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Study of the bromination of pitch-based carbon fibres
MATERIALS CHEM~STR&W&dD ELSEVIER
Materials Chemistry and Physics 56 (1998) 14-20
Study of the bromination
of pitch-based carbon fibres
I.V. Klimenko ‘,* , T.S. Zhuravleva a, V.M. Geskin ‘, T. Jawhary b * lnstiture of Biochemical Physics of Russ_ian Academy of Sciences, Kmygin St. 4, Moscow 117334, Russia bServeis Cietttifico-Tecnics, Universitat de Barcelona, E-08028 Barcelona, Spain Received 25 June 1997; received in revised form 16 February 1998; accepted 2 March 1998
Abstract The changes in the structure and properties of pitch-based carbon fibres (CF) brominated for different periods of time (up to 144 h) and the state of bromine in these fibres have been studied. Electrical conductivity in the temperature range from 10 to 300 K, IR and Raman spectra, and thermogravimetric analysis data are presented. The structural modifications in CF upon bromination, as indicated by these data,
include the growth of the fine-crystalline phase, the degradation of the graphite structure, the emergenceand growth of an amorphous phase, and an increase in the number of graphite layers adjacent to bromine. If the bromination time does not exceed 96 h, the process leads to the intercalation of bromine, i.e., the formation of charge-transfer complexes between the carbon network and bromine. More prolonged bromination results in the formation of C-Br chemical bonds. 0 1998 Elsevier Science S.A. All rights reserved. Keywords: Pitch-based carbon fibres; Bromination;
Electiical resistivity; Thernlogravimetric
1. Introduction
Synthesis of fullerenes and nanotubes is presently causing a rebirth of interest in the transport properties of carbonmaterials, in particular, those of graphitized carbon fibres (CF). Graphitized CF are electrically conducting and possess a unique combination of mechanical and thermal properties [ 11. The electrical resistivity of CF should be reduced for their application in conducting composites. To this end, intercalating agents can be introduced. In particular, bromination of pitch-based CF reduces their resistance by three to five times [ 21. Fair temporal and thermal stability of the properties of brominated fibres, along with the relative simplicity of their preparation, make these fibres especially attractive for practical applications. The properties of brominated CF were recently studied in a number of works by various experimental methods (see, e.g., [2-g]) . Nevertheless, the understanding of the CF bromination mechanism at the microscopic level (microstructural modifications in CF upon bromination, the state of bromine in CF, etc.) is still incomplete. This work aims to clarify at least some of these issues. Materials of this type usually have strong intralayer bonds and weak interlayer bonds. Three main tools are often used for the study of the lattice structure: Raman, infrared and
* Corresponding author. Tel.: + 7-095-939-7 E-mail: [email protected] 0254.0584/98/$ - see front PIIs0254-0584(98)00118-7
matter 0
197; Fax + 7-095-137-4101;
analysis (TGA);
Raman spectroscopy
inelastic neutron scattering spectroscopy. All these methods were used for intercalated graphite compounds and for some types of fibres. The graphite model based on the early results from Ref. [ lo] is widely used. Therefore the results presented here will be discussed in the framework of these previous studies on graphite structures and carbon fibres. In this paper, we present a combined study of Russian commercial pitch-based CF brominated for O-144 h. We measured the electrical conductivity of the fibres in the temperature range lo-300 K, performed the thermogravimetric analysis (TGA), recorded IR and Raman spectra in the range 100-3200 cm- ’ using a microscope and analysed their band shape by graphical Lorentzian deconvolution. Micro-Raman spectroscopy appears most informative for structural studies of graphite and graphite-like materials including CF. According to Refs. [lo-121 and from the analysis of a number of criteria, namely, the position of the principal EZg2line in the first-order spectrum and its width at half maximum, the ratio of the band intensities, and the line shapes, it is possible to characterize the material on a molecular level and determine the degree of order in graphite layers, the type of defects, the state of additives, etc. The most complete bromination is achieved in well-graphitized CF [ lo]. Based on this observation, we used in this study pitch-based CF, whose structure is closer to that of graphite than the strncture of CF prepared from polyacrylonitrile or rayon.
1998 Elsevier Science S.A. All rights rcservcd.
I.V. Kfimenko et al. / Marerials
Cheinisrv
2. Experimental
Russian commercial pitch-based CF with a heat-treatment temperature of 3000°C were taken as the initial fibres. Bromination was carried out at room temperature. Single fibres (initial diameter u 10 pm) were placed into a small glass vesselfilled with gaseousbromine and kept therefor adefinite period of time from 2 to 144h. The brorninated CF were then held in a ventilation chamber for three days at room temperature to allow for the desorption of excessivebromine and, prior to electrical measurements,evacuated at 10M5torr for three days. Electrical measurementswere carried out according to the two-probe method in a ROK-300 (Leybold) cryostat using a R3009 d.c. bridge in the temperature range lo300 K. For these measurements,silver paste contacts were applied to a single fibre fixed on a glass ceramic substrate. Measurementson a bundle of fibres confirmed that the resistance of the contactswas much lower than that of a fibre. The results obtained on a bundle using the two- and four-probe methods closely coincided. The electrical measurements were carried out with an error of 2%. The main contribution to this error resulted from the electrical instability of the contacts and the thermal instability in the holder-substrate sample assembly. The diameter of the fibres was measured using a POLAM P-3 11polarization microscope.The error in diameter determination (15%) was the principal source of error in the specific eiectrical resistance. The weights of the initial and bromine-treated CF were determined with an m-4 ( Perkin-Elmer) autobalance.The thermal stability of the fibres was studied with a TGA-7 (Perk&Elmer) thermogravimetric analyser; the samples (4-6 mg) were heated at a rate of 10 K/min in an argon flush. Micro-Raman spectra of single fibres were recorded with a T64000 spectrometer(Jobin Yvon) equippedwith a double monochromator and a cooled receiver. A 458 nm Ar laser with a spot size less than 2 pm was used as a light source. The spectra were recorded at a laser power of 1.5 mW. The line shapesin the Raman spectrawere analysedby Lorentzian deconvolution becauseof their low spectral resolution. This decomposition gave better fits ofthe experimental bandsthan decomposition into Gaussian components. II? spectra of single fibres were recorded with a 1725-X FI’IR spectrophotometerequippedwith a diffusion accessory ( Perkin-Elmer) .
and Physics
56 (1998)
14-20
15
A weak band observed in the 1360-1400 cm-’ region can be assigned to the fine crystallite phase, as in graphite [ 11,121. lR spectra of initial fibres showed a line at about 868 cm-’ usually assigned to the lattice surface mode y0 (AL,) [131. The spectrum of a fibre exposedto bromine for 48 h (Fig. 2) is markedly different. The principal bands ( 1582and 2740 cm-‘) are broadened, the bands at 1360-1400 and 2480 cm- I becomestronger, and new bands appear at about 242, 500,700, 1600-2000 and 3000 cm- ‘. Raman spectra of graphitized materials are usually analysed according to suchcriteria asthe position ofthe principal bg2 line in the first-order spectrum and its width at half maximum, the ratio of the integral intensities of the bands 1360 to 1582, 1600 to 1582, 2480 to 2740, 3000 to 2740 cm-’ , and the line shapes[ lo-12,14,15]. Table 1 showsthat the position of the 1582and 2740 cm-’ lines is virtually independentof the bromination time. However, the position of the 1360 cm- ’ line is invariant only up to 96 h of exposure to bromine, while at longer bromination times this line shifts toward high frequencies. 6 t I
I 0
IOKI
m
33 3
cm-’
Fig. 1. Micro-Raman
spectrum of a pristine pitch-based carbon fibre.
