Annihilation Of Oxygen Vacancies Research Articles

Effect of grain boundary segregation and oxygen vacancy annihilation on aging resistance of cobalt oxide-doped 3Y-TZP ceramics for biomedical applications

Abstract This study aims to investigate the diffusion stabilization process of nano-Co2O3 during the non-precursor transformation of 3Y-TZP. 3Y-TZP was set as the control group, and the experimental groups were 0.1–0.3 mol% nano-Co2O3-doped 3Y-TZP. The samples were prepared by the ball milling process, isostatic cool pressing, and sintering. All samples were hydrothermally treated at 134°C and 2 bar for different time periods. The resistance to low-temperature degradation of nano-Co2O3-doped 3Y-TZP was analyzed by X-ray diffraction. The microstructure of zirconia ceramic samples was determined by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and electron paramagnetic resonance studies. The addition of nano-Co2O3 into 3Y-TZP resulted in higher hydrothermal aging resistance than 3Y-TZP. The addition of 0.2 mol% nano-Co2O3 dopants resulted in the highest hydrothermal aging resistance among nano-Co2O3-doped 3Y-TZP ceramics. The grain sizes of 3Y-0.2Co are smaller than those in the control group. With the increase of cobaltous oxide doping contents, the segregation of Co3+ ions at the crystal boundary increased. The content of oxygen vacancies on the surface of the sample increased with the increase of the Co2O3 doping content. The oxygen vacancy concentrations of 3Y-0.2Co increased obviously after aging. 3Y-0.1Co, 3Y-0.3Co, and the control showed decreased oxygen vacancy concentrations after aging. Trivalent element doping of 3Y-TZP effectively improved the aging resistance of 3Y-TZP. The addition of 0.2 mol% nano-Co2O3 resulted in the highest hydrothermal aging resistance. Improved aging resistance is attributed to the nano-Co2O3 doping resulting in the 3Y-TZP grain size inhibition, grain boundary segregation of cobalt ions, and oxygen vacancy maintenance. This work is expected to provide an effective reference for the development and application of budget dental materials by regulating grain boundary engineering.

Tuning the superconducting performance of YBa2Cu3O7-δ films through field-induced oxygen doping.

The exploration of metal-insulator transitions to produce field-induced reversible resistive switching effects has been a longstanding pursuit in materials science. Although the resistive switching effect in strongly correlated oxides is often associated with the creation or annihilation of oxygen vacancies, the underlying mechanisms behind this phenomenon are complex and, in many cases, still not clear. This study focuses on the analysis of the superconducting performance of cuprate YBa2Cu3O7-δ (YBCO) devices switched to different resistive states through gate voltage pulses. The goal is to evaluate the effect of field-induced oxygen diffusion on the magnetic field and angular dependence of the critical current density and identify the role of induced defects in the switching performance. Transition electron microscopy measurements indicate that field-induced transition to high resistance states occurs through the generation of YBa2Cu4O7 (Y124) intergrowths with a large amount of oxygen vacancies, in agreement with the obtained critical current density dependences. These results have significant implications for better understanding the mechanisms of field-induced oxygen doping in cuprate superconductors and their role on the superconducting performance.

Strengthening chlorobenzene catalytic degradation rate over LaxSr1-xMnO3±δ by anchoring interfacial oxygen vacancy

Surficial lattice oxygen in perovskite compounds is a common participant during chloroaromatic catalytic degradation. Herein, we proposed a facile oxygen vacancy construction strategy for preparing high active LaxSr1-xMnO3±δ (LSMO) via NaBH4 treatment. Results reveal that LSMO composites are not only reduced by NaBH4 under mild condition, but also triggering a significant Sr segregation effect, which plays a key role in oxygen vacancy formation accounting to the electron donation from Sr to Mn acceptor and subsequent lattice oxygen activation. Moreover, the concentration of interfacial active oxygen species can be regulated by taming the La-Sr interaction with different Sr substitution proportions. The anchored oxygen vacancies (OVs) take advantage of electron capture or transfer and induce molecule oxygen into active oxygen species to react with chlorobenzene molecules, resulting in a preferential catalytic oxidation capacity. Amongst, reduced La0.5Sr0.5MnO3±δ catalyst displays superior chlorobenzene destruction efficiency with 90% of which converted as low as 236 °C, which also exhibits a stable performance in long-time operation. However, the OVs would be refilled by H2O molecules and results in a weaker water resistance for OVs anchored catalyst. In summary, the annihilation of OVs is the most important factor for catalyst deactivation. The research offers a new avenue to construct interfacial oxygen species in perovskite oxides during the chlorobenzene deep oxidation process.

