Density, Viscosity and Surface Tension Characterization of Supercooled Liquids and Oxide Melts Using Levitation in Microgravity

Investigation of thermophysical properties of melts and supercooled liquids provides insight needed for materials processing and manufacturing. Such property measurements are often limited by the use of containers, which prevent the formation of molten supercooled states by providing crystallization surfaces as well as opportunities for contamination and unwanted side reactions. Containers also prohibit the use of imaging techniques that can aid with measurement of properties such as density, surface tension, and viscosity of supercooled liquids. These limitations were overcome by utilizing the Electrostatic Levitation Furnace (ELF) operated by the Japan Aerospace Exploration Agency (JAXA) aboard the International Space Station (ISS). Levitation in microgravity allows in-situ density analysis and buoyancy-related convection free oscillation analysis of supercooled melts.<br/><br/>The samples were selected to cover a range of liquid fragility. In most cases the materials were relevant to development of optical technologies or investigation of geological phenomena. In ground based work, compositions based on titanium, silicon or aluminum oxides with rare earth oxides were produced as ca. 2 mm diameter spheres by melting powder mixtures with a laser beam. In the ISS experiments, an electrostatic force was applied to the sample to position it where it could be heated and melted with laser beams. Drops were imaged with a high resolution camera equipped with a bandpass filter that transmitted only at the wavelength of a bright backlight. Silhouettes of the sample were analyzed frame-by-frame by fitting to an ellipsoid using 200 points along the edge where a large brightness gradient occurs. Density was calculated from the fitted volume and known sample mass.<br/><br/>Oscillation of liquid drops was stimulated by modulating the electrostatic forces. The excitation frequency was scanned to find the characteristic resonant frequency of the drop. When the excitation was stopped the amplitude of oscillation decayed exponentially due to viscous damping. Analysis of the oscillation power spectrum enabled the surface tension and viscosity of the liquid to be determined at temperatures above and below the equilibrium melting point. The resonant frequency was determined from a Fourier transform of the power spectrum. The oscillation amplitude data were fitted with a least squares method to determine the exponential decay constant and hence viscosity. Preliminary analysis revealed Nd titanates exhibit non-Arrhenius viscosities of 15-20 mPa.s and density of 4.6-4.8 g cm^-3 in the temperature range of 1400-1700 °C. Samples are being returned to Earth where additional measurements will be made. These include X-ray and neutron diffraction to measure atomic structure. Calorimetry will be used to measure the glass transition temperature and enthalpy of vitrification..<br/><br/>This paper will describe details of the image analysis and results of measurements on selected samples.<br/><br/>This work was supported by NASA under grants 80NSSC18K0059 and 80NSSC19K1288.