How Carbon Nanotubes Will Change the World

This video was made possible thanks to 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 for the Nebula and CuriosityStream deal for the incredibly low price of $14.79/year. In 1991, a Japanese physicist, Sumio Iijima, conducted a momentous experiment. An experiment that introduced the

world

to a material so strong it could revolutionize the way engineers approach design. Taking two graphite rods as electrodes, Sumio applied a current through the rods. A spark arced between them and with it a cloud of

carbon

dioxide emerged, vaporizing the tip of the anode rod.

As the

carbon

-laden air settled on the walls of the chamber, it formed a thin layer of black soot, within which a new foreign material appeared. Small single layer carbon straws. Sumio Iijima had just created carbon

nanotubes

. Laboratory testing of these mysterious little tubes in the years that followed would reveal that these nanometer-wide hexagonal networks of carbon had the highest tensile strength known to man, and this was just 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 cornerstone of new futuristic technologies.

The most efficient computers, transformative medical devices, synthetic muscles or perhaps most ambitious of all, space elevators, the dream of countless science fiction authors, carbon nanotubes have promised to be the catalyst for the next technological revolution. But putting this revolutionary material into practice

will

not be easy. It turns out that building a fiber, which is actually a single molecule, of any significant length is incredibly difficult. To understand this fascinating molecule, let's delve into the chemical composition of carbon nanotubes. Carbon is a very familiar element. It is in everything we eat, sleep and walk on. 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 is the basis of life as we know it. Its ubiquity in our lives is the result of its versatility. Its chemical properties allow it to take many different forms, each of which affects the properties of the material in diverse and unique ways. To understand this we need to understand the basic models of how we visualize the orbits of electrons around the nucleus of an atom. To start we have the simplified Bohr model, which separates electrons into shells. The first shell can hold 2 electrons, while the next shell can hold 8.

An atom wants to fill each shell to be stable. Let's take a carbon atom, which has 6 electrons, to see how it develops. First we fill the first shell with its 2 electrons, then we have 4 electrons left to fill the next shell, leaving 4 positions open in its outer shell. The 4 open positions mean that carbon voluntarily interacts with many other elements besides itself. They often share electrons in a special type of bond, called a covalent bond. This versatility allows carbon to create many different types of molecules. Take hydrocarbons as an example. Hydrogen has 1 electron and is looking for 1 electron to fill its inner shell.

So, carbon likes to form 4 covalent bonds with 4 hydrogen atoms to form a stable 8-electron 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 of the arrangements that carbon can take. Hydrocarbons take on a wide range of shapes and configurations, but what we are interested in is how carbon bonds to itself, but this simplified Bohr model does not give us an understanding of how carbon-to-carbon bonds take radically different forms. We need to dig a little deeper before we can understand the magic of carbon nanotubes.

Electrons do not travel in ordered two-dimensional circular orbits as the Bohr model would suggest; in fact, we can't even know the position and velocity of an electron. Instead, we can make predictions about the general locations of electrons in 3D space. We call these orbitals and they are regions where we are about 90% certain that an electron is somewhere within that region. This can be quite complicated, but for now we only have to worry about two types. S and P orbitals. S orbitals are spherical in shape with the nucleus of the atom at its center. P orbitals are often called dumbbell shaped, but I don't know what gym these nerds go to, because I've never seen a dumbbell like this.

It's more like a figure 8 like the infinity symbol. In the ground state, the electrons

will

first occupy the lowest energy orbitals, which in this case is the 1S orbital. It can contain 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 axes, each capable of holding 2 electrons. Carbon in its ground state has filled 1S and 2S orbitals, with one electron in the Px orbital and another in the Py orbital. To be stable, carbon wants to fill these three p orbitals with 2 electrons each.

This is where things get a little weird 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 neighboring carbon atoms. To do this, it promotes an electron from its 2S orbital to the empty Pz orbital. This Pz orbital has higher energy than the 2S orbital and the electron does not want to stay there, so the carbon atom adopts new hybrid orbital shapes to compensate. This is called sp3 hybridization, which is a mixture of S and P orbital shapes and looks like this.

