Production of Carbon Nanotubes and Carbon Nanoclusters by the JxB Arc-Jet Discharge Method

Since the discovery of methods for the mass production of single-walled carbon nanotubes (SWNTs) [1, 2], applications of SWNTs such as transistor devices, biosensing devices, dou‐ ble-layer-type capacitors, transparent electrode films, radio wave absorbents and material hardeners have been studied [3-5]. Large-scale production and improvement of purity of SWNTs by the electric-arc techniques have been developed [6, 7]. However, the efficient production of high-quality and defect-free SWNTs, and metal/semiconductor selected or di‐ ameter-controlled production of SWNTs have not yet been achieved. Therefore, basic study of the various methods of producing SWNTs is still important, by which new high-perform‐ ance routes to producing desired SWNTs are expected to be found.


Introduction
Since the discovery of methods for the mass production of single-walled carbon nanotubes (SWNTs) [1,2], applications of SWNTs such as transistor devices, biosensing devices, double-layer-type capacitors, transparent electrode films, radio wave absorbents and material hardeners have been studied [3][4][5].Large-scale production and improvement of purity of SWNTs by the electric-arc techniques have been developed [6,7].However, the efficient production of high-quality and defect-free SWNTs, and metal/semiconductor selected or diameter-controlled production of SWNTs have not yet been achieved.Therefore, basic study of the various methods of producing SWNTs is still important, by which new high-performance routes to producing desired SWNTs are expected to be found.
Here, the production of SWNTs and carbon nanoclusters by the arc discharge method utilizing a magnetic field, known as the JxB arc-jet discharge method, has been studied [8][9][10].Although the application of a steady-state magnetic field to arc discharge is not such a popular method, electromagnetic force can change the flow of hot gas in the arc region and thus control the production process of carbon clusters.To realize the large-scale production of carbon clusters by the arc discharge method, a revolver-injection-type JxB arc-jet producer was successfully developed by our group, by which the continuous mass production of SWNTs and other carbon clusters can be carried out.
As a result, the more efficient production of SWNTs and other carbon clusters compared with conventional arc discharge methods has been achieved.Here, the development of the JxB arc-jet discharge method and results obtained using the method are described.

Theoretical model of the JxB arc jet discharge method
By applying a steady-state weak magnetic field (B 0 = 1 -5 mT) perpendicular to the discharge current in the arc discharge, the Lorentz force (JxB force) causes the ejection of the arc plasma and surrounding gas in the JxB direction as shown in Fig. 1 [8,11].In the 1960s, this force in a pulsed discharge was actively studied in relation to the electric propulsion engine of rockets [12].Here, this effect is used to eject sublimated carbon atoms in a selected direction.By controlling the magnetic field, control of the hot gas including the carbon material is possible, and suitable conditions to do hot gas reactions for the production of SWNTs and other carbon clusters can be selected.This method can also reduce the influence of the electrode direction and chamber configuration.
When the discharge current density and applied magnetic field are 50 A/cm 2 and 5 mT, respectively, the Lorentz force causing acceleration of electrons and ions is 0.25 N/cm 3 .When the gas pressure and the gas temperature around the arc are 30 kPa and 5000 K, respectively, the mean free path and collision frequency of electrons are about 0.01 mm and 10 GHz, respectively.Because of this high collision frequency, electrons frequently collide with neutral gas atoms and accelerate them in the JxB direction, resulting in the ejection of hot gas from the arc region.The acceleration time is related to the electron lifetime in the plasma.

