1. Introduction

Films materials play a special role in the development of civilizations based on high technologies. Studying optical characteristic of dielectric resulted in using thin layer as reallot light covering and interference filters. Among different material classes oxides have various functional electro physical properties (electro physical, optical, mechanical etc), which allows to use these materials as a basis for synthesis of many film materials. At the present stage of development of photonics, optoinformatics, optotechnics and laser optics the requirements to optics properties of films extend[1-2].

The most challenging issue is the absence of transparent in broad spectral zone film-forming materials with high Refractive index. Zirconia-based thin films have a high potential. Zirconia is transparent in the visible; it has high refractive index and bandgap values, good adhesion to substrates (glass, ceramics, silicon, polycor, and sapphire), thermal stability, and corrosion resistance[3-6].

Sol-gel technology, which is a synthetic method involving chemical condensation in a liquid phase, is regarded as the most efficient and simplest method for manufacturing nanoparticles. This method can provide not only dispersed powders but also thin films based on complex chemical systems having layer thicknesses of 10 to 200 nm and multilayer films having thicknesses up to 1 μm. The synthesis and characterization of nanocrystalline zirconia-based powders prepared by various methods are found in periodicals[7-12].

To use thin films materials based on dioxide zirconium in optics, particularly as reallot light covering, we should studythe influence of conditions of synthesis on thickness, phase composition and structure of resulting films, evaluate the influence of a substrate (glass, quartz, etc) on physicochemical properties of the films.

Therefore, here we study the physicochemical processes involved in the preparation of ZrO2 thin films on various substrates by sol-gel technology from film-forming solutions (FFSs) and the phase composition, structure, and properties of these films as dependent on the synthesis parameters.

2. Experimental Work

Thin films were prepared by sol-gel technology from FFSs. The FFSs were prepared from 96% ethanol and zirconium oxochloride ZrOCl₂·8H₂O (pure for analysis); the solution concentration was 0.4 mol/L. Films were prepared on glass, single-crystal silicon, quartz, polycor, and sapphire using centrifugation at 4000 rpm or pulling at 5 mm/s and subsequent heat treatment at 60℃ for 30 min and at 600-1000°C for 1 h.

The film-forming power of solutions as a function of aging time was studied viscometrically on a glass capillary viscometer (the capillary diameter was 0.99 mm; temperature was 25℃).

Reduced viscosity η_sp/c was determined as a function of zirconium oxochloride concentration by diluting the FFS with ethanol; oxochloride concentrations ranged within 0.5- 3.3 g/dL. The concentration dependences were processed after Bogoslovskii et al.[13].

The thermolysis of the dried FFS was studied on a Q-1500 derivatograph (25-1000℃, calcined A1₂O₃ as reference, air, heating at 10 K/min, Alundum crucibles). A microbalance based on a quartz piezocrystal resonator was used in the thermoanalytical experiments; the weighing accuracy was 10^-8 g[14]. IR absorption spectra were recorded for films on KBr substrates annealed at various temperatures within 400-4000 cm⁴ on a Perkin-Elmer Spectrum One spectrometer. The composition of films was determined on a DRON-3M diffractometer (CuK_α radiation, λ = 1.5418 nm, Ni filter).

The refractive index and film thickness were determined on an LEF-3M laser diffractometer. The optical parameters were calculated using the uniform nonabsorbing layer on an isotropic substrate model[15]. Reflection andtransmission spectra in the visible and IR were recorded on SF-20 and Perkin-Elmer Spectrum One spectrometers. Dielectric constants were calculated by Kramers-Kroning relations from the reflection and transmission spectra. The bandgap width was calculated from the absorption edge position. Adhesion was determined by scleroscopy on a PMT-3 microhardness tester.

3. Results and Discussion

The solutions from which thin films can be deposited by sol-gel technology should conform to several requirements. When precursors are dissolved in the solvent, some period is needed for so-called solution ripening or sol formation. This period ranges from several minutes to several days, depending on the precursor. At this stage, the true solution transforms to a colloidal one because of solvation, hydrolysis, complex formation, and condensation. The resulting associations are capable of being anchored to the surface when the solution is applied to the substrate.

