Y2O3-doped ZrO2 Research Articles

Healing behavior of microcracks on the surface of yttria-stabilized polycrystals using an ultrafast high-temperature sintering method

A technique to heal microcracks on ceramic surfaces in a relatively short time is an attractive proposition for extending the lifetime of ceramics. To this end, we used the ultrafast high-temperature sintering (UHS) method, which permits a rapid temperature increase, to heal Vickers indentation microcracks generated in a 4 mol% Y2O3-doped ZrO2 polycrystal. We examined the microstructure following microcrack healing using transmission electron microscopy and scanning electron microscopy with continuous focused ion beam slicing along the microcrack plane. This revealed that the microcrack healing process involves crack pinching and neck formation, followed by the growth of the neck. The microcracks were fully healed at the surface by UHS treatment at 2000 °C for 20 s. However, residual pores corresponding to the unhealed state were observed inside the material. We hypothesized that these pore residues were caused by significant grain coarsening during UHS. It is essential to optimize the UHS temperature and time to account for grain coarsening and facilitate the application of this method to microcrack healing.

Insight into the effect of Gd-doping on conductance and thermal matching of CeO2 for solid oxide fuel cell

GDC (Gd-doped CeO2) play a dual role in solid oxide fuel cell (SOFC), effectively blocking the reaction between YSZ (Y2O3-doped ZrO2) electrolyte and high-performance cathode materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ, and improving the low-temperature oxygen ion transport in composite cathodes. It is crucial to systematically investigate the influencing mechanism of Gd-doping on conductance and thermal matching of CeO2 for high-efficiency and stable SOFC. In this paper, the physical phase, morphology and electrochemical performance of Ce1-xGdxO2-δ (x = 0.1, 0.15, 0.2, 0.25) were characterized, and the thermal matching between GDC and other cell components was investigated by finite-element thermal stress calculations and cold-heat-cycling tests. The results indicate that Ce0.8Gd0.2O2-δ obtains the optimum conductivity, whether in the low temperature segment affected by the grain boundary with the space charge layer, or in the activation energy controlled medium temperature segment. Moreover, the finite-element calculation shows that the thermal stresses of the electrolyte facing the barrier layer and the barrier layer facing the cathode decrease gradually with the increase of Gd-doping. Consequently, the cell with Ce0.8Gd0.2O2-δ applied to the composite cathode and the barrier layer presents the optimal output performance of 0.87 W cm−2 at 750 °C, and the cell performance decreased by only 1.15 % after 50 thermal cycles between 25 and 800 °C. This is significant for improving the long-term operational stability of SOFC under thermal cycling conditions.

A generalized master sintering curve based on nucleation-limited densification kinetics

The master sintering curve was developed assuming diffusion-limited coarsening and densification kinetics. A key limitation of the model is that it is only valid for a single set of initial conditions, i.e., grain size and density. Recent in situ sintering experiments suggest that densification follows nucleation-limited kinetics. A model for sintering can be formulated based on a thermodynamic energy balance between interfacial energy dissipation and work required to overcome the nucleation barrier. A feature of this model is that it inherently relates coarsening and densification, providing a basis for formulating a generalized master sintering curve that is not sensitive to initial conditions. This paper develops the model and demonstrates its practical application to several oxide systems, including ZnO, Al2O3, and Y2O3-doped tetragonal ZrO2.

