Qing Peng | University of Michigan (original) (raw)

Papers by Qing Peng

Nanotechnology, 2020

Carbon nanotubes are outstanding reinforcements owing to their unparallel strength, while their e... more Carbon nanotubes are outstanding reinforcements owing to their unparallel strength, while their effects on the copper nanowire are still not fully understood, hampering their broad applications. Herein, we have investigated the tensile behaviors of the nanocomposite-wire of carbon nanotube-copper using molecular dynamic simulations. For the nanocomposite, both the coated and embedded carbon nanotubes increase the Young's modulus, fracture stress and toughness of the copper nanowire. A reinforcement of over fivefold in both yield strength (5.3 times) and toughness (5.1 times) has been achieved when the carbon nanotubes are coated on the copper nanowires, as well as 1.7 times in the Young's modulus. Higher temperatures and lower loading rates reduce the reinforcement. Supplementary material for this article is available online

RSC Advances, 2020

Graphyne was recently facilely synthesized with superior mechanical and electrical performance. W... more Graphyne was recently facilely synthesized with superior mechanical and electrical performance. We investigate the ballistic protection properties of a-, b-, d-, and g-graphyne sheets using molecular dynamics simulations in conjunction with elastic theory. The velocities of the in-plane elastic wave and out-of-plane cone wave are obtained by both membrane theory and molecular dynamics simulations. The specific penetration energies are approximately 83% that of graphene, indicating high impact resistance. g-Graphyne has high sound wave speeds comparable to those of graphene, and its Young's modulus is approximately 60% that of graphene. d-Graphyne has the highest cone wave speed among the four structures, while a-graphyne possesses the highest penetration energy and impact resistance at most tested projectile speeds. Our results indicate that graphyne is a good protective structural material.

Materials, 2020

Hydrogen plays a significant role in the microstructure evolution and macroscopic deformation of ... more Hydrogen plays a significant role in the microstructure evolution and macroscopic deformation of materials, causing swelling and surface blistering to reduce service life. In the present work, the atomistic mechanisms of hydrogen bubble nucleation in vanadium were studied by first-principles calculations. The interstitial hydrogen atoms cannot form significant bound states with other hydrogen atoms in bulk vanadium, which explains the absence of hydrogen self-clustering from the experiments. To find the possible origin of hydrogen bubble in vanadium, we explored the minimum sizes of a vacancy cluster in vanadium for the formation of hydrogen molecule. We show that a freestanding hydrogen molecule can form and remain relatively stable in the center of a 54-hydrogen atom saturated 27-vacancy cluster.

Journal of Alloys and Compounds, 2020

Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relati... more Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relatively high lattice thermal conductivity at modest temperatures limits their performance. Here, we investigate the lattice dynamics in a polycrystalline CuInTe 2 with a combined experimental and computational approach. The phonon dispersion and density of states are computed using the density functional theory. Raman scattering is performed to investigate the phonon dynamical properties. Together with the bulk modulus from X-ray diffraction, the mode-Grüneisen parameters are determined. The low energy B 1 1 and E 2 modes characterize the weak anharmonicity, thus responsible for the high lattice thermal conductivity. Meanwhile, B 1 1 and E 2 modes display energy redshift under pressure. The softening and enhanced anharmonicity of these modes naturally result in the reduction of the thermal conductivity. Our study suggests that pressure is a routine to reduce the phonon heat conduction at modest temperatures in chalcopyrites.

Catalysis Science & Technology, 2020

Spin polarized density functional theory computations were performed to elucidate electronic effe... more Spin polarized density functional theory computations were performed to elucidate electronic effects based on first-row transition metal doped Fe(100) and Fe 5 C 2 IJ100) surfaces for CO dissociation. Both Mn and Cr doped Fe(100) and Fe 5 C 2 IJ100) surfaces can enhance the dissociation of CO, while the Co, Ni and Cu doped ones are unfavorable for the dissociation of CO. Besides the BEP relationship, a linear relationship between activation energy and the electronegativity of the dopant atom is established. It can be deduced that the metals with lower electronegativity are favorable for the activation of CO. The reason has been analyzed by density of states and crystal orbital Hamilton population. The metals with lower electronegativity relative to Fe could donate electrons to doped sites and then activate CO with a more delocalized O 2p orbital. The electronic effects revealed herein are helpful for the understanding of the CO activation process and for the design of catalysts with desired activity.

