3D atom probe tomography study on segregation of yttrium in modified Al-Si alloys

Yttrium segregation behavior in Al-Si alloys has been studied using the three-dimensional atom probe tomography technique. Al-Si alloys were prepared by casting method, and yttrium was added to modify the eutectic silicon morphology in these alloys. The results indicated that yttrium is preferentially located within the Si phase, with the highest concentration at the interface between eutectic Al and eutectic Si

Bayesian Data Sketching for Varying Coefficient Regression Models

Varying coefficient models are popular tools in estimating nonlinear regression functions in functional data models. Their Bayesian variants have received limited attention in large data applications, primarily due to the prohibitively slow posterior computations using Markov chain Monte Carlo (MCMC) algorithms. We introduce Bayesian data sketching for varying coefficient models to obviate computational challenges presented by large sample sizes. To address the challenges of analyzing large data, we compress functional response vector and predictor matrix by a random linear transformation to achieve dimension reduction and conduct inference on the compressed data. Our approach distinguishes itself from several existing methods for analyzing large functional data in that it requires neither the development of new models or algorithms nor any specialized computational hardware while delivering fully model-based Bayesian inference. Well-established methods and algorithms for varying coefficient regression models can be applied to the compressed data. We establish posterior contraction rates for estimating the varying coefficients and predicting the outcome at new locations under the randomly compressed data model. We use simulation experiments and conduct a spatially varying coefficient analysis of remote sensed vegetation data to empirically illustrate the inferential and computational efficiency of our approach

Size induced metal insulator transition in nanostructured Niobium thin films: Intragranular and intergranular contributions

With a reduction in the average grain size in nanostructured films of elemental Nb, we observe a systematic crossover from metallic to weakly-insulating behavior. An analysis of the temperature dependence of the resistivity in the insulating phase clearly indicates the existence of two distinct activation energies corresponding to inter-granular and intra-granular mechanisms of transport. While the high temperature behavior is dominated by grain boundary scattering of the conduction electrons, the effect of discretization of energy levels due to quantum confinement shows up at low temperatures. We show that the energy barrier at the grain boundary is proportional to the width of the largely disordered inter-granular region, which increases with a decrease in the grain size. For a metal-insulator transition to occur in nano-Nb due to the opening up of an energy gap at the grain boundary, the critical grain size is ~ 8nm and the corresponding grain boundary width is ~ 1.1nm