【作者】 康建立；

【导师】 李家俊； Philip Nash；

【作者基本信息】 天津大学 ， 材料学， 2009， 博士

【摘要】 以碳纳米管(CNTs)为首的碳纳米材料因其优异的力学、电学和热学等特性自发现以来就受到了科学界的广泛关注。CNTs和碳纳米纤维(CNFs)被认为是发展高性能金属基复合材料的理想增强体。要发挥CNFs的优异性能,关键是获得结构完好的CNFs在金属基体中均匀分散且与基体之间界面结合良好,而传统的金属基复合材料制备技术难以满足上述要求,因此,寻求新的制备技术以克服现有方法的不足,是发展高性能CNFs增强金属基复合材料的关键。本文首次采用原位合成和化学共沉积相结合的方法制备了CNFs(Ni/Y)/Cu复合材料。即首先在Cu基体上获得均匀分布的活性催化剂粒子,并采用化学气相沉积法(CVD)原位合成分布均匀、含量可控的CNFs,然后将此原位复合粉末与铜离子一起共沉积,得到CNFs分布均匀且大部分进入Cu颗粒内部的CNF-Cu复合粉末,最后利用粉末冶金法制备复合材料。采用沉积-沉淀工艺在Cu粉基体上成功研制了系列新型Ni/Y和Ni/Ce复合催化剂。研究了催化剂制备和CVD合成工艺参数对产物产量、结构和形貌的影响,探讨了催化剂的催化本质和不同结构碳产物的生长机制。研究表明,Y和Ce能够稳定催化剂结构,保持催化剂活性;Y含量越高,催化剂越稳定;当WNi:WY=2:1,Ni(NO3)2·6H2O浓度为0.05mol/L,NaOH浓度为0.23mol/L,煅烧温度为250℃和400℃各2h,还原温度为450℃时,制备的催化剂活性最高;随着反应温度的升高,掺杂进Ni中的Y开始偏聚析出,并导致不同结构碳产物的生成:在较低温度下,催化剂结构稳定,CNFs和CNTs以顶端生长机制合成,随着温度升高,碳原子开始在催化剂中以体扩散为主,掺杂进催化剂中的Y开始偏聚析出,催化剂失去稳定性,当Y偏聚析出前,催化剂中的碳达到过饱和,碳层析出,形成碳包覆Ni或空心碳洋葱,这种碳洋葱在一定条件下可形成金属填充CNTs。基于此,本研究提出了球-管生长机制,当Y偏聚析出时,催化剂中的碳未达到过饱和时,基体铜开始向催化剂中扩散形成合金,当催化剂中的碳达到过饱和后,碳层析出形成碳包覆Cu-Ni合金的碳洋葱。析出的Y2O3团聚于碳洋葱或CNFs的表面。采用化学共沉积将原位合成的CNFs和铜再次混合后,CNFs在基体中分布均匀,且大部分CNFs进入Cu颗粒内。将制得的复合粉末采用真空热压工艺制备了CNFs(Ni/Y)/Cu复合材料。研究了CNFs纯度和粉末冶金工艺对复合材料微观结构与性能的影响,获得了优化的工艺参数,同时研究了CNFs含量对复合材料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,CNFs能显著提高复合材料的硬度和屈服强度,降低复合材料的热膨胀系数(CTE);当加入经800℃热处理纯化的3.4wt.% CNFs时,复合材料的硬度和屈服强度分别是纯铜的2倍和3.6倍,即使CNFs含量达到5.7wt.%时,复合材料中的CNFs未发现明显团聚,复合材料的屈服强为448MPa,是纯铜的2.8倍,CTE(30-200℃)为10.1*10-6/℃,是纯铜的57.7%。复合材料强度的提高主要是由于CNFs与基体之间强的界面结合强度使载荷在基体和增强体之间实现了有效的载荷传递。更多还原

