The aim of this paper is to examine the influence of neutron and gamma irradiation on the dissipation factor and capacitance of capacitors with polycarbonate dielectrics. The operation of capacitors subject to extreme conditions, such as the presence of ionizing radiation fields, is of special concern in military industry and space technology. Results obtained show that the exposure to a mixed neutron and gamma radiation field causes a decrease of capacitance, while the loss tangent remains unchanged

Dolićanin Edin

Jovanović Bojan

Stanković Koviljka

Vujisić Miloš

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RA  DI  A  TION  EF  FECTS  IN  POLYCARBONATE  CA  PAC  I  TORSbyMiloš VUJISI]1, Koviljka STANKOVI]1,Edin DOLI]ANIN2, and Bojan JOVANOVI]3Re  ceived on Oc  to  ber 11, 2009; ac  cepted in re  vised form on No  vem  ber 12, 2009The aim of this pa per is to ex am ine the in flu ence of neu tron and gamma ir ra di a tion onthe dis si pa tion fac tor and ca pac i tance of ca pac i tors with polycarbonate di elec trics. The op er a tion  of  ca pac i tors  sub ject  to  ex treme  con di tions,  such  as  the  pres ence  of  ion iz ingra di a tion fields, is of spe cial con cern in mil i tary in dus try and space tech nol ogy. Re sults ob tained show that the ex  po sure to a mixed neu tron and gamma ra di  a  tion field causesa de  crease of ca  pac  i  tance, while the loss tan  gent re  mains un  changed.Key words:polycarbonate,  ca pac i tance,  neu tron, gamma  ray,  ra di a tion  ef fectIN TRO DUC TIONDi elec trics  used  as  ca pac i tor  in su la tors  have  awide  range  of  phys i cal  and  struc tural  char ac ter is ticswhich give rise to dif fer ences in their elec tri cal be hav -iour. Some of the most im  por  tant char  ac  ter  is  tics of adi elec tric  ca pac i tor  are  its  po lar iza tion  prop er ties,elec tri cal  con duc tiv ity,  power  losses,  and  di elec tricstrength (max  i  mum elec  tric field strength it can with  -stand  with out  break ing  down).  Ex po sure  to  ra di a tionis among the fac  tors which may in  flu  ence these char  -ac ter is tics.  The  ef fects  of  ion iz ing  ra di a tion  on  di elec -trics  used  as  ca pac i tor  in su la tors  may  re sult  in  sig nif i -cant changes of ca  pac  i  tor per  for  mance within anelec tri cal  cir cuit.The trend of us  ing com  mer  cial off-the-shelf(COTS) cir  cuits in sat  el  lite and space  craft sys  tems,and  the  in ev i ta bil ity  of  al ter na tive  di elec trics,  such  aspoly mers re plac ing the SiO2, ne ces si tate an eval u a tion of the per for mance of such ma te ri als in ra di a tion en vi -ron  ments. Their prop  er  ties can be crit  i  cal for the per  -for  mance of the de  vice in a high-ra  di  a  tion en  vi  ron  -ment, re  gard  less of the cir  cuit ar  chi  tec  ture [1, 2].RA DI A TION  EF FECTS  IN  POLY MERDI ELEC TRICSFig ure 1 gives a sche matic dis play of ion iz ing ra di -a tion  ef fects  in  poly mer  ma te ri als.  