Pure Yttria-stabilized Zirconia Research Articles

Grazing-incidence synchrotron radiation diffraction studies on irradiated Ce-doped and pristine Y-stabilized ZrO2 at the Rossendorf beamline.

In this work, Ce-doped yttria-stabilized zirconia (YSZ) and pure YSZ phases were subjected to irradiation with 14 MeV Au ions. Irradiation studies were performed to simulate long-term structural and microstructural damage due to self-irradiation in YSZ phases hosting alpha-active radioactive species. It was found that both the Ce-doped YSZ and the YSZ phases had a reasonable tolerance to irradiation at high ion fluences and the bulk crystallinity was well preserved. Nevertheless, local microstrain increased in all compounds under study after irradiation, with the Ce-doped phases being less affected than pure YSZ. Doping with cerium ions increased the microstructural stability of YSZ phases through a possible reduction in the mobility of oxygen atoms, which limits the formation of structural defects. Doping of YSZ with tetravalent actinide elements is expected to have a similar effect. Thus, YSZ phases are promising for the safe long-term storage of radioactive elements. Using synchrotron radiation diffraction, measurements of the thin irradiated layers of the Ce-YSZ and YSZ samples were performed in grazing incidence (GI) mode. A corresponding module for measurements in GI mode was developed at the Rossendorf Beamline and relevant technical details for sample alignment and data collection are also presented.

Thermal radiation and conduction in functionally graded thermal barrier coatings. Part I: Experimental study on radiative properties

Functionally graded thermal barrier coatings (FGTBCs) are functional composites with multilayered structures whose mechanical properties are better than those of conventional ceramic thermal barrier coatings (TBCs). Although many theoretical and experimental studies concerning both the mechanical properties and heat transfer including heat conduction and thermal radiation of TBCs have been conducted, little attention was paid to heat transfer process, especially the coupled conduction-radiation heat transfer, inside FGTBCs. This two-part paper systematically studied the heat transfer inside YSZ/NiCoCrAlY FGTBCs. In Part I, we investigated their radiative properties through experimental measurements. In Part II, we presented a study on the thermal conductivity of FGTBCs and established a coupled heat transfer model. In this paper, more than thirty yttria-stabilized zirconia (YSZ)/NiCoCrAlY duplex TBC and multilayered FGTBC samples with different multilayer structures, porosities and thicknesses were fabricated via air plasma spraying. The transmittance and reflectance spectra within the 0.3–15μm wavelength range were experimentally obtained by UV–visible spectrometry and Fourier transform infrared spectroscopy. A scanning electron microscope analysis was carried out to characterize the microstructures of FGTBCs. Additionally, we studied each layer of the FGTBCs individually; the four-flux method was employed to obtain the radiative properties based on our experimental measurements. Functionally graded materials (FGM) show larger absorption coefficients and smaller scattering coefficients compared to pure YSZ. The FGTBCs show lower reflectance than YSZ TBCs. We also study the influence of the number of layers on reflectance. The results indicate that FGTBCs have worse thermal insulation ability against radiative flux tconventional ceramic TBCs of the same thickness. This paper provides practical guidance to improve the design of YSZ/NiCoCrAlY FGTBCs and other FGM coatings to reach a balance between good thermal insulation and mechanical strength.

Fabrication of functionally-graded yttria-stabilized zirconia coatings by 355 nm picosecond dual-beam pulsed laser deposition

