A New Type of Arc Plasma Reactor with Twelve-Phase Alternating Current Discharge for Synthesis of Carbon Nanotubes

Transcription

1 A New Type of Arc Plasa Reactor with Twelve-Phase Alternating Current Discharge for Synthesis of Carbon Nanotubes Tsugio Matsuura, Keiji Taniguchi and Takayuki Watanabe Industrial Technology Center of Fukui Prefecture, Kawai-Washizuka, Fukui, 9-2, Japan Professor Eeritus, Fukui Universit 2-2 Nakano, Fukui, 98-88, Japan Tokyo Institute of Technolog 4259 Nagatsuta, Midori-ku, Yokohaa, 22-82, Japan Abstract A new type of arc plasa reactor with twelve-phase alternating current (AC) discharge for synthesis of carbon nanotubes (CNTs) is proposed. A couple of six discharge electrodes by which have utually electrical connection between the to enlarge the high teperature regions in the reactor are arranged to three diensional locations. A new ethod of CNTs fabrication by this reactor which accoplishes to enlarge the suitable growth region in high-purity and at high yield was developed. Keywords Arc plasa, Twelve-phase AC discharge, Carbon nanotubes. Introduction A large nuber of applications for arc plasa have been used for the welding and cutting of etals up to this tie. Arc plasa as the energy source with effective heat efficiency has been also tried to open new technological field for better global environent with related new industrial arkets, including plasa cheical reaction. In general, the power source for generating arc plasa is accoplished by using a direct current (DC) power supply. This is the larger the capacity the higher the cost of apparatus for inverting fro AC to DC. On the other hand, the single-phase and the three-phase AC power supply have already been proposed for the sae application fields. Since these systes have a characteristic of interittent discharge, the high power arc plasa systes generated by AC power supply have not been fully developed. In order to iprove this defect, a ultiple-phase AC power supply has been developed. To obtain a ore effective arc plasa generator by expanding this concept, we have developed a twelve-phase AC power supply []. The ost iportant advantage of this syste is that there are large nuber of discharging paths aong electrodes in coparison with the case of the single-phase and the three-phase systes. Therefore, soe of the plasas always exist for continuing soothly the discharging action. It sees as if this syste was driven by the DC power supply instead of actual coercial AC power supply ( Hz) [2]. Since CNTs discovered in 99 at first tie [], any researchers have been provided its unique physical and cheical properties, and applications to nano-scale devices. Several ethods for synthesis of CNTs such as DC arc-discharge [, 4], laser ablation and theral cheical vapor deposition (CVD) have been presented. The DC arc-discharge ethod can synthesize CNTs in highest quality than the other ethods entioned above. However its yields are uch lower. Up to date, CVD ethod is the ainstrea of ass fabrication of CNTs. In order to avoid the disadvantage of the DC arc-discharge ethod, the twelve-phase AC arc-discharge ethod is proposed here. The purpose of this paper is to describe the techniques for obtaining its ethod and to show the experiental results for synthesizing CNTs by using this new type of arc plasa reactor. 2. Twelve-phase AC power supply for arc plasa reactor 2. Three-phase to twelve-phase conversion Figure shows the electrical circuit diagra of the transforers for converting fro the three-phase AC to the twelve-phase AC and the scheatic connection diagra of the plasa reactor. The input of the three-phase power supply is connected to 2V coercial power lines. The priary coils of transforers are divided into two parts: one is the connection and the other is the Y connection. The twelve-phase power supply can be realized by the cobination of these circuits. Fro the Y connection, the voltage coponents x y z ' x and ' y V of the six-phase AC are defined by the following equations: ' z n Vi = V sin( ω t π ),( i = x, n =,2,4) ()

2 n Vi ' = V sin( ω t π ),( i = x, n =,,5) (2) where V and f are the axiu value of the sinusoidal wave and the frequency of the AC ( Hz), respectively. Fro the coercial power lines R S T Vzδ Vz Vyδ Vx Vxδ Vy Vyδ V y Vz Vxδ Vzδ Vx V x Vzδ Vyδ Vxδ V z Vy Arc plasa Electrodes Fig. Electrical circuit diagra and scheatic teporary connection diagra of the plasa reactor Fro the connection, the voltage coponent V x δ, V y δ, V z δ, V xδ ', V yδ ', and V zδ ' are defined by the following equations: n Viδ = V sin( ωt π ),( i = x, n =,5,9) () n Viδ ' = V sin( ωt π ),( i = x, n = 7,,5) (4) 2.2 Advantages of the twelve-phase AC discharger The configuration of twelve electrodes are syetrically arranged to bring out the advantages of the twelve-phase discharge. The geoetrical arrangeent of the tips of electrodes is indicated in Fig. 2. In the case of the distance between electrodes No.5 and No.7 is unit the ratio of the distance aong other electrodes have the values as shown in the sae figure. Fig. 2 The geoetrical arrangeent of the tip of electrodes In general, the voltage applied between each electrodes and neutral point of the secondary coil of the transforer for the twelve-phase AC power supply can be described by following equation. ( i ) Vi = V sin( ω t π ),( i =...2) (5)

