Research on CeO2 cap layer for YBCO-coated conductor

Two groups of coated conductor samples with different thicknesses of CeO2 cap layers deposited by pulsed laser deposition （PLD） under the same conditions have been studied. Of them, one group is of CeO2 films, which are deposited on stainless steel （SS） tapes coated by IBAD-YSZ （IBAD-YSZ/SS）, and the other group is of CeO2/YSZ/Y2O3 multilayers, which are deposited on NiW substrates by PLD for the fabrication of YBCO-coated conductor through the RABiTS approach. YBCO film is then deposited on the tops of both types of buffer layers by PLD. The effects of the thickness of the CeO2 film on the texture of the CeO2 film and the critical current density （Jc） of the YBCO film are analysed. For the case of CeO2 film on IBAD-YSZ/SS, there appears a self-epitaxy effect with increasing thickness of the CeO2 film. For CeO2/YSZ/Y2O3/NiW, in which the buffer layers are deposited by PLD, there occurs no self-epitaxy effect, and the optimal thickness of CeO2 is about 50 nm. The surface morphologies of the two groups of samples are examined by SEM.     

Epitaxial growth of YSZ buffer layer for YBa2Cu3O7−δ coated conductor by reel-to-reel pulsed laser deposition

Yttria-stabilized zirconia (YSZ) buffer layers were deposited on CeO₂ buffered biaxially textured Ni-W substrate by reel-to-reel pulsed laser deposition (PLD) for the application of YBa₂Cu₃O_7−δ (YBCO) coated conductor and the influence of substrate temperature and laser energy on their crystallinity and microstructure were studied. YSZ thin films were prepared with substrate temperature ranging from 600 to 800 °C and laser energy ranging from 120 to 350 mJ. X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to investigate how thin film structure and surface morphology depend on these parameters. It was found that the YSZ films grown at substrate temperature below 600 °C or laser energy above 300 mJ showed amorphous phase, the (0 0 1) preferred orientation and the crystallinity of the YSZ films were improved with increasing the temperature, but the surface roughness increased simultaneously, the SEM images of YSZ films on CeO₂/NiW tapes showed surface morphologies without micro-cracks. Based on these results, we developed the epitaxial PLD-YSZ buffer layer process at the tape transfer speed of 3-4 m/h by the reel-to-reel system for 100 m class long YBCO tapes.     

Y2O3 stabilized ZrO2 thin films deposited by electron beam evaporation: Structural, morphological characterization and laser induced damage threshold

Four kinds of Y₂O₃ stabilized ZrO₂ (YSZ) thin films with different Y₂O₃ content have been prepared on BK7 substrates by electron-beam evaporation method. Structural properties and surface morphology of thin films were investigated by X-ray diffraction (XRD) spectra and scanning probe microscope. Laser induced damage threshold (LIDT) was determined. It was found that crystalline phase and microstructure of YSZ thin films was dependent on Y₂O₃ molar content. YSZ thin films changed from monoclinic phase to high temperature phase (tetragonal and cubic) with the increase of Y₂O₃ content. The LIDT of stabilized thin film is more than that of unstabilized thin films. The reason is that ZrO₂ material undergoes phase transition during the course of e-beam evaporation resulting in more numbers of defects compared to that of YSZ thin films. These defects act as absorptive center and the original breakdown points.     

A Ni/YSZ composite containing Ce0.9Ca0.1O2−δ particles as an anode for SOFCs

A composite material (hereafter referred to as NYC) containing Ni, Y₂O₃-stabilized ZrO₂ (YSZ) and Ce_0.9Ca_0.1O_2−δ (CC10) particles was prepared and used as the anode of solid oxide fuel cells (SOFCs). The performance of NYC was better than that of conventional Ni/YSZ anodes in terms of anodic overpotential and interface impedance. The additional CC10 particles improved the anode properties. XRD results suggest that a solid solution of YSZ and CC10 was produced. From impedance measurements, it is concluded that the solid solution exhibits substantial electronic conduction. Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ anodes exhibited the best properties over the experimental temperature range. A SOFC with an anode of Ni/YSZ/15 wt% Ce_0.9Ca_0.1O_2−δ provided the maximum power density and current density. Addition of CC10 with an average particle size of 0.3 μm was more advantageous than that with an average size of 3 μm.     