This article presents the design, fabrication, and characterization of functionally-graded yttria-stabilized zirconia (YSZ) coatings by using a novel dual-beam pulsed laser deposition method based on a 355 nm ps laser. Graded coatings were fabricated by mixing YSZ and the substrate material (stainless steel). The two materials were ablated and deposited on a substrate inside a vacuum chamber, in a way that the mixture composition gradually changes from stainless steel to YSZ, along the coating's growth direction, following a predesigned linear composition profile. The Rockwell indentation method was employed for the adhesion strength analysis based on the interfacial crack radius (c) and indentation load (P). When a load P is applied on the sample surface, a crack with a radius c will be formed. Hence, by using more than one loads and measuring the generated crack diameters, dP/dc can be estimated, which is considered as a critical parameter and is proportional to the coating's adhesion strength. The graded coating was found to provide improved adhesion strength (dP/dc = 9.41 N/μm) over pure YSZ coatings (dP/dc = 8.35 N/μm and 8.43 N/μm). In this study, the thermal barrier effects of the coatings were examined by measuring the thermal conductivity values of the coatings using a high-power multi-mode fiber laser as an impulsive heat source and an infrared camera, and the deposited YSZ coatings showed a low thermal conductivity (∼1.14 W/m·K).

Co-precipitation synthesis of alumina doped yttria stabilized zirconia

Co-precipitation method is applied to fabricate different amount of Al2O3–doped yttria stabilized zirconia (YSZ). The crystal phase, microstructure and electrical properties of the sample with access of Al2O3 are studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS), simultaneously a comparison is made with pure YSZ. When doping weight percentage of Al2O3 varies from 0 to 4%, the grain size decreases and existence of Al2O3 does not introduce new phase but pure cubic structure of YSZ can be observed. The lattice parameter increases first until doping 1 wt% Al2O3, then decreases with more doping amount. YSZ doped with different amount of Al2O3 has different ionic conductivity. As YSZ is doped with 0.5 wt%, 2 wt%, 3 wt% or 4 wt% Al2O3, the total ionic conductivity for each sample is lower than pure YSZ. Among all of the samples, the highest conductivity value belongs to the specimen of YSZ doped with 1 wt% Al2O3 owing to increased grain boundaries, reaching to 0.0334 S cm−1.

Enhanced performance of Fe2O3 doped yttria stabilized zirconia hollow fiber membranes for water treatment

Fe2O3 powders were introduced as sintering aid to fabricate yttria-stabilized zirconia (YSZ) hollow fiber membranes using a combined wet-spinning and post-sintering method. The obtained Fe2O3-YSZ hollow fiber membranes show enhanced performance for water treatment with fine crystal structure in terms of bending strength and pure water permeability. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and thermogravimetric analysis (TGA) along with mechanical tests were employed to investigate the structural evolution in the sintering process and the effect of Fe2O3. It is suggested that the Fe2O3 dopants dissolve into YSZ at elevated temperatures, providing defect sites and vacancies for fast ion migration, favoring for densification and grain growth of the YSZ, which yields dense microstructures of fine crystallites at relatively low sintering temperature. The Fe2O3-YSZ hollow fiber membranes sintered at 1150°C show a 3-fold increase of the permeate flux of pure water (F) (743Lm−2h−1) along with comparable bending strength (152MPa) compared to pure YSZ membranes. This modified method can reduce sintering costs and therefore fabrication costs which should pave the way for scale-up production for ceramic hollow fiber membranes.

Degradation of SPS-Fabricated YSZ and Nd2O3-YSZ Ceramics in Supercritical Water

Zirconia (ZrO2) ceramics are being considered as a candidate material for thermal insulating barriers in pressure tubes used in the supercritical water (SCW) nuclear reactors. However, the literature suggests that zirconia may undergo a detrimental phase transformation which is accelerated in aqueous environments. In this research, 8 mol% Yttria-Stabilized Zirconia (YSZ) ceramics with the addition of 5 and 10 mol% Nd2O3 were manufactured via spark plasma sintering (SPS) process and subsequently subjected to a SCW environment. The weight losses and microstructural evolutions of these materials during SCW exposure were studied. The results suggest that doping YSZ with Nd2O3 significantly decreased the degradation rate of the YSZ ceramic and improved its structural stability. X-ray diffraction studies revealed that after degradation testing, the Nd2O3 helped to retain the desirable cubic phase of YSZ matrix. In the case of pure YSZ ceramic, a phase change of the matrix toward the monoclinic lattice was observed and likely contributed to the ceramic’s disintegration in SCW environment.

