G. Lohöfer

VISCOSITY OF EUTECTIC PD78CU6SI16 MEASURED BY THE OSCILLATING DROP TECHNIQUE IN MICROGRAVITY

During the STS-83 Spacelab mission, a sample of Pd78Cu6Si16 was processed in the electromagnetic levitation facility TEMPUS. Surface oscillations of the levitated liquid droplet were excited, and frequency and damping of the oscillations were observed. Under microgravity conditions, the damping constant is simply related to the viscosity. This method was successfully applied. The experiments were performed in a temperature range of 400 K, including the eutectic temperature Te=1033 K. At this temperature, our data agree well with those of S. K. Lee, K. H. Tsang, and H. W. Kui [J. Appl. Phys. 70, 4842 (1991)].

Surface Tension Measurements of Liquid Metals using Levitation, Microgravity and Image Processing

An improved method for measuring the surface tension of liquid metals is proposed. It makes use of the electromagnetic levitation technique for levitating a liquid metal droplet and of digital image processing to evaluate the surface oscillations of the droplet. The oscillation frequencies determine the surface tension. This paper contains a discussion of the theoretical background and a description of the experimental setup. In addition, preliminary results on FeNi samples in 1g and microgravity are presented.

Electrical resistivity measurement of liquid metals

Two facilities for the complete noninvasive measurement of the electrical resistivity of liquid metals above the melting temperature and in the undercooled liquid state below the melting temperature are presented: a ground-based facility that was built up in our laboratory and the microgravity facility TEMPUS. Both facilities are unique in combining the containerless positioning method of electromagnetic levitation with the contactless, inductive resistivity measurement technique, thereby enabling for the first time measurements in the undercooled liquid state of a metal. The principles and technical realizations of the levitation and measurement technique of both facilities are presented. Furthermore, experimental resistivity data for Cu60Ni40 as well as Co80Pd20 alloy are presented which indicate atomic ordering effects in the undercooled melt.

Viscosity and Surface Tension Measurements in Microgravity

The viscosity and surface tension of liquid metals can be measured by observing the oscillations of a levitated drop. The frequency is related to the surface tension, while the viscosity determines the damping of the oscillations. If no external forces are present, as in microgravity, these relations are particularly simple and precise. During the recent Spacelab mission MSL-1, such experiments have been performed on Co–Pd and Pd–Cu–Si using the electromagnetic levitation facility TEMPUS. It was possible to obtain data over a wide temperature range, including the undercooled regime. While the temperature dependence of the surface tension remains linear over the complete range, the temperature dependence of the viscosity is much more pronounced and is discussed in terms of different models.

Measurements of thermophysical properties of liquid metals by noncontact techniques

With the advent of containerless processing techniques such as electromagnetic levitation, it is now possible to study the properties of high-temperature liquid metalsin situ by applying sophisticated noncontact diagnostics, such as pyrometry and high-speed videography. Thermophysical properties of interest are, e.g., specific heat, thermal conductivity, and viscosity. Applying containerless processing, it is also possible to undercool the melt because of the lack of container-induced nucleation sites. This gives access to a metastable region of the phase diagram. The knowledge of thermophysical data in this region is very important, because undercooling plays a major role in any solidification process. The degree of undercooling not only determines the growth velocity, but also is crucial in selecting the eventually obtained metastable solid phase. In this paper, some recent developments are surveyed relating to the noncontact measurements of emissivity, specific heat, electrical conductivity, density, surface tension, and viscosity, as well as a discussion of possible experiments in microgravity.

Thermophysical property measurements of liquid metals by electromagnetic levitation

Electromagnetic levitation is a useful technique for containerless processing of liquid metals. In combination with non-contact diagnostic tools it can be used for completely non-invasive measurements of thermophysical properties of metallic melts. Considerable progress has been made in this field in recent years. This paper reviews methods and results, including those obtained in microgravity.

Electrical Resistivity of Undercooled Liquid Cu-Ni Alloys

Measured values for the electrical resistivity of undercooled liquid Cu-Ni alloys of different compositions are presented. The experiments were performed in a facility that combines the containerless positioning method of electromagnetic levitation with the contactless inductive resistivity measurement technique. For high nickel concentrations, i.e., for the liquid Cu20Ni80 and Cu40Ni60 alloys, the electrical resistivity shows, as well as for pure nickel and pure copper, the typical linear temperature dependence in the whole range from above to below the liquidus temperature. A significant deviation from the linear behavior occurs for liquid Cu60Ni40 and, less distinct, also for liquid Cu80Ni20. This is explained by a formation of nickel associates in the melt that influence the scattering cross section of the conduction electrons.

Thermophysical Properties of Undercooled Liquid Co80Pd20

The surface tension, viscosity, and electrical resistivity of liquid Co80Pd20were measured containerlessly for temperatures above and, especially, below the melting point. The first two quantities were measured with the help of the oscillating drop technique, the last one by an inductive method. The experiments have been performed under low gravity in the electromagnetic levitation facility TEMPUS during the MSL-1 Spacelab mission. This environment allowed us to measure for the first time the viscosity and electrical resistivity in the deeply undercooled state, where the Co80Pd20melt shows a magnetic ordering behavior. In this paper the measurement methods and results are presented.

Viscosity measurements of metallic melts using the oscillating drop technique

By means of benchmarking reduced gravity experiments, we have verified the measured viscosity of binary Zr-Ni glass forming liquids utilizing the oscillating drop technique combined with ground-based electrostatic levitation (ESL). Reliable viscosity data can be obtained as long as internal viscous damping of a single oscillation mode of a levitated drop dominates external perturbations. This can be verified by the absence of a sample mass dependence of the results. Hence, ESL is an excellent tool for studying the viscosity of metallic glass forming melts in the range of about 10–250 mPa s, with sample masses below 100 mg. To this end, we show that, for binary Zr-Ni melts, the viscosity is qualitatively controlled by the packing density.

Aspects of high-temperature pyrometry for measurements in ultrahigh vacuum

Containerless materials processing of liquid metals with the use of electromagnetic levitation requires contactless temperature measurement by pyrometry. For high temperatures and under high-vacuum conditions, the vapor pressure of the levitated metal drop increases, leading to evaporation losses of the sample material. This flux condenses on the cold parts of the experimental apparatus including the window in front of the pyrometer. As a result, the intensity of radiation reaching the pyrometer decreases, which is erroneously interpreted as a decrease in temperature. Several methods to protect the pyrometer against contamination have been proposed. In this paper, we report experimental tests of the concepts of shielding windows and mirror optics placed into the optical path between the sample and the pyrometer. Temperature measurements with a periscopic mirror system are also presented.