Solid State Ionics 138 (2001) 191–197 www.elsevier.com / locate / ssi Deposition and characterization of nanocrystalline tetragonal zirconia films using electrostatic spray deposition T Nguyen, E Djurado* ´ Laboratoire d’ Electrochimie et de Physico-chimie des Materiaux et des Interfaces, ENSEEG-INP Grenoble, BP 75, ` 1130 Rue de la piscine, Domaine Universitaire, 38402 St Martin d’ Heres Cedex, France Received 18 July 2000; received in revised form 18 September 2000; accepted October 2000 Abstract Pure tetragonal and nanocrystalline mol.% Y O -doped ZrO thin films are deposited on stainless steel substrates by electrostatic spray deposition (ESD) technique Tetragonal phase is evidenced by Raman spectrometry and X-ray diffraction in an as-deposited film at 4008C Good homogeneity in composition is confirmed using wavelength dispersion scanning The surface microstructure of dense to porous thin films is investigated varying both deposition parameters: substrate temperature and deposition time 2001 Elsevier Science B.V All rights reserved Keywords: Nanocrystalline tetragonal yttria doped zirconia; Electrostatic spray deposition; IT-SOFC Introduction At present, efforts are focused on lowering the operating temperature of solid oxide fuel cells from | 10008C to | 7508C (IT-SOFC) Moderate operating temperatures, however, require noticeable decrease of electrolyte resistance In this context, thin films of nanocrystalline tetragonal yttria doped zirconia have been prepared by electrostatic spray deposition (ESD) In this study, tetragonal zirconia has been preferred for two reasons: (i) the excellent ionic conductivity in the low temperature domain up to 7508C compared to cubic zirconia [1,2], (ii) the better mechanical properties (high strength, high toughness and wear resistance) than the cubic form *Corresponding author Tel.: 133-4-7682-6684; fax: 133-47682-6670 E-mail address: Elisabeth.Djurado@lepmi.inpg.fr (E Djurado) Moreover, it is expected that nanometric crystallite size may show enhanced ionic conduction along the grain boundaries as it was previously shown by Mondal et al in tetragonal zirconia ceramics [3] Electrostatic atomization of liquids has been largely used in the painting industry for many years Recently, this technique has been applied mainly for a variety of functional metal oxides in SOFCs such as cubic Y O stabilized ZrO , LaCoO , BaCeO and in solid-state rechargeable lithium-ion batteries such as LiCoO , LiMn O , Li PO , TiO , MnO and SnO for example [4–10] The advantages of this technique are a simple set-up, a wide choice of precursors, relatively large film growth rate, ambient atmosphere operation, a good control of the morphology of the deposited layers and an excellent control of stoichiometry [8] compared to other deposition techniques such as RF sputtering, sol–gel, CVD or MOCVD and injection-LPCVD 0167-2738 / 01 / $ – see front matter 2001 Elsevier Science B.V All rights reserved PII: S0167-2738( 00 )00795-5 T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 192 The purpose of this work is to experimentally demonstrate the feasibility to deposit pure tetragonal ZrO –2 mol.% Y O films using ESD and to find the optimized conditions to prepare dense thin films Dense and porous films are obtained by controlling both deposition parameters: the substrate temperature and the deposition time Their microstructures have been characterized using X-ray diffraction (XRD), Raman spectrometry, scanning electron microscopy (SEM), wavelength dispersion scanning (WDS) and energy dispersion X-ray (EDX) Hereafter, films are referred to as 2Y-TZP Experimental 2.1 ESD synthesis of Y-TZP films 2Y-TZP films were deposited using a vertical ESD set-up, recently developed in our laboratory (Fig 1) The configuration and the working principle of the ESD technique have been described elsewhere [7,9] The precursor solution was prepared from a stoichiometric mixture of zirconium (IV) acetylacetonate (99%, Strem Chemicals Inc.) and hydrated yttrium (III) acetylacetonate (99.9%, Strem Chemicals Inc.) dissolved in a mixture of ethanol (C H OH, 14 vol.%) and butyl carbitol Fig Experimental ESD set-up (C H OC H OC H OH, 56 vol.