New sample levitation initiation and imaging techniques for the processing of refractory metals with an electrostatic levitator furnace

Abstract

Two new methods that substantially ease the processing and study of refractory metals, when an electrostatic levitation furnace is used, are reported. The first technique is concerned with preheating the sample on a pedestal, prior to launch, to a temperature (∼1500 K) at which thermionic emission dominates all other charging/discharging mechanisms that may be going on simultaneously. Launched into levitation at that temperature, the sample can be quickly heated to its molten state without encountering further charge loss problems. This procedure thus shortens substantially the time it takes to bring the samples to their final high temperature states at which their thermophysical properties can be measured. This technique can be applied to most materials whose melting temperatures are higher than their thermionic temperatures. The second technique described is an ultraviolet-based sample imaging configuration. Due to the excellent sample–background contrast it continuously provides during all phases of processing (from solid to overheated liquid sample), it allows the measurements of the density and the ratio of constant pressure heat capacity over hemispherical total emissivity of refractory metals. This method, compared with other imaging techniques, leads to more accurate density data at very high temperatures and to density temperature coefficients closer to those reported in the literature. This is exemplified in this article with zirconium and niobium liquid samples.