Analysis of space charge and charge trap characteristics of gamma irradiated silicone rubber nanocomposites

: Silicone rubber is widely used for electrical insulation and may be exposed to a harsh environment. The present study envisaged to improve insulation properties of silicone rubber by adding an optimised quantity of nanofillers. The fundamental space charge and charge trap characteristics were studied by adopting the pulsed electroacoustic analysis technique and through surface potential measurement. The dielectric properties of the materials were analysed through measurement of permittivity and loss factor of the material at different frequencies and temperatures. The influence of gamma irradiation on variations in fundamental properties of the material was characterised. The results of the study indicate that 5 wt.% alumina added nanocomposites had better space charge performance under gamma irradiation compared with virgin silicone rubber.


Introduction
Polymeric insulators are widely used in power transmission and distribution networks because of their superior performance over conventional ceramic and glass insulators [1]. In a specific application, silicone rubber is used as an insulant in nuclear power plant cables, where it is subjected to severe stress due to radiation. Also, silicones in various forms have been utilised in biomedical, aerospace and automotive industries. The advantages of silicone rubber include high hydrophobicity with good hydrophobicity recovery, light weight, vandal resistance, radiation resistance and remarkable pollution performance [2]. In recent times, research interest in the application of nanotechnology to high-voltage insulation has gained considerable momentum due to evidence of many advantageous performance characteristics [3,4]. Addition of appropriate nanofillers (viz. silica, alumina (Al 2 O 3 ) etc.) with optimised weight percentage into the polymer matrix improves the fundamental thermal, chemical, mechanical and electrical properties of the composite material [5,6].
The charge trapping and detrapping processes are closely related to trap distribution characteristics of the polymer. Altering the material structure by adding nanofillers causes a change in the trap characteristics and charge behaviour [7,8]. The deep trap sites present in the nanocomposites suppress charge injection into the material even under high electric field. The evident dipoles of nanoparticles build positive and negative potential wells in pairs, which capture carriers as the electrons and hole traps [9]. Su et al. [10] studied the electrical degradation of EPDM and observed that addition of antioxidant can enhance efficient charge traps between the conduction and valence bands. Hence, studying the surface potential decay behaviour provides a convenient way to understand the surface trap characteristics of a material.
Under high electrical stress, dielectric materials are prone to accumulate charge in the bulk of the material. The electrical properties of the insulating material, such as conduction and breakdown strength are greatly altered by the presence of space charge in the bulk of the material [11,12]. In addition, the electric field is distorted due to space charge accumulation, which affects the localised electric field and may lead to premature failure of the insulator [13]. To mitigate the negative impact on the insulation performance, it is essential to understand the effect of the intensity of gamma-irradiation and the proportion of nanofillers on space charge build up in silicone rubber nanocomposites.
Polymeric materials may degrade due to exposure to various forms of irradiation and stress in their service environment [14]. Therefore, understanding the characteristic changes in material properties due to such irradiation is important. Considerable research was carried out on the development of desired insulation materials for improving the insulation performance, but research on the impact of high energy irradiation on the properties of the material is limited. Sarathi et al. [15] studied the performance of corona aged silicone rubber and concluded that the charge retention capability of the insulating material is reduced drastically by degradation of the surface and distinct changes in the functional groups are also observed. Mishra et al. [16] studied the impact of gamma irradiation on epoxy nanocomposite and found that charge mobility and hardness of material increases with the irradiation dose. Fu et al. [17] analysed the space charge distribution of gamma-irradiated (100 kGy) LDPE samples and found that the intensity of irradiation is proportional to the space charge density in the insulating material. Accordingly, they concluded that due to irradiation, carbonyl and other groups are formed and act as a shallow trap for both electrons and holes. Hence, it is important to understand whether the characteristic changes in polymer properties due to such irradiation may limit the application or not. Therefore, understanding the effect of irradiation on the dielectric properties of silicone rubber nanocomposites is essential.
Considering the above aspects, experimental studies were carried out to clarify the following important aspects: (i) surface potential decay and charge trap density distribution characteristics of virgin and gamma irradiated silicone nanocomposite, (ii) space charge dynamics in nano alumina filled silicone rubber due to gamma irradiation adopting the pulsed electroacoustic (PEA) technique and (iii) the dielectric properties of silicone rubber nanoalumina composites at different frequencies and temperatures.

