Microcosmic parameter-based crosslinked polyethylene mechanical property evaluation and structure optimization method

Abstract

The invention discloses a microscopic parameter-based crosslinked polyethylene mechanical property evaluation and structure optimization method in a thermo-oxidative aging process, which comprises the following steps: S1, constructing a crosslinked polyethylene (XLPE) system model of two crosslinking modes, and carrying out geometric optimization and dynamic relaxation; s2, performing thermo-oxidative aging reaction molecular dynamics simulation on the two XLPE system models to obtain molecular structures with different thermo-oxidative aging degrees; s3, on the basis of the molecular structures of the XLPE with different aging degrees, constructing XLPE system models with different aging degrees, and performing geometric optimization and molecular dynamics simulation to obtain model motion trails; and S4, on the basis of the model motion trail, microscopic parameters associated with the mechanical properties are calculated, changes of the mechanical properties of the two XLPEs in the thermo-oxidative aging process are compared, and an XLPE structure with better mechanical properties is optimized. The invention creatively provides a micro-scale evaluation and structure optimization method aiming at the mechanical property of the crosslinked polyethylene, and is expected to break through the limitation of experimental period and cost.

Description

Technical Field

[0001] The invention relates to the technical field of insulating material performance evaluation, and more specifically to a method for evaluating the mechanical properties of cross-linked polyethylene and optimizing its structure based on microscopic parameters. Background Art

[0002] Power cables are mainly used to transmit electric energy and are the arteries of the power system. The cable insulation layer plays a role of protection and insulation, which is crucial to ensure the safe transmission of electric energy. During the operation of the cable, the insulation layer will be affected by electrical stress, thermal stress and mechanical stress, resulting in thermal oxidation aging of the insulation layer, and the degradation of its mechanical and electrical properties, which seriously affects the safe operation of the cable and the reliable transmission of electric energy.

[0003] Cross-linked polyethylene is a high molecular polymer formed by cross-linking polyethylene single chains through peroxide, irradiation, silane, etc. Due to its excellent insulation, mechanical and heat resistance properties, it is widely used as cable insulation material of various voltage levels. The performance of cross-linked polyethylene under different cross-linking methods is different. Therefore, it is of great significance to study the mechanism of performance degradation of cross-linked polyethylene during thermal oxidation aging, and compare and evaluate the performance differences of cross-linked polyethylene with different cross-linking methods during the aging process, so as to select insulation materials with better performance.

[0004] At present, the performance inspection of cable insulation materials and the state evaluation during the operation and aging process are mostly carried out through traditional test methods, which are not only costly and have a long operation cycle, but also only characterize and analyze the test results, and cannot analyze the changes in insulation material performance and the mechanism of performance differences between different insulation materials from a microscopic level. Therefore, there is a lack of a method to efficiently and accurately evaluate the performance of insulation materials and select insulation materials with better performance.

[0005] Based on this, the present invention proposes a method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene based on microscopic parameters, using reaction kinetics simulation technology to simulate the thermal oxidation aging reaction of two cross-linked polyethylenes, obtain the molecular structures of different aging degrees, and perform molecular dynamics simulation on the XLPE system models of different aging degrees, calculate the relevant microscopic parameters, evaluate and compare the changes in the mechanical properties of the two XLPEs during the aging process, and optimize the cross-linked polyethylene structure with better performance. This invention method is expected to break through the limitations of traditional methods in test cycle and cost, and establish the connection between the microstructure and macroscopic performance of insulating materials. Summary of the invention

[0006] The purpose of this method is to propose a method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters to solve the above technical problems.

[0007] To achieve the above object, the present invention adopts the following technical solution:

[0008] S1. Construction of cross-linked polyethylene models under two cross-linking modes

[0009] S11: constructing two molecular models of silane cross-linked polyethylene (Si-XLPE) and peroxide cross-linked polyethylene (p-XLPE) with different cross-linking modes, wherein both of the two XLPE molecular models contain three layers of polyethylene and six cross-linking points;

[0010] S12: based on the two XLPE molecular models constructed in S11, two XLPE periodic structure system models are established, wherein the two XLPE periodic structure system models each include 10 XLPE molecules and 10 oxygen molecules for thermal oxidation reaction simulation;

[0011] S13: Based on the XLPE system model constructed in S12, the smart algorithm is used to perform 10,000 steps of geometric optimization on the XLPE system model, the conformation and coordinate position of the molecules in the model are adjusted iteratively, and 5 cycles of annealing are performed under the NVT ensemble, the temperature range is 300-500K, the annealing time is set to 100ps, and the model after each annealing is geometrically optimized;

[0012] S13: Based on the geometrically optimized and annealed model in S13, 1000ps kinetic relaxation processes were carried out under the NPT and NVT ensembles, respectively. The pressure was set to 101.325 kPa, the temperature was set to 300 K, and the Berendsen method and Nose-Hoover method were selected for pressure and temperature control, respectively, to obtain the reaction kinetics simulation models of two XLPEs.

