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Thermal transport in CuCl2 intercalated pitch-derived carbon fibers
0038-IO98185 $3.00 + .OO Pergamon Press Ltd.
Solid State Communications, Vo1.55,No.6, pp.517-520, 1985. Printed in Great Britain.
THERMAL
TRANSPORT
IN CuC12
INTERCALATED
PITCH-DERIVED
CARBON
FIBERS
L. Piraux*, B. Nystent, J-P. Issi Universite Catholique de Louvain Laboratoire de Physico-Chimie et de Physique de 1'Etat Solide (Belgium) Place Croix du Sud, 1 - B-1348 Louvain-la-Neuve Ma&he and E.McRae Universite de Nancy 1 Laboratoire de Chimie du Solide Mineral, LA 158 B.P. 239, 54506 Vandoeuvre les Nancy Cedex (France)
J.F.
"Received
by S. Amelinckx
- April 9, 1985"
The effect of intercalation with CuC12 on the temperature variation of the thermal conductivity, thermopower and electrical resistivity of pitch-derived carbon fibers in the temperature range 2
Recently, the measurement of the temperature variation of the thermal conductivity performed on pristine pitch-derived carbon fibers in the temperature range 3
(Chemical
within the framework of the Reinforcement of the the New Technologies Office for Science Policy)
In the present work, we report the first results pertaining to the thermal conductivity and the thermopower of intercalated pitch-derived carbon fibers. The temperature dependence of the electrical resistivity measured on the same samples Is also reported, The fibers studied were Union Carbide's P-100 mesoohase Ditch-based fibers kindlv provided by Union Carbide Corporation, In these particular high-temperature heat-treated fibers, the graphitlc planes are closely parallel to the fiber axis. The transverse structure is not radial but resembles the "orlented core" structure depicted by Ng et a1.(6), in which the planes are roughly parallel to each other but approach the fiber surface at approximately a 90" angle. The average diameter is ~10 ~IIIand the interplanar distance di = 3.40 A. The electrical resistivity of the pristine fiber at 300 K lies in the range 160-360 @cm (7). The resistivlty Increases by a factor %2 at the temperature of liquid hellurn. The same kind of fiber was referenced VSC-25 in reference 8. As regards compound synthesis, a bundle of fibers was allowed to react with distllled CuC12 in a two temperature furnace (Tfiber - 515'C ; TCUC12 - 505Y) for flve days. The reaction cube was filled with *l bar gaseous chlorine ai the reaction temperature. After cooling, the fibers were transferred under argon in a glove bag to a Lindemann tube for X-ray characterisation, the reflectlons being recorded either by a counter or on a flat film. This latter method renders identification of the OO& reflections immedlate since they appear as small elongated spots in the equatorlal plane. The first method is, however, more accurate. The samples contain both first and second stage phasis. as was also the case for the Ditch-
517
518
THERMAL TRANSPORT IN CuCI, INTERCALATEDPITCH-DERIVED CARBON FIBERS L
derived fibers of reference 9. The c-axis repeat distance for first stage materials is 9.45 1, somewhat greater than the value found in intercalated natural graphite (9.37 1). In the case of the second stage intercalation compounds, the c-axis repeat distance is 12.6 8. Mention should be made of the air stability which certain authors claim for some CuC12intercalated fibers. It is shown elsewhere (10) that the materials of this study undergo a slow' increase in electrical resistivity over a period of several months upon being exposed to ambient conditions. X-ray studies show that there is no drastic modification in staging but there is a weakening in the intensity of the reflections. The measurements of the thermal conductivity. electrical resistivity and thermopower were performed on the same sample holder as that previously used to measure the thermal conductivity of a single pristine (2) or intercalated (3) benzene-derived fiber. Detailed description of the sample holder, specially designed to measure very thin samples, such as fibers or films (11). is given elsewhere (12). However, as is the case for pristine pitch-derived fibers, the thermal conductivity and the available diameters are too small to measure the thermal conductivity on a single intercalated pitch-derived fiber. So the sample we studied here consisted of a bundle of fibers. The experimental accuracy for the thermal