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Influence of the experimental parameters on the synthesis process of yttria-doped zirconia sol–gel films
Surface & Coatings Technology 204 (2010) 2257–2261
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of the experimental parameters on the synthesis process of yttria-doped zirconia sol–gel films Antonio Díaz-Parralejo a, Antonio Macías-García a, José Sánchez-González a, M. Ángeles Díaz-Díez a, Eduardo M. Cuerda-Correa b,⁎ a b
Department of Mechanical, Energetic and Materials Engineering, School of Industrial Engineering, University of Extremadura, Avda. de Elvas, s/n, E-06071 Badajoz, Spain Department of Organic and Inorganic Chemistry, Faculty of Sciences, University of Extremadura, Avda. de Elvas, s/n, E-06071 Badajoz, Spain
a r t i c l e
i n f o
Article history: Received 5 August 2009 Accepted in revised form 21 December 2009 Available online 4 January 2010 Keywords: Sol–gel Yttria-stabilized zirconia Densification Thickness
a b s t r a c t The influence of the alcohol solvent (methanol, ethanol, propanol, isopropanol or buthanol) and the proportion used in the preparation of precursor solutions to obtain ZrO2–3 mol% Y2O3 sol–gel films are investigated. In particular, the effect of the aging process of the solutions on the porosity and critical thickness of the films is analysed. Implications for the preparation of zirconia sol–gel films by the dip-coating technique are also discussed. Other aspects of the process such as the aging of the solutions, the evolution of the dimensionless parameter J, the porosity, P, and the fully dense critical thickness, tcd, are also investigated and discussed. This allows one to understand the behaviour of the precursor solutions, and to obtain dense coatings and crack-free films. The results reveal a significant influence of the alcohol used as the main solvent in the first densification stages of the films (100 °C–500 °C). Likewise, the type and amount of alcohol used also notably influence the aging process of the solution, the porosity and the fully dense critical thickness finally obtained with each solution. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The sol–gel process is nowadays being used by a growing number of researchers for the preparation of an extensive variety of new materials, including bulk materials, films, and fibres. The most attractive feature of this processing route is the possibility of tailoring unique materials by polymerization of organometallic compounds (metal alkoxides, metal acetylacetonates, etc.) to a polymer gel. Two chemical reactions are involved in this process, namely hydrolysis and polycondensation. These reactions are usually simultaneous, and are controlled by using an appropriate acid or base catalyst. Because organometallic compounds and water are not miscible, a solvent – usually an alcohol – must be used in the process [1]. Transition metal alkoxides, especially those of Ti and Zr are widely used as molecular precursors of ceramic materials, due to the fact that most of them are soluble in alcohols, thus allowing good control of the hydrolysis and condensation reactions via the parameters of the process, such as the amount of water, type of alcohol, pH of the solution, etc. [2,3]. The type of alkyl radical in the alkoxide has a strong influence on the hydrolysis and condensation reactions, affecting the properties of the final products. Also, the solvent alcohols are usually far from being inert components, but are themselves involved in the chemical process [4,5]. If the alkoxides are dissolved in alcohols containing different alkyl groups, the ⁎ Corresponding author. E-mail address: [email protected] (E.M. Cuerda-Correa). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.12.015
alcohol exchange reactions –alcoholysis–, hydrolysis and condensation take place at slower rates, yielding more complex structures [6,7]. Deposition of thin films by the sol–gel route onto different substrates offers certain advantages over other methods of obtaining thin films (e.g., physical and chemical vapour deposition, electron beam evaporation, etc.) [8]. These advantages include simple and low-cost equipment, possibility of coating large areas, etc. In addition, this method is suitable for obtaining almost any single- or multi-component oxide coating (TiO2, ZrO2, etc.). In particular, zirconia-based thin films have promising applications in optics and as protective barriers [9]. For instance, as coatings they can be used to provide specific optical properties in glasses (e.g., anti-reflection, selective reflection, photochromism, etc.) [10], as well as to prevent chemical corrosion and gas oxidation in metals [11]. All of these applications are based on the interesting combination of mechanical, chemical, and physical properties exhibited by zirconia ceramics. Also, yttria-doped zirconia can exhibit outstanding mechanical properties, combining high wear resistance with moderate toughness, frequently associated with the activation of some transformation toughening mechanism [12]. However, the structure and properties of thin films can differ significantly from those of the bulk material. In particular, thin film properties depend on porosity and thickness, making the control of both characteristics during the densification processes a critical issue in sol–gel film preparation. Likewise, the type of alcohol used as the main solvent in the process has a noticeable influence on the reactions involved in the sol–gel process, so that it can affect the structure and final properties of the films.
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Some aspects such as the type of alcohol used as the main solvent, the alcohol-to-alkoxide ratio, the evolution or aging of the solutions as well as the influence of these factors on the densification and thickness of the films require a further analysis in order to make it possible to use the sol–gel method in an optimal manner [13]. In this connection, the present study was carried out with the aim of investigating the densification of 3 mol% yttria doped zirconia thin films obtained by the sol–gel route using different types and proportions of alcohol as the main solvent. For this purpose, data obtained from transmittance spectra have been used to determine the refractive index, n, porosity, P, and thickness, t, of zirconia films subjected to heat treatment at temperatures between 100 °C and 500 °C. The fully dense critical thickness, tcd, was determined according to a method previously reported by Díaz-Parralejo et al. [14]. Other aspects of the process such as the aging of the solutions, the values of the dimensionless parameter J, the product vc · η (where vc represents the withdrawal rate and η is the viscosity of the solution), and the fully dense critical thickness, tcd, were also investigated. These aspects are essential to understand the behaviour of the precursor solutions as well as to obtain dense coatings and crack-free films. 2. Experimental 2.1. Solutions The starting solution was prepared by adding zirconium n-propoxide (ZNP) (70 wt.% of zirconium in n-propanol, Product 22989, Alfa Products, Danvers, MA) to methanol, ethanol, 1-propanol, 2-propanol, or 1-butanol in the presence of nitric acid (HNO3),under an anhydrous nitrogen-gas atmosphere. Each solution was then mixed with another solution of yttrium (III) acetate (YAc·4H2O) in 1-propanol (PrOH) and HNO3 (YAc/ PrOH/H2O/HNO3 =1/15/6/1) to obtain the precursor solution (PS) for ZrO2–3 mol % Y2O3 films. After 1 h mixing and stirring, distilled water was added carefully to each of the preparations without stopping the stirring. After 4 h, the resulting solutions were quite transparent and paleyellow and had a pH of 0.5–0.7. The final molar relationship of PS/alcohol/H2O/ HNO3 in the solutions was 1/15/6/1. The samples were designated S15m, S15e, S15p, S15i, and S15b when the alcohol used was methanol, ethanol, 1-propanol, 2-propanol and 1-buthanol, respectively. Another two solutions were prepared following the same procedure but using 1-propanol with PS/alcohol/H2O/HNO3 =1/5/6/1 (designated S5p) and PS/alcohol/ H2O/HNO3 =1/10/6/1 (designated S10p). Table 1 lists the properties of the solutions prepared as described above. 2.2. Substrate and thin films Quartz fused (99.9% SiO2) 2.5×7.6 cm sheets (supplied by Goodfellow Ltd.) were used as substrates. The sheets were previously immersed in acetic acid for 24 h, then cleaned in distilled water and finally in ethanol in an ultrasonic bath for 15 min.
