Non-dendritic Structure Research Articles

Anion-Driven Nanostructuring of Copper Electrodeposits for Tuning the Selectivity of Electrocatalytic CO2 Reduction Reaction

Electrochemical CO2 reduction (ECR) to value-added products is one of the potential ways to utilise CO2 as a feedstock, thereby decreasing its level in the atmosphere as it has harmful repercussions on planet earth. Copper (Cu)-nanostructures have demonstrated a great potential to convert CO2 into valuable higher-end hydrocarbons electrochemically, but with poor selectivity1. Therefore, novel strategies to tune Cu-based electrocatalysts' activity and selectivity toward multi-carbon products, particularly at low overpotential, are highly desirable. At the same time, keeping in mind the growing concerns regarding the environmental impact of the synthetic procedures and atom-economy among chemists, the synthesis methods employing reagents with multiple characteristics are equally essential to ensure minimal use of undesirable and excess reagents. In the present work, we report an atom-economic strategy to tune the physicochemical properties and the electrocatalytic activity of Cu-nanostructures towards ECR. The Cu-nanostructures namely CA, CN, CS and CC were synthesized via pulse electrodeposition from an electrolyte bath containing Cu-precursor salts with varying anions viz. acetate, nitrate, sulphate, and chloride respectively and are investigated for the effect of anions on the physicochemical properties and ECR performance. In the case of Cu electrodeposition process, when the pH is low as in the case of sulphate and acetate ions, electrodeposition is accompanied by hydrogen evolution reaction2 which results in the formation of nondendritic structures as in the case of CS and CA. But the presence of nitrate ions in the electrolyte bath causes nitrate reduction reaction to take place as a side reaction as well, as Cu is a well-known catalyst for nitrate reduction. Thus, this simultaneous redox reaction at the surface of the Cu electrodeposit causes an additional dissolution of the Cu nuclei3. This results in a smoother morphology as seen in CN as the dendrite formation is overridden. The very distinct cubic morphology of CC is due to Chloride ions in the bath.The presence of chloride ions during the electrodeposition of CC samples results in the formation of nanocubes by reducing Cu2+ via the formation of CuCl which is known to exist in cubic form when it precipitates4. The intermediate CuCl then quickly forms Cu2O through an equilibrium reaction in the presence of H2O when the concentration of chloride ions is not so high. The Cu-nanostructures electrodeposited from Cu-chloride demonstrated the best ECR performance with the highest total faradaic efficiency for carbon products and good selectivity for ethylene formation (30% at -0.93V vs RHE). The presence of chloride ions in the electrolyte bath strongly affects the active sites of the Cu deposits as the cubic morphology of CC catalyst exposes large number of Cu (100) faces5. Cu (100) are highly selective towards ethylene formation with very low overpotential due to the lower barrier for Cu-CO* which leads to greater CO coverage favouring CO*-CO* coupling at low bias59. In addition to that the electrochemically active surface area, the higher Cu+ content can be rationalized as the reasons for relatively higher ECR activity of CC.

Управління структурою і властивостями ливарного алюмінієвого сплаву АМ4.5Кд (ВАЛ10) модифікуванням дрібнокристалічними лігатурами

