Yttria-stabilized Zirconia Thin Films Research Articles

The relevance of interfaces for oxide ion transport in yttria stabilized zirconia (YSZ) thin films

Thin YSZ films were prepared on magnesia, sapphire and strontium titanate (STO) single crystals using pulsed laser deposition and, for comparison, by a sol-gel method on STO. The bulk and interfacial mass and charge transport properties of these films were investigated by complementary impedance spectroscopy and tracer diffusion measurements. In this context, a novel two-step tracer diffusion experiment is introduced. For YSZ films on sapphire and magnesia, grain bulk conductivities similar to those of polycrystalline samples were measured in most cases. Strongly blocking grain boundaries could be identified by impedance measurements. The films on sapphire and magnesia also exhibited good agreement between effective transport properties of impedance and tracer measurements. YSZ layers on strontium titanate single crystals, on the other hand, showed a strongly increased effective conductivity in impedance studies. However, in tracer diffusion experiments this could be unambiguously attributed to conduction in the substrate while the diffusion coefficient of YSZ on STO was comparable to that of YSZ films on other substrates. Moreover, the tracer diffusion experiments did not indicate any significant increase of oxide ion mobility on a free YSZ surface compared to a Pt|YSZ interface.

Electrochemical and Oxidation Behavior of Yttria Stabilized Zirconia Coating on Zircaloy-4 Synthesized via Sol-Gel Process

Sol-gel 8 wt.% Yttria Stabilized Zirconia (YSZ) thin films were prepared on zirconium (zircaloy-4 alloy) by dip-coating technique followed by heat treating at various temperatures (200°C, 400°C, and 700°C) in order to improve both electrochemical corrosion and high temperature oxidation properties of the substrate. Differential thermal analysis and thermogravimetric analysis (DTA-TG) revealed the coating formation process. X-ray diffraction (XRD) was used to determine the crystalline phase structure transformation. The morphological characterization of the coatings was carried out using scanning electron microscopy (SEM). The electrochemical behavior of the coated and uncoated samples was investigated by means of open circuit potential, Tafel, and electrochemical impedance spectroscopy (EIS) in a 3.5 wt.% NaCl solution. The homogeneity and surface appearance of coatings produced was affected by the heat treatment temperature. According to the corrosion parameters, the YSZ coatings showed a considerable increase in the corrosion resistance, especially at higher heat treatment temperatures. The coating with the best quality, from the surface and corrosion point of view, was subjected to oxidation test in air at 800°C. The coated sample presented a 25% reduction in oxidation rate in comparison with bare substrate.

Performance improvement of ceria-based solid oxide fuel cells with yttria-stabilized zirconia as an electronic blocking layer by pulsed laser deposition

A yttria stabilized zirconia (YSZ) layer (∼2 μm) is fabricated by pulsed laser deposition (PLD) technique on an Ce0.8Sm0.2O2−δ (SDC) electrolyte film which is prepared by a co-pressing process on a NiO-SDC anode substrate. La0.8Sr02MnO3−δ-YSZ (LSM-YSZ, 70:30 wt.%) cathode is applied onto the SDC/YSZ bilayer electrolytes to form a single cell. The open circuit voltages of the cell increase significantly compared with that of the SDC single electrolyte cell. Preparation of an SDC buffer layer (∼500 nm) on the bilayer electrolytes by PLD method is also studied for reducing the cathode polarization losses with a Sm0.5Sr0.5CoO3−δ-SDC (SSC-SDC, 70:30 wt.%) cathode and preventing the interfacial chemical reaction between YSZ and SSC. The YSZ thin film blocks electrical current leakage in the SDC layer, whereas the SDC buffer layer with the SSC-SDC cathode decreases the cathode polarization losses, which results in the overall enhanced performance.

