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Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the… Dahmardeh, Masoud; Nojeh, Alireza; Takahata, Kenichi 2011

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Possible mechanism in dry micro-electro-discharge machining of carbonnanotube forests: A study of the effect of oxygen Masoud Dahmardeh, Alireza Nojeh, and Kenichi Takahata Citation: J. Appl. Phys. 109, 093308 (2011); doi: 10.1063/1.3587158 View online: View Table of Contents: Published by the American Institute of Physics.  Related Articles Formation of graphene nano-particle by means of pulsed discharge to ethanol J. Appl. Phys. 113, 114304 (2013) Atomic structure of tensile-strained GaAs/GaSb(001) nanostructures Appl. Phys. Lett. 102, 102105 (2013) Fabrication and optical properties of large-scale arrays of gold nanocavities based on rod-in-a-tube coaxials Appl. Phys. Lett. 102, 103103 (2013) The induction of nanographitic phase on Fe coated diamond films for the enhancement in electron field emission properties J. Appl. Phys. 113, 094305 (2013) Note: Size effects on the tensile response of top-down fabricated Si nanobeams Rev. Sci. Instrum. 84, 036102 (2013)  Additional information on J. Appl. Phys. Journal Homepage: Journal Information: Top downloads: Information for Authors:  Downloaded 20 Mar 2013 to Redistribution subject to AIP license or copyright; see  JOURNAL OF APPLIED PHYSICS 109, 093308 (2011)  Possible mechanism in dry micro-electro-discharge machining of carbon-nanotube forests: A study of the effect of oxygen Masoud Dahmardeh, Alireza Nojeh,a) and Kenichi Takahataa) Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver BC, V6T 1Z4, Canada  (Received 9 February 2011; accepted 2 April 2011; published online 13 May 2011) The working principle of dry micro-electro-discharge machining of vertically aligned carbon-nanotube forests is investigated by evaluating the effect of oxygen on the process. The machining experiments with controlled oxygen/nitrogen ratios indicate a correlation between the peak current of discharge pulses and the oxygen concentration, suggesting not only a vital role for oxygen in the process, but also a removal mechanism fundamentally different from that in typical electro-discharge machining based on direct melting and evaporation of the sample material. The highest surface quality and uniformity in the machined forest microstructures as well as smooth machining without short circuiting are achieved at an approximate oxygen concentration of 20% under the discharge condition of 30 V and 10 pF, revealing that air is an optimal medium for the removal process. Elemental and molecular analyses show no evidence of significant crystalline deterioration or contamination in the nanotubes processed C 2011 American Institute of Physics. [doi:10.1063/1.3587158] with the technique. V  I. INTRODUCTION  Vertically aligned carbon nanotubes, so called CNT forests, have attracted extensive interest due to their unique electrical, mechanical, thermal, and other properties.1–3 This material provides a broad range of application opportunities, including field emitters,4 chip-cooling heat sinks,5 supercapacitors,6 biomimetic dry adhesives,7 and micro-electro-mechanical systems (MEMS).8 To design and fabricate devices based on CNT forests, there is a fundamental need to define the dimensions of the forests precisely. Lithographic techniques were reported for patterning of disordered CNT layers.9,10 For patterning of aligned CNT forests, selective chemical vapor deposition (CVD) of the material on predefined catalyst patterns has been widely used.11–13 The shapes of the grown forests are, however, primarily limited to 2-dimensional-like structures with a uniform height. To facilitate the application of this material to MEMS and other disciplines, it is essential to establish techniques to create 3-dimensional (3-D) free-form microstructures from pregrown forests. Micro-electro-discharge machining (lEDM) of carbon nanofibers14,15 as well as DC arc discharge machining of CNT forests16 have been reported for surface patterning of the materials with low aspect ratios. A pulsed lEDM process for 3-D, high-aspect-ratio micromachining of pure CNT forests has recently been developed by the authors17,18; this technique has been shown to be highly effective in forming arbitrary micro-scale geometries in the forests, as demonstrated