As-spun orientation as an indication of graphitized properties of mesophase-based carbon fiber

Carbon Vol. 36, Pergamon

No. 7-8. pp. 85%860,1998 0 1998 ElsevierScienceLtd

Printed in Great Britain. All rights reserved 0008-6223/98$19.00+ 0.00 PII: SOOOS-6223(97)00166-S

AS-SPUN ORIENTATION AS AN INDICATION OF GRAPHITIZED PROPERTIES OF MESOPHASE-BASED CARBON FIBER A. B. BARNES, F. M. DAUCHI?, N. C. GALLEGO, C. C. FAIN and M. C. THIES* Center for Advanced Engineering Fibers, Clemson University, Clemson, SC 29634-0909, U.S.A. (Received 21 February 1997; accepted in revisedfirm

30 July 1997)

Abstract-A 100% mesophase produced by supercritical fluid extraction was melt-spun into roundshaped fibers. The spinning temperature was allowed to vary while all other spinning conditions were held constant. The as-spun preferred orientation of the graphene basal planes with respect to the fiber axis was found to change as a result of a change in the spinning temperature. As-spun fibers that had developed a high degree of preferred orientation after spinning were found to have a high degree of preferred orientation, low do,,,,- sp acing, and large crystallite size after graphitization. Also, as-spun fibers with a high degree of preferred orientation were found to have high modulus and low electrical resistivity after heat treatment. Scanning electron microscopy results indicate that, regardless of spinning temperature, all fiber sets exhibit a radial-folded transverse texture. 0 1998 Elsevier Science Ltd. All rights reserved. Key Words-A. Mesophase, A. carbon fibers, B. high pressure, C. X-ray diffraction, D. mechanical properties, D. thermal conductivity.

1. INTRODUCTION

mesophase used in this study was produced by fractionating an isotropic petroleum pitch via supercritical fluid (SCF) extraction. Background information on the SCF extraction process is given in Section 2.

The processing of high Young’s modulus and high thermal conductivity mesophase-based carbon fibers is critical for heat-dissipation applications. These properties are directly related to the degree of preferred orientation of the graphene basal planes with respect to the fiber axis [ 1,2]. Thus, the development of molecular orientation during fiber formation is critical if the properties of the final fibers are to be maximized. When working with a mesophase pitch-based precursor, which can be melt-spun over a range of temperatures. it is necessary to determine the optimum spin temperature. Typically, in order to do so, fibers from a given precursor are melt-spun over a range of temperatures, producing many fiber sets. Each fiber set is then oxidized, graphitized and tested to determine which spin temperature produces the best graphitized fiber. All of this processing involves too much time, effort and money to be performed on a routine basis. Therefore, it is of great interest to develop a technique that can be used to predict the graphitized fiber properties in an efficient, cost-effective manner. Several research groups have correlated the degree of preferred orientation of carbon fibers with spinning conditions [3-61. In the present work, the use of X-ray diffraction to predict the properties of graphitized carbon fiber by measuring the degree of preferred orientation of the aromatic molecules within as-spun fiber is reported. It was hypothesized that the as-spun molecular orientation within the fibers should correlate with the final fiber properties. The

2. SCF EXTRACTION SCF extraction is a separation technique that exploits the large changes in solvent power that a compressed fluid can exhibit near its critical point. The ability of a supercritical fluid to fractionate a multicomponent mixture is based both on the solvent density and on the differences in volatilities and intermolecular forces between the solutes and the SCF solvent [7]. SCF extraction is used successfully on an industrial scale in several separation processes, several of which are described in detail elsewhere [ 81. SCF extraction is being used at Clemson to fractionate isotropic petroleum pitches with near critical and supercritical toluene [9,10]. The isotropic feed pitch used in this work is similar in nature to an Ashand A240 pitch, with a number average molecular weight of ca 530, as determined by vapor pressure osmometry, and a softening point of 103°C [9, lo]. The fractionation is performed by intimately mixing the pitch and toluene together to attain equilibrium, allowing the resulting two liquid phases to separate, and recovering these phases. The less dense liquid consists of toluene and the extracted portion of the pitch; the denser liquid consists of the unextracted portion of the pitch and toluene. Depending on the operating conditions of temperature, pressure and solvent-to-pitch ratio, the composition of each liquid phase can be dramatically varied. Typically, condi-

