Multi-scale coupling simulation method for irradiation damage of energetic material

Abstract

The invention belongs to the technical field of material property simulation, and discloses a multi-scale coupling simulation method for irradiation damage of an energetic material, which integrates first principle calculation, Monte Carlo, molecular dynamics and accelerated molecular dynamics calculation means, and performs series connection and coupling on the simulation technologies of different scales to obtain the multi-scale coupling simulation method for irradiation damage of the energetic material. By utilizing a parallel computing technology, the limitation of single-scale simulation in the prior art is overcome, and long-time and large-scale simulation of the energetic material from atomic scale defect generation to microstructure characterization under the irradiation condition is realized; the method solves the problem of limitation of single-scale simulation in irradiation damage simulation of the energetic material in the prior art, and is suitable for irradiation damage simulation of the energetic material.

Description

Technical Field

[0001] The present invention relates to the technical field of material property simulation, and in particular to a multi-scale coupling simulation method for energetic material radiation damage. Background Art

[0002] Energetic materials are substances that rapidly release a large amount of heat and gas through their own redox reactions when they are sufficiently stimulated by the outside world. They face many external environmental tests during their use, among which the irradiation environment is one of the severe application environments faced by energetic materials. Irradiation conditions will affect the performance of energetic materials. Since energetic materials themselves are high-power energy-releasing materials involving chemical reactions, they are more sensitive to the external environment. Their irradiation damage experiments are difficult, long-term, cumbersome and costly. Therefore, a small number of targeted experimental results combined with computer simulation technology are the main methods for studying the irradiation damage and aging mechanism of energetic materials.

[0003] In the prior art, the research on energetic material simulation mainly focuses on the simulation of decomposition mechanism at the microscopic scale. In the field of material irradiation, although the corresponding multi-scale coupling method for metal material irradiation has been established, the energetic material involves intermolecular chemical reactions, and its damage mechanism is quite different from that of metal materials. The analysis method for metal materials cannot be effectively applied to energetic materials. At present, there is still no simulation method and cross-scale simulation system for energetic material irradiation damage. At the same time, due to the lack of potential functions for irradiation simulation, it is still unclear how to set up a molecular dynamics physical model for the chemical reaction of energetic material irradiation. There is no clear reference for the data to be analyzed and the properties to be calculated, so it is impossible to realize the simulation research of the damage and aging mechanism of energetic materials in irradiation environment. Summary of the invention

[0004] The present invention aims to provide a multi-scale coupled simulation method for energetic material irradiation damage to solve the limitation of single-scale simulation in the existing technology for energetic material irradiation damage simulation. The method of the present invention integrates multiple computing methods such as first-principles calculation, Monte Carlo, molecular dynamics and accelerated molecular dynamics, connects and couples these simulation technologies at different scales in series, and overcomes the limitation of single-scale simulation by means of parallel computing technology, thus realizing long-term and large-scale simulation of energetic materials from atomic-scale defect generation to microstructure characterization under irradiation conditions.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] S1. First principles calculations: Use the DFT calculation method to construct the initial force field for describing the short-range interactions between atoms in molecular dynamics, and verify the constructed initial force field.

[0007] S2, Monte Carlo Step: Simulate the physical process through random sampling, and use a large number of random numbers to simulate the transport process of particles in the medium, so as to obtain the energy distribution of secondary particles, which is used as the initial input parameters for molecular dynamics simulation;

[0008] S3, Molecular Dynamics Step: Initialize the system, and based on the initial force field determined in step S1 and the secondary particle parameters determined in step S2, use the molecular dynamics method to simulate the cascade collision process triggered by secondary particles with the initial force field, and simulate multiple times under the same initial conditions. Determine the key information by analyzing the simulation results.

[0009] S4, Accelerated Molecular Dynamics Step: Conduct on the basis of the molecular dynamics simulation in step S3. According to the key data energy calculated by the molecular dynamics in step S3, realize the acceleration of the defective atoms crossing the energy barrier by continuously lifting the atomic potential energy, and extend the simulation time scale of molecular dynamics from microseconds to more than a day.

