【作者】 王海哲；

【导师】 李效东；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2012， 博士

【摘要】 碳纳米管（CNT）增强陶瓷材料是CNT应用的主要领域和发展方向之一。但是在CNT增强陶瓷材料制备过程中，存在以下几个问题：CNT的团聚，CNT在陶瓷基体中的不均匀分布，以及CNT在传统陶瓷烧成过程中高温、高压环境下的破坏等。因此，必须对CNT进行化学改性，使其在陶瓷基体中分散良好、均匀分布以及与陶瓷基体的良好相容性，同时尽量避免传统陶瓷烧成工艺的高温、高压环境。本文结合SiC陶瓷先驱体聚碳硅烷（PCS）的化学特性，对本实验室自制的CNT进行了有针对性的化学改性，得到了乙烯基取代的碳纳米管（vCNT），通过溶液共混法将vCNT引入PCS中，然后经过熔融纺丝、预氧化、裂解等工艺制备了CNT/SiC陶瓷纤维。同时通过对CNT/PCS先驱体在惰性气氛下，500~600°C进行预处理，结合球磨-模压-烧结工艺，在不添加其他粘接剂的情况下，在惰性气氛下烧结，制备了具有较高强度的CNT/SiC多孔陶瓷。在此基础上，将vCNT引入SiC陶瓷另一种重要的先驱体A-PMS中，利用PIP工艺制备了CNT/Cf/SiC复合材料。对CNT进行酸化氧化是对其进行化学改性的基础。本论文中，通过在室温下用浓硝酸对CNT进行酸化氧化，CNT内部的Fe颗粒、表面的无规碳颗粒以及杂质被清除掉，长度从几十微米减小为8~12μm，表面引入了-COH以及-COOH基团。在此基础上，利用丙烯酰氯与经过酸化氧化的CNT在66°C进行回流反应，CNT表面有包覆物出现，分析测试表明，丙烯酰氯在经过酸化氧化的CNT表面反应形成了聚丙烯酸，从而在CNT表面引入了乙烯基团。将0.1~1wt%的vCNT引入PCS的二甲苯溶液中经回流和超声后，vCNT中的乙烯基与PCS中的Si-H发生反应，使CNT与PCS通过共价键连接起来。这样CNT可以在PCS的溶液中长期稳定、均匀地分散和分布，不再发生团聚和沉降。将上述溶液蒸馏后制备的CNT/PCS先驱体即可很好地进行熔融纺丝。在熔融纺丝工艺研究时，发现由此获得的CNT/PCS可以在较宽温度范围内进行熔融纺丝。随着剪切速率的增加，纤维的无断头长度明显增加。单孔纺丝后得到均匀的深灰色的原纤维，纤维表面光滑，直径均匀。而作为对比，原CNT虽亦可混入PCS溶液，但因团聚和沉降，纺丝过程中纤维出现先黑后灰，最后变白的现象，得到的原纤维表面粗糙，直径波动很大。分析表明，由vCNT制得的CNT/PCS原纤维中，CNT的分散良好，分布均匀，而且CNT长度方向沿PCS纤维的轴向取向。对CNT/PCS原纤维进行了预氧化和热失重研究，发现vCNT的引入并不会对PCS纤维的预氧化带来本质上的变化；CNT/PCS的热失重行为与PCS没有根本区别，但陶瓷收率略有提高（3wt%左右），其原因为vCNT的引入增加了PCS的交联程度和CNT在纤维烧成过程中稳定而不发生失重。将预氧化后的CNT/PCS纤维在高温下，惰性气氛中烧成即可得到黑色带有金属光泽的较为柔软的CNT/SiC复合陶瓷纤维。对得到的CNT/SiC进行分析表征发现，由于先驱体法烧成温度相对较低，且无高压存在，CNT可以较为完整的保留下来；随着CNT引入量的增加，复合陶瓷纤维的晶粒尺寸逐渐变小，有序碳结构略有增加，得到的复合纤维的强度、模量均增加。当引入的vCNT含量为0.5wt%时，CNT/SiC纤维的抗拉强度、弹性模量分别为1.8GPa和271GPa，较同样条件下不含CNT的SiC纤维分别提高了约35%和91%。当引入的vCNT的含量为1.0wt%时，得到的CNT/SiC纤维的电阻率约为同样条件下制备的SiC纤维的电阻率的10%。对CNT/SiC多孔陶瓷的烧结过程、力学性能以及孔结构进行研究发现，随着vCNT含量的增加，烧结过程中预处理先驱体坯体的质量损失率略有减少而其线性收缩率呈逐渐增加趋势，抗弯强度逐渐增加。当引入的vCNT的含量为0.5wt%时，1300°C烧结得到的多孔CNT/SiC陶瓷的抗弯强度达到61MPa，比不含vCNT的多孔SiC提高了76.5%，而得到的多孔SiC的晶粒尺寸减小了约60%。CNT/SiC多孔陶瓷的孔径分布呈单峰分布，并无异常大孔存在，随着vCNT加入的量的增加，得到的CNT/SiC多孔陶瓷的中值孔径从12.7μm减小为10.1μm。在使用vCNT增强A-PMS制备CMC的PIP工艺研究时，发现vCNT的结构可以在得到的Cf/SiC复合材料中较为完整的保留，得到的复合材料的抗弯强度、断裂韧性分别为分别为423MPa和23.35MPam1/2，比同样条件下制备的不含vCNT的Cf/SiC复合材料的抗弯强度及断裂韧性分别提高了29.7%和27.9%。综合分析CNT在SiC陶瓷纤维、多孔陶瓷以及Cf/SiC复合材料的断裂过程，发现CNT在SiC基体中的拔出、桥接以及引起的裂纹偏转是得到的复合材料的抗弯强度以及断裂韧性提高的主要原因。同时，经过改性的CNT与SiC基体良好的结合，也提高了载荷在SiC基体与CNT之间的传递作用，在复合材料破坏时，部分CNT也被破坏，这同样改善了SiC陶瓷材料的力学性能。更多还原

