Synthesis and mechanical properties of iron-filled carbon nanotubes

Carbon forms the basis of a variety of compounds. The allotropic forms of carbon include graphene, fullerenes, graphite, carbon nanotubes and diamond. All these structures possess unique physical and chemical properties. This work focusses on the usage of carbon nanotubes (CNT), especially iron-filled CNT. An industrial application of CNT requires the understanding of the growth mechanism and the control of the synthesis process parameters. Regarding iron-filled CNT the shell formation as well as the filling process has to be understood in order to control the CNT morphology and distribution and dimension of the iron filling. The thesis involves two topics - synthesis of CNT and characterization of their mechanical properties. Chapter 2 of the present work deals with the synthesis of iron-filled CNT. In this thesis all experiments and the discussion about the growth process were conducted with respect to the demands of magnetic force microscopy probes. The experimental work was focused on the temperature profile of the furnace, the aluminum layer of the substrate, the precursor mass flow and their impact on the morphology of in-situ iron-filled CNT. By selecting appropriate process parameters for the temperature, sample position, gas flow and by controlling the precursor mass flow, CNT with a continuous filling of several microns in length were created. Existing growth models have been analyzed and controversially discussed in order to explain the formation of typical morphologies of in-situ filled CNT. In this work a modified growth model for the formation of in-situ filled CNT has been suggested. The combined-growth-mode model is capable to explain the experimental results. Experiments which were conducted with respect to the assumptions of this model, especially the role of the precursor mass flow, resulted in the formation of long and continuous iron nanowires encapsulated inside multi-walled CNT. The modified growth model and the synthesis results showed, that besides the complexity of the parameter interaction, a control of the morphology of in-situ iron-filled CNT is possible. In chapter 3 the measurements of mechanical properties of in-situ iron-filled CNT are presented. Two different experimental methods and setups were established, whereby one enabled a static bending measurement inside a TEM and another a dynamical excitation of flexural vibration of CNT inside SEM. For the first time mechanical properties and in particular the effective elastic modulus Eb of in-situ iron-filled CNT were determined based on the Euler-Bernoulli beam model (EBM). This continuum mechanic model can be applied to describe the mechanical properties of CNT and especially MWCNT in consideration of the restriction that CNT represent a macro molecular structure built of nested rolled-up graphene layers. For evaluation and determination of the elastic modulus the envelope of the resonant vibrating state was evaluated by fitting the EBM to the experimental data. The experiments also showed, that at the nanoscale the properties of sample attachment have to be taken into account. Thus, instead of a rigid boundary condition a torsion spring like behavior possessing a finite stiffness was used to model an one side clamped CNT. The extended data evaluation considering the elastic boundary conditions resulted in an average elastic modulus of Eb = 0.41 ± 0.11 TPa. The low standard deviation gives evidence for the homogeneity of the grown material. To some extend a correlation between the formation process, the morphology and the mechanical properties has been discussed. The obtained results prove the usability of this material as free standing tips for raster scanning microscopy and especially magnetic force microscopy. The developed methods provide the basis for further investigations of the CNT and the understanding of mechanical behavior in greater detail