A method for characterizing stress-aging sensitivity of a polymer under combined action of multiple fields

Abstract

The application discloses a kind of polymer stress-aging sensitivity characterization methods under multi-field combined action, belong to dielectric material aging evaluation technical field.The method includes by constructing the molecular chain fracture energy barrier reduction model based on transition state theory, obtain theoretical energy barrier reduction threshold and aging rate constant, calculate to obtain the dynamic life index under different aging time;Based on the polymer sample after aging, by thermogravimetric analysis and Coats-Redfern method constructs thermal decomposition kinetics model, and the activation energy under different aging states is calculated;By comparing and analyzing the change law of life index and activation energy, the comprehensive sensitivity evaluation under the electric-thermal coupling effect is carried out;The physical self-consistency of model is verified by introducing entropy compensation effect thermodynamic equation.The application realizes the comprehensive quantitative characterization of polymer stress-aging sensitivity under electric-thermal combined action, with the advantages of clear physical meaning, can simultaneously characterize electric and thermal stress, clear mechanism explanation and the like.

Description

Technical Field

[0001] This invention relates to the field of dielectric material aging assessment technology, specifically a method for characterizing polymer stress-aging sensitivity under multi-field combined action. Background Technology

[0002] The insulating materials of power cables, such as cross-linked polyethylene and polypropylene, are subjected to the combined effects of electric and thermal fields during operation. This inevitably leads to insulation aging. An objective law governs this aging phenomenon: polymers operating under the same conditions and on the same equipment exhibit temporal and spatial non-uniformity in their aging degree. This has become a key factor hindering the accurate characterization of polymer insulation aging. The reason for this law is that polymers show differences in aging sensitivity after being subjected to a certain stress, but existing research lacks effective means to quantitatively characterize the aging sensitivity of polymers. Therefore, the concept of "stress-aging sensitivity" is defined to characterize the response level of the polymer's aging degree to the stress it is subjected to.

[0003] Aging is the response of a polymer to electrothermal stress input; this is a universal law. If external stress is used as the input... x The degree of aging is considered as output. L Then their relationship can be expressed as L = F ( x The rate of change of this function can represent the "stress-aging sensitivity".

[0004] A common way to represent stress and insulation life is through a conventional inverse power model. ,in, D The cumulative damage required for insulation failure; L The medium is subjected to an external field strength of E Lifespan under action; n For lifespan index; in ln E -ln t coordinate system n This is the slope of the line. Because the slope represents the rate of change of the dependent variable of a function as the independent variable changes, it can be represented by a parameter. n To characterize stress-aging sensitivity.

[0005] However, utilizing n Accurately characterizing the "stress-aging sensitivity" of polymers faces two main challenges: firstly, n These are parameters in a phenomenological model, and because they lack a physical interpretation, there is no more reasonable explanation. n Value parsing expressions; secondly, n These are parameters in the inverse power model, which is typically a model of the electrical lifetime of insulating media, making it difficult to pass through. nThe value characterizes the effect of thermal stress on the degree of aging of the medium. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a method for characterizing the stress-aging sensitivity of polymers under multi-field combined action. This invention focuses on the lifetime index. n This key parameter characterizing polymer stress-aging sensitivity addresses two difficulties encountered when using the lifetime index to characterize stress-aging sensitivity under multiple combined effects through mathematical modeling and methodological innovation. This achieves the goal of establishing a polymer stress-aging sensitivity characterization method and improving the accuracy of polymer aging assessment.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for characterizing the stress-aging sensitivity of polymers under multi-field combined action, specifically including the following steps: S1. Based on the unaged polymer sample, after conducting a short-time breakdown test and determining the breakdown voltage, select at least three different external electric field strengths, conduct constant voltage breakdown tests, construct a breakdown dataset, fit a straight line in the lnE-lnt coordinate system, and obtain the initial lifetime index; and conduct aging tests on the unaged polymer sample to obtain the aged polymer sample. This invention conducts short-time breakdown tests on unaged polymer samples to determine the breakdown voltage; based on the breakdown voltage, selects i (i≥3) different applied electric field strengths to conduct constant voltage tests under an applied electric field; and based on the breakdown data, fits a curve to obtain the initial lifetime index n0. An aging test was conducted on the unaged polymer sample, and the test conditions were set as follows: E , t ), where E is the aging voltage and t is the aging time. The breakdown data (E) is obtained by breaking down the aged sample. i ,t y y≥6; S2. Based on the breakdown dataset and transition state theory, a model for the reduction of the energy barrier of molecular chain fracture under electrical stress is constructed to obtain the theoretical energy barrier reduction threshold and aging rate constant. Based on the transition state theory, the relationship between lifetime and absolute temperature, and the transition state theory, is expressed as follows: ; Theoretical energy barrier reduction threshold Δ W theory The expression is as follows: Δ W theory = eβE ; In the formula, e The amount of electron charge. β For material structure coefficients, EThe applied electric field strength; Aging rate constant α The expression is as follows: ; In the formula, For the reaction rate, Where is Boltzmann's constant, T is absolute temperature, and h is Planck's constant. For effective energy barrier.

