Introduction SiC ceramic matrix composites have excellent mechanical properties and high-temperature resistance, and hence attracts increasing research attentions in aerospace and other fields. Precursor infiltration and pyrolysis (PIP) is one of the most common methods for the preparation of SiC ceramic matrix composites. The synthesis of organic precursors is the basis of the PIP process. Liquid polycarbosilane (LPCS) is an ideal precursor for the preparation of SiC ceramic matrix composites by PIP process. LPCS can be prepared using the Grignard coupling reaction, while this method suffers from the problem of long synthesis route and high cost. The Wurtz reaction, as another method for synthesizing LPCS, has a shorter route and less cost. However, it is difficult to directly synthesize LPCS containing active groups because of the severe reaction conditions. Ultrasound can significantly promote the effective contact between sodium and chlorosilanes and reduce the reaction conditions in the Wurtz reaction, and some ether additives can improve the activity of the reaction groups. Therefore, the introduction of ultrasound and ether additives is expected to realize the efficient synthesis of LPCS containing active groups. In this work, an active LPCS was prepared directly from chlorosilane monomer containing active groups by using ultrasonic and ether additives in Wurtz reaction. The basic structure of the product, ceramic yield and pyrolytic properties of the transformed ceramics were characterized. Methods CH2Br2, MeHSiCl2 and ViMeSiCl2 were used as raw materials, and mixed with the molar ratio of 2:1:1. Under the conditions of no ultrasonic or ether auxiliary agent, ether auxiliary agent only and ultrasonic together with ether auxiliary agent, the reaction was carried out at 40 ℃ for 4 h, and the excess Si—Cl was reacted by Me3SiCl. The corresponding products were obtained by filtration and distillation. Diethylene glycol dimethyl ether (DGDME) or tetrahydrofuran (THF) was selected as ether auxiliary agent, and THF is easier to remove due to its lower boiling point, thus providing an advantage. On this basis, the molar ratio was adjusted to 3:1:1 and 4:1:1 to synthesize the products. According to the ratio of actual yield to theoretical yield, the yield of each product was calculated. The basic structures and functional groups of the synthesized products under different conditions of ultrasonic and ether were characterized by Fourier transfer infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectroscopy (1H-NMR). For the synthesized products with THF as ether auxiliary agent and the molar ratios of raw materials as 2:1:1, 3:1:1 and 4:1:1, the functional groups and structures were characterized and compared with FT-IR, and the existence of Si—Si bonds was determined and compared with UV-Vis spectroscopy. The mass change curves of the pyrolysis products at 900 ℃ were analyzed by thermogravimetric analyzer (TG). The crystal structures of the pyrolysis products prepared at 1 000, 1 200 ℃ and 1 400 ℃ were analyzed by X-ray diffractometer (XRD). Results and discussion For the precursors synthesized under different conditions of ultrasonic and ether, it was found that liquid products could not be obtained by Wurtz reaction at 40 ℃ without ultrasonic and ether. On the contrary, when ultrasonic is introduced, the reaction can take place to obtain the product, and when ultrasound and ether auxiliary agent are used simultaneously, the yield can be further improved. FT-IR and 1H-NMR spectra show that Si—H, CH= CH2 exist in all products, and Si—C—Si chain is formed, which proves that LPCS containing active groups can be successfully synthesized by this method. According to IR spectra, Si—CH2—Si in the product is the most when the molar ratio of raw materials is 3:1:1, which indicates that proper increase of CH2Br2 content may promote polymerization, which may be due to CH2Br2 affecting the concentration of reactive free radicals in the reaction. The TG curves show that the mass loss of each precursor occurs at room temperature to 250 ℃ and 500–600 ℃, respectively, due to the volatilization of small molecular oligomers and the thermal crosslinking process dominated by active groups. When the molar ratio of raw materials is 3:1:1, the highest ceramic yield is obtained up to 48.8%. UV-Vis analysis shows that Si-Si bond exists in LPCS. XRD results shows that there is no obvious diffraction peak in the pyrolysis product curve at 1 000 ℃. With the temperature increasing to 1 200 ℃, diffraction peaks of the pyrolysis products appeared at 35.7°, 59. 9° and 71.7°, corresponding to the crystal structure of β-SiC. Further, at 1 400 ℃, the characteristic diffraction peaks of β-SiC were enhanced. All above indicate that LPCS can be partially transformed into β-SiC crystal after 1 200 ℃. Conclusions Through the introduction of ultrasonic and ether auxiliary agent, LPCS containing reactive groups can be rapidly synthesized using the Wurtz reaction at a temperature of 40 ℃ with a yield of more than 90%. The synthesized LPCS takes Si—C and Si—Si bonds as the main chain structure, and is rich in Si—H, CH= CH2 active groups. When the molar ratio of CH2Br2, MeHSiCl2, ViMeSiCl2 is 3:1:1, the ceramic yield of the product reaches the highest value up to 48.8%. This study demonstrates that Wurtz reaction assisted by ultrasonic and ether auxiliary agent is simple in process, high efficient and low cost. It is an effective method to prepare liquid SiC ceramic precursors containing active groups. © 2024 Chinese Ceramic Society. All rights reserved.
