Capillary equilibrium and sintering kinetics in dispersed media and catalysts

The evolution of an aggregate of particles embedded in a fluid phase, no matter whether a liquid, a vapor, or a mixture of both, is determined by the dependence of the equilibrium interface area on porosity volume fraction. In system with open porosity, this equilibrium can be analyzed using a model representing the particles as a collection of cones of revolution, the number ofwhich is the average particle coordination number. The accuracy of themodel has been assessed using in situ X-raymicrotomography. The modelmakes possible the computation of the driving force for sintering, commonly called sintering stress. It allows the mapping of the domains of relative density, coordination number, and dihedral angle that bring about aggregate densification or expansion. The contribution of liquid/vapor interfaces is enlightened, aswell as the dependence of the equilibriumfluid phase distribution on particle size. Applied to foams and emulsions, themodel provides insight into the relationship between osmotic pressure and coordination. Interface-governed transport mechanisms are considered dominant in the macroscopic viscosity. Both sintering stress and viscosity parameters strongly depend on particle size. The capacity of modeling the simultaneous particle growth is thus essential. The analysis highlights the microstructural parameters and material properties needed for kinetics simulation

Elastic model of an entangled network of interconnected fibres accounting for negative Poisson ratio behaviour and random triangulation

A model is designed for predicting the elastic constants of a random planar network of interconnected fibres while accounting for two features of such networks: negative Poisson ratio behaviour and random triangulation. The model is based on a periodic network involving both plates and fibres segments, with possibility of reentrant cell shapes. The plates are intended to represent the effect of triangulation. Bounds for the elastic constants are obtained by calculating volume weighted averages of the elastic properties for periodic networks characterised by a uniform distribution of in-plane fibre orientations. Predictions are derived for the dependence of the in-plane Young's modulus, out-of-plane Young's modulus, and in-plane shear modulus on out-of-plane fibre orientation for a fibre volume fraction of 0.20. These predictions are assessed by reference to experimental results for transversely isotropic networks, with very low average out-of-plane fibre orientation, made by sintering compressed mats of stainless steel fibres. A comparison is also made with the predictions of a model for truss lattice core, which is liable to represent the case of a fully triangulated network. (C) 2004 Elsevier Ltd. All rights reserved

[Study of the microwave emission of a solid plasma using a hydromagnetic turbulence theory]

A linear theory of the stability of a solid plasma is insufficient for characterizing completely the phenomenon of microwave emission. A nonlinear equation is established by considering previous experimental data for emission by InSb. It is shown that it is necessary to consider the turbulent behaviour and that the Kolmogoroff theory can be applied to the determination of energy distributions in |K| space. The hypothesis of quasinormality is used to obtain an equation for the spatio-temporal correlations.Francai

Modelling of the influence of dihedral angle, volume fractions, particle size and coordination on the driving forces for sintering of dual phase systems

The equilibrium shape of solid particles in an aggregate immersed in a liquid or in a gas results from the minimization of interface energy. A model is developed for expressing the dependence of the solid-solid and solid-second phase interface areas on the system parameters: phase Volume fractions, dihedral angle, particle size and coordination. The model aims at allowing quantitative assessment of the role of these parameters on the driving force for sintering. The representative Volume element is a cone of which the apex angle accounts for the average particle coordination. In order to comply with the uniformity of interface curvature, the solid-second phase interfaces are described using the mathematics of the Delaunay surfaces. The results are compared with the Solutions obtained by approximating the interface shape by the revolution of an arc of circle around the cone axis. This approximation does not involve a significant loss of precision