Thermoelectric and magnetic properties of NaCo2-xCuxO4 (x = 0; 0,01; 0,03; 0,05)

Predmet ove doktorske disertacije je ispitivanje uticaja dveju različitih metoda sinteze keramičkih prahova i malih količina Cu2+ kao dopanta na fazni i hemijski sastav, mikrostrukturu, termoelektrična i magnetna svojstva i toplotni kapacitet keramičkog oksida natrijum-kobaltita (NaCo2O4)...The subject of this doctoral dissertation was to investigate the influence of small concentrations of Cu2+ as a dopant on phase composition, microstructure, thermoelectric and magnetic properties and heat capacity of the ceramic sodium cobaltite (NaCo2O4). Beside that, two different synthesis procedures were compared..

Mechanochemically assisted solid-state synthesis of Cu substituted thermoelectric sodium cobaltite

Polycrystalline samples of Cu substituted NaCo2-xCuxO4 (x=0, 0.01, 0.03, 0.05, 0.1) were prepared using mechanochemically assisted solid-state reaction method starting from the Na2CO3, Co3O4 and CuO powders. Each powder mixture was mechanically activated by grinding for 3 h in a planetary ball mill with ball to powder mixture ratio 20:1, at the basic disc rotation speed of 360 rpm. The asprepared powders were pressed into disc-shaped pellets and subsequently subjected to a heat treatment at 880 °C for 24 h in inert argon atmosphere. Changes in structural characteristics of the samples and particle morphology, caused by Co substitution by Cu, were characterized using X-ray diffraction and scanning electron microscopy, respectively. It should be emphasized that milling process reduced the time necessary for obtaining pure sodium cobaltite. From the results obtained in this study, observed changes in microstructure were correlated with changes in the lattice parameters, indicating the influence of Cu ion incorporation in NaCo2O4 crystal lattice

Enhancement of thermoelectric properties induced by Cu substitution in NaCo2O4

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

6. Effect of Cu dopping on microstructural, thermoelectric and mechanical properties of NaCoCuO4 ceramics

Ceramic samples of NaCo2-xCuxO4 (x = 0, 0.01, 0.03, 0.05) were obtained after calcination of powder precursors synthesized by a mechanochemically assisted solid-state reaction method (MASSR) and a citric acid complex method (CAC). Effects of small concentrations of Cu doping and the above-mentioned syntheses procedures on the microstructural, thermoelectric and mechanical properties were observed. The electrical resistivity (ρ), the thermal conductivity (κ) and the Seebeck coefficient (S) were measured simultaneously in the temperature gradient (ΔT) between hot and cold side of the sample, and the figure of merit (ZT) was subsequently calculated. ZT of the CAC samples was higher compared with the MASSR samples. The highest ZT value of 0.061 at ΔT = 473 K was obtained for the sample with 5 mol% of Cu prepared by the CAC method, and it was 1.7 times higher than the highest value obtained for the MASSR sample with 3 mol% of Cu (ZT = 0.036 at ΔT = 473 K). The CAC samples showed better mechanical properties compared to the MASSR samples due to the higher hardness of the CAC samples which is a consequence of homogeneous microstructure and higher density obtained after sintering of these samples. The results confirmed that, besides the concentration of Cu, the synthesis procedure considerably affected the microstructural, thermoelectric and mechanical properties of NaCo2O4 ceramics

Nanoindentation of nickel manganite ceramics obtained by complex polimerization method

Nickel manganite is very interesting NTC material due to its outstanding properties and wide field of applications mainly in microelectronics and optoelectronics and lately in the mobile phone industry. In this work, chemical synthesis of this material was performed by complex polymerization method (CPM). The presence of pure nickel manganite phase was confirmed by X-ray analysis. The obtained fine nanoscaled powders were uniaxially pressed and sintered at different temperatures: 1000 C -1200C for 2h. Microstructure development during sintering was observed by scanning electron microscope (SEM). Indentation experiments were carried out using a three sided pyramidal (Berkovich) diamond tip. Young’s modulus of elasticity at various indentation depths and hardness of NTC ceramics were calculated. It was found that the highest hardness and elastic modulus exhibit the ceramics sintered at highest temperature