21 0
2ow
loo0
z!mo
cm-’
Fig. 2. Micro-Raman carbon fibre.
spectrum of a bromine-intercalated
Table 1 The position of micro-Raman bromine for different times
(48 h) pitch-based
spectra lines of the carbon fibres exposed to
3. Results 3. I. Raman spectra
The micro-Raman spectrum of a pristine fibre (Fig. 1) comprises two strong bands, one at 1582 cm-’ (related to the active EZg2Raman mode), the other at 2740 cm- ’ with a shoulder at about 2480 cm-’ (corresponding to the 2730 cm- ’ band in the second-order spectrum of pure graphite).
0 2 24 48 72 96 120
1361 1361 1363 1363 1363.5 1380 1376
1582.4 1582.4 1581.8 1582.5 1582.3 1581.7 1581.8
2740 2745 2142 2742 2734 2143 2740
I.V. Klit~lenko et al. /Materials
Chemistv
Fig. 3. Raman spectra of pitch-based carbon ftbres in the region of the &,a principal line for bromination times of 48 h (a) and 120 h (b); experimental spectrum and its Lorentzian components (thin Lines).
and Physics 56 (1998) 14-20
Figs. 3 and 4 show examples of the Lorentzian deconvolution of the Raman spectra of the brominated CF. The overall fitting curve is so close to the experimental line that they are difficult to distinguish. Table 2 shows the average integral intensity ratios and the widths at half maximum based on this deconvolution for the first- and second-order spectra of the fibres brorninated for various periods of time. The integral intensity ratios for all the first-order bands increase with the bromination time, along with the width of the 1582 cm-’ band. The width of the 1360 cm-’ band, constant during the first 2 h of exposure to bromine, then sharply increases and remains unchanged afterwards. The average size of crystallites La (A) in the graphite network plane was calculated according to the empirical formula suggested for various graphitic carbons [ 111 and based on the experimental data of L, from X-ray research and from determination of S 1360/S1552from Raman spectra: (1)
La=44(~Mcl&82)-1
cm-'
Fig. 4. Low-frequency region of the Raman spectrum of a pitch-based carbon fibre brominated for 72 h; experimental spectrum and its Lorentzian components (thin lines).
t2.5
TIME
OF BROMINATION.
hrs
Fig. 5. Weight uptake P in lo-” g/cm (I), electrical resistivity pin 10m3 R cm (2) and crystallite size L, in A (3) of brominated CF vs. time of bromination, T= 300 K. Table 2 The Lorentzian components in the first- and second-order micro-Raman tBr, h
2 24 48 12 96 120
spectra of fibres exposed to bromine for different times
Fist order
Second order
SI3M)JSLIYZ 0
It is shown in Fig. 5. L, steeply decreases (by a factor of about 20) during the first 24 h of bromination and levels off at 16 A when bromination is continued. The second-order bands are wider than the first-order bands both in the pristine and brominated fibres (Table 2). The second-order spectrum is poorly resolved and the secondorder lines are thus less informative for the structural analysis. Nevertheless, the general run of the integral intensity dependence of the second-order lines follows that of the first-order lines (Table 2). New bands appear in the low-frequency region of the spectra of brominated fibres (Fig. 4)) namely, a very intense band at 242 cm- ’ with a shoulder at 160- 170 cm- ’ and less intense bands at about 500 and 750 cm- ‘. The ratio of the intensity of the low-frequency 160 cm- ’ component to the 242 cm- ’ band (equal to 0.4) and that of 500 cm- ’ to 242 cm- I (equal to 0.4 on the average) are practically independent of the bromination time. The bands at about 500 and 750 cm-’ shift slightly toward high frequencies as the bromination time increases. When the bromination time is between 48 and 72 h, the asymmetry of the low-frequency shoulder of the 242 cm- ’ band increases, and an additional band at 220 cm- ’ can be observed at room temperature. A similar multiplicity of the 242 cm-’ band was observed in the Raman spectra of
0.03 0.08 0.5
0.99 1.8 2.9 3.4
11% 3611
0.1 0.17 0.3 0.7 0.94
0.2
1.23
0.68
175 18.1 20.0 23;O 25.0 29.6 62.3
13.2 15.7 237 245 24s 240 242
&4,~&740 0.079 0.44 0.7 0.9 1.0 1.12 1.22
0.1 0.16 0.21 0.26 0.65 0.74
60
78
63 90 9.5 100
100
110 104
172 210 250 353 370
I.V. Klirnmko
et al. /Mnterials
Chemistry
bromine-intercalated graphite at low temperatures[ lo]. Note that while the bromination time did not exceed 120 h, the characteristic band of free BrZ at 323 cm- ’ was not observed.
and Physics
56 (1998)
14-20
17
p, 1O-40hm.crl-
o.10-40hm.cm
3.2. Dark conductivity
Fig. 5 also shows the dependenceof room-temperature electrical conductivity (p) and weight (P) of a fibre on bromination time. The weight increasesduring the first 30 h (curve 1, Fig. 5) and then remains practically constant up to 96 h. The maximum weight gain during this period was 22%, which is typical for pitch-based CF [ 5,6]. When the bromination time exceeded96 h, the weight increasedby 55% with respectto its initial value. The diameter of the fibre increased from 10 to 12 pm after 96 h of bromination, which was taken into account in the calculations of electrical resistivity. During the first 30 h of bromination the resistivity of the fibre decreasedby seventimes ( from 1.19to 0.17 rnfi cm). Thereafter the resistivity remained unchanged up to 96 h (curve 2, Fig. 5). However, after 96 h of bromination the resistivity of a fibre increased (up to 0.35 rn0 cm for 144 h). Fig. 6 demonstratesthe temperature dependence(lo-300 K) of the electrical resistivity for the fibres exposedto bromine vapours for various periods of time. The resistivity of all the samplesincreased asthe temperature decreasedin the range lo-270 K. This increasewas minor for the fibres brominated for 24-96 h, but it was substantial for those brominated for 120-144 h. The differences in the temperature dependencesof the resistivity for CF exposedto bromine for different times are more pronounced when their reducedresistivities, defined as the ratio of resistivities at a temperature T and 300 K, p( T>/ p( 300), are plotted (Fig. 7). The resistivity of the fibres brominated for 24-96 h has a local minimum at 270 K upon cooling. The dependenciesfor the fibres brominated for 24, 48, 72 and 96 h all passthrough a common point at 225 K. No local minimum is observed for the fibres with high bromination times ( 120 and 140 h), and the general aspect of the dependenceis somewhat different from that observedfor shorter bromination times (Fig. 7(b) ) . 3.3. Thermogrmimetric
arzalysis (TGA)
Thermogravimetric (TG) curves (Fig. 8) show a negligible weight loss below 150°Cfor all the fibres. In the range 150-250°C the weight loss was considerable, while at higher temperaturesthe weight loss was again less significant. The highest weight loss was observedfor a fibre exposedto bromine for 96 h, while above 120 h of treatment the fibres becamemore thermostable: up to 780°C the weight loss was 30% in the latter case (curve 6), while it attained 45% for the former (curve 5). 4. Discussion Fig. 5 shows that the evolution of the resistivity of the tibres as a function of bromination time correlates with the
I%-
l.s TEMPERATURE.
I
K
I
100 200 388 TEMPERATURE. K
Fig. 6. Temperature dependenciesof electrical resistivity for CF, brominated for: (a) 1,O h; (b) 2,24 h; 3,48 h; 4,72 h; 5, 96 h; 6, 120 h; 7, 144 h.
dT)/d3OO) T-
0.9g
/
100 2Q0 TEMPERATURE.
