【作者】 李斌；

【导师】 凤仪；

【作者基本信息】 合肥工业大学 ， 材料学， 2013， 博士

【摘要】 每年有成千上万吨高放射性的核废料产生，慢慢积累起来必将对人类构成巨大威胁。分离嬗变技术的提出无疑给这一难题带来了曙光。这一技术是将毒性很大的MA和LLFP等核素从高放废物中分离出来然后对其进行辐照使其变成稳定或短寿命的核素。此外，碳材料在核能和航天航空领域应用广泛，所以它们在苛刻辐射环境中的结构稳定性显得尤为重要。本文比较系统地研究了碳纤维、多壁碳纳米管和石墨在伽马射线、电子和重离子辐照下的结构和性能的变化。研究结果发现，碳纤维在伽马射线辐照后石墨化程度逐渐增加，表面润湿性增强。用强迫共振法和单丝拉伸法两种方法测量碳纤维在伽马辐照后的杨氏模量发现，碳纤维的杨氏模量均随辐照剂量的增加而增加，在2MGy时增幅达到最大。在场发射透射电镜中，汇聚的电子束轰击到碳纤维表层时会产生明显的缺口和纳米碳洋葱。小剂量的铜离子辐照能提高碳纤维的石墨化程度，然而大剂量的铜离子辐照则会对碳纤维造成损伤。碳纤维表面的润湿性随着离子辐照剂量的增加先增强后减弱，在1015ions/cm²时达到最佳。随着伽马辐照剂量的升高，多壁碳纳米管的有序化程度先降低后升高，碳纳米管表面含氧官能团增多，表面润湿性增强，碳纳米管的形貌则由管壁笔直清晰变得混乱无序再回复到相对有序。在电子辐照时，电子能量和电流密度对碳纳米管都有影响。电子辐照对碳纳米管的损伤和破坏随着电子能量和电流密度的增加而加剧。电子辐照也能在碳纳米管上形成碳洋葱，还能使两根纳米管焊接起来。碳纳米管在铜离子辐照时，随着辐照剂量的增加，有序化程度逐渐降低，表面润湿性和碳纤维一样，也呈现出先增强后减弱的趋势。在1015ions/cm²剂量时，碳纳米管表现出一种特殊的形貌，即一部分中空另一部分闭合，然而随着辐照剂量的增加，碳纳米管完全变成无定型纳米线。离子辐照也能使碳纳米管焊接，从而使碳纳米管形成无定型纳米线的网状结构。与多壁碳纳米管变化趋势相反，石墨在伽马辐照后的石墨化程度先增加后减小，在200kGy时达到最大。相反的原因主要是多壁碳纳米管具有同轴卷曲的石墨烯片层的曲率。研究发现，只有辐照速率小的伽马辐照才能提高片状石墨的有序化程度。薄层片的石墨在电子辐照时，石墨片层发生弯曲并有变成球状的趋势；厚层片的石墨在电子辐照后高分辨网格条纹变得无序紊乱，但电子衍射图没有明显变化。石墨在铜离子辐照后的表面形貌与碳纤维不同，石墨表面有很多孔洞和凹坑，局部还有熔化的痕迹，石墨高分辨网格条纹也逐渐变得紊乱。无定型碳膜在离子辐照后能逐渐观察到越来越清晰的高分辨条纹像，条纹间距大概是0.36nm，无定型碳膜的石墨化是由离子辐照引起的原子重排和加热效应造成的。对Al₂O₃-CNTs和Si₃N₄-Re嬗变模拟靶材的制造和性能研究结果显示，用放电等离子烧结的Al₂O₃-CNTs嬗变靶材的致密度、维氏硬度和断裂韧性随着CNT含量的增加均呈现先增后减的趋势，在1vol.%含量时达到最大；而用常压烧结的Si₃N₄-Re嬗变靶材的抗弯强度随着Re含量的增加呈下降趋势，致密度保持在98%左右，超过1550oC烧结时靶材中的Re将与基体发生反应。在西安脉冲反应堆中对两种靶材进行一次24h和一次48h的中子辐照嬗变实验。由于较强的感生放射性，待冷却慢化完毕后，我们将对嬗变靶材辐照后的力学性能和辐照肿胀机理等进行研究和分析。更多还原

