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Materials Chemistry and Physics 125 (2011) 161–167

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ffect of heat treatment on morphology and thermal decomposition kinetics ofultiwalled carbon nanotubes

oumya Sarkara, Probal Kr. Dasa,∗, Sandip Bysakhb

Non-oxide Ceramics and Composites Division, Central Glass and Ceramic Research Institute (CSIR), Kolkata 700032, IndiaAnalytical Facility Division, Central Glass and Ceramic Research Institute (CSIR), Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 15 February 2010eceived in revised form 2 August 2010ccepted 30 August 2010

a b s t r a c t

High temperature treatment in inert atmosphere proved to be an effective way to improve high tempera-ture stability of MWNTs in ambient condition. TEM analysis of heat-treated MWNTs confirmed successfulremoval of impurities and formation of ordered graphene layers and internal bamboo structure. TG–DTGcurves indicated that decomposition range and rate of as-received MWNTs were narrow and notably

eywords:eat treatmentlectron microscopyGA

higher, respectively, than heat-treated MWNTs mainly due to presence of impurities like metal nanopar-ticles in the former. Non-isothermal kinetic analysis revealed that the rate determining mechanism foras-received MWNTs was random nucleation and growth of active species. However, for heat-treatedMWNTs, rate controlling mechanism was chemical reaction. Higher activation energies (∼203 kJ mol−1

and 280 kJ mol−1) and reaction orders (3 and 4) of MWNTs heat-treated at 1200 ◦C and 1800 ◦C in inert,layed −1

here.

respectively, indicated deeven in oxidative atmosp

. Introduction

Since the discovery of carbon nanotubes (CNTs) by Iijima in991 [1] as multiwalled and in 1993 by Iijima and Ichihashi asinglewalled [2], this excellent allotrope of carbon with outstand-ng combination of essential properties, have gained the highestesearch interest in the field of nanomaterials [3–7]. Not onlys single component [8–10] but also as reinforcing phase in dif-erent matrix materials like polymers [11,12], metals [13,14] oreramics [15–20], CNTs have shown their outstanding capabili-ies to create engineering components with much better propertieshan achieved in conventional products. However, for structural

aterials and thermal management devices, components haveo withstand various temperatures and thermal cycling in differ-nt environments [21–23]. Further, since processing of ceramicatrix nanocomposites require high consolidation temperatures

>1400 ◦C), stability of CNTs at that temperatures in various atmo-pheres should be considered [15–20]. Thus, it is most important toave fair knowledge on thermal behavior and decomposition kinet-

cs of CNTs for their processing and real life applications. Further,

t is equally important to realize the effect of structural featuresf carbon nanotubes on their thermal stability. Thermogravimetricnalysis (TGA) is no doubt an efficient method to study decom-osition kinetics of materials up to sufficiently high temperatures

∗ Corresponding author. Tel.: +91 33 2473 3469/76/77/96; fax: +91 33 2473 0957.E-mail address: probal@cgcri.res.in (P.Kr. Das).

254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2010.08.088

thermal decomposition than as-received MWNTs (Ea ≈ 178 kJ mol ; n = 1)

© 2010 Elsevier B.V. All rights reserved.

in different atmospheres. However, limited reports are availablein literatures that dealt with decomposition kinetics and relatedparameters of CNTs [24–27]. Illeková and Csomorová [24] studieddecomposition kinetics of commercial singlewalled carbon nan-otubes (SWNTs) containing iron (Fe) catalyst particles. Activationenergy of the SWNTs was ∼120 kJ mol−1. Brukh and Mitra [25]reported activation energies of commercial SWNTs in the rangeof 120–140 kJ mol−1. For multiwalled carbon nanotubes (MWNTs),Brukh and Mitra [25] and Vignes et al. [26] reported activationenergy ∼290 kJ mol−1 and ∼150 kJ mol−1, respectively, under non-isothermal condition. While Brukh and Mitra [25] used MWNTsthat contained <2.5 wt.% catalyst particles like cobalt (Co), ironor nickel (Ni); nanotubes used by Vignes et al. [26] contained∼7 wt.% Fe-catalyst particles. Recently, Sarkar et al. [27] reportedactivation energy of thermal decomposition of commercial MWNTsclose to 180 kJ mol−1. Thus, it can be realized that within existingresults, significant variations have been noticed in kinetic featuresof CNTs. These variations may be governed by factors like numberof graphene layers in nanotubes, CNT purity level, nature of catalystparticles present and their amount and methods utilized to eval-uate kinetic parameters. In present work, thermal decompositionkinetics of different MWNTs was studied using TGA up to 1200 ◦C innormal laboratory condition. Decomposition kinetics of as-received

and heat-treated MWNTs were studied under similar experimen-tal conditions. Heat treatment of as-received nanotubes was doneat 1200 ◦C and 1800 ◦C in inert atmosphere. The purpose was tocompare the nature of degradation of as-received and heat-treatednanotubes because it has been reported that high temperature

