《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_carbon fiber_Isotropic and anisotropic microporosity development upon chemical activation of carbon fibers, revealed by microbeam small-angle X-ray scattering

Availableonlineatwww.sciencedirect.com CARBON ELSEVIER Carbon44(2006)1121-1129 www.elsevier.com/locate/carbon Isotropic and anisotropic microporosity development upon chemical activation of carbon fibers revealed by microbeam small-angle X-ray scattering D. Lozano-Castello, J. A. Macia-Agullo, D. Cazorla-Amoros A. Linares-Solano M. Muller, M. burghammer C. Riekel Departamento de quimica Inorganica, Universidad de Alicante, Ap 99, 03080 Alicante, spain Institute of Experimental and Applied Physic, Universitat Kiel, Kiel, Germany European Synchrotron Radiation Facility, Grenoble, france Received 12 August 2005; accepted 15 November 2005 Available online 9 January 2006 Abstract A sub-micrometer size beam(0.5 um diameter) in a position-resolved small angle X-ray scattering set-up (usAXS) has been used for the characterization of chemically activated carbon fibers(ACF). These materials have been prepared from isotropic carbon fibers(pitch carbon fibers)and anisotropic carbon fibers(PAN-based carbon fibers) by chemical activation with KOH and NaoH. The HSAXS experimental set-up made it possible to analyze different regions of a single fiber across its diameter and to distinguish the structural features already existing in the raw fibers or being created during the activation process. The results showed that depending on the pre- cursor, the chemical activation process produces isotropic or anisotropic development of porosity. It was observed that chemically ACF prepared from isotropic carbon fibers present an isotropic development of the porosity and that a high micropore volume is developed not only in the external region of the fiber, but also in the core. On the other hand, in the case of anisotropic PAN-based carbon fibers the existence of two regions with different structure was detected by usAXS measurements across the fiber diameter: an anisotropic external ring and a more isotropic fiber core. The results showed that these two regions remain after chemical activation and that the activating agents are reaching the fiber core. It seems that the more isotropic fiber core is activated easier by naoh than KOH 2005 Published by Elsevier Ltd Keywords: Carbon fibers; Activation; Small angle X-ray scattering: Microporosity; Microstructure 1. Introduction and pitch fibers by "physical"or chemical activation of the carbon fibers. Both the nature of the precursors and the Activated carbon fibers(ACF) are typically micropo- method of activation have a strong influence on the pe rous fibrous adsorbents, which have some advantages com- structure and adsorption properties of the resulting pared to granular and powdered adsorbents, such as high samples adsorption capacities, high mass transfer rates for both Physical"activation with CO, or steam is the usual adsorption and desorption, and they are easier to handle. procedure to obtain ACF, but chemical activation with Therefore, they have received increasing attention in recent KOH and Naoh of carbon fibers has also been reported years as adsorbents and catalyst supports [1]. ACF can be [2]. In this previous work [2] ACF with high surface area prepared from PAN fibers, cellulose fibers, phenolic fibers, were obtained using as raw material commercial carbon fibers from coal tar pitch. A deep characterization of ACF having large micropore E-mailosn ding author Fax:+34965903454 volume is strongly desired to develop advanced technolo- address, cazorla(@ uaes(D. Cazorla-Amoros gies based on this property. In our previous research, an 0008-6223/S- see front matter C 2005 Published by Elsevier Ltd doi: 10.1016/j- carbon. 2005. 11.019

Isotropic and anisotropic microporosity development upon chemical activation of carbon fibers, revealed by microbeam small-angle X-ray scattering D. Lozano-Castello´ a , J.A. Macia´-Agullo´ a , D. Cazorla-Amoro´s a,*, A. Linares-Solano a , M. Mu¨ller b , M. Burghammer c , C. Riekel c a Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, Ap 99, 03080 Alicante, Spain b Institute of Experimental and Applied Physic, Universita¨t Kiel, Kiel, Germany c European Synchrotron Radiation Facility, Grenoble, France Received 12 August 2005; accepted 15 November 2005 Available online 9 January 2006 Abstract A sub-micrometer size beam (0.5 lm diameter) in a position-resolved small angle X-ray scattering set-up (lSAXS) has been used for the characterization of chemically activated carbon fibers (ACF). These materials have been prepared from isotropic carbon fibers (pitch carbon fibers) and anisotropic carbon fibers (PAN-based carbon fibers) by chemical activation with KOH and NaOH. The lSAXS experimental set-up made it possible to analyze different regions of a single