[0008] S3. Based on the initial lifetime index, the theoretical energy barrier reduction threshold, and the aging rate constant, a time-varying model of the lifetime index is constructed to calculate the lifetime index under different aging times. The time-varying lifetime index model constructed in this invention is expressed as follows: ; In the formula, Δ W theory To lower the threshold of the theoretical energy barrier, α Let t be the aging rate constant, and t be the aging time. Where n is the Boltzmann constant, n0 is the initial lifetime exponent, and T is the absolute temperature. This represents the lifespan index at different aging times.

[0009] This invention defines As the electrothermal synergistic sensing factor, the expression for the lifetime exponential time-varying model can be expressed as: ; S4. Based on the aged polymer sample, after calculating the decomposition rate, conversion degree and ordinate value, linear fitting is performed to obtain the activation energy under different aging states, and the activation energy change curve is output. The effect of thermal stress on polymers is mainly reflected in changes in the material's microstructure (such as activation energy). The level of activation energy reflects the polymer's sensitivity to external temperature during aging. Therefore, this invention utilizes activation energy... E a As an independent characterization index of polymer aging sensitivity caused by thermal stress.

[0010] A polymer sample with mass m0 will decompose during continuous heating. t At time t, the mass becomes m1, then the expression for the decomposition rate is: ; In the formula, v The degree of decomposition can be expressed as v =(m0-m1) / (m0-m ∞ )×100%; m ∞ The final remaining substance; KThe expression for the Arrhenius reaction rate constant is: ; In the formula, A Pre-exponential factor, E a For activation energy, R The gas constant is T This refers to absolute temperature.

[0011] If we assume f ( v Only related to the degree of decomposition v If relevant, then f ( v ) = (1- v ) ψ ,make λ =d T / d t We can obtain: ; Taking ψ as 1 / 2, the activation energy under different aging states can be calculated using the Coats-Redfern method. E a The expression is as follows: ; In the formula, F ( v )=1 (1 v ) 1 / 2 For the degree of transformation, v To the degree of decomposition, λ To represent the heating rate, by fitting ln[ F ( v ) / T 2 ] and 1 / T The slope of the line can be obtained. E a Finally, by plotting the activation energy at different aging times, the sensitivity changes of the medium under thermal stress can be obtained.

[0012] The specific method for obtaining activation energy is as follows: First, determine the different degrees of conversion. v i ( i ≥5) F ( v By finding the value of ), its ordinate value can be obtained; then, based on 1 / T The value is obtained by comparing it with the value obtained in step one, i.e., (ln[ F ( v ) / T2 ], 1 / T By fitting the data and obtaining the equation of a straight line, the activation energy can be obtained from the slope. E a value.

[0013] S5. Based on the lifetime index under different aging times and the activation energy under different aging states, a comprehensive sensitivity assessment under electro-thermal coupling effect is conducted through comparative analysis and slope change rate calculation. S6. Based on the lifetime index at different aging times and the activation energy under different aging states, the changes at high temperatures are analyzed by introducing thermodynamic equations to verify the physical self-consistency of the model.

