Nugroho Adi
email: nugroho.at@gmail.com

Gilbert Teyssedre

T T N Vu

N I Sinisuka

DC field distribution in XLPE-insulated DC model cable with polarity reversal and thermal gradient

Keywords: Conductivity, HVDC cable, electric field distribution, thermal gradient, polarity reversal, space charge

The electric field distribution in polymer dielectric under DC stress is temperature and time dependent and it can be determined by using conductivity which is also dependent on field and temperature. The distribution of the field, that directly impacts the cable performances, is driven by the conduction mechanisms and settles with giving rise to a space charge distribution across the insulation. To predict conductivity-related electric field distributions, conduction current measurement under various electrical and thermal conditions in time need to be recorded. The current measurement is eventually made with polarity reversal in order to relate to the field distribution under this condition. The aim of this paper is to observe and investigate the steady state and transient electric field distribution in miniaturized HVDC power cables with construction of 1.5 mm cross-linked polyethylene (XLPE) insulation thickness. The determination of field distribution is achieved by computation using currentvoltage model for current transient in cable geometry approach. Both isothermal and thermal gradient conditions for the cable are analyzed, as a function of stressing time and applied voltage. The most critical stresses for the model cases are identified.

I. INTRODUCTION

In polymeric solid insulations, when the applied electric field exceeds the space charge accumulation threshold, injected charge from electrodes may be captured by traps distributed in insulation bulk and in this way a space charge builds-up [START_REF] Montanari | The electrical degradation threshold of polyethylene investigated by space charge and conduction current measurements[END_REF] [START_REF] Dissado | Demonstrating a threshold for trapped space charge accumulation in solid dielectrics under DC field[END_REF]. According to the work of McAllister et al [START_REF] Mcallister | Charge accumulation in DC cables: A macroscopic approach[END_REF], the accumulation of charge is considered to be the consequence of a non-uniform electrical conductivity. The effects of temperature and electric field on conductivity are considered in this case. This shows that a space charge can be generated by design into insulations. Indeed, due to this temperature dependence of the conductivity in any insulating material, a temperature gradient generates conductivity gradient, eventually in a steady state charge distribution ρ. Assuming on the temperature dependence of conductivity, by using Maxwell's equation, the electric field distribution in the cable insulation, in cylindrical geometry and under steady state condition is given by [START_REF] Fabiani | HVDC Cable Design and Space Charge Accumulation. Part 3: Effect of Temperature Gradient[END_REF]:

     r r r E r E o o o     
where Eo and σo are the electric field and conductivity at the reference position ro. Charge density associated with nonuniform conductivity is of the form:

       dr r r r E g         
where is the dielectric permittivity of the material. In the relation to Arrhenius law with activation energy Ea, the charge density can be written according to the following equation:

   r T T k E r E B a g      2 .    
where T is the absolute temperature and kB = 8.62 10 -5 eV/K is the Boltzmann's constant. These expressions show that the space charge profile is controlled by the temperature gradient into the material.

II. EXPERIMENTAL SETUP

A. Test specimen

In this paper, experiments were performed on the same single polymeric DC mini-cables consisting of an inner conductor, an inner semi-conductive layer, the cross-linked polyethylene (XLPE) insulation and an outer semi-conductive layer as depicted in Fig. 1. The mini-cable used in these experiments is with 1.5 mm cross-linked polyethylene (XLPE) insulation thickness.

This scaled down test model of a DC power cable is produced by an extrusion process, the crosslinking process of the whole process producing chemical bond at the semiconinsulation interfaces. The result is a chemical interface Cite as: N. Adi, G. Teyssedre, T.T.N. between the insulation and the semicon in the mini-cable which is similar to those found in a full scale DC power cable. With this reduced dimension, the cost of production, transportation and testing of these cables is substantially reduced.

B. Conduction current measurement on mini cable sample

Fig. 2 shows a schematic of the setup of the conduction current measurement in the mini-cable. The sample was peeled off by removing the outer SC layer for preventing surface conduction. The guard sections are prepared with 20 mm length at both ends of the measurement area and 20 mm apart from the measurement section of 250 mm length. The 1m-long specimen was installed as a ring inside the oven where both the end parts of the cable are jointed and connected to 35 kV DC high voltage generator from FuG Elektronik GmbH. The measurement and guard sections are connected to a Keithley 617 electrometer through a protecting circuit and the results are displayed in the PC screen monitor.

Measurements

were performed under different temperatures varying from 30 °C to 90 °C by step of 10 °C. For each of the temperature levels, charging/discharging current measurement were realized for 11 values of applied voltage ranging between 2 and 30 kV under isothermal conditions with polarization/depolarization steps last for 1h/1h.

III. RESULTS AND DISCUSSION

A. Conductivity measurement

Fig. 3 shows the results of conduction current measurement as a function of field at various levels of temperature applied. The presented data correspond to charging current measurement after 1-hour polarization time.

We can see that the conduction currents are strongly dependent on field and temperature as the magnitudes are rising following the increase of temperature and field. The current values range from 0.3 pA at the lowest field and temperature (2 kV / 30 °C) to 8 nA at the highest field and temperature (30 kV / 90 °C).

Regarding to the strong field and temperature dependency of the conduction currents characteristics, an analytical expression of conductivity with temperature and electric field following semi-empirical equation has been used to fit the experimental data:

     ) ( ) ( ) ( sinh exp , r E r E T C T k E A T E B a                
In order to simplify the expression, we can note that,

            T k E A B B a exp  
Equation ( 4) can be rewritten by:

     ) ( ) ( ) ( sinh , r E r E T C B T E     
So the current flowing in the outer circuit during steady state period at given temperature can be written as:

   l r r E T C B T I . . 2 . ) ( ). ( sinh . ) (    
From the equation above we can obtain the expression to determine the electric field at the radius r as follows:

            l r B I C r E  2 sinh 1 ) ( 1  
In order to determine the conductivity expression of Eq. ( 4), we first estimated B and C values for measurements at a single temperature by optimization, considering experimental quasi steady state charging currents-voltage. However, the relation between current and field cannot be applied directly as would be the case of flat geometry since the field distribution depends on the expected conductivity expression. To solve the problem we proceeded by integrating the electric field E(r) in Eq. ( 8) from the inner to outter radius which gives the relationship between the applied voltage (Vapp) and the conduction current (quasi steady state current) as follows [START_REF] Vu | Transient Space Charge Phenomena in HVDC Model Cables[END_REF]:

                                              1 1 1 1 1 1 1 1 10 log 2 1 ) ( sinh ) ( sinh ) (
M               2 ) ( , 2 ) ( , 2 ) (
Table 1 provides the conductivity model parameters. From B(T) and C(T) obtained through optimization, we first deduced the values of A constant and thermal energy activation, Ea by using Arrhenius fit to B×C product plot considering that at sufficiently low field the conductivity is approximated by: σ(T)= B(T)×C(T). As C did not appear significantly temperature dependent, we considered a constant value.

