A quantitative criterion for predicting solid-state disordering during high strain rate deformation.

A quantitative criterion for predicting the onset of disordering during high strain rate deformation is defined that is based on the potential energy (PE) per atom (PE/atom). The criterion is a necessary, but not sufficient condition to predict disorder. The stress state and loading direction of the crystal must allow deviatoric displacements that can induce disordering and the strain rate must be sufficiently high. The criterion is tested using molecular dynamics (MD) simulations for Ag over a range of a stress states and loading directions relative to the crystal axis. It is found that, above a minimum PE per atom of -2.70 ± 0.01 eV/atom, the crystal becomes unstable and disorders at temperatures well below the equilibrium melting temperature. This criterion is found to be independent of stress state and loading direction, and results suggest that it can be applied broadly to other material systems and to scenarios where deformation is non-uniform and time dependent. An example is given for its application to Au in shear. We show that the minimum critical PE for disordering under high strain rate loading is estimated by finding the equilibrium PE per atom at melting, which can be obtained from a single MD simulation for each material. An example is provided that illustrates how PE/atom can be used to predict where a simulated system is with respect to the disordering threshold without conducting multiple simulations.

In situ transmission electron microscopy observations of sublimation in silver nanoparticles.

In situ heating experiments were performed in a transmission electron microscope (TEM) to monitor the thermal stability of silver nanoparticles. The sublimation kinetics from isothermal experiments on individual nanoparticles was used to assess the actual temperatures of the nanoparticles by considering the localized heating from the electron beam. For isolated nanoparticles, beam heating under normal TEM operating conditions was found to increase the temperature by tens of degrees. For nominally isothermal experiments, the observed sublimation temperatures generally decreased with decreasing particle size, in agreement with the predictions from the Kelvin equation. However, sublimation of smaller nanoparticles was often observed to occur in discrete steps, which led to faceting of the nanoparticles. This discrete behavior differs from that predicted by conventional theory as well as from experimental observations in larger nanoparticles where sublimation was continuous. A hypothesis that explains the mechanism for this size-dependent behavior is proposed.

Determination of properties of wedged, nonuniformly thick, and absorbing thin films by using a new numerical method.

Nonuniformity in the thickness of thin films can severely distort their transmission spectra as compared with those of flat, smooth films. Methods that extract properties such as refractive index, thickness, and extinction coefficient of such films can suffer inaccuracies when they are applied to wedged or nonuniformly thick films. To accurately extract optical properties of nonuniform films, we have developed a novel numerical method and efficient constitutive relations that can determine film properties from just the transmission spectrum for films that are locally smooth with negligible scattering loss. This optimum parameter extraction (OPE) method can accommodate films with two-dimensional thickness variation that would result in significant errors in the values of refractive index and film thickness if not considered. We show that for carefully chosen test cases and for actual pulsed-laser-deposition AlN thin films, properties such as refractive index, extinction coefficient, and film thickness were very accurately determined by using our OPE method. These results are compared with previous techniques to determine the properties of thin films, and the accuracy of and applicable conditions for all these methods are discussed.