3 Fro this equation, we can obtain the difference of the voltage between the electrode No. and the others. These are explained as follows: V2 V = 2sin( π ) V cos( ωt + π ) () 2 2 V V = V cos( ω t + π ) (7) V4 V = 2V cos( ω t + π ) (8) 4 V5 V = V cos( ω t + π ) (9) 5 V V = 2cos( π ) V cos( ωt + π ) () 2 2 V7 V = 2V cos( ω t + π ) () 2 Here we should notice that the ratio of the voltage aplitudes in equations ()-() is equal to the ratio of the geoetrical distances shown in Fig. 2. As the phase shift is kept constant every oent by the cobination of transforers, the values of voltage/distance aong each electrode are also constant every oent. 2. Power supply for the twelve-phase AC discharge To realize the power supply for the generation of twelve-phase AC discharge, twelve pieces of single-phase AC arc welding transforer (DAIHEN B-25) were used. These welders are the conventional ones and have a dropping characteristic. The input voltage, the axiu non-loading output voltage, the typical loading voltage, the wattage and the range of output currents are 2 V, 8 V, 2.5 V, 2.4 kw and fro 75 A to 25 A, respectively. Two pairs of six transforers are connected to the coercial AC line (three-phase 2 V, Hz) with the figures of and Y connections. Twelve output lines fro the transforers are directly connected to the corresponding electrodes of the reactor. Fig. Setup of the twelve-phase AC power supply Fig. 4 Photograph of the twelve-phase AC arc discharge Fig. shows the setup of the twelve-phase AC power supply. The picture of the twelve-phase AC arc discharge is shown in Fig. 4. Typical voltage and current wavefors of the single-phase AC arc discharge and the twelve-phase ones are shown in Fig. 5 and Fig., respectively. Current[A] 2 5 Voltage Curren Tie[sec] Voltage[V] Current[A] 5 Voltage Current Tie[sec] Voltage[V] Power[W] Tie[sec] Fig. 5 Voltage and current wavefors Fig. Voltage and current wavefors Fig. 7 Total power wavefor in case of single -phase AC in case of twelve-phase AC in case of twelve-phase AC

4 Fig. indicates the twelve-phase AC arc discharges which do not have an interittent discharge. The wavefor of the total power in the case of twelve-phase is also shown in Fig. 7. The ripple of the total power was.%.. Synthesis of carbon nanotubes. A new type of arc plasa reactor A new type of arc plasa reactor with twelve-phase AC discharge for ass fabrication of CNTs is shown in Fig. 8. The CNTs were produced by a twelve-phase AC arc discharge aong carbon electrodes in heliu gas. The electrodes of graphite rod 2 in diaeter, 5 in length having the purity of % are configured horizontally in the reactor as shown in Fig.4. Since the electrodes are consued by evaporation, the arc gaps aong electrodes are kept constant by the otor-drive systes of which are adjusted by the voltage aong the. Fig. 8 A new type of arc plasa reactor for ass fabrication of CNTs.2 Experiental setup Figure 9 shows the scheatic diagra of the experiental setup inside of the reactor. The substrate ade of the stainless steel plate (sus4, 5 in width, 2 in length and in thickness) suspended fro the ceiling of the reactor. The tip of the substrate was apart above fro the center of twelve electrodes configuration. The diaeter of the plasa surrounded by the tips of twelve electrodes is approxiately. After purging in the chaber, heliu gas (purity 99.99%) is introduced and sets at torr. The discharging voltage and its current are 2-45 V and 7- A, respectively. The carbon electrodes contain nickels as the catalyst for CNTs synthesis at wt. %. Substrate (sus plate) 2 Electrode plasa Fig. 9 Scheatic diagra of the experiental setup inside the reactor Fig. Substrate for CNT deposition (original and after operation). Experiental results and discussion The CNTs synthesis is strongly related to the teperature of condensation and growth of carbon