We used viscosity as a measure of the film-forming power of the solution[16-18]. Solution viscosities were plotted as function of time and possibility of film preparation from these solutions. Our studies showed that a fresh zirconium oxochloride-based ethanolic solution has not film-forming properties. The film-forming ability appears 2-3 days after the solution was prepared. The solution viscosity increases strongly during this period because of the solvation of zirconium oxochloride and the formation of hydroxo complexes[Zr₄(ОН)₈(С₂Н₅ОH(Н₂O)_16-х]С1₈; as a result, a stable sol is formed in the solution. Good films with reproducible properties can be prepared if the solution viscosity reaches 2.3 × 10^-3 Pa s.

The processes occurring in the FFS during storage and operation change the composition of the complex: the number of molecules of water of coordination decreases, more OH groups appear linked to zirconium, some water molecules are displaced from the inner coordination sphere, and the hydroxo cation charge decreases:

[Zr₄(OH)₈(C₂H₅OHUH₂O)_16-х]Cl₈ →

[Zr₄(OH)₁₂(C₂H₅OH)_x(H₂O)_12x]Cl₄ + 4HC1,

[Zr₄(OH)₁₂(C₂H₅OH)_x(H₂O)_12-x]Cl₄ →

[Zr₄ (ОH)₁₆(С₂Н₅ОH)_x(H₂O)_8-x] + 4HC1,

[Zr₄(OH)₈(H₂O)_16-x(C₂H₅OH)J⁸⁺ →

[Zr₄(OH)_8+y(H₂O)_{16 -x -у}(С₂Н₅ОH)_x]^{(8 -y)+} + yH⁺.

As a result, the system loses stability because of coagulation, and the solution viscosity increases dramatically and exceeds 3.5 × 10^-3 Pa s. The films prepared from such solutions have nonuniform thicknesses and low refractive indices. Thus, the film-forming properties of the solution (the ability to form films) exist within a limited period time, namely, while the solution is ripening and aging. The optimal viscosities of zirconium oxochloride-based solutions for preparing ZrO₂ films are (2.3 to 3.5) 3.5 × 10^-3 Pa s.

The zirconium oxochloride-based FFS being a solution based on a polymeric hydroxo complex, Kolen'ko et al.' approach[12] is suitable for the quantitative description of colloid interactions in this solution. In this context, we studied the viscosity of the zirconium oxochloride-based FFS as a function of dilution and calculated the covalent and electrostatic terms of the Gibbs free energy of mixing. To describe reduced viscosity η_sp/c as a function of concentration, we used a linear extrapolation equation, namely, Huggins's equation

(1)

and nonlinear extrapolation equations, namely,

(2)

(3)

Here, a, b_i, an d_i are empirical solvation factors, which depend on the dipole moment, donor and acceptor numbers of the solvent and have the dimension of the molar volume. Various functions for the description of empirical reduced viscosity versus temperature dependences give numerical values of the characteristic viscosity r|. Variation in reduced viscosity of the zirconium oxochloride-based FFS (Fig. 1) can be described by Eqs. (l)-(3). For this solution, we calculated the Huggins constant k' and characteristic viscosity from these three equations (Table 1).

The Huggins constant has a negative value (k' = -2.41). Probably, this is explained by the effect of hydrogen bonds, which are ignored by the Huggins theory. Comparing the variance and correlation coefficients for the three equations, Eq. (3) most adequately describes the experimental dependence in question; the reliable characteristic viscosity value is 0.102 dL/g. Table 2 lists the solvation energies (in kJ/mol) for the tested FFSs derived from η according to Eq. (3). In solutions containing zirconium oxochloride, there are insignificant electrostatic interactions (ΔG_a = 3.5 kJ/mol) and specific donor-acceptor interactions (ΔG_b1 = -3.64 kJ/mol, ΔG_b2 = 2.60 kJ/mol).

