Citation: Strobl, K.; Rajab, F. Air and O 2 -Assisted Catalytic VACNT Growth Optimization for Uniformity and Throughput. Processes 2023, 11, 1585. https://doi.org/10.3390/ pr11061585 Academic Editor: Andrea Petrella Received: 12 April 2023 Revised: 12 May 2023 Accepted: 15 May 2023 Published: 23 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Article Air and O 2 -Assisted Catalytic VACNT Growth Optimization for Uniformity and Throughput Karlheinz Strobl 1 and Fahd Rajab 2, * 1 CVD Equipment Corporation, 355 S. Technology Drive, Central Islip, NY 11722, USA; kstrobl@cvdequipment.com 2 Chemical Engineering Department, Najran University, Najran 11001, Saudi Arabia * Correspondence: fahdrajab7@gmail.com or fmrajab@nu.edu.sa; Tel.: +966-175428942 Abstract: The development of an optimized air or O 2 -assisted multi-wall vertically aligned carbon nanotubes (VACNT) process that adjusts the vertical height profile of a standard H 2 O vapor-assisted VACNT process is reported. The effect of the air or O 2 chemical vapor deposition (CVD) precursor flow rate, the catalytic Fe layer thickness, the process growth temperature, and the H 2 /C 2 H 4 ratio on VACNT length was first investigated to find the optimum growth conditions. Spatial distribution height mapping of VACNT structures on six patterned 4 ′′ catalyst Si wafers prepared with a 70–90 min long O 2 -assisted growth step shows an average growth height of 1.8–2.2 mm, with a standard deviation of less than 10%. Characterization techniques included Raman spectroscopy, scanning electron microscopy (SEM), and spatial height mapping analysis for a range of Fluid channel Array Brick (FAB) components with a length of 30 mm, a width range of 2.5–15 mm, a fluid channel diameter range of d = 5–100 mm, and a fluid channel closest gap range of g = 5–50 mm. A significant finding is that the O 2 -assisted VACNT growth process optimization efforts enable 2 mm parts processing with square edges, flat top surfaces, uniform height tolerances, and maximum catalyst wafer utilization for application in engineering devices. Keywords: VACNT; O 2 -assisted; air-assisted; FAB; CVD processing; super-growth 1. Introduction Since its introduction in 2004 by Hata et al. [1]H 2 O vapor-assisted vertically aligned carbon nanotubes (VACNT) growth, commonly known as the super-growth method for carbon nanotube (CNT) synthesis, is often used to increase the growth rate, catalyst lifetime, and growth height of VACNTs. There has been substantial research into the kinetics and mechanism of VACNT growth [2]. It has been determined that the catalyst-substrate interaction in the tip growth mechanism of VACNT occurs when hydrocarbon decomposes on the metal’s top surface, allowing carbon to diffuse through the metal catalyst, causing CNT to precipitate out across the metal bottom, dislodging the entire catalyst particle from the substrate. As long as additional hydrocarbon breakdown can occur at the metal’s top, CNT will continue to grow vertically. The metal’s catalytic activity declines and CNT growth stops after it is entirely coated with excessive carbon [2–4]. The super-growth method has been used, for example, to grow thus far the tallest (up to 21 mm tall) VACNT structures with a specialized catalyst structure on cm-size catalyst wafers [5]. Although Sugime et al. reported a carbon nanotube forest with a length of 14 cm in a 26 h growth step, the carbon nanotubes are not vertically aligned [6]. Odunmaku et al. showed a chlorine-assisted chemical vapor deposition (CVD) method for preparing up to 2 mm multi-wall VACNT, filled with catalyst particles when vapor flow rates were lower than 15 mL/min, as seen in FIG 2B [7]. There is a significant lag between scientific discovery and technological implementation concerning CNTs [8–10]. Recently, we used a modified super-growth method to improve the growth height and growth rate for >4 mm VACNT Processes 2023, 11, 1585. https://doi.org/10.3390/pr11061585 https://www.mdpi.com/journal/processes