1 istry a

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bf

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wpt

wrad

k

w

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ol

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e

l

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62 S. Sarkar et al. / Materials Chem

reatment can produce defect and catalyst free, more ordered CNTshich will be able to offer superior performance than that offered

n their as-synthesized form [28–30]. Kinetic parameters of decom-osition were evaluated using three single-heating rate differentialechniques namely Friedman, Freeman–Carroll (FC) and Chang andne integral method i.e. Coats–Redfern (CR) technique. From thenalysis, the best suitable rate limiting mechanism and kineticarameters like overall activation energy (Ea), reaction order (n)nd pre-exponential factor (A) of thermal degradation of threeypes of MWNTs were evaluated and correlated with corresponding

orphology of MWNTs analyzed using electron microscope.

. Theoretical description of thermal decompositioninetics

Kinetic parameters of thermal decomposition of a material cane evaluated using a feature called degree of conversion or conversion

actor i.e. ˛ which can be expressed as:

= mi − mt

mi − mf(1)

here mi, mt and mf are initial, instantaneous and final mass of sam-le, respectively. Now, for non-isothermal decomposition process,he kinetic equation may be expressed as:

d˛

dt= ˇ

d˛

dT= k(T)f (˛) (2)

here d˛/dt = decomposition rate; ˇ = dT/dt = constant heatingate; k(T) = rate constant; T is absolute temperature in Kelvinnd f(˛) = conversion function. Temperature dependence of k(T) isescribed by the well-known Arrhenius expression:

(T) = A exp(

− Ea

RT

)(3)

here R is the universal gas constant (8.314 J mol−1 K).From Eqs. (2) and (3) one can obtain the differential form of

on-isothermal rate law:

d˛

dt= ˇ

d˛

dT= Af (˛) exp

(− Ea

RT

)(4)

Now to calculate kinetic features of thermal decomposition fromsingle-heating rate process, Friedman [31,32] proposed the fol-

owing expression derived from Eq. (4) using generalized nth ordereaction where f(˛) = (1 − ˛)n:

nd˛

dt= ln A + n ln(1 − ˛) − Ea

RT(5)

Hence, from linear plots of ln(d˛/dt) and ln(1 − ˛) against 1/Tne can calculate Ea and n, respectively. Further, from intercept ofn(d˛/dt) vs. 1/T plot, ‘A’ can be evaluated.

Another widely used single-heating rate method is thereeman–Carroll technique which uses the following expression toalculate kinetic parameters of a material [31,32]:

� ln(d˛/dt)� ln(1 − ˛)

= n − Ea

R.

(�(1/T)

� ln(1 − ˛)

)(6)

The plot of L.H.S. of Eq. (6) against �(1/T)/� ln(1 − ˛) should beinear with slope and intercept equal to −(Ea/R) and n, respectively.urther, ‘A’ can be calculated using Eq. (5).

The next differential method is the Chang method which can bexpressed as [31,33]:{ }

n

(d˛/dt)(1 − ˛)n = ln(A) −

(Ea

RT

)(7)

Now if reaction order ‘n’ is correctly chosen, a plot of L.H.S. of Eq.7) against 1/T should be a straight line. The extent of linearity of the

nd Physics 125 (2011) 161–167

line can be checked by the help of linear regression coefficient (R).From the slope and intercept of the line Ea and A can be calculated.

