Polyacrylonitrile (PAN) solutions were deposited on quartz plates by spin coating to yield 2–3 µm thick PAN films. The films were decomposed at 1000°C in N2 atmosphere into electrically conducting carbonaceous coatings. When the precursor solution contained cobalt (0.2 g Co-acetate per 1 g PAN) and/or multi-wall carbon nanotubes (MWCNTs, 2 mg MWCNT per 1 g PAN) the specific electrical resistance of the product film dropped from the original 492 Ω·cm-1 value down to 46 Ω·cm-1. By excluding all other possibilities we came to the conclusion that the beneficial effect of carbon nanotubes is related to their catalytic action in the final graphitization of condensed nitrogen-containing rings into graphitic nanocrystallites

András Erdőhelyi

Imre Kiricsi

István Sarusi

Mária Darányi

Tamás Csesznok

Zoltán Kónya

Ákos Kukovecz

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59Processing and Application of Ceramics  4 [2] (2010) 59–62Beneficial effect of multi-wall carbon nanotubes on the graphitization of polyacrylonitrile (PAN) coating#Mária Darányi1, Tamás Csesznok1, István Sarusi2, Ákos Kukovecz1,*, Zoltán Kónya1, András Erdőhelyi2, Imre Kiricsi11Department of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1., H-6720 Szeged, Hungary2Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Béla tér 1., H-6720 Szeged, HungaryReceived 15 January 2010; received in revised form 21 May 2010; accepted 31 May 2010AbstractPolyacrylonitrile (PAN) solutions were deposited on quartz plates by spin coating to yield 2–3 µm thick PAN films. The films were decomposed at 1000°C in N2 atmosphere into electrically conducting carbonaceous coat-ings. When the precursor solution contained cobalt (0.2 g Co-acetate per 1 g PAN) and/or multi-wall carbon nanotubes (MWCNTs, 2 mg MWCNT per 1 g PAN) the specific electrical resistance of the product film dropped from the original 492 Ω·cm-1 value down to 46 Ω·cm-1. By excluding all other possibilities we came to the con-clusion that the beneficial effect of carbon nanotubes is related to their catalytic action in the final graphitiza-tion of condensed nitrogen-containing rings into graphitic nanocrystallites.Keywords: PAN, carbon film, carbon nanotube, electrical conductivityI. IntroductionThin carbonaceous coatings have been studied in-tensively recently because they find important practi-cal applications in two fields: (i) hard and tough dia-mond-like carbon tool coatings [1] and (ii) electrically conductive coatings applicable in e.g. electromagnetic shielding, or flat panel display technology [2]. Conduc-tive carbonaceous coatings can be prepared by the high temperature carbonization of polymer precursor films like polyimide and its derivatives [3], phenol formalde-hyde [4], poly(furfuryl alcohol) [5], poly(vinyl alcohol) [6] and polyacrylo  nitrile (PAN) and its derivates [7,8]. The  challenge  in  converting  polymer  layers  into conductive  coatings  is  to  maximize  graphitization without damaging the integrity of the film. It has been shown  that  transition  metals  improve  the  graphitiza-tion of polymer coatings in general [9] and that the final structural order in decomposed PAN films is particular-ly sensitive to the use of a suitable metal catalyst [10]. We demonstrated earlier that the electrical conductivity of a PAN film annealed at 1000°C in N2 flow is propor-tional to the cobalt contents of the precursor PAN so-lution [11]. However, the viscosity of the polymer so-lution can also increase with the metal concentration [12]. Since increasing viscosity affects the applicabili-ty of the solution in spin coating adversely, there exists an upper limit for the electrical conductivity improve-ment achievable by metal addition only. In this contri-bution we investigate if it is possible to push this limit any further by mixing a small amount of multi-wall car-bon nanotubes (MWCNTs) into the precursor.II. ExperimentalCommercial  polyacrylo  nitrile  fiber  (ZOLTEK)  and N,N-dimethyl-formamide  (DMF,  Scharlau  Chemie S.A.)  were  used  to  prepare  the  PAN  solutions.  