Sample preparation
The nanocomposite materials were fabricated by adding different weight percentages of alumina nanofillers to the base silicone rubber matrix, adopting the following preparation procedure. First, alumina nano powder (99.9%, 100 nm particles) were placed in an oven for 24 h at 150°C to remove remnant moisture. Then, the dried nanoparticles were dispersed in ethanol using a shear mixer followed by an ultra-sonication process, each for 30 min. The ethanol-filler solution was then mixed with the base material 'Part-A' (RTV8112, Momentive, USA), by a further shear mixing process for 30 min. Then, the mixture was placed in an oven at 100°C to remove excess ethanol. Finally, the 'Part-B' (Hardener RTV9858) was added to the mixture in the ratio of 1:10. The resulting amalgam was subjected to shear mixing for 3 min, and subsequently degassed for 10 min to remove any trapped air bubbles. The mixture was cast into moulds of two sizes (100 mm × 100 mm × 0.125 and 100 mm × 100 mm × 1 mm). The samples were subjected to compression at a uniform pressure of 10 kg/cm 2 for 12 h at room temperature to ensure proper curing of the composite material. In this study, four types of silicone rubber nanocomposite samples were prepared with alumina concentrations of 0, 1, 3, 5 and 10 wt. %, respectively.

Surface potential decay analysis
Surface potential decay studies were carried out using a needleplane electrode configuration shown in Fig. 1. The prepared silicone nanocomposite samples with different filler concentrations were cut into rectangular cuboids 30 mm × 30 mm × 0.125 mm. The HV needle electrode was used for spraying the positive or negative charges over the sample surface (at position 1), by generating corona at 6 kV from the HV amplifier. After 2 min of charge spraying, the decay in surface potential of the material is measured (at Position 2) using an electrostatic voltmeter (ESV) (Trek model 341B). The gap between the material and the sensor is maintained at 5 mm.

Space charge analysis
The space charge accumulation in the bulk volume of silicone rubber was measured using PEA under the applied electric field. It is a non-destructive method of analysing the space charge behaviour in polymer materials. The typical PEA measurement system to obtain the space charge characteristics of silicone rubber is shown in Fig. 2. The main components include a pulse generator (0-500 V amplitude, 10 ns pulse width), HVDC generator (0-30 kV), oscilloscope (Tektronix DPO5034) and a piezoelectric transducer. A semi-conductive layer is placed between the sample and the high-voltage electrode to obtain impedance matching.
A short duration high-voltage pulse is applied to a polymer specimen, which is placed between the high voltage electrode and ground electrode (Al electrode). This applied pulse creates an electric force which displaces the internal charges; as a result, acoustic pressure waves are generated. The resultant pressure wave is detected and measured by a piezoelectric sensor whose output voltage is proportional to the space charge distribution in the test specimen. In order to reduce the attenuation and dispersion of acoustic waves due to the elasticity of silicone rubber, the thickness of the samples are kept as small as possible. In this study, the silicone nanocomposite samples for space charge analysis were prepared with a thickness of 125 ± 25 µm.

Dielectric relaxation spectroscopy (DRS)
DRS analysis of the nanocomposite was carried out using a broadband dielectric/impedance spectroscope (Novocontrol technologies). The variations in dielectric behaviour of the virgin and gamma irradiated samples were analysed in the frequency range of 1 Hz-10 MHz, over a temperature range from −50 to 200°C. The sample diameter and thickness were 20 and 1 mm, respectively.

