Kalra, K C

Katyal, Parveen

Nigam, R K

English

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IndianJournalofChemistryVol.27A, March 19R8,pp. ]9R-203CurrentEfficiency,DielectricMeasurements&IonMobilityStudiesof¥odic OxideFilmsFormedonTantaluminAqueousElectrolytesR K NIGAM*, K C KALRA &PARVEEN KATYALDepartmentof Chemistry,MaharashiDayanandUniversity,Rohtak124001Received6April 1987;revised10July 1987;accepted27 July 1987turrent efficiencies,dielectricconstantsandion mobilitiesof variouslyformedanodicoxidefilmson tantalminthepresenceof variouselectrolytesuchas citric,tartaric,aceticandhydrocWoricacidshavebeenevaluated.ThecUIT~ntefficienciesof suchfilms are unityupon an electrolyteconcentrationof 100molm-3• However,at higherelec*,olyteconcentration,theexperimentalmass-gainof thetantalumsampleduringanodizationis greaterthanthatexpe!::tedfor theanodicformationof TazOson tantalum,implyingthat ass-gai is probablydueto theincorpora-tionof anionsinto thefilms.The capacitancesof suchfilms dependon theconcentrationas well asnatureof theaquebuselectrolyte.The additionof cWorideions has beenfound to'inhibit film growth,the effectbeingpro-nou~cedwith increasein concentrationof cWorideions.Only tantalumions aremobileduringthegrowthof tanta-lum xidefilms.,. I" .The kin¢tics of growth of anodic oxide filmsformed~~tantalumin aqueouselectrolytes,andtheirbre down characteristicshavebeen~epor:-ed receny by heauthors1-3. The presentmvestI-gationis Ianextensionof our earlierwork1-3, andhasbeen~mdertakt:!nin viewof conflictingresults4-'Sof studid on currentefficiences,dielectricmea-surement~and ionic mobilitiesof such anodicfilms.MaterialsiandMethodsTantalujrnspecimens(32x 10-4m2 in areaand0.25mmthick)with smalltagswerecut from tan-talumshe~ts(purity99.9%)mechanicallyandwerechemicall)(polished and anodicallypolarized inaqueousdIectrolytes(at a particularcurrentdens-ity) asdeScribedearlier1• Increasein weightof thetantalum$pecimendue to its anodicoxidationtotantalumQxidewasusedto calculatecurrenteffi-ciency.T~e capacitanceand tano measurementsof the oxi~efilmsweremadeusingan electrolyticcapacitor,bridge (BPL, India) at 1KHz [tan0,knownasJosstangentor dielectriclossis theratioof the voltagedrop acrossthe hypotheticalseriesequivalent,resistance(Rs) and capacitance(Cs)'Also tan0:=WCsRs whereW=2nF, F beingthefrequencyJj Specialcarewas takento preventthetagsdipp~ intotheelectrolyte.For ionit mobilitymeasurements,two 0.25mmthicktantapumspecimensof differentarea,oneof169mm2 llrea (sample-I)and the other 100mm2area(samI1Ie-II)wereanodizedupto identicalfor-mationvoltagein 100molm- 3 aqueouscitricacidat a curreritdensityof 30A m- 2 and thenpairedtogetherto'getTa, Ta20s,Ta sandwitchgeometry.198II I ',1-11tI11 Ill"Perfectsealingof samplesI andII wasensuredbyapplying lacomite and Edward hard vacuumgreaseon the joints and the surfaces,excepttheoutersurfaceof the sampleII in the mannerde-scribedearlier13. The effectivegeometricsurfaceareaaftersealingwas1x 10- 4m2•All measurementsweremadein a well-stirred,electronicallycontrolled,thermostaticbath main-tainedat 298K (±0.5).The resultsreportedarethe mean of five repeat experiments.All thechemicalsused were of AR(BDH) grade. Thedensity of tantalum oxide fi1n1was taken as793kgm-3 asreportedbyYoung15•ResultsandDiscussionsCurrentefficiencyThe