Plasma-physics-and-technology;-industrial-applicat

Vacuum64(2002)327–336Plasmaphysicsandtechnology;industrialapplicationsG.Bonizzoni*,E.VassalloIstitutodiFisicadelPlasma,ConsiglioNazionaledelleRicerche,ViaCozzi,53,Milano20125,ItalyAbstractPlasmatechnologiesofferawidespectrumofpossibletreatmentsofmaterials.Inparticular,infunctionofthepeculiarphysicalcharacteristicsoftheplasma,producedbydifferentionisationsystems,threetypesofprocessesonthematerialscanbeactivated:(1)Destructionoftoxic/harmfulmaterials;(2)Superficialmodificationofexistingmaterials;(3)Creationofnewmaterials.Thermalplasmacanbeusedtodestroythesolid,liquidandgaseoustoxichalogenatedandhazardoussubstancesortogenerateanticorrosion,thermalbarriers,antiwearcoatings,etc.Coldplasmascanbeusedforsurfacemodificationsofmaterials,rangingfromthesimpletopographicalchangestothecreationofsurfacechemistriesandcoatingsthatareradicallydifferentwithrespecttothebulkmaterial.Eachofthethreelineshasadifferentweightwithrespecttoanimmediateindustrialtransferring.Whilethefirsttwoarereadyforanimmediateindustrialutilisationoftheprocesses,thethirdoneisstillunderinvestigations.Someindustrialprocesseswillbedescribedand,inthecaseofsometreatmentsoftoxicsubstances,acomparativecostanalysisispresented.r2002ElsevierScienceLtd.Allrightsreserved.Keywords:Plasmatreatments:Surfacemodifications:Toxicmaterialdestruction1.IntroductionTheplasmatechnologyforindustrialprocessesessentiallyusestwodifferenttypesofplasma:thefirstone,named‘‘thermalplasma’’,isproducedathighpressure(10kpa)bymeansofdirectoralternatingcurrent(dc–ac)orradiofrequency(rf)ormicrowavesources.Thesedevices,knownastorches,produceplasmaswithelectronandiontemperaturesoftheorderof1–2eVandwithaverylowgasionisation.Dcandactorchescanbedividedintotransferredandnon-transferredarcandcanberealisedwithpowerhigherthansomeMW.Theproducedplasmaismainlyutilisedtodestroytoxic-harmfulsubstancesor,asinthecaseoftheplasmaspray,toproducecoatingsofthickfilms.Thesecondtypeofplasma,namedcoldornon-equilibriumplasma,ischaracterisedbytheelectrontemperaturehigherthantheiontempera-ture;itisproducedundervacuumconditionsusinglowpowerrformicrowaveordcsources.Theinteractionsoftheplasmaparticlesonthematerialsproducethemodificationofthesurfacesinordertoadddifferentfunctionalpropertieswithrespecttothebulkmaterial.2.Thermalplasmagenerators(plasmatorches)Mostthermalplasmas,usedinmaterialsproces-sing,aregeneratedbydcoracelectricaldischarges*Correspondingauthor.Tel.:[phone];fax:[phone]-207X/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.PII:S0042-207X(01)00341-4orbyhighfrequencyinductiondischarges.Thearcdischargeprovidesahighdensityandhightemperatureregionbetweentwoelectrodesand,inthepresenceofasufficientlyhighgasflow,theplasmacanextendbeyondoneoftheelectrodesintheformofaplasmajet.Theplasmaarcgeneratorscanbedividedinto(i)non-transferredareplasmatorch(Fig.1a),(ii)transferredarcplasmatorch(Fig.1b).Inthenon-transferredarctorchthetwoelectrodesdonotparticipateintheprocessandhavethesolefunctionofplasmageneration.Thecoupleofelectrodescouldhavedifferentgeometry;acylindricalbar(cathode)andahollowcylinder(anode)ortwocoaxialcylindersthatcouldoperatewithoppositepolarities.Non-transferredacarctorchesemploycylindricalelectrodesbecauseofeasiestinversionofpolarityinthisconfiguration.Theelectrodesaregenerallywatercooledandmanydesignsusemagneticaxialfieldsgeneratedbyadirectcurrentpassingthroughacoilthatsurroundstheelectrodes.Thissolutionhelpstoforcetheattackpointofthearconthewholeelectrodessurface.Non-transferredarctorchesareavailablewithlevelsofpowerbetween1kWto6MW[1].Theheatingefficiencyrangesbetween50%and90%andincreaseswiththeincreaseofthegaseousflow.Thegaseousflowsofplasmaaremoreelevatedthanthoseusedintheprocesseswithtransferredarc;thisleadstomoreexpensivetreatmentsofthedischargegases.Useofnon-reactiveplasmas,forinstanceargon,max-imisestheelectrodeslife,butdecreasesthethermalefficiency.Non-transferredarcreactorshavebeenmainlyusedforthepyrolysisofliquidormuddywaste.Infewcases,thewastematerialsareinjecteddirectlyintothearcprovidingpartoftheplasmagas;however,theinteractionofcorrosiveelements,suchaschlorine,withtheelectrodesmayleadtohigherosionratesrequiringfrequentchangeoftheelectrodes.Therefore,thereactantsaremorefrequentlyinjectedintotheplasmajetdownstreamoftheelectrodes.Inthetransferredarcreactors,oneoftheelectrodes(anode)isexternal,sothatthearchastransferredtotheexternalanodeconstitutedbyanelectricallyconductivematerial;thisallowstheuseofextremelyelevatedthermalfluxesforthesubstancetreatments(metalmelting,slagvitrifica-tion).Thesubstanceisplacedinanelectricallygroundedmetallicvesselactingasanode.Thearcstartsbetweenthecathodeandthecontainercarryingthesubstancetolocallyelevatedtempera-tures;thesubstancesareliquefied,subsequentlyvitrified(withtheaddition,insomecases,ofsilicamixtures)reachingthecompletewasteinertisation.Fig.1.(a)Non-transferredarcplasmatorch;(b)transferredarcplasmatorch.G.Bonizzoni,E.Vassallo/Vacuum64(2002)327–336328Thecathodecanbeconstructedbyawatercooledmetalorbya‘‘consumablethroughsublimation’’conductormateria