Carbon nanotubes

The advantage of nanotubes relative to more conventional forms of carbon, such as carbon black and activated carbon, which are commonly used in industry, is that the nanotubes are much stronger, have incredibly high surface areas per unit volume and also have better thermal conductivity, properties beneficial to the chemical reactions. Carbon nanotubes have a lower density than inert dilution agents, which allows them to get easily separated by using mechanical screening once the reaction is complete. Moreover, the carbon nanotubes are left unchanged after reaction and so can be used again and again, which means their initial cost can be ignored after they have been used for many reactions.

Carbon nanotubes are used in electronic circuits, but the metals used to grow nanotubes react unfavorably with materials found in circuits and composites. Nanotubes have been grown with elements such as iron, gold and cobalt. But these can be toxic and cause problems in clean room environments. Moreover, the use of metals in nanotube synthesis makes it difficult to view the formation process using infrared spectroscopy, a challenge that has kept researchers in the dark about some of the aspects of nanotube growth.

No metal catalyst

Researchers at MIT have shown that nanotubes can grow without a metal catalyst. The researchers used zirconium oxide to grow nanotubes, but without the unwanted side effects of metal. Previous researchers have not ever proved that anything other than a metal can grow a nanotube.

Process

The concept is: A substrate is sprinkled with nanoparticle seeds made of a certain metal, of the same diameter of the desired nanotubes. The substrate and nanoparticles are heated to 600 to 900 degrees Celsius, and then a carbon-containing gas such as methane or alcohol is added. At the high temperatures, molecules break apart and reassemble. Some of these carbon-containing molecules find their way to the surface of a nanoparticle where they dissolve and then precipitate out, in nanotube form.

zirconia-grown nanotubes

The researchers found that if they just used zirconium oxide nanoparticles on the substrate, they could coax carbon into nanotubes as well. Importantly, the mechanism for growth seems to be completely different from that of metal nanoparticle-grown tubes. Instead of dissolving into the nanoparticle and precipitating out, zirconia-grown nanotubes appear to assemble directly on the surface. Images of the oxide-based nanotubes indicated that when nanotubes were formed, zirconium oxide persisted and did not form into a metal.

Composites

The finding means that carbon fiber and composites used to make different types of components could be strengthened by nanotubes. Mechanical properties and resistance to fracturing of carbon composites increase, if the composites are reinforced at the micro level with nanotubes. This would make lighter, stronger and less expensive material. The findings have far-reaching implications for commercial productions of carbon nanotubes. For example it will allow the use of a ceramic catalyst instead of a magnetic transition metal, some of which are carcinogenic.

Activating butane

Researchers from Germany have used surface-modified carbon nanotubes to activate an important industrial chemical, butane, without the need for an expensive metal catalyst. This process offers a cheaper alternative to the current industrial process for butane activation. When carbon nanotubes are used as the catalyst, it can produce olefins with four carbons under very selective, mild (low reaction temperature) and safe (low oxygen concentration) conditions. There is enormous savings in energy in this industrial process as more environment friendly than the old method.