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19.2: Spray-coating process for preparing CNT-FED cathode Chih-Che Kuo
[email protected]
Yu-An Li, Shih-Hsun Chen, Chun-Yen Hsiao, Shie-Heng Lee, Guo-Hua Chen, Jui-Ting Hsu, Li-Jen Hsu, Jin-Shou Fang, Kevin Cheng, and Te-Fong Chan R&D Department, TECO Nanotech Co., Ltd., 1560 Chung-Shan road Sec. 1, Kuang-In, Taoyuan Hsien, Taiwan
Yuan-Hsiang Yu Department of Electronic Engineering, Lan-Yan Institute of Technology, I-Lan 261, Taiwan bulk.
Abstract A novel process for fabricating field emission display (FED) devices with spray-coating method is proposed. Spray-coating method can be applied to both cathode and anode fabrication process. It is a fast and low cost process that is easily scaling up for large size panel. Mixing MWNT, which is synthesized by arcdischarge, some conductive materials, and binding with solvent makes CNT mixture for spraying. Cathode is coated with CNT film by the spray process. After sintering process, it is evaluated by electron emission. This diode type CNT-FED with spraycoating process possessed good electron emission properties.
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2.2. Solution preparation and Spray-coating process After the process of ball-milling and purifying, the abovementioned MWNT becomes powder which can be used as raw material of emitter. Spray-coating solution is then prepared by mixing MWNT powder, volatile lacquer solution, some binding and conductive media. Spray the mixture onto a silver cathode electrode of glass substrate through a mask. After a heat treatment, a cathode with a visual size of 4” diagonal, coating with CNT film can be obtained.
Introduction
Although CRT is dominating the current display market, the need for FPD has become a trend due to the mature technology of LCD and PDP. At present, LCD and PDP have been on a mass production basis, but some problems such as view-angle, high power-consumption, complicated fabrication process and costly equipments still need to be improved. The technology of field emission display (FED) is similar to that of CRT, and therefore has owned more advantages like high color saturation, high brightness, and high stability. It has been well recognized as one of FPD technologies in past few years[1][2][3]. The research of applying electron emitter overcomes various shortcomings of Spindt type FED. The structure of carbon nanotube (CNT[4]) owns the characters such as extreme aspect ratios, large field enhancement. Therefore, the carbon nanotube becomes stable electron emitter[5][6] and is very suitable to substitute microtip of Spindt type FED. Currently there have been several methods, such as screen-printing[7], spin coating[8], and thermal-CVD[9], being able to generate CNT film. However, how to create a simple and low cost fabrication process to arrange CNT on large area substrate is a topic to be investigated.
Optimum synthesis
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2. Experimental fabrication processes 2.1. CNT Synthesis CNT is produced by arc-discharge method. A stable and efficient carbon nanotube synthesis process is generated according to a set of optima conditions. High purity MWNTs can be obtained from the core part of “cathode deposition”(the “carbon bulk”). Fig. 1(a) shows the comparison of the results of applying the optima conditions. The volume of deposited carbon bulks increases obviously under optima conditions, by approximately 10 times than before. Fig. 1 (b) presents a high yield and high quality MWNT powder obtaining from the core part of deposited carbon
(b) Fig. 1 (a) The comparison of the results of CNT synthesis without and with optimized arc discharge. (b) The SEM of CNT powder.
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On the other hand, while applying screen-printing process, CNTs in a high viscosity printing paste is much difficult to reach a uniform dispersion, which leads the aggregation of CNTs on film surface. Due to the aggregation, the diameter of CNTs on screenprinting surface is larger and thus much easier to see. Either the pits or the aggregation of CNTs has a negative effect on the uniformity of FED.
Fig. 4 shows the electron emission image in a vacuum chamber with the vacuum level at 10-6 torr. The applying voltage between cathode and anode is 300 V. 12.0 0 -1
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Moreover, CNTs are less easy to be seen on screen-printing surface than on spray-coating surface. The difficulty of finding CNTs on spray-coating surface is possibly due to the smaller diameter of CNTs. Because spray-coating process makes much better uniform dispersion of CNTs, CNTs on spray-coating surface are with smaller diameter and thus hard to find.
Fig. 3 shows the field emission characteristic. The turn-on electric filed is 2.73 V/µm at current density 5.1 mA/cm2. The driving electric field 10 mA/cm2 also can be achieved at electric filed 4.8 V/um.
Ln(J/E )
Fig. 2 shows the surface images of (a) spray-coating film and (b) screen-printing film. There are two points to arise by observing these two images. First, both of these films have many pits on the surface. They are caused by the evaporation of organic materials during sintering process. The surface with pits is not smooth, which makes non-uniform distribution of electric filed.
with the anode coated with blue phosphor, forms a diode type CNT-FED device. The device is then used to evaluate electron emission and uniformity in a vacuum chamber.
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3. Results and discussion 3.1. CNTs film morphology
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Fig. 3 The field emission characteristic of spray-coating cathode. The inset shows the Fowler-Nordheim plot of emission currents. The turn-on electric filed is 2.73 V/µm at current density 5.1 mA/cm2. The driving electric field 10 mA/cm2 also can be achieved at electric filed 4.8 V/µm.
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Fig. 4 The emission image of diode type 4-inch diagonal FED forms with blue phosphor in vacuum chamber. The vacuum level during image observation is about 10-6 torr. The gap between anode and cathode is 60 µm. The applying voltage is 300 V.
3.3. Comparison of paste (b) Fig. 2 Surface morphology of (a) spray-coating film and (b) screen-printing film.
3.2. Field emission characteristics Combining the cathode and anode with a 60-um gap in between,
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Table 1 shows the different properties between spray-coating process and current screen-printing process. The spray-coating process is easier for paste preparation. The process also makes better dispersion of CNTs in the paste with less limitation of CNT content. In addition, the thickness can be controlled precisely and the shape is formed quickly under low-temperature baking in spray-coating process. The spray-coating process has been demonstrated that it is a simple and low cost way to fabricate CNT-FED panel that provides a good performance in field
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[2] W.B. Choi, et al., SID’00 Digest (2000), p324.
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[3] Y.Y. Chang, et al., ASID’00 Digest (2000), p371.
Conclusions
It makes that possible to fabricate CNT-FED in a simple and low cost way by using the spray-coating process and the optimum synthesis process of carbon nanotubes. The spray-coating process makes uniform dispersion of CNTs, which further leads efficient effect on field emission.
[4] S. Iijima, Nature 354 (1991), p56-58.
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[7] C.C. Lee, et al., SID 01 Digest (2001), p316.
Acknowledgements
This work is supported by TECO Nanotech Co.,Ltd. and Technology Research and Development Collaboration Program in Taiwan, R.O.C.
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References
[5] A.G. Rinzler, et al., Science 269 (1995),p1550. [6] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995), p1179-1180. [8] K. Yu, et al., Chemical Physics Letters 373 (2003), p109-114. [9] Y.Y. Chang, IVMC 2002, EA 152.
[1] S. Uemura, et al., SID’98 Digest (1998), p1052. Table 1 Comparison of properties of spray-coating and screen-printing processes Properties
Spray-coating
Screen-printing
Paste preparation
Easy
Complicated
Paste viscosity
10-50 cps
60000-80000 cps
CNT content
Less limitation
Limited by halftone
CNTs dispersion in paste
Easy to disperse after stirring and easy to aggregate after stay still
Hard to disperse after stirring and hard to aggregate after stay still
Thickness control
Easy to control thickness precisely
Complicated to control thickness
Coating effect
Shaped quickly and only lowtemperature baking needed
Shaped slowly and hightemperature baking needed
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