Yttria-doped Zirconia Research Articles

Cobalt-Zirconia coated monoliths fabricated by Direct Ink Writing for catalytic applications

Direct Ink Writing (DIW) is a promising technique for fabricating ceramic catalysts with complex geometries and enhanced catalytic performances, but these still require overcoming some challenges to maximize their catalytic and mechanical functionality. In this work, DIW yttria-doped zirconia (YDZ) catalysts were coated with a Co/YDZ catalyst layer. Two configurations based on a dense monolith and a porous one sintered at 1450ºC and 1300ºC were fabricated through DIW, using optimal rheological hydrogel-inks, to investigate the effect of porosity on the mechanical and catalytic performance in the ethanol steam reforming reaction. The monolith sintered at 1450°C achieved the best mechanical and catalytic performance, as it presented the highest Co concentration (2.3at.%) at the catalyst layer. In addition, this monolith exhibited excellent mechanical properties at the interface between contiguous filaments in the multilayer structure. Therefore, this simple approach is a promising method to produce enhanced catalytically and mechanically robust monoliths.

Corrosion behavior of 316L stainless steel reinforced by dispersed yttria stabilized zirconia fabricated by spark plasma sintering

This paper evaluates the corrosion behavior of 316L stainless steel (SS316L) matrix composites reinforced with 3% yttria-doped zirconia (3Y·ZrO2), fabricated using the spark plasma sintering. Four different compositions were produced (SS316L, SS316L-5%vol. 3Y.ZrO2, SS316L-10% vol. 3Y.ZrO2, and SS316L-20% vol. 3Y.ZrO2) and primarily assessed for their corrosion resistance evaluated via immersion tests and electrochemical impedance spectroscopy (EIS). Additionally, densification, surface morphology, microstructure and mechanical properties were characterized as complementary. Densification was measured using the Archimedes method, while surface and microstructural characteristics were analyzed using optical and scanning electron microscopy. Mechanical properties were determined through microhardness testing. The results indicate that higher 3Y·ZrO2 content decreases densification and induces porosity, which contributes to localized corrosion. Despite the presence of porosity, microhardness improved with increasing 3Y·ZrO2 content. Additionally, immersion and EIS tests revealed that 5% of 3Y·ZrO2 enhances corrosion resistance. However, in composites with 10% and 20% 3Y·ZrO2, porosity compromises corrosion resistance by degrading passivation. Notably, composites with 5% 3Y·ZrO2 demonstrated significantly better corrosion resistance compared to pure SS316L and the other compositions, highlighting their potential for practical applications.

Improved mechanical properties of 3YSZ ceramics prepared by pressure-less sintering assisted by cold sintering under mild conditions

It is an effective method to prepare ZrO2 ceramics by cold sintering (CS) and the subsequent pressure-less sintering (PLS), while the compacting problems are serious during the CS process with high pressure and high temperature, such as delamination, cracking and sticking to steel mold. To solve these problems, polyacrylic acid (PAA) is utilized as the organic binder, with the aid of which the 3 mol% yttrium-doped zirconia (3YSZ) nanopowder can be well compacted by CS under mild conditions (200 MPa, 100 °C). Compared to the conventional dry-pressed compacts, the particle agglomerations are observed in cold-sintered compacts as well as the slightly increased relative density, which lead to a significant improvement of Vickers Hardness (Hv) by 78 %. Consequently, the CS-PLS ceramics sintered at 1400 °C also exhibit a higher relative density (99.76 %) and better mechanical properties (Hv = 14.8 GPa, fracture toughness KIC = 6.49 MPa m1/2, flexural strength σ = 380.64 MPa) than the PLS ones without cold sintering, indicating that the CS-PLS process with mild conditions for CS is promising for preparing the denser and stronger YSZ ceramics.