Fe-doped 8YSZ at different composition for solid electrolyte in solid oxide fuel cell

Pure 8 mol% yttria stabilized zirconia (YSZ) and Fe-doped (1 mol%, 2 mol% and 3 mol%) YSZ electrolyte were prepared and sintered at 1550°C. Transition metal oxide is added into YSZ as sintering aided has a function to reduce the sintering temperature. The microstructure, crystal structure and ionic conductivity of pure YSZ and Fe-doped YSZ at different composition were investigated. The amount of cubic phase decreased as the amount of Fe increased. Fe-doped 8YSZ had higher conductivity than pure 8YSZ. The ionic conductivity of 3FeYSZ is 9.35×10 −8 S/cm higher than 1FeYSZ which is 4.72×10 −9 S/cm when operated at 300°C.

Low-temperature Sintering of Yttria-stabilized Zirconia Using Bismuth–Vanadium Oxide as a Sintering Aid at 800 °C

Abstract The effect of the addition of copper-doped bismuth–vanadium oxide to yttria-stabilized zirconia (YSZ) on the sinterability and electrical conductivity, especially in reducing atmospheres, has been investigated. When Bi2Cu0.1V0.9O5.35 was used as a sintering aid, the sintering temperature could be reduced to 800 °C, whereas the conventional sintering process requires temperatures above 1300 °C. Although the conductivity of YSZ with 1.5 mol % Bi2Cu0.1V0.9O5.35 was slightly lower than that of pure YSZ sintered at 1300 °C, no PO2 dependence was observed under reducing conditions (e.g., PO2 = 10−5–10−24 atm), which indicates that the electrolyte is applicable to solid-oxide fuel cells.

Transition metal-doped yttria stabilized zirconia for low temperature processing of planar anode-supported solid oxide fuel cell

An attempt has been made to reduce the processing temperature for fabrication of anode-supported solid oxide fuel cell using transition metal doped 8mol% yttria stabilized zirconia (YSZ) as the material for electrolyte and Ni-YSZ anode support. Through a simple mixing technique, the sintering temperature of a widely used commercial YSZ powder is possible to reduce substantially by adding 2mol% transition metal oxides e.g. cobalt, manganese and iron. More than 96% of theoretical density is achieved at 1175°C. No difference in thermal expansion behaviour is observed for the doped YSZ with respect to pure YSZ (without any additive). Using tape casting and lamination techniques, planar anode-supported half cells (Ni-YSZ/YSZ) were fabricated with such doped YSZ and co-sintered at two different temperatures viz., 1175°C and 1350°C. Finally, single cells (dia. ∼15mm) were fabricated through screen printing of La(Sr)MnO3 (LSM)–YSZ based cathode active layer and LSM current collector layer and tested with hydrogen as fuel and oxygen as oxidant in the temperature range 700–800°C. A reasonably good electrochemical performance (∼1.6A/cm2 at 0.7V and 800°C) is achieved for single cells when half-cell sintering temperature is 1350°C. However, under identical conditions, the current density dropped below 0.2A/cm2 when the corresponding half-cell is sintered at 1175°C. For such a cell, although the open circuit voltage (OCV) is found to be very stable (∼1.1V), the current drawing capability fell abruptly with increasing current load and a corresponding decrease of cell voltage. The cell microstructure revealed that while sintering half-cell at 1175°C, complete densification of the doped YSZ electrolyte film is not achieved. It is anticipated that redesigning of fabrication steps could be required to utilize the benefits of low sintering temperature of cell components.

