Researchers improve electrical and thermal properties of carbon nanotubes

Researchers improve electrical and thermal properties of carbon nanotubes

The fabrication of carbon nanotubes by flame synthesis involves a combustion system modified with a carbon source, a heat source, and an appropriate catalytic material. To this end, the assembly of carbon nanotubes on a copper (Cu) substrate by flame synthesis is a novel approach to achieve Cu-based carbon nanotube composites.

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

In a recent article in the magazine Materials Today: Proceduresresearchers fabricated Cu-based carbon nanotube composites by flame synthesis to improve the electrical and thermal properties of the Cu material. Cleaning and etching Cu substrates with concentrated sulfuric acid was the first step towards Cu-based carbon nanotubes.

The cleaned and etched Cu substrates were exposed to two different laminar methane flames. The standard diffusion flame (NDF) configuration showed a blue flame shielding 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 flames, creating two distinct temperature zones.

This flame synthesis realized favorable conditions for synthesizing the carbon nanotubes on a Cu substrate. The Cu-based carbon nanotubes produced had tube diameters of 20 to 30 nanometers. Thus, with the help of the present study, the researchers determined that a novel flame synthesis is a cost-effective approach for the construction of Cu-based carbon nanotube composites.

Synthetic strategies for carbon nanotubes

Carbon nanotubes are flat sheets of graphene folded into a tube. Their 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 powder processing approaches as primary synthetic strategies for metal-based carbon nanotubes. Studies have shown that the electrodeposition process is more advantageous than the powder processing process due to the reduced opportunities for agglomeration and the high loading factor of the carbon nanotubes. However, due to prolonged incubation and a high-temperature vacuum environment, the electrodeposition process becomes an expensive approach.

In flame synthesis, the carbon and heat sources are obtained from pyrolysis and combustion. Hydrocarbon is used as an inexpensive energy source to synthesize carbon nanotubes on metal substrates. Furthermore, previous reports mainly mentioned nickel and other transition metals as metal substrates, while copper remains relatively unexplored as a metal substrate for the synthesis of carbon nanotubes.

Among the flame structures previously reported in Flame Synthesis, NDF and IDF are safe configurations for their operation as they do not have critical explosion risks due to flame reverse currents.

Copper-carbon nanotube composites by methane diffusion flame

The present study used 3 millimeter nickel and copper grids with a pitch of 400 x 62 micrometers as metal substrates. These gratings were first cleaned with acetone and then etched with sulfuric acid. The methane-based NDF flame has been divided into molecular, growth and oxidation zones.

The molecular zone facilitated the decomposition of methane in carbon nanotube precursors. In the growth zone, carbon nanotube precursors such as carbon monoxide (CO) and pyrolyzed carbon molecules were deposited on the catalyst surface and used to form carbon nanotubes.

The growth zone is primarily concentrated in the yellow flame region, which contains a significant concentration of carbon nanotube precursors. Furthermore, these precursors were delivered to the oxidizing zone of the yellow flame, which encapsulates the blue flame. However, placing the Cu substrate in the oxidizing zone will hinder the structure of carbon nanotubes by decomposing them into carbon dioxide and water. Thus, the NDF flame growth zone is critical for carbon deposition and carbon nanotube growth.

Scanning electron microscopy (SEM) images of carbon nanotubes on a nickel substrate revealed tubular structures of 20 to 60 nanometers on a nickel lattice surface after NDF incubation for 5 minutes. In contrast, SEM images of the Cu grid incubated under the same conditions showed no formation of Cu-based carbon nanotubes despite their treatment with concentrated sulfuric acid.

The IDF consisted of an orange-yellow flame shielding the conical shaped blue flame with clear stratification between the oxidation and growth zones observed. The excess presence of fuel primarily contributes to the yellow flame and methane pyrolysis, aided by the blue flame. The yellow flame of IDF had a purpose similar to that of NDF as a growth zone of the carbon nanotubes, while the blue flame consists of an oxidation zone. The SEM images showed that, unlike in NDF, incubation in IDF resulted in carbon nanostructures with a diameter of 20 to 30 nanometers on the Cu substrate.

Conclusion

Overall, improved control over the temperature, flame shape, and appropriate concentration of carbon nanotube precursors was achieved by the diffusion flame arrangement. Consequently, the growth of carbon nanotubes in IDF and NDF configurations was controlled. The IDF configuration with clear separation of the flame zones enabled Cu incubation at higher temperatures.

These elevated temperatures provided an increased amount of carbon nanotube precursors without peening the Cu substrate. Thus, the IDF configuration was advantageous over its NDF due to the generation of a clear separation between the oxidation and growth zones, suggesting that IDF is a suitable medium for fabricating Cu-based carbon nanotubes by flame synthesis.

Relation

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

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