Engineering topics

Volz, Martin P.

Publications and source records attributed to Volz, Martin P..

23 records · Page 2

NASA NTRS1999

Use of Traveling Magnetic Fields to Control Melt Convection

An axially travelling magnetic wave induces a meridional base flow in an electrically conducting molten cylindrical zone. This flow can be beneficial for crystal growth applications. In particular, it can be effectively used to stir the melt in long cylindrical columns. It can also be tailored to modify the thermal and species concentration fields in the melt and to control the interface shape of the growing crystal. The basic theory of such an application is developed and data from a preliminary mercury column experiment are presented.

Ramachandran, Narayanan↗

NASA NTRS1999

The Effect of a Rotating Magnetic Field on Rayleigh-Benard Convection

A destabilizing vertical temperature gradient and a rotating magnetic field have been applied to a cylindrical column of liquid gallium. The convective flows which arise as a function of these driving parameters are identified. For small magnetic field strengths, a region of stationary flow is observed. This region is bounded by critical values of the Rayleigh and Hartmann numbers. As the rotating magnetic field is increased, the critical Rayleigh number can increase by more than a factor of 10. The rotating magnetic field itself induces an instability at a critical value of the Hartmann number independent of the Rayleigh number. The nature of the bifurcations (whether subcritical or supercritical) and the convective flows occurring at the critical Rayleigh numbers are dependent upon the Hartmann number. In the stationary flow region, the rotating magnetic field can induce fluid motion with velocities on the order of cm/s. Thus, a rotating magnetic field can be used to achieve the benefits of forced convection during crystal growth processes without triggering deleterious instabilities.

Volz, Martin P.↗

NASA NTRS1998

Magnetic Damping of Solid Solution Semiconductor Alloys

The objective of this study is to conduct the Earth-based research sufficient to successfully propose a flight experiment (1) to experimentally test the validity of the modeling predictions applicable to the magnetic damping of convective flows in conductive melts as this applies to the bulk growth of solid solution semiconducting materials in the reduced gravitational levels available in low Earth orbit and (2) to assess the effectiveness of steady magnetic fields in reducing the fluid flows occurring in these materials during space 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 was chosen because it 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 high growth rates compared to many other commonly studied alloy semiconductors. An important supporting investigation is determining the role, if any, that thermoelectromagnetic convection (TEMC) plays during growth of these materials in a magnetic field. Some compositional anomalies observed by us in magnetic grown crystals can only be explained by TEMC; this 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.

Szofran, Frank R.↗

NASA NTRS1998

A Numerical Investigation of the Effect of Thermoelectromagnetic Convection (TEMC) on the Bridgman Growth of Ge(1-x)Si(x)

Thermoelectric currents at the growth interface of GeSi during Bridgman growth are shown to promote convection when a low intensity axial magnetic field is applied. TEMC, typically, is characterized by a meridional flow driven by the rotation of the fluid; meridional convection alters composition of the melt, and shape of the growth interface substantially. TEMC effect is more important in micro-gravity environment than the terrestrial one, and can be used to control convection during the growth of GeSi. In this work, coupled thermo-solutal flow equations (energy, scalar transport, momentum and mass) are solved in tandem with Maxwell's equations to compute the thermo-solutat flow field, electric currents, and the growth-interface shape.

Yesilyurt, Serhat↗

NASA NTRS1998

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 micro-g 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.

Szofran, Frank R.↗