To management the structure, mechanical and operational properties of the high-strength cast aluminum alloy АМ4.5Кд (ВАЛ10), the work uses a modification method based on the principle of structural inheritance, using rapidly cooled (Vcool. ≥ 10^5 °С/s) fine-crystal ligatures AlTi5 and AlZr10, and as well as ligatures of the chemical composition of the base alloy with nanoscale size of intermetallics and Alα crystals. Studies have shown that the introduction of fine-crystalline additives into the melt leads to a transition from a dendritic to a non-dendritic structure, a significant decrease in the size of the crystals of the Alα solid solution, and an increase in its degree of supersaturation. The microstructure becomes more uniform - the difference between the maximum and minimum size of the grains decreases. The most effective reduction of the grain size and the transition from dendritic to non-dendritic structure of the aluminum solid solution at increased cooling rates occurs when alloying with fine-crystal AlTi5 ligature introduced into the melt in terms of pure titanium 0.05-0.15 wt. %. We must think that the main factor of modification by rapidly cooled ligatures is the introduction of a large number of additional crystallization centers into the melt. After T6 heat treatment, the highest strength of AM4.5Kd alloy (VAL10) is achieved when modified with AlZr10 ligature, in particular, with a mass fraction of zirconium of 0.25%. Probably, this is mainly due to the expansion of the region of the solid solution of copper and zirconium in aluminum during high-speed cooling and its subsequent disintegration during heat treatment with the release of strengthening nano-sized CuAl2 and Al3Zr phases. Tribological studies of AM4.5Kd alloy (VAL10) were carried out. The alloy modified with fine crystal ligature of the base alloy composition in the amount of 12 wt.% has the highest wear resistance. Keywords: fine crystal ligatures, AM4.5Kd (VAL10), modification, microstructure, strength, wear resistance.

The non-dendritic microstructure arising from grain boundary formation and wetting: A phase-field simulation and experimental investigation of semi-solid deformation

The semi-solid metal forming process can prevent the formation of unpopular dendritic microstructures but the mechanism remains unclear. To explore the underlying causes, a challenging phase-field simulation was performed, supported by a semi-solid experiment. The simulation managed to couple dendritic growth with melt flow and solid deformation, a process that few existing models or in situ experimental techniques could analyze. This study found that under deformation, grain boundary formation and wetting play a pivotal role in the formation of non-dendritic structures: it is the grain boundary formation and wetting that split the bending side-branch from dendrite trunk; the detached side-branch could then evolve into a non-dendritic morphology. The simulation showed that wetting could finish almost immediately after a moderate bending. This process appears similar to but is essentially distinct from the conventional explanation that describes dendrite fragmentation as mechanical breakage.

Одержання виливків із доевтектичних силумінів способом тиксолиття

Experimental studies of the method of thixocasting of AK7ч hypoeutectic silumin using cylindrical billets with different initial prepared structure (dendritic and non-dendritic – rosette-like and globular) of the primary solid phase were carried out. It is shown that after casting using billets with the initial dendritic structure of aluminum solid solution, the structure in shaped castings remains dendritic. At the same time, a certain number of destroyed dendrites are observed in the structure, predominantly melted. The thixocasting of the hypoeutectic silumin AK7ч using billets with the initial rosette-like structure of primary aluminum crystals ensures the production of shaped castings with a globular structure of the primary phase with a globular size of 80–100 μm. At the same time resulting the globular structure of the primary solid phase in castings is homogeneous and evenly distributed over the entire cross section. Thixocasting of silumin AK7ч using billets with the initial globular structure of the primary solid phase does not lead to a change of the structure of the casting. This slightly increases the size of the globules in the casting. Eutectic colonies in shaped castings obtained by thixocasting are characterized by a higher degree of differentiation compared to the initial structures of billets. Determination of mechanical properties showed an increased plasticity of the investigated hypoeutectic silumin in castings obtained after thixocasting without additional hardening heat treatment. Keywords: thixocasting, hypoeutectic silumin, non-dendritic structure, globular structure, rosette-like structure, solid-liquid billet, shaped casting.

