Yuko Inatomi

<jats:p>Planetary protection is a guiding principle aiming to prevent microbial contamination of the solar system by spacecraft (forward contamination) and extraterrestrial contamination of the Earth (backward contamination). Bioburden reduction on spacecraft, including cruise and landing systems, is required to prevent microbial contamination from Earth during space exploration missions. Several sterilization methods are available; however, selecting appropriate methods is essential to eliminate a broad spectrum of microorganisms without damaging spacecraft components during manufacturing and assembly. Here, we compared the effects of different bioburden reduction techniques, including dry heat, UV light, isopropyl alcohol (IPA), hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>), vaporized hydrogen peroxide (VHP), and oxygen and argon plasma on microorganisms with different resistance capacities. These microorganisms included <jats:italic>Bacillus atrophaeus</jats:italic> spores and <jats:italic>Aspergillus niger</jats:italic> spores, <jats:italic>Deinococcus radiodurans</jats:italic>, and <jats:italic>Brevundimonas diminuta</jats:italic>, all important microorganisms for considering planetary protection. <jats:italic>Bacillus atrophaeus</jats:italic> spores showed the highest resistance to dry heat but could be reliably sterilized (i.e., under detection limit) through extended time or increased temperature. <jats:italic>Aspergillus niger</jats:italic> spores and <jats:italic>D. radiodurans</jats:italic> were highly resistant to UV light. Seventy percent of IPA and 7.5% of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> treatments effectively sterilized <jats:italic>D. radiodurans</jats:italic> and <jats:italic>B. diminuta</jats:italic> but showed no immediate bactericidal effect against <jats:italic>B. atrophaeus</jats:italic> spores. IPA immediately sterilized <jats:italic>A. niger</jats:italic> spores, but H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> did not. During VHP treatment under reduced pressure, viable <jats:italic>B. atrophaeus</jats:italic> spores and <jats:italic>A. niger</jats:italic> spores were quickly reduced by approximately two log orders. Oxygen plasma sterilized <jats:italic>D. radiodurans</jats:italic> but did not eliminate <jats:italic>B. atrophaeus</jats:italic> spores. In contrast, argon plasma sterilized <jats:italic>B. atrophaeus</jats:italic> but not <jats:italic>D. radiodurans</jats:italic>. Therefore, dry heat could be used for heat-resistant component bioburden reduction, and VHP or plasma for non-heat-resistant components in bulk bioburden reduction. Furthermore, IPA, H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>, or UV could be used for additional surface bioburden reduction during assembly and testing. The systemic comparison of sterilization efficiencies under identical experimental conditions in this study provides basic criteria for determining which sterilization techniques should be selected during bioburden reduction for forward planetary protection.</jats:p>

Thermoelectric materials with optimum carrier concentration of the order of 1019–1020/cm3 are required to obtain a high figure of merit (ZT) value. As undoped In0.8Ga0.2Sb has a lower carrier concentration (~1016/cm3), Te impurity was doped between low (1 × 1018/cm3) and high level (1 x 1021/cm3) to understand the effects of doping on its thermoelectric properties. The undoped and Te-doped In0.8Ga0.2Sb crystals retained cubic zinc blende crystal structure irrespective of heavy doping of Te element. In addition to the optical phonon vibrational modes, acoustic phonon modes were also present when the doping concentration exceeded 1 × 1018/cm3. The carrier concentration in Te-doped In0.8Ga0.2Sb crystals were varied in the range 1018–1020/cm3. Te-doped In0.8Ga0.2Sb with concentration 1 × 1018/cm3 was recorded a higher power factor because of its lower resistivity and higher mobility than other crystals. The ZT of Te-doped In0.8Ga0.2Sb (1 × 1018/cm3) was higher than other samples at 300–450 K. This study revealed that the optimum Te dopant concentration to enhance the ZT value of InxGa1−xSb is 1 x 1018/cm3 for optimizing its properties toward mid-temperature thermoelectric applications.

Thermoelectric devices require p-type and n-type semiconductors with similar chemical, mechanical and thermoelectric properties to achieve maximum efficiency. To match with n-type In0.95Ga0.05Sb crystals for the fabrication of thermoelectric device, zinc (Zn) element was doped with In0.95Ga0.05Sb crystal intentionally to change its conductivity from n-type to p-type and its thermoelectric properties were studied. The Zn-doped In0.95Ga0.05Sb crystals grown by directional solidification were free from micro-cracks and their composition was distributed homogeneously. The carrier concentration was increased upon doping with Zn element. The resistivity of Zn-doped In0.95Ga0.05Sb increased with increasing temperature that showed degenerate semiconducting characteristics resulted from heavy doping. The Peierls distortion resulting from Sb–Sb interaction was observed in Zn-doped In0.95Ga0.05Sb crystals. The higher electron contribution and lower phonon contribution to total thermal conductivity were obtained in Zn-doped In0.95Ga0.05Sb than undoped crystals. The maximum ZT of 0.24 at 573 K was achieved by Zn-doped In0.95Ga0.05Sb with dopant concentration 1 × 1020 atoms/cm3. The ZT achieved is the highest among other reported values of p-type III–V semiconductors.

