**3. Production of carbon clusters by the** *J***x***B* **arc-jet discharge method**

#### **3.1. Heat flux**

To investigate the *J*x*B* arc-jet discharge reaction, several types of arc reactors are used. Fig‐ ure 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 <sup>d</sup>*= 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 <sup>0</sup>*= 0 - 5 mT.

**2. Theoretical model of the** *J***x***B* **arc jet discharge method**

4 Syntheses and Applications of Carbon Nanotubes and Their Composites

of rockets [12].

**Figure 1.** Schematic diagram of the *J*x*B* arc-jet discharge.

and chamber configuration.

**3.1. Heat flux**

By applying a steady-state weak magnetic field (*B <sup>0</sup>*= 1 – 5 mT) perpendicular to the dis‐ charge current in the arc discharge, the Lorentz force (*J*x*B* force) causes the ejection of the arc plasma and surrounding gas in the *J*x*B* 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

Here, this effect is used to eject sublimated carbon atoms in a selected direction. By control‐ ling 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

When the discharge current density and applied magnetic field are 50 A/cm2 and 5 mT, re‐

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, re‐ spectively. Because of this high collision frequency, electrons frequently collide with neutral gas atoms and accelerate them in the *J*x*B* direction, resulting in the ejection of hot gas from

. When

spectively, the Lorentz force causing acceleration of electrons and ions is 0.25 N/cm3

the arc region. The acceleration time is related to the electron lifetime in the plasma.

**3. Production of carbon clusters by the** *J***x***B* **arc-jet discharge method**

To investigate the *J*x*B* arc-jet discharge reaction, several types of arc reactors are used. Fig‐ ure 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 When a magnetic field is applied during the discharge, the shape of the arc flame dramati‐ cally changes, and a strong plasma flow in the *J*x*B* direction can be observed. Figures 3(a) and (b) respectively show side views of the arc flames for *B <sup>0</sup>*= 0 and *B <sup>0</sup>*= 2.0 mT, where *p*(He)= 40 kPa and *I <sup>d</sup>*= 80 A. The upper direction is the *J*x*B* 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 local‐ ized in the upper part of the arc plasma (FWHM value of about 50 mm). Above the arc plas‐ ma, the heat flux monotonically increases.

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

**Figure 3.** Arc flames for *B0*= 0 (a) and *B0*= 2.0 mT (b) (side views), where *p*(He)= 40 kPa and *Id*= 80 A.

**Figure 4.** Radial profiles (a) and vertical profiles (b) of heat flux above the arc plasma for *B0*= 0, 1.0, 2.0 and 3.0 mT. *p*(He)= 40 kPa and *Id*= 60 A.

(b)

(a) (b)

**3.2. Relations among directions of the discharge current, magnetic field and gravity**

(b) Gap distance dependence of the heat flux, where *p*= 40 kPa, *Id*= 60 A and *z*= 40 mm.

carbon cathode is 15 mm in diameter.

relative to that of gravity *G <sup>0</sup>* are changed.

**Figure 5.** (a) He pressure dependence of the heat flux, where *Id*= 60 A, *dG*= 5 mm and *z*= 40 mm from the arc center.

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In the case of gas arc discharge, gravity induces strong heat convection. Therefore, by chang‐ ing the current direction relative to that of gravity, different production characteristics of carbon can be expected [13]. Direction of the *J***x***B* force compared with that of gravity should also be an important parameter. To examine the relations among the directions of the dis‐ charge current, magnetic field and gravity for the production of fullerenes, five experimen‐ tal 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

**Figure 6.** Schematic of five experimental configurations. The directions of the discharge current *J* and magnetic field *B*

The heat flux above the arc plasma as a function of He gas pressure is measured and shown in Fig. 5(a), where *I <sup>d</sup>*= 60 A, *d <sup>G</sup>*= 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 <sup>0</sup>*= 2.0 mT. Figure 5(b) shows the heat flux above the arc plasma as a function of the gap dis‐ tance between the two electrodes *d <sup>G</sup>*, where *p*= 40 kPa, *I <sup>d</sup>*= 60 A and *z*= 40 mm. By changing *d <sup>G</sup>*, 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 <sup>0</sup>*= 2.0 mT.

