《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

D. Lozano- Castello et al. Carbon 44(2006)1121-112 -esemi-external zone Azimutal angle(degrees) zone and fiber edge)of the raw PAN-based carbon fiber xcel)). The integration was done for a g value between 0.52 and 0.57 nm narp curve obtained for the measurement done at the fiber the existence of refraction effects 80 EHx Fig. 6. Volume corrected invariant versus beam position for the raw PAN-based carbon fiber. The error bars are included. The vertical lines indicate the fiber diameter and the position of the border between the two regions existing in the fiber, as estimated by tEm the center of the fiber are more or less constant. This region lower pore volume in this region compared to the fiber corresponds to a beam position from. 5 um to +1.5 um, core For a proper quantification of the volume corrected approximately. a displacement to a beam position further invariant corresponding only to the fiber core, in those from the center of the fiber gives lower values of the vol- measurements done in regions around the center of the corrected invariant This change in the volume- cor- fiber, the volume corrected invariant have been recalcu- rected invariant for the more external scans indicate the lated by subtracting the contribution of the external ring. existence of an external ring with different characteristics In order to calculate the contribution of the external ring that the fiber core. The transition from one region to to the volume corrected invariant in each of those measure another obtained by HSAXS scans agree quite well with ments, two parameters have been estimated: (i)the Porod the dimensions of the two regions estimated by TEM(see Invariant per unit volume at the external ring and; (ii) Fig 3). The lower values of volume corrected invariant the volume of external ring analyzed in those measure- obtained in the external ring indicate the existence of a ments done in regions around the center of the fiber [20]

the center of the fiber are more or less constant. This region corresponds to a beam position from 1.5 lm to +1.5 lm, approximately. A displacement to a beam position further from the center of the fiber gives lower values of the vol￾ume-corrected invariant. This change in the volume-cor￾rected invariant for the more external scans indicate the existence of an external ring with different characteristics that the fiber core. The transition from one region to another obtained by lSAXS scans agree quite well with the dimensions of the two regions estimated by TEM (see Fig. 3). The lower values of volume corrected invariant obtained in the external ring indicate the existence of a lower pore volume in this region compared to the fiber core. For a proper quantification of the volume corrected invariant corresponding only to the fiber core, in those measurements done in regions around the center of the fiber, the volume corrected invariant have been recalcu￾lated by subtracting the contribution of the external ring. In order to calculate the contribution of the external ring to the volume corrected invariant in each of those measure￾ments, two parameters have been estimated: (i) the Porod Invariant per unit volume at the external ring and; (ii) the volume of external ring analyzed in those measure￾ments done in regions around the center of the fiber [20]. 0 0.2 0.4 0.6 0.8 1 1.2 0 20 40 60 80 100 120 140 160 180 Azimutal angle (degrees) Normalized intensity center semi-external zone fiber edge Fig. 5. Normalized angular intensity distribution curves in different regions (center, semi-external zone and fiber edge) of the raw PAN-based carbon fiber (sample Hx (Hexcel)). The integration was done for a q value between 0.52 and 0.57 nm1 . The sharp curve obtained for the measurement done at the fiber edge corroborates the existence of refraction effects. 0 20 40 60 80 100 120 140 -4 -3 -2 -1 0 1 2 3 4 Position (μm) Volume corrected invariant Hx Hx core Fig. 6. Volume corrected invariant versus beam position for the raw PAN-based carbon fiber. The error bars are included. The vertical lines indicate the fiber diameter and the position of the border between the two regions existing in the fiber, as estimated by TEM. 1126 D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129

