Takehiko Ishikawa

Rare earth aluminum garnets are important materials in optical, dielectric, and thermal barrier applications. To advance the understanding of their melt processing and glass forming ability, we report the atomic structure of molten Yb3Al5O12 over 1770–2630 K, which spans the equilibrium and supercooled liquid regimes. The melt density at Tm = 2283 K is 5.50 g cm−3, measured via silhouette imaging of electrostatically levitated drops over 1010–2420 K. Four separate structure measurements were made with aerodynamically levitated melts using x-ray and neutron diffraction with isotope substitution of Yb (172Yb, 174Yb, or natYb). Empirical potential structure refinement models were developed, which are in excellent agreement with the experiments. Coordination environments for Al–O are predominantly 4- and 5-coordinate, with a mean coordination of nAlO = 4.43(8), while Yb–O environments mostly range from 5- to 8-coordinate, with nYbO = 6.26(8). The cation–oxygen polyhedra are connected primarily by corner-sharing, with edge-sharing constituting up to ∼1/3 of the connectivity among polyhedra with Yb or higher-coordinated Al–O. Structurally, the –Al–O– network in molten Yb3Al5O12 appears conducive to glass formation: nOAl = 1.85(3), there are 1.86 AlOx–AlOx connections per Al atom (e.g., a mixture of Q3 and Q4 units), and the modal ring size is six cations. These characterize a network that is somewhat less constrained compared to SiO2 glass, yet Yb3Al5O12 cannot be quenched into crystal-free glass. Aluminum garnet compositions with larger rare earth cations do form glass, so these characterizations help reveal the structural characteristics corresponding to the limit of glass forming ability in rare earth aluminates.

Abstract Possible existence of dense iron-rich silicate melt layer above Mars’ core is important in understanding the nature and evolution of Mars. However, gravitational stability of iron-rich silicate melt in the Mars’ interior has not been well constrained, due to experimental difficulties in measuring density of iron-rich peridotitic melt. Here we report density measurements of iron-rich peridotitic melts up to 2465 K by using electrostatic levitation furnace at the International Space Station. Our experimentally obtained densities of iron-rich peridotitic melts are markedly higher than those calculated by first principles simulation, and are distinct from those estimated by extrapolating a density model for SiO₂-rich basaltic melts. Our determined density model suggests that peridotitic melt with the Fe/(Mg+Fe) ratio more than 0.4-0.5 has higher density than that at the base of the Mars’ mantle, which indicates gravitational stability of the iron-rich peridotitic melt at the core-mantle boundary in Mars.

Abstract In this study, the microstructure, hardness, density, viscosity, and surface tension of molten pure Ti with TiC particles were studied via electrostatic levitation experiments, where the electrostatic levitation experiment involved container-less processing, which can suppress heterogeneous nucleation via crucibles. Microstructural observation revealed long needle-shaped α-grains across the whole area in the pure Ti sample. On the other hand, smaller needle-shaped α-grains were found in the samples with TiC particles. However, the detailed microstructural analysis of Ti + 0.7vo l%TiC sample revealed that the fine α-grains observed in the Ti + 0.7vo l%TiC are transformed from single grain of prior β phase. This is because the TiC particles dissolve into the molten Ti during the electrostatic levitation experiment. Instead, Ti–rich TiC precipitates formed by cooling can act as pinning sites rather than heterogeneous nucleation sites, which results in a finer microstructure for the samples with TiC particles during the electrostatic levitation experiment. The density of the samples is linearly related to the temperature, and it decreases with increasing temperature. In addition, a higher density is observed for the samples with TiC particles. Although linear relationships between the surface tension and temperature were found, the addition of TiC particles had no notable effect on the viscosity of the molten pure Ti.

