Reliable characterization of organic dielectrics degraded by high electric field

Received Jun 21, 2018 Revised Dec 13, 2018 Accepted Jan 23, 2019 The study of the degradation of organic dielectrics is of great interest for industrial applications, especially in electrical industry. So great, that other applications are also being developed in the fields of optical communications, information storage, stacked memories or manufacturing new components in micro technology, mainly in the fields of MEMS and sensors. Under the same conditions of temperature and of high intensity electric field during operation, we present results on Poly Ethylene Terephthalate (PET) which is the most useful material; the degradation considered here is induced by high electric field in ambient atmosphere with a high voltage range scaled around on 12kV during 1h.The physical and thermal characterization is carried out by an implementation of an experimental technique very fine high sensitivity and high accuracy measures such as Broadband Dielectric Spectroscopy (BDS) and Differential Scanning Calorimetry (DSC). Phase transition, relaxation mode changes, molecular relaxation and trapping effect will be discussed.


INTRODUCTION
In electrical industry, whether in insulating parts, where a thin layer organic polymer-based is more and more introduced between two conductive and semiconductor layers [1], or in medium and high voltage power cables, andevenin waveguide structures [2], organic dielectrics are subjected to physical and thermal stresses that degrade their properties and hence their performances [3]- [5].This degradation is accompanied by phenomena of structural relaxation named physical aging [6] or modification of the chemical structure of the chain named chemical aging [7], [8].Characterization of degraded organic dielectrics is then necessary for the design and dimensioning of the insulating structures.Today, research focuses on some organic dielectrics used or will be used in electrical industry taking account of the bioethical standards currently in force.The degradation considered here is associated with the specific operating conditions in the electrical industry [9]; it was carried out in the High Voltage Laboratory (L.T.H.) of the University of Science and Technology of Oran.(U.S.T. Oran) and is induced by high electric field in ambient atmosphere with a high voltage range scaled around on 12kV.
Among the polymers most studied in this field are thermoplastics, silicones, polyepoxides and other thermosetting polymers.We present here the results on thePoly Ethylene Terephthalate PET which has a low dissipation factor 0.002 and a dielectric constant of εr = 3.3; Furthermore stretching can improve its mechanical strength and dimensional stability and increase its high frequency properties [10], [11].The physical and thermal characterization is carried out by an implementation of an experimental technique very fine high ISSN: 2089-3272  Reliable characterization of organic dielectrics degraded by high electric field (Djamila Abid) 77 sensitivity and high accuracy measures such as Broadband Dielectric Spectroscopy (BDS) and Differential Scanning Calorimetry (DSC).The equipment used is very efficient; it will allow the acquisition of more reliable measurements than before and thus induce an improvement of the characterization.The analysis will exploit the latest knowledge on organic dielectrics.The analysis focuses here on the comparison of a PET nondegraded sample and an irradiated PET sample to precisely show at a same time the sensitivity, accuracy of the measurements and the effects of high electric field on an organic dielectric.Phase transitions, relaxation mode changes, molecular relaxation and shape modification and trap effect will be discussed.

MATERIALS AND METHODS
The BDS method is becoming a strong tool for the analysis of molecular dynamics in short and longdistance charges and transport in polymers [12].Its principle is based on the application of a sinusoidal voltage at a sample located among two metal electrodes and the analysis of the amplitude and phase of the electrical response of the material [13].In our case the Alpha analyzer from Novocontrol covers a wide frequency range between 10 -1 and 2 10 6 Hz and we shall focus only on the analysis based on the measure of the frequency response, and the temperature response acquired isothermally in a range from to −150 °C to 50 °C.
the real (ε′ (ω)) and imaginary parts (ε″(ω)) of the complex dielectric permittivity (ε*(ω)) are fitted with the following (1)-( 4): where ω is the angular pulsation, ΔɛHN the relaxation strength of relaxation, τHN the relaxation time and αHN and βHN the symmetric and asymmetric broadening factor.To study the effect of the electric field on the PET we applied a voltage on sample as shown in Figure 1.

