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High electrical conductivity in graphite intercalated with acid fluorides
Materials Science and Engineering, 31 ( 1 9 7 7 ) 261 - 265
261
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
High Electrical Conductivity in Graphite Intercalated with Acid Fluorides
F. L. V O G E L , G. M. T. F O L E Y , C. Z E L L E R , E. R. F A L A R D E A U a n d J. G A N
Department of Electrical Engineering and Science and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (U.S.A.)
SUMMARY
Unusually high room temperature electrical conductivity, higher than that of pure copper, has been observed under certain conditions in graphite intercalated with the strong acid fluorides, antimony pentafluoride and arsenic pentafluoride. Since these results are of considerable scientific and technological importance, and likely to affect the course of research in this area, the confirming experiments are reviewed here. The first experiment demonstrating the potential of these c o m p o u n d s was measurements on a composite wire consisting of an SbF 5 intercalated graphite powder core with a copper sheath [1]. Results were obtained by a d.c. four point resistance method on samples 1 mm dia. × 10 cm long. Comparison of this composite wire with a control sample of copper demonstrated convincingly the superior conductivity of the intercalated graphite core. More recent experiments of a similar nature have confirmed those early findings. We have also made measurements of highly oriented polycrystalline graphite (HOPG) intercalated with AsF 5 [2]. In careful measurements a contactless r.f. (100 kHz) induction technique has substantiated the high electrical conductivity of these materials. While the measured conductivity of the AsF5 compounds has a maximum value only marginally greater than pure copper, this value must be regarded as conservative and any correction made to the r.f. measurements for sample imperfections, etc., would tend to increase the calculated conductivity above the quoted value. These results lend support to the notion that very high electrical conductivities are possible in the acceptor c o m p o u n d s of
intercalated graphite and that the intercalated acid is responsible for a marked increase in the density of charge carriers while the mobility remains high.
RESUME
Une haute conductivit~ ~lectrique ~ temperature ambiante, sup~rieure ~ celle du cuivre put, a ~t~ observ~e pour certains compos~s d'insertion du graphite avec des acides fluor~s forts, les pentafluorides d'antimoine et d'arsenic. Comme ces r~sultats sont d'une importance considerable dans les domaines scientifique et technologique et sont susceptibles de modifier le cours des recherches entreprises, ces experiences prometteuses sont d~crites ci-dessous. Les premieres experiences r~v~lant l'importance de ces compos~s consistaient en s~rie de mesures sur des fils composites r~alis~s partir d'un coeur de poudre de graphite intercal~ avec du SbF5 dans une gaine de cuivre. Les r~sultats ont ~t~ obtenus par la m~thode quatre points de mesure de r~sistance en courant continu sur des ~chantillons de 1 mm de diam~tre par 10 cm de long. La comparaison de ces fils composites avec un ~chantillon de contrSle en cuivre a prouv~ indiscutablement que le coeur de graphite ins~r~ avait une conductivit~ sup~rieure. Des experiences r~centes de m~me nature [3] ont confirm~ ces premiers r~sultats. Nous avons fait aussi des mesures sur du graphite polycristaUin hautement orient~ ins~r~ avec de I'AsF5 [2]. Des mesures soigneuses avec une m~thode d'induction sans contact ont confirm~ la haute conductivit~ ~lectrique de ces mat~riaux. Tandis que la conductivit~ mesur~e pour les compos~s d'AsF5 a une valeur maximale seulement un
262 peu plus grande que celle du cuivre, cette valeur peut ~tre consid~r~e comme minimale car toute amelioration faite dans le cadre de la m~thode par induction pour corriger les imperfections des ~chantillons, ...etc., ne conduirait qu'~ accroi'tre la valeur de la conductivit~ calcul~e. Ces r~sultats accr~ditent l'id~e que de tr~s hautes conductivit~s ~lectriques sont possibles dans les compos~s de graphite ins~r~s accepteurs d'~lectrons et que l'acide est responsable de l'accroissement notable de la densit~ des porteurs de charges tandis que la mobilit~ reste ~ une valeur haute.
