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Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling
Materials Chemistry and Physics 110 (2008) 363–369
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling Nono Darsono a , Parlindungan Yonathan a , Dang-Hyok Yoon a,∗ , Jaemyung Kim b , Yoonjin Kim b a b
School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea Electric Material Development Team, Corporate R&D Center, Samsung SDI, Yong-in 449-577, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 November 2007 Received in revised form 15 February 2008 Accepted 17 February 2008 Keywords: Nanostructuress Field emission Raman spectroscopy and scattering
a b s t r a c t Multi-walled carbon nanotubes (MWNTs) were high-energy milled for 2 h in texanol after a polycarboxylic acid polymeric dispersant had been added in order to enhance the dispersion. The degree of MWNT dispersion was significantly enhanced by high-energy milling compared to the intact sample, which increased the density of surface-exposed MWNTs with a screen-printed paste. However, the emission properties of high-energy milled MWNTs did not show such a high emission current density relative to their increased surface-exposed density. Further investigation using Raman spectroscopy and X-ray diffraction evidenced the milling-induced MWNT damage, which explained the relatively lower emission current density of high-energy milled MWNTs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been intensively studied for applications in electronic fields. CNTs have promising application as the electron emitters in field emission display (FED) due to their high aspect ratio and low work function [2]. CNTs as electron emitters have been fabricated by the direct growth [3], electrophoretic deposition [4], and screen printing [5] methods, of which the latter has been successfully adopted to produce a large-scale panel display in a relatively simple process [6]. Fig. 1 shows a schematic illustration of an FED structure with the CNT emitters on the cathode substrate and red–green–blue (RGB) phosphor dots on the anode side. Single-walled carbon nanotubes (SWNTs) generally show a shorter lifetime, in spite of their higher degree of structural perfection and emission current density, than multi-walled carbon nanotubes (MWNTs) do [7]. The long-term stability which ensures a uniform electron emission for more than 30,000 h is the most important challenge for the commercialization of CNT-FED [8]. Therefore, current efforts are mainly being focused on MWNTs with a diameter of several nanometers for actual applications [9]. Since MWNTs tend to form aggregates due to their high van der Waals interactions, their dispersion is a prerequisite to form uniform emission sites for FED application. MWNT agglomerates are usually broken up and dispersed into smaller units through mechanical milling for initial dispersion. The conventional ball
∗ Corresponding author. Tel.: +82 53 810 2561; fax: +82 53 810 4628. E-mail address: [email protected] (D.-H. Yoon). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.016
mills and attrition mills that have been used for this purpose require more than 100 h of milling to get an average length of 200–300 nm [10,11]. However, modern high-energy mills are known to be very efficient in milling due to their high-speed rotor rotating at up to several thousand rpm and use of small grinding media with diameters of 0.05–0.8 mm [12]. As high-energy mills have recently been introduced, however, there are few reports thus far regarding its efficiency in MWNTs dispersion. In the subsequent stabilization process, dispersion is achieved by electrostatic and steric mechanisms, both of which prevent the close approach of MWNTs by using electrostatic repulsive force and hindrance via the adsorption of polymeric molecules on the particle surface, respectively [13]. The paste for screen printing requires numerous ingredients such as CNT, binder resin, solvent, dispersant, metal filler, photo initiator, monomer and glass frit, all of which should be optimized in order to obtain a favorable emission property. Design of experiments (DOE) is a powerful tool for establishing relationships among many experimental factors and output responses for such a complex system. It utilizes a structured DOE table in which the input factors are varied in a planned manner in order to identify efficiently the factors that most influence the results as well as those that do not [14]. Based on the above background, MWNTs were milled and dispersed by high-energy milling in this study. Moreover, a full factorial DOE was performed in order to verify the effect of each factor and to determine the optimum MWNTs paste condition for FED application using three experimental input factors: the treatment of high-energy milling for MWNTs, Ag filler type, and polymer/MWNTs ratio. The emission properties such as turn-on field, current density and field enhancement factor for eight kinds
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Fig. 1. Schematic illustration of CNT-FED.
of paste were analyzed as output responses. Statistical analysis software, MINITAB, was used to generate the DOE table and to analyze the results. The effect of each input factor on the emission properties is described through main effect plots, microstructure, X-ray diffraction (XRD) and Raman spectra.
2. Experimental 2.1. Starting materials MWNTs were grown by catalytic chemical vapor deposition (CVD) method (CMP-310F, Iljin Nanotech, Korea) with a mean diameter of 3–5 nm, a length
Fig. 2. Comparison of the morphologies of as-received and 2-h, high-energy milled MWNTs in texanol: (a) and (b) optical microscope, (c) and (d) FESEM, and (e) and (f) HRTEM.
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365
Table 1 Eight types of MWNT pastes for experimental design No
High-energy milling
Silver filler (wt.%)
Polymer/CNT ratio
1 2 3 4 5 6 7 8
No Yes No Yes No Yes No Yes
Nano 10 Nano 10 Micro 20 Micro 20 Nano 10 Nano 10 Micro 20 Micro 20
4/1 4/1 4/1 4/1 6/1 6/1 6/1 6/1
of 10–20 m, and a specific surface area of 797 m2 g−1 . In order to synthesize a photosensitive organic vehicle, acrylic binder resin (Elvacite 2669 with an acid number of 124, Lucite International) was dissolved in texanol (C12 H24 O3 : 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) and mixed with photosensitive monomers, photo initiators and epoxy silane. Polycarboxylic acid polymer (BYK-P 104, BYK Chemie, USA) at a ratio of 100 wt.% with respect to the MWNT content was used as a dispersant. Silver powders with an average particle size of 80 and 400 nm were used as metal filler.
