J. Phys. Chem. 1995, 99, 5438-5444
5438
Connection between Growth Rate Dispersion of Large Rochelle Salt Crystals and Growth Rate Dispersion of Their Seeds M. M. MitroviC Faculty of Physics, P.O. Box 550, 11001 Belgrade, Yugoslavia Received: June 20, 1994; In Final Form: November 30, 1994@
It has been shown that a strong correlation exists between the growth rate of large Rochelle salt crystals and that of their seeds, although the crystal seed-crystal border has many defects. Large crystals grow at higher growth rates from seeds that grew at higher growth rates. Processes of partial dissolution and refaceting of the crystals did not essentially influence the growth rate of crystals. The connection between the growth rate dispersion of crystals and that of their seeds indicates that the mosaic structure of seeds is transmitted to crystals developed from them.
Introduction Under constant external conditions of supersaturation, temperature, and hydrodynamics of solution, different crystals of the same material grow at different rate. This phenomenon is termed growth rate dispersion (GRD). MitroviC'%2 and MitroviC et al.3.4have shown that GRD of small Rochelle salt crystals in the [OlO] direction is a function of the conditions under which the crystals are grown (solution temperature and supersaturation, presence of a magnetic field). Growth kinetic data of Rochelle salt crystals in the [OOl] direction are discussed in terms of dislocation BCF6 theory by A l e ~ a n d r u . ~ The GRD phenomenon is still not well understood. The variation of concentrations of dislocation step sources at the surface of the crystals is probably the most obvious way to explain not only the differences in growth rates but also the fluctuations therein, as was shown by Chernov et aL7 On the other hand, RistiC et aL8 have shown that the GRD in potash alum is not due to the presence of a variable number of dislocations in the propagating growth sectors. The growth rate of sodium chlorate9 and Rochelle salt',4 crystals, grown under constant external conditions, is inversely proportional to their mosaic spread, which represents a measure of the "overall" lattice strain or defects of structure in the crystal. This is in opposition to earlier observations in which crystals that grew faster but, under different external conditions, displayed more defects. On the basis of these results, van der Heijden and van der Eerden'O proposed several physical models are which relate lattice strain to the GRD phenomenon: point defects, distribution of dislocations, grain boundaries, and volume strain variations in crystals. If the reason for GRD is that the dominating dislocation groups on crystal faces have a different activity, larger crystals should grow at higher growth rates under the same external conditions, since larger crystals would probably have groups of dislocations with higher activity. It is believed that very large crystals grow at the same growth rates (maximum for the observed external conditions) because, in large crystals, groups of dislocations, having maximum activities, are always present. X-ray transmission topography was used to examine the distribution of potential dislocation sources in the bulk of the crystals. Dislocations present in the original seed did not propagate across the interface into the developing crystal, as was shown by Bhat et al.Il for the growth of potash alum and @
Abstract published in Advance ACS Abstracts, March 15, 1995.
0022-365419512099-5438$09.00/0
by RistiC et al.I2 for the growth of sodium chlorate crystals. Dislocations mostly originate at the central portion of the boundary between the seed and the newly grown portion. In preliminary experiments, TakuboI3 observed that larger seeds of ADP crystals grew faster than the smaller. The GRD is important for various reasons. Since it provides information about the growth process taking place at the crystal surface, the nature and magnitude of the GRD may give information about the growth mechanism.'-4 MitroviC et al.I4 have shown that Rochelle salt crystals grown at different rates under the same external conditions have different lattice parameters, so it is to be expected that there is a link between the GRD and the physical properties of crystals. Establishment of the link between the growth rate of crystals and that of their seeds is significant so that the appropriate seeds may be chosen to obtain large crystals with desired properties. Study of the GRD of small crystals is significant as it affects crystal size distribution, and hence the product quality, produced in industrial crystallizers. This paper describes the results of investigations of the GRD of small ('2 mm) and large ('6 mm) Rochelle salt crystals in the [OOl] direction. The connection between the GRD of large Rochelle salt crystals and the GRD of their seeds is examined. Experimental Procedure The subject of all the experiments is the study of the growth of Rochelle salt crystals from aqueous solutions, which were prepared by equilibrating an excess of crystals with distilled water for three days at saturation temperature 32.0 & 0.1 "C. Analar grade Rochelle salt (Merck, 99%) was used. The solution was then decanted and stored at 33.5 "C prior to measurement. The growth was observed at 30.5 "C in all experiments. From the solubility data of Ogawa and Satoh,I5 we calculated the relative supersaturation as o = (C - Co)/Co = 3.2%, where C and COare the concentration and the saturation concentration at the measurement temperature in mol/cm3. The crystals were nucleated, grown, and observed in a crystallization cell schematically represented in Figure 1. The cell had a solution capacity of about 12 mL. With a flow rate of about 1 d l m i n , the velocity of the solution through the cell was between about 0.005 " I s (through the widest part) and 1 mm/s (at entrance and exit points). The temperature of the aqueous solution was kept constant within f 0 . 0 3 "C by thermostatically controlled water flowing through the crystallization cell. The temperature in the cell was measured using
0 1995 American Chemical Society
Growth Rate Dispersion of Large Rochelle Salt Crystals
Figure 1. A schematic drawing of the crystallization cell: (1) needle for inserting air in the solution; (2) thermocouple.
a thermocouple ( 2 ) . Crystals were produced in the cell by keeping the solution for 10-30 min at a temperature of 29 "C or by momentarily stopping the flow and introducing the air bubbles through a needle (1) at the bottom of the cell. The crystals were observed under a microscope using transmitted light and a total magnification of 30 (for large crystals) and 65 (for small crystals) times, giving accuracy of *lo p m and f 5 pm, respectively. We observed changes in the crystal length in the [OOl] direction over time. The crystal length vs time dependence was subjected to the least-squares method in order to determine the average linear growth rate in this direction within the observation interval R . R is equivalent to twice the average (001) face growth rate only if both opposite faces have the same growth rate. The relative error of the average linear growth rate was obtained by the least-squares method and is a measure of the deviation of the data from the linear crystal size-time dependence. Density of inclusions formed during the refaceting process in the boundary zone between the seed and newly grown portions was assessed microscopically as well. Growth of Small (
