Dendritic Evolution Research Articles

Silver dendrites decorated filter membrane as highly sensitive and reproducible three dimensional surface enhanced Raman scattering substrates

We report a novel and flexible surface enhanced Raman scattering (SERS) substrate based on filter membranes decorated with silver dendritic nanostructures. The SERS-active substrate was fabricated via electrodeposition, where hierarchical silver dendrites were uniformly and firmly deposited within and on the top of the porous filter membranes. The morphological evolution of silver dendrites was investigated at different deposition times, and the effect of the components of electrolyte was also studied. Finite difference time domain (FDTD) simulations were performed to reveal the distribution of electric filed when Ag dendrites were illuminated with 785nm light. Such 3D SERS-active substrate exhibits extremely high sensitivity and excellent reproducibility. Raman signal sensitivity for rhodamine 6G was tested as high as 1×10−11M with 12% average intensity variations at the major Raman peak. Additionally, the as-synthesized robust substrate displays high stability under an ambient condition for several months. This 3D eco-friendly filter membrane-based substrate provides not only high density of SERS hot spots, but also a very large area for capturing target analytes. It has potential applications for the detection of trace organic contaminants in the environment.

Study on the microstructural evolution of AZ31 magnesium alloy in a vertical twin-roll casting process

Finite element method was employed to calculate the macroflow velocity and temperature distribution of the pool domain’s biting zone in twin-roll casting. Macroanalysis results were inducted as boundary conditions into microanalysis. Phase field method (PFM) was adopted to investigate the microstructure evolution. Based on the Kim–Kim–Suzuki model, the effect of metal flow velocity was coupled on the solute gradient item, and the real physical parameters of AZ31 were inducted into the numerical calculation. We used the marker and cell method in the discrete element solution of microstructural pattern prediction of AZ31 magnesium alloys. The different flow velocity values that predicted the columnar dendrite evolution were discussed in detail. Numerical simulation results were also compared with the experiment analysis. The microstructure obtained by PFM agrees with the actual pattern observed via optical microscopy.

Morphology simulation and mechanical analysis of primary dendrites for continuously cast low carbon steel

The initial growing dendrite is influenced significantly by the complicated solidification conditions in continuously oscillating mold. The uneven growth of dendrite causes some defects seen commonly such as internal crack, subsurface porosity, subsurface inclusion and other defects of continuous casting billet. The induced initial defects in mold can be expanded and propagated in the following process such as secondary cooling, straightening, rolling and other subsequent handling procedure and then evolve into serious defects that can restrict the development and the quality refinement of final steel products. The mechanical stress caused by mold oscillation and the melt flowing is a crucial factor that leads to the uneven microstructure growth of initial solidifying shell in continuous casting mold. In this work, we simulate the growth and the morphology evolution of primary dendrites in mold area by using the cellular automaton (CA) method in combination with the actual conditions for continuously cast low carbon billet (Fe-0.6 wt.%C). Further, the mechanical state of initial dendrite is analyzed by regarding primary dendrite as a cantilever beam and its mechanical stress is calculated by combining thermo-physical properties and flow rate of steel based on the principle of materials mechanics to shed light on the formation of initial defects formation in mold area of continuous casting process. The results show that the solute concentration of initial dendrite tip gradually increases with undercooling from 2 to 10 K, and the maximum concentration rises by 0.07% when the increment of undercooling is 2 K. The length of dendrite arm increases significantly with undercooling from 2 to 6 K. However, the length of dendrite arm remains steady in a stable growth rate of 0.08 mms-1 when the undercooling is enhanced from 6 to 10 K. The increase of undercooling reduces the bending stress at dendrite root when the flow rate of molten steel is improved from 0.13 to 0.33 ms-1, while the mechanical stress continuously increases with the growth of primary dendrite at a constant undercooling. The bending stress of dendrite root has a high possibility to exceed its critical fracture strength under the condition of undercooling below 6 K or dendrite grow up more than 1 s. The primary dendrite is likely to be fractured and form initial defects of billet.

Phase-Field Simulation of Binary Alloy Crystal Growth Prepared by a Fluid Flow

Phase field method (PFM) was employed to investigate the crystal growth of Mg-Al alloy, on the basis of binary alloy model, the fluid field equation was coupled into the phase-field models, and the marker and cell (MAC) method was used in the numerical calculation of micro structural pattern. In the cast process, quantitative comparison of different anisotropy values that predicted the dendrite evolution were discussed in detail, and when the fluid flow rate reaches a high value, we can see the remelting of dendrite arms.

