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化学气相沉积法原位合成碳纳米管增强铝基复合材料

Carbon Nanotubes Reinforced Al Matrix Composites Fabricated by In-situ Chemical Vapor Deposition

【作者】 何春年

【导师】 赵乃勤;

【作者基本信息】 天津大学 , 材料学, 2008, 博士

【摘要】 碳纳米管(CNTs)因具有优异的力学、电学和热学等特性而受到多个学科领域研究者的关注,同时在先进复合材料领域也引发了新的研究热点。作为金属基复合材料的理想增强体,要发挥CNTs优异的性能,需要解决的关键技术是如何获得CNTs在金属基体中分散均匀、结构保持完好且CNTs与基体之间界面结合良好的复合材料。而传统的金属基复合材料制备技术难以满足这一要求,因此,寻求新的制备技术以克服现有方法的不足,是发展CNTs增强金属基复合材料的关键。本论文首次采用在铝基体上原位合成CNTs方法制备了CNTs(Ni)/Al复合材料。首先采用沉积-沉淀法在铝粉基体上获得均匀分散的活性Ni纳米颗粒,以该Ni/Al粉末为催化剂,利用化学气相沉积法原位合成形态好、纯度高、分散均匀的CNTs,即获得CNTs(Ni)/Al复合粉末,并直接利用此复合粉末采用粉末冶金的方法来制备复合材料。通过沉积-沉淀工艺和化学气相沉积工艺分别制备了Ni/Al催化剂与CNTs(Ni)/Al复合粉末,研究了还原温度对Ni颗粒粒径的影响,系统探索了化学气相沉积工艺对CNTs产率、形貌与结构的影响。结果表明:铝可以作催化剂载体,并能有效防止Ni纳米颗粒的团聚;CNTs的形貌、结构与原位合成工艺条件密切相关,当催化剂中Ni含量较低,还原温度400℃、还原时间2 h,合成温度630℃,反应气氛比例VN2: VH2: VCH4 = 480:120:60 (ml/min)时,制备的CNTs形态好、纯度高且在铝粉中分散均匀;CNTs生长过程中Ni催化剂与铝基体之间作用力较弱,其生长属于顶端生长机制。采用差热分析仪、X射线衍射仪和透射电子显微镜分析了原位合成CNTs(Ni)/Al复合粉末及其复合材料块体发生相变的温度范围,研究了退火对CNTs-Al界面结构的影响,考察了CNTs在复合粉末和复合材料中的热稳定性及其与铝基体之间的界面浸润情况,进而探讨了CNTs-Al的界面润湿理论。结果表明:复合粉末中CNTs在退火温度低于800℃基本不与铝发生反应,只是CNTs表面的无定形碳薄层与铝反应生成Al4C3薄层;复合材料块体经过850℃退火后CNTs与铝基体之间界面结合好,且CNTs结构保持好;CNTs与铝基体间的界面反应润湿动力学主要是通过Al4C3薄层沿着CNTs轴向长大进行的;CNTs与铝基体间Al4C3薄层的形成,有利于改善CNTs-Al界面浸润性,提高界面结合强度,从而提高CNTs(Ni)/Al复合材料的载荷传递效率和力学性能。采用粉末冶金工艺对原位合成CNTs(Ni)/Al复合粉末进行压制-烧结后制备了CNTs(Ni)/Al复合材料。研究了成型工艺、烧结温度、时间和复压对复合材料微观结构与性能的影响,获得了优化的粉末冶金工艺参数。同时,研究了CNTs含量对复合材料力学性能和微观组织的影响,并探讨了复合材料的强化机理。结果表明:原位合成法引入的CNTs能显著提高复合材料的硬度、拉伸强度和弹性模量。当CNTs含量为5 wt%时,复合材料的硬度和拉伸强度较纯铝基体分别提高180%和300%,与传统高能机械球磨法制备的复合材料的硬度和拉伸强度相比,分别提高约100%和86%;复合材料的强化主要来自CNTs与铝基体热膨胀系数不匹配而引起的基体加工硬化,CNTs对铝基体变形的约束和对铝基体中位错运动的阻碍产生的位错强化和细晶强化。采用化学气相沉积法在Ni含量较高的Ni/Al催化剂中合成了碳纳米洋葱结构相(CNOs)。探讨了化学气相沉积工艺对CNOs产率、形貌与结构的影响,考察了CNOs的纯化、磁性能和摩擦学性能。结果表明:以氢气作载气时制得纯度高且具有超顺磁性的内包镍纳米碳洋葱(Ni@CNOs),而以氮气作载气时获得中空CNOs和Ni@CNOs的混合物;添加质量分数为1%的Ni@CNOs能显著提高润滑油的抗磨性能和承载能力,并大幅降低润滑油的摩擦系数。

