. The results show that:the thermal decomposition kinetics of ammonia-based precursor follows the random nucleation and subsequent growth model, the activation energy is148.59kJ/mol, pre-exponential factor is1.25×107, and kinetic equation is G(a)=[-ln(1-α)]0.44. The thermal decomposition kinetics of ethylenediamine-based precursor also follows the random nucleation and subsequent growth model, the activation energy is214.40kJ/mol, pre-exponential factor is4.34×1011, and kinetic equation is G(α)=[-ln(l-α)]0.95.The effects of process parameters such as reducing atmosphere, temperature, time, gas flow and heating rate on the morphology and phase structure of FeNi alloys were researched systematically. Under the conditions of N2/H2of1:1, temperature of450℃, time of30min, gas flow of0.1L/min, and heating rate of2℃/min, the porous FeNi alloy paticles with uniform pores and FeNi fibers with large aspect ratios can be obtained. After the thermal decomposition-reduction process, the particle morphology has undergone a certain distortion, and the volume is shrinked. With the increase of Fe content, the phase of FeNi alloy shifts from fcc structure to bcc structure gradually.The absorbing properties and mechanism of porous FeNi alloys and FeNi fibers at different absorber content, thickness and component were studied. When the mass fraction is30%, for porous Fe50Ni50sample with a thickness of3.0mm, a minimum reflection loss of-52.58dB can be obtained at6.82GHz; for porous Fe6oNi4o sample with a thickness of1.5mm, the reflection loss in13.98～18.00GHz range is less than-10dB, the effective bandwidth is up to4.02GHz. When the mass fraction is20%, for Fe10Ni90fiber with a thickness of2.0mm,a minimum reflection loss of-45.37dB at13.90GHz can be obtained,and the reflection loss in12.21-15.99GHz range is less than-10dB,the effective bandwidth is up to3.78GHz. The dielectric loss of the porous FeNi alloy mainly come from atomic polarization and electronic polarization, the magnetic loss is mainly from natural resonance, and the porous structure has an important effect on the absorbing property. The dielectric loss of the FeNi alloy fiber obey debye model, and the magnetic loss is mainly from natural resonance. Both porous and fibrous FeNi alloy have predominate magnetic loss, and their absorbing characteristics follow the λ/4matching model. When the thickness is2.0mm, the surface density of the porous Fe60Ni40alloy and Fe10Ni90alloy fiber is2.46g/cm2and2.10g/cm2, respectively. The porous structure and fiber morphology plays an important role in reducing the density of the absorber.