Institute for Multidisciplinary Research, University of Belgrade, Kneza Višeslava 1, 11000 Belgrade, Serbia
Abstract
Layered cobalt oxide materials have lately been the subject of considerable
fundamental and practical interest as potential candidates for thermoelectric
application. The polycrystalline samples of NaCo2–xCuxO4 (x = 0, 0.01, 0.03, 0.05)
were obtained by mechanochemically assisted solid-state reaction method (MASSR)
and the citric acid complex method (CAC). Ceramic samples were prepared by
pressing into disc-shaped pellets and subsequently sintered at 880 °C for 20 h in
inert argon atmosphere. The electrical resistivity (ρ), the thermal conductivity (κ)
and the Seebeck coefficient (S) were measured and observed in two temperature
regions: low (from 0 to 300 K) and high (from 300 K to 800 K), and the effect of
small concentrations of the dopant on the thermoelectric properties was observed.
The values of κ were lower in higher temperature region, and almost independent of
Cu concentration. S was positive above 25 K, and higher for Cu-doped samples,
reaching the highest values for both syntheses for samples with x = 0.03(145 μV/K
at 873 K for CAC sample). The highest figure of merit (ZT) at room temperature
(0.022) was obtained for x = 0.01 while at high temperature region ZT were 0.050
and 0.034 for CAC and MASSR samples, respectively. ZT values for all samples
were higher than in undoped samples, confirming that even small concentration of
Cu significantly influences the thermoelectric properties of NaCo2O4. It was found
that the samples synthesized by CAC method possess better thermoelectric
properties, confirming the fact that this type of synthesis enables obtaining fine,
homogeneous precursor powders with fine microstructures and small grains which
presents prerequisite for obtaining material with good thermoelectric performances
