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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.
[1]
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.
[2]
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.
[3]
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.
[4]
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.
[5] 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.
[5] 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.
[6] 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.
[7]
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.
[8]
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.
[9] 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.
[10]
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.
[11]
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.
[12]
In this manufacturing method, a precursor gas containing carbon, like methane (CH4)
is introduced into a vacuum chamber and heated.
As the heat increases inside the chamber the bonds between the carbon and hydrogen atoms
begin to decompose.
The carbon then diffuses into a melted metal catalyst substrate.
This then becomes a metal-carbon solution, which eventually becomes supersaturated with
carbon.
At this point the carbon starts to precipitate out and form carbon nanotubes..
While the hydrogen bi-product is vented out of the chamber to avoid an explosion.
Our research has focused on increasing the length of these nanotubes while not sacrificing
their structure, yield or quantity.
While some labs have gotten individual tubes as long as 50 cm it’s been a struggle to
get larger bundles of tubes, which are called forests, to a length greater than 2cm. [13]
This is because the catalyst is guaranteed to deactivate at some point during the growth
process, terminating the growth of the nanotube.
The key to growing longer nanotubes is minimizing the probability of the catalyst deactivation
[14]
In 2020, a research team based in Japan managed to grow a forest over 15 cms in length, 7
times longer than anyone else, using a new method of chemical vapor deposition that managed
to keep the catalyst active for 26 hours.
[15]
They did this by adding a layer of gadolinium to a conventional iron-aluminium oxide catalyst
coated onto a silicon substrate.
Then using a lower chamber temperature, small concentrations of iron and aluminum vapor
were added into the chamber.
These factors combined managed to keep the iron-aluminium oxide catalyst active for much
longer.
[16]
This method is a major leap forward 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 call for the fibres to be woven together to form a textile like yarn.
One study I found wove together 1 mm long nanotubes and into a yarn and then impregnated
that with an epoxy resin to form a composite material, which had a pretty good tensile
strength of 1.6 Gigapascals.
[17] Beating aluminium in its strength to weight capabilities, but below a traditional
carbon fibre composite.
However, these new longer nanotubes may give us stronger woven fibres in future.
It’s important to remember that carbon nanotubes aren’t just strong.
Their most exciting new term applications will come about as a result of their other
material properties.
Like their conductive abilities.
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.
Making carbon nanotubes excellent conductors.
The conducting core of cables that make up our overhead grid lines are typically made
from aluminium.
Even though aluminium is a poorer conductor than copper, and thus causes a greater loss
in power over the lines.
It’s used because it’s cheaper and lighter.
Allowing support structures for overhead lines to be spaced further apart.
Individual nanotubes are orders of magnitudes more conductive than copper, but creating
a yarn of nanotubes that could match copper has been a challenge.
Electrons move through individual nanotubes very efficiently, but when the tube comes
to an end the current meets resistance when jumping to a neighbouring tube.
So these longer tubes developed last year are opening doors to conductors that are vastly
lighter than aluminium and more conductive than copper.
[18]
These could be used for grid connections, allowing our power lines to be stretch further
without supports and minimize the loss of power to heat resistance, but for now the
price of nanotubes likely shuts that door.
Instead we could see these wires being used in super lightweight aircraft or cars.
They are even being investigated as a means of helping composite structure planes, like
the 787, survive lightning strikes.
The 787 is primarily composed of carbon and glass fibre reinforced plastics, but because
they do not conduct electricity, the plane has additional conductive structures added
to protect it from lightning strikes,
like a thin copper mesh.
[19] This mesh adds weight that increases fuel consumption.
This could be drastically reduced by including a carbon nanotube mesh on the surface of the
composite instead.
Nanotubes are quite elastic.
Capable of stretching to 18% of their original length and returning to their original shape
after.
This could allow conductive carbon fibre wires to be incorporated into wearable technology.
The carbon fibre threads can even be treated like a normal thread and sewn into a fabric
using a sewing machine.
Perhaps the most exciting application is in biomedical devices.
[20] Carbon nanotubes are biocompatible.
Meaning they are not toxic, non reactive, and do not elicit an immune response.
Combine this with their conductivity,flexibility and strength, nanotubes become extremely attractive
as neural interface material.
A large part of Neuralink, Elon Musk’s neural interface companies, efforts have been focusing
on creating smaller wires and the machines needed to implant them.
Larger, stiffer wires tear into flexible brain tissue over time and causes scar tissue to
form around the wire, preventing signals from passing from the neurons to the wires.
Nanotube wiring could be made smaller and more flexible while being accepted by the
body.
A potentially game changing material for biomedical implants.
As with every major material innovation, from age hardened aluminium ushering in a new age
of aviation, to silicon semiconductors opening up an entire new world for computers, carbon
nanotubes have the potential to open the door to design possibilities and technologies we
have yet to imagine.
New materials radically change how and what we build, and learning more about manufacturing
processes for complex machines really gives you insight on how a material like this could
change things.
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