*Corresponding author. Tel: 001 864 656 5424; Fax: 001 864 656 0784; e-mail: [email protected] 855

Cons are controlled such that a lO(& mcsophase pitch is obtained in the denser (bottom) liquid phase after removal of the toluene. 3. EXPERIMEN’I‘AI.

fractionate the feed isotropic pitch and proclucc the desired mesophases. see Fig. I. With this type of apparatus. sufficient quanttticz of mcsophase can be produced for both analytical characterization and for fiber spinning. The apparatus is described in detail clsewhcre [IO]. For 21typical experimental run. a 5O!SO by weight mixture 01‘ isotropic pitch and toluene is pumped using tither of two high-pressure feed cylinders. Each cylinder use> tolucne as a working fluid to indirectly pump the pitch: a floating piston serves to scparatc the two fluids. Pure toluene is also pumped scparatcl), at ;I predetermined flow rate to obtain ;I specified solventto-pitch (‘S,,P) ratio. The two streams arc then 171-cheated. combined in a mixing “tee”. and further mixed in a mixing coil before rcaching an equilibrium cell. which functions as a phac separator. The prcheating, mixing and separation steps arc pcrformcd in an isothermal nitrogen bath. The heavy and light phases are collected independently thl-ough the bottom and the top of the cell. rcspectivcly. The liquid liquid interface in the equilibrium cell is monitored by an a.~. impedance bridge and is controlled by opening and closing the bottom-phase valve with a computer-controlled servo motor. The prcssurc 01‘ the system is controlled by opening and closing the top-phase valve. also by using a servo motor. All

Pressure Toluene FL_

P”lmp

_A

zample lines exiting the nitrogen bath are wrapped with electrical heating tape to maintain the phases in the tluid state and arc insulated. ,\l’ter expansion to atmospheric pressure through the \,alvcs. both samples are collcctcd. The top phase contains mostly toluene (U 80% by weight) and the lower molecular weight (MW ) portion of the pitch. In contrast. the bottom phase contains the higha MW portion of the pitch and contains only UI 20 wt% iol~~t‘nc. After collection. the bottom phase is dried at I50 <‘ and I .S Torr in a vacuum oven for I IIOLII to I-CII~\~C the toluene. The resulting bottom phase. solid at ambient tempcraturc, consists of up to I()()%, ~ncsopl~ase. For this study. the mesophasc fraction LV;I~produced by operating the continuous-flow apparatus at 320 C. 9.7 bar, and a FP ratio of 3.5. The product yield. defined as the portion of the feed pitch that is recovered in the mesophase-containing bottom phase. u as 17%. After drying. the fraction was shown to consist of 100% mesophase with a softening point ()I‘ 270 C. The rheology of the mesophase produced \\;I> measured. and the resulting viscosity (p in Pa. second ) vs temperature (7‘ in K ) data wcrc corrclatcd using the following Arrhenius-type equa11011

Log/!=-

17.105+II

615’7

(1)

3 2 .Wclr .spinning Prior to spinning, the mesophase was dried at 320 c‘ under RI 200 Torr in order to remove any residual toluene still trapped in the solids. Then. the mcsophase was melt-spun over a large spinning temperature range using the batch melt-spinning apparatus shown in Fig. 2.

Graphitized properties of mesophase-based carbon fibres

B Heating and Insulation

Wlnder

Tinder Q

Fig. 2. Schematic of the batch melt-spinning apparatus. This apparatus uses a positive-displacement gear pump, which fixes the extrusion flow rate. Thus, all spinning variables can be held constant as the spinning temperature is changed in order to maintain a constant as-spun fiber diameter. The mesophase was spun into eight sets of round-shaped fibers at a constant shear rate of 10 700f250 seconds-’ and a constant drawdown ratio of 41+ 3.

