Five-fold symmetric diamond crystals（FSDCs） were synthesized by hot filament chemical vapour deposition（HFCVD） methods. Their surface morphologies and defects were characterised by scanning electron microscopy（SE...Five-fold symmetric diamond crystals（FSDCs） were synthesized by hot filament chemical vapour deposition（HFCVD） methods. Their surface morphologies and defects were characterised by scanning electron microscopy（SEM）. From the perspective of nucleation-growth, a growth mechanism for icosahedral and other five-fold symmetric diamond crystals was discussed. Computer modelling was also carried out. The results show that the dodecahedrane（C20H20） molecule is proposed as a nucleus for the growth of icosahedral diamond crystals（IDCs）, wherein the 20 {111} surface planes develop orthogonal to the direction of the original 20 C—H bonds by sequential H abstraction and CH3 addition reactions. IDC can be pictured as an assembly of isosceles tetrahedra, with each tetrahedron contributing a {111} plane to the surface of the IDC and the remainder of the tetrahedral surfaces forming twin planes with neighbouring tetrahedra. The small mismatch（1.44°） between the {111} surface dihedral angle of a perfect icosahedron and that of a twinned icosahedron reveals itself via twin planes in the IDC grain. The modelling suggests how the relief of strain induced by this distortion could lead to the formation of defects such as concave pentagonal cavities at vertices and grooves along the grain edges that accord well with those observed experimentally. Similar arguments based on growth from the hexacyclo pentadecane（C15H20） nucleus can also account for the observed formation of star and rod shaped FSDCs, and some of their more obvious morphological defects.展开更多
The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as per...The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as periodicity for crystals, for establishing the structure–property relationship in amorphous materials. In this paper, we briefly review the recent developments of the five-fold local symmetry in metallic liquids and glasses and the understanding of the structure–property relationship based on this parameter. Experimental evidence demonstrates that five-fold local symmetry is found to be general in metallic liquids and glasses. Comprehensive molecular dynamics simulations show that the temperature evolution of five-fold local symmetry reflects the structural evolution in glass transition in cooling process, and the structure–property relationship such as relaxation dynamics, dynamic crossover phenomena, glass transition, and mechanical deformation in metallic liquids and glasses can be well understood base on the simple and general structure parameter of five-fold local symmetry.展开更多
Five-fold twinned nanostructures are intrinsically strained or relaxed by extended defects to satisfy the space-filling requirement.Although both of metallic and semiconductor five-fold twinned nanostructures show inh...Five-fold twinned nanostructures are intrinsically strained or relaxed by extended defects to satisfy the space-filling requirement.Although both of metallic and semiconductor five-fold twinned nanostructures show inhomogeneity in their cross-sectional strain distribution,the evident strain concentration at twin boundaries in the semiconductor systems has been found in contrast to the metallic systems.Naturally,a problem is raised how the chemical bonding characteristics of various five-fold twinned nanosystems affects their strain-relieving defect structures.Here using three-dimensional(3D)electron diffraction mapping methodology,the intrinsic strain and the strain-relieving defects in a pentagonal Ag nanowire and a star-shaped boron carbide nanowire,both of them have basically equal radial twin-plane width about 30 nm,are nondestructively characterized.The non-uniform strain and defect distribution between the five single crystalline segments are found in both of the five-fold twinned nanowires.Diffraction intensity fine structure analysis for the boron carbide five-fold twinned nanowire indicates the presence of high-density of planar defects which are responsible for the accommodation of the intrinsic angular excess.However,for the Ag five-fold twinned nanowire,the star-disclination strain field is still present,although is partially relieved by the formation of localized stacking fault layers accompanied by partial dislocations.Energetic analysis suggests that the variety in the strain-relaxation ways for the two types of five-fold twinned nanowires could be ascribed to the large difference in shear modulus between the soft noble metal Ag and the superhard covalent compound boron carbide.展开更多
Developing efficient and robust electrocatalysts toward ethanol oxidation reaction(EOR)with high C1 pathway selectivity is critical for commercialization of direct ethanol fuel cells(DEFCs).Unfortunately,current most ...Developing efficient and robust electrocatalysts toward ethanol oxidation reaction(EOR)with high C1 pathway selectivity is critical for commercialization of direct ethanol fuel cells(DEFCs).Unfortunately,current most EOR electrocatalysts suffer from rapid activity degradation and poor C1 pathway selectivity for complete oxidation of ethanol.Herein,we report a novel electrocatalyst of five-fold twinned(FFT)Ir-alloyed Pt nanorods(NRs)toward EOR.Such FFT Pt-Ir NRs bounded by five(100)facets on the sides and ten(111)facets at two ends possess high percentage of(100)facets with tensile strain.Owing to the inherent characteristics of the FFT NR and Ir alloying,the as-prepared FFT Pt-Ir NRs display excellent alkaline EOR performance with a mass activity(MA)of 4.18 A·mgPt^(-1),a specific activity(SA)of 10.22 mA·cm^(-2),and a Faraday efficiency of 61.21%for the C1 pathway,which are 6.85,5.62,and 7.70 times higher than those of a commercial Pt black,respectively.Besides,our catalyst also exhibits robust durability.The large percentage of open tensile-strained(100)facets and Ir alloying significantly promote the cleavage of C-C bonds and facilitate oxidation of the poisonous intermediates,leading to the transformation of the dominant reaction pathway for EOR from C2 to C1 pathway,and effectively suppress the deactivation of the catalyst.展开更多