Fig. 4 shows the conductivity vs. E and T obtained by voltage-current model. We can see that a threshold like behaviour comes out from the representation, with a threshold field independent of temperature, which is Et ≈ 11.5 kV/mm. The relation to the value of C, which actually controls the threshold, can be deduced as Et ≈ 2.5/C. Below the threshold, the response tends to follow Ohm's law which means conductivity independent from field. But as the field is increasing, it begins to be a non-linear relation. The reason can be space charge effects that accumulate at high fields through e.g. space charge limited current. An alternative would be to have a hopping conduction with mobility dependent on field as temperature/field is rising. The two processes may have different temperature dependence of the threshold. With the assumption of using best average C constant value, we cannot reveal this feature in the modelled temperature dependent conductivity characteristic. Fig. 5 shows the comparison between measurement results and model. Compared to the measurement result, the model gives a fairly good fit of the current vs. field plot as shown in Fig. 5. It implies that the expression model in Eq. ( 4) is validated as an appropriate model to the data available here.

B. Electric field simulation based on conductivity data

We have followed the route consisting in using the conductivity model as input to finite element modelling of the field distribution in cables under thermal gradient [START_REF] Vu | Field distribution in polymeric MV-HVDC model cable under temperature gradient : simulation and space charge measurements[END_REF]. Such approach was applied on Medium Voltage cables to estimate space charge and field profiles as a function of time [START_REF] Vu | Field distribution in polymeric MV-HVDC model cable under temperature gradient : simulation and space charge measurements[END_REF] and also to deduce the shape of the transient current in case of mini-cables [START_REF] Vu | Transient Space Charge Phenomena in HVDC Model Cables[END_REF].

The first simulation set was run at voltage applied of 15 kV, with polarization time of 2 hours and 10 °C of thermal gradient is applied across the insulation at the mean temperature of 65 °C. The conductor temperature is 70 °C and outer temperature of insulation 60 °C. The field profiles of Fig. 6 shows decreasing trends near the conductor and increasing at the outer part of insulation. At the first 100 s, the maximum field is laid at the inner semicon-insulation interface, being 13.5 kV/mm, while the minimum field is at the insulation-outer semicon interface and is 7.4 kV/mm. As the time is rising, there is field redistribution in the insulation due to the gradient in conductivity induced by the thermal gradient. The field near the conductor tends to decrease following the polarization time whereas the field at the outer interface of insulation-semicon begins to increase. At the end of polarization, the field inversion is shown where the field near the conductor has dropped to 9.8 kV/mm which is lower than that at the outer insulation where the magnitude reached over 10 kV/mm. We consider next the case of polarity reversal simulation in isothermal condition at temperature of 65 °C, considering there is no load operating in the cable. reasonably along 2 hours polarization time. The effect is only due to the non-linear conductivity with field, which tends to homogenize the field in the insulation. This profile is similar to the Laplacian-field profile (considering at t = 0 s).

Comparing to the positive polarity steps, the field profile at the inversion step is slightly higher. This is due to the residual field built up under the positive step. We can see in Fig. 7 that the field magnitude at the inner semicon at -15 kV after 300 s of voltage inversion, that is 15.1 kV/mm is higher than the same period in the positive steps that is 13.4 kV/mm. Fig. 8 shows the simulation results of electric field profile at ±15 kV voltage polarity inversion applied with 10 °C thermal gradient and 2 hours polarization time. The mean temperature is set to 65 °C. The voltage polarity inversion does make the field profile at the negative step is higher than at the positive step, both at the inner and the outer semicon-insulation interface. Its magnitude difference is equal to the residual field after the polarization step.

When thermal gradient is applied across the insulation, while current homogenization in the insulation takes place and space charge settles, the field profile at the inner semicon decreases and increases at the outer semicon. This phenomenon of field inversion is the same as the one occurred in the first simulation, but with the influence of voltage polarity inversion, the inversion of field seems higher than at isotherm condition. The field at the inner semicon is decrease from 17 kV/mm to 9.9 kV/mm and at the outer semicon is increase from 4.7 kV/mm to 9.9 kV/mm.

IV. CONCLUSION

The field distribution within the insulation of cables under thermal gradient and polarity inversion can be obtained through simulation based on conductivity characteristics from the measurement. Owing to the field grading nature of the material the field distribution in isothermal conditions is homogenized in respect to the capacitive field distribution. The conductivity gradient under thermal gradient applied combined to the nonlinearity in field leads to very good homogenization of the field in the insulation for voltage of 15 kV. Electric field distribution is higher at the insulation-outer semicon interface than at conductor semicon-insulation interface under thermal gradient simulation. Hence, in the polarity inversion stage, electric field at the inner semicon is strongly enhanced, by about 30% of the capacitive field and 100% of the steady state field.

Fig. 1 :Fig. 2 :Fig. 3 :

 123 Fig. 1: Schematic representation of DC mini-cable

Fig. 4 :Fig. 5 :

 45 Fig. 4: Conductivity fit from computation

Fig. 6 :Fig. 7 :

 67 Fig. 6: Field distribution profile in sample cable with T gradient = 10˚C at voltage applied of 15 kV

Fig. 8 :

 8 Fig. 8: Field distribution profile in sample cable with voltage inversion applied of ±15 kV and at T gradient = 10˚C.

  IEEE Int. Conf. on High Voltage Engineering and Application (ICHVE), Chengdu, China, 19-22 Sept. 2016. Proc. ICHVE 2016, pp. 1-4, 2016. DOI: 10.1109/ICHVE.2016.7800612

Vu, N. Sinisuka, "DC field distribution in XLPE-insulated DC model cable with polarity reversal and thermal gradient",

TABLE 1 THE

 1 PARAMETERS OF CONDUCTIVITY MODEL

	Parameter	Value
	C (m/V)	2.15 x 10 -7
	A (S.V/m 2 )	3.83 x 10 4
	Ea (eV)	0.828

ACKNOWLEDGMENT

We acknowledge PLN for the financial support during research stay of N. Adi in Laplace Laboratory, University Paul Sabatier, Toulouse III, France.


