Disintegration Of Clusters Research Articles

Suppressing Metal Leaching and Sintering in Hydroformylation Reaction by Modulating the Coordination of Rh Single Atoms with Reactants.

Hydroformylation reaction is one of the largest homogeneously catalyzed industrial processes yet suffers from difficulty and high cost in catalyst separation and recovery. Heterogeneous single-atom catalysts (SACs), on the other hand, have emerged as a promising alternative due to their high initial activity and reasonable regioselectivity. Nevertheless, the stability of SACs against metal aggregation and leaching during the reaction has rarely been addressed. Herein, we elucidate the mechanism of Rh aggregation and leaching by investigating the structural evolution of Rh1@silicalite-1 SAC in response to different adsorbates (CO, H2, alkene, and aldehydes) by using diffuse reflectance infrared Fourier transform spectroscopy, X-ray adsorption fine structure, and scanning transmission electron microscopy techniques and kinetic studies. It is discovered that the aggregation and leaching of Rh are induced by the strong adsorption of CO and aldehydes on Rh, as well as the reduction of Rh3+ by CO/H2 which weakens the binding of Rh with support. In contrast, alkene effectively counteracts this effect by the competitive adsorption on Rh atoms with CO/aldehyde, and the disintegration of Rh clusters. Based on these results, we propose a strategy to conduct the reaction under conditions of high alkene concentration, which proves to be able to stabilize Rh single atom against aggregation and/or leaching for more than 100 h time-on-stream.

Effect of Humidity on the Mobilities of Small Ions in Ion Mobility Spectrometry.

Ions in the ion mobility spectrometry (IMS) are mostly hydrated. A single peak in the drift time spectrum is usually generated by a mixture of ions differing in the number of attached water molecules. Under real IMS detector operating conditions, ions change their composition during movement in the drift region due to the changes in the number of water molecules attached to the ion. The impact of water vapor on the drift times of small ions at different temperatures was studied experimentally using an ion mobility spectrometer. The experiments were carried out for hydronium, ammonium, oxygen, chloride, bromide, and iodide ions. A theoretical model was developed, allowing us to calculate the effective mobility of ions for a given concentration of water vapor and temperature. The basic assumption adopted in this model was the linear dependence of the effective mobility coefficient on the mobility of ions with a certain degree of hydration. The weighting factors in this relationship are the abundances of individual types of ions. These parameters were determined by calculations based on the thermodynamics of the formation and disintegration of ionic clusters. From the known values of temperature, pressure, and humidity, the values of effective mobilities can be predicted quite accurately. The dependencies of reduced mobilities on the average degree of hydration were also determined. For these dependencies, the measurement points on the graphs are gathered along specific lines. This means that the average degree of hydration unambiguously determines the value of reduced mobility for a given type of ions.

Spatial mapping of low pressure cluster jets using Rayleigh scattering

In this work, we report evolution of atomic clusters in a highly under-expanded supersonic jet of Argon. A high resolution and sensitive Rayleigh scattering based experimental set-up is designed to overcome the limitations encountered in conventional set-ups. Further, the measurement range could be extended from a few nozzle diameters to 50 nozzle diameters. Simultaneously, we had been able to generate 2D profiles of the distribution of clusters inside the jet. This paves the way to track the growth of clusters along the flow direction experimentally, which until now was limited to few nozzle diameters. The results show that spatial distribution of clusters inside the supersonic core deviates considerably from the prediction of the free expansion model. We exploit this to estimate cluster growth along the expansion direction. Further, it is observed that the growth of the clusters gets saturated after a certain distance from the nozzle. At the jet boundary, we see substantial cluster strengthening immediately upstream of barrel shock while the normal shock exhibits disintegration of clusters. These observations are noticed for the first time, which, we believe will further the understanding of cluster dynamics in a supersonic jet.

Molecular dynamics study of nano-indentation deformation behavior of Al/Al90Sm10 nanolaminate.