Solid solution SnSe0.75S0.25 has potential to improve thermoelectric performance via ultra-low thermal conductivity as compared to the pristine SnSe which originates from phonon scattering due to disordered atoms of selenium (Se) and sulfur (S). SnSe0.75S0.25 and Cu-doped SnSe0.75S0.25 compounds were prepared via high energy ball milling and pelletized by a spark plasma sintering (SPS) process. Dislocation and point defects were successfully introduced by SnSe0.75S0.25. The existence of S in the Se site induced mass fluctuation which favors high-frequency phonon scattering. This leads to an impressively ultra-low thermal conductivity (κT) value of 0.258 W mK−1 at 753 K for SnSe0.75S0.25. Next, the Cu dopant was selected to enhance the electrical conductivity, which improved from 514.44 S m−1 (SnSe0.75S0.25) to 725.08 S m−1 for Sn0.98Cu0.02Se0.75S0.25 at 738 K. Interestingly, the Cu dopant induced nanoprecipitates of Cu2Se inside the grains, which further strengthens the phonon scattering. The Cu2Se nanoprecipitates and various defects at the grain boundaries contributed to a lower κT of 0.295 W mK−1 at 753 K for a Sn0.94Cu0.06Se0.75S0.25 sample. Moreover, the maximum figure of merit of (ZT) ∼0.19 at 738 K was attained for the Sn0.98Cu0.02Se0.75S0.25 sample.

Just as the shapes of snowflakes provide us with information on the temperature and humidity of the upper atmosphere, the characteristics of presolar grains in meteorites place limits on their formation environment in a stellar outflow. However, even in the case of well-characterized presolar grains consisting of a titanium carbide core and a graphitic carbon mantle, it is not possible to delimit their formation environment. Here, we have demonstrated the formation of core-mantle grains in gravitational and microgravity environments and have found that core-mantle grains are formed by a nonclassical nucleation pathway involving the three steps: (i) primary nucleation of carbon at a substantially high supersaturation, (ii) heterogeneous condensation of titanium carbide on the carbon, and (iii) fusion of nuclei. We argue that the characteristics of not only core-mantle grains but also other presolar and solar grains might be accurately explained by considering a nonclassical nucleation pathway.

The growth of high-quality InGaSb crystals by Vertical Gradient Freezing (VGF) under microgravity was numerically simulated. Machine learning tools were used to optimize the growth conditions. The study focuses on controlling growth interface shape which directly affects the quality and homogeneity of the grown crystals. Initially, Bayesian optimization was utilized to search for the most favorable growth conditions that promote a desirable flatter growth interface shape. Afterward, a reinforcement learning model was developed. The system was subjected to a lower temperature gradient near the feed crystal and to crucible rotation with a rate ranging according to the obtained optimal strategy. Results showed that the interface deformation is considerably reduced, and a flatter growth interface could be maintained. The growth rate and solute concentration uniformity were also improved. This adaptive control recipe proves to hold great potential in the continuous and rapid optimization of other crystal growth processes.

Cool flame oscillation of a fuel droplet array in high-temperature air was numerically simulated by using a droplet vaporization/spontaneous ignition numerical model. The time series data of temperature and chemical species spatial distributions were obtained. The data were used to train a Variational Auto-Encoder (VAE) that reduces the dimension of the data. The cool flame oscillation phenomenon was mapped as the trajectory onto a phase plane spanned by two latent variables obtained by the VAE. The oscillation phenomenon was investigated by using the distribution patterns derived by the VAE that distinguishes the temperature and the species states. Proper orthogonal decomposition was carried out on the decoder output of the VAE. The oscillation mechanism was investigated by the spatial eigenfunctions (mode maps). The temporal eigenfunctions of the three dominant modes were shown onto the trajectory of the plane. The correlation among physical variable distributions was evaluated to investigate the cool flame dynamics. From the above investigation, the plane was confirmed to distinguish the physical states, for the trajectory did not intersect during the oscillation. The plane was treated as a state space. The physical phenomena associated with each mode were identified from the mode maps and the temporal eigenfunctions. The phase in which the associated phenomenon arises was identified by checking the temporal eigenfunctions along with the trajectory. The mechanism of the oscillation was discussed with the correlation diagrams.

To explain observations of abundant circumstellar dust and high stellar wind velocity, most models simply postulate the efficient nucleation and growth of silicate dust particles. Here, we report measurement of the SiO-(SiOx)(n) grain sticking coefficient in a microgravity sounding rocket experiment, indicating very inefficient (0.005-0.016) grain formation from the vapor. Application of this measurement to radiative-driven winds in oxygen-rich asymptotic giant branch stars indicates that the initial grain condensate population should consist of very tiny dust particles in very large numbers. Aggregation of this dust population will produce low-dimension fractal aggregates that should couple well to the stellar radiation field and efficiently drive stellar mass loss.

