Atomic-scale Insights Research Articles

Glass transition temperature of asphalt binder based on atomistic scale simulation

Glass transition is one of the most crucial physical properties for polymerical materials. As a typical complex polymerical material, the glass transition phenomenon in asphalt binder is directly related to their temperature-related properties. To investigate the glass transition characteristics, this study delves into the glass transition temperature of asphalt binder based on molecular dynamics simulations. It is found that the calculation range for the glass transition temperature sits between 100 and 400 K. The evolution of asphalt binder structure is influenced by different cooling rates, where lower cooling rates allow sufficient microstructural rearrangement, resulting in a smaller volume at the lower temperature. Model size is closely associated with the glass transition region. As the size increases, the transition region significantly expands. Increasing the model size also reduces volume fluctuations after isothermal relaxation, providing more stable volume changes. It is observed that higher cooling rates with a model size over 100 Å can well reproduce the glass transition process of asphalt binders. This work provides atomic-scale insights for the glass transition phenomenon in asphalt binder, which could be beneficial for the design of high-performance asphalt binder.

Atomic-scale insight on nanostructures and electrochemical properties of (NiCrCo)82Ti9Al9 high-entropy alloy coatings by density functional theory calculations

(NiCrCo)82Ti9Al9 high−entropy alloy was attached to act as coating material by combining of laser cladding and laser remelting. In this study, the electrochemical properties of obtained HEA coatings were comparatively investigated using an electrochemical workstation, and the corrosion mechanism was revealed by the density functional theory calculations. The results show that the laser cladded HEA coating is mainly consisted of face-centered and body-centered cubic structures, and the face-centered cubic structure of the laser cladded HEA coating is transferred to the amorphous structure by laser remelting, which provides the stronger anti−corrosion capability compared with the face-centered cubic structure of HEA coating.

Unraveling the Subsurface Damage and Material Removal Mechanism of Multi-Principal-Element Alloy FeCrNi Coatings During the Scratching Process

Multi-principal-element alloys (MPEAs) exhibit superior strength and good ductility. However, tribological properties of FeCrNi MPEAs remain unknown at nanoscale and complex environments. Here, we investigate the effects of scratching speed, depth, and temperature on microstructural and tribological characteristics of FeCrNi using molecular dynamics simulations combined with an elevated temperature tribological experiment. The scratching force experiences the increase stage, the undulated stage, and the stable stage due to chip formation. Compared to traditional alloy coatings, low force enhances the useful life. With increased speed, the friction coefficient decreases, agreeing with previous work. High speed impacting includes severe local plastic deformation, from dislocation to amorphization. As the scratching depth increases, the average scratch force and friction coefficient increases owing to material accumulation in front of the abrasive particles. The surface morphology and dislocation behavior are significantly different during the scratching process. In addition, we revealed a temperature-dependent friction mechanism. FeCrNi MPEAs have excellent wear resistance at an intermediate temperature, which is attributed to the high Cr content promoting the formation of the compact oxide layer. This work provides atomic-scale mechanistic insights into the tribological behavior of FeCrNi, and would be applied to the design of MPEAs with high performance.

Cu-Atom Locations in Rocksalt SnTe Thermoelectric Alloy.

The development of functional thermoelectric materials requires direct evidence of dopants' locations to rationally design the electronic and phononic structure of the host matrix. In this study, Cs-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy is employed at the atomic scale to identify Cu atoms' locations in a Cu-doped SnTe thermoelectric alloy. It is revealed that Cu atoms in the rocksalt SnTe form solid solutions at both Sn and Te sites, contrary to their electronegativity order and the intentional Cu doping at Sn sites. Cu atoms are also located at the tetrahedral and crowdion sites of the face-centred cubic structure, with varying degrees of correlations. Such high flexibility of Cu atoms in the rocksalt SnTe offers diverse phonon-scattering mechanisms conducive to the ultra-low lattice thermal conductivity of singly Cu-doped SnTe. This study offers atomic-scale insights for achieving more precise dopant engineering, leading to the accelerated development of functional thermoelectric materials.

