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Nickel Overlayers Modify Precursor Gases to Pattern Forests of Carbon Nanotubes Reut Yemini, Anat Itzhak, Yossi Gofer, Tali Sharabani, Mark Drela, and Gilbert Daniel Nessim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 30 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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The Journal of Physical Chemistry

Nickel Overlayers Modify Precursor Gases to Pattern Forests of Carbon Nanotubes Reut Yemini a,c, Anat Itzhak a,c, Yossi Gofer a, Tali Sharabani a, Mark Drela b, Gilbert D. Nessim a,

*

a Department of Chemistry, Bar Ilan Institute for Nanotechnology and Advanced materials (BINA), Bar Ilan University, Ramat Gan, 52900, Israel b Department of Aeronautics and Astronautics, Massachusetts Institute of Technology 77 Massachusetts Ave., Cambridge, MA, 02139. USA c These two authors contributed equally * Corresponding author. Tel: +972 3 7384540. E-mail: [email protected] (Gilbert D. Nessim)

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Abstract We analyzed the effect of nickel overlayers positioned in close proximity (bridges) or in contact (stencils) with the catalytic layer on the growth of vertically aligned carbon nanotubes (VACNTs) using thermal chemical vapor deposition (CVD). We studied the physical-chemical mechanisms, namely the interaction of the overlayer with the gases and with the catalyst. We demonstrate that nickel inhibits CNT growth by adsorbing carbon to form graphene and by interacting with the gas precursors, leading to their modification into species that do not nucleate and grow CNTs. We demonstrate that the effect of the nickel bridge extends to the length of its boundary layer. We tested overlayer patterns and showed that the patterns were replicated during CNT growth. This facile method is a valid alternative to pattern CNT forests without the need for complex lithography and lift-off of the catalyst in applications where lithographic precision is not required.


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Introduction In the last decade, researchers made significant progress in the synthesis of unpatterned forests of carbon nanotubes (CNTs) using chemical vapor deposition (CVD).1 We can now control many output parameters such as CNT height, CNT diameter, crystallinity, etc. by appropriately modulating input parameters such as materials (catalyst,2,3 underlayer,4–6 reservoir,7 substrate,8 etc.), gas precursors (hydrocarbon type,9,10 flow rates,11 etc.), temperature (annealing,12 gas preheating,13,14 and growth15), and steps (sequence,16,17 and duration18). The electrical, mechanical, and thermal properties of CNTs make them excellent candidates for

many

applications

such

as:

scanning

probes,19

nanoelectronics,20

sensors,21

microelectromechanical systems (MEMS),22 field emitters23 and membranes.24 Although unpatterned forests of CNTs are adequate for some applications, more advanced CNT-based devices require local patterning to form regions with CNTs and regions without CNTs. The most prevalent patterning technique consists of using lithography of the catalyst followed by liftoff.25,26 Other approaches include evaporating the catalyst material on porous silicon or plain silicon substrates,27 using organic molecules on a substrate to mark and guide the self-assembly of individual single-wall carbon nanotubes,28 or strain-engineered manufacturing of freeform carbon nanotube microstructures.29,30 The above-mentioned methods provide a good precision in patterning the CNT forest, although they often require long (multi-step) or complex processes, may expose the catalyst to solvents, and often require lithography equipment. We recently showed a method where we positioned a patterned copper overlayer on the catalyst surface during thermal anneal to poison the catalyst via diffusion of copper into the iron catalyst.31 The subsequent growth step led to patterned CNT forests replicating the copper overlayer pattern. We present here an alternative technique using a patterned nickel overlayer.


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We will show that the main mechanism here is the local modification of the composition of the gas precursors during CNT growth (and not via interdiffusion with the catalyst) leading to areas devoid of CNTs where the nickel pattern is present. Additionally, this technique is quicker since it is performed in one step, without the need for pre-annealing. Overall, this technique is a quicker and more effective alternative to grow patterned CNT forests for applications that do not require lithographic precision.

