Yttria-stabilized Zirconia Polycrystalline Research Articles

Load‐bearing capacity of all‐ceramic three‐unit fixed partial dentures with different computer‐aided design (CAD)/computer‐aided manufacturing (CAM) fabricated framework materials

The purpose of this in vitro study was to compare the load-bearing capacity of posterior three-unit fixed dental prostheses (FDP) produced with three different all-ceramic framework materials: glass-infiltrated alumina (ICA), glass-infiltrated alumina strengthened with zirconia (ICZ), and yttria-stabilized polycrystalline zirconia (YZ). Additionally, the influence on aging of mechanical cyclic fatigue loading and thermal cycling in water were evaluated. A total of 20 frameworks each were fabricated from ICA, ICZ, and YZ by a computer-aided design (CAD)/computer-aided manufacturing (CAM) system. The framework designs were identical for all specimens. All frameworks were veneered with porcelain and cemented with glass-ionomer. Prior to fracture testing, 10 FDP of each experimental group were subjected to thermal and mechanical cycling. Additionally, fractographic analysis was performed. Statistical analysis showed that FDP made from YZ had significantly higher load to failure, whereas no difference was found between the other two materials. Aging did not have a significant effect on the fracture load.

Relationship between Surface Residual Stress Fields and Hydrothermal Environment on Low Temperature Aging of 3Y-TZP

Phase stability of 3 mol% yttria stabilized polycrystalline zirconia ceramics (3Y-TZP) was evaluated by aging test in water vapor environment and Raman spectroscopic technique. In our previous study, it has been confirmed that phase stability was improved by controlling sintering temperature. In this study, we pointed our attention and evaluated the influence of surface conditions related to machining and to heat treatment, thus monitoring the changes in phase transformation fraction and residual stress on the material surface. From the results of aging test, an increase in monoclinic fraction was observed for grinded surfaces as compared with polished surfaces. Samples subjected to heat treatment after machining showed improved phase stability. A Vickers indentation print was also introduced on a 3Y-TZP surface, and the relationship between surface condition and low temperature aging was evaluated in the neighborhood of the print. We found that the residual stress fields induced in phase-transformed areas were enhanced during low temperature aging, and that phase stability was improved by heat treatment.

YBa 2Cu 3O 7 melt textured thick films grown by infiltration process on YSZ substrate prepared by sol–gel method

Polycrystalline yttria-stabilized zirconia (YSZ) compacts with a relative density of 97% were prepared by the two-step sol–gel synthesis from zirconium (IV)-propoxide and yttrium chloride. The obtained YSZ compacts were used as polycrystalline substrates for melt-textured YBa 2Cu 3O 7 (YBCO) films. The dense YSZ substrates show a limited reactivity against the highly corrosive Ba/Cu melt and a formation of interlayers with limited thickness between the superconducting film and the substrate can be observed. Crack-free YBCO films with a thickness of about 50 μm were prepared by melt infiltration of Ba/Cu melt into the deposited Y 2BaCuO 5 (2 1 1) layer. Since no seeds were used, spherulitic microstructure was formed. The AC magnetization measurements results indicate that the samples are without noticeable weak intergrain links, however, they contain areas with weaker superconducting properties.

Structure and impedance spectroscopy of La 0.6Ca 0.4Fe 0.8Ni 0.2O 3− δ thin films grown by pulsed laser deposition

Polycrystalline samples of La 0.6Ca 0.4Fe 0.8Ni 0.2O 3 (LCFN) were prepared by the liquid mix process at 600 °C. The structure of the polycrystalline powders was analyzed with X-ray powder diffraction data. The XRD patterns were indexed as the orthoferrite similar to that of PrFeO 3 having a single phase orthorhombic perovskite structure ( Pbnm). LCFN films have been deposited on yttria-stabilized zirconia (YSZ) single crystal and polycrystalline substrates at 700 °C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Both films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distribution. Electrochemical impedance spectroscopy (EIS) measurements of LCFN/YSZ single crystal/LCFN and LCFN/polycrystalline YSZ/LCFN test cells were conducted. The obtained ASR values at 850 °C for the single crystal and polycrystalline YSZ cell tests were 1.84 and 1.59 Ω cm 2, respectively.

