Optimization of Electrical Parameters for Production of Carbon Nanotubes

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Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 961-964

Published Online October 2012 (http://www.SciRP.org/journal/jmmce)

Optimization of Electrical Parameters for Produc tion of

Carbon Nanotubes

Sayantan Chattopadhyay, Kalyan Kumar Singh

Department of Mechanical Engineering & Mining Machinery Engineering Indian School of Mines (ISM), Dhanbad, India

Email: [email protected]

Received July 20, 2012; revised August 22, 2012; accepted August 30, 2012

ABSTRACT

For more than two decades, there had been extensive research on the production of carbon nanotubes (CNT) and opti-

mization of its manufacture for the industrial applications. It is believed that they are the strong enough but most flexi-

ble materials known to mankind. They have potential to take part in new nanofabricated materials. It is known that,

carbon nanotubes could behave as the ultimate one-dimensional material with remarkable mechanical properties. More-

over, carbon nanotubes exhibit strong electrical and thermal conducting properties. In the process of optimizing the

production in line with the industrial application, the researchers have found a new material to act as an anode i.e. coal,

which is inexpensive as compared to graphite. There are various methods such as arc discharge, laser ablation, chemical

vapour deposition (CVD), template-directed synthesis and the use of the growth of CNTs in the presence of catalyst

particles. The production of carbon nanotubes in large quantities is possible with inexpensive coal as the starting carbon

source by the arc discharge technique. It is found that a large amount of carbon nanotubes of good quality can be ob-

tained in the cathode deposits in which carbon nanotubes are present in nest-like bundles. This paper primarily concen-

trates on the optimising such parameters related to the mass production of the product. It has been shown through Sim-

plex process that based on the cost of the SWNT obtained by the arc discharge technique, the voltage and the current

should lie in the range of 30 - 42 V and 49 - 66 A respectively. Any combination above the given values will lead to a

power consumption cost beyond the final product cost, in turn leading to infeasibility of the process.

Keywords: Coal; Carbon Nanotubes; SWCNT; Simplex; Optimization

1. Introduction

In 1985, Drexler proposed a molecular bearing consisting

of two graphitic nanotubes of different diameter, which

are concentrically arranged. It was a virtual operation

inside a computer. This dream, however, has become

more realistic by the discovery of carbon nanotubes.

There had been revolutionizing researches on the pro-

duction of Carbon Nanotubes from last twenty years and

optimising its manufacture for the industrial applications.

It has been thought that they are the strongest but most

agile materials known to humankind, and thus have po-

tential to take part in new nanofabricated materials as

additives. It has been shown that carbon nanotubes could

behave as the ultimate one-dimensional material with

remarkable mechanical properties. Carbon nanotubes ex-

hibit strong electrical and thermal conducting properties.

Study of the past researches on the production of carbon

nanotubes from coal revealed that most of the researches

concentrate on producing CNT by arcing electrodes,

produced separately by mixing the crushed coal with coal

tar followed by molding process. However, the only

process till date that has shown a positive adaptation to

direct coal application is thermal plasma jet technique.

With the extensive research in the production of the

Carbon Nanotubes, the requirement of optimising the

process parameters are realised. This paper concentrates

on the optimising such parameters related to the mass

production of the product. It has been found through cal-

culation that the only determining parameter in the arc

discharge technique is the power of the equipment, in

terms of voltage and current, and it is seen that on opti-

misation of these parameters, the cost of the process re-

duces drastically.

2. CNT Production through Arc Discharge

Technique

Carbon nanotubes (CNTs) have been used in various

fields of research due to their unique properties. There

are various methods such as arc discharge, laser ablation,

chemical vapour deposition (CVD), template-directed

synthesis and the use of the growth of CNTs in the pre-

sence of catalyst particles [1]. Special ambient gas is re-

quired for the fabrication of CNTs, in order to prevent

the oxidation of carbon at high temperature.

