Activation and early life of a pressed barium scandate cathode

Applicationsof SurfaceScience2(1979)173—186© North-HollandPublishingCompany

ACTWATION AND EARLY LIFE OF A PRESSEDBARIUM SCANDATE CATHODE

A. vanOOSTROM andL. AUGUSTUSPhilipsResearchLaboratories,Eindhoven,TheNetherlands

Received9 July 1978Revisedmanuscriptreceived16 August 1978

Activation andearly life of a pressedbariumscandatecathodehavebeeninvestigated,corre-lating thermionic emission propertieswith elemental compositionand distribution, as deter-mined with scanningAuger microscopy.It is shownthat suchcathodesafteractivationhaveacurrentdensityJ 10 A/cm

2 at 950°Cwhich is due to activatedscandiumoxide (Sc203) re-

gionsin thesurface.

1. Introduction

In thermioniccathodesbarium is generally usedto obtain a low work functionemitter. This is the case for oxide-coatedcathodes,but also applies to metalsub-strate cathodes,as L-cathodes [1] and impregnatedcathodes[2] . In the lattertypesof cathodesa tungstensubstratehasabarium coverageof abouta monolayer.During operationthe cathodewill lose barium, asa result of evaporation,chemicalreactionswith the environmentor ion bombardment.Therefore,during life freshbariumhas to becomeavailable.The bariumneededfor the initial activationof thecathodeandsupply duringlife comesfrom below theemittingsurfaceby decompo-sition of a compound at high temperature,subsequentlyfollowed by diffusionthroughthe poresof thetungstenbody to the surface.In L-cathodesthecompoundis stored in a reservoirbelow the porousbody, while in impregnatedcathodesthecompoundis storedin the poroustungstenbody itself. Barium—calcium--aluminateeither in the ratio 5 3 : 2 or 4 : 1 1 is the mostwidely usedcompoundin thesecathodes.

Anotherway to makemetal substratecathodesconsistsof pressinga mixture oftungstenpowderand a barium containingcompound.A similar result is obtained,as in the case of the impregnatedcathode.The barium is directly availableat andbelow the surfaceof the cathode.Good resultshavebeenobtainedby usingbariumscandateas the barium containingcompound [31.Such cathodeshave excellentemissionproperties,a low evaporationrateand a longlife.

The presentwork is a first attempt to analyzethe activationprocessandearly

174 A. van Oostrom,L. Augustus/Pressedbarium scandatecathodes

life of a pressedbarium scandatecathodein moredetail.The cathodesare mountedin a vacuumchamberandanalyzedprior to, during andafter activation.The ther-mionic emissionis measured,aswell as theheight of the potentialbarrier at thesur-facewith a field emissionretardingpotentialgun. Theseresultsare correlatedwiththeelementalcompositionand distribution asdeterminedby scanningAugermicro-scopy.

2. Experimentalprocedure

The investigatedpressedbarium scandatecathodeswere composedof a mixtureof fine grain tungstenpowderand 7% (by weight) barium scandate.In the BaO—Sc203 systemseveralcompoundsexistwhich arestableunderatmosphericpressure.In the Sc203-rich part of the systemtwo compoundsBa3Sc409andBa5c204arefound, if a mixture of BaCO3 (Merck) andSc203(JohnsonandMatthey)is fired at1400°C[4]. In the cathodesBa3Sc4O9(3BaO—2Sc2O3)has beenused as thebarium containingcompound.The mixtureof tungstenpowderandbariumscandatewas pressedandsinteredat 1570°Cin a hydrogenatmosphereduring 15 mm. Afterthis processthe cathodeswhich had a diameterof 1.8 mm, were smoothlypolishedwith aluminapowderandultrasonicallycleanedin freon.

