## 3 Results and Discussion
### Influence of PIP Cycle Numbers on the Microstructure and Properties of the Composites
The reason for the different values of compressive strength between \(XY\) and \(Z\) directions could be attributed to the distribution anisotropy of 3D needed carbon fiber preform, among which the most fiber orientation was parallel and perpendicular to \(XY\) and \(Z\) directions, resulting in a much higher value at \(XY\) direction compared with that of \(Z\) direction. Figure 5: SEM images of as-prepared composites with different PIP cycle numbers from \(XY\) and \(Z\) directions: (**a,b**) one PIP cycle; (**c,d**) two PIP cycles; (**e,f**) three PIP cycles. Besides the mechanical properties, high-temperature oxidation resistance and thermal stability of the materials are also important for the practical application. The high-temperature oxidation resistance and thermal stability properties of as-prepared composites were evaluated by TGA curves from room temperature to 1000 \({}^{\circ}\)C both under air and Ar atmospheres, and the results of S3 sample and pristine low-density BPR-derived carbon modified 3D needed carbon fiber composite without SiOC ceramic are given in Figure 7. The results demonstrate that the introduction of SiOC would remarkedly improve the high-temperature oxidation resistance and thermal stability properties of low-density BPR-derived carbon modified 3D needed carbon fiber composites both under air and Ar atmospheres. It is worth pointing out that the weight reduction at low temperatures for the tested samples should be attributed to the absorbed moisture on the surface of the composites due to the high porosity. Delightedly, the S3 sample with SiOC ceramic presents excellent high-temperature oxidation resistance and thermal stability properties with weight retention above 95% up to about 1000 \({}^{\circ}\)C either under air or Ar atmosphere, indicating that the BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites could be as a promising candidate of thermal protection materials for application in harsh environment. However, the low-density BPR-derived carbon modified 3D needed carbon fiber composite without SiOC ceramic exhibits poor high-temperature oxidation resistance and thermal stability properties, especially in air. Based on the above analysis results, it is reasonable to believe that the BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites constructed and prepared in current work via a simple PIP method might be considered as a promising material employed in a harsh environment on account of their enhanced mechanical properties and excellent thermostatabilities. Figure 6: Compressive strengths of as-prepared composites with different PIP cycle numbers ranging from 1 to 3 times. Figure 7: TGA curves for the as-prepared composites with three PIP cycles and composites without silicon oxycarbide (SiOC) ceramic from room temperature to 1000 \({}^{\circ}\)C under (**a**) air and (**b**) Ar atmospheres. Furthermore, the functional performances of BPR-derived carbon modified 3D needed carbon fiber reinforced SiOC composites, namely EMW absorption properties, were evaluated to open up the potential application fields. According to previous literature, the EMW absorption property of an absorber is closely related to its electromagnetic parameters, namely complex permittivity (\(\varepsilon_{r}=\varepsilon^{\prime}\) - \(j\varepsilon^{\prime\prime}\)) and complex permeability (\(\mu_{r}=\mu^{\prime}-j\mu^{\prime\prime}\)), while the value of \(\mu_{r}\) could be equal to 1 due to not having a ferromagnetic phase involved [22; 36]. Noteworthily, the real and imaginary parts of complex permittivity, namely \(\varepsilon^{\prime}\) and \(\varepsilon^{\prime\prime}\) relate to the storage and consume abilities of electric energy, and thus the relative complex permittivity of as-prepared composites with different PIP cycle numbers are given in Figure 8[22; 35].