Method and system for obtaining heat conductivity coefficient of polyimide composite aerogel

Abstract

The invention belongs to the technical field of molecular simulation, and discloses a method and system for obtaining the heat conductivity coefficient of polyimide composite aerogel. The method comprises the following steps: establishing an initial structure of polyimide composite aerogel; establishing an initial structure model of the polyimide composite aerogel; establishing a nitrogen molecular structure model, and determining a heat transfer model of the polyimide composite aerogel containing nitrogen molecules; setting simulation calculation parameters, and carrying out molecular dynamics simulation calculation; calculating mean square displacement; and calculating the heat conductivity coefficient of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing the nitrogen molecules according to the mean square displacement. According to the method, gaseous heat transfer is simulated on the polyimide composite aerogel, mean square displacement of a system at different temperatures is analyzed, and the heat conductivity coefficient of the aerogel can be directly obtained. The result can provide a certain reference for the heat transfer micromechanism of various materials, and effective thermal protection is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of molecular simulation, and in particular relates to a method and system for obtaining the thermal conductivity of a polyimide composite aerogel. Background Art

[0002] Thermal conductivity is a key thermal property of polyimide (PI) aerogels, reflecting their thermal insulation performance at different temperatures. In aircraft thermal protection systems, the thermal insulation properties of PI aerogels significantly impact the quality and effectiveness of these systems. Thermal conductivity is a key parameter that directly reflects the gaseous heat transfer capability of PI aerogels. Traditional methods for determining aerogel thermal conductivity typically require complex, time-consuming experiments and extremely demanding experimental conditions, making it difficult to quickly and easily obtain accurate thermal conductivity data in a laboratory setting. Due to their porous structure, PI aerogels primarily transfer heat through the gaseous phase. Lower thermal conductivity indicates poorer heat transfer and better insulation. Therefore, accurately determining the thermal conductivity of PI composite aerogels is crucial for guiding their research and development, reducing experimental costs, and improving their thermal performance.

[0003] Currently, thermal conductivity is primarily measured experimentally, using commonly used methods such as laser thermal conductivity, steady-state hot plate method, hot wire method, and transient hot wire method. Factors affecting polyimide composite aerogels are primarily pore size and temperature changes. For polyimide composite aerogels, pore size directly influences the internal heat transfer path and efficiency. Smaller pores reduce the number of heat conduction channels, thereby improving thermal insulation. Furthermore, temperature changes also directly affect thermal conductivity. As temperature rises, the movement of gas molecules within the aerogel intensifies, potentially leading to an increase in thermal conductivity.

[0004] In recent years, with the continuous advancement of computer intelligence, numerical simulation technology has also experienced rapid development. Molecular dynamics simulation methods, in particular, can accurately simulate the microstructure of polyimide composite aerogels on computers and, by simulating intermolecular interactions, directly calculate the thermal conductivity. This method has become a powerful tool for studying material microstructures and their mechanisms, and its application in the aerogel field is also expanding. Summary of the Invention

[0005] To overcome the problems existing in the related art, the disclosed embodiments of the present invention provide a method and system for obtaining the thermal conductivity coefficient of polyimide composite aerogel. The present invention can provide a certain reference for the microscopic mechanism of the thermal insulation performance of polyimide aerogel, which is of great significance to aerogel thermal insulation and protective materials.

[0006] The technical solution is as follows: A method for obtaining the thermal conductivity of a polyimide composite aerogel comprises the following steps:

[0007] S1, establishing the initial structure of polyimide composite aerogel;

[0008] S2, establishing the initial structural model of polyimide composite aerogel;

[0009] S3, establishing a nitrogen molecular structure model and determining the heat transfer model of polyimide composite aerogel containing nitrogen molecules;

[0010] S4, set simulation calculation parameters and run Material Studio software to perform molecular dynamics simulation calculations;

[0011] S5, calculate the mean square displacement;

[0012] S6, calculate the thermal conductivity of polyimide composite aerogel in the heat transfer model of polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

[0013] In step S1, the initial structure of the polyimide composite aerogel is established using Chem Draw software. The initial components of the initial structure of the polyimide composite aerogel include PI, SiO2, C6H 18 O 24 P6 and BN; among them, BN and C6H 18 O 24 P6 is compounded into the PI molecular chain to form a composite PI macromolecular chain.

