Deep down in the clandestine hearths of the Muslim Crusaders of almost a thousand years ago, bladesmiths excelled at fashioning ‘Damascus blades’. With distinctive vortex-like patterns etched on their surface, they were completely resistant to shattering even under protracted periods of use, much to the agony of the Christian Crusaders. The mystery of the swords would have been lost unto the world if not for a group of German researchers who unlocked the enigma back in 2006. The swords’ surfaces, when analyzed by a transmission electron microscope, were found to contain cylindrical arrangements of carbon atoms f1rst discovered in 1991 and now made in laboratories all over the world: Carbon Nanotubes.
Known for their preternatural thermal conductivity, resilience as a mechanical material and electrical conductivity, carbon nanotubes is a thriving area of research today, the prime emphasis being to reduce the cost of the material and transform it into a commercially viable technology.
liT Madras is no stranger to this development. Joseph Berkmans, a final year Ph.D. student at the Carbon Nanotube (CNT) lab at the Department of Metallurgical and Materials Engineering, has been making inroads into developing such a technology. The focus of his research is developing a rotating arc method for the simultaneous manufacture and alignment of carbon nanotubes.
CNTs are typically forged by an arc-discharge method. A current of 100 Amperes flows between a pair of graphite electrodes (immersed in an inert atmosphere) spaced by a distance of 1 millimetre. When the temperature of the anode exceeds about 4000 degrees Celsius, the graphite evaporates as carbon ions which get attracted to the cathode and settle on it as soot. This deposit, when analyzed under a scanning electron microscope (SEM), reveals a haphazard arrangement of carbon nanotubes, along with spherical balls called fullerenes. Seems like a simple enough process, doesn’t it?
The real challenge, however, lies in making long nanotubes (~1.8 cm vertically aligned and 18.5 cm horizontally aligned is the record) and aligning them by having their axes parallel to one another, something that doesn’t happen using the standard arc discharge method. “A single-walled carbon nanotube possesses a current density that is 3 orders higher than that of copper, due to the greater mean free path of electrons in the former,” says Joseph. Long nanotubes are currently produced using chemical vapor deposition(CVD), but this is an expensive and time-consuming process. The orderly arrangement is what results in the magical properties of these tubes, just as the way in which an electric potential leads to an orderly arrangement of electrons in a wire.
The next question that naturally comes to mind is: how does one align nanotubes? Joseph is quick to answer, “Scraping is the crudest way of aligning CNTs. Ingenious methods, however, exist one adds an infinitesimal amount of magnetic nickel and iron to the CNT concoction and uses a magnetic f1eld to align the tubes, after which hydrochloric acid is added to wash away the nickel and iron as chloride salts”.
Joseph, however, has come up with a clever scheme wherein he both produces and aligns in one go: by using a rotating cathode instead of a stationary cathode. In this method, the cathode is an oversized cylindrical plate with a scraper held against its surface by means of an external support. When the anode is brought near it, a spark is generated as before and soot is collected on the rotating cathode. This time, however, the soot rains down as flakes on a petri-dish because of the scraper, whose action also aligns the soot. Unlike the previous method, a large yield of aligned CNTs is produced in almost no time, with the added advantage that one does not require an inert atmosphere.”One can produce as much as 10-grams in 10 minutes using impure graphite rods,” remarks Joseph unlike the chemical vapour deposition (CVD) route or the standard arc route.
“The requirement for a cost-effective in-situ alignment method holds center-stage in the electronics industry,” says Joseph. In the quest to pack ever-smaller electronic devices more densely with integrated circuits, researchers keep running up against some bitter truths: higher current density leads to phenomena which produce excessive heat and cause premature damage of ICs. Carbon nanotubes are not constrained by this (they don’t follow Joule’s law of heating in the f1rst place!) and if Moore’s law is to keep its truth, you now know how that’s going to happen.
The uses of carbon nanotubes are slowly opening up to the modern world. “One may soon see touch screens which are see-through and flexible using carbon nanotubes,” says Joseph. The material seems to have become the black gold of the aerospace industry: the new Boeing 787 Dreamliner uses it to enhance its wing strength. Even better, new aeroplanes are coated with a thin film of CNTs so that the plane acts as a conductor to discharge current during a lightning strike. The sports industry has realized its potential too; the latest badminton and tennis racquets now have strands of CNTs in their frame. The applications of CNTs are on a roll as researchers set out to discover newer secrets about them and ways to use them. So much for carbon and its allotropes, for they seem to pervade almost every aspect of our lives (Don’t forget the greenhouse effect!) and will continue to do so in the future.