Engineering topics

Edward N Sickafus

Publications and source records attributed to Edward N Sickafus.

3 records

NASA NTRS1967

Surface-spike growth on NiBr sub 2 crystals - A VLS mode of crystal growth.

Droplets associated with surface-spike growth of crystals of NiBr 2 by sublimation have been analyzed and found to contain Cu, Co, Mn, and Fe in addition to Ni and Br. These elements exist in the bulk of the original charge material in concentrations that are several orders of magnitude lower than that in the droplets. During crystal growth by sublimation the impurities collect on the surface of growing crystals, until sufficient concentrations are reached to cause the nucleation of a metastable liquid phase droplet. Once the liquid phase droplets have been nucleated, they may then become attached to growth steps and produce accelerated growth from these sites. Such processes occur in NiBr 2 , during growth by sublimation, at approximately 500 °C below the melting point of NiBr 2 .

Edward N Sickafus↗

NASA NTRS1966

Surface spikes - A perturbation of growth steps.

During the growth of NiBr 2 crystals by sublimation, it has been observed that in addition to the mechanism of growth spirals a secondary mode of nucleation can occur. This is an accelerated growth which appears in the form of surface spikes on growth steps. The surface spikes may move as much as 50 times faster than the unperturbed growth steps. The mechanism of formation is believed to begin with the nucleation of a liquid droplet which wets the growth step and produces the accelerated growth from the liquid phase. The growth occurs at 500°C below the melting point of NiBr 2 . It is believed that the nucleation of the liquid phase at these temperatures is impurity controlled.

Nickel compound↗

NASA NTRS1966

Concave-convex growth spirals.

The simple model for introduction of a screw dislocation into a crystal leaves a step in those surfaces of the crystal that intersect the axis of the screw dislocation such that the Burgers vector has a finite component perpendicular to the surfaces. When two parallel surfaces of a crystal intersect the same screw dislocation, the resultant step, on opposite sides of the crystal, face in opposite directions. Consequently, when two such surfaces grow by accretion of the molecules at the steps, the steps wind themselves into spirals that appear to have opposite senses of rotation when viewed along the axis of the screw dislocation. Furthermore, the growth structure that results is a plano convex cone or pyramid centered on the screw dislocation. When growth occurs simultaneously on both sides of a crystal, a doubly convex structure will develop under the control of a singe screw dislocation.

Crystal growth↗