Figure 1. Experimental degradation device
The electrical discharge is obtained by the point -to -plane system.The whole is placed in the air.The terminals of the flat electrode are rounded to 3cm in diameter to avoid edge effects.The high-voltage electrode is a cylinder of 10 mm diameter which ends in a tip of 0.5 mm diameter of curvature.The tip electrode is connected to the AC high voltage terminal of the test transformer, and the electrode is grounded.
In order to subject a sample of small thickness to the electrical stress, we will place a glass disk of thickness 5 mm; this glass constitutes a dielectric barrier whose role is to avoid the arc of the discharge between the two electrodes.
To do this we used a sample of poly (ethylene terephthalate) (PET) film supplied by the company Carolex and stored in air and at ambient temperature.The form of a film is thickness 130 μm, 20 mm in diameter, transparent color and flexible.The PET sample is placed in series with an air distance of 5 millimeter so that the channel of the discharge concentrates on the sample.This sample is then irradiated for 1 hour by an electric field at 12 kV.
The technique adopted for the structural characterization of PET is Differential Scanning Calorimetry (DSC).The DSC is a very fine technique allowing, from its thermo grams, to measure the temperatures of thermal vitreous transition Tg, crystallization Tc and fusion Tf as well as to calculate the enthalpies of fusion ΔHf and crystallization ΔHc with: Where f and i are respectively the final state (liquid) and the initial state (solid) and Cp (T) the thermal capacity of the material.ΔHcandΔHf for respectively the exothermic and endothermic peaks.
By exploiting the areas of the peaks, the DCS also makes it possible to determine the percentage of the crystalline phase as well as that of the amorphous phase in the semi-crystalline polymer.
X a = ∆C P ∆C p a ( 6 ) with (Δcp)a determined experimentally through a test conducted on amorphous PET and equal to 0.396 J/g°C and ( 7 ) with (Δ H f) c = 1 is taken to be 117 J/g according to [1] The determination of these parameters will be used also to characterize the effect of the electric field on our irradiated sample.
The DSC tests are carried out on an apparatus of the type Perkin-Elmer Instruments DSC 8000 in the air with cooling with liquid nitrogen.Low mass samples (~ 7mg) are weighed by a high precision scale Mettler Toledo MX5.The polymers are warmed from 0°C to 300 °C with a heating rate of 10 °C/min.

RESULTS AND DISCUSSIONS 3.1. Blank PET
In this experiment we measured the dielectric parameters, dielectric constant ε' and loss factor ε'', in the full range of the frequency and for temperature ranging between 49.9 °C to 118 °C. Figure 2 and Figure 3 show two relaxation peaks identified as α and secondary β relaxation peaks.The main α relaxation peak which appears at T = 79.8° C for f = 0.1Hz and it migrates towards the high frequencies gradually as the temperature increases.This displacement is accompanied by a decrease of its amplitude from 0.43 to 0.34 between 108 °C and 110 °C; the PET is in its amorphous state [15].Between 108 °C and 114 °C, the amplitude of the peak decrease roughly from 0.34 to 0.22 in a wide range of frequencies close to 10 5 Hz.This is attributed [16] to the phenomenon of crystallization of PET because the presence of the crystalline zones reduces the mobility of the entities responsible for conduction.For higher frequencies (f > 10 3 Hz) and for a temperature range from 49.9° C to 73.9° C, we observe the secondary β relaxation peak.
These two figures show that the PET has a dielectric stability up to the temperature 69°C for the low frequencies of 0.1 HZ up to 1000 HZ, indeed no variation for the dielectric parameters, dielectric constant ε 'and loss factor ε' '.
The Figure 4 represents the variation of the loss factor ε '' as a function of the frequency; When the temperature varies from 49.9° C to 67.9° C, the intensity decreases with increasing temperature.This is justified by the participation of water molecules in secondary relaxation [17].For high temperatures (between 91.9 ° C and 108 ° C) and low frequencies Figure 5 shows an increase in the dielectric loss factor.This increase is due to the phenomenon of conduction.Straight lines of slope -1 are characteristic of an ohmic conduction.
The result of the differential enthalpy analysis of the PET film in the temperature range 0 °C to 300 °C is illustrated in Figure 6.It shows a thermo gram of a PET which has two points of inflection, one in zone 1 at a temperature of 83 °C, namely the glass transition temperature.At the temperature of 150 ° C. there is another inflection point, in zone 2, corresponding to the recrystallization temperature.The absence of a recrystallization peak indicates that the sample has a high crystallization rate.Thus, the absence of a peak at the glass transition indicates that the non-irradiated sample is in a metastable equilibrium state.An endothermic peak appears at the temperature 245 °C corresponding to the melting temperature of the crystals.The blank PET is therefore thermally stable up to 232 ° C.