Heat flux
To investigate the JxB arc-jet discharge reaction, several types of arc reactors are used.Figure 2 shows a schematic of the reactor used to measure the heat flux of the arc plasma [11].The reactor is made of stainless steel (18 cm diameter, 20 cm height) and has a carbon anode (8.0 mm), a carbon cathode (15 mm), a viewing port and a movable calorimetric probe.The reactor is evacuated by a rotary pump to a pressure of less than 10 Pa and then closed.After introducing He gas with p(He) = 10 -80 kPa, discharge starts, where the discharge current is I d = 20 -80 A and voltage between the electrodes is V rod = 20 -35 V.At the front and back of the reactor, solenoid coils (20 cm inner diameter) are installed to produce a steady state magnetic field of B 0 = 0 -5 mT.
When a magnetic field is applied during the discharge, the shape of the arc flame dramatically changes, and a strong plasma flow in the JxB direction can be observed.Figures 3(a The upper direction is the JxB direction.By applying a magnetic field, the plasma and the hot gas are ejected in the vertically upward direction.The upward flow of carbon particles can sometimes be clearly observed.By developing a calorimeter [11] in which flowing water absorbs the heat flux, the local heat flux is measured and the results are shown in Figs. 4 (a) and (b) [11].By increasing the magnetic field, the heat flux is localized in the upper part of the arc plasma (FWHM value of about 50 mm).Above the arc plasma, the heat flux monotonically increases.To summarize these results, that the JxB arc jet is enhanced by increasing the applied magnetic field (B 0 = 0 -3.0 mT), the He pressure and the gap distance.However, under a stronger magnetic field of B 0 > 4 mT, the discharge tends to be extinguished easily by fluctuation in the discharge.

Relations among directions of the discharge current, magnetic field and gravity
In the case of gas arc discharge, gravity induces strong heat convection.Therefore, by changing the current direction relative to that of gravity, different production characteristics of carbon can be expected [13].Direction of the JxB force compared with that of gravity should also be an important parameter.To examine the relations among the directions of the discharge current, magnetic field and gravity for the production of fullerenes, five experimental configurations are prepared and a discharge experiment is carried out.The 5 configurations are shown in Fig. 6.Here, the carbon anode is 6.5 mm in diameter and the carbon cathode is 15 mm in diameter.The production rates of carbon soot W soot (g/h) as a function of B 0 for configurations (types A -E) are obtained and the results are shown in Fig. 7, where p(He) = 40 kPa, I d = 70 A and d G ~ 5 mm [13].Generally, the soot production rate increases steadily with the magnetic field.However, for type A, W soot is very low for B 0 =0 and it rapidly increases with increasing magnetic field.When B 0 = 4.0 mT, the differences in W soot are very small among the five configurations.The production rate of C 60 as a function of B 0 for configurations (types A -E) is obtained and the results are shown in Fig. 8, where the conditions are the same us those of Fig. 7. Similarly to in Fig. 7, the C 60 production rate generally increases with B 0 , except for type A. C 60 production rate of type A is very low at B 0 = 0.Moreover, for type E, the C 60 production rate does not increase monotonically with B 0 and the magnetic field does not have a positive effect on the production rate.
From these results, it can be concluded that the directions of the discharge current and magnetic field compared with that of gravity affect the production of carbon soot and fullerenes.The JxB force tends to reduce the effect of gravity when B 0 is sufficiently large.The type A and the Type E are less suitable for the production of fullerenes.

