Electrolytic deposition of ZrO2–Y2O3 films

September 2001

Materials Letters 50 Ž2001. 189–193 www.elsevier.comrlocatermatlet

Electrolytic deposition of ZrO 2 –Y2 O 3 films I. Zhitomirsky ) , A. Petric Department of Materials Science and Engineering, McMaster UniÕersity, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7 Received 26 November 2000; accepted 7 December 2000

Abstract Cathodic electrodeposition of pure zirconia and yttria-doped zirconia films was performed by means of hydrolysis of ZrOCl 2 and YCl 3 salts dissolved in mixed ethyl alcohol–water solvent. The deposits were characterized by X-ray diffraction, thermogravimetric analysis and inductively coupled plasma spectroscopy. By varying the YrZr atomic ratio in solutions and deposition time, the amount of the deposited material and its composition could be controlled. Deposits as obtained were amorphous. The crystallization behavior of the deposits was studied as a function of temperature. A possible mechanism of electrodeposition is discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Electrolytic deposition; Yttria-stabilized zirconia; Film; Hydroxide; Oxide; Cathodic deposit

1. Introduction Thin films of yttria-stabilized zirconia ŽYSZ. are of considerable interest for applications in solid-oxide fuel cells ŽSOFC., oxygen sensors and microelectronic devices. The importance of this material for SOFC applications is due to its high oxygen-ion conductivity and stability in oxidizing and reducing atmospheres. A great deal of recent research has been devoted to the development of various techniques for deposition of YSZ, including chemical vapor deposition, rf sputtering, slurry coating, plasma spraying, electrophoretic deposition, sol–gel, metal–organic deposition w1–7x. The importance of YSZ thin films for various applications has motivated the further development of processing technologies. Electrolytic deposition is an attractive alternative for the fabrication of thin YSZ films. )

Corresponding author. Fax: q1-905-528-9295. E-mail address: [email protected] ŽI. Zhitomirsky..

The formation of ceramic films by cathodic electrolytic deposition has received considerable attention during recent years. The feasibility of cathodic electrodeposition of various ceramic materials has been demonstrated, including individual oxides, complex oxide compounds and composites. Review papers describing materials science aspects, kinetics and mechanism of electrodeposition are now available w8,9x. In the cathodic electrodeposition process, metal ions or complexes are hydrolyzed by electrogenerated base to form oxide, hydroxide and peroxide deposits on cathodic substrates. Hydroxide and peroxide deposits can be converted to corresponding oxides by thermal treatment. Electrodeposition offers important advantages such as rigid control of film thickness, uniformity and deposition rate. The method offers the added attraction of simple and low-cost equipment and the possibility of film formation on substrates of complex shapes. The small particle size of electrolytic deposits enhances sinterability at low temperatures. Electrolytic deposition of zirconia

00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 2 3 - 3

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I. Zhitomirsky, A. Petricr Materials Letters 50 (2001) 189–193

w10–18x and yttria w19,20x films has recently been demonstrated. Therefore, it is important to find experimental conditions for the co-deposition of zirconia and yttria and the formation of YSZ films. This paper presents the results of electrodeposition of YSZ films and identifies various factors controlling deposit composition and crystallization behavior.

2. Experimental procedures Commercial purity ZrOCl 2 P 8H 2 O and YCl 3 P 6H 2 O ŽAlfa Aesar. were used for preparation of solutions in mixed ethyl alcohol–water Ž7:3 volume ratio. solvent. ZrOCl 2 concentration in the solutions was 0.005 M; YrZr atomic ratio was varied in the range 0–0.3. Cathodic deposits were obtained on chemically cleaned Pt Ž30 = 60 = 0.1 mm. and Ni Ž50 = 50 = 0.1 mm. substrates. The electrochemical cell for deposition in a galvanostatic regime included the cathodic substrate centered between two parallel platinum counterelectrodes. Electrodeposition experiments were performed at current density of 10 mArcm2 . Deposit weights were obtained by weighing the Pt substrates before and after deposition experiments followed by drying at room temperature for 24 h. The electrolytic deposits were scraped from the Pt electrodes for X-ray diffraction ŽXRD., thermogravimetric analysis ŽTG. and differential thermal analysis ŽDTA.. The deposits were dissolved in HNO 3 and then analyzed by inductively coupled plasma spectroscopy ŽPerkin Elmer Elan 6100 ICPMS. to determine the YrZr atomic ratio in the prepared films. The phase content was determined by X-ray diffraction ŽXRD. with a diffractometer ŽNicolet I2. using monochromatic Cu K a radiation at a scanning speed of 0.58rmin. For the XRD studies, the deposits were annealed in air for 1 h at various temperatures. TG and DTA analyses were carried out in air between room temperature and 12008C at a heating rate of 58Crmin using a thermoanalyzer ŽNetzsch STH-409..

of colloidal particles, which precipitate on the electrode. The cathodic reaction that generates OHy is: 2H 2 O q 2eym H 2 q 2OHy

Ž 1.

