  The resistivity and thermopower of polycrystalline (Na,Ca)Co_2O_4 were
measured and analyzed. Both the quantities increase with x, suggesting that the
carrier density is decreased by the substitutions of Ca^{2+} for Na^{+}.
Considering that the temperature dependence of the resistivity show a
characteristic change with x, the conduction mechanism is unlikely to come from
a simple electron-phonon scattering. As a reference for NaCo_2O_4, single
crystals of a two-dimensional Co oxide (Bi,Pb)_2M_3Co_2O_9 (M=Sr and Ba) were
studied. The Pb substitution decreases the resistivity, leaving the thermopower
nearly intact.
