Indi ; 11 .lourn;iI or Pure & Appli ed Phys ics Vol. :\') . SCjll elllher 2()()1. pp. 595-602

59 r .. {,02

. , Field-induced thermally stimulated currents

in poly (p-phenylene sulphide) s . ,

; I i)

~ c· J K Quamara. N , Singl~l & Azad ingh-

I (

Department or App lied Physics. Regiona l Engineering Co llege. Kurukshetra 136 11 9

I Department or Physics. Kurukshetra University. K n lkshetra. Haryana

Received 19 Fehruary 2001 : revised 27 April 2fl I : accepted 14 June 2001

./ ) \-4

Thermall y st imul ated polari zati on curreills (TSPC) have been investi gated in poly p-phenylene sulphide (PPS ) III

lemriCrature regioll 30-250 °c under various po lari zation lields. ranging rrom 3 to I X kY/cm and with different heatillg ra tes. The elTcct or high temperature annealing on TSPC hehaviour has also heen invest igated. The T SPC spectr;l show the presencc ()r a single peak (P I) around 110 °C. Thi s peak has been atlrihuted to a weak polar nalUrc or CoS l inkage. A phellomell al ri se or several order or magnitudes in the current dominnted hy conduction currelll is observed ahove 17() 0C. Duri n~ the successive cooling the eu rrelll retains its positive polarit y and in the 2nd TSPC cycl e the peak PI disarpears due 10 temrerature der endence or salUration rolarization eonlirming the polar nature or thi s peak. The kinks ohserved ill the TSPC speelr;1 or annealed samples and with slow heating rates around 170 °C arc due to the dipolar relaxat ion process ;ISS()Cial e<i wilh ether linkage. a cross-linked structure rormed due to high temrerature annea ling. The thermal v;lri ;ltioll ill dicleclri c l'DnSlanl. deduced rrom TSPC spectra is in good agreement at low temrerature. with the result obtained hy other

Inr lh(~

I. Introduction

Ow ing to its possible application as conducting pol yme r. po ly p-phenylene sulphide (PPS), a we ll ­know ll hi gh te mpe rature pol yme r has drawn considerable a tte ntion in. recent years 1. (>. A detailed

uncl c rstandin ~ o f its va ri ous die lectric re laxation processes and tra nsport mec hanism becomes esse ntial for the ir furthe r progress in prac ti ca l ~tppli c ati o n s o f PPS as conductin g po lyme rs . The Fie ld- Indu ced The rmall y S timulated C urre nt (FITSc) tec hnique 7 is acknowledged to be a prec ise ;IIHI powerftll too l for suc h studies. It involves two mutuall y tn ve rse processes: (i) the the rmall y ac tt va ted transiti o n from ne utral ity to pola rized stat c. I.e. huild- up of polari za tion and ( ii) the the rma ll y ac ti va ted transition from pol ari zed state to equllihrium state. i.e. dissipation of polari zation. ).; nown as the rmall y stimulated polarization current (TSPC) and the rmall y stimulated de polarization c urrc nt (TSDC) tec hnique, respective l/'J. Though IhL' a Ulhor.~1I1 already carried out ex te nsive TSDC studies in PPS. the re le vance of the corresponding TSPC in ves ti ~ations are impe rative because the compari son between the c urre nts gene rated during const ruction and destruction of internal polarization fttrn ishcs some c ruci a l c lues not available from

e ithe r of these. In fact, the TSPC tec hnique is considered a comparative ly more e ffic ie nt diagnosti c tool than TSDC in man y respec ts. For exampl e. it avoids the overheating o f the tes t pi ecc and e liminates the search for optimum po lin ~

tempe rature. Apart from this. the TSPC measureme nts g ive an idea of the te mpe rature at which ohmi c conduction becomes significant. Thi , is necessary beca use after the o nset of conduc ti on the stored charges in a high-res isti vity so lid get quic kl y ne utrali zed preventin g the subsequent TSDC or TSPC from its prope r functi o nin g .

