镧系稀土氯化物催化制备高强韧炭/炭复合材料基础研究

The further development and application of carbon/carbon (C/C) composites are limited due to the high cost and low toughness. Rare earth elements and their compounds have been used widely as catalysts in the decomposition and aromatization of hydrocarbons, and in the synthesis of carbon nanotubes, because rare earth elements have special chemical characteristics. However, no work is available on the application of rare earth elements or compounds in catalytic production of C/C composites. Our previous research shows that, as the content of lanthanum chloride in carbon fiber felts increases, the densifying time of C/C composites decreases, whereas the strength and the toughness of the composites increase obviously. In the present work, lanthanide chlorides are introduced into carbon fiber felts for catalytic production of C/C composites from xylene pyrolysis. The influences of lanthanide chloride's type and content, combined with deposition temperature, on the deposition kinetics and microstructure of pyrocarbon, as well as the mechanical, tribological and ablative properties of C/C composites are investigated. Subsequently, the composites with high strength and toughness, excellent tribological and ablative properties could be produced based on the optimization of densification parameters. Simultaneously, the formation and deposition mechanisms of pyrocarbon are discussed by analyzing the intermediate and final products of xylene pyrolysis. Combined analysis of the interface between the pyrocarbon and carbon fiber and the microstructure of pyrocarbon, the intrinsic influence mechanisms of rare earth compounds on the performances of C/C composites could be revealed, which is a basis for further performance improvement of the composites.

成本高、韧性差等问题限制了炭/炭复合材料的进一步发展与应用。稀土元素及其化合物具有独特的化学性能，作为催化剂已广泛用于烃类的裂解、芳构化和碳纳米管的制备，但国际上从未将其用于催化热解烃类制备炭/炭复合材料。我们的研究表明，预制体中添加氯化镧可大大缩短炭/炭复合材料的制备周期，并且其强度和韧性显著提高。基于此新发现，本项目提出采用镧系稀土氯化物催化热解二甲苯制备炭/炭复合材料，旨在明确稀土氯化物类型及其在预制体中的含量、沉积温度等对热解炭沉积动力学、基体显微结构、材料的力学、摩擦磨损及烧蚀性能的影响规律，获得强韧性高、摩擦磨损和烧蚀性能优异的低成本、高性能炭/炭复合材料及制备工艺。通过系统分析二甲苯热解的中间产物组分及终产物微观结构阐明热解炭的形成与沉积机理，并结合热解炭与纤维的界面结构研究揭示稀土化合物对炭/炭复合材料性能改善的内在作用机制，为深入挖掘炭/炭复合材料的性能潜力奠定理论基础。

将镧系稀土氯化物作为催化剂分散于炭纤维预制体，采用薄膜沸腾化学气相渗透以双热源加热的方法热解二甲苯制备C/C复合材料，研究纤维表面化学组成与结构、热解炭（PyC）显微结构形成与沉积机理、材料的致密化特性、力学、摩擦及烧蚀性能等，主要结果如下：.预制体中催化剂含量随其溶液浓度升高而增大，但分布均匀性降低，高含量下催化剂呈颗粒状及薄膜状，并出现纳米丝结构，纤维表面条纹更加明显；升高超声功率及延长时间可提高催化剂均匀性。随着催化剂含量的升高，纤维表面氧元素先增多后减少，10wt%下达到较大值。氧含量升高说明稀土促进了纤维表面活化。.催化剂含量由0wt%增大至15wt%时，PyC由粗糙层（RL）向各向同性（ISO）结构转变，3wt%下出现纳米丝状碳（NFC）；含量增大至6wt%后，NFC增多，催化剂表面积碳及气相形核使PyC呈RL和ISO混合结构，高催化剂含量及沉积温度下ISO层较厚。较低催化剂含量或温度下，芳香浓缩与小分子烃加成反应生成的六元环物种经表面形核与生长形成RL结构PyC，局部出现光滑层结构与含五元环芳香物种有关。.随着催化剂含量或沉积温度的升高，沉积前沿厚度拓宽，PyC初始沉积速率增大，材料平均密度先增大后减小，3-6wt%及1000-1100℃下材料平均密度较高（1.71-1.72g/cm3）。密度沿材料轴向减小；0-3wt%或900-1000℃时密度沿径向降低，其它条件下密度呈现升高的趋势。根据PyC厚度及密度分布特点建立材料致密化过程模型，催化剂含量或温度较低时沉积前沿呈凸面形，高含量或温度下为凹面形，沉积形貌的变化取决于材料内部的温度分布及前驱体传输效率。.随着催化剂含量或温度的升高，材料的弯曲强度、剪切强度及韧性先增大后减小，摩擦系数、磨损率和烧蚀率的变化与之相反。催化剂改善了材料的性能，6wt%及1000-1100℃下力学性能与摩擦稳定性高，摩擦系数、磨损率和烧蚀率较低。NFC对基体的增强及裂纹在稀土颗粒处的偏转是材料强度和韧性提高的重要原因；摩擦过程中，光滑摩擦膜的形成及NFC的增强作用使6wt%下材料摩擦系数和磨损较低；耐烧蚀性能的改善同样归功于NFC和稀土化合物。提高热处理温度，材料的强度、摩擦系数、磨损率及烧蚀率降低，韧性及摩擦稳定性升高。使用CeCl3为催化剂与LaCl3对材料的作用规律及机制相似。