Zirconia Single Crystals Research Articles

ZrO2와 Y2O3조성비에 따른 Co와 Ce 첨가 큐빅지르코니아(YSZ) 단결정의 광학적 및 구조적 특성

Co-(0.8 wt%) and Ce-(0.4 wt%) doped cubic zirconia (ZrO2 :Y 2O3 = 80 : 20, 70 : 30, 60 : 40, 50 : 50 wt%) single crystals grown by a skull melting method were heat-treated in N2 at 1000 o C for 5 hrs. The orange, yellowish brown and brown colored as-grown single crystals were changed into either brownish red, yellow and green color after the heat treatment. Before and after the heat treatment, the YSZ (yttria-stabilized zirconia) single crystals were cut for wafer form (φ6.5 mm × t2 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Ce 3+ ( 2 F5/2, 7/2(4f) → 2 Tg(5d 1 )), Co 2+ ( 4 A2( 4 F) → 4 T1( 4 F) or 4 T1( 4 P)) and Co 3+ , change of ionization energy and lattice

Dehydration kinetics of nano-YSZ ceramics monitored by in-situ infrared spectroscopy

The room-temperature dehydration kinetics of dense nano-crystalline yttria-stabilised zirconia (n-YSZ) ceramics and single crystals (s-YSZ) exposed to liquid H2O has been studied by means of near-infrared (NIR) spectroscopy. Spectra obtained for both samples showed a combination band at 5200cm−1 that was attributed to the fundamental (ν2+ν3) vibration modes of H2O molecules. No evidence was found for the presence of structurally bonded OH groups in the samples. The loss of H2O from n-YSZ exposed to air was observed to be significantly slower than from s-YSZ. Identical loss kinetics were found for n-YSZ (and s-YSZ) samples exposed to liquid CH3OH. Considered together, the results suggest that H2O (and CH3OH) are adsorbed on internal surfaces (pores, microcracks) of n-YSZ.

Hillock formation of Pt thin films on single-crystal yttria-stabilized zirconia

The stability of metal thin films on a dielectric substrate is conditioned by the magnitude of the interactive forces at the interface. In the case of a nonreactive interface and weak adhesion, the minimization of the free surface energy gives rise to an instability of the thin film. In order to study these effects, Pt thin films with a thickness of 50 nm were deposited via ion-beam sputtering on yttria-stabilized zirconia single crystals. All Pt films were subjected to heat treatments up to 973 K for 2 h. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy, atomic force microscopy, and standard image analysis techniques. Three main observations have been made: (i) The deposition method has a direct impact on the morphological evolution of the film during annealing. Instead of hole formation, which is typically observed as a response to a thermal treatment, anisotropic pyramidal-shaped hillocks are formed on top of the film. (ii) It is shown by comparing the hillocks' aspect ratio with finite element method simulations that the hillock formation can be assigned to a stress relaxation process inside the thin film. (iii) By measuring the quasiequilibrium shapes and the shape fluctuations of the formed Pt hillocks the anisotropy of the step free energy and its stiffness have been derived in addition to the anisotropic kink energy of the hillocks' edges.

Response of cubic zirconia irradiated with 4-MeV Au ions at high temperature: An X-ray diffraction study

Yttria-stabilized cubic zirconia single-crystals have been irradiated with 4-MeV Au2+ ions at fluences ranging from 1012cm−2 to 5×1015cm−2 and at three temperatures (room temperature, 500°C and 800°C). Evaluation of the irradiation-induced strain has been performed by the X-ray diffraction technique. It is found that, whatever the irradiation temperature, the elastic-strain build-up exhibits two steps. An increase of the (tensile) strain-level is observed in the first step. A drastic strain relaxation occurs at a transition fluence, which defines the beginning of the second step. Increasing the irradiation temperature induces a decrease of the strain level and a shift of the transition fluence towards low fluence. Both effects may be explained by an enhanced defect-clustering rate which occurs already at 500°C.

