conference papers The internal structure of single carbon fibers determined by simultaneous smalland wide-angle X-ray scattering

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1 The internl structure of single cron fiers determined y simultneous smllnd wide-ngle X-ry scttering Oskr Pris *, Dieter Loidl, Herwig Peterlik, Mrtin Müller c, Helg Lichtenegger, nd Peter Frtzl Erich Schmid Institute of Mterils Science, Austrin Acdemy of Sciences, nd Metl Physics Institute, University of Leoen, Jhnstrsse 12, A-8700 Leoen, Austri, Institute of Mterils Physics, University of Vienn, Boltzmnngsse 5, A-1090 Vienn, Austri, nd c Europen Synchrotron Rdition Fcility, B.P. 220, F Grenole Cedex 9, Frnce. E-mil:Pris@unileoen.c.t Simultneous smll-ngle scttering nd wide-ngle diffrction using synchrotron rdition microem ws pplied for the first time to investigte single cron fiers in position-resolved wy. Tking into ccount the exct X-ry em profile nd exmining the fiers in two scttering geometries llowed discrimintion etween different models for the internl rrngement of cron lyers nd pores. For fier sed on polycrylnitrile the cron lyers were rndomly oriented within the fier cross section, wheres in mesophse-pitch sed fier the lyers were rrnged in rdil structure. 1. Introduction Cron fiers my e considered nno-composites, consisting of three components, qusi-morphous nd crystlline regions, nd pores (Koets & Deev, 1997). Mny of the outstnding mechnicl properties re ttriuted to the internl structure of the fiers, e.g., to preferred orienttion of cron lyers nd to possile skin-core effects (Rulnd, 1990, nd references therein). In the pst three decdes, wide-ngle X-ry diffrction (WAXD) ws extensively used to determine the preferred orienttion of the cron lyers s well s the size nd shpe of the crystlline regions (Rulnd & Tomp, 1968; Perret & Rulnd, 1970; Tkku & Shioy, 1990). From smll-ngle X-ry scttering (SAXS) experiments on fier undles it is known tht long, thin pores re minly oriented long the fier xis, following the preferred orienttion of the cron lyers (Perret & Rulnd, 1969 & 1970; Gupt et l., 1994; Peterlik et l., 1994). In these studies, however, one ws not le to resolve detils of the structurl rrngement within the fier cross section. Informtion out the internl orgniztion in single fiers ws otined y Scnning Electron Microscopy (Koets & Deev, 1997) nd Trnsmission Electron Microscopy (Bennett & Johnson, 1979), the ltter technique llowing direct, position-resolved oservtion of the rrngement of cron lyers using lttice-fringe imging. In fiers sed on polycrynitrile (PAN) rndom rrngement of the cron lyers within the fier cross section ws usully oserved, while for mesophse-pitch (MPP) sed fiers the internl structure is known to depend strongly on the fier production process. Pronounced non-rndom rrngements of the cron lyers within the fier cross section, such s rdil- or onion-skin structures, were reported for MPP-sed fiers (Edie & Stoner, 1992). In recent yers, position-resolved X-ry scttering methods hve een developed for the investigtion of hierrchiclly structured mterils (e.g. Frtzl et l., 1997). X-ry microems with sizes down to less thn 1 µm hve recently ecome ville t synchrotron rdition sources, nd thus, smples which re heterogeneous on micrometer scle my e studied nowdys lso in position-resolved wy using X-ry microem scttering techniques (Riekel et l., 1997; Müller et l., 1998; Lichtenegger et l., 1999). Here, we report first experiments reveling the internl structure of two different cron fiers y mens of simultneous microem SAXS nd WAXD. The im of the present pper is to demonstrte tht this technique llows one to discriminte etween structurl models for the internl orgniztion of single cron fiers. 2. Experimentl Two commercilly ville cron fiers (PAN-sed fier HTA7 from Tenx GesmH nd MPP-sed fier FT500 from Tonen) were investigted. Both fiers were exmined y Scnning Electron Microscopy (SEM), yielding circulr fier cross sections with dimeters of 7.1 µm (HTA7) nd 9.6 µm (FT500). The scttering experiments were crried out t the microfocus emline (ID13) t the Europen Synchrotron Rdition Fcility (ESRF) in Grenole, Frnce. The monochromtic X-ry em (wvelength λ = Å) ws focussed y n ellipsoidl mirror nd glss cpillry (Riekel et l. 1997). An re detector (MAR-CCD, ctive dimeter 130 mm, pixel-size 64.5 µm 64.5 µm) ws plced t distnce of 45 mm from the specimen nd smll em-stop (0.3 mm led on thin glss-fier) ws mounted t distnce of out 20 mm from the smple. This