The properties of the Y2O3 films exposed at elevated temperature

Yttrium trioxide (Y₂O₃) thin films have been deposited on silicon (1 1 1) substrates by RF magnetron sputtering. The influences of thermal exposure at high temperature in air on the structure, the surface morphology, roughness, and the refractive index of the Y₂O₃ thin film were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE). The results indicate that chemical composition of the as-deposited Y₂O₃ film is apparently close to the stoichiometric ratio, and it has a cubic polycrystalline structure but the crystallinity is poor. The monoclinic and cubic phases can coexist in the Y₂O₃ film after thermal exposure to 900 °C, and the monoclinic phase disappears completely after 300 s exposure to 950 °C. The changes of the surface morphology, roughness, and the refractive index of the Y₂O₃ film are closely related to the crystal structure, the internal stress, and various defects influenced by thermal exposure temperature and time.     

Effect of CeO2 buffer layer thickness on the structures and properties of YBCO coated conductors

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on inclined-substrate-deposited (ISD) MgO-textured metal substrates by pulsed laser deposition. CeO₂ was deposited as a buffer layer prior to YBCO growth. CeO₂ layers of different thickness were prepared to evaluate the thickness dependence of the YBCO films. The biaxial alignment features of the films were examined by X-ray diffraction 2θ-scans, pole-figure, ?-scans and rocking curves of Ω angles. The significant influence of the CeO₂ thickness on the structure and properties of the YBCO films were demonstrated and the optimal thickness was found to be about 10 nm. High values of T_c = 91 K and J_c = 5.5 × 10⁵ A/cm² were obtained on YBCO films with optimal CeO₂ thickness at 77 K in zero field. The possible mechanisms responsible for the dependence of the structure and the properties of the YBCO films on the thickness of the CeO₂ buffer layers are discussed.     

Synthesis and characterization of thermoluminescent glass-ceramics Li2O-Al2O3-SiO2:CeO2

Vitroceramic powders of Li₂O-Al₂O₃-SiO₂ systems (LAS), doped with 1% (LAS:1Ce) and 10% (LAS:10Ce) molar of cerianite (CeO₂) were synthesized by means of the gelification technique of metal formates of aluminum and lithium, in the presence of tetraethoxy silane and CeO₂. The gels obtained were dried (120 °C, 2.5 h), calcined (480 °C, 5 h) and sinterized (1250 °C, 30 min). The sinterized samples were characterized by X-ray difraction (XRD), scanning electron microscopy (SEM) and microchemical analysis (EDS). There is evidence for a mixture of two phases of 64% β-spodumene (Li₂O-Al₂O₃-4SiO₂) and 36% β-eucryptite (Li₂O-Al₂O₃-2SiO₂). The LAS:1Ce system was enriched in aluminum, the LAS:10Ce system showed areas of heterogeneous composition; some regions with a shortage of CeO₂, while others zones with cerium cumulus. From the microscopy images it was found that CeO₂ acts as a densificant agent in LAS system, favoring the sintering in the host. The chemical route and the sintering processes utilized allow the production of samples exhibiting an acceptable linear correlation between total thermoluminescent emission intensity and the irradiation dose when the CeO₂ concentration is low (less than 1%), opening the possibility of using this kind of glass-ceramic in dosimetry.     

Conversion of covalent to ionic character of V2O5–CeO2–PbO–B2O3 glasses for solid state ionic devices

xV₂O₅–xCeO₂–(30−x)PbO–(70−x) B₂O₃ glasses are synthesized by using the melt quench technique. The number of studies such as XRD, density, molar volume, optical band gap, refractive index and FTIR spectroscopy are employed to characterize the glasses. The band gap decreases from 2.20 to 1.78 eV and density increases from 3.49 to 4.25 g/cm³. FTIR spectroscopy reveals that incorporation of V₂O₅ in glass network helps to convert the structural units of [BO₃] into [BO₄]. At higher concentration of vanadium, VO vibration of [VO₅] structural units and V–O–V vibration are present. The bond ionicity of glasses increases with incorporation of V₂O₅ contents.     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

Large room temperature magnetoresistance in YSZ doped La0.67Ba0.33MnO3 composite

The temperature dependence of the resistivity for composite samples of (1−x)La_0.67Ba_0.33MnO₃+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y₂O₃+92 mol% ZrO₂). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (T_P) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.     

