March 2006, APS: Computational band gap engineering in wide-gap MgO-ZnO alloys. Wide-gap semiconducting materials are extending critical applications in high temperature/power electronics and optoelectronics such as with the continued advancement of blue to ultraviolet LEDs and lasers. MgO-ZnO alloys have been increasingly investigated due to their UV luminescence from 150-400 nm, 3.3-7.8 eV. We have developed a first-principles model Hamiltonian that predicts band gaps of cubic MgO-ZnO alloys for any superlattice type or atomic configurations. First-principles band gap energies were used as input to construct an Ising-like cluster expansion, and the cluster types used were determined using a novel genetic algorithm. The design of specific wide-gap MgO-ZnO alloy superlattices for desired target band gaps is now possible with this resultant model Hamiltonian.
March 2005, APS: Magnesium Alloy Precipitate Formation Using Mixed Basis Cluster Expansions Unlike steel and aluminum alloys, the basic science of magnesium alloys is poorly understood. The automotive industry is driving demand for lighter structural material, and readily available magnesium alloys have a higher strength-to-weight ratio than their aluminum counterparts. We seek to predict magnesium alloy properties from first principles, particularly the hardening effect of precipitate formation. Mixed basis cluster expansions (MBCE) have successfully modeled precipitate shapes and growth in aluminum alloys. This methodology has not been extended to hcp-based materials such as magnesium alloys. In order to model binary magnesium alloys using the MBCE, particularly precipitate morphologies, we have constructed a coherency strain model for hcp structures to correctly represent the long-range strain fields around precipitates. Coupling this generalized strain model to an Ising-like expansion methodology we have developed a mixed-basis cluster expansion for hexagonal symmetries.