The analysisof theactivationand earlylife of thepressedscandatecathodeswascarriedout in a bakeablestainlesssteel UHV-chamberwith a basepressurebelow10_8 Pa. The vacuumsystemis pumpedwith a getterion pumpanda sublimationpump, the gas pressureis monitored with a quadrupolepartialpressureanalyzer.The cathodecanbe indirectly heatedandis mountedon a universaltype of mani-pulator. By rotating the manipulatorthe cathodecanbe set in different positionsfor severaltypesof analysis.

In one positionthe cathodefacesan anodeon a linearmotion drive feedthrough.The cathode—anodedistancerangesbetween200 micronsand 1 mm andcanbe setwith an accuracyof about20 micronsfor the thermiomcemissionmeasurements.Prior to taking emission data the nickel anodeis degassedat a temperatureof750°Cusing its own built-in heating filament. It is well-known that after suchatreatmentsulfur may accumulateat the nickel surface [5]. An Auger analysisshowedthat no sulfur contaminationof thecathodeoccurredduring thecollectionof electronemissiondata.Thermionicemissionmeasurementsweremadeusing5 uspulses with a maximum amplitude of 1000 V and a repetitionrate of 50 Hz. Inanotherpositionthecathodetemperaturecanbe determinedthroughaviewing portwith an infraredpyrometer,type Ircon,model 300 CH. All temperaturesgiven inthis paperaremolybdenumbrightnesstemperatures.

The potential barrierheight at the cathodesurfacecan also be determinedbyrotating the cathodein front of a retardingfield gun [6] *• In this gun electrons

* TheFERP-guncanbeobtainedfrom FEI Co., P.O. Box 654, McMinnville, Oregon.

A. vanOostrom,L. Augustus/Pressedbarium scandatecathodes 175

from a field emissionsourceare accelerated,focussedand then retardedon arrivalat the thermionic cathodesurface.The beamdiameterat the cathodesurfaceissmaller than 0.3 mm. The field emissionsourceoffers the distinct advantageoverothersourcesin that it enablesthe absolutemeasurementof thework function ofthe thermionic cathode.Sincemost of the field emittedelectronstunnel close tothe Fermi level into vacuum,thepotentialbarrierheightof the thermioniccathodeequalsthe appliedpotentialdifferencebetweenfield emitter andcathode.As suchameasurementcanonly beperformedwith thecathodeat low temperaturesandzerofield, where thereis neitherspacechargenor Schottkyreduction,thepotentialbar-rier heightequalsthework function of the cathode.

Finally, the cathodecanbeputin front of a cylindricalmirroranalyzerforAugeranalysis.On the axis of the analyzeris an electrongun with deflectionplates.Thisanalyzerfrom PhysicalElectronicscanproducesecondaiyelectron andAuger im-agesof the cathodesurface [7] . At points of interesta completeAugerspectrumcanbe madeandtheconcentrationof the elementspresentcanbedetermined.Thebestlateralresolutionof this instrumentis about3 microns.

The activationof a pressedbarium scandatecathodeis similarto an impregnatedcathode.Initially, the cathodetemperatureis raisedstepwiseovera periodof 1 h to950°Cto removesome impurities.Thenthe temperaturewas increasedto 1165°Cfor 1 mm andkept at 1125°Cfor another5 mm.After this treatmenttheelectronemissionwasmeasuredat either950°Cor 1050°C.If necessarysomefurther activa-tion was carriedout at 1125°C.During suchan activationcycle thepressurein thevacuumsystemnever roseabove10—6 Pa.

3. Electron emissionproperties

After activationpressedbarium scandatecathodesare excellentthermionicemit-ters.Their emissionlevels are for a temperatureof 950°Cabouta factor of fourhigher than an impregnatedcathodewith a 5 : 3 2 barium calciumaluminateim-pregnant.The emissionlevels aresimilar to the M-cathode,an impregnatedcathodecoatedwith 500nm of osmium [8] . If measuredat 1000V fully activatedscandatecathodeswill reacha current densityJ 10 A/cm2 at 950°Cand 35 A/cm2 at1050°C.Occasionally,evenhighercurrentdensitieshavebeenobservedundertheseconditions.

