SYNTHESIS AND CHARACTERIZATION OF ZrO THIN FILMS
659
2
We used viscosity as a measure of the film-forming
η /Ò
sp
power of the solution [12–14]. 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 dur-
ing this period because of the solvation of zirconium
oxochloride and the formation of hydroxo complexes
0.05
[
Zr (OH) (C H OH) (H O) ]Cl [2–4]; as a result, a
4 8 2 5 x 2 16 – x 8
stable sol is formed in the solution. Good films with
reproducible properties can be prepared if the solution
–3
viscosity reaches 2.3 × 10 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:
2
3
c
Fig. 1 Reduced viscosity η /Ò for the zirconium oxochlo-
sp
ride-based FFS vs. zirconium oxochloride concentration Ò
(g/dL).
[
Zr (OH) (C H OH) (H O)16 – x]Cl8
Here, a, b, and d are empirical solvation factors, which
i i
4
8
2
5
x
2
[
Zr (OH) (C H OH) (H O) ]Cl + 4HCl,
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 depen-
dences give numerical values of the characteristic vis-
cosity η. Variation in reduced viscosity of the zirco-
nium oxochloride-based FFS (Fig. 1) can be described
by Eqs. (1)–(3). For this solution, we calculated the
Huggins constant k' and characteristic viscosity from
these three equations (Table 1).
4
12
2
5
x
2
12x
4
[
Zr (OH) (C H OH) (H O)12 – x]Cl
4
12
2
5
x
2
4
[
Zr (OH) (C H OH) (H O)8 – x] + 4HCl,
4
16
2
5
x
2
8
+
[
Zr (OH) (H O)
(C H OH) ]
4
8
2
16 – x 2 5 x
(
8 – y)+
+ yH+.
[
Zr (OH) (H O)
(C H OH) ]
4
8 + y
2
16 – x – y
2
5
x
As a result, the system loses stability because of
coagulation, and the solution viscosity increases dra-
–3
matically and exceeds 3.5 × 10 Pa s. The films pre-
pared from such solutions have nonuniform thicknesses
and low refractive indices. Thus, the film-forming prop-
erties 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 zirco-
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. Com-
paring the variance and correlation coefficients for the
three equations, Eq. (3) most adequately describes the
experimental dependence in question; the reliable char-
acteristic 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 con-
taining zirconium oxochloride, there are insignificant
nium oxochloride-based solutions for preparing ZrO
films are (2.3 to 3.5) × 10 Pa s.
2
–3
The zirconium oxochloride-based FFS being a solu-
tion based on a polymeric hydroxo complex, Kolen’ko
et al.’ approach [8] 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 vis-
electrostatic interactions (∆G = 3.5 kJ/mol) and spe-
a
cific donor–acceptor interactions (∆G = –3.64 kJ/mol,
b1
∆
G = 2.60 kJ/mol). This is due to the high propensity
b2
of zirconium to form bulky polymeric complex struc-
tures; these structures, which have high characteristic
cosity η /Ò as a function of concentration, we used a
sp
linear extrapolation equation, namely, Huggins’s equa-
tion
Table 1. Viscometric parameters of the zirconium oxochlo-
ride–based FFS (calculated from Eqs. (1)–(3))
2
η /c =[η]a + k'[η] c,
(1)
sp
Equation
1)
(2)
3)
[η], dL/g
k'
and nonlinear extrapolation equations, namely,
ηsp/c = a + bexp(–dc),
(
0.059
0.137
0.102
–2.41
(2)
(3)
–
–
(
η /c = a + b exp(–d c) + b exp(–d c).
sp
1
1
2
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 54 No. 5 2009