S. CHATTOPADHYAY, K. K. SINGH

962

The production of carbon nanotubes in large quantities

and other nanomaterials as by-products is possible with

inexpensive coal as the starting carbon source by the arc

discharge technique [2,3]. It has been found that a large

amount of carbon nanotubes of good quality can be ob-

tained in the cathode deposits in which carbon nanotubes

are present in nest-like bundles. In the past decades, vari-

ous alternate synthesis strategies and methods have been

explored and developed in the hope of mass-producing

cheap and high quality Single-walled carbon nanotubes

(SWNT). Compared to other methods, the arc discharge

is the simplest, cheapest and easy to implement and it has

been used because of its potential merits to make a mas-

sive production. The mineral matter in raw coals may

also play an important part in the formation process of

carbon nanotubes. An approach of production of Single-

walled carbon nanotubes (SWNT) had been adopted in

2004 [4] where the coal is crushed and sieved with 150

µm mesh and mixed with coal tar and molded to form

coal rods used for anode with a graphite cathode for the

arc discharge process. Similar approach had been made

by the same researchers for the production of Double-

walled carbon nanotubes (DWNTs) which were synthe-

sized from coal in large quantity by arc-discharge me-

thod in hydrogen-free atmosphere [5], and was system-

atically examined using scanning electron microscopy,

transmission electron microscopy, high-resolution trans-

mission electron microscopy and Raman spectroscopy. In

the case of coal-derived carbons, the breaking-up scheme

involved in the arc-discharge process significantly differs

from that of graphite because of the striking difference in

their textures. Coal or coal-derived carbons feature mac-

romolecular structures rather than the lattice structure of

graphite. In their chemical structures there exist many

weak binding linkages between carbon polymeric units

such as polymerized aryl structures. In the fast paralyses

process under arc plasma conditions, these weak linkages

would be broken up rather easily to release a variety of

reactive fragments of hydrocarbon molecules such as

alkynes and aromatic species. However, the production

of SWNTs has seen a typical use of an iron wire mesh

utilised between the electrodes and the carbonaceous

matter is found on the wire mesh.

A possibility of the production of carbon nanotubes

from heavy hydrocarbon resources had been proposed in

2008 [6]. Before the use of heavy hydrocarbons, pure

compound, toluene was used as the pure substrate to es-

tablish the reaction system for the production of carbon

nanotubes. Toluene was fed by a mist-spray feeding sys-

tem with a carrier gas as 9:1 mixture of nitrogen and hy-

drogen at 100 ml/min, and following the reaction at

750˚C catalysed by 9.8% (by weight) ferrocene, Carbon

nanotubes were found in the carbonaceous product de-

posited on inner wall of a quartz tube and at the exit of

the tube. The product was observed by scanning electron

microscopy and analyzed by temperature-programmed

oxidation experiments to identify the presence of carbon

nanotubes. Based on the reaction system and reaction

conditions with toluene, the production of nanotubes was

examined by using heavy hydrocarbons such as asphalt-

tene and maltene fractions from natural asphalt. Under

selected reaction conditions including the reaction tem-

perature and the amount of the catalyst, carbon nanotubes

with a diameter of 30 - 60 nm were found.

3. Optimisation of Voltage and Current Used

in the Carbon Nanotube Production

It is known that Voltage and Current constitute the Power

requirements for an electrically operated machine. Same

in the case of the production of CNT from the arc dis-

charge method, the electrical power signifies the charac-

teristics of the arc that is generated between the two elec-

trodes, in this case, one coal-based and the other, graph-

ite. For the optimisation process, the total cost of the input

materials must be lesser than that of the output product.

It is seen from the past experimental research works

[2,3,7-9], the work has been done on Bituminous and

An- thracite coal samples. From the domestic coal, cost

fixed by Coal India Ltd. the coal costs are as stated in

Table 1.

For Anthracite and Bituminous coal, we choose Grade

B and F respectively [10]. Therefore, the cost would be

Rs.3590 per tonne and Rs.1130 per tonne respectively, if

we take the maximum cost of the above grades from dif-

ferent companies, from the Table 1.

The coal-based electrode specifications for the arc dis-

charge process are found from the experimental works in

the past. With the aim of the maximum cost involved in

the production of the coal-based electrode, the electrode

with maximum volume is selected, as it is the electrode

that involves the maximum amount of coal and thus the

cost. Out of the several independent researches the speci-

fication of diameter 10 mm and length 200 mm is chosen

as it gives the maximum volume as compared to the

other electrodes in the other researches.

Therefore, electrode volume = 22

ππ

dl 

20015707 mm1.5710m



 2

Bank density of coal is 1346 kg/m3

So, Bank Weight of the powdered coal used = 1.57



101346 0.021



Maximum Cost of the coal is taken to be Rs.3.5/kg

Therefore, Cost of the coal used = 3.5 × 0.021 = 0.073

The cost of power is to be calculated next. In a typical

experiment [9], the current was taken to be 100 A and the

voltage was taken to be 36 V.

Therefore, Power = V × I = 30 × 100 = 3000 W = 3

S. CHATTOPADHYAY, K. K. SINGH 963

Table 1. Basic price of run of mine non-long-flame non-

coking coal [11].

Cost of grades of coal in rupees/tonne

Field/Co. A B C D E F G

ECL 3690 3590 1680 1350 1010 790560

ECL/

Mugma 3690 3590 1950 1610 1290 960620

ECL/Rajm

ahal - - - - 1330 1130910

BCCL 3690 3590 1630 1350 1080 860610

CCL 3690 3590 1590 1300 1030 820590

NCL 3690 3590 1430 1200 960 750560

SECL 3690 3590 1370 1140 950 740560

MCL 3690 3590 1370 1140 950 740560

(A—Graphite/high quality anthracite; B—Anthracite (C:H > 30); C—An-

thracite (C:H ~ 26 - 30); D—Semi-anthracite; E—Semi-bituminous; F—

Bituminous; G—Low quality bituminous).