Fig. 3. Evolution of the average agglomerate size of MWNTs and the slurry viscosity as a function of high-energy milling time.
2.2. High-energy milling and characterization of MWNTs To 350 g of texanol were added 1.4 g of dispersant and 7 g of MWNTs (i.e., 20 wt.% of dispersant with respect to the MWNT content) to form a slurry of 2 wt.% MWNTs with respect to the solvent content. This slurry was high-energy milled at a rotor speed of 2000 rpm with 0.8 mm ZrO2 beads for 2 h. Optical microscopy (ICS 305B, Sometech, Korea), field emission scanning electron microscopy (FESEM: S-4100, Hitachi using 2 kV with the working distance of 5–8 mm), high-resolution transmission electron microscopy (HRTEM: H-7600, Hitachi), XRD (RINT 2000, Rigaku using Cu K␣ line), particle size analyzer (PSA: Zetasizer Nano ZS, Malvern), and FTRaman spectrometer (FRA 106/S, Bruker Optics, Germany) were used to characterize the intact and high-energy milled MWNTs. 2.3. Paste synthesis and characterization Eight different types of paste were prepared for DOE. The high-energy milling for MWNTs, Ag filler type, and polymer/MWNTs ratio were chosen as the three controlled factors with two levels for each factor, as shown in Table 1. In order to obtain the adequate rheology for screen printing, the amount of nano-sized Ag filler was adjusted to 10 wt.% with respect to the total paste weight, while that with micro-size is fixed at 20 wt.%.
The eight paste compositions were pre-mixed using a stirrer and subjected to the 3-roll milling. The MWNTs pastes were screen-printed onto an indium-tin oxide (ITO) coated soda lime glass to a thickness of approximately 2 m and dried at 90 ◦ C for 10 min in air. Since the residue of organic materials in the pastes causes problems such as out-gassing and arching during the field emission measurement, binder burnout was performed at 450 ◦ C in a nitrogen atmosphere, followed by an activation process comprising the vertical alignment of MWNTs using a sticky tape. The field emission characteristics of the pastes were measured in a vacuum chamber with a parallel diode-type configuration at a pressure of 10−6 Torr using a pulse generator with 1/500 duty. The gap between anode and cathode was 200 m, and each sample had an area of 1.0 cm × 1.0 cm. The surface resistivity of the screen-printed thick film was measured using the four-point probe method.
3. Results and discussion Fig. 2 compares the morphology of the as-received and 2-h, high-energy milled MWNTs using optical microscope, SEM and TEM images. As-received MWNTs showed a highly entangled struc-
Fig. 4. MWNT pastes screen-printed on ITO glass: side-view of samples (a) No. 1 and (b) No. 2; top-view of samples (c) No. 2 and (d) No. 4.
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ing the milling process [15]. However, when dispersion equates to a disentanglement of spaghetti-like MWNTs, as in this case, viscosity will be increased by dispersion due to the enhanced resistance to flow resulting from the internal friction between the nanotubes and solvent. Therefore, the degree of viscosity increase induced by milling indicates the enhanced dispersion of the system. Fig. 4 (a) and (b) compares the side-view images of the activated screen-printed pastes containing MWNTs of (a) without and (b) with 2-h, high-energy milling. Since the high-energy milling enhanced the dispersion as well as the cutting of MWNTs, the sample shown in Fig. 4 (b) has a much greater density of surfaceexposed MWNTs than that in Fig. 4 (a). Fig. 4 (c) and (d) shows the top surface morphology of the pastes containing (c) nano-sized and (d) micro-sized Ag. Based on the agglomerated Ag particles in Fig. 4 (c), it seems that 3-roll milling alone is insufficient to disperse the nano-sized Ag particles. Fig. 5 shows (a) the electric field vs. emission current density and (b) Fowler–Nordheim (F–N) plots for four selected samples: numbers 2, 6, 7 and 8 among the 8 samples described in Table 1. In order to compare the MWNT emitters, Table 2 presents a comparison of the eight samples in terms of the turn-on electric field for an emission current density of 10 A cm−2 , film surface resistivity, emission current density at 5 V m−1 , and the calculated field enhancement factor ( factor), assuming an MWNT work function of 5 eV, which is the same as that of graphite. All values were different, confirming the importance of parameter optimization. Sample No. 8, synthesized from 20 wt.% micro-sized Ag with a polymer to high-energy milled MWNT ratio of 6/1, showed the smallest turnon field of 1.95 V m−1 for 10 A cm−2 emission density with the highest emission current density of 101.0 A cm−2 under 5 V m−1 and the highest  factor of 24,255. The F–N model expresses the relationships among the emission current density, local electrical field, and work function (˚) of the emitter, where ln(J/E2 ) vs. 1/E shows a linear relationship based on the following equation [16]:
Fig. 5. (a) I–V characteristic and (b) F–N plot with field enhancement factor () for four selected MWNT emitters.
