Thermal Transport in Intercalated Graphite

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J-P. ISSI Laboratoire de Physico-Chimie et de Physique de l'Etat Solide, Universit6 Catholique de Louvain, I Place Croix du Sud, B-1348 Louvain-la-Neuve, BELGIUM

ABSTRACT Recent results on the temperature and stage dependence of the thermal conductivity of various graphite intercalation compounds are reported and briefly analyzed. In the lowest temperature range the in-plane thermal conductivity which is mainly electronic, is higher than that of pristine graphite. Around room temperature, in spite of an increased electronic conduction, a decrease in the total thermal conductivity relative to pristine graphite is observed. This shows that the dominant effect is the decrease of the lattice contribution due to increased phonon scattering by lattice defects. The c-axis conduction is attributed entirely to phonons and the anisotropy ratio is of order of 102 in the liquid helium range and increases up to several hundreds around room temperature. Some results on the temperature variation of the in-plane and c-axis thermopower for donor and acceptor compounds are also presented and discussed.

INTRODUCTION It

was already realized in the early fifties that thermal conductivity

measurements were

instrumental

in

gaining an insight into the defect

structure

of solids. However, because of the complexity of thermal conductivity measurements only a few groups have used this tool to characterize solids. Most of the work performed in this field is reviewed in references [1] to (6] and the bibliography given therein. Ubbelohde's group at Imperial College [7][8][9], Kelly [10][111, de Combarieu [12], Dreyfus and Maynard [13] and others (14][15] (16][17] studied this property on pristine HOPG. The status of our present knowledge of the thermal conductivity in this material has recently been reviewed by Kelly [18]. In pure HOPG the in-plane thermal conductivity is exclusively due to lattice waves, the phonons , except at very low temperatures where an electronic contribution becomes apparent [19][20]. Despite the relatively large electrical conductivity a in pristine graphite, above the liquid helium range the electronic contribution KE

= L

T

(1)

is negligible with respect to that of the lattice KL, in the purest material. L is the free electron Lorenz number and T is the absolute temperature. Relation 1, which is known as the Wiedemann-Franz law, holds provided that the charge carrier scattering is elastic. When lattice and electronic conductivities are both operative, the total thermal conductivity is given by (2)

K = KE + KL

Hat. Res.

Soc. Symp. Proc.

Vol.

20 (1983)

zElsevier Science Publfshing Co.,

Inc.

148 The very large in-plane lattice thermal conductivity K of pristine HOPG is due Indeed, to the strong covalent bonding and the small mass of the carbon atoms. HOPG has with diamond the largest room temperature lattice thermal conductivity 2 1 3 1 1 for pure copper). On the W m- K-I ( -2 x1_0 W m- K-I compared to - 4.xO other hand the c-axis thermal conductivity around room temperature is about two orders of m