DLR - Institute of Materials Physics in Space

Granular Matter

Granular matter is an important issue in many applications as well as a challenge topic in fundamental physics.
Due to the loss of energy during the collision of granular particles, assemblies of many granular particles are systems far from equilibrum. Under gravity, granular systems either need an external supply of energy or they sediment quickly. For investigations under microgravity, two regimes of different density are of particular interest. Dilute systems, known as granular gases, are prepared homogeneously by magnetic excitation, and one investigates their steady-state behavior after cessation of agitation. Using highspeed imaging, statistical data can be obtained on a microscopic level. Dense granular systems show dynamics reminiscent of glasses and eventually they settle into a static packing with permanent contacts between the constituent particles. The dynamical behavior on ground and under microgravity is investigated by multiple-scattering techniques. To monitor the evolution of contacts and the resulting contact forces quantitatively methods involving stress-birefringence are employed.

Atomare Dynamik und Struktur

Auf kleinen Zeit- und Längenskalen kontrollieren Prozesse auf mikroskopischer Ebene den Materietransport und die Strukturbildung bei der Erstarrung. Die atomare Dynamik und Struktur untersuchen wir an nationalen und internationalen Neutronen- und Röntgenstrahlungsquellen. Für Messungen an chemisch reaktiven oder unterkühlten Schmelzen kommen dabei unsere mobilen Levitationsapparaturen zum Einsatz. Ziel unserer Forschung ist es, die atomare Dynamik und Struktur von Schmelzen besser zu verstehen und deren Zusammenhang mit den Schmelzeigenschaften und der Phasenselektion bei der Erstarrung herzustellen. Darüberhinaus bieten unsere Experimente die Möglichkeit, atomistische Simulationsmethoden zu testen und weiterzuentwickeln.

Theory and Simulation

Molecular-dynamics simulations are used to study microscopic processes in model metallic melts, using effective interaction potentials that are calibrated against experimental data. Data from experiment and simulation are interpreted within the framework of mode-coupling theory.

Combined with simulation, theoretical modelling gives key insights into generic aspects of liquid dynamics. The relation among self- and inter-diffusion viscosity, and other transport coefficients is investigated, addessing the relation between liquid structure and dynamics. The underlying microscope processes and their origins, kinetic or thermodynamic, and entropic or energetic, are rationalized. Specially set up simulations reveal particle-sale phenomena influencing crystal growth and solidification dynamics. Strong external driving fields such as gravity-driven flow, are incorporated in order to understand their influence on the microscopic mechanism at work during slow dynamics and solidification.

Thermophysikalische Parameter

Schmelzeigenschaften wie Dichte, Viskosität, Diffusionskoeffizienten, spezifische Wärme, elektrische Leitfähigkeit sowie Grenz- und Oberflächenspannungen sind wichtige Grössen in der Materialherstellung, z.B. in der Kristallzucht oder in der Giesserei. Wir untersuchen thermophysikalische Eigenschaften von Metallen, Oxiden und Halbleitern sowohl in stabilen als auch in unterkühlten Schmelzen. Mit behälterfreien Verfahren wie der elektromagnetischen und elektrostatischen Levitation haben wir Zugang zu sehr hohen Temperaturen sowie chemisch reaktiven Schmelzen und erreichen Unterkühlungen von mehreren hundert Kelvin unter den Schmelzpunkt. Experimente unter Schwerelosigkeit erlauben hier präzise Referenzmessungen.

Solidification far from Equilibrium

Solidification requires undercooling below the melting point. Near equilibrium, stable-solid phases are formed. However, nucleation from the container walls usually limits the degree of undercooling possible.

Containerless processing techniques significantly widen the range of undercooling, maintaining a metastable liquid with large excess free enthalpy. Among a great variety of possible solidification pathways it emerges, that the crystallographic phase is selected by crystal nucleation. Subsequent crystal growth proceeds as dendritic, eutectic, or peritectic.

Levitation of liquid droplets is utilized with high-speed cameras to measure the velocity of the solid-liquid interface and the various effects of non-equilibrium solidification. To control the field-induced flow, experiments are carried out both on ground and in weigthlessness using electromagnetic levitation devices on parapolic flights, on sounding rockets, and aboard the International Space Station.

Gleichgewichtserstarrung

Die physikalischen Eigenschaften eines aus der Schmelze erstarrten Materials werden bestimmt durch seine chemische Zusammensetzung, seine atomare Struktur und seine Mikrostruktur. In vielen Produktionsvorgängen erfolgt die Erstarrung gerichtet in einem positiven Temperaturgradienten, z. B. bei der Kristallzucht. Wir wenden neuartige Aerogelöfen an, um die Erstarrung nahe dem Gleichgewicht zu untersuchen. Aerogele sind transparent, kaum wärmeleitend und besitzen schlechtes Benetzungsverhalten, alles ideale Eigenschaften um die Dynamik und Morphologie der fest-flüssig Grenzfläche bei der gerichteten Erstarrung weitgehend unbeeinflusst durch den Ofentiegel direkt zu beobachten.

Thermophysical and Microscopic Melt Properties

The knowledge of thermophysical properties of liquid metals and their liquid dynamics is of fundamental importance for the understanding of their liquid state. Thermophysical properties like viscosity and surface tension are an essential input for large-scale simulations in materials design.

In order to measure thermophysical and microscopic properties in the liquid state accurately and also for chemically reactive melts, levitation techniques have been developed. Electromagnetic levitation is appied in several ground based and microgravity experiments like parabolic flights, sounding rockets, and the International Space Station. In addition, unwanted force convection can be avoided by using electrostatic levitation. As levitation and heating are decoupled , deeper undercooling is possible and materials with lower melting points can be investigated. Also, non-conducting materials can be processed.

Electrostatic and electromagnetic levitation can be used with various diagnostic methods. Direct imaging allows for the determination of the liquiid density, while the surface tension and viscosity are measured by monitoring the induced oscillations of a levitated droplet both on ground and under microgrvity. Both neutron scattering and synchrotron radiation are employed to investigate structure and dynamics at the atomic level.

Materials Science in Space - User Support

Materials Science research aboard the International Space Station (ISS) rests on two major facilities, the Materials Science Lab (MSL) and the Electro-Magnetic Levitator (EML).

The EML is currently under developement and to be installed in the European Columbus module of the ISS. Preparartory experiments are performed on the sounding rocket TEXUS and in the facility TEMPUS on parabolic flights. Both the MSL and the EML aim at research in solidification, crystal growth, and diffusion of mass, as well as the thermophysical properties of metals.
Preparation, execution, and evaluation of the initial experiment batches in space is coordinated by the Institute of Materials Physics in Space within DLR´s Microgravity User Support Center (MUSC).