XeCl laser ablation of yttria stabilized zirconia

XeCl laser ablation of yttria stabilized zirconia (YSZ) in air and in vacuum (1.3×10^–4 Pa) is studied by means of etch depth measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy of ablated surfaces. Results on ablation rate, surface morphology, and surface chemical composition are discussed in terms of the influence of ambient atmosphere on the ablation process, rapid melting and solidification of ablated surfaces, and preferential removal of oxygen atoms from the YSZ surface.     

ESR study of color centers in yttria stabilized zirconia

The ESR of single crystals of yttria stabilized zirconia (YSZ), containing color centers due to reduction in a flowing H₂ atmosphere, has been measured at room temperature. The spin density is about 3 × 10¹⁷ cm^?3 and the strong ESR line displays axial symmetry with principal values g₆ = 1.989, g_⊥ = 1.860.The behaviour of the ESR spectra when the magnetic field is rotated about the 〈1 0 0〉 and 〈1 1 0〉 directions shows that the centers have axial symmetry, the symmetry axis being along the 〈111〉 and equivalent directions. The derivative peak to peak envelope width is g-dependent and varies from 18 Gauss at g = 1.989 to 65 Gauss at g = 1.860. The results are interpretable in terms of an electron trapped at an oxygen vacancy adjacent to an yttrium ion.     

Characterization of laser sealed coatings of yttria partially stabilized zirconia

The present paper reports an experimental investigation based on X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and wavelength dispersive spectroscopy (WDS) analyses of phases formed after laser sealing of plasma sprayed coatings of 8.5 wt% yttria partially stabilized zirconia (YPSZ). X-ray diffraction and X-ray step-scanning analyses showed that the plasma sprayed and sealed coatings consisted mainly of t′ phase with a very small amount of monoclinic phase (m phase) in the plasma sprayed coatings. It was also found that the small amounts of m and cubic phases (c phase) present in the sealed coatings were dependent on laser processing specific energies (specific energy is equal to laser power/traverse speed x beam diameter). It was also found that rhombohedral (r) phase formed after laser sealing of coatings at higher specific energies. A direct relationship between c/a ratio of transformable tetragonal phase (t phase) produced by thermal treatment of sealed coatings and nontransformable tetragonal phase (t′ phase) and the amount of Yttria was obtained. A new equation was derived, which describes the relationship between Yttria concentration and the c/a ratio of tetragonal phases (t or t′).     

Photoelectron spectroscopy on silver electrodes on yttria stabilized zirconia under applied potential

Silver electrode layers of different porosity on yttrium stabilized zirconia (YSZ) were investigated by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS resp.). By applying different potentials in a three electrode arrangement oxygen could be transported from a gaseous reservoir to the working electrode. The change in the work function of the working electrode as a function of applied potentials was monitored by UPS. It showed significant different behavior for dense and porous silver electrodes. On a dense silver electrode a change in work function of up to 0.6 eV was observed at potentials at which a considerable current passed through the electrolyte. In the UP spectra of the valence band an additional structure appeared at a binding energy of 3.3 eV. These changes were also reflected in the appearance of oxygen O 1s signals in the XP spectra at 529 eV assigned to subsurface oxygen. On a porous silver electrode the work function at equilibrium (open circuit) was higher than that of metallic silver and increased linearly with increasing potential, although yet no current through the electrolyte could be observed. With the onset of considerable currents at larger anodic potentials the work function increase became stronger. As reason for the linear increase of the measured work function contributions of the solid electrolyte YSZ are considered. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

The response of yttria stabilised zirconia oxygen sensors to carbon monoxide gas

Pressures to reduce pollution and conserve fuel have created a growing interest in emission control, highlighting the need for reliable chemical sensors to detect pollutant gases. With particular regard to automotive emission control, this study examines chemical sensors based on yttria stabilised zirconia (YSZ) solid electrolytes for detection and control of carbon monoxide. It was found that combustible gases in the exhaust interfere with O₂ measurement when a YSZ oxygen conducting solid electrolyte is used. The detected CO interference on the measurement of O₂ could be utilised in two ways. Firstly, to upgrade the performance of the oxygen sensor in an atmosphere containing combustibles by minimising the effect of the combustibles. Alternatively, the interference can be controlled and quantified for CO detection. Sensors with Pt and Au electrodes were tested in O₂ and CO atmospheres. The measured response was evaluated in relation to theoretical expectations. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

单晶YSZ的Xe+离子辐照效应研究

200keV Xe+离子辐照使单晶YSZ由无色透明变成紫色透明,结果表明,能量为200keV,注量为1×1017cm-22的Xe+离子辐照YSZ单晶产生的损伤高达350dpa,在损伤区产生高密度的缺陷,但仍然没有发生非晶化转变。吸收光谱测试结果表明,产生吸收带的注量阈值大约为1016cm-2。注量为1×1016cm-2和1×1017cm-2的样品,吸收带峰值分别位于522nm和497nm。光吸收带可能与Zr阳离子最近邻的氧空位捕获电子形成的F型色心和Y阳离子近邻的氧离子捕获空穴形成的V型色心有关。     