Effect of temperature on the behavior of ion-irradiated cubic zirconia

Yttria-stabilized zirconia (YSZ) single crystals were irradiated with 4-MeV Au ions at several temperatures from RT up to 800°C. For each temperature, the amount of damage created by irradiation was measured as a function of the ion fluence by Rutherford backscattering spectrometry in channeling geometry (RBS/C). In all cases the damage exhibits a peak around the ion projected range (∼500nm) and it accumulates via a multi-step process. Furthermore, the damage build-up strongly depends on the irradiation temperature: the occurrence of the second stage of damage accumulation, which presents a high increase of the damage fraction, is shifted towards lower fluences as the temperature is increased. This effect may be explained by enhanced defect-clustering at high temperature.

Thermo-stimulated luminescence of x-ray- and alpha-irradiated yttria-stabilized zirconia

Yttria-stabilized zirconia (ZrO2 : Y3+) single crystals (with 9.5 mol% Y2O3) were irradiated with x-rays and α particles. Thermally stimulated luminescence (TSL) data show a main broad peak centred at ∼500–550 K in the glow curves of all irradiated samples. The TSL peak maximum temperature is consistent with the characteristic recovery temperature (∼450 K) of colour centres (T centres) deduced from isochronal annealing curves measured by electron paramagnetic resonance (EPR) spectroscopy. However, the trap-depth energies (ranging between 0.8 and 1.2 eV) deduced from the initial rise of partially cleaned TSL peaks (and from a rough approximation using Urbach’s formula) are much larger than the activation energies for defect recovery of 0.3 eV deduced from the EPR data. A second TSL peak centred at ∼350–450 K found in freshly irradiated samples is seen to decay substantially in aged samples. The processes involved in TSL are discussed in relation to the defect annealing processes, and available defect-level energy and TSL data.

Oxidation and reduction behavior of Ni and NiO layers sputter deposited onto yttrium-stabilized zirconia single crystals

Thin single-crystal yttrium-stabilized zirconia (YSZ) substrate was prepared by indentation fracture and mechanical polishing. The specimen was analyzed in detail by transmission electron microscopy (TEM). The (110) edge surface was faceted, in contrast to the smooth (001) edge surface, and the facet surfaces were identified as {111}-type planes. Good cross-sectional TEM specimens comprised of crystalline Ni and NiO layers deposited on YSZ edge surface could be prepared by sputtering of a Ni support grid using Ar + ion milling and subsequent re-deposiotion on the smooth (001) fracture surface of the YSZ specimen. The epitaxial growth of a pure Ni layer on the YSZ edge planes occurred during ion milling in vacuum. However, subsequent ion milling of the specimen after exposure in air for several minutes resulted in the formation of a NiO layer on top of the first Ni layer. Reduction of the NiO layer was confirmed by electron energy-loss spectroscopy after annealing at 973 K in a vacuum of 1.2 × 10 −5 Pa. This Ni layer was re-oxidized upon annealing in air at 1073 K for 1 h. The deposition behavior of the Ni and NiO layers was discussed on the basis of the surface oxidation of Ni layer.

Strontium zirconate as silicon and aluminum scavenger in yttria stabilized zirconia

Here we report on strontium zirconate as a getter for silicon dioxide and aluminum oxide in yttria stabilized zirconia (YSZ) single crystals for cleaning purposes. YSZ single crystals were covered with strontium zirconate powder and heat treated at 1450 °C in water vapor. After treatment the YSZ single crystals showed homogeneously distributed ball-sectioned features (bumps) across the external surface of the crystal. The composition as quantified by energy-dispersive X-ray spectroscopy (EDS) showed a large enrichment of silicon and aluminum in the bumps. From cross-section analysis by transmission electron microscopy (TEM) the interface region between bump and YSZ single crystal bulk was examined. EDS showed a homogeneous distribution of silicon and aluminum through the cross section of a bump. The results suggest strontium zirconate as a good getter for silicon and aluminum from bulk materials at elevated temperatures and prove a route to remove trace bulk impurities in YSZ.

