Controlled moderative sulfidation-fabricated hierarchical heterogeneous nickel sulfides-based electrocatalyst with tripartite Mo doping for efficient oxygen evolution

An electrocatalyst with heterogeneous nanostructure, especially the hierarchical one, generally shows a more competitive activity than that of its single-component counterparts for oxygen evolution reaction(OER), due to the synergistically enhanced kinetics on enriched active sites and reconfigured electronic band structure. Here this work introduces hierarchical heterostructures into a NiMo@NiS/MoS_(2)@Ni_(2)S_(2)/MoO_(x)(NiMoS) composite by one-pot controlled moderative sulfidation. The optimal solvent composition and addition of NaOH enable NiMoS to own loose and porous structures, smaller nanoparticle sizes, optimal phase composition and chemical states of elements, improving the OER activity of NiMoS. To achieve current densities of 50 and 100 mA cm^(-1), small overpotentials of 275 and 306 mV are required respectively, together with a minor Tafel slope of 58 mV dec^(-1), which outperforms most reported sulfide catalysts and IrO_(2). The synergistic effects in the hierarchical heterostructures expose more active sites,adjust the electronic band structure, and enable the fast charge transfer kinetics, which construct an optimized local coordination environment for high OER electrocatalytic activity. Furthermore, the hierarchical heterostructures suppress the distinct lowering of electrical conductivity and collapse of pristine structures resulted from the metal oxidation and synchronous S leaching during OER, yielding competitive catalytic stability.

Microstructural,crystallographic,and mechanical characteristics in Ni-Mn-Ga alloys directionally solidified under a transverse magnetic field

In this study,the effect of transverse magnetic field-assisted directional solidification(MFADS)on the microstructures in Ni-Mn-Ga alloys has been investigated.The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field,causing the emergence of martensite clusters in the austenite matrix.Moreover,the magnetic field alleviates the microseg-regation on a dendritic scale and promotes the preferred growth of austenite dendrites.On the basis of the above investigation,several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation.The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles.The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized.In addition,the microsegregation on a dendritic scale can significantly influence the martensite variant distribution,corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor.The stress-induced superelasticity is closely dependent on orientation,well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain.The crystallographic evolution has been characterized during in-situ stress-induced transformation.The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.

Effect of the simultaneous application of a high static magnetic field and a low alternating current on grain structure and grain boundary of pure aluminum

Effect of the simultaneous application of a high static magnetic field and a low alternating electric current on the solidification structure of pure aluminum has been investigated. Results show that the refinement of the solidification structure is enhanced by the electric current under a certain magnetic field. However,when the magnetic field intensity exceeds a certain value, the refinement is impaired under a certain electric current. The observation by electron backscattered diffraction(EBSD) shows the complex fields have led to the increase of the low angle boundaries with the refinement. Moreover, the application of the static gradient magnetic field is capable of modifying the distribution of the refined grains. The above results may be attributed to the formation of the cavities during the electromagnetic vibration process and the high magnetic field.

Effect of transverse static magnetic field on radial microstructure of hypereutectic aluminum alloy during directional solidification

The effect of different scales thermoelectric magnetic convection(TEMC)on the radial solidification microstructure of hypereutectic Al alloy has been investigated under transverse static magnetic field during directional solidification,focusing on the formation of freckle.Our experimental and numerical simulation results indicate that the TEMC circulation at sample scale under transverse static magnetic field leads to the enrichment of solute Al on one side of the sample.The TEMC and the solute enrichment degree increase with the increase of magnetic field when the magnetic field increases to 0.5 T.The enrichment degree of solute elements under magnetic field is affected by temperature gradient and growth rate.The non-uniform distribution of solute Al in the radial direction of the sample results in the non-uniform distribution of primary dendrite arm spacing(PDAS).Moreover,the applied magnetic field can lead to freckle formation and its number increases with the increase of magnetic field.The change of freckle is consistent with the anisotropy TEMC caused by the anisotropy of primary dendrite or primary dendrite network under magnetic field.Finally,the mechanism of synergism effect of the anisotropy TEMC,the distribution of solute Al and the PDAS on freckle formation and evolution is studied during directional solidification under magnetic field.

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field(HMF)on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis(DTA).It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heatingcooling rates.Moreover,the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc.Regardless of magnetism,the nucleation frequency under an HMF is higher than that without an HMF.Furthermore,the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle,calculated by the functional relationship between the cooling rate and undercooling.This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc.Additionally,the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.

Controlling and adjusting the concentration distribution during solidification process using static magnetic fields

As concentration distribution changes have important effects on material structures and properties,controlling the concentration distribution is essential to alloy performance.The aim of the present work is to control and adjust the concentration distribution by the static magnetic field.It is found that the magnetic field disperses grain boundary segregation and causes the uniform distribution of concentration.Further,by the three-dimensional computed tomography(3 D-CT) reconstruction,the flow distribution is seen and the effect mechanism of the magnetic field is revealed.The present work may clarify the ambiguous understanding on the effect of the static magnetic field on solidification process.

Effect of vertical electromagnetic stirring on solute distribution in billet continuous casting process

A volume averaged columnar solidification model,which couples the flow,temperature and solute concentration fields,is applied to simulate experimental continuous casting cases with and without vertical electromagnetic stirring(V-EMS).The calculated distribution of magnetic induction intensity and final macrosegregation maps are consistent with the experimental results.Calculation results reveal that the V-EMS promotes longitudinal melt flow,accelerates heat dissipation and solidification and finally reduces the central segregation of carbon.However,when V-EMS is applied,the solute distribution becomes asymmetric because the melt flow shows opposite directions between the near and far sides from stirrer.An obvious positive segregation band is observed at about 1/4 width of the billet near the stirrer in both calculated and experimental results.The position and degree of such positive segregation could be affected by installation height of stirrer,as demonstrated by additional simulation cases.