Electronemissiondata are normally taken as a function of temperatureT andelectric field E. The appliedfield Ewill lower theheight of thepotentialbarrierandwill enhancethe electronemissionfor a constanttemperature.This effect,knownas the Schottky effect, canbe introducedas a correctionterm in the Richardsonequationwhich thenreads

J~AT2exp{_[ecI_(e3E)~”2]/kT}, (1)

176 A. vanOostrom,L. Augustus/Pressedbarium scandatecathodes

where 1 is the work function of the cathode,e the electroncharge,k Boltzmann’sconstantandA a universalconstantwith value 120 A/cm2 K2. The topof thepo-tential barrier liescloserto thesurface,the highertheelectric field. For a collectingfield E = i04 V/cm this distanceis 19 nm.

For a constantcathodetemperatureT, work function andcathode—anodedis-tanced, formula(1) reducesto the simplerelationwith constantsa andb

logJ°a+bV~2, (2)

wheretheappliedvoltage V = E/d.Let us now considerthe thermionicemissionof a particularcathodewhich after

the describedactivationprocesshadbeenkept for 90 h at a temperatureof 1050°C.Fig. I showsthe Schottkyplot log J versus v1/2 at this stagefor severaltempera-tures.The current densityat 950°Cand 1000 V amountsto about9 A/cm2. Theincreasein current densitywith appliedvoltageis more rapidthancanbe expectedfrom thecathodetemperaturemeasuredwith the infraredpyrometer.Thehighvalueof b, even at 1000V, canbe expressedas a too low valueof thetemperatureor atoo high value of the electric field strength.In the latter case a field strengthof5 X l0~V/cm has to be assumedfor an appliedvoltage of 1000V. Such a valueis

1o~

~95O~E4

~5O~C

0 10 20 30

Fig. 1. Schottky plot for an activatedpressedbarium scandatecathode.The log of thecurrentdensityJ in A/cm2 is plottedversusthe squareroot of theappliedvoltageV in volt.

A. van Oostrom,L. Augustus/ Pressedbariumscandatecathodes 177

clearly much too high, as the surfaceof a pressedcathodeis relativelysmooth.Animpregnatedcathodewith a less smoothsurfacehas a lower valueof b. Therefore,local field enhancementeffectscanbeneglected.

Considerabledeviationsfrom thenormal Schottkyplot are to be expected,if thesurfaceis emittinginhomogeneously.For a patchysurfaceit hasbeenshownthat theslopeb canbe muchhigher, particularly if the collecting field E is a few timesthepatchfield E~[9] . We shall returnto this point later.

Since the slope b is too high, the zero-fieldextrapolationvaluesof the currentdensityhave little meaningandno Richardsonplot is made.We preferto expressthe emissionlevels as currentdensity for a given temperatureand field. However,for reasonsof comparisonit is sometimesconvenientto expresscurrentdensitiesaswork functions,assumingthe currentdensitiesare measuredat saturationandtheRichardsonformulais correct.On the basisof theseassumptionswe find for theac-tivated scandatecathodea work function4= 1.78 eV at 950°C.Thermionicemis-sion datacollected at highelectric fields tend to emphasizethe low work functionregionsof a patchysurface.This is essentiallydifferent from a measurementof thepotentialbarrierheight with the field emissionretardingpotentialgun.Since this isa zero-fieldmeasurementthetop of thepotentialbarrieris muchfurther awayfromthe surfacethan for the high field case.Undersuchcircumstancesapatchysurface

(a.u.)

0 2 V(volts)

Fig. 2. Retardingfield currentI in arbitrary unitsversusthe applied potentialdifference V involt for an activatedcathodeas collector.Sincethe field emittedelectronstunnel closeto theFermi level, thework function can berapidly derivedfrom thisplot.