Bo Qiao

Gilbert Teyssedre
email: gilbert.teyssedre@laplace.univ-tlse.fr

Christian Laurent

Field and Electron Beam-Induced Luminescence Phenomena in Polypropylene Thin Films

Keywords: polypropylene thin film, electrical ageing, breakdown, electroluminescence, cathodoluminescence. I

Field-induced luminescence of insulating polymers is an exciting field of research. Being intrinsically related with the high-field electronic properties of the material, it provides a way to investigate electrical ageing and breakdown processes. It is particularly suited for those materials used in high field applications, e.g. thin films of polypropylene in capacitors. One of the puzzling aspects of electroluminescence in wide band gap polymers is that the emission spectra are only reproduced under electron beam irradiation. In this communication, we report for the first time the field and electron beam induced luminescence in polypropylene films. Measurements were performed on 17 mthick bi-oriented polypropylene films. The home-designed electron gun provides electrons of up to 10 keV in energies. Emission spectra acquired for different beam energies are compared to the electroluminescence spectra obtained under AC stress for fields in the range 150-300 kV/mm. The nature of the excitation process in field and electron beam-induced luminescence is discussed together with the nature of emitting species and their significance for ageing study.

INTRODUCTION

Insulating polymeric materials are used in a large variety of applications in electrical engineering either as bulk insulation or in thin films form, e.g. high-voltage insulated cables, windings of coils for motors, generators, transformers, capacitors, etc. They provide very good insulating solutions but are subjected to deterioration in service under the influence of electrical and thermal stresses and others depending on the working environment (e.g. irradiation, mechanical, etc.). Their use in practical applications is therefore limited by their lifetime under a given stress and not so much by their intrinsic properties. One of the main threats for the insulation is the existence of partial discharges (PD) eroding the polymer. Solutions have been implemented to strengthen the insulation against PD when they are unavoidable (e.g. large generators, motors, etc.). However, practical situations exist where the existence of PDs has to be absolutely avoided, e.g. extra-high voltage cables with synthetic insulation, capacitors, etc. Moreover, the trend towards more compact electrical systems and electrical components with higher energy density involves pushing the limit in terms of electric field in the insulation. There is therefore a renewed interest in the electronic properties of insulating polymers. One of the key challenges in investigating the electronic properties of insulating materials is to relate the basic transport mechanisms with critical transport regime where deterioration can occur, e.g. onset of hot electrons generation able to break atomic bonds and to generate defects. Detecting the optical emission from the investigated material under electric field provides a way to couple transport and dissipative processes through a possible signature of deleterious reactions at the atomic level.

Field-induced luminescence (or electroluminescence EL) of insulating polymers has been reported in a variety of materials for many years starting year 1967 [START_REF] Hartman | Electroluminescence in Organic Polymers[END_REF] where the first consistent experiments were performed on films of polyethylene, polyethylene terephthalate, etc. In spite of a firm scientific activity along the time, emission spectra were sparsely reported and when they were, large band gap optical filters were used preventing the identification of the emitting species and therefore the underlying relaxation mechanisms. Only quite recently the possibility exists to acquire wavelength-resolved spectra of EL owing to the use of specialized Charge Coupled Device -CCD, coupled with an optical dispersive system [START_REF] Mary | Understanding optical emissions from electrically stressed insulating polymers : electroluminescence in Poly(Ethylene Terephthalate) and Poly(Ethylene 2, 6-Naphthalate) films[END_REF][START_REF] Teyssedre | Identification of the electroluminescence components of polyethylene[END_REF]. What came out from these studies is that the electroluminescence spectra of different materials (polyethylene, cross-linked polyethylene, polyethylene naphthalate) cannot be interpreted on the basis of the photophysical properties of the polymer as investigated using UVinduced luminescence (photo-induced fluorescence and phosphorescence). The only way to reproduce the EL spectra is by irradiating the polymer with energetic electron beam. This suggests the processes associated with EL are degradative processes as well, which is obviously important in defining critical transport regime which marks the onset of electrical ageing. In this communication, we report for the first time the emission spectra associated with thin polypropylene films submitted to an electric field and an electron beam.

II. MATERIAL AND EXPERIMENTAL

A. Material and sample preparation

All the measurements were carried out on Bi-axially Oriented Polypropylene (BOPP) films of 17.8 μm thickness supplied by KOPAFILM, Germany. These are "hazy" films with rough surfaces to promote impregnation. The only additives contained in the BOPP film under study are pentaerythritoltetrakis(3,5-di-tert-butyl-4 hydroxyhydrocinnamate) and calcium stearate [4] which is a sterically hindered phenol used as an antioxidant to abstract unstable radicals. Calcium stearate is a lubricant and an acid scavenger to neutralize any hydrochloric acid generated from reactions between the hindered phenol and residual chloride-containing catalyst. Irganox 1010 has a phenyl ring in its chemical structure as well as a carbonyl which is also present in the structure of calcium stearate. Light absorption in the near UV is therefore mainly due to the phenyl ring and carbonyl groups.

B. Electroluminescence a cathodoluminescence excitation

For electroluminescence measurements, both surfaces of the films were metallized with semi-transparent layers of gold deposited by cold sputtering (thickness: 30 nm, diameter: 50 mm). The films were sandwiched between two brass electrodes, one having a ring shape in order to allow light emission analysis under AC or DC field up to breakdown. Light emission analysis was performed at room temperature under high vacuum (about 5x10 -7 mbar) in order to avoid partial discharges in the ambient atmosphere. Owing to the high dielectric strength of BOPP, DC electric field values up to 420 kV/mm (DC stress) and 250 kV/mm (sine AC crest field) were applied to the samples. Due to the low level of light detected under DC stress, electroluminescence spectra were realized under AC stress at different fields, spectra being recorded a short time after voltage application. For electron beam induced luminescence (also called cathodoluminescence -CL), the films were exposed directly to the beam without metallization. The electron gun, as described in [START_REF] Teyssedre | Evidence of hot electron-induced chemical degradation in electroluminescence spectra of insulating polymers[END_REF], provides electrons of energy of up to 10 keV. Electron emission is made using a tungsten filament, the flux being dependent on the current (of the order of 1 A) flowing in the filament. The sample surface is at about 40 mm from the exit of the electron gun, the axis of the electron gun being at an angle of 50° in respect to the normal of the sample foe enabling light collection. Experiments were carried out at room temperature under a vacuum of 5.10-6 mbar. The electron current was about 0.5 µA in the experiments reported therein with a spot size of about 1 mm². The beam current is independent from the energy of the electrons in the investigated energy range. Emission spectra have been recorded for different beam energies and for different times at the same beam energy.

