The 50th edition of this column examines some salient facets of the material revolution that is silently shaping our lives. Materials have always been at the forefront of our progress from prehistoric times and will continue to lead us forward in an uncertain future. We need novel materials with unique properties to solve many tough challenges facing us, for which we need to place the cart before the horse.
Fifty years ago, Dupont launched a fibre that is five times tougher than steel. Kevlar was born in a cloudy cocktail of chemistry and curiosity garnished liberally with patience and perseverance. Today, Kevlar is a de facto material in bulletproof vests and other protective apparel worn by military personnel. Kevlar is an astonishingly strong fibre that belongs to the family of aromatic polyamides (aramids) and has a myriad other applications ranging
from racing car tyres to the bow of the violin. When molten Kevlar is spun into fibres, the molecules of the polymer orient themselves in a crystalline arrangement with a high degree of order and symmetry. The fibre owes its extraordinary strength to the near perfect orientation of the molecules of poly-para-phenylene terephthalamide.
Material Revolution
The evolution of human race is inextricably tangled with materials, so much so that the three main stages of pre-historic human civilisation are categorised after materials of that
age – stone, bronze and iron. Over many centuries, our lives have been continuously reshaped by materials; materials like paper and leather, glass and concrete, alloys and plastics. Today, we are living in the silicon and lithium age. Life, without these two metals appears inconceivable. We are currently in the midst of a material revolution. Advances in
materials are fundamental to breakthroughs in science and technology that can offer remedies to some of the most challenging problems confronting us. We are seeking materials with unique and customisable mechanical, thermal, chemical, optical,electrical, electronic and magnetic properties. Materials that can solve difficult problems related to
energy storage and climate change. Materials that will improve the performance of batteries, fuel cells and photovoltaic cells. Materials that are lighter, yet stronger. Materials that are cheaper to produce and easier to process. Materials that are green and sustainable. Materials that will work in a circular economy. Materials that can be 3D printed. It is a long wish list.
Nanomaterials
Nanoparticles are interesting because they display properties that are very different from those of their parent bulk material. Ensembles of nanoparticles, in turn, exhibit properties
that are different from the discrete nanoparticles. The novel mechanical, thermal, optical, electronic and magnetic properties of nanoparticle assemblies are attributed to the coupling and interaction among individual nanoparticles and these can be tweaked and fine-tuned. Previously unknown properties can also emerge from the strong interaction between assembled nanoparticles. Nanostructures with identical composition, but different morphologies exhibit entirely different properties.
Self-assembly
Self-assembly is the process where nanoscale building blocks spontaneously organise themselves into ordered structures or patterns without any human intervention. The self-assembly process can be adjusted and modified to build nanostructures with bespoke properties. Sophisticated architectures with extremely useful properties have been achieved by influencing the self-assembly process through various types of external forces like electrical, magnetic, optical and ultrasonic. Hard templates have been used to obtain nanostructures in well-defined shapes like wires, rods, tubes, spheres, cubes, etc. Various surfactants have been used to construct nanostructures with desired porosity and texture.
Nanostructure Applications
The customised nanoparticle assemblies have several potential applications in energy storage, photocatalysis, photovoltaic cells, sensing etc. Research is underway to build nanostructures for photocatalytic water splitting and for photocatalytic capturing of carbon dioxide. Engineered nanostructures are being keenly investigated to increase the efficiency of dye-sensitised solar cells and quantum-dot sensitised solar cells. Self-assembled nanoparticle structures have been used as bio and chemical sensors. Compared to conventional sensing technologies, the techniques based on properties of self-assembled nanoparticles exhibit superior selectivity and sensitivity with an unlimited lifetime. Last year, a group of Chinese researchers obtained a new insight into nanoparticle assemblies when they discovered that they can mimic living organisms and adapt to changes in their
environment though motion, perception, self-replication, self-regulation and self-repair. These animate characteristics of nanoparticle assemblies open up an exciting future.
Self-healing Materials
Self-healing materials (SHM) have an intrinsic ability to substantially recover their mechanical properties after damage. The interest in these materials has been growing exponentially as they form an important pillar for sustainable infrastructure through a significant increase in the service life of a structural member. Most living materials exhibit a self-healing property and thus have been a great source of inspiration in the development of SHM. Many techniques have been used to impart self-healing property in concrete and polymers. A commonly adopted approach to instill self-healing in a composite material is to embed microcapsules containing healing agents. When the composite is damaged, the microcapsules rupture and release the healing agents into the network to substantively restore the original properties. Another interesting approach to self-healing is through the use of materials that have shape memory. Shape memory alloys and polymers can remember their original form and regain it when an external stimulus like heat or light is applied.
Composites
Composites characterised by high strength to weight ratio, are increasingly used by automotive and aerospace industries for achieving energy efficiency and reducing missions. They are also a growing favourite in infrastructure projects because of higher corrosion resistance, and hence reduced maintenance and onger life span. The EV evolution will provide further fillip to lightweighting technologies that are already in vogue in most manufacturing industries. The battery constitutes as much as 20 per cent of the gross weight of an electric vehicle. To overcome this weighty issue, battery developers are looking at structural batteries, where the batteries also double up as a load bearing member of the vehicle. Carbon fibre composites with excellent mechanical and electrical properties have shown great promise for structural batteries. The prototype structural battery that is currently in active trial uses carbon fibre composite anode and lithium iron phosphate cathode with a fiberglass matrix separating the electrodes and serving as the electrolyte. Composites exhibit anisotropic behaviour, with their strength, stiffness and other desirable properties strongly influenced by the way the fibres are oriented in the polymer matrix. Understanding this anisotropic performance is essential to the design and manufacture of composite parts and components.
Material Discovery
Bakelite, the first plastic, was discovered accidentally. Historically speaking, materials were discovered first; their applications came later. But, when it comes to inventing materials for the new decade, we have to put the cart before the horse. We need to find materials with an exact property for a specific application. Designing materials that possess a desired property is in the realm of fantasy. The materials genome Initiative launched by the US Government in 2011 threw a challenge to the scientific and engineering community to accelerate the pace of discovery and deployment of advanced materials. The MGI paradigm for new materials discovery combines theory, experiments and computational predictions in a synergistic fashion.
Epilogue
Advanced materials with novel properties are tumbling out of laboratories across the world at an ever faster pace. It increases the belief that there exists a panacea for every problem confronting us. We have to go out and find it before the problem consumes us.
Readers’ responses may be sent to:
k.sahasranaman@gmail.com or
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