Nanotechnology, 2020

Silicon carbide has excellent properties such as high hardness and decomposition temperature, but... more Silicon carbide has excellent properties such as high hardness and decomposition temperature, but its applications are limited by its poor toughness. Here, we investigate the enhancement of SiC's toughness by compositing silicon carbide-aluminum (SiC-Al) interpenetrating phase composites (IPCs) via molecular dynamics simulations. IPCs are a class of composites consisting of two or more phases that are topologically continuous and three-dimensionally interconnected through the microstructure. The Young's modulus and ultimate strength gradually increases with an increment of the volume fraction of SiC, opposite to the fracture strain. The interface between SiC and Al affects the mechanical properties of SiC-Al IPCs. When the volume fraction of SiC is less than 0.784, the attenuation rate of ultimate strength and fracture strain decreases. The attenuation rate increases when the volume fraction of SiC is more than 0.784. There are a minimum of ultimate strength and fracture strain at the 0.784, 0.7382 and 2.8610, respectively. The hardness of SiC-Al IPCs is about 48% of SiC. The change of SiC-Al IPCs hardness is more stable than that of SiC in the later stage of the nanoindentation test. Supplementary material for this article is available online

Applied Surface Science, 2020

The morphology change is crucial to the catalysis performance of catalyst nanoparticles in hetero... more The morphology change is crucial to the catalysis performance of catalyst nanoparticles in heterogeneous cat-alysis. Iron and iron carbide nanoparticles are used as high temperature heterogeneous catalyst, such as Fischer-Tropsch synthesis and carbon nanotube growth. Here we have investigated the effect of temperature and entropy on the surface free energy and morphology of the iron and iron carbide (θ-Fe 3 C, χ-Fe 5 C 2 , and o-Fe 7 C 3) nano-particles using molecular dynamics. The free energies of all the bulk and most of the surface systems drop following a parabolic curve with elevating temperature due to entropic effect. The nanoparticles are covered by low index surfaces at low temperature. At low temperature, the surface free energies of all surfaces usually decrease with a similar slope with increasing temperature. However, a critical temperature exists at which the high-index surfaces starts to dominate the catalyst particles. Fe 7 C 3 shows an unusual minimum surface free energy at 400 K in all the surfaces. This study provides fundamental insights into the modulation of iron-based nanocatalysts morphologies with desired catalytic performance.

Applied Surface Science, 2020

Surface feature and its variation along with complex atmosphere are of fundamental significance t... more Surface feature and its variation along with complex atmosphere are of fundamental significance to understanding the functionality of applied materials especially in heterogeneous catalysis and corrosion prevention. Here we performed a unified theoretical study on the surface structure and morphology of iron borides and their evolution under dynamic gaseous conditions by combination of density functional theory, ab initio atomic thermodynamics and Wulff construction. In particular, thermodynamic stability of iron borides and corresponding surfaces varied from the boron chemical potential (μ Δ B) of certain atmosphere, which increases with decreasing pressure and increasing temperature and concentration of boron source. The stability of boron-rich surfaces has been improved with increasing μ Δ B , while all the Fe-rich facets of iron borides are favorable at low μ Δ B condition. Accordingly, the crystallite morphology of iron borides undergoes significant evolution upon dynamic condition. Finally, the surface properties of iron borides are carefully tested by CO adsorption which indicated the activation ability of CO is closely connected with boron triggered surface charge transfer between Fe and CO. This work was expected not only to help understand the surface structure and morphology of iron borides under realistic condition, but also provides fundamental insights into rational design of corrosion resistant and catalytic materials.

Mechanics of Materials, 2020

Silicon carbide is one of the most important semiconductors with wide bandgaps and various applic... more Silicon carbide is one of the most important semiconductors with wide bandgaps and various applications including power electronics, nuclear fuel particles, hostile-environment electronics, and blue light emitting diodes. We investigate the nonlinear mechanical properties of a proposed graphene-like planar hexagonal silicon carbide (g-SiC) monolayer using first-principles calculations. The strength of g-SiC is about half that of graphene. The ultimate strain of g-SiC is 0.2, 0.25, and 0.19, in the direction of in armchair, zigzag, and biaxial, respectively. The Poisson's ratio is 1.75 times of that of graphene. In the nonlinear elasticity regime, we obtain the high order elastic constants up to fifth order. The stiffness monotonically increases with pressure, has the same trend as that of second order elastic constants but opposes to that of Poisson's ratio. There is a minimum at −4 GPa in the speed-pressure curve of compressive sound wave, different from the monotonic increment of shear waves. These theoretical mechanical properties provide elasticity limits for various applications of g-SiC.