fiber across its diameter and to distinguish the structural features already existing in the raw fibers or being created during the activation process. The results showed that depending on the pre￾cursor, the chemical activation process produces isotropic or anisotropic development of porosity. It was observed that chemically ACF prepared from isotropic carbon fibers present an isotropic development of the porosity and that a high micropore volume is developed not only in the external region of the fiber, but also in the core. On the other hand, in the case of anisotropic PAN-based carbon fibers the existence of two regions with different structure was detected by lSAXS measurements across the fiber diameter: an anisotropic external ring and a more isotropic fiber core. The results showed that these two regions remain after chemical activation and that the activating agents are reaching the fiber core. It seems that the more isotropic fiber core is activated easier by NaOH than KOH. 2005 Published by Elsevier Ltd. Keywords: Carbon fibers; Activation; Small angle X-ray scattering; Microporosity; Microstructure 1. Introduction Activated carbon fibers (ACF) are typically micropo￾rous fibrous adsorbents, which have some advantages com￾pared to granular and powdered adsorbents, such as high adsorption capacities, high mass transfer rates for both adsorption and desorption, and they are easier to handle. Therefore, they have received increasing attention in recent years as adsorbents and catalyst supports [1]. ACF can be prepared from PAN fibers, cellulose fibers, phenolic fibers, and pitch fibers by ‘‘physical’’ or chemical activation of the carbon fibers. Both the nature of the precursors and the method of activation have a strong influence on the porous structure and adsorption properties of the resulting samples. ‘‘Physical’’ activation with CO2 or steam is the usual procedure to obtain ACF, but chemical activation with KOH and NaOH of carbon fibers has also been reported [2]. In this previous work [2], ACF with high surface area were obtained using as raw material commercial carbon fibers from coal tar pitch. A deep characterization of ACF having large micropore volume is strongly desired to develop advanced technolo￾gies based on this property. In our previous research, an 0008-6223/$ - see front matter 2005 Published by Elsevier Ltd. doi:10.1016/j.carbon.2005.11.019 * Corresponding author. Fax: +34 965 903454. E-mail address: cazorla@ua.es (D. Cazorla-Amoro´s). www.elsevier.com/locate/carbon Carbon 44 (2006) 1121–1129

D. Lozano- Castello et al. Carbon 44(2006)1121-1129 important effort was dedicated to analyze the development (hydroxide/ carbon ratio(wt/wt) of 4/1). Chemical activa of porosity in isotropic pitch-based carbon fibers during tions were carried out in a horizontal furnace under nitro- physical"activation with CO2 and steam as activating gen atmosphere with a heating rate of 5 C/min up to agents. For that purpose, different techniques were used, 750C or 825C. Holding time at the maximum tempera such as gas adsorption and mechanical properties measure- ture was 2 or I h, respectively ments 3], positron annihilation lifetime spectroscopy After the heat treatment, samples were washed with 5 M (PALS)[4] and small angle X-ray scattering(SAXS)[5- HCl, vacuum filtered three times and then washed with hot 7]. In the case of SAXS experiments, the use of an X-ray distilled water(80C)and filtered until the filtrate was free microbeam of 2 um diameter(HSAXS)allowed us the char- of chloride ions. The cleaned samples were dried in an oven acterization of different regions of the same fiber with at 120oC for 24 h microscopic position resolution [6,7]. The scans across The characterization of the porosity of the ACF was the fiber diameter allowed to confirm our previous results done using physical adsorption of N2 at 77K and CO2 at [3]. Thus, in the case of CO2 activation, ACF presented 273 K(Quantachrome, Autosorb-6. The samples were scattering patterns with high intensity in everywhere across outgassed at 250C under vacuum for 4 h. Nitrogen the fiber diameter, confirming that CO2 activation takes adsorption results were used to determine BET surface area place within the fibers, generating a quite homogeneous values and Dubinin-Radushkevich micropore volumes development of porosity On the other hand, in the case (V(DR N2)). Narrow micropore volume (pore of steam activation, ACF presented scattering patterns size <0.7 nm, approximately) was obtained from CO with much higher intensity in the external zones of the adsorption data(V(DR CO2)). Table I contains the prepa fibers than in the bulk, which means that steam focuses ration conditions and pore texture characterization results. the activation at the outer parts of the fibers. These results Scattering experiments were carried out at the micro- agree with the decrease in fiber diameter observed in the focus beamline(ID13)in the European Synchrotron Radi case of steam activation compared with CO ation Facility(ESRF) in Grenoble, France. The beam In the present work, this type of study has been extended selected for the experiments of this study was a 0.5 um to ACf prepared by chemical activation with KOh and beam produced by Kirkpatrick-Baez mirrors( wavelength Naoh, using as raw material, not only isotropic pitch- 1=0.948 A). The domain of q values investigated with this based carbon fibers, but also anisotropic PAN-based car- setup is between 0. 