3 0 K
dT)/dJOO)
+73sT&d TETPERATLRE.
K
Fig. 7. Temperature dependencies of reduced resistivity for CF, brominated for: (a) I,0 h; 2,24 h; 3,48 h; 4,72 h; 5,96 h; (b) 6, 120 h; 7, 144 h.
55
35
;;;‘;‘;‘I, 1%
39% 4%
665
146
TEMPERATURE,C
Fig. 8. TGA curves for CF brominated for: 1, 2 h; 2, 24 h; 3, 48 h; 4, 72 h; 5,96 h; 6, 120 h; 7,144 h.
variation of their weight and the decreaseof their crystallite size. Furthermore, both changesin the character of the curves 1 and 2 take place at the samebromination times, namely, about 30 and 96 h. Both the resistivity and the weight of a fibre remain constant throughout the samebromination time range, between 30 and 96 h. This allows us to relate the evolution of the resistivity of a fibre to the changes in its composition and structure resulting from bromination. Three bromination periods arereadily discernedwith characteristic boundary times of 30 and 96 h. During the first period, with the bromination time up to 30 h, the resistivity undergoesa significant decrease(by seventimes) and finally attains 1.7 X 10m4R cm. This is similar to the value obtained for pitch-based P-55 and P-75 fibres but higher than the typical resistivities of other pitch-based fibres, P-100 and P-120, and also those of the fibres GM, GA and UNL obtained from the gasphase[ 2,8]. Nevertheless,the ratio pOIpBr= 7 (where p0and pBrare the specific electrical resistancesof the pristine and brominated fibres, respectively) obtained in this work is
18
I.V. Klirnenko et al. /Materials
Chemisty and Physics 56 (1998) 14-20
rather high and exceedsthe values characteristic for all the other fibres, with the UNL fiber being the only exception. The decreasein resistivity during the first bromination period must be due to the oxidation of the carbon material according to the reaction 2C+3Br2=2Br,-+2C+
(2)
This reaction yields two holes (missing electrons) on the graphite crystal&es (designated C in Eq. (2) ) adjacent to two bromine anions. Therefore, when the Br,- concentration is high enough, a quasi-metallic domain is formed within the carbon material, due to a sufficient density of holes in the Coulomb field of many anions, similarly to doping or photoexcitation processesin conducting polymers E161.It is the growth of the volume of this quasi-metallic phasethat leads to the decreasein resistivity of the brominated fibre during the first period. After 30 h of exposure to bromine, the metallization of a fibre slows down and levels off until 96 h of brornination (Fig. 5). The fibre absorbs no more bromine, the diffusion of which is apparently hindered by the oxidized surface (however, there are different opinions on this issue [ 171). Furthermore, prolonged bromination causes structural changesin the fibre. Bromine can migrate into the crystallites of the graphite lattice, thus disjoining its layers and deteriorating the bonding between them. As a result, the thermal stability of the fibres decreasestoward the end of the second bromination period (Fig. 5 and curves 3-5 in Fig. 8). After 96 h of exposureto bromine the fibre is again capable of bromine absorption,which canbe attributed to thepresence of structural defects. However, these defects cause the increase of resistance during the third bromination period, from 96 to 120h (Fig. 5, curve 2). Temperature dependenciesof the resistance of the fibres brominated for various times (Figs. 6 and 7) confirm the above schemeof structural modifications. The general view of these dependencies(in reduced coordinates) for the pristine and brominated fibres is different. There are two types of curves, the first for 24-96 h, and the other for 120-144 h of bromination. Similar temperature dependencieswithin each type can be related to similar structural modifications. The curves for the fibres treated for 30-96 h in Fig. 7 clearly indicate the increaseof the defect concentration with bromination time, though the room-temperature resistancefor this time range is invariant (Fig. 5). Crossing (at 225 K) of the reduced resistance curves (Fig. 7) of the fibres brominated for 24-96 h may be related to a phasetransition. A different character of the temperature dependencefor bromination times over 96 h reflects the presenceof new structural defects in the fibre; the resistancein such fibres increasesbecauseof accumulation of excessive bromine and the deterioration of the graphite structure. Raman spectra provide more insight into the structure of the fibres and its modification with bromination. The 15801595cm- ’ bands in the spectra of graphitized structuresare asstgnedto the E2g2active mode related to the C=C bond of
the graphite network [ lo], and the lower this frequency, the more orderedis the material’s structure [ IO]. The 1582cm- ’ band of the pristine fibre (Fig. 1) indicates a high degreeof order in the graphite structures of our fibre. Furthermore, pristine fibres show a very weak band at 1360-1400 cm;’ resulting from disordering in the graphite structure and related to the fine-crystalline structure of the fibre. Low integral intensity ratios of the bands 1360 to 1582 and 2480 to 2740 cm- ’ (Table 2) also characterizeour fibre asbeing one with a highly ordered graphite system. Eq. ( 1) provides the value of the average crystallite size in the graphite network for the pristine fibre as L, = 1470 A. The width of the 1582 and 1360cm- ’ bands also characterizesthe degreeof order in the graphite layers; this value for the principal E,,, line in our pristine fibres is close to that in highly pure fine-crystalline graphite [ 111. As the bromination proceeds, the position of the 1582 cm- ’ band remains practically invariant (Table 1)) which indicates the presenceof graphite structure in all brominated samples. The ratio of the integral intensities S1360/S1582 increases (Table 2) ; given the constant integral intensity of the 1582cm- ’ band up to 96 h of bromination, this indicates the growth of the fine-crystalline phase.As the bromination does not exceed 24 h, this ratio shows only a three-fold increase, while bromination for 24 or 48 h leads to a 16- or 30-fold increase,respectively. After 96 h the ratio S,360/S1s82grows by two orders of magnitude. This can be related to (i) a pronounced destruction of large crystallites and multiplication of the small ones (see also L, values in Fig. 5) and (ii) destruction of the graphite structure in general, leading to the appearanceand growth of the amorphousphase. When the bromination time is between 96 and 120 h, a new band appearsin the first-order spectrum at about 1530 cm-’ , which is also characteristic of amorphouscarbon [ 181; its integral intensity increaseswith brornination time (Table 2). The integral intensity of the 1530 cm- ’ band for a fibre brominated for 96-120 h increasesseventimes, also indicating the changes in the graphite structure of the fibre. The changesin the ratio Si36,,/S1582, abrupt increaseof the width of the 1360 cm-’ band at 24 h of bromination (Table 2)) and the emergenceof a new band at about 1530 cm- ’ after 96 h of bromination correlate well with our conductivity data, A certain discrepancy in the onset times of considerable changesin the graphite structure as determined by the two methodsmay result from their different nature. In fact, Raman spectroscopyprovides information on a 0.1 u,rnthick surface layer averaged on the laser beam area of 2 pm, while resistance is a bulk macroscopiccharacteristic of a fibre. All the brominated samplesshow a high-frequency unresolved 1600cm- ’ band as a symmetric shoulder of the principal line, which may result from the C=C vibration in the layer adjacentto bromine. Theselayers facilitate chargetransport in bromine-treated (for up to 100 h) fibres and their growth results in decreasedresistivity of the brominated fibres with respectto the pristine ones.The 1600cm-’ band
