space charge studies in ldpe using combined isothermal and non-isothermal current measurements

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 11, No. 1; February 2004 25

Space Charge Studies in LDPE Using CombinedIsothermal and Non-isothermal Current Measurements

M. Carmo Lanca, E. R. Neagu, R. M. Neagu, C. J. Dias, J. N. Marat-MendesDepartamento de Ciencia dos Materiaisˆ

Ž .Seccao de Materiais Electroactivos CENIMAT˜Faculdade de Ciencias e Tecnologiaˆ

Universidade Nova de Lisboa, Portugal

and D. K. Das-GuptaSchool of Informatics

University of Wales, Bangor, UK

ABSTRACTUsing a recently developed procedure combining isothermal and non-isothermalcurrent measurements space charge trapping and transport in LDPE was success-fully studied. Unaged, thermally and electrically aged samples were investigated.The samples were conditioned before each measurement in order to obtain repro-

(ducible results. In the non-isothermal measurements appeared a broad peak 40�C) ( )to 50�C that was possible to decompose into two or three peaks 35, 45 and 65�C .

( )At even higher temperature another peak was sometimes present 85�C dependingon the prior sample conditioning. The space charge is trapped near the surface in

( )deep traps maximum depth of f15 �m . Relaxation times, mobilities and activa-tion energies have been calculated for different chargingrrrrrdischarging conditions.For unaged samples the reproducibility of the results was poor while for the agedpolyethylene it was quite good, meaning that aging helps conditioning. In the elec-trically aged LDPE there is a decrease of conductivity and the broad peak of thenon-isothermal spectra shows a slight shift towards higher temperatures whencompared with the data found in the thermally aged polymer.

Index Terms — Power cables, high insulating polymers, electrical conduction,electrical aging, space charge, isothermal current, non-isothermal current, polyeth-ylene, relaxation time, mobility, activation energy.

Table of acronymsPE polyethyleneLDPE Low density polyethyleneTSDC Thermally stimulated discharge currentsICC Isothermal charging currentIDC Isothermal discharging currentFTSDC Final thermally stimulated

discharge currentFIDC Final isothermal discharging currentSC Space charge

1 INTRODUCTIONOR highly insulating polymers, such as poly-F Ž .ethylene, space charge SC can remain trapped for

Ž .very long times traps have long relaxation times . After

Manuscript recei®ed on 9 September 2002, in final form 21 May 2003.

an experiment involving sample charging and dischargingit is possible that a residual SC remains and can even goundetected in discharging current measurements. How-ever if a non-isothermal experiment, such as thermally

Ž .stimulated discharge currents TSDC , is performed theinfluence of the remnant charge is visible. Consequentlyusual TSDC measurements are highly affected even ifconventional methods for isothermal dc charging and dis-charging measurements are sometimes unable to reveal thepresence of this SC in the sample. Therefore it is difficultto obtain reproducible results if there is not a way to con-trol the remnant SC. Also the analysis of the results istroublesome. For polyethylene, especially low density

Ž .polyethylene LDPE , there have been many attempts toŽ .understand SC de trapping and transport. Single mea-

surements, either isothermal or non-isothermal, have pro-w xduced results both difficult to reproduce and interpret 1 .

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Lanca et al.: Space Charge Studies in LDPE Using Combined Isothermal Measurements26

To overcome these problems, in this work, a recentlydeveloped procedure combining isothermal charging and

w xdischarging methods and non-isothermal measurements 2w xwas used and adapted for LDPE 3,4 . This combined pro-

Žcedure is an improvement of the FTSDC final thermally. w xstimulated discharging current method 5 and will be de-

scribed in detail in the next section.

