Influence of Swelling Effect on Electrical Treeing Characteristics of Silicone Rubber under Impulse Voltage

Author： GOZ Electric Time：2024-09-05 09:44:43 Read：13

The main molecular structure of SiR is polydimethylsiloxane (PDMS), and silicone grease has a similar molecular structure to SiR. Therefore, according to the principle of similar compatibility, when silicone grease is coated on the surface of SiR, it will gradually diffuse into the interior of SiR, causing it to swell. Therefore, with the increase of swelling time, the mass and volume of SiR gradually decrease, and the density gradually increases, as shown in Figure 1.

The cross-linked network inside SiR includes a chemical cross-linked structure formed by covalent bonds and a physical cross-linked structure formed by hydrogen bonds. The hydrogen bonds are formed between the oxygen atoms on the main chain of SiR and the hydroxyl groups on the surface of nano-silica. The hydrogen bonds are densely distributed on the surface of nano-silica, which limits the relative sliding of the SiR molecular chain and plays a reinforcing role in the mechanical properties of SiR. J P COHEN-ADDAD et al. pointed out that the hydrogen bond binding sites on the internal molecular chain of SiR are a dynamic system accompanied by formation and dissociation, and cannot be regarded as a stable and unchanging rigid lattice. Although the hydrogen bonds are not completely tightly connected, their huge number plays a stabilizing role on the molecular chain to a certain extent. Since silicone grease has a similar molecular chain structure to SiR, the oxygen atoms on its main chain can also form hydrogen bonds with the hydroxyl groups on the surface of nano-silicon dioxide. Therefore, when silicone grease enters SiR, the binding sites of hydrogen bonds are constantly changing. Therefore, when some binding sites on the main chain of SiR are dissociated, the hydroxyl groups on the surface of nano-silicon dioxide may be occupied by the oxygen atoms on the main chain of silicone grease, resulting in a decrease in the number of hydrogen bonds formed on the original main chain of SiR and a certain degree of damage to the physical cross-linking structure. The chemical cross-linking structure inside SiR is formed by covalent bonds, which have a large bond energy and cannot be damaged by silicone grease. The chemical cross-linking network is not greatly affected by the swelling effect. Therefore, as the swelling degree of silicone grease increases, the physical cross-linking density of SiR decreases, which in turn leads to a decrease in its cross-linking density, as shown in Figure 2. The destruction of SiR's physical cross-linking structure will affect its free volume. After the SiR molecular chain forms a bond with nano-SiO2, the local area contains several free volumes separated by molecular chain segments. When the sites on the surface of nano-

SiO2 that were originally bonded with the SiR molecular chain are occupied by small silicone molecules, some of the separated free volumes are connected to each other, resulting in an increase in free volume.

Figure 1

Figure 2

The increase in free volume will affect the electrical tree characteristics of SiR. Figure 3 shows a schematic diagram of carrier transport inside SiR. The increase in free volume leads to an increase in the mean free path of carriers. On the one hand, after the mean free path increases, carriers can obtain higher energy under the action of electric field acceleration, and the energy generated after collision ionization is also higher, which aggravates the degree of damage to the SiR molecular chain segment; on the other hand, the energy obtained by carriers after migrating a certain distance le under the action of the electric field is recorded as Ele. If this energy is higher than the barrier height Ea of the trap, the carriers can jump over the trap and continue to migrate, thereby increasing the number of carriers, intensifying the collision ionization process, and the generated high-energy electrons hit the SiR molecular chain to intensify its fracture process. The destruction of local molecular segments leads to a further increase in free volume. When the free volume increases to a certain extent, local discharge occurs, and then a low-density area is formed, and finally electrical tree formation begins. Therefore, with the increase of swelling degree, the tree voltage of SiR gradually decreases.

Figure 3

Considering the influence of traps, when carriers are transported inside the medium, the time required for them to migrate in the extended state is much shorter than the time they stay in the localized state (trap). Therefore, the carrier transport process is mainly regulated by the traps inside the medium. Deep/shallow traps have different abilities to capture carriers. The energy level of deep traps is larger, and the energy obtained by carriers under the action of the electric field is lower than the deep trap barrier, so they are captured by the deep traps. The thermally assisted de-trapping process of carriers takes a long time, and carriers cannot be de-trapped in a short time. Therefore, the increase in the density of deep traps will lead to a decrease in the number and mobility of migrating carriers. Since the energy level of shallow traps is shallow, the energy obtained by carriers under the action of the electric field can often make them cross shallow traps without being captured. Shallow traps can provide transition sites for carriers, and carriers can migrate by jumping between shallow traps. Therefore, the increase in the density of shallow traps reduces the average distance between shallow traps, and carriers are more likely to transition between shallow traps, resulting in an increase in mobility. According to the test results of the trap characteristics in the previous article, with the increase of swelling time, the density of deep traps in SiR samples decreases, the density of shallow traps increases, and the energy levels of deep/shallow traps decrease. After swelling, the number of deep traps inside SiR decreases, and the potential barrier decreases. The probability of carriers encountering deep traps during migration decreases. At the same time, the probability of directly crossing deep traps and continuing to migrate after encountering deep traps increases, so the probability of carriers being captured by deep traps decreases. The increase in shallow trap density and the decrease in energy level are also conducive to the migration of carriers. The concentration and mobility of migrating carriers increase, which intensifies the collision ionization process and accelerates the initiation of electrical tree formation. Therefore, with the increase of swelling time, the tree formation voltage of SiR samples gradually decreases.

During the growth process, electrical tree formation mainly grows along the physical cross-linking area and free volume. Although the physical cross-linking area is densely distributed with many physical cross-linking points, it shows greater mechanical strength as a whole, but the bond energy of hydrogen bonds is much lower than the bond energy between atoms on the molecular chain and the bond energy of covalent bonds formed by chemical cross-linking. Therefore, the physical cross-linking area belongs to a weak area, and high-energy electrons can more easily destroy it. Before swelling, the cross-linking structure of SiR is intact, and the electric dendrites grow along some physical cross-linking areas and the axial direction with higher field strength, and cannot produce many branches, so they mainly form branch-like electric dendrites; after swelling, due to the destruction of the physical cross-linking structure, the free volume increases, and the electric dendrites are more likely to grow along more directions in the physical cross-linking area, and can also grow in large quantities in the radial direction with weaker field strength, producing more branches. Therefore, with the increase of swelling time, the number of electric dendrites in SiR increases, and the morphology gradually tends to be dense, changing from dendrite-like to jungle-like

.