Síntesis e investigación del mono cristal BiSBTeSe dopado con Zr obtenido mediante el método de Bridgman

Single crystal ingot of BiSbTeSe doped with Zr was synthesized using Bridgman method. Energy dispersive spectrometry (EDS) analysis was used to determine chemical composition of studied samples as well as to check and confirm samples homogeneity. X-ray diffraction (XRD) measurements proved that obtained crystal ingot is a single cristal and confirms Bi2Te3-type compound with orientation (00l) of single crystal. Melting point was determined by dilatometrically measured shrinkage during heating. Mobility, concentration, resistivity/conductivity and Hall coefficient of BiSbTeSe doped with Zr samples were determined using a Hall Effect measurement system based on the Van der Pauw method. The Hall Effect was measured at room tem-perature with an applied magnetic field strength of 0.37 T at different current intensities. The measured ingot samples were cut and cleaved from different regions. Calculated results obtained using a Hall Effect measurement system (Ecopia, HMS-3000) were mutually compared for cleaved and cut samples. Changing of transport and electrical parameters with the increase of the current intensity was also monitored.The results confirmed that electrical and transport properties of single crystal depend on crystal growth direction and mobility was also significantly improved in comparison with theoretical value of Bi2Te3 and available literature data.El lingote del mono cristal de BiSbTeSe dopado con Zr se sintetizó utilizando el método de Bridgman. La composición química se determinó mediante análisis con espectroscopía de dispersión de energía (EDS). Mediante difracción de rayos X (DRX) se demostró que el lingote de cristal obtenido es un mono cristal y confirmó que se trata de un compuesto del tipo Bi2Te3, con orientación del mono cristal (001). El punto de fusión se determinó por medidas dilatométricas. La movilidad, concentración, resistividad/conductividad, y el coeficiente de Hall del BiSbTeSe dopado con Zr, se determinaron utilizando un sistema de medición de efecto Hall basado en el método de Van der Pauw. El efecto Hall se midió a temperatura ambiente con una intensidad de campo magnético aplicada de 0,37 T a diferentes intensidades de corriente. Las muestras de lingotes medidos se cortaron y se rompieron de diferentes regiones. Las muestras de lingotes utilizadas en las medidas fueron obtenbidas mediante corte y escisión en distintas zonas. Los resultados obtenidos utilizando un sistema de medición de efecto Hall (Ecopia, HMS-3000) en las distintas muestras, se compararon entre sí. Los resultados confirmaron que las propiedades eléctricas y de transporte del mono cristal dependen de la dirección del crecimiento del cristal. La movilidad se mejoró significativamente en comparación no solo con el valor teórico de Bi2Te3 sino también con los datos existentes en la literatura

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

Influence of pH value on particle size and morphology of zinc oxide powders obtained by solvothermal synthesis

Zinc oxide powders have been synthesized from ethanolic zinc acetate solutions in the presence of lithium hydroxide by the solvothermal method. In this work we have considered the influence of pH value on morphology and size of ZnO particles for temperature 200oC and reaction time 2 h. The ZnO powder microstructure was controlled using X-ray diffraction and field emission scanning electron microscopy. Grain size of ZnO particles ranges in the interval (40–200) nm depending on pH value. Increasing of pH value result in decreasing of particle size, changing from hexagonal to round particle form and uniforming of particle shape and size

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

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

TEM and FESEM investigation of lanthanum nickelate thin films obtained by chemical solution deposition

Lanthanum nickelate (LNO) is a perovskite oxide material with metallic conductivity in a wide temperature range which makes it suitable for application as electrode material for thin films. In this paper LNO thin films were prepared by polymerizable complex method from the diluted citrate solutions. Precursor solutions were spin coated onto Si-substrates with amorphous layer of SiO2. Deposited layers were thermally treated from the substrate side with low heating rate (1°/min) up to 700°C and finally annealed for 10 hours. Results of AFM and FESEM showed that films are very smooth (Ra = 4 nm), dense, crack-free and with large square-shaped grains (170 nm). According to FESEM and TEM results the obtained four-layered film was only 65 nm thin. EBSD and XRD analyses confirmed polycrystalline microstructure of the films without preferential orientation. It was concluded that the presence of SiO2 layer on Si substrate prevents epitaxial or oriented growth of LNO

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

The influence of spark plasma sintering temperature on the properties of Sb-doped barium stannate ceramics