datafor the experimentalmass-gain(as aresult of anodic oxidation),the calculatedmass-gain due to oxidation(whencurrentefficiencyisunit) and the currenteffciency('YJ) for thegrowthof oxide films in 10, 50, 100, 500 and1000molm- 3 aqueouscitric acid at a currentdensityof 50'A m-2 (whena particularamountofchargewaspassed)arereportedinTable1.At each electrolyteconcentrationcurrenteffi-ciencywas found to be the sameirrespectiveofthechargepassed.The experimentalandcalculat-ed mass-gainsare close to each other upto anelectrolyteconcentrationof 100molm- 3 (i.e.cur-rent efficiencyis closeto unity)implyingthattheentirecurrentpassedis ionic (and no fractioniselectronic)andhas beenusedfor the film forma-tion. However,the experimentallyobservedmass-gainsweregreaterthanthosecalculatedby aboutNIGAM etal.:STUDIES OF ANODIC OXIDE FILMS FORMED ON TANTALUMTableI-ExperimentalandCalculatedMass-GainsandCurrentEfficienciesforFormationofOxideFilmsonTantaluminAqueousElectrolytesatDifferentChargesElectrolyte ConcentrationChargeMass-gain(kgx W)Current( molm-3)Cx 103 efficiencyExp.Calc.Citricacid1083206816900. 87do50403303 94108076476 3543251 51648016. 1Tartaricacid107 601 801 .Aceticacid845260. 96Hydrochloric acidS93 26 and 12%respectivelyat 500and 1000molm- 3citric acid concentrations.The data in tartaric,aceticand hydrochloricacidsas electrolytesalsoshowedsimilartrends(Table1).The dataindicateanexcessequivalento 14,10and8% respectivelyover the calculatedmass-gaindue to oxidationin1000molm- 3 tartaric,oxalicandhydrochloricac-ids. Experimentalerror and formationof higher<Jxidesof tantalumhavebeenruledoutas.thepos-siblecausesfor the observedmass-gainsinceex-perimentswererepeatedfive timesandtheexcessmass-gain obtained was highly reproducible(±2,ug),and in literatureformationsof no otherstabletantalumoxide,exceptTa20shasbeenre-ported.The onlypossibilityis theincorporationofanionsinto theoxidefilm andthis is substantiatedby the resultsreportedin our earliercommunica-tion2 thatthebreakdownvoltageof thefilmsis af-fectedby the incorporationof anions (and notcations)fromtheseelectrolytes.Further,theincor-porationof anionssuchasP, C~S andF into theoxidefilm hasbeenreportedby otherworkers4,sandthisalsosuggeststhattheenhancedmass-gainis mostprobablydue to the incorporationof an-ionsintotheoxidefilms.DielectricmeasurementsCapacitancedata for the oxidesfilms formedupto various formation voltages (V) in100molm- 3 aqueouscitricacidata currentdens-ity 50A m-2 wereobtained.The plot of recipro-cal capacitance(1/C) per m2 versusthicknessperm2 calculatedusingFaraday'slaw, is linear (Fig.1), indicatingthat the film is uniformlyformedwith constantfield and 1/C can be taken as ameasureof thicknessof the oxidefilm formedinaqueouselectrolyte.The effectof concentrationoflS001000- ••••...!j!soo0.00Fig. 1-Plot of 1/C per m2versusthicknessperm2 for anodicoxidefilmsin aqueouscitricacid.the electrolytewas studiedby forming films ofvaryingvoltagesin 10,100and 1000molm- 3 cit-ric acid.The plotsof 1/C perm2 versusV ateachelectrolyteconcentrationare linear (Fig. 2) andshowa highervalueof 1/C for a film formedatlowerelectrolyteconcentration.The plotsof tan~versusV for thesefilmsaregivenin Fig. 3.As ex-pectedtan~decreaseswith theincreasein voltageof formation at each electrolyteconcentration.The valuesof