Cambridge University Technical Services Limited (Cambridge, GB) received U.S. Patent 7,767,615 for a method for producing carbon nanotubes and/or nanofibers on a large scale by growing the nanotubes on more than one side of a catalyzed substrate.
Briefly the Cambridge method for producing aligned carbon nanotubes and/or nanofibres comprises providing finely divided substrate particle having substantially smooth faces with radii of curvature of more than 1 .mu.m and of length and breadth between 1 micron and 5 mm and having catalyst material on their surface and a carbon-containing gas at a temperature and pressure at which the carbon-containing gas will react to form carbon when in the presence of the supported catalyst, and forming aligned nanotubes and/or nanofibres by the carbon-forming reaction.
Inventors Ian Kinloch, Charanjeet Singh, Milo Sebastian Peter Shaffer, Krzysztof K. K. Koziol and Alan Windle method for producing aligned carbon nanostructures comprises: (a) providing (i) finely divided substrate particles having substantially smooth faces with radii of curvature of more than 1 micron and of length and breadth between 1 micron and 5 mm and having a catalyst material on their surface and (ii) a carbon-containing gas at a temperature and pressure at which the carbon-containing gas will react to form carbon when in the presence of the catalyst material, and (b) forming aligned nanostructures by the carbon-forming reaction, wherein the substrate particles are of silica, alumina, carbon, mica, magnesium oxide, calcium oxide, sodium chloride, or a mixture of two or more thereof, or are of graphite, aluminium, or titanium, and further the substrate particles are freshly prepared by colloidal processing, spray-drying, hydrothermal processing, or ball milling.
Also the substrate particles having the catalyst material on their surface are prepared by depositing the catalyst material on the surface of the substrate particles by electroless deposition, solvent drying, supercritical drying, sputtering, physical vapour deposition or electroplating, and further wherein the catalyst material is a transition metal, an alloy of two or more thereof, a compound of a transition metal or a mixture of two or more compounds of transition metals, where the transition metal is iron, cobalt, molybdenum or nickel.
FIG. 1 shows an SEM image of aligned nanotubes grown on quartz substrate at -700.degree. C. for 90 minutes.
The advantages of the Cambridge technique for large scale production of carbon nanotubes are summarized as follows: 1. Low production cost: the raw materials involved for forming the support, catalyst and carbon feedstock are cheap and readily available in large quantities. 2. The method results in high volumes and high yield of good quality pure nanotubes, and can readily be scaled up to an industrial level. 3. Characteristics such as nanotube/nanofibre diameter and length can be manipulated by the growth process.
The disadvantage of other known flat substrate methods of production compared with the method is that the ratio between the growing surface area and volume of support is low as a result of the geometry of the plates. Because the growth is confined to a macroscopic surface the total volumetric yield of product is low. This is apparently because the substrate particles used in that method do not have the required characteristics.
Whilst the inventors do not wish to be bound by this theory, it is believed that during growth the aligned nanotube or nanofibre arrays grow from all sides of the substrate particle, leading to a large volume of carbon product. During growth the substrate particles move apart in order to accommodate the growing nanotubes.