The present pape r is prima ril y concerned with e lucidating the di e lectric relaxation be ha viour of pol y (p-phenylene sulphide) by measurin g TSP current unde r various test conditions. like diffe re nt bias voltage. heating rates. success ive heat ing. cooling/TSDC cyc les. The effect of thi ckness a ncl annealing has also bee n studied on the patte rn of TSP- c urre nt behaviour.

2 Theoretical Considerations

While measuring the the rmall y stimulated current unde r a bi as field (£1') ' the total conduc ti vit y

(0) o f the sample at any instant comprises two components, viz. one resulting from o ri e ntatio n or

INDIAN J PURE & APPL PHYS, VOL 39, SEPTEMB ER 2001

dlpolc.\ (01)) and the other from res idual Ioni c L() nciucti vily (0i ). This ( 0 i) could he assumed to be J1 L' ~ I I ~ ihlc in view of the assumpt ion that the \ peC llllen IS thoroughl y uncharged, i.c. P(t)=O at l=() dnl! the operatin g temperature is not too hi gh.

In eLise of an ideal diel ectric consisting of free rotatin g dipoles with sin gle relaxati on mode, the hu ill -up polari zati on and the corresponding displacement current density are given by:

1'(1 1 = ex p - J- f (-)exp f- dl ( '. rtl ' J' P' I (,. ti l " ) II r II T il T

. . . ( I )

ali t!

p (T) i (E ) ( I )1 {-E) 1 .1 ".1 (1' )= _0_1 cX I - -7: - - fex __ tI IT X [ II I, I {3To I kT

, I f "'I' J

I , . 1--1 fex p

1 Ii f " , (

- I:' " ) rr ., l dF ,,] , ~ ( r ~

.. . (2)

\ liCrl' 1'1 I i ~ the freq uency facto r. E" the act ivation e n e r~v. ~ uniform heatin g rate, and Po(7) the ~ ,JllIr , Jt i() Jl polari zat ion given by the Lange vi n for 111 U I ae:

tV 1, 2 F I ' ( /' ) = " " ' "

' I 'l,k 7 ... ('1, )

whcrl' tV" i.s the ci ip() le concentrati on, ~Ld the dipole moment and k the Boltzmann ' s constant.

Eq . (2) can he re-written in d more simplified 1(lnn a.\ :

tV I I .' F .I (rl = .1.1""

1'" ' " /. .7' ,1 \ TI)

... (4)

The integral term in Eq. (4) is neg ligibl e over the initi al part of curve fo ll owi ng Garli ck and G'bb' . II TI . . E I son s assumpti ons . le ac tl vatl on energy ( ::, ,, ) could be ca lcul ated from the slope or low temperature curve of the In[J(T)j versu: rl plot.

Now, if the temperature dependence of PI) i ~

ignored. Eg. (4) gives the condition for a max imum (Till) in the TSPC thermogram as:

. .. ( ) )

and the slope of the initi al part or the In I.I(T) I against r l plot should come out to be (-E;/k).

o Interestingly, the above menti oned TSPC Eg. (4 )

possesses the same form as that for the TSDC de ri ved by Bucci e l a l .!) as well as for TSDC due to disorientation of dipol es by Shindo l

:' except the opposite sign. This is plausibl e, as the concept of the phenomena of built-up and es truct ion of polari zati on as es tab li shed and they are analogous. Therefore, from the theoretica l point of view. ,I

TSPC peak should have the same positi on. shape and ac ti vat ion energy as its TSDC counterpart.

o Another source of TSPC is number of iom. which grad ually jump fro m their equilibrium troughs due to ri se in temperature. and whi ch migrate under the influence of the app li ed fi e ld . But thi s is least signifi cant for practical purposcs, because the corresponding peak being at hi gher temperature is generall y occurred due to intense ohmi c currents.

