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Low temperature thermal conductivity of graphite-FeCl3 intercalation compounds
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Solid State Co~mnunications, Voi.38, pp. lll7-1119. Pergamon Press Ltd. 1981. Printed in Great Britain.
LOW TEMPERATURE
0038-I098/81/241117-03502.0010
THERMAL CONDUCTIVITY OF GRAPHITE-FeCI 3 INTERCALATION COMPOUNDS
J. Boxus*, B. Poulaert, J-P. Issi Universlte Catholique de Louvaln Laboratolre de Physico-Chimie et de Physique de l'Etat Sol[de Place Croix du Sud, 1 B-1348 Louvain-la-Neuve, Belgium) and H, Mazurek, M.S. Dresselhaus Massachusetts Institute of Technology Center for Materials Science and Engineering Cambridge, Massachusetts 02139, USA
(Received by S. Ame~inckx, March 20, 1981) The low-temperature variation of the in-plane thermal conductivity of graphiteFeCI 3 intercalation compounds is reported. Around room temperature, though holes are participating, there is an Important lattice contribution, but much smaller than in pristine graphite. The electronic contribution dominates in the liquid helium range. These preliminary results suggest lattice thermal conductivity measurements as a new tool to investigate defects introduced by intercalation.
During the last few years graphite intercalation compounds have attracted a great deal of attention, particularly because of their promise as materials of high conductivity and an[sotropy. Until now emphasis has been given to electrical conductivity studies, I where Increases in the electrical conductivity up to a factor of ~40 have been achieved. 2 We present here the first studies of the thermal conductivity of graphite intercalation compounds. Thes~ studies are signlficant both in showing almost an order of magnitude decrease In room temperature thermal conductivity and in providing additional insights into temperature dependent transport mechanisms for acceptor compounds. The lattice thermal conductivity, through the Interpretation of phonon scattering mechanisms, provides a new tool to investigate the lattice defects introduced during the intercalation process. Besides, the separation of the contribution of the host and intercalate layers is more obvious than in the case of the electrical conductivity. Also, unlike the behavior of pristine graphite, where the electronic contribution is very small, the low temperature In-plane thermal conductivity of intercalated graphite ts dominated by the contribution from holes. Measurements of the in-plane thermal conductIv[ty of a stage 2 graphite-FeCl 3 intercalation compound were performed from 1.7 to 300 K, and the results compared to those obtained with pristine graphite. Preliminary measurements made on two stage 4 samples are also presented. The samples were prepared using a standard two-zone furnace where the stage was controlled by the temperature difference between the graphite host (HOPG) and the FeCI 3 powder. 3 A gas
* Supported by an IRSIA grant
pressure of -500 torr C12 was used to achieve well-staged samples. After intercalation, the samples were characterized by x-ray (00£) diffractograms to determine the stage index, stage fidelity, and c-axls repeat distance, I c. After performing the thermal conductivity measurements, x-ray diffractograms were taken again. Cycling the samples from room to liquid helium temperatures did not affect the staging. The dimensions of the samples were 20 x 4 x 0.5 mm 3 for the stage 2, and 7 x 2 x 0.4 m 3 and 7 x 1.8 x 0.24 =m 3 for the stage 4 samples. The samples were mounted in a variable temperature liquid helium cryostat. A static method was used, employing a heater and heat sink. A special holder was designed to support the samples horizontally, while maintaining them strain-free. Because of the relatively small size of the samples, we carefully checked that the heat dissipation via the connecting leads was negligible throughout the entire temperature range. Electrical contacts were glued to the sample by means of silver ink SCI8S, 4 which insured good thermal contact at low temperatures. The temperature sensors were Au(O.03 at% Fe)-Chromel P thermocouples. Standard four probe thermal conductivity measurement techniques were successfully applied to graphite intercalation compounds in contrast with the case of the electrical conductivity, where the high electrical anisotropy necessitates the use of contactless measurement techniques. 5 Reproducible thermal conductivity results were obtained on different runs taken on the same sample and on runs taken on two different samples of the same stage (n - 4). Exploratory measurements on the room-temperature c-axls thermal conductivity show that the anlsotropy of this property In the intercalated material is less than two orders of magnitude. In Fig. I the temperature variation of the thermal conductivity of a stage 2 graphlte-FeCl 3 Iil7
