Electromagnetic Levitation Part III: Thermophysical Property Measurements in Microgravity

摘要

Strong inhomogeneous magnetic fields are necessary to generate a finite levitation force in ground based electromagnetic levitation techniques. External forces such as magnetic and gravitational forces influence the oscillation spectrum and counteract the surface movement resulting in a frequency shift, and making the use of electromagnetic levitation techniques in microgravity an attractive alternative to measure thermophysical properties of liquid metals. Under microgravity conditions the magnetic field strength around a liquid droplet is significantly lower than that required to position the same specimen against earth gravity. Hence, a low magnetic field strength results in a low amount of heat energy absorbed by the specimen making the deep undercooling of molten metals in UHV environment possible. There is no need to cool samples convectively using a high-purity inert gas. The low strength and uniformly distributed magnetic force fields do not change the spherical shape of the droplet, and the theories which assume spherical droplet shape can be applied to determine thermophysical properties, such as viscosity, surface tension and electrical conductivity. A low magnetic field strength slows down the stirring of the molten specimen and reduces the turbulence of fluid motion.