Yttria-stabilized Zirconia Surface Research Articles

Optimal Modeling of Ni/YSZ Triple Phase Boundary in Solid Oxide Cells Using First-Principles Calculations

Power generation with renewable energy using solid oxide cells (SOCs) has been widely researched. To solve the existing problems of SOCs, such as degradation and efficiency improvement, it is essential to understand reaction mechanisms on the surface/interface such as triple phase boundary (TPB) composed of catalysts, electrolytes, and gas phases. However, a reliable TPB model has not been uniquely defined to discuss the property. This study focused on the TPB model comprising Ni catalysts, yttria-stabilized zirconia (YSZ) electrolytes, and gas phases, and aimed to theoretically identify a reliable TPB model by using density functional theory calculations. The stable structure of YSZ surface models was first identified considering various oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and YSZ surfaces. Thereafter, a reliable Ni/YSZ interface model was discussed by evaluating various Ni structure types, Ni interfaces in contact with the YSZ surface, and interface positions. As a result, we have proposed a more reliable YSZ surface structure than previous reports and reasonable Ni/YSZ interface models considering the computational cost to discuss the properties of TPB. These findings will contribute to the improved design of SOCs as high-performance energy conversion systems for sustainable energy storage.

Antiviral and antibacterial activities of yttria-stabilized zirconia

Antiviral and antibacterial activities of yttria stabilized zirconia (YSZ) powder were evaluated against Escherichia coli, Staphylococcus aureus, bacteriophage Qβ, and bacteriophage Φ6 using ISO procedures. For comparison, the same evaluation was also conducted using Y2O3 and ZrO2 powders. Against all these bacteria and viruses, YSZ exhibited antibacterial and antiviral activities, but the extent of its activity differed depending on the bacteria and virus. Those differences in activity might be attributable to electrostatic interactions between the YSZ surface or ions and bacteria and viruses, the degree of adsorption attributable to specific interactions between Y or ZrO2 and phosphate ions, and differences in protein denaturation including lipid bilayer deformation.

Femtosecond Pulsed Laser Irradiation of Zirconia for Embedding Silver Nanoparticles in Surface Nanopores

Femtosecond pulsed laser irradiation was performed to investigate the feasibility and fundamental characteristics of embedding silver nanoparticles onto zirconia ceramic surfaces. By irradiating laser, nanopores were fabricated on the surface of the yttria-stabilized zirconia (YSZ) substrate, and silver nanoparticles were infiltrated and immobilized into the pores using a commercial nano-silver dispersion solution. Numerous nanopores embedded with silver nanoparticles were successfully obtained on the YSZ surface while keeping the grains’ shapes unchanged by controlling laser parameters. Optimizing laser fluence and scanning speed near the ablation threshold made it possible to remove only the excess dispersant that remained on the surface while keeping silver in the pores and without causing machining of the surface of the YSZ substrate. In addition, about 60% embedding in the nanopores was achieved. It was found that the shorter pulse width was suitable to avoid evaporating both dispersant and silver. Cross-sectional observation revealed that the silver nanoparticles were agglomerated to form clumps and were embedded without a gap at the bottom of the pores at a depth of about 600 nm. After laser irradiation, no significant laser-induced phase change was observed in the YSZ substrate, indicating that there was no in-process thermal damage to the bulk. These findings demonstrated the possibility of adding a metal nanoparticle to the zirconia surface by using only a laser process without damaging the properties of the base material during the process. New applications of zirconia, such as the generation of functional surfaces with antibacterial properties, are expected.

Investigation of Corrosion Resistance of YSZ Coating with Sacrificial Aluminum Oxide Protective Layer against CMAS and Composite Corrosives

An Al film was deposited onto the surface of yttria-stabilized zirconia (YSZ) coating using magnetron sputtering, followed by in situ oxidized to form Al2O3, referred to as Al2O3-YSZ (A-YSZ). The corrosion behavior of the A-YSZ coating against the attacks of calcium–magnesium–aluminum–silicon (CMAS) and a composite corrosive agent containing salt (NaVO3/Na2SO4/NaCl) and CMAS (CMAS + salt). Results demonstrate the excellent resistance of the investigated coating to the corrosion of CMAS and CMAS + salt. The Al2O3 layer plays a crucial role as sacrificial protection in effectively isolating the composite corrosive agents, substantially extending the service life of YSZ coatings. NaVO3, Na2SO4, and NaCl promote the formation of high-melting-point products, reducing their penetration into the YSZ layer. The reaction mechanisms of Al2O3 with CMAS and CMAS + salt were clarified.