All the above techniques utilize the basic differential formof non-isothermal rate law. The integral form of rate law can beobtained by integrating Eq. (4) with respect to ‘T’ i.e.:

∫0

d˛

f (˛)= g(˛) = A

ˇ

T∫0

exp(

− Ea

RT

)dT

[because, ˇ = dT

dt

](8)

where g(˛) is integral conversion function that depends on f(˛). TheR.H.S. of Eq. (8) is known as temperature integral and has no analyt-ical solution. It has many mathematical approximations [34–38].The R.H.S. of Eq. (8) may be approximated as follows:

T∫0

exp(

− Ea

RT

)dT = RT2

Eaexp

(− Ea

RT

)Q (x) (9)

where x = Ea/RT and Q(x) is a function. Therefore, from Eqs. (8) to(9) we can write:

g(˛) = A

ˇ.RT2

Eaexp

(− Ea

RT

)Q (x) (10)

Eq. (10) is the basic expression for many integral methods.Coats–Redfern expression [39] is one of the most commonly usedintegral techniques to evaluate kinetic parameters of thermaldecomposition of materials. Coats–Redfern estimated Q(x) of Eq.(10) as [39]:

Q (x) = x − 2x

= 1 − 2RT

Ea

(∵ x = Ea

RT

)(11)

Therefore, from Eqs. (10) to (11), one can obtain the followingCR expression:

ln(

g(˛)T2

)= ln

[(AR

ˇEa

).(

1 − 2RT

Ea

)]− Ea

RT(12)

However, as (1 − (2RT/Ea)) is normally close to unity because(2RT/Ea) � 1, therefore, the CR expression finally takes the followingform:

ln(

g(˛)T2

)= ln

(AR

ˇEa

)− Ea

RT(13)

Therefore, for a particular g(˛), the plot of ln(g(˛)/T2) vs. 1/Tshould be a straight line whose slope and intercept allow an assess-ment of Ea and A, respectively. Some common expressions of g(˛)are given in Table 1 [34,35,40] with associated rate limiting mech-anisms.

3. Experimental

3.1. Material used

Multiwalled carbon nanotubes used in this work was procured from Shen-zhen Nanotech Port Co., China. According to the supplier’s specifications, CNTs were≥95 wt.% pure with diameter from 60 nm to 100 nm and length from 5 �m to 15 �m.Metal catalyst particles and amorphous carbon (<3 wt.%) were present as impurities.The as-received nanotubes will be referred as ‘AC’ in rest of the article.

3.2. High temperature heat treatment

To study the effect of high temperature exposure in inert atmosphere on thestructure of MWNTs and associated changes in their thermal stability compared to

as-received one, MWNTs were heat-treated at 1200 ◦C and 1800 ◦C in Argon atmo-sphere in a graphite resistance heating furnace (Model: 1000-4560-FP20, ThermalTechnology Inc., USA). During the treatment, heating and cooling rates were kept at10 ◦C min−1 and Argon gas pressure was ∼0.05 MPa. Holding time at each peak tem-perature was 1 h. The nanotube samples heat-treated at 1200 ◦C and 1800 ◦C will bereferred as C-12 and C-18 specimens, respectively.

S. Sarkar et al. / Materials Chemistry and Physics 125 (2011) 161–167 163

Table 1Mathematical expressions of g(˛) and corresponding rate limiting mechanism.

No. Mechanism Name of function g(˛) Rate determining mechanism

1 A1, F1 Avrami-Erofeev eq. −ln(1 − ˛) Random nucleation and growth; n = 12 Fn nth order [(1 − ˛)(1−n)−1]/(n − 1) Chemical reaction; n = 2, 3, 4 and 53 An Avrami-Erofeev eq. [−ln(1 − ˛)]1/n Random nucleation and growth; n = 2, 3, 4 and 54 R1 Power law ˛ One-dimensional movement5 R2 Power law 1 − (1 − ˛)1/2 Contracting area6 R3 Power law 1 − (1 − ˛)1/3 Contracting surface

˛2 1-D diffusion˛ + (1 − ˛)ln(1 − ˛) 2-D diffusion1 − (2˛/3) − (1 − ˛)2/3 3-D diffusion[1 − (1 − ˛)1/3]2 3-D diffusion

3

1Gy1c

3

Gowwtnm

4

4

eFeXlaocHacrsA

7 D1 Parabola law8 D2 Valensi eq.9 DGB-3 Ginstling-Brounstein eq.10 DJ-3 Jander eq.

.3. Electron microscopic analysis

Fine scale morphology and nanostructure of all samples viz. AC, C-12 and C-8, were characterized by transmission electron microscopy (TEM) using a Tecnai230ST (FEI Company, USA) transmission electron microscope. To perform TEM anal-sis, MWNT samples were dispersed in isopropyl alcohol and ultrasonicated for5 min. TEM specimens were prepared by applying a drop of each suspension ontoommercially available holey carbon coated copper (Cu) grids (Ted Pella Inc., USA).

