Gravity-induced anomalies in interphase spacing reported for binary eutectics.

It has been reasoned that desirable microstructural refinement in binary eutectics could result from freezing in reduced-gravity. It is recognized that the interphase spacing in a binary eutectic is controlled by solute transport and that, on Earth, buoyancy-driven convection may enhance this. Hence, it has been presumed that the interphase spacing ought to decrease when a eutectic alloy is frozen under conditions of much-reduced gravity, where such buoyancy effects would be largely absent. The result of such speculation has been that many workers have frozen various eutectics under reduced gravity and have reported that, although some eutectics became finer, others showed no change, and some even became coarser. This reported varied behavior will be reviewed in the light of long term studies by the author at Queen's University, including recent microgravity studies in which samples of two eutectic alloy systems, MnBi-Bi and MnSb-Sb, were frozen under very stable conditions and showed no change in interphase spacing.

The study of devitrification processes in heavy-metal fluoride glasses.

Heavy-metal fluoride glasses are very promising optical fiber materials because of their predicted ultralow loss and long transparency range. Although conventional silica fibers have attained their theoretical minimum loss of 0.15 dB/km, fluoride glasses have the potential to yield losses of only 0.001 dB/km. Fluoride glasses also exhibit transparency into mid-IR frequencies, a region inaccessible to silica fibers. However, this group of glasses is very unstable to devitrification during both bulk glass synthesis and fiber-drawing. This instability has limited their commercial exploitation to a small niche market in the laser industry. The ZBLAN glass (53ZrF(4)-20BaF(2)-4LaF(3)-3AlF(3)-20NaF) is the most promising of these materials since its fiber-drawing region lies on the edge, or possibly just outside its crystallization region. It is believed that additional research into understanding the nucleation mechanics involved in the devitrification of fluoride glasses will lead to the development of technology to suppress such nucleation, or at least minimize the associated crystallization temperature region, allowing high optical quality fibers to be produced. It has recently been demonstrated that a microgravity environment can suppress devitrification in ZBLAN glass preform preparation, and that devitrification may be reduced when preparing ZBLAN terrestrially in a containerless facility. It is believed that the role of viscosity is critical in the devitrification mechanism of ZBLAN glass and in determining the optimum fiber-drawing temperature. Unfortunately, viscosity data for fluoride glasses are only available above the melting point and around the glass transition. A piezoelectric viscometer has been developed and is being used to determine the missing viscosity data in the fiber-drawing and crystallization temperature regions. Shear thinning of the glasses and/or the application of hydrostatic pressure on the glasses have been recently proposed to be responsible for devitrification during fiber-drawing at 1 g and in reduced gravity. The study we report here is to explore the extent to which such a proposal is realistic.

The influence of gravity on the precise measurement of solute diffusion coefficients in dilute liquid metals and metalloids.

It is now well known that the diffusion coefficient (D) measured in a laboratory in low earth orbit (LEO) is less than the corresponding value measured in a terrestrial laboratory. However, all LEO laboratories are subject to transient accelerations (g-jitter) superimposed on the steady reduced gravity environment of the space platform. In measurements of the diffusion coefficients for dilute binary alloys of Pb-(Ag, Au,Sb), Sb-(Ga,In), Bi-(Ag,Au,Sb), Sn-(Au,Sb), Al-(Fe, Ni,Si), and In-Sb in which g-jitter was suppressed, it was found that D proportional to T (temperature) if g-jitter was suppressed, rather than D proportional to T(2) as observed by earlier workers with g-jitter present. Furthermore, when a forced g-jitter was applied to a diffusion couple, the value measured for D increased. The significance of these results is reviewed in the light of recent work in which ab initio molecular dynamics simulations predicted a D proportional to T relationship.

ATEN: a new high temperature materials processing facility for the international space station.

Since the beginning of microgravity materials research, studies of diffusion in liquids have been performed as the typical research that efficiently uses the microgravity environment. Successful experiments in microgravity have demonstrated the ability of the Canadian Microgravity Program (QUEST I, QUELDs I and II) to make significant contributions to this field of international microgravity research. Recently, Millenium Biologix was selected to develop and build the advanced thermal environment facility (ATEN) for the International Space Station. The design of this new processing facility builds on the considerable experience gained in designing and building the QUELD II furnace and developing sealed samples for use on board a manned space platform. The system requirements for ATEN are presented, along with preliminary test data from a prototype furnace.

A molecular dynamics simulation of the diffusion of the solute (Au) and the self-diffusion of the solvent (Cu) in a very dilute liquid Cu-Au solution.

The identification of the manner in which a solute diffusion coefficient (D) might vary with temperature (T) in a fused metal or semimetal has led to considerable experimental study and some theoretical analysis. However, the conclusions of this work are inconsistent. In the present work, molecular dynamics studies of diffusion of a very dilute solute (Au) in liquid Cu are presented. Using the simple Enskog theory of diffusion, it is shown that the ratio of the diffusion constant of the solute to the diffusion constant of the solvent for a very dilute solution is approximately constant. This prediction is confirmed by molecular dynamics simulations although the values of ratios agree only within 20%-25%. In agreement with experiment, current simulations predict that within the usually investigated temperature range, the diffusion coefficient is linearly dependent on temperature. A very small contribution of parabolic behavior can only be observed for a temperature range much wider than that available for physical experiments due to materials limitations.

Estimation of the solute diffusion coefficient of a dilute liquid alloy: static structure factor and isothermal compressibility estimates obtained using the rational function approximation of the radial distribution.

A simple method for estimating the mass diffusion coefficient of a dilute binary liquid alloy that sequentially uses experimental data for the static structure factor and isothermal susceptibility of the solvent is presented, as well as another using the static structure factor alone and a method using the isothermal susceptibility alone. A fourth method that simultaneously uses the static structure factor and isothermal susceptibility is also noted. Of significance is the fact that these methods do not require information about the interatomic potential. Stability with respect to weights in the optimization process employed has been established and is reported, as well as some indication of the upper limits on the applicable solute concentration. Comparisons are made with results from a high quality capillary experiment for Pb 1 wt% Au liquid alloy performed in microgravity, and with velocity autocorrelation estimates derived from molecular dynamics simulation. The results suggest that the capillary experiments are influenced by reverse diffusion of the solvent, and actually measure an average of the mass diffusion coefficients, D(ij), weighted by the equilibrium concentrations of the solvent, x(1), and solute, x(2), defined by [Formula: see text] The three methods are required to provide upper and lower estimates for the mixed solvent-solute diffusion coefficient, which is not directly accessible from the experimental data, and demonstrate agreement with the experiment via D(tot).

Solute diffusion in nonionic liquids--effects of gravity.

We have been engaged in examining the influence of gravity on the results of experiments to measure the variation of solute diffusion coefficients (D) with temperature (T) in fused metals and semimetals since our first STS flights in 1992. These early experiments, conducted with the in situ g-jitter of the shuttle, showed the near-parabolic variation of D with T reported by others. However, with the aid of the Canadian Space Agency's microgravity isolation mount (MIM) to isolate the diffusion facility from the existing g-jitter of the Russian space station MIR, we showed that in all the alloy systems and over the temperature range studied, D increased linearly with T. If the isolating system was deactivated, then the more familiar parabolic relationship appeared. We have always assumed that the values of D measured using the MIM would be closer to the intrinsic values for the alloy system considered; to test this contention, we have been involved in two modeling activities. The first has been to estimate the effects of g-jitter-level disturbances on solute distributions in long capillary diffusion couples. The second has been to conduct various molecular dynamics modeling studies of solute diffusion. This paper presents results of these studies.