Researchers Introduce Ionic Bombardment Method to Grow Carbon Nanotubes on a Catalytic Base

Researchers at Iran’s Plasma Physics Research Centre of Islamic Azad University have developed an improved method to synthesize and grow carbon nanotubes. The new ion energy method can produce the nanotubes that have applications in manufacturing electron emitters and solid devices with high thermal conductivity.

Since the 1991 discovery of carbon nanotubes, researchers have  attempted to optimize carbon nanotube production to lower their cost and increase their application in different industries.

Majid Mojtahedzadeh Larijani, one of the researchers, undertook this study with the aim of “Introducing a novel method for carbon nanotubes' growth especially on substrate beds with a catalytic base by means of ionic bombardment."

"The bed used in this study was steel. First in the process, sub-layer underwent surface treatment by argon ionic bombardment of the surface at different ion energies and doses. Then, carbon nanotube growth on bombarded samples using hydrogen and steel gases was accomplished by TCVD method and finally samples were analyzed by TEM, SEM, AFM, and Raman spectroscopy devices," Mojtahedzadeh told the Iran Nanotechnology Initiative Council (INIC) news service  in an interview.

The results showed that ion energy and dose in which sub-layer surface turns into fine grains are very appropriate for the growth of dense and adhesive carbon nanotubes. “These nanotubes could be applied for manufacturing electron emitters and solid devices with high thermal conductivity in electronics industry,” according to Mojtahedzadeh.

The details of the present study are available at Surface & Coatings Technology, volume 203, pages 2510 to 2513, 2009.

IBM Reveals Carbon Nanotube Based Field Effect Transistor Manufacturing Process

IBM has earned U. S. Patent 7,628,974 for its method to control single wall carbon   nanotube (SWNT) diameter growth during manufacturing by either chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD).

Carbon nanotube based field effect transistors (CNTFETs) show great promise for device applications. Recently CNTFETs with excellent electrical characteristics comparable to state-of-the-art silicon MOSFETs have been demonstrated.  The electrical characteristics of CNTFETs however depends largely on the band-gap of the single wall carbon nanotube (SWNT) forming the channel of the transistor. Since the band-gap of SWNTs has a strong dependence on the diameter, accurate control of the diameter is essential to the success of any device technology based on carbon nanotubes.

A crucial difficulty in obtaining individual SWNTs by CVD is control of nanometric catalyst particle size at growth temperatures of 700-1000.degree. C. It has been theorized that the particle size of the growth catalyst used can define the diameter of as grown carbon nanotubes. This hypothesis has been supported by the observation that catalytic particles at the ends of CVD grown SWNT have sizes commensurate with the nanotube diameters Catalysts typically employed are transition metals, notably Fe, Mo, Co, NI, Ti, Cr, Ru, W, Mn, Re, Rh, Pd, V or alloys thereof. However, the synthesis of small catalyst particles with a narrow diameter distribution is complicated and difficult to control.

IBM controls the diameter of CVD or PECVD grown CNTs based on the control of the residence time of the gases in the reactor such as by controlling the pressure, or the gas flow rates, or a combination of both, independent of catalyst particle size, according to inventors Alfred Grill, Deborah Neumayer and Dinkar Singh.

The gas residence time is a measure of the average time of the gas in the reactor. Thus, if the flow is constant and the pressure increases, the residence time increases, and if the pressure is constant and the flow increases the residence time decreases. The inventors unexpectedly discovered that by varying the residence they can influence the nanotube diameter. If the residence time is too high, only pyrolytic carbon is deposited and if the residence time is too low, nothing is deposited. The residence time is typically about 1 minute to about 20 minutes and more typically about 1 to about 10 minutes. The residence time is typically determined by controlling the pressure, flow or both the pressure and flow in the reactor. By varying the residence time (e.g growth pressure and/or flow rates) of the CNT precursor gases in the CVD or PECVD reactor, nanotube diameter can be varied from about 0.2 nanometers to several nanometers to 100 nanometers.

FIGS. 1A-1B show scanning electron microscope images of CNTs grown at atmospheric pressure using identical catalysts, but different gas flows. (FIG. 1A shows that higher gas flow results in relatively thin tubes, while FIG. 1B shows that lower gas flows in result in relatively thick tubes).

 

A further aspect of the patent relates to fabricating a SWNT or array of SWNTs having well defined diameters and origins by the above disclosed processes wherein the SWNTs form the channel of a field effect transistor. A field-effect transistor having source and drain regions and a channel located between the source and drain regions is obtained by a process comprising: a) depositing a thin film of catalyst; b) lithographically patterning the thin film of catalyst to provide catalyst only in the source or drain region or both; c) removing unwanted catalyst from the channel region defined by the lithographic pattern; and d) growing nanotube with a well controlled diameter ranging from about 0.2 nanometers to about 100 nanometers by controlling the residence time of gases in the reactor used for the growing of the nanotube and wherein the channel region extends from the source region to the drain region.

