Thermoelectric properties of CaMnO3 films obtained by soft chemistry synthesis

Polycrystalline randomly oriented CaMnO3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K < T < 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO3 films at 900 °C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900 °C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavio

Structural characterization and EXAFS wavelet analysis of Yb doped ZnO by wet chemistry route

Lanthanide doped ZnO are interesting materials for optical and electrical applications. The wide band gap of this semiconductor makes it transparent in the visible range (Egap = 3.2 eV), allowing a sharp emission from intra shell transition from the lanthanides. From the electrical side, ZnO is a widely used material in varistors and its electrical properties can be tailored by the inclusion of lanthanides. Both applications are influenced by the location of the lanthanides, grain boundaries or lattice inclusion. Yb doped ZnO samples obtained by wet chemistry route were annealed at different temperatures and characterized by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Rietveld refinement of XRD data, and X-ray Absorption Fine Structure (XAFS). These techniques allowed to follow the changes occurred in the matrix and the Yb environment. The use of the Cauchy continuous wavelet transform allowed identifying a second coordination shell composed of Zn atoms, supporting the observations from XRD Rietveld refinement and XAFS fittings. The information obtained confirmed the incorporation of Yb in Oh sites of the wurtzite structure without Yb2O3 clustering in the lattice.Fil: Otal, Eugenio Hernan. Universidad Tecnológica Nacional. Facultad Regional Santa Cruz; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Sileo, Elsa Ester. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Aguirre, Myriam H.. Universidad de Zaragoza; EspañaFil: Fabregas, Ismael Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; ArgentinaFil: Kim, Manuela Leticia. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Ministerio de Defensa. Unidad de Investigación y Desarrollo Estratégico para la Defensa; Argentina. Universidad Tecnológica Nacional. Facultad Regional Santa Cruz; Argentin

On the chemical interaction of nanoscale lanthanum doped strontium titanates with common scandium and yttrium stabilized electrolyte materials

Chemical interactions between common electrolyte materials and various La doped strontium titanates (LST), which are redox-stable candidates for SOFCs anodes, were thoroughly investigated. The reactions of nanosized reagents were studied by SEM/EDX microscopy and XRD with subsequent Rietveld refinement. It was found that all A-site deficient LSTs promoted a reaction with Sc and Y stabilized zirconia, whilst stoichiometric LST was chemically stable. Detected structural and microstructural changes were solely assigned to high mobility of Ti. Diffusion of Ti into the zirconia structure promoted formation of tetragonal structures with p42/nmc-type space groups. The results indicate that the reduction of oxygen partial pressure during sintering and application of Sc-containing electrolyte material are successful strategies to hinder or even avoid reactivity

Structural and photocatalytic properties of perovskite-type (La,Ca)Ti(O,N)(3) prepared from A-site deficient precursors

Two series of oxide precursors for perovskite-type (La,Ca)Ti(O,N)(3) were prepared by adding Ca2+ to A-site deficient LaTiO3.5 heterogeneously (Ca2+-backfilling) or by substituting Ca2+ for La3+ in stoichiometric LaTiO3.5 homogeneously (Ca2+-substitution). Activity of the resultant (La,Ca)Ti(O,N)(3) for photocatalytic O-2 evolution was tested in the presence of an electron acceptor (Ag+) and a superior activity of Ca2+-backfilled LaTiO2N compared to Ca2+-substituted LaTiO2N and unsubstituted LaTiO2N was demonstrated. X-ray diffraction patterns of the precursor oxides revealed a higher degree of crystallinity in La1-xTiO3.5-3x/2 compared to La1-xCaxTiO3.5-x/2. The higher crystallinity in La1-xTiO3.5-3x/2 resulted in lower Ti3+ defect formation during the ammonolysis reaction. This was evidenced by the lower background absorption of the diffuse reflectance spectra in Ca2+-backfilled compared to Ca2+-substituted LaTiO2N. Structural refinement of the diffraction patterns revealed growing crystal sizes with the Ca2+ content, which was more pronounced for Ca2+-backfilled than for Ca2+-substituted LaTiO2N. Thus, the enhanced photocatalytic activity of Ca2+-backfilled LaTiO2N was related to the quality of the precursor oxides, which influenced the defect concentrations and crystallite sizes of the resulting oxynitrides