Simulation of the Microstructural Evolution of Pure Material and Alloys in an Undercooled Melts via Phase-field Method and Adaptive Computational Domain

The phase-field methods were developed mainly for studying solidification of pure materials, being then extended to the solidification of alloys. In spite of phase-field models being suitable for simulating solidification processes, they suffer from low computational efficiency. In this study, we present a numerical technique for the improvement of computational efficiency for computation of microstructural evolution for both pure metal and binary alloy during solidification process. The goal of this technique is for the computational domain to grow around the microstructure and fixed the grid spacing, while solidification advances into the liquid region. In the numerical simulations of pure metal, the phase-field model is based on the energy and phase equations, while, for binary alloy, the said model is based on the concentration and phase equations. Since the thermal diffusivity in the energy equation is much larger than the diffusivity term in phase equation in pure metal system, about twenty eight times the difference between them. The computational domain growth around the microstructure is controlled according with the thermal diffusivity for pure material in the liquid region. In the numerical simulation of dendritic evolution of Fe-C alloy, the idea is similar, i.e., the solute diffusivity in concentration equation is larger than the diffusivity term in phase equation in the liquid region, in this case eleven times the difference in Fe-C alloy system. The computational domain growth is controlled via solute diffusivity in the liquid region. Hence, phase-field model is proposed with an adaptive computational domain for efficient computational simulation of the dendritic growth in a system for both pure metal and binary alloy. The technique enables us to reduce by about an order of magnitude the run time for simulation of the solidification process. The results showed that the microstructure with well-developed secondary arms can be obtained with low computation time.

Lithium Metal Polymer Battery Interfaces Studied By Hard X-Ray Microtomography

Conventional Lithium-ion batteries contain volatile liquid compounds that can leak and violently react with the environment leading to a fire.1 Solid polymer electrolytes (SPEs) are one solution as these flexible materials do not contain any volatile components. Additionally, they enable the use of high energy density lithium (Li) metal anodes.2 However, Li metal battery utilization is hampered by the dendrite growth.3 Upon cycling lithium metal is plated unevenly on the lithium metal anode side. Such dendrites can grow through the electrolyte and cause the battery to fail by short circuit. Solid block copolymer electrolytes are a class of SPEs that contains a mechanical reinforced block and an ion conducting block doped with a Li salt.4 The block immiscibility induces microphase separation, producing ordered morphologies on the nanometer scale.5 The seminal work done by K.J. Harry et al. 6 on solid polystyrene-b-poly(ethylene oxide) (SEO) block copolymer electrolyte led to a better understanding of Li dendrite formation and growth. Using Li symmetric cells dendrite evolution was observed and imaged by a non-destructive tool, hard X-ray microtomography. To go toward the application, batteries were imaged by hard X-ray microtomography. The batteries are made of a Li metal anode, a SEO electrolyte layer and a composite cathode. The cathode deposited onto on Aluminum (Al) foil, contains LiFePO4 as active material, SEO electrolyte as binder, and carbon black. Hard X-ray microtomography enables to visualize the microstructural changes at the Li/SEO and SEO/cathode interfaces to get insight on the battery failure mechanisms. The batteries were cycled at 90°C at a C/20 charge rate and a C/8 discharge rate (Figure 1a). Prior and after battery cycling, the electrode-electrolyte interfaces were imaged by hard X-ray microtomography as shown in figure 1b and 1c, respectively. We found that the supposed intimate Li/SEO interface undergoes a strong delamination phenomenon that leads to the formation of voids (Figure 1b). Using a rendering image software, the void fraction can be measured all along the Li/SEO interfaces, and its effect on the battery capacity fading be estimated. References J.-M. Tarascon, M. Armand, Nature, 414 (2001) 359.M. Armand, J.-M. Tarascon, Nature, 451 (2008) 652.M. Rosso, C. Brissot, A. Teyssot, M. Dollé, L. Sannier, J.-M. Tarascon, R. Bouchet, S. Lascaud, Electrochimica Acta, 51 (2006) 5334.M. Singh, O. Odusanya, G.M. Wilmes, H.B. Eitouni, E.D. Gomez, A.J. Patel, V.L. Chen, M.J. Park, P. Fragouli, H. Iatrou, N. Hadjichristidis, D. Cookson, N.P. Balsara, Macromolecules, 40 (2007) 4578.V. Abetz, T. Goldacker, Macromolecular Rapid Communications, 21 (2000) 16.K.J. Harry, D.T. Hallinan, D.Y. Parkinson, A.A. MacDowell, N.P. Balsara, Nature Materials, 13 (2014) 69. Figure 1

A facile formation of silver dendrites on indium tin oxide surfaces using electrodeposition and amperometric sensing of hydrazine

