Formation Mechanism Of Dendrites Research Articles

The Mechanism of Dendrite Formation in a Solid-State Transformation of High Aluminum Fe-Al Alloys.

The mechanism of solid-state dendrite formation in high-aluminum Fe-Al alloys is not clear. Applying an in-situ observation technique, the real-time formation and growth of FeAl solid-state dendrites during the eutectoid decomposition of the high-temperature phase Fe5Al8 is visualized. In-situ experiments by HT-CSLM reveal that proeutectoid FeAl usually does not preferentially nucleate at grain boundaries regardless of rapid or slow cooling conditions. The critical radii for generating morphological instability are 1.2 μm and 0.9 μm for slow and rapid cooling, respectively. The morphology after both slow and rapid cooling exhibits dendrites, while there are differences in the size and critical instability radius Rc, which are attributed to the different supersaturation S and the number of protrusions l. The combination of crystallographic and thermodynamic analysis indicates that solid-state dendrites only exist on the hypoeutectoid side in high-aluminum Fe-Al alloys. A large number of lattice defects in the parent phase provides an additional driving force for nucleation, leading to coherent nucleation from the interior of the parent phase grains based on the orientation relationship {3¯30}Fe5Al8//{1¯10}FeAl, <111¯>Fe5Al8//<111¯>FeAl. The maximum release of misfit strain energy leads to the preferential growth of the primary arm of the nucleus along <111¯> {1¯10}. During the rapid cooling process, a large supersaturation is induced in the matrix, driving the Al atoms to undergo unstable uphill diffusion and causing variations in the concentration gradient as well as generating constitutional undercooling, ultimately leading to morphological instability and the growth of secondary arms.

Phase-Field Investigation of Lithium Electrodeposition at Different Applied Overpotentials and Operating Temperatures.

Li metal is an exciting anode for high-energy Li-ion batteries and other future battery technologies due to its high energy density and low redox potential. Despite their high promise, the commercialization of Li-metal-based batteries has been hampered due to the formation of dendrites that lead to mechanical instability, energy loss, and eventual internal short circuits. In recent years, the mechanism of dendrite formation and the strategies to suppress their growth have been studied intensely. However, the effect of applied overpotential and operating temperature on dendrite formation and their growth rate remains to be fully understood. Here, we elucidate the correlation between the applied overpotential and operating temperature to the dendrite height and tortuosity of the Li-metal surface during electrodeposition using phase-field model simulations. We identify an optimal operating temperature of a half-cell consisting of a Li metal anode and 1 M LiPF6 in EC/DMC (1/1), which increases gradually as the magnitude of the overpotential increases. The investigation reveals that the temperature dependence identified in the simulations and experiments often disagree because they are primarily conducted under galvanostatic and potentiostatic conditions, respectively. The temperature increase under potentiostatic conditions increases the induced current while it decreases the induced overpotential under galvanostatic conditions. Therefore, the analysis and comparison of temperature-dependent characteristics must be carried out with care.

Self-assembly growth of electrolytic silver dendrites

The atomic level assembly of silver dendrite has never been disclosed despite the numerous studies published on fractal dendrite structures. We report for the first time an HRTEM investigation of the formation of atomic embryos (< 5 nm) and the self-assembly of atoms on an atomic plane of the tip of a dendrite arm. The mechanism of dendrite formation proceeds via the sequence of amorphous embryos aggregates (5–10 nm), nuclei, crystallites (10–20 nm), dendritelets (50–100 nm) and submicron dendrite protypes. The atomic plane is an entirely atomic-level zig-zag structure with d-spacing kink steps. The zig-zag structure triggers the self-assembly of atoms and thus directional growth to produce a dendrite arm with a high aspect ratio.

A computational approach to characterize formation of a passivation layer in lithium metal anodes

AbstractLi metal anode is the “Holy Grail” material of advanced Lithium‐ion‐batteries (LIBs). However, it is plagued by uncontrollable dendrite growth resulting in poor cycling efficiency and short‐circuiting of batteries. This has spurred a plethora of research to understand the underlying mechanism of dendrite formation. While experimental studies suggest that there are complex physical and chemical interactions between heterogeneous solid‐electrolyte interphase (SEI) and dendrite growth, most of the studies do not reveal the mechanisms triggering these interactions. To deal with this knowledge gap, we propose a multiscale modeling framework which couples kinetic Monte Carlo and Molecular Dynamics simulations. Specifically, the model has been developed to account for (a) heterogeneous SEI, (b) dendrite‐SEI interactions, and (c) effect of electrolyte on Li electrodeposition and potential dendrite formation. This allows the proposed computational model to be extended to various electrolytes and SEI species and generate results consistent with previous experimental studies.

