Understanding Graphene Part 7: Graphene Fibre
If graphene fibres make you think of carbon fibre or carbon nanotubes then you are not alone. That was my thought when I first came across them. In fact graphene fibres are quite different, I’ll explain why…
Let’s take a look at carbon fibre and carbon nanotubes first
Most carbon fibre is made from a material called acrylonitrile. This is polymerised to make polyacrylonitrile. Think of this polymer as a long carbon chain made up of many repeating units. To give you an idea of scale if these polymer chains could be stretched out they would be hundreds or a few thousands of nanometres long. This is a still fraction of a millimetre.
These polymer chains are made into longer fibres by a process called spinning where the polymer mix is extruded through a narrow jet. This makes a larger scale fibre by aligning the polymer chains, much in the same way that a rope is made of myriad smaller fibres.
This polymer fibre is heated in air at 300°C to oxidise it then charred at 1,500°C in the absence of oxygen. This graphitises the polymer chains creating the carbon fibre.
Carbon nanotubes are made by a different process. For example NASA has developed a process using graphite anode and cathode and passing an electric arc discharge between them. This produces tubes that are typically two nanometres (nm) wide and several thousand nm long.
A thousand nanometres sounds big but is still only one micrometre, a millionth of a metre. The longest carbon nanotubes have been made using a floating chemical vapour deposition process with nano catalysts at the tips of growing nanotubes in hot gases made of methane and ethane. This can produce nanotubes up to 500mm long (half a metre). However this produces very small quantities of ultra-long nanotubes and is not a commercially viable method at present.
Now to graphene fibre…
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Making graphene fibre should be simple. Get some graphene, melt it then push it through a nozzle to extrude the fibre.
The problem is that another of graphene’s super properties is its very high melting point. In fact the melting point is so high, no one really knows for sure. The latest research shows that graphene can withstand temperatures exceeding 5000°C. Even then it probably does not melt but sublimes, decomposing to a gas without going through the liquid phase. Graphene looks like having the highest melting point of any known material.
So, melting graphene is not feasible. However graphene fibres have been made and were first reported in 2011. The process starts with graphite, which is exfoliated into nanoplatelets, sometimes misleadingly called graphene sheets. These nanoplatelets are oxidised to graphene oxide (GO).
Graphene oxide nanoplatelets are negatively charged so will repel one another in suspension. In 2012 A team from the University of Science and Technology of China overcame this problem by wet spinning the GO suspension into a bath containing a coagulant called CTAB This is positively charged and sticks the GO platelets together helping them form a stable fibre.
The latest research making graphene fibres in May 2017 exfoliates and oxidises graphite using hummer’s method to make them water dispersible. The suspension of GO nanoplatelets in water is subjected to ultrasound to further disperse the nanoplatelets in a stable suspension. This stabilised suspension is thought to be a Graphene Oxide Liquid Crystal (GOLC) where the nanoplatelets align with each other in the liquid.
The GOLC suspension is then injected into a coagulation bath of sodium hydroxide and calcium chloride in ethanol and water. The calcium ions are the coagulating agent and this forms a stable fibre. After washing in a bath of methanol the fibre is reduced with sodium borohydride. This turns the graphene oxide fibre back to graphene fibre. This is a continuous process for making graphene fibre. The test results showed that the graphene fibre could take a stress of 10 mega Pascals (MPa) before breaking.
All this chemistry is needed because graphene does not melt. The researchers claim that graphene fibre produced by this method conducts electricity and is less brittle than other methods.
The latest research reveals the state of the art for manufacturing graphene and graphene oxide fibres. They can be made by a continuous process that creates fibres that conduct electricity and can stand 10MPa of tensile force before they break. This is about as strong as a human hair.
To put this in context, regular readers will note that we have proposed a process for making continuous sheet graphene that will have a tensile strength of 130 giga Pascals (GPa) (or about ten thousand times stronger)
Graphene fibre will find niche applications such as helping paint form flexible films and creating conductive fabrics. However be cautious of implied claims of amazing strength properties.
Adrian Nixon began his career as a scientist and is a Chartered Chemist and Member of the Royal Society of Chemistry. As a scientist and ... <Read more about Adrian Nixon>