We present the results of systematic molecular dynamics simulations of pure aluminium melt with a well-accepted embedded atom potential. The structure and dynamics were calculated over a wide temperature range, and th...We present the results of systematic molecular dynamics simulations of pure aluminium melt with a well-accepted embedded atom potential. The structure and dynamics were calculated over a wide temperature range, and the calculated results（including the pair correlation function, self-diffusion coefficient, and viscosity） agree well with the available experimental observations. The calculated data were used to examine the Stokes–Einstein relation（SER）. The results indicate that the SER begins to break down at a temperature Tx（-1090 K） which is well above the equilibrium melting point（912.5 K）.This high-temperature breakdown is confirmed by the evolution of dynamics heterogeneity, which is characterised by the non-Gaussian parameter α2（t）. The maximum value of α 2（t）, α（2,max）, increases at an accelerating rate as the temperature falls below Tx. The development of α（2,max） was found to be related to the liquid structure change evidenced by local fivefold symmetry. Accordingly, we suggest that this high-temperature breakdown of SER has a structural origin. The results of this study are expected to make researchers reconsider the applicability of SER and promote greater understanding of the relationship between dynamics and structure.展开更多
基金Projects(51301211,21271188)supported by the National Natural Science Foundation of ChinaProject(2012M521540)supported by the China Postdoctoral Science Foundation+2 种基金Project(2013RS4027)supported by the Postdoctoral Science Foundation of Hunan Province,ChinaProject(20110933K)supported by the Open Foundation of the State Key Laboratory of Powder Metallurgy,ChinaProject supported by the Open-End Fund for Valuable and Precision Instruments of Central South University,China
文摘Five-fold symmetric diamond crystals（FSDCs） were synthesized by hot filament chemical vapour deposition（HFCVD） methods. Their surface morphologies and defects were characterised by scanning electron microscopy（SEM）. From the perspective of nucleation-growth, a growth mechanism for icosahedral and other five-fold symmetric diamond crystals was discussed. Computer modelling was also carried out. The results show that the dodecahedrane（C20H20） molecule is proposed as a nucleus for the growth of icosahedral diamond crystals（IDCs）, wherein the 20 {111} surface planes develop orthogonal to the direction of the original 20 C—H bonds by sequential H abstraction and CH3 addition reactions. IDC can be pictured as an assembly of isosceles tetrahedra, with each tetrahedron contributing a {111} plane to the surface of the IDC and the remainder of the tetrahedral surfaces forming twin planes with neighbouring tetrahedra. The small mismatch（1.44°） between the {111} surface dihedral angle of a perfect icosahedron and that of a twinned icosahedron reveals itself via twin planes in the IDC grain. The modelling suggests how the relief of strain induced by this distortion could lead to the formation of defects such as concave pentagonal cavities at vertices and grooves along the grain edges that accord well with those observed experimentally. Similar arguments based on growth from the hexacyclo pentadecane（C15H20） nucleus can also account for the observed formation of star and rod shaped FSDCs, and some of their more obvious morphological defects.
基金supported by the National Natural Science Foundation of China(Grant Nos.51271195 and 51271197)the National Basic Research Program of China(Grant No.2015CB856800)+1 种基金the Fundamental Research Funds for the Central Universities,Chinathe Research Funds of Renmin University of China(Grnat No.16XNLQ01)
文摘The structure of metallic glasses has been a long-standing mystery. Owing to the disordered nature of atomic structures in metallic glasses, it is a great challenge to find a simple structural description, such as periodicity for crystals, for establishing the structure–property relationship in amorphous materials. In this paper, we briefly review the recent developments of the five-fold local symmetry in metallic liquids and glasses and the understanding of the structure–property relationship based on this parameter. Experimental evidence demonstrates that five-fold local symmetry is found to be general in metallic liquids and glasses. Comprehensive molecular dynamics simulations show that the temperature evolution of five-fold local symmetry reflects the structural evolution in glass transition in cooling process, and the structure–property relationship such as relaxation dynamics, dynamic crossover phenomena, glass transition, and mechanical deformation in metallic liquids and glasses can be well understood base on the simple and general structure parameter of five-fold local symmetry.