Rare earth and barium titanates are useful as ferroelectric, dielectric, and optical materials. Measurements of their thermophysical properties in the liquid state can help guide melt processing technologies for their manufacture and advance understanding of fragile liquids' behavior and glass formation. Here, we report the density, thermal expansion, viscosity, and surface tension of molten BaTi2O5, BaTi4O9, and 83TiO2-17RE2O3 (RE = La or Nd). Measurements were made using electrostatic levitation and droplet oscillation techniques in microgravity, which provide access to quiescent liquid droplets and deep supercooling of 510–815 K below the equilibrium melting points. Densities were measured over 900–2400 K. Viscosities were similar for all four compositions, increasing from ∼10 mPa s near 2100 K to ∼30 mPa s near 1750 K. Surface tensions were 450–490 dyn cm−1 for the rare earth titanates and 383–395 dyn cm−1 for the barium titanates; surface tensions of all compositions had small or negligible temperature dependence over 1700–2200 K. For solids recovered after melt quenching, x-ray microtomography revealed the fracture mechanics in crystalline products and minimal internal porosity in glass products, likely arising from entrapped gas bubbles. Internal microstructures were generally similar for products processed either in microgravity or in a terrestrial aerodynamic levitator.

Abstract The relationships between materials processing and structure can vary between terrestrial and reduced gravity environments. As one case study, we compare the nonequilibrium melt processing of a rare-earth titanate, nominally 83TiO₂-17Nd₂O₃, and the structure of its glassy and crystalline products. Density and thermal expansion for the liquid, supercooled liquid, and glass are measured over 300–1850 °C using the Electrostatic Levitation Furnace (ELF) in microgravity, and two replicate density measurements were reproducible to within 0.4%. Cooling rates in ELF are 40–110 °C s⁻¹ lower than those in a terrestrial aerodynamic levitator due to the absence of forced convection. X-ray/neutron total scattering and Raman spectroscopy indicate that glasses processed on Earth and in microgravity exhibit similar atomic structures, with only subtle differences that are consistent with compositional variations of ~2 mol. % Nd₂O₃. The glass atomic network contains a mixture of corner- and edge-sharing Ti-O polyhedra, and the fraction of edge-sharing arrangements decreases with increasing Nd₂O₃ content. X-ray tomography and electron microscopy of crystalline products reveal substantial differences in microstructure, grain size, and crystalline phases, which arise from differences in the melt processes.

A study of uncertainty analysis was conducted on four key thermophysical properties of molten Platinum using a non-contacting levitation technique. More specifically, this work demonstrates a detailed reporting of the uncertainties associated with the density, volumetric thermal expansion coefficient, surface tension and viscosity measurements at higher temperatures for a widely used refractory metal, Platinum using electrostatic levitation (ESL). The microgravity experiments were conducted using JAXA’s Electrostatic Levitation Furnace (ELF) facility on the International Space Station and the terrestrial experiments were conducted using NASA’s Marshal Space Flight Center’s ESL facility. The performance of these two facilities were then quantified based on the measurement precision and accuracy using the metrological International Standards Organization’s Guide to the Expression of Uncertainty Measurement (GUM) principles.

Abstract A new method for quantifying facility performance has been discussed in this study that encompasses uncertainties associated with thermophysical property measurement. Four key thermophysical properties: density, volumetric thermal expansion coefficient, surface tension, and viscosity of liquid Au have been measured in microgravity environment using two different levitation facilities. Levitation experiments were conducted using the Electrostatic Levitation Furnace (ELF) onboard the ISS in Argon and air, and the TEMPUS Electromagnetic Levitation (EML) facility on a Novespace Zero-G aircraft parabolic flight in Argon. The traditional Maximum Amplitude method was augmented through the use of Frequency Crossover method to identify the natural frequency for oscillations induced on a molten sample during Faraday forcing in ESL. The EML tests were conducted using a pulse excitation method where two techniques, one imaging and one non-imaging, were used to study surface oscillations. The results from both facilities are in excellent agreement with the published literature values. A detailed study of the accuracy and precision of the measured values has also been presented in this work to evaluate facility performance.

This work employed the electrostatic levitation furnace (ELF) apparatus installed in the KIBO Japanese Experiment Module on the International Space Station. The ELF is able to levitate high-melting-point materials such as ceramics without containers. The heating and melting of refractory oxides in this apparatus allows the analysis of various thermophysical properties, including density, surface tension and viscosity, that are otherwise very difficult to measure in terrestrial laboratories. In the present study, the density of molten Y2O3 was determined over a wide temperature range using the ELF. A density of 4700 kg/m3 was obtained at 2712 K, in good agreement with the value previously obtained using an aero-acoustic levitator. The relationship between the ionic radius and the molar volume of liquid Y2O3 was ascertained and found to be similar to those for other non-glass-forming sesquioxides.