ture, which should be dispersed for CNT-FED application to get uniform electron emission sites with a paste form, as shown in Fig. 2 (c). The optical microscope images in Fig. 2 (a) and (b) show the more dispersed MWNTs morphology after 2-h, high-energy milling. A comparison of the SEM images in Fig. 2 (c) and (d) indicated that the MWNTs were not only dispersed but also cut by high-energy milling. TEM images in Fig. 2 (e) and (f) show that the as-received MWNTs had four straight graphene layers with a diameter of approximately 4.8 nm and with some attached impurities, while the high-energy milled sample had a somewhat curved morphology with partly collapsed ends as shown with the inset in Fig. 2 (f). Maintaing tube-shape is very important after milling in order to use MWNTs as a field emitter. Since the energy dispersive X-ray analysis (EDX) results (data not shown here) only showed a carbon peak, these impurities must have been amorphous carbon, fullerenes or nanocrystalline graphite. Fig. 3 represents the evolution of the average agglomerate size and slurry viscosity of the MWNTs as a function of the milling time. The initial agglomerate size of 1136 nm was decreased to 265 nm, while the initial slurry viscosity of 110 mPa s was abruptly increased to 11,600 mPa s after 10 min of milling and then gradually increased to 12,700 mPa s after 2-h milling. The viscosity of a slurry with spherical particles generally decreases with increasing dispersion due to the particle mobility enabled by the fluidity between the interparticulate layers, so long as a new surface is not created dur-
ln
J E2
= ln
aˇ2 ˚
−
0.95b˚3/2 ˇE
(1)
where J is the emission current density (A m−2 ), E is the applied electric field (V m−1 ), a and b are constants (a = 1.54 × 10−6 A eV V−2 , b = 6.83 × 109 eV3/2 ), ˇ is the field enhancement factor, and ˚ is the work function of CNT. The slope of ln(J/E2 ) vs. 1/E from Eq. (1) is given by: Slope =
d(ln(J/E 2 )) 0.95b˚3/2 =− d(1/E) ˇ
(2)
Therefore, ˇ can be calculated from Eq. (2) since the other parameters are known. The ˇ factors of the 8 samples ranged from 7151 to 24,255, which is relatively larger than that of theoretical or individual CNT values of few thousand [17]. Based on their experimental results, Berdinsky et al. [18] proposed that this 10–50 times higher  factor for CNT paste than that for an individual CNT was due to the addition of emission currents from the CNTs in an array. Many of other researchers also reported such a high  factor for a CNT array [2,5,19–21]. The main effects plot for emission current density at 5 V m−1 , which can verify the effect of each input factor, is shown in Fig. 6. A dashed reference line is drawn at the average of the eight samples (=84.9 A cm−2 ), and the circle symbols in the graph correspond to the emission current density at each level of the corresponding input factor. The straight line connecting between two circle symbols does not mean a real linear output response for this 2-level factorial design. The most significant factor was the Ag filler type in the paste, with a larger emission density being shown with the
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Fig. 6. Main effects plot for emission current density at an electric field of 5 V m−1 .
Fig. 7. Main effects plot for surface resistivity of MWNT thick film.
20 wt.% micro-sized Ag compared to the 10 wt.% nano-sized sample. Due to the different content, however, the size effect of the Ag filler in the emission current density could not be distinguished. CNT paste containing fillers such as Ag and Al2 O3 is known to show higher emission density and more adequate rheological properties than the paste without filler [22]. Based on the higher density of the surface-exposed MWNTs in Fig. 4, the paste containing highenergy milled MWNTs was expected to have a much larger emission density than that of the intact sample, but this was contradicted by the result shown in Fig. 6. According to our previous report with the same type of unmilled MWNTs [9], the higher surfaceexposed density resulted in the larger emission current density along with a smaller surface resistivity. Although the surface resistivity of the screen-printed film was decreased drastically with the
high-energy milled MWNTs due to their high surface-exposed density compared to the paste with intact sample in this study, as shown in Fig. 7, the paste containing high-energy milled MWNTs did not show such a high emission current density relative to the surface-exposed density. As shown in Fig. 8 (a), there was no correlation between the emission current density of the paste with high-energy milled MWNTs, while current density increased with decreasing surface resistivity for the pastes with intact MWNTs, which showed a similar trend to the results of our previous study shown in Fig. 8 (b). Even though the larger MWNT content in the paste increased the emission density of the intact MWNTs, the content was fixed at 2 wt.% in this study due to the excessively high paste viscosity for screen printing and the difficulty in dispersion for higher content.
Table 2 Values of turn-on field, surface resistivity, current density and -factor for eight samples No
Turn-on field for 10 A cm−2 (V m−1 )
Surface resistivity (k/square)
Current density at 5 V m−1 (A cm−2 )
-factor
1 2 3 4 5 6 7 8
2.75 3.35 2.25 2.65 2.90 2.90 2.65 1.95
184.97 1.72 26.81 1.26 219.67 31.99 1.47 1.97
84.7 63.8 93.0 90.1 77.0 76.5 93.3 101.0
11,180 7151 10,816 9960 10,475 10,979 10,000 24,255
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In order to explain the lower average emission current density of the paste with high-energy milled MWNTs over the surfaceexposed density, XRD patterns and Raman spectra were compared between the as-received and high-energy milled MWNTs, as shown in Fig. 9. Based on the XRD patterns of MWNTs shown in Fig. 9 (a), the peak at 2 of 25.60◦ (P1), which corresponded to the {0 0 3} plane of synthetic carbon/graphite [JCPDS Card #: 26-1079], showed an especially wide diffraction angle, indicating the existence of amorphous-like carbonaceous impurities. Miller indices for each peak were not available because JCPDS cards contain more than eighty kinds of carbonaceous materials which have different diffraction peaks. The peak at 2 of 42.80◦ (P2), which corresponded to {1 0 1} plane of the synthetic carbon/graphite material, decreased and broadened significantly after the milling. The relative intensities of this peak compared to that at 2 of 25.60◦ (=P1/P2) were 1.078 and 0.315 for the as-received and 2h, high-energy milled samples, respectively. The small peaks at 2 of 62.34 and 78.66◦ , both corresponded to {1 0 5} and {1 1 0} plane of the above material, respectively, with the as-received sample had almost disappeared after the milling. The decrease in intensity and peak broadening suggested a degraded MWNT crystallinity due to milling damage. The Raman spectra, an alternative method for checking the degree of damage, of MWNTs using Ar excitation ( = 514.5 nm) is shown in Fig. 9 (b). The G-band near 1587 cm−1 that originated from the vibration of sp2 -bonded atoms
Fig. 9. (a) XRD patterns and (b) Raman spectra of MWNTs before and after 2-h, high-energy milling.
