【作者】 李海鹏；

【导师】 赵乃勤；

【作者基本信息】 天津大学 ， 材料学， 2008， 博士

【摘要】 铝基复合材料具有密度低、尺寸稳定、比强度高、耐腐蚀性强、高温性能好等优点,在各工业领域具有广泛应用前景,是当前金属基复合材料研究开发的重点。碳纳米管(CNTs)管径小、长径比大、物理和化学性能优异,是复合材料的理想增强相,也成为新型铝基复合材料开发和CNTs应用研究的热点。但传统粉末冶金法机械混合或铸造法机械搅拌制备的CNTs/Al基复合材料存在CNTs团聚、破损和界面污染等问题,因此,开发新型复合材料制备工艺并研究相关的组织、性能是推动该材料应用的关键。本文采用原位合成工艺制备了CNTs增强铝基复合材料,即在铝基体中原位合成CNTs增强相基础上,对复合粉末中CNTs的结晶程度、氧化稳定性及不同结构CNTs-Al的异质界面结合特征进行研究,并对CNTs原位增强铝基复合材料的制备工艺、组织、性能和强化机制进行探讨。本文首次对Fe、Co、Ni催化剂在铝基体上低温化学气相沉积(CVD)合成CNTs的可行性和合成效果进行系统研究;在优选催化剂类型基础上,对金属铝与传统Al2O3陶瓷基体的CNTs合成效果进行了对比分析,并探讨了合成条件对CNTs形态、结构、结晶程度等的影响。结果表明:不同含量Ni催化剂都能够在铝基体上CVD稳定合成产率高、形态良好、晶化程度高的CNTs,但Fe/Al、Co/Al催化剂的CNTs原位合成效果不佳,即Ni是铝基体上CNTs原位合成的最佳催化剂类型;与Al2O3陶瓷基体相比,金属Al基体具有抑制Ni催化剂颗粒团聚的作用,可实现CNTs低温高产率的稳定合成;本实验条件下,600℃时Ni催化剂合成的CNTs结构和形态较好,是CNTs/Al复合粉末合成的最佳温度。对不同条件合成CNTs的氧化稳定性、结晶程度、界面反应程度进行了对比,发现了不同温度条件下可获得鱼骨状和管状结构的CNTs,探讨了鱼骨状和管状CNTs-Al异质界面的结构特征,并从固相反应热力学和动力学角度分析了Al4C3的形成机理和不同类型CNTs-Al界面反应差异的原因。对CNTs的氧化稳定性研究表明,Ni含量可对CNTs管径、结构缺陷和氧化稳定性产生作用从而影响其热稳定性,Ni含量越高,CNTs稳定性越差,因此,CNTs/Al原位复合粉末制备过程中应控制Ni含量在较低范围。对CNTs-Al界面结构的研究表明,低温合成的鱼骨状CNTs结晶程度低、结构缺陷多、化学稳定性差,导致鱼骨状CNTs-Al之间出现Al4C3脆性界面反应层;较高温度合成管状CNTs的石墨基面结构及较高的结晶程度使其具有良好化学稳定性,可与铝基体形成无不良界面反应的直接结合型界面。本文对合成的复合粉末采用粉末冶金法制备了CNTs/Al基复合材料,分别研究了冷压-烧结-复压和冷压-烧结-热挤压两种工艺对该复合材料物理和力学性能的影响,对其微观组织结构进行了深入分析并探讨了该复合材料的强化机制。物理性能方面,与纯铝相比,CNTs原位增强铝基复合材料在密度、热膨胀系数、抗软化温度方面具有明显优势,但过高的CNTs含量对复合材料致密度、高温稳定性有不利影响。力学性能方面,随CNTs含量升高,CNTs对铝基体强化作用先升后降,0.5wt%CNTs/Al基复合材料的硬度、弹性模量、抗拉强度等综合力学性能较佳,这是载荷传递、位错强化、弥散强化等强化机制综合作用的结果,但其强度提高的同时,塑性降低。此外,热挤压变形具有减少孔隙、改变铝基体晶粒取向、实现CNTs弥散分布等优点,使CNTs/Al基复合材料具有较好的综合物理和力学性能,是该新型复合材料制备的较佳工艺。更多还原

