Design of Active Materials from First Priniciples: New Transforming Materials with Unprecedented Physical and Mechanical Properties; Final rept. 1 Jun 2005-30 Nov 2006

摘要

A strong correlation has been found between the size of the hysteresis in structural phase transformations and certain special values of the lattice parameters. These special values are associated with conditions of compatibility between the phases. One such condition is seen to have an especially dramatic effect: this is the condition that the middle eigenvalue lambda(sub 2) of the transformation strain matrix is 1. The authors systematically varied the composition in the system TiNiX, X = Cu, Pd, Pt, Au, to achieve lambda(sub 2) = 1. They found a sharp drop in hysteresis in all cases. Since hysteresis is a measure of the energy dissipated by the material, there is a strong likelihood that these special conditions also relate to reversibility (i.e., the number of times one can go through the transformation without significant degradation of the material). The authors have begun to understand the relative roles of compatibility and numbers of variants/strains/interfaces. They now have evidence that the latter also are important for behavior, even though they do not have a big hysteresis effect. These discoveries open up the possibility of seeking new families of highly reversible transforming materials. This is especially interesting in cases in which the two phases have distinct electromagnetic or optical (EMO) properties. The latter can be expected because EMO properties are generally sensitive to lattice parameters and structural phase transformations have a change of lattice parameters. Promising material systems were identified, including highly reversible Cu-based shape memory materials, ferroelectrics near the morphotropic boundary, GMR/CMR materials, fuel-cell compounds, multiferroic materials, nonvolatile memory materials in the family Ge2Sb2Te5, certain high- energy density battery electrode materials, and ferromagnetic shape memory/thermomagnetic alloys.