INTRODUCTION The observation of electrical conductivity higher than that of copper in a graphite intercalation c o m p o u n d [1] is a matter of considerable scientific and technological interest. Since the original results on high conductivity were reported there have been contradictory results [2, 3]. These differences have now been resolved by the understanding of the importance of the anisotropy of conductivity in graphite which has been intercalated with a strong acid [4]. This point is well explained in the preceeding paper of this volume. Thus, it seemed worthwhile to review the evidence for unusually high electrical conductivity in graphite intercalated with antimony pentafluoride and measured by a four point d.c. method on composite wires [1], and on highly oriented pyrolytic graphite (HOPG) intercalated with arsenic pentafluoride and measured by the r.f. induction method [4] which is described in the preceeding paper of this volume.
EXPERIMENTAL RESULTS In the composite wire experiments, graphite powder, having a 10 pm dia. particle size was vacuum annealed overnight and loaded into a cylindrical copper ampoule having a 6.4 mm o.d. and a 3.8 mm i.d. The ampoules were either phosphorus deoxidized copper (p = 2.1 × 10 -6 ~2 cm) or high conductivity deoxidized copper (p = 1.7 X 10 -6 ~2 cm). Then, a weighed amount of SbF 5 was introduced into the ampoule, under the
protective cover of a N 2 glove box, in sufficient quantity to produce a stage three compound. The ampoule was tightly sealed with a copper end plug and then heated overnight at 150 °C to form the c o m p o u n d in a method similar to that described by Lalancette [5]. The tubes containing the c o m p o u n d were then cold swaged down to 2 mm or 1 mm dia. and annealed at 500 °C. As a control on the experiment, identical copper ampoules were filled with AMAX LO copper powder, which powder, when reduced to ideal density of 8.95 g/cm 3 results in a material having ideal copper resistivity of 1.7 X 10 -6 gt cm. Alternatively, a solid rod of OFHC copper was subjected to the same swaging and annealing process as a control. It became apparent that the powders in the core of the composite wires did not reach ideal density after the swaging and annealing treatment given and that a density correction would have to be made. This was done for the c o m p o u n d core wires by chemically analyzing the copper in weighed segments and back calculating to the diameter and mass of the inside. Direct current resistivity measurements of the composite wires were made using a four point method. A jig was constructed having outer or current contacts 15 cm apart and inner or voltage contacts 10.1 + 0.05 cm apart. These four contacts were connected directly to the four terminals of a Shallcross Milliohmeter which read the resistance directly. These determinations were cross checked by separately measuring the current from a d.c. supply and the voltage drop at the inner contacts. Also, the measurements were checked by using alternating current. In all cases, satisfactory agreement was found compared with copper wire of known resistivity. The results from this series of experiments are depicted in Table 1. The remarkable result of a core conductivity higher than that of copper (conductivity of copper Ocu = 5.9 X 105 gt - i cm - i ) requires some comment. It is interesting that with these separate experiments, done at different times, and with probable variations in technique, orientation, compositions, etc., the final result shown on the b o t t o m line of the Table comes o u t the same: o = 1 X 10 -6 ~ cm -+ 10%. Particularly convincing is
263 TABLE 1 Conduetivities of antimony pentafluoride intercalated graphite in a copper sheath
Composite conductance ~ - 1 Composite conductivity ~ - 1 cm-1 Apparent core density % of ideal Conductivity of sheath ~-~-1 cm-1 Conductivity of core at ideal d e n s i t y ~ - I cm-1
Phosphorus deoxidized copper sheath
Conductivity grade copper sheath
Control sample
Exptl. sample
Control sample
Exptl. sample
Control sample
Exptl. sample
4.7 x 102
4.5 x 102
4,9
x 202
2.0 x 103
2.2 × 10-3
4.5x 105
5.6x105
5.5x 105
5.95×105
5.8×105
6.5x105
75
85
73
80
(100)
85
4.8 x 105
4.8 x 105
5.9 x 105
5.9
5.8 × 105
5.8 x 105
5.9 × 105
1.1 X 106
5.9 × 105
1.0 × 106 (5.8× 105 ) .