Preparation by radio-frequency magnetron co-sputtering and characterization of thin films of lanthanum–strontium ferromanganites

(La 0.8Sr 0.2)(Mn 1 − y Fe y )O 3 ± δ films with y = 0, 0.2, 0.5, 0.8 and 1, a few hundred nanometers thick, were deposited onto polycrystalline yttria-stabilized zirconia YSZ by a magnetron co-sputtering technique using individual targets of La 0.8Sr 0.2MnO 3 and La 0.8Sr 0.2FeO 3. The deposition parameters, substrate temperature and gas pressure, were studied by scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction and interferential microscopy for their effects on the morphological, chemical, topographic and crystallographic properties of films and YSZ-film interfaces. (La 0.8Sr 0.2)(Mn 1 − y Fe y )O 3 ± δ thin films were found to be homogeneous, dense and crystallized in the perovskite structure after deposition at 600 °C. Profilometry by secondary ion mass spectrometry and observations by Auger electron spectrometry after selective dissolution of the films were also used to investigate both morphology and quality of the YSZ-film interface after deposition.

Spectral Radiative Heat Transfer Analysis of the Planar SOFC

Thermo-mechanical failure of components in planar-type solid oxide fuel cells (SOFCs) depends strongly on the local temperature gradients at the interfaces of different materials. Therefore, it is of paramount importance to accurately predict the temperature fields within the stack, especially near the interfaces. Because of elevated operating temperatures (of the order of 1000K or even higher), radiation heat transfer could become a dominant mode of heat transfer in the SOFCs. In this study, we extend our recent work on radiative effects in solid oxide fuel cells [J. Power Sources, 124, No. 2, pp. 453–458] by accounting for the spectral dependence of the radiative properties of the electrolyte material. The measurements of spectral radiative properties of the polycrystalline yttria-stabilized zirconia electrolyte we performed indicate that an optically thin approximation can be used for treatment of radiative heat transfer. To this end, the Schuster–Schwartzchild two-flux approximation is used to solve the radiative transfer equation for the spectral radiative heat flux, which is then integrated over the entire spectrum using an N-band approximation to obtain the total heat flux due to thermal radiation. The divergence of the total radiative heat flux is then incorporated as a heat sink into a three-dimensional thermo-fluid model of a SOFC through the user-defined function utility in the commercial FLUENT computational fluid dynamics software. The results of sample calculations are reported and compared against the base line cases when no radiation effects are included and when the spectrally gray approximation is used for treatment of radiative heat transfer.

Nanoscale effects on ion conductance of layer-by-layer structures of gadolinia-doped ceria and zirconia

Layer-by-layer structures of gadolinia-doped ceria and zirconia have been synthesized on Al2O3(0001) using oxygen plasma-assisted molecular beam epitaxy. Oxygen ion conductivity greatly increased with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia-doped ceria electrolytes. The conductivity enhancement in this layered electrolyte is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Structural and Compositional Characterization of Yttria-Stabilized Zirconia: Evidence of Surface-Stabilized, Low-Valence Metal Species

The structural properties of polycrystalline yttria-stabilized zirconia (YSZ) have been studied using FT-Raman spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). Yttria content was varied between 8 and 15% (by mole fraction) to determine compositional effects on YSZ phonon structure, lattice parameter, and oxidation state. The dominant feature in the low-frequency Raman spectrum correlates quite closely with the material's sole (cubic) lattice parameter. XPS measurements of typical YSZ samples show only a single species of both Y and Zr. After exposing YSZ to a reducing environment (H2) at elevated temperatures (1000 degrees C), however, the XPS spectra of YSZ show new features at lower binding energy for both Y and Zr. Angle-resolved XPS measurements suggest that these reduced forms of Y and Zr exist only within the first few molecular layers of the sample. This treatment does not effect the XRD pattern, nor does it change the low-frequency phonon structure observed in the Raman spectrum, although the Raman spectrum does experience approximately 50% reduction in overall signal intensity. These disparities are reconciled with each other based on differences in each technique's sampling depth. The impact that surface-reduced YSZ may have on the chemistry occurring within solid oxide fuel cells is discussed briefly.

Nanoscale Effects on Ion Conductance of Oxide Nanostructures

Layer-by-layer epitaxial structures of gadolinia-doped ceria and zirconia have been synthesized on Al 2 O 3 (0001) using oxygen plasma-assisted molecular beam epitaxy. Increase in oxygen ion conductivity is observed with an increasing number of layers compared to bulk polycrystalline yttria-stabilized zirconia and gadolinia doped ceria electrolytes. The conductivity enhancement in these layered electrolytes is interesting, yet the exact cause for the enhancement remains unknown. For example, the space charge effects that are responsible for analogous conductivity increases in undoped layered halides are suppressed by the much shorter Debye screening length in layered oxides. Therefore, it appears that a combination of lattice strain and extended defects due to lattice mismatch between the heterogeneous structures may contribute to the enhancement of oxygen ionic conductivity in this layered oxide system.