[0013] S2. Perform thermal oxidation aging reaction kinetics simulation to obtain the molecular configurations of two XLPE with different aging degrees

[0014] S21: Based on the XLPE system model obtained in S13, the thermal oxidation aging reaction simulation of ReaxFF force field was carried out under the NVT ensemble, with the simulation temperature of 600K, the simulation step size of 0.1fs and the simulation time of 400ps;

[0015] S22: During the thermal oxidative aging reaction simulation process, the main molecular structures of XLPE at thermal aging simulation times of 0ps, 60ps, 120ps, 200ps, and 300ps are selected as molecular structure models of different thermal oxidative aging degrees for subsequent molecular dynamics simulation calculations.

[0016] S3. Construct XLPE system models with different aging degrees and perform molecular dynamics simulations

[0017] S31: Based on the XLPE molecular structures with different aging degrees selected in S22, top and itp files of the XLPE system models with different aging degrees are generated using Sobtop software, each XLPE system model contains 30 XLPE molecules, and the top and itp files can be recognized by gromacs software;

[0018] S32: Based on the top and itp model files generated in S31, the model is energy minimized using gromacs software, and the energy minimization method is the conjugate gradient method;

[0019] S33: The energy-minimized model was subjected to a 10 ns relaxation equilibrium in the NPT ensemble, where the temperature was controlled by the velocity-rescale method, the pressure was controlled by the Berendsen method, the simulation temperature was set to 303 K, the simulation step was 1 fs, and the force field was selected as GAFF;

[0020] S34: The same ensemble as S33, velocity-rescale temperature control method and Parrinello-Rahman pressure control method were used to perform a 10 ns molecular dynamics simulation on the model after relaxation equilibrium. The simulation temperature, step size and force field were consistent with those in S33, and finally the model motion trajectory was obtained.

[0021] S4: Calculate microscopic parameters and evaluate and compare the mechanical properties of the two XLPE

[0022] S41: Based on the model motion trajectory in S34, the gromacs software is used to calculate the information of mechanical parameters such as density, gyration radius, free volume fraction and cohesive energy density of the XLPE system as well as Young's modulus (E), shear modulus (K) and bulk modulus (G);

[0023] S42: Based on the microscopic parameter information in S41, the mechanical properties of the two XLPEs during the thermal oxidative aging process are evaluated and compared, so as to select the XLPE structure with better mechanical properties.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The present invention uses molecular simulation technology to perform microscopic computational analysis on the performance changes of XLPE during the operation aging process. Compared with traditional experimental methods, it does not require a large amount of experimental materials and complex experimental equipment, which not only saves the test cycle and cost, but also can reveal the microscopic mechanism of changes in mechanical properties during aging. In addition, the method of the present invention can accurately evaluate and predict the performance of materials, and by comparing the performance differences between different materials, it can more efficiently select materials with better performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of the method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermo-oxidative aging based on microscopic parameters;

[0027] Figure 2 It is the molecular model of silane cross-linked polyethylene (Si-XLPE), peroxide cross-linked polyethylene (p-XLPE) and the simulation model diagram of XLPE reaction kinetics;

[0028] Figure 3 It is a molecular structure model diagram of two XLPE with different degrees of thermal oxidation aging;

[0029] Figure 4 This is a molecular dynamics simulation model diagram of two XLPE with different aging degrees;

[0030] Figure 5 This is the trend diagram of the radius of gyration and density of two XLPE during thermal oxidation aging;

[0031] Figure 6 It is the trend diagram of the change of Young's modulus, shear modulus and bulk modulus of two XLPE during the thermal oxidation aging process;

[0032] Figure 7 This is the trend diagram of the free volume changes of two XLPE during the thermal oxidation aging process;

[0033] Figure 8 This is the trend diagram of the cohesive energy density of two XLPEs during thermal oxidation aging;

[0034] Fig. 9 This is a conceptual diagram of the microscopic mechanism of changes in the mechanical properties of two XLPEs during thermal oxidative aging. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solution in the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0036] Example