2 THE COMBINED ISOTHERMAL ANDNON-ISOTHERMAL PROCEDURE

The procedure combines different measurements andthis combination ensures an almost complete discharge ofthe sample at the end of a complete experimental run.Reproducibility and good analysis are achieved by a care-ful control of the experimental parameters. Each run is

Žtime consuming and composed of different steps see Fig-.ure 1 :

Ž . Ž .i and ii Isothermal dc charging and discharging cur-Žrents measurement ICC - isothermal charging current and

.IDC - isothermal discharging current . The sample isŽ .charged while kept at constant temperature T and a dci

Ž .field E is applied during time t and the ICC registered.c

Then the field is switched off and the sample is short-cir-cuited, discharging during time t still at constant temper-d

ature T and the IDC is recorded. These two steps corre-i

spond to a selective charging of the sample.

Ž .iii Non-isothermal discharging current measurementŽ .FTSDC - final thermally stimulated discharge current inwhich the sample is heated at a low constant rate b.

Figure 1. Experimental procedure used to analyze space charge inLDPE. The four steps are schematically represented showing electricfield, temperature and current. Selective charging corresponds to

Ž .isothermal charging and discharging ICC and IDC . Partial dis-charge proceeds during FTSDC. Almost complete discharge is only

w xachieved during FIDC. Schematics adapted from 3 .

Ž .iv Final isothermal discharging current measurementŽ . Ž .FIDC at the final temperature of iii to allow as muchas possible a complete discharge of the sample and theFIDC is registered.

A careful selection of charging and discharging condi-Žtions field strength, temperature and ratio of charging and

.discharging times allow distinct features of SC trapsŽ .activation energy and relaxation times to be revealed inthe following FTSDC. The FIDC at the final temperatureof the FTSDC allows almost complete discharge of thesample. In this way the sample is conditioned so that itsprior measurement history does not influence the nextmeasurement impairing reproducibility and interpretationof results. The complete procedure is repeated in eachrun. Experiments following this full procedure are time

Ž .consuming particularly the FIDC step but are unfortu-nately unavoidable.

3 EXPERIMENTAL3.1 SAMPLE PREPARATION AND AGING

In this work LDPE disk shaped films of approximatly200 �m thick were hot pressed molded from Borealis pel-lets almost additive free. For aging the useful disc areahad a diameter of 35 to 40 mm. The aging configuration

w xused a modified Cigre cell and was presented in 6 .´Three different types of samples were used:

ŽA�thermally aged in solution during 1500 h at 40�C.in 1M NaCl ,

ŽB �electrically aged in solution applied ac electric field.of 6 kVrmm, 50 Hz during 1500 h at 40�C in 1M NaCl

presenting water trees andŽ .C �unaged just hot press molded .

3.2 COMBINED MEASUREMENTSPROCEDURE

Ž .The first LDPE samples type A to be studied follow-ing this procedure were also used to establish the best

Žmeasurement parameters field, temperature and charg-.ingrdischarging times ratio to be used in charging and

Ž .discharging selective charging on the subsequent sam-ples. The measurement setup has been described else-

w x Ž .where 7 . For measurements after aging Al electrodeswith 33 mm diameter were vacuum evaporated on bothsurfaces of the disk shaped samples. All experiments wereperformed under rotary pump vacuum.

The method followed was implemented using type Asamples by a thorough test of the different conditions. Forthis sample charging and discharging temperatures of 2,10, 20 and 30�C were studied. The dc field strength wasalso investigated over the range 1 to 5 kVrmm. The influ-ence of the ratio of charging and discharging times wasalso analyzed. The FTSDC was performed always at aheating rate of 1�Crmin from the charging temperature

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Ž . Ž .T up to 90�C T . This final temperature was kept longi f

enough time to ensure almost complete discharge of theremnant SC after each FIDC step.