Theoretical investigation of optical spectra and covalent effect of Cr in Y2Ti2O7 and Y2Sn2O7

In this work, the complete matrix of optical spectral levels in trigonal symmetry of 3d² (3d⁸) ions are established on basis of strong field coupling mechanism by using two spin–orbit coupling parameters model. The contribution of the spin–orbit coupling of ligand to the optical spectra has been included in these formulas. As an application, the optical spectra of Cr⁴⁺ in Y₂Ti₂O₇ and Y₂Sn₂O₇ have been studied by the complete diagonalization (energy matrix) method. The covalent effect has been studied and the difficulty about Dq parameter in explanation of optical spectra of Cr-doped Y₂Ti₂O₇ and Y₂Sn₂O₇ is removed. The theoretical results are in good agreement with observed data.     

Effect of thermal annealing on the ac impedance of Co/Al2O3/Co/Al2O3/Co double-barrier MTJs

Double-barrier magnetic tunnel junctions (DBMTJs) were prepared from Co(75 nm)/Al₂O₃(2.3 nm)/Co(5 nm)/Al₂O₃(2.3 nm)/Co(50 nm) sputtering pentalayer films. The ac electrical properties of as-deposited DBMTJs and those annealed in a vacuum at 100–350 °C for 30 min were then investigated using a complex impedance spectroscopic technique. The ac impedance responses as a function of annealing temperature were further analyzed based on Maxwell's layered dielectric barrier and Maxwell–Wagner capacitor models after considering the DBMTJs as having double-capacitor-type structures. The effect of thermal annealing on the ac transport behavior of the DBMTJs was interpreted by examining the equivalent electric circuits fitted to Nyquist plots of each different sample. The effects were found to be due to changes in the structural characteristics in both bulk and interface morphologies of Co and Al₂O₃ layers. The structural morphology determined the different ac transport modes that occurred in the DBMTJs.     

Partial electronic conductivity and electrolytic domain of bilayer electrolyte Zr0.84Y0.16O1.92/Ce0.9Gd0.1O1.95

Partial electronic conductivity and total conductivity have been determined by Hebb-Wagner polarization method and a.c. impedance spectroscopy, respectively, on bilayer electrolyte Zr_0.84Y_0.16O_1.92(YSZ)/Ce_0.9Gd_0.1O_1.95(GDC) with thickness ratios 10^− 3/1 and 10^− 4/1 at 800°, 900° and 1000 °C, respectively. While their ionic conductivities remain close to that of GDC, the electronic conductivities are suppressed the more from that of GDC towards that of YSZ the higher the thickness ratio, as expected. Even when the GDC-side is exposed to reducing atmosphere, the electronic conductivity is also suppressed, but to a less extent. It is suggested that oxygen activity distribution is discontinuous across the YSZ/GDC interface under ion-blocking condition, refuting the “continuity hypothesis” that has been usually adopted in calculating the oxygen activity distribution across a multilayer of mixed conductor oxides. The electrolytic domain widths of the bilayer electrolyte are reported depending on temperature, thickness ratio and direction of oxygen activity gradient imposed.     

Synthesis of Y2Si2O7:Eu nanocrystal and its optical properties

Nanocrystalline Y₂Si₂O₇:Eu phosphor with an average size about 60 nm is easily prepared using silica aerogel as raw material under ultrasonic irradiation and annealing temperature at 300-600 °C and this nanocrystalline decomposes into Y₂O₃:Eu and silica by heat treatment at 700-900 °C. The excitation broad band centered at 283 and 254 nm results from Eu³⁺ substituting for Y³⁺ in Y₂Si₂O₇ and Y₂O₃/SiO₂, respectively. Compared with Y₂O₃:Eu/SiO₂ crystalline, the PL excitation and emission peaks of Y₂Si₂O₇:Eu nanocrystalline red-shift and lead to the enhance of its luminescence intensity due to the different chemical surroundings of Eu³⁺ in above nanocrystallines. The decrease of PL intensity may be ascribed to quenching effect resulting from more defects in Y₂O₃:Eu/SiO₂ crystalline.     