o In general. the conducti on currents accompa ny the actual charging currents ; so it would be more appropri ate to express the observed TSP current in the form :

ip(7) =dP(T) /dT+f!, (T) Ep ... (6)

where f!,(T) is the ohmic conducti vi ty . Also, the thermally generated polarization in a polymer cou ld be represented in terms of its dielectr ic consta nt (E)

as

.. (7)

Further, the thermal va ri at ion of d ie lect ri c constant (E) with reference temperature (To) mi ght be calculated through the TSPC spectra by usin g the

. J1 ~ re lation · :

QUAMARA et a l.:THERMALL Y STIMULATED CURRENTS 597

I ' D.f ( T ) = E(T)-E(7~I)= JII(T )- / o(T) kIT

EoE"A {3 Tn

.. . (8)

where A is the sample area, I and 10 are the values of the first TSPC at temperature T with and without the e lectric fi e ld (EI') ' respec ti vely.

:3 Experimental Details

The poly p-phenylene sulphide (PPS) pell ets we re prepared from the powder (Po ly Science. US.'\ ) hy usin g. a pe ll et formation machine. A constant we ight of 5 ton was applied. The pe llets of SIi'l' 1.3 cm diameter and I mm thickness were prepa red until stated otherwi se. The samples were metalli zed on both sides by vacuum evaporation of AI for electric contac ts. The sample holder was placed inside a properly grounded metallic chamber. As the transient current at room temperature became almost negli gibl e. a linear thermal program (2 °CI min unl ess staled otherwise) was run with simu lt aneous app li cation of a constant dc fie ld 1:) i"rom :l to I g kV Icm ) across the sample in series With ,I se ns iti ve e lectrometer (Kei thlay 610 C) to llle , I ~ Ure tile TSP current. The slow heatin g rate (:2 °C/min ) wa" maintained in order to prevent temperature gradi ent within the sample: temperature

30

25

20

10 15

1 0

25 I (10'''A)

20

1 5

10 5 +-______ ~ ____ ----~----__ _,

5 10

5 b-==----

.90 1 10

lag between sample and surroundin gs . as weil as in obtaining a fine resolution In the current -

\ temperature spectrum. Mckeever and Hu ges described the initial cooling of the sampl e to very low temperature, e. g. 77 K (so as to freeze the dipoles in their random positions) prior to the sta rt of heating in the presence of dc field . However, the initial cool ing may be done up to re latively hi gher

I d b h k 1-1· 16 temperature a so, as reporte y ot er wor ers . It may altogether be eliminated if the samp le is short-circulated for a sufficient time and spuriou:­polarization is a ll owed to decay. This has been successfull y employed by the authors lD

o While performing repeated TSPC cyc les. between two successive repeated TSPC cyc les the sample was allowed to cool fr2m the hi ghest temperature (T2) to room temperature (TI ) under ambient conditions and was maintained at T I for about half an hour to allow almost all the relaxa ti on modes to become saturated at that point. The activation energy (Eil ) of the TSPC peak has been evaluated (Table I) via the Bucci-plot method"-

4 Results and Discussion

o The typica l TSPC spectra of PPS samples for various polarizing field (ranging from :3 to I ~ kY/cm) has been illustrated in Fig. I. A single pea k

130 150 1 70 190 210

r:i~ . I - Th e TSPC spectra of PPS samples under dilTcrenl polarization fi elds ( I ) 3 (2) 5 (3) 10 (4) I S (5) I Rk V /e m Inset: Peak currCIll (l1Il)as a funclion o f Polarizing Field (Ep)

