Structural and electrical properties of ZnO-doped 8 mol% yttria-stabilized zirconia

8 mol% Yttria-stabilized zirconia (8YSZ) powder was prepared by coprecipitation. ZnO (0.5, 1.0, 2.0, 5.0, 10.0 wt.%) was added to the YSZ powder through a mechanical mixing method. The densification , microstructure and electrical properties of the YSZ ceramics sintered at 1300 °C for 2 h, were investigated. It was found that the small addition of ZnO was effective in reducing the sintering temperature and promoting the densification rate of the ceramics. The 5.0 wt.% ZnO-doped YSZ has ∼ 96% relative density, as compared to ∼ 89% relative density for the undoped sample. The total conductivity of 8YSZ was evidently increased by doping small amount of ZnO. For the 0.5 wt.% doped sample, the total conductivity of 2.89 × 10^− 2 Ω^− 1 cm^− 1 and an increase of 120% in conductivity were observed at 800 °C, as compared to that of the undoped one. We also found that the grain boundary (GB) conductivity could be improved by small addition of ZnO. At intermediate temperature (∼ 300 °C), the maximum enhancement of GB conductivity was observed with 5.0 wt% ZnO dopant. Finally, the volume percentage of GB in the ceramics was estimated by the brick layer model. The possible mechanism related to the improved GB conduction of the YSZ due to the ZnO additions was discussed.     

Yttrium segregation and surface phases of yttria-stabilized zirconia (1 1 1) surface

Surface segregation of yttria-stabilized zirconia (YSZ) was studied via first-principles computations and thermodynamics. For the cubic YSZ (1 1 1) surface, yttrium can segregate only to a subsurface layer, and these segregation phases are terminated at the surface by defective oxygen layers with honeycomb structure. The segregation is independent of the bulk yttrium concentration at high oxygen partial pressures or low temperatures. At very low oxygen partial pressures and high temperatures there is no surface yttrium segregation and the surface is terminated by O–Zr. Our results provide a reasonable explanation for previous experimental work, and also a framework for extending our understanding of cation segregation in oxide surfaces.     

Ferromagnetism in defective yttria-stabilized zirconia

Significant research attention has been devoted to identifying and synthesizing new magnetic materials via doping of non-magnetic materials. The material defects offer an approach to stabilize ferromagnetism in non-magnetic materials such as oxygen-deficient HfO₂ and oxygen-deficient ZrO₂. In this study, we demonstrated room-temperature ferromagnetism via nitrogen ion implantation on yttria-stabilized zirconia (YSZ) single crystals. The results of structural and chemical analyses indicate the formation of a distinct surface layer through the implantation of nitrogen ions and potential oxygen vacancies. The lattice constant in this surface layer increased by 0.6% compared to the bulk value. Nitrogen ions were observed in this region, and their concentration was estimated to be 2.32 atoms per unit cell. In contrast to the lack of magnetic hysteresis in a YSZ single crystal, ferromagnetic hysteresis was observed in the ion-implanted YSZ crystals, owing to defects—nitrogen ions and oxygen vacancies in the surface layer.     

Photoluminescence-spectra study of yttria-stabilized zirconia

Spectral analyses of photoluminescence of yttriastabilized zirconia single crystals are carried out in the temperature range from 90 K to 330 K. Over the whole temperature range the emission spectrum can be decomposed into two broad bands. The main band is centred in the yellow-orange region of the spectrum and the secondary in the green region. The temperature dependence of the emission spectrum is tentatively analyzed in terms of the first three moments of the two band shapes. In addition, thermal quenching energy is determined to be 0.10 eV for both the yellow-orange and green bands. Results are discussed on basis of the major defects, oxygen vacancies and complexes formed by associating with the Y³⁺ ions.     

Charge kinetics in spinel and yttria-stabilized zirconia

Spinel and zirconia were studied by measuring the total secondary electron emission (SEE) yield σ in a dedicated scanning electron microscope (SEM) especially equipped to study the fundamental aspects of the charge trapping in insulating materials during a 1.1 keV electron irradiation at room temperature. The variation of the total SEE yield with the injected dose for both spinel and zirconia is different. In spinel the coefficient σ starts from its intrinsic value σ₀ = 4 and reaches a plateau at σ = 1 at the end of irradiation, which corresponds to the self-regulated regime. The continuity of the curves, shot after shot, proves that the trapped charges are stable and does not spread out in the material as injection proceeds. In this case spinel is called “trapper insulator”. In contrast with the spinel, σ in zirconia, never reaches unity while the injected charge increases: it evolves from its intrinsic yield σ₀ = 2.3 to a steady value a few percent above 1. The curve shows the relaxation of the positive generated charge. In this case zirconia is called “conductive insulator”. The difference in the charging kinetics of the two materials is attributed to the difference in conductivities.     

