Construction and experimental validation of an electromagnetic levitation facility for the measurement of thermophysical properties in liquid metal

Kurzfassung

The electromagnetic processing without crucibles is a key technology for materials science to study the formation of meta-stable phases and nucleation phenomena and it also allows the measuring of important thermo-physical material properties such as heat capacity, surface tension, thermal conductivity and viscosity of high-melting samples. In Cologne, at the DLR institute for materials science (Materialphysik) in space, methods for processing molten metal without receptacles under reduced gravity have been employed for years to analyse non-equilibrated solidification. During terrestrial levitation experiments, gravity and the magnetic field deform the sample. Therefore, an exact evaluation of the vibrational spectrum of a-spherical samples is only possible if corrections are applied. Micro-gravitation offers the possibility to measure the oscillation of ball-shaped samples because, in opposition to earthbound experiments, the samples are decoupled from the interfering positioning fields. For this kind of experiment the DLR uses the TEMPUS-apparatus. TEMPUS stands for Tiegelfreies Elektromagnetisches Prozessieren unter Schwerelosigkeit (electromagnetic processing without platens at zero gravity) and describes a scientific experimentation apparatus where fused and undercooled metals and alloys are researched. In this apparatus, electronically conducting samples with a diameter between six and ten millimetres can be brought to levitate, heat up and melt in a inductor coil with electric current. Since the samples levitate free from the coil, no container is needed, in opposition to conventional smelting in a furnace. Thus, the molten metal, which can be chemically very reactive at times, does not touch the material of the container. Consequently, the samples are not contaminated and it is even possible to keep the metals liquid below their solidification point. Comparative experiments in space and on earth allow the experiential designation of gravitational phenomena such as convection, sedimentation and uplift, which is a pre-requisite for the development of physical models to quantitatively describe solidification processes. The TEMPUS apparatus is used during parabola flights and a modified version is also employed in TEXUS-altitudinal research missiles. This produces effective processing times at zero gravity between 22 and 300 seconds. In the light of long term research, a modular mechanism is meant to be installed on the ISS (International Space Station) in the European space laboratory COLUMBUS in 2011. When samples are heated significantly above their melting point in a vacuum environment, a considerable amount of the material from the sample surface evaporates and condenses on the neighbouring cold parts of the machine. If the processing takes places in a gas atmosphere, the material flow of the condensed particles is limited due to the diffusion process. The gas flow transports the particles away from the sample and the particles then form dust, which remains in the atmosphere or sticks to surfaces, but does not form a homogenous layer. Condensation from the sample material on the water-cooled copper coil must not cause electronic short-circuits between the various coils. The cumulative effect makes the layer of the condensed sample material thicken to the point that it separates from the coils, or from the walls of the recipient respectively. Material flakes that separate from the coils can cause contamination of the sample or premature consolidation. In order to minimise these dangers, the temperature-dependent condensation rate of each material to be processed has to be determined. Hence, exact temperature and time profiles of the samples to be processed are an indispensable pre-requisite. The primary research aim of the assembled equipment to measure condensation rates (ARMA – Abdampfratenmessanlage) is the detailed analysis of the various condensation rates of material under a vacuum environment and under a protective gas atmosphere within the overall framework of the ground accompanying programme. For this purpose, the vacuum chamber of the experimentation machine has been equipped with a deposition monitor to measure the layer thickness of the condensed material. The amount of condensed material during an experiment can be determined with the help of both, an experimentation plan, and the knowledge of the different condensation rates under in a vacuum environment and 400mbar protective gas atmosphere. As a result, a detailed schedule can be set up. Opening of, or damage to, the experimentation chamber can release toxic concentrations of metal particles into the space platform. A limited amount of condensation and the resulting reduced layer thickness also diminish the processing time of the samples to be analysed. Analyses of the cumulative effect have shown that the overall thickness of the condensed material on the coils should not exceed 20μm to guarantee a stable layering. This limit has been adopted by the ESA as the research requirement. The condensation rate of the individual samples thus defines their possible processing time on the space platform. Ten experimentation containers (batches) with eighteen samples each are scheduled to be on the MSL-EML in a first stage as they can share among themselves the maximum amount of condensed layer thickness. Heavily fuming materials thus have to be processed for a shorter time than less fuming materials in order to adequately divide up this limited resource.