l 1!8
GRAPHITE-FeC 13 INTERCALATION
sample is compared to that of pristine HOPG graphite. The experimental accuracy for measurements on all samples is + 3%. However, due to the uncertainties in estimating the distance between the thermometers, the curve for the stage 2 sample might be multiplied as a whole by a factor ranging from 0.95 to 1.05. There is a much larger uncertainty in this respect with regard to the stage 4 sample, so that the stage 4 data are accurate with regard to temperature variation, but not with regard to absolute magnitudes. The measurements show that the thermal conductivity of this sample below I0 K varies linearly with temperature to within -+ 3%. Around room temperature the measured thermal conductivity is about an order of magnitude lower than that of pristine graphite. In the latter, the thermal conductivity is entirely due to lattlce waves. For intercalated graphite, the lattice contribution of the intercalate layer should be negligible at room temperature. Thus, the main effect of intercalation in reduclng the in-plane thermal conductivity ~Ig is attributed to increased scattering of the graphite phonons by an increased number of large scale defects (as opposed to point defects). Since the charge carrier density is greatly increased relative to that of pristine graphite, one might expect a significant electronic contribution
=
LOT
~I ,
(i)
and take the value of the in-plane electrical conductlvity o I given by Perrachon et al.,6 (o I " 2.15 105 R-I cm-i at 300 K), we find for
(2)
In the lowest temperature range the thermal conductivity of the stage 2 sample increases llnearily with temperature up to about 3 K. This range is extended to about I0 K for the stage 4 sample. Such a temperature dependence is indicative of an electronic thermal resistivity in the residual range, where the charge carrier mean free path A is limited either by static imperfections or by sample size, so that A remains temperature-lndependent. If we again use the Wiedemann-Franz relation to compute the value of the residual electrical resistivity from the measured
~~ ~ o.
COMPOUNDS
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Vol. 38, No.
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TEMPERATURE (K) FIGURE CAPTION ~ Log-Log plot of the temperature variation of the thermal conductivity of a stage 2 and a stage 4 FeCI 3 intercalated sample compared to that of pristine HOPG graphite. The dark circles are for several runs on the stage 2 sample. The open circles are for a stage 4 sample below I0 K and show that for T < i0 K the thermal conductivity is linear (solid llne). The crosses represent the data we obtained on an HOPG sample of about the same dimensions as the stage 2 sample. The dashed llne indicates a T 2.41 variation. The upper insert shows the temperature variation of the electrical resistivity of one of the stage 4 samples in arbitrary units (linear scale) as a function of temperature (logarithmic scale). Note that, though for a given sample the absolute value of the resistivity was found to vary from run to run, the temperature variation of the resistivity remained the same. In the insert each graduation represents approximately 1% of the residual resistivity.
Both effects will tend to decrease the measured thermal conductivity with respect to the linear behavior. We therefore propose that the additional thermal conductivity observed is due to a lattice contribution from the FeCI 3 intercalate sandwich. In the temperature range 3 < T < 30K, in constrast to room temperature, the carbon layers have low thermal conductlvlties so that a contribution from the intercalate lattice conductivlty would be observable in this range as a hump. In this context it is interesting to note that the lattice thermal conductivity humps for most solids (thus for most intercalants), are situated below 30 K, while pristine graphite is one of the few solids that has its maximum ther-
Vol. 38, No. 12
GRA~HITE-FeCI 3 INTERCAI.ATION COMPOUNDS
mal conductivity above I00 K. The introduction of large scale defects should further enhance the temperature at which the maximum occurs. This suggests that temperature dependent thermal conductivity measurements could provide an ideal tool to separate the lattice contributions from the graphite and intercalate layers, and to study the effect of defect formation through intercalation. The large temperature dependence of the thermal conductivity over a limited temperature range is in contrast to the behavior observed for the electrical conductivity of acceptor
1119
graphite intercalation compounds. Further experimental work is now in progress to separate the elec=ronic and lattice contributions to the thermal conductivity of intercalated graphite-FeCl 3 of various stages.