Kinetic studies of the electrochemical nitrogen reduction and incorporation into yttria stabilized zirconia

The kinetics of the electrochemical reduction of molecular nitrogen at gold micro electrodes on yttria stabilized zirconia (YSZ) solid oxide electrolyte is studied by steady state polarization measurements. From the η / lg i plot for both cathodic and anodic polarization the apparent transfer coefficients α_a and α_c are evaluated. The sum of α_a + α_c exceeds unity and thus a multistep electron transfer process is suggested. The concept of the stoichiometric number is applied to the electrode reaction N₂ + 6e⁻ = 2N³⁻ supposing that the overall process involves at least two intermediate species. On the basis of the evaluation of the experimental results the reaction N₂⁻ + e⁻ ⇌ N₂²⁻ is suggested as the rate determining reaction step for the cathodic nitrogen reduction and nitride formation.     

High temperature scanning tunneling microscopy of purely ion conducting yttria stabilized zirconia (YSZ)

Scanning tunneling microscopy (STM) on wide band gap insulators is generally not possible. Here we demonstrate that ionic insulators with high ion mobility, such as yttria stabilized zirconia, are an exception to this rule. The ion conductivity ensures that the sample maintains a defined potential relative to the STM tip and thus a stable tunnel junction. Consequently, ion conductors provide an opportunity for characterizing wide band gap insulators, and thin films and nanostructures supported on them by STM.     

Quasi-elastic neutron scattering of benzene in Na-mordenite

The rotational and translational dynamics of benzene adsorbed in Na-mordenite have been studied by incoherent quasi-elastic neutron scattering. The measurements were performed at two benzene coverages at 300, 400 and 450 K. The observed quasi-elastic broadenings are described by a uniaxial rotational model about the six-fold axis of benzene. The mean time between successive jumps, at 300 K, is τ=1.45 × 10^?12 s at low coverage and 2.05×10^?12 s at high coverage. The correlation times follow an Arrhenius law with E_A=4.51 kJ mol^?1, at both coverages. The translational diffusion coefficient has been measured at 300 K and was found to be 0.67 × 10^?6 cm²s^?1.     

High temperature water electrolysis: Cathodic behaviour of Pt/yttria stabilized zirconia (ysz) interface

The electrolysis of steam using a solid state cell fed by mixtures of hydrogen and water has been carried out at temperatures ranging 700 to 1000°C. The kinetics of the cathodic reduction of water at Pt/YSZ interface has been investigated.Two different processes seem rate determining depending upon the current densities. The transition between each other was found to depend on the temperature but not on the hydrogen vs. water ratio of the feeding mixtures. At low current densities the over-voltage shows a typical Tafel behaviour; the activation enthalpy of the cathodic process was evaluated around 14 Kcal/mole. At high current densities the straight line relationship between total applied voltage and current density suggests the process to occur without apparent overvoltage. This behaviour has been explained in terms of formation of large amounts of complex defects acting as charge exchange sites for the water reduction.The faradic yield of the electrolysis has been proved to be 100%.     

Electrode reactions at oxygen,noble metal / stabilized zirconia interfaces

The polarization behaviour of electrodes of the type “oxygen, noble metal / stabilized zirconia”, comprising different zirconia-based materials as electrolyte, platinum or gold as metal component and an oxygen containing gas atmosphere, was investigated at elevated temperatures under equilibrium and non-equilibrium conditions by means of impedance spectroscopy. Massive metal contacts were used as part of the working electrodes. Under non-polarized conditions, the experimental results for platinum indicate a basically uniform reaction mechanism in a vast range of temperature and oxygen partial pressure, involving the surface diffusion of dissociatively adsorbed oxygen on platinum towards the electrochemical reaction sites on the electrolyte surface as rate-determining step. The experimental findings for gold are consistent with the occurrence of two competing reaction mechanisms, namely a charge transfer controlled process and a surface diffusion controlled process, each of them prevailing in different regimes of temperature and oxygen partial pressure. Under polarized conditions, a significant decrease of the polarization resistance takes place, followed by the onset of low frequency loops in the impedance spectra. In the case of cathodic polarization, the onset voltage can be correlated with the partial electron conductivity of the electrolyte, thus confirming the hypothesis of direct participation of electronic species of the electrolyte in the electrode reaction under biased conditions. At moderate temperatures, the polarization induced changes in the electrode properties exhibit a slow relaxation behaviour. This can be attributed to the successive annihilation of additional metastable electrochemical reaction sites having been created during the preceding polarization treatment. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy     