3.8 X 102
the wire represented in the last column which exhibits in the composite state a conductivity higher than that of copper, as in terms of resistivity PCg_SbFs = 1.54 ~ 2 cm compared with copper at Pcu = 1.7 p ~ cm. Further confidence in these measurements derives from the realization that any conceivable errors from non-uniformity, contact resistance, imperfect crystal alignment, and others, would all have the effect of making the true electrical conductivity higher than the measured one. It is also significant that a comparison made along the top line of Table 1 reveals that each experimental sample showed a conductance larger than its respective control of the same geometry without correction for density, orientation, or conductivity of the sheath. For the intercalated crystal experiments, highly oriented pyrolytic graphite (HOPG) donated by Dr. A. Moore of Union Carbide was intercalated with AsF 5 using standard high vacuum and inert atmosphere techniques. The graphite was cut by air abrasion into ~ 5 × 5 mm basal plane squares and cleaved to the desired c-axis starting thickness. Initial dimensions, weight, and conductivity of each unintercalated HOPG piece were obtained. Arsenic pentafluoride (Ozark Mahoning) was generally purified by trap to trap distillation prior to use. This distillation essentially proved to be a precaution, as no differences in either vapor density of, or the intercalation reaction results were evident with undistilled materials, suggesting the AsF5 to be suitable as purchased.
x 105
1.0X 10 -6
As the previous paper has discussed, precise measurement of the basal plane conductivity, oa, in highly anisotropic c o m p o u n d s can only be satisfactorily accomplished by means of an r.f. induction technique. The technique has permitted both in situ measurements of conductivity (i.e., measurements during the intercalation reaction) and measurements on samples of fixed stage sealed off under an atmosphere of dry nitrogen. The AsF5-graphite c o m p o u n d s of fixed stage were prepared by vapor intercalation of HOPG with AsF 5 in a manner similar to that previously reported [7]. The intercalated products were transferred under dry nitrogen to 2 X 6 mm i.d. rectangular tubes and sealed. The materials were characterized by their X-ray (00l) reflections, c-axis thickness increase, and gravimetric analysis. These data correspond to those previously reported, and for stages 1 - 3 gave materials of stoichiometry C8,AsF5 where n is the stage. The in situ reactions were carried o u t in sealed pyrex tubes consisting of 8 mm o.d. round tubing which is suitable for viewing the c-axis thickness change, and 2 × 6 mm i.d. rectangular tubing which is convenient for the r.f. measurements. The AsF5 was allowed to vaporize completely, with care taken to avoid contact of the graphite with liquid AsF 5 during the warming process. The c-axis thickness and the r.f. signal were monitored with time b y shifting the HOPG from the round to the rectangular portion of the tube reactor. Figure 1 shows in situ data, AT (change in period of oscillation of the r.f. circuit) vs.
264 i
I • AT
I
~1
I --
1
r •
• •
0
oo
120 •JI8l
t
o/'._! Io
l.6 14
e• ~e AT
N •••• w•l •• • I••
o I0
•*g
(arbunlt}
eme
oo o o°
o o
t
06
STAGE 2
0.4
o
:o°°i
8o°°~ STAGE 5 STAGE 4 __ , ] I 2
oo
O8
aL to
0.2 I_ 5
I 4
REACTION
J 5 TIME
I 7
/~ s
0.0
(hours)
Fig. 1. Changes of resistivity with time while intercalating with AsF 5.
65
I --
60
Ag
q
F
o
o"
•
g o
5.5-
~o x
45
CONCLUSIONS
-
o
~"Cu
5.0--
-
o
_--Au
o" Csn As F5
4.0 -AI
_~
o IN SITU • UNDER N 2
•
I
&5 026~_
HOPG I I
2
3
oo
STAGE n
Fig. 2. Resistivity graphite.
v s.
conductivity oa = (6.3 + 0.7) × 105 (~2 cm) -z at stage 2 where the quoted error is the experimental uncertainty. This value is probably conservative however, since corrections for sample imperfections of any kind all tend to increase the calculated conductivity. The observed peak in a-axis conductivity at a concentration lower than stage I is a feature c o m m o n to many intercalation compounds, e.g., those with H2SO 4 [9], Br 2 [9, 10], and the alkali metal compounds [11].
