Yttria-stabilized Zirconia Thin Films Research Articles

Microstructural properties of multi-nano-layered YSZ thin films

We report the fabrication of submicron, multi-nano-layered, yttria-stabilized zirconia (YSZ) thin films by aerosol assisted CVD. The film consisted of a periodic stack of several layers, a few nanometers thick, of the same composition but different density; formation of voids during synthesis originate the low-density layer. Grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry, high-resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF) images were employed to analyze the microstructure of the films. GIXRD pattern showed characteristic peaks of cubic zirconia. Peak broadening in the pattern comes from a microstructure composed of nanocrystals, but principally due to the multilayered structure, that cause satellite peaks around the Bragg reflections. Lattice fringes measurement in HRTEM and HAADF images was consistent with the interplanar distance of the YSZ cubic phase. Additionally, lattice parameter obtained from selected area electron diffraction and GIXRD patterns was around 0.513 nm, in agreement to values reported in the literature for YSZ.

YSZ Free-standing Membranes for Silicon-based Micro SOFCs

The present study is devoted to analyze the compatibility of yttria-stabilized zirconia thin films prepared by pulsed laser deposition technique for developing new silicon-based micro devices for micro solid oxide fuel cells applications. Yttria-stabilized zirconia free-standing membranes with thicknesses from 60 to 240 nm and surface areas between 50x50 µm2 and 820x820 µm2 were fabricated on micromachined Si/SiO2/Si3N4 substrates. Deposition process was optimized for deposition temperatures from 200ºC to 800ºC. A mechanical study comprising thermomechanical stability, residual stress of the membranes and annealing treatment as well as a preliminary electrical characterization of ionic conductivity was performed in order to evaluate the best processing parameters for the yttria-stabilized zirconia membranes.

Nanostructured sapphire vicinal surfaces as templates for the growth of self-organized oxide nanostructures

Vicinal substrates of sapphire with miscut angle of 10° from the (0 0 1) planes towards the [1 1 0] direction have been annealed in air in the range from 1000 to 1500 °C. The behaviour of these surfaces has been characterized as a function of the temperature and the thermal treatment time by Atomic Force Microscopy observations. A thermal treatment at 1250 °C allows to stabilize a surface made of periodically spaced nanosized step-bunches. Such stepped surfaces were used as template to grow self-patterned epitaxial oxide nanoparticles by thermal annealing of yttria-stabilized zirconia thin films produced by sol–gel dip-coating. Grazing Incidence Small Angle X-ray Scattering and High-Resolution Transmission Electron Microscopy were used to study the morphology of the nanoparticles and their epitaxial relationships with the substrate.

Grain‐Size Effects in YSZ Thin‐Film Electrolytes

The transport properties of oxygen‐ion conducting yttria‐stabilized zirconia (YSZ)—featuring mean grain sizes from a few nm up to the μm regime—were studied with regard to grain‐size effects. Chemically homogeneous, 8.3 mol% YSZ thin films (thickness approximately 400 nm) were processed on single‐crystal sapphire substrates by a sol–gel method. The mean grain size d of the thin films was systematically adjusted to 5 nm≤d≤782 nm by (i) a rapid thermal annealing step for conversion into the oxide phase and (ii) a consecutive calcination step at 650°C≤Tcal (24 h) ≤1400°C for grain growth. The quality of the thin films was examined with respect to chemical homogeneity, crystal structure, grain‐size, and grain‐boundary properties. Total and specific conductivities of the thin films were characterized by means of electrical impedance spectroscopy at 200°≤T≤400°C in ambient air, where a complex nonlinear least‐squares approximation was applied to determine the bulk conductivity and the grain‐boundary conductivity. Despite grain boundaries being free of second phases, oxygen transport was observed to be impeded by the grain boundaries as the specific grain‐boundary conductivity was determined to be two orders of magnitude below the bulk conductivity for thin films with d>36 nm. The transport properties of nanoscaled YSZ thin films (5 nm≤d≤36 nm) were modeled by application of the brick‐layer model indicating the absence of beneficial grain‐size effects at the nanoscale.

