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Abstract


Contents

Introduction

Nanoscience and Nanotechnology is a new revolutionary way thinking and production, using the traditional scientific approach, based on a progressively decreasing scale. In practice, this approach makes it possible to create products and processes with improved properties. In the next ten years is the development of nanotechnology and production of new nanomaterials will be one of the main engines stimulating change in science. In connection with this research nanostructures and technologies are becoming an increasing importance, because they have the potential to create new ways to get materials, the controlled manipulation and management of properties materials at the nanoscale [ 1 ].

Analysis of the status and trends of nanotechnology facilities in the now allows us to conclude that one of the most promising field of nanotechnology is the synthesis of carbon nanomaterials (CNM). Among these materials occupy a special place Carbon nanotubes (CNTs), which with a diameter of 1 ... 50 nm and lengths up to several micrometers form a new class of nano-objects. CNTs have number of unique properties due to the ordered structure of their nanofragmentov, materials that are based on carbon nanotubes can be successfully used as a structural modifier construction materials, hydrogen storage, items, electronics, etc. The widely discussed use of carbon nano-structures in thin chemical synthesis, biology and medicine. [ 2 ]

1. Theme urgency

A number of methods for producing carbon nanomaterials: electric, thermal and laser sputtering of graphite, condensation method. These methods have some efficiency, but poorly suited for industrial applications, and have some significant drawbacks such as high cost equipment and the complexity of the organization of production.

Master's thesis is devoted to actual scientific problem of the development thermal parameters and systems to enable industrial synthesis of carbon nanotubes by catalytic pyrolysis, which is in contrast to the methods mentioned above, has a number of advantages such as relatively low energy process; the use of cheap and readily available carbonaceous materials, "soft" technological parameters of synthesis, ease of construction and manufacturability of the equipment used, the lack of need expensive purification from impurities.

2. Overview

Carbon nanotubes - a cylindrical elongated structure with a diameter 1 .. 10 nanometers and lengths up to several centimeters, consisting of one or several collapsed in a tube of hexagonal graphite planes, and usually ending hemispherical head which can be viewed as half fullerene molecule [ 3 ]. Carbon nanotubes were discovered in 1991 the Japanese Idzhimoy researcher. The first nanotube was obtained by sputtering of graphite in an electric arc. Measurements made using an electronic microscope showed that the diameter of these fibers does not exceed several nanometers and a length of from one to a few microns.

By cutting along the longitudinal axis of the nanotube, it was discovered that she consists of one or more layers, each of which represents hexagonal grid of graphite which is based on hexagons arranged at the vertices of angles of the carbon atoms. In all cases, the distance between the layers is equal to 0.34 nm, the is the same as that between the layers in crystalline graphite. Top ends of the tubes are closed by hemispherical caps, each layer of which composed of six-and pentagons, reminiscent of the structure of the halves of the molecule fullerene. [ 4 ]

Nanotubes are members of the family of fullerenes, which also includes a spherical fullerenes. The diameter of nanotubes on the order of several nm (About 1/50, 000 the width of a human hair), while they can be up to 18 centimeters in length (as of 2010) Applied quantum chemistry, In particular, orbital hybridization, best describes the type of chemical bonding in nanotubes. Chemical bonding of nanotubes is completely consist of SP2 communication, like graphite. These links are stronger than SP3, and they provide a unique strength of nanotube. In addition, nanotubes are naturally united "ropes" held together by by van der Waals [ 5 ].

3. Obtaining carbon nanomaterials by the catalytic pyrolysis of carbon-containing gases

By feedstock can be two groups of CNM synthesis processes, the first of which includes disproportionation of CO, the second pyrolysis of hydrocarbons. Works R. Smalley initiated the process of creating HiPSO (The High pressure CO) - method for the catalytic production of SWCNTs in a continuous stream of CO (Feedstock) using Fe (CO) 5 as an iron-containing catalyst. Nanotubes are, flowing CO, mixed with Fe (CO) 5 , through the heated reactor. This method produced nanotubes were diameter of 0.7 nm, which is supposed to have the lowest the size of the achievable chemically stable SWCNT.