Molecular dynamics-based investigation has been carried out to simulate the nano-indentation loading in crystalline Al-amorphous Al90Sm10 metallic glass (MG) with an aim to investigate the effect orientation of crystalline-amorphous (C/A) interface orientation on the nano-indentation behavior of the C/A Al-Al90Sm10 nanolaminate for varying indenter speeds. Post-analysis techniques like adaptive-common neighbor analysis (a-CNA), atomic strain, dislocation extraction algorithm (DXA), and Voronoi polyhedral analysis (VP) have been employed to capture the structural evolution during simulated nano-indentation loading. C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter exhibits the presence of elastic regime followed by plastic curve, whereas load versus depth curve behaves plastically since the beginning in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter. The dislocation density growth is slower in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter attributed to the sinking of dislocations into MG counterpart of the nanolaminate, thereby triggering shear transformation zone activation. Whereas, the dislocation generation is delayed in case of C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter by virtue of amorphous Al90Sm10 MG coating on crystalline Al but is extensive and rapid. The disintegration of ICO-like structures and mixed clusters and growth of crystal-like clusters is discernible in C/A Al-Al90Sm10 nanolaminate with C/A interface orientated perpendicular to the indenter. On the other hand, the VP population exhibits cyclic variation in C/A Al-Al90Sm10 nanolaminate with C/A interface orientated parallel to the indenter. A transformation pathway of VPs has been mapped out for C/A Al-Al90Sm10 nanolaminate under nano-indentation loading. The simulations have been carried out by employing Molecular Dynamics using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) platform. Post-analysis techniques like adaptive-common neighbor analysis (a-CNA), atomic strain, dislocation extraction algorithm (DXA), and Voronoi polyhedral analysis (VP) have been employed to capture the structural evolution during simulated nano-indentation loading.

Activating Mobile Dislocation in Boron Carbide at Room Temperature via Al Doping.

Dislocation glide, deformation twinning, and phase transition are critical mechanisms resulting in irreversible plastic deformations of materials. Because of the lack of dislocation movement, superhard ceramics generally exhibit brittle failure at room temperature. Here, by employing molecular dynamics simulations using a machine-learning force field, we reveal several plastic deformation mechanisms in superhard boron carbide as a small amount of aluminum (Al) is doped. Under shear deformation, dislocation nucleation and glide occur in Al-doped boron carbide (B_{12}-CAlC) due to the breakage of weakened chain bonds rather than the disintegration of icosahedral clusters. The dislocation activities then cause twin boundaries to migrate, thereby mitigating amorphization and enhancing ductility. Furthermore, the mobile dislocation with the Burgers vector of b=⟨11[over ¯]0⟩{111} is observed in the tensile nanopillar, which is well consistent with the experiment. This Letter demonstrates that mobile dislocation could be activated in superstrong covalent materials through a simple doping strategy.

Adsorption Mechanism of Benzene Alkylation System Catalyzed by an Acid Catalyst: Effect of Pressure.

Alkylbenzene is an important chemical intermediate, and its industrial production is mainly through the alkylation reaction of benzene and olefin under the action of an acid catalyst. In this article, the adsorption mechanism of benzene/propylene binary in HY zeolite was first revealed by Monte Carlo molecular simulation. It was found that the adsorption mechanism of benzene and propylene changes at the adsorbate loading of 36 molecules/UC, and benzene plays a dominant role. Below this loading, the adsorption sites of benzene and propylene mainly occupy the "ideal" adsorption sites, and benzene has a "first-hand advantage" toward those sites. Above 36 molecules/UC, benzene and propylene coform "molecular clusters" in the supercage of HY, resulting in the enhanced localization of adsorbates, suggesting that at low and intermediate adsorbate loadings the adsorption property is expected to effectively improve by introducing more superior adsorption sites. However, above 36 molecules/UC, the interaction between adsorbents in the clusters becomes dominant. At this point, it is critical to change the cage-like structure of the micropore and introduce more mesopore to facilitate the disintegration of adsorbates clusters and reduce the steric resistance so that the advantage of adsorption sites can be brought into play. These results lay the foundation for optimizing the operating conditions of alkylation reactions and can be further used to explain the effect of loading on similar adsorption separation or catalytic systems, such as catalytic cracking.

Electric-field induced modulation of amorphous protein aggregates: polarization, deformation, and reorientation

Proteins in their native state are only marginally stable and tend to aggregate. However, protein misfolding and condensation are often associated with undesired processes, such as pathogenesis, or unwanted properties, such as reduced biological activity, immunogenicity, or uncontrolled materials properties. Therefore, controlling protein aggregation is very important, but still a major challenge in various fields, including medicine, pharmacology, food processing, and materials science. Here, flexible, amorphous, micron-sized protein aggregates composed of lysozyme molecules reduced by dithiothreitol are used as a model system. The preformed amorphous protein aggregates are exposed to a weak alternating current electric field. Their field response is followed in situ by time-resolved polarized optical microscopy, revealing field-induced deformation, reorientation and enhanced polarization as well as the disintegration of large clusters of aggregates. Small-angle dynamic light scattering was applied to probe the collective microscopic dynamics of amorphous aggregate suspensions. Field-enhanced local oscillations of the intensity auto-correlation function are observed and related to two distinguishable elastic moduli. Our results validate the prospects of electric fields for controlling protein aggregation processes.