Zirconia–parylene multilayer thin films for enhanced fracture resistance of dental ceramics

Recent research has shown that the application of specific thin films can enhance the material properties of a laminate construct. In this study, the effect of different mono/multilayered films on the strength of a ceramic specimen is demonstrated. It is well established that cracks can initiate and/or propagate from the internal surfaces of all-ceramic dental restorations. Modifying that surface by thin-film deposition might help increase clinical longevity and applicability. Specimens were divided into the following groups according to different surface treatments received: uncoated (control group), 10 microm yttria-stabilized zirconia (YSZ) thin film, 10 microm parylene thin film, 9.75 microm YSZ + 0.25 microm parylene film, and a multilayered film (five layers of 1.25 microm YSZ + 0.75 microm parylene). Depositions were performed using a radio-frequency magnetron sputter system (working pressure 15 mT, 150 degrees C, 30:1 Ar/O2 gas ratio) to produce the YSZ layers, and a vapour deposition process was used to produce the parylene layers. Flexural strength measurements were carried out by three-point bending (span = 10 mm) in a servo-electric material testing system in deioinized (DI) water (37 degrees C). The results showed that the strength of the specimen significantly increased with the deposition of all types of coating, showing the greatest increase with the multilayered film (approximately 32 per cent). It is hypothesized that a multilayer thin film (brittle/ductile) can promote crack deflection, causing strength enhancement of the brittle construct.

Yttria-stabilized zirconia thin films by pulsed laser deposition: Microstructural and compositional control

The ns-laser ablation characteristics of tetragonal 3YSZ versus cubic 8YSZ have been investigated to minimize a transfer of particulates in the pulsed laser deposition process. 3YSZ is significantly less prone to the exfoliation of μm-sized fragments than 8YSZ due to its enhanced fracture toughness, which allows the deposition of particulate-free films in a fluence range of 1.2–1.5 J/cm 2. The influence of the PLD process parameters on the film microstructure and stoichiometry have been investigated with respect to the growth of dense 3YSZ layers with adequate adhesion to the c-cut sapphire single crystals. Dense 3YSZ films are obtained below a threshold pressure of ∼0.025 mbar. At 600 °C polycrystalline layers with a preferential (1 0 1) and (0 0 1) orientation and a columnar microstructure are formed while deposition at room temperature yields uniform amorphous layers. Strongly oxygen deficient films of the metastable t′′ phase are obtained at a low background pressure of 10 −3 mbar. The meta phase films exhibit a low activation energy of 0.77 ± 0.02 eV and an enhanced d.c. electrical conductivity, e.g. 9 × 10 −5 S/cm at 400 °C, comparable or even higher than for 8YSZ films and bulk at temperatures up to 500 °C.

Preparation of thin film YSZ electrolyte by using electrostatic spray deposition

Yttria-stabilized zirconia (YSZ) thin film was successfully deposited onto Ni–YSZ anode disk by the electrostatic spray deposition (ESD) technique. To deposit a dense YSZ thin film onto porous Ni–YSZ substrate, the influence of process parameter variables were examined. The relationship between process parameters and morphologies of YSZ films coated by ESD was investigated by means of SEM photography and X-ray diffraction (XRD). The result showed that dense YSZ film of average 10–15 μm thickness was deposited on the porous Ni–YSZ substrate with temperature of 400 °C, the precursor solution concentration of 0.05 M, nozzle-to-substrate distance of 30 mm, applied electric field of 18 kV, and deposition time of 90 min.

