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Spray pyrolysis deposition of YSZ and YSZ–Pt composite films
Applied Surface Science 252 (2005) 1266–1275 www.elsevier.com/locate/apsusc
Spray pyrolysis deposition of YSZ and YSZ–Pt composite films R. Todorovska b, N. Petrova a, D. Todorovsky a,* a
b
Faculty of Chemistry, University of Sofia, 1, J. Bourchier Blvd., Sofia 1164, Bulgaria Institute of Electronics, Bulgarian Academy of Sciences, 72, Tsarigradsko Shousse Blvd., Sofia 1784, Bulgaria Received 21 October 2004; received in revised form 10 February 2005; accepted 11 February 2005 Available online 5 April 2005
Abstract In the present paper procedures are described for spray pyrolysis deposition of YSZ films (0.1–30 mm in thickness) with 8 or 15 mole % of YO1.5 on different substrates. Aqueous or ethylene glycol solutions of Y–Zr-citrates were used as starting material and O2 as carrier gas. The crystal structure and the morphology of the films were studied. The optimal deposition and post-deposition annealing conditions were defined, taking into account the desired film thickness and characteristics. Substrate temperatures of 250 8C during the deposition followed by heating for 10 min to 400 8C after every spraying and to 590 8C after every three sprayings with final annealing at 590 8C for 2 h in static air atmosphere were found to be suitable for the production of dense, uniform and cracks-free films. # 2005 Elsevier B.V. All rights reserved. PACS: 81.15.-z; 81.15.Rs; 68.55.-a; 81.20.Fw Keywords: Films; Citric complexes; Platinum; Spray pyrolysis; Yttria-stabilized zirconia
1. Introduction Yttria-stabilized zirconia (YSZ) is widely used for oxygen sensors. Because of its oxygen ion conductivity and stability in both oxidizing and reducing atmospheres, it is the most commonly used electrolyte in solid oxide fuel cells. YSZ films are used as buffer layers for deposition of YBCO superconductors. The * Corresponding author. Tel.: +359 2 81 61 322; fax: +359 2 962 54 38. E-mail address: [email protected] (D. Todorovsky).
technological application of YSZ strongly depends on the concentration of yttria, the compositional homogeneity of the material and the thickness of the layer when YSZ is applied as film. Thin films are used as buffer layers but much thicker ones are necessary for the other above-mentioned purposes. The search for methods ensuring preparation of homogeneously doped ZrO2 with certain composition has been of interest in the recent years. Citric complexes formed in a medium of polyvalent alcohol (such as ethylene glycol (EG)) and immobilized in a polyester resin, resulting from the polyesterification between citric acid (CA) and EG
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.106
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(Pechini’s process) have already been applied as precursors for preparation of number of stoichiometric, phase-homogeneous, bulk materials, including polycrystalline doped zirconia. As dopants, different rare earth oxides like ceria [1], yttria [2–6] and scandia [4–6] are used. Simple aqueous solution technique utilizing CA as complexing agent (known as amorphous citrate route) has also been studied for the preparation of Ce- and Y-doped zirconia powders [7]. Varieties of methods have been proposed for YSZ films deposition, most of them vacuum: pulsed laser deposition [8–10], ion-beam assisted deposition [11–13], e-beam evaporation [14,15], rfmagnetrone sputtering [15] and vacuum plasma spray deposition [16]. On the other hand the solution based processes propose several advantages: simplicity of the process, access to a wide range of metal oxides stoichiometries, precise control of the composition, applicability to substrates of different shape and size, low cost and high throughput. In [17] a dip-coating process is used and different compounds dissolved in isopropanol are tested as source of the metals (Zr- and Y-tetrabutoxides, Y-pentanedionate, Y(NO3)3, Y2O3). The obtained films are not phase homogeneous – the same authors observe the presence of hexagonal, cubic and monoclinic Y2O3 [18]. Spin-coating using poly(ethylene glycol) solution of the respective metal ions is applied in [19,20]. Spray pyrolysis using water/isopropanol solution of Zr-oxalate precursor is used for the deposition of pure zirconia films [21]. Thin YSZ films are successfully prepared by ultrasonic spray pyrolysis using metal octylates as precursors [22]. Solutions of Y-nitrate and Zr-acetylacetonate in butyl carbitol/ ethanol mixtures are used as starting materials in the electrostatic spray pyrolysis technique, developed in [23] for production of thin films on dense and porous substrates. Most of the proposed methods aim deposition of thin films. Among the above mentioned methods only ion beam assisted deposition [12] and plasma spray [16] are reported to permit the production of layers in the range of 20 mm–1 mm. In the same time, in a few recent papers we have shown that ethylene glycol solutions of metal citrates could be successfully used as precursors for spray
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pyrolysis deposition of different mono- and polymetallic oxide films such as a-Fe2O3 [24], Y2O3 [25], Y3Fe5O12 [26], LaMnO3 and Ca-doped manganite films [27]. One of the main advantages of this approach is the fact that in many bimetallic-CA–EG systems, mixed-metal complexes are formed, strongly helping in this way the formation of stoichiometric, homogeneous layer. Such a nature of the complexes has been proven for the yttrium– zirconium citrates prepared in ethylene glycol or in water media [28]. Some preliminary results concerning spray pyrolysis deposition of YSZ thin films using EG solution of Y–Zr-citrate have been reported in [29]. Different YSZ-based composites have also been studied recently. Undoubtedly great deal of interest among them attracted the composites based on YSZ and doped lanthanum manganites in view of their potential use in SOFC [30–33]. In [33] Pechini-type process is used for preparation of such composite powders and the initial solutions are tested for dipcoating deposition of thin La1 xSrxMnO3–YSZ composite films on dense YSZ substrates. YSZ–Pt composites membrane has been used as diffusion barrier in a new type of amperometric oxygen sensor [34]. The Pt–YSZ powders have been prepared by a sol–gel process similar to that of Pechini, using PtCl4, Y(NO3)6H2O, ZrCl28H2O, CA and polyethylene glycol as starting materials. The capacitor behaviour of YSZ–Pt composite materials, obtained by impregnation of porous YSZ with Ptcontaining solution is studied in [35]. In the present paper a spray pyrolysis method for deposition of thin and thicker YSZ and YSZ–Pt films, using aqueous and mainly ethylene glycol solutions of citric complexes, is proposed. It can be expected that such a method besides the well-known advantages of the spray methods, compared to the vacuum ones (simplicity, low cost, coating of large area substrates), would ensure high quality of the produced films in respect to stoichiometry and phase homogeneity (taking in mind the proven mixed-metal nature of the Y–Zr-citric complex formed in the precursor solutions). Up to now the possibility to use EG solution of mixed-metal citrates as a starting solution for thick films deposition has not been explored and hence this was another goal of this work.