The Composition of Graded Dental Zirconias

Zirconia dental ceramics have evolved from uniform blocks of 3 mol.% yttria (3Y) to strength- and color-graded blocks containing 3 mol.% and 5 mol.% components. Relatively little is known about the graded materials’ compositions and microstructures. Concerns have been raised about aging and degradation. This study investigated the microstructure, elemental composition, and phase content of different zones of strength- and color-graded zirconia blocks using scanning electron microscopy, x-ray fluorescence, and x-ray diffraction. Specimens were made from green-state blocks using CAD/CAM machining and sintering. Two strength- and color-graded zirconia materials had different grain sizes, elemental compositions, and phase contents between their top and bottom zones, these data being internally consistent as well as being broadly consistent with prior compositional physical property data. A color-graded zirconia material did not exhibit substantial differences between its top and bottom zones, consistent with expectations and previously published data. Modeling phase content for complex yttria-doped zirconia crystal systems with multiple heterogeneous crystal lattices from XRD data was inherently difficult, which may account for the ranges among previously published data; authors should describe detailed methodologies. Detailed compositional data at the scale of the microstructure is needed to relate composition to phase content, physical behavior, including crack evolution.

Oxygen Self-Diffusion in Fluorite High Entropy Oxides

High-entropy oxides have recently attracted the interest of the community as a way of attuning the properties of oxides to energy applications. Here, we employ molecular dynamics simulations combined with empirical pair potential models to examine the predicted oxygen diffusivity of fluorite-structured high-entropy oxides. We show that lower levels of the dopants increase the overall diffusivity of the composition, but not to the levels of diffusion seen in yttria-doped zirconia. We attribute this to an increased resistance of the cation sublattice to the distortion that occurs through any multiple substitutions on the cation sublattice. To conclude, it is calculated that oxygen self-diffusion in high-entropy oxides is suppressed as compared to isostructural ternary oxides.

Sintering, mechanical properties and hydrothermal resistance of ZrO2/ZrSiO4 slip cast composites

Yttria-doped tetragonal zirconia polycrystalline (Y-TZP) is one of the most widely used materials in thermal applications and as a structural component due to its mechanical properties. Its drawbacks are the high cost of the starting raw materials and its polymorphic transformation to monoclinic phase associated with the occurrence of mechanical failures. In the present work, tetragonal zirconia materials were obtained by slip casting and also ZrO2/ZrSiO4 composites by partially replacing the zirconia with low cost and high corrosion resistance zircon. The different parts showed a high microstructural homogeneity, as well as high hardness and Young's modulus (up to 16.7 and 354 GPa, respectively), even with the incorporation of 20 vol% zircon. The zircon-containing composites subjected to hydrothermal low-temperature degradation (LTD) at 205 °C for 240 h exhibited similar mechanical properties, unaffected microstructural integrity, and a crystallographic phase composition equal to the as-sintered samples, without phase transformation from t-ZrO2 to m-ZrO2.

The Effect of Deposition Temperature on Structural, Morphological, and Dielectric Properties of Yttria-Doped Zirconia Thin Films

The aim of this study was to investigate the effect of deposition temperature on the structural, optical, morphological, and dielectric properties of yttria-stabilised zirconia (YSZ) films prepared by sol-gel spin-coating method. X-ray diffraction (XRD) measurements of YSZ films showed that the peaks of the cubic phase were prominent and the peak intensities increased with deposition temperature. The crystallite size, dislocation density, and microstrain of the thin films were identified by XRD. It was observed that the crystal size of the YSZ thin films increased from 16 nm to 22 nm with the deposition temperature. The surface roughness of the thin films was found to have changed as revealed by Atomic Force Microscopy (AFM) measurements. The roughness increased from 7.72 nm to 11.92 nm with increasing temperature. The optical transmittance of the YSZ thin films was investigated in the wavelength range 200-900 nm and was found to increase slightly with increasing deposition temperature. Metal-Oxide-Semiconductor (MOS) devices were fabricated from these YSZ materials for dielectric characterization. The dielectric properties of the Ag/YSZ/n-Si MOS structure were investigated. It was found that the capacitance, conductivity and other dielectric parameters of these structures are strongly frequency dependent.

Variation of grain boundary dihedral angles caused by voltage-to-current-based flash treatment in fully stabilized yttria-doped zirconia polycrystals

Variation of grain boundary dihedral angles caused by voltage-to-current-based flash treatment in fully stabilized yttria-doped zirconia polycrystals

Understanding oxide ion transport in cation-ordered yttria-stabilized zirconia.

