Researchers Improve Electrical and Thermal Properties of Carbon Nanotubes

The preparation of carbon nanotubes via flame synthesis involves a modified combustion system with a carbon source, heat source and suitable catalytic material. To this end, constructing carbon nanotubes on copper (Cu) substrates via flame synthesis is a novel approach to achieve Cu-based carbon nanotube composites.

Study: Synthesis of copper-carbon nanotube composites via methane diffusion flame. Image Credit: Evannovostro/Shutterstock.com

In an article recently published in the journal Today’s Material: Proceedings, researchers made Cu-based carbon nanotube composites through flame synthesis to improve the electrical and thermal properties of Cu materials. Cleaning and etching of Cu substrates with concentrated sulfuric acid is the first step towards Cu-based carbon nanotubes.

The cleaned and carved Cu substrates were subjected to two different laminar flames using methane. The standard diffusion flame configuration (NDF) shows a blue flame, shielding a carbon-rich yellow flame with no clear distinction between flames. On the other hand, the reverse diffusion flame configuration (IDF) shows a clear separation between blue and yellow flames, creating two different temperature zones.

This flame synthesis provides favorable conditions for synthesizing carbon nanotubes on Cu substrates. The Cu-based carbon nanotubes made have a tube diameter of 20 to 30 nanometers. So, with the help of this research, the researchers realized that the new flame synthesis is a cost-effective approach for the construction of Cu-based carbon nanotube composites.

Synthetic Strategy Towards Carbon Nanotubes

Carbon nanotubes are flat graphene sheets that are folded into a tube. Their incorporation into transition metals produces advanced composite materials that take advantage of the beneficial properties of carbon nanotubes such as current-carrying capacity, electrical conductivity, mechanical strength, and thermal conductivity.

Previous reports mentioned the electrodeposit approach or powder processing as the main synthetic strategy for metal-based carbon nanotubes. Studies have shown that the electrodeposition method is more advantageous than the powder processing method due to reduced opportunities for agglomeration and high carbon nanotube loading factor. However, due to the long incubation time and high temperature vacuum environment, the electrodeposition method becomes an expensive approach.

In fire synthesis, the source of carbon and heat is obtained from pyrolysis and combustion. Hydrocarbons are used as a cost-effective energy source to synthesize carbon nanotubes on metal substrates. In addition, previous reports mostly mention nickel and other transition metals as metal substrates, while copper as a metal substrate for synthesizing carbon nanotubes is still relatively unexplored.

Among the fire structures previously reported in fire synthesis, NDF and IDF are safe configurations for their operation as they do not have a critical explosion risk due to fire backflow.

Copper-Carbon Nanotube Composite Through Methane Diffusion Flame

This research uses a nickel and Cu grating measuring 3 millimeters with a pitch of 400 x 62 micrometers as a metal substrate. The grid is initially cleaned with acetone, followed by etching with sulfuric acid. The methane-based NDF flame is divided into molecular, growth and oxidation zones.

The molecular zone facilitates the decomposition of methane into carbon nanotube precursors. In the growth zone, carbon nanotube precursors such as carbon monoxide (CO) and pyrolysis carbon molecules are deposited on the catalyst surface and used to form carbon nanotubes.

The growth zone is concentrated mainly in the yellow flame region which contains significant concentrations of carbon nanotube precursors. Next, this precursor is sent to the yellow flame oxidizing zone which encapsulates the blue flame. However, placing the Cu substrate in the oxidizing zone will inhibit the carbon nanotube structure by breaking it down into carbon dioxide and water. Thus, the flame growth zone of NDF is critical for carbon deposition and growth of carbon nanotubes.

Scanning electron microscopy (SEM) images of carbon nanotubes on a nickel substrate reveal a tubular structure of 20 to 60 nanometers on the surface of the nickel grid after NDF incubation for 5 min. In contrast, the SEM images of the Cu lattice incubated under the same conditions did not show the formation of Cu-based carbon nanotubes despite being treated with concentrated sulfuric acid.

The IDF consists of an orange-yellow flame, shielding a conical blue flame, where a clear stratification is observed between the oxidation and growth zones. The presence of excess fuel mainly contributes to the yellow flame and pyrolysis of methane, supported by the blue flame. The IDF yellow flame serves the same purpose as in NDF, as a growth zone for carbon nanotubes, while the blue flame consists of an oxidation zone. SEM images revealed that, unlike in NDF, incubation in IDF resulted in carbon nanostructures with a diameter of 20 to 30 nanometers on Cu substrates.

Conclusion

Overall, improved control over temperature, flame shape, and corresponding carbon nanotube precursor concentration was achieved through the regulation of the diffusion flame. As a result, the growth of carbon nanotubes was controlled in both IDF and NDF configurations. The configuration of the IDF with different fire zone separations allows incubation of Cu at higher temperatures.

This increased temperature provides an increase in the number of carbon nanotube precursors without inhibiting the Cu substrate. Thus, the IDF configuration is more advantageous than NDF as it results in different separation between the oxidation and growth zones, indicating IDF to be a suitable medium for fabricating Cu-based carbon nanotubes via flame synthesis.

Reference

How, HC., Chow, YL., Wong, HY., Ho, JH., Law, CH. (2022). Synthesis of copper-carbon nanotube composites through methane diffusion flame. Today’s Material: Proceedings. https://doi.org/10.1016/j.matpr.2022.06.489

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