Table 1 Physicochemical properties of the solutions.
Deposition of ZrO2–3 mol% Y2O3 films was performed in air by dipcoating. The Guglielmi–Zenezini equation [15] may be written as:
tp = J ·
Molar rate alcohols/PS
Oxide concentration (g/L)
Density (g/cm3)
Viscosity (cP)
S5p S10p S15p S15m S15e S15i S15b
5 10 15 15 15 15 15
140 100 77 104 88 76 69
0.97 0.92 0.89 0.94 0.92 0.88 0.88
19.1 6.5 5.1 3.2 3.9 5.0 5.0
η· ν ρs ·g
1 = 2
ð1Þ
where η and ρs are the viscosity (Pa s) and the density (g/cm3) of the solution, respectively, ν is the substrate withdrawal rate (cm/min), g is the acceleration due to gravity (cm/ss), J is the dimensionless flow parameter, ρp is the density of the heat-treated film (g/cm3), and Cp is the solute concentration in the solution (in grams of final oxide per litre). According to this equation the film thickness can be obtained by fixing the physicochemical properties of the solutions and the substrate withdrawal rate. Thus, Eq. (1) has been used to obtain the thickness (tp) of the films here studied. Along the text, subscript “p” will refer to porous films (i.e., only partially densified). In order to investigate the influence of the physicochemical properties of solutions on film thickness and critical thickness, a set of films were deposited from the solutions and heat-treated at different temperatures between 100 °C and 500 °C in air for 2 h. For the deposition of the films, fresh solutions were used, i.e., immediately after being prepared or with very short aging time (below 5 h in all cases). 2.3. Film characterization A Thermo Spectronic spectrophotometer (Heλios α) was used to obtain the transmission spectra of the films. The thickness (tp) and refractive index (np) (at λ = 600 nm) of the films were obtained from these transmission spectra using the Swanepoel method [16] as described in a previous work [14]. From the values of the refractive index of the porous films (np) and the refractive index of dense zirconia–3 mol% yttria (nd), i.e. 2.17 [17], we calculated the porosity (P) of the films using Yoldas' equation [18]: " P = 100 · 1−
n2p −1 n2d −1
# :
ð2Þ
It is worth noting that subscript “d” refers to completely densified films that lack of a remarkable porosity, whereas, as indicated above subscript “p” refers to porous films. The porosity is related to the density through the expression: ρp P = 1− ρd
ð3Þ
where ρp is the density of the heat-treated film and ρd is the density of dense zirconia-3 mol% yttria, i.e. 5.95 g/cm3 with cubic structure [19,20]. A method based on film thickness measurements from transmittance spectra [16] was also used. According to the definition of porosity given in Eq. (3) one may write:
P = 1−
Solution
Cp · 10−3 · ρp
td tp
ð4Þ
where td is the film thickness after full densification and tp is the thickness of the partially densified films here studied. Obviously, application of this method entails a prior selection of an adequate heat treatment protocol to achieve full densification of the film. Here it has been considered that full densification is achieved when the refractive index of the film reaches the value corresponding to dense zirconia–3 mol% yttria (i.e., n = 2.17).
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3. Results and discussion 3.1. Aging of the solutions In order to evaluate the aging of the solutions, the viscosity was determined as a function of time. Fig. 1 shows the experimental values of viscosity for some of the solutions used in this work. Solutions prepared with propanol (i.e., S5p, S10p, and S15p) show larger values of viscosity than those prepared using methanol (S15m) and ethanol (S15e) as the main solvent. The viscosities of the latter solutions remained practically constant along the whole analysis time. These differences are attributable to the different alkyl group presented by methanol or ethanol with respect to the precursor alkoxide of zirconium (propanol). This would lead to slow exchange reactions between the groups corresponding to the alkoxides (propoxides) and those deriving from the solvent (methoxides and ethoxides), thus hindering the formation of interconnected molecular structures in the hydrolysis and condensation stages and leading to constant values of viscosity [22,23]. 3.2. Porosity of the samples Figs. 2 and 3 show the porosity of films annealed at different temperatures ranging from room temperature up to 500 °C for 2 h. The films were deposited from as-prepared solutions at a withdrawal rate of 10 cm/min. The porosity was calculated from the refractive index of the films (np) and the measured film thickness (tp) by using Eq. (4). With such an aim, the refractive index (nd) and thickness (td) corresponding to a fully dense ZrO2–3 mol% Y2O3 film (nd = 2.17) were attained at 1100 °C. It is worth noting that other techniques such as SEM or AFM may provide valuable information on thin films when studying the porosity of films treated at high temperatures (e.g., 1000–1100 °C and above). However, such images do not provide any remarkable information in the present study. In fact, for treatment temperatures below 700–800 °C the porosity existing in these films cannot be observed by any of the referred techniques. On the contrary, the analysis of the refractive indexes of the fully densified and porous films (nd and np, respectively) makes it possible to observe the evolution of the porosity and densifying degree of the samples in a more accurate manner. As expected, the porosities decreased with increasing temperature due to film densification. Also, porosity decreased with increasing proportion of alcohol, especially at temperatures below 300 °C. This could be due to the fact that, within the temperature range under study, the films are formed by the residues of the sol–gel synthesis inside the pores [14]. This latter also justifies the difference observed in the values of porosity of the sample treated above 300 °C, since remnants of organic residues may be found. In the case of samples
Fig. 1. Viscosity (η) vs. time for some of the solutions.
