ISSN 1068364X, Coke and Chemistry, 2012, Vol. 55, No. 2, pp. 76–81. © Allerton Press, Inc., 2012. Original Russian Text © O.I. Zelenskii, V.M. Shmal’ko, V.G. Udovitskii, A.Yu. Kropotov, 2012, published in Koks i Khimiya, 2012, No. 2, pp. 43–47.
CHEMISTRY
Production of Carbon Nanostructures by the Atomization of Solid Coking Products within an Electric Arc O. I. Zelenskiia, V. M. Shmal’koa, V. G. Udovitskiib, and A. Yu. Kropotovb a
Ukrainian State CoalChemistry Institute, Kharkov, Ukraine email:
[email protected],
[email protected] bScientific Center of Physical Technologies, Ukrainian Ministry of Education and Science and Ukrainian Academy of Sciences, Kharkov, Ukraine Received January 25, 2012
Abstract—The atomization of anodes made of solid coking products (coke, pyrocarbon, and pitch coke) within an electric arc is studied experimentally. The products of electricarc synthesis obtained in such con ditions are analogous to the nanostructures obtained from graphite electrodes. Keywords: carbon nanostructures, coke, pyrocarbon, electrodes, electrical arc. DOI: 10.3103/S1068364X1202010X
maintained below atmospheric pressure, in the range 6–20 kPa. A potential difference of ~25–30 V is main tained between the electrodes during arc combustion; the arc current is ~70–90 A. Arc combustion is accompanied by sublimation of the anode, with the formation of the following products: ⎯a cathode deposit (encrustation); ⎯soot that contains fullerenes (on the chamber walls); ⎯a light deposit (resembling cobwebs) in a few areas of the chamber. Carbon nanotubes are extracted from the cathode deposit and from all the condensation products of the
At present, various methods are used for the syn thesis of carbon nanostructures such as singlelayer and multilayer carbon nanotubes, nanofibers, and fullerenes. The most promising are electric arc dis charge, laser ablation, and catalytic pyrolysis of hydro carbons [1]. Research on these methods was reviewed in [2, 3]. Besides the extensive literature on the production and properties of fullerenes and carbon nanotubes, there has been experimental research with anodes for electricarc synthesis [4–7]. In such research, coal is the initial material for the production of C60 and C70 fullerenes. In the present work, we investigate the synthesis of carbon nanostructures in an electric arc by means of anodes made of solid coking products.
Table 1. Characteristics of solid coking products Elementary composition, %
EXPERIMENTAL METHOD To obtain carbon nanostructures in an electric arc, we employ the following samples: blastfurnace coke, pitch coke, and pyrocarbon deposits from within the coke furnace (Table 1). Anodes (diameter 10 mm, length up to 100 mm) are cut from these samples. A channel (diameter 4 mm, depth 90 mm) in the anode is filled with catalyst (car bonyliron powder). A graphite electrode (diameter 6 mm) is used as the cathode. The equipment for arc synthesis is shown in Fig. 1. The cathode and anode are positioned symmetri cally at a distance of ~1 mm within a sealed chamber, which is initially evacuated to a pressure of 2–5 Pa and the filled with inert gas (argon). The gas pressure is
Technical analysis, %
Sample Cdaf
76
Hdaf
d
Ndaf + Odaf
Ad
St
Vdaf
Blastfur 96.70 0.60 nace coke
1.60
12.1
0.95
1.7
Pitch coke
97.84 0.43
1.18
0.6
0.54
0.6
Pyrocar bon
97.4
0.8
3.1
1.46
0.9
0.3
PRODUCTION OF CARBON NANOSTRUCTURES BY THE ATOMIZATION
77
3
7
6 1 10 3
2
5
4 8
9
Fig. 1. Apparatus: (1) cathode; (2) anode; (3) viewing window; (4) tube for evacuation and inertgas introduction; (5) cooling water input; (6) water outflow; (7) vacuum chamber; (8) voltmeter; (9) power source for arc; (10) cathode deposit.
carbon vapor. The cathode deposit has a hierarchical structure and contains columns (diameter 50–250 nm) whose axes are oriented along the electrode axes. These columns contain multilayer nanotubes, most of which are not aligned with the electrode axes. It is found that the axes of the multilayer nanotubes are mainly per pendicular to the electrode axes [8]. The growth rate of the cathode deposit in the experiments is ~1 mm/min. The distance between the electrodes is maintained constant throughout the pro cess by continuous motion of the cathode toward the anode as it evaporates. RESULTS AND DISCUSSION In the atomization of anodes with no catalysts, two products are obtained: a cathode deposit containing carbon nanotubes and other structures; and fullerene soot. The carbon nanostructures may be extracted from the cathode deposit by ultrasonic dispersion and stud COKE AND CHEMISTRY
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ied on a PEM 125K transmission electron microscope (Fig. 2). The cathode deposits from all the samples contain three types of particles: carbon nanotubes; oval parti cles; and spherical particles (Fig. 2; Table 2). By means of a Jeol JSM 840 scanning electron microscope, we Table 2. Characteristics of carbon nanostructures from cathode deposits Type of nanoparticles Carbon nanotubes
Diameter d and length L of nanoparticles d = 40 nm L = 5 µm or less
Oval particles
d = 50–80 nm L = 90–200 nm
Spherical particles
d = 80 nm
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ZELENSKII et al.
