Subtitles section Play video Print subtitles This video was made possible by Curiosity Stream. Get access to exclusive content from the Real Engineering team, like the Logistic of D-Day series and our new podcast by signing up to the Nebula and CuriosityStream deal for the amazingly low price of 14.79 a year. In 1991 a Japanese physicist, Sumio Iijima, conducted a momentous experiment. An experiment that introduced the world to material so strong that it could revolutionise how engineers approach design. Taking two graphite rods as electrodes, Sumio applied a current across the rods. A spark arched between them and with it a cloud of carbon gas puffed into existence, vaporising the tip of the anode rod. As the carbon laden air settled on the chamber walls it formed a thin layer of black soot, within it a strange new material appeared. Tiny single layer straws of carbon. Sumio Iijima had just created carbon nanotubes.  Laboratory testing of these mysterious little tubes in the following years would reveal that these nanometer-wide hexagonal lattices of carbon had the strongest tensile strength known to man, and this was just As one of the many incredible material properties they displayed. Carbon nanotubes are light, conductive and biocompatible.  It soon became clear that the carbon nanotube had the potential to be the building block of futuristic new technologies. The most efficient computers, transformative medical devices, synthetic muscles, or perhaps the most ambitious of all, space elevators, the dream of countless sci-fi authors, Carbon nanotubes has promised to be the catalyst for the next revolution in technology. But, putting this revolutionary material to work will not be easy. It turns out that building a fibre, that is actually a single molecule, of any significant length is incredibly difficult. To understand this fascinating molecule, let's dive into the chemical makeup of carbon nanotubes. Carbon is a very familiar element. It's in everything we eat, sleep on and step over. It is the element that holds our DNA together. It forms the carbohydrates, proteins and lipids that we depend on to build and fuel our bodies. It's the basis of life as we know it. It's ubiquity in our lives is a result of its versatility. It's chemical properties allow it to take many different shapes, each impacting it's material properties in diverse and unique ways. To understand this we need to understand the basic models of how we visualize electron orbits around the nucleus of an atom. To start we have the simplified bohr model, which separates the electrons into shells. The first shell can contain 2 electrons, while the next shell can hold 8. An atom wants to fill each shell to be stable. Let's take an atom of carbon, which has 6 electrons, to see how this plays out.  First we fill the first shell with it's 2 electrons, then we have 4 electrons left to fill the next shell, leaving 4 open positions in its outer shell The 4 open positions mean that carbon willingly interacts with many other elements as well as itself. Often by sharing electrons in a special type of bond, called a covalent bond. This versatility allows carbon to create many different kinds of molecules. Take hydrocarbons. Hydrogen has 1 electron, and seeks 1 electron to fill it's inner shell. So, carbon likes to form 4 covalent bonds with 4 hydrogen atoms to form a stable 8 electron outer shell, while helping hydrogen form a stable 2 electron shell. This is methane, an incredibly common molecule that is the main ingredient in natural gas fuels. This is just one arrangement carbon can take. Hydrocarbons take a huge range of shapes and configurations, but what we are interested in is how carbon bonds to itself, but this simplified Bohr model doesn't give us an understanding of how carbon to carbon bonds take radically different shapes. We need to dive a little deeper before we can understand the magic of carbon nanotubes. Electrons don't travel in neat 2D circular orbits as the Bohr model would suggest, in fact we can't even know the position and speed of an electron. Instead we can make predictions about electrons' general locations in 3D space. We call these orbitals, and they are regions where we have about a 90% certainty that an electron is located somewhere within that region. This can get pretty complicated, but for now we just need to concern ourselves with two types. S and P orbitals.  S orbitals are spherical in shape with the nucleus of the atom at their centre. P orbitals are often called dumbbell shaped, but I don't know what gym these nerds are going to, because I have never seen a dumbbell like this. It's more like a figure of 8 shape like the infinity symbol. In the ground state, electrons will occupy the lowest energy orbitals first, which in this case is the 1S orbital. It can hold two electrons. Next we have the 2S orbital, which is a larger sphere, and can also hold 2 electrons. Then we have our three P orbitals, one aligned along the X, Y and Z axis, each capable of holding 2 electrons. Carbon in its ground state has the 1S and 2S orbitals filled, with one electron in the Px orbital and one in the Py orbital. To be stable, Carbon wants to fill these three p orbitals with 2 electrons each. Now this where things get a little funky and confusing, and it will be on your final exam. Carbon can bond to itself in different ways that affect these orbital shapes. Take diamonds. To fill these orbitals, carbon bonds with 4 neighbouring carbon atoms. To do this it promotes one electron from it's 2S orbital into the empty Pz orbital.  