New soft chemical routes to ferroelectric materials

A new soft chemical route has been developed in the first part of this thesis for synthesis of relaxor ferroelec tric, (1 -~)Pb(Mg~~-,Nb~~~)O~-xPbTi0~ [(I -x)PMN-xPT] (x = 0.10, 0.35) ceramics. This polyethylene glycol-based method shows some very interesting features. It allows the sol-gel reactions to be performed at room temperature, eliminating reflux steps usually required in sol-gel processes. Moreover, it does not require excess amounts of lead starting material to compensate for lead oxide loss during sintering as is usually the case in conventional synthesis. The effect of a trio1 molecule, l , l , 1-tris(hydroxy)methylethane (THOME), a known complexing agent for binding together metal ions in solutions, has also been studied. Well-sintered 0.90PMN-O.1OPT and 0.65PMN-0.35PT ceramics were prepared in a single heat-treatment step of the precursor powder. The use of THOME gives excellent dielectric and ferroelectric properties in stoichiometric 0.90PMN-O.lOPT ceramics sintered at 1050 "C. A room temperature dielectric constant, &IRT of -25,000 is observed at 1 kHz in this ceramic which is the highest value obtained so far in ceramics of the same composition. A 5% mol excess of lead in the sol-gel process without THOME improves the formation of the perovskite phase but degrades the dielectric properties. The 0.65PMN-0.35PT ceramics prepared with THOME and stoichiometric amounts of lead, and sintered at 1040 "C show pure perovskite phase and very good dielectric and ferroelectric properties (&lRT >5,000, -27,000, and = 21 p~/cm2). In the second part of this work, pure perovskite ferroelectric SrBi2Ta209 (SBT) ceramics have been synthesized by two new soft chemical techniques: a sol-gel process and a co-precipitation method. The sol-gel derived ceramics show denser and more homogeneous microstructure, and E',,, and P~,RT values (950 and 7.6 p~lcm2, respectively) higher than in ceramics prepared under the same conditions by the coprecipitation method. These values are also higher than in ceramics prepared by solid state reactions. The superior dielectric and ferroelectric properties of the sol-gel synthesized SBT ceramics are attributed to a more homogeneous microstructure with a better distribution of grain orientations, which eliminates the preferential grain orientation along the [00B]-direction.