Production of SWNTs
The production of high-quality SWNTs is one of the most important targets in advancing nanomaterial development.The growth model of SWNTs in the arc-discharge reactors has not been confirmed.Several models show importance of catalyst particles in the hot gas, carbon density and catalyst temperature.[14,15] Here, the production of SWNTs is examined using the JxB arc-jet method, which could modify the growth reactions in the hot gas.In this case, a Ni/Y catalyst is included in the carbon material rods (6.0 x 6.0 mm, rectangular type, 4.2 W% of Ni and 0.9 W% of Y included), and p(He)= 60 kPa and I d = 50 A. The soot production rate as a function of applied magnetic field is shown in Fig. 9(a) [16,17].By increasing the magnetic field, the production rate monotonically increases.However, further increasing B 0 to above 3.5 mT makes the discharge unstable.In the arc production of SWNTs, effect of gas species is examined.Ar, Ne or N 2 gas is used instead of He gas, all of which degrade production of SWNTs.When amount of H 2 gas is included in He gas, the SWNT production rate considerably decreases, which is not consistent with the previous report [19].It is conjectured that He atom has high ionization potential, and it causes almost no chemical reactions and less emission loss.When Co or Fe particles are used as catalyst material instead of Ni/Y particles, the SWNT production rate considerably decreases.However, in case of of Co catalyst, a little amount of very long bundles of SWNTs is obtained, which is about 5 cm long.Improvement of the long-SWNT yield by this method is one of our study targets.
Usually SWNTs have poor dispersibility in water, which limits their potential applications.Therefore, the development of water-dispersible SWNTs is very important.Here we attempt to dissolve a SWNT sample in pure water.First, a small amount of SWNTs is placed in 20 mL of pure water, which is then mixed using a supersonic homogenizer (Sonics Co., VC-130, 25 W) for about 40 min.Then, a small amount of surfactant is added, which is one of biopolymers [20,21].And it is mixed by sonication again for about 40 min.As a result, the SWNTs are well dispersed in water, and the dispersion remains very stable for more than 1 month.Figure 12 shows a photograph of SWNTs dispersed in water after 50 times dilution.
The study of SWNTs dispersed in water is continuing with the aim of realizing biological applications.

Production of endohedral metallofullerenes
Using the above arc-jet discharge methods, endohedral metallofullerenes (such as Gd@C 82 and La@C 82 ) [22] and carbon nanocapsules are efficiently produced.Applications of these materials are expected.
By performing arc discharge using a Gd 2 O 3 -containing carbon rod (6.0 x 6.0 mm, rectangular), metallofullerenes are produced, where p(He)= 50 kPa and I d = 58 A. The production rate of soot is about 2.5 g/h. Figure 13(a) shows a typical TEM image of the sample obtained, in which gadolinium nanoparticles with a diameter on the order of 10 nm are covered with carbon atoms, resulting in the formation of carbon capsules with a size of 10 -50 nm, which are very stable in air.By refluxing the sample with toluene, fullerenes can be extracted from the soot.After 4 h of reflux, the liquid is filtered and a reddish liquid is obtained.A mass spectrum of this sample obtained using a laser-desorption time-of-flight mass spectrometer (LD-TOF-MS) (Bruker Co., Autoflex, + ion mode, 50 shots averaged) is shown in Fig. 13(b).We can confirm the existence of not only C 60 and higher fullerenes but also endohedral metallofullerenes Gd@C 82 .Although the peak intensities are not quantitative, the relative yield of Gd@C 82 compared with that of C 60 is several mol%.The Gd@C 82 is expected to be used as a contrast agent in MRI [23].

Production of magnetic nanoparticles
Using the JxB arc-jet discharge method, many types of metal-particle-encapsulated carbon nanoparticles [24,25] can be easily produced.As examples, iron-encapsulated carbon nanoparticles and cobalt-encapsulated carbon nanoparticles have been produced.Both are ferromagnetic nanoparticles with a size of 10 -100 nm, and are very stable.
Using iron-containing carbon rods (6.0 x 6.0 mm, rectangular), iron-encapsulated carbon nanoparticles are produced, where p(He)= 50 kPa and I d = 50 A. The soot production rate is about 0.43 g/h. Figure 14 (a) shows a photograph of the produced soot suspended by a magnet, demonstrating the good ferromagnetic property.A typical TEM image of the sample is shown in Fig. 14 (b).Iron particles with a size of 1 nm to 20 nm are covered with carbon atoms, resulting in the formation of carbon particles with a size of 10 -100 nm.These particles are very stable in air and inactive in hydrochloric acid.Cobalt-encapsulated carbon nanoparticles, which also have ferromagnetic properties, are produced by the arc-jet discharge method.They are dispersed in pure water with a small amount of surfactant (gelatin etc.) and mixed using a supersonic homogenizer (Sonic Co., VX-130) for 1 h.Finally, a black inklike liquid is obtained.The dispersion is homogeneous and stable, and most of the particles do not precipitate even after one month.These watersoluble magnetic nanoparticles potentially have many applications in the fields of liquid sealing, medical diagnostics and medical treatment [26]. Figure 15 shows a photograph of the stable iron-containing carbon nanoparticles dispersed in water.