Fig. 2. Comparative plot of the porosity (P) obtained from refractive index measurements using Eq. (4), for solutions S5p, S10p, and S15p.
prepared with different alcohols (S15m, S15e, S15p, S15i, and S15b), however, no significant differences in porosity were observed. In fact, the P vs. T plots of Fig. 3 almost overlap for all samples, which is indicative of a very similar behaviour. 3.3. Analysis of the dimensionless flow parameter, J From the porosity values of Figs. 2 and 3 the density (ρp) of the heattreated ZrO2–3 mol% Y2O3 films with cubic structure [20,21] has been calculated using Eq. (3). Such values are presented in Table 2. Using the experimentally measured ρp and tp values, the initially deposited liquid film thickness, t = [103 tp ρp /Cp], can be determined [15]. Next, it is possible to calculate the dimensionless flow parameter J =t /t0 (see Eq. (1)), where t0 = (ηv /ρsg)1/2 [14]. Fig. 4 shows the t vs. t0 plots corresponding to films deposited from S15m, S15e, and S15p solutions at different substrate withdrawal rates and heat-treated at 500 °C in air for 2 h. The J values may be obtained from the slope of the plot. For the sake of clarity and in order to avoid the overlapping of the experimental data, only selected t vs. t0 values are shown in Fig. 4. Nevertheless, all of the experimental points have been used when fitting to the corresponding equation. All the J values are given in Table 3 with their corresponding error of approximately 10%. Calculations at 300 and 400 °C demonstrate that the value of J is independent of the sintering temperature within the 300–500 °C range. From the values of the parameter J listed in Table 3 it may be observed that such values slightly differ, on average being close to 0.27. Nevertheless, for solutions S5p, S10p, and S15p, a slight increase of the parameter J was appreciated with increasing quantity of solvent. This
Fig. 3. Comparative plot of the porosity (P) obtained from refractive index measurements using Eq. (4), for solutions S15m, S15e, S15p, and S15b.
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Table 2 Density (ρd) of the heat-treated ZrO2–3 mol% Y2O3 films obtained from Eq. (3). Temperature (°C) 300 400 500
Density (g/ml) S15m 3.99 4.43 4.76
S15e 4.16 4.34 4.37
S15i 4.15 4.34 4.37
S15p 4.34 4.49 4.55
S15b 4.28 4.52 4.58
latter suggests that the quantity of material deposited onto a substrate also increases. However, no clear dependence of J on the physicochemical properties (i.e., ρs, Cp and η) of the sol–gel solutions prepared with different types of alcohol was observed. Nevertheless, a slight increase in J was appreciated with increasing molar volume of the alcohol used as principal solvent. Indeed, although J is an indicator of the amount of material deposited, the final thickness reached in the deposition processes is affected by multiple factors, including the nature and quantity of solvent, the viscosity and pH of the solution, stresses in the films, etc. 3.4. Film thickness Fig. 5 shows the plot of thickness (tp) of films deposited from S15e, S10p, and S15p solutions vs. the substrate withdrawal rates for samples heat-treated at 500 °C in air for 2 h. The experimental values of tp (represented by dots) were obtained from the transmission spectra using the Swanepoel method [16]. The lines represent the best fit obtained by applying the Guglielmi–Zenezini Eq. (1). The physicochemical properties of the solutions (summarized in Table 1), the porosity, ρp, and J values (included in Tables 2 and 3, respectively), and the substrate withdrawal rate of the deposition process have been used for the calculations. It may be observed that the model is able to predict reasonably well the experimental values of tp, which corroborates the dependence of tp on v1/2 as in the Guglielmi–Zenezini expression. It is important to emphasize that in this work several solutions have been prepared using alcohols of different alkyl group to dissolve the precursor alkoxide of zirconium. This alkoxide (zirconium n-propoxide) is formed by zirconium atoms bound to propoxide groups (70% dissolved in propanol). As a consequence, exchange reactions between these groups and the alkoxides (namely methoxides, ethoxides, isopropoxides or buthoxides) of the alcohols are induced, which leads to different hydrolysis and condensation times during the sol–gel process. The critical thicknesses, tc, listed in Table 3 correspond to films deposited at the critical withdrawal rate, vc, from the different solutions,
Fig. 4. Liquid film thickness (t) vs. characteristic thickness (t0) for films deposited from S15m, S15p, and S15b with different substrate withdrawal rates and heat-treated at 500 °C for 2 h in air. The lines represent the best-fit slope (J = t / t0) through the data points.