Influence of microwave heating on grain growth behavior and phase stability of nano Y2O3/La2O3 co-doped ZrO2 ceramics

Y2O3-doped ZrO2 (YSZ) is a significant structural material and excellent functional material, it has been widely used in fields such as ceramics, thermal barrier coatings, and medical, etc. The disadvantage of YSZ is that it undergoes phase transition occurs during high-temperature cooling, leading to cracks in zirconia. It is worth mentioning that the failure phase transition of YSZ can be suppressed by the addition of rare earth oxide such as lanthanum oxide. In this paper, ZrO2 nano-powders with 0.2 mol% La2O3-2.8 mol% Y2O3 were prepared by a high-energy ball milling method. The comparison of microwave sintering and conventional sintering temperatures (800 °C–1300 °C) on the phase transition transformation of La2O3-doped Y2O3-stabilized zirconia powders was investigated by the control variable method. Meanwhile, the effects of microwave sintering technology on the microstructure and grain growth behavior of the sintered powders were further analyzed. The results show that microwave sintering is superior to conventional sintering technology. Microwave sintering promotes the rate of conversion of zirconia from monoclinic to tetragonal phase. Furthermore, the average grain size of the samples increased linearly with increasing temperature, and the grain growth further contributed to the presence of stable phases. Adding La3+ and Y3+ can reduce the grain growth activation energy and increase the contents of both the t-ZrO2 and c-ZrO2 in the sample. In this experiment, we find that the samples sintered best at 1200 °C with stable phase composition, well-defined morphology and uniform particle distribution. It is well known that the synthesis of high-quality zirconia materials is essential for good zirconia raw materials in practical production. This study provides an experimental and theoretical basis for the synthesis of zirconia powder with excellent performance.

Lu2O3–Y2O3–ZrO2: A Lu3+ co-doped YSZ system—Oxygen-ion conductor with high electrical conductivity and improved mechanical properties

This study investigates the electrical and mechanical properties of a Lu2O3–Y2O3–ZrO2 system (ZrO2 co-doped with Lu2O3 and Y2O3) and evaluates its possibility for use as a high-performance oxygen-ion conducting ceramic suitable for intermediate/high-temperature solid electrolytes. The use of electrochemical impedance spectroscopy (EIS) in the analysis and fitting of AC impedance spectra arcs on 8 mol% (Lu2O3–Y2O3)– ZrO2 has allowed to compare with the Y2O3-doped ZrO2 (YSZ) and Sc2O3-doped ZrO2 (ScSZ). (ZrO2)0.92(Y2O3)0.07(Lu2O3)0.01, with 1 mol% Lu2O3 co-doped with Y2O3, shows a bulk conductivity of 9.60 × 10−1 S/cm at 1000 °C and 3.47 × 10−2 S/cm at 800 °C, which are least several times higher than the corresponding values of any other sample in the experimental group, including Sc2O3-doped ZrO2, over the entire measured temperature range. (ZrO2)0.92(Y2O3)0.075(Lu2O3)0.005 also shows an outstanding conductivity; over two times higher than that of ScSZ. Thus, Lu2O3 co-doping can enhance mechanical properties such as flexural strength (19.5 % (1 mol% Lu2O3) and 21.5 % (2 mol% Lu2O3) higher than that of YSZ), as confirmed by the flexural strength and Vickers hardness tests. After 8 mol% sesquioxide doping of YSZ, the maximum improvement in the electrical and mechanical properties occurs at approximately 1 mol% (for electrical properties) and 2 mol% (for mechanical properties) Lu2O3 co-doping, respectively. The results of this work suggest that a small amount of Lu doping, performed during the development of materials for use in electrochemical devices (including high-efficiency energy conversion apparatus), could be a considerable approach to overcome the limitations of conventional solid oxide electrolytes.

Shell-like structure to limit further densification formed during flash sintering under alternative current electric field for 8 mol %Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub>

A phenomenon often appears that the final achieved density is limited during conventional flash sintering even under alternative current electric field. By observing the cross-sectional microstructure of the flash sintered 8 mol %Y2O3-doped ZrO2 at this state, we found that a thin surface portion with near full density surrounds a porous inside with lower density. The densified thin surface portion is shell-like structure covering a whole of the sintered compact. The formation of this characteristic shell-like structure in the early stage of flash sintering greatly restricts subsequent densification of porous inside. In order to suppress the formation of the shell that inhibit densification, a technique of controlling a steep power spike at a flash event must be necessary to be appropriately transitioned.