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2. Experimental 2.1. Materials Y(NO3)36H2O, ZrCl4, anhydrous CA, EG and H2[PtCl6], all of p.a. grade were used for the preparation of the starting solutions. The exact content of Y and Zr in the respective salts was determined by titration with EDTA. Optical grade silica glass, quartz single crystals, LiKAl2Si4O12 (b-quartz), ceramic YSZ, ceramic erbia-stabilized zirconia (ESZ) and corundum-Rubalit1 708 (96% Al2O3) and Rubalit1 710 (99.5% Al2O3) (Rosenthal Technique AG, Germany) were used as substrates (usually 20 mm 10 mm 2 mm). Before the film deposition the substrates were treated consecutively with trichlorethylene, i-propanol and acetone in an ultrasonic bath for 10 min. 2.2. Initial solutions preparation For the preparation of Pt-free YSZ two types of precursor solutions were used: (i) EG solutions of the citric complexes were prepared by addition of Y(NO3)36H2O and ZrCl4 to an EG solution of CA. The molar ratio Y3+:Zr4+:CA:EG was set to 0.15 (or 0.08):0.85 (or 0.92):10:40. The solution was agitated for 30 min at 120 3 8C. Complexes of the type: Y0.08(ZrOCl)0.92(HCit3 )0.142(CitROH3 )0.245 2.1HOROH1.7H2O (or analogous but with Y:Zr molar ratio 0.15:0.85), where R = (CH2)2, HCit = CH2COOC(OH)COOCH2COO, Cit = CH2COOC (O )COOCH2COO are formed at such conditions [28]. (ii) Aqueous solution of the citric complexes was prepared using the method proposed in [7]. CA was first dissolved at 80 8C in water, followed by addition of Zr-salt under constant stirring to bond Zr4+ in stable Zr–citric complexes. After achieving complete dissolution for 2 h at 80 8C, Y-salt was added and the mixture was additionally stirred at the same temperature to produce a transparent, colorless solution containing the complexes and used further for the spray pyrolysis deposition. The molar ratio CA:H2O:(Zr+Y) was set to 17.5:300:1.
The initial solution for YSZ–Pt composite deposition was prepared using a method, similar to the one proposed in [34] and to the procedure, used for the preparation of Pt-free EG solutions. A weighted amount of anhydrous CA was dissolved in a measured volume of ethylene glycol at 40 8C. Y(NO3)36H2O and ZrCl4 were added to the solution. The temperature was raised sharply up to 120 8C in order clear solution to be obtained and then the Pt-salt is added. The molar ratio Y3+:Pt4+:Zr4+:CA:EG was set to 0.145:0.055:0.81:7:50. The system was heated under stirring at 120 8C for 30 min and after cooling to room temperature the obtained solution was used as a starting material for the deposition of the films. 2.3. Films deposition The spray device shown in [24] was used. The initial solution was passed through a pneumatic nebulizer with a diameter of the nozzle 0.7 mm. In the case of EG initial solutions, these were diluted with EG 10 times for the thicker films production and 103 times for the thinner ones. Pressurized O2 at a flow rate of 1 dm3/min was used as a carrier gas. The substrate was placed on a heater 20 cm apart from the nozzle. The pulverization was performed at an angle of 458 for 30 s. The film thickness was controlled by the number of the spray procedures, all other parameters being kept constant. After the deposition the samples were annealed at certain, defined conditions. The other experimental details will be described below. 2.4. Films characterizations The films thickness was determined by means of Talystep profilometer. X-ray diffractograms were taken with a Siemens powder diffractometer (Germany) at 40 kV, 40 mA, 2Q step of 0.058/6 s at Co Ka radiation or using DRON-3 diffractometer (USSR) with filtered Cu Ka at 2Q step of 0.058/2 s. The film morphology was evaluated by a scanning electron microscope (JEOL JSM 5510, Japan). The optical transparency of the thin films deposited on silica glass was measured by a Specord UV–vis spectrometer (C. Zeiss, Germany). Energy dispersive X-ray microanalysis was employed to determine the films elemental composition and the uniformity of the distribution of the elements (Tractor Northern TN
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2000 X-ray microanalyser with scanning electron microscope JEOL JSM 35 CF). Raman spectra were recorded with SPEX 1403 double spectrometer, equipped with a photomultiplier working in photon counting mode. The 488 nm line of Ar laser was used for excitation (laser power of 5 mW), the spectral slit width being 4 cm 1.
3. Results and discussion 3.1. Thin YSZ films Films with 8 mole % YO1.5 and thickness up to 1 mm were deposited on substrates from b-quartz and ESZ [29], quartz single crystals and fused silica, heated at 350–370 8C using the above described aqueous or diluted EG-solutions of citric complexes. X-ray detectable crystal structure is obtained on bquartz after post-deposition annealing of the film, prepared from EG-solution at 750 8C for 2 h (Fig. 1.1) (The described deposition and post-deposition conditions are referred below in the text as regime A). Additional 2 h of heating at 850 8C are necessary for the crystal structure to be observed on silica substrate when initial aqueous solution is used (Fig. 1.2). For comparison, YSZ powder with the same Y-content (8 mole %), produced from the same initial aqueous solution and heated at the same conditions at which the film deposition (350 8C, 1 h) and post-deposition annealing (750 8C, 2 h, 850 8C, 2 h) was performed, is used as referential sample (Fig. 1.3). Since both cubic and tetragonal YSZ phases exhibit diffraction patterns at nearly overlapping angels, the diffraction patterns of the films (Figs. 1.1 and 1.2) could be attributed to either cubic or tetragonal phases. However, the Raman spectroscopy (Fig. 2.1) applied for the powder used as a referential sample confirms the formation of tetragonal phase. In more details the problem is discussed in [28] and it is shown that tetragonal YSZ is obtained using also EG–citric solution, with molar ratio of the metals in it Y3+:Zr4+ = 0.08:0.92, i.e. containing 8 mole % Y. The higher temperature necessary for the crystal structure to be revealed when aqueous solution of citrates is used shows that the formation of EG-citrates ensures better mixing of the metals on an atomic level,
Fig. 1. X-ray diffractograms (Co Ka) of YSZ (8 mole % YO1.5) films: deposited on b-quartz from EG-solution and annealed at 750 8C for 2 h (1); deposited on fused silica from aqueous solution, heated for 2 h at 750 8C and at 850 8C (2); powder prepared from the same aqueous solution (3). Miller indexes of tetragonal YSZ are shown (according to JCPDS 48-0224); the reflections labeled with (*) belong to the substrate.
Fig. 2. Raman spectra of YSZ with 8 mole % YO1.5 (1) and with 15 mole % YO1.5 (2), produced, respectively from the initial aqueous and EG-solutions, used for the films deposition.
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thus facilitating the final product formation and crystallization. Such an assumption is supported by the observation in [28], that the metal content in the isolated from the water medium bimetallic Y–Zrcomplex, slightly differs from the one in the initial solution, i.e. in water medium the desired stoichiometry in the precursor may not be readily achieved. The observed effect may also be due to the fact that the use of water solution leads to formation of a product with very small crystallites size (D101 6 nm according to the line broadening in the XRD patterns), i.e. with crystal structure, difficult for X-ray detection. Applying additional 2 h of heating at 850 8C leads to crystallites size D101 16 nm. Crystallites size D101 28 nm is observed for the film produced from EG-solution and heated only at 750 8C. It seems that the use of aqueous solution reveals possibilities nanocrystalline product to be prepared. The energy dispersive X-ray microanalysis of the film produced from EG-solution suggests the formula Y0.078 0.005Zr0.922 0.005O1.961 0.002, which confirms that the formed oxide layer represents satisfactory the content of the metals in the initial solution. The films produced by both types of starting solutions are uniform, dense, with very small morphological grains (Fig. 3), much smaller than the obtained by ultrasonic spray-pyrolysis (up to 250 nm) [22]. The difference may be due to the significantly higher temperature (750 8C) of the substrate during the performance of the latter method.