1 nm(small angle resolution of 63 nm, bon fibers. To be able to analyze by SAXS the evolution approximately) up to 10 nm-l. The carbon fibers were of porosity in different regions of a single fiber(fiber diam- mounted in an aluminium frame, and several fibers of a eter in some cases less than 8 um)across its diameter, a given sample were analyzed to check the reproducibilit beam size much smaller than the fiber diameter and with of the experimental method. An area detector (MAR high intensity is needed. In this work, a sub-micrometer CCD) with an active diameter of 130 mm was used for size beam(0.5 um diameter) much smaller than the one the measurements. The distance from the detector to the used in our previous research(2 um diameter) has been samples was 470 mm. These uSAXS measurements were used, in a position-resolved X-ray scattering experiment. carried out scanning the fiber across its diameter with a The objective of this work is to analyze the development step size of I um and with an accuracy better than of porosity by chemical activation at different regions of 0. I um. All measured data was corrected for background a single fiber. The advantage of the SAXs technique to A scheme of the experimental set-up can be found else- be sensitive to shape and orientation of the scattering where [7]. Data evaluation was done using the software objects(pores), will allow us to obtain additional informa- package FIT2D [9] tion on the structural features already existing in the raw fibers or being created during the activation process, as well as in anisotropic studies in oriented samples. It should be noted that, a study of the misalignment of objects in the fibers requires the analysis of a single fiber, because when the analysis is carried out with a bundle of parallel fibers Table I the misalignment of the diferent fibers would also contrib- Preparation conditions and porous texture characterization results corre- ute to the scattering pattern [8] sponding to the raw carbon fibers and the ACF Sample Chemical activation BEt surface V DR V DR 2. Experimental (m2/g)N2 (cm/g)(cm/g) Two different precursors were employed for the prepara- CF(Kureha tion of ACF: commercial petroleum pitch carbon fibers CFNa65 NaOH, 750C, I h, 4/11085 (Kureha)(fiber diameter between 14 and 20 um)and high Hx (Hexcel CFK60KOH,750°,lh,4/1980 performance commercial PAN-based carbon fibers(Her aOH,750°C,2h,4/1717 el)(fiber diameter between 5 and 7 um). These carbon HxK28 °C,2h fibers were chemically activated using KOH and NaOH HxK25

important effort was dedicated to analyze the development of porosity in isotropic pitch-based carbon fibers during ‘‘physical’’ activation with CO2 and steam as activating agents. For that purpose, different techniques were used, such as gas adsorption and mechanical properties measure￾ments [3], positron annihilation lifetime spectroscopy (PALS) [4] and small angle X-ray scattering (SAXS) [5– 7]. In the case of SAXS experiments, the use of an X-ray microbeam of 2 lm diameter (lSAXS) allowed us the char￾acterization of different regions of the same fiber with microscopic position resolution [6,7]. The scans across the fiber diameter allowed to confirm our previous results [3]. Thus, in the case of CO2 activation, ACF presented scattering patterns with high intensity in everywhere across the fiber diameter, confirming that CO2 activation takes place within the fibers, generating a quite homogeneous development of porosity. On the other hand, in the case of steam activation, ACF presented scattering patterns with much higher intensity in the external zones of the fibers than in the bulk, which means that steam focuses the activation at the outer parts of the fibers. These results agree with the decrease in fiber diameter observed in the case of steam activation compared with CO2. In the present work, this type of study has been extended to ACF prepared by chemical activation with KOH and NaOH, using as raw material, not only isotropic pitch￾based carbon fibers, but also anisotropic PAN-based car￾bon fibers. To be able to analyze by SAXS the evolution of porosity in different regions of a single fiber (fiber diam￾eter in some cases less than 8 lm) across its diameter, a beam size much smaller than the fiber diameter and with high intensity is needed. In