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Graphite fibers intercalated with AsF5: A comparison of PAN and pitch-based fibers
oLw&6223/87 53.00 + .oo 0 1987 Pergamon Journals Ltd.
Carbon Vol. 2% No. 6, pp. 799-801. 1987 Printed in Gnat Britain.
GRAPHITE FIBERS INTERCALATED WITH AsFg A COMPARISON OF PAN AND PITCH-BASED FIBERS S. LUSKI, I. OHANA* and H. SELIG Institute of Chemistry and Racah Institute of Physics, * The Hebrew University of Jerusalem, Jerusalem, Israel (Received 30 January 1987; Accepted in revised form 11 June 1987) Al&act-Pitch-based (P-120) and PAN-based (HMS) fibers were intercalated with AsF,. The intercalated fibers were examined by means of weight uptake, X-ray and Raman scattering measurements. The pitch based fibers are completely intercalated to uniform stage I with d = 8.12 A. The PAN-based fibers are intercalated only on the surface layers giving a mixture of stages. Key Words-Graphite
fibers, intercalation,
arsenic pentafluoride,
X-ray diffraction, Raman spectros-
6Y.
Mahoning Co.). All experiments were done with the AsF, gas maintained at a pressure of 650 Torr. Weight uptake measurements in the AsF, atmosphere were conducted on a Sartorius 4201 Magnetic Suspension Balance designed to work with corrosive atmospheres. The standard glass reaction chamber was replaced by the Sartorius stainless steel high pressure attachment, which was suitable for work with gases such as F2, HF, AsF,, etc. The chamber walls reacted somewhat with the corrosive gases causing a slow, steady pressure decrease that was monitored with an MKS Baratron pressure gauge. The gold-plated magnet assembly and Pt cup containing the sample were inert, however. The intercalation of type A fibers reached stage I with a formula &AsF, within 20 min under the above conditions. The stoichiometry was determined from nominal weight uptake by applying a buoyancy correction resulting from the slow continuous decrease of AsF, pressure. A remarkable difference was observed in the intercalation rate of the type B fibers. After an exposure of 48 h to AsF,, the weight gain corresponded to a formula (&,&SF,. For Xray measurements, the fibers were placed in thinwalled glass capillaries that were drawn from l/4-in. Pyrex tubes, which were connected to the vacuum line with an Ultratorr Seal. The two types of fibers were exposed simultaneously to an AsF, pressure of 650 Torr. The (001) diffractions were obtained immediately after the capillaries were sealed off under an AsF, atmosphere. The fibers were perpendicular to the incident X-ray beam. The diffractogram of the intercalated A fiber is shown in Fig. 1 and indicates that a stage I compound is obtained with a d spacing similar to that of intercalated HOPG. For comparison, the (002) line in fhe diffractogram of pristine fiber A is shown in the same figure. The repeat distance, d, between graphite layers was obtained from the slope of a least squares plot of the 1values vs. 2h-‘sin 0 for the stage I compound. The
1. INTRODUCTION
Graphite fibers are normally intercalated by techniques similar to those used for the corresponding
HOPG-based GIC, although the specific intercalation conditions will differ in detail[l]. The nature and extent of the intercalation reaction depend both on the chemical reactivity of the intercalant and on the type of graphite host. Usually, well-graphitized carbon fibers undergo an efficient intercalation reaction similar to HOPG. By contrast, poorly graphitized carbon fibers [low modulus fibers, (<400 GPa)] are intercalated little if at all. In this work, we studied the intercalation of AsF, into two kinds of commercial fibers: PAN- and pitch-based fibers. These fibers differ substantially from each other in their microstructure and degree of graphitization. We have conducted weight uptake measurements, (001) Xray diffraction and Raman measurements to investigate the dependence of the intercalation reaction on the degree of graphitization. 2. EXPERIMENTAL The fibers used-pitch-based P-120X (Union Carbide Co.) and PAN-based HMS (Hysol Grafil Ltd.)will be designated as type A and B, respectively. The PAN fibers are oriented preferentially as concentric graphite planes, at least in the outer part of the fibers, whereas the pitch fibers have radial graphite planes[2]. Suitable high temperature treatment (H’IT) renders the fibers more graphitized. Before intercalation, pitch-based fibers were heated to 350°C in air to remove the polyvinyl alcohol sizing. PANbased fibers were used as received. The intercalation was done by exposing the fibers to AsF, (Ozark-
*Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 799
800
s.