In-depth analysis on the mechanism and reaction efficiency of hydrogen deoxidation in ultra-low carbon steel

The reaction mechanism and efficiency of hydrogen deoxidation used in ultra-low carbon steel are explored in this work. A combination of first-principle simulations and thermodynamic calculations were performed on the reaction mechanisms of hydrogen with dissolved oxygen. Laboratory thermal state experiments were used to analyze the effects of different initial oxygen contents of steel and different hydrogen injection flow rates on the deoxidation ability. The results show that the hydrogen deoxidation mainly happens with the gas state of H2 since there are more reactive sites of H atoms in the gas bubble compared to the dissolved H. Higher initial oxygen content and larger hydrogen blowing flow rate are beneficial to the deoxidation reaction efficiency. With hydrogen deoxidation, the inclusion number can be reduced by half compared to Al deoxidation, and the finally total oxygen content can reach 6.8 × 10-6. The actual reaction hydrogen utilization efficiency fluctuates between 0.13%-1.17% in this study, which can be improved by extending the resistance time of H2 bubbles in the molten steel. This paper provides in-depth theoretical support and atomic-scale insights into the reaction between hydrogen and oxygen in steel, building a foundation for the hydrogen application in the production of ultra-low-carbon steel.

Characterization of L12-Al3Ce phase and its purification mechanism in the Al-Ce-TiCN alloy

Rare earth element Ce has a positive purifying effect on Fe and Si impurities in pure aluminum. In this study, we unveiled the mechanism of Ce purification of Fe and Si impurities in commercial pure aluminum at the atomic scale via utilizing the properties of TiCN nanoparticles, which exhibit distribution along grain boundaries and impede solute atom diffusion at specific cooling rates. The transition phase L12-Al3Ce was characterized in aluminum, which is considered to be an important transition phase to purified products. Furthermore, High Angle Dark Field Scanning Transmission Electron Microscopy (HADDF-STEM) and 3D Atom Probe Tomography (3D-APT) results showed that Ce could respectively enrich the Fe and Si impurities, resulting in the formation of Al-Ce-Fe and Al-Ce-Si clusters. Density functional theory (DFT) results indicated that Fe and Si atoms can incorporate into L12-Al3Ce crystal, forming more stable structures and therefore giving rise to the formation of Al-Ce-Si and Al-Ce-Fe nanoclusters. This study provides atomic-scale insights into the mechanism of Ce purifying Fe and Si impurities in aluminum.

Atomic-scale insights into the effect of layer thickness ratio on the tensile properties of heterostructured nano-aluminum

Gradient-structured alloys with lamellar microstructures show extraordinary potential in breaking the strength-ductility trade-off dilemma. When partial grain size exceeds the Hall-Petch limit, the deformation mechanism is not fully understood. In this study, the competing relationship between grain boundary (GB) softening and hetero-deformation-induced (HDI) strengthening in laminated nano-grained aluminum was investigated under uniaxial tensile loading. In the soft domains, the high activity of grain boundaries results in pronounced GB softening. However, when the volume fraction of the soft layer exceeds 50 %, the flow stress surpasses the predicted values of the rule of mixtures (ROM), demonstrating a significant HDI strengthening. A detailed analysis of microstructural evolution is conducted, including strain gradients, GB migration, and dislocation distribution. The effects of mechanical incompatibility between soft and hard layers on the distribution of plastic strain and overall material strength are elucidated in gradient structures. The study provides critical theoretical insights into the design and development of high-performance gradient nanocrystalline aluminum alloys, particularly in applications where a balance between strength and ductility is of paramount importance.

A machine learning–based framework for mapping hydrogen at the atomic scale

Hydrogen, the lightest and most abundant element in the universe, plays essential roles in a variety of clean energy technologies and industrial processes. For over a century, it has been known that hydrogen can significantly degrade the mechanical properties of materials, leading to issues like hydrogen embrittlement. A major challenge that has significantly limited scientific advances in this field is that light atoms like hydrogen are difficult to image, even with state-of-the-art microscopic techniques. To address this challenge, here, we introduce Atom-H, a versatile and generalizable machine learning-based framework for imaging hydrogen atoms at the atomic scale. Using a high-resolution electron microscope image as input, Atom-H accurately captures the distribution of hydrogen atoms and local stresses at lattice defects, including dislocations, grain boundaries, cracks, and phase boundaries. This provides atomic-scale insights into hydrogen-governed mechanical behaviors in metallic materials, including pure metals like Ni, Fe, Ti and alloys like FeCr. The proposed framework has an immediate impact on current research into hydrogen embrittlement and is expected to have far-reaching implications for mapping "invisible" atoms in other scientific disciplines.