5 olecules in plasa [5, ], therefore the deposition was carefully analyzed to investigate the effect of deposition teperature. During the discharge for one hour, CNTs were prepared fro the evaporation of the electrodes. A lot of soot-like deposit was obtained on the surface of the substrate as well as on the inner surface of the reactor. Figure shows the original substrate and after operation one. The surface of reained substrate was divided equally into nine blocks. The deposit on the substrate is carefully gathered fro nine sapling blocks. The products were exained using scanning electron icroscopy (SEM), transission electron icroscopy (TEM) and Raan spectroscopy with a 54.5n Ar + laser and were also characterized by using an energy dispersive X-ray analysis (EDX) (Block ) (Block 2) (Block ) (Block 4) (Block 5) (Block ) (Block 7) (Block 8) (Block 9) Fig. SEM iages of the CNTs fro gathering nine blocks on the substrate The SEM iages of crude saples at each block of the substrate are shown in Fig.. The clusters of carbon particles which are considered to be aorphous carbons are uch ore observed at the iages of blocks, 2, 7, 8, and 9 in coparison with the iages of blocks -. On the contrar CNTs with uch less aorphous carbons are observed at the iages of blocks -. The surface teperature at the tip of block of the substrate is estiated at 4 degree Celsius of its elting point. The surface teperature of the botto of block 9 was easured at 8 degree Celsius by using the therocouples. Fro these evaluations, the teperature distribution on the substrate can be roughly estiated with the linear assuption of the teperature distribution. The optiu teperature for CNTs synthesis is approxiately in the range fro to 25 degree Celsius. This result was congruous well with previous report [7]. Fro the orphological observation of blocks 2 and 8, even though their locations are not suitable for fabrication of CNTs, the length of it in block 8 is shorter than that of block 2. The ulti-walled CNTs of 2 n-4 n in diaeter shown in Fig. 2 were produced not only on the substrate but also on the wall of reactor. Nickel particle filled with the CNTs shown in Fig. was identified by the energy dispersive X-ray spectroetry.

6 Fig. 2 TEM iage of the ulti wall CNTs Fig. CNTs filled with nickel particles Raan spectru shown in Fig. 4 contains the G-band (58 c - ) and the D-band ( c - ). In general, the G-band shows the CNT oriented peak and the D-band shows the aorphous carbon or the structure defects of CNTs oriented peak. The synthesis of CNTs is not clearly found out on the block shown in Fig.4 (a). Fig.4 (c) shows CNTs and the aorphous carbon are both synthesized on the block 8. On the contrar the strong peak of G-band caused by synthesis of CNTs was appeared on the block shown in Fig.4 (b). These results were congruous very uch with the orphological observation of SEM iages entioned above D-Band G-Band 4 5 D-Band G-Band 4 2 D-Band G-Band Intensity (a.u.) 5 4 Intesity (a.u.) Intensity (a.u.) Wavenuber (/c) Wavenuber (/c) Wavenuber (/c) (a) block (b) block (c) block 8 Fig. 4 Raan spectru of the CNTs on the substrate 4. Conclusions We have described the techniques for obtaining the new type of arc plasa reactor with twelve-phase AC discharger. This configuration can provide the attractive advantages of the arc plasa in superior stability and long continuity. A new ethod of CNTs fabrication by using this reactor which accoplishes to enlarge the suitable growth region in high-purity and at high yield was developed. The optiu teperature for CNTs synthesis is approxiately in the range fro to 25 degree Celsius. The ulti-walled CNTs of 2 n-4 n in diaeter were produced 5. References [] T.Matsuura, O.Tago, H.Tshujino, K.Taniguchi, Proceedings of International Conference MODELLING, SIMULATION & IDENTIFICATION, C-22, 8 (994). [2] T.Matsuura, K.Taniguchi, H.Makida, The Japan Society of Waste Manageent Experts, 8, (997). [] S.Iijia, Nature, 54, 5 (99). [4] T.W.Ebbesen and P.M. Ajayan, Nature, 58, 22 (992). [5] M.Yudasaka, T.Ichihashi, S.Iijia, J. Phys. Che., B2, 2 (998). [] Y.Saito, New Diaond and Frontier Carbon Technolog 9, (999). [7] T.Sugai, H.Oote, S.Bandow, N.Tanaka, and H.Shinohara, J. Che. Phys., 2, (2).