Yttria-stabilized Zirconia Thin Films Research Articles

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y2O3-stabilized ZrO2 target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.

Nanostructured YSZ thin films for solid oxide fuel cells deposited by ultrasonic spray pyrolysis

Nanostructured thin films of yttria-stabilized zirconia (YSZ) have been prepared on single-crystalline silicon substrates by ultrasonic spray pyrolysis using zirconium acetylacetonate and yttrium acetylacetonate hydrate as metallo-organic precursors dissolved in anhydrous methanol. The morphology, structure and electrical properties were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and impedance spectroscopy (IS). The substrate temperature was optimized for obtaining smooth, dense and homogeneous nanocrystalline films with grains sizes as small as 10 nm. The influence of thermal annealing on structural properties of films was studied. The activation energy measured for electrical conduction through the grains (1.14 eV) was similar to that obtained in bulk of YSZ, but for conduction through the grain boundaries it acquires a value of 0.79 eV, increasing the total conductivity of the material up to 0.033 S/cm at 650 °C. These activation energy values are related to the small grain size and the close boundaries obtained at the optimized conditions. The obtained films are good candidates for applications as electrolytes in solid oxide fuel cells (SOFC) operating at relatively low temperatures.

Effect of composite pore-former on the fabrication and performance of anode-supported membranes for SOFCs

In this investigation, a composite pore-former was developed to improve the properties of the NiO–YSZ (yttria-stabilized zirconia) anode substrate for YSZ membrane used in SOFC. By utilizing the composite pore-former in the anode substrate, the requirements of both suitable shrinkage that was compatible with that of electrolyte films and rational porosity could be matched much better. The development of this composite pore-former composed of activated carbon and flour was based on the research about their characteristics as an individual pore-former, respectively, i.e., activated carbon affected the shrinkage of anode substrate more effectively comparing to the porosity, while flour could adjust the porosity more availably. A single fuel cell based on YSZ thin films was successfully fabricated using the anode with such composite pore-former. The open-circuit voltage (OCV) as high as 1.08 V was obtained at 700 °C, and the maximum power densities of 688 and 1122 mW/cm 2 were achieved at 700 and 800 °C, respectively, using hydrogen as fuel and ambient air as oxidant.

Residual stress and microstructure of as-deposited and annealed, sputtered yttria-stabilized zirconia thin films

The microstructure and residual stress of sputter-deposited yttria-stabilized zirconia (YSZ) films are presented as a function of thickness (5–1000 nm), deposition pressure (5–100 mTorr), and post-deposition temperature. The as-deposited residual stress of YSZ ranges from −1.4 GPa to 100 MPa with variations in sputtering conditions. Transitions from compressive to tensile stress are identified with variations in working pressure and film thickness. The origins and variations in as-deposited stress are determined to be from tensile stress due to grain coalescence/growth, and compressive stresses are due to forward sputtering/“atomic peening” of target atoms. The evolution of residual stress with post-deposition annealing shows a tensile stress hysteresis of up to 1 GPa for films deposited at low working pressures. This hysteresis is believed to be due to crystallization and the diffusive relief of compressive stresses initially generated by atomic peening during deposition. Discussion and evaluation of other common residual stress mechanisms are presented throughout.

Properties of YSZ thin films deposited by e-beam technique

In the present study yttrium stabilized zirconia (YSZ) thin films were deposited on the Alloy-600, optical quartz and porous Ni–YSZ substrates using e-beam deposition technique controlling deposition parameters: substrate temperature and electron gun power. These parameters are influencing the thin film deposition mechanism. The dependence of substrate temperature and electron gun power on thin film ionic conductivity, structure and surface morphology was investigated by X-ray diffraction, scanning electron microscopy (SEM). It was found that electron gun power has influence on the crystallite size and growth of YSZ films. Dominating dispersion in the deposited YSZ thin films (substrate temperature T = 250 °C, e-beam gun power P = 0.9 kW) relates to ionic transport in the crystallites in the measured frequencies and temperature ranges. The measured values of conductivity (9.6 ÷ 41.2 × 10 − 4 Sm − 1 ) and activation energies of conductivity (0.87 ÷ 1.1 eV) at 387 °C temperatures are typical to the polycrystalic ZrO 2 — 8 mol% Y 2O 3 compound. The conductivity values of crystallites of YSZ thin films and ceramic are similar. To find out about the stability of formed YSZ thin films and to understand the influence of high temperature on crystallite size was done thermal annealing at 1100 °C temperature. It was found that crystallite sizes increases in size after annealing (3 ÷ 18 nm) and that depends on substrate temperature and e-beam gun power during the formation of YSZ thin films and crystallographic orientation of crystallite.

