Calculation of partial excess energies

[nils-ellendt]

Dear all,

I am a new user to pycalphad. I have followed the tutorials and I want to implement the butler equation to estimate the surface tension of alloys. I have followed the viscosity example, which is a great help!

I am stuck with the calculation of the partial excess energies. I am using the Sn-Bi system for development because I have a working reference code here which uses the RK directly instead of a tdb.

In my model, I have calculated excess energy with:

G_E=self.excess_mixing_energy(dbe);

which gives me
1.0*(LIQUID0BILIQUID0SN(490.0 + 0.97T) + (LIQUID0BI - LIQUID0SN)LIQUID0BILIQUID0SN(-30.0 - 0.235*T))/(LIQUID0BI + LIQUID0SN)

which is fine. How can I use G_E.diff() to calculate the partial excess energies? In the viscosity example

excess_entropy = -excess_energy.diff(v.T)

was used, however, I cannot find entries in V which contain LIQUID0BI or LIQUID0SN. Or is there a smarter way to do that?

thank you for your help!

Nils

[richardotis]

Hi Nils, I'm not sure if I fully understand your question, but if you want to calculate partial quantities with respect to the temperature and the site fractions, the following should work:

G_E.diff(v.T)
G_E.diff(v.Y('LIQUID', 0, 'BI'))
G_E.diff(v.Y('LIQUID', 0, 'SN'))

[nils-ellendt]

Oh, I should have tried this notation. Then I can also grab the elements from self.pure_elements if I change the system, right?

Thank you a lot!

[richardotis]

You can use self.pure_elements, or self.nonvacant_elements if you want vacancies to be excluded (not an issue for liquids usually, unless you're using a model with complex ordering). You can also use self.site_fractions, which simply provides a list of all the site fraction variables (the v.Y(...)) already created for you, in sublattice-followed-by-alphabetical order.