Field Emission Behavior of Carbon Nanotube Yarn for Micro-Resolution X-Ray Tube Cathode

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1 Copyright 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Nanoscience and Nanotechnology Vol. 13, , 2013 Field Emission Behavior of Carbon Nanotube Yarn for Micro-Resolution X-Ray Tube Cathode Jae Won Hwang 1, Chan Bin Mo 1, Hyun Kyu Jung 2, Seongwoo Ryu 1, and Soon Hyung Hong 1 1 Department of Materials Science and Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon , Korea 2 Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong-gu, Daejeon , Korea Carbon nanotube (CNT) has excellent electrical and thermal conductivity and high aspect ratio for X-ray tube cathode. However, CNT field emission cathode has been shown unstable field emission and short life time due to field evaporation by high current density and detachment by electrostatic force. An alternative approach in this direction is the introduction of CNT yarn, which is a one dimensional assembly of individual carbon nanotubes bonded by the Van der Waals force. Because CNT yarn is composed with many CNTs, CNT yarns are expected to increase current density and life time for X-ray tube applications. In this research, CNT yarn was fabricated by spinning of a super-aligned CNT forest and was characterized for application to an X-ray tube cathode. CNT yarn showed a high field emission current density and a long lifetime of over 450 hours. Applying the CNT yarn field emitter to the X-ray tube cathode, it was possible to obtain micro-scale resolution images. The relationship between the field emission properties and the microstructure evolution was investigated and the unraveling effect of the CNT yarn was discussed. Keywords: Carbon IP: Nanotube, Field Emittion, On: Sat, X-Ray 14 Tube. Sep :16:09 1. INTRODUCTION Carbon nanotube (CNT) is an ideal field emitter due to its high aspect ratio, reasonable electrical and thermal conductivity, and high mechanical properties. Thus, many researchers have focused on effecting a technical breakthrough by replacing conventional field emitters with CNTs for applications such as field emission displays, back light units, and X-ray tube cathodes. 1 4 However, most attempts have failed due to the unstable field emission and short lifetime of CNTs, which properties stem from field evaporation by high current density and detachment by electrostatic force. 5 6 In addition, non-uniform field emission has limited the application of CNTs to field emission displays. Therefore, the critical issues in the development of a CNT based field emitter are increasing stability, lifetime, and uniformity. On the other hand, uniform field emission of electrons is not necessary for X-ray source applications because all electrons should be focused in one focal spot. Recently, many researchers have been trying to develop an X-ray tube cathode by utilizing the unique properties of CNT field emitters, such as rapid Author to whom correspondence should be addressed. response time and high current density, compared to those properties of conventional thermionic filaments. 3 4 CNT yarn is a one dimensional assembly of individual CNTs bonded by the Van der Waals force CNT yarn can be fabricated by spinning of a CNT forest 7 9 or by direct spinning of CNT aerosol from the hot zone of a tube furnace. 8 CNT yarn has outstanding mechanical properties such as high strength, modulus, and toughness, which are inherited from individual CNTs. In addition, CNT yarn is easy to manipulate for fabrication of composites. Another promising application of CNT yarn is as a field emitter It has been shown that CNT yarn can be a excellent field emitter with exceptional high current density and field enhancement factor It is technically important to investigate whether the critical issues mentioned above can be solved using CNT yarn or not. Based on this research background, CNT yarn was fabricated and characterized as a field emitter for application as an X-ray source. 2. EXPERIMENTAL DETAILS Various fabrication processes of CNT yarn from a CNT forest have been reported elsewhere by other groups J. Nanosci. Nanotechnol. 2013, Vol. 13, No /2013/13/7386/005 doi: /jnn

