Multiwall Carbon Nanotubes (MWCNT): Production, Analysis and Application
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. Analogously MWNTs include several tubes in concentric cylinders as shown in Figure 1.
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 multiwall nanotubes (MWNTs), with specific emphasis on recently introduced materials that signify the most recent advancement of the technology, and the level of its commercialization.
Figure 1. Multi-walled carbon nanotubes
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. One must note that the outer wall alone is conducting, 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 is based not just on aspect ratio, but also on the degree of entanglement and the straightness of the tubes, which in turn is a function of the 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 compounds, can significantly increase its strength.
- Thermal: MWNTs have a thermal stability more than 600 °C, based on the level of defects and to certain extent on the purity as residual catalyst in the product can also catalyze decomposition.
- Chemical: MWNTs are an allotrope of sp2 hybridized carbon similar to graphite and fullerenes and as such have high chemical stability. However, one can functionalize the nanotubes to enhance both the strength and dispersibility of composites.
The challenges in commercializing MWNTs include the following:
- Dispersion: These have better dispersability into solutions or polymers than SWNTs, however the quality of the dispersion obtained is a critical factor in the performance of the final product.
- Purity: Many MWNTs processes cause 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 much of the value of their use.
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 number of walls. Furthermore 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.
There are a large number of present and evolving applications for MWNTs. These include:
- Electrically Conductive Polymers: MWNTs are suitable for these applications especially due to its high conductivity and high aspect ratio. The needed 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 improvement in 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 increase in stiffness and strength of composite structures such as structural laminates and golf club shafts for aerospace application.
- Water filtration membranes: High aspect ration, 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; enhanced carbon fiber and others.
- A new group of MWNTs were developed by SWeNT known as specialty multi-walled CNT (SMW), by which the number of walls is controlled to vary between three to eight walls while maintaining CNT lengths more than 3 µm , hence yielding an aspect ratio in the 350 – 550 range. The lesser number of walls will result in higher purity, lesser structural defects and lesser waste of carbon material, while longer and straighter tubes provide better overall CNT morphology as shown in Figure 2.
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 also 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.
Figure 2. Commodity MWNT and SWeNT® SMW 200 (Aldrich Product No. 773840) TEM and AFM images
Figure 3 shows a comparison of conductivity data for SWeNT® SMW 200 (Aldrich Product No. 773840) when compared to other commercially available MWNTs materials. Determining the sheet resistance of CNT buckypapers (a thin solid film created by filtration of 0.15 g of CNT/m2) shows that as expected a SWNT material (in this case SWeNT® SG76, Aldrich Product No. 704121) has a very low resistivity value. Actually, the SMW 200 purified material is more than twice as conductive when compared to the best MWNT material studied (Competitor A).
Figure 3. Buckypaper resistivity measurements of various CNT products
Figure 4 compares sheet resistance buckypaper data for a range of products as a function of the CNT aspect ratio (L/D), as determined by AFM and TEM analysis. It is observed clearly that trend of lower sheet resistance with higher aspect ratios, with SWeNT® SMW 200 (Aldrich Product No. 773840) having excellent conductivity properties. Further to the CNT aspect ratio it was also observed that the tube morphology is another major factor that strongly impacts tube conductivity.
Figure 4. Sheet resistance of buckypapers as a function of the CNT aspect ratio
MWNTs have been adopted in quite a number of applications, but advancements in their properties are needed for harnessing 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:
|Aldrich Prod. No.||SWeNT® Product||Product Name||Features|
|773840||SMW 200||Carbon nanotube, multi-walled
6-8 tube walls
|724769||SMW 100||Carbon nanotube, multi-walled
6-9 nm diameter
This information has been sourced, reviewed and adapted from materials provided by SouthWest NanoTechnologies (SWeNT).
For more information on this source, please visit SouthWest NanoTechnologies (SWeNT).