%) An addition of 30 vol.% of acetic acid (HOAc) favored the dissolution of precursors in the solvent The molar ratio Zr:Y is equal to 0.961:0.039 The Zr (or Y) concentrations were 0.05 M The conductivity of the solutions was found equal to | mS / cm Typical deposition times were varied from 0.10 to h in ambient air Flow rate of precursor solution was set to ml / h using a syringe pump (Sage) A positive high voltage from 14 to 20 kV was applied to the nozzle in order to obtain a stable spray The nozzle to substrate distance was about cm Substrate temperatures, controlled by the voltage applied to the heating element, were chosen in the 300–4008C range X8Cr24 stainless steel discs (1 cm in diameter and mm in thickness) were used as substrates after a preliminary polishing down to 0.25 mm This steel was chosen as substrate for its lower thermal expansion coefficient of 12 10 26 8C 21 compared with 18 10 26 8C 21 for austenitic steels It has a closer variation in thermal expansion compared to zirconia with temperature 2.2 Characterization The as-deposited films were characterized by micro-Raman spectrometry using an XY Dilor multichannel spectrometer equipped with a CCD detector Radiation of the 514.53 nm line from an argon ion laser was used as the excitation source with 20 mW incident power We observed a continuous increasing background in the 100 to 900 cm 21 frequency range which could be attributed to fluorescence owing to presumably organic residues in the surface of as-deposited films In this respect, all films were previously heat-treated using 930 mW incident power for X-ray diffraction was carried out using a Siemens D500 u / 2u diffractometer with the Bragg Brentano geometry from 25 to 908 in 2u (0.048 in 2u step, 10 s as a counting time) with Fe K a radiation ( l 0.1936 nm) High purity silicon was used as the standard in order to precisely measure the instrumental resolution FWHM of XRD peaks were determined by deconvolution of Pseudo-Voigt-shaped peaks with ABFit software (ILL, Grenoble, France) Average crystallite size was calculated using the Debye– T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 193 Scherrer formula [11], corrected with silicon for the 111 peak of tetragonal zirconia 0.9l D ]] b cosu where D is the crystallite size (in nm), l the wavelength (in nm), b the FWHM corrected from high purity silicon (in radian) and u the diffraction angle Phases were identified using DIFFRAC-AT software systems (Socabim, Paris) Surface morphologies and their composition were analyzed using SEM (JEOL JSM 6400) equipped with an energy dispersive X-ray analyzer (EDX Princeton Gamma Technology) Operating conditions were the following: secondary electron detector and 20 kV accelerating voltage of the filament Wavelength dispersion scanning (WDS) investigations were carried out using electron probe microanalysis (CAMECA SX50) (200 nA, 15 kV) in order to verify the yttria distribution in films Results and discussion As-deposited film prepared at 4008C for h with 3.9 ml / h flow rate is polycrystalline as shown by XRD (Fig 2) Large peaks could be attributed to either the cubic and / or tetragonal phases since both exhibit diffraction patterns at nearly overlapping angles A pure tetragonal phase has been clearly Fig Typical micro-Raman spectrum of 2Y-TZP ESD film after of Ar ion laser power of 930 mW detected by Raman spectrometry (Fig 3) This crystalline film has been characterized by an average 7.860.3 nm crystallite size which has been deduced from the broadening of the 111 XRD line (FWHM5 1.368 compared to the 111 line of high-purity silicon FWHM50.158) All thin films are found amorphous with low flow rate (2 ml / h) up to 4008C deposition temperature A following annealing at 8008C for h with slow heating and cooling rates (18C / min) in air led to crystallized tetragonal deposits Stoichiometry is the same as that of precursor solution within experimental error (Table 1) which is another advantage of ESD technique The composition has been also confirmed by EDX (Fig 4) 3.1 Effect of deposition temperature Fig shows microscopic observations of 2Y-TZP films as deposited on stainless steel for different temperatures ranging from 300 to 3608C for h from 0.05 M precursor solution containing 30 vol.