Zirconia Single Crystals Research Articles

Surface structure of coherently strained ceria ultrathin films

Cerium oxide, or ceria, is an important material for solid oxide fuel cells and water splitting devices. Although the ceria surface is active in catalytic and electrochemical reactions, how its catalytic properties are affected by the surface structure under operating conditions is far from understood. We investigate the structure of the coherently strained $\mathrm{Ce}{\mathrm{O}}_{2}$ ultrathin films on yttria-stabilized zirconia (001) single crystals by specular synchrotron x-ray diffraction (XRD) under oxidizing conditions as a first step to study the surface structure in situ. An excellent agreement between the experiment data and the model is achieved by using a ``stacks and islands'' model that has a two-component roughness. One component is due to the tiny clusters of nanometer scale in lateral dimensions on each terrace, while the other component is due to slightly different $\mathrm{Ce}{\mathrm{O}}_{2}$ thickness that span over hundreds of nanometers on neighboring terraces. We attribute the nonuniform thickness to step depairing during the thin film deposition that is supported by the surface morphology results on the microscopic level. Importantly, our model also shows that the polarity of the ceria surface is removed by a half monolayer surface coverage of oxygen. The successful resolution of the ceria surface structure using in situ specular synchrotron XRD paves the way to study the structural evolution of ceria as a fuel cell electrode under catalytically relevant temperatures and gas pressures.

Insights into the enhancement of oxygen mass transport properties of strontium-doped lanthanum manganite interface-dominated thin films

Strontium-doped lanthanum manganite thin films were deposited by pulsed laser deposition on yttria-stabilized zirconia single crystals for a comprehensive electrochemical characterization of the material acting as a cathode. A physically-meaningful electrical model was employed to fit the electrochemical impedance spectroscopy results in order to extract the main oxygen mass transport parameters as a function of the temperature and oxygen partial pressure. The oxygen diffusion and surface exchange coefficients extracted from the analysis showed several orders of magnitude of enhancement with respect to the bulk values reported in the literature and an unexpectedly low dependence with the oxygen partial pressure. Different observations were combined to propose a mechanism for the enhanced incorporation of oxygen in interface-dominated thin films mainly based on the high concentration of oxygen vacancies expected in the grain boundaries.

Crystal orientation dependence of the stress-induced martensitic transformation in zirconia-based shape memory ceramics

Small volume samples of zirconia can survive stress-induced martensitic transformation without cracking, which enables in-depth explorations of martensite mechanics using micro-scale specimens. Here we present a systematic investigation of the orientation dependence of tetragonal crystals undergoing a uniaxial stress-driven martensitic transformation to the monoclinic phase, in single crystal zirconia pillars doped with yttria and titania. The Young’s modulus, martensitic transformation stress and transformation strain are highly dependent on the crystallographic orientation, and generally align with expectations based on known tensor properties and transformation crystallography. However, in some orientations, fracture or plastic slip are apparently preferred to martensitic transformation, and thus crystallography favors certain orientations if superelasticity or shape memory properties are specifically desired in zirconia ceramics.

Operando X-ray Investigation of Electrode/Electrolyte Interfaces in Model Solid Oxide Fuel Cells.

We employed operando anomalous surface X-ray diffraction to investigate the buried interface between the cathode and the electrolyte of a model solid oxide fuel cell with atomic resolution. The cell was studied under different oxygen pressures at elevated temperatures and polarizations by external potential control. Making use of anomalous X-ray diffraction effects at the Y and Zr K-edges allowed us to resolve the interfacial structure and chemical composition of a (100)-oriented, 9.5 mol % yttria-stabilized zirconia (YSZ) single crystal electrolyte below a La0.6Sr0.4CoO3−δ (LSC) electrode. We observe yttrium segregation toward the YSZ/LSC electrolyte/electrode interface under reducing conditions. Under oxidizing conditions, the interface becomes Y depleted. The yttrium segregation is corroborated by an enhanced outward relaxation of the YSZ interfacial metal ion layer. At the same time, an increase in point defect concentration in the electrolyte at the interface was observed, as evidenced by reduced YSZ crystallographic site occupancies for the cations as well as the oxygen ions. Such changes in composition are expected to strongly influence the oxygen ion transport through this interface which plays an important role for the performance of solid oxide fuel cells. The structure of the interface is compared to the bare YSZ(100) surface structure near the microelectrode under identical conditions and to the structure of the YSZ(100) surface prepared under ultrahigh vacuum conditions.

