Thermoelectric properties of Cu-doped sodium cobaltite ceramics

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

THERMOELECTRIC PROPERTIES OF NaCo2–xCuxO4 (x = 0, 0.01, 0.03, 0.05) CERAMIC

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

The effect of surface oxidation on the catalytic properties of Ga3Ni2 intermetallic compound for carbon dioxide reduction

Background: In a routine handling of a catalyst material, exposure to air can usually not be avoided. For noble metal catalysts that are resistant to oxidation, this is not an issue, but becomes important for intermetallic catalysts composed of two or more non-noble chemical elements that possess much different standard enthalpies of the oxide formation. The element with higher affinity to oxygen concentrates on the surface in the oxide form, whereas the element with lower affinity sinks into the subsurface region. This changes the number of active sites and the catalytic performance of the catalyst. We have investigated the instability of the surface composition to oxidation of the Ga3Ni2 noble metal-free intermetallic compound, a new catalyst for the CO2 reduction to CO, CH4 and methanol. Methods: The instability of the oxidized Ga3Ni2 surface composition to different heating-annealing conditions was studied by X-ray photoelectron spectroscopy (XPS), used to determine the elemental composition and the chemical bonding in the near-surface region. The dispersion of active sites available for the chemisorption of H-2 and CO on the Ga3Ni2 catalyst surface was determined by H-2 and CO temperature-programmed desorption. CO2 conversion experiments were performed by using the catalyst material reduced in hydrogen at temperatures of 300 and 600 degrees C. Results: XPS study of the Ga3Ni2 surface subjected to different heating-annealing conditions has revealed that the concentration of Ga at the oxidized surface is strongly enhanced and the concentration of Ni is strongly depleted with respect to the values in the bulk. By annealing the surface at 600 degrees C in ultra-high vacuum, the oxides have evaporated and thermal diffusion of atoms near the surface has partially reconstructed the surface composition towards the energetically more favorable bulk value, whereas annealing at a lower temperature of 300 degrees C was ineffective to change the surface composition. Catalytic tests were in agreement with the XPS results, where an increased CO2 conversion for the catalyst reduced with hydrogen at a higher temperature followed an increased Ni/Ga surface concentration ratio. Conclusions: The instability of the active surface chemical composition to oxidation in air must be taken into account when considering noble metal-free intermetallic catalysts as alternatives to the conventional catalysts based on noble metals. Ga3Ni2 and other Ga-Ni intermetallic compounds are good examples of binary intermetallic catalysts, whose catalytic performance is strongly affected by exposure to the air

Antiferromagnetism and heat capacity of NaCo2-xCuxO4 ceramics

Polycrystalline samples of NaCo2-xCuxO4 (x=0, 0.01, 0.03 and 0.05) were synthesized in two different ways: 1) by a mechanochemically assisted solid-state reaction method (MASSR) and 2) by a citric acid complex method (CAC). In this work we examined the influence of these synthesis routes and small Cu concentrations on magnetic properties and the heat capacity of sintered samples. The magnetic susceptibility (chi) of all samples followed the Curie-Weiss law in the temperature range between 50 K and 300 K, while a negative Weiss constant (theta) implied an antiferromagnetic interaction. According to the magnetic susceptibility data, a peak around 30 K indicating the presence of Co3O4 as a secondary phase appeared for all MASSR samples and CAC samples with Cu content above 1%. The effective magnetic moment (mu(eff)) of CAC samples was lower than the theoretical, spin only value obtained for the Co4+ ion in the low spin state indicating the presence of low spin Co3+(S = 0). These values were also lower compared to the values obtained for MASSR samples. The highest mu(eff) of 1.75 mu B/atom Co was obtained for the undoped MASSR sample. The heat capacity of CAC samples at 2 K decreased with Cu concentration due to lowering of the electronic specific heat coefficient (gamma). The highest gamma of 63.9 mJ/molK(2) was obtained for the undoped CAC sample. This reduction in gamma values was the result of the decrease of the density of state and/or mass enhancement factor