Characterization | Sustainable Energy and Reacting Flow

1. Synthesis setup-methane diffusion flame

*General synthesis setup for carbon nanotubes growth.*

The CNT synthesis setup employed a laminar methane diffusion flame burner as shown in Figure 1 with a concentric fuel and oxidizer outlet tube with a 17 mm and 26 mm diameter, respectively. The positioning system is installed for the sampling method, where the catalyst substrates or nickel wire were inserted horizontally into the flame at desired location and various heights above burner (HAB) with high accuracy and repeatability. The entire synthesis was done at atmospheric pressure within a burner enclosure equipped with an extractor fan to minimize the surrounding air’s entrainment and maintain continuous gas flow. The methane gas of 99.995% purity flows out of the central tube, controlled by Omron flow sensor at a fixed rate of 0.3 slpm. The oxidizer consists of a mixture of oxygen 99.9% and nitrogen 99.9% purity, controlled by a precision metering valve with HoneywellTM sensors, flows out of the concentric tube at a various flow rates as needed. The samples were exposed in the flame at various duration as well.

2. Flame characterization

Effects of parameter changes is also studied towards the characteristics of the standing flame and its effects towards CNT growth.

Images of standing methane diffusion flame at varied fuel and oxygen concentrations. Effect of fuel concentration toward standing flame with 24% oxygen concentration are represented by images (a), (b), and (e) with fuel concentration of 100%, 66%, and 50%, respectively. Images (c)–(f) shows effect of increasing oxygen concentration from 19% to 27% respectively in flame with 50% fuel concentration. The corresponding flame under wire mesh image for each flame is taken at an angle with fixed 10 mm HAB

3. Wire-based Macro-Image Analysis (WMA) and growth on nickel wire

The present study provides a detailed sensitivity and accuracy evaluation of a simple wire-based macro-image analysis (WMA) method for the measurement of steady-state and temporal change of growth region size for multiwalled carbon nanotubes (MWCNT) in a methane diffusion flame with varied air flow rate at atmospheric condition. The WMA method was developed to simplify the MWCNT deposit region identification using the image produced by digital single-lens reflex (DSLR) camera and post-processed using baseline data that was gathered through a one-time SEM analysis. Pure nickel wire was positioned in the flame with a stainless-steel wire grid placed on top of the substrate wire to redistribute the flow field. The MWCNT deposit region that is confined in the fuel-rich region at the inner side of the flame sheet shifts toward the flame centerline with the increase of the air flow rate from 1 to 10 slpm due to the shift of the flame sheet in the same direction. The inception and growth of MWCNT are consistently observed together with the formation of amorphous carbon layer, which has been verified based on detailed SEM analysis. WMA method is proven to be a simple yet reliable measurement of CNT deposit region with minimal use of SEM images processing, which is especially practical for the growth region analysis in a large environment of heterogeneous synthesis.

*SEM images at respective location on the nickel wire*

*Growth region identification at various flow rate identified through WMA method*

FESEM and TEM images of the synthesized CNT. (a) to (c) are FESEM images of deposit region after 30 s of exposure in flame before fully covered with CNT once the exposure time increases. (a) 50k magnification at the center of CNT growth region, (b) 10k magnification of CNT growth region leading-edge toward flame centerline, and (c) 50k magnification of CNT forest at the leading edge of the CNT growth region. Meanwhile, (d)–(f) are TEM images of the synthesized CNT. (d) Typical bent CNT found in most samples tested, (e) multiwall carbon cylindrical nanofibers with nickel catalyst nanoparticle, and (f) nano-onions

4. Growth of CNT on silicon substrate

*Black spots seen on silicon substrate marking the growth of CNT*

CNT has been successfully grown on silicon substrate via catalyst impregnation (titration) method. The formation of nanoparticles and growth of CNT is observed and analyzed. Physically, the black spots on the substrate indicate the area where possible CNT has grown.

*FESEM images of the grown CNT at (a) 1K magnification, (b) 10K magnification, and (c) 50 K magnification. Bright white spots indicate the catalyst particles*

FESEM images confirmed the formation of tubular, straight, and long CNT strands with average diameter of 38nm.