Classical molecular dynamics simulations are carried out on cationically ordered yttria-doped zirconia, YxZr1-xO2-x/2, at the dopant (Y3+) concentration of x = 12.5%. A variety of Zr4+/Y3+ ordered structures are examined for local migration pathways and microscopic energetics governing oxide ion transport in the system. Starting from a layer of cubic Y2O3 spanning the basal plane, the number of Y3+ layers in the simulation cell is multiplied systematically, at the expense of their coverage per layer. The study reveals that cationic ordering in YxZr1-xO2-x/2 can produce a profound impact on the oxide ion transport in the framework, wherein with the maximal dispersion of the dopant a four-fold enhancement in the ionic conductivity is observed relative to the cationically disordered matrix. We demonstrate that this improvement in ion mobility is due to the homogenization of oxide ion vacancies across the matrix. This study thus provides valuable insights for the enhancement of the electrochemical performance of solid oxide fuel cells.

Micromechanical properties of Yttria-doped zirconia ceramics manufactured by direct ink writing

Yttria-doped zirconia ceramics have many applications in a wide range of industries mainly due to their excellent mechanical properties, corrosion resistance and biocompatibility. In this study, micromechanical properties of yttria-doped zirconia produced by Direct-Ink Writing (DIW) were investigated and compared to the ones produced by Cold Isostatic Pressing (CIP). In doing so, mechanical response was assessed at different length scales, from macro- up to submicrometric-, by means of Vickers hardness, nanoindentation, and nanoscratch tests. Microstructure was also characterized by determining grain size, crystal structure and phase tetragonal to monoclinic phase transformation. Results revealed that printed samples displayed 20–25% lower hardness values compared to those exhibited by the respective CIP pairs. Differences in hardness between 3 and 8 mol% yttria content evaluated for CIP samples were slight for printed samples, due to the effect of microstructural defects like porosity, resulting from the processing parameters used. At the local level, such an effect was found to be lower. In this sense, hardness and elastic modulus achieved by nanoindentation were closer, when comparing printed and CIP samples. Scratch tests carried out from 0 to 250 mN revealed that 3 mol% Y2O3 samples developed micro-fracture events in the track length, being the printed samples the ones heavily deformed.

Yttria-zirconia electrochemical sensor for the detection of tyrosine

A simple enzyme-free electrochemical sensor, based on modified yttria-doped zirconium oxide screen-printed carbon electrode (SPCE) has been developed for the determination of tyrosine (Tyr). Firstly, the morphology, microstructure and the electrochemical characteristics of commercial yttria-doped zirconium oxides with different yttria loading (0–40 mol. %) were reported. Cyclic voltammetry (CV) tests performed in phosphate buffer solutions (0.01 M PBS; pH = 7.4) in the absence and the presence of Tyr showed that the modified Zr8Y/SPCE (containing 8 mol. % of yttria) exhibits better electrochemical properties compared to other electrodes investigated. Finally, the Zr8Y/SPCE sensor was tested for the detection of tyrosine by using the differential pulse voltammetry (DPV) method, providing a sensitivity of 0.140 μA μM−1cm−2 and a limit of detection (LOD) of 0.1 μM (at S/N = 3). The performed study shows excellent results for tyrosine detection in a real food supplement sample. The calculated relative standard deviation (RSD) was 1.41 %. To our knowledge, this is the first device developed with a commercial yttria-doped zirconia nanomaterial for the electroanalytical determination of tyrosine.

Investigation of an Impedimetric LaSrMnO3-Au/Y2O3-ZrO2-Al2O3 Composite NOx Sensor.