Selective Oxidation of Methanol to Methyl Formate on Gold: The Role of Low-Coordinated Sites Revealed by Isothermal Pulsed Molecular Beam Experiments and AIMD Simulations.

To elucidate the role of low-coordinated sites in the partial methanol oxidation to methyl formate (MeFo), the isothermal reactivity of flat Au(111) and stepped Au(332) in pulsed molecular beam experiments was compared for a broad range of reaction conditions. Low-coordinated step sites were found to enhance MeFo selectivity, especially at low coverage conditions, as found at higher temperatures. The analysis of the transient kinetics provides evidence for the essential role of Au x O y phases for MeFo formation and the complex interplay of different oxygen species for the observed selectivity. Ab initio molecular dynamic simulations yielded microscopic insights in the formation of Au x O y phases on flat and stepped gold surfaces emphasizing the role of low-coordinated sites in their formation. Moreover, associated surface restructuring provides atomic-scale insights which align with the experimentally observed transient kinetics in MeFo formation.

Unraveling the Mechanical Response of Short-Chain Branched Amorphous Polyethylene: Insights from Molecular Dynamics Simulations

The deformation behavior of short-chain branched, amorphous polyethylene (PE) was investigated using united atom molecular dynamics (MD) simulations under uniaxial stretching. Our research reported in this paper examined the internal mechanisms underlying the mechanical response of the PE by analyzing potential energies, dihedral angle distributions, chain orientation and entanglements. The findings indicated that the short-chain branching generally enhanced Young’s modulus, except for sample B60 with a critical ethyl branch content of 60/1000 (60 ethyl branches per 1000 backbone carbons). The Young’s modulus of B60 was the local minimum value in the various samples. Analysis of the evolution of dihedral angles shows that the short-chain branching hinders the transition from gauche to trans configurations, leading to a decrease in the trans population as the branch content increases. Regarding the sample B60 with critical branch content 60/1000, the orientation parameter and entanglement parameter of the molecular chains along the backbone were lowest in the initial structure, and the efficiency of chain disentanglement was at its lowest during deformation. The simulation results elucidated the deformation mechanism related to the conformational evolution of the molecular chains, which can provide atomic-scale insights into the mechanical properties and processability of branched polyethylene, thereby allowing for optimization in its design and application.

Investigating nanostructure-property relationship of WTaVCr high-entropy alloy via machine learning optimized reactive potential

In spite of the promising prospects of the WTaVCr refractory high-entropy alloy (RHEA), the nanoscale structure-property relationship remains largely unexplored. This study introduces a supervised machine learning (ML) framework to develop a charge-transfer ionic potential (CTIP) for the W/Ta/V/Cr/O multi-component system. This novel approach demonstrates exceptional advantages in optimizing numerous parameters of a highly flexible yet physically rigorous potential, leveraging a modified distributed breeder genetic algorithm (DBGA) to balance search comprehensiveness and efficiency. The robustness of developed potential is verified through cross-validation against first-principles predictions on various properties of metals, alloys and oxides. Subsequently, dynamic simulations of annealing, mechanical loading, surface oxidation and radiation collision are conducted utilizing CTIP. Results of these simulations align with previous experiments about nanoscale chemical ordering, plastic deformation behavior, oxidation mechanisms and radiation tolerance. These findings not only further corroborate the reliability of CTIP potential, but also uncover atomic-scale insights that are experimentally unattainable, thereby enhancing the understanding of the nanostructure-property relationship.

Exploring the Facet-Dependent Structural Evolution of Pt/CeO2 Catalysts Induced by Typical Pretreatments for CO Oxidation.

