About Using Volume in the Diffusion Module (DICTRA)

This section or topic is specific to Console Mode.

The following is applicable to the use of the classic models in the Diffusion Module (DICTRA). That is, all models except the homogenization model.

DICTRA uses a simplified treatment of volume, i.e. DICTRA does not use volume data from the thermodynamic databases. This implementation in the original software development was because most databases did not contain volume data at that time.

The basic rule to use when applying the classic model is that only substitutional phase constituents contribute to volume and the partial molar volume of such constituents are assumed to be constant and equal. The partial molar volume of interstitial phase constituents are assumed to be zero. The actual value used as partial molar volume of substitutional phase constituents is 10-5 [m³/mol] and this is entered in databases as a constant VS.

The databases contain definitions of zero volume species and these species can then be interstitial phase constituents. Typically, C, N and O are entered as zero volume species.

There is, however, a rule that supersedes the basic rule, that is, for a phase that consists of a single sublattice, such as graphite, all constituents are considered to be substitutional. Another case is the liquid, which in most databases is described with a single sublattice. In such a liquid all constituents are then considered to be substitutional.

Thus, a given species can be interstitial in one phase but substitutional in another.

For the homogenization model the treatment of volume is, unfortunately, but for good reason, slightly different. The homogenization model only considers the elements of the system and regards each element as being either substitutional or interstitial. This status is the same in all phases. For many cases this treatment yields the same result as the rules for the classic model described above. However, it does not work for graphite, which has zero volume.