A method for analyzing the mechanical properties of nanomaterials using molecular dynamics simulation

Abstract

Based on the GROMOS53a6 all-atom force field, the verified 12‑6LJ parameters were used to describe the atomic interactions of quintuple twinned silver nanowires, reproducing the mechanical properties of face-centered cubic metals under ambient conditions. Then, three different labeling methods were used for the quintuple twinned silver wire model using materials studio software. The stretching trajectory of the model was visualized by VMD software, and the atomic movement at any position of the model could be observed in any frame. We observed the atomic mode caused by dislocation decomposition from the side of the model and the cross-section at a specific point, and discovered the special atomic movement law guided by the quintuple twin structure. This labeling method provides ideas for studying the changes in the internal lattice structure of nanomaterials through simulation calculations, and also transforms the abstract dislocation movement into concrete atomic movement through specific model evolution, which helps to understand the more abstract basic concepts in crystallography.

Description

Technical Field

[0001] The invention belongs to the technical field of nanomaterials and relates to a method for analyzing stress hardening in a necking process of quintuple twinned silver nanowires based on molecular dynamics simulation. Background Art

[0002] As a one-dimensional nanomaterial, quintuple twinned silver nanowires have the advantages of simple synthesis methods, high electrical conductivity, good thermal conductivity, and excellent mechanical properties such as high flexibility, which make them widely used in materials, electronics, aerospace and other fields. Therefore, the research on the mechanical properties of quintuple twinned silver nanowires has received more and more attention.

[0003] Quintuple twinned silver nanowires are nanoscale materials. In terms of experiments, the study of their mechanical properties is easily restricted by the loading experimental equipment, experimental environment, and detection and analysis instruments, and the experiments are labor-intensive and costly. For example, in existing literature, the nanowires are directly stretched after being fixed at both ends by in-situ transmission electron microscopy, and the changes in the nanowires are dynamically observed during the stretching process, and the stress-strain curve is obtained. The entire process is already a very challenging task from fixing the two ends of the nanowire. In addition, it is also necessary to pay attention to applying the appropriate amount of force to the nanowire so that it does not break too quickly, so that the deformation process of the nanowire or more changes in lattice details cannot be observed clearly.

[0004] As a scientific research method of "numerical experiment", molecular dynamics simulation is widely used in materials, medicine, machinery and other fields because of its high efficiency and effective cost saving of experimental research. There have been a lot of works on simulation calculations for the study of mechanical properties of nanowires, but the observation of the movement of atoms during stretching and the generation of defects mostly require the use of the central symmetry parameter (CSP) developed by Kelchner et al. to identify defects by the color of atoms. This calculation method requires the definition of the central symmetry parameter of each atom in the calculation group. In solid-state systems, the central symmetry parameter is a useful indicator of the disorder of the local lattice around atoms, which can be used to characterize whether the atom is part of a perfect lattice, a local defect (such as a dislocation or stacking fault), or on the surface. The calculation method involved in this process of identifying defects is relatively complex.

[0005] However, if we use the same method to observe the lattice changes and atomic movements of quintuple-twinned silver nanowires during stretching, our attention will be distracted by many details. Therefore, we mark the model with different types of colors when it is not deformed to highlight the structural changes in the deformation of the quintuple-twin model, allowing us to gain a clearer understanding of the basic principles of the nanowire deformation process.

[0006] According to the applicant's understanding, the method of studying nanowires using molecular dynamics simulation has not yet gained an understanding of the stress hardening during the necking process of quintuple twinned nanowires, and there has been no report on the dislocation movement process that uses color marking on the initially deformed model to transform the abstract dislocation movement into concrete atomic movement. Summary of the invention

[0007] The technical problem solved by the present invention is to analyze the mechanical properties of nanomaterials by molecular dynamics simulation, and to analyze the principle of stress hardening during the necking process of quintuple twinned silver nanowires by combining an effective marking method.

