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Kenji Hata- Publikacja
John Hart - Publikacja
John Hart - Wywiad
Galeria
The porous nature of the CNT material allows for rapid and efficient heat dissipation, which prevents significant heat accumulation, the common cause for property degradation. We interpret that our CNT material exhibited this high level of reversible deformation from the entanglement of the long traversing CNTs throughout the material, like a network of springs creating elasticity.
To provide insight into the mechanism of viscoelasticity, we carried out SEM observations (Fig. 4A) and Herman’s orientation factor (HOF) calculations (Fig. 4B) under increasing shearing strains up to 1000%. HOF describes the degree of alignment (0 is random, and 1 is aligned) and was calculated from the fast Fourier transform (FFT) analysis of SEM images. Up to 100% strain, the traversing, randomly aligned CNTs structurally deformed into mutually aligned CNTs, and the HOF steadily increased from 0.07 to ~0.48. Beyond 100% strain, the HOF plateaued, showing no increase in alignment, and the intermittently contacting CNTs became increasingly bundled. This observation at 100% strain corresponded with the measured failure strain. From these results, we propose that the strain was absorbed at low level by reversible unfolding of the traversing CNTs and beyond 100% strain by an irreversible process of straightening, slipping, and bundling of CNTs (Fig. 4C).
Fig. 4
Energy dissipation model. (A) SEM images at varying shear strains. (B) Herman orientation factor (HOF) as a function of shear strain. (Inset) 2D FFT of the SEM images at 0% and 100% strain. (C) Schematic description of the change in intertube structure with strain. (Inset) TEM image of intertube structure at 1000% strain. (D) TEM image of the as-prepared intertube structure. (Inset) Selected section indicating nodes. (E) Schematic of the zipping and unzipping of nodes.
Transmission electron microscope (TEM) observation further revealed the intertube structure of the CNT material, which showed insight into the mechanism of energy dissipation (Fig. 4D). We found an intertube structure resembling a 3D highway network, where each CNT made contact with numerous other CNTs. This peculiar intertube structure was characterized by a high density of connections (nodes), that is, two to four CNTs intermittently contacting in parallel for only short spans (~150 nm). These nodes were separated mainly by isolated CNTs (struts). In addition, no CNT ends were observed, implying long, continuous CNTs, and the long CNTs were both randomly oriented and traversing. We interpret that this intertube structure of struts and nodes was the key for structural cohesiveness that allowed for large deformations. This structure differs from typically bundled CNT material where CNTs are straight and contacts the same CNTs over long spans.
Although not directly observed, we believe that the CNTs in a node reversibly attached and detached through zipping and unzipping (Fig. 4E). This process would dissipate energy because of the energy consumed to overcome the large van der Waals (vdW) attraction between CNTs when unzipped, yet no energy is required for zipping. We interpret that sliding among CNTs was not a significant contribution because the friction coefficient (0.003) is small. Because the CNT material possessed a very high density of these śdetachable” nodes, we interpret that they were the source of the high energy dissipation ability. Within our model, under the critical strain, the nodes perpendicular to the strain direction could reversibly zip and unzip and thereby dissipate energy. Under increased strain, the number of detachable nodes gradually decreased through either unzipping or alignment (Fig. 4C), and eventually the ability to dissipate energy decreased. Beyond the critical strain, this zipping/unzipping process was no longer reversible, and upon cycling CNTs zipped at different places and/or became bundled and aligned, resulting in degradation. A similar irreversible process caused by the slipping among CNTs under large strain has been observed in CNT yarns.
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