Crosstalk‐Free Mesa‐Etched Passive‐Matrix Microscale Light‐Emitting Diode Array Using Yttrium‐Iron Garnet P‐Type Resistive‐Switching Electrodes

AbstractMicroscale light‐emitting diodes (µLEDs) have ushered in a new era of small‐sized high‐resolution displays by integrating numerous microscale pixels within a limited panel area. However, to realize passive‐matrix‐based high‐resolution µLED displays, it is important to prevent image diffusion (crosstalk) between the pixels. Herein, p‐type resistive‐switching (RS) electrodes are introduced in a mesa‐etched passive‐matrix‐based µLED array structure to block electrical interference between adjacent pixels and eliminate cross‐talk. To distinguish between addressed and unaddressed pixels in the µLED array structure, the resistance state of the RS electrode is modulated from low (52 Ω) to high (1.2 × 107 Ω). Moreover, the RS electrode exhibits a high on/off ratio (106), good retention (105 s), and stable endurance (104 cycles). X‐ray photoelectron spectroscopy analysis reveals that the switching mechanism in the RS electrodes is achieved by the generation and annihilation of oxygen vacancies in the conductive path. Consequently, the fabricated µLEDs with RS electrodes exhibit a 93.1% higher electroluminescence intensity and 95.3% higher light output power compared to a reference device fabricated through combined mesa‐ and isolation‐etching.

Density Functional Theory Investigation of the Biocatalytic Mechanisms of pH-Driven Biomimetic Behavior in CeO2.

There is considerable interest in the pH-dependent, switchable, biocatalytic properties of cerium oxide (CeO2) nanoparticles in biomedicine, where these materials exhibit beneficial antioxidant activity against reactive oxygen species (ROS) at a basic physiological pH but cytotoxic prooxidant activity in an acidic cancer cell pH microenvironment. While the general characteristics of the role of oxygen vacancies are known, the mechanism of their action at the atomic scale under different pH conditions has yet to be elucidated. The present work applies density functional theory (DFT) calculations to interpret, at the atomic scale, the pH-induced behavior of the stable {111} surface of CeO2 containing oxygen vacancies. Analysis of the surface-adsorbed media species reveals the critical role of pH on the interaction between ROS (•O2- and H2O2) and the defective CeO2 {111} surface. Under basic conditions, the superoxide dismutase (SOD) and catalase (CAT) biomimetic reactions can be performed cyclically, scavenging and decomposing ROS to harmless products, making CeO2 an excellent antioxidant. However, under acidic conditions, the CAT biomimetic reaction is hindered owing to the limited reversibility of Ce3+ ↔ Ce4+ and formation ↔ annihilation of oxygen vacancies. A Fenton biomimetic reaction (H2O2 + Ce3+ → Ce4+ + OH- + •OH) is predicted to occur simultaneously with the SOD and CAT biomimetic reactions, resulting in the formation of hydroxyl radicals, making CeO2 a cytotoxic prooxidant.

High-pressure TiO2-II polymorph as an active photocatalyst for CO2 to CO conversion

Rutile and anatase titanium oxide phases are widely used for photocatalytic CO2 conversion, and there have been significant attempts to improve their activity by various chemical methods. In this study, we show that the nanocrystalline high-pressure TiO2-II polymorph, synthesized by a high-pressure technique, shows higher photocurrent and better photocatalytic activity for CO2 to CO conversion compared to the anatase phase. The photocatalytic activity of material is further enhanced by thermal annihilation of oxygen vacancies generated in the bulk of TiO2-II during the high-pressure treatment. The high potential of TiO2-II phase for CO2 conversion is discussed based on the light absorbance, band structure, charge carrier mobility and CO2 adsorption.

