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Home / Issues / № 2, 2014

Engineering

Carbon nanotubes for reinforcement сeramic - matrix nanocomposites: synthesis, modeling, optimization
Koltsova E.M., Skitchko Е.А., Poriyseva Е.А., Zhensa А.V.
At present 2D and 3D – ceramic-matrix  composites reinforced with carbon nanotubes have become widespread. Reinforcement with carbon nanotubes provides ceramics toughening due to inhibition of the cracks growth occurring under impact effects.

Carbon nanotubes suitable for reinforcing ceramic-matrix composites were synthesized by catalytic pyrolysis of methane- hydrogen mixture, of propane-butane mixture using iron-cobalt-aluminum oxide catalysts.

A series of experiments was carried out during which we studied the effect of iron-cobalt-aluminum catalyst active phase composition and quantity, temperature and feed gas mixture composition on the yield and quality of the carbon nanotubes by the methane-hydrogen mixture catalytic pyrolysis. The best catalyst according to the ratio of carbon nanotubes output - quality carbon appeared to be one with the iron to cobalt ratio in the active phase composition as 3:1 ([Fe0,45Co0,15Al0.40]2O3). As the temperature increased, the initial synthesis rate and the carbon nanotubes yield raised, diameter dispersion decreased. With active phase catalyst amount reducing onset synthesis temperature increased but carbon nanotubes output dropped. Methane diluting by hydrogen makes it possible to achieve nanotubes yield rise by 4 times (at hydrogen  content of 40% vol.). Thus the most promising modes for carbon nanotubes synthesis by methane catalytic pyrolysis are: catalyst - [Fe0,45Co0,15Al0.40]2O3, pyrolysis temperature - 775˚С, hydrogen content in feed gas mixture - 40 vol.%. These conditions allow us to obtain nanotubes with outer diameter 10-20 nm, inner diameter 5-10 nm.

A series of experiments was carried out to investigate temperature impact on nanotubes synthesis on the catalyst (Fe0,48Co0,12Al0,40)2O3 by propane-butane gas mixture catalytic pyrolysis, as components of oil dissolved gas. It was shown that the highest nanotubes yield is achieved at the temperature 600ºС, nanotubes have an outer diameter 5-40 nm and inner diameter being in the range 5-20 nm.

Kinetic experimental investigations of nanotubes synthesis were carried out on the developed kinetic plant. Temperature and feed mixture composition effects on  nanotubes output during the time were examined. Kinetic studies showed that nanotubes output and the initial growth rate increased as the synthesis temperature  raised and the induction period (duration of active centers formation) decreased, i.e. catalyst active centers were formed more rapidly.

To learn the mechanism of carbon nanotubes synthesis mathematical model was developed. The mathematical model includes the equations for component concentrations changes of continuous and dispersed phases participated in catalytic pyrolysis and enables to calculate their concentration at any time in any reactor point. Kinetic schemes for methane-hydrogen mixture pyrolysis were developed. The best scheme (giving the largest fit with experimental data) is the following:

Kt + CH4 → [CH3 – Kt] + H;

[CH3 – Kt] + H → [CH2 – Kt] + H2;

[CH2 – Kt] → [CH – Kt] + H2;

[CH – Kt] + H → [C – Kt] + H2;

[C – Kt] → CНТ + Kt;

[Cа – Kt] → [Cа – Kt];

Kt + H2 → [H – Kt] + H;

[H – Kt] + H → Kt2 + H2;

Kinetic scheme includes nine steps: methane adsorption on the catalyst active center with a surface compound formation [CH3 – Kt]; surface compounds dehydrogenation [CHi – Kt]; carbon nanotubes and amorphous carbon formation; amorphous carbon removal from the catalyst active centers (catalyst activation); hydrogen adsorption and desorption.

Kinetic parameters impact on nanotubes synthesis mechanism was examined. Changes in constants of adsorption and catalyst decontamination affected on extreme point position of output dependence on hydrogen concentration in the gaseous phase, dehydrogenation reaction constants impact on carbon nanotubes initial growth rate, the hydrogasification constant for amorphous carbon with methane formation affects on the entire course of nanotubes growth kinetic curve.

On the base of experimental studies and computations using mathematical models optimal modes for carbon nanotubes synthesis on the catalyst [Fe0,45Co0,15Al0.40]2O3 were obtained: pyrolysis temperature-775ºС, hydrogen content in feed gas mixture -40% vol. It allows to obtain the multilayer carbon nanotubes with outer diameter 10-20 nm, the number of layers – 13, the length ~ 1.23 mcm, the nanotubes enable to reinforce ceramic–matrix composites (based on Al2O3).

The research was fulfilled under financial support of Russian Science Foundation within scientific project 14-19-00522.



Bibliographic reference

Koltsova E.M., Skitchko Е.А., Poriyseva Е.А., Zhensa А.V. Carbon nanotubes for reinforcement сeramic - matrix nanocomposites: synthesis, modeling, optimization. International Journal Of Applied And Fundamental Research. – 2014. – № 2 –
URL: www.science-sd.com/457-24627 (21.08.2019).