stage for AsF 5 intercalated
time for an AsFs-graphite reaction. The intercalation process was found to be "well behaved" showing plateaux where little change in signal is observed over a finite time period. This is the so called "staging" phenomenon, plateaux being identifiable with a particular stage, n. A corresponding plot of c-axis thickness change, At~to, versus time shows plateaux at similar locations. The end of a plateau in At/to represents completion of a uniform stage of maximum stoichiometry as confirmed by X-ray and gravimetric analysis. This has greatly facilitated the preparation of compounds of a unique stage. Shown in Fig. 2 is a plot of oa vs. stage from r.f. data for both in situ samples and those under dry nitrogen. The scatter in the data is relatively small and we attribute much of it to differences in sample quality rather than experimental error. On this basis, we conclude that AsFs-graphite has a peak a-axis
The work described in this paper demonstrates convincingly the promising aspects of graphite intercalation compounds as practical synthetic metals. Understanding of the interactions which lead to the greatly enhanced basal plane conductivities remains poorly understood and awaits more detailed experimental studies. It is far from evident, for example, that an unusually large increase in carrier density is responsible for the remarkable conductivity. Rather, the evidence available to date [12] suggests that a relatively smaller decrease in carrier mobility over the parent graphite may be the principal contributor. Thus, there may be a need for reexamination of ideas which suggest that intercalation with increasingly electronegative species should lead to large changes in free carrier density and so to greatly enhanced conductivities. It should be viewed therefore, as most encouraging that despite our lack of real understanding of the mechanisms involved, compounds with conductivities rivalling those of any naturally occurring metal have been synthesized. ACKNOWLEDGEMENTS
The authors express their appreciation to Dr. Arthur Moore of Union Carbide for his generous donation of HOPG. Throughout most of this work G. M. T. Foley was supported under NSF Grant DMR75-04954 and MRL/DMR76-00678. E. R. Falardeau was supported by M R L / D M R 7 6 - 0 0 6 7 8 and Air Force Materials Lab Contract F33625-75-C5231 and J. Gan by MERADCOM, Ft. Belvoir under Contract DAAG53-76C-0061.
265 REFERENCES 1 F. L. Vogel, Bull. Am. Phys. Soc., 21 {1976) 26; J. Mater. Sci., 12 (1977) 982. 2 T. E. Thompson, E. R. Falardeau and L. R. Hanlon, Carbon, 15 (1977) 39. 3 H. Fuzellier, J. Melin and A. H~rold, Carbon, 15 (1977)45. 4 G. M. T. Foley, C. Zeller, E. R. Falardeau and F. L. Vogel, submitted to Solid State Commun. 5 J. M. Lalancette and J. Lafontaine, J. Chem. Soc., Chem. Commun., (1973) 815. 6 P. W. Taubenblat and G. Goller, J. Inst. Met., 98 (1970).
7 E. R. Falardeau and T. E. Thompson, submitted to Inorg. Chem. 8 E. R. Falardeau, G. M. T. Foley, C. Zeller and F. L. Vogel, J. Chem. Soc., Chem. Commun., in press. 9 A. R. Ubbelohde, Proc. R. Soc. London, Ser. A, 327 (1972) 289. 10 A. R. Ubbelohde, Proc. R. Soc. London, Ser. A, 309 (1969) 297. 11 L. C. F. Blackman, J. F. Mathews and A. R. Ubbelohde, Proc. R. Soc. London, Ser. A, 258 (1960) 339. 12 L. R. Hanlon, E. R. Falardeau and J. E. Fischer, submitted to Solid State Commun.
261
© Elsevier S e q u o i a S.A., L a u s a n n e - - P r i n t e d in t h e N e t h e r l a n d s
High Electrical Conductivity in Graphite Intercalated with Acid Fluorides
F. L. V O G E L , G. M. T. F O L E Y , C. Z E L L E R , E. R. F A L A R D E A U a n d J. G A N
Department of Electrical Engineering and Science and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (U.S.A.)
SUMMARY
Unusually high room temperature electrical conductivity, higher than that of pure copper, has been observed under certain conditions in graphite intercalated with the strong acid fluorides, antimony pentafluoride and arsenic pentafluoride. Since these results are of considerable scientific and technological importance, and likely to affect the course of research in this area, the confirming experiments are reviewed here. The first experiment demonstrating the potential of these c o m p o u n d s was measurements on a composite wire consisting of an SbF 5 intercalated graphite powder core with a copper sheath [1]. Results were obtained by a d.c. four point resistance method on samples 1 mm dia. × 10 cm long. Comparison of this composite wire with a control sample of copper demonstrated convincingly the superior conductivity of the intercalated graphite core. More recent experiments of a similar nature have confirmed those early findings. We have also made measurements of highly oriented polycrystalline graphite (HOPG) intercalated with AsF 5 [2]. In careful measurements a contactless r.f. (100 kHz) induction technique has substantiated the high electrical conductivity of these materials. While the measured conductivity of the AsF5 compounds has a maximum value only marginally greater than pure copper, this value must be regarded as conservative and any correction made to the r.f. measurements for sample imperfections, etc., would tend to increase the calculated conductivity above the quoted value. These results lend support to the notion that very high electrical conductivities are possible in the acceptor c o m p o u n d s of
intercalated graphite and that the intercalated acid is responsible for a marked increase in the density of charge carriers while the mobility remains high.