Z.V. Klimmko
et al. /Materials
Chemists
already appears after 2 h of bromination, and its intensity grows with bromination time. Apparently, the boundary graphite layers are formed almost immediately ( 1600 cm-’ band appears) but not extensively (cf. the 1600 and 1582 cm-’ intensity ratio in Table 1) . Only at 72 h of bromination is there a considerableincreasein the 1600cm-’ band intensity and its ratio to that of 1582 cm-’ indicates that the concentration of such layers grows significantly. The second-order lines evolve similarly to the first-order lines (Table 1). The dominating line in the second-order spectrum is at 2740 cm-‘, which can be represented as 2 X 1365cm- ’ = 2730 cm- ’ [ 19,201.A broad line at about 2480 cm-’ is related to the combination of the modes of high-density phonon stateswith different wave vectors (860 cm-‘+ 1620 cm-‘- - 2480 cm- ‘) [20]; a lattice surface mode AZuat 868 cm- ’ was recordedin the IR spectrum.The 3000 cm-’ band is observed only in brominated samples. This line is related to the graphite lattice disordering and results from the combination of modes about 1365 cm- ’ + 1620cm- ’ = 2985 cm- I [ 201. The ratio S2J80/S2710 is equal to 0.079 for the initial fibre (Table 1) and it increases upon bromination to 0.9, 1.Oand 1.22 after 48,72 and 120 h of treatment, respectively. The ratio Ss0001S27J0 is virtually invariant up to 72 h of bromination. Considerable changes occur at 96 h of bromination, thus confirming the deterioration of large gr$phite structures and the emergenceof smaller ones (L, = 16A, Fig. 5) asa result of prolonged bromination. The bands at about 242 cm- ’ in the spectraof brominated graphitized materials are assigned to the vibrations of bromine anions (Br,-, Br,-) [21-231. They are characteristic of bromine intercalated into graphite [ lo]. Similar lines in the spectrum of our fibre indicate the presenceof bromine anions, which could only be formed in a graphite structure due to an electron transfer from the graphite layers adjacent to bromine, as we have noted above. In some cases,the dependenciesof resistivity on bromination time (such as curve 2 in Fig. 5) may provide information on the intercalation stages.For example, the existence of four distinct stages in such a dependencepermitted the authors of Ref. [ 81 to deducefour intercalation stages.However, there is only one extendedplateau in the dependenceof resistivity on bromination time of our pitch-based fibres (Fig. 5)) as well as for those observedfor GA, P-75, P-100 and P120 brands of fibres. The concentration of bromine in the tibre can be calculated from the weight uptake upon bromination (curve 1 in Fig. 5). For bromination times of 40-96 h this concentration is 4.5 mol%, yielding an approximate formula CZ2Br2for the intercalated substance,which correspondsto the third stage of intercalation [ 15,171, In certain cases,the intercalation stagecan also be deduced from the shift of the Raman bands.For instance, a shift of the bands at about 1582 and 1600 cm-’ was observedresulting from the fibre state changes [ 241, For highly oriented pyrographite and carbon fibres obtained from the gas phase, the 1582 cm-’ band shift upon the transition from the third to
and Physics
56 (1998)
14-20
19
the fourth stage is about 1.5 cm- ’ [ 241. Unfortunately, in the conditions of our experiment, this shift would be within experimental error. Thermogravimetric curves (Fig. 8) show considerable weight loss between 150 and 250°C which can be related to the desorption of intercalated bromine. The maximum weight loss in this temperature range was 22% for a bromination time of 96 h, which is actually close to the weight changesin Fig. 5. For highly graphitized P-100 and P-120 fibres, a substantial weight loss takesplace between 100and 300°C [ 251, A temperature of 100°Cis usually consideredasthe point of in-plane melting of intercalated bromine [26]. The differences between the temperature range and kinetics of the weight loss observedin this work (Fig. 8) on the one hand, and the data [ 251 on the other hand must result from different structures of the fibres studied. More complicated weightloss kinetics in the 150-250°C range in our casemight be due to simultaneous thermal destruction of the carbon material along with the desorption of bromine. Analysis of the gaseous reaction products could provide additional information on this issue. Themlogravimetric curves in Fig. 8 can be grouped into two classes:the first group involves the fibres brominated for 24-96 h and the second,for 120-144 h. Within each group, the weight loss at lOO-200°C increases with bromination time. On the other hand, the decreasein the weight loss between the groups indicates improved thermal stability, in accordwith the results [ 251 obtained for other types of fibres. This decreasedweight loss observed for the fibres exposed to bromine for periods longer than 96 h may indicate that bonding of bromine to the graphite network is stronger than in the intercalated state, presumably due to C-Br chemical bonds. Formation of thesebonds would decreasethe amount of Br, - and Br, - ions along with the concentration of holes (charge carriers) in the carbonnetwork, in spite of the growth of the total bromine uptake by the sample (curve 1, Fig. 5). Decreased hole concentration may be the reason for the increase in resistivity as the bromination time exceeds96 h (curve 2, Fig. 5). The existence of the bands at about 500 and 750 cm-‘, which have been assigned to the valence C-Br vibrations in bromine-intercalated fullerenes [ 271, might be regarded as a corroboration of C-Br bonding. Nevertheless, an unambiguous interpretation of these bands is still lacking in the literature [ 151.
5. Conclusions We have studied brominated Russian commercial pitchbasedfibres (heat-treated at 3000°C) by micro-Raman spectroscopy, thermogravimetry and conductivity measurements. All the obtained experimental data can be accounted for by the structural changes within the fibre caused by the introduction of bromine. Bromination leads to intercalation and also to the formation of the C-Br chemical bond. If the bromination time does not exceed 96 h, the major part of the
I.V. Klimenko et al. /Materials
20
Chemistq and Physics 56 (1998) 1620
bromine is in the intercalated state, i.e., in the form of a charge-transfer complex. When the bromination time is longer than 96 h, C-Br chemical bonds are formed extensively in the fibre, leading to an increase in its thermal stability and also in its electrical resistance. We believe that the found properties of &hebrominated pitch-based fibres and the bromination process trends described in this paper could be used for optimization of the bromination process of these fibres as well as for other fibres and thus would be important for the development of non-metallic highiy conducting fibres.
Acknowledgements The authors are grateful for financial support from the Russian Foundation for Fundamental Research under the grants 95-03-08360 and 96-15-97492 and from the Intemational Science and Technology Center under the grant 015.
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Carbon Fibers and their Composires, Springer, Berlin, Stabe, N. Shaffer, Carbon Jaworske, Synth. Met. 12 Chung, Carbon 27 (1989) Chung, Carbon 28 (1990)
26 (19881 381. ( 1985) 525. 603. 831.