Permittivity and AC conductivity in yttria-stabilized zirconia

Alternating current (ac) conductivity and permittivity measurements were carried out on yttriastabilized cubic zirconia, with yttria varying from 9.6 to 25.5 mol%. In this composition range, the dielectric constant measured at 10³ Hz decreased from 29.8 to 23.2 as the yttria concentration increased. A plot of the ac conductivity versus frequency exhibited a plateau conductivity (_dc) and a frequency () dependent term '(). An analysis of '() with a suitable theoretical approach based on a pairs-approximation model, resulted in a calculated binding energy for the yttrium-associated oxygen vacancy of 0.29±0.05 eV, which compares favorably with values obtained by different means in previous studies. The experimental determination of the frequency dependence of the conductivity and its analysis provides an effective basis for understanding the ionic conductivity mechanism.     

Electrical conductivity of yttria-stabilized zirconia with cobalt addition

Yttria-stabilized zirconia is the most developed solid electrolyte for use in high-temperature solid oxide fuel cells. Commercial yttria-stabilized zirconia powders reach high densification at temperatures higher than that of the usual anode materials. Reduction of the sintering temperature of the solid electrolyte could allow for co-firing of both ceramic components, thereby reducing production costs. The main purpose of this work was to study the effect of small cobalt additions on densification and on electrical conductivity of 8 mol% yttria-stabilized zirconia. Linear shrinkage results show that the onset temperature for shrinkage decreases with increasing cobalt content. Impedance spectroscopy measurements reveal that the electrical conductivity depends on the sintering profile. For specimens sintered at 1400 °C for 0.1 h the electrical conductivity of grains and grain boundaries are almost unchanged with that of 8YSZ. In contrast, for specimens sintered at the same temperature but for 0.5 h of soaking time, the electrical conductivity is higher in 0.025 mol% samples and is lower for 1 mol% Co doped 8YSZ. Degradation of the microstructure by increased porosity was obtained for high additive contents.     

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.     

Microscopic observation of laser glazed yttria-stabilized zirconia coatings

Thermal barrier coatings (TBCs) are frequently used as insulation system for hot components in gas-turbine, combustors and power plant industries. The corrosive gases which come from combustion of low grade fuels can penetrate into the TBCs and reach the metallic components and bond coat and cause hot corrosion and erosion damage. Glazing the top coat by laser beam is advanced approach to seal TBCs surface. The laser beam has the advantage of forming a dense thin layer composed of micrograins. Plasma-sprayed yttria-stabilized zirconia (YSZ) coating was glazed with Nd-YAG laser at different operating conditions. The surface morphologies, before and after laser treatment, were investigated by scanning electron microscopy. Laser beam assisted the densification of the surface by remelting a thin layer of the exposed surface. The laser glazing converted the rough surface of TBCs into smooth micron-size grains with size of 2-9 μm and narrow grain boundaries. The glazed surfaces showed higher Vickers hardness compared to as-sprayed coatings. The results revealed that the hardness increases as the grain size decreases.     

Photoluminescence decay and time-resolved spectroscopy of cubic yttria-stabilized zirconia

The time-resolved spectra and luminescence decays of cubic yttria-stabilized zirconia single crystals were investigated in the 100–300 K temperature range. At each temperature the time-resolved spectra are dominated by a yellow-orange broad band with a shoulder in the green region, and their shapes appear similar to those displayed in fluorescence. In addition, the shapes remain almost independent of the delay times over the range between 0.04 and 0.4 ms after excitation. The luminescence decays can be satisfactorily described by the superposition of two exponential functions, as well as by two expressions commonly given for decays related to disorder. In the three cases, the temperature dependences of the time constants and the other parameters derived from these expressions are analyzed. The time constants can be accounted for by assuming a radiative decay from two metastable levels with a typical separation of 0.057±0.005 eV. Some correlations between the parameters from the luminescence-decay formulae are given. The results are in good agreement with luminescence due to radiative recombinations at donor F-type levels in which complexes formed by oxygen vacancies in a disordered sublattice are involved.     

Interactions of oxygen with yttria-stabilized zirconia at room temperature

This paper reports the results of work function and EPR studies of yttria-stabilized zirconia (10Y-ZrO₂). The experimental data are considered in terms of the formation of oxygen chemisorbed species and subsequent oxygen incorporation as well as related charge transfer. It is concluded that oxygen chemisorption on 10Y-ZrO₂ at room temperature in its initial stage results in the formation of O ₂ ⁻ species. These species are then transformed into O ₂ ²⁻ , O⁻ species and, finally, are slowly incorporated into the oxide lattice. These processes take place without presence of Pt or any other electrode material.     