[0008] In order to solve the above technical problems, the technical solution proposed by the present invention is: a method for analyzing the mechanical properties of nanomaterials using molecular dynamics simulation, the experimental steps are as follows

[0009] Step (1): Based on the structural characteristics of the quintuple twinned silver nanowires, the axial direction is

[110] and the surface is arranged in {100} crystal planes, a quintuple twinned silver nanowire model is automatically constructed by writing a script;

[0010] Step (2): Using GROMACS 5.0.7 software, based on the gromos53a6 all-atom force field and the verified 12-6LJ parameters, a balanced simulation was performed on the constructed five-fold twinned silver nanowire to obtain a relatively stable structure;

[0011] Step (3): stretching the constructed five-fold twinned silver nanowire model by stretching molecular dynamics (SMD); first, stretching the model at 300-500K for 5-20ns, and observing the stretching process using visual molecular dynamics (VMD) graphics software;

[0012] Step (4): Import the model created in step (1) into materials studio for atomic labeling. Use three labeling methods to label the atoms of the overall model vertically and parallel to the axial arrangement respectively. Then import VMD to observe the stretching trajectory, extract the atomic configuration at any time during the stretching process, and import it into materials studio for detailed observation in three-dimensional space.

[0013] In step (4), the marking steps of the three marking methods are as follows:

[0014] Step (1): Import the quintuple twinned silver nanowires into materials studio for atomic labeling. First, label the atoms in each grain boundary with five different colors and label the atoms at the twin boundary with orange. Then, import the model with each single crystal region labeled with different colors into VMD graphics software to observe the atomic movement process during stretching. At this time, the atomic arrangement of the model at any time point can be exported from VMD software and imported into materials studio software again for detailed observation.

[0015] Step (2): In the previous step, the area where the model was severely deformed was observed. Before the deformation, mark each 110-atom layer with a different color, then import it into VMD graphics software to observe the stretching process. Export the atomic arrangement of the model at any time point from VMD software, and import it into materials studio software again for detailed observation.

[0016] Step (3): When the model is not deformed, mark the 100 faces of each layer from the central axis to the outermost layer with different colors, import it into VMD graphics software to observe the stretching process, select and export the atomic arrangement at any time point, and import it into materials studio software again for detailed observation.

[0017] Preferably, the construction of the model in step (1) is based on the structural characteristics of the quintuple twinned silver nanowire, which has an axial direction of

[110] and a surface of {100} crystal plane arrangement, and the quintuple twinned silver nanowire model is automatically constructed by writing a script; the diameter of the pentagonal prism is 3.03 nanometers and the length is 25.28 nanometers, involving a total of 34,408 silver atoms, of which the axial direction is

[110] and the surface is a {100} crystal plane.

[0018] Preferably, in step (2), GROMACS 5.0.7 software is used to perform equilibrium simulation on the constructed quintuple twinned silver nanowires based on the gromos53a6 all-atom force field and the verified 12-6LJ parameters to obtain a relatively stable structure;

[0019] Preferably, in step (3), the quintuple twinned silver nanowire is first stretched at 300K for 20ns, and the stretching process is observed using visual molecular dynamics VMD graphics software, and the quintuple twinned silver nanowire is stretched by guided molecular dynamics SMD; the two ends of the 1.2nm quintuple twinned silver nanowire are fixed with a length of 1.2nm each, and the atomic spacing of this part does not change during the stretching process, and one end is reversely stretched along the axial direction at a constant speed, and the stretching speed is 1m / s.

[0020] Beneficial effects of the present invention:

[0021] (1) Based on the structural characteristics of the quintuple twinned silver nanowire, which has an axial direction of

[110] and a surface of {100} crystal plane arrangement, a quintuple twinned silver nanowire model was automatically constructed by writing a script; the mechanical properties of the face-centered cubic metal under ambient conditions were reproduced based on the gromos53a6 all-atom force field and the 12-6LJ parameters provided by Hendrik Heinz et al. Then, three different marking methods were used for the quintuple twinned silver wire model using the materials studio software. The stretching trajectory of the model was observed using the VMD software, and the atomic movement at any position of the model could be observed in any frame. We observed the atomic mode caused by dislocation decomposition from the side of the model and the cross-section at a specific point, and discovered the special atomic movement law guided by the quintuple twinned structure. This marking method also provides ideas for studying the changes in the internal lattice structure of nanomaterials through simulation calculations.

