Ba0.7Sr0.3powders with B2O3 additive prepared by the sol-gel method for use as-microwave material

Ferroelectric films based on barium strontium titanates have been widely investigated in microwave devices, such as tunable filters and phase shifters. Recent trends include integration of ferroclectric layers into multilayer dielectric modules by a tape casting procedure. The sintering temperature of BST prepared by the conventional oxide mixing method is, however, very high (1350degreesC), and only platinum or refractory metals can be used for conductors and electrodes in the co-sintering process of tapes. Decrease of the sintering temperature of BST below 1000degreesC would result in good compatibility of the ferroclectric layers with embedded electrodes made of highly conductive metals, such as silver. The effects of B2O3 addition and powder properties on the sintering behavior and the dielectric properties were investigated. The sol-gel method was used to prepare all powders because of its high purity and homogeneity control and its ability to produce fine powders. The BET results showed that all powders had high specific surface areas, but in dilatometric studies, only boron addition produced the desired shrinkage and dense microstructure at low temperatures (< 1000degreesC). This was also verified in sintered microstructures. The XRD showed an appearing of two secondary phases caused by boron addition. This decreased permittivity and thus set a limit on boron addition. (C) 2002 Elsevier Science Ltd. All rights reserved.