The mechanism of ionic conductivity in stabilized cubic zirconia

The electron-density functional method (in the gradient approximation) and the pseudopotential method are used to study the mechanism of ionic conductivity in the cubic phase of zirconia stabilized with magnesium or yttrium. The oxygen-ion migration in the stabilized zirconia is shown to be a two-stage process, which consists in the formation of active oxygen vacancies and in oxygen-ion jumps from one active vacancy to another. The total activation energy of these processes is calculated to be 1.0–1.5 eV, which agrees with experimental data.     

Quasi-localized representation of electronic band structures in cubic crystals

Tabulated results and formulae are given for the parametrization of electronic band structures in simple cubic, body-centered cubic and face-centred cubic systems. This technique is based on the decoupling transformation and does not involve least squares fitting.A band labelling scheme is introduced which allows the decoupling transformation to be used to define the energy matrix of Wannier states in copper. This approach is used to re-examine the significance of the tight binding approximation for the d-states and to relate Fermi surface parameters directly to calculated band energies.     

Stability of cubic zirconia and of stoichiometric zirconia nanoparticles

Using the electron density functional method, it is shown that the oxygen sublattice of cubic zirconia is unstable with respect to random displacements of oxygen atoms, which results in general instability of bulk cubic zirconia at low temperatures. A comparison of the equilibrium atomic structures and total energies of stoichiometric ZrO₂ nanoparticles about 1 nm in size shows that particles with cubic symmetry are more stable than those with rhombic (close-to-tetragonal) symmetry. The electronic structure of nanoparticles exhibits an energy gap at the Fermi level; however, this gap (depending on the symmetry and size of the particle) can be much narrower than the energy gap of the bulk material.     

Theoretical investigation on electronic structure and mechanical properties of cubic crystallographic structures with point defects in Al-based alloys

Electronic structure and mechanical properties of cubic crystallographic structures with point defects in Al-based alloys are investigated using the first-principles calculations. Equilibrium structural parameters and mechanical parameters such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio and anisotropy are calculated and agreed well with experimental values. Effects of point defects on the electronic structures and mechanical properties of such cubic phases are further analyzed and discussed in view of the charge density and the density of states.     

Kinetics of densification in stabilized zirconia

Densification mechanisms during the sintering of CaO-stabilized ZrO₂ have been studied using the technique of hot pressing. The dependence of the densification rate on applied pressure has been found to be linear suggesting that the kinetics are controlled by a diffusion process. The densification rate undergoes a maximum value at a composition of approximately 12 m/o CaO; this is close to the value at which the ionic conductivity maximum also occurs. Since this composition coincides with a phase boundary in this system, parallel experiments have been carried out using the CeO₂/Y₂O₃ system in which no corresponding phase boundary occurs. The similar dopant dependence for the conduction and densification processes in these oxides suggests that the fast carrier (responsible for conduction) and the slow carrier (responsible for densification) depend in a similar way on the additive content. This in turn suggests that densification is controlled by the diffusion of cation interstitials and further that the maximum is caused by increasing degrees of defect association at high dopant contents.     

Regularities of formation of cubic zirconia stabilized with calcium,yttrium, and indium oxides under mechanochemical treatment

An effect of the composition of reacting components during mechanochemical activation on the ZrO₂ structure stabilized with additives of In₂O₃, CaO, and Y₂O₃ oxides has been revealed. It is established that mechanochemical activation of oxide mixtures leads to the formation of dispersed solid solutions based on cubic zirconia. It is shown that interaction depth of the components during mechanochemical activation increases in the series of oxides In₂O₃ < CaO < Y₂O₃.     

Structure and mechanical properties of crystals of partially stabilized zirconia after thermal treatment

The phase composition and morphology of the twin structure of the Y₂O₃-stabilized zirconia crystals (from 2.8 to 4.0 mol %) after the thermal treatment at 1600°C have been investigated by X-ray diffractometry and transmission electron microscopy. It is shown that as the concentration of the stabilizing Y₂O₃ impurity increases, the character of the twin structure changes, and the amount of the untransformed phase t′ increases. The dependence of the hardness and crack resistance of the crystals of partially stabilized zirconia on the Y₂O₃ concentration and the indenter orientation is investigated using the microindentation method. The sample with the lowest concentration of the stabilizing Y₂O₃ impurity turned out the most crack resistant. This can be explained by a high content of tetragonal phase t in it, which provides the transformation strengthening mechanism of the material, and by a more multilevel character of twinning.     

Grain boundary crystallography in polycrystalline yttria-stabilised cubic zirconia

Properties of grain boundaries such as grain boundary energy, mobility and diffusion are reported to depend strongly on their crystallography. While studies on ceramic bicrystals with low Σ misorientations have shown highly ordered structures and low energies, studies on dense polycrystalline ceramics often show the significance of grain boundary planes. In the present study, grain boundary plane distributions were studied for yttria-stabilised cubic zirconia with varying grain sizes using Electron Back Scattered Diffraction technique combined with a stereological approach. Despite nearly isotropic grain boundary plane distributions, a highly anisotropic grain boundary character distribution is observed for specific misorientations. Certain low-energy symmetric tilts such as Σ3 and Σ11 are found to occur with high frequencies across the grain size range studied, leading to an inverse correlation between GB energy and frequency of occurrence, consistent with other ceramics studied in literature.     

