Multiwalled carbon nanotubes (MWCNTs) are a unique form of carbon with an extraordinary structure. They exist at the nanoscale, measured in nanometers—billions of a meter. MWCNTs are essentially tiny, hollow cylinders made entirely of carbon atoms, attracting the attention of scientists and engineers due to their potential to revolutionize various fields. They are a novel material with properties that surpass many traditional substances.
Unique Characteristics
The distinct structure of MWCNTs involves multiple concentric cylinders of graphene sheets, resembling nested tubes. The space between these layers is approximately 0.34 nanometers, similar to the spacing in graphite. This layered arrangement, typically 3 to over 20 individual carbon nanotubes, dictates their characteristics. Their diameter varies, with innermost tubes as small as 2 nanometers and outermost potentially exceeding 50 nanometers.
This unique atomic arrangement gives MWCNTs high strength, with tensile strengths ranging from 10 to 60 gigapascals (GPa), significantly stronger than mild steel’s 0.5 GPa. Their high electrical conductivity, comparable to copper, results from electron delocalization, allowing efficient charge transport. They also exhibit high thermal conductivity, exceeding 3000 W/m.k, enabling efficient heat dissipation. These properties make MWCNTs desirable for enhancing other materials.
How They Are Produced
Multiwalled carbon nanotubes are typically produced through controlled processes that create their nanoscale structures. One common method is Chemical Vapor Deposition (CVD), including variations like catalytic CVD (CCVD). This process breaks down carbon-containing gases, such as hydrocarbons, at high temperatures (500-1000°C) over a metal catalyst. Carbon atoms then reform into nanotubes.
Another method is arc discharge, using high temperatures (over 1,700°C) by generating an electric arc between graphite electrodes in an inert atmosphere like helium. This heat vaporizes graphite, and carbon atoms condense to form nanotubes. Laser ablation is a third technique where a high-power laser vaporizes a graphite target in a furnace. The vaporized carbon forms nanotubes upon condensation, often with metal catalysts. While arc discharge and laser ablation produce high-quality nanotubes, CVD is often preferred for larger-scale production due to greater yields and purity, which can reduce purification costs.
Diverse Applications
Multiwalled carbon nanotubes find wide-ranging applications due to their properties. In materials science, they are used as reinforcing agents, significantly improving the mechanical properties of composites. Incorporating MWCNTs into plastics can create stronger, lighter components for aerospace and vehicle manufacturing. They also enhance the fatigue life of composites, increasing resistance to damage from repeated stress.
In electronics, MWCNTs are instrumental in developing advanced components. Their high electrical conductivity makes them suitable for miniaturized circuits, flexible displays, and conductive inks for printing electronic components. They also contribute to thermal management in integrated circuits by improving the heat dissipation capabilities of materials like die-attach epoxies and lead-free solders. MWCNTs are also employed in the fabrication of efficient sensors due to their electrical, mechanical, and thermal properties, proving effective in strain-sensing applications.
MWCNTs are also used in energy storage systems. They are utilized as electrode materials in supercapacitors and batteries, where their large surface area and conductivity can improve performance and energy density. Beyond energy, these nanotubes have potential in biomedical applications, including drug delivery systems that can precisely target specific destinations within the body, and in enhancing medical imaging. Their biocompatible nature further expands their utility in biomedical devices. Additionally, MWCNTs play a role in environmental applications such as water and air filtration, aiding in the removal of pollutants.
Safety Considerations
The nanoscale size and fiber-like shape of MWCNTs necessitate ongoing research into their potential health and environmental interactions. Studies indicate MWCNTs share characteristics with other fibrous particulates, raising concerns about their interaction with biological systems, particularly inhalation risks. Research in animal models shows inhalation exposure to MWCNTs can lead to pulmonary inflammation, granulomas, and fibrosis in the lungs.
While some studies have not observed systemic organ toxicity beyond the respiratory tract after certain exposure durations, the persistence of MWCNTs in lung tissue has been noted. Due to these findings, efforts are being made to establish safe handling practices, develop controlled manufacturing processes, and ensure appropriate disposal methods for MWCNT-containing materials. These measures aim to mitigate risks and support the responsible development and use of these nanomaterials.