INDI AN.J PURE & APPL PHYS. VOL 39, SEPTEMB ER 200 1

( /' , I "round 1 I () °c is appearing in each current­Il'l11per;l1 ure ( / ,,-T) curve. A fter completi on o f the pGIK ;t stead y ri se in current in high temperature

re~lo n heyond 170 °C IS observed . These ohservat i o n .~ arc in conformity with the non­iso thermal charging mechanism of a dielectric as l1 ut iineci in theory. where the polari 7. ing current (/1') i., supposed to consist of two components behaving tiitTcrcntl y as a function o f temperature. The dipo le orienta ti on as a transient process gi ves rise to a IK·;IK. whereas thc conduction current that deri ves 11'0111 11lL' motion 0 1' eq uilibrium charge carrier il l c re; l ~e~ cominllously w ith temperature. T he PPS "rUClurc consists of zig-zag chain of sulphur atoms ;lIl d henzene rin gs adjacent to each other as shown he InIA :

r~

I s n

Though th L' PPS docs not contai n any major

Iml; lr group. the sulph ur atom in PPS being sp \ hvbr idi zed there w ill be some dev iation f rom the lillL'; lriI Y between C-S and S-C linkage. U nder thi s conditi lln th L' C-S lin kage in PPS may be assoc iated 10 a weak dipolar relaxation. As the polymer is he;l\ ecl in the presence o f a bi as ing fi eld , the dipolar rL'I; lxatlnn timc decreases and they are progressively (l ri cnt ed. T he 1 ~lrge r the po lari zing fi eld. thc stronger I ~ th L' l)nILlri z; l1i on T hi s is corrohorated by the fact l iI ;11 ;111 ;illTlost linear rela t ionship is obtai ned notabl y II I Ihe 11 1 ~ 11 fi eld ran~e between the fi eld strength , Ep.

;11 1(1 PC;I" amplitude. 1", ( Inset Fig. I ). This also IInpll e ~ the occurrence of uniform bulk polarization . The dipolar ori g in of peak PI is also confirmed by Ihe 1'; ICt that (il i ts locati on is very c lose to that of

dipo/;(r peak (~-peak) appearing in the TSDC ~ J1ec lr;1 of PPS thermoelectret. · ( ii ) the average .ICII \·;II I() 11 energy (0 .4 eV ) responding to thi s rl' I;I\;II I()1l th ough i ~ rnore thall the theoreti cal prL' ti lc ll cci valu t' (0 .24 eV) for this peak. is justified III till' sell se hCC<llI se the later is for side group () I'Il' nLIII!l1l whe rea~ C-S linkage is in main chain. 1-: lllL' r~ l' nc L' 0 1' thi .' peak is also consi stent with the

dl c l eCl ric~ loss max ima observed near 110 °C by I I lin ;lIld Cchc 17

o Once the d ipolar relaxati on is over. the TSPC spectra seem to be dominated by conduction curren!. In an attempt to separate the contributions from the dipolar mot ion and ohmic conducti on after the first TSPC cycle is over, the sample is coo led down to original room temperature (TI ) w ith fi eld (Ep)

retained and under identical conditi ons a 2',,1 TSPC

cyc le is carried out. It is observed from F ig. 2(a) that the peak vanishes in 2"'1 TSPC cycle and current possesses pos iti ve polarity in the entire temperature region with decreased magnitude in low temperature region and eventuall y shows a fast increase at high temperature. Since during the repeated polari zing cyc le no more dipoles are there to orient and the satu rati on polarizat ion (Po) that has alread y been saturated at the lowest temperature (T1 ) begins to decay as per Eg. (3), the displacement current in principle should remain throughout negat i ve. But thi s di spl acement current graduaHy becomes insignificant in compari son to the conducti on current with increase in operati ng temperature and it is mainly the later component. w hich is observed . Similarl y , the instantaneous polari zat ion increase~

accordin g to the increase in Po with descending temperature maintain current pos iti ve under the b i a~

through the coo ling step (Fig:. 2). A "mall ki nk

around 170 °C in second T SPC cycle is very interesting and assoc iated to a cross-li nked product to be discussed later.

o An attempt waS also made for isolating the conduction current from the actw.li polarizing current adopting different heating rates during the TSPC process as it affects the character ist ics of the later component but not that of former one. A nalogous to the case of TSDC the TSPC maxima shifts towarcl~ higher temperature (Fi~ . :1 ) with increas ing heating rate. However. the peak currenl decreases w ith heating rate, which is ~omewhat

unusual. The I,,-T curves for samp l e~ w ith Imv

heating rate (I DC/min ) show a kink at higher

temperature side around 170 DC similar to the kink in 2'1(1 TSPC cycle curve (Fig. 2).