em-stop position ws found optiml for reduced ir scttering (no evcuted em pth ws ville) s well s for reching sufficiently smll scttering ngles. The setup llowed the simultneous recording of SAXS nd WAXD dt with single 2D-detector in the rnge of scttering vectors from q = 0.1 Å -1 to 5 Å -1 (q = 4 π sin (θ ) / λ, where 2θ is the scttering ngle). Since the size of the X-ry em ws only fctor 2 to 3 smller thn the dimeter of the fiers, specil effort ws undertken to determine the exct em profile t the specimen position. This ws done y scnning circulr pinhole (Pt-Ir, 20 µm dimeter) in two orthogonl directions (y,z) perpendiculr to the em. The two-dimensionl em profile ws derived y fitting model function to oth dt sets, tking into ccount the curvture of the pinhole. The full width t hlf mximum (FWHM) of the em t the smple position ws determined to e 3.6 µm. The smples could e scnned in the two directions y nd z perpendiculr to the em with n ccurcy of 0.1 µm, nd the exct em position with respect to the smple could e determined using n online video microscope. The fiers were exmined in two different scttering geometries: (i) the usul fier geometry with the fier xis perpendiculr to the X- ry em, nd (ii) trnsverse geometry with the fier xis prllel to the em. For experiments in this geometry, single fier hd to e emedded into resin, cut y microtome to length of out 3-5 µm nd mounted on copper grid. Liner scns were performed long two perpendiculr directions of the fier cross section for the PAN-sed fier HTA7. For the MPP-sed fier FT500 two-dimensionl mesh-scn ws crried out with step size of 2 µm. Typicl mesurement times were 30 s for single scttering pttern. All mesured dt were normlized with respect to the primry em monitor nd corrected for ckground. Dt evlution ws prtly done using the softwre pckge FIT2D (Hmmersley,1999). 3. Results Two different types of cron fiers were investigted using synchrotron microem SAXS nd WAXD. Fig. 1 shows exmples of 2D scttering ptterns tken in fier geometry with the fier xis eing verticl (z-direction). On the equtor (horizontl xis), the nisotropic SAXS signl ner the incident em indictes the J. Appl. Cryst. (2000). 33, 695±699 # 2000 Interntionl Union of Crystllogrphy Printed in Gret Britin ± ll rights reserved 695

2 presence of elongted pores oriented minly prllel to the fier xis. Moreover, the 002 nd 004 reflections from the prllel pcking of the hexgonl cron lyers re visile. On the meridin (verticl xis) one identifies the 100 reflection (lyer line) from the twodimensionl structure of the cron lyers Å Å Figure 1 Two-dimensionl scttering ptterns otined in fier geometry t the center of () the PAN-sed fier HTA7 nd () the MPP-sed fier FT500. The wve vector trnsfer is given y the rs in the figures, nd the reflections 100, 002 nd 004 re indicted. The gry-level scle for the intensity t the right (in ritrry units) is the sme for oth scttering ptterns. Both ptterns were normlized with respect to the primry em monitor nd to fier thickness, so the gry-levels my e compred directly. The coordinte system on the lower right side defines the geometry of the scn experiment in rel spce: z-xis = fier xis, y-xis = scn xis, x-xis (perpendiculr to the pper plne) = em xis. There re remrkle differences in the two ptterns, illustrting the vriety of structurl fetures in cron fiers, depending on the type nd het-tretment (Rulnd, 1990). This concerns in prticulr the widths of the intensity distriutions, which re relted to structurl prmeters like size, perfection nd degree of orienttion of the crystlline regions nd size, shpe nd orienttion of the pores. Similr ptterns with the proper position resolution were up to now only otined in electron diffrction studies (Fourdeux, Perret & Rulnd, 1968; Bennett & Johnson, 1979). The min dvntges of X- rys s compred with electrons include etter resolution in reciprocl spce, no multiple scttering nd more quntittive evlution of these scttering ptterns. A detiled discussion of ll the structurl fetures is, however, eyond the scope of this pper. Similr ptterns s in Fig. 1 were derived in fier geometry s function of position y y scnning the fiers perpendiculr to the em with steps of 0.5 µm. For fully rndom rrngement of the cron lyers within the volume illuminted y the X-ry em, the intensity of the equtoril reflections would e smered z y homogeneously in rings within the x-y plne in reciprocl spce. In this cse the mesured scttering ptterns in fier geometry would show chnge of the totl intensity of the