SOL-GEL PROCESSING OF BULK AND THIN FILM LEAD ZIRCONATE

Abstract

This project studied a sol-gel synthesis method to produce bulk and film lead zirconate. Lead nitrate, Pb(NO3)2, was selected as the lead source, and zirconium alkoxide as the zirconium source. The solvent used for zirconium alkoxide was isopropanol, while dilute HNO3 and HCl acids, respectively, were tried as the solvent for lead nitrate. The chelating agent used was acetylacetone. To form the gel solution, Zr(OPr)4, isopropanol and the chelating agent were first mixed together to proportions. Then, lead nitrate and its solvent were introduced into the system, mixed thoroughly for a certain length of time (aging time), and then allowed to dry in petri­ dishes. The dried gels obtained were characterized by means of SEM and EDS, and their annealing behaviors were investigated with the aid of XRD, FTIR and DTA.

Several synthesis parameters were investigated, including the drying speed and thickness of fresh gel, the type, concentration and volume of acids used as the lead nitrate solvent, and the concentration of chelating agent. It was found that the concentration and type of lead nitrate solvent influenced the morphology and the subsequent annealing behavior of the gels, and that gels obtained from 0.lM HCl solvent showed promises. Before calcination, two phases existed in the dried gels, i.e. lead nitrate and the background phase which was equally rich in both zirconium and lead. In the thinner gel pellets, which were dried at a much faster rate, the lead nitrate crystals were smaller and distributed more homogeneously. Lead zirconate was obtained after the powder was calcined at 600°C and above.

Film gels were also prepared via the spin-coating technique. The gel solution was prepared in the same way as described above, except that the aging time used was shorter. The parameters investigated in this case included the aging time, number of coating layer, and concentration of the chelating agent. The substrate used was quartz. Spin coating was achieved by first applying a few drops of the gel solution onto the substrate and then spinned at 2000-4000 rpm for 60s. For multi-layer films, no pyrolysis was carried out between successive layers. The annea.ling behaviors of the film gels were also investigated, up to a temperature of 700°C. The heating rate used was 5°C/min. The results showed that before annealing, the same two phases were formed in the film gels. After annealing, only the pyrochlore structure of lead zirconate was obtained. Our results also showed that the aging of the sol influenced the morphology of the film gel. With ageing, the lead nitrate crystals formed in the film gels were found to be larger and less homogeneous.

The functions of acid solvents and the chelating agent in the synthesis process were discussed. The formation mechanism of Pb(NO3)2 crystals in the gel was explained, as well as measures to reduce their growth. Compared with other synthesis methods, the present synthesis process is simpler and inexpensive. The bulk lead zirconate so produced has a crystallization temperature comparable or lower than that obtained by other workers. As for the lead zirconate film, however, more work is required to understand better and prevent the formation of the pyrochlore phase. Rapid annealing process may be required in this respect.