Structural evolution of Sc-containing zirconia electrolytes

A powder of nominal composition (ZrO₂)_0.886(Sc₂O₃)_0.104(Al₂O₃)_0.01 was synthesized by spray drying with the purpose of testing the performance of solid oxide fuel cells containing scandia-stabilized zirconia (ScSZ) as electrolyte. The phases resulting from calcination and sintering at different temperatures were investigated by XRD, impedance and Raman spectroscopy. At sintering temperatures of 1200–1400 °C nearly equal amounts of cubic and rhombohedral ScSZ were detected, whereas heat treatment higher than 1500 °C led to a cubic single-phase material. A preliminary reaction mechanism of phase formation is proposed with respect to the various results depending on heat treatment.     

Influence of elastic properties on superplasticity in doped yttria-stabilized zirconia

We report the first principles calculations of elastic and electronic properties of yttria-stabilized tetragonal zirconium dioxide (YZP) doped with GeO₂, TiO₂ and SiO₂. Electronic structure and isotropic elastic properties of YZP do not change upon addition of dopants. Addition of dopants affects the shear C₆₆ elastic constant that decreases with the increasing dopant concentration. A simple model that connects elastic softening to enhancement of superplasticity in doped fine-grained zirconia ceramics is proposed.     

Molecular dynamics calculations of anion diffusion in nitrogen-doped yttria-stabilized zirconia

Anion diffusion was simulated in the system (Y_0.2Zr_0.8)–(O_1.72N_0.15) with the molecular dynamics (MD) technique using the program DL_POLY, employing empirical potentials of the Buckingham type. To describe nitrogen migration, nitrogen potentials had to be developed, assuming the interaction of charged nitrogen shells with a mass of 0.15?amu with cores. Comparing experimental and simulated anion diffusivities, the diffusion coefficients were found to be of similar order. However, nitrogen diffuses five times slower than oxygen according to the computer simulation, while experimentally, the difference is reported to be smaller. Calculated activation enthalpies were 1.2 and 1.4?eV, respectively, for the two elements, with pre-exponential factors of 10^?5 and 10^?4?cm²/s, respectively.     

Computer simulation of defects and oxygen transport in yttria-stabilized zirconia

We have used molecular dynamics simulations and energy minimization calculations to examine defect energetics and oxygen diffusion in yttria-stabilized zirconia (YSZ). Oxygen vacancies prefer to be second nearest neighbors to yttrium dopants. The oxygen diffusion coefficient shows a peak at 8 mol% yttria consistent with experimental findings. The activation energy for oxygen diffusion varies from 0.6 to 1.0 eV depending on the yttria content. The Y_Zr′–V_O^··–Y_Zr′ complex with a binding energy of − 0.85 eV may play an important role in any conductivity degradation of YSZ.     

Absorption and emission spectra of yttria-stabilized zirconia and magnesium oxide

We have investigated the luminescence and absorption spectra of doped and undoped ZrO₂-Y₂O₃ and MgO crystals at room- and low temperatures. The crystals used are partly doped with the transition metals Ni, Co, Cr and the rare earth Pr. The emission spectra were obtained under laser excitation at different wavelengths. The observed optical emission and absorption bands of the MgO crystals doped with Ni, Co and Cr correspond to transitions between spin-orbit split crystal field levels of the transition metals. Luminescence and absorption bands of undoped yttria-stabilized zirconia (YSZ) crystals are due to color centers, absorption bands of the doped YSZ correspond to the well known transitions of the Ni²⁺, Co²⁺ and Pr³⁺ ions, respectively. The emission spectra of the doped YSZ obtained under various laser excitations can be explained by an energy transfer process between the color center and the doping materials. The influence of annealing on the absorption and emission of Pr³⁺/Pr⁴⁺ is investigated.     

Gamma-gamma angular correlation study of oxygen motion in yttria-stabilized zirconia

The nuclear quadrupole relaxation of ¹⁸¹Ta in ZrO₂9 mole% Y₂O₃ has been measured by time differential angular correlations between 350 and 1150°C. The results are consistent with a distribution of the activation energy controlling the local motion of oxygen vacancies.     

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.     

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。     

单晶YSZ的Xe+辐照损伤的电子显微分析

不同注量200keV Xe+ 注入YSZ单晶样品的电子显微分析结果表明,随着辐照注量的增加,缺陷簇的密度增大,在1×1015~1×1016cm-2Xe+注量,缺陷簇密度迅速增大,形成间隙型位错环;当Xe+注量增大到1×1017cm-2,缺陷簇密度的增加变得缓慢,并且有直径为2～4nm的Xe气泡析出。选区电子衍射花样表明YSZ样品没有产生非晶化转变。在Xe+辐照的离位率高达约350dpa的情况下,YSZ晶体没有非晶化,其原因主要是由于注入的Xe+以气泡形式析出。