Effect of oxygen vacancy gradient on ion-irradiated Ca-doped YMnO3 thin films

In this study, we investigate the effect of ion irradiation on Y0.95Ca0.05MnO3 (YCMO) thin films. X-ray diffraction and Raman spectroscopy measurements show single-phase and strain/stress modifications with ion irradiation. Rutherford backscattering spectrometry confirms the variation in oxygen vacancies. The near-edge x-ray absorption fine structure shows valence state reduction of Mn ions, which is attributed to oxygen vacancies. The optimal resistive switching ratio is observed at the lowest fluence (1 × 1011 ions/cm2) of ion irradiation. At higher fluences (1 × 1012 and 1 × 1013 ions/cm2), the strain relaxation and oxygen vacancy annihilation are ascribed to the local annealing effect. The double logarithmic curve and modified Langmuir–Child's law satisfy the space charge limited conduction mechanism in all thin films. These results suggest the crucial role of irradiation-induced oxygen vacancies in modifying the electronic structure and electrical properties of YCMO thin films.

Performance of protective oxide films on Fe–Ni alloy anodes in molten KF–AlF3–Al2O3 salts at 700 °C

An oxide film containing NiFe2O4 was prepared by oxidizing Fe–Ni alloy in hot air, and the performance of the protective oxide film in molten KF–AlF3–Al2O3 salts at 700 °C was characterized by electrochemical methods, including electrochemical impedance spectroscopy, capacitance measurements, linear sweep voltammetry, and chronoamperometry to clarify the breakdown and growth process of the oxide film. For the pre-oxidized 57Fe–43Ni, 50Fe–50Ni, and 43Fe–57Ni alloys, the oxide films, with a thickness of 5 μm, are composed of Fe2O3 and NiFe2O4, according to X-ray diffraction and scanning electron microscopy analysis. The transfer resistance of ions in the oxide film increases with increasing nickel content, and reaches its maximum value of 91.39 Ω⋅cm2 for the pre-oxidized 43Fe–57Ni alloy based on electrochemical impedance spectroscopy measurements. Moreover, the carrier density in the oxide film determined from Mott–Schottky plots decreases as the nickel content increases. Under anodic polarization, the breakdown of oxide film follows the generation of metal cation vacancies and condensation of excess cation vacancies. Concurrently, a new oxide layer forms along the substrate owing to the annihilation of oxygen vacancies and diffusion of oxygen anion in the oxide film. After oxygen evolution, a new oxide film layer containing NiFe2O4 forms on the alloy substrate, leading to the passivation observed in the anodic polarization curves. The maximum polarization resistance in the passivation region is 4.86 Ω⋅cm−2 for the pre-oxidized 43Fe–57Ni alloy, indicating that the pre-oxidized 43Fe–57Ni alloy possesses better corrosion resistance to molten fluorides than the other two pre-oxidized Fe–Ni alloys. Chronoamperometry measurements show that the oxide film on the 43Fe–57Ni alloy is in a dynamically stable state during oxygen evolution.

Advanced 3Y-TZP bioceramic doped with Al2O3 and CeO2 potentially for biomedical implant applications

ABSTRACTThis research studies 3 mol% yttria-stabilized zirconia (3Y-TZP) investigating the effects of Al2O3 and CeO2 dopants on the stability of tetragonal phase and the microstructure of 3Y-TZP determined over the operating temperature ranging from 1250°C to 1550°C. It is found that the mechanical properties of 3Y-TZP are dependent on the sintering temperature and the dopant amount. The current study reveals that the optimum sintering temperature is 1450°C for all 3Y-TZP samples while attaining more than 98% of the theoretical density (6.1g/cm3). With optimum dopants, the 3Y-TZP ceramic samples demonstrate the Vickers hardness of 10.9 GPa and fracture toughness (KIC) of 10 MPa.m1/2. Fracture toughness increases with the dopant content, indicating that the annihilation of oxygen vacancies in 3Y-TZP is responsible for the instability of the t-ZrO2 lattice. To investigate the biocompatibility of 3Y-TZP, cell culture study was performed using osteoblast cells. The results demonstrate a high percentage of cell attachment and proliferation that confirmed the biocompatibility of synthesized 3Y-TZP.