J.M. Chiou, C.T. Ho, D.D.L. Chung, Carbon 27 (1988) 227. [71 M. Endo, H. Yamanashi, G.L. Doll, MS. Dresselhaus, Appl. Phys. 64 (1988) 2995. [81 J.R. Gaier, Synth. Met. 22 (1987) 15. [9] I.V. Klimenko, A.N. Shchegolikhin, T.S. Zhuravleva, Synth. Met. 71 (1995) 1773. [lOI M.S. Dresselhaus, G. Dresselhaus, in: M. Cardona, G. Guntherodt (Ed%), Light Scattering in Solids III, Springer, Berlin, 1982. [Ill D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385. t121 R.J. Nemanich, S.A. Solin, Phys. Rev. B20 (1979) 392. r131 R.J. Nemanich, G. Lucovsky, S.A. Solin, Solid State Commun. 23 (1977) 117. [141 B.S. Elman, MS. Dresselhaus, G. Dresselhaus, E.W. Maby, H. Mazurek, Phys. Rev. B24 (1981) 1027. 1151 J.J. Song, D.D.L. Chung, P.C. Eklund, MS. Dresselhaus, Solid State Commun. 20 (1976) 1111. [I61 I.A. Misurkin, T.S. Zhuravleva, V.M. Geskin, V. Gulbinas, S. Pakalnis, V. Butvilos, Phys. Rev. B49 (1994) 7178. [I71 D.D.L. Chung, MS. Dresselhaus, Physica 89B (1977) 131. [I81 J.N. Rouzand, A. Oberlin, C. Beny-Bassez, Thin Solid Films 105 (1983) 75. [I91 T.C. Chieu, M.S. Dresselhaus,M. Endo,Phys. Rev.B26 ( 1982) 5867. WI MS. Dresselhaus, G. Dresselhaus, K. Sugihara,I.L. Spain, H.A. Goldberg, Graphite Fibers andFilaments, Springer Proc. MaterialsScience, vol. 5, Springer, Berlin, 1988. [211 C.T. Ho, D.D.L. Chung, Carbon 28 (1990) 815. 1221 CT. Ho, D.D.L. Chung, Carbon 28 (1990) 521. 1231 D. Ghosh, D.D.L. Chung, Mater. Res. Bull. 18 (1983) 1179. ~24] M.S. Dresselhaus, M. Endo, in: H. Zabel, S.A. Solin (Eds.), Graphite Intercalation Compounds II, Transport and Electronic Properties, Springer, Berlin, 1993. WI CT. Ho, D.D.L. Chung, Carbon 28 (1990) 825. [261 S.H. Anderson, A. Chung, D.D.L. Chung, Carbon25 (1987) 191. 1271 P.V. Huong, Solid State Commun. 88 (1993) 23.
[a
Materials Chemistry and Physics 56 (1998) 14-20
Study of the bromination
of pitch-based carbon fibres
I.V. Klimenko ‘,* , T.S. Zhuravleva a, V.M. Geskin ‘, T. Jawhary b * lnstiture of Biochemical Physics of Russ_ian Academy of Sciences, Kmygin St. 4, Moscow 117334, Russia bServeis Cietttifico-Tecnics, Universitat de Barcelona, E-08028 Barcelona, Spain Received 25 June 1997; received in revised form 16 February 1998; accepted 2 March 1998
Abstract The changes in the structure and properties of pitch-based carbon fibres (CF) brominated for different periods of time (up to 144 h) and the state of bromine in these fibres have been studied. Electrical conductivity in the temperature range from 10 to 300 K, IR and Raman spectra, and thermogravimetric analysis data are presented. The structural modifications in CF upon bromination, as indicated by these data,
include the growth of the fine-crystalline phase, the degradation of the graphite structure, the emergenceand growth of an amorphous phase, and an increase in the number of graphite layers adjacent to bromine. If the bromination time does not exceed 96 h, the process leads to the intercalation of bromine, i.e., the formation of charge-transfer complexes between the carbon network and bromine. More prolonged bromination results in the formation of C-Br chemical bonds. 0 1998 Elsevier Science S.A. All rights reserved. Keywords: Pitch-based carbon fibres; Bromination;
Electiical resistivity; Thernlogravimetric
1. Introduction
Synthesis of fullerenes and nanotubes is presently causing a rebirth of interest in the transport properties of carbonmaterials, in particular, those of graphitized carbon fibres (CF). Graphitized CF are electrically conducting and possess a unique combination of mechanical and thermal properties [ 11. The electrical resistivity of CF should be reduced for their application in conducting composites. To this end, intercalating agents can be introduced. In particular, bromination of pitch-based CF reduces their resistance by three to five times [ 21. Fair temporal and thermal stability of the properties of brominated fibres, along with the relative simplicity of their preparation, make these fibres especially attractive for practical applications. The properties of brominated CF were recently studied in a number of works by various experimental methods (see, e.g., [2-g]) . Nevertheless, the understanding of the CF bromination mechanism at the microscopic level (microstructural modifications in CF upon bromination, the state of bromine in CF, etc.) is still incomplete. This work aims to clarify at least some of these issues. Materials of this type usually have strong intralayer bonds and weak interlayer bonds. Three main tools are often used for the study of the lattice structure: Raman, infrared and
* Corresponding author. Tel.: + 7-095-939-7 E-mail: [email protected] 0254.0584/98/$ - see front PIIs0254-0584(98)00118-7
matter 0
197; Fax + 7-095-137-4101;
analysis (TGA);
Raman spectroscopy
inelastic neutron scattering spectroscopy. All these methods were used for intercalated graphite compounds and for some types of fibres. The graphite model based on the early results from Ref. [ lo] is widely used. Therefore the results presented here will be discussed in the framework of these previous studies on graphite structures and carbon fibres. In this paper, we present a combined study of Russian commercial pitch-based CF brominated for O-144 h. We measured the electrical conductivity of the fibres in the temperature range lo-300 K, performed the thermogravimetric analysis (TGA), recorded IR and Raman spectra in the range 100-3200 cm- ’ using a microscope and analysed their band shape by graphical Lorentzian deconvolution. Micro-Raman spectroscopy appears most informative for structural studies of graphite and graphite-like materials including CF. According to Refs. [lo-121 and from the analysis of a number of criteria, namely, the position of the principal EZg2line in the first-order spectrum and its width at half maximum, the ratio of the band intensities, and the line shapes, it is possible to characterize the material on a molecular level and determine the degree of order in graphite layers, the type of defects, the state of additives, etc. The most complete bromination is achieved in well-graphitized CF [ lo]. Based on this observation, we used in this study pitch-based CF, whose structure is closer to that of graphite than the strncture of CF prepared from polyacrylonitrile or rayon.
1998 Elsevier Science S.A. All rights rcservcd.
I.V. Kfimenko et al. / Marerials
Cheinisrv
2. Experimental
Russian commercial pitch-based CF with a heat-treatment temperature of 3000°C were taken as the initial fibres. Bromination was carried out at room temperature. Single fibres (initial diameter u 10 pm) were placed into a small glass vesselfilled with gaseousbromine and kept therefor adefinite period of time from 2 to 144h. The brorninated CF were then held in a ventilation chamber for three days at room temperature to allow for the desorption of excessivebromine and, prior to electrical measurements,evacuated at 10M5torr for three days. Electrical measurementswere carried out according to the two-probe method in a ROK-300 (Leybold) cryostat using a R3009 d.c. bridge in the temperature range lo300 K. For these measurements,silver paste contacts were applied to a single fibre fixed on a glass ceramic substrate. Measurementson a bundle of fibres confirmed that the resistance of the contactswas much lower than that of a fibre. The results obtained on a bundle using the two- and four-probe methods closely coincided. The electrical measurements were carried out with an error of 2%. The main contribution to this error resulted from the electrical instability of the contacts and the thermal instability in the holder-substrate sample assembly. The diameter of the fibres was measured using a POLAM P-3 11polarization microscope.The error in diameter determination (15%) was the principal source of error in the specific eiectrical resistance. The weights of the initial and bromine-treated CF were determined with an m-4 ( Perkin-Elmer) autobalance.The thermal stability of the fibres was studied with a TGA-7 (Perk&Elmer) thermogravimetric analyser; the samples (4-6 mg) were heated at a rate of 10 K/min in an argon flush. Micro-Raman spectra of single fibres were recorded with a T64000 spectrometer(Jobin Yvon) equippedwith a double monochromator and a cooled receiver. A 458 nm Ar laser with a spot size less than 2 pm was used as a light source. The spectra were recorded at a laser power of 1.5 mW. The line shapesin the Raman spectrawere analysedby Lorentzian deconvolution becauseof their low spectral resolution. This decomposition gave better fits ofthe experimental bandsthan decomposition into Gaussian components. II? spectra of single fibres were recorded with a 1725-X FI’IR spectrophotometerequippedwith a diffusion accessory ( Perkin-Elmer) .