in two-dimensional hexagonal lattices, which is referred to as a highly ordered or crystalline carbon [23], and the D-band near 1279 cm−1 was indicative of some disruption, disorder or defects in the graphitic layers and/or carbonaceous particles [6]. The relative band height ratio of G- to D-band was 7.03 for as-received MWNTs, but was nearly halved to 3.63 after 2-h, high-energy milling. These XRD and Raman results indicated that the MWNT amorphism was increased by milling, and that such damage is inevitable due to the collision and shear forces exerted on the MWNTs during the milling process. Even though there has been no report of decreased electron emission with MWNTs after milling, we considered that the structural damage associated with the milling decreased in the emission current density despite the high density of surfaceexposed MWNTs. 4. Conclusion
Fig. 8. Relationship between emission current density and film surface resistivity for (a) the present study results with 2-h, high-energy milled MWNTs and (b) the previous study results with unmilled MWNTs.
The dispersion and field emission properties of MWNTs following 2-h, high-energy milling were examined in this paper. High-energy milling significantly enhanced the MWNT dispersion and the surface-exposed density in a paste form. Based on the field emission properties of the 8 samples investigated using DOE technique, however, the overall emission current density of pastes
N. Darsono et al. / Materials Chemistry and Physics 110 (2008) 363–369
containing 2 wt.% high-energy milled MWNTs was lower than that of the paste containing the same amount of intact sample, despite their higher density of surface-exposed MWNTs. Further characterization using XRD and Raman spectroscopy indicated that the degree of crystallinity with high-energy milled MWNTs was lower than that of the intact samples due to the collision and shear forces experienced during the milling process, which explained the lower emission current density of high-energy milled MWNTs. The paste synthesized from 20 wt.% macro-sized Ag with a polymer to high-energy milled MWNT ratio of 6/1 demonstrated the optimum condition among the 8 samples, with an emission current density of 101 A cm−2 under 5 V A−1 and a field enhancement factor of 24,255, which are satisfactory values for application to commercial CNT-FED as long as the long-term reliability is ensured. Acknowledgments This work was supported by The Ministry of Commerce, Industry and Energy of Korea through a Components and Materials Technology Development Project. (No.0401-DD2-0162). References [1] S. Ijima, Nature 354 (1991) 56. [2] Y. Saito, S. Uemura, Carbon 38 (2000) 169. [3] C.J. Lee, J. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175.
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[4] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D: Appl. Phys. 31 (1998) L34. [5] J.M. Kim, W.B. Choi, N.S. Lee, J.E. Jung, Diam. Relat. Mater. 9 (2000) 1184. [6] J.H. Park, J.S. Moon, J.W. Nam, J.B. Yoo, C.Y. Park, J.M. Kim, J.H. Park, C.G. Lee, D.H. Choe, Diam. Relat. Mater. 14 (2005) 2113. ¨ ´ A. Chˆatelain, Appl. Phys. A 69 [7] J.-M. Bonard, J.-P. Salvetat, T. Stockli, L. Forro, (1999) 245. [8] S. Itoh, M. Tanaka, T. Tonegawa, J. Vac. Sci. Technol. B 22 (2004) 1362. [9] N. Darsono, S.W. Kwon, D.H. Yoon, J. Kim, H. Moon, S.U. Park, J. Mater. Sci.: Mater. Electron 19 (2008) 17. [10] J. Li, Y. Zhang, Appl. Surf. Sci. 252 (2006) 2944. ´ Kukovecz, T. Kanyo, ´ Z. Konya, ´ [11] A. I. Kiricsi, Carbon 43 (2005) 994. [12] D.H. Yoon, J. Ceram. Proc. Res. 7 (4) (2006) 343. [13] J.A. Lewis, J. Am. Ceram. Soc. 83 (2000) 2341. [14] D.C. Montgomery, Design and Analysis of Experiments, John Wiley & Sons, New York, 1991. [15] D.H. Yoon, B.I. Lee, J. Eur. Ceram. Soc. 24 (2004) 739. [16] J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 (1973) 487. [17] J.M. Bonard, M. Croci, I. Arfaoui, O. Noury, D. Sarangi, A. Chatelain, Diam. Relat. Mater. 11 (2002) 763. [18] A.S. Berdinsky, A.V. Shaporin, J.B. Yoo, J.H. Park, P.S. Alegaonkar, J.H. Han, G.H. Son, Appl. Phys. A. 83 (2006) 377. [19] S.J. Chung, S.H. Lim, J. Jang, Thin Solid Films 383 (2001) 73. ¨ [20] J.-M. Bonard, T. Stockli, O. Noury, A. Chˆatelain, Appl. Phys. Lett. 78 (2001) 2775. [21] R.B. Sharma, D.J. Late, D.S. Joag, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 428 (2006) 102. [22] J.W. Nam, S.H. Cho, Y.C. Choi, J.S. Ha, J.H. Park, D.H. Choe, J.B. Yoo, J.H. Park, Diam. Relat. Mater. 14 (2005) 2089. [23] U. Ritter, P. Scharff, C. Siegmund, O.P. Dmytrenko, N.P. Kulish, Y.I. Prylutskyy, N.M. Belyi, V.A. Gubanov, L.I. Komarova, S.V. Lizunova, V.G. Poroshin, V.V. Shlapatskaya, H. Bernas, Carbon (2006) 2694.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Dispersion and field emission properties of multi-walled carbon nanotubes by high-energy milling Nono Darsono a , Parlindungan Yonathan a , Dang-Hyok Yoon a,∗ , Jaemyung Kim b , Yoonjin Kim b a b
School of Materials Science and Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea Electric Material Development Team, Corporate R&D Center, Samsung SDI, Yong-in 449-577, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 November 2007 Received in revised form 15 February 2008 Accepted 17 February 2008 Keywords: Nanostructuress Field emission Raman spectroscopy and scattering