Evolution of Hot Corrosion Behavior of YSZ‐Ta2O5 Composites with Different YSZ/Ta2O5 Ratios

This study compares the hot corrosion performance of yttria‐stabilized zirconia (YSZ)‐Ta2O5 composite mixtures (TaYSZ) in the presence of molten mixture of Na2SO4 + V2O5 at 1100°C. The tested compositions vary from pure YSZ (0TaYSZ) to a Ta2O5‐dominating (70 wt% Ta2O5+ balance YSZ, 70TaYSZ) composite mixture with a gradual increase of Ta2O5. 50TaYSZ (50 wt% Ta2O5 + balance YSZ) sample forms a highly stable zirconium tantalum oxide. 50TaYSZ is more stable, both thermally and chemically in Na2SO4+V2O5 media at 1100°C, than other tested YSZ‐Ta2O5 (TaYSZ) composites, and shows a good hot corrosion resistance.

Ni-based anode-supported Al2O3-doped-Y2O3-stabilized ZrO2 thin electrolyte solid oxide fuel cells with Y2O3-stabilized ZrO2 buffer layer

In order to reduce the sintering temperature of Ni-based anode-supported thin 8 mol% yttria-stabilized zirconia (YSZ) elsectrolyte solid oxide fuel cells (SOFCs), alumina, with a weight percent of 1, 3, 5, and 7, is respectively doped into YSZ as sintering aid. A pure YSZ buffer layer is introduced between the Al2O3-doped-YSZ electrolyte and Ni–YSZ anode, to prevent Al2O3 and NiO from forming non-conductive spinel NiAl2O4. The experimental results show that doping proper amount of Al2O3 doping can reduce the sintering temperature of YSZ, e.g., 1 wt.% doping decreases the temperature from 1673 K to 1573 K. Anode-supported SOFCs are prepared with Al2O3-doped-YSZ electrolytes sintered at different temperatures. Electrochemical characterization of the SOFCs shows that the single cell with 1 wt.% alumina-doped YSZ electrolyte sintered at 1573 K gives the highest output. The effect of alumina doping on sintering behavior and electrical performance of YSZ is discussed in detail.

Radiopacity and Cytotoxicity of Portland Cement Containing Zirconia Doped Bismuth Oxide Radiopacifiers

IntroductionThis study evaluates the radiopacity and cytotoxicity of Portland cements containing a radiopacifier of bismuth oxide (Bi2O3) with yttria-stabilized zirconia (YSZ) dopant. MethodsVarious radiopacifier powders of Bi2O3 with 0%, 15%, 30%, and 100% YSZ dopant were prepared by solid-state reaction at 700°C for 12 hours and characterized by x-ray diffraction. Portland cement/radiopacifier/calcium sulfate (75/20/5) were mixed and set by deionized water. Changes in radiopacity and in vitro cell viability of the hydrated cements were assessed. An average of 6 measured equivalent thickness of aluminum (N = 6) capable of producing similar radiographic density was recorded. The cytotoxicity of each material was determined in MC3T3 E1 cell-based methyl-thiazol-tetrazolium assay. ResultsThe x-ray diffraction patterns of YSZ doped Bi2O3 are different from those of pure Bi2O3 and YSZ. The cement-containing radiopacifier of Bi2O3/YSZ (85/15) presented significantly greater radiopacity (P < .05) compared with pure Bi2O3. The mouse osteoblastic cell (MC3T3-E1) viabilities of these 2 groups were statistically similar (P < .05). ConclusionsThe radiopacifier of Bi2O3/YSZ (85/15) reveals higher radiopacity but similar cell viability when compared with pure Bi2O3. It shows potential use as an alternative radiopacifier in root-end filling materials.

In Situ Analytical Electron Microscopy Studies of Redox Reactions at a YSZ/Pt Interface

Redox reactions were studied at a single yttria-stabilized zirconia (YSZ)/Pt electrode interface, in parallel with pure YSZ with no catalyst electrode, by in situ analytical electron microscopy at elevated temperatures and in an oxygen atmosphere. In situ electron holography showed that the oxide underwent reduction at elevated temperatures in a vacuum and was consequently reoxidized upon exposure to an oxygen flux at the same temperature. In situ energy loss spectroscopy measurements were in agreement with in situ electron holography observations and indicated that the oxidation state of the host cation zirconium was altered in the reduced state of the YSZ to the metastable state Zr(3+).