[0014] Compared with the prior art, the present invention provides a method for characterizing the stress-aging sensitivity of polymers under multiple combined effects, which has the following beneficial effects: (1) This invention establishes a quantitative relationship between the macroscopic lifetime index and the microscopic energy barrier by constructing a molecular chain fracture energy barrier reduction model based on transition state theory. This solves the defects of the phenomenological model that lacks physical interpretation and reasonable analytical expression of the lifetime index, and realizes the transformation of the lifetime index from a phenomenological parameter to a material property parameter with clear physical connotation.

[0015] (2) By constructing a time-varying lifetime index model and a thermal decomposition kinetic model, the present invention uses the dynamic lifetime index and activation energy to characterize the aging sensitivity under electrical stress and thermal stress, respectively. This solves the technical problem that the traditional inverse power model described in the background art is difficult to characterize the influence of thermal stress on the degree of aging, and realizes a comprehensive quantitative characterization of polymer stress-aging sensitivity under the combined action of electricity and heat.

[0016] (3) By introducing the thermodynamic equation of entropy compensation effect, this invention solves the problem of the inexplicable mechanism of the contradictory phenomenon of reduced activation energy and slowed change in aging sensitivity at high temperature, and realizes the theoretical verification of the physical self-consistency of the model. Attached Figure Description

[0017] Figure 1 This is a flowchart of a method for characterizing polymer stress-aging sensitivity under multi-field combined action according to the present invention; Figure 2 This is the overall flowchart of the present invention; Figure 3 This is a graph showing the trend of the lifespan index of the present invention; Figure 4 This is a graph showing the change in activation energy at 70°C aging temperature under a 20°C / min aging rate according to the present invention. Figure 5 This is a graph showing the change in activation energy at 90°C aging temperature under a 20°C / min aging rate according to the present invention. Figure 6This is a graph showing the change in activation energy at 105°C aging temperature under a 20°C / min aging rate according to the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] like Figure 1 and Figure 2 As shown, a method for characterizing the stress-aging sensitivity of polymers under multiple combined effects includes the following steps: S1. Based on the unaged polymer sample, after conducting a short-time breakdown test and determining the breakdown voltage, select at least three different external electric field strengths, conduct constant voltage breakdown tests, construct a breakdown dataset, fit a straight line in the lnE-lnt coordinate system, and obtain the initial lifetime index; and conduct aging tests on the unaged polymer sample to obtain the aged polymer sample. This invention conducts short-time breakdown tests on unaged polymer samples to determine the breakdown voltage; based on the breakdown voltage, selects i (i≥3) different applied electric field strengths to conduct constant voltage tests under an applied electric field; and based on the breakdown data, fits a curve to obtain the initial lifetime index n0. An aging test was conducted on the unaged polymer sample, and the test conditions were set as follows: E , t ), where E is the aging voltage and t is the aging time. The breakdown data (E) is obtained by breaking down the aged sample. i ,t y y≥6; This example uses an XLPE polymer sample: The breakdown data obtained by the present invention through aging tests are shown in Table 1. Table 1: Short-time breakdown voltage data S2. Based on the breakdown dataset and transition state theory, a model for the reduction of the energy barrier of molecular chain fracture under electrical stress is constructed to obtain the theoretical energy barrier reduction threshold and aging rate constant. Based on the transition state theory, the relationship between lifetime and absolute temperature, and the transition state theory, is expressed as follows: (1); Theoretical energy barrier reduction threshold Δ W theory The expression is as follows: ΔW theory = eβE (2); In the formula, e The amount of electron charge. β For material structure coefficients, E The applied electric field strength; Aging rate constant α The expression is as follows: (3); In the formula, For the reaction rate, Where is Boltzmann's constant, T is absolute temperature, and h is Planck's constant. For effective energy barrier.

[0020] S3. Based on the initial lifetime index, the theoretical energy barrier reduction threshold, and the aging rate constant, a time-varying model of the lifetime index is constructed to calculate the lifetime index under different aging times. The time-varying lifetime index model constructed in this invention is expressed as follows: (4); In the formula, Δ W theory To lower the threshold of the theoretical energy barrier, α Let t be the aging rate constant, and t be the aging time. Where n is the Boltzmann constant, n0 is the initial lifetime exponent, and T is the absolute temperature. This represents the lifespan index at different aging times. The desired Δ... W theory and α Substituting into equation (4) yields the degree of change in the sensitivity of the medium to external stress under electrical stress.