We proposed a simple statistical analysis method of minute concentration changes for measuring diffusion coefficient with reduced interference fringe noise effect. In the “Soret-Facet Mission,” the one-dimensional diffusion equation discretized by the finite difference method was applied for temporal homogenization processes of the minute concentration gradient induced by the Soret effect with random noises. Measured diffusion coefficient Dexp was determined by evaluating the obtained apparent diffusion coefficient distributions D. The measured value Dexp obtained by the proposed processes was found to be valid because the measured value Dexp was close to the theoretical one Dth calculated by the Darken equation and the reference one Dref calculated in” Facet Mission” in the same solution system, respectively. In addition, by analyzing about 1/16th of the total field of view, it was possible to obtain a diffusion coefficient that are more than 95% convergent for the measured value Dexp obtained from the full field of view analysis.

This study aims at expressing the velocity fields of the liquids held in a narrow-closed container heated from above as a function of the dimensions of the container to estimate the diffusion-dominant condition. The motions of tracer particles in liquid salol held in a quasi-two-dimensional glass cell were tracked in cavities of various inner width W through the series of experiments. The mean velocity of the tracer particles, Vmean, decreased almost linearly with the decrease in the values of W. Thermal flow fields were successfully reproduced by the series of numerical simulations. We found that the unintended horizontal temperature difference is realized, which induces the natural convection in the closed narrow cavity heated from above. We derived the correlation between the Reynolds and Grashof numbers, which are described as a function of the mean velocity and the unintended horizontal temperature difference, respectively. The threshold of the convection onset in the present geometry was then proposed.

The second and third microgravity experiments on SiGe crystal growth by the traveling liquidus-zone (TLZ) method were carried out aboard the Japanese Experiment Module (JEM) “Kibo” in the International Space Station (ISS) in July 2013 and February 2014. In this study, we numerically investigated the details of transport phenomena and solidification in these two experiments. We found that the deformation of the melt/SiGe crystal interface shape increased with time, and that the growth rate near the crystal edge was much larger than that near the central axis. Comparing the numerical and experimental results of the concentration distribution of Ge, the numerical concentration distributions are reasonably coincident with the experimental ones in both the axial and radial directions. In addition, both numerical and experimental results show that the radial distributions of the Ge concentration remain relatively uniform throughout the entire crystal, although the mean values depend on the growth length.

© 2020 The investigation of state-of-the-art copper chalcogenides (Cu2-yX; X = S, Se or Te) based thermoelectric (TE) materials can direct to improve the performances of TE devices. Herein, we report the synthesis of Cu2SexTe1-x solid solutions prepared by two-step ball-milling method. The effect of Se concentration on their TE performance as a function of temperature, was investigated. XRD and HRTEM analyses confirmed the size reduction and the presence of amorphous regions with polycrystalline behavior of the samples. To evaluate their TE performance, as-prepared powder samples were cold-pressed and annealed at 773 K. The high figure of merit was obtained for Cu2Se0·7Te0.3 solid solution as 0.3 at 500 K due to the synergy of high Seebeck coefficient (174 μVK−1 at 500 K) and low thermal conductivity (1.16 Wm−1K−1 at 500 K). The high Seebeck coefficient with optimum carrier concentration and low thermal conductivity was achieved due to wide range of phonon scattering. The results suggested that, Cu2SexTe1-x solid solutions were efficient for low grade (300–500 K) waste heat recovery.

As fundamental research on spray combustion, spontaneous ignition phenomenon is important and research has been conducted by various approaches. The Japan-Gennan joint research program "PLIOENIX-2" aims to ignition process accompanying the cool flame around ignition limit by utilizing a TEXUS sounding rocket and it is very important study conjunction with the vaporization and flame spread. The design of the experimental equipment has been carried out by confirming the functions of each part by conducting element prototypes and parabolic flight experiments. At present, the detailed design review has been completed and the production phase is is underway.

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.

InxGa1-xSb is a ternary alloy with tunable properties. The wavelength of InxGa1-xSb can be varied in the range of 1.7-6.8 mu m, which is in the infrared (IR) region and makes that InxGa1-xSb can be used as a substuite for IR detector and therniophotovoltaic (TPV). The phase diagram reveals that there is a large temperature gap between liquidus and solidus lines, which leads to constitutional supercooling and the formation of crystal defects during the solidification process of InGaSb crystal. Moreover, convection caused by gravity will increase the inhomogeneity of transport in the liquid region near the crystal growth interface, which makes it quite difficult to grow InxGa1-xSb crystal. The convection will be restrained in microgravity environment, and thus, it is very beneficial for crystal growth. This article introduces the effect of microgravity on the growth of InxGa1-xSb crystal and the results of the space growth experiment of InxGa1-xSb ternary crystal with a high concentration of In in SJ10 recoverable scientific experimental satellite.