In-situ regulation of zinc metal surface for Dendrite-Free Zinc-ion hybrid supercapacitors

Aqueous Zn-ion hybrid supercapacitors (ZHSCs) combine the high power density of supercapacitors with the high energy density of batteries. However, the growth of dendrite and spontaneous Zn corrosion damage the service life of ZHSCs and further seriously restrict their large-scale application in energy storage fields. Herein, we study the corrosion mechanism of Zn metal in ZnSO4 solution and report a method for the modification of Zn anode with zeolitic imidazolate framework-8 (ZIF-8) layer grown in-situ. The obtained ZIF-8 layer with hydrophobicity can not only inhibit Zn corrosion and induce Zn electroplating/peeling on the Zn surface, but also promote uniform deposition of Zn during charge/discharge processes. As a result, the prepared dendrite-free Zn electrode shows low polarization (89.0 mV at 10 mA cm−2) and high cycling stability (over 800 h at 10 mA cm−2), and the developed ZHSC maintains a high capacity retention of 96% after 13,000 cycles at 5 A g−1. Furthermore, a solar power supply system is assembled by the ZHSC and monocrystalline silicon plates to certify the utility of the device, and it can light a LED successfully. This study provides a simple and inexpensive strategy to manipulate the Zn electrodeposition behavior from dendrites to non-dendritic structures, which builds a way for the development of practical ZHSCs with mild electrolytes.

A phase-field simulation of the solidification process under compression

Semi-solid metal forming processes cover a wide variety of technologies in manufacturing, generally leading to non-dendritic microstructures typified by rosette-like or spherical grains. With the contemporary experimental methods, researchers still cannot completely understand the formation of such microstructures. However, as a process coupled with dendritic growth, deformation and flow, the microstructural evolution during the semi-solid deformation has been scarcely simulated. With the vector-valued phase-field method, two-phase flow model and an innovative design of boundary conditions, the current research numerically compressed a single dendrite into a less dendritic polycrystalline structure. The resulting microstructure was consistent with the metallographic analysis and the simulation enables a clear observation of the whole process: the ripening transformed the highly branched shape into a less dendritic one; the subsequent deformation crushed the dendrite into a stubby structure; eventually, the deformation caused the boundaries inside this structure, dividing the solid into multiple grains. The analysis on the boundaries inside the deformed solid offered further insight into the formation of non-dendritic structures. These boundaries showed a tendency towards melting with increasing misorientation, demonstrating the possibility of the grain boundary wetting, which was considered as the source of the fragmentation when the dendrite undergoes deformation, but is hard to validate in situ even with the cutting-edge X-ray imaging technique.

Effects of pouring temperature and electromagnetic stirring on semi-solid microstructure of hypoeutectic Mg2Si/Al composite

In this paper, hypoeutectic Al-Si matrix composite reinforced by 5wt. % Mg2 Si were prepared under different pouring temperature (superheat) and electromagnetic stirring (EMS), and the effects of pouring temperature and EMS on the morphology a nd size distribution of primary α-Al, (α-Al+Mg2Si) eutectic cell and eutectic Mg2Si are investigated. The experimental results show that the low superheat + EMS process not only refines the grain structure of the primary α-Al and (α-Al+Mg2Si) eutectic cell, an d promotes the formation of more non-dendritic structure of the phases, but also refine s the eutectic Mg2Si and changes the nucleation mode of (α-Al+Mg2Si) binary eutectic from the nucleation on primary α-Al under conventional casting to the one on nuclean t particles in the interdendritic liquid under low superheat + EMS.

Development and mechanical characterisation of Al-4.5%Cu alloy prepared using spark plasma sintered method

ABSTRACT Spark plasma sintering of Al-Cu composites yields a refined low segregation microstructure and the conversion of spherical grains to irregular non-dendritic structures during the cooling phase of the process. Al-4.5Cu alloy powder with an average particle size of 210 to 250 µm was gas atomised in an inert gas for physical and mechanical characterisations. The alloy powder was compacted into cylindrical specimens of 30 mm diameter and 20 mm thickness. The Al-4.5 wt. % Cu alloy powder was filled in enclosed graphite die and with the application of pressure and heat, the sintering samples were developed. For microstructural characterisation and mechanical properties, test samples were prepared by using Electrical discharge machining wire-cutting machine according to ASTM standards. The density of the alloy was improved in the range of 4.7% to 7.8% and the hardness value was improved by more than 100% when compared to the conventionally sintered Al-4.5Cu alloy. The compressive strength of the spark plasma sintered alloy is much higher than the conventionally sintered alloy samples by 307.32%, 150.15% and 47.45%.

