Analysis of the Formation of Multi-Layer Carbon Nanotubes in the Process of Mechanical Activation of the Pyrolysis Products of Vegetable Raw Materials

The carbon nanotubes are formed by pyrolytic and mechanochemical technology. Amorphous carbon is produced at 950°C and then subjected to mechanochemical treatment in a planetary mill for 1–46 h. Analysis ofinfluence of duration of mechanical activation of amorphous carbon on the morphology of moldable multilayer carbon nanotubes. It is demonstrated that prolonged mechanical activation of carbon composite in a vario-planetary mill promotes to formation of aggregates and amorphous carbon and to loss of thermal stability of nanotubeswith furtherconduct of vacuum annealing.


Introduction
Material generatedbased on carbon nanotubeshave unique propertiesand were therefore findwide application as components of radio electronics, modifiers of structural materials, additives to lubricants, varnishes and paints, hydrogen accumulators of fuel cells of electric cars, high-efficiency adsorbents, gas diffusion layers of fuel cells, etc. Use of carbon nanotubes in fine chemical synthesis, biology, and medicine is promisingas well asfor producing composites with improved mechanical and electrophysical properties, including for imparting antistatic and conducting properties to polymers [1].
Properties of multilayer carbon nanotubes (MCNT) formed in the process of mechanical activation of amorphous carbon of plant origin in a vario-planetary mill for 1-27 h were studied earlier [2][3][4]. However, the problem of maximally possible MCNT yield under fixed conditions of mechanical activation of amorphous carbon remained unclarified. This study was carried out to determine the influence of duration of mechanical activation of amorphous carbon on the nanotube content in the end product.
The carbon nanotubes were produced by employing pyrolytic and mechanochemical technologies. Rusty (brown) peat moss (Sphagnum fuscum) and Magellan's (midway) peat moss (Sphagnum magellanicum), shoots of corn of the variety Katerina SV, thorny (spiny) bamboo (Bambusablumeanaschultes), cotton plant of the variety Priozernyi (Lake region)-4, and stems (stalks) of okra (Abelmoschusexculentus) of the variety Zelenyibarkhat(Green velvet) were used as the feed material. The plant material was initially dried and sieved to remove excess moisture and foreign matters, and disintegrated to structure were obtained at 950° washing with a mixture (1:1) of 25% in an analytical autoclave from Wiegand International GmbH (Germany) for 50 min. Next, the amorphous carbon was removed from the mixture of the acid solutions on a Keramtech (Czech Republic) filter and washed with distilled water in an Elamasonic S 30 (Germany) ultrasonic washer for 30 min up to neutral pH (pH 7.0). Thereafter, it was dewatered in a Sigma Laborzetrifugen (Germany) centrifuge and then dried in a Binder drying oven for Further, the amorphous carbon was treated in a Fritsch (Germany) Pulverisette mill. The mechanoreactor of the vario container with a VK-6 (WC-6) hard alloy insert: t mm in diameter. The mill operation conditions were: rotation speed of main disk 400 rpm and of planetary pinions 800 rpm, intensity (ratio of weight of original materials to weight of pulverizing balls) 1:50.
The specific surface area of the carbon materials was studied on a Sorbtometr (CJSC) KATAKON, Novosibirsk] specific surface area analyzer and the specific surface area was determined by thermal desorption of nitrogen. The X performed on an EVO-50XVP (Carl Zeiss) scanning electron microscope in conjunction with an INCA Energy-350 (England) Xstudied on a Hitachi S5500 (Japan) high for transmission microscopy.  3 and HCl solutions at 100°C. The treatment was carried out in an analytical autoclave from Wiegand International GmbH (Germany) for 50 min. Next, the amorphous carbon was removed from the mixture of the acid solutions on a Keramtech (Czech and washed with distilled water in an Elamasonic S 30 (Germany) ultrasonic washer for 30 min up to neutral pH (pH 7.0). Thereafter, it was dewatered in a Sigma Laborzetrifugen (Germany) centrifuge and then dried in a Binder drying oven for 60 min at 125-130°C.