Preparation of (001)- and (114)-oriented epitaxial thin films of Bi 2VO 5.5 by a coating pyrolysis process

Thin films of Bi 2VO 5.5 (BVO) were prepared on polycrystalline yttria-stabilized zirconia (YSZ) and single-crystal SrTiO 3 [STO] (100) and (110) substrates by a coating-pyrolysis process. X-ray diffraction (XRD) θ–2θ scans showed that the crystallinity of the films increased with increasing heat-treatment temperature. The crystallinity of the BVO (001) films was so high that the XRD peak intensity of the 002 reflection was comparable with that of the STO 100 reflection. XRD pole-figure analysis and reciprocal-space mapping revealed that the films on YSZ are uniaxially c-axis oriented, while epitaxial BVO films with the (001) orientation have grown on STO (100). On the other hand, the occurrence of epitaxial (114) orientation in the BVO films was discovered by the present study and is similar to the case of (116)-oriented SrBi 2Ta 2O 9 films on STO (110).

Surface cleaning and characterization of yttria‐stabilized zirconia

AbstractCleaning substrate surfaces and checking the effectiveness of the cleaning method are two very important aspects of reliable surface analysis. The absence of a cleaning process leading to an yttria‐stabilized zirconia (YSZ) surface that is free of contaminants and of unchanged structure has inspired us to investigate a new surface cleaning method based on a combination of high vacuum annealing at 300°C and oxidation with atomic oxygen. The efficiency of the cleaning procedure is examined by studying the surface composition after each stage of the process with low‐energy ion scattering (LEIS or ISS) and x‐ray photoelectron spectroscopy (XPS). The cleaning method results in complete removal of contaminants adsorbed from the ambient atmosphere, such as water and hydrocarbons. No segregation or sputtering effects due to the cleaning procedure are observed. The process also is compared with more conventional methods such as thermal oxidation and plasma etching. Although this study focuses on cleaning polycrystalline YSZ, which is inherently difficult to clean, other oxides and several metals also are investigated. For all the oxides studied, the newly proposed cleaning strategy removes up to ∼90% more contamination than the commonly used method of thermal oxidation. In the case of metals, the surface carbon is replaced by oxygen, which can be removed much easier. Additional segregation of bulk carbon is avoided. Copyright © 2002 John Wiley & Sons, Ltd.

Interfacial properties of a hetero-structure YSZ/p-(1 0 0)Si prepared by magnetron sputtering

Thin films of polycrystalline YSZ (yttria-stabilized zirconia ) of 4–500 nm in film thickness were deposited on (1 0 0)Si at 800°C by rf-magnetron sputtering of a sintered YSZ target. The sputtered YSZ thin films were evaluated as an alternate gate oxide for Si-MOSFETs. It was found that the interfacial layer of SiO x was formed between YSZ and (1 0 0)Si. The formation of the SiO x layer during the sputtering deposition of the YSZ was not predominant. The surface of the Si was chiefly oxidized during a cleaning stage and/or a pre-sputtering due to possibly residual H 2O or O 2 in a sputtering chamber. The cooling process after the deposition affected the interfacial properties. The rapid cooling in Ar ambient increased a breakdown voltage in a heterostructure Au/YSZ(1 0 0)/p-Si with evaporated gold top electrodes. A small leakage current of 5×10 −8 cm 2 at 1 V and a high breakdown voltage of 9.3 MV/cm were observed for the YSZ thin films of 7.5 nm. The resultant equivalent oxide thickness (EOT) for Si-gate oxide was 3 nm. The interfacial quality of the YSZ and SiO x seems to have a close relation to the small leakage current.

