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Register a new userModeling the microstructural evolution during constrained sintering
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A numerical model able to simulate solid-state constrained sintering is presented. The model couples an existing kinetic Monte Carlo (kMC) model for free sintering with a finite element model (FEM) for calculating stresses on a microstructural level. The microstructural response to the local stress as well as the FEM calculation of the stress field from the microstructural evolution is discussed. The sintering behavior of a sample constrained by a rigid substrate is simulated. The constrained sintering results in a larger number of pores near the substrate, as well as anisotropic sintering shrinkage, with significantly enhanced strain in the central upper part of the sample surface, and minimal strain at the edges near the substrate. All these features have also previously been observed experimentally.
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A thermodynamically consistent phase-field model is developed to study the non-isothermal grain coalescence during the sintering process, with a potential application to the simulation in unconventional sintering techniques, e.g. spark plasma sintering, field-assisted sintering, and selective laser sintering, where non-equilibrium and high temperature gradient exist. In the model, order parameters are adopted to represent the bulk and atmosphere/pore region, as well as the crystallographic orientations. Based on the entropy analysis, the temperature-dependent free energy density is developed, which includes contributions from the internal energy (induced by the change of temperature and order parameters) and the order parameter related configurational entropy. The temperature-dependent model parameters are determined by using the experimental data of surface and grain boundary energies and interface width. From laws of thermodynamics, the kinetics for the order parameters and the order-parameter-coupled heat transfer are derived. The model is numerically implemented by the finite element method. Grain coalescence from two identical particles shows that non-isothermal condition leads to the unsymmetric morphology and curved grain boundary due to the gradients of on-site surface and grain-boundary energies induced by the local temperature inhomogeneity. More simulations on the non-isothermal grain coalescence from two non-identical and multiple particles present the temporal evolution of grain shrinkage/growth, neck growth, and porosity, demonstrating the capability and versatility of the model. It is anticipated that the work could provide a contribution to the research community of unconventional sintering techniques that can be used to model the non-isothermal related microstructural features.
We report a kinetic Monte Carlo modeling study of nanocrystal layer sintering. Features that are of interest for the dynamics of the layer as a whole, especially the morphology of the evolving structure, are considered. It is found that the kinetics of sintering is not entirely a local process, with the layer morphology affected by the kinetics in a larger than few-particle neighborhood. Consideration of a single layer of particles makes the numerics manageable and allows visualization of the results, as well as numerical simulations of several realizations for statistical averaging of properties of interest. We identify optimal regimes for sintering, considering several particle size distributions and temperature control protocols.
One of the main challenges of the sintering of sterling silver is the phenomenon of swelling causing de-densification and a considerable reduction of the sintering kinetics. This swelling phenomenon opposes sintering and it needs to be addressed by a well-controlled processing atmosphere. In the present study, the pressure-less sintering behavior of sterling silver is investigated in air, argon, and vacuum. A specially modified spark plasma sintering mold is designed to study the pressure-less sintering of sterling silver in a high vacuum environment. The conducted analysis is extended to the new constitutive equations of sintering enabling the prediction of the swelling phenomena and the identification of the internal equivalent pressure that opposes the sintering.
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Vincent Langlois Universite Paris-Est
, n Laboratoire de Physique des Materiaux Divises et des Interfaces
2010
Ultrasound transmission in PMMA spherical bead packings is investigated during the sintering process under stress. Velocity and amplitude measurements of coherent longitudinal waves are performed to monitor the evolution of the elastic properties of the solid frame from noncohesive packing to sintered granular material. Comparison between the experimental velocity data and the prediction by a contact model [Digby, J. Appl. Mech. 48, 803, (1981)] reveals the crucial role of the bonding effect on the mechanical behavior of granular compacts. By using the correlation technique of acoustic speckles, we also observe the important rearrangements in granular packings before the onset of sintering.
Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the hypothesis of variation in charged defects, which however are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (VO) order are identified during the fatigue process. It reveals that the isolated VO tend to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of VO within vacancy clusters is accommodated with a lower energy barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer incorporated with highly concentrated VO. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered VO cluster in the fatigue failure of BiFeO3 thin films.
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