Silver dendritic structures have been prepared using constant potential deposition on indium tin oxide (ITO) electrodes from an aqueous solution of AgNO3 and KNO3 without any surfactants or structure directing agents. The applied potential is shown to be a crucial factor in the formation of dendrites. Depending upon the magnitude of the applied potentials, morphologies vary from polygons to dendrites. The shape evolution of the silver dendrites has been investigated by varying the deposition time and metal precursors. The mechanism of formation of dendrites is interpreted using the diffusion limited aggregation (DLA) model. The fractal dimensions of the dendrites calculated using the box counting algorithm are in agreement with those predicted by the DLA model. The Ag dendrites coated ITO electrodes exhibit excellent electrocatalytic activity toward oxidation of hydrazine in alkaline medium. The sensing of hydrazine is carried out with amperometry, where from the limit of detection and linear calibration range is deduced as 0.5μM and 100–1700μM respectively. The effect of ascorbic acid, urea, ethanol, glucose, K+, Cl−, Br− and I− ions as possible interfering agents in the detection of hydrazine is analyzed.

In situ synchrotron X-ray imaging on morphological evolution of dendrites in Sn–Bi hypoeutectic alloy under electric currents

The growth behavior and morphological evolution of dendrites in solidifying Sn–Bi alloy under electric currents [e.g., direct current (DC) and electric current pulse (ECP)] are in situ studied using synchrotron radiation X-ray imaging technique. The suppression of dendrite growth, floating and rotation of dendrites, refinement and remelting of dendrites are investigated by analyzing a series of animated images captured during the experiments. The modification mechanisms of dendrite morphology by electric fields are discussed based on the in situ and real-time observations. When DC is imposed on the samples, the growth of dendrites is significantly suppressed due to the effect of Joule heat, and a small dendrite freely flows up and rotates due to the common effect of natural convection. When ECP is imposed in the whole solidification process, the outset of solidification is delayed by Joule heat. And due to the accumulation of undercooling, dendrites suddenly nucleate, grow and finally become fine primary dendrite arm spacing. When ECP is imposed during the crystal growth stage only, the dendrites are remelted at first and then reappear along the original growing trajectories, showing the hereditary feature.

Chemistry Design for Suppressing Dendritic Growth during Metal Electrodeposition – Applications in Energy Storage Devices

Electrodeposition of metals such as lithium (Li) and zinc (Zn) is central to charging of advanced energy storage devices including lithium-metal batteries and zinc-halogen flow batteries. However, a key technological hurdle facing the development of high energy density rechargeable metal anodes is the formation of dendritic morphology during battery charging and cycling [1,2]. In the present talk, key mechanisms underlying the development of dendritic morphology during Li and Zn electrodeposition will be discussed. Mechanistic effects associated with electrolyte components (additives and solvents) will be elucidated through in situ ‘live’ monitoring of the dendrite evolution process [3]. Performance of a variety of additives (polymers, sulfides, carbonates) and solvents (aqueous and organic) in the Li and Zn system will be discussed. In situ dendrite propagation studies will be complemented with electroanalytical studies such as polarization measurements on a rotating disc electrode. As shown in attached Figs. 1 and 2, polarization measurements provide valuable insights into the modulation of surface electrochemical kinetics by additives and solvents. Knowledge of surface polarization then enables quantitative modeling of the dendrite growth propensity and the additives-assisted dendrite suppression efficacy [4,5]. A combination of theory and experiment allows development of a comprehensive framework for characterizing the surface chemistry responsible for the dendritic morphology. Based on this framework, quantitative guidelines for designing chemistries to suppress dendrite formation will be presented.

Formation and evolution of nanoporous dendrites during dealloying of a ternary Al–Ag–Au precursor

Formation and evolution of nanoporous dendrites during dealloying of a ternary Al–Ag–Au precursor have been investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The rapidly solidified Al65Ag22.75Au12.25 precursor is composed of equiaxed Al2(Au,Ag) dendrites which uniformly distribute in a lamellar α-Al–Ag2Al eutectic matrix. The kind of solution has a significant influence on the dealloying of Al65Ag22.75Au12.25. The dealloying of α-Al–Ag2Al eutectic in the HCl solution leads to the formation of nanoporous Ag (np-Ag), but the Al2(Au,Ag) phase cannot be dealloyed. Nanoporous AgAu (np-AgAu) alloys with bimodal channel size distributions can be fabricated through a two-step dealloying of Al65Ag22.75Au12.25. The further dealloying of np-AgAu in a concentrated HNO3 solution results in the formation of nanoporous Au (np-Au). Moreover, the dendrite morphology can be retained in the as-dealloyed samples during the multi-step dealloying. Through the dealloying strategy, we can fabricate novel np-AgAu and np-Au ribbons composed of nanoporous dendrites.