Table 3 Dimensionless parameter (J) and critical parameters (vc · η, tc, tcd) obtained for films heat-treated at 500 °C in air for 2 h from the different solutions. Solution
J
vc · η
tc
tcd
S5p S10p S15p S15m S15e S15i S15b
0.26 ± 0.03 0.27 ± 0.02 0.28 ± 0.04 0.27 ± 0.01 0.28 ± 0.01 0.31 ± 0.02 0.32 ± 0.03
27 ± 1 55 ± 2 95 ± 3 38 ± 1 47 ± 2 57 ± 2 112 ± 4
175 ± 3 190 ± 2 195 ± 4 156 ± 3 166 ± 2 186 ± 4 225 ± 3
134 ± 1 145 ± 2 149 ± 3 122 ± 2 123 ± 2 137 ± 3 173 ± 1
followed by thermal treatment at 500 °C in air for 2 h. The values indicate that tc depends on the properties of the sol–gel solutions used for the deposition process. In the films made with propanol, tc increases as the amount of propanol used in the sol–gel synthesis does (i.e., S5p→S10p→S15p). Also, tc depends on the type of alcohol used as solvent of ZNP, increasing in the order S15m→S15e→S15i→S15p→S15b. These results evidence that the properties of the sol–gel solutions have a strong influence on the value of tc. It may be observed that tc increases as the molecular size of the solvent does. Also, from Eq. (4) one can calculate the thickness that will be reached by each film of a certain value of tc, once made fully dense by treatment at 1100 °C in air for 2 h. The values of the fully dense critical thickness, tcd, of each film are listed in Table 3. From these data it may be concluded that, as it was also the case with the critical thickness, tc, the fully dense critical thickness, tcd, increases slightly with increasing proportion of alcohol in the solution (S5p, S10p, and S15p). The results also suggest that tcd also increases with increasing molar volume of the alcohol, as it was the case with the values of tc. Nevertheless, the differences are now smaller. Furthermore, no differences are appreciable when methanol or ethanol is used. This may be explained taking into account that, even though the tc values are considerably larger when using methanol the porosities of the films are larger, too. The final result is that the largest quantity of material that can be deposited before cracks appear is similar in both cases. From a practical perspective, these results clearly suggest that the best option to obtain thick, crack-free films is from the S15b solution, which provides thicker and less porous coatings. Finally, in a sol–gel solution the values of Cp and ρs are constant. Therefore, the tp value of a treated film will be dependent on the variations of η and v (Eq. (1)). Increments of η involve increases of tp that can surpass the value of tc in the film. However, such increments can be compensated with decreases in ν during the deposition process. Therefore, the value of the parameter vc ·η in Eq. (1) is directly related to tc via the value of the critical parameter vc ·η (see Table 3), and can be used as a control parameter to avoid the formation of cracks in the films.
Fig. 5. Experimental thickness of the films deposited from S15e, S10p, and S15p with different substrate withdrawal rates and heat-treated at 500 °C in air for 2 h. The lines represent the fit with the Guglielmi–Zenezini equation.
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4. Conclusions
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in the present work. A study of these characteristics is of great interest for the authors, and is an open topic for future work.
From the results obtained in the present study the following conclusions may be drawn. • The properties of sol–gel solutions are directly related to the different components used in the synthesis process. Basically, these differences are linked to the different types and proportions of alcohols used as the solvent of ZNP. Thus, the smaller the proportion of alcohol used in the solution the larger is viscosity and the faster is the aging process of the solution. • The properties of the sol–gel solutions – and, particularly, the molecular size of the solvent – strongly influence the values of tc and tcd. Thus, the values of tc increase in the order S15m→S15e→S15i→S15p→S15b. The intrinsic and extrinsic values of the parameters of the sol–gel process (η, Cp, ρs, v, etc) affect the molecular structures formed during the synthesis and, in turn, these structures affect the tc and tcd values of the coatings. • The values of the dimensionless parameter J are useful to predict the thickness of the films formed by the sol–gel method. A slight increase of J is observed when the molecular size of the solvent grows. Likewise, from a practical perspective, it is very interesting to know the value of the (vc ·η) product for any solution, because it allows the evolution or aging of the sol–gel solution to be compensated by adjusting the withdrawal rate, v. • The analysis of the molecular complexity of these gels and their relationship with the critical thickness would involve the use of experimental characterization techniques different from those used
References [1] J.D. Wright, N.A.J.M. Sommerdijk, Sol–Gel Materials Chemistry and Applications, Taylor and Francis, London, 2001. [2] T.A. Guiton, C.L. Czekaj, C.G. Pantano, J. Non-Cryst. Solids 121 (1990) 7. [3] S. Thitinun, N. Thanabodeekij, A.M. Jamieson, S. Wongkasemjit, J. Eur. Ceram. Soc. 23 (2003) 417. [4] L. Zhonghong, Q. Nongxue, Y. Gongming, J. Membr. Sci. 326 (2009) 533. [5] R. Caruso, O. de Sanctis, A. Frattini, C. Steren, R. Gil, Surf. Coat. Technol. 122 (1999) 44. [6] W. Cuiming, W. Yonghui, X. Tongwen, Y. Weihua, J. Non-Cryst. Solids 352 (2006) 5642. [7] Y.S. Baron, A. Ruiz, G. Navas, Surf. Coat. Technol. 202 (2008) 2616. [8] C. Viazzi, J.P. Bonino, F. Ansart, Surf. Coat. Technol. 201 (2006) 3889. [9] R.L. Jones, R.F. Reidy, D. Mess, Surf. Coat. Technol. 82 (1996) 70. [10] Z. Sha, M. Fei, S. Zhongxiao, X. Kewei, Optical Mater. 30 (2008) 910. [11] L. Haibin, L. Kaiming, M. Lefu, G. Shouren, Mater. Sci. Eng. 341 (2003) 87. [12] D.J. Green, R.H.J. Hannink, M.V. Swain, Transformation and Toughening of Ceramics, CRC Press, Boca Raton, FL, 1989. [13] L. Arda, S. Ataoglu, S. Sezer, Z. Abdulaliyev, Surf. Coat. Technol. 202 (2007) 439. [14] A. Diaz-Parralejo, R. Caruso, A.L. Ortiz, F. Guiberteau, Thin Solid Films 458 (2004) 92. [15] M. Guglielmi, S. Zenezini, J. Non-Cryst. Solids 121 (1990) 303. [16] R.J. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [17] F. Horowitz, A.F. Michels, Appl. Opt. 47 (2008) C185. [18] B.E. Yoldas, Appl. Opt. 19 (1980) 1425. [19] J.I. Eldridge, C.M. Spuckler, J. Amer. Ceram. Soc. 91 (2008) 1603. [20] R. Caruso, A. Díaz-Parralejo, F. Guiberteau, P. Miranda, J. Mater. Res. 16 (2001) 2391. [21] R.L. Jones, D. Mess, Surf. Coat. Technol. 86–87 (1996) 94. [22] R. Brenier, A. Gagnaire, Thin Solid Films 392 (2001) 142. [23] Y. Castro, M. Aparicio, R. Moreno, A. Durán, J. Sol–gel Sci. Technol. 35 (2005) 41.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Influence of the experimental parameters on the synthesis process of yttria-doped zirconia sol–gel films Antonio Díaz-Parralejo a, Antonio Macías-García a, José Sánchez-González a, M. Ángeles Díaz-Díez a, Eduardo M. Cuerda-Correa b,⁎ a b
Department of Mechanical, Energetic and Materials Engineering, School of Industrial Engineering, University of Extremadura, Avda. de Elvas, s/n, E-06071 Badajoz, Spain Department of Organic and Inorganic Chemistry, Faculty of Sciences, University of Extremadura, Avda. de Elvas, s/n, E-06071 Badajoz, Spain
a r t i c l e
i n f o
Article history: Received 5 August 2009 Accepted in revised form 21 December 2009 Available online 4 January 2010 Keywords: Sol–gel Yttria-stabilized zirconia Densification Thickness
