Studies On Microwave Absorbing Properties Of GF/ACF Screen Composites

Based on impedance matching principle, the epoxy matrix composites, which contained activated carbon fiber (ACF) felts screens as absorbers and glass fiber as the impedance modifier, were successfully prepared. Microwave absorbing properties and reflection properties of the resulting composites were evaluated, and the mechanisms were also discussed. The main research contents are summarized as follows:The influences of the shape and size parameters of ACF screens on the microwave absorbing properties and reflection properties were firstly studied. For the rectangular screen composite, the effects of the aspect ratio of aperture and aperture spacing on the absorbing property of composites were investigated. When the aspect ratio was 2 and the aperture spacing was 0.5cm, there could be achieved the lowest reflection loss (about -34.9dB), and the frequency range, corresponding to the reflection loss below -10dB, covered 2.3 to 18GHz. As to the square ACF screen composite, when the aperture length was 2cm and the aperture spacing was 0.8cm, the lowest reflection loss reached -33.9dB, indicating that the absorbing property of composites was best. Furthermore, the influence of the thickness of absorber on microwave reflection properties was investigated. It was not an effective way to improve the absorbing properties only by increasing the absorber's thickness, once the thickness of absorber exceeded the skin depth of microwave in the medium. In summary, the absorbing property of screens composite could be significantly affected by the structural parameters of ACF screens. The main absorbing parameters of the composites such as frequency selectivity of screens, the dielectric loss and eddy current loss of ACF felts should be considered. It could be concluded that proper design of the structural parameters of the screens was an effective way to prepare excellent microwave absorbing materials.Microporous ACF felts were further activated at different temperatures and the resulting ACF screens were used to manufacture the epoxy matrix composites. The microwave absorbing properties of those composites were investigated as well. The results showed that the composites containing the reactivated ACF felts screens rather than untreated ones performed better microwave absorbing properties. Especially for those composites with ACF felts activated at 750℃, the reflection loss reduced to below-10dB with the range from 5.2GHz to 9.9GHz and the lowest reflection loss achieved-21.1dB. The polar relaxation ofπelectron and the electromagnetic wave scattering loss caused by ACF morphology led to the electromagnetic loss.The influence of glass fiber, as an impedance modifier, on microwave reflection and absorption properties of ACF screens was also investigated. When a 3cm x 1.5cm rectangular aperture was filled by a glass fiber sheet (2cm x 1cm), the effective bandwidth was 12.1 GHz, whereas a square aperture (the side length was 3 cm) was filled by a square glass fiber sheet (2cm x 2cm), the minimum value of reflection loss dropped to -29.8dB. Therefore, "impedance matching" is one of the most important principles for the design of the microwave absorbing materials. The good absorbing materials, possessing efficient impedance matching, high absorptions and low reflection properties can be prepared via adjusting the composition of wave-absorber (such as carbon fibers) and wave-transparent (such as glass fiber) materials.