Carbon Nanotubes (CNTs) can be used in a wide range of applications, and thanks to their myriad properties they are found in batteries, car tyres, and even next-gen quantum computers. But, CNTs have a natural tendency to agglomerate and form bundles due to strong van der Waals forces between the CNTs, so, getting the best effect from them usually means they have to be evenly dispersed in a host material. Dispersion is required for optimal performance in applications, such as improving mechanical strength, electrical conductivity, and thermal properties. 

As the number of applications for CNTs continues to rise, different dispersion techniques are being examined. Key factors to consider include:

- Will the dispersion method damage the CNTs?

- How to achieve stable, long-term dispersion without reagglomeration over time?

- How to ensure that the medium and dispersed CNTs are integrated?

Dispersion methods can be divided into two main categories: mechanical dispersion and chemical dispersion. Users must weigh the advantages and disadvantages of each method to determine which route best suits their needs.

CNT Dispersion Using Mechanical Methods

Mechanical methods for CNT dispersion include ball milling, ultrasonication, and high-shear mixing. These methods are popular due to their simplicity, cost-effectiveness, and limited post-treatment processing required.

Ball milling is commonly used for CNT dispersion in solid mediums, such as powders or polymers. The CNTs are mixed with the desired medium and placed into the ball mill chamber with milling balls. The chamber is rotated, causing the milling balls to break up the CNT agglomerates using shear and impact forces. The rotation distributes the CNTs uniformly within the medium.

Ultrasonication is often used to disperse CNTs in aqueous, organic, or polymer solutions. The CNTs are added to the medium, and the mixture is subjected to ultrasonic waves of 20-40 kHz that create microscopic bubbles, which impart a shear force on CNT bundles. The resulting turbulence effectively distributes the CNTs in the media.

High shear mixing is used for CNT dispersion in liquids, such as liquid polymers, resins, or solvents. A rotor-stator mixer generates shear forces within the mixture. The rotor's rapid spinning creates turbulence and pressure, breaking up the CNT agglomerates and distributing them.

CNT Dispersion Using Chemical Methods

Chemical methods of CNT dispersion include functionalisation and surfactant or polymer treatments. These methods are popular due to their long-term stabilisation of agglomerates and properties which reduce rates of reagglomeration.

Functionalisation uses strong acids, such as nitric or sulfuric acid, as an oxidising agent. The oxidising agents introduce chemical functional groups to the surface of the CNTs, reducing the van der Waals interactions and improving CNT dispersion. This is often combined with ultrasonication to increase the surface area of the CNTs available for the acids. The solution may also be heated to increase the chemical reactions.

Dispersion using surfactants uses their amphiphilic properties – containing hydrophobic and hydrophilic chemical functional groups. The CNT surface attaches to the hydrophobic regions, and the surrounding liquid interacts with the hydrophilic regions. Ultrasonication breaks up existing CNT bundles, increasing the surface area for the surfactants to coat. Polymers can be used similarly to surfactants, adsorbing to the CNT surface and preventing agglomeration through steric hindrance or electrostatic repulsion.

Users of CNT Dispersion

Dispersion techniques are required across industries where CNTs are used. The specific dispersion technique chosen will depend on the medium and the desired end-use.

CNT dispersions enhance the properties of a wide variety of materials: 

- Institutions may use surfactant/polymer-based dispersion to investigate the behaviour of CNTs and, through this, develop new polymer-CNT composites with applications in mechanical, thermal, and electrical fields. 

- Ultrasonication is used to disperse CNTs for liquid-based applications, such as conductive inks for use in touch screens—something we do at TrimTabs.

- Ball milling is a versatile method, often used to improve conductivity and energy density in batteries by dispersing CNTs within the lithium compounds forming the electrode materials.

- The aerospace industry uses high-shear mixing techniques to disperse CNTs within polymers, creating high-strength but lightweight materials for aeroplane components.

Incorporating CNTs into many different industries and fields of research is having a transformative effect on these areas. However, the game-changing impact of CNTs is dependent on their effective dispersion through mediums. The continuing development and optimisation of dispersion techniques for use in different applications drives the next generation of CNT-enabled technologies.