Kinetics of Charge Accumulation and Decay on Silicone Rubber/SiO₂ Nanocomposite Surface

1. Introduction

The surface charging processes and characteristics of composite dielectrics have received much interest. This is particularly relevant to analyzing surface discharge in an electric field, for insulated electrical and electronic devices [1-3]. Silicone rubber (SiR) composites are typical outdoor electrical insulating materials [4, 5]. Under high voltage (HV), electrons emitted from HV conductors, and positive and negative ions remain together. These can be injected into SiR insulating surfaces to form surface charge. This is a major cause of surface degradation and surface discharge [6-8]. Accumulated surface charge should be avoided or efficiently relaxed, to protect SiR materials against insulation failure.

The dynamic behavior of surface charge in insulating polymers has been recently investigated. Gubanski et al. reported spatial and temporal distributions of surface charge on ethylene propylene diene monomer rubber and SiR, with varying voltages and pulse numbers. They also investigated the mechanism of surface potential decay in high-temperature-vulcanized SiR in relation to air and bulk properties [9-11]. Li et al. reported the charge transport of epoxy resin/glass composition (FR4), polytetrafluoroethylene and polyimide. Their focus was on evaluating spacecraft charging, and predicting surface and internal electrostatic discharging [12]. Laurent et al. reported that charge transport and trapping in lowdensity polyethylene was influenced by surface states. An exponential distribution of traps with a maximum trap depth was used to explain this result [13]. Du et al. reported the effect of nanofiller concentration on the surface charge accumulation and decay of epoxy/TiO2, epoxy/BN and polyimide/Al2O3 nanocomposites. Charge dynamics were found to depend on localized surface states, the characteristics of which were altered by the nanoparticles [14-16].

Understanding the effect of SiO2 nanoparticles on the surface charge kinetics of SiR/SiO2 composites remains a challenge. In the current study, the accumulation and decay of surface charge in SiO2 nanoparticle-loaded SiR composites was investigated. The effect of SiO2 nanoparticle different weight ratios was investigated.

 

2. Experimental Arrangement

Neat room-temperature-vulcanized SiR (Shenzhen Anpin Silicone Material Co., Ltd.) without solid fillers was employed. Spherical SiO2 nanoparticles (Nanjing Haitai Nanomaterials Co., Ltd) of average diameter of ~20 nm were surface treated to avoid agglomeration. The nanoparticles were dried in a desiccator for >24 h, before being dispersed in SiR. Samples with dimensions of 35×35×2.5 mm were prepared by incorporating 0.1, 0.3, 0.5, and 0.7 wt.% SiO2 nanoparticles into SiR matrices. Undoped samples were investigated for comparison. Fig. 1 shows a scanning electron microscopy (SEM) image of the nanoparticle distribution. Most nanoparticles were well-dispersed in the SiR matrix.

Fig. 1.SEM image of SiR/SiO2 nanocomposite containing 0.1 wt.% SiO2

A schematic of the surface charge measurement apparatus is shown in Fig. 2. Surface charge was measured using a pair of needle-plated electrodes, coupled with a grid electrode at room temperature. The needle electrode had a diameter of 1 mm and tip radius of ~13 μm, and was connected to a +DC HV source. The needle tip was positioned 5 mm above the 35×35×0.4 mm grid electrode, the latter which was located 5 mm above the sample surface. The sample was placed on a metal transporter, which could move in one dimension along a grounded metal and epoxy guide rail. 6 kV was applied between the needle and the ground electrodes, and 4 kV was applied to the grid electrode. Corona charging tests were performed at room temperature and relative humidity of ~40 %. The charging time was 10 min.

Fig. 2.Schematic of the apparatus used to measure surface charge.

After the corona charging test, the sample was quickly transferred to the underside of a non-contacting probe, to determine the surface potential. The Kelvin-type probe (Trek 6000B-5C) coupled with electrostatic voltmeter (Trek 347-3HCE) provided a measurement accuracy of ±3 V and spatial resolution of 3 mm. The probe-to-surface distance was 3 mm. The distances between the probe and electrodes were sufficiently large to avoid any influence of electric field distribution on the surface potential measurement. Surface potential decay curves were recorded by a computer.

 

3. Results and Discussion

3.1 Surface charge accumulation of SiR/SiO2 nanocomposite

Fig. 3 shows the dependence of initial surface potential on the SiO2 nanoparticle weight ratio. The initial surface potential was decreased by the presence of the nanoparticles. Thus, the SiO2 nanoparticles suppressed surface charge accumulation in SiR. There was a significant interaction between the SiO2 nanoparticles and SiR matrix, in which the SiR chains were chemically and/or physically bound to the nanoparticle surface. Charge accumulation largely arose from intrinsic defects within the molecular structure, so charge produced by the electric field was captured in the composite. The nanoparticles had a reducing effect on defects, which restrained charge injection to the bulk.

Fig. 3.Relationship between initial surface charge and SiO2 nanoparticle weight ratio.