Experimental Thermal chemical vapor deposition To synthesize forests of CNTs, we used a three-zone furnace thermal chemical vapor deposition (CVD) system with a fused silica (quartz) tube with 25-mm external diameter (and 22 mm internal diameter). In the first two zones (upstream) we preheated the precursor gases to decompose the hydrocarbons and to form controlled ppm’s of water vapor from oxygen and hydrogen.32 We introduced the sample in the third zone (downstream) of the furnace using the fast-heat technique to start the thermal process after gas flows and temperatures have reached equilibrium.33 The gases used were Ar (99.9999%), H2 (99.9999%), C2H4 (99.999%), and Ar/O2 (a mixture with 99% Ar and 1% O2). We controlled the mass flows using electronic mass flow controllers (MKS P4B) with digital control unit (MKS model 247D). During all experiments, the two preheating zones were set at 770 °C and the growth zone was set at 755 °C.

Sample preparation and experimental procedures The substrates were prepared using e-beam evaporation (5 rpm rotation, partial pressure of 10−6 Torr), to deposit 10 nm Al2O3 (underlayer) and 1.8 nm Fe (catalyst) on polished Si wafers


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(the crystal orientation (100)). We tested two different processes in which the nickel overlayer was either positioned in contact with the catalytic surface (stencil) or above it (bridge) (Fig. 1): (1) the nickel overlayer is on the catalyst surface for the whole annealing and growth durations or (2) the nickel overlayer is on the catalyst surface only during the pre-annealing step, with subsequent CNT growth without the overlayer.

Figure 1. Schematic figure of (a) nickel overlayer on the sample (stencil) and (b) nickel overlayer 525 µm (±25 µm) above the sample (bridge).

In the first process (one step, no pre-anneal), the steps were as follows: (1) purge with Ar and H2 (1000 and 400 sccm) for 10 min followed by an additional 10 min with the same gases but with Ar flow reduced to 100 sccm (the sample was positioned in the quartz tube section outside the furnace, which is at room temperature); (2) push the quartz tube inside the furnace to introduce the sample into the third zone and anneal for 5 min under the same Ar and H2 gas flows; (3) introduce C2H4 and Ar/O2 gases (200 sccm and 100 sccm respectively) to grow CNTs (20 min.); and (4) finally push the quartz tube outside the furnace to cool the sample under only Ar flow prior to removal. In the second process (two steps: pre-anneal followed by growth), the steps were as follows: (1) pre-anneal the sample with the nickel overlayer under a flow of Ar and H2 for 30 min; (2) cool the sample and remove the nickel overlayer; and (3) process the sample a second time,


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without the nickel overlayer, to grow CNT using the first process described above (purge, anneal, and growth). We collected the output gases and performed gas chromatography–mass spectrometry (GCMS) for two processes: one with a large nickel foil inside the growth zone and one without. The steps of the thermal process replicated those of the CNT growth process: (1) purge with Ar and H2 (1000 and 400 sccm) for 10 min followed by another 10 min with the same gases, but with the Ar flow reduced to 100 sccm; (2) anneal for 5 min under the same Ar and H2 gas flows; and (3) introduce C2H4 and Ar/O2 (200 sccm and 100 sccm respectively); the two preheating zones were set at 770 °C and the growth zone was set at 755 °C during all synthesis. The end section of the quartz tube, which is located outside the furnace, was immersed in a dry ice bath. After the thermal process, we washed with hexane the inside section of the quartz tube (that was immersed in dry ice) to extract the compounds that condensed on it, poured the solution in a vial, and performed GC-MS.