conductivity measurement depends on the temperature range investigated and the thermal conductance of the sample. In our case, at high temperature, the estimated error is less than 5% while in the lowest temperature range (below 4 K) it may reach 10%. In figure.1, we present the temperature variation of the thermal conductivity of CuC12 intercalated pitch-based fibers. The results are compared to those previously obtained on pristine pitch-derived fibers taken from the same batch (1). At high temperature (above 80 K), the thermal conductivity of the CuC12 intercalation compound is reduced with respect to that of pristine fibers though the functional behavior is very similar. Below 80 K, the situation is qualitatively different : in intercalated fibers, the thermal conductivity also decreases continuously when the temperature decreases but less r p dly than the pristine fibers which exhibit a variation. Thus below 25 K. we obtain for the sample-of CuC12 intercalation compound a thermal conductivity enhancement with respect to At still lower temperathe pristine fibers. tures, around 5 K, we observe a steeper decrease followed innnediately by a quasi linear variation with temperature. As we shall see latter. this very complex behavior may be qualitatively explained if we separate the measured thermal conductivity into its electronic and lattice contributions. In order to estimate the electronic thermal conductivity we have also performed simultaneously electrical resistivity measurements on the same bundle of fibers. The results are reported in figure 2. We observe a very slight temperature dependence with a very small RRR (1.21). The temperature variation of the thermopower is illustrated in figure 3. As expected for acceptor compounds, the thermopower is positive over the whole temperature range
I
Vol. >5, No. 6 I
.
TEMPERATURE
.
.
0..
-
(K)
F~JQ Temperature dependence of the thermal conductivity of CuC12 intercalated pitchderived fibers (*) compared to that of pristine fibers coming from the same batch (m).
T8.d
Fip,.2 Temperature variation of the electrical resistivity of a bundle of CuC12 intercalated pitch-derived fibers which thermal conductivity is presented in Fig.1. investigated. At very low temperature (below 10 K), the thermopower exhibits a linear behavior while at high temperature it presents a tendency to level off. In the past it was shown that two mechanisms, at least in some temperature range, contribute to the conduction of heat in graphite intercalation compounds : the lattice vibrations and the charge carriers (4). The total thermal conductivity is given by the sum of the two contributions: k
-
“L
+
KE
Vol..55. No. 6
519
TRRRMAL TRANSPORT IN CuC12 INTRRCALATJZsD PITCH-DERIVED CARBON FIBERS
temperature ,the electronic thermal conductivity Is small with respect to the total thermal conductivity. The separatlon of the total thermal conductivity Into Its electronic and lattice contributions Is presented ln figure 4.
If we
.
/ 0
-! I I
50
I
100
I
150
NIu*~llo
1
1
200
250
300
Flg.3 Temperature dependence of the absolute thermopower of CuC12 intercalated pitch-derived The inset shows the linear variation in fibers. the lowest temperature range. where KL Is the lattice conductlvlty and KE the electronic contribution to the thermal In pristine carbon fibers, as is conductivity. the case for prlstlne graphlte. it was shown that the lattice conduction dominates the scene above the liquid hellum temperature: the electronic contrlbutlon becoming significant For the case of the only at lower temperatures. ptlstlne P-lOO-type fibers (1) the ratio KE/KL Is about 10% at about 5 K which Is larger than what Is observed in prlstlne Bdf (2). The electronic thermal conductlvlty can be directly deduced from electrlcal reslstivlty measurements via the Wiedemann-Franz law (WFL),
KE - u Lo T
(2)
where Lo (2.44 lO_8V2K-2) is the free-electron Lorenz number. The WFL Is only valid in the temperature range of elastic scattering of the charge carriers, which Is the case for scattering by static defects. The fact that the electrical reslstlvlty of the intercalated fiber is only slightly temperature dependent In the whole temperature range investlgated (Flgure 2) Is cpnslstent wlth the results previously obtained for acceptor-type (9) or donor-type (8) lntercalated pitch flbers. The total electrical reslstlvlty Is given by the sum of a temperatureIndependent residual 'term prc which Is due to static defects, and a temperature-dependent lntrlnslc contribution pi, which Is mainly due to electron-phonon Interaction, P w pr + pl