tan~arehigherat lower electrolyteconcentration.The capacitancedatawerealsoob-tainedfor thefilmsformedin 100molm- 3 tartar-ic, acetic,oxalic and hydrochloricacids and theplotsof 1/C per m2 versusV are linearandtheir199INDIAN 1.CHEM., VOL. 27A, MARCH 1988" .... (2)... (1)c= EAEoBVwhere E is the dielectricconstantsof the oxidefilm, Eo is the permitivityof the free space(8.85x 1O-1~Fm-l),A is theareaof thespecimen(usedas a counterelectrode)and d is the thick-nessof theoxidefilm.At a constantcurrentdens-ity, d increaseslinearlywith voltageandbecomesequalto B V whereB is theanodizationconstant.SubstitutingBV for d inEq. (1)wegetc= EAEod14QO1200080bI.LNE 600- ~40bslopesobtainedby themethodof leastsquaresarereportedin Table 2. The typicaldatafor a 50Vfilm (Table2) showdependenceof capacitanceon .~,thenatureof theelectrolyte.The dielectricconstantswere calculatedusingthecapacitanceformulaefor a parallelplatecapa-citorin themannerdescribedbelow:The capacitance(C)isdefinedbyEq. (1)70Voltage (V)200Fig. 2-t Variationfo lIC per m2 withvoltageof formation(Y)for anqdicoxide films on tantalumat varyingconcentrationsof aq4eouscitricacid[(e) 10molm-3; (0) 100;(L::.) 1000].voltag. IV)Fig. 3-iYariation of dielectricloss factor (tano)with voltageof fOllI1fitionfor anodic oxide films on tantalumat varyingconcenttationsof aqueouscitric acid. [(e) 10molm-3; (0)100;( L::. ) 1000]..-... (3)puttingA= 1m2 andrearrangingEq. (2),wegetThe plot of 11C versusV is linearwith theslope=B / EEo· If the anodizationconstantis knownthenfrom theslopeE canbe calculated.Anodiza-tion constantswere calculatedfrom the anodiza-tion datausingFaraday'slaw and arereportedinTable2. Using theslopesof 11C per m2 versusVplots and the anodizationconstants(Table2), thedielectricconstantswere calculated.The resultsreportedin Table2 showdependenceof dielectricconstanton thenatureaswellastheconcentrationof theelectrolyte.60oo><..c.IT4ble2- DielectricMeasurementDatafor VariouslyFormedTantalmOxideFilmsinAqueousElectrolytesElectrolyteConcentrationapacitanceAnodiatioSlopeof lICDiel ctric(mol m-3)of.a50Y fi mconstant(fl)versus plotscons ant(FxW)(my-I x lOW)(F-1 m2Y-I)Tartaricacid10010.45.726 1728 8Aceticacid931. 6.Oxal6805Hydrochloric acid29 1Citricacid110.2do1 02.383200 I III Ii''.111I'1qqlNIGAM etal.:STUDIES OF ANODIC OXIDE FILMS FORMED ON TANTALUM600of chloride ions tantalummetal combineswithchlorideions givingan adsorbedfilm of tantalumchloride(TaCls)whichthenundergoespartialdis-solutionprovidingTa+s ions in solution.The re-leasedtantalumions can combinewith the chlo-ride ions in the solutiongivingrise to a stablechloro-complex of tantalum (of the typeTasC~a-S)-)which does not dissociatesand thuspreventsfurtherparticipationof chlorideions intheanodicchargetransfer.At higherchlorideionconcentration,the chlorocomplexof tantalumisformed more efficiently,thereby inhibiting thegrowthof oxide film. A similarobservationhasbeenrecordedearlier1Z•The rateof film dissolutionwasstudiednextbymeasuringthecapacitanceof a 50Y film (formedin 100molm-3 citric acid) in 5,400 and10,800molm- 3 aqueoushydrofluoricacid solu-tionsat variousintervalsof time(t) andthe plotsof 11C per mZ versust are shownin Fig. 5. Thefilmdissolvesmorereadilyin 10,800molm- 3 thanin 5,400molm- 3 HF; for instance,the timeperi-ods requiredto dissolvehalf of thefilm in 10,800and 5,400molm-3 HF are 205 and 275 sec.,re-spectively.The rateof dissolutionis veryfastuptoa stagewhenhalf theflim dissolvesandthereafterit slowsdown.Similartrendsin the rateof disso-lution are found for a 100Y film (Fig. 5), andsincethisfilm haslargerthickness,it requireslarg-er timeto reduceto half its originalthickness.It ismostlikelythatwhenTazOsfilm is put in concen-tratedhydrofluoricacid,a surfacelayerof tanta-10007S0.. .....!