Flow behavior of cementitious-like suspension with nano-Fe3O4 particles under external magnetic field

The flow behavior of cementitious-like (limestone powder) suspension containing nano-Fe3O4 particles at constant shear rate of 10 s−1, characterized by the evolution of apparent viscosity over time, is investigated under various magnetic fields. Results show that the limestone powder suspension at flow-state exhibits remarkable magneto-rheological responses, reflected by a significant increase in the apparent viscosity after applying an external magnetic field. A higher field strength corresponds to a more rapid and pronounced response. The apparent viscosity experiences a sudden alteration with the stepwise change of the magnetic field due to the formation or disintegration of magnetic clusters. Linearly increasing magnetic field strength at low ranges (e.g. 0 T–0.3 T) shows less influences on the evolution of apparent viscosity, while at relatively high magnetic field, the apparent viscosity gradually increases with the magnetic field strength and the increase rate is comparable to that obtained under constant high magnetic field of 0.75 T. When the magnetic field is removed, the apparent viscosity exhibits a sharp reduction. If the magnetic field strength linearly decreases to zero, however, the apparent viscosity continuously increases until reaching a peak and then gradually decreases. This research shows in different ways how a desired apparent viscosity level of a cementitious-like suspension can be reached by means of an external magnetic field.

Mitigating amorphization in superhard boron carbide by microalloying-induced stacking fault formation

The abnormal brittle failure of superhard boron carbide $({\mathrm{B}}_{4}\mathrm{C})$ and other icosahedral solids arises from the shear-induced amorphization. Mitigating the amorphization in these materials remains challenging due to the lack of other deformation mechanisms such as mobile dislocations. This paper illustrates the shear-induced amorphization process of ${\mathrm{B}}_{4}\mathrm{C}$ from molecular dynamics (MD) simulations using quantum-mechanics-derived machine-learning force field. The amorphization in ${\mathrm{B}}_{4}\mathrm{C}$ initiates from the disintegration of icosahedral clusters, and then this icosahedral deconstruction propagates and merges to form an amorphous region with 2--3 nm in width, leading to the following cavitation and brittle failure. More interesting, the deformation mechanism transforms from amorphization to stacking fault (SF) formation by microalloying aluminum (Al) into ${\mathrm{B}}_{4}\mathrm{C}$. This SF formation originates from the enhanced icosahedral slip as the Al is incorporated into the C-B-C chain to form a C-Al-C chain. This paper illustrates a deformation mechanism of superhard icosahedral solids and provides a strategy for suppressing the amorphization and brittle failure of ${\mathrm{B}}_{4}\mathrm{C}$.

Kinematic Viscosity ofMulticomponent FeCuNbSiB-BasedMelts.

The work investigated the temperature dependences of the kinematic viscosity for multicomponent melts of nanocrystalline soft magnetic alloys. It is shown that there is a linear relationship between the reduced activation energy of viscous flow Ea·(RT)−1 and the pre-exponential factor ν0. This ratio is universal for all quantities, the temperature dependence of which is expressed by the Arrhenius equation. It is shown that the activation energy of a viscous flow is linearly related to the cluster size on a natural logarithmic scale, and the melt viscosity increases with decreasing cluster size. The change in the Arrhenius plot in the anomalous zone on the temperature dependence of viscosity can be interpreted as a liquid–liquid structure transition, which begins with the disintegration of clusters and ends with the formation of a new cluster structure.

Strengthening boron carbide through lithium dopant

AbstractThe high strength of boron carbide (B4C) is essential in its engineering applications such as wear‐resistance and body armors. Here, by employing density functional theory simulations, we demonstrated that the strength of B4C can be enhanced by doping lithium to boron‐rich boron carbide (B13C2) to form r‐LiB13C2. The bonding analysis on r‐LiB13C2 indicates that the electron counting rule (or Wade's rule) is satisfied in r‐LiB13C2 whose formula can be written as r‐Li+(B12)2‐(CB+C). The shear deformation on r‐LiB13C2 indicates that its ideal shear strength is larger than that of B4C because of the existing of Li dopant. The failure process of r‐LiB13C2 under ideal shear deformation initiates from breaking the icosahedral‐icosahedral B‐B bonds. Then these B atoms react with the middle B in the C‐B‐C chain, resulting in the disintegration of icosahedral clusters and brittle failure. More interesting, the nanotwinned r‐LiB13C2 is even stronger than r‐LiB13C2 because of the directional nature of covalent bonding at the twin boundaries. This suggests that the nanotwinned r‐LiB13C2 has a significant enhanced strength compared to B4C. Our simulation results illustrate the deformation mechanism of Li‐doped boron carbide and its nanotwinned microstructure. We proposed to improve the strength of boron carbide by doping Li into B13C2 and increasing its twin densities.