Composite NOx sensors were fabricated by combining partially and fully stabilized yttria-doped zirconia with alumina forming a composite electrolyte, Y2O3-ZrO2-Al2O3, and strontium-doped lanthanum manganese oxide mixed with gold to form the composite sensing electrode, La0.8 Sr0.2MnO3-Au. A surface chemistry analysis of the composite sensor was conducted to interpret defects and the structural phases present at the Y2O3-ZrO2-Al2O3 electrolyte, as well as the charge conduction mechanism at the LaSrMnO3-Au electrode surface. Based on the surface chemistry analysis, ionic and electronic transport properties, and microstructural features of sensor components, the working principle was described for NOx sensing at the composite sensor. The role of the composite materials on the NOx sensing response, cross-sensitivity to O2, H2O, CO, CO2, and CH4, and the response/recovery rates relative to sensor accuracy were characterized by operating the composite sensors via the impedimetric method. The composite sensors were operated at temperatures ranging from 575 to 675 °C in dry and humidified gas environments with NO and NO2 concentrations varying from 0 to 100 ppm, where the balance gas was N2. It was found that the microstructure of the composite NOx sensor electrolyte and sensing electrode had a significant effect on interfacial reactions at the triple phase boundary, as well as the density of active sites for oxygen reactions. Overall, the composite NOx sensor microstructure enabled a high NOx sensing response, along with low cross-sensitivity to O2, CO, CO2, and CH4, and promoted NO detection down to 2 ppm.

Investigation of Structural and Catalytic Performance of Nano Crystalline Yttria Dopped Zirconia

Nanocrystalline yttria doped zirconia (YZ) were prepared by sol-gel method and characterized by XRD, SEM, EDX, TGA and XPS techniques. The XRD results indicated the formation highly crystalline material with 2 to 4 nm in size. The tetragonal phase of the nanoparticles was confirmed by XRD analysis. Small amount of this catalysts was efficiently catalyzed acetylation of benzyl alcohol with acetic anhydride in short time under solvent free conditions. The 5 wt.% YZ gave best results at 120 ºC with 100% conversion of benzyl alcohol with 100% selectivity of the product. Different parameters were studied for optimization of acetylation, which proved the catalysts has excellent activity and can be used at large scale applications in industry.

Surface Effect of Nano-Roughened Yttria-Doped Zirconia on Salivary Protein Adhesion.

Biocompatibility of yttria (3 mol%) stabilized zirconia ceramics, 3Y-TZP, was affected to a large degree as a result of protein adsorption from human saliva that in turn depends on materials surface properties. Variable nano-roughness levels in 3Y-TZP discs were characterized and tested for specificity and selectivity with respect to size and uptake for human salivary protein.

An overview of thermotransport in fluorite-related ionic oxides

Abstract In this overview, we summarize the phenomenon of thermotransport (the close coupling of mass transport and heat transport) in two fast ion conductors: yttria-doped zirconia and gadolinia-doped ceria. We focus on two recent molecular dynamics calculations using the Green-Kubo formalism. We show that the Onsager thermotransport cross coefficient (mass-heat coupling) is negative, meaning that oxygen ions would drift, in principle, to the hot side in a temperature gradient. Simulation results presented in this overview show reasonable agreement with available experimental data for thermal conductivity. Results of this study suggest that the coupling between mass and heat transport in oxygen ion electrolytes could have significant effect for practical applications.

Surface segregation of 3 mol % yttria-doped tetragonal zirconia particle studied by atomic-resolution scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy

Atomic structure of the surfaces in commercial 3 mol % yttria-doped tetragonal zirconia (TZP) nano-particulate powders were studied by atomic resolution scanning transmission electron microscopy (STEM), and the surface segregation of Y were investigated by STEM energy-dispersive X-ray spectroscopy (EDS) at atomic scale. STEM-EDS observations revealed that the Y cations tend to segregate on the top surface layer, forming a monolayer segregation structure. Static lattice calculations revealed that the surface segregation of Y is energetically favorable. These results indicate that Y surface segregation occurs in the TZP powders, which might partially account for the grain boundary segregation in the sintered body of TZP ceramics.

