Evolutionary optimization of a charge transfer ionic potential model for Ta/Ta-oxide hetero-interfaces

Tantalum, tantalum oxide and their hetero-interfaces are of tremendous technological interest in several applications spanning electronics, thermal management, catalysis and biochemistry. For example, local oxygen stoichiometry variation in TaOx memristors comprising of metallic (Ta) and insulating oxide (Ta2O5) have been shown to result in fast switching on the sub-nanosecond timescale over a billion cycles, relevant to neuromorphic computation. Despite its broad importance, an atomistic scale understanding of oxygen stoichiometry variation across Ta/TaOx hetero-interfaces, such as during early stages of oxidation and oxide growth, is not well understood. This is mainly due to the lack of a variable charge interatomic potential model for tantalum oxides that can accurately describe the ionic interactions in the metallic (Ta) and oxide (TaOx) environment as well as at their interfaces. To address this challenge, we introduce a charge transfer ionic potential (CTIP) model for Ta/Ta-oxide system by training against lattice parameters, cohesive energies, equations of state, and elastic properties of various experimentally observed Ta2O5 polymorphs. The best set of CTIP parameters are determined by employing a single-objective global optimization scheme driven by genetic algorithms followed by local Simplex optimization. Our newly developed CTIP potential accurately predicts structure, thermodynamics, energetic ordering of polymorphs, as well as elastic and surface properties of both Ta and Ta2O5, in excellent agreement with DFT calculations and experiments. We employ our newly parameterized CTIP potential to investigate the early stages of oxidation of Ta at different temperatures and atomic/molecular nature of the oxidizing species

Comparing optimization strategies for force field parameterization

Classical molecular dynamics (MD) simulations enable modeling of materials and examination of microscopic details that are not accessible experimentally. The predictive capability of MD relies on the force field (FF) used to describe interatomic interactions. FF parameters are typically determined to reproduce selected material properties computed from density functional theory (DFT) and/or measured experimentally. A common practice in parameterizing FFs is to use least-squares local minimization algorithms. Genetic algorithms (GAs) have also been demonstrated as a viable global optimization approach, even for complex FFs. However, an understanding of the relative effectiveness and efficiency of different optimization techniques for the determination of FF parameters is still lacking. In this work, we evaluate various FF parameter optimization schemes, using as example a training data set calculated from DFT for different polymorphs of Ir$O_2$. The Morse functional form is chosen for the pairwise interactions and the optimization of the parameters against the training data is carried out using (1) multi-start local optimization algorithms: Simplex, Levenberg-Marquardt, and POUNDERS, (2) single-objective GA, and (3) multi-objective GA. Using random search as a baseline, we compare the algorithms in terms of reaching the lowest error, and number of function evaluations. We also compare the effectiveness of different approaches for FF parameterization using a test data set with known ground truth (i.e generated from a specific Morse FF). We find that the performance of optimization approaches differs when using the Test data vs. the DFT data. Overall, this study provides insight for selecting a suitable optimization method for FF parameterization, which in turn can enable more accurate prediction of material properties and chemical phenomena

Measurements of Rn-222 Concentrations In Dwellings In a City With High Population

16th International Conference on Nuclear Tracks in Solids -- SEP 07-11, 1992 -- BEIJING, PEOPLES R CHINAWOS: A1993NY99100114The contribution of radon and its daughters to the natural radiation exposure of human beings which especially increases quite excessively in the atmosphere of living quarters during winter for energy saving purposes is always at significant levels. In this study, track etch film (Kodak LR-115 type 2) was used to determine the distribution of radon levels in dwellings. The results indicate that the distribution of the radon concentration is approximately 35-80 Bq/m3.CHINA INST ATOM ENERGY, INT NUCL TRACK SO

Measurements of Rn-222 concentrations in dwellings in a city with high population

The contribution of radon and its daughters to the natural radiation exposure of human beings which especially increases quite excessively in the atmosphere of living quarters during winter for energy saving purposes is always at significant levels. In this study, track etch film (Kodak LR-115 type 2) was used to determine the distribution of radon levels in dwellings. The results indicate that the distribution of the radon concentration is approximately 35-80 Bq/m3. © 1994

The effect of organic matters on ferrous oxidation

In oxygen-free aquatic environments, such as groundwaters and hypolimnetic waters of eutrophic lakes, iron exists predominantly in the ferrous, Fe(II), state. In these waters certain natural organic compounds like humic materials also exist. The aim of this study was to find out the effects of tannic acid, acetic acid and glucose on converting/stabilizing Fe(II) to Fe(III). Experimental studies have been carried out in two-stage batch systems. In the first stage, the catalytic effect of Fe(III) on ferrous iron oxidation by atmospheric oxygen was studied at the concentration range of 0-1500 mg/l Fe(III), keeping Fe(II) constant at 25 mg/l. It has been found that kcat increases linearly with increasing Fe(III) concentration up to about 500 mg/l and this rate decreases beyond this value. In the second stage, a study has been made of how oxidation of Fe(II) and the catalytic effect of Fe(III) are affected by tannic acid, acetic acid and glucose. The results of this stage are that tannic acid completely inhibited the oxidation of ferrous iron, but acetic acid and glucose had no observable effect on the oxidation reaction