LUSKI
et d.
Table 1. X-ray constants for pristine pitch 120 x and PAN HM fibers
dw,
I+
L
(4
Pitch 120x
3.37
200 + 20
PAN
3.44
60 + 10
‘Silicon powder was used as an internal reference.
, 30
I 3s
I
40
45
D,ffrod,on
Angle
1 25
- 2%
I 1s
I
20
I
IO
(degrees)
Fig. 1. (001) diffractogram of type A fiber intercalated with AsF,. Inset: (002) line of pristine A. Note the expanded abscissa.
(001) diffractogram of “intercalated” fiber B as well as the (002) line of pristine fiber B are shown in Fig. 2. The full width half maximum (FWHM) of the (002) reflections indicate that type A fibers are much more ordered than type B fibers. The crystallite size in the c direction, L,, can be obtained from the Scherer equation L, = 0.9X/B cos 0, where B is the value of FWHM. The results are given in Table 1 and show that the more ordered fibers have a smaller d spacing. After the intercalation, the FWHM of the (002) reflection increased by less than 50%. For type B fibers, the situation is completely different. A very broad asymmetric line is observed that is centered at 28 = 24.5”. This broadening is interpreted as due to a mixture of stages. The most intense lines (i.e., the (00 n + 1) for any stage n of graphite/AsFJ are located between 28 = 22” (n = 1) and 20 = 26.5” (HOPG). The samples for the Raman measurements were prepared as follows: Two bundles of fibers (one of each type) were mounted behind an optical window fitted to a Pyrex tube that was connected to the
vacuum line. The fibers were then exposed to 650 Torr AsFS for 48 h, and then the part containing the samples and optical window was sealed off under a pressure of 400 Torr AsF,. The Raman measurements were performed in a backscattering configuration. The 4880-A incident beam was cylindrically focused on the fibers. The scattered light was collected into a versatile spectrometer used as a triple monochromator[3]. The Raman spectra are shown in Fig. 3. In both spectra of the intercalated fibers, the Raman line of the C-C vibration assigned to E, symmetry is observed at 1639 cm-‘. These lines occur at the same frequency as in the HOPG/AsF, compounds. The FWHM of the type AlAsF, is 14 cm-’ and is -50% broader than that of the corresponding HOPG compound. The high frequency Raman line for type B/AsF, is also broader. It also exhibits an asymmetric line shape skewed toward the low frequency region. The low frequency line observed at 1370 cm-‘, the same as in pristine fibers, originates from off-zone-center phonons that become Raman allowed through a disorder-induced mechanism[4]. Note that in the spectrum of the type A fibers no feature is seen in this energy region. The asymmetric shape of the high frequency line is attributed to the contribution of different stages. Stage
1639
,032,
A
xQ25 I
I
1
I
1
I
35
30
25
20
15
IO
Diffraction
Angle -20
(degrees)
Fig. 2. (001) diffractogram of type B fiber “intercalated” with AsF,. Inset: (002) line of pristine B. Note the expanded abscissa.
10
_“-J
1300
1600 I700 (cm-11 Fig. 3. Raman spectra of AsF, intercalated type A and B fibers. 1400
WAVENUMBER
1500
Graphite fibers intercalated with AsFS II exhibits a Raman line at about 1625 cm-‘. Stages higher than II exhibit two Raman lines at ~1585 cm-’ and 1600 cm -I due to the C-C vibrations in the interior and bounding graphite planes, respectively. Also note that although the maximum of the line located at 1639 cm -’ arises from stage I compound, we cannot conclude that stage I dominates within the skin-depth penetration of the electromagnetic wave. The reason is that scattering from stage I is resonantly enhanced in the blue-green region[5], whereas scattering from stages higher than I is much less enhanced in this spectral region. Bearing this in mind, we can estimate that within the skin depth, stages I and II dominate and the contribution from others is less by at least a factor of 3.
