Yttria-stabilized Zirconia Single Crystals Research Articles

Rate Determining Step in ORR of PrO x -Based Film Cathodes

Introduction Single cell performance of solid oxide fuel cell (SOFC) is largely limited by oxygen reduction reaction (ORR) at cathode, 1/2O2 + VO¨ + 2e’ → OO ×. Therefore, understanding the mechanism of ORR is fundamental issue in improving SOFC performance. The above reaction equation tells us that higher chemical potential of electron is advantageous to the ORR kinetics, which implies that catalytic activity is significantly influenced by electronic band structure of cathode material. The authors also point out that the electronic structure is related to the basicity (a(O2-)) determined by the reaction, 1/2O2 + 2e’ → O2-, which also has an impact on the cathode activity. Though the discussion based on electronic structure has been initiated, the comprehensive understanding on the relationship between ORR activity, electronic structure and basicity has not been achieved. To better understand the ORR mechanism, PrO x -based cathode is employed as a model system, which is a simple material and sensitive to doping and surface modification. In order to examine the influence of density of surface-adsorbed oxygen, the authors also investigate the PrOx-based cathode surface-modified with BaPrO x (BPO), which includes a strong basic element, Ba. The cathode reaction properties of those cathodes are evaluated and the ORR mechanism will be discussed. Experimental PrO x thin film electrodes have been prepared by an RF-sputtering technique in an oxygen-including atmosphere on both sides of the substrates (electrolyte) of single-crystal yttria-stabilized zirconia (YSZ). For surface modification with BaPrO x (BPO), a thin layer is deposited on the PrO x thin film by RF-sputtering under the same condition. A reference electrode is fabricated by painting Pt paste and firing at 1273K before the above thin film fabrication. The performance of these thin film electrodes has been evaluated in terms of interfacial conductivity by three-terminal AC impedance measurement under controlled temperature and oxygen partial pressure (Po2). The samples are analyzed by the X-ray diffractometry (XRD), Raman spectroscopy and X-ray photoemission spectroscopy (XPS). Results and Discussion The Nyquist plot typically shows, in addition to a contribution of the electrolyte resistance at highest frequency, a small semi-circle at intermediate frequency and a large one at the lowest frequency. The latter can be ascribed to the surface reaction, and the resistance is normalized as area-specific conductivity (σ LF) to serve for a measure of cathode reaction activity.In the Po2 dependence of σ LF at 700˚C, all of the examined electrodes, with two different thickness (90 and 270nm) and a BPO modification, show a 1/2 power dependence. In addition, this behavior is perfectly the same as that of (La,Sr)CoO3-based cathodes [1]. Although the bulk properties such as defect structure and electrical transport are totally different among those oxides, the universally observed Po2 dependence suggests that the rate-determining step is the following reaction, O2(ad) 2- + VO ●●→OO x + O(ad) - [2]. Figure 1 shows the temperature dependence of σ LF at Po2=1 atm. It is noted that the activation energy indicated is averaged value for the temperature above 550˚C. The PrO x electrodes with 90nm- and 270nm-thickness show different magnitude and activation energy in σ LF, suggesting that especially at lower temperatures, these may have different rate determining steps. For the 270nm-thick and 270nm+BPO electrodes, a discontinuous drop of σ LF are observed between 500-550˚C, which can be related to the phase transition of PrO x from β-phase (Pr6O11) to α-phase (PrO x ). The 270nm+BPO electrode shows higher σ LF than the single-layer PrO x electrodes. This implies that the ORR activity of the BPO-modified electrode is improved by enhancement of surface basicity and resultant increase in the density of surface-adsorbed peroxide ion. [1] A. Takeshita et al., ECS Trans. 57 (2014) 1733. [2] A. Takeshita et al., to be submitted. Figure 1

Multilayered polycrystallization in single-crystal YSZ by laser-shock compression