.4. Thermogravimetric analysis

TGA of AC, C-12 and C-18 samples were performed in Netzsch STA-409 (NETZSCH-erätebau GmbH, Germany) up to 1200 ◦C in ambient conditions at a heating rate (ˇ)f 10 ◦C min−1. In each experiment, amount of CNT taken was 4–5 mg. Higher amountas not possible due to excessive floppy structure of nanotubes. Finally, TGA dataere treated with four non-isothermal analytical models as mentioned in Section 1

o obtain the kinetic parameters of thermal decomposition and the changes in theature of thermal decomposition of the samples were correlated with correspondingorphologies.

. Results and discussion

.1. Microstructure of AC, C-12 and C-18

TEM image of AC sample (Fig. 1) shows presence of differ-nt sized nanotubes, which are essentially multiwalled in nature.ig. 2a shows high-resolution TEM (HRTEM) image of catalystntrapped end-cap of a CNT rope. Fig. 2b shows energy dispersive-ray (EDX) spectrum of that area suggesting the entrapped cata-

yst particle was nickel. The copper (Cu) peaks in EDX spectrumppeared due to TEM specimen grid used. During TEM analysisf AC sample, Ni-nanoparticles were not only observed at end-aps but were also detected within hollow channels of nanotubes.owever, after heat treatment of the tubes in Argon atmosphere

t 1200 ◦C for 1-h, existence of catalyst particles reduced signifi-antly (Fig. 3a). EDX spectrum of C-12 specimen (Fig. 3b) showsemarkable decrease in peak intensities associated with nickel andignificant increase in characteristic carbon peak as compared toC sample. For C-18 sample, Fig. 4(a and b) suggested that end-

Fig. 2. (a) HRTEM image of catalyst entrapped nano

Fig. 1. Bright field TEM image of as-received nanotubes showing size distribution.

caps and hollow channels of MWNTs were free from Ni-catalysts.However, in C-18 nanotubes, splitting of some graphene layers atend-caps were observed (Fig. 4a). From Fig. 4b, it can be visualizedthat hollow cores of C-18 specimen were enriched in extensive butirregular bamboo morphology. Formation of such internal bamboostructure during high temperature treatment of MWNTs contain-ing entrapped Ni-nanoparticles has also been reported by Wanget al. [30]. Further, development of a few nano-scale corrugations

on external surfaces of nanotubes like splitting and coiling of outergraphene layers were also noticed in HRTEM image of C-18 MWNTs(Fig. 4b). These were most possibly formed from the highly strainedregions or from the flaws already existed in nanotubes. EDX spec-trum of C-18 nanotubes (Fig. 4c) shows further increase in peak

tube end-cap; (b) EDX spectrum of that area.

164 S. Sarkar et al. / Materials Chemistry and Physics 125 (2011) 161–167

here

intmd

4

stsFt

Fw

Fig. 3. HRTEM image of MWNT heat-treated at 1200 ◦C in Argon atmosp

ntensity associated with carbon and decrease in peak related toickel compared to C-12 sample. All these observations suggestedhat high temperature heat treatment not only effectively removed

etal catalyst particles from the nanotubes but also increased theegree of ordered structure of graphene layers.

.2. TGA and DTG plots

TGA curves of three types of nanotubes i.e. AC, C-12 and C-18 are

hown in Fig. 5. From the curves, it can be noticed that during ini-ial course of TGA analysis, mass of all the three samples increasedlightly. Two possible explanations can be given for this behavior.irstly, this early weight gain (∼4.5 wt.% for C-18) might be dueo chemisorptions of oxygen atoms on to external surfaces of CNTs

ig. 4. HRTEM image of MWNT heat-treated at 1800 ◦C in Argon showing (a) catalyst free eith (b) extensive bamboo structure and external nano-scale corrugations, respectively;

showing (a) catalyst free channel; (b) EDX spectrum of C-12 nanotubes.

