X α studies of laser crystals III. Electronic structures of vacancy and impurity centers in cubic yttria stabilized zirconia hosts

The electronic structures of oxygen-ion vacancies and substitutional divalent and trivalent cobalt ions in cubic yttria stabilized zirconia hosts were calculated by the discrete-variational (DV)-X method. Various clusters account for different configurations of the oxygen-ion vacancies and different coordinations of the cobalt ions. The one-electron eigenvalues and charge distributions of the clusters were reported. The energies of optical transitions were obtained by transition state calculations, and compared with available experimental data. The effects of composition and thermal processing on the absorption spectra of the crystal were discussed.     

Preparation of cathodes for thin film SOFCs

SOFCs are expected to become competitive devices for electrical power generation, but successful application is dependent on decreasing working temperature from 1000 to 800 °C, without detrimental effects on resistance and on electrode processes. This requires a reduction of the stabilized zirconia electrolyte thickness and an optimization of the electrodes, especially the cathode, where losses are higher. Strontium doped lanthanum manganites are the most common materials tested as cathodes for SOFCs working at high temperature (1000 °C). This cathode material presents high electronic and oxygen-ion conductivities, a thermal expansion coefficient compatible with stabilized zirconia and good catalytic activity. For thin film SOFC devices working at intermediate temperatures (less than 800°C), we have studied the optimization of this type of cathode. Strontium doped lanthanum manganite has been deposited on yttria stabilized zirconia electrolyte substrates by spray-pyrolysis and by RF sputtering. The electrode performances depend strongly on cathode microstructure, influenced by processing conditions. With spray-pyrolysis processes, large porosity is expected. This is important for the supply of oxygen, via O₂ molecules through the pores to the triple phase boundaries, where the gas, the cathode and the electrolyte are in contact and where oxygen reduction may occur. However, large porosity can have a nefaste effect on electronic conductivity. With RF sputtering, denser films with higher electronic conductivity are obtained. But, in that case, the supply of oxygen occurs via adsorbed O-atoms in a diffusion process through the cathode to the electrolyte. Spraypyrolysis and RF sputtering have been compared relative to electrode properties. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Conductivity behaviour of a cubic/tetragonal phase stabilized nanocrystalline La2O3-ZrO2

La₂O₃ doped nanocrystalline zirconia (ZrO₂) was prepared by chemical co-precipitation method for the 3, 5, 8, 10, 15, 20 and 30 mol.% concentrations of La₂O₃. Structural studies were performed using X-ray diffraction (XRD). All the as-synthesized samples were found to be in monoclinic phase. As-synthesized samples were given heat treatment at higher temperatures for tetragonal/cubic structural phase stabilization. Sintering the samples at temperature 1173 K stabilized the tetragonal and cubic phases. A slight shift in the 100% peak of the cubic phase was observed towards the low diffraction angle indicating the substitution of the bigger La³⁺ ion into the ZrO₂ lattice. Grain sizes were found to lie between 10 and 13 nm. Electrical conductivity studies were performed on the cubic phase stabilized La₂O₃-ZrO₂ by complex impedance spectroscopy. The conductivity increases up to the dopant concentration 10 mol.% and then decreases with further increase in La₂O₃ concentration. Initial increase in conductivity is correlated to the stabilization of the cubic phase and the subsequent decrease in the conductivity with the dopant content is interpreted on the basis of the oxygen-ion movement model. Electrical conductivity has contributions from grain and grain boundary regions. But the grain boundary conductivity is slightly higher than the corresponding grain conductivity. Higher grain boundary conductivity shows higher diffusion coefficient for the atoms on the surface of the ZrO₂ grains. The possible mechanism of the oxygen ion conduction in the La₂O₃ stabilized zirconia (LSZ) is reported. The Barton, Nakajima and Namikawa (BNN) relation has been applied to the conductivity data and found that the d.c. and a.c. conductions have been correlated to each other by the same mechanism.     

Processing and microstructure characterization of ceria-doped yttria-stabilized zirconia powder and ceramics

Cubic stabilized zirconia is a promising material as target for the transmutation of actinides in nuclear reactors. In this concept, actinides are incorporated into an inert matrix (zirconia) to form a solid solution. The present work is focused on the synthesis of 8 mol% yttria-stabilized zirconia doped with 10 mol% ceria (10Ce–8YSZ) in which Ce is used to simulate the incorporation of tetravalent actinides. A wet chemical route powder synthesis method was applied to make homogeneous single-phase ceria-doped yttria-stabilized zirconia ceramics. The synthesis as well as the characterization of samples by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray emission Spectrometry (EDXS) and Rutherford Backscattering Spectroscopy (RBS) is presented.     

