Understanding the impact of gamma irradiation of epoxy titania nanocomposites on surface and bulk charge characteristics

Figure 5 shows the variation in surface potential of epoxy nanocomposites on the removal of the charge deposition process by DC corona activity. There are several processes by which the surface charge deposited on the sample gets dissipated such as lateral charge spreading, neutralization with the ions in the air, transportation of charges through the bulk of the sample, etc. Liang et al have indicated that charge transport through the bulk is considered as the main decay process [14]. It is observed that the magnitude of negative charge is high compared to that of the positive charge in the surface charge characteristics. Similar characteristics were observed by Du et al and have indicated that negative corona can be induced more easily than a positive corona with an identical absolute value of applied voltage thereby, the number of negative charges generated be high compared to positive charges [7]. This generated charges gets deposited on the surface of the insulating material. As the charge is transferred through the bulk, charge trapping and detrapping play an important role in the decay characteristics. The charge decay rate is more on the removal of the charge deposition process by corona activity and becomes less with time as shown in figure 5. The fast decay rate is due to the detrapping of shallow traps and later on, the charge needs to be detrapped from the deep traps. As the gamma-ray irradiation dosage is increased the decay rate gets enhanced due to structural variation occurred due to irradiation forming chain scission, oxidation on the surface of the specimen enhancing the shallow trap site [15].

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Charge trapping and detrapping play a major role in charge transportation through the sample. The demarcation energy, E_m indicates the border between the emptied and occupied traps. The trap distribution characteristics can be derived from the surface potential decay characteristics, by adopting isothermal current decay theory. From Simmons and Tam's theory [16], trap density and the energy gap in the charge trap sites are given as [17].

$$\begin{eqnarray}&&{\rm{\Delta }}{\rm{E}}={{\rm{E}}}_{{\rm{c}}}-{{\rm{E}}}_{m}={\rm{kT}}\,\mathrm{ln}\left({\rm{vt}}\right)\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&{\rm{N}}\left({\rm{E}}\right)=\displaystyle \frac{2\,{\varepsilon }_{{\rm{o}}}{\varepsilon }_{{\rm{r}}}}{{{\rm{qL}}}^{2}{{\rm{kTf}}}_{{\rm{o}}}\left({\rm{E}}\right)}{\rm{t}}\displaystyle \frac{{{\rm{dV}}}_{{\rm{s}}}}{{\rm{dt}}}\end{eqnarray} \tag{ 2 }$$

Where ${\rm{\Delta }}{\rm{E}}$ is trap depth, E_c is the conduction band energy level, T is the absolute temperature in kelvin, k is Boltzmann constant, t is time, v is an attempt to escape frequency and its value is set to 10¹², ${\rm{N}}\left({\rm{E}}\right)$ is trap density at energy level E, ${\varepsilon }_{{\rm{o}}}$ is the permittivity of free space, ${\varepsilon }_{{\rm{r}}}$ is relative permittivity of the sample, L is thickness of the sample, q is charge of an electron, V_s is the surface potential on the sample and f_o(E) is the occupancy rate of initial electrons.

Figure 6 shows the trap distribution characteristics of epoxy nanocomposites for charge deposited due to corona activity formed under positive and negative DC voltage of 12 kV. As the irradiation is increased, the initial surface potential on the sample is decreased and there is a left shift in the peak of the energy density, N(E) which shows that the energy gap is decreasing. This indicates the formation of shallow trap sites with gamma-ray irradiated samples when compared with virgin samples, as can be observed from table 1. Xing et al have studied the charge trap characteristics with gamma irradiated epoxy AlN nanocomposites and have indicated the left shift in the trap depth [18]. Haque et al have indicated that charge trap sites occur in solid insulation due to physical changes, chemical impurities or by the radiation caused damage [19]. In general, high energy irradiation can form chain scission and oxidation to the insulating material enhancing surface trap site and deep trap sites. Huang et al have indicated that the deep trap formed can enhance the surface flashover voltage [8]. However, the trapped charges can gain energy by several mechanisms including electron tunneling, impact ionisation, thermal de-trapping, and photon assisted detrapping [20]. Thus depending on the trap depth and local condition, the trapped charges cause local electric field enhancement causing localised discharges leading to catastrophic failure of insulating material [21].