C. Light detection set-up

Light detection is carried out by two systems: a cooled photomultiplier (PM) working in photon counting mode for integral light detection, and a grating monochromator (4.5 nm in resolution) coupled to a liquid nitrogen cooled charge coupled device (CCD) camera for spectral analysis which covers the wavelength range from 300 nm to 850 nm. The full set-up has been described elsewhere [START_REF] Mary | Understanding optical emissions from electrically stressed insulating polymers : electroluminescence in Poly(Ethylene Terephthalate) and Poly(Ethylene 2, 6-Naphthalate) films[END_REF][START_REF] Teyssedre | Identification of the electroluminescence components of polyethylene[END_REF].

III. RESULTS AND DISCUSSION

A. Electroluminescence and cathodoluminescence spectra

The electroluminescence spectra at different AC fields are shown in Fig. 1a. They have not been corrected for surface plasmon (SP) effects at the metallic electrodes that contribute to the EL at low electric field through a broad spectral component peaking at about 800 nm. A full discussion of the SP effect can be found in [START_REF] Laurent | Charge dynamics and its energetic features in polymeric materials[END_REF]. From samples to samples, there is some variation of the SP contribution to the light intensity and the spectra shown in Fig. 1a correspond to a case where the SP has a weak contribution. Its contribution is therefore neglected in the analysis. The EL spectra peak at about 570 nm and appear broad and mainly structureless although a shoulder can be distinguished at about 500 nm. They roughly keep their shape for different fields.

The cathodoluminescence spectra are shown in Fig. 1b. They are much better resolved in wavelength due to the intensity of the emission (x600 for CL relatively to EL). The spectral shape does not change to a great extent vs. the beam energy with a maximum at 573 nm and three secondary shoulders at 505, 415 and 328 nm. However, the relative contributions at 328 nm and 415 nm increase with the field.

As observed in a variety of polymers, CL and EL spectra of PP films peak at the same wavelength and cover the same range suggesting they are organized along the same components. A tentative interpretation of the spectral components is given in the next section.

.

B. Identification of the spectral components

As pointed out previously, the BOPP films under consideration contain some additives and their presence could play a role in the emission process. One straightforward experiment to check for a possible contribution of these additives to the emission is to record the UV-induced luminescence spectrum of the BOPP. Fig. 2a shows the UVabsorption spectrum of a film with an absorption maximum at 200 nm and two absorption shoulders at 230 nm and 280 nm. Absorptions at 200 nm and 230 nm are typical of * transitions in CC double bonds that are present in BOPP films through the phenyl ring of Irganox as well as unsaturations of the PP chain. The absorption at 280 nm is rather due to the n* transition in carbonyl compounds [START_REF]Luminescence techniques in solid state polymer research[END_REF]. Fig. 2b shows the fluorescence emission (transition from the first excited singlet down to the singlet ground state of the excited species) which is at maximum when excited at 230 nm and 280 nm (corresponding to the absorption maxima). Such kind of photoinduced emission is typical of PP fluorescence with a residual oxidation [START_REF]Luminescence techniques in solid state polymer research[END_REF][START_REF] Teyssedre | Spectral analysis of optical emission due to isothermal charge recombination in polyolefins[END_REF]. Fig. 2c shows the phosphorescence peak excited at 250 nm (forbidden transition from the first excited triplet down to the singlet ground state of the excited species). It exhibits a maximum at 472 nm and two shoulders at 440 nm and 500 nm which have been associated with the presence of unsaturated carbonyls of the di-enone type [START_REF] Teyssedre | Spectral analysis of optical emission due to isothermal charge recombination in polyolefins[END_REF]. Anyway in the context of our discussion, it appears that the EL and CL spectra are not organized around the same components as those revealed in photoluminescence, except may be for the shoulder at 328 nm visible in the CL spectra. A contribution of the natural fluorescence of PP cannot be ruled out. Following the same approach, the shoulders at 415 nm and 505 nm in the CL spectra (and hardly distinguishable in the EL spectra even if the 505 nm component can contribute) have to be compared with the emission maxima of PP films in chemiluminescence and recombination-induced luminescence -RL experiments. The emission spectra of chemiluminescence in PP have been deeply investigated along the oxidation reaction [START_REF] Tiemblo | Chemiluminescence spectral evolution along the thermal oxidation of isotactic polypropylene[END_REF]. It has been shown that at the beginning of the oxidation reaction, the emission has a quite narrow shape peaking at 415 nm and is due to the formation of carbonyl species (spectrum shown in Fig. 3b). In RL experiments where the light comes from charge recombination on the natural chromophores of PP the emission is broad and peaks at 505 nm as shown in Fig. 3c [START_REF] Teyssedre | Spectral analysis of optical emission due to isothermal charge recombination in polyolefins[END_REF]. Following this picture, the EL and CL spectra of PP could be interpreted as the superposition of four contributions: the fluorescence of the natural chromophore, a chemiluminescence component, the phosphorescence of enone compounds among the natural chromophore which act as recombination centers, and a fourth component at longer wavelengths tentatively attributed to excitation of by-products of the PP molecules.

C. Spectral reconstruction on the basis of elementary components

We have tried to reconstruct the emission spectra of CL and EL on the basis of the previous analysis. The spectra associated with fluorescence, chemiluminescence and recombinationinduced luminescence are shown in Fig. 3. The shape of the fourth component associated with PP by-products is unknown. It has been deduced from fitting the CL spectra assuming each spectral component has a Gaussian shape with the same maxima as the known components. The fit to CL and EL spectra is shown in Fig. 4a and4b respectively. Both spectra are nicely reproduced. The relative contribution of each elementary component is given in Table I. The different mechanisms do not contribute with the same weight in both processes: fluorescence and chemiluminescence have a stronger relative contribution to the CL spectrum.

