Diffusion-controlled solidification of a ternary melt from a cooled boundary

We present details of an experimental study of crystallization adjacent to a cooled boundary from an aqueous solution of potassium nitrate and sodium nitrate. This transparent system is typical of many ternary melts that do n ot form solid solutions, including examples in igneous petrology and metall urgy. We have measured the rates of advance of the front of crystallization and the eutectic front, behind which the system is completely solid. From careful measurements of the concentration and temperature fields, we have b een able to infer the location of an internal phase boundary: the cotectic front separating a region in which only one component of the ternary system forms crystals from a region in which two components form crystals. Our ex periments were conducted under conditions in which fluid flow is minimal, s o that rates of crystallization are determined principally by the diffusive transport of heat. We have confirmed that the thicknesses of the various r egions all grow in proportion to the square root of time, as is expected of diffusion-limited growth, and have determined the constants of proportiona lity for a range of different initial concentrations and boundary temperatu res. We have found evidence to suggest that there may be a significant nucl eation delay in the secondary and tertiary crystallization. Our measurement s of concentration provide much more information about the ternary phase di agram than has hitherto been available.