3. DISCUSSION
The weight uptake, X-ray, and Raman scattering measurements indicate that the intercalation reaction goes to completion in type A fibers. This is in contrast to previous reports where only stage II or, at best, mixtures of stages I and II were obtained in the intercalation with AsF,[6]. This should not be surprising as the degree of graphitization of type A is very high. The weight uptake of AsF, intercalated PAN-based fibers shows that the intercalation is very low. This is normal for low degrees of graphitization. The question raised here is how the intercalated species are distributed over the B type fibers. There are three possibilities: 1. The fibers are intercalated homogenously from the surface to the center. 2. The intercalated species is spread nonhomogenously, but its concentration decreases monotonically from the surface to the center. 3. The intercalation takes place only in a small annulus close to the surface. In this region, the concentration changes monoto~~lly from the surface to the boundary of the region. No intercalant reaches below the surface region. We will examine these models in the light of the experimental results that are summa~zed below: 1. The stoichiometric formula is C&,&SF,. 2. The X rays that penetrate the whole fiber show a mixture of stages including areas of pristine fiber as well as a high ~nt~bution of stages II and III. 3. The Raman scattering measurements, which are sensitive to a penetration depth of 500-1000 A, in-
801
dicate that within this region there are contributions from all stages (including pristine fibers) preferentially from stages I and II. The first model is excluded, because the stoichiometry excludes the presence of significant amounts of stage I. From a combination of X ray, Raman, and weight uptake, the second model can also be excluded. Hence, it appears that the intercalation reaction takes place only in the outer region of the fiber. The order below this region is very poor, not only in terms of absence of intercalation but also in the sense that X ray measurements show no periodicity in this region. The picture of the intercalation process is as follows: The pristine fibers have two regions that differ substantially from each other by the degree of graphitization. A plot of the degree of graphitization as a function of the distance from the surface may show an edge. The sharpness and location of the edge are characteristic for each fiber. The intercalation takes place from the surface up to this edge. The mixture of stages in our case indicates that for PAN fibers, this edge is not sharp and is located 3000-5000 A from the surface. It appears that the concept of an edge between the intercalated and unintercalated regions is very general, and that it should be possible to characterize fibers by two parameters: the edge and its slope. Such parameters can be characterized by other techniques such as scanning electron microscopy. Acknowledgments-This work was supported by grant 840192 of the U.S.-Israel Binational Science Foundation (BSF), Jerusalem, Israel. We wish to thank Drs. S. L. Singer and R. Bacon of Union Carbide Co. and Dr. W. D. Carleton of Hysol Graphil Ltd. for supplying the fibers. REFERENCES
1. M. S. Dresselhaus Ed., In: Intercalationin Layered Materials, p. 461, Plenum Press, New York (19i6). 2. (a) J. L. White, C. B. Nn. M. Buechler and E. J. Watts. &ended Absti. 15th B&Gal Conf. Carbon, p. 310: Philadelphia, PA (1981). (b) P. cwizera anh M..S. Dresselhaus, ibid, p. 102. 3. I. Ohana, Y. Yacoby and M. Bezalel, Rev. Sci. inst. 57, 9 (1986). 4. M. S. Dresselhaus and G. Dresselhaus, In Topics in Aoolied Phvsics (Edited bv M. Cardona and G. Gun&odt), Vbl. 51: p. 3. Sphnger, Berlin (1982). 5. I. Ohana, Y. Yacoby and D. Schmeltzer, Phys. Rev. B. (in press). 6. I. L. Kalnin and H. A. Goldberg, Extended Abstr. 15th Biennial Co@. Carbon, p. 367. Philadelphia, PA (1981).
