Thomas Laude's pages - Science de la matière et de l'énergie

II. Properties and perspectives of applications

Note : These pages are in no way a reference nor an exhaustive review of all the tremendous amount of work that as been published in the field. I welcome any remark or any proposal for links and bibliography.

Nano-structures provide interesting perspectives of applications because of their unique properties. Many possible developments have been suggested (mainly for carbon nano-tubes), although none of them is at a stage of commercialisation yet. One practical difficulty is, obviously, the manipulation of individual structures on the nanometre range (as required for nano-electronics, for instance). This is possible in laboratory conditions, at the tip of a scanning microscope for instance. However a large-scale manipulation of individual nano-objets is not achieved. Another difficulty is that, synthesis methods are still little efficient. Similar results can now be obtained for BN and carbon tubes. But, production in kg quantities at low cost is still not achieved. Also, the technical ability in synthesising selected structures is still limited, because most synthesis methods produce particles with a large structural diversity. There is a need to develop post synthesis techniques for particle segregation.
Note: POSSIBLE TOXICITY. It should be stressed that it is not yet clear if nano-tube-based material have some toxicity. As a volatile fibrous material, nano-tubes may cause damage lung cells, as it is famously the case of asbestos fibres.

II.1. Some mechanical applications

II.1.1 Composite materials

The strength of a composite material is linked to the strength of the fibres embedded in the matrix. As both carbon [6.12] and BN nano-tubes (1.2 TPa [4.10]) have an exceptional elastic modulus, using them as reinforcement fibres is a possible way to obtain ultra resistant materials. For its good chemical inertness, especially to oxygen, BN is the best candidate.
The Typical problem encountered practically is the adherence at contact surface between tubes and matrix material. Tubes may actually slip along the matrix material. This may be worse for ropes, in which tubes can slip on each other. (Tubes with a spherical extremity, as synthesised by the present method, could lower this problem, because spherical extremities may fix as anchor in the matrix.)

II.1.2 Solid lubricants

Solid lubricants are used when conditions do not allow the usage of standard lubrication oil. This is typically under vacuum or in oxidising atmosphere. h-BN, graphite and WS2 are already intensively used as a solid lubricant in industry. h-BN is especially interesting for having both a very low friction coefficient and a high range of suitable temperature in air (up to 900 °C). [1.1.5] Nano-onions powders are exceptional solid lubricants, because onions act like nanometric ball bearings. This was demonstrated for powders of WS2 nano-onions [2.1][2.2], but it is probably true for the other materials.

II.1.3 Filters, Tissues, insulator materials

As any fibres, tubes and ropes could be applied to filters, tissues, thermal or acoustic insulator, or any other fibrous material. The exceptional porosity of such a material, due to the high aspect ration of nano-tubes, enables a filtering of much smaller particles, and/or a higher crossing flux (because the efficient collision surface is very small). For instance, this could be applied on a car, to filter gases in the exhaustive pipe.
Other properties of nano-tube tissues are not obvious, like thermal conductance, or resistance to tearing. Further studies are clearly needed, but this first requires an improvement of mass production.

II.2. Some chemical applications

II.2.1 Nano-shielding

Nano-tubes and onions of carbon have the ability to shell many materials inside their structure. It has been confirmed for many simple elements (Y, Bi, Gd, Ti, Cr, Fe, Zn, Mo, Pd, Sn, Ta, W, Gd, Dy, Yb, Pb, Mn, Co, Ni, Cu, Si, Ge...) and for some compounds. [6.7.6]. This may be of some interest, to protect nano-material from their environment, especially from oxidation. For instance, magnetic particles for data storage could be protected from air. It also offers a possibility to synthesise diverse hybrid nano-object, like metallic nano-rods, inside a tube cavity.

II.2.2 Hydrogen containers [2.10]

Industrial age is causing an exponential growth of CO2 concentration in the atmosphere, mainly due to extensive use of fossil energy sources. The harmfulness of such a change on the ecosystem, first expected as a "global warming effect", makes research on non-polluting energy sources a priority. Hydrogen is the ideal candidate, because its combustion produces no other release than water. Its energy per mass is higher than usual hydrocarbons. Furthermore, it is present in high quantity on earth.
Practically, the main limit to the commercialisation of hydrogen motor is the difficulty for a safe way to store hydrogen. Nano-tubes and nano-onions are thought to be a safe storage, because their cavity can absorb hydrogen molecules. Such storage was measured with variable success, between 0 and 10 wt % in carbon tubes.

II.3. Some electronic applications

II.3.1 Nano-transistors

The conductivity of a carbon SWNT depends on its diameter and helicity, hence on its structure [6.10.4]. Different carbon nano-tubes can theoretically form a nano-sized junction, which is a first step toward a "nano-electronic". A nano-transistor was realised through the body of a carbon nano-tube. [2.4]
One practical difficulty is the manipulation on nanometre scale. A focused ion beam (FIB) can be used to make the contact electrodes, but not on a large production scale. Another problem is to synthesise carbon tubes with a specific structure. This is not affordable with present synthesis methods.
The case of BN is different, conductivity is little dependent on structure. But on the other hand, it is an insulator of large band gap (~ 5eV, like diamond). To be used as a conductor, it should be efficiently doped. Diverse possibility of hetero-structures, like C/BN or C/Si are studied at present. (See [2.6], for instance.)

II.3.2 AFM / STM Tips

Carbon nano-tubes are very thin. They can be used as a nano-tip in both Atomic force microscopy (AFM) and tunnelling microscopy, to improve the resolution of the image. Furthermore, the exceptional elasticity of the tip avoids the damage from contact surface. It also enables an improved resolution of surface irregularities, because the tip can enter small cavities. Such a carbon nano-tube tip was proved to be feasible [2.7].
One problem is the reactivity of the tip with surface material. Carbon forms bounds with many materials. BN can be used instead to avoid this problem, in the case of AFM. (BN is probably not suitable for tunnel microscope, because of its electric resistance.)

II.3.3 Electron field emission

Electron sources are essential for screens or electron microscopes. Carbon nano-tubes can emit a high electron field emission current from their tip, when submitted to a bias voltage. The threshold voltage is exceptionally low because of the tip curvature. Emitting surfaces were realised by different post-synthesis methods. A prototype display and a lighting element were already produced. Hence, this application may seem the closest to commercialisation at present. However, many technical problems are still to be solved, regarding emission surface fabrication and, the understanding of the emission phenomenon.
Carbon nano-tubes are also interesting for electron microscope emitters, because the emission form a single nano-tube is intense and very coherent. The energy dispersion of such a beam is of the order 0.2 eV, and the life length of a single tube was found in the order of 2 months, in emission conditions. See [2.12 to 15]

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The material present here is copyrighted. Key-words: nanotechnology, science, nanotubes.

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