this work, a sub-micrometer size beam (0.5 lm diameter) much smaller than the one used in our previous research (2 lm diameter) has been used, in a position-resolved X-ray scattering experiment. The objective of this work is to analyze the development of porosity by chemical activation at different regions of a single fiber. The advantage of the SAXS technique to be sensitive to shape and orientation of the scattering objects (pores), will allow us to obtain additional informa￾tion on the structural features already existing in the raw fibers or being created during the activation process, as well as in anisotropic studies in oriented samples. It should be noted that, a study of the misalignment of objects in the fibers requires the analysis of a single fiber, because when the analysis is carried out with a bundle of parallel fibers the misalignment of the different fibers would also contrib￾ute to the scattering pattern [8]. 2. Experimental Two different precursors were employed for the prepara￾tion of ACF: commercial petroleum pitch carbon fibers (Kureha) (fiber diameter between 14 and 20 lm) and high performance commercial PAN-based carbon fibers (Hex￾cel) (fiber diameter between 5 and 7 lm). These carbon fibers were chemically activated using KOH and NaOH (hydroxide/carbon ratio (wt/wt) of 4/1). Chemical activa￾tions were carried out in a horizontal furnace under nitro￾gen atmosphere with a heating rate of 5 C/min up to 750 C or 825 C. Holding time at the maximum tempera￾ture was 2 or 1 h, respectively. After the heat treatment, samples were washed with 5 M HCl, vacuum filtered three times and then washed with hot distilled water (80 C) and filtered until the filtrate was free of chloride ions. The cleaned samples were dried in an oven at 120 C for 24 h. The characterization of the porosity of the ACF was done using physical adsorption of N2 at 77 K and CO2 at 273 K (Quantachrome, Autosorb-6). The samples were outgassed at 250 C under vacuum for 4 h. Nitrogen adsorption results were used to determine BET surface area values and Dubinin–Radushkevich micropore volumes (V(DR N2)). Narrow micropore volume (pore size < 0.7 nm, approximately) was obtained from CO2 adsorption data (V(DR CO2)). Table 1 contains the prepa￾ration conditions and pore texture characterization results. Scattering experiments were carried out at the micro￾focus beamline (ID13) in the European Synchrotron Radi￾ation Facility (ESRF) in Grenoble, France. The beam selected for the experiments of this study was a 0.5 lm beam produced by Kirkpatrick–Baez mirrors (wavelength k = 0.948 A˚ ). The domain of q values investigated with this setup is between 0.1 nm1 (small angle resolution of 63 nm, approximately) up to 10 nm1 . The carbon fibers were mounted in an aluminium frame, and several fibers of a given sample were analyzed to check the reproducibility of the experimental method. An area detector (MAR￾CCD) with an active diameter of 130 mm was used for the measurements. The distance from the detector to the samples was 470 mm. These lSAXS measurements were carried out scanning the fiber across its diameter with a step size of 1 lm and with an accuracy better than 0.1 lm. All measured data was corrected for background. A scheme of the experimental set-up can be found else￾where [7]. Data evaluation was done using the software package FIT2D [9]. Table 1 Preparation conditions and porous texture characterization results corre￾sponding to the raw carbon fibers and the ACF Sample Chemical activation BET surface area (m2 /g) V DR N2 (cm3 /g) V DR CO2 (cm3 /g) CF (Kureha) – 0 0 0.18 CFNa65 NaOH, 750 C, 1 h, 4/1 1085 0.42 0.34 CFK60 KOH, 750 C, 1 h, 4/1 980 0.40 0.34 Hx (Hexcel) – 0 0 0.09 HxNa39 NaOH, 750 C, 2 h, 4/1 717 0.30 0.30 HxK28 KOH, 750 C, 2 h, 4/1 727 0.31 0.29 HxK25 KOH, 825 C, 1 h, 4/1 748 0.33 0.33 1122 D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129

D. Lozano- Castello et al. Carbon 44(2006)1121-1129 1123 3. Results and discussion diameter has been done and the scattering measurements have been normalized by the total sample volume analyzed 3. 1. Chemical activation of an isotropic pitch-based by the beam. From the corrected scattering results, a useful carbon fiber(Kureha) parameter for the analysis of porous materials, Porod Invariant(PI), has been obtained. PI is defined as [10] The results contained in table i show that the raw pitch-based carbon fiber, which does not present any PI q1(q) dq dsorption of N2 at 77 K, after chemical activation with KOH or NaoH (samples CFK60 and CFNa65, respec- PI is related to the void fraction() of the material unde tively)has a quite high volume of microporosity, reaching investigation, as indicated in Eq. ( 2) respectively. To analyze the way this microporosity devel- PI=2x(4p)'%(1-o) ops in more detail, two-dimensional scattering patterns were obtained at the center of the fiber for the isotropic