3.3 Wide angle X-ray dzjfiiaction (WAXD) A small unidirectional composite from each as-spun and graphitized fiber set was fabricated for X-ray analysis. Each unidirectional composite was prepared by dipping an ca 7.5 cm length bundle of fiber into molten paraffin wax. After dipping the fiber bundle, the excess wax was removed and uniaxial fiber alignment was achieved by hand. Upon cooling to room temperature, the rigid composite was affixed to an aluminum ring with household cement, forming the X-ray fiber sample shown in Fig. 3. This X-ray fiber sample, with an outside diameter of 2.54 cm aluminum ring

857

and a thickness of 0.8 cm, fits into the powder diffraction holder and the transmission attachment of a Scintag XDS 2000 diffractometer. The orientation of the graphene basal planes with respect to the fiber axis was quantified for as-spun and graphitized fiber by an azimuthal or chi (X)-scan of the (0002) diffraction intensity using Cu Ka radiation. The X-ray fiber sample was placed in the transmission attachment of a Scintag XDS 2000 diffractometer with the fiber/paraffin composite in a horizontal (x = 0”) position. The transmission attachment allows the fiber to rotate in a plane transverse to the incident/diffracted beam plane. A schematic of the azimuthal scan orientation is shown in Fig. 4. The position of the maximum (0002) diffraction intensity was determined by a continuous scan of the range 24” to 28” 20 at a scan rate of 0.5” minutee’ with the fiber/paraffin composite fixed in the x = 0” position. The 28 angle at which the (0002) diffraction line achieved maximum intensity was located by the background subtraction and profile-fitting functions of the diffraction management system (DMS) software. An azimuthal scan was conducted with the goniometer set as this 20 angle of (0002) maximum intensity, while the sample was rotated in the transverse plane. Intensities were collected over 360 of x rotation at 1” steps and 10 seconds per step. The full width at half maximum (FWHM) intensity, or Z-value, was determined by the profile-fitting function of the DMS software. This Z-value is used to characterize the relative misalignment of graphene basal planes with respect to the fiber axis. Each unidirectional composite fabricated from graphitized fiber was placed in the powder diffraction holder of the Scintag XDS 2000 diffractometer and adjusted to the proper focal plane level of the diffractometer setup using a sapphire disk. The sapphire was cut so that its (0001) plane was parallel to the disk surface, preventing fluorescence of the stainless steel holder during analysis. Continuous scans from 20 to 90” 20 were conducted at a scan rate of fiber set. Silicon 0.5” minute i for each graphitized reference material, NIST 640-B, was used to correct angular position and determine the instrumental component of peak breadth [ 111. The d,,,,,-spacing was calculated using the NelsonRiley method [ 121, while the crystallite stack height, L,, coherence length, L,, and non-uniform strain were calculated using the Warren-Averbach method [ 131.

3.4 Oxidation and graphitization

2.54 cm

0.8 cm

Fig. 3. Fiber composite X-ray diffraction sample.

In order to study the effects of spinning temperature and correlate as-spun Z-values to final properties, it is necessary that all fiber sets be oxidized and graphitized in the same manner. As-spun fibers from each fiber set were cut into lengths of co 7.5 cm and placed in a Thermolyne Series 9000 forced-air convection oven. They were heated from room temperature to 250°C at 2.5”C minute-’ and soaked at 250°C for 2.5 hours. At the completion of oxidation, thermo-

