L(caps)ightweight and high-strength are now one of the most essential themes of material development. It is important to make the economic use of metals and increase the longevity of any product and add new functionality to that. Carbon nanotube(CNTs) are one of the great inventions of the modern age which can be replaced by steel as it has almost two times tensile strength than steel and also extremely lightweight and thin at the same time. It is already being used in several modern age equipment but with better manufacturing technology and continuous research it can be a very essential metal like steel which can be used from small surgery tools to space stations.
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Introduction:
In the 1970s’ Arthur C. Clarke was the man who put him about the space elevator in his science fiction novel named “The Fountains of Paradise”. It was a super-strong cable that can take a spacecraft from earth to the space station. But for many years it was just a science fiction book thought until a scientist from Japan named Sumio Iijima synthesized and studied a super-strong material named Carbon Nanotube in 1991 and published his research paper in 1993. This was the time when scientists reconsidered the idea of a Space elevator. Although it’s still quite impossible to bring that science fiction idea in the light as we have to make a megastructure of 35,786 km and we have to face atmospheric turbulence, Coriolis forces and the cable should be able to handle 63 GPa but research is still going on to discover more about this extremely strong but super lightweight material.
What are CNTs?
In short carbon, Nano-tubes are made from carbon ( C ) which has an atomic number of 6 and its diameter is measured in nanometers. We can think of CNTs as a very thin sheet of graphite that is rolled into a tube with bonds. And at the end of that sheet, a bond is formatting which is closing the tube. The strength of Carbon nanotubes mainly comes from the extremely strong bond between carbon atoms.
Varieties and Mechanical Characteristics of CNTs
Carbon nanotubes can be structured single-wall or multiple-walls. But fullerite, nano knot, and a torus are also some rare types of CNTs.
Single-walled carbon nanotubes (SWCNTs) are basically one single layer of graphite roll that can make a seamless long cylindrical shape. For one dimensional structure, it can have a length of 1000m where the diameter is just 1nm. For this characteristic, it made this material really lightweight whereas the tensile strength is very high.
Multi-walled carbon nanotubes(MWCNTs) is simply a group of SWCNTs and it can have different diameter and side according to its need. The interlayers distance of Multi-walled carbon nanotubes is really low which is about 3.3Ã…. Double-walled carbon nanotubes can also be found for different purposes.
Structure and Mechanical Properties of CNTs
The structure of CNTs is really important as the tensile strength, stiffness and other mechanical properties depend on the structure although the materials are the same. In early 2000 scientists thought that it can withstand 300 GPa but later one it was discovered that the strength can vary from 60~600 GPA based on its structure(Theoretical. Nb: practical is only 25~66GPa). Depending on the structure CNTs can be divided into mainly 3 types which are Zigzag, Chiral, Armchair. These three structures depend on which direction the graphite layers are cut and rolled. As graphite is hexagonal in structure the angle of the hexagonal plays a very important role here.
Zigzag:
This structure is made by cutting the graphite layer following the red mark and rolling it over. This structure has a Modest strength which can be 30~50 GPa and diameter can be less than 2nm
Chiral:
Chiral structure is made by cutting the graphite layer at an angle marked below. The chiral structure has the lowest strength which is around 25GPa
Armchair:
This structure has the highest strength with is over 60GPa and diameter can as low as 1.5nm
Here point to be noted that the strength of CNTs fully depends on the angle of the structure and the diameter of the tube but if any defect in the structure it will weaken the tensile strength. Not only the strength it also has very good elasticity. Pressing the tip of a CNT can bend the tube without damaging it and also the original shape can be gained.
Processing
From 2009~2011 many types of research were conducted to improve the production process of carbon nanotubes but the CNT synthesis method was highly appreciated and popular as it can go for mass production. But as for greater quantity, we always have to sacrifice precious quality this method is not the best for high-quality CNT materials.
In the synthesized process of CNTs, both high and low temperature can be used. CVD withstands for Chemical Vapor Deposition is the process for using low temperature. This is basically a reaction between a carbon contaminated flowing gas and some catalyst particles. The high-temperature method is using Arc discharge and laser ablation which can occur at high temperatures of 1000~3000 degrees celsius. The high quality of CNTs purification process is done later on.
The reason behind choosing CNTs over steels:
The strength of Carbon nanotubes lays behind its sp^2 which is even higher than the diamond structure which has an sp^3 bond. Here is a chart of strength comparison among different materials based on specific strength which is force per unit area at failure divided by its density (kN.m/kg)
According to this chart, it’s clear that even the minimum strength of CNTs is way too high that the steel. Also, it has a lightweight and holding this much strength CNTs fiber can be very small in diameter. So, while building and strong rope for building any megastructure or nanostructures that need a lot of strength CNTs can be preferable. Also for its lightweight spacecraft or even modern automobile structure can be made as it can save a lot of fuel making the whole structure very lightweight.
For mechanical reinforcement CNTs have no other component as it has a huge length: diameter ratios. One research from General motors Research and development showed that adding to polypropylene 11.5% by weight of nested CNTs of 0.2Mn diameter can double the tensile strength of polypropylene.
Another research from the University of Tokyo has proved that incorporation of 5% by volumes of CNTs with Aluminum can increase the tensile strength of aluminum almost twice. Another research can be found which was done with 30% of CNTs and 70% steel. With proper processing, the tensile strength of steel can be increased 7 times from that alloy.
Futuristic Thought about CNTs
The burning question about CNTs is a space elevator. Although it’s just a science fiction thought, different research is still going on. Although it has to be a megastructure of 35,786km One Russian engineer Eury Archothanob proposed to make a satellite that will orbit the earth at the same speed as the earth and the elevator can be made from there. But as this structure will fully depend on the strength of the material CNTs can be a light to this project. Although the calculation is saying that the material has to withstand the pressure of 66GPa.
Despite all these negative facts if this futuristic project can be brought to light it will be a huge revolution of space technology as they have to spend a huge amount of money for each spacecraft. According to SpaceX's data, it costs around 3,370 $ per kg of a falcon to take space from the earth surface and for space subtle this cost is around 28,037$ per kg. But if this futuristic space elevator can be made this can reduce the cost as low as 220 $ per kg.
Conclusion
Although the history of carbon nanotubes started from a science fiction novel, significant progress has been achieved in the area of this super material. New Manufacturing and processing ideas are coming up through the study. Even if the space elevator is a futuristic project but using the mechanical properties of CNTs we can develop new useful products with huge potential.
Citations:
[2] https://www.sciencedirect.com/science/article/pii/S1026309812002477
[3] https://www.intechopen.com/chapters/72939
[4] https://www.bu.edu/gk12/marc/Lessons/cnt/cnt_talk.pdf
[5] https://www.prescouter.com/2017/03/applications-carbon-nanotubes/
[7] https://en.wikipedia.org/wiki/Carbon_nanotube