Elastic properties and electronic structures of lanthanide hexaborides

The structural, elastic, and electronic properties of a series of lanthanide hexaborides(Ln B6) have been investigated by performing ab initio calculations based on the density functional theory using the Vienna ab initio simulation package.The calculated lattice and elastic constants of Ln B6 are in good agreement with the available experimental data and other theoretical results. The polycrystalline Young's modulus, shear modulus, the ratio of bulk to shear modulus B/G, Poisson's ratios, Zener anisotropy factors, as well as the Debye temperature are calculated, and all of the properties display some regularity with increasing atomic number of lanthanide atoms, whereas anomalies are observed for Eu B6 and Yb B6. In addition, detailed electronic structure calculations are carried out to shed light on the peculiar elastic properties of Ln B6.The total density of states demonstrates the existence of a pseudogap and indicates lower structure stability of Eu B6 and Yb B6 compared with others.     

Visual observation of chemical diffusion in stabilized zirconia

Strongly reduced crystals of stabilized zirconia are black and completely opaque. These samples may be reoxidized at T200° C in air. We present results of the reoxidation process and try to explain the observed behavior through annihilation of color centers with an energy level at E>–1.60 eV. The difficulties with this model are pointed out.     

Ab-initio study of electronic and optical properties of InN in wurtzite and cubic phases

We have performed systematic first principle calculations for the electronic and optical properties of a narrow band gap semiconductor InN in cubic and wurtzite phases by ‘state-of-the-art’ DFT calculations within generalized gradient approximation (GGA) and Engel-Vosko's corrected generalized gradient approximation (EVGGA) using full potential linear augmented plane wave (FPLAPW) method as implemented in WIEN2k code. The total energy for the wurtzite phase of InN was found to be smaller by 0.0184 Ry/molecule by cubic phase which confirms the greater stability of the wurtzite structure than the cubic one. Band structure, effective masses, density of states, valence charge densities, and dielectric functions are computed and presented in detail. The critical points are extracted out of calculated dielectric function, compared with available measured data and are explained in terms of transitions occurred in the band structure along different symmetry and antisymmetry lines. The valence band maxima and conduction band minima are strongly dominated by N-2p states and located at the Γ-symmetrical line which predicts its direct band gap nature in both phases.     

XeCl laser ablation of yttria stabilized zirconia

XeCl laser ablation of yttria stabilized zirconia (YSZ) in air and in vacuum (1.3×10^–4 Pa) is studied by means of etch depth measurements, scanning electron microscopy, and X-ray photoelectron spectroscopy of ablated surfaces. Results on ablation rate, surface morphology, and surface chemical composition are discussed in terms of the influence of ambient atmosphere on the ablation process, rapid melting and solidification of ablated surfaces, and preferential removal of oxygen atoms from the YSZ surface.     

Characterization of ceria-yttria stabilized zirconia plasma-sprayed coatings

Ceria-yttria stabilized zirconia (CYSZ) coatings were prepared by air plasma-sprayed on the nickel alloy. The as-sprayed CYSZ coatings and heat-treated CYSZ coatings were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The XPS data indicated the coexistence of Ce³⁺, Ce⁴⁺, Y³⁺ and Zr⁴⁺ ions near the surface of the as-sprayed CYSZ coatings and the disappearance of Ce³⁺ ions in the CYSZ coatings after thermal treatment at 1000 °C for 15 h. From the XRD patterns, the solid solution of CeO₂-Y₂O₃-ZrO₂ formed in the CYSZ coatings because of the lack of any features from Y₂O₃ and ZrO₂ single phases. After thermal treatment, the main phases of all the samples were consistent with the characteristic peaks of cubic ZrO₂.     

The defect structure of calcia stabilized zirconia

Theoretical techniques have been used to examine the two models proposed for the defect structure of stabilized zirconias. The first model is based on simple clusters of dopant ions and the charge compensating oxygen vacancies, while the second suggests that fluorite-related microdomains form within the host lattice.

We show that the energies of formation of the clusters and microdomains are almost equal, the microdomain being favoured by 0.09 eV per dopant ion. Thus the calculations suggest that both point defects and microdomains may be present, with point defects predominating at low dopant concentration and higher temperatures.     

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.     

Investigation of the vibrational properties of cubic yttria-stabilized zirconia: A combined experimental and theoretical study

A combined experimental and theoretical investigation into the vibrational properties of cubic 8–9 mol% yttria-stabilized zirconia (YSZ) is presented. Measurements of acoustic phonon dispersion curves have been obtained from inelastic neutron scattering investigations using a triple axis spectrometer, as well as calculations of the vibrational density-of-states (vDOS) using density-functional theory. The present measurements agree closely with, and extend, previously published results. The phonons become broader and decrease in intensity as the Brillouin zone boundary is approached, particularly in the Γ–Δ–X direction. Interestingly, there is evidence of a previously unreported low energy phonon band (8–9 meV) in the Γ–Σ–X direction, which could possibly be related to the stabilization (by yttria doping) of the imaginary mode of cubic ZrO₂ about the X-point. Compared to pure cubic ZrO₂, the vDOS of YSZ are broader and extend to higher frequency. Furthermore, the prominent Zr-related feature in the vDOS of c-ZrO₂ at ≈14 meV is shifted to higher energy in the vDOS of YSZ. This behavior is consistent with the measured dispersion bands (first acoustic branch in the Γ–X direction, about the X-point) of YSZ which is higher in energy by a similar amount relative to that of c-ZrO₂, thus providing support for the structural model considered.