[0014] In step S2, the Amorphous Cell module in the Material Studio software is used to dynamically calculate the cubic simulation box of the composite PI macromolecular chain according to the molecular composition and target density of the composite material using the following formula:

[0015]

[0016] Where L is the length of each side of the cubic simulation box, N is the number of molecular fillings predicted by Monte Carlo simulation, with an error rate of <3%; V mol is the average volume of a single molecule, ρ is the density of polyimide composite aerogel;

[0017] Introducing the exclusion volume weight factor ω in the Amorphous Cell module in Material Studio software excluded , the distance between polyimide composite macromolecules was optimized by gradient descent algorithm, SiO2 was added to the established cubic simulation box in the Amorphous Cell module to determine the initial structural model of polyimide composite aerogel.

[0018] In step S3, nitrogen molecules are added according to the porosity of the aerogel. and temperature T are calculated using the modified Clausius-Clapeyron equation:

[0019]

[0020] Where, is the number of nitrogen molecules, P is the standard atmospheric pressure, V BOX is the pore volume, κ B is the Boltzmann constant, α(T) is the temperature correction factor;

[0021] The adaptive Poisson sphere algorithm was used in combination with molecular dynamics pre-simulation to optimize the nitrogen distribution. The Build module in MaterialStudio software was used to define a rectangular box of nitrogen molecules, i.e., the nitrogen molecular structure model, based on the microscopic morphology analysis data SEM of the aerogel. The initial structure model of the polyimide composite aerogel and the nitrogen molecular structure model were merged to output the merged heat transfer model of the polyimide composite aerogel containing nitrogen molecules.

[0022] In step S4, the simulation temperatures are selected as 25°C, 100°C, and 200°C in the Forcite module of the Material Studio software. During the simulation, the ensemble used in the dynamic process is set to the canonical ensemble NVT ensemble. Based on the material composition, that is, a homogeneous organic-inorganic mixed system, the Universal force field is adopted according to the force field matching algorithm, the PPPM algorithm is enabled to calculate the electrostatic effect, and the Ewald summation is combined to process the long-range part of the van der Waals force. The GPU computing module in the Material Studio software is enabled, the number of running cores is increased, and the simulation time is shortened.

[0023] Furthermore, the mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules was calculated using the Analysis function in the Forcite module of the Material Studio software. A drift correction algorithm was used to eliminate the system drift error by detrending the center of mass motion, and the final mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules was determined. The drift correction calculation formula is:

[0024]

[0025] Where r' i (t) is the original position vector drift correction value of the i-th molecule at time t, r i (t) is the original position vector of the i-th molecule at time t, N is the number of all molecules, r j (t) is the original position vector of the jth molecule at time t, is the center of mass position vector of all molecules at time t.

[0026] In step S6, the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules is calculated based on the mean square displacement as follows:

[0027]

[0028] Where MSD is the mean square displacement, k is the correction factor used to quantify the effect of pore distribution heterogeneity on thermal conductivity; n is the number of diffusing molecules, r is the i (t),r i (0) are the position vectors at time t and t = 0, respectively;

[0029]

[0030] Where D is the diffusion coefficient, K MSD is the slope of the MSD curve.

[0031] Another object of the present invention is to provide a system for obtaining the thermal conductivity of a polyimide composite aerogel, which implements the method for obtaining the thermal conductivity of a polyimide composite aerogel, and the system comprises:

[0032] An initial structure building module is used to build the initial structure of polyimide composite aerogel;

[0033] An initial structure model building module is used to build an initial structure model of polyimide composite aerogel;

[0034] Heat transfer model building module, used to establish nitrogen molecular structure model and determine the heat transfer model of polyimide composite aerogel containing nitrogen molecules;

[0035] Molecular dynamics simulation calculation module, used to set simulation calculation parameters and run Material Studio software for molecular dynamics simulation calculation;

[0036] A mean square displacement calculation module, used to calculate the mean square displacement;

[0037] The thermal conductivity calculation module is used to calculate the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

[0038] Furthermore, the coefficient system is carried on a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the functions of the above-mentioned system for obtaining the thermal conductivity coefficient of polyimide composite aerogel can be realized.

[0039] Furthermore, the system is used in the detection of thermal insulation properties of polyimide aerogel protective materials in aircraft thermal protection systems.

