Composites of carbon nanotubes and two-dimensional nanomaterials : development and technology applications

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Κωσταράς, Χρήστος

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The fabrication of carbon nanotubes (CNT) from Iijima in 1991 constitutes a landmark for the development of nanocomposites and paved the path for employing carbon-based nanomaterials as the reinforcement phase. The next decade was marked by the isolation of monolayer graphene by means of micromechanical cleavage (2004). Henceforth, composite materials that employ CNTs and graphene as reinforcement are the epicenter of intense research interest for improvement of the mechanical properties of continuous matter, mainly of polymeric nature. At the same time, carbon-based nanomaterials display a very low percolation threshold for imparting a polymer matrix with electrical properties, thus drawing comparable amounts of interest for their use in lightning protection, pressure and mechanical stress sensing and electromagnetic shielding. A particular use of graphene/CNT composites is resistive (Joule) heating – i.e. the tendency of materials to heat up under electrical current load. This property is particularly prominent in de-icing applications in the aeronautical and energy sector such as airplane wings and wind turbine blades, but also in de-fogging and personalized heating applications. The electrical and thermal properties of graphene and its allotropes render them attractive materials for use in Joule heating-related applications. Composites that employ only small amounts of nanomaterials exhibit limited flexibility, small electrical and thermal conductivities – in most cases of orders of magnitude smaller than the filler material itself – and slow heating/cooling rates resulting from the small thermal conductivity of polymer materials combined with interface resistance effects. Increasing filler content to achieve bigger weight ratios (<10 - 20 % wt. in filler content) is burdened by poor dispersion in the matrix which, in turn, downgrades the composite’s mechanical performance. Various processing methods, which aim at the enhancement of dispersion nanofiller in polymer matrices involve calendaring, shear-mixing or ball-milling. However, such methods tend to reduce nanofiller size, which also reduces property transfer. During the 90s Richard Smalley demonstrated that filtered CNT suspensions form free-standing structures consisting of the nanomaterial forming an intertwined microstructure resembling cellulose in paper. The popularity of the then-newly discovered Buckminster fullerene (C-60), commonly termed “buckyball”, inherited the name of “buckypaper” to the films. Later on, it was shown that these paper-like films can be infiltrated by a polymeric matrix allowing the formation of composites with high filler content and without the issues of poor dispersion. Since then, filtration of suspensions of well dispersed nanomaterials have also been used in hybrid structures of CNTs and graphene with great success. Graphene, well exfoliated flakes of which are hard to create stable suspensions alone, can be supported within the CNT network. A sibling material however, graphene oxide (GO), has been shown to exfoliate readily to form good aqueous suspensions. Its stacked flakes can also form paper-like materials with excellent mechanical properties that are only diminished by flake quality (degree of functionalization, flake size and defects) and the presence of voids within the lamellar structure. While the primer 4 parameter is determined and can be controlled through thorough characterization of the nanomaterial, inter-lamellar void filling can be achieved with the inclusion of polymeric material forming a biomimetic microstructure known as brick-and-mortar. This structure that is found in nacre (mother of pearl) is a composite of calcite platelets (brick) bound by a biopolymer (mortar). Thus, with a relatively smaller polymer weight ratio (10 – 20 % wt.) a strengthening of the paper-like structure is achieved, while maintaining flexibility and only slightly affecting electrical conductivity. Moreover, the polymer is part of the initial dispersion, thus the need for insertion of nanomaterial in the matrix and vice versa is circumvented. The present thesis reports the development and mechanical and resistive heating characterisation of 10 different types of hybrid paper-like films synthesized from multi-walled CNTs (MWCNT) and different types of graphenes at variable concentrations. Mechanical properties of the papers were determined by tensile testing. Small amounts of polymeric materials (10 wt. %) were added to the papers to investigate mechanical strengthening of brick-and-mortar structures in films consisting of MWCNTs with increasing graphene weight ratio. Three different types of graphene were used, as well as GO and epoxidized reduced graphene oxide erGO in order to investigate the effect of lateral size, number of layers, dispersibility in solvents and interaction between nanomaterials on the papers’ mechanical properties. Dispersion of nanomaterials in solvents was assisted by two methods, either chemical modification or a common non-ionic surfactant (Triton X-100). Furthermore, reduction processes to remove the functional groups from the materials are described along with their effects in papers that consist of the corresponding materials. Finally, the resistive heating of the produced materials was tested. Performance was gauged as the temperature achieved from the samples when working as heating elements as a function of the power drawn when under direct current load. As the materials tested are tens of microns thick and of small mass, large heating rates were expected while under load, and thus tested. Moreover, since additional mass of a material that is not a good electrical or heat conductor is included, heating performance as well as heating rates are expected to change.



Carbon nanotubes, Graphene, Chemical modification, Reduction, Mechanical characterization, Joule heating