David Jauffrès

Simulation of the elastic properties of porous ceramics with realistic microstructure

Quasi-static discrete element method (DEM) simulations are used to model the elastic behavior of porous ceramic obtained by partial sintering. The porous ceramic is modeled as a set of particles bonded by solid necks. A new approach introducing bond interactions is proposed to allow, in particular, the correct prediction of Poissons ratio. It leads to a simple formulation that takes into account the deformation of a particle subjected to multiple contact forces, without the complexity of a full finite element method (FEM) analysis. The advantage of this new formulation is demonstrated, first by a comparison with FEM calculations on a simple test case, and then by a comparison with experimental results and other models from the literature on random aggregates of partially sintered particles. The proposed model with bond interactions correctly predicts Poissons ratio of porous ceramics and improves the accuracy of Youngs modulus when compared with the DEM model without bond interactions.

Coupling in-situ X-ray micro- and nano-tomography and discrete element method for investigating high temperature sintering of metal and ceramic powders

The behaviour of various powder systems during high temperature sintering has been investigated by coupling X-ray microtomography and discrete element method (DEM). Both methods are particularly relevant to analyse particle interactions and porosity changes occurring during sintering. Two examples are presented. The first one deals with a copper powder including artificially created pores which sintering has been observed in situ at the European synchrotron and simulated by DEM. 3D images with a resolution of 1.5 μ m have been taken at various times of the sintering cycle. The comparison of the real displacement of particle centers with the displacement derived from the mean field assumption demonstrates significant particle rearrangement in some regions of the sample. Although DEM simulation showed less rearrangement, it has been able to accurately predict the densification kinetics. The second example concerns multilayer ceramic capacitors (MLCCs) composed of hundreds of alternated metal electrode and ceramic dielectric layers. The observation of Ni-based MLCCs by synchrotron nanotomography at Argon National Laboratory with a spatial resolution between 10 and 50 nm allowed understanding the origin of heterogeneities formed in Ni layers during sintering. DEM simulations confirmed this analysis and provided clues for reducing these defects.