Electrical Discharge Machining of Carbon Nanofiber for Uniform Field Emission

Electrical Discharge Machining of Carbon Nanofiber for Uniform Field Emission B. H. Kim1, J. G. Ok2, Y. H. Kim2, C. N. Chu2(2) 1 2

School of Mechanical Engineering, Andong National University, Andong, Korea

School of Mechanical and Aerospace Engineering, Seoul National University, Seoul, Korea

Abstract Electrical discharging machining (EDM) of carbon nano fiber (CNF) is introduced. CNF has been investigated as an emitter for field emission display (FED). For the uniform field emission, uneven CNFs need to be machined to uniform height. For the planarization of CNFs, EDM was used. EDM was conducted in air, unlike conventional EDM, to prevent the contamination of CNFs. For the uniformity of the machined surface, machining characteristics were investigated with applying different capacitance and voltage. With this method, the uniformity of field emission of CNFs was improved. Micro machining of CNFs was also studied. Keywords: Electrical discharge machining (EDM), Nano technology, Carbon

10 μm. As a tool electrode, a tungsten carbide (WC) rod with 1 mm in diameter was used. A smaller tool electrode was fabricated by wire electrical discharge grinding (WEDG) [8].

1 INTRODUCTION Carbon nanotubes (CNTs) have been investigated extensively since discovered in 1991 [1]. CNTs are known as a good emitter for field emission display due to their low operating voltage and high aspect ratio which leads to a larger emitting current than other field emission devices. Recently, carbon nanofiber (CNF) film has been intensively researched as a good candidate for cold cathode material [2-7]. Since the field emission uniformity determines the quality of devices, there have been many studies to improve the uniformity of field emission. In particular, controlled growth, mechanical polishing, chemical treatment, plasma treatment, and laser treatment have been investigated. For uniform field emission, the fabrication of uniform CNFs is required. Since EDM (electrical discharging machining) is able to machine any conductive materials, it can be applied to level uneven CNFs. However, since this process has not been tested before, no concrete data is available about its use. In this paper, therefore, EDM of CNFs is investigated.

(a)

2 EXPERIMENTAL SYSTEM Figure 1 shows the experimental system for the EDM of CNFs. A tilting stage is attached on an X-Y-Z stage and a CNF substrate is fixed on a tilting stage. Since dielectric fluid which is generally used in EDM can cause the damage and contamination to CNFs, EDM was carried out in air. Since minimum discharge energy is required to machine nano-sized CNFs, an RC circuit was used. During the machining process, a feedback signal was not used. Unlike in general micro EDM, it was not useful to determine the machining status and to control the tool feed. When a short circuit between the tool electrode and the CNFs was detected, CNFs were already damaged. The CNFs were machined layer-by-layer and the depth of each layer was 10 μm. The feed rate was set to 200 μm/s, which was slow enough to prevent a short between the CNFs and the tool electrode. To prevent a short, it was important to maintain a constant machining volume. Before machining, the substrate was positioned parallel to the tool feed direction as much as possible by using the tilting stage. Over the machining area with 10 mm x 10 mm, the position deviation in the z-direction was less than Annals of the CIRP Vol. 56/1/2007

(b) Figure 1: (a) schematics of EDM process for CNF planarization, (b) photo of the EDM system. Usually, in EDM of metal, the relative position between the tool and work piece is determined by electrical contact. However, CNFs are easily machined before the contact is detected. Therefore, as the tool electrode was approached to the CNF substrate, the position where a spark was observed was determined as the reference position of the CNFs. Tool rotation was not useful to machine CNFs. During the machining, machined CNFs were attached on the tool electrode. When the tool -233-

doi:10.1016/j.cirp.2007.05.055

few hundred μm. At the edge of the tool bottom, however, debris was not found and very small craters generated by electrical discharge were observed (Figure 6(d)). However, tool wear was negligible in throughout the whole experiment. Figure 7 shows the surfaces which were machined with different capacitance. When 10,000 pF was used, the size of the craters was about 50 - 100 μm in diameter, which is much larger than that in micro EDM of metal. However, when stray capacitance was used, the amount of debris was reduced and uniform surface was obtained. Therefore, stray capacitance is most suitable for CNFs with uniform surface. 100

electrode rotates, the attached CNFs scribe the CNF on the substrate. CNFs were fabricated in the area of 2 cm x 2 cm. The CNFs were grown using a thermal chemical vapor deposition (CVD) method at 600°C for 20 min from NH3/C2H2 gases with Ni catalysts [9].

