Structural investigations on lead titanate xerogels and aerogels

The preparation of highly porous piezoelectric materials offers a promising route to ultrasonic transducers with low acoustic impedancees suitable for ranging applications or gas flow measurements. Sol-gel derived lead titanate gel were dried at ambient pressure and under supercritical conditions. Electron microscopy, thermogravimetry and dilatometer measurements were made on samples of the prepared materials. Nitrogen adsorption experiments were performed to get information about the microstructure of gel samples that had been annealed for 1 h at temperatures between 110 and 550 deg C. A comprehensive model for the formation of lead titanate gels was established: primary particles are formed in the initial stage of hydrolysis and condense to yield secondary particles which are the building blocks for thr inorganic network of the wet gel. Solvent evaporation at ambient pressure leads to a complete breakdown of this hirarchy. the microstructural features of the alcogels are mainly conse rved throughout supercritical drying in carbon dioxide at 35 deg C, but shrinkage of secondary particles and their rearrangement result in a low mechanical stability of these aerogels. In contrast to the application of carbon dioxide, supercritical drying in isopropanol at 236 deg C promotes interparticle ripening and thus stabilization of the inorganic network. Under these conditions lead titanate aerogels with improved mechanical stability are obtained.