Flame-made visible light active TiO2:Cr photocatalysts: correlation between structural, optical and photocatalytic properties

TiO2:Cr nanoparticles with dopant concentration from 0.1 to 10 at.% were synthesized by a liquid-fed one-step flame spray synthesis. All investigated nanopowders were synthesized with a specific surface area of about 60 m2 g-1. Undoped flame-made TiO2 consisted mainly of anatase. The increase of Cr content in TiO2:Cr nanopowder was accompanied by a decrease of the anatase phase and a consequent increase of the rutile polymorph. The comparison of the XANES spectra of samples with different doping level of Cr showed that Cr is incorporated in the structure. The presence and the concentration of Cr significantly affected the optical properties of TiO2 and caused a red-shift of the fundamental absorption edge. Photocatalytic performance of TiO2 for the gas phase formaldehyde decomposition under visible light irradiation was enhanced by Cr doping and reached its maximum at 3 at.% Cr

High-temperature thermoelectric properties of W-substituted CaMnO 3

The thermoelectric properties of W-substituted CaMn1-xWxO3-δ (x = 0.01, 0.03; 0.05) samples, prepared by soft chemistry, were investigated from 300 K to 1000 K and compared to Nb-substituted CaMn0.98Nb0.02O3-δ. All compositions exhibit both an increase in absolute Seebeck coefficient and electrical resistivity with temperature. Moreover, compared to the Nb-substituted sample, the thermal conductivity of the W-substituted samples was strongly reduced. This reduction is attributed to the nearly two times greater mass of tungsten. Consequently, a ZT of 0.19 was found in CaMn0.97W0.03O3-δ at 1000 K, which was larger than ZT exhibited by the 2% Nb-doped sample.Fil: Alfaruq, Dimas S.. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Eilertsen, James. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Thiel, Philipp. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Aguirre, Myriam H.. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Otal, Eugenio Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Unidad de Investigación y Desarrollo Estratégico para la Defensa. Subsede Instituto Universitario Aeronáutico; ArgentinaFil: Populoh, Sascha. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Yoon, Songhak. EMPA. Solid State Chemistry and Catalysis; SuizaFil: Weidenkaff, Anke. EMPA. Solid State Chemistry and Catalysis; Suiz

The influence of defects formed by Ca excess and thermal post-treatments on the persistent luminescence of CaTiO_3:Pr

Red emitting CaTiO3:Pr phosphors with a nominal composition of Ca0.998+xPr0.002TiO3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca3Ti2O7 - Ca4Ti3O10) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO3 samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples

Perovskite-Type LaTiO2N Oxynitrides for Solar Water Splitting: Influence of the Synthesis Conditions

Oxynitrides with the nominal composition LaTiO2N were prepared from a La2Ti2O7 precursor by thermal ammonolysis of the oxide under an NH3 flow for different durations t (4 30 h). X-ray diffraction (XRD) indicated that phase-pure LaTiO2N samples were obtained when t 13 h. The material was further characterised by scanning electron microscopy (SEM), surface area measurements based on the BET method, thermogravimetric analysis (TGA) and UV-visible diffuse reflectance spectroscopy (DRS). The powders displayed an increased crystallite size and a decreased surface area with increasing t. The ratio of N/(N+O) increased with t from 0.22 0.27. Correlated to this nitrogen increase, a small variation of the bandgap energy was observed from 2.19 to 2.12 eV with t. All LaTiO2N samples oxidised H2O into O2 in the presence of an electron acceptor (Ag+). The O2 evolution was increased from 12 μmol/h (t = 13 h) to 22 24 μmol/h (16 h t 30 h). It was found that the nitrogen content and the amount of defects played a key role in the photocatalytic O2 evolution.ISSN:1876-610

LaTiO2N/In2O3 photoanodes with improved performance for solar water splitting

LaTiO2N photoanodes for solar water splitting were prepared by electrophoretic deposition and demonstrated the best photo-currents ever reported for this material. Further important enhancement of the performance was obtained by the use of a sputtered In2O3 overlayer