4 RESULTSŽ .As was stated above the thermally aged LDPE type A

was used to investigate the best conditions to perform theexperiment in order to obtain consistent and reproducibleinformation about SC trapping. Previously published re-

w xsults 3 focused mainly on the influence of theŽ .chargingrdischarging time ratio t rt and the influencec d

of the FIDC step was also analyzed. The effect of charg-Ž . Ž .ing temperature T and electric field E is presentedi

here. In Figure 2 are shown the currents measured duringthe four steps of each run for different charging fields fora charging temperature of 30�C and t rt s1hr2h. Thec d

Ž . Ž .charging ICC and discharging IDC curves in Figures 2aand 2b, respectively, are typical results for LDPE and havea magnitude in good agreement with the values found in

w xthe literature 8 . Under these measurement conditions itis usual that the FTSDC spectra obtained have a broadpeak around 40 to 60�C, as is observed approximately inFigure 2c. However if the FIDC step has not proceeded

for long enough time it does not guarantee nearly com-plete discharge of the remnant charge and another peak

w xaround 85�C appears 3 . As can be also observed in Fig-ure 2c the current is not linearly dependent on the ap-plied electric field, which is contrary to what happens with

Ž .the ICC and the IDC Figures 2a and 2b . This responseis typical of mechanisms that are SC dominated and notof dipolar origin as will be discussed in more detail in

Ž .Section 5. The FIDC data Figure 2d were recorded untilthe setup measurement limit was reached. From the plotit is seen that at higher fields this limit is achieved at longertimes. Consequently, it can be concluded that morecharges are deposited in traps with longer relaxationstimes as the charging field is increased.

The effect of chargingrdischarging temperature is seenŽ . Ž .for the FTSDC a and the FIDC b steps in Figure 3.

The maxima in the FTSDC plots are shifted towardshigher temperatures with the increase of charging temper-ature. The current noise also seems to increase with T upi

to 20�C. In order to make the residual charge vanish it isŽ .clear from the analysis of the FIDC data Figure 3b that

higher T results in larger currents and the need for longeri

discharging times.

Ž .Figure 2. Thermally aged LDPE type A results for different charging fields. Chargingrdischarging conditions: T s30�C, t rt s1hr2h andi c dEs1 to 5 kVrmm. a, ICC; b, IDC; c, FTSDC; d, FIDC.

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Ž .Figure 3. Thermally aged LDPE type A results for differentchargerdischarge temperatures. Chargingrdischarging conditions: Es2 kVrmm, t rt s1hr2h and T s30, 20, 10 and 2�C. a, FTSDC; b,c d iFIDC.

While the ICC and the IDC steps are little affected byŽa less careful control of the charging procedure as it is

.described in Section 2 , the FTSDC is greatly influenced.However a good reproducibility can be obtained for runsperformed under the same controlled measurement con-

Ž .ditions see Figure 4 .

Figure 4. FTSDC for two different runs in the same thermally agedLDPE sample with the same chargingrdischarging conditions: T si30�C, Es2 kVrmm, t rt s1hr2h. The reproducibility of the spectrac dis quite good. It is also dependent on the FIDC’s time to be longenough to guarantee an almost complete discharge of the sample.

Ž .Figure 5. FTSDC plot showing jrE thermally aged LDPE type Afor different charging fields. Chargingrdischarging conditions: T si2�C, t rt s1hr2h and Es2 and 4 kVrmm.c d

The non-linear behavior with charging field observed fortype A samples at T s30�C is, as expected, also seen wheni

Ž .the charging temperature is lower T s2�C in Figure 5 .iŽ .The data represented by the dotted line 4 kVrmm shows

the influence of the remnant charge from the previous runfor which the FIDC duration was too short. Moreover thishigh temperature contribution partially masks the lowerbroader peak making more difficult the analysis of thethermograms. Once more the importance of the last FIDCstep is perceived.

Ž .The electrically aged LDPE type B samples was stud-ied using the knowledge acquired during the analysis of

Ž .thermally aged LDPE type A samples . In Figure 6,FTSDC data for a type B sample are shownŽchargingrdischarging conditions are the same as used for

.type A data in Figure 5 . In agreement with the resultsfound for type A samples, also for type B there is a non-linear behavior with the applied field. The results marked

Ž . Ž .with i and ii correspond to two runs that share the samechargingrdischarging conditions but the results are not re-producible. During the measurement history of this sam-

ŽFigure 6. FTSDC plot showing jrE of electrically aged LDPE type.B for different charging fields. Chargingrdischarging conditions: Ti

Ž . Ž .s2�C, t rt s1hr2h and Es2 to 4 kVrmm. i and ii are runsc dŽ .made under the same conditions but run i was one of the first runs

Ž .made while run ii was one of the last ones.