[24]. However, for as-received MWNTs this slight increase in weight(∼3.4 wt.%) can also be aroused due to partial oxidation of metal cat-alysts present in the sample [26]. It is evident from Fig. 5 that theonset of decomposition temperature (Ts ≈ 730 K) of AC sample waslower than observed in other two specimens (Table 2). Impuritiesviz. Ni-nanoparticles, thus, possibly acted as oxidation acceleratorsduring thermal decomposition of as-received MWNTs. It can alsobe visualized from Fig. 5 that the end point (Te) of decompositionprocess was substantially increased with increasing temperature of

pre-thermal treatment. Therefore, it can be safely stated that hightemperature treatment of MWNTs in inert atmosphere helped insuccessful removal of impurities which has been previously con-firmed by TEM analysis (Section 4.1) and eventually making themmore resistant towards thermal degradation in normal ambient

nd-cap having external splitted graphene layers at the tip and catalyst free channels(c) EDX spectrum of C-18 nanotubes.

S. Sarkar et al. / Materials Chemistry and Physics 125 (2011) 161–167 165

Fig. 5. TGA curves of three types of nanotube samples.

Table 2TGA and DTG data obtained for three nanotube samples.

Parameter AC C-12 C-18

Ts (K) 728 779 801

cg

soTptmoochnndar(tcl

Te (K) 977 1123 1177(d˛/dT)max (%/K) 0.9999 0.4936 0.5585Tp (K) 917 949 990%Wremaining at Tp 39.14 67.16 69.90

ondition. Ts and Te of decomposition process of all specimens areiven in Table 2.

Fig. 6 shows differential thermogravimetric (DTG) plots of threepecimens. DTG plots also suggested that the decomposition ratef AC sample was much higher than heat-treated specimens.his further supports the observations made from TGA plots thatresence of impurities had significant influence in acceleratinghe decomposition process of multiwalled carbon nanotubes. The

aximum decomposition rate i.e. (d˛/dT)max of as-received nan-tubes (0.9999%/K) was nearly two times higher compared to thosebtained for C-12 and C-18 specimens (Table 2). However, DTGurves also indicated that (d˛/dT)max of C-18 specimen was slightlyigher than that of C-12. This was possibly due to formation ofano-scale defects on external surface as well as splitting of exter-al graphene layers at sharp bends of C-18 nanotubes as observeduring TEM analysis (Section 4.1) and those defects might acteds easy source of carbon atoms that had ultimately increased the

ate of decomposition of the sample. However, in spite of higherd˛/dT)max, C-18 showed much improved resistance towards highemperature exposure compared to as-received one due to suc-essful removal of impurities and ordering of defect-free grapheneayers. DTG curves further suggested that for AC sample, the nature

Fig. 6. DTG curves of three types of nanotube samples.

Fig. 7. Generalized Friedman plots of three types of nanotube samples.

of changes in d˛/dT with temperature was different from the twoheat-treated nanotubes. In case of AC sample, decomposition rateincreased steadily up to (d˛/dT)max, followed by a sharp decrease.This was achieved by the formation of reactive species i.e. carbonatoms after ∼730 K by gaining activation energy of decomposition(Fig. 6) and reacted with environmental oxygen to produce reac-tion products. Up to (d˛/dT)max rate of formation of active specieswas higher than that of d˛/dT and thus, d˛/dT increased steadilyand at (d˛/dT)max rate of formation of active species and rateof formation of decomposition products was the same. However,after (d˛/dT)max formation rate of active sites started to decreasesharply thereby decreasing overall d˛/dT of the process. However,in case of heat-treated nanotubes, although the increasing nature ofd˛/dT with temperature was quite similar to that of AC sample, thedecreasing pattern was significantly different. After reaching the(d˛/dT)max, decomposition rate of heat-treated nanotubes was notreduced sharply up to a certain temperature, suggesting that rateof formation of active sites was not that much lowered to decreasethe overall decomposition rate very quickly up to that temperatureas observed in AC sample. Therefore, DTG curves also confirmedthat heat-treated MWNTs were thermally more stable comparedto as-received nanotubes.

