Indian Journal of Pure & Appli ed Physics Vol. 30. April 2001 . pp. 259-262

Therl!1w.l~ sti!llulated Q~polarization c~rrent behaviour of 50 ~eV Li+ ion-irradiated kapton-H polyimide film --- -

M

INDIAN J PURE & APPL PHYS. VOL 39. APRIL 200 I

or a precIsi on temperature controller-cum-programmer. The thermoe lectrects were prepared by subj ectin g the sampl es (irradiated as well as pri stine) to the des ired dc fi e ld (E,,) at a certain constant temperature (~, ) for a peri od of one hour. The samples were then cooled to the roo m temperature with in fixed time (112 hr) in the pr~sence of E1 •• The fri ctional st ray su rface charges accu mu lated duri ng polari zat ion were el i mi nated by short circuiting the electrodes ror about 10 min. In order to obtain depolarization current the system was short-circuited throu gh an e lectrometer (Keithley make 6 1 () C) and the samples were depolari zed at a linear heatin g rate of 2°CI min . The correspondi ng act i vati on energy (U) has been ca lculated th rough the Bucci-plot method('.

3 Results and Discussion

The TSDC spectra of various kapton-H samples (p ri stin e as we ll as ion-irradi ated) have been illu strated in Figs 1-4 . The poling parameters i.e. the poling temperature T" and fi e ld Ep. are the governin g r;lctors in determinin g the different di electri c re la xation processes in the polymers . The chemica l structure of kapton-H polyimide is shown below.

16 r i

1L f

12

~ 10

L

J U

oL-__ ~ __ -L __ ~ __ ~L-~~--~ o 50 100 150 200 250 300

Tem perature ('C I

I:il,'. 1- T SDC spCClra or pri slinc bploll -II samp lc (1;,:

GA RG el 01. : Li+ ION-IRRAD IATED KAPTON-H POLYI MIDE FILM

from a phenyl ring requires 9.93 eV to brea k (J and IT bonds. T hi s suggests that with ion-irradi at ion severe damages occur at ca rbonyl group and ether linkages but phenyl rings will be least affec ted. Thus it is most probable that the ion-irradi ation causes the decompos iti on of the carbonyl group and ether lin kages. Thi s shoul d result a change in the vari ous relaxati on processes assoc iated with these groups. The TSDC curve fo r pri st ine kapton-H (Fig. I) consist of two max ima P, around 80"C and P2 around 15() 'C. The peak P, (~-re l axa ti o n) is associated with the di po lar nature owing to the presence of carbonyl group in the kapton-H structure7.X. " . Further, kapton-H polyimide is also know n to have absorbed water,l.l3 . The possible sites where water molecul e ca n be bound are oxygen of the ether linkage and the carbonyl groups. Thi s absorbed water in kapton-H also contributes to thi s re laxati on'2" . Thi s peak appears to be large ly affec ted by Li -i on irradi ati on (Fig. 2). The peak is ve ry broad for low Ep (80 kV/c m) and becomes shape I' at hi gh field (400 kV/cm). TRIM calcul at ions show that 50 MeV Li -ion will depos it roughl y I MeV energy in 12.5 f..un kapton-H sampl es. Thi s will cause the dimeri zati on of the carbonyl groups and wi II prov ide di stributi ve shall ow energy traps I I . The presence of multipl e peaks in low fi e ld poled sa mples (Fig. 2) and the fie ld dependence vari at ion of acti vat ion energy fo r peak P, (0. 13-0.32 eV) confirm ~ thi s type of ion in teracti on. Hi gh Ep TS DC spectra is smooth in nature. Thi s is due to the dom in ance of pronounced ~-re l axati on . The repoling of t~l e sa mpl e, which is equi va lent to the poling of the annea led sampl e, suppresses thi s ~relaxati onX ') revea ling the radi ati on induced relaxati ons mainl y ow in g to the decompos iti on of the carbonyl group, in the fo rm of small peaks (Fig. ~ ) in the TS DC spectra. This is also in the cOl1lorlll ity with the findings of the Hi oki e! ot-'., where It has been reported that ion-irrad iation dec reases nitrogen and the oxygen atom components or the film and increases the carbon number densit y.

The hi gh temperature TSDC spectra is mainl y due to space charge trappin g mec hani sm and is responsible for the peak P2 (a.- relaxat ion). Usuall y thi s relaxati on appears in sampl es poled at hi gh TilEI' (Ref. 8). An effect ive charge-trapping is pl ausible, as ka pton-H is ex pected to be very ri ch in trapp ing sites owing to the one hand, a very low

...degree of crys tallinity'-' which is also responsible for

space charge fo rmation via barrier or in terracial polari zation (Maxwell -Wagner-Sill ars e ffect). and on the other hand to the presence of the un saturateci phenylene and carbonyl groups in its st ructu re which act to a signi ficant extent as traps fo r charge carriers. The effect of hi gh-energy ion-irradiation on a.-relaxati on is illustrated in Fig. 4. An increase in

12

10

-« ~ 6 C 111 L C :J U

O r---~--~----~--~---++---~ o 50 300

-2 Temperature I'C)

Fig. 4 - TSDC spew'a or Li+ ion-irrad iated kapton- I-I

(Tp : 200"C and Ep : 400 kY /el11) ror el i rrerenl nu x: (;,) pri sline. (b) 105 io n s/em ~ and (c) 5 x 10' ions/cm2

flu x resul ts an increase in peak magni tucie. s h i rtin ~ of pea k towards hi gher temperature and ~ h arpe nin g of peak. Deloca li sed electrons, vv hic h res ult in the fo rmation of conjugated bonds2, are res pons ibl e 1'

262 INDI AN J PURE & APPL PHYS. VOL 39 , APR1L 200 I

relaxati on (dipolar ori entation) due to dimerization of carbony l groups, provides new shall ow and deep energy traps in the form of conjugated bonds and create new sub-polar groups due the cross-linking of imide groups.

Acknowledgements

The auth ors are thankful to Dr K Ashokan and Dr D K A vasthi (Scienti st. NSC. New Delhi ) for their he lp. Funds were pro vided by Nuclear Science Centre. New Delhi and All India Council for Technical Educati on. New Delhi . for carrying out the work .

References

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:I Marl clla G. Ol i vcri C. Feria G & Pingnataro S. Surf IlI lnjllct' Anal. 12 ( 198R) 447.

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( 1983)30

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10 Technica l ill /