Yttria-stabilized Cubic Zirconia Research Articles

YSZ-rGO composite ceramics by spark plasma sintering: The relation between thermal evolution of conductivity, microstructure and phase stability

Despite a high demand for green energy sources, low performance of solid oxide fuel cells (SOFC) hinders the potential for its practical application. Zirconia-graphene composites are promising candidates as solid electrolytes for SOFC enabling efficient heat transfer and effective ion charge carriers movement. Herein, we report the change in electrochemical behavior of spark plasma sintered zirconia-reduced graphene oxide (YSZ-rGO) composites. The conductivity is shown to be altered from fully ionic to mixed electronic-ionic mode upon a small amount of rGO addition. Mixture of freeze-dried nano-sized yttria stabilized cubic zirconia (YSZ) powder and microwave sintered reduced graphene oxide (rGO) was utilized for the fabrication of YSZ-rGO ceramic composites. According to SEM and EBSD data, addition of rGO resulted in a bi-modal grain size distribution. The material with 2 wt.% rGO has demonstrated a mixed electron-ion conductivity in a wide temperature range of 294–1073 K in thermal cycling experiments at oxygen partial pressure of 10−3–10−5 atm, where oxygen ion conductivity dominates over electronic one at elevated temperatures. The electrochemical stability of YSZ-2 wt.% rGO was tested by thermal cycling in nitrogen and argon atmospheres via impedance spectroscopy at 473–1073 K. The model to describe the conductivity evolution with temperature and thermal cycling has been suggested for the first time.

Direct bonding of high dielectric oxides for high-performance transistor applications

We developed a plasma activation process using a gas mixture of O2/NH3/H2O for direct bonding, which realized the combination of bulk aluminum oxide (Al2O3) and yttria-stabilized cubic zirconia (YSZ) without interlayers for the first time. The bonding could withstand multiple mechanical grinding and polishing processes and exhibited sufficient bonding strength for device micro/nanofabication. A nanoscale, sharp, and layered crystalline bonding interface with fewer grain boundaries was confirmed by transmission electron microscopy. The interfacial structure is well-suited for superior performance in high dielectric constant metal–oxide–semiconductor field-effect transistors. Moreover, this bonding method is universal for the fabrication of Al2O3/SiO2 and YSZ/SiO2 heterostructures.

(Invited) In Situ Filamentary Resistance-Switching Experiments Using Bulk Oxide Single Crystals and Polycrystals

This presentation will describe how in situ testing of bulk oxide specimens can provide useful information about nanofilament formation and resistance switching in oxide memristor devices. Past in situ experiments performed in transmission electron microscopes using electron-transparent thin films are known to be subject to artifacts because of radiolysis. Past ex situ experiments inspecting filaments in memistor devices are also problematic because of poor statistics statistics: finding a filament in a resistance-switching device is like finding a needle in a haystack. Two contrasting model oxides in single crystal and polycrystal forms were used in our study. (a) Yttria stabilized cubic zirconia (YSZ) is a fast oxygen ion conductor with a very wide electrolytic range and a very small contribution from hole conductivity especially at low temperatures. (b) Undoped as well as acceptor doped strontium titanate (STO) is a p-type semiconductor with a small amount of quenched-in oxygen vacancies that move very slowly especially at low temperatures. These oxides at the composition studied have no structural or electronic phase transition, unlike V2O5or VO2. Especially at low temperature, their cations are so immobile that they cannot migrate to form metallic filaments, unlike Ag or Cu. Therefore, they may be regarded as two extremes of prototypical oxides, and any observation common to both are likely to be general and applicable to other oxides and nitrides as well. This is important, for already there are a large number of materials—in many structures and chemistries—made into memristors and they all behave similarly. Our goal is to find the common element that explains their behavior, namely filament formation and filamentary resistance-switching. To achieve this goal, our in situ experiment on bulk specimens must specifically demonstrate at least the following: (1) a localized region can form in a virgin oxide, which results in an overall resistance that is orders of magnitude smaller, obtaining the “low resistance state” (LRS); (2) in this LRS, the overall resistance can next be increased by orders of magnitude to reach the “high resistance state” (HRS), meanwhile some features of the localized region should still be retained. These have been successfully demonstrated in our experiment providing highly reproducible observations. Having achieved (1-2) and delineated the conditions for doing so, the in situ experiment should also answer the following question: what is the thermodynamics, kinetics, geometry and electrostatics/electrochemistry of filament formation and resistance switching. We attempt to answer the above question by comparing the experimental observations with the solution of the field equation for the distribution of oxygen and electric potential, the concentrations of defects and electrons/holes, and the temperature. Given the appropriate boundary conditions and our knowledge of YSZ and STO, this is solvable although the solution procedure is quite different for two oxides because they are two rather different conductors with rather different distributions of oxygen vacancies. It is important to note that the boundary conditions, which include the flux of oxygen, the ratio of ionic current to the electronic current, and the form of the electric potential/field, are themselves observables that can be ascertained from the experiment in situ and ex situ. Moreover, varying the boundary conditions provides a further means to tune the conditions for (1-2), thus lending further support to our explanation. Despite the macroscopic size of our samples, the findings can be transferred to nanoscale devices using scaling arguments. For example, in the field equation, one scaling parameter is jL, the product of current density and the distance between the top and bottom electrodes, and in the normalized form the same distribution is obtained in different problems when their jL is the same. Since j scales with (filament area)-1and L with (filament length), it follows that the switching current is much smaller in a nanoscale device than in a bulk sample. These considerations have provided further insight to the switching parameters, such as critical voltage, of nanoscale memristors.

