Measurement of Diffusion Coefficients of Dopants in Ge and Si Melts

No abstract availabl

Reduction of Defects in Germanium-Silicon

No abstract availabl

Magnetic Damping of Solid Solution Semiconductor Alloys

The objective of this study is to: (1) experimentally test the validity of the modeling predictions applicable to the magnetic damping of convective flows in electrically conductive melts as this applies to the bulk growth of solid solution semiconducting materials; and (2) assess the effectiveness of steady magnetic fields in reducing the fluid flows occurring in these materials during processing. To achieve the objectives of this investigation, we are carrying out a comprehensive program in the Bridgman and floating-zone configurations using the solid solution alloy system Ge-Si. This alloy system has been studied extensively in environments that have not simultaneously included both low gravity and an applied magnetic field. Also, all compositions have a high electrical conductivity, and the materials parameters permit reasonable growth rates. An important supporting investigation is determining the role, if any, that thermoelectromagnetic convection (TEMC) plays during growth of these materials in a magnetic field. TEMC has significant implications for the deployment of a Magnetic Damping Furnace in space. This effect will be especially important in solid solutions where the growth interface is, in general, neither isothermal nor isoconcentrational. It could be important in single melting point materials, also, if faceting takes place producing a non-isothermal interface. In conclusion, magnetic fields up to 5 Tesla are sufficient to eliminate time-dependent convection in silicon floating zones and possibly Bridgman growth of Ge-Si alloys. In both cases, steady convection appears to be more significant for mass transport than diffusion, even at 5 Tesla in the geometries used here. These results are corroborated in both growth configurations by calculations

Reduction of Defects in Germanium-Silicon

It is well established that crystals grown without contact with a container have far superior quality to otherwise similar crystals grown in direct contact with a container. In addition to float-zone processing, detached-Bridgman growth is often cited as a promising tool to improve crystal quality, without the limitations of float zoning. Detached growth has been found to occur quite often during microgravity experiments and considerable improvements of crystal quality have been reported for those cases. However, no thorough understanding of the process or quantitative assessment of the quality improvements exists so far. This project will determine the means to reproducibly grow Ge-Si alloys in the detached mode. Specific objectives include: (1) measurement of the relevant material parameters such as contact angle, growth angle, surface tension, and wetting behavior of the GeSi-melt on potential crucible materials; (2) determination of the mechanism of detached growth including the role of convection; (3) quantitative determination of the differences of defects and impurities among normal Bridgman, detached Bridgman, and floating zone (FZ) growth; (4) investigation of the influence of defined azimuthal or meridional flow due to rotating magnetic fields on the characteristics of detached growth; (5) control time-dependent Marangoni convection in the case of FZ-growth by the use of a rotating magnetic field to examine the influence on the curvature of the solid-liquid interface and the heat and mass transport; and (6) grow high quality GeSi-single crystals with Si-concentration up to 10 at% and diameters up to 20 mm

Hot hydrogen testing of direct current sintered NbC and (ZrNbW)C for nuclear thermal propulsion

Refractory carbides are promising materials for use in nuclear thermal propulsion, either as the primary material in nuclear fuel elements or as protective coatings. In particular, NbC and ZrC have high melting points, relatively low vapor pressures at elevated temperatures, and low thermal neutron cross sections. In this study, NbC and (Zr0.4Nb0.4W0.2)C were fully consolidated by direct current sintering, and relative densities of 98% were achieved. The samples were characterized by scanning electron microscopy and X-ray diffraction before and after hydrogen testing. The samples were exposed to hot flowing hydrogen gas at a pressure of 1 atm and a flow rate of 8 SLPM, at temperatures up to 2500 K, and for durations up to 3 h. The mass loss rate (MLR) of NbC decreased with increasing exposure, before reaching a steady state value of about 1 mg/m2s. For (ZrNbW)C, a comparison was made by exposing samples to either hot flowing hydrogen or argon gas. The mass loss rates decreased with increasing exposure to either gas, but the final steady-state MLR was one to two orders of magnitude greater for those samples exposed to hydrogen. As evidenced by X-ray data, small amounts of residual oxygen in the (ZrNbW)C samples were preferentially removed upon hydrogen exposure