Strain-induced anisotropic low-field magnetoresistance of La–Sr–Mn–O thin films

Sputtered La0.71Sr0.29Mn1.01O3−δ (LSMO) thin films on (001) SrTiO3, polycrystalline yttria-stabilized zirconia (YSZ) and (112̄0) sapphire substrates demonstrate the distinctive low-field magnetoresistance (MR) correlated with the microstructure and the strain of the films. The epitaxial LSMO film on (001) SrTiO3 shows the in-plane magnetic anisotropy with [110] easy axis and the attendant anisotropic MR. The polycrystalline films on YSZ and sapphire substrates with grain sizes from 20 to 60 nm exhibit different anisotropic feature of transport: the isotropic MR of the film on YSZ and the large anisotropy on sapphire substrates. Moreover, in the (112̄0) film plane of sapphire substrate, the [1̄100]SAP magnetic easy axis appears due to a large tensile stress, and the longitudinal MR becomes pronounced along the [0001]SAP hard axis. This implies that the anisotropy of the low-field MR is attributed to the stress induced by the thermal expansion mismatch between film and substrate. These results emphasize that the low-field MR in the polycrystalline manganite can be advanced by the strain induced magnetic anisotropy.

High temperature superconducting films on flexible substrates for cryoelectronics

High temperature superconducting (HTS) films on flexible, low loss, low thermal conductivity substrates offer unique advantages to transmit multichannel digital data with low signal attenuation for cryoelectronic applications. Polycrystalline, flexible yttria stabilized zirconia (YSZ) with a low thermal conductivity of 0.015 W/cm K, a low RF loss of 4×10 −4 (at 77 K, 5 GHz) and a relatively low dielectric constant of 28 is an excellent choice as a substrate if high quality HTS films can be developed with desirable RF properties. This paper presents recent data obtained on biaxially textured Y 1B 2C 3O 7− x (YBCO) films deposited on polycrystalline YSZ substrates with a biaxially-textured YSZ template. YSZ structural templates were developed by Ion beam-assisted pulsed laser deposition (PLD). YBCO films were deposited by PLD. The biaxially textured YBCO films exhibit in-plane (Phi-scans) FWHMs of ∼7°. The transition temperatures ( T c's) of the films are in the range 88–89 K with transition widths of 0.5 K. The critical current densities ( J c's) measured at 77 K, zero field, are in the range of 2×10 6 A/cm 2. The surface resistance of the films measured at 77 K, 10 GHz is 700 μΩ indicating a low microwave loss in these films. The high materials quality obtained in these HTS films on polycrystalline substrates can be used in fabricating a variety of components for cryoelectronic applications.

Preparation of epitaxial SrBi2Nb2O9 and SrBi2Ta2O9 thin films by the coating-pyrolysis process

Epitaxial and polycrystalline thin films of bismuth layer-structured ferroelectrics, SrBi2Nb2O9 (SBN) and SrBi2Ta2O9 (SBT), were prepared on single-crystal SrTiO3(001) and polycrystalline yttria-stabilized zirconia substrates, respectively, by the coating-pyrolysis process. The epitaxial relationship of the films and substrates was SBN, SBT (001)//SrTiO3(001) and SBN, SBT [100]//SrTiO3[100],[010], where pseudotetragonal indices were adopted for SBN and SBT. The lattices of the epitaxial films were found to be slightly strained owing to stress from the substrate. Atomic force microscopy observations showed that the epitaxial films as well as polycrystalline films consisted of round-shaped, islandlike grains of submicrometer size.

Synthesis of Y–Ba–Cu–O films with ethylene diamine tetra-acetic acid complexes assisted by excimer laser ablation

Y–Ba–Cu–O films were prepared on yttria-stabilized zirconia polycrystalline substrate using KrF excimer laser irradiation with ethylene diamine tetra-acetic acid (EDTA) complexes as a target material. The results of the x-ray θ-2θ scan showed that the films grown above 750 °C were preferentially oriented with a c-axis normal to the substrate. The value of full-width at half-maximum of (005) reflection reduced from 4.4° to 2.1° by increasing the laser power density in a range between 0.8 and 3.8 J/cm2. The transcription of the compositional ratio was improved by approaching the target-substrate distance from 45 to 20 mm. The films obtained at 800 °C consisted of many islands showing well-developed spiral growth, suggesting that the droplets of metal-EDTA complexes do not form random oriented crystalline particles on the surface of the substrate.

