Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.Comment: 29 pages, 5 figure

MACHINE-LEARNING INTERATOMIC POTENTIAL FOR ULTRA-WIDE BANDGAP SEMICONDUCTOR AND X-RAY SCATTERING STUDY OF CHARGE DENSITY WAVE

28 pagesUnderstanding the role of native defects, impurities, and dopants in reducing the intrinsicallyhigh thermal conductivity of wurtzite AlN is of importance for effective thermal management of next-generation devices. A computational approach is pursued that implements a machine-learning interatomic potential (MLP) to transfer the accuracy of first principles methods to the lengthscales accessible by molecular dynamics (MD) simulations. MLPs are trained on a dataset obtained by density functional theory calculations to map atomic configurations to per atom energies, reconstructing the potential energy surface. In this work, a training dataset is generated and two styles of MLP, a neural network potential and a gaussian approximation potential, are trained and evaluated. The established framework for MLPs, which is limited to the consideration of short- range interactions, is not sufficient to reproduce the potential energy surface of wurtzite AlN due to the material’s piezoelectric nature. Transition metal dichalcogenides host charge density wave (CDW) states, a periodic modulation of electron density accompanied by a periodic lattice distortion that emerges below a transition temperature. A signature of the CDW state is the appearance of superlattice reflections in diffraction patterns. Synchrotron source x-ray scattering measurements of bulk 2H-TaSe2 were taken on a cooling-warming cycle over the range of the commensurate and incommensurate CDW transition temperatures. The modulation vector defining the periodicity of the CDW state exhibited thermal hysteresis, and the commensurate CDW transition shifted on the warming cycle. The persistence of superlattice reflections above the incommensurate transition temperature indicated local ordering prior to the onset of the CDW state

Behavior of soda-lime silicate glass under laser-driven shock compression up to 315 GPa

Shock experiments give a unique insight into the behavior of matter subjected to extremely high pressures and temperatures. Understanding the behavior of materials under such extreme conditions is key to modeling material failure and deformation dynamics under impact. While studies on pure silica are extensive, the shock behavior of other commercial silicates that contain additional oxides has not been systematically investigated. To better understand the role of composition in the dynamic behavior of silicates, we performed laser-driven dynamic compression experiments on soda-lime glass (SLG) up to 315 GPa. Using the accurate pulse shaping offered by the long pulse laser system at the Matter in Extreme Conditions end-station at the Linac Coherent Light Source, SLG was shock compressed along the Hugoniot to multiple pressure-temperature points. Velocity Interferometer System for Any Reflector was used to measure the velocity and determine the pressure inside the SLG. The U s -u p relationship obtained agrees well with the previous parallel plate impact studies. Within the error bars, no transformation to the crystalline phase was observed up to 70 GPa, which is in contrast to the behavior of pure silica under shock compression. Our studies show that the glass composition strongly influences the shock compression behavior of the silicate glasses