At the University of Oklahoma (USA) developed a process CoMoCAT. In this method of carbon materials grown disproportionation of CO at t = 700 ... 950 ° C. The technique is based on unique composition of the catalyst Co / Mo, which slows down the sintering CO particles and therefore slows down the process of formation of undesirable forms of carbon. In the reaction, CO recovered from the oxide state to metal. At the same time converted into a form Mo carbide Mo 2 C.

The essential drawbacks of HiPCO process should include hard insurmountable problem of disproportionation processes of CO, especially in large quantities, because of the need for a cold CO area with a high fever. The process is based on CoMoCAT unique and, consequently, expensive catalyst. In addition, CO is a toxic gas and poses a significant risk when it use in industrial environments. Therefore, for a more detailed research method was chosen to obtain carbon nanotubes by pyrolysis of hydrocarbons in the reactor with the chemical synthesis catalyst on a solid substrate.

Pyrolysis, in principle, may be any carbon-containing substances. Described, in particular, to obtain nanofibers by pyrolysis of the simplest hydrocarbon paraffin a number of - CH 4 (The first were the work performed at the Institute of Catalysis. GK SB RAS Boreskov and Northeastern University, Boston, USA), C 2 H 6 , C 3 H 8 and C 5 H 12 . Numerous publications are devoted to pyrolysis with 2 H 2 , studied pyrolysis of saturated hydrocarbons is not, as with 2 H 4 , With 3 H 4 , With 3 H 6 , methylacetylene. Saturated cyclic hydrocarbons are cyclohexane, aromatic hydrocarbons - C 6 H 6 , With 6 H 5 CH 3 , (CH 3 ) 2 C 6 H 4 , polyaromatic - polifeniatsetilenom, pyridine, and pyrene, ketones - Acetone, alcohol - methanol and ethanol, and so on.

From the above it follows that as the carbon source for the synthesis of CNM can be used in almost any carbon-containing gases. However, when creation of technology appropriate to the industrial synthesis of CNM to choose the most accessible and cheap gas, besides providing a high performance such as methane or propane-butane mixture.

By way of organizing the processes of pyrolysis can be divided into two groups: with the catalyst on a carrier and with a volatile catalyst. In the first case active component of the catalyst is introduced into the reaction area on a substrate or carrier in a solid form, in the second - in the form of vapor or solutions sprayed into thin drops. As the vapor use carbonyls, phthalocyanines, metallocenes and other compounds metals, as solutions - for example, metal carbonyls in toluene. "Mortar" option implemented in the injection reactors.

An example of a process with a volatile catalyst is a device. The quartz tube containing two heating zones, acts the role of the reactor. A mixture of camphor and ferrocene with quartz substrate is placed in the center of the pipe, at an equal distance from the zones heating. After heating the furnace area of ??the substrate is shifted to a lower temperature, where the camphor and ferrocene evaporated at 200 ° C and subjected to pyrolysis in an area with temperature of 900 ° C in an argon flow of 50 ml / min. After 15 minutes of heating is switched off. Upon cooling to room temperature are carbon is deposited on a quartz substrate and the inner wall of the quartz tube in an area with high temperature. These processes are not common, there are no information on their application on a large scale.

Consider a process with a catalyst on a carrier. One of their advantages a much larger number of CNTs and CNFs obtained at unit mass catalyst. This amount (specific yield) when receiving CNF may in the tens or hundreds of grams of carbon per gram of catalyst (G C / gkt). The specific yield in the synthesis of SWCNTs usually less than receipt of MWNTs. Another parameter that determines the effectiveness of processes with catalyst on a carrier, a specific performance catalyst, ie, number of CNTs or CNFs obtained per unit mass catalyst per unit time (g / (GKAT min)).

Use a variety of

how to activate the process: thermal (external heating of the reactor, hot filament, the partial combustion of hydrocarbons), plasma (Different types of discharges), laser (selective excitation vibrational modes), with the help of an electric potential on the substrate, combined (hot filament and discharge, and the selective excitation of the discharge.

Pyrolytic methods allow the synthesis of matrix by, for example, CNTs and CNFs grown on the catalyst introduced in nanopores of membranes. Only a catalytic pyrolysis, the possibility of using the chemical vapor deposition the gas phase can be structured to obtain precipitation of CNTs and CNFs in the substrates with catalysts in the form ordered islands, strips, and any figures, ie produce elements of the devices.

