SiC ceramics, known for their lightweight, high strength, high temperature resistance and oxidation resistance, find extensive applications in aerospace, energy and transportation industries. The polymer-derived ceramics (PDC) method is an important method for the preparation of SiC ceramics, employing special polymers as raw materials and converting them into inorganic ceramics via high-temperature pyrolysis. This method offers advantages such as controllable elemental composition, ease of molding, and lower processing temperatures. As one of the most important raw materials in the PDC method, hyperbranched liquid polycarbosilanes, synthesized via the Grignard coupling reaction and the subsequent reduction reaction, are prized for their low viscosity, high ceramic yield, and ease of processing and molding. Therefore, it has been widely used in the fabrication of ceramic matrix composites, coatings and membrane materials. A large number of articles have been published on the synthesis and applications of polycarbosilanes. The molecular structure of SiC polymer precursors not only influences their curing and pyrolysis processes, including weight loss and gas release behavior, but also plays an important role in determining the elemental composition (e.g., C/Si ratio), crystallization behavior and oxidation resistance. There are many studies conducted to investigate the structure of precursors affecting their curing and pyrolysis properties. However, it is very necessary to investigate the relationship between molecular structures of different types of liquid polycarbosilanes with varying C/Si ratios and their curing, pyrolysis and crystallization behaviors. In this work, the curing and pyrolysis behaviors as well as the crystallization behavior of two major types of liquid hyperbranched polycarbosilanes vinylhydridopolycarbosilane (VHPCS) and allylmethylhydridopolycarbosilane (AMHPCS)) with different carbon-to-silicon ratios and molecular structures were comprehensively characterized by a variety of testing methods. The relationship between the molecular structure and the ceramization process, as well as the properties of the ceramic products, was established using five types of liquid polycarbosilanes with varying C/Si ratios and molecular structures.

0.3% of DCP was added to the VHPCS precursor, while 0.5% of DCP was added to the AMHPCS precursor, the mixtures were then stirred at room temperature until the DCP fully dissolved. The compounds were then poured into tetrafluoroethylene coated stainless steel molds and defoamed in vacuum before being transferred to the blast oven for curing. The curing procedure of VHPCS/AMHPCS precursor was as follows: the temperature was increased from room temperature to 120 ℃/160 ℃ at a rate of 3 ℃/min, held for 1 h, then increased from 120/160 ℃ to 170/200 ℃ at a rate of 2 ℃/min, held for 2 h. Finally, the samples were cooled to room temperature at a rate of 3 ℃/min to obtain transparent cured cylindrical. The cured samples were pyrolyzed in an Argon atmosphere with a ramping rate of 3 ℃/min to 1 100/1 500 ℃ and held at the target temperature for 2 h.

The molecular structure of the VHPCS and AMHPCS precursors was characterized by ¹H-NMR and FT-IR spectroscopy. The curing temperatures of the two types of precursors were analyzed by DSC tests. The cross-linking reactions temperatures was increased with the higher methyl, vinyl and allyl content. When DCP was added to the precursor, the peak of curing temperature reduced by about 80 ℃. By comparing the DSC curves of the precursor A1-A3, it can be discovered that the contents of Si—H, Si—Me, and Si—allyl groups collectively determine the curing temperature of the five precursors. Due to the different molecular structure, significant differences were observed in the TG curves of VHPCS and AMHPCS, as represented by precursor V1 and precursor A3. The reasons for these differences were explored through TG-DTG and TG-MS curves. During the pyrolysis process, the two precursors released different amounts of gas at different temperature ranges. The crystallization behavior and antioxidant properties of the PDCs were also investigated. It was observed that the VHPCS precursors are easier to form SiC crystal compared to the AMHPCS precursors. This phenomenon was evidenced by the larger grain sizes observed in VHPCS-derived ceramics under identical pyrolysis temperatures. Additionally, an increase in carbon content in the pyrolysis products of the precursors was found to inhibit the growth of β-SiC. TG analysis tests conducted in air atmosphere revealed that designing the molecular structure of the single-source SiC precursor to adjust the carbon-to-silicon ratio of the pyrolysis products can improve the antioxidation properties of polymer-derived ceramics.

The relationship between the molecular structure and the ceramization process as well as the properties of ceramic products was established by analyzing the molecular structure characteristics, curing and pyrolysis, crystallization as well as high-temperature oxidation behavior of the pyrolyzed VHPCS and AMHPCS precursors. The results show that the type and amount of the side-chain functional groups could affect the curing temperature and ceramic yield of the precursors, and the addition of appropriate amount of DCP could reduce the curing temperature by about 80 ℃. The ceramic precursor has low viscosity and high ceramic yield, but the reduction of active groups can lead to higher curing temperature and lower ceramic yield. For precursor A3 with 40% Si—Me groups, it reached its curing peak temperature at 188 ℃ and exhibited a ceramic yield of 75%, while precursor A1 with 10% Si—Me groups reached its curing peak temperature at 160 ℃ and a ceramic yield of 84.4% when pyrolyzed at 1 000 ℃. Side groups affect the pyrolysis gas release behavior and different precursors lead to different C/Si ratios in the ceramic products. For precursor V1, the main cause of weight loss is the release of H₂, whereas for precursor A3, the main factor leading to weight loss is the release of small molecules of methane or olefins. Carbon-rich compositions inhibit the crystalline growth of SiC, and the single-source precursor with a C/Si ratio of 1.7 in the pyrolytic ceramic product has good high-temperature oxidation resistance.