Electroless Plating of Co Thin Film on Hollow Glass Microspheres and the Effect of Film Thickness on Microwave Absorbance

Abstract

Co thin film was coated on hollow microspheres by electroless plating, and the effect of its thickness on the high-frequency electromagnetic and microwave absorbing properties were investigated for application in lightweight microwave absorbers. Metal plating was carried out in a two-step electroless plating process (pretreatment of activation and plating). The thickness of the Co film can be increased up to 3 μm by repeating the batch of the plating process. Due to the metallic and ferromagnetic behavior of the Co films, a high dielectric constant and magnetic loss could be obtained in the polymer composites containing the Co-coated microspheres with increased Co film thickness. From the analysis of impedance matching, it was found that the higher the Co film thickness, the lower the matching frequency due to the increased dielectric constant and magnetic loss. For the composite of microspheres with Co film thickness of 2 μm, for example, reflection loss less than -25 dB was determined at 9 GHz with a matching thickness of 2 mm. The predicted matching thickness in X-band frequencies was quite small compared with conventional ferrite composite absorbers (~ 3 mm), and the result is attributed to the higher magnetic loss and dielectric constant from the Co films with a controlled thickness.