Carbon Vol. 2% No. 6, pp. 799-801. 1987 Printed in Gnat Britain.
GRAPHITE FIBERS INTERCALATED WITH AsFg A COMPARISON OF PAN AND PITCH-BASED FIBERS S. LUSKI, I. OHANA* and H. SELIG Institute of Chemistry and Racah Institute of Physics, * The Hebrew University of Jerusalem, Jerusalem, Israel (Received 30 January 1987; Accepted in revised form 11 June 1987) Al&act-Pitch-based (P-120) and PAN-based (HMS) fibers were intercalated with AsF,. The intercalated fibers were examined by means of weight uptake, X-ray and Raman scattering measurements. The pitch based fibers are completely intercalated to uniform stage I with d = 8.12 A. The PAN-based fibers are intercalated only on the surface layers giving a mixture of stages. Key Words-Graphite
fibers, intercalation,
arsenic pentafluoride,
X-ray diffraction, Raman spectros-
6Y.
Mahoning Co.). All experiments were done with the AsF, gas maintained at a pressure of 650 Torr. Weight uptake measurements in the AsF, atmosphere were conducted on a Sartorius 4201 Magnetic Suspension Balance designed to work with corrosive atmospheres. The standard glass reaction chamber was replaced by the Sartorius stainless steel high pressure attachment, which was suitable for work with gases such as F2, HF, AsF,, etc. The chamber walls reacted somewhat with the corrosive gases causing a slow, steady pressure decrease that was monitored with an MKS Baratron pressure gauge. The gold-plated magnet assembly and Pt cup containing the sample were inert, however. The intercalation of type A fibers reached stage I with a formula &AsF, within 20 min under the above conditions. The stoichiometry was determined from nominal weight uptake by applying a buoyancy correction resulting from the slow continuous decrease of AsF, pressure. A remarkable difference was observed in the intercalation rate of the type B fibers. After an exposure of 48 h to AsF,, the weight gain corresponded to a formula (&,&SF,. For Xray measurements, the fibers were placed in thinwalled glass capillaries that were drawn from l/4-in. Pyrex tubes, which were connected to the vacuum line with an Ultratorr Seal. The two types of fibers were exposed simultaneously to an AsF, pressure of 650 Torr. The (001) diffractions were obtained immediately after the capillaries were sealed off under an AsF, atmosphere. The fibers were perpendicular to the incident X-ray beam. The diffractogram of the intercalated A fiber is shown in Fig. 1 and indicates that a stage I compound is obtained with a d spacing similar to that of intercalated HOPG. For comparison, the (002) line in fhe diffractogram of pristine fiber A is shown in the same figure. The repeat distance, d, between graphite layers was obtained from the slope of a least squares plot of the 1values vs. 2h-‘sin 0 for the stage I compound. The
1. INTRODUCTION
Graphite fibers are normally intercalated by techniques similar to those used for the corresponding
HOPG-based GIC, although the specific intercalation conditions will differ in detail[l]. The nature and extent of the intercalation reaction depend both on the chemical reactivity of the intercalant and on the type of graphite host. Usually, well-graphitized carbon fibers undergo an efficient intercalation reaction similar to HOPG. By contrast, poorly graphitized carbon fibers [low modulus fibers, (<400 GPa)] are intercalated little if at all. In this work, we studied the intercalation of AsF, into two kinds of commercial fibers: PAN- and pitch-based fibers. These fibers differ substantially from each other in their microstructure and degree of graphitization. We have conducted weight uptake measurements, (001) Xray diffraction and Raman measurements to investigate the dependence of the intercalation reaction on the degree of graphitization. 2. EXPERIMENTAL The fibers used-pitch-based P-120X (Union Carbide Co.) and PAN-based HMS (Hysol Grafil Ltd.)will be designated as type A and B, respectively. The PAN fibers are oriented preferentially as concentric graphite planes, at least in the outer part of the fibers, whereas the pitch fibers have radial graphite planes[2]. Suitable high temperature treatment (H’IT) renders the fibers more graphitized. Before intercalation, pitch-based fibers were heated to 350°C in air to remove the polyvinyl alcohol sizing. PANbased fibers were used as received. The intercalation was done by exposing the fibers to AsF, (Ozark-
*Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. 799
800
s.