Surface potential decay test
The surface potential variations of the gamma-irradiated silicone nanocomposite due to corona charging of surface under positive and negative DC voltages are shown in Fig. 3. The potential on the surface of the material were measured by using an ESV and the decay characteristics of the insulating material followed an exponential reduction, which can be mathematically expressed as where λ is the decay rate and V o is the initial potential and the mean lifetime 'τ = 1/ λ'. Surface potential decay dynamics rely upon the local condition of the specimen, which is altered by the addition of fillers into the base matrix. The authors of [7] observed that the mean lifetime of the material increases with an increase in filler concentration with respect to the base matrix. Addition of filler increases the energy level of the trap sites, which restricts the charge de-trapping and eventually leads to an increase in the mean lifetime of the material.
For a given sample, the mean lifetime is relatively long under negative voltage. This could be due to the high affinity of the surface for negative charges [18]. Fig. 3 clearly indicates that, with an increase in gamma irradiation, the decay time of the surface potential reduces. This indicates that the surface potential decay time increases with the addition of filler. On the other hand, the decay time is reduced with an increase in gamma irradiation. Table 1 shows the mean lifetime and maximum trap energy of virgin and gamma-irradiated silicone nanocomposites due to corona charging of the sample surface under positive and negative DC voltages.
As the gamma irradiation dose increased, the mean life time of the potential drop is reduced. This drop is an indication of the reduction of the energy barrier for charge de-trapping, due to gamma irradiation. This characteristic variation is the same with silicone rubber nanocomposites, irrespective of percentage of filler content.

Charge trap characteristics:
According to the isothermal current decay theory, the surface charge distribution can be obtained from surface potential characteristics [15]. The density of trapped charges (N(E)) at different energy levels (E) within the bulk of the material can be represented as where q is the electron charge, L is the thickness of the sample, k is the Boltzmann constant, T is the absolute temperature, t is time, f o (E) is an occupancy rate of initial electrons, ɛ 0 is the permittivity of free space and ɛ r the relative permittivity of the sample. The trap depth Δ(E) can be expressed as where E C is conduction band energy and E M trap level energy. The trap distribution characteristics of positively and negatively charged silicone rubber nanocomposites under different irradiation doses of gamma rays, are shown in Fig. 4. Su et al. [10] have clearly indicated that addition of antioxidants to the polymers has significant influence on charge trapping characteristics to enhance the lifetime of the insulation structure. It is noted that the trap energy level of the pure silicone sample lies in the range from 0.8 to 1 eV. Compared with this virgin silicone sample, there is an increase in trap depth from 0.8 to 1.05 eV for the silicone sample with alumina nanofiller. The trap energy level is dependent on the stability of the trapped charge. When deep traps of nanoscale size with energy level of >0.5 eV capture free charges, they reduce the average free path for the charge transportation and hence the charge injection is reduced. A high energy level corresponds to deep traps in the sample that require a long time for de-trapping thereby increasing the mean lifetime of the material. Hence the material with higher trap energy level and trap density is more likely to hinder the migration of charge into the bulk of the sample. The trap depth variation can be predominantly observed when the wt.% of nanofiller is increased above 5 wt.%. The deep traps are enhanced due to the inclusion of nanoparticles, which restrain the charge transport by trapping more charges and influence the surface charge accumulation and decay rate. The dissipation of surface potential for the gamma-irradiated samples occurs over a time period shorter than that of the pristine samples. This could be due to the structural variation in the material due to irradiation thereby causing a variation in surface resistance of the material. While the trends of the characteristics are similar for both positive and negative polarity, the trap depth is higher under positive polarity.