Characterization We characterized the samples using an environmental scanning electron microscope (SEM, Quanta 2000, from FEI) operating at 3 keV, high-resolution SEM (HRSEM, Zeiss 982). Selected CNTs were removed, dispersed in 2-propanol with gentle sonication for 1 hour, and then placed one drop of the solution on a 300-mesh Cu holey carbon grid (from SPI) and characterized it using high resolution transmission electron microscope (HRTEM, JEM- 2100 from JEOL) operating at 200 Kev. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis HS spectrometer with monochromatized Al Kα X-ray radiation source (photon energy 1486.6 eV). All spectra were acquired at pass energy of 80 eV and source power of 150


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W. Binding energies were recalibrated by setting the CC/CH component of the C 1s peak at 285 eV. The nickel foils were analyzed using a Horiba Jobin Yvon Raman spectrometer with a frequency of 532 nm. The GC-MS samples were analyzed using gas chromatograph (Agilent 7890A) coupled to the mass selective (Agilent 5975C MSD) and FID detectors. The compounds were separated on Rxi-5ms capillary column (30m×0.25mm, 0.25 µm, Restek). Helium was used as a carrier gas at 1.3 ml/min flow rate. The mass spectrometer was operated in a positive EI mode. The compounds were identified using MS data and NIST 05 library. Semi-quantitative results were obtained using FID responses relatively to biphenyl oxide, which was added to the samples as an internal standard.

Results and discussion To analyze how a nickel overlayer influences CNT growth, we synthesized a CNT forest grown on a Fe/Al2O3/Si catalyst/underlayer/substrate system with a strip of nickel foil positioned in the middle of the sample, in contact with the catalytic surface (stencil, Fig. 2.1). We observed that no CNTs grew where the Ni overlayer was positioned. We then tested the effect of a nickel overlayer positioned as a bridge 525 μm above the substrate (Fig. 2.2). This time we observed that not only no CNTs grew under the Ni bridge, but also no CNTs grew after the bridge until the end of the sample. To examine the limits of this effect, we tested a longer sample (Fig. 2.3) and observed that, at a certain distance from the bridge, CNTs started to grow again. The results obtained point to an interaction of the nickel with the gases and not necessarily of interdiffusion with the iron catalyst.


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Figure 2. Photo images and corresponding SEM micrographs of CNT forests: (1) nickel stencil present during all synthesis, (2) nickel bridge present during all synthesis, (3) nickel bridge present during all synthesis, but with a longer sample, (4) nickel stencil present during a 30 min anneal followed by a second step of CNT growth after removal of the overlayer. The blue squares on the photos indicate where the nickel overlayer was present. The scale bars for all SEM micrographs are 500 µm except for two images that required higher magnification (with scale bar 20 µm, as indicated).

To test our hypothesis, we did an additional experiment where we annealed a sample with a nickel foil positioned in contact with the catalytic surface (stencil) for 30 min (at the same temperature used for the CNT synthesis). After cooling the sample and removing the nickel overlayer, we reintroduced the sample into the furnace for CNT growth (Fig 2.4).We now


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observed that CNTs grew on the entire sample, including the area that was under the nickel foil. This experiment supports the hypothesis that interdiffusion between Ni (foil) and Fe (catalyst) is not at play here or that, if it occurs, it does not affect the catalytic process. To test whether (a) there was no interdiffusion between Ni and Fe at this temperature, or (b) there was a diffusion of Ni into Fe but it had no effect on CNT growth, we annealed for 30 minutes a sample with a nickel stencil positioned above the catalyst and characterized it using SEM. In Fig. 3.1, we observed no visible difference in the shape or size of the catalytic dots of iron between the three areas: before, under, and after the nickel overlayer.

Figure 3. Characterization of three different areas of the sample with the nickel stencil (a) area before the nickel stencil (upstream), (b) area under the nickel stencil, (c) area after the nickel stencil (downstream). (1) HRSEM images of catalyst morphology after 30 min. anneal with nickel stencil. The scale bar is 500 nm for all images. (2) HRTEM images of the CNTs grown in a subsequent growth step without overlayer (after the 30 min. anneal with the nickel stencil). The scale bar is 2 nm unless mentioned otherwise. ACS Paragon Plus Environment

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X-ray photoelectron spectroscopy (XPS) analysis of the surface of the sample indicates no peak in a binding energy that fits the presence of Ni (fig. 4a). Furthermore, when we examined the side of the nickel overlayer that was in contact with the sample, we observed only Ni related Auger peaks that are arising from X-ray induced Auger emission and no indication for the Fe presence (fig. 4b). Therefore, no nickel diffused on the sample surface and that no iron diffused in the nickel foil.

