聚碳硅烷的分子量分布与可纺性研究

阐述了聚碳烷的合成及其分子量分布，讨论了高低分子含量对聚碳硅烷熔点及可纺性的影响，而且提出了分子量分布的控制标准与方法。     

Characteristics of polycarbosilanes produced under different synthetic conditions and their influence on SiC fibers: Part I

SiC fibers can be obtained by the spinning, curing, and heat treatment of polycarbosilane (PCS); however, the properties of the PCS precursor must be considered to set the correct spinning conditions. Although many studies have focused on the synthesis conditions, the characterization (in particular, the structural characteristics) of PCS fibers, and the polymer itself has limitations. In this study, PCS was prepared in two steps, and the growth of the polymer with respect to the reaction conditions was analyzed. We found that PCS is formed and grown by the rearrangement and subsequent condensation reactions of polydimethylsilane (PDMS). Further, fiber formation was affected by the reaction temperature, time, and pressure. Three types of PCS were obtained under different synthetic conditions, and they were all characterized. Regardless of the structural similarity of the PCS fibers (based on the spectroscopic analysis), the polymers showed different thermal and rheological properties. Our findings will be important in improving the production of PCS fibers (and subsequent SiC fibers) with finely controlled properties.     

The kinetics of oxidation curing of polycarbosilane fibers

For the oxidation curing of polycarbosilane (PCS) fibers in air, a first-order reaction based on the weight gainw (in percentage) was approached with the kinetic equationdwldt=k(w _m-w). The maximum weight gain w_m was observed to be 16% under normal oxidation conditions and the activation energyE to be 79.27 kJ/mol. The numeric integration based on the kinetics provides a precise prediction of the curing degree of PCS fibers under various heating programs and conditions.     

Mechanical and tribological properties of silicon carbide coating on Inconel alloy from liquid pre-ceramic precursor

Liquid polycarbosilane (LPCS) derived hard coatings of silicon carbide (SiC) were deposited on Inconel alloy at three different moderately high temperatures by chemical vapour deposition. The deposited films were characterized by X-ray diffractometry and Field emission scanning electron microscopy. Liquid PCS yielded a mixture of α-SiC and β-SiC during decomposition having uniform round-shaped particles of dimension around 200–300 nm without extensive cracking and few discrete shaped particles were also found to form at higher temperature (i.e. 1100 °C and 1200 °C) deposited films. The coated samples showed substantial increment in hardness and fracture toughness as compared to the uncoated sample. The fracture toughness (K_IC) values of the deposited films were in the range of 6.7–10.7 MPa(m)^1/2. The tribological properties and hardness of the films were also found to vary with deposition temperature. The scratch tracks of the films revealed that brittle failures occurred in all SiC coated substrates.     

Preparation and properties of nano-SiC strengthening Al2O3 composite ceramics

The processing and mechanical behaviors of Al₂O₃-xwt.%SiC (x = 1, 2, 5, ASx) nano-composites prepared by the in situ synthesis of SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The microstructure and mechanical properties of the sintered composites were analyzed. The results showed that a fully dense structure was obtained when a few nano-SiC were doped and that the fracture toughness and strength were highly improved compared with those of monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in AS2 composite. The maximum flexural strength was 516 MPa obtained in AS1 composite.     

Porous SiC using polycarbosilane/camphene solutions: Roles of freeze casting parameters

In order to control the pore characteristics and macroscopical performance of porous ceramics, roles of the freeze casting parameters are the key points. Herein, aligned dendritic porous SiC was fabricated by freeze casting of PCS-camphene solutions with different solid loading, freeze front velocity, temperature gradient, and freezing temperature. Influence of these parameters on the microstructure and compressive strength of porous SiC was investigated. With increasing the PCS content, freeze temperature, freeze front velocity or temperature gradient, degree of undercooling of the camphene was increased, resulting in the formation of smaller pore size, decreased porosity and increased compressive strength. Compared to variables of freeze temperature and temperature gradient, increased freeze front velocity was more efficiency in improving the compressive strength of porous SiC, owing to the formation of smaller pore size and longer secondary dendritic crystals. Promising micron-sized porous SiC with high porosity (79.93 vol%) and satisfactory strength (15.84 MPa) was achieved for 10% PCS-camphene solution under optimized freezing conditions.     

快速烧结法制备连续碳化硅纤维

通过熔融纺丝，不熔化处理制得连续聚碳硅烷（ＰＣＳ）不熔化纤维，采用快速烧结方法制备出性能较好的连续ＳｉＣ纤维。探讨了气封条件的选择，以及烧结速度对ＳｉＣ纤维的组成，结构及性能的影响。结果表明，快速烧结条件下，可以实现向纤维上施加张力以及纤维的无机化转变，烧结速度加快会降低纤维的Ｃ／Ｓｉ（原子比），同时有利于提高纤维的抗拉强度和热稳定性。     

快速烧成连续碳化硅纤维的表面分析

以聚碳硅烷（PCS）为先驱体；经熔融纺丝、不熔化处理和快速烧成工艺制备出性能较好的连续碳化硅（SiC)纤维。采用XPS、SEM、TG等方法对所得SiC纤维的表面结构组成和热稳定性进行了分析，并探讨了快速烧成方式下引起纤维抗拉强度降低的主要原因。结果表明：快速烧成的SiC纤维表层有富含游离碳的热解沉积物，对纤维的热稳定性产生不利影响，采用超声清洗可以将其快速除去；表层缺陷是引起SiC纤维抗拉强度下降的主要原因。     

电子束辐照聚碳硅烷热解合成 SiC 陶瓷材料──I . 空气中辐照产物的热解特性研究

利用加速器产生的电子束（ＥＢ）在空气中辐照聚碳硅烷（ＰＣＳ），以使之形成交联结构，然后经高温热解转化成ＳｉＣ陶瓷。结果表明：在辐照产物中形成了ＳｉＣＳｉ以及ＳｉＯＳｉ等交联结构，热解温度以及热解陶瓷产率都随辐射吸收剂量增加而明显提高。通过ＴＧＩＲ联用分析技术，在热解产物中检测到了低分子量ＰＣＳ以及ＣＨ４等小分子化合物，在吸收剂量高于２．３ＭＧｙ时，ＰＣＳ主要通过析出ＣＨ４而热解成ＳｉＣ。     

预氧化聚碳硅烷纤维的恒温烧结

本文主要针对预氧化聚碳硅烷纤维的恒温烧结过程进行了基础方面的研究。采用XPS、TG -DTG -DTA、IR对预氧化聚碳硅烷原纤维进行标定 ;采用日本精密天平连续跟踪纤维质量变化情况 ,探讨了烧结过程中温度对纤维失重及收缩的影响 ;采用IR、SEM和数码相机分析了纤维恒温烧结过程中微观结构和宏观形貌的变化情况 ,并对预烧结纤维进行了二次烧结