Flow and Temperature Effect on Thin Films of Yttria-Stabilized Zirconia by Sputtering.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Low-temperature electrochemical characterization of dense ultra-thin lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.8Fe0.2O3) cathodes synthesized by RF-sputtering on nanoporous alumina-supported Y-doped zirconia membranes

Dense ultra-thin nanocrystalline La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3 (LSCF) films with thickness of ∼50 nm, have been sputtered on nanoporous anodic alumina-supported nanocrystalline thin film yttria-stabilized zirconia and patterned by photolithography into microelectrodes. This approach enables low-temperature (425–550 °C) electrochemical properties of dense ultra-thin nanocrystalline LSCF to be characterized. The results reveal that the electrochemical resistance of nanocrystalline ultra-thin LSCF is dominated by the oxygen surface exchange reaction at the electrode surface with an activation energy of 1.1 eV. Area-specific resistance of LSCF was obtained and the results are of potential relevance to utilizing nanostructured oxide cathodes for micro-SOFCs operated at low temperatures.

A comparative study of YBa2Cu3O7−δ/YSZ bilayer films deposited on silicon-on-insulator substrates with and without HF pretreatment

Highly epitaxial YBa2Cu3O7−δ (YBCO) and yttria-stabilized zirconia (YSZ) bilayer thin films have been deposited on silicon-on-insulator (SOI) substrates by using in situ pulsed laser deposition (PLD) technique. In the experiment, the native amorphous SiO2 layers on some of the SOI substrates are removed by dipping them in a 10% HF solution for 15s. Comparing several qualities of films grown on substrates with or without HF pretreatment, such as thin film crystallinity, general surface roughness, temperature dependence of resistance, surface morphology, as well as average crack spacing and crack width, naturally leads to the conclusion that preserving the native SiO2 layer on the surface of the SOI substrate can not only simplify the experimental process but can also achieve fairly high quality YSZ and YBCO thin films.

Enhancement of the crystalline quality of reactively sputtered yttria-stabilized zirconia by oxidation of the metallic target surface

Yttria-stabilized zirconia (YSZ) thin films were fabricated on glass substrates by direct current magnetron reactive sputtering. We found out that the crystalline quality of the YSZ film was improved by an oxidation process of the metallic target surface prior to the sputtering deposition process. It is speculated that, at the initial stage of the sputtering, the sputtered particles from the oxidized target surface form a layer with higher degree of crystallization on the substrate, compared with those particles from the metallic target surface. This crystallized layer can enhance the crystallization of the film deposited subsequently. Other sputtering conditions such as sputtering pressure, oxygen flow rate, substrate temperature, and Y 2O 3 content in the film were investigated, for optimization of the crystalline quality of the deposited YSZ film.

Interface energies of(100)YSZand(111)YSZepitaxial islands on(0001)α-Al2O3substrates from first principles

We present an ab initio study of the interface energies of cubic yttria-stabilized zirconia (YSZ) epitaxial layers on a ${(0001)}_{\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}}$ substrate. The interfaces are modeled using a supercell geometry and the calculations are carried out in the framework of density-functional theory (DFT) and the local-density approximation (LDA) using the projector augmented wave (PAW) pseudopotential approach. Our calculations clearly establish the existence of competing growth mechanisms between ${(111)}_{\text{YSZ}}\ensuremath{\parallel}{(0001)}_{\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}}$ and ${(100)}_{\text{YSZ}}\ensuremath{\parallel}{(0001)}_{\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}}$ interfaces. This result is central to understanding the behavior of YSZ thin solid film islanding on ${(0001)}_{\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}}$ substrates, either flat or in presence of defects.

Low-temperature Growth of Yttria-stabilized Zirconia Thin Films by Pulsed Laser Deposition

Low-temperature Growth of Yttria-stabilized Zirconia Thin Films by Pulsed Laser Deposition

Influences of deposition rate and oxygen partial pressure on residual stress and microstructure of YSZ thin films

Yttria-Stabilized Zirconia (YSZ) thin films were deposited on borosilicate crown glass substrates using electron beam evaporation technique and controlling technological parameters: deposition rate and oxygen partial pressure. Spectrophotometry, optical interferometry and X-ray diffraction were used to investigate how the thin film optical properties, residual stresses, and structure depend on these parameters. The results showed that the deposition rate had a significant influence on the increase of the refractive index of YSZ thin films while the oxygen partial pressure had less influence on it. In all samples, the tensile stress increased with the increasing of deposition rate and the decreasing of oxygen partial pressure. Meanwhile, all deposited films were poly-crystallizations, while crystallite size and preferential orientation of YSZ thin films changed as a function of deposition rate and oxygen partial pressure. The variations of the optical spectra and residual stress corresponded to the evolution of the film structures induced by the deposition parameters.