Direct Synthesis of Yttria-Stabilized Zirconia Nanopowder by Thermal Decomposition

An easy synthesis route of YSZ (Yttria-Stabilized Zirconia) nanopowder with the formation of Zr0.92Y0.08O1.96is synthetized by using thermal decomposition of ZrOCl2and Y(NO3)3, with polyvinyl alcohol (PVA) as surface active agent and with quantitative filter paper as carrier. The roasting temperature of YSZ precursor was 650oC. The results of SEM and XRD analysis showed that the pure ZrO2and YSZ powders prepared via thermal decomposition had better crystallinity, and YSZ powders had perfect crystalline t-ZrO2phase after heating at 650 °C, and changing the amount of PVA or the reaction precursors could obtain YSZ powders sized 20-50 nm.

Aqueous slurry processing of monolithic films for SOFC – YSZ, LSM and YSZ–NiO systems

The powder precursor for yttria stabilized zirconia (YSZ) was prepared by reverse strike co-precipitation method while those for lanthanum strontium manganite (LSM) and nickel oxide (NiO) were prepared by gel combustion method. The thermal decomposition and phase evolution behavior of these powder precursors was carried out by thermogravimetry and XRD. Phase pure cubic YSZ formed around 430 °C from the dried amorphous gel and exhibited a mass loss of 23%. Even though the as formed LSM and NiO precursors exhibited nano-crystallinity, they contained some amount of volatiles (up to 8%), elimination of which required a post heat treatment. The optimum calcination temperature for these powders to obtain sintered bodies with desired densities (viz. >95% T.D. for YSZ and 65–75% T.D. for LSM, YSZ–NiO) at the desired sintering temperatures (1350, 1400 and 1500 °C respectively), was found to vary in the range of 900 1350 °C. Fine YSZ powder with size (D50) 0.7–1.2 μm was used in formation of the electrolyte film while YSZ, LSM and NiO powder with size (D50) 3–5 μm along with carbon pore former (15 wt% in LSM) were used for formation of electrode films. The conditions for slurry formation for film casting were evaluated through surface charge and rheological studies. The study of the effect of pH of aqueous suspension on zeta potential showed that YSZ and NiO were charged to sufficient extent (>20 mV) in both acidic and alkaline media while LSM and pore former exhibited sufficient surface charging only in alkaline medium. The slurries for tape casting were formulated using a polyvinyl binder solution and the composition was optimized through rheological studies. Compositions were fixed to form slurries with desired amount of pseudo-plasticity that could exhibit controlled flow to form flexible films with desirable thickness. The process conditions were optimized to form flat sintered electrolyte films possessing about 95% T.D. and electrode films possessing 65–75% T.D. Sintered bodies of the electrolytes exhibited fine-grained microstructure while the electrodes exhibited composite structure of grains and inter-connected pores.

Elaboration of thin and dense CGO films adherent to YSZ by electrostatic spray deposition for IT-SOFC applications

The aim of this work was to fabricate a dense and thin film of gadolinium doped ceria (CGO) adherent on yttria-stabilized zirconia (YSZ) using electrostatic spray deposition (ESD) for intermediate temperature solid oxide fuel cells (IT-SOFC) applications. In order to optimize the film microstructure and thickness, several ESD parameters were studied such as the total salt concentration of the solution (0.05, 0.01 and 0.005 mol L−1), the substrate temperature (range from 275 to 350 °C), the solvent composition (using different butyl-carbitol : ethanol ratios), nozzle-to-substrate distance (35 and 45 mm), the solution flow rate (0.50, 0.67 and 1 mL h−1) and the deposition time (from 15 to 60 min). The different morphologies observed by SEM are discussed. A continuous 85 nm film was successfully deposited on a YSZ substrate for the first time by ESD. Grazing incidence X-ray diffraction analysis showed two phases: the cubic CGO and YSZ phases with no secondary phases. The first impedance measurements did not show any degradation of CGO/YSZ electrical properties compared with pure YSZ.