Relationship between Cation Segregation and the Electrochemical Oxygen Reduction Kinetics of La0.6Sr0.4CoO3−δ Thin Film Electrodes

Pulsed laser deposited La0.6Sr0.4CoO3−δ (LSC) thin film electrodes on yttria stabilized zirconia (YSZ) single crystals were investigated by impedance spectroscopy, time of flight secondary ion mass spectrometry (ToF-SIMS) and inductively coupled plasma optical emission spectrometry (ICP-OES). Effects caused by different film deposition temperatures, thermal annealing and chemical etching were studied. Correlations between changes in electrode polarization resistance of oxygen reduction and surface composition were found. At high deposition temperatures and after thermal annealing an inhomogeneous cation distribution was detected in the surface-near region, most manifest in a significant Sr enrichment at the surface. An activating effect of chemical etching of LSC is described, which can lower the polarization resistance by orders of magnitude. Chemistry behind this activation and thermal degradation was analyzed by ToF-SIMS and ICP-OES measurements of in-situ etched LSC films. The latter allow quantitative depth resolved compositional analysis with nominally sub nm resolution. High resolution scanning electron microscopy images illustrate the accompanying changes in surface morphology. All measurements suggest that stoichiometric LSC surfaces intrinsically exhibit very high activity towards oxygen reduction.

Thermo-stimulated luminescence of ion-irradiated yttria-stabilized zirconia

Yttria-stabilized zirconia (ZrO2:Y3 + )single crystals (with 9.5 mol% Y2O3) were irradiated with ions (from 1 MeV He to 2.6 GeV U). Electronparamagnetic resonance (EPR) data show that two kinds of colour centres (F + -typeand T centres) are produced. Thermo-stimulated luminescence (TSL) data exhibit a quite strong peakat ∼ 500–550 K in the glow curves of all irradiated samples regardless of the ion species and energy. Moreover,the 3D-TSL measurements reveal that this peak is correlated with a light emission at a wavelength of ∼ 620 nm(i.e. photon energy ∼ 2 eV). The TSL peak maximum temperatures are consistent with characteristic temperaturesof about 500 K of annealing stages of colour centres. However, the trap-depthenergies (ranging between 0.7 and 1.4 eV) deduced from the initial rise of partiallycleaned TSL peaks, or from a rough approximation using Urbach’s formula, arerather larger than the activation energies for defect recovery, ranging between 0.3and 0.7 eV, as deduced from the EPR data. The processes involved in TSL arediscussed in relation to available photoluminescence and defect energy-level data.

Structure and phase composition studies of partially stabilized zirconia

Zirconia single crystals doped with 2.8, 3.2, 3.7, and 4.0 mol % of Y2O3 have been studied. The phase composition and structure have been studied by X-ray diffraction analysis and transmission electron microscopy. It has been established that all investigated samples has two ZrO2 tetragonal phases with tetragonality c/a = 1.006–1.007 and c/a = 1.014–1.015 independent of the stabilizing impurity content. The former phase is not transformed, whereas the latter is transformed into a monoclinic one. In all cases the samples have a developed twin structure. Twinning hierarchy is observed: there are first-, second-, third-order, etc., twins, each of them containing the next-order twins inside them. Elastic stress relaxation occurs by twinning rather than by generation of dislocations. The stabilizing impurity content affects the structure nonmonotonically; the minimum dimensions of the twin lamellas refer to the Y2O3 concentration of 3.2 mol %.

YSZ:Dy3+ single crystal white emitter

Dysprosium doped yttria stabilized zirconia single crystals grown by the laser floating zone method crystallize in the tetragonal form as confirmed by Raman spectroscopy. In addition to the role of the dysprosium ion as stabilizer, the ion optical activation constitutes an opportunity to explore the zirconia-based material in the photonics area. The spectroscopic characteristics of the doped fibers were analyzed as a function of the amount of the Dy3+ concentration. The ratio of blue and yellow transition of the Dy3+ ions indicates that lightly doped samples should be considered for the production of the white emission. The analysis of the excitation population mechanisms provides information on the ions preferential excitation paths for the desired optical applications. It was found that under ultraviolet excitation the lightly doped fibers exhibit an intense and bright white luminescence which can be observed by naked eye at room temperature.

