The effect of crystalline anisotropy on the structure and kinetics of dendritic growth.

摘要

This project studies the structure and kinetic behavior of dendritic growth in camphene, pivalic acid, and succinonitrile. These are members of a class of organic materials called "plastic crystals", which form dendrites under certain temperature and solvent conditions. We have worked with pure materials and imposed temperature gradients to bring about the desired growth. We are interested in these materials because they freeze in the way that cast metals do, and are therefore valuable for modeling purposes.;Dendrite tips are paraboloidal; the tip radius decreases and the growth velocity increases with increasing undercooling, or driving force. Current theories of free dendritic growth claim the product of velocity and tip radius squared to be constant with undercooling.;Dendrites grow in specific crystallographic orientations. A cubic material, for instance, has its main branch in the ;We selected what we believed to be three cubic materials of differing interfacial tension anisotropy, and measured their dendrite tip radii and velocities at a series of undercoolings, in the pure melt. Our work focused on pivalic acid and camphene; succinonitrile data was available from previous studies. We examined the dependence of velocity and radius on undercooling, and compared our results to theoretical predictions.;Careful anisotropy studies of camphene, which had not previously been done, revealed it to be tetragonal, rather than cubic. Its dendritic structure has some unusual features, but its kinetic behavior is similar to that of succinonitrile, as well as to dendritic growth theory. Pivalic acid, which is cubic but of a higher degree of anisotropy than the other sample materials, demonstrated a noticeably different, non-linear dependence of the product of velocity and tip radius squared on undercooling.;These results suggest that polarity of the molecules may influence molecular attachment kinetics and hence dendritic growth rates. They also imply that materials of different unit cell structure may be compared, in studying the effect of interfacial tension anisotropy on dendritic growth, if other factors are indeed strongly influential.