LLNL researchers have created a new desalination membrane that combines billions of aligned carbon nanotubes on a silicon chip. Shown here is an artist’s rendering of methane molecules flowing through a carbon nanotube less than two nanometers in diameter Source: Lawrence Livermore National Laboratory

membranes, the enhanced flowrate per pore and the high pore density makes them superior in both air and water permeability,” says Olgica Bakajin, one of the principal researchers. Preliminary estimates suggest that these permeable nanotube-based membranes could reduce the energy costs of using reverse osmosis (the prevailing method for desalination and demineralization) by up to 75%.

Today, carbon nanotubes are being incorporated into various matrices, including fluoropolymers such as ethylene tetrafluoroethylene (ETFE) and polyvinylidene fluoride (PVDF), to produce ultra-lightweight composite materials that have exceptional strength and other functional advantages over conventional materials, and to create advanced thin-film membranes, fibers and coatings. Such advanced composites are already being used in various automotive, electronics and materials-handling applications — particularly those that require improved chemical resistance, greater barrier resistance to chemical permeation, inherent lubricity, better resistance to sloughing, and improved control of static electricity compared to the non-reinforced materials.

Carbon nanotubes are also being investigated as key components in tomorrow’s advanced sensors, electronic and optical devices, catalysts, batteries and fuel cells. In industrial fuel cell applications, for example, the use of carbon nanotubes has been shown to improve fuel cell performance while greatly reducing the amount of platinum catalyst required. Millenium Cell (Eatontown, N.J.; millenniumcell.com), Altair Nanotechnologies (Reno, Nev.; altairnano.com), Samsung (Seoul, Korea; samsung.com), and Nanomix (Emeryville, Calif.; nano.com) are among the companies that are working to developing advanced fuel cells based on nanoscaled materials.

Researchers at the U.S. National Aeronautical and Space Administration (NASA; Langley, Va.; nasa.gov) are also developing polyimide nanocomposites that incorporate single-walled nanotubes to improve radiation and tear resistance, and thermal and electrical conductivity — all key material considerations for aircraft and spacecraft construction.

Production routes for carbon nanotubes and other nanoparticles

A variety of competing production routes are being pursued to allow carbon nanotubes to be mass-produced in the most cost-effective manner. The key technical challenges are to produce nanotubes that have predictable, consistent dimensions, acceptable purity levels and minimal structural defects (since these can drastically alter the anticipated behavior of the nanoscaled particles).

In general, there are six established methods for producing carbon nanotubes and other nanoscaled particles of various materials [Ref. 1, 2]:

• Plasma-arc and flame-hydrolysis methods (including flame ionization): Involves the use of a high-temperature plasma or flame-ionization reactor (involving both gas-to-particle and droplet-to-particle methods)

• Chemical vapor deposition (CVD): A starting material is vaporized and then condensed on a surface, usually under vacuum conditions

• Electrodeposition techniques: Individual species are deposited from solution in a precisely controlled manner, to form a nanoscaled surface film

• Sol-gel synthesis: A wet-chemical method that allows high-purity, high-homogeneity nanoscale materials to be synthesized at lower temperatures and milder conditions compared to competing high-temperature methods (The inorganic or “colloidal” route uses metal salts in aqueous solution, such as chloride, oxychloride nitrate, as raw materials; The metal-organic or “alkoxide” route employs metal alkoxides in organic solvents)

• Mechanical crushing via ball milling: Entails the pulverization of conventional starting materials (such as metal oxides), using conventional high-energy ball mills

• Use of naturally occurring nanomaterials:  Certain naturally occurring materials, such as zeolites, can be synthesized and modified by conventional chemistry to produce particles with nanoscaled dimensions

In addition to the proven techniques discussed above, additional technologies are also being pursued. These include:

• Flame or jet-flame reactors: These introduce an additional flame behind the reaction zone, in order to transform the aggregates into spherical particles more effectively

• Improved plasma processes: These are designed to promote more rapid cooling, in order to produce fewer agglomerates

• Sonochemical processing routes: Use an acoustic cavitation process to generate a transient localized hot zone with an extremely high-temperature gradient and pressure

• Hydrodynamic cavitation processes: Generates nanoparticles through creation and release of gas bubbles inside a sol-gel solution

• Microemulsion techniques: These show promise for the synthesis of metallic semiconductor silica, barium sulfate, magnetic and superconductor nanoparticles

One of the drawbacks associated with chemical vapor deposition (CVD) — the catalytic method most often used to produce carbon nanotubes — is that it typically requires additional purification steps, to remove unwanted catalyst particles and carbon deposits from the final product. Researchers at the National Institute of Advanced Industrial Science &

To the naked eye, carbon nanotubes look like graphite powder Credit: Reprinted with permission from Arkema

Technology (AIST; Tsukuba, Japan; aist.go.jp) have developed a more efficient CVD process. AIST claims to be able to synthesize single-walled carbon nanotubes with 99.98% purity — an improvement that can eliminate the need for additional purification, and lower production costs.

