Covalent-like Bonds Research Articles

Hydrogen Bonds-Pinned Entanglement Blunting the Interfacial Crack of Hydrogel-Elastomer Hybrids.

Anchoring a layer of amorphous hydrogel on an antagonistic elastomer holds potential applications in surface aqueous lubrication. However, the interfacial crack propagation usually occurs under continuous loads for amorphous hydrogel, leading to the failure of hydrogel interface. This work presents a universal strategy to passivate the interfacial cracks by designing a hydrogen bonds-pinned entanglement (Hb-En) structure of amorphous hydrogel on engineering elastomers. The unique Hb-En structure is created by pinning well-tailored entanglements via covalent-like hydrogen bonds, which can amplify the delocalization of interfacial stress concentration and elevate the necessary fracture energy barrier within hydrogel interface. Therefore, the interfacial crack propagation can be suppressed under single and cyclic loads, resulting in a high interfacial toughness over 1650 J m-2 and an excellent interfacial fatigue threshold of 423 J m-2. Such a strategy universally works on blunting the interfacial crack between hydrogel coating and various elastomer materials with arbitrary shapes. The superb fatigue-crack insensitivity at the interface allows for durable aqueous lubrication of hydrogel coating with low friction.

Improving fatigue property of Zr–Ni–Al metallic glass by tailoring chemical composition with high structure stability

Improving the tolerance to fatigue damage for bulk metallic glass (BMG) is of vital importance for its industrial applications during long-term mechanical service. In this work, the composition dependence of structure stability against thermal and mechanical stimuli are studied with Zr–Ni–Al BMGs from the perspective of atomic topological packing and chemical affinity. Large atomic size disparity and strong interactions between component elements facilitate the stability of glass structure, which impart BMGs with relatively higher glass transition temperature and yield strength. Fatigue failure of BMG initiates from the accumulation of inelastic shear transformation events on the sample surface under cyclic loading. The fatigue endurance limit of Zr–Ni–Al BMG is effectively improved by tailoring the chemical composition with increased structure stability, which is characterized by high mismatch entropy and mixing enthalpy. Aluminum plays a role in enhancing the fatigue tolerance of Zr–Ni–Al BMGs, which presumably arises from the covalent-like bond character of aluminum that promotes the formation of rigid atomic packing motifs with great resistance to shear rearrangement, and consequently mitigates the fatigue crack initiation process.

Effects of transition metal segregation on the thermodynamic stability and strength of Ni Σ11 [110](113) symmetrical tilt grain boundary

Precisely controlling grain boundary (GB) thermodynamic stability and strength is important for processing nanograined alloys. The segregation-induced stabilization and strengthening GB mechanisms of 20 transition metal elements in Ni Σ11 [110](113) STGB with low interfacial excess energy were clarified from first-principles total-energy calculations in this work. The results indicate that the segregation energy and GB energy are inversely related to the radius difference between TM and matrix Ni atoms. The segregation of 11 elements were identified to enable the Ni Σ11 [110](113) STGB to be stable according to Schuh criterion for GB stability. The electronic mechanism of GB stabilization through TM segregation is mainly due to the less anti-bonding states of Ni-X pairs than those of Ni–Ni pairs in unsegregated GB, from the analysis of crystal orbital Hamiltonian populations. The GB embrittlement resulted from TM segregation is mainly from the mechanical contribution caused by atomic size mismatch between segregation atom and matrix atom. The segregation caused GB strengthening comes from the chemical contribution, which is due to the fact that the segregation of X leads to the formation of stronger Ni-X covalent-like bonds instead of Ni–Ni metallic bonds. This work could provide better understanding on designing high-stable Ni nanograined alloys.