基金National Natural Science Foundation of China(Grant Nos.51201015 and U1532262).
文摘Five-fold twinned nanostructures are intrinsically strained or relaxed by extended defects to satisfy the space-filling requirement.Although both of metallic and semiconductor five-fold twinned nanostructures show inhomogeneity in their cross-sectional strain distribution,the evident strain concentration at twin boundaries in the semiconductor systems has been found in contrast to the metallic systems.Naturally,a problem is raised how the chemical bonding characteristics of various five-fold twinned nanosystems affects their strain-relieving defect structures.Here using three-dimensional(3D)electron diffraction mapping methodology,the intrinsic strain and the strain-relieving defects in a pentagonal Ag nanowire and a star-shaped boron carbide nanowire,both of them have basically equal radial twin-plane width about 30 nm,are nondestructively characterized.The non-uniform strain and defect distribution between the five single crystalline segments are found in both of the five-fold twinned nanowires.Diffraction intensity fine structure analysis for the boron carbide five-fold twinned nanowire indicates the presence of high-density of planar defects which are responsible for the accommodation of the intrinsic angular excess.However,for the Ag five-fold twinned nanowire,the star-disclination strain field is still present,although is partially relieved by the formation of localized stacking fault layers accompanied by partial dislocations.Energetic analysis suggests that the variety in the strain-relaxation ways for the two types of five-fold twinned nanowires could be ascribed to the large difference in shear modulus between the soft noble metal Ag and the superhard covalent compound boron carbide.
基金This work was supported by the National Natural Science Foundation of China(No.21908036)the China Postdoctoral Science Foundation(No.2019M662143)+1 种基金the Natural Science Foundation of Anhui Province(No.2008085QB74)the Fundamental Research Funds for the Central Universities(No.JZ2021HGTB0116).
文摘Developing efficient and robust electrocatalysts toward ethanol oxidation reaction(EOR)with high C1 pathway selectivity is critical for commercialization of direct ethanol fuel cells(DEFCs).Unfortunately,current most EOR electrocatalysts suffer from rapid activity degradation and poor C1 pathway selectivity for complete oxidation of ethanol.Herein,we report a novel electrocatalyst of five-fold twinned(FFT)Ir-alloyed Pt nanorods(NRs)toward EOR.Such FFT Pt-Ir NRs bounded by five(100)facets on the sides and ten(111)facets at two ends possess high percentage of(100)facets with tensile strain.Owing to the inherent characteristics of the FFT NR and Ir alloying,the as-prepared FFT Pt-Ir NRs display excellent alkaline EOR performance with a mass activity(MA)of 4.18 A·mgPt^(-1),a specific activity(SA)of 10.22 mA·cm^(-2),and a Faraday efficiency of 61.21%for the C1 pathway,which are 6.85,5.62,and 7.70 times higher than those of a commercial Pt black,respectively.Besides,our catalyst also exhibits robust durability.The large percentage of open tensile-strained(100)facets and Ir alloying significantly promote the cleavage of C-C bonds and facilitate oxidation of the poisonous intermediates,leading to the transformation of the dominant reaction pathway for EOR from C2 to C1 pathway,and effectively suppress the deactivation of the catalyst.
基金supported by the National Natural Science Foundation of China(11274173,51032002,61222403,11374159)FundamentalResearch Funds for the Central Universities,China(NZ2013304)~~
基金supported by the National Basic Research Program of China(Grant No.2011CB012900)the National Natural Science Foundation of China(Grant No.51171115)+3 种基金the Natural Science Foundation of Shanghai City,China(Grant No.10ZR1415700)the Research Fund for the Doctoral Program of Higher Education of China(Grant No.20100073120008)the Program for New Century Excellent Talents in Universities of Chinapartially supported by Alexander von Humboldt Foundation
文摘We present the results of systematic molecular dynamics simulations of pure aluminium melt with a well-accepted embedded atom potential. The structure and dynamics were calculated over a wide temperature range, and the calculated results（including the pair correlation function, self-diffusion coefficient, and viscosity） agree well with the available experimental observations. The calculated data were used to examine the Stokes–Einstein relation（SER）. The results indicate that the SER begins to break down at a temperature Tx（-1090 K） which is well above the equilibrium melting point（912.5 K）.This high-temperature breakdown is confirmed by the evolution of dynamics heterogeneity, which is characterised by the non-Gaussian parameter α2（t）. The maximum value of α 2（t）, α（2,max）, increases at an accelerating rate as the temperature falls below Tx. The development of α（2,max） was found to be related to the liquid structure change evidenced by local fivefold symmetry. Accordingly, we suggest that this high-temperature breakdown of SER has a structural origin. The results of this study are expected to make researchers reconsider the applicability of SER and promote greater understanding of the relationship between dynamics and structure.