Machining gap (Pm)

3 MACHINING OF CNFS In EDM of CNFs, sparks were observed between a tool electrode and a CNF substrate, The CNFs were decomposed and Figure 2 shows the CNF before and after EDM. Although the machined end of the CNFs does not have a regular shape, it appears different from the original CNF.

80 60 electrode diameter 500 Pm 1000 Pm

40 20 0

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Figure 3: The change of machining gap according to applied voltage and electrode diameter.

(a) CNF (b) EDMed CNF Figure 2: Carbon nano fibers before and after EDM.

(a) 40 V

The effect of voltage were investigated. A voltage in the range of 20 V – 150 V was tested. Tool electrodes with 500 μm and 1000 μm in diameter and stray capacitance were used. When 150 V was used, very large sparks occurred and the machining gap was also large. The machining gap according to the applied voltage is shown in Figure 3. As voltage increases, the size of craters on the surface enlarges as well due to the large discharging energy as shown in Figure 4. Since the electric field between the tool electrode and CNFs also increase, more CNFs are attracted to the tool electrode and cut by sparks. However, at 40 V, the scratched surface was observed, as shown in Figure 4(a) and at 20 V, the CNFs were not machined. The effect of capacitance was investigated. Figure 5 shows the change of the machining gap according to capacitance. As expected, it increases as capacitance amplifies. The increase in the tool size does not affect the machining gap significantly. When the CNFs were machined with high capacitance such as 10,000 pF, much CNF debris was generated and subsequently attached to the tool electrode as shown in Figure 6(a). On the bottom of the tool, short CNF debris which were shorter than 10 μm in length were attached uniformly as shown in Figure 6(b). They were found even when stray capacitance was used. On the side of the tool electrode, as the machining progresses, CNF debris link to each other and were entangled. As shown in Figure 6(c), they amassed to a

(b) 80 V

(c) 100 V (d) 150 V Figure 4: Machined surface according to voltage. 120 Machining gap (Pm)

110 100 90 electrode diameter 500 Pm 1000 Pm

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(100 V applied)

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2000 4000 6000 8000 10000 Capacitance (pF)

Figure 5: The change of machining gap according to capacitance.

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side. After machining a few grooves with 5 mm long and 20 μm depth from where a spark started, the tool wear was too small to measure accurately.

Figure 6: CNFs attached to a tool electrode.

Figure 8: Micro groove machined by EDM (Ø 25 μm tool electrode, 60 V, stray capacitance) 4

PLANERIZATION OF CNFS FOR UNIFORM FIELD EMISSION Figure 9 shows examples of CNFs before and after EDM. Original CNFs were grown irregular and entangled. Before the EDM process, their height was from 50 to 80 μm. After EDM, the CNFs were machined to a uniform height of 20 μm, which shows that their uniformity was improved. In order to investigate the improvement of the emission uniformity, The CNF surface with an area of 10 mm x 10 mm was machined by feeding a tool electrode line by line. The cylindrical tungsten carbide (WC) tool with 1 mm in diameter was used. Each toolpath has 0.5 mm overlap. Stray capacitance and 100 V were used. After finishing a tool path, debris on the tool electrode was automatically cleaned with a dust-free wiper. Since a high temperature is generated in EDM, the property of CNFs might be changed by the heat during the machining. A Raman spectra analysis was conducted to ensure that the crystalline properties of CNFs were not changed [10]. Figure 10 shows the Raman spectra of the CNF substrate before and after EDM. The ratio of ID to IG was not changed, which shows that the property of CNFs was not altered. It proves that EDM is able to cut CNFs without thermal damage on the overall CNFs substrate. Figure 11 shows the emission image of raw CNFs and leveled CNFs [10]. As shown in Figure 12(a), in the raw CNFs substrate, the distribution of emission was poor. At a low electric field, only tall emitters produced electrons because the electric field was concentrated to them. Although more emitters started to work at higher electric field, good emission distribution was not obtained (Figure 12(b)). In contrast to raw CNFs, the leveled substrate showed the uniform emission as shown in Figure 12(c) and (d). Emission spots were evenly distributed even at a low electric field. Since the height of CNFs reduces after EDM process, the intensity of emission is smaller than that of raw CNFs. However, the uniformity in field emission is more important to many applications. By EDM process, the quality of field emission can be improved.