Crystallization of 8 mol% yttria-stabilized zirconia thin-films deposited by RF-sputtering

The crystallization upon thermal treatment of 8mol% yttria-stabilized zirconia thin films deposited by RF-sputtering at room temperature is investigated. The as-deposited YSZ films are biphasic with crystallites of 5–10nm in diameter building a columnar-like microstructure. Fully crystalline YSZ thin films are obtained after isothermal dwells in the temperature range 800–1100°C. X-ray diffraction and Raman spectroscopy demonstrate that the crystalline films have a cubic structure retained even after high-temperature annealing while a strong texture is developed. Further, a stagnating grain-growth is observed, characterized by a final grain size of about 60nm and a micro-strain of 0.15%. Given the observed films' micro-structural stability, their application in miniaturized electrochemical devices such as micro-solid oxide fuel cells or sensors can be foreseen.

Optimization of DC Reactive Magnetron Sputtering Deposition Process for Efficient YSZ Electrolyte Thin Film SOFC

AbstractYttria‐stabilized zirconia (YSZ, ZrO2:Y2O3) thin films were deposited by reactive DC magnetron sputtering with a high deposition rate from a metallic target of Zr/Y in an argon/oxygen atmosphere. Plasma parameters and composition analysis of the gas phase reveal that the sputtering process in the “compound” mode is reached for a 2.5 sccm oxygen flow rate. Deposition onto silicon in “metal” mode at a flow rate close to the transition, allows obtaining at very high deposition rates (>10 μm h–1) a compact columnar stoichiometric crystallized YSZ film. When deposited on NiO‐YSZ commercial anode, the obtained coatings show the same properties. In spite of the complexity of the substrate (roughness and porosity), a compact and conformed layer was formed. Annealing treatments in air or hydrogen do not significantly alter the structure of the layers. Electrochemical test at 850 °C with a screen‐printed LSM (LaSrMnO3) cathode exhibits a satisfying gastightness (OCV = 900 mV) and a maximum power density of 350 mW cm–2.

Optimized Sol–Gel Routes to Synthesize Yttria-Stabilized Zirconia Thin Films as Solid Electrolytes for Solid Oxide Fuel Cells

Sol–gel strategies are carefully evaluated and compared to synthesize electrolyte materials for Intermediate Temperature SOFCs (Solid Oxide Fuel Cells). Robust 10–20 μm thick cubic Yttria-Stabilized Zirconia (YSZ), (Y2O3)0.08(ZrO2)0.92, films have been prepared by using nanocomposite stable sols composed with “sol–gel made” YSZ binders and YSZ powder. Two different strategies have been investigated for the synthesis of 8YSZ binders: a mixed alkoxide-nitrate route in alcoholic media, and an environment-friendly hydrothermal route, in water media. Ionic conductivities of ∼2.5 × 10–2 S cm–1 are obtained at 800 °C for powders made by the hydrothermal and the alkoxide routes. Films synthesized from composite sols that contain commercial YSZ powder and the organic binder or the water-based binder, have been deposited on YSZ-NiO cermets and integrated in an entire single fuel cell. The two optimized routes yield to gastight YSZ electrolytes. In particular with the organic-based binder, a maximum power density of 250 mW cm–2 at 850 °C has been achieved. The proposed route can be extended to other materials for different applications such as ferroelectric and dielectric materials.

