Thermodynamic and morphological transitions in crystalline and soft material interfaces

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (p. 187-208).Interfaces are defects present in all materials. Interface transitions are characterized by abrupt changes in interface structure, chemistry and/or morphology under suitable conditions. They exist in many material systems and often produce profound changes in material properties. Several interface transitions in crystalline and soft materials that have not been previously well understood were studied in this thesis. In the first part of this thesis, a diffuse-interface thermodynamic framework was developed for grain boundary structural and chemical transitions. A graphical construction method was developed to predict conditions for grain boundary transitions. A grain boundary premelting transition is predicted for systems of fixed stoichiometry. When extended to binary systems, the diffuse-interface model predicts the existence of a coupled grain boundary premelting/prewetting transition, which produces cooperative grain boundary disordering and segregation at sub-eutectic and sub-solidus temperatures. The analysis rationalizes the thermodynamic origin of intergranular glassy films (IGFs) widely observed in multi-component ceramics and alloys, for which thermodynamic stability has not been well explained in previous research. Predictions on the conditions for IGF's formation are consistent with experiments. As part of this work, a prototype of "grain boundary complexion diagrams" was constructed which delineates the stability domains of different grain boundary "complexions" on bulk phase diagrams. Morphological transitions of interfaces in soft materials such as surfactant self-assembled structures were investigated in the second part of this thesis. A phase-field model was developed for simulating morphological evolution of surfactant aggregates in solutions.(cont.) The model captures both the self-assembling behavior of surfactants and the effect of interface-curvature elastic-energy on the morphologies of self-assembled structures. Simulations of single surfactant micelle growth in dilute solutions reveal several previously unknown morphological transitions, including a disk-to-cylinder micellar shape transition and a tip-splitting instability of cylindrical micelles. It is proposed that the observed morphological instabilities provide kinetic pathways to the formation of branch points between individual cylindrical micelles, whose presence has significant effects on the rheological properties of solutions. Surface wetting transitions often display simultaneous changes in interface structure and morphology. Despite the extremely broad technical applications of the Si/SiO2 structure The equilibrium wetting properties of silicon oxide on silicon are poorly understood, This is partly due to the extremely low equilibrium oxygen activity for SiO2/Si coexistence (e.g. 10-37 torr at 700°C), which cannot be reached by current ultra-high vacuum techniques. In the third part of this thesis, a solid-state buffer method was developed to access oxygen partial pressures near the Si/SiO2 equilibrium with systematic control. It was discovered from experiments that silicon oxide does not perfectly wet Si(001) surfaces near the equilibrium oxygen activity, with the wetting morphology being oxide islands coexisting with a thin oxide layer of ~0.4nm on top of Si.by Ming Tang.Ph.D