(3)
In our case, the ratio pl/pr is less than 2.5% when T
10’ TEMPERATURE
102 (K)
Fln.4 Separation of the total measured thermal conductivity (*) of a CuCl2-pitch fiber lntercalation compound Into Its electronic (.) and lattice (0) contributions. The electronic contribution Is computed using the electrical reslstivlty data (Flg.2) taken on the same sample and using the WFL (relation 2). The lattice thermal conductivity is obtained by subtraction of the electronic thermal conductivity from the tn+al measured thermal conductivity (relation 1). 60 K we see that the lattice thermal conductivity domlnates. Such situations have already been observed In the past In intercalated HOPG (4.5) and more recently In Intercalated Bdf (3). In each case, It appears that the graphite phonons are the domlnant heat transport mechanism by the lattice In Intercalated graphlte at high temperature. Also, If we compare the estlmated KL to that of the pristine fiber we see that Intercalation reduces the lattice thermal conductivity. This Is attrlbuted to'an equivalent decrease in the size of the large scale defects (e.g. grain boundarles). However, in Intercalated pitchderived flbers, the decrease of KL relative to ,prlstine material (about a factor 4 at room temperature) Is less significant than that observed In Intercalated HOPG or Intercalated Bdf. In the latter two cases the decrease reaches one order of magnitude. This Is consistent with the fact that the structure of
520
THERMAL TRANSPORT IN CuCI, INTERCALATED PITCH-DERIVED CARBON FIBERS L
the pristine pitch-fibers is less perfect than that of Bdf or HOPG and thus the relative effect of intercalation is less important in pitchfibers. Below 60 K, the situation is qualitat'vely different : the departure from the T h law becomes more pronounced as the temperature decreases while, below 4 K. KL shows a tendency to become insignificant with respect to the total thermal conductivity. However, it may be seen from figure 4 that the lattice thermal conductivity of the intercalated fiber presents a surprising enhancement relative to the pristine fiber below 15 K. Such an additional contribution was recently observed in intercalated Bdf (3). A tentative interpretation of this phenomenon is discussed in reference 3 and minht be extended to the case of pitch derived inte&alated fibers. It attributes the excess lattice thermal conductivity to the presence of intercalant. It is very likely that the modification of the phonon-spectrum of the system due to intercalation leads to an additional lattice thermal conductivity of the compound in the lowest temperature range. Below 3.5 K, the measured values of the thermal conductivity correspond to those predicted by the WFL. So we may conclude that heat is entirely carried by the charge carriers at ultra low temperature. Such a situation was previously observed in
Vol. 55, No. 6
dilute intercalation HOPG compounds where the separation of the electronic contribution from the lattice contribution required the use of high magnetic fields (13). The thenopower results (figure 3) are in good agreement with those generally reported for acceptor compounds (14.15). with positive values typical for p-type systems over the whole temperature range investigated. At very low temperature (below 8 K) we observe a linear variation of the thermopower which could be ascribed to a typical diffusion mechanis The coefficient of the linear term (8.03 lo- 's' VK-2) is very similar to that recently obtained in the same stage CuCl2-Bdf intercalation compound (3). However, for such multicarrier systems, the estimation of the Fermi energy from the diffusion thermopower values is not straightforward as for the case of a single charge carrier system. The authors are indebted to the Union Carbide Corporation (Parma Technical Center) for kindly supplying the pristine fibers and to the Centre National de Recherche Scientifiaue (France) for financial assistance through an A.T.P. PIRMAT. They enjoyed enlightening discussion with Or L. Singer, Or J. Heremans and M. Lambricht. They are also grateful to Mr P. Coopmans for his skilful technical help.
REFERENCES 1. 2.
3.
4. 5.
6.
