Transition metal diborides based ultrahigh temperature ceramics(UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity a...Transition metal diborides based ultrahigh temperature ceramics(UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy(HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm2 s^-1 and 0.51 W m^-1K^-1, respectively. In addition, it exhibits high compressive strength of3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is a novel strategy in making UHTCs thermal insulating.展开更多
High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical...High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/A,in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) from 0℃to 2400℃were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) were analyzed by displacements of atoms from their ideal positions,which are in an order of 10^(-3) A and one order of magnitude smaller than those in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.It indicates that lattice distortions in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.展开更多
The valence electron structure (VES) of RuB2 and OsB2 were calculated by the empirical electron theory (EET) of solids and molecules and compared with the results derived from the first-principles calculations. The di...The valence electron structure (VES) of RuB2 and OsB2 were calculated by the empirical electron theory (EET) of solids and molecules and compared with the results derived from the first-principles calculations. The distributions of covalent electrons in different bonds indicate that B-B and B-Me have remarkably covalent bonding characters. Lattice electrons cruising around Me-Me layers are found to have great influences on electronic conductivity and high temperature plasticity. The ultra-high values of elastic constant Cn in the two compounds originate from close-packed covalent bonding along the c axis. Uneven bond strengths and distributions of covalent bonds, especially for B-Afe bonds, yield significant anisotropy. Low ratios of lattice electrons to covalent electrons suggest the intrinsic embrittlement in crystals. The fact that the calculated cohesive energies well agree with experimental results demonstrates the good suitability of the EET calculations in estimating cohesive energy for transition-metal borides.展开更多
High-entropy nanomaterials have been arousing considerable interest in recent years due to their huge composition space,unique microstructure,and adjustable properties.Previous studies focused mainly on high-entropy n...High-entropy nanomaterials have been arousing considerable interest in recent years due to their huge composition space,unique microstructure,and adjustable properties.Previous studies focused mainly on high-entropy nanoparticles,while other high-entropy nanomaterials were rarely reported.Herein,we reported a new class of high-entropy nanomaterials,namely(Tao2Nbo2Ti.2Wo.2Moo2)B2 high-entropy diboride(HEB-1)nanoflowers,for the first time.Formation possibility of HEB-1 was first theoretically analyzed from two aspects of lattice size difference and chemical reaction thermodynamics.We then successfully synthesized HEB-1 nanoflowers by a facile molten salt synthesis method at 1423 K.The as-synthesized HEB-1 nanoflowers showed an interesting chrysanthemum-like morphology assembled from numerous well-aligned nanorods with diameters of 20--30 nm and lengths of 100-200 nm.Meanwhile,these nanorods possessed a single-crystalline hexagonal structure of metal diborides and highly compositional uniformity from nanoscale to microscale.In addition,the formation of the as-synthesized HEB-I nanoflowers could be well interpreted by a classical surface-contolled crystal growth theory.This work not only enriches the categories of high-entropy nanomaterials but also opens up a new research field on high-entropy diboride nanomaterials.展开更多
Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2 , where X=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in ...Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2 , where X=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in the hexagonal AlB2 , ReB2 , and orthorhombic OsB2 -type structures. The lattice parameters, bulk modulus, bond distances, second order elastic constants, and related polycrystalline elastic moduli （e.g., shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities） were calculated. Our results indicate that these compounds are mechanically stable in the considered structures, and according to ＂Chen’s method＂, the predicted Vickers hardness shows that they are hard materials in AlB2 - and OsB2 -type structures.展开更多
Designing active,robust and cost-effective catalysts for the nitrogen reduction reaction(NRR) is of paramount significance for sustainable electrochemical NH3 synthesis.Transition-metal diborides(TMB_2)have been recen...Designing active,robust and cost-effective catalysts for the nitrogen reduction reaction(NRR) is of paramount significance for sustainable electrochemical NH3 synthesis.Transition-metal diborides(TMB_2)have been recently theoretically predicted to be a new class of potential NRR catalysts,but direct experimental evidence is still lacking.Herein,we present the first experimental demonstration that amorphous FeB_2 porous nanosheets(a-FeB_2 PNSs) could be a highly efficient NRR catalyst,which exhibited an NH3 yield of 39.8 μg h^(-1) mg^(-1)(-0.3 V) and a Faradaic efficiency of 16.7%(-0.2 V),significantly outperforming their crystalline counterpart and most of existing NRR catalysts.First-principle calculations unveiled that the amorphization could induce the upraised d-band center of a-FeB_2 to boost d-2π~* coupling between the active Fe site and ~*N_2 H intermediate,resulting in enhanced ~*N_2 H stabilization and reduced reaction barrier.Out study may facilitate the development and understanding of earth-abundant TMB_2-based catalysts for electrocatalytic N_2 fixation.展开更多