[0037] This embodiment proposes a method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermal oxidation aging based on microscopic parameters. The specific implementation steps are as follows:

[0038] S1. Constructing simulation models of cross-linked polyethylene under two cross-linking methods

[0039] S11: constructing two molecular models of silane cross-linked polyethylene (Si-XLPE) and peroxide cross-linked polyethylene (p-XLPE) with different cross-linking modes, wherein both of the two XLPE molecular models contain three layers of polyethylene and six cross-linking points;

[0040] S12: based on the two XLPE molecular models constructed in S11, two XLPE periodic structure system models are established, wherein the two XLPE periodic structure system models each include 10 XLPE molecules and 10 oxygen molecules for thermal oxidation reaction simulation;

[0041] S13: Based on the XLPE system model constructed in S12, the conjugate gradient method is used to perform 10,000 steps of geometry optimization on the XLPE system model, and the conformation and coordinate position of the molecules in the model are adjusted iteratively. Then, 5 cycles of annealing are performed under the NVT ensemble, with a temperature range of 300-500 K and an annealing time of 100 ps. The model is then geometry optimized after each annealing.

[0042] S13: Based on the geometrically optimized and annealed model in S13, 1000ps kinetic relaxation processes were carried out under the NPT and NVT ensembles, respectively. The pressure was set to 101.325 kPa, the temperature was set to 300 K, and the Berendsen method and Nose-Hoover method were selected for pressure and temperature control, respectively, to obtain the reaction kinetics simulation models of two XLPEs.

[0043] S2. Perform thermal oxidative aging reaction kinetics simulation to obtain the molecular configurations of the two XLPEs at different thermal oxidative aging stages

[0044] S21: Based on the XLPE system model obtained in S13, the thermal oxidation aging reaction simulation of ReaxFF force field was carried out under the NVT ensemble, with the simulation temperature of 600K, the simulation step size of 0.1fs and the simulation time of 400ps;

[0045] S22: During the thermal oxidative aging reaction simulation process, the main molecular structures of XLPE at thermal aging simulation times of 0ps, 60ps, 120ps, 200ps, and 300ps were selected as molecular structure models of five different thermal oxidative aging degrees for subsequent molecular dynamics simulation calculations.

[0046] S3. Construct XLPE system models with different aging degrees and perform molecular dynamics simulations

[0047] S31: Based on the XLPE molecular structure models with different aging degrees selected in S22, top and itp files of the XLPE system models with different aging degrees are generated using Sobtop software, each XLPE system model contains 30 XLPE molecules, and the top and itp files can be recognized by gromacs software to perform molecular dynamics simulation;

[0048] S32: Based on the top and itp model files generated in S31, the model is energy minimized using gromacs software, and the energy minimization method is the conjugate gradient method;

[0049] S33: The energy-minimized model was subjected to relaxation equilibrium for 10 ns under the NPT ensemble, where the temperature was controlled by the velocity-rescale method, the pressure was controlled by the Berendsen method, the simulation temperature was set to 303 K, the simulation step was 1 fs, and the GAFF force field was selected;

[0050] S34: The same ensemble as S33, velocity-rescale temperature control method and Parrinello-Rahman pressure control method were used to perform a 10 ns molecular dynamics simulation on the model after relaxation equilibrium. The simulation temperature, step size and force field were consistent with those in S33, and finally the model motion trajectory was obtained.

[0051] S4: Calculate relevant microscopic parameters and evaluate and compare the mechanical properties of the two XLPEs.

[0052] S41: Based on the model motion trajectory in S34, the gyration radius of the different aging degree system models of silane cross-linked polyethylene and peroxide cross-linked polyethylene were calculated using gromacs software Figure 5 a. Density, free volume fraction and cohesive energy density as well as mechanical parameter information such as Young's modulus, shear modulus and bulk modulus. Various microscopic parameter information shows that during the entire thermal oxidative aging process, the mechanical properties of XLPE show a trend of first strengthening and then weakening;

[0053] S42: Based on the microscopic parameter information in S41, the mechanical properties of the two XLPEs during the thermal oxidative aging process were evaluated and compared. The results showed that during the entire thermal oxidative aging process, Si-XLPE had a higher density, a smaller free volume, and a larger cohesive energy density than p-XLPE, showing a higher intermolecular force. Therefore, Si-XLPE formed by silane crosslinking has better mechanical properties.