Rare earth oxide CeO2 enhanced reactive air brazing of YSZ ceramics

Cerium oxide (CeO2) in the range of 0–16 mol.% is used to enhance the joint of Ag-based filler for brazing yttria-stabilized zirconia (YSZ) ceramic in the air. Ag–CeO2 filler is first adopted to braze YSZ ceramic (3 mol.% yttria). The CeO2 is found to cluster at the interface of YSZ and Ag seam, improving the wetting behavior of Ag-based fillers. The maximum shear strength of YSZ joints can reach 35.1 MPa using Ag–1CeO2 fillers at 1050 °C/30 min. Moreover, CeO2 is used to enhance the Ag–CuO filler for brazing YSZ ceramic. As the increase of CeO2 content, the contact angle of Ag–CuO on the YSZ surface decreases first and then increases. The average shear strength could reach 76.5 MPa using Ag–CuO–8CeO2 at 1050 °C/30 min. The CeO2 could decrease residual stress by adjusting the linear expansion coefficient of Ag-based fillers. Therefore, this study provides a new strategy for exploring air brazing fillers.

Electrochemical Study of Zinc Adsorption on Yttria-Stabilized Zirconia-Water Interfaces

Research has shown that zinc ions can mitigate the corrosion of the alloy materials in nuclear power plants in addition to reducing radiation contaminations. Understanding how zinc ions alter the interfacial structure of the alloy surface is therefore important in improving the life cycle of the materials and achieving higher burnups of the nuclear fuels. While the corrosion mitigation of zinc on nickel and stainless-steel alloys has been widely studied and reported, its effect on zirconium oxide surfaces, i.e. the oxide layer of zirconium alloys exposed to the aqueous solution, has rarely been reported. Meanwhile, zirconium oxide also serves as the solid oxide fuel cell electrolyte in energy storage. The study of the corrosion mitigation mechanism on zirconia could also contribute to the successful commercialization of SOFC technologies as cleaner and more efficient energy conversion electrochemical systems. In this work, we utilize the electrochemical techniques to study the interfacial structure changes under the influence of zinc ions on the yttria-stabilized zirconia (YSZ) surfaces on the (100), (110), and (111) orientations in contact with aqueous solution with and without externally applied electric potentials. The experimental measurements indicated the modification of the interfacial structures by zinc on (100) and (111) surfaces is strongly influenced by the applied external electric fields, while the modification of surface structure by zinc ions is not as evident on the (110) surface. Our study also showed that the Zn2+ adsorption layers on the oxide surfaces could be better preserved by tuning the electric potential on the oxide substrates towards the negative potentials.

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. We explore the novel combination of two methods of VO2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO2. Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

Gradual modification of the YSZ structures by Au ion implantation and high-energy Si ion irradiation

Gold nanoparticles (Au-NPs) were created in crystalline yttria-stabilized zirconia (YSZ). (100)-, (110)- and (111)-oriented YSZ samples were implanted by 1 MeV Au+ ions at room temperature and fluences ranging from 1.5 × 1016 cm−2 to 7.5 × 1016 cm−2. The prepared Au: YSZ structures were annealed at 1100 °C for 1 h on air to support the Au-NPs coalescence and YSZ structure recovery. Subsequent irradiation with the 10 MeV Si3+ ions with a fluence of 5.0 × 1014 cm−2 was performed to enable gradual modification of Au-NPs. Au-depth profiles and YSZ structure modification in the produced samples were analysed via Rutherford backscattering spectrometry in channelling mode (RBS-C) and X-ray diffraction (XRD). RBS-C showed the gold distributed in the region of about 50–300 nm below the YSZ surface. The disorder was accumulated in the region with Au-NPs and the concentration of the disorder increases as a function of ion implantation fluence. The Zr-disorder was partially decreased after the annealing, while the subsequent Si-ion irradiation increased Zr-disorder again, however, the disorder does not reach values before the annealing. XRD measurement evidenced elastic deformation of the YSZ host lattice in the Au-implanted samples. Optical absorbance showed the appearance of the new absorption band at 550 nm for the Au-ion fluences above 5.0 × 1016 cm−2 ascribed to the Au-NPs formation. After the annealing, the absorption band is shifted to the wavelength of 580 nm which could be connected to the proceeding clustering of Au. The maximum absorption peak intensity increases which is connected to the increasing amount of the Au-NPs. Transmission electron microscopy (TEM) with Energy dispersive spectroscopy (EDS) confirmed the presence of Au-NPs in the implanted layer after the annealing. The subsequent Si-ion irradiation did not change the Au-NP shape which remained spherical with a slight size increase.

