From the Carbon potential list, select the carbon potential of the exterior cext. Select Carbon potential to use the carbon potential defined at the physics interface level. Select User defined to define the carbon potential directly. From the Mass transfer coefficient list, select the mass transfer coefficient to be used to define the carbon flux. Select Arrhenius to define a thermally activated mass transfer coefficient. Define the Pre-exponential factor b0 and the Activation energy Qb. Select User defined to define the Mass transfer coefficient b directly.