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Materials Frontier No.164

Title:  1. Thin films and ultrastable metallic glasses
          2. Contactless processing of doped SiGe melts based on MSL-EML under microgravity conditions
Speaker: Prof. Yuansu Luo. Physics institute, University Göttingen, Germany
Date/Time: 2015-09-22,2pm
Venue: Room 308, Material Building A
Invitor: Mingwei CHEN
Abstract for Title 1
    Amorphous thin films prepared by vapor deposition technology are used in technologies such as nanostructured spin electronics and optic devices. For special applications we prepared recently ultrastable metallic glasses (MGs) of Zr-based alloys near the glass transition temperature Tg. Compared with ordinary MGs produced by rapid cooling of melts, the ultrastable MGs reveal a modified structure with slow dynamics of relaxation and an enhancement of Tg and elastic modulus. These natures make ultrastable MGs potentially useful for technology such as coating for corrosion and wear protection. To understand the observed behaviors, a comparison to fragile systems was made and we found that the enhanced Tg is correlated with the fragility value of the systems. This result demonstrates the forming ability of ultrastable MGs.    
Abstract for Title 2
    Electromagnetic levitation facility (EML) is designed for ISS laboratory to process melts of different kind materials under microgravity conditions, where the absence of gravity-driven convection of components allows precision measurements of thermophysical properties. As a precondition for this facility a good electric conductivity is necessary for samples. The processing of semiconductors, such as Si and Ge is therefore a challenge, because this kind materials show a poor conductivity. In this presentation we reports on results obtained recently from highly doped semiconductors Si1-xGex (x=1.0, 0.75, 0.5 and 0.25) prepared by Czochralski growth. The dopant used is B with a maximal concentration in the range of 1.5-1.9x1020at./cm3. The conductivity is found about 1.5x103 Ω-1cm-1 at room temperature. It increases upon heating and over 104 Ω-1cm-1 at melting point. The relevant processing was performed by parabola flight campaigns, using spherical samples (8mm) and the MSL-EML facility as same as designed for the ISS. The parabola flights provide a 20s microgravity condition with a g-level within ±0.05g, being more suitable for low-melting-point (Ge-rich) samples. A large overheat ∆T of about 415°C was achieved for example for the sample Si25Ge75. The temperature-dependent volume of drops was evaluated from axial and radial images recorded by high-speed cameras, giving the thermal expansion coefficient of the melts. Anormaly volume changes arising from liquid-solid phase transition was observed in this way. Surface tension and viscosity of the drops were determined by oscillating drop technique, using optic and electronic data. The results demonstrate a successful processing of the semiconductor alloys in the MSL-EML facility.

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