The overwhelming majority of scientific and patent literature on the synthesis of carbon CNT and CNF is devoted to periodic processes. They realize, as a rule, in tubular reactors, which are typical scheme is shown in Figure 1.

Scheme of the horizontal periodic reactor for the pyrolysis of carbon-containing gas

Figure 1 - Scheme of the horizontal periodic reactor for pyrolysis carbon emissions:

1 - quartz tube, 2 - insulation, furnace with resistive heating, 3 - catalyst layer, 4 - boat, 5 - Thermocouple

Heated to a temperature of pyrolysis (550 ... 1000 ° C) reaction zone was purged with an inert gas (Ar, He), then fed carbon-containing gas. Moving along the catalyst for the gas it diffuses through the layer and adsorbed on the surface of the active centers (the metal), which runs a series of sequential chemical reactions, the final products which are carbon and hydrogen.

products of this process, which is classified as chemical vapor deposition (GFHO) or CDV-process are as follows: CNM, SWNT, MWNT, and CNF. In general thermodynamic relations describing the processes CNM in the expansion of education, for example methane CH 4 Can as follows.

The overall reaction of formation of gaseous methane CH 4 (d) , Graphite - a standard state of solid carbon C (t) :

CH 4 (d) = C (t) + 2H 2 (g) K1

where K1 - a constant equilibrium reaction.

activity of methane and g can be determined by the relation

Mr. and R1 = (P CH4 / P 2 H 2 )

where P CH 4 - The equilibrium pressure of methane;

P H2 - the equilibrium pressure of hydrogen.

However, the result is education is not more thermodynamically stable graphite, and metastable form of carbon - carbon fiber.

Therefore, taking

With (t) = With (a) K2,

Gibbs energy of formation of Gv for carbon fibers and the activity

in a = exp (Gv / RT),

get the condition under which fiber formation is thermodynamically allowed.

a r > in a ,

where (r - graphite, T - solid carbon in the Fiber).

The properties of pyrolytic CNM from the properties of nanostructures, obtained arc and ablation method. As a rule, they contain greater number of defects are a wide range of scattering diametrical sizes and lengths, large interlayer distance. Therefore, despite the apparent simplicity organization Pyrolysis methods of synthesis require careful approach to the selection parameters used, the study and optimization of the kinetic characteristics of the process. In this If possible get a CNM with high quality indicators in including SWCNTs.

Analysis of the literature allows us to establish the basic parameters affecting the structure, morphology and properties of pyrolytic CNM, is:

  1. the gas mixture;
  2. nature of the catalytic systems;
  3. temperature and pressure;
  4. duration of the process;
  5. conditions for phase transformations defined by the design of the reactor.

For CNM most often use the disproportionation carbon monoxide, decomposition of methane, butane, ethylene, propylene, acetylene. Almost all the authors justify the choice of reactant gas, emphasizing its advantages. It should agree with the authors that the chemical nature of the gas used significantly impact on the morphology of the nanocarbon deposits does not have.

It is emphasized, for example, the kinetic stability of methane, which, together so requires increasing pyrolysis temperature, especially for quality of the nanotubes. The use of CO results in a tube with a smaller (<20 nm) in diameter, however, difficult to imagine the creation of environmentally friendly production in presence as a raw material CO.

When creating the conditions for CNM in significant quantities should take into account that communications output and quality of material, depending the type of raw material gas is shown kinetics of the process. It is also important as the availability of raw materials and safety. For application purposes it is important to achieve minimum presence of amorphous carbon in the product, used for this purpose dilution of hydrocarbons with hydrogen. In order to passivate the active catalyst particles, hindering their coking and loss of activity, is also used ammonia, and to increase Released CNM added CO.

A critical component of pyrolytic synthesis method is a CNM nature of the catalytic the system. It should consider not only the composition but also the way it is preparation and application to the substrate. The terms used to obtain CNM pyrolysis catalysts for hydrocarbon is quite extensive. In general used metal 3d-group - iron, nickel, cobalt and their binary mixtures and alloys with other metals: Co / Fe, Fe / Mo, Co / Mo, Fe / Cu.

