Applied Surface Science 255 (2008) 2735–2739

Space charge and electroluminescence characteristics of thermallyaged LDPE films

Kai Yang, Guan-Jun Zhang *, De-Min Tu, Zhang Yan

State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an, Shaanxi 710049, China

A R T I C L E I N F O

Article history:

Received 27 December 2007

Received in revised form 4 April 2008

Accepted 7 August 2008

Available online 31 August 2008

Keywords:

Space charge

Electroluminescence (EL)

Fourier transform infrared (FTIR)

absorption spectra

Pulsed electro-acoustic (PEA)

A B S T R A C T

Polymeric insulation aging is closely related to the change of its physical and chemical properties. An

attempt is tried to correlate the space charge and electroluminescence (EL) behaviors of aged low-density

polyethylene (LDPE) samples films. LDPE samples were aged in a thermal aging oven with five different

time intervals (120, 240, 360, 480 and 600 h). After thermal aging, their molecular structures were

analyzed via the Fourier transform infrared (FTIR) absorption spectra, and due to oxidation reaction, two

new peaks appeared at �1725 and 3380 mm corresponding to carbonyl and hydroxyl groups,

respectively. Then the pulsed electro-acoustic (PEA) method was employed to measure the space charge

distribution for each sample, and aged samples showed a higher space charge distribution. EL phenomena

from aged LDPE surface were investigated under stepped ac voltages, and aged samples showed a

relatively lower EL onset voltage and intensity than that without aging. It is considered that EL occurs due

to the radiative recombination of electrons and holes injected from both electrodes. New products due to

the oxidation reaction can result in more charge traps, which will reduce the surface contact potential

barrier and also enhance the charge accumulation inside, and hence lead to the space charge and EL

phenomena observed.

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1. Introduction

Since 1960s, polymeric materials such as low-density poly-ethylene (LDPE, which is polymerized by the single polyethyleneunder the pressure of 98–294 MPa. Its density is of 0.910–0.925 g/cm3, and it has a semi-crystal and amorphous structure, as shownin Fig. 1) have been widely used as electrical insulation because oftheir excellent physical and chemical properties. However, thelong time electrical insulation failure is an important problem,which seriously restricts their further application. Generallyelectrical treeing is responsible for the breakdown of the polymerinsulation material, and it has two distinct periods: the inductionperiod corresponding to the initiation phase, and the propagationperiod during which partial discharge (PD) takes place and the treegrows till the breakdown [1]. Before the first phase, the weakelectroluminescence (EL) phenomena have been observed bymany researchers, and some of them believed that its ultravioletcomponent will induce the degradation of insulation materials, soEL can be used as a prebreakdown warning and provide a sensitivemethod to diagnose the degradation of polymer material [2–4].

* Corresponding author. Tel.: +86 29 82668172; fax: +86 29 82668172.

E-mail address: gjzhang@mail.xjtu.edu.cn (G.-J. Zhang).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.08.070

Essentially EL emission is a dynamic response of chargestransporting in a polymer, closely related to the trap and spacecharge behaviors [5]. EL under high ac electric fields is considereddue to the recombination of electrons and holes injected intopolymer insulation material from the electrodes. Actually there arelots of physical and chemical defects or other impuritiesdistributing in polymeric materials, which will act as traps forcapturing the injected charges and form space charges, and thespace charges will distort the local field stress and affect theinjection and drawout of charges from the electrodes [6,7], also itwill affect the EL characteristics.

Space charge measurement has been considered as an importanttool to investigate the trap property of solid dielectrics. In lastdecades, its measurement techniques have been greatly developed,several techniques such as the thermal pulse method [8,9], thepressure wave propagation method [10] and the pulsed electro-acoustic (PEA) method [11,12], have been developed to nondestruc-tively measure space charge distribution. Here we use the PEAmethod to measure the space charge distribution in samples,comparing with other techniques, the PEA method can be used tomeasure the thicker dielectric material, and even the on-site spacecharge distribution [13].

In this paper, we do not focus on how the breakdown ofpolymers occurred, but on how the characteristics of aged

Fig. 1. Chemical structure and sketch map of low-density PE.

Fig. 3. Photon counting system for EL measurement.

K. Yang et al. / Applied Surface Science 255 (2008) 2735–27392736

polymers changed by measuring the infrared spectra, and therelationship of space charge and EL characteristics of aged LDPEsamples, and hope to find a sensitive diagnostic method ofpolymeric insulation aging.

2. Experimental arrangement

The raw LDPE materials (Daqing 18D) are mixed uniformlyusing a blender with the temperature of 363 K, and then thesample is pressed in a vulcanizing machine. The thickness of eachsample is 0.4 mm. Different LDPE samples are aged in a thermalaging oven with five periods of 120, 240, 360, 480 and 600 h. TheFourier transform infrared (FTIR) absorption spectrum (ShimadzuFTIR-8300) is used to analyze the component of aged samples.