沉积温度对氧化钇稳定氧化锆薄膜残余应力的影响

沉积温度对氧化钇稳定氧化锆薄膜残余应力的影响

Influence of bias voltage on the crystallographic orientation and morphology of sputter deposited yttria stabilized zirconia (YSZ) thin films

AbstractZrO2‐8% mol. Y2O3 (8YSZ) thin films were deposited onto silicon [100] and AISI 304 stainless steel substrates by r.f. (13.56 MHz) multi‐target magnetron sputtering. To improve the adhesion of a YSZ monolayer to the stainless steel substrate, a buffer layer of Al2O3 was incorporated too. Crystal structure and Infrared (IR) absorption bands of YSZ were investigated as functions of substrate bias by X‐ray diffraction (XRD) and Fourier Transformed Infrared Spectroscopy (FTIR), respectively. The influence of the bias voltage on the roughness, grain size, and microstructure of deposited thin films was determined by AFM and SEM. XRD results show the presence of a tetragonal phase with [111] and [200] orientations. On the other hand, FTIR spectra exhibit the 2Eu and F1u modes as two broad bands in the frequency range of 450 ∼ 550 cm–1 and 550 ∼ 650 cm–1, corresponding to the tetragonal and cubic phases of ZrO2, respectively. In this work we present the systematic influence of the bias voltage on the crystalline structure, the presence of the tetragonal phase and morphology of the YSZ thin films. The XRD, FTIR, and AFM results indicate that when the bias voltage increases from ‐20 V to ‐60 V the preferential crystallographic orientation of YSZ tetragonal phase changes from [111] to [200], and the percentage of the tetragonal phase diminishes, as well as the grain size of deposited films from (560 ± 5) to (470 ± 5) nm. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure and surface morphology of YSZ thin films deposited by e-beam technique

In present study yttrium-stabilized zirconia (YSZ) thin films were deposited on optical quartz (amorphous SiO 2), porous Ni–YSZ and crystalline Alloy 600 (Fe–Ni–Cr) substrates using e-beam deposition technique and controlling technological parameters: substrate temperature and electron gun power which influence thin-film deposition mechanism. X-ray diffraction, 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 crystallite size, roughness and growth mechanism of YSZ thin films are influenced by electron gun power. To clarify the experimental results, YSZ thin-film formation as well evolution of surface roughness at its initial growing stages were analyzed. The evolution of surface roughness could be explained by the processes of surface mobility of adatoms and coalescence of islands. The analysis of these experimental results explain that surface roughness dependence on substrate temperature and electron gun power non-monotonous which could result from diffusivity of adatoms and the amount of atomic clusters in the gas stream of evaporated material.

La 0.4Ce 0.6O 1.8–La 0.8Sr 0.2MnO 3–8 mol% yttria-stabilized zirconia composite cathode for anode-supported solid oxide fuel cells

The composite cathodes of La 0.4Ce 0.6O 1.8 (LDC)–La 0.8Sr 0.2MnO 3 (LSM)–8 mol% yttria-stabilized zirconia (YSZ) with different LDC contents were investigated for anode-supported solid oxide fuel cells with thin film YSZ electrolyte. The oxygen temperature-programmed desorption profiles of the LDC–LSM–YSZ composites indicate that the addition of LDC increases surface oxygen vacancies. The cell performance was improved largely after the addition of LDC, and the best cell performance was achieved on the cells with the composite cathodes containing 10 wt% or 15 wt% LDC. The electrode polarization resistance was reduced significantly after the addition of LDC. At 800 °C and 650 °C, the polarization resistances of the cell with a 10 wt% LDC composite cathode are 70% and 40% of those of the cell with a LSM–YSZ composite cathode, respectively. The impedance spectra show that the processes associated with the dissociative adsorption of oxygen and diffusion of oxygen intermediates and/or oxygen ions on LSM surface and transfer of oxygen species at triple phase boundaries are accelerated after the addition of LDC.

A Direct Ammonia Tubular Solid Oxide Fuel Cell

Abstract A Ni-anode-supported tubular solid oxide fuel cell (SOFC) with a dense 8% yttria-stabilized zirconia (YSZ) thin film was fabricated. The YSZ film was prepared using the vacuum-assisted dip-coating method. The performance of the single SOFC-based system running on ammonia was compared with that running on hydrogen. At 800°C, the peak power densities were 202 and 200 mW/cm 2 using hydrogen and ammonia as the fuel, respectively. A GC analysis of the effluent from the anode showed considerable amounts of nitrogen and hydrogen, but no trace of NO x , when using ammonia as the fuel. The ammonia performance is comparable to hydrogen, suggesting that ammonia can be an attractive alternative fuel.