Impact of the Stacking Order of HfO x and AlO x Dielectric Films on RRAM Switching Mechanisms to Behave Digital Resistive Switching and Synaptic Characteristics

Resistive random access memory (RRAM) devices with analog resistive switching are expected to be beneficial for neuromorphic applications, and consecutive voltage sweeps or pulses can be applied to change the device conductance and behave synaptic characteristics. In this paper, RRAM devices with a reverse stacking order of 6-nm-thick HfO x and 2-nm-thick AlO x dielectric films were fabricated. The device with TiN/Ti/AlO x /HfO x /TiN stacked layers exhibited digital resistive switching, while the other device with TiN/Ti/HfO x /AlO x /TiN stacked layers could demonstrate synaptic characteristics that were analog set and reset processes under consecutive positive and negative voltage sweeps or a train of potentiation and depression pulses. Moreover, this device could also implement synaptic learning rules, spike-timing-dependent plasticity (STDP). Varying temperature measurements and linear fittings of the measured data were conducted to analyze current conduction mechanisms. As a result, the variation of resistive switching behavior between these two devices is attributed to the varying effectiveness of the oxygen scavenging ability of the Ti layer when put into contact with either AlO x or HfO x . Moreover, AlO x functioned as a diffusion limiting layer (DLL) in the device with TiN/Ti/HfO x /AlO x /TiN stacked layers, and gradual modulation of the production and annihilation of oxygen vacancies is the cause of synaptic characteristics.

Synthesis and characterization of Sr0.75Y0.25Co0.9Ru0.1O3−δ perovskite as a solid oxide fuel cell cathode material

The oxygen-deficient Sr0.75Y0.25Co0.9Ru0.1O3−δ (SYCR) cathode is systematically evaluated for the application of solid oxide fuel cells. X-ray diffraction analysis indicates that SYCR presents a tetragonal structure with space group of I4/mmm (139). In the measured high oxygen partial pressure (pO2) region (0.01–0.21 atm), the conductivity increases with increasing pO2 because of the oxygen vacancy annihilation and hole creation, relating to a general p-type semiconducting mechanism. To get an insight into the rate-limiting step of SYCR cathode, behaviors of individual polarization resistance (R 1 and R 2) are investigated in different pO2. The obtained fitting results reveal that R 1 is nearly independent on the pO2, while R 2 presents a (pO2)−0.5 dependence. At 800 °C, SYCR cathode exhibits an R p value of 0.14 Ω cm2, moreover, when using the wet hydrogen (∼ 3% H2O) as fuel and ambient air as oxidant, the maximum power density of single cell Ni-YSZ (yttria-stabilized zirconia)|YSZ|SYCR reaches 452.9 W cm−2.

Resistive switching properties of polycrystalline HfOxNy films by plasma-enhanced atomic layer deposition

Polycrystalline HfOxNy films were deposited by plasma-enhanced atomic layer deposition. Bipolar resistive switching properties were observed in the Ag/HfOxNy/Pt devices (Ag-devices) which had stable SET/RESET voltages and high high/low resistance ratios of ∼105. Pt/HfOxNy/Pt structured devices (Pt-devices) was prepared for comparison. The current–voltage characterizations and resistance temperature coefficients measurement suggested that the resistance switching is dominated by formation and rupture of Ag filaments for the Ag-devices, and is governed by formation and annihilation of oxygen vacancies for the Pt-devices. HfOxNy films are promising in the application of electrochemical metallization memories.

Quantum Conductance Probing of Oxygen Vacancies in SrTiO3 Epitaxial Thin Film using Graphene.

Quantum Hall conductance in monolayer graphene on an epitaxial SrTiO3 (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate-voltage sweep range is gradually increased in the device, systematic generation and annihilation of oxygen vacancies, evidenced from the hysteretic conductance behavior in the graphene, are observed. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and noninvasive probe for understanding the electronic and ionic phenomena in complex transition-metal oxides without impairing the oxide layer underneath.