【Abstract】 Carbon nanotubes (CNTs) have attracted much attention because of their excellent mechanical, electrical, and thermal properties. Of particular interest is the use of CNTs in the reinforcement of metal matrix composites as a means of overcoming the performance limits of conventional materials. The most important issues for preparing CNTs/metal composites with high performances are the homogenous molecular-level mixing between CNTs and metal powders, the retention of perfect structure of CNTs during preparation of composites, and the strong interfacial bonding between CNTs and matrix to ensure load translation. However, these issues are very hard to be solved by means of traditional powder mixing process (high-energy ball milling). Therefore, it is very urgent to search a novel method to improve the traditional route and develop CNTs/metal composites with high performances.This thesis reports for the first time the synthesis of Al matrix composites reinforced with CNTs with a low Ni content, produced by a novel fabrication process, which involved preparation of a Ni(OH)2/Al precursor by a deposition-precipitation route, calcinations, and reduction of the precursor to yield active Ni nanoparticles spread evenly on the Al powder surfaces, and in situ synthesis of CNTs through chemical vapor deposition (CVD) followed by pressing and sintering of the CNTs(Ni)/Al composite powders. This process produced CNTs(Ni)/Al composites in which the in situ synthesized CNTs were very homogeneously dispersed within the Al powders, unlike conventional CNT-reinforced Al matrix composites. This unique in situ synthesis of CNTs in a matrix is particularly suitable for the fabrication of CNTs-reinforced metal matrix composites, because the desired dispersion of CNTs can be rather easily achieved.Ni/Al catalysts and CNTs(Ni)/Al composite powders were prepared using deposition-precipitation route and CVD method, respectively. The influence of reduction temperature on the diameters of Ni nanoparticles, and the influences of CVD technics on the yield, morphology and structure of CNTs were investigated. The results showed that the aluminum powder as a catalyst carrier could prevent the transition metal particles from agglomerating and promote the dispersion of nano-scale Ni particles after reduction, which are frequently responsible for CNT growth; the morphology and structure of CNTs were dependent on the in-situ synthesis technics, when the Ni content in the catalyst was relatively low, reduction temperature was 400 oC, reduction time was 2 h, synthesis temperature was 630 oC, and the ratio of reaction gases was VN2: VH2: VCH4 = 480:120:60 (ml/min), the CNTs synthesized were with better morphology, higher purity, and more homogenous dispersion in Al powder; the interaction between Ni catalyst and Al during CNT growth was feeble, which led to a tip growth mechanism for CNTs.The temperature ranges of phase change of CNTs(Ni)/Al composite powders and composite bulk, the influences of annealing on the CNTs-Al interfacial structure, the chemical stability of CNTs in the composite system, and the interfacial wetting theory were explored by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. The results showed that the CNTs were thermodynamically stable in Al below 800 oC, and only the locations containing an amorphous carbon coating of CNTs have reacted with Al to form Al4C3 thin film; the composite bulk annealed at 850 oC has good interfacial bonding between CNTs and Al, and the CNTs in the composite presented perfect structure; the interfacial reactive wetting kinetics in the composite system was developed by lateral growth of Al4C3 on the CNT surface. The carbide formed on the surface of the CNTs could improve the interfacial interaction between CNTs and Al. This also contributed to the enhancement of the mechanical properties of the composites.In-situ CNTs(Ni)/Al composites were fabricated by pressing and sintering of CNT(Ni)/Al composite powders. The optimal powder metallurgy technical parameters were obtained by analyzing the influences of initial pressure, sintering temperature, sintering time and repressing pressure on the microstructure and performances of composites. Meanwhile, the effects of CNT content on the mechanical properties and microstructure of composites, and the strengthening mechanism of composites were discussed. The results showed that the CNTs in-situ synthesized could remarkably improve the hardness, tensile strength and elastic modulus of the composites. The hardness and tensile strength of the Al-5%CNT-1%Ni composite were 4.3 and 2.8 times that of neat Al, respectively, and 2.0 and 1.8 times that of the same composition composites obtained by traditional methods; the strengthening mechanisms of CNTs(Ni)/Al composites were the work hardening of matrix resulted from the significant coefficient of thermal expansion mismatch between the matrix and the CNTs, Orowan and grain strengthening due to the inhibition of dislocation motion and matrix distortion.Carbon nano-onions (CNOs) were synthesized over Ni/Al catalyst with relatively high Ni content by CVD. The influences of CVD technics on the CNO yield, morphology and structure, and the purification, magnetic and friction properties of CNOs were investigated. The results showed that the Ni@CNOs with high purity and superparamagnetic behavior were obtained when using hydrogen as a carrier gas, and mixture of Ni@CNOs and CNOs with a hollow core were produced in the case of using nitrogen as a carrier gas; after addition of Ni@CNOs, the ability of resistance to wear as well as friction coefficient of the lubricant were improved remarkably.

  • 【网络出版投稿人】 天津大学
  • 【网络出版年期】2009年 07期
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