B. Nysten, L. Piraux, J-P. Issi, J. Phys. 0, in press. L. Piraux, B. Nysten, A. Haquenne. J-P. Issi, M.S. Dresselhaus, M. Endo, Solid State Comm. 50, 697 (1984) L. Piraux, B. Nysten, J-P. Issi, L. Salamanca-Riba and M.S. Dresselhaus, to be published. L. Piraux, B. Nysten, J-P. Issi, L. Salamanca-Riba and M.S. Dresselhaus, Extended Abstracts (Graphite Intercalation Compounds) of the MRS (Nov 1984), edited by P.C. Eklund, M.S. Dresselhaus and G. Dresselhaus, p. 195. J-P. Issi, J. Heremans, M.S. Dresselhaus, Phys. Rev. m, 1333 (1983) J-P. Issi, 3. Heremans, M.S. Dresselhaus, in Physics of Intercalation Compounds, edited by L. Pietronero and E. Tosatti, (Springer, Berlin, 1981), p.310 C.B. Ng, G.W. Henderson, M. Buechler and J.L. White, Extended Abstracts, 16th Biennal
7. 8. 9.
Conference p. 515. C. Manini,
on Carbon,
San Diego, CA, 1983,
These de 3e Cycle, Nancy
(1984)
C. Manini, J-F. Mar&h& E. McRae, Synthetic Metals 8. 261 (1983) V. NataFajan. J.A. Woolam, Synthetic Metals 8, 291 (1983)
Mar&he and A. 10. C. Manini, E. McRae, J.F. H&-old; Rev. Chim. MinCrale. in press. 11. B. Poulaert, Ch. Vandenhende, J-C. Chielens, p. Billaud, Solid State Corms. (in press) P. Coopmans, to be 12. L.Piraux, J-P Issi, published in J. Phys. E 13. J. Heremans, M. Shayegan, M.S. Dresselhaus. J-P. Issi, Phys. Rev. 826, 3338 (1982) Issi, J. Boxus, B.oulaert, H. 14. J-P. Mazurek, M.S. Dresselhaus, J. Phys. C : Sol. St. Phys. l4, 307 (1981) 15. M. Elringa. D.T. Morelli, C. Uher, Phys. Rev. 826, 3312 (1982)
Solid State Communications, Vo1.55,No.6, pp.517-520, 1985. Printed in Great Britain.
THERMAL
TRANSPORT
IN CuC12
INTERCALATED
PITCH-DERIVED
CARBON
FIBERS
L. Piraux*, B. Nystent, J-P. Issi Universite Catholique de Louvain Laboratoire de Physico-Chimie et de Physique de 1'Etat Solide (Belgium) Place Croix du Sud, 1 - B-1348 Louvain-la-Neuve Ma&he and E.McRae Universite de Nancy 1 Laboratoire de Chimie du Solide Mineral, LA 158 B.P. 239, 54506 Vandoeuvre les Nancy Cedex (France)
J.F.
"Received
by S. Amelinckx
- April 9, 1985"
The effect of intercalation with CuC12 on the temperature variation of the thermal conductivity, thermopower and electrical resistivity of pitch-derived carbon fibers in the temperature range 2
Recently, the measurement of the temperature variation of the thermal conductivity performed on pristine pitch-derived carbon fibers in the temperature range 3
(Chemical
within the framework of the Reinforcement of the the New Technologies Office for Science Policy)
In the present work, we report the first results pertaining to the thermal conductivity and the thermopower of intercalated pitch-derived carbon fibers. The temperature dependence of the electrical resistivity measured on the same samples Is also reported, The fibers studied were Union Carbide's P-100 mesoohase Ditch-based fibers kindlv provided by Union Carbide Corporation, In these particular high-temperature heat-treated fibers, the graphitlc planes are closely parallel to the fiber axis. The transverse structure is not radial but resembles the "orlented core" structure depicted by Ng et a1.(6), in which the planes are roughly parallel to each other but approach the fiber surface at approximately a 90" angle. The average diameter is ~10 ~IIIand the interplanar distance di = 3.40 A. The electrical resistivity of the pristine fiber at 300 K lies in the range 160-360 @cm (7). The resistivlty Increases by a factor %2 at the temperature of liquid hellurn. The same kind of fiber was referenced VSC-25 in reference 8. As regards compound synthesis, a bundle of fibers was allowed to react with distllled CuC12 in a two temperature furnace (Tfiber - 515'C ; TCUC12 - 505Y) for flve days. The reaction cube was filled with *l bar gaseous chlorine ai the reaction temperature. After cooling, the fibers were transferred under argon in a glove bag to a Lindemann tube for X-ray characterisation, the reflectlons being recorded either by a counter or on a flat film. This latter method renders identification of the OO& reflections immedlate since they appear as small elongated spots in the equatorlal plane. The first method is, however, more accurate. The samples contain both first and second stage phasis. as was also the case for the Ditch-