(a)
(b)
Fig. 2. Carbon nanostructures from cathode deposits: (a) carbon nanotubes and oval particles, ×25000; (b) spherical particles, ×50000 (transmission electron microscope).
(a)
(b) Fig. 3. Fractal surface of cathode deposit (scanning electron microscope) (a) ×800; (b) ×1000.
find that the cathode deposits have the same fractal structure, covered by carbon nanofibers (Fig. 3). The fullerene soot is studied by means of a scanning electron microscope. The surface of the soot deposits has a fractal structure (Fig. 4). The anode made from pitch coke is studied by means of a catalyst: iron. In that case, besides the cath ode deposit and the fullerene soot, we see cobweblike deposits in the upper part of the chamber. Their microstructure is shown in Fig. 5. The carbon cobwebs are formed by multilayer car bon nanotubes, according to transmission electron microscopy (Fig. 6). Fig. 4. Microphotograph of fullerene soot (scanning elec tron microscope); ×5000.
On some of the images, we see iron particles from the catalyst on the carbon cobweb (Table 3; Fig. 7). COKE AND CHEMISTRY
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PRODUCTION OF CARBON NANOSTRUCTURES BY THE ATOMIZATION
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(a)
(b) Fig. 5. Carbon cobwebs. Optical microscope: (a) ×100; (b) ×500.
The carbon cobwebs consist of tangled carbon nanotubes (Figs. 5 and 7). These nanotubes (diam eter 12–30 nm) are relatively uniformly distributed throughout the sample. We see metal particles at intersections of the carbon nanotubes or even within them.
In energydispersion Xray analysis on a scanning electron microscope, we may establish the quantitative composition of the carbon cobweb (Table 3; Fig. 8).
Table 3. Quantitative composition of carbon cobweb
Table 4. Quantitative product yield in the arc method
Element
Content, wt %
Content, at. %
Carbon (C)
59.85
79.28
Oxygen (O)
12.33
12.26
Silicon (Si)
0.35
0.20
Sulfur (S)
2.04
1.01
Manganese (Mn)
0.75
0.22
Iron (Fe)
24.68
7.03
Total
100
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Table 4 presents the quantitative content of the products obtained for different anodes.
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Product yield, % Anode material
cathode deposit
fullerene soot
carbon cobweb
Graphite
63
37
–
Coke
58
42
–
Pyrocarbon
61
39
–
Pitch coke
65
35
–
Pitch coke with catalyst
72
–
28
80
ZELENSKII et al.
(a)
(b)
Fig. 6. Multilayer carbon nanotubes from carbon cobwebs (transmission electron microscope): (a) ×60000; (b) ×200000.
(a)
(b) Fig. 7. Carbon cobweb with iron particles (transmission electron microscope): (a) ×30000; (b) ×100000.
Spectrum 7
C
Fe Fe
Mn
Mn
S
O
Fe
Si 1
Mn
2
3
4
5
6
7
Fig. 8. Energydispersion spectrum of carbon cobweb. COKE AND CHEMISTRY
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PRODUCTION OF CARBON NANOSTRUCTURES BY THE ATOMIZATION
CONCLUSIONS The experimental results indicate that, instead of expensive graphite, the anodes used in the arc synthe sis of carbon nanostructures may be made of solid cok ing products—pyrocarbon, coke, and pitch coke. Such anodes reduce the cost of the final products— fullerenes and multilayer nanotubes—and expand the range of uses for cokeplant products.
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3. Rakov, E.G., Production of Carbon Nanotubes, Usp. Khim., 2000, vol. 69, no. 1, pp. 41–59. 4. Pang, L.S.K., Vasslalo, A.M., and Wilson, M.A., Fullerenes from Coal, Nature, 1991, no. 352, p. 480. 5. Yua, J., Lucas, J., Strezov, V., and Wall, T., Coal and Carbon Nanotube Production, Fuel, 2003, no. 82, pp. 2025–2032. 6. Pang, L.S.K and Wilson, M.A., Nanotubes from Coal, Energ. Fuels, 1993, no. 7(3), pp. 436–437.
REFERENCES 1. Kozlov, G.I., Formation of Carbon Cobwebs in the Synthesis of SingleWalled Nanotubes within a Jet of LaserAblation Products Expanding in an Electric Field, Pis’ma ZhTF, 2003, vol. 29, no. 18, pp. 88–94. 2. Eletskii, A.V., Carbon Nanotubes and Their Emissive Properties, Usp. Fiz. Nauk, 2002, vol. 172, no. 4, pp. 401–438.
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7. Pang, L.S.K., Vasslalo, A.M., and Wilson, M.A., Fullerenes from Coal: A SelfConsistent Preparation and Purification Process, Energ. Fuels, 1992, no. 6(2), pp. 176–179. 8. Rakov, E.G., Nanotrubki i fullereny: uch. posobie (Nano tubes and Fullerenes: A Textbook), Moscow: Univer sitetskaya Kniga, Logos, 2006.