Fig. 2. Space charge me

PEA method is used to measure the space charge distribution inLDPE samples with different aging periods, and the measurementdevice is shown in Fig. 2. It is made up of four parts: (1) 0–20 kVcontrollable power source; (2) 1 kV pulse generator; (3) 30 mmpolyvinylidene fluoride film piezoelectricity sensor; (4) 400 MHzdigital oscilloscope and computer system. When the pulsepressure is applied on the space charge, it will bring the mechanicalstress, which is converted into electric signal. We can get the spacecharge distribution by analyzing the signal [13].

Each LDPE sample is employed to investigate its EL phenomenawith the setup shown in Fig. 3. Each sample is mounted in avacuum chamber connected to a mechanical pump and a turbomolecular pump to ensure the vacuum level of �10�4 Pa. Allexperiments are performed in vacuum to avoid the ambientinfluences. A photon counting module (Perkin–Elmer SPCM-AQR)is sensitive enough to detect the single photon with thewavelength of 400–1060 nm from EL emission, which is madeup of the avalanche photodiode as the detector, the active resetcircuit and detector temperature controller. The output TTL pulses

asurement device.

K. Yang et al. / Applied Surface Science 255 (2008) 2735–2739 2737

with 5 V high in a 50 V load and 30 ns width are recorded by a self-constructed photon counting card.

3. Results

3.1. FTIR of LDPE samples

After the LDPE samples are heated for different hours in thermalaging oven, the FTIR absorption spectra are employed todistinguish their molecular structures, as shown in Fig. 4. Fig. 4a

Fig. 4. FTIR of unaged and aged LDPE samples: (a) 0 h;

indicates the LDPE without aging, and comparing it with others, wecan find that at �1725 cm�1 between 1900 and 1650 cm�1 and atabout 3400 cm�1 between 3750 and 3000 cm�1, there are two newabsorption peaks appeared in others figures, corresponding to theabsorption peak of nC O and nOH� . They are considered due to theoxidation reaction of LDPE molecule chain and its branch chainwhen the LDPE samples are aged in the heating oven, and theoxygen gas reacting with the carbon atom or the hydrogen atomattached to the main carbon chain will produce the carbonyl andthe hydroxy.

(b) 120 h; (c) 240 h; (d) 360 h; (e) 480 h; (f) 600 h.

Fig. 6. Relation between EL intensity and applied voltage.

K. Yang et al. / Applied Surface Science 255 (2008) 2735–27392738

3.2. Space charge measurement in LDPE

The space charge distribution experiments are performedwith each LDPE samples placed as in Fig. 2. The DC electric fieldis kept at 40 kV/mm for 25 min, and at the same time the voltagepulses are applied with the amplitude of 200 V and a width of20 ns. Fig. 5a shows the typical space charge distribution in anunaged sample under gradually increased electric fields, andFig. 5b exhibits the space charge distribution in six samples(one unaged and five aged samples) under a fixed field of 40 kV/mm. In Fig. 5a, with the field enhancement, the space chargedensity obviously becomes larger. In Fig. 5b, we find that theelectron and hole space charge distribution is different fromeach other. The space charge distribution is complicated, andcompared with the LDPE without aging, with the increased agingtime, there are new charge profile appeared between thenegative and positive electrode. Space charges reflect thephysical defect existence and location in the samples [14], sowe consider that there are new traps forming in aged LDPEsamples.

3.3. EL measurement from LDPE

The LDPE samples with different aging time are in turn putinto the chamber to measure the EL intensity. The 50 HzHVAC voltage is applied for every 2 min with the step of 200 Vfrom the initial voltage of light emission from the surface ofsamples. For each sample, the experiment is repeated for four

Fig. 5. Space charge distribution in unaged and aged LDPE samples: (a) space charge

distribution under different electric fields and (b) space charge distribution of

different aging time LDPE under 40 kV/mm.

times. Then we calculate the average photon counts underdifferent voltage steps, as shown in Fig. 6. We can find that ELintensity is the function of the applied voltage, and foraged samples, their EL initial voltage is lower than that of thesample without aging. The EL intensity of aging samplesdecreased with aging periods, but they do not show a simplelinear relation.

4. Discussion

For LDPE samples, when ethylene is polymerized into macro-molecule chain, the double bond is broken forming the (–CH2–CH2–)n chain of methylene, which mainly behaves the threecharacteristic peaks of infrared spectra, i.e., the stretchingvibration peak nC–H (–CH–) with absorption peak values of 2926and 2835 cm�1, the flexural vibration peak dC–H (–CH–) with peakvalue of 1468 cm�1, and the flexural vibration peak dC–H (–CH–)n

(n > 4) with peak value of 720 cm�1. When LDPE is being aged inthe heating oven, the sample will react with the oxygen in air. Theoxidation reaction is shown in Fig. 7.