A single shot of an ultra-intense laser with 0.8 J of energy and a pulse width of 110 fs (peak intensity of W cm−2) is divided into two beams and the two beams counter-irradiated onto a 0.5 mm-thick single crystal yttria-stabilized zirconia (YSZ), changing the YSZ into a multilayered polycrystalline state. The laser-driven shock wave of the intensity 7.6 Pa penetrated the crystal as deep as 96 m, causing formation of a four-layered structure (the first layer from the surface to 12 m, the second from 12 to 28 m, the third from 28 to 96 m, and the fourth from 96 to 130 m, respectively). The grain size of the first layer was 1 m, while that of the second layer was broken into a few tens nanometers. The grain size of the third layer was a few hundred nanometers to a few ten micrometers. The area deeper than 96 m remained as a single crystal. The plasma heat wave might remelt the first layer, resulting in the grain size becoming larger than that of the second layer. The surface polycrystallization seems to maintain the residual stresses frozen in the film thickness direction. Our experimentally observed spatial profile of the grain size can be explained by this shock and heat waves model.

Evaluation of pulsed laser deposited SrNb0.1Co0.9O3−δ thin films as promising cathodes for intermediate-temperature solid oxide fuel cells

SrNb0.1Co0.9O3−δ (SNC) thin films prepared on single-crystal yttria-stabilized zirconia (YSZ) electrolytes are evaluated as promising cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Geometrically well-defined polycrystalline SNC thin films with low surface roughness and high surface oxygen vacancy concentration are successfully fabricated by pulsed laser deposition. The thin films are characterized by basic techniques, e.g., X-ray diffraction for phase structure identification, scanning electron microscopy and atomic force microscopy for microstructures measurement, and X-ray photoelectron spectroscopy for elements quantification. Electrochemical impedance spectroscopy (EIS) is used to investigate oxygen reduction reaction activities of SNC thin films in symmetric electrochemical cells. Current collectors (Ag paste, Ag strip, and Au strip) are found to have negligible impact on polarization resistances. A slight decrease of the electrode polarization resistances is observed after adding a samarium doped ceria (SDC) buffer layer between SNC and YSZ. SNC thin-film electrodes exhibit low electrode polarization resistances, e.g., 0.237 Ω cm2 (SNC/SDC/YSZ/SDC/SNC) and 0.274 Ω cm2 (SNC/YSZ/SNC) at 700 °C and 0.21 atm, demonstrating the promise of SNC materials for IT-SOFCs. An oxygen reduction reaction mechanism of SNC thin films is also derived by analyzing EIS at temperature of 550–700 °C under oxygen partial pressure range of 0.04–1 atm.

Effects of surface chemistry and microstructure of electrolyte on oxygen reduction kinetics of solid oxide fuel cells

We report systematic investigation of the surface properties of yttria-stabilized zirconia (YSZ) electrolytes with the control of the grain boundary (GB) density at the surface, and its effects on electrochemical activities. The GB density of thin surface layers deposited on single crystal YSZ substrates is controlled by changing the annealing temperature (750–1450 °C). Higher oxygen reduction reactions (ORR) kinetics is observed in samples annealed at lower temperatures. The higher ORR activity is ascribed to the higher GB density at the YSZ surface where ‘mobile’ oxide ion vacancies are more populated. Meanwhile, oxide ion vacancies concurrently created with yttrium segregation at the surface at the higher annealing temperature are considered inactive to oxygen incorporation reactions. Our results provide additional insight into the interplay between the surface chemistry, microstructures, and electrochemical activity. They potentially provide important guidelines for engineering the electrolyte–electrode interfaces of solid oxide fuel cells for higher electrochemical performance.