To summarize these results, that the *J*x*B* arc jet is enhanced by increasing the applied mag‐ netic field (*B <sup>0</sup>*= 0 - 3.0 mT), the He pressure and the gap distance. However, under a stronger magnetic field of *B <sup>0</sup>*> 4 mT, the discharge tends to be extinguished easily by fluctuation in the discharge.

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

#### **3.2. Relations among directions of the discharge current, magnetic field and gravity**

**Figure 3.** Arc flames for *B0*= 0 (a) and *B0*= 2.0 mT (b) (side views), where *p*(He)= 40 kPa and *Id*= 80 A.

6 Syntheses and Applications of Carbon Nanotubes and Their Composites

(a) (b)

**Figure 4.** Radial profiles (a) and vertical profiles (b) of heat flux above the arc plasma for *B0*= 0, 1.0, 2.0 and 3.0 mT.

The heat flux above the arc plasma as a function of He gas pressure is measured and shown in Fig. 5(a), where *I <sup>d</sup>*= 60 A, *d <sup>G</sup>*= 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 <sup>0</sup>*= 2.0 mT. Figure 5(b) shows the heat flux above the arc plasma as a function of the gap dis‐ tance between the two electrodes *d <sup>G</sup>*, where *p*= 40 kPa, *I <sup>d</sup>*= 60 A and *z*= 40 mm. By changing *d <sup>G</sup>*, 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 <sup>0</sup>*=

To summarize these results, that the *J*x*B* arc jet is enhanced by increasing the applied mag‐ netic field (*B <sup>0</sup>*= 0 - 3.0 mT), the He pressure and the gap distance. However, under a stronger magnetic field of *B <sup>0</sup>*> 4 mT, the discharge tends to be extinguished easily by fluctuation in

*p*(He)= 40 kPa and *Id*= 60 A.

2.0 mT.

the discharge.

(b)

In the case of gas arc discharge, gravity induces strong heat convection. Therefore, by chang‐ ing the current direction relative to that of gravity, different production characteristics of carbon can be expected [13]. Direction of the *J***x***B* force compared with that of gravity should also be an important parameter. To examine the relations among the directions of the dis‐ charge current, magnetic field and gravity for the production of fullerenes, five experimen‐ tal 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.

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

The production rates of carbon soot *W soot* (g/h) as a function of *B <sup>0</sup>* for configurations (types A – E) are obtained and the results are shown in Fig. 7, where *p*(He) = 40 kPa, *I <sup>d</sup>*= 70 A and *d <sup>G</sup>*~ 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 <sup>0</sup>* =0 and it rapidly increases with increasing magnetic field. When *B <sup>0</sup>*= 4.0 mT, the differences in *W soot* are very small among the five configurations.

The production rate of C60 as a function of *B <sup>0</sup>* for configurations (types A – E) is ob‐ tained 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 C60 production rate generally increases with *B <sup>0</sup>*, except for type A. C60 production rate of type A is very low at *B <sup>0</sup>*= 0. Moreover, for type E, the C60 production rate does not increase monotonically with *B <sup>0</sup>* and the magnetic field does not

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From these results, it can be concluded that the directions of the discharge current and mag‐ netic field compared with that of gravity affect the production of carbon soot and fullerenes. The *J*x*B* force tends to reduce the effect of gravity when *B <sup>0</sup>* is sufficiently large. The type A

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, car‐ bon density and catalyst temperature. [14, 15] Here, the production of SWNTs is examined using the *J*x*B* 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 <sup>d</sup>*= 50 A. The soot produc‐ tion 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 <sup>0</sup>* to above 3.5 mT makes the discharge unstable. Figure 9(b) shows the pressure depend‐ ence of the soot production rate for *B <sup>0</sup>*= 0 and *B <sup>0</sup>*= 2.2 mT. As the pressure increases and the

collisional effect of He gas increases, the *J*x*B* force clearly affects soot production.

**Figure 9.** Production rate of soot including SWNTs versus *B0* (a), where *p*(He)= 60 kPa and *I <sup>d</sup>*= 50 A, and pressure de‐

have a positive effect on the production rate.