Cholic Acid-Derived Amphiphile which Combats Gram-Positive Bacteria-Mediated Infections via Disintegration of Lipid Clusters.

Inappropriate and uncontrolled use of antibiotics results in the emergence of antibiotic resistance, thereby threatening the present clinical regimens to treat infectious diseases. Therefore, new antimicrobial agents that can prevent bacteria from developing drug resistance are urgently needed. Selective disruption of bacterial membranes is the most effective strategy for combating microbial infections as accumulation of genetic mutations will not allow for the emergence of drug resistance against these antimicrobials. In this work, we tested cholic acid (CA) derived amphiphiles tethered with different alkyl chains for their ability to combat Gram-positive bacterial infections. In-depth biophysical and biomolecular simulation studies suggested that the amphiphile with a hexyl chain (6) executes more effective interactions with Gram-positive bacterial membranes as compared to other hydrophobic counterparts. Amphiphile 6 is effective against multidrug resistant Gram-positive bacterial strains as well and does not allow the adherence of S. aureus on amphiphile 6 coated catheters implanted in mice. Further, treatment of wound infections with amphiphile 6 clears the bacterial infections. Therefore, the current study presents strategic guidelines in design and development of CA-derived membrane-targeting antimicrobials for Gram-positive bacterial infections.

The hidden disintegration of cluster heterogeneity in Fe-based glass-forming alloy melt

The evolution of liquid metal at high temperature is known much less than their solid states. This is partially due to that the message concerning clusters, metastable phase or heterogeneity in liquid is usually too slight to be traced. Here, we shed some light on the nature of structural evolution of Fe-based glass-forming alloy during overheating process by the investigation of high-temperature melt viscometry and first principles simulations. It was found that a structural transition around 1400 ℃ occurred in the melts of initial homogeneous ingot, heterogeneous ingot and amorphous ribbons jointly, and was confirmed by the results from differential scanning calorimeter (DSC), and ab initio molecular dynamics (AIMD). Combining these results with Fe-Si-B ternary phase diagram and the melting characteristics of Fe-B compounds, it is safe to conclude that the disintegration of Fe2B-type clusters to Fe3B-type clusters leads to the observed transition. These results offer a significant reference for the preparation and property control of Fe-based amorphous alloys.

Thixotropy Hysteresis and Structure Formation in Elastomeric Suspensions

A method for identifying normal and anomalous thixotropy hysteresis in elastomeric composites containing powdered reinforcement materials of different nature is proposed, and the explanation of existence of the two forms of hysteresis is provided. We assume that large superstructural elements—clusters consisting of a number of reinforcement particles, of colloidal particles, and of the matrix—may arise in such systems. The formation of clusters in the system gives rise to normal hysteresis, and hysteresis is anomalous in the absence of clusters. Instant thixotropy is observed when the rates of formation and disintegration of clusters are equal. The hysteresis changes its character in the inversion regions, which are the regions of critical shear stress that divides the stress–shear velocity field into low- and high-velocity domains.

Anomalies of dielectric properties and conductivity in single domain LiNbO3:Zn crystals

ABSTRACTA study of the temperature dependence of dielectric constant, conductivity, and piezoelectric modulus in the single-domain state of LiNbO3 crystals modified by Zn admixture at threshold concentration is reported. Unipolarity of the LiNbO3:Zn crystals is observed to increase after treatment of brand-new samples by high-temperature electro-diffusion annealing and by subsequent high-temperature annealing of short-circuited samples. The observed effects are explained as a result of meta-stable residual domains collapsing at high temperature the collapse being assisted by disintegration of charged clusters stabilizing domain walls. The rise of unipolarity is accompanied by anomalies on the σ(T) and ϵ(T) dependences and by a significant increase of the value of the piezo-modulus.

Ellipticity-dependent of multiple ionisation methyl iodide cluster using 532 nm nanosecond laser

ABSTRACTThe dependence of multiply charged ions on laser ellipticity in methyl iodide clusters with 532 nm nanosecond laser was measured using a time-of-flight mass spectrometer. The intensities of multiply charged ions Iq+(q = 2–4) with circularly polarised laser pulse were clearly higher than those with linearly polarised laser pulse but the intensity of single charged ions I+ was inverse. And the dependences of ions on the optical polarisation state were investigated and a flower petal and square distribution for single charged ions (I+, C+) and multiply charged ions (I2+, I3+, I4+, C2+) were observed, respectively. A theoretical calculation was also proposed to simulate the distributions of ions and theoretical results fitted well with the experimental ones. It indicated that the high multiphoton ionisation probability in the initial stage would result in the disintegration of big clusters into small ones and suppress the production of multiply charged ions.

