UNIVERSITY OF BELGRADE – FACULTY OF PHYSICAL CHEMISTRY Belgrade, Serbia
Abstract
Layered cobalt oxides have attracted great attention during past decade
as potential candidates for thermoelectric application. However, the scientists
are dealing with several problems concerning synthesis, Na evaporation,
changes of the stoichiometry of the ceramic, etc. In order to reduce synthesis
duration and temperature, prevent Na evaporation and improve mixing of the
precursors we applied mechanochemically assisted solid state reaction and
citric acid complex methods to obtain NaCo2–xCuxO4 (x = 0, 0.01, 0.03, 0.05)
powders. Ceramic samples were prepared by pressing into disc-shaped pellets
and subsequently sintered at 880 °C in inert argon atmosphere. The electrical
resistivity (ρ), the thermal conductivity (κ) and the Seebeck coefficient (S) were
measured simultaneously in the temperature range from 2 K to 830 K, and the
effect of small concentrations of the dopant on the thermoelectric properties
was observed. It was found that in the low temperature range ρ increased with
temperature, indicating metallic behavior. The values of κ decreased as the
temperature increased. S was higher in all Cu-doped samples, reaching 145
μV/K at 830 K for x = 0.03, and this suggested strong electron correlation in
these systems. The highest figure of merit (ZT) at room temperature (0.022)
was obtained for x = 0.01 prepared by the citric acid complex method and it was
twice higher than in undoped sample. In the temperature region between 300 K
and 830 K, higher ZT was also obtained for the samples prepared by citric acid
complex method, reaching the value of 0.056 at 830 K for x = 0.05 and it was
almost three times higher than in undoped sample. These results confirm that
even small concentration of Cu significantly influences the thermoelectric
properties of NaCo2O4