A new approach to protect and extend longevity of the thermal barrier coating by an impermeable layer of silicon nitride

AbstractA sample representation of a gas turbine engine blade, consisting of a nickel superalloy substrate with a deposited thermal barrier coating (TBC), was covered with silicon nitride, Si3N4, as an impermeable layer using plasma enhanced chemical vapor deposition (PECVD). The silicon nitride layer was used to seal the topcoat of yttria‐stabilized zirconia (YSZ) surface of the TBC to mitigate calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS testing was carried out on the covered and uncovered surfaces by melting a ratio of 25 mg/cm2 of CMAS powder onto the surface of each sample in a furnace at 1100°C for 1 h. The conformal surface reaction of the sealed layer confirmed no cracking or delamination at high temperatures. Scanning electron microscopy (SEM) micrographs confirmed that the surface of YSZ was successfully sealed. The new coating of silicon nitride was shown to be a viable solution and technique to significantly block CMAS infiltration in porous thermal barrier coatings.

XPS investigation of monoatomic and cluster argon sputtering of zirconium dioxide

The surfaces of zirconium dioxide and yttria-stabilized zirconia (YSZ) have been analyzed using x-ray photoelectron spectroscopy after ion sputtering with monoatomic Ar+ or an argon gas cluster ion beam (GCIB). The O/Z ratio and new components in the Zr 3d lines show reduction to lower oxidation states when sputtered with monoatomic Ar+, but significantly less damage is observed when GCIB sputtering is used. The damaged surface layer caused by Ar+ sputtering can be removed by subsequent GCIB sputtering. However, the depth resolution observed in depth profiles of thin YSZ films was significantly better when Ar+ sputtering is used. Differences in the Sn content in the oxidized Zr-4 specimen were also observed when comparing Ar+ and GCIB sputtering, suggesting preferential sputtering.

First-Principles Studies for Optimal Model of the Ni/YSZ Triple Phase Boundary in Solid Oxide Cells

To resolve the existing issues of solid oxide cells such as degradation and efficiency improvement, it is essential to understand reaction mechanisms on the surface/interface such as triple phase boundary (TPB) as a highly active site that consists of catalysts, electrolytes, and gas phases. However, the reliable TPB model has not been still uniquely defined to discuss the property. In this study, we have focused on the TPB model comprising Ni catalysts, yttria-stabilized zirconia (YSZ) electrolytes, and gas phases and aimed to theoretically identify a reliable TPB model. In concrete, we identified firstly the stable structure of YSZ surface models by using density functional theory calculations considering different oxygen vacancy positions, yttrium atom arrangements, yttria concentration, and YSZ surfaces. Thereafter, we discussed a reliable Ni/YSZ interface model based on our proposed YSZ model by evaluating different Ni structure types, Ni interfaces in contact with the YSZ surface, and interface positions.

Modeling the Distribution of Oxygen Partial Pressure in the Electrolyte of Solid Oxide Cells and Its Implication on Microstructure Evolution in the Hydrogen Electrode

The distribution of oxygen partial pressure in the electrolyte has an important effect on the stability of solid oxide cells (SOCs). It is well known that the high oxygen partial pressure at the oxygen electrode and electrolyte interface causes delamination, while its effect in the hydrogen electrode (HE) has received comparatively little attention. The only existing model for the distribution of oxygen partial pressure in the electrolyte of SOC is proposed by Virkar et al., which is a one-dimensional model that does not consider the Butler-Volmer equation at triple phase boundary (TPB) nor the microstructure’s effect. In this work, the Virkar’s model was extended to three dimensions and the Butler-Volmer equation was added at TPB to investigate the distribution of oxygen partial pressure in the actual electrode microstructure. The oxygen partial pressure in the yttria-stabilized-zirconia (YSZ) phase of HE near the HE-electrolyte interface was found to be significantly greater than the oxygen partial pressure in the pore phase of HE, which may lead to Ni oxidation. Furthermore, a phase field model was employed to simulate the microstructural evolution of Ni particles on YSZ surfaces with the assumption that NiO forms at the Ni-YSZ interface. The NiO formation affects the microstructure evolution in HE by changing the shape of the Ni particles.