Materials & Design, 2020

Despite outstanding and unique properties, the structure-property relationship of high entropy al... more Despite outstanding and unique properties, the structure-property relationship of high entropy alloys (HEAs) is not well established. The machine learning (ML) is used to scrutinize the effect of nine physical quantities on four phases. The nine parameters include formation enthalpies determined by the extended Miedema theory, and mixing entropy. They are highly related to the phase formation, common ML methods cannot distinguish accurately. In this paper, feature selection and feature variable transformation based on Kernel Principal Component Analysis (KPCA) are proposed, the feature variables are optimized, the distinction of phases is carried out by Support vector machine (SVM) model. The results indicate that elastic energy and atom-size difference contribute significantly in the formation of different phases. The accuracy of testing set predicted by SVM based on four feature variables and KPCA (4V-KPCA) is 0.9743. The F1-scores predicted detailedly by SVM based on 4V-KPCA for the considered alloy phases are 0.9787, 0.9463, 0.9863 and 0.8103, corresponding to solid solution, amorphous, the mixture of solid solution and intermetallic, and intermetallic respectively. The extended Miedema theory provides accurate thermodynamic properties for the design of HEAs, and ML methods (especially SVM combined KPCA) are powerful in the prediction of alloy phases.

Scripta Materialia, 2020

Dual and triple ion beam irradiations were performed on alpha-Cr using 5 MeV Fe++, 2.9 MeV He++ a... more Dual and triple ion beam irradiations were performed on alpha-Cr using 5 MeV Fe++, 2.9 MeV He++ and/or 270 keV H+ to study the synergy of hydrogen and helium on cavity formation. Results indicate that co-implantation of helium with iron enhances cavity nucleation and co-implantation of hydrogen with iron augments cavity growth. Under triple beam irradiation, hydrogen also accelerates cavity nucleation via stabilizing initial embryos. First-principles calculations reveal that the presence of helium increases the maximum number of hydrogen atoms that can be captured by a vacancy from 6 to 9, providing an atomistic explanation to the triple beam synergy.

Nanotechnology, 2020

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradia... more High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy. Supplementary material for this article is available online

Applied Surface Science, 2021

Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is... more Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is of great importance in rationally modulating the catalytic performance of hydrogenation processes. Herein, we have investigated the interaction between hydrogen and four typical surfaces, (1 0 0), (2 1 0), (2 1 1), and (3 1 1) of pyrite FeS 2 through density functional theory calculations. On (2 1 0) surface, the hydrogen dissociative adsorption on unsaturated-coordination sulfur atoms is favorable both in thermodynamics and kinetics. The hydrogen activation barrier is 0.83 eV with slight exothermic of 0.12 eV on (3 1 1). While on (1 0 0) and (2 1 1) surface, the hydrogen dissociation is unfavorable due to the high activation barriers and remarkable positive reaction energies. For high adsorption coverage, the pure molecule adsorption mode is favorable on (1 0 0) facet, opposed to the other surfaces which have temperature and pressure dependence. The saturated coverage sequence is (1 0 0) > (2 1 0) > (2 1 1) > (3 1 1) for a wide range of temperature and pressure. The remove of sulfur atoms most likely occurs on (2 1 0) surface. Our atomistic insights might be useful in engineering hydrogen-involved processes catalyzed by iron sulfide.

Macromolecular Theory and Simulations, 2020

Amorphous solids in general exhibit a volume change during plastic deformation due to microstruct... more Amorphous solids in general exhibit a volume change during plastic deformation due to microstructure change during plastic relaxation. Here the deformation dilatancy of alkane polymer glasses upon shearing is investigated using molecular static simulations at zero temperature and pressure. The dilatancy of linear alkane chains has been quantified as a function of strain and chain length. It is found that the system densities decrease linearly with respect to strain after yield point. In addition, dilatability decreases considerably with increasing chain length, suggesting enhanced cooperation of different deformation mechanisms. An analytic model is introduced for dilatability based on the atomistic study. The entanglement chain length is predicted as 43 for alkane polymers from the model, agreeing well with experiments. The study provides insights of correlations of the physical properties and chain length of polymers which might be useful in material design and applications of structural polymers.

Nanomaterials, 2020

Carbon honeycomb has a nanoporous structure with good mechanical properties including strength. H... more Carbon honeycomb has a nanoporous structure with good mechanical properties including strength. Here we investigate the adsorption and diffusion of hydrogen in carbon honeycomb via grand canonical Monte Carlo simulations and molecular dynamics simulations including strength. Based on the adsorption simulations, molecular dynamics simulations are employed to study the effect of pressure and temperature for the adsorption and diffusion of hydrogen. To study the effect of pressure, we select the 0.1, 1, 5, 10, 15, and 20 bars. Meanwhile, we have studied the hydrogen storage capacities of the carbon honeycomb at 77 K, 153 K, 193 K, 253 K and 298 K. A high hydrogen adsorption of 4.36 wt.% is achieved at 77 K and 20 bars. The excellent mechanical properties of carbon honeycomb and its unique three-dimensional honeycomb microporous structure provide a strong guarantee for its application in practical engineering fields.