Fully biofriendly, biodegradable and recyclable hydrogels based on covalent-like hydrogen bond engineering towards multimodal transient electronics

Biodegradable or recyclable transient electronics originated from natural polymers are considered as significant solution for electronic waste (e-waste) management, sustainable development, and emerging implantable devices. Yet, they are mostly struggling from mismatched mechanical features with human tissue and inferior environmental adaptability. In this work, based on the novel covalent-like hydrogen bond engineering, we developed a syrup-modified gelatin/Ag-hydrogel, which is entirely derived from natural or safe constituents and highly stretchable, yet fully biodegrades or recyclable. Meanwhile, strong covalent-like hydrogen bonds endow it with several favorable features of adhesion, rapid self-healing and antifreeze. A series of multi-modal application scenarios are further demonstrated to verify the adaptability of this platform that integrates various attributes. Overall, this work provides the feasibility of modified natural polymers to replace synthetic polymers, and provides a general toolbox for the construction of functional units for “green electronics” and the expansion of application models.

A Molybdenum Polysulfide In-Situ Generated from Ammonium Tetrathiomolybdate for High-Capacity and High-Power Rechargeable Magnesium Battery Cathodes.

Rechargeable magnesium batteries (RMBs) are a promising large-scale energy-storage technology with low cost and high reliability. However, developing high-performance cathode materials remains the most prominent obstacle because of the insufficient magnesium-storage active sites and unfavorable magnesium cation transport paths, as well as the strong interaction between the cathode material and the bivalent magnesium cation. Herein, ammonium tetrathiomolybdate is demonstrated to be a high-performance RMB cathode material. Ammonium tetrathiomolybdate exhibits a high capacity of 333 mAh g-1 at 50 mA g-1 and a good rate performance of 129 mAh g-1 at 5.0 A g-1 (∼15 C). An amorphous structure with plenty of efficient magnesium-storage active sites and open magnesium transport paths is in situ formed during the first cycle via ammonium extraction. The covalent-like bond between the molybdenum and sulfur delocalizes the negative charge, weakening the interaction with the bivalent magnesium cation and accelerating the kinetics. The covalent-like molybdenum-sulfur bond also promotes the simultaneous redox of molybdenum and sulfur, leading to a high specific capacity. The present work introduces a high-capacity and high-power RMB cathode material, elucidates the origin of the high performance, and provides insights for the design and optimization of RMB cathode materials.

The effect of solute segregation on stability and strength of Cu symmetrical tilt grain boundaries from the first-principles study

Grain boundary (GB) stability and strength are two essential points for nanocrystalline materials. In this study, the effects and underlying mechanisms of segregation-induced GB stabilization and GB strengthening in various Cu GBs for selected 8 alloying elements (Mg, Ca, Cr, Ni, Zn, Zr, Ag and Sn) were investigated through first-principles total energy calculations. The segregation energy results show that all other elements excluding Ni can segregate to GB thus improving the GB stability. The segregation driving force is associated with the volume of Voronoi cell of solute, in the trend that increases along with the volume of Voronoi cell growth. The stabilizing mechanism should attribute to the decrease in the number of antibonding electrons. As to GB strength, the embrittling potency results show that Mg, Cr, Zr and Ag are enhancers in Σ5 [001](310) GB, while for Σ5 [001](210) and Σ11 [110](113) GBs, only Cr and Zr are enhancers. The embrittling potency is concerned with segregation energy and the volume of Voronoi cell of solute, in a trend that increases with the segregation energy decline and the volume of Voronoi cell growth. The enhancing mechanism lies in the formation of covalent-like bonds between solute and matrix atoms. Furthermore, by comparing the stabilizing and strengthening effectiveness of alloying elements in various Cu GBs. The trend that the worse the stability and strength of GBs, the better the stabilizing and strengthening effects of alloying elements were revealed.

Diatomic Pd-Cu Metal-Phosphorus Sites for Complete N≡N Bond Formation in Photoelectrochemical Nitrate Reduction.