and Physics
56 (1998)
14-20
15
A weak band observed in the 1360-1400 cm-’ region can be assigned to the fine crystallite phase, as in graphite [ 11,121. lR spectra of initial fibres showed a line at about 868 cm-’ usually assigned to the lattice surface mode y0 (AL,) [131. The spectrum of a fibre exposedto bromine for 48 h (Fig. 2) is markedly different. The principal bands ( 1582and 2740 cm-‘) are broadened, the bands at 1360-1400 and 2480 cm- I becomestronger, and new bands appear at about 242, 500,700, 1600-2000 and 3000 cm- ‘. Raman spectra of graphitized materials are usually analysed according to suchcriteria asthe position ofthe principal bg2 line in the first-order spectrum and its width at half maximum, the ratio of the integral intensities of the bands 1360 to 1582, 1600 to 1582, 2480 to 2740, 3000 to 2740 cm-’ , and the line shapes[ lo-12,14,15]. Table 1 showsthat the position of the 1582and 2740 cm-’ lines is virtually independentof the bromination time. However, the position of the 1360 cm- ’ line is invariant only up to 96 h of exposure to bromine, while at longer bromination times this line shifts toward high frequencies. 6 t I
I 0
IOKI
m
33 3
cm-’
Fig. 1. Micro-Raman
spectrum of a pristine pitch-based carbon fibre.
21 0
2ow
loo0
z!mo
cm-’
Fig. 2. Micro-Raman carbon fibre.
spectrum of a bromine-intercalated
Table 1 The position of micro-Raman bromine for different times
(48 h) pitch-based
spectra lines of the carbon fibres exposed to
3. Results 3. I. Raman spectra
The micro-Raman spectrum of a pristine fibre (Fig. 1) comprises two strong bands, one at 1582 cm-’ (related to the active EZg2Raman mode), the other at 2740 cm- ’ with a shoulder at about 2480 cm-’ (corresponding to the 2730 cm- ’ band in the second-order spectrum of pure graphite).
0 2 24 48 72 96 120
1361 1361 1363 1363 1363.5 1380 1376
1582.4 1582.4 1581.8 1582.5 1582.3 1581.7 1581.8
2740 2745 2142 2742 2734 2143 2740
I.V. Klit~lenko et al. /Materials
Chemistv
Fig. 3. Raman spectra of pitch-based carbon ftbres in the region of the &,a principal line for bromination times of 48 h (a) and 120 h (b); experimental spectrum and its Lorentzian components (thin Lines).
and Physics 56 (1998) 14-20
Figs. 3 and 4 show examples of the Lorentzian deconvolution of the Raman spectra of the brominated CF. The overall fitting curve is so close to the experimental line that they are difficult to distinguish. Table 2 shows the average integral intensity ratios and the widths at half maximum based on this deconvolution for the first- and second-order spectra of the fibres brorninated for various periods of time. The integral intensity ratios for all the first-order bands increase with the bromination time, along with the width of the 1582 cm-’ band. The width of the 1360 cm-’ band, constant during the first 2 h of exposure to bromine, then sharply increases and remains unchanged afterwards. The average size of crystallites La (A) in the graphite network plane was calculated according to the empirical formula suggested for various graphitic carbons [ 111 and based on the experimental data of L, from X-ray research and from determination of S 1360/S1552from Raman spectra: (1)
La=44(~Mcl&82)-1
cm-'
Fig. 4. Low-frequency region of the Raman spectrum of a pitch-based carbon fibre brominated for 72 h; experimental spectrum and its Lorentzian components (thin lines).
t2.5
TIME
OF BROMINATION.
hrs
Fig. 5. Weight uptake P in lo-” g/cm (I), electrical resistivity pin 10m3 R cm (2) and crystallite size L, in A (3) of brominated CF vs. time of bromination, T= 300 K. Table 2 The Lorentzian components in the first- and second-order micro-Raman tBr, h
2 24 48 12 96 120
spectra of fibres exposed to bromine for different times
Fist order
Second order
SI3M)JSLIYZ 0
It is shown in Fig. 5. L, steeply decreases (by a factor of about 20) during the first 24 h of bromination and levels off at 16 A when bromination is continued. The second-order bands are wider than the first-order bands both in the pristine and brominated fibres (Table 2). The second-order spectrum is poorly resolved and the secondorder lines are thus less informative for the structural analysis. Nevertheless, the general run of the integral intensity dependence of the second-order lines follows that of the first-order lines (Table 2). New bands appear in the low-frequency region of the spectra of brominated fibres (Fig. 4)) namely, a very intense band at 242 cm- ’ with a shoulder at 160- 170 cm- ’ and less intense bands at about 500 and 750 cm- ‘. The ratio of the intensity of the low-frequency 160 cm- ’ component to the 242 cm- ’ band (equal to 0.4) and that of 500 cm- ’ to 242 cm- I (equal to 0.4 on the average) are practically independent of the bromination time. The bands at about 500 and 750 cm-’ shift slightly toward high frequencies as the bromination time increases. When the bromination time is between 48 and 72 h, the asymmetry of the low-frequency shoulder of the 242 cm- ’ band increases, and an additional band at 220 cm- ’ can be observed at room temperature. A similar multiplicity of the 242 cm-’ band was observed in the Raman spectra of
0.03 0.08 0.5
0.99 1.8 2.9 3.4
11% 3611
0.1 0.17 0.3 0.7 0.94
0.2
1.23
0.68
175 18.1 20.0 23;O 25.0 29.6 62.3
13.2 15.7 237 245 24s 240 242
&4,~&740 0.079 0.44 0.7 0.9 1.0 1.12 1.22
0.1 0.16 0.21 0.26 0.65 0.74
60
78
63 90 9.5 100
100
110 104
172 210 250 353 370
I.V. Klirnmko
et al. /Mnterials
Chemistry
bromine-intercalated graphite at low temperatures[ lo]. Note that while the bromination time did not exceed 120 h, the characteristic band of free BrZ at 323 cm- ’ was not observed.
and Physics
56 (1998)
14-20
17
p, 1O-40hm.crl-
o.10-40hm.cm
3.2. Dark conductivity
Fig. 5 also shows the dependenceof room-temperature electrical conductivity (p) and weight (P) of a fibre on bromination time. The weight increasesduring the first 30 h (curve 1, Fig. 5) and then remains practically constant up to 96 h. The maximum weight gain during this period was 22%, which is typical for pitch-based CF [ 5,6]. When the bromination time exceeded96 h, the weight increasedby 55% with respectto its initial value. The diameter of the fibre increased from 10 to 12 pm after 96 h of bromination, which was taken into account in the calculations of electrical resistivity. During the first 30 h of bromination the resistivity of the fibre decreasedby seventimes ( from 1.19to 0.17 rnfi cm). Thereafter the resistivity remained unchanged up to 96 h (curve 2, Fig. 5). However, after 96 h of bromination the resistivity of a fibre increased (up to 0.35 rn0 cm for 144 h). Fig. 6 demonstratesthe temperature dependence(lo-300 K) of the electrical resistivity for the fibres exposedto bromine vapours for various periods of time. The resistivity of all the samplesincreased asthe temperature decreasedin the range lo-270 K. This increasewas minor for the fibres brominated for 24-96 h, but it was substantial for those brominated for 120-144 h. The differences in the temperature dependencesof the resistivity for CF exposedto bromine for different times are more pronounced when their reducedresistivities, defined as the ratio of resistivities at a temperature T and 300 K, p( T>/ p( 300), are plotted (Fig. 7). The resistivity of the fibres brominated for 24-96 h has a local minimum at 270 K upon cooling. The dependenciesfor the fibres brominated for 24, 48, 72 and 96 h all passthrough a common point at 225 K. No local minimum is observed for the fibres with high bromination times ( 120 and 140 h), and the general aspect of the dependenceis somewhat different from that observedfor shorter bromination times (Fig. 7(b) ) . 3.3. Thermogrmimetric
arzalysis (TGA)
Thermogravimetric (TG) curves (Fig. 8) show a negligible weight loss below 150°Cfor all the fibres. In the range 150-250°C the weight loss was considerable, while at higher temperaturesthe weight loss was again less significant. The highest weight loss was observedfor a fibre exposedto bromine for 96 h, while above 120 h of treatment the fibres becamemore thermostable: up to 780°C the weight loss was 30% in the latter case (curve 6), while it attained 45% for the former (curve 5). 4. Discussion Fig. 5 shows that the evolution of the resistivity of the tibres as a function of bromination time correlates with the
I%-
l.s TEMPERATURE.