2 It is known that the thickness of the catalysts and the ratio of the reaction gases are critical parameters for superaligned growth of the CNT forest. Thus far, most previous results have been based on the growth of super-aligned carbon nanotube forests by using acethylene or ethylene as a carbon source. However, these gases are very unstable and toxic. In this paper, we thus attempted to use methane as a carbon source and used microwave plasma for effective decomposition of the methane. In this paper, Al (10 nm) as a buffer layer and Fe (1 3 nm) as a catalyst layer were deposited on Si wafer by sputtering. Other catalyst combinations such as an Fe/Si wafer without an Al layer and an Fe/Al 2 O 3 /Si wafer have been also evaluated, but it was found that the Fe/Al/Si wafer is the most effective combination for super-aligned growth of a CNT forest. The Fe/Al layers were oxidized at 550 C under an oxygen atmosphere followed by reduction at 720 C under a hydrogen atmosphere, resulting in Fe/Al 2 O 3 layers. This sequential process of oxidation and reduction could be an effective means of attaining good adhesion between Fe and Al 2 O 3. Typically, a super-aligned CNT forest was grown in a microwave plasma-assisted chemical vapor deposition chamber at 720 C for 20 min. The gas flow rates of H 2,CH 4, Ar, and O 2, were sccm, sccm, sccm, and 0 1 sccm, respectively. The plasma power was W. Fig. 1. SEM image of the CNT forest and yarn, (a) super-aligned CNT forest, (b) CNT sheet spin-out from the CNT forest, (c) CNT yarn, (d) high magnification image of the CNTs in the CNT yarn. the proximity and the contact area between CNTs, not to the crystallinity or cleanness of the walls. In order to fabricate an X-ray tube cathode, the CNT yarn was cut with a steel knife and mounted on a stainless steel electrode by using silver paste, as shown in 3. RESULTS AND DISCUSSION The CNT forest (Fig. 1(a)) was spinnable, as shown in Figure 1(b), and CNT yarn was obtained by spinning, as shown in Figure 1(c). In the CNT yarn, the CNTs are highly aligned (Fig. 1(d)) and tightly bonded with each other by Van der Waals force. Super-alignment of CNTs is critical for the spinning of CNTs. The Van der Waals force between CNTs is thought to be the main origin for the spinning of CNT yarn 9 and can be maximized when the distance between CNTs is small and the CNTs are parallel to each other. There is no quantitative definition of super-alignment, but there are key features such as straightness and area density of CNTs. 7 9 The SEM image of the CNT forest (Fig. 1(a)) shows quite straight CNTs; most of the CNTs are already bundled (see inset image) due to the high area density of the CNTs. This microstructure is quite comparable to that in the previously reported results. 9 Another important parameter to determine the spinnability of a CNT forest is the amorphous carbon content. Some researchers have reported that the CNT forest is not spinnable when the amorphous carbon content is high or the surface of the CNT is not clean However, the transmission electron microscopy image (Fig. 2(a)) and the Raman spectrum (Fig. 2(b)) of the CNT forest in this study revealed a very large amount of amorphous carbon on the surface of the CNT, and the CNT forest was nevertheless spinnable. This is attributed Fig. 2. to the Van der Waals force being directly proportional to of CNT forest. (a) TEM image of a CNT from the forest, (b) Raman spectrum J. Nanosci. Nanotechnol. 13, ,