LUSKI
et d.
Table 1. X-ray constants for pristine pitch 120 x and PAN HM fibers
dw,
I+
L
(4
Pitch 120x
3.37
200 + 20
PAN
3.44
60 + 10
‘Silicon powder was used as an internal reference.
, 30
I 3s
I
40
45
D,ffrod,on
Angle
1 25
- 2%
I 1s
I
20
I
IO
(degrees)
Fig. 1. (001) diffractogram of type A fiber intercalated with AsF,. Inset: (002) line of pristine A. Note the expanded abscissa.
(001) diffractogram of “intercalated” fiber B as well as the (002) line of pristine fiber B are shown in Fig. 2. The full width half maximum (FWHM) of the (002) reflections indicate that type A fibers are much more ordered than type B fibers. The crystallite size in the c direction, L,, can be obtained from the Scherer equation L, = 0.9X/B cos 0, where B is the value of FWHM. The results are given in Table 1 and show that the more ordered fibers have a smaller d spacing. After the intercalation, the FWHM of the (002) reflection increased by less than 50%. For type B fibers, the situation is completely different. A very broad asymmetric line is observed that is centered at 28 = 24.5”. This broadening is interpreted as due to a mixture of stages. The most intense lines (i.e., the (00 n + 1) for any stage n of graphite/AsFJ are located between 28 = 22” (n = 1) and 20 = 26.5” (HOPG). The samples for the Raman measurements were prepared as follows: Two bundles of fibers (one of each type) were mounted behind an optical window fitted to a Pyrex tube that was connected to the
vacuum line. The fibers were then exposed to 650 Torr AsFS for 48 h, and then the part containing the samples and optical window was sealed off under a pressure of 400 Torr AsF,. The Raman measurements were performed in a backscattering configuration. The 4880-A incident beam was cylindrically focused on the fibers. The scattered light was collected into a versatile spectrometer used as a triple monochromator[3]. The Raman spectra are shown in Fig. 3. In both spectra of the intercalated fibers, the Raman line of the C-C vibration assigned to E, symmetry is observed at 1639 cm-‘. These lines occur at the same frequency as in the HOPG/AsF, compounds. The FWHM of the type AlAsF, is 14 cm-’ and is -50% broader than that of the corresponding HOPG compound. The high frequency Raman line for type B/AsF, is also broader. It also exhibits an asymmetric line shape skewed toward the low frequency region. The low frequency line observed at 1370 cm-‘, the same as in pristine fibers, originates from off-zone-center phonons that become Raman allowed through a disorder-induced mechanism[4]. Note that in the spectrum of the type A fibers no feature is seen in this energy region. The asymmetric shape of the high frequency line is attributed to the contribution of different stages. Stage
1639
,032,
A
xQ25 I
I
1
I
1
I
35
30
25
20
15
IO
Diffraction
Angle -20
(degrees)
Fig. 2. (001) diffractogram of type B fiber “intercalated” with AsF,. Inset: (002) line of pristine B. Note the expanded abscissa.
10
_“-J
1300
1600 I700 (cm-11 Fig. 3. Raman spectra of AsF, intercalated type A and B fibers. 1400
WAVENUMBER
1500
Graphite fibers intercalated with AsFS II exhibits a Raman line at about 1625 cm-‘. Stages higher than II exhibit two Raman lines at ~1585 cm-’ and 1600 cm -I due to the C-C vibrations in the interior and bounding graphite planes, respectively. Also note that although the maximum of the line located at 1639 cm -’ arises from stage I compound, we cannot conclude that stage I dominates within the skin-depth penetration of the electromagnetic wave. The reason is that scattering from stage I is resonantly enhanced in the blue-green region[5], whereas scattering from stages higher than I is much less enhanced in this spectral region. Bearing this in mind, we can estimate that within the skin depth, stages I and II dominate and the contribution from others is less by at least a factor of 3.