Plasmonic-Based Sensing Using an Array of Au–Metal Oxide Thin Films

An optical plasmonic-based sensing array has been developed and tested for the selective and sensitive detection of H(2), CO, and NO(2) at a temperature of 500 °C in an oxygen-containing background. The three-element sensing array used Au nanoparticles embedded in separate thin films of yttria-stabilized zirconia (YSZ), CeO(2), and TiO(2). A peak in the absorbance spectrum due to a localized surface plasmon resonance (LSPR) on the Au nanoparticles was monitored for each film during gas exposures and showed a blue shift in the peak positions for the reducing gases, H(2) and CO, and a red shift for the oxidizing gas, NO(2). A more in-depth look at the sensing response was performed using the multivariate methods of principal component analysis (PCA) and linear discriminant analysis (LDA) on data from across the entire absorbance spectrum range. Qualitative results from both methods showed good separation between the three analytes for both the full array and the Au-TiO(2) sample. Quantification of LDA cluster separation using the Mahalanobis distance showed better cluster separation for the array, but there were some instances with the lowest concentrations where the single Au-TiO(2) film had separation better than that of the array. A second method to quantify cluster separation in LDA space was developed using multidimensional volume analysis of the individual cluster volume, overlapped cluster volume, and empty volume between clusters. Compared to the individual sensing elements, the array showed less cluster overlap, smaller cluster volumes, and more space between clusters, all of which were expected for improved separability between the analytes.

Deposition of YSZ Thin Films by Atmospheric Plasma-Assisted Pulsed Laser Ablation

Yttria-stabilized zirconia (YSZ) thin films have been deposited on a nickel-based substrate by a laser-assisted plasma coating technique at atmospheric pressure. The substrate is heated by an atmospheric plasma torch to 800 °C to form a thermally grown oxide layer and is increased to 1000 °C during film deposition. The deposited films show columnar structures similar to films deposited in high-vacuum deposition methods. The microstructures of the deposited YSZ films have been examined by focused-ion beam and X-ray photoelectron spectroscopy.

Precursor Decomposition, Microstructure, and Porosity of Yttria Stabilized Zirconia Thin Films Prepared by Aerosol‐Assisted Chemical Vapor Deposition

AbstractMicrostructures of yttria stabilized zirconia thin films deposited by aerosol assisted chemical vapor deposition (AA‐CVD) are correlated with the thermal decomposition behavior of the corresponding metal precursors, zirconium and yttrium 2,4‐pentanedionate. Process conditions of AA‐CVD are investigated with the aim of producing dense and compact YSZ thin films for applications as gas‐tight electrolyte. Based on systematic cross sectional scanning transmission electron microscopy (STEM) investigations and conductivity measurements, the development of percolating nanoporosity is observed in samples prepared at temperatures between 350 °C and 600 °C at standard solution throughput. Compact columnar thin films with bulk conductivity are obtained at 600 °C by reducing the metal content of the precursor solution and at 450 °C by reducing the solution throughput.

The fictional transition of the preferential orientation of yttria-stabilized zirconia thin films

The fundamental study of the microstructural and textural evolution of yttria-stabilized zirconia (YSZ) thin films is of great importance given that the crystallographic properties are intimately related to their extrinsic or functional properties. In order to study these properties, YSZ thin films were obtained using dual magnetron sputtering. The results of a polar plot graph, based on X-ray diffraction (XRD) data, seem to indicate a transition from [200] out-of-plane preferential orientation to [111], indicating a dependence on composition and yttrium target–substrate (Y T–S) distance at low pressure. However, no transition is identified at high pressure, showing only [111] out-of-plane orientation, independent of composition and Y T–S distance. Scanning electron microscopy (SEM) indicates a tilt in the columnar structure of the film but no other microstructural change is in evidence, possibly related to the growth transition from [200] to [111]. Pole figures were used to clarify the texture transition in the YSZ thin films. These results indicate that there is indeed no transition in the preferential orientation of the films from [200] to [111] but a tilt of the [200] orientation towards the zirconium source. Detailed study using pole figures and SEM, clearly indicated that no growth zone transition was present and the effect is caused by geometrical configuration, contradicting expectations from standard θ/2θ XRD measurements.