Resputtering effect during MgO buffer layer deposition by magnetron sputtering for superconducting coated conductors

In this study, MgO thin films were deposited by radio-frequency magnetron sputtering. The film thickness in the deposition area directly facing the target center obviously decreased compared with that in other areas. This reduction in thickness could be attributed to the resputtering effect resulting from bombardment by energetic particles mainly comprising oxygen atoms and negative oxygen ions. The influences of deposition position and sputtering pressure on the deposition rate were investigated. Resputtering altered the orientation of the MgO film from (111) to (001) when the film was deposited on a single crystal yttria-stabilized zirconia substrate. The density distribution of energetic particles was calculated on the basis of the measured thicknesses of the MgO films deposited at different positions. The divergence angle of the energetic particle flux was estimated to be approximately 15°. The energetic particle flux might be similar to the assisting ion flux in the ion beam assisted deposition process and could affect the orientation of the MgO film growth.

Damage processes in MgO irradiated with medium-energy heavy ions

The micro-structural modifications produced in MgO single crystals exposed to medium-energy heavy ions (1.2-MeV Au) were investigated using Rutherford backscattering spectrometry in channeling geometry coupled to Monte-Carlo analyses, secondary ion mass spectrometry, X-ray diffraction and transmission electron microscopy. The damage accumulation and the elastic strain variation were interpreted in the framework of the multi-step damage accumulation (MSDA) model. Both build-ups follow a multi-step process similar to that recently observed for ion-irradiated yttria-stabilized zirconia (YSZ) single crystals. However, in MgO, an unexpectedly high disorder level occurs far beyond the theoretical damage distribution. These results strongly suggest that the migration of defects created in the near-surface layer is most likely at the origin of the broadening of the damage depth distribution in MgO.

Effects of He Irradiation on Yttria‐Stabilized Zirconia Ceramics

Effects of He irradiation on polycrystalline yttria‐stabilized zirconia (YSZ) are studied with the focus on irradiation‐induced damage buildup, He behavior, and volume swelling. The evolution of irradiation‐induced structural damage in polycrystalline YSZ, which is independent of grain orientation, is described by a multistep damage accumulation model. A three‐step damage evolution process was found, and different types of defects were observed in the different damage steps. Compared with single‐crystal YSZ, the second damage step occurs at a lower dose in polycrystalline YSZ due to the initial defects and strain. The implanted He ions are readily trapped along the grain boundaries and the mobility of He ions is greatly increased. The enhanced He mobility along the grain boundary leads to a lower threshold irradiation dose and a larger penetration depth for bubble formation. Similar morphologies are observed for the He bubbles in the polycrystalline YSZ and in single‐crystal YSZ, and the formation of He bubbles in polycrystalline YSZ is not influenced by grain orientation. As both the extended defects and He bubbles can induce volume swelling, the variation in volume swelling as a function of dose can be divided into a two stage process.

Colour centre production in yttria-stabilized zirconia by X-ray and electron irradiations: effect of yttria content

Yttria-stabilized zirconia (YSZ) single crystals (for 9.5 and 18 mol% yttria) were irradiated at room temperature (RT) by X-rays (W white spectrum) and 2.5-MeV electrons. The growth curves of the so-called T-centre (for trigonal centre, i.e. Zr3+ sitting in a trigonal symmetry site) were studied as a function of absorbed dose, or irradiation time, by UV–visible optical absorption (OA) spectroscopy and X-band electron paramagnetic resonance spectroscopy. The defect concentration at saturation and the production rate are increased by a factor around two for 18 mol% yttria with respect to 9.5 mol%. Defect decay was then followed after irradiation by OA spectroscopy as a function of ageing time at RT. Growth and decay curves of the T-centre are modelled on the basis of rate equations of charge-exchange reactions with the zirconium lattice ions. Increase in yttrium content is thought to decrease hole trapping on Zr3+ ions, thereby enhancing T-centre formation.