a b s t r a c t Multi-walled carbon nanotubes (MWNTs) were high-energy milled for 2 h in texanol after a polycarboxylic acid polymeric dispersant had been added in order to enhance the dispersion. The degree of MWNT dispersion was significantly enhanced by high-energy milling compared to the intact sample, which increased the density of surface-exposed MWNTs with a screen-printed paste. However, the emission properties of high-energy milled MWNTs did not show such a high emission current density relative to their increased surface-exposed density. Further investigation using Raman spectroscopy and X-ray diffraction evidenced the milling-induced MWNT damage, which explained the relatively lower emission current density of high-energy milled MWNTs. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have been intensively studied for applications in electronic fields. CNTs have promising application as the electron emitters in field emission display (FED) due to their high aspect ratio and low work function [2]. CNTs as electron emitters have been fabricated by the direct growth [3], electrophoretic deposition [4], and screen printing [5] methods, of which the latter has been successfully adopted to produce a large-scale panel display in a relatively simple process [6]. Fig. 1 shows a schematic illustration of an FED structure with the CNT emitters on the cathode substrate and red–green–blue (RGB) phosphor dots on the anode side. Single-walled carbon nanotubes (SWNTs) generally show a shorter lifetime, in spite of their higher degree of structural perfection and emission current density, than multi-walled carbon nanotubes (MWNTs) do [7]. The long-term stability which ensures a uniform electron emission for more than 30,000 h is the most important challenge for the commercialization of CNT-FED [8]. Therefore, current efforts are mainly being focused on MWNTs with a diameter of several nanometers for actual applications [9]. Since MWNTs tend to form aggregates due to their high van der Waals interactions, their dispersion is a prerequisite to form uniform emission sites for FED application. MWNT agglomerates are usually broken up and dispersed into smaller units through mechanical milling for initial dispersion. The conventional ball
∗ Corresponding author. Tel.: +82 53 810 2561; fax: +82 53 810 4628. E-mail address: [email protected] (D.-H. Yoon). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.02.016
mills and attrition mills that have been used for this purpose require more than 100 h of milling to get an average length of 200–300 nm [10,11]. However, modern high-energy mills are known to be very efficient in milling due to their high-speed rotor rotating at up to several thousand rpm and use of small grinding media with diameters of 0.05–0.8 mm [12]. As high-energy mills have recently been introduced, however, there are few reports thus far regarding its efficiency in MWNTs dispersion. In the subsequent stabilization process, dispersion is achieved by electrostatic and steric mechanisms, both of which prevent the close approach of MWNTs by using electrostatic repulsive force and hindrance via the adsorption of polymeric molecules on the particle surface, respectively [13]. The paste for screen printing requires numerous ingredients such as CNT, binder resin, solvent, dispersant, metal filler, photo initiator, monomer and glass frit, all of which should be optimized in order to obtain a favorable emission property. Design of experiments (DOE) is a powerful tool for establishing relationships among many experimental factors and output responses for such a complex system. It utilizes a structured DOE table in which the input factors are varied in a planned manner in order to identify efficiently the factors that most influence the results as well as those that do not [14]. Based on the above background, MWNTs were milled and dispersed by high-energy milling in this study. Moreover, a full factorial DOE was performed in order to verify the effect of each factor and to determine the optimum MWNTs paste condition for FED application using three experimental input factors: the treatment of high-energy milling for MWNTs, Ag filler type, and polymer/MWNTs ratio. The emission properties such as turn-on field, current density and field enhancement factor for eight kinds
364
N. Darsono et al. / Materials Chemistry and Physics 110 (2008) 363–369
Fig. 1. Schematic illustration of CNT-FED.
of paste were analyzed as output responses. Statistical analysis software, MINITAB, was used to generate the DOE table and to analyze the results. The effect of each input factor on the emission properties is described through main effect plots, microstructure, X-ray diffraction (XRD) and Raman spectra.
2. Experimental 2.1. Starting materials MWNTs were grown by catalytic chemical vapor deposition (CVD) method (CMP-310F, Iljin Nanotech, Korea) with a mean diameter of 3–5 nm, a length
Fig. 2. Comparison of the morphologies of as-received and 2-h, high-energy milled MWNTs in texanol: (a) and (b) optical microscope, (c) and (d) FESEM, and (e) and (f) HRTEM.
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365
Table 1 Eight types of MWNT pastes for experimental design No
High-energy milling
Silver filler (wt.%)
Polymer/CNT ratio
1 2 3 4 5 6 7 8
No Yes No Yes No Yes No Yes
Nano 10 Nano 10 Micro 20 Micro 20 Nano 10 Nano 10 Micro 20 Micro 20
4/1 4/1 4/1 4/1 6/1 6/1 6/1 6/1
of 10–20 m, and a specific surface area of 797 m2 g−1 . In order to synthesize a photosensitive organic vehicle, acrylic binder resin (Elvacite 2669 with an acid number of 124, Lucite International) was dissolved in texanol (C12 H24 O3 : 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate) and mixed with photosensitive monomers, photo initiators and epoxy silane. Polycarboxylic acid polymer (BYK-P 104, BYK Chemie, USA) at a ratio of 100 wt.% with respect to the MWNT content was used as a dispersant. Silver powders with an average particle size of 80 and 400 nm were used as metal filler.