Thin Film Yttria-Stabilized Zirconia (YSZ) Electrolyte Fabricated by a Novel Chemical Solution Deposition (CSD) Process for Solid Oxide Fuel Cells (SOFCs)

Thin Film Yttria-Stabilized Zirconia (YSZ) Electrolyte Fabricated by a Novel Chemical Solution Deposition (CSD) Process for Solid Oxide Fuel Cells (SOFCs)

Crack-free yttria stabilized zirconia thin films by aerosol assisted chemical vapor deposition: Influence of water and carrier gas

Yttria stabilized zirconia thin films are deposited on silicon single crystal substrates by aerosol assisted chemical vapor deposition from precursor solutions of zirconium and yttrium 2,4-pentanedionate in ethanol. Continuous films are obtained using pure oxygen, pure nitrogen, or mixtures of both as carrier gas. In the simultaneous presence of water and oxygen, crack formation is observed for films deposited at intermediate substrate temperatures (450°C), while those deposited at low (300°C) and high (600°C) temperatures remain crack-free. Crack-free films can be deposited at 450°C in a water-free setting, or in the presence of water using pure nitrogen as carrier gas. The addition of water to the precursor solutions also significantly reduces film growth rates.

Influence of microstructure on the cross‐plane oxygen ion conductivity of yttria stabilized zirconia thin films

AbstractThe electrical cross‐plane conductivity of 8 mol% yttria stabilized zirconia (YSZ) thin films prepared by different deposition techniques, namely aerosol assisted chemical vapor deposition, wet spray pyrolysis (SP), and pulsed laser deposition (PLD), is correlated with their microstructure. Depending on deposition technique and process conditions, microstructures ranging from amorphous to randomly oriented nanocrystalline or columnar with preferred (111) orientation are obtained. Cross‐plane AC impedance measurements of these thin films show that the oxygen ion conductivity of randomly oriented nanocrystalline samples is determined by the grain boundaries, which show significantly lower transport properties than the grain interior. In columnar microstructures, the conductivity is determined by ionic transport through the grains only. The same conduction behavior is found for amorphous and randomly oriented microstructures with grain sizes between 3 nm and 9 nm, indicating that no true size effects occur in 8 mol% YSZ. magnified imageGrain and grain boundary conductivity determined for nanocrystalline 8 mol% yttria stabilized zirconia thin films of different microstructures.

Nanostructured ruthenium – gadolinia-doped ceria composite anodes for thin film solid oxide fuel cells

Ruthenium and gadolinia-doped ceria (Ru–CGO) composite nano-crystalline thin film anodes are demonstrated for the first time in self-supported micro-solid oxide fuel cells (μSOFCs) for direct methane utilization, targeting development of microstructurally stable anodes under operation. Nano-composite thin film anodes are synthesized by co-sputtering from Ru metal and CGO oxide targets. Detailed electrical, crystalline and microstructural analyses were carried out on composite films to characterize properties as a function of Ru/CGO relative fraction. Microstructural stability of the composite film is compared to Ru metal film from conductivity and grain size behavior during 500°C thermal treatments. Stress-relaxed composite thin film anodes were developed on self-supported μSOFCs with yttria-stabilized zirconia thin film electrolytes and porous platinum cathodes. μSOFCs were tested with room temperature humidified methane as fuel and air as the oxidant, exhibited an open circuit voltage of 0.97V and a peak power density of 275mWcm−2 at 485°C. Microstructural stability of Ru–CGO composite anodes relative to metal electrodes is discussed. The results suggest a general approach to synthesis of functional metal-oxide nano-composite thin films for electrodes in fuel cells and related energy conversion devices.

Ion Conduction in Nanoscale Yttria-Stabilized Zirconia Thin Films Fabricated by Atomic Layer Deposition

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Microstructures of YSZ and CGO Thin Films Deposited by Spray Pyrolysis: Influence of Processing Parameters on the Porosity

AbstractMicrostructures of yttria‐stabilized zirconia (YSZ) thin films deposited by spray pyrolysis at 370 °C on sapphire are investigated. The as‐deposited films are predominantly amorphous and crystallize upon heating at temperatures above 370 °C, developing grains in the range of 5 nm to several 100 nm. During post‐deposition heat treatment up to 800 °C, ∼ 50 vol% porosity develops in the center of the films with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin films consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 °C. In gadolinium‐doped‐ceria (CGO) thin films similar microstructures and porosity are found after low‐temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin films is observed in both the YSZ and in CGO thin films. Stagnating grain growth in the thin films is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densification has taken place.