1  g PAN was dissolved in 20 ml DMF and mixed with 0.2 g cobalt acetate (Reanal) and/or 2 mg multi-wall car-bon nanotubes using an ultrasonic bath. The MWCNTs were prepared in house by catalytic chemical vapor de-position as described earlier [13]. 25×25 mm quartz substrate plates were cleaned with acetone and distilled #Paper presented at 8th Students’ Meeting, SM-2009, Processing and Application of Ceramics, Novi Sad, Serbia, 2009 * Corresponding author: tel: +36 62 544 620, fax: +36 62 544 619, e-mail: kakos@chem.u-szeged.hu60M. Darányi et al. / Processing and Application of Ceramics 4 [2] (2010) 59-62water prior to spin coating them at 80 rps by PAN films to approx. 2–3 µm final thickness using a total amount of 30 droplets (approximately 50 mg) of the polymer solution. Spin-coating was followed by drying at 70°C for 120 minutes in air and then by heating to the final carbonization temperature (1000°C) in N2 flow at 4°C/min. The decomposition reaction was run for 2 hours then the products were allowed cool to room tempera-ture in N2 flow. The thickness of the carbon layers was measured by scanning electron microscopy (Hitachi S-4700 Type II cold field emission SEM instrument). The thermal de-composition was carried out in a Netzsch equipment, model STA 409, by heating from 30 to 1000°C in he-lium atmosphere at a heating rate of 10°C/min. The evolved gases were analysed by a Pfeiffer QMS 200 quadropole mass spectrometer. The conductivity of the samples was measured by a purpose-built two-probe apparatus utilizing a PicoScope ADC-216 oscilloscope with 16 bit A/D resolution and a Metex MS9150 pow-er supply.III. Results and discussionIn  Fig.  1  scanning  electron  microscopic  imag-es of a fresh and a decomposed PAN film are present-ed. Both coatings appear homogeneous and crack-free. The thickness of the coating decreased from ~2.7 µm to ~0.97 µm during the thermal treatment. These values are typical in our experiments as film shrinkage is the combined result of (i) completed drying, (ii) material loss in the chemical reactions (cyclization, dehydroge-nation, aromatization) taking place during heating and (iii) structural rearrangement within the remaining car-bonaceous layer.Let us now consider Fig. 2 in order to understand the chemistry of PAN decomposition in the presence of a transition metal catalyst [14]. The first reaction is cobalt catalyzed homopolymer PAN cyclization com-mencing in our system at ~260°C. The lone pair elec-trons of the C≡N nitrogen coordinate to Co2+ and conse-quently, the bond energy of the triple bonds in the PAN nitrile groups decreases and therefore, the cyclization reaction takes place at a lower temperature than that of the PAN without metal compounds (~285°C [15]). In the second reaction phase (300-800°C) the decomposi-tion of the cyclic intermedier takes place accompanied by the evolution of HCN, NH3 and at higher tempera-tures, H2. Decomposition is concluded at high tempera-ture (above 800°C) in the final aromatization step when N2 is released and the structure relaxes into graphit-ic domains. This mechanism was confirmed earlier by Xue et al. [16] and also agrees well with our own mass spectrometry data collected in situ during the controlled heating of PAN films (Fig. 3). It can be seen in Table 1. that without any additives the final product of the 1000°C PAN conversion reac-Figure 2. Suggested thermal degradationscheme of PANFigure 1. SEM images of PAN films before heat treatment (top) and after carbonization at 1000°C for 2 hours in N2 atmosphere (bottom)61M. Darányi et al. / Processing and Application of Ceramics 4 [2] (2010) 59-62tion exhibits a specific resistance of 492 Ω cm-1. If a suitable catalyst metal is available as discussed above, the specific resistance can drop to 86 Ω cm-1. Our earli-er experiments have shown that no additional conduc-tivity improvement can be achieved by increasing the cobalt concentration above 0.2 g Co-acetate / 1 g PAN [11]. Moreover, the overall quality of the coating ac-tually deteriorates upon further cobalt addition due to the increasing viscosity of the precursor solution. On the other hand, adding a trace amount of multi-wall car-bon nanotubes (0.2 mg MWCNT / 1 g PAN) to the Co-containing precursor solution reduces the specific