D. Lozano-Castello et al. Carbon 44(2006)1121-1129 1127 The first parameter has been estimated from the scattering tion of porosity during activation in each region, the same patterns obtained at the external ring. The second parame- type of analysis as the one done with the original PAN ter has been estimated making use of the fiber and core based carbon fiber(see Fig. 6) has been carried out for dimensions obtained by TEM(see Fig 3), the beam diam- the three chemically ACF(samples HxNa39, HxK28, and eter and using geometrical considerations. The values HxK25 obtained after this correction have been included in the Fig 8 includes the volume corrected invariant versus the plot as single points(filled symbols)(Hx I core). These beam position for the KOH and NaOH ACF For compar- results clearly show the existence of higher pore volume ison purposes, the results corresponding to the raw carbor in the more isotropic fiber core than in the anisotropic fiber(Fig. 6) are also included. The values of volume cor- external ring rected invariant obtained at the external ring are higher Once the existence and the dimensions of the two than the values corresponding to the raw fiber in the same regions with different structure in PAN-based carbon fibers region, indicating that porosity has been developed in the have been analyzed by HSAXS, the next objective is to anisotropic region of the fiber. For the samples prepared study how the chemical activation process affects to these at the same activation temperature (i.e. 750C; samples zones. Table I shows that, by chemical activation(NaOH HxNa39, HxK28) but with different activating agent or KOH) at different conditions, the raw material(Hx, (NaOH and KoH, respectively ) the porosity development Hexcel), which does not present N2 adsorption at 77K, in this region is very similar develops an important micropore volume (around An increase of porosity is also observed in the e measure- 0.30 cm /g, respectively ). The increase of porosity after ments carried out around the center of the fiber for these activation is also deduced from the increase in intensity samples, which indicates that the activating agents(KOH of the 2D-scattering patterns presented in Fig. 7, corre- and NaoH) are reaching the center of the fibers and poros- sponding to samples HxNa39 and HxK28, which have ity is also developed in the more isotropic region. The been prepared using the same activation conditions but dif- increase of volume corrected invariant in this region is ferent activating agents(NaOH and KOH, respectively ). It higher for the Naoh ACf (sample HxNa39) than for an be seen that, after activation, the equatorial fan-shaped the KOh ACF (sample HxK 28 ). This different behaviour catering remains in both samples, indicating that pores of both activating agents in the fiber core is more clearly created during activation follow the preferential orienta- seen after substracting the contribution of the external ring tion of the microvoids along the fiber axis existing in the(see single points corresponding to core). The comparison raw PAN-based carbon fiber. The orientation of the graph- of the core values obtained for the chemically ACF ene layers along the fiber axis, and the fact that the porosity (HxNa39 core and HxK28 core) with those obtained for generation occurs via removal of these layers, permits us to the raw fiber(Hx core) corroborates that the development understand the anisotropy of the porosity generated of porosity in the core with NaoH is much higher than In order to confirm the existence of the two regions in with KOH. These results indicate that, although both acti- the fiber after chemical activation and to analyze the evolu- vating agents reach the fiber core, the most disordered Intensity Fig. 7. Two-dimensional scattering patterns obtained at the center of two PAN-based ACF (scale bar :(a)HxNa39: (b)HxK28

The first parameter has been estimated from the scattering patterns obtained at the external ring. The second parame￾ter has been estimated making use of the fiber and core dimensions obtained by TEM (see Fig. 3), the beam diam￾eter and using geometrical considerations. The values obtained after this correction have been included in the plot as single points (filled symbols) (Hx 1 core). These results clearly show the existence of higher pore volume in the more isotropic fiber core than in the anisotropic external ring. Once the existence and the dimensions of the two regions with different structure in PAN-based carbon fibers have been analyzed by lSAXS, the next objective is to study how the chemical activation process affects to these zones. Table 1 shows that, by chemical activation (NaOH or KOH) at different conditions, the raw material (Hx, Hexcel), which does not present N2 adsorption at 77 K, develops an