The across-plane conductivity and microstructure of SrZr0.95Y0.05O3 − δ thin films

The across-plane electrical conductivity of the proton-conducting SrZr_0.95Y_0.05O_3???δ thin films fabricated by chemical solution deposition on single-crystalline ZrO₂ doped by 10 mol% of Y₂O₃ (YSZ) substrates was investigated using impedance spectroscopy. The average grain size of the films was found to increase considerably with thermal treatment. This change in grain size has a strong effect on the electrical behavior of films. Our results show that the electrochemical performance of the cell is strongly affected by the potential difference at the film/substrate interface. Coarse-grain film microstructure was proved to be preferable for the reduction of both the film resistance and interfacial barrier.     

The effect of alumina addition on the conductivity, microstructure and mechanical strength of zirconia — yttria electrolytes

The effect of alumina additions on the ionic conductivity and mechanical properties of 3 and 8 mol% Y₂O₃ - ZrO₂ compositions has been investigated. Such materials are of interest for use in fuel cells and other similar applications. Sintered specimens were characterised by XRD, SEM, impedance spectroscopy, four — probe DC conductivity and mechanical strength measurements. Alumina additions had no affect on the conductivity degradation behaviour at 1000 °C and the activation energy but resulted in an increase in the strength of 8 mol% Y₂O₃ - ZrO₂ composition by about 25%. However, a significant drop in the ionic conductivity well above what could be accounted for by the decrease in the volume fraction of the conducting phase, was observed.     

Development of ceria-doped zirconia electrolytes with high toughness and ionic conductivity

Development of Y₂O₃ stabilized ZrO₂ electrolytes for solid oxide fuel cell with better mechanical properties was attempted. 3 mol% Y₂O₃ stabilized ZrO₂ doped with 3–15 mol% CeO₂ was investigated in the present work. The results reveal that the toughness and the bending strength of 3–6 mol% CeO₂ doped 3 mol% Y₂O₃–ZrO₂ are much higher than that of 8 mol% Y₂O₃–ZrO₂. The best ionic conductivity observed in 6 mol% ceria-doped 3 mol% Y₂O₃–ZrO₂ electrolyte is better than that of 8 mol% Y₂O₃–ZrO₂ at 800 °C, which indicates the possibility of developing ZrO₂-based electrolyte with enhanced toughness.     

Microstructure, hardness, and electrical behavior of Y-doped CaZrO3 films prepared by chemical solution deposition

CaZr_0.9Y_0.1O_3???δ films were fabricated by chemical solution deposition on single-crystalline YSZ (ZrO₂ doped by 10 mol% of Y₂O₃) substrates. Mechanical hardness and morphology of the films were studied using nano-indentation and atomic force microscopy techniques. Grain microstructure of the films has been shown to depend upon thermal treatment duration and film thickness. Thin films with grains a few times greater than the film thickness have been obtained. It has been shown that thickness of films can be evaluated by comparing of the indentation curves for the clean and coated by the film substrates. Mechanical hardness of the film has been found to be sensitive to the film grain microstructure. Electrical behavior of CaZr_0.9Y_0.1O_3???δ films studied by impedance spectroscopy strongly depends on the film microstructure.     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

The transport and thermal properties of glassy LiPO<Subscript>3</Subscript>/crystalline Al<Subscript>2</Subscript>O<Subscript>3</Subscript> (ZrO<Subscript>2</Subscript>) composite electrolytes

Glassy LiPO₃/crystalline Al₂O₃ and glassy LiPO₃/crystalline ZrO₂ (0–12.5 vol.% of oxide fillers) composite solid electrolytes have been prepared by glass matrix softening. Their thermal and transport properties have been investigated by differential scanning calorimetry (DSC) and impedance spectroscopy methods. The addition of ZrO₂ leads to a decrease in the crystallization temperature of LiPO₃ glass. It was found that the conductivity behavior depends on the nature of the dispersed addition. In the case of the Al₂O₃ addition, the increase in the electrical conductivity is observed. The ionic conductivity of the LiPO₃/10% Al₂O₃ composite reaches 5.8 × 10^?8 S/cm at room temperature. In contrast, the conductivity in the LiPO₃/ZrO₂ composite system decreases.     