**3.3. Production of SWNTs**

pendence (b), where *I <sup>d</sup>* = 50 A.

and the Type E are less suitable for the production of fullerenes.

**Figure 7.** Production rate of carbon soot versus *B0* for the five configurations. *p*= 40 kPa, *I <sup>d</sup>*= 70 A and discharge time *T <sup>d</sup>*= 60 min

**Figure 8.** Production rate of C60 versus *B <sup>0</sup>* for the five configurations. *p*= 40 kPa, *I <sup>d</sup>*= 70 A and discharge time *T <sup>d</sup>*= 60 min.

The production rate of C60 as a function of *B <sup>0</sup>* for configurations (types A – E) is ob‐ tained 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 C60 production rate generally increases with *B <sup>0</sup>*, except for type A. C60 production rate of type A is very low at *B <sup>0</sup>*= 0. Moreover, for type E, the C60 production rate does not increase monotonically with *B <sup>0</sup>* 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 mag‐ netic field compared with that of gravity affect the production of carbon soot and fullerenes. The *J*x*B* force tends to reduce the effect of gravity when *B <sup>0</sup>* is sufficiently large. The type A and the Type E are less suitable for the production of fullerenes.

#### **3.3. Production of SWNTs**

The production rates of carbon soot *W soot* (g/h) as a function of *B <sup>0</sup>* for configurations (types A – E) are obtained and the results are shown in Fig. 7, where *p*(He) = 40 kPa, *I <sup>d</sup>*= 70 A and *d <sup>G</sup>*~ 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 <sup>0</sup>* =0 and it rapidly increases with increasing magnetic field.

**Figure 7.** Production rate of carbon soot versus *B0* for the five configurations. *p*= 40 kPa, *I <sup>d</sup>*= 70 A and discharge time *T*

**Figure 8.** Production rate of C60 versus *B <sup>0</sup>* for the five configurations. *p*= 40 kPa, *I <sup>d</sup>*= 70 A and discharge time *T <sup>d</sup>*= 60

*<sup>d</sup>*= 60 min

min.

When *B <sup>0</sup>*= 4.0 mT, the differences in *W soot* are very small among the five configurations.

8 Syntheses and Applications of Carbon Nanotubes and Their Composites

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, car‐ bon density and catalyst temperature. [14, 15] Here, the production of SWNTs is examined using the *J*x*B* 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 <sup>d</sup>*= 50 A. The soot produc‐ tion 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 <sup>0</sup>* to above 3.5 mT makes the discharge unstable. Figure 9(b) shows the pressure depend‐ ence of the soot production rate for *B <sup>0</sup>*= 0 and *B <sup>0</sup>*= 2.2 mT. As the pressure increases and the collisional effect of He gas increases, the *J*x*B* force clearly affects soot production.

**Figure 9.** Production rate of soot including SWNTs versus *B0* (a), where *p*(He)= 60 kPa and *I <sup>d</sup>*= 50 A, and pressure de‐ pendence (b), where *I <sup>d</sup>* = 50 A.

Using this *J*x*B* arc-jet method, a large amount of SWNTs is produced from carbon rods in‐ cluding a Ni/Y catalyst. Figure 10 shows a typical TEM image of the produced soot, in which many bundles of SWNTs are included. There are also carbon nanoparticles and catalyst nanoparticles in the soot, which should be removed during the purification of the SWNT products. The quality of the products is measured by a Raman spectrometer (Jasco Co., NR-1800. An Ar ion laser of λ= 488.0 nm is used.), and the results are shown in Figs. 11 (a) and 11(b) for *B <sup>0</sup>*= 0 and *B <sup>0</sup>*= 3.2 mT. In both cases, there are very small D(disorder) band peaks and large G (graphite) band peaks, from which we can estimate the content and quali‐ ty of SWNTs in the produced carbon soot. These figures show that the *J*x*B* arc discharge does not degrade the quality of the SWNTs. From the signals of the radial breathing mode in Fig. 11(a), we can evaluate the SWNT diameters [18]. The major diameter is 1.40 nm, and SWNTs with a diameter of 1.26 nm also exist in the case of *B <sup>0</sup>*= 3.2 mT. SWNTs with diame‐ ters of 1.70 nm, 1.16 nm and 1.0 nm also exist in the case of *B <sup>0</sup>*= 0.