S

Incident beam

Jifl’racted

beam

clii rotation

chi = 0”

gravimetrlc analysis (TGA) was performed to dctcrmine the wt% carbon yield of the libcr upon carboniration. Approximately 5 mg of fiber ~\as placed in a Perkin Elmer Series 7 TGA system. The fiber was heated from room tempcraturc to 1000 (‘ at 1000 <’ li,r at 20 C minute ’ and soaked 30 minutes in flowing industrial grade nitrogen at a rate of (‘(I 60 ml minute- ‘. All fiber sets yielded 84f? wt% carbon. Because all (iber sets were stabilized in a similar manner, and all fiber sets cxpcricnced a similarly high carbon yield alict carbonization. the onl) source of variability within the fiber sets is that due to a change in spinning temperature. The oxidized tibers were placed in a graphite boat and inserted into an Astro graphite resistance furnace that was purged with industrial grade helium. The fibers were heated from room temperature to 1800 C at 20 ‘C minute I. 1800 C‘ to 2400 C ill 10 ‘C minute- ’ and soaked at 2400 C for 15 minutes. Small unidirectional composites from each graphitized fiber set were fabricated for X-ray analysis as described in Section 3.3. The remainder of the libcr was subjected to tensile modulus, electrical resistivitl and electron microscopy evaluation.

3.5.1

SingleT$liimrw

t

t ~wsilc

/c.stin‘y.

\)htem compliance correction l’actor was calculated using tensile data obtained from testing fibers at 15. 25. and 35 mm gage lengths; a constant gage length 01 IS mm was subsequently used for all testing. Fiber dlametcrs were measured using a laser diffraction technique [ 141. Approximately 50 single fibers wcrc tested to estimate the average tensile strength and m~~dulus for each fiber. 1.5.7 Mtusuremctil of ~~kctricid rrsistivilc. ‘l‘hc electrical rcsistivitics of all fibers were determined using a standard four-point probe technique similar 11) that described by Coleman [15]. Approximately IC smgle fibers from each set were measured to estimate the electrical resistivity of each fiber type. 3.5.3 Scanning rktron microscopy. After graphitization, the textures and microstructure of the fibers were analyzed by scanning electron microscopy (SEM ). The SEM study was conducted on a Jcol .ISM-IC‘ 848 at an accelerating voltage of 15 kV. (‘arbon libers from all the fiber sets were mounted on aluminum stubs and gold-coated using an Antech IHummer X sputter coatcr. The SEM was operated in the secondary electron imaging (SEI) mode to study the fracture surfaces of all fiber types. A more detailed description of the procedures for sample preparation for SEM is given by Robinson and Fidie [ 161.

I‘hc

mechanical properties of the graphitized fibers were determined according to ASTM D 3379-75 (standard test method for tensile strength and Young’s modulus for high-modulus single-filament materials) using an Instron Model TM tensile-testing machine. The

4. KESULIS AND DISCUSSION -i. I Wide ctnglr X-ru>- ci~jfiuctiotl fhc L-value is used to characterize the relative misalignment of aromatic molecules with respect to

Graphitized properties of mesophase-based carbon fibres

859

Table 1. Lattice parameters and properties of as-spun and graphitized fibers Fiber set Spin temperature (“C) Mesophase viscosity (Pa. second) As-spun fiber diameter (pm) As-spun Z-value (“) Graphitized fiber diameter (pm) Graphitized Z-value (“) L, (A) L (A) Graphitized d,,,,-spacing (A) Tensile modulus (GPa) Electrical resistivity (@m)

A

B

C

346 52 13.7f0.6 25.3 8.4kO.2 17.5 160 NA* 3.382 593 & 30 3.7kO.l

351 36 13.7kO.6 23.0 10.5kO.3 13.8 160 330 3.380 888 * 50 3.4kO.3

354 30 13.7kO.6 22.9 11.2kO.3 8.2 200 340 3.379 8lOk50 3.OkO.2

*The (1120) plane of fiber set A was not developed so an k Based upon a 95% confidence interval.

Fig. 5. SEM micrographs of fibers A, B and C indicating radial-folded transverse texture after graphitization to 2400°C.

