Uncertainty Quantification and Reduction of Molecular Dynamics Models

Molecular dynamics (MD) is an important method underlying the modern field of Computational Materials Science. Without requiring prior knowledge as inputs, MD simulations have been used to study a variety of material problems. However, results of molecular dynamics simulations are often associated with errors as compared with experimental observations. These errors come from a variety of sources, including inaccuracy of interatomic potentials, short length and time scales, idealized problem description and statistical uncertainties of MD simulations themselves. This chapter specifically devotes to the statistical uncertainties of MD simulations. In particular, methods to quantify and reduce such statistical uncertainties are demonstrated using a variety of exemplar cases, including calculations of finite temperature static properties such as lattice constants, cohesive energies, elastic constants, dislocation energies, thermal conductivities, surface segregation and calculations of kinetic properties such as diffusion parameters. We also demonstrate that when the statistical uncertainties are reduced to near zero, MD can be used to validate and improve widely used theories

Apparatus and method for intra-layer modulation of the material deposition and assist beam and the multilayer structure produced therefrom

A method of producing a multilayer structure that has reduced interfacial roughness and interlayer mixing by using a physical-vapor deposition apparatus. In general the method includes forming a bottom layer having a first material wherein a first plurality of monolayers of the first material is deposited on an underlayer using a low incident adatom energy. Next, a second plurality of monolayers of the first material is deposited on top of the first plurality of monolayers of the first material using a high incident adatom energy. Thereafter, the method further includes forming a second layer having a second material wherein a first plurality of monolayers of the second material is deposited on the second plurality of monolayers of the first material using a low incident adatom energy. Next, a second plurality of monolayers of the second material is deposited on the first plurality of monolayers of the second material using a high incident adatom energy

Uniaxial pulling and nano-scratching of a newly synthesised high entropy

Multi component alloys possessing nanocrystalline structure, often alluded to be as Cantor alloys or high entropy alloys (HEAs), continues to attract great attention of the research community. It has been suggested that about 64 elements in the periodic table can be mixed in various compositions to synthesize as many as ~108 different types of HEA alloys. Most HEA's possess a face centered cubic or a body centered cubic crystal structure. The nanomechanical studies on any types of HEA combining experimental and atomic simulations are rather scarce in literature, which was a major motivation behind this work. In this spirit, a novel high entropy alloy (Ni25Cu18.75Fe25Co25Al6.25) was synthesized using arc melting method which followed a joint simulation and experimental effort to investigate dislocation mediated plastic mechanisms in HEA. The investigation takes advantage of an Embedded atomic method (EAM) type potential energy function corroborating the material composition to perform the nanoscale tensile and scratch MD simulation studies followed by experimental nano-scratching to investigate plasticity and material removal mechanisms, aspects related to nanofriction and nanotribology, side flow, pileup and crystal defects formed in the sub-surface as an elasto-plastic material response of the HEA during and after the scratch process. The major types of crystal defects associated with the plastic deformation of the crystalline face centred cubic structure of HEA were 2,3,4-hcp layered like defect coordination structure, coherent ∑3 twin boundary and ∑11 fault or tilt boundary, in combination with Stair rods, Hirth locks, Frank partials and Lomer–Cottrell (LC) locks. They formed much of the damage in the sub-surface and side-flow mechanisms. Moreover, 1/6 Shockley with exceptionally larger dislocation loops were seen to be the transporters of stacking faults deeper into the substrate than the location of the applied cutting load. The (100) orientation showed the highest value for the coefficient of kinetic friction but least amount of cutting stress and cutting temperature during HEA deformation suggesting this orientation to be better than the other orientations for its improved manufacturing

Horizons of modern molecular dynamics simulation in digitalized solid freeform fabrication with advanced materials

