查看更多>>摘要：Intermetallic compounds have the characteristics of long-range ordered structure and combination of metallic and covalent bonds,showing intrinsic brittleness and outstanding performance stability.The synthesis mechanism,pore structure characterization and material properties of powder metallurgy porous intermetallics are reviewed in this paper.Compared with traditional porous materials,porous intermetallics have good thermal impact resistance,machinability,thermal and electrical conductivity similar to metals,as well as good chemical corrosion resistance,rigidity and high-temperature property similar to ceramics.The mechanisms of preparation and pore formation of porous intermetallics mainly include four aspects: (1) the physical process based on the interstitial space between the initial particles and its evolution in the subsequent procedures;(2) the chemical combustion process based on the violent reaction between the initial powder components;(3) the reaction kinetics process based on the difference between the diffusion rates of elements;(4) the phase transition process based on the difference between the phase densities.The characterization parameters to the pore structure description for porous intermetallics include mainly overall porosity,open porosity,permeability,maximum pore size,pore size distribution and tortuosity factor.In terms of microstructure characterization of porous intermetallics,three-dimensional pore morphology scanning technology has the potential to reveal the internal characteristics of pore structures.The research on material properties of porous intermetallics mainly focuses on electrochemical catalytic activity,generalized oxidation resistivity at high temperature,resistance against chemical corrosion and mechanical properties,which have obvious advantages over traditional porous materials.In the field of the development of porous intermetallics,it is expected to expand their applications by further reducing the pore size to the nanoscale level to improve the filtration accuracy or increase the specific surface area,as well as introducing the high entropy design on the composition to improve the brittleness and enhance their material performance.
查看更多>>摘要：Electromagnetic wave (EMW) absorbing materials play a vital role in modern communication and information processing technologies to inhibit information leakage and prevent possible damages to environment and human bodies.Currently,most of EMW absorbing materials are either composites of two or more phases or in the form of nanosheets,nanowires or nanofibers in order to enhance the EMW absorption performance through dielectric loss,magnetic loss and dielectric/magnetic loss coupling.However,the combination of complex shapes/multi phases and nanosizes may compound the difficulties of materials processing,composition and interfaces control as well as performance maintenance during service.Thus,searching for single phase materials with good stability and superior EMW absorbing properties is appealing.To achieve this goal,the EMW absorbing properties of transition metal carbides TMCs (TM=Ti,Zr,Hf,Nb and Ta) and high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C which belong to ultrahigh temperature ceramics,were investigated in this work.Due to the good electrical conductivity and splitting ofd orbitals into lower energy t2g level and higher energy eg level in TMC6 octahedral arrangement,TMCs (TM=Ti,Zr,Hf,Nb and Ta) exhibit good EMW absorbing properties.Especially,HfC and TaC exhibit superior EMW absorbing properties.The minimum reflection loss (RLmin) value of HfC is -55.8 dB at 6.0 GHz with the thickness of 3.8 mm and the effective absorption bandwidth (EAB) is 6.0 GHz from 12.0 to 18.0 GHz at thickness of 1.9 mm;the RLmin value of TaC reaches-41.1 dB at 16.2 GHz with a thickness of 2.0 mm and the EAB is 6.1 GHz with a thickness of 2.2 mm.Intriguingly,the electromagnetic parameters,i.e.,complex permittivity and permeability are tunable by forming single phase solid solution or high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C.The RLmin value of high entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C is -38.5 dB at 9.5 GHz with the thickness of 1.9 mm,and the EAB is 2.3 GHz (from 11.3 to 13.6 GHz) atthickness of 1.5 mm.The significance of this work is that it opens a new window to design single phase high performance EMW absorbing materials by dielectric/magnetic loss coupling through tuning the conductivity and crystal field splitting energy of d orbitals of transition metals in carbides,nitrides and possibly borides.
