Effect of Thermal and Electric Fields on Thermal Stability and Dielectric Properties of SiO2/Cellulose Composite Materials

Molecular simulation techniques are widely used to study transformer insulation material characteristics and nano-modification mechanism. However, there are numerous researches on the thermal stability and dielectric properties of nano-modified cellulose insulation paper, and few studies have combined nano-modification with the process of electrical-thermal aging. This study established SiO2/Cellulose models with different weight percentages to calculate their average number of hydrogen bonds (AHB), glass transition temperature (Tg ) and relative permittivity (RP) under electro-thermal coupling. The study indicates that the 5wt% SiO2/Cellulose model exhibits the best modification effect. Temperature is the primary factor causing aging of the cellulose insulation paper, and the introduction of an electric field accelerates the thermal aging of the cellulose insulation paper. Increasing hydrogen bond quantity and restricting molecular chain movement are key factors that enhance insulation materials anti-aging performance.


Introduction
Insulation paper performance has impacted transformer lifespan [1][2].Thermal and electric field inside transformers reduced thermal stability and dielectric performance of cellulose insulation paper to compromise the power system safety [3].Enhancing cellulose insulating paper anti-aging performance is crucial to the power system reliability.
Nanoparticle modification improves thermal stability and dielectric properties of the insulating paper.Molecular simulation is widely used in nano-modification research [4][5].Wang Xiaobo investigated the impact of silane coupling agent-modified SiO2 on the micro-properties of the insulation paper.It has been found that when silane coupling agent KH792 is used to modify nano-SiO2, the binding force and compactness of the cellulose and nano-SiO2 interface are improved, and the binding effect on the cellulose chain is the most significant [6].Tang Chao employed molecular dynamics simulation to analyze the mechanical properties of cellulose insulation paper modified with nano-SiO2 to reveal the microscopic mechanism of the nanoparticle-induced modification [7].Qiao Lijun established a composite model of nano-SiO2 and cellulose to calculate its Tg through molecular simulation, which closely matched experimental values [8].The existing research have primarily focused on nanoparticle modification effect in insulation materials under the static condition, and there is a lack of sufficient investigation on the thermal stability and dielectric properties of nano modified polymers during the dynamic process of electrical-thermal aging.This article investigates pure cellulose models and SiO2/Cellulose models with different weight percentages (1wt%, 3wt%, 5wt%, and 7wt%) using molecular simulation technology.The models are compared based on parameters such as AHB, Tg and RP.By analyzing the mentioned parameters, the relationship between the aging of the insulating paper and the movement of cellulose molecular chains is determined.The study explores the anti-aging effect of SiO2 nanoparticles in the dynamic deterioration process of insulating paper from the atomic and molecular level, to provide theoretical support for enhancing the anti-aging capability of cellulose paper and advancing nano-modification technology in this field.

Model Building
This article has built five models based on 40 cellulose single chains with a polymerization degree of 10 and SiO2 nanoclusters: pure cellulose model, 1wt% SiO2/Cellulose model, 3wt% SiO2/Cellulose model, 5wt% SiO2/Cellulose model, and 7wt% SiO2/Cellulose model, the five models are named as P0, P1, P3, P5, and P7 respectively.The molecular structure of each model is represented in Figure 1 (a) to (e) correspondingly to P0, P1, P3, P5, and P7.The average density of the model is set to 1.5g/cm 3 .The radius of SiO2 nanoclusters is uniformly set to 5Å (1Å=0.1nm),and the SiO2 surface is hydrogenated as shown in Figure .1 (f), where unsaturated oxygen atoms are bonded to H atoms and unsaturated silicon atoms are bonded to -OH.

Simulate Details
Prior to molecular dynamics calculations, the structure of five models was optimized and the energy was minimized.The model was then annealed from 200 K to 900 K and cycled 5 times to eliminate residual stress and relax the molecular chain structure.Sequential NVT (fixed particle count, fixed volume, fixed temperature) and NPT (fixed particle count, fixed pressure, fixed temperature) molecular dynamics simulations was conducted within the temperature range of 303-683 K with increments of 20 K, each for 100ps, as illustrated in Figure 2.

Hydrogen Bond
This article examined the impact of hydrogen bonds on the thermal stability of cellulose, which can be more influential than the role of chemical bonds.This article calculated AHB through a self-designed script.The temperature range of 303-423 K was sampled at 40 K intervals, and illustrates the changing trend of AHB for the models in Figure 3. Figure 3 shows that the AHB decreases with the temperature increasing for five models.As the temperature increases, some hydrogen bonds are disrupted due to the intensified movement of the cellulose molecules.The modified models consistently have higher AHB than the P0 model, and the P5 has the highest value of AHB.The decreasing in AHB of the modified model is less than P0 Under thermal aging, AHB of the P0 decreases from 1931.285 at 303 K to 1722.155 at 423 K. AHB of the P5 decreases Comparing AHB changes among models at lower temperatures, the electric field has little effect on the AHB; As temperature increasing, the AHB decline rate is higher under combined electric and thermal aging than the one under thermal aging alone.It indicates that the electric field enhances the movement of molecular chains and intensifies the influence of the temperature on AHB.