[0054] The above is only one embodiment of the present invention and does not limit the present invention in any form. The above described embodiment is not intended to limit the present invention. Any technician familiar with the art can use the above disclosed methods and technical contents to make many possible changes and modifications to the technical solution of the present invention, or modify it into an equivalent embodiment of equivalent changes without departing from the spirit and technical solution of the present invention. Therefore, any simple modification, equivalent replacement, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of protection of the technical solution of the present invention.

Claims

1. A method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters, characterized in that: The following steps are involved: S1. constructing two cross-linking methods of cross-linked polyethylene (XLPE) molecular structures, and establishing periodic structural system models of the two XLPEs, performing geometric optimization and dynamic relaxation equilibrium, and obtaining a structurally reasonable XLPE system model; S2. Based on the Reaxff force field, the molecular dynamics simulation of the thermal oxidative aging reaction of the two XLPE system models in S1 was carried out to obtain the molecular structures of different thermal oxidative aging degrees; S3. Based on the XLPE molecular structures with different degrees of thermal oxidation aging obtained in S2, a system model of XLPE with different degrees of aging is constructed, and geometric optimization and molecular dynamics simulation are performed to obtain the motion trajectory of the model; S4. Based on the model trajectory of the molecular dynamics simulation in step S3, the microscopic parameter information associated with the mechanical properties is calculated, the mechanical properties of the two XLPEs during the thermal oxidative aging process are evaluated and compared, and the XLPE structure with better mechanical properties is selected.

2. The method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters according to claim 1, characterized in that: The two cross-linked polyethylene molecules in step S1 are silane cross-linked polyethylene (Si-XLPE) and peroxide cross-linked polyethylene (p-XLPE), and both XLPE molecular models contain 3 layers of polyethylene and 6 cross-linking points. The XLPE periodic structure system model includes 10 XLPE molecules and 10 oxygen molecules. The geometric optimization and dynamic relaxation equilibrium in step S1 includes: using a conjugate gradient algorithm to perform 10,000 steps of geometric optimization on the XLPE system model, and continuously iteratively adjusting the conformation and coordinate position of the molecules in the model; Five cycles of annealing were carried out under the NVT ensemble with a temperature range of 300-500K and an annealing time of 100ps. The model was geometrically optimized after each annealing. A 1000ps dynamic relaxation process was carried out successively under the NPT and NVT ensembles with the pressure set to 101.325kPa and the temperature set to 300K. The Berendsen method and Nose-Hoover method were selected as the pressure and temperature control methods, respectively, to obtain a system model with a reasonable structure.

3. The method for evaluating the mechanical properties and optimizing the structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters according to claim 1, characterized in that: The simulation temperature of the molecular dynamics simulation of the thermal oxidative aging reaction in step S2 is 600K, the simulation step is 0.1fs, and the simulation time is 400ps; the molecular structures of different thermal oxidative aging degrees are taken from the main molecular structures of XLPE when the thermal aging reaction simulation time is 0ps, 60ps, 120ps, 200ps, and 300ps.

4. The method for evaluating mechanical properties and optimizing structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters according to claim 1, characterized in that: Step S3 uses Sobtop software to generate top and itp files of the XLPE system models with different aging degrees. Each XLPE system model contains 30 XLPE molecules. The top and itp files can be recognized by gromacs software. The geometry optimization and molecular dynamics simulation processes described in step S3 are both performed using gromacs software, and the specific steps are as follows: first, the conjugate gradient method is used to minimize the energy of the model; then, the model is subjected to a 10 ns relaxation process under the NPT ensemble, during which the temperature is controlled using the velocity-rescale method and the pressure is controlled using the Berendsen method; finally, a 10 ns molecular dynamics simulation is performed using the same ensemble as the above relaxation process, the velocity-rescale temperature control method, and the Parrinello-Rahman pressure control method, with the simulation temperature set to 303 K, the simulation step size to 1 fs, and the GAFF force field selected to obtain the model motion trajectory.

5. The method for evaluating mechanical properties and optimizing structure of cross-linked polyethylene during thermal oxidative aging based on microscopic parameters according to claim 1, characterized in that: The microscopic parameters associated with the mechanical properties in step S4 include radius of gyration, density, free volume fraction and cohesive energy density as well as Young's modulus, shear modulus and bulk modulus. Based on the microscopic parameters, the mechanical properties of the two XLPEs during the thermal oxidative aging process are evaluated and compared, and the XLPE structure with better mechanical properties is preferably selected.

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