The spin-coating method using poly(ethylene glycol) solution of the respective metal ions ensures size of the crystallites 12 nm at post-deposition heating at 600 8C and 24–27 nm at 900 8C, respectively [19,20] which is close to the crystallite sizes obtained by us. However, the morphological grains obtained by the means of spin-coating are larger (47 nm [19,20]). The optical transparency of a 700 nm thick film, deposited from EG solution on b-quartz (Fig. 4), exceeds 70% for light with l > 600 nm. 3.2. Thick YSZ films It was found that it is impossible to obtain films thicker than 1.2 mm by the described simple procedure suitable for thin films production (regime A, using both EG-or aqueous starting solutions). Further experiments were performed using only EG-solutions with molar ratio of the metals in it Y3+:Zr4+ = 0.152:0.848, i.e. containing 15 mole % Y (aiming at the preparation of YSZ films with cubic structure). The main problem to be overcome is the cracking of the films, of primary importance for which, are the temperatures of the substrate during the sprayings as well as during the post-deposition (or intermediate) heating of the films. The temperature of the substrate during the deposition has to be high enough to ensure burning of the main part of the organic components. If sufficiently high temperature is not ensured the films crack after the third or forth spray cycle. On the other
Fig. 3. SEM images of YSZ films deposited from solutions containing 8 mole % Y on substrates heated during the deposition at 350 8C: deposited on fused quartz from aqueous solution, post-deposition annealed 2 h at 750 8C and 2 h at 850 8C (1); deposited on polycrystalline YSZ from EG-solution, post-deposition annealed 2 h at 750 8C (2).
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hand at higher substrate temperatures the productivity of the method is rather low due to the fact that the aerosol does not reach the substrate surface because of evaporation. Therefore, it is necessary to perform the spraying at relatively low temperature and after every few sprayings to increase the temperature in order burning of the organic components to be ensured. A detailed study of the thermochemical behaviour of the complexes used as initial materials for YSZ-powder preparation is reported in [28]. It has been shown that the thermal destruction of the EG-complexes starts above 80 8C with dehydration, followed by deesterification and partial decarboxylation. Stepwise separation of adduct-like bonded EG takes place between 210 and 500 8C. Up to 300 8C approximately 40% of the carbon present in the complex is released; additional 23% are released at heating up to 390 8C. Complete destruction of the organic skeleton takes place between 500 and 610 8C. At this temperature approximately 13% of the initial content of the carbon remains as elemental carbon. Only 17% of the carbon is released up to 340 8C from the aqueous complex, other 33% are released up to 400 8C. The destruction of the organic skeleton is completed at 520 8C but 50% of the carbon are still present as elemental carbon and are ready for burning at higher temperature. For this reason, despite the lower initial carbon content of the aqueous complexes, their solutions do not offer significant advantages as precursors for films production, compared to the EGones. When considering films, the above mentioned temperature intervals might be taken only as tentative ones – it has been proven [24] that the thermal decomposition processes in films take place at lower temperature compared to bulk powder samples. Accounting for these results, experiments were performed at the following deposition conditions (noted further as regime B, using EG-starting solutions): substrate temperature at the deposition 300 8C, heating up to 400 8C at a rate of 10 8C/ min, holding at this temperature for 10 min, cooling down at the same rate to 300 8C, repeating of the cycle (as many times as necessary for the desired film thickness to be obtained) and final post-deposition annealing at 590 8C for 2 h in static air atmosphere. Regime B, however, leads again to unsatisfactory results-films with some cracks are produced (Fig. 5.1). Considering
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Fig. 4. Optical transparency of YSZ film deposited on b-quartz with thickness 700 nm.
the grey color of the films, it may be assumed, that although at 400 8C the organic skeleton of the complexes is destroyed, this temperature is not sufficient for the complete burning of the residual carbon. Its burning, when the film is thicker, causes appearance of irreversible cracks. Satisfactory results were obtained at intermediate heating temperature of 500 8C. The experiments show that when this temperature is applied, the substrate temperature during the spraying could be decreased to 250 8C, thus ensuring better productivity of the spraying procedure, not affecting the final film quality. The carrier gas pressure is another important factor for the uniformity of the film and for the size of its morphological grains. Stable, almost laminar flow with medium size drops, evaporating on the substrate surface has to be ensured. The optimal gas pressure depends on the substrate temperature and the distance between the substrate and nozzle. Its increase, keeping constant the other conditions, permits (in some limits) the preparation of denser films with smaller morphological grains. Spraying at relatively high substrate temperature (300 8C) requires higher aerosol velocity, ensured by higher pressure of the carrier gas. In the opposite case (low velocity), there is enough time for evaporation of the liquid phase of the aerosol. Gas pressure, ensuring flow rate not below 1 dm3/min was found to be optimal when the substrate temperature during the deposition is 250–300 8C. Under these conditions satisfactory films were obtained on substrates with rather smooth surface (optical grade silica, quartz crystals). However, the rough surface of ceramic substrates has a
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Fig. 5. SEM images of YSZ films (from EG-solutions with 15 mole % Y) prepared by: regime B (see the text) on Rubalit1 708 ceramics (1); regime C on: Rubalit1 710 (2a, b), frosted ceramics (3), Rubalit1 708 (6 mm in thickness) (4).
repulsive effect on the aerosol, more significant at higher speed of the latter. That is why substrates with relatively rough surface require lower aerosol rates and, accordingly, lower deposition temperature, not higher than 250 8C. So the best results for all of the substrates used were obtained by the following procedure (regime C): substrate temperature during spraying 250 8C; after every three spraying heating of the sample to 400 8C at a rate of 10 8C/min and to 500 8C at a rate of 5 8C/ min, holding at this temperature for 10 min and cooling down to 250 8C at a rate of 10 8C/min. The cycle is repeated and finally, the film is heated up to 590 8C at a rate of 4 8C/min, kept at this temperature for 2 h and cooled down to room temperature at a rate of 10 8C/min. All heating procedures are performed in static air. Uniform layers, up to 25 mm in thickness, without cracks and with morphological grains size 0.6–1.2 mm were produced by this method (Fig. 5.2.a, 5.2.b and 5.3). The grain size of the thinner films (up to 6 mm in thickness) is much smaller (Fig. 5.4). The deposition of the film on already deposited oxide layer is different compared to the deposition on
clean surface. The deposition rate decreases and the morphological grains of the new deposited layers become larger, probably due to some recrystallization. The X-ray pattern (Fig. 6.4, 6.5; Table 1) shows crystal structure after post-deposition heating for only 2 h at 590 8C. The line broadening indicates crystallites size D111 28 nm for the films heated at 750 8C and D111 18 nm for the films, heated at 590 8C. The Raman spectrum of the powder, produced from the Y– Zr-citrate with 15 mole % Y, isolated from the same initial EG-solution and heated under conditions as close as possible to the ones applied for the films production (750 8C, 2 h), confirms the expected cubic structure of the obtained YSZ (Fig. 2.2). The cell constant of the YSZ thick film is 0.513 nm (0.5139 nm for the referential YSZ material according to JCPDS 30-1468). Cell constant approximately 0.508 nm is found in [19] for spin-coated thin films using poly(ethylene glycol) solution of the respective metal ions. It might be concluded that the use of mixed-metal citric complexes as precursors in our experiments favors the production of YSZ film with cell constant closer to the one of the referential material.