Investigation of the electrode effects in mixed potential type ammonia exhaust gas sensors

Mixed potential ammonia exhaust gas sensors provide a high capability for applications in harsh environments when appropriate means are conducted to stabilize the electrodes catalytic activity. The discussed sensor utilizes oxygen ion conducting yttria stabilized zirconia and gold electrodes, one of which is covered with an SCR active film to establish ammonia sensitivity and selectivity. The used SCR catalyst is well-known and developed especially for ammonia SCR exhaust gas aftertreatment systems. Electrochemical half-cell measurements vs. a Pt reference indicate that both electrode configurations are electro catalytically active for ammonia oxidation, but the activity is more pronounced at the additional SCR catalyst layer. The results of the half-cell tests including ammonia dependent voltage and oxygen interference agree very well with the performance of the ammonia sensor device. The behavior of the catalyst electrode cover is characterized separately by activity measurements of powders. The sensing mechanism combines electrochemical reactions at the three phase boundary and chemical reactions at the catalyst material.     

Spectral-luminescent investigation of absorption of phtalocyanine photosensitizers on a zirconia surface

Absorption on the surface of crystalline nanostructured, partially stabilized zirconia (PSZ) was studied by fluorescent spectroscopy. Water-soluble dissociable phtalocyanine derivatives of the anion and cation types were used as fluorescent markers. The Photosens luminescent dye was used in the case of the anion molecule, and Holosens was used in the case of the cation molecule. The absorption coefficients were evaluated by the method of comparison of the residual fluorescence of the dye solution after absorption of the crystalline powders of oxides under study (sapphire and PSZ) on the surface. It is shown that the adsorption ability of alumina (sapphire) is higher than that of zirconia (PSZ) by a factor of 6?C8 for both the anionic and cationic molecules.     

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.     

IBAD技术制备涂层导体用YSZ缓冲层长带的研究

在钇钡铜氧(YBCO)高温超导涂层导体制备路线中,离子束辅助沉积技术(IBAD)是两大主流技术路线之一,取得了最为突出的研究成果.本文简要介绍了IBAD技术制备YBCO涂层导体的最新研究进展;并采用离子束辅助沉积技术在哈氏合金(Hastelloy)基底上成功制备了1米长具有钇稳定氧化锆(YSZ)缓冲层的金属基带.采用X射线衍射仪(XRD)分析YSZ缓冲层的取向;利用原子力显微镜(AFM)和扫描电镜(FESEM)观察其表面形貌.获得了可以实际应用的IBAD-YSZ/Hastelloy缓冲层长基带,可以在该基带上研制其他缓冲层以制备YBCO高温超导涂层导体带材.     

MEASUREMENTS OF THERMAL CONDUCTIVITY AND OPTICAL PROPERTIES OF POROUS PARTIALLY STABILIZED ZIRCONIA

Experimental measurements of the high-temperature (300 < T < 800 K) thermal conductivity of highly porous (porosity greater than 80%), partially stabilized zirconia (PSZ) were performed. A method of simultaneously inverting conductivity and extinction coefficient from the experimental data is presented. The effect of natural convection within the porous plates with heating from below was found to be negligible. The thermal conductivity integral (TCI) method was incorporated into the inversion of conductivity and radiative properties from the diffusion approximation of the combined radiation and conduction heat flux measurement. The measured conductivity decreased slightly as the pore size of the PSZ increased. The extinction coefficient decreased with increased pore size, and for pore size greater than 0.6 mm the trend had good agreement with the geometric optics limit prediction.     

生物质气为燃料的燃料电池理论电动势计算

在以生物质气体为燃料的固体氧化物燃料电池体系中,通过热力学平衡分析编程计算出700-1200 K时气体中各物种例如H2、CO、CO2和CH4的平衡组成,从而得到以氧化钇稳定氧化锆(YSZ)为电解质的电池理论电动势．碳沉积对电动势有显著影响．通过计算可以推测比较适宜的操作温度和增湿条件．还计算了以掺杂氧化铈为电解质的电池电动势,相比氢气为燃料时,使用生物质气体导致的电动势的下降比使用YSZ为电解质的电池要小的多．     