Atomic-scale insights into the interactions between metals and supports play a crucial role in optimizing catalyst design, understanding catalytic mechanisms, and enhancing chemical conversion processes. The effects of oxide support on the dynamic behavior of supported metal species during pretreatments or reactions have been attracting a lot of attention; however, very less systematic integrations are carried out experimentally using real catalysts. In this study, we here utilized facet-controlled CeO2 as examples to explore their influence on the supported Pt species (1.0 wt %) during the reducing and oxidizing pretreatments that are typically applied in heterogeneous catalysts. By employing a combination of microscopy, spectroscopy, and first-principles calculations, it is demonstrated that the exposed crystal facets of CeO2 govern the evolution behavior of supported Pt species under different environmental conditions. This leads to distinct local coordinations and charge states of the Pt species, which directly influence the catalytic reactivity and can be leveraged to control the catalytic performance for CO oxidation reactions.

A TEM Study of Structural Degradation in LiFePO4 Batteries after Hybrid Vehicle Use

Establishing a robust, efficient circular economy for batteries hinges on understanding how they decay over time under various conditions. This understanding is crucial to maximize material use before recycling or disposal. A key aspect of this endeavor is the study of lithium iron phosphate (LiFePO4), a pivotal battery technology whose structural integrity over time remains a subject of inquiry.1 Known aging mechanisms in LiFePO4 (LFP) batteries include electrode degradation, SEI growth, and electrolyte decomposition.1 These processes extend to the cathode, where degradation can manifest as Fe dissolution, Li inventory loss, Fe/Li anti-site defects, and LFP amorphization.1–3 However, these mechanisms are typically studied in lab-aged cells under controlled conditions, leaving a gap in our understanding of their behavior in real-life, commercialized applications.Our research aims to bridge this gap by characterizing the degradation mechanisms of LFP cathode material in various states of health (SOH) after use in a hybrid vehicle. We sourced our cathode samples from 26650 cells extracted from a BAE ESS-A123 hybrid bus battery pack. After selecting cells with drastically different SOHs based on previous analyses,4 we employed transmission electron microscopy (TEM) for detailed characterization.In this talk, we will share insights gained from high-resolution TEM characterization, alongside chemical analysis using energy-filtered TEM and electron energy loss spectroscopy (EELS). Our focus will be on changes in the olivine structure, including lattice parameter alterations, Li and Fe migration, and Fe oxidation. By combining cell-level SOH analyses with TEM characterization, we will highlight how electrochemical test results correlate with material degradation mechanisms, enhancing our understanding of battery health, longevity, and diagnostics. Wang, L. et al. Insights for understanding multiscale degradation of LiFePO4 cathodes. eScience 2, 125–137 (2022). Li, X. et al. First Atomic-Scale Insight into Degradation in Lithium Iron Phosphate Cathodes by Transmission Electron Microscopy. J. Phys. Chem. Lett. 11, 4608–4617 (2020). Xu, P. et al. Efficient Direct Recycling of Lithium-Ion Battery Cathodes by Targeted Healing. Joule 4, 2609–2626 (2020). Ramirez-Meyers, K., Rawn, B. & Whitacre, J. F. A statistical assessment of the state-of-health of LiFePO4 cells harvested from a hybrid-electric vehicle battery pack. J. Energy Storage 59, 106472 (2023).

(Invited) OER Active Phases, Catalytic Mechanism and Reaction Centers of Ni (oxy)Hydroxide with and without Fe Impurities

