Thermal Induction Plasma Processes for the Synthesis of SOFC Materials Thermische Induktionsplasmaverfahren zur Synthese von SOFC-Materialien M. Mu È ller, E. Bouyer, M. v. Bradke, D. W. Branston, R. B. Heimann, R. Henne, G. Lins and G. Schiller This work demonstrates the potential of two recently developed plasma spray methods to deposit Sr-doped La-(Mn,Fe,Co) perovs- kite (LSM, LSF, LSCF) cathodes, and yttria-doped zirconia (YSZ) electrolytes as components of solid oxide fuel cells (SOFC). These methods are Thermal Plasma Chemical Vapour Deposition (TPCVD) and Suspension Plasma Spraying (SPS). The microstruc- ture obtained is determined by the method applied. TPCVD results in either globular or columnar coatings which were deposited with growthratesupto30 lm/min. SPS with a deposition rate in excess of 100 lm/min yields coatings of layered microstructure. The homogeneity of the dopant distribution and the phase purity of YSZisexcellent.Processmodificationswillbenecessary,however, to improve the properties in the case of perovskite coatings. Diese Arbeit demonstriert das Potenzial zweier junger Varianten desPlasmaspritzens,derdurcheinthermischesHochfrequenz-Plas- ma unterstu Ètzten chemischen Abscheidung aus der Dampfphase (Thermal Plasma Chemical Vapour Deposition, TPCVD) und desSuspensionsplasmaspritzens(SPS)zurAbscheidungvonSr-do- tierten La-(Mn,Fe,Co)-Perowskit (LSM, LSF, LSCF)-Kathoden und Yttria-dotierten Zirconia (YSZ)- Elektrolyten fu Èr SOFC. Die MikrostrukturdererzeugtenSchichtenistabha Èngigvomeingesetz- tenVerfahren.SowurdenmittelsTPCVDglobulareodersta Èngelige Schichten mit Wachstumsraten bis 30 lm/min abgeschieden. Beim SPSentstehenbeiAuftragsratengro Èûerals100 lm/minweitgehend lagige Schichten. Die Homogenita Èt der Dopandenverteilung sowie die Phasenreinheit der Schichten sind im Falle des YSZ hervorra- gend.Jedochmu Èssensiefu Èr die Perowskitschichten durch Prozess- modifikationen weiter verbessert werden. 1 Introduction Thermalspraymethodshavebeenappliedtotheproduction of solid oxide fuel cell (SOFC) components for more than ten years, e.g. to produce SOFC cathodes by the plasma spraying ofperovskitepowders[1,2].Thedrawbackofratherlowlayer porosity was addressed by the addition of carbon to the per- ovskite spray powders that was subsequently burned out [3]. Dense zirconia electrolyte layers were plasma-sprayed with the help of process modifications such as injection along the central axis or Laval-type nozzles to accelerate the flow [4±7]. Since the three-phase boundary of electrolyte, electrodeandreactiongasshouldbeaslargeaspossible,com- posite anodes comprising electrolyte and anode materials [8] aswellascompositecathodesmadeofzirconiaandperovskite [9, 10] have been formed by plasma-spraying. Economically speaking the ideal process when using va- cuum plasma spraying would be the successive fabrication of the complete three layer assembly of anode, electrolyte and cathode without breaking the vacuum. Successful at- tempts along these lines have been reported [11±20]. Usuallythetechniquesreportedforplasmasprayingofper- ovskite cathodes require specially prepared powders or post- deposition treatment of the coating to obtain the required por- osity. Other attempts tried to avoid the expensive powder pre- paration by application of liquid precursors. Perovskite coat- ingsfromaqueoussolutionsweredepositedinathermalradio frequency(rf)plasmawithaªplasmaexpandedthroughanoz- zleº(PETN)techniqueachievingagrowthrateofabout1 lm/ min [21]. Similar precursors were injected into a plasma gen- eratedwithatripledirectcurrent(d.c.)torchreactor.Withthis Thermal Plasma CVD (TPCVD) method perovskite coatings were achieved with growth rates of about 7 lm/min [22]. Mi- crowave plasma enhanced CVD of LSM coatings resulted in growth rates of only < 0.1 lm/min [23]. Sincezirconialayersareveryefficientthermalbarriercoat- ings (TBC) numerous deposition techniques have been ap- plied for their production. Plasma spray methods are both well investigated and well established for TBC production [24±27]. TBCs of superior quality are obtained by Electron Beam PVD [28], a process which requires rather expensive equipment. The production of zirconia powders by injecting an aerosol into an inductively coupled plasma was reported in [29]. Suspension plasma spraying (SPS) was first used to pro- ducecoatingsofhydroxyapatite[30].Theprocessiswellsui- ted to combine the high growth rate of conventional plasma spraying with the possibility of in-situ chemical synthesis ob- served in PETN and TPCVD. InthiscontributiontheresultsofTPCVDandSPSusingan inductively coupled thermal plasma for the deposition of per- ovskite electrodes and zirconia electrolyte coatings are pre- sented, the major part of our work being devoted to the immediate deposition of YSZ and LSM, LSF or LSCF layers from liquid precursors atomised into an inductively coupled plasma, a process which was introduced to fabricate YBCO superconducting layers with a growth rate of about 1 lm/ min [31]. 2 Experimental 2.1 Set-up and Process Conditions An inductively coupled radio frequency (rf) plasma was used because of its intrinsic properties such as the ability 322 0933-5137/02/0606-0322$17.50  .50/0 Mat.-wiss. u. Werkstofftech. 33, 322±330 (2002) Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2002