Further, the amorphous carbon was treated in a Fritsch (Germany) Pulverisette mill. The mechanoreactor of the vario-planetary mill consisted of a leak-proof corrosion 6) hard alloy insert: the pulverizing bodies were VKmm in diameter. The mill operation conditions were: rotation speed of main disk 400 rpm and of rpm, intensity (ratio of weight of original materials to weight of pulverizing The specific surface area of the carbon materials was studied on a Sorbtometr (CJSC) KATAKON, Novosibirsk] specific surface area analyzer and the specific surface area was determined by thermal desorption of nitrogen. The X-ray energodispersion microanalysis was 50XVP (Carl Zeiss) scanning electron microscope in conjunction with an -ray energodispersion spectrometer. The structure of the MCNT was studied on a Hitachi S5500 (Japan) high-resolution scanning electron microscope with an attachment The carbon composites were prepared for vacuum annealing using coal toluene (OAO Bagleikoks, Ukraine). The carbon mass (0.4 biologicalfilter and then dried in a drying oven at 60°C. The amorphous carbon was removed by three stage vacuum annealing at 220 graphite electrode from Contorr Vacuum Industries (USA). The temperature was 220°C in the first stage, 550°C in the second, and 870°C in the third stage.
At the initial stage of the study, carbon modifications with amorphous structure were obtained from the plant material at the pyrolysis temperature of 950°C. The chemical composition of the amorphous carbon obtained by pyrolytic treatment of brown (rusty) In all cases, the morphology of the amorphous carbon is represented by the original for a specific type of plant material The carbon composites were prepared for vacuum annealing using coal toluene (OAO Bagleikoks, 4 g) was mixed with 50 ml of toluene and filtered on a finely disperse biologicalfilter and then dried in a drying oven at 60°C. The amorphous carbon was removed by three stage vacuum annealing at 220-870°C in a laboratory vacuum furnace with a System VII graphite electrode from Contorr Vacuum Industries (USA). The temperature was 220°C in the first stage, 550°C in the second, and 870°C in the third stage.
At the initial stage of the study, carbon modifications with amorphous structure were obtained from the plant material at the pyrolysis temperature of 950°C. The chemical composition of the amorphous carbon obtained by pyrolytic treatment of brown (rusty) sphagnum peat moss is given in the In all cases, the morphology of the amorphous carbon is represented by the original for a specific type of plant material(figure 1).

(ad) and corn (e and f) a, b, c, e, f -SEM image; d -
The carbon composites were prepared for vacuum annealing using coal toluene (OAO Bagleikoks, ml of toluene and filtered on a finely disperse biologicalfilter and then dried in a drying oven at 60°C. The amorphous carbon was removed by three-870°C in a laboratory vacuum furnace with a System VII-series graphite electrode from Contorr Vacuum Industries (USA). The temperature was 220°C in the first At the initial stage of the study, carbon modifications with amorphous structure were obtained from the plant material at the pyrolysis temperature of 950°C. The chemical composition of the amorphous sphagnum peat moss is given in the Table 1. In all cases, the morphology of the amorphous carbon is represented by the original structure typical  To ascertain the sequence of MCNT formation, the amorphous carbon was submitted to mechanical activation for 1 to 46 h. The change in the structure of the amorphous carbon in the course of its mechanical treatment in a vario-planetary mill is shown in Figure1. As can be seen from Figure 1a, in the first 1-6 h, formation of MCNT is not discernible by electron microscopic methods (the amorphous carbon retains its lamellar form). After 8 h of treatment (see figure2 b), begins the process of formation of a nanofiber structure, which is realized in the mass of the carbon particle (see figure2 c), whereupon carbon nanotubes with diameters of 10-20 nm are formed. After 10 h of mechanical activation, the whole volume of the treated material consists of carbon nanotubes with diameters ranging from 10 to 70 nm (figure2 d).
Note that some particles up to 3 μm in size survive up to 16 h of mechanical treatment (figure2e,f) when amorphous carbon is produced by pyrolysis of corn and Magellan's peat moss and up to 27 h when amorphous carbon from cotton plant, bamboo, and okra are used.