D. Excitation mechanisms

In cathodoluminescence, the kinetic energy of electrons in the beam is the main source of excitation. They lose their energy through a diversity of channels among which impact excitation and impact ionization of the molecules. Impact excitation on the natural chromophores of PP will generate excitonic states that will further relax to their ground states (generating fluorescence and/or phosphorescence emission depending on the nature of the exciton) or react with the . Impact ionization will be a source of positive charges and there is the possibility of charge recombination (generating light) when the incoming electron has already lost its kinetic energy, being trapped in the vicinity of the positive charge. Recombination can also occur between trapped electrons and trapped holes in diffusive recombination processes. In recombination processes, the emitting species will be those able to act as recombination centers i.e. those providing deep traps for electric charges. In PP, these centers are bound to CO species. Owing to the chemical degradation of the material under the electron beam, all these recombination processes can concern by-products leading to new components in the CL spectra. The CL experiment provides therefore the full set of excitation. In EL, the all set of processes can also be involved depending on the experimental conditions. We have shown that recombination between charges of opposite polarity provides the conditions for EL in polyolefins [START_REF] Laurent | Charge dynamics and its energetic features in polymeric materials[END_REF]. It has to be realized that recombination opens the way to chemiluminescence as well through the excitonic states created by the recombination process itself that can react with dissolved oxygen. Because a chemical pathway is open through reactive excited species, recombination can further affect newly formed chemical groups. In case of hot electron processes and although the kinetic energies is not comparable in CL experiments (a few eV in EL vs. a few keV in CL), there is still the possibility for impact processes, with or without ionization. Impinging electrons with a kinetic energy of a few eV will not have the energy required for impact ionization (15 eV) but they can excite species that will relax through fluorescence and phosphorescence emission, or be stabilized to form transient ions (negative attachmentsee [START_REF] Sanche | Dissociative attachment and surface reactions induced by low-energy electrons[END_REF]) able to subsequently dissociate into molecular fragments. Comparing CL and EL spectra, it appears that the components attributed to fluorescence of the natural chromophores and to chemiluminescence have a lower relative contribution in EL. However, EL spectra with different relative contributions of the elementary components have been recorded and this could be due to difference in the excitation conditions.

IV. CONCLUSION

We have compared the electroluminescence and cathodoluminescence spectra of thin polypropylene films. Like in other polymers, they are organized around the same components that we have tentatively assigned to specific molecular processes. It is striking their main components is due to byproducts of the polymer generated under an excited form. The results provide the evidence that electroluminescence is mainly related to molecular degradation and therefore to the ageing of the material under an electric field.

  . The former is known as Irganox 1010, Cite as: B. Qiao, G. Teyssedre, C. Laurent, "Field-and electron beam-induced luminescence phenomena in polypropylene thin films", Int. Conf. Properties and Applications of Dielectric Materials (ICPADM), Sydney, Australia, 19-22 July 2015. Proc. ICPADM-11, pp. 196-199, 2015. DOI: 10.1109/ICPADM.2015.7295242

  spectra of gold metallized BOPP films under 50 Hz AC at different fields. Spectra integrated for 5min at each field level. spectra of BOPP films under electron beam at different energies. Spectra were integrated for 10s a short time after irradiation start.

Fig. 1 :

 1 Fig.1: Electroluminescence (a) and cathodoluminescence (b) spectra of BOPP films

Fig. 2 :

 2 Fig.2: UV-absorption spectra (a), photo-induced fluorescence emission (b) and photo-induced phosphorescence emission (c) of BOPP films

Fig. 3 :

 3 Fig.3: Spectral components associated with the fourth elementary processes. Fluorescence spectrum -taken from Fig.2(a), chemiluminescence spectrum -taken from ref.[START_REF] Tiemblo | Chemiluminescence spectral evolution along the thermal oxidation of isotactic polypropylene[END_REF] (b), recombinationinduced luminescence -taken from ref.[START_REF] Teyssedre | Spectral analysis of optical emission due to isothermal charge recombination in polyolefins[END_REF] (c), unknown componentderived from fitting CL spectrum (d). Note that the spectra have been smoothed for eliminating noise.

Fig. 4 :

 4 Fig.4: Reconstruction of the CL and EL spectra on the basis of four elementary components. FL stands for fluorescence, CHL for chemiluminescence, RL for recombination-induced luminescence and peak 4: unknown component associated to by-products generated in excited states dissolved oxygen. It follows the possibility to emit light through the formation of carbonyl functions or polyene sequences (chemiluminescence). Impact ionization will be a source of positive charges and there is the possibility of charge recombination (generating light) when the incoming electron has already lost its kinetic energy, being trapped in the vicinity of the positive charge. Recombination can also occur between trapped electrons and trapped holes in diffusive recombination processes. In recombination processes, the emitting species will be those able to act as recombination centers i.e. those providing deep traps for electric charges. In PP, these centers are bound to CO species. Owing to the chemical degradation of the material under the electron beam, all these recombination processes can concern by-products leading to new components in the CL spectra. The CL experiment provides therefore the full set of excitation. In EL, the all set of processes can also be involved depending on the experimental conditions. We have shown that recombination between charges of opposite polarity provides the conditions for EL in polyolefins[START_REF] Laurent | Charge dynamics and its energetic features in polymeric materials[END_REF]. It has to be realized that recombination opens the way to chemiluminescence as well through the excitonic states created by the recombination process itself that can react with dissolved oxygen. Because a chemical pathway is open through reactive excited species, recombination can further affect newly formed chemical groups. In case of hot electron

TABLE I

 I

	: RELATIVE CONTRIBUTION OF THE FOUR ELEMENTARY COMPONENTS
	IN CL AND EL SPECTRA
	Component	328 nm	415 nm	505 nm	573 nm
	CL	1.5%	25%	1.5%	72%
	EL	0%	8%	5%	87%

Cite as: B. Qiao, G. Teyssedre, C. Laurent, "Field-and electron beam-induced luminescence phenomena in polypropylene thin films", Int. Conf. Properties and Applications of Dielectric Materials (ICPADM), Sydney, Australia, 19-22 July 2015. Proc. ICPADM-11, pp. 196-199, 2015. DOI: 10.1109/ICPADM.2015.7295242

ACKNOWLEDGMENT

Thanks to Steve Boggs and Kopafilm Elektrofolien GmbH for supplying BOPP films. Thanks to the CSC for PhD grant.


