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.