Chemical Vapor Deposition Substrate Attack Angle—Implications on Vertically Aligned Carbon Nanotubes Growth Efficiency

Vertically aligned carbon nanotubes (VACNT) offer three key benefits: an exceptionally high specific surface area, outstanding thermal and electrical conductivities, and adjustable directionality. Traditionally, yield optimization research on chemical vapor deposition (CVD) for synthesizing VACNT has focused on the chemical reactions between reactive gas and catalyst surfaces, yet the fluid dynamics within CVD reactors, which govern the transportation of reactive species to the catalyst, are equally crucial. Experiments show that during VACNT synthesis, the sample's center experiences 'growth starvation' due to less carbon feedstock reaching it compared to the periphery, resulting in inhomogeneous growth with a sunken middle. Extreme cases can even lead to the center collapsing. In this work, we combine computational fluid dynamics (CFD) and experimental data to investigate what causes the collapsing phenomenon, using pragmatic, first-order approximated simulations to depict a simple physical flow and its interaction with a solid object, revealing a deflection pattern. The simulations demonstrate that increasing sample height leads to a thicker flow boundary layer. In order to reduce the boundary layer thickness, the substrates were simulated to be at positive attack angles (10° through 30°) with respect to inbound gas flow. While all attack angles reduce boundary layer thickness across all sample heights, the simulations of 15°, 20°, and 25° showed an exceptionally thin boundary layer. Based on these simulations, jigs were manufactured to hold substrates at precise attack angles (15°, 20° and 25°) during CVD growth for various durations (4, 7, 10 and 13 minutes). While all experimented attack angles resulted in better growth performance compared to horizontally oriented non-jigged samples, samples mounted at 25° exhibited the most significant improvement—25% or 600-µm CNT. This research integrates CFD simulations and experimental data, investigating the impact of substrate attack angle on the growth behavior of VACNT samples and highlighting that yield optimization for nanomaterials extends beyond nano-scientific tools alone. On a broader scale, outside the specific application of VACNT growth, the research may indicate the catalysts are not exploited for their full potential, and efficiency enhancement is within reach.