## 1 Introduction  
Carbon-fiber-reinforced ceramic matrix composites (CMCs) are advanced materials, which not only possess those outstanding properties of conventional C/C materials but also perform with the advantages of high specific strength, high specific modulus, and superb mechanical properties at room and high temperature, as well as excellent oxidation ablative performance [1]. Hence, such excellent materials have attracted the attention of many scholars worldwide. Furthermore, they are applied to numerous aspects, in both military and civilian use, such as sliding components, engines, cutters, brakes, etc. Such materials are also believed to raise the efficiency of turbines and cut emissions at the same time [2; 3; 4]. In addition, considering their specific properties, they are mainly expected to be the important and irreplaceable candidates for high-temperature structural components, such as nose cones and wing leading edges of hypersonic aircraft [5; 6; 7; 8; 9]. Generally, CMCs are composed of carbon fibers and ceramic matrix. Among these, the carbon fibers serve as strong skeletons, and the ceramic matrix serves as a supporter to connect the whole structure. It is necessary to introduce ceramic phase into carbon fiber braid in practical production; therefore, the quality of CMCs will largely depend on the properties of the ceramic matrix and the mobility of ceramic precursors. Among CMCs, C/SiC composites or ultra-high-temperature ceramic (UHTC)-modified C/SiC composites are the most important materials in the engineering application [10; 11; 12; 13], and the composites are usually fabricated by the precursor infiltration and pyrolysis (PIP) process using polycarbosilane precursors as raw materials [14; 15; 16]. Originally, polycarbosilane is mostly solid and should be dissolved by organic solvents. However, the wettability between a polycarbosilane-organic system and carbon fibers is poor, which results in the low impregnation efficiency and a long preparation period. Even worse, the high content of organic solvents may lead to a relatively low volume ratio of polycarbosilane, which means a relatively high economic cost. The polycarbosilane we use today is liquid, and compared to the previous solid polycarbosilane-organic solvents solution, its liquidity has improved a lot. Nevertheless, this liquid phase has a high viscosity, leaving the impregnation effect unsatisfactory. Therefore, reducing the viscosity of polycarbosilane (PCS) effectively and improving the wettability between carbon fibers and ceramics at the same time is of great interest. Over many years, researchers have developed a variety of methods to solve the problem of low impregnation efficiency during the PIP process and typically introduce xylene or DVB solutions [17,18]. However, those solvents are poisonous and detrimental to environmental protection [19]. Moreover, the PCS/xylene system is prone to \"foaming\" at high temperature, resulting in a decline in infiltration efficiency [20]. Researchers also found that increasing the temperature of the polycarbosilane precursor could decrease its viscosity within a certain range. However, when the temperature was high (>80 \({}^{\circ}\)C or more), it can easily lead to the rapid curing and failure of the polycarbosilane precursors, and the viscosities of the precursors will increase sharply in the several hours after heating, which extensively hinders the impregnation process [21, 22, 23]. Hence, exploring a reasonable method to effectively reduce the viscosities of precursors and greatly improve the wettability between carbon fibers and PCS that can lower the cost in time and money and can optimize impregnation process is presented as imminent and imperative work. In this work, an innovative method of adding acetone to reduce the viscosity of liquid polycarbosilane (L-PCS) and improve the wettability between L-PCS and carbon fibers was carried out.