Characterization of yttria-stabilized zirconia thin films grown by pulsed laser deposition (PLD) on various substrates

Films of yttria-stabilized zirconia (YSZ) were deposited on substrates of various surface types by pulsed laser deposition (PLD). We have investigated formation of YSZ films by X-ray diffraction, focused ion beam and scanning electron microscopy. The deposited material grows with different morphologies such as columns or grains depending on the substrate used. We observed growth of grains only on the polycrystalline substrates, while on the single crystal columnar crystallites were seen. The YSZ coatings were porous, and nanochannels throughout film layers were observed in all YSZ deposits. Regardless of the type of substrate, all films exhibit a cubic phase of YSZ with the (1 1 1) crystallographic planes parallel to the surface as the major peak.

Fabrication of YSZ thin films from suspension by electrostatic spray deposition

This work describes a novel fabrication technique to prepare yttria-stabilized zirconia (YSZ) thin films by electrostatic spray deposition (ESD) from suspension. A detailed discussion on the formulation of colloidally stable suspension to prepare dense and uniform YSZ thin films is presented in this study. X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to analyze the structure and morphology of the YSZ thin films. The results show that a mixture of acetylacetone and ethanol in a ratio of 1:1 by volume is an effective suspension medium for YSZ fine particles to produce colloidally stable suspension, and the YSZ thin films with uniform in thickness about 2 μm and densely packed can be obtained by ESD from the suspension.

Impedance Studies of Thin-film and Patterned Sr-doped LaMnO3 on Yttria- Stabilized Zirconia to Probe Oxygen Reduction Kinetics

Sr-doped LaMnO3 (LSM)/Yttria-stabilized Zirconia (YSZ) are the conventional cathode/electrolyte materials for SOFCs. However, the mechanisms that occur during oxygen reduction reaction (ORR) at the cathode/electrolyte interface are still unclear and have been a roadblock for further enhancement of SOFC performance, especially at low-temperatures. For this study, LSM is fabricated on sputtered and single crystal YSZ by thin-film techniques and patterned via photolithography to obtain precise microelectrodes. EIS results on thin-film LSM microeletrodes supported on thin-film YSZ showed at least four distinct processes during ORR, which were tentatively assigned to (i)surface chemical reaction on LSM, (ii)surface diffusion on LSM, (iii)electrolyte ion transport and (iv)bulk+three-phase boundary (TPB) charge transfer process. It should be noted that the resistance associated with bulk+three-phase boundary charge transfer was highly dependent on LSM electrode thickness. In contrast, thin-film LSM microelectrodes supported on single-crystal YSZ exhibited one distinct process, which is be attributed to bulk charge transfer process.

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La 0.7Sr 0.3MnO 3 (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm 0.2Ce 0.8O 1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm −2 at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm −2 was derived from the cell with LSM/YSZ cathode, using 100 ml min −1 oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.

Effect of YSZ thin film coating thickness on the strength of a ceramic substrate

Although ceramics are used for many different biomedical applications they are brittle materials that can be compromised by surface defects when under stress. The objective of this study was to evaluate the effect of surface modification with an yttria-stabilized zirconia (YSZ) thin film coating on the strength of a machinable dental ceramic. Fifty bars (2 mm x 2 mm x 15 mm) were cut from ProCAD (Ivoclar-Vivadent) blocks. Specimens were wet-polished through 1200-grit SiC abrasive. One surface of each bar was sandblasted with 50 microm Al(2)O(3) abrasive (0.34 MPa). Specimens were further modified through the deposition of a sputtered YSZ thin film on the sandblasted surface. Different thin film thicknesses were evaluated: 1, 3, 5, and 7 microm. Depositions were performed using a radio frequency magnetron sputter system (working pressure of 15 mT, 150 degrees C, 30:1 Ar/O(2) gas ratio). Flexural strength measurements were carried out by three-point bending (span = 10 mm) in a servo-electric material testing system in DI water (37 degrees C). The results showed that the strength of porcelain significantly increased with the deposition of a 3-microm YSZ thick coating. A nonlinear relationship was observed between film thickness and strength. Strengthening of porcelain is shown through the application of a sputtered YSZ thin film. It is presumed that the strengthening mechanism is due to modification of surface flaws and/or surface residual stress by the applied thin film.

Investigation of YSZ Thin Films on Silicon Wafer and NiO/YSZ Deposited by Ion Beam Sputtering Deposition (IBSD)

Ion-beam sputtering deposition is a physical deposited method which uses accelerated ionbeam to sputter oxide or metal targets, and deposits atoms on substrate. Thin films of yttrium-stabilized zirconia (YSZ) were deposited on Si (100) wafer and NiO/YSZ plate. Scanning electron microscopy and transmission electron microscopy with EDS were employed to study the microstructural and chemically stoichiometric results of the films and the crystal growth process by various heat treatments. X-ray diffraction was also used to analysis crystalline phase of the YSZ films. The influence of different targets, substrates deposited efficiency and the properties of the film will be presented and discussed.

