Condensed Matter Theory

Computer codes to calculate electronic structure and total energies for systems that can be modeled using periodic cells are now standard commodities. These codes can be based on muffin tin atomic spheres usually implemented as the full potential linearized augmented plane wave (FLAPW) method (Wien, Exciting) or they can use pseudoptentials with a plane wave expansions for the electronic states (CaSTEP, VASP)

We have applied these codes to a various problems in Materials Science

Strength of Materials

with JR. Alvarez, graduate student; J.S Braithwaite postdoc

Impurities segregating to grain boundaries in metals can dramatically change the mechanical properties of the material. In some cases (B in Ni₃Al, C in steel) there is a strengthening of the material, in other cases (Bi in Cu) the introduction of the impurity leads to embrittlement. In collaboration with Prof. David Williams, Department of Materials Science, Lehigh University, we applied electron energy loss spectroscopy to the study of electronic structure changes due to impurities at grain boundaries. We also investigated B in boundaries in Ni₃Al and B,C,P,S, as both substitutional and interstitial impurities in (210) tilt boundaries in Fe. We found that interstitial sites are preferred over substitutional sites. The calculations also showed that B led to cohesive enhancement , while P, S led to strong embrittlement and C gave weak embrittlement except when it was present as a thin cementite layer (even 1 monolayer thick) at the boundary

References

Li Battery Materials

Collaboration with the group of Prof. Brent Fultz at Caltech

Li based batteries are the power supply for portable electronic devices. Limitations in batteries make electric cars impractical and constrain the performance of hybrid vehicles. New and better battery materials would revolutionize tranportation.

What are the essential features of a battery?

(1)An anode which can take up large quantities of a light transportable element (Li ) in neutral form.

(2)A cathode which can take up large quantities of the same element in ionic form. Neither the anode nor the cathode can change phase when the transportable element goes in and out. They must also have as low a density as possible and be reasonable electric conductors.

(3)An electrolyte through which the transportable element moves between anode and cathode and the other way round.

Graduate student M.P. Kocher, postdoc J.S. Braithwaite

We have used a combination of electron energy loss spectroscopy and first principles DFT calculations to understand the electronic structure changes in both anode materials such as graphite and cathodes materials such as Li_xCoO₂ Li_xNiMn)_0.5O₂, Li_x(NiMnCo)_0.333O₂ and LiFePO₄. In anode materials the Li is neutral not ionized (otherwise there wouldn’t be a potential difference between cathode and anode !)

Contrary to popular belief we have shown that the cathode accommodates the insertion and removal of Li not by having the transition element ion change its charge state but by a change in bonding of the oxygen. For the phosphate system, proposed as an alternative to the cobaltate to overcome problems of heating, there is a change in the charge state of Fe going from LiFePO₄ to FePO₄.

References