Investigation of intrinsic defect magnetic properties in wurtzite ZnO materials

Theoretical and experimental investigations of the ferromagnetism induced by intrinsic defects inside wurtzite zinc oxide structures are performed using magnetic field-dependent circular dichroism (MCD-H), direct magnetization measurement (M-H) by superconducting quantum interference device (SQUID) as well as by generalized gradient density functional theory (GGA-DFT). To investigate localized magnetic moments of bulk material intrinsic defects - vacancies, interstitial atoms and Frenkel defects, various-size periodic supercells are calculated. It is shown that oxygen interstitial atoms (Oi) or zinc vacancies (Znv) generate magnetic moments of 1,98 и 1,26µB respectively, however, the magnitudes are significantly reduced when the distance between defects increases. At the same time, the magnetic moments of oxygen Frenkel defects are large (~1.5–1.8µB) and do not depend on the distance between the defects. It is shown that the origin of the induced ferromagnetism in bulk ZnO is the extra spin density on the oxygen atoms nearest to the defect. Also dependence of the magnetization of ZnO (101̅0)and(0001) thin films on the positions of Oi and Znv in subsurface layers were investigated and it is shown that the magnetic moments of both defects are significantly different from the values inside bulk material. In order to check theoretical results regarding the defect induced ferromagnetism in ZnO, two thin films doped by carbon (C) and having Zn interstitials and oxygen vacancies were prepared and annealed in vacuum and air, respectively. According to the MCD-H and M-H measurements, the film, which was annealed in air, exhibits a ferromagnetic behavior, while the other does not. One can assume annealing of ZnO in vacuum should create oxygen vacancies or Zn interstitial atoms. At that annealing of the second C:ZnO film in air leads to essential magnetization, probably by annihilation of oxygen vacancies, formation of interstitial oxygen atoms or zinc vacancies. Thus, our experimental results confirm our theoretical conclusions that ZnO magnetization origin are Oi or Znv defects.

Reconciling in vivo and in vitro kinetics of the polymorphic transformation in zirconia-toughened alumina for hip joints: III. Molecular scale mechanisms

Understanding the intrinsic reason(s) for the enhanced tetragonal to monoclinic (t→m) polymorphic phase transformation observed on metal-stained surfaces of zirconia-toughened alumina (ZTA) requires detailed knowledge of off-stoichiometry reactions at the molecular scale. In this context, knowledge of the mechanism(s) for oxygen vacancy creation or annihilation at the material surface is a necessary prerequisite. The crucial aspect of the surface destabilization phenomenon, namely the availability of electrons and holes that allow for vacancy creation/annihilation, is elucidated in this paper. Metal-enhanced alterations of the oxygen sublattice in both Al2O3 and ZrO2 of the ZTA composite play a decisive role in accelerating the polymorphic transformation. According to spectroscopic evidences obtained through nanometer-scale analyses, enhanced annihilation of oxygen vacancies triggers polymorphic transformation in ZrO2 near the metal stain, while the overall Al2O3 lattice tends to dehydroxylate by forming oxygen vacancies. A mechanism for chemically driven “reactive metastability” is suggested, which results in accelerating the polymorphic transformation. The Al2O3 matrix is found to play a key-role in the ZrO2 transformation process, with unambiguous confirmation of oxygen and hydrogen transport at the material surface. It is postulated that this transport is mediated by migration of dissociated O and H elements at the surface of the stained transition metal as they become readily available by the thermally activated surrounding.

Self-Organized Porous Anodic Oxide Formation: Role of Oxide Flow Driven By Surface Stress