Identifying atomic-scale structures of catalytically active phases, catalytic mechanism and reaction centers are crucial for establishing rigorous structure-activity relationship of highly active catalysts and for designing new catalysts with further improved performance. Due to the lack of theoretical guidance and challenges in experimental structural characterization during OER conditions, in-situ structures and stabilities of two (a-Ni(OH)2 and g-NiOOH with water and ion intercalation) out of four (oxy)hydroxide phases (another two are b-Ni(OH)2 and b-NiOOH without water and ion intercalations) remain elusive after at least half a century study, which has hindered the establishment of rigorous structure-activity relationship of Ni (oxy)hydroxides and their derivatives for the OER. As a consequence, it is still unclear regarding which phases should be the hosts and the precursors of choice for highly active catalysts. Here, by combining ab initio molecular dynamics simulation, in-situ wide-angle X-ray scattering, and electrochemical measurements, we provide direct atomic-scale and self-consistent insights into the crystal structure, water and ion intercalation, and stable potential windows of complex a-Ni(OH)2 and g-NiOOH with respect to b-phases. DFT calculations indicate that b-NiOOH and g-NiOOH with Fe impurities have similar OER activity to or even higher OER activity than that of g-NiFe LDH, with a dependence on the Fe coverage on the edges. Current work suggests that both b-NiOOH and g-NiOOH are potential hosts of highly active OER catalysts, which enlarges the phase space for the design of OER catalysts with further improved performance.We will discuss the implications of the above insights toward understanding of the active phases, reaction centers and catalytic mechanism of NiM and CoM layered double hydroxides. If time permits, we will further review our work our work regarding the promotion effects of non-covalent ligand-oxide interaction in OER and mitigation of RuO6 octahedron distortion by enhanced A-site electronegativity in pyrochlore for acidic water oxidation.

Boosted Photocatalytic Activities of Ag2CrO4 through Eu3+-Doping Process.

Ag2CrO4 is a representative member of a family of Ag-containing semiconductors with highly efficient visible-light-driven responsive photocatalysts. The doping process with Eu3+ is known to effectively tune their properties, thus opening opportunities for investigations and application. Here, we report the enhancement of the photocatalytic activity and stability of Ag2CrO4 by introducing Eu3+cations. The structural, electronic, and photocatalytic properties of Ag2CrO4:xEu3+ (x = 0, 0.25, 0.5, 1%) synthesized using the coprecipitation method were systematically discussed, and their photodegradation activity against rhodamine B (RhB), ciprofloxacin hydrochloride monohydrate (CIP), and 4-nitrophenol (4-NP) was evaluated. Structural analyses reveal a short-range symmetry breaking in the Ag2CrO4 lattice after Eu3+ doping, influencing the material morphology, size, and electronic properties. XPS analysis confirmed the incorporation of Eu3+ and alteration of the surface oxygen species. Furthermore, photoluminescence measurements indicated that the doping process was responsible for reducing recombination processes. The sample doped with 0.25% Eu3+ exhibited superior photocatalytic performance compared to pure Ag2CrO4. Scavenger experiments revealed an increase in the degradation via •OH reactive species for the sample doped with 0.25% Eu3+. DFT calculations provided atomic-scale insights into the structural and electronic changes induced by the Eu3+ doping process in the Ag2CrO4 host lattice. This study confirms that Eu3+ doping alters the band structure, enabling different degradation paths and boosting the separation/transfer of photogenerated charges, thereby improving the overall photocatalytic performance.

Atomic-scale insights into the electrochemical mechanisms of aluminum-sulfur batteries: A first-principles study of Al-S clusters on graphene

Aluminum-sulfur (Al-S) batteries, known for their high energy density, cost-effectiveness, and sustainability, face challenges due to limited understanding of their atomic-level electrochemical mechanisms. This study uses first-principles calculations to explore the geometric and electronic structures of Al-S clusters and their interaction with graphene substrates. Key findings include charge transfer from graphene to Al-S clusters, especially in C@Al2S6 and C@Al2S3, and moderate adsorption strength crucial for reversible cycling. Thermodynamic analysis identifies the free energy change in the transition from C@Al2S6 to C@Al2S3, while AIMD simulations confirm structural stability, providing insights for optimizing Al-S battery performance.

Modeling of Nanomaterials for Supercapacitors: Beyond Carbon Electrodes.