A cobalt tungstate compound sensing electrode for hydrogen detection based upon mixed-potential type sensors

An yttria-stabilized cubic zirconia (YSZ) based mixed-potential sensor coupled with CoWO4 as the sensing electrode was developed for hydrogen detection at elevated temperatures.

Hydration Structures and Water Chemistry at Zirconia-Water Interface

The life cycle of zirconium cladding is a key safety factor and design parameter for higher fuel burnups in pressurized water reactors. The degradation of the metal alloy is primarily attributed to interactions of native oxide with water. Here we probe the detailed interfacial hydration structures at the 8 mol% yttria-stabilized cubic zirconia (YSZ) surfaces as a model system using synchrotron-based high-resolution X-ray reflectivity technique. Interfacial hydration structures in the major crystallographic orientations, (100), (110), and (111), were determined with sub-angstrom resolution and compared with each other to identify common features as well as different surface chemistry effects on the interfacial processes. All three surfaces terminate with significant number of point defects originating from the metal depletion and the intrinsic oxygen vacancies. Water molecules fill those vacancies, forming an ordered, layered structure on the top surface. Above the termination layer on each surface, two additional adsorbed layers form characteristic interfacial hydration structures. The first adsorbed layers likely include metal species based on the observed local densities, whereas the second layers consist of pure water. The current results provide valuable references for future studies of water chemistry effect on the interfacial hydration structure and the high temperature stability test of the zirconia in pressurized water conditions simulating realistic environment in pressurized water reactors of nuclear power plants. We also studied the effect of zinc adsorption on the interfacial hydration structure in zinc acetate solutions. Zinc injection into the reactor coolant system has been known to be effective in reducing the radioactive wastes as well as stabilizing crud oxide layers of the cladding materials. Our X-ray reflectivity data qualitatively reveal that there are obvious hydration structure changes at (100) and (111) surfaces while (110) surface show little changes. The interfacial electron density profiles were derived from the structure factor analysis of the measured reflectivity data. The structure factor analysis indicated an inner-sphere adsorption of Zn2+ layer at YSZ (111)-water interface, while there appeared to be an outer-sphere adsorption of Zn2+ layer at YSZ (100)-water interface. We further confirmed the detailed element specific adsorption profile of Zn2+ ions on YSZ (110) and (111)-water interfaces with resonant anomalous X-ray reflectivity measurements.

Potentiometric hydrogen sensors based on yttria-stabilized zirconia electrolyte (YSZ) and CdWO4 interface

The yttria-stabilized cubic zirconia (YSZ) based potentiometric sensor coupled with CdWO4 as the sensing electrode was developed for hydrogen detection. The developed CdWO4/YSZ/Pt sensor has found significant response to different concentration of hydrogen from 0.5vol% to 3vol% at 500°C. The sensor showed good selectivity to several possible interferents such as CO, NO2 and C3H8 when the concentrations are less than 1500ppm. The dc polarization curves and ac impedance spectra of the sensor were measured to study the sensing mechanism. The mixed-potential theory was confirmed to explain the sensing results. The reproducibility and signal repeatability of the sensor was examined to test the reliability of the sensor. The possible influence of oxygen variation and humidity in the background on the sensor response was also studied.

