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introduction

Polypropylene (PP), as a thermoplastic material with the characteristics of environmental friendliness, recyclability, and high melting temperature, has become the most potential material to replace existing DC cable insulation. However, when using only polypropylene material as insulation, it has poor toughness and high storage modulus, which limits its application.

Therefore, under the premise of ensuring good dielectric properties, how to effectively improve the physical and chemical properties of polypropylene is one of the main research topics in the field of DC cable insulation.

The interface domain (or phase) formed between the polypropylene matrix and nanoparticles after the addition of nanoparticles is considered to be the key factor affecting the performance of polypropylene. Domestic and foreign scholars have proposed different models to explain the reasons for the changes in the performance of nanocomposites, such as Lewis' single-layer model based on the gel chemical double layer theory and Nelson's dielectric double layer model.

However, research on the impact of core-shell nanoparticles on the insulation performance of polypropylene is still in its early stages, and the mechanism of the influence of shell on the interface region is not yet clear.

In response to the above issues, this article uses surface precipitation method to prepare coatings with good coating effect Al203@SiO2 Core shell nanospheres were studied by adding different contents Al203@SiO2 The polypropylene nanocomposite dielectric properties of core-shell nanospheres, specifically including Al203@SiO2 The microstructure of core-shell nanospheres Al203@SiO2 /The dielectric properties and space charge characteristics of PP composite dielectrics.

Next, let's study together Al203@SiO2 What is the effect of adding core-shell nanospheres on the dielectric and space charge of polypropylene.

1. Materials

Using spherical nanoparticles produced by Aladdin Reagent Co., Ltd., with a particle size of approximately 30nm, Siwei Polyvinylpyrrolidone (PVP), analytically pure, China National Pharmaceutical Group Chemical Reagent Co., Ltd., Ethylorthosilicate (C8H20O4Si, TEOS), analytically pure, China National Pharmaceutical Group Chemical Reagent Co., Ltd.: ammonia water (NH3H20), analytically pure, China National Pharmaceutical Group Chemical Reagent Co., Ltd.: anhydrous ethanol, analytically pure, China National Pharmaceutical Group Chemical Reagent Co., Ltd.: polypropylene (iPP), Sinopec Shanghai Petrochemical Co., Ltd.

2. Preparation of core-shell nanoparticles

This article uses surface precipitation method to prepare Al203@SiO2 Core shell nanospheres, as shown in Figure 1 Al203@SiO2 The synthesis principle of core-shell nanospheres is to first use PVP as a surfactant to uniformly disperse spherical Al203 nanospheres, and then use surface deposition method to synthesize core-shell structures Al203@SiO2 Nanospheres, PVP chains, and molecular chains on the surface of Al0 can capture Si (OH) 4 monomers generated by TEOS hydrolysis in alkaline environments, thereby forming a SiO2 shell layer on the surface of Al203.

3. Detailed preparation steps

Centrifuge the mixed solution multiple times and wash it alternately with distilled water and anhydrous ethanol to obtain Al02 particles that are uniformly attached to PVP and well dispersed in the solution. Then, coat Al02 with SiO2 and add the solution containing A particles obtained by centrifugation to a beaker in a certain proportion with anhydrous ethanol. Prepare 5ml of TEOS and add it dropwise to the solution, stirring at a constant speed for h.

4. Preparation of composite dielectrics

Set the temperature of the double roll mixer barrel to about 170, put a certain amount of iPP driving wheel into one end, mix for 3mn, make the PP stick evenly, and then weigh the different contents Al203@SiO2 Core shell nanospheres were slowly added to iPP and mixed for 15 minutes to fully disperse in PP, resulting in Al203@SiO2PP Nanocomposite dielectric, the abbreviation and composition of the sample are shown in Table 1, respectively containing core-shell nanospheres with mass fractions of 0.1wt%, 0.5wt%, 1wt%, and 5wt%.

two .Al203@SiO2 Physical and chemical properties

1.Al203@SiO2 Morphological characterization and dispersibility of

As shown in Figure 3, they are Al203@SiO2 The microstructure, particle size distribution, and EDS line scan of core-shell nanospheres indicate that the synthesized Al203@SiO2 The nanoparticles exhibit a spherical like structure.

Particle size distribution display in Figure 3 Al203@SiO2 The diameter of the nanospheres ranges from about 80 to 140nm. Compared with the original spherical Al02 nanoparticles with a particle size of 30m, SiO2 with an average thickness of about 50nm was synthesized on the surface of A0 nanoparticles. The shell was analyzed as shown in the figure Al203@SiO2 The energy spectrum of nanospheres, combined with the line scan of the graph, can be obtainedThe distribution of silicon and oxygen elements on the surface and overall range of nanospheres is relatively uniform.