equtoril reflections proportionl to the ctul fier volume illuminted y the X-ry em. For ny kind of preferred orienttion of the cron lyers, however, texture effect is expected which would ffect the totl intensity of the equtoril reflections. Therefore, the totl intensity of the 002 reflection within the detector plne (denoted s I 002 ) ws clculted, nd is drwn in Figs. 2 nd 2 s function of position y for the fiers HTA7 nd FT500, respectively. In order to compre these dt with the distriution expected for rndom rrngement of cron lyers within the fier cross section, we hve clculted the convolution Ω ( y) = f ( y) d( y) (1) where f (y) is the em profile projected to the y-direction nd d( y) R y 2 2 = 2 (2) is the thickness of the fier s function of y (R = fier rdius). The function Ω (y) for the respective fiers is drwn in Figs. 2 nd 2 y solid lines. For the PAN-sed fier HTA7 (Fig. 2), Ω (y) fits the experimentl dt quite well, in greement with rndom rrngement of the cron lyers. For the MPP-sed fier FT500, however, there is considerle devition of the clculted curve from the experimentl dt (Fig. 2d), indicting non rndom orienttion of the cron lyers within the cross section of this fier. It hs lredy een demonstrted in previous investigtion on wood (Lichtenegger et l., 1999) tht in scttering geometry with the fier xis prllel to the X-ry em, it is possile to get more direct informtion out the internl structurl orgniztion within the fier cross section. Fig. 3 shows mp of scttering ptterns from 2D mesh scn through the cross section of the MPP-sed fier FT500 with the fier xis oriented prllel to the em. In the figure, scttering ptterns were zoomed to visulize the SAXS signl only. The SAXS ptterns ner the center of the fier re lmost circulr nd I 002 (r. units) Figure 2 The totl intensity of the 002 reflection is shown versus position y for the fiers () HTA7 nd () FT500 y the open circles. The solid lines in () nd () re the clculted vlues of Ω from eqution (1), i.e., the convolution of the X-ry em profile with the contour of the respective fier (see text for detils). The y-scle is different ecuse of the different dimeters of the two fiers (HTA7: D Fier = 7.1 µm, FT500: D Fier = 9.6 µm). 696 O. Pris et l. J. Appl. Cryst. (2000). 33, 695±699

3 ecome ellipticl outside, the short xes of the ellipses pointing consistently into rdil fier direction (Fig. 3). The ellipticl shpe of the SAXS ptterns is indictive of non circulr cross section of the pores. Note, tht for rndom distriution of pores the SAXS ptterns would e circulr symmetric t every position even for non-circulr pore cross section. Thus, tking model of shelf-like pores (with their longest dimension long the fier xis), the longer side of the pore cross section would point into rdil fier direction. Since shpe nd orienttion of the pores must e connected with the rrngement of cron lyers, rdil rrngement of cron lyers within the fier cross section is directly ovious from Fig. 3. Figure 3 (): Mp of SAXS ptterns comined ccording to mesh scn over the cross section of the MPP-sed fier FT500. The fier xis ws prllel to the direction of the X-ry em nd the step size ws 2 µm. Every "pixel" of the mesh scn corresponds to single scttering pttern with scttering vector rnge: -0.3 Å -1 < q < 0.3 Å -1, horizontlly nd verticlly. The dshed line indictes the order of the fier. The signls outside the dshed line re minly due to scttering contriutions from the resin nd from the gurd pinhole. (): The rrows indicte the direction of the short xis of the ellipticl SAXS signl from (). The length of the rrows is roughly proportionl to the eccentricity of the ellipses. The PAN-sed fier HTA7 ws lso investigted in this geometry y performing liner scns long two perpendiculr directions of the fier cross section. In this cse, the shpe nd orienttion of the SAXS ptterns did not chnge with position, indicting rndom distriution of the pores nd cron lyers within the fier cross section. 