[0040] Combining all of the above technical solutions, the present invention achieves the following beneficial effects: It simulates gaseous heat transfer on polyimide composite aerogels, analyzes the mean square displacement of the system at different temperatures, and directly derives the aerogel's thermal conductivity. The results provide a reference for understanding the microscopic heat transfer mechanisms of various materials and for developing effective thermal protection technologies.

[0041] This invention replaces traditional experimental testing with molecular dynamics simulation, significantly reducing the number of physical experiments required for material preparation and testing, and is expected to save 30% to 50% in R&D costs. Automated modeling and parallel computing technologies can shorten the thermal conductivity analysis cycle from traditional weeks to days, greatly improving R&D efficiency. In areas such as aerospace thermal protection systems, new energy vehicle battery insulation, and energy-saving building materials, high-precision thermal conductivity prediction technology can promote the rapid commercialization of high-performance aerogel materials.

[0042] Compared with traditional molecular dynamics simulations that often ignore the effect of pore distribution heterogeneity on thermal conductivity, this invention introduces the correction coefficient k to quantify the dynamic relationship between pore heterogeneity and thermal conductivity for the first time, filling the gap in the microscopic heat transfer model of porous materials.

[0043] Compared with the existing technology that finds it difficult to quantify the nonlinear effect of temperature on the thermal conductivity of aerogel, the present invention reveals the synergistic mechanism of temperature and porosity for the first time through parallel simulation of multiple temperature points and modified Clausius-Clapeyron equation, providing theoretical support for the design of high-temperature thermal insulation materials.

[0044] Compared to existing techniques that believe molecular dynamics simulations struggle to balance efficiency and accuracy in complex multiphase systems, this paper demonstrates the high reliability of this simulation technology in organic-inorganic composite systems through GPU acceleration and the PPPM long-range force algorithm. Secondly, molecular dynamics simulations have long assumed uniform pore distribution in aerogels, leading to deviations from actual thermal conductivity predictions. This paper introduces a pore heterogeneity correction factor k (experimentally calibrated to β = -0.1), providing a new direction for microstructure optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present disclosure and, together with the description, serve to explain the principles of the present disclosure;

[0046] Figure 1 This is a schematic diagram of a method for obtaining the thermal conductivity of a polyimide composite aerogel provided in an embodiment of the present invention;

[0047] Figure 2 This is a flow chart of a method for obtaining thermal conductivity of a polyimide composite aerogel provided in an embodiment of the present invention;

[0048] Figure 3 This is a schematic diagram of a cubic simulation box of a composite PI macromolecular chain provided by an embodiment of the present invention;

[0049] Figure 4 This is a schematic diagram of a rectangular box of nitrogen molecules provided by an embodiment of the present invention;

[0050] Figure 5 This is a schematic diagram of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules provided in an embodiment of the present invention before merging;

[0051] Figure 6 This is a schematic diagram of the combined heat transfer model of the polyimide composite aerogel containing nitrogen molecules provided in an embodiment of the present invention;

[0052] Figure 7 This is a diagram showing the variation of the MSD of the polyimide composite aerogel heat transfer model containing nitrogen molecules of the present invention at different temperatures. DETAILED DESCRIPTION

[0053] To make the above-mentioned objects, features, and advantages of the present invention more readily apparent, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings. The following description sets forth numerous specific details to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art may make similar modifications without departing from the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0054] The innovation of this invention lies in its ability to significantly improve modeling accuracy by dynamically calculating and simulating the box size and porosity-temperature coupled nitrogen filling model, combined with exclusion volume optimization and drift correction algorithms. A pore heterogeneity correction factor is introduced to quantify the impact of microstructure on thermal conductivity and reduce error rates. GPU acceleration and parallel simulation at multiple temperature points significantly improve computational efficiency. This method overcomes the high cost and time-consuming nature of traditional experiments, providing a high-precision, cross-scale theoretical tool for the efficient development of aerospace thermal protection systems and new energy materials.