I
K
I
100 200 388 TEMPERATURE. K
Fig. 6. Temperature dependenciesof electrical resistivity for CF, brominated for: (a) 1,O h; (b) 2,24 h; 3,48 h; 4,72 h; 5, 96 h; 6, 120 h; 7, 144 h.
dT)/d3OO) T-
0.9g
/
100 2Q0 TEMPERATURE.
3 0 K
dT)/dJOO)
+73sT&d TETPERATLRE.
K
Fig. 7. Temperature dependencies of reduced resistivity for CF, brominated for: (a) I,0 h; 2,24 h; 3,48 h; 4,72 h; 5,96 h; (b) 6, 120 h; 7, 144 h.
55
35
;;;‘;‘;‘I, 1%
39% 4%
665
146
TEMPERATURE,C
Fig. 8. TGA curves for CF brominated for: 1, 2 h; 2, 24 h; 3, 48 h; 4, 72 h; 5,96 h; 6, 120 h; 7,144 h.
variation of their weight and the decreaseof their crystallite size. Furthermore, both changesin the character of the curves 1 and 2 take place at the samebromination times, namely, about 30 and 96 h. Both the resistivity and the weight of a fibre remain constant throughout the samebromination time range, between 30 and 96 h. This allows us to relate the evolution of the resistivity of a fibre to the changes in its composition and structure resulting from bromination. Three bromination periods arereadily discernedwith characteristic boundary times of 30 and 96 h. During the first period, with the bromination time up to 30 h, the resistivity undergoesa significant decrease(by seventimes) and finally attains 1.7 X 10m4R cm. This is similar to the value obtained for pitch-based P-55 and P-75 fibres but higher than the typical resistivities of other pitch-based fibres, P-100 and P-120, and also those of the fibres GM, GA and UNL obtained from the gasphase[ 2,8]. Nevertheless,the ratio pOIpBr= 7 (where p0and pBrare the specific electrical resistancesof the pristine and brominated fibres, respectively) obtained in this work is
18
I.V. Klirnenko et al. /Materials
Chemisty and Physics 56 (1998) 14-20
rather high and exceedsthe values characteristic for all the other fibres, with the UNL fiber being the only exception. The decreasein resistivity during the first bromination period must be due to the oxidation of the carbon material according to the reaction 2C+3Br2=2Br,-+2C+
(2)
This reaction yields two holes (missing electrons) on the graphite crystal&es (designated C in Eq. (2) ) adjacent to two bromine anions. Therefore, when the Br,- concentration is high enough, a quasi-metallic domain is formed within the carbon material, due to a sufficient density of holes in the Coulomb field of many anions, similarly to doping or photoexcitation processesin conducting polymers E161.It is the growth of the volume of this quasi-metallic phasethat leads to the decreasein resistivity of the brominated fibre during the first period. After 30 h of exposure to bromine, the metallization of a fibre slows down and levels off until 96 h of brornination (Fig. 5). The fibre absorbs no more bromine, the diffusion of which is apparently hindered by the oxidized surface (however, there are different opinions on this issue [ 171). Furthermore, prolonged bromination causes structural changesin the fibre. Bromine can migrate into the crystallites of the graphite lattice, thus disjoining its layers and deteriorating the bonding between them. As a result, the thermal stability of the fibres decreasestoward the end of the second bromination period (Fig. 5 and curves 3-5 in Fig. 8). After 96 h of exposureto bromine the fibre is again capable of bromine absorption,which canbe attributed to thepresence of structural defects. However, these defects cause the increase of resistance during the third bromination period, from 96 to 120h (Fig. 5, curve 2). Temperature dependenciesof the resistance of the fibres brominated for various times (Figs. 6 and 7) confirm the above schemeof structural modifications. The general view of these dependencies(in reduced coordinates) for the pristine and brominated fibres is different. There are two types of curves, the first for 24-96 h, and the other for 120-144 h of bromination. Similar temperature dependencieswithin each type can be related to similar structural modifications. The curves for the fibres treated for 30-96 h in Fig. 7 clearly indicate the increaseof the defect concentration with bromination time, though the room-temperature resistancefor this time range is invariant (Fig. 5). Crossing (at 225 K) of the reduced resistance curves (Fig. 7) of the fibres brominated for 24-96 h may be related to a phasetransition. A different character of the temperature dependencefor bromination times over 96 h reflects the presenceof new structural defects in the fibre; the resistancein such fibres increasesbecauseof accumulation of excessive bromine and the deterioration of the graphite structure. Raman spectra provide more insight into the structure of the fibres and its modification with bromination. The 15801595cm- ’ bands in the spectra of graphitized structuresare asstgnedto the E2g2active mode related to the C=C bond of
the graphite network [ lo], and the lower this frequency, the more orderedis the material’s structure [ IO]. The 1582cm- ’ band of the pristine fibre (Fig. 1) indicates a high degreeof order in the graphite structures of our fibre. Furthermore, pristine fibres show a very weak band at 1360-1400 cm;’ resulting from disordering in the graphite structure and related to the fine-crystalline structure of the fibre. Low integral intensity ratios of the bands 1360 to 1582 and 2480 to 2740 cm- ’ (Table 2) also characterizeour fibre asbeing one with a highly ordered graphite system. Eq. ( 1) provides the value of the average crystallite size in the graphite network for the pristine fibre as L, = 1470 A. The width of the 1582 and 1360cm- ’ bands also characterizesthe degreeof order in the graphite layers; this value for the principal E,,, line in our pristine fibres is close to that in highly pure fine-crystalline graphite [ 111. As the bromination proceeds, the position of the 1582 cm- ’ band remains practically invariant (Table 1)) which indicates the presenceof graphite structure in all brominated samples. The ratio of the integral intensities S1360/S1582 increases (Table 2) ; given the constant integral intensity of the 1582cm- ’ band up to 96 h of bromination, this indicates the growth of the fine-crystalline phase.As the bromination does not exceed 24 h, this ratio shows only a three-fold increase, while bromination for 24 or 48 h leads to a 16- or 30-fold increase,respectively. After 96 h the ratio S,360/S1s82grows by two orders of magnitude. This can be related to (i) a pronounced destruction of large crystallites and multiplication of the small ones (see also L, values in Fig. 5) and (ii) destruction of the graphite structure in general, leading to the appearanceand growth of the amorphousphase. When the bromination time is between 96 and 120 h, a new band appearsin the first-order spectrum at about 1530 cm-’ , which is also characteristic of amorphouscarbon [ 181; its integral intensity increaseswith brornination time (Table 2). The integral intensity of the 1530 cm- ’ band for a fibre brominated for 96-120 h increasesseventimes, also indicating the changes in the graphite structure of the fibre. The changesin the ratio Si36,,/S1582, abrupt increaseof the width of the 1360 cm-’ band at 24 h of bromination (Table 2)) and the emergenceof a new band at about 1530 cm- ’ after 96 h of bromination correlate well with our conductivity data, A certain discrepancy in the onset times of considerable changesin the graphite structure as determined by the two methodsmay result from their different nature. In fact, Raman spectroscopyprovides information on a 0.1 u,rnthick surface layer averaged on the laser beam area of 2 pm, while resistance is a bulk macroscopiccharacteristic of a fibre. All the brominated samplesshow a high-frequency unresolved 1600cm- ’ band as a symmetric shoulder of the principal line, which may result from the C=C vibration in the layer adjacentto bromine. Theselayers facilitate chargetransport in bromine-treated (for up to 100 h) fibres and their growth results in decreasedresistivity of the brominated fibres with respectto the pristine ones.The 1600cm-’ band
