Yttria-stabilized Zirconias Research Articles

Instantiations of Multiscale Kinship in Pressing‐Defect Distributions in Yttria‐Stabilized Zirconias by Powder Partitioning

Modern dry pressing of ceramic powders using spray‐dried granulates cannot avoid the occurrence of defects related to persisting inter‐ and intra‐granulate interstitial voids. These constitute the parent defect size population limiting the application of polycrystalline ceramics in high‐stress conditions. The mitigation of such defects could widen the range of application in technical and biomedical engineering, reduce the safety range for design, and extend the lifetime of components. Herein, the Weibull size‐effect on strength in size‐partitioned Yttria‐stabilized zirconias (YSZ) feedstocks is used to explore the viability of changing the density distribution of granulate sizes as an effective strategy to obtain a denser particle packing that could reduce the size distribution of strength‐limiting pressing defects. In a direct assessment of critical defect size using multiscale strength testing with a dataset of ≈1300 values, the success of such an approach in increasing the strength reliability for small volume components is demonstrated, along with its ultimate failure in altering the defect size distribution in sintered YSZ ceramics across several length scales. Finally, it is shown that granule morphology (spherical or dimpled) fails to affect the defect density and size distribution in YSZ ceramics.

Damage tolerance of six dental zirconias with different translucencies

Purpose High-translucent dental zirconia has been introduced as a suitable material for anterior monolithic restorations. The material composition differs from traditional 3Y-TZP both with regard to yttria content and grain size. Little is known regarding how these alterations affect other properties than translucency and flexural strength. The aim of this study was to evaluate the crack propagation resistance and hardness of dental zirconias with different yttria content and different manufacturing methods. Materials and methods Measurement of hardness (HV2/5) and crack propagation from the indents (damage tolerance) was performed using a hardness tester(Vicker) on a flat polished surface of five crowns from six different commercial dental zirconias; one hard-machined 3Y-TZP, three soft-machined 3-5% yttria-stabilized zirconias and two soft-machined zirconias with ≥5% yttria content. Results Damage control varied greatly among dental zirconias with different compositions and fabrication methods. The hard-machined 3Y-TZP had better crack propagation resistance than soft-machined, 3-5% yttria-stabilized zirconias Conslusion The ultra-translucent zirconias with ≥5% yttria content had the lowest crack propagation resistance. Hardness is not a suitable indicator for damage tolerance.

Structural, mechanical, thermal and optical properties of Yb, Pr-doped Y4Zr3O12 ceramics

In the present work, Y4Zr3O12 bulk ceramics doped with Yb3+ and Pr3+ were characterized to assess their structural, mechanical, thermal and optical properties. Specimens were synthesized by the polymerizable complex method and sintered as pellets at 1550 °C for 2 h. Samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers-indentation method, thermodilatometry, laser flash method, diffuse reflectance and luminescence spectroscopies. All compositions exhibit cubic like-fluorite structure, low thermal conductivity, moderate fracture toughness and thermal expansion coefficient values comparable to those of other yttria-stabilized zirconias. Regarding the optical properties, diffuse reflectance spectra support the oxidation of praseodymium from Pr3+ to Pr4+ in the Pr-doped specimens. When irradiated under an IR laser (975 nm), only the pellet containing Yb3+ showed white light emission at low and room pressures, but not the undoped, and those pellets doped or codoped with Pr. These results demonstrate that it is possible to produce white light not only in nanostructured systems but also in bulk ceramic oxides, and that the introduction of additional electronic states within the band gap of the host associated with the Pr, inhibits the generation of the white light.