Lack of Relationship Between Occupational Workload and Microscopic Alterations in Lumbar Intervertebral Disc Disease

Objectives : The study investigated the impact of occupational workloads on disc surgery specimens. We report the relationship between workload and histological features.Methods : Specimens were collected prospectively from patients suffering from lumbar disc prolapse (n=90) or spinal osteochondrosis (n=19). Histomorphology and occupational workload data and histomorphological features were evaluated. Occupational data were collected in a structured, standardized patient interview assessing lifting and carrying loads. In this way the exposure was assessed for each test subject's entire working life up to surgery.Results : There was no association between cumulative workload and histological patterns. In a subgroup of patients with a workload period of 12 months prior to surgery a relevant formation of chondrocyte clusters (p=0.055) was apparent. Chondrocyte cluster formation was found in 83% (n=74) of the prolapse patients and in 58% (n=11) of the osteochondrosis patients (p=0.02). Fibrocyte mediated scar formation was found in 55% of the prolapse patients and in 45% of the spinal stenosis patients. Chondrocyte clusters and their de novo collagen matrix did not integrate biomechanically sufficient with collagen fibers of the disc. Disintegration of clusters from disc matrix and formation of intra-discal sequesters were observed.Conclusion : Matrix degeneration was common but displayed no relationship to occupational workload or other histological features. Scar formation was observed in every second specimen. Regenerative chondrocyte cluster proliferation was a common feature in disc specimens and tended to be associated in patients with a workload one year before surgery.

Nucleation, aggregation, annealing, and disintegration of granular clusters.

The processes of nucleation, aggregation, annealing, and disintegration of clusters of non-Brownian paramagnetic beads in a vibrofluidized system are experimentally investigated. The interaction among the beads is induced by a magnetic seed composed of two dipoles allocated outside the container cell. We observe a clearly differentiated nucleation stage, whose evolution (nucleation time versus acceleration strength) follows a power law. Thereafter, the beads aggregate forming 2D disordered clusters around the nucleus. Both processes (nucleation and aggregation) are determined by the competition between magnetic forces and the drag produced by a thermal bath created by glass particles. Once the agglomerates reach a final state (shape and length), they are annealed by increasing and decreasing the granular temperature. We found that the fractal dimension and the lacunarity index clearly describe the structural variations of the clusters. Our discussion on this phenomenon is addressed, making a rough analogy with the glass transition in a super-cooled liquid. Finally, we study the disintegration of the clusters as a function of time and the density of the surrounding gas. The question is not if, but how they disintegrate upon removing the external field; we find that the disintegration follows an exponential decay.

Laboratory study of gravel‐bed cluster formation and disintegration

AbstractIncreased knowledge of clusters is essential for the understanding of sediment transport behavior and the monitoring and protection of aquatic life. A physical study using graded river gravels is conducted in a laboratory environment. Using photogrammetry and painted gravels, a cluster identification tool (CIT) is developed based on image subtraction between subsequent frames, allowing identification of any stable areas and groups of particles on the bed. This is combined with digital particle tracking (DPT) to present a novel approach for monitoring the formation and disintegration of clusters. Clusters from graded gravels are formed successfully during the experimental stage, allowing investigation into the complex dynamic behavior of cluster formation and disintegration in a simulated natural environment. Various anchor stone arrangements are used in the experiments. However, only about one fifth of the potential anchor stones on the bed surface enable cluster formation. In general, clusters classified as “typical” and “heap” are most common. Inspection of temporal cluster coverage of the test‐bed surface shows that the proportion of clusters present on the surface tends to grow with time. Maximum cluster surface coverage of between 5% and 34% is observed. In addition, particles entering and departing from clusters are monitored. Most commonly, particles enter from directly upstream of the cluster, however >20% of particles approach from a direction >20 deg from the streamwise direction. Approximately 35% of all particles directly upstream of a cluster bypass the cluster.

Disintegration of argon clusters in collisions with highly vibrationally excited SF6 molecules in crossed molecular and cluster beams

It has been found that collisions of highly vibrationally excited SF6 molecules (with the vibrational energy E vib ≥ 0.5–2.0 eV) with Ar N clusters (where N ≤ 30–40 is the number of atoms in a cluster) in crossed molecular and cluster beams result in capture of molecules followed by complete disintegration of the clusters. Possible applications of the effect for selective doping of clusters with molecules, laser separation of isotopes, and selective transport of molecules to the surface are discussed.