Figure 3

Figure 4 shows Al203@SiO2 and Al203@SiO2 XRD diffraction patterns of nanospheres, Al203@SiO2 The diffraction peaks of the nanospheres located in the range of 2 between 25 and 80 correspond to the typical diffraction peaks in the Al203 (card numbers 86-1410) diffraction pattern, while the diffraction peak at 23 is a typical amorphous SiO2 diffraction peak, indicating that the dielectric properties of the Al203 surface successfully coated with SiO2 composite dielectric are closely related to the dispersion degree of the nanoparticles in the matrix.

Figure 4

Containing different additive contents Al203@SiO2 The cross-section of the polypropylene composite dielectric with nanospheres is shown in the figure, and Figure 5 shows Al203@SiO2 When the content of nanospheres added is 0.1wt%, 0.5wt%, and lwt%, it can be seen that large-scale nanospheres aggregation can hardly be observed in the matrix at low content, Al203@SiO2 The nanospheres are evenly dispersed in the polymer matrix. When the content of nanospheres increases to 5wt%, agglomeration occurs in the matrix, and the particle size of the agglomerates is within the micrometer range. The figure shows the agglomerated particles Al203@SiO2 Nanospheres, which disperse in the matrix in the form of clusters, will cause overlapping areas of the interface.

Figure 5

2. DMA experimental results and discussion

Figure 6 shows adding different contents Al203@SiO2 The storage modulus E trend of polypropylene composite dielectric with nanospheres,

Figure 6

The influence of nanoparticles on the storage modulus of the matrix is mainly determined by the degree of dispersion of particles in the matrix, the strength of interfacial interactions, and the size of particles. The core-shell structure of SiO2 coated Al0 will cause Al203@SiO2 The increase in specific surface area, decrease in surface energy, and better dispersion of nanospheres can weaken the agglomeration phenomenon.

Table 2 shows the values of A0 before and after coating calculated using the gas equation by measuring the gas adsorption capacity Al203@SiO2O2 The specific surface area of BET and Langmuir was obtained with a relative pressure range of 0.05~0.35, with BET considering multi-layer adsorption and Langmuir considering monolayer adsorption.

It can be observed that after Al02 is coated with SiO2, Al203@SiO2 The specific surface area of nanospheres significantly increases, which can enhance the compatibility between nanospheres and PP matrix. At the same time, the interface area of nanospheres in the matrix can be increased, which can inhibit the movement of polymer chains around the nanospheres and serve as an energy dissipation source after cracks occur, improving the toughness of the matrix. In terms of dynamic mechanical properties, it shows a decrease in storage modulus.

In addition, the toughening effect of nanospheres is also related to the amount of added content Al203@SiO2 When there are fewer nanospheres, the toughening effect is not very obvious, mainly due to the fewer interface areas generated. When the addition amount is large, the surface active groups between inorganic particles will self aggregate, and the size of nanospheres increases, weakening their toughening effect.

This article synthesized core-shell structures through surface precipitation method Al203@SiO2 Nanospheres, studied the addition of different contents Al203@SiO2 The physical and chemical properties, dielectric spectrum, and space charge characteristics of polypropylene nanocomposite dielectrics with nanospheres were analyzed. The influence of the interface zone generated by core-shell nanospheres on the matrix was analyzed, and the following conclusions were obtained:

(1) The core-shell structure of SiO2 coating Al02 will reduce the surface energy of Al02, improve the dispersibility of Al02, and enhance the compatibility between particles and PP matrix. Therefore, adding Al203@SiO2 The storage modulus of polypropylene composite dielectric after nanospheres is significantly reduced, and the toughness is improved.

(2) When the addition content is less than 5wt%, Al203@SiO2 The interface region between the nanospheres and the matrix has a strong interaction, effectively limiting the movement of molecular chains, leading to polarization difficulties, resulting in lower relative dielectric constant and dielectric loss factor than pure polypropylene. When the content is greater than or equal to 5wt%, the overlap of the interface region and the aggregation of the nanospheres increase the relative dielectric constant and dielectric loss factor.

(3) Under a+30kV/mm electric field, there is a phenomenon of space charge accumulation inside pure polypropylene, with a certain amount of Al203@SiO2 The addition of nanospheres can reduce the average charge density and carrier mobility of polypropylene in the body under direct current electric field, resulting in the generation of homopolar charges near the cathode of polypropylene composite dielectric, generating an additional electric field to suppress the accumulation of space charges Al203@SiO2O2 When the content of nanospheres is excessive, the inhibitory effect will weaken.

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