The  in flu ence  can  bedi  vided into two phases. In the first phase, gamma pho  -tons give rise to sec ond ary elec trons by way of Comptonscat ter ing and pho to elec tric ab sorp tion. Neu tron in duced nu clear re  ac tions give rise to al pha par  ti cles which go onto pro  duce delta elec  trons along their tracks. Sec  ond  aryelec  trons pro  duced by the gamma and neu  tron ra  di  a  tionfur  ther in  ter  act with poly  mer macromolecules, caus  ingtheir  ion iza tion  and  ex ci ta tion  [3,  4].In the sec  ond phase, the re  lax  ation of ex  citedmol e cules  and  lo cally  formed  ion iza tion  clus ters  re -sults in the for ma tion of large amounts of free rad i cals.Highly re  ac  tive free rad  i  cals cause the de  struc  tion ofpoly  mer chains, ei  ther by chain scission (ran  dom rup  -tur  ing of bonds) or cross-link  ing (for  ma  tion of large,three-di men sional mo lec u lar net works). As a re sult ofchain  scission,  low-mo lec u lar-weight  frag ments,  gasevo  lu  tion, and unsaturated bonds may appear [5].Free  rad i cals  formed  dur ing  ir ra di a tion  can  sur -vive for sev  eral weeks be  fore re  com  bin  ing. The ra  tioof  re sul tant  re com bi na tion,  cross-link ing,  and  chainscission  de pends  on  the  chem i cal  com po si tion  andM. Vujisi}, et al.:  Ra di a tion  Ef fects  in  Polycarbonate  Ca pac i tors 209Nu clear  Tech nol ogy  &  Ra di a tion  Pro tec tionTech ni cal  pa perUDC: 537.311.32:539.166DOI: 10.2298/NTRP0903209V1 Fac ulty  of  Elec tri cal  Engineering,Uni ver sity  of  Bel grade73, Bulevar Kralja Aleksandra 11000 Bel  grade, Ser  bia2 Col  lege for Pro  fes  sional Stud  ies in Econ  omy,Uni ver sity  of  Prishtinab. b., 24. novembra, 38218 Leposavi}, Ser  bia3 Fac ulty  of  Tech ni cal  Sci ences,Uni ver sity  of  Prishtina7, Knjaza Miloša, 38220 Kosovska Mitrovica, Ser  biaE-mail  ad dress  of  cor re spond ing  au thor:vujsa@ikomline.net (M. Vujisi})mor phol ogy  of  the  poly mer,  to tal  ra di a tion  dose  ab -sorbed, and the rate at which the dose was de  pos  ited.Un like  polytetrafluoroethylene,  poly propy lene,  orpolymethyl-meth ac ry lates,  the  phys i cal  prop er ties  of a polycarbonate re main vir tu ally un changed at low ab -sorbed doses (<105 Gy). This oc  curs de  spite the rel  a  -tively  large  chem i cal  trans for ma tions  and  ex tremelylong life  times of the formed free rad  i  cals [6, 7].Sta bi li za tion  tech niques,  aim ing  to  in crease  ra -di a tion tol er ance of poly mers, make use of com pounds act  ing as free rad  i  cals and elec  tron scav  en  gers. Freerad i cal  scav en gers  are  ca pa ble  of  gen er at ing  a  largenum ber of hy dro gen rad i cals which ter mi nate free rad -i  cals pro  duced in the poly  mer by ra  di  a  tion. Elec  tronscav en gers de gen er ate into rad i cal/ion pairs upon col -lid  ing with a high-en  ergy elec  tron. This mech  a  nismre  duces the en  ergy of sec  ond  ary elec  trons and can,thereby, re  duce the dam  age caused by ra  di  a  tion inpoly  mers. The com  bi  na  tion of both elec  tron and rad  i  -cal scav  en  gers in poly  mers pro  duces a com  plete pro  -tec tive  ef fect.  