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Friday, 25 May 2018

Carbon nanotubes (CNTs)

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One of the promising directions in modern materials science is carbon skeletal structures (fullerenes, nanotubes, etc.). Carbon nanotubes (CNTs), first discovered in 1991 by Japanese scientist Sumio Iijima, occupy a special place among the structures mentioned above.

CNT is hexagonal graphite plane rolled in a tube and usually have a hemispherical cap at the end which can be considered as half of a fullerene molecule. The length of the nanotubes can reach tens of microns and up to several times surpass its diameter, which generally is only a few nanometers. There are single-walled (single-layer) and multi-walled (layered) carbon nanotubes. The interlayer distance in multi-walled nanotubes is close to 3.4 Å.

 

One of the most remarkable properties of carbon nanotubes is the relationship between the geometric structure of the nanotube and its electronic properties: carbon nanotubes are metallic or semiconducting.

Interest associated with synthesis and study of carbon nanotubes (CNTs) is dictated by two important factors. Firstly, CNTs exhibit the unique physical and chemical properties (strength, stiffness, chemical resistance, thermal conductivity, electrical conductivity, etc.) which are not applicable to other nanoscale objects. The unique properties of CNTs strongly depend on their structure and geometric parameters (diameter, length, number of layers, the region of coherent scattering), which in turn depend on the synthesis technique and technological conditions. Secondly, the properties determine the possibility of CNTs applications to create new materials and devices (cold cathode field emitter display, batteries and high capacity capacitors, heavy duty ceramics, etc.).

There are many methods of carbon nanostructures synthesis. The main ones are the electric arc discharge, laser ablation, catalytic pyrolysis of hydrocarbons. High quality carbon single-walled nanotubes are synthesized using the electric arc discharge method. Laser ablation method allows to vary the type and size of CNTs depending on the catalyst and the laser wavelength. In the high pressure (30-50 atm) carbon monoxide disproportionation process (HiPCO) at 1100 - 1200 ° C basically single-walled CNTs are obtained.

The most technologically advanced method for CNT synthesis is the catalytic pyrolysis of carbon compounds (gaseous and liquid) in the presence of metal containing catalysts. This method has significant advantages over the other methods of CNT obtaining due to lower temperatures (800-1000 °C) and the possibility to choose the kind of carbon compounds and catalyst materials.

On the basis of this method we have developed another one with different technological equipment and synthesis conditions: carbon nanostructures were obtained by the catalytic pyrolysis of liquid hydrocarbons in isochoric conditions at temperatures (450 to 550 °C) in steel autoclaves. Thereby, we were able to reduce the synthesis temperatures compared with the standard method of catalytic pyrolysis. Other advantages of the method include the low cost, the possibility of varying the temperature and time of synthesis, carrying out the process in the autoclaves without external pressure sources.

As a source of carbon were used: toluene (C6H5CH3), benzene (C6H6) and isopropanol ((CH3) 2CHOH) with using nickel as the catalyst.

Characterization of nanostructures by electron microscopy

CNT and CNF obtained by pyrolysis of toluene, benzene and isopropanol at t = 500 ° C, t = 48 h

Microstructure

Outer diameter (D outer), nm

Inner diameter (D inner), nm

Length (L), µm

Number of layers (n)

CNT (Toluene)

CNT (Toluene)

 

50 – 130

 

5 – 15

 

1 – 5

 

30 – 90

CNT (Benzene)

CNT (Benzene)

 

20 – 30

 

5 – 8

 

0.3 – 1

 

10 – 20

CNF (Isopropanol)

CNF (Isopropanol)

 

250 – 300

 

 

1 – 2

 

CNF - carbon nanofiber