High-rate reactive sputtering of yttria-stabilized zirconia using pulsed d.c. power

Polycrystalline yttria-stabilized zirconia (YSZ) thin films were deposited by reactive magnetron sputtering of a Zr-Y alloy target in an Ar-O 2 atmosphere. Using a combination of pulsed d.c. power at a frequency of 70 kHz at both the target and substrate, and partial pressure control of the O 2 gas, arcing was controlled and a stable process was obtained. Fully-oxidized cubic (Y 2O 3) 0.10(ZrO 2) 0.90 films were deposited at rates as high as 210 nm/min, or 60% of the metal alloy deposition rate. For films deposited onto electrically-conducting substrates, the structure and hardness of the films were affected by the negative substrate bias V s. With V s = floating (34 V), X-ray diffraction scans showed a strong (200) texture, which switched to a strong (111) texture for V s = 80–150 V. Scanning electron microscopy showed a porous structure with no substrate bias and a dense structure with a bias of V s = 80 V. The film hardness measured using nanoindentation was ≈ 4 GPa for V s = 34 V, and reached a maximum of 17 GPa, comparable to bulk YSZ, at V s = 80–100 V. For films deposited onto glass substrates, the pulsed d.c. substrate bias had no effect, such that porous, low-hardness YSZ was obtained. This was due to the very short time constant for charging the thick glass substrate, compared with the d.c. pulse period.

Preparation of Epitaxial and Polycrystalline La0.8Sr0.2MnO3 Films by Dipping-Pyrolysis Process

ABSTRACTThe films of La0.8Sr0.2MnO3 (LSMO) were prepared on single-crystal SrTiO3(100) and polycrystalline yttria-stabilized zirconia (YSZ) substrates by dipping-pyrolysis process using metal naphthenates as starting materials. The crystallinity, grain alignment, morphology and electric properties of the films were compared. Epitaxial films were found to grow on SrTiO3, heat-treated at 1200°C, by x-ray diffraction, and to exhibit very smooth surfaces by SEM and AFM observations. On the other hand, the surfaces of the polycrystalline films grown on YSZ consisted of round-shaped LSMO grains. The resistivity of the epitaxial films was about two orders of magnitude lower than that of the polycrystalline films prepared under the same conditions. Preparation of epitaxial LSMO films on SrTiO3(110) is also presented.

Y-123 Films on technical substrates

Abstract For high-current applications, homogeneous well-textured high-temperature superconducting Y 1 Ba 2 Cu 3 O 7− δ (Y-123) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as a template for Y-123, was extended to large-area substrates as well as differently shaped long substrates. TEM, as well as XRD investigations, was performed to observe the relevant growth mechanisms. The influence of nucleation, growth selection, and homoepitaxial growth on the development or the maintenance of a preferential orientation of the YSZ buffer is discussed. Y-123 films were prepared by a modified pulsed-laser-deposition (PLD) technique based on a quasi-equilibrium substrate heating and variable azimuth scanning of the target. The method allows a PLD coating of long pieces of tapes and tubular substrates because no direct mechanical contact of a substrate with the heater is required. Critical current densities, J c , above 10 6 A cm −2 were achieved on polycrystalline Ni tapes as well as on polycrystalline YSZ sheets at 77 K and self fields. The dependence of J c on magnetic field resembles that of Y-123 deposited on single-crystalline substrates and demonstrates the absence of weak links due to grain boundaries. The thickness dependence of J c is interpreted in terms of a nucleation layer appearing in the early stage of film growth. A growth model is proposed which seems to be in agreement with the experimental observations. The measured strain tolerance of the films does not impede their application.

Thin film growth and microstructure analysis of CeO2 prepared by MOCVD

AbstractCeO2 Thin films were prepared by metal‐organic chemical vapor deposition (MOCVD) using tetrakis(2,2,6,6‐tetramethyl‐3,5‐heptanedionato)cerium(iv), [Ce(thd)4], as the precursor. The films were grown in a horizontal cold‐wall reactor at low pressure and temperatures of 400–900°C in an atmosphere of argon/oxygen. SrTiO3, yttria stabilized zirconia (YSZ), and MgO were used as substrates. Films were prepared on single crystalline substrates with a (100) orientation as well as on polycrystalline YSZ. The film microstructure was investigated by X‐ray diffraction and the surface morphology was studied by scanning electron microscopy (SEM). The growth rate was determined using data obtained by profilometry and cross‐section SEM. The films are polycrystalline with a (100) preferential orientation with respect to the substrate orientation. Low oxygen partial pressures deteriorated the crystalline quality. The SEM images of the surface and the cross sections demonstrated a columnar microstructure. The growth rates in the range of 25–250 nm/h were strongly dependent on the precursor feeding rate but less affected by the deposition temperature and substrate material.