Controlled sintering and phase transformation of yttria-doped tetragonal zirconia polycrystal material

In this paper, 3 mol% yttria-doped tetragonal zirconia polycrystal material (3 mol% Y2O3–ZrO2) was prepared using an optimised pressureless sintering process. The phase change and particle size distribution of Y2O3–ZrO2 during sintering were studied, and the effect of sintering temperature on the properties of Y2O3–ZrO2 was analysed. The raw materials and prepared samples were analysed using XRD, Raman spectroscopy, SEM, and Gaussian mathematical fitting. The results show that sintering encourages the transformation of the monoclinic phase into the tetragonal phase, thus improving the crystallinity of the sample. The relative content of the tetragonal phase in the sample increased from 57.43% to 99.80% after sintering at 1200 °C for 1 h. In the range of sintering temperatures studied in this paper (800–1200 °C), the zirconia material sintered at 1000 °C presented the lowest porosity and the best density.

Intraoral low-temperature degradation of monolithic zirconia dental prostheses: Results of a prospective clinical study with ex vivo monitoring

ObjectiveTo investigate the intraoral development and kinetics of low-temperature degradation (LTD) in second-generation 3 mol.% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) monolithic prostheses, as well as the influence of masticatory mechanical stress and glaze layer on it. MethodsA total of 101 posterior tooth elements were included in a prospective clinical study, which included ex vivo LTD monitoring (at baseline, 6 months, 1 year, and 2 years) using Raman spectroscopy (n = 2640 monoclinic phase measurement points per evaluation time) and SEM. Four types of areas (1–2 mm2 surface, 6 on molars, and 4 on premolars) were analyzed on each element surface: occlusal, axial, glazed, or unglazed. Raman depth mapping and high-resolution SEM were performed on the selected samples. ResultsLTD developed in 3Y-TZP monolithic restorations 6 months after intraoral placement and progressed with time. After two years, the tetragonal-to-monoclinic transformation was non-uniform, with the presence of localized clusters of transformed grains. In axial areas, the grain aspect was typical of the classical nucleation-growth process reported for LTD, which progresses from the surface to a depth of several tens of microns. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, which induced an underestimation of the aging process when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD. SignificanceIf LTD occurs in dental prostheses in the same way as in orthopedic prostheses, its clinical impact is unknown and needs to be further studied.

Grain-size effect on cracking accumulation in yttria-doped zirconia ceramics during cyclic martensitic transformations

Bulk polycrystalline zirconia-based ceramics generally crack as they undergo martensitic transformations, largely due to the transformation mismatch stresses. Polycrystals subjected to cyclic transformations bodily lose individual grains and progressively comminute, but it is not yet clear how the grain size of a polycrystal affects such cyclic degradation. We explore this issue in 1.5 mol% yttria-doped zirconia by varying the grain size from 0.6 to 7.9 µm in pellets of fixed composition and sample size and subjecting them to multiple thermal cycles through the transformation. A smaller grain size is found to increase the number of cycles required to disaggregate the pellet because of the larger amount of grain boundary area that must crack. Calorimetry analysis shows that the energy relieved through cracking decreases with increasing grain size and suggests an apparent material length scale of ~2 micrometers for the stress relief zone. For grain diameters below this critical length scale, complete stress relief is suggested, while at larger grain sizes, intergranular cracking apparently does not fully relieve the transformation mismatch stresses. Alternate accommodation mechanisms are required including the formation of multiple variants and even some transgranular fracture.

Direct Electric-Field Induced Phase Transformation in Paraelectric Zirconia via Electrical Susceptibility Mismatch.

Electric field driven phase transformations require two phases with a mismatch in their electric polarization, as seen in antiferroelectric-to-ferroelectric transformations, where the ferroelectric phase has a permanent polarization that is favored under field. Many other nonferroelectric dielectric materials can become electrically polarized according to their electrical susceptibility, yet such induced polarizations are not generally considered capable of enabling a phase transformation. Here we explore a susceptibility-mismatch phase transformation in a paraelectric ceramic, yttria-doped zirconia. Using insitu x-ray diffraction at 550 °C we show that the monoclinic-to-tetragonal transformation can be driven directly by an electric field, providing experimental evidence of a paraelectric-to-paraelectric phase transformation. Considering the ∼1% mechanical strain of this transformation, the resulting electromechanical coupling may have potential for solid-state electrical actuators.