# A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications
## 5 Applications Involving DWNTs and TWNTs
They found that two main frequencies describe the coupled oscillations in the TWNTs, and these frequencies range from 50-100 GHz and from 80-200 GHz, depending on the species composing the TWNT system, as shown in Figure 11a. Another very interesting prediction was made by [PERSON] _et al._[169], in which they placed the DWNT oscillator perpendicular to a surface and studied the proximity effects, which are related to the friction between the surface and the DWNT. In their calculations, they demonstrated that the DWNT oscillatory behavior is strongly affected by its proximity with the surface. Their results suggest, as shown in Figure 11b, that this friction-dependent oscillatory behavior has a great potential to make DWNT systems excellent candidates for a new generation of atomic force microscope, in which the inner tube would be probing the surfaces in the GHz regime [169]. Figure 11: (**a**) The primary (\(F_{1}\)) and the secondary (\(F_{2}\)) frequencies of a TWNT oscillator plotted against the oscillation amplitude (\(A_{1}\)) related to \(F_{1}\)[168]. (**b**) The top panel shows the design of an atomic force microsppe (AFM) system in which DWNTs would be used as surface probes. The bottom panel shows the intensity of the force between the inner tube of the DWNT and the sample plotted against the distance between the surface and the edge of the outer tube of the DWNT. The free oscillations of the DWNT system have a constant intensity around 0.62 nN, and as the DWNT starts interacting with the sample, a region of a linear force regime dominates [169]. DWNTs and TWNTs were also shown to possess excellent mechanical properties, being more resistant than SWNTs to tensile loads and strains [173]. Recently, DWNTs were also shown to have good superlubricity properties, whereby the inner tubes can easily slide across the interior of the outer tube composing the system [174]. DWNTs can also be used to reinforce polyimide composites, which may be used in electronic devices, transparent conducting films and aerospace parts. Individually-dispersed DWNTs do not perturb transparency, while the non-isolated tubes are randomly aggregated and, thus, deteriorate the visual appearance of the polyimide film, as shown in Figure 12a-c. The chemically-modified and -isolated DWNTs lead to increased modulus, tensile strength and elongational break of polyimide nanocomposites [162], as shown in Figure 12d. Figure 12: (**a**) Pure polyimide (PI) film, (**b**) polyimide/non-isolated DWNTs (PI/DWNT-OH non-isolated) film and (**c**) polyimide/isolated DWNTs (PI/DWNT-OH isolated) film. The non-isolated DWNTs perturb the transparency of the film. (**d**) Stress-strain curves of PI (black solid curve), PI/DWNT-OH non-isolated (red solid curve) and PI/DWNT-OH isolated (blue solid curve). Reproduce with permission from [162], Copyright John Wiley and Sons, 2008. The weak IL interactions observed in DWNTs and TWNTs make them excellent candidates for the fabrication of nano-motors and nano-actuators [175; 176; 177]. [PERSON] _et al_. [175] used multi-walled carbon nanotubes as bearings for rotating platforms, while [PERSON] _et al_. [176] proposed a nanomotor based on SWNTs and DWNTs in mechanical contact with each other. In their design, as shown in Figure 13a-c, the SWNT is used as a power nanotube, which can translate along the DWNTs axis, which is composed of one shaft nanotube (inner tube) and a bearing nanotube (outer tube). Their calculations suggested good controllability for translational and rotational motions in their nanomotor, and this controllability may be tuned according to SWNT species composing the DWNTs [176]. [PERSON] _et al_. [177] also reported a fully functional design that would work either as a nanometer-scale electromechanical system or as a micro-electromechanical system. Figure 13c,d shows SEM images of the motor, which consists of a plate attached to the outer SWNT of the DWNT, and various angular positions of the plate [177].