[0022] (2) By comparing with the necking structure in actual stretching, the more abstract concept of dislocation movement in actual crystals is transformed into concrete atomic movement through the evolution of a specific model, which helps to understand the more abstract basic concepts in crystallography. This method also provides new ideas for marking defects in more nanomaterial simulation calculations.

[0023] (3) Using molecular dynamics simulation to analyze the mechanical properties of nanomaterials can enable us to effectively predict the mechanical properties of actual crystalline materials. By labeling atoms to analyze the essential factors that affect the mechanical properties of materials, it is of great help to the application and development of nanomaterials. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described below in conjunction with the accompanying drawings.

[0025] Figure 1 This is a schematic diagram of the five-fold twinned silver nanowire model where the atomic layer collapses along the {111} plane toward the necking center after 1600ps (indicated by the orange arrow);

[0026] Figure 2 (a) The atoms in each grain boundary are marked with five different colors, and the atoms at the twin boundary are marked in orange; (b) The necking cross-section at 2200ps;

[0027] Figure 3 (a) The whole model in which the atoms in each grain boundary are marked with five different colors and the atoms at the twin boundary are marked with orange; (b) The whole model in which the necking occurs at 2200ps (the blue arrow is the observation position of the model cross section);

[0028] Figure 4(a) shows that each {111} plane gradually becomes smaller in the neck, and the inset shows that the redundant atoms are forced to move along the twin plane; (b) shows the color separation marking of 110 atomic layers at the undeformed front of the model. When stretched to 2200ps, the lines of the edge atoms are disrupted, and the sharp corners point to the center of the neck;

[0029] Figure 5 The model is marked with different colors along each layer of {100} plane from the axis to the outside, showing the full picture of the model at 900ps, 1600ps, 2200ps, and 5000ps (the arrow is the observation position of the model section);

[0030] Figure 6 The model is a cross-sectional view of the model with each layer {100} plane marked with different colors from the axis to the outside at (a) 900ps, (b) 1600ps, (c) 2200ps, and (d) 500ps;

[0031] Figure 7 This is the overall view and cross-sectional view of the tetrahedral single-crystal silver nanowire at three time points: 1326ps, 1362ps, and 10116ps.

[0032] Figure 8 This is the overall view and cross-sectional view of the cylindrical single-crystalline silver nanowire at four time points: 2350ps, 2370ps, 2698ps, and 8070ps.

[0033] Fig. 9 This is a spherical aberration electron microscope image of the inside of the neck, marking the face angle dislocation configuration consisting of two stacking faults and two incomplete dislocations;

[0034] Fig.10 is the curve of force and energy changing with time during the stretching of the quintuple twin;

[0035] Fig.11 is the average distance between adjacent {110} face atoms; DETAILED DESCRIPTION

[0036] Example 1

[0037] All models were automatically constructed by scripting based on the structural characteristics of the quintuple twinned silver nanowires, with the axial direction being

[110] and the surface being arranged in the {100} crystal plane. The diameter of the quintuple twins is 3.03 nm and the length is 25.28 nm, involving a total of 34,408 silver atoms, with the axial direction being

[110] and the surface atoms being arranged in {110}. GROMACS 5.0.7 software was used to describe the interactions between the atoms of the quintuple twinned silver nanowires based on the gromos53a6 all-atom force field and the verified 12-6LJ parameters. The time step was set to 2 fs, and the threshold of the van der Waals (vdW) interaction force was 1.2 nm. The velocity-rescale heat bath method was used to keep the system temperature at 300 K.