Journal of Physics and Chemistry of Solids, 2021

Graphene-reinforced nickel matrix nanocomposites with high-density interfaces are recommended as ... more Graphene-reinforced nickel matrix nanocomposites with high-density interfaces are recommended as candidate materials for advanced nuclear reactors because of the potential irradiation tolerance. Nonetheless, the mechanism that graphene damage due to irradiation affects the tolerance of the composites remains unclear. Here we report the relationships between irradiation damage behavior of graphene and defect sink efficiency of nickel–graphene interface by using atomistic simulations. With the accumulation of irradiation dose, a nickel–graphene interface exhibits enhanced trapping ability to defects despite the gradually deteriorative damage of graphene. The enhancement originates in that the damaged regions of graphene can provide abundant recombination and/or annihilation sites for irradiation defects and strengthen the energetic and kinetic driving forces of the interface to defects. This study reveals a new possible interface-mediated damage healing mechanism of irradiated materials.

Physica E: Low-dimensional Systems and Nanostructures, 2021

Due to outstanding electric conductance, copper is widely used as wires in electricity and as int... more Due to outstanding electric conductance, copper is widely used as wires in electricity and as interconnects in microchips, where the thermal conductivity could be enhanced by compositing with carbon nanotubes (CNTs). Here, we have numerically designed a class of sandwich-like CNT/Cu/CNT nanotubes which possess high thermal conductivity revealed by molecular dynamics simulations. The enhancement factor of thermal conductivity of the composite using single-walled CNT is 37.5 times of that of a 5-nm-radius copper nanowire. The enhancement factor is further enlarged to 58.2 using triple-walled CNT in the outer side. The atomic stress analysis manifests that the thermal stresses are concentrated in the region around the CNT/Cu interface. The stabilities and larger enhancement factors at high temperatures imply high temperature applications of these CNT-sandwiched tubular copper nanocomposites in heat management and electronics.

Nanomaterials, 2021

Ultra-thin and continuous metallic silver films are attracting growing interest due to the applic... more Ultra-thin and continuous metallic silver films are attracting growing interest due to the applications in flexible transparent conducting electrodes. The surface morphology and structure of silver film are very important for its electrical resistivity and optical loss. Therefore, roughness control is essential for the production of ultra-thin metallic electrode film. We have investigated the effect of aluminum doping on the improvement of surface morphology of ultra-thin silver films using molecular dynamics simulations. Al-doped silver films showed smaller surface roughness than pure silver films at various substrate temperatures. When the temperature of the substrate was 600 K, the roughness of Al-doped silver film first decreased, and then increased with the increase of the incident velocity of silver atoms. Silver atoms were more likely to agglomerate on the surface of the substrate after adding aluminum atoms, as aluminum dopants promoted the immobilization of silver atoms on SiO2 substrate due to the anchoring effect. The smoother surface could be attributable to the reduced mean free path of silver due to the cage effect by the aluminum dopant.

Nanoscale, 2019

Various types of topological defects are produced during proton irradiation, which are crucial in... more Various types of topological defects are produced during proton irradiation, which are crucial in functiona-lizing graphene, but the mechanisms of the defect generation process and the structure change are still elusive. Herein, we investigated the graphene defect generation probabilities and defect structures under proton irradiation using both ab initio and classical molecular dynamics simulations. As the proton energy increases from 0.1 keV to 100 keV, defect structures transition from single vacancy and Frenkel pairs to a rich variety of topological defects with the possibility of ejecting multiple atoms. We show that, relatively good agreement on defect generation probabilities can be reached between the two simulation approaches at a proton energy of 1 and 10 keV. However, at 0.1 keV, the single vacancy generation probability differs significantly in two methods due to the difference in the energy required to form single vacancy. Using the classical molecular dynamics simulation, we also studied the evolution of different types of defects and the dependence of their probabilities of occurrence on the proton energy and incident angle. The correlation between the impact positions and defect types allows for the convoluted relationship between the defect probabilities, geometric parameters, and proton energy to be elucidated. We show that the proton energy and incident angle can be used to effectively tune the generation probabilities of different types of defects. Our results provide insights into the controlled defect engineering through ion irradiation, which will be useful for the development of functionalized graphene and graphene electronics.