The synergistic effect of bimetallic heterogeneous catalysis in the reaction of nitrate reduction to nitrogen has been widely discussed, but it is still not clear how this effect works at the atomic scale, hindering the rational design of high-performance catalysts. Here, for the first time, 2D phosphorene was used as a giant P ligand to confine high-density PdCu dual-atom to form a unique PdCuP4 coordination structure, and this catalyst achieves 96.3 % NO3 - removal rate and 95.2 % N2 selectivity. In situ characterization combined with density functional theory (DFT) calculations show that the PdCu dual-atom form covalent-like bonds with adjacent P atoms, reducing the adsorption energy of the reactants. The synergistic effect of PdCu dual-atom promotes the breaking of N-O bond, and the short bond length of ≈3 Å between PdCu atoms accelerates the transfer of NO2 - , and eventually the two Pd-N adjacent to the surface of Pd rapidly combine to form N2 .

Dynamic crossover in metallic glass melt detected by NMR

The structure and dynamics of melts have critical influence on the glass formation and solidification of crystals, which attracts broad interest to clarify the underlying mechanisms. In this paper, the quadrupolar spin-lattice relaxation rates of 63Cu and 65Cu nuclei in a Au50Cu25.5Ag7.5Si17 metallic glass melt are studied by using a home-designed high-precision high-temperature NMR equipment. The relaxation rates follow the Arrhenius behavior at high temperatures above 870 K, but become non-Arrhenius at low temperatures, which indicates a non-Arrhenius dynamical crossover. The crossover temperature TA is about 2.3 times of the glass-transition temperature (Tg=378 K). Based on the analysis of Knight shift and magnetic relaxation rates, Cu and Si atoms are found to tend to form stronger covalent-like bonds below TA. Such an increase in bonding strength may promote the connectivity of energetically preferred short-range clusters, which is responsible for the observed sluggish dynamics.

Understanding the unusual-caged dynamics from the microstructure and interatomic interactions in binary metallic glass-forming liquids

The caged dynamics appears at time shorter than the β-relaxation but longer than the boson peak. Due to the limitations of experimental conditions, the mechanism of caged dynamics is not well studied in metallic glasses. We study a model La65Al35 glass-forming liquid with strong atomic interactions that presents an unusual caged dynamics, and the La65Ni35 GF liquid is also studied to clarify the underlying mechanisms of caged dynamics. There is a sudden drop near 0.1 ps in the partial non-Gaussian parameter of Al atoms, reflecting the development of caging of the Al atoms with time. A similar feature is also found in Ni80P20, Sm80Al20, Pd80Si20, and Y80Mg20 metallic GF liquids. The chemical order, Voronoi polyhedra, and covalent-like bonds are all related to the cage-breaking behavior of the central atoms, in turn regulating the unusual caged dynamics. These findings shed light on the complex relaxation dynamics of GF liquids.

First-principles computational tensile test of γ-Fe grain boundaries considering the effect of magnetism: Electronic origin of grain boundary embrittlement due to Zn segregation

The development of high-strength steels requires detailed understanding of the effect of solute elements on \ensuremath{\gamma}-Fe grain boundaries (GBs). In this study, first-principles computational tensile tests (FPCTT) were conducted on \ensuremath{\gamma}-Fe GBs to elucidate the mechanism of GB embrittlement due to Zn segregation. The paramagnetic \ensuremath{\gamma}-Fe GB was simulated by the $\mathrm{\ensuremath{\Sigma}}5\phantom{\rule{0.28em}{0ex}}(310)$ GB in the antiferromagnetic double-layer (AFMD) configuration. The FPCTTs revealed that the fracture stress and fracture energy of the \ensuremath{\gamma}-Fe GB were reduced by Zn segregation, which is consistent with experimental results. Crystal orbital Hamilton population analysis was also performed to investigate the change in electronic states during the tensile process, and the enhancement of GB fracture by Zn segregation is caused by the breaking of the covalent-like bonds between Fe and Zn at a relatively small strain compared to the Fe--Fe bonds. This behavior is attributed to the localized nature of the $3d$ orbitals of Zn in \ensuremath{\gamma}-Fe. The FPCTTs of \ensuremath{\gamma}-Fe GBs using the AFMD properly consider the effect of magnetism in paramagnetic \ensuremath{\gamma}-Fe under tensile strain and is useful for investigating the effects of various solute elements on GB fracture and for the development of high-strength steels.