Yttria-stabilized zirconia-based composites with adaptive thermal conductivity

Thermal conductivity trends in a “chameleon coating” thin film were characterized with a time-domain thermoreflectance (TDTR) technique. A yttria-stabilized zirconia (YSZ)-based nanocomposite material containing ∼21 vol.% silver (Ag) was employed for this study. The thermal conductivity ( k) of the as-deposited composite film was measured with TDTR and found to have a value of 7.4 ± 1.4 W m −1 K −1. The film was then annealed at 500 °C for 1 h to stimulate Ag flow from within the composite to the surface via diffusion. The Ag that coalesced on the surface during annealing was removed to expose the underlying porous YSZ matrix, and the sample was reexamined with the TDTR technique. The thermal conductivity of the porous nanocomposite YSZ material was then measured to be 1.6 ± 0.2 W m −1 K −1, which is significantly lower than a fully dense control sample of pure nanocrystalline YSZ (2.0 ± 0.1 W m −1 K −1). The annealed film displayed a 20% reduction in thermal conductivity as compared to the control sample and a 4–5-fold reduction in thermal conductivity as compared to the as-deposited material. The experiments demonstrate temperature triggering of a composite material, resulting in self-modifying thermal conductivity and diffusion-controlled porosity. These aspects can be used to enhance or restrict thermal transport (i.e., a thermal switch). The applicability of the TDTR technique to measurements of thin, nanoporous film materials is also demonstrated.

Microwave sintering of plasma-sprayed yttria stabilized zirconia electrolyte coating

In this work, microwave (MW) was used to sinter atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia (YSZ) electrolytes coatings. Three types of coatings were studied: pure YSZ coating, 5 wt% BaTiO 3 filled YSZ coating and 10 wt% BaTiO 3 filled YSZ coating. Microstructures and ionic conductivities of as-sprayed and MW-sintered coatings were characterized. The results indicated that that the pure YSZ coating can hardly be sintered by the MW sintering and it can only be heated up to 900 °C. For two composite coatings, BaTiO 3 particles distributed in the composite coating act as a good MW susceptor and thereby, a good heat source for sintering YSZ matrix. After a MW sintering at 1450 °C for about 30 min, an obvious structural modification was noticed: the lamellar and columnar crystal features of as-sprayed plasma-sprayed YSZ coatings were modified to equiax crystal in the sintered coatings. Ionic conductivity of sintered 5 wt% BaTiO 3 filled YSZ composite coatings were elevated to about twice of as-sprayed coating. A further increase of BaTiO 3 content to 10 wt% in the composite coating enhanced the sintering efficiency, however, the ionic conductivity of composite coatings was decreased.

The effect of Fe doping on the properties of SOFC electrolyte YSZ

Pure YSZ (8 mol% yttria stabilized zirconia) and Fe-doped (2 mol% and 4 mol%) YSZ electrolytes were prepared and sintered at 1200 °C and 1400 °C, respectively by solid state method. Their crystal structure, microstructure, sintering behavior, electrical conductivity, and performance as SOFC electrolyte were investigated. Fe can be dissolved into YSZ lattice and decrease the lattice parameter. When sintered at 1200 °C, Fe dopant can make the YSZ electrolyte denser and increase its total conductivity. When sintered at 1400 °C, the total conductivities of samples decrease for higher Fe dopant concentrations though the densities of samples increase. All the SOFCs with Fe-doped YSZ electrolytes show better performance than those with Fe-free electrolytes. The mechanisms of the effect of Fe doping on the properties of YSZ were analyzed.

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.