Search results for "Metas"
showing 10 items of 1693 documents
Crystallization in suspensions of hard spheres: a Monte Carlo and molecular dynamics simulation study
2011
The crystallization of a metastable melt is one of the most important non-equilibrium phenomena in condensed matter physics, and hard sphere colloidal model systems have been used for several decades to investigate this process by experimental observation and computer simulation. Nevertheless, there is still an unexplained discrepancy between the simulation data and experimental nucleation rate densities. In this paper we examine the nucleation process in hard spheres using molecular dynamics and Monte Carlo simulation. We show that the crystallization process is mediated by precursors of low orientational bond-order and that our simulation data fairly match the experimental data sets.
Solidification of a colloidal hard sphere like model system approaching and crossing the glass transition
2014
We investigated the process of vitrification and crystallization in a model system of colloidal hard spheres. The kinetics of the solidification process was measured using time resolved static light scattering, while the time evolution of the dynamic properties was determined using time resolved dynamic light scattering. By performing further analysis we confirm that solidification of hard sphere colloids is mediated by precursors. Analyzing the dynamic properties we can show that the long time dynamics and thus the shear rigidity of the metastable melt is highly correlated with the number density of solid clusters (precursors) nucleated. In crystallization these objects convert into highly…
Evaluation of metastable pitting on titanium by charge integration of current transients
2010
Abstract The metastable pitting of titanium has been studied under potentiostatic control in solutions containing chloride ions. An approach based on the charge integration of current transients was proposed for a quantitative determination of metastable pitting. A pit density (dmpit) was defined as the number of metastable pits per unit area per unit time (cm−2 h−1) with a typical size, instead of a size distribution. The calculated dmpit of titanium at 0.5 VSCE in 0.6 M NaCl was about 1.0 × 103 cm−2 h−1 with a typical radius of 0.12 μm. An exponential potential dependence of dmpit was obtained through the integration approach.
Preferred Growth Direction by PbS Nanoplatelets Preserves Perovskite Infrared Light Harvesting for Stable, Reproducible, and Efficient Solar Cells
2020
Formamidinium-based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm(-2), and a stable incident photon to current efficiency in the infr…
Polymeric scaffolds prepared via thermally induced phase separation: Tuning of structure and morphology
2008
Scaffolds suitable for tissue engineering applications like dermal reconstruction were prepared by Thermally Induced Phase Separation (TIPS) starting from a ternary solution PLLA/dioxane/water. The experimental protocol consisted of three consecutive steps, a first quench from the homogeneous solution to an appropriate demixing temperature (within the metastable region), a holding stage for a given residence time, and a final quench from the demixing temperature to a low temperature (within the unstable region). A large variety of morphologies, in terms of average pore size and interconnection, were obtained upon modifying the demixing time and temperature, owing to the interplay of nucleat…
New Schemes for the Production and Spectroscopy of Positronium
1989
The rate of positronium formation has been increased by 2–3 orders of magnitude using recently developed accelerator based slow positron sources. This opens the possibility of improvements of precision experiments on the Ps atom as well as new experiments on excited states. First evidence for enhanced metastable Ps formation is presented and future possibilities are discussed.
Lifetime and collisional depopulation of the metastable 5D 3/2-state of Yb+
1988
The lifetime and collisional depopulation rates of the metastable 5D 3/2 state of Yb+ have been determined in a radiofrequency ion trap by observation of the fluorescence count rate after ion excitation by a short laser pulse. From measurements using He, N2 and H2 as buffer gases between 10−8 and 10−6 mbar pressure and linear extrapolation to zero pressure we obtain a lifetime of τ=52.15±1.00 ms and rate constants ofR(H2)=(1.02±0.10)×10−9 cm3/s andR(N2)=(1.78±0.19)×10−10 cm3/s. The lifetime is in fair agreement with a calculated value of 74 ms.
Transient Moiré rotation patterns in thin colloidal crystals
2010
A high resolution optical microscopy study on low salt charged sphere suspensions settling under gravity reveals the formation of AB-stacked triangular layers via different transient structures. In particular, metastable triangular Moire rotation patterns (formally corresponding to 1 × 1 Rα super structures) with rotation angles α = 27.8° and α = 38.2° are identified.
Laser and radiofrequency spectroscopy of the 5d 4 6s 6 D multiplet in Ta I
1993
The hyperfine structure (hfs) of181Ta has been investigated using laser radio-frequency double resonance and high resolution laser spectroscopy on collimated atomic beams. The magnetic dipole and electric quadrupole hyperfine structure coupling constants of the 5d46s 6D3/2, 5/2, 7/2, 9/2 metastable states have been determined using radio-frequency spectroscopy. In the 5d4 6s6D1/2 metastable state and the excited 5d3 6s 6p4D3/2,6D5/2, 9/2 as well as the unidentified 28 182.6 cm−1 and 30 021.2 cm−1 states, hfs constants have been obtained from high resolution laser spectroscopy. A radio-frequency converter has been developed in order to reach the frequency region 2.7–10 GHz.
Stress-induced dislocation generation in large FZ- and CZ-silicon single crystals—numerical model and qualitative considerations
2001
When growing silicon crystals with higher diameter (presently up to 300 mm) the thermal stresses and possible dislocation generation in single crystals become a serious problem for both FZ- and CZ-methods. A two-dimensional problem oriented code for the FEM-package ANSYS has been developed to calculate the temperature field in the growing crystal considering radiation exchange with reflectors and environment and thermal stresses. Comparing calculated stresses with critical stresses, the dislocated zone is determined. A qualitative concept for the occurrence of dislocations using the metastable state is developed. In a parametric study for different thermal boundary conditions and crystal ge…