The dipolar nature of peak PI is also supported by the fact that its locati on is unaffec ted by sample thi ckness (Fig. 4) . The increase in tota l polarization (area under the curve ) with samp le thi ck nes~

indicates an overall dominance of uniform vo lume

poiari zation . A shoulder appearing around X."i DC in thinner sample may be due to a sub T" -weak

I. IX C

re axatron process .

QUA MARA el al.:THERMALL Y STIMULATED CURRENTS :illll

3 0

2 ~

2 0

<t:

Ie:> 15

~

10

~

-20 30 80 130 180 230

0 .2

0 1

()

50 1 00 150 200 250 300

- 0 1

~O ;?

<t -0 . 3

'0 - 0 .4

- 05

-0 .6

- 0 7

- 0 R

0 9

Fi ~ . 2 - (.11 Thc thcnnn~ram nr(l) I " TSPC (2) Cnolin~ 0) 2nd TSPC in PPS samp lc. (C,,=IX kV/cm:

(h) - Thc thcrmogram or TSDC aftcr 2'~' TSPC in PPS samplc (l:.'p= I X kV lem )

The tW() major differences in our experimental J'indill [!s and the theoretical model for FITSC pmIX)~ecl hy Vanderscherin c l al .

7 or the model

ca l c ulati () n ~ for pure displ acement current by Shin(i(l',c arc ( i ) absence of all Y current reversa l 1() "()\"'1Il ~ the d ipo lar peak (Fig. I ) and ( ii ) the large rna ~ tlittld t' oj' TSPC current than that of c mrc~p()nd ill~ TSDC current (Fig. 2(b). However. th e~e dev iatIons can be accounted for in the li ght of till' fact that the assumptions made In the

aforementioned modals are too simplified and rarely valid in actual practice. Further, the charging currents inherently superimposed onto the de conduction current. Another pre-requisite viz. the ex istence of a single relaxati on mode could also not be sat isfied even when the ohmic conduct ion i ~

negligible.

o The quantitati ve discrepancy in TSP and TSI) (Fig. 2(a). 2(b)) current can be understood owing to the following facts: (i) the monitored value of TSPC

60() INDI AN J PURE & APPL PHYS. VOL 39. SEPTEMB ER 200 I

11ll'l lI ci e ,\ an add iti ona l contributi on fro m conducti on phc ll o me na and ( ii ) thc monito rcd va luc of TSDC is slllalkr because or the incomplete di scharge of an c icc tre t in the ve ry first TS DC cyc le and due to the build up of an ionic space hete rocha rge during the po lar i7.at ion ste p, w hic h reduces the inte rna l e lectri c rie ld . consequentl y caus ing reducti on in the dipo lar re laxa ti on. The TS DC pea k appearing around 190 0(' (F ig. 2h ) and att r ibuted to the space cha rge re laxation I ~ not observed In the TSPC c il;!r; lc te ri sti cs. Th i~ i~ not surpri s ing. s ince during

1 ~}

10

'e::::>

5 ..

I

I

the po lari zatio n cyc le suffic ient time is not ava il ab le fo r space charge trapping and a lso to the fac t that the hi gh te mperature region is dominated hy conduc tion current.

o So me inte resting observati ons are observed in the TSPC of hi gh te mperature annealed sa mpl es (Fig. 5). Not onl y the re is a shift in the peak pos iti on

toward s hi ghe r temperature (:==1 20 DC), but a lso the peak magnitude is nearl y 20 times more th an the corresponding T SPC of pri stine sample at respec tive

otl ____ ~~~~~~ ______ ~ ________ ~----------~~T~(~DC~)----_._ o 50 1 00 1 5 0 2 0 0 250