The authors are indebted to Dr. G. Dresselhaus for valuable discussions and to AFOSR grant 77-3391 for partial support of this work. The skill of M. Paul Coopmans in mounting the samples and holder is also greatly appreciated.
REFERENCES i. J.E. Fischer, Physics and Chemistry of Materials with Layered Structure (ed. F. Levy), D. Reldel Publishing Co., Dordrecht, Holland 6, 481 (1979). 2. G.M.T. Foley, C. Zeller, E.R. Falardeau and F.L. Vogel, Solid State Commun. 24, 371 (1977). 3. C. Underhill, S.Y. Leung, G. Dresselhaus and M.S. Dresselhaus, Solid State Commun. 29, 769 (1979). 4. Microcircuits Co., New Buffalo, Michigan,
49117, U.S.A. 5. C. Zeller, A. Dennensteln and G.M.T. Foley, Rev. Scl. Instrum. 50, 602 (1979). 6. J.B. Perrachon, C. Zeller and F.L. Vogel, Ext. Abst. Biennlal Conf. Carbon, 14, 304 (1979). 7. L.A. Pendrys, T.C. Wu, C. Zeller, H. Fuzellier and F.L. Vogel, Ext. Abstr. Biennial Conf. Carbon, 14, 306 (1979).
Solid State Co~mnunications, Voi.38, pp. lll7-1119. Pergamon Press Ltd. 1981. Printed in Great Britain.
LOW TEMPERATURE
0038-I098/81/241117-03502.0010
THERMAL CONDUCTIVITY OF GRAPHITE-FeCI 3 INTERCALATION COMPOUNDS
J. Boxus*, B. Poulaert, J-P. Issi Universlte Catholique de Louvaln Laboratolre de Physico-Chimie et de Physique de l'Etat Sol[de Place Croix du Sud, 1 B-1348 Louvain-la-Neuve, Belgium) and H, Mazurek, M.S. Dresselhaus Massachusetts Institute of Technology Center for Materials Science and Engineering Cambridge, Massachusetts 02139, USA
(Received by S. Ame~inckx, March 20, 1981) The low-temperature variation of the in-plane thermal conductivity of graphiteFeCI 3 intercalation compounds is reported. Around room temperature, though holes are participating, there is an Important lattice contribution, but much smaller than in pristine graphite. The electronic contribution dominates in the liquid helium range. These preliminary results suggest lattice thermal conductivity measurements as a new tool to investigate defects introduced by intercalation.