Simulation of Grain Growth during Solidification of Twin-Roll Continuously Cast Pure Aluminum with a Modified CA Model

Abstract A new mathematic model for the nucleation and the dendrite growth of twin-roll-continuously cast pure aluminum thin strip during solidification process was developed based on cellular automaton(CA)-finite element(FE) method and the classical transfer equations during dendrite growth. The model coupled the temperature field calculation and microstructure simulation. The solute diffusion, the curvature undercooling and the latent heat releasing were also considered in the model, all of which have significant influence on the dendrite evolution. So the dendrite growth morphology can be modeled. The applications of the model in equiaxed dendrite growth, multiple equiaxed dendrite growth, columnar grain growth, and columnar dendrite transformation to equiaxed dendrite in twin-roll-continuously cast aluminum thin strip solidification were presented. The microstructure simulation results were compared with the experimental results and they were in good agreement in dendrite morphology.

Numerical Simulation of Binary Alloy Crystal Growth Using Phase-Field Method

The competitive growth of multiple dendrites and crystal growth of directional solidification in a Mg-Al binary alloy were simulated using phase-field model, and the effect of undercooling value on the microstructural dendritic growth pattern in directional solidification process was studied in the paper. The simulation results showed the impingement of the adjacent grains, which made the dendrite growth inhibited in the competitive growth of multiple dendrites, and in directional solidification process, quantitative comparison of different undercooling values that predicted the columnar dendrite evolution were carried out. With the increasing of the undercooling value, the dendrite tip radius and second dendrite arms became smaller, and the crystal structure is more uniform and dense.

Phase-Field Micro-Solidification Simulation for Dendrite Growth in Ni-Cu Binary Alloy

By optimizing the relevant dendrite growth parameters of Ni-Cu alloy undercooling melt, it has studied the effect that the dendrite evolution process of undercooled melt and the degree of undercooling melt have on the dendrite growth of undercooling melt. In the isothermal and non-isothermal solidification condition, relatively accurate result is obtained by applying the phase field method to simulate Ni-Cu alloy. Simulation results show non-isothermal simulation with Neuman boundary condition suit to the actual physical process better.

The microstructure prediction of magnesium alloy crystal growth in directional solidification

Directional solidification process can better control the grain orientation and eliminate the horizontal grain boundary, so excellent performance of alloy can be achieved by it. In the paper, the macro and micro analysis were coupled to predict the microstructure growth of magnesium alloys in directional solidification process. FEM (finite element method) was used to calculate undercooling temperature, by which the macro analysis results were coupled into the micro analysis. Phase-field method (PFM) offers the prospect of carrying out realistic numerical calculation on dendrite growth in metallic systems, and it was employed to calculate real dimension of magnesium alloy microstructure in directional solidification. Different magnitude values of anisotropy that predicted the columnar dendrite evolution were calculated. Quantitative comparison of numerical simulation results to the experiment analysis were carried out. The microstructure obtained by PFM is in good agreement with the actual pattern by the optical microscope observation.

Mathematical Modelling and Numerical Simulation of Dendrite Growth Using Phase-Field Method with a Magnetic Field Effect

AbstractIn this paper, we present a new model developed in order to analyze phenomena which arise in the solidification of binary mixtures using phase-field method, which incorporates the convection effects and the action of magnetic field. The model consists of flow, concentration, phase field and energy systems which are nonlinear evolutive and coupled systems. It represents the non-isothermal anisotropic solidification process of a binary mixture together with the motion in a melt with the applied magnetic field. To illustrate our model, numerical simulations of the influence of magnetic-field on the evolution of dendrites during the solidification of the binary mixture of Nickel-Copper (Ni-Cu) are developed. The results demonstrate that the dendritic growth under the action of magnetic-field can be simulated by using our model.

A three-dimensional cellular automata model coupled with finite element method and thermodynamic database for alloy solidification

A three-dimensional (3D) cellular automata (CA) model has been developed for the simulation of microstructure evolution in alloy solidification. The governing rule for the CA model is associated with the phase transition driving force which is obtained via a thermodynamic database. This determines the migration rate of the non-equilibrium solid–liquid (SL) interface and is calculated according to the local temperature and chemical composition. The curvature of the interface and the anisotropic property of the surface energy are taken into consideration. A 3D finite element (FE) method is applied for the calculation of transient heat and mass transfer. Numerical calculations for the solidification of Fe–1.5wt% C alloy have been performed. The morphological evolution of dendrites, carbon segregation and temperature distribution in both isothermal and non-isothermal conditions are studied. The parameters affecting the growth of equiaxed and columnar dendrites are discussed. The calculated results are verified using the analytical model and previous experiments. The method provides a sophisticated approach to the solidification of multi-phase and multi-component systems.