Using the binary compounds can lead to increased effectiveness of the growth process CNM. For example, some researchers in their work receive high-quality multi- nanotubes by catalytic decomposition of C 2 H 2 on particles of Co + Mo, deposited on Y-zeolites. Good results were obtained using the catalyst Fe / Mo and methane pyrolysis temperature of 680 ° C.

For effective growth of nanotubes necessary to the active centers catalyst mass were small size. The use of fine powders micrometer size, attainable by mechanical dispersion, it is ineffective. In Dumb papers used to the size of Ni powder 3 micron particles in the pyrolysis of benzene and the temperature to 900 ° C. It was received a number of Munmu number of layers and a diameter of up to 65 <100 nm. However, the sintering of Ni particles was observed and, as consequence, the low yield (gC / gkt) of the desired product. Therefore, the synthesis of catalysts used various media, applying the methods of coprecipitation, impregnation, application of slurry to the substrate, thermal decomposition, etc. used as carriers of non-volatile oxides and hydroxides of metals (Mg, Ca, Al, La, Ti, Y, Zr), zeolites, selikogeli, porous Si, aluminogel, etc.

The role of the media - to prevent the sintering of metal particles catalyst to ensure their uniform distribution in the catalyst mass, the promoting effect on pyrolysis.

The choice of carrier depends on several factors, chief among them - The level of difficulty removing the carrier from the CNM at the end of the synthesis process. In this sense, a very attractive magnesium oxide MgO, it is easy to remove from the product acid treatment. Of paramount importance are the nature and composition catalysts for the pyrolysis of hydrocarbons. They largely determine the temperature and pressure during the process, the nature of produced nanocarbon tubes.

So far, clearly uncertain factors determining the rate of destruction hydrocarbons and the growth of CNTs, which does not allow to obtain analytical expression of the kinetic equations. However, the most important growth factors are the following:

  1. character and nature of reactions occurring on the surface of the catalyst, and in the gas phase. However, only this can hardly explain the experimentally observed fact of influence length of the thin-film catalyst on the formation rate and the output SWCNTs. Thus, the on the catalyst Mo - Fe / Al 2 O 3 with the size of film 1 ? 1 cm CNTs from CH 4 not formed. However, the increase size up to 1 ? 15 cm leads to growth "forests" of CNTs.
  2. size of catalyst particles. The surface is energetically heterogeneous catalysts, but adsorption isotherms on them or the source of hydrocarbons, or intermediate products unknown. On the surface can occur, and secondary processes - Thickening of the nanotubes by deposition of amorphous carbon adsorption polyaromatic compounds or graphitized particles.
  3. temperature rise enhances the growth of deposits, but, naturally, did not inhibit the process only physical, and chemical adsorption. Among the published experimental data on the growth rate of CNTs are only the technological parameters and No kinetic. There are no data on the rate constants, the apparent activation energy, even of the most common reasons can not be to answer the question, What mode: kinetic, diffusion or mixed implemented the growth process of CNTs.

The kinetics of catalytic pyrolysis of hydrocarbons studied highly enough. Thus, the reaction order for CH 4 varies from 1.2 to 1.8, the average value of 1.5. It can not be explain the parallel increase in non-catalytic decomposition of methane increasing P (CH 4 ). The contribution of non-catalytic decomposition of methane at the same time in the overall rate process in both cases should be the same.

On the other hand, the obtained experimental results indicating that a formal order of pyrolytic formation of CNFs is function of temperature, increasing from 1.0 (600 ° C) to 1.3 (700 ° C). Obtained and enough strange results, indicating the independence of the rate of pyrolysis the formation of SWNTs from P (CH 4 ), ie on zero order reaction for methane. It is impossible to explain the control of process or the bulk of lateral diffusion in which the kinetic order should be is equal to 1. Most likely the limiting step is different multistep process that is not associated with carbon particles.

In the absence of kinetic regularities, in principle, be consider the stepwise mechanism of synthesis of CNTs. For him, even conjectural statements need to know the reaction order for all reactants in the constant temperature and constant activity in catalyst. Data are needed on the degree of filling the catalyst surface molecules of reactants or products destruction. Only in this case can, in first approximation, to formulate the composition of the intermediate complex.

