Study of Interesting Solidification Phenomena on the Ground and in Space (MEPHISTO)

Real-time Seebeck voltage variations in a Sn-Bi melt during directional solidification in the MEPHISTO spaceflight experiment flown on the USMP-3 mission, have been correlated with well-characterized thruster firings and an Orbiter Main System (OMS) burn. The Seebeck voltage measurement is related to the response of the instantaneous average melt composition at the melt-crystal interface. This allowed us to make a direct comparison of numerical simulations with the experimentally obtained Seebeck signals. Based on the results of preflight and real-time computations, several well-defined thruster firing events were programmed to occur at specific times during the experiment. In particular, we simulated the effects of the thruster firings on melt and crystal composition in a directionally solidifying Sn-Bi alloy. The relative accelerations produced by the firings were simulated by impulsive accelerations of the same magnitude, duration and orientation as the requested firings. A comparison of the simulation results with the Seebeck signal indicates that there is a good agreement between the two. This unique opportunity allows us to make the first quantitative characterization of actual g-jitter effects on an actual crystal growth experiment and to calibrate our models of g-jitter effects on crystal growth

Mass transport phenomena in microgravity: Preliminary results of the first MEPHISTO flight experiment

The MEPHISTO space program is the result of a cooperative effort that involves the French nuclear and space agencies (Commissariat a l'energie atomique, CEA - Centre National d'Etudes Spatiales, CNES) and the American National Aeronautics and Space Administration (NASA). The scientific studies and apparatus development were funded in the frame of the GRAMME agreement between CEA and CNES, the flight costs being taken in charge by NASA. Six flight opportunities are scheduled, with alternating French and American principal investigators. It is the purpose of this paper to briefly present MEPHISTO along with the preliminary results obtained during its first flight on USMP-1 in October 1992

Measurement of thermophysical properties of liquid metallic alloys in a ground- and microgravity based research program. The Thermolab Project

An outline of the Thermolab Project is reported with the aim of informing on the wide range of properties which are becoming available for some industrial alloys. Selected examples of experiments and properties are provided

Experimental study of Marangoni flows in molten and solidifying Sn and Sn-Bi layers heated from the side

An experimental study of Marangoni flows in horizontal layers of tin based metallic alloys heated from the side is carried out, based on the observation of the motion of tracer particles at the surface associated with a direct measurement of temperatures in the melt. A key feature of the procedure is that the experiments are performed in a high vacuum environment to limit as much as possible the pollution of the liquid vapour interface. In pure tin, depending on the intensity of the driving force measured by the Reynolds-Marangoni number, either viscous or boundary layer flow regimes are found. In addition, the onset of temperature oscillations is found to be consistent with the hydrothermal wave instability mechanism proposed by Smith and Davis. In tin-bismuth alloys, the observed flow directions and velocities are interpreted in terms of a non linear dependence of the surface tension on temperature, and successfully compared with the dependence predicted by interpolating the available experimental surface tension data with the help of a simple thermodynamic model. During the directional solidification of the layer, the concentration gradient in the solute rich boundary layer ahead of the growth front is found to locally modify the flow pattern. For instance, when thermal and solutal driving forces act in the same direction, temperature oscillations are observed at low values of the Reynolds-Marangoni number. As for opposing driving forces, the situation is that a local reversal of the flow direction is observed in correlation with the morphological instability of the growth front

Additional transport by oscillatory buoyancy driven convection in diffusion experiments

A model for the transverse g-jitters induced transport in binary diffusion experiments is presented. The flow is supposed parallel and oscillating at a single frequency supposed high compared to the solute lateral diffusion time scale. This model applies to long aspect ratio experiments. A time-averaged effective diffusivity approach allows us to recover several recently published results and to provide a better understandaing of the underlying physics. In the case of a given fully established velocity profile, the variation of the effective diffusion coefficient is such that (Deff-D)/D follows an ω-2 or an ω-7/2 law respectively for low and high frequency compared to the momentum lateral diffusion time scale. The transport by this type of flow is shown to be hardly significant in typical impurity diffusion experiment in microgravity

Thermodynamics and kinetics of dissolutive wetting of Si by liquid Cu

International audienceWetting of Si single crystals by pure copper has been studied using the dispensed drop technique under high vacuum at 1100 C. At this temperature copper dissolved large quantities of Si during spreading. CuSi droplets presaturated in silicon were also used to obtain non-reactive spreading. From the results obtained the different contributions to "dissolutive wetting" were determined. Moreover, a simple analytical model was formulated for analysing the influence on wetting of the interfacial atomic processes and of Si transport in the liquid. It is seen that, under our experimental conditions, solute transport by Marangoni convection controls the spreading kinetics

Diffusion and viscosity in molten Pd40Ni40P20 and Pd40Cu30Ni10P20 alloys

The self diffusion of 62Ni and the shear viscosity in liquid Pd40Ni40P20 and Pd40Cu30Ni10P20 have been measured. The used methods were the long capillary technique for diffusion measurement and the gas film levitation for viscosity measurement. The temperature dependence of diffusion in the equilibrium melt follows the prediction of the mode coupling theory. The Stokes Einstein relation describes well the momentum and mass transport in both melts. The 62Ni diffusion is equal in both alloys whereas the normalized bulk viscosity is higher in Pd40Cu30Ni10P20. Thermodynamic and structural considerations are invoked to propose a qualitative explanation for this behavio

Solute segregation in a lid driven cavity: Effect of the flow on the boundary layer thickness and solute segregation.

International audienceOur objective in the present work is to study the effect of convective flows, ranging from laminar to fully turbulent, on solute segregation in directional solidification configurations. To do so, numerical simulations performed in a model 2D lid driven cavity: the problem parameters, apart from the species molecular diffusion coefficient, are the lid and growth velocities. Purely diffusive to fully convective mass transport conditions are modelled in our parametric study. In parallel, a scaling analysis aiming at the determination of the solute boundary layer thickness is proposed. The results show that a single non-dimensional number, based on the interface stress, is able to capture the physics of the solute transport phenomena. (C) 2011 Elsevier B.V. All rights reserved