Analysis of space charge density
In the present study, the space charge characteristics were observed for virgin and gamma irradiated samples under a + 20 kV/mm electric field. The samples were charged for 30 min by applying + 20 kV/mm stress (voltage on) and then discharge characteristics were observed by turning the voltage off. It is observed from Fig. 5, that the space charge density at the anode is smaller and broader than that at the cathode. To analyse the variation in the magnitude of space charge density in the bulk of the sample, the space charge volume density ρ t at time t is calculated as where x 0 and x 1 represent the position of the electrodes equal to the thickness of the sample; q p x, t is space charge density at location x. For all cases, the space charge increases up to 1 min due to charge injection from the electrodes and gradually reduces and settles down due to the recombination of electrons and holes. It is noticed from Fig. 5 and 6 that the filler weight percentage and space charge accumulation have a direct correlation. This is possibly due to loading of nano-Al 2 O 3 , causing abundant carrier traps and leading to more charge accumulated. Also, the addition of nanofiller increases the volume fraction of the interface zone between the filler and the polymer matrix, which gives rise to interfacial/space charge polarisation. It is observed that for pristine samples the accumulated charge near the cathode over the time is of the same polarity as the electrode, also called homocharge. Homocharge means that the polarity of the space charge is similar to that of the neighbouring electrode and heterocharge is the opposite situation. Under applied electric field, a heterocharge near the electrode is expected to reduce the breakdown voltage, whereas a homocharge will increase it. Formation of homocharges is observed in all the nanocomposites due to the enhanced charge injection from both electrodes due to the addition of nanofiller to  silicone base matrix [19]. Whereas heterocharges may occur due to the charge separation process in the material but play only a minimal role. The presence of a large volume fraction of interfaces in nanocomposites hinders the migration and drifting of ions causing a reduction in the accumulation of heterocharges. Due to the changes in the material characteristics such as variation in permittivity and trap depth, the electrical performance of the materials is altered. This causes a variation in the space charge performance of the gamma irradiated specimens. It is observed from Figs. 5 and 6 that, regardless of the nanofiller percentage, space charge accumulation of gamma-irradiated samples is higher than for the pristine sample. The charge accumulation near to the electrodes and in the bulk of the sample is found to increase with the irradiation level. As shown in Figs. 5b and c with gamma irradiation the space charge accumulation in the '0 wt.%' specimen, heterocharge accumulation was observed in the vicinity of the anode. This is associated with the ionisation of the residues of crosslinking by-products due to the gamma irradiation [20]. These by-products may be present in the material for a long period and will undergo an ionisation process under a high electric field.
Accordingly, under the influence of the electric field, these ionised charge carriers move towards the opposite electrodes to form the heterocharge. Compared to virgin samples under gamma irradiation, the space charge accumulation in the bulk volume is not less with alumina nanocomposites. As the radiation dose increases the amount of charge migration in the bulk of the sample and formation of hetero charge decreases for the 5 wt.% sample compared to the other samples, inferring an improved performance under gamma irradiation. The space charge decay profile of virgin (0 wt.%) silicone sample during the 5 min of the depoling period is shown in Fig. 7. It can be seen from the figure that for the virgin sample the residual space charge in the bulk of the sample increases with gamma irradiated specimen. Also, formation of heterocharges was observed near to the anode, with increase in irradiation dosage. As the wt.% of nanofiller in silicone rubber is increased (Fig. 8), suppression of heterocharges was observed. Also, by comparing Figs. 7 and 8 it clearly indicates that under gamma irradiation the space charge accumulation in the bulk volume is not high with nanocomposites. Quick decay of space charge is observed with addition of nanofiller due to enhanced carrier mobility. Also, for all the nanocomposites, the rate of charge de-trapping is increased with increase in dosage of gamma-irradiation. It can be seen under irradiated conditions that silicone-alumina nanocomposites showed improved space charge decay performance.

Analysis of space charge distribution
The space charge distribution in silicone nanocomposite with 5 wt.% during poling under 20 kV/mm stress is shown in Fig. 9. Under the applied field, the positive and negative homocharges were formed during the poling period near to the anode and cathode interface.
For the virgin specimen and in all cases, a very small amount of charge accumulation was observed in the bulk volume of the specimen, because the injected charge is mostly concentrated in the vicinity of the electrodes. The homocharge formation is considered to be due to charge injection from the electrodes. The formation of homocharge is the result of the strong attachment and interaction between the nanoparticle and the polymer layer. This interaction leads to a stable interface with the free ions and defects to contribute to interfacial polarisation in the bulk of the nanocomposite. Compared with the virgin specimen (Fig. 5), the 5 wt.% sample suppresses heterocharge formation near to the anode and also accumulation of space charge in the bulk of the sample. This clearly indicates that the presence of alumina filler enhances the space charge characteristics of the insulant under gamma irradiation.

