Showing papers on "Nucleation published in 2020"

Real-time observations of TRIP-induced ultrahigh strain hardening in a dual-phase CrMnFeCoNi high-entropy alloy.

Abstract: Strategies involving metastable phases have been the basis of the design of numerous alloys, yet research on metastable high-entropy alloys is still in its infancy. In dual-phase high-entropy alloys, the combination of local chemical environments and loading-induced crystal structure changes suggests a relationship between deformation mechanisms and chemical atomic distribution, which we examine in here in a Cantor-like Cr20Mn6Fe34Co34Ni6 alloy, comprising both face-centered cubic (fcc) and hexagonal closed packed (hcp) phases. We observe that partial dislocation activities result in stable three-dimensional stacking-fault networks. Additionally, the fraction of the stronger hcp phase progressively increases during plastic deformation by forming at the stacking-fault network boundaries in the fcc phase, serving as the major source of strain hardening. In this context, variations in local chemical composition promote a high density of Lomer-Cottrell locks, which facilitate the construction of the stacking-fault networks to provide nucleation sites for the hcp phase transformation.

On the mechanism of soot nucleation.

Abstract: The mechanism of carbon particulate (soot) inception has been a subject of numerous studies and debates. The article begins with a critical review of prior proposals, proceeds to the analysis of factors enabling the development of a meaningful nucleation flux, and then introduces new ideas that lead to the fulfillment of these requirements. In the new proposal, a rotationally-activated dimer is formed in the collision of an aromatic molecule and an aromatic radical; the two react during the lifetime of the dimer to form a stable, doubly-bonded bridge between them, with the reaction rooted in a five-member ring present on the molecule edge. Several such reactions were examined theoretically and the most promising one generated a measurable nucleation flux. The consistency of the proposed model with known aspects of soot particle nanostructure is discussed. The foundation of the new model is fundamentally the H-Abstraction-Carbon-Addition (HACA) mechanism with the reaction affinity enhanced by rotational excitation.

Kinetic pathways of crystallization at the nanoscale

Abstract: Nucleation and growth are universally important in systems from the atomic to the micrometre scale as they dictate structural and functional attributes of crystals. However, at the nanoscale, the pathways towards crystallization have been largely unexplored owing to the challenge of resolving the motion of individual building blocks in a liquid medium. Here we address this gap by directly imaging the full transition of dispersed gold nanoprisms to a superlattice at the single-particle level. We utilize liquid-phase transmission electron microscopy at low dose rates to control nanoparticle interactions without affecting their motions. Combining particle tracking with Monte Carlo simulations, we reveal that positional ordering of the superlattice emerges from orientational disorder. This method allows us to measure parameters such as line tension and phase coordinates, charting the nonclassical nucleation pathway involving a dense, amorphous intermediate. We demonstrate the versatility of our approach via crystallization of different nanoparticles, pointing the way to more general applications.

Self-Nucleation Effects on Polymer Crystallization

Abstract: The existence of a “memory” of the previous crystalline state, which survives melting and enhances recrystallization kinetics by a self-nucleation process, is well-known in polymer crystallization ...

Tailoring desolvation kinetics enables stable zinc metal anodes

Abstract: Low-cost and high-safety aqueous Zn ion batteries have been considered as promising alternatives to Li-ion batteries, provided that a stable Zn metal anode could be developed. Dendrite growth and low coulombic efficiency (CE) are the primary two issues afflicting the design of advanced Zn metal anodes. Inspired by complexing agents in the electroplating industry, acetonitrile (AN) is proposed as an electrolyte additive to guide the smooth growth of Zn. The enhanced intermolecular interactions between Zn2+ and mixed H2O/AN solvents lead to the supersaturating of adatoms on the current collector, as revealed by the complementary theoretical and experimental studies. Consequently, homogeneous nucleation and smooth growth of Zn are enabled for achieving exceptional stability up to 1000 cycles with an excellent CE of 99.64% on average. Application-wise, the incorporation of a complexing agent into the electrolyte is fully compatible with the cathode while maintaining the non-flammable nature for safe operation. The solvation chemistry regulation strategy provides a promising route to stabilize Zn metal anodes.

Combining theories and experiments to understand the sodium nucleation behavior towards safe sodium metal batteries

Abstract: Rechargeable sodium (Na) based batteries have gained tremendous research interest because of the high natural abundance and low cost of Na resources, as well as electrochemical similarities with lithium (Li) based batteries. However, despite the great potential as a candidate for next-generation grid-scale energy storage, the implementation of the Na metal anode has been primarily hindered by dendritic and "dead" Na formation that leads to low Coulombic efficiency, short lifespan and even safety concerns. Na dendrite formation mainly originates from the uncontrolled Na deposition behavior in the absence of nucleation site regulation. Hence, the Na nucleation and initial stage of growth are critically important for the final morphology of Na metal. Here, this tutorial review aims to provide a comprehensive understanding of the importance of the nucleation behavior towards dendrite-free Na metal anodes. Firstly, we start with an introduction about the advantages of Na metal batteries over the Li counterpart and the challenges faced by Na metal anodes. The differences between metallic Li and Na are summarized according to advanced in situ characterization techniques. Next, we elucidate the key factors that influence the Na nucleation and growth behaviors based on the existing theoretical models. Then, we review the state-of-the-art approaches that have been applied to effectively regulate Na nucleation for dendrite-free Na deposition. Lastly, we conclude the review with perspectives on realizing safe Na metal batteries with high energy density.

