While researchers have known about carbon nanotubes (CNT) since 1991, only a limited number of CNT-based products have found their way into niche markets, such as additives to increase the strength and toughness of a composite, conductive polymeric films for touch screen displays and carbon nanotube tips for the atomic force microscope (AFM), a type of high resolution scanned-proximity probe microscope.

The major barriers for full commercialization of CNT materials in the consumer mass market are their high cost and limited availability. Considering the fact that the price of high-grade single-wall carbon nanotubes (SWNTs) is between $350,000 to $500,000 per kilogram, the possibility of emerging CNT-based consumer products such as high-capacity rechargeable lithium batteries or portable fuel cells used in mobile devices, becomes unlikely in the near future. 

In the consumer electronics sector, where the CNT manufacturing process is fully compatible with current nanoelectronic processes, CNT-based products such as high resolution low-cost flat panel displays and power semiconductor chips for automobiles are already in the final stage of development for the mass market.

The development and commercialization of CNT-based products in other sectors including biomedical devices, household appliances and sporting goods, will follow as the major CNT producers begin to ramp up their capacity and utilize advanced manufacturing technology to reduce costs. It should be pointed out that a slowdown in the macro economy due to geopolitical uncertainties and high energy costs could dampen the progress of CNT-based product development and commercialization.

Catalyzed Chemical Vapor Deposition (CCVD) and High-Pressure CO conversion (HiPCO) Processes - CVD is a widely used method in the fabrication of microelectronic devices for depositing various types of thin films including oxide and low-k dielectrics. In a catalyzed CVD process for SWNT and multi-walled nanotube (MWNT) manufacturing, reactant gases such as acetylene (C2H2) and methane (CH4), diluted with H2 or ammonia (NH3), are used as a carbon source.

The gas mixture, which enters the CVD reactor at room temperature, is heated up to temperatures ~ 700ºC using a resistance heater before reacting with a suitable metal catalyst such as nickel, iron or cobalt to form carbon nanotubes. The catalyst can be deposited directly on the surface of silicon, silicon dioxide (SiO2) or quartz substrate surface as a thin film or in the form of catalytic nanocrystalline. 

To increase the catalytic reaction and thus the process throughput, plasma from various sources including radio frequency (RF) and microwave (MW) is used. The plasma enhanced CVD (PECVD) process can be operated at temperatures as low as 330ºC and thus is more compatible with commercial-scale manufacturing, compared to the thermal CVD process.

Large-scale production of high-purity (90% or higher) SWNTs can be done using the "High-Pressure CO conversion" (HiPCO) method. In this method, a gas mix of CO with a small amount of iron pentacabonyl Fe(CO)5 catalytic particles, is continuously flowed through high-pressure heated carbon monoxide (CO) gas in a quartz flow tube reactor, operated at ~ 1200ºC. Because CO gas is used as the primary gas source, CNTs produced by this method contain no hydrocarbon and are considerably cleaner than those from other catalyzed CVD processes. Houston, TX-based Carbon Nanotechnologies, Inc., one of the leading CNT material manufacturers, uses this technique to produce high-graded CNTs and fluorinated SWNTs for supercapacitor electrodes and biomedical sensors.

Although the thermal CVD, PECVD and HiPCO methods provide feasible commercial-scale CNT manufacturing solutions with high throughput and low-cost of ownership, the key technical challenge is the control of metallic contamination originating from the catalyst as well as oxygen, hydrogen and hydrocarbon from the reactant gases. A high level of impurities can have a significant impact on CNT quality and cause additional manufacturing costs for purification.

In nanoelectronics, the CVD method is considered to be the "process of choice" because selective CNTs can be grown on substrate using  standard microlithography processes. In addition, the CNT material can be doped with elements including nitrogen, boron or potassium to alter electronic properties which enable production of new types of active switching devices such as field-effect transistors and nanotube emitters. Munich, Germany-based Infineon Technologies AG (NYSE ADR: IFX) is one of the first semiconductor manufacturers to pioneer the low-temperature CCVD process to replace the conventional metal via process, the vertical interconnects between two metal layers in the chip, with ultra high current density CNT materials.

Large equipment manufacturers including Applied Materials (NASDAQ:AMAT) and Novellus Systems (NASDAQ:NVLS) do not currently offer catalyzed CVD solutions for CNT manufacturing. In the United States, small production and research thermal CVD and PECVD reactors are available from specialized equipment companies such as a privately held Carpinteria, CA-based First Nano, Inc. and Sunnyvale, CA-based SEOCAL Inc.

Direct Spinning from Chemical Vapor Deposition - Promising novel technology for commercial-scale manufacturing of continuous CNT fiber has been developed by a group of research scientists at University of Cambridge, Ya-Li Li, et al Science 304, 276–278 (2004).  In the direct spinning CVD technique, ethanol (C2H5OH) is used as the carbon source. A gas mix of ethanol, ferrocene (Fe(C5H5)2) and thiophene (C4H4S) vapors along with a hydrogen carrier gas are injected into the top section of a vertical CVD reactor, operated in the temperature range of 1050°C to 1200°C. 

In the reactor hot zone, the catalyzed reaction causes a formation of an aerogel-like substance, an ultra low-density solid which is composed of 99.8% air. The SWNTs are then mechanically removed from the catalyzed reaction zone by continuous wind-up. According to the research scientists, unlimited length of high-grade SWNTs can be spun using this technique.