517
518
THERMAL TRANSPORT IN CuCI, INTERCALATEDPITCH-DERIVED CARBON FIBERS L
derived fibers of reference 9. The c-axis repeat distance for first stage materials is 9.45 1, somewhat greater than the value found in intercalated natural graphite (9.37 1). In the case of the second stage intercalation compounds, the c-axis repeat distance is 12.6 8. Mention should be made of the air stability which certain authors claim for some CuC12intercalated fibers. It is shown elsewhere (10) that the materials of this study undergo a slow' increase in electrical resistivity over a period of several months upon being exposed to ambient conditions. X-ray studies show that there is no drastic modification in staging but there is a weakening in the intensity of the reflections. The measurements of the thermal conductivity. electrical resistivity and thermopower were performed on the same sample holder as that previously used to measure the thermal conductivity of a single pristine (2) or intercalated (3) benzene-derived fiber. Detailed description of the sample holder, specially designed to measure very thin samples, such as fibers or films (11). is given elsewhere (12). However, as is the case for pristine pitch-derived fibers, the thermal conductivity and the available diameters are too small to measure the thermal conductivity on a single intercalated pitch-derived fiber. So the sample we studied here consisted of a bundle of fibers. The experimental accuracy for the thermal conductivity measurement depends on the temperature range investigated and the thermal conductance of the sample. In our case, at high temperature, the estimated error is less than 5% while in the lowest temperature range (below 4 K) it may reach 10%. In figure.1, we present the temperature variation of the thermal conductivity of CuC12 intercalated pitch-based fibers. The results are compared to those previously obtained on pristine pitch-derived fibers taken from the same batch (1). At high temperature (above 80 K), the thermal conductivity of the CuC12 intercalation compound is reduced with respect to that of pristine fibers though the functional behavior is very similar. Below 80 K, the situation is qualitatively different : in intercalated fibers, the thermal conductivity also decreases continuously when the temperature decreases but less r p dly than the pristine fibers which exhibit a variation. Thus below 25 K. we obtain for the sample-of CuC12 intercalation compound a thermal conductivity enhancement with respect to At still lower temperathe pristine fibers. tures, around 5 K, we observe a steeper decrease followed innnediately by a quasi linear variation with temperature. As we shall see latter. this very complex behavior may be qualitatively explained if we separate the measured thermal conductivity into its electronic and lattice contributions. In order to estimate the electronic thermal conductivity we have also performed simultaneously electrical resistivity measurements on the same bundle of fibers. The results are reported in figure 2. We observe a very slight temperature dependence with a very small RRR (1.21). The temperature variation of the thermopower is illustrated in figure 3. As expected for acceptor compounds, the thermopower is positive over the whole temperature range
I
Vol. >5, No. 6 I
.
TEMPERATURE
.
.
0..
-
(K)
F~JQ Temperature dependence of the thermal conductivity of CuC12 intercalated pitchderived fibers (*) compared to that of pristine fibers coming from the same batch (m).
