Dielectric relaxations in ferroelectric ceramics occur at various frequencies, depending on the type of chemical or physical defects. These defects depend on either intrinsic or extrinsic heterogeneities due to special heat treatments (quenching, annealing,…), ionic substitutions, grain size additives, and grain boundary nature. The value of the relaxation frequency f_r increases from about 10² to 10¹² Hz as the scale of the defect phenomenon decreases from microstructure, to nanostructure, to unit-cell, to atomic vibrations. This type of study requires a pluridisciplinary approach involving solid state chemistry, materials science, solid state physics and various industrial aspects. The applications in the area of electronic ceramics are related to the value of f_r. In absorbants, the usable frequency is close to f_r, whereas it is far from f_r in good insulators. In view of numerous experimental examples, a classification is proposed to predict which materials may be suitable for a given application.

Spark plasma sintering (SPS) is an efficient tool to obtain highly densified ferroelectric-dielectric ceramic composites with clean interfaces and tunable properties. Dielectric MgO and ferroelectric ${\mathrm{Ba}}_{0.6}{\mathrm{Sr}}_{0.4}\mathrm{Ti}{\mathrm{O}}_3$ (BST) were combined in two-dimensional multilayer and three-dimensional random powders design. Their unmodified BST Curie temperature proves the suppression of interdiffusion while dielectric losses are below 0.5% and the tunability is 40% at room temperature. The composites and pure BST with similar densities $(>95\%)$ were obtained, owing reliable comparison of their dielectric properties. Such SPS ceramics can be used as experimental input for simulation and are potential candidates for high frequency applications.

International audienceFerroelectrics materials have been tremendously attractive since the 40 s with the discovery of ferroelectricity in metal oxide perovskite materials and more precisely in barium titanate. Due to their high potential for industrial applications, intensive research has been carried out to better understand their behavior and develop processes to produce them. Trying to face the down scaling demand of high quality particles towards the nanometer range, some conventional methods such as the solid state one reach their limits. The development of other processes are thus required and the synthesis in supercritical fluids can be considered as a promising alternative. This technology exhibits very interesting characteristics such as fast continuous synthesis (few seconds) of high quality nanoparticles (well crystallized nanoparticles with narrow size distribution) with controlled composition (Ba1−xSrxTiO3 with 0 ⩽ x ⩽ 1) at intermediate synthesis temperatures (<400 °C) with the use of non-toxic solvents (water, ethanol). Reaching the nanometer size range, the intrinsic properties of ferroelectric materials change compared to the bulk. Consequently a deep study concerning the crystalline structure, the presence of defects and the surface chemistry of those nanoparticles has to be achieved to control their properties for further use in functional devices

We report a large d31 piezoelectric coefficient and corresponding electromechanical coupling factor, Kp, of 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BCTZ50) and 0.68Ba(Zr0.2Ti0.8)O3–0.32(Ba0.7Ca0.3)TiO3 (BCTZ32) lead-free piezoceramics. The piezoelectric coefficient, d31, reaches a high value of 200 pC/N for BCTZ50 at room temperature which is comparable to the one of the soft PZT. This confirms the previously reported d33 for the same material. A useful way to achieve such performances at the expense of a smaller thermal budget is suggested, enabling better control of the ceramics composition and microstructure. Based on pyroelectric and ferroelectric hysteresis loops measurements, we show that such outstanding properties are likely due to the high flexibility of polarization under thermal and electric stresses.

A systematic study of the dielectric relaxation phenomenon has been carried out for ceramics with compositions derived from barium titanate in the frequency and temperature ranges 1 MHz–1 GHz and 250–500 K, respectively. Whatever the composition may be, a minimum of the relaxation frequency and a maximum of the dielectric dispersion appears at each phase transition temperature. The dipole-type relaxation is correlated to coherent displacements of the ions in the octahedron sites ordered either along chains or in 3D-lattice.