Electrical and Piezoresistive of CNTs

The unique electrical properties of CNTs are to a large extent derived from their 1-D character and the peculiar electronic structure of graphite. Resistance occurs when an electron collides with some defect in the crystal structure of the material through which it is passing. The defect could be an impurity atom, a defect in the crystal structure, or an atom vibrating about its position in the crystal. Such collisions deflect the electron from its path. But the electrons inside a CNT are not so easily scattered. In a 3-D conductor, electrons have plenty of opportunity to scatter, since they can do so at any angle. Any scattering gives rise to electrical resistance. In a 1-D conductor, however, electrons can travel only forward or backward. Under these circumstances, only backscattering (the change in electron motion from forward to backward) can lead to electrical resistance. But backscattering requires very strong collisions and is thus less likely to happen. So the electrons have fewer possibilities to scatter. This reduced scattering gives CNTs very low resistance. In addition, they can carry the highest current density of any known material, measured as high as 109A/cm2 [Khare and Bose 2005].

Theoretical calculations have shown early on that the electronic properties of the CNTs are very sensitive to their geometric structure. Although graphite is a zero-gap semiconductor, theory has predicted that the CNTs can be metallic, semi-metallic or semi-conducting with different size energy gaps, depending very sensitively on the diameter and helicity of the tubes (Fig. 1) [Dresselhaus et al. 2000].

Field emission is an alternative mechanism to extract electrons. It is a quantum effect where under a sufficiently high external electric field, electrons can tunnel through the energy barrier and escape to the vacuum level. All the field emission sources rely on field enhancement due to sharp tips/protrusions, so they tend to have smaller virtual source sizes because of the primary role of the field enhancement factor. The larger the field enhancement factor, the higher the field concentration, and therefore the lower the effective threshold voltage for emission. CNTs possess the right combination of properties: nanometer-size diameter, structural integrity, high electrical and thermal conductivity, and chemical stability, so they have excellent emission characteristics such as a low threshold field for emission and a high current density. The emission has been observed at fields lower than 1V/m, and high current densities of over 1A/cm2 have been obtained [Meyyappan 2005, Cheng and Zhou 2003].

When CNTs are subjected to external force such as tension/compression, torsion and squashing, they are deformed. Deformations of CNTs' structures will affect the electron transport condition. As a result, CNTs present extraordinary piezoresistive properties (Fig. 26) [Tombler et al. 2000]. The change in electrical conducitivity of CNTs in response to strain can reach 0.02 S/cm with per 1% change in compressive strain [Pushparaj et al. 2010].

Dispersing conductive fillers into the nonconductive matrix can form conductive materials. The electrical conductivity of these materials is strongly dependent on the concentration of the conductive fillers. At low concentration, the conductivity remains very close to the conductivity of the pure matrix. When a certain concentration is reached, the conductivity of the materials drastically increases by many orders of magnitude. This phenomenon is known as percolation and can be well explained by a percolation theory. The electrical percolation threshold of conductive reinforcements embedded in an insulating matrix is sensitive to the geometrical shape of the conductive fillers. The small size and large aspect ratio are helpful to lower the percolation threshold. Because CNTs have tremendously large aspect ratios, many researchers have found that CNTs reinforced materials exhibit exceptionally low electrical percolation thresholds. For example, the electrical

Fig. 26 Conductance change induced by mechanical deformation of metallic CNT using AFM tip (G is conductance of CNT, a is stain of CNT, and AZc is vertical displacement).

percolation threshold was reported at 0.0025 wt.% CNTs and conductivity at 2 S/m at 1.0 wt% CNT in CNTs reinforced epoxy materials. CNTs not only can be utilized to tailor conductivity of polymer as reinforcement, but also are excellent conductive fillers to impart piezoresistivity to polymer. Much research efforts have been concentrated on piezoresistivity of CNTs reinforced polymer materials [Khare and Bose 2005].

Therefore, CNTs are expected to be more effective in producing significantly conductive and piezoresistive cement-based materials than traditional conductive fillers (e.g. carbon fibers, steel fibers and carbon black etc).

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