[0010] Step S1 is specifically as follows: Based on the energy and force calculated by DFT, by comparing the energy and force predicted by DFT and ReaxFF potential energy, smoothly connect the Ziegler - Biersack - Littmark repulsive potential energy (ZBL potential) with the ReaxFF potential energy, and use the minimization of the residual sum of squares RSS to determine the optimal parameters in the transition region, so as to generate the initial force field for describing the short - range interaction between atoms in molecular dynamics. After construction, verify the initial force field.

[0011] Step S2 is specifically as follows: Use the Geant4 program to simulate the transport process of neutrons in the material, obtain the energy spectrum and coordinate distribution of secondary particles, determine the percentage and energy range of secondary particles, and determine the energy value with the highest probability. The obtained secondary particle information is used as the initial input for molecular dynamics simulation;

[0012] Step S3 is specifically as follows: Initialize the system, based on the initial force field determined in step S1 and the secondary particle parameters determined in step S2, specify the incident direction and energy of the primary displaced atom, and at the same time set the initial coordinates, velocities and neighboring atom information for the atoms. Use the molecular dynamics method to simulate the cascade collision process triggered by secondary particles with the initial force field, and simulate multiple times under the same initial conditions. Determine the key information through in - depth statistical analysis of the molecular dynamics simulation results.

[0013] Furthermore, the verification in step S1 includes the verification of equilibrium state properties and high - energy collision verification

[0014] Furthermore, in step S3, initialize the system, and specifically set the system parameters including the system, temperature, and pressure.

[0015] Furthermore, the key information in step S3 includes the initial defect distribution, critical data energy, mechanical property changes, microstructure evolution, chemical products, reaction paths, etc.

[0016] Furthermore, in step S4, the acceleration of defect atoms crossing the energy barrier can also be achieved by means of high-temperature simulation.

[0017] The beneficial effects of the technical solution are as follows: The method of the present invention couples four different-scale simulation methods, namely the first-principles calculation method (DFT), Monte Carlo (MC), molecular dynamics (MD), and accelerated molecular dynamics (AMD). With the help of parallel computing technology, it can break through the single-scale limitation and achieve a long-time, large-scale, and relatively accurate computer simulation process of the interaction of irradiation rays, atomic-scale defect generation, defect evolution process to microstructure characterization of energetic materials in an irradiation environment. Thus, it can deeply understand the irradiation damage mechanism of energetic materials, predict the performance changes of energetic materials after irradiation, and provide a convenient and efficient scientific means for the design and performance prediction of energetic materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a flowchart of the simulation method of the present invention;

[0019] Figure 2 is a flowchart of the first-principles calculation of the present invention;

[0020] Figure 3 is a flowchart of the Monte Carlo step of the present invention;

[0021] Figure 4 is a flowchart of the molecular dynamics step of the present invention;

[0022] Figure 5 is a flowchart of the accelerated molecular dynamics step of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0023] The following further describes the present invention in detail with reference to the drawings and embodiments:

[0024] A multi-scale coupled simulation method for irradiation damage of energetic materials proposed by the present invention aims to overcome the limitations of a single simulation method by comprehensively applying various computational simulation technologies. This method combines the first-principles calculation, Monte Carlo method, molecular dynamics method, and accelerated molecular dynamics method to exert their respective advantages at different time scales and space scales. The specific flowchart is as Figure 1 shown.

[0025] First, use the first principles to calculate the atom pairs of carbon, hydrogen, oxygen, and nitrogen elements at Calculate the potential energy and force within a certain distance, and fit them by combining the ReaxFF-lg potential function and the ZBL potential function to construct a new ReaxFF-lg / ZBL force field to describe the process of high-energy collisions between atoms in molecular dynamics, resulting in energy transfer and collision cascades.

[0026] Use the Monte Carlo method to simulate the interaction between irradiated particles and atoms in energetic materials, and calculate the energy distributions of different types of primary knock-on atoms (PKAs) such as C, H, O, and N atoms during the displacement cascade process. The calculation results can provide a theoretical basis for setting the PKA energy range in molecular dynamics.

