SiC纳米线/CVD石墨烯/热解炭复合材料制备、界面结构与力学性能研究

The low strength and great brittleness of pyrocarbon (PyC) greatly limit the development of high-tech defence equipments of our country. Although adding nanofibers into PyC can improve the mechanical performance of PyC, the reinforcing efficiency of nanofibers always declines after the graphitization of PyC due to the poor interfacial interaction between nanofiber and PyC. In order to resolve this problem, our project proposes the idea about applying hybridized multidimensional nano-reinforcements fabricated by growing nano-sized graphene on SiC nanowire (SiCnanowire) by chemical vapor deposition (CVD) to stiffen and toughen PyC. Toward this end, this project will systematically investigate the effects of CVD graphene with different morphologies and microstructures on the preparation and graphitization process of SiCnanowire reinforced PyC (SiCnanowire/PyC) composites. Based on the results, the general law of action between graphene and depositing behaviors of PyC, mircostructures of SiCnanowire/PyC composite, micro- and macro-mechanical properties of SiCnanowire/PyC composites will be explored and the related mechanism will be established. After that, the findings will be used as the guidance for the optimization of the morphology and microstructure parameters of CVD graphene, aiming to the great improvements of SiCnanowire/PyC composites in mechanical performance. The present work would pave a meaningful way to the development of PyC with high strength and excellent toughness, and also enrich the reinforcement theory of graphene-based composites.

热解炭（PyC）强度低、脆性大的力学特性限制了某些国防高科技装备的发展，开发纳米纤维改性PyC是解决上述问题的有效途径。然而，纳米纤维对PyC的强韧作用随石墨化处理而显著衰减。针对这一问题，本项目借助化学气相沉积（CVD）在SiC纳米线（SiCnanowire）表面生长了纳米尺寸石墨烯，提出通过“多维杂化”途径改善纳米线对PyC的强韧效果。在对纳米石墨烯生长形貌控制和结构调整的基础上，系统研究石墨烯对SiCnanowire/PyC复合材料制备过程和石墨化过程的影响，探究石墨烯对复合材料组织结构、内界面和微观、宏观力学性能的诱导作用，建立复合材料组织和力学特性之间的映射关系，揭示纳米石墨烯界面的强韧作用。最终，通过对石墨烯形貌和结构的优化，实现其对SiCnanowire/PyC复合材料力学性能的大幅改善，为高强韧纳米改性PyC的设计开发奠定一定基础。

高强韧热解碳在航天航空高技术装备研发中具有重要价值。纳米线强韧化是提高热解碳力学性能的一个有效途径。针对纳米线/热解碳界面结合不足问题，本项目主要研究在纳米线表面可控生长石墨烯提高纳米线/热解碳复合材料的力学性能。结果发现：随着生长时间的延长，SiC纳米线表面生长的石墨烯由二维形貌转变为三维自支撑形貌，且石墨烯纳米片的层厚由1-3层提高至30层以上。自支撑石墨烯以等离子体和氧化性气氛刻蚀石墨烯水平生长层产生的纳米凹坑为起始点外延生长。高温下等离子体和水氧化消耗甲烷热解产生的低晶碳产物、提高气氛中芳香族大分子中间基团，促进石墨烯的形核与生长；同时，等离子体和水刻蚀早期生长的水平石墨烯层，导致面内结构中断并产生暴露的原子边界，为自支撑石墨烯纳米片的外延形核与生长提高活性位点。提高前驱体分压或降低水分压均会导致PECVD系统中氧化性气氛的比例失调，影响其氧化刻蚀能力，进而影响自支撑三维石墨烯纳米片的生长。在SiC表面生长石墨烯后，热解碳的初期致密化速率显著提高，但在沉积16h后这种影响消失。热处理前后石墨烯与SiC纳米线、石墨烯与热解碳界面结合状态未见显著变化，说明其间形成了强界面结构。基于石墨烯纳米片的π电子分布研究结果表明，开放原子边界结构的存在导致石墨烯内部π电子分布发生变化，电子出现一定的局域化，并在石墨烯面内产生一定的负电集中，诱导热解碳沉积单元在石墨烯面内的组装与强结合；同时，边界可以促进热解碳的外延生长，也有利于强界面的形成。石墨烯在纳米线表面的生长后，有效细化了热解碳基体，减轻了石墨化处理导致复合材料力学退化的程度，使得复合材料内应力由拉应力转变为压应力，并改善了应力分布均匀性。适量的石墨烯可以显著提高SiC纳米线-热解碳复合材料的力学性能，过量石墨烯会导致纳米多孔界面形成，降低复合材料力学强度，但可显著改善复合材料的断裂假塑性。