Ab initio phase diagram and nucleation of gallium.

Abstract: Elemental gallium possesses several intriguing properties, such as a low melting point, a density anomaly and an electronic structure in which covalent and metallic features coexist. In order to simulate this complex system, we construct an ab initio quality interaction potential by training a neural network on a set of density functional theory calculations performed on configurations generated in multithermal–multibaric simulations. Here we show that the relative equilibrium between liquid gallium, α-Ga, β-Ga, and Ga-II is well described. The resulting phase diagram is in agreement with the experimental findings. The local structure of liquid gallium and its nucleation into α-Ga and β-Ga are studied. We find that the formation of metastable β-Ga is kinetically favored over the thermodinamically stable α-Ga. Finally, we provide insight into the experimental observations of extreme undercooling of liquid Ga. Exploring nucleation processes of gallium by molecular simulation is extremely challenging due to its structural complexity. Here the authors demonstrate a neural network potential trained on a multithermal–multibaric DFT data for the study of the phase diagram of gallium in a wide temperature and pressure range.

Investigation of the properties of alkali-activated slag mixes involving the use of nanoclay and nucleation seeds for 3D printing

Abstract: This study investigated the properties of alkali activated slag (AAS) binders formulated for extrusion‐based 3D printing. The fresh properties of AAS mixes were tailored through the use of nanoclay (NC) and nucleation seeds. The printability criteria employed were the ease of extrusion (extrudability) and the stability of the layered structure (buildability). Introduction of 0.4% NC in AAS mixes led to improved thixotropic properties due to the flocculation effect, which accounted for the extrudability and shape fidelity of the binder. Inclusion of 2% hydromagnesite seeds in this mix design provided additional nucleation sites for the increased precipitation of hydrate phases, resulting in denser microstructures. This enhanced the hydration reaction and improved the structural build-up rate necessary for large-scale 3D printing. The developed AAS mix containing 0.4% NC and 2% hydromagnesite seeds was used in the printing of an actual 3D structure to demonstrate its feasibility to be used in 3D printing applications.

Lithiophilic Zn Sites in Porous CuZn Alloy Induced Uniform Li Nucleation and Dendrite-free Li Metal Deposition

Abstract: Three-dimensional (3D) lithiophilic host is one of the most effective ways to regulate the Li dendrites and volume change in working Li metal anode. The state-of-the-art 3D lithiophilic hosts are facing one main challenge in that the lithiophilic layer would melt or fall off in high-temperature environment when using the thermal infusion method. Herein, a 3D porous CuZn alloy host containing anchored lithiophilic Zn sites is employed to prestore Li using the thermal infusion strategy, and a 3D composite Li is thus fabricated. Benefiting from the lithiophilic Zn sites with a strong adsorption capacity with Li, which is based on the analyses of the nucleation overpotential, binding energy calculation, and the operando optical observation of Li plating/stripping behaviors, facile uniform Li nucleation and dendrite-free Li deposition could be achieved in the interior of the 3D porous CuZn alloy host and the 3D composite Li shows remarkable enhancement in electrochemical performance.

Selective laser melting of 304L stainless steel: Role of volumetric energy density on the microstructure, texture and mechanical properties

Abstract: The role of volumetric energy density on the microstructural evolution, texture and mechanical properties of 304L stainless steel parts additively manufactured via selective laser melting process is investigated. 304L is chosen because it is a potential candidate to be used as a matrix in a metal matrix composite with nanoparticles dispersion for energy and high temperature applications. The highest relative density of 99 %±0.5 was achieved using a volumetric energy density of 1400 J/mm3. Both XRD analysis and Scheil simulation revealed the presence of a small trace of the delta ferrite phase, due to rapid solidification within the austenitic matrix of 304L. A fine cellular substructure ranged between 0.4–1.8 μm, was detected across different energy density values. At the highest energy density value, a strong texture in the direction of [100] was identified. At lower energy density values, multicomponent texture was found due to high nucleation rate and the existing defects. Yield strength, ultimate tensile strength, and microhardness of samples with a relative density of 99 % were measured to be 540 ± 15 MPa, 660 ± 20 MPa and 254 ± 7 HV, respectively and higher than mechanical properties of conventionally manufactured 304L stainless steel. Heat treatment of the laser melted 304L at 1200 °C for 2 h, resulted in the nucleation of recrystallized equiaxed grains followed by a decrease in microhardness value from 233 ± 3 HV to 208 ± 8 HV due to disappearance of cellular substructure.

