Size-dependent Transition Research Articles

Understanding water slippage through carbon nanotubes.

It is a formidable challenge to understand water slippage through carbon nanotubes (CNTs), despite its great significance in fundamental research and technology. Herein, we propose an effective scheme to describe water slippage properties by extending two friction models - the phononic friction model and Einstein's diffusion model, both relying on the potential corrugation of water slippage. Our scheme effectively captures the tube-size effect on the viscosity and slippage of water molecules through CNTs. It also identifies the experimentally reported size-dependent transition from continuum to sub-continuum flow and further reveals that this transition is likely to be determined by the hydrogen bond instead of the structural transition or entropic change. Besides, the size-dependence of slip lengths is found to be controllable by temperature. Our methods are thus expected to be a useful basis for further studies on substance transport under confinement.

Size-temperature equivalence in tensile deformation of metallic glass

Abstract Tensile fracture of irradiation-free metallic glass specimens with diameters ranging from 100 nm to 500 μm is investigated at different temperatures. A gradual change in fracture morphology from vein-pattern to completely smooth fracture surface to necking is observed with decreasing sample size and testing temperature. The size-temperature equivalence in the entire length scale can be described by considering the thermal effects in shear localization of metallic glasses. We construct an empirical model based on the shear band heating and velocity formulations to qualitatively describe the size and temperature effects on the fracture morphology. Our results suggest that the widely reported size-dependent transition from shear-localized to homogeneous flow in metallic glasses is fundamentally different from the high temperature homogeneous viscous flow. The plastic deformation in nanoscale samples is spatially localized in embryonic shear bands, which never mature to the propagation stage due to lack of heat content.

Grain Size Altering Yielding Mechanisms in Ultrafine Grained High-Mn Austenitic Steel: Advanced TEM Investigations

The underlying mechanism of discontinuous yielding behavior in an ultrafine-grained (UFG) 31Mn-3Al-3Si (wt.%) austenitic TWIP steel was investigated by the use of advanced TEM technique with taking the plastic deformation mechanisms and their correlation with grains size near the macroscopic yield point into account. The discontinuous yielding with noticeable yield drop has been reported in various ultrafine grain materials. However, typical yield drop mechanisms such as the dislocation locking by the Cottrell atmosphere due to the presence of interstitial impurities cannot explain the origin of this phenomenon in the UFG high-Mn austenitic TWIP steel. Here, we experimentally reveal by TEM that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, shows an obvious grain size dependence, more specifically, the main mechanisms shift from the conventional in-grain slip deformation to deformation by twinning nucleated from grain boundaries as the grain size decreases in the high-Mn steel.Planar dislocation substructure was initially generated from the grain interior in over-1μm grains, whereas stacking faults and deformation twins were directly nucleated at grain boundaries without the presence of dislocations pile-ups in under-1μm grains. This observation indicates that the grain size dependent deformation mechanism transition is also deeply associated with the discontinuous yielding behavior as it could govern the changes in the grain interior dislocation density around the macroscopic yield point.

Influence of size, shape and dimension on glass transition and Kauzmann temperature of silver (Ag) and tantalum (Ta) nanoparticles

A simple model is developed for the investigation of size, shape and dimension dependent glass transition temperature (Tg) and Kauzmann temperature (TK) of nanoparticles. The model is based on thermodynamical quantity cohesive energy and is free from fitting parameters and approximations. To check the validity of the model, calculations on the size, shape and dimension dependent glass transition (Tg) and Kauzmann temperature (TK) are performed for silver (Ag) and tantalum (Ta) nanoparticles (NPs) of different shapes. The considered shapes are spherical, tetrahedral, octahedral and icosahedral accompanied with zero-, one- and two-dimensional geometries. Our results reveal that the Tg and TK strongly depend on the size of the nanoparticles. As the size of the NPs decreases, Tg and TK decrease. It is observed that both temperatures follow the trend as (icosahedral, D) > (spherical, D) > (octahedral, D) > (tetrahedral, D) for selected Ag and Ta nanoparticles. However, in terms of dimension, they show the d = 0 < d = 1 < d = 2 trend. The calculated values of glass transition and Kauzmann temperatures for both considered nanoparticles have good agreement with available molecular dynamics (MD) simulation and experimental data.