[0055] Example 1, as Figure 1 Schematic diagram of the method for obtaining the thermal conductivity of polyimide composite aerogel provided in an embodiment of the present invention;

[0056] For example, Figure 2 As shown, the method for obtaining the thermal conductivity of the polyimide composite aerogel provided by the embodiment of the present invention includes:

[0057] S1, establishing the initial structure of polyimide composite aerogel;

[0058] S2, establishing the initial structural model of polyimide composite aerogel;

[0059] S3, establishing a nitrogen molecular structure model and determining the heat transfer model of polyimide composite aerogel containing nitrogen molecules;

[0060] S4, set simulation calculation parameters and run Material Studio software to perform molecular dynamics simulation calculations;

[0061] S5, calculate the mean square displacement;

[0062] S6, calculate the thermal conductivity of polyimide composite aerogel in the heat transfer model of polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

[0063] For example, the initial structure of the polyimide composite aerogel in step S1 is established using Chem Draw software, and the initial components of the initial structure of the polyimide composite aerogel include PI, SiO2, C6H 18 O 24 There is no mass ratio limit between P6 and BN. 18 O 24 P6 is compounded into the PI molecular chain to form a composite PI macromolecular chain. SiO2 is not chemically linked to the PI macromolecular chain, but is physically compounded, free around the PI molecular chain and evenly wrapped in the aerogel.

[0064] For example, the initial structural model of the polyimide composite aerogel in step S2 is obtained by dynamically calculating the cubic simulation box of the composite PI macromolecular chain using the "Amorphous Cell" module in the MaterialStudio software according to the molecular composition and target density of the composite material using the following formula: Figure 3 As shown, the calculation formula is:

[0065]

[0066] Where L is the length of each side of the cubic simulation box, N is the number of molecular fillings predicted by Monte Carlo simulation, with an error rate of <3%; V mol is the average volume of a single molecule, ρ is the density of polyimide composite aerogel;

[0067] In the "DMol3" module of Material Studio software, the exclusion volume weight factor (ω excluded ), and then introduced the gradient descent algorithm script to optimize the spacing between polyimide composite macromolecules to ensure the uniformity of molecular spatial distribution and avoid local accumulation. In the "Amorphous Cell" module, SiO2 was added to the established cubic simulation box to determine the initial structural model of the polyimide composite aerogel.

[0068] Exemplarily, in step S3, the number of nitrogen molecules is calculated according to the aerogel porosity (φ) and temperature (T) by using the modified Clausius-Clapeyron equation:

[0069]

[0070] Where, is the number of nitrogen molecules, P is the standard atmospheric pressure, V BOX is the pore volume, κ B is the Boltzmann constant, α(T) is the temperature correction factor;

[0071] Adaptive Poisson sphere algorithm is used in combination with molecular dynamics pre-simulation to optimize nitrogen distribution, ensuring that the distance between nitrogen molecules meets the van der Waals exclusion radius and avoids overlap;

[0072] Use the "Build" module in Material Studio software to define the nitrogen molecule rectangular box, i.e. the nitrogen molecule structure model, based on the microscopic morphology analysis data (SEM) of the aerogel. Figure 4 As shown;

[0073] The initial structure model of the polyimide composite aerogel and the nitrogen molecule structure model are merged, and the merged heat transfer model of the polyimide composite aerogel containing nitrogen molecules is output.

[0074] Among them, such as Figure 5 , Schematic diagram of the heat transfer model of polyimide composite aerogel containing nitrogen molecules before merging,

[0075] like Figure 6 , schematic diagram of the combined heat transfer model of polyimide composite aerogel containing nitrogen molecules.

[0076] Exemplarily, in step S4, the simulation temperature is selected as 25°C, 100°C and 200°C in the "Forcite" module of the Material Studio software. During the simulation, the ensemble used in the dynamic process is set to the canonical ensemble NVT ensemble. Based on the material composition, that is, a homogeneous organic-inorganic mixed system, the Universal force field is adopted according to the force field matching algorithm, the PPPM (Particle-Particle Particle-Mesh) algorithm is enabled to calculate the electrostatic effect, and the Ewald summation is combined to process the long-range part of the van der Waals force. The truncation error is reduced to <1%, the step size is 500,000 steps, and the interval between each step is 1.0 fs. The "GPU" computing module in the Material Studio software is enabled, the number of running cores is increased to 12, and the simulation time of 500,000 steps is shortened.

[0077] For example, in step S5, the Analysis function in the Forcite module of the Material Studio software calculates the mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules, and uses a drift correction algorithm to eliminate the system drift error by detrending the center of mass motion to determine the final mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules. The drift correction calculation formula is:

[0078]

[0079] Where r' i (t) is the original position vector drift correction value of the i-th molecule at time t, r i (t) is the original position vector of the i-th molecule at time t, N is the number of all molecules, r j (t) is the original position vector of the jth molecule at time t, is the center of mass position vector of all molecules at time t.