Effect of cooling slope casting and partial remelting treatment on microstructure and mechanical properties of A319-xMg2Si In-Situ composites

The present study is aimed at evaluating the effect of cooling slope casting and partial remelting on hypoeutectic A319- 8Mg2Si and hypereutectic A319- 16 Mg2Si in-situ composites on the microstructural evolution and mechanical and wear properties. The characteristics are compared with those of conventional gravity castings. The microstructural features have been characterized by optical and scanning electron microscopy and X-ray diffraction analysis. Mechanical properties viz. bulk hardness, tensile properties and dry sliding wear characteristics with change in applied normal loads have been evaluated. The fractographs and worn surfaces are examined under scanning electron microscope. From the experimental findings it is observed that, the cooling slope casting can effectively refine and globularize α-Al grains, primary and eutectic Mg2Si and promote non-dendritic structures. Partial remelting further globularizes the matrix structures but develops microporosities. Improved hardness, tensile strength and dry sliding wear properties are obtained in cooling slope castings and in combined cooling slope casting with partial remelting. The improved properties are attributed to refinement and globularization of microstructural constituents with non-dendritic structures. Positive effects of microstructural features overshadowed the negative effect of porosity to the improvement in properties.

Phase field lattice Boltzmann model for non-dendritic structure formation in aluminum alloy from LSPSF machine

The formation of non-dendritic structures in the primary phase of an aluminum alloy solidified using low superheat pouring with a shearing field (LSPSF) machine was investigated by numerical simulation. The growth and motion of a dendrite during solidification was simulated by a combination of the lattice Boltzmann method and the phase field method. The simulation results indicated that enough shear flow helped homogenize the concentration fields, rotate crystals and altere microstructures from dendritic to non-dendritic. The interaction of grains was also discussed. A fragmentation criterion was established based on partial remelting of dendrite arms; fragmentation was enhanced by a strong shear flow and larger inclined angles. The simulation results were verified experimentally.

Numerical simulation of equiaxed growth of Ni-based alloy in multi-directional flowing melt

A model for crystal growth in multi-directional flowing melt was built to study the crystal morphology evolution during solidification under forced convection. Based on the model, the formation of non-dendritic structure of Ni-based alloy solidified with forced convection was numerically investigated by the cellular automata method. The results indicate that the crystal morphology strongly depends on the flow directions. The change of flow directions brings a much different condition for crystal growth, including heat transfer and solute diffusion in front of the solid-liquid interface, which eventually results in the formation of non-dendritic morphology. Besides, it is found that the influence of forced convection on the crystal growth is closely related to the cooling condition. The forced convection has to be intensive enough to achieve the non-dendritic morphology for the solidification condition with high cooling rate.

Correction to: Numerical Simulation and Experimental Validation of Nondendritic Structure Formation in Magnesium Alloy Under Oscillation and Ultrasonic Vibration

In this study, the formation of nondendritic structures in the primary phase of magnesium alloy solidified under oscillation and ultrasonic vibration was investigated by numerical simulation and experimentally. The growth and motion of a dendrite during solidification was simulated by a combination of the lattice Boltzmann method and the phase-field method. The simulation and experimental results indicated that higher oscillation amplitudes and acoustic streaming made the microstructures change from dendritic to nondendritic in the α-Mg primary phases. A sufficient shear stress and an appropriate flow time in the barrel should be satisfied when a given inclined angle is selected. The effects of the flow and the thermal field on the nucleation behavior, the constitutional undercooling, and the growth morphology are also discussed. It was found that a high shear rate and high turbulence can help homogenize the temperature and the concentration fields and collide and rotate the α-Mg primary phases.

