The many faces of carbon

Graphene placed on top of boron nitride, a step in forming a superlattice. The first discovery on the properties of this lattice were made by a team at Berkeley. Credit: Berkeley Lab

Diamond nano thread visualization: Credit: Penn State University

Diamond anvils malformed during synthesis of ultrahard fullerite. Note the dent in the center. Credit: MIPT

Just yesterday I reported the finding of a "magic" paint made of carbon flakes that can make any surface impermeable to fluids and gases. Carbon is indeed a most interesting atoms and scientists are learning to exploit its many configurations that are possibile, some naturally occurring on the Earth (or in the interstellar space, like the CNT - Carbon Nano Tubes), others manufactured by them.

Today I share with you some other progress in the creation of materials with interesting properties by creating specific carbon structures.

One example is the research of a joint team at MIT and Manchester University reported on a paper published on Science where the researchers describe the semiconductor properties of a superlattice (a thin layer of material, a few atoms thick) created by overlaying a graphene layer onto a boron nitride layer. The overlay is made in such a way that the "rings" of graphene are overlaid on the "rings" of boron nitride. 

Now come the interesting part. When charges move on graphene they behave like massless particles (neutrinos) moving at the speed of light (they are massless....). However, when one overlay the graphene on the boron nitride layer the charges behave like neutrinos that have acquired a significant mass. This looks like a new kind of electrons (in terms of behaviour), a kind that can be easily be controlled by applying an electric field. Thus it opens the way to switch on and off current, that is making the superlattice behave as a transistor.

It is not completely clear what is going on, it is an example where technology advances go beyond the present capability of science to explain.

We know that carbon comes in different forms, different aggregations of its atoms that lead to completely different physical characteristics. Graphite and diamonds are exactly the same in terms of chemical composition, both are pure carbon, but try to propose to your girlfriend with a wonderful slab of graphite rather than with a diamond and you see what I mean.

Now researchers at the Penn State University have published a paper showing how to produce diamond nano threads that have amazing physical properties in terms of strengths and stiffness, much better than anything produced so far.

Such characteristics will come handy in the production of lighter vehicles, leading to lower fuel consumption and in several other applications. One of the most interesting, and challenging one, is the development of a space elevator
A space elevator requires a cable that is tied to one point on the Earth surface at the equator and extend into space well beyond satellite geostationary orbit. At the other end of the cable, the one in space, you have it tied to a mass that being beyond the geostationary orbit is being subjected to the centrifugal force, hence the cable is pulled tight by the two forces, the one of the Earth gravitational field and the centrifugal force.

The problem is finding a cable that has the required strength to resist these two forces. The bigger the cable the more it weights and the worse the problem is. Materials like kevlar would be good enough to support a space elevator on the Moon and probably on Mars but the Earth gravity is too much for them.

This discovery by Penn researchers seems to be just what is needed to make a Earth space elevator feasible. That would dramatically decrease the cost of moving things into orbit since what you need is the power to push them onto the cable up to the geostationary orbit and you basically don't need any power to carry the loading vehicle into space, which by far is what makes space voyage expensive. The payload of a Saturn rocket (the biggest payload so far) was 118,000 kg and the weight of the rocket was 2,950 tons. Hence the ratio between vehicle weight and payload was 1:25, and so was the power required. Indeed, the possibility of having a space elevator (whose idea goes back to 1895!) would change the rules of the game for space travel.

A third advance in material science on carbon is the result of a research carried out at the Technological Institute for superhard and novel carbon materials in Troitsk, Russia, and MIPT (Moscow). 

They have been able to create a new material, fullerite, made by tiny balls of carbon atoms (fullerene - each ball made up by 60 carbon atoms), that has a hardness between 150 and 300Gpa whilst the diamond is below 150GPa (Giga Pascal), 115 on the average and between 70 and 150 depending on the purity. Notice that materials having a GPa over 40 are considered super-hard materials, for comparison iron has a 0.6 GPa value.

The problem in creating the fullerite is the high pressure needed to squeeze the fullerene balls together, in the order of 130,000 atmospheres. We do not have industrial equipment to achieve that kind of pressure. The researchers at MIPT have found that by using Carbon disulphide they can obtain the fullerite with pressures lower than 80,000 atmospheres and at room temperature (rather than having to heat the fullerene at 820° Celsius).

This advance makes the development of ultra hard material within sight. That would allow the production of manufacturing tools that will last much longer and hence lower the cost of production. So it is not just about Industry 4.0 and the benefits brought by the mingling of Internet with production processes, it is also about new materials that can change production processes and lower their cost.

Author - Roberto Saracco

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