Electrodeposition resulted in the formation of cathodic deposits from all the solutions prepared. The cathodic deposits were analyzed by inductively coupled plasma spectroscopy. Obtained results revealed co-deposition of yttrium and zirconium species. Fig. 1 shows YrZr atomic ratio in the deposits versus that in the solutions used for electrodeposition. Nearly linear dependence was observed. However, the YrZr atomic ratio in deposits was found to be lower than that in solutions. Thin films with YrZr ratio in the range 0–0.22 were prepared. Fig. 2 shows deposit weight versus deposition time dependence for deposits containing Y and Zr in the ratio of YrZr s 0.2. Deposit weight increases with deposition time in agreement with Faraday’s law. The experimental dependencies shown in Figs. 1 and 2 pave the way in which the amount and composition of the deposited material could be controlled. In zirconyl chloride solutions, tetramers wZr4ŽOH. 8 ŽH 2 O.16 x8q can be considered as a main zirconium species. The tetramers are hydrolyzed by electrogenerated base to form colloidal particles of Zr4 O 8yx ŽOH. 2 x w17x. It is known w20,21x that positively charged Y 3q, YŽOH. 2q, Y2 ŽOH. 24q species exist in yttrium salt solutions; however, Y 3q can be considered as a main yttrium species in acid and

3. Results and discussion In the cathodic electrodeposition method, the high pH of the cathodic region brings about the formation

Fig. 1. YrZr atomic ratio in deposits versus YrZr atomic ratio in YCl 3 qZrOCl 2 solutions.

I. Zhitomirsky, A. Petricr Materials Letters 50 (2001) 189–193

Fig. 2. Deposit weight versus deposition time for yttria-doped zirconia deposits ŽYrZr s 0.2..

neutral solutions. It is supposed that positively charged yttrium species are hydrolyzed by electrogenerated base to form yttrium hydroxide particles w20x. Hydrolysis reactions result in the accumulation of colloidal particles near the electrode. Deposit formation is achieved via particle coagulation. Recent advances w9,22x in understanding the mechanism of electrolytic deposition have come from the application of the Derjaguin–Landau–Verwey–Overbeek ŽDLVO. w23,24x and Sogami–Ise w25x theories of colloidal stability and other experimental and theoretical data related to the interaction of charged particles in an electric field near the electrode surface. It was supposed w9x that the formation of electrolytic deposits is caused by flocculation introduced by the electrolyte. However, the coagulation of colloidal particles near the electrode can be enhanced by the electric field and electrohydrodynamic flows arising from the passage of ionic current through the solution w9x. As pointed out in Ref. w22x, particle coagulation could also be influenced by Coulombic attraction resulting from ion correlation and depletion forces w25x. In this work, electrodeposition experiments were performed from mixed ethyl alcohol–water solutions. The choice of the solvent was prompted by the following reasons. The addition of ethyl alcohol to aqueous solutions reduces the total dielectric constant of the solvent, thus reducing the solubility of zirconium and yttrium species in solutions. It is in

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this regard that precipitation experiments w26x performed in a mixed water–alcohol system indicate significant enhancement of the particle-formation kinetics. Repulsion between colloidal particles formed near the electrode is related to the diffuse layer charge on the particles. The thickness of the double layer decreases with the decreasing dielectric constant of the solvent, promoting particle coagulation w9x. The deposition process needs a sufficient amount of water for base generation Žreaction 1.. However, control of the rate of OHy generation is of paramount importance for the deposition process. When the rate of OHy generation is faster than the rate of OHy consumption by hydrolysis reaction at the electrode, a fraction of the OHy ions generated at the cathode is transported away by the electric current and diffusion. In this case, the high pH boundary moves away from the electrode surface, resulting in lower adhesion of the deposits w9x. Adsorbed water in green deposits leads to drying shrinkage and cracking. Mixed solutions were found to be preferable in order to reduce cracking and porosity in the deposits w15,16,27–29x. The X-ray diffractograms of fresh deposits display their amorphous nature ŽFigs. 3 and 4.. Deposits

Fig. 3. X-ray diffraction patterns for pure zirconia deposits: as prepared Ža. and after thermal treatment for 1 h at 4008C Žb., 6008C Žc., 8008C Žd., 10008C Že. and 12008C Žf., v —tetragonal zirconia.