important micropore volume (around 0.30 cm3 /g, respectively). The increase of porosity after activation is also deduced from the increase in intensity of the 2D-scattering patterns presented in Fig. 7, corre￾sponding to samples HxNa39 and HxK28, which have been prepared using the same activation conditions but dif￾ferent activating agents (NaOH and KOH, respectively). It can be seen that, after activation, the equatorial fan-shaped scattering remains in both samples, indicating that pores created during activation follow the preferential orienta￾tion of the microvoids along the fiber axis existing in the raw PAN-based carbon fiber. The orientation of the graph￾ene layers along the fiber axis, and the fact that the porosity generation occurs via removal of these layers, permits us to understand the anisotropy of the porosity generated. In order to confirm the existence of the two regions in the fiber after chemical activation and to analyze the evolu￾tion of porosity during activation in each region, the same type of analysis as the one done with the original PAN￾based carbon fiber (see Fig. 6) has been carried out for the three chemically ACF (samples HxNa39, HxK28, and HxK25). Fig. 8 includes the volume corrected invariant versus the beam position for the KOH and NaOH ACF. For compar￾ison purposes, the results corresponding to the raw carbon fiber (Fig. 6) are also included. The values of volume cor￾rected invariant obtained at the external ring are higher than the values corresponding to the raw fiber in the same region, indicating that porosity has been developed in the anisotropic region of the fiber. For the samples prepared at the same activation temperature (i.e. 750 C; samples HxNa39, HxK28) but with different activating agent (NaOH and KOH, respectively), the porosity development in this region is very similar. An increase of porosity is also observed in the measure￾ments carried out around the center of the fiber for these samples, which indicates that the activating agents (KOH and NaOH) are reaching the center of the fibers and poros￾ity is also developed in the more isotropic region. The increase of volume corrected invariant in this region is higher for the NaOH ACF (sample HxNa39) than for the KOH ACF (sample HxK28). This different behaviour of both activating agents in the fiber core is more clearly seen after substracting the contribution of the external ring (see single points corresponding to core). The comparison of the core values obtained for the chemically ACF (HxNa39 core and HxK28 core) with those obtained for the raw fiber (Hx core) corroborates that the development of porosity in the core with NaOH is much higher than with KOH. These results indicate that, although both acti￾vating agents reach the fiber core, the most disordered Fig. 7. Two-dimensional scattering patterns obtained at the center of two PAN-based ACF (scale bar 1 nm1 ): (a) HxNa39; (b) HxK28. D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129 1127

D. Lozano- Castello et al. Carbon 44(2006)1121-112 ■ -e- HxNa39 -AHXK28 ▲HxK28 -e-HxK25 ●HxK25core Position(um) Fig 8. Volume corrected invariant versus beam position for the chemically ACF (samples HxNa39, HxK28 and HxK25, including the error bars. The results corresponding to the raw fiber are also included region of the fiber is activated easier by naoh than KoH, show that depending on the precursor the chemical activa which agrees with a previous work carried out with carbon tion process produces isotropic or anisotropic development nanotubes [15]. At this activation temperature(750C), PI of porosity. It was observed that chemically ACf prepared in the bulk is higher than in the external zones, indicating a from isotropic pitch-based carbon fibers present an isotro- higher activation in the more isotropic fiber core pic development of the porosity. The results obtained from In the case of the sample HxK25(sample prepared at the scans across the fiber diameter indicate that chemical 825C using KOH), the values of volume corrected invari- activation(KOH and NaoH) of an isotropic carbon fiber ant obtained at the external ring are higher than those develops a high micropore volume not only in the external obtained with the samples prepared at lower activation region of the fiber, but also in the core, which is opposite to temperature(HxK28 and HxNa39), which indicates that the results obtained for"physically" ACF in a previous a higher activation temperature produces a higher develop- work, where the maximum porosity development was ment of porosity in the