Bo Qiao

G Teyssedre

C Laurent

N Shimizu
email: shimizu@meijo-u.ac.jp.

The significance of electroluminescence in polyolefins

Keywords: Insulating polymers, electroluminescence, polyethylene, divergent field, uniform field, electrical ageing. I

Electroluminescence has been widely reported in large band gap insulating polymers under any kind of electrical stress, being ac, dc, impulse etc. but its significance is still the matter of debate. It was first investigated in the late seventies in divergent field experiments (i.e. using needle electrodes embedded in the material) as a precursor of electrical treeing. Later on in the nineties, uniform field experiments (i.e. using semi-transparent or transparent electrodes on top of a polymer film) were carried out. The main advantage of the divergent field configuration is the possibility to focus physical/chemical microanalysis in the tiny high field region around the needle tip for characterizing material degradation, which is not easy under uniform field owing to the low level of deterioration and its likely local character. The main advantage of the uniform field configuration is the possibility to correlate electroluminescence with real-time space charge distribution measurements as well as to acquire wavelength-resolved emission spectra. In this communication, we discuss the consistency of the experimental results obtained in divergent/uniform field experiments and the cross-fertilization between the two as regards the relationship between electroluminescence and material degradation.

INTRODUCTION

Insulating polymeric materials are used in a large variety of applications in electrical engineering either as bulk insulation or in thin films form, e.g. high-voltage insulated cables, windings of coils for motors, generators, transformers, capacitors, etc. They provide very good insulating solutions but are subjected to deterioration in service under the influence of electrical and thermal stresses, and other factors depending on the working environment (e.g. irradiation, humidity, etc.). Their use in practical applications is limited by their lifetime under a given stress, and not that much by their intrinsic properties [START_REF] Sun | 50+ Years of Intrinsic Breakdown[END_REF]. One of the main threats for the insulation is the existence of partial discharges (PD) eroding the polymer; solutions have been implemented to strengthen the insulation against PD when they are unavoidable (e.g. large generators, motors, etc.). However, there are practical situations where the existence of PD must absolutely be avoided, e.g. extra-high voltage cables with synthetic insulation, capacitors, etc. Moreover, the trend towards more compact electrical systems and electrical components with higher energy density involves pushing the limit in terms of electric field in the insulation.

There is therefore a renewed interest in the electronic properties of insulating polymers and in the driving factors in their intrinsic limits. One of the key challenges in investigating the electronic properties of this class of materials is to relate the basic transport mechanisms with critical transport regime where deterioration can occur, e.g. onset of hot electrons generation able to break atomic bonds and to generate defects [START_REF] Th | Field-enhancing Defects in Polymeric Insulators Causing Dielectric Aging[END_REF]. Detecting the optical emission from the investigated material under electric field (electroluminescence -EL) provides a way to couple transport processes and dissipative processes as a possible signature of deleterious reactions at the atomic level [START_REF] Roy | Charge transport and dissipative processes in insulating polymers: joining experiments and model[END_REF].

Electroluminescence of insulating polymers has been reported for a variety of materials for many years starting in 1967 [START_REF] Hartman | Electroluminescence in Organic Polymers[END_REF] where the first consistent experiments were performed on films of polyethylene, polyethylene terephthalate, etc. The experiments were carried out under vacuum for avoiding gaseous discharges in the atmosphere. The samples were pressed between two electrodes, one of them being a transparent and conductive glass. The emission spectrum was realized for polyethylene films using optical filters. In their conclusion, the authors propose that electrons injected from the electrodes or generated from bound states into the material are accelerated by the field and collide with some luminescent centers. Owing to the similarity with the luminescence excited by UV and by shorter radiation, they concluded that carbonyl groups should be at the origin of the emission itself. In spite of the novelty of their observation and interpretation, there was no report on EL under uniform field in the next few years. The topic was reactivated in the late 70ths when different groups started to work on the initiation phase of electrical treeing trying to understand the formation of trees in the absence of partial discharges [START_REF] Teyssedre | Advances in high field insulating polymeric materials over the past 50 years[END_REF]. Divergent field experiments dominate until the early 80ths when uniform field was reinvestigated for correlating EL and space charge. Since then, each type of experiment brought a piece of the puzzle with the physicalchemical analysis of the material deterioration in the luminescent volume around a needle electrode, and a clear correlation between electroluminescence occurrence, the existence of space charge and some specific regime of charge transport for uniform field experiments.

In this paper, we present a short summary of electroluminescence experiments carried out in divergent and in uniform fields and give the main conclusions drawn in each Cite as: Bo Qiao, G. case. In a second part, we introduce spectral analysis as a step forward in the understanding of electroluminescence. In a third part, we cross-correlate the results, show their consistency and propose an interpretation of the processes at play in both cases, leading to a unique picture of the physicochemical mechanisms involved in polyethylene under electrical stress.

II. ELECTROLUMINESCENCE FEATURES

A. Divergent field

A review of the topic has been published in [START_REF] Laurent | Optical Emissions due to Space Charge Effects in Electrically Stressed Insulating Polymers[END_REF][START_REF] Shimizu | Electrical Tree Initiation[END_REF] where the references cover the period up to the 1998, i.e. a time window where most of the divergent field experiments where developed. We just summarize below the salient features of the results:

-the emission comes from a tiny region around the needle electrode (which size is of the order of magnitude of the curvature radius at needle tip) and can be detected under ac, impulse and other time varying voltage waveforms (including grounding the electrode after dc charging). EL has never been reported under constant dc stress in divergent field. This was explained in terms of excitation conditions: EL needs either hot electrons production or bipolar recombination, or both processes acting together. Time varying stress provides conditions for both whereas dc stress under a divergent field leads to the formation of a stable space charge cloud around the needle which screens the field at the tip preventing further injection.

-EL is detected at any temperature but is more easily analyzed at low temperature (e.g. liquid nitrogen temperature): this is due partly to the increase of breakdown strength with a decreasing temperature allowing a higher stress to be applied without tree initiation, and partly to the quenching of excited states by the dissolved oxygen, a process which is inhibited at low temperature.

