Applications

The spherical shape of the fullerene-like nanoparticles and their inert sulfur-terminated surface suggest that MoS2 particles could be used as a solid-lubricant additive in lubrication fluids, greases, and even in solid matrices. Applications of a pure IF-MoS2 powder could be envisioned in high vacuum and microelectronics equipment, where organic residues with high vapor pressure can lead to severe contamination problems [84,85]. Since the MoS2 layers are held together by weak van der Waals forces, they can provide easy shear between two close metal surfaces, which slide past each other. At the same time, the MoS2 particles, which come in the form of platelets, serve as spacers, eliminating contact between the two metal surfaces and minimizing the metal wear. Therefore MoS2 powder is used as a ubiquitous solid-lubricant in various systems, especially under heavy loads, where fluid lubricants cannot support the load and are squeezed out of the contact region between the two metal surfaces. Unfortunately, MoS2 platelets tend to adhere to the metal surfaces through their reactive prismatic (1010) edges, in which configuration they "glue" the two metal surfaces together rather than serve as a solid lubricant. During the mechanical action of the engine parts, abrasion and burnishing of the solid lubricant produces smaller and smaller platelets, increasing their surface area and consequently their tendency to stick to the metal surfaces through their reactive prismatic edges. Furthermore, the exposed prismatic edges are reactive sites, which facilitate chemical oxidation of the platelets. These phenomena adversely affect the tribological benefits of the solid lubricant. In contrast, the spherical IF-MS2 nanoparticles are expected to behave like nano-ball bearings and upon mechanical stress they would slowly exfoliate or mechanically deform to a rugby-shape ball, but would not lose their tribological benefits, until they are completely gone, or oxidized. To test this hypothesis, various solid-fluid mixtures were prepared and tested under standard conditions [86]. The beneficial effect of IF powder as a solid lubricant additive has been thus confirmed through a long series of experiments [87].

The mechanism of the action of the IF nanoparticles as additives in lubrication fluids is more complicated than was initially thought. First, it is clear that the more spherical the nanoparticles and the fewest structural defects they include, the better is their performance as solid lubricant additives [88]. Three main mechanisms responsible for the onset of failure of the nanoparti-cles in tribological tests have been clearly identified. They include: exfoliation of the nanoparticles; deformation into a rugby ball shape, and explosion. The partially damaged nanoparticles are left with reactive edges, which can undergo further oxidation and can lead to a complete loss of their tribological action. Recent nanotribological experiments, using the surface force apparatus with the lubricant between two perpendicular mica surfaces, revealed that material transfer from the IF nanoparticles onto the mica surface is a major factor in reducing the friction between the two mica surfaces [87]. This experiment and many others, carried out over the last few years, suggest an important application for these nanoparticles both as an additive in lubrication fluids or greases, as well as in composites with metals, plastics, rubber, and ceramics.

Another important field where inorganic nanotubes can be useful is as tips in scanning probe microscopy [13]. Here applications in the inspection of microelectronics circuitry have been demonstrated and potential applications in nanolithography are being contemplated. A comparison between a WS2 nanotube tip and a microfabricated Si tip indicates, that while the microfabricated conically-shaped Si tip is unable to probe the bottom of deep and narrow grooves, the slender and inert nanotube can go down and image the bottom of the groove faithfully [13]. This particular tip has been tested for a few months with no signs of deterioration1, which is indicative of its resilience and passive surface. Although other kinds of tips have been in use in recent years for high resolution imaging using scanning probe microscopy, the present tips are rather stiff and inert, and consequently they are likely to serve in high resolution imaging of rough surfaces having features with large aspect ratios. Furthermore, inorganic nanotubes exhibit strong absorption of light in the visible part of the spectrum and their electrical conductivity can be varied over many orders of magnitude by doping and intercalation. This suggests numerous applications, in areas such as nanolithography, pho-tocatalysis and others.

The shape of the BœCyNz IF nanoparticles (onion or cocoon like) again suggests important tribological applications (such as lubricants), and the inherently strong covalent bond of some of the IF nanotubes, such as those composed of BœCyNz, suggest high-strength, high stiffness fiber applications.

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