4.3. Thermal decomposition kinetics

Friedman plots of three nanotubes samples are shown in Fig. 7.Most of the data points were found to be deviated from their cor-responding linear fit and consequently, the correlation coefficients(R2) were small (0.97665–0.9865). The nature of these plots sug-gested that this generalized technique was not the best suited forstudying decomposition kinetics of present CNTs. However, differ-ential FC technique showed far better linearity of data points forall three types of MWNTs (Fig. 8) and higher R2 (Table 4). Thus, FCmethod was suitably utilized to evaluate the oxidative decomposi-tion parameters of the nanotubes from slope and intercept of theircorresponding FC plots (Fig. 8). The third differential method wasthe Chang method where one can set various order of reaction toget the best linearity of data points. In present study, range of reac-tion order values were chosen based on ‘n’ values obtained from theFC method. For as-received nanotubes, the FC plot (Fig. 8) showed‘n’ value to be 1.05. Thus, for Chang method we have checked order

of reactions from 0.80 to 1.10. Among all those n values, the high-est R2 (0.9977) i.e. the best linearity of data points was obtained atn = 0.85. Therefore, the other kinetic parameters i.e. overall Ea and Avalues of AC sample were calculated from the Chang plot at n = 0.85(Fig. 9). Similar process was adopted for selecting appropriate ‘n’

166 S. Sarkar et al. / Materials Chemistry and Physics 125 (2011) 161–167

Fig. 8. Freeman–Carroll plots of three types of nanotube samples.

vos30aC

mC(fii

TSp

Fig. 9. Chang plots of three types of nanotube samples.

alues from the Chang technique for analyzing the decompositionf C-12 and C-18 specimens. Among all tested ‘n’ values, decompo-ition reaction order of C-12 and C-18 nanotubes were found to beand 3.3, respectively, with the highest R2 values of 0.99868 and

.99968, respectively, and thus, the kinetic parameters were evalu-ted from corresponding Chang plots at n = 3 (for C-12) and 3.3 (for-18) (Fig. 9).

For selecting the best feasible rate limiting mechanism of ther-al decomposition of these three types of nanotubes, integral

oats–Redfern method was utilized. 16 different g(˛) expressionsTable 1) were used to get resultant CR plots. R2 value of lineart versus rate limiting mechanism chart of as-received nanotubes

s shown in Table 3. From the table, it can be easily understood

able 3election of appropriate rate limiting mechanism from integral CR method. The bold fontoints (i.e. with highest R2 values) and rest of the kinetic parameters are calculated from

Mechanism AC C-12 C-18R2 R2 R2

A1F1 0.99912 0.97153 0.97393F2 0.97426 0.99335 0.99118F3 0.92379 0.99922 0.99856F4 0.87731 0.99622 0.99986F5 0.84235 0.98985 0.99804A2 0.99892 0.96199 0.96662A3 0.99866 0.94737 0.95601A4 0.99826 0.92391 0.94012

Fig. 10. Integral Coats–Redfern plots of three types of nanotube samples.

that for AC sample, A1F1 mechanism had the highest R2 (0.99912)i.e. the best linearity of data points among all mechanisms stud-ied. Thus, decomposition of AC sample should follow a first orderreaction with random nucleation of reactive species and their sub-sequent growth as rate deciding step. Related kinetic parametersof decomposition of as-received nanotubes were thus, evaluatedfrom corresponding CR plot using g(˛) = −ln(1 − ˛) (Fig. 10). Sim-ilarly, CR plots for all g(˛) expressions were plotted using TGAdata of C-12 and C-18 specimens. It was observed that for C-12and C-18 specimens, the highest R2 were obtained for F3 and F4mechanisms (Table 3), respectively, suggesting that rate decid-ing step was chemical reaction having reaction order of 3 and 4,respectively. Thus, kinetic parameters of thermal decomposition ofC-12 specimen were evaluated from corresponding CR plot usingg(˛) = 0.5[(1 − ˛)−2 − 1] (Fig. 10). For C-18 MWNTs, Ea and A ofdecomposition process were calculated from corresponding CR plotusing g(˛) = 0.333[(1 − ˛)−3 − 1] (Fig. 10).

Table 4 shows overall kinetic parameters of thermal decom-position of three types of MWNTs studied in this work. It can beseen from the table that kinetic features obtained from Friedmanmethod had quite lower values in each case compared to others.The lowest R2 obtained using this method also suggested the high-est possible errors in results. Results evaluated from rest threemethods were comparable. It can be noted that with increasing pre-heat treatment temperature in inert atmosphere, MWNTs showedincreased overall activation energy of decomposition suggestinghigher thermal resistance of heat-treated nanotubes. Ea of C-18MWNTs (Ea-CR = 280.77 kJ mol−1) was ∼100 kJ mol−1 higher thanthat obtained for AC sample (Ea-CR = 178.79 kJ mol−1). Further it canbe noted that although AC sample followed first order reaction, forC-12 and C-18 specimens order of reaction was 3 and 4, respectively.

