查看更多>>摘要：In this study,we developed a facile method to fabricate three-dimensional(3D)structures composed of FeNi alloy nanoparticles encapsulated in N-doped carbon nanotubes that grafted on the SiO2 spheres(FexNiy@NCNT@SiO2)for electromagnetic wave(EMW)absorption.The experimental results suggest that the impedance matching characteristic can be tuned by the introduction of SiO2 spheres in the 3D struc-ture.Density functional theory(DFT)calculations showed that the introduction of Ni improved the polar-ization and conductive losses of the FexNiy@NCNT@SiO2.As a result,the optimal 3D structure exhibits ex-cellent EMW absorption property with a reflection loss and effective absorption bandwidth are-49.39 dB and 4.32 GHz,respectively,even though the matching thickness is only 1.6 mm,superior to most mag-netic carbon-based composites.Thus,our current approach opens up an effective way to the development of low-cost,high-performance EMW absorbers.
查看更多>>摘要：Metal-organic frameworks(MOFs)are considered as a novel type of microwave absorption(MA)material owing to the sufficient pore structure,diverse configurations,and easy-to-control magnetic properties.However,their evolution is limited by the imperfect impedance matching conditions caused by the un-desirable microstructure.Herein,two types of novel porous coral-like carbon/Co3O4 and carbon/Co com-posites have been effectively fabricated for the first time by a facile heat treatment process of precursor Co-MOF-71.The graphitization degree,magnetic property and MA ability of the product can be effort-lessly tuned by altering the heat treatment temperature of Co-MOF-71.Remarkably,S500(Co-MOF-71 calcined at 500℃)composite displays strong and multi-frequency absorption performance,whose min-imum reflection loss(RL)value achieves-36.4 dB with an absorbing thickness of 3.0 mm and attains an effective absorbing bandwidth(RL≤-10 dB)of 5.76 GHz(almost covers whole Ku band)at a thin-ner coating thickness of 2.5 mm.Such superb MA ability has roots in the coral-like structure derived from the layer Co-MOF-71,sufficient electromagnetic loss.This work ameliorates the MA ability of MOFs through a special nanostructural design,which provides a fresh way for the preparation of novel MA materials.
查看更多>>摘要：Solar-to-hydrogen conversion through photocatalysis is a sustainable and promising strategy for storing solar energy.Recently,elemental red phosphorus(RP)with broad light absorption has been recognized as a potential candidate for photocatalytic hydrogen evolution,while challenges remain due to the rapid recombination of photogenerated carriers.In this work,RP modified TiO2 hollow spheres were designed and fabricated through the chemical vapor deposition method.The optimal hydrogen production rate reaching 215.5 μmol/(g h)over TiO2@RP heterostructure was obtained under simulated solar light irra-diation.Experimental results evidenced that the hollow sphere structure and RP light absorber extended light absorption ability,and the heterostructure induced interfacial charge migration facilitated photoin-duced charge separation,which benefited the photocatalytic hydrogen production performance.
查看更多>>摘要：Our previous studies have demonstrated that underwater friction stir additive manufacturing(FSAM)could effectively suppress the macroscale softening of the fabricated Al-Zn-Mg-Cu alloy build from top to bottom.However,the accompanying local softening problem,i.e.,a low-hardness region at the bot-tom of each stir zone,becomes prominent.In this study,an Al-Zn-Mg alloy with low quench sensitivity was used to fabricate a multilayered build via underwater FSAM.In-process water cooling could effec-tively solve the macroscale and local softening problems in the FSAM of the Al-Zn-Mg alloy and improve the mechanical performance of the build.The microhardness and ultimate tensile strength(UTS)of the water-cooled build in as-fabricated and aged states were more uniform along the building direction and higher than those of their counterparts.After 90 days of natural aging,the UTS of the water-cooled build in building and traveling directions reached 398 and 400 MPa,respectively,slightly higher than that of the base metal(392 MPa).The enhancement in the mechanical performance of the water-cooled build was attributed to a high degree of supersaturation and age-strengthening ability because of a high cool-ing rate of the underwater FSAM process and low quench sensitivity of the base metal.
查看更多>>摘要：Dielectric polarization performance induced by defect engineering approach has been proved to be an ef-fective way for improving the microwave absorption property of carbon-based materials.Assisting by the solid-state reaction,the structural integrity of multi-walled carbon nanotubes(MWCNTs)would be bro-ken along with the volume expansion and etching process of cobalt oxides.Therefore,the defects could be obtained and result in the enhancement of microwave absorption property.Ascribing to the broken wall structures,the defect-distances(LD)and concentrations(nD)have been optimized to be 9.80 nm and 3.37×1011 cm-2.The minimum reflection loss(RL)had reached-54.6 dB at 4.5 GHz with a thickness of 4.13 mm and the corresponding effective absorption bandwidth(EAB＜-10 dB)was analyzed to be 14.6 GHz.Such enhancement is correlated to the dielectric polarization and the permeability-to-permittivity trans-formation raised from the defect structures and concentrations.The present work demonstrates an effec-tive strategy for tailoring the microwave absorption property of MWCNTs by engineering defect concen-trations,and could be further extended to other carbon-based absorbents.