Pulsed Laser Deposition of Epitaxial and Polycrystalline Bismuth Vanadate Thin Films

We report pulsed laser deposition (PLD) synthesis of epitaxial and polycrystalline monoclinic bismuth vanadate (BiVO4, BVO) thin films. X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy were used to characterize the samples. Epitaxial, c-axis oriented growth was achieved using single crystal yttria-stabilized zirconia (100), a substrate temperature of 575–600 °C, and an oxygen pressure of 7.8 mTorr. The volatility of Bi necessitated a large excess (Bi:V = ∼6:1) of this element in the ceramic targets to obtain stoichiometric films. XRD confirmed a BVO (001)||YSZ (001) and BVO [100]||YSZ [100] epitaxial relationship. Film growth was 3-D, and the morphology was discontinuous, consisting of irregular, smooth grains. Additionally, dense, continuous polycrystalline films were deposited on fluorine-doped tin oxide (FTO) on glass substrates at room temperature with stoichiometric targets and postdeposition annealing in air. Evaluation of these samples as photoanodes yielded photocurrents of ∼0.15 and ∼0.05 mA cm–2 at 1.23 V vs RHE under backside AM1.5G illumination with and without a hole scavenger (Na2SO3), respectively. We argue that the photocurrents are due to the high oxygen content inherent in the PLD process and suggest that these continuous films may be well-suited to investigating oxygen-related defects in BVO.

Electrical conductivity of CaZr0.9Y0.1O3−δ films deposited from liquid solutions

CaZr0.9Y0.1O3−δ (CZY) films were fabricated by chemical solution deposition (CSD) from ethanol-based liquid solutions. The films were deposited on single crystals of yttria-stabilized zirconia (YSZ) in (100), (110) and (111) crystallographic orientations. The structural, mechanical and electrical properties of the films have been studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation and impedance spectroscopy methods. XRD and SEM results have shown that the films were polycrystalline. Effect of the substrate orientation on the microstructural, mechanical and electrical properties of CZY films has been examined. The films deposited on (110)-substrates consist of the largest grains; show the highest values of hardness and electrical conductivity. Conductivity of CZY film on (110)-substrates increases with rise in humidity which proves proton transport in the films.

스컬용융법에 의해 성장시킨 Co와 Pr이 첨가된 이트리아안정화큐빅지르코니아(YSZ) 단결정의 연구

Co-(0.7 wt%) and Pr-(2.0, 3.5 or 5.0 wt%) doped cubic zirconia (ZrO :Y O = 50 : 50 wt%) single crystals grown by a skull melting method were heat-treated in N at 1150 C for 5 hrs. The brown colored as-grown single crystals were changed into either dark brownish green, greenish blue and light green color after the heat treatment. Before and after the heat treatment, the YSZ (yttria-stabilized zirconia) single crystals were cut for wafer form (o7.5 mm × t3 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Co (≒589 nm: Γ ( A ( F)) →Γ +Γ ( T ( F)), ≒610 nm: Γ ( A ( F)) →Γ ( T ( F)), ≒661 nm: Γ ( A ( F)) →Γ ( T ( F))) and Pr (≒450 nm: H − P , ≒473 nm: H → P , ≒484 nm: H → P ), change of ionization energy and lattice parameter were confirmed.

Liquid antimony anode direct carbon fuel cell fueled with mass-produced de-ash coal

A liquid antimony (Sb) anode DCFC (direct carbon fuel cell) is fabricated on a smooth single crystal YSZ (Yttria Stabilized Zirconia) electrolyte substrate with porous Pt cathode to reveal the intrinsic reaction kinetics of electrochemical oxidation of liquid Sb and the reduction reaction characteristics of Sb2O3 with the reaction mass-produced Taixi de-ash coal fuel. The reduction kinetics of Sb2O3 with the de-ash coal is obtained using a temperature programmed reaction testing system. The reaction kinetics of the Sb2O3 with the de-ash coal can be enhanced by decreasing the coal particle size, and by adding de-ash coal into the anode chamber. The Sb2O3 accumulation at the interface between anode and electrolyte lead to the increase of ohmic resistance. While effective reaction active sites increase when the mole content of oxygen ion conductor Sb2O3 increase at the earlier stage of the cell discharging processes which further decrease the electrode polarization. The Si and Fe in the ash possibly accumulate at the interface between anode and electrolyte.

