In this work polycrystalline samples of NaCo2–xCuxO4 (x = 0, 0.01, 0.03, 0.05)
were obtained from the powder precursors synthesized in two different ways: 1) by a
mechanochemically assisted solid-state reaction method (MASSR) and 2) a citric
acid complex method (CAC). Ceramic samples were prepared by pressing into discshaped
pellets and subsequently sintered at 880 °C in inert argon atmosphere. The
electrical resistivity (), the thermal conductivity () and the Seebeck coefficient (S)
were measured simultaneously in the temperature range from 330 K to 830 K, and
the effect of small concentrations of the dopant and syntheses procedures on the
thermoelectric properties was observed. According to the temperature dependence of
, all MASSR samples showed metal-insulator transition. The values of were
lower for undoped samples in both syntheses. S increased with temperature and it
was higher in all Cu doped samples, reaching 145 V/K at 830 K for the sample
with 3 mol% of Cu prepared by the CAC method. High thermopower is the
consequence of the strong electron correlation, present in this type of compounds.
The CAC samples showed higher ZT compared with the MASSR samples of the
same composition. The highest figure of merit (ZT = 0.056) was obtained for the
sample with 5 mol% of Cu prepared by the CAC method, and it was 1.5 times
higher than the highest value obtained for the MASSR sample (ZT (NCO3-MASSR)
= 0.036). This result confirmed that, beside the dopant concentration, synthesis
procedure considerably affected the thermoelectric properties of NaCo2O4