[0080] For example, in step S6, the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules is calculated based on the mean square displacement as follows:

[0081]

[0082] Where MSD is the mean square displacement, k is the correction factor used to quantify the effect of pore distribution heterogeneity on thermal conductivity; n is the number of diffusing molecules, r is the i (t),r i (0) are the position vectors at time t and t = 0, respectively;

[0083]

[0084] Where D is the diffusion coefficient. Since the main conduction mode of aerogel is gaseous heat transfer, its thermal conductivity is directly proportional to D. The diffusion coefficient can be used to directly determine the change law of the thermal conductivity of polyimide aerogel; K MSD is the slope of the MSD curve. The more uneven the pore distribution is, the smaller the k value is, that is, the smaller the thermal conductivity is, and the thermal insulation performance is improved.

[0085] The present invention also provides a system for obtaining the thermal conductivity of a polyimide composite aerogel, which implements the method for obtaining the thermal conductivity of a polyimide composite aerogel. The system comprises:

[0086] An initial structure building module is used to build the initial structure of polyimide composite aerogel;

[0087] An initial structure model building module is used to build an initial structure model of polyimide composite aerogel;

[0088] Heat transfer model building module, used to establish nitrogen molecular structure model and determine the heat transfer model of polyimide composite aerogel containing nitrogen molecules;

[0089] Molecular dynamics simulation calculation module, used to set simulation calculation parameters and run Material Studio software for molecular dynamics simulation calculation;

[0090] A mean square displacement calculation module, used to calculate the mean square displacement;

[0091] The thermal conductivity calculation module is used to calculate the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

[0092] Furthermore, the coefficient system is carried on a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the functions of the above-mentioned system for obtaining the thermal conductivity coefficient of polyimide composite aerogel can be realized.

[0093] Furthermore, the system is used in the detection of thermal insulation properties of polyimide aerogel protective materials in aircraft thermal protection systems.

[0094] To further illustrate the effects of the embodiments of the present invention, the following experiments were conducted.

[0095] Experimental Example 1: A method for obtaining the thermal conductivity of polyimide composite aerogel using molecular dynamics.

[0096] In the first step, according to the reaction of hexagonal nano-boron nitride and phytic acid in polyamic acid solution and the molecular chain characteristics of polyamic acid, BN and C6H 18 O 24 P6 is linked into its molecular chain to obtain the initial structure of polyimide composite aerogel.

[0097] In the second step, the initial structure of the constructed polyimide composite aerogel was imported into the Material Studio software, and the geometry optimization and energy minimization were performed in the "Forcite" module. The Monte Carlo simulation was used in the "Amorphous Cell" module to predict the molecular filling number, and the molecular composition and density of the polyimide composite aerogel were calculated by equations to establish the A cubic simulation box for polyimide composite aerogels was created. The exclusion volume weight factor was introduced, and the gradient descent method was used to optimize the spacing between composite macromolecules. SiO2 was added to the simulation box to establish the initial structural model of the polyimide composite aerogels. The initial structural model of the polyimide composite aerogels was geometrically optimized and energy minimized in the "Forcite" module.

[0098] Step 3: In the "Amorphous Cell" module of Material Studio, determine the number of nitrogen molecules according to the modified Clausius-Clapeyron equation and create A nitrogen molecule cubic simulation box is constructed; according to the adaptive Poisson sphere method, a molecular dynamics simulation of 10 ps is performed on the nitrogen simulation box to optimize the molecular distribution; in the "Build" module, according to the aerogel SEM data, the rectangular box is defined as The initial structure model of polyimide composite aerogel and the nitrogen molecule simulation box were combined to establish a heat transfer model of polyimide composite aerogel.

[0099] In the fourth step, molecular dynamics simulation of the above heat transfer model was performed in the "Forcite" module of Material Studio. The simulation temperature was 298K, the ensemble was the NVT ensemble, the force field was determined to be the Universal force field according to the force field matching algorithm, the PPPM algorithm was enabled to calculate the electrostatic effect, and the Ewald summation was combined to process the long-range part of the van der Waals force. The truncation error was reduced to <1%, the step size was 500,000 steps, and the interval between each step was 1.0 fs. The number of running cores in the "GPU" computing module was increased to 12 to shorten the simulation time.