Fig. 3. Evolution of the average agglomerate size of MWNTs and the slurry viscosity as a function of high-energy milling time.
2.2. High-energy milling and characterization of MWNTs To 350 g of texanol were added 1.4 g of dispersant and 7 g of MWNTs (i.e., 20 wt.% of dispersant with respect to the MWNT content) to form a slurry of 2 wt.% MWNTs with respect to the solvent content. This slurry was high-energy milled at a rotor speed of 2000 rpm with 0.8 mm ZrO2 beads for 2 h. Optical microscopy (ICS 305B, Sometech, Korea), field emission scanning electron microscopy (FESEM: S-4100, Hitachi using 2 kV with the working distance of 5–8 mm), high-resolution transmission electron microscopy (HRTEM: H-7600, Hitachi), XRD (RINT 2000, Rigaku using Cu K␣ line), particle size analyzer (PSA: Zetasizer Nano ZS, Malvern), and FTRaman spectrometer (FRA 106/S, Bruker Optics, Germany) were used to characterize the intact and high-energy milled MWNTs. 2.3. Paste synthesis and characterization Eight different types of paste were prepared for DOE. The high-energy milling for MWNTs, Ag filler type, and polymer/MWNTs ratio were chosen as the three controlled factors with two levels for each factor, as shown in Table 1. In order to obtain the adequate rheology for screen printing, the amount of nano-sized Ag filler was adjusted to 10 wt.% with respect to the total paste weight, while that with micro-size is fixed at 20 wt.%.
The eight paste compositions were pre-mixed using a stirrer and subjected to the 3-roll milling. The MWNTs pastes were screen-printed onto an indium-tin oxide (ITO) coated soda lime glass to a thickness of approximately 2 m and dried at 90 ◦ C for 10 min in air. Since the residue of organic materials in the pastes causes problems such as out-gassing and arching during the field emission measurement, binder burnout was performed at 450 ◦ C in a nitrogen atmosphere, followed by an activation process comprising the vertical alignment of MWNTs using a sticky tape. The field emission characteristics of the pastes were measured in a vacuum chamber with a parallel diode-type configuration at a pressure of 10−6 Torr using a pulse generator with 1/500 duty. The gap between anode and cathode was 200 m, and each sample had an area of 1.0 cm × 1.0 cm. The surface resistivity of the screen-printed thick film was measured using the four-point probe method.
3. Results and discussion Fig. 2 compares the morphology of the as-received and 2-h, high-energy milled MWNTs using optical microscope, SEM and TEM images. As-received MWNTs showed a highly entangled struc-
Fig. 4. MWNT pastes screen-printed on ITO glass: side-view of samples (a) No. 1 and (b) No. 2; top-view of samples (c) No. 2 and (d) No. 4.
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ing the milling process [15]. However, when dispersion equates to a disentanglement of spaghetti-like MWNTs, as in this case, viscosity will be increased by dispersion due to the enhanced resistance to flow resulting from the internal friction between the nanotubes and solvent. Therefore, the degree of viscosity increase induced by milling indicates the enhanced dispersion of the system. Fig. 4 (a) and (b) compares the side-view images of the activated screen-printed pastes containing MWNTs of (a) without and (b) with 2-h, high-energy milling. Since the high-energy milling enhanced the dispersion as well as the cutting of MWNTs, the sample shown in Fig. 4 (b) has a much greater density of surfaceexposed MWNTs than that in Fig. 4 (a). Fig. 4 (c) and (d) shows the top surface morphology of the pastes containing (c) nano-sized and (d) micro-sized Ag. Based on the agglomerated Ag particles in Fig. 4 (c), it seems that 3-roll milling alone is insufficient to disperse the nano-sized Ag particles. Fig. 5 shows (a) the electric field vs. emission current density and (b) Fowler–Nordheim (F–N) plots for four selected samples: numbers 2, 6, 7 and 8 among the 8 samples described in Table 1. In order to compare the MWNT emitters, Table 2 presents a comparison of the eight samples in terms of the turn-on electric field for an emission current density of 10 A cm−2 , film surface resistivity, emission current density at 5 V m−1 , and the calculated field enhancement factor ( factor), assuming an MWNT work function of 5 eV, which is the same as that of graphite. All values were different, confirming the importance of parameter optimization. Sample No. 8, synthesized from 20 wt.% micro-sized Ag with a polymer to high-energy milled MWNT ratio of 6/1, showed the smallest turnon field of 1.95 V m−1 for 10 A cm−2 emission density with the highest emission current density of 101.0 A cm−2 under 5 V m−1 and the highest  factor of 24,255. The F–N model expresses the relationships among the emission current density, local electrical field, and work function (˚) of the emitter, where ln(J/E2 ) vs. 1/E shows a linear relationship based on the following equation [16]:
Fig. 5. (a) I–V characteristic and (b) F–N plot with field enhancement factor () for four selected MWNT emitters.
