## 3 Results and Discussion  
### Effect of Pyrolysis Temperature on the Composition and Microstructure of the Composites  
According to previous literature, the composition and microstructure of composites with the addition of SiOC precursor were strongly dependent on the pyrolysis temperature. Therefore, the effect of the pyrolysis temperature on the composition and microstructure of the as-obtained composites was investigated first [11; 13; 27]. Figure 1 shows the XRD patterns collected on the composites pyrolyzed at 1200, 1400, and 1600 \({}^{\circ}\)C for 60 min. At low pyrolysis temperature 1200 \({}^{\circ}\)C, apart from the sharp diffraction peak at about 15\({}^{\circ}\) from carbon, the term pattern of the products presents a completely amorphous structure only with the presence of amorphous silica centered at about 2\(\theta\) = 25\({}^{\circ}\). After the pyrolysis temperature increased to 1400 \({}^{\circ}\)C, the (111) diffraction peak of cubic SiC at about 35\({}^{\circ}\) starts to appear, and this diffraction peak becomes more evident when the pyrolysis temperature further increased to 1600 \({}^{\circ}\)C, and other characteristic peaks of SiC could also be observed, including at about 42\({}^{\circ}\), 60\({}^{\circ}\), and 72\({}^{\circ}\), which could be attributed to the (200), (220), and (311) planes of standard cubic SiC (JCPDS No. 29-1129), respectively [28; 29; 30]. The above results confirm that the crystallization of \(\beta\)-SiC would be promoted by the increasing of pyrolysis temperature, and the pyrolysis temperature plays an important role in the composition of the composites after the introduction of SiOC. During the high-temperature treatment, the following reactions would occur within the structural evolution of the SiOC preceramic based on previous results [11; 13; 27]:  
\[\mathrm{SiOXCy\toSiO_{2}\ (amorphous)+SiC+C_{free}} \tag{3}\]  
\[\mathrm{SiO_{2}\ (amorphous)+3C_{free}\toSiC+2 CO} \tag{4}\]  
Figure 1: The XRD patterns of as-prepared composites after different pyrolysis temperatures. From the results shown, it can be seen that Reaction (3) occurs below 1400 \({}^{\circ}\)C, if the temperature exceeds 1400 \({}^{\circ}\)C, Reaction (4) takes place. Raman spectroscopy was carried out to evaluate the structural evolution of the free carbon phase within the as-prepared composites under different pyrolysis temperatures, and the results are shown in Figure 2. From the results, two obvious peaks could be observed, approximately 1350 and 1600 cm\({}^{-1}\), referring to the typical D and G band corresponding to the disordered and graphitized carbon state, respectively [31]. The relative parameters for the above two bands are listed in Table S1, and the intensity ratio of the D band to the G band (\(I_{D}\)/\(I_{G}\)) could be employed to evaluate the relative content of free carbon within the sample, and the higher value implies the higher relative content of free carbon compared with that of the graphitized carbon [32; 33]. From the results listed in Table S1, the value of \(I_{D}\)/\(I_{G}\) increases with the increase of pyrolysis temperature, suggesting that the relative content of free carbon increases as the pyrolysis temperature is increased. The value of \(I_{D}\)/\(I_{G}\) could also be used to analyze the content of defects in the sample. The higher value indicates the larger content of defects in the sample, and thus it is reasonable to believe that the number of defects in the composites increases with pyrolysis temperature, which would be beneficial to improve the EMW absorption properties of the composites. In addition, the band around 2700 cm\({}^{-1}\) could be assigned to two-phonon (2D) band, which refers to the graphitic ordering, which appears at 1400 \({}^{\circ}\)C and becomes more evident at 1600 \({}^{\circ}\)C, demonstrating that the composites after above 1400 \({}^{\circ}\)C heat-treatment would own relatively high graphitization degree, which would result in enhanced EMW absorption performance [27].