[0038] Example 2

[0039] The five-fold twinned silver nanowires were stretched by stretching molecular dynamics (SMD). The atoms within a length range of 1.2nm were fixed at both ends of the 1.2nm five-fold twinned silver nanowire, and the atomic spacing of this part did not change during the stretching process. One end was stretched in the opposite direction along the axial direction at a constant speed, and the stretching speed was 1m / s.

[0040] Example 3

[0041] First, the five-fold twinned silver nanowires were subjected to NVT relaxation for 50 ps at 300 K to obtain the equilibrium configuration. Then, the five-fold twinned silver nanowires were stretched for 20 ns at 300 K, and the stretching process was observed using the visual molecular dynamics (VMD) graphics software.

[0042] It can be observed that the five-fold twin model begins to deform at 1600ps. As the deformation deepens, the atomic layer moving along the {111} plane can be seen to collapse toward the center of the deformation area. Figure 1 )

[0043] Example 4

[0044] The quintuple twinned silver nanowires were imported into the Materials Studio for atomic labeling. First, the atoms within each grain boundary were labeled with five different colors, and the atoms at the twin boundaries were labeled orange ( Figure 2 a), in order to observe the position change of grain boundaries during the deformation of the silver wire.

[0045] Example 5

[0046] Then, the model with each single crystal region marked with different colors is imported into the VMD graphics software, and stretched in the same stretching method as in Example 3. Figure 3) observed that at 2200ps, the necking part of the quintuple twin was already very obvious.

[0047] Example 6

[0048] At this point, you can export the atomic arrangement of the model at any time point from the VMD software and import it into the materials studio software for detailed observation. Figure 3 The arrow above the model is the mark point. By deleting the atoms on the left side of the necking part, the movement of atoms inside the necking can be observed in three-dimensional space. It can be found that although the necking causes serious deformation of the model, the atoms on both sides of the grain boundary do not melt into each other ( Figure 2 b).

[0049] Example 7

[0050] Before the model is deformed, the atoms that will be deformed during the stretching process are marked with different colors for each 110 atomic layer and imported into the VMD graphics software to observe the stretching process. It can be seen that each {111} face is a triangle, and in the necking section ( Figure 4 The excess atoms are forced to move along the twin planes and crowd around the central core, disrupting the lines of the edge atoms and pointing to the center of the neck ( Figure 4 b).

[0051] Example 8

[0052] When the model is not deformed, mark the {100} plane of each layer from the central axis to the outermost layer with different colors, import it into VMD graphics software to observe the stretching process, and then import it into materials studio software for detailed observation. Figure 5 The arrow above the model is the mark point. By deleting the atoms on the left side of the necking part, the movement of atoms inside the necking part can be observed in three-dimensional space. It can be found that from the cross-section ( Figure 6 a), as the {111} crystal plane moves inward and gradually becomes smaller, the excess atoms gather near the grain boundary. The necking starts at 1600ps (Fig. 6b), and the product is at 2200ps (Fig. Figure 6 c) presents a typical star shape, with the preserved twin boundary passing through the star tip. The inner atomic arrangement near the central core is much more fragmented than the outer atomic arrangement. The concave boundary of the star corresponds to Figure 6 The location of the partial dislocation observed in Figure 6 As can be seen in d, even after the silver nanowire is completely broken at 5000ps, there is no further advancement of the concave boundary as seen from the cross section, which is consistent with the behavior of face angle dislocations.

