The history of study on electromagnetic levitation under microgravity is described. Joining the TEMPUS symposium in DLR in December 1988, the Japanese science community was fascinated by the potential of an electromagnetic levitation technique for thermophysical property measurement for metallic melts not only under microgravity but also on earth. The first levitation experiment of molten silicon was successfully attempted on board the KC-135 under the NASA-MIT-DLR-NEC collaboration in May 1994. In terrestrial experiments surface tension and density of molten silicon were measured. The SEMITHERM program was proposed by European and Japanese scientists. This application stimulated further development of thermophysical property measurement techniques. Thermal conductivity of molten metals were successfully measured by superimposing a strong static magnetic filed. Also a technique for measurement of oxygen partial pressure dependence of surface tension was developed. The THERMOLAB-ISS and OXYTHERM were proposed to ESA under international collaboration of scientists.
The ElectroMagnetic Levitator (EML) is a facility for containerless processing of liquid metals and semiconductors under microgravity conditions. It shall be launched with the ATV and installed in the European Module COLUMBUS of the ISS for utilisation as of 2012. In preparation for the scientific utilisation of this facility, first experiments have been selected by the funding agencies ESA, DLR, JAXA and NASA. The scientific teams backing the selected experiments have representatives in the Investigators Working Group, IWG. The IWG monitors the progress of the hardware development and the mission planning. In addition, the IWG is the scientific coordination forum where maximisation of the scientific return on the resources allocated to an EML increment is sought for. This paper briefly describes the tasks of the IWG and gives a short report on the present status.
EML is an electromagnetic levitator for containerless processing of conductive samples between 5 and 8mm in diameter up to 2100°C in an ultraclean environment. Based on many years of experience with various missions of the TEMPUS facility, comprising Spacelab, sounding rocket and parabolic flights the EML model for the International Space Station (ISS) is currently being developed by ASTRIUM Space Transportation under ESA and DLR contracts. The characteristics of EML are a separate control of RF heating and positioning power, extensive optical diagnostic devices in the visible and near infrared regime, combined with sample stimulation ability. These tools offer unique experiment capabilities in research fields comprising study of phase formation, nucleation phenomena, and thermophysical properties. Extensive commanding and reprogramming capabilities allow telescience experiments as well as quasi-automatic processing.
The ThermoLab Project is concerned with the measurement of the thermophysical properties of industrial alloys in the liquid phase. The project combines long and short duration microgravity measurements based on containerless processing with an electromagnetic levitation device and a ground-based experimental programme using conventional and containerless processing techniques. An overview of the project and representative results are given.
Development of the electrostatic levitation furnace (ELF) for the International Space Station has been conducted by Japan Aerospace Exploration Agency (JAXA) for around 20 years. Its design and target research area has been changed by many reasons; by progress of ground based research and laser technology, by restriction of ISS, and by financial reason. This paper briefly describes the history of ELF development and current design concept.
Viscosity of liquid boron was measured over the temperature range from 2325 K to 2556 K using an electrostatic levitation method combined with an oscillation drop technique. The results obtained revealed that the viscosity increases slowly with decreasing temperature from 2.2 mPa s at 2550 K to 2.6 mPa s at 2370 K, and substantially increases with further decrease of temperature below the melting temperature (Tm＝2360 K), becoming as large as 6.4 mPa s at 2325 K. The increase of the viscosity suggests that clusters with extension may appear in supercooled liquid of boron.
Thermophysical properties of liquid metals are necessary for numerical process modeling for casting, welding, refining and crystal growth. However, measuring thermophysical properties is a difficult task because of existence of convections in liquids and contamination from contact materials. To overcome these experimental difficulties, we have developed the noncontact laser modulation calorimetry using electromagnetic levitation in a static magnetic field. A static magnetic field is superimposed to a metallic droplet levitating electromagnetically. Convections in the droplet and oscillation of the droplet can be well suppressed by the Lorenz force caused by a static magnetic field. The droplet with suppressed convections is subjected to the modulated laser calorimetry. Heat capacity and thermal conductivity are successfully measured using this method.
The electromagnetic levitator for microgravity experiment called PFLEX (Parabolic Flight Levitation Experiment Facility) was used for the surface tension measurement of molten copper in consideration of the influence of oxygen partial pressure of ambient atmosphere under microgravity. The sample was levitated under normal gravity followed by a parabolic flight. When the electromagnetic force was significantly decreased under microgravity, the surface oscillation of the droplet gradually degenerated. Consequently the surface oscillation showed a single frequency of / = 2 mode. The surface tension calculated from the single frequency for the microgravity experiment corresponded to that calculated from the frequencies of the oscillations of m=0, ± 1, and ±2 on the ground.