Figure 1. Reduced viscosity ηsp/c for the zirconium oxochloride-based FFS vs. zirconium oxochloride concentration с (g/dL)

Table 1. Viscometric parameters of the zirconium oxochloride-based FFS (calculated from Eqs. (l)-(3)) Equation [η], dL/g k' (1) 0.059 -2.41 (2) 0.137 - (3) 0.102 -

This is due to the high propensity of zirconium to form bulky polymeric complex structures; these structures, which have high characteristic viscosities, are responsible for the good film-forming properties of alcoholic solutions of zirconium oxochloride.

Figure 2. Data of differential thermal analysis а) Zirconium oxochloride powder b) Powder prepared from film-forming solutions based on zirconium oxochloride

We used a set of mutually supplementing methods to study the physicochemical processes involved in the formation of thin films and powders of zirconia from the FFS during heat treatment. Our studies showed that the powder prepared from the zirconium oxochloride-based FFS is thermolyzed differently than ZrOCl₂ · 8H₂O (Fig. 2).

ZrOCl₂ · 8H₂O is thermolyzed as follows:

The thermolysis of the powder prepared from the FFS differs in that it contains the product of zirconium oxochloride thermolysis:

ZrO₂ film formation is a more intricate process, as shown by piezocrystal weighing, IR spectroscopy, and X-ray powder diffraction. This process has the following distinguishing feature: when the solution is applied to the substrate, chloride ions remain in the solution and are not contained in the thin FFS layer on the substrate surface. The solution is anchored to the substrate via the interaction of zirconium hydroxo complexes with the surface hydroxide groups of the substrate as a result of hydrolysis. Schematically, subsequent dehydration in a thin layer can be represented as follows:

The IR spectra of a freshly applied film contain peaks corresponding to zirconium hydroxide and physisorbed water (Table 3). Film adhesion to the substrate is low; the refractive index is about 1.6, which is not characteristic of zirconia (Fig. 3). The refractive index of the film systematically increases during heat treatment as temperature increases, while the film thickness decreases to reach a steady-state value at 400℃ (Fig. 3). The values of 2.0-2.1 for the refractive index mean that the film composition corresponds to zirconia.

Table 2. Worksheet for the calculation of solvation energies a b1 b2 ΔGa, ΔGb1 ΔG b2 d1 d2 [η], dL/g ΔGcm kJ/mol kJ/mol 0.145 -0.150 0.107 3.5 -3.64 2.60 0.063 0.686 0.102 2.46

Figure 3. (1) Refractive index and (2) thickness vs. temperature for ZrO2 films

In cases where a powder is formed, zirconia crystallization is observed above 450℃ and accompanied by two exotherms, at 480 and 720℃. The former is due to the transition of the amorphous phase to the cubic and tetragonal phases; the latter, to the transition to the monoclinic phase (Fig. 4).

In cases where a film is formed (with thicknesses up to 100 nm), X-ray powder diffraction detects cubic ZrO₂ even at 400℃. This correlates with the Ostwald phase rule, which says that the phase with the lowest thermodynamic stability is the first to crystallize[8]. IR spectra show that the absorption bands associated with the vibrations of water and OH groups disappear as temperature elevates. Table 3 makes it clear that dehydration is accompanied by polymerization and the formation of infinite -Zr-O-Zr- chains whose vibration frequency is about 610-620 cm^-1. About ~400℃, the film is restructured to form a regular crystal structure.

This is made clear by the disappearance of the bands associated with the vibrations of chains and the appearance, instead, of the vibrations of ZrO⁴⁺ tetrahedra at 1415 cm^-1.