Space charge accumulation during poling and De-poling
The accumulation of space charge with time for different silicone nanocomposites with virgin and gamma irradiated specimens is shown in Fig. 10. For pristine samples, it is observed that the charge density increases as the percentage weight of filler increases. The total amount of charge almost reaches a steady-state within 10 min during poling. The 10 wt.% sample accumulates the most charge due to high interfacial polarisation, while there is no significant difference between the 3 and 5 wt.% samples. Also, it is observed that with gamma irradiation, the space charge density is increased for all the samples. This is due to the gamma irradiation induced defects in the bulk of polymer matrix, which establish traps of various depths and the formation of free radicals. The charge density of the 10 wt.% alumina filled sample is higher than that of the other samples, while the 5 wt.% sample showed the least space charge accumulation at a high dose (8 kGy). Also, it is found that there is only a small difference in space charge accumulation between the 0, 1 and 5 wt.% samples at high radiation (8 kGy). As the gamma irradiation increases, the increment rate in space charge accumulation decreases for the nanocomposites compared with the pristine sample. Consequently as the radiation level increases, the nano alumina filled samples exhibit better space charge characteristics (less space charge accumulation) compared with the virgin silicone sample (0 wt.%).
During de-poling, the charge density decays to a minimum value within 1 min, as shown in Fig. 11. The injected space charges were found to decay exponentially after the removal of voltage due to the conduction, diffusion due to the concentration gradient, and recombination of charge. The decay is faster with the gammairradiated specimen compared with the pristine specimen. Also, compared with other samples, the discharge rate is faster for the 10 wt.% sample, while it is slowest for virgin sample. Thus the space    The comparison between the average space charge density of virgin and gamma-irradiated samples at 20 kV/mm stress is shown in Fig. 12. The value is calculated taking the average of ten charge density measurements after reaching the steady-state. The space charge density is high for the alumina filled samples due to the enhancement of the interfacial polarisation. As the filler percentage increases, the incremental rate of charge density before and after gamma irradiation decreases up to 5 wt.% of filler addition. For the 10 wt.% sample, the increase in charge density is high possibly due to agglomeration of the filler. Compared with nanocomposites the virgin sample experiences more structural alterations, which result in charge trap sites and space charge accumulation. Hence, the addition of filler up to a certain weight percentage, in this case 5 wt.%, enhances the performance under gamma irradiation, while beyond that range (10 wt.%), the space charge performance of the samples deteriorates.
The trap depth of alumina silicone nanocomposites due to space charge accumulation (as calculated based on (2) and (3)) is shown in Fig. 13 and it was observed that its value varied in the range of 0.7-0.95 eV. It is observed that silicone nanocomposites showed both shallower and deeper trap formation. The shallower trap density is higher than the deeper trap density, which is the main reason for fast detrapping of space charge in silicone nanocomposites. The deep trap depth is reduced by inclusion of nanoparticles, which enhances the charge transport and the space charge decay rate. It indicates that the charge trapping is easy in nanocomposites. Table 2 shows the highest trap depth observed in silicone nanocomposites and it can be seen that the trap depth decreased with increase in irradiation. For pristine samples the trap depth of pure silicone rubber is high compared with the aluminasilicone nanocomposite. After irradiation, silicone nanocomposites up to 5 wt.% showed a reduction in trap energy. This could be due to the structural variation in the material due to irradiation thereby causing a variation in surface resistance of the material. This indicates that the higher the intensity of irradiation the easier the charge detrapping becomes in nanocomposites.
It is known that the space charge increases with an increase in the applied field, but the rate of rise in charge accumulation varies when the applied field exceeds threshold [13]. In order to calculate the threshold field of virgin samples, space charge analysis was carried out at 5, 8, 10, 15, 16 and 20 kV/mm for 30 min of poling period. Similarly for gamma-irradiated samples a space charge analysis was carried out for 8, 10, 12, 14, 16, 20 kV/mm fields. The respective values of charge density versus electric field are plotted in Fig. 14 and linear fitting is applied to the data over two specific ranges of applied field. The point of intersection of the two lines is taken as a threshold value for a given sample [13]. The characteristic curve of space charge density of the pristine specimen for different electric fields is shown in Fig. 14a. It is observed that the variation in space charge is gradual up to a certain electric field value and then a steep increase is noticed beyond the knee point which is considered as a threshold field. The inclusion of nanofiller to the base matrix lowers the threshold value of space charge accumulation. The interaction zone between the nanofiller and the polymer layer enhances the space charge accumulation at a minimum applied electric field. This is due to the reduction in the Coulomb barrier near the electrode by the combined effect of multilayer nanofiller particles [6]. Hence the addition of filler increases the injection of charge carriers in the bulk of the samples at the reduced electric field. Similarly, due to gamma induced cross-linking and chain scission, the charge  injection into the bulk of the sample takes place at a lower electric field. Thus the threshold value is further reduced due to the inclusion of filler and gamma irradiation, and the charge density increases with the inclusion of filler and irradiation. Table 3 shows the space charge threshold field of silicone nanocomposites under different irradiation doses, deduced from the results of Fig. 14.