Capacitive storage devices allow for fast charge and discharge cycles, making them the perfect complements to batteries for high power applications. Many materials display interesting capacitive properties when they are put in contact with ionic solutions despite their very different structures and (surface) reactivity. Among them, nanocarbons are the most important for practical applications, but many nanomaterials have recently emerged, such as conductive metal-organic frameworks, 2D materials, and a wide variety of metal oxides. These heterogeneous and complex electrode materials are difficult to model with conventional approaches. However, the development of computational methods, the incorporation of machine learning techniques, and the increasing power in high performance computing now allow us to tackle these types of systems. In this Review, we summarize the current efforts in this direction. We show that depending on the nature of the materials and of the charging mechanisms, different methods, or combinations of them, can provide desirable atomic-scale insight on the interactions at play. We mainly focus on two important aspects: (i) the study of ion adsorption in complex nanoporous materials, which require the extension of constant potential molecular dynamics to multicomponent systems, and (ii) the characterization of Faradaic processes in pseudocapacitors, that involves the use of electronic structure-based methods. We also discuss how recently developed simulation methods will allow bridges to be made between double-layer capacitors and pseudocapacitors for future high power electricity storage devices.

Atomic-Scale Insights into the High-Pressure Superconductor La3Ni2O7 and Topotactically Reduced LaNiO2 Single Crystals

Atomic-Scale Insights into the High-Pressure Superconductor La3Ni2O7 and Topotactically Reduced LaNiO2 Single Crystals

Atomic-scale insights into precipitates and their interfacial properties in CuCrZr alloys induced by cryogenic treatment

In this study, high-resolution TEM was utilized to provide a detailed characterization of the precipitates in CuCrZr alloys induced by cryogenic treatment. Building on these findings, first-principles computational models were developed to investigate the mechanical properties of various precipitates and their interfacial bonding characteristics with the matrix. The results demonstrated that cryogenic treatment generated two submicron-sized secondary phases in the matrix: pure Cr particles with a BCC structure and Cu5Zr particles with an FCC structure. Additionally, we observed that nano-sized pure Cr particles were dispersed throughout the matrix and exhibited an orientation relationship of (−111) Cu//(0–11) Cr and [0–11] Cu//[−1−1-1] Cr. First-principles calculations revealed that all the above precipitates exhibited higher modulus and hardness compared to the matrix, with Cr particles being particularly notable. Additionally, the adhesion strength of the Cr/Cu interface was exceptional, surpassing that of the fully coherent CuSlab and CuTwin. Furthermore, the analysis of the electronic properties of the interface revealed that the strengthening effect could be attributed to the higher charge density and localized nature of the Cr atoms near the interface. In summary, precipitates with high hardness and strong adhesion to the matrix can more effectively hinder dislocation movement, which is crucial for enhancing the mechanical properties of the alloys.

Atomic-scale insights into grain boundary-mediated plasticity mechanisms in a magnesium alloy subjected to cyclic deformation

Sustaining grain boundary (GB) plasticity is important to prevent the premature GB cracking during plastic deformation of hexagonal-close-packed (HCP) materials. Here, we investigated atomic structures of GBs and their deformation modes in the form of motion, twin nucleation, dislocation emission, and dissociation in a HCP magnesium alloy subjected to cyclic deformation, according to atomic-resolution observations and crystallographic analyses. Besides migration and sliding, symmetric tilt GBs tended to move to form local facets, resulting in generation of asymmetric tilt low-angle GBs (LAGBs) parallel to {101¯2} lattice plane or serrated asymmetric tilt high-angle GBs (HAGBs). Crystallographic analyses indicated that local facets of the resultant asymmetric tilt GBs were often parallel to {101¯2} plane, basal and/or prismatic planes of adjacent grains. Interestingly, abundant {101¯2} twin embryos and recrysatllized nanograins nucleated from local facets of the asymmetric tilt GBs, accompanying with emission of arrays of basal 〈a〉 dislocations. A {101¯2} twin lamella along a tilt LAGB could be formed either by the coalescence of {101¯2} twin embryos, or by the combination of recrystallized nanograins, followed by {101¯2} twin nucleation from triple junctions of GBs and twin growth. Furthermore, the twin lamella could be formed through dissociation of one tilt LAGB into two tilt HAGBs, followed by {101¯2} twin nucleation from the HAGBs and triple junctions of GBs. Importantly, activation of GB deformation modes mentioned above can release local stress concentrations at GBs and sustain the ability of GB-mediated plasticity, playing important roles in plastic deformation and mechanical properties of HCP materials.