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Preparation and Electromagnetic Properties of a Few Kinds of Dielectric/Ferromagnetic Composite Powder

Author: JiangJianTang
Tutor: ZhenLiang
School: Harbin Institute of Technology
Course: Materials Science
Keywords: Dielectric / ferromagnetic composite powders Electroless plating Heat treatment Microstructure Microwave electromagnetic properties Electromagnetic wave absorption performance
CLC: TB383.3
Type: PhD thesis
Year: 2008
Downloads: 325
Quote: 8
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Abstract


This paper to obtain good performance of the electromagnetic wave absorption material for the purpose, was prepared by plating in the surface chemistry of the the Al18B4O33 whiskers and SiC whiskers submicron Co or CoFe alloy coating Al18B4O33w/Co Al18B4O33w/CoFe, SiCw / Co three dielectric / iron magnetic composite powder, and heat treatment to its composition and structure of regulation. XRF, SEM, XRD, EDS microstructure of the composite powders were certificates of analysis table; VSM, PPMS, Microhmmeter, VNA test the electromagnetic properties of the composite powders, and based on the test results by the The transmission line theorem evaluation composite powder coatings electromagnetic wave absorption performance. Obtained by studying the impact of the structural factors of each component of the electromagnetic properties and preliminary analysis of the impact of the mechanism. In order to calculate the permeability of the material is accurate and reliable, but also established the MRC-BEMT method. The chemical plating process, the study shows that the pH value and temperature of the bath is not only affect the chemical plating process efficiency and speed of response, but also significantly affect the composition of the coating and thus affect its crystal structure. With increase in the relative content of Plating Bath Fe2, chemical plating the CoFe process efficiency decreased; the CoFe alloy composition is controlled by the bath Co2, Fe2 the relative content of all ingredients of the the CoFe alloy coatings are amorphous. Each component coating microstructure showed the characteristics of the particle packing; coating growth behavior and morphology, the structure is also influenced by the matrix whisker surface wettability and pretreatment. Hydrogen heat treatment of the chemical coating becomes dense and smooth, reduced to form metal atom diffusion coating morphology and structure evolution, while the drop in surface energy is the driving force of the morphology evolution. Thin and easy to destroy and isolated particles in hydrogen heat treatment process, combined with the low strength of the coating. Heat treatment in Ar plating the Co layer morphology did not change significantly. Electroless Co layer and a CoFe layer plated crystallization more rapidly, when the heat treatment temperature is high, or a longer time, the phase change will occur in the coating. Co introduction of the ceramic whisker / cobalt composite powders with good electromagnetic properties, the electromagnetic properties of the dielectric / ferromagnetic composite powder after appropriate heat treatment to improve further. Analysis that the the composite powders permittivity improved in the heat treatment process, and dielectric relaxation appears mainly due to the decrease in the coating resistivity, while coating the saturation magnetization increased and coatings in the enhancement of the exchange coupling interaction is the magnetic permeability The main reason for the rate increased. Coating resistance is higher than 5 × 10-3? · Cm middleweight, eddy permeability is not obvious, the resistivity of less than 5 × 10-2? · Cm when the composite powders dielectric relaxation. The crystal structure of the coating spectral characteristics of its permeability. Composite powders in Al18B4O33w/CoFe at 400 ° C hydrogen heat treatment, electromagnetic properties little changed; its permeability is significantly improved in the higher temperature heat treatment, but the resistivity decreases slowly, little change in dielectric constant. The unique shape and electrical properties of SiCw whisker triggered a strange the electric magnetic resonance behavior, its specific cause is not known. Al18B4O33w/Co composite powders electromagnetic wave absorption performance increased with the increase of the Co content and heat treatment time prolonged first reducing the Co content is too high or low resistivity of the coating compound powder absorbing less efficient. Appropriate the heat treatment Al18B4O33w/Co composite powder prepared coating for the absorption of medium having excellent electromagnetic wave absorption properties, this paper the typical coating maximum loss of 78 dB greater than 10 dB effective absorption bandwidth of greater than 3.5 GHz, matching a thickness of approximately 2 mm. Absorbing Properties of SiCw / Co composite powders in similar Al18B4O33w/Co, composite Al18B4O33w/CoFe powder absorbing performance is poor. The MRC-BEMT established effectively eliminates the instability, when the the Bruggeman equation in the low filling factor of the composite material application to improve the consistency and accuracy of the calculation of permeability. The tests showed that the method is better than existing methods, can be used for the calculation of material permeability, but its intrinsic permeability prediction accuracy needs further verification.

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