Thermoelectric power generators require semiconductor materials with controlled phonon and free charge carrier transport properties. This could be achieved by changing their molecular and lattice dynamics through introducing/ controlling structural imperfections (defects engineering). The structural imperfections such as point defects and compositional segregations in a multicomponent alloy are observed experimentally, and their impact on electron and phonon transport properties was explained. The thermoelectric properties of a III-V ternary alloy InGaSb was improved by the presence of point defects and compositional segregations. The compositions were segregated randomly, and they had a major impact on the phonon contribution to the thermal conductivity. The point defects affected electrical resistivity, and the Seebeck coefficient was influenced by carrier concentration. The figure of merit (ZT) of In0.95Ga0.05Sb is enhanced to 0.62 at 573 K, and it is the highest among any other reported values of binary/ternary III-V semiconductor alloys. The enhancement in the ZT of InGaSb from the viewpoints of point defects and compositional segregations are explained. This experimental defect engineering study could be helpful to understand and improve the thermoelectric properties of many other crystalline materials.

In order to determine the optimum heating rate for the planned microgravity experiments on the International Space Station (ISS), axisymmetric 2D numerical simulations were performed under the zero-gravity condition to investigate the effect of heating rate on the dissolution process of the seed and feed crystals in a sandwich system of InGaSb. The simulation results showed that the dissolution lengths of the seed and feed crystals are strongly affected by heating rate. A higher heating rate leads to larger feed and seed dissolutions. Simulation results suggest that the microgravity experiments on the ISS should not adopt a heating rate higher than 3.6 K/h in order to avoid a complete dissolution of the feed and/or seed crystals in this sandwich system.

Microgravity crystal growth experiment for the growth of In0.11Ga0.89Sb was performed at the Chinese recoverable satellite through the space program SJ-10. This experiment is aimed to understand the melt formation and growth kinetics of InxGa1-xSb solid solution with higher indium composition, because their segregation coefficient was higher than the crystals with lower indium compositions. The target composition and uniformity were achieved with higher growth rate under microgravity, whereas the uniformity in composition was not achieved under normal gravity. The growth and dissolution were affected mainly by the steady state equilibrium in the melt composition because of the convection under normal gravity. The non-steady state equilibrium in the melt composition under microgravity helped to achieve a higher growth rate and compositional homogeneity at higher indium composition of InxGa1-xSb solid solution.

© 2018 Elsevier Ltd In/Ga elements were doped in GaSb/InSb crystals respectively, and their thermoelectric performances were studied. The crystals had cubic zinc blende structure with change in lattice parameters. The charge transfer occurred between all the three (In, Ga and Sb) elements when In was doped with GaSb. Whereas in Ga doped InSb crystals, the charge transfer occurred only among Ga and Sb elements. In/Ga doped GaSb/InSb crystals exhibited degenerate and non-degenerate electrical properties, respectively. Optical modes of phonon vibrations were present, and their transverse mode was dominant over longitudinal mode in all the samples. The thermoelectric figure of merit (ZT) of In doped GaSb crystals were low because of their low power factor and high thermal conductivity. The highest power factor (59.5 μW/cmK2) and ZT (0.56) at 573 K were achieved by Ga doped (1 × 1021/cm3) InSb crystal. The ZT 0.56 at 573 K is thus far the highest among other reported values of InSb crystals.

The Soret-Facet Mission was conducted in microgravity to measure the Soret coefficient ST using a two-wavelength Mach-Zehnder Interferometer (2-MZI). Interferometer analysis generally uses procedures like fringe tracing (thinning method) that impede processing speed and accuracy. In the present study, an automated phase analysis using spatio-temporal images was developed based on the simple conversion of intensity I(t) into phase phi(t). This automatically analyzes steps, starting with obtaining the spatio-temporal images of moving interference fringes to calculating phase change Delta phi(t). It dramatically improved processing speed and larger analyzing areas compared to the thinning method. Applying filters and a threshold to the fringes using suitable conditions-which the method determined automatically and quickly-was confirmed to be efficient for the correct unwrapping of phase.

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.

The Soret-Facet mission was conducted under microgravity conditions to measure the Soret coefficient (ST) for salol/tert-butyl alcohol using a two-wavelength Mach-Zehnder Interferometer (2-MZI). The 2-MZI is useful in the simultaneous measurement of temperature and concentration in binary mixtures. However, the simultaneous analysis of the 2-MZI had the limitation in accurate determination of ST because of the uncertainties in the experimental values of coefficients of refractive indices. To reduce the uncertainties in the measurement of coefficients of refractive indices, this paper describes an alternative method to measure temperature and concentration individually, using the 2-MZI. This alternative method was applied to analyze the microgravity data of Soret-Facet mission and the changes of temperature and concentration were shown at each wavelength. The coefficients of refractive indices and ST were corrected based on matching the two changes of temperature or concentration so that two or three of the following constraints were satisfied: fulfill the deviation ranges of coefficients; minimize the difference between two gradients; and match with thermocouples. This correction led to the reduction in dispersions of analyzed values in the simultaneous analysis, and clarified that it is necessary to improve not only the coefficients of refractive indices but also the ratio between phase changes in the simultaneous analysis. The results indicated that the separate analysis for the 2-MZI can estimate the coefficients of refractive indices and is useful for measuring the Soret coefficient in binary mixtures.