Nanotechnology, 2020

Carbon nanotubes are outstanding reinforcements owing to their unparallel strength, while their e... more Carbon nanotubes are outstanding reinforcements owing to their unparallel strength, while their effects on the copper nanowire are still not fully understood, hampering their broad applications. Herein, we have investigated the tensile behaviors of the nanocomposite-wire of carbon nanotube-copper using molecular dynamic simulations. For the nanocomposite, both the coated and embedded carbon nanotubes increase the Young's modulus, fracture stress and toughness of the copper nanowire. A reinforcement of over fivefold in both yield strength (5.3 times) and toughness (5.1 times) has been achieved when the carbon nanotubes are coated on the copper nanowires, as well as 1.7 times in the Young's modulus. Higher temperatures and lower loading rates reduce the reinforcement. Supplementary material for this article is available online

RSC Advances, 2020

Graphyne was recently facilely synthesized with superior mechanical and electrical performance. W... more Graphyne was recently facilely synthesized with superior mechanical and electrical performance. We investigate the ballistic protection properties of a-, b-, d-, and g-graphyne sheets using molecular dynamics simulations in conjunction with elastic theory. The velocities of the in-plane elastic wave and out-of-plane cone wave are obtained by both membrane theory and molecular dynamics simulations. The specific penetration energies are approximately 83% that of graphene, indicating high impact resistance. g-Graphyne has high sound wave speeds comparable to those of graphene, and its Young's modulus is approximately 60% that of graphene. d-Graphyne has the highest cone wave speed among the four structures, while a-graphyne possesses the highest penetration energy and impact resistance at most tested projectile speeds. Our results indicate that graphyne is a good protective structural material.

Materials, 2020

Hydrogen plays a significant role in the microstructure evolution and macroscopic deformation of ... more Hydrogen plays a significant role in the microstructure evolution and macroscopic deformation of materials, causing swelling and surface blistering to reduce service life. In the present work, the atomistic mechanisms of hydrogen bubble nucleation in vanadium were studied by first-principles calculations. The interstitial hydrogen atoms cannot form significant bound states with other hydrogen atoms in bulk vanadium, which explains the absence of hydrogen self-clustering from the experiments. To find the possible origin of hydrogen bubble in vanadium, we explored the minimum sizes of a vacancy cluster in vanadium for the formation of hydrogen molecule. We show that a freestanding hydrogen molecule can form and remain relatively stable in the center of a 54-hydrogen atom saturated 27-vacancy cluster.

Journal of Alloys and Compounds, 2020

Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relati... more Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relatively high lattice thermal conductivity at modest temperatures limits their performance. Here, we investigate the lattice dynamics in a polycrystalline CuInTe 2 with a combined experimental and computational approach. The phonon dispersion and density of states are computed using the density functional theory. Raman scattering is performed to investigate the phonon dynamical properties. Together with the bulk modulus from X-ray diffraction, the mode-Grüneisen parameters are determined. The low energy B 1 1 and E 2 modes characterize the weak anharmonicity, thus responsible for the high lattice thermal conductivity. Meanwhile, B 1 1 and E 2 modes display energy redshift under pressure. The softening and enhanced anharmonicity of these modes naturally result in the reduction of the thermal conductivity. Our study suggests that pressure is a routine to reduce the phonon heat conduction at modest temperatures in chalcopyrites.

Catalysis Science & Technology, 2020

Spin polarized density functional theory computations were performed to elucidate electronic effe... more Spin polarized density functional theory computations were performed to elucidate electronic effects based on first-row transition metal doped Fe(100) and Fe 5 C 2 IJ100) surfaces for CO dissociation. Both Mn and Cr doped Fe(100) and Fe 5 C 2 IJ100) surfaces can enhance the dissociation of CO, while the Co, Ni and Cu doped ones are unfavorable for the dissociation of CO. Besides the BEP relationship, a linear relationship between activation energy and the electronegativity of the dopant atom is established. It can be deduced that the metals with lower electronegativity are favorable for the activation of CO. The reason has been analyzed by density of states and crystal orbital Hamilton population. The metals with lower electronegativity relative to Fe could donate electrons to doped sites and then activate CO with a more delocalized O 2p orbital. The electronic effects revealed herein are helpful for the understanding of the CO activation process and for the design of catalysts with desired activity.

Nanotechnology, 2020

Silicon carbide has excellent properties such as high hardness and decomposition temperature, but... more Silicon carbide has excellent properties such as high hardness and decomposition temperature, but its applications are limited by its poor toughness. Here, we investigate the enhancement of SiC's toughness by compositing silicon carbide-aluminum (SiC-Al) interpenetrating phase composites (IPCs) via molecular dynamics simulations. IPCs are a class of composites consisting of two or more phases that are topologically continuous and three-dimensionally interconnected through the microstructure. The Young's modulus and ultimate strength gradually increases with an increment of the volume fraction of SiC, opposite to the fracture strain. The interface between SiC and Al affects the mechanical properties of SiC-Al IPCs. When the volume fraction of SiC is less than 0.784, the attenuation rate of ultimate strength and fracture strain decreases. The attenuation rate increases when the volume fraction of SiC is more than 0.784. There are a minimum of ultimate strength and fracture strain at the 0.784, 0.7382 and 2.8610, respectively. The hardness of SiC-Al IPCs is about 48% of SiC. The change of SiC-Al IPCs hardness is more stable than that of SiC in the later stage of the nanoindentation test. Supplementary material for this article is available online

