Effect of B4C content and annealing on complex permittivity and microwave-absorption properties of B4C/Al2O3 coatings

The B4C/Al2O3 coatings were fabricated by air plasma spraying technology, and their complex permittivity and microwave absorption properties in the X-band were investigated before and after annealing (500 °C/2 h). Both the real and imaginary parts of the complex permittivity of the coatings decreased after annealing, which can be attributed to the weakening of polarization relaxation intensity and the reduction of electrical conductivity caused by the escape of carbon atoms. In addition, the density of B4C/Al2O3 coatings decreased from 3.01 to 2.16 g/cm3 with increasing B4C content. The B4C/Al2O3 coatings exhibit a minimum reflection loss (RL) value of −39.58 dB and the effective absorption bandwidth (RL<−10 dB, EAB) covers 1.9 GHz at a thickness of 1.6 mm. After annealing, the above coatings still had an EAB of 1 GHz. Therefore, the B4C/Al2O3 coatings can be considered as a promising microwave-absorption candidate with good high-temperature microwave-absorbing performance and low density.