Figure 4. (a) binding energy range of nickel in the XPS spectra of the sample after 30 min annealing with the nickel stencil, (b) binding energy range of iron in the XPS spectra of the side of the nickel stencil in contact with the catalytic surface.

Using HRTEM, we analyzed the CNTs grown (with a subsequent CNT growth step) for the three areas and observed CNTs with comparable diameter and number of walls (Fig. 3.2). From this analysis, we can conclude that no detectable Fe/Ni interdiffusion occurred. The results of the experiments described confirm that the CNT deactivating mechanism depends solely on the interaction of the gases with the nickel overlayer. Since the gas compounds reaching the Fe catalyst are critical for CNT growth,14,26,28,33,34 we can now consider two possibilities regarding the effect of the nickel overlayer with the incoming gas compounds:


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(a) the carbon-containing gases adsorbed on the nickel foil, thus preventing carbon to interact with the Fe catalyst to nucleate and grow CNTs, and/or (b) the nickel interacted with the carboncontaining gases and modified them into species that, although they did reach the iron catalyst, did not nucleate CNTs. To test the first assumption, namely that the carbon-containing gases adsorb on the nickel overlayer, as nickel is known to be as a good catalyst for graphene synthesis,35 thus preventing carbon to interact with the FGCe catalyst to nucleate and grow CNTs, we did Raman on the

Figure 5. Raman spectra of: (a) the nickel stencil after CNT synthesis; several measurements on different spots on the same nickel foil confirm the presence of graphene; (b) sample of Fig. 2.3 in different growth areas after mild oxygen post-anneal; (c) sample of Fig. 2.4 in different growth areas


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nickel side that was in contact with the Fe catalyst (Fig. 5). Raman showed the typical G, D and 2D peaks of graphene, indicating that graphene grew on the nickel foil surface (albeit with defects and most likely few-layer, consistent with our past results using Ni to grow graphene.36 Thus, carbon adsorption on the Ni foil is indeed a factor that contributed to reduce the amount of carbon reaching the Fe catalyst in the proximity of the bridge (we will discuss later about the boundary layer region due to the bridge). The Raman spectra of the the CNTs exhibit D peaks that are higher than the G peaks, as can be expected in MWCNTs and very different the graphene spectra obtained. When we first did the Raman spectra of the sample in figure 2.3, we observed mild differences in the Raman spectra in the three areas. This indicates of some carbon residues and probably some amorphous carbon are present on the area where CNT growth was suppressed. In order to remove these carbon residues, we performed a short anneal in Ar/O2 (Ar with 1% O2) for up to 5 minutes as we showed in a previous publication31. This post-process cleaned the area with noticeable impact on the Raman spectra as can be seen in figure 5b with no observable G peak (blue line). For the sample pre-annealed with the nickel stencil (Fig. 2.4), we observed that the three zone present a similar Raman spectrum (Fig. 5.c). The major effect of aromatic compounds on CNT growth was shown in numerous papers.13,14,36,37 To test our second assumption, namely that nickel interacts with the carboncontaining gases and modifies them into species that do not nucleate CNTs, we performed gas chromatography - mass spectrometry (GCMS) to analyze the concentration of aromatic compounds in the gas composition with and without nickel in the CVD reactor. The results are summarized in the graphs in Figure 6 and in Table S1 (in supporting information).


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a

b

Figure 6. GC-MS analysis of the hydrocarbons collected from a synthesis with and without a nickel foil. (a) The distribution by number of aromatic rings of the hydrocarbons collected. (b) The distribution of aromatic compounds collected by name.