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The surface charge is deposited on the sample by corona charging using a series of switching impulse voltage, 100 μs/1000 μs at 100 Hz for both positive and negative polarities with a peak voltage of 12 kV. Figure 7 shows the surface potential decay of epoxy nanocomposites after the sample surface is charged for 2 min. With an increase in irradiation dosage, the initial surface potential is decreased and the decay rate is increased, irrespective of the polarity of charging through switching impulse voltage (table 1). Due to gamma irradiation of polymer composites, matrix structure gets altered due to chain scission, oxidation, and crosslinking. As a consequence, carbonyl (C=O) and hydroxyl (–OH) groups are observed in epoxy composites. The adjacent radicals can cross link thereby reducing the surface charge trap sites leading to a reduction in surface potential [22]. On removal of charging the surface, the recombination and lateral motion of charges occur and the increase in conductivity due to shallow traps formed enhances the decay rate. Thus, with an increase in irradiation dosage, the initial surface potential is decreased and the decay rate is increased, irrespective of the polarity of charging through switching impulse voltage. The initial surface potential is less and the decay rate is increased with charge deposited under switching impulse voltage compared to the charges deposited under DC voltage, irrespective of polarity, and level of irradiation.

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Du et al have investigated the trap distribution characteristics of epoxy resin for a repetitive pulse voltage and have observed reduction in trap depth, irrespective of the polarity of the applied voltage pulse [23]. With an increase in irradiation dosage, there is a reduction in the trap depth as given in table 1, which indicates that shallow traps are increased. The characteristics are much similar to the trend observed under DC voltage (table 1). Comparing trap depths under DC and switching impulse voltages, the charge gets trapped deeper in insulating material under negative DC voltages. When the epoxy nanocomposite is gamma ray irradiated, structural changes occur to the surface of the material. Pulikkalparambil et al carried out accelerated weathering studies with the bio epoxy ionic liquid blends and have indicated an increased contact angle [24]. In the present study, to check the surface condition of the insulating material for hydrophobicity, contact angle measurements are performed. A typical photograph of the water droplet for contact angle measurement with epoxy nanocomposites is shown in figure 8. The results of the study indicate a marginal reduction in contact angle (table 2). The characteristics of the trap depth and contact angle shows a direct correlation.

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Figure 11 shows the variation in space charge profiles of epoxy nanocomposites stressed with 12 kV mm⁻¹ at different instants of time during the poling period. It is observed only the homo charges are present at the electrodes. Also, not much of a peak shift is noticed with an increase in poling time, especially with the virgin epoxy nanocomposites. This indicates that no charge transport into the bulk of the insulating material has occurred. With gamma irradiated specimen, a marginal spread in the homo charges is observed, near to the surface of the specimen. On comparing with figure 6, increase in the surface charge trap predominated than the deep trap formation in the bulk of the insulating material. Canadas et al studied space charge behaviour with UV irradiated PET material and have observed shallow trap sites in irradiated material than deep traps [27]. As time progresses, the homocharge formed near to the electrode opposes the applied electric field thereby reducing the further charge injection from the contact electrode [28].

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Figure 12 shows the space charge profile in epoxy nanocomposites, at different time instants, during depoling. As time progresses, the homo charge near the electrodes and the trapped charge in the bulk of the specimen is detrapped or recombined. Chen et al have said that space charge decay is determined generally by the concentration of carriers, their mobility and by the local electric stress [29]. The rate of dissipation depends on the detrapping time constants of the shallow and deep traps [30]. As irradiation causes the structural variations in the sample, the mobility of the carriers in the virgin sample is different from the gamma-ray irradiated samples.

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Figure 13 shows the space charge, ${Q}_{m}$ decay after being poled for 1 h. In general, the depoling process involves three different processes which include charge de-trapping, charge transport, and charge injection, and each mechanism has a different time period of operation [31]. They also have indicated that the space charge decay and the initial charge density at the start of the depoling stage increase with an increase in irradiation dosage. From figure 13, we can observe two different trends in the decay process of space charge.

$$\begin{eqnarray}&&{Q}_{m}\left(t\right)={q}_{s}{e}^{-{\tau }_{s}t}+{q}_{d}{e}^{-{\tau }_{d}t}\end{eqnarray} \tag{ 4 }$$

By fitting the space charge decay curve to the double exponential as given in equation (4) we can find the characteristic parameters during depoling as given in table 3.

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Table 3.  Space charge decay parameters.

Sample	${q}_{s}$ (C m−3)	${\tau }_{s}$ (s−1)	${q}_{d}$ (C m−3)	${\tau }_{d}$ (s−1)
Virgin	1.341	0.035	0.235	0.0002
4 kGy	1.592	0.049	0.169	0.000 21
8 kGy	1.725	0.057	0.251	0.000 23
12 kGy	1.992	0.066	0.285	0.000 28

In table 3, the parameters ${q}_{s}$ and ${q}_{d}$ (C m⁻³) represents the amount of trapped space charge. The parameters ${\tau }_{s}$ and ${\tau }_{d}$(s⁻¹) represent the space charge decay rates. As the absolute values of ${q}_{s}$ and ${\tau }_{s}$ are large when compared with ${q}_{d}$ and ${\tau }_{d},$ it indicates the shallow traps are more. Zhou et al have studied the space charge decay process in composite insulators using the double exponential function and have indicated similar characteristics as observed with epoxy titania nanocomposites [30].