Alumina (Al2O3) is believed to be the first major condensate to form in the gas outflow from oxygen-rich evolved stars because of the refractoriness and that alpha-Al2O3. (corundum, most stable polymorph) is a potential origin of a 13 mu m feature that appears close to stars. However, no one has directly reproduced the 13 mu m feature experimentally, and it has remained as a noteworthy unidentified infrared band. Here, we report nucleation experiments on Al2O3 nanoparticles monitored by a specially designed infrared spectrometer in the microgravity environment of a sounding rocket. The conditions approximate to those around asymptotic giant branch (AGB) stars. The measured spectra of the nucleated Al2O3 show a sharp feature at a wavelength of 13.55 mu m and comparable in width to that observed near oxygen-rich AGB stars. Our finding that alpha-Al(2)O(3 )nucleates under certain condition provides a solid basis to elaborate condensation models of dust around oxygen-rich evolved stars.

InxGa1−xSb crystals were grown from (1 1 0), (1 1 1)A, and (1 1 1)B planes of GaSb under microgravity and their dissolution and growth kinetics were discussed. The dissolution geometry is independent of orientation even when the rate of dissolution is varied. The growth rate of (1 1 0) was lied in-between (1 1 1)B and (1 1 1)A experiments. The growth kinetics are largely affected by the dissolution process through the establishment of a concentration gradient in the melt.

A Si-1_Ge-x(x) (x similar to 0.5) crystal measuring 10 mm in diameter was grown by the traveling liquidus zone method using the Gradient Heating Furnace aboard the International Space Station. We quantitatively investigated the composition and shape of the crystal/melt interface during growth by analyzing the grown crystal. The growth interface shapes were obtained by tracing the striations that varied as a result of modulation of the interface near the Si seed to a smooth convex shape as the SiGe crystal growth proceeded. We also successfully measured the Ge concentrations on the interface using an electron-probe microanalyzer. Several growth interfaces showed a highly uniform Ge composition within mole fraction +/- 0.0005. Detailed measurements of the shapes and the Ge concentrations on those interfaces revealed that millimeter-scale Ge concentration gradient fluctuations existed for more than 16 h in a nonsteady and unsaturated SiGe melt in front of a crystal growth interface grown under microgravity conditions.

Plants play an important role in bio-regenerative life support systems (BLSSs) for long-term manned space missions. During parabolic airplane flights, we investigated stem sap flow without forced air movement and water vapor conductance with forced air movement using sweetpotato plants. Stem sap flow was promoted under microgravity, but only when forced air movement was applied to the plants. The water vapor conductance of the plant leaves increased under microgravity at an air velocity of 0.25 m.s(-1). Leaf temperatures also increased under microgravity at an air velocity of 0.02 m.s(-1). This suggests that forced air movement is important in maintaining long-term, healthy plant growth in BLSSs.

We studied crystallization process of charged colloids under microgravity (mu G) by performing time resolved reflection spectroscopy, during aircraft parabolic flight experiments. We used dilute colloidal silica samples (specific gravity of the particles = 2.1, the particle concentration = 1 - 2 vol.%) that crystallized from shear-melt colloids, within the duration time of mu G (approximately 20 seconds). An influence of mu G was found to be less significant for the heterogeneous nucleation process with the aid of the cell walls. The incubation times of the homogeneous nucleation, tau, at mu G was longer than that at 1G. Furthermore, the half-widths of the Bragg diffraction peaks from the crystals were narrower by 30% at the largest, under mu G. This suggests that the crystal grain size (cross-sectional area) became larger under mu G environment, according to the Scherrer's relation.

Convection flows induced by formation of solute depletion zone around growing protein crystals are visualized to confirm whether growth of crystals at a ceiling position (top wall of a growth container) can really suppress this convection flow effectively or not. First, we observed the convection flows around the crystal at the ceiling position at 1.0 G. Second, parabolic flight experiments revealed that flow rates of polystyrene particles (as marker particles, 500 nm in diameter) around a crystal at the ceiling position did not indicate zero at high-gravity conditions, and the particles almost stop at zero gravity. Thus, stable microgravity experiments are still indispensable to attain complete convection-free conditions.

Cosmic dust, which is composed of nanometer-sized particles and is ubiquitously distributed in the universe, is formed in a gas outflow from evolved stars under a microgravity environment. Its formation processes have been studied on the basis of knowledge obtained under the 1 G environment on Earth and is thus not fully understood under realistic conditions. To better understand the process, here, we performed nucleation experiments of dust analogs under a microgravity environment. We show the details of our experiments using an aircraft including results of in-situ observation employing an interferometer and ex-situ transmission electron microscopy to reveal the difficulty of nucleation and variability of nucleation processes. Of particular note is the size distribution of the produced particle, which was monotonical in microgravity experiments against a double peak for particles produced in the laboratory. Under a microgravity environment, nucleation tends to suppress because of smaller density fluctuation due to convection free and less turbulence due to smaller Reynolds number which gives atoms/molecules (particles) smaller chance to collide with each other for nucleation (coagulation).