re-sistance to 46 Ω·cm-1, a 47 % improvement over the cobalt-only sample.It is interesting to contemplate on the role of MW-CNTs in the graphitization process of the PAN film. For example, nanotubes could form continuous conduction channels in the polymer matrix. Since pure MWCNTs are among the best known electrical conductors, a nan-otube chain would certainly decrease the resistance of the film. However, the fact that in the absence of Co2+ ions the nanotube induced resistance drop is less than 5 % quickly proves this hypothesis wrong. It should also be noted that the applied 0.2 mg MWCNT / PAN load-ing is well below the typical percolation threshold of MWCNTs in polymers [17,18]. Another possibility is that the nanotubes themselves could decompose into graphite nanocrystallites distributed evenly in the car-bonized coating and thus improving its electrical con-ductivity. Arguments against this explanation are (i) the absence of the beneficial effect in the cobalt-free sam-ple and (ii) the well-known thermal stability of MW-CNTs under our experimental conditions. Carbon nan-otubes were reported to withstand temperatures up to 3000°C without structural damage in inert atmosphere. In fact, such elevated temperatures are beneficial for the spontaneous healing of nanotube wall defects [19].A closer examination of the temperature programmed mass spectrometric data presented in Fig. 3 reveals that the onset of the HCN and H2 evolution processes is un-affected by the presence of nanotubes. These products both originate from Co2+ catalyzed cyclization and de-hydrogenation reactions. N2 formation at high temper-ature is due to the final graphitization step. Apparently, this process takes place at a significantly lower temper-ature (825°C vs. 859°C) and thus, must have lower ac-tivation energy when carbon nanotubes are also pres-ent in the matrix. We suggest that the perfect graphitic structure of the MWCNT walls may serve as a “tem-plate” for the graphitization of the N-containing con-densed rings via stacking interactions. Although fur-ther work is certainly required to rigorously prove this hypothesis, our experiments have clearly demonstrated that there exists a synergetic catalytic effect between Co2+ ions and MWCNTs in the thermal conversion of PAN films into electrically conductive carbonaceous coatings.IV. ConclusionsIn this paper we investigated the effect of multi-wall carbon nanotubes (MWCNTs) on the thermal decompo-sition of polyacrylonitrile (PAN) films into conductive carbonaceous films of submicron thickness. The addi-tion of MWCNTs decreased the specific electrical resis-tance of the film by 47 % compared to a reference film containing the same amount of Co2+ catalyst but no nan-otubes. Since the MWCNTs are thermally stable, their loading was below the percolation threshold and low Table 1. Specific electrical resistance (Ω cm-1) of PAN films carbonized at 1000°C for 2 hours in N2 atmosphereSpecific electrical resistance [Ω cm-1]MWCNT (mg) / PAN (g)Co-acetate (g) / PAN (g)0 0.20 492 862 470 46Figure 3. Evolution of HCN, H2 and N2 during the thermal decomposition of PAN in the absence (top) and presence (bottom) of multi-wall carbon nanotubes62M. Darányi et al. / Processing and Application of Ceramics 4 [2] (2010) 59-62temperature chemical reactions were unaffected by the presence of the nanotubes, we suggest that MWCNTs may catalyze the final graphitization step of PAN deg-radation. Acknowledgment: The financial support of the Hun-garian Scientific Research Fund (OTKA) through proj-ects NNF-78920 and 73676 is acknowledged.ReferencesJ.  Robertson,  “Diamond-like  amorphous  carbon”,  1. Mater. Sci. Eng. R-Rep., 37 (2002) 129–281.S.S. Azim, A. Satheesh, K.K. Ramu, S. Ramu, G. Ven- 2. katachari, “Studies on graphite based conductive paint coatings”, Prog. Org. 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Processing and Application of Ceramics

Beneficial effect of multi-wall carbon nanotubes on the graphitization of polyacrylonitrile (PAN) coating

Beneficial effect of multi-wall carbon nanotubes on the graphitization of polyacrylonitrile (PAN) coating

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