~sooI II 1/ / .....'13S0,fjI:j!2S012S0The effectof chlorideions on the capacitanceof TazOs formedin 100molm-3 citric acid wasstudiedby measuringthecapacitancesof thefilmsin the mixedsolutionsof hydrochloricand citricacids in the molar ratios of 1:1200,1:120,1:12and 1:2.4.The capacitanceof the film increasedwith the increasein the amountof chlorideions.The plots of 11C per mZ versusvoltageof filmformationare linear(Fig. 4). The slopesof theselinearplotsdecreasefrom6.20to 5.30,4.30,3.95and3.45mZF-1y-l whentheconcentrationof hy-drochloric acid is increasedsuccessively.Thecorrespondingdielectricconstantsare,foundto in-creasefrom 29.0to 33.9,41.8,45.6and52.2,re-spectively.Direct anodizationof tantalumwasalsocarriedout in all themixedsolutions.It is found,thoughthe maximumfilm formationvoltageis 365Y incitricacidyetthefilmscouldbeformedonlyupto340, 320, 280 and 220Y respectivelyin the re-spectivemixedsolutionscontainingchlorideionsin increasingconcentration.It appearsthatduringthe growthof tantalumoxidefilm in thepresenceO<lo 20 SO 100 150VOllag. (V)200 2S0 o 200 1000till2000 3000Fig. 4-Effect of chlorideion concentrationon anodicoxidefilm growthon tantalum.[(8) 100molm-3 citric acid;mixedsolutionof hydrochloricacid and citric acid in the molar ra-tiosof 1:\) 1:1200:Ie' 1'120(x) 1:12;(,:.) 1:2.4j.Fig. 5-Rate of dissolutionof anodicallyformedfilm on tanta-lum in hydrofluoricacid.[(0) 100V in 5400molm-3 hydrof-luoric acid; (.) 50V in 5400molm- 3 hydrofluoricacid;and(b.) SOVin 10800molm-3 hydrofluoricacid].201INDIAN J. CHEM., YOL. 27A, MARCH 1988Fig. 6-Plot of variationof voltageof formationversustimeof anodizationfor tantalumoxidefilms of variousthicknesses[(0) 30Y; (e)40Y; (A) 50Y].AcknowledgementOne of theauthors(PK) thankstheCSIR, NewDelhi for the awardof a junior researchfellow-ship.References1 NigamR K, Kalra K C & Katyal P, Indian J Chern,25A(1986)1080.2 NigamR K, Kalra K C & Katyal P, Indian J Chern,26A(1987)17.3 NigamR K, Kalra K C & Katyal P, Indian J Chern,26A(1987)819.4 RandallJ J Jr, ElectrochirnActa,20(1975)663.5 Arifuku F, IwakuraC, YoneyamaH & TamuraH, DenkiKogaku,46(1978)19.6 NigamR K & ChaudharyR S, Indian J Chern,11 (1973)356.7 Amsel G, Cherki C, FeiulladeG & Nadai J P, J physChernsolids,30(1969)2117.~.t .•504030tts)20106050:':40.!3~ 2010voltageot the film alreadypresenton it (Pig. 6).This showsthat thereis no possibilityof oxygenion movingthroughthe electrolyteto specimen-Ito producelayersinsidethe existingoxidelayers.The normalrateof anodizationcomparewellwiththe anodizationrate duringthis experiment.Thisindicatesthat the field of Ta20sfilm betweenspecimensI andII of thisexperimentremainsun-changedand tantalumions producedat specimenI aretransportedto specimen-IIthroughthetanta-lumoxide(wherethefield is constant)andthentosolutionto producefreshlayeron the outersur-faceof thespecimen-II.Thus it canbe