A versatile hydrogel network-repairing strategy achieved by the covalent-like hydrogen bond interaction.

Hydrogen bond engineering is widely exploited to impart stretchability, toughness, and self-healing capability to hydrogels. However, the enhancement effect of conventional hydrogen bonds is severely limited by their weak interaction strength. In nature, some organisms tolerate extreme conditions due to the strong hydrogen bond interactions induced by trehalose. Here, we report a trehalose network–repairing strategy achieved by the covalent-like hydrogen bonding interactions to improve the hydrogels’ mechanical properties while simultaneously enabling them to tolerate extreme environmental conditions and retain synthetic simplicity, which proves to be useful for various kinds of hydrogels. The mechanical properties of trehalose-modified hydrogels including strength, stretchability, and fracture toughness are substantially enhanced under a wide range of temperatures. After dehydration, the modified hydrogels maintain their hyperelasticity and functions, while the unmodified hydrogels collapse. This strategy provides a versatile methodology for synthesizing extremotolerant, highly stretchable, and tough hydrogels, which expand their potential applications to various conditions.

The occupancy preferences of transition metal elements and its effects on ductility and brittleness of γ/β phases in TiAl alloys: a first-principles investigation

ABSTRACT The occupancy tendency of M elements in γ/β-TiAl and the bonding strength, electronic structure, and tough-brittle transition of γ/β-TiAl-M were systematically investigated by first-principles calculations. According to the point defect anti-site-formation energy, charge density difference and electron localization function simulations, in the fourth and sixth cycles, M = Fe, Co, Os, Ir, Pt and Au (Ru in β-TiAl) elements preferentially occupy Al sites and lead to the formation of strong covalent-like directional bonds between themselves and surrounding Ti or Al elements. The electronic structure and chemical bonding features are analyzed. γ/β-TiAl-M compounds consist of covalent and metallic bonds, and M elements in the intermediate region give rise to strong covalent bonding features. In addition, transition metal elements that form metallic bonds with Ti or Al will contribute to the ductility of TiAl-based alloys. The diagram of B/G reflects this very well.

Structure of monolayer 2H−TaS2 on Au(111)

We determined the structure of epitaxial $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moir\'e superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the $\mathrm{Ta}{\mathrm{S}}_{2}$ layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.

Co-alloying effects on the site preference and the elastic properties of γ′-Ni3Al from first-principles calculations

The influences of co-alloying atom pairs Ta–X (X: Ta, Mo, W, Cr, Co, and Re) on the site preference and elastic properties of γ′-Ni3Al precipitate phase were studied using first-principles calculations, and the underlying mechanism for the strengthening effect is determined. It is found that the site preferences of Ta–X co-alloying atom pairs are in the sequence of Al–Al, Al–Ni, Ni–Al, and Ni–Ni sites except for Ta–Co. The atom pair Ta–Co does not display a strong site preference tendency. The elastic properties for the Al–Al configuration were examined, and the results were compared with those of single-alloying to investigate the co-alloying effect. It is shown that the elastic modulus and hardness are obviously improved by co-alloying. Moreover, the effect of co-alloying is stronger than that of single-alloying. The further analyses by the charge density difference and the electronic density of states reveal that the strong effect of the co-alloying is attributed to the enrichment of the charge density between X and its nearest-neighbor Ni, as well as the orbital hybridization between Ta/X-d and Ni-d which acts as covalentlike bonds. Our results can provide an in-depth insight into the physics in the strengthening effect of co-alloying elements on the mechanical properties of single crystal superalloys.

Not All Fluorination Is the Same: Unique Effects of Fluorine Functionalization of Ethylene Carbonate for Tuning Solid-Electrolyte Interphase in Li Metal Batteries.

