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Pure iron grains are rare in the universe

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Science Advances  18 Jan 2017:
Vol. 3, no. 1, e1601992
DOI: 10.1126/sciadv.1601992
  • Fig. 1 Schematic of the configuration and optical path of the double-wavelength Mach-Zehnder–type laser interferometer with a nucleation chamber.

    The red and green lines show the optical paths of the red and green lasers, respectively. The resulting images of interference fringes are recorded with a charge-coupled device camera (cam). The evaporation source of Fe wire wrapped around a tungsten filament 0.3 mm in diameter and 68 mm in length is shown as the black solid line (es) in the nucleation chamber (n). The other labels are as follows: b, beam splitter; c, collimator; d, dichroic mirror; e, electrode; g, gas line; l, lens; m, mirror; o, optical fiber; p, polarizer; s, short-pass filter; t, thermocouple; v, vacuum gauge; gl, green laser; py, pyrometer; rl, red laser; va, valve; vp, view port.

  • Fig. 2 Photographs of interference images and the temperature and partial pressure during the experiment on Fe nucleation under microgravity.

    Colored images of the interference fringes (see fig. S4 for monochromatic images) at representative times for the experiment with an initial pressure of Ar buffer gas of 4 × 104 Pa: (A) before heating of the evaporation source, (B) 0.4 s before the nucleation of Fe grains, and (C) at the time of nucleation. Scale bar, 3 mm. In (C), the dotted line in the upper right corner indicates the nucleation front of the Fe grains above which the disappearance of interference fringes is due to scattering of light by abundantly formed Fe grains. (D) Profiles of the temperature (squares), partial pressure (circles), and number density (triangles) of the Fe gas from the evaporation source to the nucleation front in (B). The error for the x axis is within the symbols. The solid black and blue curves are the temperature profile and the initial number density of Fe atoms, respectively, used in the calculation. The temperature was expressed by Eq. 10 with a time t = x2 D−1, where x is the distance from the evaporation source and D is the diffusion coefficient of Fe atoms. The labels are as follows: e, electrode; t, thermocouple; es, evaporation source.

  • Fig. 3 Temperatures and partial pressures of Fe gas at the nucleation front, obtained from the microgravity experiments.

    (A) The blue and orange points with error bars indicate the temperatures and partial pressures of Fe gas just before the nucleation for the experiments at initial Ar gas pressures of 2.0 × 104 Pa and 4.0 × 104 Pa, respectively. The green and red lines show the relationship between the temperature and the partial pressure of Fe gas to explain the shifts in the interference fringes for the green and red laser beams; the solid and dashed lines are the results for Ar gas at 2.0 × 104 Pa and 4.0 × 104 Pa, respectively. (B) The equilibrium vapor pressure of Fe between the vapor and solid is shown by the solid curve. The two square symbols are the same as the points in (A). The temperatures measured at the evaporation source are shown by the blue and orange vertical lines for experiments in Ar gas at 2.0 × 104 Pa and 4.0 × 104 Pa, respectively. The large gap between the two square symbols and the solid curve shows the presence of a very large supersaturation at nucleation.

  • Fig. 4 Estimation of the sticking probability by simulations to explain the results of the experiments.

    (A) Result of calculations for the formation of Fe grains for a sticking probability of α = 1.8 × 10−5 and a surface tension of σ = 2.48 N m−1, which was obtained by applying the SP nucleation model. The dashed and solid curves show the time variation in the nucleation rate J and the number density of gas-phase Fe atoms n1(t), respectively. The vertical dotted line shows the nucleation temperature derived from microgravity experiments. The thin gray curve shows the number density of Fe atoms for α = 1 and σ = 2.48 N m−1, for which nucleation occurs at a much higher temperature (1700 K) than the experimental result. (B) Plot of the sticking probability α against the surface tension σ estimated from a comparison of the results of experiments and the simulations. The blue and orange points plot results for experiments in Ar gas at 2.0 × 104 Pa and 4.0 × 104 Pa, respectively. The vertical dot-dashed green line shows the bulk surface tension of molten Fe (20).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/1/e1601992/DC1

    Supplementary Text

    fig. S1. Time evolution of the acceleration gravity in the sounding rocket during the microgravity experiment.

    fig. S2. Photographs of the experimental systems.

    fig. S3. Examples of nucleated particles in a microgravity environment.

    fig. S4. Images of interference fringes during the Fe nucleation experiment under microgravity.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Time evolution of the acceleration gravity in the sounding rocket during the microgravity experiment.
    • fig. S2. Photographs of the experimental systems.
    • fig. S3. Examples of nucleated particles in a microgravity environment.
    • fig. S4. Images of interference fringes during the Fe nucleation experiment under microgravity.

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