**Figure 11.** Raman spectra of the produced samples for magnetic fields of *B0*= 3.2 mT and 0 T. *p*(He)= 60 kPa and *Id*= 50

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In the arc production of SWNTs, effect of gas species is examined. Ar, Ne or N2 gas is used instead of He gas, all of which degrade production of SWNTs. When amount of H2 gas is included in He gas, the SWNT production rate considerably decreases, which is not consis‐ tent with the previous report [19]. It is conjectured that He atom has high ionization poten‐ tial, 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 bun‐ dles of SWNTs is obtained, which is about 5 cm long. Improvement of the long-SWNT yield

A. λ = 488.0 nm.

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

by this method is one of our study targets.

**Figure 10.** Typical TEM image of SWNTs produced by the *J*x*B* arc-jet discharge method.

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**Figure 11.** Raman spectra of the produced samples for magnetic fields of *B0*= 3.2 mT and 0 T. *p*(He)= 60 kPa and *Id*= 50 A. λ = 488.0 nm.

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

Using this *J*x*B* arc-jet method, a large amount of SWNTs is produced from carbon rods in‐ cluding a Ni/Y catalyst. Figure 10 shows a typical TEM image of the produced soot, in which many bundles of SWNTs are included. There are also carbon nanoparticles and catalyst nanoparticles in the soot, which should be removed during the purification of the SWNT products. The quality of the products is measured by a Raman spectrometer (Jasco Co., NR-1800. An Ar ion laser of λ= 488.0 nm is used.), and the results are shown in Figs. 11 (a) and 11(b) for *B <sup>0</sup>*= 0 and *B <sup>0</sup>*= 3.2 mT. In both cases, there are very small D(disorder) band peaks and large G (graphite) band peaks, from which we can estimate the content and quali‐ ty of SWNTs in the produced carbon soot. These figures show that the *J*x*B* arc discharge does not degrade the quality of the SWNTs. From the signals of the radial breathing mode in Fig. 11(a), we can evaluate the SWNT diameters [18]. The major diameter is 1.40 nm, and SWNTs with a diameter of 1.26 nm also exist in the case of *B <sup>0</sup>*= 3.2 mT. SWNTs with diame‐

ters of 1.70 nm, 1.16 nm and 1.0 nm also exist in the case of *B <sup>0</sup>*= 0.

10 Syntheses and Applications of Carbon Nanotubes and Their Composites

**Figure 10.** Typical TEM image of SWNTs produced by the *J*x*B* arc-jet discharge method.

In the arc production of SWNTs, effect of gas species is examined. Ar, Ne or N2 gas is used instead of He gas, all of which degrade production of SWNTs. When amount of H2 gas is included in He gas, the SWNT production rate considerably decreases, which is not consis‐ tent with the previous report [19]. It is conjectured that He atom has high ionization poten‐ tial, 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 bun‐ dles 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.

**3.5. Production of magnetic nanoparticles**

containing carbon nanoparticles.

Using the *J*x*B* arc-jet discharge method, many types of metal-particle-encapsulated carbon nanoparticles [24, 25] can be easily produced. As examples, iron-encapsulated carbon nano‐ particles and cobalt-encapsulated carbon nanoparticles have been produced. Both are ferro‐

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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 <sup>d</sup>*= 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 mag‐ net, 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 parti‐

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

magnetic nanoparticles with a size of 10 - 100 nm, and are very stable.

cles are very stable in air and inactive in hydrochloric acid.

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

#### **3.4. Production of endohedral metallofullerenes**

Using the above arc-jet discharge methods, endohedral metallofullerenes (such as Gd@C82 and La@C82) [22] and carbon nanocapsules are efficiently produced. Applications of these materials are expected.