【Abstract】 With wide application potential in various industrial fields, aluminium matrix composite is always the focus of metal matrix composite development and research work, due to its distinct advantages including low density, size stability, high specific strength, corrosion resistance, excellent high temperature performance, and so on. Carbon nanotubes (CNTs) have been introduced as an ideal reinforcement phase of composites by their virtues of small diameter, high slenderness ratio, and outstanding physical and chemical properties. However, there are some inevitable problems, such as the agglomeration and breakage of CNTs and the interface pollution between CNTs and Al when using the traditional methods including powder metallurgy and casting. Thus, the development of new preparation process and the investigation about structure and property of CNTs/Al composite are essential to promote its application.The emphasis of this study was to prepare aluminum matrix composite reinforced by CNTs by the method of synthesis in-situ. On the basis of successful synthesis in-situ of CNTs over aluminium matrix, the crystallization and oxidative stability of CNTs in the composite powders, and the heterogeneous interfaces between different structural CNTs and Al were investigated. Further, the preparation process, structure, property and strengthening mechanism of aluminum matrix composite reinforced by CNTs in-situ were discussed.In this paper, the feasibility and effectiveness of CNTs synthesized using Fe, Co, Ni as catalysts over Al matrix at low temperature by chemical vapor deposition (CVD) were researched systematically for the first time. In addition to the optimization of catalyst type, a comparative analysis for CNTs synthesized over metal Al and the traditional Al2O3 ceramic matrix was investigated, and the influences of synthesis parameters on the morphology, structure and crystallization of CNTs were also studied. The results showed that, CNTs with high yield, good shape and high crystallization could be always obtained using different Ni contents by CVD, while Fe/Al and Co/Al catalysts did not work well, that is, Ni is the fittest catalyst for the synthesis in-situ of CNTs over aluminum matrix. Compared with Al2O3 matrix, Al matrix realized the high yield of CNTs at low temperature by preventing Ni catalyst particles from agglomerating. Under our experimental conditions, CNTs synthesized by Ni catalyst at 600℃always have ideal structure and morphology, which is the fittest temperature for the preparation of CNTs/Al composite powders.The oxidative stability, crystallization and interface reaction of CNTs synthesized at different conditions were compared. It was found that herringbone and tubular CNTs could be synthesized under different temperatures and the heterogeneous interface characteristics between herringbone and tubular CNTs with Al were investigated. And the formation mechanism of Al4C3 and the difference in interface reaction between different structural CNTs with Al were analyzed by both solid-state reaction thermodynamics and kinetics. The study on oxidative stability showed that, Ni content could affect its thermal stability by controlling CNTs diameter, structural defect and oxidative stability, namely, the higher Ni content, the poorer of CNTs stability. So CNTs/Al composite powders should be prepared by low Ni content. The results also showed lower crystallization, more structural defects and poorer chemical stability of herringbone CNTs synthesized at low temperature resulted in obvious formation of Al4C3, the brittle interface reaction layer, along CNTs-Al interface. In reverse, the graphite basis plane and high crystallization of tubular CNTs obtained at high temperature led them outstanding chemical stability, and the direct bonding interface was formed without detected interface reaction.Using composite powders synthesized in-situ, CNTs/Al composite was prepared by powder metallurgy. The influences of two different processes, multiple compression or hot extrusion after cold molding and sintering, on the physical and mechanical properties of the composite were systematically studied, as well as the microstructures and strengthening mechanisms. To the physical properties, CNTs/Al composite gains the advantages over pure aluminum in density, coefficient of thermal expansion and resistive softening temperature, but too high content of CNTs will negatively affect the tightness and high temperature stability of the composite. To the mechanical properties, the strengthening effect of CNTs on aluminum matrix went up firstly and then down with the content of CNTs rising. The composite with 0.5wt%CNTs showed the best performance on hardness, elastic modulus, tensile strength, and so on, which is the result of combined effects of several reinforcement mechanisms, including effective load transmission, dislocation strengthening, dispersion strengthening, etc. But the plasticity of the composite decreased with its strength rising. Furthermore, the process of hot extrusion deformation could improve the physical and mechanical properties of CNTs/Al composite by reducing the porosity, changing grain orientation of aluminum and dispersing the CNTs, so that, it is a positive technology for the preparation of this new composite.更多还原