“Oxygen quenching” in Eu-based thermographic phosphors: Mechanism and potential application in oxygen/pressure sensing

The effect of ambient oxygen concentration on thermographic phosphor thermometry was investigated to uncover the “oxygen quenching” mechanism of Eu-based phosphors using a lifetime-based measurement system. The phosphors being studied were the Eu doped yttria stabilized zirconias (YSZs) with Y3+ concentrations of 0, 8, 12 and 57mol%, having monoclinic, tetragonal, cubic and δ crystalline lattice, respectively. To vary the ambient oxygen concentration, three different gas phases (air, oxygen and nitrogen with methane) were used. It was found that the phosphorescent lifetimes and intensities of all four phosphors were sensitive to the ambient oxygen concentration, while the reference phosphor, Y2SiO5:Eu, was not influenced. The “oxygen quenching” phenomenon observed in Eu-doped YSZs was attributed to the oxygen vacancies in the phosphors. In addition, the four YSZ:Eu phosphors exhibited different sensitivity to ambient oxygen concentration. The oxygen sensitivity was found to be closely related to the site symmetry of Eu3+, which was previously determined by the phosphorescent spectra. The site symmetry of Eu3+ strongly affected the charge-transfer states and therefore determined the non-radiative decay rate. The current findings provided guidance for the phosphor selection and sensor design for oxygen/pressure sensing in high-temperature environment.

Fabrication of Porous Yttria-Stabilized Zirconias Controlled by Additives

To fabricate a thick, porous yttria-stabilized zirconia without cracking and warping, a method for the simultaneous control of the porosity and shrinkage was designed. As a pore former, a potato starch was used. For the control of shrinkage, the oxidation of Al metal particles was used. For the sintering of the above powder mixtures, a partial sintering technique was used at 1300℃ for 10 min in air. Upon adding the additives, high open porosity above 53% and a low shrinkage level were obtained. As a result, cracking and warping of the sintered body were deterred. This outcome most likely resulted from the compensation of sintering shrinkage due to the volume expansion caused by oxidation of the Al metal particles during heat-treatment.

Local order and thermal conductivity in yttria-stabilized zirconia. II. Numerical and experimental investigations of thermal conductivity

The thermal conductivity of yttria-stabilized zirconias has been determined numerically, by applying a nonequilibrium molecular dynamic calculation. Comparing random configurations and Monte Carlo configurations in which chemical correlations are taken into account, we have established that the short range order developing in yttria-stabilized zirconia gives rise to an increase in lattice thermal conductivity for high dopant concentrations and temperatures below 800 K. At higher temperatures or for lower concentrations, the short range order does not influence the thermal conductivity. This conclusion is consistent with experiment. The results are discussed in relation with the various phonon scattering mechanisms.

Thermocyclic Behavior of Variously Stabilized EB-PVD Thermal Barrier Coatings

The demand for increasing gas inlet temperatures in modern gas turbines up to 1500°C and above is the main reason for the need for more reliable thermal barrier coatings. New ceramics should provide higher phase stability and better resistance against chemical attack by pollutants in the combustion gas. Electron-beam physical vapor deposition (EB-PVD) processed, ZrO2-based TBCs were generated on bond-coated superalloy directionally solidified (DS) samples. Common yttria-stabilized zirconias of two different compositions, as well as novel stabilizers like CeO2 and La2O3, were investigated. A columnar structure was established during high-rate deposition in all cases. Diameter, degree of ordering of the columns, and phase composition depended on stabilizer oxide and content. The role of differences of vapor pressures is addressed with regard to chemical homogeneity of the coatings. The performance of the TBCs having various stabilizers was investigated in a cyclic oxidation furnace test and in a burner rig at Mach 0.3. The results were correlated to the type and content of stabilizer with special emphasis on phase analyses. Evaporation of new ceramic compositions necessitates special precautions because the vapor pressures of the components may differ too much. A new dual-source evaporation coater allows the production of these innovative TBCs with close control of chemistry. The potential of the equipment will be discussed.

Oxide-ion conductivity of calcia- and yttria-stabilized zirconias prepared by a rapid-combustion route

Calcia- and yttria-stabilized zirconias have been synthesised by a novel rapid-combustion route. The oxide-ion conductivities of these stabilized zirconias are comparable to these of commercially prepared samples. Variations in grain boundary impedance of calcia- and yttria-stabilized zirconias with composition and temperature are presented.