% Table Measured stoichiometry of elements by WDS in a dense 2Y-TZP thin film deposited for 0.25 h at 3578C on stainless steel Fig Typical XRD pattern of 2Y-TZP as-prepared ESD film at 4008C for h with 3.9 ml / h flow rate on stainless steel Element (wt.%) Theoretical Measured O Y Zr 25.8 2.8 71.3 28.5 2.6 69.0 194 T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 the following step consists in studying the morphology of films deposited for different times at the optimized deposition temperature that we selected: 3578C 3.2 Effect of deposition time Fig Typical EDX spectrum of 2Y-TZP ESD film deposited at 4008C on stainless steel for h HOAc with a flow rate of ml / h and applied voltage of 14–20 kV At low temperature (3008C), a dense but cracked bottom layer covered by a reticular network of particles is formed (Fig 5a and e) When temperature is increased to 3508C, cracks are clearly observed and agglomerated particles are growing on top of this first layer (Fig 5b and f) Formation of cracks in the layer is probably due to stresses during the drying process since Zr and Y precursors contain large fractions of organic components [12] Indeed, a lower substrate temperature leads to an incomplete solvent evaporation in the incoming spray droplets The wet deposit is then fast dried on the heated substrate Severe cracking results owing to stresses during this too fast drying process When temperature is increased up to 3578C (Fig 5c and g), the main point is that cracks have disappeared and some agglomerates are growing on top of a relatively dense layer (Fig 5g) This morphology corresponds to type III as previously reported by Schoonman et al [13] Then, morphology of doped zirconia films shifts to a highly porous microstructure when temperature is up to 3608C (Fig 5d and h) This dense to porous surface morphology is similar to that previously observed by Schoonman et al [13] by electrospraying when deposition temperature is increased We have found that surface morphologies are observed to be very sensitive to a deposition temperature shift of |58C This work is focused on the achievement of a denser morphology free of cracks In this respect, Surface morphologies of 2Y-TZP films deposited on stainless steel are shown in Fig for different times from 0.10 h to h at 3578C The shorter the deposition time, the thinner the layer thickness and the denser the deposited film (Fig 6a–d) Once again, this dense layer with incorporated particles corresponds to type II morphology as previously reported by Schoonman et al [13] This dense morphology can be explained by a more rapid spreading of the solution droplets which takes place initially on the metal substrate as it was already reported for LiCoO [6] It is well known that the surface tension of a metal is usually much greater than that of a metal oxide Furthermore, evaporation of ethanol and butyl carbitol has to occur simultaneously with the decomposition of acetylacetonates onto the substrate to obtain a continuous film [6] SEM micrographs of the cross-section of two 2YTZP thin films deposited for 0.25 h (Fig 7a) and h (Fig 7b) show a non linear growth rate which can be estimated from mm / h to mm / h, respectively As expected, when time is increased, the charged droplets are no longer spreading on a metallic substrate but on the bottom oxided layer leading to a lower growth rate Good adhesion of zirconia on stainless steel has been found using the tape test for deposition time shorter than h When deposition time is increased to h, the porous top layer is growing on the dense bottom layer (type III, [13]) which starts to crack (Fig 6e) The roughness of this earlier layer surface influences the morphology of the top layer leading to more particle agglomeration With a deposition time of h (Fig 6f), a completely porous film is formed as type III morphology [13] At this point, the spreading of solution droplets now occurs on zirconia bottom layer which presents a smaller surface tension than on stainless steel Consequently, the wettability of ethanol solution on zirconia surface is decreased and discrete particles grow on the surface owing to the slow spreading At this time, agglomeration of further particles is favored by the T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 195 Fig Surface morphologies of 2Y-TZP films deposited on stainless steel at (a, e) 3008C, (b, f) 3508C, (c, g) 3578C, (d, h) 3608C for h from 0.05 M precursor solution containing 30 vol.