Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

Strength Analysis of Alternative Airframe Layouts of Regional Aircraft on the Basis of Automated Parametrical Models

This publication presents the results of complex parametrical strength investigations of typical wings for regional aircrafts obtained by means of the new version of the four-level algorithm (FLA) with the modified module responsible for the analysis of aerodynamic loading. This version of FLA, as well as a base one, is focused on significant decreasing time and labor input of a complex strength analysis of airframes by using simultaneously different principles of decomposition. The base version includes four-level decomposition of airframe and decomposition of strength tasks. The new one realizes additional decomposition of alternative variants of load cases during the process of determination of critical load cases. Such an algorithm is very suitable for strength analysis and designing airframes of regional aircrafts having a wide range of aerodynamic concepts. Results of validation of the new version of FLA for a high-aspect-ratio wing obtained in this work confirmed high performance of the algorithm in decreasing time and labor input of strength analysis of airframes at the preliminary stages of designing. During parametrical design investigation, some interesting results for strut-braced wings having high aspect ratios were obtained

Optical properties of tellurite glasses elaborated within the TeO2-Tl2O-Ag2O and TeO2-ZnO-Ag2O ternary systems

The optical properties of glasses elaborated within the TeO2-Tl2O-Ag2O and TeO2-ZnO-Ag2O ternary systems are presented. The paper is articulated in two parts. The first part concerns the TeO2-Tl2O-Ag2O system, where the linear refractive index, the absorption coefficient and the optical band gap are measured as a function of the Ag2O content. From ellipsometry measurements, it is noticed that the linear refractive index n0 remains constant up to 5 mol% in AgO0.5 and then slightly increases at higher concentrations (10 mol%). Such evolution of the refractive index was related to the higher electronic polarizability of Ag+ cations compared to that of Te4+ cations, assuming that the electronic polarizability of O2 anions remains constant. The reduction in the optical band gap value is attributed for some part to the higher quantity of non-bridging oxygen atoms induced by the depolymerization of the glass. The third order non-linear optical properties (c and Re(v(3)) values) of these TeO2-Tl2O-Ag2O glasses are then extracted from Z-scan measurements. For glass compositions ranging from 0 to 10 mol% in AgO0.5, the Re(v(3)) value shifts from roughly 60 to more than 80 times that of silica, which testify to the excellent non-linear optical properties of such TeO2-Tl2O glasses containing silver (tellurite glasses possess intrinsic high optical non-linearity). The addition of Tl2O induces the depolymerization of the glass framework but, on the other hand, helps to maintain the amplitude of the optical non-linearity, through the presence of highly polarizable Tl-based entities. Here, below 5 mol% in AgO0.5, it is believed that the activity of the electronic lone pair of Tl+ ions is inhibited. Over this limit, such lone pair would become active, explaining the clear increase of the Re(v(3)) value. Finally, the second part of the paper is devoted to the study of photoluminescence properties of new TeO2-ZnO-Ag2O glasses. Indeed, probably due to the creation of Ag+-Tl+ pairs, there is absolutely no photoluminescence detected for glasses elaborated in the TeO2-Tl2O-Ag2O system. At the opposite, the temperature dependent photoluminescence properties of TeO2-ZnO-Ag2O glasses are reported. Under UV-visible excitation at 385 nm, the samples emit orange light, with a broad emission wavelength peaking around 580-585 nm. The nature of the light emitting centers is discussed and two main hypotheses are debated: the creation of Ag+-Ag+ or Ag+-Zn2+ pairs, with stronger arguments more in favor of the last one