in the produced sample structures shown in Fig. 1. lEDM utilizes pulses of thermomechanical impact induced by a miniaturized electrical discharge generated between a microscopic electrode and a workpiece that can essentially be any electrically conductive material.19 The miniaturized arc discharge locally melts and evaporates the material at the arc spot, and a)  Authors to whom correspondence should be addressed. Electronic addresses: and  0021-8979/2011/109(9)/093308/4/$30.00  micromachining is performed by repeating the unit removal by a single pulse at high frequencies while controlling the relative position between the electrode and the workpiece. The typical lEDM process for bulk materials is conducted in a dielectric liquid (such as oil or de-ionized water). On the other hand, to the best of the authors’ knowledge, all the lEDM processes of pure CNTs have been performed in dry ambient, specifically in air, mainly in order to avoid the collapse of the forest structures due to the capillary force resulted when the wetted structures are dried (regular lEDM in oil for CNT-polymer composites has been reported in Wan et al.20). However, the effects of the ambient gas in the dry lEDM for CNT-forest micromachining have not been studied. Kunieda et al. showed that the efficiency of macro-scale dry EDM for metal machining could be improved by supplying oxygen to the ambient.21 In the present paper, the effect of oxygen in CNT-forest lEDM is investigated, revealing a strong dependence of the process on the oxygen concentration in the ambient of nitrogen. II. SAMPLE PREPARATION AND EXPERIMENTAL SET-UP  The CNT forest samples used in this study were grown on silicon substrates with iron catalyst through an ethylenebased CVD process at 750 C; the catalyst-substrate preparation and the CVD process followed the same conditions reported in Khalid et al.17 Forests of vertically aligned multiwalled CNTs (MWNTs) with lengths of up to several 100 lm were obtained. lEDM experiments were carried out with a 3-axis lEDM machine (EM203, SmalTec International, USA) that employed relaxation-type resistor-capacitor (R-C) circuitry for pulse generation/timing.22 As opposed to the DC arc technique,16 this lEDM process, which uses nanosecond pulses of arc discharge for machining, potentially enables precise control of discharge energy delivered to a CNT forest while protecting the sample from overheating.23  109, 093308-1  C 2011 American Institute of Physics V  Downloaded 20 Mar 2013 to Redistribution subject to AIP license or copyright; see  093308-2  Dahmardeh et al.  J. Appl. Phys. 109, 093308 (2011)  FIG. 3. SEM images of lEDMed CNT forests in N2 gas mixed with (a) 0% O2 (oxygen free), (b) 10% O2, (c) 21% O2, (d) 50% O2. Drilling results in (e) pure N2 and (f) air.  voltage 30 V; capacitance 10 pF; electrode rotation speed 3000 rpm; X–Y feed rates 1 mm/min) that was previously developed for lEDM of CNT forests in air.17 FIG. 1. Needlelike structures with controlled sidewall angles created by dry C 2011 IEEE. lEDM18 V  The experimental set-up used for dry lEDM characterization is shown in Fig. 2. Oxygen is first mixed with nitrogen, an inert dilute gas, inside a buffer chamber, and the mixed gas is introduced to the machining chamber, where the O2 concentration is measured using an oxygen sensor (VN202, Vandagraph Co., UK). The flow rates of O2 and N2 are adjusted so that the O2 concentration reaches the target value and is stabilized in the machining chamber for at least 10 mins prior to machining. In this study, the O2 concentrations of 0% (oxygen free), 6%, 10%, 21% (approximately equal to the ratio in air), and 50% were tested. A tungsten electrode (32–100 lm diameter) and the CNT forest are connected as the cathode and the anode, respectively, with the R-C circuit as shown in Fig. 2. The electrode feed rate in the vertical (Z) direction was set to be 0.5 lm/s. The lEDM machine has a feedback control system such that when a short circuit between the electrode and the sample (which prevents the discharge generation, i.e., material removal) is detected, the system retracts the electrode up while checking the status of the short circuit and resumes machining by feeding the electrode as soon as the circuit is opened. The machining experiments were conducted with an optimal condition (machining  FIG. 2. (Color online) Experimental set-up for dry lEDM of CNT forests with controlled oxygen concentrations in O2/N2 ambient.  