% HOAc with a flow rate of ml / h and applied voltage: 14 to 20 kV 196 T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 Fig Surface morphologies of 2Y-TZP films deposited on stainless steel at 3578C for (a) 0.10 h, (b) 0.17 h, (c and d) 0.25 h, (e) h and (f) h from 0.05 M precursor solution containing 30 vol.% HOAc with a flow rate of ml / h and applied voltage of 14 to 20 kV T Nguyen, E Djurado / Solid State Ionics 138 (2001) 191 – 197 197 fully prepared by ESD for the first time The film composition is homogeneous and is quite similar to that of the precursor solution Surface morphology of films has been well controlled versus two main deposition ESD parameters: the substrate temperature and the deposition time A monolithic IT-SOFC based on dense 2Y-TZP thin film as a new electrolyte material is in progress Acknowledgements The authors are grateful to Prof Dr J Schoonman (Delft University of Technology) for stimulating discussions about the ESD set-up B Meester (Delft University of Technology) and M Vaujany are thanked for their technical help The European Science Foundation is acknowledged for financial support References Fig SEM micrographs of the cross-section of 2Y-TZP thin films deposited on stainless steel at 3578C for (a) 0.25 h and (b) h from 0.05 M precursor solution containing 30 vol.% HOAc with a flow rate of ml / h and applied voltage of 14 to 20 kV presence of surface roughness The cracks formed in the bottom layer are now repaired by the subsequent solution droplets which preferentially penetrate into these defects by capillary action [6] The ESD optimized conditions in order to obtain dense tetragonal zirconia thin films (0.5 mm thick) on polished stainless steel are the following: 3578C for to 15 from 0.05 M precursor solution containing 30 vol.% HOAc with a flow rate of ml / h and applied voltage: 14 to 20 kV Conclusion Dense and pure tetragonal mol.% yttria-doped zirconia thin films (0.5 mm thick) have been success- [1] W Weppner, Solid State Ionics 52 (1992) 15 [2] S.P.S Badwal, Solid State Ionics 52 (1992) 23 [3] P Mondal, A Klein, W Jaegermann, H Hahn, Solid State Ionics 118 (1999) 331 [4] E.M Kelder, O.C.J Nijs, J Schoonman, Solid State Ionics 68 (1994) [5] A.A van Zomeren, E.M Kelder, J.C.M Marijnissen, J Schoonman, J Aerosol Sci 25 (1994) 1229 [6] C.H Chen, E.M Kelder, P.J.J.M van der Put, J Schoonman, J Mater Chem (1996) 765 [7] C.H Chen, E.M Kelder, M.J.G Jak, J Schoonman, Solid State Ionics 86 (1996) 1301 [8] C.H Chen, A.A.J Buysman, E.M Kelder, J Schoonman, Solid State Ionics 80 (1995) [9] C.H Chen, E.M Kelder, J Schoonman, J Mater Sci 31 (1996) 5437 [10] C.H Chen, E.M Kelder, J Schoonman, Thin Solid Films 342 (1999) 35 ´ [11] A Guinier, in: Dunod (Ed.), 3rd Edition, Theorie et Technique de la radiocristallographie, 1964, p 482, Chapter 12 [12] C.H Chen, K Nord-Varhaug, J Schoonman, J Mater Synth Process (3) (1996) 189 [13] N.H.J Stelzer, C.H Chen, L.N Van Rij, J Schoonman, in: A.J McEvoy, K Nisancioglu (Eds.), Materials and Processes, 10th I.E.A SOFC Workshop, Annex VII (Switzerland), Vol 2, 1997, p 236 ... homogeneous and is quite similar to that of the precursor solution Surface morphology of films has been well controlled versus two main deposition ESD parameters: the substrate temperature and the deposition. .. with a flow rate of ml / h and applied voltage of 14–20 kV At low temperature (3008C), a dense but cracked bottom layer covered by a reticular network of particles is formed (Fig 5a and e) When temperature... observed and agglomerated particles are growing on top of this first layer (Fig 5b and f) Formation of cracks in the layer is probably due to stresses during the drying process since Zr and Y precursors