[0027] Use molecular dynamics to simulate the cascade collision process initiated by PKA. This process is a key step in understanding the microscopic damage mechanism of energetic materials under irradiation. The initial defect trajectory results generated by molecular dynamics simulation are used as the input file for accelerated molecular dynamics (AMD). Set the acceleration parameters (such as temperature, acceleration barrier, and cut-off distance) and start the simulation of accelerated molecular dynamics based on the cohesive energy and bond energy data obtained from MD simulation.

[0028] Through this coupling method, it is possible to simulate the characterization of the microstructure of energetic materials from atomic-scale defects to large spatio-temporal scales, achieve accurate prediction of the performance changes of energetic materials under service conditions, not only improve the accuracy of the simulation, but also expand the time and space ranges of the simulation, providing a strong theoretical basis and prediction tool for the design and performance optimization of energetic materials.

[0029] The specific implementation process is as follows, and the specific parameters can be adjusted according to the actual situation:

[0030] First-principles calculation (DFT) steps:

[0031] 1. Use non-spin-polarized density functional theory to calculate the interaction energy and force between each pair of C, H, O, and N atoms (a total of 10 pairs) at short-range distances. Among them, use the Fourier transform to calculate the PAW potential and increase the FFT grid points to help the GGA (generalized gradient approximation) calculation converge.

[0032] 2. Take the first-principles calculation results as the reference standard, construct a new ReaxFF-lg / ZBL force field, and determine the optimal parameters for the transition region [R1, R2] by minimizing the sum of squared residuals to smoothly connect the ZBL potential energy and the ReaxFF-lg potential energy to improve the description of the short-range interaction between atoms in energetic materials.

[0033] 3. Verification of equilibrium properties. The first-principles calculations were used to obtain the unit cell parameters and the equation of state of energetic materials at equilibrium, and the results were compared and verified with those of ReaxFF-lg / ZBL and ReaxFF-lg force fields. The results showed that ReaxFF-lg / ZBL was very close to the experimental results in describing the equilibrium properties of energetic materials and was superior to the ReaxFF-lg force field.

[0034] 4. Verification of high-energy particle collisions. The displacement cascades caused by the modified ReaxFF-lg / ZBL force field were compared and verified with the results of first-principles molecular dynamics. The results indicated that the results of the ReaxFF-lg / ZBL force field in describing energy transfer in high-energy collisions were more consistent with the first-principles calculation results.

[0035] The flow chart of the first-principles calculation is as Figure 1 shown.

[0036] Monte Carlo (MC) steps:

[0037] 1. Setting up the particle transport model. The Geant4 program was used to simulate the transport process of neutrons in the C4H8N8O8 material. During the simulation of neutrons incident on the target, a hemispherical incidence method was adopted to simulate the point source situation. Since the neutron energy was set to 0.1 MeV, the pre-compiled physical model FTFP.BERT.HP was selected. This model can effectively simulate the process of neutron-matter interaction in the energy range from 0.025 eV to 20 MeV. Specifically, it can accurately simulate the inelastic hadron-nucleus process by integrating the Fritiof-parton model (FTF), Bertini model, and Precompound model. At the same time, for the elastic scattering of protons and neutrons, the G4ChipsElasticModel was used. This model uses the Kossov parameterized cross-section and combines high-precision neutron models and cross-sections to be able to describe the elastic scattering, inelastic scattering, capture, and fission behaviors at low energies in detail, and finally successfully obtains the energy distribution and density distribution results of PKAs.

[0038] 2. Generation of secondary particles. Prepare a model of energetic material (such as octogen, HMX) with a volume of 2.5×3.25×3 cm 3 , and determine its negative y-axis (i.e., the b crystal axis) direction. Set the neutron source energy to 0.1 MeV and the neutron incidence direction to the negative y-axis, and conduct 10 6 simulations of the interaction between neutrons and the atoms of the material in the model, and calculate the energy distribution and coordinate distribution of 168,900 primary knock-on atoms (PKAs).

