Thick PZT films via Sol-Gel Process
The maximum film thickness achievable without cracking via non-repetitive, single-step sol-gel ceramic film deposition is often called "critical thickness." From the viewpoint of industrial production, (1) the small values of the critical thickness (< 0.1 µm in many cases) and (2) the short lifetime of the sols due to increase in viscosity with time are serious drawbacks of sol-gel ceramic thin film deposition. Achievement of thickness of submicron or over by non-cycled gel film deposition and firing is desired especially for PZT films to be applied as pyroelectric infrared detectors, piezoelectric actuators, and so on.
Here we announce the single-step deposition of PZT via "PVP-assisted sol-gel method" (Ref. 9). PZT films as thick as 0.75 µm could be prepared via single-step deposition from a Pb(NO3)2 - Zr(OC3H7n)4 - Ti(OC3H7i)4 - CH3COCH2COCH3 - n-C3H7OH - CH3OC2H4OH solution containing polyvinylpyrrolidone (PVP, average molecular weight 6.3 x 105) (Fig. 1). The gel films were deposited on silica glass substrates by spin-coating, and then were heat-treated successively at 80°, 300° and 700°C, the last step of which was conducted in flowing oxygen. The gel film deposition and heat-treatment was not cycled, but performed just once. The films were optically transparent, crack-free, and PZT in single phase (Fig. 2), exhibiting P-E hysteresis loop with remanent polarization of 24.6 ± 1.6 °C cm-2 and coercive field of 95.6 ± 9.8 kV cm-1 (Fig. 3). The coating solution was stable in viscosity for over 1300 h at room temperature (Fig. 4).
For these few years, we have demonstrated that inclusion of PVP (Fig. 5) in sols significantly enhances the critical thickness of sol-gel-derived ceramic coating films. Single-layer, crack-free BaTiO3 and PZT films over 1 µm could be obtained via the PVP-assisted sol-gel method. Although we still do not know exactly the details on how and why PVP could enhance the critical thickness, we believe that PVP allows the structural relaxation or plastic deformation of the films, suppressing the increase in tensile stress in films subjected to heat-treatment. Probably the hydrogen bonds between the C=O group of PVP and the OH groups of the metalloxane polymers play an important role, and not only PVP but also polyvinylacetamide, which also has amide groups, has been demonstrated to enhance the critical thickness.
In order to make dense ceramic thin films, the PVP-containing solutions should be transparent and homogeneous. In some cases the compositions of the solutions should be tailored so that the solutions be homogeneous. The ceramic thin films obtained through the PVP-assisted sol-gel method are optically transparent in general, but are not necessarily fully dense. We have recognized, however, that calcination of gel films beforehand at 300 - 400°C, where PVP is thermally decomposed, is effective in increasing the final density of the fired films. The densification through calcination works in BaTiO3 films, where the 1.2 µm thick BaTiO3 film became a denser film of 0.7 µm in thickness.
Before the present announcement, we reported that single-layer 1.7 µm thick PZT films can be prepared via the PVP-assisted route from Pb(CH3COO)2.3H2O - Zr(OC3H7n)4 - Ti(OC3H7i)4 - CH3COOH - n-C3H7OH - CH3OC2H4OH solutions. However, the calcination did not work for densification and rather induced crack formation. Then we replaced Pb(CH3COO)2.3H2O and CH3COOH by Pb(NO3)2 and CH3COCH2COCH3, ending up with the successful densification.
Contact information: Hiromitsu KOZUKA, Ph.D., Associate Professor
Department of Materials Science and Engineering, Kansai University
3-3-35 Yamate-cho, Suita, 564-8680, Japan