[0100] Step 5. After the simulation is completed, export the Mean square displacement (MSD) curve in "Analysis" in the "Forcite" module, eliminate the system drift error according to the drift correction algorithm, and determine the final MSD curve.

[0101] Step 6: Calculate the thermal conductivity of the polyimide composite aerogel based on the MSD curve and the modified Einstein equation. The calculation results of Example 1 are shown in Table 1.

[0102] Table 1 Calculation results of Example 1

[0103] Experiment No. <![CDATA[K MSD ]]> <![CDATA[D(×10 -9 mm 2 / s)]]> Experimental Example 1 16.00 2.67

[0104] Experimental Example 2: A method for obtaining the thermal conductivity of polyimide composite aerogel using molecular dynamics.

[0105] In the first step, according to the reaction of hexagonal nano-boron nitride and phytic acid in polyamic acid solution and the molecular chain characteristics of polyamic acid, BN and C6H18 O 24 P6 is linked into its molecular chain to obtain the initial structure of polyimide composite aerogel.

[0106] In the second step, the initial structure of the constructed polyimide composite aerogel was imported into the Material Studio software, and the geometry optimization and energy minimization were performed in the "Forcite" module. The Monte Carlo simulation was used in the "Amorphous Cell" module to predict the molecular filling number, and the molecular composition and density of the polyimide composite aerogel were calculated by equations to establish the A cubic simulation box for polyimide composite aerogels was created. The exclusion volume weight factor was introduced, and the gradient descent method was used to optimize the spacing between composite macromolecules. SiO2 was added to the simulation box to establish the initial structural model of the polyimide composite aerogels. The initial structural model of the polyimide composite aerogels was geometrically optimized and energy minimized in the "Forcite" module.

[0107] Step 3: In the "Amorphous Cell" module of Material Studio, determine the number of nitrogen molecules according to the modified Clausius-Clapeyron equation and create A nitrogen molecule cubic simulation box is constructed; according to the adaptive Poisson sphere method, a molecular dynamics simulation of 10 ps is performed on the nitrogen simulation box to optimize the molecular distribution; in the "Build" module, according to the aerogel SEM data, the rectangular box is defined as The initial structure model of polyimide composite aerogel and the nitrogen molecule simulation box were combined to establish a heat transfer model of polyimide composite aerogel.

[0108] In the fourth step, molecular dynamics simulation of the above heat transfer model was performed in the "Forcite" module of Material Studio. The simulation temperature was 373.15K, the ensemble was the NVT ensemble, and the force field was determined to be the Universal force field according to the force field matching algorithm. The PPPM algorithm was enabled to calculate the electrostatic effect. The Ewald summation was combined to process the long-range part of the van der Waals force. The truncation error was reduced to <1%. The step size was 500,000 steps, and the interval between each step was 1.0 fs. The number of running cores in the "GPU" computing module was increased to 12 to shorten the simulation time.

[0109] Step 5. After the simulation is completed, export the mean square displacement (MSD) curve in "Analysis" in the "Forcite" module, eliminate the system drift error according to the drift correction algorithm, and determine the final MSD curve.

[0110] Step 6: Calculate the thermal conductivity of the polyimide composite aerogel based on the MSD curve and the modified Einstein equation. The calculation results of Example 2 are shown in Table 2.

[0111] Table 2 Calculation results of Example 2

[0112] Experiment No. <![CDATA[K MSD ]]> <![CDATA[D(×10 -9 mm 2 / s)]]> Experimental Example 2 16.09 2.68

[0113] Experimental Example 3: A method for obtaining the thermal conductivity of polyimide composite aerogel using molecular dynamics.

[0114] In the first step, according to the reaction of hexagonal nano-boron nitride and phytic acid in polyamic acid solution and the molecular chain characteristics of polyamic acid, BN and C6H 18 O 24 P6 is linked into its molecular chain to obtain the initial structure of polyimide composite aerogel.