Thermal Stability Analysis
The Tg is a critical temperature for assessing the thermal stability of polymers.It marks the transition from a glassy to an elastic state.The glass transition process can significantly reduce the thermal stability of cellulose insulation paper.Enhancing Tg is vital to improve its anti-aging properties.Tg is determined by using the specific volume temperature curve method.The density parameters were extracted for every 20 K in the temperature range of 303-683 K. Figure 4   It shows that the SiO2 modified cellulose model has a higher Tg than the P0 under both thermal aging and electric heating aging conditions in 4.This is because the hydrogenated SiO2 surface forms hydrogen bonds with cellulose molecular chains to increase intermolecular forces and thus increasing the Tg of the model.The Tg of the SiO2 modified cellulose model increases with a higher weight percentage, as more hydrogen bonds are formed.Under thermal aging conditions, the Tg of P5 is the highest at 491 K.However, excessive adding leads to SiO2 aggregation, interface layer overlap, and a decreasing in hydrogen bonds between SiO2 and cellulose chains.It weakens the restriction on molecular chains to result in a low Tg for the P7 compared to the P5.
From Figure 4 (b), it can be seen that the introduction of an electric field reduces the Tg of all models.The introduction of an electric field enhances the polarization of cellulose molecules to reduce the number of hydrogen bonds and the interaction forces between chains in the model, make the movement ability of cellulose molecular chains stronger, make it easier to overcome movement obstacles, and transit to a high elastic state at lower temperatures.The Tg of P5 under electrical thermal aging conditions is 480K, which is 11 K less than that under thermal aging conditions.

Dielectric Properties
Reducing the RP material can help mitigate surface charge accumulation and reduce the probability of partial discharge, which is highly significant.The relative RP is calculated by analyzing the fluctuations in dipole moments of the model, as depicted in equation (1).
In equation ( 1): M is the dipole moment of molecules in each frame; ... is the average value of the system; V is the volume of the model, Å 3 ; T is the thermodynamic temperature, K; KB is the Boltzmann constant; ε0 is the vacuum dielectric constant.
Record dipole moment fluctuations of 5 models between 303-423 K and calculate the RP by using equation (1).The variations in the RP for the 5 models are depicted in Figure 5.
Under thermal aging conditions, the RP of the P0 is 2.803 at 303 K.As the temperature increasing, molecular thermal motion is intensified, to reduce the number of hydrogen bonds and increase molecular polarization.At 383 K, the RP increases to 3.238.With further temperature rising, molecular motion hampers the cellulose chain orientation to lead to the dipole reorientation.Consequently, the RP of the P0 decreases to 3.0839 at 423 K. From Figure 5 (b), it can be seen that the introduction of an electric field amplifies the impact of the temperature on RP.The RP of P0 under electrical thermal aging conditions is 3.041 at 303 K, 3.6403 at 383 K, and 3.3749 at 423 K.The higher the temperature, the greater the impact of the electric field.Observing the other four groups models can also summarize similar variation.The RP of the modified model is smaller than that of the P0.This is mainly because after adding SiO2 particles, an interface effect is formed between SiO2 and the cellulose, and the properties of the interface layer dominate the properties of the material.Due to the small RP of the interface layer, the total RP of the composite system decreases.At the same time, the addition of SiO2 particles increases the number of hydrogen bonds to make it difficult for polymer chains or side groups in the interface region to turn and resulting in a weakening of molecular polarization.As the SiO2 nanoparticles content increases, the number of interface layers increases, and the value of RP decrease.In this experiment, the RP of the P5 is the lowest.Under thermal aging conditions, the RP of the P5 at 303 K is 2.338, which is 16.6% lower than that of the P0.Due to the high content, SiO2 particles aggregate, interface regions overlap with each other, the number of hydrogen bonds decreases, and molecular polarization increases.Therefore, the value of RP for the P7 is larger than that of the P5.

Conclusions
This article uses molecular simulation technology to study the thermal stability and dielectric properties of the pure cellulose models and SiO2/Cellulose models with different weight percentages (1wt%, 3wt%, 5wt%, and 7wt%) under thermal aging and electro-thermal aging conditions.Based on the research results, the conclusions are below: • Adding an appropriate amount of SiO2 nanoparticles can increase the hydrogen bonding between cellulose molecular chains to improve the thermal stability and dielectric properties of nanoparticles can lead to aggregation and have a negative impact on the modification effect of cellulose.Adding an appropriate weight percentage of the nanoparticles to enhance compatibility with cellulose is crucial to improve the thermal stability and dielectric properties of the insulating paper.• The thermal stability and dielectric properties of cellulose are mainly affected by temperature.
Introducing an electric field can enhance molecular polarization, reduce hydrogen bonds and intermolecular interactions, increase molecular chain mobility, and accelerate the aging of the cellulose model.The impact of the electric field intensifies with more severe thermal aging.

Figure 1 .
Figure 1.Molecular structure of each model.

Figure 3 .
Figure 3. Changes in hydrogen bond numbers of various models under different aging conditions.
coupling field from 2303.165 at 303 K to 2150.295 at 423 K.The addition of SiO2 nanoparticles create strong interfacial interactions with cellulose molecular chains to result in increasing hydrogen bonding and reducing the mobility of the chains.Slight AHB decreasing for the P5 is due to the aggregation and interface layer overlap for SiO2.
presents the glass transition temperature of each model under different aging conditions.(a) Thermal aging conditions (b) Electrical-thermal aging conditions

Figure 4 .
Figure 4. Glass transition temperature of each model under different aging conditions.

Figure 5 .
Figure 5. Changes in relative permittivity of various models under aging conditions.
P5 shows the best modification effect in this study.Under thermal aging conditions, compared with P0, the value of Tg for P5 increases by 54 K; At 303 K, the value of RP for P5 decreases by 16.6% and the value of AHB increases by 19.3%.Excessive SiO2