Fig. 3 shows a peak in initial surface potential at 0.3 wt.% SiO2 nanoparticle loading, for the investigated SiR/SiO2 nanocomposites. The potential at 0.1 wt.% was lower, and the potential decreased with increasing weight ratio from 0.3 to 0.7 wt.%. It was thought that charge was injected into the bulk through the specimen surface. With increasing weight ratio, more nanoparticles were bound to SiR chains through van der Waals forces, which could easily be destroyed by the outside electric field. The higher surface energy of nanoparticles caused injected charge to be electrostatically captured around the nanoparticles. This resulted in homogeneous charge layers, which restricted further charge injection. This accounted for the decreasing charge accumulation from 0.3 to 0.7 wt.%.

3.2 Decaying characteristics of surface charge of SiR/SiO2 nanocomposite

Fig. 4(a) shows the decay of surface charge with SiO2 nanoparticle weight ratio. Surface charge initially rapidly decayed, and then slowly eased off with increasing lapse time. This indicated that injected charge induced a high electric potential. Accumulated charge was released from the composite, and then passed across the surface and/or through the bulk into the ground electrode.

Fig. 4.Decay of surface charge with SiO2 nanoparticle weight ratio

The average decay velocity during the decay time was calculated from Eq. (1):

where VDecaying is the average decay velocity (V/s), and Ut is the surface potential at time t.

The relationship between average decay rate and weight ratio is shown in Fig. 4(b). The rate of decay in potential of the SiR/SiO2 nanocomposites was slower than that of SiR. The lowest average decay velocity of surface potential occurred at 0.1 wt.% weight ratio. The decay velocity was largely constant from 0.3 to 1.0 wt.% SiO2 nanoparticle content. This influence was considered to result from interaction zones formed by the nanoparticles. The increasing weight ratio may have led to overlapping interaction zones around nanoparticles, which were more reactive than those at lower mass fraction.

Fig. 5 shows the trap distributions of samples containing different SiO2 nanoparticle weight ratios. Charge transport is typically associated with carrier trapping and de-trapping. The energy of traps is related to the lapse time by the demarcation energy, Em, which indicates the border between unoccupied and occupied traps. With increasing time, Em moves away from the conduction band, and the relationship between the lapse time and energy gap is defined as:

Fig. 5.Trap distributions of SiR/SiO2 nanocomposites with different SiO2 nanoparticle weight ratios

where Ec is the conduction band energy level, v is the attempt to escape frequency, tdv/dt is proportional to the trap energy density N(Em) at energy level Em(t), Nt and Nc are the trap and conduction state densities, respectively, and τ0 is the carrier lifetime of conduction states.

3.3 Effects of Nano-SiO2 particles on trap depth of nanocomposite

Fig. 5 shows that the trap depth in the SiR/SiO2 nanocomposites was deeper than that in SiR. The shallow traps represented low energy levels, which could not obstruct the mobility of trapped charge. Thus, charge easily moved out from shallow traps. The well-dispersed nanoparticles in the matrix restrained charge transport in bulk SiR, and influenced the decay velocity of surface charge, as shown in Fig. 4. With increasing nanoparticle weight ratio, the overlap of interaction zones surrounding nanoparticles increased, and became more reactive than those at lower mass fraction. When many interaction zones overlapped, many conductive paths formed through the overlap of the transition region in the bulk, thus reducing charge trapping. The minimum trap depth was at ~0.834 eV for neat SiR, and the average decay velocity was ~1.8 V/s, as shown in Fig. 4(b).

Fig. 6 shows a schematic of the interaction between SiO2 nanoparticles and SiR matrix. Chemical species generated by association with nanoparticles can significantly influence charge accumulation and transport in nanocomposites. Fig. 6 shows that the SiR/SiO2 interface contained electronic states associated with oxygen. The electronic structure and arrangement of SiR matrix and nanoparticles suggested that charge injection was restrained, by capture in the bulk composite. The nanoparticles increased the trap energy level, as shown in Fig. 5. Charge de-trapping could occur by thermally-activated electrons hopping from occupied oxygen sites to nearest-neighbor unoccupied oxygen sites. This lowered the delay velocity, compared with that of SiR. Increasing the SiO2 nanoparticle weight ratio from 0.1 to 0.7 wt.% increased the delay velocity, as shown in Fig. 4. This was associated with overlapping interaction zones of the nanoparticles. The resulting overlapping layers were more reactive than those at lower weight ratio [17]. Many conductive paths formed through the overlap of the transition region in the bulk nanocomposite. These also reduced charge accumulation, and accelerated the de-trapping of surface charge.

Fig. 6.Schematic of the interaction between SiO2 nanoparticles and SiR matrix

 

4. Conclusion

In summary, the addition of SiO2 nanoparticles of up to 0.7 wt.% improved the accumulation and decay of surface charge in SiR/SiO2 nanocomposites, compared with SiR. The addition of 0.1 wt.% SiO2 resulted in the lowest surface charge. The SiO2 nanoparticles restricted surface charge from accumulating in the SiR composites, and resulted in a low surface charge after decay. This approach has application in the surface flashover of outdoor SiR electrical insulators.