Experimental Research on the Regeneration of Incomplete Crystallographic KH₂PO₄ Crystal Enclosed by Singular Face

Abstract

The phenomenon of thin surface layer growth during the regeneration of incomplete crystallographic potassium dihydrogen phosphate crystal has not been thoroughly elucidated to date. In the present study, four sets of incomplete crystallographic KH 2 PO 4 crystals enclosed by singular faces are designed to observe their regeneration characteristics in supersaturated solution. In all the crystals, even though every crystal face is singular surface, thin surface layers still grow from crystal edges or concave angles to repair themselves to convex polyhedrons. Through surface energy calculation, it is found that the thin surface layer growth encounters higher growth barrier than ordinary layer-by-layer growth. Here, an assumption is proposed that the incomplete crystal possesses a "potential of morphology repair", which drives the thin surface layers growing to repair itself into complete form. This assumption is confirmed to some extent by a thin surface layer fragments experiment. The experiment implies that thin surface layer is the same as the ordinary crystal seed once separated from incomplete crystal, and its growth rate is one order slower than in incomplete crystal.KDP raw material (purity of 99.9%, Sinopharm Chemical Reagent Co., Ltd., Shanghai) and extra-pure water (resistance of 18.2 M ) were used for the solution preparation. The solution was filtered through filters with pore size of 0.1 µm and then overheated at a temperature of 65°C for 24 h. The temperature of crystallizer was controlled by a thermostat with accuracy of ±0.05°C. Supersaturation σ was created by temperature reduction and expressed in relative units, σ = (C − C 0 )/C 0 , where C and C 0 are the real and the equilibrium concentrations, respectively.