The control of very low oxygen partial pressures with stabilized zirconia cell

Control of very low oxygen partial pressures was performed in the range of 10⁻²¹-10⁻²⁴ Pa at 750°C by pumping oxygen into the purified hydrogen stream with a stabilized zirconia cell. The oxygen partial pressures were monitored by a stabilized zirconia sensor. The stabilized zirconia oxygen sensor was calibrated by H₂-CO₂ gas buffer mixture in the range of oxygen pressure from 10⁻¹⁷ to 10⁻²¹ Pa, and oxygen partial pressures below 10⁻²¹ Pa were measured by extrapolating the calibration line to very low oxygen partial pressures. The lowest oxygen partial pressure controlled was 10⁻²⁴ Pa at 750°C, which was limited by gas leaks in the system and also by the reduction of the ionic transference number in solid electrolyte used as the oxygen pump.     

Quasi-elastic neutron scattering study of dynamics in condensed matter

Quasi-elastic neutron scattering (QENS) technique, known to study stochastic motions has been successfully used to elucidate the molecular motions and physical properties related to them, in a variety of systems. QENS is a unique technique that provides information on the time-scale of the motion as well as the geometry of the motions. In this paper, results of some of the systems studied using the facility available at Dhruva, Trombay and other mega-facilities are discussed. Emphasis is given on the results obtained from three different systems studied using QENS, namely, (1) alkyl chain motions in monolayer protected metal clusters, (2) molecular motions of propane in Na-Y zeolitic systems and (3) the study of reorientational motions of liquid crystal innO.m series in different mesophases.     

Kinetics of densification in stabilized zirconia

Densification mechanisms during the sintering of CaO-stabilized ZrO₂ have been studied using the technique of hot pressing. The dependence of the densification rate on applied pressure has been found to be linear suggesting that the kinetics are controlled by a diffusion process. The densification rate undergoes a maximum value at a composition of approximately 12 m/o CaO; this is close to the value at which the ionic conductivity maximum also occurs. Since this composition coincides with a phase boundary in this system, parallel experiments have been carried out using the CeO₂/Y₂O₃ system in which no corresponding phase boundary occurs. The similar dopant dependence for the conduction and densification processes in these oxides suggests that the fast carrier (responsible for conduction) and the slow carrier (responsible for densification) depend in a similar way on the additive content. This in turn suggests that densification is controlled by the diffusion of cation interstitials and further that the maximum is caused by increasing degrees of defect association at high dopant contents.     

About ionic conductivity/diffusion relationships in yttria doped zirconia

The values of the oxygen self-diffusion coefficients (measured using¹⁸O tracer) are compared to the ionic conductivity (measured by impedance spectroscopy) of 9.5 mol% yttria doped zirconia single crystals in the temperature range 240–800 °C in air. Electrical conductivity measurements in polycrystals, exhibiting a grain boundary contribution to the ionic conductivity, are furthermore discussed in the frame of a “brick/boundary model”. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Effects of assisting and sputtering ion current on ion beam assisted deposition textured yttria stabilized zirconia buffer layers of coated conductors

Biaxially textured yttria stabilized zirconia (0 0 1) thin films were fabricated on untextured hastelloy substrates by ion beam assisted deposition method. The effects of assisting beam current density J_a and sputtering beam current density J_s on the textures of the films were studied. The results indicate that as J_a or J_s increase, both the out-of-plane and the in-plane textures are improved initially, and then degrade. The results can be attributed to anisotropic damage and selective sputtering effect of assisting ions. At the same ion-to-atom arrival ratio r, which is reflected with J_a/J_s value, lower deposition rate can enhance the biaxial texture.     

Luminescence of oxygen molecular ion in neodymium-doped yttria

Pulsed cathodoluminescence spectra of Nd³⁺:Y₂O₃ compacts registered after their annealing in air at a temperature above 950°C exhibit a structured band in the range 610–660 nm with four well-resolved components located at wavelengths of 620.6, 630.6, 645.3, and 655.6 nm. At the same time, the lattice parameter of the cubic yttria increases, and the color of samples changes from light blue to yellowish. In addition, the nearly complete absorption of the intrinsic luminescence band of yttria is observed in the range of 380–600 nm with a center at λ = 485 nm. We assume that these effects are caused by oxygen molecular ions O₂⁻, which are formed in cubic yttria due to the penetration of oxygen into natural anion vacancies. The frequencies of vibrations of the ground and excited states of the oxygen molecular ion has been determined.     

Visual observation of chemical diffusion in stabilized zirconia

Strongly reduced crystals of stabilized zirconia are black and completely opaque. These samples may be reoxidized at T200° C in air. We present results of the reoxidation process and try to explain the observed behavior through annihilation of color centers with an energy level at E>–1.60 eV. The difficulties with this model are pointed out.