[0021] This invention defines As the electrothermal synergistic sensing factor, the value of the latter part of equation (4) reflects the field strength. E and temperature T The change in the dielectric lifetime index, i.e. the susceptibility of the dielectric to aging after being subjected to electrical and thermal stress, can be expressed by the time-varying model of the lifetime index as follows: (5).

[0022] This invention selects aging t The breakdown field strength at time Δ can be obtained by substituting the data into equations (1) and (2). W theory and α ; Substituting the obtained data into equations (1) and (2), the data is shown in the table below.

[0023] Table 2: Characteristic Parameter Values Based on the construction of a time-varying lifespan index model, the lifespan index under different aging times is calculated, such as... Figure 3 As shown.

[0024] S4. Based on the aged polymer sample, after calculating the decomposition rate, conversion degree and ordinate value, linear fitting is performed to obtain the activation energy under different aging states, and the activation energy change curve is output. The effect of thermal stress on polymers is mainly reflected in changes in the material's microstructure (such as activation energy). The level of activation energy reflects the polymer's sensitivity to external temperature during aging. Therefore, this invention utilizes activation energy... E a As an independent characterization index of polymer aging sensitivity caused by thermal stress.

[0025] A polymer sample with mass m0 will decompose during continuous heating. t At time t, the mass becomes m1, then the expression for the decomposition rate is: (6); In the formula, v The degree of decomposition can be expressed as v =(m0-m1) / (m0-m ∞ )×100%; m ∞ The final remaining substance; K The expression for the Arrhenius reaction rate constant is: (7); In the formula, A Pre-exponential factor, E a For activation energy, R The gas constant is T This refers to absolute temperature.

[0026] If we assume f ( v Only related to the degree of decomposition v If relevant, then f ( v ) = (1- v ) ψ ,make λ =d T / d t Substituting equation (6) into equation (7), we get: (8); Taking ψ as 1 / 2, the activation energy under different aging states can be calculated using the Coats-Redfern method. Ea The expression is as follows: (9); In the formula, F ( v )=1 (1 v ) 1 / 2 For the degree of transformation, v To the degree of decomposition, λ To represent the heating rate, by fitting ln[ F ( v ) / T 2 ] and 1 / T The slope of the line can be obtained. E a Finally, by plotting the activation energy at different aging times, the sensitivity changes of the medium under thermal stress can be obtained.

[0027] The specific method for obtaining activation energy is as follows: First, determine the different degrees of conversion. v i ( i ≥5) F ( v Substituting the value of ) into equation (8) yields its ordinate value; then, based on 1 / T The value is obtained by comparing it with the value obtained in step one, i.e., (ln[ F ( v ) / T 2 ], 1 / T By fitting the data and obtaining the equation of a straight line, the activation energy can be obtained from the slope. E a value.

[0028] In this embodiment, the activation energies under different aging states are obtained as follows: Figure 4 , Figure 5 and Figure 6 As shown.

[0029] Based on the above analysis, this embodiment determines the parameters characterizing the medium sensitivity under single stress. The following is the characterization method of stress-aging sensitivity.

[0030] From equation (3), it can be seen that to obtain the change with aging time, n The values ​​that need to be obtained first include: (1)Δ W theory The value; (2) Reaction rate constant α The value; (3) Initial life index value of unaged media n 0.

[0031] Similarly, from equation (9), it can be seen that for activation energy... E a To obtain it, the following values ​​need to be calculated: (4) ln[ for different degrees of transformation] F ( v ) / T 2 The value of ]; (5) Values ​​of 1 / T at different temperatures.

