Investigation and structuring of the ceramic composite system barium strontium titanate and magnesium borate

This work investigates the material system barium strontium titanate/magnesiumborate, with a particular focus regarding the occurrences during the sintering process and the dielectric properties. Intially, the structural and dielectric properties of conventional composites (homogeneous mixture) were measured dependant on the composition and sintering temperature. Noteworthy in particular was the observed diffusion of titanium from the BST into the MBO, mostly at the interfaces of the two phases. These processes did already occur at 1050 °C, the typical sintering temperature of the composite. At higher temperatures, a titanium-rich secondary phase formed. The dielectric properties featured a decline of relative permittivity, dielectric loss and tunability with increase of MBO as expected, but particularily around 70 Vol.-% MBO the samples showed a very good tunability to loss ratio. Two different structures were realized. First, layered structures were made, which feature low losses with low MBO content. Their realization proved to be difficult and the reduction of losses could not be realized for thicker intermediate layers. Furthermore, a BST micro network was made via fabrication of core-shell structured spray composites. The aim here was to obtain the formation of thin but percolated BST structures with a high overall MBO content, which is supposed to result in a good tunability to loss ratio. This was succesfully realized with a two step spray granulation process. The tunability of the structured composites surpassed that of the unstructured ones significantly while featuring similar losses. This composite materials is therefore very suitable for application in tunable electrical devices.