T8.d
Fip,.2 Temperature variation of the electrical resistivity of a bundle of CuC12 intercalated pitch-derived fibers which thermal conductivity is presented in Fig.1. investigated. At very low temperature (below 10 K), the thermopower exhibits a linear behavior while at high temperature it presents a tendency to level off. In the past it was shown that two mechanisms, at least in some temperature range, contribute to the conduction of heat in graphite intercalation compounds : the lattice vibrations and the charge carriers (4). The total thermal conductivity is given by the sum of the two contributions: k
-
“L
+
KE
Vol..55. No. 6
519
TRRRMAL TRANSPORT IN CuC12 INTRRCALATJZsD PITCH-DERIVED CARBON FIBERS
temperature ,the electronic thermal conductivity Is small with respect to the total thermal conductivity. The separatlon of the total thermal conductivity Into Its electronic and lattice contributions Is presented ln figure 4.
If we
.
/ 0
-! I I
50
I
100
I
150
NIu*~llo
1
1
200
250
300
Flg.3 Temperature dependence of the absolute thermopower of CuC12 intercalated pitch-derived The inset shows the linear variation in fibers. the lowest temperature range. where KL Is the lattice conductlvlty and KE the electronic contribution to the thermal In pristine carbon fibers, as is conductivity. the case for prlstlne graphlte. it was shown that the lattice conduction dominates the scene above the liquid hellum temperature: the electronic contrlbutlon becoming significant For the case of the only at lower temperatures. ptlstlne P-lOO-type fibers (1) the ratio KE/KL Is about 10% at about 5 K which Is larger than what Is observed in prlstlne Bdf (2). The electronic thermal conductlvlty can be directly deduced from electrlcal reslstivlty measurements via the Wiedemann-Franz law (WFL),
KE - u Lo T
(2)
where Lo (2.44 lO_8V2K-2) is the free-electron Lorenz number. The WFL Is only valid in the temperature range of elastic scattering of the charge carriers, which Is the case for scattering by static defects. The fact that the electrical reslstlvlty of the intercalated fiber is only slightly temperature dependent In the whole temperature range investlgated (Flgure 2) Is cpnslstent wlth the results previously obtained for acceptor-type (9) or donor-type (8) lntercalated pitch flbers. The total electrical reslstlvlty Is given by the sum of a temperatureIndependent residual 'term prc which Is due to static defects, and a temperature-dependent lntrlnslc contribution pi, which Is mainly due to electron-phonon Interaction, P w pr + pl
(3)
In our case, the ratio pl/pr is less than 2.5% when T
10’ TEMPERATURE
102 (K)
Fln.4 Separation of the total measured thermal conductivity (*) of a CuCl2-pitch fiber lntercalation compound Into Its electronic (.) and lattice (0) contributions. The electronic contribution Is computed using the electrical reslstivlty data (Flg.2) taken on the same sample and using the WFL (relation 2). The lattice thermal conductivity is obtained by subtraction of the electronic thermal conductivity from the tn+al measured thermal conductivity (relation 1). 60 K we see that the lattice thermal conductivity domlnates. Such situations have already been observed In the past In intercalated HOPG (4.5) and more recently In Intercalated Bdf (3). In each case, It appears that the graphite phonons are the domlnant heat transport mechanism by the lattice In Intercalated graphlte at high temperature. Also, If we compare the estlmated KL to that of the pristine fiber we see that Intercalation reduces the lattice thermal conductivity. This Is attrlbuted to'an equivalent decrease in the size of the large scale defects (e.g. grain boundarles). However, in Intercalated pitchderived flbers, the decrease of KL relative to ,prlstine material (about a factor 4 at room temperature) Is less significant than that observed In Intercalated HOPG or Intercalated Bdf. In the latter two cases the decrease reaches one order of magnitude. This Is consistent with the fact that the structure of
520
THERMAL TRANSPORT IN CuCI, INTERCALATED PITCH-DERIVED CARBON FIBERS L
