Nanostructured ferroelectric and dielectric ceramics. Processing and properties

Many electronic devices, such as multilayer ceramic capacitors, microwave resonators and filters, ferrites cores, antennae, actuators, NTC and PTC thermistors, contain functional ceramics as key components. Devices have continued to evolve towards increasing performances (higher capacitance, lower dielectric losses, better temperature stability, improved tunability, etc.) and smaller size (miniaturization). As an example, high-end capacitors have now dielectric layers with a thickness <1 micron and grains of 100-200 nm and, consequently, their processing requires very fine powders and controlled sintering. The aim of this lecture is to give some examples about the synthesis of BaTiO3 and SrTiO3-based particles, their sintering in dense ceramics and the influence of grain size and microstructure on functional properties. Particles with size of 10-100 nm and narrow size distribution can be produced by means of solution-based synthesis methods, including precipitation, hydrothermal and solvothermal synthesis, using relatively cheap inorganic precursors such as BaCl2, SrCl2 and TiOCl2. Particle size and morphology can be tuned by controlling temperature, concentration and solvent composition during synthesis as well as the nature of the titanium precursor. Good densification of these particles with limited grain growth can be achieved using rapid sintering methods such as spark plasma sintering. The grain size can be tailored by varying the sintering conditions and the particle size. The temperature and electric field dependence of permittivity of BaTiO3 ceramics as well as the ferroelectric loops are strongly influenced by the grain size. The observed behavior is largely of extrinsic origin and related to the increasing density of low-permittivity grain boundaries with decreasing grain size. The grain boundaries are also responsible for the rapid decrease of thermal conductivity with decreasing grain size observed in La-doped SrTiO3 ceramics. The functional properties can be further controlled through the fabrication of ceramic composites, combining ferroelectric BaTiO3 with paraelectric SrTiO3 or antiferroelectric PLZT. Rapid sintering methods are essential to preserve the morphology of the original particles and avoid complete interdiffusion between the components while achieving a good densification. Although thermodynamically unstable, the final materials are kinetically stable and show high, temperature independent permittivity and reasonable linear tunability.