Molecular Dynamics Simulation of Oxygen Ion Diffusion in Yttria Stabilized Zirconia Single Crystals and Bicrystals

AbstractMolecular dynamics simulation studies have been performed to study the oxygen ion diffusion in yttria stabilized zirconia single crystals and bicrystals separated by tilt grain boundaries (GBs). Two types of GBs which are Σ 5 (3 1 0) and Σ 13 (5 1 0) are studied at temperatures between 1,000 K and 2,000 K. The effect of grain size, which is the distance between two GBs, and the effect of GB orientations are systematically investigated in this study. The oxygen diffusion in the bicrystals is found to be blocked by the GB, and the blocking effect increases with decreasing grain size and is affected by different grain orientations. In addition, the oxygen diffusion along the GB plane is most hindered.

Implantation of high concentration noble gases in cubic zirconia and silicon carbide: A contrasted radiation tolerance

The modifications of the microstructure of yttria-stabilized cubic zirconia and silicon carbide single crystals implanted with high concentrations of noble gas ions and subsequently annealed at high temperature were characterized using RBS/C, XRD and TEM. It is found that the annealing behavior is strongly dependent on both the material and the implanted noble gases. Ar-implanted yttria-stabilized zirconia shows no significant microstructural modification upon annealing at 800°C, e.g. dislocations are still present and the size of the Ar bubbles does not evolve. This is in strong contrast with previous observations on helium-implanted zirconia, where the formation of bubbles and elongated fractures were observed. In the case of SiC, thermal annealing at 1000°C shows an enhanced damage recovery when He is implanted as compared to Ar implantation and the recrystallization of the matrix is accompanied with the release of noble gas atoms. This difference can be ascribed to different atomic radii, and thus mobility of implanted species.

Determination of strain and damage profiles in irradiated materials: Application to cubic zirconia irradiated at high temperature

A methodology is presented that allows to retrieve strain and damage profiles in irradiated single crystals. The approach makes use of high-resolution X-ray diffraction θ–2θ scans coupled with numerical simulations of the diffraction profiles. The potential of the method is illustrated with cubic yttria-stabilized zirconia single crystals, irradiated with 4MeV Au2+ ions at different temperatures (25, 500 and 800°C). The simulations reveal that upon increasing ion fluence, the width of the damaged region increases and both the strain and damage levels inside this region increase. The damage build-up occurs according to a two-step mechanism: in the first step, the damage increases slowly up to a critical fluence, above which the second step takes place and is characterized by dramatic increase of the damage. The transition fluence is shifted towards lower values at higher temperatures.

Effects of thermal annealing on the evolution of He bubbles in zirconia

Single crystals of yttria-stabilized zirconia were implanted with 100keV He ions at two fluences of 9×1016 and 3×1017cm−2 (5 and 17 He at.%). In order to investigate the effect of thermal annealing on the evolution of both zirconia lattice and implanted He, the samples were annealed at several temperatures ranging from 500°C to 1400°C. Three complementary analysis techniques, RBS/C, AFM and TEM were used to study structural damage and surface morphology of the crystal before and after implantation. Results show different He evolution phenomena under the two implantation fluences. It is inferred that, at the lower fluence, the migration and agglomeration of He ions lead to bubble formation after annealing. These bubbles jack up sample surface causing the deformation of surface region and the damage level of surface region increase accordingly. As the temperature continues to increase, He gradually releases and the damage recovers. However, at the higher fluence, the He concentration is sufficient to induce bubble precipitation without annealing. He release and damage recovering is less efficient upon annealing.