[0053] Example 9

[0054] A rectangular cube (6nm×6nm×25nm) was built, involving 58381 silver atoms, where the model surface was {100} and the axial direction was

[110] . The time step was set to 2fs, and the threshold of the van der Waals (vdW) interaction force was 1.2nm. The velocity-rescale heat bath method was used to keep the system temperature at 300K. The tetrahedral single-crystalline nanowire was stretched by guided molecular dynamics (SMD). The two ends of the 6nm thick silver nanowire were fixed and stretched in the z direction at a constant speed, and the stretching speed was 1m / s. The silver nanowire was subjected to NVT relaxation for 50ps at 300K to obtain the equilibrium configuration. Then the tetrahedral single-crystalline nanowire was stretched for 20ns at 300K, and the stretching process was observed using the visual molecular dynamics (VMD) graphics software. In order to further observe the movement of atoms, we color-coded each layer of atoms in the tetrahedral single-crystal silver nanowire and then stretched it. In the simulation calculation results, we found that when the tetrahedral single-crystal silver wire entered the necking stage, stacking faults also occurred, showing very obvious atomic dislocations in the model, and without any obstruction, running through the entire model along the {111} plane. However, as the stretching progressed, necking began to occur at 1362ps, accompanied by the interweaving of stacking faults transmitted from nucleation at various locations on the surface, resulting in face angle dislocations, which led to the ultra-large plastic deformation of the single-crystal tetrahedral prism, and it did not break until 10116ps ( Figure 7 ).

[0055] Example 10

[0056] A cylindrical single-crystal silver nanowire model was established to observe the changes in its lattice structure during the stretching deformation process. The {(111} plane is perpendicular to the wire axis. A total of 41,480 silver atoms are involved. The wire is stretched in the z direction at a constant speed. In order to further observe the movement of atoms, we color-code each layer of the cylindrical single-crystal silver nanowire. The deformation of the cylindrical single-crystal silver nanowire model at different times is observed, as shown in Figure 2. Figure 8 From the main view, before necking occurs, the single crystal nanowire keeps its original appearance, but in the initial time of necking, that is, 2370ps, it can be seen that the stacking faults nucleated from the surface cause the silver wire to deform, and the stacking faults running through the entire silver wire can be seen in the cross-sectional view. However, unlike the quintuple twins, the single crystal cylindrical nanowires lack the obstruction of the twin boundaries and are obviously not so easy to get stuck. Therefore, we captured the model image at 2698ps some time after the necking occurred, and saw more parallel stacking faults in the cross-sectional view.

[0057] Comparative Example 1

[0058] Through the method of electrospinning fiber membrane wrapping and stretching, the same necking structure as in the simulation calculation results can be obtained. After the necking is photographed by a high-resolution electron microscope, Fig. 9 In the figure, a structure consisting of two stacking faults slipping along the {111} plane intertwined can be observed inside the neck. Incomplete dislocations decomposed from full dislocations are marked near the two stacking faults. After meeting, these two incomplete dislocations will merge according to the energy and geometric principles. The slip plane of the new dislocation after the merger is no longer the {111} plane, so the new dislocation cannot move in the FCC lattice structure. This configuration formed by two stacking faults and three incomplete dislocations is called a face angle dislocation. Face angle dislocation is an immovable defect in the lattice structure, so it will also serve as a Frank dislocation source, causing the crystal to further accumulate defects in the same place during deformation and harden.

[0059] Comparative Example 2

[0060] The force and energy changes over time during the simulation and calculation of the stretching process were statistically analyzed (Figure 10). There was a large rise before the neck was formed, starting from 1600ps. As plane slip and bond stretching occurred in the neck area, the energy and force dropped sharply. The second kinetic barrier occurred at 2200ps, which may be due to the strain hardening caused by the face angle dislocation. Comparing the atomic positions at the concave surface of the star at 2200 ps and after the fracture, it was found that the atoms on the concave surface after the fracture did not move inward, which corresponds to the movement law of the face angle dislocation.

[0061] Comparative Example 3

[0062] Measure the average distance between adjacent {110} face atoms ( Fig.11 ) shows that before the necking begins, the bond distance increases continuously, and the bond length in the intact area returns to the initial length, which is consistent with the sharp drop in the energy / force curve. For the neck region where the structure is deformed, the average atomic distance no longer reflects the bond length, but reflects the degree of deformation. After 2200ps, the changes mainly occur in the atoms between the 50th and 54th layers. In other words, the middle neck region is formed by the downward sliding of the middle layers, such as Figure 1 shown.