Cooling Behavior and Microstructure of Semisolid A201 Aluminum Alloy Prepared by the SEED Process

The biggest challenge in semisolid processing of high-performance aluminum alloys is the narrow temperature processing windows of these alloys, and as a result, the preparation of qualified semisolid slurries is very important. High solid fraction slurries of high-strength A201 alloy were prepared by the Swirled Enthalpy Equilibration Device (SEED) process. The cooling behavior and microstructures of the A201 slurries produced by the standard, as well as a modified, SEED process were investigated. The results show that qualified A201 slurry can be produced by decreasing the pouring temperature and controlling the processing time in the SEED process. The modified SEED process significantly reduced the radial temperature gradient of the melt, due to the slow cooling rates involved, with the resulting slurries being more uniform, with more spherical microstructures, as compared to those produced by the standard SEED process. The formation of the nondendritic grain structure in the SEED process is attributed to the uniformly distributed large number of nuclei within the melt and the slow cooling of the melt in the containing crucible.

Numerical Simulation and Experimental Validation of Nondendritic Structure Formation in Magnesium Alloy Under Oscillation and Ultrasonic Vibration

In this study, the formation of nondendritic structures in the primary phase of magnesium alloy solidified under oscillation and ultrasonic vibration was investigated by numerical simulation and experimentally. The growth and motion of a dendrite during solidification was simulated by a combination of the lattice Boltzmann method and the phase-field method. The simulation and experimental results indicated that higher oscillation amplitudes and acoustic streaming made the microstructures change from dendritic to nondendritic in the α-Mg primary phases. A sufficient shear stress and an appropriate flow time in the barrel should be satisfied when a given inclined angle is selected. The effects of the flow and the thermal field on the nucleation behavior, the constitutional undercooling, and the growth morphology are also discussed. It was found that a high shear rate and high turbulence can help homogenize the temperature and the concentration fields and collide and rotate the α-Mg primary phases.

Semisolid Casting of Short Freezing Range Alloys

Semisolid casting and non-dendritic solidification of commercially pure tin (about 1 °C freezing range) and Zamak 3 alloy (about 10 °C freezing range) by a modified serpentine channel method were studied. It was shown that semisolid casting of very small freezing range metals with a non-dendritic structure was possible using this method. The results showed that the wall of the copper serpentine channel mold acted as a substrate for heterogeneous copious nucleation of primary solid particles and the channel provided sufficient self-steering action to disperse the nuclei in the melt. The average diameter and shape factor of the primary particles in the semisolid cast CP-Sn sample was measured to be about 107 μm and 0.75, respectively. The average diameter and shape factor of the primary particles in the semisolid cast Zamak 3 alloy was measured to be about 16 μm and 0.8, respectively. Hardness of semisolid samples was slightly higher than those of conventional gravity cast samples.

Preparation of mesoporous silica nanoparticles with controlled pore size, particle diameter, morphology, and structure by two-step process of chlorosilane residue

In this study, mesoporous silica nanoparticles (MSNs) were successfully prepared in two steps using chlorosilane residue. First, chlorosilane residue was subjected to alcoholysis with n-propanol to synthesize tetrapropoxysilane (TPOS), followed by the synthesis of MSNs using TPOS as silicon source. Alcoholysis experiment was designed and optimized using orthogonal experimental method. In addition, effects of four factors (i.e., molar ratio of n-propanol to chlorosilane residue; feed rate of n-propanol; reaction temperature; and molar ratio of n-hexane to chlorosilane residue) on the yield of TPOS were investigated. The optimum alcoholysis conditions were determined. Yield of TPOS was 56% under the optimum alcoholysis conditions. Results obtained from single factor analysis of variance revealed that molar ratio of n-propanol to chlorosilane residue was the most significant factor affecting TPOS yield. Next, a series of spherical MSNs with dendritic structure and irregular mesoporous/microporous silica nanoparticles with non-dendritic structure were successfully synthesized via precise regulation of various experimental parameters in emulsion systems. In this regard, the adjustment of n-hexanol/1,3,5-triisopropylbenzene (TIPB) molar ratio or n-octane/n-hexanol/TIPB molar ratio was found to be effective for achieving controllable regulation of morphology (spherical or irregular); structure (dendritic or non-dendritic); pore size (mesoporous (4.6–9.2 nm) or microporous (1.6–1.7 nm)); and particle diameter (60–134 nm) of silica nanoparticles. However, the adjustment of n-octane/TIPB molar ratio marginally affected spherical morphology, dendritic structure, and pore size (4.0–4.6 nm) of synthesized MSNs, with considerable effect on particle diameter (61–151 nm). In addition, as-synthesized MSNs exhibited large specific surface area (708–857 m2/g) and large pore volume (1.5–3.6 cm3/g).