查看更多>>摘要：Confined metal clusters as sub-nanometer reactors for electrocatalytic N2 reduction reaction(eNRR)have received increasing attention due to the unique metal-metal interaction and higher activity than single-atom catalysts.Herein,the inspiration of the superior capacitance and unique microenvironment with regular surface cavities of the porous boron nitride(p-BN)nanosheets,we systematically studied the catalytic activity for NRR of transition-metal single-clusters in the triplet form(V3,Fe3,Mo3 and W3)confined in the surface cavities of the p-BN sheets by spin-polarized density functional theory(DFT)calculations.After a two-step screening strategy,Mo3@p-BN was found to have high catalytic activity and selectivity with a rather low limiting potential(-0.34 V)for the NRR.The anchored Mo3 single-cluster can be stably embedded on the surface cavities of the substrate preventing the diffusion of the active Mo atoms.More importantly,the Mo atoms in the Mo3 single-cluster would act as"cache"to accelerate electron transfer between active metal centers and nitrogen-containing intermediates via the intimate Mo-Mo interactions.The cooperation of Mo atoms can also provide a large number of occupied and unoccupied d orbitals to make the"donation-backdonation"mechanism more effective.This work not only provides a quite promising electrocatalyst for NRR,but also brings new insights into the rational design of triple-atom NRR catalysts.
查看更多>>摘要：WMoTaNbTi RHEAs formed by SEBM with negative defocus distance were investigated.Four scanning speeds were applied,an electron beam with scanning speed at 2.5 m/s completely fused the premixed WMoTaNb alloyed powder and pure Ti powder.Significant vaporization of Nb and Ti elements happened during the formation of WMoTaNbTi RHEAs,however,the single BCC phase remains stable.Weakened solid-solute strengthening caused by elemental vaporization,dropping percentage of Nb and Ti solutes in the matrix as well as improved ductilizing effects with decreasing scanning speeds leads to falling microhardness and better local ductility.Microhardness of scanning speed at 4.0 m/s,3.5 m/s,3.0 m/s and 2.5 m/s is 578±17 HV,576±12 HV,573±10 HV and 511±2 HV,respectively.The as-deposited WMoTaNbTi RHEA formed at a scanning speed of 2.5 m/s displays ultimate strength of 1312 MPa.
查看更多>>摘要：NiFe2O4/polypyrrole(NiFe2O4/PPy)nanocomposites are prepared by a simple surface-initiated polymer-ization method and demonstrate negative permittivity in the low frequency regions.These nanocompos-ites also exhibit significantly enhanced electromagnetic wave(EMW)absorption property in the high frequency regions.Compared with pure PPy,the enhanced negative permittivity is observed in the NiFe2O4/PPy nanocomposites with a NiFe2O4 loading of 5.0,10.0,20.0 and 40.0 wt％,indicating the for-mation of metal-like electrical conducting network in NiFe2O4/PPy nanocomposites.Moreover,the neg-ative permittivity could be tuned by changing the NiFe2O4 loading.The minimum reflection loss(RL)of-40.8 dB is observed in the 40.0 wt％NiFe2O4/PPy composites with a thickness of only 1.9 mm.The effective absorption bandwidth below-10.0 and-20.0 dB reaches 6.08 and 2.08 GHz,respectively.The enhanced EMW absorption performance benefits from the improved independence matching,EMW at-tenuation capacity,and synergistic effects of conduction loss,dielectric loss(interfacial and dipole polar-izations)and magnetic loss(exchange and natural resonances).This research work provides a guidance for the fabrication of nanocomposites with an excellent EMW absorption.
查看更多>>摘要：The intrinsic origins and formation of atomic-scale structure in multicomponent alloys remain largely unknown owing to limited simulations and inaccessible experiments.Herein,we report the formation of three-dimensional periodicity from a disordered atomic-scale structure to an imperfect/perfect ordered cluster and finally to long-range translational and rotational symmetry coupled with Nb heterogeneity.Significant atomic-scale structural clustering and atomic arrangements involving solvent or solute atoms simultaneously occurred during isothermal annealing.A close relationship between atomic-scale struc-tural evolution and composition variation has important implications in depicting the chemical and topo-logical packing during the early crystallization stage in metallic glasses.This work can provide a compre-hensive understanding of how short-range orders evolve into long-range periodicity and will further shed light on the origins and nature of metallic glasses.
查看更多>>摘要：Significantly enhanced varistor properties via tailoring interface states were obtained in Ca1-2x/3YxCu3Ti4O12-SrCu3Ti4O12 composite ceramics.The breakdown field was improved to 35.8 kV cm-1 and the nonlinear coefficient in 0.1-1 mA cm-2 was enhanced to 14.6 for Ca0.67Y0.5Cu3Ti4O12-SrCu3Ti4O12.Noticeably,the withstand voltage of single grain boundary reached up to 24 V while the reported ones were constant to about 3 V.Greatly improved properties were attributed to the formation of superior grain boundary rather than the reduced grain size.Surprisingly,with distinct discrepancy of nonlinear performance in the composites,the resistance and activation energy of grain boundary exhibited little differences.Based on the double Schottky barrier at grain boundary and the field-assisted thermal emission model,it was found that the excellent electrical nonlinearity arose from the formation of deper and broader interface states at grain boundary.In this case,interface states were not easily entirely filled and the barrier could maintain its height under applied voltage.This work provides a novel routine for enhancing the varistor properties of CaCu3Ti4O12 based ceramics by manipulating interface states at grain boundary.
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