Spectroscopic investigation of oxygen vacancies in solid oxide electrolytes

The primary species in both solid-state conduction and heterogeneous electrocatalysis of solid oxide electrolytes is the anion vacancy. The nature and effect of the local environment on anion vacancies in 10 m/o yttria stabilized zirconia (YSZ) and 20 m/o erbia stabilized bismuth oxide (ESB) was studied using uv-visible absorption and fluorescence spectroscopy. Partial reduction of YSZ and ordering of the oxygen sublattice in ESB is discussed. The species common to both of these phenomena, anion vacancies, was found to be luminescent and the absorption and fluorescence spectra attributable to F-center type defects is described.Current address: Materials Research Laboratory, SRI International, 333 Ravenswood Ave., Menlo Park, CA 94025, USA     

A review of zirconia-based solid electrolytes

Zirconia-based electrolyte is considered to be the most reliable candidate as oxide-ion electrolyte for oxygen sensor, oxygen pump, and solid-oxide fuel cell. The electrical property and stability of zirconia-based electrolyte depend strongly on dopant type and concentration. In this review, phase diagrams, electrical properties, and the latest developments of zirconia-based electrolyte with different dopant are discussed. The methods used to increase oxide-ion conductivity and decrease the electronic conductivity of stabilized zirconia are also discussed.     

Improvement of Cu–CeO2 anodes for SOFCs running on ethanol fuels

Ethanol is considered to be an attractive green fuel for solid oxide fuel cells (SOFCs) due to several advantages. In this paper, we presented recent progress of our group in Cu–CeO₂ anodes for SOFCs with ethanol steam as a fuel. Cu–CeO₂–ScSZ (scandia stabilized zirconia)anodes with different ratios of copper versus ceria were fabricated and the impedance spectra of symmetric cells were measured to optimize the anode composition. Area specific resistance (ASR) of these anodes was examined to prove the thermal stability of them, and possible reasons for degradation were analyzed. Furthermore, a Ni–ScSZ interlayer was added between Cu–CeO₂–YSZ (yttria stabilized zirconia) anode and ScSZ electrolyte to improve the anode performance, and the three-layer structure was fabricated by acid leaching of nickel and wet impregnation method. The maximum power density of the single cell reached 604 mW cm^? 2 and 408 mW cm^? 2 at 800 °C in hydrogen and ethanol steam respectively, and the cell obtained stable output in ethanol steam over an operation period of 50 h.     

Crystallization of amorphous zirconium thin film using ion implantation by a plasma focus of 1 kJ

In this work preliminary results of amorphous zirconium crystallization using ion beam pulses are presented. Energetic argon- and oxygen-ion beams generated by a plasma focus device were used to promote crystallization on amorphous ZrO₂-2.5 mol% Y₂O₃ film deposited by chemical solution deposition onto silica glass substrate. The films were burnt at 370 °C for 1 h in normal atmosphere previous to plasma irradiation. The irradiation was performed by means of successive pulses of ion beams. The evolution of the surface morphology and crystallization was followed by AFM and X-rays diffraction in a grazing incidence asymmetric Bragg geometry (GIAB), respectively. Argon-irradiated films showed highly nucleated cubic zirconia after 10 pulses. On the other hand, oxygen-irradiated films showed a delayed and less extensive cubic nucleation, but a more ordered structure and well-defined grains.     

Mechanisms of conductivity ceiling in YSZ

The conductivity ceiling phenomena of O²⁻ ions in stabilized zirconia are studied in terms of the path probability method and Monte Carlo simulation. It is shown that there exist two kinds of possible mechanisms. One of them is due to an order–disorder transition of O²⁻ ions, which is caused by the repulsive interaction between O²⁻ ions, and the other is owing to a percolation transition of dopant cations network. It is proposed that the YSZ case originates in the percolation transition.     

Surface influence on the behavior of stabilized zirconia nanoparticles under pressure

The surface state of partially stabilized zirconia with nanoparticles of sizes 10–30 nm after temperature and pressure treatments was investigated by Fourier transform infrared spectroscopy, X-ray diffraction and small-angle X-ray scattering. It is shown that the synthesized nanoparticles are surface fractals and the fractal dimensions non-monotonically change with nanoparticles size change. The martensite tetragonal-to-monoclinic transition of the partially stabilized zirconia nanoparticles under hydrostatic pressure (100–1000 MPa) was investigated. It was shown that the character of the martensite transition in nanoparticles’ system depends on the pressure values. Three ranges of pressures were revealed. It was shown that the stability of martensite tetragonal–monoclinic transition decreases with the increase in size of the nanoparticles only for the pressures range of 300–500 MPa. Below 200 MPa, the character of the martensite transition is extreme and has a maximum for the particle size of 17 nm. In pressure range of 600–1000 MPa, the degree of martensite transition is dependent on the fractal dimension of the surface.     