-EL is sensitive to the nature of the gas dissolved in the amorphous parts of polyethylene. Notably, degassing the sample before testing leads to a substantial increase of the time-to-tree initiation under a given stress. This has been explained by the reduction of the degradation rate of the material when the concentration of oxygen is lowered. Oxygen molecules in the polymer under stress act as a catalyst for the degradation.

-long term application of the stress (before electrical tree initiation) leads to the formation of a spherical region at the needle tip with different optical properties than the rest of the material. Microanalysis of the material properties in this region called for physical chemical deterioration characterized by: (i) an increase of C=C and C=O bonds (using FTIR); (ii) the apparition of free radicals (using ESR); (iii) the formation of small voids (< 0.5 m) (using SEM); (iv) damage to the structure of the lamellae (using TEM). This clearly indicates a degradation of the material in the region of higher stress. This domain has been called "deteriorated region".

-the diameter of the EL emitting zone and the diameter of the deteriorated region increase with the increase of the stress (see Fig. 1). Moreover, emitting region and deteriorated region have the same diameter for a given stress. This allows to associate degradation and EL in a framework where both processes occur in the same volume. This does not demonstrate that EL is the source or a consequence of the degradation but that both phenomena are linked.

-the question of a threshold for EL onset has been discussed at length in the literature because it was believed it could be used as a material intrinsic characteristic in that above this field, rapid degradation takes place from hot electrons and carrier recombination. Within the Field Limiting Space Charge theory [START_REF] Hibma | Direct measurement of space-charge injection from a needle electrode into dielectrics[END_REF], the EL threshold corresponds to a critical field above which injected charges become highly mobile at needle tip leading to the expansion of a space charge cloud which density increases as the square of the applied voltage. This has been checked in different materials [START_REF] Hibma | Direct measurement of space-charge injection from a needle electrode into dielectrics[END_REF][START_REF] Cao | Guarded needle for charge injection measurement[END_REF] and preliminary measurements presented in [START_REF] Baumann | Charge Injection and Electroluminescence in the Prestate of Dielectric Aging[END_REF] tend to show the EL onset is the critical field. However, measurements on polyethylene contrast with this view and the existence of a steep increase in carrier mobility has not been proven. EL is explained in terms of recombination between charges alternatively injected in positive and negative half cycles of the ac voltage. In that case, a physical threshold can be defined if the measurement is not limited by the sensitivity of the set-up (experimental limitation) because the process of charge injection is highly non-linear. Space charge could be injection limited as opposed to the FLSC model where it is mobility limited. Diameter of luminescence region and deteriorated region as a function of applied voltage. From [START_REF] Shimizu | Electrical Tree Initiation[END_REF].

B. Uniform field

A review of the topic has been published recently [START_REF] Laurent | Charge dynamics and its energetic features in polymeric materials[END_REF] where the references cover the period up to the year 2012, i.e. a time window where most of the uniform field experiments were developed. We just summarize below the salient features of the results:

-EL is detected at any temperature and under any kind of stress. Contrary to the case of a divergent field, the uniform field provides conditions for charge injection at both electrodes. Bipolar recombination can therefore occur after the injected charges have transited through the thickness of the sample (being usually 20 m to several 100 m thick in lab's experiments).

-Combined EL and real-time spatially-resolved space charge measurements under dc field show that bipolar charge recombination is the main driving factor in EL excitation under dc stress. Numerical models incorporating bipolar charge injection, transport, trapping and recombination have been developed that reproduced nicely the main features of EL. -Combined EL and real-time spatially-resolved space charge measurements under 50 Hz ac field demonstrate that the conditions for EL excitation exist in the vicinity of the injecting electrodes. Numerical modelling based on the same physical processes as under dc shows that a much higher density of states has to be introduced at the interface electrode/polymer when compared to the bulk density of states. ac stress provides the conditions for charge recombination in a thin region (a few microns thick) in contact with the electrodes.

-The question of a threshold for EL emission has also been discussed in the situation of a uniform field. Under ac stress, the same conclusion is drawn as in the case of a divergent field: EL seems injection limited rather than mobility limited and a threshold can be defined at the onset of nonlinearity in the charge/field characteristic. Under dc stress and in the absence of any internal space charge prior to the application of the stress, a threshold in time can be defined at a given stress, being the transit time of injected carriers that need to interact for recombination to occur [START_REF] Roy | Charge transport and dissipative processes in insulating polymers: joining experiments and model[END_REF]. Once the time under stress is greater than the carrier transit time, there is a non-zero probability for bipolar charge recombination and therefore no threshold for EL emission. However, the recombination rate is highly non-linear in field and a threshold can be defined at the onset of the non-linearity.

III. SPECTRAL ANALYSIS

A. Divergent field

Interference optical filters (typically 50 nm width) covering the entire visible range (300 nm to 800 nm) were most often used in the light path of a photomultiplier (PM) in order to get a rough evaluation of the emission spectrum of polyethylene. Wavelength-resolved spectral analysis was seldom used because of the faint emission, but some data were collected at liquid nitrogen and room temperature. These data have been replotted in Figure 2. It can be seen that the spectra obtained by different groups are consistent with an emission maximum between 500 nm and 600 nm, and a low contribution at lower wavelengths.

B. Uniform field

Wavelength-resolved emission spectra from polyethylene films under a uniform field have been investigated thoroughly by coupling a low dispersion grating to very sensitive detectors based on CCD (charge-coupled device) technology [START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF]. An example of such spectra acquired under ac stress is shown in Figure 3. They exhibit similar structures as the spectra obtained under divergent field with a maximum of emission at 573 nm and a small contribution at lower wavelengths. A contribution is seen in the domain 700 nm-800 nm and this was ascribed to radiative surface plasmons excited at the surface of the metallic electrodes, a process well documented in Metal-Insulator-Metal tunnel junctions and first reported in Metal-Polymer-Metal sandwich in 1999 [START_REF] Teyssedre | Identification of the components of the electroluminescence spectrum of PE excited in uniform fields[END_REF]. Emission spectra from LDPE samples tested under a divergent field configuration and ac stress, from different groups: square from [START_REF] Bamji | Degradation of Polymeric Insulation Due to Photoemission Caused by High Electric Fields[END_REF], triangle from [START_REF] Shimizu | The Space Charge Behavior and Luminescence Phenomena in Polymers at 77 K[END_REF], circle from [START_REF] Pietsch | Influence of Space Charges and Content of Oxygen on the Luminescent Light and Dielectric Aging Effects[END_REF], and diamond from [START_REF] Laurent | Mechanisms of Electroluminescence during Aging of Polyethylene[END_REF]. All the spectra were obtained using interference filters, except the data by Pietsch [START_REF] Pietsch | Influence of Space Charges and Content of Oxygen on the Luminescent Light and Dielectric Aging Effects[END_REF] which were acquired with a spectrometer. Fig. 3.

