Electromagnetic property of SiO₂-coated carbonyl iron/polyimide composites as heat resistant microwave absorbing materials

Heat resistant microwave absorbing materials were prepared by compression molding method, using polyimide resin as matrix and SiO₂ coated carbonyl iron (CI) as filler. The SiO₂ coated CI particles were prepared by Stober process. The microwave absorbing properties and the effect of heat treatment on the electromagnetic properties of SiO₂ coated CI/polyimide composites were investigated. When the content of SiO₂ coated CI is 60 wt%, the value of minimum reflection loss decreases from -25 dB to -33 dB with the thickness increases from 1.5 mm to 2.1 mm. According to the thermal-gravimetric analyses (TGA) curves, the polyimide matrix can be used at 300 °C for long time. The complex permittivity of the composites slightly increases while the complex permeability almost keeps constant after heat treatment at 300 °C for 10 h, which indicating that the composites can be used at elevated temperature as microwave absorbing materials at the same time have good heat resistance and microwave absorption.