Pierre Abelard

Heteroaggregation between Al2O3 Submicrometer Particles and SiO2 Nanoparticles: Experiment and Simulation

The aggregation process of a two-component dilute system (3 vol %), made of alumina submicrometer particles and silica nanoparticles, is studied by Brownian dynamics simulations. Alumina and silica particles have very different sizes (diameters of 400 and 25 nm, respectively). The particle-particle interaction potential is of the DLVO form. The parameters of the potential are extracted from the experiments. The simulations show that the experimentally observed aggregation phenomena between alumina particles are due to the silica-alumina attraction that induces an effective driving force for alumina-alumina aggregation. The experimental data for silica adsorption on alumina are very well reproduced.

Self-assembly of oppositely charged particles in dilute ceramic suspensions: predictive role of simulations

Ceramic suspensions composed of oppositely charged alumina and silica particles are studied experimentally and by means of Brownian dynamics simulations. Alumina and silica particles have quite similar sizes, the former having an average diameter larger by a factor 1.6. The suspension behavior is studied as a function of composition. The aggregation state, the aggregate composition, structural aspects at several length scales and the aggregate growth kinetics are analysed. A good agreement between numerical and experimental results is obtained. The simulations allow us to describe in detail the aggregation process and mechanisms. Simulations appear as an important tool to predict and control the particle assembly in such binary suspensions, whose behaviour depends on several parameters.

Dielectric relaxation in alkali silicate glasses: A new intepretation

Abstract The electric and dielectric properties of a ternary alkali sillicate glass (0.14 Na 2 0−0.11 Mg0−0.75 SiO 2 ) have been studied as a function of temperature (350 K − 600 K) and frequency (1 Hz − 100 khz) using the impedance spectroscopy. It is proved that dispersion arises from the motion of sodium ions. Experimental data are interpreted with the helo of the CTRW, Continuous Time Random Walk , formalism developed by Scher and Lax which assumes that all the alkali ions are mobile but with different mobilities.

Simulation of the heteroagglomeration between highly size-asymmetric ceramic particles

Aggregation phenomena of dilute suspensions composed of two kinds of oxide particles (alumina d(1)=400 nm and silica d(2)=25 nm) have been studied by computer simulations. These particles are oppositely charged and so are prone to heteroagglomerate. The interaction between particles has been modeled by the DLVO potential. Two kinds of simulations have been performed: Brownian dynamics simulations to study the aggregation kinetics and global minimization searches that permit the examination of the most stable configurations of agglomerates. We demonstrate that aggregation should occur also for quite large fractions of added silica (even when 200 silica particles are adsorbed on each alumina particle) and that aggregates are likely to present chainlike shapes. Both findings are in agreement with experiments.

An approach to the defect chemistry of barium titanate

Abstract In ternary oxides, the concentrations of point defects that are responsible for departures from stoichiometry depend upon three thermodynamic variables under a constant total pressure. These are the temperature and two activities that may be chosen as the oxygen partial pressure and the activity of one oxide component. A new representation of majority defects pairs is developed in a two-dimensional space, the coordinates of which are related to these activities. The effect of a constant cationic composition is analysed with the help of such diagrams, which appear complementary to those used in the case of simpler binary metal oxides. Barium titanate has been chosen for a specific application of these ideas.

Experimental survey of dopant ions in ZnO: nonlinearity and degradation

The addition of 0.1 at% Bi to ZnO yields a ceramic with nonlinear current-voltage behaviour (I=KVα) characterized by a maximum α value of 5. The effect of further addition of 0.1 at% of a 3d transition element was tested through the series Sc, Ti, V, Cr, Mn, Fe, Co, Ni or Cu. This low level of doping does not provoke any marked variation in the microstructure. Mn followed by Cu gave the most significant enhancement to nonlinearity achieving α=50 and α=30 respectively. The strong performances due to Mn doping are counterbalanced by degradation of the electrical behaviour under repeated high-voltage cycles whereas the samples doped with other 3d elements were much stable. This is related to the capability of Mn to adopt several oxidation states.