the pristine pitch-fibers is less perfect than that of Bdf or HOPG and thus the relative effect of intercalation is less important in pitchfibers. Below 60 K, the situation is qualitat'vely different : the departure from the T h law becomes more pronounced as the temperature decreases while, below 4 K. KL shows a tendency to become insignificant with respect to the total thermal conductivity. However, it may be seen from figure 4 that the lattice thermal conductivity of the intercalated fiber presents a surprising enhancement relative to the pristine fiber below 15 K. Such an additional contribution was recently observed in intercalated Bdf (3). A tentative interpretation of this phenomenon is discussed in reference 3 and minht be extended to the case of pitch derived inte&alated fibers. It attributes the excess lattice thermal conductivity to the presence of intercalant. It is very likely that the modification of the phonon-spectrum of the system due to intercalation leads to an additional lattice thermal conductivity of the compound in the lowest temperature range. Below 3.5 K, the measured values of the thermal conductivity correspond to those predicted by the WFL. So we may conclude that heat is entirely carried by the charge carriers at ultra low temperature. Such a situation was previously observed in
Vol. 55, No. 6
dilute intercalation HOPG compounds where the separation of the electronic contribution from the lattice contribution required the use of high magnetic fields (13). The thenopower results (figure 3) are in good agreement with those generally reported for acceptor compounds (14.15). with positive values typical for p-type systems over the whole temperature range investigated. At very low temperature (below 8 K) we observe a linear variation of the thermopower which could be ascribed to a typical diffusion mechanis The coefficient of the linear term (8.03 lo- 's' VK-2) is very similar to that recently obtained in the same stage CuCl2-Bdf intercalation compound (3). However, for such multicarrier systems, the estimation of the Fermi energy from the diffusion thermopower values is not straightforward as for the case of a single charge carrier system. The authors are indebted to the Union Carbide Corporation (Parma Technical Center) for kindly supplying the pristine fibers and to the Centre National de Recherche Scientifiaue (France) for financial assistance through an A.T.P. PIRMAT. They enjoyed enlightening discussion with Or L. Singer, Or J. Heremans and M. Lambricht. They are also grateful to Mr P. Coopmans for his skilful technical help.
REFERENCES 1. 2.
3.
4. 5.
6.
B. Nysten, L. Piraux, J-P. Issi, J. Phys. 0, in press. L. Piraux, B. Nysten, A. Haquenne. J-P. Issi, M.S. Dresselhaus, M. Endo, Solid State Comm. 50, 697 (1984) L. Piraux, B. Nysten, J-P. Issi, L. Salamanca-Riba and M.S. Dresselhaus, to be published. L. Piraux, B. Nysten, J-P. Issi, L. Salamanca-Riba and M.S. Dresselhaus, Extended Abstracts (Graphite Intercalation Compounds) of the MRS (Nov 1984), edited by P.C. Eklund, M.S. Dresselhaus and G. Dresselhaus, p. 195. J-P. Issi, J. Heremans, M.S. Dresselhaus, Phys. Rev. m, 1333 (1983) J-P. Issi, 3. Heremans, M.S. Dresselhaus, in Physics of Intercalation Compounds, edited by L. Pietronero and E. Tosatti, (Springer, Berlin, 1981), p.310 C.B. Ng, G.W. Henderson, M. Buechler and J.L. White, Extended Abstracts, 16th Biennal
7. 8. 9.
Conference p. 515. C. Manini,
on Carbon,
San Diego, CA, 1983,
These de 3e Cycle, Nancy
(1984)
C. Manini, J-F. Mar&h& E. McRae, Synthetic Metals 8. 261 (1983) V. NataFajan. J.A. Woolam, Synthetic Metals 8, 291 (1983)
Mar&he and A. 10. C. Manini, E. McRae, J.F. H&-old; Rev. Chim. MinCrale. in press. 11. B. Poulaert, Ch. Vandenhende, J-C. Chielens, p. Billaud, Solid State Corms. (in press) P. Coopmans, to be 12. L.Piraux, J-P Issi, published in J. Phys. E 13. J. Heremans, M. Shayegan, M.S. Dresselhaus. J-P. Issi, Phys. Rev. 826, 3338 (1982) Issi, J. Boxus, B.oulaert, H. 14. J-P. Mazurek, M.S. Dresselhaus, J. Phys. C : Sol. St. Phys. l4, 307 (1981) 15. M. Elringa. D.T. Morelli, C. Uher, Phys. Rev. 826, 3312 (1982)