III. RESULTS AND DISCUSSION  Figures 3(a)–3(d) show the microstructures machined with different O2 concentrations and the machining conditions noted above, by scanning a 100-lm-diameter electrode along a rectangular pattern (300 lm  400 lm) in the X–Y directions while feeding the tool in the Z direction to a depth of 40 lm. The results indicate that the process at 0% O2 [Fig. 3(a)] resulted in the lowest removal quality, and that the uniformity and the surface smoothness of the machined structures were consistently improved as the O2 concentration was increased to 21% [Fig. 3(c)]. As can be seen in the drilling results in N2 [Fig. 3(e)] and in air [Fig. 3(f)] performed under the same EDM conditions, the CNT removal in air is as effective as the 21% O2 case. Machining in 50% O2 [Fig. 3(d)], however, led to rougher surfaces compared to those at 21% O2 as apparent from the SEM images. To visualize the progression of electrode feeding and the impact of oxygen on the feeding, the Z position of the electrode was tracked during drilling of a forest to a depth of 50 lm with different O2 concentrations [Fig. 4]. The results with the 21% and 50% O2 concentrations showed smooth feeding with no short-circuit detection, reaching the target depth  FIG. 4. (Color online) Progressions of electrode feeding in lEDM of a CNT forest with different O2 concentrations in N2 at 30 V and 10 pF.  Downloaded 20 Mar 2013 to Redistribution subject to AIP license or copyright; see  093308-3  Dahmardeh et al.  FIG. 5. (Color online) Measured average peak current of discharge pulses as a function of the O2 concentration at 30 V and 10 pF (the inset shows a typical pulse observed in 21% O2 at 30 V and 10 pF).  within the ideal machining time of 100 seconds (i.e., [50 lm]/[0.5 lm/s]). As the concentration was lowered, zigzag patterns appeared, due to the occurrence of short circuits and the resultant controlled retraction of the electrode. It is clearly seen that lowering the O2 concentration below 21% deteriorates the machining efficiency. For the oxygen-free case, machining beyond 10 lm of depth was not achieved. To probe the EDM dependence on oxygen, the discharge current was measured under identical EDM conditions (30 V, 10 pF) with varying O2 concentrations using a current probe (CT-1, Tektronix, USA) inserted in the discharge circuit, as shown in Fig. 2. Figure 5 plots the average peak current of discharge pulses (n ¼ 600) measured as a function of the O2 concentration. It can be seen that the average peak current drops as the O2 concentration increases and saturates ($13 mA) at around 21% O2. This saturation is likely related to the results in Fig. 4, which show similar straight feeding paths for the O2 concentrations of 21% and 50%. The discharge current is the highest ($23 mA) at 0% O2, while proper machining at this condition is barely feasible. This condition does not follow the typical relationship between the discharge current and the material removal rate in regular EDM, in which larger discharge currents lead to higher removal rates in general. It has been suggested that the lower current carrying capacity of MWNTs in the presence of oxygen is mainly because of the loss of individual carbon shells due to thermal oxidation.24,25 This characteristic is consistent with the measured result in Fig. 5, which shows the highest current in the absence of oxygen and little removal of CNTs. Based on the results observed, the removal of CNTs in the lEDM process may be related to the thermally enhanced oxidation, rather than direct melting/evaporation due to heat provided by the discharge pulses as the typical removal mechanism in EDM; in other words, this CNT lEDM may essentially be a pulsed process of local oxygen plasma etching of the nanotubes. To evaluate the structures of the processed nanotubes, high-resolution scanning electron microscopy (SEM) was performed for the surfaces of the machined structures shown in Fig. 3 and for an unprocessed area near the structures  J. Appl. Phys. 109, 093308 (2011)  FIG. 6. SEM images of (a) original CNT forest before lEDM, and lEDMed CNT forest (b) in air, and with O2 concentrations of (c) 0%, and (d) 50% in N2 (scale bar size in each image is 200 nm).  [Figs. 6(a)–6(d)]. For low concentrations of O2 (up to 21%), some of the CNTs developed sharp tips (indicated by arrows in Fig. 6(c)), resembling the needle-shaped bundles of CNTs after plasma etching reported in Liu et al.,26 while other CNTs exhibited different morphologies with rougher surfaces compared to those of the original CNTs with smooth surfaces (similar results were reported in Yu et al.27). For 50% O2, thickening of CNTs is evident; this could be related to the formation of thicker bundles of individual CNTs and/or the adsorption of the carbon debris produced in the machining process.17 The analysis of the machined samples with energydispersive X-ray spectroscopy (EDX) showed no noticeable difference between the results with different O2 concentrations. Tungsten was not observed in the EDX results, suggesting negligible electrode consumption during the lEDM process under the employed machining conditions. In order to evaluate the impact of lEDM on the crystalline properties of the CNTs, Raman spectra were collected from the lEDMed regions in the structures shown in Figs. 3(a), (c), and (d) machined in the forest with the O2 concentrations of 0%, 21%, and 50%, respectively, as well as from the original CNTs in the same forest for comparison [Fig. 7]. The G mode that arises from the sp2 C crystalline structures and the D mode related to crystalline defects28 are seen in the collected data. As shown in the graph, the ID/IG ratios in the original and lEDMed forest surfaces are very close (0.76–0.79), suggesting that the impact of lEDM on the CNT’s crystalline properties under the used conditions is minimal. The DC arc discharge machining of CNT forests has been reported to lower the ID/IG ratio.16 It is also known that high-density oxygen plasma treatment of CNTs causes defects in them.29 The Raman analysis in the present study provided no evidence of significant ID/IG ratio reduction or increased defects in the CNTs processed with the pulsed lEDM used. A closer look at Fig. 7 reveals that for the oxygen-free (0%) condition, the G and D peaks are shifted by $22 and $12 cmÀ1 from the peaks (at 1570 and 1340 cmÀ1, respectively) for the nonzero O2 concentrations to the higher wavenumber side. The shifts were observable in Raman data from some locations on the surfaces  Downloaded 20 Mar 2013 to Redistribution subject to AIP license or copyright; see  093308-4  Dahmardeh et al.  J. Appl. Phys. 109, 093308 (2011)  ported by the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation, and the British Columbia Knowledge Development Fund. 1  FIG. 7. (Color online) Raman spectra of the CNT forest before and after lEDM with different O2 concentrations in N2.  machined under the oxygen-free condition but were not consistent over the entire area of the surfaces. It has been suggested that the compressive thermal strain causes shifts of the G-mode peak.30–32 In the machining process, uncontrolled spontaneous large sparks were occasionally observed at low concentrations (0–10%) of oxygen; the observed Raman shifts in the oxygen-free case could be related to high current stressing of the carbon nanotubes caused by the large sparks. IV. CONCLUSIONS  The effect of oxygen in dry lEDM of CNT forests was investigated. The experimental results revealed that oxygen plays a critical role in the removal process, suggesting that localized oxygen plasma etching induced by pulsed arc discharges, unlike the direct thermal removal in regular lEDM, may be the main removal mechanism of the process. It was found that the use of oxygen-free, 100%-nitrogen ambient prevents proper CNT removal. The process at the O2 concentration of $21% in nitrogen achieved not only the highest machining quality among all O2 concentrations tested but also efficient CNT removal, without suffering from short circuiting during the process. This suggests that air may be a suitable, and in fact, optimal medium for dry lEDM of the forests. Thickening of the tips of the processed CNTs was observed, which could be related to the adsorption of the removed carbon atoms and bundling of the CNTs. The Raman and EDX analyses, however, suggest that the lEDM process may not cause significant crystalline deterioration in the CNTs and contamination with the electrode elements. ACKNOWLEDGMENTS  The authors thank Parham Yaghoobi for his assistance in CNT sample preparation. This work was partially sup-  N. Hamada, S. Sawada, and A. Oshiyama, Phys. Rev. Lett. 68, 1579 (1992). 2 J. P. Lu, Phys. Rev. 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