RESUME
Une haute conductivit~ ~lectrique ~ temperature ambiante, sup~rieure ~ celle du cuivre put, a ~t~ observ~e pour certains compos~s d'insertion du graphite avec des acides fluor~s forts, les pentafluorides d'antimoine et d'arsenic. Comme ces r~sultats sont d'une importance considerable dans les domaines scientifique et technologique et sont susceptibles de modifier le cours des recherches entreprises, ces experiences prometteuses sont d~crites ci-dessous. Les premieres experiences r~v~lant l'importance de ces compos~s consistaient en s~rie de mesures sur des fils composites r~alis~s partir d'un coeur de poudre de graphite intercal~ avec du SbF5 dans une gaine de cuivre. Les r~sultats ont ~t~ obtenus par la m~thode quatre points de mesure de r~sistance en courant continu sur des ~chantillons de 1 mm de diam~tre par 10 cm de long. La comparaison de ces fils composites avec un ~chantillon de contrSle en cuivre a prouv~ indiscutablement que le coeur de graphite ins~r~ avait une conductivit~ sup~rieure. Des experiences r~centes de m~me nature [3] ont confirm~ ces premiers r~sultats. Nous avons fait aussi des mesures sur du graphite polycristaUin hautement orient~ ins~r~ avec de I'AsF5 [2]. Des mesures soigneuses avec une m~thode d'induction sans contact ont confirm~ la haute conductivit~ ~lectrique de ces mat~riaux. Tandis que la conductivit~ mesur~e pour les compos~s d'AsF5 a une valeur maximale seulement un
262 peu plus grande que celle du cuivre, cette valeur peut ~tre consid~r~e comme minimale car toute amelioration faite dans le cadre de la m~thode par induction pour corriger les imperfections des ~chantillons, ...etc., ne conduirait qu'~ accroi'tre la valeur de la conductivit~ calcul~e. Ces r~sultats accr~ditent l'id~e que de tr~s hautes conductivit~s ~lectriques sont possibles dans les compos~s de graphite ins~r~s accepteurs d'~lectrons et que l'acide est responsable de l'accroissement notable de la densit~ des porteurs de charges tandis que la mobilit~ reste ~ une valeur haute.
INTRODUCTION The observation of electrical conductivity higher than that of copper in a graphite intercalation c o m p o u n d [1] is a matter of considerable scientific and technological interest. Since the original results on high conductivity were reported there have been contradictory results [2, 3]. These differences have now been resolved by the understanding of the importance of the anisotropy of conductivity in graphite which has been intercalated with a strong acid [4]. This point is well explained in the preceeding paper of this volume. Thus, it seemed worthwhile to review the evidence for unusually high electrical conductivity in graphite intercalated with antimony pentafluoride and measured by a four point d.c. method on composite wires [1], and on highly oriented pyrolytic graphite (HOPG) intercalated with arsenic pentafluoride and measured by the r.f. induction method [4] which is described in the preceeding paper of this volume.