Study of the Morphological and Structural Features of Inert Matrices Based on ZrO2–CeO2 Doped with Y2O3 and the Effect of Grain Sizes on the Strength Properties of Ceramics

This article is devoted to the study of the mechanical and strength properties of Y2O3-doped ZrO2–CeO2 composite ceramics. The choice of these ceramics is due to their prospects in the field of nuclear energy, structural materials and as the basis for materials of dispersed nuclear fuel inert matrices. The choice as objects for research is due to their physicochemical, insulating and strength properties, the combination of which makes it possible to create one of the promising types of composite ceramics with high resistance to external influences, high mechanical pressures and crack resistance. The method of mechanochemical synthesis followed by thermal annealing of the samples at a temperature of 1500 °C; was used as a preparation method; to study the effect of Y22O3 doping, scanning electron microscopy methods were used to determine morphological features. The X-ray diffraction method was applied to determine the structural features and phase composition. The mechanical methods of microindentation and single compression for determination were applied to determine the strength characteristics. During the tests, it was found that the most resistant materials to external mechanical influences, and thermal heating for a long time of testing, are ceramics, in which the CeZrO4 phase dominates. At the same time, the strengthening of ceramics and an increase in crack resistance is due to a change in the phase composition and to a decrease in the grain size, leading to the formation of a large dislocation density, and, consequently, the appearance of the dislocation strengthening effect. The relevance and novelty of this study lies in obtaining new types of ceramic materials for inert matrices of nuclear fuel, studying their morphological, structural, strength and thermophysical properties, as well as assessing their resistance to external influences during prolonged thermal heating. The results obtained can later be used as fundamental knowledge in assessing the prospects for the use of oxide ceramics as nuclear materials.

Influence of Sintering Temperature on the Structural, Morphological, and Electrochemical Properties of NiO-YSZ Anode Synthesized by the Autocombustion Route

In this study, nickel oxide–Y2O3-doped ZrO2 (NiO-YSZ) composite powder as an anode material was synthesized using a cost-effective combustion method for high-temperature solid oxide fuel cell (SOFC). Further, the effects of sintering temperatures (1200, 1300, and 1400 °C) were studied for its properties in relation to the SOFC performance. The prepared and sintered NiO-YSZ materials were characterized for their surface morphology, composition, structure, and conductivity. The cubic crystalline nature of NiO and YSZ was sufficed by X-ray diffraction, and SEM images revealed an increase in the densification of microstructure by an increase in the sintering temperature. EDX spectrum confirmed the presence of nickel, yttrium, and zirconia without any impurity. Conductivity measurements, under a hydrogen environment, revealed that NiO-YSZ, sintered at 1400 °C, exhibits better conductivity compared to the samples sintered at lower temperatures. Electrochemical performance of button-cells was also evaluated and peak power density of 0.62 Wcm−2 is observed at 800 °C. The citrate combustion method provided peak performance for cells containing anode sintered at 1200 °C, which was previously reported at higher sintering temperatures. Therefore, the citrate combustion method is found to be a suitable route to synthesize NiO-YSZ at low sintering temperature.

Excess oxygen-vacancy formed by FAST regime of direct-current electric field during flash sintering for 3 mol%–10 mol% Y2O3-doped ZrO2

The amount of excess oxygen vacancies that are generated during FAST regime of direct-current flash sintering was estimated semi-quantitatively for 3 mol%–10 mol% Y2O3-doped ZrO2 (3–10YSZ) using the temperature dependence of the electrical conductivity. For the estimation, the electrical conductivity in the temperature range, where shrinkages are below a few percent, was used to avoid the effect of large microstructural changes on the electric conductivity. For all Y2O3 contents except 3YSZ, the amount of excess oxygen vacancies increased above a compact temperature of approximately 840 °C, which depended on Y2O3 content, and was highest in 8YSZ. Approximately 0.13% excess oxygen vacancy was generated by direct-current flashing in 8YSZ in the present electric field condition. In contrast, the increase in the excess oxygen vacancies is negligible in 3YSZ. The dependence of the amount of excess oxygen vacancies on the Y2O3 content was related possibly to the dependence of the electrode overvoltage on the Y2O3 content.