From the GC-MS results, we can make four key observations: 1. The concentration of aromatic compounds with one ring significantly increased when the Ni foil was present in the CVD reactor (e.g., toluene by 9X, ethylbenzene by 6X, and styrene by 4X).


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2. Xylene and propylbenzene, also one aromatic ring compounds, were only detected when the nickel foil was present during the thermal process. 3. The concentration of aromatic compounds with two or more rings was significantly reduced when the Ni foil was present in the CVD reactor (e.g. 2-vinylnaphtalene by 4X, naphthalene and acenaphthylene by 3X). 4. The largest aromatic compounds detected such as fluorene (3 rings), pyrene (4 rings), and fluoranthene (5 rings), which have been shown to favor both CNT growth33 and graphene growth34 were not detectable when the Ni foil was in the reactor during the thermal process.

From the above-mentioned observations, we can infer that the aromatic compounds with multiple rings, which initially constituted most the gas composition, were significantly reduced when Ni was introduced. More specifically, from the GC-MS data we observe that, by interacting with the nickel foil, approximately 2/3 of the large aromatic compounds converted into one-ring aromatic compounds. It is interesting that, based on a thermodynamic study, individual aromatic rings with less than 12 carbon atoms do not adsorb on nickel as they are not stable on the nickel surface.34 From the above, we can infer that an abundance of one-ring compounds is hindering CNT nucleation.


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Aerodynamic flow considerations can explain why CNTs start growing again far away from the Ni bridge: when the gas flow that did not interact with the nickel overlayer reaches again the catalyst surface somewhere away from the bridge, CNT growth restarts. We used boundary layer approximations to estimate the length of the “tail”, downstream to the bridge, where CNT growth was suppressed (ld in figure 7 b). Figure 7 a shows a sketch of the expected flowfield features, and Figure 7 b indicates the relevant dimensions. The estimated maximum height of the boundary layer is only 4 mm, much smaller

a

b

Figure 7. Sketched data for the boundary layer approximations. (a) expected flowfield features (b) relevant dimensions.


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compared to the furnace inside tube diameter of 22 mm, leaving plenty of volume for the flow of preheated hydrocarbon gas mix that was not in contact with the Ni overlayer to reach the catalytic surface at the end of the boundary layer, and restart CNT growth.

We took into account two cases: in the first case the transverse concentration gradient stays constant over the diffusion zone. This assumption is expressed in equation 1. . ⋅ ∇ ≃

1 1 ≃ ⇒ = = = 1.51 = 7.6## 6 6 0.332

In the second case, that is expressed in equation 2 we assumed that the transverse gradient linearly decays to zero, its average value over the diffusion front will be half the initial value: $. % ⋅ ∇&'()

1 2 ≃ ⟹ = = 3.02 = 15.1## 2 6

The measured length of the area of our sample with suppressed CNT growth is approximately16 mm; by subtracting the 5 mm width of the Ni bridge, the measured ld is 11mm, which fits in the range between the two limits described above. A more elaborate description of our estimate of the boundary layer can be found in the supporting information (Fig. S2).


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Based on all the above-described characterizations (morphology, Raman, XPS, GC-MS), the estimated boundary layer, and the results obtained, we can formulate a plausible mechanism for the results obtained based on three key points: 1.

Given the geometry of our reactor, gas flows, and position of the sample and of the bridge, there is a region, the boundary layer, where the incoming gases interacted with the bridge. The CNT growth in that area may be affected depending on the physical-chemical interaction between the gases and the bridge material.

2.

Nickel did affect the gases in the region of the boundary layer, hindering CNT growth. This was supported by the evidence that when we tried other metals, such as Ta, Ti, and Mo, we did not observe any effect on CNT growth. Also, the Raman detection of graphene nucleating and growing on the nickel foil confirms that nickel strongly interacted with the incoming hydrocarbons (no graphene was observed with bridges made with other metals). A final confirmation of the role of nickel in affecting the gas mix is given by our GC-MS results that clearly show specific enhancement or reduction of specific types of aromatic compounds.