This Pz orbital is higher energy than the 2S orbital, and the electron doesn't want to stay there, so the carbon atom takes on new hybrid orbital shapes to compensate. This is called sp3 hybridisation, which is a mixture of S and P orbital shapes and looks something like this. Where one side of the figure of 8 expands while the other contracts. The 2S and 3 P orbitals are transformed into these new SP3 orbital shapes. They repel each other equally in this 3D space to form this four lobed tetrahedral shape with 109.5 degrees between each lobe. Covalent bonds now form between the carbon molecules where these orbital lobes overlap head on in what's called a sigma bond. This creates a repeating structure like this and it's this rigid framework of carbon atoms that makes diamond extremely hard. Now, what's fascinating to me, is that you can take the same carbon atoms and now form graphite, a material so soft that we use it as pencil lead and as a lubricant. How does that work? Here a different hybridisation occurs. Once again 1 electron from the 2S orbital is promoted into the Pz orbital, but this time the S orbital hybridizes with only 2 of the P orbitals, giving us the name SP2 hybridization.  This gives us three SP hybrid orbitals and 1 regular P orbitals. This new arrangement causes the orbitals to take a new shape, with the 3 SP orbitals arranging themselves in a flat plane separated by 120 degrees, with the P orbital perpendicular to them. Now, when the carbon atoms combine, the heads of the SP orbitals overlap once again to form this flat hexagonal shape. A hexagon pattern is naturally a very strong and energy-efficient shape. For example, bees don't intentionally build honeycombs in hexagons. They form as a result of the warm bee bodies melting the wax and the triple junction hardens in the strongest formation.  The shape is frequently used in aerospace applications where high strength and low weight is a priority. These SP2 bonds are stronger than SP3 bonds, because they have a higher s character. This sounds complicated, but all it means is that they are more like S orbitals than a P orbitals. Because there are 3 SP bonds, they have a 33% S character, whereas SP3 orbitals have 4 SP bonds giving them 25% S character. S orbitals are closer to the nucleus, making SP2 bonds shorter and more electronegative than SP3 bonds, and thus stronger.  This hexagonal structure and strong bonds make graphene exceedingly strong. Laboratory testing of graphene using atomic force microscopes has shown graphene has a young's modulus of 0.5 TPa and an ultimate tensile strength 130 gigapascals.  So strong that if we could somehow create a large perfect sheep of graphene, which we can't, we could build an invisible single atom deep hammock that could support the weight of a cat.  Imagine the amount of cats we could confuse. That's the world I want to live in. That's an entertaining, but not terribly useful application, but graphene is a very common material and the form we are used to, graphite, is not strong. This hexagonal shape itself is extremely strong, but because graphite forms these single atom layer sheets with only weak van der waal forces holding them together, the sheets can easily slide over each other, which is the reason graphite is so soft.  Now what is interesting is that carbon nanotubes take the same repeating hexagonal structure as graphite. The ends of the sheets are simply loops and connect with themselves to form a tube, and this structure is what gives carbon nanotubes their incredible strength. Researchers found that single-walled nanotubes have strength similar to that of graphite, about 130 Gigapascals.  For the non-engineers in the crowd, let me rephrase that. It's a lot. About 100 times greater than steel, and to boot it's vastly lighter. If this material could be feasibly manufactured into a single extremely long fibre, it could potentially open up entirely new design possibilities. Like the space elevator. I'd explain exactly why carbon fibres would make space elevators possible now, but I already did that in a past video that I will link at the end of this one. So where are there space elevators? Here lies the difficulty. Manufacturing carbon nanotubes. Carbon nanotubes strength relies on creating a continuous perfect lattice of carbon atoms in a long tube, and that process is not something we have yet developed. So how can we create carbon nanotubes? Things have changed a bit since the days of Sumio Ijima's first discovery. The most promising method for industrial scale production of high purity carbon nanotubes is chemical vapor deposition.