178 A. van Oostrom,L. Augustus/Pressedbariumscandatecathodes

with a variation in work function will exhibit an averagevalue for the work func-tion whichis muchhigher than for thethermioniccase.

An advantageof the measurementwith the retardingfield gun is that theworkfunction at low temperaturescanbe determinedalsoprior to activation.Theinitialwork function of an unheatedcathodeis about5.5 eV. On heating it will slowlystart to drop, after heatingto 950°Cit is still 3.3 eV. Fig. 2 shows the retardingfield currentversusappliedvoltagedifferencebetweenfield emitterandthermioniccathodefor the samepressedbariumscandatecathodeof fig. 1. From this curvewederivethat thework functionof the cathodeis about2.0eV.

4. Elementaldistribution

In contrastto a standardimpregnatedtungstencathode,thesurfaceof a pressedbarium scandatecathodeis relativelysmooth.However, a more detailedexamina-tion of the surfacein a SEM showsa largenumberof lessperfectlysmoothregions.One of theseregionsis shown in fig. 3. Cathodesmountedin theUHV-systemcanbe examinedwith the scanningAuger analyzer.Secondaryelectron imagescanbedirectly comparedwith Augerimagesof the samesurfacearea.It is then foundthatthe less smooth regionson the cathodesurface,as seenin the secondaryelectron

Fig. 3. SEM-imageof anunactivatedcathode.Magnification5000 X.


image,correspondto scandium-richareas.This is presumablycausedby imperfectmixing of the tungstenpowderandthe barium scandate.Fig. 4 illustratesthe in-homogeneousdistribution of scandiumin the tungstenmatrix.The Augerimagesofscandium and tungstenare shown, characteristicAuger line transitionshavebeenselectedfrom theAuger spectrum.In comparingtheseimagesit mustberememberedthat Augerelectronspectroscopyis strictly a surfacesensitivetechnique.This is il-lustratedat the two tungstenimagesof fig. 4 takenat the samed.c. level settings.

Sc,333eV

V

W,169 eV

Fig. 4. Auger-imagesof an unactivatedcathodeof tungstenand scandium.Imagedarea130X 170 ~m

2.Primary beamenergy5 keY, primary beamcurrent 5 nA, diameterelectronbeam4 ~m, recordingtime 5 mm.

180 A. van Oostrom,L. Augustus/Pressedbariumscandatecathodes

The Auger imagecorrespondingto theW 169 eV transitionis morediffuse andlessbright. Since the escapedepth of Augerelectronsof this energyis about0.5 nmcomparedwith 1.6nm for theW 1738eV transition,the lowerpeakis moresurfacesensitive and decreasesmore on adsorption.The scandiumimage is essentiallyanimageof barium scandatein the surfaceregion. The position of the LMM-transitionof scandiumhasshifted from 340 eV in themetal to 333 eV in theboundstate.

Changesin theelementalcompositionanddistributioncouldnow be monitoredduring the activationprocess.After eachstep in the activationprocessthetempera-ture is reducedto room temperature.The work function is measuredwith the re-tarding field gun and an Augeranalysismade.TheAuger analysisconsistsof takingan Auger spectrumby rapidly scanningwith a small beamsize over an area130X 170jim2. Augerpeaksbelongingto elementsof interest,asbarium,scandiumortungsten,were then usedto makeelementalimagesof particularregionsof the sur-face. This procedureoften leadto a closerexaminationof areas with a high con-centrationof a particularelement.

Initially, the Auger spectrumindicatesthe presencenot only of tungsten,bari-um, scandiumand oxygen,but also of impurities as carbon,sulfur, chlorineandfluorine. Already prior to activation at relatively low temperaturecarbonis beingremovedas carbon monoxide from the surface.At 950°Cthe carbon removaliscomplete,someoxygenhasalsobeenlost from thesurfaceandthebarium scandatestartsto decompose.The amountof sulfurhasincreasedat this stageandmay reachmonolayercoverageof the tungstensubstratelocally. The presenceof sulfur is cor-relatedwith tungsten,it is found only in regionswherethescandateis absent.Thissituationis illustratedin fig. 5, wheretheAugerimagesof tungsten,scandium,sul-fur andoxygenare compared.