On the nature of the structural transitions between anti-Mackay stacking, chiral stacking and their thermal stability in AgCu nanoalloys

AgCu nanoalloys are known to exhibit a size dependent transition to the chiral stacking from the anti-Mackay stacking. In the current work, the role of composition on the structural transitions between the anti-Mackay and the chiral stacking in AgCu nanoalloys has been studied computationally using a combination of global optimization searches and heating simulations. It is observed that in addition to the size, composition also affects the surface stacking in these nanoalloys. Further, results show that the nature of the recently reported thermally induced transition to the chiral stacking is strongly dependent on the Cu content in the AgCu nanoalloys. The amount of Cu in the nanoalloy affects both the transition temperature and the “sharpness” of the transition. By analyzing the frequencies of the vibrational modes calculated using the Harmonic Superposition Approximation (HSA), the thermal stability of the chiral stacking is attributed to the vibrational entropic contribution. Finally, the findings in the current work have been consolidated in the form of a size-composition map. By referring to this size-composition map, the preference for the chiral stacking can be understood for the nanoalloys belonging to the size window between the “magic” sized nanoalloys.

Role of core-shell energetics on anti-Mackay, chiral stacking in AgCu nanoalloys and thermally induced transition to chiral stacking

In AgCu nanoalloys a size-dependent transition to the chiral stacking from the anti-Mackay stacking has been predicted previously. This trend is explained by considering the interplay between the core-shell energetics. Results indicate that the energy changes in the Ag shell alone is not sufficient to explain the stability of the chiral stacking and the energy changes in the Cu core also need to be considered. In addition to this, thermally induced transition to chiral stacking was observed at sizes where anti-Mackay stacking is energetically favourable. On transition to the chiral stacking, the Ag-Ag, Ag-Cu and Cu-Cu bond lengths change significantly. These observations are also applicable for AgCu nanoalloys with incomplete Ag shells.

Size-dependent critical transition in the origin of light emission from core–shell Si–SiO2 nanoparticles

Here, it is established that the origin of the photoluminescence of Si–SiO2 core–shell nanoparticles is strongly size-dependent, with large nanoparticles emitting mostly from the core, whereas small nanoparticles emit predominantly from the shell.

Structural Evolution of Palladium Clusters Pd55--Pd147-: Transition to the Bulk.

We present a study of the structural evolution of palladium cluster anions in a size range from 55 to 147 atoms using a combination of trapped ion electron diffraction and density functional theory computations. We show that Pdn- clusters (n = 55, 65, 75, 85, 95, 105, and 147) change from an icosahedral motif at Pd55- to the bulk fcc motif at Pd147-. This size-dependent structure transition is probed experimentally at a temperature of 95 K and characterized by a continuously increasing fraction of fcc isomers over the considered size range showing a crossover to the fcc motif at n ≈ 90.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical, electronic and optoelectronic properties of the materials, especially low-dimensional materials. Two-dimensional (2D) organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field; nevertheless, studies on this remain elusive. Here we report on how the surface depletion field affects the structural phase transition, quantum confinement and Stark effect in 2D (BA)2PbI4 perovskite microplates by the thickness-, temperature- and power-dependent photoluminescence (PL) spectroscopy. Power dependent PL studies suggest that high-temperature phase (HTP) and low-temperature phase (LTP) can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates. With the decrease of the microplate thickness, the structural phase transition temperature first gradually decreases and then increases below 25 nm, in striking contrast to the conventional size dependent structural phase transition. Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase, the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs. This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes. Along with the thickness dependent phase transition, the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

A unified analysis of nano-to-microscale particle dispersion in tubular blood flow

Transport of solid particles in blood flow exhibits qualitative differences in the transport mechanism when the particle varies from nanoscale to microscale size comparable to the red blood cell (RBC). The effect of microscale particle margination has been investigated by several groups. Also, the transport of nanoscale particles (NPs) in blood has received considerable attention in the past. This study attempts to bridge the gap by quantitatively showing how the transport mechanism varies with particle size from nano-to-microscale. Using a three-dimensional (3D) multiscale method, the dispersion of particles in microscale tubular flows is investigated for various hematocrits, vessel diameters, and particle sizes. NPs exhibit a nonuniform, smoothly dispersed distribution across the tube radius due to severe Brownian motion. The near-wall concentration of NPs can be moderately enhanced by increasing hematocrit and confinement. Moreover, there exists a critical particle size (∼1 μm) that leads to excessive retention of particles in the cell-free region near the wall, i.e., margination. Above this threshold, the margination propensity increases with the particle size. The dominance of RBC-enhanced shear-induced diffusivity (RESID) over Brownian diffusivity (BD) results in 10 times higher radial diffusion rates in the RBC-laden region compared to that in the cell-free layer, correlated with the high margination propensity of microscale particles. This work captures the particle size-dependent transition from Brownian-motion dominant dispersion to margination using a unified 3D multiscale computational approach and highlights the linkage between the radial distribution of RESID and the margination of particles in confined blood flows.

