This reaction is carried out in a solution, which contains Pb2+, Zr4+ and Ti4+. This solution and base material is kept at high temperature and high pressure. The ratio Zr : Ti of the film can be controlled by the ratio of ions in solutions.
We used the PZT thin film formed by a single process hydrothermal method . The former hydrothermal method needed two different processes, the nuclear process and the nuclear growth process. This reaction was carried out twice, so that the thickness of the film was 10µnm.
The piezoelectric constant d31 of PZT film deposited by the single process hydrothermal method was -24 pC/N. This value is a little larger than one fourth of that of PZT balk material (Zr:Ti = 52:48, d31 = -93.5 pC/N ). However, this will be enough to fabricate transducers.
The piezoelectric constant e31 of PZT of the film was 0.3 C/m2. This value is about one tenth of that of PZT balk (e31 = -3.06 C/m2 ). This value is too small compared with d31 above.
We have fabricated devices using hydrothermally deposited PZT thin films, such as a micro ultrasonic motor, a touch probe sensor for surface shape measuring, and a ear phone for a hearing aid.
 T.Morita, T.Kanda, M.Kurosawa, and T.Higuchi: "Single process to deposit lead zirconate titanate (PZT) thin film by a hydrothermal method", Jpn. J. Appl. Phys., Vol.36, No.5B, pp.2998-2999, 1997.
 B.Jaffe, W.R.Cook, and H.Jaffe: "Piezoelectric Ceramics", Academic Press, London, 1971.