where p is the electronic density, (Ap)- is the contrast term itch-based CF, before and after chemical activation with and V is the sample volume. Thus, Pi gives a useful com- KOH and NaOH. Fig. I presents, as an example, the parison of how the void fraction of materials changes fol- 2D-scattering pattern corresponding to the center of the lowing the activation treatment. Fig. 2 includes the KOH ACF(sample CFK60) and the Naoh ACF(sample volume corrected PI versus the beam position, for the CFNa65). Comparing with the 2D-scattering pattern ACF prepared by NaoH and KOH activation(samples obtained for the original pitch-based CF, which was pub- CFNa65 and CFK60, respectively ). In this plot, the beam lished elsewhere [6, 7], an increase of scattering intensity position equal to zero corresponds to the center of the can be observed after activation of the original fiber, as fiber. Then, this figure can be considered as a map of the expected due to the increase of pore volume in the ACF. distribution of porosity across the fiber diameter. It is seen Similar to the results obtained by usAXs for "physically" that the PI values obtained for each sample across the fiber ACF [6, 7], the development of porosity in chemical ACF diameter are similar, indicating a high concentration of prepared from the same raw material (petroleum pitch- pores even in the inner regions of the fiber. Additionally, based carbon fiber)is isotropic, i.e. the porosity develop- the scattering profiles, as a function of the position of the ment does not present any preferential orientation along fibers, are similar for the Naoh and KOH activated mate- the fiber axis. It should be mentioned that all the two- rials. The different fiber diameter of both samples is in the dimensional scattering patterns obtained at different range of the measured Kureha carbon fiber diameter(fiber regions of the chemically ACF, across its diameter, gave diameter from 14 to 20 um) also the same isotropic scattering. A analysis This trend observed for chemically ACF is very differ- of each of those 2D scattering patterns the fiber ent to that obtained for ACf prepared by"physical Fig. I. Two-dimensional scattering patterns obtained at the center of two pitch-based ACF (scale bar I nm ): (a) CFK60: (b)CFNa65. Exposure time

3. Results and discussion 3.1. Chemical activation of an isotropic pitch-based carbon fiber (Kureha) The results contained in Table 1 show that the raw pitch-based carbon fiber, which does not present any adsorption of N2 at 77 K, after chemical activation with KOH or NaOH (samples CFK60 and CFNa65, respec￾tively) has a quite high volume of microporosity, reaching values of micropore volume of 0.40 cm3 /g and 0.42 cm3 /g, respectively. To analyze the way this microporosity devel￾ops in more detail, two-dimensional scattering patterns were obtained at the center of the fiber for the isotropic pitch-based CF, before and after chemical activation with KOH and NaOH. Fig. 1 presents, as an example, the 2D-scattering pattern corresponding to the center of the KOH ACF (sample CFK60) and the NaOH ACF (sample CFNa65). Comparing with the 2D-scattering pattern obtained for the original pitch-based CF, which was pub￾lished elsewhere [6,7], an increase of scattering intensity can be observed after activation of the original fiber, as expected due to the increase of pore volume in the ACF. Similar to the results obtained by lSAXS for ‘‘physically’’ ACF [6,7], the development of porosity in chemical ACF prepared from the same raw material (petroleum pitch￾based carbon fiber) is isotropic, i.e. the porosity develop￾ment does not present any preferential orientation along the fiber axis. It should be mentioned that all the two￾dimensional scattering patterns obtained at different regions of the chemically ACF, across its diameter, gave also the same isotropic scattering. A detailed analysis of each of those 2D scattering patterns across the fiber diameter has been done and the scattering measurements have been normalized by the total sample volume analyzed by the beam. From the corrected scattering results, a useful parameter for the analysis of porous materials, Porod Invariant (PI), has been obtained. PI is defined as [10]: PI ¼ Z q2 IðqÞdq ð1Þ PI is related to the void fraction (/) of the material under investigation, as indicated in Eq. (2), 1 V PI ¼ 2pðDqÞ 2 /ð1 /Þ ð2Þ where q is the electronic density, (Dq) 2 is the contrast term and V is the sample volume. Thus, PI gives a useful com￾parison of how the void fraction of materials changes fol￾lowing the activation treatment. Fig. 2 includes the volume corrected PI versus the beam position, for the ACF prepared by NaOH and KOH activation (samples CFNa65 and CFK60, respectively). In this plot, the beam position equal to zero corresponds to the center of the fiber. Then, this figure can be considered as a map of the distribution