Advances in Science and Technology Vol. 45

Abstract: TiO2 has been intensively studied as a wide band-gap transition metal oxide partially due to the multi-valence nature of its cation. Here, density-functional theory calculations within the supercell approximation are carried out to determine the preferred charge state of point defects in rutile TiO2. The first component of this work is to investigate the dependence of the defect formation energies on supercell size and the electrostatic Makov-Payne correction. The results show that the Makov-Payne correction improves the convergence of defect formation energies as a function of supercell size for positively charged titanium interstitials and negatively charged titanium vacancies. However, in the case of positively charged oxygen vacancies, applying the Makov-Payne correction gives the wrong sign for the defect formation energy correction. This is attributed to the shallow nature of the transition levels for this defect in TiO2. Finally, we combine the calculated defect formation energies with thermodynamic data to evaluate the influence of temperature on the relative stabilities of point defects. The results indicate that when the Makov- Payne correction is applied, a stable charge transition occurs for titanium interstitials. In addition, as the temperature increases, the dominant point defect in TiO2 changes from oxygen vacancies to titanium interstitials.

Abstract: In many elaboration furnaces, ceramics are used with other materials in vacuum atmospheres. If temperatures are sufficiently high, vaporizations occur. Depending on the thermodynamic stability of the investigated oxide, this vaporization can be congruent or not. In this last case, chemical reactions with the ceramic can take place which can lead to the destruction of it. One way to study these processes is the use of the High Temperature Mass Spectrometry (HTMS). This technique allows determining the composition of the gas phase and the partial pressures of the different gaseous species. By combining the spectrometer to a multiple Knudsen cell furnace, it is also possible to determine activities by direct comparison of the partial pressures of each gaseous species in equilibrium with a mixture and with pure components or compounds in the same experiment. Another recent development of the above technique is to characterize non equilibrium states by determining evaporation and condensation coefficients. These coefficients describe the difference between the real vaporization state of the system and the equilibrium state. To perform such determinations it is necessary to vary the shape of the Knudsen cell orifices in order to change the net evaporation process at the surface of the samples. Theses coefficients can be used to obtain better modeling in processing.

Abstract: This paper shows how phase equilibria and thermodynamic properties of various multicomponent oxide systems can be represented with the use of model equations for the Gibbs energies of all phases as functions of temperature and composition. Starting from the stoichiometric phases and random solutions, the complexity of the models increases tremendously when ordering tendencies or certain defects in the lattice of ionic crystals, such as vacancies and/or variable oxidation states have to be taken into account.

Abstract: A formulation belonging to the CaO-MgO-SiO2-Al2O3 system was characterized in order to found a monophase system based on indialite (Mg2Al4Si5O18), known for its good mechanical characteristics. The classical kinetic studies performed in the muffle kiln show an initial surface crystallization followed from immediate bulk crystallization. The kinetic results obtained via differential scanning calorimetry (DSC) were treated with isothermal method, Kissinger, Ozawa and Matusita and Sakka approaches and compared. These results partially explain the macroscopic behavior, because the studies performed on sample of different grain size show a typical trend for a surface crystallization, while the value of Avrami parameter calculated from the kinetics is 2.3, demonstrating a bi-dimensional crystallization. The value of crystallization energy EC calculated with the isothermal method is 517.8 kJmol-1 and the comparison with nonisothermal studies suggests bulk crystallization to occur with an increasing number of nuclei and a two-dimensional growth of crystals.

Abstract: Five-component B-phase may be readily formed through the nucleation and crystallisation heat treatment of nitrogen-rich parent glasses with composition (e/o) 35R:45Si:20Al:83O:17N. This paper is focussed on the B-phase structure where R stands for ytterbium, erbium or yttrium. Fine probe EDX analysis in the TEM has shown that the lenticular B-phase crystals take up a substantial range of composition and that the element R is always clearly anti-correlated with silicon. A larger R3+ cation radius moves the B-phase composition range to lower R contents, and as a consequence of the anti-correlation with silicon, the silicon solid solution range goes to higher values. The EDX results lend support to a B-phase structure consisting of two-dimensional network of randomly linked (Si,Al)(O,N)4 tetrahedra between layers of R3+ cations. It is suggested that, in addition to the random substitution of silicon by aluminium in the (Si,Al)(O,N)4 tetrahedra, a locally increased density in the bi-dimensional network of randomly oriented tetrahedra is associated with an increased density of vacancies in the R3+ cation lattice.

Abstract: The nature of SHS process was studied, and conditions under which equilibrium and non-equilibrium regimes take place for wave propagation and structure formation of SHS products are under consideration. The important role of cooling down time is shown. Depending on its magnitude, either autoannealing or autoquenching processes can take place in the products after combustion front propagation. The correlation between transition from non-equilibrium to equilibrium state of the product with prolongation of cooling time was examined as well as character of products and processes was indicated in dependence on relationship between the characteristic times of combustion, time of cooling down, and structure formation in SHS product.

Abstract: Sintering of nickel ferrite is strongly enhanced by carbonaceous debinding. It is shown that this effect can be easily explained through thermodynamic consideration on oxygen potential and content evolution during the sintering operation. DTG and dilatometry experiment are in good agreement with driving force calculation and phase diagrams analysis.

Abstract: The sintering map of an ultra fine and ultra pure α-alumina powder, doped with MO and DO2 (M for mono-valence cations, D for bi-valence cations), has been built from isothermal sintering experiments. It has been compared with the one obtained in a previous study with the same un-doped α-alumina raw powder. It seems that a transient liquid phase appears above a critical temperature in the co-doped material during sintering. It is proposed that this transient liquid phase is promoting densification and minimizing grain growth, as confirmed by transmission electron microscopy (TEM).