anisotropic region. In order to see the external zone [6]. the effect of the temperature on the activation of the fiber In the case of anisotropic PAN-based carbon fibers the core, the core values(HxK25 core)have been calculated existence of two regions with different structure was as explained earlier for other samples. It can be observed detected by uSAXS measurements across the fiber diame that these values are very similar or slightly lower than ter. It was seen that the raw Pan fiber has an anisotropic the core values for the raw fiber(Hx core). This fact can outer ring, with microvoids oriented along the fiber axis, be understood if we take into account the nature of the and a center with less oriented porosity. The use of a very PI parameter. This parameter is more sensitive to small narrow microbeam(0. 5 um) has also allowed us to detect porosity(high values of scattering vector, q) than larger the transition between both regions and make an estima porosity (low values of scattering vector, q). Therefore, tion of the dimensions of the external ring(2 um, approx the fact that the volume corrected invariant values for imately) and the core(3 um diameter, approximately) the HxK25 core are very similar or slightly lower than which agrees with the dimensions obtained by tEM. The those for the raw fiber(Hx core), could be explained saying analysis of the 2D scattering patterns have shown that that the core experiences greater activation to the point of these two regions remain after chemical activation with pore wall collapse of some of the smaller pores in this KOH or NaoH, and that porosity is developed in both region, while the fiber as a whole developed more small regions, indicating that the activating agents(KOH and porosity in the periphery, so that the overall micropore vol- NaOH) are reaching the center of the fibers. The porosity ume increased developed in the external anisotropic region of the fiber is very similar for both activating agents. However, the devel 4. Conclusions opment of porosity in the fiber core with NaoH is much Information on the chemical activation process from the although both activating agents reach the fiber core, the oint of view of the porosity development was deduced most disordered region of the fiber is activated easier by from the two-dimensional scattering patterns. The results NaoH than KOH

region of the fiber is activated easier by NaOH than KOH, which agrees with a previous work carried out with carbon nanotubes [15]. At this activation temperature (750 C), PI in the bulk is higher than in the external zones, indicating a higher activation in the more isotropic fiber core. In the case of the sample HxK25 (sample prepared at 825 C using KOH), the values of volume corrected invari￾ant obtained at the external ring are higher than those obtained with the samples prepared at lower activation temperature (HxK28 and HxNa39), which indicates that a higher activation temperature produces a higher develop￾ment of porosity in the anisotropic region. In order to see the effect of the temperature on the activation of the fiber core, the core values (HxK25 core) have been calculated as explained earlier for other samples. It can be observed that these values are very similar or slightly lower than the core values for the raw fiber (Hx core). This fact can be understood if we take into account the nature of the PI parameter. This parameter is more sensitive to small porosity (high values of scattering vector, q) than larger porosity (low values of scattering vector, q). Therefore, the fact that the volume corrected invariant values for the HxK25 core are very similar or slightly lower than those for the raw fiber (Hx core), could be explained saying that the core experiences greater activation to the point of pore wall collapse of some of the smaller pores in this region, while the fiber as a whole developed more small porosity in the periphery, so that the overall micropore vol￾ume increased. 4. Conclusions Information on the chemical activation process from the point of view of the porosity development was deduced from the two-dimensional scattering patterns. The results show that depending on the precursor the chemical activa￾tion process produces isotropic or anisotropic development of porosity. It was observed that chemically ACF prepared from isotropic pitch-based carbon fibers present an isotro￾pic development of the porosity. The results obtained from the scans across the fiber diameter indicate that chemical activation (KOH and NaOH) of an isotropic carbon fiber develops a high micropore volume not only in the external region of the fiber, but also in the core, which is opposite to the results obtained for ‘‘physically’’ ACF in a previous work, where the maximum porosity development was in the external zone [6]. 