[0115] In the second step, the initial structure of the constructed polyimide composite aerogel was imported into the Material Studio software, and the geometry optimization and energy minimization were performed in the "Forcite" module. The Monte Carlo simulation was used to predict the molecular filling number in the "Amorphous Cell" module, and the molecular composition and density of the polyimide composite aerogel were calculated by equations to establish the A cubic simulation box for polyimide composite aerogels was created. The exclusion volume weight factor was introduced, and the gradient descent method was used to optimize the spacing between composite macromolecules. SiO2 was added to the simulation box to establish the initial structural model of the polyimide composite aerogels. The initial structural model of the polyimide composite aerogels was geometrically optimized and energy minimized in the "Forcite" module.

[0116] Step 3: In the "Amorphous Cell" module of Material Studio, determine the number of nitrogen molecules according to the modified Clausius-Clapeyron equation and create A nitrogen molecule cubic simulation box is constructed; according to the adaptive Poisson sphere method, a molecular dynamics simulation of 10 ps is performed on the nitrogen simulation box to optimize the molecular distribution; in the "Build" module, according to the aerogel SEM data, the rectangular box is defined as The initial structure model of polyimide composite aerogel and the nitrogen molecule simulation box were combined to establish a heat transfer model of polyimide composite aerogel.

[0117] In the fourth step, molecular dynamics simulation of the above heat transfer model was performed in the "Forcite" module of Material Studio. The simulation temperature was 473.15K, the ensemble was the NVT ensemble, and the force field was determined to be the Universal force field according to the force field matching algorithm. The PPPM algorithm was enabled to calculate the electrostatic effect, and the Ewald summation was combined to process the long-range part of the van der Waals force. The truncation error was reduced to <1%, the step size was 500,000 steps, and the interval between each step was 1.0 fs. The number of running cores in the "GPU" computing module was increased to 12 to shorten the simulation time.

[0118] Step 5. After the simulation is completed, export the Mean square displacement (MSD) curve in "Analysis" in the "Forcite" module, eliminate the system drift error according to the drift correction algorithm, and determine the final MSD curve.

[0119] Step 6: Calculate the thermal conductivity of the polyimide composite aerogel based on the MSD curve and the modified Einstein equation. The calculation results of Example 3 are shown in Table 3.

[0120] Table 3 Calculation results of Example 3

[0121] Experiment No. <![CDATA[K MSD ]]> <![CDATA[D(×10 -9 mm 2 / s)]]> Experimental Example 3 18.74 3.12

[0122] In summary, the increase in temperature will intensify the thermal motion of molecules and gradually break the macromolecular chains, resulting in an increase in the slope of the MSD curve, which in turn affects the increase in the thermal conductivity of the polyimide composite aerogel. Therefore, the higher the temperature, the greater the thermal conductivity of the polyimide composite aerogel.

[0123] Among them, in the above experimental example, the MSD of the polyimide composite aerogel heat transfer model containing nitrogen molecules changes at different temperatures as shown in the following example: Figure 7 shown.

[0124] The above description is only a preferred specific implementation method of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with this technical field within the technical scope disclosed by the present invention and within the spirit and principles of the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for obtaining the thermal conductivity of a polyimide composite aerogel, characterized in that: The method comprises the following steps: S1, establishing the initial structure of polyimide composite aerogel; S2, establishing the initial structural model of polyimide composite aerogel; S3, establishing a nitrogen molecular structure model and determining the heat transfer model of polyimide composite aerogel containing nitrogen molecules; S4, set simulation calculation parameters and run Material Studio software to perform molecular dynamics simulation calculations; S5, calculate the mean square displacement; S6, calculate the thermal conductivity of polyimide composite aerogel in the heat transfer model of polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

2. The method for obtaining the thermal conductivity of polyimide composite aerogel according to e1, characterized in that: In step S1, the initial structure of the polyimide composite aerogel is established using Chem Draw software. The initial components of the initial structure of the polyimide composite aerogel include PI, SiO2, C6H 18 O 24 P6 and BN; among them, BN and C6H 18 O 24 P6 is compounded into the PI molecular chain to form a composite PI macromolecular chain.

3. The method for obtaining the thermal conductivity of polyimide composite aerogel according to claim 1, characterized in that: In step S2, the Amorphous Cell module in the Material Studio software is used to dynamically calculate the cubic simulation box of the composite PI macromolecular chain according to the molecular composition and target density of the composite material using the following formula: Where L is the length of each side of the cubic simulation box, N is the number of molecular fillings predicted by Monte Carlo simulation, with an error rate of <3%; V mol is the average volume of a single molecule, ρ is the density of polyimide composite aerogel; Introducing the exclusion volume weight factor ω in the Amorphous Cell module in Material Studio software exluded , the distance between polyimide composite macromolecules was optimized by gradient descent algorithm, SiO2 was added to the established cubic simulation box in the Amorphous Cell module to determine the initial structural model of polyimide composite aerogel.