Rapid sintering of 3 mol % Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub> by a combined rapid furnace heating and shrinkage-controlled flash sintering protocol

Flash sintering has attracted much attention as a useful sintering technique that is able to lower the sintering furnace temperature and shorten the sintering time. To further improve this original flash sintering technique, we have developed a technique to facilitate shrinkage at a constant shrinkage rate by controlling the power dissipation during a flash state, which is termed as shrinkage-controlled flash (SCF) sintering. Here, we combine this SCF sintering method with a rapid furnace heating process to shorten the entire sintering process time. Accordingly, we succeed in sintering 3 mol % Y2O3-doped ZrO2 (3YSZ) polycrystals with a high density of 6.05 g/cm3 and a uniform grain size of approximately 0.47 µm with a short sintering time of approximately 40 min. The obtained 3YSZ polycrystals are confirmed to exhibit a Vickers hardness and fracture toughness [by an indentation fracture method] of approximately 1295 HV and 5.3 MPa m1/2, respectively.

Oxygen-ion conductivity and mechanical properties of Lu2O3-doped ZrO2 as a solid electrolyte

The characteristics of Lu2O3-doped ZrO2 as a solid electrolyte material were investigated in terms of its oxygen ion conductivity and flexural strength to realize its electrolytic function at intermediate and high temperatures. The effect of doping Lu3+, which has a high nuclear charge electric field strength, was examined through impedance spectroscopy, open-circuit potential measurements, and bending tests. The results with Lu2O3 dopant were compared with those obtained with a widely used dopant, Y3+, having a similar ionic radius with Lu3+, as well as a dopant that provides high ionic conduction, Sc3+, having a smaller ionic radius with Zr4+. The results revealed that, at the same dopant concentration, both the ionic conductivity and the flexural strength of Lu2O3-doped ZrO2 are higher than those of the widely used Y2O3-doped ZrO2. The conductivity of 8 mol% Lu2O3-doped ZrO2 surpassed that of 8 mol% Sc2O3-doped ZrO2 in the range of 800–950 °C (0.153 S/cm vs. 0.121 S/cm at 900 °C). These results indicate the potential of Lu3+ as a dopant for enhancing the performance of ZrO2 solid electrolytes.

Near complete densification of flash sintered 8YSZ: controlled shrinkage rate effects

Rate-controlled sintering of 8 mol%Y2O3-doped ZrO2 (8YSZ) polycrystals was performed by rating the current limit of alternating current (AC) electric field during a flash event; this method promotes constant linear shrinkage of the green compact with respect to the sintering time. A power dissipation peak associated with the flash event was suppressed by setting the initial current limit at 100 mA; after AC current hits 100 mA, the current limit was rated to a maximum value of 1100−2000 mA to keep shrinkage rate constant. The power dissipation behavior during the current-rating process exhibited a characteristic inverted S-shaped curve as a function of time. 8YSZ polycrystal with a near-theoretical density of 5.97 g/cm3 and a grain size of approximately 3.4 μm was produced in AC electric field (150 V/cm at 1000 Hz) with a final maximum current limit of 1150 mA as a rectangular bulk style.

