Synthesis And Electromagnetic Properties Of Co/sio₂ Composite Powder

Dendritic cobalt particles have been synthesized by the selective absorption of the stabilizing agents (OH-). Co composite particles were then prepared by cladding SiO₂ on surface of particles through sol-gel method. Then, the prepared composite particles were heat-treated to tailor the composition and microstructure. Morphology, composition and microstructure of particles were analyzed and characterized through SEM, EDS and XRD. The electromagnetic properties of composite particles were measured by using VSM, microhmmeter and VNA. According to the transmit-line theory [4], the reflection loss (RL) was calculated from the electromagnetic parameters to evaluate the electromagnetic wave absorbing (EMA) performance of coating using the composite particles as filler.It is found that morphology, size and thesymmetry of Co particles were influenced by concentration of reactant and reaction time. Branch-like Co particles could be obtained through concentration controlling of NaOH. As concentration of reducing agent increased, nucleation rate increased but particles size decreased. XRD patterns indicates that the Co particles were composed withα-Co andβ-Co.SiO₂ shell of the Co/SiO₂ composite particles was of amorphous state. After heat-treated, the composite particles can still keep branch-like, but part ofβ-Co transformed toα-Co. The oxidation temperature of composite particles increased about 200℃. After heat-treated, The oxidation temperature increased 200℃more. With the protection of SiO₂ shell, the corrosion resistance was also improved.Dendritic Co particles which were shape anisotropic have evidently higher complex permittivity and complex permeability than spheic particles. Moreover, a dielectric relaxation takes place in the dendritic Co particles. Due to the existence of SiO₂ shell, the effect of eddy current in the Co/SiO₂ composite particles was efficiently suppressed. The introduction of SiO₂ shell also brought lots of ferromagnetic/dielectric interphase which caused new loss mechanism and improved the electromagnetic properties.With a proper heat treatment, Ms and the enhanced exchange coupling of Co increased. At the same time, part ofβ-Co transformed toα-Co. Both of these two reasons increased the complex permeability. The occurrence of the dielectric relaxation was also attributed to heat treatment, so the spectra characteristic was improved.The coatings using Co/SiO₂ composite particles with proper heat treatment as fillers present excellent EMA performance with a maximum reflection loss of 35 dB, an effective-absorption bandwidth of 4 GHz and a typical thickness of only 1 mm.