Barium-stannate (BaSnO3, BSO) is a member of the perovskite-type alkaline earth stannates ASnO3 (A = Ca, Sr, Ba) with an ideal cubic crystal structure (space group: ). Doping with antimony (Sb5+) can change this wide band-gap semiconductor (Eg = 3.1-3.4 eV) into an n-type semiconductor with high electrical conductivity at room temperature. The major drawbacks in the BSO-based ceramics synthesis are phase composition and low density of final ceramic materials. These problems could be solved using spark plasma sintering (SPS), a current and pressure-assisted technique, which enables the preparation of dense ceramics at significantly lower temperatures and for a shorter time. To investigate the influence of spark plasma sintering temperature on the structural, microstructural and electrical properties of BaSn1-xSbxO3 (BSSO, x = 0.00; 0,04; 0.06; 0.08; and 0.10) ceramics samples, BSSO powders were spark plasma sintered at 1100 °C, 1200 °C and 1250 °C for 5 min. X-ray diffraction (XRD) analysis confirmed that all ceramic samples sintered at 1100 °C crystallized in a single-phased cubic BSO structure. Their relative densities were in the range of 72–82% ρt. Sintering at 1200 °C increased the samples’ relative densities to 79–96% ρt, but also induced the formation of a barium-rich secondary phase, Ba2SnO4. Rising the sintering temperature further to 1250 °C induced the melting of all samples except BaSn0.92Sb0.08O3. Field emission scanning electron microscopy (FE-SEM) revealed that doping with antimony decreased the grain sizes in BSSO samples sintered at 1100 °C and 1200 °C up to the concentration x = 0.08. Electrical measurements revealed the typical semiconductor behavior of the undoped samples, showing nonlinear current-voltage characteristic and the existence of one semicircle in their impedance spectra, characteristic for materials with double Schottky barrier at the grain boundaries. However, samples with higher dopant concentrations (x = 0.08 and 0.10) showed significantly lower electrical resistivity and linear current-voltage characteristic. The lowest and almost constant value of electrical resistivity in the temperature range of 25–150 °C, and complete loss of the semicircle in its impedance spectrum revealed the metallic-like behavior of sample BaSn0.92Sb0.08O3 sintered at 1200 °C

Structural, microstructural and electrical properties of Sb-doped BaSnO3 ceramics

BaSnO3 (BSO) belongs to the perovskite-type oxides with a cubic crystal structure. It exhibits interesting electrical, optical and photocatalytic properties. BSO has a potential application as transparent conductor, gas sensor, photocatalyst or dielectric capacitor. It is an insulating material with wide band gap (Eg = 3.1–3.4 eV), but its electrical properties can be adjusted by doping with aliovalent cations. In this work, we investigated the influence of antimony (Sb3+), as a dopant, on crystal structure, microstructural and electrical properties of BSO ceramics. Stoichiometric mixture of powders BaCO3, SnO2 and Sb2O3 was mechanochemically activated in a planetary ball mill and afterwards calcined at 900 ºC for 4 h. As-prepared powders were sintered by spark plasma sintering technique (1200 ºC for 5 min) in order to produce ceramic samples BaSn1-xSbxO3 (x = 0.00, 0.04, 0.06, 0.08 and 0.10). All samples were characterized using X-ray Diffraction (XRD) analysis, High Resolution Transmission (HRTEM) and Field Emission Electron Microscopy (FESEM). Electrical conductivity of BaSn1-xSbxO3 ceramics was determined by measuring of the current-voltage (I–U) characteristics in different mediums and at different temperatures. XRD analysis confirmed the existence of cubic BSO, as a dominant phase, and tetragonal Ba2SnO4, as a secondary phase. FESEM analysis revealed homogenous microstructure in all samples and noticeable decrease of the grain size in doped samples compared to BSO. HRTEM micrographs of the undoped sample showed less ordered microstructure with amorphous phase in the grain boundary region. Doped samples revealed much higher crystallinity, especially in the grain boundary regions without presence of defects. Low angle grain boundaries (LAGB) are observed (the angle equals 2.08°) on the HRTEM micrographs of BaSn0.92Sb0.08O3. It was observed that all doped samples are n-type semiconductors, having linear I–U characteristics up to 150 °C. Sample BaSn0.92Sb0.08O3 showed the highest conductivity, most likely due to the presence of the LAGB, which allow easier charge carrier transfer between grains and greater carrier mobility [1]. [1] Y. Furushima, A. Nakamura, E. Tochigi, Y. Ikuhara, K. Toyoura, K. Matsunaga, J. Appl. Phys., 120 (2016) 1421071–1421079