Ultrafast laser processing of ceramics: Comprehensive survey of laser parameters

The productivity and quality of laser micromachining depend on multiple laser parameters that are intricately correlated. For these optimizations, a quick survey of laser parameters is vital. Recently, the authors developed a Yb-doped fiber chirped-pulse amplification system that can control various laser parameters in a wide range (pulse duration: 0.4–400 ps, repetition: single shot to 1 MHz, etc.). In this work, using this laser system, percussion microdrilling of three types of advanced ceramics, AlN, Al2O3, and Y2O3-doped ZrO2, was explored. In the case of the microdrilling of the Al2O3 ceramic, the ablation volume increased about 2–3 times as the pulse repetition increased from 100 Hz to 1 MHz. This suggests a different mechanism because the volume removal became dominant at 1 MHz. Scanning electron microscope observation confirmed a drastic melt formation at 1 MHz. From these, there is an additional volume removal due to the heat accumulation by multipulse irradiation on the Al2O3 ceramic at a higher repetition rate. It was also found that the variation of ablation volume with the pulse duration and fluence exhibited a big difference among these ceramics. A comprehensive survey of ultrafast laser ablation of ceramics was demonstrated.

Mechanical properties and thermal shock in thin ZrO2–Y2O3–Al2O3 films obtained by the sol-gel method

Thin multilayer coatings of ZrO2–Y2O3–Al2O3 were prepared using the sol-gel method and dip-coating technique in order to advance in the study of what influence the incorporation of Al2O3 has on films of Y2O3-doped ZrO2, investigating its role in the synthesis of the solutions and in the characteristics and properties of the coatings. After the characterization of the solutions used in the process, the microstructure of the films was studied and their mechanical behaviour and resistance to thermal shock were determined so as to optimize the characteristics and functionality of these coatings. With increased alumina content, 3YSZ-Al2O3 (20 mol%), the cubic phase of the zirconia disappeared completely at the sintering temperature used (700 °C), resulting in the tetragonal phase with Al in solution. There was also a decrease in the coatings' hardness and Young's modulus, and an increase in toughness and resistance to thermal shock. These results allow guidelines to be established for the design of multilayer structures that are, tougher, more resistant, and have improved surface properties.

Blue photoluminescence at room temperature from Y2O3-doped ZrO2 polycrystals sintered by flash sintering

Photoluminescence band variation of Y2O3-doped ZrO2 (YSZ) polycrystals was investigated for flash-sintered YSZ polycrystals. Nearly full density could be obtained by flash sintering under an alternating current field of 100 V cm−1, 100 Hz, limiting current of 1000 mA and at 1200 °C for 5 min. The flash-sintered YSZ polycrystals emitted blue photoluminescence even at room temperature under excitation of a handheld ultraviolet light, which could be observed with the naked eye. By contrast, conventionally sintered YSZ polycrystals emitted no photoluminescence despite using the same raw powders. In addition, the photoluminescence bands were tunable by varying the Y2O3 content.

Blue photo luminescence from 3 mol%Y2O3-doped ZrO2 polycrystals sintered by flash sintering under an alternating current electric field

We investigated the photoluminescence (PL) properties of 3 mol%Y2O3-ZrO2 (3YSZ) polycrystals. The 3YSZ polycrystals were prepared by an isothermal flash sintering technique in which an alternating electric field was applied at a predetermined furnace temperature. After flash sintering, clear, visibly blue PL at 2.82 eV (440 nm) was confirmed under an ultraviolet light (3.4 eV (365 nm)); however, no visible PL was observed from 3YSZ polycrystals conventionally sintered using the same raw powders. Thus, the emission of blue PL was induced by the electric field applied during sintering. The obtained PL band was related to excitation at 3.87 eV (320 nm), which is possibly assigned to the singly ionized associated oxygen vacancy defects (AOD + centers). Furthermore, the blue emission was stable after high temperature annealing at 1400 °C for 1 h in air. We possibly attribute the emission of this PL to the stabilization of AOD + centers by the electric field loading during flash sintering.