Development of automatic JxB arc-jet producer
To produce SWNTs and carbon nanoclusters at a commercial scale by the JxB arc-jet discharge method, a revolver-injection-type arc jet producer (RIT-AJP) has been developed by collaboration with Daiavac Ltd.(Japan) [9].
A schematic and photograph of RIT-AJP are shown in Fig. 16.The left side of the machine is an arc discharge chamber, which consists of a vacuum vessel made of stainless steel 25 cm in diameter and 70 cm high that is uniformly cooled by water jackets.About 5 L of water is stored in the jackets and cooling water is slowly supplied to the jackets.In the central part of the chamber, a cathode electrode (30 mm in diameter), an anode electrode, an exhaust port, a viewing port and an electrode-cleaning hand are mounted.The top and bottom parts of the chamber are soot collectors, with an inner diameter of about 25 cm and a height of 24 cm, in which produced soot is deposited.Using these collectors, as much as 25 L of soot can be easily collected after a single operation.
The right side of the apparatus is a revolver-type carbon rod magazine.In the cylindrical metal vacuum vessel, which is 34 cm in diameter and 49 cm long, there is a rotatable cylindrical magazine, in which as many as 50 carbon rods of 6 -10 mm diameter and 300 mm length can be loaded.A schematic figure of the material-feeding mechanism of the magazine is shown in Fig. 17(a), and a photograph of a rotatable carbon rod magazine (for 50 carbon rods) is shown in Fig. 17(b).Under the vacuum chamber, there is a vacuum pump, an electrical controller and a microcomputer.Discharge power is supplied by an inverter-type DC power supply (Daihen Co., ARGO-300P).
The production sequence is as follows.First, up to 50 carbon rods are loaded in the magazine, and the chamber is evacuated by the vacuum pump.After evacuation to a pressure of less than 10 Pa, pure helium gas is introduced and the chamber is closed.Upon turning on the electrical controller, a metal striker pushes one of the carbon rods towards the cathode, and the discharge starts upon electrical contact.As the discharge conditions are determined by the discharge voltage and discharge current, the carbon rod is automatically moved until both parameters match the set values.Once the carbon rod is consumed, the cylindrical carbon magazine rotates 1/50 of a turn and the next carbon rod is inserted by the striker.A magnetic field can be applied by a block-type ferrite magnet located outside the chamber, by which a magnetic field of about 2 mT is applied horizontally to the discharge space.Carbon deposited on the cathode is removed by a cathode-cleaning hand.After the discharge, produced soot that has been deposited is carefully collected.
As an example of the continuous production of carbon clusters, fullerenes are produced.Using 134 carbon rods of 8 mm diameter, continuous JxB arc-jet discharge is carried out, where p(He)= 40 kPa, the discharge current is I d = 120 A, the voltage between electrodes is V rod ~ 33 V and the gap distance is d G ~ 5 mm.The insertion speed of the carbon rods is about 30 cm/h.After the discharge, carbon soot from three parts (the top collector, central chamber and bottom collector) is collected separately and their masses are measured.The amount of soot deposited on the top wall is considerably increased by applying the magnetic field, because the carbon molecules are blown upward onto the top wall.After sufficient mixing, the C 60 content in the soot is measured by a UV/visible spectrometer (Shimadzu Co. UV-1200).
At the top collector, the C 60 content is the highest and about 14 W% of C 60 is present, whereas, 4.2 W% is present on the center wall and 2.9 W% is present on the bottom wall.In total, about 105 g of soot containing about 7 g of C 60 is produced in 12 h.
The contents of higher fullerenes in the soot are measured using a high-pressure liquid chromatograph (HPLC) (Jasco Co., Gulliver Series, PU980) [27].The collection rates of C 60 , C 70 , C 76 , C 78 and C 84 for two different magnetic fields are shown in Fig. 18. White rectangules in the graphs show the measurement errors.By applying a magnetic field, the collection rates of these fullerenes considerably increase.