查看更多>>摘要：Electrolyte formulation with high stability towards both Li metal anode and high-voltage cathode is considered as one of key points for the high-energy density lithium metal batteries (LMBs).In our previous study,by adding only 2％ of 2-fluoropyridine (2-FP) as the additive in the carbonate and ether-based electrolyte formulations effectively suppressed Li dendrite growth.In this study,we further found that the main fluoropyridine (FP) family members can serve as not only the effective additive but also the excellent electrolyte solvent in the electrolyte formulations to enhance the performance of LMBs.For the 2-FP,when it was also used the electrolyte solvent and paired with single-salt lithium bis(trifluoromethylsulfonyl)imide (LiTFSI),the obtained electrolyte formulation of 1 M LiTFSI in pure 2-FP solvent not only allowed faster ion transport though solvation effect,but also possessed impressive oxidation stability window over 4.3 V.As a result,the high-voltage LiNi1/3Mn1/3Co1/3O2 (1.5 mA h cm-2)|Li metal battery with it exhibited a capacity retention of more than 80 ％ over a long-term cycle even at 0.45 mA cm-2 with a lean electrolyte (30 μL).Meanwhile,for another FP family member (i.e.,3-FP) as the electrolyte additive,the 4.3 V LMBs with the carbonate-based electrolyte containing only 1 ％ of 3-FP maintained 83.9 ％ of initial capacity after 200 cycles at 0.75 mA cm-2.Density functional theory (DFT) calculations and experiments confirmed that three typical FPs,i.e.,2-FP,3-FP and 4-FP can not only regulate the initial Li nucleation process,but more importantly also induce a protective layer,leading to a uniform and dendrites-free Li deposition.This bifunction of the FP family member as either electrolyte solvent or additive in the electrolyte formulations should be promising for the achieving of dendrites-free high-energy density LMBs.
查看更多>>摘要：Graphitic carbon nitride (g-C3 N4) with the merits of high visible light absorption,proper electronic band structure with high conduction band edge and variable modulation,is viewed as a promising photocatalyst for practical use.To alleviate its high recombination rate of photo-excited charge carriers and maximize the photocatalytic performances,it is paramount to design highly effective transfer channels for photo-excited charge carriers.Ferroelectric materials can have the charge carriers transport in opposite directions owing to the internal spontaneous polarization,which may be suitable for constructing the heterostructure with g-C3N4 for efficient charge separation.Inspired by this concept,herein ferroelec tric PbTiO3,which can be the visible-light absorber,is coupled with g-C3N4 to construct PbTiO3/g-C3N4 heterostructure with close contact via Pb-N bond by the facile post thermal treatment.The optimized PbTiO3/g-C3N4 heterostructure exhibited excellent photocatalytic and photoelectrochemical activities under visible light irradiation.Moreover,the simultaneous application of ultrasound-induced mechanical waves can further improve its photocatalytic activities through reinforcing the built-in piezoelectric field.This work proposes a widely applicable strategy for the fabrication of high-performance ferroelectric based photocatalysts and also provides some new ideas for developing the understanding of ferroelectric photocatalysis.
查看更多>>摘要：Additive manufacturing is a very promising manufacturing method widely used in various industries.In this study,for the first time,a new type of combined cable wire (CCW) with multi-element composition has been designed and developed for arc additive manufacturing (AAM) of non-equiatomic Al-Co-Cr-Fe-Ni high-entropy alloy.CCW composed of 7 filaments and 5 elements has the advantages of high deposition efficiency,self-rotation of welding arc and energy saving capability.Thin HEA walls were fabricated under pure argon gas using cold metal transfer technology.Microstructural observations of the developed HEA reveal (i) BCC and FCC phases,(ii) Good bonding between layers and (iii) defect-free microstructure.The developed alloy exhibits high compression strength (～2.8 GPa) coupled with high plastic strain (～42 ％) values (possess both strength and ductility).It has been identified that by varying the heat input via torch travel speed,the microstructure and mechanical properties of the HEA can be controlled.From this feasibility study,it has been proved that the innovative CCW method can be used to manufacture HEAs with CCW-AAM.Further,the study highlights the advantage of the rapid cooling involved in the CCW-AAM process which gives rise to superior mechanical properties.
查看更多>>摘要：Investigation about the corrosion behavior of Ti alloys in different ambient environment is of great significance for their practical application.Herein,we systematically investigate the corrosion behavior of a newfound Ti-6Al-3Nb-2Zr-1 Mo (Ti80) alloy in hydrochloric acid (HCI) ranging from 1.37 to 7 M,and temperature ranging from 25 to 55 ℃,by means of electrochemical measurements,static immersion tests and surface analysis.Results manifest that increasing either HCI concentration or temperature can accelerate the corrosion of Ti80 alloy via promoting the breakdown of native protective oxide film and then further facilitating the active dissolution of Ti80 matrix.According to potentiodynamic polarization curves,Ti80 alloy displays a spontaneous passive behavior in 1.37 M HCI at 25 ℃,compared to a typical active-passive behavior under the other conditions.As indicated by cathodic Tafel slope,the rate determining step for cathodic hydrogen evolution reaction is likely the discharge reaction step.The apparent activation energies obtained from corrosion current density and maximum anodic current density for Ti80 alloy in 5 M HCI solution are 62.4 and 55.6 kJ mol-1,respectively,which signifies that the rate determining step in the corrosion process of Ti80 alloy is mainly determined by surface-chemical reaction rather than diffusion.Besides,the electrochemical impedance spectroscopy tests demonstrate that a stable and compact oxide film exists in 1.37 M HCl at 25 ℃,whereas a porous corrosion product film forms under the other conditions.Overall,the critical HCI concentration at which Ti80 alloy can maintain passivation at 25 ℃ can be determined as a value between 1.37 and 3 M.Furthermore,the corroded surface morphology characterization reveals that equiaxed α phase is more susceptible to corrosion compared to intergranular β3 phase due to a lower content of Nb,Mo,and Zr in the former.