a b s t r a c t The influence of the alcohol solvent (methanol, ethanol, propanol, isopropanol or buthanol) and the proportion used in the preparation of precursor solutions to obtain ZrO2–3 mol% Y2O3 sol–gel films are investigated. In particular, the effect of the aging process of the solutions on the porosity and critical thickness of the films is analysed. Implications for the preparation of zirconia sol–gel films by the dip-coating technique are also discussed. Other aspects of the process such as the aging of the solutions, the evolution of the dimensionless parameter J, the porosity, P, and the fully dense critical thickness, tcd, are also investigated and discussed. This allows one to understand the behaviour of the precursor solutions, and to obtain dense coatings and crack-free films. The results reveal a significant influence of the alcohol used as the main solvent in the first densification stages of the films (100 °C–500 °C). Likewise, the type and amount of alcohol used also notably influence the aging process of the solution, the porosity and the fully dense critical thickness finally obtained with each solution. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The sol–gel process is nowadays being used by a growing number of researchers for the preparation of an extensive variety of new materials, including bulk materials, films, and fibres. The most attractive feature of this processing route is the possibility of tailoring unique materials by polymerization of organometallic compounds (metal alkoxides, metal acetylacetonates, etc.) to a polymer gel. Two chemical reactions are involved in this process, namely hydrolysis and polycondensation. These reactions are usually simultaneous, and are controlled by using an appropriate acid or base catalyst. Because organometallic compounds and water are not miscible, a solvent – usually an alcohol – must be used in the process [1]. Transition metal alkoxides, especially those of Ti and Zr are widely used as molecular precursors of ceramic materials, due to the fact that most of them are soluble in alcohols, thus allowing good control of the hydrolysis and condensation reactions via the parameters of the process, such as the amount of water, type of alcohol, pH of the solution, etc. [2,3]. The type of alkyl radical in the alkoxide has a strong influence on the hydrolysis and condensation reactions, affecting the properties of the final products. Also, the solvent alcohols are usually far from being inert components, but are themselves involved in the chemical process [4,5]. If the alkoxides are dissolved in alcohols containing different alkyl groups, the ⁎ Corresponding author. E-mail address: [email protected] (E.M. Cuerda-Correa). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.12.015
alcohol exchange reactions –alcoholysis–, hydrolysis and condensation take place at slower rates, yielding more complex structures [6,7]. Deposition of thin films by the sol–gel route onto different substrates offers certain advantages over other methods of obtaining thin films (e.g., physical and chemical vapour deposition, electron beam evaporation, etc.) [8]. These advantages include simple and low-cost equipment, possibility of coating large areas, etc. In addition, this method is suitable for obtaining almost any single- or multi-component oxide coating (TiO2, ZrO2, etc.). In particular, zirconia-based thin films have promising applications in optics and as protective barriers [9]. For instance, as coatings they can be used to provide specific optical properties in glasses (e.g., anti-reflection, selective reflection, photochromism, etc.) [10], as well as to prevent chemical corrosion and gas oxidation in metals [11]. All of these applications are based on the interesting combination of mechanical, chemical, and physical properties exhibited by zirconia ceramics. Also, yttria-doped zirconia can exhibit outstanding mechanical properties, combining high wear resistance with moderate toughness, frequently associated with the activation of some transformation toughening mechanism [12]. However, the structure and properties of thin films can differ significantly from those of the bulk material. In particular, thin film properties depend on porosity and thickness, making the control of both characteristics during the densification processes a critical issue in sol–gel film preparation. Likewise, the type of alcohol used as the main solvent in the process has a noticeable influence on the reactions involved in the sol–gel process, so that it can affect the structure and final properties of the films.
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Some aspects such as the type of alcohol used as the main solvent, the alcohol-to-alkoxide ratio, the evolution or aging of the solutions as well as the influence of these factors on the densification and thickness of the films require a further analysis in order to make it possible to use the sol–gel method in an optimal manner [13]. In this connection, the present study was carried out with the aim of investigating the densification of 3 mol% yttria doped zirconia thin films obtained by the sol–gel route using different types and proportions of alcohol as the main solvent. For this purpose, data obtained from transmittance spectra have been used to determine the refractive index, n, porosity, P, and thickness, t, of zirconia films subjected to heat treatment at temperatures between 100 °C and 500 °C. The fully dense critical thickness, tcd, was determined according to a method previously reported by Díaz-Parralejo et al. [14]. Other aspects of the process such as the aging of the solutions, the values of the dimensionless parameter J, the product vc · η (where vc represents the withdrawal rate and η is the viscosity of the solution), and the fully dense critical thickness, tcd, were also investigated. These aspects are essential to understand the behaviour of the precursor solutions as well as to obtain dense coatings and crack-free films. 2. Experimental 2.1. Solutions The starting solution was prepared by adding zirconium n-propoxide (ZNP) (70 wt.% of zirconium in n-propanol, Product 22989, Alfa Products, Danvers, MA) to methanol, ethanol, 1-propanol, 2-propanol, or 1-butanol in the presence of nitric acid (HNO3),under an anhydrous nitrogen-gas atmosphere. Each solution was then mixed with another solution of yttrium (III) acetate (YAc·4H2O) in 1-propanol (PrOH) and HNO3 (YAc/ PrOH/H2O/HNO3 =1/15/6/1) to obtain the precursor solution (PS) for ZrO2–3 mol % Y2O3 films. After 1 h mixing and stirring, distilled water was added carefully to each of the preparations without stopping the stirring. After 4 h, the resulting solutions were quite transparent and paleyellow and had a pH of 0.5–0.7. The final molar relationship of PS/alcohol/H2O/ HNO3 in the solutions was 1/15/6/1. The samples were designated S15m, S15e, S15p, S15i, and S15b when the alcohol used was methanol, ethanol, 1-propanol, 2-propanol and 1-buthanol, respectively. Another two solutions were prepared following the same procedure but using 1-propanol with PS/alcohol/H2O/HNO3 =1/5/6/1 (designated S5p) and PS/alcohol/ H2O/HNO3 =1/10/6/1 (designated S10p). Table 1 lists the properties of the solutions prepared as described above. 2.2. Substrate and thin films Quartz fused (99.9% SiO2) 2.5×7.6 cm sheets (supplied by Goodfellow Ltd.) were used as substrates. The sheets were previously immersed in acetic acid for 24 h, then cleaned in distilled water and finally in ethanol in an ultrasonic bath for 15 min.
