Design of New Tunable Ferroelectric Composites.

Non-linear dielectrics which display electric-field dependent dielectric properties can be used in microwave devices such as tunable resonators or phase shifters. The required performances are essentially moderate permitivity (100-1000), low dielectric losses (≤1%) in the GHz frequency range, good thermal stability and high tunability of the dielectric susceptibility (>10%). The mixed perovskite Ba 0.6 Sr 0.4 TiO 3 [BST] is an attractive candidate as its ferroelectric transition occurs close to room temperature and it displays high tunability but also strong temperature sensitivity and high frequency dielectric losses. To overcome these two deficiencies, ceramic/ceramic composites based on BST and a non-ferroelectric oxide such as MgO, MgTiO 3 , SnO 2 and SiO 2 were synthesized and compared to BST. For a better control of the losses a new concept based on a core-shell principle is presented. Through soft chemistry routes, a core made of BST grains is coated by a simple binary oxide shell. Modulation of the dielectric properties is expected by changing the core-shell structure. High frequency experiments are underway in order to check the GHz losses of these composites. [ABSTRACT FROM AUTHOR]