## 3 Results and Discussion
### Influence of PIP Cycle Numbers on the Microstructure and Properties of the Composites
The EMW absorption properties of all samples with different PIP cycle numbers were evaluated, and the two-dimensional (2D) RL contour plots are revealed in Figure 9. The pristine low-density BPR-derived carbon modified 3D needed carbon fiber composite without SiOC ceramic possesses a poor EMW absorption performance just with an RL\({}_{\text{min}}\) value of \(-7\) dB, and the BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites achieve substantially enhanced microwave absorption performances. From Figure 9a, the RL\({}_{\text{min}}\) value for the S1 sample with one PIP cycle is about \(-12.22\) dB at 18 GHz with a thin thickness of 1.00 mm, and the EAB value is about 2.1 GHz (15.9-18.0 GHz). However, after increasing the PIP cycle number to 2, the RL\({}_{\text{min}}\) value is just about \(-9.81\) dB at the frequency of 18.00 GHz and the matching thickness of 1.00 mm. Obviously, the composite with three PIP cycle numbers (S3 sample) exhibits the best EMW absorption property, whose RL\({}_{\text{min}}\) value is as low as \(-24.31\) dB at 18 GHz with an ultra-thin thickness of 1.05 mm (Figure 9c), and the largest EAB value for the S3 sample is about 4.9 GHz ranging from 13.08 GHz to 17.98 GHz with the thickness of only 1.21 mm. Combined with the above results, it is reasonable to believe that the introduction of SiOC ceramic would be beneficial for the enhancement of the EMW absorption properties of low-density BPR-derived carbon modified 3D needed carbon fiber composites, and their EMW absorption performances could be effectively controlled by just adjusting the PIP cycle numbers. Figure 4:0 Frequency dependence of (**a**) the real part (\(\epsilon\)’) and (**b**) imaginary part (\(\epsilon\)”) of complex permittivity, and (**c**) dielectric loss (tan \(\delta\)e) and (**d**) Cole–Cole curves for as-prepared composites with different PIP cycle numbers in the frequency range of 2–18 GHz. Figure 4:1 The 2D calculated reflection loss (RL) value maps of as-prepared composites with different PIP cycle numbers within 0.5–5 mm and 2–18 GHz: (**a–c**) one, two, and three PIP cycles. Reasons for the improved EMW absorption performance are described in the following: Firstly, the dipole and interfacial polarizations would be mainly responsible for the improved properties due to the presence of numerous Cole-Cole semicircles and interfaces [42; 43]. Secondly, conductive and dielectric losses also tightly affected the EMW absorption performance because of the different conductivities among carbon fibers, BPR-derived carbon, and SiOC ceramic [42; 43]. Moreover, the defects within the composites might also play an important role in increasing the microwave absorption abilities [42; 43]. Overall, the enhanced EMW absorption properties might be mainly due to the matched impedance and improved loss ability that arose from the synergetic effect among carbon fibers, BPR-derived carbon, and SiOC ceramic. To date, lots of efforts have been made to investigate and improve the EMW absorption properties of carbon fibers and PDCs, especially in construction of composites, and thus we further compared the EMW absorption capabilities between our products and absorbers recently reported and the results were displayed in Figure 10 and Table S2 [6; 9; 10; 11; 12; 13; 15; 16; 17; 18; 19; 20; 24; 25; 26]. Although the RL\({}_{\rm min}\) value for our products, namely BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites is not the lowest, the value is comparable to those of most of PDCs (Figure 10a). Notably, our BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites possess the widest EAB value among all the absorbers listed in the references, which is vitally important for design and construction of next-generation EMW absorption absorbers.