Density, thermal expansion coefficient, surface tension and viscosity of Ni-based CMSX-4� Plus have been measured for a range of liquid temperature by utilizing two Electrostatic levitation (ESL) facilities. Ground-based tests were conducted using the NASA MSFC ESL facility in Ultra High Vacuum and space-based tests were conducted using JAXA ELF in a 172 kPa Argon gas atmosphere. The measured values were compared to the available literature data from various other facilities. This study focuses on a detailed uncertainty analysis of the experimental data to measure the accuracy and precision of the measured properties using Guide to the expression of Uncertainty Measurement (GUM) principles. The findings from this study have been used to quantify the performance of the two ESL facilities.

Abstract Round-robin measurement of surface tension of high-temperature liquid platinum was conducted free of any contamination from the supporting materials and oxygen adsorption, using an electrostatic levitator (ESL), two electromagnetic levitator (EML), and an aerodynamic levitator (ADL). The measured temperature dependences of the surface tension using ESL and two EMLs were in good agreement and were expressed as σ=1,798±74.3−(0.12±0.0445)×(T−2,041)\sigma =\mathrm{1,798}\pm 74.3-(0.12\pm 0.0445)\times (T-\mathrm{2,041}) [10^–3 N·m^–1] (1,900–2,600 K). However, the surface tension values measured with ADL were slightly lower than those exceeding the uncertainty of the measurement plots at high temperatures.

Due to their high melting temperatures and the risk of contamination from the crucibles, molten oxides which melting temperatures are above 2000 °C can hardly be processed using conventional methods. This explains that their thermophysical properties are very scarce. Containerless methods with gas flows have been developed and several thermophysical properties such as density, surface tension, and viscosity have been reported. However, the gas flow has detrimental side effects such as deformation of the sample and induction of internal flows in the molten sample, which affect the accuracy of the measurements. The electrostatic levitation furnace onboard the International Space Station (ISS-ELF), which utilizes the Coulomb force to levitate and melt samples in microgravity, has several advantages for thermophysical property measurements of refractory oxide melts. Levitation without a gas flow coupled to a reduced gravity environment minimizes the required levitation (positioning) force and reduces the deformation as well as the internal flow. This report briefly introduces the ISS-ELF facility and the thermophysical property measurement methods. The measured density, surface tension, and viscosity of molten Al₂O₃ are then presented and compared with the ones obtained by other methods. Finally, the measured data of refractory oxides whose melting temperatures are above 2,400 °C are summarized.

Abstract We measured the thermophysical properties of molten gallium oxide (Ga₂O₃) in a contamination-free and microgravity environment by using the electrostatic levitation furnace in the International Space Station. The density of molten Ga₂O₃ was obtained over a wide temperature range of 2001–2174 K including the undercooled state and found to be expressed as 5004.8–0.4478(T − T_m) (kg m⁻³), where T_m, the melting point, is 2066 K. Measurements of its viscosity and surface tension were also performed by using the drop oscillation method and these values were found to be 337.0 (10⁻³ N m⁻¹) and 13.6 (10⁻³ Pa·s) at 2228 K, respectively.