4. The method for obtaining the thermal conductivity of polyimide composite aerogel according to claim 1, characterized in that: In step S3, nitrogen molecules are added according to the porosity of the aerogel. and temperature T are calculated using the modified Clausius-Clapeyron equation: Where, is the number of nitrogen molecules, P is the standard atmospheric pressure, V BOX is the pore volume, κ B is the Boltzmann constant, α(T) is the temperature correction factor; The adaptive Poisson sphere algorithm was used in combination with molecular dynamics pre-simulation to optimize the nitrogen distribution. The Build module in Material Studio software was used to define a rectangular box of nitrogen molecules, i.e., the nitrogen molecular structure model, based on the microscopic morphology analysis data SEM of the aerogel. The initial structure model of the polyimide composite aerogel and the nitrogen molecular structure model were merged to output the merged heat transfer model of the polyimide composite aerogel containing nitrogen molecules.

5. The method for obtaining the thermal conductivity of polyimide composite aerogel according to claim 1, characterized in that: In step S4, the simulation temperatures are selected as 25°C, 100°C, and 200°C in the Forcite module of the Material Studio software. During the simulation, the ensemble used in the dynamic process is set to the canonical ensemble NVT ensemble. Based on the material composition, that is, a homogeneous organic-inorganic mixed system, the Universal force field is adopted according to the force field matching algorithm, the PPPM algorithm is enabled to calculate the electrostatic effect, and the Ewald summation is combined to process the long-range part of the van der Waals force. The GPU computing module in the Material Studio software is enabled, the number of running cores is increased, and the simulation time is shortened.

6. The method for obtaining the thermal conductivity of polyimide composite aerogel according to claim 5, characterized in that: The Analysis function in the Forcite module of Material Studio software calculates the mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules. A drift correction algorithm is used to eliminate the system drift error by detrending the center of mass motion to determine the final mean square displacement of the heat transfer model of the polyimide composite aerogel containing nitrogen molecules. The drift correction calculation formula is: Where r' i (t) is the original position vector drift correction value of the i-th molecule at time t, r i (t) is the original position vector of the i-th molecule at time t, N is the number of all molecules, r j (t) is the original position vector of the jth molecule at time t, is the center of mass position vector of all molecules at time t.

7. The method for obtaining the thermal conductivity of polyimide composite aerogel according to claim 1, characterized in that: In step S6, the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules is calculated based on the mean square displacement as follows: Where MSD is the mean square displacement and k is the correction factor, which is used to quantify the effect of pore distribution heterogeneity on thermal conductivity. n is the number of diffusing molecules, r i (t),r i (0) are the position vectors at time t and t = 0, respectively; Where D is the diffusion coefficient, K MSD is the slope of the MSD curve.

8. A system for obtaining thermal conductivity of polyimide composite aerogel, characterized in that: The system implements the method for obtaining the thermal conductivity of polyimide composite aerogel according to any one of claims 1 to 7, and the system comprises: An initial structure building module is used to build the initial structure of polyimide composite aerogel; An initial structure model building module is used to build an initial structure model of polyimide composite aerogel; Heat transfer model building module, used to establish nitrogen molecular structure model and determine the heat transfer model of polyimide composite aerogel containing nitrogen molecules; Molecular dynamics simulation calculation module, used to set simulation calculation parameters and run Material Studio software for molecular dynamics simulation calculation; A mean square displacement calculation module, used to calculate the mean square displacement; The thermal conductivity calculation module is used to calculate the thermal conductivity of the polyimide composite aerogel in the heat transfer model of the polyimide composite aerogel containing nitrogen molecules based on the mean square displacement.

9. The system for obtaining thermal conductivity of polyimide composite aerogel according to claim 8, characterized in that: The coefficient system is carried on a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the functions of the above-mentioned system for obtaining the thermal conductivity coefficient of polyimide composite aerogel can be realized.

10. The system for obtaining thermal conductivity of polyimide composite aerogel according to claim 8, characterized in that: The system is used in testing the thermal insulation properties of polyimide aerogel protective materials in aircraft thermal protection systems.

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