Ni-Y/SWNTs

Figure 3.44 TEM images of the different samples tested (a) Ni/Y-SWNTs, (b) SWNTs, (c) DWNTs, (d) MWNTs...

Figure 3.46 Raman spectrum of Ni/Y-SWNTs at two different laser wavelengths 632.817 and...

Figure 3 Contact pressure effect on the friction coefficient for a 1 wt % Ni/Y-SWNT dispersion...

Figure 349 Comparison of the friction reducing properties of Ni/Y-SWNTs and other carbon forms tested in the same conditions (1 wt% concentration, P = 0.83 GPa, v = 2.5 mm/s, T = 20 °C)...

Figure 3.50 Wear scars observed on pins after the friction test with (a) pure PAO, (b) lwt% Ni/Y-SWNTs, (c) lwt% graphite and Iwt % Cgo...

A comparison of nanotubes without a catalyst (SWNTs) showed a key role for catalysts in the tribological properties of the single-walled nanotubes. Addition of purified nanotubes (SWNTs) in PAO does not lead to a reduction in the friction coefQcient (Figure 3.51). A slight diminution of the wear on the pin is observed with SWNTs, the wear scar diameter being 115 fim, against 170 fim in the case of PAO. However, this reduction is nevertheless lower than the one observed with Ni/Y-SWNTs (Figure 3.52). 

Figure 3 1 Comparison of friction coefficients obtained with Ni/Y-SWNTs and SWNTs dispersed at 1 wt% in PAO...

It is difficult to remove these catalytic particles from the sample because they are at the end of the nanotubes or are surrounded by amorphous carbon. Energy filtered transmission elec-hon microscopy (EFTEM) realized on Ni/Y-SWNTs shows the presence of catalytic particles not only at the end of nanotubes but also inside the amorphous carbon present in the sample (Figure 3.54). 

Figure 3.54 (a) TEM image of Ni/Y-SWNTs and (b) the corresponding chemical mappings of Ni, (c) C and (d) Y. These images show the presence of catalytic particles made of Ni and Y at the extremity of a nanotube and in the amorphous carbon phase... 

Figure 3.55 XPS analysis of Ni-Y/SWNTs indicating that yttrium is in the oxide form and nickel is only metalhc...

Figure 3 6 Fit of the Ni 2p3/2 of the XPS analysis performed on the wear track after the friction test with Ni/Y-SWNTs...

The contact pressure has an important influence on the friction coefficient (Figure 3.60). At 0.83 GPa, friction is relatively high (0.15). At a contact pressure of 1.12 GPa or higher, friction becomes low, reaching a value lower than 0.06 for contact pressures of 1.42 and 1.72 GPa. Friction coefficients at these pressures are lower than those observed in the case of Ni/Y-SWNTs tested under the same conditions. 

Arc discharge [25] is initially used for producing C60 fullerenes. Nanotubes are produced by arc vaporization of two carbon rods placed in a chamber that is filled with low pressure inert gas (helium, argon). The composition of the graphite anode determines the type of CNTs produced. A pure graphite anode produce preferably MWNT while catalyst (Fe, Co, Ni, Y or Mo) doped graphite anode produces mainly SWNT. This technique normally produces a complex mixture of components, and requires further purification to separate the CNTs from the soot and the residual catalytic metals present in the crude product. 

Some combinations of metals improve remarkably the yield of SWNTs, even though the respective metals show no or very low catalytic activity by themselves. Typical combinations showing such an effect are Fe-Ni, Rh-Pt, and Ni-Y. 

Figure 6 (a) SEM image of SWNTs produced by arc-discharge method using Ni Y (4.2 1 at.%) catalyst (scale bar 1 pm). (Reprinted with permission from Ref. 20. 1997 Macmillan Magazines Ltd.) (h) Electron diffraction pattern of carbon nanotubes produced by arc-discharge method. (Reprinted with permission from Y. Saito, T. Yoshikawa, S. Bandow, M. Tomita, and T. Hayashi, Phys. Rev. B., 1993, 48, 1907. 1993 by the American Physical Society)... 

Figure 8 TEM of SWNTs bundle synthesized using Ni Y (2 0.5 at.%) catalyst. (Reprinted from Ref 47. 1998, with permission from Elsevier)...

The laser ablation of the graphite target produces high quality SWNTs with narrow size distribution in high yields. Among various catalysts studied, Ni, Co, and the mixture of Co/Ni and Ni/Y have produced higher yields of SWNTs. 

NiA"-SWNTs contain a mixture of catalyst Ni/Y (38 wt %) with an atomic ratio of 85/15. With this ratio, nanotubes in long bundles are obtained [110]. With an XPS analysis carried out on nanotubes deposited on a substrate covered by a thin gold film, it was possible to check the hypothesis of a segregation of yttrium on the periphery of the catalytic particle (Figure 3.55). Indeed, yttrium was observed only in the oxide form (Y 3d3 at 159 eV) while nickel was only in metallic form (Ni 2p3/2 at 853 eV). This confirms the growth mechanism of carbon nanotubes previously described. 

SWNTs Arc discharge (<1 wt% Ni and Y) HaCaT Reduced cell proliferation [26]... 

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