As reported by Chemical Engineering magazine (January 2005), AIST’s CVD process uses ethylene and hydrogen in an argon or helium carrier gas, and a small (roughly 100 ppm) amount of water vapor. The growth of the single-walled carbon nanotubes is catalyzed by nanoparticles of iron, which are formed by sputtering metal films with FeCl₃ onto a substrate of Si, quartz or metal foil, at 500-1,200°C under high vacuum. The presence of water in the CVD process removes amorphous carbon deposits on the catalyst, thus preserving the catalyst activity and increasing its lifetime.

Meanwhile, scientists at Stanford University (Stanford, Calif.; stanford.edu) have discovered a method for growing vertical, single-walled carbon nanotubes on a large scale. The technique offers an improvement of plasma-enhanced CVD (PECVD), a standard fabrication method used by the semiconductor industry. According to the researchers, early attempts to use PECVD to grow single-walled carbon nanotubes have resulted in poor yields, because hydrogen radicals (which form when hydrocarbon feedstocks, such as methane decompose upon exposure to a catalyst-laden substrate) react with the carbon nanotubes, interfering with growth. The Stanford researchers found that by adding oxygen to the gas mixture, the oxygen scavenges the hydrogen radicals, enabling the carbon nanotubes to grow more successfully.

 In early 2006, CEVP Ltd. (Newhaven, U.K.; cevp.co.uk) introduced a fabrication tool — dubbed the NanoGrowth system — that allows carbon nanotubes to be grown at room temperatures, using a process developed by the University of Surrey’s Advanced Technology Institute (ATI; Surrey; ati.surrey.ac.uk). Using PECVD and vacuum conditions, the NanoGrowth system promotes the growth of nanotubes with closely controlled density, length and position, across areas up to three inches in dia., says the firm.

Arkema’ reactor can produce up to 10 m.t/year of multi-walled nanotubes using a continuous process Credit: Reprinted with permission from Arkema

According to CEVP, the ability to maintain the substrate at room temperature allows the nanotubes to be grown either horizontally or vertically, even on heat-sensitive materials, such as plastic or metallized paper. The tool is also being investigated for use in growing other nanomaterials, such as nanowires of doped silicon or tungsten oxide. Commercialization plans are underway.

Bayer Material Science AG is currently perfecting its new continuous process for producing multi-wall carbon nanotubes (dubbed “Baytubes” by the company). The process involves injecting a carbon-bearing gas (such as methane or ethane) into a high-temperature reactor that contains a newly developed, proprietary catalyst. Since late 2005, the company has been producing Baytubes at a rate of 2 kg/d of nanotubes (at a ratio of 150 grams of 99%-pure nanotubes for each gram of catalyst), at its pilot plant in Leverkusen. The company says it expects little difficulty in scaling up the capacity to several kilograms per hour, because the reactants can be fed to the reactor continuously, and the products can be removed as needed without having to shut off the system or alter the process temperature or pressure. Bayer Material Science claims that the price-per-kilogram of its Baytubes product is “significantly lower” than the prevailing $1,250/ton cost associated with most carbon nanotubes. According to the company, custom-made carbon nanotubes with different diameters, lengths and wall thicknesses can be produced for any application by selecting the corresponding catalyst.

Like Bayer, Arkema brought a pilot plant online at its Lacq Research Center in Aquitaine, France, in January 2006, to produce 10 metric tons/year (what the firm calls “semi-commercial quantities”) of multi-walled carbon nanotubes, using a patented catalysis process. These nanotubes will be suitable for use as additives in thermoplastics, coatings, batteries and fuel cells. Arkema has also formed a strategic partnership with Zyvex Corp. (Richardson, Tex.; zyvex.com), whose NanoSolve products already include nanotube-reinforced composites with enhanced thermal, electrical and mechanical properties.

 

< Previous Page     Next Page >