Thermodynamics of Nanomaterials
Contact: Jörg Weissmüller
Alloy phase diagrams document equilibrium states of multicomponent systems, for instance in the temperature-composition domain. They form an indispensable engineering data base for materials selection and optimization in technological applications. For nanoscale materials, the energetics of the many interfaces has a significant impact on the driving forces for phase transformations and on the resulting equilibrium states. Phase diagrams of nanoscale alloys therefore depend on the grain- or particle size as an additional dimension in configuration space. Many of the relevant issues still await a systematic study. Our experiments on matrix-isolated nanoparticles and on nanocrystalline metal hydrides provide insights that can be generalized to nanoscale phase equilibria in a wider context.
| The free energy, G, of nanocrystalline alloys with a pronounced tendency for grain boundary segregation exhibits a minimum when plotted as the function of the grain size, D. The minimum corres- ponds to a metastable state of the alloy in which grain growth is suppressed and the nanoscale grain structure remains stable to elevated temperature. | |
The Gibbs phase rule imposes restric- tions on the topology of alloy phase diagrams. For instance, three phases can only coexist in a single point in the temperature-composition domain. These rules are apparently violated in certain nanoscale systems, such as the Bi-Cd alloy nanoparticle in the figure. |