查看更多>>摘要：Microsegregation formed during solidification is of great importance to material properties.The conventional Lever rule and Scheil equation are widely used to predict solute segregation.However,these models always fail to predict the exact solute concentration at a high solid fraction because of theoretical assumptions.Here,the dynamics of microsegregation during polycrystalline solidification of refined Al-Cu alloy is studied via two-and three-dimensional quantitative phase-field simulations.Simulations with different grain refinement level,cooling rate,and solid diffusion coefficient demonstrate that solute segregation at the end of solidification (i.e.when the solid fraction is close to unit) is not strongly correlated to the grain morphology and back diffusion.These independences are in accordance with the Scheil equation which only relates to the solid fraction,but the model predicts a much higher liquid concentration than simulations.Accordingly,based on the quantitative phase-field simulations,a new analytical microsegregation model is derived.Unlike the Scheil equation or the Lever rule that respectively overestimates or underestimates the liquid concentration,the present model predicts the liquid concentration in a pretty good agreement with phase-field simulations,particularly at the late solidification stage.
查看更多>>摘要：Developing low-cost,efficient,and stable non-precious-metal electrocatalysts with controlled crystal structure,morphology and compositions are highly desirable for hydrogen and oxygen evolution reactions.Herein,a series of phosphorus-doped Fe7S8 nanowires integrated within carbon (P-Fe7S8@C) are rationally synthesized via a one-step phosphorization of one-dimensional (1D) Fe-based organicinorganic nanowires.The as-obtained P-Fe7S8@C catalysts with modified electronic configurations present typical porous structure,providing plentiful active sites for rapid reaction kinetics.Density functional calculations demonstrate that the doping Fe7S8 with P can effectively enhance the electron density of Fe7S8 around the Fermi level and weaken the Fe-H bonding,leading to the decrease of adsorption free energy barrier on active sites.As a result,the optimal catalyst of P-Fe7S8-600@C exhibits a relatively low overpotential of 136 mV for hydrogen evolution reaction (HER) to reach the current density of 10 mA/cm2,and a significantly low overpotential of 210 mV for oxygen evolution reaction (OER) at 20mA/cm2 in alkaline media.The work presented here may pave the way to design and synthesis of other prominent Fe-based catalysts for water splitting via electronic regulation.
查看更多>>摘要：Bimetallic additively manufactured structures (BAMSs) can replace traditionally-fabricated functionally-graded-components through fusion welding processes and can eliminate locally-deteriorated mechanical properties arising from post-processing.The present work fabricates a BAMS by sequentially depositing the austenitic stainless-steel and Inconel625 using a gas-metal-arc-welding (GMAW)-based wire + arc additive manufacturing (WAAM) system.Elemental mapping shows a smooth compositional transition at the interface without any segregation.Both materials being the face-center-cubic (FCC) austenite,the electron backscattered diffraction (EBSD) analysis of the interface shows the smooth and cross-interface-crystallographic growth of long-elongated grains in the <001> direction.The hardness values were within the range of 220-240 HV for both materials without a large deviation at the interface.Due to the controlled thermal history,mechanical testing yielded a consistent result with the ultimate tensile strength and elongation of 600 MPa and 40 ％,respectively,with the failure location on the stainless-steel side.This study demonstrates that WAAM has the potential to fabricate BAMS with controlled properties.
查看更多>>摘要：The key to develop high specific energy rechargeable batteries is development of new electrode materials.The existing electrode materials still have many problems: the shuttle effect and poor conductivity of the sulfur cathode,the inevitable volume expansion of the silicon anode and the lithium dendrite of the lithium metal anode that cause short circuits,etc.Nanofibers,as active electrical materials,conductive additives and electrode bodies,can play multiple roles in electrode design.More interestingly,nanofibers can be functionalized to obtain better controllable properties (i.e.,electrolyte affinity,pore size distribution and surface electronic structure),thereby further enhancing electrochemical performance.In this article,the latest research progress in electrode design based on nanofibers is reviewed,including processing methods,structure,morphology and electrochemical performance.The key problems affecting the electrochemical performance of the electrode are also discussed,such as the preparation process,atomic structure,electrical conductivity,surface area and pore distribution of nanofibers,to provide reference points for nanofibers in excellent electrode design.
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