Reaction order of as-received and heat-treated nanotubes stronglysupports the nature of DTG plots since a lower order reaction musthave higher reaction rate (d˛/dt) than that observed in reactionwith higher orders. This also suggested that heat-treated MWNTs

used to emphasize the fact that this mechanisms have the highest linearity of datathose corresponding equations only.

Mechanism AC C-12 C-18R2 R2 R2

A5 0.99770 0.88298 0.91476R1 0.97816 0.92914 0.94358R2 0.99349 0.95297 0.96054R3 0.99664 0.95977 0.96531D1 0.98055 0.93939 0.95078D2 0.93979 0.88702 0.91439DGB-3 0.99277 0.95695 0.96352DJ-3 0.99702 0.96503 0.96938

S. Sarkar et al. / Materials Chemistry and Physics 125 (2011) 161–167 167

Table 4Kinetic parameters of AC, C-12 and C-18 specimens obtained from the four methods.

Sample Method Ea (kJ mol−1) n A (s−1) R2 Rate limiting mechanism

AC FRa 131.37 1.07 6.93 × 104 0.98569 –FC 174.37 1.05 3.99 × 107 0.99864 –CHb 159.91 0.85 4.73 × 106 0.99770 –CR 178.79 1.00 5.83 × 107 0.99912 A1F1

C-12 FRa 177.97 1.14 1.14 × 107 0.97665 –FC 200.95 3.24 3.64 × 108 0.99984 –CHb 194.62 3.00 1.45 × 108 0.99868 –CR 203.22 3.00 4.06 × 108 0.99922 F3

C-18 FRa 216.73 2.12 6.09 × 108 0.98309 –FC 274.15 4.05 1.55 × 1012 0.99974 –CHb 236.70 3.30 1.16 × 1010 0.99968 –

httitwHaarscf

5

pkrifoFhapikica

A

tpc(tc

R

[[

[[

[

[

[[

[

[[

[[[

[[[

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[

[

[[[

[[

[[36] C.D. Doyle, J. Appl. Polym. Sci. 5 (1961) 285.[37] R.K. Agrawal, M.S. Sivasubramanian, AIChE J. 33 (1987) 1212.[38] J. Cai, F. Yao, W. Yi, F. He, AIChE J. 52 (2006) 1554.

CR 280.77 4.00

a Friedman method.b Chang method.

ad much delayed decomposition than as-received MWNTs. Fromhe table it is also clear that among all four methods studied CRechnique showed the highest linearity of data points suggestingts better applicability to study decomposition kinetics of CNTs dueo its inherent versatility and simplicity. In CR method, workingith only ‘˛’ data notably minimizes calculation induced errors.owever, in case of differential techniques, working with both ‘˛’nd d˛/dt needs huge calculation and truncation of derivatives afterfew decimal digits that produces calculation generated errors in

esults. Furthermore, differential Friedman method is the most sen-itive towards rate of heating particularly when heat of reactionhanges significantly with ‘ˇ’ that can also alter the results obtainedrom this technique [41].

. Conclusions

Presence of impurities like metal catalyst particles and amor-hous carbon showed significant effect on thermal decompositioninetics of CNTs in ambient condition. TEM and EDX analysisevealed that high temperature treatment of as-received nanotubesn inert atmosphere seemed to be an effective way in success-ul removal of such impurities from MWNTs with formation ofrdered graphene layer structure compared to as-received one.ormation of a few nanoscale corrugations and splitting of someighly strained external graphene layers of MWNTs heat-treatedt 1800 ◦C resulted in slight increase in rate of decomposition com-ared to nanotubes heat-treated at 1200 ◦C in inert atmosphere

n which such morphological changes were absent. TGA data andinetic analysis suggested that heat-treated MWNTs not only hadmproved thermal stability in ambient condition but also tracedompletely different decomposition mechanisms than observed ins-received nanotubes.

cknowledgements

The authors wish to express their gratitude to the Director, Cen-ral Glass and Ceramic Research Institute (CGCRI), India for his kindermission to publish this work. The first author of this paper sin-erely acknowledges the Council of Scientific and Industrial ResearchCSIR), India for financial support. The authors are also grateful tohe members of Central Characterization Unit of CGCRI, India forarrying out the TGA experiments.

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