During the last few years graphite intercalation compounds have attracted a great deal of attention, particularly because of their promise as materials of high conductivity and an[sotropy. Until now emphasis has been given to electrical conductivity studies, I where Increases in the electrical conductivity up to a factor of ~40 have been achieved. 2 We present here the first studies of the thermal conductivity of graphite intercalation compounds. Thes~ studies are signlficant both in showing almost an order of magnitude decrease In room temperature thermal conductivity and in providing additional insights into temperature dependent transport mechanisms for acceptor compounds. The lattice thermal conductivity, through the Interpretation of phonon scattering mechanisms, provides a new tool to investigate the lattice defects introduced during the intercalation process. Besides, the separation of the contribution of the host and intercalate layers is more obvious than in the case of the electrical conductivity. Also, unlike the behavior of pristine graphite, where the electronic contribution is very small, the low temperature In-plane thermal conductivity of intercalated graphite ts dominated by the contribution from holes. Measurements of the in-plane thermal conductIv[ty of a stage 2 graphite-FeCl 3 intercalation compound were performed from 1.7 to 300 K, and the results compared to those obtained with pristine graphite. Preliminary measurements made on two stage 4 samples are also presented. The samples were prepared using a standard two-zone furnace where the stage was controlled by the temperature difference between the graphite host (HOPG) and the FeCI 3 powder. 3 A gas
* Supported by an IRSIA grant
pressure of -500 torr C12 was used to achieve well-staged samples. After intercalation, the samples were characterized by x-ray (00£) diffractograms to determine the stage index, stage fidelity, and c-axls repeat distance, I c. After performing the thermal conductivity measurements, x-ray diffractograms were taken again. Cycling the samples from room to liquid helium temperatures did not affect the staging. The dimensions of the samples were 20 x 4 x 0.5 mm 3 for the stage 2, and 7 x 2 x 0.4 m 3 and 7 x 1.8 x 0.24 =m 3 for the stage 4 samples. The samples were mounted in a variable temperature liquid helium cryostat. A static method was used, employing a heater and heat sink. A special holder was designed to support the samples horizontally, while maintaining them strain-free. Because of the relatively small size of the samples, we carefully checked that the heat dissipation via the connecting leads was negligible throughout the entire temperature range. Electrical contacts were glued to the sample by means of silver ink SCI8S, 4 which insured good thermal contact at low temperatures. The temperature sensors were Au(O.03 at% Fe)-Chromel P thermocouples. Standard four probe thermal conductivity measurement techniques were successfully applied to graphite intercalation compounds in contrast with the case of the electrical conductivity, where the high electrical anisotropy necessitates the use of contactless measurement techniques. 5 Reproducible thermal conductivity results were obtained on different runs taken on the same sample and on runs taken on two different samples of the same stage (n - 4). Exploratory measurements on the room-temperature c-axls thermal conductivity show that the anlsotropy of this property In the intercalated material is less than two orders of magnitude. In Fig. I the temperature variation of the thermal conductivity of a stage 2 graphlte-FeCl 3 Iil7
l 1!8
GRAPHITE-FeC 13 INTERCALATION
sample is compared to that of pristine HOPG graphite. The experimental accuracy for measurements on all samples is + 3%. However, due to the uncertainties in estimating the distance between the thermometers, the curve for the stage 2 sample might be multiplied as a whole by a factor ranging from 0.95 to 1.05. There is a much larger uncertainty in this respect with regard to the stage 4 sample, so that the stage 4 data are accurate with regard to temperature variation, but not with regard to absolute magnitudes. The measurements show that the thermal conductivity of this sample below I0 K varies linearly with temperature to within -+ 3%. Around room temperature the measured thermal conductivity is about an order of magnitude lower than that of pristine graphite. In the latter, the thermal conductivity is entirely due to lattlce waves. For intercalated graphite, the lattice contribution of the intercalate layer should be negligible at room temperature. Thus, the main effect of intercalation in reduclng the in-plane thermal conductivity ~Ig is attributed to increased scattering of the graphite phonons by an increased number of large scale defects (as opposed to point defects). Since the charge carrier density is greatly increased relative to that of pristine graphite, one might expect a significant electronic contribution
=
LOT
~I ,
(i)
and take the value of the in-plane electrical conductlvity o I given by Perrachon et al.,6 (o I " 2.15 105 R-I cm-i at 300 K), we find for
(2)
In the lowest temperature range the thermal conductivity of the stage 2 sample increases llnearily with temperature up to about 3 K. This range is extended to about I0 K for the stage 4 sample. Such a temperature dependence is indicative of an electronic thermal resistivity in the residual range, where the charge carrier mean free path A is limited either by static imperfections or by sample size, so that A remains temperature-lndependent. If we again use the Wiedemann-Franz relation to compute the value of the residual electrical resistivity from the measured
~~ ~ o.