3.

Based on all our characterizations and past reports on nickel interacting with hydrocarbons, we postulate that the suppression of CNT growth is caused by the nickel (a) adsorbing ethylene or other incoming hydrocarbons produced by the decomposition of ethylene and (b) converting ethylene or other incoming hydrocarbons produced by the decomposition of ethylene into specific aromatic compounds that do not favor CNT growth. Although the chemical-physical mechanisms at play are numerous and complex, we will attempt below to describe some plausible mechanisms based on our results and on past reports.


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The interaction of nickel with hydrocarbons is well documented. Kim et al. performed insitu GC-MS by flowing hydrocarbon mixtures over Ni particles in a tube reactor. They found that when a flow C2H4 and H2 in a ratio of 4:1 interacted nickel particles, almost 60% of the ethylene precipitated as solid carbon and another 20% was hydrogenized to form C2H6.38 Vattuone et al. studied the interaction of acetylene and ethylene with Pd{100} and Ni{100}. They found that with sufficient energy, complete dehydrogenation is observed for all the systems and hydrogen desorption takes place.39 Shah et al. used Ni for hydrogen production by catalytic decomposition of methane. They found pure nickel to be a very active catalyst, exhibiting the lowest initial methane decomposition temperature of all catalysts tested.40 Our GC-MS results, supported by our previous work from Somekh et al.36, showed that large polyaromatic compounds adsorb to the nickel to form graphene or convert into smaller onering aromatic compounds. However, since large aromatic ring compounds are also critical for CNT growth,13,34 this can explain why CNT growth is suppressed in proximity of the nickel foil. Ethylene decomposition is thermodynamically allowed at all temperatures.41 When there is sufficent activation energy, ethylene can self-decompose into carbon, dihydrogen and a large variety of volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs).41 When the preheated hydrocarbon mix and especially PAHs come in contact with the Ni foil bridge, they adsorbs onto of the Ni foil surface. Consequently the gas flow that is close to the sample in the post-bridge position (green area in figure 7a) has fewer PAHs and a lower carbon partial pressure in general, with subsequent hindrance to CNT growth.


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Since we introduce a small amount of oxygen in the system during the thermal process, it is also possible that the mechanism of this decomposition is similar to what happens in combustion systems. In combustion systems, carbon-based gases decompose mainly by a radical route and then undergo recombination to more complex carbon compounds such as VOCs, PAHs, amorphous carbon, and soot.36,39–46

Finally, we tested our technique with a simple pattern, with the synthesis results shown in Fig. 5. Overall, the technique presented is a simple, non-lithographic way to pattern CNT forests for applications that do not require lithographic precision.

a

b

Figure 8. Patterned CNT growth using nickel stencils. (a) Sample before synthesis with Ni stencils visible; (b) sample after synthesis of CNTs, with no CNTs growing where the Ni stencils were positioned during CNT growth.


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Conclusions In summary, we demonstrate a simple method of patterning CNT forests using nickel overlayers that locally suppress CNT growth in the areas where they are positioned. Multiple experiments and characterizations showed that the mechanism of the nickel overlayer for preventing CNT growth is by interacting with the incoming gases by adsorbing large aromatic hydrocarbons and by transforming them into one-ring aromatic hydrocarbons that do not catalyze CNTs. This study provides a facile one-step synthesis to grow patterned CNT forests for applications that do not require lithographic precision and opens the door to future research focused on understanding the effect of overlayers on gas precursors for the growth of other 1D and 2D nanostructures.

Supporting information Table S1: A table of GC-MS analysis of the hydrocarbons collected from a synthesis with nickel foil and a synthesis without. Figure S1: Detailed reactor structure Figure S2: Full description of aerodynamic assumptions and estimate of boundary layer.

Acknowledgements This research was partially funded by the Israel Science Foundation (ISF) through the Israel National Research Center for Electrochemical Propulsion (INREP) and I-CORE Program (number 2797/11).


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