Modeling of Thermal Gradient Sublimation

Many electronic applications, such as organic light-emitting devices (OLEDs), require low molecular-weight organic materials of high purity. Industrially, this purification is typically accomplished via thermal gradient sublimation. In this process, the crude material is heated and sublimed, and the resulting vapor is transported through a temperature controlled tube until it deposits downstream on a cooler section of the tube, where it is later collected manually. Scale-up and optimization of thermal gradient sublimation have been hindered by difficulties in modeling the transport and deposition of the rarified vapors involved.

In this presentation, we consider the challenges in predicting the deposition location and rate for a given tube setup and temperature profile. We present an overview of the various regimes experienced during transport of the vapor from sublimation to deposition, including viscous flow, volume diffusion, and Knudsen diffusion, and we develop an overall transport model that incorporates each of these mechanisms based on the kinetic theory of gases. Finally, we explore the predicted deposition behavior of the developed model and demonstrate how it can be used to tune and optimize sublimation processes to maximize both purity and yield of high-value materials.