Applied Surface Science, 2020

The morphology change is crucial to the catalysis performance of catalyst nanoparticles in hetero... more The morphology change is crucial to the catalysis performance of catalyst nanoparticles in heterogeneous cat-alysis. Iron and iron carbide nanoparticles are used as high temperature heterogeneous catalyst, such as Fischer-Tropsch synthesis and carbon nanotube growth. Here we have investigated the effect of temperature and entropy on the surface free energy and morphology of the iron and iron carbide (θ-Fe 3 C, χ-Fe 5 C 2 , and o-Fe 7 C 3) nano-particles using molecular dynamics. The free energies of all the bulk and most of the surface systems drop following a parabolic curve with elevating temperature due to entropic effect. The nanoparticles are covered by low index surfaces at low temperature. At low temperature, the surface free energies of all surfaces usually decrease with a similar slope with increasing temperature. However, a critical temperature exists at which the high-index surfaces starts to dominate the catalyst particles. Fe 7 C 3 shows an unusual minimum surface free energy at 400 K in all the surfaces. This study provides fundamental insights into the modulation of iron-based nanocatalysts morphologies with desired catalytic performance.

Applied Surface Science, 2020

Surface feature and its variation along with complex atmosphere are of fundamental significance t... more Surface feature and its variation along with complex atmosphere are of fundamental significance to understanding the functionality of applied materials especially in heterogeneous catalysis and corrosion prevention. Here we performed a unified theoretical study on the surface structure and morphology of iron borides and their evolution under dynamic gaseous conditions by combination of density functional theory, ab initio atomic thermodynamics and Wulff construction. In particular, thermodynamic stability of iron borides and corresponding surfaces varied from the boron chemical potential (μ Δ B) of certain atmosphere, which increases with decreasing pressure and increasing temperature and concentration of boron source. The stability of boron-rich surfaces has been improved with increasing μ Δ B , while all the Fe-rich facets of iron borides are favorable at low μ Δ B condition. Accordingly, the crystallite morphology of iron borides undergoes significant evolution upon dynamic condition. Finally, the surface properties of iron borides are carefully tested by CO adsorption which indicated the activation ability of CO is closely connected with boron triggered surface charge transfer between Fe and CO. This work was expected not only to help understand the surface structure and morphology of iron borides under realistic condition, but also provides fundamental insights into rational design of corrosion resistant and catalytic materials.

Mechanics of Materials, 2020

Silicon carbide is one of the most important semiconductors with wide bandgaps and various applic... more Silicon carbide is one of the most important semiconductors with wide bandgaps and various applications including power electronics, nuclear fuel particles, hostile-environment electronics, and blue light emitting diodes. We investigate the nonlinear mechanical properties of a proposed graphene-like planar hexagonal silicon carbide (g-SiC) monolayer using first-principles calculations. The strength of g-SiC is about half that of graphene. The ultimate strain of g-SiC is 0.2, 0.25, and 0.19, in the direction of in armchair, zigzag, and biaxial, respectively. The Poisson's ratio is 1.75 times of that of graphene. In the nonlinear elasticity regime, we obtain the high order elastic constants up to fifth order. The stiffness monotonically increases with pressure, has the same trend as that of second order elastic constants but opposes to that of Poisson's ratio. There is a minimum at −4 GPa in the speed-pressure curve of compressive sound wave, different from the monotonic increment of shear waves. These theoretical mechanical properties provide elasticity limits for various applications of g-SiC.

Materials & Design, 2020

Despite outstanding and unique properties, the structure-property relationship of high entropy al... more Despite outstanding and unique properties, the structure-property relationship of high entropy alloys (HEAs) is not well established. The machine learning (ML) is used to scrutinize the effect of nine physical quantities on four phases. The nine parameters include formation enthalpies determined by the extended Miedema theory, and mixing entropy. They are highly related to the phase formation, common ML methods cannot distinguish accurately. In this paper, feature selection and feature variable transformation based on Kernel Principal Component Analysis (KPCA) are proposed, the feature variables are optimized, the distinction of phases is carried out by Support vector machine (SVM) model. The results indicate that elastic energy and atom-size difference contribute significantly in the formation of different phases. The accuracy of testing set predicted by SVM based on four feature variables and KPCA (4V-KPCA) is 0.9743. The F1-scores predicted detailedly by SVM based on 4V-KPCA for the considered alloy phases are 0.9787, 0.9463, 0.9863 and 0.8103, corresponding to solid solution, amorphous, the mixture of solid solution and intermetallic, and intermetallic respectively. The extended Miedema theory provides accurate thermodynamic properties for the design of HEAs, and ML methods (especially SVM combined KPCA) are powerful in the prediction of alloy phases.