[0032] S5. Based on the lifetime index under different aging times and the activation energy under different aging states, a comprehensive sensitivity assessment under electro-thermal coupling effect is conducted through comparative analysis and slope change rate calculation. S6. Based on the lifetime index under different aging times and the activation energy under different aging states, the changes at high temperatures are analyzed by introducing thermodynamic equations to verify the physical self-consistency of the model. Regression single-stress model and mechanism verification based on entropy-induced compensation effect: This invention innovatively utilizes the activation energy under different aging states. E a Data Explanation of Lifespan Index at Different Aging Times n ( t Pattern of change: Contradictory phenomenon: TGA displays at high temperatures E a The temperature decreases (the material becomes weaker), but model calculations show that the increment of the n value at high temperatures is actually smaller (the sensitivity change is suppressed).

[0033] Physical mechanism: Introducing the thermodynamic equation ΔG= E a - T Δ S ,in T Δ S This is the entropy term. Although high temperature leads to the enthalpy term... E a While the energy loss decreases, it also stimulates conformational motion of the polymer chains, leading to a significant increase in the entropy term TΔS. This entropy increase compensates for the bond energy loss and dissipates the energy of the applied electric field.

[0034] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for characterizing the stress-aging sensitivity of polymers under multi-field combined action, characterized in that, Includes the following steps: S1. Based on unaged polymer samples, after conducting short-time breakdown tests and determining the breakdown voltage, select a preset number of different applied electric field strengths and conduct constant voltage breakdown tests to construct a breakdown dataset; fit the breakdown dataset to obtain the initial lifetime index, and conduct aging tests on the unaged polymer samples to obtain aged polymer samples. S2. Based on the breakdown dataset and transition state theory, a model for reducing the energy barrier of molecular chain fracture under electrical stress is constructed to calculate the theoretical energy barrier reduction threshold and aging rate constant. S3. Based on the initial lifetime index, the theoretical energy barrier reduction threshold, and the aging rate constant, a time-varying model of the lifetime index is constructed to calculate the lifetime index under different aging times. S4. Based on the aged polymer sample, after calculating the decomposition rate, conversion degree and ordinate value, linear fitting is performed to obtain the activation energy under different aging states, and the activation energy change curve is output. S5. Based on the lifetime index under different aging times and the activation energy under different aging states, a comprehensive sensitivity assessment under electro-thermal coupling effect is conducted through comparative analysis and slope change rate calculation. S6. Based on the lifetime index at different aging times and the activation energy under different aging states, the changes at high temperatures are analyzed by introducing thermodynamic equations to verify the physical self-consistency of the model and complete the polymer stress-aging sensitivity characterization method.

2. The method for characterizing polymer stress-aging sensitivity under multi-field combined action according to claim 1, characterized in that, In step S3, the constructed lifetime index time-varying model expression is as follows: In the formula, Δ W theory To lower the threshold of the theoretical energy barrier, α Let t be the aging rate constant, and t be the aging time. Where n is the Boltzmann constant, n0 is the initial lifetime exponent, and T is the absolute temperature. This represents the lifespan index at different aging times.

3. The method for characterizing polymer stress-aging sensitivity under multi-field combined action according to claim 2, characterized in that, In S2, the theoretical energy barrier reduction threshold Δ W theory The expression is as follows: D W theory = eβE ; In the formula, e The amount of electron charge. β For material structure coefficients, E The applied electric field strength is denoted as .

4. The method for characterizing polymer stress-aging sensitivity under multi-field combined action according to claim 3, characterized in that, In S2, the aging rate constant α The expression is as follows: ； In the formula, For the reaction rate, Where is Boltzmann's constant, T is absolute temperature, and h is Planck's constant. For effective energy barrier.

5. The method for characterizing polymer stress-aging sensitivity under multi-field combined action according to claim 4, characterized in that, In step S4, the expressions for the activation energy under different aging states are obtained by linear fitting as follows: in, F ( v )=1 (1 v ) 1 / 2 For the degree of transformation, v To the degree of decomposition, λ For the heating rate, E a For activation energy, R The gas constant is... T Absolute temperature A It is a pre-exponential factor.

6. The method for characterizing polymer stress-aging sensitivity under multi-field combined action according to claim 5, characterized in that, In S6, the expression for the thermodynamic equation is: ΔG= E a - T D S ; in, T Δ S Let ΔG be the entropy term and ΔG be the change in Gibbs free energy.

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