To accurately measure the surface tension of liquid titanium free of contamination from chemical reaction with the supporting materials and dissolution of atmospheric oxygen, the measurement was performed by using electromagnetic levitation (EML) and electrostatic levitation (ESL) in consideration of the influence of oxygen partial pressure of the measurement atmosphere, PO2. When liquid titanium was maintained at 2000 K under Ar–He gas with PO2 of 10 Pa flowing at 2 L·min−1 using EML, the surface tension decreased with time due to the dissolution of atmospheric oxygen into the sample. When the PO2 of the gas was decreased to 10−2 Pa, the oxygen content and the surface tension were confirmed to not vary, even after 120 min. Even though PO2 further decreased to 10−11 Pa under Ar–He–H2 gas, the surface tension slightly increased with time due to gas phase equilibrium between H2 and H2O that allowed for a continuous dissolution of atmospheric oxygen into the liquid titanium. The surface tension of liquid titanium measured by ESL, which prevents contamination of the sample from supporting materials and the high 10−5 Pa vacuum inhibits the dissolution of oxygen, showed almost the same value as that measured under Ar–He gas at PO2 of 10−2 Pa by EML. From the measurement results of EML and ESL, the surface tension of the 99.98 mass % pure liquid titanium, free from any contaminations from chemical reactions, with the supporting material and dissolved oxygen was expressed as σ99.98%=1613−0.2049T−1941 (10−3 N·m−1).

Liquid densities of three lanthanoid sesquioxides (Tm2O3, Yb2O3, and Lu2O3), whose melting temperatures are above 2400 °C, were measured using an electrostatic levitation furnace onboard the International Space Station (ISS). Each sample was positively charged, and its position was controlled by Coulomb forces between the sample and the surrounding electrodes. Following heating and melting of the sample by high-power lasers, its volume was calculated from its spherical shape in its liquidus phase. After weighing the mass of the sample returned to Earth, its density was determined. The densities (ρ) of Tm2O3, Yb2O3, and Lu2O3 can be expressed as ρTm2O3 = 8304 − 0.18 × (T − Tm), ρYb2O3 = 8425 − 0.55 × (T − Tm), and ρLu2O3 = 8627 − 0.43 × (T − Tm), respectively, where Tm is their melting temperatures.

Co-Cr-Mo (CCM) alloys, which are used in biomedical implants, are currently produced by additive manufacturing, for which accurate modeling of the process is required to attain the desired thermophysical properties of the melts. For the purpose of modeling, the density, surface tension, and viscosity of two CCM melts of distinct carbon content (0.05 and 0.25% by mass) were measured using an electrostatic levitation technique. The temperature dependence of both density and surface tension of the melts were assumed to be linear, whereas that for viscosity was assumed to have Arrhenius form, from which the activation energy corresponding to the viscous flow of each CCM melt was obtained. Using the Szyzkowski model along with our present and previous results, the influence of the partial pressure of oxygen on the surface tension of the two CCM melts was evaluated. No substantial difference in surface tension and viscosity between these melts was found.

<title>Abstract</title>The Faraday forcing method in levitated liquid droplets has recently been introduced as a method for measuring surface tension using resonance. By subjecting an electrostatically levitated liquid metal droplet to a continuous, oscillatory, electric field, at a frequency nearing that of the droplet’s first principal mode of oscillation (known as mode 2), the method was previously shown to determine surface tension of materials that would be particularly difficult to process by other means, e.g., liquid metals and alloys. It also offers distinct advantages in future work involving high viscosity samples because of the continuous forcing approach. This work presents (1) a benchmarking experimental method to measure surface tension by excitation of the second principal mode of oscillation (known as mode 3) in a levitated liquid droplet and (2) a more rigorous quantification of droplet excitation using a projection method. Surface tension measurements compare favorably to literature values for Zirconium, Inconel 625, and Rhodium, using both modes 2 and 3. Thus, this new method serves as a credible, self-consistent benchmarking technique for the measurement of surface tension.

<title>Abstract</title>Understanding the liquid structure provides information that is crucial to uncovering the nature of the glass-liquid transition. We apply an aerodynamic levitation technique and high-energy X-rays to liquid (<italic>l</italic>)-Er₂O₃ to discover its structure. The sample densities are measured by electrostatic levitation at the International Space Station. Liquid Er₂O₃ displays a very sharp diffraction peak (principal peak). Applying a combined reverse Monte Carlo – molecular dynamics approach, the simulations produce an Er–O coordination number of 6.1, which is comparable to that of another nonglass-forming liquid, <italic>l</italic>-ZrO₂. The atomic structure of <italic>l</italic>-Er₂O₃ comprises distorted OEr₄ tetraclusters in nearly linear arrangements, as manifested by a prominent peak observed at ~180° in the Er–O–Er bond angle distribution. This structural feature gives rise to long periodicity corresponding to the sharp principal peak in the X-ray diffraction data. A persistent homology analysis suggests that <italic>l</italic>-Er₂O₃ is homologically similar to the crystalline phase. Moreover, electronic structure calculations show that <italic>l</italic>-Er₂O₃ has a modest band gap of 0.6 eV that is significantly reduced from the crystalline phase due to the tetracluster distortions. The estimated viscosity is very low above the melting point for <italic>l</italic>-ZrO₂, and the material can be described as an extremely fragile liquid.