L,

value could not be measured.

the as-spun fiber axis. As the Z-value increases, the degree of preferred orientation decreases. In order to determine if a correlation between as-spun Z-values and final fiber properties exists, and to determine the proper melt-spinning temperature of the tested mesophase, all eight fiber sets were X-rayed to determine their as-spun orientation. Three of these sets were chosen for further testing. These three fiber sets are referred to as A, B and C. Of the eight as-spun fiber sets, A was found to have the highest as-spun Zvalue. B and C were found to have similar low as-spun Z-values, with the orientation of C being slightly better than that of B. Therefore, if the present premise is correct, the potential for these fibers to develop high moduli and high thermal conductivities upon similar heat treatment should improve from A to B, and B and C should exhibit similar properties. Lattice parameter measurements of as-spun and graphitized fiber were made using WAXD. The results are given in Table 1. Judging from the as-spun Z-values obtained from an azimuthal scan of the (0002) diffraction line, the degree of misorientation decreases sequentially from A to B. The as-spun orientation of B and C are similar, with the orientation of C being slightly better than that of B. This indicates that the molecular orientation that occurs in the capillary increases as the spin temperature increases from 346 to 354°C. Upon graphitization to 2400°C the graphitized Z-values of the three fiber sets were measured. They follow a similar trend to the as-spun Z-values, with the degree of misorientation of the graphitized fiber decreasing from A to C. This correlation between as-spun and graphitized orientation was expected because all fiber sets were heat-treated in the same manner. The orientation of all fiber sets improved after graphitization, but the extent of basal plane alignment was limited due to the amount of orientation achieved during spinning. The crystallite coherence length, L,, increases from A to B, while B and C have similar values of L,. It should be noted that the L, measurement is based on the development of the (lOlO) and the (1120) diffraction lines. The L, size for A was not reported

because it did not develop a ( 1120) diffraction line upon graphitization as did B and C. This indicates that a-direction crystal growth is poor in A and should be detrimental to transport properties in the a-direction. Because B and C crystallite development in the a-direction was similat-. they should possess similar transport capabilities in the 21.direction. Additionally. the crystallite stack height. I+,. was significantly larger for C than A or B. Judging l‘rom the d,,,,,2,-spacing obtained from the (0002) and (0004) diffraction lines, the degree of graphitization increases sequentially from A to B. while B and C Overall. the have a similarly low d(,,,,,-spacing. X-ray data of the graphitized fiber support the present premise that as-spun orientation can bc used to determine the final fiber properties and appropriate spin temperature.

4.2 Testing Young’s modulus was determmed for the graphitized fiber. As expected from the X-ray data. the modulus of A is significantly lower than those of B and C. which achieved high moduli upon graphiti/ation. This is because the modulus is highly dependent on the degree of basal plane orientation with respect to the fiber axis [ 17. IS]. Because the preferred aricntation increases from A to C. the full benefits of the C C bond can be exploited when tensile loads arc applied, causing the modulus to increase. Due to the strong relationship found to cxlst between electric.al resistivity and thermal conductivity [ 191. electrical resistivity measurements were made to determine the transport properties of the three fibct sets. The

electrical

resistivity

decreases

from

A to (‘.

in preferred orientation. larger L, size, and smaller intcrplaner d,,,,,,,,-spacing that occurs sequentially from A to C [ 20.2 I 1. Incidentally. the effect of fiber diameter on the present results is believed to be insignificant. The diameters of B and C were identical within statistical limits. and the smaller diameter of A would have improved its properties. Thus. the differences observed between A and B/C are probably conservative. This is a result

of the increase

5. CONCI,USIONS

4 100% mesophase produced by SCF extraction wa\ melt-spun into round-shaped fiber sets. The spinning temperature was the only spinning parametcr allowed to vary while all other spinning conditions hc’rc held constant. An improved degree of preferred orientation with respect to the as-spun fiber axis was observed as the spinning temperature was increased. .fruc statistical differences in the as-spun Z-values of [UC) tiber sets yielded, upon identical heat treatment. true ditrercnces in tensile properties, electrical resistivity and crystallographic parameters for the associated final fibers. The reproducibility of the correlation between low as-spun Z-values and improved propcrties 01‘ the final fibers was tcstcd using two sets 01 fibers having essentially identical as-spun %-values; Icnsile and clcctrical propertics of the final fibers wcrc identical within the experimental errors associaled with the measurements. It is therefore concluded that the measurement of the degree of preferred orientation within as-spun fibers can be used to monitor the potential of the final fibers to develop improved tensile and thermal properties.

