First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram

First-principles calculations predict a surface phase stability diagram for the polar perovskite KTaO3.

Influence of Interfaces on the Phase Stability in Nanostructured Thin Film Multilayers

Nanostructured thin film multilayers, comprising of alternating A/B layers, can exhibit metastable structures in one or both layers. From a classical thermodynamic viewpoint, the reduction interfacial energy is primarily responsible for this stabilizing effect. Based on this idea, a model has been constructed in which phase stability regions are represented as functions of both the bilayer thickness and volume fraction of the one the layers. Applying this classical thermodynamic model to a single, previously reported hcp to bcc transformation in Zr for Zr/Nb multilayers, a phase stability diagram was proposed. Various Zr/Nb multilayers with different bilayer thicknesses and volume fractions have been sputtered deposited. hcp to bcc transformations in the Zr layer were confirmed by x-ray and electron diffraction. Furthermore the Zr/Nb stability diagram predicted a novel hcp Nb phase which was subsequently verified experimentally. Using Zr/Nb as a guide, a similar phase stability diagram was constructed and experimentally determined for Ti/Nb multilayers. For each multilayer system, the reduction in interfacial energy was calculated from the experimentally determined diagram. These values were then compared to estimations of the structural component of the interfacial energy. The structural component was based on the energy per unit area of a misfit dislocation network constructed by an o-lattice. This simple assesment suggests that the reduction of the structural component of the interfacial energy is sufficient to drive the transformation.

Thermodynamic analysis of combustion synthesis of Al2O3–TiC–ZrO2 nanoceramics

Combustion synthesis of Al2O3–TiC–ZrO2 nanoceramics by reactions in the TiO2–Al–C–ZrO2 system is a new method with advantages of simplicity and efficiency. In this work, the effect of ZrO2 nanoparticles on the thermodynamics of combustion synthesis of the TiO2–Al–C system is studied to obtain desired phases. The result of thermodynamic analysis shows that the adiabatic temperature Tad of the system stays at about 2327 K (the melting point of Al2O3) with the addition of ZrO2in the range of 0–15 wt% and the fraction of molten Al2O3 varied from 100% to 78%. The possible combustion products are discussed with an approach of an overlapped phase stability diagram of the Al–O–N, Ti–O–N, Zr–O–N, and C–O–N systems at 2300 K. It has been shown that the combustion product is a mixture of Al2O3–TiC–ZrO2, which coincides with the results obtained by x-ray diffraction and transmission electron microscopy.

Raman Studies of Stress-Induced Phase Transformations in Titania Films

Time-resolved micro-Raman spectroscopy is used to follow the amorphous to crystalline phase transformation in sol-gel deposited titania films induced thermally or through the action of applied hydrostatic pressure in a diamond anvil cell. Time-dependent phonon intensities intrinsic to the growing phase are related to the volume fraction of crystallite present at any time. The sigmoidally generated curves can be modeled in terms of modified Avrami ingrowth kinetics in which diffusion of the amorphous phase to the nucleation center is restricted by the morphology of the evolving phase. Phonon frequency and linewidth measurements during the course of the transformation probe changes in film stress and particle size which are used to understand the mechanistics of the transformation. Raman measurements also are used to derive a phase stability diagram for titania films.