3 Fig. 4. X-ray images obtained using the CNT yarn field emitter (a) X-ray image of line pairs, (b) X-ray image of line pair #20 (scale unit = m). of the yarn. It seems that a synergy effect of CNT yarn and CNTs that were located on the CNT yarn is at work here, increasing the field enhancement factors more than those of a point type CNT field emitter.17 The maximum current density was 251 A (51 A/cm2 at 1.45 V/ m. Such a high current density in a low electrical field and with a high field enhancement factor are remarkable compared to the previous results obtained from screen-printed or CVD-grown carbon nanotube field emitters.1 4 Fig. 3. (a) Photo of the CNT yarn field emitter assembly as an X-ray High current density from a small cross-section area of tube cathode, (b) a field emission curve (cathode to anode space = CNT yarn, different from the case of film-type CNT field 175 m) and a Fowler-Nordheim plot. Delivered by Publishing Technology to: Korea Advanced Institute of advantageous Science & Technology (KAIST) emitters, is quite in the fabrication of a high resolution X-ray tube cathode. X-ray images were obtained American Figure 3(a). The field emission curvecopyright: was measured for a Scientific Publishers using an X-ray inspection machine (X-eye 3000B, SEC.) diode structure, as shown in Figutre 3(b). Field emission with the CNT yarn cathode; the focal spot was 5 um. An curves were measured using a DC power supply controlled X-ray image was obtained using a CNT yarn field emitter, by computer software, which enables current voltage feedas shown in Figure 4. The black and white line pair is back. Vacuum chamber pressure was typically under 3 clearly separated. 20 line pairs in Figure 4(b) correspond 6 10 Torr. Diameter of CNT yarn was 25 m and length of to 1 mm, that is, the width of one black line is 25 m. CNT yarn was 3 mm. Space between the CNT yarn tip and Thus, the resolution of the image is less than 25 m; this the anode was 175 m. Field emission current increases would be further improved by adopting a triode structure exponentially and fits very well with the Fowler-Nordheim with a gate that limits the divergence of the electrons. plot, as shown in the inset image of Figure 3(b). The field The most important issue for CNT field emitters is their enhancement factor was 61,111. Edgcombe et al.15 fabrilifetime, as mentioned in the introduction. Figure 5 shows cacated a cylinder type carbon field emitter and simulated the stability of the current and voltage for 450 hours. In field enhancement factors according to various geometries this measurement, the current was kept constant at 50 ua of electrode. In the case our model, we obtained relations (corresponding to 10 A/cm2 and the current was mainas follows. tained at a constant rate using voltage feedback. For the = 1 2 L/r (1) feedback, the voltage goes sensitively up and down to where obtain objective current or to suppress over-current. An interesting feature of voltage evolution is that it shows hill L: length of cylinder and valley shapes not only on a large time scale but also r: radius of cylinder. on a small time scale, as shown in Figure 5(b) and in the However, there is large discrepancy between the experiinset image. The transition from a valley to a hill means an mental data and the estimated data, 61,111 and 91. There increase of the voltage to increase the current; a transition are CNT tips on the end part of the yarn. In that case, the from a hill to a valley means a decrease of the voltage to field enhancement factor of the CNT tip itself should also decrease the current. be considered. Thus, this high field enhancement factor is This unique behavior is attributed to the nature of the attributed to the combinational effect of the high aspect CNT yarn. CNT yarn consists of several thousand CNTs ratio of the CNT yarn and that of the CNTs on the end part bound to each other by Van der Waals force. Thus, new 7388 J. Nanosci. Nanotechnol. 13, , 2013

