A Stabilization Mechanism of Zirconia Based on Oxygen Vacancies Only

The microscopic mechanism leading to stabilization of cubic and tetragonal forms of zirconia (ZrO$_2$) is analyzed by means of a self-consistent tight-binding model. Using this model, energies and structures of zirconia containing different vacancy concentrations are calculated, equivalent in concentration to the charge compensating vacancies associated with dissolved yttria (Y$_2$O$_3$) in the tetragonal and cubic phase fields (3.2 and 14.4% mol respectively). The model is shown to predict the large relaxations around an oxygen vacancy, and the clustering of vacancies along the $$ directions, in good agreement with experiments and first principles calculations. The vacancies alone are shown to explain the stabilization of cubic zirconia, and the mechanism is analyzed.Comment: 19 pages, 6 figures. To be published in J. Am. Ceram. So

Magnetic tight-binding and the iron-chromium enthalpy anomaly

We describe a self consistent magnetic tight-binding theory based in an expansion of the Hohenberg-Kohn density functional to second order, about a non spin polarised reference density. We show how a first order expansion about a density having a trial input magnetic moment leads to the Stoner--Slater rigid band model. We employ a simple set of tight-binding parameters that accurately describes electronic structure and energetics, and show these to be transferable between first row transition metals and their alloys. We make a number of calculations of the electronic structure of dilute Cr impurities in Fe which we compare with results using the local spin density approximation. The rigid band model provides a powerful means for interpreting complex magnetic configurations in alloys; using this approach we are able to advance a simple and readily understood explanation for the observed anomaly in the enthalpy of mixing.Comment: Submitted to Phys Rev

Theory and simulation of ultra-high-temperature ceramics

At Imperial College our group contributes theory and simulation advances to the Materials for Extreme Environments (XMat) project. Our research supports experiment and industry by developing and applying new high-temperature modelling techniques. These techniques are broad-ranging, from CALPHAD and DFT, to interatomic potentials and analytic models. Here we present advances on each approach and re-cover highlights including: - the release of MEAMfit, the interatomic potential fitting code - the development of the TU-TILD approach, for fast and full-order anharmonic thermodynamics [1] - a new first-principles-assisted CALPHAD assessment of ZrC - analytic models of strain and anharmonicity in carbides and borides - ab initio prediction of intrinsic defects at ultra-high temperatures - first principles heat and charge transport predictions for carbides Further, we summarise ongoing developments from the theory and simulation group, such as on first principles MAX phase thermodynamics Please click Additional Files below to see the full abstract

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

Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia

The high-temperature cubic-tetragonal phase transition of pure stoichiometric zirconia is studied by molecular dynamics (MD) simulations and within the framework of the Landau theory of phase transformations. The interatomic forces are calculated using an empirical, self-consistent, orthogonal tight-binding (SC-TB) model, which includes atomic polarizabilities up to the quadrupolar level. A first set of standard MD calculations shows that, on increasing temperature, one particular vibrational frequency softens. The temperature evolution of the free energy surfaces around the phase transition is then studied with a second set of calculations. These combine the thermodynamic integration technique with constrained MD simulations. The results seem to support the thesis of a second-order phase transition but with unusual, very anharmonic behaviour above the transition temperature

Flash spark plasma sintering of UHTCs

During the five year XMat research project supported by EPSRC (Engineering and Physical Sciences Research Council, UK) at Queen Mary we developed a novel sintering technique called Flash Spark Plasma Sintering (FSPS[1]) which is particularly suitable for the ultrarapid (a few seconds) consolidation of UHTCs. As in the case of incandescent lamps, flash sintering techniques use localized Joule heating developed within the consolidating particles using typically a die-less configuration. Heating rates are extreme (104–106 °C/min), and the sintering temperature is therefore reached extremely rapidly. The research covered mostly metallic conductors (ZrB2[2], HfB2,TiB2) and semiconductors (B4C, SiC and their composites). The talk will summarize the joint XMat team efforts to: -Identify the FSPS consolidation mechanism using modelling and transmission electron microscopy, -Characterise the structural properties for the bulk materials and redefine the structure-property relationships of FSPSed materials Please click Additional Files below to see the full abstract