Mass Transport and Heterogeneous Kinetics

We are looking at developing the one-dimensional stagnation flow geometry as a tool for measuring heterogeneous kinetics. At right are concentration contours of species A generated using a computational fluid dynamic model. This is a simple system with no gas phase chemistry and a first order surface reaction A => B. Rapid surface reaction produce concentration gradients above the substrate as shown in the figure at right. In this system transport and kinetics are coupled. In order to quantify kinetics one must first understand transport, and vice versa. We have recently shown that this can be accomplished using the following equations. The first relates the gas phase mole fractions at the surface inlet to the surface rate constant (k_s) and a modified mass transfer coefficient (h_m).

The second is a correlation that we have developed to numerically determine h_m. Using equation (2) h_m can be determined and controlled explicitly for any combination of operating conditions. As such surface kinetics (k_s) may be determined by measurements of gas-phase concentrations (x_s). For heterogeneous reactions that lead to deposition or etching, k_s can be determined by measurements of growth/etch rates.  We are currently working on demonstrating this capability in a number of model systems.