In-situ Synthesis Of Carbon Nanotubes (Fibers) Supported On Copper And The Properties Of Their Composites

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.