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 diame￾ter. It was seen that the raw PAN fiber has an anisotropic outer ring, with microvoids oriented along the fiber axis, and a center with less oriented porosity. The use of a very narrow microbeam (0.5 lm) has also allowed us to detect the transition between both regions and make an estima￾tion of the dimensions of the external ring (2 lm, approx￾imately) and the core (3 lm diameter, approximately), which agrees with the dimensions obtained by TEM. The analysis of the 2D scattering patterns have shown that these two regions remain after chemical activation with KOH or NaOH, and that porosity is developed in both regions, indicating that the activating agents (KOH and NaOH) are reaching the center of the fibers. The porosity developed in the external anisotropic region of the fiber is very similar for both activating agents. However, the devel￾opment of porosity in the fiber core with NaOH is much higher than with KOH. These results indicate that, although both activating agents reach the fiber core, the most disordered region of the fiber is activated easier by NaOH than KOH. 0 20 40 60 80 100 120 140 160 180 -4 -3 -2 -1 0 1 2 3 4 Position (μm) Volume corrected invariant Hx Hx core HxNa39 HxNa39 core HxK28 HxK28 core HxK25 HxK25 core Fig. 8. Volume corrected invariant versus beam position for the chemically ACF (samples HxNa39, HxK28 and HxK25), including the error bars. The results corresponding to the raw fiber are also included. 1128 D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129

D. Lozano- Castello et al. Carbon 44(2006)1121-1129 1129 Acknowledgments fiber by microbeam small-angle X-ray scattering. Macromolecules 1998:31:3953-7 The authors thank ESRF(Experiment number ME-366) 9 Hammersley AP ESRF Internal Report, ESRF97HAO2T, FIT2D: an introduction and overview. 1997 and MCYT (Project PPQ2003-03884)for financial support. (10) Guinier A, Fournet G. Walker CB. Small angle scattering of J.A. M.-A. thanks universidad de Alicante for Thesis X-rays. New York: Wiley: 1955, p 5-78 grant. Authors would also like to thank Achim Klein- [Il] Berger D, Carton B, Metrot A, Herold A Interactions of potassium Hoffmann for the TEM image Chemistry and physics of carbon, vol. 12. New York: Dekker: 1975 [12] Marsh H, Murdie N, Edwards IAS, Boehm HP. Interactions of References carbons, cokes, and graphites with potassium and sodium. In: Thrower PA, editor. Chemistry and physics of carbon, vol. [1 Bonnet JB, Wang TK, Peng JCM. Carbon fibers. New York: 20. New York: Dekker: 1987. p. 213-72. Dekker;1998,p.524-33 [13] Herinckx C, Perret R, Ruland w. Interstitial com 2 Macia-Agullo JA. Moore BC, Cazorla-Amoros D, Linares-solano A sium with carbon fibers. Carbon 1972- 10: 711-22 Activation of coal tar pitch carbon fibres: physical activation vS. [14] Macia-AgulIo JA, Moore BC. Cazorla-Amoros D, Linares-Solano A. hemical activation. Carbon 2004; 42(7): 1367-70 Chemical activation by KOH and NaOH of carbon materials with 3] Alcaniz. Monge J, Cazorla-Amoros D, Linares-Solano A, Yoshida S, ifferent crystallinity. Extended abstracts, Carbon 2003, Oviedo, OyaA. Carbon1994:32(7):1277-83 4 Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, Hall PJ, [15] Raymundo-Pinero E, Azais P, Cacciaguerra T, Cazorla-Amoros D, Fernandez jJ. Characterization of activated carbon fibers by positron Linares-Solano A. Beguin F. KOH and NaOH activation mecha- 2000: 128. ,time spectroscopy (PALS. Stud Surf Sci Catal nihilatio nisms of multiwalled carbon nanotubes with different structural organisation. Carbon 2005: 43(4): 786-95 [] Cazorla-Amoros D, Salinas-Martinez de Lecea C, Alcaniz.Monge [16] Reynolds WN. Structure and physical properties of carbon fibers. In: Gardner M. North A. Dore J Characterization of activated carb Walker Jr PL, Thrower PA, editors. Chemistry and physics of carbon, fibers by small angle X-ray scattering. Carbon 1998: 36(3): 309-12 voL. ll. New York: Dekker: 1973. P. 1-6 [6 Lozano-Castello D, Raymundo-Piniero E, Cazorla-Amoros D [7] Ruland w. X-ray determination of crystallinity and diffuse disorder Linares-Solano A, Muller M, Riekel C. Characterization of pore scattering. Acta Cryst 1961; 14: 1180-5 distribution in activated carbon fibers by microbeam small angle [18] Hentschel MP, Hosemann