Nanosecond laser irradiation of Yttria stabilized Zirconia: Pulsed laser ablation and surface treatment

Nanosecond pulsed laser irradiation resulting in material ablation and surface micro structuring of Yttria Stabilized Zirconia (YSZ) targets, has been investigated. Our study aims to evaluate suitable ablation conditions enabling either, creation of desired YSZ surfaces having micro porosity for directed use, or generation of YSZ vapors for deposition of homogenous, dense and particle free YSZ films via conventional Pulsed Laser Deposition (PLD) technique. The work being reported here involves a thermal model based theoretical simulation describing heat transport, melting and vaporization of a laser irradiated YSZ target. Our simulation has been validated against our experimental data on nanosecond laser ablation from a sample target of ceramic YSZ. Surface porosity generated on laser irradiated YSZ targets for low laser fluence levels ∼ 1 ​J/cm2 has been explained in terms of pore formation via entrapment of gas bubbles in the solidifying interface on laser melting and re-solidification thereafter, of the YSZ target. To understand surface crater formation on laser irradiated YSZ targets at high laser fluence levels ∼10 ​J/cm2 maximum temperature reached by the laser irradiated YSZ target has been compared with the thermodynamic critical temperature of YSZ. Our study confirms that judicious choice of laser fluence enables either, creation of surfaces having micro porosity of desired nature on YSZ target, or controlled generation of YSZ vapors for deposition of good quality YSZ films and coatings using laser deposition technique.

The mechanism in surface morphology of YSZ ceramics influencing its corrosion resistance to CMAS melt: Molecular dynamics research

In this paper, several surface morphologies such as smooth style, or mesh & stripe style containing nanostructure were constructed and simulated by molecular dynamics simulation. The results show that the diffusion area of molten calcium-magnesium-alumina-silicate (CMAS) droplets on yttria-stabilized zirconia (YSZ) surface with stripe nanostructure is smaller than that on smooth or mesh style surfaces, and its contact angle obtains the largest value. The diffusion coefficient points out that the best corrosion resistance of YSZ surface comes from the lowest diffusivity of elements in molten CMAS on stripe nanostructure. Elements radial distribution function of Ca-O verified that they mainly keep in CMAS droplets instead of permeating into YSZ ceramics. And, the influence of nanostructure on the depth of Mg, Al, Si infiltration layer in CMAS is greater than that of Ca ion. Therefore, YSZ surfaces with stripe nanostructures exhibit the best corrosion resistance.

Generation of nanopore structures in yttria-stabilized zirconia by femtosecond pulsed laser irradiation

Femtosecond pulsed laser irradiation of yttria-stabilized zirconia (YSZ) was performed to investigate the feasibility and fundamental characteristics of nanopore structure fabrication. Numerous nanopores were successfully generated on the YSZ surface with only a single laser pulse shot by controlling laser power near the ablation threshold. The nanopore generation mechanism involves light focusing by the convex shape of a crystal grain and ablation inside the grain. By increasing laser power, the number of nanopores increased. Cross-sectional observation revealed that the depth of nanopores was about 500 nm, and nanopores were generated inside the YSZ grains of the top layer while keeping the grains' shapes unchanged. It was found that higher scanning speed and a small number of scans enabled nanopores fabrication without changing the original surface shape, avoiding the removal of the surface layer. Laser-irradiated surfaces of YSZ were characterized by Raman spectroscopy, and it was found that there was no significant phase change in the specimen after nanopore generation compared with the original YSZ surface, confirming that there was no thermal damage to the bulk material. The surface with nanopores showed an enhancement in surface hydrophilicity. These findings demonstrated the possibility of selectively generating nanopores in the top surface of YSZ for use in functional surfaces without serious thermal damage to the workpiece. It is expected that the nanopore generation will lead to new applications of YSZ.

Operando observations of active three phase boundary of patterned nickel - Yttria stabilized zirconia electrode in solid oxide cell

In solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), the substantial performance degradation of nickel (Ni) - yttria-stabilized zirconia (YSZ) fuel electrode has been a growing challenge for commercialization. The microstructure of Ni-YSZ electrode is not stable during long-term operation due to Ni phase redistribution. In the present study, operando observations of patterned Ni film electrodes under SOFC, SOEC and reversible solid oxide cell (RSOC) operation modes are conducted. Confocal laser scanning microscopy is applied to acquire real-time microscopy images of the three phase boundary (TPB) under three operation modes to study the Ni mobility on the YSZ surface. The operando observation showed that the Ni phase dynamically spreads and splits up only under the SOFC mode, while this phenomenon is not observed under the SOEC mode. Ni coarsening occurs regardless of the operation mode. The dynamic motion of active TPB qualitatively correlates with the variation in the current density during the operation. The contact angles of Ni on flat YSZ surface after operation are the smallest for SOFC, followed by RSOC and the largest for SOEC. Competition between the altered Ni wettability and the Ni agglomeration finally determines the complicated Ni morphological changes.