Li metal batteries (LMBs) are crucial for electrifying transportation and aviation. Engineering electrolytes to form desired solid-electrolyte interphase (SEI) is one of the most promising approaches to enable stable long-lasting LMBs. Among the liquid electrolytes explored, fluoroethylene carbonate (FEC) has seen great success in leading to desirable SEI properties for enabling stable cycling of LMBs. Given the many facets to desirable SEI properties, numerous descriptors and mechanisms have been proposed. To build a detailed mechanistic understanding, we analyze varying degrees of fluorination of the same prototype molecule, chosen to be ethylene carbonate (EC) to tease out the interfacial reactivity at the Li metal/electrolyte. Using density functional theory (DFT) calculations, we study the effect of mono-, di-, tri-, and tetra-fluorine substitutions of EC on its reactivity with Li surface facets in the presence and absence of Li salt. We find that the formation of LiF at the early stage of SEI formation, posited as a desirable SEI component, depends on the F-abstraction mechanism rather than the number of fluorine substitution. The best illustrations of this are cis- and trans-difluoro ECs, where F-abstraction is spontaneous with the trans case, while the cis case needs to overcome a nonzero energy barrier. Using a Pearson correlation map, we find that the extent of initial chemical decomposition quantified by the associated reaction free energy is linearly correlated with the charge transferred from the Li surface and the number of covalent-like bonds formed at the surface. The effect of salt and the surface facet have a much weaker role in determining the decompositions at the immediate electrolyte/electrode interfaces. Putting all of this together, we find that tetra-FEC could act as a high-performing SEI modifier as it leads to a more homogeneous, denser LiF-containing SEI. Using this methodology, future investigations will explore -CF3 functionalization and other backbone molecules (linear carbonates).

First-principles study of mechanical and thermodynamic properties of intermetallic Pt3M (M = Al, Hf, Zr, Co, Y, Sc)

Pt-based alloys are a potential high-temperature structural material. The formation of second-phase intermetallic Pt3M (M = Al, Hf, Zr, Co, Y, Sc) can further stabilize the high-temperature mechanical properties of Pt-based alloys. Here, we systematically studied the equilibrium geometry, elastic constants, electronic characteristics, Debye temperature and elastic wave velocity of intermetallic Pt3M using first-principles density function theory calculations. Elastic properties are estimated using the Voigt-Reuss-Hill approximation. Pt3Hf has the highest moduli and hardness, while Pt3Y has the lowest values. Pt3Co exhibits the strongest elastic anisotropy, while Pt3Zr is the weakest. Pt3Zr and Pt3Y have the highest and lowest Debye temperature. The metallicity and conductivity of these alloys mainly arise from d-state electronic contribution according to the electronic density of states. The charge density difference analysis shows that electrons flow from Pt to M atoms and accumulate into covalent-like bonds, which results in structural stability and mechanical anisotropy.

Atomic level bonding mechanism in steel/aluminum joints produced by cold pressure welding

Cold pressure welding of aluminum alloys and steels offers an attractive, cost-effective opportunity of joining the two most important structural materials. Aim of this work is to investigate the bonding mechanism between aluminum alloy EN AW6082 and carbon steel C15, joint by cold extrusion welding after targeted heat treatment. Mechanical testing under tensile loads revealed site specific interface strengths between 30 and 60% of the constituent materials. However, investigation of fracture surfaces indicates that interface strengths can exceed the AW6082 strength in areas where the bonding process substantially enlarged the Fe-Al interface region, due to the welding geometry. Near atomic scale investigations of the interface using aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy disclose two different regions along the cold pressure welded interface. The majority of the interfacial area is oxygen free with Fe-based and Al-based crystals joining. No intermetallic phase was identified. Adhesion is attributed to a more covalent bond characteristic at the Fe-Al interface evidenced in the Al-L2,3 edge. A small fraction of the steel/aluminum interface was covered with oxidic regions of about 10 nm thickness. This oxide is predominantly an amorphous aluminum oxide, with only few Fe-O bonds. The observed metallurgical bond formation is explained by an extended Bay model through (i) cracking of the brittle, nanoscopic native oxide layer on the materials during forming, (ii) formation of metallic contact between Al and Fe, under reduction of iron oxides by aluminum, and (iii) formation of covalent-like Al-Fe bonds across the interface.