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substrates with morphological grains of medium size like Rubalit1 710 (2 mm). The films prepared on glazed ceramics and on silica glass or crystals have little worse adhesion and are more inclined to cracking. Most suitable for the films preparation by the proposed method are silky-like mat substrates. 3.3. Thick YSZ–Pt films Experiments were performed with EG solutions, containing 15 mole % of Y (Y3+/Zr4+ molar ratio 0.152/0.848) aiming at the preparation of YSZ films with cubic structure. As can be expected, according to the SEM data, only regime C leads to production of dense, uniform films up to 30 mm in thickness (Fig. 7). As to elucidate the crystal structure of the obtained composite materials, X-ray diffractograms of the films and of the powder material (prepared from the initial EG-solution by charring the gel at 350 8C for 2 h and annealing of the precursor thus obtained at 750 8C for 2 h) were taken (Figs. 6 and 8). In Table 1 the interplanar distances and the relative intensities of the reflections of the samples are compared with referential materials. The introduction of Pt in YSZ does not change its cubic structure. The cell constant of the doped powder material and the one of the Pt-free YSZ thick film are practically the same ( 0.513 nm). In the powder material Pt presents as a separate phase. Its introduction significantly decreases the crystallinity of the
Fig. 6. X-ray diffractograms of Rubalit1 710-substrate (1) and films deposited on it from: YSZ–Pt annealed at 590 8C (2) and at 750 8C (3); YSZ annealed at 590 8C (4) and at 750 8C (5). Miller indexes of YSZ are shown, the bars represent the reflections of Pt and the unlabelled reflections belong to the substrate.
The X-ray energy dispersive microanalysis data for five randomly chosen points in the film leads to the formula Y0.154 0.008Zr0.846 0.009O1.923 0.004, indicating that the Y-distribution is more uniform than the one obtained by another method permitting thick (20 mm) YSZ films preparation [16]. In the latter films Y2O3 content varies between 6.11 and 7.41 mole %. The adhesion of the films is very good – the qualitative test with ‘‘Scotch’’ tape practically does not damage them. Nevertheless, cracks-free films with best adhesion are obtained on frosted ceramics
Table 1 ˚ ), relative intensity (I, %) and Miller indexes of YSZ thick film, YSZ–Pt composites and of referential YSZ- and PtInterplanar distances (d, A materials Pt JCPDS 04-0802
Y0.15Zr0.85O1.93 JCPDS 30-1468 d
I
hkl
2.968 2.571
100 25
111 200
d
2.265 1.962 1.818 1.550 1.484
55 40 6
220 311 222
1.285
5
400
1.387
I
YSZ film, annealed at 750 8C hkl
100 53
31
YSZ–Pt film annealed at 750 8C
d
I
d
I
d
I
2.963 *
100 *
100 * 100
40 30 6
100 21 100 43 47 30 6 25 5
2.966 * 2.264
1.814 1.547 1.480
2.952 2.560 2.255 1.955 1.810 1.544 1.480 1.384 1.282
1.816 1.549
60 40
111 200
220
YSZ–Pt powder sample
* The position of the corresponding YSZ diffraction coincides with a diffraction peak of the substrate.
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deviations are shown) which leads to the formula Y0.153 0.003Zr0.844 0.004O1.922 0.004 + Pt0.055 0.0016. Obviously, the film composition reproduces very well the composition of the starting solution. The presence of Pt does not significantly influence the films adhesion, compared to the Pt-free layers.
4. Conclusions
Fig. 7. SEM image of YSZ–Pt films (15 mole % YO1.5, 4.5 mole % Pt) prepared by regime C on Rubalit1 710.
samples (Fig. 6) – it leads to a decrease of the YSZ crystallites size (D111, evaluated from the X-ray pattern) from 24 nm in the Pt-free polycrystalline powder material to 17 nm in the Pt-containing one and from 28 nm in the Pt-free YSZ-film to 15 nm in the Ptcontaining one. In the powder the Pt crystallites are 40 nm in size. The X-ray energy dispersive microanalysis performed for eight randomly chosen points gives the following data for the layer composition (in mole % related to the sum Y + Zr + Pt): Zr – 80.27 0.40, Y – 14.48 0.31, Pt – 5.25 0.15 (the standard
The results reported show that spray pyrolysis using solution of citric complexes as starting material can be applied for deposition of YSZ and Pt-doped YSZ films. Along with the well-known advantages of the solution-based methods and especially of the spray pyrolysis ones, the proposed method ensures preparation of dense, uniform films with good adhesion, easily controlled stoichiometry and satisfactory uniform distribution of Y and Pt due to the nature of the precursor used. The films are denser, with smaller morphological grains than the ones obtained by some spin-coating methods. Nanocrystalline films (crystallite size 6 nm) can be prepared using aqueous solution of the mixed-metal citric complexes. The procedures worked out, permit the preparation of cracks-free thick films, more uniform compared to films with similar thickness prepared by a plasmaspray method. The thermal regime in the course of the deposition and the post-deposition treatment is of primary importance for the films quality. The Pt introduced in the YSZ–Pt composite bulk material forms a separate phase. In the composition of the film it disturbs the YSZ crystallinity.
Acknowledgement The work is financially supported by the Bulgarian National Science Fund under Contract X-1210. References
Fig. 8. X-ray diffractogram of YSZ–Pt composite powder; bar diagram of cubic YSZ (JCPDS 30–1468) and of cubic Pt (dashed, JCPDS 04-0802) and the corresponding Miller indexes are shown.
