DC electrical conductivity and elastic moduli of cubic samples made of two
kinds of compressed expanded graphite are measured as a function of their a
pparent density. Different percolation thresholds at which the physical pro
perties under study are found to vanish are determined. The accuracy of the
values is confirmed by the application of the usual scaling laws above but
near these thresholds: the corresponding critical exponents are indeed fou
nd to be very close to their universal values. The relationships between th
e connectivity d(c) (i.e., scalar) and rigidity d(r) (i.e., vectorial) crit
ical densities are discussed. It is shown that the ratio d(r)/d(c) is alway
s almost equal to 8/5, which fact may be accounted by the so-called Kirkwoo
d-Keating (KK) model. To our knowledge, it is the first time that such a co
nstant value is observed in real materials. The KK model is also consistent
with critical exponents of elasticity found in both kind of samples. Hence
, compressed expanded graphite appears to behave like a disordered heteroge
neous medium in which elastic forces are mostly of central nature but inclu
de also bond-bending contributions. (C) 2001 Elsevier Science B.V. All righ
ts reserved.