Structural and lattice dynamical properties of ReB2,RuB2,and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation.The present results show ...Structural and lattice dynamical properties of ReB2,RuB2,and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation.The present results show that these compounds are dynamically stable for the considered structure.The temperature-dependent behaviors of thermodynamical properties such as internal energy,free energy,entropy,and heat capacity are also presented.The obtained results are in good agreement with the available experimental and theoretical data.展开更多
We have performed low temperature resistivity p（T） and specific heat C（T） measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperatu...We have performed low temperature resistivity p（T） and specific heat C（T） measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is -4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat γn=4.51 mJmol^-1K^2 and the Debye temperature θD=419 K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/γnTc = 2.84 exp（-1.21Tc/T）. This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.展开更多
The potential applications of transition metal diborides(TMB_2) in extreme environments are particularly attractive but still blocked by some intrinsic properties such as poor resistances to thermal shock and oxidatio...The potential applications of transition metal diborides(TMB_2) in extreme environments are particularly attractive but still blocked by some intrinsic properties such as poor resistances to thermal shock and oxidation. Since surface plays a key role during grain growth and oxygen adsorption, an insight into the surface properties of TMB_2 is essential for understanding the materials performance and accelerating the development of ultra-high temperature ceramics. By employing two-region modeling method, the stability and oxygen adsorption behavior of TMB_2 surfaces were investigated by first-principles calculations based on density functional theory. The effects of valance electron concentration on the surface stability and oxygen adsorption were studied and the general trends were summarized. After analyzing the anisotropy in surface stability and oxygen adsorption, the observed grain morphology of TMB_2 were well explained, and it was also predicted that YB_2, HfB_2 and TaB_2 may have better initial oxidation resistance than ZrB_2.展开更多
The recent discovery of superconductivity up to 32 K in the pressurized MoBreignites the interest in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the...The recent discovery of superconductivity up to 32 K in the pressurized MoBreignites the interest in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5 d transition-metal diborides.Here we systematically investigate the responses of both structural and physical properties of WBand ReBto external pressure,which possess different types of boron layers. Similar to MoB, the pressure-induced superconductivity was also observed in WBabove 60 GPa with a maximum Tcof 15 K at 100 GPa, while no superconductivity was detected in ReBin this pressure range. Interestingly, the structures at ambient pressure for both WBand ReBpersist to high pressure without structural phase transitions. Theoretical calculations suggest that the ratio of flat boron layers in this class of transition-metal diborides may be crucial for the appearance of high Tc. The combined theoretical and experimental results highlight the effect of the geometry of boron layers on superconductivity and shed light on the exploration of novel high-Tcsuperconductors in borides.展开更多
Transition metal diborides(TMB_(2)s)are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities.However,the degradation of oxidation resistance of TMB_(2)s at elevated t...Transition metal diborides(TMB_(2)s)are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities.However,the degradation of oxidation resistance of TMB_(2)s at elevated temperature still hinders their applications.To cope with this challenge,it is effective to incorporate rare earth elements to form high-entropy transition and rare-earth metal diborides(HE TMREBs).To obtain thermodynamically stable single-phase structures for HE TMREB_(2)s,a“16×16 mixed enthalpy matrix”is constructed using first-principles calculations to predict the single-phase formation ability of120 two-component diborides(TCBs).Through the use of the“16×16 mixed enthalpy matrix”of TCBs,specific combinations of TMB_(2)s and REB_(2)s that are most likely to form single-phase HE TMREB_(2)s are confirmed.Subsequently,based on the energy distribution of the local mixing enthalpies of all possible configurations,the enthalpy and entropy descriptors of HE TMREB_(2)s(RE=Sc,Lu,Tm,Er,Ho and Dy)are investigated.It is found that the mixing enthalpy plays a critical role in the stability of the single-phase HE TMREB_(2)s,i.e.,HE TMREB_(2)s are enthalpy-stabilized materials.The experimental results further confirm that enthalpy dominates the thermodynamic domain and drives the stability of REB_(2)s in HE TMREB_(2)s.This study validates that enthalpy-stabilized HE TMREB_(2)s can further expand the compositional space of ultrahigh temperature ceramics(UHTCs)and is expected to further improve the oxidation resistance and high temperature properties of UHTCs.展开更多