Bridging the local configurations and crystalline counterparts of bulk metallic glass by nanocalorimetry

The structural understanding of crystallization in bulk metallic glasses (BMGs) has attracted much attention while rapid crystallization occurring under controllable conditions is less involved. In this study, a Ce68Al10Cu20Co2 (at.%) BMG was thermally devitrified by differential scanning calorimetry (DSC) and nanocalorimetry. At a heating rate of 10K/min by DSC, AlCe3 and Ce are the major crystalline phases after devitrification while Al13Co4 quasicrystals and Ce are the dominant phases in the crystallization products at a heating rate of 5000K/s by nanocalorimetry. Attributing to the covalent-like bond in Al–Co atom pairs, Al13Co4 quasicrystals precipitate in the primary crystallization and work as the precursors associating local atomic configurations in the glassy state with crystalline phases after crystallization. Attributing to the enhanced mobility of Cu atoms, compositional redistribution occurs in the as-cast sample. On nanocalorimetry heating, an unambiguous discrepancy in the nucleation and growth of the nano-sized Al13Co4 quasicrystals is thus triggered, contributing to an obvious difference in the crystal size. This research unveils the distinct crystallization behaviors of Ce-based BMG on rapid heating. The formation of quasicrystals demonstrates the multi-stage crystallization on rapid heating and bridges the structural gap between local atomic configurations of metallic glasses and crystalline phases.

Sc-Centered Octahedron Enables High-Speed Phase Change Memory with Improved Data Retention and Reduced Power Consumption.

Phase change memory (PCM) with advantages of high operation speed, multilevel storage capability, spiking-time-dependent plasticity, etc., has wide application scenarios in both Von Neumann systems and neuromorphic systems. In the automotive application, intelligent system not only needs high efficiency to handle massive data processing but also good robustness to retain the existing data against high working temperature. In this work, Sc-doped GeTe is developed for PCM, which has achieved 120 °C data retention for 10 years, 6 ns operation speed, and 7 nJ low power consumption. The high data retention is attributed to the high coordination number of Sc and its strong bonds with Te atoms in the amorphous phase, which enhances the robustness of the atomic matrices. Sc-centered octahedrons in amorphous state provide a nucleation center, leading to fast crystallization. In the crystalline phase, Sc atoms occupy Ge vacancies to form a homogenous GeTe-like rhombohedral phase. The strong covalent-like Sc-Te bonds weaken the neighboring Ge-Te bonds, lowering energy for melting. Together with the increased energy efficiency originated from confined grain size, the reduced power consumption has been achieved. The improvements in data retention, speed, and power efficiency have made Sc-doped GeTe a promising candidate for high-performance automobile electronics application.

Polyamorphic transition in a transition metal based metallic glass under high pressure

Pressure-induced glass-glass transition (GGT) has been reported in a few rare-earth based and main-group metallic glasses (MGs), a process during which the compressibility decreases with the transformation from a low-density to a high-density amorphous state. Herein, we report an unexpected GGT behavior in Pd-based MGs under pressure, which is characterized by an anomalous increase in the compressibility with the pressure. State-of-the-art high-energy synchrotron technique, coupled with theoretical simulations, reveals that this unique GGT behavior is primarily caused by the change in bonding characteristics, i.e., some covalentlike atomic bonds in the as-prepared state change to metallic ones under high pressure, which also leads to the increase in the compressibility. Our current findings shed light on the understanding of the nature of polyamorphic transitions in MGs.