Materials Science and Engineering, A178 (1994) 73-76 73 Containerless processing of undercooled melts: measurements of surface tension and viscosity Ivfin Egry and Stefan Sauerland Institute of Space Simulation, DLR, 51140, Cologne (Germany) Abstract Electromagnetic levitation is an elegant way of processing highly reactive liquid metals. It avoids contact with any container walls and thus protects the sample's surface from contamination. This is important not only for obtaining reliable surface tension data for the pure metals or alloys, but also for accessing the metastable, undercooled regime. With electromagnetic levitation, the undercooled state can be maintained for extended periods of time, allowing in sire diagnostics, i.e. non-contact measurements of thermophysical properties. The oscillating drop technique can be used to determine surface tension and viscosity as a function of temperature. The frequencies of the oscillations of liquid drops are related to the restoring force, the surface tension, while the damping of the oscillations is related to the viscosity. 1. Electromagnetic levitation An inhomogeneous, alternating electromagnetic field has two effects on a conducting, diamagnetic body: firstly, it induces eddy currents within the material, which, owing to ohmic losses, eventually heat up the sample (inductive heating) and, secondly, it exerts a force on the body pushing it towards regions of lower field strength (Lorentz force). The latter effect can be used to compensate the gravitational force acting on the body. The fundamental parameter of levitation theory is q =R/a (1) Here R is the radius of the sample and 6 is the skin depth, given by ~ ) 1/2 a = (2) where o is the electrical conductivity and w is the angular frequency of the electromagnetic field. For frequencies in the megahertz range, the skin depth is about 0.1 mm for most metals, whereas usually R=3-5 mm. The time-averaged absorbed power by a sample of radius R in a homogeneous magnetic field of amplitude B is given by [1] P=3~R 2 H(q)B 2 (3) oflo where sinh(2q) + sin(2q) H (q) = q cosh(2q)- cos(2q) - 1 (4) For a weakly non-homogeneous magnetic field, the time-averaged force acting on a conducting sample can be calculated from ~R ~ F = --- G(q)VB 2 (5) 2~, where 3 sinh(2q)-sin(2q) C(q)=l (6) 2q cosh(2q)- cos(2q) Equations (3) and (5) are the (approximate)working relations of electromagnetic levitation. Figure 1 shows the field distribution inside a typical conical levitation coil. 1. I. Microgravity On earth, strong magnetic fields are needed to compensate the lg gravitational force. The micro- gravity environment offers the unique possibility of minimizing the magnetic positioning fields. Therefore, after melting, in the cooling phase, only negligible forces act on the sample. There is practically no defor- mation of the sample, and the spherical shape is main- tained. This is important for measurements of the 0921-5093/94/$7.00 © 1994 Elsevier Sequoia. All rights reserved SSDI 0921-5093(93)04514-I