【Abstract】 To prepare carbon nanotubes (CNT) reinforced ceramic composites is one of the most important application of CNT. However, several key issues must be considered. Firstly, the CNT should be dispersed uniformly in the matrices without any agglomeration. Secondly, a compatible interface between the CNT and the matrices is needed to achieve an enhanced stress transfer capability from the matrices to the CNT. Thirdly, the harsh conditions required in the traditional preparation of ceramic composites need to be avoided as such high temperature and high pressure may destroy the CNT. Up to date, chemical modification of CNT is one of most effective way to obtain ceramic/CNT composites with uniformly dispersed CNT and compatible interface.In this study, vinyl modified carbon nanotube (vCNT) was made by chemically modifying CNT and used in the preparation of ceramic/CNT composites. The premade CNT was treated with concentrated HNO3at ambient temperature for8h, and then reacted with acryloyl chloride at66℃for12h. The length of the CNT decreased from20-100μm to about8-12μm after the reactions. It was found that the polymerization of acryloyl chloride occurred on the surface of CNT and the product with ethylene groups encapsulated the surface. The chemical modification of CNT effectively promoted its dispersivity in the toluene solution of polycarbosilane (PCS). The vCNT was homogeneously dispersed in the obtained hybrid precursor without any entanglement and agglomeration.The vCNT-reinforced SiC ceramic fibers were successfully prepared by mixing vCNT with PCS, followed by melt spinning, curing, and pyrolysis. The vCNT/PCS precursors were melted and spun into continuous green fibers in N2stream at about305-325℃. The vCNT were aligned to the axis-orientation of the fibers after melt spinning. Oxidative cross-linking of the CNT/PCS green fibers was carried out in hot air to make these fibers infusible prior to pyrolysis. Then the cured fibers were heated in N2at1300℃and the CNT/SiC ceramic fibers were obtained. It was observed that the aligned vCNT remained in the ceramic fibers after curing and pyrolysis. Significant improvement in Young’s modulus and tensile strength was achieved by incorporating the vCNT into the SiC ceramic fibers. The addition of0.5wt%CNT led to an increase of93.6%in the Young’s modulus and an increase of38.5%in the tensile strength.Besides, vCNT reinforced porous SiC ceramics were successfully obtained by milling, molding and pyrolyzing the pretreated CNT/PCS precursor through the polymer derived ceramic (PDC) process. The vCNT was dispersed well in the precursors, and the flexural strength of the obtained porous CNT/SiC ceramics was improved. The addition of0.5wt%vCNT resulted in an increase of76.5%in the flexural strength (60.9MPa). The increase in mechanical properties can be attributed to the homogenous dispersion of vCNT within the matrix, the strong interface connections and the enhanced stress transfer capability from the matrix to the vCNT.Moreover, the vCNT reinforced Cf/SiC composites were fabricated by PIP method using antimony substituted polymethlysilane (A-PMS) as the precursor. It was found that CNT was well-dispersed in the precursor after ultrasonic treatment, and remain in the obtained ceramic composites after the PIP process. The vCNT embedded in the composites increased the pullout resistance, bridged the crack gaps, and caused crack deflection in the final composites. The vCNT broke in the sword-in-sheath fracture mode, in which the outer shells broke and the inner core was then pulled away, leaving fragments of the outer shells in the matrix. These fragments enabled the vCNT in the composites to have a much higher load carrying capacity. As a result, the mechanical properties of the obtained composites were improved. The addition of1.5wt%of CNT resulted in29.7%and27.9%increases in the flexural strength and the fracture toughness, respectively. Large-scale CNT/Cf/SiC/composites can be conveniently prepared by using this method.更多还原