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[19] Y. Zhang, S. Jin, C. Liao, C.H. Yan, Mater. Lett. 56 (2002) 1030. [20] Y.W. Zhang, Y. Yang, S. Jin, C.S. Liao, C.H. Yan, J. Mater. Si. Lett. 21 (2002) 943. [21] P. Peshev, I. Stambolova, S. Vassilev, P. Stefanov, V. Blaskov, K. Starbova, N. Starbov, Mat. Sci. Eng. 96 (2003) B106. [22] Y. Matsizaki, M. Hishinuma, I. Yasuda, Thin Solid Films 340 (1999) 72. [23] D. Peredniz, M. B. Joerger, K. Honegger, L. J. Gauckler, in: Proc. 7th Int. Symp. On Solid Oxide Fuel Cells, SOFC VII, Tsukuba, Japan, 16, 2001. p. 989. [24] R.V. Todorovska, St. Groudeva-Zotova, D.S. Todorovsky, Mater. Lett. 56 (2002) 770. [25] R. Todorovska, N. Petrova, D. Todorovsky, C.R. Acad. Bulg. Sci. 56 (2003) 41. [26] R. Todorovska, St. Groudeva-Zotova, D. Todorovsky, G. Tzvetkov, P. Stefanov, J. Mat. Synth. Proc. 10 (2002) 283. [27] R. Todorovska, N. Petrova, D. Todorovsky, St. Groudeva – Zotova ‘‘Spray pyrolysis deposition of LaMnO3 and La1 xCaxMnO3 thin films’’, Appl. Surf. Sci. (submitted). [28] N. Petrova, D. Todorovsky ‘‘Thermal decomposition of zirconium–yttrium citric complexes prepared in ethylene glycol and water media’’, Mat. Res. Bull (submitted). [29] N. Petrova, R. Todorovska, D. Todorovsky, D. Shopova, Key Eng. Mat. 264–268 (2004) 427. [30] F. Chen, M. Liu, J. Eur. Ceram. Soc. 21 (2001) 127. [31] Z.C. Xia, S.L. Yuan, W. Feng, J.L. Zhang, G.H. Zhang, J. Tang, L. Liu, S. Liu, G. Peng, D.W. Niu, L. Chen, Q.H. Zheng, Z.H. Fang, C.Q. Tang, Solid State Comm. 128 (2003) 291. [32] D. Das, A. Saha, S.E. Russek, R. Raj, D. Bahadur, J. Appl. Phys. 93 (10) (2003) 8301. [33] M. Gaudon, Ch.-L. Robert, F. Ansart, A. Rousset, Solid State Sci. 5 (2003) 1377. [34] Z. Peng, M. Liu, Ed. Balko, Sensors and Actuators B 72 (2001) 35. [35] M.G.H.M. Hendriks, B.A. Boukamp, J.E. ten Elshof, W.E. van Zyl, H. Verweij, Solid State Ionics 146 (2002) 123.
Spray pyrolysis deposition of YSZ and YSZ–Pt composite films R. Todorovska b, N. Petrova a, D. Todorovsky a,* a
b
Faculty of Chemistry, University of Sofia, 1, J. Bourchier Blvd., Sofia 1164, Bulgaria Institute of Electronics, Bulgarian Academy of Sciences, 72, Tsarigradsko Shousse Blvd., Sofia 1784, Bulgaria Received 21 October 2004; received in revised form 10 February 2005; accepted 11 February 2005 Available online 5 April 2005
Abstract In the present paper procedures are described for spray pyrolysis deposition of YSZ films (0.1–30 mm in thickness) with 8 or 15 mole % of YO1.5 on different substrates. Aqueous or ethylene glycol solutions of Y–Zr-citrates were used as starting material and O2 as carrier gas. The crystal structure and the morphology of the films were studied. The optimal deposition and post-deposition annealing conditions were defined, taking into account the desired film thickness and characteristics. Substrate temperatures of 250 8C during the deposition followed by heating for 10 min to 400 8C after every spraying and to 590 8C after every three sprayings with final annealing at 590 8C for 2 h in static air atmosphere were found to be suitable for the production of dense, uniform and cracks-free films. # 2005 Elsevier B.V. All rights reserved. PACS: 81.15.-z; 81.15.Rs; 68.55.-a; 81.20.Fw Keywords: Films; Citric complexes; Platinum; Spray pyrolysis; Yttria-stabilized zirconia
1. Introduction Yttria-stabilized zirconia (YSZ) is widely used for oxygen sensors. Because of its oxygen ion conductivity and stability in both oxidizing and reducing atmospheres, it is the most commonly used electrolyte in solid oxide fuel cells. YSZ films are used as buffer layers for deposition of YBCO superconductors. The * Corresponding author. Tel.: +359 2 81 61 322; fax: +359 2 962 54 38. E-mail address: [email protected] (D. Todorovsky).
technological application of YSZ strongly depends on the concentration of yttria, the compositional homogeneity of the material and the thickness of the layer when YSZ is applied as film. Thin films are used as buffer layers but much thicker ones are necessary for the other above-mentioned purposes. The search for methods ensuring preparation of homogeneously doped ZrO2 with certain composition has been of interest in the recent years. Citric complexes formed in a medium of polyvalent alcohol (such as ethylene glycol (EG)) and immobilized in a polyester resin, resulting from the polyesterification between citric acid (CA) and EG
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.02.106
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(Pechini’s process) have already been applied as precursors for preparation of number of stoichiometric, phase-homogeneous, bulk materials, including polycrystalline doped zirconia. As dopants, different rare earth oxides like ceria [1], yttria [2–6] and scandia [4–6] are used. Simple aqueous solution technique utilizing CA as complexing agent (known as amorphous citrate route) has also been studied for the preparation of Ce- and Y-doped zirconia powders [7]. Varieties of methods have been proposed for YSZ films deposition, most of them vacuum: pulsed laser deposition [8–10], ion-beam assisted deposition [11–13], e-beam evaporation [14,15], rfmagnetrone sputtering [15] and vacuum plasma spray deposition [16]. On the other hand the solution based processes propose several advantages: simplicity of the process, access to a wide range of metal oxides stoichiometries, precise control of the composition, applicability to substrates of different shape and size, low cost and high throughput. In [17] a dip-coating process is used and different compounds dissolved in isopropanol are tested as source of the metals (Zr- and Y-tetrabutoxides, Y-pentanedionate, Y(NO3)3, Y2O3). The obtained films are not phase homogeneous – the same authors observe the presence of hexagonal, cubic and monoclinic Y2O3 [18]. Spin-coating using poly(ethylene glycol) solution of the respective metal ions is applied in [19,20]. Spray pyrolysis using water/isopropanol solution of Zr-oxalate precursor is used for the deposition of pure zirconia films [21]. Thin YSZ films are successfully prepared by ultrasonic spray pyrolysis using metal octylates as precursors [22]. Solutions of Y-nitrate and Zr-acetylacetonate in butyl carbitol/ ethanol mixtures are used as starting materials in the electrostatic spray pyrolysis technique, developed in [23] for production of thin films on dense and porous substrates. Most of the proposed methods aim deposition of thin films. Among the above mentioned methods only ion beam assisted deposition [12] and plasma spray [16] are reported to permit the production of layers in the range of 20 mm–1 mm. In the same time, in a few recent papers we have shown that ethylene glycol solutions of metal citrates could be successfully used as precursors for spray
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pyrolysis deposition of different mono- and polymetallic oxide films such as a-Fe2O3 [24], Y2O3 [25], Y3Fe5O12 [26], LaMnO3 and Ca-doped manganite films [27]. One of the main advantages of this approach is the fact that in many bimetallic-CA–EG systems, mixed-metal complexes are formed, strongly helping in this way the formation of stoichiometric, homogeneous layer. Such a nature of the complexes has been proven for the yttrium– zirconium citrates prepared in ethylene glycol or in water media [28]. Some preliminary results concerning spray pyrolysis deposition of YSZ thin films using EG solution of Y–Zr-citrate have been reported in [29]. Different YSZ-based composites have also been studied recently. Undoubtedly great deal of interest among them attracted the composites based on YSZ and doped lanthanum manganites in view of their potential use in SOFC [30–33]. In [33] Pechini-type process is used for preparation of such composite powders and the initial solutions are tested for dipcoating deposition of thin La1 xSrxMnO3–YSZ composite films on dense YSZ substrates. YSZ–Pt composites membrane has been used as diffusion barrier in a new type of amperometric oxygen sensor [34]. The Pt–YSZ powders have been prepared by a sol–gel process similar to that of Pechini, using PtCl4, Y(NO3)6H2O, ZrCl28H2O, CA and polyethylene glycol as starting materials. The capacitor behaviour of YSZ–Pt composite materials, obtained by impregnation of porous YSZ with Ptcontaining solution is studied in [35]. In the present paper a spray pyrolysis method for deposition of thin and thicker YSZ and YSZ–Pt films, using aqueous and mainly ethylene glycol solutions of citric complexes, is proposed. It can be expected that such a method besides the well-known advantages of the spray methods, compared to the vacuum ones (simplicity, low cost, coating of large area substrates), would ensure high quality of the produced films in respect to stoichiometry and phase homogeneity (taking in mind the proven mixed-metal nature of the Y–Zr-citric complex formed in the precursor solutions). Up to now the possibility to use EG solution of mixed-metal citrates as a starting solution for thick films deposition has not been explored and hence this was another goal of this work.