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, and other metals in baths that dissolve the oxide. Procedures to grow films with highly ordered and controllable arrangements of nanoscale pores have led to the extensive use of PAO films in nanofunctional devices. Experiments and calculations show that transport in the oxide occurs by electrical migration coupled with plastic flow driven by mechanical stress in the oxide, and suggest that flow may play a role in formation of self-ordered PAO (1-3). Pores initiate by a morphological instability at the oxide-solution interface when the initially formed barrier oxide reaches a critical thickness (4). Results of a combined experimental and modeling investigation of the formation and self-ordering of PAO are discussed. Experiments were carried out to identify stress generation mechanisms during galvanostatic formation of Al barrier films in 0.4 M H3PO4. Stress distributions in these films were measured by monitoring stress-induced sample curvature changes during oxide formation followed by open circuit dissolution of the films (5). Elevated compressive stress reaching several GPa was found to build up within a few nanometers of the oxide-solution interface, coinciding with high phosphorus contamination in the same layer revealed by GDOES (6). The compressive surface stress is likely due to electric field-induced phosphate incorporation. The experimentally characterized surface stress was incorporated in an anodizing model based on transport by coupled viscous flow and electrical migration (2). Morphological stability analysis of the model revealed pattern formation consistent with the characteristic pore spacing to voltage relationship of PAO (7). Pore formation is governed by competition between stabilizing oxide formation at the solution interface and destabilizing flow driven by gradients of anion incorporation-induced surface stress. The model successfully predicts the narrow ranges of anodizing efficiency characterizing PAO formation on many metals (3,4). A compelling analogy exists between the mechanisms of PAO ordering and formation of hexagonally-ordered Marangoni patterns in thin liquid films. Electrochemical impedance spectroscopy of steady-state porous anodic alumina formed in sulfuric and oxalic acid solutions are presented. It is shown that the characteristic inductive loop of impedance spectra of anodic oxide can be understood quantitatively in terms of electric field-induced homogeneous formation and annihilation of oxygen vacancies. We argue that this reaction can produce oxide displacement in the presence of sufficiently large stress gradients. The possible explanation of viscous oxide flow by such a vacancy creep mechanism is discussed.

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

AbstractMethanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu‐ZnO‐Al2O3 catalyst, which is unselective and experiences rapid deactivation. In‐depth characterization of the In2O3‐based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co‐feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2 O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu-ZnO-Al2 O3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In2 O3 -based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.

Effect of cation dopant radius on the hydrothermal stability of tetragonal zirconia: Grain boundary segregation and oxygen vacancy annihilation

The hydrothermal aging stability of 3Y-TZP-xM2O3 (M = La, Nd, Sc) was investigated as a function of 0.02–5 mol% M2O3 dopant content and correlated to the overall phase content, t-ZrO2 lattice parameters, grain size distribution, grain boundary chemistry and ionic conductivity.The increased aging stability with increasing Sc2O3 content and the optimum content of 0.4–0.6 mol% Nd2O3 or 0.2–0.4 mol% La2O3, resulting in the highest aging resistance, could be directly related to the constituent phases and the lattice parameters of the remaining tetragonal zirconia.At low M2O3 dopant contents ≤0.4 mol%, the different aging behavior of tetragonal zirconia was attributed to the defect structure of the zirconia grain boundary which was influenced by the dopant cation radius. It was observed that the grain boundary ionic resistivity and the aging resistance followed the same trend: La3+ > Nd3+ > Al3+ > Sc3+, proving that hydrothermal aging is driven by the diffusion of water-derived mobile species through the oxygen vacancies. Accordingly, we elucidated the underlying mechanism by which a larger trivalent cation segregating at the zirconia grain boundary resulted in a higher aging resistance.

Ionic Conductivity of Mesostructured Yttria-Stabilized Zirconia Thin Films with Cubic Pore Symmetry—On the Influence of Water on the Surface Oxygen Ion Transport.

Thermally stable, ordered mesoporous thin films of 8 mol % yttria-stabilized zirconia (YSZ) were prepared by solution-phase coassembly of chloride salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly process. The resulting material is of high quality and exhibits a well-defined three-dimensional network of pores averaging 24 nm in diameter after annealing at 600 °C for several hours. The wall structure is polycrystalline, with grains in the size range of 7 to 10 nm. Using impedance spectroscopy, the total electrical conductivity was measured between 200 and 500 °C under ambient atmosphere as well as in dry atmosphere for oxygen partial pressures ranging from 1 to 10(-4) bar. Similar to bulk YSZ, a constant ionic conductivity is observed over the whole oxygen partial pressure range investigated. In dry atmosphere, the sol-gel derived films have a much higher conductivity, with different activation energies for low and high temperatures. Overall, the results indicate a strong influence of the surface on the transport properties in cubic fluorite-type YSZ with high surface-to-volume ratio. A qualitative defect model which includes surface effects (annihilation of oxygen vacancies as a result of water adsorption) is proposed to explain the behavior and sensitivity of the conductivity to variations in the surrounding atmosphere.