Structural and optical properties of europium doped zirconia single crystals fibers grown by laser floating zone

Yttria stabilized zirconia single crystal fibers doped with europium ions were developed envisaging optical applications. The laser floating zone technique was used in order to grow millimetric high quality single crystal fibers. The as-grown fibers are completely transparent and inclusion free, exhibiting a cubic structure. Under ultraviolet (UV) excitation, a broad emission band appears at 551 nm. The europium doped fibers are translucent with a tetragonal structure and exhibit an intense red emission at room temperature under UV excitation. The fingerprint transition lines between the D50 and F7J(0–4) multiplets of the Eu3+ ions are observed with the main emission line at ∼606 nm due to D50→F72 transition. Photoluminescence excitation and wavelength dependent the photoluminescence spectra confirm the existence of different Eu3+ optical centers.

Characterization of point defects in yttria stabilized zirconia single crystals

Characterization of point defects in a fully stabilized ZrO2 + 9 mol.% Y2O3 single crystal with cubic structure was performed in this work. It was found that the crystal contains a high density of vacancy-like defects characterized by a lifetime of 175 ps. First principle theoretical calculations showed that this lifetime is comparable with lifetime of positrons trapped in zirconium vacancies associated with hydrogen. In particular, in the vicinity of the zirconium vacancy hydrogen forms an O-H bond with one of the nearest neighbour oxygen atoms. The calculated bond length is close to 1 Å. Using nuclear reaction analysis it was found that the hydrogen concentration in the crystal is 0.3 at.%. This amount of hydrogen is sufficient to form zirconium vacancy – hydrogen complexes capable of saturated positron trapping.

Deformation mechanisms in yttria-stabilized cubic zirconia single crystals

Abstract This work attempts to give a unified picture of the microscopic mechanisms which control the plastic deformation in yttria-stabilized cubic zirconia single crystals, particularly for the soft orientation (uniaxial compression along the crystallographic axis) at low, intermediate and high temperatures while also reviewing and updating the existing data for different yttria contents (9.4 – 32 mol.%). The controlling deformation mechanisms are: internal friction within the crystal lattice (Peierls mechanism at low temperatures), pinning of dislocations by localized obstacles and long-range interaction between dislocations (intermediate temperatures) and dislocation viscous glide and climb (recovery creep) at higher temperatures. New aspects of the Portevin–le Chatelier phenomenon during viscous glide, due to the pinning and unpinning of dislocations from their defect clouds (yttrium substitutional atoms), are included.

Co와 Ce를 첨가한 큐빅지르코니아(YSZ) 단결정의 이온화에너지 및 격자상수

Co-(0.5 mol%) and Ce-(0~0.3 mol%) doped cubic zirconia (ZrO :Y O = 64 : 36 mol%) single crystals grown by a skull melting method were heat-treated in N at 1200 C for 3 hrs. The brown-colored as-grown single crystals were changed into either green or blue color after the heat treatment. Before and after the heat treatment, the YSZ (yttria- stabilized zirconia) single crystals were cut for wafer form (φ7 mm × t2 mm) and round brilliant cut (φ12 mm). The optical and structural properties were examined by UV-VIS spectrophotometer and X-ray diffraction. Absorption by Ce ( F (4f) → T (5d )), Co ( A ( F) → T ( F) or T ( P)) and Co , change of ionization energy and lattice parameter were confirmed.

