Nearly 3 decades after its discovery, the much-hyped Carbon Nanotubes have failed to live up to its potential. Our columnist examines the reasons for this unfulfilled promise. Despite the material’s unique properties, there are still large gaps in understanding how it is formed. A lot needs to be done before we can have a robust process for manufacturing Carbon Nanotubes of consistent quality on an industrial scale.
The discovery of Carbon Nanotubes in June 1991 was serendipitous. Sumio Iijima was examining various carbon materials under an electron microscope when he noticed peculiarly elongated needle-like structures. He subsequently gave these filaments the name of Carbon Nanotubes. Carbon Nanotube is a single sheet of a honeycomb network of carbon atoms rolled up seamlessly into a cylinder. Later it began to be widely regarded as the fourth allotrope of Carbon, after diamond, graphite and fullerene. The diameter of Carbon Nanotube, in the range of few nanometres, is almost comparable to that of an individual molecule. Because of their size, Carbon Nanotubes behave as both molecule and solid and hence exhibit unique physical and chemical properties.
Properties
The tensile strength of Carbon Nanotube is 400 times that of steel, but its density is only one-sixth of steel. Carbon Nanotubes are possibly the best heat-conducting material ever discovered. They have an aspect ratio of more than 1000; they are extremely thin in relation to their length. This high aspect ratio means that they can form a percolating matrix at low loadings and impart the desired properties to the composite. Addition of low concentrations of Carbon Nanotubes to plastic makes the latter electrically conductive. Carbon Nanotubes also possess astonishing electrical properties which allow them to switch between a fully conducting material and a semiconductor.
Applications
With a slew of such unique properties, a host of interesting applications have been envisioned for Carbon Nanotubes. These include advanced composites for aerospace, coaxial cables for power transmission, fuel cells, electronic devices, and biosensors. Carbon Nanotube-based composites have received much attention because of their superior mechanical, thermal, electrical and chemical properties. In addition to significant weight reduction, these composites promise to offer increased tensile strength, improved wear resistance, flame retardancy and antistatic properties. Automobiles and aircrafts made from carbon nanotube-based composites are expected to weigh 30% less and the consequent saving in fuel consumption would reduce carbon dioxide emission by about 4 billion tons. Use of carbon nanotube wiring in power distribution would decrease transmission losses significantly and shrink the carbon footprint of power companies.
Because of their high surface area, excellent chemical stability and polyaromatic structure, carbon nanotubes are able to adsorb or conjugate with a variety of therapeutic molecules. They have proved to be excellent vehicles for drug delivery by penetrating directly into cells while keeping the drug intact without metabolism during transport through the body. carbon nanotubes have been used as scaffolds in tissue generation and artificial implants. They have also been used successfully for enantiomer separation of chiral molecules in the pharmaceutical industry.
Manufacture
For material with such spectacular properties and applications, Carbon Nanotubes still lacks a robust and cost-effective method of production on an industrial scale. The problem is mainly due to a lack of clear understanding of the growth mechanism, in the absence of which it is not easy to synthesise nanotubes of a specific structure. Chemical Vapour Deposition is currently the most popular method of producing carbon nanotubes. It involves the thermal decomposition of hydrocarbon vapour in the presence of a metal catalyst. Carbon nanotubes are grown on the catalyst surface. Most commonly used hydrocarbons are methane, ethylene, acetylene and benzene. Iron, Cobalt and Nickel are the most common catalysts because of their strong adhesion to the growing nanotubes. The carbon precursor, the metal catalyst and its particle size are among the key parameters influencing the growth of carbon nanotubes. Even minor changes in these parameters affect the product characteristics in inexplicable ways. Large scale production continues to be plagued by poor yields, rapid catalyst deactivation and inconsistent product quality.
Unfulfilled Promise
Because of their remarkable mechanical, electrical and thermal properties, carbon nanotubes sparked considerable interest immediately after their discovery in 1991. Investments poured into research on carbon nanotubes over the next two decades. Plenty of material was made available for research and product development. The tiny nanotubes were added to polymer matrices as fillers or modifiers to improve mechanical strength and impart specific electrical properties. Carbon nanotube-based products like fibres, cables and composites were produced for testing and development. Many companies aggressively scaled up production in anticipation of growing demand, which however failed to materialise.
The unfulfilled promise gradually dampened the enthusiasm for the wonder material. In 2013, Bayer MaterialScience decided to exit the carbon nanotube business. Starting with a pilot facility in 2007, the company had grown to become the world’s largest producer of carbon nanotubes in 2010. Bayer had invested and collaborated with others in developing and scaling up processes for the manufacture of specific carbon nanotubes for different applications. But obviously, the company was disillusioned that the potential areas of application that had seemed promising failed to take off.
In September 2014, the National Nanotechnology Initiative in the USA sponsored a study in collaboration with NASA to identify technical barriers to the production of Carbon Nanotube-based materials and composites with optimal mechanical and electrical properties and to explore ways to overcome these barriers.
Entangled Mess
Carbon nanotubes have a tendency to entangle and aggregate into clumps. This presents a great challenge in dispersing them into a polymer matrix. The way in which the nanotubes are dispersed and aligned in the bulk matrix and the way these nanotubes interact with each other make a huge impact on the final properties. Because of the clumping and entanglement, the properties of the bulk materials and composites fall much short of what is obtained with the individual nanotubes. The difficulty in achieving adequate disentanglement of the carbon nanotubes and their dispersion into the matrix is one of the main impediments in the realisation of the material’s full potential. Researchers have tried various methods to unentangle the clumpy aggregates of nanotubes. These include high-pressure washing, treating with surfactants etc.
In 2015, Molecular Rebar, a start-up, achieved a breakthrough in disentangling and straightening the carbon nanotubes. The nanotubes modified by Molecular Rebar’s proprietary technology has been successfully used to enhance the performance of lead-acid and lithium-ion batteries. Earlier this year, SABIC of Saudi Arabia took a controlling interest in Molecular Rebar’s proprietary technology. More of such initiatives and efforts are needed to fulfil the potential of carbon nanotubes.
Roadmap
From time immemorial advances in materials have paved the road of human civilisation. From stone to Silicon it has been a fascinating journey. Carbon nanotubes are yet another addition to the long list of materials that have made our technologies leapfrog periodically. It is a material of incredible potential, but one fraught with challenges in both manufacturing and purification. There are still large gaps in our understanding of how the material is formed. Replication of the material’s remarkable nanoscale properties on a macroscale has been largely elusive. A lot of research needs to be done to unlock the full promise of carbon nanotubes.
Readers’ responses may be sent to k.sahasranaman@gmail.com or chemindigest@gmail.com
