High-temperature microwave bilayer absorber based on lithium aluminum silicate/lithium aluminum silicate-SiC composite

The microwave absorbing properties of lithium aluminum silicate (LAS) and LAS–SiC double layer composite absorbers were investigated within the frequency range of 8.2–12.4 GHz at 300–500 °C. The composite absorbers for use as high-temperature radar wave-absorbing materials were developed by hot-pressing LAS glass–ceramic and LAS–SiC composite, which were used as the impedance transformer layer and the low-impedance resonator layer, respectively. Nanometer-size β-SiC powders were fabricated at the relatively low sintering temperature of 1450 °C in argon atmosphere. The structure and morphology of the SiC powders were characterized using thermogravimetry-differential scanning calorimetry (TG-DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The electromagnetic absorbing properties of the double layer at different temperatures and SiC contents were measured at normal temperature (27 °C) and high temperature by a vector network analyzer. The results indicate that the contents of nanometer-size β-SiC can increase the relative permittivity and dielectric loss of the double layer microwave absorbers. At high temperatures, the microwave absorber consisting of a 2-mm-thick layer of LAS ceramic and a 2-mm-thick layer of LAS–SiC composite with 10 wt% SiC content exhibited excellent performance with a minimum absorption at −42.8 dB at 10.5 GHz and the absorption bandwidth (reflection loss less than −10 dB) of 3.5 GHz in the X-band. The minimum reflection loss measured at 300–500 °C was less than −24 dB.