© 2016 Elsevier B.V. In order to elucidate the cause of the morphological transition of crystals growing in an undercooled melt of semiconducting materials, we carried out the containerless solidification of undoped Si and Si-1 at%Sn using a CO2 laser-equipped electromagnetic levitator (EML). The crystallization of these materials was successfully achieved under controlled undercooling. The relation between the shape of growing crystals and the degree of undercooling in Si-1 at%Sn was similar to that in undoped Si; that is, plate-like needle crystals were observed at low undercooling, whereas at medium and high undercooling the shape of growing crystals changed to massive dendrites. The grain-size of as-solidified samples of Si-1 at%Sn was remarkably small compared with that of undoped Si. The surface morphologies of samples solidified by dropping the melt onto a chill plate of mirror-polished silicon consisted of typical twin-related <110> dendrites. On the other hand, samples that were dropped from the undercooled state consisted of twin-free <100> dendrites. The nucleation rate of two-dimensional nuclei calculated on the basis of two mechanisms, which are the twin-plane re-entrant edge mechanism and the twin-free mechanism, suggested that the morphological transition to twin-free <100> dendrites from twin-related <110> dendrites occurs when the degree of undercooling becomes larger than the critical value. These results indicate that the cause of the morphological transition of Si growing in the undercooled melt is not the roughening transition of the crystal–melt interface but the transition of the nucleation kinetics to the twin-free mechanism from the twin-related mechanism.

InxGa1-xSb bulk crystals have been grown on the International Space Station using a GaSb (feed) / InSb / GaSb (seed) sandwich -structured sample. In order to gain a deeper insight into the transport phenomenon and the relevant fundamental mechanisms during the dissolution process of InGaSb in this system, four numerical simulations with different temperature conditions and under the assumption of zero gravity were performed by the volume -averaging continuum model. Simulation results showed the heat loss through the bottom wall did not affect the final feed/seed dissolution lengths and the grown crystal interface shape. The final dissolution lengths of the feed and seed crystals were determined by the temperature calculated along the seed interface. The results also indicate that the actual temperature of the growth ampoule should be around 3K lower than that measured on the outside the protective cartridge.

The abundant forms in which the major elements in the universe exist have been determined from numerous astronomical observations and meteoritic analyses. Iron (Fe) is an exception, in that only depletion of gaseous Fe has been detected in the interstellar medium, suggesting that Fe is condensed into a solid, possibly the astronomically invisible metal. To determine the primary form of Fe, we replicated the formation of Fe grains in gaseous ejecta of evolved stars by means of microgravity experiments. We found that the sticking probability for the formation of Fe grains is extremely small; only a few atoms will stick per hundred thousand collisions so that homogeneous nucleation of metallic Fe grains is highly ineffective, even in the Fe-rich ejecta of type Ia supernovae. This implies that most Fe is locked up as grains of Fe compounds or as impurities accreted onto other grains in the interstellar medium.

© 2016 Elsevier B.V. Si0.5Ge0.5 crystals were grown at two different temperature gradients on board the International Space Station (ISS) using the traveling liquidus-zone (TLZ) method and effects of temperature gradient on crystal quality were investigated. Although average axial Ge concentration profile was not affected by the temperature gradient, crystal quality was affected greatly. Single crystal length was shortened and constitutional supercooling (CS) is shown to occur more easily at higher temperature gradient. The calculated degree of CS based on the solute concentration profile in the melt and phase diagram data is about 4 times larger when the temperature gradient is twice, which supports the experimental results. Instability at high temperature gradient is unique to the TLZ method and is not common to other crystal growth methods such as the directional solidification method and Czochralski method.

© 2016, Springer-Verlag Berlin Heidelberg. InxGa1−xSb (x = 0–1), a III–V ternary alloy, was grown by melt solidification process. The effects of varying indium composition on the thermoelectric properties of InxGa1−xSb polycrystals were studied for the first time. The segregations of indium and gallium elements were observed in the grown crystals, and the defects present in crystals were revealed by etching process. Room-temperature Raman measurement revealed that the dominant optical modes of phonon vibrations in InSb and GaSb binaries were suppressed in InxGa1−xSb ternaries. The in-phase vibrations of acoustic mode phonons were scattered more effectively in InxGa1−xSb by the present defects, and the relative value of lattice thermal conductivity was reduced. Thus, the thermal conductivity of InSb and GaSb binaries was drastically reduced in InxGa1−xSb by alloy scattering. InSb indicated the highest ZT 0.51 because of its higher power factor 70 µW/cmK2. Next to InSb, In0.8Ga0.2Sb had higher ZT value of 0.29 at 600 K among the InxGa1−xSb ternaries. The ZT of In0.8Ga0.2Sb was increased about 30 times than that of GaSb by the increase of power factor as well as the decrease of thermal conductivity.