Fig, 3-The TSPC speclra or pps samples using diffe rent healing rates ( I ) I (2) 2 (3) 3 (4) 4 DC/min. (I~'r= 1 8 kY/em )

~o

l B

1 F, -

,,,

« ' 2

-S 1 0

~

8

6

'" 2

0 +-------- ... . -------~------~------___r-----

() 50 1 00 1 5 0 200 250

Fig. 4 - The TS PC spectra or PPS sampl es or diffe rent thickness ( I ) 0. 7 (2) 1.00 (3) I ,2 III III , (t:p=5 kY fCIll)

QUAMARA ef a l.:THERMALLY STIMULATED CURRENTS ClO I

he lcls . Hi gh tempe rature annea lin g of PPS in presence of ox ygen leads to the formation of c ross­I III ked strucrure with new ethe r linkage ' \) as:

o

Tahle I - The acti vation energy for the TSPC peak (P ,) in PI'S samples

1:I'( kY /c m ) I~'a (eY ) 3

5 050 I() 0 .52 15 042 IX 0 37

':.J Authors l!) have shown earli e r throu gh the ir

TSDC investi gat ions that this et he r linkage provides ne w po lar groups with maxima in TSDC curves at hi ghe r te mpe rature than the corresponding C-S linkage dipo lar re laxations. It is very likely that in the TSPC of annealed sa mple. the polarization curre nt fro m the C-S linkage polar group will be added with the polarization current of e the r linkage g:roup .~ . Thi s accounts for the hi ghe r TSP currents in anne;ti ecl samp les. T he small kink appearing around 17() °C in 2 lid TSPC cyc le (Fig. 2b) can be assoc iated with the pol a r nature to thi s new cross­linked structure.

o Similarly the TSPC at very s low heat in g rate ( I

DC/min) will g ive some annealing like effects in hi gh temperature region, resulting in the formati on of some ethe r linkage. respon sibl e for the kinb appearing around 170 DC (Fig. 3).

Table 2 - The temperalUre variation or dielectri c constan t of PI'S samples wi th respect to the reference temperature T" =30 °C tL'>E. deduced from TSPC spectra. and L'>'£ from direct measurements)

M

1::,,='3 £ 1'=5 C,,= I (I

kY/cm kY/em kY/clll

3D 342 5. 16 4 .RO '2.56

50 2.R2 4 .02 4 .32 4 .()R

RO 2.6'i 11 .79 20.67 16.24

100 2.56 17.20 3 1.20 2'3()4

120 2.4R 4 1.20 3R .2() 29. 'iO

o Table 2 gives the variation of die lec tri c constant

(L'l£) from the room temperature va lue o r PPS as ;1

function of temperature as deduced from the i I' respective TSPC curve along with those measured

L'l£' independently us ing precis ion LCZ meter. At

low temperature, L'l£ and L'l£' ex hibit a s li ght agreement but dev iation becomes more pronounced at high te mperature with the onset of o hmic conduction. It is very like ly in the present case that the dipolar peak (PI) may include so me contribution s from charge trapping as well , because the unsaturated pheny l group acts as traps for c harge

. ) () ') I TI' . I d' I carne rs - .- . liS IS a so supporte tra m t 1('

activation energy values for thi s peak (Table I )

which are somewhat larger than those recogni zed for pure dipole.

Acknowledgement

Funds were provided by the Ministry of Human Reso urces & Development . Government of Indi a. New Delhi. One of the author (AS) is thankful to the Department of Tec hnical Educat ion. Harya na (India) fo r grantin g study leave.

602 INDIAN J PURE & APPL PHYS. VOL 39. SEPTEMBER 200 1

250

200

150 <{

'= -100

50

o ~l------.--~~~==~==~ ______ ~ o 50 1 00 150 200

---. 250

Fi~.(5) - The TSPC spectra of annealed PPS samples (annealed at 280 °C lor I hr) under different po larizing field s ( I ) 3 (2) 5 (3 ) 10 kV/cm

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