COMPOUNDS
-_ _ _
l
Vol. 38, No.
I I llllll
aO o
oo
o
l
I l lJ~1111
o o
o
/
/"/J"
I
12
'
a
m~g X~tX
//
/,
IG ---
/ i
/
TI
10 Q
'~ ~"
-
o.g
=/ °/,'
oa O / /
0
-
I 2
I I I IIII 5 10
I I I IIIIII I r 20 50 100 200
TEMPERATURE (K) FIGURE CAPTION ~ Log-Log plot of the temperature variation of the thermal conductivity of a stage 2 and a stage 4 FeCI 3 intercalated sample compared to that of pristine HOPG graphite. The dark circles are for several runs on the stage 2 sample. The open circles are for a stage 4 sample below I0 K and show that for T < i0 K the thermal conductivity is linear (solid llne). The crosses represent the data we obtained on an HOPG sample of about the same dimensions as the stage 2 sample. The dashed llne indicates a T 2.41 variation. The upper insert shows the temperature variation of the electrical resistivity of one of the stage 4 samples in arbitrary units (linear scale) as a function of temperature (logarithmic scale). Note that, though for a given sample the absolute value of the resistivity was found to vary from run to run, the temperature variation of the resistivity remained the same. In the insert each graduation represents approximately 1% of the residual resistivity.
Both effects will tend to decrease the measured thermal conductivity with respect to the linear behavior. We therefore propose that the additional thermal conductivity observed is due to a lattice contribution from the FeCI 3 intercalate sandwich. In the temperature range 3 < T < 30K, in constrast to room temperature, the carbon layers have low thermal conductlvlties so that a contribution from the intercalate lattice conductivlty would be observable in this range as a hump. In this context it is interesting to note that the lattice thermal conductivity humps for most solids (thus for most intercalants), are situated below 30 K, while pristine graphite is one of the few solids that has its maximum ther-
Vol. 38, No. 12
GRA~HITE-FeCI 3 INTERCAI.ATION COMPOUNDS
mal conductivity above I00 K. The introduction of large scale defects should further enhance the temperature at which the maximum occurs. This suggests that temperature dependent thermal conductivity measurements could provide an ideal tool to separate the lattice contributions from the graphite and intercalate layers, and to study the effect of defect formation through intercalation. The large temperature dependence of the thermal conductivity over a limited temperature range is in contrast to the behavior observed for the electrical conductivity of acceptor
1119
graphite intercalation compounds. Further experimental work is now in progress to separate the elec=ronic and lattice contributions to the thermal conductivity of intercalated graphite-FeCl 3 of various stages.
The authors are indebted to Dr. G. Dresselhaus for valuable discussions and to AFOSR grant 77-3391 for partial support of this work. The skill of M. Paul Coopmans in mounting the samples and holder is also greatly appreciated.
REFERENCES i. J.E. Fischer, Physics and Chemistry of Materials with Layered Structure (ed. F. Levy), D. Reldel Publishing Co., Dordrecht, Holland 6, 481 (1979). 2. G.M.T. Foley, C. Zeller, E.R. Falardeau and F.L. Vogel, Solid State Commun. 24, 371 (1977). 3. C. Underhill, S.Y. Leung, G. Dresselhaus and M.S. Dresselhaus, Solid State Commun. 29, 769 (1979). 4. Microcircuits Co., New Buffalo, Michigan,
49117, U.S.A. 5. C. Zeller, A. Dennensteln and G.M.T. Foley, Rev. Scl. Instrum. 50, 602 (1979). 6. J.B. Perrachon, C. Zeller and F.L. Vogel, Ext. Abst. Biennlal Conf. Carbon, 14, 304 (1979). 7. L.A. Pendrys, T.C. Wu, C. Zeller, H. Fuzellier and F.L. Vogel, Ext. Abstr. Biennial Conf. Carbon, 14, 306 (1979).