Scanning electron microscope (SEM) analysis
The filler dispersion was observed using a SEM. Fig. 15 shows the SEM images for different wt.% of nano alumina added silicone nanocomposite samples. The images show that the method adopted for the preparation of nanocomposites gave good dispersion up to 5 wt.%. However, some agglomeration of alumina nanoparticles was observed when 10 wt.% of alumina was added to the silicone base matrix, as can be seen in Fig. 15d.

Contact angle measurement
The variation in static contact angle of silicone nanocomposite with a different weight percentage of alumina nanofiller and irradiation dose (0, 4 and 8 kGy) is shown in Fig. 16. The static contact angle was measured at five different locations on the flat plate sample and the average value with deviations, is indicated. It is observed that the contact angle is >90° for all samples, which indicates that the material is hydrophobic. This is due to the flexible Si-O backbone, allowing the substituted methyl groups to reorient readily to the surface thereby minimising the surface energy.
Silicone rubber on addition of nano fillers, results in a marginal variation in static contact angle. The reason for this could be the change in surface energy due to the addition of nano particles [21]. The contact angle decreased for the gamma-ray irradiated specimens, which could be due to irradiation-induced chain scission and cross-linking [16]. However, there was a marginal reduction in the contact angle with alumina content and for gamma-irradiated samples.

Dielectric relaxation spectroscopy
3.7.1 Effect of permittivity on space charge: The variation in the relative permittivity of silicone rubber nanocomposites in the temperature range of −50 to 200°C over a frequency range from 1 Hz to 10 kHz is shown in Fig. 17. For a given temperature, the relative permittivity reduces with increase in supply voltage frequency and forms a plateau above a certain frequency value. It is observed from Fig. 18 that the dielectric constant increases with temperature and with irradiation intensity. For a given sample, irrespective of filler concentration, at high temperature, the magnitude of ɛ′ is high over a low frequency range and low at high  frequency. When the external field is applied, the free dipolar functional groups in the silicone nanocomposite tend to orient themselves easily at low frequencies resulting in an increased permittivity value. At high frequencies the space charge polarisation is not very dominant, hence the orientation of the dipolar groups takes place at a slower rate compared with low frequencies, resulting in a gradual reduction in permittivity [22]. As the temperature increases, the charge carriers become thermally excited making the dipolar groups orient in the direction of the applied field more easily. The rise in temperature enhances the dipole moment orientation; hence, the dipolar group can easily orient along the external electric field. As a result, an increment in permittivity at lower frequencies with increasing temperature is seen. Over a lowtemperature range (−50 to 0 °C), the dipoles become frozen which inhibits the orientation polarisation and resulting in a decrease in permittivity. However, at high temperatures, polarisation occurs at a faster rate, hence irrespective of the filler percentage there is no significant variation in permittivity with frequency. A similar trend was observed for all nanocomposite samples (Fig. 17b).
The efficient dispersion of nanofiller in the base matrix reduces the defect sites resulting in less space charge accumulation, hence the lower permittivity over the low-temperature range (−50 °C to 0°C ). Since the dipolar and interfacial polarisations are dependent on temperature and frequency, the interfacial polarisation is predominant at low frequencies resulting in higher values of effective permittivity (Fig. 17b). The nanocomposite samples showed a higher permittivity at high temperature and low frequency compared with the virgin (0 wt.%) silicone sample, in accordance with what was found in [23], except at the highest nanofiller concentration and for temperatures of 100°C and above. At this high, 10 wt.%, filler concentration possible agglomerations in the bulk volume of the material may explain the reduction in permittivity due to the restriction of polymer chain movement. Besides, the inclusion of nanofiller into base matrix creates a large volume fraction of interfaces between nanofiller and polymer layer [24], enhancing the overall interfacial polarisations occurring at low frequency and high temperature which leads to an increase in overall permittivity of nanocomposites.
It is observed from Figs. 17c and d, that there is a gradual increase in permittivity for the gamma irradiation dose, irrespective of wt.% of alumina in silicone rubber nanocomposites. However, the trend is similar to the pristine nanocomposites. Exposure of the material to gamma irradiation causes crosslinking, chain scission and the formation of ions and polar groups. This leads to the formation of oxidised products and the modification in the material's electrical characteristics. The rise in permittivity is due to an increase in carrier polarisation indicating that charge carriers move by irregular hopping movements between localised sites (Figs. 17c and d). The irregularity in the polymer chains due to irradiation may give rise to the hopping mechanism which enhances the polarisation in the polymer matrix. The steep increment in permittivity is noticed as result of enhancement in the magnitude of polarisation at high temperature or at low frequency.