Effect of a Strong Magnetic Field on Dendritic Growth of Ti-Ni Alloy

A phase field model has been developed to simulate the dendritic growth of Ti-Ni alloy subjected to a strong magnetic field. The influence of a strong magnetic field on the microstructure morphology and its evolution was successfully investigated by the model. The effect of the magnetic field intensity on the dendritic evolution has been further discussed. The simulating results revealed that with greater magnetic field intensity, the primary dendritic arms and the side branches were easier to coarsen. Besides, the dendritic tip growth rate increased with increasing magnetic field intensity, while the curvature radius had an opposite tendency. The microstructure evolution under a strong magnetic field was also studied combined with solidification thermodynamics theory. The results indicate that, the temperature of equilibrium solidification of Ti-Ni alloy changes with the presence of a strong magnetic field, and the morphology of dendritic grains will be affected eventually.

Computer-aided cooling curve thermal analysis of near eutectic Al–Si–Cu–Fe alloy

The effects of bismuth (Bi), antimony (Sb) and strontium (Sr) additions on the characteristic parameters of the evolution of aluminium dendrites in a near eutectic Al–11.3Si–2Cu–0.4Fe alloy during solidification at different cooling rates (0.6–2 °C) were investigated by computer-aided cooling curve thermal analysis (CA-CCTA). Nucleation temperature (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \)) is defined with a new approach based on second derivative cooling curve. The results showed that \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \) increased with increasing cooling rate but both the growth temperature (\( T_{\text{G}}^{{\alpha {\text{ - Al}}}} \)) and the coherency temperature (TDCP) decreased. Increase in the temperature difference for dendrite coherency (\( T_{\text{N}}^{{\alpha {\text{ - Al}}}} - T_{\text{DCP}} \)) with increasing cooling rate indicate a wider range of temperature before the dendrite can impinge on each other and higher fraction solid (\( f_{\text{S}}^{\text{DCP}} \)). Additions of Bi, Sb and Sr to the base alloy produced only a minor effect on \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \). Additions of Bi and Sb resulted in an increase in fraction solid and an increase of 30 % in the value of \( T_{\text{N}}^{{\alpha {\text{ - Al}}}} \, - \,T_{\text{G}}^{{\alpha {\text{ - Al}}}} \) to almost 13 °C.

The Morphological Evolution of Low Volume Fraction Tin Dendrites During Coarsening

Tin dendrites tend to grow in closely spaced, parallel sheets; thus, within a certain range of solid volume fraction, it is possible to obtain samples with both dense dendritic regions and dendrite-free regions of liquid. Such samples were produced by directionally solidifying Pb-69.1 wt pct Sn, allowing us to compare tin dendrite structure and coarsening in a traditional dense mushy zone with the same dendrites in much lower volume fraction solid regions. The morphology of the dendrites, both in the dense and less-dense regions is analyzed using three-dimensional reconstructions obtained by serial sectioning. Quantitative measurements of these complex structures were obtained by calculating interfacial curvature and interfacial normal distributions, and the spatial correlations of interfacial curvature. We find that the spatial correlation measurement can be used to determine average secondary or tertiary arm length. We find also that coarsening proceeds in this system by both welding of secondary arms and dissolution and growth via long-range diffusional interactions and that the microstructure becomes more morphologically anisotropic as coarsening proceeds.

An implicit parallel multigrid computing scheme to solve coupled thermal-solute phase-field equations for dendrite evolution

An implicit, second-order space and time discretization scheme together with a parallel multigrid method involving a strip grid domain partitioning has been developed to solve fully coupled, nonlinear phase field equations involving solute and heat transport for multiple solidifying dendrites. The computational algorithm has been shown to be stable and monotonously convergent, and allowed time marching steps that were 3–4 orders of magnitude larger than those employed in similar explicit approaches, resulting in an increase of 3–4 orders of magnitude in computing efficiency. Full solute and thermal coupling was achieved for metallic alloys with a realistic, high Lewis number of >10 4. The parallel multigrid computing scheme is shown to provide a scalable methodology that allowed the efficient use of distributed supercomputing resource to simulate the evolution of tens of complex shaped 2D dendrites in a computational domain containing tens or even hundreds of millions of grid points. The simulations have provided insight into the dynamic interplay of many growing dendrites in a more realistic fully coupled thermal-solute condition, capturing for the first time fine scale features such as dendrite splitting.