Solvothermal synthesis of Ti doped ZnO

Titanium doped zinc oxide powders were synthesized by solvothermal method. Polycrystalline powders of ZnO with different amounts of Ti -Zn1-xTixO (x=0, 1, 2, 5, 7.5, 10 at%) were obtained from ethanolic solution of zinc acetate dihydrate in the presence of lithium hydroxide and titanium citrate. Reaction was conducted in autoclave at 225 0 C and 42 bar for 6 h. Detailed structural analysis was carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Based on obtained results mechanism of Ti incorporation in ZnO lattice was discussed

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

Tailoring of BaCe0.9Y0.1O3-d Electrolyte Properties by Co-Doping

BaCe0.9Y0.1O3–δ has been known as one of the best proton conducting electrolyte, which enables its application in intermediate-temperature solid oxide fuel cells (IT-SOFC) operating between 500 °C and 700 °C. The main disadvantage of this material is its instability in a CO2-rich atmosphere that limits its application with respect to fuel selection. Therefore, many attempts has been made to improve its stability by replacing yttrium with other dopants, or by co-doping. In this study, we compared BaCe0.9Y0.1O3–δ and BaCe0.85Y0.1M0.05O3–δ (M = {In, Zr, Nb}) electrolytes by taking into consideration the dopant properties (primarily the valence, electronegativity and ionic radius) and how they influenced the microstructure, conductivity and chemical stability of doped BaCeO3. The samples were synthesized by the citric-nitric autocombustion method. BaCe0.85Y0.1In0.05O3–δ was sintered at 1400 °C for 5 h in air, while the temperature of 1550 °C was required for the other materials to complete the sintering. This makes the doping with In a preferable method since sintering temperatures above 1500 °C can lead to a certain materials degradation resulting in BaO loss. The total conductivities (σ) measured at 700 °C in wet hydrogen decreased in the following order: BaCe0.9Y0.1O3–δ > BaCe0.85Y0.1Zr0.05O3–δ > BaCe0.85Y0.1Nb0.05O3–δ > BaCe0.85Y0.1In0.05O3–δ. By comparing the stability of the ceramics exposed to a 100% CO2 atmosphere at 700 °C for 5 h and examined by X-ray analysis, it was observed that only BaCe0.85Y0.1In0.05O3–δ could sustain the aggressive environment. The exposed sample contained only traces of secondary phases, while the other compositions were partially or significantly decomposed. By taking into account the values of the Goldschmidt tolerance factor (t) and dopant electronegativit

Microstructural, Thermoelectric and Mechanical Properties of Cu Substituted NaCo2O4

Polycrystalline samples of NaCo2−xCuxO4 (x = 0, 0.01, 0.03, 0.05) were obtained from powder precursors synthesized by a mechanochemically assisted solid-state reaction method (MASSR) and a citric acid complex method (CAC). Ceramic samples were prepared by pressing into disc-shaped pellets and subsequently sintering at 880 °C in an argon atmosphere. Effects of low concentrations of Cu doping and the above-mentioned synthesis procedures on the thermoelectric and mechanical properties were observed. The electrical resistivity (ρ), the thermal conductivity (κ) and the Seebeck coefficient (S) were measured simultaneously in the temperature gradient (ΔT) between the hot and cold side of the sample, and the figure of merit (ZT) was subsequently calculated. The ZT of the CAC samples was higher compared with the MASSR samples. The highest ZT value of 0.061 at ΔT = 473 K was obtained for the sample with 5 mol% of Cu prepared by the CAC method. The CAC samples showed better mechanical properties compared to the MASSR samples due to the higher hardness of the CAC samples which is a consequence of homogeneous microstructure and higher density obtained during sintering of these samples. The results confirmed that, besides the concentration of Cu, the synthesis procedure considerably affected the thermoelectric and mechanical properties of NaCo2O4 (NCO) ceramics

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