Explanation

The transfer of mass and heat governs numerous materials preparation process of technological importance. An accurate knowledge of mass and thermal diffusivities is important for improving model predictions in materials processing. However, currently, even self-diffusion in liquid elements is poorly understood, much less binary diffusion. Besides this technological interest, reliable diffusivity data over a wide temperature range is also important for the verification of microscopic theories of transport in liquids and correlation to the structure of liquids.

Diffusivities obtained in liquids at normal gravity are prone to be contaminated by uncontrolled convection. Simple scaling illustrates the difficulty of obtaining purely diffusive transport in liquids. In a system of diffusivity 10^-5 cm²/sec and a typical diffusion distance of 1 cm the characteristic diffusion distance is of order 10^-5 cm/sec, hence is true diffusion is to be observed, convective flow velocities parallel to the concentration gradient must be of order 10^-7 cm/sec or less. Thus, in liquids at normal gravity, the attainment of diffusion-dominated transport over macroscopic distances is not a simple task.

Presently, low-gravity conditions appear to offer the only means to obtain reliable liquid diffusion data. However, in view of the limited flight opportunities that exist, more efficient experimental scheme need to be developed.

Personnel at the CMMR have development a novel, in-situ, methodology for determination of the (mass) diffusivity and its' temperature dependence. This technique allows us to measure diffusivities at several temperatures utilizing a single sample. Employing this technique diffusivities can be determined essentially in real-time without the need for sample solidification and post-flight analysis. Thus, (low-gravity) diffusivity determinations can be conducted with higher frequency and immediate data feedback/analysis. A low temperature version of this hardware was flown on Mir. Future versions will be flown on the Shuttle or the international space station.

For our self-diffusivity studies eight elements have been chosen (In, Cd, Sn, Te, Se, Rb, Al and Ga; Sb and Zn may be added), based on their expected temperature dependence. In addition, determining the diffusivity and its temperature dependence of these elements will provide a strong theoretical underpinning for our diffusivity studies in the II-VI semiconductors.

Thermal diffusion studies in the liquid II-VI semiconductors have been very limited. This is due to several technological problems associated with these systems; such as their low thermal conductivity and aggressive (chemically reactive) melts. Thermal diffusion measurements are prone to convective contamination similar to the mass-diffusion measurements. However, in these measurements the temperature (and hence, density) gradient is established as part of the measurement process. We are developing experimental schemes and techniques to deal with these problems. Again, low gravity experiments appear to offer a means to obtain reliable results.

The thermal expansion of liquids, particularly at their melting points, is difficult to measure. Because of the importance of these values in our thermal and mass diffusion measurements in the II-VI materials we are developing an interferometric technique to determine these values.

• Development a novel, in-situ, methodology for determination of the (mass) diffusivity and its' temperature dependence. This technique allows us to measure diffusivities at several temperatures utilizing a single sample. Employing this technique diffusivities can be determined essentially in real-time without the need for sample solidification and post-flight analysis. Thus, (low-gravity) diffusivity determinations can be conducted with higher frequency and immediate data feedback/analysis. A low temperature version of this hardware was flown on Mir. Future versions will be flown on the Shuttle or the international space station.

• Characterized the influence of the low (10^-6 - 10^-4 g) steady state and g-jitter inputs on fluid systems. This work has led to the capability of predicting the effect of various shuttle orbital maneuvers and environment on fluid experiments. These results were most recently confirmed in the MESPHISTO hardware.

• Design instrumentation to determine the thermal diffusivities of low thermal conduction semiconductor melts. In these systems thermal "short circuits" through the container wall often mask the true values. This hardware will be flown on the international space station.

Research Group