Enhancing the flame stability in a slot burner using yttrium-doped zirconia coating

Quenching experiments using a slot burner and surface analysis were conducted to analyze the effects of two materials (ZrO2 and 13.5 wt%Y2O3-doped ZrO2 (13.5YSZ)) deposited on stainless steel 304 (STS304) substrate on the flame stability of premixed methane/air mixtures of varying equivalence ratio and flow velocity at a wall temperature range of 373 K to 773 K. Results showed that these two types of coatings are beneficial for decreasing the quenching distance and extending the flame stability limit. Moreover, doping 13.5 wt%Y2O3 into ZrO2 coating was more conducive to reduce quenching data and broaden lower flammability limits in the fuel-lean side. Surface analyses reflected that the superior mobility of lattice oxygen for 13.5YSZ coating plays a crucial role in improving the quenching characteristics due to the formation of extrinsic active oxygen vacancies. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) verified that the methane oxidation process can be accelerated over the two coating surfaces due to the high catalytic performance. Finally, one migration route of oxygen ions consisting of the dynamic oxygen diffusion and replenishment pathways was proposed to discuss the underlying mechanism of flame stability improvement when using yttrium-stabilized zirconia composite coatings in a narrow channel.

Effects of WO3 electrode microstructure on NO2-sensing properties for a potentiometric sensor.

Planar potentiometric NO2 sensors based on 8YSZ (8 mol% Y2O3-doped ZrO2) were prepared with WO3 sensing electrode material. The various electrode microstructures prepared by different sintering temperatures were characterized by field emission scanning electron microscopy (SEM), and the microstructure influences on the sensors' performances were investigated. The sensor sintered at 800°C, with the most reaction sites, moderate adsorption sites and appropriate electrode thickness, exhibits the highest NO2 voltage response. While the sensor sintered at 750°C exhibits the lowest NO2 sensitivity because of the strongest gas-phase catalytic consumption in the WO3 sensing electrode. Based on the results of SEM characterization and electrochemical impedance spectroscopy tests, the difference in NO2-sensing performance was attributed to different amounts of electrochemical reaction sites at three-phase boundary, adsorption sites and different degrees of gas-phase catalysis.

The Sensing Mechanism Study of Potentiometric NO2 Sensor Based on Varying WO3 Electrode Thickness

In order to understand the NO2 sensing mechanism, YSZ-based (Y2O3-doped ZrO2 electrolyte) potentiometric sensors with varying WO3 sensing electrode thickness were investigated. The NO2 sensing performances were evaluated at 500 °C, 550 °C and 600 °C. All sensors showed linear relationships between voltage response and NO2 concentration. The sensor with moderate WO3-SE thickness (34.9 μm) exhibited higher sensitivity than any other sensors with thinner or thicker WO3-SE. This behavior was attributed to different gas adsorption sites and a different degree of catalytic decomposition of NO2 to NO occurring at the WO3 (Pt)/YSZ interface.

YSZ-based mixed-potential type highly sensitive acetylene sensor based on porous SnO2/Zn2SnO4 as sensing electrode

abstract Here, the porous SnO2/Zn2SnO4 sensing electrode material prepared by facile hydrothermal method was applied to fabricate the mixed-potential-type gas sensor based on yttrium-stabilized zirconia (8 mol% Y2O3-doped ZrO2) solid electrolyte plane substrate for effectively detecting acetylene (C2H2) at 700 °C. In terms of the sensing characteristics of the C2H2 gas sensor, the response value toward 100 ppm C2H2 was -82.3 mV, and the detection limit of C2H2 was lowered to 500 ppb. The ΔV of the sensing device varied piecewise linearly with the logarithm of C2H2 concentration gradient range of 0.5–2 and 5–1000 ppm, with sensitivities of -12 and −56 mV/decade, respectively. In addition, the sensor exhibited good humidity stability, long stability, and selectivity to C2H2 at 700 °C. More interestingly, after high-temperature measurement (700 °C) for 20 consecutive days, the sensor continued to present good response transients, sensitivity, and reproducibility to C2H2. Furthermore, the sensing mechanism of the mixed-potential-type sensor was verified by measuring the polarization curves and complex impedance curves.