Influence of copper- and iron-doping on cubic yttria-stabilized zirconia

Cubic yttria-stabilized zirconias (YSZs) (15 mol% Y2O3) doped with 2 wt% CuO and 0.3 wt% Fe2O3, respectively, have been compared with undoped YSZ. The lattice constants were found to increase in the sequence:aYSZ <aYSZ/Fe <aYSZ/cu. Vickers microhardness of polycrystalline YSZ/Fe exceeds that of polycrystalline YSZ/Cu, whereas the hardness of the single crystalline materials YSZ/Fe and YSZ/Cu are nearly identical and consistently lower than the polycrystalline values. Raman and infrared spectra reveal a breakdown of the selection rules, i.e. these techniques probe the local, non-cubic arrangement of oxygen vacancies. The findings are discussed in terms of a substitutional versus an interstitial-doping model.

A cluster-modeling study of the structure of yttria-stabilized zirconias

The Zr and Y coordination polyhedra, as found in the room-temperature phases of their pure oxides, are used as building blocks in modeling the structures of yttria-stabilized zirconias (YSZ). It is found that, in order to obtain the right coordination geometry, three clusters of four Zr- and Y-based building blocks can be constructed. When Y building blocks are present in a cluster they form a couple, characterized by a common structural vacancy, a neutral defect which can be defined when an oxygen is missing with respect to the case of the Zr seven coordination in monoclinic zirconia. It is shown that this model fairly well justifies the experimental results obtained by others. In conclusion, this kind of cluster modeling is proposed as a general method for solving structural problems in other mixed oxides.

Site selective spectroscopy and structural analysis of yttria-doped zirconias

Yttria-stabilized zirconias Zr 1− x Y x O 2−0.5 x (tetragonal, cubic fluorite, and cubic C Ln 2O 3-type forms) are investigated by spectroscopic methods. Optically active europium ions that partially substitute yttrium ions act as a structural microprobe. Both conventional ultraviolet and dye laser (site-selective) excitation are used. The yttria-stabilized zirconias may be well described by a cationic sublattice in which tetravalent and trivalent cations are statistically distributed. The anionic sublattice accommodates vacancies at random on oxygen positions. These vacancies in turn cause complex oxygen displacements giving rise to what may be described as a “glass of anions.”

181Ta gamma-gamma angular correlation study of oxygen self-diffusion in heavily defective oxides

The results of gamma-gamma angular correlation experiments performed on the 181Ta nucleus in calcia-and yttria-stabilized zirconias and in calcia-doped cerias between 20–1150°C are reported. At high temperatures, the angular correlation spectra indicate a relaxation of the 181Ta spins induced by the diffusive motions of oxygen vacancies. The slow and fast relaxation regimes are successively observed when the temperature is raised. The behaviour of the relaxation constant is fairly well reproduced by assuming the existence of fluctuations in the potential energy barrier which controls the migration of oxygen vacancies. For stabilized zirconias the mean activation energies deduced from angular correlation experiments agree with the conductivity data. The disagreement observed in doped cerias is attributed to impurity effects produced by the 181Ta probe itself. The local investigation of oxygen self-diffusion give evidence for the homogeneous behaviour of these highly defective oxides at a microscopic level. The 181Ta relaxation data, especially the comparison of the respective data relative to aged and deaged calcia-stabilized zirconia, strongly suggest the existence of short-range ordering phenomena in these solid solutions.

Statistical aspects of gross anion sub-stoichiometry in fluorite-related systems

The current theory as to the nature of grossly sub-stoichiometric fluorite-related systems, of which the well known lime- and yttria-stabilized zirconias form an important segment, assumes the presence of randomly distributed vacancies in the anion lattice. Some necessary consequences of a completely random arrangement of vacancies in sub-stoichiometric fluorite-related systems can be obtained from statistical methods. The results show that the postulated random arrangement would entail cations having energetically very unstable environments, and indicate that a degree of ordering of the vacancies must occur in physical systems. Some simple models involving partially or completely ordered vacancies are analysed and discussed.