Characterization of ceramic sol–gel coatings as an alternative chemical conversion treatment on commercial carbon steel

Sol–gel yttria-stabilized zirconia (YSZ) thin films were prepared on commercial carbon steel sheets by dip-coating technique followed by a low temperature heat treatment (473, 573, and 673 K for 1 h) in order to improve both corrosion properties and adhesion. For comparison, zirconia (ZrO 2) coatings have been also analyzed. Electrochemical techniques, Fourier Transform Infrared (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the anticorrosion behavior of the coatings in a 3.5 wt% NaCl solution. The adhesion with a polyester organic coating was evaluated by the pull-off technique. The typical thickness of the deposited layers ranged from 1 to 1.3 μm depending on process parameters. The obtained results indicated that sol–gel ZrO 2 and YSZ coatings without an organic coating can act as protective barriers against wet corrosion during the first hours, but they fail when the time exposure is longer than 1 day. However, when synthesized films were used as a pre-treatment and an organic coating was added (top-coated), the anticorrosive and adhesion properties were strongly affected by the temperature of the treatment, and an increase in both properties was observed at higher temperatures. The structural and morphological characteristics of the coating provide an explanation of the role of each film in the electrochemical behavior in this aggressive medium. Comparing both systems, YSZ displayed greater protective and adhesion values than exhibited for ZrO 2 which can be correlated with the stabilization of the cubic phase.

Ultra-thin nanocrystalline lanthanum strontium cobalt ferrite (La 0.6Sr 0.4Co 0.8Fe 0.2O 3− δ) films synthesis by RF-sputtering and temperature-dependent conductivity studies

Nanocrystalline lanthanum strontium cobalt ferrite (LSCF) ultra-thin films with high in-plane electrical conductivity have been deposited by RF sputtering from composite targets. The films, with nominal thickness of 54 nm, are crystalline when annealed or deposited at temperatures above 450 °C. Effects of annealing temperature, annealing time, and substrate temperature on crystallization, microstructure, and room temperature lateral electrical conductivity have been systematically studied. No interfacial reaction products between the LSCF and single crystalline yttria-stabilized zirconia (YSZ) were observed from X-ray diffraction studies upon annealing until 750 °C. In-plane electrical conductivity as high as 580 S cm −1 at 650 °C has been observed for LSCF thin films deposited on single crystalline YSZ substrates and sputtered nanocrystalline YSZ thin films; while activation energy for conductivity were determined to be 0.15 eV and 0.10 eV for the former and latter films, respectively, in 650–400 °C range. The high in-plane electrical conductivity for the nanocrystalline LSCF ultra-thin films is likely attributed to their low level of porosity. Micro-solid oxide fuels cells using 15 nm thick LSCF films as cathodes and sub-100 nm yttria-doped zirconia thin film electrolytes have been fabricated successfully and demonstrated to achieve peak power density of 60 mW cm −2 at 500 °C. Our results demonstrate that RF sputtering provides a low-temperature synthesis route for realizing ultra-thin nanocrystalline LSCF films as cathodes for intermediate- or low-temperature solid oxide fuel cells.