Table 2. Carbon nanotube price list [12].

SWNT DWNT

Package

(gram) High purity*

($)

Arc CNT*

($) High purity* ($)

1 210 83 210

10 1600 700 1600

50 6850 3050 6850

100 12400 5500 12400

500 Call Call Call

1000 Call Call Call

*The High Purity CNT (more than 90% pure) is not achievable by Arc

Discharge Method, as per different researches, and thus the cost of the Arc

CNT, made from Arc Discharge Method, is always less than the former.

However, DWNTs are not the common by-product from the Arc Discharge

Method.

The rate of anode feed in the experiment is taken to be

10mm for 2 hrs. The length of the electrode in the ex-

periment is 75 mm. So, the total experiment time equals

15 hrs. For that time the 45 units of electricity is con-

sumed (1 KWhr = 1unit of consumed electricity).

As per the regulations of Calcutta Electricity Supply

Corporation (CESC), the rate is Rs.2.7/unit for the first

25 units consumed, then at the rate of Rs.3.3/unit for the

next 35 units.

Therefore, Total cost = 25 2.720 3.3



  Rs.133.5

So, the total input cost for the production of CNT is

Rs.133.5 (neglecting the cost of coal as too small as

compared to the power consumption cost)

Now, the cost of the Carbon Nanotubes as per stan-

dards are stated in Table 2.

The amount of CNT produced in experiment is about

20mg [9]. The cost above is given in $, so we consider

1$ = Rs 50 and the package unit is in grams.

Thus, the total cost comes to be 20 83 50

1000

 

83.

Therefore, it can be understood that as the input cost is

greater than the output cost, optimisation is essential in

this case.

Optimising the Power

Power = PVI

loglog logPVI





Taking, logP = x1; logV = x2; logI = x3

In all the experiments minimum voltage and current

are taken as 30 V and 50 A respectively.

30,50VI

log1.47, log1.69VI

1.47, 1.69xx

Therefore, 1232 3

1.47,1.6,9xxxx x



 

Considering the power consumption rate as 2.7/unit,

The total power can be calculated as 15 × 2.7 × P =

40.5 P

For optimum result,

40.5 83P

83 2.05 KW2050 W

40.5

P

log 3.31P

13.31x



Applying Simplex Algorithm,

Min 3





Subjected to the constraints,

3.31xx





21.47x

31.69x

Solving by graphical method, we get the following

plotted and shown in Figure 1.

We plot the graph by taking equalities for the above

constraints,

3.31xx





21.47x



31.69x



The arrows shown on the particular line denoted the

lesser-than-equal-to and greater-than-equal-to zone for

the particular constraint line. The area enclosed by the

three constraint lines gives the range of feasibility of the

variables.

Therefore, we can conclude that, the enclosed region

S. CHATTOPADHYAY, K. K. SINGH

964

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Figure 1. Solution graph of simplex optimisation process.

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with the three lines, fit the feasible solution range.

Max 1.62

Max 1.82



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The optimal range of the voltage and current can be

defined as (by antilog of the results obtained):

29.5 ~41.68V

48.97~66.07A



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4. Summary and Future Trends

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velopments in Formation of Nanotubes from Coal,” Fuel,

Vol. 81, No. 1, 2002, pp. 5-14.

doi:10.1016/S0016-2361(00)00192-7

An optimisation of the main process parameters i.e. vol-

tage and current, pertaining to the arc discharge process

discussed in the paper. The result focuses on the feasible

and optimum range of the parameters, based on the cost

of the SWNT obtained as the product. It can be certainly

perceived through calculation that if the voltage or the

current cross the optimal range, the power consumption

is increased and thus, the input cost is aggravated result-

ing in infeasibility of the production line.

[9] K. A. Williams, M. Tachibana, J. L. Allen, L. Grigorian,

S. C. Cheng, S. L. Fang, G. U. Sumanasekera, A. L.

Loper, J. H. Williams and P. C. Eklund, “Single-wall

Carbon Nanotubes from Coal,” Chemical Physics Letters,

Vol. 310, No. 1-2, 1999, pp. 31-37.

doi:10.1016/S0009-2614(99)00725-3

[10] M. S. Krishnan “Classification of Coal,” Geological Sur-

vey of India, Vol. 4, No.3, 1940, p. 552.

However, still more scopes remain, as the major as-

sumptions viz. the cost of power consumption is only

confined to the unit power consumption cost fixed by

CESC, can be made universal by programming the algo-

rithm mentioned above, and a detailed analysis of the

optimization by various other processes may lead to

many valuable information in this virgin field.

[11] “Domestic Coal Price Fixation,” Coal India Ltd., 2011.

[12] http://www.helixmaterial.com/Ordering.html