of porosity across the fiber diameter. It is seen that the PI values obtained for each sample across the fiber diameter are similar, indicating a high concentration of pores even in the inner regions of the fiber. Additionally, the scattering profiles, as a function of the position of the fibers, are similar for the NaOH and KOH activated mate￾rials. The different fiber diameter of both samples is in the range of the measured Kureha carbon fiber diameter (fiber diameter from 14 to 20 lm). This trend observed for chemically ACF is very differ￾ent to that obtained for ACF prepared by ‘‘physical’’ Fig. 1. Two-dimensional scattering patterns obtained at the center of two pitch-based ACF (scale bar 1 nm1 ): (a) CFK60; (b) CFNa65. Exposure time 20 s. D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129 1123

D. Lozano- Castello et al. Carbon 44(2006)1121-112 ,+ 亠CFK60 Fig. 2. Volume corrected invariant versus beam position for the chemically ACF prepared from pitch-based carbon fiber(samples CFK60 and CFNa65) The vertical lines indicate the fiber diameter( CFK60 dashed lines and CFNa65 lines ation with CO2 and steam of the same raw carbon layer-plane orientation. Because the crystals are optically where the maximum scattering corresponded to the active, regions can be identified in polished transverse fiber measurements carried out at the external zone of the fiber cross-sections where the basal planes are mainly oriented [6]. In the case of NaOH and KOH activation, the results radially, circumferentially or a mixed type with the center indicate that the activating agents penetrate inside the fiber, radial and the outer region circumferential. Depending which is a new interesting observation, which points out on the starting material and production technique, these that alkaline hydroxides penetrate better than CO2 and patterns appeared to be different, as shown in Ref. [16] the chemically ACF compared to physically ACF, could the PAN-based carbon fibers used in the present study be explained considering the important differences between point out the existence of two different regions in the the mechanisms of both activation methods In our case, PAN-based carbon fiber. A comparison with the optical potassium and sodium hydroxides are used and, after melt- micrographs presented in Ref. [16] illustrating various eg, they react with isotropic carbon fibers producing sev- types of layer-plane orientation suggested that, the PAN ral compounds and reducing cations to the based carbon fibers of the present study have a central corresponding metals, which are preserved towards oxida- region isotropic and an outer layer circumferential. The ion by the inert atmosphere. It is known that both K and existence of these two regions has been also observed by la metals can form intercalation compounds, not only TEM. Fig 3 presents the TEM image of the cross-sections with well-ordered materials, but also with disordered car- corresponding to the raw PAN-based carbon fibers embed bon materials [11-14], as it is the case of isotropic carbon ded in a resin. From this image it can be said that PAN fibers. It has been shown that this intercalation has a con- based carbon fibers consists of an external ring of 2 tribution to the development of porosity by chemical acti- and a core of 3 um diameter vation [15]and that it may be related to an increase of the The objectives of this section are two: (i) to check the graphene layers distance, which could allow a better acces- existence of two regions with different structure in PAN bility of the activating agent. The position resolved fibers(7 um diameter), taking the advantage of the use of uSAXS measurements suggest that penetration of the a very small microbeam(0.5 um diameter ); and(ii)to ana liquid-state hydroxide into the bulk of the fibers is quite lyze by saXs how the chemical activation process affects effective, developing porosity in all the regions across the to the two existing regions fiber diameter Fig 4 contains the two-dimensional scattering patterns corresponding to the PAN-based carbon fiber in the two 3. 2. Chemical activation of an anisotropic PAN-based different regions:(a) center; and(b) at the fiber edge. In carbon fiber( Hexcel) the pattern of Fig. 4a, very different characteristics can b bserved to that d in Fig. 1. It can be seen th The research carried out by Knibbs [16] using in the case of the raw PAN-based carbon fiber, anisotropic xele idea a resin matrix, illustrated various tion microscopy with PAN-based carbon scattering develops in a fan-shape perpendicular to the fiber axis. As demonstrated in the literature [17] a fan like