Fabrication of microstructured yttria-stabilized zirconia electrolyte surface using centrifugal molding for high-performance solid oxide fuel cells

This study replicated the surface of yttria-stabilized zirconia (YSZ) microstructures by centrifugal filling to improve the performance of solid oxide fuel cells. A slurry with YSZ nanoparticles were filled with centrifugal force in polymer replica mold, solvent dried, and then transferred and sintered onto a bulk YSZ electrolyte plate. 3 m-pitch pillar structure was successfully replicated with a centrifugal acceleration of 2,500 G, and the polarization resistance of the electrodes was reduced by approximately 25%. Similarly, a line-and-space structure was formed to investigate the structure dependence, and resistance reduction was confirmed. Furthermore, a distribution relaxation time analysis confirmed that the reduction in resistance was due to decreasing tortuosity factor of YSZ.

Metallization by Sputtering to Improve the Bond Strength between Zirconia Ceramics and Resin Cements

Zirconia has been used as a prosthesis material for over a decade because of its excellent mechanical properties and esthetics. The surface treatment for zirconia generally involves sandblasting and the application of primers for favorable bond strength between the surface and resin. However, sandblasting causes the microcracking and chipping of the zirconia surface. To overcome these challenges, the metallization of the zirconia surface was performed. Ti and Au were sputtered on yttria stabilized zirconia (YSZ) disks and heated to 800 °C for 15 min in air. These disks were bonded to stainless-steel rods using resin cement. Then, shear bond strength tests were performed using an Instron-type testing machine. The shear bond strength of the Ti sputtering group was significantly higher than that of the other groups. According to the results of X-ray photoelectron spectroscopy and electron probe microanalysis, the Ti-sputtered YSZ surface contained both sub-titanium oxide and titanium oxide before heating. Sub-titanium oxide was converted to titanium oxide by heating. These results suggest that metallization using Ti is effective for zirconia surface treatment to improve the shear bond strength between YSZ and resin cement. This metallization technique for YSZ has potential in clinical applications.

Activation of Porous Pt Electrodes Deposited on YSZ Electrolyte by Nitric Acid Treatment.

The effect of nitric acid treatment on the electrochemical performance of porous Pt electrodes deposited on YSZ (abbreviation from yttria stabilized zirconia) electrolyte was investigated. Two identical symmetrical Pt/YSZ/Pt cells with porous Pt electrodes were fabricated, after which the electrodes of the first cell were kept as sintered, while those of the second cell were impregnated with HNO3 solution. The electrochemical behavior of the prepared electrodes was studied using impedance spectroscopy and cyclic voltammetry. Significant reduction of the polarization resistance of the HNO3-treated electrodes was revealed. The observed enhancement of the electrochemical performance of porous Pt electrodes was assumed to be caused by adsorption of NOx-species on YSZ and Pt surfaces, which promotes oxygen molecules dissociation and transport to the triple phase boundary by the “relay-race” mechanism. The obtained results allow for considering the nitric acid treatment of a porous Pt electrode as an effective way of electrode activation.

Multidisciplinary approach for investigating the hot corrosion behavior of thermal barrier coatings

Herein, we propose a new multidisciplinary approach for investigating the corrosion behavior of thermal barrier coatings, combining the thermal-gradient mechanical fatigue method with hot corrosion tests. Corrosive salts (Na2SO4; V2O5) of varying concentrations (10−20 mg/cm2) were deposited on the surface of yttria-stabilized zirconia (YSZ) coatings to evaluate the microstructural changes occurring during the reaction, via thermal graded mechanical fatigue tests. The tests were conducted at a surface temperature of 1150 °C in 10 min cycles and underwent applied uniaxial tensile loads. The corrosion tests show that the ZrV2O7 plays an important role as an intermediate in the collapse of the lamellar structures during the reaction, which results from the repetitive precipitation of V2O5 and m-ZrO2 from the ZrV2O7. The microstructure of corroded YSZ coatings exhibited a different degradation mechanism under a thermomechanical environment, compared with the testing under isothermal atmospheric conditions.