Effect of loss factor variations on space charge:
The variation of the loss factor ɛʺ of pure and nanocomposites over the temperature range −50 to 200 °C is shown in Fig. 19. Characteristic variation in loss factor is observed with increase in wt.% of alumina in silicone rubber nanocomposites. Over the low temperature range (−50 °C to 50 °C to), with the inclusion of nanofiller there is a reduction in ɛʺ due to the strong interaction between the nano particles and the polymer chain. This reduces the number of available mobile charge carriers which leads to a reduction in electrical conductivity. It is well known that, based on (5), ɛʺ is directly proportional to the conductivity. Therefore, with addition of nanofiller a reduction in ε′′ is observed where σ conductivity of the material, ε′′ is the loss factor of the material and ω is the frequency. It is noted that ε′′ increases with decreasing frequency. This is possibly due to the increase in carrier traps with the addition of filler, causing more charge accumulation and increasing the volume fraction of the interface zone between the filler and the polymer matrix. This increases the magnitude of interfacial polarisation at low frequency. The interfacial polarisation increases with increasing temperature, leading to a rise in ε′′ for all the nanocomposite samples at high temperature. A steep increase in ε′′ is observed with increase in temperature at low-frequency range. Thermal excitation of the charge carriers resulting from the rise in temperature aids the orientation of polar groups with the external applied electric field. This enhances the polarisation and increases the dielectric values of the nanocomposite. Since the dielectric polarisation in metal oxides is mainly due to the conduction mechanism, the effect of temperature is dominant and hence the electron exchange occurs more than the displacement of charge carriers which leads to an increase in the values of dielectric loss with temperature [25]. The rise in dielectric constant with temperature at low frequencies indicates that temperature has a predominant influence on interfacial polarisation rather than on dipolar polarisation. From Figs. 19c and d, it is observed that the trend in loss factor for gamma irradiated samples is similar to the pristine nanocomposites. However, there is an increase in dielectric loss for the gamma irradiated silicone nanocomposites. Exposure of the material to gamma irradiation causes crosslinking, chain scission and the formation of ions and polar groups. This leads to the formation of oxidised products and the modification in the material's electrical characteristics.

Conclusions
The following important conclusions can be made based on the present study: • Addition of nanoparticles to the base matrix increases the deep trap energy level. • Addition of alumina nanofiller to the base matrix increases the space charge for pristine samples. Regardless of the filler percentage, the space charge density increases for gamma irradiated samples. The space charge performance of 5 wt.% silicone nano composite improved following gamma-irradiation. • Inclusion of alumina nanofiller reduces the threshold electric field for pristine as well as for the gamma irradiated samples. • The addition of nanofiller to the base polymer matrix causes reduction in the static contact angle. Also, the mean lifetime and initial surface potential of the nanocomposites were increased significantly. • Dielectric permittivity of the material increased with increase in wt.% of alumina in silicone rubber at lower frequency while the converse was found at higher frequency. Also, it is observed that the temperature of the specimen enhances the permittivity of the material. • Dielectric loss factor increases with decreasing frequency due to the increase in carrier traps with the addition of filler. Also an increase in dielectric loss is observed for the gamma irradiated silicone nanocomposites.