activation with CO2 and steam of the same raw carbon fiber, where the maximum scattering corresponded to the measurements carried out at the external zone of the fiber [6]. In the case of NaOH and KOH activation, the results indicate that the activating agents penetrate inside the fiber, which is a new interesting observation, which points out that alkaline hydroxides penetrate better than CO2 and steam. The different evolution of porosity obtained for the chemically ACF compared to physically ACF, could be explained considering the important differences between the mechanisms of both activation methods. In our case, potassium and sodium hydroxides are used and, after melt￾ing, they react with isotropic carbon fibers producing sev￾eral compounds and reducing cations to the corresponding metals, which are preserved towards oxida￾tion by the inert atmosphere. It is known that both K and Na metals can form intercalation compounds, not only with well-ordered materials, but also with disordered car￾bon materials [11–14], as it is the case of isotropic carbon fibers. It has been shown that this intercalation has a con￾tribution to the development of porosity by chemical acti￾vation [15] and that it may be related to an increase of the graphene layers distance, which could allow a better acces￾sibility of the activating agent. The position resolved lSAXS measurements suggest that penetration of the liquid-state hydroxide into the bulk of the fibers is quite effective, developing porosity in all the regions across the fiber diameter. 3.2. Chemical activation of an anisotropic PAN-based carbon fiber (Hexcel) The research carried out by Knibbs [16] using optical reflection microscopy with PAN-based carbon fibers embedded in a resin matrix, illustrated various types of layer-plane orientation. Because the crystals are optically active, regions can be identified in polished transverse fiber cross-sections where the basal planes are mainly oriented radially, circumferentially or a mixed type with the center radial and the outer region circumferential. Depending on the starting material and production technique, these patterns appeared to be different, as shown in Ref. [16]. The optical micrographs, taken in polarized light, from the PAN-based carbon fibers used in the present study point out the existence of two different regions in the PAN-based carbon fiber. A comparison with the optical micrographs presented in Ref. [16] illustrating various types of layer-plane orientation suggested that, the PAN￾based carbon fibers of the present study have a central region isotropic and an outer layer circumferential. The existence of these two regions has been also observed by TEM. Fig. 3 presents the TEM image of the cross-sections corresponding to the raw PAN-based carbon fibers embed￾ded in a resin. From this image it can be said that PAN￾based carbon fibers consists of an external ring of 2 lm and a core of 3 lm diameter. The objectives of this section are two: (i) to check the existence of two regions with different structure in PAN fibers (7 lm diameter), taking the advantage of the use of a very small microbeam (0.5 lm diameter); and (ii) to ana￾lyze by SAXS how the chemical activation process affects to the two existing regions. Fig. 4 contains the two-dimensional scattering patterns corresponding to the PAN-based carbon fiber in the two different regions: (a) center; and (b) at the fiber edge. In the pattern of Fig. 4a, very different characteristics can be observed to that presented in Fig. 1. It can be seen that, in the case of the raw PAN-based carbon fiber, anisotropic scattering develops in a fan-shape perpendicular to the fiber axis. As demonstrated in the literature [17], a fan like 0 50 100 150 200 250 300 350 400 450 -11 -9 -7 -5 -3 -1 1 3 5 7 9 11 Position (μm) Porod Invariant/volume CFK60 CFNa65 Fig. 2. Volume corrected invariant versus beam position for the chemically ACF prepared from pitch-based carbon fiber (samples CFK60 and CFNa65). The vertical lines indicate the fiber diameter (CFK60 dashed lines and CFNa65 lines). 1124 D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129

D. Lozano- Castello et al. Carbon 44(2006)1121-1129 1125 In order to check the existence of two regions with dif- ferent structure in the original PAN fiber, intensity mea surements as a function of the azimuthal angle()at constant g, were carried out in different regions across its fiber diameter. This type of measurements allows to deter- mine the degree of orientation of the pores [19]. For this analysis two different regions of the fiber have been selected: the center of the fiber and a region located at 2.5 um from the center of the fiber (the so-called"sem external "zone in this work ). In Fig. 5 the angular intensity distribution curves (normalized intensity versus azimuthal angle) are plotted for the semi-external zone and the center In addition, the angular intensity distribution curve corre- sponding to the measurement done at the fiber edge is also included for comparison purposes. The distributions have been calculated for angles from 0 to 180%and, as expected the maximum