By performing arc discharge using a Gd2O3-containing carbon rod (6.0 x 6.0 mm, rectangular), metallofullerenes are produced, where *p*(He)= 50 kPa and *I <sup>d</sup>*= 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 gado‐ linium nanoparticles with a diameter on the order of 10 nm are covered with carbon atoms, re‐ sulting 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 re‐ flux, the liquid is filtered and a reddish liquid is obtained. A mass spectrum of this sample ob‐ tained 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 C60 and higher fullerenes but also endohedral metallofullerenes Gd@C82. Although the peak intensities are not quantitative, the relative yield of Gd@C82 compared with that of C60 is several mol%. The Gd@C82 is expected to be used as a contrast agent in MRI [23].

**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.

#### **3.5. Production of magnetic nanoparticles**

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

Using the above arc-jet discharge methods, endohedral metallofullerenes (such as Gd@C82 and La@C82) [22] and carbon nanocapsules are efficiently produced. Applications of these

By performing arc discharge using a Gd2O3-containing carbon rod (6.0 x 6.0 mm, rectangular), metallofullerenes are produced, where *p*(He)= 50 kPa and *I <sup>d</sup>*= 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 gado‐ linium nanoparticles with a diameter on the order of 10 nm are covered with carbon atoms, re‐ sulting 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 re‐ flux, the liquid is filtered and a reddish liquid is obtained. A mass spectrum of this sample ob‐ tained 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 C60 and higher fullerenes but also endohedral metallofullerenes Gd@C82. Although the peak intensities are not quantitative, the relative yield of Gd@C82 compared with that of C60 is

several mol%. The Gd@C82 is expected to be used as a contrast agent in MRI [23].

(a) (b)

(b) LD-TOF-MS spectrum of a carbon sample extracted from soot using toluene.

**Figure 13.** (a) TEM image of gadolinium-continuing carbon nanoparticles produced by the arc-jet discharge method.

applications.

materials are expected.

**3.4. Production of endohedral metallofullerenes**

12 Syntheses and Applications of Carbon Nanotubes and Their Composites

Using the *J*x*B* arc-jet discharge method, many types of metal-particle-encapsulated carbon nanoparticles [24, 25] can be easily produced. As examples, iron-encapsulated carbon nano‐ particles and cobalt-encapsulated carbon nanoparticles have been produced. Both are ferro‐ magnetic 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 <sup>d</sup>*= 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 mag‐ net, 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 parti‐ cles are very stable in air and inactive in hydrochloric acid.

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

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

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.

deposited on the cathode is removed by a cathode-cleaning hand. After the discharge, pro‐

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As an example of the continuous production of carbon clusters, fullerenes are produced. Us‐ ing 134 carbon rods of 8 mm diameter, continuous *J*x*B* arc-jet discharge is carried out, where *p*(He)= 40 kPa, the discharge current is *I <sup>d</sup>*= 120 A, the voltage between electrodes is *V rod*~ 33 V and the gap distance is *d <sup>G</sup>*~ 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, be‐ cause the carbon molecules are blown upward onto the top wall. After sufficient mixing, the C60 content in the soot is measured by a UV/visible spectrometer (Shimadzu Co. UV-1200). At the top collector, the C60 content is the highest and about 14 W% of C60 is present, where‐ as, 4.2 W% is present on the center wall and 2.9 W% is present on the bottom wall. In total,

The contents of higher fullerenes in the soot are measured using a high-pressure liquid chro‐ matograph (HPLC) (Jasco Co., Gulliver Series, PU980) [27]. The collection rates of C60, C70, C76, C78 and C84 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

**Figure 18.** Collection rates of C60 and C70 (a), and C76, C78 and C84 (b) for two different magnetic fields.

**1.** By applying a steady state magnetic field perpendicular to the discharge current, *J*x*B* arc-jet discharge is successfully produced. The flow of hot gas and the heat flux are con‐

**2.** Carbon atoms sublimated from the anode are continuously ejected from the arc plasma in the *J*x*B* direction. As a result of the *J*x*B* force, the effect of gravity (heat convection)

duced soot that has been deposited is carefully collected.

about 105 g of soot containing about 7 g of C60 is produced in 12 h.

of these fullerenes considerably increase.

centrated in the *J*x*B* direction.

can be reduced.

**5. Summary**