Scripta Materialia, 2020

Dual and triple ion beam irradiations were performed on alpha-Cr using 5 MeV Fe++, 2.9 MeV He++ a... more Dual and triple ion beam irradiations were performed on alpha-Cr using 5 MeV Fe++, 2.9 MeV He++ and/or 270 keV H+ to study the synergy of hydrogen and helium on cavity formation. Results indicate that co-implantation of helium with iron enhances cavity nucleation and co-implantation of hydrogen with iron augments cavity growth. Under triple beam irradiation, hydrogen also accelerates cavity nucleation via stabilizing initial embryos. First-principles calculations reveal that the presence of helium increases the maximum number of hydrogen atoms that can be captured by a vacancy from 6 to 9, providing an atomistic explanation to the triple beam synergy.

Nanotechnology, 2020

High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradia... more High entropy alloy has attracted extensive attention in nuclear energy due to outstanding irradiation resistance, partially due to sluggish diffusion. The mechanism from a defect-generation perspective, however, has received much less attention. In this paper, the formation of dislocation loops, and migration of interstitials and vacancies in CoNiCrFeMn high entropy alloy under consecutive bombardments were studied by molecular dynamics simulations. Compared to pure Ni, less defects were produced in the CoNiCrFeMn. Only a few small dislocation loops were observed, and the length of dislocation was small. The dislocation loops in Ni matrix were obviously longer and so was the length of dislocation. The interstitial clusters had much smaller mean free path during migration in the CoNiCrFeMn. The mean free path of 10-interstitial clusters in CoNiCrFeMn was reduced over 40 times compared to that in pure Ni. In addition, CoNiCrFeMn had a smaller difference of migration energy between interstitial and vacancy, which increased the opportunity of recombination of defects, therefore, led to less defects and much fewer dislocation loops. Our results provide insights into the mechanism of irradiation resistance in the high entropy alloy and could be useful in material design for irradiation tolerance and accident tolerance materials in nuclear energy. Supplementary material for this article is available online

Applied Surface Science, 2021

Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is... more Elucidating the interactions between hydrogen and catalysts under complex realistic conditions is of great importance in rationally modulating the catalytic performance of hydrogenation processes. Herein, we have investigated the interaction between hydrogen and four typical surfaces, (1 0 0), (2 1 0), (2 1 1), and (3 1 1) of pyrite FeS 2 through density functional theory calculations. On (2 1 0) surface, the hydrogen dissociative adsorption on unsaturated-coordination sulfur atoms is favorable both in thermodynamics and kinetics. The hydrogen activation barrier is 0.83 eV with slight exothermic of 0.12 eV on (3 1 1). While on (1 0 0) and (2 1 1) surface, the hydrogen dissociation is unfavorable due to the high activation barriers and remarkable positive reaction energies. For high adsorption coverage, the pure molecule adsorption mode is favorable on (1 0 0) facet, opposed to the other surfaces which have temperature and pressure dependence. The saturated coverage sequence is (1 0 0) > (2 1 0) > (2 1 1) > (3 1 1) for a wide range of temperature and pressure. The remove of sulfur atoms most likely occurs on (2 1 0) surface. Our atomistic insights might be useful in engineering hydrogen-involved processes catalyzed by iron sulfide.

Macromolecular Theory and Simulations, 2020

Amorphous solids in general exhibit a volume change during plastic deformation due to microstruct... more Amorphous solids in general exhibit a volume change during plastic deformation due to microstructure change during plastic relaxation. Here the deformation dilatancy of alkane polymer glasses upon shearing is investigated using molecular static simulations at zero temperature and pressure. The dilatancy of linear alkane chains has been quantified as a function of strain and chain length. It is found that the system densities decrease linearly with respect to strain after yield point. In addition, dilatability decreases considerably with increasing chain length, suggesting enhanced cooperation of different deformation mechanisms. An analytic model is introduced for dilatability based on the atomistic study. The entanglement chain length is predicted as 43 for alkane polymers from the model, agreeing well with experiments. The study provides insights of correlations of the physical properties and chain length of polymers which might be useful in material design and applications of structural polymers.