InGaSb ternary alloys were grown from GaSb (111)A and B faces (Ga and Sb faces) under microgravity conditions on board the International Space Station by a vertical gradient freezing method. The dissolution process of the Ga and Sb faces of GaSb and orientation-dependent growth properties of InGaSb were analysed. The dissolution of GaSb(111)B was greater than that of (111)A, which was found from the remaining undissolved seed and feed crystals. The higher dissolution of the Sb face was explained based on the number of atoms at that face, and its bonding with the next atomic layer. The growth interface shape was almost flat in both cases. The indium composition in both InGaSb samples was uniform in the radial direction and it gradually decreased along the growth direction because of segregation. The growth rate of InGaSb from GaSb (111)B was found to be higher than that of GaSb (111)A because of the higher dissolution of GaSb (111)B.

© 2016 Elsevier B.V. All rights reserved. Compositionally homogeneous Sb-doped (5×1018 and 1×1019 cm-3) Si0.73Ge0.27 bulk crystals were grown by a vertical gradient solution growth method. The sandwich sample Si (seed)/Sb-doped Ge/ Si(feed) was set up inside a furnace under a mild temperature gradient 0.57 °C/mm for homogeneous growth. The Si composition was analyzed by electron probe micro- analysis (EPMA). It revealed that the Si composition was homogeneous and the lengths of the Sb-doped (5×1018 and 1×1019 cm-3) Si0.73Ge0.27 bulk crystals were 18.3 and 15.1 mm, respectively. Grain distribution was investigated by electron backscattered diffraction spectrum (EBSD). The Seebeck coefficients (-440 and -426 μV/K) of Sb-doped (5×1018 and 1×1019 cm-3) Si0.73Ge0.27 were higher than the reported value (-211 μV/K) of P-doped (5×1019 cm-3) Si0.8Ge0.2 at room temperature. Thermal conductivity of Ga and Sb-doped SiGe was decreased with temperature due to scattering of phonon at the temperature range between 313 and 913 K. The maximum ZT values of Ga and Sb-doped SiGe were 0.34 and 0.44 at 820 K, respectively. The ZT values of Ga and Sb-doped SiGe were higher (0.07 and 0.13) than the reported value of Ga-doped Si0.81Ge0.19 (0.05) and P-doped (5×1019 cm-3) Si0.8Ge0.2 bulk crystals at room temperature. The improvement in ZT value was caused by a decrease of thermal conductivity which related to a composition of the alloy and doping concentration in the crystal.

The paper reviewed the previous microgravity experiment using Chinese recovery satellite, the in-situ measurement of composition profile in the solution by X-ray penetration method and homogeneous growth of InGaSb by temperature freezing method under terrestrial condition for making clear the effect of gravity on the growth of InGaSb ternary alloy semiconductor crystals. The previous experimental results showed that the shape of solid/liquid interfaces and composition profile in the solution were significantly affected by gravity. Based on the previous microgravity experimental results, experimental conditions were investigated to grow homogeneous In Ga-x Sb1-x with higher indium composition at Chinese recovery satellite SJ-10 in near future.

InxGa1−xSb bulk crystal was grown using a GaSb (seed)/InSb/GaSb (feed) sandwich-structured system onboard the International Space Station (ISS). In order to investigate the transport phenomena especially in terms of interface shapes and dissolution heights, the dissolution process was simulated under a micro-gravity level of the ISS. Simulation results showed that the seed/melt interface was concave towards the seed due to the temperature distribution of the system. This prediction is in good agreement with the results of our previous experimental study.

The volume-averaging continuum technique has been utilized to obtain numerical predictions for the transport phenomena occurring during the dissolution process of GaSb into InSb melt in a sandwich system. Dissolution and subsequent growth in this system are achieved by the application of a temperature gradient. The developed model was first verified for two test cases [(i) fluid/solid conjugate heat transfer and (ii) the solidification process of the binary system]. The code was then utilized to simulate the dissolution process of GaSb into InSb in the GaSb/InSb/GaSb sandwich system. The present results show that the developed volume-averaging model provides accurate predictions.