Growth of highly oriented YSZ and CeO2 films with Tasker-forbidden surfaces in oxygen-deficient environments

Yttria-stabilized zirconia and cerium dioxide thin films are deposited in oxygen-deficient environments and compared to those deposited in oxygen-rich environments. Oxygen-deficient films are highly (001)-oriented, which corresponds to a surface that should be forbidden based on Tasker’s theoretical calculation. Oxygen-vacancy-induced surface reconstruction and relaxation are proposed to explain the phenomenon. The analysis is consistent with preferred orientations previously observed in indium tin oxide, SnO2, and NiO films. Detailed studies of preferred orientations of these oxygen-deficient metal oxide films are of importance for their use in solid oxide fuel cells, gas sensors, and optical devices, etc.

Grain Boundary Blocking Effect in Yttria Stabilized Zirconia Thin Films

The cross-plane oxygen ion conductivity of yttria stabilized zirconia thin films prepared by aerosol assisted chemical vapor deposition is strongly influenced by the thin film microstructure. Thin films with highly textured columnar grains oriented parallel to the current direction exhibit significantly higher conductivity than thin films with randomly oriented nanocrystalline microstructure, where ionic transport proceeds through the numerous grain boundaries. The total conductivity of columnar AA-CVD thin films is consistent with literature values on oxygen ion conduction through chemically pure YSZ grains as determined for microcrystalline samples, while that of the nanocrystalline specimens is similar to corresponding specific grain boundary conductivities.

Residual Stress and Buckling Patterns of Yttria-Stabilised-Zirconia Thin Films for Micro-Solid Oxide Fuel Cell Membranes

Free-standing yttria-stabilised-zirconia (YSZ) thin films can be found in today's miniaturised gas sensors and as electrolytes in micro-solid oxide fuel cell membranes. 8 mol.% YSZ thin films prepared by pulsed laser deposition on silicon substrates are investigated by wafer curvature and nanoindentation. The 300nm thin 8YSZ films deposited at 700°C have a compressive stress of -1100±150 MPa and a Young's modulus of 205±20 GPa at 25 °C. The corresponding free-standing 8YSZ membranes are investigated by light microscopy and white light interferometry. The 8YSZ membranes deposited at 700°C have a buckling shape with a C4z-rotational symmetry and buckling amplitude of 6.5μm at 25°C. Numerical simulations of the buckling patterns using the Rayleigh-Ritz-method are in good agreement with the experimental data. These simulated buckling patterns are used to extract the local stress distribution. This is important regarding the application of YSZ membranes in micro-solid oxide fuel cells which must be thermomechanically-stable during device operation.

Reactive magnetron sputtering of uniform yttria-stabilized zirconia coatings in an industrial setup

Yttria-stabilized zirconia (YSZ) thin films were deposited by reactive magnetron sputtering in an industrial scale setup on silicon wafers as well as commercial NiO-YSZ fuel cell anodes. The texture, morphology, and composition of the deposited films were investigated as a function of deposition parameters. Homogeneous coatings could be deposited over large areas within the coating zone, which is important for industrial applications. The use of substrate bias during film growth was identified as a key parameter to promote less columnar coatings and made it possible to tailor the texture of films deposited on Si. Bias voltages≤−40V resulted in highly <200> textured YSZ films, intermediate bias voltages of −50V to −70V in <220> textured films and high bias voltages (≥−90V) in a mixed orientation. In contrast, films grown on NiO-YSZ were seen to be randomly orientated when deposited at substrate bias voltages≤−30V. When bias was further increased the film took over the orientation of underlying substrate due to substrate template effects.