Growth of ammonium nitrate phase I and II dendrites

Abstract

Ammonium nitrate (AN) phase I and phase II crystallize as dendrites from a melt containing respectively 0–4.5 and 4.5–12 wt% water. In-situ microscopic measurements show that the, for dendrites, characteristic ratio S / ϱ (where S = side branch spacing near the tip and ϱ = dendrite tip radius) varies between 2.3 and 5.1 for both AN(I) and AN(II). This agrees reasonably well with the theoretically derived value of 2.8 reported in the literature. The water concentration has a strong influence on the relation between the side branch spacing near the tip, S, and the tip growth rate, v, of the dendrites. The findings satisfy at least qualitatively the theoretically expected relationship between S and v.