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2. Experimental 2.1. Materials Y(NO3)36H2O, ZrCl4, anhydrous CA, EG and H2[PtCl6], all of p.a. grade were used for the preparation of the starting solutions. The exact content of Y and Zr in the respective salts was determined by titration with EDTA. Optical grade silica glass, quartz single crystals, LiKAl2Si4O12 (b-quartz), ceramic YSZ, ceramic erbia-stabilized zirconia (ESZ) and corundum-Rubalit1 708 (96% Al2O3) and Rubalit1 710 (99.5% Al2O3) (Rosenthal Technique AG, Germany) were used as substrates (usually 20 mm 10 mm 2 mm). Before the film deposition the substrates were treated consecutively with trichlorethylene, i-propanol and acetone in an ultrasonic bath for 10 min. 2.2. Initial solutions preparation For the preparation of Pt-free YSZ two types of precursor solutions were used: (i) EG solutions of the citric complexes were prepared by addition of Y(NO3)36H2O and ZrCl4 to an EG solution of CA. The molar ratio Y3+:Zr4+:CA:EG was set to 0.15 (or 0.08):0.85 (or 0.92):10:40. The solution was agitated for 30 min at 120 3 8C. Complexes of the type: Y0.08(ZrOCl)0.92(HCit3 )0.142(CitROH3 )0.245 2.1HOROH1.7H2O (or analogous but with Y:Zr molar ratio 0.15:0.85), where R = (CH2)2, HCit = CH2COOC(OH)COOCH2COO, Cit = CH2COOC (O )COOCH2COO are formed at such conditions [28]. (ii) Aqueous solution of the citric complexes was prepared using the method proposed in [7]. CA was first dissolved at 80 8C in water, followed by addition of Zr-salt under constant stirring to bond Zr4+ in stable Zr–citric complexes. After achieving complete dissolution for 2 h at 80 8C, Y-salt was added and the mixture was additionally stirred at the same temperature to produce a transparent, colorless solution containing the complexes and used further for the spray pyrolysis deposition. The molar ratio CA:H2O:(Zr+Y) was set to 17.5:300:1.
The initial solution for YSZ–Pt composite deposition was prepared using a method, similar to the one proposed in [34] and to the procedure, used for the preparation of Pt-free EG solutions. A weighted amount of anhydrous CA was dissolved in a measured volume of ethylene glycol at 40 8C. Y(NO3)36H2O and ZrCl4 were added to the solution. The temperature was raised sharply up to 120 8C in order clear solution to be obtained and then the Pt-salt is added. The molar ratio Y3+:Pt4+:Zr4+:CA:EG was set to 0.145:0.055:0.81:7:50. The system was heated under stirring at 120 8C for 30 min and after cooling to room temperature the obtained solution was used as a starting material for the deposition of the films. 2.3. Films deposition The spray device shown in [24] was used. The initial solution was passed through a pneumatic nebulizer with a diameter of the nozzle 0.7 mm. In the case of EG initial solutions, these were diluted with EG 10 times for the thicker films production and 103 times for the thinner ones. Pressurized O2 at a flow rate of 1 dm3/min was used as a carrier gas. The substrate was placed on a heater 20 cm apart from the nozzle. The pulverization was performed at an angle of 458 for 30 s. The film thickness was controlled by the number of the spray procedures, all other parameters being kept constant. After the deposition the samples were annealed at certain, defined conditions. The other experimental details will be described below. 2.4. Films characterizations The films thickness was determined by means of Talystep profilometer. X-ray diffractograms were taken with a Siemens powder diffractometer (Germany) at 40 kV, 40 mA, 2Q step of 0.058/6 s at Co Ka radiation or using DRON-3 diffractometer (USSR) with filtered Cu Ka at 2Q step of 0.058/2 s. The film morphology was evaluated by a scanning electron microscope (JEOL JSM 5510, Japan). The optical transparency of the thin films deposited on silica glass was measured by a Specord UV–vis spectrometer (C. Zeiss, Germany). Energy dispersive X-ray microanalysis was employed to determine the films elemental composition and the uniformity of the distribution of the elements (Tractor Northern TN
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2000 X-ray microanalyser with scanning electron microscope JEOL JSM 35 CF). Raman spectra were recorded with SPEX 1403 double spectrometer, equipped with a photomultiplier working in photon counting mode. The 488 nm line of Ar laser was used for excitation (laser power of 5 mW), the spectral slit width being 4 cm 1.
3. Results and discussion 3.1. Thin YSZ films Films with 8 mole % YO1.5 and thickness up to 1 mm were deposited on substrates from b-quartz and ESZ [29], quartz single crystals and fused silica, heated at 350–370 8C using the above described aqueous or diluted EG-solutions of citric complexes. X-ray detectable crystal structure is obtained on bquartz after post-deposition annealing of the film, prepared from EG-solution at 750 8C for 2 h (Fig. 1.1) (The described deposition and post-deposition conditions are referred below in the text as regime A). Additional 2 h of heating at 850 8C are necessary for the crystal structure to be observed on silica substrate when initial aqueous solution is used (Fig. 1.2). For comparison, YSZ powder with the same Y-content (8 mole %), produced from the same initial aqueous solution and heated at the same conditions at which the film deposition (350 8C, 1 h) and post-deposition annealing (750 8C, 2 h, 850 8C, 2 h) was performed, is used as referential sample (Fig. 1.3). Since both cubic and tetragonal YSZ phases exhibit diffraction patterns at nearly overlapping angels, the diffraction patterns of the films (Figs. 1.1 and 1.2) could be attributed to either cubic or tetragonal phases. However, the Raman spectroscopy (Fig. 2.1) applied for the powder used as a referential sample confirms the formation of tetragonal phase. In more details the problem is discussed in [28] and it is shown that tetragonal YSZ is obtained using also EG–citric solution, with molar ratio of the metals in it Y3+:Zr4+ = 0.08:0.92, i.e. containing 8 mole % Y. The higher temperature necessary for the crystal structure to be revealed when aqueous solution of citrates is used shows that the formation of EG-citrates ensures better mixing of the metals on an atomic level,
Fig. 1. X-ray diffractograms (Co Ka) of YSZ (8 mole % YO1.5) films: deposited on b-quartz from EG-solution and annealed at 750 8C for 2 h (1); deposited on fused silica from aqueous solution, heated for 2 h at 750 8C and at 850 8C (2); powder prepared from the same aqueous solution (3). Miller indexes of tetragonal YSZ are shown (according to JCPDS 48-0224); the reflections labeled with (*) belong to the substrate.