EXPERIMENTAL RESULTS In the composite wire experiments, graphite powder, having a 10 pm dia. particle size was vacuum annealed overnight and loaded into a cylindrical copper ampoule having a 6.4 mm o.d. and a 3.8 mm i.d. The ampoules were either phosphorus deoxidized copper (p = 2.1 × 10 -6 ~2 cm) or high conductivity deoxidized copper (p = 1.7 X 10 -6 ~2 cm). Then, a weighed amount of SbF 5 was introduced into the ampoule, under the
protective cover of a N 2 glove box, in sufficient quantity to produce a stage three compound. The ampoule was tightly sealed with a copper end plug and then heated overnight at 150 °C to form the c o m p o u n d in a method similar to that described by Lalancette [5]. The tubes containing the c o m p o u n d were then cold swaged down to 2 mm or 1 mm dia. and annealed at 500 °C. As a control on the experiment, identical copper ampoules were filled with AMAX LO copper powder, which powder, when reduced to ideal density of 8.95 g/cm 3 results in a material having ideal copper resistivity of 1.7 X 10 -6 gt cm. Alternatively, a solid rod of OFHC copper was subjected to the same swaging and annealing process as a control. It became apparent that the powders in the core of the composite wires did not reach ideal density after the swaging and annealing treatment given and that a density correction would have to be made. This was done for the c o m p o u n d core wires by chemically analyzing the copper in weighed segments and back calculating to the diameter and mass of the inside. Direct current resistivity measurements of the composite wires were made using a four point method. A jig was constructed having outer or current contacts 15 cm apart and inner or voltage contacts 10.1 + 0.05 cm apart. These four contacts were connected directly to the four terminals of a Shallcross Milliohmeter which read the resistance directly. These determinations were cross checked by separately measuring the current from a d.c. supply and the voltage drop at the inner contacts. Also, the measurements were checked by using alternating current. In all cases, satisfactory agreement was found compared with copper wire of known resistivity. The results from this series of experiments are depicted in Table 1. The remarkable result of a core conductivity higher than that of copper (conductivity of copper Ocu = 5.9 X 105 gt - i cm - i ) requires some comment. It is interesting that with these separate experiments, done at different times, and with probable variations in technique, orientation, compositions, etc., the final result shown on the b o t t o m line of the Table comes o u t the same: o = 1 X 10 -6 ~ cm -+ 10%. Particularly convincing is
263 TABLE 1 Conduetivities of antimony pentafluoride intercalated graphite in a copper sheath
Composite conductance ~ - 1 Composite conductivity ~ - 1 cm-1 Apparent core density % of ideal Conductivity of sheath ~-~-1 cm-1 Conductivity of core at ideal d e n s i t y ~ - I cm-1
Phosphorus deoxidized copper sheath
Conductivity grade copper sheath
Control sample
Exptl. sample
Control sample
Exptl. sample
Control sample
Exptl. sample
4.7 x 102
4.5 x 102
4,9
x 202
2.0 x 103
2.2 × 10-3
4.5x 105
5.6x105
5.5x 105
5.95×105
5.8×105
6.5x105
75
85
73
80
(100)
85
4.8 x 105
4.8 x 105
5.9 x 105
5.9
5.8 × 105
5.8 x 105
5.9 × 105
1.1 X 106
5.9 × 105
1.0 × 106 (5.8× 105 ) .
3.8 X 102
the wire represented in the last column which exhibits in the composite state a conductivity higher than that of copper, as in terms of resistivity PCg_SbFs = 1.54 ~ 2 cm compared with copper at Pcu = 1.7 p ~ cm. Further confidence in these measurements derives from the realization that any conceivable errors from non-uniformity, contact resistance, imperfect crystal alignment, and others, would all have the effect of making the true electrical conductivity higher than the measured one. It is also significant that a comparison made along the top line of Table 1 reveals that each experimental sample showed a conductance larger than its respective control of the same geometry without correction for density, orientation, or conductivity of the sheath. For the intercalated crystal experiments, highly oriented pyrolytic graphite (HOPG) donated by Dr. A. Moore of Union Carbide was intercalated with AsF 5 using standard high vacuum and inert atmosphere techniques. The graphite was cut by air abrasion into ~ 5 × 5 mm basal plane squares and cleaved to the desired c-axis starting thickness. Initial dimensions, weight, and conductivity of each unintercalated HOPG piece were obtained. Arsenic pentafluoride (Ozark Mahoning) was generally purified by trap to trap distillation prior to use. This distillation essentially proved to be a precaution, as no differences in either vapor density of, or the intercalation reaction results were evident with undistilled materials, suggesting the AsF5 to be suitable as purchased.