Table 1 Physicochemical properties of the solutions.
Deposition of ZrO2–3 mol% Y2O3 films was performed in air by dipcoating. The Guglielmi–Zenezini equation [15] may be written as:
tp = J ·
Molar rate alcohols/PS
Oxide concentration (g/L)
Density (g/cm3)
Viscosity (cP)
S5p S10p S15p S15m S15e S15i S15b
5 10 15 15 15 15 15
140 100 77 104 88 76 69
0.97 0.92 0.89 0.94 0.92 0.88 0.88
19.1 6.5 5.1 3.2 3.9 5.0 5.0
η· ν ρs ·g
1 = 2
ð1Þ
where η and ρs are the viscosity (Pa s) and the density (g/cm3) of the solution, respectively, ν is the substrate withdrawal rate (cm/min), g is the acceleration due to gravity (cm/ss), J is the dimensionless flow parameter, ρp is the density of the heat-treated film (g/cm3), and Cp is the solute concentration in the solution (in grams of final oxide per litre). According to this equation the film thickness can be obtained by fixing the physicochemical properties of the solutions and the substrate withdrawal rate. Thus, Eq. (1) has been used to obtain the thickness (tp) of the films here studied. Along the text, subscript “p” will refer to porous films (i.e., only partially densified). In order to investigate the influence of the physicochemical properties of solutions on film thickness and critical thickness, a set of films were deposited from the solutions and heat-treated at different temperatures between 100 °C and 500 °C in air for 2 h. For the deposition of the films, fresh solutions were used, i.e., immediately after being prepared or with very short aging time (below 5 h in all cases). 2.3. Film characterization A Thermo Spectronic spectrophotometer (Heλios α) was used to obtain the transmission spectra of the films. The thickness (tp) and refractive index (np) (at λ = 600 nm) of the films were obtained from these transmission spectra using the Swanepoel method [16] as described in a previous work [14]. From the values of the refractive index of the porous films (np) and the refractive index of dense zirconia–3 mol% yttria (nd), i.e. 2.17 [17], we calculated the porosity (P) of the films using Yoldas' equation [18]: " P = 100 · 1−
n2p −1 n2d −1
# :
ð2Þ
It is worth noting that subscript “d” refers to completely densified films that lack of a remarkable porosity, whereas, as indicated above subscript “p” refers to porous films. The porosity is related to the density through the expression: ρp P = 1− ρd
ð3Þ
where ρp is the density of the heat-treated film and ρd is the density of dense zirconia-3 mol% yttria, i.e. 5.95 g/cm3 with cubic structure [19,20]. A method based on film thickness measurements from transmittance spectra [16] was also used. According to the definition of porosity given in Eq. (3) one may write:
P = 1−
Solution
Cp · 10−3 · ρp
td tp
ð4Þ
where td is the film thickness after full densification and tp is the thickness of the partially densified films here studied. Obviously, application of this method entails a prior selection of an adequate heat treatment protocol to achieve full densification of the film. Here it has been considered that full densification is achieved when the refractive index of the film reaches the value corresponding to dense zirconia–3 mol% yttria (i.e., n = 2.17).
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3. Results and discussion 3.1. Aging of the solutions In order to evaluate the aging of the solutions, the viscosity was determined as a function of time. Fig. 1 shows the experimental values of viscosity for some of the solutions used in this work. Solutions prepared with propanol (i.e., S5p, S10p, and S15p) show larger values of viscosity than those prepared using methanol (S15m) and ethanol (S15e) as the main solvent. The viscosities of the latter solutions remained practically constant along the whole analysis time. These differences are attributable to the different alkyl group presented by methanol or ethanol with respect to the precursor alkoxide of zirconium (propanol). This would lead to slow exchange reactions between the groups corresponding to the alkoxides (propoxides) and those deriving from the solvent (methoxides and ethoxides), thus hindering the formation of interconnected molecular structures in the hydrolysis and condensation stages and leading to constant values of viscosity [22,23]. 3.2. Porosity of the samples Figs. 2 and 3 show the porosity of films annealed at different temperatures ranging from room temperature up to 500 °C for 2 h. The films were deposited from as-prepared solutions at a withdrawal rate of 10 cm/min. The porosity was calculated from the refractive index of the films (np) and the measured film thickness (tp) by using Eq. (4). With such an aim, the refractive index (nd) and thickness (td) corresponding to a fully dense ZrO2–3 mol% Y2O3 film (nd = 2.17) were attained at 1100 °C. It is worth noting that other techniques such as SEM or AFM may provide valuable information on thin films when studying the porosity of films treated at high temperatures (e.g., 1000–1100 °C and above). However, such images do not provide any remarkable information in the present study. In fact, for treatment temperatures below 700–800 °C the porosity existing in these films cannot be observed by any of the referred techniques. On the contrary, the analysis of the refractive indexes of the fully densified and porous films (nd and np, respectively) makes it possible to observe the evolution of the porosity and densifying degree of the samples in a more accurate manner. As expected, the porosities decreased with increasing temperature due to film densification. Also, porosity decreased with increasing proportion of alcohol, especially at temperatures below 300 °C. This could be due to the fact that, within the temperature range under study, the films are formed by the residues of the sol–gel synthesis inside the pores [14]. This latter also justifies the difference observed in the values of porosity of the sample treated above 300 °C, since remnants of organic residues may be found. In the case of samples
Fig. 1. Viscosity (η) vs. time for some of the solutions.