Fortunately,the sulfur contaminationis not a permanentone. After the hightemperatureactivation at 1165 and 1125°Cthe sulfur concentrationhas alreadydecreased.After 90 h of operationat 1050°C, sulfur hasdroppedbelowthedetec-tion limit of the instrument(10—2%)and thecurrentdensityhasincreased.Chlorineand fluorine have alreadydisappearedjust after the high temperatureactivation.The Auger spectrumobtainedby scanningoveran areaof 260 X 340 pm2 after90hat 1050°Cis shown in fig. 6. The cathodeis now well-activated,asthethermionicemission resultsindicate and is free of contaminants.Only the elementstungsten,barium,scandiumand oxygenare foundin thespectrum.

As remarkedearlier the barium scandatein a new pressedcathodeis notevenlydistributed.At low temperaturesthe scandateremainsunaltered,but afterheatingto 950°Cthescandatestartsto decompose.Thebariumhasstartedto diffuse awayfrom thescandateareas,scandiumandoxygen,are still localized,aswe havealreadyseenin fig. 5. After activation the barium concentrationis aboutthe sameevery-where,while oxygenhasalso becomemobile.The differencebetweenthesituationprior to andafter activationcanbe clearly seenby comparingfig. 5 with fig. 7. Infig. 7 theAuger imagesof barium,scandiumandoxygenare shownjustafter activa-tion of the samesurfacearea, asin fig. 5. After activationthe scandiumis stifi in

A. vanOostrom,L. Augustus/ Pressedbarium scandatecathodes 181

____ Sc

Fig. 5. Auger-imagesof tungsten,scandium,sulfur andoxygenafterheatingof thecathodefor15 mm at 950°C.Note thesimilarity betweenthescandiumandoxygenimagesandtheapparentcorrelationbetweenthe tungstenandsulfur images.Otherconditionsasin fig. 4.

0

0 1000 E (eV)

Fig. 6. Auger spectrumof an activatedpressedbarium scandatecathode.Scannedarea260X 340Mm

2.Otherconditionsof theelectronbeamasin fig. 4.


~Ba

Sc

~4.

Fig. 7. Auger-imagesof barium, scandiumand oxygenafter the high temperatureactivationprocess,illustrating theimmobility of scandiumin comparisonwith bariumand oxygen.Condi-tionsasin fig. 4.

the boundstate,ascanbe seenfrom the Auger spectrum.Apparently,barium scan-datedecomposesduring the activationprocessandsurfacediffusion of barium andto a lesserextent oxygenhasset in. The scandiumappearsto be the leastmobileandis presentasscandiumoxide.

Auger spectroscopyis a techniquewhich givesinformationon the elementaldis-tribution and concentrationwith the exceptionof hydrogenandhelium. In somespecificcasesline shifts or peakshapechangesmay give informationonthechemical

A. van Oostrom,L. Augustus/Pressedbarium scandatecathodes 183

bond of the element.A well-known exampleis the oxidesof the seriesMg, Al, SiandP [10]. In generalcalibration has to carried outby taking spectrafrom thecompoundsinvolved. The informationobtainedmay taketheform of lineshifts,butmore often a changein peak amplitudes.Sensitivityfactors,as publishedfor thepure elements[11] , may changedrasticallyfor elementsin compounds,if the elec-tronsinvolved in theAuger transitionare also tied up in the chemicalbond.