Electrodeposition and characterization of Ni-Y2O3 composite

Nano-sized Y₂O₃ particles were codeposited with nickel by electrolytic plating from a nickel sulfate bath. The effects of the incorporated Y₂O₃ on the structure, morphology and mechanical properties (including microhardness, friction coefficient and wear resistant) of Ni-Y₂O₃ composite coatings were studied. It is observed that the addition of nano-sized Y₂O₃ particles shows apparent influence on the reduction potential and pH of the electrolyte. The incorporated Y₂O₃ increases from 1.56 wt.% to 4.4 wt.% by increasing the Y₂O₃ concentration in the plating bath from 20 to 80 g/l. XRD results reveal that the incorporated Y₂O₃ particles favour the crystal faces (2 0 0) and (2 2 0). SEM and AFM images demonstrate that the addition of Y₂O₃ particles causes a smooth and compact surface. The present study also shows that the codeposited Y₂O₃ particles in deposits decrease the friction coefficient and simultaneously reduce the wear weight loss. Ni-Y₂O₃ composite coatings reach their best microhardness and tribological properties at Y₂O₃ content 4.4 wt.% under the experiment conditions.     

Electrical conductivity of the system Y2O3CeO2

The electrical conductivity of the system Y₂O₃CeO₂ was measured in the temperature range 500–1100°C and P_o2 range 10^–7?10^?1 atm. Possible defect models were suggested on the basis of conductivity data, which were investigated as a function of temperature and of P_o2. The observed activation energies were 0.40 eV and 1.79 eV in the low- and high-temperature regions, respectively. The observed conductivity dependences on P_o2 were in the temperature range 500–750°C and at temperatures from 750–1100°C. It is suggested that the system Y₂O₃CeO₂ shows a mixed ionic plus hole conduction due to an O^″_i defect and an electronic hole conduction due to a V^'''_Y defect in the low- and high-temperature regions, respectively.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

Synthesis of Y2O3:(Li,Eu) films using phosphor powders coated with SiO2 nano particles

Y_1.9−xLi_0.1Eu_xO₃ (x=0.02, 0.05, 0.08, and 0.12) films were fabricated by spin-coating method. A colloidal silica suspension with Y_1.9−xLi_0.1Eu_xO₃ phosphor powder was exploited to obtain the highly stable and effective luminescent films onto the glass substrate. After heating as-prepared Y_1.9−xLi_0.1Eu_xO₃ films at 700 °C for 1 h, the phosphor films exhibit a high luminescent brightness as well as a strong adhesiveness on the glass substrate. The emission spectra of spin-coated and pulse-laser deposited Y_1.82Li_0.1Eu_0.08O₃ films were compared. The cathodoluminescence of the phosphor films was carried out at the anode voltage 1 kV.     

Epitaxial growth of La2Zr2O7 buffer layers for YBa2Cu3O7−δ coated conductors on metallic substrates using pulsed laser deposition

In the present work, La₂Zr₂O₇ (LZO) buffer layers were deposited using pulsed laser deposition (PLD) on various metallic substrates including epitaxial pure Ni on a LaAlO₃ (LAO) substrate as well as highly textured Ni–5 at.%W tapes. It is shown that the LZO deposited on pure Ni-buffered LAO exhibits a mixed orientation while LZO on Ni–5 at.%W grows epitaxially. This difference may be explained by the existence of a sulphur superstructure on the surface of Ni–5 at.%W tapes, promoting the epitaxial (0 0 l) nucleation of seed layers. Highly textured YBa₂Cu₃O_7?δ layers were prepared either by using a single buffer layer of LZO or bilayer buffers of CeO₂/LZO on Ni–5 at.%W. The superconducting transition temperature (T_c) increases with the LZO thickness, reaching a value of 90 K with a very narrow transition width (1.5 K) for 240 nm thick LZO layers. Inductive J_c measurements at 77 K in self-field show a value of about 0.96 MA/cm² for the thickest LZO layers, which is comparable to the value observed on standard buffer architectures such as CeO₂/YSZ/Y₂O₃.     

Preparation and characteristics of three-layer YSZ-(YSZ/Al2O3)-YSZ TBCs

Novel three-layer YSZ-(YSZ/Al₂O₃)-YSZ (6 wt.% Y₂O₃ partially stabilized ZrO₂) thermal barrier coatings (TBCs) were successfully prepared on Ni-based superalloy substrate using composite sol-gel and pressure filtration microwave sintering (PFMS) techniques. The coatings were evaluated for the cyclic oxidation resistance, thermal barrier effect and the presences of phases and microstructures. FE-SEM results indicate that the coatings were dense and crack-free. The coatings maintained their structural integrity when they were exposed at 1100 for 100 h. They exhibited superior oxidation resistance, spallation resistance and thermal insulation property compared with single-layer YSZ coatings. Moreover, the detailed mechanisms were discussed in order to understand the improved performance of the three-layer TBCs.