Biological and physical properties of pulsed-Laser-deposited zirconia/hydroxyapatite on titanium: In vitro study

Coating dental implants with hydroxyapatite (HA) may give certain advantages such as active encouragement of new bone growth, a lower rejection rate, and an improved long-term prosthesis fixation. This study examined the mechanical and biological properties of titanium alloy implant cores with an interlayer of zirconia and a coating of HA created using pulsed Laser deposition (PLD). The thickness of the zirconia layer was 50–100 nm, and the HA layer was ～600 nm. The crystallinity, morphology, wettability, and Ca/P ratio of the HA layer were investigated by electron microscopy, X-ray diffraction, goniometric measurement of contact angle, and wavelength dispersive X-ray analysis. The physical tests indicated adequate mechanical properties and a satisfactory adhesion to a titanium core modified with zirconia and HA. Cell proliferation and metabolic activity of human embryonal lung fibroblasts were determined using counting of harvested cells and providing an MTT assay. it was demonstrated that none of the samples were cytotoxic and their surfaces promoted cell colonization. PLD was found to be a promising method of applying coatings to a metal core for dental implants, and the in vitro biological tests suggest that the crystalline HA coating can improve the biological properties of titanium covered with zirconia.     

考虑空间电荷层效应的氧离子导体电解质内载流子传输特性

晶界或异质界面诱发的空间电荷层(space charge layer,SCL)效应,被认为是氧离子导体电解质内界面附近区域载流子传输特性显著区别于体相区域的关键原因之一.现有研究多采用Poisson-Boltzmann(PB)方程预测SCL效应的影响规律,但其基于载流子电化学平衡假设,无法用于载流子存在宏观运动的工况,极大限制了相关传输机理研究.本文耦合Poisson方程和载流子质量守恒方程,建立了适用于载流子具有宏观运动时氧离子导体内载流子传输过程的模型,推导了控制SCL效应的关键无量纲参数.聚焦固体氧化物燃料电池中常用的AO2-M2O3氧离子导体电解质,对比研究了传统PB方程和本文建立的Poisson-载流子质量守恒耦合方程的预测结果可靠性.进一步采用耦合模型深入分析了考虑SCL效应时氧离子导体内部氧空位传输机理,发现导体界面电流密度增大导致SCL电阻先减小后增大.增大无量纲Debye长度(表征空间电荷层厚度与导体厚度的比值)可显著增大SCL电阻.当驱动氧空位移动的过电势与热势数量级相当时,增大无量纲电势(表征过电势与热势的比值)导致SCL电阻增大;当过电势远小于热势时,改变无量纲电势对氧空位传输过程几乎无影响.本文研究结论可为通过合理设计晶界或异质界面以改善氧离子导体内载流子传输能力及最终提高相关电化学器件性能提供理论依据.     

Structure and mechanical properties of crystals of partially stabilized zirconia after thermal treatment

The phase composition and morphology of the twin structure of the Y₂O₃-stabilized zirconia crystals (from 2.8 to 4.0 mol %) after the thermal treatment at 1600°C have been investigated by X-ray diffractometry and transmission electron microscopy. It is shown that as the concentration of the stabilizing Y₂O₃ impurity increases, the character of the twin structure changes, and the amount of the untransformed phase t′ increases. The dependence of the hardness and crack resistance of the crystals of partially stabilized zirconia on the Y₂O₃ concentration and the indenter orientation is investigated using the microindentation method. The sample with the lowest concentration of the stabilizing Y₂O₃ impurity turned out the most crack resistant. This can be explained by a high content of tetragonal phase t in it, which provides the transformation strengthening mechanism of the material, and by a more multilevel character of twinning.     

Stabilization of the (1 1 0) tetragonal zirconia surface by hydroxyl chemical transformation

Different hydroxyl coverages on the (1 1 0) and (1 0 1) surfaces of tetragonal zirconia have been studied with periodic density functional theory. The tetragonal zirconia (1 1 0) surface is polar and intrinsically unstable. It is however very efficiently stabilized by hydroxyl formation which decreases the effective charge of surface oxygen atoms and hence avoids the electrostatic instability. The hydroxylation induces a strong stabilization of the (1 1 0) surface with respect to the non-polar (1 0 1) termination, and explains why the (1 1 0) surface of ZrO₂ can be found in some catalytic preparations. Surface chemical transformation appears to be a more efficient way to stabilize the (1 1 0) surface in comparison with the surface reconstruction processes.