Fig. 2. Raman spectra of YSZ with 8 mole % YO1.5 (1) and with 15 mole % YO1.5 (2), produced, respectively from the initial aqueous and EG-solutions, used for the films deposition.
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thus facilitating the final product formation and crystallization. Such an assumption is supported by the observation in [28], that the metal content in the isolated from the water medium bimetallic Y–Zrcomplex, slightly differs from the one in the initial solution, i.e. in water medium the desired stoichiometry in the precursor may not be readily achieved. The observed effect may also be due to the fact that the use of water solution leads to formation of a product with very small crystallites size (D101 6 nm according to the line broadening in the XRD patterns), i.e. with crystal structure, difficult for X-ray detection. Applying additional 2 h of heating at 850 8C leads to crystallites size D101 16 nm. Crystallites size D101 28 nm is observed for the film produced from EG-solution and heated only at 750 8C. It seems that the use of aqueous solution reveals possibilities nanocrystalline product to be prepared. The energy dispersive X-ray microanalysis of the film produced from EG-solution suggests the formula Y0.078 0.005Zr0.922 0.005O1.961 0.002, which confirms that the formed oxide layer represents satisfactory the content of the metals in the initial solution. The films produced by both types of starting solutions are uniform, dense, with very small morphological grains (Fig. 3), much smaller than the obtained by ultrasonic spray-pyrolysis (up to 250 nm) [22]. The difference may be due to the significantly higher temperature (750 8C) of the substrate during the performance of the latter method.
The spin-coating method using poly(ethylene glycol) solution of the respective metal ions ensures size of the crystallites 12 nm at post-deposition heating at 600 8C and 24–27 nm at 900 8C, respectively [19,20] which is close to the crystallite sizes obtained by us. However, the morphological grains obtained by the means of spin-coating are larger (47 nm [19,20]). The optical transparency of a 700 nm thick film, deposited from EG solution on b-quartz (Fig. 4), exceeds 70% for light with l > 600 nm. 3.2. Thick YSZ films It was found that it is impossible to obtain films thicker than 1.2 mm by the described simple procedure suitable for thin films production (regime A, using both EG-or aqueous starting solutions). Further experiments were performed using only EG-solutions with molar ratio of the metals in it Y3+:Zr4+ = 0.152:0.848, i.e. containing 15 mole % Y (aiming at the preparation of YSZ films with cubic structure). The main problem to be overcome is the cracking of the films, of primary importance for which, are the temperatures of the substrate during the sprayings as well as during the post-deposition (or intermediate) heating of the films. The temperature of the substrate during the deposition has to be high enough to ensure burning of the main part of the organic components. If sufficiently high temperature is not ensured the films crack after the third or forth spray cycle. On the other
Fig. 3. SEM images of YSZ films deposited from solutions containing 8 mole % Y on substrates heated during the deposition at 350 8C: deposited on fused quartz from aqueous solution, post-deposition annealed 2 h at 750 8C and 2 h at 850 8C (1); deposited on polycrystalline YSZ from EG-solution, post-deposition annealed 2 h at 750 8C (2).
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hand at higher substrate temperatures the productivity of the method is rather low due to the fact that the aerosol does not reach the substrate surface because of evaporation. Therefore, it is necessary to perform the spraying at relatively low temperature and after every few sprayings to increase the temperature in order burning of the organic components to be ensured. A detailed study of the thermochemical behaviour of the complexes used as initial materials for YSZ-powder preparation is reported in [28]. It has been shown that the thermal destruction of the EG-complexes starts above 80 8C with dehydration, followed by deesterification and partial decarboxylation. Stepwise separation of adduct-like bonded EG takes place between 210 and 500 8C. Up to 300 8C approximately 40% of the carbon present in the complex is released; additional 23% are released at heating up to 390 8C. Complete destruction of the organic skeleton takes place between 500 and 610 8C. At this temperature approximately 13% of the initial content of the carbon remains as elemental carbon. Only 17% of the carbon is released up to 340 8C from the aqueous complex, other 33% are released up to 400 8C. The destruction of the organic skeleton is completed at 520 8C but 50% of the carbon are still present as elemental carbon and are ready for burning at higher temperature. For this reason, despite the lower initial carbon content of the aqueous complexes, their solutions do not offer significant advantages as precursors for films production, compared to the EGones. When considering films, the above mentioned temperature intervals might be taken only as tentative ones – it has been proven [24] that the thermal decomposition processes in films take place at lower temperature compared to bulk powder samples. Accounting for these results, experiments were performed at the following deposition conditions (noted further as regime B, using EG-starting solutions): substrate temperature at the deposition 300 8C, heating up to 400 8C at a rate of 10 8C/ min, holding at this temperature for 10 min, cooling down at the same rate to 300 8C, repeating of the cycle (as many times as necessary for the desired film thickness to be obtained) and final post-deposition annealing at 590 8C for 2 h in static air atmosphere. Regime B, however, leads again to unsatisfactory results-films with some cracks are produced (Fig. 5.1). Considering
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Fig. 4. Optical transparency of YSZ film deposited on b-quartz with thickness 700 nm.
the grey color of the films, it may be assumed, that although at 400 8C the organic skeleton of the complexes is destroyed, this temperature is not sufficient for the complete burning of the residual carbon. Its burning, when the film is thicker, causes appearance of irreversible cracks. Satisfactory results were obtained at intermediate heating temperature of 500 8C. The experiments show that when this temperature is applied, the substrate temperature during the spraying could be decreased to 250 8C, thus ensuring better productivity of the spraying procedure, not affecting the final film quality. The carrier gas pressure is another important factor for the uniformity of the film and for the size of its morphological grains. Stable, almost laminar flow with medium size drops, evaporating on the substrate surface has to be ensured. The optimal gas pressure depends on the substrate temperature and the distance between the substrate and nozzle. Its increase, keeping constant the other conditions, permits (in some limits) the preparation of denser films with smaller morphological grains. Spraying at relatively high substrate temperature (300 8C) requires higher aerosol velocity, ensured by higher pressure of the carrier gas. In the opposite case (low velocity), there is enough time for evaporation of the liquid phase of the aerosol. Gas pressure, ensuring flow rate not below 1 dm3/min was found to be optimal when the substrate temperature during the deposition is 250–300 8C. Under these conditions satisfactory films were obtained on substrates with rather smooth surface (optical grade silica, quartz crystals). However, the rough surface of ceramic substrates has a
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Fig. 5. SEM images of YSZ films (from EG-solutions with 15 mole % Y) prepared by: regime B (see the text) on Rubalit1 708 ceramics (1); regime C on: Rubalit1 710 (2a, b), frosted ceramics (3), Rubalit1 708 (6 mm in thickness) (4).