Summary
1.By applying a steady state magnetic field perpendicular to the discharge current, JxB arc-jet discharge is successfully produced.The flow of hot gas and the heat flux are concentrated in the JxB direction.

2.
Carbon atoms sublimated from the anode are continuously ejected from the arc plasma in the JxB direction.As a result of the JxB force, the effect of gravity (heat convection) can be reduced.

3.
By increasing the magnetic field, the production rate of carbon soot including SWNTs considerably increases, in which the quality of the SWNTs remains high.Water-soluble SWNTs can be obtained by additional processing.

5.
For the continuous and large-scale production of carbon clusters, a revolver-injectiontype arc-jet producer (RIT-AJP) has been developed.Using this machine, the automatic mass production of SWNTs and carbon clusters is realized.We are currently attempting to fabricate many new types of carbon clusters using this machine.
) and (b) respectively show side views of the arc flames for B 0 = 0 and B 0 = 2.0 mT, where p(He)= 40 kPa and I d = 80 A.

Figure 2 .
Figure 2. Schematic side view of the arc reactor with a calorimetric probe installed.

Figure 4 .
Figure 4. Radial profiles (a) and vertical profiles (b) of heat flux above the arc plasma for B 0 = 0, 1.0, 2.0 and 3.0 mT.p(He)= 40 kPa and I d = 60 A. The heat flux above the arc plasma as a function of He gas pressure is measured and shown in Fig. 5(a), where I d = 60 A, d G = 5 mm, and the distance from the arc center is 40 mm.The heat flux increases monotonically with the pressure, which is particularly in the case of B 0 = 2.0 mT. Figure 5(b) shows the heat flux above the arc plasma as a function of the gap distance between the two electrodes d G , where p= 40 kPa, I d = 60 A and z= 40 mm.By changing d G , the effect of the arc jet changes, which can be observed from the viewing port.The heat flux gradually increases with the gap distance, and this effect is greatly enhanced when B 0 = 2.0 mT.

Figure 5 .
Figure 5. (a) He pressure dependence of the heat flux, where I d = 60 A, d G = 5 mm and z= 40 mm from the arc center.(b) Gap distance dependence of the heat flux, where p= 40 kPa, I d = 60 A and z= 40 mm.

Figure 6 .
Figure 6.Schematic of five experimental configurations.The directions of the discharge current J and magnetic field B relative to that of gravity G 0 are changed.

Figure 7 .Figure 8 .
Figure 7. Production rate of carbon soot versus B 0 for the five configurations.p= 40 kPa, I d = 70 A and discharge time T d = 60 min

Figure 9 (
b) shows the pressure dependence of the soot production rate for B 0 = 0 and B 0 = 2.2 mT.As the pressure increases and the collisional effect of He gas increases, the JxB force clearly affects soot production.

Figure 9 .
Figure 9. Production rate of soot including SWNTs versus B 0 (a), where p(He)= 60 kPa and I d = 50 A, and pressure dependence (b), where I d = 50 A.

Figure 10 .Figure 11 .
Figure 10.Typical TEM image of SWNTs produced by the JxB arc-jet discharge method.

Figure 12 .
Figure 12.Photograph of SWNTs dispersed in pure water.

Figure 13 .
Figure 13.(a) TEM image of gadolinium-continuing carbon nanoparticles produced by the arc-jet discharge method.(b) LD-TOF-MS spectrum of a carbon sample extracted from soot using toluene.

Figure 14 .
Figure 14.(a) Photograph of iron-containing carbon nanoparticles suspended by a magnet.(b) TEM image of ironcontaining carbon nanoparticles.

Figure 15 .
Figure 15.Photograph of iron-containing carbon nanoparticles dispersed in water.

Figure 18 .
Figure 18.Collection rates of C 60 and C 70 (a), and C 76 , C 78 and C 84 (b) for two different magnetic fields.