Phase separation of InxGa1-xN alloys into Ga-rich and In-rich regions was observed by a number of research groups for samples grown with high indium content, x. Due to the radiation sensitivity of InGaN to beam damage by fast electrons, high-resolution imaging in transmission electron microscopy (TEM) or core-loss electron energy-loss spectroscopy (EELS) may lead to erroneous results. Low-loss EELS can yield spectra of the plasmon loss regions at much lower electron fluxes. Unfortunately, due to their delayed edge onset, the low energetic core losses of Ga and In partially overlap with the plasmon peaks, all of which shift with indium content.

Here we demonstrate a method to quantify phase separation in InGaN thin films from the low-loss region in EELS by simultaneously fitting both plasmon and core losses over the energy range of 13-30eV. Phase separation is shown to lead to a broadening of the plasmon peak and the overlapping core losses, resulting in an unreliable determination of the indium concentration from analyzing the plasmon peak position alone if phase separation is present. For x=0.3 and x=0.59, the relative contributions of the binary compounds are negligibly small and indicate random alloys. For x nom.=0.62 we observed strong broadening, indicating phase separation.

Near real-time visualization of complex two-phase flow in a porous medium was demonstrated with dynamic 4-dimensional (4D) (3D + time) imaging at the 2-BM beam line of the Advanced Photon Source (APS) at Argonne National Laboratory. Advancing fluid fronts through tortuous flow paths and their interactions with sand grains were clearly captured, and formations of air bubbles and capillary bridges were visualized. The intense X-ray photon flux of the synchrotron facility made 4D imaging possible, capturing the dynamic evolution of both solid and fluid phases. Computed Tomography (CT) scans were collected every 12 s with a pixel size of 3.25 μm. The experiment was carried out to improve understanding of the physics associated with two-phase flow. The results provide a source of validation data for numerical simulation codes such as Lattice-Boltzmann, which are used to model multi-phase flow through porous media.

Recent advances in studies of local mechanical properties of polymers with different atomic force microscopy techniques (contact, Hybrid and amplitude modulation modes) are described in interplay between experiment and theory. Analysis of force curves and time dependencies of probe response to sample compliance, which were recorded on a number of polymer materials at various temperatures, leads to quantitative mapping of specific mechanical properties (elastic modulus, work of adhesion, etc). High spatial resolution of elastic modulus mapping (10-20 nm) is illustrated in measurements of lamellar structures of several polymers. Challenges of examination of viscoelastic properties are pointed out and a possible solution is presented.

The interface between the adhesive toes of geckos and a substrate consists of an array of regularly sized, densely packed, and elastically coupled nanoscopic contacts. The velocity-dependent friction exhibited by this system hints at a convolution of various material and structural effects. We explore the dynamics of frictional sliding in these materials using models based on arrays of coupled masses driven by external forces that can become pinned and unpinned to a potential energy landscape. The model system is driven at normalized velocities spanning 6 orders of magnitude, and the output of this model captures both the low-V and high-V behavior of the actual gecko materials. We explore modifications to the essential model that incorporate features more representative of the structure and behavior of the natural gecko system. These results have implications in the design of materials with custom frictional properties.

Zinc oxide has recently been touted as a material that may prove useful for high-power and high-frequency electron device applications. Unfortunately, at the present moment at least, zinc oxide’s electron transport results are based upon material parameter selections that remain disputed, i.e., their exact values have yet to be satisfactorily resolved. In order to establish how the expected range of disputed material parameter values influence the corresponding electron transport results, this paper assesses the sensitivity of the electron transport results associated with zinc oxide to variations in these disputed material parameters. The disputed material parameters that we focus on for the purposes of this particular analysis include the non-parabolicity coefficient associated with the lowest energy conduction band valley, the conduction band inter-valley energy separation, and the effective mass associated with the electrons in the upper energy conduction band valleys. For the purposes of this analysis, steady-state electron transport results are the focus of this sensitivity analysis, the velocity-field characteristic associated with zinc oxide being the principal metric of concern. We find that increases in the non-parabolicity coefficient associated with the lowest energy conduction band valley lead to increases in the peak field of the velocity-field characteristic and initially an increase and then a decrease in the peak electron drift velocity of this material. Increases in the conduction band inter-valley energy separation are instead found to result in increases in the peak field and concomitant increases in the peak electron drift velocity. Finally, increases in the effective mass associated with the electrons in the upper energy conduction band valleys are found to lead to a sharpening of the slope of the velocity-field characteristic in the region beyond the peak field, greater effective mass leading to a greater magnitude slope. Based on the magnitude of these variations, we conclude that zinc oxide may indeed be considered as a material for high-power and high-frequency electron device applications even when the variations in these disputed material parameters have been accounted for.

We present a thermodynamically consistent phase-field model describing the free energy of perovskite-based BCT-BZT solid solution containing an intermediate morphotropic phase boundaries. The Landau coefficients are derived as functions of composition of zirconium. The electrostrictive and elastic constants are appropriately chosen from experimental findings. The resulting Landau free energy is constructed to describe the stable polarization states as a function of composition. The evolution of the polarization order parameters at a particular composition is described by a set of time-dependent Ginzburg-Landau (TDGL) equations. Additionally, we solve Poisson’s equation and mechanical equilibrium equation to account for the ferroelectric/ferroelastic interactions. We have performed two dimensional and three-dimensional simulations with appropriate electrical boundary conditions to study the effect of external electric field on domain dynamics in BCT-BZT system at the equimolar composition.

An a posteriori error-estimation framework is introduced to quantify and reduce modeling errors resulting from approximating complex mesoscale material behavior with a simpler macroscale model. Such errors may be prevalent when modeling welds and additively manufactured structures, where spatial variations and material textures may be present in the microstructure. We consider a case where a <100> fiber texture develops in the longitudinal scanning direction of a weld. Transversely isotropic elastic properties are obtained through homogenization of a microstructural model with this texture and are considered the reference weld properties within the error-estimation framework. Conversely, isotropic elastic properties are considered approximate weld properties since they contain no representation of texture. Errors introduced by using isotropic material properties to represent a weld are assessed through a quantified error bound in the elastic regime. An adaptive error reduction scheme is used to determine the optimal spatial variation of the isotropic weld properties to reduce the error bound.