Where one side of the figure 8 expands while the other contracts. The 2S and 3P orbitals transform into these new SP3 orbital forms. 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 are now formed between carbon molecules where these orbital lobes overlap head-on in what is called a sigma bond. This creates a repeating structure like this and it is this rigid structure of carbon atoms that makes diamond extremely hard. Now, what I find fascinating is that you can take the same carbon atoms and form graphite, a material so soft that we use it as a pencil tip and as a lubricant.

How does it work? Here a different hybridization occurs. Once again, 1 electron from the 2S orbital is promoted to the Pz orbital, but this time the S orbital hybridizes to only 2 of the P orbitals, giving us the name SP2 hybridization. This gives us three SP hybrid orbitals and 1 regular P orbital. This new arrangement causes the orbitals to take on a new shape, with the 3 SP orbitals arranged in a plane 120 degrees apart, 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 hexagonal pattern is naturally a very strong and energy efficient shape. For example, bees do not intentionally build honeycombs in hexagons. They form when the bees' hot bodies melt the wax and the triple bond hardens into the strongest formation. The shape is frequently used in aerospace applications where high strength and low weight are a priority. These SP2 bonds are stronger than SP3 bonds because they have a larger s character. This sounds complicated, but all it means is that they are more like S orbitals than P orbitals. Because there are 3 SP bonds, they have a 33% S character, while SP3 orbitals have 4 SP bonds giving them a 25% S character.

The S orbitals are closer to the nucleus, making the SP2 bonds shorter and more electronegative than the SP3 bonds and therefore stronger. This hexagonal structure and its strong bonds make graphene extremely strong. Laboratory testing of graphene using atomic force microscopes has shown that graphene has a Young's modulus of 0.5 TPa and an ultimate tensile strength of 130 gigapascals. So strong that if we could somehow create a perfect big sheep out of graphene, which we can't, we could build an invisible deep hammock from a single atom that could support the weight of a cat. Imagine the number of cats we could confuse.

That's the

world

I want to live in. It is an entertaining application, but not very useful, 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 sheets of single-atom layers with only weak van der Waal forces holding them together, the sheets can easily slide over each other, which is why which graphite is so soft. Now, the interesting thing is that carbon nanotubes adopt the same repetitive hexagonal structure as graphite. The ends of the sheets are simply loops and connect together to form a tube, and this structure is what gives carbon nanotubes their incredible strength.

The researchers found that single-walled nanotubes have a resistance similar to that of graphite, about 130 gigapascals. For the non-engineers in the crowd, let me rephrase that. Its alot. Approximately 100 times larger than steel and is also much lighter. If this material could be made in a single, extremely long fiber, it could open up entirely new design possibilities. Like the space elevator. I would explain exactly why carbon fibers would make space elevators possible now, but I already did that in a previous video that I will link at the end of this one. So where are there space elevators? Here lies the difficulty.

Manufacture of carbon nanotubes. The strength of carbon nanotubes depends on creating a continuous, perfect network of carbon atoms in a long tube, and that process is not something we have developed yet. So how can we create carbon nanotubes? Things have

change

d 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. In this manufacturing method, a carbon-containing precursor gas, such as methane (CH4), is introduced into a vacuum chamber and heated. As the heat inside the chamber increases, the bonds between the carbon and hydrogen atoms begin to break down.

The carbon then diffuses into a molten metal catalyst substrate. This then becomes a metal-carbon solution, which eventually becomes supersaturated with carbon. At this point, the carbon begins to precipitate and form carbon nanotubes. While the hydrogen byproduct is vented out of the chamber to prevent an explosion. Our research has focused on increasing the length of these nanotubes without sacrificing their structure, performance or quantity. While some laboratories have achieved individual tubes up to 50 cm long, it has been difficult to obtain bundles of larger tubes, called forests, longer than 2 cm. This is because the catalyst is guaranteed to deactivate at some point during the growth process, ending the growth of the nanotube.

