The development of oblique preferred orientations in zeolite films and membranes

We propose a model that can explain the development of 101 and other oblique crystallographic preferred orientations in thin MFI-type zeolite films and membranes. The model is a refinement of the competitive growth model. In the classic competitive growth model the preferred orientation is determined by the fastest growth direction of the crystals. We show that the lateral growth component also plays a role. By the combination of outward growth and lateral growth, those crystals that have their fastest growth direction at an angle to the normal to the membrane will dominate in the early stages of growth, and an oblique orientation develops. With prolonged growth the crystals with the fastest growth direction perpendicular to the membrane will dominate and the classic competitive growth model applies again. This concept has been verified using two-dimensional growth simulations. The new model also applies to other types of thin films. (C) 2003 Elsevier Inc. All rights reserved.