Adsorption-controlled growth of BiVO4 by molecular-beam epitaxy

Single-phase epitaxial films of the monoclinic polymorph of BiVO4 were synthesized by reactive molecular-beam epitaxy under adsorption-controlled conditions. The BiVO4 films were grown on (001) yttria-stabilized cubic zirconia (YSZ) substrates. Four-circle x-ray diffraction, scanning transmission electron microscopy (STEM), and Raman spectroscopy confirm the epitaxial growth of monoclinic BiVO4 with an atomically abrupt interface and orientation relationship (001)BiVO4 ∥ (001)YSZ with [100]BiVO4 ∥ [100]YSZ. Spectroscopic ellipsometry, STEM electron energy loss spectroscopy (STEM-EELS), and x-ray absorption spectroscopy indicate that the films have a direct band gap of 2.5 ± 0.1 eV.

Industrial growth of yttria-stabilized cubic zirconia crystals by skull melting process

We reported the development of a Φ100 cm growth apparatus for skull melting growth of yttria-stabilized cubic zirconia (YSZ) crystals and more than 1000 kg crystals have been grown in the furnace each time. The growth conditions were optimized and the structure of the as-grown crystals was characterized by X-ray diffraction. The transmittance of 15 mol.% yttria-stabilized cubic zirconia crystal was nearly 80% in the range of 400–1600 nm. The refractive indices were measured and fitted the Sellmeier equation which was obtained by least-squares method. The results showed that the apparatus was very suitable for industrial growth of YSZ crystals.

Computational Modeling Study of Bulk and Surface of Yttria-Stabilized Cubic Zirconia

Interatomic potential simulation techniques have been applied to both the bulk and surfaces of yttrium-stabilized cubic zirconia (YSZ). In the Mott−Littleton bulk calculations representing the infinitely dilute system, the yttrium dopants tend to exist as a pair with two yttriums close to each other and preferentially occupying the next-nearest neighbor (NNN) sites to the oxygen vacancy. However, the energy of the YSZ system as well as the configuration of defect cluster depends on the dopant concentration level. The calculated lattice energy for supercell models linearly increases with yttria content. At around 10% mol Y2O3, the simulations indicate the existence of two stable cubiclike phases differencing in term of the detached arrangement of oxygen ions. Surface-energy calculations confirm the dominance of the (111) surface in c-ZrO2 and YSZ. The yttrium solution energy at the surface has been estimated as a function of the Y dopant-vacancy cluster depth. The calculations demonstrate that for yttrium segregation to the top layers of the (111) surface. However, there is no evidence for strong segregation to the (110) surface.

Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation

We have succeeded in growing epitaxial and highly stoichiometric films of EuO on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation process during the molecular beam epitaxy assisted growth enables the consistent achievement of stoichiometry. We have also succeeded in growing the films in a layer-by-layer fashion by fine tuning the Eu vs oxygen deposition rates. The initial stages of growth involve the limited supply of oxygen from the YSZ substrate, but the EuO stoichiometry can still be well maintained. The films grown were sufficiently smooth so that the capping with a thin layer of aluminum was leak tight and enabled ex situ experiments free from trivalent Eu species. The findings were used to obtain recipes for better epitaxial growth of EuO on MgO (001).

Microstructural Changes Induced by Heating at High Temperatures in the Single Crystal of Yttria-Stabilized Cubic Zirconia Implanted with Ag Ions

Ag ions were implanted into single crystals of yttria-stabilized cubic zirconia (YSZ) with largely different energies of 20 keV, 128 keV, and 1.5 MeV, and the cross sections of the implanted layer were examined. After implantation, thermal treatments were performed for the samples at temperatures up to 1000 °C in air. The surface of the sample implanted at 20 keV with a fluence of 3×1016 ions/cm2 was violet and showed an absorption peak at 508 nm. The samples implanted with 128 keV or 1.5 MeV with fluences of 3×1016 and 2×1016 ions/cm2, respectively, were light brown. However, after heating at 800–1000 °C, their color changed from light brown for the as-implanted state to violet. The absorption spectra of the samples implanted with markedly different energies showed different aspects of changes when the samples were heated at high temperatures. Scanning transmission electron microscopy (STEM) and high-angle annular dark field (HAADF) observations were conducted. Ag nanoparticles of approximately 6–12 nm were densely formed in the as-implanted state for both samples implanted with the energies of 20 and 128 keV, but they could not be observed for the sample implanted with 1.5 MeV. Ag nanoparticles were formed after heating at 1000 °C for the sample implanted at 1.5 MeV. While Ag nanoparticles remained stably even when heated at 900–1000 °C for the samples implanted at 128 keV or 1.5 MeV, those particles for the sample implanted at 20 keV gradually reduced in number when heated at high temperatures.