The key to growing longer nanotubes is to minimize the probability of catalyst deactivation. In 2020, a Japan-based research team managed to grow a forest over 15 cm in length, 7 times longer than any other, using a new chemical vapor deposition method that managed to keep the catalyst active for 26 hours. . They did this by adding a layer of gadolinium to a conventional aluminum iron oxide catalyst coated on a silicon substrate. Then, using a lower chamber temperature, small concentrations of iron and aluminum vapor were added to the chamber. These combined factors managed to keep the iron-aluminum oxide catalyst active for much longer.

This method is a breakthrough that could allow carbon nanotube products to begin entering the market, but we are still a long way from a space elevator. Most products today require fibers to be intertwined to form a textile-like yarn. One study I found wove 1mm long nanotubes into a thread and then impregnated it with an epoxy resin to form a composite material, which had a pretty good tensile strength of 1.6 gigapascals. Exceeding aluminum in its strength and weight capacity, but below a traditional carbon fiber composite. However, these new longer nanotubes could give us stronger woven fibers in the future.

It is important to remember that carbon nanotubes are not only strong. TheirMore interesting applications of new terms will emerge as a result of the properties of other materials. Like his driving skills. Like graphite, nanotubes are highly conductive, because each carbon atom is only bonded to 3 other carbon atoms, each atom has 1 free valence electron available for electrical conduction. Make carbon nanotubes excellent conductors. The conductive core of the cables that make up our overhead power lines is usually made of aluminum. Although aluminum is a worse conductor than copper and, therefore, causes greater power loss in the lines. It is used because it is cheaper and lighter.

Allow greater separation of overhead line support structures. Individual nanotubes are orders of magnitude more conductive than copper, but creating a nanotube strand that can match copper has been a challenge. Electrons move through individual nanotubes very efficiently, but when the tube comes to an end, the current encounters resistance by jumping to a neighboring tube. So these longer tubes developed last year are opening doors to conductors that are much lighter than aluminum and more conductive than copper. These could be used for grid connections, allowing our power lines to stretch further without supports and minimize energy loss due to heat resistance, but for now the price of nanotubes will probably close that door.

Instead, we could see these cables being used in superlight airplanes or cars. They are even being investigated as a means to help composite airframe aircraft, like the 787, survive lightning strikes. The 787 is composed primarily of carbon and fiberglass-reinforced plastics, but because they do not conduct electricity, the plane has additional conductive structures added to protect it from lightning, such as fine copper mesh. This mesh adds weight that increases fuel consumption. This could be drastically reduced by instead including a mesh of carbon nanotubes on the surface of the composite. Nanotubes are quite elastic. Able to stretch up to 18% of its original length and return to its original shape afterwards.

This could enable the incorporation of carbon fiber conductive cables into wearable technology. Carbon fiber threads can even be treated like normal thread and sewn into fabric with a sewing machine. Perhaps the most interesting application is biomedical devices. Carbon nanotubes are biocompatible. Which means they are non-toxic, non-reactive and do not provoke an immune response. Combining this with their conductivity, flexibility and strength, nanotubes become extremely attractive as a neural interface material. Much of Neuralink, Elon Musk's neural interface company, has focused on creating smaller cables and the machines needed to implant them. Larger, stiffer cables tear flexible brain tissue over time and cause scar tissue to form around the cable, preventing signals from passing from the neurons to the cables.

Nanotube wiring could be made smaller and more flexible while still being accepted by the body. A potentially revolutionary material for biomedical implants. As with all major material innovations, from toughened aluminum ushering in a new era of aviation to silicon semiconductors opening up a whole new world for computers, carbon nanotubes have the potential to open the door. door to design possibilities and technologies that we have yet to imagine. . New materials radically

change

how and what we build, and learning more about the manufacturing processes of complex machines will really give you an idea of ​​how a material like this could change things.

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