Exploring novel deformation mechanisms in aluminum–copper alloys using in situ 4D nanomechanical testing

Even after nearly a century of extensive use of aluminum alloys in structural applications, our understanding of such precipitation-strengthened materials is far from complete. With the advent of next generation advanced characterization techniques, our ability to probe materials in unique ways and at different length scales has established a new paradigm for devising new pathways to alloy design by engineering materials and tailoring specific properties at the nanoscale. Here, we perform in situ nanomechanical testing in conjunction with synchrotron-based hard X-ray nanotomography to capture initiation and evolution of damage in 3D in Al–Cu alloys. Precipitates in these alloys are seen to exhibit unprecedented localized deformation in compression, which is attributed to novel observations of kinking in these brittle second-phase particles, accompanied with the generation of a fine polycrystalline texture in the adjacent matrix. We observe a size-dependent transition in precipitate deformation behavior that has been thoroughly investigated using a comprehensive correlative approach.

Size-dependent solid-solid phase transition process of Ag2S nanoparticles

It is well known that for a nanoparticle the solid-solid phase transition begins with the appearance of a high temperature disordered phase at the surface and the phase interface moves inward gradually with the increase of temperature. However, the size-dependent phase transition behavior remains unclear. Here we report an in-situ TEM study of the phase transition process of different-sized Ag2S nanoparticles at atomic resolution. The onset temperature of disordered phase of the small nanoparticle is found to be lower than that of the big nanoparticle. And, the disordered phase thickness of small nanoparticle is always thicker than that of big nanoparticle. By considering surface and interface free-energy, a phenomenological model based on the minimization of system free-energy is established, which could well explain our experimental results. These discoveries extend our understanding of size dependent phase transition mechanism.

Size-dependent structural transition thermodynamics of octahedral nanoparticles: Theoretical and experimental study

The structural transition of nanoparticles is greatly different from corresponding bulk materials, where particle size plays an important role. However, the effect of particle size on structural transition thermodynamics of octahedral nanoparticles has not been reported. Herein, the relations of structural transition temperature and thermodynamic properties with length of octahedral nanoparticles were derived, the effects of length on structural transition temperature and thermodynamic properties of octahedral nanoparticles were discussed. In experiment, the structural transition of octahedral nano-Fe2O3 from phase γ to α was taken as a system to research the size-dependent structural transition thermodynamics; and the structural transition temperature and thermodynamic properties were determined by DSC. The experimental results are consistent with the theoretical relationships.The results show that there are significant effects of the length on the temperature, the enthalpy and the entropy of structural transition of octahedral nanoparticles, and phase transition temperature and thermodynamic properties decrease with the length decreasing. Besides, within the length range of the experimental study, there exist linear relationships of phase transition temperature and thermodynamic properties with the reciprocal of the length, respectively. This research will play guiding roles for the preparation, research and application of octahedral nanoparticles.

Pressure Induced Nanoparticle Phase Behavior, Property, and Applications.

Nanoparticle (NP) high pressure behavior has been extensively studied over the years. In this review, we summarize recent progress on the studies of pressure induced NP phase behavior, property, and applications. This review starts with a brief overview of high pressure characterization techniques, coupled with synchrotron X-ray scattering, Raman, fluorescence, and absorption. Then, we survey the pressure induced phase transition of NP atomic crystal structure including size dependent phase transition, amorphization, and threshold pressures using several typical NP material systems as examples. Next, we discuss the pressure induced phase transition of NP mesoscale structures including topics on pressure induced interparticle separation distance, NP coupling, and NP coalescence. Pressure induced new properties and applications in different NP systems are highlighted. Finally, outlooks with future directions are discussed.

An unexpected phase transformation of ceria nanoparticles in aqueous media.

Cerium oxide Nanoparticles (CNPs) are of significant interest to the scientific community due to their wide spread applications in a variety of fields. It is proposed that size dependent variations in the extent of Ce3+ and Ce4+ oxidation states of cerium in CNPs determines the performance of CNPs in application environments. To obtain greater molecular and structural understanding of chemical state transformations previously reported for ceria ≈ 3 nm nanoparticles (CNPs) in response to changing ambient conditions, microXRD and Raman measurements were carried out for various solution conditions. The particles were observed to undergo a reversible transformation from a defective ceria structure to a non-ceria amorphous oxy-hydroxide/peroxide phase in response to the addition of 30% hydrogen peroxide. For CNPs made up of ~8 nm crystallites, a partial transformation was observed and no transformation was observed for CNPs made up of ~ 40 nm crystallites. This observation of differences in size dependent transition behavior may help explain the benefits of using smaller CNPs in applications requiring regenerative behavior.