Fig. 2. Comparative plot of the porosity (P) obtained from refractive index measurements using Eq. (4), for solutions S5p, S10p, and S15p.
prepared with different alcohols (S15m, S15e, S15p, S15i, and S15b), however, no significant differences in porosity were observed. In fact, the P vs. T plots of Fig. 3 almost overlap for all samples, which is indicative of a very similar behaviour. 3.3. Analysis of the dimensionless flow parameter, J From the porosity values of Figs. 2 and 3 the density (ρp) of the heattreated ZrO2–3 mol% Y2O3 films with cubic structure [20,21] has been calculated using Eq. (3). Such values are presented in Table 2. Using the experimentally measured ρp and tp values, the initially deposited liquid film thickness, t = [103 tp ρp /Cp], can be determined [15]. Next, it is possible to calculate the dimensionless flow parameter J =t /t0 (see Eq. (1)), where t0 = (ηv /ρsg)1/2 [14]. Fig. 4 shows the t vs. t0 plots corresponding to films deposited from S15m, S15e, and S15p solutions at different substrate withdrawal rates and heat-treated at 500 °C in air for 2 h. The J values may be obtained from the slope of the plot. For the sake of clarity and in order to avoid the overlapping of the experimental data, only selected t vs. t0 values are shown in Fig. 4. Nevertheless, all of the experimental points have been used when fitting to the corresponding equation. All the J values are given in Table 3 with their corresponding error of approximately 10%. Calculations at 300 and 400 °C demonstrate that the value of J is independent of the sintering temperature within the 300–500 °C range. From the values of the parameter J listed in Table 3 it may be observed that such values slightly differ, on average being close to 0.27. Nevertheless, for solutions S5p, S10p, and S15p, a slight increase of the parameter J was appreciated with increasing quantity of solvent. This
Fig. 3. Comparative plot of the porosity (P) obtained from refractive index measurements using Eq. (4), for solutions S15m, S15e, S15p, and S15b.
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Table 2 Density (ρd) of the heat-treated ZrO2–3 mol% Y2O3 films obtained from Eq. (3). Temperature (°C) 300 400 500
Density (g/ml) S15m 3.99 4.43 4.76
S15e 4.16 4.34 4.37
S15i 4.15 4.34 4.37
S15p 4.34 4.49 4.55
S15b 4.28 4.52 4.58
latter suggests that the quantity of material deposited onto a substrate also increases. However, no clear dependence of J on the physicochemical properties (i.e., ρs, Cp and η) of the sol–gel solutions prepared with different types of alcohol was observed. Nevertheless, a slight increase in J was appreciated with increasing molar volume of the alcohol used as principal solvent. Indeed, although J is an indicator of the amount of material deposited, the final thickness reached in the deposition processes is affected by multiple factors, including the nature and quantity of solvent, the viscosity and pH of the solution, stresses in the films, etc. 3.4. Film thickness Fig. 5 shows the plot of thickness (tp) of films deposited from S15e, S10p, and S15p solutions vs. the substrate withdrawal rates for samples heat-treated at 500 °C in air for 2 h. The experimental values of tp (represented by dots) were obtained from the transmission spectra using the Swanepoel method [16]. The lines represent the best fit obtained by applying the Guglielmi–Zenezini Eq. (1). The physicochemical properties of the solutions (summarized in Table 1), the porosity, ρp, and J values (included in Tables 2 and 3, respectively), and the substrate withdrawal rate of the deposition process have been used for the calculations. It may be observed that the model is able to predict reasonably well the experimental values of tp, which corroborates the dependence of tp on v1/2 as in the Guglielmi–Zenezini expression. It is important to emphasize that in this work several solutions have been prepared using alcohols of different alkyl group to dissolve the precursor alkoxide of zirconium. This alkoxide (zirconium n-propoxide) is formed by zirconium atoms bound to propoxide groups (70% dissolved in propanol). As a consequence, exchange reactions between these groups and the alkoxides (namely methoxides, ethoxides, isopropoxides or buthoxides) of the alcohols are induced, which leads to different hydrolysis and condensation times during the sol–gel process. The critical thicknesses, tc, listed in Table 3 correspond to films deposited at the critical withdrawal rate, vc, from the different solutions,
Fig. 4. Liquid film thickness (t) vs. characteristic thickness (t0) for films deposited from S15m, S15p, and S15b with different substrate withdrawal rates and heat-treated at 500 °C for 2 h in air. The lines represent the best-fit slope (J = t / t0) through the data points.
