MWCNT bundling

To prepare the CNT/MOX liquids for spin-coating, the SWCNT bundles were dispersed in the organometallic solutions (Sn [OOCCH(C2H5)-QH9]2,aq. tin (H) 2-ethylhexanoate -90% in 2-ethylhexanoic acid) by ultrasonic vibration. Alternatively, MWCNTs bundles with Sn02 nanoparticles and cetyltrimethyl ammonium bromide can be dispersed in water. ° In... 

The formation of the MWCNTs bundles restrict the phonon transport in composites, which may be attributed to two reasons (i) the MWCNTs aggregation reduces the aspect ratio, consequently, decreasing the contact area between the MWCNTs and the TPNR matrix (ii) the MWCNTs bundles cause the phenomenon of reciprocal phonon vector, which acts like a heat reservoir and restricts heat flow diffusion. ... 

Li et al. prepared arrays of vertically aligned individual MWCNTs and MWCNT bundles from plasma-enhanced chemical vapor deposition (PECVD) on 10-30 nm thick nickel films... 

The bundle of MWCNT can be released in ultrasonic cleaner using ethanol as the solvent. The scanning tunnelling microscope (STM) image of thus released MWCNT is shown in Fig. 2. 

Fig, 6. EEL spectra of bundle of four SWCNTs, MWCNT and graphite in the energy ranges (a) from 0 to 45 eV (plasmon region) and (b) from 280 to 300 eV (carbon K-edge) (modified from ref. 14). 

Some explanations could be possible for these contradictory results. One is that a various types of CNTs may be obtained by different methods, since SWCNTs as much as 50 % are chiral and nonmetallic [42]. The other is that the result may be attributable to the contact condition of SWCNT bundles. When the bundles closely contact each other, the SWCNT system will likely become a three-dimensional one just as in the case of contacted MWCNTs. 

After briefly introducing the main electronic features of CNTs (Sec. 2) and some general aspects of electronic conduction and transmission (Sec.. 1), we will show how complex electrical measurements to perform on such tiny entities are (Sec. 4). Then we will present the main experimental results obtained on the electrical resistivity of MWCNT and SWCNT and the very recent data relative to the thermopower of SWCNT bundles (Sec. 5). We will also discuss the effect of intercalation on the electrical resistivity of SWCNT bundles (Sec. 6). Finally, we will present some potential applications (Sec. 7). 

We will discuss below the reeent experimental observations relative to the eleetrieal resistivity and magnetoresistance of individual and bundles of MWCNTs. It is interesting to note however that the ideal transport experiment, i.e., a measurement on a well eharacterised SWCNT at the atomic scale, though this is nowadays within reaeh. Nonetheless, with time the measurements performed tended gradually eloser to these ideal eonditions. Indeed, in order to interpret quantitatively the eleetronie properties of CNTs, one must eombine theoretieal studies with the synthesis of well defined samples, which structural parameters have been precisely determined, and direet electrical measurements on the same sample. 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

Fig. 1. (a) Comparison of normalised electrical conductivity of individual MWCNTs (Langer 96 [17], Ebbesen [18]) and bundles of MWCNTs (Langer 94 [19], Song [20]). (b) Temperature dependence of resistivity of different forms (ropes and mats) of SWCNTs [21], and chemically doped conducting polymers, PAc (FeClj-doped polyacetylene [22]) and PAni (camphor sulfonic acid-doped polyaniline [2. ]) [24]. 

As-synthesized MWCNT and SWCNT exist as bundles or ropes and tend to agglomerate due to strong van der Waals forces (13) (Figure 7.1). Unless the CNTs are separated in to individual tubes and dispersed in the polymer matrix, the interactions of the nanotubes with the polymer will be weak. The mechanical failure of such composites will occur due to slippage of the tubes in the bundle that are not bonded to the matrix. In addition, the aggregates or bundles reduce the aspect ratio of the reinforcement which affects electrical properties as well (15). Because of these factors the first step will be to open up these bundles to separate individual tubes by using different techniques to increase the volume of interface between the CNT and the matrix (40). 

It is indisputable that ILs can be widely applied to functionalize CNTs due to their excellent dispersing power. Recently, liquid-phase exfoliation of CNTs has been reviewed by Coleman [61] however, the dispersion mechanism for CNTs in ILs is still controversial. A cation-jt interaction between nanotubes and imidazolium ions was first proposed by Fukushima and Aida [55,57] to account for the exfoliation of entangled SWCNTs into smaller bundles, even individuals. On the basis of combined results of differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectrum, and X-ray diffraction (XRD) analysis, Gilman et al. [81] provided the evidence for a cation-Ti interaction in the imidazolium-treated MWCNTs. Some researchers also cited this interpretation to support their cases [82]. 

Figure 9.1 Different types of filamentous carbon (a) CNF (b) MWCNT (c) DWCNTs and bundle of SWCNTs.

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