High-entropy diborides(HEBs)have attracted extensive research due to their potential ultra-high hardness.In the present work,the effects of transition metals(TM)on lattice parameters,electron work function(EWF),bondin...High-entropy diborides(HEBs)have attracted extensive research due to their potential ultra-high hardness.In the present work,the effects of transition metals(TM)on lattice parameters,electron work function(EWF),bonding charge density,and hardness of HEBs are comprehensively investigated by the first-principles calculations,including(TiZrHfNbTa)B_(2),(TiZrHfNbMo)B_(2),(TiZrHfTaMo)B_(2),(TiZrNbTaMo)B_(2),and(TiHfNbTaMo)B_(2).It is revealed that the disordered TM atoms result in a severe local lattice distortion and the formation of weak spots.In view of bonding charge density,it is understood that the degree of electron contribution of TM atoms directly affects the bonding strength of the metallic layer,contributing to the optimized hardness of HEBs.Moreover,the proposed power-law-scaled relationship integrating the EWF and the grain size yields an excellent agreement between our predicted results and those reported experimental ones.It is found that the HEBs exhibit relatively high hardness which is higher than those of single transition metal diborides.In particular,the hardness of(TiZrNbTaMo)B_(2)and(TiHfNbTaMo)B_(2)can be as high as29.15 and 28.02 GPa,respectively.This work provides a rapid strategy to discover/design advanced HEBs efficiently,supported by the coupling hardening mechanisms of solid solution and grain refinement based on the atomic and electronic interactions.展开更多
High-entropy ceramics attract more and more attention in recent years.However,mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensive...High-entropy ceramics attract more and more attention in recent years.However,mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensively reported.In this work,high-entropy(Ti0.2Zr0.2Hf0.2Nb0.2Ta 0.2)B2(HEB)monolithic and its composite containing 20 vol%SiC(HEB–20SiC)are prepared by hot pressing.The addition of SiC not only accelerates the densification process but also refines the microstructure of HEB,resulting in improved mechanical properties.The obtained dense HEB and HEB–20SiC ceramics hot pressed at 1800℃exhibit four-point flexural strength of 339±17 MPa and 447±45 MPa,and fracture toughness of 3.81±0.40 MPa·m1/2 and 4.85±0.33 MPa·m1/2 measured by single-edge notched beam(SENB)technique.Crack deflection and branching by SiC particles is considered to be the main toughening mechanisms for the HEB–20SiC composite.The hardness Hv0.2 of the sintered HEB and HEB–20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa,respectively.With the increase of indentation load,the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N,due to the indentation size effect.Based on the current experimental investigation it can be seen that the room temperature bending strength and fracture toughness of the high-entropy diboride ceramics are within ranges commonly observed in structure ceramics.展开更多
In order to improve the lifespan of spot-welding electrodes used for welding zinc coated steel sheets, titanium diboride was deposited onto their surface after precoating nickel as an intermediate layer. The microstru...In order to improve the lifespan of spot-welding electrodes used for welding zinc coated steel sheets, titanium diboride was deposited onto their surface after precoating nickel as an intermediate layer. The microstructures and phase compositions of TiB2 and Ni coatings were characterized by SEM and XRD. The coating hardness was measured using a microhardness tester. The results indicate that a satisfactory TiB2 coating is obtained as a result of the intermediate nickel layer acting as a good binder between the TiB2 coating and the copper alloy substrate. Owing to its capacity of deforming, the precoated nickel layer is dense and crack free, while cracks and pores are observed in the TiB2 coating. The hardness of the TiB2/Ni coating decreases with the increase of voltage and capacitance because of the diffusion of copper and nickel and the oxidation of the coating materials. Because of the good thermal and electrical conductivities and high hardness properties of TiB2, the deformation of the electrode with TiB2/Ni coating is reduced and its spot-welding life is by far prolonged than that of the uncoated one.展开更多
The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys ...The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys （6061 and 7015） at high temperatures were studied. Titanium diboride （TiB2） particles were used as the reinforcement. All the composites were produced by hot extrusion. The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy. TiB2 particles provide high stability of the alumin- ium alloys （6061 and 7015） in the fabrication process. An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys. Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure, and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.展开更多
基金supported by the National Natural Science Foundation of China (Nos. 51672064 and U1435206)
文摘Transition metal diborides based ultrahigh temperature ceramics(UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy(HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm2 s^-1 and 0.51 W m^-1K^-1, respectively. In addition, it exhibits high compressive strength of3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE(Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2 is a novel strategy in making UHTCs thermal insulating.
