Understanding Materials Behavior from Atomistic Simulations: Case study of Al-containing High Entropy Alloys and Thermally Grown Aluminum Oxide

Atomistic simulation refers to a set of simulation methods that model the materials on the atomistic scale. These simulation methods are faster and cheaper alternative approaches to investigate thermodynamics and kinetics of materials compared to experiments. In this dissertation, atomistic simulation methods have been used to study the thermodynamic and kinetic properties of two material systems, i.e. the entropy of Al-containing high entropy alloys (HEAs) and the vacancy migration energy of thermally grown aluminum oxide. In the first case study of the dissertation, a computational scheme for evaluating the entropy of HEAs has been developed. Entropy is a key factor for the phase stability of HEAs. However, it has not been well understood yet. In this study, atomistic simulation methods have been used to quantify the configurational and vibrational entropy of HEAs for the first time. Modified embedded atom method was used to describe the interatomic interactions in HEAs. Monte Carlo simulation and thermodynamic integration method were used to calculate the thermodynamic properties such as entropy and free energy. This scheme has been tested on AlxCoCrFeNi HEAs. The results show that a reasonable evaluation of the entropy of AlxCoCrFeNi HEAs can be obtained by the developed scheme. The FCC to BCC phase transition in this alloy system has also been captured by the calculated free energy. Importantly, it is found that atomic vibrations have an important effect on the quantitative prediction of the compositional boundary of the FCC-BCC duplex region in the AlxCoCrFeNi HEA system. The calculated entropy has been validated by comparing the atomic ordering in the simulated HEAs to the HEAs in experiments. The good agreement between the simulations and experiments indicates that the developed computational scheme captured the non-ideality in HEAs which is the key to understand the entropy of HEAs. In the second case study of this dissertation, the charge effect on the vacancy diffusion in α-Al2O3 has been investigated. It has been known that the charge state has an effect on the formation energy of vacancies. However, the relation between the charge state and the migration energy of vacancies is unknown yet. In this study, density functional theory calculations have been used to investigate the charge effect on the vacancy migration energy. It is found that the vacancy migration energy depends strongly on the charge state of the vacancy. This dependency is explained by the shift of the defect levels associated with the vacancy and the electron occupancy on the defect levels. These findings for the first time built a link between the electronic structure and the migration of vacancy in metal oxides. This information indicates a novel approach to tune the diffusion kinetics by modifying the electronic structure of metal oxides

SUPPLEMENTARY INFORMATION Dislocation nucleation facilitated by atomic segregation DOI: 10.1038/NMAT5034

This is a set of supplementary data and information supporting the Journal Publication 'Dislocation nucleation facilitated by atomic segregation', DOI: 10.1038/NMAT5034, and available at Journal article in Nature Materials

Bivalence Mn5O8 with hydroxylated interphase for high-voltage aqueous sodium-ion storage.

Aqueous electrochemical energy storage devices have attracted significant attention owing to their high safety, low cost and environmental friendliness. However, their applications have been limited by a narrow potential window (∼1.23 V), beyond which the hydrogen and oxygen evolution reactions occur. Here we report the formation of layered Mn5O8 pseudocapacitor electrode material with a well-ordered hydroxylated interphase. A symmetric full cell using such electrodes demonstrates a stable potential window of 3.0 V in an aqueous electrolyte, as well as high energy and power performance, nearly 100% coulombic efficiency and 85% energy efficiency after 25,000 charge-discharge cycles. The interplay between hydroxylated interphase on the surface and the unique bivalence structure of Mn5O8 suppresses the gas evolution reactions, offers a two-electron charge transfer via Mn2+/Mn4+ redox couple, and provides facile pathway for Na-ion transport via intra-/inter-layer defects of Mn5O8

Molecular and Electronic Structures of Transition-Metal Macrocyclic Complexes as Related to Catalyzing Oxygen Reduction Reactions: A Density Functional Theory Study

Transition-metal (TM) macrocyclic complexes have potential applications as nonprecious electrocatalysts in polymer electrolyte membrane fuel cells. In this study, we employed density functional theory calculation methods to predict the molecular and electronic structures of O₂, OH, and H₂O₂ molecules adsorbed on TM porphyrins, TM tetraphenylporphyrins, TM phthalocyanines, TM fluorinated phthalocyanines, and TM chlorinated phthalocyanines (here TM = Fe or Co). Relevant to their performance on catalyzing oxygen reduction reaction (ORR), we found for the studied TM macrocyclic complexes: (1) The type of the central TM is the most determinant factor in influencing the adsorption energies of O₂, OH, and H₂O₂ (chemical species involved in ORR) molecules on these macrocyclic complexes. Specifically, the calculated adsorption energies of O₂, OH, and H₂O₂ on the Fe macrocyclic complexes are always distinguishably lower than those on the Co macrocyclic complexes. (2) The peripheral ligands are capable of modulating the binding strength among the adsorbed O₂, OH, and H₂O₂, and the TM macrocyclic complexes. (3) A N–TM–N cluster structure (like N–Fe–N) with a proper distance between the two ending N atoms and a strong electronic interaction among the three atoms is required to break the O–O bond and thus promote the efficient four-electron pathway of the ORR on the TM macrocyclic complexes

Atomic-scale phase separation induced clustering of solute atoms.

Dealloying typically occurs via the chemical dissolution of an alloy component through a corrosion process. In contrast, here we report an atomic-scale nonchemical dealloying process that results in the clustering of solute atoms. We show that the disparity in the adatom-substrate exchange barriers separate Cu adatoms from a Cu-Au mixture, leaving behind a fluid phase enriched with Au adatoms that subsequently aggregate into supported clusters. Using dynamic, atomic-scale electron microscopy observations and theoretical modeling, we delineate the atomic-scale mechanisms associated with the nucleation, rotation and amorphization-crystallization oscillations of the Au clusters. We expect broader applicability of the results because the phase separation process is dictated by the inherent asymmetric adatom-substrate exchange barriers for separating dissimilar atoms in multicomponent materials

Atomic-scale phase separation induced clustering of solute atoms

Dealloying usually relies on chemical dissolution of an alloy component. By contrast, the authors demonstrate an atomic-scale phase separation process that differs completely from the chemical dealloying mechanism and thus represents a significant departure from the well-known Hume-Rothery rules

Structure-based discovery of dual pathway inhibitors for SARS-CoV-2 entry

Abstract Since 2019, SARS-CoV-2 has evolved rapidly and gained resistance to multiple therapeutics targeting the virus. Development of host-directed antivirals offers broad-spectrum intervention against different variants of concern. Host proteases, TMPRSS2 and CTSL/CTSB cleave the SARS-CoV-2 spike to play a crucial role in the two alternative pathways of viral entry and are characterized as promising pharmacological targets. Here, we identify compounds that show potent inhibition of these proteases and determine their complex structures with their respective targets. Furthermore, we show that applying inhibitors simultaneously that block both entry pathways has a synergistic antiviral effect. Notably, we devise a bispecific compound, 212-148, exhibiting the dual-inhibition ability of both TMPRSS2 and CTSL/CTSB, and demonstrate antiviral activity against various SARS-CoV-2 variants with different viral entry profiles. Our findings offer an alternative approach for the discovery of SARS-CoV-2 antivirals, as well as application for broad-spectrum treatment of viral pathogenic infections with similar entry pathways