Nanomaterials, 2020

Carbon honeycomb has a nanoporous structure with good mechanical properties including strength. H... more Carbon honeycomb has a nanoporous structure with good mechanical properties including strength. Here we investigate the adsorption and diffusion of hydrogen in carbon honeycomb via grand canonical Monte Carlo simulations and molecular dynamics simulations including strength. Based on the adsorption simulations, molecular dynamics simulations are employed to study the effect of pressure and temperature for the adsorption and diffusion of hydrogen. To study the effect of pressure, we select the 0.1, 1, 5, 10, 15, and 20 bars. Meanwhile, we have studied the hydrogen storage capacities of the carbon honeycomb at 77 K, 153 K, 193 K, 253 K and 298 K. A high hydrogen adsorption of 4.36 wt.% is achieved at 77 K and 20 bars. The excellent mechanical properties of carbon honeycomb and its unique three-dimensional honeycomb microporous structure provide a strong guarantee for its application in practical engineering fields.

Journal of Physics and Chemistry of Solids, 2021

Graphene-reinforced nickel matrix nanocomposites with high-density interfaces are recommended as ... more Graphene-reinforced nickel matrix nanocomposites with high-density interfaces are recommended as candidate materials for advanced nuclear reactors because of the potential irradiation tolerance. Nonetheless, the mechanism that graphene damage due to irradiation affects the tolerance of the composites remains unclear. Here we report the relationships between irradiation damage behavior of graphene and defect sink efficiency of nickel–graphene interface by using atomistic simulations. With the accumulation of irradiation dose, a nickel–graphene interface exhibits enhanced trapping ability to defects despite the gradually deteriorative damage of graphene. The enhancement originates in that the damaged regions of graphene can provide abundant recombination and/or annihilation sites for irradiation defects and strengthen the energetic and kinetic driving forces of the interface to defects. This study reveals a new possible interface-mediated damage healing mechanism of irradiated materials.

Physica E: Low-dimensional Systems and Nanostructures, 2021

Due to outstanding electric conductance, copper is widely used as wires in electricity and as int... more Due to outstanding electric conductance, copper is widely used as wires in electricity and as interconnects in microchips, where the thermal conductivity could be enhanced by compositing with carbon nanotubes (CNTs). Here, we have numerically designed a class of sandwich-like CNT/Cu/CNT nanotubes which possess high thermal conductivity revealed by molecular dynamics simulations. The enhancement factor of thermal conductivity of the composite using single-walled CNT is 37.5 times of that of a 5-nm-radius copper nanowire. The enhancement factor is further enlarged to 58.2 using triple-walled CNT in the outer side. The atomic stress analysis manifests that the thermal stresses are concentrated in the region around the CNT/Cu interface. The stabilities and larger enhancement factors at high temperatures imply high temperature applications of these CNT-sandwiched tubular copper nanocomposites in heat management and electronics.

Nanomaterials, 2021

Ultra-thin and continuous metallic silver films are attracting growing interest due to the applic... more Ultra-thin and continuous metallic silver films are attracting growing interest due to the applications in flexible transparent conducting electrodes. The surface morphology and structure of silver film are very important for its electrical resistivity and optical loss. Therefore, roughness control is essential for the production of ultra-thin metallic electrode film. We have investigated the effect of aluminum doping on the improvement of surface morphology of ultra-thin silver films using molecular dynamics simulations. Al-doped silver films showed smaller surface roughness than pure silver films at various substrate temperatures. When the temperature of the substrate was 600 K, the roughness of Al-doped silver film first decreased, and then increased with the increase of the incident velocity of silver atoms. Silver atoms were more likely to agglomerate on the surface of the substrate after adding aluminum atoms, as aluminum dopants promoted the immobilization of silver atoms on SiO2 substrate due to the anchoring effect. The smoother surface could be attributable to the reduced mean free path of silver due to the cage effect by the aluminum dopant.

Nanoscale, 2019

Various types of topological defects are produced during proton irradiation, which are crucial in... more Various types of topological defects are produced during proton irradiation, which are crucial in functiona-lizing graphene, but the mechanisms of the defect generation process and the structure change are still elusive. Herein, we investigated the graphene defect generation probabilities and defect structures under proton irradiation using both ab initio and classical molecular dynamics simulations. As the proton energy increases from 0.1 keV to 100 keV, defect structures transition from single vacancy and Frenkel pairs to a rich variety of topological defects with the possibility of ejecting multiple atoms. We show that, relatively good agreement on defect generation probabilities can be reached between the two simulation approaches at a proton energy of 1 and 10 keV. However, at 0.1 keV, the single vacancy generation probability differs significantly in two methods due to the difference in the energy required to form single vacancy. Using the classical molecular dynamics simulation, we also studied the evolution of different types of defects and the dependence of their probabilities of occurrence on the proton energy and incident angle. The correlation between the impact positions and defect types allows for the convoluted relationship between the defect probabilities, geometric parameters, and proton energy to be elucidated. We show that the proton energy and incident angle can be used to effectively tune the generation probabilities of different types of defects. Our results provide insights into the controlled defect engineering through ion irradiation, which will be useful for the development of functionalized graphene and graphene electronics.