It should be noted that it is necessary to explore and features of the lateral diffusion of particles adsorbed on the catalyst surface as a function of its nature, the temperature of the system and partial (not total in the system) the pressure of the reagent. Without detailed study of these processes is still too early to raise the issue of actual mechanism of sound process. Moreover, he himself, or at least, the nature of limiting step, of course, are a function of the nature of catalyst. So far, the literature says about the alleged Gross-process or, rather, some stages, though, perhaps limiting.

All the approaches they need to clarify statement special studies, sometimes very expensive. But without relevant data, and not on one but on a number of systems can not create a scientific basis synthesis of CNTs by catalytic pyrolysis. In such a case would be missing base and forecast any scientific work is doomed to the use of the successive approximation - the method is extremely costly and long-term.

As a basic method, implemented by us to obtain a CNM industrial scope, the methodology described in [ 10 ], and studies conducted in RCTU them. D. Periodic catalyst obtained by reduction in hydrogen atmosphere at 873 K precursor NiO / MgO, prepared by coprecipitation in acidic medium salts of Ni and Mg. The approximate equality of the ion radii of Mg 2 + and Ni 2 + promotes the fact that the NiO and MgO have good mutual solubility in the binary system NiO / MgO form solid solution of Ni x Mg a - X O. Because of this, Ni ions are distributed sparsely and uniformly over the volume of the lattice of MgO and the interaction precursor to the H 2 only a small portion of Ni reduced to metallic Ni, and full recovery of Ni and also prevents the stabilization of the valence crystal field of MgO. As a result, clusters of metallic Ni rare and uniformly distributed on the surface and have a small nositlya dimensions.

Once again, noting the crucial role played by the catalyst GFHO in the process, you must also state that the number of active metal in the catalyst mass may be a factor regulating the parameters obtained by CNM and, Specifically, their diameters. Studies in Chemical Engineering to them. D. Periodic studies on the Ni / MgO catalyst in the pyrolysis of CH 4 identified the following effect:

Ni / Mg 2:1 1:3 1:5 1:10 1:20
T, ° C 510 580 620 630 650
D MWNTs , nm 35 - 21 17 13

Conclusions

Synthesis of carbon nanomaterials and carbon nanotubes Specifically, is one of the most promising areas of nanoscience and is not only theoretical research but also practical interest. Development of optimal methods of synthesis leads to a simplification industrial production, increase the output of suitable products and reduction of material costs.

Master's thesis is devoted to actual scientific problem of the development thermal characteristics of the method for producing carbon nanotubes by catalytic pyrolysis of carbon-containing gases. In the trials carried out:

  1. Based on the analysis literature highlights the main parameters that can be used in the proposed method for the synthesis of nanotubes.
  2. A number of experiments to increase the intensity of heat and mass transfer, analyzed the results.

    3. Further studies focus on the following aspects:

    1. Quality improvement of the proposed method, its complement and expand.
    2. design allows the device to increase the intensity of the heat and mass transfer processes.

    In writing this essay Master's work is not yet complete. Final completion: December In 2013. The full text of the and materials on the topic can be obtained from the author or his head after that date.

    References

    1. Вісник придніпровської державної Академії будівництва та архітектури, №9, вересень 2009 (138) Приходько А. П., Сторчай Н. С. Нанотехнологии: состояние, направления и тенденции развития в производстве строительних материалов. с.12
    2. Мищенко С.В., Ткачев А.Г. Углеродные наноматериалы. Производство, свойства, применение. – М.: Машиностроение, 2008. – 320 с.
    3. http://ru.wikipedia.org/wiki/Углеродные_нанотрубки
    4. http://www.portalnano.ru/read/prop/pro/part2/c-nanotubes
    5. http://en.wikipedia.org/wiki/Carbon_nanotube
    6. http://en.wikipedia.org/wiki/Mechanical_properties_of_carbon_nanotubes
    7. Popov, M.; et al. (2002). "Superhard phase composed of single-wall carbon nanotubes". Physical Review B 65 (3): 033408.
    8. http://www.berkeley.edu/news/media/releases/2003/07/23_motor.shtml
    9. http://landrefs.land.ru/ref/20/4.html
    10. http://www.patentgenius.com/patent/7001586.html