Precise Control of Perovskite Crystallization Kinetics via Sequential A-Site Doping.

Abstract: Two-step-fabricated FAPbI3 -based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI3 -based perovskite film with sequential A-site doping of Cs+ and GA+ is revealed by in situ X-ray scattering and first-principles calculations. Incorporating Cs+ in the first step induces an alternative pathway from δ-CsPbI3 to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI2 . However, pinholes are formed due to the nonuniform nucleation with sparse δ-CsPbI3 crystals. Fortunately, incorporating GA+ in the second step can not only promote the phase transition from δ-CsPbI3 to the perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two-step process over the one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.

Unusual activated processes controlling dislocation motion in body-centered-cubic high-entropy alloys

Abstract: Atomistic simulations of dislocation mobility reveal that body-centered cubic (BCC) high-entropy alloys (HEAs) are distinctly different from traditional BCC metals HEAs are concentrated solutions in which composition fluctuation is almost inevitable The resultant inhomogeneities, while locally promoting kink nucleation on screw dislocations, trap them against propagation with an appreciable energy barrier, replacing kink nucleation as the rate-limiting mechanism Edge dislocations encounter a similar activated process of nanoscale segment detrapping, with comparable activation barrier As a result, the mobility of edge dislocations, and hence their contribution to strength, becomes comparable to screw dislocations

Controlled Growth of Single-Crystal Graphene Films

Abstract: Grain boundaries produced during material synthesis affect both the intrinsic properties of materials and their potential for high-end applications. This effect is commonly observed in graphene film grown using chemical vapor deposition and therefore caused intense interest in controlled growth of grain-boundary-free graphene single crystals in the past ten years. The main methods for enlarging graphene domain size and reducing graphene grain boundary density are classified into single-seed and multiseed approaches, wherein reduction of nucleation density and alignment of nucleation orientation are respectively realized in the nucleation stage. On this basis, detailed synthesis strategies, corresponding mechanisms, and key parameters in the representative methods of these two approaches are separately reviewed, with the aim of providing comprehensive knowledge and a snapshot of the latest status of controlled growth of single-crystal graphene films. Finally, perspectives on opportunities and challenges in synthesizing large-area single-crystal graphene films are discussed.

Understanding the conversion mechanism and performance of monodisperse FeF2 nanocrystal cathodes.

Abstract: The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(ii) fluoride nanorods. Near theoretical capacity (570 mA h g−1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2. The application of metal fluorides as cathodes for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities. Reversible conversion reaction in iron fluoride nanocrystals is shown to be due to topotactic cation diffusion and nucleation of metallic particles.

Revisiting nucleation in the phase-field approach to brittle fracture

Abstract: Twenty years in since their introduction, it is now plain that the regularized formulations dubbed as phase-field of the variational theory of brittle fracture of Francfort and Marigo (1998) provide a powerful macroscopic theory to describe and predict the propagation of cracks in linear elastic brittle materials under arbitrary quasistatic loading conditions. Over the past ten years, the ability of the phase-field approach to also possibly describe and predict crack nucleation has been under intense investigation. The first of two objectives of this paper is to establish that the existing phase-field approach to fracture at large — irrespectively of its particular version — cannot possibly model crack nucleation. This is so because it lacks one essential ingredient: the strength of the material. The second objective is to amend the phase-field theory in a manner such that it can model crack nucleation, be it from large pre-existing cracks, small pre-existing cracks, smooth and non-smooth boundary points, or within the bulk of structures subjected to arbitrary quasistatic loadings, while keeping undisturbed the ability of the standard phase-field formulation to model crack propagation. The central idea is to implicitly account for the presence of the inherent microscopic defects in the material — whose defining macroscopic manifestation is precisely the strength of the material — through the addition of an external driving force in the equation governing the evolution of the phase field. To illustrate the descriptive and predictive capabilities of the proposed theory, the last part of this paper presents sample simulations of experiments spanning the full range of fracture nucleation settings.

Black phosphorus quantum dots in inorganic perovskite thin films for efficient photovoltaic application.

Abstract: Black phosphorus quantum dots (BPQDs) are proposed as effective seed-like sites to modulate the nucleation and growth of CsPbI2Br perovskite crystalline thin layers, allowing an enhanced crystallization and remarkable morphological improvement. We reveal that the lone-pair electrons of BPQDs can induce strong binding between molecules of the CsPbI2Br precursor solution and phosphorus atoms stemming from the concomitant reduction in coulombic repulsion. The four-phase transition during the annealing process yields an α-phase CsPbI2Br stabilized by BPQDs. The BPQDS/CsPbI2Br core-shell structure concomitantly reinforces a stable CsPbI2Br crystallite and suppresses the oxidation of BPQDs. Consequently, a power conversion efficiency of 15.47% can be achieved for 0.7 wt % BPQDs embedded in CsPbI2Br film-based devices, with an enhanced cell stability, under ambient conditions. Our finding is a decisive step in the exploration of crystallization and phase stability that can lead to the realization of efficient and stable inorganic perovskite solar cells.