基金supported by Natural Sciences Foundation of China under Grant No.51972089 and No.51672064。
文摘High entropy diborides are new categories of ultra-high temperature ceramics,which are believed promising candidates for applications in hypersonic vehicles.However,knowledge on high temperature thermal and mechanical properties of high entropy diborides is still lacking unit now.In this work,variations of thermal and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) with respect to temperature were predicted by molecular dynamics simulations.Firstly,a deep learning potential for Ti-Zr-Hf-Nb-Ta-B diboride system was fitted with its prediction error in energy and force respectively being 9.2 meV/atom and 208 meV/A,in comparison with first-principles calculations.Then,temperature dependent lattice constants,anisotropic thermal expansions,anisotropic phonon thermal conductivities,and elastic properties of high entropy(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) from 0℃to 2400℃were evaluated,where the predicted room temperature values agree well with experimental measurements.In addition,intrinsic lattice distortions of(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) were analyzed by displacements of atoms from their ideal positions,which are in an order of 10^(-3) A and one order of magnitude smaller than those in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))C.It indicates that lattice distortions in(Ti_(0.2)Zr_(0.2)Hf_(0.2)Nb_(0.2)Ta_(0.2))B_(2) is not so severe as expected.With the new paradigm of machine learning potential,deep insight into high entropy materials can be achieved in the future,since the chemical and structural complexly in high entropy materials can be well handled by machine learning potential.
文摘The valence electron structure (VES) of RuB2 and OsB2 were calculated by the empirical electron theory (EET) of solids and molecules and compared with the results derived from the first-principles calculations. The distributions of covalent electrons in different bonds indicate that B-B and B-Me have remarkably covalent bonding characters. Lattice electrons cruising around Me-Me layers are found to have great influences on electronic conductivity and high temperature plasticity. The ultra-high values of elastic constant Cn in the two compounds originate from close-packed covalent bonding along the c axis. Uneven bond strengths and distributions of covalent bonds, especially for B-Afe bonds, yield significant anisotropy. Low ratios of lattice electrons to covalent electrons suggest the intrinsic embrittlement in crystals. The fact that the calculated cohesive energies well agree with experimental results demonstrates the good suitability of the EET calculations in estimating cohesive energy for transition-metal borides.
基金We acknowledge financial support from the National Key R&D Program of China(No.2017YFB0703200)National Natural Science Foundation of China(Nos.51802100 and 51972116)+1 种基金Young Elite Scientists Sponsorship Program by CAST(No.2017QNRC001)Guangdong Basic and Applied Basic Research Foundation(No.2019A1515012145).
文摘High-entropy nanomaterials have been arousing considerable interest in recent years due to their huge composition space,unique microstructure,and adjustable properties.Previous studies focused mainly on high-entropy nanoparticles,while other high-entropy nanomaterials were rarely reported.Herein,we reported a new class of high-entropy nanomaterials,namely(Tao2Nbo2Ti.2Wo.2Moo2)B2 high-entropy diboride(HEB-1)nanoflowers,for the first time.Formation possibility of HEB-1 was first theoretically analyzed from two aspects of lattice size difference and chemical reaction thermodynamics.We then successfully synthesized HEB-1 nanoflowers by a facile molten salt synthesis method at 1423 K.The as-synthesized HEB-1 nanoflowers showed an interesting chrysanthemum-like morphology assembled from numerous well-aligned nanorods with diameters of 20--30 nm and lengths of 100-200 nm.Meanwhile,these nanorods possessed a single-crystalline hexagonal structure of metal diborides and highly compositional uniformity from nanoscale to microscale.In addition,the formation of the as-synthesized HEB-I nanoflowers could be well interpreted by a classical surface-contolled crystal growth theory.This work not only enriches the categories of high-entropy nanomaterials but also opens up a new research field on high-entropy diboride nanomaterials.