(a) Stray capacitance

(b) 10,000 pF Figure 7: Machined surface according to capacitance. If a significant amount of debris is accumulated on the tool, it could cause a short circuit and prevent the uniformity of the machined surface. It is considered that all debris does not attach to the tool and some might fall on the surface of the CNF substrate. Since the debris on the CNF substrate can deteriorate the uniformity of field emission, it is important to minimize debris. Therefore, in large area machining, not only the use of small capacitance, but also a process which removes the debris on the tool electrode frequently so that more debris can be attached on the tool is required. EDM can be used to remove CNFs or CNTs in selective area or machine them in a pattern. With Ø 25 μm tool electrode, micro grooves were machined on CNF substrate. Since the diameter of the tool electrode is diminutive and the machining volume is relatively low, the machining was stable even at a small voltage, 60 V. Figure 8 shows the groove which was machined at 60 V. The width is 40 μm at the bottom and 80 μm on the upper

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5 CONCLUSIONS The machining of CNFs by EDM was investigated to improve the uniformity of field emission. Dielectric fluid was not used to prevent contamination. In dry EDM, high speed gas is used instead of dielectric fluid [11]. However, it can damage CNFs. The use of high speed gas is considered to remove debris which is attached on a tool electrode for future work. To obtain uniform machining surface, machining conditions should be carefully determined. From the experiments, stray capacitance and 80 – 100 V are suitable for the uniform machining surface. Since debris can deteriorate the field emission property, it is important to minimize them. The machining condition which produces small discharging energy is useful to reduce debris. EDM is able to machine selective areas in CNFs without damage or contamination. Aside from the planarization of CNFs, the applications such as the micro patterning of CNFs are expected [5, 7].

(b)

6 REFERENCES [1] Iijima, S., 1991, Helical Microtubules of Graphitic Carbon, Nature, 354/6348:56-58. [2] de Heer, W. A., Châtelain, A., Ugarte, D., 1995, A Carbon Nanotube Field-Emission Electron Source, Science, 270:1179-1180. [3] Chen, Y., Patel, S., Ye, Y., Shaw, D. T., Guo, L., 1998, Field Emission from Aligned High-density Graphitic Nanofibers, Applied Physics Letters, 73/15:2119-2121. [4] Zhu, W., Bower, C., Zhou, O., 1999, Large Current Density from Carbon Nanotube Field Emitters, Applied Physics Letters, 75/6:873-875. [5] Sohn, J. I., Choi, S. Y., Cho, K. I., Nam, K. S., 2001, Patterned Selective Growth of Carbon Nanotubes and Large Field Emission from Vertically Wellaligned Carbon Nanotube Field Emitter Arrays, Applied Physics Letters, 78/7:901-903. [6] Sveningsson, M., Morjan, R. E., Nerushev, O. A., Campbell, E. E. B., Malsch, D., Schaefer, J. A., 2004, Highly Efficient Electron Field Emission from Decorated Multiwalled Carbon Nanotube Films, Applied Physics Letters, 85/19:4487-4489. [7] Chang-Jian, S. K., Ho, J. R., Cheng, J. W., Sung, C. K., 2006, Fabrication of Carbon Nanotube Field Emission Cathodes in Patterns by a Laser Transfer Method, Nanotechnology, 17/5:1184-1187. [8] Masuzawa, T., Fujino, M., Kobayashi, K., Suzuki, T., 1985, Wire Electro-Discharge Grinding for MicroMachining, Annals of the CIRP, 34/1:431-434. [9] Sung, W. Y., Kim, W. J., Yeon, S. C., Lee, S. M., Lee, H. Y., Kim, Y. H., 2006, Field Emission Characteristics of Carbon Nanocoils Grown on Copper Micro-tips, Proceedings of the 19th International Vacuum Nanoelectronics Conference (IVNC) & 50th International Field Emission Symposium (IFES), 101-102. [10] Ok, J. G., Kim, B. H., Sung, W. Y., Chu, C. N., Kim, Y. H., 2006, Uniformity Enhancement of Carbon Nanofiber Emitters via Electrical Discharge Machining, Applied Physics Letters, accepted. [11] Kunieda, M., Lauwers, B., Rajurkar, K. P., Schumacher, B. M., 2005, Advancing EDM through Fundamental Insight into the Process, Annals of the CIRP, 54/2:599-622.

(c) Figure 9: SEM images of CNF films: (a) before and (b), (c) after the EDM process.

Intensity (arb. unit)

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after EDM ID/IG = 0.819

800 before EDM ID/IG = 0.817

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Raman shift (cm ) Figure 10: Raman spectra of CNF substrate before and after EDM [10].

Figure 11: Emission Images at a low electric field range: (a), (b) before and (c), (d) after the EDM treatment. (a) and (c), (b) and (d) were taken at the same electric field of 2.3 V/μm, 2.7 V/μm, respectively [10].

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