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Ž .ple the run marked as i was one of the first made whileŽ .ii data corresponds to one of the last performed on the

Žsame sample with more than 20 runs in between corre-. Žsponding to about 40 days . The differences found higher

Ž ..current and lower maximum peak temperature for i re-flect the influence of previous aging history on the films.It should be remarked that it is not a reproducibility prob-lem since other runs made on the same sample followingthe same measurement conditions show a very good re-producibility.

Finally the attempts made to use this method on un-Ž .aged LDPE type C samples revealed that it was very

difficult to produce reproducible results for the FTSDCstep. However for the ICC and the IDC measurementslower current values were registered for the virgin sam-ples when compared with aged ones. In the FIDC step forthe unaged polymer the current drops below the measure-ment range much faster than for the aged LDPE. Thisindicates that less charge is available in deeper traps inthe virgin films.

5 DISCUSSIONFrom the analysis of the data it is possible to under-

stand the origin of the peaks observed in FTSDC spectra.As it was stated in the previous section the non-linear de-pendence of the current with the applied field suggeststhat the peaks are due to SC and not to dipole reorienta-tion. Further evidence is found in the FTSDC spectra sincethe peak is very broad and its maximum is well above thecharging temperature. For example, for T s2�C the tem-i

perature of maxima currents are above 40�C while dipolarpeaks are not expected more than 20�C above the poling

w x w xtemperature 9 . Vanderschueren et al. 10 more specifi-cally state that one or more peaks can appear at tempera-tures much higher than the charging temperature when

they are due to injected carriers deeply trapped. Alsocomparing the ICC and the IDC, it is observed that thetwo currents are not mirror images of each other speciallyfor high fields and long enough t , which also can be at-c

w xtributed to trapped SC 1,11 .

An estimation of relaxation times was made for ICC,ŽIDC and FIDC steps for the aged polymer either ther-

.mally or electrically aged . The longest relaxation timefound for the IDC stage spans from 1000 s to 6000 s. Anincrease in the relaxation times is observed when thecharging temperature decreases while the change with thefield is not significant if Figure 2b is analyzed no mean-ingful change in the slopes is seen. In FTSDC the peak

Žposition is not much affected by the field amplitude Fig-.ures 2c and 6 .

The total charge in each step was calculated and somerepresentative results can be seen in Table 1 for the runspresented in Figure 3. Seen in the ICC and the IDC of allthe runs is the decrease of the total charge with decreas-

Žing temperature. The ratio Q rQ third column inIDC ICC.Table 1 in % is lower than the one found by adding the

contribution from all the discharging processes, Q rQd ICCŽ .where Q sQ qQ qQ fifth column . Con-d IDC FTSDC FIDCŽsequently at the end of the IDC step and even at the end

.of the FTSDC step there is still trapped charge remain-ing in the sample. In this way it is shown the importanceof the FIDC step for reproducibility and analysis of theresults in high insulating polymers.

The zero field plane of the charge centroid was calcu-w xlated using the equation in Neagu et al. 12 and from

Žthese values the mobility was obtained see last column of.Table 1 . The values found for the mobilities span from

6�10y17 to 6�10y15 m2 Vy1sy1 and when compared withw xthe literature are of similar order of magnitude 13�16 .