ture, which should be dispersed for CNT-FED application to get uniform electron emission sites with a paste form, as shown in Fig. 2 (c). The optical microscope images in Fig. 2 (a) and (b) show the more dispersed MWNTs morphology after 2-h, high-energy milling. A comparison of the SEM images in Fig. 2 (c) and (d) indicated that the MWNTs were not only dispersed but also cut by high-energy milling. TEM images in Fig. 2 (e) and (f) show that the as-received MWNTs had four straight graphene layers with a diameter of approximately 4.8 nm and with some attached impurities, while the high-energy milled sample had a somewhat curved morphology with partly collapsed ends as shown with the inset in Fig. 2 (f). Maintaing tube-shape is very important after milling in order to use MWNTs as a field emitter. Since the energy dispersive X-ray analysis (EDX) results (data not shown here) only showed a carbon peak, these impurities must have been amorphous carbon, fullerenes or nanocrystalline graphite. Fig. 3 represents the evolution of the average agglomerate size and slurry viscosity of the MWNTs as a function of the milling time. The initial agglomerate size of 1136 nm was decreased to 265 nm, while the initial slurry viscosity of 110 mPa s was abruptly increased to 11,600 mPa s after 10 min of milling and then gradually increased to 12,700 mPa s after 2-h milling. The viscosity of a slurry with spherical particles generally decreases with increasing dispersion due to the particle mobility enabled by the fluidity between the interparticulate layers, so long as a new surface is not created dur-
ln
J E2
= ln
aˇ2 ˚
−
0.95b˚3/2 ˇE
(1)
where J is the emission current density (A m−2 ), E is the applied electric field (V m−1 ), a and b are constants (a = 1.54 × 10−6 A eV V−2 , b = 6.83 × 109 eV3/2 ), ˇ is the field enhancement factor, and ˚ is the work function of CNT. The slope of ln(J/E2 ) vs. 1/E from Eq. (1) is given by: Slope =
d(ln(J/E 2 )) 0.95b˚3/2 =− d(1/E) ˇ
(2)
Therefore, ˇ can be calculated from Eq. (2) since the other parameters are known. The ˇ factors of the 8 samples ranged from 7151 to 24,255, which is relatively larger than that of theoretical or individual CNT values of few thousand [17]. Based on their experimental results, Berdinsky et al. [18] proposed that this 10–50 times higher  factor for CNT paste than that for an individual CNT was due to the addition of emission currents from the CNTs in an array. Many of other researchers also reported such a high  factor for a CNT array [2,5,19–21]. The main effects plot for emission current density at 5 V m−1 , which can verify the effect of each input factor, is shown in Fig. 6. A dashed reference line is drawn at the average of the eight samples (=84.9 A cm−2 ), and the circle symbols in the graph correspond to the emission current density at each level of the corresponding input factor. The straight line connecting between two circle symbols does not mean a real linear output response for this 2-level factorial design. The most significant factor was the Ag filler type in the paste, with a larger emission density being shown with the
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Fig. 6. Main effects plot for emission current density at an electric field of 5 V m−1 .
Fig. 7. Main effects plot for surface resistivity of MWNT thick film.
20 wt.% micro-sized Ag compared to the 10 wt.% nano-sized sample. Due to the different content, however, the size effect of the Ag filler in the emission current density could not be distinguished. CNT paste containing fillers such as Ag and Al2 O3 is known to show higher emission density and more adequate rheological properties than the paste without filler [22]. Based on the higher density of the surface-exposed MWNTs in Fig. 4, the paste containing highenergy milled MWNTs was expected to have a much larger emission density than that of the intact sample, but this was contradicted by the result shown in Fig. 6. According to our previous report with the same type of unmilled MWNTs [9], the higher surfaceexposed density resulted in the larger emission current density along with a smaller surface resistivity. Although the surface resistivity of the screen-printed film was decreased drastically with the
high-energy milled MWNTs due to their high surface-exposed density compared to the paste with intact sample in this study, as shown in Fig. 7, the paste containing high-energy milled MWNTs did not show such a high emission current density relative to the surface-exposed density. As shown in Fig. 8 (a), there was no correlation between the emission current density of the paste with high-energy milled MWNTs, while current density increased with decreasing surface resistivity for the pastes with intact MWNTs, which showed a similar trend to the results of our previous study shown in Fig. 8 (b). Even though the larger MWNT content in the paste increased the emission density of the intact MWNTs, the content was fixed at 2 wt.% in this study due to the excessively high paste viscosity for screen printing and the difficulty in dispersion for higher content.
Table 2 Values of turn-on field, surface resistivity, current density and -factor for eight samples No
Turn-on field for 10 A cm−2 (V m−1 )
Surface resistivity (k/square)
Current density at 5 V m−1 (A cm−2 )
-factor
1 2 3 4 5 6 7 8
2.75 3.35 2.25 2.65 2.90 2.90 2.65 1.95
184.97 1.72 26.81 1.26 219.67 31.99 1.47 1.97
84.7 63.8 93.0 90.1 77.0 76.5 93.3 101.0
11,180 7151 10,816 9960 10,475 10,979 10,000 24,255
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In order to explain the lower average emission current density of the paste with high-energy milled MWNTs over the surfaceexposed density, XRD patterns and Raman spectra were compared between the as-received and high-energy milled MWNTs, as shown in Fig. 9. Based on the XRD patterns of MWNTs shown in Fig. 9 (a), the peak at 2 of 25.60◦ (P1), which corresponded to the {0 0 3} plane of synthetic carbon/graphite [JCPDS Card #: 26-1079], showed an especially wide diffraction angle, indicating the existence of amorphous-like carbonaceous impurities. Miller indices for each peak were not available because JCPDS cards contain more than eighty kinds of carbonaceous materials which have different diffraction peaks. The peak at 2 of 42.80◦ (P2), which corresponded to {1 0 1} plane of the synthetic carbon/graphite material, decreased and broadened significantly after the milling. The relative intensities of this peak compared to that at 2 of 25.60◦ (=P1/P2) were 1.078 and 0.315 for the as-received and 2h, high-energy milled samples, respectively. The small peaks at 2 of 62.34 and 78.66◦ , both corresponded to {1 0 5} and {1 1 0} plane of the above material, respectively, with the as-received sample had almost disappeared after the milling. The decrease in intensity and peak broadening suggested a degraded MWNT crystallinity due to milling damage. The Raman spectra, an alternative method for checking the degree of damage, of MWNTs using Ar excitation ( = 514.5 nm) is shown in Fig. 9 (b). The G-band near 1587 cm−1 that originated from the vibration of sp2 -bonded atoms
Fig. 9. (a) XRD patterns and (b) Raman spectra of MWNTs before and after 2-h, high-energy milling.
