The preparation of carbon nanotubes by flame synthesis involves a modified combustion system with a suitable carbon source, heat source and catalytic material. To this end, building carbon nanotubes on a copper (Cu) substrate by flame synthesis is a new approach to achieve Cu-based carbon nanotube compounds.
Study: Synthesization of copper and carbon nanotube compounds by methane diffusion flame. Image credit: Evannovostro / Shutterstock.com
In a recent article in the journal Materials Today: Proceedings, researchers fabricated Cu-based carbon nanotube compounds by flame synthesis to improve the electrical and thermal properties of the Cu material. The cleaning and etching of Cu substrates with concentrated sulfuric acid was the initial step towards Cu-based carbon nanotubes.
The cleaned and etched Cu substrates were subjected to two different laminar flames using methane. The standard diffusion flame (NDF) configuration showed a blue flame, protecting the carbon-rich yellow flame without a clear distinction between the flames. On the other hand, the inverse diffusion flame (IDF) configuration showed a clear separation between the blue and yellow flame, creating two different temperature zones.
This flame synthesis achieved favorable conditions for synthesizing carbon nanotubes on Cu substrate. The Cu-based carbon nanotubes manufactured had tubular diameters of 20 to 30 nanometers. Thus, with the help of the present study, the researchers realized that a new flame synthesis is a cost-effective approach for the construction of Cu-based carbon nanotube compounds.
Synthetic strategies towards carbon nanotubes
Carbon nanotubes are flat sheets of graphene folded into a tube. Its incorporation into transition metals results in an advanced composite material that takes advantage of the favorable properties of carbon nanotubes such as current carrying capacity, electrical conductivity, mechanical strength, and thermal conductivity.
Previous reports mentioned electrodeposition or dust processing approaches as primary synthetic strategies for metal-based carbon nanotubes. Studies have shown that the electrodeposition method is more advantageous than the powder processing method due to the reduced possibilities of agglomeration and a high carbon nanotube loading factor. However, due to incubation for an extended period and a high temperature vacuum environment, the electrodeposition method becomes an expensive approach.
In flame synthesis, carbon and heat sources are obtained from pyrolysis and combustion. Hydrocarbon is used as a cost-effective energy source to synthesize carbon nanotubes on metal substrates. In addition, previous reports primarily mentioned nickel and other transition metals as metal substrates, while copper as a metal substrate for synthesizing carbon nanotubes remains relatively unexplored.
Among the flame structures reported above in flame synthesis, NDF and IDF are safe configurations for their operations, as they are devoid of critical explosion risks due to flame returns.
Composites of copper and carbon nanotubes through the methane diffusion flame
The present study used 3 mm nickel and Cu grids with a pitch of 400 x 62 micrometers as metal substrates. These grids were initially cleaned with acetone and then etched with sulfuric acid. The methane-based NDF flame was divided into molecular, growth, and oxidation zones.
The molecular zone facilitated the decomposition of methane into carbon nanotube precursors. In the growth zone, precursors of carbon nanotubes such as carbon monoxide (CO) and pyrolyzed carbon molecules were deposited on the surface of the catalyst and were used to form carbon nanotubes.
The growth zone is mainly concentrated in the yellow flame region which contains a significant concentration of carbon nanotube precursors. In addition, these precursors were delivered to the oxidizing zone of the yellow flame that encapsulates the blue flame. However, placing the Cu substrate in the oxidizing zone will make it difficult to structure the carbon nanotubes by breaking them down into carbon dioxide and water. Thus, the growth zone of the NDF flame is critical for carbon deposition and the growth of carbon nanotubes.
Scanning electron microscopy (SEM) images of carbon nanotubes on nickel substrate revealed tubular structures 20 to 60 nanometers on the surface of the nickel grid after incubation of NDF for 5 minutes. In contrast, SEM images from the Cu grid incubated under the same conditions did not show the formation of Cu-based carbon nanotubes despite their treatment with concentrated sulfuric acid.
The IDF consisted of an orange-yellow flame, which protected the blue flame conically, where a clear stratification was observed between the oxidation and growth zones. Excess fuel presence mainly contributes to the yellow flame and the pyrolysis of methane, supported by the blue flame. The yellow flame of the IDF had a purpose similar to the NDF, as the growth zone of carbon nanotubes, while the blue flame consists of an oxidation zone. SEM images revealed that, unlike NDF, incubation in IDF resulted in carbon nanostructures 20 to 30 nanometers in diameter on the Cu substrate.
Conclusion
In general, improved control of temperature, flame shape, and proper concentration of carbon nanotube precursors was achieved by the arrangement of the diffusion flame. Consequently, the growth of carbon nanotubes was controlled in IDF and NDF configurations. The IDF configuration with different separation of flame zones allowed the incubation of Cu at higher temperatures.
These high temperatures provided an improved amount of carbon nanotube precursors without compromising the Cu substrate. Thus, the configuration of the IDF was advantageous over its NDF due to the generation of a different separation between the oxidation and growth zones, suggesting that the IDF was a suitable medium for manufacturing carbon nanotubes. Cu-based by flame synthesis.
Reference
Com, HC., Chow, YL., Wong, HY., Ho, JH., Law, CH. (2022). Synthesization of copper and carbon nanotube compounds by methane diffusion flame. Materials Today: Proceedings. https://doi.org/10.1016/j.matpr.2022.06.489
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