Comparative properties of ternary oxides of ZrO2-TiO2-Y2O3 obtained by laser ablation, magnetron sputtering and sol-gel techniques

Thin films of ternary oxides of zirconium (Zr), yttrium (Y) and titanium (Ti) were obtained by the sol-gel, magnetron sputtering and laser ablation techniques. Zirconium propoxide, titanium isopropoxide and yttrium nitrate hexahydrate reagents were used as precursors in the sol-gel process. The Zr/Ti and Zr/Y molar ratios have been controlled by varying the precursors and surfactant concentration. The obtained gels were supported by dip coating on α-alumina supports, dried at 373 K and calcinated at 873 K. The powders obtained after the removal of carbon residuals were subsequently pressed and calcinated for using as targets for the magnetron sputtering and pulsed laser deposition techniques. The chemical composition was determined by Auger Electron Spectroscopy. The surface topography was investigated by Atomic Force Microscopy, while the crystallinity was evaluated from X-ray diffraction. The electrical response of the deposits to nitrogen oxides (NO_x) toxic gas is discussed according to the experimental conditions.     

Properties of ytterbium and neodymium doped alkali metal yttrium double phosphates of the M3Y1−xLnx(PO4)2 type

The spectroscopic behaviour of the Nd³⁺ and Yb³⁺ doped alkaline metal yttrium double phosphates, M₃Y_1−xLn_x(PO₄)₂ (M=Na, Rb; x=0.01–0.3) were studied for both powder and single crystal samples. The high resolution absorption and emission spectra were measured in the visible and IR regions. Spectral changes with the Nd³⁺ and Yb³⁺ concentration were interpreted. The absorption strengths of the 4f–4f transitions were analysed and used to assess the structural modifications of the two double phosphates. Based on the 4 K absorption spectra the number of metal sites occupied by the dopants was investigated.

Strong emission from Na₃Y_1−xNd_x(PO₄)₂ involving the ⁴F_3/2→⁴I_9/2,⁴I_11/2,⁴I_13/2,⁴I_15/2 transitions were observed whereas the corresponding emission from the rubidium phosphate was presumably quenched by multiphonon processes due to the water molecules absorbed in the channel-like structure.

The IR spectra were used to assign the vibronic components of the electronic transitions. The Yb³⁺ emission bands were broadened depending on the Yb³⁺ concentration (1–10 mol%). The tentative energy level scheme of the ground and excited ²F_J (J=7/2, 5/2) levels was described.     

Er- and Nd-doped yttrium aluminosilicate glasses: Preparation and characterization

The undoped, and Nd-, Er-doped low silica high alumina yttrium aluminosilicate (YAS) glasses were prepared by flame synthesis in the form of transparent glass microbeads with diameters ranging from a few to several tens of micrometers. The silica content ranged between 5 and 20 mol.%. The prepared glass microbeads were characterized by optical microscopy, SEM XRD, FT-IR and UV–VIS–NIR spectroscopy. The glass forming ability of glasses, expressed in terms of the difference between the glass transition temperature, T_g, and the onset of crystallization, T_x, improved with increasing silica content. Doping of YAS15 glass with neodymium or erbium at the level of 1–5 mol.% leads to decrease of both the T_g and T_x. However, the glass forming ability was not affected. The UV–VIS–NIR reflectance spectra in the spectral range from 300 to 1800 nm shows characteristic absorptions, due to the optically active Nd³⁺ and Er³⁺ ions in the host glass.     

Growth and structural investigations of epitaxial hexagonal YMnO3 thin films deposited on wurtzite GaN(001) substrates

Epitaxial hexagonal YMnO₃ (h-YMnO₃) films having sharp (00l) X-ray diffraction peaks were grown above 700 °C in 5 mTorr O₂ via pulsed laser deposition both on as-received wurtzite GaN/AlN/6H-SiC(001) (w-GaN) substrates as well as on w-GaN surfaces that were etched in 50% HF solution. High-resolution transmission electron microscopy revealed an interfacial layer between film and the unetched substrate; this layer was absent in those samples wherein an etched substrate was used. However, the substrate treatment did not affect the epitaxial arrangement between the h-YMnO₃ film and w-GaN substrate. The epitaxial relationships of the h-YMnO₃ films with the w-GaN(001) substrate was determined via X-ray diffraction to be (001)_YMnO3 ‖ (001)_GaN : [11¯0]_YMnO3 ‖ [110]_GaN; in other words, the basal planes of the film and the substrate are aligned parallel to one another, as are the most densely packed directions in planes of the film and the substrate. Interestingly, this arrangement has a larger lattice mismatch than if the principal axes of the unit cells were aligned.     

Effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrochemical performance of anode-supported solid oxide fuel cells

Lowering operating temperature and optimizing electrolyte thickness, while maintaining the same high efficiencies are the main considerations in fabricating solid oxide fuel cells (SOFCs). In this study, the effect of yttrium-stabilized bismuth bilayer electrolyte thickness on the electrical performance was investigated. The yttrium-stabilized bismuth bilayer electrolyte was coated on the nickel–samarium-doped composite anode/samarium-doped ceria electrolyte substrate with varying bilayer electrolyte thicknesses (1.5, 3.5, 5.5, and 7.5 μm) via dip-coating technique. Electrochemical performance analysis revealed that the bilayer electrolyte with 5.5 μm thickness exhibited high open circuit voltage, current and power densities of 1.068 V, 259.5 mA/cm² and 86 mW/cm², respectively at 600 °C. Moreover, electrochemical impedance spectroscopy analysis also exhibited low total polarization resistance (4.64 Ωcm²) at 600 °C for the single SOFC with 5.5 μm thick yttrium-stabilized bismuth bilayer electrolyte. These findings confirm that the yttrium-stabilized bismuth bilayer electrolyte contributes to oxygen reduction reaction and successfully blocks electronic conduction in Sm_0.2Ce_0.8O_1.9 electrolyte materials. This study has successfully produced an Y_0.25Bi_0.75O_1.5/Sm_0.2Ce_0.8O_1.9 bilayer system with an extremely low total polarization resistance for low-temperature SOFCs.     

Efficient strategy to boost the electrochemical performance of yttrium stabilized zirconia electrolyte solid oxide fuel cell for low-temperature applications

Yttrium stabilized zirconia (YSZ) used as the state-of-the-art electrolyte for solid oxide fuel cells (SOFCs) requires high temperature (over 800 °C) to realize sufficient oxygen ion conductivity. Thus, the high operational temperature is the main restriction for the commercial process of YSZ-based SOFCs. To obtain decent ionic conductivity at intermediate-low temperatures, Sr-free cathode LaNiO₃ is introduced into YSZ to construct a novel LaNiO₃-YSZ composite electrolyte, which is sandwiched by two Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) electrodes to assemble systematical fuel cells. This device presents an excellent peak output of 1045 mW cm^-2 at 600 °C and even 399 mW cm^-2 at 450 °C. A series of characterizations indicates that the oxygen ion conductivity of the LaNiO₃-YSZ composite is significantly promoted in comparison with that of pure YSZ, and the LaNiO₃ component has certain proton conductivity after hydrogenation. Both of the two factors contributes to the superior performance of such devices at intermediate-low temperatures. Furthermore, the sharp decrease in electronic conductivity for LaNiO₃ in hydrogen atmosphere combined with Schottky junction at the anode-electrolyte interface eliminates the short-circuiting problem. Our work demonstrates that incorporating Sr-free cathode LaNiO₃ into the YSZ electrolyte is an efficient strategy to boost the performance and reduce the operational temperature of YSZ-based SOFCs.