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Fig. 4. X-ray diffraction patterns for yttria-doped zirconia deposits ŽYrZr s 0.2.: as prepared Ža. and after thermal treatment for 1 h at 4008C Žb., 6008C Žc., 8008C Žd., 10008C Že. and 12008C Žf..

prepared from pure ZrOCl 2 solutions displayed crystalline peaks after thermal treatment at 4008C in agreement with the results of other investigations w11,12x. The main crystalline phase at 4008C was tetragonal zirconia. In addition, faint peaks of monoclinic zirconia were also observed ŽFig. 3.. However, it is difficult to distinguish between the cubic and tetragonal zirconia phases owing to peak broadening. On exposure of the deposits to higher temperatures, transformation to monoclinic phase was observed. In Fig. 3, it can be seen that the deposits possess the monoclinic structure up to 12008C. The X-ray diffractogram of yttria-doped zirconia deposits ŽYrZr s 0.2. thermally treated at 4008C exhibited a very broad peak near 2 u ; 308, but the deposits contained a significant amount of amorphous phase ŽFig. 4.. After thermal treatment at higher temperatures, crystallization of cubic zirconia was observed. Thermogravimetric analysis of the deposits revealed weight loss during heating, which is attributed to the gradual decomposition of hydroxides. Fig. 5 shows the results of a TG study of yttria-doped zirconia deposits ŽYrZr s 0.2.. A sharp reduction of sample weight was observed up to ; 4008C, and no appreciable weight change was recorded at tempera-

tures higher than 6008C ŽFig. 5.. The total weight loss at 12008C was ; 31.6%. Similar weight losses were observed for other materials obtained via hydroxide precursors w8,17x. The DTA curve exhibits a broad endotherm around 1408C and an exotherm at 4758C. The observed endothermic peak is associated with the weight loss. The exothermic peak at 4758C is considered to be due to crystallization of yttriadoped zirconia and is in agreement with the X-ray data. Pure ZrO 2 exhibits monoclinic, tetragonal and cubic crystalline forms. The monoclinic phase is the stable form at room temperature, whereas tetragonal and cubic phases are metastable. However, crystallization of the tetragonal phase was observed at 4008C ŽFig. 3.. In the literature, many hypotheses have been proposed regarding the factors controlling the stabilization of the tetragonal and cubic phases. It was supposed that the metastable tetragonal phase could be formed when the size of crystallites is lower than the critical value of about 10 nm w30x. When the size of tetragonal crystallites exceeds the critical value, transformation to the monoclinic phase is observed. However, monoclinic zirconia particles as small as 6 nm were obtained w31x. Moreover, tetragonal zirconia powders were precipitated with particle size exceeding 100 nm w32x. Co-deposition experiments performed from mixed ZrOCl 2 –YCl 3 solutions resulted in the formation of a cubic yttriastabilized zirconia phase ŽFig. 4. in agreement with

Fig. 5. TG Ža. and DTA Žb. data for yttria-doped zirconia deposits ŽYrZr s 0.2..

I. Zhitomirsky, A. Petricr Materials Letters 50 (2001) 189–193

the phase diagram for ZrO 2 –Y2 O 3 w33x. Therefore, experimental results of this work pave the way for the electrodeposition of thin films of yttria-stabilized zirconia for various applications. However, co-deposition of yttria and zirconia is influenced by deposition conditions. Therefore, more detailed study, currently under way, would give very useful information on the composition and crystallization of the deposits prepared under various experimental conditions.

4. Conclusions Thin films of ZrO 2 –Y2 O 3 ŽYrZr s 0–0.22. have been prepared using the cathodic electrodeposition process from solutions containing ZrOCl 2 and YCl 3 salts dissolved in mixed ethyl alcohol–water solvent. By variation of the concentration of Zr and Y species in the starting solutions, the composition of the deposited material could be controlled. The amount of the deposited material could be controlled by variation of the deposition time. The films as obtained were amorphous. The effect of thermal treatment on the phase content of the obtained deposits has been studied. The feasibility of electrodeposition of yttria-stabilized zirconia films has been demonstrated.

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