The Japan-German joint research program "PHOENIX-2" on the cool flame dynamics using TEXUS sounding rocket is planned and in progress. The reference data of spontaneous ignition of fuel droplet arrays and pairs in hot air and those of succeeding cool flame combustion arc to he obtained through the microgravity experiments. The present review introduces the blown mechanism of cool flame and the importance of the flame in various practical combustion applications. The recent research revealed the significant scientific needs for reliable modelling of cool flame chemistry, especially for the case of non-uniform field where heat and species dissipate. The droplet combustion system is suited for controlling the dissipative field, and the experimental data are promised to be the reference for the chemical modelling. The scope of the experiments is explained in view of this needs.

Understanding liquid structures provides crucial information for uncovering the nature of glass transition. We adopted a combination of the aerodynamic levitation technique and the synchrotron hard X-rays to reveal the structure of high-temperature liquid oxides. To achieve accurate diffraction measurements, the Iwo-axis diffractometer at the high-energy X-ray diffraction beamline 13L04B2 of SPring-8 was upgraded. By installing four CdTe detectors and three Ge detectors, we can measure diffraction signals from levitated liquids approximately three times faster than the previous set-up. We have measured liquid (l)-Y2O3, l-Gd2O3 and l-Ho2O3. No first sharp diffraction peak (FSDP) was observed in these non-glass forming liquids but a sharp principal peak (PP) was observed. Density data on these liquids necessary for further analysis are currently measured using the electrostatic levitation furnace in the International Space Station (ISS).

© 2020 Author(s). The phase relation between supercooled liquid silicon (l-Si) and amorphous silicon (a-Si) is discussed based on experimental results. Electrostatically levitated l-Si samples were supercooled down to low temperatures, 300 K below the melting temperature (Tcl: 1683 K), and solidified accompanied by the release of latent heat. It was found that solidified Si samples melted again at 1480 K caused by the latent heat. Also, it was found that the Si samples that rapidly quenched near the solidification temperature contained a large amount of a-Si with tetrahedral coordination. These two findings show that the supercooled l-Si samples solidified into a-Si and a-Si melted, confirming the idea of a first-order phase transition between two metastable phases proposed by Turnbull et al. [Metall. Mater. Trans. A 29, 1825 (1998)].

Density of gadolinium oxide in its liquid phase was measured using a containerless technique under microgravity environment in the International Space Station (ISS). An electrostatically levitated sample was melted using high power semiconductor lasers. Pictures of a molten spherical sample were analyzed and corresponding volumes were obtained as afunction of temperature. After weighing the returned sample mass, the density of the Gd2O3 was found to be 7240 kg/m3 at its melting temperature (Tm = 2693 K).

A microgravity experiment utilizing sounding rocket is going to be held in 2021 to clarify cool flame occurrence from n-decane droplet array near spontaneous ignition limit. As a preliminary study for the experiment, the 2D numerical simulation is carried out. The almost identical numerical geometry to the actual furnace for the rocket experiment is used to predict the cool flame occurrence in the rocket experiment. The interference of the droplets is validated by calculation with different inter-droplet distance. The employed fuel is n-decane of 1.0 mm in diameter. The initial temperature and pressure are 550 K and 1 atm respectively. The results shows that the cool flame occurs from the outside of the fuel droplet array. A fuel concentration at inter-droplet is higher than the outer side, whereas the temperature at inter-droplet is lower than the outer side. It is thought that the lower temperature yields increase in the spontaneous ignition delay time surpassing the shorten effect of the ignition delay time due to higher fuel concentration.