Carbon nanotube fibers with martensite and austenite Fe residual catalyst: room temperature ferromagnetism and implications for CVD growth

We report on the room temperature ferromagnetic properties of continuous macroscopic fibers made up of carbon nanotubes grown by floating catalyst chemical vapor deposition. Their ferromagnetic behavior originates from the presence of residual catalyst nanoparticles: martensite with 0.77 wt% C content and FCC Fe. The first is intrinsically ferromagnetic, but the latter only due to severe lattice distortion as a consequence of C supersaturation. The stabilization of martensite and austenite occurs mainly because of the small diameter of the nanoparticles, in the range of 4-20 nm. This is smaller than the embryonic nucleus of the relevant equilibrium phases, but also implies that large C concentrations can build up in FCC Fe before C can be segregated as a stable graphitic nucleus. The room temperature remanence ranges from 10% to 25% and the coercivity from 55 to 300 Oe, depending on the choice of promoter for fiber synthesis (S or Se). Superparamagnetic behavior is only observed in S-grown samples on account of the smaller diameter of residual catalyst particles. The results of this work provide an explanation for the widespread observation of magnetic properties in oxide-free CNT samples produced by catalytic growth under a wide range of synthesis conditions

Threading Through Macrocycles Enhances the Performance of Carbon Nanotubes as Polymer Fillers

In this work we study the reinforcement of polymers by mechanically interlocked derivatives of single-walled carbon nanotubes (SWNTs). We compare the mechanical properties of fibers made of polymers and of composites with pristine single-walled carbon nanotubes (SWNTs), mechanically interlocked derivatives of SWNTs (MINTs) and the corresponding supramolecular models. Improvements of both Young's modulus and tensile strength of up to 200 % were observed for the polystyrene-MINTs samples with an optimized loading of just 0.01 wt.%, while the supramolecular models with identical chemical composition and loading showed negligible or even detrimental influence. This behavior is found for three different types of SWNTs and two types of macrocycles. Molecular dynamics simulations show that the polymer adopts an elongated conformation parallel to the SWNT when interacting with MINT fillers, irrespective of the macrocycle chemical nature, whereas a more globular structure is taken upon facing with either pristine SWNTs or supramolecular models. The MINT composite architecture thus leads to a more efficient exploitation of the axial properties of the SWNTs and of the polymer chain at the interface, in agreement with experimental results. Our findings demonstrate that the mechanical bond imparts distinctive advantageous properties to SWNT derivatives as polymer fillers.Comment: 39 pages, 19 figure