C arbon N anotubes : B earing S tress L ike N ever B efore Aditya Limaye B S J C arbon, element number six, is often considered the backbone of life on Earth. With four valence elec- trons and many different bonding geometries, carbon is present in all biological macromolecules and plays an integral role in fundamental biological processes, making it truly deserving of its own field, organic chemistry. While carbon is usually anecdotally known for its abundance in biological systems, carbon’s many bonding geometries and versatile electronic configu- rations make it an exceptional material for synthetic molecules for physical applications, such as building materials or semiconductors. Serious investigation into carbon for physical applications began in 1985, when a group of researchers at Rice university de- signed a “buckyball,” a molecule known as buckmin- sterfullerene with the chemical formula C60 arranged in a structure akin to a soccer ball, with six and five- membered rings positioned adjacent to each other. In fact, buckminsterfullerene, named after the American architect R. Buckminster Fuller, who built geodesic domes resembling the molecule’s shape, was only one molecule in a class of many fullerenes, molecules made entirely out of carbon, arranged in the shape of a hollow sphere or tube. After the 1996 Nobel Prize in Chemistry was awarded to a team for the discovery of fullerenes, research into the fullerenes was taken up in earnest by much of the scientific community. D uring this time of high interest in the fullerene molecules, a Japanese team of scientists led by Dr. Sumio Ijima designed a tubular fullerene designed entirely out of six-membered carbon rings; forming a large cylindrical structure they termed the “carbon nano- tube” (Popov, 2004). Since this fortuitous discovery in 1991, research into carbon nano- tubes increased rapidly, spanning from the origi- nal field of chemistry into related disciplines such as physics, materials science, and biology. Research into the properties of carbon nanotubes continues in full force even today, and new applications for carbon nanotubes are currently being studied at the forefront of scientific research. O ne of the most important properties of the carbon nanotube is its incredible ability to withstand applied Buckminsterfullerene Ball tensile forces. When choosing the appropriate material for structural applications, materials engineers often need to consider the way in which a material responds to outside stresses, such as the tensile forces applied in cabling for bridges or the compressive forces applied against reinforcement beams in buildings. In these cases, it is important to select a material that can with- stand an appreciable amount of stress without frac- turing, and the carbon nanotube presents quite an enticing choice. The stress response of materials is often quantified using the Young’s modulus or elastic modulus, which is the ratio of the stress applied to a material to the subsequent strain, either compressive or expansive, that the applied stress causes. Materi- als with high elastic moduli, such as a steel beam, are stiff, while materials with low elastic moduli, such as rubber bands, are flexible. For most build- ing applications, a delicate balance must be struck between stiffness and flexibility, since very stiff materials such as ceram- ics can break very easily, while very flexible materi- als support little weight. Based on these constraints, the carbon nanotube pres- ents a very good choice for a structural material, with an elastic modulus five to ten times greater than high-strength steel, but an ability to flex under certain stress condi- tions. Based on these properties, carbon nanotubes have been studied in many different stress-bearing applications, with the goal of exploiting the molecu- lar structure and mechanical properties of the carbon 6 • B erkeley S cientific J ournal • S tress • F all 2013 • V olume 18 • I ssue 1