4. Discussion It is well documented in the literture (e.g. Rulnd, 1990) tht in PAN-sed fiers the internl rrngement of cron lyers within the fier cross section is rndom, while pronounced non-rndom structures re generlly expected for MPP-sed fiers. Consistent with the literture, the SAXS ptterns otined from the geometry with the fier xis prllel to the em indicte rdil rrngement of pores for the MPP-sed fier FT500 (see Fig. 3), nd rndom distriution for the PAN-sed fier HTA7. For the mesurements performed in stndrd fier geometry, the orienttion of cron lyers nd pores is not directly visile in the scttering ptterns. However, when compred with the totl volume illuminted y the em (s expressed y eqution 1), the position dependence of I 002 is consistent with rndom structure only for HTA7 (Fig. 2). For FT500 the experimentl dt devite clerly from the curve expected for rndom rrngement (Fig. 2). Nevertheless, it would e quite importnt to otin more detiled informtion lso from stndrd fier geometry experiments, since it provides the possiility to investigte the fiers in non destructive wy. In-situ tension experiments for instnce, which would give new insights in the reltionship etween structure nd mechnicl properties of cron fiers could only e performed in this geometry. In the following, we present simple picture which qulittively reltes the informtion otined from the fier geometry experiments (Fig. 2) to different models for the rrngement of cron lyers within the fiers. For certin fier volume illuminted y the X-ry em, the Brgg condition for the 002 reflection is fulfilled only for those cron lyers which re tilted y n ngle of θ with respect to the incident em (2θ = Brgg ngle). The sitution is sketched for n onion-skin nd rdil rrngement in Fig. 4. Assuming certin distriution of tilt ngles of the cron lyers with respect to min direction, only regions within the shded res in Fig. 4 would fulfill the Brgg condition. Thus, the totl intensity of the 002 reflection for certin position is proportionl to the intersection of the X-ry em with the shded re. Qulittively this would led to decrese of the intensity from the orders towrd the center of the fier for n onionskin rrngement (Fig. 4, left), while for rdil distriution (Fig. 4, right), vnishing signl t the orders nd strong increse towrd the center of the fier should pper. Oviously, the signl would not depend on position for fully rndom structure. In order to compre these considertions with the experimentl dt shown in Fig. 2, we hve chosen two model functions M(y), representing qulittively the expected trend of the 002 intensity for the cse of the onion-skin nd the rdil model. These two functions re shown in Figs. 5 nd 5 for the fiers HTA7 nd FT500, respectively. J. Appl. Cryst. (2000). 33, 695±699 O. Pris et l. 697

4 onion-skin rdil Model Functions M 1.0 D Fier = 7.1 µm D Fier = 9.6 µm D θ = 8 D θ = 8 I 002, Ω 1.0 c d Figure 4 Sketch of the geometricl constrints for the Brgg condition of the 002 equtoril reflection (2θ = 16 ) in fier geometry for two different models. The orienttion of the grphite lyers within the fier cross section is indicted y circles nd sectors for n onion-skin (left) nd rdil morphology (right), respectively. The shded re corresponds to the regions which fulfill the Brgg condition, ssuming distriution of tilt ngles of out 20 (corresponding to the zimuthl ngulr width of the 002 reflections in Fig. 1). The verticl lines indicte the volume illuminted y the X-ry em with dimeter D s it is scnned cross the fier. Only the intersection of the em with the shded res contriutes to the 002 reflection. It is ovious from this sketch tht the intensity would decrese versus the center of the fier for the onion-skin morphology, wheres it would increse versus the center of the fier for the rdil morphology Figure 5 Model functions M(y) for () HTA7 nd () FT500 representing the increse of intensity for the rdil model (dshed lines) nd the decrese of intensity for the onion-skin model (dshed-dotted lines) versus the center of the fiers ccording to the sketch in Fig. 4. As in Fig. 2, the experimentl dt I 002 re shown y the open circles for (c) HTA7 nd (d) FT500. The clculted distriutions Ω from eqution (3) re shown for the rdil model (dshed lines) nd for the onion-skin model (dshed-dotted lines). The full lines recll Ω for the cse of M(y) = constnt, i.e. the rndom model. To clculte I 002 for these model functions, eqution (1) ws rewritten { M ( y) d( ) } Ω ( y) = f ( y) y (3) nd Ω ws clculted for oth models. The results re shown in Figs. 5c nd 5d together with the experimentl dt. It is ovious from the figure tht the onion-skin model (dshed-dotted lines) cn e discrded for oth fiers. For FT500 (Fig. 5d) the rdil model descries the dt much etter thn the rndom model. Since the fier HTA7 ws much thinner, the dt re lmost resolution limited, nd the curves re not very sensitive to discrimintion etween the rndom nd the rdil model (Fig. 5c). However, reconsidering the results from the experiments with the em prllel to the fier xis, consistent picture