Al though  elec tron  scav en gers  help  sta -bi  lize the poly  mer macromolecule, they nev  er  the  lesscan  not pre  vent the for  ma  tion of rad  i  cals. These rad  i  -cals must still be ter mi  nated with the help of free rad  i  -cal scav  en  gers [6, 8].Many  im por tant  phys i cal  and  chem i cal  prop er -ties of poly  mers – such as mo  lec  u  lar weight, chainlength,  en tan gle ment,  polydispersity,  branch ing,  andchain  ter mi na tion  –  can  be  mod i fied  with  ra di a tion.These struc  tural changes al ter the elec tri  cal prop  er tiesof poly mers used as di elec trics in ca pac i tors, af fect ingthe loss tan  gent and ca  pac  i  tance [6].EX PER I MEN TAL  RE SULTS  ANDDIS CUS SIONPolycarbonate  di elec tric  ca pac i tors  used  in  thisre  search had a cus  tom  ary ex  tended foil de  sign withmetallized elec  trodes.Polycarbonate ca  pac  i  tors used in the  ex  am  i  na  -tion of  ra di a tion tol er ance had the nom i nal ca pac i tanceof 1 mF, nom i nal im ped ance of 1014 W, and nom i nal loss tan  gent of 15×10–4. The ca  pac  i  tors were ex  posed to amixed neu  tron and gamma (n+g) field from a 252Cfsource, en  cap  su  lated in the form of Cf2O3. The mass ofthe used 252Cf radionuclide was 2.265 mg, its spe  cificneu  tron and gamma emis  sion rates 2.34×106 (mg s)–1and 5.3×109 (mg s)–1,  re spec tively.  The  av er age  neu tronen  ergy of the 252Cf source was 2.14 MeV, av  er  agegamma pho  ton en  ergy 0.88 MeV. All post-ir  ra  di  a  tionmea sure ments of ca pac i tance were per formed at a fixed volt  age fre  quency of 100 kHz.Polycarbonate ca  pac  i  tors were ex  posed to threedif  fer  ent lev  els of to  tal neu  tron and gamma fluences(Fn and Fg, re  spec  tively), given in tab. 1, in as  cend  ingor  der. Each fluence level is marked by a num  ber (nF)shown in the first col  umn.Val ues of ca pac i tance mea sured im me di ately af -ter ir ra di a tion are pre sented in fig. 2. The graph showsthe de crease of the mea sured ca pac i tance with fluencelevel num  ber nF, i. e. with the rise of fluence. The ca  -pac i  tance was mea  sured again 120 hours af  ter ir ra di a  -tion. The ob tained val ues were all close to the nom i nalca  pac  i  tance, lead  ing to the con  clu  sion that the ef  fectsof ra  di  a  tion had mean  while been re  versed.210 Nu clear  Tech nol ogy  &  Ra di a tion  Pro tec tion  –  3/2009 Fig ure 1. Ion iz ing ra di a tion ef fects in poly mer ma te ri alsTa  ble 1. Val  ues of neu  tron (Fn) and gamma (Fg) fluencesused for polycarbonate ca  pac  i  tor vs. the fluence levelnum bernF Fn [1010 cm–2] Fg [1013 cm–2]1233.557.1010.668.6617.326Fig ure  2.  Ca pac i tance  C of the polycarbonate ca  pac  i  torver sus the fluence level num  berAs op posed to ca pac i tance, no mea sur able in flu -ence of n + g    ra  di  a  tion on the loss tan  gent has been de  -tected.The de  crease of ca  pac  i  tance of polycarbonate ca  -pac i tors  when  ex posed  to  n + g. ra di a tion  can  be  ex -plained by the for ma tion of ionic struc tures in side the di -elec  tric vol  ume. A higher con  cen  tra  tion of ions in thedi elec  tric causes a greater in flu ence of ionic po  lar  iza  tionon the di  elec  tric con  stant of the ma  te  rial, lead  ing to thede crease  of  ca pac i tance.  