Properties and processing of direct-spun carbon nanotube mats
The mechanical and electrical properties of direct-spun carbon nanotube mats are investigated. Processing techniques which enhance their performance are developed, and their effects are characterised and understood through experiment and micromechanical modelling.
Macroscopic carbon nanotube material properties are surmised with material property charts that elucidate the relationships between processing, microstructure and properties, and identify the contribution of different carbon nanotube morphologies to material-property space.
The mechanical and electrical properties of a direct-spun carbon nanotube mat are determined. Formed from a 2D network of interconnected nanotube bundles, the measured stress-strain response is elasto-plastic, with orientation hardening. In-situ microscopy reveals foam-like deformation of the bundle network due to macroscopic strain. A micromechanical model is developed to relate the macroscopic mechanical properties to those of the nanotube bundles. Direct-spun carbon nanotube mat-epoxy composites are manufactured with varying volume fractions of air, epoxy, and nanotube bundles. Their electrical conductivity relates proportionally to the nanotube bundle volume fraction, whereas their strength and modulus depend nonlinearly upon the nanotube bundle and epoxy volume fractions. A unit cell idealisation of the composite microstructure captures the variation in modulus and strength over the compositional range.
The stress-strain response of a direct-spun mat is measured whilst immersed in organic solvents and in chlorosulfonic acid. Softening observed upon immersion in organic solvents is attributed to a reduced contact area between bundled nanotubes. Upon chlorosulfonic acid immersion nanotubes separate, resulting in ductile behaviour. A tensile drawing process based upon solutions of chlorosulfonic acid and chloroform is capable of enhancing the modulus and strength of direct-spun mat samples; the properties of the drawn mats are investigated as a function of draw strain and chosen fluid.
The thesis concludes with recommendations for future work — in furthering the understanding of the relationship between direct-spun mat microstructure and properties, and in enhancing performance.