concludedthat only tantalumions are mobile during thegrowthofTa20sfilm.lum f' uoride (TaPS) is formed which dissolvedcomp telyin concHF andhencecausesdissolu-tion0 thefilm.Ion m(JbilitystudiesTh1speCimensI and II wereanodizedat filmforma'on voltages of 30, 40 and 50V in100m I m- 3 citric.acid at.a curren,t density.of3 A - 2 and theIr cap CItancesbefor sealingwererpeasuredand found to be 1.92,1.44, 1.15(forfl'cimenI) and 1.14,0.85,0.68px 10-6 (forspec' n II) at the abovefilm formationvoltagesrespec·vely.In the combinedassemblythe capa-citanceLof both specimensI and II are the same(0.57,p.43, 0.34x 1O-6P at 30, 40 nd 50V re-spectivfly)and the valuesare in proportion toth irr1ducedareasof 1X 1O-4m2,andtheassem-bly act~as oneunit (area1x 1O-4m2).The tagofthesPeFimenI in eachof thepairedassemblywasconnectedto thepositiveterminalof theconstantcurrentIgeneratorand the specimenII whichwasea li r~odized upto 30, 40 and 50V pa redas-semblie~was further anodizedupto 60, 70 and80V, Ifspectively. The capacitancesof thesepaired~ystemswere againmeasuredtakingsam-lesI ard II in thecircuitrespectively.The capacitances!of specimensI andII wereagainfoundtobe the $ame(0.30,0.26 and 0.22x 1O-6p at 60,70 andfl80Vrespectively)and this ensuredcom-pletese . g of I and II and no possibilityof theelectrol, e cre pingthroughtheoxid .The magni-tudesof capacitanceswerefurtherreducedduetofurther~rmationof thefilm.The assembliesweredismantllidby dissolvinglacomitein acetoneandwashingIwithconductivitywater.The capacitancesof separ~tedspecimensI andII weremeasuredasalsotho~eof theouterand innersurfacesof boththespedmensafterapplyinglacomiteto theotherside.Th~capacitancesof the specimenI afterse-parationl(1.90,1.42,1.17x x 1O-6p at 30, 40 and50V, resbectively)and beforesealingwere ident-ical indi4atingthatspecimen-Ir mainedunaffect-ed. The Icapacitancesper m2for both the innerand out~rsurfacesof the spec~en-I we~e0.56,0.42andl0.35x 1O-2Pm-2 at film formatIOnvol-tagesof 130,40 and 50V respectivelywhile thecorrespo*dingvaluesfor the inner surfaceof thespecimen~II were 0.57, 0.43 and0.34x lOi2p m-2. The magnitudesof capacitanceperm2fqr thespecimenI comparewellwiththoseof the ll$er surfaceof the specipen-II.This im-pliesthatI thesandwitchedoxidefilm is not affect-ed.Purth~ranodizationof the separatedspecime-n-I incre;lsesthe film formationvoltageimmedi-ately andlthe film formationvoltageattainsthe202II I 'I 'lltll 1111" I 'Iii'·II III!'1'1 'Ill·III IINIGAM etal.:STUDIES OF ANODIC OXIDE FILMS FORMED ON TANTALUM8 CherkiC, ElecrrochimActa,16 (1971)1227.9 YoungL, Anodic oxidefilms,(AcademicPress,New York)1961.10 KlererJ, J electrochemSoc,112(1965)896.11 VijhAK, Co"Science,11(1971)161.12 Vijh A K, Oxide and oxidefilms, Vol 2 editedby J WDiggle(MarcelDekker,NewYork) 1973.13 Nigam R K & ChaudharyR S, ElectrochimActa, 16(1971)1719.14 Amsel G & SamuelD, J phys chem Solids, 23 (1962)1707.15 YoungL ProcRoySocLondon,A244 (1958)41.203

Current Efficiency, Dielectric Measurements & Ion Mobility Studies of Anodic Oxide Films Formed on Tantalum in Aqueous Electrolytes

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Current Efficiency, Dielectric Measurements & Ion Mobility Studies of Anodic Oxide Films Formed on Tantalum in Aqueous Electrolytes

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