Synthesis of Carbon Nanomaterials on Preoxidized Steel Substrates by means of Plasma Discharge

Abstract

Carbon nanotubes (CNTs) are emerging and critical nanomaterials for advanced energy storage systems due to their exceptional, structural and electrical properties. This thesis explores a novel method of CNT synthesis via plasma-assisted chemical vapor deposition (PACVD), focusing on the effects of substrate composition, oxidation pretreatment, and furnace power density on CNT morphology. The objective was to develop a controlled, scalable synthesis route that enables high-quality CNT growth tailored for energy-related applications. A systematic experimental investigation was conducted using two steel substrates—EN 1.4301 and EN C15— under varying oxidation regimes and four plasma power densities: M2074 (40 W/m²), M2101 (92 W/m²), M2105 (275 W/m²), and M2108 (550 W/m²). Additionally, a modified gas composition was tested by replacing argon (Ar) with nitrogen (N₂) under the same conditions as M2101; this experiment is referred to as M2114. The results revealed a pronounced influence of substrate composition on CNT growth efficiency. EN C15 consistently outperformed EN 1.4301, exhibiting longer and thicker bamboo-like CNTs. This behavior was attributed to EN C15’s simpler alloy structure and enhanced catalytic responsiveness, particularly under optimized oxidation conditions at 250 °C for 30 minutes and 500 °C for 3 minutes. In contrast, the chromium-rich surface of EN 1.4301 inhibited catalyst mobility, leading to reduced CNT length and diameter. The influence of plasma power density added another dimension to the morphological control of CNTs. Moderate power settings (M2105 – 275 W/m²) provided the most consistent and stable growth environment, balancing catalyst activation and structural precision. Higher power densities (M2108 – 550 W/m²) accelerated growth and diameter expansion, especially for EN C15, but introduced instability with prolonged exposure. Conversely, low power input (M2074 – 40 W/m²) proved insufficient for effective CNT formation on both substrates. The use of nitrogen instead of argon in M2114 likely failed to remove the oxide layer, which may have hindered proper CNT formation. Oxide particle size analysis further highlighted the critical role of thermal oxidation in determining catalyst morphology. Lower temperatures yielded smaller, more uniform particles favorable for fine CNT growth, whereas high-temperature oxidation induced rapid particle coarsening, enhancing CNT thickness but at the cost of structural uniformity. These findings underscore the importance of synchronizing substrate chemistry, oxidation parameters, and plasma energy input to enable targeted control over CNT characteristics. This thesis contributes a customizable framework for PACVD-based CNT synthesis, demonstrating that strategic tuning of substrate material, oxidation conditions, and power density enables precise control over nanotube growth. The results help improve how CNTs are made and can be used in many energy-related areas. This also helps create nanomaterials that work better and are more environmentally friendly.

Date of Award	2025
Original language	English
Supervisor	Christian Forsich (Supervisor)