Melt Properties
The knowledge of thermophysical properties of liquid metals and oxides as well as of their dynamics is of fundamental importance for the understanding of their liquid state. Thermophysical property data are an essential input for large-scale simulations in materials design. In order to accurately measure thermophysical and microscopic properties also for chemically reactive liquids, levitation techniques have been developed. Electromagnetic levitation (EML) experiments are carried out on ground and under microgravity conditions during parabolic flights, on sounding rockets, and aboard the ISS. Electrostatic levitation (ESL) is an alternative method that has been developed more recently. As levitation and heating are decoupled, deeper undercooling is possible, and materials with lower melting points can be investigated. Also, nonconducting materials can be processed. Electrostatic and electromagnetic levitation are used with various diagnostic methods: direct high-speed video imaging allows for the determination of the liquid density, while surface tension and viscosity are measured by monitoring the induced oscillations of a levitated droplet both on ground and under microgravity. Both neutron scattering and synchrotron radiation are employed in combination with dedicated levitation devices to investigate structure and dynamics at the atomic level.
Diffusion processes in melts are vital to understand liquid dynamics, vitrification, nucleation and solidification. Yet, their experimental determination in conventional capillary experiments suffers from large errors due to gravity-induced flow- and sedimentation. The institute has developed an entire suite of advanced and complementary techniques: Self-diffusion coefficients are measured on absolute scale by quasielastic neutron scattering with EML or ESL as well as in conventional furnace experiments. Long capillary experiments are combined with X-ray or neutron radiography to monitor the entire interdiffusion process. Diffusion coefficients in multicomponent alloys are obtained by the use of multisegmented shear cells that allow defined experimental conditions at melting and solidification. To circumvent the influence of buoyancy convection, long capillary diffusion experiments are performed under microgravity conditions, e.g. on sounding rockets, including the use of shear cells and X-ray radiography.
Topics addressed in diffusion research include:
- Measurement of liquid structure, thermophysical properties, and atomic dynamics
- Relation between self-, impurity-, and interdiffusion, dependence on temperature, structure and alloy composition
- Relation to viscous flow and crystal growth