Microstructure and Ionic Conductivity of Yttria‐Stabilized Zirconia Thin Films Deposited on MgO

A number of reports have suggested that nanometric thin films of yttria‐stabilized zirconia (YSZ) deposited on MgO can support high ionic conductivity, but the results remain controversial and difficult to repeat. In this work, sub‐100‐nm‐thick YSZ films have been deposited on single‐crystal MgO substrates with different crystallographic orientations and sourced from different companies. The growth of YSZ on MgO (100) was found to be unstable: both (111)‐oriented films with polycrystalline structure and (100)‐oriented films with cube‐on‐cube epitaxy were observed despite seemingly identical deposition conditions. On MgO (110) and MgO (111) substrates, the growth of YSZ was more stable with high degrees of texture in the (110) and (111) film directions, respectively. Ionic conductivities of the films were measured with impedance spectroscopy, and conductivity values were consistently near or slightly below that of a YSZ single crystal.

Strain evolution and morphological transition of magnetron-sputtered CeO2 thin films induced by deposition parameters

Cerium dioxide (CeO2) films were fabricated on yttria-stabilized zirconia (YSZ) single crystals using unbalanced radio frequency (RF) magnetron sputtering. X-ray diffraction measurements revealed film strain discrepancies under different deposition parameters. Strain evolution was induced by varying sputtering pressure, RF power, and sputtering gas. A distinct morphological transition from a granular surface to an interwoven surface was also realized by varying the above parameters. On the basis of the “atomic peening” mechanism, the influence of different parameters on film strain was discussed. The film surface characteristics were revealed to be highly correlated with film strain. YBa2Cu3O7−δ (YBCO) films were post-deposited on the as-grown CeO2/YSZ(001) stack by using the trifluoroacetate metal-organic deposition (TFA-MOD) method. The superconducting property of the YBCO layer varied significantly with the morphology of the CeO2 buffer films.

Cobalt-free polycrystalline Ba0.95La0.05FeO3−δ thin films as cathodes for intermediate-temperature solid oxide fuel cells

Ba0.95La0.05FeO3−δ (BLF) thin films as electrodes for intermediate-temperature solid oxide fuel cells are prepared on single-crystal yttria-stabilized zirconia (YSZ) substrates by pulsed laser deposition. The phase structure, surface morphology and roughness of the BLF thin films are characterized by X-ray diffraction, scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy is used to analyze the compositions of the deposited thin film and the chemical state of transition metal. The dense thin film exhibits a polycrystalline perovskite structure with a low surface roughness and a high oxygen vacancy concentration on the surface. Ag (paste or strip) and Au (strip) are applied on both surfaces of the symmetric cells as current collectors to evaluate electrochemical performance of the thin films. The electrode polarization resistances of the symmetric cells are found to be lower than those of most cobalt-free thin-film electrodes, e.g., 0.437 Ω cm2 at 700 °C and 0.21 atm. The oxygen reduction reaction mechanism of the BLF cathode in symmetric cells is studied by electrochemical impedance spectroscopy thanks to the equivalent fitting analysis. Both the oxygen surface exchange reaction and charge transfer are shown to determine the overall oxygen reduction reaction.

Effects of interface roughness on a liquid-Sb-anode solid oxide fuel cell

Liquid Sb anodes are examined at 1073 K in solid-oxide fuel cells with single-crystal yttria-stabilized zirconia (YSZ) electrolytes. Two types of cells with different anode–electrolyte interface morphologies are operated in “battery” mode, with dry Ar flow in the anode chamber, to characterize the effects of the interface roughness on the electrochemical oxidation of the metals with oxygen ions. When the effects of the electrolyte surface morphology are minimized by using an extremely smooth (Ra = 0.69 nm) anode–electrolyte interface, the intrinsic exchange current density is found to be 1.5 mA cm−2 by fitting the Tafel equation. When the surface roughness is increased to Ra = 540 nm, the effective contact area of the liquid Sb anode increases by 33% compared to the smooth cell. The product Sb2O3 can increase the effective reaction boundary by acting as an oxygen ion conductor. However, the accumulation of Sb2O3 at the interface weakens the effect of the surface morphology.