repulsive effect on the aerosol, more significant at higher speed of the latter. That is why substrates with relatively rough surface require lower aerosol rates and, accordingly, lower deposition temperature, not higher than 250 8C. So the best results for all of the substrates used were obtained by the following procedure (regime C): substrate temperature during spraying 250 8C; after every three spraying heating of the sample to 400 8C at a rate of 10 8C/min and to 500 8C at a rate of 5 8C/ min, holding at this temperature for 10 min and cooling down to 250 8C at a rate of 10 8C/min. The cycle is repeated and finally, the film is heated up to 590 8C at a rate of 4 8C/min, kept at this temperature for 2 h and cooled down to room temperature at a rate of 10 8C/min. All heating procedures are performed in static air. Uniform layers, up to 25 mm in thickness, without cracks and with morphological grains size 0.6–1.2 mm were produced by this method (Fig. 5.2.a, 5.2.b and 5.3). The grain size of the thinner films (up to 6 mm in thickness) is much smaller (Fig. 5.4). The deposition of the film on already deposited oxide layer is different compared to the deposition on
clean surface. The deposition rate decreases and the morphological grains of the new deposited layers become larger, probably due to some recrystallization. The X-ray pattern (Fig. 6.4, 6.5; Table 1) shows crystal structure after post-deposition heating for only 2 h at 590 8C. The line broadening indicates crystallites size D111 28 nm for the films heated at 750 8C and D111 18 nm for the films, heated at 590 8C. The Raman spectrum of the powder, produced from the Y– Zr-citrate with 15 mole % Y, isolated from the same initial EG-solution and heated under conditions as close as possible to the ones applied for the films production (750 8C, 2 h), confirms the expected cubic structure of the obtained YSZ (Fig. 2.2). The cell constant of the YSZ thick film is 0.513 nm (0.5139 nm for the referential YSZ material according to JCPDS 30-1468). Cell constant approximately 0.508 nm is found in [19] for spin-coated thin films using poly(ethylene glycol) solution of the respective metal ions. It might be concluded that the use of mixed-metal citric complexes as precursors in our experiments favors the production of YSZ film with cell constant closer to the one of the referential material.
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substrates with morphological grains of medium size like Rubalit1 710 (2 mm). The films prepared on glazed ceramics and on silica glass or crystals have little worse adhesion and are more inclined to cracking. Most suitable for the films preparation by the proposed method are silky-like mat substrates. 3.3. Thick YSZ–Pt films Experiments were performed with EG solutions, containing 15 mole % of Y (Y3+/Zr4+ molar ratio 0.152/0.848) aiming at the preparation of YSZ films with cubic structure. As can be expected, according to the SEM data, only regime C leads to production of dense, uniform films up to 30 mm in thickness (Fig. 7). As to elucidate the crystal structure of the obtained composite materials, X-ray diffractograms of the films and of the powder material (prepared from the initial EG-solution by charring the gel at 350 8C for 2 h and annealing of the precursor thus obtained at 750 8C for 2 h) were taken (Figs. 6 and 8). In Table 1 the interplanar distances and the relative intensities of the reflections of the samples are compared with referential materials. The introduction of Pt in YSZ does not change its cubic structure. The cell constant of the doped powder material and the one of the Pt-free YSZ thick film are practically the same ( 0.513 nm). In the powder material Pt presents as a separate phase. Its introduction significantly decreases the crystallinity of the
Fig. 6. X-ray diffractograms of Rubalit1 710-substrate (1) and films deposited on it from: YSZ–Pt annealed at 590 8C (2) and at 750 8C (3); YSZ annealed at 590 8C (4) and at 750 8C (5). Miller indexes of YSZ are shown, the bars represent the reflections of Pt and the unlabelled reflections belong to the substrate.
The X-ray energy dispersive microanalysis data for five randomly chosen points in the film leads to the formula Y0.154 0.008Zr0.846 0.009O1.923 0.004, indicating that the Y-distribution is more uniform than the one obtained by another method permitting thick (20 mm) YSZ films preparation [16]. In the latter films Y2O3 content varies between 6.11 and 7.41 mole %. The adhesion of the films is very good – the qualitative test with ‘‘Scotch’’ tape practically does not damage them. Nevertheless, cracks-free films with best adhesion are obtained on frosted ceramics
Table 1 ˚ ), relative intensity (I, %) and Miller indexes of YSZ thick film, YSZ–Pt composites and of referential YSZ- and PtInterplanar distances (d, A materials Pt JCPDS 04-0802
Y0.15Zr0.85O1.93 JCPDS 30-1468 d
I
hkl
2.968 2.571
100 25
111 200
d
2.265 1.962 1.818 1.550 1.484
55 40 6
220 311 222
1.285
5
400
1.387
I
YSZ film, annealed at 750 8C hkl
100 53
31
YSZ–Pt film annealed at 750 8C
d
I
d
I
d
I
2.963 *
100 *
100 * 100
40 30 6
100 21 100 43 47 30 6 25 5
2.966 * 2.264
1.814 1.547 1.480
2.952 2.560 2.255 1.955 1.810 1.544 1.480 1.384 1.282
1.816 1.549
60 40
111 200
220
YSZ–Pt powder sample
* The position of the corresponding YSZ diffraction coincides with a diffraction peak of the substrate.
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deviations are shown) which leads to the formula Y0.153 0.003Zr0.844 0.004O1.922 0.004 + Pt0.055 0.0016. Obviously, the film composition reproduces very well the composition of the starting solution. The presence of Pt does not significantly influence the films adhesion, compared to the Pt-free layers.
4. Conclusions
Fig. 7. SEM image of YSZ–Pt films (15 mole % YO1.5, 4.5 mole % Pt) prepared by regime C on Rubalit1 710.
samples (Fig. 6) – it leads to a decrease of the YSZ crystallites size (D111, evaluated from the X-ray pattern) from 24 nm in the Pt-free polycrystalline powder material to 17 nm in the Pt-containing one and from 28 nm in the Pt-free YSZ-film to 15 nm in the Ptcontaining one. In the powder the Pt crystallites are 40 nm in size. The X-ray energy dispersive microanalysis performed for eight randomly chosen points gives the following data for the layer composition (in mole % related to the sum Y + Zr + Pt): Zr – 80.27 0.40, Y – 14.48 0.31, Pt – 5.25 0.15 (the standard
The results reported show that spray pyrolysis using solution of citric complexes as starting material can be applied for deposition of YSZ and Pt-doped YSZ films. Along with the well-known advantages of the solution-based methods and especially of the spray pyrolysis ones, the proposed method ensures preparation of dense, uniform films with good adhesion, easily controlled stoichiometry and satisfactory uniform distribution of Y and Pt due to the nature of the precursor used. The films are denser, with smaller morphological grains than the ones obtained by some spin-coating methods. Nanocrystalline films (crystallite size 6 nm) can be prepared using aqueous solution of the mixed-metal citric complexes. The procedures worked out, permit the preparation of cracks-free thick films, more uniform compared to films with similar thickness prepared by a plasmaspray method. The thermal regime in the course of the deposition and the post-deposition treatment is of primary importance for the films quality. The Pt introduced in the YSZ–Pt composite bulk material forms a separate phase. In the composition of the film it disturbs the YSZ crystallinity.
Acknowledgement The work is financially supported by the Bulgarian National Science Fund under Contract X-1210. References
Fig. 8. X-ray diffractogram of YSZ–Pt composite powder; bar diagram of cubic YSZ (JCPDS 30–1468) and of cubic Pt (dashed, JCPDS 04-0802) and the corresponding Miller indexes are shown.
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