Atomic Structures and Electronic Properties of Large-Sized GeN Clusters (N = 45, 50, 55, 60, 65, 70) by First-Principles Global Search

A two-step unbiased global search was performed to explore the lowest-energy structures of large GeN clusters with N = 45–70 atoms. It has been revealed that the most stable structures for these large-sized Ge clusters are stuffed cages. Based on the lowest-energy structures, the theoretical results for the size-dependent structural transition, binding energy and ionization potential compare well with the available experimental data. Overall speaking, the structural characteristics and electronic properties of GeN clusters in the considered size range gradually approach the bulk limits, but still with certain deviations.

Size and shape dependent melting temperature of metallic nanomaterials

This study aims to characterize the size and shape dependent melting temperature of nanomaterials. Considering that surface atoms and interior atoms affect the melting of materials in different manners, we thus define an equivalent relationship between the contribution of surface atoms and interior atoms. Based on this definition, a criterion of melting is proposed through introducing a critical energy storage density of melting, the sum of the contribution of surface atoms and the interior atoms. According to the proposed criterion, a new theoretical model without any adjustable parameters is developed to characterize the size effect of melting temperatures of nanomaterials. The model predictions are in good agreement with the available experimental data or molecular dynamics simulations. This model uncovers the quantitative relationship between the melting temperature, size, atomic diameter and shape of nanomaterials. In addition, this model is extended to predict the size dependent glass transition temperatures of polymers. This study can help to better understand and characterize the size dependent melting temperatures of nanomaterials, as well as the size dependent glass transition temperatures of polymers.

Surface compressive and softening effect on deformation mode transition in Ni-Nb metallic glassy thin films: A molecular dynamics study

Size-dependent deformation mode transition in metallic glasses (MGs) attracts a lot of interest due to potential application in micro-devices, but the underlying mechanisms are still unclear from the perspective of structure, e.g., how the chemical composition affects the deformation mode transition in a particular system is mysterious as well and needs to be addressed. Here, a series of NixNb100−x (x = 30, 50, 62, and 70 at. %) MG thin films has been studied by molecular dynamics simulations for better understanding the thickness-dependent tensile behaviors. The results show that the deformation mode transition from highly-localized to non-localized occurs as the film thickness (t) approaches the critical size, tc, which strongly correlates with the chemical composition, i.e., a Ni-rich specimen with higher modulus has smaller tc. It is revealed that the softening and compressive effect of surface layers with about 0.4 nm thickness in terms of Voronoi volume is the key factor for this transition regardless of composition. We illustrate the surface softening effect in various MG thin films by introducing a softening coefficient (Ψ) parameter reflecting the structural difference between the surface layer and the internal part. It is found that the higher the Ψ, the severer the surface softening effect, and the larger the tc in the Ni-depleted specimen, indicating the importance of Ψ as an indicator for the deformation mode transition.

Size-dependent phase transition of Er2O3 under high pressure

The size effect on the structural and optical properties of cubic Er2O3 was investigated under pressure by in-situ angular dispersive synchrotron x-ray diffraction (AD-XRD), Raman scattering, photoluminescence (PL), and impedance spectroscopy. Contrary to the phase transition sequence of cubic→monoclinic→hexagonal in bulk Er2O3, a transformation from cubic directly to hexagonal was observed in Er2O3 nanoparticals. Compared with bulk Er2O3, nano-Er2O3 showed an obvious elevation of phase transition pressure and larger bulk module. A third-order Birch-Murnaghan fitting yields zero pressure bulk moduli (B0) of 181(5), and 226(4) GPa and their pressure derivatives (B′0) of 4.0(7), 1.9(5) for the cubic and hexagonal phases, respectively. The multiple PL lines of 4S3/2→4I15/2 originating from the cubic phase are also altered due to phase transformation. The impedance spectroscopy indicated that the nano-Er2O3 is an insulator up to 30 GPa. These findings give a fresh understanding of size influence on the phase transition sequences and sheds light on the applications of nano-Er2O3.

Selection of trilateral continuums of life history strategies under food web interactions

The study of life history strategies has a long history in ecology and evolution, but determining the underlying mechanisms driving the evolution of life history variation and its consequences for population regulation remains a major challenge. In this study, a food web model with constant environmental conditions was used to demonstrate how multi-species consumer–resource interactions (food-web interactions) can create variation in the duration of the adult stage, age of maturation, and fecundity among species. The model included three key ecological processes: size-dependent species interactions, energetics, and transition among developmental stages. Resultant patterns of life history variation were consistent with previous empirical observations of the life history strategies of aquatic organisms referred to as periodic, equilibrium, and opportunistic strategies (trilateral continuums of life history strategies). Results from the simulation model suggest that these three life history strategies can emerge from food web interactions even when abiotic environmental conditions are held constant.