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Thick PZT films via Sol-Gel Process

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.

Fig. 1 SEM picture of the PZT film prepared via single-step deposition.	Fig. 2 XRD pattern of the PZT film.

Fig. 3  P-E hysteresis of the 0.75 mm thick PZT film.	Fig. 4  Viscosity-time relation of the PZT precursor coating solution that was kept standing at room temperature.

Related works:

1. H. Kozuka and M. Kajimura, "Achievement of crack-free BaTiO_{3 films over 1 mm in thickness via non-repetitive dip-coating," Chem. Lett., 1029 (1999).}

2. H. Kozuka, M. Kajimura, T. Hirano and K. Katayama, "Crack-free, thick ceramic coating films via non-repetitive dip-coating using polyvinylpyrrolidone as stress-relaxing agent," J. Sol-Gel Sci. Techn., 19, 205 (2000).

3. H. Kozuka and M. Kajimura, "Single-step dip-coating of crack-free BaTiO3 films > 1 mm thick: Effect of poly(vinylpyrrolidone) on critical thickness," J. Am. Ceram. Soc., 83, 1056 (2000).

4. H. Kozuka, M. Kajimura, K. Katayama, Y. Isota and T. Hirano, "Deposition of Crack-free BaTiO3 and Pb(Zr,Ti)O3 films over 1 mm thick via single-step dip-coating," Mat. Res. Symp. Proc., 606, 187 (2000).

5. H. Kozuka, Y. Isota, A. Higuchi and T. Hamatani, "Cracks in gel-derived ceramic coatings and thick film formation," Proceedings of International Symposia on Materials Science for the 21th Century, The Society of Materials Science, Japan, Suita, Japan, May 21-22, 2001, vol. B., pp. 122-125.

6. H. Kozuka, K. Katayama, Y. Isota and S. Takenaka, "Achievement of crack-free ceramic coatings over 1 mm in thickness via single-step deposition," Ceramic Transactions, Volume 123, "Sol-Gel Commercialization and Applications," ed. by X. Feng, L.C. Klein, E.J.A. Pope, and S. Komarneni, The American Ceramic Society, Westerville, 2001, pp. 105-110.

7. H. Kozuka, Y. Isota and M. Hosokawa, "Single-step deposition of crack-free, thick ceramic films aided by polyvinylpyrrolidone," Ceramic Transactions, Volume 112, "Ceramic Processing Science VI", ed. by S.-I. Hirano, G.L. Messing, and N. Claussen, The American Ceramic Society, Westerville, 2001, pp.335-340.

8. H. Kozuka and A. Higuchi, "Single layer submicron thick BaTiO3 coatings from PVP-containing sols: gel-to-ceramic film conversion, densification and dielectric properties," J. Mater. Res., 16(11), 3116 (2001).

9. S. Takenaka and H. Kozuka, "Sol-gel preparation of single-layer, 0.75 mm thick lead zirconate titanate films from lead nitrate-titanium and zirconium alkoxide solutions containing polyvinylpyrrolidone," Appl. Phys. Lett., 79 (21), 3485 (2001).

10. H. Kozuka, S. Takenaka, H. Tokita, T. Hirano, Y. Higashi and T. Hamatani, "Stress and cracks in gel-derived coatings and thick film formation," J. Sol-Gel Sci. Techn. (Special Issue, Proceedings of 11th International Workshop on Glasses, Ceramics, Hybrids and Nanocomposites from Gels, Abano Terme, Italy, Sept. 16-21, 2001), in press.

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