Table 3 Dimensionless parameter (J) and critical parameters (vc · η, tc, tcd) obtained for films heat-treated at 500 °C in air for 2 h from the different solutions. Solution
J
vc · η
tc
tcd
S5p S10p S15p S15m S15e S15i S15b
0.26 ± 0.03 0.27 ± 0.02 0.28 ± 0.04 0.27 ± 0.01 0.28 ± 0.01 0.31 ± 0.02 0.32 ± 0.03
27 ± 1 55 ± 2 95 ± 3 38 ± 1 47 ± 2 57 ± 2 112 ± 4
175 ± 3 190 ± 2 195 ± 4 156 ± 3 166 ± 2 186 ± 4 225 ± 3
134 ± 1 145 ± 2 149 ± 3 122 ± 2 123 ± 2 137 ± 3 173 ± 1
followed by thermal treatment at 500 °C in air for 2 h. The values indicate that tc depends on the properties of the sol–gel solutions used for the deposition process. In the films made with propanol, tc increases as the amount of propanol used in the sol–gel synthesis does (i.e., S5p→S10p→S15p). Also, tc depends on the type of alcohol used as solvent of ZNP, increasing in the order S15m→S15e→S15i→S15p→S15b. These results evidence that the properties of the sol–gel solutions have a strong influence on the value of tc. It may be observed that tc increases as the molecular size of the solvent does. Also, from Eq. (4) one can calculate the thickness that will be reached by each film of a certain value of tc, once made fully dense by treatment at 1100 °C in air for 2 h. The values of the fully dense critical thickness, tcd, of each film are listed in Table 3. From these data it may be concluded that, as it was also the case with the critical thickness, tc, the fully dense critical thickness, tcd, increases slightly with increasing proportion of alcohol in the solution (S5p, S10p, and S15p). The results also suggest that tcd also increases with increasing molar volume of the alcohol, as it was the case with the values of tc. Nevertheless, the differences are now smaller. Furthermore, no differences are appreciable when methanol or ethanol is used. This may be explained taking into account that, even though the tc values are considerably larger when using methanol the porosities of the films are larger, too. The final result is that the largest quantity of material that can be deposited before cracks appear is similar in both cases. From a practical perspective, these results clearly suggest that the best option to obtain thick, crack-free films is from the S15b solution, which provides thicker and less porous coatings. Finally, in a sol–gel solution the values of Cp and ρs are constant. Therefore, the tp value of a treated film will be dependent on the variations of η and v (Eq. (1)). Increments of η involve increases of tp that can surpass the value of tc in the film. However, such increments can be compensated with decreases in ν during the deposition process. Therefore, the value of the parameter vc ·η in Eq. (1) is directly related to tc via the value of the critical parameter vc ·η (see Table 3), and can be used as a control parameter to avoid the formation of cracks in the films.
Fig. 5. Experimental thickness of the films deposited from S15e, S10p, and S15p with different substrate withdrawal rates and heat-treated at 500 °C in air for 2 h. The lines represent the fit with the Guglielmi–Zenezini equation.
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4. Conclusions
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in the present work. A study of these characteristics is of great interest for the authors, and is an open topic for future work.
From the results obtained in the present study the following conclusions may be drawn. • The properties of sol–gel solutions are directly related to the different components used in the synthesis process. Basically, these differences are linked to the different types and proportions of alcohols used as the solvent of ZNP. Thus, the smaller the proportion of alcohol used in the solution the larger is viscosity and the faster is the aging process of the solution. • The properties of the sol–gel solutions – and, particularly, the molecular size of the solvent – strongly influence the values of tc and tcd. Thus, the values of tc increase in the order S15m→S15e→S15i→S15p→S15b. The intrinsic and extrinsic values of the parameters of the sol–gel process (η, Cp, ρs, v, etc) affect the molecular structures formed during the synthesis and, in turn, these structures affect the tc and tcd values of the coatings. • The values of the dimensionless parameter J are useful to predict the thickness of the films formed by the sol–gel method. A slight increase of J is observed when the molecular size of the solvent grows. Likewise, from a practical perspective, it is very interesting to know the value of the (vc ·η) product for any solution, because it allows the evolution or aging of the sol–gel solution to be compensated by adjusting the withdrawal rate, v. • The analysis of the molecular complexity of these gels and their relationship with the critical thickness would involve the use of experimental characterization techniques different from those used
References [1] J.D. Wright, N.A.J.M. Sommerdijk, Sol–Gel Materials Chemistry and Applications, Taylor and Francis, London, 2001. [2] T.A. Guiton, C.L. Czekaj, C.G. Pantano, J. Non-Cryst. Solids 121 (1990) 7. [3] S. Thitinun, N. Thanabodeekij, A.M. Jamieson, S. Wongkasemjit, J. Eur. Ceram. Soc. 23 (2003) 417. [4] L. Zhonghong, Q. Nongxue, Y. Gongming, J. Membr. Sci. 326 (2009) 533. [5] R. Caruso, O. de Sanctis, A. Frattini, C. Steren, R. Gil, Surf. Coat. Technol. 122 (1999) 44. [6] W. Cuiming, W. Yonghui, X. Tongwen, Y. Weihua, J. Non-Cryst. Solids 352 (2006) 5642. [7] Y.S. Baron, A. Ruiz, G. Navas, Surf. Coat. Technol. 202 (2008) 2616. [8] C. Viazzi, J.P. Bonino, F. Ansart, Surf. Coat. Technol. 201 (2006) 3889. [9] R.L. Jones, R.F. Reidy, D. Mess, Surf. Coat. Technol. 82 (1996) 70. [10] Z. Sha, M. Fei, S. Zhongxiao, X. Kewei, Optical Mater. 30 (2008) 910. [11] L. Haibin, L. Kaiming, M. Lefu, G. Shouren, Mater. Sci. Eng. 341 (2003) 87. [12] D.J. Green, R.H.J. Hannink, M.V. Swain, Transformation and Toughening of Ceramics, CRC Press, Boca Raton, FL, 1989. [13] L. Arda, S. Ataoglu, S. Sezer, Z. Abdulaliyev, Surf. Coat. Technol. 202 (2007) 439. [14] A. Diaz-Parralejo, R. Caruso, A.L. Ortiz, F. Guiberteau, Thin Solid Films 458 (2004) 92. [15] M. Guglielmi, S. Zenezini, J. Non-Cryst. Solids 121 (1990) 303. [16] R.J. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 1214. [17] F. Horowitz, A.F. Michels, Appl. Opt. 47 (2008) C185. [18] B.E. Yoldas, Appl. Opt. 19 (1980) 1425. [19] J.I. Eldridge, C.M. Spuckler, J. Amer. Ceram. Soc. 91 (2008) 1603. [20] R. Caruso, A. Díaz-Parralejo, F. Guiberteau, P. Miranda, J. Mater. Res. 16 (2001) 2391. [21] R.L. Jones, D. Mess, Surf. Coat. Technol. 86–87 (1996) 94. [22] R. Brenier, A. Gagnaire, Thin Solid Films 392 (2001) 142. [23] Y. Castro, M. Aparicio, R. Moreno, A. Durán, J. Sol–gel Sci. Technol. 35 (2005) 41.