CBE Seminar - Edward Cussler Jr.

Distinguished Institute of Professor, Chemical Engineering & Materials Science, University of Minnesota

Sublimation vs. Diffusion

Edward L. Cussler

Distinguished Institute of Professor

Chemical Engineering & Materials Science
University of Minnesota

Abstract

Organic light emitting diodes (OLEDs) are valuable in light-emitting devices.  The inexpensive purification of these materials by sublimation represents a critical step in their high throughput processing. In this sublimation, OLED vapor is transported from high temperature to colder temperature. The pressure at high temperature is close to the equilibrium vapor pressure of the material being sublimed, and the vapor pressure in the colder region equals or exceeds the vapor pressure at the wall temperature. The sublimation rates seem to be controlled by diffusion. If diffusion is rate limiting, the reciprocal of the flux should vary linearly with the length. If diffusion is rate limiting, the total amount sublimed should vary with the square of the tube diameter.  These expectations are supported by experiment.  Predictions for convective flow and Knudsen diffusion are not supported by experiment.  Diffusion does seem the key.

However, the success of these predictions does not mean that the sublimation mechanism results from intermolecular diffusion or even from self-diffusion. In intermolecular diffusion, there are two solutes; here, there is one. In self-diffusion, there is one solute, but a portion of this solute can be separately identified by radioactivity, activation, or even optical rotation; here, no such identification is made. Moreover, intermolecular diffusion must be measured relative to some form of velocity, like a mass average or a volume average velocity. In many intermolecular diffusion experiments, this velocity is zero. In sublimations like those studied here, there is a real non-zero velocity. While diffusion is a quick label for what is occurring, diffusion in the conventional sense is not involved.

This paper discusses the causes of this behavior.  They may arise from a mass balance, from inertia, or from bulk viscosity.  Which of these mechanisms is most likely is discussed.

Bio

Edward L. Cussler, currently Distinguished Institute Professor at the University of Minnesota, received his B.E. with honors from Yale University in 1961, and his M.S. and Ph.D. in Chemical Engineering from the University of Wisconsin in 1963 and 1965, respectively, working with E. N. Lightfoot.  After thirteen years teaching at Carnegie-Mellon University, Cussler joined the University of Minnesota in 1980.  He has written over 250 articles and five books, including Diffusion, Bioseparations, and more recently, Chemical Product Design.  Cussler has received the Colburn and Lewis Awards from the American Institute of Chemical Engineers (AIChE), for whom he served as Director, Vice President, and President.  He has received the Separations Science Award from the American Chemical Society, the Merryfield Design Award from the American Society of Engineering Education, and honorary doctorate degrees from the Universities of Lund and Nancy.  Cussler is a Fellow of the American Association for the Advancement of Science and a member of the National Academy of Engineering.