## 3 Results and Discussion
### Influence of PIP Cycle Numbers on the Microstructure and Properties of the Composites  
According to the above results, the BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites were achieved after annealing at 1400 \({}^{\circ}\)C, and three PIP cycle numbers (1, 2, and 3 times) were selected to investigate the effect of PIP cycle numbers on the microstructure and properties of BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites, and the three samples were marked as S1, S2, and S3, respectively. Firstly, the influence of PIP cycle numbers on the densities of the as-prepared composites was evaluated, and the dependence of density on the numbers of the PIP cycle, as depicted in Figure 4. The pristine low-density BPR-derived carbon modified 3D needed carbon fiber composites was about 0.28 g/cm\({}^{3}\), and it can be clearly seen that the density of as-prepared composites increases with increasing PIP cycle number; this is in line with previous literature [4; 5]. Concretely, the values of density for the obtained composites increased from 0.28 g/cm\({}^{3}\) for pristine low-density BPR-derived carbon modified 3D needed carbon fiber composite to 0.59 g/cm\({}^{3}\) for S1 sample, to 0.77 g/cm\({}^{3}\) for S2 sample and further to 0.88 g/cm\({}^{3}\) for S3 sample, respectively. In addition, the dependence between density and PIP cycle number can be used to determine the microstructure and mechanical properties of the as-prepared composites. Figure 4: The densities of as-prepared composites with different pyrolysis process (PIP) cycle numbers. Figure 3: (a) TEM image of as-prepared composites obtained at 1600 \({}^{\circ}\)C. (b) HRTEM image of SiOC within as-prepared composites obtained at 1600 \({}^{\circ}\)C. SAED images of (c) free carbon and (d) SiC within as-prepared composites obtained at 1600 \({}^{\circ}\)C.  
Secondly, the influence of PIP cycle numbers on the microstructure of as-prepared composites was observed by the SEM images listed in Figure 5. From the results, there the number of open pores within the composites decreases with increasing PIP cycle number, leading to an increase in density. Large open pores can be found in the composites after one PIP cycle. In contrast to this, only small voids could be observed after two PIP cycles. Then, the content of small voids decreases when the PIP cycle number increases to 3, and some local regions exhibit relatively dense. In addition, the presence of pores and voids would be beneficial to decrease the thermal conductivity of composites and improve the efficiency of thermal insulation, and further to tailor the impedance matching of composites. Moreover, the effect of PIP cycle numbers on the compressive strength of as-prepared composites was investigated. Notably, the compressive strength of low-density composites with high porosities attracts more attention than that of tensile strength from the point of view of practical application, especially in the thermal protection system field, and thus the compressive strengths of as-prepared composites were evaluated and are displayed in Figure 6 [4]. The pristine compressive strength low-density BPR-derived carbon modified 3D needed carbon fiber composites without SiOC ceramic was just about 3.96 \(\pm\) 0.24 MPa in XY direction and 1.86 \(\pm\) 0.13 MPa in Z direction. From the results revealed in Figure 6, incorporation of SiOC ceramic would strongly improve the compressive strength of the composites, and the value of compressive strength increases both in the \(XY\) and \(Z\) directions as the number of PIP cycle increases. Specifically, the compressive strengths for as-achieved samples with different PIP cycle numbers are about: 12.94 \(\pm\) 1.52 MPa in \(XY\) direction and 6.65 \(\pm\) 0.98 MPa in \(Z\) direction for S1 sample, 14.48 \(\pm\) 1.68 MPa in \(XY\) direction and 8.62 \(\pm\) 1.03 MPa in \(Z\) direction for S2 sample, and 17.18 \(\pm\) 1.92 MPa in \(XY\) direction and 9.01 \(\pm\) 1.25 MPa in \(Z\) direction for S3 sample, respectively.