Pulsed laser deposition of high temperature protonic films

Pulsed laser deposition has been used to fabricate nanostructured BaCe_0.85Y_0.15O_3−δ films. Protonic conduction of the fabricated BaCe_0.85Y_0.15O_3−δ films was compared to the sintered BaCe_0.85Y_0.15O_3−δ. Sintered samples and laser targets were prepared by sintering BaCe_0.85Y_0.15O_3−δ powders derived by solid state synthesis. Films 1 to 8 μm thick were deposited by KrF excimer laser on porous Al₂O₃ substrates. Thin films were fabricated at deposition temperatures of 700 to 950 °C at O₂ pressures up to 200 mTorr using laser pulse energy densities of 1.4–3 J/cm². Fabricated films were characterized by X-ray diffraction, electron microscopy and electrical impedance spectroscopy. Single phase BaCe_0.85Y_0.15O_3−δ films with a columnar growth morphology are observed with preferred crystal growth along the [100] or [001] direction. Results indicate [100] growth dependence upon laser pulse energy. Electrical conductivity of bulk samples produced by solid state sintering and thin film samples were measured over a temperature range of 100 to 900 °C. Electrical conduction behavior was dependent upon film deposition temperature. Maximum conductivity occurs at deposition temperature of 900 °C; the electrical conductivity exceeds the sintered specimen. All other deposited films exhibit a lower electrical conductivity than the sintered specimen. Activation energy for electrical conduction showed dependence upon deposition temperature, it varied from 115 to 54 kJ. Film microstructure was attributed to the difference in electrical conductivity of the BaCe_0.85Y_0.15O_3−δ films.     

Electrical properties of modified TZP(+TiO2,FeOx)

1 to 24 mol% TiO₂ and FeO_X were added to 3.2 mol% Y₂O₃ doped ZrO₂ (Z3Y) to obtain tetragonal zirconia polycrystals (TZP) with modified electronic properties. The materials were prepared by coprecipitation which allows to obtain fine, homogeneous and sinteractive powders. The solubility of TiO₂ in Z3Y can reach 24 mol%, while the maximum solubility of FeO_x is about 8 mol%. The impedance results show a decrease of the bulk and total conductivity of titania doped TZP with increasing titania concentration, while those of FeO_x doped TZP show only minor changes. The Hebb-Wagner polarization method was applied to evaluate the partial hole and electron conductivities. Three models are presented to interpret the polarization curves. 1.6 mol% FeO_x doped TZP has higher hole conductivity in air, while titania doped TZP has higher electronic conductivity at low oxygen partial pressures as compared to pure Z3Y. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, 11–18 Sept.1994     

Electrochemical characterization of nanostructured zirconias

Redox stability of cubic nanostructured zirconia ceramics, free of any secondary phases, has been investigated experimentally as a function of grain size. Pure 8 mol% Y₂O₃-doped ZrO₂ powders were synthesized by a spray pyrolysis process and then compacted by uniaxial pressing, followed by cold isostatic pressing. Using appropriate thermal treatments, average grain sizes ranging from 25 to 242 nm and relative densities from 71% up to 98% were obtained. An electrochemical characterization was performed with comparison on ceramics of 3.2 and 7.6 μm and 98% of theoretical density starting from commercial YSZ powder.     

Correlation between microstructure and degradation in conductivity for cubic Y2O3-doped ZrO2

The application of Yttria-stabilized Zirconia (YSZ) as solid electrolyte in high-temperature solid oxide fuel cells (SOFC) is well established. However, the strong decrease of the ionic conductivity in 8.5 mol% Y₂O₃-doped ZrO₂ at high temperature has not yet been clarified completely. To contribute to the understanding of the degradation process, transmission electron microscopy (TEM) was applied to analyze the microstructure in YSZ electrolyte substrates in as-sintered and aged material. Selected area electron diffraction and conventional TEM imaging were performed to investigate the evolution of different phases and phase transitions in YSZ. Grain boundary charging and the possible formation of a glassy phase at grain boundaries after aging were investigated using transmission electron holography and high-resolution TEM. The ionic conductivity was characterized by dc-conductivity measurements and impedance spectroscopy.     

Structure,phase composition,and spectral luminescence properties of partially stabilized zirconium dioxide crystals doped with Yb<Superscript>3+</Superscript> ions

Investigation of partially stabilized zirconium dioxide crystals via transmission electron microscopy has revealed a developed twin structure therein. The following compositions of the above crystals have been selected for this study: 97.2 mol % ZrO₂–1.0 mol % Y₂O₃–1.8 mol % Yb₂O₃; 97.2 mol % ZrO₂–2.0 mol % Y₂O₃–0.8 mol % Yb₂O₃; 97.2 mol % ZrO₂–2.5 mol % Y₂O₃–0.3 mol % Yb₂O₃; and 96.3 mol % ZrO₂–3.4 mol % Y₂O₃–0.3 mol % Yb₂O₃. X-ray diffraction analysis of these crystals indicate the presence of transformable (t) and nontransformable (t') tetragonal phases. Optical spectroscopy measurements of ZrO₂–Y₂O₃–Yb₂O₃ crystals with tetragonal and cubic structures have highlighted in Yb³⁺-doped zirconium dioxide samples the formation of the optical centers of the Yb³⁺ ions is observed, whose crystal surrounding is similar to those in cubic zirconium dioxide crystals.     