Figure 4. Data of X-ray powder diffraction a)ZrO2 powder b) ZrO2 thin 1 – on glass; 2 – on quartz

Table 3. Assignment of IR bands for films and powders annealed at various temperatures

Table 4. Kinetic parameters of stages in the formation of zirconia films and powders Parameter Film Powder Temperature range, К 25-125 - Relative process rate, g/min 3.1 - Activation energy, kJ/mol 36 - Order of reaction 0.7 - Temperature range, К 125-230 110-200 Relative process rate, g/min 2.9 7.8 Activation energy, kJ/mol 54 66 Order of reaction 1.1 2.2 Temperature range, К 230-300 - Relative process rate, g/min 2.6 - Activation energy, kJ/mol 89 - Order of reaction 0.7 - Temperature range, К 300-355 250-350 Relative process rate, g/min 3.9 6.3 Activation energy, kJ/mol 130 157 Order of reaction 1.7 0.6 Temperature range, К 355-375 450-720 Relative process rate, g/min 2.4 7.1 Activation energy, kJ/mol 119 200 Order of reaction 0.9 1.8

The dehydration character is also revealed by the activation energies and orders of reactions for these stages calculated after Metzer and Horowitz[19] (Table 4). The activation energy of the initial stage does not exceed 70 kJ/mol; such values imply that the products are desorbed from the surface. The order of reaction at this stage for films is less than unit; therefore, diffusion processes are dominant. The activation energies of subsequent stages (100-200 kJ/mol) are indicative of the occurrence of chemical processes; the order of reaction is about unity or higher. The examination of the data in Table 4 makes it clear that processes in a thin layer occur at lower temperatures. The activation energies of stages in thin films are lower than in disperse powers. The rate-controlling stage of oxide formation in thin layers is also different, first, because of an insignificant diffusion retardation in thin layers and, secondly, because of a specific condition of the thin surface film and the influence of the surface energy of the substrate on kinetic parameters.

Films in our experiments have good adhesion to glass, quartz, silicon, polycor, and sapphire substrates. These films are insulators with a high bandgap width. Table 5 compiles selected physicochemical properties of the films.

The transmittance spectra of dioxide zirconium films on quartz is defined (Fig. 5). The absorption band of ZrO₂ films is within 220 nm, which allows use it as UV-filters.

One important feature of thin films is the dependence of their fundamental properties not only on the composition but also on the physicochemical parameters of the substrate material. The films on glass or amorphous quartz have the lowest refractive indices because they are amorphous on these substrates. On polycrystal (polycor) and single crystal (quartz) substrates, films in part contain a crystalline phase and an amorphous phase, depending on their formation temperature.

Table 5. Physicochemical properties of ZrO2 films Film property Substrate Glass Quartz Silicon Polycor Sapphire Film thickness, nm 100-120 100-120 40-100 40-100 40-100 Refractive index 1.85 1.93 2.08 2.06 2.08 Dielectric constant - - 4.0 - - Bandgap width, eV - - 5.1 - - Adhesion strength, MPa 0.68 0.84 0.93 0.98 0.94

At high annealing temperatures, polycrystalline films are formed on such substrates, as verified by their higher electrical conductivities and refractive indices[19].

Figure 5. Transmittance spectrum T (%) depending on wave length λ, nm 1-Quartz; 2- ZrO2 thin film on quartz

4. Conclusions

Zirconia thin films with thicknesses of 40-120 nm on glass, single-crystal silicon, quartz, polycor, and sapphire substrates have been prepared from zirconium oxochloride and ethanol FFSs. The physicochemical processes involved in film formation and the phase composition and properties of the films have been studied. The film-forming power of the FFS is controlled by the formation of zirconium hydroxo complexes in the solution; the optimal viscosities of FFSs for the deposition of quality films are within 2.3-3.5 × 10^-3 Pa s. Thermally induced zirconia formation occurs in several stages involving removal of solvolysis products and ZrO₂ crystallization. The films prepared on glass or quartz substrates are amorphous; those on silicon, polycor, or sapphire have a crystal structure depending on the annealing temperature and film thickness. The resulting ZrO₂ films have refractive index indicator 1,86 – 2,08, are insulators, with high indicators of bandgap width 5,0 – 5,2 eV, absorption edge is limited by 220 nM, which allows to use it as reallot light covering. ZrO₂ films have high refractive indices, are insulators, and have good adhesion to substrates.