Understanding the physical requirements for a broad bandwidth is vital for the design of high-efficiency microwave absorber. Our recent works on silicon carbide (SiC) fiber mats-based absorbers imply that metal modification (e.g., Fe or Hf) could benefit their bandwidth effectively. For verification, we fabricated a Co/SiC fiber mat via a similar electrospinning process and subsequent pyrolysis at 1400 °C in Ar atmosphere. The results indicate that after Co modification, the SiC fiber mats show elevated permittivity and tangent loss. With a proper amount of Co adding, the mats could exhibit a wide bandwidth of around 8 GHz (ranging from 10 to 18 GHz) for effective absorption (reflection loss (RL) less than -10 dB) at 2.8 mm thickness. This is similar to our previous findings, confirming that metal modification could be an effective approach to extend the bandwidth of SiC mat absorbers. Explanations can be found through theoretical analysis with the quarter wavelength (λ/4) cancellation theory. It suggests that the declining permittivity (with the increase of frequency) is the key to keep the wavelength in material (λm) nearly unchanged within a frequency range. As a result, in this range, λ/4 cancellation could still be satisfied without changing thickness, which could explain the reasons for the broad bandwidth of metal-modified SiC fiber mats. With this model, it is further predicted that the effective absorption bandwidth could be even extended to be around 12 GHz with appropriate tangent loss. It should be emphasized that the implications obtained in this study could also be applicable to other dielectric absorbers. The requirement of permittivity and the proposed approach could serve as guidelines to achieve a wide bandwidth on a dielectric absorber relying on the λ/4 cancellation principle.
Keywords: Co/SiC fiber; broadband; dielectric absorbers; electrospinning; λ/4 cancellation.