x 105
1.0X 10 -6
As the previous paper has discussed, precise measurement of the basal plane conductivity, oa, in highly anisotropic c o m p o u n d s can only be satisfactorily accomplished by means of an r.f. induction technique. The technique has permitted both in situ measurements of conductivity (i.e., measurements during the intercalation reaction) and measurements on samples of fixed stage sealed off under an atmosphere of dry nitrogen. The AsF5-graphite c o m p o u n d s of fixed stage were prepared by vapor intercalation of HOPG with AsF 5 in a manner similar to that previously reported [7]. The intercalated products were transferred under dry nitrogen to 2 X 6 mm i.d. rectangular tubes and sealed. The materials were characterized by their X-ray (00l) reflections, c-axis thickness increase, and gravimetric analysis. These data correspond to those previously reported, and for stages 1 - 3 gave materials of stoichiometry C8,AsF5 where n is the stage. The in situ reactions were carried o u t in sealed pyrex tubes consisting of 8 mm o.d. round tubing which is suitable for viewing the c-axis thickness change, and 2 × 6 mm i.d. rectangular tubing which is convenient for the r.f. measurements. The AsF5 was allowed to vaporize completely, with care taken to avoid contact of the graphite with liquid AsF 5 during the warming process. The c-axis thickness and the r.f. signal were monitored with time b y shifting the HOPG from the round to the rectangular portion of the tube reactor. Figure 1 shows in situ data, AT (change in period of oscillation of the r.f. circuit) vs.
264 i
I • AT
I
~1
I --
1
r •
• •
0
oo
120 •JI8l
t
o/'._! Io
l.6 14
e• ~e AT
N •••• w•l •• • I••
o I0
•*g
(arbunlt}
eme
oo o o°
o o
t
06
STAGE 2
0.4
o
:o°°i
8o°°~ STAGE 5 STAGE 4 __ , ] I 2
oo
O8
aL to
0.2 I_ 5
I 4
REACTION
J 5 TIME
I 7
/~ s
0.0
(hours)
Fig. 1. Changes of resistivity with time while intercalating with AsF 5.
65
I --
60
Ag
q
F
o
o"
•
g o
5.5-
~o x
45
CONCLUSIONS
-
o
~"Cu
5.0--
-
o
_--Au
o" Csn As F5
4.0 -AI
_~
o IN SITU • UNDER N 2
•
I
&5 026~_
HOPG I I
2
3
oo
STAGE n
Fig. 2. Resistivity graphite.
v s.
conductivity oa = (6.3 + 0.7) × 105 (~2 cm) -z at stage 2 where the quoted error is the experimental uncertainty. This value is probably conservative however, since corrections for sample imperfections of any kind all tend to increase the calculated conductivity. The observed peak in a-axis conductivity at a concentration lower than stage I is a feature c o m m o n to many intercalation compounds, e.g., those with H2SO 4 [9], Br 2 [9, 10], and the alkali metal compounds [11].
stage for AsF 5 intercalated
time for an AsFs-graphite reaction. The intercalation process was found to be "well behaved" showing plateaux where little change in signal is observed over a finite time period. This is the so called "staging" phenomenon, plateaux being identifiable with a particular stage, n. A corresponding plot of c-axis thickness change, At~to, versus time shows plateaux at similar locations. The end of a plateau in At/to represents completion of a uniform stage of maximum stoichiometry as confirmed by X-ray and gravimetric analysis. This has greatly facilitated the preparation of compounds of a unique stage. Shown in Fig. 2 is a plot of oa vs. stage from r.f. data for both in situ samples and those under dry nitrogen. The scatter in the data is relatively small and we attribute much of it to differences in sample quality rather than experimental error. On this basis, we conclude that AsFs-graphite has a peak a-axis
The work described in this paper demonstrates convincingly the promising aspects of graphite intercalation compounds as practical synthetic metals. Understanding of the interactions which lead to the greatly enhanced basal plane conductivities remains poorly understood and awaits more detailed experimental studies. It is far from evident, for example, that an unusually large increase in carrier density is responsible for the remarkable conductivity. Rather, the evidence available to date [12] suggests that a relatively smaller decrease in carrier mobility over the parent graphite may be the principal contributor. Thus, there may be a need for reexamination of ideas which suggest that intercalation with increasingly electronegative species should lead to large changes in free carrier density and so to greatly enhanced conductivities. It should be viewed therefore, as most encouraging that despite our lack of real understanding of the mechanisms involved, compounds with conductivities rivalling those of any naturally occurring metal have been synthesized. ACKNOWLEDGEMENTS
The authors express their appreciation to Dr. Arthur Moore of Union Carbide for his generous donation of HOPG. Throughout most of this work G. M. T. Foley was supported under NSF Grant DMR75-04954 and MRL/DMR76-00678. E. R. Falardeau was supported by M R L / D M R 7 6 - 0 0 6 7 8 and Air Force Materials Lab Contract F33625-75-C5231 and J. Gan by MERADCOM, Ft. Belvoir under Contract DAAG53-76C-0061.
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