Coloration Related to Nanostructure of Yttria-Stabilized Cubic Zirconia Single Crystal Implanted with Ag Ions

Ag ions were implanted into single crystals of yttria-stabilized cubic zirconia (YSZ) at 1.5 MeV to a fluence of 2×1016 ions/cm2. After implantation, heat treatments were performed at 500, 900, and 1000 °C in air. The surface color tone markedly changed from brown to a beautiful violet after heat treatments. Scanning transmission electron microscope (STEM) observations revealed that Ag nanoparticles were formed when samples were heated at temperatures higher than about 900 °C. The absorption spectrum in the visible region was measured. A large absorption peak appeared at 510 nm after heating at 900 °C. The peak could be attributed to the surface plasmon resonance of Ag nanoparticles.

X-ray photoemission spectroscopy determination of the InN/yttria stabilized cubic-zirconia valence band offset

The valence band offset of wurtzite InN(0001)/yttria stabilized cubic-zirconia (YSZ)(111) heterojunctions is determined by x-ray photoemission spectroscopy to be 1.19±0.17eV giving a conduction band offset of 3.06±0.20eV. Consequently, a type-I heterojunction forms between InN and YSZ in the straddling arrangement. The low lattice mismatch and high band offsets suggest potential for use of YSZ as a gate dielectric in high-frequency InN-based electronic devices.

Molecular dynamics simulations of basal and pyramidal system edge dislocations in sapphire

Directionally solidified ceramic eutectics containing alumina as a topologically continuous majority component with, e.g., yttria-stabilized cubic-zirconia or with YAG as minority components have become of considerable interest as members of a new family of potential high temperature structural materials. The attractive creep resistance of such eutectics in fiber-form is based on their remarkably tight [0 0 0 1] texture of the alumina component in which neither the basal plane nor the prism plane can be activated to result in glide that can extend the eutectic by creep in tension. Under such conditions creep in such eutectics must be governed by climb of the (1/3) 1 ¯ 101 edge dislocations out of either the ( 11 2 ¯ 0 ) prism plane or the ( 1 1 ¯ 02 ) pyramidal plane. To develop better understanding of the core structure of these dislocations and their potential role in the creep resistance of the alumina component in eutectics with tight [0 0 0 1] textures, a molecular dynamics (MD) simulation was carried out of the comparative core structures of both the (1/3) 2 1 ¯ 1 ¯ 0 basal edge dislocations and the (1/3) 1 ¯ 101 pyramidal edge dislocations on the ( 1 1 ¯ 02 ) plane in sapphire. The MD simulation revealed that the equilibrium structure of the core of the pyramidal edge dislocations undergo a dissociation into two half strength partial edge dislocations displaced vertically out of the best glide plane of cation holes with weak covalent bonding and possible fair glide resistance into two adjacent pyramidal planes of very strong covalent bonding, and consequent, very high glide resistance. While this explains the immobility in glide of such dislocations on the pyramidal system, no important structural impediment was found for their climb motion out of the pyramidal planes.

A Two-Step Thermochemical Water Splitting by Iron-Oxide on Stabilized Zirconia

The thermochemical two-step water splitting cycle was examined by using an iron oxide supported on yttrium-stabilized, cubic zirconia (YSZ) as the working material with a view toward direct conversion of solar high-temperature heat to clean hydrogen energy. In the first step of the cycle, the YSZ-supported Fe3O4 was thermally decomposed to the reduced phase at 1400°C under an inert atmosphere. The reduced solid phase was oxidized back to the original phase (the YSZ-supported Fe3O4) with steam to generate hydrogen below 1000°C. A new redox pair, which is different from the Fe3O4-FeO pair previously examined by others, served as the working solid material on this YSZ-supported Fe3O4. Our new redox reaction proceeded as follows. The Fe3O4 reacted with YSZ to produce an Fe2+-containing ZrO2 phase by releasing oxygen molecules in the first step: The Fe2+ ions entered into the cubic YSZ lattice. In the second step, the Fe2+-containing YSZ generated hydrogen via steam splitting to reproduce Fe3O4 on the cubic YSZ support. This cyclic reaction could be repeated with a good repeatability of the reaction below 1400°C.