Ž .Table 1. Total charge calculated for each of the four stages of a run with different chargingrdischarging temperature T for a thermally agediLDPE sample - type A. Chargingrdischarging conditions: Es2 kVrmm, t rt s1hr2h and T s30, 20, 10 and 2�C.c d i

Chargingr Q QIDC F IDCy9 y9Ž . Ž .discharging Q 10 C Q 10 C Charge MobilityIC C F T S DC

�y9 y9 y16 2 y1 y1Ž . Ž . Ž . Ž . Ž .conditions 10 C %Q rQ 10 C %Q rQ Centroid 10 m V sIDC IC C d IC C

Es2 kVrmmT s30�C 3.37 y2.77 y0.28 y0.11 0.06 L 17I

Ž . Ž .t rt s1hr2h 82% 94%c dt s14h30f d

Es2 kVrmmT s20�C 2.47 y2.06 y0.29 y0.04 0.03L 8.3I

Ž . Ž .t rt s1hr2h 83% 97%c dt s14hf d

Es2 kVrmmT s10�C 1.85 y1.53 y0.27 y0.07 0 0I

Ž . Ž .t rt s1hr2h 83% (100%c dt s42hf d

Es2 kVrmmT s3�C 1.51 y1.26 y0.16 y0.01 0.05L 14I

Ž . Ž .t rt s1hr2h 79% 95%c dt s22hf d

� Ž .L is the sample thickness around 200 �m .

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The charge centroid position and mobility values decreasewith decreasing temperature T except for the lowest tem-i

Ž .perature 2�C which exhibits a considerable increase. ForT s10�C a zero value was found, meaning that the chargei

remains mostly at the surface or very near the surface. AsŽ .is known for LDPE, the � or � glass transition peak isaŽfound near 0�C while the � or � glass transition in thec

. w xcrystalline regions lies well above this temperature 17,18 .At both transitions there are important structural changesthat would result in creation and destruction of traps, sothe measurements with T s10�C would be in an interme-i

diate zone where less traps might exist. If the temperatureis lowered the influence of the � transition would starta

to be felt, while advancing towards higher temperaturesŽthe influence of the � transition would appear this isc

also reflected in the broad peak seen in the FTSDC at.40�50�C .

The effect of the dc applied field on the charge cen-troid and the mobility is presented in Table 2. In terms ofthe charge centroid, the charge penetrates more in the

Ž .sample for the lower field 1 kVrmm and attains a mini-mum at 3 kVrmm, increasing again between 3 and 4kVrmm and then stabilising for the highest field strengthsŽ .the mobility follows the same trend . Considering a trap-pingrdetrapping process it seems that initially, with fieldincrease, deeper traps are being filled. This represents asmaller penetration in the sample since deeper traps are

w xexpected to be located more near the surface 13,19 . FromŽ .a certain field value 3 kVrmm the available traps seem

to be completely occupied and the charge is free to movefurther in the bulk. Moreover this charge has gained moreenergy supplied by the higher electrical field and it willmove faster so that an increase in mobility is observedwith the field strength rise. More charge accumulates un-til there is enough to produce a screening effect that stopsthe charge from moving further inside. Therefore the cen-troid and the mobilities become approximately constantw x16 .

The more striking feature of the FTSDC plots is thepresence of a broad peak roughly at 40�50�C. The spectra

w xof polyethylene obtained by Fukuzawa et al. 20 , in a TSCexperiment using needle electrodes, also shows a broad

w xpeak around 50�C. Kim et al. 21 have found a similarpeak in TSC of �-irradiated LDPE that was related to SCand also another one at 90�C.

Activation energies were calculated by the initial risew xmethod 10,22 . Some results for thermally and electrically

aged LDPE are presented in Table 3. With this method itis difficult to obtain consistent data since the peak is com-posed of individual peaks and their relative influence isdependent on the experimental parameters. Neverthelessit should be remarked that the low values found are simi-

w xlar to some in the literature 19 .

Individual peaks can sometimes be perceived as ashoulder or a small peak superimposed on the resulting

Table 2. Charge centroid and mobility for different charging dcŽ .electric fields E for a thermally aged LDPE sample - type A.