Glass transition or crystallization during solidification of Zr-based alloys has been investigated by a time-resolved synchrotron x-ray diffraction under containerless cooling conditions using a conical nozzle levitation technique. To observe relative variations of structure in undercooled liquid alloys, we have performed millisecond order time-resolved x-ray diffraction experiments with a 2-dimensional detector. Diffraction patterns on the glass transition of Zr50Cu40Al10 alloy, which is a typical bulk metallic glass former, were obtained on the containerless cooling process as well as the crystallization. By comparing scattered x-ray intensity profiles between the glassy state and the undercooled liquid state, we have identified formation of icosahedral short range ordering in the undercooled liquid of Zr50Cu40Al10 alloy. In a series of this study, we have found microcrystalline dispersion in the Zr-Pd-Al alloys by the containerless solidification. Time-resolved x-ray intensity profiles on solidification of Zr50Pd40Al10 alloy show that precipitation of a bcc-ZrPd compound as a primary phase around 1100K from the undercooled liquid. Then, with decreasing temperature, it was revealed that a martensitic phase transformation from the bcc-phase to the orthogonal-ZrPd phase occurred below 670K.
The electrostatic levitator (ESL) is one of the most fruitful facilities for the experiments of liquid metals, for example, observation of solidification or measurements of thermophysical properties. Recently, ESL is tried to apply to the structural analysis of high temperature liquids with the combination of X-ray or Neutron scattering techniques. For this purpose, some of characteristic elemental techniques must be concerned. In this report, the development of sample position detector with high precision, the optimization of shape of electrodes and data analysis of structure analysis of high temperature melts are present.
Undercooling a melt often facilitates a metestable phase to preferentially nucleate. In the present paper, the formation of a metastable phase from undercooled melts was developed from the point of the competitive nucleation criterion. The classical nucleation theory shows that the most critical factor for forming a critical nucleus is the interface free energy g. In fact, on the simple liquid such as the melt of a mono-atomic metal, Spaepen's negentropic model suggests that a scaling g with the entropy of fusion is to be the decisive factor for forming the critical nucleus. However, recent numerical simulations such as the molecular dynamics or density functional theory show ambiguous relations between a and the crystal structures. Furthermore, in compound materials such as oxides, in which polyhedrons of oxygen are the structural units both in the solid and liquid phases, it is suggested that the decisive factor for forming the critical nucleus isn't a but the entropy offusion. According to this idea, the entropy-undercooling regime criterion for metastable phase formation was reviewed by using REFeO3 (RE: Rare-earth element) as the model material.
In response to the European Space Agency announcement for proposals for research in physical sciences on sounding rockets and the International Space Station (AO-2009 PHYS-BIOSR), an international team was assembled to investigate the influence of convection on phase selection during rapid solidification of commercially important peritectic alloys. Ground-based experiments and modeling activities culminate in experiments which are to be run in microgravity using the MSL-EML facility. Electromagnetic levitation is used to melt reactive metal samples in a containerless manner while imposing varied conditions of induced convection within the molten droplets. Metastable-tostable phase transformations will be monitored using pyrometry and high speed digital video leading to the development of new solidification models to allow better microstructural control during processing on earth.
The application of cntainerless processing by electromagnetic levitation gives access to liquid drops of metals and alloys to be undercooled over a large temperature range below their melting points. In such a way a metastable liquid is created, which possesses an enhanced free enthalpy. This can be used by the system to choose between different solidification pathways in various metastable solid states which may differ in their physical properties from the stable counterpart. In the present, the velocity of crystal growth in undercooled melts is investigated. Crystal growth in undercooled melts takes place by rapid propagation of dendrites into the liquid. Dendrite growth is controlled by heat-and in case of alloys by mass-redistribution in front of the solid / liquid interface. Since heat-and mass transport are influenced by convection it is essential to determine the influence of convection on dendrite growth dynamics. Comparative experiments on measurements of dendrite growth velocity as a function of undercooling under terrestrial conditions and in reduced gravity are presented. Mesoscopic models of dendrite growth are extended in order to describe growth dynamics without and with convection. These models are applied to analyze the experimental results. Finally, an outlook is given for future experiments to be performed on board the International Space Station, which are currently in preparation.