Diffusion and Migration
The Diffusion and Migration node sets up the model inputs for the physics interface. Choose how to define the density of the gas mixture.
The node name changes to Diffusion, Diffusion and Migration, Convection and Diffusion or Convection, Diffusion, and Migration based on the check boxes selected in the Transport Settings section. If Global is selected in Diffusion model the node name changes to Global Model.
Model Inputs
This section has fields and values that are inputs to expressions that define material properties. If such user-defined property groups are added, the model inputs display here. Initially, this section is empty. The input depends on which Transport Settings are selected on The Heavy Species Transport Interface.
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Enter a Temperature T (SI unit: K) for the gas mixture used to compute the gas density and species transport properties.
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Enter an Absolute pressure pA (SI unit: Pa) to compute the gas density and the binary diffusion coefficients.
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When the default Migration in electric field check box is selected, enter an Electric potential V (SI unit: V). This is used to compute the electric field that results in migration of any charged species.
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If the Convection check box is selected, enter coordinates for the Velocity field μ (SI unit: m/s).
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If the Compute tensor electron transport properties check box is selected, enter coordinates for the Magnetic flux density B (SI unit: T). This usually comes from a magnetostatic model and is used to compute anisotropic transport properties for electrons.
If Global is selected in Diffusion model this section has the following fields:
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Enter the temperature of the background gas in Temperature T (SI unit: K).
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If the Reactor type is set to Closed reactor or Constant mass enter an Initial pressure p0 (SI unit: Pa). If the Reactor type is set to Constant pressure enter an Absolute pressure pA (SI unit: Pa).
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Enter an Absolute electron Flux |Γe| (SI unit: 1/(m2·s)) if there are reaction rate constants defined using Townsend coefficient.
Density
Select an option from the Mixture density list — Ideal gas (the default) or User defined. For User defined enter a value for ρ (SI unit: kg/m3).
For Ideal gas it computes the expression for the gas density automatically using the ideal gas law, based on the composition of the mixture. The density is a function of the mean molecular weight
where pA is the absolute pressure, T is the temperature, R is the universal gas constant, and Mn is the mean molar mass given by
Total Mass flow
If Global is selected in Diffusion model and the Reactor type is set to Constant mass or Constant pressure enter the Number of SCCM units for the reactor feed. The Feed mole fraction of individual species can be set in the Species node ().
Mean electron energy
If Global is selected in Diffusion model enter the Mean electron energy (unit: V) to compute the Bohm velocity:
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In the Surface Reaction node () in the section Reaction formula the ion forward rate constant is set equal to the Bohm velocity if Bohm velocity is selected in Specify reaction using.