【Abstract】 Every year tens of thousands tons of high radioactive nuclear waste has been produced,which will pose a great threat to humanity when they accumulate gradually. Presenting ofPartitioning and Transmutation technique bring dawn and hope to this problem. The technique isthat, first separating MA and LLFP nuclides from high radioactive nuclear waste and thenirradiating them to make them change to stable and short life nuclide. In addition, carbonmaterials have been widely applied in nuclear power and aerospace field. So the structurestability of them in severe radiation environment is quite important.In this paper, we studied the structure and property change of carbon fiber, MWCNT andgraphite under gamma ray, electron and Cu²⁺irradiation systematically. The results revealed thatthe degree of graphitization of the carbon fiber gradually increased and the surface wettabilityenhanced after gamma ray irradiation. Forced resonant peak method and single filament drawingmethod were used to determine the Young’s modulus of the carbon fibers before and aftergamma irradiation, which showed that the Young’s modulus of the fiber increased withincreasing gamma dose and it reached the maximum at a dose of2MGy. In FETEM, focusedelectron beam was performed on the carbon fiber surface. It could create a gap and nano-sizedcarbon onions. Low dose Cu2+irradiation could raise the degree of graphitization of the carbonfiber while high dose ion irradiation damaged the carbon fiber. The carbon fiber surfacewettability was first enhanced and then down with increasing the ion dose and reached themaximum at the dose of1015ions/cm².With increasing gamma irradiation dose, the degree of structural order in MWCNTs wasdecreased first and then increased, the surface wettability of the nanotube was enhanced, and theclear straight fringe of the nanotube was changed to amorphous structure before it changed backto ordered structure. The damage degree to the nanotube after electron irradiation increased withincreasing the electron energy and current density. Electron irradiation could also create carbononions on MWCNTs. In addition, it could make two nanotubes weld together. When carbonnanotubes were irradiation by Cu2+, with the increase of irradiation dose, the structure ordergradually reduced, and the surface wettability presented enhancement trend before weakening like carbon fibers. At the ion dose of1015ions/cm², the carbon nanotubes showed a specialmorphology which contained part of the hollow structure and part of closure structure. However,the carbon nanotubes completely transformed to the amorphous nanowires with the increase ofirradiation dose. Ion irradiation could also make carbon nanotubes welding, which made theyform the amorphous nanowire reticular structure.In contrast to MWCNTs, the degree of graphitization of graphite was increased beforedecreased after gamma irradiation and it reached the maximum at the dose of200kGy. The mainreason for the different evolution of structural order between graphite and MWCNTs was thecurvature-induced strain. Studies have found that only low rate gamma irradiation could raise thestructure order in flaky graphite we used. When graphite with thin graphene layer was underelectron irradiation, the layers curled and showed the trend to sphere structure. The highresolution images became disorderly structure after electron irradiation of graphite with thickgraphene layer. However, the electron diffraction pattern did not change after electron irradiation.Unlike carbon fiber after Cu2+irradiation, there were a lot of holes, pits and some melting traceon the graphite surface, and the high resolution images turned to be disorderly. The amorphouscarbon film can be observed gradually clear high resolution images of fringes after ionirradiation and the fringe distance is about0.36nm. Graphitization of the amorphous carbon filmis caused by rearrangement of atoms and irradiation heating effect.The fabrication and properties of Al₂O₃-CNTs and Si₃N₄-Re transmutation simulationtargets were investigated. The results showed that the relative density, Vickers hardness andfracture toughness of the Al₂O₃-CNTs transmutation target sintered by SPS increased beforedecreased with increasing the content of CNTs, and they reached the maximum when the contentof CNTs was1vol.%. The bending strength of the Si₃N₄-Re transmutation target sintered bypressureless sintering method decreased with the increasing of the Re content. The relativedensity was maintained at about98%. The Si₃N₄matrix would react with Re if the sinteringtemperature exceeding1550oC. The two kinds of target were irradiated for24h and48h inXi’an pulse reactor to perform transmutation experiments. Due to the strong inducedradioactivity, we will investigate mechanical properties and irradiation swelling mechanism ofthe transmutation targets after cooling and moderation of the seal cans.更多还原