There are two phases during the aging process of polymericmaterial, i.e., the first is the accumulation of hydroperoxides, andthe oxygen absorption is not great in this process, and the secondis the auto-oxidation of hydroperoxides with the carbonylappeared. After the oxidation reaction, the new element nC O

and nOH� occured, compared with methylene, there are obviousdifferences in molecular mass, electron structure and atomicvalence among them. The different molecular weight will locallyaffect the crystal lattice vibration, and the different structure andatomic valence will induce new energy eigenstate, which will actas traps and decrease the density of charge carriers. As a kind ofsemi-crystal and amorphism materials, the macromolecule chainof LDPE has the crystal property, and there will be definitelysome structure defects in the crystal above 0 8C [15]. Under alower temperature, the thermal defect density is not large, butwhen it is put into the aging oven, the thermal defects increaseand affect the dielectric performance. Henceforth, the new

Fig. 7. Oxidation reactions occurred in LDPE.

K. Yang et al. / Applied Surface Science 255 (2008) 2735–2739 2739

produced traps will capture the injected holes or electrons andthus change the space charge distribution, as shown in Fig. 5b.

From Fig. 5b, we can find that, the space charges are hetero-charges near the Al(+) electrode, and near the Cu(�) electrode isthe homo-charges. The hetero-charges occur because of the dipolepolarization and the impurity ionization migration, and the homo-charge appearance is because of the charge injection [16]. We alsofind that space charges of aging samples show different profileshapes. For samples aged with 0, 240 and 360 h, the space chargesnear two electrodes are almost symmetrical, and for samples agedwith 120 and 480 h, the charges near the Cu electrode is larger thanthat near the Al electrode, while it is contrary to the space charge ofsample aged with 600 h. The possible reason is due to the change ofbulk trap distribution inside polymeric material during the agingprocess, and also we think that there must be some change ofsurface states, that will change the contact potential barrierbetween electrode and material, and hence affect the injection ofholes and electrons from electrodes.

Fig. 6 shows different EL characteristics of samples with differentaged time under stepped voltages. Above 4.6 kV, the EL intensity ofLDPE aged for 0, 120, 240 and 360 h decreases with the aging time,indicating the inverse proportion, but the EL intensity of samplesaged for 480 and 600 h display is larger than that of sample aged for360 h and is less than that of sample without aging. EL under acvoltage is resulted from the recombination of holes and electronsinjected with the trapped charges, especially in a thin surface layer ofthe polymer [17]. It can be shown in Fig. 8 [18]. Under the alternativeelectric fields, when the applied field stress value is beyond thecontact potential barrier between electrode and material surface,the holes and electrons accelerated by the high field obtaining theenough energy and injected into the polymer through the band gap,some of them recombine each other and the released energydisplays by photon emission. After a LDPE sample is aged in agingoven, its surface barrier is considered to be reduced due to theincrease of surface traps after the oxidation reaction, which willgreatly affect the injection of charges. On the other hand, the bulktraps in material also increase during the aging process, whichchanges the space charge distribution and further distorts theelectric field, and also affects the charge injection. So the ELcharacteristics will be greatly changed due to the two effects. Thelower contact potential barrier after aging makes the chargeinjection easier at the area near electrodes, which is responsiblefor the decrease of EL initial voltage for aged LDPE. Moreover, thehigher bulk traps after aging consequentially lead to the enhance-ment of charge trapping and higher space charge accumulation,which would reduce the recombination probability of electrons andholes injected, and also greatly distort the local electric field stress,

Fig. 8. Schematic representation of EL mechanism.

and hence restrict the injection of charges, and finally lead to thelower EL intensity. Thus the experimental results in Figs. 4–6 areactually consistent with each other. However, the abnormal ELphenomena of LDPE samples aged for 480 and 600 h in Fig. 6 need tobe further researched.

5. Conclusions

Thermally aged LDPE samples with different time intervals areemployed to explore the relation between space charge and ELcharacteristics of polymeric insulation. The appearance of newbranch chain nC O and nOH� due to oxidation reaction leads to theenhancement of charge traps and further the change of spacecharge distribution inside LDPE samples. The experimental resultsof aged LDPE samples reveal a lower critical voltage and ELintensity than that of unaged, which can be ascribed to thereduction of surface potential barrier and the trapping effect ofinjected charges, respectively. It is considered that EL is apromising method to reveal polymeric insulation aging condition.

Acknowledgment

This work is in part supported by the Program for New CenturyExcellent Talents in University (NCET-04-0943) and ExcellentYoung Teachers Program of MOE, China.

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