文摘Structural and mechanical properties of several rare-earth diborides were systematically investigated by first principles calculations. Specifically, we studied XB2 , where X=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Lu in the hexagonal AlB2 , ReB2 , and orthorhombic OsB2 -type structures. The lattice parameters, bulk modulus, bond distances, second order elastic constants, and related polycrystalline elastic moduli （e.g., shear modulus, Young’s modulus, Poisson’s ratio, Debye temperature, sound velocities） were calculated. Our results indicate that these compounds are mechanically stable in the considered structures, and according to ＂Chen’s method＂, the predicted Vickers hardness shows that they are hard materials in AlB2 - and OsB2 -type structures.
基金supported by the National Natural Science Foundation of China (51761024)the CAS ‘‘Light of West China” Program, ‘‘Feitian Scholar” Program of Gansu Provincethe Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong University。
文摘Designing active,robust and cost-effective catalysts for the nitrogen reduction reaction(NRR) is of paramount significance for sustainable electrochemical NH3 synthesis.Transition-metal diborides(TMB_2)have been recently theoretically predicted to be a new class of potential NRR catalysts,but direct experimental evidence is still lacking.Herein,we present the first experimental demonstration that amorphous FeB_2 porous nanosheets(a-FeB_2 PNSs) could be a highly efficient NRR catalyst,which exhibited an NH3 yield of 39.8 μg h^(-1) mg^(-1)(-0.3 V) and a Faradaic efficiency of 16.7%(-0.2 V),significantly outperforming their crystalline counterpart and most of existing NRR catalysts.First-principle calculations unveiled that the amorphization could induce the upraised d-band center of a-FeB_2 to boost d-2π~* coupling between the active Fe site and ~*N_2 H intermediate,resulting in enhanced ~*N_2 H stabilization and reduced reaction barrier.Out study may facilitate the development and understanding of earth-abundant TMB_2-based catalysts for electrocatalytic N_2 fixation.
基金supported by the National Key Research and Development Program of China(2022YFB3708600)the National Natural Science Foundation of China(52122204 and 51972116)Guangzhou Basic and Applied Basic Research Foundation(202201010632).
文摘Structural and lattice dynamical properties of ReB2,RuB2,and OsB2 in the ReB2 structure are studied in the framework of density functional theory within the generalized gradient approximation.The present results show that these compounds are dynamically stable for the considered structure.The temperature-dependent behaviors of thermodynamical properties such as internal energy,free energy,entropy,and heat capacity are also presented.The obtained results are in good agreement with the available experimental and theoretical data.
基金Supported by the National Natural Science Foundation of China, the Knowledge Innovation Project of Chinese Academy of Sciences, and National Basic Research Programme of China under Grant Nos 2003CCC01000, 2005CB32170X and 2006CB921301.
文摘We have performed low temperature resistivity p（T） and specific heat C（T） measurements on a superconducting polycrystalline Nb0.75Mg0.25B2 sample. The results indicate that the superconducting transition temperature is -4.6 K. The zero temperature upper critical field determined from the resistivity and specific heat is 3123 Oe. The electronic coefficient of specific heat γn=4.51 mJmol^-1K^2 and the Debye temperature θD=419 K are obtained by fitting the zero-field specific heat data in the normal state. At low temperatures, the electronic specific heat in the superconducting state follows Ces/γnTc = 2.84 exp（-1.21Tc/T）. This indicates that the superconducting pairing in Nb0.75Mg0.25B2 has s-wave symmetry.