3. DISCUSSION
The weight uptake, X-ray, and Raman scattering measurements indicate that the intercalation reaction goes to completion in type A fibers. This is in contrast to previous reports where only stage II or, at best, mixtures of stages I and II were obtained in the intercalation with AsF,[6]. This should not be surprising as the degree of graphitization of type A is very high. The weight uptake of AsF, intercalated PAN-based fibers shows that the intercalation is very low. This is normal for low degrees of graphitization. The question raised here is how the intercalated species are distributed over the B type fibers. There are three possibilities: 1. The fibers are intercalated homogenously from the surface to the center. 2. The intercalated species is spread nonhomogenously, but its concentration decreases monotonically from the surface to the center. 3. The intercalation takes place only in a small annulus close to the surface. In this region, the concentration changes monoto~~lly from the surface to the boundary of the region. No intercalant reaches below the surface region. We will examine these models in the light of the experimental results that are summa~zed below: 1. The stoichiometric formula is C&,&SF,. 2. The X rays that penetrate the whole fiber show a mixture of stages including areas of pristine fiber as well as a high ~nt~bution of stages II and III. 3. The Raman scattering measurements, which are sensitive to a penetration depth of 500-1000 A, in-
801
dicate that within this region there are contributions from all stages (including pristine fibers) preferentially from stages I and II. The first model is excluded, because the stoichiometry excludes the presence of significant amounts of stage I. From a combination of X ray, Raman, and weight uptake, the second model can also be excluded. Hence, it appears that the intercalation reaction takes place only in the outer region of the fiber. The order below this region is very poor, not only in terms of absence of intercalation but also in the sense that X ray measurements show no periodicity in this region. The picture of the intercalation process is as follows: The pristine fibers have two regions that differ substantially from each other by the degree of graphitization. A plot of the degree of graphitization as a function of the distance from the surface may show an edge. The sharpness and location of the edge are characteristic for each fiber. The intercalation takes place from the surface up to this edge. The mixture of stages in our case indicates that for PAN fibers, this edge is not sharp and is located 3000-5000 A from the surface. It appears that the concept of an edge between the intercalated and unintercalated regions is very general, and that it should be possible to characterize fibers by two parameters: the edge and its slope. Such parameters can be characterized by other techniques such as scanning electron microscopy. Acknowledgments-This work was supported by grant 840192 of the U.S.-Israel Binational Science Foundation (BSF), Jerusalem, Israel. We wish to thank Drs. S. L. Singer and R. Bacon of Union Carbide Co. and Dr. W. D. Carleton of Hysol Graphil Ltd. for supplying the fibers. REFERENCES
1. M. S. Dresselhaus Ed., In: Intercalationin Layered Materials, p. 461, Plenum Press, New York (19i6). 2. (a) J. L. White, C. B. Nn. M. Buechler and E. J. Watts. &ended Absti. 15th B&Gal Conf. Carbon, p. 310: Philadelphia, PA (1981). (b) P. cwizera anh M..S. Dresselhaus, ibid, p. 102. 3. I. Ohana, Y. Yacoby and M. Bezalel, Rev. Sci. inst. 57, 9 (1986). 4. M. S. Dresselhaus and G. Dresselhaus, In Topics in Aoolied Phvsics (Edited bv M. Cardona and G. Gun&odt), Vbl. 51: p. 3. Sphnger, Berlin (1982). 5. I. Ohana, Y. Yacoby and D. Schmeltzer, Phys. Rev. B. (in press). 6. I. L. Kalnin and H. A. Goldberg, Extended Abstr. 15th Biennial Co@. Carbon, p. 367. Philadelphia, PA (1981).