Nanocomposites and nanomaterials 
Carbon nanotubes synthesis by CVD method
Khovavko A.I.1, Nebesniy A.A.1, Svyatenko A.M.1, Filonenko D.S.1, Kotov V.G.1, Bondarenko B.I.1, Nie G2
1Department of Thermo-chemical Processes and Nanotechnologies,
Gas Institute of the NAS of Ukraine. 
Degtyarivska, 39, Kyiv-03113, Ukraine. 
2School of Physics and Telecommunication Engineering, Yulin Normal University, Yulin 537000, China.
E-mail: ahova2005@ukr.net

The Gas Institute of NAS of Ukraine in recent years has been exploring and developing new approaches that would allow continuous production of nanotubes of uniform quality [1,2]. The problem of improving the method of carbon nanotubes (CNTs) obtaining on a catalytic substrate have successfully solved. 
The process of forming carbon nanotubes by the CVD (chemical vapor deposition) method on different catalysts at different hydrodynamic and temperature modes was investigated on the created equipment. A new plate-type catalyst has been developed and successfully tested. The efficiency of its use is confirmed by the quality and quantity of the obtained carbon material.
The most common carbon-containing gaseous source – natural gas was used as the feedstock. Natural gas was pre-converted on a nickel catalyst, whereupon the resulting conversion products were used to synthesize carbon nanotubes.
In order to improve the quality of the resulting carbon product, we have proposed that natural gas be pre-converted, converting hydrogen-bound carbon into a less strong carbon-oxygen compound – carbon monoxide. And only then, the obtained conversion products containing carbon in the form of CO are sending to the process of CNTs synthesis. It has been found that at temperatures above 700° C, the released carbon has a scaly structure and, at lower temperatures, it is synthesized as CNTs. This leads to the conclusion that in the temperature range below 700° C, carbonization-decarbonation cycles in the Feγ- Feα-Fe3C system can play a crucial role in CNTs formation.
Our technological solutions have made it possible to increase the yield of CNTs per volume unit of carbon-containing atmosphere. The highest content of nanotubes in carbon material was approximately 90%.
It is advisable to recirculate spent gases to increase carbon nanomaterial yield. The use of gas recirculation increases the specific yield of carbon material, but in this case, due to the accumulation of nitrogen in the gas phase, the recirculation rate is limited and cannot exceed n = 0,8-1,0.
We have an opportunity to reduce the cost and increase the reactor productivity by creating the modular equipment. The price of obtained CNTs is much lower than the world analogues in addition obtained by us nanotubes are quite high in purity (almost amorphous carbon is absent).
1. A. Nebesnyi, V. Kotov, A. Sviatenko, D. Filonenko, A. Khovavko, B. Bondarenko. Carbon nanomaterial Formation on Fresh Reduced Iron by Converted Natural Gas // Nanoscale Research Letters (2017) 12:107 DOI 10.1186/s11671-017-1882-6.
2. Bondarenko B, Sviatenko O, Kotov V, Khovavko A, Filonenko D, Nebesniy A, Vishnevsky A (2013) Research of the technology of carbon nanotubes production in gas mixtures contained CO // International Journal of Energy for a Clean Environment 14(2–3):177–182. doi:10.1615/ InterJEnerCleanEnv2014006802.