Three-dimensional numerical study of the behavior of thermoelectric and mechanical coupling during spark plasma sintering of a polycrystalline material

Spark plasma sintering (SPS) is a promising modern technology that sinters a powder, whether it is ceramic or metallic, transforming it into a solid. This technique applies both mechanical pressure and a pulsed direct electric current simultaneously. This study presents a three-dimensional (3D) numerical investigation of the thermoelectric (thermal and electric current density fields) and mechanical (strain-stress and displacement fields) couplings during the SPS process of two powders: alumina (ceramic) and copper (metallic). The ANSYS software was employed to solve the conservation equations for energy, electric potential, and mechanical equilibrium simultaneously. Initially, the numerical findings regarding the thermoelectric and mechanical coupling phenomena observed in the alumina and copper specimens were compared with existing numerical and experimental results from the literature. Subsequently, a comprehensive analysis was conducted to examine the influence of current intensity and applied pressure on the aforementioned coupling behavior within the SPS device. The aim was to verify and clarify specific experimental values associated with these parameters, as reported in the literature, and identify the optimal values of applied pressure (5 MPa for alumina and 8.72 MPa for copper) and electric current (1000 A for alumina and 500 A for copper) to achieve a more homogeneous material

Mathematical modeling of the dissolution process of silicon into germanium melt

Numerical simulations were carried out to study the thermosolutal and flow structures observed in the dissolution experiments of silicon into a germanium melt. The dissolution experiments utilized a material configuration similar to that used in the Liquid Phase Diffusion (LPD) and Melt-Replenishment Czochralski (Cz) crystal growth systems. In the present model, the computational domain was assumed axisymmetric. Governing equations of the liquid phase (Si-Ge mixture), namely the equations of conservation of mass, momentum balance, energy balance, and solute (species) transport balance were solved using the Stabilized Finite Element Methods (ST-GLS for fluid flow, SUPG for heat and solute transport). Measured concentration profiles and dissolution height from the samples processed with and without the application of magnetic field show that the amount of silicon transported into the melt is slightly higher in the samples processed under magnetic field, and there is a difference in dissolution interface shape indicating a change in the flow structure during the dissolution process. The present mathematical model predicts this difference in the flow structure. In the absence of magnetic field, a flat stable interface is observed. In the presence of an applied field, however, the dissolution interface remains flat in the center but curves back into the source material near the edge of the wall. This indicates a far higher dissolution rate at the edge of the silicon source.We gratefully acknowledge the financial support provided by the Canadian Space Agency (CSA), Canada Research Chairs (CRC) Program, and the Natural Sciences and Engineering Research Council (NSERC) of Canada.Publisher's Versio

Thermal Behavior of Mesoporous Aramid Fiber Reinforced Silica Aerogel Composite for Thermal Insulation Applications: Microscale Modeling

This paper explores the incorporation of aramid fibers, recognized for their high mechanical flexibility and low thermal conductivity (TC), to serve as reinforcing agents within the highly porous aerogel matrix in order to overcome their fragility and weak mechanical structure that impose limitations on their practical utility especially in piping insulation. The thermal properties are determined using a micromechanical modeling approach that considers parameters such as temperature, fiber volume fraction, thermal conductivity, and porosity of the silica aerogel. For specific conditions, including an Aramid fiber radius of 6 microns, a silica aerogel thermal conductivity of 0.017 W.m-1.K-1, and a porosity of 95%, the resulting AFRA composite exhibits an Effective Thermal Conductivity (ETC) of 0.0234 W.m-1.K-1. Notably, this value is lower than the thermal conductivity of air at ambient temperature. The findings are further validated through experimental and analytical techniques. A response surface methodology (RSM) based on Box-Behnken design (BBD) is employed. This approach leads to the development of a quadratic equation intricately relating the key parameters to the ETC of the AFRA. The aim is optimization, identifying target optimal values for these parameters to further enhance the performance of AFRA composites

Numerical investigation on the thermal behavior of a Tubular-PCM-Enclosure: impact of HTF-inner tube location

The present paper summarizes a numerical study investigating the behavior of a phase change material PCM filling up a cylindrical container. The PCM is located between two tubes, the inner tube cares for the Heat Transfer Fluid HTF and it is proposed to produce a heat flux. The investigation focuses mainly on comparing the effect of changing the inner tube position, i.e., up, center, and down, in the melting process. Results showed that moving the inner tube downward leads to a decreased melting time and the rate of the thermal energy storage becomes better

Modeling of combined natural/thermocapillary convection flows in germanium melt at high temperatures

A mathematical model for the description of combined natural/ thermocapillary convection melt flows in cylindrical (3D) geometry is developed. It is utilized to model germanium melt convective flows in an isothermal experimental crucible setup. This experimental setup is devised exclusively to study the dissolution phenomenon of silicon into germanium melt. In this system, the germanium melt is subjected to higher temperatures. Using a simplified axisymmetric (2D) model, numerical simulations are carried out to examine the combined natural/thermocapillary flows developing in the germanium melt. Pure thermocapillary and pure natural convective flows are also studied numerically.Publisher's Versio

Modélisation de la convection au cours des changements de phase liquide-solide (effet d un champ magnétique)

Les principaux phénomènes physiques de transport par convection à la base de la formation des macros ségrégations lors d un processus à changement de phase liquide solide (notamment au cours de la solidification des alliages métalliques) sont présentés. A l échelle microscopique, il s agit de la micro ségrégation résultant du rejet de solutés dans la phase liquide et de la diffusion dans la phase solide. À l échelle macroscopique, les espèces chimiques ainsi rejetées sont transportées dans la pièce sous l effet des mouvements de convection dans la phase liquide. Ces mouvements de convection sont causés par les gradients de masse volumique, eux-mêmes générés par les gradients de température et de concentration en solutés. C est cette convection thermo-solutale qui donne naissance aux macros ségrégations, hétérogénéités de concentration à l échelle de la pièce ou du lingot de fonderie, qui vont affecter diverses propriétés (mécaniques, chimiques ) en service ou lors de transformations ultérieures. D autre part, l effet de stabilisation par champ magnétique est examiné. Les phénomènes d instabilités causés par les transports convectifs sont illustrés et décrits numériquement dans le travail présenté.The essential feature in the solidification of a metallic alloy is the liquid-solid phase change associated with the release of latent heat and the solute redistribution. The solutes are often redistributed non-uniformly in the fully solidified casting, giving birth to what is usually called segregation. Segregation occurring on a microscopic scale (i.e., between and within dendritic arms) is known as micro segregation. While segregation occurring on a macroscopic scale (i.e., in a range from several millimeters to centimeters or even meters) is called macro segregation. Micro segregation can be controlled or reduced by a high temperature treatment (homogenization). However, macro segregation occurring on the macroscopic dimensions of the casting cannot be eliminated by homogenization. Magnetic field effects are examined in order to stabilizing the flow field. The work presented here illustrates this objective. The symmetry breaking instability is a phenomenon well known associated with convection. The present work illustrates this phenomenon and describes it numerically.LIMOGES-BU Sciences (870852109) / SudocSudocFranceF