[0039] 3. Determination of secondary particle energy. Based on the energy distribution data, a probability density curve is fitted using a probability density function, and the most probable energy value corresponding to the probability density curve is selected as the energy of the secondary particle.

[0040] The flowchart of the Monte Carlo steps is as Figure 2 shown.

[0041] Molecular dynamics (MD) steps:

[0042] 1. Construct an initial model of the energetic material to obtain the types and initial coordinates of the atoms;

[0043] 2. Initialize the system settings and atomic information, including the system, temperature, pressure, incident direction, energy of the first lattice atom (PKA) displaced by a neutron, the initial coordinates of the atoms, and neighboring atoms, etc.

[0044] 3. Define the force field and parameters. To better describe the short-range interactions between atoms, the ReaxFF-lg / ZBL force field is used for molecular dynamics simulation; at the same time, according to Nordlund's friction model, the electron stopping effect is considered. When the kinetic energy of an atom is greater than the threshold energy (Ec), the atom will be subject to a frictional force proportional to its velocity during the implantation process, as shown in the following equation:

[0045]

[0046] To match the energy loss of electrons (the Ec value used in the simulation was tested to be 9 eV), the proportionality parameter β can be obtained from the linear fitting function of the particle velocity and the electron stopping energy calculated by SRIM).

[0047] 4. Energy minimization. The conjugate gradient (CG) algorithm is used to relax the energetic material model to find the lowest energy configuration of the system, with an energy tolerance of 10 -6 kcal / mol.

[0048] 5. Equilibrate the system. The Nose-Hoover thermostat method is used to perform relaxation for 10 ps in the canonical ensemble (NVT) at 300 K, with a time step set to 0.1 fs; subsequently, the energetic material model is relaxed for 60 ps in the NPT ensemble to eliminate internal stress and bring the system to thermodynamic equilibrium; after relaxation, it is judged whether the structure is stable and whether the potential energy and density curves tend to converge. If equilibrium is reached, the model can be used for the next cascade process simulation;

[0049] 6. Cascade process simulation. During the displacement cascade simulation, the selected PKA atoms are randomly distributed within the PKA region on one side of the simulation box; to increase the collision probability with other atoms, the movement direction of the PKA atoms is set to point towards the center of the box, and in each case, only one PKA atom is triggered and tracked at a time; a cooling layer with a thickness of is set on the outermost layer of the simulation box, and the internal region is the cascade collision zone. The cascade collision zone is simulated under the microcanonical ensemble (NVE), and the cooling layer is simulated under the NVT ensemble by velocity scaling to allow energy dissipation, thereby eliminating the spurious pressure waves reflected from the boundary cells and finally cooling to the target temperature of 300K. In addition, to avoid the influence of PKA on the cooling layer, the displaced atoms are removed once they reach this region; the time step is automatically adjusted between 10 -5 fs and 0.1fs to ensure that the maximum displacement distance and the maximum energy transfer are less than and 0.1 kcal / mol within one time step respectively. After the system temperature is annealed to 300K, further NPT relaxation is carried out for 30 ps with a fixed time step of 0.1fs to obtain a model without internal stress.

[0050] 7. For each PKA energy case, PKA will be randomly selected and the simulation process will be repeated 8 - 10 times under the same initial conditions to obtain statistical results.

[0051] 8. Information collection and analysis. During the cascade collision process, the system's kinetic and thermodynamic information is statistically analyzed every 20 fs, and the molecular species and their compositions of the products are identified, analyzed, and statistically counted according to the minimum bond order of different molecules to provide detailed information on the chemical reactions during the cascade process.

[0052] The flow chart of the molecular dynamics steps is as shown in Figure 3 Figure.

[0053] Accelerated molecular dynamics (AMD) steps:

[0054] From the initial defect structure finally generated by molecular dynamics, the bond lengths in the system and the system potential energy are statistically analyzed.

[0055] 1. Set the acceleration parameters. Set the atomic critical displacement distance to 10% of the acceleration barrier parameter; set the critical distance increment to 5% of the atomic critical displacement distance; set the acceleration time to 10 15 fs.