The effects of heat treatment on microstructure and mechanical properties of rheocasting ADC12 aluminum alloy

The effects of heat treatment on microstructure and mechanical properties of rheocasting ADC12 aluminium alloy produced by gas induced semi-solid (GISS) technique were studied. The solution heat treatment performed at the temperature of 520°C under various solution treatment times for 2, 4, 6, 8, 10, and 12 h and followed by water quenching at room temperature. The results show that the optimum solution heat treatment condition for the non-dendritic structure of ADC12 aluminium alloy was 520°C for 8 h. The age hardening was also carried out at 170°C for different aging times of 6, 8, and 10 h, respectively. It was found that the artificial aging at 170°C for 6 h was sufficient to achieve the highest hardness of 73.2 HRB. This was due to the appropriate refinement of acicular eutectic structure.

Effects of stirring parameters on the rheocast microstructure and mechanical properties of aluminum alloy ADC12

In this study, effects of stirring time and stirring speed on the microstructure of semisolid rheocast (SSR) aluminum alloy ADC12 were examined. The research method was done by gravity casting using a metal mold. The aluminum ADC12 slurry is stirred by a mechanical stirrer (round rod stirrer) at 610OC with a variation of speed 0, 250, 300, 350 rpm for 0, 20, 40, 60 seconds. Furthermore, the aluminum slurry of ADC12 is poured on a metal mold with temperature 600OC. The microstructure characteristics were examined by direct observation using optical microscopy, secondary -Al phase dendrite arm spacing, shape factor, and Si eutectic phase were identified. The mechanical properties were investigated by hardness test and tensile test. A short period of stirring below the liquidus temperature to form a non-dendritic structure. The final morphology of the primary particles after a short period of stirring time has little impact as the stirring time increases. The optimal mechanical properties (hardness and tensile strength) were obtained at 20 seconds of stirring time and 300 rpm of stirring rate.

Numerical simulation of non-dendritic structure formation in Mg-Al alloy solidified with ultrasonic field

The formation of non-dendritic structure of Mg alloy solidified with ultrasonic treatment was investigated by numerical simulation and experiment. The models of nucleation and crystal growth involved the effects of ultrasonic cavitation and acoustic streaming were built. Based on the models, the grain refinement and the microstructure change from dendrite to non-dendritic structure of a Mg-Al alloy were numerically simulated by cellular automata method. The simulation and experimental results indicated that the ultrasonic cavitation strongly contributes to the grain refinement by improving nucleation, while the acoustic streaming is mainly responsible for the formation of non-dendritic structure.

Novel casting TiAl alloy with fine microstructure and excellent performance assisted by ultrasonic melt treatment

A novel casting TiAl alloy with fine homogeneous microstructure and excellent performances is obtained assisted by the ultrasonic melt treatment. The coarse columnar crystal is modified into non-dendritic structure and the large grain is refined from 3098μm to 88μm; the micro-segregation, micro-porosities and precipitates are eliminated as well. After ultrasonic melt treatment for 90s, the highest compressive strength is improved up to 1705MPa, about 12% higher than the “quasi-oriented” TiAl alloy.