In the Soret-Facet performed on the International Space Station, the Soret coefficient S-T for salol/tert-butyl alcohol was measured by using a two-wavelength Mach-Zehnder interferometer. The temperature difference between the sides of the solution was set to 10 degrees C so that its mean temperature was 45 degrees C. The refractive index changes in a narrow observation field were measured by using a charge-coupled device camera. We improved the interference fringe analysis used to determine the refractive index changes by determining the interference fringe shifts in a wide area of the solution rather than in the narrow observation field. The interference fringe shifts outside the observation field were measured by moving the field of view and comparing the interference fringe positions. The fringes were found to shift linearly in the wide area. Then, the values of S-T in the observation field and the wide area, S-Tnarrow and S-Twide, respectively, were determined based on the interference fringe shifts. The measurement error delta(S-T) was caused by the standard deviation of the slopes of the fit lines, and values of delta(S-T)(narrow) = +/- 0.34 K-1 and delta(S-T)(wide) = +/- 0.024 K-1 were obtained for the observation field and the wide area, respectively. Based on the fit lines, which satisfied two constraints, S-Tnarrow and S-Twide were determined to be -0.17 K-1 and -0.06 K-1, respectively, for tert-butyl alcohol in salol. Consequently, delta(S-T)(narrow)/S-Tnarrow = 190% and delta(S-T)(wide)/S-Twide = 40% were obtained. Thus, the error delta(S-T)/S-T decreased from 190% to 40% when the interference fringe shifts were measured in the wide area in 0.25 mm intervals rather than in the narrow observation field.

Total of four SiGe crystal growth experiments aboard the ISS were successfully performed for evaluating a two-dimensional growth model of the traveling liquidus zone (TLZ) method and for obtaining insights into large homogeneous SiGe crystal growth conditions. The TLZ growth requires diffusion limited mass transport in a melt and experiments in microgravity are essential. Although a little deviation from the expected compositional uniformity due to emissivity change of the cartridge surface is observed, homogeneous SiGe crystals are grown. Over all axial growth rate is consistent with the one-dimensional TLZ growth model prediction. However, radial growth rates are different from the two-dimensional growth model prediction. The difference is closely related to the flat interface shape in space grown crystals compared with the terrestrial ones and the radial compositional uniformity is much better than those of terrestrially grown crystals. Suppression of convection in a melt is favorable for obtaining flat freezing interface and is beneficial to large homogeneous SiGe crystal growth. It is expected that the obtained results are utilized and large homogeneous crystal growth is realized on the ground and electronic devices using SiGe substrates are developed.

The hexagonal structure with a space group of P6(3)cm has been known to be stable in the RMnO3 system (R: rare earth element) of the rare earth elements having smaller ionic radius. The hexagonal RMnO3 (h-RMnO3) has attracted great interest towards their wide applications in the field of electronic industry, because h-RMnO3 shows multiferroic properties such as ferroelectricity, ferromagnetism and ferroelasticity in the same phase. Nevertheless, the materials for practical applications remain undeveloped, because h-RMnO3 shows anti-ferromagnetism as well as low magnetic transformation temperature below 100 K. To solve this problem, we considered that a composite of a ferromagnetic phase and a ferroelectric phase is more realistic than a single-phase material. On the basis of this idea, we attempted to synthesize the multiferroic composite consisting of ferroelectric h-RFeO3 and ferromagnetic Fe3O4 by utilizing the containerless technique. The experimental result showed that it is possible to exhibit a fine composite structure when h-LuFeO3 and Fe3O4 are equimolar amounts. However, the orthorhombic phase (o-RFeO3) as well as h-RFeO3 was observed in the samples in which the mole-fraction of Fe3O4 was increased. The reason for forming the o-RFeO3 phase is attributed to the decrease of the driving force for forming a metastable phase due to the fact that the solute atom or molecule is redistributed at the solid-liquid interface of the growing crystal.

© 2015, Springer Science+Business Media Dordrecht. InxGa1−xSb bulk crystals are to be grown using a GaSb(seed)/InSb/GaSb(feed) sandwich-structured sample onboard the International Space Station (ISS). The InGaSb crystals will be grown on top of GaSb seed single crystals with different orientations viz., (111)A, (111)B, (110), (100) in order to examine and understand the growth kinetics of the crystals. In the present work, a numerical model of the crystal growth system has been developed to investigate the interface kinetics effects on the growth process by taking kinetics coefficient into account. The proposed numerical model was applied to evaluate the effect of crystal orientation on growth rate. Simulation results showed that the kinetics coefficient, whose value depends on crystal orientation, affected the growth rate of InGaSb crystal and the dissolution rate of GaSb feed crystal in the sandwich system.

© 2015 Elsevier B.V. All rights reserved. A Si0.5Ge0.5 crystal was grown on board the International Space Station (ISS) using the traveling liquidus-zone method. Average Ge concentration was 49±2 at% for the growth length of 14.5 mm. Radial compositional uniformity was excellent especially between the growth length of 3 and 9 mm; concentration fluctuation was less than 1 at%. In this experiment, cartridge surface temperatures were monitored and heater temperatures were adjusted based on the monitored temperatures for improving compositional uniformity of a grown crystal. A step temperature change by 1 °C was imposed for adjusting heater temperatures. This procedure made it possible to observe growth interface shape; striations due to heater temperature change were observed by a backscattered electron image. Growth rates were precisely determined by the relation between interval of heater temperature change and the distance between striations. Based on the measured growth rates, two-dimensional growth model for the traveling liquidus-zone method was discussed.