R, Lange A, Uther B, Bruckner R. X-ray -ray scattering Carbon 2002: 40: 2727-35 small angle scattering from metallic wires, glass fibers, and hard [7 Lozano- Castello D, Raymundo-Pinero E, Cazorla-Amoros D, elastic polypropylene. Acta Crystallogr A 1987: 43(4): 506-13 Linares.s C. Microbeam small angle [19] Gupta A, Harrison IR, Lahijani J. Small-angle X-ray scattering in carbon fibers. J Appl Cryst 1994: 27: 627-3 on fibers. Stud ci Cata2002;144:51-8. [20]Moller M, Riekel C, Vuong R, Chanzy H. Skin/core micro-structure [8 Muller M, Czihak C, Vogl G, Schober H. Riekel C. Direct n viscose rayon fibers analysed by X-ray microbeam and electron observation of microfibril arrangement in a single native cellulose diffraction mapping. Polymer 2000: 41: 2627-32

Acknowledgments The authors thank ESRF (Experiment number ME-366) and MCYT (Project PPQ2003-03884) for financial support. J.A.M.-A. thanks Universidad de Alicante for Thesis grant. Authors would also like to thank Achim Klein– Hoffmann for the TEM image. References [1] Donnet JB, Wang TK, Peng JCM. Carbon fibers. New York: Dekker; 1998, p. 524–33. [2] Macia´-Agullo´ JA, Moore BC, Cazorla-Amoro´s D, Linares-Solano A. Activation of coal tar pitch carbon fibres: physical activation vs. chemical activation. Carbon 2004;42(7):1367–70. [3] Alcan˜iz-Monge J, Cazorla-Amoro´s D, Linares-Solano A, Yoshida S, Oya A. Carbon 1994;32(7):1277–83. [4] Lozano-Castello´ D, Cazorla-Amoro´s D, Linares-Solano A, Hall PJ, Fernandez JJ. Characterization of activated carbon fibers by positron annihilation lifetime spectroscopy (PALS. Stud Surf Sci Catal 2000;128:523–32. [5] Cazorla-Amoro´s D, Salinas-Martı´nez de Lecea C, Alcan˜iz-Monge J, Gardner M, North A, Dore J. Characterization of activated carbon fibers by small angle X-ray scattering. Carbon 1998;36(3):309–12. [6] Lozano-Castello´ D, Raymundo-Pin˜ero E, Cazorla-Amoro´s D, Linares-Solano A, Mu¨ller M, Riekel C. Characterization of pore distribution in activated carbon fibers by microbeam small angle X-ray scattering. Carbon 2002;40:2727–35. [7] Lozano-Castello´ D, Raymundo-Pin˜ero E, Cazorla-Amoro´s D, Linares-Solano A, Mu¨ller M, Riekel C. Microbeam small angle X-ray scattering (lSAXS): a novel technique for the characterization of activated carbon fibers. Stud Surf Sci Catal 2002;144:51–8. [8] Mu¨ller M, Czihak C, Vogl G, Fratzl P, Schober H, Riekel C. Direct observation of microfibril arrangement in a single native cellulose fiber by microbeam small-angle X-ray scattering. Macromolecules 1998;31:3953–7. [9] Hammersley AP. ESRF Internal Report, ESRF97HA02T, FIT2D: an introduction and overview, 1997. [10] Guinier A, Fournet G, Walker CB. Small angle scattering of X-rays. New York: Wiley; 1955, p. 5–78. [11] Berger D, Carton B, Me´trot A, He´rold A. Interactions of potassium and sodium with carbons. In: Walker Jr PL, Thrower PA, editors. Chemistry and physics of carbon, vol. 12. New York: Dekker; 1975. p. 1–37. [12] Marsh H, Murdie N, Edwards IAS, Boehm HP. Interactions of carbons, cokes, and graphites with potassium and sodium. In: Thrower PA, editor. Chemistry and physics of carbon, vol. 20. New York: Dekker; 1987. p. 213–72. [13] He´rinckx C, Perret R, Ruland W. Interstitial compounds of potas￾sium with carbon fibers. Carbon 1972;10:711–22. [14] Macia´-Agullo´ JA, Moore BC, Cazorla-Amoro´s D, Linares-Solano A. Chemical activation by KOH and NaOH of carbon materials with different crystallinity. Extended abstracts, Carbon 2003, Oviedo, Spain, 2003. [15] Raymundo-Pin˜ero E, Azaı¨s P, Cacciaguerra T, Cazorla-Amoro´s D, Linares-Solano A, Be´guin F. KOH and NaOH activation mecha￾nisms of multiwalled carbon nanotubes with different structural organisation. Carbon 2005;43(4):786–95. [16] Reynolds WN. Structure and physical properties of carbon fibers. In: Walker Jr PL, Thrower PA, editors. Chemistry and physics of carbon, vol. 11. New York: Dekker; 1973. p. 1–67. [17] Ruland W. X-ray determination of crystallinity and diffuse disorder scattering. Acta Cryst 1961;14:1180–5. [18] Hentschel MP, Hosemann R, Lange A, Uther B, Bru¨ckner R. X-ray small angle scattering from metallic wires, glass fibers, and hard elastic polypropylene. Acta Crystallogr A 1987;43(4):506–13. [19] Gupta A, Harrison IR, Lahijani J. Small-angle X-ray scattering in carbon fibers. J Appl Cryst 1994;27:627–36. [20] Mu¨ller M, Riekel C, Vuong R, Chanzy H. Skin/core micro-structure in viscose rayon fibers analysed by X-ray microbeam and electron diffraction mapping. Polymer 2000;41:2627–32. D. Lozano-Castello´ et al. / Carbon 44 (2006) 1121–1129 1129