Regrowth of NaCl whiskers from pure and poisoned solutions

Abstract

The growth behaviour of NaCl was studied by using seed crystals in different states of perfection; i.e. as-grown, kinked and irregularly deformed whiskers. The original whiskers were grown in air by the Gyulai method. This work is a continuation of the previous investigations by the present authors 1,2). As-grown whiskers can grow in pure solution, provided that the degree of supersaturation (σ) is larger than about 0.6%. By applying Kossel's formula on the growth rate ( R) for perfect crystals to the R-σ relation obtained in this experiment, the effective edge-energy of a two-dimensional nucleus is estimated to be 3.4 ± 2 x 10 -15 erg/ion. Deformed whiskers can grow in pure supersaturated solution of any degree. The parabolic law of Frank et al. 4) was confirmed. The growth rate is drastically decreased by adding chlorides of heavy metals (Pb, Hg, Cd and Sn) to the solution as impurities, whereas practically no effect is detected for the chlorides of lighter metals (Zn, Cu and Ca). The growth behaviour in Pb-poisoned solution was extensively studied. It is explained by a coupling effect of the distrinution of lattice imperfections, which may be the growth-step source, with impurity poisoning resulting in a suppression of the step movement.