4 effect differentiates a CNT yarn from CNTs grown on metal tips,3 16 and from other CNT films deposited on a substrate Copyright: American Publishers 4. CONCLUSIONS CNT tips can be generated by electrical perturbations such Scientific as a sudden increase of the voltage when the current is In conclusion, it was possible to fabricate a CNT yarn by decreased below an objective value in the feedback circuit. spinning of a super-aligned CNT forest. CNT yarn showed The voltage increases as the CNT tips are degraded and good field emission behavior with high efficiency and a decreases as new CNT tips are generated. Several degralong lifetime due to the unraveling effect. By applying a dation mechanisms of CNTs are known, but it is believed CNT yarn field emitter to an X-ray tube cathode, microthat the main mechanisms are field evaporation and detachscale resolution images were obtained. ment of CNTs. Field evaporation is a gradual degradation causing a gradual increase of the voltage on a large time Acknowledgments: This work was financially supscale; detachment is a catastrophic degradation causing ported by a grant (code#:10k ) from Censtep-wise increase of the voltage. In Figure 5(a), one can ter for Nanostructured Materials Technology under 21st observe both a gradual increase of the voltage on a large Century Frontier R&D Programs of the Ministry of Edutime scale and a step-wise increase of the voltage, often on cation, Science nad Technology in Republic of Korea. a short time scale. Thus, it can be deduced that degradait also suppoted by Priority Research Centers Program tion of the CNT yarn occurs by both field evaporation and through the National Research Foundation of Korea (NRF) detachment. In spite of the degradation, the current can funded by the Ministry of Education, Science and Techbe maintained at a constant rate because there are many nology ( ). available CNTs underneath the CNT yarn surface. Generation of new CNT tips is attributed to an unravreferences and Notes eling (or debundling) effect. Figure 6(a) shows an SEM image of the as-cut CNT yarn before field emission. Ini1. J. M. Kim, W. B. Choi, N. S. Lee, and J. E. Jung, Diam. Relat. tially, there are not so many CNT tips on the surface of the Mater. 9, 1184 (2000). 2. H. Murakami, M. Hirakawa, C. Tanaka, and H. Yamakawa, Appl. yarn. However, many CNT tips were generated by unravelphys. Lett. 76, 1776 (2000). ing during field emission, as shown in Figures 6(b) and (c). 3. S. H. Heo, A. Ihsan, and S. O. Cho, Appl. Phys. Lett. 90, Electrostatic force asserted on the CNTs is enough to (2007). deform or to detach them irreversibly.6 18 Thus, the CNTs 4. Z. Liu, G. Yang, Y. Z. Lee, D. Bordelon, J. Lu, and O. Zhou, Appl. Phys. Lett. 89, (2006). can be unraveled during field emission. Such an unraveling Fig. 5. (a) Stability of field emission current and voltage with time in diode mode, (b) voltage fluctuation at small time scale. J. Nanosci. Nanotechnol. 13, , Fig. 6. Microstructure evolution of the CNT yarn (a) SEM image of the as-cut CNT yarn, (b) SEM image of the CNT yarn after field emission.

5 5. P. Vincent, S. T. Purcell, C. Journet, and V. T. Binh, Phys. Rev. B 66, (2002). 6. Y. Saito, K. Seko, and J. I. Konoshita, Diam. Relat. Mater. 14, 1843 (2005). 7. M. Zhang, K. R. Atkinson, and R. H. Baughman, Science 306, 1358 (2004). 8. Y. L. Li, I. A. Kinloch, and A. H. Windle, Science 304, 276 (2004). 9. K. Liu, Y. Sun, L. Chen, C. Feng, X. Feng, K. Kiang, Y. Zhao, and S. Fan, Nano Lett. 8, 700(2008). 10. Y. Wei, D. Weng, Y. Yang, X. Zhang, K. Jiang, L, Liu, and S. Fan, Appl. Phys. Lett. 89, (2006). 11. AL. A. Zakhidov, R. Nanjundaswamy, A. N. Obraztsov, M. Zhang, S. Fang, V. I. Klesch, R. H. Baughman, and A. A. Zakhidov, Appl. Phys. A: Mater. 88, 593 (2007). 12. Y. Wei, K. Jiang, L. Liu, Z. Chen, and S. Fan, Nano Lett. 7, 3792 (2007). 13. X. Zhang, K. Jiang, C. Feng, P. Liu, L. Zhang, and J. Kong, Adv Mater. 18, 1505 (2006). 14. Q. Li, X. Zhang, R. F. DePaula, L. Zheng, Y. Zhao, and L. Stan, Adv Mater. 18, 3160 (2006). 15. C. J. Edgcombe and U. Valdre, Philos. Mag. B 82, 987 (2002). 16. G. Chen, S. Neupane, and W. Z. Li, Diam. Relat. Mater. 25, 134 (2012). 17. H. S. Lee, J. C. Goak, J. S. Choi, B. Y. Kong, C. H. Lee, K. B. Kim, J. Y. Park, Y. H. Seo, Y. C. Choi, Y. H. Songm, and N. S. Lee, Carbon 50, 2126 (2012). 18. Y. Saito, K. Seko, and J. I. Kinoshita, Diam. Relat. Mater. 14, 1843 (2005). Received: 1 May Accepted: 20 December J. Nanosci. Nanotechnol. 13, , 2013