Tunable composites with low permittivity for microwave applications
Synthesis and Properties of Functional Materials
Final Report Abstract
This project as a whole has intended to further the understanding and performance of tunable dielectric materials. For this, dielectric/paraelectric composites were investigated by means of simulation and experimental work. The particular focus of this project was the influence of microstructuring of the two composite materials on the dielectric properties. For the experimental part, BST and MBO were chosen as paraelectric and dielectric, respectively. This was done due to their favourable dielectric properties, namely high tunability in the case of the BST and low dielectric loss in the case of the MBO. This system was studied in regard to the interaction during sintering. It was found that titanium migrates from the BST into the MBO in interface regions. This is of importance, since the titanium did not seem to be substituted in any way, while being majorly responsible for the tunability of the BST, indication a certain passivation in the interface regions. Therefore, structuring processes require a robustness of the BST phase, as too filigree networks might become completely passivated and not show great tunability. At the same time, this effect can be utilized, since the intended increase of tenability relies on changing canal widths of the BST. The actual structuring was done with a two-step granulation process. This proved advantageous, since the thickness of the BST layer could be varied this way, ensuring a sufficiently thick active canal. The structured samples were characterized in regard to their microstructure and dielectric properties, leading to a measureable clustering of MBO and an according formation of a BST network. This was confirmed in the high frequency measurements, which showed a significant increase in tunability when compared to conventional composites, while only featuring a slight increase in dielectric loss, resulting in an increase of the ratio of the two. The structuring process therefore led to an increase of performance overall, making it highly attractive for tunable dielectric composites. Regarding the ternary composites, various conductive phases were investigated. Silver composites showed high dielectric losses due to the high necessary sintering temperatures of the BST/MBO composite. Metal and ceramic conductive materials showed heavy degradation or secondary phase formation during sintering. The simulation analysis partially explains the experimental phenomenon, as far as untuned relative permittivities are concerned with addition of second and third phases in BST in the form of MBO and metal. But still the tunabilities and dielectric losses effect are still need to be investigated in the simulation environment. Also, acoustic resonance phenomenon has to be established in the simulation environment to have insights about the origination of such resonances. Depending upon funding in the future, the focus will shift on making efforts on expanding the simulation environment vigorously.
Publications
- NDK and HDK composite networks for optimization of dielectric behavior. 44th International Conference and Expo on Advanced Ceramics and Composites, Daytona Beach, 2020
K. Haeuser, P. Agrawal, R. Jakoby, H. Maune, J.R. Binder
- NDK and HDK composite networks for optimization of dielectric behavior. Electroceramics XVII, Darmstadt, 2020
K. Haeuser, P. Agrawal, R. Jakoby, H. Maune, J.R. Binder
- Sintering behavior and electrical properties of the paraelectric/dielectric composite system BST/MBO. Journal of the European Ceramic Society 2021, 41, 7022-7028
K. Haeuser, R. Azmi, P. Agrawal, R. Jakoby, H. Maune, M. J. Hoffmann, J. R. Binder
(See online at https://doi.org/10.1016/j.jeurceramsoc.2021.07.008) - Suppression of Acoustic Resonances in BST-Based Bulk- Ceramic Varactors by Addition of Magnesium Borate. Crystals 2021, 11, 786
Agrawal, P.; Kienemund, D.; Walk, D.; Matic, S.; Bohn, N.; Häuser, K.; Fink, T.; Abrecht, M.; Bigler, W.; Binder, J.R.; Jakoby, R.; Maune, H.
(See online at https://doi.org/10.3390/cryst11070786)