Irradiated PET
The Figure 7 shows the thermo gram of the irradiated PET where we can see the effect of the electric field on the structure of the PET.Zone 1 is the glass-transition region, there is a peak characterizing physical aging or structural relaxation of PET [18], [19].The glass transition temperature is 77 °C.
Figure 7 shows also that PET, when under irradiation, becomes unstable [20] and [21].In zone 2, an exothermic peak characteristic of the crystallization of a material is seen.The crystallization temperature is at T = 150 °C.The second endothermic peak at 245 °C corresponds to the melting temperature.

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The Table 1 below presents the comparison of the structural characteristics measured in DSC of the blank PET and the irradiated PET at 12 KV for 1 hour.The irradiated PET which has a semi crystalline structure with Xc equal to 16.8 %.Blank PET is more amorphous with Xa equal to 91%.The decrease in Tg and the increase in Δcp heat capacity jump between non-irradiated PET and irradiated PET is then observed.The decrease in Tg could be the result of chain breaks which have the effect of reducing the number of junction points and thus increasing molecular mobility [22].In fact, irradiation in the presence of oxygen generally leads to more chain splitting than under neutral gas [23].The cross linking involves radical coupling, and in a medium rich in oxygen, these molecules react rapidly with the radicals and prevent recombination and therefore the cross linking of the polymer [24].
The appearance of an endothermic peak near the glass transition temperature reflects the physical aging of the irradiated PET as exposed by Vigier [20].This aging is attributed to the formation of radicals within the amorphous material after irradiation.These radicals are trapped in the vitreous material and can cause slow and gradual changes in the network when maintained at a temperature below the glass transition temperature [25].
We present now the main results obtained by comparing the two types of samples from the BDS analysis.Figure 8 shows the loss factor as a function of frequency for temperatures below the glass transition temperature for irradiated PET.The values of ε '' are high towards the high frequencies.These values are the consequence of the existence of an extrinsic parasitic series resistance to the sample as suggested by Runt [12].This increase does not exist for non-irradiated PET as shown in Figure 4. Comparison of the two figures shows closer spectra for irradiated PET that could be associated with a dehydration of PET during irradiation.
Figure 9 shows the relaxation time temperature dependence.We have an Arrhenius trend characterizing molecular localized mobility for the blank PET.And a tendency Vogel-Fulcher-Tammann law characterizing a cooperative mobility of molecules and so a large molecular mobility in the irradiated PET.It would reveal a molecular chain scission due to irradiation.
The isochronous spectrum in Figure 10

CONCLUSION
The structural, electrical and thermal properties of PET samples are investigated using BDS and DSC measurements with under electric field within the volume of the irradiated samples.The equipment used is very efficient; it will allow the acquisition of more reliable measurements than before and thus induce an improvement of the characterization.
For non-irradiated of PET the measurements show that the water molecules are participated in the β secondary relaxation of PET.These samples are then characterized as homogeneous structure, ohmic conduction at high temperature, dielectric stability up to 69°C in the frequency range from 0.1Hz to 1KHz and thermal stability up to T = 232°C.
Electric field irradiation affects structural properties of PET so that it is characterized in this case by semi-crystalline and heterogeneous structure depending on the phenomenon of charge injection into the volume because it presents a place of trapping for the load to inject.The closer thermal spectra of loss factor for irradiated PET could be associated with a dehydration of PET during irradiation.The tendency Vogel Fulcher -Tammann law in the irradiated PET characterize a cooperative mobility of molecules; it would reveal a molecular chain scission due to electric field irradiation.

Figure 5 .
Figure 5. Loss factor with frequency: for the temperature varies between 91.9°C and 108°C.blank PET.

Figure 8 .
Figure 8. Frequency loss factor for temperatures below the glass transition temperature for irradiated PET reveals a charge injection into the PET after irradiation of 1 hour; it justifies the increase in the amplitude of the peak close to 108 ° C and the appearance of another peak around 116 ° C which represents the peak of the electrode polarization.

Figure 9 .
Figure 9. Frequency-Temperature diagram for blank and irradiated PET

Table 1 .
PET: Structural characteristics from DSC