Oxygen detection using junctions based on thin films of yttria-stabilized zirconia doped with platinum nanoparticles and pure yttria-stabilized zirconia

Here we develop new types of ceramic junctions composed of two plane-parallel ceramic electrodes: one of them consists of yttria-stabilized zirconia (YSZ), the other one is composed of YSZ doped with platinum nanoparticles. The bottom electrode is either of pure YSZ or of doped YSZ + Pt with the top electrode being of the opposite material. The concentration of platinum has been varied at 2.5, 4.5 and 6.5 mol%. The tests are made in an experimental setup allowing the exchange of gas atmosphere with variable oxygen content at fixed temperature or the gradual change in the temperature up to about 400 °C in a pure nitrogen medium. For YSZ/YSZ + Pt junctions the potential difference is positive, while for YSZ + Pt/YSZ junctions it is negative. At a fixed temperature we investigate the rough response to oxygen of different structures by periodically exchanging the oxygen from 0.9% (technical grade nitrogen) to 21% (air). Good reproducibility of the results is maintained both in the regime of nitrogen flowing and air flowing. The detailed investigation of the kinetics of potential relaxation revealed that it is governed by the diffusion of oxygen in the ceramics. The fastest response is detected with the junctions YSZ + Pt/YSZ with 6.5 mol% of platinum. We also study the potential difference at varying oxygen concentration by tuning the ratio between air and nitrogen. The functional dependence is Nernstian type like in potentiometric gas sensors, which is well-described by the respective equation if the junction is considered as a sort of thermocouple with the electrodes being the contact point. This makes our junctions promising as oxygen sensors at low working temperature and very low oxygen content.

Electrophoretic deposition of YSZ electrolyte coatings for SOFCs

The main purpose of this study was to create crack-free, thin-film electrolyte coatings for solid oxide fuel cells (SOFCs) at low cost. The electrophoretic deposition (EPD) technique was employed to prepare dense yttria-stabilized zirconia (YSZ) thin films on stainless steel and porous strontium-doped lanthanum manganite (LSM) substrates. The LSM substrates were produced by slurry casting and subsequent sintering. The suspensions of YSZ were prepared in pure acetylacetone, ethanol, and in mixtures of the two. The deposition experiments were performed with the YSZ suspensions in different solvents to examine the effects of the solvent on the EPD behavior. The effects of current density and powder concentration on the EPD rate were also studied. The deposited films were dried at room temperature and then sintered at high temperatures. The microstructures of the sintered coatings were observed with scanning electron microscopy. By using a partially sintered LSM substrate and an optimized set of deposition conditions, dense and continuous YSZ thin-film coatings with thicknesses less than 10 μm were achieved.

Effect of SDC-impregnated LSM cathodes on the performance of anode-supported YSZ films for SOFCs

Sm 0.2Ce 0.8O 1.9 (SDC)-impregnated La 0.7Sr 0.3MnO 3 (LSM) composite cathodes were fabricated on anode-supported yttria-stabilized zirconia (YSZ) thin films. Electrochemical performances of the solid oxide fuel cells (SOFCs) were investigated in the present study. Four single cells, i.e., Cell-1, Cell-2, Cell-3 and Cell-4 were obtained after the fabrication of four different cathodes, i.e., pure LSM and SDC/LSM composites in the weight ratios of 25/75, 36/64 and 42/58, respectively. Impedance spectra under open-circuit conditions showed that the cathode performance was gradually improved with the increasing SDC loading. Similarly, the maximum power densities (MPD) of the four cells were increased with the SDC amount below 700 °C. Whereas, the cell performance of Cell-4 was lower than that of Cell-3 at 800 °C, arising from the increased concentration polarization at high current densities. This was caused by the lowered porosity with the impregnation cycle. This disadvantage could be suppressed by lowering the operating temperature or by increasing the oxygen concentration at the cathode side. The ratio of electrode polarization loss in the total voltage drop versus current density showed that the cell performance was primarily determined by the electrode polarization. The contribution of the ohmic resistance was increased when the operating temperature was lowered. When a 100 ml min −1 oxygen flow was introduced to the cathode side, Cell-3 produced MPDs of 1905, 1587 and 1179 mW cm −2 at 800, 750 and 700 °C, respectively. The high cell outputs demonstrated the merits of the novel and effective SDC-impregnated LSM cathodes.

Preparation of nitrogen-doped YSZ thin films by pulsed laser deposition and their characterization

The pulsed laser deposition technique was applied to deposit nitrogen-doped yttria stabilized zirconia (YSZ) thin films. The working parameters were varied in order to achieve a maximal nitrogen content. The films were characterized by SIMS, XPS, X-ray diffraction and optical spectroscopy. The surface topography was studied by AFM and HRSEM. The influence of the deposition parameters on the film properties is discussed.