The Power of Black
A carpet of carbon nanotubes may be the perfect black material to trap solar energy.
A solar panel atop a house in Calcutta vertically aligned carbon nanotubes
vertically aligned carbon nanotubes
Rising population, especially in the developing countries, and the ever increasing energy demand by industries have had futurologists worrying over the power scenario in the coming years. Conventional methods of power generation are fraught with problems, not to mention the cost factor. Power transmission through wires has its own perils. In a country like ours line losses — read thefts — are rampant, often making the poor consumer wish there were no wires at all.
But there seems to be a solution — and it lies in tapping solar energy. This is already being done in some places, but not on a commercial scale. Recent scientific developments, however, hold the promise of change, paving the way for a “magic carpet” that would generate power for a household, group of houses, and so on.
In order to trap solar energy, a black surface that can absorb it efficiently is needed. An ideally black substance absorbs perfectly light of all wavelengths and at all angles, serving as a storehouse of energy. Such a substance would be immensely useful in future contraptions for tapping solar energy.
Physicists have been on the lookout for the blackest material since decades. In 1985, Buckministerfullerence — an allotrope of carbon — was discovered by Robert Curl, Harold Kroto and Richard Smalley of the University of Sussex, UK, and Rice University, US. It was named after Richard Buckminister Fuller, a famous American architect. Actually the molecular arrangement of this substance strongly resembles the structure of the geodesic dome discovered by Fuller, hence the name. A geodesic dome is an almost spherical structure based on a network of circles lying approximately on the surface of a sphere.
Curl, Kroto and Smalley’s discovery ultimately led to the discovery of carbon nanotubes (CNTs) in 1991 by Sumio Iijima, a Japanese physicist. In 1992 Pulickel Madhavapanicker Ajayan, a graduate from Banaras Hindu University (BHU) and currently professor of materials science at Rensselaer Polytechnic Institute in Troy, New York, joined Sumio’s laboratory and further advanced the work on CNTs.
Miniscule carbon nanotubes had become a fascination for Ajayan. He and his team-mate Thomas Ebbesen were able to produce CNTs in bulk by placing two graphite rods millimetres apart, wired to a power source. As 100 ampere current sparked through the gap, carbon present between the rods vaporised and produced hot plasma. Some of this plasma — rich with carbon vapour — formed CNTs upon condensation. This arc-evaporation method paved the way for the bulk production of CNTs.
Carbon paints and graphite appear black to us. But in the strict physical sense, they are not perfect blacks. They have a moderate reflection of 5-10 per cent. Still better is a hollow sphere with a blackened inner surface and a tiny hole. Light entering through the hole is repeatedly reflected from the walls of the sphere and is ultimately fully absorbed. But this kind of phenomenon has only experimental value. A Nickel-Phosphorous (NiP) film holds the record for lowest reflectance (0.16–0.18 per cent) in the Guinness Book.
In their pursuit of the blackest material, physicist Zu-Po Yang of the Rensselaer Polytechnic Institute and his colleagues, including Ajayan of the materials science department of the same university, tried to see if an array of vertically aligned CNT (VA-CNT) films would act as real black matter. Their work, reported in the journal Nano Letters in January 2008, was based on the theory that like in the blackened sphere, any radiation that enters a CNT array will be reflected repeatedly within the walls till all the energy is absorbed. They also made a low-density VA-CNT film with thickness ranging from 10 to 100 micrometres (one micrometre is one millionth of a metre). The total reflectance of the film was 0.045 per cent, which is about three times less than that of an NiP film. The biggest advantage of a VA-CNT film is that it can be easily peeled off the substrate.
VA-CNT films are the blackest of materials produced so far. They have a wide range of application — from solar energy conversions to pyroelectric (materials that produce electricity on heating) detectors. The latter are used in infrared sensors because of their capacity to respond to minute temperature changes. Apart from being of great value in industry, such sensors are useful in the military too. Infrared sensors are often used to detect the movement of camouflaged enemy at night in a dense forest from a plane or even a satellite.
The discovery of the blackest material promises to be the most beautiful human creation as it can provide a solution to many problems. With hydrocarbons being of limited availability and also earning the wrath of environmentalists, and hydro and nuclear powers beseeched with fears of disaster, solar energy seems to be the only way out of the global energy crisis. Perhaps the day is not far when a strip of VA-CNT film on the terrace will illuminate our homes and offices.
Sources: The Telegraph (Kolkata, India)