Chargingrdischarging conditions: T s30�C, t rt s1hr2h and Es1i c dto 5 kVrmm.

Chargingrdischarging Charge Mobility

� y16 2 y1 y1Ž .conditions Centroid 10 m V s

Es1 kVrmmT s30�C 0.11 L 61I

t rt s1hr2hc dt s18hf d

Es2 kVrmmT s 30�C 0.06 L 17I

t rt s1hr2hc dt s14h30f d

Es3 kVrmmT s30�C 0.02 L 0.6I


Es4kVrmmT s30�C 0.05L 0.7I


Es5 kVrmmT s30�C 0.06 L 0.7I


� Ž .L is the sample thickness around 200 �m .

Ž .broad peak see, for instance, Figure 6 . For some se-lected runs, a peak decomposition was attempted using an

w ximprovement of the method by Neagu et al. 23 and con-sidering SC as the origin of the peaks.

In Table 3 the first three columns show the data for theŽbroad peak T is the peak temperature maximum, j them m

current density at the peak maximum and E the activa-a.tion energy calculated by the initial rise method . Fittings

done by decomposition of the spectra into individual peaksŽcan be seen in the last columns of Table 3 T is themi

individual peak temperature maximum, E the activationaiŽ .energy, Q the SC density at T and � T the relax-oi i eq mi

.ation times . As it was observed the high temperature peakŽ .above 80�C may appear or not, depending mostly on theFIDC duration and consequently on the almost completedischarge of the sample. The presence of this peak stronglyaffects the analysis of the lower temperature broad peak.

In the literature the thermally stimulated spectra ofpolyethylene above RT show the presence of many peaksat temperatures around 35 to 70�C and are attributed to

w xSC but considering different mechanisms 24�29 . Part ofthis range is a consequence of poor control of experimen-tal conditions and the difficulty of analysis for highly insu-lating polymers.

For charging temperature T s2�C it was possible toiŽ .decompose the thermogram into three four individual

peaks roughly at 35, 45 and 65�C and, when appearing, ahigh temperature peak at 85�C. The choice of these tem-peratures was supported by some extra experimental datasuch as heating and cooling the sample without previous

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charging and also by values found in the literature. Just bycooling or heating the sample sharp peaks appeared at

Ž .temperatures close to the ones selected 45, 65 and 85�Cand detected mostly during cooling. Because these peaksare very sharp it is expected that they result from struc-tural changes. Peaks at similar temperatures were re-

w xported by other authors, such as Kobayashi et al. 30 whofound the same three lower temperature peaks. It is inter-esting to notice that, even if the experiment was done us-ing elongated LDPE, the conditions were similar.Kobayashi attributed the first two peaks to structural

Ž .changes cavities in the interface crystallineramorphousinterface caused by melting and recrystallization. Blake et

w xal. 31 have also found a peak at 35�C in �-irradiatedHDPE which was attributed to trapped electrons.

Some authors also report another peak very close tow x40�C 19,27 , which can be related either with the 35�C or

the 45�C peaks. According to these authors this peak isŽdue to electron traps of chemical origin chemical impuri-

.ties, oxidation products, broken chains, etc. . At 45�C Kaow xet al. 26 have related the peak to traps that can be both

in the surface and in the bulk of the polymer. Sawa et al.w x24,25 attribute a peak at 50�C to SC detrapping causedby the onset of the molecular motion of the crystallites.