Sincewe would like to make the resultsmore quantitative,we havetakenstan-dardAugerspectraof barium scandate(Ba3Sc4O9),scandiumoxide (Sc203)andbarium oxide (BaO). Thesespectrawere calibratedagainsta silver targetcleanedbyargonion bombardment.Fig. 8 is the standardspectrumobtainedfrombarium scan-date powderpressedin a copper matrix, similar spectrawere obtainedfrom theothercompoundsand the silver target.From thesespectrasensitivity factorsweredetermined,assumingthe sensitivity for silver to be one.Table 1 summarizestheresultfor thecompounds,aswell asfor the pureelements.

If the surfaceof anactivatedpressedbariumscandatecathodeis sputter-cleanedby argonion bombardment,the Auger spectrumwill only indicatethepresenceoftungsten,scandiumandoxygen at the surface.The tungstensurfacewill be clean,the scandiumwill be in the oxidized state.A comparisonwith the standardspec-trum shows that the remainingscandiumoxide is indeed5c203.No barium orbarium oxide is left on the surface.We canusethe W 179 eV and the Sc 333 eVpeak to determinethe concentrationsof the tungstenandthe scandiumoxidewiththe aid of the sensitivity factorsof table 1. Wethen find the sputter-cleanedsurface

dNdE

Ba Ba

Sc

0

I -- J~~f ~ 1

0 500 E(eV)

Fig. 8. StandardAuger spectrumof barium scandate(Ba3Sc4O9).Scannedarea26 X 34 ~m2.

Otherconditionsof theelectronbeamasin fig. 4.


Table 1Sensitivity factorsfor the elementsandcompoundsof interestin thiswork calibratedagainstAg (S = 1.0).Primary beamenergy5.0 keY

Element BaO Sc2O3 Ba3Sc4O9

Ba(600eV) 0.12 0.21 0.180(510 eV) 0.40 0.37 0.14 0.19Sc (333,340 eV) 0.28 0.14 0.19

to consist of 72% tungstenand 28% scandium-oxide,in good agreementwith the7% by weight additionof barium scandateto the tungstenpowderprior to pressing.

An examinationof the compositionof the activatedcathodeafter90 h of opera-tion gives an interestingresult.The tungstensubstrateis coveredwith aboutequalamountsof BaO andSc203. In regions,wherebarium scandatewas presentin thematrix prior to activation, decompositionof the scandatehasoccurred.In suchre-gions the ratio BaO/Sc203is about0.25.A considerabledepletionof BaO is foundin the formerscandateregions,thesurfacediffusion of BaO wasalsoseenin fig. 7.

20°C

_ 950tFig. 9. Secondaryelectronimagesof cathodesurfaceat 20°Cand950°C.Scandium-oxiderichareasshow up as dark patchesat room temperatureandbright onesat950°C.Conditionsasinfig. 4.


Finally, the activatedcathodeshowsan interestingphenomenon,if thetempera-ture is switchedfrom room temperatureto operatingtemperatures.The scandium-oxide rich regionsbecomehighly activatedat a temperatureof 950°C.The Augerpeaksof barium,scandiumandoxygen shift towardshigherenergy,while thetung-stenpeakdoes not showa line shift. Scandium-oxiderich regionsnow havea lowerwork function than the tungstensubstrateareas.This is also demonstratedby thesecondaryemissionimages at room temperatureand950°C.Fig. 9 illustratesthisphenomenonwhich showsthe drastic changein the secondaryemissionyield for asurfaceareaof 130 X 170pm2 at 20°and950°C.

5. Discussion

The surfaceareaof an activatedpressedbarium scandatecathodeafter 90h ofoperationstifi showsthe distinction visible in examiningtheSEM-pictureof an un-activatedcathode.The regionswith the tungstensubstrateare stifi different in be-haviour from regionswith barium scandate,despitechangesin both areas.