Stress-induced cubic-tetragonal transformation in partially stabilized ZrO2: Raman spectroscopy study

Regularities of the cubic-tetragonal transformation (C→t′) in partially stabilized zirconia were studied with Raman spectroscopy and high-temperature Raman spectroscopy techniques. New ‘low temperature’ mechanism of tetragonal nanoparticles formation in a volume of cubic solid solution was revealed in ZrO₂-Gd₂O₃ (Eu₂O₃) (6-8 mol%) single crystals. This mechanism includes nucleation of the tetragonal nanoparticles due to diffusionless C→t′ phase transformation at the first stage and gradual decrease of the stabilizer concentration inside t′-domains after subsequent low-temperature annealing. Predominant orientation of tetragonal domains due to the stress-induced C→t′ transformation was registered in ZrO₂-Gd₂O₃ (8 mol%) single crystals.     

Impedance spectroscopy study of the sintering of yttria-stabilized zirconia/magnesia composites

In the present study, the sintering of (ZrO₂:8 mol%Y₂O₃)_1 ? m–(MgO)_m, YSZ–mMg composites, with m in the 0–30 mol% range, has been investigated by impedance spectroscopy (IS), dilatometry, and X-ray diffraction. Impedance diagrams were collected at 400 °C after heating the green compacts up to a selected sintering temperature, which was increased stepwise from 800 to 1400 °C. The combined experimental results revealed that the samples can be separated in two categories: below and above the solubility limit of MgO in the YSZ (m ~ 10). Moreover, important microstructural features associated with both the sintering process and solid solution formation of YSZ–mMgO samples were correlated to the electrical properties inferred by IS.     

Electrochemical characterization of mixed conducting and composite SOFC cathodes

The electrochemical performance of La_0.58Sr_0.4Co_0.2Fe_0.8O_3−δ (L58SCF), La_0.9Sr_1.1FeO_4−δ (LS2F) and LSM (La_0.65Sr_0.3MnO_3−δ)/LSM–YSZ (50 wt.% LSM–50 wt.% ZrO₂ (8 mol% Y₂O₃)) cathode electrodes interfaced to a double layer Ce_0.8Gd_0.2O_2−δ (CGO)/YSZ electrolyte was studied in the temperature range of 600 to 850 °C and under flow of 21% O₂/He mixture, using impedance spectroscopy and current density–overpotential measurements. The L58SCF cathode exhibited the highest electrocatalytic activity for oxygen reduction, according to the order: LS2F/CGO/YSZ ≤ LSM/LSM–YSZ/CGO/YSZ < L58SCF/CGO/YSZ.     

Oxygen diffusion studies on (Y2O3)2(Sc2O3)9(ZrO2)89

The oxygen tracer diffusion coefficient (D?) has been measured for 9 mol% scandia 2 mol% yttria co-doped zirconia solid solution, (Y₂O₃)₂(Sc₂O₃)₉(ZrO₂)₈₉, using isotopic exchange and line scanning by Secondary Ion Mass Spectrometry, as a function of temperature. The values of the tracer diffusion coefficient are in the range of 10^? 8–10^? 7 cm² s^? 1 and the Arrhenius activation energy was calculated to be 0.9 eV; both valid in the temperature range of 600–900 °C. Electrical conductivity measurements were carried out using 2-probe and 4-probe AC impedance spectroscopy, and a 4-point DC method at various temperatures. There is a good agreement between the measured tracer diffusion coefficients (D?, Ea = 0.9 eV) and the diffusion coefficients calculated from the DC total conductivity data (D_σ, Ea = 1.0 eV), the latter calculated using the Nernst–Einstein relationship.     

Photoluminescence of Y2O3:Eu thin film phosphors by sol-gel deposition and rapid thermal annealing

Y₂O₃:Eu³⁺ phosphor films have been developed by using the sol-gel process. Comprehensive characterization methods such as Photoluminescent (PL) spectroscopy, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy were used to characterize the Y₂O₃:Eu³⁺ phosphor films. In this experiment, the XRD profiles show that the Y₂O₃:Eu³⁺ phosphor films crystallization temperature and optimum annealing temperature occur at about 650 and 750 °C, respectively. The optimum dopant concentration is 12 mol% Eu³⁺ and the critical transfer distance (R_c) among Eu³⁺ ions is calculated to be about 0.84 nm. Vacuum environment is more efficient than oxygen and nitrogen to eliminate the OH content and hence yields higher luminescent phosphor films. The PL emission intensity of Y₂O₃:Eu³⁺ phosphor films is also dependent on the annealing time. It was found that the H₂O impurities were effectively eliminated after annealing time of 25 s at 750 °C in vacuum environment. From the experiment results, the schematic energy band diagram of Y₂O₃:Eu³⁺ phosphor films is constructed.     