Radiation-induced defect clusters in fully stabilized zirconia irradiated with ions and/or electrons

Microstructure evolution of yttria-stabilized cubic zirconia (YSZ), ZrO 2–13 mol% Y 2O 3, was investigated through transmission electron microscopy under irradiation with electrons and/or ions. Anomalous formation of large defect clusters was found under electron irradiation subsequent to ion irradiation, such as 300 keV O +, 100 keV He + and 4 keV Ar + ions. Such defect clusters were not formed solely with ion irradiation. The extended defect clusters possess strong black/black lobes contrast, and are observed preferentially around a focused electron beam at or near dislocations at temperature less than 520 K. The defect clusters were transformed into dislocation network when they reached a critical diameter of about 1.0–1.5 μm, and processes of nucleation, growth and transformation were repeated under electron irradiation. The defect clusters are assumed to be oxygen platelets induced through selective displacements of oxygen ions in YSZ with electron irradiation. An important role of the accumulation of electric charges due to the selective displacements in YSZ is also discussed.

Optical properties of YSZ implanted with Ag ions

Ag ions were implanted into YSZ (yttria stabilized cubic zirconia) single crystals in both keV and MeV energy regions. For samples with 6 × 10 16 ions/cm 2 at 20 keV, a large absorption peak appears at 536 nm. This absorption peak gradually decreases with increasing temperature. For samples implanted at 3 MeV, a broad absorption peak at around 430 nm was observed. The absorption gradually decreases with increasing temperature, and the sample turns colorless at 700 °C. As the sample is further heated at higher temperatures of 800–1000 °C, a new absorption peak appears at 520 nm and grows with the heating time. This peak appears after several minutes heating at 1000 °C. This absorption peak at 520 nm dose not appear after heating in an Ar atmosphere.

Structure and magnetic properties of Co grown on yttria-stabilized cubic zirconia substrates

Cobalt films have been grown on yttria-stabilized cubic zirconia (YSZ) (100) and (110) substrates by molecular beam epitaxy. Both hcp(0001) and fcc(111) twin structures and fcc(110) films have been successfully fabricated on the YSZ(100) and (110), respectively. For the Co on YSZ(100) case, the Co films possess either hcp(0001) or fcc(111) crystals with in-plane 30° rotation. For the Co on YSZ(110) case, the structural relationship is YSZ(110)[100] ‖ Co(110)[1–10]. All the films display an isotropiclike magnetic anisotropy with the coercivity increasing abruptly above its martensitic transition temperature. The coercivity decreases with increasing the thickness of Co films from 100 Å to 500 Å; and increases as the deposited temperature above 500 °C. Co films grown on YSZ(100) are in favor of the layer by layer growth, and Co films grown on YSZ(110) are in favor of the island growth.

Modelling and simulation of the mechanical properties of YSZ/Al 2O 3 composites: a preliminary study

In recent years, a model based on the Voronoi tessellation and the random electrical networks (EN) has been developed to simulate the influence of the microstructure on the electrical properties of polycrystalline ionic conductor composites and encouraging results have been obtained. By the same approach, the elastic modulus of yttria-stabilized cubic zirconia (YSZ)/Al 2O 3 has been tentatively simulated exploiting the analogy between the electrical and mechanical dynamic responses. A random network constituted of discrete elastic elements (springs) was used to sketch the mechanical behaviour of these polycrystalline composites. Preliminary results and remarks are presented and discussed.

Synthesis of yttria stabilized cubic zirconia (YSZ) powders by microwave-hydrothermal route

Yttria stabilized cubic zirconia (YSZ) ceramic powders are prepared by microwave-hydrothermal (MH) treatment (200°C, 194 psi, 30 min) using ZrO(NO 3 ) 2 · x H 2 O and YCl 3 as starting precursors and KOH as mineralizer. Solutions of ZrO(NO 3 ) 2 · x H 2 O (0.018 M) and YCl 3 (0.002 M) were mixed in appropriate proportions and were added to 1.2 M KOH solution. Resulting mixture was treated in specially designed vessels, under high pressure using microwave accelerated reaction system (MARS-5). The resultant precipitates are characterized by XRD, IR, EDAX, BET and SEM techniques. The results showed that the as-precipitated particles consisted of YSZ having cubic fluorite structure and thermal stability up to 800°C. The surface area of the microwave-hydrothermally prepared powder at 200°C (194 psi), 30 min was found to be 125 m 2 g −1 . The SEM photograph shows agglomerated YSZ particles having varied size distribution (0.2–3 μm). The cubic phase is obtained at a temperature/pressure and time as low as 125°C/18 psi and 5 min, respectively.