in two-dimensional hexagonal lattices, which is referred to as a highly ordered or crystalline carbon [23], and the D-band near 1279 cm−1 was indicative of some disruption, disorder or defects in the graphitic layers and/or carbonaceous particles [6]. The relative band height ratio of G- to D-band was 7.03 for as-received MWNTs, but was nearly halved to 3.63 after 2-h, high-energy milling. These XRD and Raman results indicated that the MWNT amorphism was increased by milling, and that such damage is inevitable due to the collision and shear forces exerted on the MWNTs during the milling process. Even though there has been no report of decreased electron emission with MWNTs after milling, we considered that the structural damage associated with the milling decreased in the emission current density despite the high density of surfaceexposed MWNTs. 4. Conclusion
Fig. 8. Relationship between emission current density and film surface resistivity for (a) the present study results with 2-h, high-energy milled MWNTs and (b) the previous study results with unmilled MWNTs.
The dispersion and field emission properties of MWNTs following 2-h, high-energy milling were examined in this paper. High-energy milling significantly enhanced the MWNT dispersion and the surface-exposed density in a paste form. Based on the field emission properties of the 8 samples investigated using DOE technique, however, the overall emission current density of pastes
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containing 2 wt.% high-energy milled MWNTs was lower than that of the paste containing the same amount of intact sample, despite their higher density of surface-exposed MWNTs. Further characterization using XRD and Raman spectroscopy indicated that the degree of crystallinity with high-energy milled MWNTs was lower than that of the intact samples due to the collision and shear forces experienced during the milling process, which explained the lower emission current density of high-energy milled MWNTs. The paste synthesized from 20 wt.% macro-sized Ag with a polymer to high-energy milled MWNT ratio of 6/1 demonstrated the optimum condition among the 8 samples, with an emission current density of 101 A cm−2 under 5 V A−1 and a field enhancement factor of 24,255, which are satisfactory values for application to commercial CNT-FED as long as the long-term reliability is ensured. Acknowledgments This work was supported by The Ministry of Commerce, Industry and Energy of Korea through a Components and Materials Technology Development Project. (No.0401-DD2-0162). References [1] S. Ijima, Nature 354 (1991) 56. [2] Y. Saito, S. Uemura, Carbon 38 (2000) 169. [3] C.J. Lee, J. Park, S.Y. Kang, J.H. Lee, Chem. Phys. Lett. 326 (2000) 175.
369
[4] K. Yamamoto, S. Akita, Y. Nakayama, J. Phys. D: Appl. Phys. 31 (1998) L34. [5] J.M. Kim, W.B. Choi, N.S. Lee, J.E. Jung, Diam. Relat. Mater. 9 (2000) 1184. [6] J.H. Park, J.S. Moon, J.W. Nam, J.B. Yoo, C.Y. Park, J.M. Kim, J.H. Park, C.G. Lee, D.H. Choe, Diam. Relat. Mater. 14 (2005) 2113. ¨ ´ A. Chˆatelain, Appl. Phys. A 69 [7] J.-M. Bonard, J.-P. Salvetat, T. Stockli, L. Forro, (1999) 245. [8] S. Itoh, M. Tanaka, T. Tonegawa, J. Vac. Sci. Technol. B 22 (2004) 1362. [9] N. Darsono, S.W. Kwon, D.H. Yoon, J. Kim, H. Moon, S.U. Park, J. Mater. Sci.: Mater. Electron 19 (2008) 17. [10] J. Li, Y. Zhang, Appl. Surf. Sci. 252 (2006) 2944. ´ Kukovecz, T. Kanyo, ´ Z. Konya, ´ [11] A. I. Kiricsi, Carbon 43 (2005) 994. [12] D.H. Yoon, J. Ceram. Proc. Res. 7 (4) (2006) 343. [13] J.A. Lewis, J. Am. Ceram. Soc. 83 (2000) 2341. [14] D.C. Montgomery, Design and Analysis of Experiments, John Wiley & Sons, New York, 1991. [15] D.H. Yoon, B.I. Lee, J. Eur. Ceram. Soc. 24 (2004) 739. [16] J.W. Gadzuk, E.W. Plummer, Rev. Mod. Phys. 45 (1973) 487. [17] J.M. Bonard, M. Croci, I. Arfaoui, O. Noury, D. Sarangi, A. Chatelain, Diam. Relat. Mater. 11 (2002) 763. [18] A.S. Berdinsky, A.V. Shaporin, J.B. Yoo, J.H. Park, P.S. Alegaonkar, J.H. Han, G.H. Son, Appl. Phys. A. 83 (2006) 377. [19] S.J. Chung, S.H. Lim, J. Jang, Thin Solid Films 383 (2001) 73. ¨ [20] J.-M. Bonard, T. Stockli, O. Noury, A. Chˆatelain, Appl. Phys. Lett. 78 (2001) 2775. [21] R.B. Sharma, D.J. Late, D.S. Joag, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 428 (2006) 102. [22] J.W. Nam, S.H. Cho, Y.C. Choi, J.S. Ha, J.H. Park, D.H. Choe, J.B. Yoo, J.H. Park, Diam. Relat. Mater. 14 (2005) 2089. [23] U. Ritter, P. Scharff, C. Siegmund, O.P. Dmytrenko, N.P. Kulish, Y.I. Prylutskyy, N.M. Belyi, V.A. Gubanov, L.I. Komarova, S.V. Lizunova, V.G. Poroshin, V.V. Shlapatskaya, H. Bernas, Carbon (2006) 2694.