intensity corresponds to 90(equator). It can be seen that the angular intensity distribution is wider fo Fig3. TEM image of the cross-sections of PAN-based carbon fibers the center than for the semi-external zone, which confirms scale bar 2 um). the existence of a more anisotropic outer region and a higher misorientation of the pores at the center of the fiber. These results agree with the type of structure deduced from scattering along the equator is produced by a dilute system the optical micrographs [16] of microvoids with preferred orientation along the fiber The use of a very narrow microbeam(0.5 um)in the axis. This type of anisotropic scattering is observed for present study made possible to scan the very thin PAN- all the 2D patterns obtained scanning across the fiber diam- based carbon fiber(7 um in diameter) across its diameter, eter. It must be mentioned that the 2D scattering pattern obtaining information about the existing porosity. Fig. 6 obtained at the fiber edge(Fig 4b )has a very well defined contains the volume-corrected invariant versus beam posi- fan shape with a thin equatorial streak perpendicular to the tion for the raw PAN-based carbon fiber. In this plot, the axis fiber. This streak is an artifact due to refraction effects fiber dimensions and the position of the border between at the fiber edge, as indicated by Hentschel et al. [ 18]. Thus, the two regions existing in the fiber estimated by tEM the information obtained by HSAXS measurements at the(see Fig 3), have been indicated by vertical lines. The vol- fiber edges has not been considered ume corrected invariant values obtained in a region around Columns Fig. 4. Two-dimensional scattering patterns corresponding to the raw PAN-based carbon fiber(Hx) in different position across its diameter(scale bar I nm ):(a) at the center of the fiber, (b) at the fiber edge showing the streak, which is an artefact due to refraction effects

scattering along the equator is produced by a dilute system of microvoids with preferred orientation along the fiber axis. This type of anisotropic scattering is observed for all the 2D patterns obtained scanning across the fiber diam￾eter. It must be mentioned that the 2D scattering pattern obtained at the fiber edge (Fig. 4b) has a very well defined fan shape with a thin equatorial streak perpendicular to the axis fiber. This streak is an artifact due to refraction effects at the fiber edge, as indicated by Hentschel et al. [18]. Thus, the information obtained by lSAXS measurements at the fiber edges has not been considered. In order to check the existence of two regions with dif￾ferent structure in the original PAN fiber, intensity mea￾surements as a function of the azimuthal angle (u) at constant q, were carried out in different regions across its fiber diameter. This type of measurements allows to deter￾mine the degree of orientation of the pores [19]. For this analysis two different regions of the fiber have been selected: the center of the fiber and a region located at 2.5 lm from the center of the fiber (the so-called ‘‘semi￾external’’ zone in this work). In Fig. 5 the angular intensity distribution curves (normalized intensity versus azimuthal angle) are plotted for the semi-external zone and the center. In addition, the angular intensity distribution curve corre￾sponding to the measurement done at the fiber edge is also included for comparison purposes. The distributions have been calculated for angles from 0 to 180 and, as expected, the maximum intensity corresponds to 90 (equator). It can be seen that the angular intensity distribution is wider for the center than for the semi-external zone, which confirms the existence of a more anisotropic outer region and a higher misorientation of the pores at the center of the fiber. These results agree with the type of structure deduced from the optical micrographs [16]. The use of a very narrow microbeam (0.5 lm) in the present study made possible to scan the very thin PAN￾based carbon fiber (7 lm in diameter) across its diameter, obtaining information about the existing porosity. Fig. 6 contains the volume-corrected invariant versus beam posi￾tion for the raw PAN-based carbon fiber. In this plot, the fiber dimensions and the position of the border between the two regions existing in the fiber estimated by TEM (see Fig. 3), have been indicated by vertical lines. The vol￾ume corrected invariant values obtained in a region around Fig. 3. TEM image of the cross-sections of PAN-based carbon fibers embedded in a resin (scale bar 2 lm). Fig. 4. Two-dimensional scattering patterns corresponding to the raw PAN-based carbon fiber (Hx) in different position across its diameter (scale bar 1 nm1 ): (a) at the center of the fiber, (b) at the fiber edge, showing the streak, which is an artefact due to refraction effects. D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129 1125