Dynamic Fatigue Behavior of Dental Porcelain Modified by Surface Deposition of a YSZ Thin Film

The objective of this study was to evaluate the basic fatigue parameters of a dental porcelain modified by deposition of a yttria-stabilized zirconia (YSZ) thin film and to compare the data to that of an unmodified control. Two hundred bars (2 x 2 x 15 mm(3)) were cut from ProCAD blocks. Specimens were wet-polished with 1200-grit SiC abrasive. One surface of each bar was sandblasted with 50 microm Al2O3 abrasive (50 psi). Half the specimens were further modified through deposition of a 3-microm YSZ thin film on the sandblasted surface. Depositions were performed using an radio frequency magnetron sputter system (working pressure of 15 mT, 150 degrees C, 30:1 Ar:O2 gas ratio). Specimens were tested at different stressing rates: 5.0, 0.1, and 0.01 MPa/s (n = 25/group) in deionized water (37 degrees C), and inert strength was determined in air (25 degrees C, 70 MPa/s). All strength measurements were carried out by three-point bending (span = 10 mm) in a servo-electric test system. The mean flexural strength values (MPa) and standard deviation for the uncoated sandblasted group were: 98.6 (5.5), 90.7 (5.9), and 84.2 (8.5), and for the sandblasted + YSZ thin film group: 125 (9.4), 119.3 (7.8), and 102.8 (7.0), for the highest to the lowest stressing rates, respectively. The fatigue parameters n and lnB were calculated by linear regression of dynamic fatigue data. For the uncoated group, n = 38 and lnB = 4.7 MPa(2)/s, and for the coated group, n = 33 and lnB = 10.8 MPa(2)/s. Weibull analysis was also performed showing that the characteristic parameter (sigma(o)) was 113.3 and 125.7 MPa for the uncoated and coated group, respectively. There was an increase in strength for specimens modified by application of a YSZ thin film. It is hypothesized that thin-film application modifies flaws or residual surface stress states.

Thin porous Ni–YSZ films as anodes for a solid oxide fuel cell

Porous Ni–YSZ (YSZ—yttria-stabilized zirconia) films were fabricated by reactive co-sputtering of a Ni and a Zr–Y target, followed by sequentially annealing in air at 900 °C and in vacuum at 800 °C. The Ni–YSZ films comprised small grains and pores that were tens of nanometers in size. The porous Ni–YSZ films were used as an anode on one side of a YSZ electrolyte disc and a La 0.7Sr 0.3MnO 3 thick film was used as a cathode on the other side of the disc to form solid oxide fuel cells (SOFCs). The voltage–current curves of the SOFCs with single- and a triple-layered porous anodes were measured in a single-chamber configuration, in a mixture of CH 4 and air (CH 4:O 2 volume ratio=2:1). The maximum power density of the SOFC using the single-layered porous Ni–YSZ thin films as the anode was 0.38 mW cm −2, which was lower than that of 0.76 mW cm −2, obtained using a screen-printed Ni–YSZ thick anode. The maximum power density of the SOFC with a thin anode was increased, but varied between 0.6 and 1.14 mW cm −2 when a triple-layered porous Ni–YSZ anode was used.

Fabrication of an anode-supported yttria-stabilized zirconia thin film for solid-oxide fuel cells via wet powder spraying

The wet powder spraying (WPS) technique was used for the deposition of dense and thin (Y 2O 3) 0.08(ZrO 2) 0.92 (YSZ-8) films on an anode substrate being used for fuel cell applications. Both agglomeration of the powder and the presence of organics in the substrate have a significant effect on the quality and densification of the thin electrolyte layer. High-energy ball milling effectively broke up the agglomerates and enhanced the packing density of the green layer. Pre-calcination of the substrate at ∼1000 °C enhanced the match of sintering shrinkage between the electrolyte layer and the substrate and improved the quality of the YSZ-8 thin film significantly. Crack-free dual-layer assembly with a highly densified YSZ-8 film as thin as 10 μm was successfully fabricated by optimizing the fabrication parameters. The cells with a La 0.8Sr 0.2MnO 3 cathode showed a high open circuit voltage of 1.071 V and a peak power density of 894 mW cm −2 at 850 °C operated with hydrogen fuel.

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.