Gerald W. Sears

Effect of Poisons on Crystal Growth

The effect of impurities is inexplicable by the accepted theory of crystal growth. In this report a mechanism is presented which describes the specific action of crystal‐growth poisons.By this model the step motion in growth is hindered and nucleation rate on an ideal surface is increased. To rationalize the effect of growth poisons it is necessary to postulate that complete monostep adsorption occurs at growth steps. In addition, the poison ion or molecule must present energetic and/or steric barriers to the addition of solute atoms so as to form an unpoisoned step by burying the adsorbed line of molecules.

Growth of Aluminum Oxide Whiskers by Vapor Deposition

The growth of aluminum oxide crystals has been observed in progress at 80X magnification with a stereo‐microscope. Hydrogen is passed over an aluminum‐oxide rod heated by a surrounding tungsten coil. As the hot products diffuse to the cooler region around the reaction zone, the reaction reverses to grow aluminum‐oxide crystals. This system is of particular interest since it allows the screw dislocation theory of crystal growth to be examined in great detail on a high‐temperature system.

Evaporation of Perfect Crystals

Evaporation from a perfect face of a paratoluidine crystal does not occur until a critical undersaturation is reached. The vaporization kinetics may be rationalized by postulating that the evaporation rate is limited by two‐dimensional nucleation of monolayer holes.

Growth of Mercury Platelets from the Vapor

A mechanism for the vapor deposition of mercury platelets is presented. The growth behavior is accounted for on the basis of a single screw dislocation in a [110] crystallographic direction. The growth of whiskers versus platelets is determined by preferential nucleation of (100) or (110) planes on the active sites.

Influence of Adsorbed Films on Crystal Growth Kinetics

The growth kinetics of crystals are appreciably altered by strongly adsorbed films. The growth steps are inoperative or poisoned below a critical supersaturation. Since this supersaturation for poisoning is much less than the supersaturation level for two‐dimensional nucleation, the effect should only be observable for the growth of imperfect crystals.

Growth of Potassium Chloride Crystals from Aqueous Solution

It is shown that the growth behavior of potassium chloride crystals from aqueous solution is consistent with the Frank growth mechanism. The growth behavior is altered by the presence of a few parts per million of lead chloride. It is imputed that the rate of growth at unit steps is inhibited and that the rate of two‐dimensional nucleation is markedly increased by the presence of lead chloride. The effects are consistent with complete line adsorption of a complex lead containing ion at growth steps.

Growth Mechanism of Near‐Perfect Crystals

Direct evidence is presented to show that whisker growth by vapor deposition occurs at the tip rather than the base. Platelets are shown to grow at constant thickness by the action of a set of crossed‐screw dislocations, and whiskers are shown to bear no screw dislocations normal to their axis.

Interaction of Condensable Gases with Cold Surfaces

Deposition from a molecular beam onto a cold target surface only occurs when the beam pressure is many orders of magnitude larger than the vapor pressure of the depositing phase at the substrate temperature. The critical beam pressure for deposition has been interpreted as a critical supersaturation for heterogeneous nucleation of a condensed phase. It is shown that the critical beam pressure is much too large to be accounted for as a critical value for nucleation if the adsorbate temperature and the substrate temperature are alike. The interpretation of critical deposition phenomena as a nucleation event must include the additional assumption that the adsorbate temperature is somewhat higher than the substrate temperature.

Melting and freezing of p-toluidine

Abstract Studies have been made of the melting and freezing of near-perfect platelets of p -toluidine. It has been shown that a perfect face of a p -toluidine platelet can be heated appreciably above its melting point in contact with its own melt without detectably melting. It is implied that the free-energy curve of a solid phase does not have an abrupt discontinuity at the melting point as would be required by the L indemann concept of melting.

Twist in Lithium Fluoride Whiskers

Eshelby twists have been observed microscopically on very fine lithium fluoride whiskers grown from aqueous solution.