The most interestingpart of the surfaceareais wherethe barium scandatehasdecomposedduring the activation process.Scandiumoxide (Sc

203)is the mainconstituentwith some barium oxide and free barium also present.However,theinitial concentrationratio BaO/Sc203= 1.5 hasbeenreducedto about0.25. Abouta quarterof the available cathodesurfaceareaconsistsof suchscandium-oxiderichregionswhich vary in size from less thanonemicron up to morethantenmicrons.At room temperaturescandium-oxidebehavesas an insulator, but at high temper-aturethe oxide becomesa semiconductor.From the secondaryelectronemissionpropertiesit is concludedthat the scandium-oxideregionsemitconsiderablybetterthan the surroundingregions.The surroundingregions consistof a tungstensub-stratewith equalamountsof BaO andSc2O3.The total coverageis aboutamono-layerin theseregions.

The patchinessof thecathodesurfaceis also reflectedin the anomolousbehav-iour of the Schottkyplot. The essentialconditionsfor sucha behaviourasa resultof patchinessreguiresthe collectingfield E ? 3Ev, if E~is thepatchfield.Thepatchfield is unknownsincewe do notknow thecontactpotentialdifferencesbetweenpatchesand their surroundingregions.The patcheson the surface are relativelysmall and fall within the resolution of the probe-holetechnique,as appliedprevi-ously to oxide andimpregnatedcathodes[12] . Therefore,we havenotbeenable tomeasurethe contactpotentialdifferences.If we assumea valueof 0.5 eV, thepatchfields will vary between5 X 102 and 5 X l0~V/cm. The collecting field in our ex-perimentsis about 2 X l0~V/cm and the conditionfor the anomolousSchottkyplot is fulfilled.

The impurities initially presentduringheatingandtheactivationprocessdo notseemto play an important role. Chlorine, fluorine and carbonrapidly disappearprior to and during activation,sulfuris moredifficult to removeand also interferes


with the surfacemigration of barium oxide. In generalsulfur did notstopthe acti-vation of thecathode.

The experimentsdescribedrefer to the pressedscandatecathodeduring activa-tion and early life. Wehave seenthat themobility of scandiumis considerablylessthen for barium or oxygen.This featurewill be an importantfactorfor considera-tion, if scandatecathodesare operatingunderion bombardment,despitethe factthat after ion sputter cleaningscandiumis still visible in theAugerspectrum.If thesurface region is depletedfrom scandium during operation, reactivationof thecathode will take many hours. This aspectof scandatecathodescertainly needsfurther examination.Generallyspeaking,the pressedbarium scandatecathodewitha currentdensityJ 10 A/cm2 at 950°Cis an interestingnewtype of cathodewithmanyuseful applications.

Acknowledgment

The authorswish to thank Mr. A. van Stratumfor preparingandmanufacturingthe cathodesandDr. P. Zalm for stimulatingdiscussions.

References

[11 H.J.Lemmens,MJ. Jansenand R. Loosjes,Philips Tech.Rev. 11(1949)341.[2] R. Levi, J. AppI. Phys.24 (1953) 233; 26 (1955)639.[3] U.S. Patent3, 358, 178 (1967).[41L.N. KomissarovaandB.I. Pokrovskii,RussianJ. Inorg. Chem. 10 (1965) 1532.[5] L.A. Harris, J. Appl. Phys.39 (1968)1419.[6] R.W. Strayer,W. Mackieand L.W. Swanson,SurfaceSci. 34 (1973)225.[7] N.C. MacDonald,AppI. Phys.Letters 16 (1970) 76.[81P. ZaIm andA.J.A. van Stratum,Philips Tech.Rev. 27 (1966)69.

[9] C.Herringand M. Nichols,Rev.Mod. Phys.21(1949)185.[10] L.E. Davisand A. Joshi,ASTM STP 596 (1976)52.[11] L.E. Davisetal., Handbookof AugerElectron Spectroscopy,2nd Ed. (1976).[12] C.G.J.Jansen,A. Venemaand Th.H. Weekers,J. Appi. Phys.37 (1966)2234.

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Activation and early life of a pressed barium scandate cathode