[0063] The comparative examples prove that the conclusions obtained by this atomic labeling method have good reliability and reference value. Combining it with the molecular dynamics simulation analysis method can provide a more intuitive analysis of the factors affecting the mechanical properties of nanomaterials.

[0064] The present invention is not limited to the specific technical solutions described in the above embodiments, and all technical solutions formed by equivalent replacement are within the protection scope required by the present invention.

Claims

1. A method for analyzing the mechanical properties of nanomaterials using molecular dynamics simulation, characterized in that The experimental steps are as follows: Step (1): Based on the structural characteristics of the quintuple twinned silver nanowires, which have an axial direction of [110] and a surface arrangement of {100} crystal planes, a quintuple twinned silver nanowire model is automatically constructed by writing a script; Step (2): Using GROMACS 5.0.7 software, based on the gromos53a6 all-atom force field and the verified 12-6 LJ parameters, a balanced simulation was performed on the constructed five-fold twinned silver nanowires to obtain a relatively stable structure; Step (3): stretching the constructed quintuple twinned silver nanowire model by stretching molecular dynamics (SMD); first, stretching the model at 300-500K for 5-20 ns, and using visual molecular dynamics (VMD) graphics software to observe the stretching process; Step (4): Import the model created in step (1) into materials studio for atomic labeling. Use three labeling methods to label the atoms of the entire model vertically and parallel to the axial arrangement, respectively. Import VMD to observe the stretching trajectory, extract the atomic configuration at any time during the stretching process, and import it into materials studio for detailed observation in three-dimensional space. In step (4), the marking steps of the three marking methods are as follows: Step (1): Import the quintuple twinned silver nanowires into materials studio for atomic labeling. First, label the atoms in each grain boundary with five different colors and label the atoms at the twin boundary with orange. Then, import the model with each single crystal region labeled with different colors into VMD graphics software to observe the atomic movement process during stretching. At this time, the atomic arrangement of the model at any time point can be exported from VMD software and imported into materials studio software again for detailed observation. Step (2): In the previous step, mark the area where the model is severely deformed with different colors before deformation, and then import it into VMD graphics software to observe the stretching process. Export the atomic arrangement of the model at any time point from VMD software and import it into materials studio software again for detailed observation; Step (3): When the model is not deformed, mark the {100} plane of each layer from the central axis to the outermost layer with different colors, import it into VMD graphics software to observe the stretching process, select and export the atomic arrangement at any time point, and import it into materials studio software again for detailed observation; The model in step (1) is constructed based on the structural characteristics of the quintuple twinned silver nanowire, which has an axial direction of [110] and a surface of {100} crystal plane arrangement. The quintuple twinned silver nanowire model is automatically constructed by writing a script; the diameter of the pentagonal prism is 3.03 nanometers, the length is 25.28 nanometers, and a total of 34,408 silver atoms are involved, of which the axial direction is [110] and the surface is a {100} crystal plane; In step (2), a relatively stable structure was obtained by performing equilibrium simulation on the constructed quintuple twinned silver nanowire using GROMACS 5.0.7 software based on the gromos53a6 all-atom force field and the verified 12-6LJ parameters; the time step was set to 2 fs, and the velocity-rescale heat bath method was used to keep the system temperature at 300 K.

2. The method for analyzing the mechanical properties of nanomaterials using molecular dynamics simulation according to claim 1, characterized in that: Step (3) firstly stretches the quintuple twinned silver nanowire at 300 K for 20 ns, and observes the stretching process using visualized molecular dynamics (VMD) graphics software, and stretches the quintuple twinned silver nanowire by stretching molecular dynamics (SMD); fixes the atoms within a length range of 1.2 nm at both ends of the quintuple twinned silver nanowire, so that the atomic spacing of this part does not change during the stretching process, and reversely stretches one end along the axial direction at a constant speed, and the stretching speed is 1 m / s.

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