In  ad di tion,  the  lo cal  elec tricfield of ionic struc  tures per  forms a par  tial screen  ing ofthe ca  pac  i  tor’s elec  tric field, al  beit the change in the ca  -pac i tance at trib uted to the screen ing is rel a tively small. Aper  ma  nent pres  ence of ionic struc  tures in side the di  elec  -tric can cause its di  elec  tric strength to de  crease and thusac cel er ate  the  ag ing  of  the  ca pac i tor  [6,  8].The re vers ibil ity of the ob served ra di a tion ef fectsis a re  sult of the re  com  bi  na  tion pro  cesses in  side thepolycarbonate di  elec  tric. The fact that the ef  fects werere  versed af  ter a pe  riod of time shows that the ab  sorbeddoses were not high enough to pro  duce per  ma  nentchanges in the mo  lec  u  lar struc  ture of the di  elec  tric.CON CLU SIONSThe  re sults  of  re search  on  ra di a tion  har di ness  ofca pac i tors  with  solid  polycarbonate  di elec trics  werepre sented in this pa per. It was shown that the ex po sureto a mixed neu tron and gamma ra di a tion field causes ade crease  of  ca pac i tance  in  polycarbonate  ca pac i tors.Ra di a tion  ef fects  in  the  polycarbonate  in su la tor  werere vers ible at fluence lev els used for this pa per, but may lead to a per ma nent deg ra da tion at higher doses, ac cel -er at ing  the  ag ing  of  ca pac i tors.  Fur ther  re searchshould ad  dress the com  bined ef  fects of high tem  per  a  -tures  and  ra di a tion  ex po sure,  as  well  as  sta bi li za tiontech  niques for the hard  en  ing of polycarbonate di  elec  -trics  to  ra di a tion.AC KNOWL EDG EMENTThis work was sup ported by the Min is try of Sci -ence  and En vi ron men tal Pro tec tion  of  the Re pub lic ofSer bia  un der  con tract  141046.REF ER ENCES[1] Petkov, M. P., Lynn, K. G., Rodbell, K. P., Volksen, W.,Miller, R. D., Ra di a tion Ef fects in Low Di elec tric Con -stant Methyl–Silsesquioxane Films, IEEE Trans  ac  -tions on Nu clear Sci ence, 49 (2002), 6, pp. 2724-2728[2] Vujisi}, M., Stankovi}, K., Vasi},  A.,  Com par i son  ofGamma Ray Ef  fects on EPROMs and E2PROMs, Nu  -clear Tech nol ogy and Ra di a tion Pro tec tion, 24 (2009),1, pp. 61-67[3] Ashcroft, N. W., Mermin, N. D., Solid State Phys  ics,Holt, Rinehart & Winston, New York, USA, 1976[4] Bunget, I., Popescu, M., Phys  ics of Solid Di  elec  trics,Elsevier  Pub lish ing  Com pany,  Am ster dam,  The  Neth -er lands,  1984[5] Dole,  M.,  Ra di a tion  Chem is try  of  Macromolecules,Vol. 1, Ac  a  demic Press, New York, USA, 1973[6] Clegg, D. W., Collyer, A. A., Ir  ra  di  a  tion Ef  fects onPoly  mers, Springer Verlag, Berlin, 1991[7] Lundy,  Ch.,  Polycarbonate  Sta bi li za tion  dur ingHigh-En ergy  Ir ra di a tion,  Med i cal  Plas tics  andBiomaterials  Mag a zine,  3  (1996), 4, pp. 38-41[8] Holmes-Siedle, A. G., Ad ams, L., Hand book of Ra di a -tion Ef fects, Ox ford Uni ver sity Press, New York, USA, 2002M. Vujisi}, et al.:  Ra di a tion  Ef fects  in  Polycarbonate  Ca pac i tors 211Milo{ VUJISI], Koviqka STANKOVI],Edin DOLI]ANIN, Bojan JOVANOVI]EFEKTI  ZRA^EWA  U  POLIKARBONATNIM  KONDENZATORIMACiq ovog rada je da ispita uticaj neutronskog i gama zra~ewa na faktor gubitaka ikapacitivnost kondenzatora sa polikarbonatnim dielektricima. Rad kondenzatora izlo`enihekstremnim uslovima, kao {to je prisustvo jonizuju}eg zra~ewa, od posebnog je zna~aja za vojnuindustriju i tehnologiju svemirskih letelica. Dobijeni rezultati pokazuju da izlagawekombinovanom poqu neutrona i gama zraka dovodi do smawewa kapacitivnosti polikarbonatnihkondenzatora, dok tangens ugla gubitaka ostaje nepromewen.Kqu~ne re~i:  polikarbonat, kapacitivnost, neu  tron, gama zra~ewe, efekat zra~ewa

Radiation effects in polycarbonate capacitors

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