Multi-Walled Carbon Nanotubes: Production, Analysis, and Application

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Single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) are similar in certain respects but they also have striking differences. SWNTs are an allotrope of sp2 hybridized carbon similar to fullerenes. The structure is a cylindrical tube including six-membered carbon rings similar to graphite. Analogous MWNTs have several tubes in concentric cylinders.

The number of these concentric walls may vary from 6 to 25 or more. The diameter of MWNTs may be 30 nm when compared to 0.7–2.0 nm for typical SWNTs. The unique properties of carbon nanotubes enable a wide range of novel applications and improvements in the performance of existing ones.

This article offers a brief overview of the physico-chemical nature and characterization of MWNTs, with specific emphasis on recently introduced materials that signify the most recent advancement of the technology and the level of its commercialization.

Properties of MWNTs

MWNTs have excellent properties and are being employed in a large number of commercial applications. The properties of MWNTs are:

• Electrical: MWNTs are highly conductive when properly integrated into a composite structure, however, the outer wall alone has conductive properties, but the inner walls are not instrumental to conductivity.
• Morphology: MWNTs have a high aspect ratio with lengths typically more than 100 times the diameter, and in certain cases much higher. Their performance and application are based not just on their aspect ratio, but also on the degree of entanglement and the straightness of the tubes; which in turn is a function of both the degree and dimension of defects in the tubes.
• Physical: Defect–free, individual, MWNTs have an excellent tensile strength and when integrated into a composite, such as a thermoplastic or thermoset compound can significantly increase its strength.
• Thermal: MWNTs have a thermal stability above 600 °C, as a result of the level of defects - and to a certain extent on the purity; as a residual catalyst in the product can also accelerate decomposition.
• Chemical: MWNTs are an allotrope of sp2 hybridized carbon similar to graphite and fullerenes, and therefore have high chemical stability. However, the nanotubes can be functionalized to enhance both the strength and dispersibility of composites.

Challenges in Commercialization of MWNTs

The challenges in commercializing MWNTs include:

• Dispersion: This property in MWNTs is better in solutions or polymers than SWNTs, however, the quality of the dispersion is a critical factor in the performance of the final product.
• Purity: Many MWNTs processes cause a considerable residual metallic catalyst which can be detrimental to performance.
• Defects: The number of defects is dependent on the number of layers within MWNTs. The high aspect ratio of MWNTs contributes significantly to its use.

Characterization of MWNTs and Quality Assurance Parameters

Observational techniques such as SEM, TEM and AFM are used for characterizing MWNTs and can be used to obtain data such as length, diameter and the number of walls. In addition, thermogravimetric analysis (TGA) is used to measure the residual mass, the temperature at the onset of oxidation and the temperature of the maximum oxidation rate.

The shape of the derivative curve provides qualitative information with respect to the uniformity of the sample with reference to polydispersity of the material. A high, narrow peak indicates a narrow distribution of diameters and minimal tube defects.

Applications of MWNTs

There are a large number of present and evolving applications for MWNTs. These include:

• Electrically Conductive Polymers: MWNTs are suitable for these applications – in particular as a result of their high conductivity and high aspect ratio. The required conductivity level can be achieved with much lesser loadings than for conventional solutions such as metal particulates or carbon black. Applications include electrostatic discharge protection in wafer processing fabrication, antistatic elastomeric and plastic components for automobile fuel line components, plastics rendered conductive to enable electrostatic spray painting of automobile body parts, RFI shielding materials, and more.
   
• Battery Cathodes: Novel MWNT materials from SouthWest NanoTechnologies (SWeNT®) have shown considerable improvements when integrated into cathodes.
   
• Improved Structural Composites: MWNTs in the form of non-woven or woven fabrics or resin infused buckypaper, when saturated with thermoset resins have shown considerable increases in the stiffness and strength of composite structures, such as structural laminates, golf club shafts and aerospace applications.
   
• Water filtration membranes: High aspect ratio, high mechanical strength and large specific surface enable very efficient filtration media.

Other development applications include spray-coatable heater elements; thermal interface and other heat conduction materials and enhanced carbon fiber.

A new group of MWNTs has been developed by SWeNT, and are called specialty multi-walled CNT (SMW). In these, the number of walls is controlled to vary between three to eight walls while CNT lengths are maintained above 3 µm, which yields an aspect ratio of between 350 – 550. The lower number of walls results in higher purity, less structural defects and reduced carbon material waste; while longer and straighter tubes provide better overall CNT morphology.

Certain features of the SMW product are:

• The SMW product has a considerably higher aspect ratio (length/diameter) when compared to either of the other grades and an increased aspect ratio is needed in order to develop a conductive network in the polymer matrix at a low loading of additive.
• The SMW tubes are straight, which is beneficial in establishing a conducting network.

Competitive materials show defects and impurities. Tubes may fracture during dispersion at defect sites, bringing down the number of electrical pathways and the resulting conductivity.

Conclusion

MWNTs have been adopted in quite a number of applications, but advances in their properties are required to harness their potential. Improved MWNTs have been developed that will considerably expand the market reach of this unique category of advanced materials. CoMoCAT® high purity MWNTs manufactured by SWeNT®, Inc. and available in research quantities exclusively from Aldrich® Materials Science are provided in the table below:

This article was updated 11^th September, 2019.