Further extension of mechanical treatment time to 27 h leads only to increased defects in the carbon nanotubes with partial formation of "enclosednanocones" and "bamboo" type of structures. The MCNTs produced by mechanical activation of amorphous carbon have a fairly large specific surface area (S sp = 400-510 m 2 /g and a low ash content (~ 1.5 wt. %).
It was shown earlier that the quantity of nanotubes formed upon mechanical activation of amorphous carbon depends linearly on the time of mechanical treatment (1-27 h) of carbon composite in a vario-planetary mill [2][3][4].To determine the nanotube content in the carbon composite, we carried out in this work three-stage vacuum annealing of the latter, which facilitates removal of amorphous carbon. It is well known that carbon nanotubes retain thermal stability during high-temperature vacuum annealing up to 1900°C and even above and that no change in morphology of the nanotubes occurs after thermovacuum treatment, which allows one to effectively purify CNTs and get a product with a purity of no less than 99.9 wt. % [5].
As will be seen from Table 2, a substantial quantity of CNT is formed in just 4 h of mechanical activation of the amorphous carbon, although electron microscopic methods do not show them up until 8 h of mechanical activation. The reason for this is CNT formation inside the amorphous carbon particles. The yield of nanotubes reaches the maximum for all the studied amorphous carbon modifications when the duration of mechanical activation is 36 h. In this case, the MCNT content is found to be maximum (79.48 wt. %) after vacuum annealing of carbon composite obtained by mechanical activation of rusty (brown) sphagnum peat moss pyrolysis products, which is twice as much as the yield of CNTs obtained by similar treatment of another sphagnum moss, namely, Magellan's peat moss. The results of scanning electron microscopic study of CNTs formed upon 36-and 48-h of mechanical activation of the amorphous carbon produced from rusty (brown) sphagnum peat moss are shown in Figure 3. In this case, the carbon composites were not submitted to vacuum annealing. As can be seen, the carbon nanotubes formed in 36 h of mechanical activation are segregated from each other, and formation of agglomerates and aggregates in the carbon material mass is not observed. Extension of mechanical activation time to tomentoseaggregates (nanocomposites) consisting of carbon nanotubes and amorphous carbon. The aggregates are formed apparently due to electrostatic interaction of MCNTs. If this carbon nanocomposite is submitted to vacuum annealing for removing amorphous carbon, the nanotube content in the annealing products decreases several folds (

Conclusions
Thus, the maximum carbon nanotube yield is observed after amorphous carbon obtained by pyrolysis of plant materials. Longer mechanical activation produces MCNT + amorphous carbon nanocomposite aggregates, which annealing to loss of thermal stability of the carbon nanotubes constituting the aggregate. This fact has to be taken into consideration while optimizing the technology of production and purification of b d Structure (SEM) of carbon nanotubes obtained by mechanical activation for: Extension of mechanical activation time to 46 h leads to formation of aggregates (nanocomposites) consisting of carbon nanotubes and amorphous carbon. The aggregates are formed apparently due to electrostatic interaction of MCNTs. If this carbon nanocomposite is submitted to vacuum annealing for removing amorphous carbon, the nanotube content in the annealing products decreases several folds (see table 2).
Thus, the maximum carbon nanotube yield is observed after 36 h of mechanical activation of amorphous carbon obtained by pyrolysis of plant materials. Longer mechanical activation produces amorphous carbon nanocomposite aggregates, which subsequently leads upon vacuum annealing to loss of thermal stability of the carbon nanotubes constituting the aggregate. This fact has to be taken into consideration while optimizing the technology of production and purification of nanotubes obtained by mechanical activation for:a, b -36 h; c,dh leads to formation of 20-100 nm aggregates (nanocomposites) consisting of carbon nanotubes and amorphous carbon. The aggregates are formed apparently due to electrostatic interaction of MCNTs. If this carbon nanocomposite is submitted to vacuum annealing for removing amorphous carbon, the nanotube h of mechanical activation of amorphous carbon obtained by pyrolysis of plant materials. Longer mechanical activation produces subsequently leads upon vacuum annealing to loss of thermal stability of the carbon nanotubes constituting the aggregate. This fact has to be taken into consideration while optimizing the technology of production and purification of