Ž .The values for relaxation times, � T , are of the or-eq mi

der of magnitude expected and can be related to a peaky3 w xfound in dielectric spectroscopy below 10 Hz 32 . The

w x Žactivation energies found by Sawa et al. 25 and obtained.by two different methods are higher than the ones calcu-

Žlated in this work 0.11 to 0.48 eV at 35�C and 0.21 to 0.80

. w xeV at 45�C and 50�C . On the other hand von Seggern 19Ž .has found values 0.15eV more similar to the ones re-

ported here.The peak at 65�C is usually attributed to electron traps.

w xKobayashi et al. 30 consider a peak at 60�C occurring inw xthe crystalline region while von Seggern 19 attributes it

Ž .to chemical defects as for the peak at 40�C . Dorlanne etw xal. 28 have found a similar peak in AC stressed polyeth-

ylene. The high temperature peak has also been reportedin the literature, however the temperature maximum en-

w x w xcountered spreads from 85�C 24,25,28 , 95�C 26 to nearw xthe melting point at 110�C 19,33 . The activation energy

ranges from 0.87 to 1.85 eV. Nevertheless the values ob-Ž .tained in the work presented here 1.34 to 1.55 eV agreew xwell with others found in literature 19,25,26,28 .

Ž .The electrically aged LDPE type B was analyzed in aŽ .comparable way to the thermally aged type A polymer.

The behavior is relatively similar; for instance, bothFTSDC spectra show a similar broad peak. However ifthe temperatures of the maximum of those peaks arecompared there is a slight shift towards higher tempera-tures for the electrically aged polyethylene, as can be seenin Table 3.

Ž . Ž . ŽThe differences in Figure 6 for the runs i and ii al-.ready referred above are very important in understanding

the influence of the aging history. As can be observed, runŽ .i has a shoulder around 60�C, that is seen as well whenthe applied field is 4 kVrmm. Also the mobility values

Ž .showed that run i has a much lower mobility than runŽ .ii suggesting that more SC remains in the sample indeeper traps.

Ž . Ž .Table 3. FTSDC results for thermally type A and electrically type B aged LDPE.

Charging EaŽand initial

. Ž .Sample discharging T J rise T E Q � Tm m m i ai o i eq i m iy11 y2 y8 y2 2Ž . Ž . Ž . Ž . Ž . Ž . Ž .Ageing conditions �C 10 Am eV �C eV 10 Cm 10 s

Thermal Es2 kVrmm 35 0.48 7.6 10T s3�C 40 6.5 0.22 45 0.59 5.6 8.9I

t rt s1hr2h 65 0.52 10 11c dt s22hf d

Thermal Es4 kVrmm 35 0.11 29 44T s2�C 45 17 0.21 45 0.38 42 14I

t rt s1hr2h 65 0.42 9.7 14c dt s19h 85 1.46 15 6.7f d

Electrical Es2 kVrmm 35 0.45 5.0 11T s2�C 45 8.0 0.29 45 0.39 14 14I

t rt s1hr2h 65 0.33 15 18c dŽ .t s61h if d

Electrical Es4 kVrmm 35 0.29 10 17T s2�C 50 14 0.23 45 0.47 10 12I

t rt s1hr2h 65 0.30 4.2 18c dt s14hf d

Electrical Es3 kVrmm 35 0.29 14 17T s2�C 50 7.5 0.19 45 0.10 0.1 �I

t rt s1hr2h 65 0.35 24 17c dt s21hf d

Electrical Es2 kVrmm 35 0.32 6.0 15T s2�C 50 3.8 0.22 45 0.39 1.6 14I

t rt s1hr2h 65 0.46 0.9 13c dŽ .t sy iif d

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The activation energies calculated by the initial risemethod as for the thermally aged polymer, do not show

Ž .any significant change with the field, even for runs i andŽ .ii .

As for the thermally aged LDPE some selected thermo-grams of the FTSDC spectra were decomposed in individ-ual peaks and the results are presented in Table 3. The

Ž .values chosen for the peaks temperature 35, 45 and 65�Care the same as the ones used in the decomposition of theFTSDC of the thermally aged polyethylene.