Horizontally networked carbon nanotubes grown on Au–Fe catalyst nanoparticles

Undoped and doped indium tin oxide (ITO) with different concentrations (2, 4, and 6 mol%) of different dopants (CuO, Cr₂O₃, and ZrO₂) were prepared in the nano-size scale (19–33 nm) using Pechini method. The thermal decomposition of the precursors was studied. The electrical properties of all the prepared samples were investigated. All the investigated systems have higher conductivity than that of ITO. For ITO doped with CuO, as the concentration of CuO increases, the conductivity increases. The highest conductivity was obtained for ITO doped with 6 mol% of CuO. For ITO doped with Cr₂O₃, as the concentration of Cr₂O₃ increases the conductivity increases and above 4 mol% Cr₂O₃ the conductivity decreases. For ITO doped with ZrO₂, as the ZrO₂ concentration increases, the conductivity increases up to 4 mol% of ZrO₂ and then decreases. The band gap was detected for all the investigated systems.     

Effect of ion nitriding on the crystal structure of 3 mol% Y2O3-doped ZrO2 thin-films prepared by the sol-gel method

We investigated the effect of ion nitriding on the crystal structure of 3 mol% Y₂O₃-doped ZrO₂ (3YSZ) thin-films prepared by the sol-gel method. For this purpose, we used X-ray diffractometry to determine the crystalline phases, the lattice parameters, the crystal sizes, and the lattice microstrains, and glow discharge-optical emission spectroscopy to obtain the depth profiles of the elemental chemical composition. We found that nitrogen atoms substitute oxygen atoms in the 3YSZ crystal, thus leading to the formation of unsaturated-substitutional solid solutions with reduced lattice parameters and Zr_0.94Y_0.06O_1.72N_0.17 stoichiometric formula. We also found that ion nitriding does not affect the grain size, but does generate lattice microstrains due to the increase in point defects in the crystalline lattice.     

Sinterability of 8Y–ZrO2 powders hydrothermally synthesized at low temperature

Nanometric yttria (8 mol%)-stabilized zirconia powders were hydrothermally synthesized at 110 °C for 7 days in the presence of dilute (0.20 M) or concentrated (2.0 M) solutions of (KOH+K₂CO₃) mineralizer. Zirconia xerogel, crystalline Y(OH)₃, crystalline Y₂O₃ and a xerogel of coprecipitated (Y–Zr) hydroxide were used as starting materials. Setting the content of yttria constant and equal to 8 mol%, three types of mixtures were tested. Zirconia xerogel in mixture with crystalline Y₂O₃, zirconia xerogel in mixture with crystalline Y(OH)₃ and, finally, a xerogel of coprecipitated (Y–Zr) hydroxide were hydrothermally treated.The different characteristics of the resulting powders are discussed in terms of both the mineralizer concentration and the type of Y-based precursor used in the hydrothermal treatments, respectively.Weakly agglomerated cubic ZrO₂ powders with primary particles bigger in size and without any preliminary treatment show better performances when they are directly sintered at 1500 °C.     

Intrinsic luminescence centers in scandia,yttria, and their solid solutions

Pulsed cathodoluminescence (PCL) of Y₂O₃ and Sc₂O₃ powders, as well as of ceramic samples of binary (11 mol % Sc₂O₃–ZrO₂ and 10 mol % Y₂O₃–ZrO₂) and ternary (xSc₂O₃–(10–x)Y₂O₃–ZrO₂) (x = 5, 6, 7, 8 mol %) solid solutions are studied in the range of 300–850 nm at room temperature. In Y₂O₃ and Sc₂O₃, series of strong narrow luminescence bands emitted by surface bound radicals _...0^...0>-Y=O and _...0^...0>-Sc=O are found. The PCL spectra of xSc₂O₃–(10–x)Y₂O₃–ZrO₂ ceramic samples showed the same series of narrow bands at 543, 551, 555, 572, 583, 594, 614, and 639 nm as the yttrium oxide spectra. The existence of these luminescence bands, which correspond to the emission of the _...0^...0>-Y=O radical, and the absence of the emission lines of the _...0^...0>-Sc=O radical indicate that yttrium ions, due to their larger radius, are the first that are displaced to the surface of crystallites in these systems, which is accompanied by the formation of the second phase in subsurface layers.