Heat transfer mechanisms in solidification

Solidification as a heat transfer process accompanying a phase transition plays a major role in manufacturing processes and material evolution processes. However, the complex combinations of solidification mechanisms at interfacial boundaries prevent us from its full understanding. A statistical thermodynamic transport theory for heat transfer has been proposed. We here demonstrate the validity of the heat transfer theory for internal convection and external conduction by examining the solidification processes at the interface of two different phase materials. Three variable separation constants designate particle number constants and play the major roles in exploring the distinct solidification mechanisms. In solidification, the four regimes of convection, nucleation, transient, and film solidification are illustrated and their heat flux curves are explored as a function of temperature difference between electrode and saturation temperature. Three limiting heat fluxes and four activation temperatures are implemented as input parameters. The heat transfer theory pictures that the formation mechanism of dendrites takes place under the control of nucleation solidification, and the relevant parameters in the control mechanism are predicted. Theoretical heat flux profiles in solidification agree with experimental heat flux data, and the theory is capable of predicting the heat flux mechanisms in full temperature regimes of solidification as well as in the spatial and temporal regimes.

Preparation of Dendritic Copper Nanostructures and Their Characterization for Electroreduction

Dendritic copper nanostructures of different morphologies were synthesized by a surfactant-free electrochemical method. Single crystal nature of the nanostructures was revealed from their X-ray diffraction and electron diffraction patterns. Mechanism of dendrite formation was discussed from thermodynamic aspects using the concept of supersaturation. Supersaturation of the copper metal reduced on the surface of the electrode was the crucial factor for the generation of different morphologies. Effects of applied potential, temperature, and the solution concentration on the supersaturation were studied. The NO3− and H2O2 electroreduction ability of the dendritic materials was tested. Use of copper dendrite-modified electrode as NO3− sensor was demonstrated.

A simple mixed surfactant route for the preparation of noble metal dendrites

A simple but novel mixed surfactant route was designed to fabricate noble metal (Au, Ag) nanocrystals with novel dendritic structures in mixed cetyltgrimethlammonium bromide and sodium dodecyl sulfate solutions. The obtained dendrites have interesting morphologies that consist of a long central backbone and parallel symmetry rather than randomly ramified secondary branches. The products were characterized by X-ray diffraction, transmission electron microscopy, and field emission scanning electron microscopy and selected area electronic diffraction. The purity and surface composition were measured using X-ray photoelectron spectra. The influence of surfactants on the formation of dendrites has been investigated in detail and it is found that surfactant with a proper concentration present in the reaction system is the key factor for the formation of dendritic nanostructures. A possible formation mechanism of dendrites is discussed.

Template-based synthesis of nanoscale Ag2E (E = S, Se) dendrites

Herein, well-defined nanoscale Ag2S and Ag2Se dendrites, which represent a novel morphology of silver chalcogenides, were successfully synthesized using a template-based method at room temperature and ambient pressure. It was found that the template and ultrasonic wave played important roles in the formation of well-defined dendrites. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and elemental analysis. The optical properties of the products were also recorded by means of UV-Vis absorption spectroscopy, indicating that the products were within a quantum confinement regime. Finally, the formation mechanism of dendrites was also investigated.

Al-Si合金の固-液共存層における空隙のhealing現象

Using the authors’ method that an artificial hole was made in the solid-liquid coexisting zone, the interdendritic liquid was allowed to flow into the cylindrical hole or the wedgewise concavity for different fractions of solid. The healing phenomena in relation to hot tearing were pursued by the formation mechanism of dendrites within the liquid flowing into the hole.The results on an Al-3 mass%Si alloy are summarized as follows.(1) When the diameter of the cylindrical hole was above 4 mm, the occurrence of crystals was recognized in the hole and the hole was healed. In case of the hole of 2 mm diameter, a narrow shrinkage pipe was observed in the central part of the hole.(2) The formation of the dendrites within the hole becomes possible with increasing diffusion distance of solute in the liquid in front of the dendrites developing from the wall of the hole towards the center of it, because the degree of constitutional undercooling in the central part of the hole increases with increasing the diffusion distance. In the hole of 2 mm diameter without crystallization, the liquid in front of growing dendrites is promoted to be enriched in solute and the solute-rich liquid is utilized as the source of feeding against the solidification contraction of the specimen, so that a shrinkage pipe is formed.(3) The experimental results of the healing phenomena in case with holes of several diameters could be reappeared with the wedgewise concavity which was made by the procedure similar to the formation of the cylindrical hole in the solid-liquid coexisting zone.

Electrodes de zinc en milieu alcalin

The various theories on the behaviour of zinc in alkaline solutions, and some aspects of actual problems arising from the manufacturing of alkaline cells using a zinc electrode, are analysed.The thermodynamic constants and the conditions of formation of zinc oxides and hydroxides existing in alkaline medium are given. The dissolved species of zinc in alkaline solutions are considered and the relation between electrode potential, activities of dissolved species and pH is given.Anodic and cathodic behaviour of the zinc electrode is discussed, special attention being drawn to dissolution in the active state and to passivation phenomena. The conditions of formation and the nature of passive anode layers are described.Problems arising from utilization of zinc electrodes in primary and secondary alkaline cells are presented. Means to defer anodic passivation are discussed and the mechanism of dendrite formation during cathodic polarization is analysed. Possible interferences between positive and negative electrodes are also noted.