The results for the broad peak, in the first three columnsof Table 3, show that the maximum is around 50�C and

Ž .run i is the only one with a slightly lower value. Thisdifference will be reflected on the decomposition resultsthat have proved to be very important to understand thechanges with ac aging and also during the course of thesuccessive experimental runs. Basically the three peaksrelevance varies through the series of measurements. Theform in which the results are presented in Table 3 corre-sponds to the chronological order in which the runs were

Ž . Ž .done, with run i as one of the first and run ii one of thelast. The peak at 45�C is the one that suffers more changesduring the course of the experiments. Its higher magni-

Ž .tude is for run i , 2 kVrmm, decreasing for the next onewhich has a higher charging field, 4 kVrmm and almost

Ž .disappears for the remaining two 3 kVrmm and ii 2kVrmm, which is seen by the changes in the values of Q .oi

Ž .Analysis of other FTSDC results not presented here alsoindicate this behavior. It seems that the charge responsi-

Ž .ble for the 45�C peak by detrapping has not been re-placed by the dc charging process. It seems possible toassume that the charge was trapped during the previoushistory of the sample and that it is mostly a consequenceof the ac aging process. One possibility can be the pres-ence of chemical traps created during the ac electrical ag-ing that can be related to ions which have also diffused induring the same process. It is known that oxidation occurs

w xduring electrical aging 34 . These ions can act as trappingcenters for carriers andror recombination centers. If theseions can diffuse andror recombine with carriers injectedby the dc process it would result in both a decrease ofcurrent and the gradual disappearance of the correspond-ing peak in the course of the combined experiment. Fur-thermore for the thermally aged LDPE it appears that theindividual peak at 45�C does not vanish so easily as for theac aged samples. The electrically aged material could alsobe a more open structure facilitating the transport of car-riers. This diffusion might not be seen in the results formobility because the traps would remain more or less atthe same depth in the material.

For unaged samples, which are just partially condi-tioned, the reproducibility is not good. Hence it is possi-ble that the aging itself acts as a conditioning process.

6 CONCLUSIONSHE combined isothermal and non-isothermalTmethod allows reproducible results to be obtained for

LDPE. It was possible to study the FTSDC spectra and todecompose the complex broad peak around 40 to 50�C.Depending mainly on the FIDC a peak at higher temper-ature close to the melting point can also appear.

In the region above room temperature the peaks foundare attributed to trapped SC and most probably to elec-tron traps. The broad peak in the range of 40 to 50�C isthe superposition of two or three individual peaks and itis due to modifications caused in the trap states by changesin the amorphousrcrystalline interface. This is likely to be

Žrelated to the � transition glass transition for the crys-c.talline region . The origin of the traps themselves can be

either structural such as cavities or chemical defects. Thehigh temperature peak has a strong dependence on theFIDC step and corresponds to the deeper traps with longerrelaxation times and closer to the surface. Since it is nearthe melting temperature it must be related to changes re-sulting from the onset of melting. The origin of the trapsis probably similar in nature to the more shallow ones butno conclusive evidence is available.

Comparing the results obtained for the ac electricallyaged and thermally aged samples is not easy. The maindifference observed is the current values, which are usu-ally higher in the thermally aged polymer. If the relativeamplitudes of peak current for the 35�C and 65�C arecompared, it is seen that the higher temperature peak ismore important in ac aged than in thermally aged LDPE.As a consequence the complex peak temperature wouldbe shifted towards higher temperatures. These two differ-ences could be understood if recombination was more im-portant andror if the number of traps was greater and thetraps had longer relaxation times in the electrically agedpolyethylene. Also it is possible that the ac aged materialis a more open structure with more andror enlarged cavi-

Ž .ties such as the water trees presence indicates allowingan easier path to carriers and thus facilitating diffusionand recombination.

ACKNOWLEDGMENTSThe authors would like to thank Borealis for supplying

the LDPE pellets and BICC-Celcat-Portugal, especiallyMr. F. Pedroso. Furthermore, the authors are grateful to

Ž .Professor J. Fothergill Univ. of Leicester, UK and co-workers for gently lending some modified Cigre cells.´

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1070-9878rrrrr1rrrrr$17.00 � 2004 IEEE32


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space charge studies in ldpe using combined isothermal and non-isothermal current measurements

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