Microstructure evolution and mechanical behavior of Ni-rich Ni-Mn-Ga alloys under compressive and tensile stresses

The microstructure evolution and mechanical behavior in directionally solidified Ni-rich Ni-Mn-Ga alloys with nominal compositions of Ni_(58)Mn_(25)Ga_(17) and Ni_(60)Mn_(25)Ga_(15) under compressive and tensile stresses have been investigated.The composition distribution shows the element Ni segregates in gamma phase,while elements Mn and Ga segregate in martensite phase.Furthermore,the microstructure orientation examined by electron backscatter diffraction(EBSD)indicates that beta phase has a preferred growth orientation of(001)_(A) in Ni_(58)Mn_(25)Ga_(17) alloys,while gamma phase has a preferred growth orientation of(001)_(γ) in Ni_(60)Mn_(25)Ga_(15) alloys.The fracture morphology suggests that the existence of ductile y phase can reduce the crack propagation and promote fracture strain,particularly in the Ni_(60)Mn_(25)Ga_(15) alloys.Finally,Schmid factor and deformation gradient tensor were calculated to well explain the crystallographic evolution during the detwinning under compressive and tensile stresses.The present findings not only elucidate the mechanism ofγphase on the mechanical behavior of Ni-rich Ni-Mn-Ga alloys,but also shed light on the composition design of high temperature Ni-Mn-Ga shape memory alloys.

Effect of a High Magnetic Field on the Microstructure During Directionally Solidified Sn-20wt.%Pb Hypoeutectic Alloy

The microstructures of Sn-20wt.%Pb hypoeutectic alloy directionally solidified under a longitudinal magnetic field were investigated.The results show that the application of a high magnetic field has a great influence on the morphology of primary β-Sn phase at a temperature gradient of G_L=52 K/cm.At a certain growth speed,with the increase of magnetic field intensity,the magnetic field causes the primary β-Sn phase irregular and to be deformed,further,the magnetic field promotes the columnar to equaixed transition(CET).Further,the thermoelectric magnetic force(TEMF) imposed on the dendrite under a high magnetic field has been calculated and the results show that the numerical magnitude of the TEMF during directional solidification under a 10 T high magnetic field is about 10~4N/m^3 and this force should be responsible for the occurrence of the CET in the Sn-Pb alloy.This may act as an experimental proof that the coupling of temperature gradient and high magnetic field will induce the occurrence of the CET in Sn-Pb alloy.Above phenomena may be attributed to the thermoelectric magnetic force(TEMF)in solid.

In-Situ Analysis of Bulk Solidifying the Hyper-Monotectic Alloy Under a Compound Electric-Magnetic Field by Physical Simulation

A physical simulation was carried out to investigate the realistic experiment of bulk solidifying the Zn-Bi hyper-monotectic alloy under various compound electric-magnetic fields(CEMF).For this experiment,two crucial parameters determinate the cast microstructure,the one is electric-magnetic force(EMF)and the other is the frequency of AC current.Results show that the minor phase could be mixed in the other phase from the initial layered structure when the EMF above a specific value under fixed frequency,and the average diameter of minor phase droplet decreases with increasing EMF.The evolution of the liquid phases structure is reasonable agree with the realistic experiment of Zn-Bi hyper-monotectic alloy,which suggests that the mechanism revealed by the physical simulation could represent the one in the realistic experiment.

Effect of γ phase on mechanical behavior and detwinning evolution of directionally solidified Ni-Mn-Ga alloys under uniaxial compression

Specimens with single-phase martensite, net-like γ/martensite mixed structure and lamella-likeγ/martensite mixed structure were designed to investigate the effect of the γ phase on the mechanical behavior and the detwinning of non-modulated(NM) martensitic variant in Ni-rich Ni-Mn-Ga alloys under uniaxial compression. It can be found the existence of the γ phase significantly enhances the compressive stresses, and the net-like γ phase specimen presents a higher value of compressive strain than that of the lamella-like γ phase specimen. Especially, the detwinning plateau of the lamella-like γ phase specimen is almost invisible due to the martensite colonies with low Schmid factors. Finally, according to the calculation of deformation gradient tensor, we found that the tensors along compression direction(εzz) of net-like γ phase/martensite mixed structure specimen and single-phase martensite specimen are lower than that of the specimen with lamella-like γ phase/martensite mixed structure, which well explained the detwinning strain for these specimens. The present study not only highlights the role ofγ phase on the mechanical behavior, but also provides more guidelines for the mechanical training of Ni-Mn-Ga shape memory alloys.

Columnar-to-Equiaxed Transition During Directional Solidification Under Strong Magnetic Field

Effects of strong magnetic fields on the columnar-to-equiaxed transition(CET) have been investigated experimentally.Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET.The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically.Numerical results reveal that a torque is created on cells/dendrites and equiaxed grains and the value of the thermoelectric magnetic force increases as the magnetic field intensity increase.This torque breaks cells/dendrites and drives the rotation of equiaxed grains.As a consequence,the CET will occur during directional solidification under a strong magnetic field.This may initiate a new method to induce the CET via an applied strong magnetic field during directional solidification.