Measurement of surface tension and density by means of levitation

With the development of tools and techniques for computer assisted calculations and simulations, knowledge of thermophysical properties at elevated temperatures up into the liquid phase became even more important for the metalworking industry and related fields. Advances in computer-based simulations allow modeling of heat transport, melting and remelting processes, solidification shrinkage, residual stress or even predictions of microstructures, to name a few. The most relevant properties needed for such simulations are heat of fusion and for the liquid phase heat capacity, electrical resistivity, density, thermal conductivity and thermal diffusivity, thermal expansion, hemispherical emittance, viscosity and surface tension. For about 25 years the Subsecond Thermophysics Group in Graz is operating an ultra fast ohmic pulse-heating system which allows the investigation of thermophysical properties such as heat capacity and the mutual dependencies between enthalpy, electrical resistivity, temperature and volume, thermal conductivity and thermal diffusivity. They are performed from room temperature up to superheated liquid states. Our working group has rendered outstanding services in the field of dynamic pulse calorimetry, but measurements of surface tension and viscosity are unfortunately missing. Main focus of this project is to build up a working levitation facility for determining surface tension and density of liquid metals and alloys in the range between the actual melting temperature and about 200 K into the liquid phase. Most parts of the experimental equipment for this project we have already obtained used, but functional, from DLR Cologne including a high frequency generator with an induction coil, a vacuum-system, pyrometers for measuring the sample temperature and a gas-system for protecting the levitated drop against environmental influences, for cooling and locating it inside the levitation coil. The radius of a levitated drop undergoes spherical harmonic oscillations. While angular frequency of these oscillations can be related to the surface tension, the damping of such oscillations is due to the viscosity of the sample material. Data evaluation is drastically simplified if the equilibrium shape of the droplet is spherical, allowing usage of the formulae of Rayleigh and Kelvin to determine the surface tension, provided that the mass of the sample and its radius are known. The main limitation is that a spherical shape can only be obtained under force-free conditions, which can only be satisfied in microgravity but not under terrestrial conditions. Therefore corrections for electromagnetic levitation have to be applied. The proposed measurements are possible by digital image processing and frequency analysis of induced surface waves of a levitated liquid sample. After installing this new experiment we will be able to obtain data of surface tension and density for liquid metals and alloys, up to now such data are sparse.

The workgroup Thermophysics and Metal physics is interested in obtaining thermophysical properties of liquid Metals and Alloys. These can be seen as a selection of temperature dependent electrical, optical, and thermal material properties, with a certain relevance to industrial, scientific, and metallurgical applications.The focus of this project was, after setting up the electromagnetic levitation-device, to obtain the thermophysical properties surface tension and density as a function of temperature for liquid metals and alloys. With our well-introduced pulse-heating system we are already able to obtain heat of fusion and heat capacity, and for the liquid phase electrical resistivity, density, thermal conductivity and thermal diffusivity, thermal expansion and normal spectral emissivity at 684,5 nm as function of temperature.The results achieved have been presented at scientific conferences and published in reviewed scientific journals.