This review gathers detail on the processing of piezo‐ferroelectric ceramic materials by spark plasma sintering for the first time. The results reported here clearly indicate that it is a powerful technique and opens the possibility of processing ceramics with controlled sub‐micron or even nanoscale grain sizes.

Ba 1-x Sr x TiO 3 (x ) 0, 0.25, 0.5, 0.75, and 1) nanocrystalline powders were prepared by mechanosynthesis in a planetary mill, from stoichiometric mixtures of BaO 2 , SrO, and TiO 2 at room temperature. Evolution during the mechanical treatment and subsequent annealings was investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Formation of the solid solution between BaTiO 3 and SrTiO 3 was observed in the whole range of compositions. The spark plasma sintering (SPS) technique was used to prepare dense ceramics. Densification occurred within a very short time (about 10 min). The combination of mechanosynthesis and spark plasma sintering has been used for the first time to process ceramics of the Ba-Sr-Ti-O system with very high density and homogeneous microstructure, at a temperature 300-400 °C lower than the conventional sintering of Ba 1-x Sr x TiO 3 phases obtained by solid-state reaction. This approach allows grain growth to be controlled and opens the possibility of processing fully dense nanostructured Ba 1-x Sr x TiO 3 ceramics. Dielectric permittivity as a function of temperature was characterized for a series of samples across the solid solution and confirmed that dense, fine-grained ceramics can be processed by this novel approach for all compositions. Ferroelectric hysteresis loops were recorded for BaTiO 3 ceramics with decreasing grain size. The variations of remanent polarization and coercitive field are consistent with the transition from the lamellar 90°ferroelectric domain to grains with only 180°domains.

We have investigated the occurrence of phase-change functional responses in the BiFeO3-PbTiO3 perovskite solid solution, analogous to those anticipated by a recent first-principles study of BiFeO3-BiCoO3. Like the former system, BiFeO3-PbTiO3 shows a morphotropic phase boundary (MPB) between multiferroic polymorphs of rhombohedral and tetragonal symmetries. MPB BiFeO3-PbTiO3 is a high temperature ferroelectric with the phase transition around 900 K, and a room temperature square-shape hysteresis loop with remnant polarization as high as 62 μC cm−2. Strain under the electric field was studied, and a phase-change response was found. Analogous magnetoelectric effects are expected from the multiferroic nature of this MPB.

Among the recently investigated magnetoelectric materials, those belonging to the solid solution xBiFeO 3 -(1 À x)PbTiO 3 (xBF-(1 À x)PT) seem to be the most promising. However, there is not much information about the characteristics of this system in the whole range of compositions. In this work, the preparation of the system for compositions with 0 # x # 1 is described. The synthesis has been successfully achieved by mechanosynthesis in a high-energy planetary mill. A structural characterization of the mechanosynthesized phases, before and after different thermal treatments, has been carried out by X-ray powder diffraction (XRD), in order to analyze the crystallographic evolution of the phases, correlated to the phase transitions detected by differential thermal analysis (DTA). The structural characterization has allowed the range of compositions of the morphotropic phase boundary (MPB) to be established. First order ferro-paraelectric transitions have been detected by DTA measurements in the whole range of the system and the corresponding Curie temperatures have been determined. Moreover, a comparative microstructural study of the ceramic materials corresponding to the x ¼ 1, 0.7 and 0.6 compositions, prepared by two different processing methods (conventional sintering and spark plasma sintering (SPS)), is reported.