Our ability to shape and finish a component by combined methods of fabrication including (but not limited to) subtractive, additive, and/or no theoretical mass-loss/addition during the fabrication is now popularly known as solid freeform fabrication (SFF). Fabrication of a telescope mirror is a typical example where grinding and polishing processes are first applied to shape the mirror, and thereafter, an optical coating is usually applied to enhance its optical performance. The area of nanomanufacturing cannot grow without a deep knowledge of the fundamentals of materials and consequently, the use of computer simulations is now becoming ubiquitous. This article is intended to highlight the most recent advances in the computation benefit specific to the area of precision SFF as these systems are traversing through the journey of digitalization and Industry-4.0. Specifically, this article demonstrates that the application of the latest materials modelling approaches, based on techniques such as molecular dynamics, are enabling breakthroughs in applied precision manufacturing techniques

Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property non-uniformity that causes low manufacturing yield. While tremendous efforts have been made in the past to reduce Te inclusions / precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbation and the associated property non-uniformity. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material, and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals

A Validated Liquid Chromatographic Method for Berberine Analysis in Tissue and Application

Simple and rapid high-performance liquid chromatography methods were developed for the determination of berberine (BB) in various rat tissues so as to evaluate a P-gp inhibitor, glycyrrhetinic acid (GA), on BB’s oral bioavailability. Acetonitrile was used to extract BB from tissues and showed different extraction recoveries in diverse tissues. The intra- and interday precision and accuracy were less than 10%. Long-term stability, pre (post) -preparative stability, and freeze-thaw stability were evaluated, and the results showed it could meet the need of this study. The proposed methods were subsequently applied to investigate the possible drug-drug interaction of GA and BB in vivo from the aspect of tissue distribution. The results showed that no significant difference was found between the group of low dose and high dose at the same time point. The tissue distributions show a saturated model, i.e., the content of BB in tissue tends to be constant while its dose is more than 200 mg/kg. Besides, the contents of BB ranged from high to low according to the order of the liver, kidney, and spleen. The BB content in the liver is especially high as compared to other tissues

Reliability Assessment of Multiprocessor System Based on (N, K)-Star Network

As the size and complexity of a multiprocessor system increases, reliability evaluation becomes an important issue. The performability of a multiprocessor system heavily depends on the application program and the underlying architecture. In multitasking multiprocessor system, the problem of dynamically assigning a given dimensional subsystem to a special task is considered as a reallocation in the presence of node and/or link failures. This paper takes the generalization of star graph, (n, k)-star graph, as an empirical object. In order to measure the reliability of (n, k)star graph, the analytical model introduces mean time to failure (MTTF) to show the time that the appearance of a certain number of faulty S n −1 ,k− 1 costs. The higher the MTTF, the better the robustness. So, the way to evaluate the robustness of an (n, k)-star is to count how much the MTTF is. In fact, an (n, k)-star can be partitioned along any dimension (except the first one) with corresponding identification code. So, we will explore the reliability of (n, k)-star graph when it is partitioned along any dimension (except the first one) under node and/or link fault model. Comparisons among the simulation results under two partitioning models reveal that the MTTF is higher under liberal partition model, which better reflect the steady state of an interconnection network that can persist when the network is destroyed

Molecular Dynamics Calculations of Grain Boundary Mobility in CdTe

Molecular dynamics (MD) simulations have been applied to study mobilities of Σ3, Σ7 and Σ11 grain boundaries in CdTe. First, an existing MD approach to drive the motion of grain boundaries in face-centered-cubic and body-centered-cubic crystals was generalized for arbitrary crystals. MD simulations were next performed to calculate grain boundary velocities in CdTe crystals at different temperatures, driving forces, and grain boundary terminations. Here a grain boundary is said to be Te-terminated if its migration encounters sequentially C d · T e − C d · T e … planes, where “·” and “−” represent short and long spacing respectively. Likewise, a grain boundary is said to be Cd-terminated if its migration encounters sequentially T e · C d − T e · C d … planes. Grain boundary mobility laws, suitable for engineering time and length scales, were then obtained by fitting the MD results to Arrhenius equation. These studies indicated that the Σ3 grain boundary has significantly lower mobility than the Σ7 and Σ11 grain boundaries. The Σ7 Te-terminated grain boundary has lower mobility than the Σ7 Cd-terminated grain boundary, and that the Σ11 Cd-terminated grain boundary has lower mobility than the Σ11 Te-terminated grain boundary