【Abstract】 Since their discoveries, carbon nanotubes (CNTs) and its relative nanomaterials has been attracted much attention due to their excellent mechanical, electrical, and thermal properties. CNTs and carbon nanofibers (CNFs) have been regarded as ideal reinforcements for composites. In order to tap into the advantage of these excellent properties of CNFs, there are two important processing issues in the fabrication of CNFs/metal composites: a homogeneous distribution of CNFs with perfect structure and a high interfacial bonding strength between CNFs and matrix. However, these issues are hard to be solved by current traditional methods. It is necessary and urgent to pursue new approaches to overcoming the limits of traditional methods for CNFs/metal composites.A combination of in situ synthesis and chemical co-deposition was introduced to fabricate CNFs/Cu composites for the first time. CNFs with a controllable content and homogeneous distribution on Cu powders were in situ synthesized by chemical vapor deposition (CVD) using a novel catalyst supported on Cu. The in situ CNF-Cu composite powders were mixed with Cu again by chemical co-deposition, which caused CNFs imbedded into Cu particles and produced a high dispersion. Finally, the composite was fabricated by powder metallography (PM) using the final CNF-Cu composite powders.The novel Ni/Y and Ni/Ce catalysts supported on Cu were prepared by a deposition-precipitation method. The effect of the process parameters of the catalyst preparation and carbon nanostructure growth by CVD on the yield, structure and morphology of the products was investigated. The mechanisms of the novel catalyst and different carbon nanostructure growth were discussed. The results show that Y and Ce can stabilize the structure of the catalyst at high temperature and keep the activity of the catalyst. The higher Y content is, the stabler the catalyst is. The catalyst with WNi:WY=2:1 got the best catalytic activity when prepared with a proper solution concentration of Ni(NO3)2·6H2O (0.05mol/L) and NaOH (0.05mol/L), calcined at 250℃and 400℃for 2h respectively and reduced at 450℃for 3h. With the reaction temperature increasing, Y doped in Ni begins to aggregate, which causes different carbon nanostructure growth. At relative low temperature, the catalyst is stable and catalyzes the CNF and CNT growth by tip growth mechanism. With the temperature increasing, carbon atoms begin to diffuse through the catalyst body and Y doped in Ni aggregates. Before Y separated from the catalyst, carbon atoms in the catalyst were over saturated and carbon onions with a hollow core or Ni nanoparticle were formed. The carbon onions obtained have a trend to coalesce to form metal filled CNTs under a sphere-tube mechanism. If the catalyst was not over saturated with carbon after Y separated from the catalyst, Cu began to diffuse into Ni. Carbon onions with a Cu-Ni alloy particle were formed after the over-saturation of the catalyst. Y2O3 separated from the catalyst aggregated on the surface of carbon onions or CNTs.The in situ synthesized CNFs have been embedded in Cu particles with a high dispersion after mixing with Cu by chemical co-deposition. CNFs(Ni/Y)/Cu composites were fabricated by vacuum hot pressing using the final powders obtained. The effect of the purity of CNFs and PM parameters on the structure and property of the composites was investigated, and the optimized parameters of CNF purification and PM were obtained. Meanwhile, the effect of the CNF content on the structure and property of the composites was also investigated. The strength mechanism of the CNFs/Cu composite was discussed in detail. The results show that the introduction of CNFs can improve the hardness and yield strength of the composite significantly and reduce the coefficient of thermal expansion (CTE) of the composite. The hardness and yield strength of the composite with 3.4wt.% of the CNFs heat treated at 800℃are 2 times and 3.6 times higher than that of copper matrix, respectively. Even with 5.7wt.% of CNFs, the composite has a good dispersion of CNFs in the matrix, which exhibits a CTE of 10.1*10-6/℃( 57.7% of that of Cu) and a compressive yield strength of 448MPa, 2.8 times higher than that of Cu. Such excellent strength of the composite was mainly due to the effective load transfer between CNFs and matrix by high interfacial bonding.更多还原