Multiscale composites based on carbon fibers and carbon nanotubes

The observation of hierarchical structures in nature has shown how great mechanical performances can be achieved by relatively weak constituents if the matter is distributed along multiple length scales[1]. In order to reproduce a biomimetic multiscale material in laboratory[2] pure carbon material have been chosen both for of the intrinsic strength of such materials and for the availability of carbon materials with different length scales (micro and nano). The chemical grafting was performed by dispersing the oxidized nanotubes (CNTs) in acetone by mean of an ultrasound probe, and by pouring the obtained dispersion over the carbon fibers (CFs) drop-by-drop, allowing the solvent evaporation. Then the CF-CNT system was heated in order to create the chemical bonding, finally the thermally treated fibers were washed with water and dried. The composite preparation was done by tape casting technology using a polyvinyl butyral matrix and modifying the type and content of the fillers: at first, only CFs or CNTs in different concentrations in order to asset the properties of the single filler; secondly, a physical mixture of CFs and CNTs; finally, the mixture of CFs and CNTs with hierarchical structure. The composite tapes were cut into specimens that underwent tensile testing. Further characterizations were done by optical microscopy and SEM observation of the fracture surfaces. The work allowed the assessment of mechanical properties and the collection of a wide and statistically sound range of data that is possible to consider reliable for modelling purposes, both because of the rigorous methodology followed in preparation and testing and because of the matching between the collected data and the ones expected by theory.