Ion beam synthesis of ZrCxOy nanoparticles in cubic zirconia

{110}-oriented yttria-stabilized zirconia single crystals have been implanted with low-energy C ions in an axial direction, at room temperature and at 550 °C. Room temperature ion implantation generated a damage layer that contains the expected dislocation loop clusters. Strikingly, the high temperature implantation produced zirconium oxycarbide nanoparticles (ZrCxOy) at a shallow depth in the yttria-stabilized cubic zirconia crystal, with a diameter in the range of 4–10 nm. Moreover, in the high concentration region of implanted C ions, between 100 and 150 nm below the surface, a number of large precipitates, up to 20 nm, were observed.

Relationship between charge carrier relaxation and peculiarities of electric response in some solid oxygen ion conductors

The electrical properties (impedance, dielectric permittivity, and electric modulus) of various zirconia and ceria based oxygen ion conductors have been investigated in a broad frequency (from 1Hz to 10GHz) and temperature (from ambient to about 1100K) ranges by impedance spectroscopy. A plateau of dielectric permittivity in frequencies lower than the frequency of impedance's imaginary part's peak was observed in the case of yttrium and calcium stabilized zirconia single crystals. The temperature dependence of the dielectric permittivity's plateau's value has been determined, which was found to decrease with increasing temperature. The temperature behaviour of characteristic frequencies, corresponding to peak values of the imaginary parts of electric modulus fm and impedance fz, was also investigated in single crystals and ceramics. The ratio r=fm/fz was found to decrease with increasing temperature and decreasing dopant concentration in all compounds. A sharp peak of r was detected in the vicinity of the phase transition region in scandium stabilized zirconia. A simple interpretation, based on the behaviour of a conductive system with a variable distribution of relaxation times, was proposed to explain the observed phenomena.

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.

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.

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.

Comprehensive study of the effect of the irradiation temperature on the behavior of cubic zirconia

Cubic zirconia single-crystals (yttria-stabilized zirconia (YSZ)) have been irradiated with 4 MeV Au2+ ions in a broad fluence range (namely from 5 × 1012 to 2 × 1016 cm−2) and at five temperatures: 80, 300, 573, 773, and 1073 K. Irradiated samples have been characterized by Rutherford backscattering spectroscopy in channeling mode, X-ray diffraction and transmission electron microscopy techniques in order to determine the disordering kinetics. All experimental results show that, whatever is the irradiation temperature, the damage build-up follows a multi-step process. In addition, the disorder level at high fluence is very similar for all temperatures. Thus, no enhanced dynamic annealing process is observed. On the other hand, transitions in the damage accumulation process occur earlier in fluence with increasing temperature. It is shown that temperature as low as 573 K is sufficient to accelerate the disordering process in ion-irradiated YSZ.

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.

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.

Oxygen-Ion Transfer between YSZ/YSZ and YSZ/LSCF under Mechanical Contact Stress

The present study concerns oxygen-ion transfer between yttria-stabilized zirconia single crystals (YSZ/YSZ) under mechanical contact stress. Two YSZ single crystal plates were piled up between platinum meshes, and contact stress was mechanically applied. The interfacial conductivity as well as the bulk conductivity was successfully measured with this setup. The interfacial conductivity is greatly influenced by the contact stress, whereas the bulk conductivity is almost independent of the stress. The interfacial conductivity was significantly increased with increasing contact stress. A similar improvement in the interfacial conductivity between YSZ and lanthanum strontium cobalt ferrite (YSZ/LSCF) was also observed.

Time-dependent analysis of agglomerating Pt thin films on YSZ single crystals

Abstract The controlled assembly of nanostructures via shape instability mechanisms is a potential alternative to traditional top-down processes like e-beam lithography for nanostructuring surfaces. In this contribution, the dynamics of the nanostructuresʼ assembly via thin film agglomeration have been analyzed. Pt thin films with a thickness of 50 nm were deposited via magnetron sputtering on yttria-stabilized zirconia (YSZ) single crystals and subjected to heat treatments at 1023 K for times ranging from 10 to 130 min. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM), obtaining the hole growth dynamics and morphological parameters like the lateral correlation length and the nanostructuresʼ Minkowski functionals. The experimentally obtained morphology evolution is compared to the simulated evolution of thin film structures resulting from a cell dynamical system (CDS) model. Three main observations have been made. (i) The hole radius is found to scale as function of time t with a rate proportional to t − 3 4 [ log 3 t ] . This is in agreement with Srolovitzʼs instability theory describing hole growth predominated by surface diffusion. (ii) The morphological evolution of the Pt thin films has been analyzed as function of time t by means of Minkowski measures and the lateral correlation length. A discontinuity in the lateral correlation length and a significant deviation of the Minkowski functionals from the expected Gaussian behavior was found to be coupled with the coalescence of holes. (iii) By using the Ginzburg–Landau equation for the description of the fundamental diffusion process, the CDS model allows a computational reproduction of the experimentally obtained film morphologies in the early stages of agglomeration.