I

B.. H&d/wokof” C&po.~ites, Vol. I, Strong ‘7. Rand. Fibers ( Edited bv W. Walt and B. V. Perov). Elsevier. New York, 1985: KurtL. D. J.. D’Abatc, G. D., Tzcng, S. S. and Diefendorf. R. J . Proceeding.s of 2nd Japan Internationul S.4MPE Symposium, Chiba, Japan, 1991, pp. I58 165. 1 1Iamada. T., Furuyama, M., Saiidi. Y.. Tomioka, T. and Endo, M.. J. Mater. Rrs., IY”90, 5, 1271. Y _. Sekivu-Gakaku-shi. 1987. 30. 29 I 7. Matsumura. Y.. Kag;du-to-Kogyo, 1988, 41, ‘133. 6. Matsumurd, 7. tlutchenson, K. W., Roebers, J. R. and Thics, M. C.. Chrhon, 1991, 29, 215. x. McHugh, M. A. and Krukonis, V. J.. Suprrcri/rccd Fhlid E,xtrac~tinn, 2nd edn. Butterworth Heinemann, 1994. 0. Bolanos. C., Liu, G. 2. and Thies, M. C., Fluid Phaw f5/uilihriu, 1993, 82, 303. IO. DauchC. F. M., Bolaiios,

I I. 17. Ii.

I-1.

Figure 5 shows the typical gross textures of the graphitized fibers. Note that all fibers exhibit a radiafolded transverse texture, regardless of the spinning temperature. Fibers B and C exhibit a sharper fracture surface than fiber A. which is an indication of a more brittle material. that is, a higher modulus. Findings by other research groups [22.23] indicate that the texture of graphitized fibers produced from coal-tar pitches and from naphthalene-derived mesophases changes from radial to onion texture a> the spinning temperature increases. However. such changes in texture were not observed for the supercritically extracted mesophasc used in this study.

IS.

16. 17.

G.. Blasig, A. and Thies, M. C.. Curhon. in press. National Bureau of Standards Certificate, Standard Reference Material 640B, 1987. Nelson, J. B. and Riley, D. P., Prowedings o/‘the Physi~1 Society of‘ London, 1945, 57, 160. Warren, B. E. and Avcrbach, B. L.. J. Appl. Phys.. 1950. 21, 59.5. Tzeng. S. S. and Diefendorf, R. J., Proceedings o/ 2/.tr Biennial Cor@enw on Carbon. Buffalo, New York. 1993, on. I2 13. Cole&an. L. B., Rev. Sci. Instrum., 1975, 46(8), 1125. Robinson, K. E. and Edie, D. D.. Carbon, 1996, 34, 13. Brydges, W. T., Badami, D. V. and Joiner, J. C., App/. Polvmrr

Symp.,

1969, 9, 255.

18. Johnson. W.-and Watt, W., Nuture, 1967, 215, 384. SAMPE Symposium and IY. Kowalsky, 1. M., International Exhibition Advanced Materials Technology, Anaheim, CA. U.S.A. (Edited by R. Carson, M. Burg, K. J. Kjoller and F. J. Riel), Vol. 32. 1987; pp. 953 963. 20. Bright, A. A. and Singer, L. S., C&on, 1979, 17, 59. J. and Rahim, I.. Ploys. Rev. B., 1985. 21. Heremans, 32( IO), 6742. 22. Matsumoto, T., Purr uwd App1. C’/wm.. 1985, 57, 1553. 2;. Hamada. T.. Nishida, T. and Koyama, M., Curhon, I Y88. 26, 837.