Emission spectra from LDPE film at different fields (uniform field, 50 Hz ac). From [START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF].

C. Interpretation of the electroluminescence spectra

Electroluminescence spectra have been recently interpreted in polyethylene and polypropylene with the help of a panoply of luminescence spectroscopies (including photo-, chemi, cathodo-and recombination-induced luminescence) [START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF]. They can be reconstructed from three elementary components being (i) chemiluminescence emitted during carbonyl functions formation with a typical signature at 415 nm, (ii) recombination-induced luminescence on deep traps acting as recombination centers (natural chromophores of the polymer) with a typical signature at 505 nm and (iii) a component peaking at 573 nm and tentatively assigned to by-products of field-induced reactions. This is shown in Figure 4 where the spectrum of Figure 3, once corrected from the contribution of surface plasmons, is reconstructed on the basis of the elementary components -see [START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF]. Surprisingly, there is no contribution of fluorescence of the polymer chromophores contrary to the case of cathodoluminescence spectra. This has been explained by a quenching of the singlet exciton (weakly bounded) by the electric field. Reconstruction of the electroluminescence spectrum of polyethylene on the basis of three elementary components, as explained in Table I. Taken from [START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF]. CHL stands for chemiluminescence; RL stands for recombinationinduced luminescence; EL stands for electroluminescence; Peak 4 is associated with field-induced by-products of PE. situations are the same. Emission spectra are reconstructed using three elementary components. One of those is chemiluminescence (C=O* > C=O) which indicates that fieldinduced oxidation is taking place. This is consistent with a degradation scheme already proposed for polyethylene where radicals generated under stress react with oxygen initiating an auto-oxidation process [START_REF] Shimizu | Electrical Tree Initiation[END_REF]. It follows the formation of excited species, radicals, hydroperoxide and C=O bonds. The scheme is supported by the physical chemical analysis of the deteriorated region. One other component of the EL spectrum is due to charge recombination on the natural chromophores of the polymer (conjugated C=C and C=O unsaturations of the polymer chain). Bipolar recombination has been advocated as the source of electroluminescence under divergent and uniform fields. This gives a totally consistent picture of the physical processes at play. Finally, the third component of the EL spectrum has been ascribed to some radiative reaction involving by-products. With the opening of a chemical route, by-products could be produced in the excited state and be responsible for emission in a wavelength range that is not typical of the initial chemistry of the polymer. The main peak in electroluminescence could reveal such degradation process.

A still open question is the localized or delocalized character of the degradation under uniform field. Under dc stress, the recombination domains -where degradation is occurring, is within the bulk. Under ac stress, the EL is occurring at the interfaces electrode/polymer where the electronic properties control the build-up of local currents which are possible sources of local degradation.

V. CONCLUSION

The significance of electroluminescence from insulating polymers has been discussed since a long time, especially in polyethylene-based materials. Excitation mechanisms and the relationship to electrical degradation were the most tackled questions. We demonstrated that divergent and uniform field experiments carried out on polyethylene provide consistent and complementary results leading to a global picture. EL tells us that chemical reactions induced by the field are ongoing and as such constitutes a warning for the material withstanding. It also opens several routes for investigating physical chemical processes associated with charge injection and space charge.

Fig. 1 .

 1 Fig. 1.Diameter of luminescence region and deteriorated region as a function of applied voltage. From[START_REF] Shimizu | Electrical Tree Initiation[END_REF].

  Cite as: Bo Qiao, G. Teyssedre, C. Laurent, N. Shimizu, "The significance of electroluminescence in polyolefins", IEEE Int. Conf. on Dielectrics (ICD), Montpellier, 3-7 July 2016. Proc. ICD, pp. 856-859, 2016. DOI: 10.1109/ICD.2016.7547751Location of the domains where the recombination rate is the highest changes as a function of the stress (which controls charge density and their location).

Fig. 2 .

 2 Fig. 2.Emission spectra from LDPE samples tested under a divergent field configuration and ac stress, from different groups: square from[START_REF] Bamji | Degradation of Polymeric Insulation Due to Photoemission Caused by High Electric Fields[END_REF], triangle from[START_REF] Shimizu | The Space Charge Behavior and Luminescence Phenomena in Polymers at 77 K[END_REF], circle from[START_REF] Pietsch | Influence of Space Charges and Content of Oxygen on the Luminescent Light and Dielectric Aging Effects[END_REF], and diamond from[START_REF] Laurent | Mechanisms of Electroluminescence during Aging of Polyethylene[END_REF]. All the spectra were obtained using interference filters, except the data by Pietsch[START_REF] Pietsch | Influence of Space Charges and Content of Oxygen on the Luminescent Light and Dielectric Aging Effects[END_REF] which were acquired with a spectrometer.

  Fig.[START_REF] Hartman | Electroluminescence in Organic Polymers[END_REF].Reconstruction of the electroluminescence spectrum of polyethylene on the basis of three elementary components, as explained in TableI. Taken from[START_REF] Qiao | Electroluminescence and cathodoluminescence from polyethylene and polypropylene films: spectra reconstruction from elementary components and underlying mechanisms[END_REF]. CHL stands for chemiluminescence; RL stands for recombinationinduced luminescence; EL stands for electroluminescence; Peak 4 is associated with field-induced by-products of PE.

  Teyssedre, C. Laurent, N. Shimizu, "The significance of electroluminescence in polyolefins", IEEE Int. Conf. on Dielectrics (ICD), Montpellier, 3-7 July 2016. Proc. ICD, pp. 856-859, 2016. DOI: 10.1109/ICD.2016.7547751

TABLE I .

 I CONTRIBUTION OF EACH ELEMENTARY COMPONENT IN LDPE EL

		SPECTRUM RECONSTRUCTION
	Chemiluminescence	Recombination-induced	Polymer by-
	in its early stage	luminescence	products
	Peak max at 415 nm	Peak max at 505 nm	Peak max at 573 nm
	11.5%	8.5%	80%