[0056] 2. Start the accelerated molecular dynamics simulation. Read the initial defect trajectory generated by the MD simulation, and use LAMMPS to perform the accelerated molecular dynamics simulation according to the parameters set in step 1. The relaxation temperature is set to 300K, the initial time step is 1fs, and the barrier - raising acceleration is turned on. The acceleration time is obtained through transition state theory:

[0057]

[0058] Among them, t total is the acceleration time, n is the number of times of performing acceleration simulation, and t MD is the simulation time in each acceleration simulation, V is the acceleration barrier, k b is the Boltzmann constant, T is the simulation temperature, and relaxation simulation is continuously performed under this simulation setting until the preset acceleration time is reached.

[0059] The above are only embodiments of the present invention, and specific technical solutions or common knowledge such as characteristics well known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several deformations and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicability of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners and the like recorded in the specification can be used to interpret the content of the claims.

Claims

1. A multi-scale coupled simulation method for energetic material radiation damage, characterized in that: The following steps are involved: S1. First principles calculation: Use DFT calculation method to construct the initial force field for describing the short-range interaction between atoms in molecular dynamics, and verify the constructed initial force field; S2, Monte Carlo step: simulate the physical process through random sampling, use a large number of random numbers to simulate the transport process of particles in the medium, so as to obtain the energy distribution of secondary particles, which is used as the initial input parameters of molecular dynamics simulation; S3, molecular dynamics step: initialize the system, and according to the initial force field determined in step S1 and the secondary particle parameters determined in step S2, use the molecular dynamics method, use the initial force field to simulate the cascade collision process caused by the secondary particles, and simulate multiple times under the same initial conditions, and determine the key information by analyzing the simulation results; S4, accelerated molecular dynamics step: based on the molecular dynamics simulation in step S3, according to the key data energy and defect structure obtained by the molecular dynamics calculation in step S3, the defect atoms are accelerated to cross the energy barrier by continuously raising the atomic potential energy, and the simulation time scale of molecular dynamics at the microsecond level is extended to more than a day; The step S1 specifically comprises: taking the energy and force calculated by DFT as a benchmark, by comparing the energy and force predicted by DFT and ReaxFF potential energy, smoothly connecting the Ziegler-Biersack-Littmark repulsive potential energy and the ReaxFF potential energy, minimizing the residual sum to determine the optimal parameters in the transition region, thereby generating an initial force field for describing the short-range interaction between atoms in molecular dynamics, and verifying the initial force field after the construction is completed; The step S2 specifically includes: using the Geant4 program to simulate the transport process of neutrons in the material, obtaining the energy spectrum and coordinate distribution of the secondary particles, determining the percentage and energy range of the secondary particles, and determining the energy value with the highest probability, and the obtained secondary particle information is used as the initial input of the molecular dynamics simulation; The step S3 is specifically as follows: initializing the system, specifying the incident direction and energy of the primary delocalized atom according to the initial force field determined in step S1 and the secondary particle parameters determined in step S2, setting the initial coordinates, velocity and neighboring atom information for the atom, using the molecular dynamics method, using the initial force field to simulate the cascade collision process caused by the secondary particles, and simulating multiple times under the same initial conditions, and determining key information through in-depth statistical analysis of the molecular dynamics simulation results.

2. A multi-scale coupled simulation method for energetic material radiation damage according to claim 1, characterized in that: The verification in step S1 includes equilibrium property verification and high energy collision verification.

3. The multi-scale coupled simulation method for energetic material radiation damage according to claim 1, characterized in that: In step S3, the system is initialized and system parameters including system, temperature and pressure are set in detail.

4. The multi-scale coupled simulation method for energetic material radiation damage according to claim 1, characterized in that: The key information in step S3 includes initial defect distribution, key data energy, mechanical property changes, microstructure evolution, chemical products and reaction paths, etc.

5. The multi-scale coupled simulation method for energetic material radiation damage according to claim 1, characterized in that: In step S4, the defect atoms may be accelerated to cross the energy barrier by means of high temperature simulation.

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