Oxygen-ion transfer between yttria stabilized zirconia single crystals under mechanical contact stress

The present study concerns oxygen-ion transfer between yttria-stabilized zirconia (YSZ) single crystals under mechanical contact stress. Two YSZ single crystal plates were piled up between platinum meshes, and contact stress was mechanically applied. The interfacial conductivity as well as the bulk conductivity was successfully measured with this setup. The interfacial conductivity is greatly influenced by the contact stress, whereas the bulk conductivity is almost independent of the stress. The interfacial conductivity is significantly increased with increasing contact stress, especially from 0 to 5MPa, and is saturated around 56kΩcm2 with the stress around 40MPa. The interfacial capacitance increases with contact stress. The activation energy for the interface conductivity is equal to 105–111kJ/mol, similar to that for the bulk, and appears to change slightly with stress.

Temperature-dependent 2-D to 3-D growth transition of ultra-thin Pt films deposited by PLD

During the growth of thin metal films on dielectric substrates at a given deposition temperature Td, the film’s morphology is conditioned by the magnitude and asymmetry of up- and downhill diffusion. Any severe change of this mechanism leads to a growth instability, which induces an alteration of the thin film morphology. In order to study this mechanism, ultrathin Pt films were deposited via pulsed laser deposition onto yttria-stabilized zirconia single crystals at different deposition temperatures. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy, atomic force microscopy and standard image analysis techniques. The experimentally obtained morphologies are compared to simulated thin film structures resulting from a two-dimensional kinetic Monte Carlo approach. Two main observations have been made: (i) thin Pt films deposited onto ZrO2 undergo a growth transition from two-dimensional to three-dimensional growth at Td>573K. The growth transition and related morphological changes are a function of the deposition temperature; and (ii) a critical cluster size of i∗=4 in combination with an asymmetric Ehrlich–Schwoebel barrier favoring the uphill diffusion of atoms allows for a computational reproduction of the experimentally obtained film morphologies.

Growth and characterization of low yttria-doped fully cubic stabilized zirconia-based single crystals

For the first time, fully cubic stabilized zirconia-based single crystals with large nitrogen contents and optimized anion vacancy concentrations, interesting for new electrochemical devices such as nitrogen sensors, were prepared by a two-step approach. This two-step approach combines growth of nitrogen-free zirconia single crystals via the skull melting method and their subsequent direct nitridation to form cubic nitrogen doped zirconia-based single crystals. In the initial step, zirconia crystals with various yttria contents (2, 3, 4 and 6mol%) were prepared by the skull-melting technique. As determined by X-ray powder diffraction, tetragonal stabilized crystals were obtained, containing monoclinic segregations, when the yttria content of the grinded crystals was below 4mol%. Subsequently, the crystals were nitrided with nitrogen gas at 1500°C using different reaction times. Nitrogen content and crystallographic identity of the nitrided samples were investigated, revealing successful nitrogen incorporation and, hence, full cubic stabilization of the initial nitrogen-free samples also for the lowest yttria content. The degree of deviation from the cubic fluorite-type structure is discussed with respect to crystallographic parameters (‘pseudo-cubic factor’ etc.). The stabilization of cubic or tetragonal phases to ambient temperature is related to the anion vacancy concentration with respect to different impacts of the anion vacancies generated either by yttria or nitrogen incorporation (effective vacancy concentration [Veff]).

Influence of elastic properties on the strain induced by ion irradiation in crystalline materials

Cubic zirconia, an elastically anisotropic material, has been irradiated with 300 keV Cs+ ions and the irradiation-induced elastic strain has been measured using x-ray diffraction (XRD). In order to highlight the influence of the elastic properties on the strain level, two different cases have been investigated: (i) {1 0 0} versus {1 1 1}-oriented zirconia single crystals and (ii) {1 0 0} zirconia versus {1 0 0} strontium titanate. In the former case, raw experimental results show that the {1 0 0} planes deform less than the {1 1 1} planes. However, with a modelling that takes into account the effect of the elastic properties, it is demonstrated that the strain due to irradiation defects only is the same for both orientations (with a maximum value of ∼0.8%). Combined use of this modelling and of a dedicated computer program for fitting XRD data allowed determining both defect concentration (∼1%) and relaxation volume (∼0.5 atomic volume). In the latter case, it is shown that elastic properties affect the measured strain in irradiated zirconia and strontium titanate. However, a difference between the two materials remains even taking into account this effect, confirming that different compounds of a same class of materials (namely ceramic oxides) can exhibit a different response to ion irradiation.