Robert A. Lefever

Convection phenomena during the growth of sodium chlorate crystals from solution

Abstract Schlieren observations of sodium chlorate crystals suspended in supersaturated solutions showed that, as supersaturation or crystal size increased, the boundary layer on the side faces remained stable while the top boundary layer and emerging plume changed from stable to partially oscillatory, with localized eddies or pulses rising periodically from portions on the top face. The transition was a function of the Grashof number Grtop (in which the characteristics length was defined as the area/perimeter of the top face) and the crystal height z, the critical Gr∗top being equal to 70 + (17 mm-1)z. Grtop correlated well with plume width, number of streamlines, eddy density, and plume velocity. The oscillation period, however, correlated best with the weight-increase growth rate. The mass transfer rate, as measured by the Sherwood number, correlated with Grtop according to the relation Sh=0.27 (Grtop) 1 4 . The linear growth rate varied as the inverse 1 4 power of the characteristic length and the 5 4 power of t he supersaturation. The vertical faces grew more rapidly than the horizontal faces, the ratio (1.1 to 1.6) having no apparent correlation with Grtop but increasing with decreasing supersaturation. A rough estimate of boundary layer flow versus growth rate for near-critical stable convection indicated that growth of the side faces depleted over half the excess solute in the boundary layer. External impulses initiated transient instabilities and/or oscillations in stable plumes, the threshold impulse energy being a function of Gr∗top minus Grtop. The duration of the transients increased with impulse energy until a maximum decay time was reached. Impulse energies greatly in excess of the threshold value caused transient separation of the boundary layer on the top face of in extreme cases over the entire crystal surface. Plume wavering, due to background-induced fluid motion, was observed only at the lowest supersaturations.

Influence of magnetic field on vertical Bridgman-Stockbarger growth of InxGa1 − xSb

Abstract Ingots of In x Ga 1 − x Sb 9 mm diameter x 60 mm long showed 50% fewer straight boundaries and 30% fewer curved boundaries when solidified vertically with a 4 kilogauss horizontal magnetic field. The magnetic field almost entirely stopped formation of curved boundaries and greatly reduced formation of twins. There was no noticeable change in the normal rate of growing-out of grains and twins formed in the initial nucleation of the solid on the end of the silica ampoule. The reduction is attributed to the elimination of temperature fluctuations, which probably cause formation of grains and twins during freezing rate fluctuations due to transient constitutional supercooling.

Thermocapillary convection in floating zone melting: Influence of zone geometry and prandtl number at zero gravity

Abstract For silicon, changes in zone size and shape altered only slightly the appearance of the streamlines for a fixed value of the dimensionless thermocapillary flow parameter. However significant changes were noted in the heat and mass transfer and in the magnitude of the dimensionless vorticity. Steady state solutions were also obtained for sapphire with electron beam heating, but could not be obtained with radiant heating. This suggests that oscillatory flow might easily be produced with radiant heating of high Prandtl number materials.

Epitaxy versus oriented heterogeneous nucleation of organic crystals on ionic substrates

Abstract It is plausible to assume that epitaxy is a special case of heterogeneous nucleation in which a restrictive crystallographic relationship exists between substrate and deposit orientations. This would mean that epitaxial substrates should always induce a perceptible reduction in the critical supercooling for nucleation of the deposit. To test this hypothesis, the critical supercoolings of six organic compounds were measured on glass and 11 single-crystal cleaved substrates including (0001) graphite, (001) mica, (111) BaF2, SrF2, and CaF2, and (100) KCl, KBr, KI, NaCl, NaF, and LiF. Reductions in supercooling (with reference to glass substrates) were checked many times for repeatability and reproducibility and shown in almost all cases to have a standard deviation of 1 C or less. Acetanilide, benzoic acid, and p-bromochlorobenzene showed a wide range of supercooling reductions and were oriented on all crystalline substrates. Naphthalene and p-dibromobenzene showed only slight supercooling reductions but were oriented on all substrates, including glass. Benzil showed strong supercooling reductions only for mica and KI but was oriented not only in these cases but also with KI, BaF2, CaF2, and graphite. There was little correlation between degree of lattice match and either supercooling reduction or degree of preferred orientation. These results suggest that, for the systems and geometry studied, forces such as molecular dipole binding and growth anisotropy had a stronger effect than lattice match.

Composition variations in directionally solidified InSb-GaSb alloys

Abstract Melts containing 10%, 30% and 50% InSb were directionally solidified horizontally, vertically and in Skylab 3 and 4. The resulting concentration profiles were much more uniform in the ingots solidified horizontally. Fluctuations in composition occurred sooner as the InSb content increased and later in the horizontally processed ingots. In the 10% InSb ingot processed in SL-3 a large orientation-dependent segregation was observed at about 2 cm from the initial position of the interface. A radial variation in composition was also observed, and tentatively attributed to surface-tension driven convection.

Orientation, twinning and orientation-dependent reflectance in InSb-GaSb alloys

Abstract InSb-GaSb solid solution ingots prepared by directional solidification aboard Skylab and on earth consist primarily of twins with (111) twin planes parallel to the direction of solidification and growth directions that lie along 〈211〉, 〈110〉, or 〈321〉 crystallographic directions. The optical bireflectance exhibited by polished samples when observed by reflected plane-polarized white light is symmetrical with respect to twin boundaries, regardless of twin orientation relative to solidification direction, and displays a two-fold rotational symmetry rather than the four-fold symmetry normally associated with uniaxial strain birefringence.