Arkema Combines Ball Milling with Vapor Deposition to Continuously Produce Highly Pure Carbon Nanotubes

Arkema France (Colombes, FR) reveals in U.S. Patent 7,622,059 a method for synthesis of carbon nanotubes of the highest carbon purity by combining the process of vapor phase chemical deposition. with ball milling.  The nanotubes produced can be used to advantage in all known applications of carbon nanotubes. Inventors Serge Bordere, Patrice Gaillard and Carole Baddour say the process produces 15 grams of carbon nanotubes for each gram of catalyst used.

FIG. 1 is a scanning electron micrograph of the CNT agglomerates obtained according to the prior manufacturing methods.

FIG. 2 is a scanning electron micrograph of the milled CNTs obtained from step b) according to Arkema's invention; and by comparing FIG. 2 with FIG. 1, it may be clearly seen that the process according to the invention results in a very small number of CNT agglomerates with a diameter greater than 200 .mu.m. The final product thus formed is therefore more easily dispersed within a material, in particular a polymer.

FIG. 3 illustrates a milling device according to Arkema's invention which may be installed either within an actual synthesis reactor (6) for synthesizing CNTs by CVD (in situ milling) or in an external loop allowing possible recycling of all or part of the CNTs milled within the reactor (ex situ milling).

The milling device shown in FIG. 3 comprises a system of high-velocity gas jets generated through injectors (2) which entrain the CNT powder onto one or more targets (5) held by a support (4), that has to be subjected to the bombardment of the CNT agglomerates thus reducing the particle size by impact. The fluidization may be carried out by just these injectors (2) and/or in combination with a gas stream diffused by the distributor (3) around these injectors (2). The dimensions of the milling system and the flow rates of incoming gas (1) and (2a) used are suitable for obtaining good fluidization and the desired particle size, depending on the hardness and the density of the catalyst substrate. The distributor (3) is designed to support the catalyst, which is in powder form, at the time T0 of the synthesis.

The form of the milling device will advantageously be adapted according to the materials used and/or the behavior of the fluidized bed. The process may be carried out semi-continuously or in batch mode, but preferably continuously. At least part of the entangled CNT/catalyst network resulting from step a) may be extracted from the synthesis reactor to a milling device operating continuously, semi-continuously or in batch mode, then injected (step c)) either into the same synthesis reactor of step a) or into a second CNT synthesis reactor by fluidized-bed CVD (finishing reactor).

It is also possible to carry out the milling (step b) in the synthesis reactor of step a), provided with milling means as shown by the device in FIG. 3, which avoids having to extract the powder from the reactor and therefore reduces the head losses and the risk of powder fly-off.

Step b) is carried out inside the CNT synthesis reactor (6) by injecting some of the reactive gas or gases and/or an additional gas through injection nozzles (2) distributed over the surface of the distributor (3), the vertical gas jet or jets (1) entraining the particles toward a target (5). The particles consist of CNT agglomerates and/or catalyst. The target (5) is in the form of a cone, made of stainless steel, preventing deposition of particles at the top of the target (5).

This milling makes catalytic CNT growth sites accessible, thereby making it possible, during step c), to grow further CNTs on these now accessible sites, but also on the CNT agglomerates formed during step a), the size and/or the number of which have been reduced thanks to the milling. Growth of the CNTs during step a) and step c) may take place using identical gas sources (which is the case during a process involving in situ milling) or sources that differ both in terms of nature and flow rate (which is especially the case during a process involving ex situ milling). The CNTs synthesized during introduction of synthesis gas and fresh catalyst, during step c), may be subjected to a further milling step d) under the conditions described above. The CNTs thus obtained after step c) or d) are finally recovered.

These CNTs have improved properties, especially their dispersion in a material, in particular a polymer. It is thus possible to introduce a higher quantity of CNTs compared with the prior art, with better distribution and/or homogeneity, thereby improving the final properties of the material containing the CNTs.

These CNTs can be used in all applications in which CNTs are employed, especially in fields in which their electrical properties are desired (depending on the temperature and their structure, they may be conductors, semiconductors or insulators), and/or in fields in which their mechanical properties are desired, for example for the reinforcement of composites (the CNTs are one hundred times stronger and six times lighter than steel) and in electromechanical applications (they can elongate or contract by charge injection). For example, mention may be made of the use of CNTs in macromolecular compositions intended for example for the packaging of electronic components, for the manufacture of fuel lines (gasoline or diesel), antistatic coatings, in thermistors, electrodes, especially in the energy sector, for supercapacitors, etc