Z.V. Klimmko
et al. /Materials
Chemists
already appears after 2 h of bromination, and its intensity grows with bromination time. Apparently, the boundary graphite layers are formed almost immediately ( 1600 cm-’ band appears) but not extensively (cf. the 1600 and 1582 cm-’ intensity ratio in Table 1) . Only at 72 h of bromination is there a considerableincreasein the 1600cm-’ band intensity and its ratio to that of 1582 cm-’ indicates that the concentration of such layers grows significantly. The second-order lines evolve similarly to the first-order lines (Table 1). The dominating line in the second-order spectrum is at 2740 cm-‘, which can be represented as 2 X 1365cm- ’ = 2730 cm- ’ [ 19,201.A broad line at about 2480 cm-’ is related to the combination of the modes of high-density phonon stateswith different wave vectors (860 cm-‘+ 1620 cm-‘- - 2480 cm- ‘) [20]; a lattice surface mode AZuat 868 cm- ’ was recordedin the IR spectrum.The 3000 cm-’ band is observed only in brominated samples. This line is related to the graphite lattice disordering and results from the combination of modes about 1365 cm- ’ + 1620cm- ’ = 2985 cm- I [ 201. The ratio S2J80/S2710 is equal to 0.079 for the initial fibre (Table 1) and it increases upon bromination to 0.9, 1.Oand 1.22 after 48,72 and 120 h of treatment, respectively. The ratio Ss0001S27J0 is virtually invariant up to 72 h of bromination. Considerable changes occur at 96 h of bromination, thus confirming the deterioration of large gr$phite structures and the emergenceof smaller ones (L, = 16A, Fig. 5) asa result of prolonged bromination. The bands at about 242 cm- ’ in the spectraof brominated graphitized materials are assigned to the vibrations of bromine anions (Br,-, Br,-) [21-231. They are characteristic of bromine intercalated into graphite [ lo]. Similar lines in the spectrum of our fibre indicate the presenceof bromine anions, which could only be formed in a graphite structure due to an electron transfer from the graphite layers adjacent to bromine, as we have noted above. In some cases,the dependenciesof resistivity on bromination time (such as curve 2 in Fig. 5) may provide information on the intercalation stages.For example, the existence of four distinct stages in such a dependencepermitted the authors of Ref. [ 81 to deducefour intercalation stages.However, there is only one extendedplateau in the dependenceof resistivity on bromination time of our pitch-based fibres (Fig. 5)) as well as for those observedfor GA, P-75, P-100 and P120 brands of fibres. The concentration of bromine in the tibre can be calculated from the weight uptake upon bromination (curve 1 in Fig. 5). For bromination times of 40-96 h this concentration is 4.5 mol%, yielding an approximate formula CZ2Br2for the intercalated substance,which correspondsto the third stage of intercalation [ 15,171, In certain cases,the intercalation stagecan also be deduced from the shift of the Raman bands.For instance, a shift of the bands at about 1582 and 1600 cm-’ was observedresulting from the fibre state changes [ 241, For highly oriented pyrographite and carbon fibres obtained from the gas phase, the 1582 cm-’ band shift upon the transition from the third to
and Physics
56 (1998)
14-20
19
the fourth stage is about 1.5 cm- ’ [ 241. Unfortunately, in the conditions of our experiment, this shift would be within experimental error. Thermogravimetric curves (Fig. 8) show considerable weight loss between 150 and 250°C which can be related to the desorption of intercalated bromine. The maximum weight loss in this temperature range was 22% for a bromination time of 96 h, which is actually close to the weight changesin Fig. 5. For highly graphitized P-100 and P-120 fibres, a substantial weight loss takesplace between 100and 300°C [ 251, A temperature of 100°Cis usually consideredasthe point of in-plane melting of intercalated bromine [26]. The differences between the temperature range and kinetics of the weight loss observedin this work (Fig. 8) on the one hand, and the data [ 251 on the other hand must result from different structures of the fibres studied. More complicated weightloss kinetics in the 150-250°C range in our casemight be due to simultaneous thermal destruction of the carbon material along with the desorption of bromine. Analysis of the gaseous reaction products could provide additional information on this issue. Themlogravimetric curves in Fig. 8 can be grouped into two classes:the first group involves the fibres brominated for 24-96 h and the second,for 120-144 h. Within each group, the weight loss at lOO-200°C increases with bromination time. On the other hand, the decreasein the weight loss between the groups indicates improved thermal stability, in accordwith the results [ 251 obtained for other types of fibres. This decreasedweight loss observed for the fibres exposed to bromine for periods longer than 96 h may indicate that bonding of bromine to the graphite network is stronger than in the intercalated state, presumably due to C-Br chemical bonds. Formation of thesebonds would decreasethe amount of Br, - and Br, - ions along with the concentration of holes (charge carriers) in the carbonnetwork, in spite of the growth of the total bromine uptake by the sample (curve 1, Fig. 5). Decreased hole concentration may be the reason for the increase in resistivity as the bromination time exceeds96 h (curve 2, Fig. 5). The existence of the bands at about 500 and 750 cm-‘, which have been assigned to the valence C-Br vibrations in bromine-intercalated fullerenes [ 271, might be regarded as a corroboration of C-Br bonding. Nevertheless, an unambiguous interpretation of these bands is still lacking in the literature [ 151.
5. Conclusions We have studied brominated Russian commercial pitchbasedfibres (heat-treated at 3000°C) by micro-Raman spectroscopy, thermogravimetry and conductivity measurements. All the obtained experimental data can be accounted for by the structural changes within the fibre caused by the introduction of bromine. Bromination leads to intercalation and also to the formation of the C-Br chemical bond. If the bromination time does not exceed 96 h, the major part of the
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Chemistq and Physics 56 (1998) 1620
bromine is in the intercalated state, i.e., in the form of a charge-transfer complex. When the bromination time is longer than 96 h, C-Br chemical bonds are formed extensively in the fibre, leading to an increase in its thermal stability and also in its electrical resistance. We believe that the found properties of &hebrominated pitch-based fibres and the bromination process trends described in this paper could be used for optimization of the bromination process of these fibres as well as for other fibres and thus would be important for the development of non-metallic highiy conducting fibres.
Acknowledgements The authors are grateful for financial support from the Russian Foundation for Fundamental Research under the grants 95-03-08360 and 96-15-97492 and from the Intemational Science and Technology Center under the grant 015.
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