Catalytic Activity Enhancement for Oxygen Reduction on Epitaxial Perovskite Thin Films for Solid‐Oxide Fuel Cells

The active ingredient: La0.8Sr0.2CoO3−δ (LSC) epitaxial thin films are prepared on (001)-oriented yttria-stabilized zirconia (YSZ) single crystals with a gadolinium-doped ceria (GDC) buffer layer (see picture). The LSC epitaxial films exhibit better oxygen reduction kinetics than bulk LSC. The enhanced activity is attributed in part to higher oxygen nonstoichiometry. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Diffusion of Hydrogen in Single Crystals of Monoclinic-ZrO<sub>2</sub> and Yttrium Stabilized Cubic Zirconia

Transport of hydrogen through oxide layers formed on zirconium alloys has been the focus of studies associated with hydrogen ingress into nuclear reactor core components. The studies have shown that microstructure and microchemistry of the underlying alloy can affect the characteristics of the oxide and in turn the transport of hydrogen through the oxide and into the underlying metal. In certain cases the oxide layer can be a homogeneous medium for hydrogen diffusion, while in most cases it is found to be heterogeneous and comprised of homogeneous, nonporous and fully oxidized, cells surrounded by a network of fast diffusion paths for hydrogen. Since in such heterogeneous systems the derived diffusion parameter and the cell size are interdependent, an absolute diffusion parameter could only be derived if the cell size were known. However, no such information is available from the microstructural studies on the oxides grown on these alloys. Another alternative is to carry out the measurements in a material, which is homogeneous at least within the dimensions of a few μm - the range of the measurements. With this in mind, hydrogen diffusion measurements were carried out in a set of single crystal zirconia specimens with the prospect that the single crystals would provide a non-porous and homogeneous medium to study the diffusivity of hydrogen. These measurements show that the single crystal specimens, contrary to the initial thinking, are not entirely homogeneous and the results do not yield an absolute diffusion coefficient for hydrogen in zirconium oxides. The details and analyses of the results from the single crystal zirconia specimens are discussed in this paper.

Investigation of hydrogen interaction with defects in zirconia

Defect studies of a ZrO2 + 9 mol. % Y2O3 single crystal were performed in this work using a high resolution positron lifetime spectroscopy combined with slow positron implantation spectroscopy. In order to elucidate the nature of positron trapping sites observed experimentally, the structural relaxations of several types of vacancy-like defects in zirconia were performed and positron characteristics for them were calculated. Relaxed atomic configurations of studied defects were obtained by means of ab initio pseudopotential method within the supercell approach. Theoretical calculations indicated that neither oxygen vacancies nor their neutral complexes with substitute yttrium atoms are capable of positron trapping. On the other hand, zirconium vacancies are deep positron traps and are most probably responsible for the saturated positron trapping observed in yttria stabilized zirconia single crystals. However, the calculated positron lifetime for zirconium vacancy is apparently longer than the experimental value corresponding to a single-component spectrum measured for the cubic ZrO2 + 9 mol. % Y2O3 single crystal. It was demonstrated that this effect can be explained by hydrogen trapped in zirconium vacancies. On the basis of structure relaxations, we found that zirconium vacancy – hydrogen complexes represent deep positron traps with the calculated lifetime close to the experimental one. In zirconium vacancy – hydrogen complexes the hydrogen atom forms an O-H bond with one of the nearest neighbour oxygen atoms. The calculated bond length is close to 1 Å.

Preparation and Characterization of Unique Foamy Zirconia Single Crystals via Sol-Gel-Hydrothermal Method

Porous ZrO2 single crystals were prepared through a sol-gel-hydrothermal method and characterized by XRD, HR-TEM, SEM and FT-IR. The results show that the dried sol gel was converted into monoclinic zirconia crystals after calcined above 550 °C. The zirconia single crystal has a porous structure with the crystal sizes of 20-70 nm. The origin of the pores was the gaseous hydrolysates of the superstoichiometric urotropine. The gel structure and the hydrothermal treatment are the required conditions resulting in the pores in crystals. After sintered at 1400 °C, the disc of the obtained ZrO2 crystals was dense. However, Y2O3 should be added to avoid fracture due to phase change if the disc was used as heat insulating layer.