High temperature electrostatic sample levitator as a future containerless materials processing facility in space

Abstract

Investigation of deeply undercooled melts will open up the possibilities of studying basic phenomena of thermodynamics, nucleation and solidification processes. Such research work is particularly important to understand the processes of metastable solids which are formed from the non-equilibrium state of undercooled melt. There are a wide variety of metastable states, ranging from metastable crystalline phases (supersaturated and grain-refined alloys) to amorphous metals. A detailed understanding of the thermodynamics, the nucleation and crystal growth conditions can lead to comprehensive understanding of the criteria for the formation of such metastable states. However, at the present time, only limited information is available about the thermophysical parameters as a function of undercooling. The demands for accurate thermophysical property values have also been strong in the electronics industry which constantly demands high quality semiconductor materials for high density integrated circuit devices. In order to simulate the crystal growth for optimization of the growth process, the accurate thermophysical properties of molten semiconductors are essential input parameters. Thermophysical properties of high temperature molten materials are difficult to determine accurately because of the experimental problems associated with taking measurements at high temperatures in the presence of gravity. In the presence of gravity, convective flows are generated if there exist density gradients in a melt. In the high temperature materials processing, for reasons of maintaining purity of sample materials and attaining deeply undercooled states of melts, the sample has to be isolated from the container walls using some kind of levitators. However, the levitation of a high density melt against the gravity requires strong levitation forces which in turn induce undesirable flows in the melt. Such flows in melts would make measurements of certain thermophysical properties either impossible or at best erroneous. In microgravity environment of space, these disturbing effects are greatly reduced, therefore, more accurate measurement results are expected. Different kinds of sample positioning (levitating) devices have been investigated in the past, some of which have even gained a few space experiences. However, the only serious sample positioner which was designed for high temperature materials processing experiments in space is the TEMPUS, the German electromagnetic sample positioner. In this presentation I would like to introduce a new sample positioning device and discuss about its capabilities of measuring various thermophysical properties and studying non-equilibrium solidification process. At Jet Propulsion Laboratory, we have developed a high temperature electrostatic levitation system for the ground based applications (Rhim et al. 1993, and Rhim 1997). The system is operated in a high vacuum level (∼10−8 torr) and can heat up a sample of 2000 K. A typical sample diameter is 2∼3 mm. Advantages of the electrostatic levitation technique over other levitation techniques, especially the electromagnetic levitation is decoupling of levitation and heating elements and a wide selection of samples to be levitated. The sample can be levitated at any temperature between room and maximum temperatures. Both conductive and non-conductive (including semiconductive) materials can be levitated. For any thermophysical property measurements, diagnostic devices must be incorporated with the levitation technique. The devices must be based on non-invasive techniques and at the same time compatible with the levitation mechanism. We have developed the diagnostic techniques such as a high speed pyrometer, static as well as oscillating sample imaging and analysis. These techniques allow us to measure the thermophysical properties which include the true temperature, the emissivities, the density (specific volume) (Chung et al. 1996, and Ohsaka et al. 1997), total hemispherical emissivity (Rulison et al. 1995, and Rhim et al. 1997), specific heat (Rulison et al. 1995, and Rhim et al. 1997), surface tension, and viscosity (Rhim et al., submitted for publication, and Ohsaka et al., submitted for publication). We also envision adding the capabilities which will allow us to measure the thermal and electrical conductivities and to determine the liquid structures in the near future.