Processing and microstructure of lead-based and lead-free complex perovskites in bulk, thick and thin film form for  electrocaloric applications

The electrocaloric effect (ECE) in ferroelectric materials has been recognized for years, but due to its small magnitude it has not attracted much attention until recently. The effect is described as a reversible temperature change in a material under an applied electric field at adiabatic conditions and it could be used in solid-state cooling devices.

Lead-based relaxor-ferroelectric bulk ceramics, such as xPbMg_1/3Nb_2/3O₃-(1-x)PbTiO₃ (PMN-PT) and (Pb_1-3x/2La_x)Zr_0.65Ti_0.35O₃ (PLZT), exhibit  high values of ECE temperature change (ΔT) at temperatures near the dielectric permittivity vs. temperature maximum. In the case of bulk PMN-PT and PLZT  ΔT  = 2-3 K were obtained at the electric field amplitude up to 90 kV/cm. Due to environmental concerns, lead-free relaxors, such as sodium potassium niobate-strontium titanate, represent a group of materials which could potentially replace lead-based electrocalorics.

In the contribution we summarize the results of the study of various relaxor ferroelectrics based on selected compositions within the PMN-PT, PLZT and lead free xK_0.5Na_0.5NbO₃-(1-x)SrTiO₃ (KNN-STO) solid solutions.  The bulk materials were prepared by conventional or mechanochemically activated solid state synthesis and sintering to high relative density. The PMN-PT thick films were prepared by screen printing the as-synthesized powders in a suitable organic vehicle on platinized alumina substrates. The PLZT thin films were prepared by Chemical Solution Deposition by spin-coating the alkoxide-acetate coating solutions on platinized silicon substrates and rapid thermal annealing.

The materials were characterized in terms of chemical composition, phase composition, microstructure, dielectric and ferroelectric properties. The EC response i.e. the change of temperature ΔT upon application of electric field of bulk, thick and thin films was measured directly by a high resolution calorimeter.

In the contribution we discuss the processing – microstructure  properties relation of the studied lead-based and lead-free solid solutions in bulk, thick and thin film form.