cn finlly e drwn for the two fiers investigted: - A rndom rrngement of cron lyers nd pores is consistent with the dt for the PAN-sed fier HTA7. A different structure t the skin nd in the core of PAN-sed fiers hs frequently een oserved (e.g., Rulnd, 1990). We could, however, not resolve such n effect in the present experiment, proly ecuse the position resolution ws not sufficient (i.e., the skin could e very thin). - The structure of FT500 corresponds to the rdil model, i.e., the cron lyers nd the (shelf-like) pores re rrnged rdilly within the fier cross section. Even though the qulittive picture for the fier geometry experiments presented in Figs. 4 nd 5 is le to distinguish roughly etween different structurl models, it hs to e further improved. For detiled clcultion of I 002, not only the em profile ut lso the em divergence hs to e tken into ccount. Furthermore, the threedimensionl distriution of tilt ngles of the cron lyers illuminted in certin volume hs to e modeled from detiled nlysis of the Brgg reflections. Finlly, it hs to e mentioned tht the ove picture ssumes constnt volume frction of cron nd pores within the fiers. The volume frction s function of position within the fier cross section my e derived y clculting the integrted intensity of the SAXS signls. The detiled nlysis of ll reflections nd the SAXS dt should enle quntittive modeling of the internl structure of the fiers from the stndrd fier geometry experiments. This work is currently still in progress nd will e pulished elsewhere. 5. Conclusions We investigted the internl rrngement of cron lyers nd pores in two different single cron fiers using synchrotron rdition microem SAXS nd WAXD. It could e demonstrted clerly, tht this novel technique is le to discriminte etween different models for the internl structure of single cron fiers. In prticulr, it ws possile to otin the informtion lso from stndrd fier geometry experiments. This opens the possiility to investigte single fiers in non-destructive wy, nd thus to correlte structurl chnges with mechnicl properties y performing in situ-tension experiments. 698 O. Pris et l. J. Appl. Cryst. (2000). 33, 695±699

5 Severl importnt fctors contriuting to the success of the present investigtion should e stressed once gin: (i) A em-size of only out 3 µm llowed the imging of the nnostructure with micrometer resolution y scnning the fiers through the em. (ii) Even though the size of the X-ry em ws only fctor 2 to 3 smller thn the dimeter of the fiers, position-resolved informtion ws otined y tking the exct em profile into ccount. (iii) Acquiring WAXD- nd SAXS dt with one single 2D-detector provided complementry informtion out the cron lyer structure nd the pores simultneously. (iv) Two dimensionl mesh-scns using scttering geometry with the fier xis prllel to the em yielded more direct interprettion of the internl rrngement of pores nd could therefore support the interprettion of the stndrd fier geometry experiments. In conclusion, the present investigtion demonstrted once more the potentil of X-ry microem scttering techniques to investigte complex, hierrchiclly structured mterils. Importnt questions in mterils science my e ddressed in future studies, such s in-situ tensile tests of single cron fiers nd the investigtion of the interfcil structure etween fiers nd mtrix in cron/cron composites. References Bennett, S. C. & Johnson, D. J. (1979). Cron 17, Edie, D. D. & Stoner, E. G. (1992). Cron-Cron Mterils nd Composites, edited y J. D. Buckley & D. D. Edie, pp Prk Ridge, NJ, USA: Noyes Pulictions. Fourdeux, A., Perret, R. & Rulnd, W. (1968). J. Appl. Cryst. 1, Frtzl, P., Jko, H. F., Rinnerthler S., Roschger, P. nd Klushofer K. (1997). J. Appl. Cryst. 30, Gupt, A., Hrrison, I. R. & Lhijni J. (1994). J. Appl. Cryst. 27, Hmmersley, A. (1999). ESRF wepge, hyperlink: Koets, L. P. & Deev, I. S. (1997). Comp. Sci. Techn. 57, Lichtenegger, H., Müller, M., Pris, O., Riekel, C. & Frtzl, P. (1999). To pper in J. Appl. Cryst. 32. Müller, M., Czihk, C., Vogl, G., Frtzl, P., Schoer, H. & Riekel, C. (1998). Mcromolecules 31, Perret, R. & Rulnd, W. (1969). J. Appl. Cryst. 2, Perret, R. & Rulnd, W. (1970). J. Appl. Cryst. 3, Peterlik, H., Frtzl P. & Kromp K. (1994). Cron 32, Riekel, C., Cedol, A., Heidelch, F. & Wgner, K. (1997). Mcromolecules 30, Rulnd, W. & Tomp H. (1968). Act Cryst. A24, Rulnd, W. (1990). Adv. Mter. 2, Tkku A. & Shioy M. (1990). J. Mt. Sci. 25, J. Appl. Cryst. (2000). 33, 695±699 Received 12 June 1999 Accepted 20 Octoer