基金supported by the National Natural Sciences Foundation of China under Grant No.51672064 and No.U1435206Beijing Municipal Science & Technology Commission under Grant No.D161100002416001
文摘The potential applications of transition metal diborides(TMB_2) in extreme environments are particularly attractive but still blocked by some intrinsic properties such as poor resistances to thermal shock and oxidation. Since surface plays a key role during grain growth and oxygen adsorption, an insight into the surface properties of TMB_2 is essential for understanding the materials performance and accelerating the development of ultra-high temperature ceramics. By employing two-region modeling method, the stability and oxygen adsorption behavior of TMB_2 surfaces were investigated by first-principles calculations based on density functional theory. The effects of valance electron concentration on the surface stability and oxygen adsorption were studied and the general trends were summarized. After analyzing the anisotropy in surface stability and oxygen adsorption, the observed grain morphology of TMB_2 were well explained, and it was also predicted that YB_2, HfB_2 and TaB_2 may have better initial oxidation resistance than ZrB_2.
基金supported by the National Key R&D Program of China(Grant Nos.2018YFA0704300,2018YFE0202600,and 2017YFA0302903)the National Natural Science Foundation of China(Grant Nos.U1932217,11974246,12004252,12174443,and 11774424)+5 种基金the Natural Science Foundation of Shanghai(Grant No.19ZR1477300)the Science and Technology Commission of Shanghai Municipality(Grant No.19JC1413900)the Shanghai Science and Technology Plan(Grant No.21DZ2260400)the Beijing Natural Science Foundation(Grant No.Z200005)the Fundamental Research Funds for the Central Universities and Research Funds of Renmin University of China(RUC)(Grant Nos.18XNLG14,19XNLG13,19XNLG17,and 22XNKJ40)support from Analytical Instrumentation Center(Grant No.SPSTAIC10112914),SPST,Shanghai Tech University。
文摘The recent discovery of superconductivity up to 32 K in the pressurized MoBreignites the interest in exploring high-Tc superconductors in transition-metal diborides. Inspired by that work, we turn our attention to the 5 d transition-metal diborides.Here we systematically investigate the responses of both structural and physical properties of WBand ReBto external pressure,which possess different types of boron layers. Similar to MoB, the pressure-induced superconductivity was also observed in WBabove 60 GPa with a maximum Tcof 15 K at 100 GPa, while no superconductivity was detected in ReBin this pressure range. Interestingly, the structures at ambient pressure for both WBand ReBpersist to high pressure without structural phase transitions. Theoretical calculations suggest that the ratio of flat boron layers in this class of transition-metal diborides may be crucial for the appearance of high Tc. The combined theoretical and experimental results highlight the effect of the geometry of boron layers on superconductivity and shed light on the exploration of novel high-Tcsuperconductors in borides.
基金the National Natural Science Foundation of China(Nos.51772027 and 51972089)。
文摘Transition metal diborides(TMB_(2)s)are the materials of choice in extreme environments due to their excellent thermal and chemical stabilities.However,the degradation of oxidation resistance of TMB_(2)s at elevated temperature still hinders their applications.To cope with this challenge,it is effective to incorporate rare earth elements to form high-entropy transition and rare-earth metal diborides(HE TMREBs).To obtain thermodynamically stable single-phase structures for HE TMREB_(2)s,a“16×16 mixed enthalpy matrix”is constructed using first-principles calculations to predict the single-phase formation ability of120 two-component diborides(TCBs).Through the use of the“16×16 mixed enthalpy matrix”of TCBs,specific combinations of TMB_(2)s and REB_(2)s that are most likely to form single-phase HE TMREB_(2)s are confirmed.Subsequently,based on the energy distribution of the local mixing enthalpies of all possible configurations,the enthalpy and entropy descriptors of HE TMREB_(2)s(RE=Sc,Lu,Tm,Er,Ho and Dy)are investigated.It is found that the mixing enthalpy plays a critical role in the stability of the single-phase HE TMREB_(2)s,i.e.,HE TMREB_(2)s are enthalpy-stabilized materials.The experimental results further confirm that enthalpy dominates the thermodynamic domain and drives the stability of REB_(2)s in HE TMREB_(2)s.This study validates that enthalpy-stabilized HE TMREB_(2)s can further expand the compositional space of ultrahigh temperature ceramics(UHTCs)and is expected to further improve the oxidation resistance and high temperature properties of UHTCs.
基金financially supported by the Science Challenge Project(No.TZ 2018002)。
文摘High-entropy diborides(HEBs)have attracted extensive research due to their potential ultra-high hardness.In the present work,the effects of transition metals(TM)on lattice parameters,electron work function(EWF),bonding charge density,and hardness of HEBs are comprehensively investigated by the first-principles calculations,including(TiZrHfNbTa)B_(2),(TiZrHfNbMo)B_(2),(TiZrHfTaMo)B_(2),(TiZrNbTaMo)B_(2),and(TiHfNbTaMo)B_(2).It is revealed that the disordered TM atoms result in a severe local lattice distortion and the formation of weak spots.In view of bonding charge density,it is understood that the degree of electron contribution of TM atoms directly affects the bonding strength of the metallic layer,contributing to the optimized hardness of HEBs.Moreover,the proposed power-law-scaled relationship integrating the EWF and the grain size yields an excellent agreement between our predicted results and those reported experimental ones.It is found that the HEBs exhibit relatively high hardness which is higher than those of single transition metal diborides.In particular,the hardness of(TiZrNbTaMo)B_(2)and(TiHfNbTaMo)B_(2)can be as high as29.15 and 28.02 GPa,respectively.This work provides a rapid strategy to discover/design advanced HEBs efficiently,supported by the coupling hardening mechanisms of solid solution and grain refinement based on the atomic and electronic interactions.
基金This work was supported by the National Natural Science Foundation of China(Nos.51532009,51872045)Science and Technology Commission of Shanghai Municipality(No.18ZR1401400)+1 种基金the Fundamental Research Funds for the Central Universities(Nos.2232018D3-32,2232019A3-13)State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,Donghua University(No.19ZK0113).
文摘High-entropy ceramics attract more and more attention in recent years.However,mechanical properties especially strength and fracture toughness for high-entropy ceramics and their composites have not been comprehensively reported.In this work,high-entropy(Ti0.2Zr0.2Hf0.2Nb0.2Ta 0.2)B2(HEB)monolithic and its composite containing 20 vol%SiC(HEB–20SiC)are prepared by hot pressing.The addition of SiC not only accelerates the densification process but also refines the microstructure of HEB,resulting in improved mechanical properties.The obtained dense HEB and HEB–20SiC ceramics hot pressed at 1800℃exhibit four-point flexural strength of 339±17 MPa and 447±45 MPa,and fracture toughness of 3.81±0.40 MPa·m1/2 and 4.85±0.33 MPa·m1/2 measured by single-edge notched beam(SENB)technique.Crack deflection and branching by SiC particles is considered to be the main toughening mechanisms for the HEB–20SiC composite.The hardness Hv0.2 of the sintered HEB and HEB–20SiC ceramics is 23.7±0.7 GPa and 24.8±1.2 GPa,respectively.With the increase of indentation load,the hardness of the sintered ceramics decreases rapidly until the load reaches about 49 N,due to the indentation size effect.Based on the current experimental investigation it can be seen that the room temperature bending strength and fracture toughness of the high-entropy diboride ceramics are within ranges commonly observed in structure ceramics.
基金Project (50575069) supported by the National Natural Science Foundation of China
文摘In order to improve the lifespan of spot-welding electrodes used for welding zinc coated steel sheets, titanium diboride was deposited onto their surface after precoating nickel as an intermediate layer. The microstructures and phase compositions of TiB2 and Ni coatings were characterized by SEM and XRD. The coating hardness was measured using a microhardness tester. The results indicate that a satisfactory TiB2 coating is obtained as a result of the intermediate nickel layer acting as a good binder between the TiB2 coating and the copper alloy substrate. Owing to its capacity of deforming, the precoated nickel layer is dense and crack free, while cracks and pores are observed in the TiB2 coating. The hardness of the TiB2/Ni coating decreases with the increase of voltage and capacitance because of the diffusion of copper and nickel and the oxidation of the coating materials. Because of the good thermal and electrical conductivities and high hardness properties of TiB2, the deformation of the electrode with TiB2/Ni coating is reduced and its spot-welding life is by far prolonged than that of the uncoated one.
文摘The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys （6061 and 7015） at high temperatures were studied. Titanium diboride （TiB2） particles were used as the reinforcement. All the composites were produced by hot extrusion. The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy. TiB2 particles provide high stability of the alumin- ium alloys （6061 and 7015） in the fabrication process. An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys. Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure, and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.
